A METHOD FOR PRODUCING A CHANNEL REGION LAYER IN A VOLTAGE CONTROLLED SEMICONDUCTOR DEVICE

Abstract: In a method for producing a channel region layer in a SiC-layer for producing a voltage controlled semiconductor device a layer (1) of silicon being one of a) polycrystalline and b) amorphous is applied on top of the SiC-layer, an aperture (4) is etched in said silicon layer extending to the SiC-layer, a surface layer of a certain thickness of said silicon layer is oxidised, and the lateral extension of said channel region layer is determined by removing said oxidized layer and carrying out a further implantation into the area exposed by the so formed enlarged aperture.

Description: A METHOD FOR PRODUCING A CHANNEL REGION LAYER IN A VOLTAGE CONTROLLED SEMICONDUCTOR DEVICE TECHNICAL FIELD OF THE INVENTION AND PRIOR ART The present invention relates to a method for producing a channel region layer of an opposite doping type, one of a) p and b) n, than the regions of laterally opposite sides thereof in a SiC-layer for producing a voltage controlled semiconductor device, as well as a voltage controlled semiconductor device produced by using such a method. Such a method may be used for producing any voltage con- trolled semiconductor device, such as for instance MISFETs, MOSFETs and IGBTs. This type of semiconductor devices 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 devices made of SiC are particularly well suited for high power applications, since such appli¬ cations make it possible to benefit from the superior properties of SiC in comparison with especially Si, namely the capability of SiC to function well under extreme conditions. These properties involving a high thermal stability, a high thermal conductivity and a high breakdown voltage are well known in the art, but another well known feature of SiC, namely the extremely low diffusivity of dopants at convenient temperatures therein, has until now made it very difficult to produce a well defined channel region layer for a voltage controlled semiconductor device of SiC in a simple and reliable manner. For for instance Si there will be no problem to produce such channel region layers, since the diffusion technique functions excellent for Si. SUMMARY OF THE INVENTION The object of the present invention is to provide a method of the type defined in the introduction, which makes it possible to produce a well defined channel region layer in a voltage controlled semiconductor device of SiC in a simple and thereby commercially interesting way. This object is according to the invention obtained by providing such a method with the steps of: 1) applying a layer of silicon being one of a) polycrystal¬ line and b) amorphous on top of the SiC-layer, 2 ) etching an aperture in said silicon layer extending to the SiC-layer, 3 ) oxidizing a surface layer of a certain thickness of said silicon layer, 4) implanting dopants of a first doping type, which corre¬ sponds to the doping type of the SiC-layer, into an area of the SiC-layer defined by said aperture in said silicon layer for obtaining a high doping concentration of said first type in a first surface near layer of the SiC-layer under said area, 5) removing said oxidized surface layer by etching and thereby increasing said aperture by the thickness of said oxidized layer, 6 ) implanting dopants of a second dopant type into an area of the SiC-layer defined by the aperture formed by step 5) to such a degree that the doping type of said surface near layer created by carrying out step 4) is maintained but the doping type of a second surface near layer under the area exposed through step 5) is changed for forming a channel region layer with a lateral extension determined by the thickness of the oxidized silicon layer, wherein the steps are carried out in one of a) the order mentioned and b) the order mentioned, except for step 4) before step 3). Thus, the invention is based on the idea to take advantage of the fact that polysilicon or amorphous silicon has a much greater oxidation rate than SiC for forming a well de¬ fined channel region layer, which may be given a very short length in the sub-micron area when desired. Thanks to the fact that the oxidation rate for polysilicon and amorphous silicon is almost two orders of magnitude faster than for SiC, a negligible thickness of the SiC layer will be oxidized. A very important feature of the oxide layer zone created in the silicon layer is that it may be easily removed very accurately without touching any other struc- ture of the device, so that this very precise way of enlarging said aperture in a mask for implantation will in combination with the use of the self-aligned technique for forming the channel region layer by implantation makes it possible to obtain a well defined channel region layer having exactly the length desired for each particular device. The thickness of the oxidized silicon layer removed before said implantation will determine the lateral exten¬ sion of the