Substrate Structure and Assembly Method for Reduced Spatial Light Modulator Size

1. Technical Field

This invention discloses a spatial light modulator which is designed to use up a minimum surface area on a silicon substrate. More specifically, by sealing the glass cover on the silicon substrate in a special way, the area needed on the surface of the silicon substrate is greatly reduced.

2. Description of the Prior Art

Typically, for a reflective or transmissive Spatial Light Modulator (SLM), the liquid crystal (LC) cell is formed by gluing a cover glass to the substrate containing the electrical circuitry and injecting LC into the gap (1). For SLMs fabricated with semiconducting wafers as substrates, a significant portion of the Si substrate or wafer is occupied by the extra area needed for the glue seal. This limits the number of Si substrates that can be fabricated per wafer. Typically, for spatial light modulators used for direct view liquid crystal (LC) displays, the liquid crystal layer is confined by a glue seal between a substrate containing the addressing circuitry and a cover glass with an Indium-Tin oxide (ITO) transparent counter electrode. A minimum spacing is needed between the glue seal and active elements to avoid potential contamination of the liquid crystal by the glue. This contamination is believed to be due to diffusion of glue components into the liquid crystal and results in a reduced resistivity, and hence lower charge retention time. The required separation between the glue seal and active elements is approximately 2.5 mm. Additional space of about 1.5 mm is also needed along each edge for the glue seal itself For SLMs with small array sizes built on semiconducting wafers, the requirement of an additional 4 mm distance beyond the pixels on each side sharply reduces the number of Si substrates which can be built on a wafer and hence increases the cost of each SLM. It should also be mentioned that at least one edge of the SLM must extend beyond the glue seal area for electrical connections.

As an example, consider a VGA array (480.times.640) with 17 mm pixels. The array size is 8.16.times.10.88 mm and with the additional 4 mm per side, the chip is 16.16.times.18.88 mm. Further, one side needs to extend about 1.5 mm beyond the glue seal to provide an area for bonding, so the total chip size is 16.16.times.20.38 mm. This corresponds to an area of 329 mm2. Just in terms of area, a total of only 37 SLMs could be fit on a 125 mm wafer, assuming no wasted area due to the square chip.

Accordingly, this invention is a spatial light modulator having a silicon substrate on a backing plate, where the backing plate extends beyond the Si wafer on three sides. The Si substrate or wafer has the reflecting pixels. A glass cover is placed over the Si substrate on the backing plate and liquid crystal fills gap between the cover glass plate and the Si substrate. The liquid crystal is kept in place by a glue seal bead on the four edges of the cover glass plate.

In one embodiment of the invention, the glue sealant is placed only on one edge of the surface of the substrate so as to minimize the surface area on the silicon substrate that is needed.

In one embodiment of the invention the substrate is recessed into the back plate while in another embodiment the silicon substrate is raised above the surface of the back plate.

With the current invention, the chip extends 1 mm beyond the array on three sides to provide room for circuitry and wiring and 5.5 mm (2.5 mm array to glue seal, 1.5 mm glue seal, and 1.5 mm bonding area) beyond the array on the remaining side for electrical connections. This results in a chip size of 10.16.times.17.38 mm, or an area of 177 mm2. Dividing the area of a 125 mm wafer by this area results in a maximum number of chips per wafer of 69, or nearly twice as many as using the traditional glue seal method. This is equivalent to about a 50% cost reduction from the 329 mm2 described above.

FIG. 1 is a top view of the spatial light modulator assembly according to the invention.

FIG. 2 is a schematic illustration of the side view of the spatial light modulator in accordance with the invention.

FIG. 3 graphically illustrates an embodiment of the invention where the silicon substrate is recessed into the back plate.

Shown in FIG. 1 is a top view and in FIG. 2 is the side view of spatial light modulator assembly 10 (SLM) according to the invention. Shown is backplate 5 which is recessed to accommodate the silicon substrate 6 which is attached to a supporting piece 8 which is typically made of glass and provides for rigidity for electrical connections. Also shown in FIG. 2 is top glass 2 which is attached over the backplate S with a glue seal 4 to form a cavity 7 which will contain an electro-optic material such as liquid crystal. Electrical interconnection pads 3 are shown along one edge of the silicon substrate and are used to electrically connect the silicon substrate to external electronics. Also shown in this figure are spacers 11 which are used to define and maintain the cell gap.

Shown in FIG. 1 is the plan view of the SLM 10 with one dimension Ds of the silicon substrate is being smaller than the corresponding dimension Dg of the top glass. Shown is glue seal 4 where the seal is in contact with only one edge 1 on the silicon substrate surface.

In this embodiment where the Si substrate is recessed into the backplate as shown in FIG. 3, the surface of the Si substrate 12 is approximately coplanar with the surface of the back plate 13. In this embodiment the volume of liquid crystal used is much less than in the case where the Si substrate sits on the backplate. In all embodiments pieces 5 and 8 can be combined into one piece.

In a typical assembly process, the Si is bonded to an optically flat glass plate. It is then coated with a polymer layer that is rubbed to facilitate the alignment of the liquid crystal to be filled later. The top glass which has the transparent electrode is also coated with the same polymer layer and rubbed for the alignment ›2!. The Si assembly is then placed into the recess in the back plate. Spacer balls are then sprinkled over the Si substrate. These serve to define and control the cell gap between Si and the top glass plate. In lieu of spacer balls, insulating spacers can be directly fabricated on the Si substrate. The glue for the seal is then dispensed either on the top glass or on the Si/back plate assembly and the two pieces are mated to form the cell. A small gap, about 10 mm or so in length, is left unglued so that the liquid crystal may be injected into the cell. After the liquid crystal is filled through the gap, the fill gap is then sealed with an epoxy glue. This assembled cell is then annealed at 120 degrees Celsius for 30 minutes to complete the liquid crystal alignment.

For the electro-optic material, any material such as, for example, the following may be used: liquid crystal, ferro-electric liquid crystal, polymer dispersed liquid crystal, and polymer stabilized cholesteric texture liquid crystal.

The silicon substrate has on its surface thousands of individual pixels over an area as small as one square inch. Under each pixel there can be transistor circuitry to make the display an active matrix type (1). In one embodiment, each pixel has a reflecting metallic top layer that is used in conjunction with a counter electrode to apply electrical potentials to electro-optic material. The transparent top plate 2 has on its inside surface a transparent conducting layer that is the counter electrode for the metallic top layer of the pixels on the silicon substrate surface. The electric field thereby generated at each pixel is used to alter the electro-optic material above the pixel. When an appropriate potential is applied across a chosen pixel the electro-optic material locally, for example, changes either the polarization of the light going through it or changes from being transparent to being scattering. The bottom plate material can be, for example: a metal, ceramic, glass, or plastic.

The reinforcing plate 8 can be, for example: glass, ceramic, metal or plastic.

The top plate 2 can be, for example: glass or any transparent plastic.

The glue can be, for example: Ultra Violet (UV) curable adhesive or thermally curable adhesive material.

References ›1! Thin Film Transistor/liquid crystal display technology--Introduction by W. E. Howard, IBM Journal of Research and Development, Vol. 36, No. 1, January 1992. ›2! Flat Panel Displays and CRTs, Edited by L. E. Tarras, Jr., Pub. Van Nostrand Reinhold Co., NY, 1985, Chapter 11.