Patent Publication Number: US-9842906-B2

Title: Semiconductor device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of and claims the benefit of priority under 35 U.S.C. §120 for U.S. Ser. No. 13/299,882 filed Nov. 18, 2011, and claims the benefit of priority under 35 U.S.C. §119 from Japanese Patent Application Nos. 2010-262120 filed Nov. 25, 2010 and 2011-232666 filed Oct. 24, 2011. The entire contents of each of the above are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a semiconductor device, and more specifically to a semiconductor device with a transistor cell with a polysilicon gate electrode and an interconnect line containing aluminum. 
     Description of the Background Art 
     Semiconductor elements (MOSFETs (metal oxide semiconductor field effect transistors), IGBTs (insulated gate bipolar transistors) and others) formed by using silicon carbide (SiC) are promising candidates for next-generation switching elements capable of achieving high breakdown voltage, low loss, and high resistance to heat, and are expected to be applied to power semiconductor devices such as inverters. 
     A generally employed structure of a conventional MOSFET using SiC (SiC-MOSFET) is such that a silicide layer to make ohmic contact is formed on a source region, and a source electrode made of aluminum (Al) is formed on the silicide layer (see for example Japanese Patent Application Laid-Open No. 2009-194127). In the structure disclosed in Japanese Patent Application Laid-Open No. 2009-194127, a metal layer made of Ti is interposed between a silicide layer on a source region and an aluminum source electrode, and the metal layer functions as a barrier metal to suppress diffusion of Al. 
     A reliability test such as an HTGB (high temperature gate bias) test conducted by continuously applying a voltage between the gate and the source shows that the conventional SiC-MOSFET suffers from reduction with time in a gate-to-source threshold voltage (VGSth). 
     Reduction in the threshold voltage increases the transfer characteristics (ratio of output to input) of the MOSFET to generate flow of overcurrent during the actual use of the MOSFET, resulting in a fear of breakage of the MOSFET. Reduction in the threshold voltage also increases a switching speed during turn on. This generates nonuniformity of the operations of a plurality of MOSFET cells of a semiconductor chip, resulting in a fear of breakage of the semiconductor chip. Further, even if no problem in electric characteristics is found as a result of the test, the threshold voltage may be reduced due to prolonged application of voltage stress between the gate and the source, so the aforementioned problem is likely to occur. 
     SiC devices achieve excellent electric characteristics at high temperatures, so they are expected to be used in a high-temperature condition. Meanwhile, Al forming a source electrode may cause corrosion of an interlayer insulating film intended to maintain the isolation between the gate and the source, or may diffuse into polysilicon forming a gate interconnect line that is what is called “Al spike,” resulting in a fear of formation of a short circuit between the gate and the source. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a semiconductor device capable of suppressing reduction with time in a threshold voltage, preventing corrosion of an insulating film to be generated by an aluminum interconnect line, and preventing a short circuit to be generated between a gate and a source by Al spike. 
     The semiconductor device of the present invention includes: a main transistor cell with a polysilicon gate electrode formed on a semiconductor layer, and a source region that is an impurity region formed in an upper portion of the semiconductor layer; and an interlayer insulating film covering the gate electrode. A source electrode which contains aluminum and is connected to the source region extends on the interlayer insulating film. A gate pad containing aluminum is connected to the gate electrode. A barrier metal layer that prevents diffusion of aluminum is interposed between the source electrode and the interlayer insulating film, and between the gate pad and the gate electrode. 
     The barrier metal layer that prevents diffusion of aluminum is interposed between the source electrode and the interlayer insulating film, and between the gate pad and the gate electrode. This prevents reduction in a threshold voltage to be generated by voltage stress applied to the gate electrode of the transistor, thereby enhancing the stability of a MOSFET. Further, the interlayer insulating film is not corroded by Al contained in the source electrode and the gate pad, and Al spike is not generated in the polysilicon gate electrode even in a high-temperature condition, thereby preventing generation of a short circuit between the gate and the source. 
