GB2426341A

GB2426341A - Measuring nano-scale mechanical displacements by the compression or expansion of a compliant insulator

Info

Publication number: GB2426341A
Application number: GB0512657A
Authority: GB
Inventors: Marko Dorrestijn; Alexander Bietsch; Ernst Meyer; Christoph Gerber
Original assignee: Universitaet Basel
Current assignee: Universitaet Basel
Priority date: 2005-05-19
Filing date: 2005-05-19
Publication date: 2006-11-22
Also published as: WO2006123239A1; GB0512657D0

Abstract

A sensor measuring nanometre-scale mechanical displacements, such as those used in readout mechanisms for MEMS (Micro-Electro-Mechanical System) and NEMS (Nano-Electro-Mechanical System). The sensor uses a compliant insulator 3 located between a translating member 6 and a pickup member 5 of the device, whereby the insulator 3 is compressed or expanded respectively when the translating member 6 moves. A measuring device determines the change in an electrical value of the insulator 3 upon its compression or expansion, for example by measuring the capacitance or the tunnel current through the insulator 3. The translating member 6 may comprise a sharp tip for use as an atomic force microscope or may comprise a sensitive detection layer for sensing chemical or physical properties.

Description

DESCRIPTION

Displacement sensor, its use, and method for making such a sensor

Field of the Invention

This invention concerns a novel, highly sensitive mechanism to measure nanometer-scale mechanical displacements, especially applicable as readout mechanism for MEMS (Micro-Electro-Mechanical System) and NEMS (NanoElectro-Mechanical System) devices, e.g. motion sensors, physical sensors, cantilever sensors, etc.

Background and Prior Art

Whereas other uses for a readout mechanism according to the invention are * possible, a deflecting microcantilever sensor was chosen as exemplary em- :.: 15 bodiment because of its simplicity and because of the large amount of avail- *.*.

able literature. This simplifies the comparison of various readout mechanisms. * ** * * *

Microcantilevers are applied as sensing element in various devices, predomi- nantly in scanning probe microscopy (SPM), as described some time ago by n... G. Binnig, H. Rohrer, Ch. Gerber, E. Weibel, "Tunneling through a controllable *:*. vacuum gap", Appl. Phys. Left. 40, 178-80 (1981) and by G. Binnig, C. F. Quate, Ch. Gerber, "Atomic Force Microscope", Phys. Rev. Left. 56, 930-3 (1986). Microcantilevers are also used in cantilever-based (bio)chemical rec- ognition, as described by H.P. Lang, M.K. BaIler, R. Berger, Ch. Gerber, J.K.

Gimzewski, F.M. Battiston, P. Fornaro, J.P. Ramseyer, E. Meyer, and H.-J.

GUntherodt, "An Artificial Nose Based on a Micromechanical Cantilever Array", Analytica Chimica Acta, 393, 59 (1999).

For cantilever-based sensors, readout of the surface deflection is commonly solved optically (laser beam deflection), piezoresistively, or piezoelectrically.

Less often used techniques include capacitive readout, interferometry, optical ring resonator, tunneling (through a vacuum/air gap), thermal detection, or in- terdigital detection (optical diffraction grating). The latter are rather impractical methods for common applications.

One restriction of optical detection, including laser beam deflection, interfer- ometry, optical ring resonator, and interdigital detection, is that it is rather ex- pensive, it is complicated to position the optics, and it limits the miniaturization.

Integrated sensors appear to be favorable. However, piezoresistive, piezoelectric, and thermal detection are not sensitive enough. Especially, piezore- sistive detection needs rather large detection currents, which create heat and cause elevated temperatures on the cantilever, compromising sensitivity and reliability. Naturally, the same problem occurs with thermal detection. In ef- : forts to use capacitive readout, potential snap-in between the cantilever and the counter electrode has prevented working devices. For capacitive readout, * *. smaller gaps and higher readout voltages are preferred for increased sensitiv- ity. However, since an applied voltage causes an electrostatic attraction force, this combination for increased sensitivity also promotes snap-in. Tunneling readout is limited to measurements in vacuum and air, is sensitive to contami- nation, comprises a difficult approach mechanism, and is sensitive to vibrations since the approach mechanism mechanically decouples the cantilever from the counter electrode, i.e. the device is not well integrated.

