JPH07235491A - Thin film semiconductor element and manufacture thereof - Google Patents

Abstract

PURPOSE:To provide a polysilicon film, comprising a thin film semiconductor element for use in liquid crystal displays, with a high field effect mobility (muFE). CONSTITUTION:An amorphous silicon film is crystallized by heating and turned into a polysilicon film 2. In this process the heating energy is so adjusted that projections and recesses will be formed on its surface 2a. The inclination angle (8) of the recesses and projections on the surface is controlled to the range of 4.8-30 deg.C with 9.6 deg. taken as the center. This obtains a XFE of 50-150cm<2>/V.sec or above.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a thin film semiconductor element used in a liquid crystal display device and the like and a method for manufacturing the same.

[0002]

2. Description of the Related Art At present, a thin film semiconductor element used in a display device such as a liquid crystal display is composed of an amorphous silicon thin film or a polysilicon thin film, and the field effect mobility (hereinafter referred to as μFE) of the thin film semiconductor element. ) Depends on the constituent material of the thin film semiconductor element, but μFE of amorphous silicon is 0.1 to ¹ cm ² / V ·
It is as small as sec, and its application range is limited. The reason that the thin film semiconductor element made of amorphous silicon has a small μFE is that dangling bonds (unpaired electron pairs of silicon atoms) are mainly present in the amorphous silicon in a large amount. Therefore, it is considered difficult to improve the μFE of the thin film semiconductor element as long as the silicon thin film forming the thin film semiconductor element is in the amorphous state.

On the other hand, by the solid phase growth method at 600 ° C. or higher, polysilicon exceeding 40 cm ² / V · sec
It has been reported that a thin film semiconductor device having FE can be manufactured (S. Morozumi et al., SID'84 Digest p312).
However, according to the conventional solid phase growth method, the thin film semiconductor element can be formed only on a substrate of about 30 cm square, and when a thin film semiconductor element for a large area and high pixel display device is manufactured due to the problem of shrinkage of the substrate, There was an inconvenience that it could not be used.

As a method for solving such a problem, it has been attempted to improve the μFE by crystallizing the surface of an amorphous silicon thin film capable of forming a large area by thermal annealing. This method is widely used at the time of manufacturing a thin film semiconductor element at 600 ° C. or lower where a glass substrate or the like can be used. However, this method has a μFE exceeding 50 cm ² / V · sec. There is a disadvantage that it is difficult to obtain a thin film semiconductor device.

[0005]

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to obtain a thin film semiconductor device having a high mobility of μFE of 50 cm ² / V · sec or more.

[0006]

This problem is solved by heating the amorphous silicon film and crystallizing it to make the surface of the polycrystalline silicon film uneven. Then, by setting the inclination angle of the surface within the range of 4.8 to 30 degrees centering on 9.6 degrees, a higher μFE can be obtained. Further, since this inclination angle is obtained from the intensity measurement of the primary Raman line of the polycrystalline silicon film, if the intensity measurement of the primary Raman line is used as a process monitor, it is possible to manufacture a polycrystalline silicon film having high μFE and unevenness. You can

The present invention will be described in detail below. Figure 1
FIG. 1 is a cross-sectional view schematically showing an example of the thin film semiconductor device of the present invention, in which reference numeral 1 is an insulating substrate made of glass, quartz glass, ceramics or the like. On this insulating substrate 1, a polycrystalline silicon film (hereinafter referred to as a P-Si film) 2
Are formed. The P-Si film 2 has a surface 2a.
Is not flat as shown in FIG. 1, but is uneven. In the figure, reference numeral 3 indicates a grain boundary of each crystal of the P-Si film 2, and each convex portion 2b of the irregularities on the surface 2a is a crystal grain boundary 3.
… Located on top.

Then, the convex portion 2b on the surface 2a and the substrate 1
The inclination angle (indicated by θ in the figure) formed with the surface of the is about 9.6 to 4.8 to 30 degrees. The unevenness having the inclination angle of 9.6 degrees corresponds to a state in which a large number of cones having a bottom diameter of 600 nm and a height of 50 nm are gathered. If the tilt angle is less than 4.8 degrees, μFE is 5
When it is less than 0 cm ² / V · sec, the high mobility which is the object of the present invention cannot be obtained. Also, the inclination angle is 30
If it exceeds the above range, the mobility becomes high, but the surface of the P-Si film 2 is roughened, which causes practical inconvenience and P-.
The uniformity of μFE in the plane of the Si film 2 is reduced.

