METHOD FOR MANUFACTURING A SEMICONDUCTOR COMPONENT AND SEMICONDUCTOR COMPONENT

Abstract

Method for fabricating a semiconductor device (1) comprising the following steps: forming a semiconductor device (2a, 2b) in a semiconductor substrate (3) having a first major surface and a second major surface and a first conductivity type with a first charge carrier concentration, wherein the semiconductor device (2a, 2b) serves to conduct a current between the first major surface and the second major surface, wherein the formation of a semiconductor device (2a, 2b) includes: implanting acceptor ions from the second major surface of the semiconductor substrate (3), performing a wet process for accumulating hydrogen atoms in an impurity region (23, 35) of the semiconductor substrate (3) in which the acceptor ions are implanted from the second major surface of the semiconductor substrate (3),Applying charged particle radiation from the second main surface of the semiconductor substrate (3) and, after carrying out a wet process and applying charged particle radiation, carrying out a heat treatment process on the semiconductor substrate (3) to form a field stop layer (21) of the first conductivity type with a second charge carrier concentration that is higher than the first charge carrier concentration in the semiconductor substrate (3), wherein the hydrogen atoms have a distribution in which the concentration of hydrogen atoms decreases continuously from the surface of the impurity region (23, 35) to a boundary between the impurity region (23, 35) and the field stop layer (21).

Description

TECHNICAL FIELDThe present invention relates to a method for manufacturing a semiconductor device and to a semiconductor device, in particular to a method for manufacturing a semiconductor device including a field stop layer and to a semiconductor device.PRIOR ARTSome semiconductor devices, e.g. an insulated gate bipolar transistor (IGBT) or a diode, for example as power semiconductor devices, have a structure including a field stop layer as an improving measure for achieving lower losses.A field stop layer is provided within a drift layer at a position farther from a breakdown voltage principal point of the drift layer than a lower resistance layer than the drift layer and the same conductivity type (normally n-type) as the drift layer. The field stop layer is formed after a semiconductor functional layer such as a semiconductor element is formed on a front side of a silicon wafer. As boundary conditions in forming the field stop layer, therefore, the field stop layer needs to be formed under conditions that do not affect the semiconductor functional layer formed on the front surface of the silicon wafer.In the formation of a field stop layer, the heat treatment is performed after introduction of an impurity. In recent years, silicon wafers (semiconductor substrates) are reduced in thickness to reduce power consumption of the power device. The reduced thickness of the silicon wafer can lead to cracks in the silicon wafer as a result of the heat treatment. Thus, methods have been proposed in which the heat treatment is performed at lower temperatures in order to suppress the cracking of the silicon wafer by the heat treatment in the formation of the field stop layer. That is, a method of forming a field stop layer by conversion into donors by proton irradiation (Patent Document 1 and Patent Document 2) has been proposed.Patent Document 1 proposes a method for recovering, by heat treatment, radiation-induced defects generated by proton irradiation (hydrogen atoms) of a single-crystal silicon wafer and forming composite defects (hydrogen-related donors) by reaction of the radiation-induced defects and the hydrogen atoms near a center portion of the protons, thereby forming a highly concentrated field stop layer.Patent Document 2 proposes a method of repeating proton irradiation from the back side of an n-type semiconductor substrate a plurality of times to thereby form an n-type field stop layer. In the plurality of proton irradiations, the sites of radiation-induced defects left from previous proton irradiation are used as targets for the next proton irradiation. At this time, hydrogen atoms are supplied to a region having many radiation-induced defects formed by the previous proton irradiation to eliminate free bonds, resulting in the conversion of the radiation-induced defects to donors. Thereby, according to Patent Document 2, an n-type field stop layer having a highly concentrated hydrogen-related donor layer can be formed while suppressing the increase in leakage current due to residual radiation-induced defects.US 2014 / 0 175 620 A1 discloses a method for manufacturing a semiconductor device, the method comprising: forming a first semiconductor region on a front surface of a substrate, the first semiconductor region including an active element that regulates current flowing in a thickness direction of the substrate; grinding a back surface of the substrate; after the grinding, performing a first etching that etches the back surface of the substrate with a chemical solution containing phosphorus; after the first etching, performing a second etching that etches the back surface with an etching method with a lower etching rate than the first etching; and after the second etching, forming a second semiconductor region through which the current is to flow by implanting impurities from the back surface of the substrate.From US 2017 / 0 271 447 A1 a semiconductor device is known, comprising: a semiconductor substrate doped with impurities; an electrode on the front surface side provided on a side of the front surface of the semiconductor substrate; and an electrode on the back surface side provided on a side of the back surface of the semiconductor substrate. Here, the semiconductor substrate includes: a peak region disposed on the back surface side of the semiconductor substrate and having one or more peaks of the impurity concentration; a high concentration region disposed closer to the front surface than the peak region and having a lower impurity concentration than the one or more peaks; and a low concentration region disposed closer to the front surface than the high concentration region and having a lower impurity concentration than the high concentration region.PRIOR ARTPatent Document 1: Japanese Patent Application Laid-Open JP 2001-160 559 APatent Document 2: Japanese Patent Application Laid-Open JP 2015-130 523 ABRIEF DESCRIPTION OF THE INVENTIONProblems to be Solved by the InventionA problem in a method of forming a field stop layer by proton irradiation is that radiation-induced defects generated in a silicon wafer by proton irradiation of the silicon wafer remain after performing a heat treatment process.The present invention has been made in view of such problems, and has an object to provide a method of manufacturing a semiconductor device in which residual radiation-induced defects are reduced, and another object to provide a semiconductor device in which residual radiation-induced defects are reduced.Means for Solving the ProblemsThe object is achieved by a method for producing a semiconductor component having the features of claim 1. advantageous developments of the method result from dependent claims 2 to 8.Effect of the InventionAccording to the method for manufacturing a semiconductor device according to the present invention, a semiconductor device having reduced radiation-induced defects remaining in the field stop layer can be manufactured by the wet process and the proton irradiation.In the semiconductor device according to the present invention, radiation induced defects remaining in the field stop layer can be reduced.BRIEF DESCRIPTION OF THE DRAWINGSFIG. 