WO2015015672A1 - 炭化珪素半導体装置及びその製造方法 - Google Patents
炭化珪素半導体装置及びその製造方法 Download PDFInfo
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- WO2015015672A1 WO2015015672A1 PCT/JP2014/001262 JP2014001262W WO2015015672A1 WO 2015015672 A1 WO2015015672 A1 WO 2015015672A1 JP 2014001262 W JP2014001262 W JP 2014001262W WO 2015015672 A1 WO2015015672 A1 WO 2015015672A1
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- silicon carbide
- interface
- insulating film
- gate insulating
- semiconductor device
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Definitions
- the present invention relates to a silicon carbide semiconductor device and a method for manufacturing the same.
- Silicon carbide is expected as a next-generation semiconductor material capable of realizing a semiconductor device with high breakdown voltage and low loss.
- gate insulating semiconductor devices such as MOSFETs (Metal-Oxide-Semiconductors, Field Effect Transistors) and IGBTs (Insulated Gate Bipolar Transistors) capable of switching operation are particularly expected. is there.
- MOSFETs Metal-Oxide-Semiconductors, Field Effect Transistors
- IGBTs Insulated Gate Bipolar Transistors
- a threshold voltage (V th ) that is a gate voltage at which an on-current starts to flow needs to be high to some extent.
- a typical threshold voltage of an IGBT using silicon (Si) that is generally commercialized is 5 V, and the threshold voltage is at least several volts or more in consideration of malfunction and operation at a high temperature. It is necessary to be high.
- the threshold voltage is greatly influenced by fixed charges in the gate insulating film and interface traps at the so-called MOS interface between silicon carbide and the gate insulating film. It is known that when silicon carbide is used, more interface traps are generated at the MOS interface than in the case of silicon (Si), which is generally used as a material for conventional semiconductor devices, and the quality of the MOS interface is low. Yes.
- heat treatment hydrogen annealing
- heat treatment nitrogen monoxide (NO) gas or dinitrogen monoxide (N 2 O) gas
- phosphorus oxychloride POCl 3
- a method of reducing interface traps at the MOS interface and improving channel mobility by heat treatment in gas (POCl 3 annealing) has been disclosed (see, for example, Patent Document 1).
- Reducing interface traps at the MOS interface improves channel mobility, but at the same time lowers the threshold voltage.
- there is a trade-off between improving channel mobility and threshold voltage, and maintaining a high threshold voltage results in lower channel mobility, and increasing channel mobility lowers threshold voltage, resulting in normally-on characteristics. turn into. That is, if the interface trap is reduced in order to increase the channel mobility, there is a problem that it is difficult to obtain a normally-off characteristic because the threshold voltage is lowered.
- the present invention has been made to solve the above-described problems, and an object thereof is to provide a silicon carbide semiconductor device that can improve the trade-off relationship between channel mobility and threshold voltage and realize normally-off characteristics.
- a silicon carbide semiconductor device includes a first conductivity type drift layer formed on a surface of a silicon carbide substrate, and a plurality of second conductivity types formed on the surface layer portion of the drift layer at intervals.
- Well region a first conductivity type source region formed in part of the surface layer of the well region, a gate insulating film formed on the surface of the well region and the source region, and a source on the surface of the gate insulating film and a gate electrode formed so as to be opposed to the end portion and the well region of the area, the density of the formed interface traps at the interface region between the gate insulating film and the well region D it [cm -2 eV - 1 ], the interface trap energy level which is the depth from the energy level E c of the conduction band of silicon carbide is (E c -E) [eV], and the interface trap energy level is ⁇ [eV] Trap density it coefficient values for asymptotic A [cm -2 eV -1], the coefficients B [
- the threshold voltage can be effectively increased while suppressing a decrease in channel mobility. That is, the trade-off relationship between channel mobility and threshold voltage can be improved.
- FIG. 1 It is sectional drawing which shows the silicon carbide semiconductor device which concerns on Embodiment 1 of this invention. It is an enlarged view near the MOS interface of the silicon carbide semiconductor device according to the first embodiment of the present invention. It is a figure which shows typically the defect containing the coupling
- the silicon carbide semiconductor device which concerns on Embodiment 1 of this invention it is sectional drawing for demonstrating the manufacturing method until drift layer formation.
- the silicon carbide semiconductor device which concerns on Embodiment 1 of this invention it is sectional drawing for demonstrating the manufacturing method until well region formation.
- the silicon carbide semiconductor device which concerns on Embodiment 1 of this invention it is sectional drawing for demonstrating the manufacturing method until source region formation.
- Embodiment 1 is a cross sectional view showing a silicon carbide semiconductor device according to Embodiment 1 of the present invention.
- an n-channel silicon carbide MOSFET will be described as an example of a silicon carbide semiconductor device.
- first conductivity type drift layer 2 is formed on the surface that is the first main surface of first conductivity type silicon carbide substrate 1, and the surface layer portion of drift layer 2 is spaced from each other.
- Two well regions 3 of the second conductivity type are provided.
- a source region 4 of the first conductivity type is formed on a part of the surface layer portion of the well region 3, and a gate insulating film 5 is formed on part of the surface of the well region 3 and the source region 4.
- a gate electrode 6 is formed on the surface of the gate insulating film 5 so as to face the end of the source region 4 and the well region 3.
- Source electrode 7 is formed on the surface of source region 4, and drain electrode 8 is formed on the back surface, which is the second main surface of silicon carbide substrate 1.
- an n-channel silicon carbide MOSFET in which the first conductivity type is n-type and the second conductivity type is p-type will be described.
- the first conductivity type is p-type and the second conductivity type is n-type.
