METHOD FOR PRODUCING A SEMICONDUCTOR DEVICE HAVING A SEMICONDUCTOR LAYER OF SiC COMPRISING A MASKING STEP

Abstract: A method for producing a semiconductor device having a semiconductor layer (1-3) of SiC comprises at least the steps of: a) appliction of a mask (4) on at least a portion of said SiC layer and b) a heat treatment of said SiC layer. The mask (4) is made of a material having crystalline AIN as only component or AIN as a major component of a crystalline alloy constituting said material.

Description: Method for producing a semiconductor device having a semiconductor layer of SiC comprising a masking step TECHNICAL FIELD OF THE INVENTION AND PRIOR ART The present invention relates to a method for producing a semiconduc¬ tor device having a semiconductor layer of SiC, said method compris¬ ing at least the steps of a) application of a mask on at least a portion of said SiC layer and b) a heat treatment of said SiC layer. The production of all types of semiconductor devices is comprised, such as for example different types of diodes, transistors and thyris- tors. Such devices are particularly used in applications in which it is 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. 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. Further- more, it has a high thermal conductivity, so that SiC devices can dissipate a high power without overheating. 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. The heat treatment in the semiconductor device production method mentioned in the introduction may be for instance a heating of the SiC layer for obtaining a diffusion of dopants thereinto or an annealing af¬ ter ion implantation in the SiC layer so as to activate the dopants im- planted. Due to the character of SiC such a heat treatment has to be carried out at comparatively high temperatures, mostly above 1500°C, which makes it impossible to use traditional mask materials, such as Siθ2 or Si3N4, as used for Si, inter alia for diffusion in Si. A possible mask material for producing semiconductor devices having a semiconductor layer of SiC would be obtained by using the so called self-making technique, namely by employing SiC itself as a mask. In the diffusion case this will mean that the region which should be masked from diffusion would be covered with a sufficiently thick SiC layer, so that the diffusion front may not reach the device region which must be protected from diffusion. The disadvantage of the self-mask¬ ing technique is the requirement of obtaining a thick masking layer on the crystal surface. In fact, the thickness of the masking layer of SiC cannot be lower than the dopant diffusion depth provided the diffusion coefficient is the same in the region where the diffusion shall be obtained and in the masking layer of SiC, so that the self-masking technique is not a real planar technique. Besides, the planar or nearly planar geometry is highly desirable from view point of device techno¬ logy, because the device patterning, etching and metallization can be performed much more accurately on a flat wafer surface. Furthermore, the self-masking technique involves lapping and polishing on a flat polisher to remove the masking layer of SiC, which has been found to severely deteriorate the device performance due to mechanical damages which penetrate into the crystal bulk. In particular, dealing with the application of the diffusion technique for fabrication of diffused guard rings in SiC power devices turns out to be not practical, because of the deep dopant penetration required for these guard rings. Thick¬ ness of the masking SiC layers consequently turns out to be unrea¬ sonably high. SUMMARY OF THE INVENTION The object of the present invention is therefore to find a remedy to the problems mentioned above and provide a method for producing a semiconductor device having a semiconductor layer of SiC in which the masking may be carried out in such a way as to make it possible to produce high quality semiconductor devices, especially high-voltage power devices. This object is in accordance with the invention obtained by providing a method according to the introduction, in which a mask made of a material having crystalline AIN as only component or AIN as a major component of a crystalline alloy constituting said material is applied on said layer portion before said heat treatment. AIN has a very good lattice match with SiC with a misfit of only 0,7%, it has nearly the same coefficient of thermal expansion as SiC and it is stable at the high temperatures (up to 2200°C) used when heat treatment of SiC layers takes place during the production of semicon- ductor devices. Due to said lattice match high quality AIN layers may be grown on SiC for producing a mask, which can be patterned if desired for diffusion of dopants into the SiC layer. The good lattice match of AIN to SiC is important for preventing enhanced diffusion along the interface via defects and from the surface through defects during said heat treatment of the SiC layer. Some other compounds, like the other group 3B-nitrides and silicon carbide itself may be added in minor amounts to AIN for forming a mask of a crystalline alloy constituting said material, although such group 3B-nitrides have a considerably inferior lattice matching and a much higher decomposi- tion rate at elevated temperatures than AIN. However, the addition of such minor amounts of said compounds to the masking material may appear to be desirable, for example from view point of the deposition process convenience and on the other hand will not deteriorate the masking properties or high temperature stability of the masking material. Another advantage of using AIN as mask material is that it has a good blocking tolerance for diffusion, especially for diffusion of boron, which probably will be the most interesting dopant to introduce into said SiC layer by diffusion due to the high diffusitivity of boron in SiC at elevated temperatures. This is explained by the fact that boron is an isoelectronic impurity in AIN, so that a reaction between boron and nitrogen results in boron nitride, a refractory compound chemically close to AIN. Intermixing of BN and AIN is very slow at any tempera¬ ture below the melting