JP5809317B2 - Manufacturing method of semiconductor device - Google Patents

Description

  The present invention relates to a method for manufacturing a semiconductor device using a silicon carbide single crystal substrate.

  Conventional power semiconductor devices (power semiconductor elements) are manufactured using a silicon single crystal substrate. Advances in device design and manufacturing technology have reduced the resistance (on-resistance) during energization while maintaining a high breakdown voltage, and have achieved a low loss. However, since the breakdown voltage and the on-resistance are in a trade-off relationship determined by the band gap of the substrate material, there is a limit to improving the performance as long as conventional silicon is used. For this reason, wide gap semiconductors typified by silicon carbide and gallium nitride have attracted attention as materials that can replace silicon. This is because if a wide gap semiconductor is used, a high-performance power semiconductor element can be realized as compared with the case where silicon is used. In particular, development of power semiconductor devices using silicon carbide has been active. The first reason is that the quality and size of silicon carbide single crystal substrates are increasing, and it is relatively easy to obtain substrates necessary for manufacturing semiconductor devices. The second reason is that silicon carbide can easily form both n-type and p-type conductivity regions by epitaxial growth and ion implantation, and can form a film mainly composed of silicon dioxide by thermal oxidation. This is because there is a possibility that various power semiconductor devices can be manufactured. So far, unipolar elements such as MOSFET (Metal Oxide Semiconductor Field Effect Transistor) and JFET (Junction Field Effect Transistor) as well as BJT (Bipolar Junction Field Effect Transistor) have been studied as switching elements. As rectifying elements, pn junction diodes and Schottky barrier diodes have been developed. In particular, a diode element called JBS (Junction Barrier Shottoy), which is a combination of a pn junction diode and a Schottky barrier diode, has been developed for high withstand voltage.

Japanese Patent Laid-Open No. 5-17229 JP 2003-146778 A

  Many of the manufacturing technologies cultivated in the manufacture of silicon semiconductor devices can be applied to the manufacture of power semiconductor devices using silicon carbide single crystal substrates, but some of the manufacturing technologies will be newly developed for silicon carbide. There is a need. In general, silicon carbide single crystal substrate manufacturing technology, silicon carbide processing technology, silicon carbide epitaxial layer formation technology, high temperature ion implantation technology, impurity activation technology, ohmic contact formation technology, MOSFET gate insulating film formation technology, etc. This is said to be the case. However, as a result of investigations by the inventors, it has been found that diversion of the technology established for silicon is insufficient for the cleaning technology, and a technology specialized for silicon carbide is necessary. In particular, regarding metal contamination removal, it has become clear that a sufficient effect cannot be obtained even if the manufacturing method for a silicon semiconductor is directly applied to a silicon carbide semiconductor.

  In the manufacture of silicon semiconductor elements, a chemical cleaning process is frequently used. In an oxidizing chemical such as ammonia / hydrogen peroxide mixture, sulfuric acid / hydrogen peroxide mixture, hydrochloric acid / hydrogen peroxide mixture, and nitric acid, a film mainly composed of silicon dioxide is formed on the silicon surface. . In many cases, the metal element present on the silicon surface is lifted onto the oxide film or taken into the oxide film as the oxide film is formed. After the oxidation, when the oxide film is removed with a hydrofluoric acid aqueous solution, the metal elements on the oxide film and in the oxide film are also removed at the same time.

  In the conventional method for manufacturing a semiconductor element using a silicon carbide single crystal substrate, cleaning with a chemical solution used for manufacturing a silicon semiconductor element is used in an appropriate combination. However, unlike silicon, silicon carbide uses silicon Even when immersed in a chemical solution that oxidizes, a film containing silicon dioxide as a main component is hardly formed. For this reason, it is considered that a mechanism for removing the metal element is not realized by forming the oxide film as described above. In silicon, silicon itself can be etched with hydrofluoric acid or the like, but silicon carbide can be etched only with a special chemical such as a potassium hydroxide melt. For this reason, it is impossible to apply a technique of simultaneously removing the metal element by etching silicon carbide itself with a chemical solution.

  Several cleaning methods have been devised for silicon carbide sintered bodies. For example, Patent Document 1 (Japanese Patent Laid-Open No. 5-17229) describes a method using blast cleaning with silica abrasive grains. However, since damage such as defects is introduced by blast cleaning, manufacturing of a semiconductor device is described. It cannot be used as it is for cleaning the silicon carbide single crystal substrate used in the above. Further, Patent Document 2 (Japanese Patent Application Laid-Open No. 2003-146778) describes a method of cleaning a sintered body by exposing it to plasma. It is described that the plasma generation gas may be anything as long as it does not react with silicon carbide, and the cleaning mechanism is physical contamination removal by plasma. Therefore, the power for generating plasma is large, and the time for exposing the sintered body to plasma is also long. This method also causes great damage to the substrate, and cannot be used as it is for cleaning a silicon carbide single crystal substrate used for manufacturing a semiconductor element. Any of them is considered to be an effective method only for a member of a semiconductor manufacturing apparatus or a sintered body of silicon carbide used as a dummy substrate.

  Although the diffusion coefficient of the metal in the silicon carbide single crystal is small, there is an undeniable possibility that the metal is captured by defects in the single crystal and the breakdown voltage is lowered or the long-term reliability is lowered. In addition, the metal element taken into the gate insulating film or interlayer insulating film containing silicon dioxide as a main component easily diffuses. It can be easily estimated that the initial characteristics and long-term reliability of the gate insulating film and the interlayer insulating film will be adversely affected. In silicon semiconductor elements, it is known that metal contamination adversely affects the initial characteristics of the manufactured elements, reduces the yield rate, and reduces long-term reliability. . Development of an effective method for removing metal contamination that can be applied to the manufacture of a semiconductor device using a silicon carbide single crystal substrate is desired.

  The problem of the prior art to be solved is that the metal contamination on the silicon carbide surface is not sufficiently removed in the method of manufacturing a semiconductor device using a silicon carbide single crystal substrate. Thereby, there is a high possibility that the initial characteristics of the manufactured silicon carbide semiconductor element are deteriorated and the yield rate is reduced. Metal contamination is also considered to have an adverse effect on the long-term reliability of semiconductor devices.

  The main object of the present invention is to provide a method for manufacturing a semiconductor device having a metal contamination removal step for removing metal contamination on the surface of a silicon carbide single crystal substrate, wherein the metal contamination removal step oxidizes the surface of the silicon carbide single crystal substrate. And a thin film forming step for forming a film containing silicon dioxide as a main component on the surface of the silicon carbide single crystal substrate, and a thin film removing step for removing the film containing silicon dioxide as a main component. .

  By being exposed to oxygen plasma, a thin film mainly composed of silicon dioxide is formed on the substrate surface. The metal element originally present on the surface of the substrate is often lifted on the oxide film or taken into the oxide film. When this oxide film is removed with a hydrofluoric acid aqueous solution, metal elements on the oxide film and in the oxide film are also removed at the same time. Thereby, the surface of silicon carbide can be cleaned.

