A FIELD EFFECT TRANSISTOR OF SiC AND A METHOD FOR PRODUCTION THEREOF

Abstract: A transistor of SiC comprises superimposed a drain (13), a highly doped substrate layer (1), a low doped n-type drift layer (2), a p-type base layer being divided into a first lower highly doped base sub-layer (3) and an upper low doped second base sub-layer (4) on top thereof, a highly doped n-type source region layer (6) and a source (11). It also has an insulating layer (8) with a gate electrode (9) thereon arranged on top of the base layer and extending laterally from the source region layer to a n-type layer (7) connected to the drift layer.

Description: A field effect transistor of SiC and a method for productjloπ thereof TECHNICAL FIELD OF THE INVENTION AND PRIOR ART The present invention relates to a transistor of SiC having an insulated gate and being one of a) a MISFET and b) an IG¬ BT, said transistor comprising superimposed in the order mentioned a drain, a highly doped substrate layer being for a) of n-type and for b) of p-type, for b) on top thereof one of c) a highly doped n-type buffer layer and d) no such layer, a low doped n-type drift layer, a p-type base layer, a highly doped n-type source region layer and a source, said transistor further comprising an insulating layer with a gate electrode thereon arranged on top of the base layer and extending substantially laterally from at least said source region to a n-type layer being one of e) the drift layer and f) an additional n-type layer connected to the drift layer, for, upon applying a voltage to the gate electrode, forming a conducting inversion channel extending substantially lat¬ erally in the base layer at the interface to the insulating layer for electron transport from the source to the drain, as well as a method for producing such a transistor. Such transistors of SiC may especially be used as switching devices in power applications owing to the possibility to turn them on and off very rapidly. Such transistors made of _ SiC are particularly well suited for high power applica¬ tions, since such applications make it possible to benefit from the superior properties of SiC in comparison with espe¬ cially Si, namely the capability of SiC to function well un- der extreme conditions. SiC has a high thermal stability due to a large bandgap energy, such that devices fabricated from said material are able to operate at high temperatures, namely up to 1000 K. Furthermore, it has a high thermal con¬ ductivity so that SiC devices may be arranged at a high den¬ sity. SiC also has a more than five times higher breakdown field than Si, so that it is well suited as a material in high power devices operating under conditions where high voltages may occur in the blocking state of a device. An advantage of this type of MISFET and IGBT having an in¬ version channel extending substantially laterally is that the charge carrier mobility in such an inversion channel will be much higher than in such devices having a vertical inversion channel along a trench, since the trap density at the interface between the base layer and the insulating layer will be much lower with this location of the surface channel, since the surface of the base layer may be formed by epitaxial growth, whereas the vertical trench wall of a device with a vertical channel has to be formed by etching or the like giving rise to a higher concentration of traps. Another advantage of this type of device structure is the absence of reliability problems connected with the high electric field occuring at the trench corner of a transistor having such a trench. MISFET is to be interpreted broadly and shall comprise every type of field effect transistor having an insulated gate, and accordingly also MOSFETs. A transistor according to the introduction in the form of a MOSFET is known through US patent 5 397 717. However, this transistor does not make it possible to fully benefit from the excellent property of SiC to withstand high reverse voltages in the blocking state thereof, so that it will not be suited for use in high power applications. The reason for this is that the base layer of p-type has to be low doped for keeping the threshold voltage, which has to be applied on said gate for creating said inversion channel, compara¬ tively low and by that obtain a high carrier mobility in the channel, but this low doping of the base layer will result in a depletion thereof already at a low reverse voltage, so that this device cannot withstand high reverse voltages, which occur in high power applications. SUMMARY OF THE INVENTION The object of the present invention is to provide a transis¬ tor of the type defined in the introduction, through which the problem of the prior art transistor described above is solved to a large extent making it well suited for high power applications. This object is in accordance with the invention obtained by making said p-type base layer low doped in a region next to the interface to said insulating layer at which said