- the present invention can also be applied to the p-channel MOSFET.
- the conductivity type of silicon carbide substrate 1 is the first conductivity type, but the present invention can also be applied to an IGBT having the second conductivity type.
- FIG. 2 shows an enlarged view of the vicinity of the MOS interface, which is an interface region between the gate insulating film 5 and the well region 3, which is surrounded by a dotted line in FIG.
- FIG. 2 is an enlarged schematic view of the vicinity of the MOS interface of the silicon carbide semiconductor device in the present embodiment.
- the gate insulating film 5 has a defect 10 including a bond of silicon and hydrogen in the gate insulating film 5 and a MOS interface that is an interface region with the well region 3.
- FIG. 3 shows a schematic diagram of an atomic structure showing a bonding state of the defect 10 including a bond between silicon and hydrogen.
- hydrogen atoms replace oxygen vacancies that should originally contain oxygen atoms (O).
- a region surrounded by a dotted line indicates oxygen vacancies.
- FIG. 3A shows a defect including a bond between one of two silicon atoms (Si) adjacent to an oxygen vacancy and a hydrogen atom (H).
- the bond of the other silicon atom becomes unbonded and is electrically unstable. It becomes.
- the dangling lines of silicon atoms are indicated by a one-dot chain line. When such a dangling bond exists, it becomes electrically unstable and becomes an interface trap having an energy level deeper than the conduction band edge of silicon carbide.
- the entire system including FIG. 3A takes a stable structure. In other words, the system including FIG. 3A captures electrons existing in the conduction band in an attempt to become an electrically stable structure.
- the defect shown in FIG. 3 (b) is a defect in which a hydrogen atom is bonded to one of two silicon atoms adjacent to the oxygen vacancies, similarly to the defect shown in FIG. 3 (a).
- the other silicon atom to which no hydrogen atom is bonded tries to directly bond to the silicon atom present at the nearest position in an attempt to become an electrically stable structure.
- the bond is indicated by a two-dot chain line.
- One of the silicon atoms bonded by the two-dot chain line forms a fifth bond. Since there are four bonds of the original silicon atom, a negative charge must be charged in order to form the fifth bond. That is, the structure shown in FIG. 3B acts as an interface trap having an energy level deeper than the conduction band edge of silicon carbide and captures electrons existing in the conduction band in an attempt to become an electrically stable structure.
- FIG. 3 shows an energy level at a depth of 1.1 eV from the conduction band edge of silicon carbide
- FIG. 3B shows an energy level at a depth of 0.72 eV. Since it is deeper than the conductive band edge, it becomes an electrically active first trap to capture electrons.
- the first trap formed by the defect 10 including the bond between silicon and hydrogen contributes to the improvement of the threshold voltage.
- FIG. 4 shows the gate characteristics of the MOSFET using this embodiment.
- the characteristics in the case of having the defect shown in FIG. 3 using this embodiment are shown by black circles, and as a comparative example, the case in which this embodiment is not used, that is, the conventional case having no defect 10 including a bond of silicon and hydrogen.
- An example is indicated by a white circle. From FIG. 4, it can be seen that the gate voltage at which the drain current rises, that is, the threshold voltage Vth , shifts to a large positive value when this embodiment is used.
- the slope of the linear region after the drain current rises which is the region surrounded by the dotted line, depends on the channel mobility, but it can be seen that the case of using this embodiment and the conventional example are almost equal.
- the threshold voltage can be effectively increased while suppressing a decrease in channel mobility.
- Figure 5 shows the energy level dependence of interface trap density D it in the MOS interface MOSFET using this embodiment.
- the characteristics in the case of having the defect shown in FIG. 3 using this embodiment are shown by black circles, and as a comparative example, the case in which this embodiment is not used, that is, the conventional case having no defect 10 including a bond of silicon and hydrogen.
- An example is indicated by a white circle.
- the energy level of the interface trap is represented by the depth (E c ⁇ E) from the conduction band edge E c as indicated by the horizontal axis in FIG.
- the characteristics when this embodiment is used are indicated by black circles, and the characteristics when this embodiment is not used are indicated by white circles as comparative examples.
- FIG. 6 is a schematic cross-sectional view of the gate controlled capacitor used in this embodiment. Volume was determined by applying a voltage to the gate of FIG. 6 - By fitting the voltage characteristic and the theoretical curve, it is possible to determine the energy level dependence of D it in FIG.
- the interface trap obtained using the gate controlled capacitor of FIG. 6 is obtained with the p-type well region at the MOS interface inverted, it can be evaluated with the same structure as the actual MOSFET operating state. Therefore, unlike the case where evaluation is performed using a simple n-type capacitor, the relationship between the interface trap density and the channel mobility and threshold voltage of the MOSFET is accurate when calculated using a gate-controlled capacitor. It becomes.
- the interface trap density is increased.
- the interface trap that is deeper than the conventional band at the deep energy level from the conduction band edge corresponds to the first trap.
- the difference between the interface trap density Dit is as small as about twice when the present embodiment is used and when it is not used. It can be seen that there is a difference of about one digit at an energy level deeper than 6 eV. That is, the energy level distribution of the interface trap of the present embodiment is obtained as shown in FIG. 5 by adding the first trap to the interface trap of the conventional example.
- Interface traps at the MOS interface greatly affect the channel mobility and threshold voltage of the MOSFET.
- the quality of the MOS interface is worse than in MOSFETs using silicon, and the interface trap density is very high, resulting in low channel mobility.
- the channel mobility of the MOSFET using silicon carbide is greatly influenced by the interface trap density existing from the conduction band edge to a depth of about 0.2 eV at the MOS interface.