channel region layer or the length of said channel, so that the latter may be controlled by control- ling the degree of oxidation of said silicon layer. Thus, it is possible to produce very short channels by a self- aligned technique that does not require any delicate process steps of a very high accuracy. By using the forma¬ tion of an oxide layer and removal thereof for defining the width of a channel region layer, a non-directional process is utilized for obtaining said channel region layer, which is of great importance. Thus, the advantages of a structure produced by this method is that it will be possible to obtain a small cell size (pitch) of the channel region layer enabling a high channel density without using any delicate technique requiring any means able to carry out high-precisely process steps. According to a preferred embodiment of the invention the steps l)-6) are carried out in the order mentioned, so that the channel region layer formed in step 6) gets the same lateral extension as the thickness of the oxidized silicon layer. This is advantageous due to the fact that the oxide layer will occupy a greater volume than the layer thereun¬ der of polysilicon or amorphous silicon, so that material will in this way be saved, since less polysilicon or amorphous silicon has to be oxidized for a certain channel width. Provided that the oxidized layer is thick enough this may then function as a mask for the implantations according to steps 4) and 6), which will reduce the number of masking steps and thereby result in a lower production cost of the device produced by utilizing this method. According to another preferred embodiment of the invention the dopants implanted in step 6 ) are of p-type for forming a doped p-type channel region layer laterally surrounded by n-type layers. This will mostly be convenient, since the SiC-layer will when no dopants have been introduced therein during the epitaxy thereof be n-doped. According to another preferred embodiment of the invention said method comprises a step carried out after step 2) but before step 5), and in which dopants of said second doping type are implanted deeply into the area defined by the aperture existing in said silicon layer for creating a deep layer of said second doping type under the first surface- near layer of the first doping type created in step 4). It will in this way by such a deep implantation be possible to create a device having a DMOS-type of structure inspite of the absence of the possibility to use the diffusion tech¬ nique for obtaining such a device in SiC. A more appropri- ate name for such a device would be a VIMOS (a vertical im¬ planted MOS ) , but it is emphasized that claim 4 also of course covers the case of producing other types of voltage controlled semiconductor devices of SiC having a deeply im¬ planted layer, such as for instance IGBTs. The on-state performance of a device produced according to this method will be limited (for blocking voltages below 1 kV) by the carrier mobility of the channel formed in the implanted re¬ gion. According to another preferred embodiment of the invention the steps l)-6) are carried out in the order mentioned, ex¬ cept for that step 4) is carried out before step 3), and an additional masking layer is supplied on top of said silicon layer after carrying out step 1 ) . Such an additional masking layer may be advantageous to have for preventing the ions accelerated towards the SiC-layer to reach the SiC-layer in regions lying outside said aperture, since such ions may travel between grain boundaries in the silicon layer and reach the SiC-layer. Thus, it may be preferred to use such an extra masking layer, since a layer of polysilicon or amorphous silicon may in some cases not be a totally efficient mask. This is particularly valid when this embodiment is combined with the embodiment including the deep implantation due to the high accelera- tion energies used for such deep implantation. According to another preferred embodiment of the invention it is polycrystalline silicon that is applied on top of the SiC-layer in step 1). The use of polycrystalline silicon for forming an oxidized layer for defining the width of a channel region layer is preferred, since there are a lot of chemical processes developed for polycrystalline silicon. Furthermore, there is a very diffuse boarder between poly¬ crystalline silicon and amorphous silicon, and there may often be a mixture of these two types of silicon structure in a silicon layer applied in step 1), so that the defini¬ tions polycrystalline silicon and amorphous silicon will in this disclosure also cover the case in which the other one of the two is present and it will only mean that the one mentioned is in majority. The invention also comprises a method for producing a voltage controlled semiconductor device of SiC being one of a) a MISFET and b) an IGBT, comprising the steps of: 8 ) 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 dope n-type drift layer, 