JP3185790B2 - Method for manufacturing thin film semiconductor device - Google Patents

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 a thin film semiconductor device, and more particularly, to an insulated gate field effect transistor or a thin film transistor (TFT).
(Istor) of the gate insulating film.

[0002]

2. Description of the Related Art In recent years, SOI (Silicon On) has been developed.
Insulator) or a three-dimensional IC, a large-sized liquid crystal display panel, a high-speed, high-resolution contact type image sensor, and the like, realize a high-performance thin film semiconductor device on an insulating amorphous material. Technology has become important. Further, a technique for forming a high-quality gate insulating film at a low temperature has also become important. An oxide film (SiO ₂ ) is generally used as the gate insulating film. As an oxide film forming method, there are a thermal oxidation method, a CVD method, and a photo CVD method.
Method, a plasma CVD method, an anodic oxidation method, a high pressure oxidation method, and the like. Among the above methods, the electrical characteristics of the silicon oxide film interface formed by the thermal oxidation method are the most excellent. Since the thermal oxidation method is a high-temperature process of about 900 to 1200 ° C., (1) an element cannot be formed on an inexpensive glass substrate. (2) Lateral diffusion of impurities. (3) Three-dimensional I
C has a bad influence (diffusion of impurities, etc.) on the element in the lower layer. (4) The thermal oxide film of poly-Si has an insufficient withstand voltage and a high interface state density. (5) There is a problem that a large stress is applied to the Si / SiO ₂ interface.

[0003]

In the conventional thermal oxidation method,
When the substrate was inserted into the oxidation furnace, the heating rate of the substrate was not controlled. A predetermined oxidation temperature, for example, 1000
In the case of performing thermal oxidation at a temperature of 1000 ° C.,
It was suddenly inserted into the oxidation furnace set to ° C.
Therefore, the oxide film had grown rapidly. It is known that the oxidation rate is increased particularly at the beginning of the oxidation. Therefore, an oxide film formed by a conventional thermal oxidation method has a problem that it has a very large strain and a high interface state density. Further, there is a problem that the gate withstand voltage is low and the reliability is adversely affected. When a semiconductor device such as a thin film transistor is manufactured using such a gate oxide film, there is a problem that the ON current and the mobility are reduced, and the threshold voltage (Vth) and the OFF current are increased.

An object of the present invention is to solve such a problem and to realize a thin film semiconductor device such as a thin film transistor having excellent transistor characteristics by forming an excellent gate oxide film by a thermal oxidation method. .

[0005]

According to the present invention, there is provided a method of manufacturing a thin film semiconductor device having a gate insulating film formed by a thermal oxidation method, comprising the steps of: forming a substrate on which a thin film made of amorphous silicon or polycrystalline silicon is formed; The thin film is inserted into an oxidation furnace at a temperature lower than the temperature at which the thin film is oxidized, the temperature of the oxidation furnace is increased to 20 ° C./min or less, and the thin film is solid-phase grown and oxidized to form a gate insulating film. Forming
Thereafter, the temperature of the oxidation furnace is lowered to a temperature lower than the temperature at which the gate insulating film is formed, and then the substrate is taken out of the oxidation furnace. In the present invention, the rate of raising the temperature of the oxidation furnace is 20 ° C./min or less,
The cooling rate for lowering the temperature of the oxidation furnace is lower than the heating rate.

[0006]

[0007]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments will be described by taking as an example a case where the present invention is applied to a thin film transistor. FIG. 1 is a process sectional view showing a method for manufacturing a thin film transistor according to an embodiment of the present invention.