inver¬ sion channel is obtainable and highly doped in a region thereunder next to the drift layer. By doping different regions of the base layer differently in this way a high channel mobility thanks to a low threshold voltage may be combined with a high breakdown voltage in the blocking state of the transistor. The region of low doping next to the interface to the insulating layer results in a low threshold voltage, i.e. a low voltage necessary to apply to the gate for creating the inversion channel and by that a high charge mobility in said channel, and the highly doped region thereunder next to the pn-junction to the drift layer will make it possible to apply high reverse voltages to the transistor without depleting the base layer and causing a breakdown of the device. Thus, this transistor will be well suited for high power applications. According to a preferred embodiment of the invention said p- type base layer is divided into two sub-layers, a highly doped lower first base sub-layer and a low doped upper sec¬ ond base sub-layer on top thereof. The combination of a low threshold voltage, which means a high channel mobility, and a high breakdown voltage may in this way be advantageously obtained by making the base layer of two differently strong doped layers. According to another preferred embodiment of the invention said transistor comprises an additional n-type layer ar¬ ranged laterally of the base layer on said base layer at a lateral distance from said source region layer, said addi- tional n-type layer extends to the drift layer so as to con¬ duct electrons moving in said conducting inversion channel from the source region layer to the drift layer, and said additional n-type layer is highly doped. In this way a low resistance of the transistor in the on-state may be achieved at the same time as the blocking capacity thereof will be high when reverse biased. According to a further preferred embodiment of the invention said transistor comprises an additional highly doped p-type layer on top of said base layer and located laterally at the opposite side of the source region layer with respect to said interface for forming said conducting inversion chan¬ nel, and this additional p-type layer is connected to the source. In this way a good contact from the source to the p- base may be obtained. Furthermore, if the transistor is an IGBT this additional layer will collect the holes injected into said drift layer from the substrate layer as a conse¬ quence of the electron current flowing through said inver¬ sion channel and towards the drain. These holes are moving towards the source for recombining with electrons from the source. A further object of the invention is to provide a method for producing a transistor of SiC having an insulated gate and being one of a) a MISFET and b) an IGBT, for which the draw- back of the prior art transistor of this type discussed above is considerably reduced. Such a method comprises according to the invention the steps of: 1) epitaxially growing on top of each other the following semiconductor layers of SiC: a highly doped substrate layer being for a) of n-type and for b) of p-type, for b) on top thereof one of c) a highly doped n-type buffer layer and d) no such layer, and a low doped n-type drift layer, 2) implanting p-type dopants into a surface layer of said drift layer for forming a highly doped p-type first base sub-layer, 3) epitaxially growing a low doped p-type second base sub-layer on top of said first base sub-layer, 4) implanting n-type dopants into a surface layer of said second base sub-layer for forming a highly doped n-type source region layer, 5) etching away said second base sub-layer at a lateral distance from said source region layer for forming a wall extending to the drift layer, 6) implanting n-type dopants into said second base sub¬ layer at a lateral distance from said source region layer and in connection with said wall and into said wall for forming an additional n-type layer extending from the surface of said second base sub-layer to the drift layer, 7) applying an insulating layer with a gate electrode thereon on the second base sub-layer at least from the source region layer to said additional n-type layer and a source on said source region layer, the steps 4) and 5) being carried out in one of the follow¬ ing orders: a) 4 and then 5) and b) 5) and then 4) . In this way a transistor with a lateral inversion channel well suited for high power applications thanks to a low threshold voltage in combination with a high breakdown volt¬ age may be obtained in an advantageous way. The key to ob¬ taining such a transistor is the combination of implanting of p-type dopants into a surface layer of the epitaxially grown drift layer for forming a highly doped p-type first base sub-layer with an epitaxial regrowth of a low doped p- type second base sub-layer on top of said first base sub¬ layer. Thanks to the use of this regrowth technique it will be possible to use the preferred implantation