JP2013045789A - Manufacturing method of silicon carbide semiconductor device - Google Patents

Abstract

Increase in interface state density even when polysilicon is formed on a gate insulating film oxidized in a wet atmosphere on a silicon carbide (000-1) surface by a low pressure CVD method using an inert gas. To suppress the deterioration of the MOS interface characteristics and improve the quality of the silicon carbide semiconductor device.
A gate insulating film is formed so as to be in contact with a semiconductor region having a surface inclined by 0 ° to 8 ° with respect to a (000-1) plane of a silicon carbide semiconductor, and is formed on the gate insulating film by using a low pressure CVD method. When the polysilicon gate conductive film is formed in contact with the substrate, the target furnace temperature to be stabilized while supplying an inert gas is set to 450 ° C. or more and 550 ° C. or less, and the polysilicon gas is injected while injecting the source gas. The furnace temperature was consistently maintained at the target furnace temperature, including the step of generating the gate conductive film, and the step of replacing the source gas with an inert gas after the generation of the gate conductive film.
[Selection] Figure 1

Description

  The present invention relates to a method for manufacturing a semiconductor device using a silicon carbide substrate, and more particularly to a method for manufacturing a silicon carbide semiconductor device characterized by a step of forming a conductive film on a gate insulating film by a low pressure CVD method.

Research and development of semiconductor devices using silicon carbide substrates are underway. Silicon carbide can form an insulating film by thermal oxidation like silicon, but has a characteristic that the channel mobility at the MOS interface differs depending on the crystal plane and the oxidation method.
It is said that the (000-1) plane of a silicon carbide substrate exhibits higher channel mobility than the (0001) plane when oxidized in a wet atmosphere. Note that an interface state density is an alternative index for evaluating channel mobility. Generally, it is known that the channel mobility tends to increase as the interface state density decreases.

  With regard to a method for manufacturing a semiconductor device using such a silicon carbide substrate, Patent Document 1 listed below shows a high value by oxidizing the silicon carbide (000-1) surface in a wet atmosphere in order to reduce the interface state density. A method for obtaining channel mobility is shown. Specifically, annealing is performed in a hydrogen or water vapor atmosphere after gate oxidation.

Japanese Patent No. 4374437

The interface state density of the MOS interface obtained by oxidizing the (000-1) surface of the silicon carbide substrate in a wet atmosphere is a heat treatment in an inert gas atmosphere in a later step, for example, for forming an ohmic contact of metal. It is known that the MOS interface characteristics deteriorate due to an increase in annealing in an inert gas.
That is, it is said that the interface state density is reduced by oxidation in a wet atmosphere because hydrogen or a hydroxyl group terminates the interface state, but the hydrogen terminated by annealing in an inert gas. Alternatively, it is presumed that the interface state density is increased by desorption of the hydroxyl group, and the MOS interface characteristics are deteriorated.

  This MOS interface characteristic degradation due to thermal annealing does not occur in MOS devices fabricated on the silicon carbide (0001) surface, so the device process differs between the MOS devices on the silicon carbide (000-1) surface and the (0001) surface. Need to be devised.

By the way, as the conductive film (gate electrode) formed on the gate insulating film, a metal such as aluminum, polysilicon, or a molten material of polysilicon and metal is used. It is formed on the gate insulating film using a method. In the low pressure CVD method, the inside of a reaction furnace is depressurized with a vacuum pump, and silane gas (SiH ₄ ) is supplied as a source gas (reaction gas) to achieve a uniform film thickness. At that time, the film was formed at a film forming temperature of about 600 ° C., which is the optimum temperature, so as to obtain a desired film forming speed.

With respect to the MOS interface obtained by oxidizing the (000-1) plane in a wet atmosphere under the same conditions, the interface state density when the gate electrode is polysilicon deposited by the low pressure CVD method is There is a problem that it becomes larger than the interface state density in the case of using aluminum deposited by the above method.
In polysilicon film formation in an actual low pressure CVD furnace, in order to stabilize the temperature before flowing SiH ₄ to form polysilicon, an inert gas is flowed to keep it for a predetermined time. The cause is considered to include a process of replacing SiH ₄ with an inert gas after film formation. In other words, it is considered that a high-temperature heat treatment in an inert gas is substantially included, and this is considered to be a factor of desorbing the terminating hydrogen or hydroxyl group and increasing the interface state density.

  Therefore, an object of the present invention is to form an interface state even if polysilicon is formed on the gate insulating film oxidized in a wet atmosphere on the silicon carbide (000-1) surface by low pressure CVD using an inert gas. An object of the present invention is to provide a manufacturing process of a silicon carbide semiconductor device that suppresses an increase in unit density and prevents deterioration of MOS interface characteristics while maintaining film formation efficiency.