1 is a cross-sectional view of a semiconductor device including an IGBT according to a first embodiment; FIG. 2 is a flow chart showing the steps of manufacturing the semiconductor device in the first embodiment; FIG. 3 is a flow chart showing the steps of forming a diffusion layer on a back surface of a semiconductor substrate in the first embodiment; FIG. 4 shows a distribution of boron and a distribution of hydrogen atoms in the semiconductor substrate after the acceptor ion implantation and a wet process in the first embodiment; FIG. 5 is a diagram showing a distribution of hydrogen atoms and a distribution of radiation-induced defects in the semiconductor substrate after proton irradiation in the first embodiment; FIG. 6 is a diagram showing a distribution of hydrogen atoms introduced by proton irradiation and a distribution of hydrogen atoms introduced by the wet process in the semiconductor substrate after performing a heat treatment process in the first embodiment; FIG. 7 is a graph showing calculated distributions of carrier concentration in a field stop layer in the first embodiment; FIG. 8 is a graph showing distributions of carrier concentration in the field stop layer in a completed semiconductor device in the first embodiment; FIG. 9 is a flow chart showing the steps of forming a diffusion layer from the back side of a semiconductor substrate in a semiconductor device according to a second embodiment; FIG. 10 is a diagram showing a distribution of hydrogen atoms and a distribution of radiation-induced defects in the semiconductor substrate after application of helium ion radiation in the second embodiment; FIG. 11 is a graph showing a distribution of hydrogen atoms introduced by a wet process after a heat treatment process is performed in the second embodiment; FIG. 12 is a graph showing calculated distributions of carrier concentration in a field stop layer in the second embodiment; FIG. 13 is a diagram showing distributions of carrier concentration in the field stop layer in a completed semiconductor device in the second embodiment; FIG. 14 is a cross-sectional view of a semiconductor device including a diode according to a third embodiment; FIG. 15 is a flow chart showing the steps of forming a diffusion layer on a back surface of a semiconductor substrate in the third embodiment; and FIG. 16 is a flow chart showing the steps of forming a diffusion layer on a back surface of a semiconductor substrate in a semiconductor device according to a fourth embodiment.DESCRIPTION OF THE EMBODIMENTSEmbodiment 1A semiconductor device including an IGBT will be described here as an example of a semiconductor device. First, a region denoted by "n" in the present specification and drawings means that electrons are the majority carriers. A region marked "p" means that holes are the majority carriers. In addition, "+" or "-" is attached to "n" or "p". The "+" indicates that the carrier concentration is relatively high. The "-" indicates that the carrier concentration is relatively low.As shown in FIG. 1, in a semiconductor device 1 including an IGBT 2 a, a p-type base layer 9 is formed from a first main surface of a semiconductor substrate 3 to a certain depth (depth A), an n+type emitter layer 11 and a p+type contact layer 7 are formed in the p-type base layer 9. The n+ type emitter layer 11 is formed from a surface of the p type base layer 9 to a depth (depth B) less than the depth A. The p+ type contact layer 7 is formed to a position less than the depth A and less than the depth B from the surface of the p type base layer 9.A trench 3 aextending from the first main surface of the semiconductor substrate 3 to a position deeper than the depth A has a gate electrode 15 formed therein, between which a gate insulating film 13 is interposed (MOS gate structure). An interlayer insulating film 17 is formed to cover the gate electrode 15. An emitter electrode 19 is formed to cover the interlayer insulating film 17. The emitter electrode 19 is in contact with the n+ type emitter layer 11 and the p+ type contact layer 7.A p+ type collector layer 23 is formed to a certain depth (depth C) from a second main surface of the semiconductor substrate 3. The n-type field stop layer 21 is formed from the depth C to a position deeper than the depth C. A collector electrode 25 is formed in contact with the p+ type collector layer 23. An n-type drift layer 5 (a portion of the semiconductor substrate 3) is provided between the n-type field stop layer 21 and the p-type base layer 9.As described later, the semiconductor device 1 including the above-described IGBT 2 ahas a profile of the carrier concentration in the n-type field stop layer 21 and a manufacturing method different from a profile of the carrier concentration in the n-type field stop layer and a manufacturing method of a semiconductor device including a conventional IGBT.In a conventional method for forming an n-type field stop layer with hydrogen-related donors by proton irradiation, it is necessary to leave radiation-induced defects generated by the proton irradiation without completely recovering them by a heat treatment method after the proton irradiation. In the conventional method, therefore, the radiation-induced defects remain in the n-type field stop layer.Thus, photoluminescence examination of an n-type field stop layer from proton irradiation revealed that radiation-induced defects remain between interstitial silicon pairs in the n-type field stop layer. It was also found that a longer heat treatment time or temperature taken into account in the recovery of the radiation-induced defects resulted in the recovery of the radiation-induced defects but resulted in a lower activation level of donors.Accordingly, in forming an n-type field stop layer only with proton irradiation, it is inevitable that a large number of radiation-induced defects remain, resulting in a high leakage current in an IGBT. Moreover, in the formation of an n-type field stop layer only with proton irradiation, the activation ratio of donors is low. Accordingly, the formation of an n-type field stop layer is associated with an increased radiation dose of protons and increased manufacturing cost.To solve these