Active matrix color display with multiple cells and connection through substrate

A color active matrix display system including a plurality of active matrix arrays that are stacked around a liquid crystal material, vertically aligned and interconnected to provide a high resolution color display system.

BACKGROUND OF THE INVENTION 
Flat-panel displays are being developed which utilize liquid crystals to 
produce high quality images. These displays are expected to supplant 
cathode ray tube (CRT) technology and provide a more highly defined 
television picture. The most promising route to large scale high quality 
liquid crystal displays (LCDs), for example, is the active-matrix approach 
in which thin-film transistors (TFTs) are co-located with LCD pixels. The 
primary advantage of the active matrix approach using TFTs is the 
elimination of cross-talk between pixels, and the excellent grey scale 
that can be attained with TFT-compatible LCDs. 
Flat panel displays employing LCDs generally include five different layers: 
a white light source, a first polarizing filter that is mounted on one 
side of a circuit panel on which the TFTs are arrayed to form pixels, a 
filter plate containing at least three primary colors arranged into 
pixels, and finally a second polarizing filter. A volume between the 
circuit panel and the filter plate is filled with a liquid crystal 
material. This material will rotate the polarization of light when an 
electric field is applied across it between the circuit panel and a ground 
affixed to the filter plate. Thus, when a particular pixel of the display 
is turned on, the liquid crystal material rotates polarized light being 
transmitted through the material so that it will pass through the second 
polarizing filter. 
SUMMARY OF THE INVENTION 
The present invention relates to the use of stacked active matrix arrays 
utilizing thin film transistors in a single crystal material to provide 
improved transmission color display panels. A preferred embodiment of the 
invention includes three active matrix arrays where each array controls 
transmission of light through a separate layer of liquid crystal material. 
The three layers or cells are vertically registered to form a subtractive 
color display. Drive circuitry for each of the three active matrix arrays 
can be formed in the thin film of single crystal material in which the 
TFTs are formed. Each array and its drive circuitry is transferred onto a 
glass substrate during fabrication. The drive circuitry on each layer can 
be electrically or optically interconnected with the other layers of the 
display. A plurality of such displays can be fabricated and can be 
optionally separated to form separate displays. 
Another preferred embodiment utilized an active matrix formed on two sides 
of a single substrate to control two layers of liquid crystal material. 
The drive circuitry for both layers can be interconnected through the 
substrate. The use of two active matrix arrays mounted on opposite sides 
of a single substrate can also be stacked with a third active matrix array 
to provide a three cell structure. 
Another preferred embodiment includes two active matrix arrays mounted on 
opposite sides of a single layer of liquid crystal material so that both 
arrays can be used to control light transmission through the layer. Each 
pixel electrode on one side of the liquid crystal material has a pixel 
electrode aligned on the opposite side of the liquid crystal material. 
This permits control of the electric field along three different axes. 
This also permits control of the field strength to adjust greyscale and 
field direction. By controlling the electric field in three directions, 
the electric field can be used to select the color.