The figure for demonstrating the Example of this invention. The figure which shows the measuring point of the silicon carbide single crystal substrate demonstrated in Example 1 of this invention. The figure which shows the measurement point of the metal contamination demonstrated in Example 1 of this invention. The figure which shows the analysis result of the metal contamination demonstrated in Example 1 of this invention. The figure which shows the analysis result of the metal contamination demonstrated in Example 1 of this invention. Sectional drawing which shows the manufacturing process of the pn junction diode of the present Example 1. FIG. Sectional drawing which shows the manufacturing process of the pn junction diode of the present Example 1. FIG. Sectional drawing which shows the manufacturing process of the pn junction diode of the present Example 1. FIG. Sectional drawing which shows the manufacturing process of the pn junction diode of the present Example 1. FIG. Sectional drawing which shows the manufacturing process of the pn junction diode of the present Example 1. FIG. Sectional drawing which shows the manufacturing process of the pn junction diode of the present Example 1. FIG. Sectional drawing which shows the manufacturing process of the pn junction diode of the present Example 1. FIG. The figure which shows the pressure | voltage resistant distribution of the pn junction diode by the manufacturing method of this invention. The figure which shows the pressure | voltage resistant distribution of the pn junction diode by the conventional manufacturing method. The figure which shows the analysis result of the metal contamination demonstrated in Example 2 of this invention. The figure which shows the analysis result of the metal contamination demonstrated in Example 2 of this invention. The figure which shows the analysis result of the metal contamination demonstrated in Example 2 of this invention. The figure which shows the analysis result of the metal contamination demonstrated in Example 2 of this invention. The figure which shows the analysis result of the metal contamination demonstrated in Example 2 of this invention. Sectional drawing which shows the manufacturing process of the JBS diode of the present Example 3. FIG. Sectional drawing which shows the manufacturing process of the JBS diode of the present Example 3. FIG. Sectional drawing which shows the manufacturing process of the JBS diode of the present Example 3. FIG. Sectional drawing which shows the manufacturing process of the JBS diode of the present Example 3. FIG. Sectional drawing which shows the manufacturing process of the JBS diode of the present Example 3. FIG. Sectional drawing which shows the manufacturing process of the JBS diode of the present Example 3. FIG. Sectional drawing which shows the manufacturing process of the JBS diode of the present Example 3. FIG. Sectional drawing which shows the manufacturing process of MOSFET of the present Example 4. FIG. Sectional drawing which shows the manufacturing process of MOSFET of the present Example 4. FIG. Sectional drawing which shows the manufacturing process of MOSFET of the present Example 4. FIG. Sectional drawing which shows the manufacturing process of MOSFET of the present Example 4. FIG. Sectional drawing which shows the manufacturing process of MOSFET of the present Example 4. FIG. Sectional drawing which shows the manufacturing process of MOSFET of the present Example 4. FIG. Sectional drawing which shows the manufacturing process of MOSFET of the present Example 4. FIG. Sectional drawing which shows the manufacturing process of MOSFET of the present Example 4. FIG. Sectional drawing which shows the manufacturing process of MOSFET of the present Example 4. FIG. Sectional drawing which shows the manufacturing process of MOSFET of the present Example 4. FIG. Sectional drawing which shows the manufacturing process of MOSFET of the present Example 4. FIG. Sectional drawing which shows the manufacturing process of MOSFET of the present Example 4. FIG. Sectional drawing which shows the manufacturing process of MOSFET of the present Example 4. FIG. Sectional drawing which shows the manufacturing process of MOSFET of the present Example 4. FIG. Sectional drawing which shows the manufacturing process of MOSFET of the present Example 4. FIG. The figure which shows the result of the reliability evaluation of the gate insulating film produced by this invention and the conventional method. The figure which shows the result of the reliability evaluation of the gate insulating film produced by this invention and the conventional method. Sectional drawing which shows the manufacturing process of the trench type MOSFET of the present Example 5. FIG. Sectional drawing which shows the manufacturing process of the trench type MOSFET of the present Example 5. FIG. Sectional drawing which shows the manufacturing process of the trench type MOSFET of the present Example 5. FIG. Sectional drawing which shows the manufacturing process of the trench type MOSFET of the present Example 5. FIG. Sectional drawing which shows the manufacturing process of the trench type MOSFET of the present Example 5. FIG. Sectional drawing which shows the manufacturing process of the trench type MOSFET of the present Example 5. FIG. Sectional drawing which shows the manufacturing process of the trench type MOSFET of the present Example 5. FIG. Sectional drawing which shows the manufacturing process of the trench type MOSFET of the present Example 5. FIG. Sectional drawing which shows the manufacturing process of the trench type MOSFET of the present Example 5. FIG. Sectional drawing which shows the manufacturing process of the trench type MOSFET of the present Example 5. FIG. Sectional drawing which shows the manufacturing process of the trench type MOSFET of the present Example 5. FIG. Sectional drawing which shows the manufacturing process of the trench type MOSFET of the present Example 5. FIG.

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

  By exposing the surface of the silicon carbide single crystal substrate to oxygen plasma, a film mainly composed of silicon dioxide having a thickness of less than 5 nm is formed on the surface of the silicon carbide single crystal substrate, and the silicon dioxide formed on the silicon carbide surface is mainly used. A metal contamination removal step of removing the silicon carbide single crystal substrate by immersing the silicon carbide single crystal substrate in an etching solution containing hydrofluoric acid is applied to a method for manufacturing a semiconductor device using the silicon carbide single crystal substrate. Thereby, the initial characteristics of the semiconductor element to be manufactured are improved, and the yield rate is increased. Long-term reliability is also improved.

This will be described with reference to FIGS. This embodiment is an embodiment applied to the manufacture of a pn junction diode. FIG. 1 shows a process flow of a method of manufacturing a pn junction diode according to the present invention. In order to make a silicon carbide single crystal substrate, first, an ingot made of a single crystal of silicon carbide (4H—SiC) was produced. Currently, the most common sublimation method is used for manufacturing the ingot, but other silicon carbide single crystal growth techniques such as a melting method may be used. As the single crystal, in addition to 4H—SiC, other crystal forms of silicon carbide single crystal such as 2H—SiC, 6H—SiC, and 3C—SiC can also be used. The ingot single crystal silicon carbide contains 3 × 10 ¹⁸ cm ^{−3 of} nitrogen and has a high concentration of n-type. Next, a single crystal substrate was cut into a thin plate shape from the manufactured ingot. This process is generally called a slicing process. The surface of the single crystal substrate was cut out so as to have an off angle of 4 ° in the [11-20] direction from the (0001) plane. In addition to the (0001) plane, the (000-1) plane or the (11-20) plane may be cut out. Further, the off angle is not limited to 4 °, and may be any other angle as long as it is about 0 to 8 °. The maximum diameter is 76.2 mm, and two main and sub orientation flats are formed. The diameter is not limited to 76.2 mm, and other dimensions may be used.

  Next, the cut substrate was roughly cut to an appropriate thickness in a grinding process. A polishing process may be used instead of the grinding process. Thereafter, the ground and polished surfaces were polished in a polishing process in order to finish them flat and mirror-finished. As a polishing method, chemical mechanical polishing (CMP; Chemical Mechanical Polishing) was applied. In this example, this polishing process also served to remove defects and scratches introduced into the substrate in the slicing process and grinding process, but if necessary, a special process such as dry etching using plasma was added. The altered layer on the surface may be removed. In that case, these steps are inserted before or after the polishing step. Polishing by CMP was performed in three stages, and the flatness of the substrate surface was gradually increased by selecting the slurry and polishing conditions to obtain a mirror surface by the third stage polishing. Polishing of the back surface of the substrate was performed in a second stage under different conditions from the front surface. Although the thickness of the substrate after polishing is 350 μm, the thickness may be other dimensions. During the process from slicing to polishing, chemical cleaning, water washing, and drying processes for removing organic substances and metal contamination adhered to the substrate were appropriately performed. The final cleaning was performed after the polishing process. Realized by applying decontamination process. The surface and back surface of the polished silicon carbide single crystal substrate were oxidized by exposing them to oxygen plasma, and then the substrate was immersed in an etchant containing hydrofluoric acid to remove the oxide film.

Details of the metal contamination removal process after polishing will be described. The oxygen plasma treatment was performed using a dry etching apparatus using microwaves. The processing conditions are an oxygen flow rate of 200 sccm, a pressure in the reaction chamber of 5 Pa, a microwave source power of 800 W, and an RF bias power of 5 W, and the time is 60 seconds for both the front and back surfaces. The electrode shape of the device is a circle with a diameter of approximately 200 mm. Therefore, the RF bias power per unit area is 0.016 W / cm ² . First, the surface side treatment was performed with the surface of the silicon carbide single crystal substrate facing up, and then the back surface treatment was performed with the back surface of the silicon carbide silicon substrate facing up. By this oxygen plasma treatment, a film mainly composed of silicon dioxide is formed on the front and back surfaces of the silicon carbide silicon substrate. When the thickness of the film mainly composed of silicon dioxide formed on the surface composed of the silicon surface was measured, it was less than 2 nm.

