JP7641872B2 - Method for manufacturing ferroelectric liquid crystal panel - Google Patents

Description

The present invention relates to a method for manufacturing a ferroelectric liquid crystal panel.

Liquid crystal panels installed in liquid crystal display devices have the advantage of being able to be made thin and compact, and can be operated with low power consumption, so in recent years they have been widely used in the field of small electronic devices such as mobile devices, projectors, viewfinders, HMDs, and HUDs.

Among the liquid crystals used in liquid crystal panels, ferroelectric liquid crystals have a much faster response than the nematic liquid crystals commonly used in many products, because the strong spontaneous polarization of the liquid crystal molecules creates a strong interaction with the electric field applied between the substrates.

In ferroelectric liquid crystal, alignment defects called "zigzag defects" can occur at the boundaries of the layer structure formed by the liquid crystal molecules. Zigzag defects can be removed by performing an alignment process in which the ferroelectric liquid crystal injected into the liquid crystal panel is heated to a temperature at which the liquid crystal undergoes a phase transition to an isotropic phase, and then gradually cooled to a temperature at which the ferroelectric liquid crystal undergoes a phase transition to an SmC phase while a voltage is applied to the ferroelectric liquid crystal. (See, for example, Patent Document 1)

JP 2020-52081 A

However, even if the above-mentioned alignment process is performed, it is not always possible to completely remove the zigzag defects. Even if the zigzag defects are completely removed, it may be necessary to repeat the same alignment process many times until they are completely removed, which poses the problem that the zigzag defects cannot be removed efficiently.

The present invention aims to provide a method for manufacturing a ferroelectric liquid crystal panel that can efficiently remove zigzag defects.

This is a method for manufacturing a ferroelectric liquid crystal panel, which comprises a first alignment process in which a ferroelectric liquid crystal filled between a first substrate and a second substrate bonded together is heated to a first temperature at which the ferroelectric liquid crystal undergoes a phase transition to an isotropic phase, and the ferroelectric liquid crystal is cooled from the first temperature to a temperature at which the ferroelectric liquid crystal undergoes a phase transition to an SmC phase while applying a DC voltage or an AC voltage containing a DC component to the ferroelectric liquid crystal, and a second alignment process in which the ferroelectric liquid crystal is heated to a second temperature at which the ferroelectric liquid crystal undergoes a phase transition to an SmA phase or an N phase, and the ferroelectric liquid crystal is cooled from the second temperature to a temperature at which the ferroelectric liquid crystal undergoes a phase transition to an SmC phase while applying an AC voltage not containing a DC component to the ferroelectric liquid crystal or without applying a voltage to the ferroelectric liquid crystal, and the first alignment process and the second alignment process are performed in this order.

The method for manufacturing a ferroelectric liquid crystal panel may also include maintaining the ferroelectric liquid crystal at a temperature at which it is in the SmC phase from the end of the first alignment process until the start of the second alignment process.

The method for manufacturing a ferroelectric liquid crystal panel may be such that the second temperature is a temperature at which the ferroelectric liquid crystal undergoes a phase transition to the SmA phase.

The method for manufacturing a ferroelectric liquid crystal panel may be such that the second temperature is a temperature at which the ferroelectric liquid crystal undergoes a phase transition to the N phase.

The present invention makes it possible to efficiently remove zigzag defects.

FIG. 1 is a vertical cross-sectional view of a ferroelectric liquid crystal panel according to an embodiment of the present invention; FIG. 2 is a vertical cross-sectional view showing the alignment state of liquid crystal molecules in a ferroelectric liquid crystal panel according to an embodiment of the present invention. Diagram showing the switching of liquid crystal molecules in a ferroelectric liquid crystal A diagram showing the state in which the liquid crystal molecules (domains) of a ferroelectric liquid crystal are uniformly aligned. A diagram showing a state in which the orientation (domains) of ferroelectric liquid crystal molecules are not uniform (mixed). A flowchart showing a method for manufacturing a ferroelectric liquid crystal panel according to an embodiment of the present invention. FIG. 1 is a diagram showing a temperature profile and transition of the alignment state of liquid crystal molecules when performing alignment treatment according to an embodiment of the present invention.

The following describes an embodiment of the present invention.

Figure 1 is a vertical cross-sectional view showing a ferroelectric liquid crystal panel according to an embodiment of the present invention. The liquid crystal panel shown in Figure 1 includes a first substrate 101 (e.g., a glass substrate), a second substrate 102 (e.g., a silicon substrate), electrode films formed on the opposing surfaces of the first substrate 101 and the second substrate 102, organic alignment films 103 formed on each of these electrode films, a frame-shaped sealant that bonds the first substrate 101 and the second substrate 102, and ferroelectric liquid crystal filled in the gap (cell gap) between the first substrate 101 and the second substrate 102 in the area surrounded by the sealant.

