Active solid-state devices (e.g. – transistors – solid-state diode – Schottky barrier
Reexamination Certificate
2000-08-23
2002-07-30
Flynn, Nathan (Department: 2826)
Active solid-state devices (e.g., transistors, solid-state diode
Schottky barrier
C257S109000, C257S149000, C257S409000, C257S472000, C257S480000, C257S481000
Reexamination Certificate
active
06426540
ABSTRACT:
In semiconductor components having at least one blocking p-n junction, the latter appears on the surface of the substrate, on which and in which the semiconductor component is realized, somewhere between the live contacts. In said surfacing areas high electric field strengths, in case the p-n junction is blocking, i.e. a higher voltage is applied to the cathode than to the anode or a controllable semiconductor device is not yet connected through its gate terminal, result in undesired leaking currents flowing between the anode and the cathode, which currents are designated as positive or negative reverse bias current depending on the direction of polarization of the voltage to be blocked or reversed. For reducing that portion of the reverse bias currents, which is caused by high field strengths in the termination portion, so-called “junction terminations” are employed in the prior art, which, for example in the form of specially formed field plates, cause an optimized course of equipotential lines and thus avoid high field strengths in the termination portion of such components, cf. DE-A 195 35 332 (Siemens), column 3, line 58 to column 4, line 35; or “Multistep Field Plates . . .”, IEEE Transactions on electron. devices, Vol. 39, No. 6, June 1992, from page 1514 onwards; “The Contour of an Optimal Field Plate”, IEEE Transactions on electron. devices, Vol. 35, No. 5, May 1988, from page 684 onwards; and finally “Theoretical Investigation of Planar Junction Termination”, Solid-State Electronics, Vol. 39, No. 3, pages 323 to 328, 1996. The planar junction terminations described therein are optimized with regard to their geometry, on the one hand as a field plate having steps and on the other hand as an optimized steadily curved field plate having a modified elliptical geometry. However, the prior art has not yet succeeded in economically fabricating the optimized geometric structure of a field plate in the termination portion of a semiconductor component capable of blocking, especially of such a component to which a high voltage of more than 500 V can be applied.
It is the object of the invention to fabricate the aforementioned semiconductor components, which are in particular highly blocking or capable of blocking, on an economic basis, i.e. at low costs, and nevertheless utilize their maximum blocking capacity.
This is achieved by the invention, if a termination portion of the inner anode metallic coating comprises an insulator profile, having a shape which begins flat and is curved outwards and upwards in a steadily increasing manner, which portion is the “curved portion” of the insulator profile, and having a “base portion”, which is located directly adjacent thereto and is virtually planar, said base portion together with the curved portion determining the cross-section of the insulator profile.
The insulator profile is designed such that, between the curved inner metallic coating, outwardly extending the anode, and the outer metallic coating, located outwards of and adjacent to the base portion of the insulator profile, which will in most cases be the cathode, peak values of an electric field generated during operation can be avoided. The insulator profile is produced by a method, in which an at first deposited insulator layer having a thickness is additionally covered with a resist layer over the entire substrate, which resist layer is illuminated through a mask in a structured manner, which mask changes in its gray-tone value in accordance with the desired course of curvature in the curved portion of the respective insulator profile. The gray-tone value in the mask is transferred into the resist layer by exposure, which layer can be structured subsequent thereto, especially by developing, in order to then transfer the structure of the developed resist layer into the insulator layer having a thickness by an etching process, such as RIE (reactive ion etching), wherein it is an advantage if the etching rate of the insulator layer and the etching rate of the resist remainders, remaining after developing the exposed resist layer, are about equal in order to prevent a not-to-shape transfer of the resist profile into the insulator.
The resulting insulator profiles can either surround the anode in the form of a wall or a plurality of insulator profiles may be provided which are arranged in an outwards staggered manner and the curved surface of which is differently shaped. If a plurality of staggered insulator profiles is provided (claim
2
, claim
3
), the curvature of the surfaces of the curved portions is not equal, but steadily increases with each profile being located further outwards (claim
3
).
On the mentioned respective curved surfaces metallic coatings are deposited, which, for the insulator profile outwardly adjoining the inner anode, conductingly pass over into the anode metallic coating.
The structuring, which is coded in its gray-tone value, is performed, during exposure, such that a desired light-intensity profile is coded into the mask by the semitone process, i.e. via a pixel screen, and that the pixel sizes are transferred below the resolving limit of a reducing projection exposure in an almost continuous course of exposure of the resist layer, by which it is thus possible to produce continuous surfaces curving outwards and upwards; the insulator profiles formed according to the invention thus have at least one continuous surface (without steps) steadily extending across a substantial area, which surface is designed in a manner which theoretical calculations for an optimized course of flux lines, when a reverse bias voltage load or conducting-state blocking load is applied, imply to be favorable.
By use of the invention the thickness of the insulator layer can be continuously varied in a process in a predetermined and controlled manner over a wide area of up to 10 &mgr;m; it is not necessarily required to give the surface curvature an ideal course as long as it is ensured that the substantial increases of field strength can be avoided and that the reverse bias voltage load at the termination of the anode towards the cathode does not include substantial peak values.
Even with semiconductor components having reverse bias voltages of more than approx. 500 V, the theoretically maximally possible reverse bias voltage can almost be achieved at minimum space requirements for the junction termination, i.e. the “blocking capacity” of the occupied space can be fully utilized. The minimum space requirements are important in said components, a plurality of which is produced from one wafer, and wherein utilizing the blocking capacity to a maximally possible extent becomes the more important the higher the reverse bias voltages are. Said aspects are of particularly great importance for highly blocking IGBTs.
Even with Schottky diodes, which are not based on a p-n junction, but which utilize the blocking capacity of a metal-semiconductor junction, the insulator profiles produced according to the invention may be employed in an advantageous manner. At the edge of the metal-semiconductor junction the small effective radii of curvature would result in excessive field increases. For preventing said increases, diffused guard rings are employed in the prior which, however, at strong conducting-state loads, cause an undesired injection of minority charge carriers. By use of the insulator profiles and field plates produced according to the invention such an injection of charge carriers does not occur and the diffusion process during production can be omitted.
The use of the invention, which is limited to measures performed at the surface of the semiconductors, is particularly advantageous even if the power semiconductors are to be improved on the basis of silicon carbide (SiC), as an example for a semiconductor having a high band width (claim
12
). In said semiconductors a very low diffusion constant for doting substances must be put up with and this being the reason why termination portions can virtually not be produced by diffusion.
The component, with the insulator profiles prod
Dudde Ralf-Ulrich
Nagel Detlef
Reimer Klaus
Sittig Roland
Wagner Bernd
Duane Morris LLP
Flynn Nathan
Forde Remmon R.
Fraunhofer-Gesellschaft zur Foerderung der angewandten Forschung
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