WO2011016392A1 - Method for manufacturing silicon carbide semiconductor device - Google Patents

Abstract

Disclosed is a method for manufacturing a silicon carbide semiconductor device wherein warpage of a substrate is suppressed, and a silicon carbide surface smoother than conventional silicon carbide surfaces is provided, while maintaining a high impurity activation rate. The method for manufacturing the silicon carbide semiconductor device wherein an impurity-doped region is provided on the front surface of the silicon carbide substrate is characterized in that the following steps are sequentially performed: a step of implanting impurity ions into the front surface of the silicon carbide substrate; a step of forming carbon films on the front surface and the rear surface of the silicon carbide substrate, respectively; a step of performing activation heat treatment to the silicon carbide substrate using the carbon film as a protection film; and a step of removing the carbon films formed on the front surface and the rear surface after the step of performing activation heat treatment.

Description

Method for manufacturing silicon carbide semiconductor device

The present invention relates to a method for manufacturing a silicon carbide semiconductor device, and more particularly to activation heat treatment after impurity ions are implanted into a silicon carbide substrate.
This application claims priority based on Japanese Patent Application No. 2009-181564 filed in Japan on Aug. 4, 2009, the contents of which are incorporated herein by reference.

Silicon carbide semiconductors have superior characteristics such as higher breakdown voltage, wider energy band gap, and higher thermal conductivity than silicon semiconductors, so light emitting elements, high power power devices, high temperature resistant elements, radiation resistant elements Applications to devices, high frequency devices, etc. are expected.

In order to form an element (SiC semiconductor element) using the silicon carbide semiconductor, for example, an epitaxial growth layer is formed as an active region of a semiconductor element on a silicon carbide substrate (SiC substrate), and this epitaxial growth layer is selected. It is necessary to control the conductivity type and carrier concentration in the region. Therefore, it is possible to form various p-type or n-type impurity doped regions by partially injecting impurity dopant atoms into the epitaxial growth layer, which is an active region, and to configure semiconductor elements such as transistors and diodes. Become.

Incidentally, in order to activate the impurities ion-implanted into the active region of the silicon carbide substrate, it is necessary to perform an annealing process at a very high temperature (for example, 1600 ° C. to 2000 ° C.). It is known that this annealing process at high temperature vaporizes Si atoms on the surface of the silicon carbide substrate, making the surface rich in carbon (hereinafter referred to as C), causing surface roughness and bunching, which adversely affects device characteristics. It has been. Therefore, even when a transistor or a diode is formed using such a silicon carbide substrate having such a surface, there is a problem that it is difficult to obtain electrical characteristics expected from the excellent physical properties of SiC. .

Therefore, a high-temperature annealing method that can suppress surface roughness of the silicon carbide substrate has been proposed (Patent Documents 1 to 3). Specifically, Patent Document 1 discloses that an SiC substrate is formed by depositing a diamond-like carbon (DLC) film or an organic film as a protective film on an epitaxial layer serving as an active region and performing activation annealing (activation heat treatment). A high-temperature annealing method that suppresses surface roughness is disclosed.

Patent Document 2 discloses a high-temperature annealing method that prevents the occurrence of surface roughness by performing activation annealing using a film obtained by carbonizing a resist layer formed on an active region as a protective film.
Patent Document 3 discloses a high-temperature annealing method for preventing the occurrence of surface roughness due to activation annealing by forming a carbon film by sputtering on an active region and using it as a protective film, and defining the purity of the carbon film. Is disclosed.

JP 2001-68428 A JP 2007-281005 A JP 2005-353771 A

In the inventions disclosed in Patent Documents 1 to 3 described above, a protective film is formed only on the front surface (activated surface or epi surface) of the silicon carbide substrate before the high-temperature annealing treatment that is the activation heat treatment. It was. However, in this case, thermal stress due to the difference in thermal expansion coefficient between the protective film and the silicon carbide substrate exists only on the front surface side of the silicon carbide substrate, and there is no thermal stress on the back surface. Further, the front surface side of the silicon carbide substrate is affected by the residual stress (internal stress) of the protective film, but is not affected by the back surface side having no protective film. Thus, when the protective film is formed only on the front surface of the silicon carbide substrate, stress due to the film formation exists asymmetrically on both surfaces of the substrate, resulting in high temperature activation annealing. Sometimes the substrate warps. In addition, as will be described in detail later, when there is a protective film only on the front surface side of the silicon carbide substrate, the front surface and the back surface of the silicon carbide substrate are heated differently, so the front surface A difference in temperature occurs between the back surface and the back surface, and this temperature difference causes a difference in thermal expansion, which causes warpage of the substrate. The warpage of the substrate becomes particularly noticeable as the diameter of the substrate increases.
Furthermore, when the back surface is used as an element function, such as the back electrode of the silicon carbide substrate, it is necessary to suppress surface roughness on the back surface side of the silicon carbide substrate.