     These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a top view of a semiconductor chip with a semiconductor device of a first preferred embodiment; 
         FIGS. 2A and 2B  respectively are sectional views of a MOSFET cell portion and a gate pad region of the semiconductor chip of the first preferred embodiment; 
         FIG. 3  shows a relationship between time of a negative HTGB test and the change of a threshold voltage of a MOSFET; 
         FIG. 4  shows a relationship between the thickness of a Ti barrier metal layer and the change of the threshold voltage of the MOSFET; 
         FIG. 5  shows a relationship between the thickness of a TiN barrier metal layer and the change of the threshold voltage of the MOSFET; 
         FIG. 6  is a top view of a semiconductor chip with a semiconductor device of a second preferred embodiment; 
         FIG. 7  is a sectional view of a current sensing cell portion of the semiconductor device of the second preferred embodiment; 
         FIG. 8  shows a relationship between the thickness of a Ti barrier metal layer and the change of the threshold voltage of the MOSFET; 
         FIG. 9  shows a relationship between the thickness of a TiSi barrier metal layer and the change of the threshold voltage of the MOSFET; 
         FIGS. 10A and 10B  respectively are sectional views of a MOSFET cell portion and a gate pad region of a semiconductor chip of a fifth preferred embodiment; 
         FIG. 11  shows a relationship between time of a negative HTGB test and the change of a threshold voltage of a MOSFET; 
         FIGS. 12A and 12B  are cross-sectional views of a MOSFET cell portion and a gate pad region of a semiconductor chip of a sixth preferred embodiment; 
         FIG. 13  shows a relationship between time of a negative HTGB test and the change of a threshold voltage of a MOSFET; 
         FIG. 14  is a top view of a semiconductor chip with a semiconductor device of a seventh preferred embodiment; and 
         FIG. 15  is a sectional view of a temperature sensing diode portion of the semiconductor device of the sixth preferred embodiment. 
     
    
    
     EMBODIMENTS FOR CARRYING OUT THE INVENTION 
     First Preferred Embodiment 
       FIG. 1  is a top view of a semiconductor chip with a semiconductor device of a first preferred embodiment of the present invention. An SiC-MOSFET is shown as an example of the semiconductor device. A source electrode  101  and a gate pad  102  connected to a gate electrode are formed on the upper surface of a MOSFET chip  100  to hold the MOSFET. A field limiting ring  103  is provided as a termination structure on the outer circumference of the MOSFET chip  100 . 
       FIGS. 2A and 2B  are sectional views of the MOSFET chip  100 .  FIG. 2A  is a sectional view of a MOSFET cell portion (cross section taken along a line A-A of  FIG. 1 ).  FIG. 2B  is a sectional view of a gate pad portion (cross section taken along a line B-B of  FIG. 1 ). The MOSFET chip  100  includes parallel connection of a plurality of cells of the structure shown in  FIG. 2A , and the gate electrode of each of the cells is connected to the gate pad  102 . 
     The MOSFET includes an SiC substrate to become an n + -buffer layer  1 , and an epitaxial substrate with an epitaxial growth layer to become an n − -drift layer  2 . As shown in  FIGS. 2A and 2B , a p-base region  3  is formed in an upper portion of the n − -drift layer  2 , and an n + -source region  4  and a p + -contact layer  13  are formed in a surface portion of the p-base region  3 . A gate insulating film  5  which is constructed of a thermally oxidized film and covers the n + -source region  4 , the p-base region  3 , and part of the n − -drift layer  2  adjoining the p-base region  3  is formed in the upper surface of the epitaxial growth layer. A polysilicon gate electrode  6  is formed on the gate insulating film  5 . 
