GB2462604A

GB2462604A - Thin film structure for use in bioassay applications

Info

Publication number: GB2462604A
Application number: GB0814659A
Authority: GB
Inventors: Carl Edward Norman; Katja Franziska Kastl
Original assignee: Toshiba Research Europe Ltd
Current assignee: Toshiba Europe Ltd
Priority date: 2008-08-11
Filing date: 2008-08-11
Publication date: 2010-02-17
Anticipated expiration: 2028-08-11
Also published as: GB2462604B; GB0814659D0

Abstract

A thin film structure comprising first and second non-metallic layers 1, 3 having a tendency to curl in opposite directions to one another. The thickness of the first and second films is chosen to balance the internal stress of the layers. The layers can be deposited by an evaporation technique and are preferably oxides of the same element but with different stoichiometry e.g. SiO, SiO2. This may be achieved by varying the oxygen pressure during the evaporation technique. Further magnetic and metallic layers may be provided. The structure may also be patterned in order to produce a diffraction grating. The structure may be used as an encoded carrier, apparatus for reading the encoded carrier also being claimed.

Description

Thin Film Structure, Method For Fabricating A Thin Film Structure and Apparatus for Reading a Thin Film Structure The present invention relates to the field of thin film structures and methods for their fabrication. More specifically, the present invention relates to a thin film structure suitable for Bioassay applications.

There has always been a need to be able to quickly and efficiently monitor reactions between different chemicals in order to both identify unknown molecules and to study the reactions of both known and unknown molecules. This need has become all the more acute with the substantial and numerous discoveries being made in the

biotechnology field.

DNA has the capacity to "hybridise", in other words, a single strand of DNA is capable of pairing to its complementary single strand but not pairing to an unrelated sequence.

Using this technique it is possible to identify and monitor the reactions of an unknown strand of DNA by attempting to hybridise it with known DNA strands.

Currently, there are a number of methods known for performing this analysis. One particular method is taught in GB 2 404 918. Tn this patent, encoded carriers are formed.

The encoded carriers have a first region which is a reaction region and a second region which is a code region. The reaction region has a variation in its refractive index so that a surface plasmon resonance measurement may be made.

In a surface plasmon resonance measurement, if radiation of a certain wavelength, polarisation angle and incident angle irradiates a structure which has a periodic variation in its refractive index, the radiation will couple to the surface plasmon mode causing a decrease in the amplitude of the reflected light. The periodic variation in the refractive index serves to couple radiation with a correct incident angle and properties to the surface plasmon mode.

The incident angle at which surface plasmon resonance occurs will be dependent on any chemical species on the surface. In GB 2 404 918, probe molecules are attached to the surface of the reaction region. If the probe molecules then react with a further chemical species, the incident angle of the radiation necessary to excite surface plasmon resonance will vary. Therefore, it is possible to determine if a reaction has taken place.

In GB 2 404 918, encoded carriers are provided with a code corresponding to the different types of probe molecules used in the reaction region.

However, the measurement of the incident angle will be affected by any curvature of the surface or other strain related phenomena.

Thus, in a first aspect, the present invention provides a thin-film structure comprising a non-metallic first layer and a non-metallic second layer, the first layer and the second layers having a tendency to curl in opposite directions to one another, the thickness of the first film and second layers being chosen such that the internal strain of the layers balances to produce a flat structure.

The above structure does not bend, and the stored elastic energy within the strain-compensated structure acts so as to discourage (by lowering the chemical driving force) the diffusion of ambient materials such as water and oxygen into the structure. Such materials tend to change the internal stresses within mixed stoichiometry oxides.

Preferably, the layers will be formed using a growth tecimique such as evaporation.

Thus, the boundary between the first and second layers is diffuse as opposed to sharp if the layers were formed and then bonded together.

