JP6108588B2 - Method for manufacturing silicon carbide semiconductor element - Google Patents

Description

  The present invention relates to a method for manufacturing a silicon carbide single crystal semiconductor element, and more particularly, to a method for manufacturing a silicon carbide semiconductor element that prevents a SiC semiconductor surface from being roughened during heat treatment in a semiconductor device using an SiC semiconductor.

  In recent years, silicon carbide (SiC) has attracted attention as one of semiconductor materials that can replace silicon (hereinafter referred to as Si). Since SiC has a band gap of 4H-SiC, which is 3.25 eV, which is nearly three times larger than the Si band gap (1.12 eV), the upper limit temperature of operation can be increased. Also, the dielectric breakdown electric field strength is 3.0 MV / cm for 4H-SiC, which is about an order of magnitude higher than the dielectric breakdown electric field strength of Si (0.25 MV / cm). The resistance (ON resistance) of the element that works in the reciprocal is reduced, and the power loss in the steady state can be reduced.

Furthermore, the thermal conductivity is 4.9 W / cmK for 4H-SiC, which is more than three times higher than the thermal conductivity of Si (1.5 W / cmK). It also has the advantage of being able to. Since the saturation drift speed is as large as 2 × 10 ⁷ cm / s, it is excellent in high-speed operation. For these reasons, SiC is expected to be applied to power semiconductor elements (hereinafter referred to as power devices), high-frequency devices, high-temperature operating devices, and the like.

  Currently, MOSFETs, pn diodes, Schottky diodes, etc. are manufactured using SiC, and devices that exceed the characteristics of Si in terms of withstand voltage and on-resistance (ie, forward voltage / forward current during energization) have been developed one after another. The mass production is gradually being accelerated.

  The fabrication of these SiC elements requires a technique for controlling the conductivity type and carrier concentration in selected regions. As the method, there are a thermal diffusion method and an ion implantation method. In SiC, since the diffusion coefficient of impurities is very small, the thermal diffusion method widely used in Si semiconductors is difficult to apply to SiC. Therefore, an ion implantation method is usually used for SiC (for example, see Patent Document 1 below). As ion species to be implanted, nitrogen (N) and phosphorus (P) are used for n-type, and aluminum (Al) or boron (B) is often used for p-type.

  After ion implantation for SiC, in order to activate the ions, it is necessary to perform an activation heat treatment at a temperature of, for example, around 1700 ° C. However, it is known that surface roughening occurs when the heat treatment is performed without protecting the surface. One cause of surface roughness is sublimation of Si atoms near the surface at high temperatures. The other is surface roughness due to step bunching. Step bunching refers to, for example, an epitaxial layer grown on a SiC substrate tilted by about 8 ° in the [11-20] direction from the (0001) plane of 4H—SiC (this angle is referred to as an off angle). Since the crystal grows in the lateral direction, the growth step at the end of each atomic layer is integrated under certain conditions, and the surface becomes uneven.

  After the activation heat treatment, for example, a thermal oxide film is formed in the case of a MOS element having an insulated gate structure, and a Schottky electrode is formed in the case of a Schottky diode. However, there is a problem that the above-described surface roughness causes dielectric breakdown or leakage and causes deterioration of element characteristics.

  As one method for preventing this surface roughness, it has been reported that the surface roughness can be reduced by heat treatment in Ar gas to which SiH4 gas is added. In this method, special specifications for safely using the special material gas are required for each part such as the supply of the material gas, the apparatus main body, and the exhaust of the residual gas, which raises a problem that the apparatus cost increases. As another method for preventing the surface roughness, it is known that the surface roughness is reduced by performing a heat treatment after forming a carbon film on the surface after ion implantation. The carbon protective film is formed by sputtering, chemical vapor deposition (CVD), or carbonizing by applying a resist.

  In the case of the CVD method, the cost of each part such as supply of a hydrogen material gas used for a carrier gas or the like, apparatus main body, exhaust of residual gas, and the like becomes high. In the case of the method of carbonizing by applying a resist, a large amount can be formed at one time, but the utilization efficiency of the material itself is low and the number of steps for carbonization increases, so it is difficult to say that the method is generally effective. For this reason, generally, sputtering is performed with Ar gas using a carbon target, and a heating film formation temperature of, for example, 200 to 600 ° C. is used.

JP 2001-68428 A

  However, in the above sputtering method, sublimation of Si atoms near the surface (the front side of the SiC substrate) and surface roughness due to step bunching can be suppressed, but the depth from the front side is 2 to 10 nm, the width There is a problem that a linearly curved streak-like dent such as a worm is generated at a location of about 100 nm having a length of about 1 μm.

