WO2008010219A1

WO2008010219A1 - Liquid crystal display with a microlens array and method of fabrication thereof

Info

Publication number: WO2008010219A1
Application number: PCT/IL2007/000898
Authority: WO
Inventors: Golan Manor; Michael Golub; David Rosenblatt; Baruch Shifman
Original assignee: Explay Ltd.
Priority date: 2006-07-17
Filing date: 2007-07-17
Publication date: 2008-01-24

Abstract

A liquid crystal (LC) panel is presented. The LC panel is configured as an integrated multi-layer structure comprising an LC pixel matrix configured and operable for spatially modulating light passing therethrough, and at least one microlens array (MLA). The MLA is configured such that every pixel of the pixel matrix is associated with its corresponding microlens from said at least one MLA. The MLA has a curved surface defining an array of convex or concave surface regions of the microlens in the MLA, respectively, and an opposite planar surface which is common for all the microlenses in the MLA. The MLA by either one of its surface is spaced from an active surface of the pixel matrix or from another MLA by a spacer layer structure. The spacer layer structure has at least 10 microns thickness and at most 150 microns thickness, and is made either from an inorganic material by a sacrificial layer technology or from one or more organic polymer materials including an BCB layer.

Description

LIQUID CRYSTAL DISPLAY WITH A MICROLENS ARRAY AND METHOD OF FABRICATION THEREOF

FIELD OF THE INVENTION

This invention is in the field of liquid crystal display (LCD) devices, and relates to an LCD utilizing one or more microlens arrays, and a method of fabrication the microlens array suitable to be used in the LCD manufacture. The invention is particularly useful in projection displays.

BACKGROUND OF THE INVENTION

Image projection systems are becoming the essential part of various electronic devices such as computers, television and cinema systems. The use of video or digitally controlled image projecting systems was disclosed in numerous publications. Another class of devices of the kind specified utilizes rear projection TVs for home theatre systems. Flight simulator systems deal with upright projection and projection in a different direction. Compact projection systems allow for displaying, in a user-friendly form, a digital contact tapped inside a laptop and desktop computers, cellular phones, compact memory devices.

Micro-lens arrays (MLA) are widely used in illumination and imaging systems, as well LCD desktop screens and television systems. US7, 128,420 and WO2004/064410, both assigned to the assignee of the present application, disclose an image projecting device including a Spatial Light Modulator (SLM) unit (a liquid crystal panel) utilizing microlens array(s) as a part of the pixel arrangement of the SLM unit.

Known processes for manufacturing MLAs include resist writing and resist reflow processes. The planarizing and spacer layer are deposited on top of the MLA layer for achieving both mechanical and optical properties. The planarizing layer planarizes the MLA layer, and the spacer layer creates a longitudinal spacing after the MLA to focus light onto a region within the open area on the TFT layer. The planarizing layer is to have as low as possible index of refraction to enable high power of the lens with minimum curvature. The spacer layer defines a distance between the MLA layer and its focal plane and is to be formed with a thickness of up to dozens of microns. The conventional techniques utilize a variety of polymer materials as planarizing and spacer layers.

GENERAL DESCRIPTION OF THE INVENTION

There is a need in the art in compact image projecting systems, utilizing one or more microlens arrays within an LCD panel of the projecting system, and preferably capable of projecting colored images. The MLA containing layer is associated with a spacer layer structure. Using the conventional approach of spin coating of known resist layers, it is difficult to form layers with adequate thickness of at least 10 microns (e.g. 10 -150 microns) without high stress, cracking or delamination and layer uniformity. Also, layers with thermal properties compatible with LCD manufacturing processes (processes under temperatures over 250 degrees C) are difficult to be formed. It is also difficult to form polymer layers that are compatible with further processing steps including deposition of ITO and metals with good adhesion. Further to that, using the conventional approach of etching of MLAs of a ready fused silica or glass substrate it is difficult to form layers with adequate thickness of at most 150 microns (e.g. 10 -150 microns) without high stress, cracking or delamination and layer uniformity, because thin layers of glass are nor mechanically stable without carrier substrate.

The invented technique provides solution for the above problems by providing a novel technique of the LC-with-MLA(s) structure, as will described in the description below The system of the present invention can effectively utilize a light source unit operable with different wavelengths (colors), where at least one of light sources in the light source system may be a laser. Laser light sources provide higher ^"brightness, large depth of focus for a projected image and efficient electrical to optical conversion. They may be used in extra-compact portable arrangements like cellular phones, pocket computers. However the portable configuration demands special solutions in mechanics, packaging, light sources, illumination optics, projection lens and spatial light modulator for the entire projection system. Therefore designs of all the devices are operating in desktop configuration and are not directly transferable to a portable "pocket" configuration due to dimensions, power consumption and heat dissipation problems. The present invention solves this problem by providing a novel compact projecting system (a so-called "nano-projector") based on novel physical and engineering approaches.

Highly efficient liquid crystal transmissive SLMs are the key components of laser projection microdisplays with extended depth of focus. Specifically, MLAs are required for achieving higher efficiency of the transmissive LCDs. Collimated and spatially coherent laser light sources provide for improving a throughput efficiency of microdisplays with the aid of MLAs, which reduce lateral dimensions of a light beamlet passing through each pixel of an SLM down to dimensions of an TFT layer micro- diaphragm. Diffraction spread effects and complicated surface shapes of MLAs impact on the microdisplay performance. Physical optics propagation combined with geometrical ray-tracing are used for both minimizing a diffraction spread and accounting for aberrations in each lenslet.

The inventors have modeled an MLA enabling to predict such parameters as focal spot size and its longitudinal variations, transmission efficiency for a given clear aperture of each pixel, output divergence, a cross talk between adjacent pixels. Simulation data for spherical, elliptical and asymmetrical aspherical lenslet shapes was used as a base for designing advanced SLMs with high throughput efficiency.

The present invention provides high efficient transmissive LCD panels exploiting MLA arrays, for highly efficient projection system comprising laser and LED light sources. As indicated above, the use of an MLA in the transmissive LCD is aimed at reducing lateral dimensions of the light spot down to the diaphragm that is created by a TFT layer. This purpose defines some specific requirements to the MLA configuration and fabrication process. MLAs for LCD devices require a spacer layer to allow the lenses to properly focus the incident light through the open area in the LCD device, where lenslets of the MLA are to be configured with relatively small focal length, e.g. of about 10-150 μm. The MLA layer, namely the MLA and spacer, should be compatible with an TFT or ITO layer used in the LCD structure manufacture. To this end, the MLA layer and the microlens shape should stand high temperatures (i.e. those of ITO and polyimide deposition and cure, e.g. temperatures reaching 250-700⁰C for periods of 30 minutes or longer. Also, an appropriate alignment between ITO and TFT - A -

pixels from one side and MLA array at the other side should be provided. The arrangement of MLA should be such as to provide large (about 100%) fill factor of clear aperture of micro-lenses.

Generally, microlens array surfaces can be made by a number of known techniques including laser writing or grey scale lithography accompanied by resist smoothing followed by an etching step such as reactive ion etching (RIE) to transfer the pattern into the lens material. The microlens array surfaces can also be made by embossing in polymers or plastic. As to further components of the microlens arrays in LCD devices, it is possible to fabricate the planarizing layer and spacer layer from polymers by applying the layer coating on top of the microlens surface. Such polymer coatings however must be able to withstand the temperatures of the LCD processing. Another option is to use inorganic spacer layer, which is insensitive to temperature. However, such layers with thickness of 10 -150 microns are difficult to process and handle. The present invention provides novel solutions for the MLA configuration and fabrication to be suitable to the LCD manufacture. This can be achieved by forming the MLA in glass, quartz or fused silica, rather than in a plastic material. The MLA containing substrate, which is as thin as the MLA focal length (which condition is hard to satisfy in the conventional LCD production process), is a relatively thin transparent layer of thermal oxide on the base Silicon SSP wafer. The MLA surface is then attached to another glass window which serves also as the cover glass of the LCD panel, whereas thin transparent layer of thermal oxide serves as an inorganic spacer layer. Finally, etching or cleaving removes the sacrificial Silicon substrate. The microlens array is configured such that every and each pixel of the LCD is provided with the microlens; each microlens has a separate convex surface and has a planar surface common for all the microlenses in the array. Hence, the lateral spot size of light is reduced separately for each pixel, as a combined effect of both surfaces of the microlens array.

As for the spacer layer, in some embodiments of the invention, an inorganic spacer layer is utilized, which can be fabricated by a sacrificial layer process, as described above. In some other embodiments of the invention organic spacer layer is utilized. Specifically, a new class of cross-linked polymer materials is used, such as benzocyclobutenes (BCB), as the both the planarizing and organic spacer layer, which have properties that are compatible with the requirements described above. Such materials include but are not limited to Cyclotene manufactured by Dow Chemical. BCB polymers are known in the art for use in electronics where the low dielectric constant and thick layers are useful in interlayer dielectric applications. BCB materials are compatible with processing in temperature range of 250° C and have been used for planarization of LCD devices prior to deposition of ITO.

