Method for manufacturing diffraction grating

Abstract

The invention relates to a method for manufacturing a diffraction grating. The method includes providing a first substrate and fabricating a first surface profile on the first substrate using a temporary grating material. Next, the first surface profile is fully covered by the final grating material layer, and the second substrate is bonded to the final grating material. Finally, the first substrate and the temporary grating material are removed for creating a second surface profile on the final grating material, the second surface profile being a negative of the first surface profile. The invention allows for the use of, for example, inorganic materials to conveniently produce high quality gratings and height and / or fill factor modulation for diffraction efficiency control.

Description

[0001] This application is a divisional application of the international application number PCT / FI2018 / 050369, the international application date of May 18, 2018, the national phase application number 201880038472.2, and the application named “Method for Manufacturing Diffraction Grating”. Technical Field

[0002] The present invention relates to the fabrication of micro- and nanostructures for optical purposes. In particular, the present invention relates to the fabrication of optical diffraction gratings which may be used, for example, in display applications such as near-eye displays. Background Art

[0003] Near-eye displays (NEDs) and heads-up displays (HUDs) often include diffraction gratings to generate a visible image. The required gratings are an in-coupling grating, which couples the image from the image source to the waveguide; an out-coupling grating, which generates the final visible image for the user; and an exit pupil expander (EPE), which increases the size of the display exit pupil.

[0004] The quality and characteristics of the grating determine the quality of the resulting image. In addition to having clear and consistent grating lines, in advanced applications it is also desirable to be able to locally control the diffraction efficiency of the grating. This can be achieved by changing the grating line height or fill factor within the grating (i.e., using height or fill factor modulation). In order to achieve the maximum possible efficiency adjustment range, both height and fill factor should be modulated. Therefore, a robust and cost-effective manufacturing method for diffraction gratings is needed, in which the diffraction efficiency can be freely controlled and the method is suitable for large-scale production. Furthermore, in some cases, non-polymer materials are required, which increases the process complexity compared to direct polymer modulation.

[0005] The manufacture of height modulated elements is generally accomplished by repeating manufacturing cycles, wherein a height is defined within a cycle. Specifically, it is difficult to manufacture microstructures and nanostructures with varying heights on the same substrate, especially in the case of inorganic materials that are difficult to process. This generally requires several aligned manufacturing cycles, wherein each element height is defined separately during a cycle. This also requires highly optimized and often complex processing of the material. In order to obtain vertical sidewalls in the material, highly anisotropic etching is required in currently available methods. A known processing method is discussed in C. David's "Fabrication of stair-case profiles with high aspect ratios for blazed diffractive optical elements", Microelectronic Engineering, 53 (2000). Due to the complexity of the method, the process has a low yield. In addition, overlay exposure requires lateral placement accuracy at the nanometer level, and any deviation from the optimum results in a loss of optical performance. When both height modulation and fill factor modulation are desired in order to achieve the maximum efficiency adjustment range, specific challenges are faced.

[0006] Therefore, there is a need for novel industrial-scale techniques for producing diffraction patterns, and in particular methods that allow height and / or fill factor modulation to achieve diffraction efficiency control using challenging materials. Summary of the invention

[0007] It is an object of the present invention to overcome at least some of the disadvantages of the known art and to provide a novel method for manufacturing a diffraction grating.

[0008] The invention is based on avoiding micro- or nano-scale processing of the final grating material by using a temporary material on a substrate to define the final grating and overgrow the final grating material thereon. The temporary material is removed after laminating a new support on the other side of the structure and peeling off the original substrate. The surface profile of the temporary material is replicated as a negative of the final grating material.

[0009] In more detail, the method comprises manufacturing a diffraction grating, the method comprising:

[0010] - providing a first substrate,

[0011] - producing a first surface profile on a first substrate using a temporary grating material,

[0012] - completely covering the first surface profile with a final layer of grating material,

[0013] - bonding the second substrate to the final grating material,

[0014] - removing the first substrate, and

[0015] - Removing the temporary lenticular material for producing a second surface profile on the final lenticular material, the second surface profile being the negative of the first surface profile.

[0016] Specifically, the present invention is characterized by what is stated in claim 1.

[0017] The present invention provides significant benefits.

[0018] In the present method, the final grating material or set of materials can be freely selected, since it does not require further processing. In particular, the material may include an inorganic compound. This is because the final profile is completely determined by processing a more easily processable material, such as a resist or polymer material, in the case of preparing the surface profile of the sacrificial layer. Thus, the final grating material can be applied in one process step using a deposition method that does not require spatial precision or control, and the surface profile of the sacrificial layer is replicated as a negative of the final grating layer.