Preferably, the first layer and second layers are oxides constituted of the same elements and wherein the first layer has a different stoichiometry to the second layer. In a preferred embodiment, Silicon oxides are used, for example, the first layer may primarily comprise Si02 and the second layer may primarily comprise SiO. In general, one layer will be SiO, and the other will be SiO, where 1<x �=2, 0.5<y�=2 and y<x. y will usually be 1. However, as it is sometimes difficult to obtain perfect stoichiometry, ymay vary from 1.

Other layers may also be present, for example, a metal layer. A metal layer may be desirable for surface plasmon resonance experiments. It may be desirable to add a magnetic layer to control the structures using a magnetic field. Further layers which provide some function may also be provided. For example layers with an electrical charge may be controlled by an external electric field. Such "functional layers" may be additional layers or may be provided by the oxide layers of the carrier which may carry an electric charge.

Also, the structure may comprise 3 or more layers whose thicknesses have been chosen to balance the internal strain of the layers.

The present invention may be applied to any technology or apparatus that requires flat, stable, chemically inert, free-standing structures including tethered structures such as cantilevers in MEMS. However, the primary use of the present invention is envisaged to be for encoded carriers in bioassay applications. For such applications, the structure may comprise a diffraction grating. A code may also be provided on the structure.

In a second aspect, the present invention provides a method of fabricating a thin film structure, the method comprising evaporating a first non-metallic layer and evaporating a second non metallic layer, the layers having a tendency to curl in opposite directions to one another, the thickness of the first film and second layers being chosen such that the curling forces balance to produce a flat structure.

The first and second layers may be oxides which are grown from the same evaporation source and differences in the stoichiometry are achieved by varying the oxygen pressure. Alternatively, the layers may be evaporated from different sources.

In a preferred embodiment, the method further comprises: providing a substrate having a raised pattern formed on a surface of said substrate, evaporating said first and second layers over said substrate; dissolving said substrate to release said deposited material to form a patterned thin film structure from said released deposited material.

The substrate pattern preferably comprises sidewalls which are parallel to the deposition direction of the layers.

In a directional deposition process, there is always a danger of partial sidewall coverage in pits or on mesas, which would result in "wings" of extraneous material attached to the structures. These can prevent the structures from lying down flat in the flow cell during an analysis, or may act as "sails" which allow the fluid flow to lifi structures off the flow cell floor. To avoid this, the total thickness of the layers evaporated is substantially equal to the depth of the pattern provided on said substrate.

As previously mentioned, the carrier may be advantageously provided with a magnetic layer. However, such a layer is advantageous even if the carrier does not comprise multiple non-magnetic layers. Therefore, in a third aspect, the present invention provides an encoded carrier comprising a code region having a code and a reaction region separate from said code region, said reaction region having a variation in its refractive index or dielectric constant in a direction generally parallel to the surface of the reaction region, the carrier having a layered structure comprising at least one magnetic layer.

The addition of a magnetic layer provides a way of making the encoded carriers lie flat on a surface and prevents movement of the carriers during measurement. Therefore, in a fourth aspect, the present invention provides an apparatus for reading an encoded carrier, said encoded carrier comprising a code region having a code and a reaction region separate from said code region, said reaction region having a variation in its refractive index or dielectric constant in a direction generally parallel to the surface of the reaction region, the carrier comprising at least one layer which is attracted or repelled by an external field, said apparatus comprising means to perform a measurement of a physical property of the reaction region; and means to read the code provided on said coding area, the apparatus further comprising means to apply an external field to control the movement of said carrier.

For example, the external field may be used to anchor said carrier to a surface during measurement of the carrier. External fields may also be used for orientation of the carriers, for example, to align the gratings, ensuring that the gratings all lie with the same side uppermost and compacting the gratings closer together.

The layer which is attracted or repelled by an external field maybe a magnetic layer and

the external field is a magnetic field.

An external electric field could be used in a further embodiment to manipulate carriers with an electric charge.

