SiC SEMICONDUCTOR DEVICE COMPRISING A PN JUNCTION

Abstract: A semiconductor device of planar structure, consisting of doped silicon carbide (SiC), comprising a pn junction, formed of a first conducting type layer (1) and on top thereof of a second conducting type layer (2), the edge of the second of said layers being provided with an edge termination (JTE) (3), enclosing stepwise or continuously decreasing effective sheet charge density towards the outer border of the termination, where the pn junction and its JTE are covered by a third layer (4).

Description: SiC semiconductor device comprising a pn junction TECHNICAL FIELD The invention relates to a semiconductor device of planar structure, comprising doped silicon carbide (SiC) and exhibiting a pn junction, formed of a first conducting type layer and a second conducting type layer, the edge of at least one of said layers being provided with an edge termination (JTE), enclosing stepwise or continuously decreasing effective sheet charge density towards the outer border of the termination. BACKGROUND ART Semiconductor devices based on silicon carbide as base material are continuously developed to be used in connection with high temperatures, high power applications and under high radiation conditions. Under such circumstances conventional semiconductors do not work satisfactorily. Evaluations indicate that SiC semiconductors of power MISFET-type and diode rectifiers based on SiC would be able to operate over a greater voltage and temperature interval, e.g. up to 650 - 800 °C, and show better switching properties such as lower losses and higher working frequencies and nevertheless have a volume 20 times smaller than corresponding silicon devices. These possible improvements are based on the favorable material properties that silicon carbide possesses in relation to silicon, such e.g. a higher breakdown field (up to 10 times higher than silicon), a higher thermal conductivity (more than 3 times higher than silicon) and a higher energy band gap (2.86 eV for 6H-SiC, one of the crystal structures of SiC). SiC semiconductor technology is relatively young and in many aspects immature. There are many critical manufacturing problems that are to be solved until SiC semiconductor devices may be realized experimentally and large scale manufacturing may become a reality. This is especially true of devices intended for use in high-power and high-voltage applications. One of the difficulties to overcome when manufacturing high voltage diodes or other types of semiconductor devices comprising a voltage absorbing pn junction is CONFIRMATION COPY to produce a proper junction termination at the edge of the junction. The electric field at the periphery of the junction is normally enhanced compared to the electric field in the bulk of the junction. This field increase at the periphery of the junction may be further reinforced in the presence of surface charge. A high electric field at the edge of the pn junction implies a great risk of voltage breakdown or flash-over at the edge of the junction as well as giving rise to an instability of the blocking voltage known as voltage drift. To avoid the above-mentioned disadvantages it becomes very important to reduce the field concentration, where the junction reaches the surface. Combined with efforts to passivate the surface of the device, measures are taken to flatten out the electric field at the surface e.g. by acting on how the pn junction emerges at the surface. As an example it is known from silicon power devices to lap (grind, sandblast, etch) the surface of the edge to a certain angle in relation to the pn junc- tion to thereby flatten out the field. Another known technique is to gradually decrease the charge content on the highly doped side of the junction, in such a way that the charge content of the highly doped layer is reduced towards the outermost edge of the junction, a so called Junction Termination Extension, (JTE). The methods, known from silicon technology, used to achieve a JTE of an Si device cannot always be directly transferred to devices with SiC as base material, primarily due to the great hardness of SiC and extremely low diffusivity of proper SiC dopants. Ion implantation of doping elements, a common technique when manufacturing Si devices, is difficult to master and not yet fully developed for SiC but is still perhaps the most promising method for doping of SiC. However, ion implantation introduces damage to the SiC crystal, which is a factor to be considered. High voltage diodes from 6H-SiC with epitaxially formed pn and Schottky junctions have been made experimentally (see e.g. M. Bhatnagar and B. J. Baliga, IEEE Trans. Electron Devices, Vol. 40, No. 3 pp 645 - 655, March 1993 or P. G. Neudeck, D. J. Larkin, J. A. Powell, L. G. Matus and C. S. Salupo, Appl. Phys. Lett. Vol 64, No 11, 14 March 1994, pp 1386-1388). Some of the problems related to SiC devices have thus been solved, but no reliable solution to the problems connected with electric field concentration at the edges of the junction has been presented as yet. Any method or device to accomplish a semiconductor device corresponding to the principle of Junction Termination Extension at a pn junction composed of Si is not publicly known for a device, where SiC constitutes the base material of the junction. Solutions for arriving at SiC devices comprising pn junctions with planar JTEs are described in the unpublished patent application US 08/683 059, which is hereby included in this description by reference. The solutions described there involve stepwise decreasing