JPH04313272A - Manufacture of thin-film transistor - Google Patents

Abstract

PURPOSE:To obtain a thin-film transistor with improved electrical characteristics, a wide operation temperature, and a high yield by performing selective oxidation of a semiconductor layer which becomes an activation layer for forming a specific film thickness. CONSTITUTION:A silicon film 2 is laminated on an insulation substrate 1 and an area other than a surface of a silicon oxide film 2 for performing selective oxidation is covered with a nitriding silicon film 4 and is oxidized locally, thus enabling a silicon dioxide film 5 to be formed. When performing this local and selective oxidation, a silicon atom within the silicon film 2 is realigned and the silicon film 2 changes into a polycrystalline film 3. After removing a silicon dioxide film 5 and a nitriding silicon film 4, the polycrystalline silicon film 3 is oxidized for constituting a gate insulation film 7 and then the polycrystalline film 3 is converted into a polycrystalline silicon film 6 at the time of this oxidation. A source region and a drain region of a thin-film transistor and a channel region which becomes an activation layer are formed at the polycrystalline silicon film 6 which is formed by this oxidation. Then, a film thickness of a region which becomes an activated layer is set to 200Angstrom to 700Angstrom .

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing polycrystalline silicon thin film transistors.

0002

2. Description of the Related Art As a conventional method for manufacturing polycrystalline silicon thin film transistors, an example described in Japanese Patent Application Laid-open No. 119269/1983 is known. FIGS. 7(a) to 7(c) are process-order sectional views showing a conventional example. A detailed explanation will be given below based on the drawings.

First, as shown in FIG. 7A, a polycrystalline silicon thin film is formed into an island shape on an insulating substrate such as a quartz substrate 21 or an insulating film, and then a gate insulating film 24 and a gate electrode are formed. Impurity ions are implanted into a part of the silicon thin film 22 to form a source electrode region 9 and a drain electrode region 10. Thereafter, as shown in FIG. 7B, after forming an interlayer insulating film 23 and aluminum wiring 11, hydrogen ions 20 are irradiated onto the surface of the thin film transistor. Furthermore, as shown in FIG. 7C, a protective film 13 such as a silicon nitride film is deposited to complete a polycrystalline silicon thin film transistor. As is known from the example described in JP-A-63-119270, hydrogen ion irradiation may be performed after the protective film 13 shown in FIG. 7(c) is formed. This has the advantage that damage to the wiring 11 due to hydrogen ion irradiation can be prevented.

0004

However, in the above conventional technique, the active layer, source electrode region 9, and drain electrode region 10 of the thin film transistor are formed of the same silicon thin film 22, which has a large effect on the characteristics. The problem is that it is difficult to optimize the thickness of the silicon thin film provided. In fact, the contact resistance formed at the contact surface between the aluminum wiring 11 and the source electrode region 9 and the contact surface between the aluminum wiring 11 and the drain electrode region 10 and the sheet resistance of both the source and drain electrode regions themselves are different from that of the source electrode region 9. It has been found that the thicker the silicon thin film of the drain electrode region 10 is, the smaller it is, and the thinner it is, the larger it is. Generally, it is known that if the impurity concentration is constant, the sheet resistance of the source electrode region 9 and the drain electrode region 10 is inversely proportional to the thickness of the silicon film forming both electrode regions. Therefore, when the thickness of the silicon thin film constituting the source electrode region 9 and the drain electrode region 10 is about several hundred Å, the total of these contact resistances and sheet resistances ranges from several kilohms to several tens of kilohms. This caused deterioration of electrical characteristics in the current region. However, in order to improve the electrical characteristics of thin film transistors when a low voltage is applied, which is called the saturation region, it is necessary to increase the thickness of the silicon thin film that will become the active layer by 2.
It is desirable to set the thickness between 00 Å and 700 Å. This is because when the active layer is made thinner than when it is thick, the maximum depletion layer width in the active layer extending from the interface between the gate insulating film and the active layer to the substrate side is suppressed, and the inversion layer where carriers flow is reduced. This is thought to be because it is easier to configure under relatively low voltage. For these reasons, in order to improve the electrical properties in the saturation region, it is best to keep the thickness of the silicon thin film to a few hundred Å or less, and on the other hand, in order to improve the electrical properties in the high current state, the film thickness should be increased. It turns out that the thicker it is, the better. For this reason, it is desirable to change the film thicknesses of the silicon thin films that make up both the source and drain regions and the silicon thin film that makes up the active layer, but if they were to be formed separately, There are various problems such as an increase in contact resistance due to defects and contaminants at the contact interface between the source and drain electrode regions and the active layer, and the need to secure extra area for alignment between the source and drain electrode regions and the active layer. This will create new problems. In addition, in order to control contact resistance to a low level of several tens of kilohms or less, it is difficult to reduce the thickness of the active layer of a silicon thin film to a few hundred Å or less, so hydrogen ions are used to improve electrical characteristics. Another problem is that the irradiation is not effective unless it is carried out for a long period of time. The main reason why the electrical characteristics of polycrystalline silicon thin film transistors improve by irradiation with hydrogen ions is that the irradiated hydrogen ions terminate dangling bonds made of unpaired electrons and reduce the barrier to conduction carriers. It depends on the fact that there is. For this reason, when the silicon thin film that serves as the active layer is as thick as several hundred angstroms, the number of lattice defects and unpaired electrons at crystal boundaries in the silicon thin film increases, and a large amount of unpaired electrons are required to terminate them. This is because hydrogen ions must be implanted. Moreover, most of the hydrogen ions implanted into a silicon thin film with a thickness of several hundred Å are concentrated in a region several tens of Å to several hundred Å from the gate insulating film side interface of the silicon thin film. It does not reach the interface of the thin film on the substrate side. For this reason, other unpaired electrons existing in the silicon thin film are not terminated and form trap levels, carrier scattering occurs, and the electrical characteristics of the thin film transistor cannot be sufficiently improved. have.

