Abstract:
SiC field-effect transistors with source, gate and drain contacts and in which the source contacts are located on the surface of the semiconductor wafer, the drain contacts on the underside of the wafer and the gate contacts in trench-like structures. The trench-like structures surround the source electrodes of the transistors in the shape of a ring and the gate contacts are connected to each other on the floors of the trenches.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to an arrangement of SiC field-effect transistors and a method of producing them. 
     Because of its large band gap, SiC has proven suitable for high voltage, and different proposals have been made for embodying field-effect transistors for high voltages. 
     In silicon technology, it is known to use the &#34;surface-gate technology&#34; described in the book by B. Jayant Baliga, &#34;Modern Power Devices,&#34; N.Y., 1987. In order to produce the gate regions of power JFETs, it is not particularly favorable to use the planar technique. Regardless of whether the silicon technique or SiC technique is used, an arrangement of the masks which does not provide precise coverage leads to short circuits if the typical dimensions of the components are used. Nevertheless, in order to maintain the necessarily small dimensions of the components for good blocking ability, it is proposed in the above-cited book to sink the gate. As is readily apparent, this arrangement of the gate contact in a vertical trench, into whose walls boron diffuses, has technological advantages. In particular, the attachment of the gate contact by means of metallization is facilitated by the fact that the metal hardly precipitates on the walls, which saves a separate masking step. 
     of course, it is desirable to transfer this technique to SiC material. However, there are various difficulties associated with this. 
     1. The diffusion of the dopant is not possible, because practically no diffusion occurs in SiC at the conventional processing temperatures. 
     2. Because a higher field critical intensity is possible in SiC, the structures can be reduced by a factor of 5 to 10. A corresponding lateral scaling approaches the limits of the masking technique. If the geometrical channel width cannot be scaled down accordingly, the pinch-off behavior worsens. 
     The object of the invention is to provide a structure for field-effect transistors in which a sufficient pinch-off behavior can be achieved in the channel zone by the smallest possible gate voltage. 
     SUMMARY OF THE INVENTION 
     The above object generally is achieved according to the present invention by an arrangement of SiC field-effect transistors having a source contact, a gate contact and a drain contact disposed on a semiconductor wafer, wherein the source contacts are disposed on the surface of the semiconductor wafer, the drain contacts are disposed on the underside of the semiconductor wafer, the gate contacts are disposed in trench-shaped structures, with the trench-shaped structures surrounding the individual source electrodes of the field-effect transistors in ring fashion, and with the gate contacts being connected to one another on the bottom of the trenches. 
     Modifications of the invention and a method of producing the field-effect transistors according to the invention are likewise disclosed. 
     The essential nature of the invention is that the components no longer have a striated structure, but are configured to be rotationally symmetrical. Because of this, the pinch-off of the channel can be effected from all sides of the annular gate contact on the trench bottom, beginning from the side walls. Because diffusion is not possible, the doping is preferably performed by means of ion implantation. This is particularly simple if the mesa-shaped component has inclined side walls. Because of the inclined side walls, doping is easily possible through ion implantation with boron or other trivalent dopants. The mesa-shaped components are switched in parallel in order to attain greater outputs. The gate contacts are guided in the trenches, the source contacting is effected on the top side of the silicon wafer and the drain connection is attached to the underside. 
    
    
     The invention is described in detail below in conjunction with the drawing. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic representation of an embodiment of the components in plan view; 
     FIGS. 2a-2h schematically illustrate the production method in different phases; 
     FIG. 3 shows a simplified structure of the individual component for simulation calculations, and 
     FIGS. 4a and 4b shows the results of simulation calculations for two different geometries. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In a preferred embodiment of the invention, as shown schematically in a transitional stage in FIG. 1, for example, the individual components are hexagonal and have a diameter of, for example, approximately 6 μm. The source contact 1 is raised, and the side walls 2 of the annular trench are inclinded. According to one example, the trenches have side walls 2 which form an angle of 60° to 90° with the horizontal, while according to another example, the side walls 2 of the trenches form an angle of 70° to 85° with the horizontal. The gate contacts 4 on the bottom of the trench 3 surround the individual components and connect them at the same time. 
     The production of the JFET by means of a self-adjusting masking process is described in detail in conjunction with FIG. 2. This method represents a preferred example. However, the underlying processes can also be easily performed in modified form, for example with an SiC or a silicon wafer being used as substrate D. 
