MIS device and method of manufacturing the same

Abstract: A vertical type semiconductor device having a semiconductor substrate and at least one gate electrode structure formed in the semiconductor substrate for controlling a current flow across the semiconductor substrate. The semiconductor substrate is formed by a silicon substrate (11), a silicon carbide (12) or diamond layer epitaxially deposited on the silicon substrate (11), and a silicon layer (13) epitaxially deposited on the silicon carbide (12) or diamond layer. Recesses are formed in the silicon layer (13) and gate electrodes (15) are provided in the recesses via silicon oxide films (14). Source or emitter regions (16) are formed in portions of the silicon layer (13) which are brought into contact with the silicon oxide films (14) by inverting the conductivity of these portions. Source or emitter electrodes (17) are provided on the source or emitter regions (16), and a drain or collector electrode (18) is provided on a rear surface of the silicon substrate (11).

Description: Background of the Invention Field of the Invention The present invention relates to a semiconductor device and a method of manufacturing the same, and more particularly to a vertical type power semiconductor device such as a power MOSFET (Metal Oxide Silicon Field Effect Transistor), an SI (Static Induction) thyristor, and an IGBT (Insulated Gate Bipolar Transistor) power transistor, and to a method of manufacturing such a vertical type power semiconductor device. Description of Related Art The above mentioned vertical type power semiconductor devices have been widely used as a power semiconductor device and has been described in the following documents. 1. Junichi Nishizawa: "High Power Vertical Type Junction FET having Triode Characteristics", Nikkei Electronics, September 27, 1971, pp. 50-61 2. J. Nishizawa, T. Terasaki and J. Shibata: "Field-Effect Transistor versus Analog Transistor (Static Induction Transistor)", IEEE Trans. on Electron Device, ED-22(4), 185 (1975) 3. J. Nishizawa and K. Nakamura: Physiquee Appliquee, T13, 725 (1978) 4. J. Nishizawa and Y. Otsubo: Tech. Dig. 1980 IEDM, 658 (1980) 5. J. Nishizawa, T. Ohmi, T. Sha and K. Mototani: Technological Report of the Electron and Communication Society, ED81-84 (1981) 6. M. Ishidoh, et al: "Advanced High Frequency GTO", Proc. ISPSD, 189 (1988) 7. B. J. Baliga, et al: "The Evolution of Power Device Technology", IEEE Trans. on Electron Device, ED-31, 1570 (1984) 8. M. Amato, et al: "Comparison of Lateral and Vertical DMOS Specific On-resistance", IEDM Tech. Dig., 736 (1985) 9. B. J. Baliga: "Modern Power Device", John Wiley Sons, 350 (1987) 10. H. Mitlehner, et al: "A Novel 8kV Light-Trigger Thyristor with Over Voltage Self Protection", Proc. ISPSD, 289 (1990) In the semiconductor device of the type mentioned above, a current passing between main electrodes can be controlled by controlling a gate voltage, so that such semiconductor device may be advantageously used in a power transducer and switching power source as a high speed switching element. Particularly, in a vertical type semiconductor device in which a current flows vertically within a substrate, it is possible to control a very large current. In such a semiconductor device, a silicon substrate has been generally used as a semiconductor substrate. In the silicon substrate there are formed channels and currents pass through these channels and semiconductor regions adjacent to the channels. In the power semiconductor device, a channel and adjoining semiconductor regions have certain resistance even in a conductive state (generally termed as an on-resistance). Therefore, heat is generated during the conduction state. In case of using the silicon substrate, a threshold temperature below which the device operates safely is about one hundred and several tens degree in Celsius's temperature scale. Therefore, when a large current flows, a breakdown voltage of semiconductor device might be decreased and an efficiency of the device might be reduced by a loss due to heat. Therefore, it has been proposed to decrease the on-resistance as low as possible. For instance, in order to reduce a resistance of the channel, it has been proposed to reduce a length of the channel. However, this solution is limited by a fine working, so that the channel resistance could not be decreased to a desired value. It has been further proposed to make a semiconductor substrate of a material having a higher threshold temperature for safety operation. Such a solution has been described in the following reference. 11. M. Bhatnagar and B. J. Baliga: "ANALYSIS OF SILICON CARBIDE POWER DEVICE PERFORMANCE" Proc. 3rd International Symposium on Power Semiconductor Devices and Ics, 176-180 (1991) In this reference, it is proposed to use a silicon carbide substrate in a vertical type semiconductor device. As compared with the silicon substrate, the silicon carbide substrate has several advantages such that a wide band gap, high electron mobility, high saturation drift velocity, high thermal conduction and high breakdown voltage. Such properties show that the silicon carbide substrate is particularly suitable for the power semiconductor device. However, it is practically very difficult to manufacture a semiconductor device comprising the silicon carbide substrate. Therefore, in the above reference, characteristics of SiC power MOS FET are derived by theoretical calculations. This simulation indicates that the on-resistance of the SiC power MOS FET is smaller than that of the Si power MOS FET by about 2000 times and a breakdown voltage of the SiC power MOS FET amounts to about 5000V. As stated above, it is practically difficult to form the silicon carbide substrate having good property, so that it could not be possible to realize the power semiconductor device having excellent characteristics even if the theoretical calculation indicates such excellent characteristics. Furthermore, if in future a suitable silicon carbide substrate could be manufactured, the cost of the silicon carbide substrate would become higher than the silicon substrate. Since the silicon carbide substrate can be treated or machined only with difficulty, a manufacturing cost of the silicon carbide substrate would become very expensive. GB-A-2267781 discloses a vertical-type diamond field effect transistor having a substrate composed of doped silicon and a diamond layer and a gate electrode on the diamond layer. US-A-5371378 discloses a similar diamond transistor having a heavily doped monocrystalline silicon substrate, and on this doped and undoped diamond layers there is a Schottky contact within the substrate. Summary of the Invention The present invention provides a novel and useful semiconductor device which uses a hybrid type semiconductor substrate, in which a high breakdown voltage can be attained even if a thickness of the substrate is small and thus the on-resistance can be reduced and heat generation can be suppressed, loss due to heat generation can be reduced, and the substrate can be manufactured easily at a low cost. According to the