The present invention will now be described with reference to the following non-limiting embodiments in which: Figure 1 a schematically illustrates two layers which may form a thin film structure in accordance with an embodiment of the present invention and figure lb is a thin film structure in accordance with an embodiment of the present invention; Figure 2a is a schematic of a thin film structure in accordance with an embodiment of the present invention, figured 2b is a graph illustrating the ratio of thicknesses of the layers the layers and the effect on the curvature of the structure; Figure 3a is a thin film structure in accordance with a further embodiment of the present invention comprising four layers and figure 3b is a thin film structure in accordance with a further embodiment of the present invention comprising three layers; Figure 4 is a thin film structure, in accordance with an embodiment of the present invention configured as an encoded carrier; Figures 5a and b schematically illustrate fabrication steps for fabricating a thin film structure on a patterned substrate in accordance with an embodiment of the present invention; Figures 6a and 6b schematically illustrate fabrication steps for fabricating a thin film structure on a patterned substrate in accordance with a further embodiment of the present invention; Figure 7a is an encoded carrier in accordance with an embodiment of the present invention and figure 7b is a plot of the reflected amplitude against angle of incidence of radiation impinging on said carrier; Figure 8a is a schematic of an encoded carrier in accordance with a further embodiment of the present invention and figure 8b is the corresponding plot of reflected amplitude against angle of incidence; and Figure 9 is an apparatus which may be used to read the encoded carrier of figures 4 to 6.

Figure la shows a first layer 1 and a second layer 3. Both layers are formed by evaporation. If the first layer was formed on its own it would start to curve such that the end of the layer curved upward with respect to the plane of the paper. If layer 3 was formed on its own, it would also start to curve, but in this case, layer 3 would curve in the opposite direction to that of layer 1.

Therefore, by a growing layer 1 followed by the second layer 3, it is possible to obtain a flat structure, where the internal stress of the first layer which causes it to curve is balanced by the internal stress of the second layer. Thus, a flat structure may be formed.

In other words, a multi-layered structure is formed with the internal stress configured in such a way that the structure does not bend overall and the stored elastic energy within the strain-compensated structure acts to discourage (by lowering the chemical driving force) the diffusion of ambient materials such as water and oxygen into the structure.

From a practical fabrication point of view, it is desirable for the material of layer 1 to be as similar as possible to the material of layer 3. Certain oxides have been found to modify the internal stresses dependent on their stoichiometry.

A particular example of this is silicon. Silicon monoxide when deposited by e-beam evaporation tends to curl the opposite direction to silicon dioxide when deposited by e beam evaporation.

Thus, it is possible to fabricate a thin-the film structure contain a SiO layer of a specific thickness and a SiO, layer (1<x �=2) of a specific thickness by evaporating SiO and changing the oxygen overpressure during growth to change one of the layers from a Silicon monoxide layer a silicon dioxide layer during growth. The SiO layer may not have a perfect stoichiometry of 1 Si to 1 0 and thus the stochiometry may vary.

However, the stochiometry of the SiO layer is set such that it will have a tendency to curl in the opposite direction to that of the SiO, layer.

Figured 2a schematically illustrates such a structure. To avoid in any unnecessary repetition, like reference numerals will be used to denote like features. Although it is possible to balance the internal stresses to achieve a flat structure, the layer thicknesses must be carefully chosen.

Figure 2b is a plot of the curvature of a two layer structure against the ratio of layer thicknesses TA/TB where TA is the thickness of the Si0, layer, TB is the thickness of the SiO layer and 1<x �=2. A particle deflection of zero indicates a flat structure.

It can be seen from figured 2b that the growth rate also affects the relative thicknesses of the layers required in order to produce a zero deflection. In practice, experiments may be performed to determine the exact values for the graph of figure 2b and then the device can be fabricated using the growth rate and layer thicknesses which correspond to a zero deflection. It has been found that for a SiO growth rate of 0.5 nmls and a SiO growth rate of 0.1 mm's, a flat structure is produced if the ratio of SiO to Si0, is 955:250.

Figures lb and 2a show two layer thin film systems. However, the present invention may also be extended to multiple layer systems of the type shown in figures 3a and 3b In figure 3a, a three layer structure is shown. The first layer 11, provides a curvature in a first direction and the second layer 13 provides curvature in the opposing direction.