The present invention has been made in view of the above circumstances, and not only surface roughening and bunching due to high-temperature annealing treatment, but also warping of the substrate is suppressed, and carbonization is smoother than before while maintaining a high impurity activation rate. It is an object of the present invention to provide a method for manufacturing a silicon carbide semiconductor device having a silicon surface.

In FIG. 1, the conceptual diagram of the heating of the board | substrate by a radiant heat is shown about the case where it has on both sides when it has a protective film (carbon film) on one side of a silicon carbide substrate.
In FIG. 1, reference numeral 1 is a silicon carbide substrate provided with a protective film, reference numeral 2 is a susceptor, reference numeral 2A is a susceptor body, reference numeral 2B is a susceptor lid, and reference numeral 2a is a sample stage.
When the silicon carbide substrate 1 is heat-treated in the annealing furnace, when a protective film is formed only on one surface, the radiant heat from the susceptor 2 is first applied to the protective film on the surface having the protective film (front surface). The protective film is absorbed and heated, and the front surface is heated by heat conduction from the heated protective film. On the other hand, since silicon carbide substrate 1 is translucent on the surface (back surface) that does not have a protective film, radiant heat passes through the back surface. Therefore, the back surface is heated by heat conducted from the front surface through the substrate. As described above, the surface of the silicon carbide substrate 1 that has the protective film and the surface that does not have a different mode of heating, resulting in a temperature difference between the two surfaces. Since the difference in thermal expansion between the respective surfaces occurs in accordance with this temperature difference, as described above, it causes the warpage of the substrate during high-temperature annealing.
On the other hand, when the protective film is provided on both surfaces of silicon carbide substrate 1, there is no difference in the mode of heating by the surface, so that the substrate can be uniformly heated.

The present invention provides the following means. (1) A method of manufacturing a silicon carbide semiconductor device having an impurity doped region on a front surface of a silicon carbide substrate, the step of implanting impurity ions into the front surface of the silicon carbide substrate, and the silicon carbide substrate After the step of forming a carbon film on the front surface and the back surface, the step of activating heat treatment of the silicon carbide substrate using the carbon film as a protective film, and the step of activating heat treatment, the front surface And a step of removing the carbon film on the back surface in order. (2) The foregoing item, wherein the carbon film is any one of a carbon film formed by sputtering or CVD, a DLC (diamond-like carbon) film, or a carbon film formed by carbonizing an organic film ( A method for manufacturing a silicon carbide semiconductor device according to 1). (3) The method for manufacturing a silicon carbide semiconductor device according to any one of (1) and (2), wherein the activation heat treatment is performed at a heating temperature of 1600 to 2000 ° C. (4) The activation heat treatment is heating by any one of a high-frequency heating method, a lamp heating method, and a vacuum thermoelectron impact method, according to any one of the above items (1) to (3), A method for manufacturing a silicon carbide semiconductor device. (5) The silicon carbide semiconductor device according to any one of (1) to (4), wherein the activation heat treatment is performed in an argon atmosphere or a reduced pressure atmosphere of 1 × 10 ⁻² Pa or less. Method.

Even if impurity ions are implanted into the silicon carbide substrate and then a high temperature activation heat treatment is performed, the warpage of the substrate is suppressed. This effect is particularly effective for large-diameter substrates. Further, since the back surface of the substrate is smooth and the surface roughness is suppressed, the back surface can be used as an element function like a back electrode.

It is a conceptual diagram of the heating of the board | substrate by a radiant heat about the case where a protective film is formed only on the single side | surface of a silicon carbide substrate, and the case where it forms into a film on both surfaces. (A)-(d) is process sectional drawing which shows the manufacturing method of the silicon carbide semiconductor device of this embodiment. It is a figure which shows the curvature before and behind annealing of a 3 inch silicon carbide substrate, and the measurement result of WARP. It is a figure which shows the result of the surface morphology by atomic force microscope (AFM) observation of the front surface and back surface of a silicon carbide substrate.