It is thus desirable to combine the high sensitivity of optical readout with the integrated nature of piezoresistive, piezoelectric, and thermal detection. Also, low power consumption is advantageous since it is a prerequisite for mobile applications. Further, insensitivity to vibrations and contaminants is necessary for many environments. If heat generation is limited or even absent, sensitivity and reliability can be increased. Finally, the drawbacks of existing tunneling and capacitive readout mechanisms may be overcome by defining a small dis- tance or thin gap between the cantilever and the readout system which design circumvents snap-in and complicated alignment mechanisms.

International patent application WO 2004/053782 to Danfoss A/S discloses a tactile sensor element comprising an elastomeric body beween two pressure transfer layers. Two electrodes, one on each side of the body, form a capaci- tor with the body as dielectric. Activation of, i.e. pressing, the sensor element squeezes the elastomeric body and thus changes the capacitance of the de- vice. This change of capacitance indicates that the element has been acti- vated. This WO publication briefly mentions that measurement of the capaci- tance may provide information on the pressure exerted to the sensor, but it does not address any measuring function of the disclosed sensor device. In other words, the WO publication does not give a hint towards the object of the present invention, namely to devise a simple and sensitive readout method and mechanism for nanometer-scale measurements. S... * S S. ..

* The present invention devises a novel, very sensitive readout method and mechanism for measuring nanometer-scale mechanical displacements, which S..

* 20 is applicable for MEMS and NEMS devices, e.g. motion sensors, physical sen- sors, or cantilever sensors, resolving the above-mentioned shortcomings. In * *.: principle, the novel readout method measures the translation of a surface us- ing a compliant insulator located between the translating surface and a counter electrode. Examplary methods include measuring the capacitance or the tun- neling current through the compliant insulator. This leads to a very reliable, low cost, compact, and easily operable measurement device.

The Invention As mentioned above, the basic principle of the present invention consists of using a compliant or squeezable insulator located between the translating sur- face, e.g. a cantilever, and a counter electrode for determining mechanical translations or movements of said surface. This squeezable insulator, which may be seen as a tunnel barrier or a dielectric layer, provides the readout for any out-of-plane surface translation.

The compliant insulator preferably has two opposite, at least approximately parallel sides bounded by an electric contact on either side. One of these con- tacts is positioned on the translating surface, the other one is relatively station- ary. Translation of the surface now causes the insulator to deform, either squeezing it or stretching it, whereby the insulator changes its electrical prop- erties, which change in turn serves to identify the amount of translation. * ,* * * * S.. *

*::: One preferred method of measuring the change of electrical properties of the * insulator is by using current tunneling. Generally, tunneling currents are very sensitive to thickness changes of a tunelled insulator (tunnel barrier): the varia- tion is exponential. Since the currents are small, typically in the nA regime, : . small dissipation occurs in the cantilever, which is of advantage for sensor ap- plications.

As an alternative to the tunneling current approach, capacitance of the sand- wich structure may be determined. This can be done by measuring the im- pedance using an applied AC voltage for one or more frequencies, according to l=d(CV)/dt=CdV/dt. For oscillatory motion of the translating surface, meas- urement of the response to an applied DV voltage may be used, according to l=d(CV)/dt=VdC/dt. Although the disclosed technology is applicable to many more NEMS or MEMS devices, three such devices will be discussed: * anSPM, * a cantilever-based chemical sensor, and * a pressure sensor or microphone.

The three preferred groups of embodiments described further down are based on these devices.

The invention also includes a new use for such a novel displacement sensor and a method for making such a novel displacement sensor.

The basic principle and details of the invention shall be explained in the follow- ing.

