JPH07168172A - Liquid crystal display and its production - Google Patents

Abstract

PURPOSE:To fully satisfy the electrical characteristic in the peripheral circuit and the dynamic characteristic in the picture display part especially in a small- sized and high-definition display by making the embedded insulating part corresponding to the picture display part thiner than the other embedded parts. CONSTITUTION:A counter substrate 2 and an element substrate 1 are stuck together with a sealing material 9 and divided into respective liq. crystal cells 10. A liq. crystal 5 is injected and selaed, and only the region corresponding to a picture display part 6 is etched from the rear of the substrate 1. The etching is continued until the rear of a thermally oxidized film as an embedded insulating film is exposed. An almost transparent thin film 7 formed on the thermally oxidized film in the substrate 1 and having a PMOSFET, etc., is formed in the region of the display part 6. The rear of the thermally oxidized film of the display part 6 is etched off by buffered aq. hydrofluoric acid, etc., having selectivity to silicon, and the film thickness is reduced to a specified thickness to satisfy the dynamic characteristic.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a liquid crystal image display device, and more particularly to a liquid crystal image display device used for a small flat panel display, a projection television, a viewfinder of a video camera and the like.

[0002]

2. Description of the Related Art A method for producing a small-sized, high-definition liquid crystal display device using a silicon-on-insulator (SOI) type substrate is described in, for example, Japanese Patent Application Laid-Open No. 3-194115.

That is, as shown in FIG. 8, a generally transparent thin film is laminated by a conventional LSI technique on a single crystal silicon SOI substrate 1 having a buried silicon oxide film to form an active matrix and peripheral circuits. . Then, the liquid crystal cell is assembled by laminating it with the glass substrate 4 having the color filter 3 serving as the counter substrate 2, cutting and injecting the liquid crystal 5. Then, the back surface side of the silicon substrate in the region 6 to be the image display portion is etched by an anisotropic alkali etching solution having a high selectivity until the buried oxide film is exposed, and the active matrix is substantially transparent. Forming a thin film 7 on which

After that, a normal electrical mounting is performed, and wiring is drawn out from the bonding pad 8 formed on the semiconductor substrate to the outside to complete the liquid crystal display device.

[0005]

However, in the conventional example, the film thickness of the buried silicon oxide film in the region where the peripheral circuit exists and the back surface side of the substrate is not removed by etching, and Since it is equal to that of the region 6 removed by etching, it is very difficult to satisfy the electrical characteristics of the peripheral circuit part and to realize a uniform liquid crystal cell gap in the image display part. This point will be described below in order.

Generally, a PMOSFET having a short process and a high yield and being stable in reliability is mainly used for the electric element. A bootstrap circuit or the like is used for the PMOS shift register that drives the active matrix, which is a part of the peripheral circuit, for the purpose of suppressing power consumption. Since this bootstrap circuit is accompanied by a rise in voltage, the PMOSFET has a high breakdown voltage (~
20V) is required. Further, in order to suppress the leak current, it is necessary to always apply a positive bias voltage to the substrate terminal of the PMOS.

However, since the active layer (single crystal silicon layer) on which the PMOS is formed on the SOI substrate is thin, depletion is likely to occur, and in order to apply a correct back gate bias, the bias voltage is applied. It's not enough. Therefore, it is necessary to apply a voltage that effectively acts as a back gate bias also to the back surface side of the silicon substrate existing via the buried oxide film. Therefore, this effective back gate bias (V _BG ) depends on the film thickness (capacitance) of the buried oxide film in addition to the applied voltage.

On the other hand, by applying a back gate bias, the threshold voltage (| V
_th |) rises. This increase in | V _th | deteriorates the element characteristics, and in the worst case, the performance required for the element may not be satisfied.

Generally, the inversion voltage V _T of the MOS structure is

The charge amount (Q _B ) of the depletion layer is 1 of the V _BG .
/ For approximately proportional to the square, the smaller the V _BG | V _th |
It can be seen that the rise of the can be suppressed small.

Therefore, how small V _BG is the PMOS
The key is to be able to suppress the leakage current of.