The present invention is intended to solve these problems, and its purpose is to provide thick film source and drain electrode regions with good contact resistance and sheet resistance. It is an object of the present invention to provide a method for manufacturing a thin film transistor that has an active layer and can easily obtain excellent electrical characteristics by hydrogenation for a short time.

[0006]

Means for Solving the Problems The method for manufacturing a thin film transistor of the present invention is a method for manufacturing a thin film transistor in which a MOS field effect transistor is formed on an insulating substrate, in which a thin film semiconductor layer to be an active layer is selectively oxidized to a thickness of 70 Å.
It is characterized by including the step of making the film thickness as follows. The method for manufacturing a thin film transistor according to the present invention is characterized by including a step of irradiating and heating the thin film semiconductor layer serving as the active layer with a laser.

[0007] Furthermore, the method for manufacturing a thin film transistor according to the present invention is characterized in that it includes a step of implanting hydrogen ions into the thin film semiconductor layer that will become the active layer.

[0008]

EXAMPLES The method for manufacturing a thin film transistor according to the present invention will be described in detail below based on examples.

First, an example of manufacturing a thin film transistor based on the first aspect of the present invention will be described. IntroductionFigure 1 (
As shown in a), a silicon film 2 is laminated on an insulating substrate 1 such as a quartz substrate. This silicon film 2 is exposed to plasma C.
2 by VD (chemical vapor deposition using plasma excitation) method.
It may be an amorphous silicon film formed in a heated state of 00° C. to several hundreds of degrees Celsius, or a polycrystalline silicon film formed in a heated state of several hundred degrees Celsius or higher by low pressure CVD. However, for silicon films with a hydrogen content of several percent or more, hydrogen is removed by heating at 300° C. to 500° C. after deposition to increase the stability of the silicon film. Then,
As shown in FIG. 1B, the surface of the silicon film 2 other than the surface to be selectively oxidized is covered with a silicon nitride film, and is locally oxidized as shown in the figure in an oxygen atmosphere to form a silicon dioxide film 5. The heat applied during this local selective oxidation causes the silicon atoms in the silicon film 2 to be rearranged, and the silicon film 2 changes into a polycrystalline silicon film 3. At the same time, the thickness of the oxidized portion of the polycrystalline silicon film 3 becomes thinner. Furthermore, after removing the silicon dioxide film 5 and silicon nitride film 4 formed by selective oxidation, the polycrystalline silicon film is processed into an island shape by photolithography, and the polycrystalline silicon film is oxidized, as shown in FIG. 1(c). The gate insulating film 7 is configured as shown in FIG. During this oxidation, the thickness of polycrystalline silicon film 3 is reduced and converted into polycrystalline silicon thin film 6. In the polycrystalline silicon thin film 6 formed by this oxidation, a source region, a drain region, and a channel region which becomes an active layer of a thin film transistor are formed. The film thickness of the region to be the active layer is 200 Å to 700 Å, and the film thickness of the regions to be the source electrode region 9 and the drain electrode region 10 is 1000 Å to 2000 Å. Next, Figure 1 (
As shown in d), after forming the gate electrode 12, a source electrode region 9 and a drain electrode region 10 are formed by a self-alignment method. Furthermore, as shown in the figure, a part of the gate insulating film covering the source electrode region 9 and drain electrode region 10 is removed by photolithography so that the material of the aluminum wiring 11 comes into contact with the source electrode region 9 and drain electrode region 10. A protective film 13 is formed by processing. The thin film transistor formed in this way has a thickness of 200 Å.
Thin channel region from 700 Å and from 1000 Å to 20
It has a thick source electrode and drain electrode region of 00 Å. Therefore, the maximum depletion layer in the channel region is suppressed to a shallow extent, and an inversion layer that becomes a conductive layer during transistor operation can be easily formed even when the voltage applied to each electrode is low. Therefore, the electrical characteristics of the thin film transistor are significantly improved compared to the case where the thin film transistor has a thick active layer. The thickness of the active layer is 1000 in Figure 3.
The electrical characteristics representing the relationship between the gate applied voltage and drain current of the thin film transistor are shown when the thickness is 400 Å and when the thickness is 400 Å. In FIG. 3, the electrical characteristics indicated by a solid line 16 are based on a conventional example in which the active layer and the source/drain regions are made to have the same thickness. The thickness of the silicon thin film is 1000 Å. In contrast to this characteristic, a broken line 17 shows the electrical characteristic of the thin film transistor when the thickness of the active layer is set to 500 Å based on the embodiment of the present invention. As shown in this example, reducing the thickness of the active layer has the effect of significantly improving the electrical characteristics of the thin film transistor. Moreover, since the film thicknesses of the source electrode region 9 and the drain electrode region 10 are thick, the sheet resistance of these electrode regions and the contact resistance with the aluminum wiring 11 are kept sufficiently low, which adversely affects the electrical characteristics of the thin film transistor as a parasitic resistance. No problem.