     The production process is divided into eight individual steps a through h. An Epi layer A made of SiC and having a thickness of approximately 20 μm is deposited on the substrate D with a doping of 10 16  cm -3 . This substrate preferably comprises a 200-300 μm thick SiC wafer and is doped with a donor of 10 16  cm -3 . The underside of the SiC layer adjacent thereto is metallized, and forms the drain electrode (not shown). Generally the range of thickness of the layer A between the drain D and source S is 3 to 1000 μm. However, in one example the thickness of the layer A between the drain D and source S is in the range of 3 to 100 μm, while in another example the thickness of the layer A between the drain D and source S is in the range of 3 to 25 μm. 
     The layer B is an n doped zone which has been deposited in situ or ion-implanted (N=10 18  cm -3 ). 
     FIG. 2a shows an oxide mask C which is subsequently applied and in this example has a web width of approximately 2 μm, is 1 μm thick and comprises, for example, SiO 2 . The next step is shown in part b of FIG. 2. First, the SiO 2  is structured, which is effected with light underetching of the oxide mask C in accordance with wet-chemical technology. A so-called RIE process of the inclined trenches is effected in the next step by means of a gas mixture. The gas mixture comprises Cl 2  /SiCl 4  /O 2  /Ar, or N 2  instead of Ar. The concentrations are in the sequence of the enumeration: 40/20/4.2/10 sccm. The depth of the trench in the described and illustrated embodiment is approximately 4 to 5 μm. It is therefore approximately twice as large as the diameter. However, the depth of the trench generally may be in the range of 2 to 10 μm, and more specifically the depth of the trench is in the range of 4 to 8 μm. 
     Part c of FIG. 2 shows the p +   ion implantation. This implantation is performed with, for example, Al, B or another trivalent element. The depth is approximately 0.5 μm and is attained through multiple implantation with energies E&lt;500 keV. The implantation can be effected from above, i.e., perpendicular to the surface of the silicon wafer, or at an angle and with continuous rotation. In this instance, the doped region 2 is created in the region of the trench wall, and the region 3 is created in the region of the bottom. 
     The metallization of the gate contacts 4 is shown in part d of FIG. 2. They are produced by sputtering of, for example, Ti, Ni, NiCr or other metallization systems. The production of Schottky contacts is effected simply by not doping the region 2. 
     FIG. 2e illustrates the result of the so-called lift-off process. Here the thin metal layer on the trench walls is also etched away, possibly with an additional etching process. 
     The filling of the trenches with an insulator 5 is shown in the next FIG., 2f. A CVD SiO 2  is typically used for this filling process. 
     In FIG. 2g the wafer is shown following mechanical polishing of the surface of the oxide 5, with the extreme hardness of the SiC providing a natural polishing stop. An etching process may be inserted as an intermediate step in order to prepare the metallic contacting of the next step. If the oxide layer is relatively level and does not project significantly beyond the layer 1 doped for the contact, a method which solely employs etching and does not include polishing is also possible. This layer 1 comprises islands which are created by structuring the layer B. 
     As a last step, a total-surface metallic coating is applied in order to produce the source contact 6. 
     A large-surface parallel connection of the elementary components can be achieved in, for example, a hexagonal structure, as shown in FIG. 1. The gate contacts 4 in the trenches are indicated here in thin lines, and are guided outwardly and on contact paths. The source contacting is effected over the entire surface of the top side of the semiconductor wafers, while the third connection (drain connection) is provided on the underside of the semiconductor wafer. 
     To optimize the different parameters, a computer simulation was performed for a maximum blocking voltage of 2 kV. The electrical characteristic curves of the elementary component shown in FIG. 3 were determined with the aid of a computer simulation. The pinch-off properties, which were improved considerably in a rotationally-symmetrical layout, are shown in FIG. 4. A blocking voltage of 1900 V can be attained in a rotationally-symmetrical structure with a gate voltage of -10 V. As illustrated for striated geometry in the left part of FIG. 4, this does not occur with a gate voltage of -32 V. 
     For drain voltages U D&lt;  3 V, an on-resistance of R on  =0.013 Qcm 2 , which is therefore smaller by a factor of approximately 100 than in an Si component of identical blocking capability, is read out of the simulations. If maximum power density in a range of 100 to 200 W/cm 2  is used as the basis, maximum current densities in a range of 130 to 180 A/cm 2  result.