JP4375154B2 - Method for manufacturing silicon carbide semiconductor device - Google Patents

Description

  The present invention relates to a method for manufacturing a silicon carbide semiconductor device having an impurity region formed by forming a trench in a silicon carbide substrate and then embedding an epitaxial (hereinafter simply referred to as epi) layer therein.

  Conventionally, Patent Document 1 discloses a selective embedding epi technique in silicon carbide. Specifically, an epitaxial technique is disclosed in which a trench is formed on the surface of a silicon carbide substrate by dry etching using an oxide film or the like as a mask, and then the epitaxial region is selectively grown using the mask as it is as a selective growth mask. Has been. Problems caused when the impurity region is formed by the conventional ion implantation method by this epi technique, that is, low activation rate, existence of ion implantation residual defect, increase of leakage current, depth limit of impurity layer It is said that these problems can be solved.

In addition, when the inventors previously formed a trench in a silicon carbide semiconductor having an off angle in the Japanese Patent Application No. 2003-102831, and buried the trench by epi, the carbon film serving as a selective growth mask is formed in a region other than the trench region. It proposes the technology of forming into This describes that the formation of the (0001) facet surface due to the off angle can be prevented.
Japanese Patent Laid-Open No. 11-16840

However, in the technique of Patent Document 1, the use of SiO ₂ is described as a mask serving as both a trench etching mask and a selective growth mask, but the substrate heating of about 1500 degrees is used for the epitaxial growth of α-type silicon carbide such as 4H. There is a problem that the SiO ₂ is melted and the mask pattern is easily lost when the temperature of the substrate is raised before the burying growth.

  In the technique of Japanese Patent Application No. 2003-102831, the use of a carbon film is described as a selective growth mask. However, when epitaxial growth is performed by a normal method, β-type silicon carbide (3C -SiC defects) occurred, and 3C-SiC defects were mixed in the buried region. Further, depending on the growth conditions, there has been a problem that the selective mask after selective growth cannot be easily removed by thermal oxidation.

  In view of the above points, the present invention relates to a method for manufacturing a silicon carbide semiconductor device in which a trench is formed in a silicon carbide substrate and the trench is filled to form an impurity region, so that 3C-SiC defects are not mixed in the impurity region. At the same time, it is an object to make it possible to remove the selection mask steadily.

To achieve the above object, according to the first aspect of the present invention, in the impurity region forming step, the upper surface of the impurity region (7) formed in the trench (5) is a carbon film (2 It is characterized by growing so as to be lower than the upper surface of).

For example, FIG. 9 shows a case where the upper surface of the growth layer in the trench exceeds the upper surface of the carbon film which is a selective growth mask. In such a case, according to experiments by the present inventors, 3C-SiC defects are generated in the growth region where there is no step information of the silicon carbide substrate (1) due to the off-angle of the silicon carbide substrate (1). confirmed.

For this reason, by suppressing the upper surface of the impurity region (7) formed in the trench (5) so as to be lower than the upper surface of the carbon film (2), the lateral expansion of the impurity region (7) is suppressed. 3C-SiC polymorphism that occurs when lateral growth occurs can be suppressed.

Further, in the first aspect of the present invention, after removing the trench etching mask (3), the silicon carbide substrate (1) is heated to an epitaxial growth temperature in an inert gas atmosphere, and then deactivated. The impurity region forming step is performed by switching the gas atmosphere to a mixed gas in which a source gas is mixed with hydrogen as a carrier gas .

  Thus, by making the temperature raising step for raising the temperature to the epitaxial growth temperature an inert gas atmosphere, the etching of the carbon film (2) is eliminated, and the carbon film (2) is retracted from the opening end of the trench (5). It can be lost. As a result, generation of 3C-SiC defects can be prevented.

