CA2141561A1 - Thin film gas sensor and method of fabrication thereof - Google Patents
Thin film gas sensor and method of fabrication thereofInfo
- Publication number
- CA2141561A1 CA2141561A1 CA 2141561 CA2141561A CA2141561A1 CA 2141561 A1 CA2141561 A1 CA 2141561A1 CA 2141561 CA2141561 CA 2141561 CA 2141561 A CA2141561 A CA 2141561A CA 2141561 A1 CA2141561 A1 CA 2141561A1
- Authority
- CA
- Canada
- Prior art keywords
- thin film
- gas sensor
- recited
- ionic conductor
- heating element
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Abandoned
Links
Classifications
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
- G01N27/26—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating electrochemical variables; by using electrolysis or electrophoresis
- G01N27/403—Cells and electrode assemblies
- G01N27/406—Cells and probes with solid electrolytes
- G01N27/407—Cells and probes with solid electrolytes for investigating or analysing gases
- G01N27/4071—Cells and probes with solid electrolytes for investigating or analysing gases using sensor elements of laminated structure
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
- G01N27/26—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating electrochemical variables; by using electrolysis or electrophoresis
- G01N27/403—Cells and electrode assemblies
- G01N27/406—Cells and probes with solid electrolytes
- G01N27/407—Cells and probes with solid electrolytes for investigating or analysing gases
- G01N27/4073—Composition or fabrication of the solid electrolyte
- G01N27/4074—Composition or fabrication of the solid electrolyte for detection of gases other than oxygen
Abstract
2141561 9428403 PCTABS00034 An integrated monolithic gas sensor comprises a substrate and thin films deposited on this substrate. The thin films include a thin film electrically conductive heating element, a thin film conductive reference electrode, and a second thin film conductive electrode, these electrodes and heating element being electrically isolated from each other. A thin film ionic conductor and a thin film reactive gas sensitive layer are placed between the reference electrode and the second conductive electrode to form an electrolytic cell in which an electrolytic reaction including as reagent the gas to be detected produces between the two conductive electrodes an electromotive force indicative of the concentration of the gas. Also deposited is a micro-thermometer formed of a thin film wire having a temperature-dependent resistance. When the gas to be detected is CO2, the ionic conductor may comprise NASICON of formula Na3Zr2Si2PO12, and the reactive gas sensitive layer may comprise Na2CO3.
Description
2 t ~ 1 PCT/CAg4/00312 THIN ~ AS SENSOR AND :.
,~' .
. METHOD OF FABRICATION THEREF ;~ ~
lo BACKGROUND OF THE INVENTION ,.-~
. .
1. Field of the invention:
~
The present invention relates to an -integrated monolithic thin film gas sensor capable of continuously detecting and monitoring low concentration of gas with a sensitivity of 100 p.p.m.
; 20 or less. The present invention further relates to a ~;
method of fabricating this sensor, with thin film fabrication techniques.
2. Brief descr~tion of the Prior art: ~
`
Many techniques have been proposed for detecting concentrations of gases such as CO2 in the air. ~or example, spectroscopic techniques including infrared and photoacoustic spectroscopies carry out ~`~
detection through electronic transitions produced in , the gas ~when illuminated. ~nother~ ex~mpl~ is cQncerned with an electrochemical cell provided with an electrode chemically sensitive to the gas to be ~`
sensed. The gas sensors that ha~e been constructed on ;
` 35 ~the basis of ~hese prior art techniques are bulky, :
: ::
fraglle, not integrable with the electronic circuit -~
~ through whlch they are controlled, and, generally, .
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W094/28407i PCT/CA9410031~
2l~56~ I
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they are not adapted to inexpensive mass production.
In particular, spectroscopic sensors are as a whole I expensive and quite fragile.
¦ 5 Another prior art gas senso~ is described in U.S. patent N 4,668,374 granted to Bhagat et al. on l May 26, 1987. More specifically, this patent discloses a fast response gas sensor fabricated through microelectronics technology to form multiple thin film solid-electrolyte pump and sense cells within a hermetically sealed sensor cavity. This prior art sensor requires an airtight chamber which must be free of any chemical and any source of current leakage. The electrolyte is in the form of a membranej and the response time is related to the ! thickness of the electrolyte, and thus to the thickness of the membrane. To obtain very fast response times, the membrane of the ~electrolyte must be so thin that it suffers from mechanical integrity;
the resulting membrane is fragile and can rupture easily in normal service to render the sensor ; inoperative. Moreover, the sensor of~Bhagat et~a1. i6 designed to be used in an ambient~temperature of 300 ~C, and comprises an ionic conductor which, at that temperature, has a sufficient conductivity and does not suffer from interference due to water adsorbed at the su~face of the sensor.~ Therefore,`a dr!awbaibk ofi`
the sensor of Bhagat et al. is that it cannot be~used at room temperature;. Also, noth~ing is disclosed ~ regarding methods to increase sensltivity of the gas detection, ~or to integrate the ~sensor with an ~` ~ electronic control c1rcuit.
WO 94128403 PCT/CA94/0031~ ~
21~1~61 , ;:
,~' .
. METHOD OF FABRICATION THEREF ;~ ~
lo BACKGROUND OF THE INVENTION ,.-~
. .
1. Field of the invention:
~
The present invention relates to an -integrated monolithic thin film gas sensor capable of continuously detecting and monitoring low concentration of gas with a sensitivity of 100 p.p.m.
; 20 or less. The present invention further relates to a ~;
method of fabricating this sensor, with thin film fabrication techniques.
2. Brief descr~tion of the Prior art: ~
`
Many techniques have been proposed for detecting concentrations of gases such as CO2 in the air. ~or example, spectroscopic techniques including infrared and photoacoustic spectroscopies carry out ~`~
detection through electronic transitions produced in , the gas ~when illuminated. ~nother~ ex~mpl~ is cQncerned with an electrochemical cell provided with an electrode chemically sensitive to the gas to be ~`
sensed. The gas sensors that ha~e been constructed on ;
` 35 ~the basis of ~hese prior art techniques are bulky, :
: ::
fraglle, not integrable with the electronic circuit -~
~ through whlch they are controlled, and, generally, .
;'`. . . .
!~:, ~, ~
'~`. . ' '',~`.
W094/28407i PCT/CA9410031~
2l~56~ I
.!
they are not adapted to inexpensive mass production.
In particular, spectroscopic sensors are as a whole I expensive and quite fragile.
¦ 5 Another prior art gas senso~ is described in U.S. patent N 4,668,374 granted to Bhagat et al. on l May 26, 1987. More specifically, this patent discloses a fast response gas sensor fabricated through microelectronics technology to form multiple thin film solid-electrolyte pump and sense cells within a hermetically sealed sensor cavity. This prior art sensor requires an airtight chamber which must be free of any chemical and any source of current leakage. The electrolyte is in the form of a membranej and the response time is related to the ! thickness of the electrolyte, and thus to the thickness of the membrane. To obtain very fast response times, the membrane of the ~electrolyte must be so thin that it suffers from mechanical integrity;
the resulting membrane is fragile and can rupture easily in normal service to render the sensor ; inoperative. Moreover, the sensor of~Bhagat et~a1. i6 designed to be used in an ambient~temperature of 300 ~C, and comprises an ionic conductor which, at that temperature, has a sufficient conductivity and does not suffer from interference due to water adsorbed at the su~face of the sensor.~ Therefore,`a dr!awbaibk ofi`
the sensor of Bhagat et al. is that it cannot be~used at room temperature;. Also, noth~ing is disclosed ~ regarding methods to increase sensltivity of the gas detection, ~or to integrate the ~sensor with an ~` ~ electronic control c1rcuit.
WO 94128403 PCT/CA94/0031~ ~
21~1~61 , ;:
3 `-OBJECTS OF THE INVENTIQN
,,,., ;.
~,An object of the present invention is .~.
~5 therefore to overcome the above discussed drawbacks .:;.
!and to provide an integrated monolithic thin film gas .: .
Isensor which is free o~ hermetic "referencel' cavity .and is mechanically robust.
:, .
10Another object of the invention is tG
provide an integrated monolithic thin film gas sensor which is compatible with mass production.
; A third object of the present invention is to provide a monolithic thin film gas sensor : comprising an integrated thin film heating element for heating the sensor and improve the detection performance while reducing the susceptibility to interference from gases other than the target one.
A fourth object of the invention is to provide a monoli~hic thin film gas sensor comprising an integrated thermometer ~o enable measurement and control o the temperature of operation of the sensor.
A further object of the present invention j1 '! is to ,provide integrated monol`ithic t~in !film! gas sensors which can be connected in cascade to improve sensi~ivity of gas detection.
. 30 :
A sixth object~of the subject invention is to provide:an integrated monol.ithic gas sensor . ~ comp~ising a thin ~ilm ionic conductor, and a "dry"
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W094/~8403 PCTICAs4l00312 2~ 6~
~` fabrication techni~ue for producing this thin film ~i~ ionic conductor having an improved ionic canducti~ity.
~;
A still further object of the invention is to provide a thermal process which significantly improves ionic conduction of the thin film solid state .-electrolyte.
,, SllM~RY OF THE INVENTIQN
:' .
Nore specifically, in accordance with the -:
present invention, there is provided a monolithic gas 15 sensor comprising first and second electrodes each. ~;
made of a film of electrically co~ductive material, a reactive layer made of a film of material sensitive to ~;
the gas to be detected, and an ionic conductor being .
under the form of thin ~ilm to efficiently conduct ..
ions therein. The~first and second electrodes, the ::
reactiv~ layer, and the thin film ionic conductor are : .
deposited on ea~h other to ~orm an electrolytic cell in which a chemical reaction involving as reagent the : -, . . .
- gas to be detected produces an electromotive force 25 betwèen the first and second electrodes. ~ : -In accordance withia pre~erred embodlmentl : o~ the i~vention, the monolithic gas sensor comprises: ~ :
a:;substrate;~
~ a thin film~ heating e:lement made: of ~ ~ ' ~: electrically conductive ;material deposited on the . ~ ~: ` substrate;
, :
~:
i a thin film of dielectric material , deposited on the thin film heating element;
a reference electrode consisting of a thin ~:
film of electrically conductive material deposited on .:
¦ 5 the thin ~ilm of dielectric material;
~ a thin film ioniG conductor deposited on :~
! the thin film referenc~ electrode;
a reactive gas sensitive layer consisting `~
of a thin film of reacti~e gas sensitive material :~
deposited on the thin film ionic conductor;
a second electrode formed of a thin film ~
of electrically conductive material deposited on the : :
thin film reactive gas sensitive electrode; and ~:
a thin film micro-thermometer consisting ;~
of a portion of the thin film of electrically conductive material constituting the reference electrode, the thin film portion forming the micro-thermometer having a temperature-dependent resistance .
than can be converted to temperature. ,;
;~:
In accordance with another preferred : embodiment of the present inventiorl, the monolithic gas sensor comprises:
a substrate;
a thin film of dielectric material deposited on the substrate; :
' a:thin film~of electrically conductive' ~::
material deposited on the thin film of dielectric - ~
i material and divided into four separate thin film ~;
¦ 30 portions respectively forming (a) a thin film ..
¦ reference electrode, (b) a second electrically I conductive elec~rode, ~c) a thin film heating elementJ
and ~d) a thln film microthe~mometer;
` : ' '1 ' ; W094/28403 PCT/CA94/00312 2~4i5 ..
., ' . ~.
a thin film ionic conductor deposited onto ';
the thin film reference electrode; and a reactive gas sensitive thin film layer deposited on the second electrical;ly conductive ¦ 5 electrode and on the thin film ionic ~conductor.