Hereinafter, a method for manufacturing a silicon carbide semiconductor device according to an embodiment to which the present invention is applied will be described in detail with reference to the drawings. In addition, in the drawings used in the following description, in order to make the features easy to understand, there are cases where the portions that become the features are enlarged for the sake of convenience, and the dimensional ratios of the respective components are not always the same as the actual ones. Absent. *

2 (a) to 2 (d) are process cross-sectional views illustrating the method for manufacturing the silicon carbide semiconductor device of the present embodiment. The method for manufacturing a silicon carbide semiconductor device of this embodiment includes a step of implanting impurity ions into the front surface of a silicon carbide substrate (impurity implantation step), and a step of forming carbon films on both surfaces of the silicon carbide substrate (protection). Film formation step), a step of activating heat treatment of the silicon carbide substrate using the carbon film as a protective film (activation heat treatment step), and a step of removing the carbon film (protective film removal step). These steps are sequentially performed to manufacture a silicon carbide semiconductor device having an impurity doped region on the front surface of the silicon carbide substrate.

(Impurity implantation process)
First, in the impurity implantation step, impurities are implanted into the front surface of the silicon carbide substrate. Specifically, first, as shown in FIG. 2A, an epitaxial substrate 1 in which an n type epitaxial layer 2 is grown on an n ⁺ type silicon carbide substrate 3 is used as a silicon carbide substrate. The epitaxial substrate 1 is preferably a smooth surface with a small surface roughness, for example, Ra <1 nm or less.

Next, a mask for impurity ion implantation is formed on the surface of the epitaxial layer. This mask covers a part of the surface of the epitaxial layer, and an opening is provided in a region where a p-type region (impurity region) is to be formed by impurity ion implantation. Then, impurity ions, for example, aluminum (Al) ions 6 for forming a p-type region are implanted into the surface layer of the epitaxial layer exposed from the opening in multiple stages using six types of acceleration voltages. Specifically, a total of six stages of ion implantation are performed with acceleration voltages of 240 kV, 150 kV, 95 kV, 55 kV, 27 kV, and 10 kV (six-stage implantation method). The implanted Al concentration is, for example, 2 × 10 ¹⁹ cm ⁻³ or 2 × 10 ²⁰ cm ⁻³ . By such an impurity implantation step, an impurity ion implantation layer is formed as shown in the figure.

(Protective film formation process)
Next, as shown in FIG. 2B, carbon films 4 and 5 are formed on the front surface and the back surface of the epitaxial substrate (silicon carbide substrate) in the protective film forming step. Specifically, first, the mask used for impurity ion implantation is removed. Subsequently, a carbon film as a protective film is formed on both the front surface and the back surface of the epitaxial substrate.

The carbon films 4 and 5 as protective films are formed by sputtering or CVD, formed by DLC (diamond-like carbon) film by high-frequency plasma CVD, or carbon film obtained by carbonizing an organic film such as a resist. Can be used.

The film thickness of the carbon film is preferably 10 to 500 nm, more preferably 30 to 200 nm, and particularly preferably 50 to 150 nm when formed by sputtering or CVD. If the film thickness of the carbon film is less than 10 nm, the function as a protective film becomes insufficient in the activation heat treatment step described later, which is not preferable. Further, if the carbon film thickness exceeds 500 nm, it is not preferable because the substrate is warped or cracked. Furthermore, it is not preferable because it becomes difficult to remove the carbon film in the protective film removing step described later. On the other hand, if the film thickness of the carbon film is in the above range, the substrate can be prevented from warping or cracking during the activation heat treatment, and the sublimation of Si atoms from the surface of the epitaxial substrate 1 can be suppressed and protected. It is preferable because removal is easy in the film removal step.

The carbon films 4 and 5 are formed on the front surface and the back surface of an epitaxial substrate (silicon carbide substrate) as follows, for example.
When forming a carbon film by sputtering, first install the epitaxial substrate 1 with the front side facing the sputtering source and the back side contacting the substrate mounting side. A carbon film is formed on the substrate. Thereafter, the substrate is turned over, and the back surface side is directed to the sputtering source side, the front surface side is placed in contact with the substrate mounting side, and film formation is performed on the back surface side.
When forming a carbon film by the CVD method, first install the epitaxial substrate 1 so that the front surface side faces the gas phase reaction atmosphere side (plasma atmosphere side) and the back surface side contacts the substrate mounting side. Then, a carbon film is formed on the front surface side. Thereafter, the substrate is turned over, and the back side is directed to the gas phase reaction atmosphere side (plasma atmosphere side) and the front side is in contact with the substrate placement side, and film formation is performed on the back side.
In the case of a carbon film obtained by carbonizing an organic film such as a resist, the organic film is applied to both the front surface side and the back surface side of the epitaxial substrate 1 to about 3 μm and baked under predetermined conditions. Thereafter, a carbon film is formed under a predetermined heating condition in a heating furnace in an argon atmosphere.