Thus, according to one broad aspect of the invention, there is provided a liquid crystal (LC) panel, which is configured as an integrated multi-layer structure comprising an LC pixel matrix configured and operable for spatially modulating light passing therethrough, and at least one microlens array (MLA) configured such that every pixel of the pixel matrix is associated with its corresponding microlens from said at least one MLA, the MLA having a curved surface defining an array of convex or concave surface regions of the microlens in the MLA, respectively, and an opposite planar surface which is common for all the microlenses in the MLA, the MLA by either one of its surface being spaced from an active surface of the pixel matrix or from another MLA by a spacer layer structure, the LC panel with said at least one MLA being configured for focusing input light beams of different incident directions onto spaced-apart spots, respectively, within the same pixel by the corresponding microlens of the focusing MLA.

In some embodiments of the invention, the MLA is carried by the spacer layer structure defining the planar surface of the MLA. The curved surface of this MLA may face one of the two substrates (quartz or silica) enclosing the LC panel therebetween. The MLA carrying layer may be patterned to define at least one substantially planar surface in between the convex or concave surface regions to thereby facilitate attachment of the curved surface of the MLA to a layer of the LC panel or the substrate. In some other embodiments of the invention, the MLA is carried by a substrate defining the planar surface of the MLA, while the curved surface of the MLA is attached to the spacer layer structure. The MLA carrying layer may be patterned to define at least one substantially planar surface in between the convex or concave surface regions to thereby facilitate attachment of the curved surface of the MLA to the spacer layer structure.

The LC panel may include the focusing MLA and the collimating MLA, which are symmetrically identical with respect to the active surface of the pixel matrix, being located at opposite sides thereof. In some other embodiments, the LC panel includes the focusing MLA and the field MLA₅ accommodated in a spaced-apart relationship in an optical path of light passing through the LC panel towards the active surface of the pixel matrix. The configuration may be such that the planar surfaces of the focusing and field MLAs are spaced from one another by the first spacer layer structure, and a curved surface of the field MLA is located close to the active surface of the pixel matrix.

Preferably, the LC panel includes the focusing and collimating MLAs arranged as described above, and first and second field MLAs configured symmetrically identical with respect to the active surface of the pixel matrix and located close thereto at opposite sides thereof. The focusing MLA and the first field MLA are accommodated in a spaced-apart relationship in an optical path of light passing through the LC panel towards the active surface of the pixel matrix such that the planar surfaces of the focusing and the first field MLAs are spaced from one another by the first spacer layer structure and a curved surface of the first field MLA is located close to the active surface of the pixel matrix. The second field MLA is located close to the active surface of the pixel matrix at the opposite side thereof. The collimating and the second field MLAs are accommodated in a spaced-apart relationship in an optical path of light emerging from the pixel matrix such that the planar surfaces of the focusing and the second field MLAs are spaced from one another by a second spacer layer structure. The inorganic spacer layer structure may be made of fused silica, silicon oxide, quartz or glass, fabricated by a sacrificial layer technology, described below.

The inorganic spacer layer structure may alternatively comprise an air gap supported by discrete spacers such as ball spacers or line spacers located outside the active surface of the pixel matrix, at the borders of the liquid crystal (LC) panel. The organic spacer layer structure may comprise one or more layers made of one or more cross-linked polymer materials. The cross-linked polymer material is preferably benzocyclobutenes (BCB) type material.

The spacer layer structure may be made of a polymer material serving as a spacer and planarizer. The spacer layer structure may be fabricated by a single layer coating of the MLA surface. Alternatively, the spacer layer structure is fabricated by at least two layer coatings of the same polymer material one directly on top of the other., which may or may not include at least one additional polymer layer between the at least layer coatings. The at least one additional polymer layer may include a polyimide layer, or a rubber or elastomer type material layer; or a stack of rubber or elastomer type material layer and a polyimide layer.

The spacer layer structure may include layers of different organic polymer materials serving as spacer and planarizer layers, respectively. For example, the polymer planarizer layer at one side thereof interfaces with the MLA surface and at the other side thereof interfaces with the spacer layer. The planarizer layer is preferably made of a material with lower refractive index than the spacer layer material. The planarizer layer may be made of Cytop, and the spacer layer may be a BCB type material. The spacer layer may be fabricated by a single layer coating of the planarizer layer; or by at least two layer coatings of the same polymer material one directly on top of the other, both on the planarizer layer. In the latter case, the spacer layer may at least one additional polymer layer (e.g. polymide, or rubber or elastomer type material, or both) between the two layer coatings.

According to another broad aspect of the invention, there is provided a liquid crystal (LC) panel configured as an integrated multi-layer structure enclosed between two cover substrates, the LC panel comprising an LC pixel matrix configured and operable for spatially modulating light passing therethrough, and at least one microlens array (MLA) configured such that every pixel of the pixel matrix is associated with its corresponding microlens from said at least one MLA, the MLA having a curved surface defining an array of convex or concave surface regions of the microlens in the MLA, respectively, and an opposite planar surface which is common for all the microlenses in the MLA, said curved surface of the MLA being made either as a pattern on an inner surface of one of the cover substrates and being attached to the spacer layer structure, or as a pattern on a surface of the spacer layer which is attached to one of the cover substrates, the spacer layer structure being either an inorganic structure or an organic structure including a benzocyclobutenes (BCB) layer.

According to yet another aspect of the invention, there is provided a liquid crystal (LC) panel configured as an integrated multi-layer structure enclosed between two cover substrates, the LC panel comprising an LC pixel matrix configured and operable for spatially modulating light passing therethrough, and at least one microlens array (MLA) configured such that every pixel of the pixel matrix is associated with its corresponding microlens from said at least one MLA, the MLA having a curved surface defining an array of convex or concave surface regions of the microlens in the MLA, respectively, and an opposite planar surface which is common for all the microlenses in the MLA, said curved surface of the MLA being made either as a pattern on an inner surface of one of the cover substrates and being attached to the spacer layer structure, or as a pattern on a surface of the spacer layer which is attached to one of the cover substrates, the spacer layer structure having a thickness of at least 10 microns and being either an inorganic structure or an organic structure including a benzocyclobutenes (BCB) layer.

According to yet further aspect of the invention, there is provided a method for use in manufacture of a liquid crystal (LC) panel in the form of a multi-layer integrated structure, the method comprising: creating a microlens array (MLA) having a curved surface defining an array of convex or concave surface regions of the microlens in the MLA, respectively, and an opposite planar surface which is common for all the microlenses in the MLA, where the MLA is coupled to a respective layer of the multilayer structure via a spacer layer structure of a desired thickness of at least 10 microns. In some embodiments, the MLA is created by patterning an inner surface of a substrate to form said curved surface of the MLA₅ an opposite planar surface of the substrate defining the planar surface of the MLA, and coating said curved surface of the MLA by the polymer-based spacer layer structure. The substrate may be a cover substrate of the LC panel. In some other embodiments, the MLA is created by patterning one surface of the polymer-based spacer layer structure to form the curved surface of the MLA, an opposite planar surface of the spacer-layer structure defining said planar surface of the MLA. In this case, a silicon substrate of a sacrificial wafer containing the spacer layer is provided, and a lithographic process is applied to create the curved surface of the MLA on the spacer. Then, the sacrificial wafer with the MLA-on-spacer therein is bonded to a carrier wafer using a wafer to wafer bonding adhesive; and a material removal or backgrinding and polishing is applied to remove the sacrificial wafer layer, resulting in a structure containing the spacer layer with the MLA, the bonding material layer, and the carrier wafer. In yet another embodiment, the MLA is created by patterning a surface of a base substrate, containing the spacer layer, to form said curved surface of the MLA, an opposite planar surface of said base substrate defining the planar surface of the MLA. In this case, the base substrate is created on a surface of a sacrificial wafer, and a lithographic process is applied to the base substrate. The sacrificial wafer may be a silicon layer, and the substrate - a silicon oxide layer. The lithographic process includes resist spinning on the silicon oxide layer, exposing the photoresist to light to create a surface relief in the photoresist, and developing the photoresist, resulting in the formation of a pattern corresponding to curved surface of the MLA in the silicon oxide layer. Then, a top cover wafer is attached to the curved surface of MLA, and the sacrificial silicon wafer is removed. Preferably, the MLA containing layer is sealed.