When the gas to be detected is CO2, the thin film ionic conductor may comprise NASICON of ``~
formula Na3Zr2Si2po12, and the reactive ~as sensitive 10 thin film layer may comprise Na2CO3.
The monolithic gas sensor in accordance with the subject invention may comprise a plurality of x said elPctrolytic cell mounted in cascade in order to ~;
15 improve gas detection sensitivity of the sensor.
The present invention further relates to a method of fabricating a monollthic gas s~ensor, comprising the step of depositing onto a substrate (a)~
20 first and sè~ond thin film electrodes màde of electrically~conductive material, (bj a reactive thin film layer made of material sensitive to the gas~to~be detected, and~(c~) an ionic~conductor under the~form of thin film to~effici~ently conduct ions therein,~wherein 25~ the dép~siting step ~comprises stacking the first an~d~
second thin film electrodes! the reactive thin ~ilm i layer,, ahdlthe thin film lonic cond~!ctor to fo~m a~s ~ electrolytic~ cell ~in which a chemi~cal reactlon~
;; ~ `in~olving as~reagent the~gas to be~detected produ~ce~an ~- ~ 3;0 ~ el~ectromotive force~between the first and se~ond thin~
~` ~ film~eleatrodes~
WO 94/28403 2 1 ~1 5 fi 1 PCT/CA94/00312 ~ .
::
.
; In accordance with a preferred embodiment :~
. of the invention, the method of fabricating a monolithic gas sensor further comprises the step of depositing on the substrate a thin film heating element made of electrically conductive materiaI, and the step of depositing on the substrate a micro-thermometer made of a thin film of electrically conductiYe material having a temperature-dependent resistance which can ~P converted to temperature.
, Patterning of the first and second thin film electrodes, the reactive thin film layer, the thin film ionic conductor, the thin film heating element, and the thin film micro-thermometer can be .
carried out by means of proximity masking during the thin film deposition process or by means of laser ablation following this thin film deposition process.
In accordance with another preferred `
~0 embodiment of the method of fabricating a monolithic -~
gas sensor, rapid thermal annealing is carried out on the thin film ionic conductor to favor crystalline ;~:~
microstructure formation and thereby improve ionic conductivity of this thin film ionic conductor.
.
The objects, advantages and other features of the present:invention will become more apparent upon reading of the following non restrictive :~description of preferred embodiments thereof, given by j30 way of example ~nly with~reference to the accompan~ing . drawings. ~ :
.
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'W094/28403 PCT/CA9410031~
2 ~ $ 6 ~ !
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BRIEF DESCRIPTION OF THE DRAWINGS
,."...
In the appended drawings: ..
" `
Figure l is a side elevational view of a `"
first preferred embodiment of an integrated monolithic ; ;
thin film gas sensor in accordance with the present `:~.
invention, capable o~ sensing CO2 ~and comprising I0 integrated thin film ionic conductor, heating eIement and micro-thermometer: .
. .....
Figure 2 is a top plan view of the thin ;-~
film gas sensor of Figure l; ~, :-~
Figure 3 is a side elevational, cross ~ :
. sectional view, taken aIong line 3-3 of Figure 4,~ of a ~second preferred embodiment of the integrated j, monolithic thin film gas sensor in accordance with the :present inventi~n, capable of sensing CO2 and including integrated thln film ionic conductorl heating element . and micro-thermometer~
F}gure 4: is a top plan view of the;th1n film gas sensor of Figure 3; and ~, ,..;
Figure 5 ~is a side elëva~ional !.~iew showing two qas~sensors as:illustrated in Figures:3 and 4:mounted in::cascade. ~
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':
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¦_ETAILED DESCRIPTION OF THE PREFERRED EM~ODIMENTS
.,~ ';',' ! `
The first preferred embodiment of the 15 integrated monolithic thin fiIm C2 s~nsor in ¦accordance with the present invention is generally !identified by the re~erence 11 in Figures 1 and 2 of the appended drawings.
As illustrated in Figure 1, the sensor 10 comprises a stack of six thin films (see 12-17) , deposited onto a substrate 11.
:- .
Su~strate 11:
The substrate 11 is made of a piece of crystalline silicon, glass, metal, ceramic, or plastic material capable of resisting to high temperatures.
Fabrication and operation of the thin film CO2 sensor 10 requires a substrate 11 of which~the material i5 resistant to temperatures higher than 200 C, to water and water vapour, to any organic or inor`ganic chemical contaminants, and to ~ust accumulation.
.
Ideally, the elastic constants (Young's modulus and Poisson's ratio) and the thermal expansion coefficient of the material constitutin`g the~subs~rate 11 are ~early those of silicon ~or quartz to a~oid --cracking due to temperature cycling and ultimately problems related to~reliability and short-lifetime of the sensor 10. ~ ~-, ~:
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W094/28403 PCT/CA94/00312 ``
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~Heatina element 12 `.,1 . .
The first thin film 12, deposited on the -~
~top surface l8 of the substrate ll, is made of metal i5 such as tungsten or of highly conductive silicon to form a thin film heating element Hëating element 12 ,;`j is an electrically conductive thin film supplied with a current I The thickness, width and length of the ~~
thin film heating element 12 are adjusted to obtain a io resistance R adapted to an external current source `;
supplying the current I Power (I2R) is dissipated~in the resistive heating element 12 by the Joule ~ffect to thereby heat the sensor 10 ; 15The sandwich geometry of the thin film sensor lO of Figures l and 2 reguires no electrical insulation between the heating element 12 and the `~
substrate l1 as long as the material constituting ~his substrate 2 is not substantially conductiv~; it is ~he 20 case for a substrate ll made of~ silicium, glass, ; ,~
plastic or ceramic material When the substrate ll is made of metal and is therefore electrically conductive, an insulating thin film (not shown in Figures 1 and 2)~is required to prPvent the heating ; 25~ current, normally flowing through the heating element 12, to deviate in the substrate 1l Obviously, the~
? I ! curren~ flowing~through the isubst~ate lll doés~no~' contribute in heating the sensor 10 ~Thin film Qf die1ectric~materia1~13~
: The second thin film 13, deposited on the ~ -thin f llm beaclng element 12, cons~sts of dieleceric WO 94l284~3 21~1 S 61 pcrlcAs4loo3l2 J material such as SiO2 t~ electrically insulate the thin film heating element 12 from the thin film reference ' electrode 14. Other insulating dielectric materials, j for example Si3N~ could also be used as the material e 5 of the thin film of dielectric material 13.
.
Other functions of the thin film of dielectric material 13 are to ensure go~d adhesion between the thin film heating element 12 and the 10 su~se~lent thin films 14-17, and to provide a diffusion or alloying barrier between the metallic heating element 1~ and the metallic reference electrode 14. The thin film 13 should ha~e yood thermal properties, in particular a good thermaI
15 expansion coefficient, and should be chemically neutral so as not to react with gases to which the sensor 10 is exposed and so as to protect the substrate 11 from these gases.
20 Reference electrode 14~
The reference electrode 14 is metallic and formed of a thin film of pure metal such as platinum or other noble metal deposited on the thin film~13 of 25 dielectric material. As will be seen in the foIlowing description, the metallic thin film 14 is also used to form the integrated micro-thermometèr.
As the function of the thin film reference 30 metal~lic electrode 14 is to collect electric current, ` its thickness must be ~sufficient to make this electrode continuous (with no electrical interruptlons) and conductive with a series resistance .
;
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~ 12 .
smaller than 10 n. The thin film reference electrode i 14 should not contact any other metallic thin film of ~ the sensor 10, including the second metallic electrode ~`:
', 17 and the heating element 12.
Ionic conductor 15 !
A further thin film 15 of NASICON (~a :;
8uper lonic Conductor), of formula Na3Zr2Si2POt2, is 10 deposited on the thin film reference electrode 14 to .~
form a thin film ionic conductor. The thin film ionic ~.
conductor 15 may have a homogeneous compositioni, or : may include a plurality of layers which through their composition and/or structure serve to act as passivation, stabilisation and/or diffusion-barrier layers. ~
~ ,,.
Reactive C2 sensitive layer 16~
A further thin film, deposited on the thin ~;
film ionic conductor 15, is made of sodium carbonate .~-;
alone or stabiliz~d by barium carbonate to form a .
reactive thin film CO2 sensitive layer 16.
, 25 Second metallic electrode 17: .~
. .
1 I. ! ` i ` ~ i 1 ' A second metallic electrode 17'is made, i ;~.
. as the rererence electrode~ 14~ of a thin frilm of l . platinum or o~her noble metal deposited on the sodium carbonate alone or stabilized by barium carbonate, of the reactive tbln fil- CO2 sensitlve layer 16, ' : ,~
WO 94/28403 21415 6 1 FC:T/CA~4/00312 13 :
, ~ .
I Inteqrated_micro-thexmometer:
.,~ .
'~ A micro-thermometer 19 is integrated to j the thin film CO2 sensor 10. This micro-thermo~eter 19 ¦ 5 comprises a portion 20 of the thin film of dielectric material 13 deposited onto the thin film heating t element 12, and a portion ~1 of ~he metallic thin film 14 deposited onto the thin film portion 20. As illu~trated in Figure 2, the thin film portion 21 10 ~orms an elongate wire 22 having a temperature- ~
dependent resistance. ~;
:`
Mechanism of detection of sensor 10:
The thin film ionic conductor 15 is a rather porous structure which is rich in sodium ~Na).
In the presence of air which contains about 16~ of oxygen (2) and some humidity, sodium reacts with oxygen to form Na20 at the surface of the thln film -~
ionic conductor 15, especially in the proximity of ~he interface with the "Pt" reference electrode 14, also known as the cathode of the electrolytic cell. Oxygen therefore plays a role in the chemical reaction of concern and is sometimes called the dri~ing species or elem,ent in this chemical rieaction.
Interposed between thé otheri"Pt"l thin~
film metallic electrode 17, or anode, ~nd thie thiin i~ film ionic conduçtor 15 is the thin film CO2 sensitive ;1 30 layer 16 made of sodium~ carbonate (NazCO~) alone or ~
~, ~
stabilized by barium carbonate. The difference in the -;~
electronegativities of the two substances, Na2O and Na2C03, gives~ rise to an electromotive force (EMF) . , .:~
: .
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~, :;
W094/28403 PCT/CA94/0031' 2~ 4~S 6 1 14 `~
'~ '.
,. .
! between the thin film metallic electrodes 14 and 17.
Mobile ions are subjected to the ~ield produced by this electromotive force and drift along the field , lines, thereby generating an ionic current. It should i 5 be noted here that NASICON is electrically insulating, so that electrons stay-put and do not ~ravel in the fieldl but can be collected at the conductive thin film metallic electrodes 14 and 17 and circulated in external electric circuits such as resistive and capacitive loads, voltmeters, ammeters, etc.
Under normal conditions, an equilibrium exists between oxidised sodium, and ionic sodium in the thin film ionic conductor 15 (solid electrolyte).
Sodium ions can also travel easily within the electrolyte by a special mass transport mechanism through the crystalline structure. Sites in the crystal lattice at which only sodium ions can sit are connected together to form paths or channels capable of conducting? the sodium ions. The to and fro coupled rocking of the stacked oxide tetrahedra around zircon or silicon atoms "push" the sodium ions along these paths or channels and contribute to the "super ionic conductivity" referred to in the acronym NASICON ~?~
8uper ~onic Co~ductor).
~ . .
In the presence iof carbon diC??Xide/ a chemical reaction ~ccurs in which sodiu~ oxide is `
"reduced" by the carbon diox1de (CO2). This reduction causes sodium ions to be "liberated" from the oxide, to travel across the NASICON and react to form sodium ~`
.; carbonate (NazCO3j. The ionic transport generates the , ~ , ~;.