9) carrying out the steps according to claim 1 according to one of two orders defined there, in which the first doping type is n and the second is p, 10) a step carried out after step 2) but before step 5), and in which dopants of p-type are implanted deeply into the area defined by the aperture existing in said silicon layer for creating a deep p-type base layer under the layer of n-type created in step 4), and forming a source region layer of the device, 11) removing the remaining silicon layer by etching, 12) applying an insulating layer onto the SiC-layer extend¬ ing at least over said channel region layer, 13 ) applying a gate electrode on top of said insulating layer at least over said channel region layer and a source on said source region layer created in step 4) . A high quality MISFET or IGBT in SiC having a short channel and thereby a high channel density may in this way be produced in an easy and reliable manner. The invention also involves voltage controlled semiconduc- tor devices of SiC produced by the method comprising the steps according to any of the appended method claims. The advantages of such a device will clearly appear from above. Further advantages and preferred features of the invention will appear from the description and the other dependent claims. BRIEF DESCRIPTION OF THE DRAWINGS With reference to the appended drawings, below follows a specific description of preferred embodiments of the invention, cited as examples. In the drawings: Figs 1-3 are schematic cross section views illustrating different steps of a method for producing a channel region layer in a SiC-layer for producing a voltage controlled semiconductor device according to a first preferred embodi- ment of the invention, Fig 4 is a schematic cross section view illustrating how a voltage controlled semiconductor device produced by utiliz¬ ing the method steps illustrated in Figs 1-3 may look like, and Fig 5 is a view illustrating how the steps shown in Figs 1 and 2 may be carried out in a method according to a second preferred embodiment of the invention. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION A method for producing a channel region layer in a SiC- layer for producing a voltage controlled semiconductor device will now be described with reference to Figs 1-3. Conventional semiconductor device producing steps having nothing to do with the invention, such as further steps of masking and demasking and so on, have for the sake of clearness not been shown in the Figures. First of all a layer 1 of silicon being polycrystalline or amorphous is applied on top of a substantially monocrystalline SiC-layer 2, which here is a lightly doped layer of n-type. On top of the layer 1 a further layer 3 of a masking material, such as a metal, is applied. An aperture 4 is then etched through the layers 1 and 3 and to the surface of the SiC- layer 2. Then n-type dopants are implanted into an area of the SiC-layer defined by said aperture for obtaining a high doping concentration of donors in a surface near layer 5 of the SiC-layer under said area, p-type dopants are also implanted into the area under said aperture 4, and this implantation is carried out while using high acceleration energies, so as to form a deep layer 6 with a high concentration of acceptors. The layers 5 and 6 may in this way be produced in an arbitrary order, and the layers 1 and 3 will function as a mask for producing the layers 5 and 6 in alignment with each other. The masking layer 3 is then removed and a surface layer 7 (see Fig 2) of a certain thickness of said silicon layer 1 is then oxidized at high temperature, so that a layer 7 of oxide (Siθ2) is formed. The oxidation temperature for the polycrystalline or amorphous silicon is below 1000°C at which temperature SiC does not oxidize significantly. It is indicated in Fig 2 that the oxide occupies a somewhat greater volume than the silicon from which it derives, so that said oxide layer 7 will reduce the area of the aperture a little bit. The oxidized surface layer 7 is then removed by wet etch¬ ing, which may be done very accurately without touching any other structure. Then dopants of p-type are implanted into an area of the SiC-layer defined by the aperture 8 formed by the removal of the oxidized layer 7 to such a degree that the doping type of said surface near layer 5 previ¬ ously created is maintained but the doping type of a second surface near layer 9 exposed through said removal is changed for forming a p-doped channel region layer 9 with a lateral extension determined by the thickness of the oxidized silicon layer 7. The remaining part of the layer la is during this implantation functioning as a mask. Thus, the thickness of the oxidized surface layer 7 will deter¬ mine the width of the channel region layer 9, but in this case the width of the channel region layer will be a little bit smaller than said thickness due to the greater volume occupied by the Siθ2 than said polycrystalline or amorphous