JPH0451070B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to thin film transistors. Thin film transistors are active elements used in large-area devices such as liquid crystal displays and contact image sensors, and the semiconductor films used include cadmium sulfide, cadmium selenide, single crystal silicon, polycrystalline silicon, and amorphous silicon. It is being said.
All of these semiconductor materials are photoconductors to varying degrees, and light shielding is required in liquid crystal display devices and the like where thin film transistors are exposed to light. Conventionally, the method described below has been adopted as a method of blocking light.

FIG. 1 shows an example of a conventional element in which the light-shielding film is made of metal. In the figure, 1 is a glass substrate, 2 is a gate electrode, 3 is a gate insulating film, 4 is a source electrode,
5 is a drain electrode, 6 is a semiconductor film, and 7 is an insulating film for insulating the semiconductor film and the light-shielding metal 8. In this example, the structure is such that light entering the source and drain electrodes of the thin film transistor is blocked by a metal light shielding film 8. It is true that metal has a large absorption coefficient, and a film thickness of several thousand angstroms can provide sufficient light-shielding ability, but this method has the following drawbacks.

FIG. 2 shows an equivalent circuit when the thin film transistor shown in FIG. 1 is applied to a liquid crystal display device. In the figure, 9 is a drain electrode, 10 is a gate electrode, 11 is a source electrode, 12 is an equivalent capacitance of the liquid crystal, and 13 is a capacitance between the source and the drain. The source-drain capacitance is mainly composed of metals 4, 5, and 8 and an insulating film 7 in FIG. In a liquid crystal display device, as shown in Figure 2, a pulse is applied to the drain, so the liquid crystal capacitance is 1.
2 and the source-drain capacitor 13 are accumulated in the liquid crystal capacitor 12, causing a decrease in the display contrast of the pixel. There are various other structures for thin film transistors, and the positions where the light shielding film is formed are also various, but in any case, the parasitic capacitance increases and the contrast decreases. This effect can be alleviated by increasing the thickness of the insulating film 7, but the formation time increases, and the increased annealing effect caused by the increased time to heat the substrate adversely affects the device.

Next, FIGS. 3 and 4 show conventional examples in which the light shielding film is an amorphous semiconductor. In this case, the semiconductor is limited to an amorphous silicon film. In the figure 1
4 and 16 are light-shielding films made of amorphous silicon doped with impurities of 0.1% or more, and 15 is an insulating film. This insulating film is made by doping amorphous silicon with impurities to increase the absorption coefficient and thereby increase the light-shielding film. However, as can be seen from the examples, doping with impurities lowers the resistance and requires isolation by an insulating film. This indicates that the metal light shielding film has the same drawbacks as those of the metal light shielding film. In addition, even with such a film, the absorption coefficient is about 10 ³ to 5 × 10 ⁵ cm ^-1 in the visible light range, and it takes a film of about 5 μm to make the transmitted light 0.01% of the incident light over the entire visible light range. Thickness is necessary. Thin film transistor thickness is 1μm
This value is quite large considering that it is a small amount.

Next, we will discuss an example of using amorphous silicon formed by the argon sputtering method as a light-shielding film, and an example of applying it to an inverted staggered thin film transistor.
As shown in FIG. In the figure, 14 and 16 are light shielding films. This film is said to be made by doping amorphous silicon with argon to increase its defect level density and increase its light absorption coefficient and resistivity. However, since this film is formed by the sputtering method, when the semiconductor film and the light-shielding film are in direct contact, for example in the inverted staggered type shown in Figure 6 or the inverted coplanar type shown in Figure 4, the interface is formed by the sputtering method. This has the disadvantage that the interface level increases and off-leakage current increases. This is particularly noticeable when the semiconductor material is other than amorphous silicon or when it is amorphous silicon formed by a glow discharge method.

An object of the present invention is to eliminate the drawbacks of the conventional structure described above and to provide a thin film transistor equipped with a light shielding film that does not increase parasitic capacitance or deteriorate the characteristics of the thin film transistor.

According to the present invention, an insulating substrate, a silicon film provided on the insulating substrate, source and drain electrodes provided at a predetermined interval on one of the main surfaces of the silicon film, and the silicon film a gate electrode provided on one main surface of the silicon film via a gate insulating film, and a gate electrode or a source or drain electrode provided on at least one main surface of the silicon film, or a gate electrode or a source When the drain electrodes are all on the same side of the main surface, a thin film transistor is obtained that includes a multilayer reflective film in which an amorphous silicon film and an insulating film are alternately laminated on the opposite main surface.