- silicon carbide has many defects due to C in the MOS interface and oxide film, and these defects become interface traps having a shallow energy level of about 0.2 eV from the edge of the conduction band.
- Some C and Si dangling bonds also form interface traps at shallow energy levels near the conduction band edge.
- the interface trap of about 0.1 eV has a greater influence on the channel mobility.
- FIG. 5 it can be seen that the interface trap density having an energy level of 0.1 eV depth is comparable between the case where this embodiment is used and the conventional example which does not use this embodiment.
- the MOSFET having the defect shown in FIG. 5 by increasing the number of first traps that do not significantly affect the channel mobility while suppressing the increase of interface traps at levels shallower than 0.2 eV that affect the channel mobility, Only the interface traps with energy levels deeper than .6 eV are specifically increased.
- the interface trap density of this embodiment when the interface trap density of this embodiment is compared with the conventional example, the energy other than the depth of 1.1 eV of the defect of FIG. 3A and the depth of 0.72 eV of the defect of FIG. Even at the level, the interface trap density increases. This is partly because the defect shown in FIG. 3 has not only a single energy level but also an energy level with a certain width around each energy level. For example, when generating these defects, different energies other than 0.72 eV and 1.1 eV are generated by distorting the atomic arrangement or generating a bond defect in which the defect shown in FIG. 3 is partially deformed. An interface trap having a level is also generated. However, the rate of occurrence of interface traps gradually decreases at energy levels shallower than 0.6 eV.
- the energy level which is the depth from the energy level E c at the conduction band edge of silicon carbide, is the interface trap density D it (E) at (E c -E) [eV]
- D it (E) at (E c -E) [eV] When [cm ⁇ 2 eV ⁇ 1 ], B [cm ⁇ 2 eV ⁇ 1 ], and X [eV] are coefficients, fitting is performed using Equation 1.
- the slope X is obtained by the least square method using Equation (1).
- the coefficient X was determined to be 0.08 eV.
- the coefficient X was obtained as 0.13 eV from FIG.
- the existence ratio of interface traps having a deep energy level is increased, the purchase of the transition region from the shallow energy level to the deep energy level can be reduced. A certain coefficient X can be reduced.
- the rate of increase of the interface trap density at the shallow energy level can be suppressed so that the coefficient X increases. Therefore, it is possible to increase the number of deep energy level interface traps while suppressing the increase of shallow energy level interface traps that greatly affect channel mobility.
- the threshold voltage depends on the interface trap density regardless of the energy level, and the threshold voltage increases by the amount of electrons trapped at the MOS interface. At room temperature, for example, there is a high probability that electrons are captured in an interface trap deeper than 0.1 eV from the conduction band edge of silicon carbide. Therefore, the threshold voltage increases as the number of interface traps increases at any energy level deeper than 0.1 eV. Will increase. That is, even when an interface trap having an energy level deeper than 0.6 eV from the conduction band edge occurs, the threshold voltage increases.
- an increase in interface traps at an energy level shallower than 0.2 eV from the conduction band edge is suppressed, and an energy level sufficiently deeper than 0.2 eV.
- an interface trap having an energy level deeper than 0.6 eV from the edge of the conductive band may be increased.
- the interface trap at a level shallower than 0.2 eV from the conduction band edge hardly increases as compared with the comparative example.
- the increase in the interface trap at an energy level deeper than 0.2 eV, particularly an energy level deeper than 0.6 eV suppresses a decrease in channel mobility.
- the threshold voltage can be greatly increased.
- the inventors have shown that the defect 10 including the bond of silicon and hydrogen shown in FIG. 3 is less than 0.6 eV while suppressing the generation of interface traps having an energy level shallower than 0.2 eV from the conduction band edge. It has been found that the deep energy level interface traps can be greatly increased.
- FIG. 7 is a cross-sectional view for explaining the manufacturing method up to formation of drift layer 2 in the silicon carbide semiconductor device according to the present embodiment.
- first conductivity type drift layer 2 made of silicon carbide is formed on the surface which is the first main surface of silicon carbide substrate 1 by epitaxial crystal growth.
- 4H—SiC is used as silicon carbide substrate 1 and the plane orientation of the first main surface is a (0001) plane with an off angle of 4 ° in the ⁇ 11-20> direction.
- FIG. 8 shows a cross-sectional view for explaining a manufacturing method up to formation of well region 3 in the silicon carbide semiconductor device according to the present embodiment.
- Impurities are ion-implanted into a portion of the surface layer portion of the drift layer 2 spaced apart by a predetermined interval, using a resist as a mask, to form a pair of second conductivity type well regions 3.
- FIG. 8 is a cross-sectional view after removing the resist.
- Examples of the p-type impurity at the time of ion implantation, that is, the second conductivity type include boron (B) and aluminum (Al).
- FIG. 9 is a cross-sectional view for explaining the manufacturing method up to formation of source region 4 in the silicon carbide semiconductor device according to the present embodiment. Impurities are ion-implanted into the surface layer portion of the well region 3 using a resist as a mask to form a first conductivity type source region 4. FIG. 9 is a cross-sectional view after removing the resist. Examples of the n-type, that is, the first conductivity type impurity during ion implantation include phosphorus (P) and nitrogen (N).
- the implanted ions are electrically activated.
- FIG. 10 is a cross-sectional view for illustrating the manufacturing method up to formation of gate insulating film 5 in the silicon carbide semiconductor device according to the present embodiment.
- a silicon dioxide film SiO 2 film
- the SiO 2 film that is the gate insulating film 5 may be a thermal oxide film formed by thermal oxidation, or may be a deposited film formed by a CVD (Chemical Vapor Deposition) method. Since the SiO 2 film on silicon carbide has good insulation characteristics, a highly reliable gate insulating film 5 can be formed.