  After the oxygen plasma treatment, first, the silicon carbide single crystal substrate was immersed in aqua regia for 180 seconds, washed with water, and then immersed in a sulfuric acid / hydrogen peroxide mixture kept at 120 ° C. for 180 seconds. The mixing ratio of sulfuric acid and hydrogen peroxide solution (31% aqueous solution) is 7: 3. After washing with water, it was immersed in a hydrofluoric acid aqueous solution for 180 seconds. The mixing ratio of hydrofluoric acid (55% aqueous solution) and water is 1:20. By dipping in this hydrofluoric acid aqueous solution, the film mainly composed of silicon dioxide formed on the front and back surfaces of the silicon carbide single crystal substrate by oxygen plasma treatment is removed. Finally, after being immersed in a mixed solution of ammonia and hydrogen peroxide for 120 seconds, the substrate was washed with water and dried with a spin dryer. For comparison, a silicon carbide single crystal substrate by a conventional manufacturing method was also produced. After cutting out from the ingot, the process of polishing the front and back surfaces by chemical mechanical polishing is the same as that of the silicon carbide single crystal substrate according to the manufacturing method of the present invention in this embodiment. For final cleaning, a conventional metal decontamination process was applied after the polishing process. That is, unlike the present invention, the treatment of exposing the substrate to oxygen plasma is not used, and only chemical cleaning with aqua regia, sulfuric acid / hydrogen peroxide mixture, hydrofluoric acid aqueous solution, ammonia / hydrogen peroxide mixture is performed on the substrate. gave. The components of the chemical solution used and the cleaning conditions are the same as those of the silicon carbide single crystal substrate according to the present invention in this example.

FIG. 2C shows the result of analyzing metal contamination on the surface of the silicon carbide single crystal substrate of this example to which the present invention is applied. FIG. 3 shows the result of analyzing the metal contamination on the surface of the silicon carbide single crystal substrate by the conventional manufacturing method. The total reflection X-ray fluorescence analysis was used for the analysis, and titanium, chromium, iron, nickel, copper, and zinc were measured at five points on the substrate surface. A main orientation flat 22 and a sub-orientation flat 23 are formed on the surface of the silicon carbide single crystal substrate 21 of FIG. 2A. As shown in FIG. 2B, the center 24 is the measurement point 1. Measurement points 2, 3, 4, and 5 are evenly arranged on a circle having a diameter of 20 mm with the measurement point 1 as the center. Measurement points 1 to 5 correspond to 24, 25, 26, 27, and 28 on the wafer surface in FIG. The surface concentration of each metal element at each measurement point is displayed in the format indicated by 29 in FIG. 2B. The unit is x10 ¹⁰ atoms / cm ² as shown in the figure. The management reference value is 3 × ^{10 10} atoms / cm ² .

  Also for FIG. 3, the surface concentration of each metal element at each measurement point is displayed for the same measurement point as in FIG. 2C in the same format as in FIG. 2C. As shown in FIG. 3, iron and copper elements remain on the substrate surface by the conventional manufacturing method, but as can be seen from FIG. 2C, iron, copper are formed on the substrate surface by the manufacturing method of the present invention. All elements including are present below the control standard limit. It is unclear at which stage iron or copper adheres to the substrate surface, but contamination from tools in the slicing process, grinding process, slurry in the polishing process, etc. is suspected. The difference in the remaining metal contamination is considered to be due to the presence or absence of oxidation on the surface of the silicon carbide single crystal substrate.

  That is, according to the manufacturing method of the present invention, a film mainly composed of thin silicon dioxide is formed on the substrate surface by being exposed to oxygen plasma. The metal element originally present on the surface of the substrate is often lifted on the oxide film or taken into the oxide film. When this oxide film is removed with a hydrofluoric acid aqueous solution, metal elements on the oxide film and in the oxide film are also removed at the same time. The conventional manufacturing method is a diversion of the manufacturing method of the silicon semiconductor element. Unlike the silicon single crystal substrate, the silicon carbide single crystal substrate is immersed in a chemical solution containing hydrogen peroxide, A film composed mainly of silicon dioxide is hardly formed. For this reason, it is considered that a mechanism for removing the metal element is not realized by forming the oxide film.

  In this example, the metal contamination removal process of the present invention was applied after chemical mechanical polishing of the silicon carbide single crystal substrate itself. However, it may be applied after polishing a silicon dioxide film, a polycrystalline silicon film, or the like during device fabrication. It is valid. As in this embodiment, there is an effect of removing metal contamination.

In order to apply the silicon carbide single crystal substrate produced as described above to the production of a power device, a silicon carbide n-type epitaxial layer containing nitrogen was formed on the substrate surface as a drift layer (electric field relaxation layer). A substrate produced by the production method of the present invention and two substrates produced by the conventional production method were simultaneously installed in the epitaxial growth apparatus, the hydrogen flow rate was adjusted to 10 slm, and the pressure in the reaction chamber was adjusted to 10 kPa. The substrate was heated to 1400 ° C. by high frequency induction heating and kept for 10 minutes. At this time, the substrate surface is etched and introduced during the processing of the substrate, and the damaged layer including the remaining defects and scratches is removed. After raising the substrate temperature to 1500 ° C., in addition to hydrogen 10 slm, propane 0.6 sccm, monosilane 2.5 sccm, and nitrogen 0.2 sccm were simultaneously supplied to the reaction chamber. After maintaining this state for 140 minutes, the supply and heating of propane, monosilane, and nitrogen were stopped, and the substrate was cooled in hydrogen. Thereafter, two silicon carbide single crystal substrates were taken out, and the concentration and thickness of the epitaxial layer were measured. In all four sheets, the nitrogen concentration of the epitaxial layer was 2 × 10 ¹⁶ cm ⁻³ and the thickness was 8 μm.

By observing the surface of the substrate with an optical microscope and visually counting epi defects such as comet defects and carrot defects, the silicon carbide single crystal substrate according to the manufacturing method of the present invention is a silicon carbide single crystal substrate according to the conventional manufacturing method. It was found that the density of epi defects decreased to about 60%. Next, etch pits were formed using a potassium hydroxide melt for each of the substrate produced by the production method of the present invention and the substrate produced by the conventional production method, and the basal plane dislocation densities were compared. This method is a well-known method for observing dislocations. The epitaxial layer on the silicon carbide single crystal substrate according to the present invention was 260 cm ⁻² , and the epitaxial layer on the silicon carbide single crystal substrate according to the conventional manufacturing method was 380 cm ⁻² . The basal plane dislocation density was reduced by about 30% on the silicon carbide substrate according to the present invention than on the silicon carbide substrate according to the conventional manufacturing method.

A pn diode was fabricated using each of the substrate according to the manufacturing method of the present invention in which an n-type epitaxial layer was formed and the substrate according to the conventional manufacturing method for comparison. 4A to 4F are cross-sectional views showing the manufacturing process of the pn junction diode of this example. A termination region for relaxing the electric field is formed around the pn junction diode, but the termination region is omitted in FIGS. 4A to 4F. As shown in FIG. 4F, the pn junction diode of this example includes an n-type drift layer 42 that is provided on the main surface of single-crystal silicon carbide substrate 41 and has a thickness of 8 μm, and includes a surface of drift layer 42. A p-type doped layer 43 having a thickness of about 0.5 μm including aluminum and a high-concentration p-type layer 44 having a thickness of 0.1 μm including aluminum provided on the surface of the p-type doped layer 43. With layers. The single crystal silicon carbide substrate 41 and the n-type drift layer 42 have nitrogen concentrations (donor concentrations) of 3 × 10 ¹⁸ cm ⁻³ and 2 × 10 ¹⁶ cm ⁻³ , respectively, and the p-type doped layer 43 and the high-concentration p-type. The aluminum concentration (acceptor concentration) of the layer 44 is 2 × 10 ¹⁸ cm ⁻³ and 5 × 10 ¹⁹ cm ⁻³ , respectively. As shown in FIG. 4B, aluminum ions are implanted into part of the surface of the drift layer 42 to form a p-type doped layer 43.

  Subsequently, as shown in FIG. 4C, aluminum ions are implanted into the surface of the p-type doped layer 43 so as to have a higher concentration than the p-type and drift layer 43, thereby forming a high-concentration p-type layer 44. After ion implantation of the p-type doped layer 43 and the high-concentration p-type layer 44, activation heat treatment was performed at 1850 ° C. for 1 minute in an argon atmosphere. Thereafter, as shown in FIG. 4D, a silicon dioxide film 45 was formed by plasma CVD (Chemical Vapor Deposition), and the electrode portion of the diode was opened. Next, a nickel film having a thickness of 50 nm is formed on the front and back surfaces using a sputtering apparatus, and heat treatment is performed at 800 ° C. for 1 minute in an argon atmosphere using an RTA (Rapid Thermal Anneal) apparatus. After that, when the silicon carbide single crystal substrate 41 is immersed in a mixed solution of ammonia and hydrogen peroxide to remove the unreacted nickel film, the result is as shown in FIG. 4E. Then, layers 46 and 47 mainly composed of nickel silicide were formed. Here, using the RTA apparatus once again, this time heat treatment was performed at 1000 ° C. for 1 minute in an argon atmosphere. As shown in FIG. 4F, an aluminum electrode 48 having a thickness of 3 μm was formed on the surface side. For the formation of the aluminum film, a sputtering apparatus was used, and patterning was performed by a known lithography process and wet etching technique. On the back surface, a nickel film 49 having a thickness of 100 nm was formed using a sputtering apparatus to form a back electrode. Finally, a protective film 50 made of polyimide resin was formed, and an opening was made on the aluminum film 48 of the diode electrode. As described above, the pn junction diode of this example was manufactured by the manufacturing method of the present invention.

