WO2002082130A2

WO2002082130A2 - Optical grating structures and method for their manufacture

Info

Publication number: WO2002082130A2
Application number: PCT/GB2002/001625
Authority: WO
Inventors: Nikolai Mikhailovitch Lyndin
Original assignee: Thermo Fast U.K. Limited
Priority date: 2001-04-05
Filing date: 2002-04-05
Publication date: 2002-10-17
Also published as: GB0108557D0; AU2002251250A1; WO2002082130A3

Abstract

A method is disclosed for the fabrication of an optical grating structure. An intermediate layer (3) of a relatively low refractive index material is deposited on a planar substrate (1) of relatively low refractive index or upon a base layer (2) of relatively high-refractive index material applied to the substrate (1). The intermediate layer (3) is of a material capable of being formed precisely into a pattern corresponding to a desired grating pattern. Material is selectively removed from the intermediate layer (3) so as to create a corrugation in the intermediate layer (3) corresponding to the desired grating pattern. A surface layer (5) of relatively high refractive index material of uniform thickness is deposited on the intermediate layer (3), whereby the surface layer (5) is formed with a corrugation corresponding to the corrugation formed in the intermediate layer (3).

Description

Title - Optical grating structures and method for their manufacture

This invention relates to optical grating structures, and in particular to optical gratings of high precision and very shallow depth. Such grating structures are of particular utility in sensor devices, especially biosensor devices.

Devices are known with a surface on which is immobilized a layer of biomolecules having an affinity for other molecules ("the analyte") in a sample under test. Such devices are commonly referred to as biosensors. The immobilized biomolecules and the analyte may, for example, constitute a specific binding pair such as an antigen- antibody pair. Interaction of the two members of the pair causes a change in the physical properties of the device. This change can be used as an indicator of the presence and/or concentration of the analyte, the strength and/or progress of the interaction etc.

In many biosensors, it is the optical properties of the device that are monitored. One class of optical biosensor comprises a waveguide in the form of a thin layer of relatively high refractive index material coated on a substrate of optically transparent lower refractive index material. Biomolecules are immobilized on the surface of the waveguide and the interface between the substrate and the waveguide is irradiated with a beam of light.

Means are generally provided to facilitate coupling of light into the waveguide. The optical properties of the device will depend on the nature of those means, as well as on other factors including the wavelength of the incident light, the materials used for the waveguide and the substrate, the thickness of the waveguide etc. In general, however, incident light is coupled into the waveguide to a greater or lesser extent. Chemical binding events at or in the vicinity of the waveguide surface will cause a localized change in refractive index, which in turn causes a change in the coupling characteristics of the device. This provides a means for monitoring interactions between the immobilized biomolecules and the analyte molecules. One form of coupling means which has been proposed is a grating structure formed, for instance, in the interface between the substrate and the waveguide and/or at the surface of the waveguide. In general, light incident on the grating will be reflected, transmitted or scattered into the various diffraction orders of the grating. Further, at certain angles of incidence, where a diffraction order matches the waveguide propagation condition, light will be coupled into the waveguide. This light propagates within the waveguide, parallel to the substrate surface, and continues to interact with the grating. The light will couple back out of the waveguide via the various diffraction orders and into free-space beams. The outcoupled light will include beams in the same direction as the transmitted and reflected, uncoupled beams. As a result of this, attempts to measure the coupling condition are hampered by overlap of the waveguide- derived beams and the uncoupled, transmitted or reflected components. This leads to measurements of low contrast.

WO 97/29362 describes an experimental approach in which near-total reflection of light is achieved at a certain angle of incidence. This arises as a result of destructive interference between the zeroth order transmitted beam and the beam radiated from the out-coupled guided wave into the material beyond the waveguide. The "anomalous" or "abnormal" reflection so obtained is relatively easily monitored.

