CA1155967A

CA1155967A - Medium film deposition of electric circuits

Info

Publication number: CA1155967A
Application number: CA000371636A
Authority: CA
Inventors: Carl M. Anderson; Hussein M. Naguib; Lawrence H. Hobbs; Richard Sue
Original assignee: Northern Telecom Ltd
Current assignee: Nortel Networks Corp
Priority date: 1981-02-24
Filing date: 1981-02-24
Publication date: 1983-10-25

Abstract

MEDIUM FILM DEPOSITION OF ELECTRIC CIRCUITS
Abstract of the Disclosure A process for forming microcircuits by exposing a photosensitive organic layer on a substrate to ctive radiation to render part of the layer soft and particle receptive and part of the layer hard and particle non-receptive. Powder having conductive or resistive component is then spread over the layer 9 embedded into the soft material as a multilayer, and removed from the hard material.
Finally, the substrate is fired to burn off the organic layer, to sinter the powder particles together and to fuse them to the substrate.
The resin must be from 5 to 10 m thick and the powder from 0.5 to 10 m in diameter in order that the film produced by firing does not have voids or blisters. Circuits can be made having much higher resolution than can be achieved using thick film techniques and at a much lower cost than fabricating thin film circuits.

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Description

~5~7 This invention relates to a process for depositing electrical circuit components, particularly conductors and resistors on an insulating substrate. The invention relates also to microcircuits including components or interconnects formed using such a process.
The common techniques for deposi~ing such circuit components are the so-called thick and thin film processes.
Conventional screen-printed thick film methods are capable of resolving conductor line spacing as small as 5 mil (125 microns) using mesh screens and approximately 3 mil (75 microns) using more expensive screens and metal masks. However, the current design trends in electronic packaging techniques require complicated geometry and fine line resolution which are beyond the limits of conventional screen printing technology. Recently a number of photolithographic adaptations of these techniques have been developed to satisfy the new demands but they require many additional processing steps with corresponding increases in processing and equipment costs. High resolution is attainable using thin film techni~ues but only at additional expense incurred at having to develop and etch photo-resistivè materials.
A medium film process is now proposed which produces films intermediate thin and thick films in terms of resolution and, to some extent, process steps. The process is loosely based on a colography9 a known method of applying powdered material to solid substrates to obtain multi-coloured images on ceramic materials, United States Patent 3,637,385, Hayes et al, issued on January 25, 1972. In this imaging process, a solid, positive-acting or negative-acting radiation sensitive oryanic layer having a thickness of Ool to 40 microns is exposed to actinic radiation in an image t . ' ' : , ' ' . .
' , ' ' ' , ' ' . ..

receiving manner. As a result of selective exposure-related hardening or softening, the organic layer is divided into particle receptive and particle non-receptive regions. A layer of free~flowing powder particles having a diameter along at least one axis of at least 0.3 microns up to 25 times the thickness of the organic layer is then applied to the layerO While at a temperature below the melting points of the powder and of the organic layer, the particles are physically embedded as a monolayer in a stratum at the sur-face of the light sensitive layer to yield images having portions varying in density in proportion to the light exposure of each portion. Subsequently, non-embedded particles are removed from the organic layer to develop an image.
The processing conditions, especially the relat;onship between powder size and organic layer thickness, described in the above patent to produce multi-coloured monolayer images on a ceramic - substrate were found to be unsuitable for adaptation of the me~hod to fabrication of conductor interconnect and resistor patterns for hybrid micro-electronic applications.
In order to achieve desired predictable electrical characteristics, the resin should be appreciahly softer than that used in colography so as to ensure the de-position of a multilayer; in addition the resin should have a thickness T in the range of 5-20 microns with a powder particle si2e P in the range 0.5-10 microns where T is not less than P. It is recogni~ed that powder particles will rarely be perfectly spherical. A reference made in this specification to a powder particle having a particular diameter indicates that the particle has this diameter along one axis at leastO

