JPH04137765A - Polycrystal silicon thin film transistor - Google Patents

Abstract

PURPOSE:To form a high-quality polycrystal silicon thin film without using an expensive heat-resisting glass substrate by forming the polycrystal silicon thin film on a light penetrable substrate at low temperatures, using the specified film formation gas and etching gas, not using hydrogen for a dilution gas. CONSTITUTION:In the manufacture of a polycrystal silicon thin film transistor, a light penetrable substrate 1 is used. Also, silicon hydride is used for a film formation gas for forming a polycrystal silicon film and silicon fluoride or silicon chloride gas is used for an etching gas. Such reaction gas is discharged at the specified power intensity into a plasma and a polycrystal silicon thin film 2 is formed on the light penetrable substrate 1 which is kept to a constant temperature between about 100 deg.C and 700 deg.C. Under such condition, the thin film 2 of (100) orientation 30% or above, hydrogen content 2.5 atom % or below and fluorine content 3 atom % or below can be obtained. On the thin film 2, a source electrode 4 and a drain electrode 3 are formed and a gate electrode 6 is formed on or under a part of the polycrystal silicon thin film between the source and drain electrodes through an insulated film 5. Thus, the device is completed.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a thin film transistor formed on a light-transmitting substrate and composed of a polycrystalline silicon thin film with a large (100) orientation.

[Conventional technology]

A polycrystalline silicon thin film is a state in which a large number of crystalline silicon particles with a size of several hundred to several tens of micrometers are aggregated, and thin film transistors using this film have higher performance compared to amorphous silicon transistors.
The mobility is high by about one to two orders of magnitude, and it has the characteristics that even peripheral scanning circuits can be fabricated on the same substrate. Conventionally, this thin film has been mainly manufactured by thermal CVD.

[Problem to be solved by the invention]

By the way, conventional polycrystalline silicon thin film transistors require a high film formation temperature, which limits the number of substrates that can be used.Generally, the higher the process temperature, the better the device performance and stability. ℃
An image sensor manufactured using a high-temperature process,
Liquid crystal televisions and the like have been put into practical use.

In this way, conventional polycrystalline silicon thin film transistors use heat-resistant glass such as quartz glass when formed on a glass substrate, making them extremely expensive and potentially unsuitable for mass production. Ta.

In addition, conventional polycrystalline silicon thin films have poor orientation, especially (1
00) Due to the low orientation, the flatness of the thin film surface deteriorates, and when used in thin film transistors, there was a problem that the yield after microfabrication decreased.

Furthermore, recent research has shown that in the case of polycrystalline silicon produced by thermal CVD, there are many gaps between silicon particles due to the high temperature during production, which prevents them from adhering to each other, as well as dangling grain boundaries. It has been found that the bond deteriorates the electrical properties, and there are known drawbacks such as the need to perform hydrogen tepassivation on the grain boundaries to correct the inconvenience of cutting.

For this reason, the plasma CVD method (Japanese Unexamined Patent Publication No. 15787/1986), which can produce polycrystalline silicon thin films at low temperatures, has recently been
2, No. 63-175417) is attracting attention. In these methods, a large amount of hydrogen gas is used as a component of the reaction gas, so the polycrystalline silicon obtained contains hydrogen between the silicon particles compared to when using the thermal CVD method. It has the advantage that there is no need to passivate it.

However, the polycrystalline silicon thin film obtained by the above method contains about 2.5 atomic percent or more of hydrogen, and the silicon crystal grain size can only be about 500 atomic percent.

Therefore, as a result of further research on polycrystalline silicon thin film transistors, the inventors of the present invention discovered that without using hydrogen as a diluent gas, silicon hydride was used as a film forming gas, and silicon fluoride or silicon chloride gas was used as an etching gas. A polycrystalline silicon thin film transistor can be formed on a transparent substrate by a low-temperature process using plasma CVD or photoCVD, and the thin film has a (100) orientation of 30% or more, preferably 50% or more, and a hydrogen content of 2. The present invention was achieved by discovering that it is possible to obtain a polycrystalline silicon thin film transistor with a fluorine content of 5 atomic % or less and a fluorine content of 3 atomic % or less.

Therefore, an object of the present invention is to provide a high quality polycrystalline silicon thin film transistor formed on a transparent substrate.

Another object of the present invention is to provide a thin film transistor made of a high-quality polycrystalline silicon thin film with a large (100) orientation.

Furthermore, the object of the present invention is to have a hydrogen content of 2.5 at% or less,
Fluorine content is 3 at% or less, crystal grain size is large,
An object of the present invention is to provide a thin film transistor made of a high-performance polycrystalline silicon thin film.