BRIEF DESCRIPTION OF THE DRAWINGS In order to understand the invention and to see how it may be carried out in practice, embodiments will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which:

Fig. 1 is a block diagram of an image projection system of the present invention utilizing a liquid crystal panel operable as an SLM unit; Fig. 2 illustrates a light propagation scheme in an image projection system of the present invention utilizing a liquid crystal panel including a focusing microlens array inside an SLM unit;

Fig. 3 illustrates an optical scheme of the liquid crystal panel including a focusing microlens array and a field microlens array inside the SLM unit; Fig. 4A illustrates an optical scheme of a liquid crystal panel including a focusing microlens array and a collimating microlens array inside the SLM unit;

Fig. 4B illustrates an optical scheme of a liquid crystal panel including a focusing microlens array, a pair of field microlens arrays, and a collimating microlens array, all inside the SLM unit; Figs. 5a to 5c illustrate the geometrical layout for a lenslet model No. 6 in Table

I₅ Fig. 5a showing the surface shape, Fig. 5b showing the ray-tracing, and Fig. 5c showing the geometrical spot diagram at the SLM pixel;

Figs. 6a to 6d shows diffraction calculations for the lenslet model No.6, Figs. 6a-6c showing the intensity distribution, and Fig. 6d showing ensquared power being a fraction of power in square with given semi-dimension compared with the total power in the plane.

Fig. 7 illustrates a ray tracing layout with the single ideal MLA; Fig. 8 shows the footprint on aperture stop of a projection lens with the single ideal MLA;

Fig. 9 is a spot diagram of the pixel image (target <453 μm ) obtained with the single ideal MLA; Fig. 10 shows a ray tracing layout of MLA 1 and MLA 2 with a pair of ideal

MLAs;

Fig. HA shows an MLA 2 aperture with a pair of ideal MLAs;

Fig. HB shows the footprints on MLA 1 and MLA 2;

Fig. HC shows the footprint on the system aperture with a pair of ideal MLAs; Fig. HD is a spot diagram of the pixel image obtained with a pair of ideal

MLAs;

Fig. 12 illustrates the process of glass MLA fabrication according to the invention;

Fig. 13 illustrates an LCD arrangement with a single glass-MLA, and a light propagation scheme from the TFT side;

Fig. 14 illustrates an LCD arrangement with a single glass-MLA, and a light propagation scheme from the opposite to TFT side;

Figs. 15a to 15c illustrate an example of a method of the present invention for manufacturing an MLA-with-spacer structure suitable for use in an LC panel, where an MLA is formed on a spacer layer;

Fig. 16 shows more specifically a structure of a glass MLA on inorganic spacer, produced by the method of Figs. 15a-15c;

Figs. 17a-17c shows illustrates an example of a method of the present invention for manufacturing an MLA-with-spacer structure suitable for use in an LC panel, where an MLA is formed on a carrier wafer;

Fig. 18 a structure of a glass MLA on a carrier glass wafer, with planar inorganic spacer, produced by the method of Figs. 17a- 17c;

Fig. 19A illustrates the wafer layout for the glass MLA on inorganic spacer;

Fig. 19B illustrates the wafer layout for the glass MLA on carrier glass wafer, with planar inorganic spacer;

Fig. 20 yet another example of an MLA structure with an inorganic spacer layer, where the spacer layer is formed by an air gap supported by discrete spacers located outside the active surface of the pixel matrix; Fig. 21 illustration of a process of the MLA fabrication with, planar inorganic spacer;

Fig. 22 illustrates an LC panel with a single glass MLA on inorganic spacer, and light propagation scheme when incident from the opposite to TFT side; Fig. 23 illustrates an LC panel with a single glass MLA on the carrier glass wafer, planar inorganic spacer, and light propagation scheme when incident from the opposite to TFT side;

Fig. 24 shows a single layer polymer spacer layer;

Fig. 25 shows a single layer polymer spacer, composed of several sublayers with the same material;

Fig. 26 shows a planarization layer made of a lower index of refraction polymer material and a BCB spacer layer;

Fig. 27 shows a polymer spacer layer stack with a BCB layer with a polyimide layer; i Fig. 28 shows a polymer spacer layer stack with a BCB layer with rubber material layer;

Fig. 29 shows a polymer spacer layer stack of a BCB with rubber materials and polyimide layers;

Fig. 30 shows a polymer spacer layer stack with a low index spacer layer and polyimide and rubber layers; and

Fig. 31 shows planarization of concave lanes with high index polymer, followed by BCB spacer layer.

DETAILED DESCRIPTION OF EMBODIMENTS Referring to Fig. 1, there is illustrated an image projection system 1 of the present invention. The system 1 includes a light source unit IA, an LC panel IB configured as an SLM, and a projection lens unit 1C. The light source unit IA includes one or multiple light emitters and possibly includes a light splitting unit for producing input light in the form of multiple light beams. The system 1 operates to produce a magnified image of the active pixel matrix of the LC panel onto a projection surface or screen. Fig. 2 illustrates an example of the LC panel IB configuration and a corresponding light propagation scheme. As shown, multiple beams (three beams in the present example), e.g. of different wavelengths, e.g. corresponding to primary colors R, G, and B, impinge onto the LC panel IB with different angles of incidence Θ_l5 Θ₂ and 0₃, or the angle of incidence is continuously varying in the range from Θi up to Θ₃ with a chief (central) ray angle Θ₂.

The LC panel IB is actually configured and operable as an SLM unit having an LC-based pixel unit, which defines an active plane 16 or surface of the liquid crystal and includes an array of spaced-apart pixels or pixel apertures 17. The pixel unit together with its corresponding electrodes-containing layers (not shown here) at opposite sides of the active plane 16 is enclosed between two substrates, typically glass substrates which are also not shown here. The LC panel or SLM unit IB is of a kind utilizing a focusing microlens array (MLA) 12 in an optical path of input light entering the SLM unit (through the substrate) and propagating towards the active plane 16. The focusing MLA 12 is spaced from the active plane by a spacer layer structure 19 of a thickness L. The spacer layer structure 19 may include inorganic (e.g. quartz) material, or organic (polymer) material, as will be described more specifically further below.

As shown in the present example, the input light beams Θi, Θ₂ and Θ₃ (which may or may not be of the same wavelength) impinge into a lens 15 of the focusing MLA 12 at different angles of incidence, respectively, and pass through the lens 15 towards the pixel aperture 17 in the active plane 16. The focusing MLA 12 is configured to focus each of the beams Θ₁-Θ₃ onto a small spot on the pixel aperture 17, and the focused beams pass the active plane 16 without obscuration through the pixel aperture. Different angles of incidence result in different positions of the small spots along the pixel aperture 17. It is critical, for achieving a high efficiency of SLM, to have actual spot dimensions and positions compatible with the pixel apertures 17. It should be noted that the beams passed through the pixel apertures 17 still have different angles of propagation in accordance with different angles of incidence. Each lens of the MLA has one curved (convex or concave) surface and an opposite planar surface. In the present example, each lens of the focusing MLA 12 has a first convex surface 12A by which it faces a respective glass substrate (not shown), and a second substantially planar surface 12B by which it faces the active plane 16. The focusing MLA 12 is carried by the spacer structure 19, which defines the planar surface of the MLA. Fig. 3 shows a light propagation scheme in an LC panel 10, which is configured generally similar to the above-described panel IB, but additionally includes a field MLA 14. The MLA 14 is located close to (up to a physical contact with) an active pixel plane 16. The field microlens array 14 is configured to appropriately change a direction of propagation of each of all the beams with an angle of incidence continuously varying in the range from Θj up to Θ₃ with a chief (central) ray angle Θ₂, and preferably to make chief rays of all the beams parallel to each other and all together parallel to the optical axis. Then, light passes through the liquid crystal layer essentially parallel to the optical axis, that is beneficial for the transmittance and contrast of the LC panel. Accordingly, the beams passed through the pixel apertures 17 have exactly the same angles of propagation, despite of their different angles of incidence. The lens of the field MLA 14 may for example have a first planar surface 14 A, and a second convex surface 14B by which it faces the active pixel plane 16. The focusing MLA 12 and the field MLA 14 have a common spacer layer 19, whereas may have different planarizing layers as will be described more specifically further below. The field MLA 14 is placed in contact with the active plane 16 and the focusing MLA 12 is spaced from the active plane 16 by the spacer 19 of a thickness L.

Fig. 4 A shows a light propagation scheme in an LC panel 100, which, in distinction to the previously described panel IB, includes a collimating MLA 18 spaced from an active pixel plane 16 at opposite side thereof by a spacer 19' of the same thickness L as the spacer 19. The collimating microlens array 18 converts each of the diverging beams Θj, Θ₂ and Θ₃, as passed through the pixel plane 16, to essentially collimated beams. It should be noted that beams that have passed through the pixel apertures 17 still have different angles of propagation in accordance with different angles of incidence. The lens of the collimating MLA may, for example, have a first planar surface 18A by which is faces the pixel arrangement 16 and a second convex surface 18B.