In the arrangement just described, the optical characteristics, notably the incidence angle at which the reflection maximum occurs, the maximum value of the reflection and the reflection peak width depend critically on the dimensions of the waveguide and the grating structure. The uniformity of the effective thickness of the waveguide is important, as is the uniformity of the grating structure (long-term and stochastic short- term uniformity). In practice, however, the formation of uniform grating structures in the high refractive index materials typically used for the waveguide is difficult to achieve. The grating structures formed in them are therefore susceptible to variation in terms of the depth of the grating troughs, and hence the waveguide effective thickness, and also in the ratio of the peak width and trough width with the same effect.

WO 97/29362 describes a grating fabrication method which involves etching a grating pattern in the surface of a substrate and then depositing the waveguide layer on top of the substrate. The deposited grating follows the contour of the substrate, with the result that both the interface between the waveguide and the substrate and the surface of the waveguide are corrugated.

However a disadvantage of such a double-corrugated waveguide is that for outcoupling of light from a waveguide at near normal direction with respect to the structure plane the most important components of the field are the "Transverse Electric" component or TE mode and the "Longitudinal Electric" component or TM mode. For the fundamental TE₀ mode the essential component has the same sign in the whole cross-section of the waveguide while the essential component for the fundamental TM₀ mode undergoes a change of sign and has a net intensity of zero at the centre of the waveguide. This can lead to a difference in diffraction efficiencies and as a consequence to a difference in resonance peak width for the TE and the TM modes of more than 50 times.

If it is necessary or desired to measure both the TM and TE components of the radiation then a grating structure with just a single corrugation must be used, the corrugation normally being at the surface of a waveguide deposited on a planar substrate. For such a waveguide, variations in the depth of the grating troughs are particularly serious, since they lead to variations in the effective thickness of the waveguide, which is critical to effective performance of the device.

There has now been devised a method for the fabrication of a grating structure that overcomes or substantially mitigates the above-mentioned or other disadvantages of the prior art.

According to a first aspect of the invention, there is provided a method for the fabrication of an optical grating structure, which method comprises the steps of a) depositing an intermediate layer of a relatively low refractive index material on a planar substrate of relatively low refractive index or upon a base layer of relatively high- refractive index material applied to the substrate, said intermediate layer being of a material capable of being formed precisely into a pattern corresponding to a desired grating pattern; b) selectively removing material from said intermediate layer so as to create a corrugation in said intermediate layer corresponding to the desired grating pattern; and c) depositing on said intermediate layer a surface layer of relatively high refractive index material of uniform thickness, whereby the surface layer is formed with a corrugation corresponding to the corrugation formed in the intermediate layer.

The method according to the invention is advantageous primarily in that it overcomes or substantially mitigates the difficulties in forming precise corrugations in the upper surface of a layer of relatively high refractive index material. By forming the corrugation in an intermediate layer of material from which it is relatively easy to remove material so as to create a precisely defined corrugation, and then depositing the surface layer of high refractive index material on top of that intermediate layer, the surface layer is caused to follow the corrugation of the intermediate layer. A waveguide is thereby formed which has a corrugation with troughs of uniform depth. The effective thickness of the waveguide is thus less susceptible to grating imperfections.

In a preferred embodiment of the invention, the intermediate layer is deposited upon a base layer of relatively high refractive index material which has previously been applied to the substrate. In such a case, the method comprises the steps of a) depositing a base layer of relatively high refractive index material upon a planar substrate of relatively low refractive index; b) depositing upon said base layer an intermediate layer of a material capable of being formed precisely into a pattern corresponding to a desired grating pattern; c) selectively removing material from said intermediate layer so as to create a corrugation in said intermediate layer corresponding to the desired grating pattern; and d) depositing on said intermediate layer a surface layer of relatively high refractive index material of uniform thickness, whereby the surface layer is formed with a corrugation corresponding to the corrugation formed in the intermediate layer.

The materials used for the various components of the grating structure according to the invention are described herein as having refractive indices that are "relatively high" or "relatively low". It will be appreciated that these are relative terms, and the absolute values of the respective refractive indices may vary within quite wide ranges. In general, all that is essential is that the refractive index of the surface layer and the refractive index of the base layer (where present) are higher than that of the substrate.

