DE4340593A1 - LCD using electro-optic bistable effect of nematic liquid crystal - has insulating film between substrate and alignment film and switching device to charge optical axis of liquid crystal - Google Patents

Abstract

In the liquid crystal display a pair of substrates, (1a,1b) on which transparent electrodes (2a,2b) are formed, are aligned to be approximately parallel to each other. A pair of alignment films (4a,4b), on which the transparent electrodes are respectively formed, and a nematic liquid crystal (6) between the pair of substrates form a liquid crystal cell. A switching device is attached to switch the optical axis of the liquid crystal by selectively applying a voltage to the electrodes. There is an insulating film (3a,3b) between the substrate and the alignment film. USE/ADVANTAGE - Displays for watches, scientific calculators, wordprocessors, PCs, and TVs. Can be reliably switched from one bistable state to other over large display surface.

Description

The invention relates to a liquid crystal display, the electro-optically bistable effect of a nematic liquid crystals used.

To previous display methods using a river sigcrystals belong to the method with dynamic scattering (DS) for converting one placed on a liquid crystal electrical signal into optical data, the process with electrically controlled birefringence (ECB = Electrically Controlled Birefrigence), the phase change (PC = phase Change) process, the storage pattern process, the guest Host (GH = guest-host) method and the method with one surface stabilized ferroelectric (SSF = Surface Stabilized Ferroelectric).  

Among the methods used for displays in products nissen like clocks, scientific calculators, word processors, PCs and televisions are used especially the twisted nematic liquid process crystal (TN = Twisted Nematic) and the process with super twisted nematic liquid crystal (STN = Supertwisted Nematic), which is an improvement on the TN process, by Importance.

These processes all use single-stable liquid crystals, the same under such an orientation treatment be pulled that the liquid crystal molecules parallel to Substrate are aligned in one direction. It exists already have a patent application for liquid crystal displays an insulating layer between the glass substrate and the Aus directional treatment layer (see publication no. 51 (1976) -124941 to a Japanese patent application). Further there is a patent application related to a method in which, as part of an alignment treatment, oblique separation vapor phase or an insulating, anor ganic thin film on a substrate with an electrode is worn (see publication no. 57 (1982) -112714 a Japanese patent application).

As bistable liquid crystal displays that have a nematic Use liquid crystal as ge in the invention G. Durand has proposed two different types gene.

One of these types uses a chiral ion to generate against a twisting force (publication WO-91/11747 an international patent application). With one Device will be both clockwise and counterclockwise chiral ions mixed with a liquid crystal to form a Deviation of the ion distribution caused by a voltage  to cause a twisting force. At a Such procedures allow the creation of an impulsive moderate electric field, like this from a liquid crystal stall display with surface stabilized ferroelectric (SSFLCD) recognizes the liquid crystal molecules in one Switch position parallel to the substrate surface. However this process has a major disadvantage its reliability since it does not contain pure ions ben used the device.

The other type of liquid crystal display uses a Verbie polarization as driving torque. As an alignment The film becomes an oblique vapor deposition phase produced SiO film used. This device takes advantage of the fact that there is an appropriate selection of film Manufacturing conditions allowed that the nematic liquid crystal stall shows a stable orientation in two directions (Publication WO-92/00546 on an international Pa registration). It is believed that there is high reliability expected without the difficulty of contamination can be because it results from an alignment distortion bending polarization as driving torsional force is used. With this method, it allows creation of a pulsed electrical field, as is the case with SSFLCD can be seen switching the liquid crystal molecules in a direction parallel to the surface of the sub strats. The response time is approximately 100 µsec. Since the Liquid crystal in a direction parallel to the substrate surface is switched, there can be no dependency on the consideration angle can be recognized. There is also this device no alignment problem as with an SSFLCD, since this Kind of liquid crystal display a nematic liquid crystal stall used, and it is a sufficiently large area the operating temperature possible. The river according to the invention Sig crystal display belongs to the latter type.  

