JPS63283013A - Forming method for polycrystalline silicon thin film - Google Patents

Abstract

PURPOSE:To form a large particle sized polycrystalline silicon thin film in which orientation and plane azimuth are considerably made uniform by forming a silicon thin film on an amorphous substrate, forming a thin film having a larger thermal expansion coefficient than silicon in a strip state on the silicon thin film, and heat treating it. CONSTITUTION:A silicon thin film 12 is formed on an amorphous substrate 10. A thin film 13 having larger thermal expansion coefficient than silicon is formed in a strip state on the film 12. Then, it is heat treated to recrystallize the film 12 formed on the substrate 10 in a solid phase. Since a tensile stress is applied to the film 12 at the time of heating a sample but the film 13 exists, the stress is concentrated at the step of the film 13, and unified in a direction perpendicular to the step of the film 13. Crystal grains in a direction (112) through a twin having fast growing speed are grown along the direction of the stress. Thus, the film 12 in which the orientation and the plane azimuth are considerably made uniform is formed.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention is directed to field effect thin film transistors (FET).
Regarding improvements in the method of forming polycrystalline silicon thin films that constitute the main parts of semiconductor devices such as
The present invention relates to a method for forming a polycrystalline silicon thin film that constitutes a main part of a highly reliable semiconductor device.

Conventional technology Polycrystalline silicon thin films have traditionally been MOS LSI
(metal-oxide film-insulator type large-scale integrated circuit), and in recent years, S○I (Sil 1con), which uses a polycrystalline silicon thin film as the active region,
Active research is being carried out on applications such as applications to (on 1 nsulator) devices and thin film transistors (T P T ) for liquid crystal display devices.

These device characteristics are greatly influenced by the quality of the polycrystalline silicon thin film used in the f1 dynamic region. The quality of this film is mainly determined by the grain size of the crystal grains forming the thin film and the carrier trap density consisting of silicon dangling bonds existing between the crystal grains, but it is important to construct a device that provides good characteristics. The problem is that the applied polycrystalline silicon thin film has a larger grain size and a lower carrier Trump density.

Traditionally, polycrystalline silicon thin films have been mainly produced using low pressure chemical vapor phase (L
PGVD) method. According to this method, a polycrystalline silicon thin film is formed on an amorphous substrate at a film formation temperature of around G , the crystal grain size is at most several 1 Orr.
It's +11+. Furthermore, although this polycrystalline silicon thin film has a weak orientation in the <110> direction, the plane orientation is completely arbitrary. If a TPT is constructed using this thin film, the field effect mobility (μeff) obtained from the TPT characteristics
has a height of 1 to 20 c+n2/V·sec, which is sufficient for display pixel display transistors, but is insufficient for constructing drive circuits such as shift renostars that drive these transnostars.

On the other hand, if it becomes possible to form a polycrystalline silicon thin film with an average grain size of 0.1 μI11 or more, μeff
> 100 can2/\1-sec -(
A TPT having the characteristics 1) can be obtained, and a liquid crystal display device with a built-in driving circuit can be realized. Furthermore, if a polycrystalline silicon thin film with orientation and uniform plane orientation is formed, the mobility will be further improved. Furthermore, since the interface level at the interface between the polycrystalline silicon thin film and the date insulating film is reduced, the threshold voltage (\7■H) of the TPT can be reduced.

As a method for obtaining large-grain polycrystalline silicon thin films, a method using an energy beam or a method using solid phase growth is used. Those who wish to do so irradiate a silicon thin film with an energy beam from a laser or accelerated electrons.
It melts and recrystallizes silicon thin films. According to this method, it is possible to form a polycrystalline silicon thin film with a grain size of several μI or more, but the recrystallized region is limited to the energy beam irradiation region, and the large-grained polycrystalline silicon thin film is uniformly formed over the entire substrate area. is difficult to form.

In the solid phase growth method, an amorphous or polycrystalline silicon thin film formed on an amorphous substrate is heated to grow crystal grains in the solid phase to obtain a grain size polycrystalline silicon thin film. In order to increase the grain size by heating a polycrystalline silicon thin film, a high temperature of around 1000°C is required in order to diffuse silicon atoms over the barrier that exists between crystal grains. When heating a thin silicon film, since there is no barrier as described above, the grain size can be easily increased at a relatively low temperature of 500 to 600°C.

At this time, since the crystal grains grow using microcrystals present in the amorphous silicon thin film as nuclei, the orientation of the crystal grains after growth is determined by the orientation of the microcrystals.

Therefore, if the microcrystals have an arbitrary orientation, there is a problem in that it is impossible to obtain a polycrystalline silicon thin film with uniform orientation after solid phase growth.

