Method of manufacturing a master plate and a master plate

Abstract

The invention concerns a method of manufacturing a master plate for fabrication of diffractive structures, and a corresponding master plate. The method comprises providing a substrate comprising a stack of selective etch layers and providing an etch mask layer on the substrate. Further, the method comprises etching the substrate in a multi-step etching process by exposing the substrate piecewise at different mask zones of the mask layer and using said selective etch layers to produce to the substrate a height-modulated surface profile defined by the mask zones in lateral dimensions and by said stack in height dimension of the substrate, and, finally, providing a height-modulated master grating onto the surface profile, the height modulation of the master grating being at least partly defined by said surface profile of the substrate.

Claims

1. A method of manufacturing a master plate for fabrication of diffractive structures, the method comprising: providing a substrate comprising a stack of selective etch layers, providing an etch mask layer on the substrate, etching the substrate in a multi-step etching process by exposing the substrate piecewise at different mask zones of the mask layer and using said selective etch layers to produce to the substrate a height-modulated surface profile defined by the mask zones in lateral dimensions and by said stack in height dimension of the substrate, and providing a height-modulated master grating onto the surface profile, the height modulation of the master grating being at least partly defined by said surface profile of the substrate.

2. The method according to claim 1, wherein the etch mask layer comprises a plurality of mask zones having different thicknesses measured from the surface of the substrate.

3. The method according to claim 2, wherein said stepwise exposing of the substrate comprises thinning the mask layer uniformly until the substrate is exposed at least at one mask zone.

4. The method according to claim 1, wherein said stepwise exposing of the substrate comprises locally thinning the mask layer in one or more steps until the substrate is exposed at least at one mask zone.

5. The method according to claim 4, wherein said stepwise exposing of the substrate comprises repeating a lithography step for each region of the mask layer until the substrate is exposed at least at one mask zone.

6. The method according to claim 1, wherein the multi-step etching process comprises, for each successive etch layer pair of the substrate, removing the upper etch layer by selective etching at regions of the mask layer, where the substrate is exposed, removing the lower etch layer by selective etching at regions where the upper etch layer is removed, and optionally, exposing the substrate at the region of another mask zone.

7. The method according to claim 3, wherein removing the lower etch layer and uniformly thinning the mask layer occurs at least partly simultaneously in a single etching phase.

8. The method according to claim 1, wherein the stack of selective etch layers comprises a stack of selective etch layer pairs comprising a lower modulation layer having a first thickness and an upper etch stop layer arranged onto the modulation layer, the first and second thicknesses of the layers together defining the height modulation of the surface profile.

9. The method according to claim 8, wherein the second thickness is smaller than the first thickness.

10. The method according to claim 1, wherein said stack comprises at least three etch layer pairs, such as at least five etch layer pairs, and the etch mask layer at least a corresponding number of mask zones having different heights.

11. The method according to claim 1, wherein said providing a height-modulated master grating comprises: coating the substrate with planar coating layer such that the surface profile is covered, and removing, for example by anisotropic etching, zones of the coating layer in according to a periodic pattern in order to produce the height-modulated master grating onto the substrate.

12. The method according to claim 1, wherein the master grating is additionally fill factor modulated.

13. The method according to claim 1, wherein the etch mask layer is provided by: coating the substrate with a uniform etchable mask layer and patterning the mask zones thereon by a microfabrication method, for example by embossing or etching, such as grayscale etching, or providing the mask zones on the substrate by depositing the etchable material, for example by printing.

14. The method according to claim 1, wherein a master plate is manufactured, that has a lateral area of at least 1 cm.sup.2, such as 2-500 cm.sup.2, and wherein the period of the master grating is 10 μm or less, in particular 1 μm or less, such as 200-800 nm.

15. A master plate for fabrication of diffractive structures, comprising: a substrate, and a master grating manufactured on the substrate, wherein the substrate comprises a stack of selectively etchable layers and has been provided with a surface profile whose height characteristics are determined by the thicknesses of the etchable layers, and the height characteristics of the master grating are at last partly defined by said surface profile of the substrate.