invention, there is provided a vertical type semiconductor device as set out in claim 1. The wide band-gap semiconductor material layer may be formed of a silicon carbide, diamond or composite material of silicon carbide and diamond. In case where the wide band-gap layer is formed by such a composite material, it is preferable to use a silicon carbide substrate having a diamond layer applied thereon, because diamond can be epitaxially deposited on the silicon carbide substrate. According to the invention, the semiconductor device may be advantageously constructed as MOS FET by using the hybrid-type semiconductor substrate. In an embodiment of the semiconductor device according to the invention, the semiconductor substrate is formed by a silicon substrate of one conductivity type, the wide band-gap semiconductor material layer is epitaxially grown on a surface of said silicon substrate and made of a semiconductor material of the same conductivity type and having a low impurity concentration, and the first silicon layer epitaxially deposited on said wide band-gap semiconductor material layer and made of silicon having a high impurity concentration. In said silicon layer there are formed recesses having a such depth that the surface of said wide band-gap semiconductor material layer is exposed or a part of said silicon layer remains under the recesses to such an extent that a depth of the remaining part of the silicon layer amounts to a depth of an inverted channel to be formed in the silicon layer during operation. A silicon oxide film is formed on an inner surface of said recess, a gate electrode is formed on said silicon oxide film, a source region is formed by inverting the conductivity type of a part of the silicon layer which adjoins said silicon oxide film, a source electrode is formed on said source region, and a drain electrode is formed on a rear surface of said silicon substrate. In another embodiment of the semiconductor device according to the invention, the semiconductor device is constructed as IGBT. The semiconductor substrate is formed by the silicon substrate of one conductivity type, a further silicon layer of the opposite conductivity type formed on a surface of said silicon substrate and having a high impurity concentration, the wide band-gap semiconductor material layer epitaxially grown on a surface of said further silicon layer and made of a semiconductor material of the opposite conductivity type and having a low impurity concentration, and the first silicon layer epitaxially deposited on said wide band-gap semiconductor material layer and made of silicon of the opposite conductivity type having a high impurity concentration. In the first silicon layer there are formed recesses having such a depth that the surface of said wide band-gap semiconductor material layer is exposed or a part of said first silicon layer remains under the recesses to such an extent that a depth of the remaining part of the first silicon layer amounts to a depth of an inverted channel which is formed in the second silicon layer during operation. A silicon oxide film is formed on an inner surface of said recess, a gate electrode is formed on said silicon oxide film, an emitter region is formed by inverting the conductivity type of a part of the second silicon layer which adjoins said silicon oxide film, an emitter electrode is formed on said emitter region, and a collector electrode is formed on a rear surface of said silicon substrate. In another embodiment of the semiconductor device constituted as IGBT according to the invention, the semiconductor substrate is formed by the silicon substrate of one conductivity type, the wide band-gap semiconductor material layer epitaxially deposited on a surface of said silicon substrate and made of a semiconductor material of the opposite conductivity type and having a low impurity concentration, and the first silicon layer epitaxially grown on said wide band-gap semiconductor material layer and made of silicon having a high impurity concentration. In said silicon layer there are formed recesses having a depth such that the surface of said wide band-gap semiconductor material layer is exposed or a part of said silicon layer remains under the recesses to such an extent that a depth of the remaining part of the silicon layer amounts to a depth of an inverted channel to be formed in the silicon layer during operation. A silicon oxide film is formed on an inner surface of said recess, a gate electrode is formed on said silicon oxide film, an emitter region is formed by inverting the conductivity type of a part of the silicon layer which adjoins said silicon oxide film, an emitter electrode is formed on said emitter region, and a collector electrode is formed on a rear surface of said silicon substrate. In the semiconductor device according to the invention, the semiconductor substrate is formed by, for instance, a silicon substrate, a wide band-gap semiconductor material layer epitaxially deposited on said silicon substrate and a silicon layer epitaxially deposited on the wide band-gap semiconductor material layer. Since the wide band-gap semiconductor material layer has a higher breakdown property than the silicon substrate, the wide band-gap semiconductor material layer can be thin, and therefore on-resistance can be decreased. Therefore, the amount of heat generated by the semiconductor device can be decreased. Moreover, the upper silicon layer can be easily worked or machined and can be formed to have excellent characteristics at a low cost as compared with a case in which a whole substrate is made of a wide band-gap semiconductor material such as silicon carbide and diamond. Therefore, the cost of the semiconductor device can be reduced. Brief Explanation of the Drawings Fig. 1 is a cross sectional view showing an embodiment of the semiconductor device formed as MOS PET according to the invention; Fig. 2 is a cross sectional view illustrating a first embodiment of the semiconductor device constituted as IGBT according to the invention; Fig. 3 is a cross sectional view depicting a second embodiment of the semiconductor device formed as IGBT according to the invention; Fig. 4 is a cross sectional view showing an embodiment of the semiconductor device according to the invention formed as a joint-type MOS FET; Fig. 5 is a cross sectional view depicting an embodiment of the semiconductor device according to the invention constituted as joint-type IGBT; Figs. 6A-6F are cross sectional views showing successive steps for manufacturing the semiconductor device shown in Fig. 1 according to the invention; Fig. 7 is a cross sectional view illustrating a modification of the embodiment shown in Fig. 1; Fig. 8 is a cross sectional view showing a modified embodiment of the semiconductor device depicted in Fig. 3 according to the invention; Fig. 9 is a cross sectional view showing an modification of the embodiment of the semiconductor device according to the invention shown in Fig. 2; Fig. 10 is a cross sectional view showing another modification of the embodiment shown in Fig. 1; Fig. 11 is a cross sectional view