Problems to be Solved by the Invention The present invention was invented in an attempt to solve the above-mentioned conventional problems. It is an object of the present invention to provide a method for forming a polycrystalline silicon thin film having a uniform diameter orientation and plane orientation.

Means for Solving the Problems The method for forming a polycrystalline silicon thin film of the present invention includes a step of forming a silicon thin film on an amorphous substrate, and a thin film having a coefficient of thermal expansion larger than that of silicon on the silicon thin film. The method includes a step of recrystallizing the silicon thin film formed on the amorphous substrate in a solid phase by performing heat treatment after forming it into a band shape.

By this method, a large-grain polycrystalline silicon thin film with fairly uniform orientation and surface orientation can be formed on an amorphous substrate.

Note that the silicon thin film formed in the step of forming a silicon thin film on the amorphous substrate described above is an amorphous thin film containing microcrystals.

In addition, the process of recrystallizing the silicon thin film consists of one or more heat treatments, and the first treatment temperature is 500°C.
It is desirable that the temperature be above 650°C.

As a method for forming a silicon thin film, C', ID (Ch
eIIIical Vupour Del+osi
tion)! L P CV D (Lou+Pr
essure Che+n1cal Vapor D
The method for forming a silicon thin film on an amorphous substrate in the present invention may be any of the above methods, but is limited to any one of them. isn't it.

The cluster ion beam method will be explained below as an example, but the present invention is not limited to this method.

As shown in FIG. 1, the formation of a silicon thin film by the cluster ion beam method involves heating and vaporizing silicon 2 filled in a closed crucible 1 having a nozzle 3, and releasing the silicon vapor at a pressure of at least one-tenth of the pressure of the silicon 2. Inject into an atmosphere having a pressure of: The silicon vapor is ionized by being bombarded with electrons emitted from the filament 4, accelerated by an accelerating electrode 5, and deposited together with neutral particles on the substrate surface 7 to obtain a silicon thin film. In addition, in Figure 1,
6 is a shell, 8 is an infrared lamp for heating the substrate 7, and 9 is a gas introduction tube.

According to the above method, by effectively utilizing the chemical activity of the ionic species itself and the kinetic energy given by the acceleration of the ions, bonding between atoms on the substrate surface is promoted, and the surface The migration effect realizes the formation of a homogeneous and dense thin film.

The structure and properties of the silicon thin film obtained by this method are affected by the degree of vacuum in the vacuum chamber, substrate temperature, silicon evaporation rate, ionization rate, accelerating voltage, etc., but important factors determine the crystallinity of the silicon thin film. are the substrate temperature, ionization rate and acceleration voltage. i.e. 4
At a substrate temperature of 00° C. or higher, the silicon thin film formed is polycrystalline regardless of the presence or absence of ions. Reverse 1ko30
At a substrate temperature of less than 0° C., it is amorphous without a crystalline structure. On the other hand, when the film is formed at a substrate temperature of 300.degree. to 400.degree. C., an amorphous thin film containing microcrystals of 100 .ANG. or less is obtained.

The density and crystallinity of the fine particles contained in this thin film can be controlled by the amount of ions and accelerating voltage. The density of microcrystalline grains is an important factor that determines the grain size of the finally obtained large-grained polycrystalline silicon thin film. In other words, by heat treatment performed after the formation of a silicon thin film, crystal grains grow by combining with atoms in the surrounding amorphous region using microcrystalline grains as nuclei, but this growth causes collisions with adjacent crystal grains. The grain size is determined at this point.

Therefore, the lower the density of microcrystalline grains, the larger the polycrystalline silicon grain size. This is not very desirable because the number of crystal grains in this case is 1, which causes variations in properties. Therefore, the crystal grain density is about 5 x 10'/cu+2- so that the crystal grain size of the polycrystalline silicon thin film finally obtained is 0.1 to 1 μm.
It is preferable to use an amorphous silicon thin film having a thickness of I x 10''7 cm2.

Note that as the substrate, an amorphous substrate such as a silica film, trisilicon tetranitride, or heat-resistant glass can be used.

As described above, in the method of the present invention, the silicon thin film formed on the amorphous substrate is a non-ff thin film containing microcrystals at the time of forming the thin film.

Next, the amorphous silicon thin film containing microcrystals is recrystallized,
A method for forming a large-grain polycrystalline silicon thin film with fairly uniform orientation and plane orientation will be described.

Heat treatment of amorphous silicon thin films causes rearrangement of silicon atoms, leading to grain growth in the solid phase.

However, simply heating an amorphous silicon thin film formed on an amorphous substrate makes it possible to increase the grain size of the crystal grains, but it is difficult to control the crystal orientation. This is because microcrystals that serve as growth nuclei have random crystal orientations, and crystal grains grow by taking over this crystal orientation.

