Height-modulated diffractive master plate and method of manufacturing thereof

Abstract

The invention relates to a method of fabricating a master plate for producing diffractive structures and a master plate obtainable therewith. The method comprises providing a substrate having a periodic surface profile, filling the surface profile uniformly at least partly with filling material, and partially removing the filling material in order to produce a master plate having a periodic height-modulated surface profile formed by said substrate and said filling material. The invention allows for producing master plates capable of further producing gratings with variable diffraction efficiency.

Claims

1. A method of fabricating a master plate for producing diffractive structures, the method comprising: providing a substrate having a periodic initial surface profile, filling the initial surface profile uniformly at least partly with filling material, and partially removing the filling material in order to produce a master plate having a periodic height-modulated surface profile formed by said substrate and said filling material, wherein the initial surface profile comprises fill factor modulation, which is persisted in the height-modulated surface profile, and wherein said removing of the filling material partially comprises: applying a physical mask layer having an uneven height profile on the filling material, and removing at each location said mask layer and an underlying filling material, so as to replicate the height profile of the mask layer to corresponding sections of the filling material in order to produce said height-modulated surface profile.

2. The method according to claim 1, wherein said removing of the filling material partially comprises using grayscale lithographic removal of said filling material in order to produce said height-modulated surface profile.

3. The method according to claim 1, wherein said height-modulated surface profile comprises at least two distinct segments having different profile heights.

4. The method according to claim 1, wherein said height-modulated surface profile comprises a lateral height gradient profile, such as linear profile.

5. The method according to claim 1, wherein the height modulation of said height-modulated surface profile takes place at least in a periodic dimension of the surface profile.

6. The method according to claim 1, wherein the initial surface profile is entirely filled with said filling material in order to planarize the substrate before said removal.

7. The method according to claim 1, further comprising providing said periodic initial surface profile to the substrate by providing a substrate plate and removing material from the substrate plate, for example by electron beam lithography, or adding material to the plate, for example by nanoimprinting.

8. The method according to claim 1, wherein the initial surface profile is a binary profile.

9. The method according to claim 1, wherein the initial surface profile is a non-binary profile, such as a triangular profile or slanted profile.

10. The method according to claim 1, wherein said filling material is removed different amounts within different lateral segments in one dimensions only so as to produce a one-dimensionally height-modulated surface profile.

11. The method according to claim 1, wherein said filling material is removed different amounts within different lateral segments in two lateral dimensions so as to produce a two-dimensionally height-modulated surface profile.

12. The method according to claim 1, wherein the initial surface profile is periodic in one dimension or in two dimensions.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIGS. 1A-1C illustrate in cross-sectional views fabrication of a master plate according to one embodiment of the present method in different stages of fabrication.

(2) FIGS. 2A-2C illustrate in cross-sectional views fabrication of a master plate according to another embodiment of the present method in different stages of fabrication.

(3) FIG. 3 shows a scanning electron micrograph of a binary structure planarized with resist material (a) and after etching to two different heights (b and c) using the method illustrated by FIGS. 1A-C.

(4) FIG. 4A shows an example how diffraction efficiency of the first transmission order of a binary 1D grating changes as a function of the grating height.

(5) FIG. 4B 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

Definitions

(6) The term “binary surface profile” herein essentially means a surface with a relief structure consisting of two heights. In a line grating the possible heights are defined by tops of ridges and bottoms of grooves adjacent to the ridges. The profile therefore consists of cross-sectionally essentially rectangular surface features (with entirely vertical sidewalls). Binary surface profile is used an example in the following discussion and drawings, although other profiles are also possible, as discussed above.

(7) Filling with filling material, unless otherwise mentioned, covers full filling and partial filling. “Planarization” means full filling such that enough filling material is provided on the surface profile to embed the binary features of the profile so that a planar surface made of the filling material is formed.

(8) “Partial removal” of filling material means that at least some thickness of filling material is left on at least some location of the substrate.

(9) “Height modulation” refers to variation of dimension of grating features in the normal direction of the substrate. For example, in the case of a line grating, height is measured from the bottom of a groove adjacent to a ridge to the top of the ridge. A height-modulated master plate thus comprises at least two lateral segments, either in one or two dimensions, with different feature heights between the regions.

(10) The term “fill factor” refers to the proportion of grating structure material to surrounding material (e.g. air or other solid material) within a grating period. In the typical case of rectangular grating lines, this equals to the proportion of line width to period width. Consequently, “fill factor modulation” refers to variation of fill factor in the lateral dimensions of the grating, i.e. between periods of the periodic structure.

Description of Selected Embodiments

(11) In its preferred embodiments, the present method combines highly anisotropic high resolution binary processing of the substrate and height modulation by lower lateral resolution grayscale lithography of the added filling material. Thus, it enables better control of the high-resolution vertical features and the sidewall profiles in comparison with direct grayscale lithography, where vertical sidewalls are difficult to achieve. Two basic embodiments are described below.

(12) FIGS. 1A-1C illustrate the first basic embodiment, which uses a grayscale lithography step to produce the height modulation on the binary substrate.