illustrating another modified embodiment of the semiconductor device shown in Fig. 1; Fig. 12 is a cross sectional view showing a modification of the embodiment shown in Fig. 4; Fig. 13 is a cross sectional view representing a modification of the embodiment illustrated in Fig. 5; Figs. 14A-14D are cross sectional views depicting several embodiments of the substrate to be used in the embodiments shown in Figs. 4 and 5; and Figs. 15A and 15B are cross sectional views showing still another modification of the embodiment of the semiconductor device depicted in Fig. 1 according to the invention. Description of the Preferred Embodiments Fig. 1 is a cross sectional view showing an embodiment of the semiconductor device according to the invention. In the present embodiment, the semiconductor device is formed as a power MOS PET. The semiconductor device comprises a semiconductor substrate formed by an N + type silicon substrate 11 having an impurity concentration of 3x10 18 atoms/cm 3 and a thickness of about 300 µm. The semiconductor substrate further comprises an N - type silicon carbide layer 12 epitaxially grown on a surface of the silicon substrate 11. The silicon carbide layer 12 has an impurity concentration of 5x10 16 atoms/cm 3 and having a thickness of about 10 µm. The semiconductor substrate further includes a P + type silicon layer 13 epitaxially deposited on a surface of the silicon carbide layer 12. The silicon layer 13 has an impurity concentration of 1x10 18 atoms/cm 3 and a thickness of about 20 µm. In the present embodiment, there are formed recesses in the silicon layer 13 such that the recesses reach the silicon carbide layer 12 and partially penetrate into the silicon carbide layer. On a surface of the recesses is covered with a silicon oxide film 14, The silicon oxide film 14 may be formed by thermal oxidation or CVD using a TEOS (tetra ethoxysilane) gas. On the silicon oxide film 14, there are deposited gate electrodes 15. N + type source regions 16 are formed in the P + type silicon layer 13 by inverting the conductivity type of surface parts of the silicon layer which are brought into contact with the silicon oxide film 14. The source regions 16 have an impurity concentration of about 1×10 19 atoms/cm 3 . On the source regions 16 are formed source electrodes 17 made of aluminum. On a rear surface of the silicon substrate 11 is formed a drain electrode 18 by depositing aluminum. During the operation of the MOS FET having the construction explained above, a bias voltage is applied across the drain electrode 18 and the source electrode 17 such that the drain is positive and the source is negative. When no gate voltage is applied to the gate electrode 15, no inverted channel is formed at portions of the P + type silicon layer 13 which are brought into contact with the silicon oxide film 14, so that electrons supplied from the source electrode 17 are not conducted to the silicon carbide layer 12. Therefore, no current flows between the source electrode 17 and the drain electrode 18 and the semiconductor device is in an off- or non-conductive condition. In this case, a breakdown voltage is determined by a depletion layer produced by the P + type silicon layer 13 and N - silicon carbide layer 12. Silicon carbide has a wider band-gap than the silicon and a breakdown voltage of silicon carbide is higher than that of silicon by about one order. Therefore, a thickness of the silicon carbide layer can be reduced. When a positive voltage is applied to the gate electrode 15, negative charge is induced in portions of the silicon layer 13 adjoining the silicon oxide film 14 and an inverted channel is formed therein. Therefore, a current passes from the drain electrode 18 to the source electrode 17 via the silicon substrate 11, N - silicon carbide layer 12, the inversion layer formed in the P + type silicon layer 13 and the source region 16. As stated above, the silicon carbide layer 12 has a high breakdown voltage and can be thin. Therefore, on-resistance can be decreased. This results in that an amount of heat generated by the semiconductor device can be reduced and a loss due to heat can be decreased. Furthermore, the inverted channel is formed within the silicon layer 13 whose impurity concentration can be easily and precisely controlled, and thus a desired property can be realized. Moreover, the recess for forming the inversion layer is formed within the silicon layer 13, and therefore the recess can be easily formed. In order to realize a high speed device for switching a current at a high speed, it is required to extinguish carriers at a high speed. To this end, a process for reducing a life time of carriers has to be performed for the semiconductor substrate For instance, gold ions have to be added or electron bombardment process has to be performed. However, such a process might increase the on-resistance and a loss due to heat might be increased. According to the invention, the silicon substrate includes the wide band-gap semiconductor material layer having a small thickness, so that the on-resistance can be reduced. Therefore, even though the above mentioned process for reducing life time of carriers is performed to increase extinguishing speed of carriers, it is possible to realize the semiconductor device having low loss due to on-resistance. Moreover, silicon carbide or diamond has a higher heat conduction than silicon, heat generated at the channel can be dissipated speedily and a heat resistance of the device can be improved. In the present invention, by using the semiconductor substrate of hybrid structure having a silicon substrate and a wide band-gap semiconductor material layer, the semiconductor substrate according to the invention has both a property of silicon such as low cost, easy workability and easy formation and a property of wide band-gap semiconductor material such as a high breakdown voltage. Fig. 2 is a cross sectional view showing an embodiment of the semiconductor device constructed as IGBT according to the invention. In the present embodiment, a semiconductor substrate is formed by a P + type silicon substrate 21 having an impurity concentration of about 3x10 18 atoms/cm 3 , N type first silicon layer 22 epitaxially deposited on a surface of the silicon substrate 21 and having an impurity concentration of about 1x10 17 atoms/cm 3 and a thickness of about 10 µm, N - type silicon carbide layer 12 epitaxially grown on a surface of the first silicon layer 22 and having an impurity concentration of 5×10 16 atoms/cm 3 and having a thickness of about 10 µm, and a second P + type silicon layer 13 epitaxially deposited on a surface of the silicon carbide layer 12. The second silicon layer 13 has an impurity concentration of 1x10 16 atoms/cm 3 and a thickness of about 20 µm. In a surface region of the second silicon layer 13 are formed N + type emitter regions 46 and emitter electrodes 47 are formed on the emitter regions. On a rear surface of the silicon substrate 11 is formed a collector electrode 48. In IGBT of the present embodiment, a bias voltage is applied across the collector electrode 48 and the emitter