~S~67 Conductor and resistor powders are preferably prepared by firing a quantity of a conductive or resistive thick film ink or paste and then pulverizing the fired material to a powder of the desired particle size. Typically thick Film inks include about 5% glass and each particle produced has a like glass content. The glass content of the solid component of a thick film ink or paste may, however, vary from 0 to 90% depending on the nature of the conductor or resistor and the nature of ~he substra~e material.
Another way of achieving the powder is by dry mixing finely divided resistive or conductive material together with finely divided frit and then adding a charge of organic adhesive to produce consolidated particles of size 0.5 to 10 microns from the finely divided material.
Following removal of non-embedded particlesg ~he substrate is fired at a temperature at which the organic layer is burned off and the powder is sintered to create a homogeneous electrical film.
Embodiments of the invention will now be described by way oF example with reference ~o the accompanying drawings in which;-Figures la to ld are sectional views, not to scale, of part of a microclrcuît substrate showing stages in a medium film process according to the invention;
Figure 2 is a block schematic representation of a sequence of medium film process steps;
Figure 3 is a sectional view to an enlarged scale of a substrate supporting a resin coating having embedded powder.

9~7 Referring in detail to Eigure 1, a substrate 10 is coated with a thin layer of an organic photo-sensitive resin 12 (Figure la) which is then exposed to UV light through a positive photo mask (Figure lb)o The exposed portion o-F the region 12 cross-links~ hardens and becomes particle non-receptive. A powder 16 consisting of particles of fused metal and glass is then applied to the substrate surface and becomes physically embedded in the soft or unexposed portions 18 o-f the light sensitive film 12 (Figure lc). Excess powder is removed and process completed by -firing the substrate 10 at a medium temperature to drive off t~le organic resin 12 by oxidation and, at a high temperature, tc. bond the powder particles 16 to each other and to the substrate 10 by sintering and fusion mechanisms (Figure ld).
In the following sections, the materials used and the processing steps are described in more detail.
MATERIALS
1. Substrates The medium film technique has been used successfully on substrates of glass, ceramic and porcelain--on-steel. Generally, ceramic and porcelain on-steel substrates are cleaned using a process developed for thick film substrates (P.G. Creter and. D.E. Peters, Proceedings ISHM ~1977) p. 281), with subsequent ultrasonic cleanirg in FreonR solvent follc.wed by firing at h;gh temperature (900C
for ceramic and 600C for porcelain-on-steel). Glass substrates are cleaned us;ng processes developed for thin films (L. Maissel, R. &lang, "Handbook of Thin Film Technology"~, McGraw-Hi7.1 Inc., New York (1970) Chapter 6.). The use of contaminated substrates can result in poor film adhesion and the formation of blisters.

~5~B7 ~ The Resi n .
An exalnple of photoresist resin is as supplied by Ferro Corporation (Cleveland, Ohio) under specification no. RV3566E. It is a positive acting, light sensitive organic polymer. The surface properties of this polymer can be varied between a soft particle receptive condition and a hard particle non-receptive condition upon expos~re to UV light. A photo-catalyzed hardening mechanism such as photo-polyrnerization, photo-crosslinking or phcto-oxidation occurs. A
plasticizer and/or a photo-activator agent is added to the photo-sensitive organic polymer for adjusting its powder receptivity and sensitivity to UV light. The surface receptivity of exposed and unexposed portions of the polymer film depend on such parameters as the size of the powder particles, the thickness of the polymer layer, the UV exposure time, the temperature and humidity of the environment and the amount of force used in applying the powder. Other resins which can be used in th~ medium film process are disclosed in the United States Patent 3,731,831, issued on January 30~ 1973 to Hayes et al, United States Patent 3,676~121, issued on July 11, 1972 to Jones et al, and United States Patent 3,723,123, issued on March 27, 1973 to Jones et al. The resins include resins which are negative acting, the resin layer being initially particle non~receptive but being softened and made particle receptive by irradiation, As these a~orementioned patents indicate, resins can be softened and made more particle receptive by the addition of a plasticizer or softener. Such resins can also be made more photosensiti~e by the addition of a photo-activator. Colography requires a resin which is relatively hard so that powder particles do not adhere merely on contact with a particle receptive region. To obtain a monolayer the powder must be applied to the resin surface and then forcibly embedded. The resin for electrical circuit fabrication must be appreciably softer and tackier.
Initially, on spreading powder on the particle receptive areas a monolayer adheres to the resin surface. Subsequent use of force both presses powder particles down closer to the substrate and builds powder layers above the level initially occupied by the resin surface thereby forming a multilayer.
3. The Powders a) Conductor Powders .
Medium film powders required for the fabrication of conductor interconnects in hybrid microcircuits consist of particles which have sub-particles of metal or metal alloy and glass. The metallic component of the powder determines the characteristics of the conductors for microelectronic applications. These characteristics include line cross-section, conductivity, solderability, bondability and compatibili~y with resistor materials in the hybrid microcircuit.
The mos-t commonly used metallic conductors in hybrid microelectronics are Au, Pt-Au, Au-Pd, Ag and Ag-Pd. Au and its alloys exhibit excellent properties for microelectronic applications, but are the most expensive materials. Ag and its alloys are the most widely used conductor materials for most industrial and commercial hybrid microelectronic applications. Medium film powders base on these conductors may be readily prepared using the methods discussed below.
In addition, powders based on cheaper conductors such as copper, have been made and successFully applied to substrates using a process according to the inventionO

