TEMPERATURE COMPENSATION IN ACTIVE SUBSTRATE ELECTRO-OPTIC DISPLAYS

Abstract: In an active substrate electro-optic display (10) such as a liquid crystal display utilizing a metal-insulator-metal (MIM) device (14) in series with each picture element (12), temperature compensation techniques are employed to control the useful operating range of the display. For example, a temperature feedback scheme is utilized to modulate the frame period of the line driver circuits and may be used separately or in combination with pulse polarity repetition driving.

Description: TEMPERATURE COMPENSATION IN ACTIVE SUBSTRATE ELECTRO-OPTIC DISPLAYS Background of the Invention This invention relates to displays utilizing an electro-optic material and, more particularly, to such displays which include active substrates to enable high levels of multiplexing. 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 employing alternative electro-optic materials, specifically cells based on electropheretic or electrochromic materials. In a matrix multiplexed addressing scheme for a liquid crystal display (LCD) of N rows and M columns, a train of scan pulses V s is, for example, applied sequentially to the row conductors, while a series of data pulses (either _+ V^) is applied to the column conductors. To turn on a picture element (pel) at a selected row and column intersection, the difference between V s and -V^ applied to the selected row and column, respectively, is made great enough to alter the LC molecular orientation, and thus the cell's optical transmissivity, in a manner known in the art. That is, the voltage difference V.-(-VuJ) is made greater than V " on, the minimum voltage required to turn on the selected pel. In contrast, in the addressed row the voltage difference V s -(+ ^) is made less than V ff , the maximum voltage required to insure that a nonselected pel in that row is in an off- state. As described by D. R. Baraff et al in U.S. patent 4,223,308, several factors combine to limit the number of lines that can be multiplexed in a LCD. Firstly, at the instant at which a pel is selected, other, nonselected pels in the selected column (i.e., in the nonaddressed rows) experience a voltage +V3. When one row is addressed, the RMS value of a.c. voltage experienced by nonselected pels is insufficient to turn them on. But, after the remaining (N-1 ) rows in a frame period are addressed, an off pel will experience additionally the voltage V^ for (N-1 ) times. The cumulative voltage may be enough to turn the pel on. It can be shown that as N increases, the ratio of the RMS voltage seen by an on pel to that seen by an off pel becomes smaller, and, since liquid crystals do not have a sharp threshold separating the on and off states, 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, the LC electro-optic response is temperature dependent. Consequently, if the LC is to be off at V -- at high temperatures, and on at V at low temperatures, the difference between Vo c ff c and Von must be g 3 reater than for constant temperature operation. These deficiencies can largely be remedied by incorporating a nonlinear electronic device in series with each pel to provide a sharper threshold characteristic. For devices fabricated on glass -1 , the active elements may be thin film transistors, amorphous silicon back-to-back diodes, or metal-insulator-metal (MIM) devices. MIM devices incorporated into LCDs are described, for example, by D. R. Baraff et al in the Proceedings of the SID, Vol. 22/4, pages 310-3 1 3 (1981). A MIM is essentially a two-terminal capacitor comprising a metallic base electrode, an oxide layer formed by anodization, and a metallic counter electrode. The attractiveness of the MIM device arises from its simple structure and the high yield anodization process .which produces oxides of excellent uniformity. Nevertheless, the MIM device is temperature sensitive because its conduction mechanism (the Poole-Frenkel effect) is a thermal excitation process. See, D. E. Castleberry, 1980 Biennial Display Research Conference Record, pages 89-92. The impact of the temperature sensitivity is felt primarily in the contrast and viewing characteristics of the display. Thus, while it may be possible to operate a MIM LCD at 40°C for a particular drive voltage frame period, the display characteristics deteriorate substantially when the temperature is decreased to, say, 10°C. Obviously, it would be desirable to be able to operate the display over a wide range of temperatures without sacrificing significantly either the contrast or viewing characteristics. Summary of the Invention In accordance with one aspect of the invention, each