JPH04246864A - Thin film transistor - Google Patents

Abstract

PURPOSE:To enable a thin film transistor where a polycrystalline semiconductor is made to serve as a channel layer to be enhanced in electric field effect mobility. CONSTITUTION:An amorphous semiconductor formed on the rugged surface of a substrate 1 is irradiated with a beam of high energy to turn into a polycrystalline semiconductor 3, and the semiconductor layer 3 concerned is made to serve as a channel layer of a thin film transistor.

Description

Description: TECHNICAL FIELD The present invention relates to a thin film transistor using a polycrystalline semiconductor. [0002] In recent years, devices using thin film semiconductors as transistor elements and the like have been significantly developed, and typical devices such as liquid crystal displays and image sensors have even been commercialized. These devices usually use insulating materials such as glass or quartz as substrates, and therefore, unlike conventional manufacturing methods in which elements are formed on single crystal semiconductor substrates, it is difficult to obtain high-quality thin film semiconductors. is difficult. especially,
When applied to devices, improving the field effect mobility of the thin film semiconductor becomes a problem. Under these circumstances, in recent years, thin film polycrystalline semiconductors have been used as a method of producing thin film semiconductors of relatively good quality. This general formation method is as follows:
Vapor phase growth method using normal pressure CVD method or low pressure CVD method, laser annealing method which irradiates a thin film semiconductor which is initially in an amorphous state with a high energy beam such as a laser to form a polycrystalline semiconductor,
There is also a solid phase growth method in which a thin film semiconductor in an amorphous state formed on a flat substrate is heat-treated at high temperature to change its quality and become a polycrystalline semiconductor. In particular, when using the laser annealing method, compared to other methods, thermal energy is applied by the laser only to the necessary parts, so thermal damage to the entire element is avoided. It is attracting particular attention because it is basically suitable for forming large-area devices. Regarding the manufacturing method of such a polycrystalline semiconductor, for example, IEEE Electron Device
eLet. EDL-1 1980, P159~P
161, IEEE Electron Device
Lett. EDL-8 1987. P425-P4
27 is described in detail. [0007]However, with the conventional laser annealing method, a polycrystalline semiconductor having relatively large crystal grains can be produced, and even in thin film transistors using this as a channel layer, good characteristics cannot be achieved. Although this has been achieved, the properties are still not comparable to those of single crystal semiconductors. Therefore, an object of the thin film transistor of the present invention is to provide a thin film transistor made of a polycrystalline semiconductor having higher field effect mobility. [Means for Solving the Problems] The thin film transistor of the present invention is characterized in that an amorphous semiconductor film formed on a substrate having an uneven surface is subjected to heat treatment by irradiation with a high-energy beam. The purpose is to use a polycrystalline semiconductor formed by applying this as a channel layer. [Operation] By forming the amorphous semiconductor film on a substrate having an uneven shape on its surface and subjecting it to heat treatment by irradiation with a high-energy beam, the crystal grain size is relatively large. In addition, it becomes possible to form a polycrystalline semiconductor with a large value of carrier mobility measured by utilizing the Hall effect. Therefore, in a thin film transistor using such a polycrystalline semiconductor as a channel layer, the field effect mobility can be improved. EXAMPLE First, the physical properties of the polycrystalline semiconductor used in the thin film transistor of the present invention will be explained. FIG. 5 shows the n-type semiconductors of the polycrystalline semiconductor (a) used in the thin film transistor of the present invention and the conventional polycrystalline semiconductor ((b) and (c)) formed on a flat substrate. It shows the relationship between carrier concentration and carrier mobility in the case of . The carrier mobility is measured by the Hall effect. [0014] The polycrystalline semiconductor (a) has a height of 10 μm.
The film is formed by irradiating n-type amorphous silicon formed by plasma CVD on an insulating substrate with an uneven shape of m with a high-energy beam, and the high-energy beam is an argon laser ( wavelength 51