In order to solve the above problems, the present invention employs the following semiconductor device manufacturing method. That is,
(1) forming a gate insulating film in contact with the semiconductor region; forming a polysilicon gate conductive film in contact with the gate insulating film in a furnace by using a low pressure CVD method; In the method of manufacturing a semiconductor device having the above structure, the semiconductor region is a region formed by a plane inclined by 0 ° to 8 ° from the (000-1) plane of the silicon carbide semiconductor, and at least a part of the step of forming the gate insulating film. And a thermal oxidation step in a gas containing moisture, and in the film formation step of the gate conductive film, the target furnace temperature to be stabilized while supplying an inert gas is set to 450 ° C. or higher and 550 ° C. or lower, The step of generating the polysilicon gate conductive film while injecting the source gas, and further, after the generation of the polysilicon gate conductive film is completed, the source gas is replaced with the inert gas. The furnace temperature was consistently maintained at the target furnace temperature.

(2) In forming the silicon gate conductive film by the low pressure CVD method, the source gas contains silane (SiH ₄ ) or disilane (SiH ₈ ).

(3) The step of forming the gate insulating film is a step in which thermal oxidation is performed in dry oxygen not containing moisture and then thermal oxidation in a gas containing moisture is combined.

(4) The step of forming the gate insulating film is a step in which after the insulating film is deposited, thermal oxidation in a gas containing moisture is combined.

  According to the present invention, a silicon gate electrode is formed on a gate insulating film formed on a (000-1) plane of a silicon carbide semiconductor by a low pressure CVD method by a thermal oxidation process in a gas containing moisture. In some cases, the target furnace temperature is set to 450 ° C. or higher and 550 ° C. or lower, and the process of generating the polysilicon gate conductive film while injecting the raw material gas is performed. In the entire process for producing a polysilicon gate conductive film by maintaining the furnace temperature consistently at the target furnace temperature, including the step of replacing the source gas with an inert gas, Therefore, it is possible to reliably suppress an increase in the MOS interface state density accompanying the high-temperature annealing at, and to achieve high channel mobility while ensuring appropriate film formation efficiency.

The figure which shows the structure of a MOS capacitor. In the MOS capacitor fabricated on the silicon carbide (000-1) surface shown in FIG. 1, CV measurement was performed for each of the cases where the silicon gate electrode was formed at 570 ° C. and at 530 ° C. The figure which shows distribution of the obtained interface state density.

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

FIG. 1 is a diagram showing a configuration of a MOS capacitor as an example of a silicon carbide semiconductor device, and a manufacturing process thereof will be described below.
[Step 1]
An n-type epitaxial film 2 having a donor density of about 1E16 cm ^{3 is} grown on a (000-1) substrate 1 (0-8 ° off substrate) having a crystal structure of 4H—SiC, grown to 5 to 10 μm, washed, and then subjected to moisture and oxygen. Wet oxidation at 1000 ° C. was performed for 30 minutes in the contained gas to form an insulating film 3 having a thickness of about 50 nm.

[Step 2]
Next, a polysilicon film is formed as the gate electrode 4.
The polysilicon film was formed by the following process. First, in order to stabilize the furnace temperature at the target furnace temperature after introducing the silicon carbide substrate on which the gate insulating film 3 is formed into a low-pressure CVD furnace maintained at a predetermined target furnace temperature described later, The inert gas was held for 30 minutes while flowing N ₂ at 1080 sccm (1080 cc per minute when converted to a reference temperature of 0 ° C.).

Next, while maintaining this target furnace temperature, while introducing N ₂ as a carrier gas at 220 sccm, SiH ₄ was introduced at 1000 sccm for forming a polysilicon film, and PH ₃ was introduced as a doping gas at 80 sccm, and phosphorus-doped poly Silicon was deposited until the film thickness reached 500 nm. The pressure during film formation was kept at 50 Pa. After film formation, the supply of SiH ₄ and PH ₃ was stopped, N ₂ was passed through the furnace at 800 sccm while maintaining the target furnace temperature, and the film was taken out.
In this embodiment, silane (SiH ₄ ) is used as the reaction gas, but disilane (SiH ₈ ) may be used. Further, N ₂ is used as the inert gas, but Ar or He may be used.

[Step 3]
After forming the gate electrode 4, an aluminum pad 5 for electrical measurement was formed to 310 nm by resistance heating vapor deposition, and after patterning by photolithography and etching, an aluminum back electrode 6 was formed to 700 nm by resistance heating vapor deposition.

The results of investigating the influence of the polysilicon gate electrode deposition temperature on the MOS interface characteristics are shown below. The polysilicon gate electrode is formed under various temperature conditions, and the results under two conditions of 530 ° C. and 570 ° C. are shown in FIG. Since the polysilicon film formation rate depends on the film formation temperature, a film formation time of 470 minutes at 530 ° C. and 155 minutes at 570 ° C. was required to obtain a film thickness of 500 nm.
When the completed MOS capacitor was measured by CV and the interface state density was calculated, as shown in FIG. 2, the case where the polysilicon was formed at 530 ° C. was compared with the case where the polysilicon was formed at 570 ° C. The interface state density was lowered.
This is considered to be because the MOS interface deterioration due to the heat treatment in the polysilicon forming process was suppressed by lowering the polysilicon film forming temperature. Since it is known that the interface state density and the channel mobility of the MOSFET have a correlation, by using the present invention, a MOSFET on a silicon carbide (000-1) plane having a high channel mobility is manufactured. be able to.