technique for obtaining the lower highly doped p-type base sub-layer with¬ out causing any damages in the upper base sub-layer, so that this may have a high quality at the interface to the insu¬ lating layer. Accordingly, this method enables the produc¬ tion of a transistor of this type of SiC, in which the dif- fusitivity of dopants is remarkably reduced in comparison to especially Si. The dopants implanted will automatically be made electrically active thanks to the high temperature used for the epitaxial growth following upon the implantation step. Another method through which said object of the present in¬ vention may be obtained comprises the steps of: 1) epitaxially growing on top of each other the following semiconductor layers of SiC: a highly doped substrate layer being for a) of n-type and for b) of p-type, for b) on top thereof one of c) a highly doped n-type buffer layer and d) no such layer, a low doped n-type drift layer, a highly doped p-type first base sub-layer and a low doped p-type second base sub-layer, 2) etching through the two base sub-layers to said drift layer for forming a wall on said two base sub-layers, 3) epitaxially growing an additional n-type layer on said wall and onto said drift layer laterally to said base sub-layers so that this additional n-type layer will have an upper surface next to said second base sub-layer at substantially the same level as the upper surface of said second base sub-layer, 4) implanting n-type dopants into a surface layer of said second base sub-layer for forming a highly doped n-type source region layer separated laterally from the addi- tional n-type layer, 5) applying an insulating layer with a gate electrode thereon on said second base sub-layer at least from the source region layer to said additional n-type layer and a source on said source region layer, the step 4) being carried out one of a) after steps 2) and 3), b) before steps 2) and 3) and c) after step 2) but be¬ fore step 3) . By using this method a transistor of SiC having an insulated gate and being a MISFET or an IGBT with a lateral inversion channel and the same preferred characteristics as that pre¬ viously mentioned may be produced. An advantage of this method with respect to the method previously defined is that the thickness of the highly doped first base sub-layer may be chosen arbitrarily as a function of the voltage which the particular transistor has to block when reverse biased. Further advantages and preferred features of the invention will appear from the description and the other claims. BRIEF DESCRIPTION OF THE DRAWINGS With reference to the appended drawings, below follows a specific description of preferred embodiments of the inven- tion cited as examples. In the drawings: Fig. 1-4 are schematic cross-section views illustrating different steps of a method for producing a MISFET shown in Fig. 4 and according to a first preferred embodiment of the invention, Fig. 5-8 are views corresponding to Figs. 1-4 illustrating different steps of a method for producing an IGBT shown in Fig. 4 and according to a second pre¬ ferred embodiment of the invention, and Fig. 8-11 are schematic cross-section views illustrating different steps of a method for producing an IGBT shown in Fig. 3 and according to a third preferred embodiment of the invention. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION A method for producing a MISFET having a lateral conducting inversion channel will now be described with reference to Fig. 1-4. Conventional semiconductor device producing steps having nothing to do with the invention, such as masking, demasking and so on, have for the sake of clearness not been shown in the figures. First of all a highly doped n-type substrate layer 1 and on top thereof a low doped n-type drift layer 2 are epitaxially grown (see Fig. 4). After that p-type dopants, preferably aluminium or boron, but also other dopants are conceivable, are implanted into the sur¬ face layer of said drift layer for forming a highly doped p- type first base sub-layer 3. That step is followed by a step of epitaxially growth (see Fig. 2) of a low doped p-type second base sub-layer 4 on top of said first base sub-layer. This epitaxial growth is preferably carried out by using the Chemical Vapour Deposition technique at a high temperature, preferably above 1500"C, and an annealing of the implanted layer 3 for making the dopants implanted electrically active may be carried out in connection therewith by raising the temperature to 1700°C or more. After that a MESA etch is carried out in said second base sub-layer 4 for forming a wall 5 extending to the drift layer 2. Then are n-type dopants implanted into the surface layer of said second base sub-layer 4 for forming a highly doped n-type source region layer 6 and n-type dopants are implanted into the second base sub-layer 4 at a lateral dis¬ tance from the source region layer and in connection with said wall and into said wall for forming an additional n- type highly doped