JPS63280222A - liquid crystal display device - Google Patents

Abstract

PURPOSE:To reduce a threshold voltage by shielding light at a joint between source and drain layers generating current based upon light and a channel layer in a channel semiconductor layer. CONSTITUTION:When a substrate 1 is transmission type liquid crystal display device, a glass or quartz transparent insulating substrate 1 is used. A non-doping layer 4 containing no impurity and drain and source layers 5, 6 implanting phosphorus or the like into p-Si by ion implantation e.g. and having low resistance are formed in a p-Si channel layer. When a light shielding electrode 9 is divided in the channel direction and light shielding electrode structure including an area with length L1 having no light shielding electrode is formed, light shielding capacity equivalent to a fully shielded element is obtained. Since an element having a light shielding effect, a low threshold voltage, small capacity between the source and drain and low crosstalk can be formed in said element structure, a liquid crystal display device having clear image characteristics even at the time of radiating light can be obtained.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to the structure of a liquid crystal display device, and relates to a liquid crystal display device that has thin film transistors that have good element characteristics when irradiated with light and achieves good image quality characteristics. .

[Conventional technology]

In recent years, flat liquid crystal display devices have been developed as display devices for televisions and computer terminals, or as large-screen transmissive display devices. The structure of these display devices is to form thin film transistors and thin film diodes on a substrate with high transmittance, such as glass or quartz, and to connect these thin film transistors and thin film diodes to an upper plate equipped with red, blue, and green color filters. 1 For example, T N
(Twisted Nematic) Liquid crystal is sealed, a light source is placed on the back or top of the substrate, and each pixel is displayed using the transistor or diode described above as a switch for driving the liquid crystal. The display contrast and image quality of this device largely depend on the characteristics of the thin film transistor formed on the substrate. For example, if the on-state current is large enough to invert the liquid crystal, and the off-current is small enough to maintain one screen, a display device with high contrast can be realized. Among the characteristics of a transistor, the off-state current is greatly affected by light, and in many cases, the off-state current increases during light irradiation, resulting in a decrease in image quality. In order to prevent the image quality from deteriorating when irradiated with light, a technique is used in which only thin film transistors whose current increases with light are shielded from light with a metal such as AQ or Cr, which is difficult for light to pass through. Conventionally, as a display device with a structure, there is a display device such as Society for Information Display, International Symposium, Digest Off-Technical Papers, (1986), pp. 289 to 292 (SID, Inter
nationa QSymposium Digest
of Technica Q, Papers.

(1986) pp 289-292).

FIG. 2 shows a cross-sectional view of the element structure of a thin film transistor section in a display device having a conventional structure. FIG. 2 shows a thin film transistor with a staggered structure in which hydrogen-doped amorphous silicon (a-3i:H) is used as the semiconductor layer of the channel portion of the thin film transistor.

1 is a substrate, 2 is a gate electrode, and 3 is a gate insulating film.

4 is a channel layer formed of a-8i:H.

5.6 are the source and drain layers, respectively, which are a-S
i: This is a highly concentrated semiconductor layer in which H is sufficiently doped with phosphorus. Reference numeral 7 denotes a source contact electrode, 8 a protective insulating film, and 9 a light-shielding electrode, forming a structure in which the channel layer 4 is not directly irradiated with light by shielding it from light irradiation as indicated by arrows in the figure. 10 is a drain contact electrode, and 11 is a display electrode, which is usually a transparent electrode made of ITO (indium tin oxide) or the like, and a transistor current is passed through this electrode to invert the liquid crystal. The light-shielding electrode structure shown in FIG. 2 can be easily applied to a self-aligned coplanar thin film transistor using polycrystalline silicon for the gate electrode and polycrystalline silicon for the channel layer. FIG. 3 shows a cross-sectional structure of a coplanar thin film transistor using the conventional technology. Polycrystalline silicon (p-8i), which has a higher formation temperature than a-8i:H, is used as a semiconductor material for the thin film transistor having the structure shown in FIG. The names of each part of the structure in FIG. 3 are the same as those in FIG. 2, and 9 is a light-shielding electrode. Usually, in a liquid crystal display device formed with a thin film transistor using p-3i in the channel portion 4, the light shielding electrode 9 is not formed in many cases.