[Means to solve the problem]

The polycrystalline silicon thin film transistor of the present invention has (100
) Orientation is 30% or more, hydrogen content in the film is 2.5 at%
Hereinafter, a source electrode and a drain electrode are provided on a polycrystalline silicon thin film having a fluorine content of 3 atomic % or less, and a gate electrode is provided on or below the polycrystalline silicon thin film between the two electrodes via an insulating layer; Further, the present invention is characterized in that a polycrystalline silicon thin film is formed on a light-transmitting substrate, and that the polycrystalline silicon thin film is formed by plasma CVD or photoCVD.

The polycrystalline silicon thin film transistor of the present invention will be described in detail below.

In order to obtain the polycrystalline silicon thin film transistor of the present invention, for example, a glass having a low strain point, such as Corning 70
A transparent substrate such as 59 glass, HOYA (7) NA40 glass, neutral borosilicate glass, or Af silicate glass with a relatively high strain point is used, and a polycrystalline silicon thin film transistor is formed on this substrate by plasma CVD. form.

Film-forming gases for forming polycrystalline silicon films include:
StH,X4-@(m is 1 to 4, preferably 2 to 4,
is a C/or F atom, preferably an F atom. ), Si
Examples include *H* and 5isH-. These film-forming gases may be used alone or in combination of two or more.

In addition, etching gases include 5iFa and 5i2Fs.
, Sice's, Ft and C12. These etching gases may be used alone or in combination of two or more.

Then, a suitable mixture of film-forming gas and etching gas is used to balance crystal growth and etching. This balance can be easily adjusted by appropriately mixing the above-mentioned film-forming gas and etching gas.
It is preferable to use each individually or in combination. or,
As the etching gas, it is preferable to use SiF4 and F, either alone or in combination.

The pressure inside the chamber that generates plasma is related to the amount of energy possessed by atoms etc. that reach the substrate, so
The pressure needs to be 0.02 Torr to 15 Torr, and particularly preferably 0.3 Torr to 5 Torr.

An inert gas such as a rare gas, preferably helium, neon, argon, etc., may be added as a diluent gas to the film-forming gas and etching gas, and hydrogen gas is not used. The dilution gas is 1 to 100% of the etching gas.
It is preferable to use 0 times the amount, especially 5 to 100 times the amount.

Regarding the film-forming gas and the etching gas, the amount of the etching gas is about 1 to 500 times, preferably 5 to 200 times that of the film-forming gas. If the etching gas is less than 1 times the film forming gas, the substrate surface cannot always be kept in the best condition to facilitate the growth of polycrystalline silicon.
If it is 500 times or more, the etching rate becomes too high and the silicon crystal growth rate decreases.

A power density of 001-1 is applied to the reaction gas that meets the above conditions.
0W/cof, preferably 0. The reaction gas is turned into plasma by discharging at 1 to 5 W/cd, and the temperature is about 100°C to 700°C.
A polycrystalline silicon thin film is formed on a glass substrate maintained at a constant temperature, preferably between about 300°C and 600°C. The discharge may be high frequency discharge, direct current discharge, microwave discharge, or the like.

When the substrate temperature is lower than 100° C., an amorphous phase appears, resulting in a microcrystalline structure, resulting in poor quality.

The power density may vary depending on the type and pressure of the reaction gas, or if the power density is less than 0.01 W/cd, the pressure of the reaction gas must be sufficiently lowered, resulting in a slow film formation rate; This is not preferable because the quality of the thin film cannot be maintained at a high level.

In this way, an intrinsic polycrystalline silicon thin film can be produced, and by mixing a dopant gas of group Ⅰ or group V of the periodic table of elements into the reaction gas, the formed polycrystalline silicon thin film can be made into n-type or It can be n-type. In this case, examples of the dopant gas include diborane,
Examples include hydrides such as phosphine and arsine.

Regarding polycrystalline silicon thin films formed under predetermined conditions,
When analyzed by X-ray diffraction, data as shown in FIG. 1 was obtained.

FIG. 1(a) shows the X-ray diffraction intensity of polycrystalline silicon used in the present invention, and FIG. 1(b) shows the X-ray diffraction intensity of an A37M card (standard sample).