(13) In the first step illustrated by FIG. 1A, a binary structure with a possible fill factor modulation (not shown here) is fabricated using for example optical, electron beam lithography, embossing or nanoimprint lithography. The structure comprises a base plate 12A and binary elements 14A extending from the base plate 12A. At this point all the elements 14A have the same height. Depending on the fabrication method, the base plate 12A and binary elements 14A may comprise a unitary piece of single material or be made of different materials. For example, the elements 14A may comprise polymer added on an inorganic or polymeric base plate 12A. Alternatively, the substrate can be polymeric or inorganic unitary entity into which the features are processed by lithographic methods. Fill factor modulation, including line width modulation, gap width modulation or both, can also be included. This layer is referred to as grating layer.

(14) Next, as illustrated by FIG. 1B, the binary structure 12A, 14A is coated with an electron beam or optical lithography resist material, to at least partially fill the gaps between the elements 14A, and typically so that the grating layer is fully covered with the resist material and the surface of the structure is planarized with a fill layer 16A. A spin-coating, spray-coating, casting or dip-coating method, for example, can be used.

(15) Next, illustrated by FIG. 1C, a grayscale lithography is performed to the fill layer 16A. This kind of an overlay grayscale exposure results in different development speeds caused by different exposure doses and different heights arising from the surface. As a result, several regions S1, S2, S3 of different element heights h1, h2, h3, respectively, are formed, defined by the thickness of the fill layer 16A′ at each region.

(16) As mentioned above, the grayscale lithography may be carried out using direct writing with a laser, for example, according to an exposure scheme corresponding to the desired modulation or using an optical mask producing an illumination pattern having simultaneously a plurality of distinct regions of different intensities corresponding to the desired modulation. Instead or in addition to distinct regions, a continuous gradient may be formed.

(17) FIGS. 2A-2C illustrate the second basic embodiment, which uses a physical mask to produce the height modulation.

(18) In the first step illustrated by FIG. 2A, a binary structure 12B is fabricated using in a similar manner as described above with reference to the first basic embodiment.

(19) Next, as shown in FIG. 2B, the binary structure is planarized with a fill layer 16B as described above.

(20) On top of the fill layer 16B, a variable-height mask layer 18B is applied by any lithography method. Here, significantly lower lateral resolution is required in comparison with the grating layer resolution.

(21) Next, as illustrated in FIG. 2C, a dry or wet etch method is used to transfer modulation of the mask layer 18B into the fill layer 16B′. Again, due to high selectivity between the fill layer 16B, 16B′ and the grating layer 12B, high control over grating profile is retained.

(22) In this example, a gradient mask is used, but, like in the previous example, the mask can comprise distinct regions of different heights. A combination of these approaches can also be used, depending on the desired height characteristics of the resulting master plate.

(23) Both of the described basic embodiments allow for different diffraction efficiencies in a single high-quality diffractive structure as the vertical sidewall characteristics of the structure are defined by the substrate and height characteristics are defined by the masked etch process. Due to high selectivity between filling layer 16A, 16B and grating layer 12A-B, 14A-B, the full control over the grating line profiles is retained even if the etch process was isotropic.

(24) It should be noted that also the binary surface profile of the substrate can be a resist structure or it can be transferred to the substrate by wet or dry etching. The structure is then typically coated with an electron beam or optical lithography resist which has selectivity with respect to the binary structure.

(25) If a segmented plate is produced, the period of the grating is typically a fraction of minimum lateral dimensions of the mask zones, i.e. the segments have a considerably higher size with respect to the period. For example, the mask zones, which determine diffraction efficiency segments in the final product, can have dimensions of 10 μm or more, typically 1 mm or more, whereas the grating period is typically 10 μm or less, in particular 1 μm or less.

(26) The substrate in all embodiments can be a standard silicon wafer or SiO.sub.2 wafer, for example. Also, any other rigid or flexible substrate that can be applied in optical or electron beam lithography can be used.

(27) The binary surface profile, i.e., a relief structure, can be etched to the aforementioned substrate or it can be processed into a thin film deposited on the substrate. In case of etching, this layer can be any thin film that can be processed in dry or wet etching methods.

(28) Alternatively, the surface profile can be formed using an optical or electron beam resist, in which case the binary structure can be fabricated by optical or electron beam exposure and developed to the layer. The resist material can also be etched in order to fabricate the binary structure.

(29) The filling layer can be an optical or electron beam resist. It can be applied by spin-coating, spray coating or dip-coating to mention some examples.

(30) Alternatively, the filling layer can be deposited by using physical vapor deposition (PVD) or chemical vapor deposition (CVD) or atomic layer deposition (ALD). This can be for example a metal oxide such as Al.sub.2O.sub.3 or TiO.sub.2 or other. It can also be SiO.sub.2.

(31) It is preferred that the filling layer has high selectivity with respect to the binary surface profile material, taking into account the removal method used (e.g. high etch selectivity). In other words, the filling material has to be selected so that during partial removal of this layer, the original binary structure remains intact.

(32) The present height modulation can take place in the periodic dimension of the surface profile, as illustrated in the drawings. However, it is not excluded that it additionally or instead of that, takes place in the perpendicular dimension of the grating, for example along the grating lines of a line gradient. The present method is also equally applicable to two-dimensional gratings having periodicity in two different directions.

(33) 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.

(34) 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.

(35) FIGS. 4A and 4B 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. 4A, the grating fill factor is 0.5 and in FIG. 4B, the grating height is 250 nm.

CITATIONS LIST

Non-Patent Literature

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