picture element of a multiplexed electro-optic display, such as a LC display, is connected in series with a separate nonlinear electronic device, such as a MIM, which has a temperature-sensitive characteristic that tends to adversely affect the contrast, viewing or other characteristics of the display. A temperature sensor is provided to sense- the temperature of the display and to generate a feedback signal which is used to change the frame period of the drive signals. By appropriately adjusting the frame period, the voltage on each pel can, for example, be increased so that the desired display characteristics are obtained even though the display temperature changes. In accordance with another aspect of the invention, the polarity of the drive pulses of the display may be repeated one or more times in adjacent frame periods in order to increase the voltage level on each pel, before the usual polarity reversal which is employed to prevent degradation of the LC material. This technique, known as pulse polarity repetition, may be used separately or in combination with the frame period modulation aspect described above. Brief Description of the Drawing The invention, together with its various features and advantages, can be readily understood from the following more detailed description taken in conjunction with the accompanying drawing, in which: FIG. 1 is a schematic circuit of a LCD in accordance with one embodiment of the invention; FIG. 2 is a schematic, cut-away, cross-sectional view of a LC cell showing a LC pel electrode connected to a MIM device; FIG. 3 is a graph of the absolute temperature dependence of the Poole-Frenkel conduction parameters β and k for an illustrative MIM device; FIG. 4 is a table listing the Poole-Frenkel conduction parameters for three different temperatures: 10°C, 25°C, and 50°C for a particular, typical full page display design; FIG. 5 is a table of voltages for 10%, 50%, and 90% transmission for normal viewing as a function of temperature for the nematic LC material ROTN-701 which is commercially available from F. Hoffman-LaRoche & Co., Basle, Switzerland; and FIG. 6 is a graph of on-voltage V don u vs. drive voltage frame period indicating the worst case RMS on voltages for polarity reversal after every frame (1X) and after every two frames (2X). Acceptable voltage levels for a given temperature are determined from the table of FIG. 5. Drive voltage levels of 13.6 volts for the rows and 3.4 volts for the columns were assumed. Additional curves, not shown, exist for Vof G4 t Z . Detailed Description With reference now to FIG. T, there is shown a LCD in accordance with one embodiment of the invention. A suitable cell structure 10 includes a matrix of LC pels 12 each of which has connected in series therewith a nonlinear electrical device 14 such as a MIM device. The cell structure, shown in more detail in FIG. 2, includes a pair of parallel glass plates 22 and 24 which contain therebetween a LC material 26 such as a nematic liquid. Metal conductors forming bus bars, the pels and the MIM devices are deposited on the interior major surfaces of the glass plates. For example, a transparent column electrode 28 is deposited on the interior surface of glass plate 22, and a row bus bar 30 is deposited on the interior surface of glass plate 24. In addition, a transparent electrode 32 on the lower glass plate 24 is used to define a pel 12. Illustratively, the transparent material of electrodes 28 and 32 is indium tin oxide. Electrode 32 typically takes the shape of a square or rectangle and has a cutout portion 34 in which a MIM device 14 is formed. Illustratively, MIM device 14 includes a finger-like member 36 which extends from the row bus bar 30 into the opening 34. The bus bar and the finger, both of which illustratively comprise tantalum, are oxidized (e.g., to form Ta2θ 5 ). The active region of the MIM 14 lies in the region of overlap between the finger 36 and a counter electrode 38. The electrode 38, illustratively comprising chromium, overlays both the electrode 32 and the finger 36. Thus, the MIM device 14 connects the electrode 32, and hence the pel 12, to the bus bar 30. Of course, the display includes a matrix of such pels each connected in series with a MIM device and to an array of bus bars 30 as shown in FIG. 1. In a practical device, the transparent electrode 32 of each pel is relatively large, being about 20-50 mils square with only about 1-2 mil channels between adjacent pels. The column electrodes 28 are about as wide as the pel electrodes 32. On the other hand, the bus bar 30 may be only 1-5 mils wide and the MIM device 14 is considerably smaller, measuring