Liquid crystal cells – elements and systems – Particular structure – Interconnection of plural cells in parallel
Reexamination Certificate
2000-06-21
2003-05-20
Parker, Kenneth (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Interconnection of plural cells in parallel
C349S074000
Reexamination Certificate
active
06567138
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to electronic flat panel displays and, more particularly, to a method for assembling tiled semiconductor microdisplays into a single composite flat-panel display, the projected image of which has visually imperceptible seams.
BACKGROUND OF INVENTION
Microdisplays (&mgr;Ds) are the most recent addition to the family of flat-panel displays. While &mgr;Ds are based on a number of different techniques to generate or modulate light, all use microfabrication to produce a rectangular array of pixels on a semiconductor back plane, usually silicon. Examples of demonstrated &mgr;Ds include liquid crystal displays (LCDs), field emission displays (FEDs), and digital micro-mirror displays (DMDs). Pixels in &mgr;Ds can be fabricated to have pitches in the range of approximately 10 &mgr;m×10 &mgr;m to 30 &mgr;m×30 &mgr;m. For a 10 &mgr;m pixel pitch, the display array size is approximately 10.24 mm×7.68 mm assuming XGA resolution (1024×768 pixels). Control, driver and image processing circuits may be embedded into the back plane. When viewed through a suitable magnifying lens, such &mgr;D pixel arrays can be designed to appear to the human observer as equivalent to a desktop monitor (e.g., a 15″ diagonal monitor) when viewed at a distance of approximately twice the diagonal dimension. In such magnified applications, &mgr;Ds are suitable for use, for example, in portable television (TV), compact disc (CD), digital video disc (DVD), personal digital assistant (PDA) applications, and the like.
In contrast, when projected on the front or back side of a large screen through an optical system, &mgr;Ds have the potential to produce images that rival conventional projectors using polycrystalline LCDs. In this projection configuration, &mgr;Ds may be used in applications such as large screen TV, multi-user (multi-viewer) computer, multi-media, and home theater. However, many design and fabrication problems have heretofore prevented the realization of this potential. For example, reflective microdisplays generally have a rather low geometric efficiency, which makes high brightness projected images difficult to achieve. Microdisplays also force the use of small size arc lamps or special lamps that provide high luminous output at small source size in order to maintain the geometric efficiency. High resolution &mgr;Ds with larger pixel pitches also would require unacceptably large chip sizes.
The construction of a typical reflective LCD &mgr;D device is described hereinafter and serves as a starting point for describing tiled &mgr;D assemblies. The tiling structures, fabrication methods, and circuits for other reflective or emissive &mgr;Ds are essentially the same and are not described separately.
The back plane of a typical &mgr;D is formed from a crystalline silicon chip which includes integrated circuits (ICs). Typically these are of the CMOS family. Therefore CMOS is used here to represent all other suitable integrated circuit families. The CMOS ICs used in &mgr;Ds are fabricated using a typical SRAM-like process as is well known to those skilled in the art. Minimum feature sizes of less than 1 &mgr;m are typical on such chips. No significant difference in the fabrication process compared to standard CMOS chips occurs until the application of the upper levels of metal interconnect. Multi-layer Al/SiO
2
metalization is still used but the topmost metal layer forms a two-dimensional array of rectangular mirrors, each about 10-30 &mgr;m on the side with a gap of about 1 &mgr;m between each pair. These mirror elements serve as the pixels of the &mgr;D; the topmost Al layer being polished to a mirror finish in order to serve as a highly effective optical mirror with a reflectivity generally greater than 80%.
The gaps between the mirror elements are filled with a dielectric material, typically SiO
2
, with low optical reflectivity. The ratio of the optically active area of each mirror to the entire area of the mirror plus any optically inactive areas, such as the gaps, is called the aperture ratio. Typical &mgr;D aperture ratios are on the order of 85%, much higher than is possible with direct view transmissive active matrix liquid crystal displays (AMLCDs). The metal layer immediately under the mirrors forms light shields under the gaps that prevent light from reaching the light sensitive CMOS circuitry in the back plane.
Each mirror element is connected to a CMOS driver circuit through single or multiple vias that provide the voltage to that particular pixel. The rest of the metal interconnect in/on the back plane is used for conventional addressing of the pixels (e.g., matrix addressing) and for regular circuit functions and services for the CMOS circuitry. The CMOS back plane can also contain some or all of the circuits needed for display addressing (e.g., row and column drivers for matrix addressing), control circuits, and any desired image processing circuits.
Given the CMOS back plane with the mirror elements, the &mgr;D is assembled as follows. A passivation layer and an optional LCD alignment layer are applied to the top of the mirror plane. A seal bead with a small fill port is next dispensed around the pixel array in the periphery using screen-printing or a dedicated dispensing system. Separately, a glass cover plate is fabricated having on its lower side a conductive transparent electrode film (e.g., indium-tin-oxide (ITO)), and possibly another alignment layer for the LC material. Large area microfabrication techniques may be used to make arrays of cover plates, which may be scribed and broken into appropriate sizes.
The cover plate is placed on the seal, aligned to the CMOS back plane mirror array and then bonded to the seal bead. The display is next filled with a suitable liquid crystal (LC) material, such as a twisted nematic liquid crystal (TN-LC) or a ferroelectric liquid crystal (FLC) material, and then the fill port is sealed. This LC fill may also contain spacer particles that are dispensed throughout the fill, unless spacers have optionally already been fabricated on top of the mirror array or cover plate using microfabrication techniques. Electrical connections from the conductive electrode on the underside of the common cover plate to the silicon back plane are made at the same time, for example using conductive adhesive.
A polarizer film may be applied on the top surface of the cover plate or placed elsewhere into the optical system. Next the &mgr;D component is mounted on an interconnect substrate, usually flex, and electrical connections are made from the edge of the CMOS back plane to the substrate. Finally, the &mgr;D component is suitably encapsulated, thus providing environmental protection. Plastic encapsulation is typically used in consumer products. The resulting &mgr;D modules produced in this manner are compact, lightweight, and relatively inexpensive.
The optical systems for use with &mgr;Ds provide three separate functions: (a) provision of light, (b) formation of color, and (c) magnification of the image to the desired size. There are several ways to produce color. Most direct view transmissive AMLCDs form color by placing a color triad (e.g., red, green, and blue) into each pixel, using white back light and patterned color filters. This is called the spatial color generation technique. Since this increases the pixel pitch by a factor of three compared to a monochromatic pixel array, this technique is not usually preferred in &mgr;Ds. The second way for producing color is to use a separate display unit for each color and then to combine the colors into a single, final image. This so-called three channel approach is the favored technique in commercial front-projection displays with polysilicon transmissive LCDs. However in &mgr;D applications, this approach may be acceptable only in large rear projection systems. In the third method, the mirror array is illuminated sequentially with the different primary colors, one at the time, thus forming the proper color mix as a time average in the human
Krusius J. Peter
Seraphim Donald P.
Parker Kenneth
Qi Mike
Rainbow Displays Inc.
Salzman & Levy
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