electrode 47 such that the collector electrode becomes positive and the emitter electrode becomes negative. When a voltage is not applied to the gate electrode 15, an inverted channel is not generated at portions of the P + type second silicon layer 13 which are brought into contact with the silicon oxide film 14, so that electrons supplied from the source electrode 17 are not conducted into the silicon carbide layer 12. Therefore, no current flows between the collector electrode 48 and the emitter electrode 47, so that the semiconductor device is in a cut-off state. When a positive voltage is applied to the gate electrode 15, negative charge is induced in portions of the second silicon layer 13 adjoining the silicon oxide film 14 and an inverted channel is formed therein. Therefore, an electron current flows into the N - type silicon carbide layer 12 serving as a drift layer. Then, holes are injected from the P + type silicon layer 21 and minority carriers are stored in the N - silicon carbide layer 12. In this manner, a large current passes from the collector electrode 48 to the emitter electrode 47 via the silicon substrate 21,N type first silicon layer 22, N - silicon carbide layer 12, the inversion channel and emitter region 46. In this case, the N - silicon carbide layer 12 serves as the drift layer having a high breakdown electric field strength, and therefore a desired breakdown voltage can be attained by using a thin silicon carbide layer. Thus, on-resistance can be decreased and loss due to heat generation can be reduced. On-resistance of the power MOS FET is mainly determined by the N - silicon carbide layer 12 having a high resistance. When minority carriers, i.e. holes are stored in this silicon carbide layer, a concentration of majority carriers, i.e. electrons is also increased (conductivity modulation) , and thus the on-resistance of IGBT becomes lower than that of MOS PET. Fig. 3 is a cross sectional view showing another embodiment of the semiconductor device formed as IGBT according to the invention. Also in this embodiment, portions similar to those shown in Fig. 2 are denoted by the same reference numerals used in Fig. 2. In the present embodiment, a semiconductor substrate is formed by an P + type silicon substrate 21 having an impurity concentration of about 3×10 18 atoms/cm 3 , an N - type silicon carbide layer 12 epitaxially grown on a surface of the silicon substrate 21 and having an impurity concentration of 5×10 16 atoms/cm 3 and a thickness of about 10 µm, and a P + type silicon layer 13 epitaxially deposited on a surface of the silicon carbide layer 12. The silicon layer 13 has an impurity concentration of 3×10 18 atoms/cm 3 and a thickness of about 20 µm. On a rear surface of the silicon substrate 21 is formed a collector electrode 48. The present embodiment has similar advantages to those of the embodiment illustrated in Fig. 2. Fig. 4 is a cross sectional view depicting another embodiment of the semiconductor device according to the invention constituted as MOS FET. In the present embodiment, portions similar to those of the embodiment shown in Fig. 1 are denoted by the same reference numerals used in Fig. 1. In the present embodiment, a semiconductor substrate is formed by an N + type silicon substrate 11 having a high impurity concentration of about 3×10 18 atoms/cm 3 , an N - type silicon carbide layer 12 epitaxially grown on a surface of the silicon substrate 11 and having an impurity concentration of about 5×10 15 atoms/cm 3 , and a P + type silicon layer 13 epitaxially grown on a surface of the silicon carbide layer 12 and having an impurity concentration of about 1×10 18 atoms/cm 3 . In the silicon layer 13, there are formed recesses which extend through the silicon layer 13 and partially penetrate into the silicon carbide layer 12. On a surface of the recesses is covered with a silicon oxide film 14, and gate electrodes 15 is formed on the silicon oxide film 14. N + type source regions 16 are formed in surface regions of the P + type silicon layer 13 which are brought into contact with the silicon oxide film 14. The source regions 16 have a high impurity concentration of about 1x10 19 atoms/cm 3 . On the source regions 16 are formed source electrodes 17 made of aluminum. On a rear surface of the silicon substrate 11 is formed a drain electrode 18 by depositing aluminum. The construction so far explained is similar to that of the first embodiment illustrated in Fig. 1. In the present embodiment, an N + type second silicon substrate 25 having a high impurity concentration of about 1x10 19 atoms/cm 3 is joined on a surface of the silicon layer 13, and a source electrode 17 is formed on a surface of the second silicon substrate 25. It should be noted that the gate electrodes 15 are formed such that they do not contact with the second silicon substrate 25. That is to say, the recesses are not fully filled with the gate electrodes 15, but spaces are formed on the gate electrodes, In the present embodiment, since the second silicon substrate 25 is joined on the silicon layer 13, it is possible to form the single source electrode 17 commonly for all elements. Moreover, a depth of the recesses can be shallow, so that the construction is preferable for the fine working. Fig. 5 is a cross sectional view showing another embodiment of the semiconductor device formed as IGBT according to the invention. In the present embodiment, portions similar to those illustrated in Fig. 2 are denoted by the same reference numerals used in Fig. 2. In the present embodiment, a semiconductor substrate is formed by a P + type silicon substrate 21 having an impurity concentration of about 3×10 18 atoms/cm 3 , an N - type silicon carbide layer 12 epitaxially grown on a surface of the silicon substrate 21 and having an impurity concentration of 5×10 15 atoms/cm 3 , and a P + type silicon layer 13 epitaxially deposited on a surface of the silicon carbide layer 12 and having an impurity concentration of 3×10 18 atoms/cm 3 , said silicon layer 13 serving as a base region. In the silicon layer 13 there are formed recesses such that the recesses partially penetrate into the silicon carbide layer 12. A silicon oxide film 14 is formed on an inner wall of the recesses and gate electrodes 15 are formed on the silicon oxide film. N + type emitter regions 46 are formed in surface regions of the silicon layer 13 which are brought into contact with the silicon oxide film 14 and emitter electrodes 47 are formed on the emitter regions 46. A collector electrode 48 is formed on a rear surface of the silicon substrate 21. In the present embodiment, an N + type second silicon substrate 25 having a high impurity concentration of about 1×10 19 atoms/cm 3 is joined on a surface of the silicon layer 13, and an emitter electrode 47 is formed on a surface of the second silicon substrate 25. Like as the embodiment shown in Fig. 4, the gate electrodes 15 are formed such that they are not brought into contact with the second silicon substrate 25. In the present embodiment, the emitter electrode 47 can be formed commonly for all the elements, and a