Note: Descriptions are shown in the official language in which they were submitted.
1~;064~1
This invention relates to display cells utilizing electro-
optic liquid, specifically to such display cel1s matrix multiplexed to a
high level. Although the invention is described primarily in conjunction
with a liquid crystal (LC) cell which represents its chief intended
application, it will be appreciated that it can be used to advantage with
display cells using alternative electro-optic materials, specifically cells
based on electropheretic or electrochromic liquids.
In a matrix multiplexed addressing scheme for a LC display
cell, a series of scan pulses, Vs~ is, for example, applied sequentially
to each of a series of row conductors, (scan lines), while a series of
data pulses, Vd, is applied to selected ones of a series of column
conductors, (data lines). To turn on a LC picture element, (pel), at a
selected row and column intersection, the difference between Vs and Vd
applied to the selected row and column respectively, is made great enough
to alter the LC molecular orientation, and thus the cell optical
transmissivity, in a manner known in the art.
Several factors combine to limit the number of lines that
; can be multiplexed in a LC display cell.
Firstly, at the instant at which a pel is selected, other,
non-selected pels in the selected column also experience a pulse Vd. For
one address period, the RMS value of a.c. voltage experienced by these
pels is insufficient to turn them on, but if N pels in a column are
switched on and off in a single field scan, an off pel will experience
Vd for N address periods. This may be enough to turn the pel on. It
can be shown that the ratio of RMS voltage seen by an on pel to that seen by
an off pel is:
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11069~81
V M5 ~ (Vs + Vd) ~ Vd
vRMs (Vs ~ Vd)2 - Vd2 (N-l)
As N increases, the ratio becomes smaller and, since liquid crystals do not
have a sharp threshold separating on and off, the contrast ratio between on
and off pels becomes poorer. At a certain number of row conductors, the
contrast ratio becomes unacceptable.
The problem is compounded as the angle from which the cell
is viewed deviates from an optimum value. Also, since the LC electro-optic
response is temperature dependent, then if the LC is to be off at Voff at
high temperature, and on at Von at low temperature, the difference between
Voff and Von must be greater than for constant temperature operation.
For the above reasons, prior art limits multiplexing to
about 4 lines (or 8 lines for temperature compensated display cells).
A suggestion for solving this problem proposes placing a
switch in series with each pel at the intersections of the scan and data
lines, such that pulses Vd do not activate the switch nor the pel
controlled by it whereas a selection pulse Vs + Vd does activate the
switch whereupon the LC or other electro-optic material experiences
voltage. Such a switch should be symmetrical with respect to zero voltage
since, for the purpose of preventing irreversible electro-chemical
degradation of the LC, net d.c. bias should be avoided.
In its broadest aspect, the invention proposes the use of a
pair of back-to-back, thin film diodes as the switsh. In operation,
when the voltage on the switch is less than the breakdown point of the
reverse-biased diode of the pair, only the saturatlon or sub-breakdowr
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current is passed with most of the voltage drop being across the reverse-
biased diode. When the voltage on the switch is greater than breakdown,
the reverse-biased diode conducts as much as 8 orders of magnitude more
current than in the "off" state, the limit being imposed by heat
dissipation and damage to the diode. This turn-on is sufficiently sharp to
increase the number of multiplexed lines compared to the number permitted
when no switch is used by at least a factor of 10. If, on the other hand,
the number of multiplexed lines is maintained, then using thin film diode
switches of this type provides a greatly increased viewing angle, contrast
ratio and permitted temperature range.
Thin film diodes used in this application may be of known
structure. Certain examples of homojunctions, heterostructures,
metal-semiconductor junctions and metal-insulator-semiconductor devices all
have demonstrably sharp enough turn-on thresholds for use in LC display
matrix crosspoints.
Embodiments of the invention will now be described by way of
example with reference to the accompanying drawings, in which:-
Figure 1 shows, in schematic form, a matrix multiplexed
addressing scheme for a LC display; and
Figure Z is a sectional view (not to scale) showing part of
a liquid crystal display cell using one form of thin film back-to-back diode
switch.
