MANUFACTURE OF SURFACE RELIEF STRUCTURES

Abstract

A method and apparatus for the etching of variable depth features in a substrate is described. Movement of the substrate relative to an etchant (e.g. into or out of the etchant) during the etching process is utilised to provide a varying etch time, and hence depth, across the substrate, and in various examples this is enabled without requiring a varying mask.

Claims

1. An etching system to etch a surface-relief structure on a substrate, the etching system comprising: an etching vessel; and a support apparatus configured to move the substrate between a first position and a second position in the etching system, in the first position only a first part of an area of the substrate to be etched is exposed to an etchant with the etchant comprising an anisotropic etchant, in the etching vessel, and in the second position an additional portion of the area to be etched is exposed to the etchant such that the first part of the area to be etched is exposed to the etchant for a longer period of time than at least one other part of the area to be etched, the movement being defined according to an etching profile.

2. The etching system of claim 1, wherein the surface-relief structure comprising a variable-depth etching profile.

3. The etching system of claim 1, wherein the support apparatus is configured to move the substrate between the first position and the second position in the etching system between one of moving the substrate from the first position to the second position and moving the substrate from the second position to the first position.

4. The etching system of claim 1, wherein the surface-relief structure comprises a blazed surface-relief structure.

5. The etching system of claim 1, wherein the support apparatus is configured to move the substrate between the first position and the second position along an axis substantially perpendicular to a surface of the etchant.

6. The etching system of claim 1, wherein the support apparatus is further configured to move the substrate between the first position and the second position in a series of discrete steps.

7. The etching system of claim 1, wherein the support apparatus is further configured to move the substrate in steps in which a time between steps is variable.

8. The etching system of claim 1, wherein the support apparatus is further configured to move the substrate in between the first position and the second position in a continuous manner.

9. The etching system of claim 1, wherein the surface-relief structure comprises an optical grating.

10. The etching system of claim 1, wherein the support apparatus is further configured to move the substrate in an axis of movement between the first position and the second position that is aligned with an axis of the grating.

11. An etching system to etch a nanostructure-based, surface-relief structure on a substrate, the etching system comprising: an etching vessel; and a support apparatus configured to move the substrate between a first position and a second position in the etching system, in the first position only a first part of an area of the substrate to be etched is exposed to an etchant in the etching vessel, and in the second position an additional portion of the area to be etched is exposed to the etchant such that the first part of the area to be etched is exposed to the etchant for a longer period of time than at least one other part of the area to be etched, the movement of the support apparatus being defined according to an etching profile.

12. The etching system of claim 11, wherein the surface-relief structure comprises a variable-depth etching profile.

13. The etching system of claim 11, wherein the etchant comprises an anisotropic etchant.

14. The etching system of claim 11, wherein the support apparatus is configured to move the substrate according to an etching profile, wherein both the movement of the substrate and an orientation of the substrate are relative to a surface of the etchant according to the etching profile.

15. The etching system of claim 11, wherein the support apparatus is configured to move the substrate between the first position and the second position in the etching system between one of moving the substrate from the first position to the second position and moving the substrate from the second position to the first position.

16. The etching system of claim 11, wherein the surface-relief structure comprises a nanostructure-based, blazed surface-relief structure.

17. The etching system of claim 11, wherein the support apparatus is configured to move the substrate between the first position and the second position along an axis substantially perpendicular to a surface of the etchant.

18. The etching system of claim 11, wherein the support apparatus is further configured to move the substrate between the first position and the second position in a series of discrete steps.

19. The etching system of claim 11, wherein the support apparatus is further configured to move the substrate in steps in which a time between steps is variable.

20. The etching system of claim 11, wherein the support apparatus is further configured to move the substrate in between the first position and the second position in a continuous manner.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Embodiments of the invention will be described, by way of example, with reference to the following drawings, in which:

[0033] FIG. 1 shows a schematic diagram of a waveguide for a Head Up Display;

[0034] FIGS. 2 and 3 show charts of grating properties;

[0035] FIG. 4 shows a variable thickness coating;

[0036] FIG. 5 shows a schematic diagram of an example etching apparatus;

[0037] FIG. 6 shows a flow chart of an etching method;

[0038] FIGS. 7 and 8 show example steps from the etching method of FIG. 6;

[0039] FIG. 9 shows three example surface relief structures formed using the apparatus of FIG. 5 and/or the method of FIG. 6;

[0040] FIG. 10 shows example steps from the etching method of FIG. 6; and

[0041] FIG. 11 shows three examples of a substrate mounted in different orientations in an example etching apparatus.

DETAILED DESCRIPTION

[0042] Further details, aspects and embodiments of the invention will now be described, by way of example only, with reference to the drawings. Elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. Like reference numerals have been included in the respective drawings to ease understanding.