- the interface trap present at the MOS interface of silicon carbide which causes a decrease in channel mobility, is an interface trap caused by C as described above.
- the interface trap caused by C is considered to be generated by surplus C generated when thermal oxidation of silicon carbide proceeds. It is known that when a SiO 2 film is formed by thermal oxidation of silicon carbide, thermal oxidation is promoted at a high temperature and surplus C increases, so that many interface traps due to C are generated and channel mobility is lowered. It has been.
- the deposited film by the CVD method is formed by depositing a SiO 2 film on silicon carbide by reacting a silicon supply gas and an oxygen supply gas at a temperature of 900 ° C. or less. Also in such a CVD method, the silicon carbide substrate 1 having the structure of FIG. 8 is exposed to a certain high temperature in an atmosphere containing an oxygen supply gas, and thus thermal oxidation proceeds. However, since it can be formed at a low temperature of 800 ° C. or lower, oxidation of silicon carbide is suppressed, leading to reduction of interface traps.
- a silicon oxynitride film, Al 2 O 3 , HfO 2 , or the like may be used as the gate insulating film 5, or a laminated film thereof may be used.
- Oxygen atoms need to be supplied also when forming these films, but thermal oxidation of silicon carbide proceeds slightly due to oxygen atoms, so that a slight SiO 2 film is formed in the vicinity of the MOS interface.
- the temperature for depositing Al 2 O 3 , HfO 2, or the like can be lower than 500 ° C. and lower than the SiO 2 film formed by the deposition method, generation of interface traps can be further suppressed.
- the silicon carbide substrate 1 obtained in FIG. 9 is subjected to a reoxidation process in a water vapor atmosphere.
- the first trap can be generated by the defect 10 including the bond of silicon and hydrogen at the MOS interface as shown in FIGS.
- the heat treatment temperature in the re-oxidation treatment step in a steam atmosphere is preferably 500 ° C. or higher and 1000 ° C. or lower, and particularly preferably 600 ° C. or higher and 950 ° C. or lower.
- the heat treatment temperature is less than 500 ° C.
- the effect of the reoxidation treatment cannot be obtained sufficiently. That is, the first trap cannot be generated by the defect 10 including the bond of silicon and hydrogen as described in FIGS. This is because OH ⁇ described later does not sufficiently diffuse into the gate insulating film 5 and the MOS interface.
- the generation rate of the first trap depends on the temperature, and the higher the temperature, the shorter the first trap can be generated.
- the first trap can be efficiently generated by the defect 10 including the bond of silicon and hydrogen as shown in FIGS.
- the progress of thermal oxidation of silicon carbide during the treatment can be suppressed.
- the heat treatment time in the reoxidation treatment process is about 10 minutes to 5 hours, and in this example, the heat treatment time was 30 minutes to 1 hour.
- the reoxidation process is performed in a water vapor atmosphere in which oxygen gas (O 2 ) and hydrogen gas (H 2 ) are subjected to a combustion reaction.
- oxygen gas (O 2 ) and hydrogen gas (H 2 ) are subjected to a combustion reaction.
- the flow rate ratio of hydrogen to oxygen (H 2 / O 2 flow rate ratio) is 0.7 or more and 1.9 or less.
- Figure 11 shows the H 2 O / O 2 ratio after combustion reaction to the flow rate ratio of H 2 / O 2.
- H 2 O water vapor
- H 2 generated by the combustion reaction contributes to the generation of the defect 10 including a bond in silicon and hydrogen shown in FIG.
- O 2 generated by the combustion reaction causes only oxidation of silicon carbide. That is, O 2 does not generate a first trap due to the defect 10 including a bond of silicon and hydrogen, but generates an interface trap due to surplus C.
- H 2 O has a faster chemical reaction rate than O 2 . This is because, for example, since hydrogen atoms are smaller than oxygen atoms, they easily diffuse into the gate insulating film 5 and the MOS interface. Therefore, if the flow rate of H 2 O is larger than the flow rate of O 2 , the reaction of H 2 O is promoted more than the thermal oxidation reaction of O 2 , and the generation of surplus C is suppressed, and a bond between silicon and hydrogen is included. Many first traps can be generated by the defect 10.
- the temperature of the reoxidation treatment step in the present embodiment to 600 ° C. or more and 950 ° C. or less, there is an effect of suppressing the thermal oxidation reaction of O 2 itself, and the flow rate of H 2 O is set to O 2. The effect of suppressing the generation of surplus C is synergistically increased by increasing the flow rate.
- H 2 / O 2 ratio In order to completely burn hydrogen gas during the combustion reaction, the H 2 / O 2 ratio needs to be 1.9 or less. When the flow rate ratio exceeds 2, all of hydrogen gas (H 2) is not completely burned, H 2 O in the heat treatment atmosphere, is H 2 contained in addition to O 2.
- H + is a positive Since it has an electric charge, a force that separates it from positively charged oxygen vacancies acts and it is difficult to replace the oxygen vacancies.
- H 2 O replaces oxygen vacancies to form the defects shown in FIG. 3, and contributes to the generation of interface traps having energy levels deeper than 0.6 eV from the conduction band edge.
- H 2 has the effect of reducing the interface trap having a shallow energy level of 0.2 eV or less from the end of the conduction band by terminating the dangling bond, but at the same time, the second defect due to the defect of FIG. 3 formed by H 2 O. 1 traps are also reduced.
