WO2005100965A1

WO2005100965A1 - Cantilever with polymer composite strain sensor

Info

Publication number: WO2005100965A1
Application number: PCT/DK2005/000254
Authority: WO
Inventors: Anja Boisen; Lauge Gammelgaard; Calleja Gomez Montserrat; Peter Andreas Rasmussen
Original assignee: Dtu
Priority date: 2004-04-13
Filing date: 2005-04-13
Publication date: 2005-10-27

Abstract

A cantilever with an integrated read-out-such as an integrated read-out based on polymer-based sensor means, is disclosed. The cantilever may be part of a cantilever array. The invention further relates to methods of fabricating a cantilever and/or a cantilever array, including a photolithography method and a micromoulding method. The integrated polymer-based sensor being, or comprising, a polymer-based material mixed with particles, such as conducting particles so as to obtain a piezo-resistive strain sensor. Connection means are provided for allowing electrical access to the polymer-based sensor means. A deformation of the cantilever is detected as a change of the resistivity of the strain sensor. The cantilever/cantilever array may be used as a sensor, e.g. as a bio/chemical sensor.

Description

Cantilever with polymer composite strain sensor

Field of the invention

The invention relates to a cantilever with an integrated read-out, including a cantilever array, and to methods of fabricating a cantilever and/or a cantilever array. The invention relates in particular to a cantilever or a cantilever array being, or comprising, a polymer- based cantilever mixed with particles.

Background of the invention Cantilever-based sensors have been used to monitor different physical and chemical processes by transducing changes in temperature, mass, electromagnetic field or surface stress into a mechanical response. Cantilever-based sensors have a wide range of applications in real-time local monitoring of chemical and/or biological applications. Normally, cantilevers used in cantilever-based bio/chemical detection are micrometer-sized cantilevers fabricated in silicon and designed for atomic force microscopy (AFM) imaging.

The fabrication of silicon-based sensors is rather complicated due to the comprehensive process sequence required in order to fabricate such sensors. A consequence of the comprehensive process sequence is directly reflected in the fabrication costs, causing silicon-based sensors to be very expensive.

The working principle of a cantilever sensor is often based on the optical leverage principle, to provide a simplified system where the optical aspect of the cantilever-based sensor is avoided, cantilever-based sensor systems with integrated read-out have been proposed.

In WO 03/022731 a polymer-based flexible structure with integrated sensing/actuator is disclosed. By incorporating a gold resistor in an SU-8 based polymer structure an SU-8 based cantilever sensor, which is almost as sensitive to stress changes as a silicon-based cantilever with piezo-resistive read-out is achieved.

Summary of the invention

The present invention seeks to provide an im proved cantilever-based sensor with integrated read-out. It is an object of the invention to provide local, high resolution and label-free molecular recognition measurements on a stable and sensitive porta ble device.

It is a further object of the invention to provide means for a real-time local monitoring of chemical and biological interactions.

It is an even further object of the invention to provide a sensor capable of simultaneous detection of several chemical, biological and/or biochemical substances, both gaseous substances as well as liquid substances, for example in co nnection with high-throughput screening (HTS).

It is an even further object of the invention to provide a sensor which is cheap to fabricate.

It is an even further object of the invention to provide a sensor with an improved stability and sensitivity compared to silicon-based cantilever senso rs.

According to the above and other objects of the invention, there is provided a polymer- based cantilever, a cantilever array as well as methods of fabricating such a cantilever and/or cantilever array.

According to a first aspect, there is provided a polymer-based cantilever comprising a polymer composite structure of a first polymer-based material mixed with particles so as to form a polymer-based sensor means, the cantilever further comprising connection means allowing electrical access to the polymer-based sensor means.

Cantilevers used in cantilever-based bio/chemical detection have traditionally been fabricated in silicon and designed for atomic force microscopy (AFM). It is an advantage to provide cantilevers that are designed and fabricated with the intent to be used only as bio/chemical sensors. A further advantage is that by use of a polymer material, the fabrication process is rendered simple, cheap, fast and fle-xible. It is an advantage that a cheap device may be provided, e.g. since a cantilever array may be used to measure chemical and/or biological substances which may be difficult to, time consuming to or even toxic to clean off a device, rendering it desirable to provid e a disposable device.

A cantilever may be a micrometer-sized beam attached to a platform in one end of the beam. However, the term cantilever should be construed more broadly, and should be construed to at least include a bridge structure, where at beam is clamped in two ends, as well as to include a diaphragm or membrane structure attached to a platform along parts of, or the entire, periphery. However, in general the term should be construed to include a sub-micrometer or micrometer-sized flexible structure coupled to a sensing means so that deformations of the structure may be deduced. The deform ation may be any type of deformation, such as any type of movement of a cantilever. The deformation may be a static or dynamic deformation, such as bending, twisting, oscillating, etc.

Sensors based on the cantilever principle offer the possibility of performing local, high- resolution measurements on a portable device in real-time. By providing a polymer-based sensor means optical read-out is avoided. It is an advantage to provide a system where optical read-out is avoided since a more compact system may be provided, and further a system where the detector signal is more easily accessible since aligning and light-to- voltage conversion are avoided. The polymer-based sensor means may be an integral part of the cantilever, the cantilever may even be made of, or constituted of, the polymer- based sensor, alternatively the polymer-based sensor may constitute a part of the cantilever. It is an advantage that the integrated sensor means is polymer based, since this may facilitate the production of the device.

The polymer-based sensor is a polymer composite structure of a first polymer-based material mixed with particles. A polymer composite may be defined as a mixture of any given polymer with a non-polymer material. The word parti cles should be construed broadly and at least to include any type of shape, such as substantially spherically shaped objects including grain-shaped objects or pellet-shaped objects and substantially elongated objects, such as rod-like objects. Further, the particles may also be agglomerates or aggregates of particles, such aggregates may possess very complicated shapes.

