JPH04116826A - Formation of silicon nitride film - Google Patents

Abstract

PURPOSE:To make it possible to form efficiently a silicon nitride film, which is sufficiently high in dielectric breakdown strength, by a method wherein a power density of RF discharge is controlled at a specified film-forming temperature. CONSTITUTION:A multitude of striped lower electrodes 2 are formed on the surface of a glass substrate 1 in parallel to each other and a silicon nitride film 3 is formed on these electrodes. This film 3 is controlled and formed at a film-forming temperature of 250 deg.C and in a discharge power density of 60 to 100mW/cm<2>. When the discharge power density is made low by 60 to 100mW/cm<2> or thereabouts, defects, such as pinholes, a weak spot and the like, stop hardly generating because the film 3 is evenly grown and even if the film-forming temperature is a low temperature of 250 deg.C and the film quality of the film 3 can not be densified, the dielectric breakdown strength of the film 3 can be made sufficiently high.

Description

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

[Conventional technology]

Silicon nitride films are used as gate insulating films for thin film transistors, MO5 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. Furthermore, 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 coefficients of thermal expansion, and cracks will occur in the substrate and the silicon nitride film. It is necessary to cool 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]

The present invention is characterized in that in forming a silicon nitride film using a plasma CVD apparatus, the film forming temperature is approximately 250° C., and the power density of RF discharge is controlled to 60 to 100 mW/cm 2 .

[Effect]

In this way, the power density of RF discharge is reduced to 60-100mW.
The silicon nitride film formed under the control of /cm2 has sufficient dielectric breakdown voltage even if the film forming temperature is as low as about 250°C. In addition, in the present invention, a silicon nitride film having sufficient dielectric breakdown voltage can be formed at a low film forming temperature of about 250°C, so the heating time for the substrate is shortened when forming the silicon nitride film. Since cooling of the substrate after forming the silicon nitride film can also be shortened, the silicon nitride film can be formed efficiently.

〔Example〕

An embodiment of the present invention will be described below.

In this example, the silicon nitride film was deposited at substrate temperature: 250°C and process gas: SIH, 30SCC14NHs.
60SCCN N2 390SCCH Pressure; 0, 5 TorrRF discharge frequency; 13.56MHz discharge power density + 8
Plasma CV under film formation conditions of 4 mW/cm2
The film was formed using D apparatus.

Note that the composition ratio of silicon atoms Sl and nitrogen atoms N in the silicon nitride film formed under these film forming conditions is S1/N-0,
83, and has a composition close to the stoichiometric ratio (Si/N-0.73).

Silicon nitride films formed under these conditions have a low film formation temperature (substrate temperature) of 250°C, so the density of the film quality is reduced to some extent, but compared to films formed using conventional film formation methods. It has sufficient dielectric breakdown voltage, almost the same as silicon nitride film.

This brings the power density of the RF discharge to 84 mW/a
This is because the film forming temperature (substrate temperature) is set to 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 deposited silicon nitride film will deteriorate when the discharge power density is set as low as 0°C, but if the discharge power density is set to 84 mW/cm '', a silicon nitride film with sufficient dielectric breakdown voltage can be obtained even if the film formation temperature is as low as 250°C.

That is, Fig. 1 shows the dielectric breakdown voltage histogram of the silicon nitride film formed under the above-mentioned film-forming conditions, and Fig. 2 shows the breakdown voltage histogram of the silicon nitride film formed under the above-mentioned film-forming conditions.
A dielectric breakdown voltage histogram of a silicon nitride film formed instead of m2 is shown. This dielectric breakdown voltage histogram was obtained by measuring the dielectric breakdown voltage of the silicon nitride film of the specimen shown in FIGS. 3 and 4.

In this test object, a large number of striped lower electrodes 2 are formed in parallel to each other on a glass substrate 1, a silicon nitride film 3 is formed on the striped lower electrodes 2, and the lower electrodes 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 electrode facing 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 a 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.

At a film formation temperature of 250°C, the discharge power density was 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 the test object on which the silicon nitride film 3 was deposited under a control of 7 mW/cm2. As a result, the dielectric breakdown occurrence rate of this silicon nitride film at each applied electric field strength (the ratio of the number of electrode facing parts where dielectric breakdown occurred to the total number of electrode facing parts) is:
It was as shown in Figure 2. In addition, here, lXl0-6
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 2, when the film formation temperature is 250°C, the discharge power density is 127°C.
In the silicon nitride film formed under control of mW/c ~ 2, A-mode defects (initial defects due to pinholes) that occur at weak applied electric field strengths of 3 MV/cm 2 or less are l MV/cm 2 or less. About 5% in cm 2, 2 M
B-mode failures (defects due to weak spots) occur at a large rate of about 2.5% at V/cm2 and at applied field strengths greater than 3MV/cm2, but Approximately 5.2% in m2,
At 6 MV/cm2, it occurred at a fairly large rate of about 14.3%. Note that although Fig. 2 shows the dielectric breakdown voltage histogram of the silicon nitride film formed by controlling the discharge power density to 127 mW/cm2, the film forming temperature was changed to 2.
When the temperature is 50°C, the discharge power density is 120 to 130
Even when the voltage was changed within the range of mW/cm2, the silicon nitride film formed showed a dielectric breakdown voltage almost the same as that shown in FIG. 2.