depth of the recesses can be made small. Figs. 6A to 6F are cross sectional views illustrating successive steps of an embodiment of the method of manufacturing the semiconductor device according to the invention. In the present embodiment, the power MOS FET shown in Fig. 1 is to be manufactured. As shown in Fig. 6A, an N + type silicon substrate 11 having an impurity concentration of 3×10 18 atoms/cm 3 and a thicKness of about 300 micron meters is prepared. Then, an N type silicon carbide layer 12 having a low impurity concentration of 5×10 16 atoms/cm 3 is epitaxially grown on a surface of the silicon substrate 11 such that the silicon carbide layer 12 has a thickness of about 10 micron meters. Next, as illustrated in Fig. 6B, a P + type silicon layer 13 having an impurity concentration of 1×10 18 atoms/cm 3 and a thickness of about 20 micron meters is epitaxially deposited on a surface of the silicon carbide layer 12. Then, after forming a mask having a desired pattern is formed on the silicon layer 13, recesses 31 are formed in the silicon layer 13 by anisotropic etching as shown in Fig. 6C. In the present embodiment, during the formation of the recesses 31, a surface of the silicon layer 13 is partially etched away, but according to the invention, it I not always necessary to perform such etching. This will be explained further in detail. After removing the mask, a silicon oxide film 32 having a thickness of 5000Å is formed on an inner wall of the recesses 31 and on a surface of the silicon layer 13 as depicted in Fig. 6D. Further an aluminum film 33 is formed such that the recesses 31 are fully filed with aluminum. The silicon oxide film 32 is formed by thermal oxidation. The silicon oxide film 32 may be formed by CVD using a TEOS gas. The aluminum film 33 is formed by CVD method, but may be formed by sputtering. Next, the aluminum film 33 and silicon oxide film 32 are partially removed by etching or chemical mechanical polishing to expose a surface of the silicon layer 13 as shown in Fig. 6E. Then, after forming a given mask, N type impurity ions are implanted and diffused into the surface of the silicon layer 13 to form N + type source regions 16 having an impurity concentration of about 1×10 19 atoms/cm 3 . Finally, as shown in Fig. 6F, source electrodes 17 are formed on the source regions 16 and a drain electrode 18 is formed on a rear surface of the silicon substrate 11 by aluminum deposition. In the above mentioned embodiment, the semiconductor substrate is formed by the silicon substrate, silicon carbide layer epitaxially deposited on the silicon substrate, and silicon layer epitaxially deposited on the silicon carbide layer. In this case, the inversion channel has to be formed within the silicon layer and silicon carbide layer. However, if the inversion channel extends up to a boundary between the silicon substrate and the silicon carbide layer, a breakdown voltage might be reduced due to crystal incommensurate structure at the boundary. The present invention provides a novel structure which can mitigate such a problem. Now several embodiments of the semiconductor device according to the invention will be explained. Fig. 7 is a cross sectional view showing an embodiment of the semiconductor device according to the invention, in which the above explained problem of the crystal defect in the boundary between the silicon substrate and the silicon carbide layer can be solved. The basic structure of the present embodiment is identical with that of the first embodiment shown in Fig. 1. In the present embodiment, an N type diffusion layer 41 having a higher impurity concentration than that of the silicon carbide layer 12 is formed between the N + type silicon substrate 11 and the N - type silicon carbide layer 12. The N type diffusion layer 41 may be formed by diffusing impurity ions from the silicon substrate 11 into the silicon carbide layer 12 after epitaxially depositing the silicon carbide layer 12. In the present embodiment, the N type diffusion layer 41 has an impurity concentration of 5×10 17 atoms/cm 3 and a thickness of about 10 µm. By providing the N type diffusion layer 41 in the surface region of the silicon carbide layer 12 adjoining the silicon substrate 11, the depletion layer formed at the P + N - junction could not spread beyond the boundary between the bulk portion of the silicon carbide layer 12 and the N type diffusion layer 41. In this manner, the depletion layer having the maximum electric field is formed within the same crystal without no crystal incommensurate structure, i.e. within the silicon carbide crystal. Therefore, the above mentioned decrease in the breakdown voltage due to the crystal incommensurate structure can be effectively removed. Fig. 8 is a cross sectional view showing a modification of the embodiment of the semiconductor device according to the invention, in which the above mentioned decrease in the breakdown voltage due to the crystal incommensurate structure can be avoided. In the present embodiment, after forming an N type silicon carbide layer 43 having a thickness of about 10 µm on the silicon substrate 11 by epitaxial growth, an N - silicon carbide layer 12 having a thickness of about 20 µm is formed on the N type silicon carbide layer 43 by epitaxial growth. The N type silicon carbide layer 43 has an impurity concentration Of 5×10 17 atoms/cm 3 which is higher than that of the N - type silicon carbide layer 12. Fig. 9 is a cross sectional view showing a modification of the embodiment of the semiconductor device according to the invention shown in Fig. 3, in which the decrease in the breakdown voltage due to the crystal incommensurate structure can be mitigated. In the present embodiment, an N type silicon layer 22 having an impurity concentration of about 5×10 17 atoms/cm 3 and a thickness of about 10 µm is epitaxially deposited on the P + type silicon substrate 21. Then, N - type silicon carbide layer 12 having a thickness of about 20 micron meters is epitaxially grown on the N type silicon carbide layer 22. Then, N type impurity ions are diffused from the N type silicon layer 22 into the N - type silicon carbide layer 12 to form an N type diffusion layer 41 having a thickness of about µm. This N type diffusion layer 41 has an impurity concentration of about 5×10 17 atoms/cm 3 . In the embodiments illustrated in Figs. 7-9, the decease in the breakdown voltage due to crystal incommensurate structure at the boundary between the silicon substrate and the silicon carbide layer can be avoided. It should be noted that a similar problem might occur at a boundary between a silicon carbide layer and a silicon layer having a high impurity concentration. According to the invention, this problem can be also solved by a similar measure. Fig. 10 is a cross sectional view showing a modification of the embodiment shown in Fig. 1, in which the above mentioned problem can be removed by providing a diffusion layer in a surface of the silicon carbide layer adjoining the silicon layer. On an N + type silicon substrate 11 there is epitaxially formed an N + type silicon carbide layer 12 having a