The particles may be conductive. Thus, a single particle may be conductive, however also an ensemble of particles may be conductive, e.g. a film or l ayer of the particles may be conductive. A polymer-based layer with the particles embedded in the polymer matrix may be rendered conductive by the presence of the particles in the polymer matrix. The conductivity of a polymer-based layer with embedded particles may be due to a percolation effect or any other effect providing conductivity through a polymer-based layer. The conductivity may depend upon the concentration of particles in the layer such that for a given concentration a given conductivity of the layer may be established. By straining the layer, the conductivity may be changed e.g. from a changed local concentration. In a stretching of the layer the conductivity may be reduced from a reduced particle concentration in the strained area, whereas in a compression of the layer the conductivity may be increased from an increased particle concentration in the strained area. The mentioning of a layer should only be taken as an example of the form of the sensor means, any given form may be envisioned such as a bar, a rod, a sheet, etc. A particle concentration may be provided so that the density of the particles in the first polymer-based material, i.e. in the polymer-based sensor means, is above the percolation threshold for the conductive particles in the first polymer-based material.

It is an advantage to provided conductive particles above the percolation threshold since a cantilever with integrated read-out may be provided, by providing a piezo-resistive sensor means, where a deformation of the cantilever is detected by detecting a change in the resistance of the polymer-based sensor means, the resistance change being induced by the deformation. The detection may be obtained by monitoring th e resistance between two connection means.

The connection means may be such means as one or more electrodes contacted to the sensor means, e.g. by embedding a first end of the connection means in the polymer composite material of the sensor means. A second end of the con nection means may be connected to an externally accessible terminal. The connection means may be at least one conductive or semiconductive structure, however normally two or more connection means are provided. At least two connection means may often be necessary in order to provide a measurement of e.g. a resistance change of an area spanned between the connection means in the sensor means. The connection means may be of any suitable shape, such as an L-shape, a rod-shape, etc.

At least part of the polymer-based sensor means may be provided so that it may form contact with a chemical substance. The contact between the chemical substance and at least part of the polymer-based sensor means may be a chemical reaction such as adsorption or absorption of the chemical substance in such as way that a change of the resistance of the polymer-based sensor means is induced and thereby enabling the detection of the presence of a chemical substance in contact with the cantilever.

The cantilever may typically be a layered structure comprising a layer of the polymer- based sensor means encapsulated in a polymer-based structure. The cantilever may however also be a layered structure comprising at least two layers of the polymer-based sensor means separated by at least a non-conductive polymer-ba sed separation layer. It may be an advantage to provide at least two layers of the sensor means, where a first layer is provided in a top region of the cantilever, whereas a seco nd layer is provided in a bottom region of the cantilever. A bending of the cantilever will consequently induce a tensile strain in one of the layers and a compressive strain in the other layer, thereby enabling at least two independent measurements of the bending ^■facilitating a better measurement certainty. Further since the sensor is fabricated by means of polymer technology, connection means in separate layers may be provided. The fabricating of connection means in separate layers in a silicon-based technology would be very difficult due to the number of process steps required and due to the need for electrica l connections in buried layers.

At least part of the cantilever may be encapsulated in a non-conducting second polymer- based material. It may be an advantage to encapsulate the cantilever in a non-conducting second polymer-based material especially in connection with measurements in a conductive liquid. The encapsulation may also be provided in order to protect the sensor means from the environment or due to a better control of the properties of tri e cantilever.

The conductive particles may be such particles as carbon particles or carbon nanotubes or any type of suitable particles.

The particles may be crystalline particles, such as nanosize, microsize or mesoscopic sized particles. It may be an advantage to use crystalline particles since specific properties may be provided. Such nanocrystals as ZnO-nanocrystals or semiconducting nanocrystals of the main groups IV or III-V in the periodic table may be used.

The polymer-based sensor means may be a piezo-electric sensor. A piezo-electric sensor may be provided by using ZnO-nanocrystals as the particles of the polymer- ased sensor means.

The average size of particles may be of a size so that the average diameter is between 2 nm and 1 μm, such as between 5 nm and 500 nm, such as between 10 nm and 250 nm, such as between 20 and 100 nm, such as between 25 and 75 nm, such as between 30 and 60 nm, such as between 40 and 50 nm. The average diameter may be the average diameter of an envelope sphere in case of an irregularly shaped object or it may be the average cross-sectional diameter of a spherically-shaped object, an elongated object or rod-like object.

The particles may preferably be substantially homogeneously distributed in t »e polymer composite structure. It may be an advantage to provide substantially homogeneously distributed particles since the conductivity or other measurable quantity may depend upon the concentration of the particles, by providing substantially homogeneously distributed particles a large number of cantilevers may be fabricated with at least substa ntially uniform parameters.

The substantially homogeneously distributed particles may be mixed directly i n a substance of the first polymer-based material by means of ultrasound, such a s by use of a sonicator or other ultrasound producing means. The particles may however by distributed by means of any suitable method.

Apart from the cantilever dimension it is the relationship between the gauge factor and Young's modulus of a material that determines the sensitivity of the canti lever sensor. By providing a polymer-based sensor material a small Young's modulus may be provided resulting in that a ratio between the gauge factor and the Young's modulus of the cantilever may be between 0.25 and 25 of the ratio of a silicon-based cantilever, such as between 0.5 and 15, such as between 1 and 10, such between as betwee n 2 and 5 may be provided. The gauge factor reflects the deformation sensitivity, it may be an advantage to provided a cantilever with a ratio between the gauge factor and the Young's modulus which is comparable to or better than a silicon-based cantilever. Thereby providing a cantilever with a better sensitivity for surface stress measurements than a silicon-based cantilever.

