JPH07122709B2 - Optical modulator - Google Patents

Description

The present invention relates to a method for driving an optical modulator for a display panel, and more particularly to a display using a liquid crystal substance having bistability, particularly a ferroelectric liquid crystal. The present invention relates to a driving method of a liquid crystal optical element suitable for a panel, particularly a gradation display.

[Conventional technology]

In a liquid crystal television panel using the conventional active matrix driving method, thin film transistors (TFTs) are arranged in a matrix for each pixel, and a gate-on pulse is applied to the TFTs to establish a conduction state between the source and the drain. Is applied from a source and is accumulated in the capacitor. A liquid crystal (for example, twisted nematic; TN-liquid crystal) is driven in response to the accumulated image signal, and at the same time, the voltage of the image signal is modulated to display a gradation. Has been done.

[Problems to be solved by the invention]

However, in such an active matrix drive type television panel using TN liquid crystal, since the TFT used has a complicated structure, the number of structural steps is large and the high manufacturing cost is a problem. In addition, there is a problem that it is difficult to form a thin film semiconductor (for example, polysilicon or amorphous silicon) forming a TFT over a wide area.

On the other hand, a display panel of a passive matrix drive system using a TN liquid crystal is known as one that can be manufactured at a low manufacturing cost. In this display panel, one screen (one frame ) While scanning
The time (duty ratio) during which an effective electric field is applied to one selected point is reduced by a ratio of 1 / N, which causes crosstalk and has the drawback of not producing a high-contrast image. Moreover, when the duty ratio becomes low, it becomes difficult to control the gradation of each pixel by voltage modulation, and therefore it is not suitable for a display panel with a high-density wiring number, particularly a liquid crystal television panel.

[Means for Solving Problems] and [Operation] An object of the present invention is to solve the above-mentioned drawbacks, and more specifically, a driving method of a display panel having high-density pixels over a large area, particularly gray scale. An object is to provide a driving method of an optical modulation element suitable for display.

That is, according to the present invention, a plurality of pixels in which an optical modulation material is arranged between a pair of conductive films facing each other are arranged in a matrix, and a plurality of pixels are commonly connected to each row, and a plurality of pixels are connected to each column. In the optical modulation element including an information electrode group in which the pixels are commonly connected, each of the scanning electrodes has the conductive film and a transmission line having a resistance lower than that of the conductive film. Driving means for driving the optical modulation element so that a potential difference gradient is generated between the pair of conductive films forming the pixel by applying different voltages to each other. One of the lines, a scan electrode drive circuit for performing scan selection by applying a voltage from one end of the transmission line to the other and the other end of the transmission line to the other, and an end of the transmission line to the information electrode group. Depending on the distance from An optical modulation element characterized by comprising a data electrode driving circuit for applying a broadcast signal. According to the present invention, it is possible to accurately perform gradation display based on the potential difference gradient method by suppressing the fluctuation of the potential along the transmission line and compensating for the fluctuation that still occurs by the information signal.

EXAMPLES The present invention will be described below with reference to the drawings. As the optical modulator used in the driving method of the present invention, a first optically stable state (for example, a bright state is formed) and a second optically stable state (for example, a dark state) depending on an applied electric field are used. A substance having a stable state against an electric field, that is, a liquid crystal having such a property, is used.

As a liquid crystal having bistability that can be used in the driving method of the present invention, a chiral smectic liquid crystal having ferroelectricity is most preferable, and a chiral smectic C among them is most preferable.
Phase (SmC *), H phase (SmH *), I phase (SmI *), F phase (S
mF *) and G-phase (SmG *) liquid crystals are suitable. About this ferroelectric liquid crystal, "LE JOURNAL DE PHYSIQUE LETTR"
E ") Volume 36 (L-69) 1975" Fuerro Electric
Liquid Crystals "(" Ferroelectric Liquid C
rystals ");" Applied Physics Letters "Vol. 36, No. 11, 1
980 "Submicro Second Bistable Electrifying Switching In Liquid Crystals"("Submicro Second Bistable Elect
rooptic Switching in Liquid Crystals ”);“ Solid State Physics 16 (141) 1981 “Liquid Crystal” and the like, and the ferroelectric liquid crystals disclosed therein can be used in the present invention.