However, if the growth of crystal grains in the solid phase is controlled in a fixed direction, it becomes possible to align the orientations of different crystal grains. The stress that causes thin films to cling is mainly due to the intrinsic stress of the thin film itself, the external stress brought about by the substrate on which the thin film is formed and other thin films, or the stress that occurs when solid phase growth is performed by heat treatment. This is an external stress caused by the difference between the coefficient of thermal expansion of the thin film to be grown and the coefficient of thermal expansion of the substrate and other thin films, and the intrinsic stress can be ignored.

If the coefficient of thermal expansion of the substrate and other thin films is higher than that of the thin film, tensile stress will be applied to the thin film during heat treatment. Therefore, it is desirable that the coefficient of thermal expansion of the substrate and other thin films is greater than that of the thin film to be grown.

For the above reason, if a heat treatment is performed after forming the thin film A having a larger coefficient of thermal expansion than silicon on an amorphous silicon thin film, the growth of silicon crystal grains will be promoted. Moreover, i
# When A is formed in the form of islands or bands on an amorphous silicon thin film and heat-treated, the tensile stress that causes the amorphous silicon thin film to break is due to the step difference in the thin film A, as shown in Figure 2 (a). The direction of the stress can be controlled in the direction from the area covered by the thin film A to the uncoated area.

Therefore, if the thin film A is formed into a strip shape, the stress applied to the amorphous silicon film will be perpendicular to the step surface at the stepped portion of the thin film A, so the direction of stress that causes the growth of crystal grains will be fixed in a certain direction. It can be aligned to

When amorphous silicon grows in the solid phase, it grows fastest in the direction of 2〉, forming twins, but when the stress is aligned in a certain direction, the <112
＞Direction・＼ growth can also be improved by aligning it with the direction of stress. As a result, the direction perpendicular to the step surface of thin film A is <112
Since the crystal grains having the axis grow within the amorphous silicon thin film, a polycrystalline silicon thin film with a fairly oriented (#J) orientation can be realized even within the plane of the thin film.

The thin film material may be any substance having a coefficient of thermal expansion greater than that of silicon, such as silica (SiO2). The thin film is used in the form of a strip, and its preferred thickness and width are approximately 0.1 to 1.0 .mu.I11 and approximately 1 to 5 .mu.[fl, respectively, and preferably formed at intervals of approximately 1 to 5 .mu.In. Specifically, C was applied to the previously formed amorphous silicon thin film.
A thin film is formed by a VD method or the like, and then formed into a band shape by patterning or the like.

Heat treatment at about 500-700°C, preferably about 500-700°C
It is preferable to carry out the process at a relatively low temperature of 650°C.

When the heat treatment is performed at a temperature below 500° C., the growth rate of crystal grains is very slow, and the heat treatment takes a long time, which is not practical. On the other hand, when heated at a high temperature of 700℃ or higher, new microcrystals that become nuclei for grain growth are generated within the amorphous material, so the grain density increases and the grain size increases. be hindered.

On the other hand, 500-700℃, more preferably 500℃
At a treatment temperature of ~650°C, it is possible to suppress the generation of new microcrystals and to generate crystal grains in the solid phase using existing microcrystals as nuclei.

The heat treatment is performed one or more times, and the first treatment is preferably performed at 500'C or more and less than 650C for about 5 to 70 hours. As described above, the crystal grains continue to grow until they collide with adjacent crystal grains, forming polycrystalline silicon grains having a grain size greater than the thickness of the thin film.

Example Practical embodiments of the present invention are shown below.

In the apparatus shown in Figure 1, the vacuum chamber is 1 x 10"lea
After evacuating to the following temperature, the crucible 1 filled with silicon 2 was heated to about 2000° C., and a film was formed on the substrate at a rate of 100 to 200 λ/min.

As the substrate 7, a (100) silicon single crystal substrate 10 was thermally oxidized at 1000°C, and a 5-inch)2 film 11 was formed on the surface.
000 people were used. The board 7 is an infrared lamp 8
The substrate temperature was maintained at 350°C. The accelerating voltage applied between Rubber 1 and substrate 7 was 2 KV.

At this time, the ion current density incident on the substrate 7 is approximately 3 μA/
It was cI112. The thickness of the formed silicon thin layer 12 is 0.1 μm.

As a result of observing the silicon thin film formed by the above method using a transmission electron microscope, it was found that the silicon thin film 12 had a thickness of 100 Å.
It was an amorphous thin film in which the following microcrystals were mixed at an areal density of 108/coo'.