The invention according to claim 2 is characterized in that the inert gas is He. Since He has high thermal conductivity, the source gas can be efficiently decomposed. He is a gas that can also be used for epitaxial growth of silicon carbide, and does not react with the carbon film (2) even at high temperatures. Therefore, it is possible to prevent the carbon film (2) from retreating from the opening end of the trench (5).

The invention according to claim 3 is characterized in that the inert gas is Ar. Although Ar is inferior in heat conductivity to Ar, it is a gas advantageous in cost as compared with He. Since Ar does not react with the carbon film (2) at a high temperature, it is possible to prevent the carbon film (2) from retreating from the opening end of the trench (5) when the temperature is raised.

In the invention according to claim 4 , in the impurity region forming step, the impurity region (7) formed in the trench (7) and the 3C-SiC film (8) grown on the upper surface of the carbon film (2) are not connected. Thus, the impurity region (7) is grown.

In this case, since 3C-Si grown on the upper surface of the impurity region (7) and the carbon film (2) is not connected, oxygen atoms of the oxidizing species start from the exposed carbon film (2). Since it penetrates into the entire area of the carbon film (2), the carbon film (2) can be easily oxidized, and the carbon film (2) can be easily removed. For this reason, the 3C—SiC film (8) grown on the upper surface of the carbon film (2) can be easily removed from the silicon carbide substrate (1).

In the invention according to claim 5 , in the selective buried layer forming step, the impurity region (7) formed in the trench (5) and the 3C- grown on the upper surface of the carbon film (2) outside the trench (5). The impurity region (7) is grown so that the distance to the SiC film (8) is larger than the film thickness of the carbon film (2).

By thus greater than the distance the thickness of the carbon film (2) between the impurity regions (7) and the 3C-SiC film grown on the top surface of the carbon film (2) (8), the oxidized species The entire region of the carbon film (2) can be easily oxidized without reducing the oxygen atom entry rate. For this reason, the carbon film (2) can be easily removed.

In the invention according to claim 6 , in the impurity region forming step, the growth temperature is 1600 ° C. or higher, the C / Si ratio is 2 or lower, the growth pressure is 1.33 × 10 ⁴ Pa or higher, and the growth rate is 2.5 μm / h. It is characterized by the following.

When grown under these conditions, the migration of the source gas on the carbon film (2) is activated, and the 3C-SiC film grown on the impurity region (7) in the trench (5) and the upper surface of the carbon film (2). Epi growth can be performed so that (8) is not connected.

  In addition, the code | symbol in the bracket | parenthesis of each said means shows the correspondence with the specific means as described in embodiment mentioned later.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. In the following embodiments, the same or equivalent parts are denoted by the same reference numerals in the drawings.

(First embodiment)
A method for manufacturing a silicon carbide semiconductor device in the first embodiment of the present invention will be described. FIGS. 1A to 1F show manufacturing process diagrams of the silicon carbide semiconductor device in the present embodiment, and will be described with reference to these drawings.

[Step shown in FIG. 1 (a)]
First, a silicon carbide substrate 1 is prepared, and an organic photoresist film is uniformly applied on the upper surface thereof by about 3 μm. Next, the organic photoresist film formed on the silicon carbide substrate 1 is carbonized into a carbon film 2 at a temperature of 700 ° C. or higher in a non-oxidizing atmosphere, for example, an Ar atmosphere. At this time, the film thickness of the carbon film 2 is about 1.5 μm, which is half that of the original organic photoresist film.

Unlike the SiO ₂ film, the carbon film 2 does not melt and lose its mask pattern even in an atmosphere of 2000 ° C. or higher, and does not react with the silicon carbide substrate 1 epitaxially. Since it can be easily removed later in an oxidizing atmosphere, it functions sufficiently as a selective growth mask.

  Next, an LTO film (oxide film) 3 as a trench etching mask is formed on the carbon film 2 by 1.5 μm.

