MAKING AND USING

FIELD The invention relates to non-lithographic methods for manufacturing devices, such as polarizers, that have a grid of metal conductors located on the surface of a curved substrate. The invention also relates to methods and devices for forming periodic wire grids with a period of, for example, 150 nm or less.

BACKGROUND Wire grid polarizers are widely used in, for example, devices for graphic information imaging (e.g., see U.S. Pat. No. 6,452,724, incorporated herein by reference). The commonly- used technology for manufacturing these devices is based on optical or interference lithography. However, the cost associated with the use of the tools designed for these applications is considered very significant. The existing approach and tools make it difficult to fabricate wire grid polarizers on curved substrates such as spherical or other more complex optical shapes. In addition, the existing approach makes it is very difficult to create wire grid structures with a period of 150 nm or less. While different applications have different requirements, structures with smaller feature size are usually associated with higher performance.

A method for nanorelief formation on a film surface, utilizing plasma modification of a wave-ordered structure (WOS) formed on amorphous silicon layer, was disclosed in Russian Patent Application RU 2204179, incorporated herein by reference.

An example of this approach is schematically illustrated on FIGS. 1 A, IB and 1C. First, a layer of amorphous silicon 2 is deposited on top of the target thin film layer 4. Then, the silicon layer is sputtered with a flow of nitrogen ions 31 to create a wave ordered nanostructure 1. The resultant wave-ordered nanostructure has relatively thick regions of amorphous silicon nitride 10 and relatively thin regions of amorphous silicon nitride 20 situated respectively on the front and back sides of the waves in the wave-ordered structure 1. The waves (elements) of the wave- ordered structure are oriented substantially in one direction along one axis. As shown, the wave troughs are spaced from the surface of the film layer 4 by a distance D that is usually less than the nanostructure period 3. After the wave-ordered nanostrucrure 1 is formed, its planar pattern, which is shown in FIG. 1 A, is transferred into the underlying film layer 4 by selectively etching the amorphous silicon layer 2 while using regions 10 and 20 as a nanomask. The intermediate etched structure, which is shown in FIG. 1C as an array of nanostructures 1 1 , is composed from amorphous silicon nanostripes 2 covered by the regions of amorphous silicon nitride 10. The layer 4 may be etched in exposed areas 12 between silicon nanostripes 2.

Ion beam techniques have been used to shape and polish curved optical surfaces through precise material removal by ion sputtering process, as disclosed in Michael Zeuner, Sven Kiontke, Ion Beam Figuring Technology in Optics Manufacturing: An established alternative for commercial applications, Optik & Photonik, Vol. 7, No. 2, 2012, pp. 56-58; and Vladimir Chutko, Ion Sources, Processes, Design Issues: Ion beam figuring, Control Parameters, Vacuum

Technology & Coating Magazine, September 2013, pp. 2-10., both of which are incorporated herein by reference. However, these ion beam tools cannot be used directly to form wave-ordered structures on curved surfaces such as, for example, spherical surfaces, with orientation of waves along the latitude lines (parallel lines) on the globe, for fabricating a linear polarizer.

BRIEF SUMMARY

A variety of optoelectronic and other applications can benefit from efficient methods for forming arrays of nanowires with a period of 150 nm or less on curved surfaces. To manufacture such structures, a curved hard nanomask is formed by irradiating a curved layer of a first material with an ion flow. The curved hard nanomask may be used in transferring a substantially periodic pattern onto a curved substrate. This nanomask includes a substantially periodic array of substantially parallel elongated elements having a wavelike cross-section and oriented along the lines of intersections of the curved substrate surface with a set of parallel planes. At least some of the elements have the following cross-section: an inner region of first material, a first outer region of a second material covering a first portion of the inner region, and a second outer region of the second material covering a second portion of the inner region and connecting with the first outer region at a wave crest. The first outer region is substantially thicker than the second outer region. The second material is formed by modifying the first material using an ion flow. The substantially parallel, elongated elements having the wavelike cross-section are positioned on the curved layer of the first material along the lines of intersections of the curved layer surface with a set of parallel planes.

In at least some embodiments, the average period of the substantially periodic array is in a range from 20 to 150 nm. In at least some other embodiments, the curved substrate is a lens having a diameter in a range of 4-200 mm.

