JPH04120733A - Method for forming silicon nitride film - Google Patents

Abstract

PURPOSE:To enable a silicon nitride film with a fully high dielectric breakdown voltage be formed efficiently at a low film-forming temperature by controlling film-forming temperature, power density of RF discharge, and flow ratio of process gas to specific conditions. CONSTITUTION:When forming a silicon nitride film onto a surface of a glass substrate using a plasma CVD device, the film-forming temperature (substrate temperature) is controlled to approximately 250 deg.C and a power density of RF discharge is controlled to 60-100mW/cm<2>, and a flow ratio of process gases (SiH4, NH4, N2) is controlled to SiH4:NH4:N2=1:1:14-1:2.7:12.3 for forming the silicon nitride film. A composition ratio (Si/N) of silicon atom Si of the silicon nitride film which is formed under these film-forming conditions to nitrogen atom N is Si/N=0.75-0.85. Then, the silicon nitride film which is formed under these film-forming conditions has a low film-forming temperature of approximately 250 deg.C, thus achieving a sufficient dielectric breakdown voltage although density of the film quality is reduced to some extent.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method for forming a silicon nitride film.

[Conventional technology]

Silicon nitride films are used as gate insulating films of thin film transistors, MOS type integrated circuit elements, and the like.

This silicon nitride film is generally formed using a plasma CVD apparatus. Conventionally, in order to obtain a silicon nitride film with excellent dielectric breakdown voltage characteristics, the power density of RF discharge was increased to 120°C at a film formation temperature of approximately 350°C. ~130mW/cm2
The silicon nitride film is deposited under controlled conditions.

[Problem to be solved by the invention]

However, although the conventional method for forming a silicon nitride film described above yields a silicon nitride film with dense film quality and high dielectric breakdown voltage, on the other hand, it is necessary to raise the film formation temperature to a high temperature of about 350°C. However, there was a problem in that the silicon nitride film could not be formed efficiently.

In other words, in film formation using a plasma CVD apparatus, a substrate (usually a glass substrate) on which a silicon nitride film is to be formed is carried into a vacuum chamber, and then this substrate is heated to a predetermined film formation temperature, and the temperature inside the chamber is also adjusted. The process starts after heating the substrate to an extent that does not affect the temperature of the substrate, but in this case, heating the substrate too quickly may cause cracks in the substrate, so it is necessary to heat the substrate over a certain amount of time. be. In addition, the temperature of the substrate on which the silicon nitride film has been formed is lowered until it is close to the temperature of the atmosphere to which the substrate will be exposed next (atmospheric temperature if the substrate is taken out into the atmosphere) before being taken out of the chamber. In this case, if the substrate is rapidly cooled, thermal distortion will occur between the substrate and the silicon nitride film due to the difference in their thermal expansion coefficients, and cracks will occur in the substrate and the silicon nitride film. It is necessary to cool down the substrate more slowly than when heating the substrate.

For this reason, the conventional film forming method described above, which forms a silicon nitride film at a high film forming temperature of approximately 350°C, not only requires time to heat the substrate, but also requires cooling the substrate after forming the silicon nitride film. Therefore, the silicon nitride film could not be efficiently formed.

The present invention has been made in view of the above-mentioned circumstances, and its purpose is to produce a silicon nitride film with sufficiently high dielectric breakdown voltage at a much lower film-forming temperature than conventional film-forming methods. An object of the present invention is to provide a method for forming a silicon nitride film, which can efficiently form a silicon nitride film.

[Means to solve the problem]

In the present invention, when forming a silicon nitride film using a plasma CVD apparatus, the film forming temperature is approximately 250°C, and the power density of R and F discharge is 60 to 100 mW/cm2, and the process gas flow rate ratio is 5jH4:NH4:N2-1+1. :14~1:
2.7:12.3.

[Effect]

In this way, the power density of RF discharge can be increased from 60 to 100 m
A silicon nitride film formed under controlled W/cm2 has sufficient dielectric breakdown voltage even when the film forming temperature is as low as approximately 250°C.
H4: NH4: N2-1: 1:14-1:2
．． The silicon nitride film formed by controlling the ratio of 7:12.3 was
High dielectric breakdown voltage. have. In the present invention, about 2
Since a silicon nitride film with sufficient dielectric breakdown voltage can be formed at a low film formation temperature of 50°C, the heating time for the substrate is shortened when forming a silicon nitride film, and the silicon nitride film can be formed easily. Since the subsequent cooling of the substrate can also be shortened, the silicon nitride film can be formed efficiently.