:~ ` '.
~.
WOg4/28403 PCT/CA94/00312 21~1S61 ~5 I current and the electromotive force which can be `¦ detected by external circuits.
¦ The sodium ions have such high mobility, and such a short distance (in the thin film 15~ to travel that the reaction is almost instanta~eous. It has been observed in laboratories fast response times of less than a second, this compared to several minutes in bulk sensor devices operating at much 10 higher temperatures. ~-For a CO2 sensitive thin film layer 16 "
made of sodium carbonate and a thin film ionic ;:
conductor 15 made of NASICON, the CO2 detecting ~15 electrolytic cell of the sensor 10 is made of a combination of Na2CO3 and of N~SICON (Na3Zr2Si2PO12) and the CO2 detecting mechanism can therefore be described by the following electrolytic cell: .
",:
20 Pt~ C2 ~ 2 / Na2Co3 // NASICoN / 2~ Pt The anodic reaction is: ~`
Na2CO3 ---> 2 Na + CO2 + ~ 2 ~ 2 e The cathodic reaction is:
2 Na' ~ ~ 2 + 2 e ~--> Na2O
: :
:30 The glo~al reaction is:
~O3 <===>: Na2O + CO
~` ::
.
.
WO941~8~3 PCT/CA94/00312 2 ~ 6 l i .
~ 16 '. ~'.-.
' ~:
IThe electromotive force of this cell is :~
¦measured between the thin film metallic electrodes 14 tand 17 and is proportional to the concentration of CO2 jin the air. ~ `
1 5 ~.` `
¦ The alectrochemistry behindthisprinciple .:
of operation is well established and accordingly, will . :~
not be further described in the present disclosure. :
It is similar to that used in some commercially , available sensors of higher cost (based on sintered ceramic plates of solid ionic conductors, and on thick . metal electrodes pressed in contact~, and of lesser performance.
:.' lSThe integrated thin film heating element 12 is supplied with electric current to heat the sensor lO in or~er to minimize water absorpt}on at the surface of the electrolyte thin film 15 (above water's boiling point (lOO C) water (H2O) is driven off). The heating element 12 improves the CO2 detection performance (ionic conductivity which depends on collective crystalline vibration is increased) and reduces the susceptibility of the sensor lV to interference from gases other than C02. Since the behavior of the sensor lO is dependent on the temperature, the temperature-dependent resistive wire : i 22, made o~ platinum or other noble metal, of'the micro-thermometer 19 is used to ~easure the j temperature of operation of the sensor lO and to control supply of current I to the thin film heating element 12. Temperature control of the sensor lO is khereby carried out.
, ~' , ':,.
17 ~ ' '~
.
, Fabricatlon of sensor 10:
! `
The different steps conducted during ~abrication of the thin film Co2 sensor 10 will now be 1 5 described.
r, The surface 18 of the substrate 11 is first chemically etched to clean this surface 18 prior to thin film deposition. The substrate 11 must be properly cleaned to avoid organic and inorganic contamination (to levels of p.p.m.) of the thin films to be deposited thereon. Such contamination may lead to reliability problems such as slow degradation of the sensor 10.
Tungsten or other refractory metal is deposited onto the etched surface 18 of the substrate 11 to form the thin film heating element 12. The heating element 12 can be deposited by thin film techniques including RF reactive magnetron sputtering, reactive evaporation in an ultra high vacuu~ chamber, electroplating, electroless plating, screen printing, plasma enhanced or normal chemical vapour deposition, etc.
The thin film heating element 12 may also ~ ! be produced by forming on the surface 13 of a sillcon ; substrate 11 a thin film of highly conductive silicon ! by means of conventional techniques~
~ ~ The thin film of dielectric material 3 ¦ is then formed by depositing sio~ on ~he thin film ¦ heating element ~12~either by RF reactive magnetron . ~ ~
:
' 2 ~ 4 1 PCT/CA94/00312 .
sputtering, or plasma enhanced or normal chemical vapour deposition.
Platinum or another~, noble metal is i 5 deposited onto the thin film of ~ielectric material 13 to produce the thin film reference electrode 14.
Again, the metallic electrode 14 can be deposited by means of thin film techniques including reactive evaporation in an ultrahigh vacuum chamber~ RF
reactive magnetron sputtering, electroplating, electroless plating, screen printing, or plasma enhanced or normal chemical vapour deposition.
Ionic conductor material (solid electrolyte) is sputtered onto the thin film reference electrode 14 to form the thin film ionic conductor 15.
Sputtering of the ionic conductor material forming the thin layer 14 can be done from a single target of NASICON or by co~sputtering of two targets J which are ærsio4 and Na2CO3. The sputtered thin film is amorphous~ ~
RF reactive magnetron sputtering is a .~ .
technique that is increasingly used in the manufacture of microelectronics integrated circuit to deposit metallic thin films. Reference is made to the very recent !'Handbook of Sputter ~epositioniTechnology'i~ by Wasa and Hayakawa, tl991, Noyes Publishing), for a complete state-of-the-art exposé of sputtering j~
pheinomena, sputtering systems, thin film families, micro-fabrication technology and even future -:~
processing and materials. -:
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W094/28403 21 4 ~ 5 61 PCTICA94100312 ' .
It appears that the inventors of the present in~ention are the first to apply this technology to the production of thin ~ilm ionic conductors which are ~etween O.Ol and 1 ILm thick.
Thin film ionic conductors have been produced in ultrahigh vacuum by sputtering constituent material in a reactive atmosphere with trace pure oxygen, using highly focused argon bombardment and high sputtering rat.es associated with magnetic plasma confinement at ~0 the sputtering cathode (known as magnetr~n sputtering). Ultrahigh purity .of the sputtered material(s) and, generally, of the sputtering probess is required to avoid any contamination of the thin film as the structure builds up atom by atom.
Rapid thermal annealing of ~he thin film ionic conductor 15 (and also of the sensor 10 completed up to this point) is carried out so as~ to obtain~
(a) low mechanical stresses in the thin films during fabrication; ~
(b) high ionlc conductivity and hence lower sensor operation temperatures. Rapid thermal annealing or oxidation in forming gas (H2/N2) favors formation of crystalline micros~ructures that improve ionic conducti~ity of the thin film;
(c~ good adhesion~between the thin ~ayers~
d) good chemical stability;:
W094/2840 1 ~15 61 PCT/CA94/00312 ~, `~
., f , ,'' I(e) good crack resistance; and -~
T,~f) goodthexmalexpansion~characteristics ~;
~matched to tho~e of the substrate and ultimately to ',5 the package.
, ' ., The advantage of a thin film ionic conductor 15 is the increase of mobility of ions Na~ in thin film to thereby reduce the temperature of operation of the sensor. The thin film ionic conductor enables passage of the ions Na' while blocking the other ions.
Sodium carbonate alone or stabilized by barium carbonate is then deposited on the thin film ionic conduc~or 15 to form the reactiYe thin film CO
sensitive layer 16. ~F magnetron sputtering i5 probably the best method to deposit the thin film C2 sensitive layer 16. However, other methods such as sol-gel coating, r~active evaporation in an ultra hiyh vacuum cha~ber and chemical vapour deposition r etc.
can also be contemplated.
., Platinum or another noble metal is finally ~ ~
, deposited onto the thin film Co~ sensitive layer 16 to form the second thin film metallic electrode 17. It can ~be deposited by thin film techniques~ inc1uding reactive evaporation in àn ultrahigh vacuum chamber, RF reactive magnetron sputtering, electroplating, 30 electroless plating, screen printing, or plasma ;-enban~ed or normal chemical vapour deposition.
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W094/28403 2 1 ~ 1 5 6 1 PCT/CA94/003l2 I To enable wiring of the sensor lO using conventional bonding techni~ues, no material is deposited on surface portions 23 and 24 (Figure 2) of the thin ~ilm heating element 1~, and surface portion j 5 25 (Figure 1) of the thin film reference electrode 14.
Electric wires 26 and 27 (Figure ~) can then be connected to surface portions 23 and 24 of the heating element 12, respectively, while electric wire 28 is connected to surface portion 25 of the thin film reference electrode 14.
,.
As no material is deposited on the metallic thin film electrode 17 and on the metallic ~.
thin film portion 21 of the micro-thermometer 19, a 15 wire 29 is easily connected to the metallic electrode :
17, and the micro-thermometer easily wired by means of .;
electric wires 30 and 31. :
. .
: No material is deposited on surface :
portion 32 (Figure 1) of the reactive CO2 sensitive lay~r 16 to increase the surface of contact of th~ CO2 with this layer 16. ~ .
;,,, :Proximity masking can be u ed during deposition of each thin ~ilm 12-17 to produce~ the ~ 1.`
structure shown in Figure 1, comprising exposed "!''~' surface ;portions 23 and 24 of the thin ;film hea~ing.
: e}~ment 12, exposed surface portion 25 of the thin ~.
: :film reference~ electrode 14,: and :~exposed sur~ace portion 32 of the reactive layer 16. One of ordinary ` skill in the art will also appreciate~that the same proximlty masklng opeeat:o~s;can be adapted to depos~it : : ~: : i,-.
W0~4/2840~ PCT/C~94/0031~
~4~6i 22 :.
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simultaneously the thin film portions 20 and 21 forming the micro-thermometer 19.
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The CO2 sensor 10 can .also be fabricat~d ~.
through deposition of thin films 12-17 having a same area. Patterning of the thin film C2 sensor 10 and -~
micro-thermometer 19 is then carried out by laser ablation to cut and remove selectively parts of thin films so as tu expose the above mentioned surf~ce portions 23, 24, 25 and 32, and to cut the thin ~ilm portions 20 and 21 of the micro-thermometer 19. This approach for cutting the stack of thin films 12-17 is necessary due to both the high solubility of the ~' carbonaceous materials which are used, and to the electrochemical interference of water molecules penetrating the thin films. ~
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Finally, the substrate 11 is cut by means of laser scribing, cleaving or diamond saw techniques, or a combination of these techniques, and the thin ~ilm C2 sensor 10 is then packaged using conventional hybrid packaging techniques.
: The second preferred embodiment of the integrated monolithic thin ~ilm CO2 sensor in accordance: with the present invention ls gener~lly identified by the reference 40 in Figuxes 3 and 4 o~
the appended drawings~ ..
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W094/28403 2 1 41 s 6 1 FCT/CA94/0031' :
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Substrate 4l:
; As illustrated in Figures 3 and ~, this ~.' ~ sensor 40 comprises a substrate 41 made of a piece of !, 5 crystalline silicon, glass, metal, ceramic, or plastic material càpable of resisting to high~ temperatures higher. Fabrication and operation~of the sensor 40 : -.
requires that the material of the~;substrate 41 be capable of resisting to temperatures h;igher than 200 `
C, to water and water vapour, to any organic or ~`
inorganic chemical contaminants, and to dust '"
accumulation. '"~,' , ;.
Ideally, the elastic constants (Young's ::' 15 modulus and Poisson's ratio) and the coeffic1ent of `~"
thermal expansion of the~ matexial fQrming the ~`'`.' substrate 41 are nearly those of silicon or quartz to : '.
avoid cracking due to temperature :cycling and : .
ultimately problems of reliability and short-lifetime 20 of the sensor 40. ~ r`'~, '~ ; : Thin film dieIect`ri.c material 42 .The first thin film 42, ~eposited on the~
:substrate 41, consists of dielectric material such as SiO2. Use of other insulating materials, for example i 'Si3N4, lmay~also be'contemplatedl. The'~functions~df:~he~ ~;
` thin ~ilm 42.are~to ensure:good adhesion between:the~
substrate~41 and:the~subsequent thin films '43-45,~ to~
' 30~ ensure~e1ectrical insulat:ion~between the subs~trate 41 and~ the- metallic~ thin~film 43, ~ and to~ provide ~ a~
diffusion~or alloy1ng~barrier between~the substrate:41 : :and the:;subsequent: metallic :thin film 43. The:thin : ':~
WOg4/2B403 PCT/CA94/00312 2~41S6~ , , '`;
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film 42 has good thermal properties, in particular a i~ good thermal expansion coefficient, and is chemically ~:
neutral so as not to react with gases to which the ~; de~ice is exposed, and so as to protect the substrate 42 from these gases.
