JPH0330308B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an active matrix substrate for a display using an MIS (metal-insulator-semiconductor) transistor array.

Conventional display panels using active matrices can have a much larger matrix size than the dynamic method, and are attracting attention as a method that can realize large panels with a large number of dots. Particularly in the case of light-receiving elements such as liquid crystals, there is a limit to the drive duty of the dynamic method, and the application of active matrices is being considered for television displays and the like.

FIG. 1 shows one cell of a conventional active matrix. Address line X is transistor 2
is input to the gate of the transistor and turns on the transistor.
This causes the signal on the data line Y to be stored as a charge in the holding capacitor 3. This capacitor 3 holds the data until data is written again, and at the same time drives the liquid crystal 4. Here VC is the common electrode signal. Since liquid crystal leakage is very low, it is sufficient to hold charge for a short period of time.

The manufacturing of the transistor and capacitor 1 here is exactly the same as the normal IC process.

FIG. 2 is an example in which the cell shown in FIG. 1 is produced by a silicon gate process. A transistor 10 and a capacitor 11 are constructed on a single crystal silicon wafer. Address line X and capacitor upper electrode 11
is made of polycrystalline silicon (polysilicon), and the data line Y and the liquid crystal drive electrode 13 are made of Al, and the contact holes 7, 8, and 9 connect the substrate to the Al,
Polysilicon and Al are connected respectively.

Matrix substrates that follow this type of conventional IC process have the following major drawbacks.

One is that the manufacturing process of the matrix substrate is IC.
Since this is the same, the process is complicated and the process cost is high, and at the same time, the yield decreases due to junction leakage with the substrate silicon, resulting in a high total cost. In particular, the junction between the silicon substrate and the diffusion layer that becomes the source/drain is considerably affected by crystal defects in the single crystal, and in a normal cell, this leakage current must be kept below 100 PA. It is difficult to suppress leaks from all cells. The junction leak generated here discharges the charge accumulated in the capacitor 3 and reduces the contrast.

Second, the light incident on the silicon substrate through the gap between the Al electrodes generates electron-hole pairs and is diffused to generate a photocurrent and discharge the charge in the capacitor 3, resulting in a decrease in contrast.

The purpose of the present invention is to provide a method for improving this drawback, and the structure of the present invention is based on glass, quartz,
Alternatively, a thin film transistor using a silicon thin film as a channel on a silicon wafer is constructed, and a specific example will be explained below.

FIG. 3 shows a matrix cell used in the present invention, and the difference from the conventional one shown in FIG. 1 is that the capacity is 18
However, the basic data writing and retention are the same. In this case
The GND potential means a constant bias voltage, and the bias level or signal level does not matter. moreover,
Since the counter electrode of the charge holding capacitor is electrically insulated from the gate line, the charge held in the capacitor is not adversely affected by the voltage state of the gate line. Therefore, it is easy to accurately control the operating voltage, and a stable display can always be provided regardless of the voltage state of the gate line.

Further, as the capacitance for the data line Y to sample and hold the input of display data, the capacitor 21 between the data line Y and the GND line or the capacitor 22 between the address line X is used.

An example of the cell structure is shown based on the plan view of the cell in FIG. 4A and the sectional view taken along AB in FIG. 4B. A silicon thin film 28 forming the source, drain, and channel of the transistor on the transparent substrate 33 and a silicon thin film forming the gate line serving as the gate of the transistor, or a wiring layer 26 and a GND line 27 at the same time,
Further, a data line 25 and a liquid crystal drive electrode 31 are formed of a transparent low resistance material, for example, a Nesa film such as BnO ₂ , or a metal having a thickness of several hundred square meters or less. or
The overlapping portion of the GND line 27 and the liquid crystal drive electrode becomes a charge holding capacitor (Fig. 3-18). Transistor source/drain 34, 35
N ⁺ diffusion (P ⁺ in the case of a P channel) is performed, and a channel 30 exists below the gate electrode 38 with a gate insulating film 36 interposed therebetween.

FIG. 5 shows a manufacturing process for the active matrix substrate shown in FIG. 4. A transparent conductive film such as Nesa film, In ₂ O ₃ , or a very thin metal film is formed on the transparent substrate 40 as an electrode material for the gate in order to increase the area of the capacitor, and after patterning, the gate electrode 41 is formed. make. Next, a gate insulating film 42 is formed on the gate electrode. The method for forming the gate insulating film is to form an oxide _of the gate electrode, for example, by anodic oxidation, thermal oxidation, plasma oxidation, etc., or by _CVD , _etc.
It is a nitride such as Si ₃ N ₄ . (The example shown in FIG. 5 is a method of oxidizing the gate electrode.) These insulators are transparent.

Next, a silicon thin film that will form a transistor channel is deposited and patterned to form a silicon layer 4 that will form a source, drain, and channel.
form 3. (FIG. 5A) In this state, a negative resist 44 is applied to the upper surface and patterned to form a mask portion 45 for doping impurities into the source and drain portions except for the channel portion. (5th
Figure C) When impurity ions are implanted using this resist 45 as a mask, the ions are implanted into the source/drain region 46 to form a low resistance layer, but the ions are not implanted into the lower part of the resist 45 and remain as a channel layer 47. . (FIG. 5C) Next, a transparent conductive film is deposited and patterned to form data lines 48 and drive electrodes 49, and the source/drain 46 of the transistor is formed using an insulating film and contactor holes for the insulating part. Make contact without opening it.