     An interlayer insulating film  7  made for example of TEOS (tetraethyl orthosilicate) is formed on the gate electrode  6 . To be specific, the interlayer insulating film  7  is removed on part of the n + -source region  4  and part of the p-base region  3  (parts on which the gate electrode  6  is not formed). A silicide layer  8  (compound layer of SiC and metal such as nickel silicide (NiSi)) in ohmic contact with the n + -source region  4  and the p + -contact layer  13  in the p-base region  3  is formed in these parts. A barrier metal layer  9  to suppress diffusion of aluminum (Al) is formed on the interlayer insulating film  7  and the silicide layer  8 . A source electrode  101  made of Al or an Al alloy (such as AlSi) is formed on the barrier metal layer  9 . The barrier metal layer  9  is made of titanium (Ti) or titanium nitride (TiN). A drain electrode  10  is formed on the lower surface of the n + -buffer layer  1 . 
     As shown in  FIG. 2B , a field oxide film  14  is formed on an upper surface of the epitaxial growth layer, and the gate electrode  6  extends over the field oxide film  14 . The interlayer insulating film  7  is removed in the gate pad portion to expose the gate electrode  6 . The barrier metal layer  9  is formed on the exposed upper surface of the gate electrode  6 , and an Al gate pad  102  is formed on the barrier metal layer  9 . The source electrode  101  and the gate pad  102  are formed in the same process. To be specific, the source electrode  101  and the gate pad  102  are electrically isolated from each other after being patterned together with the barrier metal layer  9 . 
       FIG. 3  is a graph showing a relationship between time of an HTGB test conducted by applying a negative voltage to the gate of the MOSFET (negative HTGB test), and the change of a gate-to-source threshold voltage (VGSth) (change from an initial threshold voltage). This graph includes a result obtained in a conventional structure with no barrier metal layer  9 , a result obtained in a structure with the barrier metal layer  9  made of Ti, and a result obtained in a structure with the barrier metal layer  9  made of TiN. TiN can be formed by depositing Ti to have a desired thickness and then performing lamp annealing at 800° for about 30 seconds under a nitrogen (N 2 ) atmosphere. The method of forming TiN used in the embodiments below may be similar to the above as well. For example, in a case of forming TiN having a thickness of 75 nm, Ti is deposited to have a thickness of 75 nm and then is subjected to lamp annealing as described above. The barrier metal layer  9  used in the test has a thickness of 75 nm. The HTGB test was conducted with a gate-to-source voltage set at −20 V, and with an ambient temperature set at 125° C. 
     As shown in  FIG. 3 , the conventional MOSFET experienced reduction in a threshold voltage of about 5 V from its initial value in the HTGB test conducted for 240 hours. The MOSFET with the TiN barrier metal layer  9  experienced lower reduction in a threshold voltage of about 2 V. The MOSFET with the Ti barrier metal layer  9  experienced substantially no reduction in a threshold voltage. This shows that the MOSFET of the present invention suppresses reduction with time in a threshold voltage, so that the operation of the MOSFET of the present invention is given enhanced stability. 
     In the semiconductor device of the first preferred embodiment, the barrier metal layer  9  to suppress diffusion of Al is interposed between the interlayer insulating film  7  and the source electrode  101  in the MOSFET cell portion, and between the gate electrode  6  and the gate pad  102  in the gate pad portion. This prevents corrosion of the interlayer insulating film  7  to be generated by Al contained in the source electrode  101 , and Al spike to be generated in the polysilicon gate electrode  6  even in a high-temperature condition, thereby preventing a short circuit between the gate and the source. Al spike in the gate electrode  6  in the gate pad portion to be generated by Al contained in the gate pad  102  is also prevented. Al spike in the gate electrode  6 , if generated especially at a temperature of 300° C. or higher, comes through the gate electrode  6  to reach the gate insulating film  5 . This may result in a problem of lower reliability of breakdown voltage of the gate insulating film  5 , and this problem is avoided by the first preferred embodiment. 
       FIG. 4  shows a relationship between the thickness of the Ti barrier metal layer  9  and the change of the threshold voltage (VGSth) of the MOSFET. The relationship is obtained as a result of an HTGB test conducted for 240 hours that is the same test as that conducted to obtain the results shown in  FIG. 3 . Here, the HTGB test was conducted on a MOSFET with the barrier metal layer  9  having a thickness of 30 nm, and on a MOSFET with the barrier metal layer  9  having a thickness of 75 nm. 