Metallic thin film 43:
The thin film 43 is metallic and made of pure metal such as platinum or other noble metal deposited on the thin film of dielectric material 43. ;~
The metallic thin film 43 is divided into four separate portions 46-49.
`~
Reference electrode 46:
The first thin film portion 46 is a thin ::~
film reference electrode whose function is to collect electrical current. The thickness of the reference electrode 4fi must be sufficient to make this ~lectrode ~
continuous (with no electrical interruptions) and :l.
conductive with a series resistance smaller than 10 Q.
It is important that the metallic electrode 46 25 contacts no other metallic thin film portions 37-39. ~:
'. ~ ' ,,`', ~ S_cond metallic electrode 47~
:: .
The second thin film:portion 47 forms a : :
1 30 second metallic electrode used, with the thin film , reference electrode 46, to measure the electromotive :force representative of the Co2 concentration.
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` wos4l28403 21 ~15 61 PCT/CA94/00312 .. ` ' ' ' I ' ~ .
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~Heating element 48:
'~Thin ~ilm portion 48 defines an open (see ' `
¦54) metallic peripheral loop constituting a heating -;
15 element. This heating element 48 is supplied~wi~h a ';-'~current I. The thicknes~,~ width and length of the '~
heating element 48 are~adjusted~to~obtain a resistance~
'~ R adapted to an external current~;~souroe~supplying the~
~heating~'el~ment~ with~the current~ Power (I2R) is~ ' dissipated in the resistive heating~element 48 by the Joule effect;to heat the sensor 40.~Accordingly, the ~
heating element 4~ loops around the sensor 40 at its ~ ~;';' peri~hery in a simple or~ snaking ;configurat~ion as required ~for~ efficient~ distri~ution~ of the~ heat~
15~ generated by the~current I.~
Inteqr~ted mioro-thermometer 49 Thin~film~port~i~on 49 constitute~a~micro~
20~ thermometer~integrated~to~the~thin film~ C02~ sensor~40;.
As illustrated~in'~Figure~'4;,~ the thin`film portion~49 ~: .
defines` an~elong~ate~wire 59' having a temperature~
dependent~resistance~ t~o~enable ~measurement~o~f~;the~
t~mpe~atur~of~he~sensor~0 Ionic~conductor 44 A~thin~ lm; ~4 ~of~NASIC~N~ a3Zr2~i~Ol2)~
's de it d~ n~the'thi~ ilm ~referenc~e~electr e~46 30~ and~ a~p ''` ~ the ~s~'face~of ~he~hin~ lm~of~
ele ~ ~et~een the e~oc~rod~s 46~and~
' W094/28403 PCT/CA94/0~312 2141~61 .~ , .
... .
,include a plurality of layers which through their .
composition or structure serve to act as passivation, ~stabilisation and/or diffusion-barrier films. ;.
!5 Reactive CO sensltive lay~r 45: ~ ;
.'.. ~ .' The last thin film 45, deposited on the electrode 47, ~he surface portion of the thin film of .:
diele.ctric material 42 between the metallic electrode 47 and the thin film ionic conductor 44, is made of sodium carbonate alone or stabilized by barium carbonate to form a reactive thin film C2 sensltive :.
layer.
`'`
Mechanism of detection of sensor 40:
.:
The mechanism of detection used by the .
sensor 40 is exactly the same as described in relation `~
: .to the ~en~or 10 and, therefore, will not be further .I 20 described in the present de~cription.
Fabrication of sensor 40:
The different steps conducted during 25 ~ fabrication of the thin film C2 sensor ~0 are explained hereinafter. :-~: ~ The surface 62 of the substrate 41 is i ~first chemical~ly etched to clean this surface 62 prior :~.30 to thin film deposition. :~The substra~e 41 must be ~
- : :
:~ properly cleaned to~ avold :organic and inorganic ~-: contamination (to:levels of p.p.m.~ of the thin layer .-~
: ~ 42 to be deposited. Such contamination may lead to .
i~i ~
,, ~~ WQ94/28~03 PCTICA91/0031' ~.
21~1561 27 .:-. reliability problems such as slow degradation of the '.' : sensor 40.
. The thin film:of dielectric material 42 `.' 5 is tben deposited on the substrate 41 either by RF '."
reactive magnetron sputtering, or plasma enhanced or ;~.'' :normal chemical'vapour deposition. When the substrate '''.
is made of silicon, thermal oxidation of the silicon '.. :.
may be used to~produce the-insulating:thin film 4~
::lO directly on the surface of the substrate 41. ~ ~;
~ . ,; ' ~.',.
Platinum or another noble meta~ is .~.'' . . .
: deposited onto the:thin film of dielectric material 42 to form the thin film reference electrode~46, second :15 thin film metallic electrode 47, heating element 48 : ~ and micro-thermometer 49.~ It can be deposited by~thin film technigues including reactive evaporation in an ultrahigh 'vacuum chamber,' RF reactive mag~etron sputte~ing, ~:~electroplating, electrol~ess plating,:
20~ : screen~printing,`or plasma enhanced;or normal chemical vapour~deposition.`
Use~of proximity~masking can be~used 'during'~depos:ition~to produce~from the s1ngle thin~film~
::; 25~ 43 the~ refer~ence electrode : 46, second metallic ~:'electrode~47, heating element 48~and micro-thermometer `;j` !49.~ Al~ernat:ivelyj laser abl:at'~ion~canibelused td ~ut' in~'the~;thin:film.43~the~:two el~ectrod:es~46 and 47,~ the~
. ~ h~ating;element:48:and~he:~'mlcr~o-thermometer~49.
Ionic~conductor material is sputt~ere~d ont~o~
the th;in:fi~lm~referen~e~e~lectrode~4~6~and a port~ion~of the;~surface of~the thln~film of dlelectric materlal~42 WO94/284Q3 PCTiCA94/0031' . ::
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between the two electrodes 46 and 47 to form the thin film ionic conductor 44. The method for depositing this thin film 44 is the same as for thin film 15 of ¦ sensor 10 (Figures 1 and 2).
~
Sodium carbonate a~one or stabilized by barium carbonate i5 then deposited on the second thin -~
film metallic electrode 47, the thin film ionic conductor 44 and th~ surface of the thin film 42 ` ;
~0 between the ionic conductor 44 and the electrode 47 to form a reactive thin film Co2 sensitive layer. RF
magnetron sputtering can be used to deposit thin film 45. Other methods such as sol-gel coating, reactive evaporation in an ultra high vacuum chamber, plasma enhanced or normal chemical vapour deposition, etc.
can also be contemplated. ;~
To enable wiring~of the sensor 40 using conventional bonding techniques, no material is deposited on surface portion 50 of the thin film reference electrode 46 whereby an electric wire 51 can be connected ~hereto. Alsot no material is deposited on surface portion 52 of the second thin film metallic electrode 47 whereby an electric wire 53 (Figure 4) , can be connected thereto. As no material is deposited on the thin ~ilm heating element 48 and micro th~rmometer 49, electric wires 57~ and 58 can be connected~to the ends 55 and~56 of the heating element 48, respectively, and electric wires 60 and 61 can be `
30 connected to the respectlve ends of the integrated ~`
micro-thermometer 49.
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W094/28403 21 ~15 61 PCT/CA94/0~31~ ~
Proximity masking can be used during :~
deposition of eiach thin film 42-45 to produce the structure shown in Figures 3 and 4. :
~aser ablation of the thin films 42-45 may also be used to give to the C02 sensor 40 the structure o~ Figures 3 and 4. -.-;
Finally, the substrate 41 is cut by means of laser scribing, cleaving or diamond saw techniques, or a combination of these techniques, and the thin film C02 sensor 10 is then packaged using conventional hybrid packaging techniques~
The thin film C02 sensor 10,40 ac¢ording to the invention is capable of continuously detecting and monitoring low concentrations of C02 with a sensitivity of loO p.p.m. or less. It uses a solid electrolyte and may be fabricated by means of thin film techniques. The sensor o~ the invention has numerous applications in the field of air quallty control, biomedical monitoring and industrial processes and control.
Several thin film C02 sensors 10,40 can be mounted in cascade to increase the amplitude level of the detected~electromotive force and `thereby/ impro~te the C02 detection~ sensitiYity. Figure 5 illustrates two thin ~ilm sensors 40~(Figures 3 and t4) mounted in cascade. In this particular case, the thin film rèference electrode of the first sensor and the second thîn fllm metallic electrode of the second sensor form : a common electrode 63. The peripheral thin film . ~: ~
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heating element 64 then surrounds the two sensors 40 mounted in cascade.
.
,Also, the thin film~CQ2 sensor lO, 40 can !5 be provided with an outer CO2 permeable membrane. When such a sensor lO is immersed in blood, the membrane enables passage of Co2 through it to thereby allow the sensor lO to sense the CO2 concentration in this blood.
lOAn electronic system (not shown) dedicated to the conversion of the electromotive force (EMF) between the two metallic electrodes to chemical concentration of CO2, as well as to periodic self~
verification of the performance of the sensor lO, regeneration of the ionic conductor thin film 15, ; thermostating the temperature of operation, auto-recalibration etc. can be integrated to the substrate 11 along with the sensor 10,40. A smart electrochemical gas sensor is then obtained~
Last of all, it should be pointed out that the chemistry of the thin ~ilms 15,44 and 16,45 of the sensor 10,40 in accordance with the present in~ention can be modified to enable detection of the concentration of gases other than C2 '' , ; ` Although the present invention hasibeen described hereinabove by way of preferred embodiments ~-i~ thereof, these embodiments~can be modi~ied at will, `30 within the scope of the appended claim~, without departing from thè spirit and~nature of thie subject `~ invention.
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~,An object of the present invention is .~.
~5 therefore to overcome the above discussed drawbacks .:;.
!and to provide an integrated monolithic thin film gas .: .
Isensor which is free o~ hermetic "referencel' cavity .and is mechanically robust.
:, .
10Another object of the invention is tG
provide an integrated monolithic thin film gas sensor which is compatible with mass production.
; A third object of the present invention is to provide a monolithic thin film gas sensor : comprising an integrated thin film heating element for heating the sensor and improve the detection performance while reducing the susceptibility to interference from gases other than the target one.
A fourth object of the invention is to provide a monoli~hic thin film gas sensor comprising an integrated thermometer ~o enable measurement and control o the temperature of operation of the sensor.
A further object of the present invention j1 '! is to ,provide integrated monol`ithic t~in !film! gas sensors which can be connected in cascade to improve sensi~ivity of gas detection.
. 30 :
A sixth object~of the subject invention is to provide:an integrated monol.ithic gas sensor . ~ comp~ising a thin ~ilm ionic conductor, and a "dry"
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W094/~8403 PCTICAs4l00312 2~ 6~
~` fabrication techni~ue for producing this thin film ~i~ ionic conductor having an improved ionic canducti~ity.
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A still further object of the invention is to provide a thermal process which significantly improves ionic conduction of the thin film solid state .-electrolyte.