  The initial characteristics of 200 pn junction diodes per sheet fabricated on two single crystal silicon carbide substrates were evaluated. The breakdown voltage distributions of the pn junction diode according to the manufacturing method of the present invention and the pn junction diode according to the conventional manufacturing method are shown in FIGS. 5A and 5B, respectively. Assuming that a withstand voltage of 600 V or more (defined as current <0.1 mA) is a non-defective product, the yield rate is 87% for the pn junction diode according to the manufacturing method of the present invention (FIG. 5A) and 47% for the pn junction diode according to the conventional manufacturing method ( FIG. 5B). This is mainly due to the fact that in the pn junction diode according to the manufacturing method of the present invention, the density of epi defects in the drift layer is reduced to about 60% of the pn junction diode according to the conventional manufacturing method. Conceivable.

Next, a current of 50 A / cm ² was passed through the pn junction diode and held for 10 hours, and the increase in on-voltage was examined. In the pn junction diode manufactured by the conventional manufacturing method, an increase in the on-voltage of about 1 V was observed in 8 of the 25 measured diodes, but the measurement was performed in the pn junction diode using the manufacturing method of the present invention. All 25 diodes were suppressed to an increase of 0.1 V or less. This is considered to reflect the difference in the effect of the metal contamination removal process performed at the end of producing the single crystal silicon carbide substrate. The mechanism of such characteristic fluctuations during energization is still under study, and not all has been elucidated, but in general, basal plane dislocations are involved in fluctuations in the on-voltage of pn junction diodes. It is considered. It is thought that this difference was caused by the fact that the epitaxial growth layer formed by the manufacturing method of the present invention has a lower basal plane dislocation density than the epitaxial growth layer formed by the conventional manufacturing method. Furthermore, it is said that it is not all basal plane dislocations but part of the basal plane dislocations that affect the characteristic fluctuation during energization. There is also a theory that basal plane dislocations that affect fluctuations are modified with some metal impurities. If this theory is correct, rather than the difference in the basal plane dislocation density itself, in the manufacturing method of the present invention, after polishing the substrate surface, the metal contamination remaining on the substrate surface is greatly reduced, so that it is incorporated into the epitaxial layer. This may have the effect of reducing the density of basal plane dislocations acting on energization fluctuations.
The reduction in the density of basal plane dislocations in which metal impurities are trapped may have contributed to the reduction in the number of defective pressure-resistant products and the improvement of the yield rate.

  By applying the manufacturing method of the present invention, the yield of non-defective pn junction diodes was improved as compared with the case of using the conventional manufacturing method, and the characteristic fluctuation during energization was greatly suppressed. In this embodiment, oxygen plasma treatment using a dry etching apparatus is used to form a film containing silicon dioxide as a main component. However, oxygen plasma treatment may be performed using another apparatus such as an asher apparatus. Is possible. The reason why the dry etching apparatus is used in this embodiment is that the reaction chamber can be cleaned. In the apparatus of this example, after the oxygen plasma treatment of this example was completed, the reaction chamber was cleaned with plasma of a mixed gas of chlorine and oxygen. When a clean silicon substrate was subjected to argon plasma treatment to inspect metal contamination, it was found to be at the same level as the cleanliness before the treatment, and there was no influence of device contamination due to this treatment. When an apparatus without a cleaning mechanism is used, it is difficult to maintain the cleanliness of the apparatus. As the oxidation method, anodic oxidation, thermal oxidation, oxidation with ozone, or the like can be used. However, in the case of thermal oxidation, since the temperature is as high as about 1000 ° C., there is a drawback that it becomes more difficult to maintain the cleanliness of the apparatus. In the case of oxidation using ozone, the temperature can be lowered compared to thermal oxidation using ordinary oxygen. In addition to being used for thermal oxidation, ozone has an effect of promoting oxidation of the surface of the silicon carbide single crystal substrate when used in combination with ultraviolet irradiation or in combination with oxygen plasma treatment.

  Moreover, it is desirable that the thickness of the film mainly composed of silicon dioxide formed on the surface is less than 5 nm. When the film is oxidized to a thickness of 5 nm or more, slight unevenness is generated on the surface of the silicon carbide single crystal substrate when the oxide film is subsequently removed using a hydrofluoric acid aqueous solution. This is because the unevenness generated on the substrate surface is known to cause an increase in reverse leakage current of the pn junction diode. Oxidation with a thickness of less than 5 nm could be easily realized by selecting RF bias power or the like in the oxygen plasma treatment. Even when other oxidation methods are used, oxidation with a film thickness of less than 5 nm is possible as in this embodiment, if the processing conditions are appropriately selected.

  This will be described with reference to FIGS. This embodiment is an embodiment applied to the manufacture of a pn junction diode. The 4H-SiC, 4 ° -off single crystal silicon carbide substrate used in this example is a product purchased from a certain substrate manufacturer with specifications. The crystal face is a silicon face ((0001) face) and the back face is a carbon face ((000-1) face), but the front face has an off angle of 4 ° in the [11-20] direction from the (0001) face. The substrate layer manufacturer also performed epitaxial growth of the drift layer on the substrate. When the substrate after delivery was inspected, the maximum diameter was 76.2 mm, and two main and sub orientation flats were formed. The thickness is 350 μm.

The epitaxial layer had a nitrogen concentration of 2 × 10 ¹⁶ cm ⁻³ and a thickness of 8 μm as specified. FIG. 6 shows the result of analyzing metal contamination on the surface of the substrate after delivery. The measurement method, measurement location, and result display format are the same as those in FIG. 2C in the first embodiment. All elements of titanium, chromium, iron, nickel, copper and zinc remained on the substrate surface. It is unclear at which stage these metal elements have adhered to the substrate surface, but contamination from slicing and grinding tools for manufacturing a single crystal silicon carbide substrate, polishing process slurry, etc. is suspected. Is called. Furthermore, contamination may be introduced during the epitaxial growth. After the substrate polishing step, the metal contamination removal step of the present invention as described in Example 1 is not performed, so it is considered that the metal element remaining on the substrate surface was taken into the epitaxial layer, In the epitaxial growth apparatus, the substrate on which the metal element remains on the surface is repeatedly heated, so that the epitaxial growth apparatus itself is likely to be contaminated by the metal element, and the metal element may be taken in during the epitaxial growth from there. it is conceivable that. Further, the metal element is contained in the member of the epitaxial growth apparatus, and the member becomes high temperature during heating for epitaxial growth, and the metal element may jump out of the member, and the substrate may take it in.

  First, both were immersed in aqua regia for 180 seconds, washed with water, and then immersed in a sulfuric acid / hydrogen peroxide mixture maintained at 120 ° C. for 180 seconds. The mixing ratio of sulfuric acid and hydrogen peroxide solution (31% aqueous solution) is 7: 3. After washing with water, it was immersed in a hydrofluoric acid aqueous solution for 180 seconds. The mixing ratio of hydrofluoric acid (55% aqueous solution) and water is 1:20. By dipping in this hydrofluoric acid aqueous solution, the film mainly composed of silicon dioxide formed on the front and back surfaces of the silicon carbide single crystal substrate by oxygen plasma treatment is removed. Finally, after being immersed in a mixed solution of ammonia and hydrogen peroxide for 120 seconds, the substrate was washed with water and dried with a spin dryer. This series of cleaning steps is not necessarily essential in terms of removing metal contamination, but was performed for the purpose of preventing contamination of a dry etching apparatus using microwaves that perform oxygen plasma treatment. FIG. 7 shows the result of analyzing the metal contamination on the surface of one of the two substrates again after a series of cleaning steps. Cleaning removed the contamination of titanium, chromium, nickel, and copper, but iron and zinc remained after cleaning.