The P-Si film 2 having such a surface shape
In that case, μFE is 50 to 150 cm ² / V · sec,
Alternatively, it has a value higher than that, and has an extremely high field effect mobility. The reason for this will be described below.

The inventors of the present invention firstly examined the μF of a polysilicon film.
Proposed to use Raman scattering method as a method for evaluating characteristics such as E, and applied for a patent (Japanese Patent Application No. 3-84).
636). As described in the above-mentioned prior specification, this evaluation method includes Raman shifts obtained by Raman spectroscopy of a polysilicon film, half width ratio of optical phonons of silicon, and Raman peak of 519 cm ⁻¹ and 480 cm.
^One of the intensity ratios (Ic / Ia) with the Raman peak of ^-1 is uniquely related to the μFE of the polysilicon film.
If any one of the parameters is obtained, the μFE of the polysilicon film can be known.

FIG. 2 shows the relationship between μFE and Raman shift of the polysilicon film, FIG. 3 similarly shows the relationship between μFE and half width ratio, and FIG. 4 similarly shows μFE and intensity ratio (Ic / Ia).
It shows the relationship with. As is apparent from these graphs, the μFE of the polysilicon film can be uniquely obtained by measuring the Raman spectrum.

Based on the teaching of this prior invention, FIG. 5 shows the relationship between the .mu.FE and Raman intensity (contrast of crystalline silicon) of the P--Si film 2 shown in FIG. 1 and the tilt angle, and FIG. From this graph, it can be seen that the inclination angle takes a value of 50 to 150 cm ² / V · sec in the range of 9.6 ± 4.8 degrees. Of course, it has been confirmed that the μFE becomes 150 cm ² / V · sec or more when the inclination angle exceeds 14.4 degrees.

The point to be noted in the graph of FIG. 5 is that the Raman intensity exceeds the Raman intensity of crystalline silicon when the tilt angle is 9.6 degrees or more. The reason is that something that increases the Raman intensity occurs on the surface of the P-Si film. It is known that the Raman intensity is generally proportional to the scattering volume of laser light of probe light. The absorption coefficient of general polysilicon is 4 × 10 ^4.
Since it is about cm ⁻¹ (488 nm), the Raman intensity of polysilicon with a thickness of 50 nm should theoretically be only about 1/5 that of single-crystal silicon with the same thickness. . However, in the actual a-Si thin film, the Raman intensity of single crystal silicon is exceeded, and as described above,
In effect, a scattering volume greater than 0 nm is created, and this increase in scattering volume contributes to the realization of high μFE.

FIG. 6 shows the Raman intensity of the Raman scattering intensity exceeding that of single crystal silicon of 50 nm, and the Raman intensity of the surface of this polysilicon after being thinly etched to a thickness of 5 nm. After etching, the Raman intensity before etching is reduced to 1/3, and the position of phonons also moves to the crystalline silicon side.
This result shows that a 10% reduction in film thickness reduces Raman intensity by 1/3.
It is believed that the outermost surface layer of polysilicon improves the Raman intensity and thus the mobility.

FIGS. 7 and 8 are scanning electron micrographs of the surface of the polysilicon film shown in FIG. 6 before and after etching, FIG. 7 before etching and FIG. 8 after etching. . Before etching, many pyramidal protrusions and many ridges connected to this are observed on the surface, but after etching, the pyramidal protrusions disappear, and the ridges become deep grooves that serve as boundaries of crystal grains. I understand. Therefore, it is concluded that the significant decrease in Raman scattering intensity is due to this change in surface structure.

As a result of a more detailed analysis of this phenomenon, it was found that the increase in Raman intensity was related to the pyramid-shaped projections of the polysilicon film, that is, the uneven shape. When the inclination angle of the irregularities on the surface of the polysilicon film becomes larger than a certain value, the light (Raman probe light) incident from directly above the film is due to the large refractive index (n = 3.5) of polysilicon. , It does not go out from the surface, complete reflection occurs at the interface, and it is confined in the polysilicon film. As a result, the scattering volume increases substantially and exceeds the value in crystalline silicon. The angle of incidence that causes this perfect reflection is calculated to be 9.6 degrees or more, and this angle coincides with the tilt angle. This is the reason why the tilt angle is determined in the present invention.