On the other hand, the thin film existing in the transparent image display portion needs to satisfy certain mechanical characteristics before satisfying the above electrical characteristics. That is, in order to keep the liquid crystal cell gap uniform, the tension of the thin film existing in the image display section needs to be a certain value or more. Generally, a liquid crystal cell gap of about 1 cm square is ± 0.
The tension required to maintain the accuracy of 2 μm is approximately 2 × 10
It is about ⁹ dyne / cm ² , which can be realized by LP (Low-Pressure) -SiN having a thickness of about 6000 Å.

However, when the thickness of the other compressive film constituting the thin film is large, the tension of the LP-SiN is reduced, so that a uniform liquid crystal cell gap cannot be obtained. Generally, LP-SiN has a film thickness of 6000Å
If it exceeds, cracks will increase abruptly, making it difficult to form a semiconductor element on this. At this time, the maximum thickness of the buried oxide film, which is compressive, is 400 at maximum.
It is about 0Å. Therefore, these values serve as a tentative guide for the thin film structure.

The specifications required for the PMOS switching Tr forming the active matrix existing on the thin film are such that the leak characteristic is P of the peripheral circuit.
Although it is similar to the MOS, the breakdown voltage is not so great because there is no problem such as voltage rise. Therefore, the constraint on the buried oxide film thickness is small (though it may be thin).

However, the maximum of 40 in the image display section is
A buried oxide film of about 00Å has a problem that it is difficult to realize a high breakdown voltage (up to 20 V) required for the buried oxide film in the peripheral circuit section in order to suppress power consumption.

In view of the above-mentioned problems, an object of the present invention is to sufficiently satisfy the above-mentioned electrical characteristics in the peripheral circuit section and mechanical characteristics in the image display section in a particularly small and high-definition liquid crystal display device. It is to blame.

[0017]

Means and Actions for Solving the Problems The present invention made to achieve the above object is

A liquid crystal display device using an element substrate having an electric element on a buried insulating film on a semiconductor substrate and having a thin film surface portion formed by removing a portion corresponding to the image display portion of the semiconductor substrate. ,

A liquid crystal display device characterized in that the thickness of the embedded insulating film in the portion corresponding to the image display portion is smaller than the thickness of the embedded insulating film in the other portions,

Further, in manufacturing the liquid crystal display device of the present invention, there is provided a method of manufacturing a liquid crystal display device, characterized in that the embedded insulating film in a portion corresponding to the image display portion is thinned by etching.

The semiconductor substrate used in the present invention is
A single crystal silicon substrate, a polycrystalline silicon substrate, an amorphous silicon substrate, a compound semiconductor substrate or the like can be applied, and the single crystal silicon substrate is preferable.

As the buried insulating film on the semiconductor substrate, an oxide film, a nitride film, a nitride oxide film, etc. of the substrate material can be cited.

According to the present invention, the thickness of the buried insulating film in the portion corresponding to the image display portion is made thinner than the thickness of the buried insulating film in the portion corresponding to the other peripheral circuit portion, thereby displaying the image. The mechanical characteristics required for the parts are compatible with the electrical characteristics required for the peripheral circuit part. This point will be described with a specific example.

FIG. 3 shows the relationship between the film thickness of the buried oxide film on the single crystal silicon substrate and the source / drain breakdown voltage of the switching Tr existing thereon. For example, withstand voltage 10V
In the image display section that requires some degree, the thickness of the buried oxide film is set to 300.
In the peripheral circuit section that requires 0 Å or more and a withstand voltage of about 20V,
It should be 6500Å or more. However, if this film thickness is too thick, the time required to form the buried oxide film becomes enormous, which is not realistic. Therefore, at most 20,0
It is preferably about 00Å.

FIG. 4 shows the relationship between the film thickness of the buried oxide film and the internal stress (residual stress). For example, in order to maintain the liquid crystal cell gap of about 1 cm square in the image display unit with an accuracy of ± 0.2 μm, a tension of about 2 × 10 ⁹ dyne / cm ² is required, so that the film thickness should be 4000 Å or less. Just do it.

That is, in order to satisfy the electrical and mechanical characteristics shown in the above example, the film thickness of the buried oxide film in the image display section is 3000Å to 4000Å and the buried oxide film in the peripheral circuit section is It suffices to set the film thickness within the range of 6500Å to 20,000Å.