Brief Description of the Drawings

:.:. Various examples and modifications for carrying out and using the invention * 1 shall be explained together with the drawings. These show in * S. Fig. I schematic side view of a first embodiment of the invention; I, Fig. 3 a second embodiment of the invention; S. * 0 * I S. Fig. 4 a third embodiment of the invention; Fig. 5 a fourth embodiment of the invention; Fig. 6 a fifth embodiment of the invention; Fig. 7 assembling multilayer SAMs of MHDA; Fig. 8 schematic of a device for measuring tunneling currents; Fig. 9 force-distance curves on a double-layer SAM of MHDA, using tunneling current approach; Fig. 10 current-distance curves on a double-layer SAM of MHDA, using tunneling current approach; Fig. 11 SEM image of a convex counter electrode; Fig. 12 schematic of a device for a capacitive readout, using a triple-layer MHDA SAM; Fig. 13 measurement circuitry for a device for a capacitive readout; and Fig. 14 results for two different diameters of the droplet in a capacitive readout device. * a. * I I Is. .

I I..

* 20 Detailed Description of Several Exemplary Embodiments Several implementations of the invention are disclosed in the following. They differ in shape and arrangement, but the same reference numbers in the draw- ings always refer to the same functional parts of the different embodiments, though they sometimes may look very different.

Figs.1 to 3 show a first group of embodiments of the invention, based essen- tially on a cantilever design.

Fig. I is a schematic side view of a first design of an embodiment of the inven- tion, generally depicted as scanning probe microscope 1. This device may be used for more or less standard scanning probe microscopy, as will be easily apparent for a person skilled in the art.

A flexible, micro- or nanostructure cantilever 6 is positioned under and in physical contact via a conductive sphere or ball 8 with a fixed, conducting arm 5, serving as a counter electrode. An insulating block 4 and a compliant, insu- lating film 3 electrically separate the cantilever 6 and the arm 5.

The tip 7 scans the surface of sample 2 whereby the cantilever 6, following the surface of the sample 2, moves in a vertical direction parallel to the paper plane. The film 3 between the solid and stationary ball 8 and cantilever 6 is- thereby deformed, i.e. is squeezed or compressed or it is stretched or ex- panded depending on the direction of movement of the cantilever 6. This de- : * formation may now be used in several ways as described in the following.

*** * 15 **..

A voltage, generated by voltage source 9, is applied via leads 10 and 11 across the arm 5 and the cantilever 6. S..

It should be obvious to a person skilled in the art, that the basic design of this :.,. 20 and most subsequently described embodiments may be reversed, i.e. that the * .. sphere or ball may be fixed to the translating member instead of the fixed arm.

Fig. 2 is, like Fig. 1, a schematic side view of a second design of an embodi- ment of the invention. Different from Fig. I is that it lacks a tip, but has instead a sensitive detection layer 12 for sensing (bio)chemicals, physical properties, etc. Again, this device may be used for more or less standard detector sys- tems as will be easily apparent for a person skilled in the art.

Fig. 3 finally concludes the first group of embodiments, i.e. the embodiments having or using a cantilever design. For measurements performed in electri- cally conducting liquid media, the fixed arm 5 and the ball 8 may be insulated by coating them with a material 13, which preferably is the same material used for the compliant film 3.

The function of the novel devices shown in Figs. I to 3 is as follows.

As shown, the mechanically fixed arm 5 carries the ball 8 which obviously has a convex shape. Deflection or translation of the cantilever 6 is then translated into a change in thickness of the compliant insulating film 3 due to indentation by the ball 8. The radius of the ball 8 may be adapted to the forces used in the application, i.e. the contact area can be adapted to the preferred pressure range of the compliant film 3. To adapt the travel range of the film 3 to the travel range of the translating surface, i.e. the cantilever 6, the position of the ball 8 can be chosen anywhere along the cantilever 6 from the site of maxi- * mum deflection to the site of zero deflection, if such a zero deflection exists is within in a given geometry.

.:" The thickness change of the compliant film 3 is now measured by establishing and evaluating a tunnel current from the conducting ball 8 to the conducting cantilever 6. A tunneling current is known to vary exponentially with the thick-

. 20 ness change, and the capacitance varies inversely proportional. Exponential behavior offers the highest sensitivity and has been used in scanning probe microscopy in the so-called scanning tunneling microscope (STM), disclosed e.g. by G. Binnig, H. Rohrer, Ch. Gerber, E. Weibel, "Tunneling through a con- trollable vacuum gap", Appl. Phys. Lett. 40, 178-80 (1981) and in early ver- sions of the atomic force microscope (AFM) having STM feedback on the can- tilever as disclosed by G. Binnig, C. F. Quate, Ch. Gerber in "Atomic Force Microscope", Phys. Rev. Left. 56, 930-3 (1986).