Referring to Fig. 4B, there is illustrated a light propagation scheme in an LC panel 200. The LC panel 200 includes a focusing MLA 12 spaced from an active pixel plane 16 a spacer 19 of a thickness distance L, a collimating MLA 18 spaced the distance L from the active pixel plane 16 at the opposite side thereof via a spacer layer 19', and a pair of field MLAs 14 and 14' at opposite sides of and close to the pixel plane 16. The field MLAs 14 and 14' are configured symmetrically identical with respect to the plane 16, namely for example the lens 14 has the first planar surface 14A and the second convex surface 14B and, accordingly, the lens 14' has a first convex surface 14A' and a second planar surface 14B\ The focusing MLA 12 focuses each of beams Θi-Θs to a small spot on the pixel plane. The field microlens array 14 is configured to appropriately change a direction of propagation of all the beams with angle of incidence continuously varying in the range from Θi up to Θ₃ with a chief (central) ray angle Θ₂ and preferably to make chief rays of all the beams parallel to each other and all together parallel to the optical axis. Then light passes through the liquid crystal layer essentially parallel to the optical axis, which feature is beneficial for the transmittance and contrast of the LCD. As indicated above, this configuration provides that beams passed through the pixel apertures 17 have the same angles of propagation despite of their different angles of incidence. The field microlens array 14' is configured to appropriately change, after passing an active pixel plane 16, a direction of propagation of all the beams from the state of being parallel to the optical axis, to a direction of propagation which is mirror inversed to those of the incidence beam, i.e. Continuously varying in the range from (-Θj) up to (-Θ₃) with a chief (central) ray angle (-Θ₂). The field MLAs 14 and 14' therefore operate together to appropriately change direction of beams' propagation so that the beams, emerging from the pixel plane 16, enter the aperture of the collimating MLA 18 without any lateral shift. The MLA 18 converts diverging beams to the essentially collimated beams. Finally, the optical scheme of Fig. 4B achieves the goal of small spots on the pixel plane 16 and returns the beams with the same angles of propagation as .the respective incident beams, except of having the angles mirror inversed to those of the incidence beam, i.e. continuously varying in the range from (- Θi) up to (-Θ₃) with a chief (central ) ray angle (-Θ₂). In analysis of interaction of the focal spot produced by the microlens array with the pixel aperture (Fig. 2 or Fig. 3), both ray-tracing and diffraction effects have to be considered. For rough estimations, a light beam focusing phenomena is treated as featuring two main issues: geometrical focusing and diffraction spread. Geometrical focusing defines the spot diameter SQ on the solid pixel aperture plane as:

S_n = D (1)

L

where LQ is effective focal length, and D is a lateral pitch of the MLA. Diffraction spread Sdrf on the solid pixel aperture plane is determined from the equation:

IA s*f = ^Lo ^ (2)

where λ is the beam wavelength.

The sum (so + Sd,β provides estimation for the total spot on the solid pixel aperture plane.

Numerical aperture NA_MLA of the propagation of a spherical beam, diverging after passage through a single MLA₅ may be estimated as the sum of three terms, as follows:

NA₁^ = NA_ιnc + NA_geom + NA_dlf , (3) where

D_

NΛ_geom = — (4)

^~2L

is a geometrical term,

^NA« =4- ⁽5⁾

is a diffraction term, Do is the dimension of the pixel apertures 17 and N_ιnc is the numerical aperture of the beam incident onto the MLA.

Assuming Z>=15μm, Z)ø=10μm, Zø⁼50μm, Z=120μm, 2=0.532μm yields

Therefore, the total spot size is ($o+Sdιj) =12.4μm, which exceeds the aperture Do- Hence, secondary diffraction takes place. For example, an ideal MLA with lOOμm focal length and 15μm diameter at λ = 532nm (green), have NA_geom=0.075 and

Iμm. If the input to the SLM has an illumination NA_mc=0.05, then the total NAM_LA of the beam incident to the projection lens will need to be larger than 0.175. This configuration might not be acceptable in several embodiments because the compact projection-lens design is usually limited to NA of about 0.1-0.15.

In order to match restrictions on the NA, the optical scheme of Fig. 3 or Fig. 4B can be used with the MLA pair (14-14' in Fig. 4B) with a reduced focal length of about 50μm re-collimates the beam. The numerical aperture of the re-collimated beam is determined by diffraction on the lenslet aperture, as follows:

XA^ = NA₁₁₁₀ +^ (6)

Assuming

5μm which fits the limitations of the projection lens design. For even more detailed and precise calculations of light intensity focused by the

MLA onto a plane at the exit of a spacer 19 (in Figs. 2-4B) and at the active SLM pixels' plane, geometrical optics and physical optics propagation are combined. Geometrical optics propagation predicts sharp focus at a focal distance from the lens and geometrically defocused spot at the exit of the spacer. However, small lenslets cannot be adequately described by only geometrical optics propagation. This is because light diffraction on the pixel aperture increases the focal spot and changes the shape and dimensions of the defocused spot. Furthermore, virtual splitting of highly aberrated aspherical and non-symmetrical lenslets into small regions with variable local focal length adds to diffraction phenomena. Hence, physical optics propagation accounting for light diffraction is to be used.

The inventors performed diffraction calculations for a spot produced on an active pixel aperture by a single lenslet at the output of the spacer 19, i.e. right before the single field MLA 14 and the active SLM plane 16. In these calculations, the lenslet of 13μm x 13μm was used as a clear aperture. Several options of a lenslet model were considered, as summarized in Table 1 below. For lenslets 1-6, effective focal length (EFL), paraxial, was calculated at wavelength 532nm in air and in the spacer 19 material. The spacer thickness of 40μm was considered. The optimal best focus spacer position was also considered, in sampled cases. In these simulations, input light incidence in air with angles of incidence of 0°, 20°, 60° was considered, where the wavelengths of light were 464nm, 532nm and 635nm. Refractive index of adhesive Π_A at wavelength 532nm was 1.5. Refractive indices of the lenslet resist and the spacer material were both modeled with ΠA= n_s =1.61, at wavelength 532nm. Table 1

Incident light passed through the layer of glass (substrate), adhesive layer and then that of the lenslet 12, and continued to propagate in the spacer 19 material. A spot was modeled at the exit of the spacer 19, between the microlens array and the liquid crystal plane. The center of the spot bears a transverse shift of 0.44μm for each incident angle, i.e. 0.87μm for the 20° incident beam and 2.6μm for the 60° incident beam. After the transverse shift, the spot should go through the solid pixel aperture of 10μmxl3.4μm dimensions.

Reference is made to Figs. 5a-5c, 6a-6d and 7-13 showing the surface shape, ray-tracing, geometrical spot diagram and diffraction propagation results relative to the spot center. More specifically, Figs. 5a-5c show the geometrical layout for the lenslet model 6, where Fig. 5a shows the surface shape, Fig. 5b - the ray-tracing and Fig. 5c — the geometrical spot diagram at the SLM pixel. Figs. 6a-6d show the diffraction calculations for lenslet model 6. Figs. 6a-6c show the intensity distribution. A frame superimposed on the intensity distribution depicts the SLM solid pixel aperture position for different incident angles. Fig. 6d shows ensquared power being a fraction of power in square with given semi-dimension compared with the total power in the plane.

The quantitative effect of angles of incidence and the SLM lenslet aberration on the system optical performance was modeled by ray-tracing. To this end, the system configuration of Fig. 1 was considered, including an SLM with MLAs IB, sources of uniform collimated light IA, and additional projection lens 1C, which projects a magnified image of the active pixels of the SLM on the screen. The inventors have considered the cases of single MLA and a pair of MLAs. The output of the simulation is the light distribution of one white pixel projected on a screen and percentage of light inside and outside of the projected pixel dimensions on the screen. The input parameters were the following: Projection lens 1C provides a 30^x magnification of the SLM active area of 4.8mm x 3.6mm and is used as an extended object for imaging with RGB wavelengths of 466nm, 532nm, 658nm. Input light impinges with incident angle of 1° / option 2°, as measured in air media in the optical path towards the SLM. The SLM active area consists of pixels with a pitch of 15μm x 15μm and clear aperture of 13μm x 13μm. The main layers of the SLM unit, for ray tracing purposes, include a polarizer layer of a thickness of 0.2mm, a front glass of the 0.7mm thickness, and then the focusing MLA layers 12 of Fig. 1 specified in Table 1 above, a spacer fabricated of SU8 polymer and again a glass substrate of the 0.7mm thickness 0.7 mm and polarizer of the 0.2mm thickness. Ideal paraxial lenslets and aberrated lenslets (see Fig. 2) defined by an appropriate polynomial profile were considered. The optimal focal length of the lenslets material is found to be 82μm (51μm in air). Fig. 7 illustrates the ray tracing layout with the single ideal focusing MLA. Fig. 8 shows the footprint (density of the rays) on the aperture stop of the projection lens when using the single ideal focusing MLA. Fig. 9 shows a spot diagram of an image created with 30^x projection lens from a single 15μm SLM active pixel in the case of the single ideal MLA. A target value for the dimension of the image is 15μm x 30=450μm.