This is because p-8i is not provided because the current generated by light is smaller than that of a-8i:H. This is because current applications are limited to pocket televisions with diagonal dimensions of only 2 to 3 inches. When this is used in a large liquid crystal device or a transmission type display device using a powerful light source, the off-state current specification of 9 is strict and a light-shielding electrode is indispensable.

[Problem that the invention seeks to solve]

In the prior art shown in FIG. 2, 9 light-shielding electrodes and 7.10
The effect of shielding the channel layer 7 from light irradiation by the contact electrode 4 is sufficiently recognized.

However, in the shielded electrode structure using conventional technology,
The following new problems arise. Normally, in a transistor having the structure shown in FIG. 2, the distance i1 between the source and drain (La in FIG. 2) is 5 to 20 μm, and the thickness tF of the insulating film 8 is 21 μm.
It is m. Therefore, by providing 9 light-shielding electrodes, a new drain-source capacitance Cos.

esc+Coc 2 tp occurs. Here, esc and Coc are the capacitances of the source and light-shielding electrodes, respectively, and Coc is the capacitance of the drain and light-shielding electrodes.
ε is the dielectric constant of the insulating film of 8. When such Cos occurs, crosstalk occurs in which the potential of the display electrode No. 11 changes as a pulsed signal voltage ΔVs propagating through the signal line that drives other pixels connected to the source contact electrode No. 7. . Therefore, problems arise in that the voltage applied to the liquid crystal fluctuates, noise occurs, and image quality deteriorates.

The problems with the thin film transistor having the structure shown in FIG. 3 are as follows. As for crosstalk, as in the structure shown in FIG. 2, a capacitance problem occurs through the thickness tp (~1 μm). In addition, problems unique to polycrystalline silicon occur. This is a new finding obtained by the applicant himself who produced a thin film transistor having the structure shown in FIG. Thin film transistors using p-Si contain crystalline components in the film compared to a-3i:H, and their mobility is about 10 to 100 times higher.
Therefore, there is an advantage that the on-state current becomes large. However, the manufacturing temperature of this p-8i is a-3i: H+
7) Higher temperature at 600°C compared to 300°C.

Therefore, Si defects (dangling bonds) in the film
The method of substituting hydrogen with hydrogen to lower the defect density during film formation is not easy to use because hydrogen separates from silicon at temperatures above 400°C. Therefore, the threshold voltage is p
The -Si element has a disadvantage that it is more than twice as high as the a-8i:H element, increasing the power supply voltage. To this end, in a normal manufacturing method, the threshold voltage can be lowered by, for example, exposing the element shown in FIG. 3 to plasma hydrogen discharge after the manufacturing process is completed at 400'C or higher. The final step at 400° C. or higher in the manufacturing process of the device shown in FIG. 3 is an annealing step for making good contact between the cositact electrodes 7 and 10 and the high-concentration phosphorus semiconductor layers 5 and 6. The light-shielding electrode 9 in Figure 3 is 7 and 1.
For example, AQ and its alloy are formed in the same process as o. When the applicant himself manufactured the device shown in Figure 3 and subjected it to plasma hydrogen treatment, the threshold voltage of the device with the light-shielding electrode 9 did not decrease, but the threshold voltage of the device without the 9 was halved. The facts have been revealed. This is because hydrogen does not pass through the light-shielding electrode 9 and does not reach the channel layer 4. There is no problem if the light-shielding electrode 9 is formed in a separate process after the plasma hydrogen treatment, but this increases the number of manufacturing steps and increases the manufacturing cost.