~ ~5~7 Most conductor powders contain glass to pro~ide cohesion of conductor particles and adhesion of these particles to the substrate. Almost any low melting point glass can be used, but alkali-free glasses3 such as high lead glasses are preferableu The properties of the glass frit are crucial to the characteristics, compatibility and performance of the conductors in hybrid microcircuits. Thuss iF the thermal expansion coefficient of the glass is significantly different from that of the substrate, cracks and blisters develop in the conductor during firing. Also adhesion at conductor/resistor interfaces is inFerior if ~he glass in the conductor powder is not compatible with the glass in the resistor powder. Dry mixed powders tend to show non-uniform distribution of glass frit in metallic powder thus contributing further to the problems of poor adhesion and blister formations. Satisfactory powders may be prepared from proprietary thick film conductor inks by burning off the organic vehicle in the ink and fusing the metal and glass frit at temperatures betwen 400 and 700C. The resulting glassy mass is then pulverized to obtain homogeneous powders. The medium film conductors prepared from these powders are found to be reproduciblle, exhibit good adhesion to ceramic substra~es, and are naturally compatible with thick film resistors used in hybrid microcircuits.
Using this method, Au and Au alloy powders can be prepared from commercially available inks such as Dupont Au-9791 and Pt/Au-9596 or Engelhard Au A-3360 and Pt/Au A-3395. The powder is prepared by stirring the conductor paste thoroughly, pouring the conductor paste into a platinum boat and thoroughly air drying to remove the organic solvents. Subsequently, the dried paste is fired in 5~9~7 a -furnace at a peak temperatllre oF 400C and with an air flow sufficient to completely oxidize all organic components during the firing cycle. Following firing, the glassy matrix is removed from the platinum boat and is ground using a mill until the particles can be sieved to 0.5 to 10 microns. A similar process can be used to prepare Ag and Ag alloy powders from commercial thick film pastes such as Dupont Pd/Ag-9061 and Engelhard Pd/Ag A-3809 except for the firing step. A firing temperature of 700C is required in the case of Pd-Ag thick film paste to improve the properties of the conductor since powders prepared by Firing the paste at lower temperature produce conductive films which are thin, porous and exhibit high resistivity.
So-called fritless thick film inks have recently been developed and from them, corresponding solid powder equivalents can be prepared for use in the medium film process. In these materials, cohesion is provided by chemical bonding.
An alternatiYe ~o fabricating the powder by pulverizing a fused mass of metal particles and glass Frit is to thoroughly dry mix very small particles of glass and metallic powder and then bring them up to the required powder particle diameter (0,5 to 10 microns) by adding an organic bonding agent. A lecithin based gum has been found ; suitable for this purpose. This method is especially suitable for producing powder particles using malleable metals such as gold s;nce any attempt to pulveri~e a gold-glass matrix often resul~s in thin flakes of material rather than particles approximating to sphericity, ~he latter being the ideal shape for the medium film process. It will be appreciated that in composition, though not in physical nature, a particle produced by this process may be practically identical with one produced by fusing and pulverizing.

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' ' b) Resistor Powders Another common component required ~or hybrid microcircuits is a resistor. Commercially available bismuth ruthenate thick film resistor pastes such as Dupont Biron 1400 series or Engelhard Rely-Ohm series are converted to powders for medium film applications using a process similar to that described for the preparation o~ conductor powders. In the case o~ Pd/Ag conductors~ it is necessary to fire the resistor paste at 700C to lock the ruthenate particles in the glass component o~ the resistor. The ~ired paste is then ground to a particle size of 0.5 to 10 microns. The measured sheet resistivity of medium films produced ~rom these resistive powders is Found to be close to screen printed thick film resistors. As in thick film~ the exact resistance values of medium film resistors can be adjusted by laser trinming. The medium ~ilm process is capable of producing 10 mil sq. resistor elements which are too small for ~abrication by screen printing thick film tehcniques. These small resistors coulds for example, be used as heating elements in moving or fixed head thermal printers.
The powders produced from thick ~ilm pastes retain most of the properties of screen printed thick -films such as conductivity or resistivity, adhesion, solderability, bondability and compatibility with thick ~ilm components. However, the medium film components exhibit better line resolution of less than 2 mils (S0 microns). Such line resolution currently cannot be obtained by thick film screen printing technology, hut only by expensive thin film techniques~
Although medium and ~hick film tehcniques use about the same amount of material in producing conductor interconnects and ~5596~