In the present invention, the method for forming the buried insulating film having different film thicknesses in each region as described above is not particularly limited, but the method by etching is the PMO.
There is no deterioration in the electrical characteristics of S, and the switching Tr
This is preferable because the degree of deterioration of the electrical characteristics of (3) is small and the cost can be reduced. The etching method is not particularly limited as long as it is a low damage etching method that does not deteriorate the liquid crystal display quality, and for example, chemical etching is preferably applied.

In the case of etching, in order to increase the thickness of the buried insulating film in the peripheral circuit region, the thickness of the buried insulating film on the semiconductor substrate is formed in advance to correspond to the image display portion of the semiconductor substrate. After removing the portion to be etched from the back surface by etching, the embedded insulating film in this portion may be etched to reduce the film thickness.

[0029]

EXAMPLES The present invention will now be described in detail with reference to examples, but the present invention is not limited to these examples.

Example 1 FIG. 2 shows the element substrate used in this example. This is thickness 6
On a single crystal silicon substrate 21 having a thickness of about 25 μm, a thickness of about 8
It has a thermal oxide film 22 of 000 Å, and further has an N-type epitaxial layer 23 having an impurity concentration of 2 × 10 ¹⁵ cm ⁻³ and a thickness of about 0.5 μm. The thermal oxide film 22 as a buried insulating film has a film thickness satisfying a withstand voltage of 20 V as can be seen from FIG. Further, on the N-type epitaxial layer 23, a PMOSFET having a gate film thickness of 500Å, a pixel electrode, and an alignment film (none of which are shown) are formed by using a known LSI technique or the like.

Next, a method of manufacturing the liquid crystal display device of this embodiment will be described with reference to the manufacturing process chart of FIG.

First, the color filter 3 is formed on the glass substrate 4.
Further, a counter electrode and an alignment film (not shown) were formed to form a counter substrate 2. The counter substrate 2 and the element substrate 1 described above are sealed with a sealing material 9
Then, the liquid crystal cell was divided into respective liquid crystal cells by laminating with each other and then cutting. Subsequently, after injecting the liquid crystal 5 and sealing with a sealing material, only a region corresponding to the image display portion 6 is etched from the back surface of the substrate 1 with a known anisotropic alkali etching solution. This etching is performed until the back surface of the thermal oxide film 22, which is a buried insulating film, is exposed, and the substantially transparent thin film 7 having the PMOSFET or the like on the thermal oxide film.
Are formed in the area of the image display unit 6. As a result, the liquid crystal cell 10 having the transparent image display section is completed (see FIG. 1A).

Next, the back surface side of the thermal oxide film 22 of the image display portion is etched with a buffered hydrofluoric acid aqueous solution having selectivity for silicon, and the film thickness is set to 4
Decrease to 000Å (see Fig. 1 (b)). This film thickness has a residual stress of the thin film 7 of approximately 2 × 1 as shown in FIG.
It is a value to keep the tensile stress of 0 ⁹ dyne / cm ² . Also,
The film thickness is a value that satisfies the withstand voltage 10V required for the switching Tr existing in the image display unit 6 (see FIG. 3).

After that, the opening portion of the image display portion 6 was covered with the cover glass 11 and the lead electrode 12 was provided on the bond pad 8 to obtain a displayable liquid crystal panel (see FIG. 1C). The size of the image display unit 6 of the liquid crystal panel manufactured in this example is 14 mm × 10 mm. The liquid crystal display device thus obtained was able to sufficiently satisfy the electrical characteristics of the peripheral circuit portion even when a bootstrap circuit or the like for suppressing power consumption was formed on the element substrate. In addition, the liquid crystal cell gap in the image display unit is ± 0.2
It was possible to maintain the accuracy of μm or less and display a high-definition image.

Example 2 In Example 1, except that the thermal oxide film shown in FIG. 1B was not etched over the entire surface of the image display portion 6.
A liquid crystal display device was produced in the same manner as in Example 1.

When the thermal oxide film 22 is etched by using the single crystal silicon body 21 portion of the element substrate 1 as a mask as in Example 1, as shown in FIG. There is an undercut 51 in the part. In general, the stress on the film surface is concentrated on the edges and corners, so the mechanical strength of the edge 52 where the undercut 51 is present decreases.
It may break easily.