Fig. 10 illustrates the ray tracing layout for the configuration with the ideal focusing and collimating MLAs 12 and 18 as in the example of Fig. 4A. Fig. HA shows the image at the aperture of MLA 18. Fig. HB shows the footprints (the density of the rays) just in front of MLA 18. Fig. HC shows the footprint on the aperture of the projection lens in the case of a pair of ideal MLAs. Fig. HD is a spot diagram of an image created with 30^x projection lens from a single 15μm SLM active pixel in the case of a pair of ideal MLAs. hi view of the above, it is clear that the use in an LC panel focusing and/or collimating microlens arrays, and preferably also additional field microlens arrays, significantly improves the LC panel operation. Specifically, the light propagation scheme of Fig. 4B provides better system performance as compared with those of Figs. 2-4A, in that efficiency is high and divergence of output beams is kept the same as for the incident beams. However, the configuration of Fig. 4B includes two spacer layers and four MLAs, which might complicate a fabrication process. Optical schemes of Figs. 2-4A give a partial compromise for the system performance and complexity of its fabrication process.

The present invention provides the image projection system operable with different colors, and separately spatially modulated by sub-pixels of the common pixel aperture. Reduction of the pixel size towards the sub-pixel size is achieved by exploiting microlens arrays and additional field microlens arrays. As indicated above, an MLA containing layer, located inside an LC panel, is always associated with a spacer layer structure, e.g. is carried by a spacer by its planar surface and/or its curved surface is spaced by such spacer from other layers in the LC structure. The present invention provides a novel technique for manufacture of an LC- with-MLA(s) panel as an integrated structure, allowing to form a spacer layer structure of at least 10 microns and up to 150 microns thickness, made of materials with thermal properties compatible with LCD manufacturing processes (processes under temperatures over 250 degrees C, deposition of ITO and metals with good adhesion, etc.).

Reference is now made to Figs. 12-14 exemplifying the manufacture of the LC panel according to the invention.

Fig. 12 shows the fabrication of the generic MLA, which may serve as a focusing or collimating MLA, or either one of two field MLAs. A silicon SSP wafer is used as a base substrate 300 (step 1). A silicon oxide layer 302 of a desired thickness (e.g. of 10-100μm) is created on the silicon substrate 300 by thermal oxidizing process (step 2). Then lithographic process is applied to create the MLA arrangement, as follows: Resist spinning 304 is applied to the silicon oxide layer 302 (step 3). The structure obtained in step 3 is exposed to UV light through a chrome mask 400 (step 4), and then resist development is performed (step 5), resulting in the formation of a resist pattern 306, corresponding to convex surfaces 306' of the MLA arrangement, in the silicon oxide layer 302 which is actually a quartz or silica. It should be understood that step 4 can be implemented using a single continuous level mask or diffractive mask, or by repeating the procedure with a set of binary masks. The silicon oxide layer 302 then undergoes dry etching followed by smoothing development of the etched surface (step 6). A top cover wafer 308 (glass, quartz or fused silica) is processed and attached to the MLA 306 by vacuum diffusion bonding under pressure (step 7). Then, the sacrificial silicon wafer 300 is etched or cleaved by batch processing wet etching to obtain the fabricated silicon oxide MLA 306 with a spacer 310 and cover wafer 308 (step 8). Thus, in this specific example, inorganic spacer 310 is used, being made in silicon oxide which is actually quartz. Alternatively, as will be described below, organic spacer (a polymer layer structure) can be used. Optionally, the MLA layer may be sealed (step 9). A pixel matrix with TFT 312 is formed on a planar surface 306", which is used as a substrate, by a standard high temperature TFT technology (step 10). The surface 306" may be either the surface of a regular cover glass or the planar surface of the MLA layer attached to the cover glass, depending on the embodiment. It should be noted that a frame may be used around the panel to seal the MLA as part of step 9, or when there is no step 9, such a frame may be used when applying sealant at step 10.

It should be noted, although not specifically shown, that fabrication of the field MLA can be implemented by etching process, similar to that of the main MLA. More specifically, resist spinning is applied on the planar side of the fabricated glass MLA₅ which is already attached to the cover wafer; and the structure is exposure to UV light through a mask followed by resist development process, dry etching of the silicon oxide layer and smoothing development of the etched surface. Reference is made to Figs. 13 and 14 illustrating two examples of an LC panel with a single glass MLA as per optical scheme of Fig. 2, and two examples for the light propagation scheme when incident from, respectively, the opposite to TFT side and from the TFT side.

As shown in Fig. 13, the SLM LC panel, generally designated 400, includes front and rear covers 402 and 404 (e.g. glass covers) each of the 600-900μm thickness, and enclosed therebetween a structure formed by a sealed glass focusing MLA 12, a spacer 19 of about 10-100μm thickness, a 4-6μm thick polysilicon and TFT layer 406, a front ITO layer 408a of a 0.1-0.15μm thickness, a front buffing layer (e.g. polyimide) 410a of a 0.03-0.05μm thickness, a 2μm thick LC layer 412, a rear 0.03-0.05μm thick buffing layer 410b, and a rear ITO layer 408b of a 0.1-0.15μm thickness. In the present example, the TFT structure is preferably fabricated in a process independent of the MLA fabrication and is then attached together, the same independent fabrication and attachment is used for glass covers 402 and 404. Light is incident from the side of front cover 402, being focused by MLA 12 to first pass TFT layer 406 and then pass active pixels of the LC layer 412, resulting in spatial modulation of light. After passing through subsequent layers, the light emerges from the SLM LC panel via the rear cover 404. An SLM LC panel 500 shown in Fig. 14 includes front and rear glass covers 502 and 504 (600-900μm thickness), sealed glass MLA 12 and spacer 19 (10-100μm thickness), a front ITO layer 508a (0.1-0.15μm thickness), a front buffing layer 510a (0.03-0.05μm thickness), an LC layer 512 (2μm thickness), a polysilicon and TFT layer 506 (4-6μm thickness), a rear buffing layer 510b, and a rear ITO later 508b. Here, the TFT is fabricated after the MLA fabrication and sealing, after step 10 in Fig. 12. Light is incident from the side of front cover 502, being focused by MLA 12 to successively pass through the active pixels of the LC layer 512 and the TFT layer 506 (resulting in spatial modulation), and after passing through further subsequent layers, emerges from the SLM LC panel via the rear cover 504. The present invention, in its other aspect, provides for a microlens arrangement including an MLA, a planarizing layer and an inorganic spacer layer. This arrangement is suitable for fabricating an LC panels with MLA(s) where the spacer layer with a thickness of up to dozens of microns is required (in order to define a distance between the MLA layer and its focal plane). The structure fabrication preferably utilizes a sacrificial wafer process. The

MLA itself may be first fabricated (from inorganic material) by the known technique with careful control of the profile and fill factor, e.g. laser writing or grey scale lithography and RIE processes. The MLA can be etched directly into the wafer substrate (such as quartz or silicon), or it can be etched into a thin-film layer deposited on the wafer.

Suitable layer materials for making the MLA include but are not limited to: Silicon Oxynitride, Silicon Carbide, Aluminum Oxide (Sapphire). These layers can be deposited by PECVD low stress processes with thickness of several microns up to 15 microns (using the technique developed by Strataglas), and are compatible with the temperature requirements for the LC processing. The MLA can be formed on either the sacrificial wafer or the carrier wafer.

The following are some examples of the microlens arrangement fabrication with the formation of an inorganic spacer layer structure, suitable for use in an LC panel. The inorganic spacer layer structure may be made of fused silica, silicon oxide, quartz or glass, fabricated by a sacrificial layer technology.

Figs. 15a-15c exemplify the structure manufacture where an MLA 600 with a spacer layer 602 is formed on a sacrificial wafer 604. A silicon oxide layer with MLA 600 and spacer layer is created on the silicon substrate of the sacrificial wafer 604 by thermal oxidizing process, and lithographic process is applied to create the MLA arrangement, as was explained above with reference to Fig. 12. As shown in Fig. 15a, a sacrificial wafer (of quartz or silicon) 604 carries the MLA-on-spacer structure thereon. Then, the sacrificial wafer with the MLA-on-spacer therein is bonded to a carrier wafer 606 using a wafer to wafer bonding adhesive 608 (Fig. 15b). The lenses are face down in contact with the carrier wafer 606. A material removal or backgrinding and polishing are then applied to remove the sacrificial wafer layer leaving the spacer layer 602 of the desired thickness. This results in a final structure containing the spacer layer 602, MLA 600, bonding material layer 608, and the carrier wafer 606 (Fig. 15c). The bonding adhesive 608 material is selected to have as low as possible index or refraction and to be compatible with stress requirements for backgrinding or removal of the sacrificial wafer, and also to be able to withstand the temperature used in LC processing. It should be noted that in this respect the process is different than the normal carrier wafer process, since there is no release step for releasing the sacrificial wafer from the carrier wafer. The bonding material can therefore be a crosslinking material that creates a permanent bond.