〔Example〕

An embodiment of the present invention will be described below.

This method of forming a silicon nitride film uses a plasma CVD apparatus to form a silicon nitride film on a glass substrate surface at a film forming temperature (substrate temperature) of approximately 250°C and an RF discharge power density of 60 to 100 m W/. cm2, and the flow rate ratio of the process gases (SI H4, NH4, N2) was set to SI H4: NH4: N2-1: 1.
:14 to 1:2.7:12.3 to form a silicon nitride film, and the composition of silicon atoms S1 and nitrogen atoms N of the silicon nitride film formed under these film formation conditions is The ratio (St/N) is Si/N-0.75 to 0.85
It is.

The silicon nitride film deposited under these deposition conditions has a low deposition temperature of approximately 250°C, so the density of the film quality decreases to some extent, but silicon nitride film deposited using conventional deposition methods does not. It has sufficient dielectric breakdown voltage, almost the same as nitride film.

This reduces the power density of the RF discharge to 60-100mW
/cm2, and the film forming temperature was 25
If the discharge power density is set to 120 to 130 mW/cm2 as in the conventional film forming method, the dielectric breakdown voltage of the silicon nitride film formed will deteriorate, or the discharge power density will be lowered to 60 to 100 mW/cm2 when the temperature is as low as 0°C. If it is as low as mW/cm2, a silicon nitride film with sufficient dielectric breakdown voltage can be obtained even if the film formation temperature is as low as 250°C.

Here, to explain the dielectric breakdown voltage of a silicon nitride film formed by controlling the film forming temperature to about 250°C and the power density of RF discharge to 60 to 100 mW/cm2, Fig. 1 shows , Film-forming temperature: 250℃ Gas flow rate: SiH<30SCCMNH
360800M N2 390SCCM Pressure; 0. 5TorrRF
Discharge frequency; 13.56MH2 discharge power density;
This figure shows a dielectric breakdown voltage histogram of a silicon nitride film deposited to a thickness of 1000 people under a deposition condition of 84 mW/cm2. Note that the composition ratio of the silicon nitride film formed under these film forming conditions is Si/N-0.83.

Furthermore, FIG. 2 shows the film-forming conditions in which only the power density of RF discharge is increased among the above-mentioned film-forming conditions, for example, the film-forming temperature
Degree; 250℃ Gas flow rate; 5iE4 30SC
CMN H, 60SCCM N2 390SCCM Pressure; 0. 5TorrRF
Discharge frequency; 1B, 56MH2 discharge power density;
This figure shows a dielectric breakdown voltage histogram of a silicon nitride film formed to a thickness of 1000 people at 127 mW/cm2. Note that the composition ratio of the silicon nitride film formed under these film forming conditions is Sj/N-0.69.

The dielectric breakdown voltage histograms shown in FIGS. 1 and 2 were obtained by measuring the dielectric breakdown voltage of the silicon nitride film of the specimen shown in FIGS. 4 and 5.

In this test object, a large number of striped lower electrodes 2 are formed parallel to each other on a glass substrate 1, a silicon nitride film 3 is formed thereon, and the lower electrodes 2 are formed on the silicon nitride film 3. The dielectric breakdown voltage of the silicon nitride film 3 is determined by sequentially applying a voltage to each lower electrode 2 and applying a voltage to one lower electrode 2. Measurements were made for all of the electrode opposing parts where the lower electrode 2 and the upper electrode 4 intersect by a method of sequentially checking the presence or absence of current flowing through each upper electrode 4 each time the voltage was applied. In this case, the total number of electrode facing parts was 691,200, the total area of all electrode facing parts was 2.07 cm2, and the silicon nitride film 3 was deposited on 1000 people using a parallel plate plasma CVD apparatus. A thickly formed film was used.