For capacitive readout, sensitivity can be increased by reducing stray capaci- tance. A larger distance could be designed between the conducting leads on the cantilever 6 and the fixed arm 5, preferably by introducing a vertical inter- C' connect between the ball 8 and the "top" surface of the fixed arm, or by con- necting the ball 8 to the fixed arm 5 via a spacer.

Figs. 4 and 5 show a second group of embodiments of the invention, using a double-clamped beam or membrane design. Both figures show sideviews of these embodiments of the invention. The single insulating block of Figs. I to 3 is replaced by two insulating blocks 4a and 4b. A micro- or nanostructure, a double-clamped flexible beam 6a or a flexible membrane (not separately shown, but easily imaginable from the design shown in Fig. 4), is positioned under and in contact with a fixed conducting bar 5a, also clamped by the two insulating blocks 4a and 4b, and a ball 8 on the bar 5a. An insulating and compliant film 3 separates the ball 8 from the flexible beam 6a or a flexible membrane, resp. S.

Again, the structure of the embodiment shown in Figs. 4 and 5 is relatively thin (compared to its other two dimensions) and forms a kind of sandwich struc- * : ture. An advantage of a double-clamped beam compared to a cantilever beam is the well-defined position of the beam surface with respect to the counter electrode, i.e. the ball/fixed beam structure. It is known that thin cantilevers, i.e. the structures shown in Figs. Ito 3, tend to bend after asymmetric coating, i.e. bend to on one side which may be undesirable in certain applications.

Similar to the embodiment shown in Fig. 2, a sensor film 12 may be covering one side of the flexible beam 6a or the respective membrane.

Whereas a beam structure may be better suited for sensing (bio)chemicals, physical properties, etc., a membrane structure may be used for measuring an electrostatic or a magnetic force, or a pressure of a medium (including sound waves). For measurements performed in electrically conducting liquid media (in particular aqueous media), the counter electrode can be insulated by coat- K) ing it with a compliant film, Cf. Fig. 5. The person skilled in art will easily know which structure best suits the particular needs of an application.

For measurements performed in electrically conducting liquid media, the fixed counter electrode, i.e. the balI 8 and the fixed bar 5a can be insulated by coat- ing it with the compliant film 13. Such an embodiment is shown in Fig. 5 which shows a structure essentially derived from the structure of Fig. 3.

Similar to the embodiment shown in Fig. 2, a sensor film 12 may be covering one side of the flexible beam 6a or the respective membrane. The sensor film I 2a depicted in Fig. 5 is somewhat shorter than the free space on the flexible beam 6a between the two insulating blocks 4a and 4b to indicate that this sen- sor film need not cover the whole surface of the beam. The same applies, mu- * tatis mutandis, if a membrane is used instead of a flexible beam.

:.: * 15 * *** Fig. 6 shows a third embodiment consisting of a membraneshaped micro- or nanostructure which is again thin compared to its othertwo dimensions. As can be seen in the depicted side view, the device comprises a flexible cantile- ver 6 with a tip 7 and a planar counter electrode 14 which is a conductive flexi- ble layer deposited on the compliant insulator film 3. The device shown is an SPM cantilever device, but such planar counter electrode may similarly applied to doubly clampled beams and membranes.

Several aspects of and variations on the above described preferred embodiments shall be discussed in the following.

The insulating and compliant film 3 may be I to 10 nm thick for tunneling read- out, and I nm to I pm thick for capacitive readout. Film thicknesses of 2 to 20 nm can be built up from several stacked, self-assembled monolayers (SAM5) grown by a method described by Evans et al., J. Am. Chem. Soc. 113, 5866-8 (1991). Films thicker than 50 nm can be made by spin-coating polymers, which are preferably low-modulus polymers to limit the force required for indentation. Films of intermediate thickness (5 to 50 nm) can be grown by surface-initiated polymerization (SIP) of oligo(ethylene glycol), as described by Ma et al. I H. Ma, J. Hyun, P. Stiller, and A. Chilkoti, "Non-fouling" oligo(ethylene glycol)-functionalized polymer brushes synthesized by surface- initiated atom transfer radical polymerization", Adv. Mater. 16, 338-341 (2004).