charges of the JTE towards the edge by ion implantation of concentric regions, also called zones, of the JTE in order to control the surface doping and surface fields. A consecutive passivating layer has to be grown or deposited on a surface which is structurally damaged by the ion implantation. DESCRIPTION OF THE INVENTION The present invention aims at describing a JTE structure of an SiC device where the pn junction and its JTE has a planar structure and where the described problems are avoided. Another aim of the invention is also to minimize the electric field at the surface of the component. A low lateral electric field at the interface of a device surface to a passivation layer is vital to the proper function of the device. Unnecessary electric field peaks may be detrimental to the short- and long-term stability of the device. This aim is reached by providing a semiconductor device of the type defined in the introduction in which the pn junction and its JTE are covered by a third layer. This layer can be a layer formed or applied by any arbitrary method known in the art and suitable for the type of layer desired but is according to one preferred embodiment an epitaxially grown layer. One aspect of the invention is composed of a planar structure semiconductor device which comprises a pn junction. Both a p-conducting layer and an n- conducting layer forming the pn junction consist of doped silicon carbide. The edge of the higher doped conducting layer of the pn junction exhibits a charge profile with a stepwise or uniformly decreasing effective sheet charge density. The effective sheet charge density is radially decreasing from an initial value at the periphery of a main junction area to a zero or almost zero effective sheet charge density at the outermost edge of a junction extension. Characteristic of the invention is that the mentioned pn junction with its junction termination extension is buried under a third layer, which might be a layer of doped or undoped SiC. This third layer is preferably an epitaxially grown layer. The effective sheet charge density is equal to the integral of the volume doping concentration over the thickness of the JTE zone or third layer. In the case of constant doping concentration, the effective sheet charge density is equal to the product of the doping concentration and the thickness of the doped layer. By manufacturing a device with the features as specified in claim 1 a damage free third layer, e.g. an epitaxial layer is provided for the passivation step in the manufacturing of, e.g., a transistor. The third layer will result in lower peaks in the electric field and reduced stress on the passivation layer. Characteristic of the invention is that the third layer may be either p-conducting or n-conducting and according to one preferred embodiment epitaxially grown. Another aspect of the invention is that the third layer may itself be used as a zone of the JTE as specified in claim 4. Thus, the number of process steps may be minimized for a given reduction of the electric field. Further aspects of the invention include the fact that a number of zones may be ion implanted into the third layer as specified in claim 5. Yet another aspect of the invention is that the device may be covered with a passivating layer. When manufacturing e.g. a MISFET, JFET, FCT or an IGBT, the third layer may be used as the drift region, the base region or the channel region, depending on device type. Other aspects of the invention further comprise methods for producing the above-mentioned SiC semiconductor device. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows a planar pn junction with ion implanted zones as JTE of the device. The pn junction and its JTE are buried under a third layer, in the figure exemplified as an epitaxial layer, being of the same conducting type as the JTE. Figure 2 shows a planar pn junction with ion implanted zones as parts of the JTE of the device. The pn junction is buried under a third layer, in the figure exemplified as an epitaxial layer, being of the same conducting type as the JTE, where the third layer serves as the outermost zone of the JTE. Figure 3 shows a planar pn junction with ion implanted zones as parts of the JTE of the device. The pn junction is buried under a third layer being of the same conducting type as the JTE. The outermost zone of the JTE is implanted into the third layer. Figure 4 shows a device similar to the one in fig. 1, where the third layer is of the same conducting type as the substrate. Figure 5 shows a device similar to the one in fig. 2, where the third layer is of the same conducting type as the substrate. Figure 6 shows a device similar to the one in fig. 3, where the third layer is of the same conducting type as the substrate. Figure 7 shows the electric field at the surface of a device, as a function of the distance from the pn junction, without the third layer. Figure 8 shows the electric field at the surface of a device, as a function of the distance from the pn junction, for a two-zone JTE when a third layer, e.g. an epitaxially grown layer, is present. Figure 9 shows the electric field at the surface of a device, as a function of the distance from the pn junction, for a three-zone JTE when the third layer constitutes the third zone. DESCRIPTION OF EMBODIMENTS The invention will be described in a number of embodiments with reference to the drawings. Figure 1 illustrates a planar device having a JTE buried under third layer, such as but not limited to an epitaxial layer, according to the invention. The semiconductor device is exemplified by