silicon. Channel region layers with a very short channel, well in the sub-micron region, may in this way be exactly produced in a very simple and reliable manner. The acceptors implanted are preferably B or Al, wherein P and N may be mentioned as possible donors for the n-type implantation. It is shown in Fig 4 how a so called vertical implanted MOS (VIMOS) produced by carrying out a method according to a preferred embodiment of the invention may look like. This device is created by epitaxially growing a low doped n-type drift layer 2 on top of a highly doped n-type substrate layer 10, which is adapted to be connected to a drain indicated by a line 11 of the device. A highly doped p-type base layer 6, a n-type highly doped source region layer 5 and a p-type channel region layer 9 are formed by using the inventional method for producing a channel region layer in a SiC-layer for producing a voltage controlled semiconduc¬ tor device. The device also includes a source contact 12 arranged in a groove 13 etched into the source region layer 5 through the base layer 6. Furthermore, an insulating layer 14, for instance of Siθ2, is applied on top of the SiC-layer over at least the entire lateral extension of the channel region layer 9. Finally, a gate electrode 15 is ap¬ plied upon said insulating layer 14 at least over the channel region layer 9. The function of such a device is well known to those skilled in the art, but it may be summarized here. When a voltage is applied to the gate electrode 15 a lateral conducting inversion channel will be created at the interface 16 between the channel region layer 9 and the insulating layer 14 between the source region layer 5 and the drift layer 2, so that, provided that a voltage is applied over the source 12 and the drain 11 in the forward direction of the device an electron current will flow from the source to the drain. It is illustrated in Fig 4 how several such devices may be placed next to each other and in this way form a device together. An IGBT may be designed in the same way, but there will be an extra highly doped p-type substrate layer below the drift layer, possibly with a highly doped n-type buffer layer therebetween. It is illustrated in Fig 5 how the method for producing a channel region layer in a SiC-layer for producing a voltage controlled semiconductor device may be modified with respect to the method described above. In this case the oxidation of a surface layer 7 is carried out directly after producing the aperture 4 therein, and no further masking layer, as the layer 3 in Fig 1, is applied in this case. The implantations forming the layers 5 and 6 are carried out after said oxidation step. The advantage of this procedure is that the thickness of the oxidized surface layer 7 will be identical to the width of the channel region layer formed by removing said surface layer 7 and carrying out a further implantation of acceptors, so that there will be a minimum of waste of material with respect to the layer 1 of polycrystalline or amorphous silicon. It may be mentioned that the length of the channel created according to the invention may typically be about 0,5 μm, and the deep implantation may be carried out to a depth of for example 2 μm. The invention is of course not in any way restricted to the preferred embodiments described above, but many possibili¬ ties to modifications thereof will be apparent for a man skilled in the art without departing from the basic idea of the invention. It is emphasized that the thicknesses of different layers in the Figures cannot be interpreted as limiting the scope of the protection, but any thickness relations are intended to be covered by the claims. The dopant used may be of any material suitable as dopants for this application, and it is well possible to change the doping type to the opposite ones than those shown in the preferred embodiments described above should this be desired. The methods according to the invention of course also include annealing steps for making the dopants implanted electrically active, but how and when these are carried out has nothing to do with the invention. The number of layers mentioned in the claims is a minimum number, and it is within the scope of the invention to arrange further layers in the device or dividing any layer into several layers by selective doping of different regions thereof. The deep implantation step as well as the layer created thereby may be left out or a corresponding layer may be produced by another technique, such as epitaxial regrowth, although the implantation according to the preferred embodiment described above is very advantageous. "Substrate layer" is in this disclosure to be interpreted 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 substrate layer may be any of the layers and is mostly the thickest one, which may be the drift layer. The layers located on top of each other does not have to be epitaxially grown in the order they are mentioned in the claims, 