45 Å) was used. The polycrystalline semiconductor (b) is made by polycrystallizing the n-type amorphous silicon formed by plasma CVD on a substrate with a flat surface under the same conditions as the laser. The polycrystalline semiconductor (c) was formed with the only difference from the formation of the polycrystalline semiconductor (b) in that the intensity of the laser was increased by 30%. be. In particular, the reason why the polycrystalline semiconductor (c) was included as a sample for comparative evaluation is that by using a substrate with an uneven shape, the evaluation based on the light scattering of the high-energy beam on the substrate surface can be achieved. This is because the amount of light transmitted to the substrate is likely to increase. That is, normally, when light is incident on a substrate having an uneven shape, multiple scattering of light occurs between adjacent convex portions of the unevenness on the surface of the substrate due to the unevenness, compared to the case of a flat substrate. This is because the amount of light incident on the substrate increases by about 30%. [0017] Therefore, considering the amount of light incident due to the light scattering, we investigated the characteristics when a polycrystalline semiconductor is formed on a flat substrate by increasing the intensity of the high-energy beam by the amount of light incident. . The formation conditions of the plasma CVD method are conventionally well known, and in changing the carrier concentration, the amount of conductivity type determining impurities in the initially formed conductive amorphous silicon is adjusted. It was done by doing. According to the figure, the carrier mobility of the n-type polycrystalline semiconductor (a) is about 3 to 6 times higher than that of the conventional polycrystalline semiconductor (b), and the carrier mobility is particularly low. Within the range, 1 x 1016cm-3 to 1 x 1018c
It can be seen that a polycrystalline semiconductor formed on a substrate having an uneven shape exhibits extremely high carrier mobility in the range of m-3, and has extremely good film quality. Next, the intensity of the high energy beam is reduced by 30%.
Comparing the increased polycrystalline semiconductor (c) and the polycrystalline semiconductor (a), it can be seen that the polycrystalline semiconductor (a) is significantly improved compared to the polycrystalline semiconductor (c). This means that the method of forming a polycrystalline semiconductor using a high-energy beam on a substrate having an uneven shape is not simply a method of heat treatment using the high-energy beam; This shows that the use of polycrystalline semiconductors can dramatically improve the properties of polycrystalline semiconductors. Although the inventors speculate that such dramatic improvements in properties are due to improvements in physical properties due to improvements in mechanisms during the crystallization process, they say that the specific cause is not clear. Although an n-type polycrystalline semiconductor was used as an example, the same applies to a p-type polycrystalline semiconductor, and compared to a conventional p-type polycrystalline semiconductor formed on a flat substrate, It has been confirmed that the carrier mobility formed on a substrate having an uneven shape is about 5 to 10 times higher. FIG. 6 is a characteristic diagram showing the relationship between the height of the uneven shape and the carrier mobility. A typical shape of the uneven shape is a truncated cone shape or a truncated pyramid shape, and is relatively densely distributed. The sample used in the figure is amorphous silicon with a film thickness of 3000 Å that has been polycrystallized using a high-energy beam. As can be seen from the figure, by increasing the height of the uneven shape, the carrier mobility increases rapidly, and when the height is increased to 0.4 μm or more, the value becomes saturated. The height at which such saturation occurs varies depending on the film thickness of the amorphous semiconductor to be polycrystallized, and has a value that roughly matches the film thickness. FIGS. 1 to 3 are device structure diagrams for each manufacturing process to explain the thin film transistor of the present invention. In the example, silicon was used as the semiconductor. In the first step shown in FIG. 2, the surface is coated with 0.
An intrinsic amorphous semiconductor (not doped with impurities) (
2) Form. In the example, an insulating substrate such as glass or ceramics was used as the substrate (1), and intrinsic amorphous silicon formed by a conventionally well-known plasma CVD method was used as the intrinsic amorphous semiconductor (2). As a method for producing the uneven shape, for example, the surface of the substrate itself is processed by chemical or physical etching, or the surface of the substrate is coated with SiO2.
In some cases, an insulating film such as a Si3N4 film or a Si3N4 film is formed, and the film itself is formed into an island shape, thereby making the surface uneven. Whichever method is adopted, it can be used as a substrate for the thin film transistor of the present invention. The method for forming the amorphous semiconductor (2) is not limited to the plasma CVD method, but other sputtering methods, vapor deposition methods, and even thermal CVD methods may be used. [0029] As the formation conditions for the plasma CVD method,
As the reaction gas, silane gas was used at a flow rate of 5 to 50 cc/min, the degree of vacuum during discharge was 0.1 to 0.2 Torr, and the discharge power was 15 to 30 W. The substrate temperature is 200-550℃,
The film thickness is 500 Å to 1 μm. Next, in the second step shown in FIG. 3, the substrate (1) on which the amorphous semiconductor (2) is formed is placed in a vacuum or in an atmosphere filled with an inert gas such as nitrogen gas. The amorphous semiconductor (2) is polycrystallized by irradiating the substrate side with an argon laser as a high-energy beam. The high energy beam is not limited to the argon laser as in the embodiment, but may also be a Xe laser.