a pn junction, where silicon carbide constitutes the base material. The manufacturing of a planar JTE, as described below, is not new. The planar JTE as described below is included in the description of the invention so as to clarify the use of the invention. The devices shown in figures 1 and 4 exemplify a first embodiment of the invention. The device in figure 1 is built on a substrate 1 consisting of doped n- conducting SiC. A highly doped p-conducting zone 2 is established employing ion implantation. As implants aluminum, boron or gallium can be used. The n- doped substrate 1 with its p-conducting zone 2 compose a pn-junction where zone 1 constitutes the first layer and zone 2 is a part of the second layer. In a second stage, a JTE 3 is formed employing masking of concentric zones followed by ion implantation. A first zone 3a of the JTE is formed by masking the entire area of the substrate outside zone 3a. During the following ion implantation, zone 3a will become p-doped and zone 2 will become more highly doped. In the next step, everything outside zone 3b is masked and the highly doped zone 2, zone 3a and zone 3b are ion implanted with the result that the resulting doping concentration of zone 3a will be higher than in zone 3b. The doping concentration of zone 3a is a result of two consecutive ion implantations. Further zones of the JTE extension 3c may be formed in consecutive steps by masking everything outside zone 3c followed by ion implantation. Alternatively, a reversed method may be employed. Thus, in a first stage the entire area of the JTE is left unmasked and is ion implanted with a dose equal to the final dose of the outermost zone 3d. In the next step, everything but the outermost zone is being left unmasked and is ion implanted so that the total dose is equal to the final dose in the outermost zone but one. The masking is then repeated for all of the zones of the JTE. In an alternative embodiment, the substrate 1 may be p-conducting and the implanted zone 2 with its JTE n-conducting. The resulting charge density of the different zones of the JTE will be decreasing, in a direction from the pn junction to the edge of the device, from an initial value at the pn junction. At the zone boundaries of the JTE, the electric field will reach peak values during operation. The height of these peaks is dependent on the number of zones used and will decrease with an increasing number of zones. In the example above, the JTE consisted of 4 zones. The number of zones is ultimately dependent on the allowed number of process steps. After the JTE has been formed, a third layer p-doped layer 4, is grown on top of the device, suitably the third p-doped layer 4 is an epitaxially grown layer. As an alternative, the third layer may consist of n-doped material as shown in figure 4 and it might be formed or otherwise applied by any arbitrary method considered suitable in the trade. One of the aspects of the invention is that a damage-free layer is supplied for the passivation step in which an optional passivating layer 5 is formed on the third layer 4, this third layer 4 is preferably an epitaxially grown layer. The passivating layer 5 may consist of, e.g., Si0 2 or A1N. A second preferred embodiment of the invention is exemplified in figure 2. In this embodiment the area extension of the JTE is smaller than the extension of the third layer covering zone 2 and its JTE 3a-3c. In forming the zones 3a-3c, the same methods as described above may be employed. The third layer, having a lower concentration of doping atoms than the last implanted zone of the JTE 3, itself serves as the outermost zone of the junction termination extension. In this embodiment the number of process steps is decreased compared with the first embodiment. This embodiment of the invention can also incorporate the passivating layer 5, for further reduction of the electric field, as described above. In this second embodiment, the third layer may consist of either n-doped or p- doped SiC and be an epitaxially grown layer or layer formed or otherwise applied by any arbitrary method considered suitable in the trade. A third embodiment of the invention can be exemplified with reference to figures 3 and 5. A device according to this embodiment can be arrived at by starting with the same steps as described for the second preferred embodiment. After the growth or formation of the third layer, a last zone 3d of the JTE is formed by ion implantation, of a dopant of the same conducting type as in zones 3a-3c, into the epitaxial layer. In this third embodiment, the third layer may consist of either n- doped or p-doped SiC and be an epitaxially grown layer or layer formed or otherwise applied by any arbitrary method considered suitable in the trade.. The pn-junction and its JTE in the examples above may be a part of a buried grid in any of an MISFET, an IGBT, a JFET or an FCT where the third layer constitutes a part of the drift region of said component. The pn-junction and its JTE in the examples above may be a part of a buried grid in any of a JFET or an FCT where the third layer constitutes a part of the channel region of said component. Alternatively, the pn junction and its JTE in the examples above may be a part of a buried grid in any of an MISFET or an IGBT and that the third layer constitutes a part of the base region of said component. In the figures the area extension of the device is terminated by etching. A device according to the invention may also be terminated by an implanted channel stopper. Figure 7 shows the electric field at the surface of a device, as a function