Mirror array light valve

A mirror array light valve is described comprising a transparent substrate, a plurality of post members arranged in a regular array on said substrate, and a plurality of deflectable square, rectangular, hexagonal or the like light-reflecting elements arranged in a regular array on said post members such that a post member is positioned under a corresponding corner of each element; methods for making the mirror array light valve are also described.

BACKGROUND OF THE INVENTION 1. Field of the Invention 
This invention relates to a mirror array light valve adapted for use with 
e.g., a cathode-ray tube in conjunction with Schlieren optics forming a 
system for projecting images upon a display screen. 2. Description of the 
Prior Art 
In forming an image pattern corresponding to a cathode-ray pattern for 
projection, several techniques are employed. One common technique employs 
a fluorescent screen containing phosphors which are excited at high energy 
levels in order to produce an image on the screen. Such a system is 
employed in commercial television. However, the area of utility for such 
fluorescent screens is limited, and as the area over which it is desired 
to project a display becomes larger, other systems must be used. 
One such system is the oil film eidophor system, in which an external light 
source is spatially modulated by an oil film the surface of which is 
rippled by an electron beam in the cathode-ray tube. The oil eidophor 
system is very complex, expensive, and subject to cathode deterioration 
due to the presence of the oil film in the vacuum tube. 
An alternative to the eidophor system is to employ an array of deflectable 
mirror elements in conjunction with an external light source; the mirror 
elements are deflectable in response to the cathode-ray signal. Such 
systems are described, for example, in U.S. Pat. No. 2,681,380, issued 
June 15, 1954, and in IBM Technical Disclosure Bulletin, Volume 13, number 
8, August 1970, pages 603-604, both of which describe rectangular 
edge-mounted mirror elements. 
More recently, U.S. Pat. No. 3,746,911, issued July 17, 1973 describes an 
electrostatically deflectable light valve for use in a large area 
projection display in which the elements of the array are comprised of a 
centrally-located post supporting a reflective element, and U.S. Pat. No. 
3,886,310, issued May 27, 1975 describes similar electrostatically 
deflectable light valves in which each element is divided into four wing 
portions which are oriented so as to be deflectable in four different 
directions, and which thereby reflect external light to four separate 
quadrants. 
SUMMARY OF THE INVENTION 
The present invention relates to a novel mirror array light valve 
comprising (a) a transparent substrate, (b) a plurality of post members 
arranged in a regular array on said substrate, and (c) a plurality of 
deflectable square, rectangular, hexagonal or the like light-reflecting 
elements arranged in a regular array on said post members such that a post 
member is positioned under a corresponding corner of each element. The 
invention also relates to methods for making such a mirror array light 
valve. 
A mirror array light valve according to the present invention provides 
improved image resolution and contrast compared with prior art systems. 
The foregoing and other objects, features and advantages of the invention 
will be apparent from the following more particular description of 
preferred embodiments of the invention, as illustrated in the accompanying 
drawings.

DETAILED DESCRIPTION 
According to the present invention, the mirror array light valve includes a 
plurality of deflectable square, rectangular, hexagonal or similar 
generally symmetrical, tightly packable light reflecting elements arranged 
in a regular array, with each light reflecting element being supported on 
a post member, and with the post members being positioned under a 
corresponding corner of each element. Thus the light reflecting elements 
are essentially cantilever mounted, resulting in favorable deflection 
properties for the elements, due to the restricted area occupied by the 
post member and its location under a corner of the element. 
The square shape is preferred for the light reflecting elements in view of 
human factor requirements and because the square shape is well adapted to 
provision of a dense rectilinear array for ease of addressing via a raster 
system, but the rectangular and other shapes are potentially useful. 
By the positioning of the supporting post members under corresponding 
corners of each element, each of the light reflecting elements is rendered 
deflectable in the same direction, so that the reflected light is directed 
to a single quadrant. By use of Schlieren optics, light can be blocked in 
three quadrants, to permit transmission only of the light shifted by the 
deflection of the light reflecting elements in the mirror array light 
valve of the invention through the fourth quadrant, thereby significantly 
reducing background light in the display. 
The mirror array light valve of the invention can be prepared by several 
methods. 
In general, first a silicon surface layer is formed on a substrate 
comprising a transparent vitrous material, such as quartz, sapphire, or 
spinel. The silicon layer may be an epitaxial, polycrystalline, or 
amorphous layer formed by chemical vapor deposition of silicon. 