(Activation heat treatment process)
Next, as shown in FIG. 2C, the impurity doped region is formed by activating heat treatment of the epitaxial substrate 1 using the carbon films 4 and 5 as protective films on both sides. The activation heat treatment is performed by a vacuum annealing method of less than 1 × 10 ⁻² Pa. The heating temperature is preferably in the range of 1600 to 2000 ° C, more preferably in the range of 1700 to 1900 ° C, and most preferably in the range of 1700 to 1850 ° C. When the heating temperature is less than 1600 ° C., the activation of the implanted impurities becomes insufficient, which is not preferable. On the other hand, if the temperature exceeds 2000 ° C., the surface of the epitaxial substrate 1 may be carbonized and roughened even if there is a protective film.

The heating time is preferably 1 to 10 minutes, more preferably 1 to 7 minutes, and particularly preferably 1 to 5 minutes. When the heating time is less than 1 minute, the activation of impurities becomes insufficient, which is not preferable. Also, if the heating time exceeds 10 minutes, the surface of the epitaxial substrate may be carbonized and roughened even if a protective film is present, which is not preferable.

(Protective film removal process)
Next, as shown in FIG. 2D, the carbon film used as the protective film is removed. The carbon film is removed by ashing the carbon film by thermal oxidation in an oxygen atmosphere. Specifically, the epitaxial layer 2 and the impurity ion-implanted layer are formed by using a condition that a substrate is placed in a thermal oxidation furnace and oxygen is supplied at a flow rate of 3.5 L / min and heated at 1125 ° C. for 90 minutes. 7 and the carbon film 5 on the back surface of the epitaxial substrate can be removed.
The epitaxial substrate 1 is placed on a substrate mounting (quartz boat or the like) in an oxidation furnace so that both surfaces of the substrate are sufficiently exposed to an oxygen atmosphere, and the carbon films on both surfaces of the substrate can be simultaneously ashed and removed. In this embodiment, the activation rate of aluminum is about 80%, and sufficient activation is performed. By such a protective film removing step, a silicon carbide semiconductor substrate (wafer) having an impurity doped region 8 with a high activation rate as shown in FIG. 1 and a smooth surface can be manufactured. A silicon carbide semiconductor device can be manufactured, for example, by forming a Schottky diode on the silicon carbide semiconductor substrate including such a surface.

The technical scope of the present invention is not limited to the above-described embodiment, and various modifications can be made without departing from the spirit of the present invention. For example, in the present embodiment, the activation heat treatment step is performed using a reduced pressure heating furnace, but a heating furnace in an inert gas atmosphere such as argon (Ar) may be used. As the heating method, a lamp heating or a high frequency method may be used, or an electron beam heating method may be used.

In this embodiment, the carbon film is removed using thermal oxidation, but the carbon film can also be removed by plasma treatment using oxygen or ozone treatment.

Hereinafter, the effects of the present invention will be described in detail with reference to examples. The present invention is not limited to these examples.

The growth of the epitaxial film on the SiC single crystal wafer was performed using a high-frequency induction heating horizontal CVD (Chemical Vapor Deposition) apparatus.
Specifically, a SiC single crystal wafer was placed horizontally on a susceptor, heated to 1620 ° C. in a 200 mbar hydrogen gas decompression atmosphere, and a SiC epitaxial film having a thickness of 8 μm was formed. Hydrogen was used as the carrier gas, and a mixed gas of SiH ₄ and C ₃ H ₈ was used as the source gas.
Thus, an epitaxial substrate in which an epitaxial film having a thickness of 8 μm was formed on a SiC single crystal substrate having a diameter of 3 inches and a thickness of 380 μm was produced.

Next, Al ions were implanted into the SiC epitaxial substrate having a diameter of 3 inches. As the Al ion implantation conditions, a six-stage implantation method (acceleration voltages of 240 kV, 150 kV, 95 kV, 55 kV, 27 kV, and 10 kV in total 6 stages) was used. The Al concentration after the implantation was 2 × 10 ¹⁹ cm ⁻³ .
After the implantation of Al ions, carbon films were formed on both the front surface and the back surface of the SiC substrate by carbon films obtained by carbonizing the resist coating film. As the above conditions, first, a resist film was applied to the front surface of the SiC substrate by about 3 μm and then baked, and then a resist film was applied to the back surface of the SiC substrate by about 3 μm and baked.
Thereafter, carbonization was performed under the following conditions.
The conditions for the carbonization treatment were as follows: in an argon atmosphere, the temperature was raised from room temperature to 800 ° C. over 1 hour, then held at 800 ° C. for 10 minutes, and then lowered to room temperature in about 7 hours.