of the distance from the pn junction, with a JTE as shown in figure 2, having only 2 zones, without the epitaxially grown layer 4. Figure 8 shows the electric field at the surface of the device, as a function of the distance from the pn junction, for a JTE according to Figure 5 having only 2 zones, when a third layer 4, such as an epitaxially grown layer, of the opposite conducting type as the JTE, is present. It is evident from the figures that the third layer reduces the height of the peaks in the electric field at the surface of the device. Figure 9 shows the electric field at the surface of the device, as a function of the distance from the pn junction, for a JTE according to Figure 2 having only 3 zones, when the third layer, being of the same conducting type as the JTE, constitutes the third zone of the JTE. Generally, for a four-zone JTE the charge densities may be varied according to the intervals of the relation Qi:Q 2 :Q3:Q4 = (50-85):(30-60):(15-40):(5-25) where Qi denotes the effective sheet charge density of the innermost zone of the JTE, Q2 the next to the innermost zone 3b and so on. The value 100 corresponds to a doping such that the zone with this charge density is completely depleted at full design voltage. This charge density is called the characteristic charge density Qo and is expressed in Coulombs/cm^. With other alternative number of zones: three zones: Qi:Q2:Q3 = (40-85): (25-50): (5-35). two zones: Qi:Q2 = (30-85): (5-50). one zone: Qi = (30-80). For a device with more than 4 zones in the JTE, the ranges as stated above may serve as a guidance. The doping of the highly doped area of the pn junction being surrounded by the JTE may be several times higher than the characteristic effective sheet charge density Qθ- The effective sheet charge density is equal to the integral of the volume doping concentration over the thickness of the JTE zone or third layer. In the case of constant doping concentration, the effective sheet charge density is equal to the product of the doping concentration and the thickness of the doped layer. The term SiC is used in the following text to refer to any of the principal crystal polytypes of this material known as 6H, 4H, 2H, 3C and 15R.

Claims: CLAIMS 1. A semiconductor device of planar structure which comprising doped silicon carbide (SiC), exhibiting a pn junction, formed of a first conducting type layer and on top thereof a second conducting type layer, the edge of the second of said layers being provided with an edge termination (JTE), enclosing stepwise or continuously decreasing effective sheet charge density towards the outer border of the termination, characterized in that the pn junction and its JTE are covered by a third layer. 2. A semiconductor device according to claim 1, characterized in that the third layer is of the first conducting type. 3. A semiconductor device according to claim 1, characterized in that the third layer is of the second conducting type. 4. A semiconductor device according to any of claims 1,2 or 3, characterized in that the third layer extends, in a direction radially outwards from the center of the pn junction, beyond the extension of the JTE in the second layer, and that the third layer itself serves as the outermost zone of the JTE. 5. A semiconductor device according to any of claims 1,2 or 3, characterized in that the third layer extends, in a direction radially outwards from the center of the pn junction, beyond the extension of the JTE in the second layer, and that there is implanted a last zone of the JTE in the third layer. 6. A semiconductor device according to any of claims 1 to 5, characterized in that the surface of the device is covered with a passivating layer (5). 7. A semiconductor device according to any of claims 1 to 5, characterized in that the pn junction and its JTE are a part of a buried grid in any of a MISFET, an IGBT, a JFET or an FCT and that the third layer constitutes a part of the drift region of said component. 8. A semiconductor device according to any of claims 1 to 5, characterized in that the pn junction and its JTE are a part of a buried grid in any of a JFET or an FCT and that the third layer constitutes a part of the channel region of said component. 9. A semiconductor device according to any of claims 1 to 5, characterized in that the pn junction and its JTE is a part of a buried grid in any of an MISFET or an IGBT and that the third layer constitutes a part of the base region of said component. 10. A semiconductor device according to any of preceding claims, characterized in that the third layer (4) is an epitaxially grown layer. 11. Method of manufacturing a semiconductor device comprising a planar structure pn junction, starting from an object having on top of each other a lower doped first conducting type silicon carbide layer (2) and a higher doped second conducting type silicon carbide layer (3) forming the pn junction, the edge of the higher doped layer being provided with an edge termination, characterized in that a third layer of SiC is formed on top of the JTE. 12. Method according to claim 11, characterized in that said third layer is formed on top of the JTE and on an extension of the second layer radially, outwards from the center of the pn junction, beyond the JTE in the second layer. 13. Method according to claim llor 12, characterized in that said third layer is epitaxially grown on top of the JTE.

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 LTD., BAKOWSKI, MIETEK, GUSTAFSSON, ULF, HARRIS, CHRISTOPHER
Application year: 1998
Main classification: H01L29
IPC codes: H01L29