Hereinafter, the thin film transistor of the present invention will be explained in detail.

First, an insulating multilayer reflective film using amorphous silicon, which is the core of the present invention, will be explained. The multilayer reflective film itself is a well-known technology.
This is a method in which films with different refractive indexes are alternately laminated and light interference due to the difference in refractive index is used to create a reflection-enhancing film. The center wavelength of reflection is determined by the refractive index and film thickness. Generally, the greater the difference in refractive index, the greater the reflectance can be obtained with a thinner film. Typical films used for multilayer reflective films include zinc sulfide and magnesium fluoride, cerium oxide and magnesium fluoride,
There are combinations such as silicon dioxide and titanium dioxide. Of course, these films can be applied to the present invention, but the present invention particularly uses a multilayer reflective film suitable for thin film transistors using amorphous silicon as a semiconductor film. An example of a light-shielding film suitable for amorphous silicon is a combination of an amorphous silicon film with a bright conductivity of 10 ^-7 (Ωcm) ^-1 or less and a SiO ₂ or SiN _x film. _SiO2
and SiN _x are insulators, and amorphous silicon can also be used to form films with sufficiently low conductivity if conditions are chosen. Therefore, this multilayer reflective film can be regarded as an insulator. An experiment conducted to investigate the characteristics of a multilayer reflective film made of this combination is shown below. FIG. 7 shows the structure of the multilayer reflective film used in the experiment. In the figure, 17 is a glass substrate, 18,
20, 22, 24 are the gas mixture ratios of hydrogen and SiF ₄
SiF ₄ /H ₂ is 30, substrate temperature is 300℃, pressure is 0.1Torr
Amorphous silicon containing fluorine and hydrogen was formed using the glow discharge method under the following conditions, each with a film thickness of 1.
8 is 390 Å, 20 is 330 Å, 22 is 430 Å, 24 is
It is 5000Å. 19, 21, 23 are SiH ₄ , NH ₃ ,
Mixing ratio of _N2 : 3:4:60, substrate temperature: 300℃, pressure
The SiN _x films were formed by the glow discharge method at 0.1 Torr, and the film thicknesses were 540 Å for 19, 470 Å for 21, and 470 Å for 21, respectively.
23 is 610 Å. 25 is incident light, and 26 is reflected light. In this case, the role of each layer is that 24 is a base instead of a thin film transistor, 18, 1
Group 9 is 550nm, Group 20, 21 is 480nm, 2
Groups 2 and 23 are reflection-enhancing films that mainly reflect light with a wavelength of 620 nm. The refractive index of amorphous silicon is
3.6, SiN _x is 2.53. FIG. 8 shows the reflectance of this multilayer reflective film in the visible range. As is clear from the figure, the reflectance is approximately 90% over the entire visible light range.
Thus, despite the film thickness being as thin as 0.277 μm, it exhibits good reflection characteristics. moreover,
This multilayer reflective film has the feature that the transmittance is further reduced because amorphous silicon has a large absorption coefficient. The dotted line in the figure is 24
It shows the percentage of light that passes through amorphous silicon.Because amorphous silicon has a large absorption coefficient at short wavelengths, the transmittance is less than 1% below 500 nm. Therefore, by adding a reflection-enhancing film with a center wavelength of 660 nm to this multilayer reflective film, it is possible to reduce the transmittance to 1% or less over the entire visible light range. In this way, an insulating multilayer reflective film with sufficient light-shielding ability can be obtained with a film thickness of only 0.μm or less. However, if an increase in parasitic capacitance is allowed to some extent, it is also possible to use an amorphous silicon film with good photoconductivity. From this experiment, it was found that in a thin film transistor under illuminance comparable to indoor light, 20,21
It can be seen that the 480nm reflective film is not necessary. This is because the absorption coefficient of amorphous silicon in the short wavelength range is extremely large, and even excluding the reflective films 20 and 21, the absorption coefficient is 1.
This is because short wavelength light is sufficiently absorbed by amorphous silicon of No. 8 and No. 22. Furthermore, if the semiconductor film is limited to amorphous silicon, even if about 10% of the transmitted light in the long wavelength range is present, it will not be a problem because the absorption coefficient of amorphous silicon itself is small. Of course, the stronger the incident light, the more layers are required.