Third layer 15 provides curvature in the direction of layer 11 and finally fourth layer 17 provides curvature in the same direction as that of layer 15. Providing the layer thicknesses and growth rates are balanced so that the internal stresses of the individual layers cancels out, the structure will remain flat and have the same advantages as those described with reference to figures 1 and 2.

Also, it is not necessary to have an even number of layers. Figure 3b shows a device where the first and third layers 11, 15 provide curvature of the first direction, and the middle layer 13 located between the first and third layers provides curvature in the opposing direction.

The structures of figures 3a and 3b may be fabricated by modulating the oxygen supply while growing the layers. During growth, the growth rate often varies. Providing that the thickness ratio of the oxygenated to de-oxygenated layers is kept constant, any variations in the growth rate should have minimal affect on the overall curvature.

Figure 4 is a planar view of a thin film structure in accordance with an embodiment of the present invention.

Figure 4 illustrates an encoded carrier to be used in surface plasmon resonance experiments of the type described with reference to figures 7, 8 and 9. The encoded carrier has a reaction region 43 and a coding region 45 comprising a code 47.

The reaction region 43 comprises a diffraction grating 49. In this embodiment, the diffraction grating comprises elongate rectangular holes 51 which are oriented parallel to one another and extend through the whole of the encoded carrier 41. Thus, the diffraction grating 49 comprises a plurality of stripes which extend through the two layers. Although in this particular embodiment, the holes 51 extend through the whole of the encoded carrier, they may also extend just part of the way into the reaction region, such that the diffraction grating 49 comprises alternating stripes of different thickness in the direction into the plane of the encoded carrier 41. Alternatively, the carrier may be corrugated, possessing a series of ridges in the upper surface, each of which correspond exactly to a groove in the lower surface, and vice-versa. Thus the overall thickness of the carrier may be approximately constant, with corresponding undulations in the two larger surfaces of the carrier providing the diffraction grating.

The code 47 is a bar code provided at the edge of the encoded carrier 41 and extends over the edge of the carrier 41. The code has been designed so that if the encoded carrier 41 is read from the opposite side, i.e. from underneath the plain of paper, each numeral can still be uniquely identified.

Corner 53 of encoded carrier 41 has been removed so that the orientation of the carrier can be determined. Confirmation of the orientation of the carrier 41 is necessary in order to correctly measure the reaction area since the orientation of the diffraction grating 49 with respect to the laser should be the same for all encoded carriers 41. The orientation of the carrier 41 may also be useful for reading the code 47.

Typically, the encoded carrier 41 will be 50 to 100 tm square. Each of the characters of the bar code will typically have a length between 5 and 10 jim.

Figures 5A and SB are cross sectional views, showing two stages in the production of an encoded carrier according to an embodiment of the invention. In the embodiments described with reference to the previous figures, the thin film structure which forms the carrier has at least two layers which are selected such that their internal stresses balance one another. However, for simplicity, only a single layer is shown in Figures 5A and SB.

In figure 5A, a soluble substrate 301 has a raised pattern 303 provided on lower substrate level 305. The pattern 303 defines an upper level 307 which is separated from the lower level 305 by substantially vertical side walls 309.

The soluble substrate may be positioned by wet or dry etching techniques such as RIlE.

Alternatively, the substrate may be made from a mould.

Figure 5A is a cross section through pattern 303, such that the pattern 303 appears as a plurality of vertical pillars. In reality the pillars are elongated into the plane of the paper and are connected at their ends to form, in this embodiment, a continuous structure as indicated by the dotted lines.

In figure 5A, a first dielectric layer of SiO and a second layer of SiO, are evaporated using e-beam evaporation on the substrate 301. The thickness and the growth rates of the two dielectric layers are selected so that the internal stresses balance as described with reference to figure 2.

The two dielectric layers are grown using only one solid source (SiO) and employ an overpressure of 02 gas to change the stoichiometry of the oxide during growth. This method minimises the number of crucibles which need to be used in the growth apparatus and allows the main body of the particle to be grown without interruption -the 02 is simply turned on and off, and the growth rate adjusted by changing the electron beam current.