     As also shown in  FIG. 3 , the threshold voltage did not reduce after the HTGB test conducted for 240 hours when the Ti barrier metal layer  9  had a thickness of 75 nm. In contrast, the threshold voltage reduced by about 0.5 V when the Ti barrier metal layer  9  had a thickness of 30 nm. Reduction in the threshold voltage is suppressed more effectively with the greater thickness of the barrier metal layer  9 . The thickness of the barrier metal layer  9  of 60 nm or more is especially effective as it can limit reduction of a threshold voltage to about 0.2 V or less. 
       FIG. 5  shows a relationship between the thickness of the TiN barrier metal layer  9  and the change of the threshold voltage (VGSth) of the MOSFET. The relationship is obtained as a result of an HTGB test conducted for 240 hours that is the same test as that conducted to obtain the results shown in  FIG. 3 . Here, the HTGB test was conducted on a MOSFET with the barrier metal layer  9  having a thickness of 30 nm, and on a MOSFET with the barrier metal layer  9  having a thickness of 75 nm. 
     As also shown in  FIG. 3 , the threshold voltage reduced by about 2 V after the HTGB test conducted for 240 hours when the TiN barrier metal layer  9  had a thickness of 75 nm. In contrast, the threshold voltage reduced by about 6.6 V when the TiN barrier metal layer  9  had a thickness of 30 nm. In a case where the barrier metal layer  9  is made of TiN, the thickness of the barrier metal layer  9  of 90 nm or more is effective as it can limit reduction of the threshold voltage to about 0.2 V or less. 
     Second Preferred Embodiment 
     Some MOSFETs include a current sensing cell that detects current flowing in the MOSFETs. An exemplary object of the current sensing cell is to detect overcurrent so that the MOSFETs can be protected from breakage due to overcurrent. The current sensing cell generally shares a gate and a drain with a MOSFET cell normally used (main MOSFET cell). The current sensing cell diverts part of main current flowing in a MOSFET to obtain minute current that is in proportion to the main current. 
       FIG. 6  is a top view of a MOSFET chip  100  of a second preferred embodiment. Some of MOSFET cells of the MOSFET chip  100  is used as a current sensing cell  110 . A source electrode (current sensing electrode)  111  of the current sensing cell  110  is separated from a source electrode  101  of a main MOSFET cell, whereas the gate electrode of the current sensing cell  110  is shared with the main MOSFET cell and is connected to a gate pad  102 . 
       FIG. 7  is a sectional view of the current sensing cell  110  of the MOSFET chip  110  (cross section taken along a line C-C of  FIG. 6 ). A cross section of the main MOSFET cell (cross section taken along a line A-A of  FIG. 6 ) is the same as that shown in  FIG. 2A , and a cross section of a gate pad portion (cross section taken along a line B-B of  FIG. 6 ) is the same as that shown in  FIG. 2B . Constituents shown in  FIGS. 6 and 7  corresponding to those shown in  FIGS. 1 and 2  are designated by the same reference numerals, and are not described in detail below. 
     As shown in  FIG. 7 , the current sensing cell  110  has the same structure as that of the main MOSFET cell ( FIG. 2A ). To be specific, a barrier metal layer  9  is interposed between an interlayer insulating film  7  and the source electrode (current sensing electrode)  111 . So, like the main MOSFET cell, the current sensing cell  110  prevents generation of corrosion of the interlayer insulating film  7  and reduction in a threshold voltage (VGSth) as described in the first preferred embodiment. 
     Current cannot be detected precisely so protection from overcurrent cannot be achieved well if the main MOSFET cell and the current sensing cell  110  have different threshold voltages. Meanwhile, in the second preferred embodiment, the current sensing cell  110  and the main MOSFET cell both have the barrier metal layer  9 , so that the current sensing cell  110  and the main MOSFET cell are allowed to have the same threshold voltage, thereby achieving precise current detection. As a matter of course, the thickness of the barrier metal layer  9  is preferably the same in the main MOSFET cell and the current sensing cell  110 . 