The counter electrode shown in Figs. I through 5 is mechanically fixed and the ball 8, being the significant part of the electrode, has a convex shape. Deflec- tion of the beam or cantilever 6 is then translated into a change in thickness of the compliant insulator film 3 due to indentation by the ball 8. It must be un- derstood that the counter electrode need not be a ball, but may have any con- * * vex shape, whereby the radius of curvature of this convex counter electrode can be adapted to the forces and the materials used in the application, i.e. the * contact area can be adapted to the preferred pressure range of the compliant * : film 3. To adapt the travel range of the compliant film 3 to the travel range of :. the translating surface, the position of the counter electrode can be chosen anywhere along the translating surface from the site of maximum deflection to the site of zero deflection (if such a site is present in the geometry).

With the thickness change of an intermediate insulator like the compliant film, a tunneling current is known to vary exponentially, and the capacitance varies inversely proportional. Exponential behavior offers the highest sensitivity and has therefore been used in scanning probe microscopy, i.e. in the Scanning Tunneling Microscope (STM) as disclosed by G. Binnig, H. Rohrer, Ch. Ger- ber, E. Weibel, in "Tunneling through a controllable vacuum gap", AppI. Phys. Lett. 40, 178-80 (1981) and in "Surface studies by scanning tunneling micros- copy", Phys. Rev. Lett. 49, 57-61 (1982) by the same authors. Early versions of the Atomic Force Microscope (AFM) used STM feedback on the cantilever as disclosed by G. Binnig, C. F. Quate, Ch. Gerber in "Atomic Force Micro- scope", Phys. Rev. Lett. 56, 930-3 (1986).

For capacitive readout, sensitivity can be increased by reducing stray capaci- tance. A larger distance could be designed between the conducting leads on the cantilever 6 and the fixed arm 5, preferably by introducing a vertical inter- connect between the ball 8 and the "top" surface of the fixed arm, or by con- necting the ball 8 to the fixed arm 5 via a spacer.

The counter electrode can be a planar, electrically conducting structure that is fixed to the translating surface; a preferred implementation is shown in Fig. 6.

It should be thin enough to not compromise the mechanical operation of the beam, and thick enough to form a continuous film. Such a counter electrode * :* could be produced in one of the following ways. 15

The first method consists of inkjet spotting a liquid pattern onto the compliant insulator. The liquid could be a suspension of metal particles, forming a de- *. posit upon drying, or it could be a solution of complexed metal ions that are deposited by action of a reducing agent, which is known as ElectroLess Depo- sition (ELD) as disclosed in "Electroless Plating: Fundamentals and Applica- tions" by Mallory, G. 0., Hajdu, J. B., Eds.; American Electroplaters and Sur- face Finishers Society: Orlando, FL, 1990 or by I P. C. Hidber, W. Helbig, E. Kim, G. M. Whitesides, Langmuir 12, 1375-1 380 (1996).

The second method comprises the transfer of a catalyst onto the compliant insulator via microcontact printing (pCP) from a soft stamp, followed by elec- troless deposition (ELD) of a metal as disclosed by M. Geissler, H. Wolf, R. Stutz, E. Delamarche, U.-W. Grummt, B. Michel, A. Bietsch in "Fabrication of Metal Nanowires Using Microcontact Printing", Langmuir 19, 6301-11 (2003).

The third method is to stamp a metal contact directly onto the compliant insula- tor, disclosed by H. Schmid, H. Wolf, R. Alienspach, H. Riel, S. Karg, B. Mi- chel, E. Delamarche, in "Preparation of Metallic Films on Elastomeric Stamps and Their Application for Contact Processing and Contact Printing", Advanced Functional Materials 13, 145-53 (2003) and by Y.-L. Loo, D. V. Lang, J. A. Rogers, and J. W. P. Hsu, "Electrical Contacts to Molecular Layers by Nanotransfer Printing", Nano Letters 3, 913-7 (2003).