Next, using a vacuum annealing furnace, the pressure was reduced to 5 × 10 ⁻⁴ to 5 × 10 ⁻³ Pa or lower, and an impurity activation heat treatment was performed at a temperature of 1830 ° C. and a holding time of 5 minutes. Finally, the carbon film was ashed and removed by thermal oxidation (1125 ° C., 90 minutes) in an oxygen atmosphere, and the silicon carbide semiconductor device of Example 1 was manufactured.

(Result of substrate warpage)
A substrate flatness measuring instrument for the 3-inch silicon carbide semiconductor device of the first embodiment (the present invention) and the 3-inch silicon carbide substrate (conventional example) having the same manufacturing conditions as those of the first embodiment and having a carbon film only on one side. “Warp” and “WARP” were measured by (manufacturer name: Corning Tropel, device name: Ultrasort). FIG. 3 shows the results of Example 1.
In the conventional example, “warp” and “WARP” before annealing are 26.215 μm and 27.215 μm, respectively, but after annealing, they are 76.293 μm and 76.377 μm, respectively. Warpage is recognized.
On the other hand, in the case of this example, “warp” and “WARP” before annealing are 15.593 μm and 14.408 μm, respectively, and 16.498 μm and 14.001 μm after annealing, respectively. It was confirmed that the warpage was suppressed.

(Results of the front and back surface conditions of the substrate)
FIG. 4 shows the results of surface morphology (Rms) observed by atomic force microscope (AFM) observation of the front and back surfaces of the 3-inch silicon carbide semiconductor device of Example 1 above. The scanning area in FIG. 4 is 2 μm × 2 μm. The height scale is shown in the figure. The Rms of the front surface and the back surface of the substrate were both 0.3 nm or less, and the same surface roughness suppressing effect as that of the front surface was confirmed for the back surface.

The method for manufacturing a silicon carbide semiconductor device of the present invention is not limited to surface roughness and bunching due to high-temperature annealing treatment, but also suppresses warping of the substrate, while maintaining a high impurity activation rate and smoothing silicon carbide. It can be used for manufacturing a silicon carbide semiconductor device having a surface. In particular, a substrate having a large diameter can be kept free from warping after the activation heat treatment, and thus is particularly effective for manufacturing a silicon carbide semiconductor device using a large diameter substrate. Further, the back surface of the silicon carbide substrate can also be used for manufacturing a silicon carbide semiconductor device in which the element function is used.

DESCRIPTION OF SYMBOLS 1 Epitaxial substrate 2 Epitaxial layer 3 Silicon carbide substrate 4 Carbon film on the front surface 5 Carbon film on the back surface 6 Impurity ions 7 Impurity ion implantation layer 8 Impurity doped layer

Claims (5)

A method for manufacturing a silicon carbide semiconductor device having an impurity doped region on a front surface of a silicon carbide substrate,
Implanting impurity ions into the front surface of the silicon carbide substrate;
Forming a carbon film on the front and back surfaces of the silicon carbide substrate;
Activating heat treatment of the silicon carbide substrate using the carbon film as a protective film;
A method for manufacturing a silicon carbide semiconductor device, wherein the step of removing the carbon film on the front surface and the back surface is sequentially performed after the step of performing the activation heat treatment. The carbon film is any one of a carbon film formed by sputtering or CVD, a DLC (diamond-like carbon) film, or a carbon film formed by carbonizing an organic film. A method for manufacturing a silicon carbide semiconductor device. 2. The method of manufacturing a silicon carbide semiconductor device according to claim 1, wherein the activation heat treatment is performed at a heating temperature of 1600 to 2000 ° C. 2. The method of manufacturing a silicon carbide semiconductor device according to claim 1, wherein the activation heat treatment is heating by any one of a high-frequency heating method, a lamp heating method, and a vacuum thermoelectron impact method. 2. The method for manufacturing a silicon carbide semiconductor device according to claim 1, wherein the activation heat treatment is performed in an argon atmosphere or a reduced pressure atmosphere of 1 × 10 ⁻² Pa or less.

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