In a conventional matrix multiplexed addressing scheme for a
LC display cell, as shown at bottom left in Figure 1, a series of scan pulses
Vs is, in use, applied sequentially to each row of a series of row
conductors 10, called scan lines, while a series of data pulses Vd is applied
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to selected ones of a series of column conductors 12, called data lines. If
an "on" pulse is desired at a LC pel 14 at a selected row and column
intersection, the difference between Vs and Vd applied to the selected row
and column respectively is made great enough to turn on the LC pel in a
manner known in the art.
As previously explained, since LC's do not have a sharp
threshold separating on and off, then a pel may turn on even though not
specifically addressed because it experiences data pulses Vd driving other
pels in the same column.
As shown at top right in Figure 1, the invention proposes the
formation of thin film switches 16 in series with each LC pel 14, each switch
having a pair of series-connected back-to-back diodes, 18, 20.
Referring to Figure 2, the liquid crystal cell comprises a
pair of glass plates 22, 24 with a layer of twisted nematic LC 26 sealed
between them. The inner surfaces of the plates 22, 24 are treated in a
manner known in the art so as to properly orientate the LC molecules. As
is well known, by applying a voltage across selected regions of the LC layer
26, the LC can be caused to undergo localized molecular reorientation with
consequent alteration in optical transmissivity of the cell.
A switch 16 is sited adjacent the position of each pel 14,
the pels being defined by a row-column array of transparent electrodes 28
on the inside surface of plate 22 and by a corresponding array of
transparent electrodes 30 on the inside surface of plate 24. Although not
illustrated, switches can be series-connected to one or both electrodes of
each pel, the pels thus having an associated thin film fabricated switch
device on either or both of the plates 22 and 24. In another alternative,
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a pair of back-to-back diodes controlling a pel can, in fact be split by
the pel, one diode connected to an electrode 28, and the other to an
electrode 30. The benefit of this structure is that the two plates 22 and
24 are identical and manufacture is consequently cheaper and easier.
In one embodiment, to obtain hydrogenated amorpous
silicon diodes of the homojunction type, the glass plates are first cleaned
ultrasonically in soap and water, rinsed in de-ionized water and dried in
isopropyl alcohol vapour. An etch protectant 32 is then deposited onto
the inner surfaces of the glass plates. A vapour-deposited nickel layer
subsequently formed on plate 22 is photodefined into row conductors lO
and bottom ohmic contact regions 34. Two layers 36, 38 of amorphous silicon
having predetermined, and different, dopant levels are next deposited by
glow discharge decomposition of silane with an appropriate dopant gas.
An n-type layer is made using phosphine (PH3) dopant gas and p-type layer is
made using diborane (B2H6) dopant gas. The layers are photodefined into
spaced pads, 40, 42, each pad partially covering one of the contact
regions 34. Ohmic conductors 44, are thin film deposited on pads 40 of
each switch. Detailed fabrication steps are described by Spear and LeComber
in Philosophical Magazine, 1976, Vol. 33, No. 6, pp 935-949. Each
conductor 44 extends to, and overlies, a row conductor lO, each row
conductor lO connecting one pole from all the switches in that row to
drive circuitry (not shown). Ohmic conductors 46 are thin film deposited
on pads 42 of each switch, each conductor 46 extending to and overlying a
respective transparent pel electrode 28.
In operation, the diodes l8, 20 of each switch are alternately
reverse-biased since, for the purpose of preventing irreversible electro-
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chemical degradation of the LC 26 net d.c. bias is avoided. ~hus when,
say, switch l8 is reverse-biased and the voltage on it is below the break-
down point, only the saturation current is passed with most of the voltage
drop occurring across diode l8. When the switch voltage is increased above
breakdown, the diode l8 conducts as much as 8 orders of magnitude more
current than in the "off" state, the limit being imposed by heat
dissipation and damage to the diodes.
The electrodes 30 common to a particular column on the glass
plate 24 are electrically connected by thin film connecting leads l2 which
enable pulses to be selectively applied to LC pels l4 by applying data
and scanning pulses, Vd and Vs~ to the appropriate conductors lO and l2.