[Step shown in FIG. 1B]
Next, the LTO film 3 and the carbon film 2 are sequentially patterned by etching to form, for example, an opening 4 having a width of about 2 μm. For example, the LTO film 3 is dry etched in a CHF ₃ plasma atmosphere, and the carbon film 2 is dry etched in an O ₂ plasma atmosphere.

[Step shown in FIG. 1 (c)]
Next, trench 5 having a depth of, for example, 4 μm is formed in silicon carbide substrate 1 by reactive dry etching using LTO film 3 as a mask.

[Step shown in FIG. 1 (d)]
Next, the LTO film 3 used in the trench etching is completely removed with HF or the like.

[Step shown in FIG. 1 (e)]
The impurity region 7 is formed by epi-growing a buried layer in the trench 5 using a growth apparatus. At this time, epi growth is performed according to the growth profile shown in FIG.

  First, Ar gas is introduced into the growth apparatus to form an Ar atmosphere, and in this state, a temperature raising process is performed in which heating is performed to 1600 ° C. or higher, for example, 1650 ° C. Then, the atmosphere of Ar is maintained until the source gas is introduced, that is, until the substrate heating and holding period after the temperature raising step is completed. Thus, if an inert gas atmosphere such as an Ar atmosphere is used, unlike the hydrogen atmosphere, the carbon film 2 is not etched by hydrogen, so that the carbon film 2 does not recede from the opening of the trench 5.

Subsequently, the impurity region 7 is epitaxially grown by switching the Ar gas to a mixed gas in which propane (C ₃ H ₈ ) and silane (SiH ₄ ), which are source gases, are mixed with hydrogen, which is a carrier gas, and the inside of the trench 5 is grown. It is buried by the impurity region 7. At this time, the impurity region 7 is grown to the lower surface (downward position) of the carbon film 2, and the impurity region 7 that grows in the trench 5 and the 3C—SiC film 8 that grows on the upper surface of the carbon film 2 outside the trench 5. The impurity region 7 is epitaxially grown so that the distance L between them is larger than the film thickness t of the carbon film 2.

Specifically, if the growth temperature is 1600 ° C. or more, the C / Si ratio is 2 or less, the growth pressure is 100 Torr or more, and the growth rate is 2.5 μm / h or less, 3C—SiC grown on the impurity region 7 and the upper surface of the carbon film 2. For example, the growth temperature is 1600 ° C. or more (eg, 1625 ° C.), C / Si is 2 or less (eg, 1.0), and the growth pressure is 1.33 × 10 ⁴ Pa ( = 100 Torr) or more (for example, 2.66 × 10 ⁴ Pa (= 200 Torr)) and the growth rate is 2.5 μm / h or less, the impurity region 7 in the trench 5 and the carbon film 2 outside the trench 5 The distance L between the 3C—SiC film 8 grown on the upper surface is about 10 μm, and the impurity region 7 and the 3C—SiC film 8 can be prevented from being connected.

  Further, when the epitaxial growth is performed, the conductivity type and impurity concentration of the impurity region 7 can be controlled by controlling the flow rate of the mixed gas and mixing nitrogen gas or TMA serving as N-type and P-type dopants into the mixed gas.

[Step shown in FIG. 1 (f)]
Next, thermal oxidation at about 1000 ° C. is performed for about 10 minutes. By this thermal oxidation, oxygen atoms of the oxidizing species enter the entire area of the carbon film 2 starting from the exposed carbon film 2, and the carbon film 2 is easily oxidized. Therefore, if the oxidized carbon film 2 is removed, the 3C—SiC film 8 grown on the upper surface of the carbon film 2 is easily removed from the silicon carbide substrate 1 accordingly.

  Through the above steps, impurity region 7 in which the impurity type and the impurity concentration are controlled is formed in silicon carbide substrate 1.

  Then, the effect by the manufacturing method of the silicon carbide semiconductor device shown by this embodiment is demonstrated.