A non-single-crystal semiconductor thin film is formed on an insulating amorphous material. As the insulating amorphous material, a quartz substrate, a glass substrate, a nitride film, a SiO ₂ film, or the like is used. When a quartz substrate is used, the process temperature is 1200
Although it is allowable up to about ℃, when using a glass substrate,
Limited to low temperature processes below 600 ° C. In some cases, a quartz substrate or a glass substrate on which an oxide film or a nitrided film is deposited in order to suppress emission and diffusion of impurities is used.
The present invention will be described by way of an example in which a quartz substrate is used and a Si thin film is used as the non-single-crystal semiconductor thin film. Using a plasma CVD apparatus, a mixed gas of SiH ₄ and H ₂ was placed on a quartz substrate 1-1 as shown in FIG.
The amorphous Si film 1-2 is deposited by being decomposed by a 3.56 MHz high frequency glow discharge. SiH of the mixed gas
₄ partial pressure 10-20%, internal pressure in depot 0.5-1.5
about torr. Substrate temperature is 250 ° C or less, 180
C is suitable. The amount of bonded hydrogen determined by infrared absorption measurement was about 8 atomic%.

Subsequently, the amorphous Si film is formed at a temperature of 400.degree.
Heat treatment at 00 ° C. to release hydrogen. This step is intended to prevent explosive desorption of hydrogen.

Next, the amorphous thin film 1-2 is solid-phase grown. For the solid phase growth method, furnace annealing using a quartz tube is convenient. As an annealing atmosphere, a nitrogen gas, a hydrogen gas, an argon gas, a helium gas, or the like is used. 1x1
Annealing may be performed in a high vacuum atmosphere of 0 ⁻⁶ to 1 × 10 ⁻¹⁰ Torr. Solid phase growth annealing temperature is 500
C. to 700.degree. In such low-temperature annealing, only crystal grains having a crystal orientation with a small activation energy for crystal growth grow selectively and grow slowly and slowly. In an experiment by the inventor, a large-diameter silicon thin film of 2 μm or more was obtained by solid phase growth at an annealing temperature of 600 ° C. and an annealing time of 16 hours. In FIG. 1B, reference numeral 1-3 denotes a solid-phase grown silicon thin film. There is no problem if a laser annealing method is used instead of the solid phase growth method.

Next, the solid-phase-grown silicon thin film 1-3 is patterned by photolithography into an island shape as shown in FIG. 1 (c).

Next, as shown in FIG. 1D, a gate oxide film 1-4 is formed by a thermal oxidation method. The thermal oxidation method includes a dry oxidation method performed in a dry oxygen atmosphere and a wet oxidation method performed by mixing water vapor or the like. In the dry oxidation method, the oxidation temperature is as high as 1000 ° C. or higher, but the film quality is excellent. The wet oxidation method can oxidize at a large oxidation rate even at a low temperature of 800 ° C. or less. In the present invention, both methods can be used without any problem. Where d
An example will be described using a ry oxidation method.

FIG. 2 shows a first embodiment of the oxidation method according to the present invention. This is a diagram showing a heating curve of the oxidation furnace. The vertical axis indicates the oxidation furnace temperature, and the horizontal axis indicates the process time. The insertion of the substrate with the silicon thin film 1-3 as a sample is started at time to. High purity dry oxygen gas is introduced into the oxidation furnace. Ta indicates the insertion oxidation furnace temperature. For example, Ta is set to a low temperature of 600 ° C.
The substrate arrives at the oxidation furnace center time t _1. At this time, the insertion speed of the substrate is set so that the temperature rise rate of the substrate surface is 20 ° C./min. When the distance from the entrance of the oxidation furnace to the center is 43 cm, the insertion speed is 100 mm.
/ Min, the rate of temperature rise on the substrate surface was 20 ° C / min or less. I checked it with a thermocouple and found that it followed well. Next, the temperature of the oxidation furnace is raised to a predetermined oxidation temperature Tb at a rate of 20 ° C./min or less. In the case of dry oxidation, Tb is usually set at 900 ° C. or higher, but may be lower. Since the oxidation rate is increased at the beginning of oxidation, the silicon thin film is slowly oxidized even when inserted at a low temperature of 600 ° C. By raising the temperature of the oxidation furnace temperature, a predetermined oxidation temperature Tb on the substrate surface time t ₂
Has been reached. At Tb = 1150 ° C., a very excellent gate oxide film was formed. The gist of the present invention is:
This means that the temperature of the insertion oxidation furnace is lower than the oxidation temperature. That is, Tb> Ta. When the substrate temperature reaches Tb, keeping the oxidation furnace temperature constant, the oxidation process by setting the time t ₃ to form a predetermined oxide thickness. At time t _3, switch to a nitrogen gas of the gas to be introduced into the oxidation furnace of an oxygen gas. Until time t ₄ is a nitrogen annealing, but this is not necessary. When taking out the end of the substrate, Slow cooling over to time t ₅ in order to prevent the occurrence of defects due to rapid cooling of the substrate. It is desirable that the temperature decreasing rate at this time be smaller than the temperature increasing rate.

