Applicant : ABB RESEARCH LTD A METHOD FOR PRODUCING A PN-JUNCTION TECHNICAL FIELD OF THE INVENTION AND PRIOR ART The present invention relates to a method for producing a pn- junction in a semiconductor layer of crystalline SiC for a semi- conductor device, in which a sub-layer of an n-type doped semi- conductor layer of crystalline SiC is provided with acceptors in the form of boron atoms for making this sub-layer p-type doped, and in which said sub-layer is provided with a doping through bo- ron atoms by supplying boron to the surface of said semicon- ductor layer and heating this layer at a temperature above 1400°C for a sufficient time for diffusion of boron atoms into said semiconductor layer for forming a p-type doped sub-layer the- rein, as well as a semiconductor device produced by using such a method.

Such a method may be used for producing any type of semicon- ductor devices having a pn-junction, for instance diodes, thyris- tors and IGBT-s."Crystalline"also includes a poly-crystalline structure, in which larger regions of the material are mono-crys- talline. However, it is mostly preferred that a mono-crystalline structure having no defects is provided, although that is in the practice not achievable.

Semiconductor devices of SiC are in particular interesting for ap- plications in which it is possible to benefit from the superior properties of SiC in comparison with especially Si, namely the capability of SiC to work well under extreme conditions. For in-

stance, the large bandgap between the valence band and the conduction band of SiC makes devices fabricated from said ma- terial able to operate at high temperatures, namely up to 1000 K.

SiC has also a high breakdown field strength, so that compara- tively thin layers of SiC may hold very high voltages in a blocking state of a semiconductor device of SiC. However, the"tough- ness"of SiC also causes problems and put particular demands on the processing techniques used for producing semiconductor devices of SiC.

Two ways of producing a p-type doped sub-layer of said type for producing a pn-junction in a semiconductor layer of crystalline SiC have primarily been used so far. The first consists in the introduction of p-type dopants into the crystal during the epitaxial growth thereof, preferably by Chemical Vapour Deposition.

However, it has turned out that this option results in a crystal structure associated with on-state characteristics and blocking capabilities of the device desired to be improved. The other way to proceed involves implantation of the acceptors into the semi- conductor layer of crystalline SiC by using very high acceleration energies. A high temperature anneal is then required for making the dopants electrically active, but this also can result in an outdiffusion of the dopants from the SiC surface severely chang- ing the implantation profile. Furthermore, a damage of the crystal is caused by the implantation.

A method of the type defined in the introduction is already known through US patent 5 654 208. Although the method disclosed there enables a production of a pn-junction of a semiconductor device with a very high quality making it substantially improved with respect to a device produced according to any of the two previous techniques, there is a desire to simplify the method by reducing the number of process steps for making it more cost- efficient and commercially interesting.

SUMMARY OF THE INVENTION The object of the present invention is to provide a method of the type defined in the introduction making it possible to produce a pn-junction in a semiconductor layer of crystalline SiC and especially, but not exclusively, a diode of SiC while satisfying said desire.

This object is according to the invention obtained by providing method which is started by etching a trench into said semi- conductor layer of crystalline SiC followed by a supply of boron atoms to the surface of said semiconductor layer and said heat- ing step for forming the pn-junction, a contact layer is applied over the entire surface of said semiconductor layer formed by said sub-layer, and finally said semiconductor layer is polished for removing such a thick layer thereof that the contact layer and the p-type sub-layer disappear except for in said trench for forming a restricted pn-junction in said semiconductor layer. This results in a very simple and cost efficient process for obtaining a pn-juncion in SiC and primarily a diode in SiC and in a"curved" pn-junction, which will sometimes be preferred. Another advan- tage of this method is that polishing may sometimes be a very favourable and from the process technique point of view simple process step for removing excess layers.

The invention also relates to a method in which it is started by etching a trench into said semiconductor layer of crystalline SiC followed by a supply of boron atoms to the surface of said semi- conductor layer and said heating step for forming the pn-junction, a contact layer is provided over a restricted surface region of said semiconductor layer including said trench, and said sub- layer is etched away around said contact layer for forming a pn- junction substantially restricted to the area under said contact layer for creating a discrete semiconductor device. This also con- stitutes a very simple and cost efficient way to produce pn- junctions in SiC.