thickness of about 20 µm, and a P + type silicon layer 13 is epitaxially deposited on the silicon carbide layer 12. Then, P type impurity ions are diffused from the silicon layer 13 into the silicon carbide layer 12 to form a P type diffusion layer 42 having a thickness of about 5 micron meters. This P type diffusion layer 42 has an impurity concentration of 5×10 17 atoms/cm 3 . Fig. 11 is a cross sectional view depicting another embodiment of the semiconductor device according to the invention, in which both the N type diffusion layer 41 shown in Fig. 7 and the P type diffusion layer 42 illustrated in Fig. 10 are provided. In the present embodiment, on an N + type silicon substrate 11 an N - type silicon carbide layer 12 having an impurity concentration of 5×10 15 atoms/cm 3 is epitaxially deposited. Then, an N type silicon carbide layer 41 having an impurity concentration of 3×10 18 atoms/cm 3 and a thickness of about 5 micron meters is formed by diffusing N type impurity ions from the silicon substrate 11 into the silicon carbide layer 12. Next, an P + type silicon layer 13 is epitaxially deposited on the silicon carbide layer 12, and then a P type diffusion layer 42 having a thickness of.about 5 µm is formed in a surface region of the silicon carbide layer 12 by diffusing P type impurity ions from the P + type silicon layer 13 into the surface region of the silicon carbide layer 12. The P type diffusion layer 42 has an impurity concentration of about 5×10 17 atoms/cm 3 . In the present embodiment, the bulk of the silicon carbide layer 12 becomes about 10 µm. In the present embodiment, the N type silicon carbide layer 41 is provided between the N + type silicon substrate 11 and the N - type silicon carbide layer 12 and the P type diffusion layer 42 is existent between the and the P type diffusion layer 42 is existent between the N - type silicon carbide layer 12 and the P + type silicon layer 13. Therefore, upon the operation it is possible to avoid the decrease in breakdown voltage due to the crystal defect within the silicon carbide layer 12 at which an electric field of the depletion layer formed by the reverse bias becomes maximum. Fig. 12 is a cross sectional view showing a modified embodiment of the power MOS FET of joint type shown in Fig. 4. In the present embodiment, an N type diffusion layer 41 is formed in a surface of an N - type silicon carbide layer 12 which adjoins with an N + type silicon substrate 11, and a P type diffusion layer 42 is formed in a surface of the N - type silicon carbide layer 12 which is brought into contact with a P@ type silicon layer 13. On the N + type silicon substrate 11 an N - type silicon carbide layer 12 is epitaxially deposited, and then, an N type diffusion layer 41 is formed by diffusing N type impurity ions from the silicon substrate 11 into the silicon carbide layer 12. Next, a P + type silicon layer 13 is epitaxially deposited on the silicon carbide layer 12, and then a P type diffusion layer 42 is formed in a surface region of the silicon carbide layer 12 by diffusing P type impurity ions from the P + type silicon layer 13 into the surface region of the silicon carbide layer 12. Each of the N type diffusion layer 41 and P type diffusion layer 42 has a thickness of 5 micron meters, so that the bulk of the silicon carbide layer 12 becomes about 10 micron meters. Pig. 13 is a cross sectional view depicting a modification of the joint type IGBT illustrated in Fig. 5. In the present embodiment, on a P + type silicon substrate 21 is epitaxially formed an N type silicon carbide layer 43 having an impurity concentration of about 3×10 18 atoms/cm 3 and a thickness of about 5 µm. Then, an N - type silicon carbide layer 12 having a thickness of about 15 µm is epitaxially deposited on the N type silicon carbide layer 43, and a P + type silicon layer 13 is epitaxially deposited on the N - type silicon carbide layer 12. Next P type impurity ions are diffused from the P + type silicon layer 13 into the surface region of the N - type silicon carbide layer 12 to form a P type diffusion layer 42 having a thickness of about 5 µm. Figs. 14A-14D are cross sectional views showing several embodiments of the second semiconductor substrate used in the joint type semiconductor device. In these embodiments, lower surfaces of the substrates are to be brought into contact with the first silicon substrate. In Fig. 14A, a metal film 52 is applied on a surface of a silicon substrate 51. The metal film 52 may be formed by a vapor deposition of aluminum. In Pig. 14B, a substrate 53 is wholly formed by a metal plate. In an embodiment of Fig. 14C, recesses 55 are formed in a surface of a silicon substrate 54 and an insulating films 56 such as oxide films are formed on inner walls of the recesses. In Fig. 14D, a metal film 58 having a given pattern is formed on a surface of a silicon substrate 57 and openings of the metal film 58 are filled with insulating films 59 such as oxide films, said insulating film having a thickness smaller than that of the metal film. The insulating films 56 and 59 of the embodiments shown in Figs. 14C and 14D correspond to the recesses in which the gate electrodes 15 are formed, and the metal films 58 of the embodiment illustrated in Fig. 14D are joined to the source or emitter regions 16. The present invention is not limited to the embodiments explained above, but various alternations and modifications may be conceived by those skilled in the art within the scope of the invention. For instance, in the above embodiments, the recesses for forming the gate electrodes 15 are formed such that the recesses fully penetrate the P + silicon layer 13 and partially penetrate into the surface of the silicon carbide layer 12. According to the invention, the recesses may be formed such that the surface of the silicon carbide layer 12 is just exposed as shown in Fig. 15A or the recesses do not reach the boundary between the silicon carbide layer 12 and the P + silicon layer 13 as illustrated in Fig. 15B. In the embodiment shown in Fig. 15A, the recesses can be formed without fine control, because the silicon carbide layer 12 is hardly etched than the silicon layer 13. In the embodiment shown in Fig. 15B, the recesses are formed to such a depth that the inversion channel formed by applying a voltage to the gate electrode 15 extends just up to the boundary between the silicon carbide layer 12 and the silicon layer 13. It should be noted that Figs. 15A and 15b show the modifications of the embodiment formed as MOS FET shown in Fig. 1, but according to the invention, similar modifications may be conceived for the remaining embodiments. As explained above in detail, according to the invention, the semiconductor substrate having the hybrid structure is formed by the silicon substrate and the wide band-gap semiconductor material layer such as silicon carbide layer and diamond layer, so that a required breakdown voltage can be attained by reducing a thickness of the semiconductor substrate. Therefore, it is possible to reduce the on-resistance, and thus an amount of generated heat can be reduced and a loss due