U.S. patent number 3,886,310 [Application Number 05/390,470] was granted by the patent office on 1975-05-27 for electrostatically deflectable light valve with improved diffraction properties.
This patent grant is currently assigned to Westinghouse Electric Corp.. Invention is credited to Jens Guldberg, Harvey C. Nathanson.
United States Patent |
3,886,310 |
Guldberg , et al. |
May 27, 1975 |
Electrostatically deflectable light valve with improved diffraction
properties
Abstract
An electrostatically deflectable light valve adapted for use in
an array for producing television pictures as a projected image
upon a large display screen. The light valve structure is such that
a plurality of reflective wing portions are free to be deflected
along directional axes which are at an angle to the prime
directional axes of the overall array, so that light which is
predominantly diffracted along the array axes may be decoupled or
separated from the signal light produced by activated light valves
and used to project the image. The contrast ratio of signal light
to background light for the system is significantly improved, using
this method of discrimination.
Inventors: |
Guldberg; Jens (Pittsburgh,
PA), Nathanson; Harvey C. (Pittsburgh, PA) |
Assignee: |
Westinghouse Electric Corp.
(Pittsburgh, PA)
|
Family
ID: |
23542587 |
Appl.
No.: |
05/390,470 |
Filed: |
August 22, 1973 |
Current U.S.
Class: |
348/771;
348/E5.14; 313/465; 315/372; 359/291; 315/374 |
Current CPC
Class: |
H01J
29/12 (20130101); H04N 5/7425 (20130101); G09F
9/372 (20130101); G02B 26/0841 (20130101) |
Current International
Class: |
G09F
9/37 (20060101); H01J 29/10 (20060101); H01J
29/12 (20060101); G02B 26/08 (20060101); H04N
5/74 (20060101); H04n 003/16 (); H01j 029/12 ();
G02f 001/28 () |
Field of
Search: |
;315/21R ;313/91,465
;178/7.5D,5.4BD ;350/161 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Wilbur; Maynard R.
Assistant Examiner: Montone; G. E.
Attorney, Agent or Firm: Sutcliff; W. G.
Claims
1. An electrostatically deflectable light valve system comprising
an array of spaced apart, deformable, light reflective elements
supported upon a substrate, each of said reflective elements
comprising a central core portion supported by a centrally located
post member which extends from one side of the reflective elements
to the supporting substrate, a plurality of generally planar,
symmetrically spaced apart and shaped independently deformable and
reflective wing portions extending outwardly from the central core
portion, which wing portions are defined by an equal plurality of
thin slits provided between said wing portions which slits extend
from the central core portion to the outer edge of the wing
2. The system specified in claim 1, wherein an electrode grid is
disposed
3. The system specified in claim 1, wherein the substrate is a
light
4. The system specified in claim 1 wherein four generally square
wing
5. The system specifified in claim 4, wherein the slits are aligned
with
6. The system specified in claim 5 when the planar portion of the
light valve is substantially square and the slits extend from the
edges of each
7. The system specified in claim 1, wherein a light reflective
coating is provided on the top surface of the central core portion
and the wing
8. The system specified in claim 3, in combination with a light
source, an optical system for directing light through the
transmissive substrate to the surface of the reflective light
valves, said optical system including an opaque stop disposed in
the optical path between the array of light valves and a display
screen, so that when the wing portions of the light valves are
nondeflected the light reflected is substantially totally reflected
off or blocked by the stop and no light reaches the display screen,
while when the wing portions are deflected a portion of the light
reflected therefrom passes around the central stop and produces a
large,
9. A light reflective element adapted for use as an
electrostatically deflectable light valve which element
comprises;
a central core portion supported by a post member which extends
from one side of the core portion, and
a plurality of generally planar, symmetrically spaced and shaped
independently deformable reflective wing portions extending
outwardly from the central core portion, which wing portions are
defined by an equal plurality of thin slits provided between said
wing portions which slits extend from the central core portion to
the outer edge of the wing
10. The light reflective element specified in claim 9, with four
generally square wing portions provided, with thin slits separating
the wing
11. An electrostatically deflectable light valve system comprising
an array of spaced apart, deformable light reflective elements
supported upon a substrate, the elements being arrayed along X and
Y orthogonal axes, each of said reflective elements comprising a
central core portion supported by a centrally located post member
which extends from one side of the reflective elements to the
supporting substrate, and four spaced apart, generally planar,
symmetrically spaced and shaped deformable and reflective wing
portions extending outwardly from a common central core portion,
which four wing portions are spaced apart by slits extending in the
direction of the X and Y axes of the array pattern.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to electrostatically deflectable light
valves which are adpated for use with a cathode ray tube, and in
conjunction with schlieren optics forming a system for projecting
television images upon a large display screen, and which may be
operated either as a real-time or a storage mode device.
