Highly-reflective liquid crystal on silicon panel comprising a continuous reflective coating covering pixel electrodes and an inter-pixel coating

A highly-reflective liquid crystal on silicon (LCOS) panel includes pixel electrodes on a substrate, each pixel electrode having a top surface with a first reflectivity. A continuous reflective coating covers the pixel electrodes and substrate surfaces therebetween, forming a plurality of coated pixel electrodes having an enhanced reflectivity that exceeds the first reflectivity. A method for increasing pixel reflectivity in a LCOS panel includes depositing a continuous reflective coating covering both (1) a plurality of pixel electrodes on a substrate and (2) a plurality of inter-pixel substrate surfaces, and depositing a layer on the continuous reflective coating.

BACKGROUND

This invention relates to the manufacture of liquid-crystal-on-silicon (LCOS) displays, and particularly, LCOS displays with enhanced reflectivity.

LCOS displays are used in consumer electronics, such as hand-held projectors and near-eye displays, and also have applications in optical communications technologies. LCOS displays include a reflective LCOS panel that contains a pixel array formed on a semiconductor wafer.

SUMMARY OF THE INVENTION

In an embodiment, a highly-reflective LCOS panel includes pixel electrodes on a substrate, each pixel electrode having a top surface with a first reflectivity. A continuous reflective coating covers the pixel electrodes and substrate surfaces therebetween, forming a plurality of coated pixel electrodes having an enhanced reflectivity that exceeds the first reflectivity.

In another embodiment, a highly-reflective LCOS panel includes pixel electrodes on a substrate and having a top surface having a first reflectivity, a reflective coating on each pixel electrode and having a top surface at a first height above the substrate, and an anti-reflective coating on substrate regions between adjacent electrodes and having a second height equal to the first height, the anti-reflective coating and pixel electrodes forming a planar surface. The pixel electrodes with the reflective coating thereon have an enhanced reflectivity that exceeds the first reflectivity. The substrate with the anti-reflective coating thereon has a reduced reflectivity less than a reflectivity of the substrate.

In an embodiment, a method for increasing pixel reflectivity in a LCOS panel includes depositing a continuous reflective coating covering both (1) a plurality of pixel electrodes on a substrate and (2) a plurality of inter-pixel substrate surfaces, and depositing a layer on the continuous reflective coating.

DETAILED DESCRIPTION

FIG. 1shows one exemplary use of a reflective LCOS panel100within a projector assembly141of a hand-held image projector140that projects an image160onto a screen150. Reflective LCOS panel100is illuminated by an illuminator110. Reflective LCOS panel100may alternately be employed in a different display device, such as in a see-through head-mounted display system.

FIG. 2illustrates a detailed view of projector assembly141. Illuminator110emits an s-polarized output beam290toward projector assembly141. InFIG. 2, s-polarization and p-polarization refer to electric field components normal to the figure plane, and parallel to the figure plane, respectively. Projector assembly141includes reflective LCOS panel100, a polarizing beamsplitter (PBS) cube254, and a projector lens256.

Output beam290is incident on PBS cube254. PBS cube254reflects output beam290to reflective LCOS panel100, which spatially modulates and reflects output beam290as p-polarized beam298that is transmitted through PBS cube254and projected by projector lens256as a projected beam299. Projected beam may be imaged as image160.

The quality of image160depends in part on the intensity of p-polarized beam298, which depends on the reflectivity of reflective LCOS panel100. Reflective LCOS panel100includes a pixel array226formed a plurality of pixels each having a pixel electrode216, the reflectivity of which largely determines the reflectivity of reflective LCOS panel100, and hence the quality of the projected image.

FIG. 3is a perspective view of a prior-art reflective LCOS panel300, which may replace LCOS panel100in projector assembly141. Prior-art reflective LCOS panel300includes a cover glass340on a semiconductor wafer310. A liquid crystal layer330is between cover glass340and semiconductor wafer310. Pixel array226is between the liquid crystal layer330and semiconductor wafer310. A corner portion of liquid crystal layer330is not shown to reveal pixel array226beneath it. Semiconductor wafer310includes a plurality of bond pads387that control each pixel of pixel array226.

