JPH044747B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for manufacturing a polycrystalline silicon thin film, which is effective for forming a polycrystalline silicon thin film containing silicon atoms as a matrix on a predetermined substrate using discharge energy such as glow discharge or arc discharge. Recently, scanning circuit parts of image reading devices such as long one-dimensional photo sensors and large-area two-dimensional photo sensors, liquid crystals (abbreviated as LC), and electrochromic materials (abbreviated as EC) have recently been developed for image reading. ) Or, as the drive circuit section of image display devices using electroluminescent materials (abbreviated as EL) is becoming larger, it has been proposed to use a silicon thin film formed on a predetermined substrate as the material. has been done. Such a silicon thin film is desired to be polycrystalline rather than amorphous in order to realize large-scale image reading devices and image display devices with higher speed and higher functionality. One of the reasons for this is the effective carrier mobility (effective
carrier mobility) μeff is required to be large, but in amorphous silicon thin films obtained by normal discharge decomposition, it is at most about 0.1 cm ² /V・sec, compared to single crystal silicon. However, it is far inferior and does not meet the desired requirements. This small mobility μeff is a characteristic unique to amorphous silicon thin films, so amorphous silicon thin films have the inherent disadvantage of not being able to take advantage of the ease of thin film production and low production costs. There is. On the other hand, the mobility μeff of a polycrystalline silicon thin film is much larger than that of an amorphous silicon thin film, based on actually measured data, and theoretically it has a much higher mobility μeff than the currently obtained value. There is a possibility that one having an even larger value of mobility μeff can be created. However, in order to create a polycrystalline silicon thin film over a large area on a given substrate, it is necessary to use a considerably high temperature, for example, a chemical vapor deposition method.
In the case of Deposition (abbreviated as CVD), low pressure chemical deposition film formation method (Low presure
Chemical Vapor Deposition: Abbreviated as LPCVD)
In this case, it is necessary to form a film on a substrate maintained at a temperature of approximately 650°C or higher while maintaining such a temperature uniformly over the entire substrate area until the film formation is completed. Maintaining such a high temperature uniformly over a large area with considerable time precision consumes a large amount of power for heating, so it is not suitable for energy saving, and the equipment itself is oriented towards high power consumption. However, this increases the size and is inconvenient in terms of equipment. Furthermore, especially in the case of LPCVD, in order to reduce the pressure inside the reaction chamber, it is necessary to add a cooling device to protect the means for maintaining the reduced pressure from the above-mentioned high temperatures, so the equipment as a whole is complicated. This is undesirable because it increases the cost of the device. In addition, as mentioned above, in order to maintain a substrate for film formation at a fairly high temperature for a long time, a substrate material with excellent heat resistance that is not affected by the historical temperature is required. This placed significant restrictions on the selection of substrate materials. Forming a polycrystalline silicon thin film, which is the material for forming the thin film transistor (abbreviated as TFT) elements that make up the scanning circuit section and drive circuit section, in such a high temperature region requires a large equipment. In addition, significant restrictions have been placed on the selection of substrate materials for film formation, making it impossible to use inexpensive materials such as general glass and heat-resistant plastics.
This is not desirable from an industrial perspective because it does not allow for cost reduction from the viewpoint of product materials. On the other hand, as a method for forming a polycrystalline silicon thin film in a relatively low temperature range compared to the above-mentioned temperature range, there is a film forming method (discharge decomposition method) that utilizes discharge such as glow discharge. Among the reports that say that polycrystalline silicon thin films have been created by such discharge decomposition methods, one with a low substrate temperature of about 400°C is mentioned.
In this way, if the substrate temperature is around 400℃, you can also include the aforementioned general glass and heat-resistant plastics in the selection of substrate materials for film formation.
The constraints on the selection of substrate materials are expanded. However, conventionally, the substrate temperature was reduced to 400℃ using the discharge decomposition method.