problems, the semiconductor device described above is characterized by using a wet method in combination with irradiation of charged particles (protons or helium ions) in a back surface step to reduce radiation-induced defects remaining in the n-type field stop layer 3.Thus, an example of a method for manufacturing the above semiconductor device will be described below. Proton irradiation as charged particles is described herein. Hereinafter, steps for manufacturing parts functioning as IGBTs, such as a MOS gate structure, can be realized by a known manufacturing method and thus can be easily described.First, a flowchart of the method for manufacturing the semiconductor device will be described with reference to FIG. 2. First, in step S 1, various diffusion layers are formed on a front surface side (first main surface side) of the semiconductor substrate. After the introduction of the semiconductor substrate, a trench MOS gate structure and the like including a p+ type contact layer 7, a p type base layer 9, an n+ type emitter layer 11, a gate insulating layer 13 and a gate electrode 15 is formed on the first main surface side of the semiconductor substrate 3 by a photolithography step, an ion implantation step, a heat treatment step and the like (see FIG. 1 ).Next, in step S 2, an electrode is formed on the front surface side of the semiconductor substrate. A metal layer is formed to cover the p+ type contact layer 7, the n+ type emitter layer 11 and the like by, for example, a vacuum sputtering method. The metal layer is patterned by a photolithography and an etching step to form the emitter electrode 19. The emitter electrode 19 is in ohmic contact with the p+ type contact layer 7 and the n+ type emitter layer 11 (see FIG. 1 ).Next, in step S 3, a surface protection layer covering the first main surface of the semiconductor substrate is formed. A silicon nitride film and the like are formed as a surface protection film, for example. Alternatively, the protective layer may also be formed by, for example, a spin coating method. Further, a protective layer in the form of a layer may be used. Subsequently, in step S 4, the semiconductor substrate is reduced in thickness. The thickness reduction of the semiconductor substrate is performed by grinding the back surface (second main surface) of the semiconductor substrate. The thickness of the semiconductor substrate is determined depending on the breakdown voltage.Next, in step S 5, a diffusion layer is formed on the back surface (second main surface side) of the semiconductor substrate. Ions of a group 13 element are implanted from the back side of the semiconductor substrate. The element from "Group 13" is represented in the Periodic Table as element "Group III". After the protective layer is formed on the back surface (second main surface side) of the semiconductor substrate, a wet process including chemical solution and proton irradiation is performed. Subsequently, a heat treatment process is performed to form the n-type field stop layer 21 and the p+type collector layer 23 (see FIG. 1 ). This step is an important part of this invention and will be described in detail later.Next, in step S 6, an electrode is formed on the back surface of the semiconductor substrate. A metal layer is formed to cover the p+ type collector layer 23 by a vacuum sputtering method and the like to form the collector electrode 25. The collector electrode 25 is in ohmic contact with the p+ type collector layer 23, and the semiconductor device including the IGBT having the n type field stop layer 21 is completed therewith (see FIG. 1 ).In this method, by the inclusion of step S 5, radiation-induced defects in the n-type field stop layer are reduced, and moreover, the n-type field stop layer is formed with a high activation degree of hydrogen-related donors. Next, the step of forming the diffusion layer on the back surface of the semiconductor substrate will be described in detail.In a hydrogen-related donor formation step using proton irradiation, the balance between the concentration of radiation-induced defects and the concentration of hydrogen atoms introduced into the semiconductor substrate affects the activation ratio of the hydrogen-related donors. The "activation" as used herein means the reaction of the radiation-induced defects and the hydrogen atoms formed in the semiconductor substrate by the proton irradiation. In particular, it is believed that a negatively charged hydrogen atom binds to a free silicon bond of a radiation-induced defect to form a covalent bond, and that an additional electron is released from the free bond or hydrogen atom to form a donor. The activation ratio is the ratio of the amount of newly generated donors (denominators) to the amount of emitted protons (denominators).In proton irradiation, the distribution of hydrogen atoms is concentrated around a central portion of proton irradiation. Thus, there is a deficiency of hydrogen atoms in a region closer to the back surface of the semiconductor substrate than the average region. In other words, only radiation-induced defects are formed in this region. This lowers the activation ratio of hydrogen-related donors.A radiation-induced defect whose free bonding has not been terminated by a hydrogen atom has a degree of impurity in the forbidden band of a semiconductor energy band, causing a restricted mobility or an increased leakage current in the semiconductor device. The leakage current is a current flowing when a reverse bias voltage is applied to a semiconductor chip (semiconductor device), and the current flows by generation of electrons and holes from a radiation-induced defect.Thus, it has been found by the inventors that a larger amount of hydrogen atoms should be diffused into a semiconductor substrate to reduce the residual radiation-induced defects whose free bonds are not terminated and form an n-type field stop layer having a high activation ratio. Based on this result, the inventor found a method of introducing hydrogen atoms into a semiconductor substrate in wet process in addition to proton irradiation in step S 5.As illustrated in FIG. 3, first, in step T 1, acceptor ions are implanted from the back surface of the semiconductor substrate. The acceptor ions may be, for example, boron ions, but are not limited to the boron ions as long as they are ions of a group 13 element. Subsequently, a local heat treatment that does not affect the front surface of the semiconductor substrate is performed by laser heat treatment to activate the implanted acceptor ions.Next, in step T 2, a wet process is performed by dipping the semiconductor substrate in a chemical solution. In this step T 2, a protection layer is applied to the front surface of the semiconductor substrate to protect the emitter electrode 19 (see FIG. 1 ) of the IGBT formed on the front surface of the semiconductor substrate from the chemical solution. As the protective layer, a protective layer protecting the front surface