  (1) In the method for manufacturing the silicon carbide semiconductor device of this embodiment, the growth upper surface of the impurity region 7 formed in the trench 5 is set to be lower than the lower surface of the carbon film 2 serving as a selection mask. That is, by suppressing the growth upper surface of the impurity region 7 formed in the trench 5 up to the lower surface of the carbon film 2, the lateral expansion of the impurity region 7 can be suppressed, and occurs when the lateral growth is performed. It becomes possible to prevent polymorphic incorporation of 3C-SiC.

  (2) Also, in the method for manufacturing the silicon carbide semiconductor device of this embodiment, before introducing the source gas for the epitaxial growth of the impurity region 7, that is, until the substrate heating and holding period after the temperature raising step is completed. Ar atmosphere is used.

In general, when impurity region 7 is formed on silicon carbide substrate 1, epi growth is performed with the growth profile shown in FIG. That is, the substrate is exposed to an atmosphere of hydrogen or HCL gas during the temperature rising process and the substrate etching until the source gases silane (SiH ₄ ) and propane (C ₃ H ₈ ) are flowed.

  For this reason, as shown in FIG. 4A, the carbon film 2 serving as a selective growth mask is partially etched by hydrogen or HCL gas, and the carbon film 2 recedes from the opening end of the trench 5. As a result, in the process of selectively burying and forming the impurity region 7 in the next step, the present inventors have found that a 3C-SiC defect occurs in the region where the carbon film 2 has receded as shown in FIG. It was confirmed by the experiment. This is because there is no step information of silicon carbide substrate 1 due to the off-angle of silicon carbide substrate 1.

  On the other hand, in this embodiment, since the Ar atmosphere is used during the temperature raising step and the substrate heating and holding period after the temperature raising step, the carbon film 2 is not etched. For this reason, it is possible to eliminate the receding of the carbon film 2 from the opening end of the trench 5, and as a result, it is possible to prevent the occurrence of 3C-SiC defects.

  (3) Further, in the method for manufacturing the silicon carbide semiconductor device of this embodiment, when the impurity region 7 is epitaxially grown, 3C—SiC formed on the carbon film 2 in which the impurity region 7 is used as a selection mask; It is not connected.

  If the impurity region 7 in the trench 5 and the 3C-SiC film 8 grown on the carbon film 2 are connected and the exposed region of the carbon film 2 disappears, the carbon film 2 even if thermal oxidation is performed thereafter. Is covered with silicon carbide having a low oxidation rate, oxygen atoms of oxidizing species do not reach the carbon film 2. For this reason, the carbon film 2 used as the selection mask cannot be removed.

  On the other hand, when the epitaxial growth is performed so that the impurity region 7 in the trench 5 and the 3C-SiC film 8 grown on the carbon film 2 are not connected, the oxygen atoms of the oxidizing species start from the exposed carbon film 2. , It penetrates into the entire area of the carbon film 2 and the carbon film 2 is easily oxidized. For this reason, the carbon film 2 can be removed easily, and the 3C—SiC film 8 grown on the upper surface of the carbon film 2 can be easily removed from the silicon carbide substrate 1.

  (4) In the method for manufacturing the silicon carbide semiconductor device of the present embodiment, the distance between the impurity region 7 in the trench 5 and the 3C—SiC film 8 grown on the upper surface of the carbon film 2 outside the trench 5 is The epitaxial growth is performed so as to be larger than the film thickness of the carbon film 2 which is a selection mask.

  Thus, by making the distance between the impurity region 7 and the 3C—SiC film 8 grown on the upper surface of the carbon film 2 larger than the film thickness of the carbon film 2, the entry rate of oxygen atoms of the oxidizing species is reduced. Therefore, the entire area of the carbon film 2 can be easily oxidized, and the carbon film 2 can be easily removed.