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 method for producing a channel region layer (9) of an opposite doping type, one of a) p and b) n, than the regions (5, 2) on laterally opposite sides thereof in a SiC-layer for producing a voltage controlled semiconductor device, comprising the steps of: 1) applying a layer (1) of silicon being one of a) poly- crystalline and b) amorphous on top of the SiC-layer (2), 2) etching an aperture (4) in said silicon layer extending to the SiC-layer, 3) oxidizing a surface layer (7) of a certain thickness of said silicon layer, 4) implanting dopants of a first doping type, which corre¬ sponds to the doping type of the SiC-layer, into an area of the SiC-layer defined by said aperture in said silicon layer for obtaining a high doping concentration of said first type in a first surface near layer (5) of the SiC- layer under said area, 5) removing said oxidized surface layer (7) by etching and thereby increasing said aperture by the thickness of said oxidized layer, 6 ) implanting dopants of a second dopant type into an area of the SiC-layer defined by the aperture (8) formed by step 5 ) to such a degree that the doping type of said surface near layer ( 5 ) created by carrying out step 4 ) is main¬ tained but the doping type of a second surface near layer (9) under the area exposed through step 5) is changed for forming a channel region layer (9) with a lateral extension determined by the thickness of the oxidized silicon layer, wherein the steps are carried out in one of a) the order mentioned and b) the order mentioned, except for step 4) before step 3) . 2. A method according to claim 1, characterized in that the steps l)-6) are carried out in the order mentioned, so that the channel region layer (9 ) formed in step 6 ) gets the same lateral extension as the thickness of the oxidized silicon layer (7). 3. A method according to claim 1 or 2, characterized in that the dopants implanted in step 6) are of p-type for forming a doped p-type channel region layer (9) laterally surrounded by n-type layers (2, 5). 4. A method according to any of claims 1-3, characterized in that it comprises a step carried out after step 2) but before step 5 ) , and in which dopants of said second doping type are implanted deeply into the area defined by the aperture (8) existing in said silicon layer for creating a deep layer (6) of said second doping type under the first surface-near layer (5) of the first doping type created in step 4) . 5. A method according to any of claims 1-4, characterized in that the steps l)-6) are carried out in the order mentioned, except for that step 4) is carried out before step 3), and that an additional masking layer (3) is applied on top of said silicon layer (1) after carrying out step 1 ) . 6. A method according to claim 5, characterized in that said additional masking layer (3) is removed after step 4) and before step 3) is carried out. 7. A method according to any of claims 1-6, characterized in that it is polycrystalline silicon that is applied on top of the SiC-layer (2) in step 1). 8. A method according to any of claims 1-7, characterized in that said surface layer (7) of said silicon layer (1) is in step 3) oxidized while heated at a high temperature. 9. A method for producing a voltage controlled semiconduc- tor device of SiC being one of a) a MISFET and b) an IGBT, comprising the steps of: 8) epitaxially growing on top of each other the following semiconductor layers of SiC: a highly doped substrate layer (10) 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), 9 ) carrying out the steps according to claim 1 according to one of two orders defined there, in which the first doping type is n and the second is p, characterized in that it also comprises the following steps: 10) a step carried out after step 2) but before step 5), and in which dopants of p-type are implanted deeply into the area defined by the aperture (4) existing in said silicon layer ( 1 ) for creating a deep p-type base layer ( 6 ) under the layer ( 5 ) of n-type created in step 4 ) and forming a source region layer of the device, 11) removing the remaining silicon layer (la) by etching, 12) applying an insulating layer (14) onto the SiC-layer extending at least over said channel region layer (9), 13) applying a gate electrode (15) on top of said insulat- ing layer at least over said channel region layer and a source (12) on said source region layer (5) created in step 4). 10. A voltage controlled semiconductor device of SiC produced by a method comprising the steps according to any of claims 1-9.

Inventors: BAKOWSKI MIETEK, BIJLENGA BO, HARRIS CHRISTOPHER, ZDANSKY LENNART, BAKOWSKI, MIETEK, BIJLENGA, BO, HARRIS, CHRISTOPHER, ZDANSKY, LENNART
Applicants: ABB RESEARCH LTD, BAKOWSKI MIETEK, BIJLENGA BO, HARRIS CHRISTOPHER, ZDANSKY LENNART, ABB RESEARCH LIMITED, BAKOWSKI, MIETEK, BIJLENGA, BO, HARRIS, CHRISTOPHER, ZDANSKY, LENNART
Application year: 1997
Main classification: H01L21
IPC codes: H01L21, H01L29