of the semiconductor substrate in the step of reducing the thickness of the semiconductor substrate may be used.Next, with the protective layer provided on the front surface of the semiconductor substrate, a wet process for diffusing hydrogen atoms into the semiconductor substrate from the back surface of the semiconductor substrate is performed. A chemical solution used for this wet process is preferably a chemical solution including an acid such as hydrofluoric acid. This wet process can be performed at room temperature, but is preferably performed while the chemical solution is heated to a temperature of about 50 to 90°C to increase the amount of diffused hydrogen atoms. The acid used may also be hydrochloric acid, sulfuric acid or nitric acid, other than hydrofluoric acid.This wet process introduces hydrogen atoms into the semiconductor substrate. It is known that a hydrogen atom chemically bonds with a boron atom introduced during proton implantation to form a B-H bond. B-H binding is considered stable at room temperature. Accordingly, the hydrogen atoms are not evaporated from inside the semiconductor substrate when the semiconductor substrate is kept at room temperature after the wet process. In this way, the wet process is performed to enrich hydrogen atoms in a region of the semiconductor substrate in which the acceptor ions have been implanted.Next, in step T 3, the proton irradiation is applied to the back surface of the semiconductor substrate. Since the proton irradiation is provided only once in the sequence of steps of the semiconductor device, the average range of the protons is the location of the depth of the n-type field stop layer that is the farthest from the back surface of the semiconductor substrate. Therefore, the energy depth of proton irradiation needs to be determined in consideration of the breakdown voltage required by the IGBT. After calculation with simulation software of the inventor, it was estimated that, at an incident proton energy of, for example, 1 MeV, the distance from the back surface of the semiconductor substrate to the end (depth) of the n-type field stop layer is about 16 μm.A high radiation dose of protons is desirable because the carrier concentration in the n-type field stop layer is thereby increased. However, as the radiation dose increases, radiation-induced defects and manufacturing costs also increase. A low radiation dose of protons, on the other hand, causes a reduction of the radiation-induced defects, which leads to the desired charge carrier concentration in the n-type field stop layer not being able to be adjusted. For this reason, the inventor has set the proton irradiation to 5×10 11 / cm 2 to 1×10 15 / cm 2.In this way, the wet process and the proton irradiation are performed to form radiation-induced defects and enrich hydrogen atoms in a region on the back surface of the semiconductor substrate. Although the proton irradiation has been described as being performed after the wet process is performed in the above method, the wet process may be performed after the proton irradiation.Next, in step T 4, a heat treatment process is performed on the semiconductor substrate to diffuse the hydrogen atoms accumulated in the semiconductor substrate. The heat treatment process is carried out, for example, in a diffusion furnace. The heat treatment process is performed on the semiconductor substrate for 0.5 hour to 5 hours at a temperature of 300° C. to 500° C. under a shielding gas atmosphere. The heat treatment process is preferably performed at a temperature of 350° C. to 450° C. in order to efficiently activate the hydrogen-related donors. The inventor has found that the hydrogen-related donors cannot be formed efficiently when the temperature is below 300°C or above 500°C and when the time is below 0.5 hours or above 5 hours.By this heat treatment method, the hydrogen atoms supplied from the proton irradiation are diffused from the inside of the semiconductor substrate toward the back side of the semiconductor substrate and also diffused to the side opposite to the back side of the semiconductor substrate from the inside of the semiconductor substrate. In addition, the hydrogen atoms supplied by the wet process are diffused into the semiconductor substrate from the back surface of the semiconductor substrate.As described above, in a region (region R) from the back surface of the semiconductor substrate to the position of the average proton region, radiation-induced defects are formed by the proton irradiation. In the heat treatment method, among the hydrogen atoms supplied and diffused by the proton irradiation, hydrogen atoms diffused from the center region location to the back surface of the semiconductor substrate and radiation-induced defects formed in the region R, hydrogen atoms diffused from the wet process to the semiconductor substrate from the back surface of the semiconductor substrate are detected.Due to the trapping of the hydrogen atoms by the radiation-induced defects, the free silicon bonds of the radiation-induced defects are eliminated. This converts some of the radiation-induced defects into hydrogen-related donors and renders radiation-induced defects leading to mobility restriction electrically inactive. The radiation-induced defects can thus be reduced. Moreover, by providing the proton irradiation once, the manufacturing cost can also be reduced compared to an example in which the proton irradiation is performed multiple times.Next, a supplementary explanation will be given of the above-described step S5 with respect to concentration profiles. First, FIG. 4 shows distributions of the concentration of impurity (boron) and the concentration of hydrogen atoms in the depth direction from the back surface of the semiconductor substrate after the acceptor ions are implanted and the wet process is performed. As shown in FIG. 4, it can be seen that the boron (acceptor ions) is distributed over a relatively flat region of up to about 1 μm from the back side of the semiconductor substrate (see BP, HWP). Boron traps a hydrogen atom to form a B-H bond. Accordingly, the hydrogen atoms have a depth-directional distribution analogous to the depth-directional distribution of boron.Further, FIG. 5 shows distributions of the concentration of impurity (boron) and the concentration of hydrogen atoms in the depth direction from the back side of the semiconductor substrate after the proton irradiation. Proton irradiation results in accumulation of hydrogen atoms and formation of radiation-induced defects in the semiconductor substrate. The accumulated hydrogen atoms correspond not to the presence of the minute amount of hydrogen mixed in a semiconductor manufacturing process, but to hydrogen in a concentration in the range of 1×10 16 to 1×10 21 atoms / cm 3, which corresponds to the same level as the implanted acceptor ions. As illustrated in FIG. 5, the distribution of the hydrogen atoms is concentrated at the location of an average