Fig. 3 shows the structure of a nematic, bistable Dis plays based on the Verbiegungspolarisation, as reported by Durand G. (s. SID documents, pp 606-607, Appl. Phys. Lett. 60 (9), 2 March 1992, pp. 1085-1086). In Fig. 3 glass substrates 1 a, a liquid crystal layer 6 , transparent electrodes 2 a, 2 b, SiO alignment films 4 a, 4 b and spacers 5 a, 5 b are shown. The vapor deposition angle for the SiO alignment film is oriented at approximately 74 ° to normal on the substrate, the thickness of the film is approximately 3 nm and the diameter of the spacers is approximately 1 to 3 µm. Under these circumstances, as shown in Fig. 4, the liquid crystal molecules are inclined by the angle R ° against the substrate surface. In addition, the direction projected onto the substrate surface is twisted by α ° to -α ° to produce directions A and B in which the liquid crystal molecules show a bistable state. In addition, when the thickness of the cell is sufficiently reduced to 1 to 3 μm, the liquid crystal molecules show a stable state in the direction perpendicular to the vapor deposition direction of the SiO and in the direction C parallel to the substrate surface.

Fig. 4 shows the SiO vapor deposition direction and the direction in which the liquid crystal molecule orientation can be stable. The alignment treatment directions for the upper and lower substrates are determined so that the vapor deposition direction for the SiO is rotated by 45 ° against the antiparallel direction. A liquid crystal material is used which is doped with a chiral material so that it causes a twist of 22.5 ° between the upper and lower substrates. Incidentally, the drilling direction of the liquid crystal molecules is set so that it is opposite to the twisting direction according to the SiO vapor deposition between the upper and lower substrates.

In the drawings, the dashed arrow indicates the SiO vapor deposition direction. Reference numerals ( 1 ) to ( 3 ) and ( 1 ) 'to ( 3 )' denote directions of stable alignment that the liquid crystal molecules can occupy on any substrate surface. In this way, the structure of the liquid crystal display is due to the fact that the SiO vapor deposition is rotated by 45 ° against the antiparallel orientation. Therefore, when a liquid crystal material having the above conditions is introduced into a cell, the orientation which is possible in the stable state is limited by the action of the chiral material. The liquid crystal molecules can be stable in combinations, as shown with ( 2 ) to ( 3 ) 'and ( 3 ) to ( 1 )'.

Figure 5 is a cross section through a cell. The symbol a corresponds to the orientation ( 2 ) to ( 3 ) ', while the symbol b corresponds to the orientation ( 3 ) to ( 1 )'. If a liquid crystal molecule is wedge-shaped, the alignment distortion on the sprayed material creates a bending polarization. The arrow in Fig. 6 indicates the direction of bending polarization. Here, the vertical components of the bending polarities designated with a and b are aligned in mutually opposite directions. This makes it possible to change the bistable state by the fact that the vertical component of the supply polarization is reversed by applying a pulse-shaped electrical field.

However, if the SiO alignment film is directly on the substrate is formed, as is the case with the above method the directions of orientation for the bistable condition on the glass surface and the transparent  term electrode due to the step between the glass top surface of the substrate and the formed through it term electrode from each other. In addition, the bista bile alignment from the glass surface or the see through term electrode, because the thickness of the SiO alignment tion film is low. The condition is also more stable Alignment changed to one of the following stable states changed: an alignment state I before creating one electric field or an alignment state II after the application of the electrical field in which the electrical Field is strengthened. So no bistable switching he can aims to be. G. Durand's report describes that Um switch in a liquid crystal domain with a size of 1 mm was realized. No bistable switching is over the entire area of the display screen is achieved. Accordingly it is very difficult to get a steady alignment state to achieve over a large part of an area. Furthermore the application of an electric field leads to short circuits between the top and bottom substrates because the thickness of the Liquid crystals with 1.5 µm to 3.0 µm is very small.

The invention has for its object a bistable To specify liquid crystal display that over large display area Chen reliably from one bistable state to another can be switched.

The liquid crystal display according to the invention is characterized by the Teaching of appended claim 1 given. This display points u. a. an insulating film between the substrate and the Alignment film on.

The insulating film is preferably used in such a liquid undergo alignment treatment.

In addition, the thickness of the insulating film is preferably in  Range from 0.01 to 1 µm or 0.5 to 3.0 times the Electrode thickness.

It is also preferred to produce the insulating film either from an inorganic thin film, which is selected from the group consisting of SiO ₂ , SiN _x , Al ₂ O ₃ , or from an organic thin film, which is made from a polyimide, a photoresist resin or a group containing polymeric liquid crystal is selected.