,, SllM~RY OF THE INVENTIQN
:' .
Nore specifically, in accordance with the -:
present invention, there is provided a monolithic gas 15 sensor comprising first and second electrodes each. ~;
made of a film of electrically co~ductive material, a reactive layer made of a film of material sensitive to ~;
the gas to be detected, and an ionic conductor being .
under the form of thin ~ilm to efficiently conduct ..
ions therein. The~first and second electrodes, the ::
reactiv~ layer, and the thin film ionic conductor are : .
deposited on ea~h other to ~orm an electrolytic cell in which a chemical reaction involving as reagent the : -, . . .
- gas to be detected produces an electromotive force 25 betwèen the first and second electrodes. ~ : -In accordance withia pre~erred embodlmentl : o~ the i~vention, the monolithic gas sensor comprises: ~ :
a:;substrate;~
~ a thin film~ heating e:lement made: of ~ ~ ' ~: electrically conductive ;material deposited on the . ~ ~: ` substrate;
, :
~:
i a thin film of dielectric material , deposited on the thin film heating element;
a reference electrode consisting of a thin ~:
film of electrically conductive material deposited on .:
¦ 5 the thin ~ilm of dielectric material;
~ a thin film ioniG conductor deposited on :~
! the thin film referenc~ electrode;
a reactive gas sensitive layer consisting `~
of a thin film of reacti~e gas sensitive material :~
deposited on the thin film ionic conductor;
a second electrode formed of a thin film ~
of electrically conductive material deposited on the : :
thin film reactive gas sensitive electrode; and ~:
a thin film micro-thermometer consisting ;~
of a portion of the thin film of electrically conductive material constituting the reference electrode, the thin film portion forming the micro-thermometer having a temperature-dependent resistance .
than can be converted to temperature. ,;
;~:
In accordance with another preferred : embodiment of the present inventiorl, the monolithic gas sensor comprises:
a substrate;
a thin film of dielectric material deposited on the substrate; :
' a:thin film~of electrically conductive' ~::
material deposited on the thin film of dielectric - ~
i material and divided into four separate thin film ~;
¦ 30 portions respectively forming (a) a thin film ..
¦ reference electrode, (b) a second electrically I conductive elec~rode, ~c) a thin film heating elementJ
and ~d) a thln film microthe~mometer;
` : ' '1 ' ; W094/28403 PCT/CA94/00312 2~4i5 ..
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a thin film ionic conductor deposited onto ';
the thin film reference electrode; and a reactive gas sensitive thin film layer deposited on the second electrical;ly conductive ¦ 5 electrode and on the thin film ionic ~conductor.
When the gas to be detected is CO2, the thin film ionic conductor may comprise NASICON of ``~
formula Na3Zr2Si2po12, and the reactive ~as sensitive 10 thin film layer may comprise Na2CO3.
The monolithic gas sensor in accordance with the subject invention may comprise a plurality of x said elPctrolytic cell mounted in cascade in order to ~;
15 improve gas detection sensitivity of the sensor.
The present invention further relates to a method of fabricating a monollthic gas s~ensor, comprising the step of depositing onto a substrate (a)~
20 first and sè~ond thin film electrodes màde of electrically~conductive material, (bj a reactive thin film layer made of material sensitive to the gas~to~be detected, and~(c~) an ionic~conductor under the~form of thin film to~effici~ently conduct ions therein,~wherein 25~ the dép~siting step ~comprises stacking the first an~d~
second thin film electrodes! the reactive thin ~ilm i layer,, ahdlthe thin film lonic cond~!ctor to fo~m a~s ~ electrolytic~ cell ~in which a chemi~cal reactlon~
;; ~ `in~olving as~reagent the~gas to be~detected produ~ce~an ~- ~ 3;0 ~ el~ectromotive force~between the first and se~ond thin~
~` ~ film~eleatrodes~
WO 94/28403 2 1 ~1 5 fi 1 PCT/CA94/00312 ~ .
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; In accordance with a preferred embodiment :~
. of the invention, the method of fabricating a monolithic gas sensor further comprises the step of depositing on the substrate a thin film heating element made of electrically conductive materiaI, and the step of depositing on the substrate a micro-thermometer made of a thin film of electrically conductiYe material having a temperature-dependent resistance which can ~P converted to temperature.
, Patterning of the first and second thin film electrodes, the reactive thin film layer, the thin film ionic conductor, the thin film heating element, and the thin film micro-thermometer can be .
carried out by means of proximity masking during the thin film deposition process or by means of laser ablation following this thin film deposition process.
In accordance with another preferred `
~0 embodiment of the method of fabricating a monolithic -~
gas sensor, rapid thermal annealing is carried out on the thin film ionic conductor to favor crystalline ;~:~
microstructure formation and thereby improve ionic conductivity of this thin film ionic conductor.
.
The objects, advantages and other features of the present:invention will become more apparent upon reading of the following non restrictive :~description of preferred embodiments thereof, given by j30 way of example ~nly with~reference to the accompan~ing . drawings. ~ :
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'W094/28403 PCT/CA9410031~
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BRIEF DESCRIPTION OF THE DRAWINGS
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In the appended drawings: ..
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Figure l is a side elevational view of a `"
first preferred embodiment of an integrated monolithic ; ;
thin film gas sensor in accordance with the present `:~.
invention, capable o~ sensing CO2 ~and comprising I0 integrated thin film ionic conductor, heating eIement and micro-thermometer: .
. .....
Figure 2 is a top plan view of the thin ;-~
film gas sensor of Figure l; ~, :-~
Figure 3 is a side elevational, cross ~ :
. sectional view, taken aIong line 3-3 of Figure 4,~ of a ~second preferred embodiment of the integrated j, monolithic thin film gas sensor in accordance with the :present inventi~n, capable of sensing CO2 and including integrated thln film ionic conductorl heating element . and micro-thermometer~
F}gure 4: is a top plan view of the;th1n film gas sensor of Figure 3; and ~, ,..;
Figure 5 ~is a side elëva~ional !.~iew showing two qas~sensors as:illustrated in Figures:3 and 4:mounted in::cascade. ~
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¦_ETAILED DESCRIPTION OF THE PREFERRED EM~ODIMENTS
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The first preferred embodiment of the 15 integrated monolithic thin fiIm C2 s~nsor in ¦accordance with the present invention is generally !identified by the re~erence 11 in Figures 1 and 2 of the appended drawings.
As illustrated in Figure 1, the sensor 10 comprises a stack of six thin films (see 12-17) , deposited onto a substrate 11.
:- .
Su~strate 11:
The substrate 11 is made of a piece of crystalline silicon, glass, metal, ceramic, or plastic material capable of resisting to high temperatures.
Fabrication and operation of the thin film CO2 sensor 10 requires a substrate 11 of which~the material i5 resistant to temperatures higher than 200 C, to water and water vapour, to any organic or inor`ganic chemical contaminants, and to ~ust accumulation.
.
Ideally, the elastic constants (Young's modulus and Poisson's ratio) and the thermal expansion coefficient of the material constitutin`g the~subs~rate 11 are ~early those of silicon ~or quartz to a~oid --cracking due to temperature cycling and ultimately problems related to~reliability and short-lifetime of the sensor 10. ~ ~-, ~:
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W094/28403 PCT/CA94/00312 ``
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~Heatina element 12 `.,1 . .
The first thin film 12, deposited on the -~
~top surface l8 of the substrate ll, is made of metal i5 such as tungsten or of highly conductive silicon to form a thin film heating element Hëating element 12 ,;`j is an electrically conductive thin film supplied with a current I The thickness, width and length of the ~~
thin film heating element 12 are adjusted to obtain a io resistance R adapted to an external current source `;
supplying the current I Power (I2R) is dissipated~in the resistive heating element 12 by the Joule ~ffect to thereby heat the sensor 10 ; 15The sandwich geometry of the thin film sensor lO of Figures l and 2 reguires no electrical insulation between the heating element 12 and the `~
substrate l1 as long as the material constituting ~his substrate 2 is not substantially conductiv~; it is ~he 20 case for a substrate ll made of~ silicium, glass, ; ,~
plastic or ceramic material When the substrate ll is made of metal and is therefore electrically conductive, an insulating thin film (not shown in Figures 1 and 2)~is required to prPvent the heating ; 25~ current, normally flowing through the heating element 12, to deviate in the substrate 1l Obviously, the~
? I ! curren~ flowing~through the isubst~ate lll doés~no~' contribute in heating the sensor 10 ~Thin film Qf die1ectric~materia1~13~
: The second thin film 13, deposited on the ~ -thin f llm beaclng element 12, cons~sts of dieleceric WO 94l284~3 21~1 S 61 pcrlcAs4loo3l2 J material such as SiO2 t~ electrically insulate the thin film heating element 12 from the thin film reference ' electrode 14. Other insulating dielectric materials, j for example Si3N~ could also be used as the material e 5 of the thin film of dielectric material 13.
.
Other functions of the thin film of dielectric material 13 are to ensure go~d adhesion between the thin film heating element 12 and the 10 su~se~lent thin films 14-17, and to provide a diffusion or alloying barrier between the metallic heating element 1~ and the metallic reference electrode 14. The thin film 13 should ha~e yood thermal properties, in particular a good thermaI
15 expansion coefficient, and should be chemically neutral so as not to react with gases to which the sensor 10 is exposed and so as to protect the substrate 11 from these gases.
20 Reference electrode 14~
The reference electrode 14 is metallic and formed of a thin film of pure metal such as platinum or other noble metal deposited on the thin film~13 of 25 dielectric material. As will be seen in the foIlowing description, the metallic thin film 14 is also used to form the integrated micro-thermometèr.
As the function of the thin film reference 30 metal~lic electrode 14 is to collect electric current, ` its thickness must be ~sufficient to make this electrode continuous (with no electrical interruptlons) and conductive with a series resistance .
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'~ ~
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~ 12 .
smaller than 10 n. The thin film reference electrode i 14 should not contact any other metallic thin film of ~ the sensor 10, including the second metallic electrode ~`:
', 17 and the heating element 12.
Ionic conductor 15 !
A further thin film 15 of NASICON (~a :;
8uper lonic Conductor), of formula Na3Zr2Si2POt2, is 10 deposited on the thin film reference electrode 14 to .~
form a thin film ionic conductor. The thin film ionic ~.
conductor 15 may have a homogeneous compositioni, or : may include a plurality of layers which through their composition and/or structure serve to act as passivation, stabilisation and/or diffusion-barrier layers. ~
~ ,,.
Reactive C2 sensitive layer 16~
A further thin film, deposited on the thin ~;
film ionic conductor 15, is made of sodium carbonate .~-;
alone or stabiliz~d by barium carbonate to form a .
reactive thin film CO2 sensitive layer 16.
, 25 Second metallic electrode 17: .~
. .
1 I. ! ` i ` ~ i 1 ' A second metallic electrode 17'is made, i ;~.
. as the rererence electrode~ 14~ of a thin frilm of l . platinum or o~her noble metal deposited on the sodium carbonate alone or stabilized by barium carbonate, of the reactive tbln fil- CO2 sensitlve layer 16, ' : ,~
WO 94/28403 21415 6 1 FC:T/CA~4/00312 13 :
, ~ .
I Inteqrated_micro-thexmometer:
.,~ .
'~ A micro-thermometer 19 is integrated to j the thin film CO2 sensor 10. This micro-thermo~eter 19 ¦ 5 comprises a portion 20 of the thin film of dielectric material 13 deposited onto the thin film heating t element 12, and a portion ~1 of ~he metallic thin film 14 deposited onto the thin film portion 20. As illu~trated in Figure 2, the thin film portion 21 10 ~orms an elongate wire 22 having a temperature- ~
dependent resistance. ~;
:`
Mechanism of detection of sensor 10:
The thin film ionic conductor 15 is a rather porous structure which is rich in sodium ~Na).