A transparent conductive layer338is on the surface of cover glass340adjacent to liquid crystal layer330. For clarity of illustration,FIG. 3shows only a portion of conductive layer338on overhang region329of cover glass340. A dam327contains liquid crystal layer330. Transparent conductive layer338is deposited on cover glass340, and is, for example, formed of indium titanium oxide (ITO).

FIG. 4is a cross-sectional view of a portion of prior-art reflective LCOS panel300showing portions of semiconductor wafer310, pixel array226, liquid crystal layer330, transparent conductive layer338, and cover glass340. Pixel electrodes216(1-3) of pixel array226are separated by inter-pixel gaps206. Pixel electrodes216are typically formed of aluminum, which has a refractive index of n=0.95+6.4 i at λ0=546 nm and a corresponding normal-incidence reflectivity of RAl=91% when the incident medium has a refractive index n=1.0.

Increasing the reflectivity of pixel electrode216of prior-art reflective LCOS panel300can enhance the quality of image160formed by projector assembly141. Existing methods of increasing pixel reflectivity include depositing a reflective thin-film coating on each pixel electrode216. For example, U.S. patent publication number US2008/0106677 to Kuan et al. describes three different reflection layers on pixel electrodes of each of a red, a green, and a blue sub-pixel included in each pixel. A disadvantage of this approach is the process requires applying a different reflective coating to each type of sub-pixel electrode. For example, referring toFIG. 4, pixel electrodes216(1),216(2), and216(3) would each have a different narrow-band coating designed to reflect one of red, green, and blue light. Since a typical width of a pixel electrode of a reflective LCOS panel is less than ten microns, the process of forming a patterned array of different narrow-band reflective films aligned to pixel electrodes216is not trivial.

Forming a single broadband reflective coating spanning the visible portion of the electromagnetic spectrum over all pixel electrodes would be a much simpler and cost-effective method of increasing pixel electrode reflectivity. In US2008/0106677, the color selectivity of the reflective LCOS panel results from each sub-pixel electrode reflecting one of red, green, and blue light. By contrast, color selectivity of an LCOS panel with a broadband reflective coating common to all pixel electrodes would not differentiate pixels by color reflectivity. Rather, reflectivity of different colors may be achieved by a field-sequential method, as known in the art.

FIG. 5is a perspective view of a highly-reflective LCOS panel500that may replace LCOS panel100in projector assembly141. Highly-reflective LCOS panel500includes a cover glass540on a semiconductor wafer510. A liquid crystal layer530is between cover glass540and semiconductor wafer510. A pixel array526is between the liquid crystal layer530and semiconductor wafer510. A corner portion of liquid crystal layer530is not shown to reveal pixel array526beneath it. Pixel array526includes a plurality of pixels each with a respective pixel electrode516, not shown inFIG. 5. Semiconductor wafer510includes a plurality of bond pads587that control each pixel of pixel array526.

A transparent conductive layer538is on the surface of cover glass540adjacent to liquid crystal layer530. For clarity of illustration,FIG. 5shows only a portion of transparent conductive layer538on overhang region529of cover glass540. A dam527contains liquid crystal layer530. Semiconductor wafer510, liquid crystal layer530, transparent conductive layer538, and cover glass540, are similar to semiconductor wafer310, liquid crystal layer330, transparent conductive layer338, and cover glass340ofFIG. 3, respectively.

FIG. 6is a cross-sectional view of a portion of a highly-reflective LCOS panel600. Highly-reflective LCOS panel600is an embodiment of highly-reflective LCOS panel500, and includes semiconductor wafer510, an isolation layer612, pixel array526, a reflective coating620, a bottom alignment layer628, liquid crystal layer530, transparent conductive layer538, a top alignment layer639, and cover glass540. In certain embodiments, isolation layer612may be formed of silicon nitride and includes a light-blocking layer614, which may be formed of an optically opaque material such as a metal. Pixel array526includes pixel electrodes516(1-3). Pixel electrodes516(1-3) of pixel array526are separated by inter-pixel gaps506. Light-blocking layer614may be formed entirely, or in part, of non-metallic materials without departing from the scope hereof.