The invention is hereinafter in connection with the beige added drawings in which:

Fig. 1 is a schematic section through a liquid crystal display according to the invention;

Fig. 2 is a schematic illustration of the relationship between the vapor deposition direction and the alignment of liquid crystal molecules;

Fig. 3 is a schematic cross section through a conventional liquid crystal display;

Fig. 4 is a general diagram for illustrating the bistable state of a liquid crystal;

Fig. 5 is a diagram illustrating the relationship between the vapor deposition direction and the alignment of liquid crystal molecules; and

Fig. 6 is a diagram illustrating the alignment of liquid crystals across the cross section of a liquid crystal cell.

The invention provides a liquid crystal display under Ver  application of a nematic liquid crystal with uniform bistable condition over the entire display area both in a first alignment state before creating an elec tric field as well as in a second alignment stood after applying the electric field, which is same is separated.

In the invention, a translucent, insulating substrate, e.g. B. glass used. On this insulating substrate, a transparent electrode with a predetermined pattern of an electrically conductive thin film such as that of InO ₃ , SnO ₂ or indium tin oxide (ITO = indium tin oxide) is formed.

An insulating film is formed on this transparent electrode. An inorganic material such as SiO ₂ , SiN _x or Al ₂ O ₃ or an organic material such as polyimide, a photoresist resin or a polymeric liquid crystal can be used as the insulating film material. If the insulating film is made of an inorganic material, it can be formed by vapor deposition, sputtering, chemical vapor deposition (CVD) or solution coating. In particular, the insulating film is best formed by electron beam (EB = electron beam) vapor deposition. If an organic material is used for the insulating film, z. B. uses a solution in which an organic material or a precursor thereof is dissolved in order to apply the aforementioned thin film by a method such as the spin coating method, the dip coating method, the screen printing method, the roll coating method or a method in which the material is cured under predetermined curing conditions (such as the application of heat or light radiation) to form the insulating film. It is also possible to use methods such as vapor deposition, sputtering, CVD or the Langmuir-Blodgett (LB) method.

Alignment treatment procedures for the insulating film hear the rubbing process and the oblique bob process separation from the vapor phase. The rubbing process is in that Case preferred that large area liquid crystal displays che be manufactured. In the rubbing process, the iso lier film after its production of a rubbing treatment educated. Here, for. B. rubbed in parallel (both Sides of a pair of substrates subjected to the rubbing treatment and they are stacked so that the Rubbing directions are aligned in the same direction), the anti-parallel rubbing process (in which both sides of the Substrates are subjected to rubbing treatment and so on be layered together that the rubbing directions in ent opposite directions), and the one-sided grater procedure in which one side of the substrates of the rubbing treatment is subjected).

The insulating film preferably has a thickness of 0.01 to 1 µm, preferably 0.02 to 0.5 µm. If the fat of the insulating film is less than 0.01 µm, it is subject to an adverse influence from the surface of the glass or the surface of the transparent electrode. If the fat is larger than 1 µm, this may be the reason for the generation a step between the substrate and the insulating film, which is very undesirable. In addition, the insulating film 0.5 to 3 times the thickness of the transparent electrode. If the insulating film is less than 0.5 times the thickness of the has transparent electrode, it tends to cut out conditions and protrusions of the transparent electrode to become. Then the alignment adjustment becomes difficult. If the insulating film is more than 3 times the thickness of the has transparent electrode, the drive voltage he be raised. In addition, electrical charge collects  in the insulating film, causing an electric field to reverse direction is generated.

An alignment film is formed on the insulating film. Either an inorganic or an organic material can be used for the alignment film. If an organic material is used, oblique deposition of silicon oxide from the vapor phase is preferred. In addition, a method such as rotary vapor deposition can be used. If an organic material is used, materials such as nylon, polyvinyl, polyimide or the like can be used. Usually, the alignment film is subjected to alignment treatment by rubbing the same. In addition, alignment treatment can be carried out by using a material such as a polymer liquid crystal, an LB film or a magnetic film, or by using a method such as the spacer edge method or the like. In addition, SiO ₂ and SiN _x can be deposited from the vapor, followed by rubbing the film to achieve alignment.