The substrate is typically of glass, eg in the form of a glass chip or slide. Depending on the particular glass used, the refractive index of the substrate is typically in the range 1.4 to 1.6. A particularly preferred form of glass is 1.1mm float glass, which has a refractive index of about 1.5.

The base layer and surface layer typically have refractive indices in the range 1.80 to 2.50. Suitable materials for these layers include hafnium dioxide, silicon nitride, titanium dioxide and, particularly, tantalum pentoxide.

The intermediate layer is of a material that can be formed precisely into a grating structure by selective removal of material from it. Suitable materials might include calcium fluoride, polymeric materials such as polymethylmethacrylate, and lithium fluoride. However, the most preferred material for the intermediate layer is silicon dioxide.

Selective removal of material from the intermediate layer is preferably carried out by etching. To achieve this the intermediate layer is preferably first coated with a protective material, preferably a layer of photoresist. The protective layer (photoresist) is preferably applied by spin coating, ie by application of the photoresist in liquid form to a rapidly rotating disc that carries the device that is to be coated.

The photoresist is then exposed to radiation, eg u.v. or visible light, in such a manner as to define the desired grating pattern, normally as a regular pattern of bands or stripes. Techniques for achieving this are well known. The areas of photoresist covering the intermediate material that are to be selectively removed are then removed, again in a manner and using agents that are known perse, to expose areas of the underlying intermediate layer. The exposed areas of intermediate layer can then be etched away by suitable means, eg reactive ion etching, ion-beam etching or wet etching. Wet etching is the preferred method for selective removal of intermediate layer material, well-known etching agents such as fluorosilicic acid and hydrofluoric acid/ammonium fluoride being among those suitable for the purpose.

The etching of the intermediate layer is preferably carried out until the whole of the exposed areas of intermediate layer are etched away completely. The etching conditions are preferably such that the material of the intermediate layer is attacked but not the underlying substrate or base layer. This prevents any etching of the substrate or base layer. In such a case, the depth of the corrugation corresponds precisely to the thickness of the intermediate layer.

After etching of the intermediate layer, the protective layer (photoresist) is removed. Again, suitable agents for effecting such removal will be familiar to those skilled in the art. The surface layer is then deposited upon the remaining areas of intermediate layer material.

Most commonly, the pattern formed in the intermediate layer comprises alternating bands of material with a width equal to their separation.

Deposition of the surface layer, and also deposition of the intermediate layer and base layer (where present) may be carried out by known techniques, particularly sputtering techniques. Such techniques may be employed very precisely so that the thickness of the deposited layers may be controlled to very high levels of accuracy.

According to a second aspect of the invention, there is provided an optical grating structure comprising a surface layer of relatively high refractive index material and a substrate of relatively low refractive index material, wherein an intermediate layer of a material having formed therein a corrugation corresponding to a desired grating pattern is interposed between said surface layer and said substrate, and wherein the surface layer is formed with a corrugation corresponding to the corrugation formed in the intermediate layer. As described above, the corrugation in the intermediate material preferably takes the form of bands of material separated by regions from which the material of the intermediate layer has been completely removed.

As further described above, in preferred embodiments the optical grating structure comprises a base layer and a surface layer, both of relatively high refractive index material, and a substrate of relatively low refractive index material, the base layer being applied to the substrate and interposed between the substrate and the surface layer, and wherein an intermediate layer of a material having formed therein a corrugation corresponding to a desired grating pattern is interposed between said surface layer and said base layer, and wherein the surface layer is formed with a corrugation corresponding to the corrugation formed in the intermediate layer.

In such a structure, the base layer and the surface layer together form a composite waveguide, the corrugated intermediate layer being embedded within that structure.

The optimal physical dimensions of the components of the grating structure will depend on a number of factors, including the wavelength of the light with which the grating is to be irradiated. In the following description, the values given for the waveguide thickness, grating depth and period, encompass those suitable for commonly-used wavelengths, eg 633nm.

