JPH0135352B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to liquid crystal (LC) display devices, particularly matrices of such display cells that are highly multiplexed.

In a matrix multiplexed addressing arrangement for LC display cells, a series of scan voltages V _s are applied sequentially to each of a series of row conductors (scan lines), for example, while a series of data pulses is applied to each of a series of row conductors (scan lines). V _d is a series of column conductors (data
line) is added to the selected one. In order to turn on the LC picture element (bel, pel) at the intersection of the selected matrices, the difference between V _s and V _d added to the selected matrices, respectively, is large enough to In methods known in the art, the molecular orientation of the liquid crystal is changed, thus changing the optical transmission of the cell.

Several factors limit the number of lines that can be multiplexed in an LC display cell.

First, when a pel is selected, other unselected pels in the selected column also experience a pulse V _d . 1
For one address period, the RMS value of the alternating voltage these pels experience is insufficient to turn them on, but the N pels in the column turn on and off in a single field scan. If switched, the off pel will experience V _d for N address periods. This may be enough to turn on Pell. RMS voltage seen by a pel on vs. seen by a pel off
The ratio of RMS voltage is It can be shown that As N increases, this ratio becomes smaller, and since the liquid crystal does not have a sharp threshold separating on and off, the contrast ratio between on and off pels becomes smaller. At a certain number of row conductors, the contrast ratio becomes unacceptable.

This problem is compounded when the angle at which the cell is viewed deviates from the optimal value. Also, since the LC electron-photoresponse is temperature dependent, if the LC is turned off at high temperature with V _pff and turned on at low temperature with V _po , the difference between V _pff and V _po is must be larger than

For the above reasons, the prior art limits multiplexing to approximately 4 lines (or 8 lines for temperature compensated display cells).

One proposal to solve this problem is to place a switch in series with each liquid crystal pel at the intersection of the scan and data lines, whereby the pulse V _d activates the switch or the pel controlled by it. Instead, the selection pulse V _s +V _d activates the switch when the LC experiences a voltage. Such a switch should be symmetrical about zero voltage. Because, in order to prevent irreversible electro-chemical degradation of the liquid crystal, net DC bias should be avoided.

In its broadest aspects, the invention proposes the use of thin metal-insulator-metal (MIM) devices as switches. MIM devices work by tunneling or trap depth modulation. In the former, the carrier passes through a thin insulator by field-enhanced quantum mechanical tunneling. In the latter, the carrier is released from its trap in the insulator when the field generated between the metal layers reduces the potential barrier to a current. Such devices are known which, in the control of a switch, exhibit an increase of 500 to 10,000 times the original current passed to double the voltage. This on switch should be sharp enough to allow at least eight times as many multiplexed switches as would be possible without the switch.
Increase the number of lines. On the other hand, if the number of multiplexed lines is maintained, using the MIM switch greatly increases the viewing angle,
Contrast ratio and possible temperature range are obtained.

Thin film MIMs can have insulators such as aluminum oxide, tantalum pentoxide, silicon nitride, and silicon dioxide. The thickness of the dielectric layer determines the conduction process. Below 50-100 Å, tunneling is possible, and below 100-1000 Å, trap depth modulation conduction processes dominate. The MIM metal can be any material that forms an ohmic or weak blocking contrast.

Some aspects of the invention will now be described with reference to the accompanying drawings.

In a conventional matrix multiplexed addressing configuration for an LC display _cell , shown at the bottom left of FIG. while a series of data pulses V _d
are applied to selected ones of the series of column conductor 12 interrogation data lines. When an "on" pulse is desired at a selected row and column intersection in the LC pel 14, the difference between V _s and V _d applied to the selected row and column, respectively, is LC
The pel is made large enough to be turned on in a manner known in the art.

As explained earlier, LC does not have a sharp threshold separating on and off, so one pel is a data pulse that drives other pels in the same column.
To experience V _d , it is not specifically addressed, but can be turned on.

As shown in the upper right corner of FIG.
We propose the formation of

Referring to FIG. 2, the LC cell consists of a pair of glass plates 18, 20 with a layer of twisted nematic LC 22 sealed between them. The inner surfaces of plates 18, 20 are treated in a manner known in the art to properly orient the LC molecules. As is well known, by applying a voltage across selected areas of the LC layer,
LC can perform local molecular reorientation,
As a result, the optical transmission through the cell changes.