Since the thermal decomposition of SiH ₄ starts at about 400 ° C., a temperature of at least 400 ° C. is necessary for the film formation of polysilicon, but the film formation speed decreases as the film formation temperature decreases. In order to apply to a practical process, a film forming temperature of 450 ° C. or higher is required.
On the other hand, as a result of experiments, it has been found that it is effective to set the temperature to 550 ° C. or lower in order to effectively suppress the deterioration of the MOS interface and realize high channel mobility.
Therefore, while the target furnace temperature is stabilized to 450 ° C. or higher and 550 ° C. or lower while supplying N ₂ as an inert gas, a polysilicon gate conductive film is formed while injecting silane gas and PH ₃ as a doping gas. In addition, after the generation of the polysilicon gate conductive film is completed, the furnace temperature is always maintained at 450 ° C. or higher and 550 ° C., including the step of replacing silane (SiH ₄ ) with N _2. From the viewpoint of channel mobility and film formation rate, it can be seen that this is optimal. That is, the optimum temperature is selected from the range of 450 ° C. or more and 550 ° C. or less based on the design target value of the film formation rate and the interface state density to be obtained (FIG. 2). do it.
In this embodiment, the deposited silicon is referred to as polysilicon (polycrystalline silicon) for convenience, but the crystallization of silicon does not proceed at a growth temperature of around 530 ° C., and amorphous (amorphous) silicon. Contains many ingredients. However, even if a large amount of amorphous is contained, the gate electrode characteristics are not affected.

  In this example, the gate insulating film is formed by wet oxidation, but the present invention can also be applied to the case where a gas containing moisture is used in a part of the gate insulating film forming step. For example, a process of forming a gate insulating film by re-oxidizing in a gas containing moisture after thermal oxidation with dry oxygen, or re-oxidizing in a gas containing moisture after depositing an oxide film or nitride film by a CVD method The present invention can also be applied to the case where the gate insulating film is formed by a process or a process of thermally oxidizing in a gas containing moisture and then depositing an oxide film or a nitride film by a CVD method.

  In this embodiment, the polysilicon of the gate electrode is doped at the same time as the film formation, but the film may be formed in an undoped state and then doped by thermal diffusion or ion implantation. Alternatively, a metal may be laminated after the polysilicon film is formed, and a silicide metal may be formed by thermal annealing.

  As described above, according to the present invention, the low pressure CVD method is used on the gate insulating film formed on the (000-1) plane of the silicon carbide semiconductor by the thermal oxidation process in the gas containing moisture. In the step of forming the gate conductive film, the target furnace temperature to be stabilized while supplying the inert gas is set to 450 ° C. or higher and 550 ° C. or lower, and the polysilicon gate conductive film is formed while the source gas is injected. In addition, after the formation of the polysilicon gate conductive film is completed, the furnace temperature is consistently maintained at the target furnace temperature, including the step of replacing the source gas with an inert gas. While using an active gas, it is possible to reliably reduce the interface state density at the time of generating the gate conductive film while maintaining a certain degree of film formation efficiency. It can be expected to be widely adopted as a process that allows for granulation.

1 n-type 4H-SiC (000-1) substrate 2 n-type epitaxial film 3 insulating film 4 gate electrode 5 aluminum pad 6 aluminum back electrode

Claims (4)

A semiconductor having a step of forming a gate insulating film so as to be in contact with a semiconductor region and a step of forming a polysilicon gate conductive film in a furnace by using a low pressure CVD method so as to be in contact with the gate insulating film In the device manufacturing method,
The semiconductor region is a region composed of a plane inclined by 0 ° to 8 ° from the (000-1) plane of the silicon carbide semiconductor,
At least part of the step of forming the gate insulating film includes a thermal oxidation step in a gas containing moisture,
In the film formation process of the gate conductive film, the target furnace temperature to be stabilized while supplying an inert gas is set to 450 ° C. or higher and 550 ° C. or lower, and the polysilicon gate conductive film is generated while the source gas is injected. And, after the formation of the polysilicon gate conductive film, the furnace temperature is consistently maintained at the target furnace temperature, including the step of replacing the source gas with the inert gas. A method for manufacturing a silicon carbide semiconductor device, comprising: 2. The method of manufacturing a silicon carbide semiconductor device according to claim 1, wherein in forming the silicon gate conductive film by the low pressure CVD method, silane (SiH ₄ ) or disilane (SiH ₈ ) is contained in a source gas.   The step of forming the gate insulating film is a step in which thermal oxidation is performed in dry oxygen containing no moisture, and then thermal oxidation in a gas containing moisture is combined. 3. A method for producing a silicon carbide semiconductor device according to 2. 3. The silicon carbide semiconductor according to claim 1, wherein the step of forming the gate insulating film is a step of combining the thermal oxidation in a gas containing moisture after depositing the insulating film. Device manufacturing method.

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