The polycrystalline silicon thin film prepared at a certain level has poor crystallinity, uneven crystal grain size, and poor crystal orientation, and has unsatisfactory semiconductor properties as a thin film material for forming TFT elements. Problems remain as a material for forming TFT devices. In order to solve this problem, it is conceivable to raise the substrate temperature, but raising the substrate temperature is not preferable because it causes the same problem as before. Furthermore, in the discharge decomposition method that uses glow discharge, etc., maintaining the substrate temperature at around 400°C to form a polycrystalline silicon thin film makes it possible to uniformize the film properties over a large area. This is extremely difficult and not good for productivity or mass production. For this reason, even in the discharge decomposition method, there is a desire for a manufacturing method that can further reduce the substrate temperature and obtain a polycrystalline silicon thin film having a large area and having desired characteristics. In addition, the method for manufacturing polycrystalline silicon thin films, which can be used as a material for creating faster, more highly functional scanning circuits and drive circuits, and is competitive as a commercial product, is when creating films on a given substrate. In addition to solving the above problems, it is necessary to have a large mobility μeff as described above (normally, in order to create the above-mentioned scanning circuit section and drive circuit section, ^an effective The film creation conditions must be met to obtain the required mobility μeff. However, the film formation conditions for obtaining a polycrystalline silicon thin film with a mobility μeff of 0.5 cm ² /V・sec or more are extremely strict, and therefore, such film formation conditions must be met over a large area. It is extremely difficult to maintain a constant temperature, and conventional methods are not suitable for productivity or mass production. The present invention has been made in view of the above, and is capable of producing a polycrystalline silicon thin film with desired characteristics in a lower temperature region and under relatively mild film forming conditions than in the past. The main object of the present invention is to provide a method for manufacturing a polycrystalline silicon thin film, which can be produced and whose semiconductor properties are uniform over a large area. Another object of the present invention is to provide a method for producing a polycrystalline silicon thin film that can easily produce a polycrystalline silicon thin film that has excellent crystallinity, uniform crystal grain size, and good orientation. It is. Another purpose is to have a mobility of 0.5cm ² /V・sec or more, and to make many of the thin film transistors (abbreviated as TFT) that constitute high-speed, high-performance scanning circuits and drive circuits made of the same film. Another object of the present invention is to provide a method for manufacturing a polycrystalline silicon thin film that can be formed with high precision as a polycrystalline silicon thin film. In the method for producing a polycrystalline silicon thin film of the present invention, a main raw material material whose constituent atoms are silicon atoms for forming a polycrystalline silicon thin film is introduced in a gaseous state into a deposition chamber that has been reduced to a desired pressure. A polycrystalline silicon thin film is deposited on a substrate disposed in the deposition chamber and heated to a substrate temperature of 350° C. or more and 400° C. or less, by discharging the raw material with discharge power to produce a polycrystalline silicon thin film. In a manufacturing method, if the minimum discharge power required to saturate the deposition rate of the polycrystalline silicon thin film in the manufacturing method is P min, then the discharge power is
The method is characterized in that a polycrystalline silicon thin film is formed by setting the discharge power to be one-tenth or more of min and less than P min. In this way, the relationship between the gas flow rate and discharge power of the main raw material material whose constituent atoms are silicon atoms for thin film formation introduced into the vacuum deposition chamber for polycrystalline silicon thin film formation is set, and the raw material material is A polycrystalline silicon thin film formed on a predetermined substrate by discharging and decomposing the polycrystalline silicon is extremely effective as a material for TFTs constituting the above-mentioned scanning circuit and drive circuit, as will be specifically shown later. However, according to the method of the present invention, the substrate temperature (Ts) during film formation can be lowered compared to the conventional method not only when obtaining a film with the same characteristics, but also when obtaining a film with even better characteristics. Since the temperature can be significantly lowered, the selection range of substrate materials is expanded, and cheaper scanning circuits and drive circuits can be provided. In the present invention, the discharge decomposition of the raw material is carried out within the range set as described above for the relationship between the gas flow rate of the main raw material for film formation and the discharge power. It is desirable to set it to one-fifth or more of P min. In the conventional method, when the substrate temperature Ts is about 400°C, only poor quality polycrystalline silicon thin films are produced, and the above-mentioned problems arise.
While it cannot be used as a material for TFT at all, under the conditions of the present invention, the substrate temperature
Even if Ts is set at 350° C., a polycrystalline silicon thin film that can be used sufficiently as a material for TFTs constituting the scanning circuit and drive circuit can be formed. Therefore, in the present invention, of course, the substrate temperature Ts is not changed as in the conventional method.