range of about 16 μm from the back surface of the semiconductor substrate (see HPP). The radiation-induced defects are distributed so as to be concentrated around the location of the central region and to decrease toward the back surface of the semiconductor substrate around the location of the central region (see DP).Next, FIG. 6 shows distributions of the concentration of hydrogen atoms by the proton irradiation and the concentration of hydrogen atoms by the wet process in the depth direction from the back surface of the semiconductor substrate after the heat treatment method. As illustrated in FIG. 6, it can be seen that the distributions of the concentration of hydrogen atoms by the proton irradiation and the concentration of hydrogen atoms by the wet process broaden in the depth direction of the semiconductor substrate.By the heat treatment method, the radiation-induced defects and the hydrogen atoms around the central portion position from the back surface of the semiconductor substrate react with each other to generate hydrogen-related donors. Moreover, the hydrogen atoms introduced by the wet process and diffused into the semiconductor substrate from the back surface of the semiconductor substrate and some of the radiation-induced defects formed between the back surface of the semiconductor substrate and around the center region location react with each other to generate hydrogen-related donors, and moreover, radiation-induced defects causing mobility restriction are made electrically ineffective. The radiation-induced defects are thus reduced.In the method for manufacturing the semiconductor device described above, a large amount of hydrogen atoms are supplied from the back surface of the semiconductor substrate by the wet method, as compared with the methods proposed in Patent Document 1 and Patent Document 2. Thereby, according to the heat treatment method, a desired amount of hydrogen-related donors can be generated by the reaction of the hydrogen atoms and the radiation-induced defects, and the radiation-induced defects can be effectively canceled to suppress the restriction of the mobility and the leakage current in the semiconductor device.Next, each concentration of carriers in the semiconductor device fabricated by the above fabrication method will be described. FIG. 7 shows a calculated profile of the carrier concentration in each region of the semiconductor device, and FIG. 8 shows a profile of the carrier concentration in each region of an actually manufactured semiconductor device. The profile of the charge carrier concentration can be detected using a common SR method (spreading resistance profiling).As illustrated in FIGS. 7 and 8, the carrier concentration in the n-type field stop layer 21 has a peak at the position of the average proton region. The carrier concentration in the n-type field stop layer 21 decreases continuously and gradually toward the back side of the semiconductor substrate from the position of the central region. In the n-type field stop layer 21, the hydrogen atoms fill the free silicon bonds of radiation-induced defects, thus eliminating radiation-induced defects that cause a restriction of carrier mobility.Embodiment 2Another example of the method for manufacturing the semiconductor device according to the first embodiment will be described in a second embodiment. A manufacturing method using helium ions as charged particles is described herein.FIG. 9 is a flow chart showing steps for forming a diffusion layer on the back surface of the semiconductor substrate. As shown in FIG. 9, acceptor ions are implanted in step V 1. A wet process is performed in step V 2. Helium ion irradiation is performed in step V 3. A heat treatment process is performed in step V 4. Except for step V 3 of the series of steps V 1 to V 4, step V 1 is substantially the same as step T 1, step V 2 is substantially the same as step T 2, and step V 4 is substantially the same as step T 4. Therefore, step V 3 will be described herein, and description of step V 1, step V 2, and step V 4 will be repeated only as needed.In step V 3, helium ion radiation is applied to the back side of the semiconductor substrate. The energy depth of the helium ions must be determined in consideration of the required breakdown voltage of the IGBT. After calculations with the simulation software of the inventor, it was estimated that, in He4suming to be abundant among the isotopes, as an example of the helium ions, when the incident He4energy is 3.75 MeV, the distance from the back surface of the semiconductor substrate to the end (depth) of the n-type field stop layer is about 16 μm.Also in helium ions, a high radiation dose of helium ions is desirable because it increases the carrier concentration in the n-type field stop layer. However, as the radiation dose increases, radiation-induced defects and manufacturing costs also increase. A low radiation dose of helium ions, on the other hand, causes a reduction of the radiation-induced defects, which leads to a desired charge carrier concentration in the n-type field stop layer not being able to be adjusted. For this reason, the inventor has set the helium ion dose to 5×10 10 / cm 2 to 1×10 14 / cm 2. The reason that the helium ion irradiation dose is set lower than the proton irradiation dose is that more defects are formed by helium ions than by protons.In this way, the wet process and the helium ion radiation are performed to form radiation-induced defects and accumulate hydrogen atoms in a region on the back surface of the semiconductor substrate. Although it has been described that the irradiation with helium ions is performed after the wet process is performed according to the above method, the wet process may be performed after the helium ion irradiation is performed.Next, in step V4, a heat treatment process is performed on the semiconductor substrate to diffuse the hydrogen atoms accumulated in the semiconductor substrate. The heat treatment process is carried out, for example, in a diffusion furnace. The heat treatment process is performed on the semiconductor substrate for 0.5 hour to 5 hours at a temperature of 300° C. to 500° C. under a shielding gas atmosphere. The heat treatment process is preferably performed at a temperature of 350° C. to 450° C. in order to efficiently activate the hydrogen-related donors.By this heat treatment method, the hydrogen atoms supplied from the wet process are diffused from the back surface of the semiconductor substrate into the semiconductor substrate. In contrast to proton irradiation, hydrogen does not enter the substrate in helium ion irradiation.As described above, in the region (region R) from the back surface of the semiconductor substrate to the position of the central region of the helium ions, radiation-induced defects are formed by the helium ion radiation. In the heat treatment method, hydrogen atoms supplied by the wet process and diffused into the semiconductor substrate from the back surface of