More specifically, examples of the ferroelectric liquid crystal compound used in the method of the present invention include desiloxybenzylidene-P'-amino-2-methylbutylcinnamate (DOBAMBC) and hexyloxybenzylidene-P'-amino-. 2-chloropropyl cinnamate (HOBACPC) and 4-o- (2
-Methyl) -butyl resorcylidene-4'-octylaniline (MBRA8) and the like.

When an element is constructed using these materials, the element is heated by a heater as necessary in order to keep the liquid crystal compound in a temperature state where it becomes SmC *, SmH *, SmI *, SmF *, SmG *. It can be supported by embedded copper blocks or the like.

FIG. 1 is a schematic drawing of an example of a ferroelectric liquid crystal cell. 11 and 11 ′ are substrates (glass plates) coated with transparent electrodes such as In ₂ O ₃ , SnO ₂ and ITO (Indium-Thein-Oxide), between which the liquid crystal molecular layer 12 is perpendicular to the glass surface. A liquid crystal of SmC * phase oriented so that it is enclosed. A thick line 13 represents a liquid crystal molecule, and this liquid crystal molecule 13 has a dipole moment (P⊥) 14 in a direction orthogonal to the molecule. When a voltage above a certain threshold is applied between the electrodes on the substrates 11 and 11 ', the helical structure of the liquid crystal molecules 13 is unraveled, and all dipole moments (⊥) 14 are oriented in the electric field direction. Can be changed. The liquid crystal molecule 13 has an elongated shape,
Liquid crystal optical modulation that exhibits refractive index anisotropy in the major axis direction and the minor axis direction, and therefore, for example, if polarizers arranged in a crossed Nicols position above and below the glass surface are placed, the optical characteristics change depending on the voltage applied polarity. It is easy to understand that the device is an element. Further, when the thickness of the liquid crystal cell is made sufficiently thin (for example, 1 μ), the helical structure of the liquid crystal molecules is unraveled (non-helical structure) even when no electric field is applied, as shown in FIG. The moment P or P'takes either an upward (24) or downward (24 ') orientation state. When an electric field E having a polarity equal to or larger than a certain threshold is applied to such a cell as shown in FIG. 2, the dipole moment electric field E is directed upwards corresponding to the electric field vector of E '.
Or, the liquid crystal molecules are turned downward 24 ', and the liquid crystal molecules are corresponding to the first stable state 23 (bright state) or the second stable state 23'.
Oriented to either one of (dark state).

There are two advantages of using such a ferroelectric liquid crystal as an optical modulation element. First, the response speed is extremely fast,
Secondly, the alignment of liquid crystal molecules has bistability.
The second point will be explained, for example, with reference to FIG. 2. When the electric field E is applied, the liquid crystal molecules are aligned in the first stable state 23.
In this state, the first stable state 23 is maintained even when the electric field is cut off, and when an opposite electric field E'is applied, the liquid crystal molecules are oriented to the second stable state 23 'and the orientation of the molecule is changed. Although it changes, it keeps this state even when the electric field is cut off, and has a memory function in each stable state. In order to effectively realize such a high response speed and bistability, it is preferable that the cell be as thin as possible, and generally 0.5% or less.
μ to 20 μ, particularly 1 μ to 5 μ are suitable. A liquid crystal-electro-optical device having a matrix electrode structure using a ferroelectric liquid crystal of this kind has been proposed by Clarke and Lagabal in US Pat. No. 4,367,924.

Next, details of the liquid crystal optical element used in the present invention will be described with reference to FIG.

Reference numeral 31 in FIG. 3 denotes one of the substrates. Reference numeral 32 denotes a display conductive film, which is laminated on the substrate of 31. Reference numeral 33 denotes a transfer electrode made of a low-resistance metal film, which is laminated on the display conductive film 32 in parallel at equal intervals. The other substrate (not shown) faces the substrate 31, and a counter conductive film (counter electrode) 34 is arranged in a region on the other substrate corresponding to the region of the pixel A in the figure. The above-mentioned optical modulator is sandwiched between the display conductive film 32 and the counter electrode 34.