The polymer of one or more of the polymer-based materials, i.e. the first , second and/or third polymer-based material, may be any suitable type of natural or synthetic polymer- based material or co-polymer-based material which may sustain a stable structure in the micrometer or sub-micrometer range. The polymer-based material may be a plastic material, such as a thermoplastic or a thermoset, or such as a so-called photoplastic, i.e. a plastic material that may be photolithographically processed. The material of the polymer- based cantilevers may be selected from the group consisting of: SU-8 based polymers, such as XP SU-8 polymer, polyimides or BCB cyclotene polymers and parylene, but many other polymer-based materials or plastic materials could be used. The chemical name of SU-8 is glycidyl ether of bisphenol A. SU-8 may be a suitable component for fabricating a cantilever array since it has a high chemical resistance, it is compatible with conventional microfabrication techniques, capable of supporting very high aspect ratios and it is relatively easy and fast to process.

It is an advantage to use a polymer-based material as the cantilever material, since compared to a silicon-based cantilever a more stable as well as a more sensitive sensor system may be provided.

By using a polymer material which may be patterned by means of photolithography also a polymer composite structure based on this material may be provided so that it may be patterned by means of photolithography. It may be an advantage to provide a polymer composite that may be patterned by means of photolithography since the polymer-based sensor means thereby may be fabricated using the same techniques as for the rest of the cantilever. In order to use the cantilever in a sensor device for detecting the presence of one or more substances it may be an advantage to coat an outer surface of the cantilever with one or more immobilization layers. The outer surface may e.g. be, or be a part of, a top or a bottom surface of a cantilever. The one or more immobilization layers may comprise a metal layer and/or a molecular layer, the metal layer may be a first layer, whereas the molecular layer may be a second layer.

The first layer may more specifically be a layer of Au, Ag, Pt, Pd, Al Cr, Ti, Cu, Ru, Rh, or any combination or alloying of such or other metals. The first layer may be provided by vapour deposition, precipitation, laser ablation, electron beam evaporation, electro-plati ng, shadow masking, or any other suitable technique for providing a metal layer. The first layer may have a thickness between a few nanometres to a few hundreds nanometres.

The first layer may be capable of immobilization of relevant molecules, such as by means of van der Waals interactions, electrostatic interactions, steric interaction, chemisorption, physisorption, or any other way of binding between a relevant molecule and the surface of the first layer. The relevant molecule may be of a molecular species which is desirable to detect by the sensor, or the relevant molecule may be the molecular constituents of a second layer. A second layer may be provided onto a first layer provided on an outer surface of a cantilever in the cantilever array. The second layer may also be provided directly onto an outer surface of a cantilever in the cantilever array.

An outer surface of a cantilever may be surface treated prior to, or as an alternative to, providing the first layer and/or the second layer. For example, an outer surface may be roughened. A rough surface may promote attachment of chemical substances to the surface, such as promote immobilization of molecular species directly on the polymer.

The at least second layer may be a molecular layer, such as a layer of receptor molecules, capable of selectively binding specific molecules, such as macro-molecules, bio-molecules, DNA, amino acids, proteins, cells, various drug molecules, constituents of explosives, traces of toxins, warfare agent, etc. The at least second layer may be a self-assembled layer, such as a self-assembled monolayer.

The cantilever as described above may be part of a chip, the chip further comprising a platform to which the cantilever is attached or protrude from, the platform comprising a t least one conductive structure, the conductive structure being electrically connected to, or an integral part of, the connection means of the cantilever. The material of the chip may be a third polymer-based material, the material of the chip may be the same material as the polymer-based cantilever, i.e. those materials as already mentioned.

Two or more cantilevers may be attached to, or protruding from the platform, so as to form a cantilever array.

By providing an array of cantilevers it is rendered possible to detect multiple different targets simultaneously, such as detecting different chemicals in a solution or a gas. The polymer fabrication provides a convenient way for realising arrays of multiple sensors and to integrate them into a miniaturized bio/chemical analysis system.

The platform may be of any shape and/or size suita le to carry a cantilever or a cantilever array in a situation of use. The platform may be a ad apted to be fastened to a sensor system, such as adapted to be attached in a measuri ng chamber such as in a liquid cell system and/or in a gas handling system, or any system in which actual measurements are to take place.

The cantilever array may comprise a multitude of cantilevers, such as 2-3, such as 2-5, such as 5-10, such as 10-20, such as 20-50 or even more cantilevers, such as more than hundred cantilevers or even more than thousand cantilevers. The cantilevers in the array may all be similar in terms of mechanical and chemical properties, groups of similar cantilevers may be provided, or all of the cantilevers in an array may be provided with different mechanical and/or chemical properties. It may be an advantage to provide similar cantilevers e.g. to compare the signal from different cantilevers with similar properties to obtain a better certainty of the measurements. However, it may also be an advantage to provide different properties of the cantilevers in order to provide a versatile sensor device.

The first and the at least second layer, may be at lea st two different first layers and/or second layers that are provided onto at least two different cantilevers in the cantilever array. It may be an advantage to be able to adjust the chemical and/or biological properties of the individual cantilevers in accordance with the desired substance to detect.

At least one cantilever of the at least two cantilevers may in a situation of use be a reference cantilever used to obtain a reference signal . It may be an advantage to be able to obtain a reference signal simultaneously with obta ining a detection signal, since the reference signal may be used to filter away noise, such as artificial cantilever signals arising form e.g. transient phenomena such as fluctuations in flow speed, fluctuations in concentration, etc. The individual cantilevers in an array may during fabrication be provided with different mechanical properties by providing different dimensions to the cantilevers. It may be an advantage to provide a cantilever array wherein at least two of the cantilevers are provided with different geometrical dimensions, since a more versatile device may be provided.