- the defect shown in FIG. 3 forms a first trap, captures electrons, and is negatively charged. For this reason, H + having a positive charge is attracted, and for example, the Si dangling bond indicated by a one-dot chain line in FIG. Further, the bond indicated by the two-dot chain line in FIG. 3B is broken and bonded to Si formed by unbonded hands to form an electrically stable structure. That is, the first trap formed by the defect 10 including the bond of silicon and hydrogen in FIG. 3 is electrically inactivated.
- the first trap for increasing the threshold voltage formed by H 2 O is used as H 2.
- the effect of improving the threshold voltage is suppressed.
- the threshold voltage is effectively increased only by the reaction with OH ⁇ without reducing the defect of FIG. 3, that is, the first trap by H + during the reoxidation process. I can do it.
- Conventionally known re-oxidation treatment is generally performed for the purpose of reducing the total number of interface traps in order to improve channel mobility. Therefore, it is performed for the purpose of terminating dangling bonds of silicon or carbon and inactivating interface traps, and conditions for increasing deep level interface traps are not known as in this embodiment. It was. Even if the threshold voltage may increase due to the conventional re-oxidation treatment, the main factor is that the fixed charge is increased in the gate insulating film 5, and in this case, the density of the fixed charge that can be generated is generated at the MOS interface. There is a limit to increasing the fixed charge from the viewpoint of the interface trap density that is smaller than that possible and from the standpoint of the insulating properties of the gate insulating film 5.
- the fixed charge in the gate insulating film 5 often includes single H or OH that is not bonded to silicon, and the gate insulating film 5 is subjected to heat treatment at 1000 ° C. or less performed in an electrode process or the like.
- the threshold voltage fluctuates. Therefore, it has been desired to improve the threshold voltage more effectively and stably.
- the threshold voltage can be effectively increased while suppressing the decrease in the degree.
- the threshold voltage obtained in this embodiment also provides stability against heat treatment performed in an electrode process or the like because the defect 10 including a bond of silicon and hydrogen is stable against heat.
- FIG. 12 shows the results of thermal desorption gas analysis (thermal dissolution spectroscopy) of the gate insulating film 5 subjected to the most oxidation treatment in the present embodiment.
- the temperature was raised from room temperature to 1100 ° C., and the amount of hydrogen desorbed from the gate insulating film 5 was evaluated.
- the background detected from the measurement system is indicated by a dotted line (a).
- the measurement result of the gate insulating film 5 of the silicon carbide semiconductor device according to the present embodiment indicated by the solid line (b) shows that hydrogen is not detected other than hydrogen near 1000 ° C. detected from the background. . This indicates that hydrogen does not exist in a single atom or in a weakly bonded state, but exists in a stable bonded state bonded to silicon as shown in FIG. ing.
- the H 2 O gas may be diluted with an inert gas such as N 2 or Ar.
- the threshold voltage can be controlled within a predetermined range while maintaining the channel mobility at a substantially constant value. I can do it.
- the energy level dependence of the interface trap density was determined by changing the reoxidation temperature from 600 ° C to 900 ° C.
- the energy level is 0.1 eV and the depth D it is the reference value D it 1 [cm ⁇ 2 eV ⁇ 1 ], and the difference between D it and D it 1 when the energy level is 0.2 eV is D If it 2 [cm ⁇ 2 eV ⁇ 1 ] and the difference between D it and D it 1 when the energy level is 0.8 eV is D it 8 [cm ⁇ 2 eV ⁇ 1 ], the ratio R is a number 2 to obtain.
- FIG. 13 shows the reoxidation temperature dependency of the ratio R obtained from the interface trap density of 0.2 eV and 0.8 eV.
- a dotted line indicates a conventional example in which reoxidation is not performed.
- the ratio R is 0.71 when reoxidation is not performed, whereas it is 0.67 when reoxidation is performed at 600 ° C., and the ratio R increases linearly as the reoxidation temperature is increased.
- the ratio R when the reoxidation temperature was 900 ° C. was 0.54.
- FIG. 14 shows the threshold voltage of the MOSFET corresponding to FIG. In FIG. 14, the threshold voltage was 1.3 V when reoxidation was not performed, whereas the threshold voltage was increased to 2.3 V when reoxidation was performed at 600 ° C., and the reoxidation temperature was further increased. As the threshold voltage increases, the threshold voltage increases.
- the threshold voltage When the threshold voltage is less than 2V, normally-on characteristics may occur due to high-temperature operation or changes in the threshold voltage over time, and it is necessary to take measures against normally-on characteristics in the peripheral circuit of the semiconductor device.
- the ratio R by setting the ratio R from 0.54 to 0.67, a threshold voltage of 2 V or higher that can ensure a normally-off characteristic even at high temperatures can be obtained. If the ratio R is smaller than 0.54, interface traps with energy levels shallower than 0.2 eV increase, which leads to a decrease in channel mobility, which is not desirable.
- interstitial hydrogen atoms were present in a single atom not bonded to silicon release It becomes easy to do. Interstitial hydrogen atoms can easily move through the oxide film, which is the gate insulating film 5, and can become mobile ions, etc., so that the reliability of the oxide film is improved by being released.
- FIG. 15 is a cross-sectional view for explaining the process up to completion of gate electrode 6 in the method for manufacturing the silicon carbide semiconductor device according to the present embodiment.
- a gate electrode 6 is formed on the gate insulating film 5 and patterned.
- the gate electrode 6 is patterned in such a shape that the pair of base region 3 and source region 4 are located at both ends, and the drift layer 2 exposed between the base regions 3 is located at the center.
- a source electrode 7 is formed and patterned at a portion where the source region 4 is exposed on the surface.
- drain electrode 8 is formed on the back surface side which is the second main surface of silicon carbide substrate 1, a silicon carbide semiconductor device according to the present embodiment as shown in FIG. 1 is completed.