Another embodiment is a method of forming a curved hard nanomask for transferring a substantially periodic pattern onto a curved substrate. The method includes depositing a first material to form a curved surface layer on top of a surface of a curved substrate; and irradiating the surface of the curved surface layer with a flow of ions until a curved hard nanomask is formed, the nanomask including a substantially periodic array of substantially parallel elongated elements having a wavelike cross-section and oriented along the lines of intersections of the layer surface with a set of parallel planes, at least some of the elements having the following structure in cross-section: an inner region of first material, a first outer region of a second material covering a first portion of the inner region, and a second outer region of the second material covering a second portion of the inner region and connecting with the first outer region at a wave crest, where the first outer region is substantially thicker than the second outer region, and where the second material is formed by modifying the first material by the ion flow, where the ion flow is arranged so as a local plane of ion incidence is substantially perpendicular to the set of parallel planes and oriented along a local surface normal of the surface layer.

In at least some embodiments, the curved substrate is a lens, during the formation of curved hard nanomask the lens is moved with respect to the ion flow, and the ion flow is shaped to the ion beam having diameter (D), which is determined by radius of curvature (R) of curved lens surface as D < R/6 or D < R/3. In at least some other embodiments, the curved substrate is positioned stationary with respect to a specially arranged ion flow. BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 A shows a SEM top view image of a conventional WOS hard nanomask having a period of about 80 nm formed by irradiation of amorphous silicon layer with N ₂ ⁺ ions having energy E = 5 keV at the angle of bombardment Θ = 53° from surface normal.

FIG. IB is a perspective view of elongated ridge elements of a conventional WOS hard nanomask and their wavelike cross-section.

FIG. 1C is a perspective view of elongated ridge elements formed in amorphous silicon layer from a conventional WOS hard nanomask by wet etching.

FIG. 2 is a perspective view of the arrangement of a curved hard nanomask on a spherical surface, according to the invention.

FIG. 3 illustrates perspective views of a fixed ion beam and relative positions of a spherical segment during its movement under the ion beam, according to the invention.

FIG. 4 schematically illustrates an ion beam tool for forming a curved hard nanomask on a moving curved substrate, according to the invention.

FIG. 5 shows a SEM top view image of a curved WOS hard nanomask having a period of 74 nm formed by irradiation of an amorphous silicon layer with N ₂ ⁺ ions having energy E = 4,5 keV at the angle of bombardment Θ = 52° from surface normal, according to the invention.

FIG. 6 schematically illustrates steps in one embodiment of a method for formation of a device, such as a wire grid polarizer, using a curved hard nanomask formed in an amorphous silicon layer, according to the invention.

FIG. 7 is a perspective view of a special grid surface of an ion source to manufacture a curved hard nanomask on a fixed curved substrate, according to the invention.

FIG. 8 shows cross-sectional views of polymeric structures with elongated nanoridges formed on curved surfaces formed using nanoimprint lithography, according to the invention. DETAILED DESCRIPTION

Detailed descriptions of the preferred embodiments are provided herein. It is to be understood, however, that the present inventions may be embodied in various forms. Therefore, specific implementations disclosed herein are not to be interpreted as limiting. FIG. 2 is a perspective view of a curved hard nanomask on a spherical surface. In at least some embodiments, the polarizer substrate is a spherical segment 21 . A thin layer of amorphous silicon is deposited onto the spherical segment 21. The elongated elements of a WOS nanomask is positioned along the parallel lines 22 (circle arcs), which are lines of intersections of the curved layer surface with a set of parallel planes 23 that, in the illustrated embodiment, are parallel to the YZ plane.