There is no problem even if the temperature is high (approximately 550℃ or higher), but
In order to uniformly heat a large area of the substrate, and considering power consumption and cooling of vacuum holding members (O-rings such as Viton), the temperature is preferably as low as possible, and optimally below 400°C. Yes. Being able to set the substrate temperature Ts to a lower temperature in this way guarantees that the selection range of substrate materials can be further expanded, contributing to a reduction in material costs, and also contributing to lower material costs. Since the apparatus for heating and temperature control for film formation can be easily and miniaturized, there is an advantage that the film forming apparatus itself can also be miniaturized and simplified. In the present invention, the gas flow rate of the main raw material gas for film formation introduced into the deposition chamber for film formation is as follows:
In relation to the discharge power, it may be determined as appropriate to satisfy the above conditions, but it may be determined based on factors such as better film properties, improved film deposition rate, and economical efficiency of raw material gas. It will be done. According to the method of the present invention, the film characteristics of the obtained polycrystalline silicon thin film itself are that, in terms of its crystal structure, the crystal grain sizes are uniform;
It has an appropriate size as a material for TFT elements,
It has excellent polycrystallinity expressed by crystal orientation. For example, the polycrystalline silicon thin film obtained by the method of the present invention has a variation of ±50 in crystal grain size.
%, the particle size is extremely uniform, and
The crystal grain size itself is within the range of about 400 to 1300 Å, making it suitable for TFT materials. Next, specific features of the manufacturing method of the present invention will be described with reference to FIG. FIG. 1 is an explanatory diagram specifically showing the film forming conditions that are characteristic of the present invention _.
SiH diluted to 10 vol% with ₄ ) gas flow rate,
It is illustrated against RF discharge power (glow discharge energy). In the power range lower than line B in Figure 1,
The electron beam diffraction pattern revealed that a microcrystalline silicon film with poor crystal orientation could be obtained. or,
In the higher power range than line A, a film with good grain size and orientation but with a film conductivity of 10 ^-4 (Ωcm) or less and which seems to have many levels at the grain boundaries is obtained, making it suitable for semiconductor devices. It is inappropriate as a material. The film obtained under the conditions between A and B (area between lines A and B) has electron mobility μeff
(effective mobility obtained from MOSFET) is 0.5
This is a good polycrystalline silicon thin film that provides characteristics of cm ² /V·sec or higher. The period between A and B shown in FIG. 1 is for a film produced at Ts=350°C. At Ts=450°C, a polycrystalline silicon film with μeff>0.5 (cm ² /V·sec) can be produced between A and B'. As shown in Figure 1, by selecting the substrate temperature Ts, RF power, and SiH ₄ flow rate, a well-oriented polycrystalline silicon film can be obtained, and its electrical properties are generally well-known. Even on substrates as low as 350°C, which cannot be obtained using conventional CVD methods, vapor deposition methods, or glow discharge, the results are comparable to or higher than polycrystalline silicon films produced using these methods at substrate temperatures of 500°C or higher. polycrystalline silicon thin films can be produced. In the region where the precipitation rate of polycrystalline silicon is saturated with respect to RF power (to the right of line A), we have described a tendency to include levels in the grain boundaries formed;
The closer the power is to line A, the stronger the orientation in the electron beam diffraction pattern, and the larger the grain size. On the lower power side, orientation and grain size growth are insufficient, and on the higher power side than line B, polycrystalline silicon with good semiconductor properties can be obtained. The present invention is based on data obtained from such systematic experiments, for example, as shown in FIG. This is based on the fact that the material was found to be extremely effective as a material for TFTs constituting the above-mentioned circuit section. That is, by producing a polycrystalline silicon film under the limited conditions between A and B, it is possible to easily and inexpensively produce a high-quality polycrystalline silicon thin film even at Ts = 350°C, which has not been achieved in the past. It has now become possible to fabricate it. In the present invention, since the substrate temperature Ts can be kept in a lower temperature range than in the conventional method, heat-resistant glasses such as high melting point glass and hard glass, heat-resistant ceramics, and sapphire, which are used in the conventional method, can be used. , spinel, silicon wafer, etc., general low melting point glass, heat resistant plastics, etc. may also be used. A TFT, which is an example of an active device made from a polycrystalline silicon thin film obtained by the method of the present invention, is known as a field effect transistor using a semiconductor layer, an electrode layer, and an insulating layer.
That is, a voltage is applied between a source electrode and a drain electrode that have an ohmic contact adjacent to the semiconductor layer, and the channel current flowing therethrough is modulated by a bias voltage applied to a gate electrode provided through an insulating layer. FIG. 2 shows an example of a typical basic structure of such a TFT. A source electrode 203 and a drain electrode 204 are provided in contact with a semiconductor layer 202 provided on an insulating substrate 201, an insulating layer 205 is provided to cover these, and a gate electrode is provided on the insulating layer 205. There are 206. In the TFT having the structure shown in FIG. 2, the semiconductor layer 202 is composed of a polycrystalline silicon thin film, and is formed between the semiconductor layer 202 and two electrodes, namely, a source electrode 203 and a drain electrode 204. has a first n ⁺ composed of amorphous silicon.