- the threshold voltage can be increased without reducing the channel mobility, and the trade-off relationship between the channel mobility and the threshold voltage can be improved.
- FIG. 16 shows the energy level dependence of the interface trap density at the MOS interface when the temperature condition in the reoxidation process is changed in the silicon carbide semiconductor device according to the present embodiment.
- the white triangle plot shows a conventional case where this embodiment is not used.
- FIG. 17 shows the result of obtaining the coefficient X in Equation 1 using the least square method from FIG.
- FIG. 17 shows the dependency of the coefficient X on the reoxidation temperature.
- the coefficient X was 0.09 or more and 0.14 or less in the range where the reoxidation temperature was 600 ° C. or more and 950 ° C. or less.
- D it at an energy level of 0.8 eV is conventionally 6 ⁇ 10 10 cm ⁇ 2 eV ⁇ 1 , but in this embodiment, 1 ⁇ 10 11 cm ⁇ 2. It was sought to be greater than eV- 1 .
- the coefficient X can be increased, so that the interface trap density at an energy level deeper than 0.6 eV can be increased while suppressing the increase in the shallow level interface trap density, the depth of D it of 0.8eV may be 1 ⁇ 10 11 cm -2 eV -1 or more. With such an energy level distribution of the interface trap, it is possible to realize a MOSFET having a high threshold voltage and a high channel mobility.
- FIG. 18 shows a relationship between coefficient X and threshold voltage in the silicon carbide semiconductor device according to the present embodiment.
- the coefficient X is determined to be 0.08 in FIG. This is as described above.
- the threshold voltage was 1.7V.
- the threshold voltage Vth is gradually improved when the coefficient X is 0.09 or more. That is, in order to improve the threshold voltage, the coefficient X needs to be 0.09 or more.
- the threshold voltage was 2.01V. Further, it can be seen that the threshold voltage increases as the coefficient X increases.
- the coefficient X is larger than 0.14, as described above, the increase in the interface traps at the energy level whose depth from the conduction band edge is shallower than 0.2 eV is also increased.
- the coefficient X is desirably 0.09 or more and 0.14.
- FIG. 19 shows the relationship between channel mobility ⁇ ch and threshold voltage V th of the silicon carbide semiconductor device according to the present embodiment.
- a case where this embodiment is used is indicated by a black circle, and a conventional example which does not use this embodiment is indicated by a white circle.
- As a characteristic of the semiconductor device it is desirable to realize low on-resistance, that is, high channel mobility and high threshold voltage.
- FIG. 19 conventionally, there has been a trade-off relationship in which a threshold voltage is lowered when a high channel mobility is realized and a channel mobility is reduced when a high threshold voltage is realized. That is, it is difficult to obtain characteristics that deviate from the trade-off curve plotted on the one-dotted line curve in FIG. Note that different channel mobility can be obtained by changing the concentration of the well region 3. That is, the white circle data in FIG. 19 was obtained on the alternate long and short dash line by changing the concentration of the well region 3.
- the threshold voltage V th is 0.5 V in the past, and it has been necessary to take measures for normally-on characteristics in the peripheral circuit.
- the threshold voltage is 3.5 V, and a normally-off characteristic that does not require a countermeasure for normally-on characteristics in the peripheral circuit can be realized.
- the interface trap existing at the MOS interface has been described.
- the defect shown in FIG. 3 may be formed in the gate insulating film 5. Even when the defect shown in FIG. 3 is formed in the gate insulating film 5, since it becomes electrically unstable, a second trap is formed in the gate insulating film 5, and electrons are captured. That is, in order to be negatively charged, the second trap that has captured the electrons acts as a negative fixed charge, and raises the threshold voltage. Even when it is formed in the gate insulating film 5, the channel mobility is not affected.
- the gate insulating film 5 has an effect of increasing the threshold voltage if a trap due to a defect 10 including a bond of silicon and hydrogen is formed at the MOS interface and the gate insulating film 5.
- the defect 10 including the bond of silicon and hydrogen is considered to exist in addition to the defect structure shown in FIG. 3, it has an energy level deeper than the conduction band edge of silicon carbide and forms an interface trap. Only defects having an energy level deeper than 0.6 eV from the end of the conductive band.
- FIG. 20 shows the gate leakage current with respect to the electric field of the gate insulating film 5 in the MOSFET according to the present embodiment. Note that FIG. 20 is obtained by measuring the gate current when a voltage is applied to the gate insulating film 5. That is, the horizontal axis represents the electric field obtained by dividing the applied gate voltage by the thickness of the gate insulating film 5.
- FIG. 20 shows that the gate leakage current is reduced when this embodiment is used. Furthermore, the leakage current is reduced as the temperature of the reoxidation treatment is higher.
- the re-oxidation treatment reduces the size, and the higher the temperature, the smaller the size, and the reliability of the gate insulating film 5 is improved.
- the defect 10 including a bond of silicon and hydrogen has been described as the defect that forms the first trap having a deep energy level from the conduction band edge.
- Na, P, V, N, As, K, Li, or the like may be used. That is, any defect may be used as long as an interface trap can be formed at a deep energy level without forming an interface trap at a shallow energy level from the conduction band edge at the MOS interface.
- a so-called inversion type MOSFET in which the channel is formed by inverting the conductivity type of the well region 3 from the second conductivity type to the first conductivity type is described.
- the channel of the well region 3 is formed.
- the present embodiment can also be applied to a storage MOSFET in which a channel region of the first conductivity type is previously provided in the region.
- the threshold voltage is particularly likely to decrease, and it is difficult to realize normally-off characteristics with good controllability. Therefore, the effect of applying this embodiment is great, and a normally-off characteristic can be obtained with good controllability by increasing the threshold voltage.