The surface of the organic alignment film 103 is given anisotropy by rubbing or the like. There are several methods for forming the organic alignment film 103, such as spin coating and vapor deposition. In place of the organic alignment film 103, it is also possible to use a photo-alignment film made of a photosensitive organic material, or an inorganic alignment film made of an inorganic material.

Figure 2 is a vertical cross-sectional view showing the alignment state of liquid crystal molecules in a ferroelectric liquid crystal panel according to an embodiment of the present invention, and Figure 3 is a diagram showing the switching of liquid crystal molecules in a ferroelectric liquid crystal. As shown in Figure 2, in a ferroelectric liquid crystal panel, by controlling the substrate distance (cell gap) to, for example, 1 μm or less, the helical structure characteristic of ferroelectric liquid crystal disappears, and the liquid crystal molecules 104 are stabilized at a predetermined angle on each side of the direction approximately perpendicular to the layer direction of the ferroelectric liquid crystal (the direction of rubbing, etc.). By switching between the two directions, the liquid crystal molecules 104 change the polarization state of the incident light and allow the polarizer to decide whether to transmit the light or not. The ferroelectric liquid crystal panel can control the polarization state of light with an extremely simple drive that only turns on and off the drive voltage for the liquid crystal molecules 104.

In a so-called SSFLCD (surface stabilized ferroelectric liquid crystal display element) with a small cell gap of 1 μm or less, a strong alignment control force is required to align the liquid crystal molecules in a predetermined direction due to the strong interaction between liquid crystal molecules caused by spontaneous polarization, and the interaction from the alignment film 103 is important. In the SSFLCD, a twisted structure of the liquid crystal molecules 104 occurs. At this time, the gap between the liquid crystal layers becomes wider, but in order to fill the gap, a so-called "chevron structure" that has a "L" shape is generated in the liquid crystal layer. As shown in FIG. 3, the chevron structure 105 can have an alignment state called C2 alignment in which the center line of the cone angle orbit 106 of the liquid crystal molecule 104 is inclined from the cone apex toward the substrate side, and an alignment state called C1 alignment in which the center line of the cone angle orbit 106 of the liquid crystal molecule 104 is inclined from the cone apex toward the center of the cell gap. The liquid crystal molecules 104 of the ferroelectric liquid crystal sealed between the first substrate 101 and the second substrate 102 are appropriately oriented according to the anisotropy of the alignment film 103 formed on the first substrate 101 and the second substrate 102. At this time, the liquid crystal molecules 104 near the surface of the alignment film 103 take either the C1 orientation or the C2 orientation based on the pretilt caused by the alignment film 103 and the pretilt of the liquid crystal molecules 104 themselves, and a chevron structure 106 is formed based on that orientation. When the C1 orientation and the C2 orientation are mixed, zigzag defects occur at the boundary between the C1 orientation and the C2 orientation.

4(a) shows a state where the directions (domains) of the liquid crystal molecules of the ferroelectric liquid crystal are uniformly aligned, and FIG. 4(b) shows a state where the directions (domains) of the liquid crystal molecules of the ferroelectric liquid crystal are not uniformly aligned (mixed). The alignment state of the liquid crystal molecules 104 is different near the surface of the alignment film 103 and near the center of the cell gap, and the liquid crystal molecules 104 near the surface of the alignment film 103 cannot move along the original cone angle orbit 106 due to the influence of the surface of the alignment film 103, and their movement is restricted. Normally, the liquid crystal molecules 104 of SSFLC are aligned in one of two directions, left and right, from the center line of the cone angle orbit 106, but which direction is chosen is uncertain. In one liquid crystal panel, the directions of the liquid crystal molecules 104 may be uniform as shown in FIG. 4(a), but may not be uniform as shown in FIG. 4(b) (two directions of alignment are mixed). In the field of liquid crystals, a region with a specific orientation direction is sometimes called a domain. Usually, when a voltage is applied, the domains align in one direction or the other depending on the direction of the electric field. However, in regions where the cone angle is fixed, the voltage (threshold) that changes the domain is high, making switching difficult. When domains are mixed, as shown in Figure 4(b), this is perceived by the user as unevenness in the image display (domain unevenness), and can be a factor in compromising the display quality. When domains are mixed, the birefringence (polarization state) of light differs in different domain parts, resulting in the visible unevenness of the display.