According to the A37M card, the ratio of diffraction intensities in the case of polycrystalline silicon with no orientation is as shown in Figure 1 (bl): (111) + (220): (311): (
400)=100:55:30:5. On the other hand, the ratio of X-ray diffraction intensities of polycrystalline silicon used in the present invention is (111): (220): (311
): (400)=0:23+10+50, and when the orientation is determined from the ratio of the standard value and the measured value, (111
): 0%, (220): 4%, (311): 3%, (
400): 93%. (400') is (100)
It can be seen that the thin film polycrystalline silicon used in the present invention has a very strong (100) orientation. As a result of manufacturing the polycrystalline silicon under different manufacturing conditions as shown in Examples below, the polycrystalline silicon of the present invention had a (100) orientation of 30% or more.

Since the (100) orientation is 30% or more, the thin film surface has excellent flatness, and the silicon crystal grains are also approximately 0.01
Since the crystal grows to a large size of μm to 5 μm, and the surface area of the crystal in the thin film decreases, the amount of hydrogen to seal the dangling bonds existing at the crystal interface is less than 2.5 at%, which is 0.2 to 1 atom. % or less. In addition, fluorine is also a terminating material for dangling bonds.
i, and the amount contained in the thin film is 3 atomic % or less,
It can be easily reduced to 0.2 to 1 atomic % or less, contributing to improvement of electrical properties.

In the present invention, the (100) orientation is preferably 30% or more, more preferably 50% or more, still more preferably 70% or more.

In addition, in order to form a thin film in a low-temperature process, Na
It becomes easy to prevent impurities such as , Ca, and B from eluting to the silicon crystal side.

Further, the polycrystalline silicon thin film used in the present invention can also be produced by a photo-CVD method.

In this case, a low-pressure mercury lamp with resonance lines at 185r+m and 254r++n is used as a light source, and as a film forming gas and an etching gas, for example, SiH4 and 5iFt are used, respectively.
The temperature of the substrate (near glass 7 o 59 glass) is set at 360° C. and the pressure is about 1.5 Torr. In advance, SiH, 5SCCM, SiF 25SCCM, and He as a carrier gas for the sensitizer (Hg) were added to a reaction chamber that was evacuated to 1X10-'Torr.
M (mercury temperature 80"C) was introduced, and once the pressure was stabilized, a low-pressure mercury lamp was turned on. The obtained polycrystalline silicon thin film had a growth rate of 0.3 people/see and a grain size of 1000 people (100). The degree of orientation was 30% or more, as in the case of plasma CVD, and there was almost no elution of impurities from the substrate into the silicon crystal.

It has been described above that the polycrystalline silicon thin film of the present invention is manufactured by the plasma CVD method and the photo-CVD method, and that a film-forming gas and an etching gas are used as raw material gases. However, the present invention can also be carried out using hydrogen and a raw material gas that has both film-forming properties and etching properties. In this case, the raw material gas is SiF4, SftF
* is selected, hydrogen gas is 1/3 to 1 of the raw material gas
/1000, especially 115 to 1/400.

Next, a top gate coplanar type polycrystalline silicon thin film transistor of the present invention will be explained with reference to FIG.

An intrinsic polycrystalline silicon thin film 2 is formed on the entire surface of a transparent substrate l by the aforementioned plasma CVD or optical CVD, and an insulating film 5 of 5iCL, SiNx, etc. is formed thereon. Next, a resist is applied and masked, and a predetermined resist pattern is formed by exposing and developing. Then, dry etching is performed by plasma discharge of an etching gas such as CF4, CHF, etc., and the insulating film in the area where there is no resist is removed. Then, as described above, by mixing a dopant gas into the reaction gas, an N-type thin film is formed in the region from which the insulating film has been removed, thereby forming N-type regions (source and drain regions). After forming the N-type region in this manner, a source electrode 4 and a drain electrode 3 are formed by sputtering and selectively formed by lift-off, and a gate electrode 6 is formed on the insulating film 5 to constitute a thin film transistor.

By using the thin film transistor of the present invention to construct an address decoder, a memory switch, an inverter, etc., a thin film drive circuit for a liquid crystal or the like can be constructed.

[Effect]

The polycrystalline silicon thin film transistor of the present invention is formed on a transparent substrate by plasma CVD or photoCVD,
(100) orientation is 30% or more. Since the substrate is light-transmissive and the thin film has good flatness, illumination with a backlight is possible, and it is suitable for microfabrication, so it is suitable for liquid crystal television drive circuits and the like.

〔Example〕

Examples of polycrystalline silicon used in the transistor of the present invention will be described in detail below, but the polycrystalline silicon used in the present invention is not limited thereto.

Example 1 A mixed gas of SiH,:Fx:He=1:5:100 was supplied as a reaction gas at 503 CCM into a reaction chamber which had been made to have a high vacuum of 1X10"' Torr in advance, and the pressure of the reaction gas was set at 1.2 Torr. Adjusted to.