For this reason, in this embodiment, as shown in FIG. 6, an etching resistant mask 61 is formed on the end portion by using a known photolithography technique (FIG. 6A), and then etching is performed (FIG. 6A). 6 (b)) eliminates the above-mentioned problem.

The mask 6 starts from the edge 62 of the image display area.
Since the distance to 1 is about several μm, the display area is not consumed unnecessarily.

Further, as shown in FIG. 7 showing the image display section viewed from the back side, since stress is likely to be concentrated at the corner portion 71 of the thin film surface of the image display section 6 as described above, the etching mask 61 is used. The corner pattern 72 may be formed into a continuous smooth curve, for example, a circle. As a result, the phenomenon of thin film destruction due to stress concentration is alleviated.

[0040]

As described above, according to the present invention,
In particular, since it is possible to optimize the film thickness of the embedded insulating film in the peripheral circuit section and the display section of a small-sized and high-definition liquid crystal display device, respectively, the electrical and mechanical characteristics required for both regions can be optimized. Can be fully satisfied.

[Brief description of drawings]

FIG. 1 is a schematic cross-sectional view for explaining the method for manufacturing the liquid crystal display device of the present invention shown in Example 1.

2 is a diagram showing an element substrate used in Example 1. FIG.

FIG. 3 is a diagram showing an example of film thickness-withstand voltage characteristics of a buried insulating film.

FIG. 4 is a diagram showing an example of film thickness-stress characteristics of a buried insulating film.

5 is a cross-sectional view of a display unit in the liquid crystal display device of Example 1. FIG.

FIG. 6 is a schematic cross-sectional view for explaining an etching method in a manufacturing process of the liquid crystal display device of the present invention shown in Embodiment 2.

FIG. 7 is a schematic view for explaining an etching method in the manufacturing process of the liquid crystal display device of the present invention shown in Example 2.

FIG. 8 is a sectional view of a conventional example.

[Explanation of symbols]

 1 Element Substrate 2 Counter Substrate 3 Color Filter 4 Glass Substrate 5 Liquid Crystal 6 Image Display 7 Thin Film 8 Bonding Pad 9 Sealing Material 10 Liquid Crystal Cell 11 Cover Glass 12 Extraction Electrode 21 Single Crystal Silicon Substrate 22 Thermal Oxide Film 23 Epitaxial Layer 51 Undercut 52 Edge 61 Mask 62 Edge 71 Corner 72 Mask Corner Pattern

Claims (10)

[Claims] 1. A liquid crystal display using an element substrate having an electric element on a buried insulating film on a semiconductor substrate and having a thin film surface portion formed by removing a portion corresponding to an image display portion of the semiconductor substrate. In the device, the thickness of the embedded insulating film in the portion corresponding to the image display unit is
A liquid crystal display device, characterized in that it is thinner than the thickness of the buried insulating film in other portions. 2. The liquid crystal display device according to claim 1, wherein the semiconductor substrate is single crystal silicon. 3. The liquid crystal display device according to claim 1, wherein the buried insulating film is a silicon compound. 4. The liquid crystal display device according to claim 1,
The semiconductor substrate is single crystal silicon, the embedded insulating film is a thermal oxide film of the substrate, and the thickness of the embedded insulating film of the portion corresponding to the image display portion is 3000Å to 4000Å, and the other portions are embedded. Insulation film thickness is 6500Å ~ 20,000
Liquid crystal display device characterized by being Å. 5. The liquid crystal display device according to any one of claims 1 to 4, wherein the embedded insulating film in a portion corresponding to the image display portion is thinned by etching. Manufacturing method. 6. The method of manufacturing a liquid crystal display device according to claim 5, wherein the etching is chemical etching. 7. The method of manufacturing a liquid crystal display device according to claim 5, wherein the etching is performed using an etching liquid having selectivity. 8. The method for manufacturing a liquid crystal display device according to claim 5, wherein the etching is performed using an etching solution containing hydrofluoric acid as a component. 9. The method of manufacturing a liquid crystal display device according to claim 5, wherein the etching is selective etching performed after forming an etching mask. 10. The method for manufacturing a liquid crystal display device according to claim 9, wherein the etching mask has a curve with a smooth corner portion.