The fourth method comprises floating a patterned metal film off a master in a liquid bath and subsequently transferring it to the compliant insulator by micromanipulation - a technique commonly used in Transmission Electron Mi- croscopy (TEM). Upon bending of the beam, the density and thickness of the film change, which changes the energy and thickness of the tunnel barrier, and also the capacitance according to the change in permittivity and thickness.

*S*S 15 * * * ** . Deflection of a beam/membrane structure can be static or dynamic, either in- : : * duced by the environment, or by an actuator, e.g. a piezo-electric element.

* * MEMS or NEMS sensors can consist of an array of equally designed sensors, as to allow for differential readout. Differential readout is used to suppress 0 background effects by using one or more sensors as references, improving * reliability of the signal as described by J. Fritz, M. K. Bailer, H. P. Lang, H. Rothuizen, P. Vettiger, E. Meyer, H.-J. Guntherodt, Ch. Gerber, and J. K. Gimzewski, Science 288, 316 (2000) and byY. Arntz, J. D. Seelig, H. P. Lang, J. Zhang, P. Hunziker, J. P. Ramseyer, E. Meyer, M. Hegner, and Ch. Gerber, Nanotechnology 14, 86 (2003).

Experimental Verification Fig. 7 shows the assembling multilayer SAMs of MHDA in a schematic. The compliant insulator was grown as a multilayer SAM of mercapto-hexadecanoic acid (MHDA) by a method of Evans et al., J. Am. Chem. Soc. 113, 5866-8 (1991) was used. In this method, copper ions are used as "glue" to stack the monolayers, binding the carboxylic acid (COOH) moieties to the thiol (SH) groups of the next monolayer. The SAMs were assembled on a 5 nm palla- dium film that was evaporated onto Si<100> using titanium as an adhesion layer.

Fig. 8 depicts schematically an arrangement for measuring tunneling currents.

A 4 nm thick double-layer SAM of MHDA was chosen and the tunneling currents were observed. By gently varying the pressure of a convex counter elec- trode with a radius of 70 nm onto the surface of the double-layer SAM, cf. Fig. 9, an exponential decay distance of 1.7 nm was found, cf. Fig. 10. The tip of a suitable tip 7 is shown as SEM picture in Fig. 11. Deflections as small as I nm could be detected, even with non-dedicated measurement electronics (located outside) and with a counter electrode that was mechanically unstable, i.e. not : ** integrated. In the literature, current vs. distance and current vs. force curves :::.:. 15 on molecular monolayers have been limited to a single monolayer, as described e.g. by I A. M. Becka and C. J. Miller, J. Phys. Chem. 96, 265768 (1992) or by X. D. Cui, X. Zarate, J. Tomfohr, 0. F. Sankey, A. Primak, A. L. Moore, T. A. Moore, D. Gust, G. Harris and S. M. Lindsay, "Making electrical :. contacts to molecular monolayers", Nanotechnology 13, 5-14 (2002).

. .: Conductance of a single molecule within such a monolayer has been recorded using a break junction, cf. M. A. Reed, C. Zhou, C. J. Muller, T. P. Burgin, and J. M. Tour, "Conductance of a molecular junction", Science 278, 252-4 (1997).

There are several recent review papers available on molecular electronics in general, e.g. D. K. James and J. M. Tour, "Electrical Measurements in Molecu- lar Electronics", Chem. Mater. 16, 4423-35 (2004) and R. L. McCreery, "Mo- lecular Electronic Junctions", Chem. Mater. 16, 4477-4496 (2004) and one specifically on metal-SAM-metal junctions by Salomon et al. "Comparison of electronic transport measurements on organic molecules", Adv. Mater. 15, 1881-90 (2003).