In the liquid crystal optical element configured as described above, a potential gradient is generated in the electric field between the counter electrode 34 and the potential gradient in the plane of the display conductive film 32 by the signal voltage applied to the transmission electrode 33. . At this time, the transfer electrode 33b is connected to the reference voltage point V _E (for example, 0 volt), and the transfer electrodes 33a and 33a are connected.
When a predetermined signal voltage Va is applied to the transmission electrode adjacent to the transmission electrode connected to the reference potential point as in 3c, the conductive film 32 between the transmission electrodes 33a and 33b or 33b and 33c is applied as shown in FIG. 11 (a). A potential gradient of Va can be applied in the in-plane length directions l ₁ and l ₂ . At this time, when the inversion threshold voltage Vth of the ferroelectric liquid crystal is set to Va, if -Vb is applied to the counter electrode 34, FIG.
Will be inversion threshold voltage Vth or higher potential difference Va + Vb to the ferroelectric liquid crystal corresponding to the length direction m ₁ and m ₂ in the plane of the conductive film 32 as shown in is applied, in such m ₁ and m ₂ The corresponding area can be inverted, for example from a bright state to a dark state. Therefore, in the present invention, the gradation can be expressed by applying Vb with a value according to the gradation for each pixel. On this occasion,
The voltage value of the voltage signal −Vb applied to the counter electrode 34 may be modulated according to the gradation information, or its pulse width may be modulated according to the gradation information, or the number of its pulses may be modulated. Therefore, the gradation can be controlled.

Further, in the present invention, prior to applying the above-mentioned gradation signal,
After the erasing step for setting the pixel to either the bright state or the dark state, the inversion voltage for inverting the state is controlled according to the gradation and applied to the ferroelectric liquid crystal. It is necessary.

Furthermore, preferred specific examples of the present invention will be described.

In FIG. 3, a SnO ₂ film, which is a transparent conductive film having a thickness of about 200 Å, was formed on a glass substrate 31 by a sputtering method to form a display conductive film 32. The sheet resistance of this SnO ₂ film is 105Ω
It was / □. Then, Al having a thickness of 1000 Å was vacuum-deposited on the above-mentioned SnO ₂ film and patterned again to form a plurality of transfer electrodes 33 as shown in FIG. In this example, the transmission electrode
The interval of 33 was 230μ. The sheet resistance of this transmission electrode 33 is about 0.4Ω / □, and its width is about 20μ. on the other hand,
An ITO film covering the area A was provided as the counter electrode 34 on the counter substrate. The sheet resistance of the ITO film serving as the counter electrode 34 was about 20Ω / □.

A polyvinyl alcohol layer of about 500 liters was formed as a liquid crystal alignment film on each surface of the two substrates thus manufactured, and subjected to rubbing treatment.

Next, the two substrates are opposed to each other and the gap is adjusted to be about 1 μ, and the ferroelectric liquid crystal (pn-octyloxybenzoic acid-P '-(2-methylbutyloxy) phenyl ester and p-n-octyloxybenzoic acid-p- A liquid crystal composition containing η-nonyloxybenzoic acid-P ′-(2-methylbutyloxy) phenyl ester as a main component was injected. The shape of the partial pixel A in which the display conductive film 32 and the counter electrode 34 overlap was 230 μ × 230 μ, and the electrostatic capacity after liquid crystal injection was about 3 PF. However, the width of pixel A is And

Polarizing plates were arranged in crossed Nicols on both sides of the liquid crystal cell thus formed, and optical characteristics were observed.

FIG. 4 schematically shows a method of applying an electric signal, and FIGS. 5 and 6 show electric signals. FIG. 5 shows the waveform of the signal (a) generated by the drive circuit 43 of FIG.
FIGS. 6 (i) to 6 (v) show the waveforms of the signal (b) generated in the drive circuit 44 of FIG.