layer 7 extending from the surface of sec¬ ond base sub-layer 4 to the drift layer 2. This step is fol¬ lowed by an annealing step for making said n-type dopants implanted electrically active. An insulating layer 8, for instance of Siθ2, with a gate electrode 9 thereon is applied on the second base sub-layer 4 at least from the source region layer 6 to said additional n-type layer 7. Said gate electrode is preferably of poly- crystalline silicon. After that a passivating additional in¬ sulating layer 10 is applied over the gate electrode and the upper surface of the device leaving an area of said source region layer 6 exposed, and a source 11 is then applied on the layer 6. This device will function in the following way: When a volt¬ age is applied on the gate electrode 9 a lateral conducting inversion channel will be created at the interface 12 be¬ tween said second base sub-layer and the insulating layer 8 between the source region layer 6 and said additional n-type layer 7, so that, provided that a voltage is applied over the source 11 and a drain 13, only indicated through a line, in the forward direction of the device an electron current will flow from the source to the drain. By making the base layer of two differently strong doped layers in this way a high channel mobility thanks to a low threshold voltage may be combined with a high breakdown voltage in the blocking state of the device. The second base sub-layer 4 having a low doping, which typically but not always means in the range of 10^^-10-'-° cm-A next to the interface to the insu¬ lating layer 8 results in a low threshold voltage, i.e. a low voltage necessary to apply on the gate electrode for creating the inversion channel and by that a high charge mo¬ bility in said channel, and the highly doped, which typi- cally but not always means a doping concentration of 10-L9 cm ~ 3 - 10^1 crrT-^ first base sub-layer 3 thereunder next to the pn-junction to the drift layer will make it pos¬ sible to apply high reverse voltages to the device without depleting the base layer and causing a breakdown of the de- vice. This preferred structure was possible to obtain for SiC by using the technique for epitaxial regrowth after im¬ plantation. Fig. 5-8 illustrates the different steps for producing an IGBT (insulated gate bipolar transistor) according to a sec¬ ond preferred embodiment of the invention, and this method comprises the same steps as the method described above with reference to Fig. 1-4 except for the initial growth of a highly doped p-type substrate layer 1 and a highly doped n- type buffer layer 14 on top thereof, and the additional step of implanting p-type dopants into a surface layer of said second base sub-layer 4 for forming an additional highly doped p-layer 15 located laterally at the opposite side of the source region layer 6 with respect to said wall 5. The source or the cathode 11 is applied in contact also with this additional p-type layer. In this way a good contact from the source to the p-base may be obtained. Furthermore, this additional layer will collect the holes injected into said drift layer 2 from the substrate layer 1 as a conse¬ quence the electron current flowing through said inversion channel and towards the drain in an on-state of the IGBT. A MISFET could also be produced according to the steps shown in Fig. 5-8, but then without said buffer layer and with highly doping of the substrate layer with n-type dopants. A method for producing an IGBT having a lateral conducting inversion channel according to a third preferred embodiment of the invention is illustrated in Fig. 9-11, and this method may also be used for producing a MISFET by the same changes in process steps as just described with respect to Fig. 5-8. First of all the following semiconductor layers of SiC are epitaxially grown on top of each other: a highly doped p-type substrate layer 1, a highly doped n-type buffer layer 15 (this layer is optional), a low doped n-type drift layer 2, a highly doped p-type first base substrate layer 3 and a low doped p-type second base sub-layer 4. By producing the highly doped p-type first base sub-layer 3 through epi¬ taxial growth it will be possible to make it just as thick as desired depending on the magnitude of the voltage to be held by the device in the blocking state thereof. After that a MESA-etch is carried out through the two base sub-layers to said drift layer for forming a wall 5 of said two base sub-layers. An additional n-type layer 7 is then epitaxially grown on said wall and onto said drift layer 2 laterally to said base sub-layers so that this additional n- type layer will have an upper surface next to said second base sub-layer at substantially the same level as the upper surface of said second base sub-layer. This step is followed by a conventional planarisation step for removing the part being grown on top of the second base sub-layer. Then n-type dopants are implanted into the surface layer of the second base sub-layer for forming a