the semiconductor substrate are trapped by the radiation-induced defects formed in the region R. In the helium ion radiation, there are no hydrogen atoms diffused from the location of the middle helium ion region to the back side of the semiconductor substrate.Due to the trapping of the hydrogen atoms by the radiation-induced defects, the free silicon bonds of the radiation-induced defects are eliminated / filled. This converts some of the radiation-induced defects into hydrogen-related donors and renders radiation-induced defects leading to mobility restriction electrically inactive. The radiation-induced defects can thus be reduced. Moreover, by applying helium ion irradiation once, the manufacturing cost can also be reduced compared to the example in which proton irradiation is performed multiple times.The above-described steps V 1 to V 4 correspond to step S 5 illustrated in FIG. 2. Next, a supplementary explanation will be given of step S5 with respect to concentration profiles. First, as described above, FIG. 4 shows distributions of the concentration of impurity (boron) and the concentration of hydrogen atoms in the depth direction from the back side of the semiconductor substrate after the acceptor ions are implanted and the wet process is performed. As shown in FIG. 4, it can be seen that the boron (acceptor ions) is distributed over a relatively flat region of up to about 1 μm from the back side of the semiconductor substrate (see BP, HWP). Boron traps a hydrogen atom to form a B-H bond. Accordingly, the hydrogen atoms have a depth-directional distribution analogous to the depth-directional distribution of boron.Further, FIG. 10 shows distributions of the concentration of impurity (boron) and the concentration of hydrogen atoms in the depth direction from the back surface of the semiconductor substrate after the helium ion irradiation. The helium ion irradiation results in the formation of radiation-induced defects in the semiconductor substrate. The radiation-induced defects are distributed so as to be concentrated around the location of the central region and to decrease toward the back surface of the semiconductor substrate around the location of the central region (see DP).Next, FIG. 11 shows the distribution of the concentration of hydrogen atoms by the wet process after the heat treatment process in the depth direction from the back surface of the semiconductor substrate. As illustrated in FIG. 11, it can be seen that the distribution of the concentration of hydrogen atoms becomes wider in the depth direction of the semiconductor substrate by the wet process after the heat treatment process.The hydrogen atoms introduced from the back surface of the semiconductor substrate and diffused into the semiconductor substrate by the wet process and some of the radiation-induced defects formed between the back surface of the semiconductor substrate and around the center region position react with each other as a result of the heat treatment process around the center region position from the back surface of the semiconductor substrate to generate hydrogen-induced donors, and moreover, radiation-induced defects causing mobility restriction are made electrically ineffective. The radiation-induced defects are thus reduced.In the method for manufacturing the semiconductor device described above, a large amount of hydrogen atoms are supplied from the back side of the semiconductor substrate by a wet method, as compared with the methods proposed in Patent Document 1 and Patent Document 2. Thereby, according to the heat treatment method, a desired amount of hydrogen-related donors can be generated by the reaction of the hydrogen atoms and the radiation-induced defects, and the radiation-induced defects can be effectively eliminated to suppress the restriction of the mobility and the leakage current in the semiconductor device.Next, each concentration of carriers in the semiconductor device fabricated by the above fabrication method will be described. FIG. 12 shows a calculated profile of the carrier concentration in each region of the semiconductor device, and FIG. 13 shows a profile of the carrier concentration in each region of an actually manufactured semiconductor device.As illustrated in FIGS. 12 and 13, the carrier concentration in the n-type field stop layer 21 has a peak at the position of the average proton region. The carrier concentration in the n-type field stop layer 21 decreases continuously and gradually toward the back side of the semiconductor substrate from the position of the central region. In the n-type field stop layer 21, the hydrogen atoms eliminate the free silicon bonds of radiation-induced defects, thus eliminating radiation-induced defects which cause a restriction of carrier mobility.Embodiment 3A semiconductor device including a diode will be described here as an example of a semiconductor device. Irradiation with protons as charged particles will now be described as a production method. As shown in FIG. 14, in the semiconductor device 1 (a diode 2 b), a p+ type anode layer 31 is formed to a certain depth from the first main surface of the semiconductor substrate 3, an anode electrode 33 is in contact with the p+ type anode layer 31.An n+ type cathode layer 35 is formed to a certain depth (depth D) from the second main surface of the semiconductor substrate 3. An n-type field stop layer 21 is formed from the depth D to a position deeper than the depth D. A cathode electrode 37 is formed in contact with the n+ type cathode layer 35. An n-type drift layer 5 is provided between the n-type field stop layer 21 and the p+type anode layer 31. In the semiconductor device including the diode, a termination structure (not shown) for holding the breakdown voltage is formed in an outer peripheral portion, adjacent to a cell region where the diode is formed.Next, a method of manufacturing the semiconductor device will be described. The semiconductor device including the diode is manufactured by substantially the same manufacturing method as the method for manufacturing the semiconductor device including the above-described IGBT (see FIG. 2 ) except for the back surface diffusion step. The difference in the diffusion step of the back surface will be described later.First, a diffusion layer is formed on the front surface side (first main surface side) of the semiconductor substrate 3. After the semiconductor substrate 3 (into the manufacturing apparatus) is inserted, the p+ type anode layer 31 is formed on the first main surface side of the semiconductor substrate 3 by a photolithography step, an ion implantation step, a reactive ion etching step, and the like (see step S 1 in FIGS. 2, 14 ).Next, an electrode is formed on the front surface of the semiconductor substrate 3, and a metal layer for covering the p+ type anode layer 31 and the like is formed by, for example, a vacuum sputtering method. The metal layer is patterned by a photolithography and an etching step to form the