In the presence of air which contains about 16~ of oxygen (2) and some humidity, sodium reacts with oxygen to form Na20 at the surface of the thln film -~
ionic conductor 15, especially in the proximity of ~he interface with the "Pt" reference electrode 14, also known as the cathode of the electrolytic cell. Oxygen therefore plays a role in the chemical reaction of concern and is sometimes called the dri~ing species or elem,ent in this chemical rieaction.
Interposed between thé otheri"Pt"l thin~
film metallic electrode 17, or anode, ~nd thie thiin i~ film ionic conduçtor 15 is the thin film CO2 sensitive ;1 30 layer 16 made of sodium~ carbonate (NazCO~) alone or ~
~, ~
stabilized by barium carbonate. The difference in the -;~
electronegativities of the two substances, Na2O and Na2C03, gives~ rise to an electromotive force (EMF) . , .:~
: .
.,'. : ~ ~
~, :;
W094/28403 PCT/CA94/0031' 2~ 4~S 6 1 14 `~
'~ '.
,. .
! between the thin film metallic electrodes 14 and 17.
Mobile ions are subjected to the ~ield produced by this electromotive force and drift along the field , lines, thereby generating an ionic current. It should i 5 be noted here that NASICON is electrically insulating, so that electrons stay-put and do not ~ravel in the fieldl but can be collected at the conductive thin film metallic electrodes 14 and 17 and circulated in external electric circuits such as resistive and capacitive loads, voltmeters, ammeters, etc.
Under normal conditions, an equilibrium exists between oxidised sodium, and ionic sodium in the thin film ionic conductor 15 (solid electrolyte).
Sodium ions can also travel easily within the electrolyte by a special mass transport mechanism through the crystalline structure. Sites in the crystal lattice at which only sodium ions can sit are connected together to form paths or channels capable of conducting? the sodium ions. The to and fro coupled rocking of the stacked oxide tetrahedra around zircon or silicon atoms "push" the sodium ions along these paths or channels and contribute to the "super ionic conductivity" referred to in the acronym NASICON ~?~
8uper ~onic Co~ductor).
~ . .
In the presence iof carbon diC??Xide/ a chemical reaction ~ccurs in which sodiu~ oxide is `
"reduced" by the carbon diox1de (CO2). This reduction causes sodium ions to be "liberated" from the oxide, to travel across the NASICON and react to form sodium ~`
.; carbonate (NazCO3j. The ionic transport generates the , ~ , ~;.
:~ ` '.
~.
WOg4/28403 PCT/CA94/00312 21~1S61 ~5 I current and the electromotive force which can be `¦ detected by external circuits.
¦ The sodium ions have such high mobility, and such a short distance (in the thin film 15~ to travel that the reaction is almost instanta~eous. It has been observed in laboratories fast response times of less than a second, this compared to several minutes in bulk sensor devices operating at much 10 higher temperatures. ~-For a CO2 sensitive thin film layer 16 "
made of sodium carbonate and a thin film ionic ;:
conductor 15 made of NASICON, the CO2 detecting ~15 electrolytic cell of the sensor 10 is made of a combination of Na2CO3 and of N~SICON (Na3Zr2Si2PO12) and the CO2 detecting mechanism can therefore be described by the following electrolytic cell: .
",:
20 Pt~ C2 ~ 2 / Na2Co3 // NASICoN / 2~ Pt The anodic reaction is: ~`
Na2CO3 ---> 2 Na + CO2 + ~ 2 ~ 2 e The cathodic reaction is:
2 Na' ~ ~ 2 + 2 e ~--> Na2O
: :
:30 The glo~al reaction is:
~O3 <===>: Na2O + CO
~` ::
.
.
WO941~8~3 PCT/CA94/00312 2 ~ 6 l i .
~ 16 '. ~'.-.
' ~:
IThe electromotive force of this cell is :~
¦measured between the thin film metallic electrodes 14 tand 17 and is proportional to the concentration of CO2 jin the air. ~ `
1 5 ~.` `
¦ The alectrochemistry behindthisprinciple .:
of operation is well established and accordingly, will . :~
not be further described in the present disclosure. :
It is similar to that used in some commercially , available sensors of higher cost (based on sintered ceramic plates of solid ionic conductors, and on thick . metal electrodes pressed in contact~, and of lesser performance.
:.' lSThe integrated thin film heating element 12 is supplied with electric current to heat the sensor lO in or~er to minimize water absorpt}on at the surface of the electrolyte thin film 15 (above water's boiling point (lOO C) water (H2O) is driven off). The heating element 12 improves the CO2 detection performance (ionic conductivity which depends on collective crystalline vibration is increased) and reduces the susceptibility of the sensor lV to interference from gases other than C02. Since the behavior of the sensor lO is dependent on the temperature, the temperature-dependent resistive wire : i 22, made o~ platinum or other noble metal, of'the micro-thermometer 19 is used to ~easure the j temperature of operation of the sensor lO and to control supply of current I to the thin film heating element 12. Temperature control of the sensor lO is khereby carried out.
, ~' , ':,.
17 ~ ' '~
.
, Fabricatlon of sensor 10:
! `
The different steps conducted during ~abrication of the thin film Co2 sensor 10 will now be 1 5 described.
r, The surface 18 of the substrate 11 is first chemically etched to clean this surface 18 prior to thin film deposition. The substrate 11 must be properly cleaned to avoid organic and inorganic contamination (to levels of p.p.m.) of the thin films to be deposited thereon. Such contamination may lead to reliability problems such as slow degradation of the sensor 10.
Tungsten or other refractory metal is deposited onto the etched surface 18 of the substrate 11 to form the thin film heating element 12. The heating element 12 can be deposited by thin film techniques including RF reactive magnetron sputtering, reactive evaporation in an ultra high vacuu~ chamber, electroplating, electroless plating, screen printing, plasma enhanced or normal chemical vapour deposition, etc.
The thin film heating element 12 may also ~ ! be produced by forming on the surface 13 of a sillcon ; substrate 11 a thin film of highly conductive silicon ! by means of conventional techniques~
~ ~ The thin film of dielectric material 3 ¦ is then formed by depositing sio~ on ~he thin film ¦ heating element ~12~either by RF reactive magnetron . ~ ~
:
' 2 ~ 4 1 PCT/CA94/00312 .
sputtering, or plasma enhanced or normal chemical vapour deposition.
Platinum or another~, noble metal is i 5 deposited onto the thin film of ~ielectric material 13 to produce the thin film reference electrode 14.
Again, the metallic electrode 14 can be deposited by means of thin film techniques including reactive evaporation in an ultrahigh vacuum chamber~ RF
reactive magnetron sputtering, electroplating, electroless plating, screen printing, or plasma enhanced or normal chemical vapour deposition.
Ionic conductor material (solid electrolyte) is sputtered onto the thin film reference electrode 14 to form the thin film ionic conductor 15.
Sputtering of the ionic conductor material forming the thin layer 14 can be done from a single target of NASICON or by co~sputtering of two targets J which are ærsio4 and Na2CO3. The sputtered thin film is amorphous~ ~
RF reactive magnetron sputtering is a .~ .
technique that is increasingly used in the manufacture of microelectronics integrated circuit to deposit metallic thin films. Reference is made to the very recent !'Handbook of Sputter ~epositioniTechnology'i~ by Wasa and Hayakawa, tl991, Noyes Publishing), for a complete state-of-the-art exposé of sputtering j~
pheinomena, sputtering systems, thin film families, micro-fabrication technology and even future -:~
processing and materials. -:
,,~ ''`.
-, ~, ,~
L`~ ~ ~ .
.~ ':
W094/28403 21 4 ~ 5 61 PCTICA94100312 ' .
It appears that the inventors of the present in~ention are the first to apply this technology to the production of thin ~ilm ionic conductors which are ~etween O.Ol and 1 ILm thick.
Thin film ionic conductors have been produced in ultrahigh vacuum by sputtering constituent material in a reactive atmosphere with trace pure oxygen, using highly focused argon bombardment and high sputtering rat.es associated with magnetic plasma confinement at ~0 the sputtering cathode (known as magnetr~n sputtering). Ultrahigh purity .of the sputtered material(s) and, generally, of the sputtering probess is required to avoid any contamination of the thin film as the structure builds up atom by atom.
Rapid thermal annealing of ~he thin film ionic conductor 15 (and also of the sensor 10 completed up to this point) is carried out so as~ to obtain~
(a) low mechanical stresses in the thin films during fabrication; ~
(b) high ionlc conductivity and hence lower sensor operation temperatures. Rapid thermal annealing or oxidation in forming gas (H2/N2) favors formation of crystalline micros~ructures that improve ionic conducti~ity of the thin film;
(c~ good adhesion~between the thin ~ayers~
d) good chemical stability;:
W094/2840 1 ~15 61 PCT/CA94/00312 ~, `~
., f , ,'' I(e) good crack resistance; and -~
T,~f) goodthexmalexpansion~characteristics ~;
~matched to tho~e of the substrate and ultimately to ',5 the package.
, ' ., The advantage of a thin film ionic conductor 15 is the increase of mobility of ions Na~ in thin film to thereby reduce the temperature of operation of the sensor. The thin film ionic conductor enables passage of the ions Na' while blocking the other ions.
Sodium carbonate alone or stabilized by barium carbonate is then deposited on the thin film ionic conduc~or 15 to form the reactiYe thin film CO
sensitive layer 16. ~F magnetron sputtering i5 probably the best method to deposit the thin film C2 sensitive layer 16. However, other methods such as sol-gel coating, r~active evaporation in an ultra hiyh vacuum cha~ber and chemical vapour deposition r etc.
can also be contemplated.
., Platinum or another noble metal is finally ~ ~
, deposited onto the thin film Co~ sensitive layer 16 to form the second thin film metallic electrode 17. It can ~be deposited by thin film techniques~ inc1uding reactive evaporation in àn ultrahigh vacuum chamber, RF reactive magnetron sputtering, electroplating, 30 electroless plating, screen printing, or plasma ;-enban~ed or normal chemical vapour deposition.
. ~ . ,.
.' '.J ; ' `.,:
~ : ~ : , .'.'' ! ~' : ~ ` :
W094/28403 2 1 ~ 1 5 6 1 PCT/CA94/003l2 I To enable wiring of the sensor lO using conventional bonding techni~ues, no material is deposited on surface portions 23 and 24 (Figure 2) of the thin ~ilm heating element 1~, and surface portion j 5 25 (Figure 1) of the thin film reference electrode 14.
Electric wires 26 and 27 (Figure ~) can then be connected to surface portions 23 and 24 of the heating element 12, respectively, while electric wire 28 is connected to surface portion 25 of the thin film reference electrode 14.
,.
As no material is deposited on the metallic thin film electrode 17 and on the metallic ~.
thin film portion 21 of the micro-thermometer 19, a 15 wire 29 is easily connected to the metallic electrode :
17, and the micro-thermometer easily wired by means of .;
electric wires 30 and 31. :
. .
: No material is deposited on surface :
portion 32 (Figure 1) of the reactive CO2 sensitive lay~r 16 to increase the surface of contact of th~ CO2 with this layer 16. ~ .
;,,, :Proximity masking can be u ed during deposition of each thin ~ilm 12-17 to produce~ the ~ 1.`
structure shown in Figure 1, comprising exposed "!''~' surface ;portions 23 and 24 of the thin ;film hea~ing.
: e}~ment 12, exposed surface portion 25 of the thin ~.