The polymer-based cantilever(s) and the platform may be made from, or based on, the same material, e.g. made from, or based on, SU-8. It may be an advantage to use as few materials as possible, or to use similar materials, since by using as few materials as possible for the complete device, the number of steps necessary to fabricate the device may by reduced.

According to a second aspect, the invention relates to a method of measuring deformations of a polymer-based cantilever, the polymer-based cantilever comprising a polymer composite structure of a first polymer-based material mixed with particles so as to form polymer-based sensor means, the cantilever further comprising connection means allowing electrical access to the polymer-based sensor means, wherein the method comprising the step of measuring the deformation of the cantilever by detecting a change of the electrical properties of the polymer-based sensor means.

The changes of electrical properties may be due to a deformation of the cantilever e.g. due to particle adsorption on at least an outer surface of the cantilever.

It may be an advantage to measure cantilever deformation according to the method of the second aspect of the invention, since the method is compatible with integrated read-out of a cantilever deformation.

According to a third aspect of the invention is provided, a method of fabricating a cantilever comprising a polymer composite structure of a first polymer-based material mixed with particles so as to form a polymer-based sensor means, the method being a lithography process including the steps of spin-coating a layer of the polymer composite structure on a support and in a subsequent step patterning the layer in accordance with a desired final form of the cantilever.

According to a fourth aspect of the invention is provided, a method of fabricating a cantilever comprising a polymer composite structure of a first polymer-based material mixed with particles so as to form a polymer-based sensor means, the method being a moulding process where polymer-based material is provided in to a mould and pressed so as to obtain a polymer-based structure.

It is an advantage that the device of the present invention, i.e. the cantilever array, may be fabricated by means of different fabrication methods. In the fabrication method according to the second aspect of the invention, photosensitive polymer-based materials may be processed, whereas in the fabrication method according to the third aspect of the invention, non-photosensitive polymer-based materials may be processed. Thus, the fabrication method may be chosen e.g. on the basis of desired properties, or desired material choice, of the device in a situation of use.

Alternatively, a cantilever or a chip comprising one or more cantilevers in accordance with the present invention may also be fabricated by means of a lamination technique where the constituents are fabricated separately and combined to a single unit by means of heating the system or by stamping the layers together by applying a small force.

The device as disclosed in connected with the present invention may in addition of being a sensor device also be an actuator device, where the one or more cantilevers are actuated by means of applying a voltage between at least two connections means, or alternatively where the one or more cantilevers are actuated by means of an external electromagnetic field.

These and other aspects, features and/or advantages of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. Features and/or advantages which have been described in connection with a specific aspect of the invention may likewise be relevant features and/or advantages of other aspects of the invention, as may be recognized by the skilled person. It will therefore be appreciated that features of the invention are susceptible to being combined in any combination without departing from the scope of the invention.

Brief description of the drawings

Preferred embodiments of the invention will now be described in details with reference to the drawings in which:

FIG. 1 illustrates an optical photograph of a cantilever according to the present invention as well as schematic illustrations of alternative embodiments. FIG. 2 illustrates the resistivity of a polymer composite for three different samples as a function of carbon filler (weight percent).

FIG. 3 illustrates the force sensitivity of a cantilever sensor according to the present invention.

FIG. 4 illustrates the slope of the force curve illustrated in FIG. 3.

FIG. 5 illustrates the sensor principle for molecular recognition of a cantilever.

FIG. 6 illustrates two embodiments of a cantilever including a platform.

FIG. 7 illustrates a cantilever array where the cantilevers have been provided on an outer surface with a first layer and for some of the cantilevers also with a second layer.

FIG. 8 illustrates a cantilever array immersed in a solution comprising different molecules.

FIG. 9 illustrates a first method of fabricating an SU-8 cantilever according to the present invention.

FIG. 10 illustrates a second method of fabricating an SU-8 cantilever according to the present invention.

Description of preferred embodiments Cantilevers according to different embodiments of the present invention are shown in FIG. 1. FIG. 1A is in optical photograph of a cantilever 1 attached to a platform 4. The cantilever is fabricated in the polymer SU-8 and the polymer-based sensor means 2, hereafter referred to as a strain sensor, is a polymer composite structure of SU-8 mixed with carbon-black particles. Four connection means 3 provide electrical connection from the outside to the strain sensor, the connection means being four gold electrodes 3. The connection means may be terminated in conducting terminals or be connected to an electrical system for signal handling. The strain sensor 2 is encapsulated in two SU-8 layers — a top layer and a bottom layer. The strain sensor is in the form of a layer. The shown cantilever is approximately 7 μm thick, and approximately 200 μm long and 200 μm wide. A typical cantilever has a thickness between 1 and 10 μm, a typical width between 10 and 300 μm and a typical length between 100 and 200 μm. The size of the platform 4 may typically be in the millimetre or centimetre range. The size ranges provided are only provided to illustrate a typical situation and should not be taken as a limiting factor of the dimension of a cantilever and/or cantilever array and/or platform, any suitable dimension may be used.

In the FIGS. IB to IE alternative embodiments are shown in schematic form. In FIG. IB and 1C L-shaped electrodes 5 are illustrated for a cantilever comprising a small strain sensor 6 and a long strain sensor 7, whereas FIGS. ID and IE illustrate an alternative form of the electrodes 8 being rod-shaped, again illustrated for at small strain sensor and a long strain sensor. It is to be understood that the embodiments illustrated in connection with FIG. 1 does not provide an exhaustive list, any suitable form or shape of either the strain sensor or the electrodes is within the scope of the invention and may be envisioned.

In FIGS. 1A and ID four electrodes are illustrated whereas in FIGS. IB, 1C and IE only two electrodes are illustrated. In most situations only two electrodes are needed, for example for measuring a resistance change between the two electrodes or for detecting a piezo-resistive effect between two electrodes. The presence of four electrodes may be to avoid or minimise the risk of a contact failure between an electrode and the polymer composite structure. Such contact failures may arise from a local defect in the mixing of the particles in the polymer material carrying the particles. In a situation where such a failure is present one may avoid discarding the entire device if a successful contact may be established between at least two of the number of electrodes present. It is to be understood that a different number of electrodes may be provided for various reasons.