The optimum 02 overpressure is around 1 e-5 Torr. Half an order of magnitude lower than this there is insufficient oxygen to saturate the layer structure (at practical growth rates), whereas one order higher and the high voltages in the e-beam kit may cause the filament to spontaneously incandesce, risking burn-out.

A metal layer is also deposited onto the substrate 301 with the dielectric layers. If the structure is to be used as an encoded carrier, the metal layer is preferably Au. A thin Cr layer may also be provided between the dielectric and the Au or other metal to aid adhesion of the metal to the dielectric.

One or more further layers may also be provided, for example, a layer which provides some physical characteristic or function to the structure. For example, a magnetic layer, e.g. Ni may be provided to allow the motion or position of the particles to be controlled by a magnet.

The deposited layers provide an upper coating 313 formed on upper level 307 and a lower coating 317 formed on lower level 305. The total thickness of the deposited layers is chosen such that it is close to the height of the side walls 309. The deposition is directional such that neither of the layers are provided on the side walls 309. The side walls 309 cause the coating 313 to be discontinuous from the lower coating 317. Due to the overall shape of pattern 303, the sections of upper coating 313 are all connected to each other to form an essentially planar structure.

The substrate 301 is dissolved in order to release the upper coating 313 as shown in Figure SB. Making the total thickness of the layers to be equal to the height of the side walls 309 avoids partial sidewall coverage, which results in "wings" of extraneous material attached to the particles.

This upper coating forms a continuous mesh. By appropriate patterning of the substrate 301, both the code area and the reaction region may be patterned using this technique.

Figure SB shows the stage of the process after the substrate 301 has been partially dissolved to release the mesh 313. The substrate is dissolved by placing it in a flowing solvent. The flow direction of the solvent is indicated by the arrow.

As the substrate continues to dissolve, the lower coating 317 is released which forms the camer.

Figures 6A and 6B show two stages in a method of making a thin film structure as a carrier with a corrugated diffraction grating. A substrate 321, as shown in perspective view in figure 6A, is provided with a three dimensional relief pattern 323 on its surface.

The pattern 323 comprises "V" shaped indents 325 and indents with vertical side walls 327.

Figure 6B shows an example of material to form an encoded carrier deposited onto the substrate of Figure 6A. The material is deposited over the relief of the substrate 321.

Material deposited over the V-shaped indents 325 takes on a corrugated form so that it may function as a diffraction grating. Material deposited over the indents with vertical side walls 327 may form part of the code region or may be used to separate adjacent encoded carriers.

The indents with the steep side walls in both Figures 5 and 6 may be used to form holes which are bounded on each side or may be used to form holes which are open to the edge of the carrier. To avoid debris getting caught in the holes, it is preferable if the code is formed from bars or characters which are open to the edge of the carrier.

Figure 7 will be used to explain how the encoded carrier fabricated as describe above is used.

Figure 7a is a schematic cross-section of an encoded carrier 1 of the type intended to be measured using apparatus in accordance with an embodiment of the invention. The encoded carrier 1 comprises a first metal layer 205 of Au, a first dielectric layer 206 of SiO,, and a second dielectric layer 207 of SiO. The order of these layers may be changed. For example SiO and SiO, can be swapped. Also, the Au layer may be covered with a thin layer of SiO, on the surface.

Region 203 is the reaction region of the encoded carrier 1, in this region, the first and second dielectric layers 206, 207 and the metal layer 205 are corrugated to form a diffraction grating.

A beam of radiation 210 is incident on said carrier 201 at angle 0. The radiation passes through dielectric medium 215 which is provided by a liquid or gas overlying the top of the carrier. If the carrier is provided within a flow cell, the medium 215 can be provided by the liquid with the flow cell. When the radiation has the appropriate wavelength, polarisation, direction and angle of incidence, the energy from the incident radiation couples to the electrons of the metal layer 205 to excite surface plasmons at the interface 217 between dielectric medium 215 and metal layer 205. At this "resonance" condition, there is a steep fall in radiation reflected from the reaction region since this radiation is now absorbed by coupling to the surface plasmon mode.