- the channel region of the first conductivity type of the storage MOSFET may be formed by epitaxial growth or ion implantation.
- the (0001) plane provided with an off angle of 4 ° is used for the first main surface of silicon carbide substrate 1, but it may be a (11-20) plane, It may be a (000-1) plane. Further, the off angle is not limited to 4 °.
- a so-called vertical type in which an on-current is caused to flow from the surface of drift layer 2 formed on the first main surface of silicon carbide substrate 1 to the back surface that is the second main surface of silicon carbide substrate 2.
- a so-called lateral type semiconductor device in which an on-current is allowed to flow laterally from the surface of the drift layer 2 to the surface of the drift layer 2 may be used.
- the silicon carbide MOSFET has been described as an example.
- a semiconductor device using silicon carbide having a gate insulating type structure in which a gate insulating film 5 is formed on silicon carbide is also used. It goes without saying that the same effect can be obtained by applying the present embodiment.
- FIG. FIG. 21 is an enlarged cross-sectional view of the vicinity of the MOS interface in the silicon carbide semiconductor device according to the second embodiment.
- the present embodiment is characterized in that the gate insulating film 5 contains nitrogen atoms. The rest is the same as in the first embodiment. According to this embodiment, the threshold voltage can be further improved.
- Nitriding in which heat treatment is performed on the gate insulating film 5 in a nitriding gas atmosphere such as nitrogen monoxide gas (NO) or dinitrogen monoxide gas (N 2 O) for the purpose of increasing the channel mobility of the MOSFET using silicon carbide. Processing is drawing attention.
- a nitriding gas atmosphere such as nitrogen monoxide gas (NO) or dinitrogen monoxide gas (N 2 O)
- NO nitrogen monoxide gas
- N 2 O dinitrogen monoxide gas
- silicon carbide substrate 1 having the structure of FIG. 10 is introduced into a nitriding furnace before re-oxidation treatment.
- the temperature in the nitriding furnace is raised in an inert gas atmosphere, and when it reaches the processing temperature, it is switched to a nitrogen monoxide gas or dinitrogen monoxide gas atmosphere, and the nitriding gas atmosphere and the processing temperature are set for a predetermined time. By maintaining, nitriding is performed.
- nitric oxide gas or dinitrogen monoxide gas diluted with an inert gas such as nitrogen, argon, helium or krypton may be used.
- An atmosphere in which nitrogen oxide gas and dinitrogen monoxide gas are mixed may be used.
- the nitriding temperature is preferably 900 ° C. or higher and 1450 ° C. or lower. This is because the nitriding rate is very slow at a low temperature of less than 900 ° C., and the inactivation of the interface trap at the MOS interface by nitrogen atoms hardly proceeds. Further, under high temperature conditions higher than 1450 ° C., thermal oxidation of silicon carbide by oxygen generated by decomposition of nitrogen monoxide gas or nitrous oxide gas proceeds, and new interface traps increase at the MOS interface. .
- the nitriding time is preferably about 10 minutes to 10 hours.
- the atmosphere in the reaction furnace is switched to an inert gas atmosphere, the temperature is lowered to the take-out temperature, and the silicon carbide substrate 1 is taken out of the reaction furnace. Thereby, the nitriding treatment step is completed.
- the reoxidation process described in the first embodiment is performed.
- the channel mobility is improved by the reduction of the shallow level interface trap density, the threshold voltage is also lowered and the normally-on characteristic is likely to be obtained.
- Nitriding is performed at a relatively high temperature of 900 ° C. or higher and 1450 ° C. or lower.
- oxygen atoms are desorbed from the gate insulating film 5 and oxygen vacancies are easily generated. Since the oxygen vacancies are positively charged, the threshold voltage of the MOSFET decreases as the oxygen vacancies increase. That is, the threshold voltage decreases when nitriding is performed.
- FIG. 22 schematically shows the relationship between channel mobility and threshold voltage when this embodiment is used.
- a one-dot chain line shows a case where the conventional reoxidation treatment and nitriding treatment are not performed, and a case where only the reoxidation treatment is performed on the conventional example shown by the one-dot chain line is shown by a dotted line.
- a dotted line corresponds to the first embodiment, and a higher threshold voltage can be realized as compared with the conventional example.
- the case where only the nitriding treatment is performed on the conventional example indicated by a one-dotted line is indicated by a solid line
- the case where both the nitriding treatment and the reoxidation treatment are performed is indicated by a two-dot difference line. Comparing the one-point difference line and the solid line, the solid line is shifted to the high channel mobility and low threshold voltage side by nitriding. This is because positively charged oxygen vacancies increase in addition to the effect of reducing interface traps.