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 127 mW/cm2, especially in the early stage of film formation, silicon nitride is scattered ( The silicon nitride film grows unevenly, causing pinholes and cracks. This is thought to be due to an increase in defects such as spots. If the film-forming temperature is as high as about 350°C as in the conventional film-forming method, the silicon nitride film formed will be a dense film without defects such as pinholes and weak spots, resulting in dielectric breakdown. Although the breakdown voltage is sufficient, the film quality of the silicon nitride film cannot be made dense when the film formation temperature is about 250° C., so the above-mentioned defects occur and the dielectric breakdown voltage decreases.

On the other hand, at a film formation temperature of 250°C, the discharge power density was 84 m
When the dielectric breakdown voltage of the silicon nitride film 3 was measured in the same manner as above on a specimen on which the silicon nitride film 3 was deposited under the control of W/cm2, the dielectric breakdown voltage of the silicon nitride film at each applied electric field strength was measured. The incidence rate is shown in Figure 1. In this case as well, the electrode facing part in which a current of 1X10-6 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 1, even when the film formation temperature is 250°C, the discharge power density is 84°C.
The silicon nitride film deposited under the control of mW/cm2 has almost completely eliminated the A-mode defects that occur at weak applied electric field strengths of 3Mv/cm2 or less, and has a film density of 3MV/cm2. B-mode failures that occur at higher applied field strengths are also less than 5M V/cm2
about 0.4% at 6 MV/cm2, about 0.6 at 6 MV/cm2
It occurred only at a very small percentage.

This means that the power density of the RF discharge is 84 mW/c
It is thought that this is because, when it is as low as d, the deposition state of silicon nitride on the substrate surface is averaged, and the silicon nitride film grows uniformly. If the silicon nitride film grows uniformly in this way, defects such as pinholes and weak spots will hardly occur, so the film formation temperature will be as low as 250 degrees Celsius, thus making the silicon nitride film denser. Even if this is not possible, the dielectric breakdown voltage of this silicon nitride film can be made sufficiently high.

As described above, the silicon nitride film formed by the film forming method of the above embodiment has an extremely high dielectric breakdown voltage. Therefore, if this silicon nitride film is used as a gate insulating film of a thin film transistor or a MOS type integrated circuit element, It is possible to significantly reduce the incidence of insulation defects in thin film transistors and MOS type integrated circuit elements, improve their manufacturing yield and reliability, and also increase the dielectric breakdown voltage of the gate insulator ml (silicon nitride film). Since the film thickness is high, the film thickness can be made thinner, so 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.

In the above example, the power density of the RF discharge was set to 84
m W / cm ', but this discharge power density has a good tear lever in the range of 60 to 100 mW/cm2 (7), and the silicon nitride film deposited with the discharge power density in this range has a deposition temperature of approximately Even if it is as low as 250"C, the first
It shows almost the same dielectric breakdown voltage as the dielectric breakdown voltage histogram shown in the figure.

According to this film-forming method, a silicon nitride film with sufficiently high dielectric breakdown voltage can be obtained 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/c.
m2, the deposition rate of the silicon nitride film is R
This is lower than the conventional film formation method in which the F discharge power density is 120 to 130 mW/cm2, but compared to this reduction in deposition rate, the reduction in heating and cooling time of the substrate is much greater, so the deposition The slowdown is not an issue.

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. Therefore, for example, in the manufacture of thin film transistors, the silicon nitride film that becomes the gate insulating film is used. If the above-described film formation method is applied to film formation, 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 an i-type semiconductor layer are successively deposited on top of it, and then an n-type semiconductor layer and a source and drain electrode are formed on top of that. Manufactured by 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 i-type semiconductor layer is patterned into the shape of a transistor element. has been done.

In manufacturing this thin film transistor, the silicon nitride film and the i-type 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 i-type semiconductor layer are successively formed on the substrate by sequentially transferring the substrate to each of the chambers. In addition, hydrogenated amorphous silicon (a-5IH) is generally used for the i-type semiconductor layer, and the semiconductor layer made of hydrogenated amorphous silicon is formed by RF radiation at a deposition temperature of about 250'C. The film is formed by controlling the discharge power density to 40 to 50 mW/cm2. The reason why the i-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 an i-type semiconductor layer are successively formed using the above-mentioned plasma CVD apparatus, conventionally the silicon nitride film is formed at a film-forming temperature of about 350"C. The substrate on which the silicon nitride film has been deposited in the nitride film deposition chamber is transferred to the next semiconductor layer deposition chamber while adjusting the temperature until the temperature reaches the deposition temperature of the i-type semiconductor layer (approximately 250°C). must be transported.

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 a silicon nitride film was formed was cooled to the temperature at which the i-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 almost 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 i-type 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〕

The present invention is characterized in that, in forming a silicon nitride film using a plasma CVD apparatus, the film forming temperature is set at approximately 250°C, and the power density of RF discharge is controlled to 60 to 100 mW/cm2. A silicon nitride film with sufficiently high dielectric breakdown voltage can be obtained at a much lower film forming temperature than the film forming method described above. 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 the drawing]

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
3 and 4 are a plan view and an enlarged cross-sectional view of a portion of the test object used for measuring the dielectric breakdown voltage of a silicon nitride film.

Claims (1)

[Claims] A method for forming a silicon nitride film using a plasma CVD apparatus, characterized in that the film forming temperature is about 250° C., and the power density of RF discharge is controlled to 60 to 100 mW/cm^2.