The pre-tilt angle is the liquid tilt angle crystal molecules against the direction perpendicular to the sub defined strat. This can change the pre-tilt angle that a polyimide type alignment film with a means of changing the vertical orientation is like N, N-octadecyl-3-amino-propyltrimethoxysilyl chloride (DMOAP) after rubbing this film or silicon oxide applied by oblique deposition from the vapor phase has been. In the case of rubbing treatment, the pre-inclination angle by changing the length of the cloth used for rubbing of a field and the number of revolutions of a roller be changed. In addition, the steam trap conditions depending on the separation angle for the  Silicon oxide and the desired film thickness can be changed.

According to the invention, the alignment film on one sub strat and the one on the other preferably trained so that their directions of orientation differ from each other the, in particular by 0 to 90 °. The angle is more preferred set to 15 to 60 °.

More specifically, a vapor phase SiO film is used as the alignment film so that the deposition direction on the two substrates is set to a smaller degree against the parallel position, that is, less than the twist direction of 90 °, or preferably to approximately 45 ° (see Fig. 2).

The liquid crystal according to the invention includes nematic ones Schiff chlorine type liquid crystals, azo type, Azoxy type, benzoate type, biphenyl type, pyrimidine type and of the dioxane type and mixtures thereof, in particular Multi-component liquid crystals. To concrete liquid crystals tall blends, as are commercially available, belong the following: Z series manufactured by Merck, such as Z-1625, Z-1565, Z-1780, Z-1800, Z-1840, Z-1825 and SCB, from BDH manufactured series E such as E-7, E-37, E-31LV, E-80, E-44, the R series manufactured by Roche such as R-200, R-623, R-701, R-619, R-627C, the L series manufactured by Chisso as L-GR45, L-9106, L-EN24, L-P23NN23 and those from Dai Nippon Ink manufactured series D such as D-601T, D-X01A and D-800. Furthermore these liquid crystals can be mixed appropriately.

Then, the above-mentioned liquid crystal is chi rales auxiliary material (optically active compound) added. By doing this, the screw pitch of the Liquid crystal phase set. Chira that can be used specifically le auxiliaries are the following: cholesteryl bromide, chole  steryl-n-hexyl ether, cholesteryl benzoate, cholesteryl-n-hepta noat, cholesterylunanoate, 4,4- (2-methylbutyl) phenylbenzoe 4′-cyanophenyl acid, 4-n-hexyloxybenzoic acid 4 ′ - (2- butoxycarbonyl) phenyl ester, t-4- (2-methylbutyl) cyclohexyl carboxylic acid-cyanobiphenyl, 4- (4-methylbutyl) -4′-cyano-p- terphenyl, N- (4-ethoxybenzylidene) -4- (2-methylbutyl) aniline, 4- (2-methylbutyl) 4'-n-hexyloxyphenyl benzoate, 4-n- Heptoxy-4 ′ - (methylbutyloxycarbonyl) biphenyl, 4- (2-methyl butyl) -4'-carbonylphenyl and 4-4- (2-methylbutyl) phenylbene 4'-butylphenyl zoate.

Products available for purchase include S-8100 (from Manufactured by Merck).

In addition, compounds other than those above can be used mentioned nematic liquid crystal compounds suitable be mixed. Such connections do not have to be liquid have crystal phase. These include:

• a) Connections to adjust the temperature range of the Liquid crystal phase;
• b) optically active compounds that have a large spontaneous pole rization in the ferroelectric liquid crystal phase gene or evoke.

After the liquid crystal is introduced into the cell, becomes a filler opening with a UV-curable acrylic resin filled.

In addition, a liquid crystal display can be produced be placed that a polarizer is arranged so that its polarization axis is approximately perpendicular to the vertical len direction of the liquid crystal cell and a pola risator roughly one of the optical axes of the liquid crystal stable in the cell.  

example 1

Fig. 1 is a main section through a liquid crystal display according to the invention.

On glass substrates 1 a and 1 b are transparent electrodes 2 a, 2 b with a thickness of 100 nm formed. An SiO ₂ film 3 a, 3 b is deposited on the glass substrate as an insulating film by separation from the vapor phase with a thickness of 100 nm. Then SiO alignment films are deposited on the vapor phase by oblique deposition. The conditions for this are specified as follows: vapor deposition angle of 70 ° against the normal on the substrate, with a film thickness of 7 nm. The upper and lower substrates are stacked so that they are 45 ° against the anti-parallel directions with regard to the vapor deposition direction be twisted. The cell thickness is 1.5 µm.