[0019] The described method allows the use of optical materials (such as inorganic optical materials) with high refractive index (such as 1.7 or higher, especially 2.0 or higher, such as 2.2 or higher) to simultaneously combine structure height and fill factor modulation for diffraction efficiency adjustment. Thus, the efficiency of the grating can be spatially varied while maintaining high optical quality.

[0020] The method can be well integrated with the manufacturing process of diffractive displays. In particular, the second substrate attached to the stack during the process can be a waveguide of the display, and the manufactured grating is an incoupling or outcoupling grating of the display, or alternatively a diffractive exit pupil expander. The optical coupling of the grating and the substrate can be ensured by the bonding process and the material selection.

[0021] The invention is particularly suitable for gratings fabricated on waveguide display elements of near-eye displays.

[0022] The dependent claims are directed to selected embodiments of the invention.

[0023] The temporary layer may feature a line structure suitable for producing a one-dimensional grating, or a dot structure suitable for producing a two-dimensional grating, for example.

[0024] The surface profile may contain variations in feature (line) width (ie, fill factor modulation) and / or variations in feature (line) height (ie, height modulation). Using both modulation techniques provides the maximum tuning range of diffraction efficiency.

[0025] The first surface profile can be made using nanoimprint lithography, electron beam lithography, optical lithography or embossing, while the final grating material can be applied using atomic layer deposition, chemical vapor deposition or physical vapor deposition or variations thereof. High refractive index polymer materials can be applied by spin coating, spray coating or inkjet printing. Typically, these materials are cured by UV light or heat, and they may contain inorganic compounds.

[0026] According to some embodiments, the grating material layer has a flat free surface after deposition, and a second substrate as part of the final product is bonded to the flat surface.

[0027] In some embodiments, the surface of the final grating material layer is processed, such as polished or otherwise smoothed, before the second substrate is applied thereto. Improving the smoothness of the surface ensures good contact and bonding between the layers. Alternatively or in addition, the process may include some other type of surface preparation for the purpose of applying the next layer.

[0028] The final grating material may include inorganic materials such as Si3N4, TiO2, SiO2, HfO2.

[0029] In some embodiments, the final grating material has a refractive index that deviates no more than 10% from the refractive index of the second substrate. In some embodiments, the refractive index of the final grating material is more than 10% higher than the refractive index of the second substrate, such as 10-30%.

[0030] In some embodiments, the second substrate is bonded to the grating material using an adhesive layer. Since the second substrate and the final grating material are typically rigid materials, the adhesive layer ensures physical and optical coupling of the substrate and the grating. In some embodiments, the adhesive layer has a refractive index that deviates by no more than 10% from the refractive index of the second substrate (and sometimes also from the refractive index of the final grating material). The final grating material may also have a refractive index that is more than 10% higher (such as 10-30%) than the refractive index of the adhesive layer.

[0031] In some embodiments, bonding of the second substrate to the grating material is performed using physical or chemical bonding.

[0032] Next, embodiments of the present invention and their advantages are discussed in more detail with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1A -1E illustrates in cross-sectional view a structure generated by an exemplary method according to one embodiment of the present invention.

[0034] Figure 2A An example is shown of how the diffraction efficiency of the first transmission order of a binary 1D grating varies depending on the grating height.

[0035] Figure 2B An example is shown of how the diffraction efficiency of the first transmission order of a 1D grating varies depending on the grating filling factor. DETAILED DESCRIPTION

[0036] An exemplary method is described herein that provides a feasible means to manufacture microstructures and nanostructures with varying structure heights (and line widths or fill factors) using any desired material that conforms to a selected deposition method. In summary, the method includes filling a mold with a possibly varying structure height and line width that is manufactured in a sacrificial material or copied into a sacrificial material. The sacrificial mold is typically on a first substrate, such as a support wafer or plate. Filling is performed by vapor deposition using one or more desired materials. The structure with the first substrate is laminated to a second substrate, such as a support plate, by, for example, adhesive bonding, physical or chemical bonding. Next, the original support plate is peeled off, exposing the sacrificial layer. The sacrificial layer is subsequently removed by a wet or dry etching method. This results in a negative copy of the original structure in the desired material.