In at least some embodiments, uniform and parallel alignment of the nanomask elements on a spherical surface is achieved through the movement of the substrate under a fixed ion beam as shown in FIG. 3. In 301 , the ion beam 31 irradiates the near central area of spherical segment 21. The segment 21 is a part of the sphere. The segment 21 moves under the ion beam along parallel lines 22. In 302, the beam is positioned near the periphery of the segment 21. This segment motion is due to the rotation of the sphere around the Z axis to the angle β = 20°. In at least some embodiments, the segment 21 moves continuously along the lines 22 with a velocity of, for example, about 140 μηι/s to form one linear stripe of nanomask elements and stops. Then segment 21 makes a step of, for example, about 1 mm for about of 0.1 s to form the next linear stripe of nanomask elements along the lines 22. The position of the spherical segment 21 in structure 303 is rotated around Y axis to the angle γ = 20° with respect to the position in 302. In at least some embodiments, the speed of segment movement along lines 22 depends on the ion beam current and the larger the beam current the greater is the speed. For example, the speed of 140 μητ/s may correspond to the beam current of about 30 uA and the speed of 4.7 mm/s may correspond to the beam current of about 1 mA. The step movement in the direction perpendicular to the lines 22 may be, for example, in the range from 0.02 to 0.1 of the ion beam diameter D and the smaller the step the faster should be the speed along the lines 22 to keep the ion fluence constant. In some embodiments, the ion beam incidence angle with respect to the local surface normal of spherical segment is in the range Θ = 49°-55°. The formation of a curved WOS nanomask was implemented in an ion beam system, which is schematically illustrated in FIG. 4. The system includes a vacuum chamber 42, a pump system 43, and a gridded ion source 41 , which forms the ion beam 31. A moving mechanism 45 enables rotation of the spherical segment 21 around the Z axis (fast motion) and the Y axis (slow stepwise motion). Elements of a curved WOS nanomask are aligned substantially parallel along the lines 22.

In some embodiments, the spherical segment is a lens having the radius of curvature of external surface R. The collimated ion beam depending on its diameter D has different angles of ion incidence Θ across the diameter on the curved surface. In some embodiments, the variation of Θ may be within the range Θ = 44°- 60° and ion beam diameter D < R/6. Taking into account the ion dose dependence of the WOS formation process, this ion beam diameter restriction may be relaxed. In at least some embodiments, the ion beam diameter D is limited by the surface curvature radius R as D < R/3.

In some embodiments, depending on beam focusing, the substrate (lens) diameter may be in the range 4 to 200 mm.

FIG. 5 shows the SEM top view of a curved WOS nanomask 51 with a period of 74 nm obtained as a result of moving the substrate under the fixed ion beam D ~ 10 mm in diameter. The substrate curvature radius was R = 60 mm.

FIG. 6 illustrates one embodiment of a method to manufacture a curved wire grid polarizer or other device on a transparent quartz spherical substrate. It shows a structure 610, including a substrate (e.g., quartz) 604 and a layer of amorphous silicon 602 (for example, approximately 220 nm thick).

The amorphous silicon layer 602 may be deposited onto the curved substrate surface, for example, by magnetron sputtering of a silicon target, by silicon target evaporation with an electron beam in high vacuum, or by any other method known in art. The thickness of the layer 602 is selected to enable the formation of a nanostructure with desired period, λ, (for example, a period of approximately 70— 80 nm). A curved WOS is formed on the surface of substrate 604, which results in the structure 61 1. In this example, the curved WOS is formed using an oblique ion beam 31 of nitrogen N ₂ ⁺ ions positioned at the local ion incidence plane XZ (the plane which is defined by a local normal to the surface of the material and a vector oriented in the direction of the ion flow) at angle Θ to the surface normal (Z-axis). In this particular example, to achieve a WOS period (wavelength) equal to 74 nm the nitrogen ion bombardment angle Θ is approximately equal to 52° and the ion energy is approximately equal to 4,5 keV. The WOS formation depth DF is approximately equal to 144 nm. The thick silicon nitride regions 10 and thin silicon nitride regions 20 on the opposite slopes of the waves are mostly elongated in the Y-axis direction. The top view of this curved WOS pattern is shown in FIG. 5. As shown in the structure 61 1, the wave troughs are spaced from the quartz substrate surface 604 by a distance D, which in a given particular example was of about 60 nm.

Depending on the chosen thickness of the modified layer 20 on the back side of waves of the wavelike nanostructure, a preliminary breakthrough etching step might be performed using argon ion sputtering or sputtering by ions of etching plasma for a relatively short period of time to remove the modified layer 20 from the back side. To remove regions 20 one can also perform wet etching in HNO3-HF solution for a short period of time.

In at least some embodiments, the structure 61 1 may be optionally wet-etched to form the structure 61 la having nanotrenches 12a in place of regions 20. This optional wet etching may improve further etching steps in plasma.