point. According to a preferred embodiment of the invention concerning a method for producing a suitable junction termination technique for a semiconductor power device said mask is through step a) and a possible following step applied to only coat a first portion of said SiC layer leaving a second portion thereof uncoated, the dopants are supplied to the SiC layer during said heat treatment for diffusion of said dopants into the SiC layer at said second portion thereof for doping the SiC layer there, the peripheral regions of the SiC layer are left uncoated through said mask for obtaining diffusion of dopants at the periphery thereof, and the heat treatment and thereby the diffusion is carried out under such conditions that the dopants may penetrate through a junction creating a graded pn-interface of said device for creating a guard ring arranged to smoothe out the electric field in the peripheral region in blocking state of the power device so created. Thus, the use of a mask according to the invention makes it possible to form guard rings for smoothing out the electric field in the peripheral region in the blocking state of a power device having a SiC layer, so that the electric field at the periphery can remain well below the breakdown threshold. This kind of use of the guard ring technique for device termination has not been used for silicon power devices, because they are so thick that it is technically difficult to diffuse the dopants deep enough. However, silicon carbide power devices have a considerably lower base thickness due to the high breakdown fields of SiC, and the guard ring technique may therefor be suitable for such power devices. According to another preferred embodiment of the invention said method comprises a step used in conjugation with ion implantation in a portion of said SiC layer not coated by said mask, said step being carried out before said heat treatment is carried out, and said heat treatment constitutes an annealing for recrystallizing the ion-implanted layer of the SiC layer, said mask preventing impurity diffusion into areas covered thereby from occurring. Such annealing will take place at high temperatures, well above 1500°C, and the mask according to the invention will efficiently prevent impurity diffusion into areas covered thereby from occurring because of the possibility to grow high quality AIN layers on SiC and the stability and the blocking tolerance for diffusion of AIN. According to a further preferred embodiment of the invention said method according to the two embodiments lastly described comprises a step of ion implantation in said peripheral region of the SiC layer, in which ions of the same type, P or N, as the dopants to be diffused into the SiC layer are implanted while creating a thin highly doped layer close to the surface of the SiC layer and then first an annealing heat treatment and then a heat treatment combined with a supply of dopants for guard ring formation is carried out, the diffusion for said guard ring formation being enhanced by the existence of said highly doped thin layer. This means that the method including the application of a mask according to the invention will make it possible to create guard rings for SiC power devices while using the ability thereof to function as a mask for annealing heat treatment as well as diffusion heat treatment. It is obvious that the production steps mentioned in the claims are only some of a great number of production steps well established within the technique of semiconductor device fabrication, such as the application of ohmic contacts, patterning or removal of the mask by for instance reactive ion etching and so on. Thus, "the application of a mask on at least a portion of said SiC layer" does not normally mean that the mask in one step is applied on a portion of the SiC layer, but the mask may be first applied onto the entire SiC layer and then removed by a suitable technique from parts of the SiC layer leaving it on said portion. Further advantages and preferred features of the invention will appear from the following description and the other claims. BRIEF DESCRIPTION OF THE DRAWING With reference to the appended drawing, below follows a specific de¬ scription of a preferred embodiment of the invention cited as an example. In the drawing: Figs 1-5 illustrate schematically different steps of a method for producing a semiconductor power device in the form of a rectifier diode of SiC according to a preferred embodiment of the invention. DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION Figs 1-5 illustrate a few important steps of a large number of steps of a method according to the invention for producing a power diode made of SiC by using a planar-diffusion technology. All the layers of the this device are made of SiC, but hetero-junction devices are also within the scope of the invention. The device will have three differ¬ ently doped layers, namely a third highly doped layer 1 of N-type for making a low resistance contact with an ohmic contact metal plate of the device not shown in the figures, a second low-doped N-type thicker layer 2 arranged thereon and a first thinner P-type layer 3 arranged on the top of the second layer 2 and forming a pn-junction at the interface therewith. In a rectifier diode having these three layers the space charge region is primarily provided by said second low-doped layer, which will take the main part of the voltage applied on the device in the reverse operating direction thereof, and such a device may have a breakdown voltage in the kV-range. However, the breakdown voltage will be lower in the peripheral region of the device than in the bulk thereof due to electric field concentraion, surface defects and the risks of spark formation and the like, so that different passivation techniques are necessary. A serious problem with the peripheral breakdown is due to its localized nature, since it only occurs at a single point at the periphery and therefore results in a very high current density and specific power dissipation. This may lead to an irreversible device degradation even in the case of a rather low reverse current. Accordingly, it would be better if the possible breakdown would occur uniformly over