Typically, the waveguide may have a thickness of the order of 50nm to 300nm, more preferably 100nm to 200nm. It is particularly preferred that the effective thickness of the waveguide is in the range 140nm to 200nm.

Where, as in the preferred embodiment, the waveguide is a composite structure made up of the base layer and the surface layer, the thicknesses of those two layers are preferably unequal, so that the residual material of the intermediate layer material is displaced from the centre of the waveguide in the finished structure.

Most preferably, the base layer has a thickness in excess of one-half of the thickness of the waveguide and the surface layer a thickness of less than one-half the thickness of the waveguide. Most preferably, the base layer has a thickness in the range 100nm to 150nm, and the surface layer a thickness of 30nm to 80nm.

The depth of the troughs in the grating structure (ie the corrugations in the surface of the surface layer) are preferably less than 50nm, eg about 30nm. Since the depth of these troughs is determined by the depth of material selectively removed from the intermediate layer, and since that material is preferably removed entirely, it follows that the intermediate layer is preferably deposited to a depth of less than 50nm, eg about 30nm.

The period of the grating structure is typically 300nm to 1000nm, more preferably 400 to 800nm.

The invention will now be described in greater detail, by way of illustration only, with reference to the accompanying drawings, in which

Figures 1 to 9, which are not to scale, are sectional views showing stages in the manufacture of a first embodiment of an optical grating structure in accordance with the invention; and

Figure 10 is a similar sectional view of a second embodiment of an optical grating structure according to the invention.

Referring in turn to Figures 1 to 9, the stages in the manufacture of a first embodiment of an optical grating structure according to the invention are as follows.

Figure 1

The starting point for the manufacturing process is a substrate 1 , eg in the form of a glass slide or chip. The substrate 1 has a planar upper (as viewed in the drawings) surface on which further layers are deposited as described below. Figure 2

In a first manufacturing stage, a base layer of relatively high-refractive index material 2 is deposited upon the planar upper surface of the substrate 1. The base layer 2 may, for instance, be of tantalum pentoxide and in one embodiment has a thickness of approximately 120nm.

The base layer 2 may be deposited by any suitable method, eg any of the methods conventionally used for the deposition of such materials on a substrate, such as sputtering.

Figure 3

A layer 3 of silicon dioxide is next deposited on top of the high-refractive index material base layer 2. The silicon dioxide layer 3 has a thickness of 30nm.

Figure 4

A layer of photoresist 4 is then applied to the silicon dioxide layer 3. Any suitable form of photoresist may be used, the photoresist typically comprising a polymeric material containing a photo-active compound. The photo-active compound undergoes a chemical change upon exposure to radiation (eg visible radiation). This change results in the regions of the photoresist layer 4 that have been so exposed to radiation being able to be stripped away.

Figure 5

The layer of photoresist 4 is exposed to radiation through a mask that defines the desired pattern on the surface of the photoresist 4. In the present case, the pattern is a series of bands with a separation equal to their width, the width of one band together with its separation from the next corresponding to the period of the desired grating.

The exposure of the photoresist 4 to light in this way creates bands 4a of material that can be stripped away to expose the underlying silicon dioxide layer 3, these bands 4a being separated by resistant bands 4b of photoresist. Figure 6

After stripping away of the bands 4a of photoresist 4, a regular pattern of resistant photoresist bands 4b is present on the surface of the silicon dioxide layer 3. The agents used to strip away the strippable photoresist 4a in this manner can be of any suitable form, including such agents as are conventionally used for similar purposes.

Figure 7

An etching agent is then applied to the surface of the assembly. The etching agent can be of any suitable form, provided that it is effective in etching away the exposed silicon dioxide but not the bands 4b of photoresist that protect the silicon dioxide material beneath them, and also provided that it does not attack the tantalum pentoxide material exposed by removal of the silicon dioxide.

After etching of the silicon dioxide material in this manner, the structure comprises bands of silicon dioxide 3a with protective coverings of photoresist 4b.