An erasing step for providing a −12 V 200 μsec pulse as the signal (a) and a 8 V 200 μsec pulse in synchronism beforehand (this is called an erase pulse) is provided. Then, the liquid crystal is switched to the first stable state, and the entire pixel A is in the bright state (the cross polarizing plate is arranged in this way). From this state, when various pulses as shown in FIGS. 6 (i) to 6 (v) are given to the counter electrode 34 in synchronization with the pulse of FIG. 5 applied to the transmission electrode 33 as the signal (b). FIG. 7 shows the optical state of the pixel A of FIG.

There is no change from the bright state 71 (corresponding to FIG. 7 (a)) at the pulse applied voltage −2V (corresponding to FIG. 6 (a)) and −5V (corresponding to FIG. 6 (b)). , Pulse applied voltage −8V
In (corresponding to FIG. 6 (c)), the liquid crystal in the vicinity of the transmission electrode 33 switches to the dark state 72 (corresponding to FIG. 7 (b)).
Further, when the applied voltage is increased to −14 V (corresponding to FIG. 6 (d)), the region of the dark state 72 becomes wide as shown (corresponding to FIG. 7 (c)) and the applied voltage 20V (corresponding to FIG. 7 (c)). Figure 6 (e)
The entire pixel A is switched to the dark state 72 (corresponding to FIG. 7 (d)). In this way, an image with gradation can be formed.

Further, various signals (b) having different pulse widths as shown in FIGS. 9 (a) to (e) and a signal (a) having a triangular wave as shown in FIG. 8 were given in synchronization. At any time, the optical state change illustrated in FIG. 7 can be shown above. At this time, the pulse shown in FIG. 8 is applied to the transmission electrode, and the pulses shown in FIGS. 9 (a) to 9 (e) are applied to the counter electrode 34 according to the gradation in synchronization with this pulse. It can express tonality.

In FIG. 4, reference numeral 41 is a ferroelectric liquid crystal, preferably a chiral smectic liquid crystal in a bistable state, and 42 is a counter substrate.

Further, in the present invention, the aluminum (Al) used in the above-mentioned example
In addition to the transmission electrode 33, a metal such as silver, copper, gold, or chromium can be used as the transmission electrode 33, and the sheet resistance thereof can be preferably 102Ω / □ or less. Further, the conductive film 32 to which the potential gradient is applied is 10Ω / □ to 1KΩ /
A transparent conductive film having a sheet resistance of □ can be used. Such sheet resistance can be designed to an appropriate value by adjusting the film thickness of the transparent conductive film.

FIG. 10 shows a specific example when the gradation expression method according to the present invention is applied to matrix driving.

In the expression panel shown in FIG. 10, a plurality of stripe-shaped conductive films 101 (101a, 101b, 101c) are arranged on a glass substrate 31,
Further, the low-resistance transmission electrodes 102 (102a, 102b, 102) are provided on both ends of each stripe-shaped conductive film 101 in the longitudinal direction.
c) and 103 (103a, 103b, 103c) are wired. Board 31
A counter electrode 104 (104a, 104b) made of a stripe-shaped conductive film provided on a counter substrate (not shown) facing the above is disposed, and the ferroelectric film is provided between the stripe-shaped conductive film 101 and the counter electrode 104 described above. A liquid crystal is arranged.

FIG. 12 shows an electrical equivalent circuit of the display panel shown in FIG. 10, 121 is the resistance of the stripe-shaped conductive film 111,
122 and 123 are resistances of the transmission electrodes that are electrically connected and wired to both ends of the stripe-shaped conductive film 111 in the longitudinal direction. Incidentally, the respective resistances are not concentrated but are uniformly distributed. Now, the sheet resistance of the striped conductive film 111 is 106Ω / □, and the transmission electrode lines 112 and 1
The sheet resistance of 13 is 0.4Ω / □. In addition, the width of the transmission electrode lines 112 and 113 is 10 μm and the length is 200 mm.
The distance between 12 and 113 was 250 μm, the terminal 124 was grounded, 1 V was applied to the terminal 125, and the terminals 126 and 127 were opened.