Fig. 12 depicts the general setup for measuring the capacitances of a multi- layer SAM on a solid substrate, which measurement has been done for the first time. By lowering a mercury droplet (Hg) onto such a SAM, a contact area dependent capacitance was measured using AC voltages. For a contact area of 0.16 mm2, the capacitance of a triple monolayer was 0.3 nF, determined by the cutoff frequency of the impedance. An increase of the contact area caused a linear increase of the capacitance, consistent with the formula for parallel- plate capacitors C = c0 c Aid, wherein A is the plate area and d the distance between the parallel plates. When, instead of the area, the distance d would be varied, the capacitance would follow according to the mentioned equation as well. The relative permittivity found is relatively high (Cr = 5.3) compared to literature values for conventional alkanethiol SAMs (c,. = 2.6). This is probably due to the high polarizability of the carboxylic acid moieties (COOH) and the copper ions in the multilayers used.

Ia.. 15 * S *...

In the existing literature, capacitances of alkanethiol SAM5 have been meas- :.:. ured for double-layer SAM5 between two mercury droplets, cf. M. A. Rampi, 0.

J. A. Schueller, and G. M. Whitesides, "Alkanethiol self-assembled monolayers as the dielectric of capacitors with nanoscale thickness", AppI. Phys. Left. 72, :.:. 1781-1783 (1998). For single-layer SAMs on gold, A. Demoz and D. J. Harri- * S son describe results in "Characterization of extremely low defect density hexa- decanethiol monolayers on Hg surfaces" Langmuir 9, 1046-1 050 (1993) and for single layers on mercury results were published M. D. Porter, T. B. Bright, D. L. Allara, and C. E. D. Chidsey, "Spontaneously organized molecular assemblies. 4. Structural characterization of n-alkyl thiol SAM5 on gold by optical ellipsometry, infrared spectroscopy and electrochemistry", J. Am. Chem. Soc. 109, 3559-3568 (1987).

A new use of a sensor according to the invention is the measurement of dis- placements as readout for micro-electromechanical system (MEMS) or nano- electromechanical system (NEMS) devices, in particular motion sensors or cantilever sensors. Thereby, the deflection or translation of the cantilever or beam or membrane may be static or dynamic, depending on the requirements, and is induced either externally or by an actuator. Piezo- electric actuators, as widely used in the MEMS and NEMS technology are an example for such ac- tuators.

The invention has been described using some detailed and some exemplary preferred embodiments. However, it is to be understood that the scope of the invention is not limited to the disclosed embodiments and that other applica- tions and modifications of the invention by a person skilled in the art are en- compassed by the following claims. * ** * * S as. 5 *.*. * . *5I* * 5* * * * I.. * a S.. I. * * **. ** U * * S 55

Claims

1. A sensor for measuring nanometer-scale mechanical displacements, in particular as readout mechanism for micro-electro-mechanical systems, said sensor having a translating member and a pickup member, characterized by a compliant insulator located between said translating member and said pickup member, said compliant insulator being compressed or expanded, resp., when said translating member moves, - a measuring device for determining the change in an electrical value of said insulator upon compression or expansion of said insulator.

2. The sensor according to claim 1, wherein the compliant insulator is a tunnel barrier and the electrical value is the tunnel is current through said insulator. I.e. * . I...

3. The sensor according to claim 1, wherein * the compliant insulator is a dielectricum of a capacitor and the electrical value *11 is the capacity of said insulator. I. * 20 * S..

4. The sensor according to any of the preceding claims, wherein the pickup member is a solid and/or fixed member having an essentially convex, preferably spherical, extension or an essentially convex shape, said convex extension or shape interacting with the compliant insulator on the translating member.

5. The sensor according to any of the claims I to 3, wherein the translating member has an essentially convex, preferably spherical, extension or an essentially convex shape, said convex extension or shape interacting with the compliant insulator on the fixed member.

6. The sensor according to any of the preceding claims, wherein the sensor is a sandwich structure.

7. The sensor according to claim 1, wherein the compliant insulator directly contacts the pickup member, preferably without a convex extension or shape in-between.