: :film reference~ electrode 14,: and :~exposed sur~ace portion 32 of the reactive layer 16. One of ordinary ` skill in the art will also appreciate~that the same proximlty masklng opeeat:o~s;can be adapted to depos~it : : ~: : i,-.
W0~4/2840~ PCT/C~94/0031~
~4~6i 22 :.
'":
simultaneously the thin film portions 20 and 21 forming the micro-thermometer 19.
'~
The CO2 sensor 10 can .also be fabricat~d ~.
through deposition of thin films 12-17 having a same area. Patterning of the thin film C2 sensor 10 and -~
micro-thermometer 19 is then carried out by laser ablation to cut and remove selectively parts of thin films so as tu expose the above mentioned surf~ce portions 23, 24, 25 and 32, and to cut the thin ~ilm portions 20 and 21 of the micro-thermometer 19. This approach for cutting the stack of thin films 12-17 is necessary due to both the high solubility of the ~' carbonaceous materials which are used, and to the electrochemical interference of water molecules penetrating the thin films. ~
..-.
Finally, the substrate 11 is cut by means of laser scribing, cleaving or diamond saw techniques, or a combination of these techniques, and the thin ~ilm C2 sensor 10 is then packaged using conventional hybrid packaging techniques.
: The second preferred embodiment of the integrated monolithic thin ~ilm CO2 sensor in accordance: with the present invention ls gener~lly identified by the reference 40 in Figuxes 3 and 4 o~
the appended drawings~ ..
.
,: ,.
~, ~
.
. ~ , .
W094/28403 2 1 41 s 6 1 FCT/CA94/0031' :
. .
Substrate 4l:
; As illustrated in Figures 3 and ~, this ~.' ~ sensor 40 comprises a substrate 41 made of a piece of !, 5 crystalline silicon, glass, metal, ceramic, or plastic material càpable of resisting to high~ temperatures higher. Fabrication and operation~of the sensor 40 : -.
requires that the material of the~;substrate 41 be capable of resisting to temperatures h;igher than 200 `
C, to water and water vapour, to any organic or ~`
inorganic chemical contaminants, and to dust '"
accumulation. '"~,' , ;.
Ideally, the elastic constants (Young's ::' 15 modulus and Poisson's ratio) and the coeffic1ent of `~"
thermal expansion of the~ matexial fQrming the ~`'`.' substrate 41 are nearly those of silicon or quartz to : '.
avoid cracking due to temperature :cycling and : .
ultimately problems of reliability and short-lifetime 20 of the sensor 40. ~ r`'~, '~ ; : Thin film dieIect`ri.c material 42 .The first thin film 42, ~eposited on the~
:substrate 41, consists of dielectric material such as SiO2. Use of other insulating materials, for example i 'Si3N4, lmay~also be'contemplatedl. The'~functions~df:~he~ ~;
` thin ~ilm 42.are~to ensure:good adhesion between:the~
substrate~41 and:the~subsequent thin films '43-45,~ to~
' 30~ ensure~e1ectrical insulat:ion~between the subs~trate 41 and~ the- metallic~ thin~film 43, ~ and to~ provide ~ a~
diffusion~or alloy1ng~barrier between~the substrate:41 : :and the:;subsequent: metallic :thin film 43. The:thin : ':~
WOg4/2B403 PCT/CA94/00312 2~41S6~ , , '`;
t , ~:~
film 42 has good thermal properties, in particular a i~ good thermal expansion coefficient, and is chemically ~:
neutral so as not to react with gases to which the ~; de~ice is exposed, and so as to protect the substrate 42 from these gases.
Metallic thin film 43:
The thin film 43 is metallic and made of pure metal such as platinum or other noble metal deposited on the thin film of dielectric material 43. ;~
The metallic thin film 43 is divided into four separate portions 46-49.
`~
Reference electrode 46:
The first thin film portion 46 is a thin ::~
film reference electrode whose function is to collect electrical current. The thickness of the reference electrode 4fi must be sufficient to make this ~lectrode ~
continuous (with no electrical interruptions) and :l.
conductive with a series resistance smaller than 10 Q.
It is important that the metallic electrode 46 25 contacts no other metallic thin film portions 37-39. ~:
'. ~ ' ,,`', ~ S_cond metallic electrode 47~
:: .
The second thin film:portion 47 forms a : :
1 30 second metallic electrode used, with the thin film , reference electrode 46, to measure the electromotive :force representative of the Co2 concentration.
, -,:, .
: ~
!.' .~ ;'.
., : : , :', ~}
~1 ~ ` : ' . ' ':` ' ~ `
; :
` wos4l28403 21 ~15 61 PCT/CA94/00312 .. ` ' ' ' I ' ~ .
.,.
~Heating element 48:
'~Thin ~ilm portion 48 defines an open (see ' `
¦54) metallic peripheral loop constituting a heating -;
15 element. This heating element 48 is supplied~wi~h a ';-'~current I. The thicknes~,~ width and length of the '~
heating element 48 are~adjusted~to~obtain a resistance~
'~ R adapted to an external current~;~souroe~supplying the~
~heating~'el~ment~ with~the current~ Power (I2R) is~ ' dissipated in the resistive heating~element 48 by the Joule effect;to heat the sensor 40.~Accordingly, the ~
heating element 4~ loops around the sensor 40 at its ~ ~;';' peri~hery in a simple or~ snaking ;configurat~ion as required ~for~ efficient~ distri~ution~ of the~ heat~
15~ generated by the~current I.~
Inteqr~ted mioro-thermometer 49 Thin~film~port~i~on 49 constitute~a~micro~
20~ thermometer~integrated~to~the~thin film~ C02~ sensor~40;.
As illustrated~in'~Figure~'4;,~ the thin`film portion~49 ~: .
defines` an~elong~ate~wire 59' having a temperature~
dependent~resistance~ t~o~enable ~measurement~o~f~;the~
t~mpe~atur~of~he~sensor~0 Ionic~conductor 44 A~thin~ lm; ~4 ~of~NASIC~N~ a3Zr2~i~Ol2)~
's de it d~ n~the'thi~ ilm ~referenc~e~electr e~46 30~ and~ a~p ''` ~ the ~s~'face~of ~he~hin~ lm~of~
ele ~ ~et~een the e~oc~rod~s 46~and~
' W094/28403 PCT/CA94/0~312 2141~61 .~ , .
... .
,include a plurality of layers which through their .
composition or structure serve to act as passivation, ~stabilisation and/or diffusion-barrier films. ;.
!5 Reactive CO sensltive lay~r 45: ~ ;
.'.. ~ .' The last thin film 45, deposited on the electrode 47, ~he surface portion of the thin film of .:
diele.ctric material 42 between the metallic electrode 47 and the thin film ionic conductor 44, is made of sodium carbonate alone or stabilized by barium carbonate to form a reactive thin film C2 sensltive :.
layer.
`'`
Mechanism of detection of sensor 40:
.:
The mechanism of detection used by the .
sensor 40 is exactly the same as described in relation `~
: .to the ~en~or 10 and, therefore, will not be further .I 20 described in the present de~cription.
Fabrication of sensor 40:
The different steps conducted during 25 ~ fabrication of the thin film C2 sensor ~0 are explained hereinafter. :-~: ~ The surface 62 of the substrate 41 is i ~first chemical~ly etched to clean this surface 62 prior :~.30 to thin film deposition. :~The substra~e 41 must be ~
- : :
:~ properly cleaned to~ avold :organic and inorganic ~-: contamination (to:levels of p.p.m.~ of the thin layer .-~
: ~ 42 to be deposited. Such contamination may lead to .
i~i ~
,, ~~ WQ94/28~03 PCTICA91/0031' ~.
21~1561 27 .:-. reliability problems such as slow degradation of the '.' : sensor 40.
. The thin film:of dielectric material 42 `.' 5 is tben deposited on the substrate 41 either by RF '."
reactive magnetron sputtering, or plasma enhanced or ;~.'' :normal chemical'vapour deposition. When the substrate '''.
is made of silicon, thermal oxidation of the silicon '.. :.
may be used to~produce the-insulating:thin film 4~
::lO directly on the surface of the substrate 41. ~ ~;
~ . ,; ' ~.',.
Platinum or another noble meta~ is .~.'' . . .
: deposited onto the:thin film of dielectric material 42 to form the thin film reference electrode~46, second :15 thin film metallic electrode 47, heating element 48 : ~ and micro-thermometer 49.~ It can be deposited by~thin film technigues including reactive evaporation in an ultrahigh 'vacuum chamber,' RF reactive mag~etron sputte~ing, ~:~electroplating, electrol~ess plating,:
20~ : screen~printing,`or plasma enhanced;or normal chemical vapour~deposition.`
Use~of proximity~masking can be~used 'during'~depos:ition~to produce~from the s1ngle thin~film~
::; 25~ 43 the~ refer~ence electrode : 46, second metallic ~:'electrode~47, heating element 48~and micro-thermometer `;j` !49.~ Al~ernat:ivelyj laser abl:at'~ion~canibelused td ~ut' in~'the~;thin:film.43~the~:two el~ectrod:es~46 and 47,~ the~
. ~ h~ating;element:48:and~he:~'mlcr~o-thermometer~49.
Ionic~conductor material is sputt~ere~d ont~o~
the th;in:fi~lm~referen~e~e~lectrode~4~6~and a port~ion~of the;~surface of~the thln~film of dlelectric materlal~42 WO94/284Q3 PCTiCA94/0031' . ::
,j ~ .
between the two electrodes 46 and 47 to form the thin film ionic conductor 44. The method for depositing this thin film 44 is the same as for thin film 15 of ¦ sensor 10 (Figures 1 and 2).
~
Sodium carbonate a~one or stabilized by barium carbonate i5 then deposited on the second thin -~
film metallic electrode 47, the thin film ionic conductor 44 and th~ surface of the thin film 42 ` ;
~0 between the ionic conductor 44 and the electrode 47 to form a reactive thin film Co2 sensitive layer. RF
magnetron sputtering can be used to deposit thin film 45. Other methods such as sol-gel coating, reactive evaporation in an ultra high vacuum chamber, plasma enhanced or normal chemical vapour deposition, etc.
can also be contemplated. ;~
To enable wiring~of the sensor 40 using conventional bonding techniques, no material is deposited on surface portion 50 of the thin film reference electrode 46 whereby an electric wire 51 can be connected ~hereto. Alsot no material is deposited on surface portion 52 of the second thin film metallic electrode 47 whereby an electric wire 53 (Figure 4) , can be connected thereto. As no material is deposited on the thin ~ilm heating element 48 and micro th~rmometer 49, electric wires 57~ and 58 can be connected~to the ends 55 and~56 of the heating element 48, respectively, and electric wires 60 and 61 can be `
30 connected to the respectlve ends of the integrated ~`
micro-thermometer 49.
~' ~ : ., -., .~..
f1; . ,"':
~ : .~
....
W094/28403 21 ~15 61 PCT/CA94/0~31~ ~
Proximity masking can be used during :~
deposition of eiach thin film 42-45 to produce the structure shown in Figures 3 and 4. :
~aser ablation of the thin films 42-45 may also be used to give to the C02 sensor 40 the structure o~ Figures 3 and 4. -.-;
Finally, the substrate 41 is cut by means of laser scribing, cleaving or diamond saw techniques, or a combination of these techniques, and the thin film C02 sensor 10 is then packaged using conventional hybrid packaging techniques~
The thin film C02 sensor 10,40 ac¢ording to the invention is capable of continuously detecting and monitoring low concentrations of C02 with a sensitivity of loO p.p.m. or less. It uses a solid electrolyte and may be fabricated by means of thin film techniques. The sensor o~ the invention has numerous applications in the field of air quallty control, biomedical monitoring and industrial processes and control.