The switches are located adjacent to each pel 14 location, and the pels are arranged in rows and columns of indium tin oxide transparent electrodes 24 on the inner surface of plate 18 and on plate 20.
and a corresponding array of transparent electrodes (not shown) on the inner surface of the. Although not illustrated, a switch can be connected in series to each pel electrode,
Each pel thus has an associated thin film MIM device on each plate 18 and 20.

To fabricate the MIM on the inside of the plate 18, a thin film 26 of tantalum is deposited by sputtering. This layer is thermally oxidized at 465℃,
Protect the glass for the next etching step.
A second layer of tantalum is deposited by sputtering and photodefined into row conductors 10 that are 2 to 25 mils wide and run the width of the plate. A conductor serving as one side of a metal-insulator-metal (MIM) device can be locally reduced to a width of 0.5 mil in the MIM active region. The conductor 10 is anodized in a mildly acidic citric acid bath at an anodizing voltage of 30-60 V to produce a surface layer 28 of tantalum pentoxide, which acts as an insulator for the MIM device. The crossed conductors were then deposited across lines of tantalum pentoxide with a width of 0.5 to 5.0 mils in clear gold bars 30, each bar overlying a respective electrode 24;
make electrical contact with it. MIM typically 1.0
A square mill active area is formed in the area of the intersection of layer 28 and bar 30. Thus each MIM has one of the electrodes 24 and one of the tantalum conductors 1
0 and connected in series. Cell is glass plate 1
8 and 20 by sealing a layer 22 of nematic liquid crystal. Electrodes 24 common to a particular column on glass plate 20 are electrically connected by thin film conductive leads 32 (or alternatively formed as a continuous strip) to provide data and scanning data. Pulses can be selectively applied to the LC pels 14 by applying pulses V _d and V _s to the appropriate row conductors 10 on plate 18 and column conductors 32 on plate 20.

Other examples of MIM devices are tantalum oxynitride,
It has insulators of aluminum oxide (Al ₂ O ₃ ), silicon nitride (Si ₃ N ₄ ), silicon dioxide (SiO ₂ ), silicon oxynitride and silicon monoxide (SiO). Another example of metallization is aluminum.

Although called a metal-insulator-metal device, an important performance characteristic of this device is that it should be fabricated as a thin film device, and that it is capable of using the aforementioned field-enhanced quantum mechanical tunneling effect. Alternatively, it should function as a switch by modulating the trap depth. Thus, in another aspect of the invention, the "metal" on one side of the MIM is indium tin oxide, which has the advantage of being transparent in nature and therefore not significantly attenuating the light transmitted through it. Because of this property advantage, other embodiments (not shown) use a single thin film indium tin oxide region to form liquid crystal electrodes and
It acts both as one "metal" layer of MIM. Other materials used in MIM equipment include:
For example, NiCr, which is only a few hundred Å
It is effectively transparent to a certain degree and can also be used as a bonded electrode and metallization.

The particular thin film technique used in forming the MIM layer (sputtering, vacuum evaporation, anodization, etc.) is chosen to be compatible with the material being formed and the glass support material.

In another embodiment, the fabrication order is reversed and the crossed conductors 30 are deposited and anodized, and then the ionic conductors are subsequently deposited. Conveniently, the insulating layer is obtained by anodization, but it can also be formed in a separate deposition step.

Another embodiment is shown in FIG. A thin film layer 26 of etch protectant is formed as described above on the inside of a piece of flat glass 18 which serves as one side of the liquid crystal cell. A thin film of tantalum is then formed on the etching protectant,
Distinct regions 34 are then photoetched. The tantalum areas are anodized to form a surface layer 36 of tantalum pentoxide. A thin film layer of gold is then formed on the support. From this layer, two gold pads 38a and 38b are photoformed onto each region 34 to produce a structure equivalent to a pair of back-to-back MIMs. Leads 40a and 40b are integrally formed with a gold pad. Each lead 40a extends between one of the regions 38a and the electrode 24 of the MIM controlled pel. Each lead 40b interconnects pads 38b on the MIM in the same row. The advantage of this embodiment is that the characteristics of the switch do not depend on the polarity of the current because the device is symmetrical. The advantage of using gold on one side of a MIM device is that it allows a low resistance connection with the device's circuitry.

In another method of fabricating this embodiment, leads 40a and 40b are tantalum and are formed at the same time as region 34. The leads are protected from anodization by coating with photoresist, which is subsequently removed.

As previously described with reference to the embodiment of FIG.
The number of steps in fabricating the illustrated cell is reduced if pad 38, lead 40 and electrode 24 are simultaneously formed as a partially transparent thin film of indium tin oxide.