The polymer composite structure may be a polymer substance mixed with particles with a given property, such as being conducting, charged, piezo-electric, etc.

In a preferred embodiment the particles are conducting particles and the polymer composite is fabricated along the following lines.

The polymer composite may be fabricated by mixing SU-8 with carbon particles, such as the carbon particles sold by the Colombian Chemical Company

(http://www.colombianchemcals.com) under the name Conductex 975 Ultra. Conductex 975 Ultra is a high structure carbon black with an average particle diameter of 21 nm and is highly conductive. This composite material can be spun on wafers and structured by standard UV-lithography and is compatible with most clean room processes. It can form thin film layers of less than 1.5 μm thickness and structures of 10x10 μm are possible to achieve with the existing fabrication process. The composite samples are prepared by placing a high power ultrasonic source directly into the SU-8 with the added carbon particles. During the mixing the samples are cooled down to avoid evaporation of the SU-8 solvent cyclopentanone. The resistivity of a polymer composite fabricated along the above-mentioned lines is shown for three different samples as a function of the carbon filler (weight percent) in FIG. 2. These measurements were made using test structures prepared on Si wafers with an oxide layer on top. Gold electrodes were provided on the insulating oxide and the polymer composite was deposited on top of the Au electrodes, in this way a good contact between the electrodes and the composite material is achieved.

The transformation from an insulating "clean" polymer to a conducting polymer composite with increasing conductivity (reducing resistivity) for increasing content of conducting particles is a well know effect described by percolation theory. The threshold or critical point below which the polymer composite is insulating is known as the percolation threshold.

Percolation theory describes the conductivity of polymer composite materials based on a conducting network of randomly dispersed conducting particles in the insulating polymer matrix. The resistivity of the composite material with carbon particles above the percolation threshold can generally be modelled by the power law relation

P. \

where p is the resistivity, p₀ is the resistivity scale factor, P is the carbon concentration, P_c is the carbon concentration at the percolation threshold, and t is the critical exponent.

The percolation threshold was determined to be close to 12% (vertical dashed line 20). A polymer composite with a carbon content of approximately 16% was chosen for the fabrication of the cantilever sensor. This was done in order to operate well above the percolation threshold such that large fluctuations in the resistance were avoided. At the same time the composite should have as low a resistance as possible and the gauge factor should be maximized. From FIG. 2 it can be seen that the conducting properties of the SU- 8/carbon composite material are in good agreement with standard percolation theory used to obtain the fitted line 21. It should be noted that a carbon content of 16% was not chosen for a specific reason other than mentioned, other concentrations may likewise be used.

The force sensitivity of the cantilever sensors fabricated by the process as described above has been determined by deflecting the cantilever. The results can be seen in FIG. 3. The cantilever was deflected in one end with a sharp needle controlled by a micrometer screw in steps of 5 μrn while the resistance was measured between two electrodes. At first the cantilever was left alone to check if the signal was stable, then it was deflected and then slowly released again. This gave the first peak 30 on FIG. 3. This was repeated (second peak 31) except that the needle was moved away from the cantilever after the full deflection distance of 50 μm.

The force sensitivity of a rectangular cantilever can be described by the following equation

where ΔR /R is the relative change in resistance and K is the gauge factor of the cantilever respectively, E is Young's modulus, I, w, h are the length, width, and thickness of the cantilever, d is the distance from the neutral axis and F is the force applied at the apex of the cantilever. As may be seen from the equation that, apart from the cantilever dimensions it is the relation between the gauge factor and Young's modulus of a material which determines the sensitivity of the cantilever sensor.

The slope of the three steps: deflection 40, release 41, and second deflection 42 as described in connection with FIG. 3 are plotted in FIG 4. This gives according to the above stated equation a value for the gauge factor K between 15 and 20 for the SU-8/carbon polymer composite material. The values have been reproduced on other cantilevers. Since the calculation of the gauge factor is based on a simple model and at the same time very dependent on the exact location of the micrometer screw during the deflection it leaves an error margin. The error margin is estimated to be at least 15%.

Young's modulus of SU-8 is approximately 5 GPa. Together with a gauge factor of 15-20 this gives a material that is 5-10 times more force sensitive than Si when integrated in an SU-8 cantilever.

In FIG. 5, the principle for molecular recognition of a cantilever is illustrated. In the figure a cantilever 52 attached to a platform 51 is seen from the side. One side of a cantilever is covered by a fi rst layer 53, such as a metal layer, or more specifically such as a gold layer. In the figure the metal layer is provided on the top side of the cantilever, however the layer may alternatively be provided on the bottom side. It is, however important that the two sides are provided with two different surfaces in order to induce a differential surface stress on the two sides, since it is the differential surface stress that will cause the cantilever to bend (FIG. 5C). In FIG. 5B the cantilever array is immersed in a solution of e.g. a self-assembling alkanethiol. The gold layer allows for adsorption, or immobilization of different molecules specifically on one side of the cantilever through S-Au bond.

In general molecular layers are known to induce surfa ce stress when they bind to a surface, due to van der Waals, electrostatic or steric interactions. A monolayer of receptor molecules is immobilized on one side of the cantilever so the molecules to be detected bind specifically to the immobilization layer, such as a gold layer, and then difference in surface stress on opposite sides of the cantilever is measured, e.g. by measuring the induced resistance change in the strain sensor according to the present invention. The cantilever deflection, Δz, resulting from this difference in surface stress can be approximated by (J. E. Sader, J. of Applied Physics, vol. 89, p. 2911 (2001)): . 3(l - v)E² ,_A . . Et^{2 (Δσ"}ι ^{~ Δσ}^ where Ε is the Young's modulus, v is Poisson ratio, t a nd L are thickness and length of the beam and Aσ_t - Δσ₂ accounts for the differential surface stress.