Then, at a film formation temperature of 250°C, the discharge power density was increased to 12
The dielectric breakdown voltage of the silicon nitride film 3 was measured while continuously changing the electric field strength applied between the electrodes 2 and 4 on a test object on which the silicon nitride film 3 was deposited under a control of 7 mW/cm2. The dielectric breakdown occurrence rate (the ratio of the number of electrode facing parts where dielectric breakdown occurred to the total number of electrode facing parts) of this silicon nitride film at each applied electric field strength was as shown in FIG. In addition, here, I X 10-
The electrode facing part through which a current of 6 A or more flowed was determined to be a defective part where dielectric breakdown occurred.

As shown in the dielectric breakdown voltage histogram in Figure 2, when the film formation temperature is 250°C, the discharge power density is 127 m
The silicon nitride film formed by controlling the rate of W/cm2 is 3
A-mode failures (initial failures due to pinholes) that occur at weak applied electric field strengths of MV/cm2 or less are
About 5% in I MV/cm2, 2 MV/cm2
B-mode defects (defects due to weak spots) occur at a large rate of approximately 2.5% at cm2 and at applied electric field strengths greater than 3MV/cm2.
Approximately 5.2% in V/cm2, 6MV/c
It occurred at a fairly large rate of about 14.3% in m2.

The reason why the dielectric breakdown voltage of the silicon nitride film formed in this manner deteriorates is that when the silicon nitride film is formed at a high discharge power density of 120 to 130 mW/cm2, especially in the early stage of film formation, the silicon nitride film deteriorates. The silicon nitride film is deposited unevenly on the substrate surface in a scattered state (similar to the adhering state of water droplets when water is sprayed onto the board surface), and as a result, the degree of growth of the silicon nitride film becomes uneven, resulting in pins. This is thought to be due to an increase in defects such as holes and weak spots.

And, unlike the conventional film forming method, the film forming temperature is approximately 350°C.
If the temperature is higher than 250℃, the silicon nitride film formed will be a dense film without defects such as pinholes or weak spots, and its dielectric breakdown voltage will be sufficient.
If the temperature is about .degree. C., the quality of the silicon nitride film cannot be made dense, so the above-mentioned defects occur and the dielectric breakdown voltage decreases.

In addition, in Figure 2, the discharge power density is 127mW70~2
The dielectric breakdown voltage histogram of the silicon nitride film deposited under the control of
Even when the temperature was changed within the range of 2, the silicon nitride film formed showed almost the same dielectric breakdown voltage as shown in FIG.

On the other hand, the silicon nitride film 3 was deposited in the same manner as described above for a test object in which the silicon nitride film 3 was deposited at a deposition temperature of 250°C and the discharge power density was controlled to 84 mW/cm2. When the dielectric breakdown voltage was measured, the dielectric breakdown occurrence rate of this silicon nitride film at each applied electric field strength was as shown in FIG. In addition, here too, lX1O-6
The electrode facing part where a current of A or more flowed was determined to be a defective part where dielectric breakdown occurred.

As shown in the dielectric breakdown voltage histogram in Figure 1, even when the film formation temperature is 250°C, the discharge power density is 84°C.
In the silicon nitride film deposited under the control of mW/cm2, the A-mode defects that occur at weak applied electric field strengths of 3MV/cm2 or less are almost completely eliminated, and the 3MV/cm2 or less A-mode defects are almost completely eliminated. B-mode failures that occur at applied field strengths greater than 5 MV/cm2 also
/ cm 2 and about 0.4%, 6 MV / cm
2, it occurred at a very small rate of about 0.6%.

This means that the power density of RF discharge is 84 mW/cm
It is thought that this is because when the temperature is lowered to about 2.2, the deposition state of silicon nitride on the substrate surface is averaged, and the silicon nitride film grows uniformly.

If the silicon nitride film is grown uniformly in this way, defects such as pinholes and weak spots will hardly occur, so the film formation temperature is as low as 250°C, and the quality of the silicon nitride film is therefore dense. Even if this cannot be achieved, the dielectric breakdown voltage of this silicon nitride film can be made sufficiently high.

In addition, in Fig. 1, the discharge power density is 84 mW/c.
The dielectric breakdown voltage histogram of the silicon nitride film formed by controlling the discharge power density is 60 to 10 m2.
In the range of 0 mW/cm2, even if the film formation temperature is as low as about 250°C, the deposited silicon nitride film exhibits a dielectric breakdown voltage almost similar to the dielectric breakdown voltage histogram shown in Figure 1. .