the diode area, so that the current density will be very low and the flow of a high reverse current would not deteriorate the device parameters. Thus, it is highly desir¬ able to remove the weak spots of the diode, since this would not only increase the operation voltage but also make it unnecessary to provide a circuitry for preventing the rectifier from overloading. Ac¬ cordingly, it is preferred that a possible breakdown should occur first in the bulk instead of near the periphery. The method for producing a semiconductor device to be described now attends to this problem. The method comprising the step of deposition of a crystalline AIN layer 4 on the top of the P-type SiC sub-layer 3. It will be possible to epitaxially grow such a crystalline AIN layer with a very high quality on the SiC layer, since the lattice match of crystalline AIN and crystalline SiC is very good, which will also result in a very high quality at the interface between the layers 3 and 4 with a very low concentration of defects. A removal of the AIN layer 4 in the selected areas to be doped is then carried out by reactive ion etching (RIE) in the peripheral regions 5 as shown in Fig 2. It is thereby important that a portion of the P-type first layer 3 is retained in these peripheral regions 5. Accordingly, a patterned mask 6 is now obtained. In the next step a p-type layer 7 heavily doped with aluminium is applied on the top of the regions subject to boron diffusion (discussed below) to enhance boron penetration. Such a layer is prepared, for ex¬ ample, by Al ion implantation with an energy of 50-100 KeV and a dose 2-3 10 16 cm -3 which gives a layer thickness in the region 0.05- 0.2 μm. The layer is initially amorphized as implanted, however the high temperature treatment in course of boron diffusion results in its recrystallization. An amount of Al diffusion into the crystal actually happens to occur however the diffusivity of Al in SiC is very low and the main effect of the Al-doped layer is the enhancement of boron diffusion. It is clear that a thin layer heavily doped with Al can as well be prepared by other techniques for example by epitaxial growth, however the ion implantation in this case seems to provide the easiest solution from the viewpoint of technical complexity. In the next step boron is supplied to the surface of the SiC layer while heating the SiC layer at 1600-1750°C. It has been found that boron has a reasonably high diffusivity in SiC at these temperatures, and the thin highly doped P-type layer 7 will enhance the diffusion of boron in the peripheral regions 5 of the SiC layer, whereas the AIN layer will efficiently block diffusion of boron thereinto and accordingly into the central active region of the semiconductor layers 2 and 3. Thus, a suitable choice of the diffusion regimes (temperature, diffusion time, supply rate of boron) will provide a sufficiently thick compensated layer 9 in the form of a guard ring in the peripheral region of the wafer and the penetration of boron through the mask 6 will be prevented. The guard rings thus obtained will extend the pn-junction of the device thus created in the peripheral regions thereof, so that the electric field in the peripheral region will be smoothed out in a blocking state of such a power device, preventing a possible breakdown from occurring firstly at the periphery of the device. Finally, it is shown in Fig 5 how the structure has been etched by RIE for removing the AIN layer 4 from the active region and to define the diode area. Then several steps of the usual sequence of device termination follows, such as oxidation or deposition of a dielectric, contact metal deposition and so on. The real method for producing the power rectifier diode will of course involve numerous additional steps, which may be performed in a conventional way, and only the steps of importance for the invention have been described above. In another preferred embodiment the boron dopant is introduced into SiC by ion implantation before the heat treatment stage. The regions in which boron penetration is undesired can be protected by AIN and by an additional metal mask which is removed after the implantation. However, annealing away the implantation induced structural disorder requires nearly same as high temperatures as diffusion in case of p- type dopants in SiC and a considerable dopant diffusion happens to occur. This inevitably brings in contamination of the regions where ac- ceptor penetration must be avoided, for example of the central active area of a power rectifier diode, unless those regions are appropirately masked during the heat treatment. It is clear, that in the present embodiment the AIN mask works quite similarly as in case of conven¬ tional thermal diffusion protects from boron diffusion during the annealing stage. Apart from the boron implantation stage and that the Al implantation is left out all the steps are the same as considered in the first embodiment. Boron may be not added to the load of the diffusion crucible, however it may be as well added to suppress its outdiffusion during the heat treatment. The invention is of course not in any way restricted to the embodiment described above, but several possibilities to modifications thereof would be apparent to a man skilled in the art without departing from the basic idea of the invention. The method may be used for the production of any type of semicon¬ ductor device having a SiC layer. The definition "SiC layer" will also include that this SiC layer may consist of several sub-layers as shown in the Figures. Furthermore, the definition layer is to be interpreted broadly and comprise all types of volume extensions and shapes. Although the first and second portion is defined as being singulars in the claims, each of these portions may comprise several separated sub-portions. All definitions concerning the material of the different layers of course also include inevitable impurities as well as intentional doping when SiC is concerned. Boron has been found to be most suitable as dopant for defining the guard rings, but diffusion of other dopants may be envisaged, such as for instance beryllium, which has a reasonably high diffusivity in SiC but is highly toxic.