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Abstract
Description
まず、この発明の実施の形態1における炭化珪素半導体装置の素子構造を説明する。図1は、この発明の実施の形態1に係る炭化珪素半導体装置を示す断面図である。本実施の形態では、炭化珪素半導体装置の一例として、nチャネル炭化珪素MOSFETを説明する。
図21は、本実施の形態2に係る炭化珪素半導体装置においてMOS界面付近を拡大した断面図である。本実施の形態では、ゲート絶縁膜5が、窒素原子を含むことを特徴とする。それ以外については、実施の形態1と同様である。本実施の形態によれば、閾値電圧をより向上することが出来る。
Claims (11)
- 炭化珪素基板の表面上に形成された第1導電型のドリフト層と、
前記ドリフト層の表層部に、互いに間隔をあけて形成された複数の第2導電型のウェル領域と、
前記ウェル領域の表層部の一部に形成された第1導電型のソース領域と、
前記ウェル領域と前記ソース領域の表面に形成されたゲート絶縁膜と、
前記ゲート絶縁膜の表面に、前記ソース領域の端部と前記ウェル領域とに対向するように形成されたゲート電極と、
を備え、
前記ゲート絶縁膜と前記ウェル領域との界面領域に形成された界面トラップの密度をDit[cm-2eV-1]、炭化珪素の導電帯のエネルギー準位Ecからの深さである前記界面トラップのエネルギー準位を(Ec-E)[eV]、前記界面トラップのエネルギー準位が∞[eV]において前記界面トラップの密度Ditが漸近する値を係数A[cm-2eV-1]、係数B[cm-2eV-1]を前記界面トラップのエネルギー準位が0[eV]のときに前記界面トラップの密度Ditが(A+B)[cm-2eV-1]と等しくなる値とし、さらに、X[eV]を係数とすると、数1において、前記界面トラップのエネルギー準位が0.1eV以上0.4eV以下の範囲内での前記係数X[eV]が、0.09eV以上0.15eV以下であること、
を特徴とする炭化珪素半導体装置。
- 前記ゲート絶縁膜は、前記ウェル領域との界面領域において、珪素と水素の結合を含む欠陥を有し、
前記界面トラップは、前記欠陥によって形成される炭化珪素の導電帯端より深いエネルギー準位をもつ第1のトラップを含むこと、
を特徴とする請求項1に記載の炭化珪素半導体装置。 - 前記界面トラップのエネルギー準位が0.1eVのときの前記界面トラップの密度を基準値Dit1[cm-2eV-1]とし、前記界面トラップのエネルギー準位が0.2eVのときの前記界面トラップの密度とDit1[cm-2eV-1]との差をDit2[cm-2eV-1]とし、前記界面トラップのエネルギー準位が0.8eVのときの前記界面トラップの密度とDit1[cm-2eV-1]との差をDit8[cm-2eV-1]、Dit2[cm-2eV-1]とDit8[cm-2eV-1]との比を比率Rとすると、数2において、比率Rが0.54以上0.67以下であること、
を特徴とする請求項1又は2に記載の炭化珪素半導体装置。
- 炭化珪素基板の表面上に形成された第1導電型のドリフト層と、
前記ドリフト層の表層部に、互いに間隔をあけて形成された複数の第2導電型のウェル領域と、
前記ウェル領域の表層部の一部に形成された第1導電型のソース領域と、
前記ウェル領域と前記ソース領域の表面に形成されたゲート絶縁膜と、
前記ゲート絶縁膜の表面に、前記ソース領域の端部と前記ウェル領域とに対向するように形成されたゲート電極と、
を備え、
前記ゲート絶縁膜は、前記ウェル領域との界面領域において、炭化珪素の導電帯端より深いエネルギー準位をもつ第1のトラップを形成し、珪素と水素の結合を含む欠陥を有すること、
を特徴とする炭化珪素半導体装置。 - 前記第1のトラップは、炭化珪素の導電帯端から0.6eV以上1.5eV以下の深さにエネルギー準位を有すること、
を特徴とする請求項1乃至4のいずれか1項に記載の炭化珪素半導体装置。 - 前記ゲート絶縁膜は、第2のトラップを形成し、珪素と水素の結合を含む欠陥をさらに有すること、
を特徴とする請求項1乃至5のいずれか1項に記載の炭化珪素半導体装置。 - 前記ゲート絶縁膜は、前記ウェル領域との界面領域に窒素原子を含有すること
を特徴とする請求項1乃至6のいずれか1項に記載の炭化珪素半導体装置。 - 前記ゲート絶縁膜は、SiO2膜であること
を特徴とする請求項1乃至7のいずれか1項に記載の炭化珪素半導体装置。 - 前記ウェル領域は、チャネルが形成される領域に第1導電型のチャネル領域を備えたこと
を特徴とする請求項1乃至8のいずれか1項に記載の炭化珪素半導体装置。 - 炭化珪素基板の表面上に第1導電型のドリフト層を形成する工程と、
前記ドリフト層の表層部に、互いに間隔をあけて複数の第2導電型のウェル領域を形成する工程と、
前記ウェル領域の表層部の一部に第1導電型のソース領域を形成する工程と、
前記ウェル領域と前記ソース領域の表面にゲート絶縁膜を形成する工程と、
前記ゲート絶縁膜を、H2/O2流量比が0.7以上1.9以下で燃焼反応させた水蒸気雰囲気中、600℃以上950℃以下で熱処理する工程と、
前記ゲート絶縁膜の表面に、前記ソース領域の端部と前記ウェル領域とに対向するようにゲート電極を形成する工程と、
を備えた炭化珪素半導体装置の製造方法。 - 前記ゲート絶縁膜を形成する工程の後、前記熱処理する工程の前に、前記ゲート絶縁膜を窒化ガス雰囲気中、900℃以上1450℃以下で窒化処理する工程を備えたこと
を特徴とする請求項10に記載の炭化珪素半導体装置の製造方法。
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JP2019050294A (ja) * | 2017-09-11 | 2019-03-28 | 株式会社豊田中央研究所 | 炭化珪素半導体装置 |
JP2020047666A (ja) * | 2018-09-14 | 2020-03-26 | 株式会社東芝 | 半導体装置、半導体装置の製造方法、インバータ回路、駆動装置、車両、及び、昇降機 |
JP2020061475A (ja) * | 2018-10-11 | 2020-04-16 | 株式会社豊田中央研究所 | 炭化珪素半導体装置とその製造方法 |
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US11282950B2 (en) * | 2017-05-31 | 2022-03-22 | Mitsubishi Electric Corporation | Method for manufacturing semiconductor device |
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