The wafer to wafer bonding material can be a such as Cytop commercially available from Asahi, or BCB type materials commercially available from Dow Chemicals, or GenTak 330 commercially available from General Chemicals. Adhesive forces of 3-20 Mpa have been achieved with these polymer systems. The extent of cure prior to lamination has to be weighed against the possibility of void formation for polymer layers that have not undergone sufficient curing ("Adhesive Wafer Bonding Using Partially Cured Benzocyclobutene for Three-Dimensional Integration", F. Niklaus, R.Kmar et al, J. Electrochem. Soc, Volume 153, Issue 4, pp. G291-G295, 2006; "Low-temperature full wafer adhesive bonding", F. Niklaus, P. Enoksson et al., Journal of Micromechanics and Microengineering, Volume 11, Issue 2, pp. 100-107, 2001; "Low-temperature bonding technique using spin-on fluorocarbon polymers to assemble Microsystems", K W Oh et al., 2002, J Micromech. Microeng. 12 187- 191, doi:l 0.1088/0960-1317/12/2/313).

The removal of the sacrificial wafer after bonding can be done by several alternative processes: backgrind and polish wet etch and dry RIE plasma etch. Conventional backgrinding processes are capable of thinning wafers to a minimum thickness of 50 microns with the thickness variation of 5 microns ("Manufacturing Issues in Memory Modules", Wafer Thinning Step 3 W. KOH₅ E. BALDWIN, Advanced Packaging, March, 2004). Backgrinding process damages the backside of the wafer, which can lead to stress nonuniformity and a tendency to bow. The microcracks cannot support tensile stress, so wafers with front side under tension will bow so that the backside curves outward. To eliminate bowing, the wafers are polished after grinding to eliminate the microcracks. Latent stresses remaining after polishing can lead to delamination of the wafer from the carrier wafer at elevated temperatures, if the adhesive layer softens. An alternative to grinding and polishing is etching by plasma or wet etching.

Under certain conditions the yield loss during backgrinding and polishing may make it advantageous to make the lens array on the carrier wafer and mount a plain featureless wafer as the sacrificial wafer that is to be background. In case the sacrificial wafer cracks, it can potentially be reworked to save the MLA carrier wafer. Using a plain wafer is also closer to standard backgrinding in that wafers to be background generally do not have topography.

Fig. 16 shows the structure formed by an MLA on an inorganic spacer and a bonded carrier wafer, produced by the above described method.

Figs. 17a-17c exemplify another technique for producing an MLA containing structure and an inorganic spacer. As shown in Fig. 17a, an MLA corresponding pattern 700 is formed on a carrier wafer 702. A sacrificial wafer layer 704, comprising a spacer layer, is formed on top of the convex surfaces of the MLA 700, using a planarizing bonding layer 706 (Fig. 17b). Then, the sacrificial layer is partially removed defining the remaining spacer layer 704' (Fig. 17c). Fig. 18 shows the structure formed by an MLA on a carrier wafer with a bonded spacer layer, produced by the above described method of Figs. 17a- 17c.

In some embodiments, the peripheral part of the MLA contains a facet, parallel to the wafer surface, which provides a small gap between a spacer layer and a carrier substrate, and consequently results in an improved sealing. This is shown in Fig. 19A and Fig. 19B for the structures with MLA-on-spacer and MLA-on-carrier wafer, respectively. As shown in the figures, the surface of the MLA formed with convex regions has spaces-apart facets, generally at 800, serving for sealing the MLA layer to, respectively, the carrier glass wafer 802 and the spacer layer 902.

An inorganic spacer layer structure may be formed by an air gap supported by discrete spacers, such as ball spacers or line spacers. Such discrete spacers are located outside the active surface of the pixel matrix, i.e. at the borders of the LC panel. This is exemplified in Fig. 20, showing a multi-layer structure 960 including a carrier wafer layer 962, and inorganic spacer layer structure 964, and a next sealed layer 968. The inorganic spacer layer structure is formed by discrete spacers 966 located in a spaced- apart relationship within an air gap 965 between the layers 962 and 968. These layers are thus supported by the discrete spacers 966. The spacers 966 are located at the borders of the liquid crystal (LC) panel located to be outside the active surface of the pixel matrix.

Considering the use of MLAs in spatial light modulator (LC panels), the following should be noted. Lenslets of the MLA should preferably have a, relatively high numerical aperture (NA), e.g. of about 0.08 -0.15, in order to minimize diffraction limit of the focal spot size. Lateral ray aberrations are preferably below 2-5μm, in the MLA focal plane. Large, about 100%, fill factor of the MLA clear aperture should preferably be achieved. The MLA surface is preferably planarized with a planarizing layer, which is the interface between the MLA and other layers of the SLM. Light incidence angles up to ±9 degrees are to be tolerated with a relative cross talk between adjacent pixels below 1%. The arrangement is to be such that a light beam, after passing an SLM, has limited output divergence, with NA below 0.17. The MLA layer and the shape of micro-lenses should preferably stand chemical and high temperature conditions compatible with the technological process of TFT layers. Alignment marks are to be provided for alignment between TFT/ ITO pixels and MLA array, on the wafer level.

Reference is made to Fig. 21 showing an example of manufacture of an LC panel utilizing an MLA-on-spacer configuration. The process starts with a silicon SSP wafer 1000 as a base substrate (step A). Then, a designed silicon oxide layer 1002, with thickness sufficient for MLA-on-spacer structure is created. Specifically the thickness of the layer 1002 includes 10-100μm for spacer layer and 2-10μm for MLA layer. More specifically: a silicon oxide layer 1002 is formed on the substrate 1000 (step B), resist spinning 1004 is applied to the silicon oxide layer 1002 (step C)₅ the photoresist surface is exposed to UV light through a chrome mask 1006 (step D), resist development process is performed (step E) to form a convex surfaces' pattern 1008 of an MLA, followed by dry etching of the MLA layer on cover glass and smoothing development of the etched surface (step F). This results in the formation of an MLA-on-spacer structure 1010 on the substrate 1000. As indicated above, the exposure may be performed through a single continuous level mask or by repeating the procedure with a set of binary masks. Then, a top cover wafer 1012 (glass, quartz or fused silica) is attached to the MLA-on-spacer structure 1010 with sacrificial layer (by thermal bonding or gluing) (step G). Thereafter, the sacrificial wafer is removed by polishing, etching or cleaving, to obtain the fabricated glass MLA on the cover glass with inorganic spacer on top of it (step H). Optionally, the MLA layer is sealed to the top wafer (step I).

Figs. 22 and 23 illustrate LC panels 2000 and 3000 with, respectively, a single glass MLA on inorganic spacer and a single glass MLA on the carrier glass wafer with a planar inorganic spacer, and light propagation scheme in these panels.

The LC panel 2000 includes an upper cover glass 2001a (quartz or fused silica) of a 600-900μm thickness, a sealed glass MLA and a 10-100μm thick spacer 2002, an upper ITO layer 2004a of a 0.1~0.15μm thickness, an upper 0.03-0.05μm thick buffing layer (e.g. polyimide) 2006a, a liquid crystal layer 2008 of a 2μm thickness, a lower buffing layer 2006b, a lower ITO layer 2004b, a polysilicon and TFT layer of a 4-6μm thickness, and a lower cover (glass, quartz or fused silica) 2001b. This LC panel is exposed to input light by its upper cover glass 2001a. Light is incident from the side of front cover 2001a, being focused by MLA 2002 to pass through further subsequent layers, TFT layer 2010 and then active pixels of the LC layer 2008, providing spatial modulation, and emerges from the SLM LC panel via the rear cover 2001b.

The LC panel 3000 includes an upper cover glass 3001a (quartz or fused silica) of a 600-900μm thickness with micro lens array, and a 10-100μm thick spacer 3002, an upper 0.1~0.15μm thick ITO layer 3004a, an upper buffing layer (e.g. polyimide) 3006a of a 0.03-0.05μm thickness, a 2μm thick liquid crystal layer 3008, a lower buffing layer 3006b of a 0.03-0.05μm thickness, a lower ITO layer 3004b a 0.1-0.15μm thickness, a polysilicon and TFT layer of a 4-6μm thickness 3010, and a lower cover 3012. Light is incident from the side of front cover 2001a, being focused by MLA on cover glass 3001a to pass through further subsequent layers, TFT layer 3010 and then active pixels of the LC layer 3008, providing spatial modulation, and emerges from the SLM LC panel via the rear cover 3001b. As indicated above, a spacer layer, which defines a distance between the MLA layer and its focal plane, should preferably be very thin, up to dozens of microns, which might be difficult to fabricate, as well as make it with thermal properties that are compatible with the high-temperature LCD processes and good adhesive properties. These problems may advantageously be solved by using a novel class of cross-linked polymer materials, such as benzocyclobutenes BCB as the both planarizing and spacer layer. Such materials include Cyclotene commercially available from Dow Chemical.

The present invention provides for using the new materials as a thick spacer layer, rather than as a simple planarization layer. As BCB has a relatively low dielectric constant and corresponding low index of refraction (in the range of 1.53) it is suitable as a spacer layer. Such a spacer layer may be a single layer of cross-linked, preferably BCB type, polymer material, or it may be a composite of several layers. The BCB may be deposited in a single step, or it may deposit in multiple steps.