The occurrence of zigzag defects and domain unevenness is related to the interaction between the liquid crystal molecules 104 and the alignment film 103, and is particularly dependent on the inclination of the liquid crystal molecules 104 relative to the alignment film 103, the so-called "pretilt". Normally, when the pretilt is high, the area of the liquid crystal molecules 104 fixed to the alignment film 103 becomes wider, and the display unevenness tends to become stronger. In addition, in the chevron structure, the C2 orientation is relatively stable, but when the pretilt becomes high, the C1 orientation becomes prominent, and the C1 orientation and the C2 orientation tend to coexist, making it easier for zigzag defects to occur. Therefore, while controlling the pretilt is important to improve the alignment quality, the pretilt is highly dependent on the material of the alignment film 103 and the method of imparting anisotropy, such as the rubbing method, and since there are limitations on the selection of these materials and methods, it is often difficult to obtain a low pretilt. Furthermore, in liquid crystal panels with an SSFLC structure, the liquid crystal molecules 104 near the surface of the alignment film 103 have a high threshold voltage for switching, so zigzag defects and domain irregularities cannot be eliminated by simply applying a normal voltage.

In contrast, the manufacturing method for a ferroelectric liquid crystal panel according to the embodiment of the present invention can efficiently remove zigzag defects and domain irregularities. The specific steps are described below.

Figure 5 is a flowchart showing a method for manufacturing a ferroelectric liquid crystal panel according to an embodiment of the present invention. In the method for manufacturing a ferroelectric liquid crystal panel shown in Figure 5, the processes of STEPs 1 to 4 are carried out in sequence.

<STEP 1>
First, a substrate bonding step is carried out in which the first substrate 101 and the second substrate 102 are bonded together via a frame-shaped seal material having a liquid crystal injection port.

<STEP 2>
Next, a liquid crystal filling step is performed in which ferroelectric liquid crystal is filled into the gap between the first substrate 101 and the second substrate 102 through the liquid crystal filling port. At this time, the ferroelectric liquid crystal is filled in a state heated to a temperature at which the liquid crystal transitions to an isotropic phase, and then gradually cooled to room temperature. The ferroelectric liquid crystal is appropriately oriented while being cooled to room temperature, but zigzag defects and domain irregularities may occur during this process (it is assumed here that these have occurred). After the ferroelectric liquid crystal is filled, the liquid crystal filling port is sealed with a sealant as necessary.

<STEP 3>
Next, a mounting process is performed in which the second substrate 102 is mounted on a circuit board. The first substrate 101 and the second substrate 102 are electrically connected to the circuit board by a conductive adhesive, wires, or the like.

<STEP 4>
Next, an alignment treatment step is carried out in which a first alignment treatment and a second alignment treatment are carried out in this order on the ferroelectric liquid crystal filled between the first substrate 101 and the second substrate 102. Specifically, the alignment treatment step is as follows.

Figure 6 is a diagram showing the transition of the temperature profile and the alignment state of the liquid crystal molecules when performing the alignment treatment according to the embodiment of the present invention. In STEP 4, first, a first alignment treatment is performed to align the domains of the ferroelectric liquid crystal in one direction. In the first alignment treatment, the ferroelectric liquid crystal is first heated to a temperature at which the phase transition occurs from room temperature to an isotropic phase. The temperature at which the ferroelectric liquid crystal transitions to an isotropic phase is, for example, 110°C. A box-shaped heating furnace or a plate-shaped heater can be used as a means for heating the ferroelectric liquid crystal. Next, a DC voltage or an AC voltage containing a DC component is applied to the ferroelectric liquid crystal while maintaining the ferroelectric liquid crystal at a temperature at which the phase transition occurs for a certain period of time. The magnitude and waveform of the voltage are appropriately selected. Then, while applying the voltage to the ferroelectric liquid crystal, the ferroelectric liquid crystal is gradually cooled to a temperature at which the phase transition occurs to the SmC phase. The temperature range at which the ferroelectric liquid crystal transitions to the SmC phase is, for example, -35°C to 80°C. Then, the application of the voltage to the ferroelectric liquid crystal is stopped at any timing after the ferroelectric liquid crystal has undergone a phase transition to the SmC phase. The domains are formed while the ferroelectric liquid crystal is undergoing a phase transition from the isotropic phase to the SmC phase, and the liquid crystal molecules 104 near the surface of the alignment film 103 are fixed in a uniform state oriented in one direction due to the action of the DC component of the voltage applied to the ferroelectric liquid crystal at this time. This completely removes domain irregularities. However, even if the zigzag defects are somewhat removed, they are not completely removed. In the first alignment process, a DC voltage or an AC voltage containing a DC component is applied to the ferroelectric liquid crystal when the ferroelectric liquid crystal is cooled, so that the domains can be efficiently aligned in one direction. This effect is relatively greater when a DC voltage is applied to the ferroelectric liquid crystal than when an AC voltage containing a DC component is applied to the ferroelectric liquid crystal.

