JPS6010682A - Forming method for thin film - Google Patents

Abstract

PURPOSE:To form a high resistance hydrogen amorphous silicon film having excellent heat resistance at a low substrate temperature by utilizing chemical activity and accelerating energy of ions, controlling atmospheric gas pressure, and adding impurity. CONSTITUTION:A forming method for a thin film heats an evaporation source silicon, ionizes, accelerates silicon vapor and hydrogen when evaporating the silicon in an atmosphere containing hydrogen as main ingredient, implants them to a substrate, and controls a film structure. The hydrogen gas pressure of the atmosphere affects the hydrogen and hydrogen ion amount incident to the film forming surface, and specifies the number of collisions of silicon atoms or cluster and hydrogen in vapor phase. Accordingly, they are important parameters in controlling the quality of the film, and 2X10<-3>-2X10<-2> Pa is particularly suitable.

Description

Detailed Description of the Invention [Technical Field] The present invention relates to a method for forming an amorphous silicon thin film, particularly a hydrogenated amorphous silicon thin film that can be used as a photoconductive film for image sensors, etc. It is formed using an ion beam method.

<Prior art> As is well known, a hydrogen amorphous silicon film has a low density of localized levels within the bandgap.

Valence electrons can be controlled by adding impurities such as phosphorus. This is because silicon-hydrogen bonds are formed during film formation, and dangling bonds of silicon atoms, which were present in large quantities in the non-hydrogenated amorphous silicon film, are significantly reduced. For this reason, its application as a thin film functional material is attracting attention, and research on its application to thin film solar cells, image sensor photoconductive films, 7IV film transistors for liquid crystal display elements, etc. is being actively conducted.

However, in hydrogenated amorphous silicon films, due to the presence of silicon-hydrogen bonds, the band gap is 16 to 1.8 eV, which is larger than that of crystalline silicon, resulting in high resistance.
The basic feature is that the spectral sensitivity to light is close to that of the human eye. In fact, it also has the advantage of having a slightly higher localized level density and higher photoconductivity. Taking advantage of these features, applications to photoconductive films for -dimensional image sensors, imaging devices, etc. have been investigated using hydrogenated amorphous silicon produced by the chive sputtering method and the glow discharge method.

In the cutting sputtering method, for example, silicon is sputtered in a mixed gas atmosphere of hydrogen and argon, and it is inevitable that a certain amount of argon will be included in the film. By increasing the amount of hydrogen in the film, it is possible to form a film with very high resistance, but at the same time there is a drawback that a sufficiently high photoconductive film cannot be obtained due to the increase in 5i (-I2 bonds). The discharge method uses glow discharge using, for example, monosilane or a mixture of monosilane and hydrogen or rare gases (He, Ar gas) to form a film. high resistance,
It is possible to obtain a highly photoconductive film.

However, hydrogenated amorphous silicon films produced by these methods have thermal instability in that hydrogen in the film is released by heating to about 200° C., resulting in deterioration of film properties, which is an obstacle to practical use. Still, there is a problem in that in order to obtain a film of relatively high quality, the film forming process must be carried out while maintaining the substrate temperature at a relatively high temperature of 250° C. to 300° C.

<Object of the Invention> In view of the conventional film forming methods as described above, the present invention provides a hydrogenated film that has high heat resistance, high resistance, and high photoconductivity at a relatively low substrate temperature by a thin film forming method using ions. The purpose is to obtain an amorphous silicon film.

<Example> The thin film formation method using ions referred to here is a method of forming hydrogenated amorphous silicon by heating silicon as an evaporation source and evaporating it into an atmosphere containing hydrogen as the main component. In this method, silicon vapor and hydrogen are ionized, accelerated, and bombarded onto a substrate to control the film structure, and includes an ion blating method, an ion beam evaporation method, a cluster ion beam method, and the like. In the following, only the cluster ion beam method that can realize denseness, high purity, and high heat resistance will be explained.

A technique for obtaining an amorphous silicon film using the cluster ion beam method is already known from Japanese Patent Publication No. 57-54.930. This type of method, as shown in Figure 1,
Vacuum chamber 2 maintained at high vacuum by vacuum pump 1
Inside, a closed crucible 9 having a nozzle 8 is filled with silicon 10, and the silicon is heated and melted by irradiating an electron beam from a filament 11, and silicon vapor is injected from the nozzle to generate a silicon vapor flow 12. let At this time, the silicon vapor becomes supercooled due to adiabatic expansion, and is said to form a cluster of 500 to 2,000 silicon atoms loosely bonded together. Furthermore, a part of the silicon vapor flow along with the gas mainly composed of hydrogen introduced into the vacuum apparatus through the pipe 13,
When the electron beam drawn from the filament 6 is irradiated through the electrode 7, it is ionized and the electrode 6 and crucible 9 are ionized.
By applying a voltage between them, it is accelerated and made to collide with the substrate 4. The kinetic energy obtained by the ions and the collapse of the cluster actively migrate silicon atoms on the substrate surface, promoting the formation of bonds between silicon atoms even at low substrate temperatures. The dangling bonds of silicon atoms generated on the surface of the film form bonds with hydrogen through interaction with hydrogen incident from the gas phase. As described above, in the cluster ion beam method, a high quality hydrogenated amorphous silicon film can be formed at a relatively low substrate temperature due to the formation of clusters and the presence of ions.