Specifically, the growth temperature in epi growth is 1600 ° C. or higher, C / Si is 2 or lower, the growth pressure is 1.33 × 10 ⁴ Pa (= 100 Torr) or higher, and the growth rate is 2.5 μm / h or lower. With such epi growth, migration of the source gas on the carbon film 2 is activated, and the impurity region 7 in the trench 5 and the 3C—SiC film 8 grown on the upper surface of the carbon film 2 outside the trench 5 The distance between the impurity region 7 and the 3C-SiC film 8 can be prevented from being connected.

(Other embodiments)
In the above embodiment, an example has been shown in which the upper surface of the impurity region 7 subjected to the buried growth coincides with the surface of the silicon carbide substrate 1, but in reality it is difficult to make it completely coincide with each other. As shown in FIGS. 5A, 5 </ b> B, and 5 </ b> C, the trench 5 is not completely filled with the impurity region 7.

  Specifically, FIGS. 5A to 5C show the degree of dents remaining in the impurity region 7 when the trench 5 is not completely filled with the impurity region 7. As described above, the degree of the dent remaining in the impurity region 7 can be adjusted according to the growth time at the time of epitaxial growth, and therefore the degree of the dent in the impurity region 7 can be adjusted according to the use of the silicon carbide semiconductor device. That's fine.

  Moreover, in the said embodiment, the impurity region 7 without a 3C-SiC defect is formed by making the growth upper surface of the impurity region 7 not exceed the lower surface of the carbon film 2. This is because when the upper surface of the buried growth layer exceeds the upper surface of the carbon layer, the 3C-SiC polymorph is mixed and lateral growth occurs. However, when the growth upper surface of the impurity region 7 is located between the lower surface of the carbon film 2 and the upper surface of the carbon film 2, the 3C-SiC polymorph is only slightly mixed into the impurity region 7. It can be applied if the use is limited. That is, it is only necessary that the growth upper surface of the impurity region 7 is positioned at least below the upper surface of the carbon film 2.

  Further, as shown in FIG. 6, it is possible to control the growth rate (growth temperature and source gas concentration) of the bottom surface to be larger or smaller than the side surface of the trench 5.

  As another embodiment, the impurity region 7 may be composed of two or more layers, or the impurity concentration may be continuously changed. FIG. 7 shows a case where the impurity region 7 is grown in two layers of a region 7a and a region 7b. Thus, when the impurity region 7 is constituted by two layers, if the region 7a to be the first layer has a low concentration and the region 7b to be the second layer has a high concentration, the resistance is low as a whole. A PN junction with little leakage current can be formed. Further, as another example of two layers, if silicon carbide substrate 1 is N-type, silicon carbide substrate can be obtained by growing first region 7a as P-type and second layer 7b as N-type. Thus, an N-type region 7b electrically isolated from 1 can be formed.

  8 shows an example in which the impurity concentration of the impurity region 7 is gradually changed. FIG. 8A is a cross-sectional view of the silicon carbide substrate 1, and FIG. 8B is a cross-sectional view of FIG. The impurity concentration distribution in the AA 'cross section in FIG. As shown in this figure, for example, by increasing the impurity concentration in accordance with the growth of the impurity region 7, the impurity concentration of the impurity region 7 is high in the central portion of the trench 5, and as the distance from the impurity region increases, The impurity concentration of 7 can be reduced. In this case, it goes without saying that the same function as when a plurality of layers are formed is realized more efficiently than the case where a plurality of layers are formed.

  In the above embodiment, Ar is used as the inert gas, but He may be used.

  Since He has high thermal conductivity, the source gas can be efficiently decomposed. He is a gas that can also be used for epitaxial growth of silicon carbide, and does not react with the carbon film 2 even at high temperatures. Therefore, the receding of the carbon film 2 from the opening end of the trench 5 can also be prevented.

  However, when Ar is used as in the above embodiment, although Ar is inferior in heat conductivity to He, it is a gas advantageous in terms of cost compared to He, so that the manufacturing cost of the silicon carbide semiconductor device is suppressed. Is advantageous.