In connection with FIG. 5 static bending (static mode) of the cantilever is discussed. However, the cantilever may also be operated in the dynamic mode. In dynamic mode detection, the cantilever is oscillated, e.g. by means of a piezo-oscillator attached to the platform, the bending of the cantilever is monitored e. g. by measuring the oscillation frequency of the measured resistivity. From the measured signal, the changes in the resonant frequency of the cantilever from the immobil ization of molecules onto the cantilever is deduced. The resonance frequency of a cantilever depends, inter alia, on the cantilever mass, near environment viscosity and surfa ce stress.

In FIG. 6 two embodiments of a cantilever including a platform 60 according to the present invention are shown. In FIG. 6A one layer 61 of a polymer composite is provided as the strain sensor, whereas in FIG. 6B two layers 62, 63 are provided as the strain sensor. The strain sensor are connected with connection means 64-, 65, 66 such as gold electrodes. The electrodes include terminals 67 for electrical access to the sensor means through the connection means from the outside. The terminals or the connection means may also be directly attached to a signal processor, such as an electronic circuit. The cantilever chips shown are illustrated in cross-section, by comparison with FIG. 1 where the cantilevers are illustrated in top-view it may be seen that at least two electrodes and hence at least two terminals are connected to each sensor means. This may also be seen in FIG. 7 where two terminals are illustrated for each cantilever in a cantilever array. For the embodiment illustrated in FIG. 6B the cantilever bending may be deduced by two independent measurements. For example if the cantilever bends downwards, the upper sensor means 62 detects a stretching of the layer, whereas the lower sensor means 63 detects a compression of the layer. By comparing the two measurements a better measurement certainty may be achieved. This may especially be important for a cantilever operated in the dynamic mode.

In FIG. 7 a cantilever array 70 where in addition to a first layer being a metal layer 72 provided on one side of a cantilever 71, a second layer 73 is provided onto the first layer. The second layer is a molecular layer ca pable of selective bonding to specific molecules. For the cantilever array in FIG. 7, the first layer 72 is typically a gold layer. The gold layer being used to immobilize a second layer, being a molecular layer, the second layer being used as a sensor or receptor layer.

In the embodiment illustrated in FIG. 7, the first layer is of the same material for all of the cantilevers in the cantilever array. On two of the cantilevers, a first type of a second layer 73 is provided, on other two of the canti levers, a second type of a second layer 75 is provided, and on yet other of the cantilevers 77, a second layer is not provided. By not providing a second layer on one or more cantilevers, the signal from such cantilevers may be used as reference cantilever(s), used to filtrate background noise away. Alternatively, a reference cantilever may be provided with a layer which does not act as a detector. In this way the reference cantilever may be made as identical to the other cantilevers as possible. For each of the cantilevers two terminals 78 for external electrical access to the strain sensor 79 are illustrated.

By providing different types of second layers on different cantilevers, the presence of different molecules in a solution may be detected. This is illustrated in FIG. 8.

In FIG. 8 a cantilever array 80 as described in connection with FIG. 7 is immersed in a solution comprising different molecules 81, 82, 83. By selecting appropriate second layers, the presence of various molecules in the solution may be selectively detected.

Different polymer materials may be used to fabricate the polymer-based cantilever array. In a preferred embodiment the photosensitive polymer SU-8 is used. In FIGS. 9 and 10 two methods of fabricating a cantilever array are illustrated. It is to be understood, that alternative polymer materials may be used, such as parylene or any other suitable material. In FIG. 9 a first method of fabricating an SU-8 cantilever with a strain sensor is described. The method being a photolithographic method.

First, as illustrated in FIG. 9A a release layer of Cr/Au/Cr with a thickness of 50/500/500 A respectively is deposited on a 4" Si wafer. Subsequently, in FIG. 9B, a thin layer 92 of 800 nm SU-8 is deposited and structured to use as the topside of the cantilever. Next (FIG. 9C) a 500 nm thick layer of Au 93 is e-beam evaporated on top of a 1.5 μm thick photoresist pattern and the gold electrodes are structured with a lift-off process. A 1.4 μm thick layer

94 of the SU-8/carbon composite material is spun on top of the Au-wires and structured by UV-lithography (FIG. 9D). To encapsulate the strain sensor 94 a 4.0 μm thick SU-8 layer

95 for the backside of the cantilever is deposited (FIG. 9E) — at the same time this layer roughly determines the distance from the strain sensor to the neutral axis in the cantilever. Finally as illustrated in FIG. 9F, the support structure, i.e. the platform 96 consisting of a 200 μm thick SU-8 layer is deposited and structured by UV-lithography and the chips are released from the wafer in a Cr-etch (FIG. 9G).

A second method of fabricating a cantilever device according to the present invention is illustrated in FIG. 10. In the second method the fabrication of a cantilever array is illustrated, in the method a micromoulding technique is used. In FIG. 10A a cross-section of the micromould 100 is illustrated, whereas a top view of the mould is illustrated in FIG. 10B. The mould may be fabricated in silicon or any other suitable material. In FIG. 10C polymer material 101 is filled into the mould and pressure is applied to shape the polymer and to get a flat surface. The pressure is applied (FIG. 10D) by pressing a plate 102 against the micromould and thereby applying pressure to the polymer material encapsulated by the mould and the plate. The moulded cantilever array may be released from the mould by using a sacrificial layer etch or by coating the mould with an anti- sticktion layer such as Teflon. The strain sensor, or other layers, may be provided by using sequential moulding.