Fig. 24 shows an example of an MLA containing structure 4000 utilizing a single-layer polymer spacer. The structure 4000 includes a carrier wafer 4002 with a convex MLA 4004 thereon, a polymer spacer and planarizer layer 4006 made of a BCB type material, and ITO and buffing (e.g. polyimide) layers 4008 and 4007.

It may be desirable to form a spacer layer from a number of coating steps. This can be used to increase the layer uniformity, since spin coating processes sometimes results in a thicker layer at the center of the wafer as opposed to the regions away from the center (not including edge beading effects). Fig. 25 exemplifies a structure 5000 utilizing a spacer layer made of more than one sub-layer made of the same material. The structure 5000 includes a carrier wafer 5002 carrying a convex MLA 5004, a polymer spacer and planarizer layer 5006 formed by a first coating BCB material sub-layer 5006a and a second coating BCB material sub-layer 5006b, an ITO layer 5008 and a buffing layer 5007. The first coating 5006a can be partially polymerized and then tihe second coating 5006b is applied. A solvent will partially dissolve the material that has not undergone full polymerization at the center of the wafer and redistribute it away from the center. Extent of polymerization and spin speed profile can be determined by methods known in the art.

It may further be desirable to make a spacer layer using multiple coatings to improve planarization. A single coating may leave some lens topography for lenses with sharp radius of curvature. Residual topography features may then be removed with a second coating deposited on the first polymerized coating. Depending on the lens material it may be desirable to use an adhesion promoter such as aminosilanes for surface treatment prior to coating ("Chemical and Structural Characterization of Silane

Adhesion Promoting Films for Use in Microelectronic Packaging", M. Jenkins, J. Snodgrass, R. Dauskardt, J. Bravman, Mat. Res. Soc. Symp. Vol. 629 pFF512.1-12.6,

Material Research Society, 2000).

As indicated above, a spacer layer can be made from multiple layers of different polymer materials. This might be useful to further improve optical and physical properties of the spacer structure. The multi-layer spacer can be implemented in various ways.

Fig. 26 exemplifies a structure 6000 utilizing a planarization layer made from lower index of refraction polymer material. There are lower index materials such as fluorinated siloxanes (Example Cytop from Asahi with index of 1.34), which have index of refraction of less than 1.53. Such a material can be used in the structure of the invention for planarization to improve the lens power, prior to building the bulk of the spacer layer required thickness from the higher index of refraction BCB type material.

The structure 6000 includes a carrier wafer 6002 with MLA 6004, a planarization layer

6010, on top of the MLA 6004, a BCB spacer layer 6006, and ITO and buffing layers

6008 and 6007. The planarization layer 6010 is made from a polymer material with lower index of refraction than the coating BCB spacer material 6006.

Another approach for multi-layer spacer structure is to form a polymer spacer layer in the form of a stack structure. Such laminate spacer layer structures with multiple layers of different materials can serve a number of purposes.

Fig. 27 exemplifies a structure 7000 utilizing a polymer spacer layer in the form of a stack structure. The structure 7000 includes a carrier wafer 7002 with an MLA

7004; a polymer spacer layer stack 7006; and ITO and buffing layers 7008 and 7007.

Here, the spacer layer stack 7006 is formed by a first coating BCB material layer 7006a and a second BCB material layer 7006b spaced by a buffing, polyimide layer 7009. BCB type materials are suitable for thick layer formation but have a relatively high thermal expansion coefficient (60 ppm/C) relative to glass. High modulus thin layers of polyimide type materials such as DuPont PI 2611 ("Planarization techniques for vertically integrated metallic MEMS on silicon foundry circuits", J.B. Lee, J. English, CH. Ahn, M.G. Allen, J Micromech. Microeng. 7 p 44-54, 1997) have been used to contain the expansion of the BCB materials. Polyimide layers have higher index of refraction of 1.6 or higher, but have little effect on the optical performance if the layers are kept thin.

Yet further implementation of a multi-layer spacer is aimed at further strain relieving the spacer layer of BCB type materials. To this end, a rubber or elastomer type spacer layer is applied between the successive BCB layers. Polystyrene-polybutadiene- polystyrene Elastomer BCB copolymers have been investigated by Dow Chemical as a way of improving the energy absorbing properties of BCB ("Divinylsiloxane- bisbenzocyclobutene-based polymer modified with polystyrene-polybutadiene- polystyrene triblock copolymers" YH So , P. Foster, J. Im, P. Garrou, J. Hetzner, E. Stark, K. Baranek, J Polym Sci Part A: Polym Chem 44: 1591-1599, 2006). The rubber material layer with its low modulus can relieve the strain of the thermal expansion of the BCB layers within the spacer. The elastomer layer may have a larger thermal expansion coefficient than the BCB₅ but since it is thin its overall effect is limited. The rubber layer is selected to be capable of withstanding processing temperatures of 250° C to 300° C for time periods of greater than 30 minutes.

An example of the above approach is shown in Fig. 28, illustrating a structure 8000, including a carrier wafer 8002 with an MLA 8004; a polymer spacer layer stack 8006 formed by first and second BCB layers 8006a and 8006b spaced by a rubber material layer 8009; and ITO and buffing layers 8008 and 8007.

To achieve further control of stress and deformation in a spacer layer, a composite spacer layer can be contemplated, where a BCB layer is applied with a rubber layer followed by a polyimide layer. The next BCB layer in the spacer structure is then applied to the polyimide layer. The stress and deformation of the upper BCB layer is then isolated from the lower BCB layer by the rubber and polyimide layers. Such a structure 9000 is illustrated in Fig. 29. The structure includes a carrier wafer 9002 with an MLA 9002, a spacer layer stack 9004, and ITO and buffing layers 9008 and 9008. The spacer layer stack 9006 includes a first BCB layer 9006a, a rubber layer 9009a, a polyimide layer 9009b, and a second BCB layer 9006b.

It is further possible to form a polymer spacer layer as a stack that includes all of the layers described above including a low index planarization layer as well as polyimide and rubber layers as required for optical design and for spacer mechanical performance. This is exemplified in Fig. 30, showing a structure 9100 including a carrier wafer 9102 with MLA 9104; a planarization layer 9110 being a low index polymer like Cytop structure; a spacer layer structure 9106, an ITO layer 9108 and a buffing layer 9107. The spacer layer structure 9106 includes first and second BCB layers 9106a and 9106b spaced from each other by a stack of rubber and polyimide layers 9109a and 9109b.

Yet another embodiment of the invention utilizes a concave MLA that is filled with a high index polymer, such as Brewer Scientific Optindex materials, followed by a BCB material as a spacer. This is illustrated in Fig. 31. A structure 9200 includes a carrier wafer 9202 patterned to form a concave MLA 9204; a high index polymer planarization layer 9212; a spacer layer stack 9206, formed by first and second BCBs 9206a and 9206b and rubber and polyimide layers 9209a and 9209b between the BCB layers; and ITO and buffing layers 9208 and 9207. This type of structure may be advantageous in case the concave MLA is efficiently manufactured for the desired lens properties. High index polymer materials such as Optindex with organometallic complexes containing metals such as Titanium can achieve index of refraction of 1.7 or higher.

As can be envisioned, all combinations of high index polymer planarization with polymer spacer layer are possible. The above described layer stacks may be periodically repeated in order to meet the optical, mechanical and temperature requirements with reduced thickness of each layer.

Thus, the present invention provides a novel configuration of a spatial light modulator unit of a kind utilizing a liquid crystal active medium, and methods of its fabrication meeting the requirement of the LC panel manufacture. The SLM unit includes one or more MLAs, serving as focusing, collimating and field MLAs. The invention provides for appropriately planarizing the MLA surface and spacing the planarized or convex/concave surface of the MLA from the other layers in the LC panel.

Claims

CLAIMS:

1. A liquid crystal (LC) panel, which is configured as an integrated multilayer structure comprising an LC pixel matrix configured and operable for spatially modulating light passing therethrough, and at least one microlens array (MLA) configured such that every pixel of the pixel matrix is associated with its corresponding microlens from said at least one MLA, the MLA having a curved surface defining an array of convex or concave surface regions of the microlens in the MLA, respectively, and an opposite planar surface which is common for all the microlenses in the MLA, the MLA by either one of its surface being spaced from an active surface of the pixel matrix or from another MLA by a spacer layer structure, the LC panel with said at least one MLA being configured for focusing input light beams of different incident directions onto spaced-apart spots, respectively, within the same pixel by the corresponding microlens of the focusing MLA.

2. The LC panel of Claim 1, wherein the spacer layer structure has a thickness of at least 10 microns.

3. The LC panel of Claim 2, wherein the spacer layer structure has a thickness of up to 150 microns.

4. The LC panel of any one of preceding Claims, wherein the spacer layer structure in made of an inorganic material.

5. The LC panel of Claim 4, wherein the spacer layer structure is made of fused silica, silicon oxide, quartz or glass.

6. The LC panel of Claim 4 or 5, comprising the MLA formed in the spacer layer structure.

7. The LC panel pf Claim 6, comprising a bonding adhesive layer for bonding the curved surface of the MLA to a layer of the LC panel or a cover substrate.