[0037] In more detail, Figure 1A A first substrate 10 is shown comprising a patterned sacrificial layer 12 comprising features of varying heights and / or widths forming a non-planar surface profile of the structure. Here, the features are rectangular in cross-section, but profiles with non-rectangular corners can also be made using the present method. For example, if a one-dimensional grating is desired, the features can be straight lines with rectangular or triangular cross-sections in a plane perpendicular to the line length.

[0038] The substrate may be a wafer or any other film structure capable of carrying the sacrificial layer and being separated from the top layer at a subsequent stage of the process.

[0039] The sacrificial layer 12 is made using a temporary grating material that is also removed in a subsequent stage of the process. The material may be, for example, a resist, a polymer or other patternable and removable thin film material. Patterning may be performed using nanoimprint lithography, electron beam lithography, optical lithography or embossing, to name a few examples.

[0040] In the illustrated example, the first substrate 10 is visible between the protruding features of the sacrificial layer 12, ie forming the bottom of the gaps between the features. However, the application and patterning of the sacrificial layer may also be performed such that the entire substrate 10 is covered.

[0041] exist Figure 1B In the next step described, Figure 1AThe surface profile of the grating is covered with a desired final grating material (such as an inorganic transparent material) layer 14. Examples include Si3N4, TiO2, SiO2 and HfO2. Covering preferably includes filling the grooves of the profile at least to the level of the highest features. Typically, the profile is overfilled so that all features of the profile are completely embedded in the final grating material layer. Preferably, after this stage, the top surface of the layer is substantially flat.

[0042] Atomic layer deposition (ALD), chemical vapor deposition (CVD) or physical vapor deposition (PVD) are all suitable methods for applying the final grating material. UV or thermally curable high refractive index polymer materials can be applied by spin coating, spray coating or inkjet printing.

[0043] Any remaining unevenness of the surface may be polished away using mechanical or chemical polishing or a combination thereof after filling the temporary grating structure with the final grating material layer 14. This ensures good bonding of the grating layer to the second substrate provided in the next step.

[0044] Next, if Figure 1C As illustrated, the second substrate 18 is attached to the free surface of the final grating layer 14. An adhesive layer or other intermediate layer 16 may be used to ensure the bonding of the layers. However, depending on the material properties, the layers may also be directly bonded using suitable physical or chemical bonding methods (e.g. using heat and / or pressure and / or radiation).

[0045] The flatness of the top surface of the grating layer 14 ensures good bonding of the second substrate 18 to the grating layer 14 and optical coupling of the layers.

[0046] Therefore, the second substrate 18 and optionally the adhesive layer 16 are preferably made of a transparent material.

[0047] It should be noted that if an adhesive is used, the adhesive material preferably has the same refractive index as the waveguide to improve optical performance. In near-eye applications, the second substrate can be a high refractive index material and can act as a waveguide layer. It should also be noted that the final grating material can be selected so that it has the same refractive index as the second substrate acting as a waveguide layer, so that light incident from the waveguide layer interacts with the modulated layer regardless of any bias layer thickness, i.e., the thickness of any unmodulated layer between the waveguide layer and the modulated grating layer.

[0048] like Figure 1D As shown, the first substrate 10 is then removed from the stack, i.e. peeled off. Thus, the temporary grating layer 12 and, optionally, the final grating layer 14 between the sacrificial material features are exposed. Depending on the material properties and the bonding between the layers, the removal may include mechanical, physical or chemical material removal.

[0049] Finally, the sacrificial material is removed from one side of the removed first substrate. This results in a negative copy of the original surface profile in the final grating material layer 14 on the second substrate 18. Chemical or physical etching methods that are selective for the sacrificial material may be used.

[0050] General considerations and variations

[0051] The final material may be an inorganic transparent material, in particular a metal compound, such as a metal oxide or a metal nitride. Specifically, the final material may include a material having a refractive index of 2.0 or more (such as 2.2 or more). The material may be, for example, TiO2, SiO2, Si3N4 or HfO2.

[0052] The second substrate 18, like the optional adhesive layer 16, is preferably optically transparent, such as a glass substrate or a polymer substrate. Transparent herein means a transmittance higher than 50%, especially higher than 95%. For display applications, it is preferred that the substrate can be used as a waveguide for visible wavelengths (i.e., as a light guide). The substrate can be planar or curved.

[0053] In a typical embodiment, the final grating material has a higher refractive index than the second substrate material. This allows light traveling in the substrate via total internal reflection to leave the substrate at the location of the grating and be diffracted. For example, the refractive index of the substrate may be less than 2.0, while the refractive index of the grating material may be greater than 2.0.