highly doped n-type source region layer 6 separated laterally from the additional n- type layer 7. P-type dopants are implanted into a surface layer of said second base sub-layer for forming an addi- tional highly doped p-type layer 15 located laterally at the opposite side of the source region layer with respect to said wall. Finally, an insulating layer 8, a gate electrode 9, an additional insulating layer 10 and a source 11 are ap¬ plied in the same way as described above for the other two methods according to the invention. This device shown in Fig. 11 will have the same function as the device shown in Fig. 8, but said highly doped first base sub-layer 3 may be maid thicker as a consequence of the different production steps. The highly doped p-type additional layer 15 may be omitted when a MISFET is produced in this way. The invention is of course not in any way restricted to the preferred embodiments described above, but many possibili¬ ties to modification thereof would be apparent for a man skilled in the art without departing from the basic idea of the invention. It is emphasised that the thicknesses of different layers in the figures can not be interpreted as limiting the scope of protection, but any thickness relations are intended to be covered by the claims. The dopants used may be of any material suitable as dopants for this application. It is well possible to make said additional n-type layer forming a connection between the inversion channel created and the drift layer moderately or low doped, but the on- state losses will be reduced when it is highly doped. The number of layers mentioned in the claims is a minimum number, and it is within the scope of the invention to ar¬ range further layers in the device or dividing any layer into several layers by selective doping of different regions thereof. "Substrate layer" is in this disclosure to be inter¬ preted as the layer closest to the drain of the layers mentioned and it has not to be a substrate layer in the strict sense of this word within this field, i.e. the layer from which the growth is started. The real sub¬ strate layer may be any of the layers and is mostly the thickest one, which may be the drift layer. The method claims are to be interpreted as not re¬ stricted to a growth of the layers located on top of each other in the order they are mentioned, but any other order of growth of these layers is within the scope of the claims. For instance, the method may be started from the drift layer and the so called substrate layer and the drain may be grown at the very end of the method.

Claims: Claims 1. A transistor of SiC having an insulated gate and being one of a) a MISFET and b) an IGBT, said transistor compris¬ ing superimposed in the order mentioned a drain (13), a highly doped substrate layer (1) being for a) of n-type and for b) of p-type, for b) on top thereof one of c) a highly doped n-type buffer layer (14) and d) no such layer, a low doped n-type drift layer (2), a p-type base layer (3, 4), a highly doped n-type source region layer (6) and a source (11), said transistor further comprising an insulating layer (8) with a gate electrode (9) thereon arranged on top of the base layer and extending substantially laterally from at least said source region layer to a n-type layer being one of e) the drift layer and f) an additional n-type layer (7) connected to the drift layer, for, upon applying of a volt¬ age to the gate electrode, form a conducting inversion chan¬ nel extending substantially laterally in the base layer at the interface (12) to the insulating layer for electron transport from the source to the drain, characterized in that said p-type base layer is low doped in a region (4) next to the interface to said insulating layer at which said inversion channel is obtainable and highly doped in a region (3) thereunder next to the drift layer (2). 2. A transistor according to claim 1, characterized in that said p-type base layer is divided into two sub-layers, a highly doped lower first base sub-layer (3) and a low doped upper second base sub-layer (4) on top thereof. 3. A transistor according to claim 1 or 2, characterized in that said source region layer (6) is partially immersed into said base layer, so that the upper surface thereof is at the same level as the surface of the base layer at said inter¬ face (12) to the insulating layer (8). 4. A transistor according to any of claims 1-3, character¬ ized in that it comprises an additional n-type layer (7) ar¬ ranged laterally of the base layer (3, 4) on said base layer at a lateral distance from said source region layer (6), that said additional n-type layer extends to the drift layer (2) so as to conduct electrons moving in said conducting in¬ version channel from the source region layer to the drift layer, and that said additional n-type layer is highly doped. 