to heat can be decreased. Particularly, when the diamond layer having a high thermal conduction is used in the hybrid type semiconductor substrate, the diamond layer serves as a heat sink for dissipating heat generated by the channel, so that the heat dissipation property can be improved. It has been proposed to make the whole substrate of a wide band-gap semiconductor material, but the wide band-gap semiconductor substrate having good characteristics is very expensive and a workability of the wide band-gap semiconductor substrate is rather low. According to the invention, the semiconductor substrate includes cheap silicon having a good workability, and thus the semiconductor device having a good property can be easily manufactured at a low cost. In for example the construction of Fig. 4, there is a second substrate (25) formed by a silicon plate. This second substrate may alternatively be formed by metal plate. In another embodiment, the silicon plate constituting the second substrate has at least one recess formed in a surface of the silicon plate which is joined to the first silicon layer, at a position corresponding to the gate electrode, and an inner wall of said recess is covered with an insulating film. In yet another embodiment, the silicon plate constituting the second substrate has at least one insulating film provided on a surface of the silicon plate at a position corresponding to said gate electrode and at least one metal film provided on said surface of the silicon plate, said metal film having thickness larger than said insulating film.

Claims: A vertical type semiconductor device comprising: a hybrid type semiconductor substrate including a silicon substrate (11,21) having first and second surfaces and a wide band-gap semiconductor material layer (12) provided on one said surface of the silicon substrate and made of a semiconductor material having a band-gap wider than that of a silicon; and a control electrode region (14,15) controlling a current flowing across the semiconductor substrate; characterised in that said hybrid type semiconductor substrate includes a first silicon layer (13) epitaxially grown on said wide band-gap semiconductor material layer (12) and having a higher impurity concentration than said wide band-gap semiconductor material layer; and said control electrode region is a gate electrode region including at least one recess formed in at least said first silicon layer (13), a silicon oxide film (14) formed on the inner wall of the or each said recess and a gate electrode (15) formed in the silicon oxide film (14) of the or each said recess. A semiconductor device according to claim 1, wherein said wide band-gap material layer (12) is made of silicon carbide, diamond or complex material of silicon carbide and diamond. A semiconductor device according to claim 1 or claim 2, wherein said hybrid type semiconductor substrate is formed by said silicon substrate (11) of a first conductivity type having first and second major surfaces, said wide band-gap semiconductor material layer (12) of said first conductivity type epitaxially grown on the second surface of the silicon substrate (11) and having an impurity concentration lower than said silicon substrate (11), and said first silicon layer (13) of the other conductivity type, and the semiconductor device further comprises at least bne source region (16) formed in a surface of the first silicon layer (13) by changing the conductivity type of a surface portion of the silicon layer (13) which adjoins said at least one silicon oxide film (14), at least one source electrode (17,25) formed on said at least one source region (16) and a drain electrode (18) formed on said first surface of the silicon substrate. A semiconductor device according to claim 1 or claim 2, wherein said hybrid type semiconductor substrate is formed by said silicon substrate (21) of a first conductivity type having first and second major surfaces, a further silicon layer (22) of the other conductivity type epitaxially grown on the second surface of the silicon substrate (21) and having an impurity concentration higher than that of the silicon substrate(21), said wide band-gap semiconductor material layer (12) of said other conductivity type epitaxially grown on said first silicon layer (22) and having an impurity concentration lower than that of the silicon substrate (21), and said first silicon layer (13) of said first conductivity type, and the semiconductor device further comprises at least one emitter region (46) formed in a surface of the second silicon layer by changing the conductivity type of a surface portion of the first silicon layer (13) which adjoins said at least one silicon oxide film, at least one emitter electrode (47) formed on said at least one emitter region(46) and a collector electrode (48) formed on said first surface of the silicon substrate. A semiconductor device according to claim 1 or claim 2, wherein said hybrid type semiconductor substrate is formed by said silicon substrate (21) of one conductivity type having first and second major surfaces, a wide band-gap semiconductor material layer (12) of the other conductivity type epitaxially grown on the second surface of the silicon substrate and having an impurity concentration lower than said silicon substrate(21) and said first silicon layer (13) of said first conductivity type, and the semiconductor device further comprises at least one emitter region (46) formed in a surface of said first silicon layer (13) by changing the conductivity type of a surface portion of the silicon layer (13) which adjoins said at least one silicon oxide film (14), at least one emitter electrode (47) formed on said at least one emitter region and a collector electrode (48) formed on said first surface of the silicon substrate. A semiconductor device according to claim 3, 4 or 5, wherein said recess is formed such that the recess penetrates into a surface of the wide band-gap semiconductor material layer (12). A semiconductor device according to claim 3, 4 or 5, wherein said recess is formed such that the recess extends just to a surface of the wide band-gap semiconductor material layer (12). A semiconductor device according to claim 3, 4 or 5, wherein said recess is formed within said first silicon layer (13) such that an inversion channel produced in the silicon layer (13) during operation of the semiconductor device reaches the wide band-gap semiconductor material layer. A semiconductor device according to claim 3, 4 or 5, wherein the semiconductor device further comprises a second substrate (25) joined to said first silicon layer (13) and having a higher impurity concentration, said second substrate (25) constituting ohmic contact with said source or emitter region (16, 46) formed by inverting the conductivity type of a portion of said first silicon layer (13) which is in contact with said silicon oxide film. A semiconductor device according to claim 9, wherein said second substrate (25) is formed by a silicon plate having the same conductivity type as that of said source or emitter region (16, 46) and an impurity concentration