  In view of the above problems, an object of the present invention is to provide a method for manufacturing a silicon carbide semiconductor element capable of preventing the surface roughness of the silicon carbide substrate after the activation heat treatment, keeping the surface smooth, and manufacturing a device having good characteristics at low cost. And

In order to achieve the above object, a method for manufacturing a silicon carbide semiconductor device according to the present invention provides impurities by ion implantation in a surface layer of a silicon carbide substrate on which a silicon carbide single crystal substrate and a silicon carbide epitaxial film are formed as a semiconductor device material. Forming a region, forming a carbon protective film by depositing non-graphitized amorphous carbon on the surface of the silicon carbide substrate using a room temperature sputtering apparatus that does not heat the silicon carbide substrate, and The carbon protective film includes a step of generating a carbon bond of graphite by activating heat treatment of the silicon carbide substrate and a step of removing the carbon protective film, whereby the carbon protective film is subjected to Raman spectroscopic measurement before the activation heat treatment. the peak of scattering intensity at a wavenumber 1280～1380Cm ^-1 and 1550～1650Cm ^-1 by Is not, the peak of the scattering intensity possess respectively after the activation heat treatment, characterized by having a peak of scattering intensity in 1500～1550Cm ^-1 and 1700 cm ^-1 before and after the activation heat treatment.

  The carbon protective film formed on the silicon carbide substrate has a thickness of 10 nm to 1000 nm, preferably 10 to 100 nm, and more preferably 10 to 30 nm.

Further, the sputtering power density of the sputtering apparatus, 1W / cm ² or less, preferably 0.5 W / cm ² or less, further preferably characterized in that 0.35 W / cm ² or less.

  According to the above configuration, it is possible to prevent local cracks in the carbon protective film by depositing amorphous carbon that has not been graphitized during the formation of the carbon protective film and performing the activation heat treatment. Further, it is possible to suppress streak-like dents, prevent the SiC substrate surface after activation heat treatment from being rough and keep it smooth, and manufacture a device having good characteristics at low cost. In addition, since the carbon protective film is formed at room temperature, the temperature rise time to the set substrate temperature can be shortened, and the heating mechanism of the sputtering equipment body is not required, improving the productivity and enabling low-cost device fabrication. It becomes.

  According to the present invention, it is possible to prevent the surface roughness of the silicon carbide substrate after the activation heat treatment from being kept smooth and to produce a device having good characteristics at low cost.

It is a figure which shows the manufacturing process of the silicon carbide semiconductor element of this invention. It is a graph which shows the Raman spectrum state before the heat activation process of the carbon protective film produced by the Example of this invention. It is a graph which shows the Raman spectroscopy state after the heat activation process of the carbon protective film produced by the Example of this invention. It is a graph which shows the Raman spectrum state of the carbon protective film in the comparative example which varied the sputtering conditions of the carbon protective film. It is an AFM image which shows the surface roughness of the SiC substrate after the removal of the carbon protective film produced by the Example of this invention. It is an AFM image which shows the surface roughness after the removal of the carbon protective film in a comparative example. It is a graph which shows the result of QBD measurement.

  Exemplary embodiments of a method for manufacturing a silicon carbide semiconductor device according to the present invention will be explained below in detail with reference to the accompanying drawings.

  The inventors have found that the streak-like dent described in the above-mentioned problem exists before the carbon protective film is removed by ashing, and therefore, due to the difference in linear expansion coefficient between the carbon protective film and SiC, It was found that local cracks occurred and Si atoms were sublimated at the locations where cracks occurred.

For this reason, in this invention, the amorphous carbon which is not graphitized is deposited at the time of carbon protective film formation, and the local crack at the time of activation heat processing is prevented. The non-graphitized amorphous carbon is formed by, for example, room temperature sputtering, a sputtering power density of 0.5 W / cm ² and a film thickness of 20 nm.

Heating the SiC substrate during sputter formation of the carbon protective film generates graphite carbon bonds. The carbon bond of graphite can be confirmed by measurement by Raman spectroscopy. One of the carbon bonds is a Raman active mode (vibration mode) of graphite, which is a so-called G band, a peak of scattering intensity of 1550 to 1650 cm ⁻¹ , and the other is a so-called D band, 1280 to 1380 cm ^−1, which is derived from graphite defects. The peak of the scattering intensity can be confirmed. When the carbon bond is present in the carbon protective film, local cracks are generated by a rapid temperature increase rate of 1 ° C./sec or more during the activation heat treatment.

When the sputtering power density is too high, such as 5 W / cm ² , similarly to the heating of the SiC substrate, the SiC substrate is heated by plasma to generate carbon bonds of graphite, resulting in local generation of the carbon protective film. . However, this is not the case in the case of a sputtering apparatus having a SiC substrate cooling mechanism.