8. The sensor according to any of the preceding claims, wherein - the translating member is a micro-mechanical cantilever having a first io surface or electrode, - the pickup member is a solid bar having a second surface, located opposite, preferably approximately parallel, to said first surface or electrode, and - the compliant insulator is located between said first and said second surface.

i

9. The sensor according to any of the claims Ito 6, wherein I...

- the translating member is a flexible membrane, preferably a micromechanical membrane, having a first surface or electrode, - the pickup member is solid having a second surface or electrode located I., opposite, preferably approximately parallel, to said first surface, and - the compliant insulator is located between said first and said second surface. 4 S *. * SS

10. The sensor according to any of the claims I to 6, wherein - the translating member is a flexible bar clamped at both its ends, said flexible bar having a first surface or electrode, - the pickup member is a solid bar having a second surface or electrode located essentially parallel opposite said first surface, and - the compliant insulator is located between said first and said second surface, said bars preferably being micro-mechanical bars.

II. The sensor according to any of the claims I to 6, wherein - the translating member is a micro-mechanical cantilever having a first surface or electrode, - the compliant insulator is located on said first surface, and - the pickup member is a conductive flexible layer located on said compliant insulator acting as a second electrode.

12. The sensor according to claim 11, wherein the bending of the cantilever changes density and/or thickness of the compliant insulator, which changes the energy and thickness of the tunnel io barrier, and/or the capacitance according to the change in permittivity and thickness.

13. The sensor according to any of the claims 7, 8 or 10, wherein both the translating member and the pickup member extend longitudinally and ::. is have at least one end clamped to a common insulating block. S.. * S S...

14. The sensor according to claim 9, wherein * both the translating flexible membrane and the pickup member are mounted to I..

a common insulating block.

15. The sensor according to any of the preceding claims, further comprising an insulating layer covering both the translating member and the pickup member, thus allowing use of said sensor in a liquid, said insulating layer being preferably of the same material as the compliant insulator.

16. The sensor according to any of the preceding claims, further comprising a voltage source providing a voltage between the translating member and the pickup member for determining the tunnel current through or the capacity of the insulator.

17. The sensor according to any of the preceding claims, wherein the translating member includes a sharply pointed tip and works as an atomic force microscope.

18. The sensor according to any of the preceding claims, wherein the translating member includes a sensitive detection layer for sensing (bio)chemicals, physical properties, etc.

19. The sensor according to any of the preceding claims, wherein the compliant insulator has a layered structure, preferably a self-assembled io monolayer (SAM) structure of mercapto-hexadecanoic acid (MHDA).

20. The sensor according to any of the preceding claims, wherein the compliant insulator is I to 10 nm thick for tunneling readout.

21. The sensor according to any of the claims I to 19, wherein : :* the compliant insulator is I nm to 1 pm thick for capacitive readout. S.

: ****

22. A sensor structure comprising two or more, in particular an array of, : sensors according to any of the preceding claims for allowing differential * 20 readout, thus suppressing background effects and improving reliability of S..

measurement. I. * * * S..

23. A method for making a sensor according to claim 10, comprising the following steps - manufacturing the micro-mechanical cantilever having a first surface or electrode, - fabricating the compliant insulator on said cantilever, and - producing a metallic pattern on said compliant insulator to form the conductive flexible layer representing the second electrode.

24. The method for making a sensor according to claim 11, wherein the metallic pattern is produced by inkjet spothng a liquid, said liquid either being a suspension of metal particles forming the metallic pattern upon drying, or being a solution of complexed metal ions deposited by electroless s deposition (ELD).

25. The method according to claim 24, wherein a catalyst is transferred onto the compliant insulator via microcontact printing (pCP) from a soft stamp, followed by electroless deposition (ELD) of a metal to io produce the metallic pattern.

26. The method according to claim 24, wherein the metallic pattern is produced by stamping a metal contact directly onto the compliant insulator.

27. The method according to claim 24, wherein :.: : a patterned metal film is floated off a master in a liquid bath and subsequently

S

transferred to the compliant insulator by micromanipulation. * **

: .: 20

28. Use of a sensor according to any of the preceding claims, whereby displacements are measured as readout for micro-electromechanical system (MEMS) or nano-electromechanical system (NEMS) devices, in particular * :. motion sensors or cantilever sensors.

29. Use of a sensor according to any of the claims I to 26, whereby the deflection or translation of the cantilever or beam or membrane is static or dynamic and is induced either externally or by an actuator, in particular a piezo-electric actuator.