Further, on this silicon thin film, atmospheric pressure CVD (APCXI
D) SiO with a film thickness of 0.3 μl + 1 using the method
! A film 13 was formed. The 5 in 2 film 13 is patterned by 7 otrinography leaving a band shape of υ11J1μ■ with an interval of 1 μm.

FIG. 2 is a diagram showing the sample structure, and 10 is silicon (1
00) Substrate, 11 is an amorphous insulating thin film such as SiO2 or Si3N formed on the substrate 10, 12 is a silicon thin film formed by cluster ion beam method, 1
3 is a SiO2 thin film formed by atmospheric pressure CVD and patterned into a band shape.

Next, the sample shown in FIG.
A heat treatment was performed for 170 hours at °C to grow polycrystalline silicon in the solid phase. The thermal expansion coefficients of silicon and SiO2 are each 3.65 x 10-6 + 5.5 x 1
0'(deg'), tensile stress is applied to the silicon thin film 12 when the sample is heated. Furthermore, since the 5in2 film 13 on the silicon thin film 12 is formed in a band shape, the stress is concentrated in the 3rd part of the SiO2 film 13, and the direction is as shown by the arrow in FIG. It is unified in the direction perpendicular to the three planes. Along the direction of this stress,
This results in crystal grain growth in the <112> direction via twins with a high growth rate.

FIG. 4 is a schematic cross-sectional view of the silicon thin film 12 after growth and a plan view observed with a transmission electron microscope.

The crystal grains 17 extend in a direction perpendicular to the stepped surface of the SiO2 thin film 13, and the grain size is approximately 1 μm in the long axis direction and approximately 0 μm in the short axis direction.
．． 5 μ [fl paste (grain is growing. In the center of each crystal grain 17, it is confirmed whether the twin crystal 14 having the (111) plane as the twin plane extends in the direction of the crystal grain. 5th
The figure shows a transmission electron diffraction image when electrons are incident perpendicularly to the thin film surface. Strong diffraction points 15 of the (110) plane arranged in a diamond shape are observed along with a diffraction ring indicating polycrystal. In addition to this strong diffraction point, a somewhat weaker diffraction point 16 is also observed, but this is due to twin diffraction with the (111) plane as the twin plane.

From this transmission electron diffraction image, the silicon thin film 12 is (11
It is confirmed that the film is a large-grain polycrystalline silicon thin film with a uniform orientation of 0) and a uniform film surface orientation.

Effects As described above, according to the present invention, it is possible to form a polycrystalline silicon thin film with large grain size and uniform orientation and surface orientation on an amorphous substrate.

Therefore, by constructing a thin film transistor (T P T ) using this multi-crystalline silicon thin film, it is possible to have high carrier mobility and reduce the threshold voltage. Furthermore, when considering application to a liquid crystal display device, it becomes possible to incorporate not only a liquid crystal switching transistor but also a drive circuit such as a shift lens star.

By forming a polycrystalline silicon thin film using this method in this way, it becomes possible to expand the range of application and improve the characteristics of a semiconductor device formed using the above-mentioned thin film.

[Brief explanation of drawings]

Figure 1 is a diagram showing the basic configuration of the cluster ion beam device, Figure 2 (,) is a cross-sectional schematic diagram showing the schematic structure of the sample,
2(1)) is a schematic plan view thereof, FIG. 3(a) is a schematic cross-sectional view showing the direction of stress in the silicon thin film 12, FIG. 3(b) is a schematic plan view thereof, and FIG. Figure (,) is a schematic cross-sectional view of the silicon thin film 12 after growth, Figure 4 (b) is a wedge diagram of the crystal grain state of the multi-crystalline silicon thin film 12 observed by transmission electron microscopy, and Figure 5 is a polycrystalline silicon thin film. 12 represents a transmission electron diffraction image of No. 12. DESCRIPTION OF SYMBOLS 1... Crucible, 2... Vapor source silicon, 3... Nozzle, 4... Ionization filament, 5... Accelerating voltage, 6... Nyanta, 7... Substrate, 8... Infrared lamp, 9
...Silicon (100) substrate, 11...Amorphous insulating thin film such as 3iO2 or Si3N-, 12...Silicon thin film formed by cluster ion beam, 13 ...A thin film with a higher coefficient of thermal expansion than silicon such as SiO2, 14 ・(111)
Twin crystal with plane, 15... (110) silicon diffraction point, 1G... Diffraction point from twin crystal

Claims (1)

[Claims] A process of forming a silicon thin film on an amorphous substrate, and forming a band-shaped thin film having a coefficient of thermal expansion larger than silicon on the same silicon thin film, and then applying heat treatment to form the thin film on the amorphous substrate. A method for forming a polycrystalline silicon thin film, comprising the step of recrystallizing the silicon thin film in a solid phase.