2. Description of the Prior Art
A considerable effort has been expended in researching and
developing systems for projecting television images upon a large
screen. Such systems are aimed at expanding the usage of closed
circuit television entertainment systems, as well as simply
providing a greatly increased display area with its obvious
advantages. The present commercial systems utilize an oil film
surface as the target of a cathode ray tube, with the electron beam
being used to produce a diffraction pattern on the film. An
external light source is directed upon the film and an optical
system with schlieren bars is utilized to project the desired image
upon the display screen correspondence to the informational pattern
upon the film. This oil film system is expensive and suffers from
the inherent problem of having a fluid film operate within an
evacuated cathode ray tube.
A variety of other display systems have tried to circumvent the
problems associated with the use of oil films. Substitution of
elastomer layers for the oil film results in very small deflections
and consequently requires a sophisticated and expensive optical
projection system. Techniques utilizing electro-optical materials,
primarily KD.sub.2 PO.sub.4 crystals, suffer from a basic problem
of incompatibility in using the material in a vacuum. Another
approach uses a taut metal membrane suspended on thin metal ribs
and segmented to form an array of picture elements addressable by
an electron beam. Energetic electrons of about 20 kilovolt
potential penetrate the film and deposit a charge on the
transparent glass substrate. The resulting electrostatic forces
will deflect the metal membrane and the image is read out with
ordinary schlieren optics.
Another feasible system utilizes an array of electrostatically
deflectable light valves or very small mirrors, which are
deflectable corresponding to the informational pattern. Such a
system is disclosed in U.S. Pat. No. 3,746,911. In such systems an
array of the deflectable light valves form the target of a cathode
ray tube. An external light source is directed onto this target
which is modulated to deflect individual valves of the array in an
informational pattern. The light is reflected from the light
valves, and for deflected valves the light is stopped while for
deflected valves the light passes the schlieren stop and is
projected upon the display screen with suitable magnification.
A problem with these prior art television image projection systems
is the poor screen contrast caused by optical diffraction effects
related to the target array. In general, such arrays have rows and
columns of light valves which define an array having an X and a Y
axes. The electrostatic deflection of each light valve modifies the
diffraction pattern of the reflected light by tilting of the
reflective plane as well as bending or bowing of the reflective
surface. Because of constructional constraints the deflection of
the light valves is also along one of these major axes. This
diffraction effect causes light to pass about the schlieren optical
stop primarily along one of the major axes of the array.
Consequently, a high contrast ratio with low background
illumination can only be achieved with extremely large deflection
angles.
This same problem exists for arrays other than rectangular arrays
and for light valves other than simple square elements.
SUMMARY OF THE INVENTION
A reflective light valve element is provided having a structure
which permits the elimination of transmission of background light
to the screen to provide significantly improved contrast for the
display. The light reflective element is adapted for use in an
array as part of a projection system for displaying television
images. The light reflective element comprises a central core
portion supported by a central post member which extends from the
underside of the core portion. A plurality of symmetrically spaced
and shaped deformable reflective wing portions extend outwardly
from the central core portion. The extending ends of the wing
portions have slits therebetween. When such reflective elements are
arrayed as a target, the slits are aligned with the predominant
axes of the array. The wing portions are then free to bend or
deflect along an axis which is at an angle to the predominant axes
of the array. A method of providing this improved light valve
structure is also detailed.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic overall view of a projection system of the
present invention.
FIG. 2 is an enlarged plan view of a single light valve of the
present invention.
FIG. 3 is a sectional view taken along line III--III of FIG. 2 of
the light valve.
FIG. 4 illustrates the various stages of preparation of the light
valve of the present invention.