Next, this reaction gas was turned into plasma using a 13.56 MHz high frequency power source at a power of 0.7 W/al, and heated to 450°C.
400 on a Corning 7059 glass substrate heated to
A silicon thin film was formed until it reached a thickness of 0.

When the obtained thin film was subjected to Raman spectroscopy, a very sharp spectrum based on crystalline silicon with a half-width of 5ao-' was observed at a position of 520an-'.

In addition, when the X-ray intensity was measured by X-ray diffraction and the ratio of (100) orientation was determined, it was 97%, and when the particle size was measured using a transmission electron microscope, the average particle size was 200.
There were 0 people. Furthermore, the electron mobility of this thin film was determined using a Hall effect measuring device and was found to be 18 an-”V-ゝ・S.
-1 was there. Also, the hydrogen content is 0. 1%, and the fluorine content was 0.2%.

Furthermore, when a top-gate Cobranar type TPT was fabricated using this polycrystalline silicon thin film by the method described above and the field effect mobility was determined, it was found to be 38aIr/V·see, confirming that it exhibited sufficient characteristics. Thus, good characteristics were achieved even when Corning 7059 glass, which has a low strain point, was used as a substrate.

Of course, good characteristics can also be obtained by using a glass with a relatively high strain point such as Af silicate glass.

Examples 2 to 7 A polycrystalline silicon thin film was obtained in the same manner as in Example 1 under the conditions shown in Table 1, and the average crystal grain size, hydrogen content, and electron mobility were measured, and the results shown in Table 2 were obtained. Ta.

(Varnish bottom molding) Table 1 Table 2 Comparative example 1゜Substrate temperature 300°C, plasma reaction chamber pressure 5To
r r, SiH4: H2=1:10 as raw material gas
A sample was prepared in exactly the same manner as in Example 1, except that plasma CVD was performed using a mixed gas of 0 and a power density of 2 W/-. Many microcrystals were present like islands in the amorphous silicon phase of the obtained silicon thin film. The hydrogen concentration in this silicon layer is as high as about 8 atomic percent.
The electron mobility is small at 0°1eaf-V-'・S-1,
(100) was 8%, and the film was not suitable for a top-gate Cobranar type TPT.

Comparative Examples 2 and 3 A silicon thin film was formed on a glass substrate in the same manner as in Comparative Example 1 under the conditions shown in Table 1. The obtained silicon thin film is
As shown in the table, it was confirmed that the film was inferior to the polycrystalline silicon thin film of the present invention and was unsuitable for TPT.

〔Effect of the invention〕

As described above, according to the present invention, a thin film with a high proportion of (100) orientation can be obtained even when a light-transmitting substrate, particularly a low strain point glass, is used. Furthermore, since the substrate is translucent and the thin film is suitable for microfabrication, it is easy to form thin film transistors, and a liquid crystal drive circuit can be easily constructed, making it possible to apply it to liquid crystal televisions. Further, since the thin film can reduce the hydrogen content and fluorine content and increase the crystal grain size, it is possible to significantly improve the electrical characteristics. Furthermore, even if a substrate containing impurities is used, the low-temperature process makes it easy to prevent the impurities in the substrate from diffusing into the formed silicon thin film. Not only can the manufacturing cost of the thin film be significantly reduced, but also the applications of the thin film can be greatly expanded along with this cost reduction.

[Brief explanation of drawings]

Figure 1 shows (100) polycrystalline silicon used in the present invention.
FIG. 2, which is a diagram for explaining orientation, is a diagram showing the structure of a polycrystalline silicon thin film transistor of the present invention. l... Glass substrate, 2... Polycrystalline silicon thin film, 3... Drain, 4... Source, 5... Gate insulating film, 6... Gate electrode. Application Patent attorney Masanobu Hiruyo Tonen Co., Ltd. (7 others)

Claims (3)

[Claims] (1) A source electrode and a drain electrode are provided in a polycrystalline silicon thin film having a (100) orientation of 30% or more, a hydrogen content of 2.5 at% or less, and a fluorine content of 3 at% or less, and both A polycrystalline silicon thin film transistor characterized in that a gate electrode is provided on or below a polycrystalline silicon thin film between electrodes with an insulating film interposed therebetween. (2) The polycrystalline silicon thin film transistor according to claim 1, wherein the polycrystalline silicon thin film is formed on a transparent substrate. (3) The polycrystalline silicon thin film transistor according to claim 1 or 2, wherein the polycrystalline silicon thin film is formed by plasma CVD or photoCVD.