Further, in the above film forming method, the flow rate ratio of the process gas is set to SiH4:NH4:N2''1:1:1.
Since the ratio is controlled to 4 to 1:2.7+12.3, the formed silicon nitride film has a very low defect density and a higher dielectric breakdown voltage.

That is, FIG. 3 shows the silicon nitride film 3 of the object to be tested at a deposition temperature of 250°C and a pressure of 250°C.
Power; 0. 5TorrRF discharge frequency;
13.56MHz discharge power density; 84mW/c
Silicon nitride films with various composition ratios were formed to a film thickness of 750 mm by changing the flow rate ratio of the process gas under film formation conditions of 2 m2, and for the silicon nitride films with each composition ratio,
It shows the results of examining the density of defects that occur when a high electric field of 3 MV/cm2 is applied between electrodes 2 and 4.

As shown in Figure 3, the density of defects that occur when a high electric field is applied to a silicon nitride film varies depending on the composition ratio (St/N) of the silicon nitride film, but when the gas flow rate ratio is
H4:NH4:N2-1: 1:14 (Gas flow rate; SI H430SCCM, NH) 30SC
The silicon nitride film with a composition ratio Sj/N-0.85 was also formed by controlling the gas flow rate to SiH4:NH4:N2~1:2.
．． 7:12.3 (Gas flow rate; S i H430S
C.C.M.

A silicon nitride film with a composition ratio of Si/N-0.75, which was formed by controlling the Si/N concentration (NH380SCCM, N2370SCCM), also had a fairly small defect density of less than 50 defects/cm2, and the composition ratio Si/N was 0.75-0.8
For example, if the silicon nitride film has a gas flow rate in the range of 5.
H4:NH4:N2-1:2:1B (gas flow rate,
S i H430SCCM, NH360SCCM
.

30 pieces/cm2 with a silicon nitride film with a composition ratio of St/N-0.83, which was formed by controlling the
, the defect density is further reduced as shown below.

As described above, the silicon nitride film formed by the above film-forming method has an extremely high dielectric breakdown voltage. Therefore, if this silicon nitride film is used as the gate insulating film of a thin film transistor or MO5 type integrated circuit element, this thin film transistor YaM
It is possible to significantly reduce the incidence of insulation defects in O5 type integrated circuit devices, improve manufacturing yield and reliability, and also because the dielectric breakdown voltage of the gate insulating film (silicon nitride film) is high. Since the film thickness can be reduced, even if the gate voltage applied to the gate electrode is the same, a stronger electric field can be applied to the semiconductor layer to increase the on-current.

According to this film-forming method, it is possible to obtain a silicon nitride film with sufficiently high dielectric breakdown voltage at a film-forming temperature (approximately 250°C) that is approximately 100°C lower than that of conventional film-forming methods. The heating time for the substrate during the formation of the nitride film is shortened, and the cooling of the substrate after the silicon nitride film is formed can also be shortened, so the silicon nitride film can be formed efficiently. can. In addition, in the above film forming method, the power density of RF discharge is set to 60 to 100 mW.
/cm2, the deposition rate of the silicon nitride film is 120-130 m2 with the power density of the RF discharge.
This is lower than the conventional film deposition method, which uses W/cm2, but compared to this decrease in deposition rate, the reduction in substrate heating and cooling time is much greater, so the decrease in deposition rate is not a problem. It won't happen.

In addition, the above film formation method forms a silicon nitride film with sufficiently high dielectric breakdown voltage at a film formation temperature of about 250°C. By applying the above film formation method to the film, the time required for depositing the silicon nitride film and the semiconductor layer can be shortened, and the manufacturing efficiency of thin film transistors can be improved.

In other words, for example, an inverted staggered thin film transistor is
After forming a gate electrode on the glass substrate surface, a silicon nitride film (gate insulating film) and a type 1 semiconductor layer are successively deposited on top of it, and then an n-type semiconductor layer and source and drain electrodes are formed on top of that. A manufacturing method in which a metal film is sequentially deposited, and then the metal film and the n-type semiconductor layer thereunder are patterned into the shape of source and drain electrodes, and the l-type semiconductor layer is patterned into the shape of a transistor element. Manufactured.