about 5-10 μm square. In the case where the LC material is a nematic liquid, the display also includes a pair of crossed polarizers 40 and 42 on opposite sides thereof as shown in FIG. 2. In addition, well-known alignment layers (not shown) are formed over the transparent electrodes 28 and 32. For simplicity of illustration, only two of the picture elements of the first row are depicted in FIG. 1. The remaining rows and the columns of pels are not shown explicitly but are implied by the broken and dotted lines in the figure. The row bus bars 30 and the column electrodes 28, respectively, are connected to a line driver circuit 16 which supplies suitable trains of pulses thereto. The frame period of the pulse trains is controlled by a control circuit 18 which is coupled to a temperature sensor 20. In practice, the location of the sensor relative to the cell structure is determined by the need to measure the display temperature, and hence the MIM device temperature, relatively accurately while at the same time not blocking the transmission of light through the display. A variety of temperature-sensing techniques can be employed. For example, a thermistor may be located on the cell in a region which is out of the field of viewing. Alternatively, a diode having a temperature dependence similar or proportional to that of the nonlinear elements (e.g., MIM devices) can be positioned in proximity to a surface of the cell. In addition, changes in a temperature dependent parameter (e.g., resistivity, capacitance) of the LC material itself can be sensed to provide the desired feedback signal. This sensing function can be performed simply by means of electrodes (e.g., ITO) placed on the interior surfaces of the glass plates of the cell. Illustratively, the line driver circuit 16 supplies a train of scan pulses of voltage amplitude V s to the row bus bars sequentially; one pulse is applied to one row at a time. Simultaneously, circuit 16 supplies data pulses of voltage amplitude +_ ^ to the column electrodes in order to select which pels will be turned on and which will remain off. To turn on a pel at a selected row and column intersection, the difference between the V s and -V d is made great enough to alter the LC molecular orientation and thus the cell's optical transmissivity in a manner well known in the art. Conversely, the difference between the V s and +V d is applied to a nonselected pel in the addressed row and is made small enough that the nonselected element remains off. As is well known in the art, one mode of addressing the rows and columns is as follows: The first row is addressed with a pulse V s , and then the second row through the Nth row are sequentially addressed with V s . The time required to address the first row through the Nth row is known as the frame period. In order to avoid d.c. degradation of the LC material, the polarity of the pulses is reversed during the next frame period so that over two adjacent frame periods the average d.c. voltage on each LC pel is zero. In accordance with a significant aspect of my invention, the frame period is controlled in response to a feedback signal produced by the temperature sensor 20. By controlling the frame period, the LCD is capable of operating over a relatively wide range of temperatures without experiencing significant deterioration in its contrast, viewing or other characteristics, as described hereinafter. Alternatively, or in combination, these characteristics can be enhanced by pulse polarity repetition in accordance with another aspect of the invention. Pulse polarity repetition means that the polarity of the pulses applied in one frame period is repeated in at least the immediately succeeding frame period. Then the polarity of the pulses is reversed for a time equal to the sum ofthe consecutive frame periods during which the polarity was not reversed. Each of these techniques is effective to increase the on-voltage V Qn at low temperatures, so that the values of V over a broad operating range is sufficient to maintain selected pels on. Note, at high temperatures V - f may increase sufficiently to turn on a pel which should be off. However, the feedback scheme described above should take this phenomenon into account to select frame periods for which Von and -Vof_f- are both suitable, For Ta-Ta 2 0 5 -Cr MIM devices, I-V-T measurements were made in the range 20-60 β C. Conduction in the MIM devices was found to obey the Poole-Frenkel equation I = kV exp (β V) , β indicates the degree of nonlinearity and k_ the low- voltage conductivity. Both are temperature-dependent parameters, and both β and k_ agreed well with a 1/T dependence