anode electrode 33. The anode electrode 33 is in ohmic contact with the p+ type anode layer 31 (see step S 2 in FIGS. 2, 14 ).Next, a surface protection layer covering the first main surface of the semiconductor substrate 3 is formed. A polyimide layer is formed as a surface protection layer, for example (see step S 3 in FIGS. 2, 14 ). Subsequently, the semiconductor substrate 3 is reduced in thickness. The thickness reduction of the semiconductor substrate 3 is performed by grinding the back surface (second main surface) of the semiconductor substrate 3, the thickness of the semiconductor substrate 3 being determined depending on the breakdown voltage (see step S 4 in FIG. 2 ).Next, the n-type field stop layer 21 and the n+type cathode layer 35 are formed as a diffusion layer on the back surface (second main side) of the semiconductor substrate 3 (see step S 5 in FIG. 2 ). The step of forming the n-type field stop layer 21 will be described later.Next, an electrode is formed on the back surface of the semiconductor substrate 3, and a metal layer for covering the n+type cathode layer 35 is formed by a vacuum sputtering method or the like to form the cathode electrode 37. The cathode electrode 37 is in ohmic contact with the n+type cathode layer 35 The semiconductor device with the diode with the n-type field stop layer 21 shown in FIG. 14 is thus completed (see step S 6 in FIGS. 2, 14 ).Next, the step of forming the diffusion layer on the back surface of the semiconductor substrate will be described in detail. Generally, a step of implanting acceptor ions into a back surface of a semiconductor substrate for a diode as a semiconductor device is not provided, unlike an IGBT as a semiconductor device. Thus, for a semiconductor device having a diode, a step of implanting acceptor ions into a back surface of a semiconductor substrate is added.Therefore, as illustrated in FIG. 15, first, in step U 1, boron ions are implanted from the back surface of the semiconductor substrate, for example, as acceptor ions.Next, in step U 2, donor ions are implanted, which become an n+ type cathode layer. The donor ions may be, for example, phosphorus ions. Subsequently, a local heat treatment that does not affect the front surface of the semiconductor substrate is performed by laser heat treatment to activate the implanted donor ions. Here, the implantation conditions (implantation energy, dose amount) are set such that, when a distribution of the acceptor ions and a distribution of the donor ions are superimposed, the concentration of the donor ions in a region where the n+type cathode layer is formed is 100 times or more higher than the concentration of the acceptor ions in this region.By establishing such a concentration relationship, the n+type cathode layer capable of accumulating hydrogen atoms therein can be formed by the wet process performed subsequently while the conductivity type of the n+type cathode layer is maintained as the n-type. In other words, the n+ type cathode layer in which B-H bonds can be provided can be formed.Although the implantation conditions are set here so that the acceptor ions remain in the region where the n+type cathode layer is formed, the acceptor ions may be distributed to a region farther from the back surface of the semiconductor substrate than the region where the n+type cathode layer is formed. In this case, the concentration of acceptor ions in the region farther than the region where the n+ type cathode layer is formed is preferably set to one hundredth or less of the carrier concentration in the n type drift layer. Accordingly, carrier compensation or defects in the n-type drift layer caused by the introduction of acceptor ions can be prevented from having a substantial influence on the characteristics of the semiconductor device.Next, in step U 3, a wet process is performed by dipping the semiconductor substrate in a chemical solution. Here, a protective layer is deposited on the front surface of the semiconductor substrate to protect the anode electrode 33 (see FIG. 14 ) of the diode formed on the front surface of the semiconductor substrate from the chemical solution.Next, in step U 4, the proton irradiation is applied to the back surface of the semiconductor substrate. Here, as described with reference to step T 3 (see FIG. 3 ), the proton irradiation dose is preferably set to 5×10 11 / cm 2 to 1×10 15 / cm 2.Next, in step U5, a heat treatment process is performed on the semiconductor substrate to diffuse the hydrogen atoms accumulated in the semiconductor substrate. Here, as described in step T 4 (see FIG. 3 ), the heat treatment process is performed on the semiconductor substrate for 0.5 hours to 5 hours at a temperature of 300° C. to 500° C. under a shielding gas atmosphere.In the semiconductor device having the diode fabricated through the above-described steps, the distribution of the carrier concentration in the n-type field stop layer 21 is the same as the distribution of the carrier concentration in the n-type field stop layer in the semiconductor device having the above-described IGBT. Therefore, in the n-type field stop layer, the hydrogen atoms eliminate the free bonds of radiation-induced defects, thus eliminating radiation-induced defects, which result in a limitation of carrier mobility.Embodiment 4Another example of the method for manufacturing the semiconductor device according to the third embodiment is described in a fourth embodiment. A manufacturing method using helium ions as charged particles is described herein.FIG. 16 is a flow chart showing steps for forming a diffusion layer on the back surface of the semiconductor substrate. As shown in FIG. 16, acceptor ions are implanted in step W 1. Donor ions are implanted in step W 2. A wet process is performed in step W 3. Helium ion irradiation is performed in step W 4. A heat treatment process is performed in step W 5. Except for step W 4 of the series of steps W 1 to W 5, step W 1 is substantially the same as step U 1, step W 2 is substantially the same as step U 2, step W 3 is substantially the same as step U 3, and step W 5 is substantially the same as step U 5. Therefore, step W 4 will be described herein, and a description of step W 1 to step W 3 and step W 5 will be repeated only as needed.In step W 4, helium ion radiation is applied to the back side of the semiconductor substrate. At this time, as described with respect to step V 3 (see FIG. 9 ), the helium ion dose is preferably set to 5×10 10 / cm 2 to 1×10 14 / cm 2.Next, in step W 5, a heat treatment process is performed on the semiconductor substrate to diffuse the hydrogen atoms accumulated in the semiconductor substrate. Here, as described in step V 4 (see FIG. 9 ), the heat treatment process is performed on the semiconductor substrate for 0.5 hours to 5 hours at a temperature of 300° C. to 500° C. under a shielding gas atmosphere.In the semiconductor device having the diode fabricated through the above-described steps, the distribution