[0043] The following disclosure sets out a new method for producing surface relief structures on a substrate. Depending upon the size of features of the surface relief structure, the structure may be a nanostructure or alternatively the features may be on a larger scale (e.g. features that are tens or hundreds of microns wide and/or deep).

[0044] The surface relief structures may comprise an array of facets which may, for example, form an optical component that may be used for diffraction (e.g. optical gratings, such as a graded optical grating), reflection (e.g. mirror arrays) or refraction (e.g. prismatic arrays) or any combination thereof. In various examples, the surface relief structure may form an optical master which is then replicated (e.g. by moulding) to form a plurality of identical optical elements (e.g. copies of the optical master). The optical components formed using the surface relief structures (or comprising the surface relief structures) may be used in HUDs, HMDs or other head-worn displays (e.g. to provide a more uniform intensity across a display). The facets may be blazed.

[0045] The substrate in which the surface relief structure is formed by means of a wet etch, as described in detail below, may be formed from a crystalline material, such as silicon, or a non-crystalline material, such as glass or metal. In examples where the substrate is formed from a crystalline material (e.g. silicon), the etching may preferentially etch along some crystal planes. For example, where the substrate is silicon, it may be an off-axis silicon wafer (or part thereof), such as a wafer with a <100> orientation top plane where the etching exposes the {111} planes within the silicon. These exposed planes may form the facets of an array of facets or grating lines within the surface relief structure. A silicon wafer is an example of a semiconductor wafer and in various examples, the substrate may be another type of semiconductor wafer.

[0046] The etchant (or etching liquid) used may be any suitable liquid. In an example the substrate may be a semiconductor wafer, for example silicon, and the etchant may be any liquid known for etching such wafers (e.g. acids such as hydrofluoric acid, HF, or buffered HF, nitric acid, or strongly alkaline solutions).

[0047] As described in more detail below, the method comprises gradually inserting the substrate into or removing the substrate from the etchant, such that different parts of the substrate are etched for different amounts of time. This results in a controlled variation in etch depth across the resulting surface relief structure, which is proportional to the time spent within the etchant.

[0048] For the purposes of the following description, any reference to a part of a substrate or area to be etched, refers to a part that is smaller than the entirety of the substrate or area to be etched respectively.

[0049] FIG. 5 shows a schematic diagram of an example apparatus which may be utilised to implement the method described hereinbelow.

[0050] An etching vessel 50 (e.g. an etch tank) is provided for containing a liquid for etching a substrate. For example the vessel may be a conventional etch tank as known in the art and be used to contain conventional or proprietary etching liquids. Other apparatus typically associated with an etching system is also present as required, but is not shown for clarity.

[0051] A suspension apparatus 51 is provided to suspend a substrate 52 for etching within the vessel 50. The suspension apparatus 51 enables the height of the substrate within the vessel to be varied. In the specific example of FIG. 5 a winch system is provided utilising a wire to suspend the substrate. The wire is attached to the substrate at its distal end, and at its proximal end to a control system for extending and retracting the wire. A pulley arrangement may be utilised to guide the wire and support the substrate in the required location. As will be appreciated any appropriate mechanical and control system may be utilised and in other examples, the substrate 52 may not be suspended but may be otherwise supported or held in place and moved into and out of the etchant (in the etching vessel 50) and in such examples, the suspension apparatus 51 may be replaced by an alternative substrate support or mounting apparatus. The suspension, or mounting apparatus, enables the gradual movement of the substrate along an axis, which may be in the plane of the substrate, into and out of the etchant.

[0052] The substrate has a etch resist mask 53 (e.g. in the form of an oxide layer) deposited on one or more surfaces such that areas to be etched are exposed to the etchant upon placement in the vessel, and areas not to be etched are protected from the etchant by the mask, as is known in the art.

[0053] FIG. 6 shows a flow chart of a method of etching using the apparatus of FIG. 5 and this can be described with reference to the two examples shown in FIGS. 7 and 8 and with reference to FIG. 10.

[0054] Prior to the etching, an etch-resist mask is applied to the substrate to be etched (block 602, FIG. 10a) and this may be applied in the conventional manner. This application of the etch-resist mask may be part of the method or alternatively, pre-coated substrates (i.e. with the etch-resist mask already applied) may be received. In various examples a constant linewidth mask is utilised as is known in the art for producing constant efficiency gratings. Since this mask is regular no special techniques are required for its production. In other examples, a different mask may be used to create other surface relief features and in various examples the mask may not have constant linewidth and/or constant pitch. In various examples, the mask may define openings (i.e. areas for etching) that are all rectangular and in other examples, the openings may be of different (or varying) shape.