An inter-pixel coating604is between each pair of adjacent pixel electrodes516. Each electrode516may be formed of aluminum and has a width586and a height584. Width586may be between five microns and ten microns, 6.4 microns for example. A distance693separates light-blocking layer614and inter-pixel coating604. Electrodes516may be formed of metals other than aluminum without departing from the scope hereof.

Reflective coating720may be a multilayer coating with more or fewer than seven layers without departing from the scope hereof. At least one layer of reflective coating720may be formed of a material other than silicon dioxide and titanium dioxide without departing from the scope hereof.

Referring toFIG. 6and along axis691therein, inter-pixel coating604is between isolation layer612and layer701of reflective coating620. In an embodiment of highly-reflective LCOS panel500, the material of inter-pixel coating604is the same as the material of layer701. For example, inter-pixel coating604and layer701are both silicon dioxide.

The quality of images produced by a reflective LCOS panel depends in part on its having a low gap reflectivity, also known as “leakage,” as high gap reflectivity decreases image contrast. For example, when LCOS panel600operates in projector assembly141, a contiguous pixel group within pixel array526may be instructed to display black, such that the pixel group reflects no light incident thereon from illuminator110. Yet, the ability of LCOS panel500to display black in the region corresponding to the pixel group is limited by a gap reflectivity associated with the inter-pixel region695between pixels of the pixel group, which reflect light from illuminator110regardless of what signals neighboring pixels receive.

In embodiments of highly-reflective LCOS panel600that follow, inter-pixel region695differs as a result of different approaches to reduce gap reflectivity Rg1.

In a first embodiment of highly-reflective LCOS panel600, inter-pixel coating604is an absorptive material at visible wavelengths. For example, inter-pixel coating604may be formed of silicon, which has a refractive index n=4.1+0.043 i at λ0=546 nm and a corresponding normal-incidence reflectivity of R=37% when the incident medium has a refractive index n=1.0.

In a second embodiment of highly-reflective LCOS panel600, inter-pixel coating604is a multi-layer coating designed to minimize reflectivity of visible light when placed between isolation layer612and reflective coating620. For example,FIG. 9is a cross-sectional view of a low-reflective coating900between two adjacent pixel electrodes516of highly-reflective LCOS panel600. Low-reflective coating900includes an inter-pixel coating604and reflective coating620, and occupies inter-pixel region695. Inter-pixel coating604may have a height984equal to height584of pixel electrodes516. In such a case, reflective coating620has a top surface921that is planar across pixel array526, and bottom alignment layer628may be formed on a planar surface. For clarity of illustration,FIG. 9does not include bottom alignment layer628, liquid crystal layer530, top alignment layer639, and cover glass540.

As illustrated inFIG. 9, a low-reflector950includes light-blocking layer614, isolation layer612, inter-pixel coating604, and reflective coating620. Low-reflector950has a reflectivity R950that is less than that of light-blocking layer614with isolation layer612on it. Low-reflector950has a low reflectivity by virtue of inter-pixel coating604being optimized to minimize reflectivity of low-reflective coating900given predetermined properties (thickness and refractive index, for example) of light-blocking layer614, isolation layer612, and reflective coating620, which are locked during an optimization process. Inter-pixel coating604may be designed using commercially available thin-film coating design software such as Essential Macleod.

FIG. 10is a cross-sectional view of a highly-reflective LCOS panel1000, which is equivalent to highly-reflective LCOS panel600without inter-pixel coating604and with a coating1020deposited directly on inter-pixel substrate surfaces694. Coating1020is similar to reflective coating620. InFIG. 10, the degree of conformality of coating1020to surfaces of pixel electrode516is shown to illustrate that coating1020need not be perfectly conformal, and is not intended to indicate a preferred or actual degree of conformality.