In manufacturing this thin film transistor, the silicon nitride film and the type 1 semiconductor layer are formed using a plasma CVD apparatus in which a chamber for forming a silicon nitride film and a chamber for forming a semiconductor layer are arranged in series. , the silicon nitride film and the l-type semiconductor layer are successively deposited on the substrate by sequentially transferring the substrate to each of the chambers. In addition, hydrogenated amorphous silicon (a-3iH) is generally used for the l-type semiconductor layer, and the semiconductor layer made of hydrogenated amorphous silicon is formed at a film formation temperature of about 250°C to form an RFi electrode. The film is formed by controlling the power density of 40 to 50 mW/cm2. The reason why the l-type semiconductor layer is deposited at a deposition temperature of approximately 250°C is that if the deposition temperature is increased, the amount of hydrogen in the hydrogenated amorphous silicon decreases, resulting in poor semiconductor properties. It's for a reason.

When a silicon nitride film and a type 1 semiconductor layer are successively formed using the above-mentioned plasma CVD apparatus, conventionally the silicon nitride film is formed at a film formation temperature of about 350°C. The temperature of the substrate on which the silicon nitride film has been deposited is adjusted in the film deposition chamber until the temperature reaches the deposition temperature for type 1 semiconductor layer (approximately 250°C), and then transferred to the next semiconductor layer deposition chamber. Must.

For this reason, in the past, a substrate cooling chamber was provided between a chamber for forming a silicon nitride film and a chamber for forming the next semiconductor layer, and the substrate on which the silicon nitride film was formed was cooled to the temperature at which the l-type semiconductor layer was formed. However, as explained in the [Problem to be Solved by the Invention] section, this substrate must be cooled slowly, so the substrate heated to approximately 350°C during the formation of the silicon nitride film is It takes a considerable amount of time to lower the temperature to about 250°C.

On the other hand, if the silicon nitride film which becomes the gate insulating film is formed by the film forming method of the present invention, the film forming temperature of the silicon nitride film will be approximately 250 nm, which is approximately the same as the film forming temperature of the i-type semiconductor layer.
℃, the substrate on which the silicon nitride film is formed in the silicon nitride film forming chamber can be transferred as it is to the next semiconductor layer forming chamber without temperature adjustment. Therefore, the time required for depositing the silicon nitride film and the type 1 semiconductor layer can be shortened, and the manufacturing efficiency of thin film transistors can be improved. Since there is no need to provide a substrate cooling chamber between the layer deposition chamber and the layer deposition chamber, the configuration of the plasma CVD apparatus can also be simplified.

〔Effect of the invention〕

In the present invention, in forming a silicon nitride film using a plasma CVD apparatus, the film forming temperature is approximately 250°C, the power density of RF discharge is 60 to 100 mW, 70 m2, and the flow rate ratio of process gas is 5jH4:NH4:N2''1: 1
Since this method is characterized by controlling the ratio to +14 to 1:2.7:12.3, it is possible to form a silicon nitride film with sufficiently high dielectric breakdown voltage at a much lower film-forming temperature than with conventional film-forming methods. Obtainable. Therefore, the heating time for the substrate when forming the silicon nitride film is shortened, and the time required to cool the substrate after forming the silicon nitride film can also be shortened, so that the silicon nitride film can be formed efficiently. can be done.

[Brief explanation of drawings]

Fig. 1 is a diagram showing a dielectric breakdown voltage histogram of a silicon nitride film formed in an embodiment of the present invention, and Fig. 2 is a diagram showing a dielectric breakdown voltage histogram at 250°C.
A diagram showing a dielectric breakdown voltage histogram of a silicon nitride film formed by increasing the power density of RF discharge at a film forming temperature of
Figure 3 is a diagram showing the relationship between the composition ratio of a silicon nitride film and the density of defects generated by the application of a high electric field. Figures 4 and 5 are specimens used to measure the dielectric breakdown voltage of silicon nitride films. FIG. 2 is a plan view and an enlarged sectional view of a portion thereof. Applicant: Casio Computer Co., Ltd. (Kaikiya (MV/cm2)) Figure 2 Figure 3

Claims (1)

[Claims] When forming a silicon nitride film using a plasma CVD device, the film forming temperature is approximately 250°C, and the power density of RF discharge is set to 6.
0 to 100mW/cm^2, process gas flow rate S
iH_4:NH_4:N_2=1:1:14 to 1:2.
A method for forming a silicon nitride film characterized by controlling the ratio to 7:12.3.