The individual cantilevers and/or groups of cantilevers may be provided with different shape, by providing a mould where the shape of the individual cantilever moulds are different. By using a mould for the fabrication of the cantilever array, a large freedom in the possible shapes of the resulting cantilevers is obtained. The plate 102 may be provided with a contoured surface facing the polymer material, so that a top shape of the cantilevers and/or the platform may be provided.

The cantilevers fabricated by one of the fabrication methods described in connection with FIGS. 9 and 10 are adequate to be used in both the static and dynamic modes of operation. Possible applications of the present invention include:

Molecular diagnosis - DNA biosensors

- Immunoassays

- Point-of-care diagnosis

- Screening of food supplies

- Veterinary diagnosis

Proteomics

- Localisation of proteins with in living cells

- Study of cell signalling pathways

- Detection of protein-protein interactions

Drug discovery

- Real-time monitoring of environmental toxins

Detection of biological and chemical warfare agents - Detection of explosives

The various applications may be rendered possible by choosing the a ppropriate first and/or second layer. The second layers may be provided using conventional immobilisation chemistry, including coating, photo-activated binding site, inkjet-prin ter principle, etc. Examples of second layers that may be used in connection with the a bove-mentioned applications include: single stranded DNA e.g. from disease-associated genes, antigens, nucleic acids, protein, cells, TNT, polymers etc.

It is to be understood that the application of the present invention is not limited to the above-listed examples.

Although the present invention has been described in connection with preferred embodiments, it is not intended to be limited to the specific form set forth herein. Rather, the scope of the present invention is limited only by the accompanying claims.

In this section, certain specific details of the disclosed embodiment such as material choices, geometry or architecture of the device or parts of the device , techniques, measurement set-ups, etc., are set forth for purposes of explanation rather than limitation, so as to provide a clear and thorough understanding of the present invention. However, it should be understood readily by those skilled in this art, that the present invention may be practised in other embodiments which do not conform exactly to the deta ils set forth herein, without departing significantly from the spirit and scope of this disclosure. Further, in this context, and for the purposes of brevity and clarity, detailed descriptions of well-known apparatus, circuits and methodology have been omitted so as to avoid unnecessary detail and possible confusion.

It will be appreciated that reference to the singular is also inten ed to encompass the plural and vice versa, and references to a specific numbers of features or devices are not to be construed as limiting the invention to that specific number of features or devices. Moreover, expressions such as "include", "comprise", "has", "have", "incorporate", "contain" and "encompass" are to be construed to be non-exclusive, namely such expressions are to be construed not to exclude other items being present.

Claims

1. A polymer-based cantilever comprising a polymer composite structure of a first polymer-based material mixed with particles so as to form a polymer-based sensor means, the cantilever further comprising connection means allowing electrical access to th e polymer-based sensor means.

2. A cantilever according to claim 1, wherein the particles are conductive particles.

3. A cantilever according to any of the claims 1 or 2, wherein a deformation of the cantilever is detected by the sensor means by detecting a change in the resistance of the polymer-based sensor means.

4. A cantilever according to any of the claims 1 or 2, wherein a change of the resistance of the polymer-based sensor means is induced from a chemical reaction between at l east a part of the polymer-based sensor means and a chemical substance.

5. A cantilever according to any of the preceding claims, wherein the cantilever is a layered structure comprising at least two layers of the polymer-based sensor means separated by at least a non-conductive polymer-based separation layer.

6. A cantilever according to any of the preceding claims, wherein at least part of cantilever is encapsulated in a non-conducting second polymer-based material.

7. A cantilever according to any of the preceding claims, wherein the conductive particles are carbon particles.

8. A cantilever according to any of the preceding claims, wherein the conductive particles are crystalline particles.

9. A cantilever according to any of the preceding claims, wherein the polymer-based sensor means is piezo-resistive.

10. A cantilever according to any of the preceding claims, wherein the polymer-based sensor means is piezo-electric.

11. A cantilever according to any of the preceding claims, wherein the average size of particles are of a size so that the average diameter is between 2 nm and 1 μm, such as between 5 nm and 500 nm, such as between 10 nm and 250 nm, such as between 20 and 100 nm, such as between 25 and 75 nm, such as between 30 and 60 nm, such as between 40 and 50 nm.

12. A cantilever according to any of the preceding claims, wherein the particles are 5 substantially homogeneously distributed in the polymer composite structure.

13. A cantilever according to claim 12, wherein the substantially homogeneously distributed particles are mixed with the first polymer-based material by means of ultrasound.

10 14. A cantilever according to any of the claims 2-13, wherein the density of the particl es in the first polymer-based material is above the percolation threshold for the conductive particles in the first polymer-based material.

15 15. A cantilever according to any of the claims 2-14, wherein the ratio between the gauge factor and the Young's modulus of the cantilever is between 0.25 and 25 of the ratio of a silicon-based cantilever, such as between 0.5 and 15, such as between 1 and 10, such between as between 2 and 5.

20 16. A cantilever according to any of the preceding claims, wherein the connection means is at least one conductive or semiconductive structure.

17. A cantilever according to any of the preceding claims, wherein the polymer composite structure can be patterned by means of photolithography.

25 18. A cantilever according to any of the preceding claims, wherein the polymer of one or more of the polymer-based materials are selected from the group consisting of photosensitive polymers, such as: SU-8 based polymers, such as an XP SU-8 polymer, polyimides or BCB cyclotene polymers and parylene.

30 19. A cantilever according to any of the preceding claims, wherein an outer surface of the cantilever is coated with one or more immobilization layers.

20. A cantilever according to claim 19, wherein a first layer of the one or more 35 immobilization layers is a metal layer.

21. A cantilever according to claim 19, wherein a second layer of the one or more immobilization layers is a molecular layer.