Figure 8

The photoresist coverings 4b are then removed, again using an agent known to be suitable for such purpose.

Figure 9

The fabrication of the grating according to the invention is completed by the deposition of a surface layer 5 of high refractive index material of constant thickness (in the illustrated embodiment approximately 60nm). The upper (as viewed in Figure 9) surface of the surface layer 5 follows the contour of the bands 3a of silicon dioxide, leading to a corrugation corresponding in form to that of the silicon dioxide bands 3a present on the surface of the base layer 2.

The material of the second high refractive index surface layer 5 may be the same as the material of the base layer 2, ie in the present example tantalum pentoxide, or it may be a different high refractive index material. The relative ease and high degree of precision with which deposition of the surface layer 5 of high refractive index material can be carried out means that the thickness of the surface layer 5 can be controlled very accurately. Furthermore, the optical properties of the intermediate product as shown in Figure 8 can be measured before deposition of the surface layer 5 takes place. Such a measurement enables a calculation to be made of the precise thickness of surface layer 5 necessary to achieve the desired properties in the finished assembly. Thus, the thickness of the surface layer 5 can be customised for each individual assembly, leading to a high degree of consistency in the finished products. Thus, adjustment of the thickness of the surface layer 5 at this final manufacturing stage can compensate for any variations introduced in preceding stages (eg variations in thickness of the base layer 2 or in the dimensions of the silicon dioxide bands 3a). This is in contrast to the application of a constant thickness of surface layer 5, followed by testing of the finished products and rejection of those that do not match the desired optical properties.

The finished grating assembly comprises a composite high refractive index layer made up of the base layer 2 and surface layer 5. As stated above, the base and surface layers 2,5 may be of the same or different relatively high refractive index material. Embedded within the composite high refractive index layer are bands of silicon dioxide 3a. The overall thickness of the composite high refractive index layer is 180nm, the relatively narrow silicon dioxide bands 3a being offset from the centre of that composite layer.

The embodiment shown in Figure 10 differs from that of Figure 9 in that there is no high refractive index base layer corresponding to the base layer 2 of the first embodiment. Instead, a layer of silicon dioxide is applied directly to the substrate 21 , which may for example be of calcium fluoride or lithium fluoride. Subsequent processing steps are as described for the first embodiment, leading to a structure comprising a waveguide 25 having a corrugation corresponding to a series of bands 23a of silicon dioxide applied to the substrate 21. This gives rise in effect to a waveguide 25 having a corrugation on each of its major surfaces.

Claims

1. A method for the fabrication of an optical grating structure, which method comprises the steps of

a) depositing an intermediate layer of a relatively low refractive index material on a planar substrate of relatively low refractive index or upon a base layer of relatively high- refractive index material applied to the substrate, said intermediate layer being of a material capable of being formed precisely into a pattern corresponding to a desired grating pattern;

b) selectively removing material from said intermediate layer so as to create a corrugation in said intermediate layer corresponding to the desired grating pattern; and

c) depositing on said intermediate layer a surface layer of relatively high refractive index material of uniform thickness,

whereby the surface layer is formed with a corrugation corresponding to the corrugation formed in the intermediate layer.

2. A method as claimed in Claim 1 , which method further comprises the step of depositing a base layer of relatively high refractive index material upon the substrate, prior to deposition of the intermediate layer.

3. A method as claimed in Claim 1 or Claim 2, wherein the substrate is glass.

4. A method as claimed in any preceding claim, wherein the substrate has a refractive index in the range 1.4 to 1.6.

5. A method as claimed in any preceding claim, wherein the base layer (where present) and surface layer have refractive indices in the range 1.80 to 2.50.

6. A method as claimed in any preceding claim, wherein the base layer (where present) and surface layer are formed from a material selected from the group consisting of hafnium dioxide, silicon nitride, titanium dioxide and tantalum pentoxide.

7. A method as claimed in any preceding claim, wherein the intermediate layer is of a material that can be formed precisely into a grating structure by selective removal of material from it.