Coating1020may be a multilayer coating jointly optimized to maximize reflectivity when on a pixel electrode516(a first substrate) while minimizing reflectivity when above isolation layer612and light-blocking layer614(a second substrate).

FIG. 11is a cross-sectional view of a portion of a highly-reflective LCOS panel1100. Highly-reflective LCOS panel1100is an embodiment of highly-reflective LCOS panel500and resembles highly-reflective LCOS panel600with the exception of inter-pixel region1195. In highly-reflective LCOS panel1100, a reflective coating620is above each pixel electrode516and does not span inter-pixel regions1195, as illustrated inFIG. 11. An anti-reflective coating1104is between adjacent pixel electrodes516of highly-reflective LCOS panel1100and has a height1110. Anti-reflective coating1104may be designed with a thin-film design tool known in the art. Anti-reflective coating1104may be a multi-layer coating designed to minimize reflectivity of visible light incident on LCOS panel1100that reaches coating1104. Anti-reflective coating1104may be optimized to minimize reflectivity of visible light thereon at a single incident angle, normal incidence for example, or for a range of incident angles.

FIG. 12is a cross-sectional view of highly-reflective LCOS panel1100showing anti-reflective coating1104and two adjacent pixel electrodes516in greater detail. For clarity of illustration,FIG. 12does not include bottom alignment layer628, liquid crystal layer530, top alignment layer639, and cover glass540. Anti-reflective coating1104occupies an inter-pixel region1195and has a top surface1101. In an embodiment, height1110that may equal the sum of height584of pixel electrodes516and height685of reflective coating620, such that top surfaces921and1101are coplanar. In such a case, bottom alignment layer628may be formed on a planar surface that includes top surfaces921and1101. In an embodiment, reflective coating620is the same as reflective coating720, shown inFIG. 7.

FIG. 13is a flow chart illustrating an exemplary method1300for increasing pixel reflectivity of an LCOS panel. The LCOS panel includes a plurality of pixel electrodes on a substrate and a plurality of inter-pixel substrates surface between respective adjacent pixel electrodes.

Step1302is optional. In optional step1302, method1300forms, between each inter-pixel substrate surface and a reflective coating thereover, an inter-pixel coating having a height equal to a pixel electrode. The inter-pixel coating and the reflective coating thereon form a low-reflective coating. The substrate with the low-reflective coating thereon has a reduced reflectivity less than a reflectivity of the substrate. In an example of step1302, inter-pixel coating604is formed between each inter-pixel substrate surface694and reflective coating620. Inter-pixel coating604and reflective coating620thereon form low-reflective coating900. Inter-pixel coating604may be confined to the region shown inFIG. 9via thin-film deposition and photolithography techniques known in the art.

In step1304, method1300deposits the continuous reflective coating covering both (1) the plurality of pixel electrodes on the substrate and (2) the plurality of inter-pixel substrate surfaces. The continuous reflective coating may be a multilayer thin-film coating, and be deposited with methods known in the art. In an example of step1304, reflective coating620is deposited over pixel electrodes516and inter-pixel substrate surfaces694, as shown inFIGS. 6 and 9.

In step1306, method1300deposits a layer directly on the continuous reflective coating. In an example of step1306, bottom alignment layer628is deposited on reflective coating620.

FIG. 14is a flow chart illustrating an exemplary method1400for increasing pixel reflectivity of an LCOS panel. In step1410, method1400forms a reflective coating over each of a plurality of pixel electrodes on a substrate. In an example of step1410, reflective coating620is formed over each pixel electrode516on isolation layer612, as shown inFIG. 11.

In step1420, method1400forms an anti-reflective coating on a substrate surface between adjacent pixels. In an example of step1420, anti-reflective coating1104is formed on inter-pixel substrate surface694.