To fabricate the cell, inner surfaces of the glass plates 22,
24 are then coated with organic polymer which is undirectionally rubbed to
ensure LC alignment. The plates are assembled into a 90 twist cell of
l2 ~m spacing which is filled with BDH E-7 cyano biphenyl-terphenyl mixture
having a resistivity of 2 x lOlO ohm cm to give a twisted nematic LC
layer 26.
If desired, the diode structure can be reversed, the
conductors 44 and 46 being deposited before formation of the homojunctions
and the common contact area 34 being thin film deposited subsequently.
One of the advantages of using thin film technology is that
conducting regions can be sputtered or vacuum deposited with a thickness
such that they are substantially transparent. Depending on the order of
fabrication, the pel electrodes 28, 30 can be formed as regions of, for
O , O
example, lO ANi.Cr, - lOAAu simultaneously with the formation of one or
more of the regions lO, l2, 34, 55 and 46.
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A second suitable class of diode structures are hetero-
junctions of which one embodiment is a CdS-Cu2S diode. Such diodes are
formed by vacuum deposition of a 2500A thin fîlm of CdS follo~ed by a film
of CuCl. The structures are converted to heterojunctions by heating to
100-200C for a short period of time. Ohmic bottom and top contacts are -
provided as described for the first embodiment. Detailed fabrications steps
are described by Das et al in Thin Solid Fi~m , Vol. 51 pp. 257-264, 1978.
A third suitable class of diode structures are metal-
semiconductor junctions, one embodiment of which is fabricated by firstly
depositing CdSe on a configuration of ohmic bottom contact regions as
described for the first embodiment. This may be annealed in air to reduce
resistivity. Gold blocking contact regions are deposited over the semi-
conductor regions as described for the first embodiment.
A fourth suitable class of diode structures are metal-
insulator-semiconductor devices which are similar to the third class of
devices except that an insulator layer is deposited between the metal and
the semiconductor which has the advantage of minimizing capacitance which
would otherwise reduce the effectiveness of the switch. In one embodiment
A1203 insulator is formed by vacuum depositing Al at slight 2 pressure.
In an alternative embodiment, the insulator layer is vacuum deposited
CdTe. Detailed fabrication steps are described by Muller and Zuleeg in
Journal of Appl. Phys. Vol. 35, No. 5, 1964.
The important characteristics of the back-to-back diode
switches are that they should be prepared as thin film devices and should
functicn as switches. The particular thin film techniques (sputtering,
vacuum evaporation, anodization, etc.) used in the formation of the
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back-to-back diode layers is chosen to be compatible with the material
being formed and the glass substrate material.
~ sing switches at matrix crosspoints, high level multi
plexing (>lO lines~ of a matrix addressed array of liquid crystal display
picture elements can be obtained without the prior art problems of narrow
viewing angle~ low contrast ratio between off and on elements, and greatly
limited operating temperature ranges. The back-to-back diode switches
may be used both in transmissive and reflective displays depending on the
character of the plates 22 and 24 and the electro-optic liquid used.
Since the thin film switches are very much less than lO
microns in thickness, i.e. at most, l micron, their presence on the
transparent plates flanking the LC material does not prevent the use of a
correspondingly thin layer of LC material as would thick film devices. In
turn, and assuming the resistance of the LC material is very high, of the
order of lOlO ohm cm., then charge through the switches is limited by the
LC. Coupled with the fact that such switches show their threshold switching
characteristics at a very low current, of the order of na-~a, it will be
appreciated that the back-to-back diode switches can be operated in a very
low current regime which reduces the chance of their failing through
excess heat dissipation. In the intended application to a large area
(e.g. 9" x 9") high pel density ~e.g. pel area of less than 25 mil sq.
display), fabrication of the thin film diode devices offers significant
cost benefits over thin film transistor switches since fabrication
techniques for the latter are more complex and are characterized by poor
yield. In addition, the fabrication techniques proposed are vastly preferred
to silicon IC techniques, again because of cost and, further, because by
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using known techniques, accurately planar glass surfaces can be achieved
which ensure little variation in LC cell thickness.
Although the embodiment described uses a twisted nematic
LC, other examples may be used. For example, a cholesteric LC operated
in a scattering mode is particularly applicable to a reflective display.
As indicated previously, other electro-optic materials, for example,
electropheretic and electrochromic materials may also be used.
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