FIG. 3 shows an embodiment of the second oxidation method according to the present invention. This is an embodiment in which the temperature of the insertion oxidation furnace is increased from Ta to Tc in the method described with reference to FIG. 2 (Ta <Tc). For example, by increasing the temperature of the insertion oxidation furnace from 600 ° C. to 900 ° C., the initial oxidation proceeds.

An embodiment of the third oxidation method according to the present invention is shown in FIG. This is an embodiment in which the heating rate of the oxidation furnace is reduced in the method described with reference to FIG. Heating rate 2
By increasing the temperature from 0 ° C./min to 2 ° C./min, the time required for raising the temperature is increased tenfold. Therefore, since the processing time at a relatively low temperature becomes longer, the effect of solid phase growth appears and the crystal grain size of silicon increases.

FIG. 5 shows a conventional oxidation method. The oxidation furnace temperature was always set to a predetermined oxidation temperature Tb. Then, a silicon thin film was inserted into the oxidation furnace at a stretch.
Therefore, the temperature of the substrate surface could be controlled only by the insertion speed of the substrate. When the rate of temperature rise on the surface of the silicon thin film at this time was measured, it was about 36 ° C./min. The field effect mobility of the thin film transistor having the gate oxide film formed by this conventional oxidation method was a small value of 10 cm ² / V · s or less. In addition, the off current is a drain voltage 5
The voltage was as large as about 50 pA. That is, sufficient characteristics cannot be obtained by the oxidation method using a heating rate of 36 ° C./min.

Thus, the oxidation step is completed. Returning to FIG. 1, a description will be given again of the manufacturing process of the thin film transistor. As shown in FIG. 1E, a gate electrode 1-5 is formed. As the gate electrode material, a polycrystalline silicon thin film, molybdenum silicide, a metal film such as aluminum or chromium, or a transparent conductive film such as ITO or SnO ₂ can be used. As a film forming method, there are methods such as a CVD method, a Spack method, a vacuum evaporation method, and a plasma CVD method. A low-resistance silicon thin film formed by solid phase growth of doped a-Si: H can also be used. Detailed description here is omitted.

Subsequently, as shown in FIG. 1F, impurities are ion-implanted using the gate electrode 1-5 as a mask.
The source region 1-6 and the drain region 1-
7 is formed. As the impurity, when manufacturing an Nch transistor, P ⁺ or AS ⁺ is used.
When a transistor is manufactured, B ⁺ or the like is used. As an impurity doping method, there are a laser doping method, a plasma doping method, and an ion shower method, in addition to the ion implantation method. Arrows indicated by 1-8 represent ion beams of impurities. When a quartz substrate is used as the insulating amorphous material 1-1, a thermal diffusion method can be used. The impurity concentration is 1 × 10 ¹⁵ to 1
It is about × 10 ²⁰ cm ⁻³ . The gate oxide film 1-
For example, when the film thickness of No. 4 is 1200 Å, phosphorus ions are about 80 to 120 keV, and boron ions are 3
Ion implantation is performed at an acceleration voltage of about 0 to 80 keV.

Subsequently, as shown in FIG. 1 (g), an interlayer insulating film 1-9 is laminated. As the material of the interlayer insulating film, an oxide film or a nitride film is used. The thickness is not particularly limited as long as the insulating property is good, but the thickness is usually from several thousand angstroms to several μm. As a method for forming a nitride film, an LPCVD method, a plasma CVD method, or the like is simple. For the reaction, a mixed gas of ammonia gas (NH ₃ ), silane gas and nitrogen gas, a mixed gas of silane gas and nitrogen gas, or the like is used.