8. The LC panel of Claim 6 or 7, wherein said curved surface of the MLA has at least one substantially planar surface region in between the convex or concave surface regions to thereby facilitate attachment of said curved surface of the MLA to a layer of the LC panel or a cover substrate.

9. The LC panel of any one of Claims 4 or 5, comprising the MLA formed in a carrier layer, the curved surface of said MLA interfacing with the spacer layer structure.

10. The LC panel of Claim 9, wherein the curved surface of the MLA has at least one substantially planar surface region in between the convex or concave surface regions to thereby facilitate attachment of said curved surface of the MLA to the spacer layer structure.

11. The LC panel of any one of Claims 4 to 10, wherein said spacer layer structure is fabricated utilizing a sacrificial layer technology.

12. The LC panel of Claim 9, wherein the inorganic spacer layer structure is formed by discrete spacers spaced by air gap between them.

13. The LC panel of any one of Claims 1 to 3, wherein the spacer layer is made of at least one organic, polymer material.

14. The LC panel of Claim 13, wherein the spacer layer structure comprises one or more layers made of one or more cross-linked polymer materials.

15. The LC panel of Claim 14, wherein said cross-linked polymer material is a benzocyclobutenes (BCB) type material.

16. The LC panel of any one of Claims 13 to 15, wherein the spacer layer structure is made of a polymer material serving as a spacer and planarizer.

17. The LC panel of Claim 16, wherein said spacer layer structure is fabricated by a single layer coating of the MLA surface.

18. The LC panel of Claim 16, wherein said spacer layer structure is fabricated by at least two layer coatings of the same polymer material one directly on top of the other.

19. The LC panel of Claim 16, wherein the spacer layer structure comprises two layer coatings of the same polymer material with at least one additional polymer layer between them.

20. The LC panel of Claim 19, wherein said at least one additional polymer layer includes a polyimide layer.

21. The LC panel of Claim 19, wherein said at least one additional polymer layer includes a rubber or elastomer type material layer.

22. The LC panel of Claim 21 , wherein the additional polymer layers include a rubber or elastomer type material layer and a polyimide layer.

23. The LC panel of any one of Claims 13 to 15, wherein the spacer layer structure comprises layers of different polymer materials serving as spacer and planarizer layers, respectively.

24. The LC panel of Claim 23, wherein the polymer planarizer layer at one side thereof interfaces with the MLA surface and at the other side thereof interfaces with the spacer layer, the planarizer layer being made of a material with lower refractive index than the spacer layer material.

25. The LC panel of Claim 24, wherein the planarizer layer is made of Cytop and the spacer layer is made of a BCB type material.

26. The LC panel of Claim 24, wherein said spacer layer is fabricated by a single layer coating of the planarizer layer.

27. The LC panel of Claim 24, wherein said spacer layer is fabricated by at least two layer coatings of the same polymer material one directly on top of the other, both on the planarizer layer.

28. The LC panel of Claim 27, wherein the spacer layer comprises two layer coatings of the same polymer material with at least one additional polymer layer between them.

29. The LC panel of Claim 28, wherein said at least one additional polymer layer includes a polyimide layer.

30. The LC panel of Claim 28, wherein said at least one additional polymer layer includes a rubber or elastomer type material layer.

31. The LC panel of Claim 28, wherein the additional polymer layers include a rubber or elastomer type material layer and a polyimide layer.

32. The LC panel of any one of preceding Claims, comprising the focusing MLA and the collimating MLA, which are symmetrically identical with respect to the active surface of the pixel matrix being located at opposite sides thereof.

33. The LC panel of any one of preceding Claims, comprising the focusing MLA and the field MLA, accommodated in a spaced-apart relationship in an optical path of light passing through the LC panel towards the active surface of the pixel matrix, the planar surfaces of focusing and field MLA being spaced from one another by the first spacer layer structure, and a curved surface of the field MLA being located close to the active surface of the pixel matrix.

34. The LC panel of Claim 32, comprising first and second field MLAs configured symmetrically identical with respect to the active surface of the pixel matrix and located close thereto at opposite sides thereof, the focusing MLA and the first field MLA being accommodated in a spaced-apart relationship in an optical path of light passing through the LC panel towards the active surface of the pixel matrix such that the planar surfaces of the focusing and the first field MLAs are spaced from one another by the first spacer layer structure and a curved surface of the first field MLA is located close to the active surface of the pixel matrix, the second field MLA is located close to the active surface of the pixel matrix, the collimating and the second field MLAs being accommodated in a spaced-apart relationship in an optical path of light emerging from the pixel matrix such that the planar surfaces of the focusing and the second field MLAs are spaced from one another by a second spacer layer structure.

35. The LC panel of any one of preceding Claims, comprising an LC layer enclosed by its first and second surfaces between a first cover substrate which is to be exposed by its external surface to input light and a second cover substrate, an inner surface of the first substrate being attached to the curved surface of the MLA, the planar surface of which is defined by the spacer layer structure, which is attached to a first stack formed by a first ITO layer and a first buffing layer located on top of the first ITO layer and interfacing the first surface of the LC layer, the second surface of the LC layer being attached to the a second buffing layer which is located on top of the second ITO layer, which at its opposite side interface an a polysilicon and TFT layer which is attached to an inner surface of the second substrate.

36. The LC panel of any one of Claims 1 to 34, comprising an LC layer enclosed by its first and second surfaces between a first cover substrate which is to be exposed by its external surface to input light and a second cover substrate, an inner surface of the first substrate being patterned to define the curved surface of the MLA, located between said curved surface and the first surface of the LC layer are the spacer layer structure which by its one side interfaces said MLA surface and by the opposite side interfaces a first ITO layer, and a first buffing layer, located between the second surface of the LC layer and an inner surface of the second cover substrate are a second buffing layer, a second ITO layer, and a polysilicon and TFT layer.

37. A method for use in manufacture of a liquid crystal (LC) panel in the form of a multi-layer integrated structure, the method comprising: creating a microlens array (MLA) having a curved surface defining an array of convex or concave surface regions of the microlens in the MLA, respectively, and an opposite planar surface which is common for all the microlenses in the MLA, where the MLA is coupled to a respective layer of the multi-layer structure via a spacer layer structure of a desired thickness of at least 10 microns.

38. The method of Claim 37, wherein the MLA is formed in the inorganic spacer layer structure.

39. The method of Claim 37, wherein the MLA is formed in a carrier layer and its curved surface interfaces the inorganic spacer layer structure.

40. The method of Claim 38, wherein the MLA is created by patterning a surface of a sacrificial layer, containing the spacer layer structure, to form said curved surface of the MLA, thereby enabling removal of the sacrificial layer to define the MLA on the spacer layer structure of the desired thickness.

41. The method of Claim 39, wherein the MLA is created by patterning a surface of the carrier layer to form the curved surface of the MLA, and coating said curved surface by a sacrificial layer containing the spacer-layer structure, thereby enabling removal of the sacrificial layer to define the spacer layer structure of the desired thickness.

42. The method of Claim 40, comprising bonding the sacrificial layer with the patterned surface to a carrier wafer using a wafer to wafer bonding adhesive, and applying a material removal or cleaving or backgrinding and polishing to remove the sacrificial layer.

43. The method of Claim 40, comprising bonding the patterned surface of the carrier layer to the sacrificial layer with the spacer layer structure a planarizing bonding layer.

44. The method of Claim 38, wherein the MLA is created by patterning a surface of a carrier layer to form the curved surface of the MLA, and providing the spacer layer structure on said curved surface in the form of discrete spacers spaced from one another by air gap.

45. The method of Claim 37, wherein the MLA is formed in a carrier layer and its curved surface interfaces the organic polymer spacer layer structure.

46. The method of Claim 45, comprising coating the curved surface with a polymer material serving as a spacer and planarizer.

47. The method of Claim 45, wherein said spacer layer structure is fabricated by at least two layer coatings of the same polymer material one directly on top of the other.

48. The method of Claim 45, wherein the spacer layer structure comprises two layer coatings of the same polymer material with at least one additional polymer layer between them.

49. The method of Claim 48, wherein said at least one additional polymer layer includes at least one of polyimide and a rubber or elastomer type material layer.

50. The method of Claim 45, wherein the spacer layer structure comprises layers of different polymer materials serving as spacer and planarizer layers, respectively.

51. The method of Claim 50, wherein the planarizer layer is made of Cytop and the spacer layer is made of a BCB type material.

52. The method of Claim 50, wherein said spacer layer structure is fabricated by a single layer coating of the planarizer layer.

53. The method of Claim 50, wherein said spacer layer structure is fabricated by at least two layer coatings of the same polymer material on the planarizer layer.

54. The method of Claim 53, wherein the spacer layer structure is fabricated by said two layer coatings of the same polymer material with at least one additional polymer layer between them.

55. The method of Claim 54, wherein said at least one additional polymer layer includes at least one of polyimide and a rubber or elastomer type material layer.