  Moreover, in the said embodiment, although the carbon film 2 is formed using the organic photoresist film, you may form using the deposited film by sputtering method, or the deposited film by CVD method.

It is a manufacturing process figure of the silicon carbide semiconductor device in a 1st embodiment of the present invention. It is a figure which shows the growth profile when carrying out epi growth of the impurity region. It is a figure which shows the growth profile generally used when performing epi growth. (A) is sectional drawing of the silicon carbide semiconductor device which shows a mode that the carbon film was partially etched by hydrogen and HCL gas, (b) is a 3C-SiC defect formed in the place where the carbon film retreated. It is sectional drawing of the silicon carbide semiconductor device which shows an appearance. It is sectional drawing of the silicon carbide semiconductor device which shows a mode when a trench is not completely embedded by the impurity region. It is sectional drawing of the silicon carbide semiconductor device which shows a mode when the growth rate of an impurity region is changed with the side surface and bottom face of a trench. It is sectional drawing of the silicon carbide semiconductor device at the time of growing an impurity region by two layers. (A) is sectional drawing of the silicon carbide substrate 1, (b) is the figure which showed the impurity concentration distribution in the A-A 'cross section in Fig.8 (a). It is sectional drawing of the silicon carbide semiconductor device which shows a mode when the upper surface of the growth layer in a trench exceeds the upper surface of the selective growth mask.

Explanation of symbols

1 ... silicon carbide substrate, 2 ... carbon film (mask for selective growth),
3 ... LTO film (mask for trench etching), 4 ... opening, 5 ... trench,
7 ... impurity region, 8 ... 3C-SiC film.

Claims (7)

A selective mask forming step of forming a carbon film (2) as a selective growth mask on the upper surface of the silicon carbide substrate (1);
A trench mask forming step of forming a trench etching mask (3) on the upper surface of the carbon film (2);
A mask pattern process for continuously patterning the trench etching mask (3) and the carbon film (2), and dry etching using the trench etching mask (3) to form trenches on the silicon carbide substrate (1). A trench forming step of forming (5);
A temperature raising step of heating the silicon carbide substrate (1) to an epitaxial growth temperature in an inert gas atmosphere after removing the trench etching mask (3);
The inert gas atmosphere is switched to a mixed gas in which a source gas is mixed with hydrogen as a carrier gas, and an impurity region (7) is formed in the trench (5) by an epitaxial growth method so that its upper surface is the upper surface of the carbon film (2). An impurity region forming step of forming so as to be lower than,
And a selective mask removing step for removing the carbon film (2). The method for manufacturing a silicon carbide semiconductor device according to claim 1 , wherein He is used as the inert gas. The method for manufacturing a silicon carbide semiconductor device according to claim 1 , wherein Ar is used as the inert gas. In the impurity region forming step, the impurity region (7) is prevented from connecting the impurity region (7) in the trench (5) and the 3C—SiC film (8) grown on the upper surface of the carbon film (2). the method for manufacturing the silicon carbide semiconductor device according to any one of claims 1 to 3), characterized in that for the growth. In the selective buried layer forming step, the distance between the impurity region (7) in the trench (5) and the 3C-SiC film (8) grown on the upper surface of the carbon film (2) is the carbon film. The method for manufacturing a silicon carbide semiconductor device according to any one of claims 1 to 4 , wherein the impurity region (7) is grown so as to be larger than a film thickness of (2). In the impurity region forming step, the growth temperature is 1600 ° C. or higher, the C / Si ratio is 2 or lower, the growth pressure is 1.33 × 10 ⁴ Pa or higher, and the growth rate is 2.5 μm / h or lower. A method for manufacturing a silicon carbide semiconductor device according to claim 4 or 5 . The method for manufacturing the silicon carbide semiconductor device according to any one of claims 1 to 6, characterized in that the selection mask removing step by thermal oxidation.

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