Next, oxygen plasma treatment was performed on one of the two sheets. The oxygen plasma treatment was performed using a dry etching apparatus using microwaves. The processing conditions are an oxygen flow rate of 200 sccm, a reaction chamber pressure of 5 Pa, a microwave source power of 800 W, and an RF bias power of 7 W, and the time is 60 seconds on both the front and back surfaces. The electrode shape of the device is a circle with a diameter of approximately 200 mm. Therefore, the RF bias power per unit area is 0.022 W / cm ² . First, the surface side treatment was performed with the surface of the silicon carbide single crystal substrate facing up, and then the back surface treatment was performed with the back surface of the silicon carbide silicon substrate facing up. By this oxygen plasma treatment, a film mainly composed of silicon dioxide is formed on the front and back surfaces of the silicon carbide single crystal substrate. The thickness of the film mainly composed of silicon dioxide was less than 3 nm on the surface composed of the silicon surface. FIG. 8 shows the result of analyzing the metal contamination on the substrate surface again at this stage. Oxygen plasma treatment alone did not reduce metal contamination and iron and zinc remained. In the manufacturing method of the present invention, the RF bias power is set sufficiently low so that damage is not introduced into the silicon carbide single crystal substrate. For this reason, it is thought that the mechanism by which metal contamination is physically removed by ion collision hardly develops. The etching apparatus used for the treatment was subjected to cleaning in the reaction chamber using a mixed gas of chlorine and oxygen, and then the metal contamination was inspected using a silicon substrate. There was no effect of the contamination of the equipment due to the use.

  Thereafter, chemical cleaning with aqua regia, sulfuric acid / hydrogen peroxide mixture, hydrofluoric acid aqueous solution, and ammonia / hydrogen peroxide mixture was performed on the substrate again. The components of the chemical solution used and the cleaning conditions are the same as the cleaning of the silicon carbide single crystal substrate according to the present invention in this example. This series of chemical cleaning was performed on a substrate not subjected to oxygen plasma treatment, and the surface was analyzed again.

  FIG. 9 shows the result of analyzing metal contamination on the surface of the silicon carbide single crystal substrate of this example. FIG. 10 shows the result of analyzing metal contamination on the surface of a silicon carbide silicon carbide single crystal substrate by a conventional manufacturing method that does not use oxygen plasma treatment. Although the elements of iron and zinc remain on the substrate surface by the conventional manufacturing method, all the elements including iron and zinc exist below the detection limit on the substrate surface by the manufacturing method of the present invention. The difference in the remaining metal contamination is considered to be due to the presence or absence of oxidation on the surface of the silicon carbide single crystal substrate. That is, according to the manufacturing method of the present invention, a film mainly composed of thin silicon dioxide is formed on the substrate surface by being exposed to oxygen plasma. The metal element originally present on the surface of the substrate is often lifted on the oxide film or taken into the oxide film. When this oxide film is removed with a hydrofluoric acid aqueous solution, metal elements on the oxide film and in the oxide film are also removed at the same time. On the other hand, when the conventional manufacturing method used in the manufacture of silicon semiconductor elements is applied to the manufacture of silicon carbide semiconductor elements as they are, in the case of silicon silicon single crystal substrates, in silicon carbide single crystal substrates, Even when immersed in a chemical solution containing hydrogen oxide, a film containing silicon dioxide as a main component is hardly formed. For this reason, it is considered that the mechanism for removing metal contamination as described above is not realized.

  A pn junction diode similar to that of Example 1 was fabricated using each one substrate from which metal contamination was removed by the manufacturing method of the present invention and the conventional manufacturing method, and the initial characteristics of the pn junction diode were evaluated. The number of pn diodes evaluated is 200 per substrate. Assuming that a withstand voltage of 600 V or higher (defined as current <0.1 mA) is a non-defective product, the non-defective product rate was 75% for the pn junction diode according to the manufacturing method of the present invention and 45% for the pn junction diode according to the conventional manufacturing method. The density of epi defects in the drift layer should not be significantly different between the pn junction diode according to the manufacturing method of the present invention and the pn junction diode according to the conventional manufacturing method. Similarly, the density of basal plane dislocations in the drift layer should not be significantly different. As described in Example 1, there is a theory that a basal plane dislocation modified with some metal impurities causes a decrease in breakdown voltage. By reducing the metal contamination on the surface of the drift layer, the metal elements taken into the drift layer during activation heat treatment during the production of the pn junction diode are reduced, and as a result, the base in which the metal impurities are trapped in the drift layer. Although the reduction in the density of plane dislocations may have contributed to the reduction in the number of defective pressure-resistant products and the improvement of the non-defective product rate, details of the mechanism are unknown.

Next, a current of 50 A / cm ² was passed through the pn junction diode and held for 10 hours, and the increase in on-voltage was examined. As a result of measuring 25 pn junction diodes manufactured by the conventional manufacturing method, an increase in the on-voltage of about 1 V was observed with 13; however, in the pn junction diode using the manufacturing method of the present invention, all 25 pn junction diodes were observed. The increase was suppressed to 0.2 V or less. This difference is also considered to be due to a decrease in the density of basal plane dislocations in which the metal impurities are trapped in the drift layer in the pn junction diode according to the manufacturing method of the present invention, as with the breakdown voltage, but the details of the mechanism are unknown. .

  In addition to the (0001) plane used in this embodiment, a (000-1) plane or (11-20) plane substrate may be used. Further, the off angle is not limited to 4 °, and may be any other angle as long as it is about 0 to 8 °. Also, other dimensions may be used for the diameter and thickness of the substrate. Even if these substrates are used, the same effects as those of the present invention are brought about.

It demonstrates using FIG. 11A-G. The present embodiment is an embodiment in which the present invention is applied to the manufacture of a JBS diode in which a Schottky barrier and a pn junction are combined. 11A to 11C are cross-sectional views illustrating the manufacturing process of the JBS diode of the example. A termination region for relaxing the electric field is formed around the JBS diode, but the termination region is omitted in FIG. As shown in FIG. 11C, the JBS diode of this example is provided on the main surface of a silicon carbide single crystal substrate 111, an n-type drift layer 112 containing nitrogen and having a thickness of about 20 μm, and the surface of the drift layer 112 P-type doped layer 113 having a thickness of 1 μm containing Al and a high-concentration p-type layer 114 having a thickness of 0.1 μm provided on top of p-type doped layer 113. I have. The donor concentrations of silicon carbide single crystal substrate 111 and n-type drift layer 112 are 3 × 10 ¹⁸ cm ⁻³ and 2 × 10 ¹⁵ cm ⁻³ , respectively, and p-type doped layer 113 and high-concentration p-type layer are used. The acceptor concentrations of 114 are 2 × 10 ¹⁸ cm ⁻³ and 5 × 10 ¹⁹ cm ⁻³ , respectively.

  A method of manufacturing the JBS diode of this example will be described. First, a silicon carbide single crystal substrate 111 as shown in FIG. 11A is prepared. The silicon carbide single crystal substrate 111 is a 4H—SiC, 4 ° off silicon carbide single crystal substrate having an off angle of 4 ° in the [11-20] direction. The maximum diameter is 76.2 mm and the thickness is 350 μm. The surface is a (0001) plane. A drift layer 112 is formed on the surface of the substrate 111.

  Next, in the step of FIG. 11B, Al ions are implanted into part of the surface of the drift layer 112 to form the p-type doped layer 113. Subsequently, Al ions are implanted into the surface of the p-type doped layer 113 at a higher dose to form a high-concentration p-type layer 114. After forming the p-type doped layer 113 and the high-concentration p-type layer 114, the metal contamination removal process of the present invention was applied. The purpose is to clean silicon carbide single crystal substrate 111 before high-temperature heat treatment for impurity activation.

First, oxygen plasma treatment was performed on the front and back of the silicon carbide single crystal substrate 111. The oxygen plasma treatment was performed using an asher device. The processing conditions are an oxygen flow rate of 100 sccm, a pressure in the reaction chamber of 10 Pa, an RF power of 50 W, and the time is 30 seconds for both the front and back surfaces. The RF power per unit area was 0.1 W / cm ² . The RF power is set sufficiently low so that damage is not introduced into the silicon carbide single crystal substrate. First, the surface side treatment was performed with the surface of the silicon carbide single crystal substrate facing up, and then the back surface treatment was performed with the back surface of the silicon carbide single crystal substrate 111 facing up. By this oxygen plasma treatment, a film mainly composed of silicon dioxide is formed on the front and back surfaces of the silicon carbide single crystal substrate 111. The thickness of the film mainly composed of silicon dioxide was less than 2 nm on the surface composed of the silicon surface.