[0054] The present invention can be used to fabricate gratings for display applications, such as wearable display applications, e.g. virtual reality or augmented reality glasses. In these applications, the area of ​​the gratings fabricated is typically at least 1 cm 2 , such as 2–500 cm 2 .

[0055] The diffraction grating may be, for example, an out-coupling grating, an in-coupling grating or an exit pupil expander (EPE) of a near-eye display (NED) or a head-up display (HUD).

[0056] The period of the pattern is typically 10 μm or less, especially 1 μm or less, such as 200-800 nm. It should be noted that in addition to constant period gratings, the present invention can also be used to produce period modulated gratings. That is, the period does not have to be constant in the lateral dimension of the grating.

[0057] If desired, the grating may be embedded in the optical structure, ie covered or coated with one or more additional layers.

[0058] Figure 2A and 2BIt is shown how the diffraction efficiency of the first transmission order of a dielectric binary grating can be modulated using height and fill factor modulation. The numerical results are obtained using the Fourier modal method (also known as rigorous coupled wave analysis). The binary grating resides at the interface between air and a glass substrate of refractive index 2.0, the grating period is 500nm, the fill factor is 0.5, and the grating is made of the same material as the substrate. The grating is illuminated with a plane wave of free space wavelength of 450nm at normal incidence. Results are shown for both transverse electric (TE) and transverse magnetic (TM) polarizations. Figure 2A In the example, the grating filling factor is 0.5, while in Figure 2B In the figure, the grating height is 250nm.

[0059] Reference List

[0060] Non-patent literature

[0061] C. David, "Fabrication of stair-case profiles with high aspect ratios for blazed diffractive optical elements," Microelectronic Engineering, 53 (2000).

Claims

1. A method for manufacturing a diffraction grating, comprising: providing a first substrate, fabricating a first surface profile on said first substrate using a temporary grating material, completely covering said first surface profile with a final layer of grating material, bonding a second substrate to the final grating material layer, exposing the temporary grating material by removing the first substrate, and After revealing the temporary grating material, the temporary grating material is removed for producing a second surface profile on the final grating material, the second surface profile being a negative of the first surface profile.

2. The method according to claim 1, characterized in that The first substrate is removed by lift-off, including mechanical, physical or chemical material removal.

3. The method according to claim 1 or 2, characterized in that: The temporary grating material is removed using a chemical or physical etching method that is selective for the sacrificial material.

4. The method according to claim 1, characterized in that: The first and second surface profiles include features of different heights for producing a height modulated diffraction grating.

5. The method according to claim 1 or 2, characterized in that: The first and second surface profiles include features of different widths for fabricating a fill-factor modulated diffraction grating.

6. The method according to claim 1 or 2, characterized in that: The method includes using nanoimprint lithography, electron beam lithography or optical lithography to produce the first surface profile.

7. The method according to claim 1 or 2, characterized in that: This includes applying the grating material using atomic layer deposition, chemical vapor deposition, physical vapor deposition, spin coating, spray coating or inkjet printing or variations thereof.

8. The method according to claim 1 or 2, characterized in that: The grating material layer has a flat free surface after deposition, and the second substrate is bonded to the free surface.

9. The method according to claim 1 or 2, characterized in that: The grating material includes inorganic materials, such as Si3N4, TiO2, SiO2, and HfO2.

10. The method according to claim 1 or 2, characterized in that: Included is bonding the second substrate to the grating material using an adhesive layer.

11. The method according to claim 10, characterized in that The adhesive layer has a refractive index that deviates from the refractive index of the second substrate by no more than 10%.

12. The method according to claim 1 or 2, characterized in that: Included is bonding the second substrate to the grating material using physical or chemical bonding.

13. The method according to claim 1 or 2, characterized in that: The final grating material has a refractive index that deviates from the refractive index of the second substrate by no more than 10%.

14. The method according to claim 1 or 2, characterized in that: The final grating material has a refractive index at least 10%, such as 10-30%, higher than the refractive index of the second substrate.

15. The method according to claim 1 or 2, characterized in that: The second substrate is a waveguide, or the method further comprises bonding the second substrate to a waveguide.

16. The method according to claim 15, characterized in that The waveguide is a waveguide display element of a near-eye display.

17. Use of a method according to any preceding claim for an in-coupling grating, pupil expander grating or out-coupling grating of a diffractive waveguide display.

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