higher than said silicon substrate (11, 21). A semiconductor device according to claim 10, wherein said second substrate (25) further comprises a metal layer applied on a surface of the silicon plate which is joined to said first silicon layer. A semiconductor device according to claim 9, wherein said second substrate (25) is formed by a metal plate. A semiconductor device according to claim 10, wherein said silicon plate (25) constituting the second substrate has at least one recess formed in a surface of the silicon plate which is joined to the first silicon layer, at a position corresponding to the gate electrode, and an inner wall of said recess is covered with an insulating film. A semiconductor device according to claim 10, wherein said silicon plate (25) constituting the second substrate has at least one insulating film provided on a surface of the silicon plate at a position corresponding to said gate electrode and at least one metal film provided on said surface of the silicon plate, said metal film having a thickness larger than said insulating film. A semiconductor device according to claim 3, 4 or 5, wherein said hybrid type semiconductor substrate includes a further semiconductor layer (41) formed in a surface of the wide band-gap semiconductor material layer (12) which is joined to the silicon substrate (11, 21), said further semiconductor layer (41) having the same conductivity type as the wide band-gap semiconductor material layer and an impurity concentration higher than the wide band-gap semiconductor material layer. A semiconductor device according to claim 15, wherein said further semiconductor layer (41) is constituted by a diffusion layer formed by diffusing impurity ions from the silicon substrate into the wide band-gap semiconductor material layer. A semiconductor device according to claim 3, 4 or 5, wherein said semiconductor substrate includes a further semiconductor layer (42) formed in a surface of the wide band-gap semiconductor material layer (12) which is joined to said first silicon layer (13) having a high impurity concentration, said further semiconductor layer (42) having the same conductivity type as said first silicon layer and an impurity concentration lower than said first silicon layer (13). A semiconductor device according to claim 17, wherein said further semiconductor layer (42) is constituted by a diffusion layer formed by diffusing impurity ions from said first silicon layer having the higher impurity concentration into the wide band-gap semiconductor material layer. A semiconductor device according to claim 3, 4 or 5, wherein said hybrid type semiconductor substrate includes a first semiconductor layer (41) formed in a surface of the wide band-gap semiconductor material layer (12) which is joined to the silicon substrate (11, 21), said first semiconductor layer (41) having the same conductivity type as the wide band-gap semiconductor material layer (12) and a first impurity concentration higher than the wide band-gap semiconductor material layer (12), and a second semiconductor layer (42) formed in a surface of the wide band-gap semiconductor material layer (12) which is joined to said first silicon layer (13) having a high impurity concentration, said second semiconductor layer (42) having the same conductivity type as said first silicon layer (13) and a second impurity concentration lower than said first silicon layer (13). A semiconductor device according to claim 19, wherein said first semiconductor layer (41) is constituted by a first diffusion layer formed by diffusing impurity ions from the silicon substrate (11, 21) into the wide band-gap semiconductor material layer (12), and said second semiconductor layer (42) is constituted by a second diffusion layer formed by diffusing impurity ions from said first silicon layer (13) having the higher impurity concentration into the wide band-gap semiconductor material layer (12). A method of manufacturing a semiconductor device comprising the steps of: preparing a silicon substrate (11, 21) having first and second major surfaces and a first impurity concentration; epitaxially growing, on the second surface of the silicon substrate (11, 21), a semiconductor material having a band-gap wider than the silicon substrate to form a wide band-gap semiconductor material layer (12), said wide band-gap semiconductor material layer having a second impurity concentration lower than said first impurity concentration of the silicon substrate; epitaxially growing a silicon layer (13) on said wide band-gap semiconductor material layer, said silicon layer (13) having a third impurity concentration higher than said second impurity concentration of the wide band-gap semiconductor material layer (12); forming at least one recess at least in said silicon layer by anisotropic etching; forming at least one silicon oxide film (14) on an inner wall of said at least one recess; forming at least one gate electrode (15) on said at least one silicon oxide film; forming at least one source or emitter region (16, 46) in a surface of the silicon layer by inverting the conductivity type of a portion of the silicon layer (13) which adjoins the silicon oxide film (14); forming at least one source or emitter electrode (17, 47) on said at least one source or emitter region (16, 46); and forming a drain or collector electrode (18, 48) on said first major surface of the silicon substrate. A method according to claim 21, wherein said at least one recess is formed such that the recess partially penetrates into the wide band-gap semiconductor material layer (12). A method according to claim 21, wherein said at least one recess is formed such that a surface of said wide band-gap semiconductor material layer (12) is just exposed at a bottom of the recess. A method according to claim 21, wherein said at least one recess is formed within said silicon layer (13) such that a distance between a bottom of the recess and a surface of the wide band-gap semiconductor material layer (12) is equal to a depth of an inversion channel formed in the silicon layer (13) during operation of semiconductor device. A method according to claim 21, wherein after forming said wide band-gap semiconductor layer (12), impurity ions are diffused from the silicon substrate (11, 21) into the wide band-gap semiconductor material layer to form a diffusion layer (41) in a surface of the wide band-gap semiconductor material layer, said diffusion layer (41) having an impurity concentration lower than the silicon substrate (11, 21), but higher than the wide band-gap semiconductor material layer (12). A method according to claim 21 or 25, wherein after forming the silicon layer (13) on the wide band-gap semiconductor material layer (12), impurity ions are diffused from the silicon layer (13) into the wide-band gap semiconductor material layer (12) to form a diffusion layer (42) in a surface of the wide band-gap semiconductor material layer.

Inventors: TERASAWA YOSHIO, TERASAWA, YOSHIO
Applicants: NGK INSULATORS LTD, NGK INSULATORS, LTD.
Application year: 1996
Main classification: H01L29
IPC codes: H01L29, H01L21