22. A chip comprising a cantilever according to any of the claims 1-21, the chip further comprising a platform to which the cantilever is attached or protrude from, the platform comprising at least one conductive structure, the conductive structure being electrically connected to, or an integral part of, the connection means of the cantilever. 5 23. A chip comprising a cantilever according to claims 5, the chip further comprising a platform to which the cantilever is attached or protrude from, the platform comprising at least two conductive structures, the at least two conductive structures being electrically connected to, or an integral part of, the at least two the connection means connected to

10 the at least two layers of the polymer-based sensor means, the at least two conductive structures being provided in at least two layers of the platform.

24. A chip according to any of the claims 22 or 23, wherein the material of the chip is a third polymer-based material.

15 25. A chip according to claim 24, wherein the polymer of the third polymer-based materials is selected from the group consisting of photosensitive polymers, such as: SU-8 based polymers, such as an XP SU-8 polymer, polyimides or BCB cyclotene polymers and parylene.

20 26. A chip according to any of the claims 23-25, comprising two or more cantilevers according to any of the claims 1-21 so as to form a cantilever array, the two or more cantilevers being attached to, or protruding from the platform.

25 27. A chip according to claim 26, wherein at least one cantilever of the at least two cantilevers in a situation of use is a reference cantilever used to obtain a reference signal.

28. A chip according to claim 26, wherein at least two of the at least two cantilevers are provided with different mechanical properties.

30 29. A chip according to claim 26, wherein at least two of the at least two cantilevers are provided with different chemical properties.

30. A method of measuring deformations of a polymer-based cantilever, the polymer- 35 based cantilever comprising a polymer composite structure of a first polymer-based material mixed with particles so as to form polymer-based sensor means, the cantilever further comprising connection means allowi ng electrical access to the polymer-based sensor means, wherein the method comprising the step of measuring the deformation of the cantilever by detecting a change of the electrical properties of the polymer-based sensor means.

31. A method according to claim 30, wherein the changes of electrical properties is due to 5 a deformation of the cantilever.

32. A method according to claim 30, wherein the changes of electrical properties is due to particle adsorption on at least an outer surface of the cantilever.

10 33. A method of fabricating a cantilever comprising a polymer composite structure of a first polymer-based material mixed with particles so as to form a polymer-based sensor means, the method being a lithography process including the steps of spin-coating a layer of the polymer composite structure on a support and in a subsequent step patterning the layer in accordance with a desired final form of the cantilever.

15 34. A method according to claim 33, further including the step of encapsulating the polymer composite structure in a non-conducting second polymer-based material.

35. A method according to any of the claims 33 or 34, further including the step of 20 providing a connection means in contact with the polymer composite structure.

36. A method according to any of the claims 33 to 35, wherein the particles are conductive particles.

25 37. A method according to any of the claims 33-36, wherein the polymer of one or more of the polymer-based materials is selected from the group consisting of photosensitive polymers, such as: SU-8 based polymers, such as an XP SU-8 polymer, polyimides or BCB cyclotene polymers and parylene.

30 38. A method according to any of the claims 33-37, further including the step of providing an outer surface with a one or more immobilization layers.

39. A method according to any of the claims 33-38, further including the step of proving a platform to which the cantilever is attached.

35 40. A method according to any of the claims 33-39, wherein two or more cantilevers are provided so as to provide a cantilever array.

41. A method according to claims 40, wherein at least two of the cantilevers in the cantilever array are provided with different mechanical properties by providing the cantilevers with different geometrical dimensions in the patterning process.

5 42. A method according to any of the clai ms 40 or 41, wherein at least two of the cantilevers in the cantilever array are provided with different chemical properties by providing the cantilevers with different immobilization layers, the immobilization layers being provided to an outer surface of the cantilevers.

10 43. A method of fabricating a cantilever comprising a polymer composite structure of a first polymer-based material mixed with particles so as to form a polymer-based sensor means, the method being a moulding process where polymer-based material is provided in to a mould and pressed so as to obtain a polymer-based structure.

15 44. A method according to claim 43, wherein the cantilever is a layered structure and wherein the moulding process is a sequential moulding process.

45. A method according to any of the clai ms 43 or 44, wherein the polymer of one or more of the polymer-based materials is selected from the group consisting of photosensitive

20 polymers, such as: SU-8 based polymers, such as an XP SU-8 polymer, polyimides or BCB cyclotene polymers and parylene.

46. A method according to any of the clai ms 43-45, further including the step of providing an outer surface with a one or more im obilization layers.

25 47. A method according to any of the clai ms 43-46, further including the step of proving a platform to which the cantilever is attached.

48. A method according to any of the clai s 43-47, wherein two or more cantilevers are 0 provided so as to provide a cantilever array.

49. A method according to claim 48, wherein at least two of the cantilevers in the cantilever array are provided with different mechanical properties by using a mould with different geometrical dimensions for at least two of the at least two cantilevers in the 5 cantilever array.

50. A method according to claim 48, wherein at least two of the cantilevers in the cantilever array are provided with different chemical properties by providing the cantilevers with different first or second layers.

51. A method of fabricating a cantilever or a chip comprising one or more cantilevers according to any of the claims 1-29, wherein the cantilever or the chip comprising one or more ca ntilevers are fabricated by means of lamination technique where the constituents 5 are fabricated separately and combined to a single unit by means of heating the system or by stam ping the layers together by applying a small force.

52. An actuator according to any of the preceding claims, wherein the one or more cantilevers are actuated by means of applying a voltage between at least two connections

10 means.

53. An actuator according to any of the preceding claims, wherein the one or more cantilevers are actuated by means of an external electromagnetic field.

15 54. A sensor for measuring the presence of a substance in a fluid or in a gaseous ambient, the sensor being provided in accordance with any of the preceding claims.

55. Use of the sensor according to claim 54, wherein the sensor is a biosensor.

20 56. Use of the sensor according to claim 54, wherein the sensor is a disposable item.