[0055] The substrate is mounted in or above the etching vessel 50 (e.g. suspended by the suspension apparatus 51 in or above the etching vessel 50), but not within the etchant. In the orientation shown in the examples of FIGS. 7 and 8, the substrate is mounted above the surface 702 of the etchant 704. The substrate is mounted (e.g. suspended) such that the areas which require more etching are positioned closest to the etchant. For example, the mask layout and suspension arrangement may be such that the end of an optical grating to be formed with higher efficiency are closest to the etchant.

[0056] The substrate is then moved (e.g. lowered) to a first position, P1, in the etchant (block 606) and etching of the areas of the substrate exposed to the etchant commences. The time at which the substrate is first moved into the etchant is denoted time, t=0, in FIGS. 7 and 8. Depending upon the implementation, in the first position only part of the overall area to be etched (i.e. not the entirety of the overall area to be etched) may be exposed to the etchant (e.g. as in FIG. 7) or all of the area to be etched may be exposed to the etchant (e.g. as in FIG. 8).

[0057] After a predetermined time (e.g. m or n in FIGS. 7 and 8), the position of the substrate in the etchant is changed, e.g. to a second position, P2, in the etchant (block 608). This comprises moving the substrate into or out of the etchant by a predefined amount, such that a different proportion (or portion) of the substrate is exposed to the etchant. The example in FIG. 7 shows the substrate being moved further into the etchant (i.e. such that more of the substrate is exposed to the etchant) and the example in FIG. 8 shows the substrate being moved further out of the etchant (i.e. such that less of the substrate is exposed to the etchant). In various examples, the motion of the substrate will be along a defined axis 706 that is in the plane of the substrate and substantially perpendicular to the surface of the etchant 702 (e.g. the axis is defined by the suspension apparatus 51 in FIG. 5) such that the motion of the substrate (in block 608) is into or out of the etchant, instead of laterally within the etchant.

[0058] In various examples, as shown in FIG. 7, in the first position, P1, only a part of the overall area to be etched is exposed to the etchant and the substrate is then lowered further into the etchant such that more of the area to be etched is exposed the etchant. Etching then commences to the newly exposed area, and continues at the previously exposed areas (which are now deeper within the etchant). In other examples, as shown in FIG. 8, where the movement of the substrate (in block 608) moves the substrate partially out of the etchant (whilst still leaving a portion of the substrate exposed to the etchant), etching continues on the exposed areas and stops on those areas which are no longer exposed to the etchant.

[0059] The repositioning of the substrate in the etchant (in block 608), by moving the substrate by a predefined amount (and at a predefined time) into or out of the substrate, may be repeated multiple times (as indicated by the arrow from block 608 back to block 608 in FIG. 6). For example, the substrate may be incrementally lowered into the etchant until the whole area to be etched is exposed or incrementally raised out of the etchant until none of the substrate is exposed to the etchant. Once etching is complete, the substrate is removed from the etchant (block 610) and processed in the normal manner (e.g. including removal of the etch-resist mask, block 612). FIG. 10b shows an example substrate following removal from the etchant (in block 610) and FIG. 10c shows the same example substrate following removal of the etch-resist mask (in block 612).

[0060] The result of the process of FIG. 6 is that different areas of the substrate (or of the area to be etched) are exposed to the etchant for different amounts of time and are therefore etched by different amounts, i.e. there is a variation in etch depth across the substrate, as shown in the examples in FIG. 9 (described below). Where the surface relief structure being formed is an optical grating, the axis of optical grating may be aligned with the axis of movement of the substrate into the etching fluid 706 and hence a graded depth of grating features is achieved along the grating and hence a graded efficiency. This is achieved without the need for non-conventional masks or specific processing techniques. A graded efficiency grating can thus be provided without significant additional complexity.

[0061] By varying the time intervals (e.g. m and n in FIGS. 7 and 8) between each repositioning of the substrate (in block 608) and/or by varying the amount by which the substrate is moved, the etch depth variation across the substrate (and hence across the surface relief structure) can be accurately controlled. In various examples, a linear depth profile may be obtained and in other examples a non-linear (e.g. polynomial) depth profile may be obtained.

[0062] The depth profile across the area to be etched may also be controlled by controlling the angle at which the substrate is moved into and/or out of the etchant with respect to the features on the etch-resist mask and three examples are shown in FIG. 11. This control of the angle may be achieved by mounting the substrate with the appropriate orientation into the apparatus (in block 604). FIG. 11a shows an example arrangement in which the movement into/out of the etchant is orthogonal to the lines in the etch-resist mask that form the etched facets. FIG. 11b shows an example arrangement shows an example arrangement in which the movement into/out of the etchant is angularly offset to the lines in the etch-resist mask that form the etched facets. FIG. 11c shows an example arrangement shows an example arrangement in which the movement into/out of the etchant is parallel to the lines in the etch-resist mask that form the etched facets.