Claims: Claims 1. A method for producing a semiconductor device having a semicon¬ ductor layer of SiC (1-3), said method comprising at least the steps of a) application of a mask (4) on at least a portion of said SiC layer and b) a heat treatment of said SiC layer, characterized in that a mask made of a material having crystalline AIN as only component or AIN as a major component of a crystalline alloy constituting said material is applied on said layer portion before said heat treatment. 2. A method according to claim 1 , characterized in that a mask (4) made of crystalline AIN is applied on said layer portion before the heat treatment. 3. A method according to claim 1 , characterized in that the mask (4) applied on said layer portion before the heat treatment is made of a material having AIN as a major component of a crystalline alloy, which further comprises one or a combination of group 3B-nitrides and SiC. 4. A method according to any of claims 1-3, characterized in that said heat treatment is carried out above 1500°C. 5. A method according to any of claims 1-4, characterized in that said mask (4) is through step a) and a possible following step applied to only coat a first portion of said SiC layer leaving a second portion (5) thereof uncoated, and that dopants are supplied to the SiC layer during said heat treatment for diffusion of said dopants into the SiC layer at said second portion thereof for doping the SiC layer there. 6. A method according to claim 5, characterized in that boron is the dopant supplied for said diffusion. 7. A method according to claim 5 or 6 for producing a semiconductor power device, characterized in that the peripheral regions (5) of the SiC layer are left uncoated through said mask (4) for obtaining diffusion of dopants at the periphery thereof, and that the heat treat¬ ment and thereby the diffusion is carried out under such conditions that the dopants may penetrate through a junction creating the space charge region of said device for creating a guard ring (9) arranged to smoothe out the electric field in the peripheral region in a blocking state of the power device so created. 8. A method according to claim 7, characterized in that said SiC layer is made of at least two sub-layers, namely a first uppermost layer (3) of P-type and a second layer (2) of N-type onto which the first layer is arranged. 9. A method according to any of the preceding claims, characterized in that a step of ion implantation is carried out in a portion (5) of said SiC layer (1-3) not coated by said mask (4) before said heat treatment is carried out, and that said heat treatment constitutes an annealing for re-crystallizing the ion-implanted layer of the SiC layer, said mask (4) preventing impurity diffusion into areas covered thereby from occur¬ ring. 10. A method according to any of the preceding claims, characterized in that it comprises at least the following steps carried out in the following order: 1) deposition of the mask material on the entire surface of the SiC layer, 2) patterning the mask for producing an uncoated second portion of the SiC layer, 3) heat treatment of the SiC layer and 4) removal of the mask in a non-mechanical way. 11. A method according to claim 7 and 9, characterized in that said ion implantation is carried out in said peripheral region (5) of the SiC layer, that ions of the same type, P or N, as the dopants to be diffused into the SiC layer are implanted while creating a thin highly doped layer (7) close to the surface of the SiC layer and then first an annealing heat treatment and then a heat treatment combined with a supply of dopants for guard ring formation is carried out, the diffusion for said guard ring formation being enhanced by the existence of said highly doped thin layer. 12. A use of a layer of a material having crystalline AIN as only component or AIN as a major component of crystalline alloy constitut- ing said material as a mask (4) against diffusion into areas coated thereby when a diffusion step is carried out into a SiC layer in the pro¬ duction of a semiconductor device having a semiconductor layer (1-3) of SiC. 13. A use of a layer of a material having crystalline AIN as only component or AIN as a major component of crystalline alloy constitut¬ ing said material as a mask (4) for preventing diffusion of impurities from taking place into SiC layer areas coated thereby when a SiC layer is annealed after ion implantation in the production of a semi¬ conductor device having a semiconductor layer of SiC.

Inventors: HARRIS CHRISTOPHER, JANZEN ERIK, KONSTANTINOV ANDREI, HARRIS, CHRISTOPHER, JANZEN, ERIK, KONSTANTINOV, ANDREI
Applicants: ABB RESEARCH LTD, HARRIS CHRISTOPHER, JANZEN ERIK, KONSTANTINOV ANDREI, ABB RESEARCH LIMITED, HARRIS, CHRISTOPHER, JANZEN, ERIK, KONSTANTINOV, ANDREI
Application year: 1996
Main classification: H01L21
IPC codes: H01L21, H01L29