Active semiconductor backplanes

Details Active semiconductor backplanes Active semiconductor backplanes

2002-03-01

2004-11-02

Nelms, David (Department: 2818)

Computer graphics processing and selective visual display system

Plural physical display element control system

Display elements arranged in matrix

C345S097000, C345S098000

Reexamination Certificate

active

06812909

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to active semiconductor backplanes suitable for use with a spaced opposed substrate, commonly a counterelectrode, to form a cell, and to devices comprising such backplanes.
2. Discussion of Prior Art
The device which is particularly described in this specification in connection with a preferred embodiment is a spatial light modulator in the form of a smectic liquid crystal layer disposed between an active semiconductor backplane and a common front electrode. It was developed in response to a requirement for a fast and, if possible, inexpensive, spatial light modulator comprising a relatively large number of pixels with potential application not only as a display device, but also for other forms of optical processing such as correlation and holographic switching. Our copending International Patent Applications even filing and priority dates (PCT/GB99/04285, U.S. Ser. No. 09/868,219, priority GB9827952.4; PCT/GB99/04286 and PCT/GB99/04276, U.S. Ser. No. 09/868,239 and U.S. Ser. No. 09/868,220, both priority GB9827965.6; PCT/GB99/04282, U.S. Ser. No. 09/446,325, priority GB9827900.3; PCT/GB99/04274, U.S. Ser. No. 09/868,218, priority GB9827964.9; PCT/GB99/04275, U.S. Ser. No. 09/868,217, priority GB9827945.8; and PCT/GB99/04260 and PCT/GB99/04277, U.S. Ser. No. 09/868,241 and U.S. Ser. No. 09/868,242, both priority GB 9827944.1) relate to other inventive aspects associated with the spatial light modulator.
During the course of development of the spatial modulator, a series of problems were encountered and dealt with, and the solutions to these problems (whether in the form of construction, function or method) are not necessarily restricted in application to the embodiment, but will find other uses. Thus not all of the aspects of the present invention are limited to liquid crystal devices, nor to spatial light modulators.
Nevertheless, it is useful to commence with a discussion of the problems encountered in developing the embodiment to be described later.
The liquid crystal phase has been recognised since the last century, and there were a few early attempts to utilise liquid crystal materials in light modulators, none of which gave rise to any significant successful commercial use. However, towards the end of the 1960's and in the 1970's, there was a renewed interest in the use of liquid crystal materials in light modulating, with increasing success as more materials, and purer materials became available, and as technology in general progressed.
Generally speaking, this latter period commenced with the use of nematic and cholesteric liquid crystal materials. Cholesteric liquid crystal materials found use as sensors, principally for measuring temperature or indicating a temperature change, but also for responding to, for example, the presence of impurities. In such cases, the pitch of the cholesteric helix is sensitive to the parameter to be sensed and correspondingly alters the wavelength at which there is selective reflection of one hand of circularly polarised light by the helix.
Attempts were also made to use cholesteric materials in electro-optic modulators, but during this period the main thrust of research in this area involved nematic materials. Initial devices used such effects as the nematic dynamic scattering effect, and increasingly sophisticated devices employing such properties as surface induced alignment, the effect on polarised light, and the co-orientation of elongate dye molecules or other elongate molecules/particles, came into being.
Some such devices used cells in which the nematic phase adopted a twisted structure, either by suitably arranging surface alignments or by incorporating optically active materials in the liquid crystal phase. There is a sense in which such materials resemble cholesteric materials, which are often regarded as a special form of the nematic phase.
Initially, liquid crystal light modulators were in the form of a single cell comprising a layer of liquid crystal material sandwiched between opposed electrode bearing plates, at least one of the plates being transparent. The thickness of the liquid crystal layer in nematic cells is commonly around 20 to 100 microns, and there is a correspondingly small unit capacitance associated with a nematic liquid crystal cell. Furthermore, the switching time from a wholly “OFF” state to a wholly “ON” state tends to be rather long, commonly around a millisecond. Relaxation back to the “OFF” state can be somewhat longer, unless positively driven, but the “OFF” state is the only stable one.
At the same time, electro-optic nematic devices comprising a plurality of pixels were being devised. Initially, these had the form of a common electrode on one side of a cell and a plurality of individually addressable passive electrodes on the other side of the cell (e.g. as in a seven-segment display), or, for higher numbers of pixels, intersecting passive electrode arrays on either side of the cell, for example row and column electrodes which were scanned. While the latter arrangements provided considerable versatility, there were problems associated with cross-talk between pixels.
The situation was exacerbated when analogue (grey scale) displays were required by analogue modulation of the applied voltage, since the optical response is non-linearly related to applied voltage. Addressing schemes became relatively complicated, particularly if dc balance was also required. Such considerations, in association with the relative slowness of switching of nematic cells, have made is difficult to provide real-time video images having a reasonable resolution.
Subsequently, active back-plane devices were produced. These comprise a back plane comprising a plurality of active elements, such as transistors, for energising corresponding pixels. Two common forms are thin film transistor on silica/glass backplanes, and semiconductor backplanes. The active elements can be arranged to exercise some form of memory function, in which case addressing of the active element can be accelerated compared to the time needed to address and switch the pixel, easing the problem of displaying at video frame rates.
Active backplanes are commonly provided in an arrangement very similar to a dynamic random access memory (DRAM) or a static random access memory (SRAM). At each one of a distributed array of addressable locations, a SRAM type active backplane comprises a memory cell including at least two coupled transistors arranged to have two stable states, so that the cell (and therefore the associated liquid crystal pixel) remains in the last switched state until a later addressing step alters its state. Each location electrically drives its associated liquid crystal pixel, and is bistable per se, i.e. without the pixel capacitance. Power to drive the pixel to maintain the existing switched state is obtained from busbars which also supply the array of SRAM locations. Addressing is normally performed from peripheral logic via orthogonal sets (for example column and row) addressing lines.
In a DRAM type active backplane, a single active element (transistor) is provided at each location, and forms, together with the capacitance of the associated liquid crystal pixel, a charge storage cell. Thus in this case, and unlike a SRAM backplane, the liquid crystal pixels are an integral part of the DRAM of the backplane. There is no bistability associated with the location unless the liquid crystal pixel itself is bistable, and this is not normally the case so far as nematic pixels are concerned. Instead, reliance is placed on the active element providing a high impedance when it is not being addressed to prevent leakage of charge from the capacitance, and on periodic refreshing of the DRAM location.
Thin film transistor (TFT) backplanes comprise an array of thin film transistors distributed on a substrate (commonly transparent) over what can be a considerable area, with peripheral logic circuits for addressing the transistors, thereby facilitating the provision of

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