Next, an embodiment related to claim 2 of the present invention will be described. 2(a) to 2(c) are step-by-step cross-sectional views for explaining a method for manufacturing a thin film transistor according to an embodiment of claim 2 of the present invention. First, a silicon film 2 is formed on an insulating substrate 1, as shown in FIG. 2(a), similarly to the embodiment of claim 1. Next, as shown in FIG. 2B, a partial region of the silicon film 2 is selectively oxidized using the silicon nitride film 4 as a mask to form a silicon dioxide film 5.
A laser beam 15 is irradiated to convert the silicon film 2 into a crystalline silicon film 14. The irradiation with this laser beam 15 instantaneously melts the silicon within the silicon film 2, and after the irradiation, the silicon rapidly solidifies again. During this solidification, silicon crystals grow and a crystalline silicon film 14 is formed. A thin film transistor in which the crystalline silicon film 14 is used as an active layer and source/drain region can exhibit superior electrical characteristics with high mobility compared to a transistor in which the silicon film in the active layer is simply made thinner. know. The electrical characteristics diagram in FIG. 4 shows the characteristics of a thin film transistor in which the channel was made thinner and irradiated with a laser beam, and the characteristics of a conventional thin film transistor with an active layer of 1000 Å. The broken line 18 shows the electrical characteristics of a thin film transistor made of a crystalline silicon film 14 formed by laser beam irradiation with a thin active layer of 500 Å, and the solid line 16 shows the electrical characteristics of the thin film transistor with an active layer of 1000 Å. It can be seen that not only making the active layer thinner but also improving the crystallinity by irradiating it with a laser beam increases the mobility and significantly improves the electrical properties when high voltage is applied. Furthermore, crystallization of the active silicon layer not only increases the electrical mobility of conduction carriers but also improves the temperature characteristics of the thin film transistor. This is because the crystallization of the silicon film in the active layer
This is because it has the effect of reducing crystal defects that act as carrier conduction barriers. Therefore, the activation energy of the current decreases to almost zero, allowing stable operation of the thin film transistor even at high temperatures.