The invention also relates to a method in which a contact layer is applied on a restricted surface region of said semiconductor layer after said heating step, and the p-type sub-layer created by said diffusion is etched away around said contact layer for sub- stantially restricting said sub-layer to the region under said con- tact layer for creating a discrete semiconductor device. This con- stitutes a simple and cost efficient way to produce a discrete semiconductor device by using few process steps.

According to a preferred embodiment of the invention the time of the duration of said heating is controlled for controlling the diffu- sion depth of boron atoms and thereby the distance of the pn- junction defined by the thickness of said sub-layer to the surface of said semiconductor layer. Another preferred embodiment pro- vides for a control of the temperature for controlling the same pa- rameter, and it is of course possible to control said time and the temperature simultaneously. This is preferably combined with a control of the supply rate of boron to said semiconductor layer of crystalline SiC, for instance by controlling the partial pressure of boron gas in an atmosphere surrounding said semiconductor layer when arranging this in an owen or furnace and creating an atmosphere with a boron content in the inner space of the owen.

By controlling these different parameters it is possible to obtain a pn-junction at a desired distance from the surface of the semi- conductor layer and accordingly a desired thickness of the p-type layer, as well as a preferred doping concentration of the p-type layer. It is also possible to coat said semiconductor layer of SiC by a material containing boron as doping source.

According to a preferred embodiment of the invention said heat- ing is carried out at a temperature of 1500-2500°C. Such high temperatures may be used for said diffusion, since SiC may with- stand these temperatures, and the diffusion speed will increase with the temperature. It has turned out that this diffusion speed will be really commercially interesting when the temperature is above 1800°C.

According to another preferred embodiment of the invention said heating is carried out for a time period of 1-4 hours. It has turned out that this time period for said heating is suitable for obtaining the thickness of said p-type layer normally aimed at for semicon- ductor devices of SiC by using the temperature mentioned above.

This thickness is according to another preferred embodiment of the invention 0,5-2 p. m, which is suitable for semiconductor de- vices of SiC.

According to another preferred embodiment of the invention said pn-junction is produced for a semiconductor device in the form of a pn-diode. This method is particularly adapted for the production of such semiconductor devices.

The invention also relates to a semiconductor device of SiC pro- duced by using a method according to any of the method claims of the present invention. The features and the advantages of such a device appear from the discussion above.

Further advantages and advantageous features of the present invention appear from the following description and the other de- pendent claims.

BRIEF DESCRIPTION OF THE DRAWINGS With reference to the appended drawings, below follows a spe- cific description of methods according to preferred embodiments of the invention cited as examples.

In the drawings: Fig 1 is an enlarged very schematic view illustrating how boron atoms are deposited on a semiconductor layer of crystalline SiC in a method according to a preferred embodiment of the inven- tion,

Figs 2-4 illustrate different steps of a method for producing a pn- junction in a semiconductor layer of crystalline SiC for a semi- conductor device according to a first preferred embodiment of the invention, Figs 5-8 illustrate different steps of a method according to a sec- ond preferred embodiment of the invention, and Figs 9-12 illustrate different steps of a method according to a third preferred embodiment of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION It is very schematically illustrated in Fig 1 how an atmosphere with boron content is provided inside an owen or a furnace above a semiconductor layer 1 of crystalline SiC being n-type doped.

The SiC may be of any polytype and is preferably of 6 H or 4 H, but also other polytypes as 3 C are conceivable. The boron atoms 2 are very schematically indicated. The atmosphere and thereby also said semiconductor layer 1 is heated to a high tem- perature, namely above 1400°C, and preferably above 1800°C.

The upper limit for said heating is 2500°C. The boron atoms will deposit on the surface 3 of the semiconductor layer 1 and mi- grate into the layer 1 as indicated by the arrows 4. The layer 1 has a moderate doping concentration of 5x1013-5x10"cm-3, pref- erably of nitrogen (N). This nitrogen has been introduced during the epitaxial growth of the semiconductor layer 1. Thus, it is nec- essary to provide a higher concentration of acceptors in a sub- layer of the semiconductor layer 1 for turning this sub-layer to p- type doped. The doping concentration of a sub-layer produced in this way may be influenced by controlling the partial pressure of boron in said atmosphere and thereby the surface concentration of dopants on the surface 2 of the layer 1. The diffusion process may also be influenced by adjusting the temperature inside the owen and the period of time for which the diffusion process is carried out.