Thereafter, the substrate was subjected to chemical cleaning with aqua regia, sulfuric acid / hydrogen peroxide mixture, hydrofluoric acid aqueous solution, and ammonia / hydrogen peroxide mixture. The components of the chemical solution used and the cleaning conditions were the same as those for cleaning the silicon carbide single crystal substrate according to the present invention in Example 1. Next, in order to prevent the front and back surfaces of the silicon carbide single crystal substrate 111 from being deteriorated at high temperatures, carbon films 115 and 116 having a thickness of 100 nm are formed on the front and back surfaces of the substrate 111 as shown in FIG. 11C. did. The carbon film is formed by sputtering, but may be formed by other film forming methods such as plasma CVD using methane as a raw material. The reason why the carbon film 116 is formed on the back surface of the substrate is that it has the effect of reducing the contact resistance of the back surface and is effective in enhancing the adhesive force of the back electrode to the substrate 111. In this state, activation annealing was then performed at 1800 ° C. in vacuum. The time kept at 1,800 ° C. is 1 minute. Thereafter, the carbon film was removed by dry etching using oxygen, and the result was as shown in FIG. 11D. The treatment conditions are an oxygen flow rate of 100 sccm, a pressure in the reaction chamber of 10 Pa, an RF power of 50 W, and the time is 90 seconds for both the front and back surfaces. The RF power per unit area was 0.1 W / cm ² . The RF power is set sufficiently low so that damage is not introduced into the silicon carbide single crystal substrate. After the carbon film is removed, the surface of silicon carbide single crystal substrate 111 is exposed to oxygen plasma. The thickness of the film mainly composed of silicon dioxide formed on the surface of silicon carbide single crystal substrate 111 was less than 2 nm on the surface composed of the silicon surface. Thereafter, chemical cleaning with a hydrofluoric acid aqueous solution and a mixed solution of ammonia and hydrogen peroxide was performed on the substrate. The components of the chemical solution used and the cleaning conditions were the same as those for cleaning the silicon carbide single crystal substrate according to the present invention in Example 1.

  Next, as shown in FIG. 11E, a silicon dioxide film 117 is formed by a plasma CVD method, a back nickel film is further formed, and heat treatment is performed at 1000 ° C. for 1 minute in argon using an RTA apparatus. . A layer 118 mainly composed of nickel silicide was formed on the back surface. Next, when an opening is formed in the silicon dioxide film 117, the result is as shown in FIG. 11F. A wet etching technique was used for etching the silicon dioxide film 117. Next, as shown in FIG. 11G, an electrode composed of a laminated film of a nickel film 119 having a thickness of 50 nm and an aluminum film 119 having a thickness of 3 μm was formed on the surface side. A nickel film and an aluminum film were formed using a sputtering apparatus and patterned by a known lithography process and a wet etching technique. On the back surface, a nickel film 121 having a thickness of 100 nm was formed using a sputtering apparatus to form a back electrode. Finally, a protective film made of polyimide resin was formed, which is not shown in FIG. 9F. As described above, the JBS diode of this example was manufactured by the manufacturing method of the present invention.

  The initial characteristics of the JBS diode by the manufacturing method of the present invention were evaluated. The number of evaluated JBS diodes is 100. Assuming that a withstand voltage of 3.5 kV or more (defined as current <0.1 mA) was a non-defective product, the non-defective product rate was 63%. In the case of the conventional manufacturing method, it was 45%, so it was greatly improved. When the reverse leakage current of the diode was measured at 2 kV, the average value of 100 diodes was reduced to about 40% of that obtained by the conventional manufacturing method. It is considered that the leakage current mainly flows through the Schottky barrier diode portion. In the manufacturing method of the present invention, by removing metal contamination on the surface of the silicon carbide single crystal substrate before and after the activation heat treatment, etc., a cleaner electrode / silicon carbide single crystal substrate interface is realized than in the case of the conventional manufacturing method. ing. This is likely to have contributed to the reduction of leakage current.

Next, a current of 70 A / cm ² was applied to the JBS junction diode and held for 10 hours, and the increase in the on-voltage was examined. When 25 JBS diodes manufactured by the conventional manufacturing method were measured, an increase in on-voltage of about 1 V was observed with 14 of them. However, in the JBS diode using the manufacturing method of the present invention, all 25 were 0. It was suppressed to an increase of 3 V or less. Details of the mechanism are unknown, but by applying the manufacturing method of the present invention, the yield rate of JBS diodes is improved compared to the case of using the conventional manufacturing method, the reverse leakage current is reduced, The characteristic fluctuation was also greatly suppressed. In addition to the (0001) plane used in this embodiment, a (000-1) plane or (11-20) plane substrate may be used. Further, the off angle is not limited to 4 °, and may be any other angle as long as it is about 0 to 8 °. Also, other dimensions may be used for the diameter and thickness of the substrate. Even if these substrates are used, the same effects as those of the present invention are brought about.

This will be described with reference to FIGS. 12A-H and 13A-G. This embodiment is an embodiment in which the present invention is applied to manufacture of a MOSFET (Metal Oxide Semiconductor Field Effect Transistor). This is an n-channel play type MOSFET. 12A-H and 13A-G are cross-sectional views showing manufacturing steps of the MOSFET of the embodiment. A silicon carbide single crystal substrate 131 as shown in FIG. 12B is prepared. In order to confirm the effect of the present invention, two substrates 131 were used. The silicon carbide single crystal substrate 131 is a 4H—SiC, 4 ° off silicon carbide single crystal substrate having an off angle of 4 ° in the [11-20] direction. The maximum diameter is 100.0 mm and the thickness is 380 μm. The surface is a (0001) plane. A drift layer 132 having a thickness of 10 μm is formed on the surface of the substrate 131. Substrate 131 is a ^{n +} substrate, the drift layer 132 the n ^- a layer. In the step of FIG. 12B, boron ions are implanted into part of the surface of the drift layer 132 to form the p-type base layer 133. The implantation was performed with silicon dioxide as a mask and the substrate temperature kept at 500 ° C. The dose amount is 1.5 × 10 ¹⁶ cm ⁻² and the concentration is about 1.5 × 10 ¹⁷ cm ⁻³ . As a dopant, aluminum can be used in addition to boron. After implantation, the silicon dioxide in the mask was removed by immersing the substrate in an aqueous hydrofluoric acid solution. Next, the metal contamination removal process of the present invention was applied to one of the two substrates 131, and oxygen plasma treatment was first performed. The oxygen plasma treatment was performed using a dry etching apparatus using microwaves as in Example 2. The treatment conditions were an oxygen flow rate of 200 sccm, a reaction chamber pressure of 5 Pa, a microwave source power of 800 W, an RF bias power of 7 W, and a time of 60 seconds on the surface. The electrode shape of the device is a circle with a diameter of approximately 200 mm.

Therefore, the RF bias power per unit area is 0.022 W / cm ² . By this oxygen plasma treatment, a film containing silicon dioxide as a main component is formed on the surface of the silicon carbide single crystal substrate 131. The thickness of the film mainly composed of silicon dioxide was less than 3 nm on the surface composed of the silicon surface. The dry etching apparatus used for the oxygen plasma treatment on the silicon carbide single crystal substrate was cleaned using chlorine and oxygen mixed gas after the treatment. This is a treatment for preventing the inside of the dry etching from being contaminated by the treatment on the silicon carbide single crystal substrate 131 and the contamination from adhering to another substrate to be treated later. Thereafter, chemical cleaning with aqua regia, sulfuric acid / hydrogen peroxide mixture, hydrofluoric acid aqueous solution, and ammonia / hydrogen peroxide mixture was performed on the substrate again. The components of the chemical solution used and the cleaning conditions were the same as those for the silicon carbide single crystal substrate according to the present invention in Example 2. This series of chemical cleaning was performed on another substrate 131 that was not subjected to oxygen plasma treatment. Next, activation heat treatment was performed on the cleaned substrate 131. It was performed under reduced pressure of about 1 Pa while flowing 100 sccm of Ar at 1850 ° C. for 3 minutes. During the heat treatment, a carbon film having a thickness of 100 nm was used as a protective film as in Example 3. Unlike Example 3, the protective film was formed only on the surface side of the substrate 131 and was removed by the same method and conditions as in Example 3 after the heat treatment.