A nematic liquid is placed in the cell thus produced filled in crystal (at which 0.36% by weight of the chiral mate rials S-811 are added to the host liquid crystal SCB).

The result is an even and stable alignment free of deviations in the bistable state. The contrast is 20 to 1.

Example 2

Example 2 of a liquid crystal display according to the invention has the same basic structure as that of example 1. Transparent electrodes with a thickness of 100 nm are formed on glass substrates. After vapor deposition of SiO ₂ with a thickness of 100 nm as an insulating film on the substrates, rubbing takes place in the same direction. Then, SiO is deposited from the vapor phase by oblique evaporation to produce an alignment film. The evaporation conditions are the same as mentioned above. The upper and lower substrates are stacked so that they are twisted with respect to the anti-parallel direction with respect to the vapor deposition direction. The thickness of the liquid crystal cell is again 1.5 µm. The same liquid crystal as specified in Example 1 is filled. This again results in a uniform and stable alignment state without deviation from the bistable state.

Example 3

Example 3 of a liquid crystal dis plays has the same basic structure as that of the Be play 1. Transparent elec trodes with a thickness of 100 nm. After opening wearing and aligning a photoresist resin (OCD P-59310; manufactured by Tokyo Applied Chemistry) by a sluice The device as an insulating film is followed by sintering at 350 ° C. SiO is formed on the insulating film by oblique deposition Vapor phase applied to make an alignment film len. The two substrates produced in this way are thus matched layered so that it is 45 ° opposite to that for steam direction of separation are twisted antiparallel direction. The cell thickness and the liquid crystal material are diesel ben as in the previous embodiments. It results an even and stable alignment free of ab departures from the bistable state.

Example 4

Example 4 of a liquid crystal display according to the invention corresponds essentially to Example 1. An SiO ₂ film is deposited on glass substrates by oblique evaporation from the vapor phase and aligned to form an alignment film. The vapor deposition angle is 74 ° from the normal on the substrate; the film thickness is 7 nm. The substrates are layered on one another as in Example 1 (see FIG. 2). The cell thickness is again 1.5 µm. A nematic liquid crystal is filled into the finished cell (in which 0.36% by weight of the chiral material is mixed for 9 s to form the host liquid crystal SCB). A uniform and stable alignment state can be obtained. When a pulsed electric field of 20 V is applied to the liquid crystal panel, a stable state is shown. The contrast is 20 to 1.

Example 5

The basic structure in Example 5 of a liquid crystal display according to the invention is the same as in Example 1. Transparent electrodes with a thickness of 100 nm are formed on glass substrates. SiO _{2 is} deposited on the substrate from the vapor phase with a thickness of 50 nm as the base film. The vapor phase is then deposited on this SiO to form an alignment film. A vapor separation angle of 74 ° is used; the film thickness is 7 nm. The two substrates are layered on top of one another as in Example 1. The cell thickness is again 1.5 µm. A nematic liquid crystal (in which 0.36% by weight of a chiral material has been mixed into the host liquid crystal 5 CB) is filled into the cell thus finished. A uniform and stable alignment state is obtained. If a pulsed electric field is applied by applying 20 V. A stable bistable state is produced on the panel produced as described above; the contrast is 20 to 1.

Example 6

Example 6 has the same basic structure as the liquid crystal cell in example 1. On glass substrates are formed by visible electrodes with a thickness of 100 nm. SiO ₂ is deposited on the substrates as a base film with a thickness of 300 nm from the vapor phase. Then, SiO is vapor-deposited thereon to form an alignment film. The vapor deposition angle is 74 ° against the normal on the substrate; the film thickness is 70 °. The substrates are layered on top of each other as in example 1, the cell thickness again being 1.5 μm. A ne matic liquid crystal is filled into the cell thus finished (in which 0.36% by weight of a chiral material is added to the host liquid crystal SCB). A uniform and stable alignment without deviation in the bistable state is obtained.