Here, when hydrogen ions are introduced by a method such as a hydrogen plasma method, a hydrogen ion implantation method, or a method of diffusing hydrogen from a plasma nitride film, the hydrogen ions are present at the interface between the gate oxide film and the crystal grain boundaries in the silicon film. Defects such as dangling bonds are inactivated, and electrical characteristics are dramatically improved. Such a hydrogenation step may be performed before stacking the interlayer insulating films 1-9.

Next, as shown in FIG. 1 (h), contact holes are formed in the interlayer insulating film and the gate insulating film.
A contact electrode is formed to form a source electrode 1-10 and a drain electrode 1-11. The source electrode and the drain electrode are formed of a metal material such as aluminum. Thus, a thin film transistor is formed.

The thin film transistor manufactured by the oxidation method of the present invention has a field effect mobility of 20 to 30 cm ² / V.
S, which is 2-3 times improved compared to the conventional method. Further, when the drain voltage is 5 V, the off-state current is about 5 pA.
And one order of magnitude smaller.

Conventionally, a silicon thin film having a large crystal grain size of 2 μm or more has been used by the solid-phase growth method. However, in the oxidation step, the gate oxide formed at once was inserted into an oxidation furnace at 1000 ° C. or more. The film had a very large strain, and the interface state at the Si / SiO ₂ interface was very large. Therefore, although the thin film transistor manufactured by the conventional method uses a silicon thin film having a large grain size, the off current is large and the on current is small. Further, Vth was also large. The reliability was low due to poor interface characteristics. Field effect mobility is 10c at most
It was as small as about m ² / V · s. Further, the off-state current was as large as about 50 pA when the drain voltage was 5 volts.

In this embodiment, a substrate is inserted at a certain insertion speed into an oxidation furnace set at a temperature lower than a predetermined oxidation temperature. Then, from the place where the substrate is set in the furnace, the temperature is raised to a predetermined oxidation temperature at a certain rate.
Further, the insertion speed and the heating rate are controlled so that the heating rate of the substrate surface is 20 ° C./min or less. Therefore, the oxidation reaction proceeds slowly, the strain is very small, and the Si / Si
A gate oxide film having excellent O ₂ interface characteristics is formed. As a parameter for controlling the oxide film and interface characteristics, a parameter called the heating rate of the oxidation furnace has been newly added in addition to the conventional conditions, so the characteristics of the gate oxide film must be controlled more precisely than in the past. Is now possible. In addition, 6
Since the process can be performed at a low temperature of about 00 ° C., the silicon thin film is oxidized and becomes a silicon thin film having a small number of defects and a large crystal grain size by the effect of solid phase growth. In the case of a silicon film deposited as a polycrystalline thin film from the beginning, the effect of solid phase growth is not so expected, but as described in the examples, if a silicon film deposited as an amorphous thin film is used first, A large effect of solid phase growth is expected. Of course, there is no problem if a polycrystalline silicon thin film is applied to the present invention.

It has become possible to produce a silicon thin film having excellent crystallinity on an amorphous insulating substrate, and to form an even more excellent gate oxide film. Therefore, it greatly contributes to the development of thin film transistors and SOI technology.
The number of photo steps does not increase at all. Although an excellent silicon thin film can be obtained, the cost does not increase.

When a thin film transistor is manufactured by using the gate oxide film and the large-grain polycrystalline silicon thin film obtained in this embodiment, excellent characteristics can be obtained. Compared with the related art, the ON current of the thin film transistor increases and the OFF current decreases. Also, the threshold voltage is reduced, and the transistor characteristics are greatly improved. The unbalance between the characteristics of the N channel and the P channel is also improved. Field-effect mobility is about the art of ^{10cm 2 / V · s 20~30cm 2} /
V · s. Further, the off-state current when the drain voltage was 5 volts was reduced from the conventional 50 pA to 5 pA.