Next, according to the present invention, the surface of the P-S having an uneven surface is used.
A method of forming the i film will be described. First, an amorphous silicon film (hereinafter referred to as an a-Si film) is formed on the insulating substrate 1. As the insulating substrate, a glass, quartz glass, ceramics is used, SiO ₂ thereon
An insulating film such as the above may be formed. The insulating base material in the present invention includes these insulating substrates and insulating films.

The method for producing the a-Si film is not particularly limited, and plasma CV at 600 ° C. or lower is used.
Usual film forming means such as D method, sputtering method, and LPCVD method can be used. In order to manufacture an amorphous silicon thin film using the plasma CVD method, for example, a silane-based gas such as silane (SiH ₄ ) or disilane (Si ₂ H ₆ ) is decomposed with a direct current in a high electric field up to 2.45 GHz. You can use the method.

When the sputtering method is used, a direct current or magnetron type RF applied sputtering method can be used. At that time, a high-purity silicon single crystal, a high-purity silicon polycrystal, or the like can be used as a target, and the atmosphere thereof can be an argon atmosphere or an argon atmosphere of hydrogenation, or a 100% hydrogen atmosphere. As the LPCVD method, for example, a method of bringing a silane-based gas such as silane gas or disilane gas into contact with an inert gas on a substrate heated under reduced pressure of 10 Torr or less can be used. In any of the above methods, the oxygen content in the atmosphere should be 10 ²¹ / cm ³ or less, preferably 10 ²⁰ / cm ³ .

The thickness of the a-Si film is not particularly limited, but is usually about 10 to 100 nm. Then, this product can be patterned by subjecting it to patterning if necessary. After that, the a-Si film is heated and crystallized to form a P-Si film. This heating has a wavelength in the ultraviolet region of an ultraviolet light source, such as an ArF excimer laser or a KrF excimer laser, and 15 pulses per pulse.
In addition to pulsed lasers having energies of 0 millijoules or more, lasers having wavelengths in the visible and ultraviolet regions such as Ar ion lasers and Kr ion lasers, lasers having wavelengths in the infrared region such as carbon dioxide lasers, and the like. It is possible to use a pulse-modulation laser or the like in which an oscillation laser is Q-switched. Although heating means such as an electric furnace can be used for heating, it is not preferable because it is difficult to control energy intensity.

When the a-Si film is heated, the heat energy applied to the a-Si film is kept within a certain range (E ₀ ), whereby irregularities are formed on the surface shown in FIG. 1 and P having a high μFE is formed. -Si film is obtained. This energy E ₀ is a
-Since it varies depending on the thickness of the Si film, the planar spread (patterning shape), etc., and cannot be uniquely determined,
The patterned narrow a-Si film is changed into a P-Si film on which irregularities are formed with lower energy than a wide a-Si film which is not patterned. Also, as the film thickness increases, higher energy is required.

For example, when an excimer laser (KrF) is irradiated to an a-Si film obtained by the low pressure CVD method with a film thickness of 50 nm in the atmosphere at atmospheric pressure and room temperature, the irradiation frequency is 4 times. , Laser energy is 250-280 mJ / c
When the irradiation frequency is 8 times in the range of m ² , the laser energy is 265 to 290 mJ / cm ² when the irradiation frequency is 16 times in the range of 260 to 290 mJ / cm ^2.
If the irradiation number of cm ² is 32 times, the energy is 2
By performing the treatment in the range of 70 to 290 mJ / cm ^2, the P-Si film having a high μFE having the structure shown in FIG. 1 can be obtained. Moreover, it has been confirmed that a P-Si film having irregularities can be obtained by only slightly changing the energy intensity under laser irradiation conditions of reduced pressure, vacuum, and atmosphere.

Then, when the a-Si film is heated by an excimer laser or the like, Raman spectroscopic measurement of the film is performed to measure the μFE of the phase-changed P-Si film and the heating conditions and the like. It is possible to control and obtain a P-Si film having a desired μFE value. Raman spectroscopic spectrum measurement can be performed immediately by irradiating a probe light with an extremely small beam diameter, so that μFE of the P-Si film can be known at any time during heating, and can be effectively used as a process monitor.

