Note: Descriptions are shown in the official language in which they were submitted.
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BACKG~OUND OF THE INVENTION
This invention relates to video display systems and,
more particularly, to improvements in the type of video
display which employs a deformable material having
deformations that depend upon a stored charge pattern on a -~
solid state device and an optical subsystem for converting
the deformations into a viewable image.
In recent years there have been developed techniques
for displaying video information by storing a charge pattern
representative of a video frame in a frame store and
u-tilizing the charge pattern to modify a characteristic of a
material. The modified characteristic of the material is
then used to obtain a viewable image. In my U.S. Patent No.
3,882,271, there is disclosed an apparatus wherein a charge
pattern on a special semiconductor frame store is used to
obtain deformations of a deformable material. The
deformable material is disposed between an array of
electrodes on the surface of the semiconductor device and a
thin conductive layer which serves as both a common
electrode and a light reflective layer. The deformable
material and the thin conductive layer are deformed in
response to the charge pattern on the array of electrodes
with respect to the conductive layer. An optical subsystem,
such as a Schleiren optical system, is then utilized to
convert the pattern of deformations (or "ripples") in the
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thin conductive layer into a viewable image.
An important aspect of the described type oE system is
the operation of the conductive and reflective layer which
is mounted on the deformable layer and must reliably and
repea-tably provide precise patterns of deformations in the
reflective layer which correspond to the charge pattern on
the array of electrodes. It has been found that the
materials used for the deformable and conductive layers, as
well as their dimensions, have a profound effect upon
operating characteristics, and prior attempts to make solid
state light modulators have me-t with one or more of the
following deficiencies: insufficient sensitivity to the
applied voltages; inadequate response characteristics as a
function of time; and/or instability over the short and long
term.
It is an object of the present invention to provide a
solid state light modulator structure, and method of making
same, which overcomes these problems.
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SUMMARY OF THE INVENTION
An aspect of the present invention is directed to
improvements in an apparatus for generating an image from an
input video signal. A semiconduc-tor device is provided, the
device having an array of spaced charge storage electrodes
thereon. An elastomer layer is disposed on the
semiconductor device, over the array of charge storage
electrodes. The elastomer layer has a thickness in the
range between one-tenth and one times the average center-to-
center spacing between adjacent active charge storage
electrodes. At least one conductive layer is disposed over
the elastomer layer. The semiconductor device is responsive
to the input video signal to selectively apply voltages
between the charge storage electrodes and the at least one
conductive layer to cause deformations of the conductive
layer and the elastomer layer. An optical subsystem is
provided for converting deformations of the at least one
conductive layer into an image, for example using a
Schleirin optical system.
Preferably, a plastic pellicle layer is disposed
between the elastomer layer and the at least one conductive
layer. This plastic pellicle layer is useful during the
fabrication of the devlce, and also serves to protect the at
least one conductive layer against elements in the elastomer
layer that might have a degrading effect thereon. In a
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pre~erred embodiment, the at least one conductive layer
comprises a thin layer o~ gold, which is applied over the
plas-tic pellicle layer, and a thin layer of silver, which is
applied over the gold layer. The gold layer applies very
uniformly on the pellicle, and provides an excellent
conductive surface and base for the silver layer. The
silver layer applies easily and uniformly over the gold
layer and provides an excellen~ light reflector, as is
necessary in conjunc-tion with the optical subsystem.
Further features and advantages of the invention will
become more readily apparent from the following detailed
description when taken in conjunction with the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram, partially in schematic form,
of a systern for displaying images and which includes
improvements in accordance with the invention and as
described in conjunction with the subsequent FIGURES.
FIG. 2 is a cross sec-tional view of the semiconductor
device of the FIG. 1 embodiment, including an elastomer
layer, pellicle layer, and conductive layers in accordance
with improvements of the invention.
FIG. 3 is a diagram illustrating the steps used in
fabricating the solid-state light modulator structure,
including the semiconductor device, elastomer layer,
pellicle layer, and conductive layers in accordance with a
preferred embod'ment of the invention.
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DESCRIPTION OF THE PREFERRED EMBODIM~T
Referring to FIG. 1, there is shown a simplified
schematic diagram of an apparatus 10 which includes
improvements in accordance wi-th an embodiment of the
invention. A solid state light modulator structure 100
includes a semiconductor substrate 105 having an array of
semiconductor controlled storage units formed in a surface
110 thereof, and a layer of deformable material 200 which
covers the surface 110. At least one reflective layer 300
of conductive material is disposed over the deformable layer
200. The semiconductor controlled storage unit array, and
associated circuitry 80, receives and samples a video
signal, the samples ultimately being stored as a charge
pattern as between electrodes on the surface 110 of
semiconductor 105 and the layer 300, which serves as a
common electrode. The electric fields associated with the
charge pattern result in forces which deform the deformable
material 200 so that information is contained in the
reflective layer 300 in the form of depressions or ripples
which are similar to a phase diffraction grating. This
information is then displayed on a screen 15, such as by
using a Schlieren type of optical system.