Next, some embodiments of the thin film transistor of the present invention will be shown. FIGS. 9 and 10 show a thin film transistor using amorphous silicon as a semiconductor film and using an insulating multilayer reflective film made of the amorphous silicon film and SiN _x film for light shielding. In the figure, 27 is a glass substrate, 28, 29, and 30 are gate, source, and drain electrodes made of Mo, respectively.
0.3 Torr, mixing ratio of SiH ₄ , NH ₃ , N ₂ 2:1:9
32 is a SiN _x film formed at a substrate temperature of 300℃ and a pressure of
8 is an amorphous silicon film formed by glow discharge method of SiH ₄ at 0.3 Torr, 33 is amorphous silicon, SiN _x , amorphous silicon, etc. from the glass surface in this order.
It is an insulating multilayer reflective film made of stacked layers, and the amorphous silicon has a substrate temperature of 300℃, a pressure of 0.1Torr, and a gas mixture ratio.
It was formed by glow discharge method under the condition of SiF ₄ /H ₂ of 30, and the film thickness was 390 Å, 330 Å, 430 Å,
It is 430Å. In addition, SiN _x has a substrate temperature of 300℃ and a pressure of
0.1 Torr, SiH ₄ , NH ₃ , N ₂ mixing ratio 3:4:60
The film thicknesses are 540 Å, 470 Å, 610 Å, and 610 Å, respectively. 34 is 3
With an insulating multilayer reflective film similar to 3, amorphous silicon,
SiN _x , amorphous silicon, etc. are laminated in this order.

In a thin film transistor with this structure, the multilayer reflective films 33 and 34 are insulators and have a light shielding ability, so parasitic capacitance does not occur.Moreover, the thickness of the multilayer reflective film is as thin as approximately 0.4 μm, so when forming the multilayer reflective film, Characteristic changes in the amorphous silicon semiconductor film due to heat are also less likely to occur. Furthermore, since this multilayer reflective film is formed by a glow discharge method, it causes less damage than spatter and does not expose the semiconductor surface. All the drawbacks of the conventional structure are therefore solved.

In this example, the semiconductor is amorphous silicon, but other materials such as polycrystalline silicon or single crystal silicon may also be used. Furthermore, if the substrate 27 is an opaque material such as ceramic, the multilayer reflective film 33 can be peeled off. Additionally, the light-shielding film that does not come into contact with the semiconductor can be a conventional high-resistance amorphous silicon film.

As is clear from the above description, the thin film transistor of the present invention has excellent features compared to conventional thin film transistors, such as no increase in parasitic capacitance, stability, and no photosensitivity.

[Brief explanation of drawings]

Figure 1 is an example of a thin film transistor using a conventional metal light-shielding film, Figure 2 is an equivalent circuit when the thin film transistor in Figure 1 is used in a liquid crystal display element, and Figure 3 is an example of a thin film transistor using a conventional metal light-shielding film.
4 shows an example of a thin film transistor using a conventional semiconductor light-shielding film, FIGS. 5 and 6 show an example of a thin-film transistor using a light-shielding film made of amorphous silicon whose resistance has been particularly high by sputtering, and FIG. The figure shows a multilayer reflective film formed to investigate the characteristics of the insulating multilayer reflective film used in the thin film transistor of the present invention, Figure 8 shows the reflection and transmission characteristics of the multilayer reflective film shown in Figure 7, Figure 9,
FIG. 10 shows a thin film transistor of the present invention. 1...Glass substrate, 2,10...Gate electrode, 3...
Gate insulating film, 4, 11... Source electrode, 5, 9...
Drain electrode, 6... semiconductor film, 7... insulating film, 8...
Metal light-shielding film, 12... Liquid crystal capacitance, 13... Source-drain capacitance, 14, 16... Semiconductor light-shielding film, 15
...Insulating film, 17...Glass substrate, 18, 20, 2
2, 24...Amorphous silicon film, 19, 21, 23
...SiN _x film, 25... Incident light, 26... Reflected light, 27
...Glass substrate, 28...Gate electrode, 29...Source electrode, 30...Drain electrode, 31...SiN _x film, 3
2...Amorphous silicon film, 33, 34...Insulating multilayer reflective film.

Claims (1)

[Claims] 1. An insulating substrate, a silicon film provided on the insulating substrate, source and drain electrodes provided at a predetermined interval on the main surface of one of the silicon films, and a silicon film provided on one of the silicon films. A gate electrode is provided on the main surface via a gate insulating film, and a gate electrode or source and drain electrodes are provided on at least one main surface of the silicon film, or all of the gate electrode, source and drain electrodes are provided on the main surface of the silicon film. A thin film transistor comprising a multilayer reflective film formed by alternately laminating an amorphous silicon film and an insulating film formed on the opposite main surface when they are on the same side of the main surface.