The curved WOS and nanostructures formed from the curved WOS by etching with the use of regions 10 and 20 as a nanomask can be characterized as a quasi-periodic, anisotropic array of elongated ridge elements having a WOS pattern, each ridge element having a wavelike cross- section and oriented substantially in one direction (Y-axis). An example of the pattern of a WOS is shown in FIG. 1A, and an example of the pattern of a curved WOS is shown in FIG. 5. Ridge elements may be waves or other features having tilted tops or sidewalls. In the wave

nanostructure 1 of FIG. IB, the ridge elements are waves with regions 10 and 20 on opposite wave slopes. In the nanostructure 1 1 of FIG. 1C, the ridge elements are stripes covered by tilted regions 10 and spaced by trenches 12. One can see that the ridge elements are elongated and mostly oriented in the direction of the Y-axis as shown in FIGS. 1A and 5. Referring again to FIG. 6, the structures 611 or 61 la can be modified by applying a reactive-ion plasma (Ch, Ch-Ar, HBr-0 ₂ or Ο ₂ -Ηε-0 ₂ or by any other method known in art) to the amorphous silicon layer 602, using the curved nanomask having regions 10 of silicon nitride. In at least some embodiments, the process results in a modified nanomask having silicon nitride regions 10a formed on top of stripes of amorphous silicon 602, as shown in the structure 612 of FIG. 6. The thickness of regions 10a may become thinner than the thickness of original regions 10 during plasma etching.

In the next step, anisotropic etching is applied to the substrate 604. Depending on the type of the substrate material, different types of plasma can be used (for example, for a quartz substrate, CF ₄ -H ₂ , CHF ₃ , C ₄ F6-Ar or C ₄ F ₈ -Ar based plasma can be used). The resulting structures 613-615 may include trenches 607, 607a, and 607b. During etching the amorphous silicon stripes 602 may be modified by plasma to the structures 602a and 602b or may be fully etched, which may result in tilted sidewalls 608 of quartz nanoridges 609.

Oblique deposition of aluminum can be performed on the array of quartz nanoridges as shown in the structure 616 to produce a curved wire-grid polarizer. For uniform deposition the curved substrate may be moved under the flow of metal atoms 610. In some embodiments, special masks may be applied across the flow of metal atoms to obtain uniform metal nanowires 61 1. In some embodiments, the the mask may include a slit. During the aluminum deposition the slit mask is positioned along the quartz nanoridges, and the quartz substrate is rotated under the mask in a direction perpendicular to the mask extension for uniform aluminum deposition to the nanoridges.

In at least some embodiments, a curved hard WOS nanomask may be manufactured on fixed spherical substrates by a gridded ion source having a special grid surface, which is shown in FIG. 7. The grid 71 provides microbeams 31 that irradiate the surface of the spherical segment 21 with the same local incidence angle (for example, in the range Θ = 49°-55°). The surface of grid 71 may be constructed from an arc circle AB 72 having a center 73 (the quasi focus of the microbeams in ZY plane). The surface of the grid 71 may be produced through the rotation of the arc 72 around the Z axis thus forming the surface of revolution to which each microbeam is normal. The border of the surface of the grid 71 is determined by the microbeams positioned at the border of the spherical segment. In at least some embodiments, the grid 71 may be used to shape the ion flow for manufacturing the curved WOS nanomasks on fixed spherical substrates. In at least some embodiments, the complex grid shape is approximated by a spherical shape.

In at least some embodiments, the spherical surface of a quartz substrate with quartz nanoridges may be used as a master mold to transfer a nanoridge pattern to the surface of a curved polymeric substrate. In structure 810 of FIG. 8 the quartz master mold 81 with nanoridges on its surface is used to transfer the nanoridge pattern into the surface of an intermediate hard polymeric mold 82 of, for example, polycarbonate. In structure 811 of FIG. 8, the intermediate hard polymeric mold 82 is applied to the UV curable surface layer 83 on the surface of the polymeric substrate 84 during the process of soft nanoimprint lithography as known in the art. In structure 812 of FIG. 8, the resultant nanoridge pattern is formed on the surface layer 83, which is positioned on the polymeric substrate 84. The oblique deposition of metal onto the polymeric nanoridges results in a curved wire-grid polarizer.

The invention can be used, for example, for forming curved nanowire arrays for nanoelectronics and optoelectronics devices.