16. The method according to claim 6, wherein removing the lower etch layer and uniformly thinning the mask layer occurs at least partly simultaneously in a single etching phase.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0032] FIGS. 1A-1G illustrate in cross-sectional side views exemplary fabrications steps according to one embodiment of the invention.

[0033] FIG. 2A shows an example how diffraction efficiency of the first transmission order of a binary 1D grating changes as a function of the grating height.

[0034] FIG. 2B shows an example how diffraction efficiency of the first transmission order of a 1D grating changes as a function of the grating fill-factor.

DETAILED DESCRIPTION OF EMBODIMENTS

[0035] The term “lateral” herein refers to dimensions parallel to the plane of the substrate surface, i.e. directions along the surface of the substrate. “Height” and “thickness” refer to dimension transverse to the lateral dimensions. A “surface profile” refers to variation of height of the surface in one or both lateral dimensions. Both one- and two-dimensional surface profiles, and therefore diffraction efficiency modulation in one or two dimensions of the grating, can be produced using the present method.

[0036] “Mask zones” are lateral regions of the mask layer processed during the present process in order to expose the substrate piecewise. The mask zones thus determine the order in which the different regions of the substrate are subjected to etching and therefore the surface profile.

[0037] FIG. 1A shows a substrate 10 comprising a base layer 11 and a stack 12 superimposed on the base layer 11. The stack 12 comprises a plurality, in this exemplary case three, pairs of alternating etch layers 12A-C, and etch stop layers 14A-C. The layer pairs have thicknesses corresponding to the desired height modulation characteristics of the final grating. That is, the etch layers and etch stop layers together define a possible section modulation step height, as described below in more detail. The pairs can have equal or different thicknesses.

[0038] In general, the stack 12 may include layers of two or more different materials, which can be selectively etched down layer by layer. In other words, the stack 12 contains two or more different materials, which are oppositely selective for subsequent etch processes. For simplicity, a two-layer process is illustrated and described herein. A desired number of such layer pairs can be applied in the stack.

[0039] The stack 12 can be manufactured e.g. by deposition techniques, such as chemical vapor deposition (CVD), atomic layer deposition (ALD) or physical vapor deposition (PVD) using a material combination suitable for selective etching. In practice, ready-made stacks with known layer thicknesses can be used. Alternatively, the stack can be manufactured as a preparatory part of the present process.

[0040] On top of the stack 12, there is an etch mask layer 16 having a plurality, in this exemplary case five, of mask zones S1-S5. At least some of the mask zones S1-S5 have different heights h.sub.S1-h.sub.S5 with respect to each other. The heights are chosen according to the requirements of the multi-step etching process as described below in more details. The widths, or if two-dimensional segmentation is desired in the height modulation pattern, lateral shapes in general, of the mask zones S1-S5 are chosen according to the desired lateral segmentation characteristics of the end product.

[0041] The mask layer 16 may be e.g. a grayscale etch mask provided using any coating or deposition method capable of providing the required height modulation. Examples include optical lithography, electron beam lithography and imprinting, to mention some alternatives. The mask can be of relatively low in vertical, i.e. height resolution.

[0042] It should be noted that in practice the lateral dimensions of the mask zones are significantly larger than the dimensions of the vertical nodulation, although illustrated for clarity reasons in the Figures otherwise.

[0043] As illustrated in FIG. 1B, the uppermost etch stop layer 14C is opened from regions exposed by the mask layer 16 (herein at mask zone S4). If the mask layer 16 does not initially expose any region of the stack 12, it can be first uniformly etched down until the surface of the stack 12 is accessible.

[0044] Next, as shown in FIG. 1C, the underlying modulation layer 12C is etched down to the level of the next etch stop layer 14B, while simultaneously etching the mask layer 16. What is notable here is that the vertical resolution of the mask layer 16 does not need to be very accurately defined because of the etch stop layers 14A-C. This allows significant safe margins in the process to ensure that each height plane is well established. The end situation after one etch cycle is shown in FIG. 1D.