However, in the cluster ion beam method, the hydrogen gas pressure in the atmosphere affects the amount of hydrogen and hydrogen ions that enter the film-forming surface, and determines the number of collisions between silicon atoms or clusters and hydrogen in the gas phase. Therefore, it has now been found that -1- is an extremely unimportant parameter that controls film quality.

That is, in the apparatus shown in FIG.
High purity hydrogen gas 14 after exhausting to below X 10-'Pa
is introduced at a predetermined pressure P, i24, and the crucible is heated to 2000°C.
The film was formed at a speed of 100 Xim. The substrate 4 is made of molten quartz that has been boiled in sulfuric acid and then washed with dilute hydrofluoric acid. It is heated with an infrared lamp 3 to bring the substrate temperature to 200℃.
It was kept at ℃. During ionization, a voltage of 300 V was applied to the filament 6 and the electrode 7 (ionization voltage), and a current of 100 mA (ionization current) was passed between them. The accelerating voltage applied between the crucible 9 and the electrode 5 was 3 KV. The thickness of the formed hydrogenated amorphous silicon film is 0.1 μm.
A comb-shaped electrode was formed by vapor-depositing aluminum to a thickness of about 0.471 m, and the dark conductivity tld and photoconductivity Δ6 p h were measured. For photoconductivity, an argon laser with an irradiation intensity of 100 mW/ca was used.

Figure 2 shows 'd (solid line) and Δ6 p h (dashed line) and PH
2, hydrogen gas pressure is 6.5 x 1O-3P
It is possible to obtain a hydrogenated amorphous silicon film having a high resistance even in the vicinity of a. The hydrogenated amorphous silicon film formed under the above hydrogen pressure exhibits activated conduction to the conduction band even near room temperature, indicating that the local level density within the band gap is small.

A film produced under a hydrogen pressure lower than the above conditions exhibits hopping conduction at room temperature, and it is considered that hydrogenation of the film has not been sufficiently performed and the local level density is large. Furthermore, even at higher hydrogen pressures, hopping conduction still occurs at room temperature. It is thought that while hydrogen in the film increases, silicon cybide (Si=H2) increases at the same time, and this increases the localized level. Even at higher board temperatures, the same effect can be achieved. Good luck Yuo
The steaming hydrogen pressure range is slightly expanded.

The degree of photoconductivity does not show a remarkable dependence on hydrogen pressure as does the dark conductivity, and a hydrogen pressure around 6.5×10 Pa is most effective for producing a high-resistance, high-photoconductivity film using the cluster ion beam method. became clear.

The aforementioned hydrogen pressure 6.5X1. In the OPa sample, its optical bandgap was l, 67 eV, while the activation energy of dark conductivity was 0.66 eV. This suggests that the Fermi level is closer to the conduction band side (approximately 1, 1, 8 eV) than the center of the band gap. Therefore, by mixing a small amount of ciboron gas into the hydrogen gas, boron is added to the hydrogenated amorphous silicon film. By doping with , a higher resistance film can be obtained.

Furthermore, in order to obtain hydrogenated amorphous silicon with high resistance, it is effective to increase the band gap and decrease the number of carriers. One way to do this is to increase the amount of hydrogen in the film, but this will also increase the amount of silicon sibide (Si-■(2)) in the film, which may deteriorate the carrier transport properties and reduce the photoconductivity. Therefore, in addition to hydrogen, nitrogen, oxygen, etc. are introduced into the film to form bonds with silicon, thereby widening the band gap and increasing the resistance. Even if it enters, there is little risk of creating dangling bonds, and if the carrier transport characteristics are deteriorated, the resistance can be significantly increased.

The film forming process will be specifically explained below.

Example 1: Fig. 1 In a cluster ion beam device,
A hydrogenated amorphous silicon film was produced by the method described above. The hydrogen pressure was maintained at 6.5 x 10-8 Pa, the substrate temperature was maintained at 200°C, the ionization voltage was maintained at 300μ, and the ionization current was varied from 100mA to 300mA.