[Means for solving problems]

The above purpose is to create a structure in which the light-shielding electrode is divided in the channel direction or has a slit shape to shield the junction between the source and drain layers and the channel layer, where current is generated by light in the channel semiconductor layer. This is achieved by

[Effect]

By dividing the light-shielding electrode into channel shapes and dividing the light-shielding electrode on the channel, the capacitance between the source and drain can be reduced. Further, since the light-shielding electrode blocks light at least at the junction between the source and drain layers and the channel layer, the current generated by light is equivalent to when the entire surface is shielded. Also, since there is a slit part on the channel without a light-shielding electrode, polycrystalline silicon is used.

Even if a plasma hydrogenation treatment is performed on a thin film transistor using hydrogen, hydrogen enters through the slit portion, and the threshold voltage can be lowered than when the light-shielding electrode is on the entire surface.

[Example] Hereinafter, an example of the present invention will be described with reference to the sectional view of FIG. Reference numeral 1 denotes a substrate, which is a transparent insulating substrate made of glass or quartz in the case of a transmission type liquid crystal display device. 2 is the gate electrode, and in the case of a self-aligned MO3 transistor, p
-5i, which allows hydrogen ions and hydrogen radicals generated in plasma hydrogen to pass through. 4 is a p-5i channel layer, which is a non-doped layer containing no impurities. 5
and 6 are low resistance drain and source layers in which, for example, phosphorus or the like is implanted into p-8i by ion implantation.

In the case of this structural element, p-Si is formed in the same manner for 4, 5.6, and 2 is formed in a separate process.

Of course, like 5.6, phosphorus ions are implanted in 2 to lower the resistance. 3 is a gate insulating film.

7 is a source contact electrode, 8 is a protective insulating film, 9 is a light shielding electrode, and 10 is a drain contact electrode. 7, 9, and 10 are formed as the same metal film of AQ, its alloy, or chromium, for example, and are separated by a photo-etching process. 11 is a display electrode using a transparent electrode such as ITO. In the figure, t, pe Lo+ Lt+ LZ are symbols representing the thickness and length of each part. The present invention is characterized by a light-shielding electrode structure in which the light-shielding electrodes 9 are divided in the channel direction, and there is a region of length Ll on the channel layer 4 where there is no light-shielding electrode. FIG. 4 shows the relationship between the light intensity and the off-state current of the element when the structural element is irradiated with light in the direction of the arrow in the figure. This characteristic diagram is obtained by the applicant himself who produced and measured an element having the structure of the present invention. The white circle (0) and the solid line are the device without a light-shielding electrode, and the triangle (Δ) and the solid line are the device with the structure of the present invention, where Lo = 5 μm, L2 = 10 μm, Lz = 40 μm, the channel length of the device is 50 μm, and the channel width is 10 μm. be. The black circle (.) and the broken line indicate a conventional light-shielding element structure with Lx=0 μm and a light-shielding electrode on the entire surface. The off-state current was measured with a source-drain voltage of 5V and a gate voltage of OV. As is clear from the figure, it can be seen that the element of the present invention has the same light-shielding ability as a completely light-shielded element.

In addition, the threshold voltages of the three structural elements shown in the figure after plasma hydrogen treatment were 6 V for the element without light shielding, 6.5 V for the element of the present invention, and 12 V for the element with the conventional structure completely shielded from light. . This is because in the device in which light was completely shielded by setting L1=0 μm, hydrogen did not reach the channel layer 4 and the threshold value did not decrease. Further, the source-drain capacitance due to the light-shielding electrode is determined as follows. In the case of total light shielding with LL = O μm, the capacitances of the source layer and the light shielding electrode, and the drain layer and the light shielding electrode are connected in series, resulting in tp. Here, εOx is the dielectric constant of the insulating film of 8.

In the device of the present invention with a slit of Ll a CL・□・t.