Using MIM switches at matrix intersections allows high levels of multiplexing (100 to 1000 lines) of the matrix address calling array of LCD picture elements, narrow viewing angles, and low contrast ratios between on and off elements. and without giving rise to the prior art problems of a highly restricted temperature range of use. MIM
Since the switch can be made on the transparent side of the cell and is not large enough to interfere with the picture elements, it can be used in both transmissive and reflective displays.

Thin film MIM devices are much thinner than 10 microns, about 1 micron, so they are placed on a transparent plate that is in contact with the LC material on the side.
The presence of MIM devices does not preclude the use of corresponding thin layers of LC material as do thick film devices. Therefore, the resistance of LC material is very high,
10 Assuming it is on the order of ¹⁰ ohms/cm,
Charge through the MIM device is limited by the LC resistance. Combined with the fact that the MIM device used exhibits switching characteristics at very low currents, i.e., on the order of −10 μA, the MIM device can be used at very low currents, thereby avoiding excessive heat. The chance of destruction is reduced by dissipating. In intended applications for large areas (e.g., 9 inches by 9 inches) with high pel densities (e.g., pel areas smaller than 25 mils square),
Fabrication of MIM devices offers significant cost benefits over thin film transistor switches because thin film transistor switch fabrication is characterized by greater complexity and lower yield. Moreover, the proposed fabrication technique is suitable for silicon, since it is possible to form a precisely flat glass surface by using known techniques, thus almost eliminating the variations in the thickness of the LC cell, due to the cost. It is much superior to other IC technology.

According to the invention, the MIM device 16, which is a switch element, as clearly shown in FIGS.
is substantially smaller than the area of picture element 14, thereby causing the capacitance of the switch element to be substantially smaller than the capacitance of the picture element.

Since the MIM device 16 and the picture element 14 are connected in series, by configuring as described above, the voltage applied to the MIM device 16 can be increased, and the MIM device can be operated appropriately. can.

[Brief explanation of drawings]

FIG. 1 is a schematic diagram illustrating a matrix multiplexed addressing arrangement for an LC display.
FIG. 2 is a partial cross-sectional, partial perspective view, not to scale, of a portion of a liquid crystal display cell employing one form of MIM. Figure 3 is similar to Figure 2, but
Figure 2 shows one plate of display cells using another form of MIM. 10...Series of row conductors, 12...Series of column conductors, 1
4...LC image element (pel), 16...thin film
MIM device, 28... surface layer, 30... gold bar, 32
...thin film conductive lead, 34...defined area, 36...surface layer.

Claims (1)

[Scope of Claims] 1. A liquid crystal display cell comprising a pair of plates in side contact with a liquid crystal material, the cell having a plurality of picture elements, each of the picture elements having a surface on the inner surface of the respective plate. 1
each having a series connected thin film metal-insulator-metal (MIM) switch device defined by a pair of opposed electrodes and formed on an inner surface of one of the plates; the area of the switching device is substantially smaller than the area of the picture element, the picture element and the switching device are laterally offset from each other and arranged to form rows and columns; and Further, first lead means electrically connects each MIM switch device to its serially connected picture elements, second lead means electrically connects a row of MIM switch devices, and a column picture element. and third lead means for electrically connecting the liquid crystal display cell. 2 the first lead means electrically connects the metal on one side of each MIM switch device to one electrode of its series connected picture elements;
The third lead means electrically connects the metal on the other side of the MIM switch device in the row, and the third lead means electrically connects the other electrode of the picture element in the column. The liquid crystal display cell according to the range 1 above. 3. The metal on one side of each MIM device is formed in the shape of first and second distinct regions;
lead means electrically connect each of the first regions to the series connected picture elements; and the second lead means electrically connect the second regions of the row together. A liquid crystal display cell according to claim 1. 4. Claim 1, wherein the insulator of the MIM switch device is one selected from the group consisting of tantalum pentoxide, aluminum oxide, silicon nitride, silicon dioxide, silicon oxynitride, and silicon monoxide.
The liquid crystal display cell according to item 1, 2 or 3. 5. The metal on at least one side of the MIM switch device is one selected from the group consisting of tantalum, aluminum, gold, indium tin oxide, and NiCr, or The liquid crystal display cell according to item 3. 6 Claim 1, wherein the MIM switch device is a tantalum-tantalum pentoxide-gold device;
The liquid crystal display cell according to item 2 or 3. 7. The liquid crystal display cell according to claim 1, 2 or 3, wherein the MIM switch device is an aluminum-aluminum oxide-gold device.