Next, between the end of the first alignment process and the start of the second alignment process, the ferroelectric liquid crystal is maintained at a temperature at which the domains are aligned in one direction and the liquid crystal enters the SmC phase. This further stabilizes the domains. However, this process is not essential and can be omitted.

Next, a second alignment process is performed to completely remove the zigzag defects. In the second alignment process, the ferroelectric liquid crystal is first heated to a temperature at which the ferroelectric liquid crystal undergoes a phase transition to the SmA phase. The temperature range at which the ferroelectric liquid crystal undergoes a phase transition to the SmA phase is, for example, 80°C to 90°C. Next, while the ferroelectric liquid crystal is held at the temperature at which the ferroelectric liquid crystal undergoes a phase transition to the SmA phase for a certain period of time, an AC voltage that does not contain a DC component is applied to the ferroelectric liquid crystal, or the ferroelectric liquid crystal is left without applying a voltage. When applying a voltage, the magnitude and waveform of the voltage are appropriately selected. Then, while applying the voltage to the ferroelectric liquid crystal, or without applying a voltage, the ferroelectric liquid crystal is gradually cooled to a temperature at which the ferroelectric liquid crystal undergoes a phase transition to the SmC phase. Then, if a voltage has been applied to the ferroelectric liquid crystal, the application of the voltage to the ferroelectric liquid crystal is released at any timing after the ferroelectric liquid crystal undergoes a phase transition to the SmC phase. The zigzag defects disappear when the ferroelectric liquid crystal is heated to a temperature at which the ferroelectric liquid crystal undergoes a phase transition to the SmA phase, and are unlikely to occur again when the ferroelectric liquid crystal subsequently undergoes a phase transition from the SmA phase to the SmC phase. Therefore, when the ferroelectric liquid crystal becomes the SmC phase, the zigzag defects are completely removed. The domains are maintained in a state of being aligned in one direction unless the ferroelectric liquid crystal is heated to a temperature at which the ferroelectric liquid crystal becomes an isotropic phase. By performing the second alignment process in a state in which the domains are aligned in one direction, the occurrence of new zigzag defects is effectively suppressed. In addition, in the second alignment process, an AC voltage not including a DC component is applied to the ferroelectric liquid crystal when the ferroelectric liquid crystal is cooled, or no voltage is applied, so the occurrence of new zigzag defects is effectively suppressed. This effect is relatively greater when an AC voltage not including a DC component is applied to the ferroelectric liquid crystal than when no voltage is applied to the ferroelectric liquid crystal.

By performing the first alignment process and the second alignment process in this order as described above, it is possible to efficiently remove zigzag defects while removing domain unevenness. The state in which the domain unevenness and zigzag defects have been removed will be sustained unless the ferroelectric liquid crystal is heated again to a temperature above the temperature at which the liquid crystal transitions to the SmA phase.

The present invention is not limited to the above examples. For example, in the second alignment treatment, the ferroelectric liquid crystal may be heated to a temperature at which the phase transition occurs to the N phase (nematic phase). In addition, the second alignment treatment may be repeated multiple times as necessary.

101 First substrate 102 Second substrate 103 Alignment film 104 Liquid crystal molecule 105 Chevron structure 106 Cone angle orbital

Claims (2)

a first alignment treatment in which a ferroelectric liquid crystal filled between a first substrate and a second substrate bonded together is heated to a first temperature at which the ferroelectric liquid crystal undergoes a phase transition to an isotropic phase, and the ferroelectric liquid crystal is cooled from the first temperature to a temperature at which the ferroelectric liquid crystal undergoes a phase transition to an SmC phase while applying a DC voltage or an AC voltage containing a DC component to the ferroelectric liquid crystal;
a second alignment treatment of heating the ferroelectric liquid crystal to a second temperature at which the ferroelectric liquid crystal undergoes a phase transition to an SmA phase or an N phase, and cooling the ferroelectric liquid crystal from the second temperature to a temperature at which the ferroelectric liquid crystal undergoes a phase transition to an SmC phase while applying an AC voltage not including a DC component to the ferroelectric liquid crystal;
The first alignment treatment and the second alignment treatment are performed in this order.
4. A method for manufacturing a ferroelectric liquid crystal panel comprising the steps of:
2. A method for manufacturing a ferroelectric liquid crystal panel as described in claim 1, characterized in that in the first alignment treatment, the ferroelectric liquid crystal is heated to the first temperature and then maintained at the first temperature for a certain period of time, and in the second alignment treatment, the ferroelectric liquid crystal is heated to the second temperature and then maintained at the second temperature for a certain period of time.

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