5. A transistor according to any of claims 1-4, character- zs . in that it comprises an additional highly doped p-type layer (15) on top of said base layer (3, 4) and located lat- erally at the opposite side of the source region layer (6) with respect to said interface (12) for forming said con¬ ducting inversion channel, and that this additional p-type layer is in contact with the source (11). 6. A method for producing a transistor of SiC having an in¬ sulated gate and being one of a) a MISFET and b) an IGBT, comprising the steps of: 1 ) epitaxially growing on top of each other the following semiconductor layers of SiC: a highly doped substrate layer (1) being for a) of n-type and for b) of p-type, for b) on top thereof one of c) a highly doped n-type buffer layer (14) and d) no such layer, and a low doped n-type drift layer (2), 2) implanting p-type dopants into a surface layer of said drift layer for forming a highly doped p-type first base sub-layer (3), 3) epitaxially growing a low doped p-type second base sub-layer (4) on top of said first base sub-layer, 4) implanting n-type dopants into a surface layer of said second base sub-layer for forming a highly doped n-type source region layer ( 5), 5) etching away said second base sub-layer at a lateral distance from said source region layer for forming a wall (5) extending to the drift layer, 6) implanting n-type dopants into said second base sub- layer at a lateral distance from said source region layer and in connection with said wall and into said wall for forming an additional n-type layer (7) extending from the surface of said second base sub-layer to the drift layer, 7) applying an insulating layer (8) with a gate electrode (9) thereon on the second base sub-layer (4) at least from the source region layer to said additional n-type layer and a source (11) on said source region layer, the steps 4) and 5) being carried out in one of the follow¬ ing orders: a) 4) and then 5) and b) 5) and then 4) . 7. A method according to claim 6, characterized in that the etching step 5) is carried out before step 4) and the im¬ plantations according to step 4) and 6) are carried out at the same time as one single process step. 8. A method for producing a transistor of SiC having an in¬ sulated gate and being one of a) a MISFET and b) an IGBT, comprising the steps of: 1 ) epitaxially growing on top of each other the following semiconductor layers of SiC: a highly doped substrate layer (1) being for a) of n-type and for b) of p-type, for b) on top thereof one of c) a highly doped n-type buffer layer (14) and d) no such layer, a low doped n- type drift layer (2), a highly doped p-type first base sub-layer (3) and a low doped p-type second base sub¬ layer (4), 2) etching through the two base sub-layers to said drift layer for forming a wall (5) on said two base sub-layers, 3) epitaxially growing an additional n-type layer (7) on said wall and onto said drift layer laterally to said base sub-layers so that this additional n-type layer will have an upper surface next to said second base sub-layer at substantially the same level as the upper surface of said second base sub-layer, 4) implanting n-type dopants into a surface layer of said second base sub-layer (4) for forming a highly doped n- type source region (6) layer separated laterally from the additional n-type layer (7), 5) applying an insulating layer (8) with a gate electrode (9) thereon on said second base sub-layer at least from the source region layer to said additional n-type layer and a source (11) on said source region layer, the step 4) being carried out one of a) after steps 2) and 3), b) before steps 2) and 3) and c) after step 2) but be¬ fore step 3) . 9. A method according to any of claims 6-8, characterized in that said additional n-type layer (7) is made hignly doped when created. 10. A method according to any of claims 6-9, characterized in that it comprises the step of implanting p-type dopants into a surface layer of said second base sub-layer (4) for forming an additional highly doped p-type layer (15) located laterally at the opposite side of the source region layer with respect to said wall, and that the source (11) is ap¬ plied also in contact with this additional p-type layer. 11. A method according to any of claims 6-9, characterized in that it comprises the step of etching through said second base sub-layer (4) to the first one (3) laterally at the op¬ posite side of the source region layer with respect to said wall, and that the source (11) is applied also in contact _ with the first base sub-layer in this trench. 12. A method according to claim 6, characterized in that one of a) boron and b) aluminium is implanted as p-type dopant in step 2) . 13. A transistor formed by carrying out the method according to any of claims 6-12.

Inventors: BAKOWSKI MIETEK, GUSTAFSSON ULF, HARRIS CHRISTOPHER, BAKOWSKI, MIETEK, GUSTAFSSON, ULF, HARRIS, CHRISTOPHER
Applicants: ABB RESEARCH LTD, BAKOWSKI MIETEK, GUSTAFSSON ULF, HARRIS CHRISTOPHER, ABB RESEARCH LIMITED, BAKOWSKI, MIETEK, GUSTAFSSON, ULF, HARRIS, CHRISTOPHER
Application year: 1997
Main classification: H01L
IPC codes: H01L21, H01L29