The Raman spectrophotometer that can be used in the present invention uses a visible to ultraviolet laser such as an argon ion laser or a krypton laser as a light source, and a stray light ratio of about 10 ^-9 is used as a spectroscope. Anything is needed, but a measurement system that has a photomultiplier tube (PM) or solid-state detector (SSD) and can measure the light intensity near 520 cm ^-1 which is an optical phonon of silicon at high speed and over a wide wavelength range is required. Is.

(Example) The starting film used in this experiment was S
An i ₂ H ₆ raw material having a film forming temperature of 460 ° C. by the LPCVD method was used. Further, the substrate is a synthetic quartz substrate, and the base film is a SiO ₂ film 300 formed by APCVD (normal pressure CVD).
nm was used. In addition, by integrating the above results, the crystallization parameters are set to the values shown in the table below in the thin film transistor (TF
T) was produced.

[0026]

FIG. 9 shows the ID-VG characteristics of the obtained TFT. Maximum field effect mobility is 160 cm ² / V · s
ec exceeded. In addition to the mobility, the threshold voltage of 3.6
Also in other characteristics such as V and S values of 0.38 V / decade, excellent characteristics were exhibited. This is probably because good crystals were obtained by the optimization using the process monitor by Raman spectroscopy.

[0028]

As described above, according to the present invention, μ
When FE is 50 cm ² / V · sec or more, 150 cm ² / V
It is possible to obtain a thin film semiconductor element made of a P-Si film having a high mobility exceeding sec.

[Brief description of drawings]

FIG. 1 is a sectional view schematically showing an example of a thin film semiconductor element of the present invention.

FIG. 2 is a graph showing the relationship between Raman shift and field effect mobility of a polysilicon film.

FIG. 3 is a graph showing a relationship between a half width ratio of a polysilicon film and a field effect mobility.

FIG. 4 is a graph showing a relationship between an amorphous silicon intensity ratio of a polysilicon film and a field effect mobility.

FIG. 5 is a graph showing the relationship between the tilt angle, mobility and Raman intensity of the P-silicon film of the present invention.

FIG. 6 is a graph showing the relationship between Raman intensity and Raman shift before and after etching the surface of the P-silicon film of the present invention.

FIG. 7 is an electron micrograph showing the surface structure of the P-silicon film of the present invention before etching.

FIG. 8 is an electron micrograph showing the surface structure of the P-silicon film of the present invention after etching.

FIG. 9 is an I of a TFT to which the P-silicon film of the present invention is applied.
It is a graph which shows a D-VG characteristic.

[Explanation of symbols]

 1 Insulating substrate 2 P-Si film

Claims (7)

[Claims] 1. A thin film semiconductor element comprising a polycrystalline silicon film obtained by heating and crystallizing an amorphous silicon film formed on an insulating substrate, the surface of the polycrystalline silicon film having irregularities. 2. A polycrystalline silicon film formed by heating and crystallizing an amorphous silicon film formed on an insulating substrate, the surface of the polycrystalline silicon film having irregularities, and the Raman intensity of which is single crystal silicon. Thin film semiconductor device that exceeds the Raman intensity of 3. A polycrystalline silicon film formed by heating and crystallizing an amorphous silicon film formed on an insulating substrate, wherein the surface of this polycrystalline silicon film has a Raman intensity of at least the Raman intensity of single crystal silicon. A thin film semiconductor device. 4. A thin film semiconductor device comprising a polycrystalline silicon film obtained by crystallizing an amorphous silicon film formed on an insulating base material by laser irradiation, the surface of the polycrystalline silicon film having irregularities. 5. The method according to any one of claims 1 to 4,
The tilt angle between the surface of the polycrystalline silicon film and the surface of the base material is
A thin-film semiconductor device having a pitch of 4.8 degrees or more and 30 degrees or less around 9.6 degrees. 6. In the step of heating an amorphous silicon film formed on an insulating base material to produce a crystallized polycrystalline silicon film, the inclination angle of the irregularities on the surface is obtained from Raman intensity, and the primary Raman A method of manufacturing a thin film semiconductor characterized by using the intensity of a wire as a process monitor. 7. The Raman intensity is used to determine the tilt angle and crystallinity of the surface irregularities in the step of heating the amorphous silicon film formed on the insulating base material to produce a crystallized polycrystalline silicon film. A method of manufacturing a thin film semiconductor, which is characterized in that it is manufactured under conditions that maximize strength.