Dark conductivity I x 10 at accelerating voltage O to 3KV
1 Ac? A high resistance film of 1×101/r)tm was obtained from n. The photoconductivity -.gamma. was in the range of 1.times.10@1 to 1.times.10 Vr'L, and a value more than 4 orders of magnitude higher than the dark conductivity was obtained.

Example 2: In the cluster ion beam apparatus shown in FIG. 1, a trace amount of diborane (CB2'H'6) was added to hydrogen to form a poron-doped hydrogenated amorphous silicon film. The film forming conditions such as hydrogen pressure are the same as in Example 1. Diborane was 7111 (500 ppm relative to hydrogen).

As a result, the dark conductivity decreased by about one order of magnitude compared to the case without diborane added, and the minimum value was 9 x 10""14
A high-resistance film of Ω/Ωσ was obtained. Although the photoconductivity was almost halved, the ratio to dark conductivity was improved.

Example 3: In the cluster ion beam apparatus shown in FIG. 1, trace amounts of diborane and ammonia (NH3) were added to hydrogen to form a boron-doped hydrogenated amorphous silicon nitrogen film. The film forming conditions such as hydrogen pressure were the same as in the example. As in Example 2, diborane was added at 500 ppm relative to hydrogen, and ammonia was introduced from lX10Pa to lX10Pa, thereby increasing the dark conductivity from lXl0I/Ωm to lXl0
A high resistance film of 1/Ωm was obtained. The photoconductivity was about I x 10 x/nan, which was not much different from Example 2.

The characteristics of each thin film shown in the above examples are shown in FIG. The vertical axis is dark conductivity, and the horizontal axis is S. r S + + S
2+S3 is the case where the hydrogen gas pressure was not optimized, So, and Example 1 where the hydrogen gas pressure was optimized. Example 2 where 500 ppm of Sl+diborane was added.
The case corresponds to case 83 in Example 3 in which S2+ and ammonia of 1×10 Pa to 1×10 Pa were added. As is clear from the figure, by optimizing the atmospheric hydrogen pressure and adding appropriate amounts of diborane, ammonia, etc., the dark conductivity was increased to 10.
A high resistance film of 101/Ω can be obtained from the above.

<Effect> As described above, by effectively utilizing the chemical activity and acceleration energy of ions, precisely controlling the atmospheric gas pressure, and adding appropriate impurities, we can create high-temperature high-temperature high-temperature products with excellent heat resistance at relatively low substrate temperatures. A resistive hydrogenated amorphous silicon film can be formed. In particular, the cluster ion beam method allows for the production of thin films with structural stability and excellent properties due to its excellent controllability, and the ripple effect is significant as a technology for providing highly reliable, high-resistance, and high-photoconductivity hydrogenated amorphous silicon films. .

4. Wi. 1. Figure 1 shows the outline of a cluster ion beam device, and Figure 2 shows the relationship between the dark conductivity and photoconductivity of a hydrogenated amorphous silicon film and atmospheric hydrogen gas pressure in one embodiment of the present invention. FIG. 3 is a diagram showing the relationship between the dark conductivity of the hydrogenated amorphous silicon film of each example according to the present invention.

2: Vacuum chamber, 4: Substrate, 5° ion acceleration electrode,
6: Filament for ionization, 7: Electron beam extraction electrode for ionization, 8: Nozzle, 9: Crucible, 10: Silicon, 11: Filament for heating the crucible, 12: Silicon vapor flow, 13: Atmospheric gas introduction management agent, patent attorney Yoshihiko Fukushi (and 2 others) 15
/b /7 1θ j"7-X, -'"ge. ~＼--m- and Fig. 3 & Claws IL spear (Pa) Fig. 2 395-

Claims (1)

[Claims] (1) In the step of heating the evaporation source silicon to evaporate it in a hydrogen atmosphere to obtain a hydrogenated amorphous silicon thin film, the atmosphere mainly consisting of hydrogen is changed from 2×10 Pa to 2
A thin film forming method characterized by obtaining a high resistance and high photoconductivity thin film by maintaining the pressure at X10Pa and accelerating ionization of silicon vapor and hydrogen by evaporating in the atmosphere and colliding with the substrate surface to grow a thin film. (Katana) The evaporation source silicon is filled in a closed crucible having a nozzle, the crucible is heated to vaporize, and the silicon is injected from the nozzle to form a thin film. (3) The thin film forming method according to claim 1, characterized in that the hydrogen atmosphere contains 0 to 2000 pprn of diborane (B2H6). ←) The hydrogen atmosphere contains hydrogen. In addition, from 1 × 10 Pa to l
2. The method for forming a thin film according to claim 1, wherein the gas A is introduced from a compound of oxygen or nitrogen such as ammonia, nitrogen, oxygen, or nitrous oxide at a pressure of 10 Pa.