of the carrier concentration in the n-type field stop layer 21 is the same as the distribution of the carrier concentration in the n-type field stop layer 21 in the semiconductor device having the above-described IGBT. In the n-type field stop layer 21, therefore, the hydrogen atoms eliminate the free bonds of radiation-induced defects, thus eliminating radiation-induced defects which cause a restriction of carrier mobility.The semiconductor devices described in the respective embodiments may be combined in various ways as needed.The embodiments disclosed herein are illustrative and not restrictive.The present invention can be advantageously applied to a semiconductor device having a field stop layer.List of reference characters1 Semiconductor device 2 a IGBT 2 b Diode 3 Semiconductor substrate 3 a Graben 5 n-type drift layer 7 p+Type contact layer 9 pType base layer 11 n+Type emitter layer 13 Gate insulating layer 15 Gate electrode 17 Interlayer insulating layer 19 Emitter electrode 21 nType field stop layer 23 p+Type collector layer 25 Collector electrode 31 p+Type anode layer 33 Anode electrode 35 n+Type cathode layer 37 Cathode electrode

Claims

A method of manufacturing a semiconductor device (1) comprising the steps of: forming a semiconductor element (2a, 2b) in a semiconductor substrate (3) having a first main surface and a second main surface and a first conductivity type having a first carrier concentration, the semiconductor element (2a, 2b) for guiding a current between the first main surface and the second main surface, wherein forming a semiconductor element (2a, 2b) comprises: implanting acceptor ions from the second main surface of the semiconductor substrate (3), performing a wet process of accumulating hydrogen atoms in an impurity region (23, 35) of the semiconductor substrate (3) in which the acceptor ions are implanted from the second main surface of the semiconductor substrate (3), Applying charged particle radiation from the second main surface of the semiconductor substrate (3) and after performing a wet process and applying the charged particle radiation, performing a heat treatment process on the semiconductor substrate (3) to form a first conductivity type field stop layer (21) having a second carrier concentration higher than the first carrier concentration in the semiconductor substrate (3), wherein the hydrogen atoms have a distribution in which the concentration of the hydrogen atoms continuously decreases from the surface of the impurity region (23, 35) toward a boundary between the impurity region (23, 35) and the field stop layer (21).The method for manufacturing a semiconductor device (1) according to claim 1, wherein ions of a group 13 element are implanted as acceptor ions in the implantation of acceptor ions.The method for manufacturing a semiconductor device (1) according to claim 1 or 2, wherein a chemical solution including hydrofluoric acid is used in performing the wet process.The method for manufacturing a semiconductor device (1) according to any one of claims 1 to 3, wherein in the case of applying charged particle radiation, the irradiation with protons as charged particles is performed under the condition that the dose amount is 5×10 11 / cm 2 to 1×10 15 / cm 2.The method for manufacturing a semiconductor device (1) according to any one of claims 1 to 3, wherein, in the case of applying the charged particle radiation, the radiation of helium ions is performed as the charged particles under the condition that the dose amount is 5×10 10 / cm 2 to 1×10 14 / cm 2.The method for manufacturing a semiconductor device (1) according to any one of claims 1 to 5, wherein in performing the heat treatment process, the heat treatment process is performed for a heat treatment time of 0.5 hour to 5 hours at a heat treatment temperature of 300°C to 500°C under a shielding gas atmosphere.The method for manufacturing a semiconductor device (1) according to any one of claims 1 to 6, wherein forming the semiconductor element (2a, 2b) includes forming an IGBT (2a), and forming the IGBT (2a) includes forming a collector layer (23) of a second conductivity type in a region from the second main surface of the semiconductor substrate (3) to the field stop layer (21).The method for manufacturing a semiconductor device (1) according to any one of claims 1 to 6, wherein forming a semiconductor element (2a, 2b) includes forming a diode (2b), wherein forming a diode (2b) includes implanting donor ions from the second main surface of the semiconductor substrate (3) to form a cathode layer (35) of the first conductivity type in a region from the second main surface of the semiconductor substrate (3) to the field stop layer (21).A semiconductor device (1) comprising a semiconductor element (2a, 2b) that conducts current between a first main surface and a second main surface of a first conductivity type semiconductor substrate (3) having a first carrier concentration, the semiconductor device (1) comprising: an impurity region (23, 35) formed to a first depth from the second main surface of the semiconductor substrate (3); and a first conductivity type field stop layer (21) formed from a position of the first depth to a second depth deeper than the first depth in the semiconductor substrate (3) and having a second carrier concentration higher than the first carrier concentration in the semiconductor substrate (3), wherein in the field stop layer (21), the maximum value of the second carrier concentration is between the first depth and the second depth, the second carrier concentration has a distribution in which the second carrier concentration decreases toward the second main surface from a region where the maximum value of the second carrier concentration is located, the concentration of radiation-induced defects remaining in the field stop layer (21) is lower than the first carrier concentration in the semiconductor substrate (3), and hydrogen atoms are accumulated in the impurity region (23, 35), the hydrogen atoms having a distribution in which the concentration of the hydrogen atoms decreases continuously from the surface of the impurity region (23, 35) toward the boundary between the impurity region (23, 35) and the field stop layer (21).The semiconductor device (1) according to claim 9, wherein the concentration of the radiation-induced defects remaining in the field stop layer (21) is one hundredth or less of the first carrier concentration in the semiconductor substrate (3).The semiconductor device (1) according to any one of claims 9 to 10, wherein the semiconductor element (2a, 2b) includes an IGBT (2a), and the impurity region (23, 35) is a collector layer (23) of a second conductivity type.The semiconductor device (1) according to any one of claims 9 to 10, wherein the semiconductor element (2a, 2b) includes a diode (2b), and the impurity region (23, 35) is a cathode layer (35) of the first conductivity type.

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