The surface plasmon mode travels along the plane of the interface and extends for approximately 200nm into the dielectric medium 215 provided on top of the interface 217.. The characteristics of the surface plasmon mode can thus be governed by physical properties of layers which are within the range of the surface plasmon. Thus chemical species 211 adhering to the surface of the reaction region may affect the surface plasmon characteristics.

The carrier is primarily intended for determining if molecules of a first type or "probes", 211, react with molecules of a second type or "targets", 213.

In figure 8a, the probes 211 have adhered to the targets 213 and thus both chemical species are attached to metal layer 205. The presence of the targets 213 will further affect the angle at which incident radiation couples to the surface plasmons, or the "resonance angle".

In figure 7a, where just the probes 211 adhere to the surface of the reaction region, the resonance angle is 0 to the surface normal. In figure 8a, where both the probes 211 and targets 213 adhere to the surface of the reaction region, the resonance angle is 0' to the surface normal. This is illustrated in figures 7b and 8b respectively which show a plot of the reflected amplitude of radiation against the incident angle of radiation.

Thus, since the reaction between the probes 211 with the targets 213 causes a variation in resonance angle, measurement of the "resonance" angle, can be used to determine if the second type of molecules 213 has reacted with the first type of molecules 211.

In use, probes will be attached to reaction region 203. The molecules may be attached using a number of methods, but will preferably be attached by placing the encoded carriers in a solution of probes.

The encoded carriers 201 will then be introduced to molecules of a different type, "target molecules". Typically, this will again be performed by placing the encoded carriers with the probes in a solution or suspension of the target molecules.

If the probes attached to reaction area 203 react with the targets, the surface plasmon resonance characteristics of the reaction area will change as described with reference to Figures 7 and 8.

In practice, a plurality of encoded carriers 201 will be prepared, by attaching a first type of probes to a first plurality of encoded carriers having the same "first" code, a second type of probes (different to the first type of probes) to a second plurality of encoded carriers having the same "second" code. The second code is different to the first code.

Further encoded carriers with third, fourth, fifth etc codes and types of probes will be prepared. The reaction region 203 for each different type of encoded carrier having a different probe will be analysed as explained with reference to figure 7 and 8 to determine the "resonance angle" prior to reaction. The different types of encoded carriers 201 will then be introduced into a solution containing the target molecules. The encoded carriers may then be removed from the solution and measured. Alternatively, the carriers may be read while in the solution.

Each type of encoded carrier may be determined from its code, An SPR measurement can then be performed to determine if the probes on the carrier have reacted with the target molecule.

The encoded carriers 201 may also be used to monitor the progress of a reaction between a molecule attached to the carrier and a target molecule. At the start of the reaction, only a few of the probes attached to the encoded carrier will react with target molecules.

Figure 9 is a schematic of the apparatus used for measuring a carrier in accordance with an embodiment of the present invention.

The apparatus comprises a motorised stage 101 which is capable of moving in the x, y and z directions. A flow cell 103 is provided overlying the motonsed stage such that movement of the motorised stage causes movement of the flow cell, The flow cell has an inlet 105 and an outlet 107. Carriers 109 of the type described with reference to figures 4 to 8 are provided in the flow cell such that they distribute themselves within the flow cell so that they can be moved into the optical path by movement of the flow cell using the motorised stage.

The carriers 109 are introduced onto the flow cell via a liquid medium. Therefore, although the carriers rest on the base of the flow cell they are not all oriented in the same way and some movement may also occur during measurement.

In a preferred embodiment, the carriers comprise a magnetic layer and a magnetic field is applied to the carriers to force them towards the base of the flow cell. This helps the carriers to lie flat along the base of the flow cell and helps to anchor the carriers 109 during measurement.