A layer 207 and a second n ⁺ layer 208 are provided to form an ohmic contact. The insulating layer 205 is made of a material such as silicon nitride, SiO ₂ , Al ₂ O ₃ , etc. formed by CVD or PCVD. The reactive gases (raw materials whose constituent atoms are silicon) used to manufacture polycrystalline silicon films include silane (SiH ₄ ), SiF ₄ (silicon fluoride),
SiH ₂ F ₂ (difluorosilane), SiHCl ₃ (trichlorosilane), silicon tetrachloride (SiCl ₄ ), disilane (Si ₂ H ₆ )
These can also be used diluted with H ₂ , Ar, He, He, etc. An example in which a polycrystalline silicon thin film is formed according to the manufacturing method of the present invention and a TFT is created using this as a material will be specifically described below. Example 1 In this example, a TFT was fabricated by forming a polycrystalline silicon thin film on a substrate, and the apparatus shown in FIG. 3 was used. Corning glass was used for the substrate 300 because the substrate temperature Ts can be kept below its softening point. First, after cleaning the substrate 300, (HF+HNO ₃
The surface was lightly etched with a mixed solution of +CH ₃ COOH), dried, and then placed in the Bergier vacuum deposition chamber 3.
Substrate heating holder 3 placed on the anode side in 01
Installed on 02. Thereafter, the Belgear 301 was evacuated to a background vacuum of 3×10 ^-6 Torr or less using a diffusion pump 309. At this time, if the degree of vacuum is low, the reactive gas not only does not effectively deposit a film and work, but also causes O and N to be mixed into the film, significantly changing the resistance of the film.
Next, increase the substrate temperature Ts to bring the temperature of the substrate 300 to 500.
It was kept at ℃. (The substrate temperature is monitored by a thermocouple 303.) Next, H ₂ gas is controlled by a mass flow controller 306 and introduced into the bell gear 301 to clean the surface of the substrate 300, and then a reactive gas is introduced. did. Substrate temperature Ts is 350℃
It was set to The pressure inside the bell jar 301 during discharge was maintained at 0.1 Torr. When the pressure is low, the energy given to the plasma is large, and the degree of orientation of the polycrystalline silicon formed is higher than when the pressure is high even at the same substrate temperature, so it is preferable.
It was preferable to conduct the test at 0.2 Torr or less. In this example, SiH ₄ gas (abbreviated as SiH ₄ (10)/H ₂ ) diluted to 10 vol % with easily handled H ₂ gas was used as the reactive gas to be introduced. The gas flow rate was controlled by a mass flow controller 304 so that it was 30 SCCM. The pressure inside the bell gear 301 is adjusted by adjusting the pressure regulating valve 310 on the exhaust side of the bell gear 301, and by using the absolute pressure gauge 31.
2 was used to set the desired pressure. bell gear 3
After the pressure inside 01 becomes stable, the cathode electrode 313
was applied by a 13.56 MHz high frequency electric field power source 314 to start glow discharge. The voltage at this time is
The voltage was 1.0 KV, the current was 80 m, and the ARF discharge power was 1.5 x 10 ^-3 W/cm ³ in density. The silicon film deposition rate under these conditions was 1 Å/sec. The thickness of the formed film was 4000 Å, and its uniformity was within ±10% for a substrate size of 3 inches by 3 inches when a circular ring-shaped outlet was used. Also, this polycrystalline silicon film is n-type and has a mobility of
It was confirmed by electron beam diffraction and X-ray diffraction that it was polysilicon with strong orientation and uniform grain size, with a resistance value of 0.8 cm ² /V·sec and a resistance value of ˜10 ⁷ Ω·cm. Next, a TFT was fabricated using this film as a material according to the process outlined in FIG. As shown in step a, after depositing the polycrystalline silicon film 401 formed as described above on the glass substrate 300, PH ₃ gas (PH ₃
(abbreviated as SiH ₄ (100 ppm)/H ₂ ) to SiH 4 (abbreviated as SiH ₄ (10)/H ₂ ) diluted to 10 vol% with H ₂ in a molar ratio of 5 × 10 ^-3 . Flow into the bell gear 301 at a certain rate, adjust the pressure inside the bell gear 301 to 0.12 Torr, and perform glow discharge.