Subsequently, as shown in FIG. 12C, an n ⁻ epitaxial layer 134 was formed on the surfaces of the two substrates 131 by epitaxial growth. A MOSFET channel layer is formed in the n ⁻ epitaxial layer 134. Epitaxial growth conditions are common to the two substrates 131. The epitaxial growth layer contains nitrogen as a dopant, and its concentration is 1 × 10 ¹⁶ cm ⁻³ . After the epitaxial growth, as shown in FIG. 12D, an n ⁺ source layer 135 was formed in a partial region by nitrogen ion implantation. This implantation was also performed while maintaining the substrate temperature at 500 ° C., and the dose amount was 1 × 10 ¹⁵ cm ² . Next, as shown in FIG. 12E, a part 136 of the epitaxial layer 134 was removed by reactive ion etching. At this time, a silicon dioxide film was used as an etching mask. Thereafter, when the deep base layer 137 is formed again by ion implantation, the result is as shown in FIG. 12F. After removing the etching mask with an aqueous hydrofluoric acid solution, the metal contamination removal process of the present invention was applied to one of the two substrates 131 again. The applied substrate is the same substrate as the substrate to which the metal contamination removal process of the present invention is applied before epitaxial growth. The apparatus used for the oxygen plasma treatment and the treatment conditions are the same as before the epitaxial growth. The cleaning of the apparatus after the plasma treatment was the same. The series of chemical cleaning performed after the oxygen plasma treatment was performed not only on the substrate 131 that was subjected to the oxygen plasma treatment, but also on another substrate 131 that was not subjected to the oxygen plasma treatment.

  Next, as shown in FIG. 12G, a gate oxide film 138 was formed. The oxidation method is dry oxidation, and the oxidation temperature is 1250 ° C. The thickness of the gate oxide film 138 was 50 nm. After gate oxidation, the substrate 131 was heat-treated at 1150 ° C. for 10 minutes in a nitrogen monoxide atmosphere. It has been found that this heat treatment reduces interface state density and improves channel mobility. Next, a polycrystalline silicon film 139 shown in FIG. 12H was formed and processed by an ordinary reactive dry etching method to form a gate electrode 140 as shown in FIG. 13A. When unnecessary portions of the gate oxide film were removed by etching in a pattern surrounding the gate electrode, the gate oxide film 141 of the MOSFET remained as shown in FIG. 13B. When a silicon dioxide film 142 is formed by plasma CVD as shown in FIG. 13C and the portions other than the film 143 on the gate electrode are removed by etching, the result is as shown in FIG. 13D. Subsequently, nickel films 144 and 145 were formed on the front and back surfaces of the substrate by sputtering as shown in FIG. 13E. In this state, a heat treatment in argon at 900 ° C. for 3 minutes is performed, and the films 146 and 147 mainly composed of nickel silicide are in contact with the silicon carbide single crystal substrate 131 or the silicon carbide epitaxial layer 134 and the nickel films 144 and 145. After removing the unreacted nickel films 144 and 145, the result is as shown in FIG. 13F. Unreacted nickel films 144 and 145 were selectively removed using a mixed solution of sulfuric acid and hydrogen peroxide aqueous solution. Thereafter, when a source electrode 148 made of aluminum is formed on the front surface side and a drain electrode 149 made of nickel is formed on the back surface side, the result is as shown in FIG. 13G. Although not shown, a contact hole is opened on the gate electrode 140 and connected to a gate electrode pad made of aluminum.

The characteristics and reliability of the MOSFET fabricated as described above were evaluated. The MOSFET on the substrate to which the metal contamination removing process of the present invention was applied and the MOSFET on the substrate by the conventional method were compared. The chip size of the MOSFET is 5.2 mm × 5.2 mm and the active area is 5.0 mm × 5.0 mm. First, the breakdown voltage was examined. Assuming that the withstand voltage> 1200 V is a non-defective product, the yield rate of the MOSFET according to the method of the present invention is 77%, and the yield rate of the MOSFET according to the conventional method is 39%. By applying the metal contamination removal process of the present invention before the epitaxial growth, it is considered that the density of critical defects contained in the epitaxial layer is reduced, and the yield rate is improved. The channel mobility was equal to the MOSFET according to the present invention and the MOSFET according to the conventional method, and was about 60 cm ² / Vs at the maximum. Next, in order to evaluate the reliability of the gate insulating film, TZDB (Time Zero Dielectric Breakdown) and TDDB (Time Dependent Dielectric Breakdown) were measured for each of the 30 chips of the MOSFET according to the present invention and the MOSFET according to the conventional method. . The results are shown in FIGS. As for TZDB, all the MOSFETs according to the present invention showed high dielectric breakdown strength of 10.0 MV / cm or more. On the other hand, in the MOSFET by the conventional method, a considerable number of chips having a lower value of 9.0 MV / cm or less were included. Further, when comparing the result of TDDB performed at a stress electric field of 9.5 MV / cm with the total amount of dielectric breakdown charges at which the cumulative dielectric breakdown rate is 63%, the MOSFET of the present invention is about 4 times as large as the MOSFET by the conventional method. The result was 6 C / cm ² .

This reflects that many of the MOSFETs according to the conventional method have reached dielectric breakdown with a relatively short time stress. By applying the metal contamination removal process of the present invention before the gate oxidation, it is considered that the reliability of the gate oxide film is improved, and the dielectric breakdown strength of TZDB and the lifetime of TDDB are improved.
In addition to the (0001) plane used in this embodiment, a (000-1) plane or (11-20) plane substrate may be used. Further, the off angle is not limited to 4 °, and may be any other angle as long as it is about 0 to 8 °. Also, other dimensions may be used for the diameter and thickness of the substrate. Even if these substrates are used, the same effects as those of the present invention are brought about.

This will be described with reference to FIGS. 16A-G and 17A-E. In this embodiment, the present invention is applied to the manufacture of a trench MOSFET. 16A to 16G and FIGS. 17A to 17E are cross-sectional views illustrating manufacturing steps of the MOSFET of the example. A silicon carbide single crystal substrate 171 as shown in FIG. 16A is prepared. The silicon carbide single crystal substrate 171 is a 4H—SiC, 4 ° off silicon carbide single crystal substrate having an off angle of 4 ° in the [11-20] direction. The maximum diameter is 100.0 mm and the thickness is 380 μm. The surface is a (0001) plane. A drift layer 172 having a thickness of 7 μm is formed on the surface of the substrate 171. The substrate 171 is an n ⁺ substrate, and the drift layer 172 is an n ⁻ layer having an impurity concentration of 1 × 10 ¹⁶ cm ⁻³ . On the drift layer 172 has a thickness of 1 [mu] m, the impurity concentration of 1 × ¹⁰ 17 ^cm p-type base layer 173 ^-3, thickness 0.5 [mu] m, the impurity concentration of 1 × ¹⁰ 19 ^{cm -3} of ^{n +} -type source layer 174 Is formed. All of these layers are formed by epitaxial growth, but at least a part of the layers can be formed by ion implantation. In that case, after the ion implantation, the substrate 171 is subjected to the activation heat treatment similar to that in Examples 3 and 4. Subsequently, a trench 175 as shown in FIG. 16B is formed. The trench 175 was formed by a dry etching method. The apparatus used for the etching is a high-density plasma etching apparatus using ICP (Inductively Coupled Plasma). Although the etching gas is sulfur hexafluoride and silicon dioxide is used for the mask, the same processing can be performed using a mixed gas obtained by adding oxygen to sulfur hexafluoride or a mixed gas of chlorine and oxygen. After the silicon dioxide of the mask was removed with a hydrofluoric acid aqueous solution, the metal contamination removal process of the present invention was applied to the substrate 171. First, oxidation treatment with ozone was performed. An oxidation furnace was used for the treatment. On the substrate 171 kept at 700 ° C., ozone having a volume concentration of about 50% was supplied from a high concentration ozone generator to oxidize the surface of the substrate 171. In this embodiment, oxidation by ozone is applied, but the same effect can be obtained by the oxygen plasma treatment similar to that of the second embodiment. Further, the oxide film may be formed by thermal oxidation such as dry oxidation, wet oxidation, ISSG (In-situ Steam Generation) oxidation, or other methods such as anodic oxidation. It is desirable that the thickness of the film mainly composed of silicon dioxide formed on the surface of the silicon carbide single crystal substrate 171 is less than 5 nm by adjusting the oxidation temperature and the oxidation time according to each method. Thereafter, chemical cleaning with aqua regia, sulfuric acid / hydrogen peroxide mixture, hydrofluoric acid aqueous solution, and ammonia / hydrogen peroxide mixture was performed on the substrate again. The components of the chemical solution used and the cleaning conditions were the same as those for the silicon carbide single crystal substrate according to the present invention in Example 2. Next, as shown in FIG. 16C, a first gate insulating film 176 was formed. The first gate insulating film 176 is a thin thermal oxide film having a thickness of 7 nm, and as shown in FIG. 16D, a thick CVD film 177 serving as a second gate insulating film is stacked thereon. The thermal oxide film 176 of this example was formed by dry oxidation at 1300 ° C. The CVD film 177 was formed by a low pressure CVD method using TEOS (Tetra Ethyl Ortho Silicate) and oxygen as raw materials. The temperature of the substrate 171 at the time of forming the CVD film is 700 ° C. After the stacked gate insulating films 176 and 177 were formed, heat treatment was performed at 1200 ° C. for 5 minutes in nitrogen monoxide. The total thickness of the gate insulating films 176 and 177 after the heat treatment is 55 nm. Although the gate insulating films 176 and 177 of this embodiment are formed by the above method, the method for forming the gate insulating films 176 and 177 is not limited thereto. For example, instead of a thermal oxide film, an amorphous silicon film formed in advance or a film formed by oxidation of a polycrystalline silicon film is used, and a CVD film similar to that of this embodiment is laminated thereon. It is also possible to do. In the case of using an amorphous silicon film or a polycrystalline silicon film, the metal contamination removal process of the present invention similar to this embodiment is applied before the formation of the amorphous silicon film or the polycrystalline silicon film.