Cl (308 nm), KrF (248 nm), ArF (
A similar effect can be obtained even when using an excimer laser (e.g., 193 nm) or other electron beams. [0032] As a result, the amorphous semiconductor (2) changes into an intrinsic polycrystalline semiconductor (3). (4) in the figure indicates a grain boundary of the polycrystalline semiconductor (3). According to the inventors, it is better in terms of characteristics to perform the laser irradiation after heat-treating the amorphous semiconductor (2) at 500°C for about 30 minutes prior to the second step. It has been discovered that stable polycrystalization can be achieved. This is because hydrogen is inevitably mixed into the amorphous semiconductor (2) to be polycrystallized due to the reactive gas used in the film formation process of the first step, so irradiation with the high-energy beam is necessary. This is because the hydrogen is released explosively due to the rapid temperature rise caused by the oxidation, causing damage to the amorphous semiconductor (2). Therefore, when the intensity of the high-energy beam is relatively high or when the hydrogen content contained in the film of the amorphous semiconductor (2) used is high, specifically, the formation temperature is The heat treatment is an effective means when the temperature is low. The grain size of the polycrystalline semiconductor (3) obtained in the example is 0.8 μm to several μm, which is 0.1 μm to several μm compared to that of a conventional polycrystalline silicon film formed on a flat substrate. 0.
It can be seen from the fact that it is 3 μm that it is relatively large. The effect due to the uneven shape of the substrate surface is as follows.
The starting material is not limited to the intrinsic amorphous semiconductor of the embodiments, but other n-type amorphous semiconductors or p-type amorphous semiconductors exhibit exactly the same behavior. Next, in the third step shown in FIG. 1, the surface of the polycrystalline semiconductor (3), which has become rough due to polycrystalization, is flattened by lapping or etching, and then a semiconductor for ohmic contact is formed by a conventionally well-known method. Membrane (5
), a gate insulating film (6), a gate metal film (7), a drain metal electrode (8), and a source metal electrode (9), respectively, to complete the device. As a result, the polycrystalline semiconductor (3) under the gate insulating film (6) functions as a channel layer. Although the flattening treatment is not necessarily required in the process, it is better to add it in cases where the degree of unevenness on the surface is about the same as the channel length of the device, and the characteristics will be stabilized. It is preferable in terms of chemical composition. In the example, chromium was used as the metal film for the gate, aluminum was used as the metal film for the source and drain, and a SiO2 film was used as the gate insulating film. FIG. 4 is a characteristic diagram showing the relationship between drain current and gate voltage of the example thin film transistor (a). For comparison, the figure also shows a conventional thin film transistor (b) using a polycrystalline semiconductor formed on a flat substrate.
The above relationship is also shown at the same time. From the figure, it can be seen that the thin film transistor of the example has better characteristics than the conventional one, with a steep rise characteristic and a sufficiently large change between the on state and the off state. Furthermore, the field effect mobility of the conventional thin film transistor was 30 cm2/V·s, whereas the thin film transistor of the present invention had a good field effect mobility of 100 cm2/V·s. Further, in the embodiment, the high-energy beam was irradiated from the substrate side, but the thin film transistor of the present invention is not limited to this, and the thin film transistor of the present invention can be formed by irradiating the high-energy beam from the side of the amorphous semiconductor to be polycrystallized. Needless to say, a polycrystalline semiconductor may also be used. According to the present invention, a thin film transistor having a characteristic of large field effect mobility can be obtained.

[Brief explanation of the drawing]

FIG. 1 is a sectional view of an element structure of a thin film transistor according to the present invention.

FIG. 2 is a cross-sectional view of an element structure explaining one step in manufacturing the thin film transistor.

FIG. 3 is a cross-sectional view of the device structure for explaining other steps of the thin film transistor.

FIG. 4 is a characteristic diagram of the thin film transistor of the present invention.

FIG. 5 is an electrical characteristic diagram of the polycrystalline semiconductor.

FIG. 6 is a characteristic diagram showing the relationship between the electrical characteristics of the polycrystalline semiconductor and the height of irregularities on the substrate surface.

Claims (1)

[Claims] Claim 1: The channel layer is a polycrystalline semiconductor formed by heat-treating an amorphous semiconductor formed on a substrate with an uneven shape on its surface by irradiation with a high-energy beam. A thin film transistor featuring: