JPH0686660B2 - Method for forming silicon oxide thin film by plasma CVD method - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for forming a silicon oxide thin film by a plasma CVD method, and in particular, plasma CVD capable of forming a silicon oxide thin film having improved electrical characteristics in terms of leak current. The present invention relates to a method for forming a silicon oxide thin film by the method.

[Conventional technology]

In the method for forming a silicon oxide thin film by the plasma CVD method, as a raw material gas, a raw material gas containing silicon atoms and a raw material gas containing oxygen atoms, and further a carrier gas if necessary,
Used as mixed in the reaction chamber. In particular, in the conventional method for forming a silicon oxide thin film by the plasma CVD method,
The flow rates of all the above-mentioned source gases are controlled so as to be maintained at a required constant flow rate value according to the environment of the apparatus from the discharge start point to the discharge end point.

[Problems to be Solved by the Invention]

By the way, according to the above-mentioned conventional method for forming a silicon oxide thin film by the plasma CVD method, in consideration of production efficiency and practicality, the flow rate of each raw material gas flowing through the reaction chamber is set relatively high in order to increase the film formation rate. Is set to a high value. Therefore, in the reaction chamber, silicon atoms are supplied to the substrate in a state where the partial pressure of the raw material gas containing silicon is higher in the transient state immediately after the start of discharge in the reaction chamber, compared to the partial pressure in the steady state. The situation occurs where layers are formed. The silicon oxide thin film formed in such a state has a problem that electrical characteristics are poor. Specifically, ions are generated at locations where there is a large amount of leakage current and electrons have escaped. Therefore, for example, such a silicon oxide thin film is used in a TFT.
When it is used as a gate insulating film of (thin film transistor), there arises a problem that the operating characteristics of the TFT vary.

On the other hand, in consideration of the above-mentioned problems, in order to improve the electrical characteristics of the silicon oxide thin film, the flow rate of silicon atoms at the time of occurrence of discharge may be set to a low value. In fact, J. Batey et al. In that paper suggest that it is effective to keep the deposition rate low in order to improve the electrical properties of silicon oxide films (J. Appl. Phys. 60 (9), 1 Nvember 1986). According to the above paper, it is concluded that a silicon oxide thin film having good electrical characteristics cannot be formed unless the film formation rate is suppressed to 0.13 nm / s or less.

However, as described above, it takes a long time to form a thin film at such a film forming rate, and therefore it cannot be adopted from the viewpoint of practicality.

An object of the present invention is to provide a method for forming a silicon oxide thin film by a plasma CVD method, which solves the above-mentioned contradictory problems and is capable of producing a silicon oxide thin film which is practically effective and has good electrical characteristics. It is in.

[Means for Solving the Problems]

A first method for forming a silicon oxide thin film by a plasma CVD method according to the present invention is a method for forming a silicon oxide thin film by mixing at least a source gas containing silicon atoms and a source gas containing oxygen atoms. Regarding the control of the flow rate of the raw material gas containing the gas, it is characterized in that the flow rate in the reaction chamber at the start of the discharge is limited, and then the flow rate is controlled to increase.

The second method for forming a silicon oxide thin film by the plasma CVD method according to the present invention is characterized in that, in the first method, the flow rate of the raw material gas containing silicon atoms in the reaction chamber at the start of discharge is zero. .

A third method for forming a silicon oxide thin film by a plasma CVD method according to the present invention is characterized in that, in the first method, a flow rate of a raw material gas containing a silicon raw material gas in a reaction chamber is small at the start of discharge. And

A fourth method for forming a silicon oxide thin film by plasma CVD according to the present invention is the method for forming a silicon oxide thin film according to the third method, wherein the small flow rate of the raw material gas containing silicon atoms causes the film forming rate of the silicon oxide thin film on the substrate to be 0.2 nm. It is characterized in that the flow rate is less than / s.

A fifth method for forming a silicon oxide thin film by plasma CVD according to the present invention is the method for forming a silicon oxide thin film according to any one of the first to fourth methods, wherein the flow rate of the source gas containing silicon atoms continuously increases after the start of discharge. It is characterized by doing.

A sixth method for forming a silicon oxide thin film by plasma CVD according to the present invention is the method for forming a silicon oxide thin film according to any one of the first to fourth methods, wherein the flow rate of the source gas containing silicon atoms is increased stepwise after the start of discharge. It is characterized by doing.

In the method for forming a silicon oxide thin film in the seventh plasma CVD method according to the present invention, in the method according to any one of the first to sixth methods, the period during which the flow rate of the source gas containing silicon atoms increases is the entire discharge period. It is characterized by

The silicon oxide thin film forming method in the eighth plasma CVD method according to the present invention is the method according to any one of the first to sixth methods, wherein a period during which the flow rate of the raw material gas containing silicon atoms increases is a predetermined period after the start of discharge. It is a period, and the flow rate is maintained at a constant value after a lapse of a predetermined period.

[Operation] According to the method for forming a silicon oxide thin film by the plasma CVD method of the present invention, in any of the above cases, the source gas containing silicon atoms during the period when the discharge is in a transient state at the time of starting the discharge and immediately thereafter By suppressing the flow rate, the state in which the partial pressure of the raw material gas containing silicon was higher than the partial pressure in the steady state was avoided, thereby preventing the formation of a layer containing excessive silicon in the silicon oxide thin film. This reduced the pool-Frenkel emission current, reduced the leakage current, and improved the electrical properties of the silicon oxide thin film. In addition, during the period other than the transitional period in the discharge, the flow rate of the raw material gas containing silicon atoms is increased, thereby increasing the film forming rate to improve the production efficiency of the thin film and satisfy the practical level.

〔Example〕

Embodiments of the present invention will be described below with reference to the accompanying drawings.

FIG. 1 schematically shows an example of a main configuration of a plasma CVD reaction chamber for carrying out the method for forming a silicon oxide thin film by the plasma CVD method according to the present invention. In FIG. 1, reference numeral 1 is a reaction chamber container having a reaction chamber therein, a heater 2 and plates 3 located on both sides of the heater 2 are arranged in the center of the inside thereof, and a plurality of plates are provided on the plate 3 on the right side of the drawing. The substrate 4 is arranged. The reaction chamber container 1 is equipped with two rf electrodes 5A and 5B inside. In this embodiment, it is configured to supply rf power only to the rf electrode 5B on the right side in the figure,
Only 5B is involved in the deposition process. Further, the raw material gas 7 is supplied from the outside into the reaction chamber through a hole 6 formed in a part of the rf electrode 5B and a shower head-shaped gas introduction portion. Further, in the figure, an exhaust port 8 is formed in, for example, the lower wall of the reaction chamber container 1, and the gas inside the reaction chamber is exhausted through the exhaust port 8 by an external pump (not shown).

In the present embodiment, in the above apparatus configuration, for example, silane SiH _{4 is used} as a source gas containing silicon atoms, and laughing gas N ₂ O is supplied as a source gas forming oxygen atoms into the reaction chamber in the reaction chamber container 1, and the substrate 4 A thin film of silicon oxide is formed on the surface of.
No carrier gas was used in this example. In the conventional plasma CVD method, the flow rates of SiH ₄ and N ₂ O in the reaction chamber are maintained at a known constant flow rate from the time when the discharge starts to the time when the discharge ends. On the other hand, in the embodiment of the present invention, the flow rate of N ₂ O is set to the same constant flow rate as the conventional one, but the flow rate of SiH ₄ is not set to the constant flow rate as described below, and is changed depending on the discharge state. The flow rate is changed so that the flow rate is completely different from the conventional one.

How to set the flow rate of the source gas SiH ₄ containing silicon atoms in the reaction chamber is described. In this embodiment, the flow rate of the source gas SiH ₄ flowing into the reaction chamber is controlled so as to change according to the discharge state. Fig. 2 shows an example of changes in the flow rate of the source gas SiH ₄ . In Fig. 2, at time t ₁ , the rf electrode
5B is supplied with rf power, discharge is started in the front space of the substrate 4 and plasma 9 is generated, and at time t ₂ , the supply of rf power is stopped and the discharge is stopped. The time interval T ₁ determined by the times t ₁ and t ₂ is the entire discharge period, which is the film formation time of the silicon oxide thin film according to this embodiment. In this embodiment, the discharge period T ₁
Is set to 200s, for example. During this discharge period T ₁ , N ₂
The flow rate of O is set to a constant flow rate. On the other hand, the SiH ₄ flow rate is varied as shown in FIG. That is, the flow rate is 0 at the discharge start time t ₁ (= 0), and the flow rate of SiH ₄ is, for example, a quadratic function change characteristic during the predetermined period T ₂ in which the discharge state immediately after the discharge start time is transient. And is set to a flow rate of, for example, 30 sccm at time t ₃ (= 90 s), and then the discharge state is maintained at a constant value of 30 sccm until the discharge state is stable and steady. Thus, the flow rate of the source gas SiH _{4 in} the reaction chamber was changed according to the timing of the discharge period, that is, the discharge state.

Predetermined period T given as the difference between the above times t ₁ and t _3
2 is 90 seconds (seconds) in the illustrated embodiment. However, the predetermined period T ₂ is not limited to 90 s, and can be changed within the range of 40 to 100 s depending on the conditions or requirements of the film formation or the apparatus. Since the time from the transitional state to the steady state after the start of discharge is about 40 to 100 s, it is appropriate to select T2 in this range.

As described above, the plasma CV is controlled by controlling the flow rate of the source gas SiH _4.
When the silicon oxide thin film forming method by the D method is executed, the silicon oxide thin film formed on the substrate 4 has a significantly reduced leak current, as is clear from comparison with a conventional method described later,
For example, when used as an insulating film, the electrical characteristics are extremely improved in terms of leak current. at the same time,
Since the flow rate of SiH ₄ was controlled to increase from the middle to the second half of the film formation time, the film formation speed was improved and the practical requirement in terms of production efficiency could be sufficiently satisfied.

Here, the reason why the leakage current of the silicon oxide thin film is reduced by using the method for forming a silicon oxide thin film by the plasma CVD method according to the present invention will be described. The electrical conductivity of a silicon dioxide film formed by thermal oxidation of silicon is Fowler-Nordh
eim) is known to be described. On the other hand, the electrical conductivity of the silicon oxide thin film formed by the plasma CVD method cannot be completely described by the above equation. Particularly in a low electric field region, a current larger than that described by the above equation flows, and a leak current is generated. A study of its physical cause revealed that this incremental amount of leakage current was Poole-Frenkele emission current. Furthermore, the level that emits electrons that contributes to this Pool-Frenkel emission current is due to the portion of the layer of silicon oxide containing excess silicon atoms, which is formed when the discharge state is transient immediately after the start of discharge. It was found to be localized. Therefore, by considering such a cause, it is possible to exclude the formation of a silicon oxide portion having excess silicon atoms at the time of starting the discharge and immediately thereafter.
It is possible to improve the electrical characteristics of the silicon oxide thin film in terms of leakage current. Plasma CV of the present invention
The method for forming a silicon oxide thin film by the method D was invented based on the above idea.

Next, FIG. 3 compares the characteristics of the silicon oxide thin film formed by the conventional plasma CVD method and the silicon oxide thin film formed by the plasma CVD method according to the present invention. Figure 3 shows the plasma SiH _x current In (where the vertical axis
In is 1In = J / E = current density (A / cm ² ) / electric field strength (V / c
3 is a graph showing a current-voltage characteristic between an applied voltage and a current defined by the equation m). In FIG. 3, a is the characteristic by the conventional film forming method, and b is the characteristic by the film forming method of the present invention. As is apparent from this figure, when a voltage V ₁ is applied, for example, there is a difference in current value I ₁ between the conventional film forming method a and the film forming method b according to the present invention. According to the membrane method, the Pool-Frenkel current is reduced by ₁ minute. As described above, according to the film forming method of the present invention, the current value in the low electric field region is low as is clear from the characteristic b. This is the pool
This means that the total number of electron emission centers contributing to the Frenkel current was reduced, and the electrical properties of the silicon oxide thin film formed by the plasma CVD method were improved.

FIG. 4 is a diagram comparing the results of quantitative analysis in the depth direction of a silicon oxide thin film formed on a silicon substrate by the conventional film forming method (a) and the film forming method (b) according to the present invention. In the figure, the horizontal axis represents the sputtering time (corresponding to the depth of the silicon oxide thin film), and the vertical axis represents the signal density by Auger analysis, which shows the distribution structure of the constituent atoms at the interface of the silicon oxide thin film. ing. As is clear from this figure, in the case of the conventional film forming method, the partial pressure of the source gas containing silicon Si formed during the period in which the discharge is transient is higher than the partial pressure in the steady state. Area A that is in a state
However, in the case of the film forming method of the present invention, such a region does not exist.

In the above embodiment, other materials can be used as the source gas. For example, as a raw material gas containing oxygen atoms, NO ₂ , CO ₂ , O _2, etc., as a raw material gas containing silicon atoms, inorganic silicon compounds such as Si ₂ H ₆ , SiF ₄ , organic silicon compounds such as TEOS, and As a carrier gas, Ar, He, N ₂ or the like which is inert or has low reactivity can be used.

Further, the method for forming a silicon oxide thin film by the plasma CVD method according to the present invention can be modified as follows.

The film forming method according to the present invention is characterized in that the flow rate of SiH ₄ is narrowed at the start of discharge and immediately after the start of discharge to limit the flow rate. Therefore, it is not necessary to completely set the flow rate to 0 at the time of starting the discharge, and the flow rate may be a small flow rate that satisfies the required condition. The required condition is a case where the film forming rate on the substrate 4 expected from the low flow rate of SiH ₄ is 0.2 nm / s or less. If this condition is satisfied, the same effect as in the above embodiment can be produced.

In the period of increasing SiH ₄ immediately after the start of discharge, the method of increasing SiH ₄ may be linear, or other functional characteristics may be used. Further, it can be increased by a continuous change or can be increased by a stepwise change. In addition, regarding the period to increase,
As in the above-mentioned embodiment, it is possible to increase only for a predetermined period and then keep it at a constant value.
It is also possible to increase gradually over the entire T ₁ .
In any case, it is possible to obtain the same effect as that of the above embodiment.

〔The invention's effect〕

As is apparent from the above description, the present invention has the following effects.

In the method of forming a silicon oxide thin film by the plasma CVD method, the flow rate of the raw material gas containing silicon atoms in the reaction chamber is limited in a transient state immediately after the start of discharge, thereby forming an excessive SiO region of silicon in the silicon oxide thin film. Since it is prevented, it is possible to reduce the number of emission centers of the pool-Frenkel current, thereby reducing the leak current and forming a silicon oxide thin film with extremely improved electrical characteristics.

In addition, the flow rate of the source gas containing silicon atoms is limited only for the transient period immediately after the start of discharge, and for other periods, the increased required flow rate is set, so that the film formation can be performed quickly. It is extremely effective in terms of thin film production efficiency and practical use.

[Brief description of drawings]

FIG. 1 is a configuration diagram of a main part of an apparatus for carrying out a method for forming a silicon oxide thin film by a plasma CVD method according to the present invention, FIG. 2 is a characteristic diagram showing an example of changes in a flow rate of a source gas SiH ₄ , and FIG. FIG. 4 is a characteristic diagram showing a comparison between the conventional film forming method and the film forming method according to the present invention. FIG. 4 is a structural diagram of a thin film formed by the conventional film forming method and a thin film formed by the film forming method of the present invention. It is a component distribution diagram showing a difference. [Description of reference symbols] 1 ... Reaction chamber container 2 ... Heater 4 ... Substrate 5B ... RF electrode 6 ... Raw material gas supply hole 8 ... Exhaust port

Claims (8)

[Claims] 1. A method of forming a silicon oxide thin film by a plasma CVD method by mixing a raw material gas containing at least silicon atoms and a raw material gas containing oxygen atoms, wherein the raw material gas containing silicon atoms is formed at the start of discharge. The method for forming a silicon oxide thin film by the plasma CVD method, characterized in that the flow rate in the reaction chamber is restricted and then the flow rate is increased. 2. The method for forming a silicon oxide thin film by the plasma CVD method according to claim 1, wherein the flow rate of the source gas containing silicon atoms in the reaction chamber at the start of the discharge is zero. Method for forming silicon oxide thin film by. 3. The plasma CVD method according to claim 1, wherein the source gas containing silicon atoms has a small flow rate in the reaction chamber at the start of the discharge. Method for forming silicon oxide thin film by the method. 4. The method for forming a silicon oxide thin film by the plasma CVD method according to claim 3, wherein the small flow rate of the raw material gas containing silicon atoms is such that the deposition rate of the silicon oxide thin film on the substrate is 0.2 nm / s or less. A method for forming a silicon oxide thin film by a plasma CVD method, which is characterized by a variable flow rate. 5. The method for forming a silicon oxide thin film by the plasma CVD method according to claim 1, wherein the flow rate of the raw material gas containing silicon atoms is continuously increased after the start of the discharge. A method for forming a silicon oxide thin film by a plasma CVD method, which is characterized in that: 6. The method for forming a silicon oxide thin film by the plasma CVD method according to claim 1, wherein the flow rate of the raw material gas containing silicon atoms is increased stepwise after the start of the discharge. A method for forming a silicon oxide thin film by a plasma CVD method, which is characterized in that: 7. The method for forming a silicon oxide thin film by the plasma CVD method according to claim 1, wherein the period during which the flow rate of the raw material gas containing silicon atoms increases is the entire discharge period. Method for forming silicon oxide thin film in plasma CVD method. 8. The method for forming a silicon oxide thin film by the plasma CVD method according to claim 1, wherein the period during which the flow rate of the source gas containing silicon atoms increases is a predetermined period after the start of discharge. The plasma CVD characterized in that the flow rate is maintained at a constant value after the lapse of the predetermined period.
Method for forming silicon oxide thin film in the method.