     Like in the first preferred embodiment, in the second preferred embodiment, the barrier metal layer  9  preferably has a thickness of 60 nm or more in a case where it is made of Ti, and preferably has a thickness of 90 nm or more in a case where it is made of TiN. 
     Third Preferred Embodiment 
     It has been derived from the first preferred embodiment that the thickness of the barrier metal layer  9  of 60 nm or more can limit reduction of a threshold voltage to be about 0.2 V or less from the results ( FIG. 4 ) of the HTGB test conducted in the cases where the barrier metal layer  9  had a thickness of 30 nm and 75 nm. 
     However, the inventors have conducted further experiments and found that the effect of suppressing reduction of a threshold voltage has already become saturated in the state in which the thickness of the Ti barrier metal layer  9  is smaller than 75 nm and reduction of a threshold voltage can be suppressed sufficiently even in a case where the thickness of the barrier metal layer  9  is smaller than 60 nm. The experimental results thereof are described below. 
       FIG. 8  shows a relationship between the thickness of the Ti barrier metal layer  9  and the change of the threshold voltage (VGSth) of the MOSFET. Here, the HTGB test similar to that of  FIG. 3  was conducted for 240 hours on a MOSFET with the barrier metal layer  9  having a thickness of 30 nm, a MOSFET with the barrier metal layer  9  having a thickness of 50 nm and a MOSFET with the barrier metal layer  9  having a thickness of 75 nm. 
     The threshold voltage did not reduce after the HTGB test conducted for 240 hours when the Ti barrier metal layer  9  had a thickness of 50 nm. In contrast, as also shown in  FIG. 4 , the threshold voltage reduced by about 0.5 V when the barrier metal layer  9  had a thickness of 30 nm. The results above reveal that the thickness of the barrier metal layer  9  of 40 nm or more is especially effective as it can limit reduction of a threshold voltage to about 0.2 V or less. 
     Further, also in the second preferred embodiment, the thickness of the barrier metal layer  9  is preferably 40 nm or more when the barrier metal layer  9  is made of Ti. 
     Fourth Preferred Embodiment 
     While the barrier metal layer  9  that prevents diffusion of Al is Ti or TiN in the first to third preferred embodiments, similar effects can be achieved as well when TiSi is used. 
       FIG. 9  shows a relationship between the thickness of a TiSi barrier metal layer  9  and the change of the threshold voltage (VGSth) of the MOSFET. TiSi can be formed by depositing Ti to have a desired thickness and then performing lamp annealing at 800° for about 30 seconds under an argon (Ar) atmosphere. The method of forming TiSi used in the embodiments below may be similar to the above as well. For example, in a case of forming TiSi having a thickness of 75 nm, Ti is deposited to have a thickness of 75 nm and then is subjected to lamp annealing as described above. Here, the HTGB test similar to that of  FIG. 3  was conducted for 240 hours on a MOSFET with the barrier metal layer  9  having a thickness of 75 nm and a MOSFET with the barrier metal layer  9  having a thickness of 150 nm. 
     As shown in  FIG. 9 , the threshold voltage did not reduce after the HTGB test conducted for 240 hours when the TiSi barrier metal layer  9  had a thickness of 150 nm, but the threshold voltage reduced by about 1.0 V when the TiSi barrier metal layer  9  had a thickness of 75 nm. In the case where the barrier metal layer  9  is made of TiSi, the thickness of the barrier metal layer  9  of 130 nm or more is effective as it can limit reduction of the threshold voltage to about 0.2 V or less. 
     The above shows that in the case where the TiSi barrier metal layer  9  is used, a reduction of the threshold voltage can be sufficiently suppressed when the thickness thereof is 130 nm or more, so that the operation of the MOSFET is given enhanced stability. 