Several thin film C02 sensors 10,40 can be mounted in cascade to increase the amplitude level of the detected~electromotive force and `thereby/ impro~te the C02 detection~ sensitiYity. Figure 5 illustrates two thin ~ilm sensors 40~(Figures 3 and t4) mounted in cascade. In this particular case, the thin film rèference electrode of the first sensor and the second thîn fllm metallic electrode of the second sensor form : a common electrode 63. The peripheral thin film . ~: ~
....
:
~ ' ',t''~
.' ''~
2 1 ~15 6 1 30 ' , .
heating element 64 then surrounds the two sensors 40 mounted in cascade.
.
,Also, the thin film~CQ2 sensor lO, 40 can !5 be provided with an outer CO2 permeable membrane. When such a sensor lO is immersed in blood, the membrane enables passage of Co2 through it to thereby allow the sensor lO to sense the CO2 concentration in this blood.
lOAn electronic system (not shown) dedicated to the conversion of the electromotive force (EMF) between the two metallic electrodes to chemical concentration of CO2, as well as to periodic self~
verification of the performance of the sensor lO, regeneration of the ionic conductor thin film 15, ; thermostating the temperature of operation, auto-recalibration etc. can be integrated to the substrate 11 along with the sensor 10,40. A smart electrochemical gas sensor is then obtained~
Last of all, it should be pointed out that the chemistry of the thin ~ilms 15,44 and 16,45 of the sensor 10,40 in accordance with the present in~ention can be modified to enable detection of the concentration of gases other than C2 '' , ; ` Although the present invention hasibeen described hereinabove by way of preferred embodiments ~-i~ thereof, these embodiments~can be modi~ied at will, `30 within the scope of the appended claim~, without departing from thè spirit and~nature of thie subject `~ invention.
?
.
Claims (20)
1. A monolithic gas sensor comprising:
first and second electrodes each made of a film of electrically conductive material;
a reactive layer made of a film of material sensitive to the gas to be detected; and an ionic conductor being under the form of thin film to efficiently conduct ions therein;
wherein said first and second electrodes, said reactive layer, and said thin film ionic conductor are deposited on each other to form an electrolytic cell in which a chemical reaction involving as reagent the gas to be detected produces an electromotive force between said first and second electrodes.
first and second electrodes each made of a film of electrically conductive material;
a reactive layer made of a film of material sensitive to the gas to be detected; and an ionic conductor being under the form of thin film to efficiently conduct ions therein;
wherein said first and second electrodes, said reactive layer, and said thin film ionic conductor are deposited on each other to form an electrolytic cell in which a chemical reaction involving as reagent the gas to be detected produces an electromotive force between said first and second electrodes.
2. A monolithic gas sensor as recited in claim 1, further comprising an integrated heating element made of a film of electrically conductive material, said heating element being supplied with electric current to heat said sensor.
3. A monolithic gas sensor as recited in claim 2, wherein said heating element is a thin film heating element.
4. A monolithic gas sensor as recited in claim 2, further comprising an integrated thermometer formed of a film of electrically conductive material having a temperature-depending resistance which can be converted to temperature.
5. A monolithic gas sensor as recited in claim 2, further comprising an integrated thermometer formed of a thin film of electrically conductive material having a temperature-dependent resistance which can be converted to temperature.
6. A monolithic gas sensor as recited in claim 1, comprising:
a substrate;
a thin film heating element made of electrically conductive material deposited on said substrate;
a thin film of dielectric material deposited on said thin film heating element;
a reference electrode consisting of a thin film of electrically conductive material deposited on said thin film of dielectric material, said reference electrode constituting said first electrode;
said thin film ionic conductor deposited on said thin film reference electrode;
said reactive gas sensitive layer consisting of a thin film of reactive gas sensitive material deposited on said thin film ionic conductor;
and said second electrode formed of a thin film of electrically conductive material deposited on said thin film reactive gas sensitive electrode.
a substrate;
a thin film heating element made of electrically conductive material deposited on said substrate;
a thin film of dielectric material deposited on said thin film heating element;
a reference electrode consisting of a thin film of electrically conductive material deposited on said thin film of dielectric material, said reference electrode constituting said first electrode;
said thin film ionic conductor deposited on said thin film reference electrode;
said reactive gas sensitive layer consisting of a thin film of reactive gas sensitive material deposited on said thin film ionic conductor;
and said second electrode formed of a thin film of electrically conductive material deposited on said thin film reactive gas sensitive electrode.
7. A monolithic gas sensor as recited in claim 6, further comprising another thin film of dielectric material deposited on said substrate, the thin film heating element being deposited on said other thin film of dielectric material.
8. A monolithic gas sensor as recited in claim 6, further comprising a thin film micro-thermometer consisting of a portion of the thin film of electrically conductive material constituting the reference electrode, said thin film portion forming the micro-thermometer having a temperature-dependent resistance than can be converted to temperature.
9. A monolithic gas sensor as recited in claim 1, comprising:
a substrate;
a thin film of dielectric material deposited on said substrate;
a thin film of electrically conductive material deposited on said thin film of dielectric material and divided into three separate thin film portions respectively forming (a) said first electrode constituting a thin film reference electrode, (b) said second electrically conductive electrode, and (c) a thin film heating element;
said thin film ionic conductor d posited onto said thin film reference electrode; and said reactive gas sensitive layer deposited in thin film on said second electrically conductive electrode and on said thin film ionic conductor.
a substrate;
a thin film of dielectric material deposited on said substrate;
a thin film of electrically conductive material deposited on said thin film of dielectric material and divided into three separate thin film portions respectively forming (a) said first electrode constituting a thin film reference electrode, (b) said second electrically conductive electrode, and (c) a thin film heating element;
said thin film ionic conductor d posited onto said thin film reference electrode; and said reactive gas sensitive layer deposited in thin film on said second electrically conductive electrode and on said thin film ionic conductor.
10. A monolithic gas sensor as recited in claim 9, wherein said thin film of electrically conductive material comprises a fourth portion constituting a thin film micro-thermometer having a temperature dependent resistance which can be converted to temperature.
11. A monolithic gas sensor as recited in claim 1, wherein said ionic conductor comprises NASICON.
12. A monolithic gas sensor as recited in claim l, wherein said gas to be detected is CO2, said ionic conductor comprises NASICON of formula Na3Zr2Si2PO12, and said reactive gas sensitive layer comprises Na2CO3.
13. A monolithic gas sensor as recited in claim 1, comprising a plurality of said electrolytic cell mounted in cascade in order to improve gas detection sensitivity of said monolithic gas sensor.
14. A method of fabricating a monolithic gas sensor comprising the step of depositing onto a substrate (a) first and second thin film electrodes made of electrically conductive material, (b) a reactive thin film layer made of material sensitive to the gas to be detected, and (c) an ionic conductor under the form of thin film to efficiently conduct ions therein, wherein said depositing step comprises stacking said first and second thin film electrodes, said reactive thin film layer, and said thin film ionic conductor to form an electrolytic cell in which a chemical reaction involving as reagent the gas to be detected produces an electromotive force between said first and second thin film electrodes.
15. A method of fabricating a monolithic gas sensor as recited in claim 14, further comprising the step of depositing on said substrate a thin film heating element made of electrically conductive material, and the step of depositing on said substrate a micro-thermometer made of a thin film of electrically conductive material having a temperature-dependent resistance which can be converted to temperature.
16. A method of fabricating a monolithic gas sensor as recited in claim 14, wherein said depositing step comprises patterning at least one of said first and second thin film electrodes, said reactive thin film layer, and said thin film ionic conductor though proximity masking.
17. A method of fabricating a monolithic gas sensor as recited in claim 14, further comprising the step of patterning the stacked first and second thin film electrodes, reactive thin film layer, and thin film ionic conductor through laser ablation.
18. A method of fabricating a monolithic gas sensor as recited in claim 15, wherein said depositing step comprises patterning, by means of proximity masking, at least one of said first and second thin film electrodes, said reactive thin film layer, said thin film ionic conductor, said thin film heating element, and said thin film micro thermometer.
19. A method of fabricating a monolithic gas sensor as recited in claim 15, further comprising the step of patterning, by means of laser ablation, the stacked first and second thin film electrodes, reactive thin film layer, and thin film ionic conductor, and said thin film heating element and thin film micro-thermometer.
20. A method of fabricating a monolithic gas sensor as recited in claim 14, further comprising a step of rapid thermal annealing of said thin film ionic conductor to favor crystalline microstructure formation and thereby improve ionic conductivity of the thin film tonic conductor.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US7026793A | 1993-06-02 | 1993-06-02 | |
| US08/070,267 | 1993-06-02 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2141561A1 true CA2141561A1 (en) | 1994-12-08 |
Family
ID=22094235
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA 2141561 Abandoned CA2141561A1 (en) | 1993-06-02 | 1994-06-01 | Thin film gas sensor and method of fabrication thereof |
Country Status (5)
| Country | Link |
|---|---|
| EP (1) | EP0653059A1 (en) |
| JP (1) | JPH07509567A (en) |
| AU (1) | AU6966894A (en) |
| CA (1) | CA2141561A1 (en) |
| WO (1) | WO1994028403A1 (en) |
Families Citing this family (7)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| DE19525764A1 (en) * | 1995-07-14 | 1997-01-16 | Siemens Matsushita Components | CO¶2¶ sensor |
| DE19623487C2 (en) * | 1996-06-12 | 1999-01-07 | Max Planck Gesellschaft | Javanese cell and its use as a gas sensor |
| US6073478A (en) * | 1998-02-02 | 2000-06-13 | Her Majesty The Queen In Right Of Canada, As Represented By The Minister Of Natural Resources | Hydrogen sensor using a solid hydrogen ion conducting electrolyte |
| US20040084308A1 (en) * | 2002-11-01 | 2004-05-06 | Cole Barrett E. | Gas sensor |
| US8813539B2 (en) * | 2007-07-10 | 2014-08-26 | National Taiwan University Of Science And Technology | Electrochemistry apparatus |
| US20170324119A1 (en) * | 2016-05-06 | 2017-11-09 | GM Global Technology Operations LLC | Reference electrode implementation with reduced measurement artifacts |
| DE102017200156A1 (en) * | 2017-01-09 | 2018-07-12 | Robert Bosch Gmbh | Manufacturing method for a micromechanical sensor device and corresponding micromechanical sensor device |
Family Cites Families (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JPH04230842A (en) * | 1990-05-11 | 1992-08-19 | Toshio Maruyama | Gas sensor using solid electrolyte |
| GB9116385D0 (en) * | 1991-07-30 | 1991-09-11 | British Gas Plc | Oxygen sensor |
| DE4131503A1 (en) * | 1991-09-21 | 1993-04-01 | Bosch Gmbh Robert | EXHAUST GAS SENSOR AND METHOD FOR THE PRODUCTION THEREOF |
-
1994
- 1994-06-01 EP EP94918253A patent/EP0653059A1/en not_active Withdrawn
- 1994-06-01 CA CA 2141561 patent/CA2141561A1/en not_active Abandoned
- 1994-06-01 WO PCT/CA1994/000312 patent/WO1994028403A1/en not_active Application Discontinuation
- 1994-06-01 JP JP7500061A patent/JPH07509567A/en active Pending
- 1994-06-01 AU AU69668/94A patent/AU6966894A/en not_active Abandoned
Also Published As
| Publication number | Publication date |
|---|---|
| WO1994028403A1 (en) | 1994-12-08 |
| JPH07509567A (en) | 1995-10-19 |
| EP0653059A1 (en) | 1995-05-17 |
| AU6966894A (en) | 1994-12-20 |
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