Since a thin film transistor having excellent characteristics can be manufactured on an amorphous insulating substrate, a sufficiently high-speed operation can be realized even when the driver circuit is applied to an active matrix substrate integrated on the same substrate. You. Furthermore, there is a great effect on reduction of power supply voltage, reduction of current consumption, and improvement of reliability.

When the present invention is applied to a contact-type image sensor in which a photoelectric conversion element and its scanning circuit are integrated on the same chip, it is very difficult to increase the reading speed, increase the resolution, and obtain gradation. To produce great effects. When high resolution is achieved, application to a contact type image sensor for color reading becomes easy. Of course, the effect is great also for reduction of power supply voltage, reduction of current consumption, and improvement of reliability.

Further, since it can be manufactured by a low-temperature process, the length of the contact-type image sensor chip can be increased, and a single facsimile apparatus such as A4 size or A3 size can be realized. Therefore, it is possible to avoid troublesome techniques such as double splicing of sensor chips and unreliable technology, and the mounting yield is improved.

Not only quartz substrates and glass substrates but also sapphire substrates (Al ₂ O ₃ ) or MgO.Al
A crystalline insulating substrate such as ₂ O ₃ , BP, or CaF ₂ can also be used.

Although a thin film transistor has been described as an example, an element using a thin film such as a bipolar transistor or a heterojunction bipolar transistor can also be used.
The present invention can be applied. Further, the present invention can be applied to an element using an SOI technology such as a three-dimensional device.

According to the present invention, the above-mentioned constitutional requirements can provide the following remarkable effects. (I) Since the substrate is taken out of the oxidation furnace after the temperature is lowered to a temperature lower than the temperature at which the thermal oxide film of the silicon thin film is formed, the occurrence of defects such as distortion and distortion of the substrate due to rapid cooling can be prevented. (II) Since the temperature of the substrate is slowly increased to 20 ° C./min or less in the oxidation furnace, solid phase growth of the silicon film can be performed at the same time, and a semiconductor device having a polycrystalline silicon film with a large grain size is provided. be able to.

[Brief description of the drawings]

FIGS. 1A to 1H are process sectional views of a thin film transistor showing an embodiment of the present invention.

FIG. 2 is a diagram showing a temperature rise curve of an oxidation furnace in a first oxidation method of the present invention. The vertical axis indicates the oxidation furnace temperature, and the horizontal axis indicates the process temperature.

FIG. 3 is a diagram showing a temperature rise curve of an oxidation furnace in a second oxidation method of the present invention. The vertical axis indicates the oxidation furnace temperature, and the horizontal axis indicates the process temperature.

FIG. 4 is a view showing a temperature rise curve of an oxidation furnace in a third oxidation method of the present invention. The vertical axis indicates the oxidation furnace temperature, and the horizontal axis indicates the process temperature.

FIG. 5 is a diagram showing a temperature rise curve of an oxidation furnace in a conventional oxidation method. The vertical axis indicates the oxidation furnace temperature, and the horizontal axis indicates the process temperature.

[Explanation of symbols]

 1-1: Insulating amorphous material 1-2; Amorphous Si film 1-3; Si film grown by solid phase 1-4; Gate oxide film

Continuation of the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) H01L 29/786 H01L 21/316 H01L 21/336

Claims (2)

(57) [Claims] In a method of manufacturing a thin film semiconductor device having a gate insulating film formed by a thermal oxidation method, a substrate on which a thin film made of amorphous silicon or polycrystalline silicon is formed is heated at a temperature lower than the temperature at which the thin film is oxidized. The thin film is solid-phase grown and oxidized to form a gate insulating film, and then the oxidizing furnace is inserted into a low-temperature oxidizing furnace. Is lowered to a temperature lower than the temperature at which the gate insulating film is formed, and then the substrate is taken out of the oxidation furnace. 2. The heating rate for heating the oxidation furnace is 20.
2. The method according to claim 1, wherein the temperature is lower than or equal to ° C./min, and a temperature lowering rate for lowering the temperature of the oxidation furnace is lower than the temperature increasing rate. 3.