over the temperature range investigated as shown in FIG. 3. Values of β and k at temperatures as low as 10°C were obtained by extrapolation of this data. To determine what effect the observed change in β and k with temperature has on display performance, and also to evaluate possible compensation techniques, computer simulation of the MIM-LCD combination was employed. On and off RMS voltages were determined for worst case driving conditions. Calculations were carried out assuming a full page display 250 rows by 480 columns, suitable for displaying 25 lines of 80 characters. Data on the odled display are summarized as follows: size - 250 x 480 lines; resolution - 60 lpi (columns)and 50 lpi (rows); LC layer thickness - 8 μm MIM size - 25 μm ; MIM capacitance - 0.138 pF; LC dielectric constant - 27 (on) and 8.4 (off); Q LC resistivity at 25°C - 7 x 10 Ω-cm (on) and 1.4 x 10 10-cm (off); LC resistivi.ty variation with temperature -2.34%/°C (on) and 3.24%/°C (off). Conduction parameter pairs (β,k) consistent with a temperature range 10-50°C were used. These are given in FIG. 4. As mentioned earlier, the values at 10°C were obtained by extrapolation. LC materials designed to operate at 1.5 volts (RMS) are readily available. Their threshold voltage, however, varies with temperature. In a typical commercial material, ROTN-70 , for example, the threshold voltages V 1Q , V 5Q and V 9Q , at 10%, 50% and 90% optical transmission, respectively, change with temperature approximately as given in the table of FIG. 5 for a viewing angle of 90°. In addition, the room temperature Freedericksz threshold is 0.82 volts. Nonselected elements with voltages below the latter threshold will be off regardless of viewing angle. These values serve as references against which the results of the computer simulation can be compared. Assume that an acceptable range of frame periods at a given temperature is that for which the V Q0 voltage level is equaled or exceeded. In FIG. 6 the three horizontal dashe lines indicated V gΩ for 50°C( 1.43V) , 25°C( 1.57V) and 10°C (1.65 V). The voltage on a pel for no polarity repetition (labeled 1X) equals or exceeds V q0 only in a narrow range of frame periods: about 1.3-9.0 msec at 50°C; about 4.0 - >14 msec at 25°C; and about 9.7 - >14 msec at 10°C. No overlap of these ranges occurs at all three temperatures. If the RMS on-voltages across the pels can be increased, however, the range of useful frame periods would broaden. In accordance with one aspect of the invention, broader ranges of useful frame periods are attained by pulse polarity repetition; i.e., by applying sequentially to the row bus bars scan pulses V s of one polarity for one frame period and then repeating the same polarity of pulses for at least the next succeeding frame period. Then, in order to avoid d.c. degradation of the LC material, the polarity of the pulses applied to the row bus bars is reversed for a time substantially equal to the sum of the consecutive frame periods during which the polarity was not changed. FIG. 6 shows RMS worst case on-voltages assuming the usual polarity reversal after every frame (1X) and after every two frames (2X) in accordance with the invention. For the case of polarity reversal after every frame, the excellent contrast and viewing characteristics consistent with the V-. voltage levels are not obtained for any single frame period at all three temperatures. In contrast, the V-^ levels of FIG. 5, which are less desirable, but acceptable for some applications, are equaled or exceeded in the range of about 7-15 msec. If, however, the V 5 CΛ 0 level is unsuitable for a particular application, the V gn levels can be satisfied simultaneously for all three temperatures in the range 6.5-9.0 msec if, in accordance with this aspect of the invention, the polarity of the row scan pulses is repeated twice (2X) before reversal. This range is evident from the overlap of the ranges at 2X: about 0.8-9.0 msec for V 90 (50°C), 2.5 - >14 msec for V 9Q (25°C), and 6.5 - >14 msec for V qn (10°C). Thus, the upper limit of the overall range is determined by the smallest upper limit of the three (9.0 msec), whereas the lower limit of the overall range is determined by the largest lower limit (6.5 msec). Note, however, due to the ability of the eye to detect changes slower than about 30 msec (i.e., flicker), frame periods greater than 7.5 msec should not be used at 2X. Thus, in practice the upper limit of 9.0 msec is actually only about 7.5 msec. At 3X the upper limit on the frame period is about 5 msec, and at 5X it is 3 msec. In contrast, frame periods less than about 3 msec press the speed limits of available