The optical system depicted in FIG. 1 is of the type
disclosed in my U.S. Patent No. 3,084,590. In this system
arc electrodes 20 generate an intense light source that is
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directed by a curved mirror 21 over a masking system 22
which includes a plurality of reflec-ting bars 23 separated
by transparent areas or slits 24. The masking system 22 is
positioned so that the light reflected from the bars 23 is
transmitted toward a lens 25 where it is collimated into
relatively parallel rays. These rays are incident on the
deformed reflective surface 300 and are reflected back
through the lens 25 toward the masking system 22. If, at a
glven instant, there were no depressions on the surface 300
to divert the light rays, the rays effectively emanating
from each bar 23 would be focused by lens 25 on a bar on the
opposite side from which it originated, and no light would
be incident on the screen 15. However, if there are
depressions in the surface 300 they will act as diffraction
gratings and some light will be diffracted and ultimately
pass through the slits 24. The portion of a light ray
transmitted through a particular slit depends on the
amplitude of the infinitesimal diffraction grating from
which the ray was diffracted. Thus, an appropriate pattern
of diffraction gratings, resulting from a selected charge
pattern, will yield a desired image on the screen 15.
Referring to FIG. 2, there is shown a cross-sectional
view of the solid state light modulator 100 in accordance
with an embodiment of the invention, and which had been
shown generally in the FIG. 1 diagram. The basic structure
of the semiconductor substrate 105 and an array of
semiconductor controlled storage units formed therein, each
including a display electrode 106, is known in the art and
is not, per se, a feature of the present invention.
Reference can be made, for example, to the above referenced
U.S. Patent No. 3,084,590. A deformable elastomer layer 200
is disposed on the semiconductor device 105, and covers the
display electrodes 106 thereof, as well as any areas of
surface 110 between the display electrodes. It will be
understood that a thin protective oxide layer (not shown)
can be provided over the display region electrodes during
manufacture of the semiconductor device, if desired.
Disposed over the top surface of the elastomer 200 layer is
a plastic pellicle layer 500. At least one, and pre~erably
two conductive metal layers are disposed over the pellicle
layer 500. In this embodiment these layers are a gold layer
310 which is covered by a silver layer 320.
In the present embodiment, the elastomer layer 200 is a
silicone gel layer, for example of the type sold by Dow
Corp. or General Electric Corp. under the trade names Dow
Gel 3-6527 or G.E. RTV-6157, respectively. The silicone gel
layer 200, can be applied by dipping of the semiconductor
device into silicone gel and then drawing it out.
Alt~rnatively, the silicone gel can be applied to the
surface of the semiconductor device and then a doctor blade
used to obtain a uniform surface, or the material can be
applied and then distribu-~ed by spinning the device.
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It is important that the elastomer layer be neither too
limber nor -too stiff. I-t is important that the elastomer
layer be neither too limber nor too stiff. The compliance
of the gels are normally measured with a universal
penetrometer. This has a shaft of 6.35 mm diameter and a
weight of 19.5 gms. The penetration is measured in
millimeters. These gels have a penetration tha-t ranges from
3mm to 30mm. The thickness of the elastomer layer affects
the sensitivity of the solid state light modulator, since it
determines the distance (and therefore the force for a given
electrical charge) between the display electrodes 106 and
the common electrode (310 and 320 in FIG. 2) and is also
determinative of the volume of deformable material on which
the common electrode "floats". Applicant has discovered
that the thic~ness of -the elastomer layer should be in the
range between one-tenth and one times the average center-to-
center spacing between adjacent active charge storage
electrodes 106, and preferably about one-half such spacing.
For an average electrode spacing of about 20 microns, the
elastomer layer thickness should be between 2 and 20
microns, and preferably around 10 microns.
In the present embodiment, the pellicle layer 500 is
formed separately by dissolving a cellulose nitrate material
in a solvent, such as amylacetate and ethyl acetate, and
then dropping the solution on water. After drying, the
pellicle can be laid over the elastomer layer 200. The
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pellicle serves two purposes. First, applican-t has found
that residual oils and/or other constituents invariably
present in the silicone gel material render it difficult to
apply an appropriate thin uniform layer of conductive metal,
such as gold, directly on top of the elastomer layer.
Second, after application of the metal layer, the pellicle
serves to isolate the metal layer from components of the
elastomer that could attack and degrade the metal layer or
layers disposed thereon. The pellicle layer should be
suf~iciently thin so as not to add undue stiffness to the
light modulator structure, a thickness of less than about
two microns being preferred.
In the present embodiment, a gold layer 310 is applied
over the pellicle layer by evaporation, such as by placing
the structure in a vacuum evaporation chamber and heating a
gold source with a filament to obtain a layer of suitable
thickness. A silver layer is then applied in the same way.
The thicknesses of the gold and silver layers are preferably
in the ranges 50 to 100 Angstroms and 100 to 1000 Angstroms,
respectively. While a single layer could be used as the
common electrode and reflective layer (e.g. layer 300 in
FIG. 1), the silver layer does not evaporate directly onto
the plastic pellicle layer with sufficient uniformity. The
initial evaporation of gold provides an excellent base layer
upon which a uniform and highly reflective layer of silver
can be evaporated. It will be understood that alternative
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techniques of applying the one or more conductive layers can
be employed, for exampIe sputtering or electroless
deposition.
Referring to FIG. 3, there is shown a diagram of the
method steps for making the solid state light modulator in
accordance with the described embodiment. The block 611
represents the formation of the semiconductor device having
an array of charge storage electrodes on a surface thereof.
The block 612 represents the application of the elastomer
layer to the semiconductor surface, and the block 613
represents the application of the pellicle layer, as
described. The blocks 614 and 615 -then represent the
application of the conductive and reflective layers 310 and
320, as described.