A doped n ⁺ layer 402 was formed to a thickness of 500 Å [Step b]. Next, as in step c, the n ⁺ layer 402 was removed except for the source electrode 403 region and the drain electrode 404 region by photoetching. Next, in order to form a gate insulating film, the above-mentioned substrate was again placed in the heating holder 302 on the anode side within the bell jar 301. As in the case of manufacturing polycrystalline silicon, the Bergier 301 is evacuated, the substrate temperature Ts is set to 250°C, and NH ₃ gas is introduced.
20SCCM, SiH ₄ (SiH ₄ (10)/H ₂ ) gas
Introducing 5SCCM to generate glow discharge to generate SiNH
Film 405 was deposited to a thickness of 2500 Å. Next, the source electrode 4 is etched by a photoetching process.
03. Open contact holes 406-1 and 406-2 for the drain electrode 404, and then,
Al is deposited on the entire surface of the SiNH film 405 to form an electrode film 407.
After forming, the Al electrode film 407 is processed by a photoetching process to form the source electrode extraction electrode 4.
08, a drain electrode extraction electrode 409 and a gate electrode 410 were formed. After this, heat treatment was performed at 250° C. in an H ₂ atmosphere. TFT formed according to the above conditions and process (channel length L =
10μ・channel width W=500μ) showed stable and good characteristics. Figure 5 shows an example of the characteristics of the TFT prototyped in this manner. Figure 5 shows the relationship between drain current I _D and drain voltage V _D using gate voltage V _G as a parameter.
Examples of TFT characteristics are shown. The gate threshold voltage is as low as 5V, and V _G = 0 at V _G = 20V.
The ratio of current values is more than three digits. In this example, Corning #7059 glass was used as the substrate, but similar characteristics could be achieved by using ultra-hard glass or quartz glass as the substrate even if the heat treatment temperature and substrate temperature were increased. Therefore, according to the present invention, the substrate temperature Ts can be freely selected from a wide range from a low temperature side to a high temperature side according to the substrate material. TFT integrated circuits can be easily produced at lower cost and using simpler equipment. Example 2 Next, the characteristics of a polycrystalline silicon thin film made by changing various conditions according to the same procedure as in Example 1, and the characteristics of a TFT made from the same are shown in Table 1. It is shown in Table 3. Table 1 shows RF power 50W, silane flow rate (SiH ₄
(10)/H ₂ ) When the substrate temperature Ts during polycrystalline silicon film fabrication is changed from 250 to 550°C under 20SCCM conditions, the average crystal grain size of the obtained polycrystalline silicon film and the same as in Example 1 are The effective carrier mobility μeff when a TFT is created for each sample by operation is shown. Table 2 shows that the substrate temperature is 350℃, silane (SiH ₄
(10)/H ₂ ) Under the condition that the flow rate is constant at 20SCCM
Similar results are shown when the RF power is varied from 5 to 200W. Table 3 shows that the silane (SiH ₄ (10)/H ₂ ) flow rate was 5 to 5 at the substrate temperature of 350℃ and RF power of 50W.
Similar results are shown when changing to 60SCCM. Even at a substrate temperature of 350℃, the average crystal grain size is 600 to 900 Å, and the μeff is 0.9 to 1.6 (cm ² /V sec).
It has been shown that this result can be produced by selecting the RF power and silane flow rate.

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【table】

【table】 [Brief explanation of drawings]

FIG. 1 is an explanatory diagram for explaining the present invention, FIG. 2 is a schematic structural cross-sectional diagram for explaining the structure of a TFT, and FIG. 3 is a schematic explanatory diagram of a device embodying the present invention.
FIG. 4 is a process diagram for explaining the manufacturing process of a TFT according to the present invention, and FIG. 5 is an explanatory diagram for explaining one result of an embodiment of the present invention.

Claims (1)

[Scope of Claims] 1. A main raw material whose constituent atoms are silicon atoms for forming a polycrystalline silicon thin film is introduced in a gaseous state into a deposition chamber that is reduced to a desired pressure, and is placed in the deposition chamber in advance. In a method for producing a polycrystalline silicon thin film, the source material is discharge-decomposed by discharge power on a substrate which is heated to a substrate temperature of 350°C or more and 400°C or less, thereby producing a polycrystalline silicon thin film. , P is the minimum discharge power required to saturate the deposition rate of polycrystalline silicon thin film in the manufacturing method.
min, the discharge power is P min
1. A method for producing a polycrystalline silicon thin film, characterized in that the polycrystalline silicon thin film is formed by setting the discharge power to one-tenth or more of P min and less than P min. 2. Before introducing the main raw material into the deposition chamber,
2. The method of manufacturing a polycrystalline silicon thin film according to claim 1, wherein the substrate is heated in a hydrogen atmosphere.