  The film thickness of the carbon protective film is preferably 10 nm or more, which can uniformly cover the SiC substrate surface by random deposition, and is preferably 1000 nm or less from the tact time in consideration of productivity.

(Example)
Next, a method for forming a carbon protective film that does not generate local cracks during the activation heat treatment will be described with reference to examples. In addition, a present Example is an example for obtaining the effect of this invention, such as a film forming method and conditions, and does not represent all conditions.

FIG. 1 is a diagram showing a process for manufacturing a silicon carbide semiconductor element of the present invention. As shown in FIG. 1 (a), the silicon carbide semiconductor device 1 of the present invention has an n-type SiC epitaxial structure with a 4H-SiC substrate (4 ° off substrate) 11 and a donor density of 1 × 10 ¹⁶ / cm ^3. A silicon carbide (SiC) substrate on which the film 12 is grown by 5 μm is used. In the following description, the 4H-SiC substrate 11 and the SiC epitaxial film 12 are referred to as the SiC substrate 11.

As shown in FIG. 1B, the SiC substrate 11 is subjected to organic cleaning and RCA cleaning, and the concentration of the box profile is 1 × 10 ¹⁸ / cm ³ up to 0.5 μm on the SiC substrate 11 by an ion implantation apparatus. Thus, aluminum (Al ⁺ ) ions 13 are implanted in the range of 30 keV to 350 keV to form impurity regions. Other ions to be implanted include phosphorus (P) and nitrogen (N).

Next, as shown in FIG. 1C, a carbon protective film 14 is formed on a SiC substrate 11 subjected to ion implantation using a carbon target by a DC magnetron sputtering apparatus and using pure argon (Ar) gas as a sputtering gas. To do. The sputtering conditions are a sputtering power density of 0.5 W / cm ² , a sputtering pressure of 0.6 Pa, and a film thickness of 20 nm. At this time, the SiC substrate 11 is not heated.

The carbon protective film 14 is formed with a thickness of 10 nm to 1000 nm, preferably 10 to 100 nm, and more preferably 10 to 30 nm. Further, the sputtering power density of the sputtering apparatus, 1W / cm ² or less, preferably 0.5 W / cm ² or less, further preferably 0.35 W / cm ² or less.

Next, the SiC substrate 11 on which the carbon protective film 14 is formed is inserted into an activation heat treatment apparatus, evacuated to 1 × 10 ⁻² Pa or less, Ar gas is introduced, and 1700 at a pressure of 1 × 10 ⁵ Pa. A thermal activation process is performed at 5 ° C. for 5 minutes.

  Next, as shown in FIG. 1D, the carbon protective film 14 is removed (ashed) from the activated heat-treated SiC substrate 11 using an ashing apparatus. Ashing is performed in oxygen plasma by introducing oxygen (O2) with a reactive ion etching apparatus and applying a pressure of 12 Pa and RF power of 500 W. Ashing time is 5 minutes.

(Comparative example)
As a comparative example, a SiC substrate was manufactured in the same process except for the sputtering condition of the carbon protective film in the manufacturing process shown in the above-described example. The sputtering conditions for the carbon protective film in this comparative example were as follows: a carbon target was used in a DC magnetron sputtering apparatus, the SiC substrate 11 was heated at 600 ° C. using pure Ar gas as a sputtering gas, a sputtering power density of 5 W / cm ² , and a sputtering pressure of 0.6 Pa. The film thickness was 20 nm.

(Evaluation 1 of Example and Comparative Example 1)
About the produced carbon protective film, the Raman spectroscopic measurement by a Raman spectroscopy was performed about the carbon protective film of the Example and the comparative example. FIG. 2 is a chart showing a Raman spectral state before the thermal activation process of the carbon protective film manufactured according to the embodiment of the present invention, and FIG. 3 is a thermal activation of the carbon protective film manufactured according to the embodiment of the present invention. It is a graph which shows the Raman spectral state after a process. FIG. 4 is a chart showing the Raman spectral state of the carbon protective film in the comparative example in which the sputtering conditions of the carbon protective film are varied. In these figures, the horizontal axis represents the wave number, and the vertical axis represents the Raman scattering intensity.

In FIG. 2 of the example, there is no peak of G band (1580 cm ⁻¹ ) and D band (1360 cm ⁻¹ ) due to carbon bonding of graphite before the activation heat treatment of the carbon protective film 14, which is shown in FIG. 3. Thus, it can be seen that peaks of the G band and the D band are generated after the activation heat treatment of the carbon protective film 14. In FIG. 4 of the comparative example, peaks of G band due to the Raman active mode of graphite and D band due to graphite defects are generated by sputtering.