[0045] In a preferred embodiment, the next mask zone or zones (herein zone S3) of the mask layer 16 is etched completely away before reaching the bottom of the modulation layer 12C. This ensures that the etch stop layers at respective mask zones are correctly exposed for the next steps of the process to succeed.

[0046] The two-step etch cycle is repeated until all layers are etched as desired. FIG. 1E shows a situation where the etch mask layer 16 has been “consumed” and its height modulation has been transferred to the surface profile of the stack 12 as defined by its layer structure.

[0047] Next, as illustrated by FIG. 1F, a planarization layer 18 is added onto the profiled substrate such that it fills the profile. The planarization layer 18 can be for example a resist layer, spin-on glass layer or spin-on carbon layer.

[0048] Lastly, the planarization layer 18 is transformed into a height-modulated grating 18′ by an appropriate lithographic methods and/or etching capable of producing a periodic structure with lateral feature dimensions the optically diffractive scale. This is depicted in FIG. 1G. In the exemplary situation, the fill factor of the grating is constant.

[0049] In some embodiments, also the fill factor of the grating is modulated at this stage. Thus, the grooves and ridged need not be of the same width throughout the grating, but may differ to further alter the diffraction efficiency.

[0050] In some embodiments, the period of the grating is modulated in addition to the height modulation and, optionally, the fill factor modulation.

[0051] Suitable lithographic methods for producing the lateral modulation include optical lithography, electron beam lithography and etching, to mention some possibilities. Also imprinting can be used, whereby the coating step of FIG. 1F can be omitted.

[0052] The described method enables improved control of high-resolution vertical features and sidewall profiles in comparison with direct grayscale lithography, where vertical sidewalls are difficult to achieve.

[0053] It should be noted that the present method is not limited to binary profiles. Thus, the profile can be slanted, mixed binary-slanted, etc. Such profiles can be achieved by e.g. using appropriate slanted grayscale etch profiles in the appropriate steps of the process.

[0054] Also the initial height modulation can be either positive or negative.

[0055] The period of the master grating is typically a fraction of minimum lateral dimensions of the mask zones. For example, the mask zones, which determine diffraction efficiency segments in the final product, can have dimensions of 0.5 mm or more, whereas the grating period is typically 10 μm or less, in particular 1 μm or less, such as 200-800 nm.

[0056] To give one exemplary suitable material combination of the present structure, the mask layer can consist of photoresist, the etch stop layers can consist of SiO.sub.2 (applicable using PVD or CVD, for example), the modulation layers can consist of aluminum (applicable using PVD, for example), the planarization layer and the grating layer can consist of electron beam lithography resist and the substrate can be a silicon substrate.

[0057] The present master plate can be used to produce diffractive optical elements having laterally non-constant diffraction efficiency for various needs. In particular, the plate can be used to produce large elements, typically having an area of 1 cm.sup.2 or more, for example for NEDs or HUDs. Variable diffraction efficiency provides advantages in in-coupling gratings, exit pupil expanders and/or out-coupling gratings of diffractive waveguide displays, such as smart glasses and virtual reality and augmented reality displays.

[0058] The master plate produced using the present method can be used in stamping processes, which are known per se in the art of producing diffractive gratings.

[0059] FIGS. 2A and 2B show how the diffraction efficiency of the first transmission order of a dielectric binary grating can be modulated using height and fill-factor modulation. Numerical results were obtained with the Fourier modal method (also known as rigorous coupled wave analysis). The binary grating resides on an interface between air and a glass substrate having refractive index of 2.0, the grating period is 500 nm, fill factor 0.5, and the grating is made of the same material as the substrate. The grating is illuminated with a plane wave with 450 nm free space wavelength at normal incidence. Results are shown for both transverse electric (TE) and transverse magnetic polarizations (TM). In FIG. 2A, the grating fill factor is 0.5 and in FIG. 2B, the grating height is 250 nm.

CITATIONS LIST

Non-Patent Literature

[0060] C. David, “Fabrication of stair-case profiles with high aspect ratios for blazed diffractive optical elements”, Microelectronic Engineering, 53 (2000).