FIG. 13 shows the potential distribution at that time. A curve 131 in FIG. 13 (a) is a potential curve showing the potential distribution in the longitudinal direction (between the terminals 124 and 126) of the transmission electrode line 112. Further, the curve 132 in FIG. 13 (b) is the transmission electrode line 1
13 is a potential curve showing a potential distribution in the longitudinal direction of 13 (between terminals 125 and 127). A curve 133 in FIG. 13C shows a stripe-shaped conductive film 11 narrowed by the transmission electrode lines 112 and 113.
6 is a potential difference curve showing a state in which the potential difference in the lateral direction of 1 is distributed in the longitudinal direction of the stripe-shaped conductive film 111. As can be seen from FIG. 13C, the potential difference in the lateral direction of the striped conductive film 111 on the terminals 124 and 125 side is 1V.
However, the potential difference decreases as the distance from the terminals 124 and 125 increases, and the potential difference approaches 0 V on the terminals 126 and 127 side.

By the way, in a display panel in which a high-density potential gradient is formed by wiring the transmission electrode lines at a high density, the resistance value of the stripe-shaped conductive film becomes so small that it cannot be ignored compared to the resistance value of the transmission electrode lines. As the leak current flowing through the film increases and moves away from the contact end, the ground potential rises as shown in FIG. 13 (a), and at the same time the applied potential drops as shown in FIG. 13 (b). First
As shown in FIG. 13 (c), the potential difference between the transmission electrode lines decreases as the distance from the contact end increases.

In order to solve this, there are two methods, for example, (1) lowering the resistance value of the transmission electrode line and (2) increasing the resistance value of the stripe-shaped conductive film. As a result, the width of the transmission electrode line is increased, which causes a reduction in the aperture ratio, and when the thickness is increased, the flatness is lost and liquid crystal alignment defects are induced. In the case of (2), the thickness of the conductive film is reduced, so that the unevenness of the resistance value increases, and if the film thickness is too small, it takes a lot of time to charge the liquid crystal, resulting in instability. Will increase.

FIG. 14 shows the potential distribution when the signals transmitted to the two transmission electrode lines are transmitted in opposite directions. A curve 141 in FIG. 14 (a) is a potential curve showing a potential distribution in the longitudinal direction of the transmission electrode line. Also,
A curve 142 in FIG. 14B is a potential curve showing a potential distribution in the longitudinal direction of the other transmission electrode line. First
Reference numeral 143 in FIG. 14C is a potential difference curve showing a state in which the potential difference in the lateral direction of the striped conductive film narrowed by two transfer electrode lines is distributed in the longitudinal direction of the striped conductive film.

In this way, by making the transmission directions 141a and 142a from the potential point applied to give the potential gradient opposite to each other,
As shown in FIG. 14 (c), it is possible to suppress a sudden change in the potential difference. On the other hand, as shown in FIG. 13, when the transmission directions 131a and 132a from the potential point are in the same direction, a sharp change in the potential difference occurs.

By the way, as shown in FIG. 14 (c), the potential difference between the two transmission electrode lines is not a constant value but a function of distance. That is, if the distance from the applied voltage point is χ, the potential difference between the transmission electrode lines is Vd (χ). This means that the dynamic range of the modulation signal changes, resulting in uneven display quality.

FIG. 15 is a diagram for explaining the present invention. FIGS. 15 (a) and 15 (b) show the voltage at a distance χ1 and the potential difference is Vd.
(Χ1), the information signal is V _E1 (m
in), for example, a blank state, and in FIG. 15B, the information signal is V _E2 (max), for example, an all black state. Further, FIGS. 15 (c) and 15 (d) show the voltage at the distance χ2 point, and the potential difference is Vd (χ2). In FIG. 15 (c), the information signal is V _E2 (min), for example, the margin. State, Fig. 15 (d)
In, the information signal is V _E2 (max), which indicates, for example, the state of all black. Vth in FIG. 15 indicates the threshold voltage of the liquid crystal.