Comparative Example 1

The basic structure of the liquid crystal display according to Comparative Example 1 is the same as that of Example 1. SiO ₂ is deposited on the substrate as a base film with a thickness of 30 nm from the vapor phase in order to produce an alignment film. The vapor deposition angle is again 74 °, the thickness of the film 7 nm. As in Example 1, the substrates are put together with a cell thickness of 1.5 μm. A nematic liquid crystal is filled into the cell thus produced (in which 0.36% by weight of a chiral material is mixed in the host liquid crystal SCB). An evaluation of the alignment state shows that it is unstable on the transparent electrode. Applying a voltage does not lead to a bistable state. After a while, there is a short circuit between the upper and lower substrates.

Comparative Example 2

Comparative example 2 of a liquid crystal display has the same basic structure as example 1 of the invention. Transparent electrodes with a thickness of 100 nm are formed on the substrate. An SiO ₂ base film with a thickness of 500 nm is then deposited from the vapor phase. Then SiO is deposited as an alignment film from the vapor phase. The vapor deposition angle is 74 ° against the normal on the substrate; the film thickness is 7 nm. The substrates are layered on top of one another as in Example 1, with a cell thickness of 1.5 μm. The same liquid crystal as in Example 1 is also filled. The orientation was relatively stable. When a pulse-shaped electrical field is applied by means of a voltage of 20 V, the device is not switched. On the other hand, it is partially switched when a pulsed electric field is generated by applying 30 V.

Using the same procedure, the thickness of the transparent Electrode set to 100 nm (fixed). Then the Thickness of the insulating film according to examples of 75, 100, 150, 200, 250 nm changed. An An is used as a comparative example Pointing device made in which the thickness of the insulating film is 40 nm to determine the liquid crystal orientation evaluate. Table 1 shows the evaluation for the alignment state in the nematic liquid crystal when the The thickness of the insulating film changes in the range from 30 to 500 nm.

As shown in Table 1, the liquid crystal shows then when the thickness of the insulating film is between 60 and 300 nm lies, a stable alignment state, and switching is stable.

Table 1

Relationship between the thickness of the insulating film and the alignment state

In this example, SiO ₂ or a photoresist resin (OCD P-59310; manufactured by Tokyo Appl. Chem.) Was used. In the invention, only the entire substrate needs to be coated uniformly to provide bistable alignment. Materials such as silicon dioxide, titanium oxide, aluminum oxide, polyimide or the like can be used as the insulating film. In addition, any process can be used to form the insulating film, such as sputtering, deposition from the vapor phase, CVD or coating by spinning. The invention is not limited to the previous examples. The thickness of the insulating film can be set in the range from 0.02 to 0.5 μm, preferably in the range from 0.05 to 0.2 μm.

The invention enables the production of a liquid crystal stall displays with a large area, that with increased speed can be operated. It shows a steady Orientation state over a larger area than in ago conventional displays. The display is made of depressions and Protrusions on the surface of the transparent electrodes not affected, and it is a stable condition in bista quick switching operations possible.

Claims (6)

1. Bistable liquid crystal display in which a pair of substrates ( 1 a, 1 b), on which transparent electrodes ( 2 a, 2 b) are formed, are aligned approximately parallel to one another, with an alignment film ( 4 a, 4 b) each of the transparent electrodes is formed and a nematic liquid crystal ( 6 ) is filled between the pair of substrates to form a liquid crystal cell, and a switching device is attached to switch the optical axis of the liquid crystal by selectively applying a voltage to the electrodes, characterized in that an insulating film ( 3 a, 3 b) is present between the substrate and the alignment film. 2. Display according to claim 1, characterized in that the insulating film ( 3 a, 3 b) has undergone an alignment treatment. 3. Display according to one of claims 1 or 2, characterized in that the insulating film ( 3 a, 3 b) has a thickness of 0.01 to 1 µm. 4. Display according to one of claims 1 to 3, characterized in that the insulating film ( 3 a, 3 b) has a thickness which is 0.5 to 3.0 times the thickness of the electrodes ( 2 a, 2 b ) is. 5. Display according to one of the preceding claims, characterized in that the insulating film ( 3 a, 3 b) is a film made of an inorganic material which is selected from the group comprising SiO ₂ , SiN _x and Al ₂ O ₃ . 6. Display according to one of claims 1 to 4, characterized in that the insulating film ( 3 a, 3 b) consists of an orga African material selected from the group consisting of polyimide, a photoresist resin and a polymeric liquid crystal.