Combinations of features:

Features described above as well as those claimed below may be combined in various ways without departing from the scope hereof. The following examples illustrate some possible, non-limiting combinations:

(A1) A highly-reflective LCOS panel may include pixel electrodes on a substrate, each pixel electrode having a top surface with a first reflectivity, and a continuous reflective coating covering the pixel electrodes and substrate surfaces therebetween, forming a plurality of coated pixel electrodes having an enhanced reflectivity that exceeds the first reflectivity.

(A2) In the highly-reflective LCOS panel denoted as (A1), each top surface may be at a first height above the substrate and further comprising on substrate regions between adjacent pixel electrodes, an inter-pixel coating between the substrate and the continuous reflective coating, the inter-pixel coating may have a second height equal to the first height such that the inter-pixel coating and pixel electrodes form a planar surface.

(A3) In at least one of the highly-reflective LCOS panels denoted as (A1) and (A2), the inter-pixel coating and the continuous reflective coating thereon may form a low-reflective coating on the substrate, the substrate and the low-reflective coating collectively having a reduced reflectivity less than a reflectivity of the substrate.

(A4) In the highly-reflective LCOS panels denoted as (A3), the inter-pixel coating may include at least one of (1) multiple layers and (2) an absorptive material

(A5) In any of the highly-reflective LCOS panels denoted as (A1) through (A4), the enhanced reflectivity may exceed the first reflectivity at all wavelengths between 425 nm and 675 nm.

(A6) In any of the highly-reflective LCOS panels denoted as (A1) through (A5), the continuous reflective coating may be a multi-layer coating.

(A7) Any of the highly-reflective LCOS panels denoted as (A1) through (A6) may further include a bottom alignment layer on a top surface of the continuous reflective coating.

(B1) A highly-reflective LCOS panel may include pixel electrodes on a substrate and having a top surface having a first reflectivity, a reflective coating on each pixel electrode and having a top surface at a first height above the substrate, and an anti-reflective coating on substrate regions between adjacent electrodes and having a second height equal to the first height, the anti-reflective coating and pixel electrodes forming a planar surface. The pixel electrodes with the reflective coating thereon may have an enhanced reflectivity that exceeds the first reflectivity. The substrate with the anti-reflective coating thereon may have a reduced reflectivity less than a reflectivity of the substrate.

(B2) In the highly-reflective LCOS panel denoted as (B1), the anti-reflective coating may include at least one of (1) multiple layers and (2) an absorptive material

(B3) In at least one of the highly-reflective LCOS panels denoted as (B1) and (B2), the reflective coating may be a multi-layer coating.

(B4) Any of the highly-reflective LCOS panels denoted as (B1) through (B3) may further include a bottom alignment layer on a top surface of the reflective coating.

(B5) In any of the highly-reflective LCOS panels denoted as (B1) through (B4), the enhanced reflectivity may exceed the first reflectivity at all wavelengths between 425 nm and 675 nm.

(C1) A method for increasing pixel reflectivity in a LCOS panel may include depositing a continuous reflective coating covering both (1) a plurality of pixel electrodes on a substrate and (2) a plurality of inter-pixel substrate surfaces. The method may also include depositing a layer on the continuous reflective coating.

(C2) In the method denotes as (C1), the layer may be an alignment layer.

(C3) At least one of the methods denoted as (C1) and (C2) may further include, before the step of depositing the continuous reflective coating, forming, between each inter-pixel substrate surface and the continuous reflective coating thereover, an inter-pixel coating having a height equal to a pixel electrode. The inter-pixel coating and the reflective coating thereon may form a low-reflective coating on the inter-pixel substrate surfaces, the substrate with the low-reflective coating thereon having a reduced reflectivity less than a reflectivity of the substrate.

(C4) Any of the methods denoted as (C1) through (C3) may further include steps of removing portions of the continuous reflective coating between adjacent pixel electrodes and forming an anti-reflective coating between adjacent pixel electrodes.

(C5) In the method denoted as (C4), the continuous reflective coating may have a top surface at a first height above the substrate, and the anti-reflective coating may have a top surface at a second height above the substrate equal to the first height.