resistors, the powders can be manuFactured more cheaply than thick film inks since in the former, no organic vehicle is necessary and flow properties are not critical.
Processing Steps 1. Resin Coating Four possible coating techniques for applying a uniform 5 to 20 microns medium film resin to various substrates are spinning, spraying, roller coating and dip coating. Whichever technique is chosen depends on such factors as substrate size and the nature of the resin9 but spraying is probably the most versatile of these techniques.
Especially for coating large area substrates up to 12"
square, an air sprayer at a pressure of 30 psi is used. The resin is diluted with a thînner (1-1-1 trichloroethane) in a ratio of 1:2 (resin:thinner). A shiny uniform coating is obtained for a 4"-6"
separation distance between the substrate and nozzle. A pebbly appearance occurs if the sprayer is held at a large distance whereas running edges are formed with a short separation.
It should be mentioned that in contrast to the colography patent 3,637,385 mentioned previously, which cites a resin thickness of 0~1 to 40 microns with an optimum thickness of 0.5 to 2.5 microns~ the thickness of the light sensitiYe resin for making medium film microcircuits should be wi~hin the range of 5-20 microns, the optimum thickness being dependent on the powder and the substrate material as will be described presently. If resin thickness is less than 5 microns, the powder particles subsequently applied may be only partially embedded and may be ripped off when subsequently removing non-embedded powder. If resin thickness is greater than 20 microns, 96~7 bulk resin may be left under the subsequently applied powder and powder particles may be dislodged as the resin volatilizes on firing. In the first case voids result and in the second the film blisters and fragments.

2. Resin Exposure The most effective radiation for the photo~polymerization of the medium film resin RV3566E is in the 365nm region. The optimum exposure time is determined by experiment with the particular powder to be deposited. For hybrid micro-electronic applications, the exposure time must be from ~-40 seconds~ the actual exposure time depending on resin thickness, suhstrate, and powder materials used. In general, if the exposure time is less than 5 seconds3 the powder is found subsequently to be retained on the apparently exposed resin regions.
The electrical properties of the resulting medium film circuit consequently become uncontrollable. On the other hand, exposure times - longer than 40 seconds causes poor edge definition and a decrease in the desired geometrical dimensions of the printed elemen~s. A typical exposure time for 10 micron thick resin is eight seconds using a powder X d nsity of 41mw/cm2 from a Tamarack UV exposure system with a 1000 watt Hg lamp for ceramic and porcelain steel substrake. However, glass substrates, owing to increased reflection of UV light, require additional exposure time.
~ Mask-substrate separation must be kept as small as possible commensurate with not touching the coated substrate. If the photomask touches the substrateg it either lifts off resin or roughens its surface, creating voids and/or poor adhesive areas. A large separation produces poor edge definition. A separation distance of 4 mil (100 micron) is usually satisfactory.

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It is preferable that resin coating and exposure be carried out in a clean yellow room. However, this is not necessary provided substrates are shielded from UV light and the powder application is followed within one hour of exposing the medium film resin to UV radiation.

3, Powder Application A soft pad or Fine brush can be used to distribute the medium film powder over the substrate coated with the selectively exposed resin. The pad or brush should not be so stiff as to score the film nor so loose fibered as to contaminate the resin. The quantity oF
the powder is not critical provided there is an excess available beyond that required for full development of the pattern. The development of the image seems to depend primarily on particle-to-particle interaction rather than pad-to surface or brush-to-brush Forces. A circular buffing motion is used to apply the powder to unexposed resin.
Excessive mechanical -force is avoided since this may result in loss of resolution on subsequent firing.
AFter the development of the image, excess powder is removed by firmly wiping the sur-Face with a clean brush or pad or is blown off using an air gun. The powdeY particles must be well embedded into the unexposed resin otherwise when wiping the surface, conductor or resistor particles will be ripped out oF the unexposed layerO In additiong unlike the colography requirements of a surface monolayer nt I ~ r e cl p ~ ~ v, ~ :1 s / ~
discussed in U.S. patent No. 3,637,385,~the particle size and resin thickness must be so related as to ensure a multilayer oF powder extending through the resin layer to the suhstrate as shown in Figure 3. The multilayer requirement must be satisfied in order to obtain a ..