It is illustrated in Fig 2 how a p-type sub-layer 5 has been pro- duced in this way in said first layer 1. The thickness of the sub- layer 5 is preferably 0,5-2 j. m, which may be obtained by heating the layer 1 and the atmosphere above 1500°C for 1-4 hours. The doping concentration of boron in said sub-layer 5 is preferably 5x10"-1020cm-3. It is illustrated how said first semiconductor layer also comprises a highly doped n-type layer 6 on the oppo- site side thereof with respect to the sub-layer 5 for forming a good contact to the contact layer of the device. The sub-layer 7 of the first layer 1 separating the layers 5 and 6 is preferably much thicker, for instance about 40, um.

A contact layer 8 is then applied on the sub-layer 5 followed by a mesa etching of said sub-layer 5 away around the contact layer for substantially restricting the sub-layer 5 to the region under said contact layer for created a discrete semiconductor device, namely in the form of a pn-diode as shown in Fig 4. It is obvious that the process for producing this diode also comprises other steps, which, however, will not be explained here, since they have nothing to do with the present invention. A contact layer is for instance to be applied next to the sub-layer 6. This device will have a very well ordered structure in the layers thereof, espe- cially close to the pn-junction reducing the conduction losses thereof in the on-state and any leakage current in the blocking state of the device and raising the breakdown voltage of the de- vice.

Figs 5-8 illustrate a method according to another very advanta- geous embodiment of the present invention according to which in a first step a mesa etching of said semiconductor layer 1 is car- ried out for forming a trench 9 therein. This is followed by a diffu- sion step as described above for forming a sub-layer 5. A contact layer 8 is then applied over the entire sub-layer 5. Finally, it is illustrated in Fig 8 how the structure according to Fig 7 is pol- ished for removing such a thick layer thereof that the contact

layer and the p-type sub-layer disappear except for in said trench for forming a restricted pn-junction 10 in the semiconductor layer.

A method according to another preferred embodiment of the in- vention is illustrated in Figs 9-12, and this differs from the one according to Figs 5-8 by an application of the contact 8 only over a restricted surface region of the sub-layer 5. This is followed by a mesa etching for etching the sub-layer away around the contact layer for forming the pn-junction substantially restricted to the area under said contact layer for creating a discrete semicon- ductor device as shown in Fig 12.

A semiconductor device of this type is particularly advantageous when high powers and/or high voltages and/or high currents are to be handled, since it is then possible to fully utilize the physical properties of SiC. A diode of the type illustrated in the Figures may preferably be used in for instance current valves in con- verter stations as free-wheeling diode reducing the number of such diodes to be connected in series for holding a certain volt- age in comparison to the use of diodes of for instance Si. Such a voltage may then well be in the order of 100-500 kV, and the diode may preferably hold a voltage above 2 kV, and most pre- ferred above 10 kV.

The methods described above and illustrated in the figures make it possible to produce especially pn-diodes of SiC through a very simple manufacturing process involving a lower number of masking steps than standard methods known so far.

The invention is of course not in any way restricted to the pre- ferred embodiments described above, but many possibilities to modifications thereof will be apparent to a man with ordinary skill in the art without departing from the basic idea of the invention as defined in the appended claims. It is pointed out that the defi- nition layer is to be interpreted broadly and comprises all types of volume extensions and shapes.

Furthermore,"etching"and"polishing"are to be interpreted broadly and comprise all types of material removal of the respec- tive type."Etching"may for example be etching by gas flows, for instance of H2, or etching by applying chemical solutions upon the layer in question or reactive ion etching. pn-junction and pn-diode are to be interpreted to also covering the case of a very low doping concentration of any of the layers next to the junction, so that this would often in the practice be called intrinsic. The diodes described above may for example just as well be called pin-diodes as a consequence of the low doping concentration of the n-type sub-layer 7.