drive circuits. However, for frame periods of 6.5-7.5 msec pulse polarity repetition driving at 2X clearly leads to improved performance for this particular display. Another aspect of the invention, which may be used separately or in combination with pulse polarity repetition, utilizes a temperature sensor 20 to monitor the display temperature, and hence the MIM temperature, and to adjust the frame period via control circuit 18 to raise the voltage level on each pel. For example, a 8.5 msec frame period might be optimal for a display at room temperature because the peak voltage (V = 1.99 V) of the 1X curve at 25°C occurs there. In contrast, at 50 β C the frame period at 1X might be changed to 3 msec and at 10°C to about 14 msec. However, limitations on circuit speed may lead to a compromise choice for the frame period at 50°C. For some display designs, the best strategy may be to employ both of these approaches to maximize tolerance to device nonuniformity and to provide the widest operating temperature range. Lack of device uniformity shifts the position of the curves of FIG. 6. The shifts narrow, and may even be large enough to eliminate, the regions of frame period overlap for a desired operating temperature range. Consequently, polarity repetition driving alone may not be adequate. In this case, frame period modulation would be used to permit operation over the desired temperature range. As a result, frame period modulation provides some tolerance for device nonuniformity and thus permits manufacturing with more lenient specifications. This may improve yield.

Claims: Claims 1. A multiplexed display apparatus comprising a display cell containing an electro-optic material, transparent conductor means defining a matrix of picture elements, and a nonlinear electrical device in series with each picture element, said nonlinear device having a temperature sensitivity which tends to adversely affect a characteristic of said cell, row and column conductors for applying voltage to said picture elements, driver means for applying voltage pulses to said conductors, said pulses being applied sequentially to said row conductors one at a time, the time to apply said pulses to all row conductors defining a frame period, sensor means for detecting the temperature of said cell and for generating a feedback signal proportional t to said temperature, and control means for changing the duration of said frame period in response to said feedback signal. 2. The apparatus of claim 1 wherein said electro- optic material comprises a liquid crystal. 3. The apparatus of claim 2 wherein said sensor means detects the change in an electrical parameter of said liquid crystal with temperature to generate said feedback signal. 4. The apparatus of claim 1, 2 or 3 wherein said nonlinear electrical device comprises a metal-insulator- metal capacitor. 5. The apparatus of claim 1, 2 or 3 wherein said driver means applies pulses with one polarity to each of said row conductors for one frame period, again applies pulses with said one polarity for at least one more frame period, and then applies pulses of the opposite polarity for a time substantially equal to the sum of said one frame period plus said at least one more frame period. 6. A multiplexed display apparatus comprising a display cell containing an electro-optic material, transparent conductor means defining a matrix of picture elements, and a nonlinear electrical device in series with each picture element, said nonlinear device having a temperature sensitivity which tends to adversely affect a characteristic of said cell, row and column conductors for applying voltage to said picture elements, and driver means for applying voltage pulses to said conductors, said pulses being applied sequentially to said row conductors one at a time, the time to apply said pulses to all row conductors defining a frame period, said driver means further applying said pulses with one polarity to each row conductor for one frame period, again applying said pulses with said one polarity for at least one more frame period, and then applying pulses of the opposite polarity for a time substantially equal to the sum of said one frame period plus said at least one more frame period. 7. The display of claim 6 wherein said electro- optic material comprises a liquid crystal. 8. The display of claim 6 or 7 wherein said nonlinear electrical device comprises a metal-insulator- metal capacitor.

Inventors: ZOLNOWSKI DENNIS RONALD, ZOLNOWSKI, DENNIS, RONALD
Applicants: AMERICAN TELEPHONE & TELEGRAPH, AMERICAN TELEPHONE & TELEGRAPH COMPANY
Application year: 1986
Main classification: G02F1
IPC codes: G02F1, G09G3