     Also in the case where the barrier metal layer  9  is made of TiSi, as in the first preferred embodiment, it is possible to prevent corrosion of the interlayer insulating film  7  by Al contained in the source electrode  101  and the generation of Al spike in the polysilicon gate electrode  6 , thereby preventing a short circuit between the gate and the source. Al spike in the gate electrode  6  in the gate pad portion caused by Al contained in the gate pad  102  is also prevented. 
     The TiSi barrier metal layer  9  is also applicable to the second preferred embodiment. That is, TiSi may be used for the main MOSFET cell and the barrier metal layer  9  of the current sensing cell  110 . This allows the main MOSFET cell and the current sensing cell  110  to have the same threshold voltage, thereby achieving precise current detection. Also in that case, the thickness of the barrier metal layer  9  is preferably 130 nm or more. 
     Fifth Preferred Embodiment 
     A fifth preferred embodiment shows an example in which the barrier metal layer  9  has a two-layer structure formed of a TiSi layer and a Ti layer. 
       FIGS. 10A and 10B  are sectional views of a MOSFET chip  100  of the fifth preferred embodiment, where  FIG. 10A  shows the cross section of a MOSFET cell portion (cross section taken along the line A-A of  FIG. 1 ) and  FIG. 10B  shows a cross section of a sectional MOSFET cell portion of a gate pad portion (cross section taken along the line B-B of  FIG. 1 ). 
     As shown in  FIGS. 10A and 10B , in the MOSFET chip  100  of this preferred embodiment, the barrier metal layer  9  has a two-layer structure formed of a TiSi layer  91  being a lower layer and a Ti layer  92  being an upper layer. The other configuration is similar to that of the first preferred embodiment, which is not described below. 
       FIG. 11  is a graph showing a relationship between time of a negative HTGB test on a MOSFET and the change (change from its initial threshold voltage) of a threshold voltage (VGSth) between the gate and the source of a MOSFET. This graph shows the case of a conventional structure that does not include the barrier metal layer  9  and the case in which the barrier metal layer  9  having a two-layer structure  9  formed of the TiSi layer  91  and the Ti layer  92  is provided. Here, the thicknesses of the TiSi layer  91  and the Ti layer  92  were respectively 75 nm (the thickness of the barrier metal layer  9  was 150 nm). As in  FIG. 3 , the HTGB test was conducted with a gate-to-source voltage set at −20 V, and with an ambient temperature set at 125° C. 
     As shown in  FIG. 11 , the MOSFET with the barrier metal layer  9  having a two-layer structure formed of the TiSi layer  91  and the Ti layer  92  experienced little reduction in a threshold voltage after the HTGB test conducted for 240 hours. As understood from the comparison with  FIG. 3 , the effects thereof are similar to those of the Ti barrier metal layer  9  having a thickness of 75 nm. Meanwhile, the conventional MOSFET experienced reduction in the threshold voltage of about 5 V from its initial value, as also shown in  FIG. 3 . 
     As described above, it is possible to suppress reduction of the threshold voltage of the MOSFET even in a case where the barrier metal layer  9  has a two-layer structure formed of the TiSi layer  91  and the Ti layer  92 . Accordingly, the operation of the MOSFET is given enhanced stability. 
     Also in the case where the barrier metal layer  9  has a two-layer structure formed of the TiSi layer  91  and the Ti layer  92 , as in the first preferred embodiment, it is possible to prevent corrosion of the interlayer insulating film  7  by Al contained in the source electrode  101  and the generation of Al spike in the polysilicon gate electrode  6 , thereby preventing a short circuit between the gate and the source. Al spike in the gate electrode  6  in the gate pad portion to caused by Al contained in the gate pad  102  is also prevented. 
     The barrier metal layer  9  having a two-layer structure formed of the TiSi layer  91  and the Ti layer  92  is also applicable to the second preferred embodiment. That is, the barrier metal layer  9  of the main MOSFET cell and the current sensing cell  110  may have a two-layer structure. This allows the main MOSFET cell and the current sensing cell  110  to have the same threshold voltage, thereby achieving precise current detection. 
     Sixth Preferred Embodiment 
     A sixth preferred embodiment shows an example in which the barrier metal layer  9  has a two-layer structure formed of a TiN layer and a Ti layer. 