Where Vd (χ1) ≠ Vd (χ2), V _E1 (min) ≠ V _E2 (mi
In n), V _E1 (max) ≠ V _E2 (max).

Thus, the maximum amplitude of the information signal is Vd (χ
1) and the maximum amplitude of the information signal is Vd (χ
2), and at the distance χ1, for example, V _E1 (mim) in the margin,
V _E1 (max) in all black, and V _E2 (mi
m), all black and V _E2 (max).

FIG. 16 is a block diagram of the electric circuit of the present invention. In FIG. 16, 161 is an information input terminal, 162 is a variable amplifier and amplification degree, A is a function A (χ) of the distance χ, 163 Is an offset superimposing circuit, and the offset S is a function S (χ) of the distance χ.
And 164 is an information line drive circuit.

The present invention enables high-quality display without display unevenness.

In addition to the above-mentioned ferroelectric liquid crystals, twisted nematic liquid crystals, guest-host liquid crystals and the like can be used in the present invention, but the most preferable are ferroelectric liquid crystals, especially ferroelectric liquids having at least two stable states. Dielectric liquid crystals are suitable.

〔The invention's effect〕

According to the present invention, high-quality display without display unevenness is possible, and gray scale display is performed by applying a gray scale signal modulated by a voltage value, pulse width, pulse number or the like as an input signal. Can be done.

[Brief description of drawings]

1 and 2 are perspective views schematically showing a ferroelectric liquid crystal element used in the present invention. FIG. 3 is a perspective view showing one of the substrates used in the present invention. FIG. 4 is a sectional view of a liquid crystal optical element used in the present invention. 5 and 6 (a) to 6 (e) are explanatory views showing pulse waveforms used in the present invention. 7A to 7D are schematic diagrams showing the gradation of pixels. 8 and 9 (a)-
(E) is an explanatory view showing the waveform of another pulse used in the present invention. FIG. 10 is a perspective view showing another liquid crystal optical element used in the present invention. 11 (a) and (b) show
It is explanatory drawing which represents typically the electric potential gradient used by this invention. FIG. 12 is an equivalent circuit diagram of an element other than the present invention, and FIGS. 13 (a) to 13 (c) are explanatory diagrams showing the potential distribution at that time. FIGS. 14 (a) to 14 (c) are explanatory diagrams showing potential distributions generated in two transmission electrode lines to which information signals are applied in opposite transmission directions. FIG. 15 is an explanatory diagram for explaining the signal amplitude. FIG. 16 is a block diagram of the device used in the present invention.

Front page continued (72) Inventor Shuzo Kaneko 3-30-2 Shimomaruko, Ota-ku, Tokyo Canon Inc. (72) Inventor Tsutomu Toyono 3-30-2 Shimomaruko, Ota-ku, Tokyo Canon Inc. (56) References JP 61-201217 (JP, A) JP 52-20851 (JP, A) JP 52-122098 (JP, A)

Claims (2)

[Claims] 1. A scanning electrode group in which a plurality of pixels in which an optical modulation substance is arranged between a pair of conductive films facing each other are arranged in a matrix, and a plurality of pixels are commonly connected in each row, and a plurality of pixels in each column. In an optical modulation element including an information electrode group in which pixels are commonly connected, each of the scanning electrodes includes the conductive film and a transmission line having a resistance lower than that of the conductive film. Driving means for driving the optical modulation element is provided so that a potential difference gradient is generated between the pair of conductive films forming the pixel by applying different voltages, and the driving means includes the two adjacent transmission lines. Out of
A scanning electrode drive circuit for performing scanning selection by applying a voltage from one end of the transmission line to one side and the other end of the transmission line to the other side, and a distance from the end of the transmission line to the information electrode group. An optical modulation element comprising: an information electrode driving circuit for applying a corresponding information signal. 2. The optical modulation element according to claim 1, wherein the optical modulation substance is a ferroelectric liquid crystal.