By a sequence of treatment steps including resist masking, etching, 
chemical conversion, and metallization a regular array of elements, 
supported by corresponding corner-position posts, in the desired pattern, 
e.g., square, is obtained, as is described in detail below. 
FIG. 1 shows a preferred method according to the invention. As illustrated 
in Step (1) of FIG. 1, a layer of etchable silicon 11, e.g., single 
crystal silicon, is applied to a transparent substrate 10, e.g., sapphire 
substantially free of optical distortion, by, e.g., chemical vapor 
deposition. 
Then, in Step (2) an n-dopant diffusion barrier layer 13 is formed on the 
silicon layer 11. By an n-dopant is meant an element which, in its 
electronically neutral state, has one more electron in its highest 
occupied orbital than does the element being doped. With respect to 
silicon, the n-dopants are phosphorous, antimony, and arsenic. The 
n-dopant diffusion barrier layer may be thermally grown silicon dioxide, 
or vapor deposited silicon dioxide or silicon nitride for example. 
The barrier layer is then masked with a first resist pattern 15 so that no 
resist is present in areas 16 corresponding to the desired pattern of the 
plurality of post members. This is shown in Step (3). 
Then, in Step (4), the portions of the barrier layer 13 underlying the 
areas 16 and thus unprotected by the resist pattern 15, i.e., areas 
corresponding to the desired post member locations, are removed, e.g., by 
reactive ion etching. This yields an n-dopant diffusion barrier layer 
having openings corresponding to the locations at which it is desired to 
form the plurality of post members. Following this, the resist pattern 15 
is removed. 
Then, in Step (5), the silicon layer 11 portions unprotected by the barrier 
layer 13 are etched through to the substrate layer 10 using reactive ion 
etching to obtain straight sided holes 17 perpendicular to the substrate 
10. 
In Step (6), an n-dopant, which is preferably phosphorous, is diffused 
laterally into the silicon layer at the post locations where the silicon 
layer is not protected by the n-dopant diffusion barrier and where it is 
desired to form the plurality of post members. By this step, cylindrical 
layers 18 of n.sup.+ -doped silicon are formed in the silicon body 11, in 
the portions thereof defining the side walls of the holes 17. 
The n-dopant diffusion mask 13 is then removed, Step (7). In Step (8), 
thermal oxide having tubular and flat portions 19p and 19e is grown on the 
silicon layer 11 at a temperature where the oxidation rate of the n.sup.+ 
-doped silicon is approximately three times that of the undoped silicon. 
This temperature is preferably about 800.degree. C. This step results in 
slight narrowing of the holes 17 but, more importantly, a relatively thick 
cylindrical or tubular post structure 19p of oxide in and at the walls of 
the holes 17. The posts 19p are continuous with thinner oxide flat 
portions 19e and form the support for the same in the final structure. 
Step (9) is shown in both plan and section (9a) diagrams. In this step, a 
second resist pattern with the resist areas defining an array of discrete 
areas 20 arranged such that a corresponding corner of each area is 
positioned over a location of the silicon layer containing the thermal 
silicon dioxide cylindrical post 19p previously formed by oxidation of the 
n.sup.+ -doped silicon. The areas 20 shown have the preferred square shape 
and orthogonal grid arrangement. 
Then the thermal silicon dioxide surface layer 13 is (preferably reactive 
ion) etched to form a plurality of elements 21 therein corresponding to 
the mask areas 20, arranged in a regular array with a silicon dioxide post 
19p under a corresponding corner of each of the silicon dioxide elements. 
The resist is then removed and the silicon is etched away using an etchant 
which attacks the silicon but not the silicon dioxide to leave the desired 
structure, Step (10). A suitable etchant is pyrocatechol ethylenediamine, 
for example. 
Lastly, as shown in Step (11), a thin film of a reflective metal, 
preferably aluminum, is deposited onto the array to form a plurality of 
electrically isolated, individually electrostatically deflectable light 
reflecting elements 23, with a conductive counter-electrode grid 24 of the 
reflective metal being formed on the substrate surface corresponding to 
the interstices between the light reflecting elements. Preferably this 
metallization step is by vacuum deposition. The metal should be as thin as 
possible, consistent with the desired reflectivity and conductivity, 
typically about 300 .ANG.. Some of the metal is deposited within the posts 
as well, but this is of no significant consequence. As set forth below, 
the Schlieren stop will intercept most of the scattered light resulting 
from the presence of such structures. 