       FIGS. 12A and 12B  are sectional views of a MOSFET chip  100  of the sixth preferred embodiment, where  FIG. 12A  shows the cross section of a MOSFET cell portion (cross section taken along the line A-A of  FIG. 1 ) and  FIG. 12B  shows a cross section of a sectional MOSFET cell portion of a gate pad portion (cross section taken along the line B-B of  FIG. 1 ). 
     As shown in  FIGS. 12A and 12B , in the MOSFET chip  100  of this preferred embodiment, the barrier metal layer  9  has a two-layer structure formed of a TiN layer  93  being a lower layer and a Ti layer  94  being an upper layer. The other configuration is similar to that of the first preferred embodiment, which is not described below. 
       FIG. 13  is a graph showing a relationship between time of a negative HTGB test on a MOSFET and the change (change from its initial threshold voltage) of a threshold voltage (VGSth) between the gate and the source of a MOSFET. This graph shows the case of a conventional structure that does not include the barrier metal layer  9  and the case in which the barrier metal layer  9  having a two-layer structure  9  formed of the TiN layer  93  and the Ti layer  94  is provided. Here, the results of the HTGB tests are shown in the case where the TiN layer  93  and the Ti layer  94  each had a thickness of 75 nm (the thickness of the barrier metal layer  9  was 150 nm), the case where the TiN layer  93  had a thickness of 25 nm and the Ti layer  94  had a thickness of 75 nm (the thickness of the barrier metal layer  9  was 100 nm), and the case where the TiN layer  93  had a thickness of 25 nm and the Ti layer  94  had a thickness of 150 nm (the thickness of the barrier metal layer  9  was 175 nm). As in  FIG. 3 , each of the HTGB tests was conducted with a gate-to-source voltage set at −20 V, and with an ambient temperature set at 125° C. 
     As shown in  FIG. 13 , the MOSFET with the barrier metal layer  9  having a two-layer structure formed of the TiN layer  93  and the Ti layer  94  experienced little reduction in a threshold voltage after the HTGB test conducted for 240 hours in all of the above-mentioned three cases. As understood from the comparison with  FIG. 3 , the effects thereof are similar to those of the Ti barrier metal layer  9  having a thickness of 75 nm. Meanwhile, the conventional MOSFET experienced reduction in the threshold voltage of about 5 V from its initial value, as also shown in  FIG. 3 . 
     As described above, it is possible to suppress reduction of the threshold voltage of the MOSFET even in a case where the barrier metal layer  9  has a two-layer structure formed of the TiN layer  93  and the Ti layer  94 . Accordingly, the operation of the MOSFET is given enhanced stability. 
     Also in the case where the barrier metal layer  9  has a two-layer structure formed of the TiN layer  93  and the Ti layer  94 , as in the first preferred embodiment, it is possible to prevent corrosion of the interlayer insulating film  7  by Al contained in the source electrode  101  and the generation of Al spike in the polysilicon gate electrode  6 , thereby preventing a short circuit between the gate and the source. Al spike in the gate electrode  6  in the gate pad portion to caused by Al contained in the gate pad  102  is also prevented. 
     The barrier metal layer  9  having a two-layer structure formed of the TiN layer  93  and the Ti layer  94  is also applicable to the second preferred embodiment. That is, the barrier metal layer  9  of the main MOSFET cell and the current sensing cell  110  may have a two-layer structure. This allows the main MOSFET cell and the current sensing cell  110  to have the same threshold voltage, thereby achieving precise current detection. 
     Seventh Preferred Embodiment 
       FIG. 14  is a top view of a MOSFET chip  100  of a seventh preferred embodiment. The MOSFET chip  100  includes a temperature sensing diode  120  as a temperature sensor that detects the chip temperature. The MOSFET cell portion and the gate pad portion of the MOSFET chip  100  have similar structures to those of the first preferred embodiment ( FIG. 2 ), which are not described below. Alternatively, the MOSFET chip  100  may further include the current sensing cell  110  of the second preferred embodiment. 