Further, the capacitor can be formed by sandwiching the same material as the gate insulating film 42 between the gate electrode 41 and the drive electrode 49 made of a transparent conductive film.

The advantage of this method is that although it forms a transparent capacitor, the process is simple.

In FIG. 4, the intersection of the data line 25 and the gate line 26 needs to be insulated from each other, but as in the present invention, it is reversed so that the gate electrode is on the bottom and the channel is on the top.
When making a MOS transistor, it is naturally insulated using the same material as the gate insulator without using a special insulating film, so the process of attaching an insulating film, which was used only for insulation as in the past, and the process of applying the insulating film The process of opening a contact hole becomes unnecessary. Furthermore, the disadvantage that the light transmittance of a transparent capacitor decreases due to the special insulating film can be prevented.

Another advantage of the present invention is that the capacitor is made using a transparent conductive film made of the same material as the gate electrode, a transparent conductive film that serves as the liquid crystal drive electrode, and a gate insulating film as a dielectric film. No extra steps are required.

As described above, the present invention provides an active matrix having silicon transistors and silicon capacitors on a substrate, and has the following advantages over the prior art.

Since the counter electrode of the charge holding capacitor is electrically insulated from the gate line, the charge held in the capacitor is not affected by the voltage state of the gate line. Therefore, it is easy to accurately control the operating voltage, and a stable display can always be provided regardless of the voltage state of the gate line.

Furthermore, unlike the case of Japanese Patent Publication No. 2-10955 in which the opposing electrode of the capacitor occupies the entire pixel electrode, the area of the opposing electrode of the capacitor is smaller than the area of the pixel electrode.
When used as a transmissive display device, not only a transparent conductive film but also an opaque conductive film can be used as the counter electrode of the capacitor. The advantages of being able to use an opaque conductive film are shown below.

(a) Opaque conductive films have lower resistance than transparent conductive films, so they have a smaller time constant and are advantageous in increasing screen size and capacity.

(b) In the process, the transparent conductive film requires a special etching solution, which can corrode parts other than the conductive film, so the gate and capacitor opposing electrodes must be made of the same material. On the other hand, in the case of opaque conductive films, no special etching liquid is required, so corrosion is less likely to occur.
The material of the gate and the opposing electrode of the capacitor do not necessarily have to be the same.

(c) In the case of transparent conductive films, a low resistance transparent conductive film can be obtained by increasing the film formation temperature to a high temperature, but the thin film transistor portion will be damaged. On the other hand, with opaque conductive films, low resistance conductive films can be obtained without damaging devices even at low temperatures, and there are many types of materials and manufacturing methods that can be used.

The manufacturing process is simple; the conventional bulk silicon type requires 6 photo etching steps, but the method of the present invention requires only 4 photo etching steps, resulting in low process costs and unlike bulk silicon, P-N
The cross-sectional area of the joint is very small, so there is little joint leakage, and an improvement in yield can be expected.

In addition, more than 90% of the light incident from above passes through, and the diffusion length of carriers in the silicon thin film is short, so almost no photocurrent is generated, and the leakage value for light is 1.0 even under 10,000 lux. PA or less, and it was possible to prevent the display image from disappearing due to the incidence of light.

Furthermore, by using a transparent liquid crystal drive on a transparent substrate, it is possible to use an FE type liquid crystal with the highest contrast, improving screen brightness and dramatically improving display quality.

At the same time, using glass or a similar material for the substrate makes it easier to assemble the panel, which improves the assembly yield and simplifies the process compared to the conventional bulk silicon type.

As described above, the present invention provides a liquid crystal that includes a liquid crystal sealed within a pair of glass substrates, pixel electrodes arranged in a matrix on one of the substrates, and a thin film transistor connected to the pixel electrode. The display device includes a gate electrode on the glass substrate, a gate insulating film on the gate electrode, a semiconductor thin film on the gate insulating film, a source/drain/channel region provided on the semiconductor thin film, and a source/drain channel region connected to the semiconductor thin film. Since the structure includes a transparent pixel electrode and a capacitance forming electrode provided between the transparent pixel electrode and the glass substrate via an insulating film, even if strong light such as external light is incident, A transparent display having a transistor that does not malfunction can be realized.

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram of a cell used in a conventional active matrix, Fig. 2 is a plan view of a cell using bulk silicon, and Fig. 3 is a cell diagram of the present invention.
Figures 4A and 4B are a plan view and a cross-sectional view of an example of its implementation.
Figures 5A to 5D show the manufacturing process. 11... Polysilicon upper electrode of capacitor 3, 10... Polysilicon gate, 7, 8, 9...
... Contact hole, 13 ... Drive electrode made of Al, 33, 40 ... Transparent substrate, 38, 41 ... Gate electrode, 36, 42 ... Gate insulating film, 34,
35,46...source/drain, 30,47...
... Channel, 25, 31, 48 ... Transparent conductive film, 45 ... Resist.

Claims (1)

[Claims] 1. A liquid crystal display device comprising a liquid crystal sealed in a pair of glass substrates, pixel electrodes arranged in a matrix on one of the substrates, and a thin film transistor connected to the pixel electrode. It has a charge retention capacitor formed by a first electrode made of a part of the pixel electrode connected to the drain of the pixel electrode, and a second electrode provided through an insulating film, and the area of the second electrode is equal to the area of the pixel electrode. 1. A liquid crystal display device characterized in that the charge holding capacitor has an area smaller than the area of the capacitor and the charge holding capacitor is electrically insulated from the gate.