FIG. 5 illustrates the various stages of another method of
construction of the present light valve.
FIG. 6 is a perspective view of the schlieren stop used in the
projection system of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention can be best understood by reference to the
exemplary embodiment shown in the drawings.
The projection system comprises an electron beam tube 10. A high
intensity light source 12 of preferably well balanced light is
provided for illumination. The light from source 12 is focused by a
lens 14 and refracted by a 45.degree. angle reflective schlieren
mirror 15. The reflected light is collimated by lens 18 onto target
20 associated with the faceplate of cathode ray tube 10. In the
absence of actuation or deformation of individual light valve
elements 22, the light will be reflected back from element 22 to be
focused on the schlieren mirror 15 and remain within the original
light cone. Deflection or deformation of a light valve element due
to electrostatic forces as will be explained, will result in the
light passing the schlieren mirror 15 and being projected through
lens system 24 to the enlarged display screen 26. In this manner, a
light image will be produced upon the screen 26, which corresponds
to the informational pattern established upon target 20 by
deformation of the individual light valve elements 22. The
deformation of the elements 22 corresponds to the applied video
signal.
The schlieren mirror or stop 15 is shown in an enlarged view in
FIG. 6, and comprises a generally square shaped mirror member 16
and rod support means 17. The mirror member is inclined at the
proper angle to direct the light onto the light valve target. The
support means 17 here comprise opaque rods which facilitate support
of the mirror member 16, and extend from the ends of each side of
mirror member 16 along the major X, Y axes corresponding to the X,
Y directional axes of the mirror array. The opaque rods 17 then
serve to block scattered light which can be expected along these
axes, which light represents a background level that substantially
degrades the display contrast.
The electron beam tube 10 comprises an outer envelope 30 having a
tubular body portion 32, and a base portion 34. The base portion 34
is provided with lead-ins 36 for applying potential to the
operative electrodes. The faceplate portion 38 is sealed to the
opposite end of the body portion 32. The target 20 is disposed on
the interior surface of the faceplate 38. An internal focusing
electrode 27 is provided within envelope 30, and external focusing
and deflection means 28 are disposed about envelope 30.
The target structure 20 comprises a plurality of light valve
elements 22, such as seen in greater detail in FIGS. 2 and 3. The
light valve elements 22 form an array, which is typically rows and
columns of identical elements 22, with the total array including
typically hundreds of thousands of the very small elements 22,
which have a dimension of about 25-70 microns on a side. The
elements 22 seen in FIG. 1, are thus shown greatly enlarged in a
schematic sense to facilitate understanding of the device.
The light valve elements 22, seen in greater detail in FIGS. 2 and
3 comprise a generally planar reflective portion 40, which
comprises a central core portion 42, and a plurality of
symmetrically spaced and shaped, deformable, reflective wing
portions 44, which extend outwardly from the central core portion
42. An opening or slit 46 separates the wing portions 44. In this
embodiment, the portion 40 is generally of square configuration,
and the four slits 46, define four wings or quadrants. The slits 46
extend in the direction of the axes of the array of elements 22,
which for this embodiment would consist of horizontal rows and
vertical columns. The elements 22 are supported upon a light
transmissive substrate 38, which serves as the faceplate of the
cathode ray tube 10. The substrate 38 may be formed of a vitreous
material such as quartz, sapphire or spinel. A support or spacer
post member 48 extends from the substrate to the underside of the
central core portion 42. The post member 48 is typically a
semi-insulator such as silicon, but can also be a conductor or
insulator, and has a cross-sectional dimension of less than about 5
microns on a side, and a height of about 1.5 to 10 microns. The
reflective array elements 22 are typically comprised of an
electrical insulator, such as silicon dioxide, with a thin
reflective layer 52 of metal, such as aluminum thereon. The
material can also be metal or semiinsulating. The thickness of the
planar portion 40 of element 22 is about 1,000-5000 Angstroms.