DETAILED DESCRIPTION 
A preferred embodiment of the invention is illustrated in the 
cross-sectional view of FIG. 1 of a substractive color display 10. The 
display 10 includes three levels 15, 17, 19 or cells, where each cell has 
an active matrix 36, 38, 40 and a liquid crystal material 30, 32, 34, 
respectively. 
The active matrix for each level operates in the manner described in 
greater detail in U.S. Pat. application Ser. No. 07/872,297 filed on Apr. 
22, 1992. The three liquid crystal levels 30, 32, 34 where each contain a 
dye. Cell 15, for example, can contain a magenta dye, cell 17 can contain 
a cyan dye and cell 19 can contain a yellow dye. Other colors can also be 
used to provide a substractive color system. By stacking the three levels 
in serial registration, the cells form a subtractive full color active 
matrix display in which single crystal or essentially single crystal 
layers of silicon are used in each active matrix area to provide the pixel 
and circuits and electrodes of each pixel element of the display. The 
peripheral regions 12, 14, 16 of each single crystal layer, in which the 
active matrix of thin film transistors is also formed, contains CMOS 
driver circuitry to drive the rows and columns of each active matrix. 
The upper cell 15 has liquid crystal material 30 sandwiched between upper 
transmissive superstrate 24 and substrate 18. The middle cell 17 has upper 
superstrate 26 and substrate 20 and the lower cell 19 has superstrate 28 
and substrate 22. A broad band light source directs light 42 through 
substrate 22. Each cell, in selective combination with the other cells can 
remove selected spectral bands of light from incident light 42 to produce 
a selected image exiting the display at superstrate 24. Polarizing plates 
(not shown) are inserted into the stack of cells to define the 
polarization of light being transmitted through display 10. 
Another preferred embodiment is illustrated in FIG. 2 where a three layer 
stack of cells is formed. Three layers of liquid crystal material 60, 62, 
and 64 are positioned adjacent to three active matrix arrays 66, 68, and 
70, respectively. An adhesive can be used to join the three cells at 82, 
84 and 86, respectively. 
Electrical and/or optical interconnects can be used in drive circuit layer 
elements 90, 92 and 94 formed on both sides of the display. For example, 
electrical interconnect 78 serves to electrically connect circuit 90 and 
92. Electrical interconnect 80 is used to electrically connect circuit 
elements 92 and 94. Additionally, light emitting and detector elements 72, 
74 and 76 can be formed in or on circuits 90, 92 and 94, respectively. The 
light emitting components can be LED's, lasers, or electroluminescent 
elements for example. The light from elements 72, 74 and 76 can be 
conducted through substrates 54, 56 and adhesive layers 82, 84 and 86. 
Optical waveguides such as optical fibers 88 or fiber optic plates can 
also be used. 
A further embodiment is shown in FIG. 2A where two active matrix arrays 95, 
97 are formed on opposite side of substrate 83. The matrix arrays 95, 97 
can be formed along with peripheral drive circuitry 93 and 91, 
respectively, in thin films of single crystal silicon and transferred onto 
opposite sides of substrate 83 using techniques described in the above 
referenced related applications. 
Optically transmissive superstrates 81 and 85 can then be secured by an 
adhesive to substrate 83 to form cavities in which a liquid crystal 
material 99 can be inserted. The material 99 in each cavity can contain 
different colors to provide a multicolor display. 
A third active matrix can be aligned over superstrate 81 to provide a third 
cavity in which a third color liquid crystal material can be inserted to 
provide a full color display. 
A process sequence is shown in FIGS. 3A-3E for forming a plurality of 
subtractive color displays on a single semiconductor wafer or substrate. 
A single crystal silicon substrate 100 is provided as shown in FIG. 3A on 
which an insulating layer 102, such as an oxide, is formed. A single 
crystal silicon layer 104 can be formed over the oxide 102 by several 
known processes including recrystallization of an amorphous or 
polycrystalline silicon layer. 
Active matrix elements 106, 108 and 110 are formed in layer 104 as shown in 
FIG. 3B. Drive circuitry (not shown) can also be formed in layer 104 for 
each active matrix of each display as described in connection with FIG. 
2A. A second level of active matrix elements 120, 122 and 124 are formed, 
and transferred onto optically transmissive substrates 112, 114 and 116 
which are aligned over matrix elements 106, 108 and 110. A third level of 
active matrix elements 136, 138 and 140 are formed and transferred onto 
substrates 130, 132 and 134 as shown in FIG. 3C. matrix elements 136, 138 
and 140 are aligned with underlying matrix elements. Optically transparent 
superstrates 142, 144 and 146 are then mounted over each display as shown 
in FIG. 3D. Cavities 150, 152 and 154 have been provided during 
fabrication in which liquid crystal material can be inserted either just 
after cavity formation or after further processing of the display. 
Substrate 100 can then be removed by an etching procedure 148 described in 
the above referenced parent applications. All three displays can then be 
mounted on a single substrate to form a multi display system, or as shown 
in FIG. 3E, separate displays 170, 172 and 174 can be mounted on 
transparent substrates 156, 158 and 160. 
Another preferred embodiment of the invention relates to an improvement in 
liquid crystal display employing two active matrix arrays on opposite 
sides of a liquid crystal material as illustrated in FIG. 4. 
Standard displays limit the electric field to directions perpendicular to 
the electrodes of the display. In such a pixel, the polarization can be 
rotated by hanging the potential across the electrodes. This yields only 
one degree of freedom in the control of light through the pixel. Usually, 
this degree of freedom is used to control greyscale. FIG. 4 illustrates a 
system that broadens the number of degrees of freedom in the electric 
field to three where a second active matrix 150 is aligned opposite the 
first active matrix 152 around liquid crystal 158. The active matrix 
arrays 150 and 152 are mounted onto transparent substrates 154 and 156 
respectively. Each display pixel 162 comprises six electrodes (two shown) 
which can be biased to create an electric field that has components in 
three axes. Thus, in addition to electric field strength (which can be 
used to control greyscale), the field direction can be altered. 
The three degrees of freedom in the electric field can be used to select 
colors, and thereby transform a single monochrome display cell into a 
color display cell. FIG. 5 illustrates one way in which this can be 
achieved. Three polarized light sources 184, 186 and 188 (red, green, and 
blue) are oriented such that the light approaches the LCD 180 in 
orthogonal directions. The display cell has a top diffuser 182. The 
electric field is then oriented so that it is parallel to one of the 
colors and perpendicular to the other colors, and in this way selects the 
light source. By shifting to other orthogonal directions, other light 
sources can be selected. By appropriately choosing an intermediate 
direction with components is all directions, a particular color can be 
selected. 
The field direction is determined by the relative potentials applied to the 
electrodes. The field strength (the magnitude of the components) can be 
adjusted without changing the relationship between the potentials and thus 
without changing the field direction. Thus, greyscale and color selection 
can be attained. 
This type of color display pixel has advantages over the color filter 
approach, the main advantage is that the pixel has much higher aperture, 
owing to the separation of light into components before incidence on the 
LCD. For example, in a conventional color filter LCD, white light is 
equally incident on red, green, and blue filters. Each filter receives one 
third of the incident light, and each filters out an additional two-thirds 
in the full-on condition. For example, the blue filter rejects all 
incident red and green light, making the best-case throughput 33%. Thus, 
in the full on state, the color filter approach has an optical throughput 
of 33%. For single color operation, the throughput is one third, or 11%. 
The pixel system described here has a throughput of 33% for single color 
operation and about 70% for the full on condition. 
The double matrix approach has other advantages when used in conventional 
approach. These include (1) enhanced greyscale, (2) enhanced reliability 
and (3) enhanced contrast ratio. In addition, the second matrix 150 can be 
used to correct for nonuniformities, or to add an overlay of a second 
image onto a first image generated by the first active matrix 150. 
Equivalents 
While this 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 details may be made 
therein without departing from the spirit and scope of the invention are 
defined by the appended claims.