       FIG. 15  is a sectional view (sectional view taken along a line D-D of  FIG. 14 ) of the temperature sensing diode  120  of the MOSFET chip  100 . As shown in  FIG. 15 , the temperature sensing diode  120  is formed of a p-type polysilicon  123  and an n-type polysilicon  124  adjacent thereto and is disposed on a silicon oxide film  11  formed on an epitaxial growth layer serving as the n − -drift layer  2  of the MOSFET. An anode electrode  121  is disposed on the p-type polysilicon  123  via the barrier metal layer  9 , and the cathode electrode  122  is disposed on the n-type polysilicon  124  via the barrier metal layer  9 . 
     The barrier metal layer  9  of the temperature sensing diode  120  is formed in the same process for the barrier metal layer  9  disposed below the source electrode  101  and the gate pad  102  of the MOSFET and is made of titanium (Ti) or titanium nitride (TiN). The anode electrode  121  and the cathode electrode  122  are formed in the same process for the source electrode  101  and the gate pad  102  of the MOSFET and is formed of Al or an Al alloy (for example, AlSi). 
     As described above, the barrier metal layer  9  is disposed in the contact part between the p-type polysilicon  123  and the anode electrode  121  and the contact part between the n-type polysilicon  124  and the cathode electrode  122  in the temperature sensing diode  120 , which improves electrical contact in those contact parts. As a result, temperature characteristics of the temperature sensing diode  120  become stabilized, and the temperature of the MOSFET chip  100  is detected with accuracy, which contributes to the stabilization of the operation of a MOSFET. 
     Further, as in this preferred embodiment, one same as the barrier metal layer  9  disposed below the source electrode  101  and the gate pad  102  of the MOSFET is used as the barrier metal layer  9  provided below the anode electrode  121  and the cathode electrode  122  of the temperature sensing diode  120 , leading to an effect that a rise of a manufacturing cost is prevented. 
     While the barrier metal layer  9  is made of Ti or TiN as in the first preferred embodiment in the description above, it may be made of TiSi as in the fourth preferred embodiment, may have a two-layer structure formed of a TiSi layer and a Ti layer as in the fifth preferred embodiment, or may have a two-layer structure formed of a TiN layer and a Ti layer as in the sixth preferred embodiment. 
     In particular, in the case where the barrier metal layer  9  made of TiSi or TiN, the barrier metal layer  9  having a two-layer structure formed of a TiSi layer and a Ti layer, or the barrier metal layer  9  having a two-layer structure formed of a TiN layer and a Ti layer is disposed on the p-type polysilicon  123  and the n-type polysilicon  124 , electrical contact between the p-type polysilicon  123  and the anode electrode  121  and electrical contact between the n-type polysilicon  124  and the cathode electrode  122  are enhanced further, whereby the temperature of the MOSFET chip  100  can be detected with more accuracy. 
     While the MOSFET having a structure in which the drift layer  2  and the buffer layer  1  (substrate) have the same conductivity type has been described above, the present invention is also applicable to an IGBT having a structure in which the drift layer  2  and the substrate  1  have different conductivity types. For example, the configuration of an IGBT is achieved when the buffer layer  1  is changed to p-type one in the configuration shown in  FIG. 2A . In that case, the source region  4  and the source electrode  101  of the MOSFET correspond to the emitter region and the emitter electrode of the IGBT, respectively, and the drain electrode  10  of the MOSFET corresponds to the collector electrode. 
     The semiconductor devices described in the respective preferred embodiments are formed by using SiC that is a wide-bandgap semiconductor having high resistance to heat. Semiconductor devices using different wide-bandgap semiconductors are also used effectively for application of the present invention as they have relatively high resistance to heat. Examples of the different wide-gap semiconductors include gallium nitride (GaN) based materials and diamond. 
     While the invention has been shown and described in detail, the foregoing description is in all aspects illustrative and not restrictive. It is therefore understood that numerous modifications and variations can be devised without departing from the scope of the invention.