I C2=□ ε a Cs +□・t3. Here, εa is the dielectric constant of air, and ts is the thickness of the light shielding electrode. Here, each εox=4×8.85
X10-” (F/am), tp=IX10-’Country, L
o=5X10''''cm, εa=8.85X10''''
No. 1 (F/am), ts=o, 5 X 10-'c
In the structural element of the present invention where m, Ll = 4 X10''''3a11, C1 = 0.885 (PF), Cz = O, 0
O11 (PF), and the source-drain capacitance due to the light-shielding electrode can be reduced by more than two orders of magnitude. As described above, the device structure using the present invention not only has a light shielding effect, but also has a low threshold voltage, and also has a low source-drain capacitance and low crosstalk, so even when irradiated with light, A liquid crystal display device with clear image characteristics can be realized.

Furthermore, the reason why the element of the present invention has the same light-shielding characteristics as the conventional completely light-shielding element is because the region where a current due to light is generated is narrower than L2, such as the region 12 in FIG. 1. The width of this region varies depending on the phosphorus concentration of the source/drain layers 5 and 6, and the concentration of the channel layer 4, which is a non-doped layer in this case, so depending on the concentration, Lo and L
It is necessary to adjust the length of x.

Another embodiment of the invention is shown in cross-section in FIG.

This structural element is an MO8 type transistor with a staggered structure, and one channel layer (usually four) is formed of an a-8i:H semiconductor layer. The feature of the invention of this structure is that, like the first invention, the light-shielding electrode is divided into slit shapes to reduce the source-drain capacitance of the light-shielding electrode and thereby reduce crosstalk. However, when forming with a-5i:H, since the formation temperature is 400°C or lower, the hydrogen mixed in during the formation of the antinode remains in the channel layer 4 without being released, so it is difficult to form the device of the present invention and the entire light shielding. There is no difference in threshold voltage in conventional structural elements.

In the embodiment of the present invention, an N-channel MO8 type device in which phosphorus is implanted into the source and drain layers is shown, but this can of course also be applied to a P-channel MO8 device in which boron or the like is implanted. In addition, amorphous silicon can also be used as a semiconductor layer.
Although the description has been made using only polycrystalline silicon, this can be similarly applied to elements using other semiconductors that are significantly affected by light.

〔Effect of the invention〕

According to the present invention, while having the same light-shielding effect as a thin film transistor with a conventional light-shielding structure, the light-shielding electrode is divided into slits, so that the source-drain capacitance due to the light-shielding electrode is lower than that of the conventional full-thickness structure. In addition to being able to reduce the threshold voltage to less than 1/100 of that of other devices, for example, devices whose channel layer is made of p-Si have the advantage that the threshold voltage can be effectively lowered by hydrogen treatment, and even during light irradiation. The off-state current is small, and crosstalk can be reduced even with a light-shielding electrode.

[Brief explanation of drawings]

FIG. 1 is a cross-sectional view of a thin film transistor for driving a liquid crystal display according to an embodiment of the present invention, FIG. 2 is a cross-sectional view of a conventional thin film transistor, and FIG. 3 is a cross-sectional view of a thin film transistor with a conventional structure different from that in FIG. 4 is a comparison diagram of off-current characteristics due to light between a thin film transistor having the structure of the present invention and a conventional example, and FIG. 5 is a diagram showing an example of the present invention. DESCRIPTION OF SYMBOLS 1...Substrate, 2...Gate electrode, 4...Channel layer, 5#1 light radiation/-J) Rice grains 2---Cushion electrode 4''---h 5--- Toy] B---/-; t4 q-y electrode #Z diagram Figure 3 4 mouth light area (1,) rise 5 mouth

Claims (1)

[Claims] 1. In a liquid crystal display device in which a thin film transistor having a gate electrode, a source electrode, and a drain electrode is formed on a transparent substrate and the thin film transistor is used to operate a liquid crystal, a source region of a semiconductor layer located on a current path constituting at least the thin film transistor. A liquid crystal display device comprising a light-shielding electrode on a junction between a drain region and a channel region, and a light-shielding electrode on a junction between a drain region and the channel region, and at least a portion of the light-shielding electrode above the channel region is cut off.