conductor or resistor layer of uniform thickness. Dispersal throughout the resin layer must be achieved in order that subsequent firing does not produce voids in the conductor or the resistor layer.

4. Firing The substrate, patterned with the medium film powder, is fired in air in a thick film conveyor furnace. During firing the organic resin is burned out completely. As can be imagined, the resulting gases affect the powdered distribution less if the powder is distributed throughout the thickness of resin than if it forms a monolayer shield overlying a bulk layer of resin. Firing is then continued at a higher temperature at which the glass content of the powder softens to wet the substrate and sinter the metal particles together in a glassy matrix. Ag and Au conductors have been fired at 600C on glass and porcelain steel substrates and at 850C on ceramic substrates, the substrates being kept at a peak temperature for 5 to 15 minutes.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY
OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A medium film process for forming electrical circuit components, the method comprising:-(i) depositing a layer of radiation sensitive material on an insulating substrate, the layer having a uniform thickness T in the range of 5 to 20 microns, (ii) selectively exposing a region of the layer to actinic radiation to promote an exposure related change in hardness of the material, thereby to render part of the layer hard and particle non-receptive and the rest of the layer soft and particle receptive;
(iii) embedding into the particle receptive part of the layer a multilayer of powder particles, the particles having diameter P
in the range 0.5 to 10 microns where T is not less than P;
(iv) removing non-embedded particles, and (v) firing, thereby to burn off the resin and to sinter the powder to produce a homogeneous layer adhering to the substrate at that region initially occupied by the particle receptive part of the photo sensitive layer said homogeneous layer having prescribed electrical characteristics corresponding to the composition of the powder particles.

2. A medium film process as claimed in claim 1, in which the particles each consist of a fused mass of metallic conductor and glass.

3. A medium film process as claimed in claim 1, in which the particles each consist of a fused mass of electrically resistive material and glass.

4. A medium film process as claimed in claim 1, in which the particles each consist of a mixture of sub-particles of conductive material and glass adhering together by means of a bonding agent.

5. A medium film process as claimed in claim 1, in which the particles each consist of a mixture of sub-particles of resistive material and glass adhering together by means of a bonding agent.

6. A medium film process as claimed in claim 1, in which the particles are prepared from a thick film ink incorporating an organic vehicle and a glass frit by heating the ink to drive off the organic vehicle.

7. A medium film process as claimed in claim 6, in which a solid derived by driving off the organic vehicle from said thick film ink is fired at high temperature to fuse the glass frit and an electrical component of the thick film ink.

8. A medium film process as claimed in claim 7, in which a glassy matrix prepared by firing at said high temperature is pulverized to produce particles of diameter between 0.5 and 10 microns.

9. A method according to claim 1, in which the resin incorporates a first component characterized by a photo-catalyzed hardening mechanism and a plasticizer determining particle receptivity of the resin.

10. A medium film process as claimed in claim 1, in which the layer of resin is deposited by spraying.

11. A medium film process as claimed in claim 1 in which the resin is exposed for a time just sufficient for all exposed regions to become completely particle non-receptive.

12. A medium film process as claimed in claim 1, in which non-embedded particles are removed by rubbing the coated substrate.

13. A medium film process as claimed in claim 1, in which firing is performed in two stages, the first stage in which the substrate is fired at a low temperature to burn off the resin and a second stage at a higher temperature to sinter particles of the powder together and to fuse the particles to the substrate.

14. A medium film process as claimed in claim 1, in which said photo-sensitive material is positive acting and hardens on exposure to radiation.

15. A medium film process as claimed in claim 1, in which the photosensitive material is negative acting and softens on exposure to radiation.

16. A medium film process as claimed in claim 1, in which the radiation sensitive material is photosensitive.

17. A medium film process as claimed in claim 16, in which the material is positive acting.

18. A medium film process as claimed in claim 16, in which the material is negative acting.