An electrically conductive grid 50 is provided on the interior
surface of substrate 38 running between the spaced apart light
valve elements 22. The grid 50 is typically formed of a thin metal
film of suitable material, such as gold or aluminum, which is
preferably thin enough to be light transmissive. The grid is
connected to an external potential source 54. The potential source
54 may be a video signal source when the grid 50 is utilized to
modulate the target voltage with the electron beam merely being a
flood beam which charges the individual planar portion 40 of the
light valves 22 to or near equilibrium with grid 62 which is
typically located in close proximity to the target array. The grid
62 is connected to potential source 64.
The electron beam may also be modulated, with the video signal
applied via grid 56 proximate the cathode or beam source 60. A
fixed bias would then be applied to electrode 27, grid 62, and grid
50. In this case the grid 50 on the target 20 merely serves as a
reference electrode necessary to produce electrostatic deflection
of the planar portion 40 of light valves 22. The electrostatic bias
is determined by the amount of charge deposited by the beam in
accordance with the instantaneous value of the video signal.
The array of light valve elements 22 may be fabricated according to
the method outlined in the aforementioned U.S. Pat. No. 3,746,911,
but with the modification as outlined in FIG. 4. The substrate 38
has a heteroepitaxially grown layer of silicon 68 which is oxidized
to a thickness of 3,000-8,000 Angstrom to provide silicon dioxide
layer 70 thereon. The oxide layer is delineated by a photoresist
process to define the generally square configuration of element 22,
and the slits 46 within elements 22 as seen in FIG. 4C. A
reoxidation is carried out to a thickness of 2,000-4,000 Angstroms
to partially close the slits and the spacing between elements 22 as
seen in FIG. 4D. The oxide between elements 22 is then removed by
another photoresist operation which does not effect the oxide at
the central core of the elements 22 or of the slit areas as seen in
FIG. 4E. The silicon dioxide is then undercut by etching with a
solution of nitric, acetic, and hydrofluoric acid in a ratio of
approximately 25/10/1. A slight oxide etch follows to ensure that
any oxide between the slits is removed to provide the structure of
FIG. 4F. The reflective metal layer 52 is then deposited upon the
planar portions 40, and also upon substrate 38 to form grid 50 as
seen in FIG. 4G.
The above described process requires accurate successive alignment
of the exposure mask in first delineating the elements, and then
redelineating them following the reoxidation process, which ensures
proper junction of the central post 48 and the planar portion 40.
The fabrication can also be arranged with other materials and
deposition materials, and the material temporarily closing the
slits does not have to coincide with the material chosen for the
planar portion 40.
Another process which obviates this realignment problem is to use a
self-alignment technique which is generally depicted in FIG. 5. The
substrate 38 has the silicon layer 68 thereon.
Through a process of successive depositions and appropriate
photolithographic operations a mask matrix can be built upon the
silicon 40. The mask comprises a thin silicon dioxide layer 72, a
silicon nitride layer 74 thereon, and a top thin layer 76 of
silicon dioxide. These three mask layers are typically vapor
deposited over the substrate and the silicon. The mask is
photolithographically delineated, removing selected portions of the
three layers to provide the matrix as seen in FIG. 5A. The silicon
layer is then thermally oxidized to provide the planar portions 40
for light valve 22 as seen in FIG. 5B. The remaining mask portions
can then be etched away, and the silicon dioxide undercut to the
final shape and to form the support post as seen in FIGS. 5C and
5D. The slits are opened by a separate etch, which is followed by
metallization of the surface of light valve 22 to ensure high
reflectivity.
In the self-aligning mask technique the silicon dioxide layer 72,
silicon nitride layer 74, and silicon dioxide layer 76, have
typical layer thicknesses of 200-500 Angstroms, 1,000-3,000
Angstroms, and 1,000-3,000 Angstroms, respectively. The etching is
carried out with conventional solutions. The generally planar
silicon dioxide portion 40 is produced by thermal oxidation to a
thickness of for example 3,000-10,000 Angstroms. This thermally
grown layer 40 is only produced in windows in the nitride mask. In
the presence of the thin intermediate silicon dioxide layer 72 the
edges of portions 40 will grow and merge in the slit area 46 as
seen in FIG. 5B. After removal of the silicon nitride layer 74, the
structure can be further etched to open the slit areas 46, while
also forming the post 48 from layer 68. In general the slit widths
should be less than 2.mu., while the grids between light valve
elements is larger than 2-3.mu. for this technique to work.
* * * * *