  In both of these Examples and Comparative Examples, after removing the carbon protective film, the surface roughness of the SiC substrate 11 was evaluated in the range of 5 μm □ using an atomic force microscope (AFM). The evaluation was performed with respect to 10 SiC substrates 11 with 5 points per SiC substrate 11. FIG. 5 is an AFM image showing the surface roughness of the SiC substrate 11 after removal of the carbon protective film produced according to the example of the present invention, and FIG. 6 is the surface roughness after removal of the carbon protective film in the comparative example. FIG.

  As shown in FIG. 5, in the example, no streak was formed on the SiC substrate 11 at all. On the other hand, as shown in FIG. 6, in the comparative example, a slightly straight and curved streak like a worm having a depth of 2 to 3 nm, a width of 100 nm, and a length of about 1 μm occurs. The number of 1 μm streak-like depressions is 3 to 25. On the average of 10 SiC substrates 11, about 10 are generated per one.

(Evaluation 2 of Example and Comparative Example 2)
Next, the SiC substrate 11 from which the carbon protective film of the example and the comparative example is removed is subjected to organic cleaning and RCA cleaning, and is subjected to wet oxidation at 1000 ° C. for 30 minutes in a gas containing water vapor and oxygen to have a thickness of about 50 nm An insulating film was formed. On the insulating film, 300 nm of Al was deposited using a vacuum deposition apparatus, and an aluminum pad for electrical measurement of φ100 μm was formed by patterning using photolithography and etching. Thereafter, Al was vapor-deposited on the back side of the SiC substrate 11 using a vacuum vapor deposition apparatus, and a back electrode having a thickness of 70 nm was formed.

The completed MOS capacitor was measured for the breakdown charge amount (QBD) at a current density of 25 mA / cm ² . FIG. 7 is a chart showing the results of QBD measurement. The horizontal axis is the QBD value, and the vertical axis is the cumulative failure probability. In the example, although the breakdown probability is shown with a slightly small charge amount, the breakdown probability of the oxide film itself is almost shown, and a better result was obtained than in the comparative example.

  As described above, according to the present embodiment, by forming an amorphous carbon film that is not heated during sputtering and has no graphite bond, cracks in the carbon protective film during heating can be suppressed, and the surface of the SiC substrate can be suppressed. The generation of a dent in the shape can be prevented. Since activation heat treatment is performed by depositing amorphous carbon that has not been graphitized when forming the carbon protective film, local cracks are prevented in the carbon protective film.

  By suppressing streak-like dents on the surface of the SiC substrate, the surface of the SiC substrate after activation heat treatment is prevented and kept smooth, preventing dielectric breakdown and leakage, improving device characteristics, and having good characteristics at low cost Can be made. In addition, since the carbon protective film is formed at room temperature, the temperature rise time to the set substrate temperature can be shortened, and the heating mechanism of the sputtering equipment body is not required, improving the productivity and enabling low-cost device fabrication. It becomes. The semiconductor device using the SiC semiconductor of the present invention can be applied to MOSFET, SBD, IGBT and the like.

  As described above, the method for manufacturing a silicon carbide semiconductor element according to the present invention is useful for power semiconductor devices such as power devices, and power semiconductor devices used for industrial or automotive motor control and engine control. is there.

DESCRIPTION OF SYMBOLS 1 Silicon carbide semiconductor element 11 Silicon carbide (SiC) substrate 12 SiC epitaxial film 13 Implanted ion 14 Carbon protective film

Claims (3)

A step of forming an impurity region by ion implantation in a surface layer of a silicon carbide substrate on which a silicon carbide single crystal substrate and a silicon carbide epitaxial film are formed as a semiconductor device material;
Using a room temperature sputtering apparatus that does not heat the silicon carbide substrate, depositing amorphous carbon that has not been graphitized on the surface of the silicon carbide substrate, and forming a carbon protective film;
Generating a carbon bond of graphite by activating heat treatment of the silicon carbide substrate;
Removing the carbon protective film,
The carbon protective layer does not have a peak of scattering intensity at a wavenumber 1280～1380Cm ^-1 and 1550～1650Cm ^-1 by Raman spectroscopic measurement before the activation heat treatment, each peak of the scattering intensity after the thermal activation Yes, and method for manufacturing a silicon carbide semiconductor device characterized by having a peak of scattering intensity in 1500～1550Cm ^-1 and 1700 cm ^-1 before and after the activation heat treatment.   2. The method for manufacturing a silicon carbide semiconductor element according to claim 1, wherein the carbon protective film formed on the silicon carbide substrate has a thickness of 10 nm to 1000 nm. The method for manufacturing a silicon carbide semiconductor element according to claim 1, wherein the sputtering power density of the sputtering apparatus is 1 W / cm ² or less.

Images