A magnetic field may also be used to orientate the carriers, for example to align all the gratings, thereby increasing throughput as many carriers could be imaged at once. A magnetic field could also be used to make sure that all carrier rest with the side to be measured uppermost. Also a magnetic field could be used to move the carriers close together for measurement thus maximising throughput.

The SPR is measured using collimated polansed light source 117. This is directed via mirror 113 through lens 115 to produce a cone of radiation of the type described with reference to figure 3 directed onto the central carrier 109 of the flow cell 103. The radiation which is reflected from the carrier is then directed via beamsplitter 117 into SPR detector 119.

The SPR technique requires the accurate measurement of angle. Therefore, variations in the profile of the carrier, for example if the carrier 109 is curved in some way will vary the angle measured in SPR detector 119.

Claims

CLAIMS: 1. A thin-film structure comprising a non-metallic first layer and a non-metallic second layer, the first layer and the second layers having a tendency to curl in opposite directions to one another, the thickness of the first film and second layers being chosen to balance the internal stress of the layers.
2. A thin film structure according to claim 1, wherein the boundary between the first and second layers is diffuse.
3. A thin film structure according to either of claims 1 or 2, wherein both the first layer and second layers are oxides constituted of the same elements and wherein the first layer has a different stoichiometry to the second layer.
4. A thin film structure according to claim 3, wherein the first layer primarily comprises SiO, and the second layer primarily comprises SiO, where 1<x �=2.
5. A thin film structure according to any preceding claim, further comprising a metal layer.
6. A thin film structure according to any preceding claim, comprising a layer configured to impart a physical function to said structure.
7. A thin film structure according to claim 6, wherein the layer configured to impart a physical function is a magnetic layer.
8. A thin film structure according to any preceding claim, wherein the layers comprise a diffraction grating.
9. A method of fabricating a thin film structure, the method comprising evaporating a first non-metallic layer and evaporating a second non metallic layer, the layers having a tendency to curl in opposite directions to one another, the thickness of the first film and second layers being chosen such that the curling forces balance to produce a flat structure.
10. A method of fabricating a thin film structure according to claim 9, wherein said first and second layers are oxides constituted of the same elements and wherein the first layer has a different stoichiornetry to the second layer.
11. A method of fabricating a thin film structure according to claim 10, wherein the first and second layers are grown from the same evaporation source and differences in the stoichiometry are achieved by varying the oxygen pressure.
12. A method of fabricating a thin film structure according to any of claims 9 to 11, comprising: providing a substrate having a raised pattern formed on a surface of said substrate, evaporating said first and second layers over said substrate; dissolving said substrate to release said deposited material to form a patterned thin film structure from said released deposited material.
13. A method according to claim 12, wherein said pattern is configured to produce a diffraction grating in the patterned thin film structure once it has been released from the deposited material.
14. A method according to either of claims 12 or 13, wherein the substrate pattern comprises sidewalls which are parallel to the deposition direction of the layers.
15. A method according to any of claims 11 to 14, wherein the total thickness of the layers evaporated is substantially equal to the depth of the pattern provided on said substrate.
16. An encoded carrier comprising a code region having a code and a reaction region separate from said code region, said reaction region having a variation in its refractive index or dielectric constant in a direction generally parallel to the surface of the reaction region, the carrier having a layered structure comprising at least one magnetic layer.
17. An apparatus for reading an encoded carrier, said encoded carrier comprising a code region having a code and a reaction region separate from said code region, said reaction region having a variation in its refractive index or dielectric constant in a direction generally parallel to the surface of the reaction region, the carrier comprising at least one layer which is attracted or repelled by an external field, said apparatus comprising means to perform a measurement of a physical property of the reaction region; and means to read the code provided on said coding area, the apparatus further comprising means to apply an external field to control the movement of said carrier.
18. An apparatus according to claim 16, wherein the external field is configured to anchor said carrier to a surface during measurement of the carrier.
19. An apparatus according to claim 16, wherein the external field is used to control the orientation of the carriers.
20. An apparatus according to claim 19, wherein said layer which is attracted or repelled by an external field is a magnetic layer and said external field is a magneticfield.