Furthermore, an embodiment related to claim 3 of the present invention will be described. In the process shown in FIG. 5, the thin film transistor is irradiated with hydrogen ions 20. In conventional hydrogen ion irradiation of thin film transistors, either the active layer and the source/drain electrode regions have the same film thickness, or the source/drain electrode regions are laminated in a separate process from the active layer. It was applied. In the latter thin film transistor laminated by separate steps, defects and resistance occurring between the source/drain electrode regions and the active layer tend to adversely affect electrical characteristics, which is undesirable. Additionally, it is difficult to completely prevent contaminants from entering during the photolithography process used to form the island-shaped electrode regions, and contaminants that do enter can reduce the reliability of thin film transistors. It was also becoming. Furthermore, in the case of the former thin film transistor in which the active layer and both electrode regions have the same film thickness, it is difficult to optimize the film thickness to improve the electrical characteristics of the thin film transistor, as described above, and it is said that the characteristics cannot be sufficiently improved. There was a problem. On the other hand, hydrogen irradiation in a thin film transistor having both electrode regions formed at once and an active layer thinned as shown in FIG. 5 can be performed in an extremely short period of time and at low power in order to thin the film. Not only that, but there are no other contaminants mixed in. When the active layer is a thick film, terminating unpaired electrons by hydrogen irradiation requires irradiation at a high power of nearly 10 KeV for more than one hour. However, in a thin film transistor having an optimized active layer thickness of 200 Å to 700 Å as in this embodiment described above, the thickness of several K
Sufficiently effective hydrogen irradiation can be performed with low power of eV or less and in a short time of about 30 minutes. If it is possible to irradiate with low power, there will be less damage to the thin film transistor during irradiation, and destruction of the thin film transistor due to hydrogen ion irradiation can be prevented, resulting in a high-yield process. The electrical characteristics of the thin film transistor shown in Figure 6 are 30 at 5KeV.
The characteristics obtained by irradiation with hydrogen ions for a minute and the characteristics obtained by irradiation with the laser beam in the above example are shown. A solid line 18 indicates the characteristics of the thin film transistor irradiated with the laser beam 15 in FIG. 2b,
A broken line 19 indicates the characteristics of a thin film transistor obtained by subjecting the thin film transistor exhibiting the characteristics shown in the solid line 18 to the hydrogen ion irradiation shown in FIG. As is clear from the characteristic comparison in FIG. 6, hydrogen ion irradiation significantly improves the electrical characteristics of the thin film transistor. The effect is seen not only in increasing the operating current of thin film transistors, but also in reducing leakage current in particular. This is considered to be because the amount of defect levels composed of unpaired electrons near the drain electrode, which causes leakage current, is terminated and reduced by the hydrogen ion irradiation. This reduction in leakage current increases the on-off ratio defined as the ratio of the amount of operating current to the amount of leakage current, broadening the allowable operating range of thin film transistors and making them applicable to a variety of high-performance application products. For example, it can be applied to drive elements for high-definition LCD televisions and high-definition color sensors.

0012

Effects of the Invention As explained above, the method for manufacturing a thin film transistor of the present invention makes it easy to optimize the film thickness of the active layer and the source/drain electrode regions by using a selective oxidation process. It has the effect of speeding up the formation, lowering the resistance of both electrodes, and dramatically improving the electrical characteristics. Further, by laser beam irradiation, the characteristics can be further improved and the operable temperature range can be expanded. moreover,
By making the active layer thinner, it is possible to reduce the power and time of the hydrogen irradiation process, and hydrogen irradiation can be performed effectively, further improving electrical characteristics, as well as weakening the damage caused by hydrogen irradiation and improving the manufacturing yield of thin film transistors. By reducing the leakage current, it is possible to apply the thin film transistor to various products requiring high performance thin film transistors.

[Brief explanation of drawings]

FIG. 1 is a step-by-step sectional view showing an embodiment of the method for manufacturing a thin film transistor of the present invention.

FIG. 2 is a step-by-step cross-sectional view showing an embodiment of the thin film transistor manufacturing method of the present invention.

FIG. 3 is an electrical characteristic diagram of a thin film transistor manufactured by the process according to claim 1 of the method for manufacturing a thin film transistor of the present invention.

FIG. 4 is an electrical characteristic diagram of a thin film transistor manufactured by the process according to claim 2 of the method for manufacturing a thin film transistor of the present invention.

FIG. 5 is a process sectional view showing an embodiment of the method for manufacturing a thin film transistor of the present invention.

FIG. 6 is an electrical characteristic diagram of a thin film transistor manufactured by the process according to claim 3 of the method for manufacturing a thin film transistor of the present invention.

FIG. 7 is a step-by-step cross-sectional view showing an example of a conventional thin film transistor manufacturing method.

[Explanation of symbols]

1 Insulating substrate 2 Silicon film 3 Polycrystalline silicon film 4 Silicon nitride film 5 Silicon dioxide film 6 Polycrystalline silicon thin film 7 Gate oxide film 8 Polycrystalline silicon thin film 9 Source electrode area 10 Drain electrode area 11 Aluminum wiring 12 Gate electrode 13 Protective film 14 Crystalline silicon film 15 Laser beam 20 Hydrogen ion 21 Quartz substrate 22 Silicon thin film 23 Interlayer insulation film 24 Gate insulating film

Claims (3)

[Claims] 1. A method for manufacturing a thin film transistor in which a MOS field effect transistor is formed on an insulating substrate, comprising:
1. A method for manufacturing a thin film transistor, comprising the step of selectively oxidizing a thin film semiconductor layer serving as an active layer to a thickness of 700 Å or less. 2. The method of manufacturing a thin film transistor according to claim 1, further comprising the step of irradiating the thin film semiconductor layer serving as the active layer with a laser and heating it. 3. The method of manufacturing a thin film transistor according to claim 1, further comprising the step of implanting hydrogen ions into the thin film semiconductor layer serving as the active layer.