8. A method as claimed in Claim 7, wherein the intermediate layer is formed from a material selected from the group consisting of calcium fluoride, polymeric materials, lithium fluoride and silicon dioxide.

9. A method as claimed in Claim 8, wherein the intermediate layer is formed from silicon dioxide.

10. A method as claimed in Claim 8, wherein the intermediate layer is formed from polymethylmethacrylate.

11. A method as claimed in any preceding claim, wherein the selective removal of material from the intermediate layer is carried out by etching.

12. A method as claimed in any preceding claim, wherein the etching comprises the steps of

a) coating the intermediate layer with a protective material;

b) exposing the protective material to radiation in such a manner as to define the desired grating pattern;

c) removing the areas of protective material covering the intermediate material that is to be selectively removed to expose areas of the underlying intermediate layer; d) etching the exposed areas of intermediate layer away; and

e) removing the protective layer.

13. A method according to Claim 12, wherein the protective material is applied by spin coating.

14. A method according to Claim 12 or Claim 13, wherein the exposed areas of intermediate layer are etched away by wet etching.

15. A method according to any one of Claims 12 to 14, wherein the etching of the intermediate layer is carried out until the whole of the exposed areas of intermediate layer are etched away completely.

16. A method according to Claim 15, wherein the etching of the intermediate layer is carried out such that the material of the intermediate layer is attacked but not the underlying substrate or base layer.

17. A method according to any preceding claim, wherein the pattern formed in the intermediate layer comprises alternating bands of material with a width equal to their separation.

18. A method according to any preceding claim, wherein the deposition of the surface layer, and also deposition of the intermediate layer and base layer (where present) is carried out by sputtering techniques.

19. A method according to any preceding claim, wherein the intermediate layer is deposited to a depth of less than 50nm.

20. An optical grating structure comprising a surface layer of relatively high refractive index material and a substrate of relatively low refractive index material, wherein an intermediate layer of a material having formed therein a corrugation corresponding to a desired grating pattern is interposed between said surface layer and said substrate, and wherein the surface layer is formed with a corrugation corresponding to the corrugation formed in the intermediate layer.

21. An optical grating structure as claimed in Claim 20, further comprising a base layer of relatively high refractive index material, the base layer being applied to the substrate and interposed between the substrate and the surface layer.

22. An optical grating structure according to Claim 21 , wherein the corrugation in the intermediate material takes the form of bands of material separated by regions from which the material of the intermediate layer has been completely removed.

23. An optical grating structure according to Claim 21 or Claim 22, wherein the base layer and the surface layer together form a composite waveguide, the corrugated intermediate layer being embedded within the composite waveguide.

24. An optical grating structure according to Claim 23, wherein the waveguide has a thickness in the range 50nm to 300nm.

25. An optical grating structure according to Claim 24, wherein the waveguide has a thickness in the range 100nm to 200nm.

26. An optical grating structure according to Claim 25, wherein the waveguide has a thickness in the range 140nm to 200nm.

27. An optical grating structure according to any one of Claims 23 to 26, wherein the thicknesses of the base layer and the surface layer are unequal, so that the residual material of the intermediate layer material is displaced from the centre of the waveguide in the finished structure.

28. An optical grating structure according to Claim 27, wherein the base layer has a thickness in excess of one-half of the thickness of the waveguide and the surface layer a thickness of less than one-half the thickness of the waveguide.

29. An optical grating structure according to Claim 28, wherein the base layer has a thickness in the range 100nm to 150nm, and the surface layer a thickness of 30nm to 80nm.

30. An optical grating structure according to any one of Claims 20 to 29, wherein the depth of the corrugations in the surface of the surface layer is less than 50nm.

31. An optical grating structure according to any one of Claims 20 to 30, wherein the period of the grating structure is in the range 300nm to 1000nm.

32. An optical grating structure according to Claim 31 , wherein the period of the grating structure is in the range 400nm to 800nm.