[0063] In various examples, for example to create more complex variations in etch depth across the substrate (e.g. U or V shaped depth profiles, where there is an inflection in the etch depth across the substrate), having removed the substrate from the etchant (in block 610), the substrate may then be remounted on the apparatus in a different orientation (in block 604) and the method repeated (e.g. as indicated by the dotted arrow from block 610 to block 604 in FIG. 6). In this example, both (or all) passes of the method of FIG. 6 use the same etch-resist mask and hence result in the etching of the same areas of the substrate. In other examples, however, the method may involve removing the first etch-resist mask (in block 612) and applying a second, different etch-resist mask (in block 602) before repeating the etch process described above (e.g. as indicated by the dotted arrow from block 612 to block 602 in FIG. 6).

[0064] In the examples above, it is assumed that the properties of the etchant 704 are substantially constant throughout the body of the etchant. In other examples, however, the properties of the etchant may vary within the etching vessel 50, e.g. there may be a temperature gradient within the etching vessel 50 that results in the etching characteristics of the etchant (e.g. the etch rate) varying within the etching vessel 50. In further examples, the etchant may be differentially agitated within the etching vessel 50 such that the etching characteristics of the etchant (e.g. the etch rate) vary within the etching vessel 50.

[0065] The resulting surface relief structure on the substrate (which may, for example, be a silicon master grating) may then be replicated and, where needed, coated (with a uniform thickness) to provide multiple optical components (e.g. multiple gratings) for use.

[0066] As described above, the movement of the substrate may comprise a series of discrete steps, with the size of the steps and/or the time between them defined as required to etch the desired profile. For example constant step and/or time gaps may be utilised, or variable step and/or time gaps may be utilised. Also, the movement may be continuous, rather than discrete steps, and can progress at any constant or variable speed. Controlling the rate of movement of the substrate thus allows any desired etch depth profile to be created, thus allowing the creation of optimum efficiency profiles as required for applications. Even complex profiles may, in various examples, still utilise a conventional linear mask. The movement of the substrate into the etchant thus defines the etching profile required.

[0067] FIG. 9 shows schematic diagrams of three example etched substrates where the substrate is formed from a crystalline material such as silicon and the etch preferentially etches some planes within the crystal. The first example, shown in FIG. 9a, shows binary grooves (e.g. which may be formed in non-crystalline substrates). The second example, shown in FIG. 9b, shows v-shaped grooves, although as shown in FIG. 9b, dependent upon the line width of the openings in the etch-resist mask and the etch times used, some of the etched features may etch to termination (e.g. as shown by the v-shaped groove 902) and other features may not etch to termination (e.g. as shown by the more u-shaped features 904). The third example, shown in FIG. 9c, shows blazed grooves, which may, for example, be achieved through use of an appropriate off-axis crystalline substrate. Use of such blazed grooves may further enable the tailoring of the surface relief structure to improve diffraction efficiency (e.g. where the surface relief structure formed is a blazed optical grating). It will be appreciated that the shape of the etched regions will be dependent on the substrate material and the choice of etchant as well as the etch depth. In all examples, there is a variation in the etch depth of the etched features across the substrate (e.g. d.sub.1<d.sub.2<d.sub.3). The use of an anisotropic etchant may improve the formation of the blazed structure allowing such a structure to be formed from a blank or un-etched substrate.

[0068] An un-etched substrate may refer to a substrate that has a substantially flat surface. An un-etched or blank substrate may be cleaned prior to etching, for example to remove contaminants or oxide layers.

[0069] The surface relief structures formed by the methods described herein (e.g. as shown in FIG. 6) may form many different types of optical elements, including optical gratings, optical facets and optical mirror arrays, etc. As noted above, the resultant structure may not itself be used as an optical element but may be used as a master to form (e.g. via moulding) a plurality of copies that are then used as optical elements. The optical elements may themselves then form part of a HUD, HMD or other head-worn device. The method described above may, for example, be used to generate a variable efficiency optical grating that may be used to improve the uniformity of intensity output over the length of the grating. In examples where the surface relief structure results in an array of mirrored facets operating in reflection, the variation in etch depth results in a variation in sampling of incident light (e.g. as a consequence of the differing lengths of the mirror facets).

[0070] Although the present invention has been described in connection with some 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. Additionally, although a feature may appear to be described in connection with particular embodiments, one skilled in the art would recognize that various features of the described embodiments may be combined in accordance with the invention. In the claims, the term comprising does not exclude the presence of other elements or steps.

[0071] Furthermore, the order of features in the claims does not imply any specific order in which the features must be performed and in particular the order of individual steps in a method claim does not imply that the steps must be performed in this order. Rather, the steps may be performed in any suitable order. In addition, singular references do not exclude a plurality. Thus, references to a, an, first, second, etc. do not preclude a plurality. In the claims, the term comprising or including does not exclude the presence of other elements.