  Next, when a polycrystalline silicon film 178 doped with boron is formed and patterned as shown in FIG. 16E, a gate electrode 179 as shown in FIG. 16F is formed. Further, when a silicon dioxide film 180 serving as an interlayer insulating film is formed by a plasma CVD method using TEOS and oxygen as raw materials, the result is as shown in FIG. 16G. When portions 181 on both sides of the trench are etched until the p-type base layer 173 on the surface of the silicon carbide single crystal substrate 171 is exposed, the result is as shown in FIG. 17A. Both silicon dioxide etching and silicon carbide etching were performed by dry etching. Subsequently, nickel films 182 and 183 were formed on the front and back surfaces of the substrate 171 by sputtering, and heat treatment was performed at 1000 ° C. for 1 minute in argon using an RTA apparatus. When unreacted nickel was removed with a mixed solution of sulfuric acid and hydrogen peroxide, the result was as shown in FIG. 17C. Layers 184 and 185 mainly composed of nickel silicide were formed on the exposed surface of the p-type base layer and on the back surface. After a nickel film 186 to be a drain electrode was additionally formed on the back surface by sputtering, an aluminum film 187 to be a source electrode was formed by sputtering as shown in FIG. 17E. Although not shown, a contact hole is opened on the gate electrode 179 and connected to a gate electrode pad made of aluminum.

  The current-voltage characteristics of the trench MOSFET manufactured as described above were evaluated. The MOSFET on the substrate to which the metal contamination removing process of the present invention was applied and the MOSFET on the substrate by the conventional method were compared. The chip size of the MOSFET is 4.2 mm × 4.2 mm, and the active area is 4.0 mm × 4.0 mm. The yield rate of MOSFETs by the method of the present invention was 63%, and the yield rate of MOSFETs by the conventional method was significantly improved from 35%. By applying the metal contamination removal process after the trench processing and before the formation of the gate oxide film, it is considered that the density of critical defects contained in the gate oxide film is reduced and the yield rate of MOSFETs is improved. Similarly to Example 4, TZDB and TDDB were also measured for the MOSFET according to the present invention and the MOSFET according to the conventional method. As a result, as in Example 4, the MOSFET of the present invention showed higher dielectric breakdown strength than the conventional MOSFET. In addition, the total amount of breakdown charge at which the cumulative breakdown rate of the MOSFET of the present invention is 63% was about five times that of the MOSFET according to the conventional method. By applying the metal contamination removal process of the present invention after trench processing and before gate oxidation, the reliability of the gate oxide film is improved, and the dielectric breakdown strength of TZDB and the life of TDDB are improved. It is thought.

  In general, since a dry etching apparatus uses a plasma of a reactive gas, it is difficult to maintain a cleanness. Therefore, it is difficult to eliminate any metal contamination transferred from the dry etching apparatus to the substrate during etching. For this reason, there is a high possibility that metal contamination exists on the substrate surface after the trench processing. The reason that the metal contamination removal process of the present invention was effective in improving the yield rate and reliability of the trench type MOSFET is considered to be because metal contamination was removed at this stage.

In this embodiment, the trench type MOSFET is described. However, the trench type junction FET as well as the MOSFET brings about the effect of improving the yield rate and increasing the reliability.
In addition to the (0001) plane used in this embodiment, a (000-1) plane or (11-20) plane substrate may be used. Further, the off angle is not limited to 4 °, and may be any other angle as long as it is about 0 to 8 °. Also, other dimensions may be used for the diameter and thickness of the substrate. Even if these substrates are used, the same effects as those of the present invention are brought about.

DESCRIPTION OF SYMBOLS 41 ... Silicon carbide single crystal substrate, 42 ... Drift layer, 43 ... P-type doped layer, 44 ... High concentration p-type layer, 45 ... Silicon dioxide film, 46, 47 ... Layer mainly composed of nickel silicide, 48 ... Aluminum film, 49 ... Nickel film, 50 ... Protective film made of polyimide resin, 111 ... Silicon carbide single crystal substrate, 112 ... Drift layer, 113 ... P-type doped layer, 114 ... High concentration p-type layer, 114, 115 ... Carbon 116, silicon dioxide film, 117 ... layer containing nickel silicide as a main component, 118 ... nickel film, 119 ... aluminum film, 120 ... nickel film, 131 ... silicon carbide single crystal substrate, 132 ... drift layer, 133 ... p-type base layer, 134 ... channel layer, 135 ... n ⁺ source layer, 138 ... gate oxide film, 140 ... gate electrode, 148 ... source electrode, 149 ... drain 171 ... Silicon carbide single crystal substrate, 172 ... Drift layer, 173 ... P-type base layer, 174 ... n ⁺ source layer, 176, 177 ... Gate oxide film, 179 ... Gate electrode, 186 ... Drain electrode, 188 ... Source electrode.

Claims (8)

Preparing a silicon carbide substrate having an epitaxial layer on which ion implantation is performed on the surface side ;
Performing activation annealing in a state where a carbon film is provided on the front and back surfaces of the silicon carbide substrate ;
An oxidation step of oxidizing the front and back surfaces of the silicon carbide substrate to form an oxide film;
And a step of removing the oxide film formed in the oxidation step by immersing in an etching solution containing hydrofluoric acid . The method of manufacturing a semiconductor device according to claim 1 , wherein the carbon films provided on the front surface and the back surface each have a film thickness of about 100 nm.   The method of manufacturing a semiconductor device according to claim 1, wherein the activation annealing is performed at a temperature of 1800 ° C. in a vacuum for about 1 minute.   The method of manufacturing a semiconductor device according to claim 1, wherein the substrate has an off angle in a range of 0 to 8 °. The method of manufacturing a semiconductor device according to claim 4 , wherein the substrate has an off angle of 4 °. 2. The method of manufacturing a semiconductor device according to claim 1 , wherein a main surface of the substrate is any one of a (0001) plane, a (000-1) plane, and a (11-20) plane. Forming a p-type doped layer by ion-implanting Al into part of the epitaxial layer;
The method of manufacturing a semiconductor device according to claim 1, wherein a high concentration p-type layer is formed by ion-implanting Al into a part of the p-type doped layer. The method of manufacturing a semiconductor device according to claim 7 , wherein the p-type doped layer has a thickness of about 1 μm, and the high-concentration p-type layer has a thickness of 0.1 μm.

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