FIG. 2 shows the resulting light valve structure, including the elements 
23, the grid 24, and the posts 19p on the substrate 10. In the 
configuration shown in FIG. 2, the cylindrical posts are hollow and of 
generally triangular cross-section. They could be solid cylinders as set 
forth below. In either case, the illustrated triangular shape facilitates 
placement of the posts near the corner of corresponding element 23. The 
showing of FIG. 2 is more representative of desirable geometry, while the 
showings of FIG. 1 and FIG. 3 are distorted to illustrate the various 
process steps clearly. 
Because the cylindrical posts 19p are straight-sided and perpendicular to 
the substrate, they do not interfere with the deflection of adjacent 
elements 23 or the formation of the counter-electrode grid 24. Moreover, 
as will be set forth hereinafter, a preferred optical arrangement for 
utilization of the light valve structure involves a light path through the 
substrate 10 and the valve element components 19e to the underside of the 
reflective elements 23. In such case it is an advantage that the posts are 
shaped and disposed to allow light applied to the elements through the 
substrate to be reflected from substantially the entire available 
reflective area of each element 23. 
The straight-sided configuration results from the fact that the posts are 
formed by wall modification, from the inside outwardly into the mass of 
undoped silicon; see Step (6). This results in hollow or tubular 
cylindrical posts. If desired, hole dimensions and oxide growth duration 
may be made such that the cylinders are vertically filled, or the 
apertures therein can be filled by appropriate masking and deposition 
steps with another substance such as chemically vapor deposited silicon 
dioxide, in either case resulting in solid cylindrical posts. 
An alternative method of producing the valve structure is to use a p-dopant 
such as boron instead of phosphorous to produce p.sup.+ -doped sidewalls, 
and perform the oxidation at 1000.degree. C. FIG. 3 shows the result. An 
anisotropic etchant such as pyrocatechol ethylenediamine (R. M. Finne and 
D. L. Klein, J. Electrochem. Soc. 114:965 (1965)) will preferentially 
dissolve the undoped silicon (not shown) away from the volume 28 under the 
silicon dioxide valve element 19e' and up to the p.sup.+ -doped silicon 
19p". The final structure, which is mechanically similar and optically 
equivalent to that previously described, will be produced with a post 19p' 
and a silicon dioxide deformable member 19e'. The post 19p' will consist 
essentially of p.sup.+ -doped silicon 19p" coated with thin oxide, 
continuous with element 19e', in its hollow portion. 
FIG. 4 shows the use of the light valve structure in a display. A cathode 
ray tube 30 has deflection means 31 whereby electron beam 32 is directed 
to various locations on light valve array 34. Typically, this is done in 
an overall pattern such as a raster pattern. Concurrently, beam current is 
modulated by control grid 36, with the result that charges of various 
amplitudes are placed on the individual array elements. Light passes from 
the light source, through the Schlieren lens and the face plate of tube 30 
to the light valve structure 34. Light reflected from valve structure 34 
passes through the projection lens and the aperture 40 of the Schlieren 
stop 42 to the projection screen; as in conventional Schlieren systems, 
the stop is located in the focal plane of the projection lens. 
However, in the system of the present invention, the stop has a specially 
shaped aperture to cooperate with the light valve structure of the 
invention. The placement of the post at the same corner of each and every 
deflectable light valve element 23 causes light reflected from the 
deflected elements to be directed toward the same quadrant 44 of the 
Schlieren stop 42. The stop aperture 40 is located in that quadrant. The 
stop 42 receives the diffraction pattern caused by the various reflecting 
structures of the light valve. Light from the undeflected elements 23 and 
the fixed structures of the light valve, such as the grid 24, are imaged 
onto the stop in all four quadrants. 
By transmitting the light from the deflected elements 23 through only one 
quadrant, that having the aperture 40, and blocking all other light that 
is incident on the other quadrants of the stop 42, the contrast ratio is 
enhanced over that achievable with the conventional stop. Such a 
conventional stop blocks primarily only the diffraction pattern caused by 
the undeflected elements. Light diffracted by the finer structures such as 
the grid passes such conventional stops substantially unblocked. 
While the invention has been particularly shown and described with 
reference to preferred embodiments thereof, it will be understood by those 
skilled in the art that various changes in form and detail may be made 
therein without departure from the spirit and scope of the invention.