Semiconductor device manufacturing: process – Making field effect device having pair of active regions... – Having insulated gate
Reexamination Certificate
2003-03-10
2004-12-21
Elms, Richard (Department: 2824)
Semiconductor device manufacturing: process
Making field effect device having pair of active regions...
Having insulated gate
C438S268000, C257S341000
Reexamination Certificate
active
06833298
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The present invention relates to a method for fabricating a semiconductor component having at least one transistor cell and an edge cell arranged adjacent to the transistor cell.
It is sufficiently known for power MOSFETs to be constructed in a cellular fashion. That is to say, to provide a multiplicity of transistor structures arranged next to one another, which in each case form a transistor. These transistors are connected in parallel to outwardly form the power MOSFET, whose current-carrying capacity substantially depends on the number of transistor structures connected in parallel. The cellular construction of a vertical power MOSFET is described, for example, in Stengl/Tihanyi: “Leistungs-MOSFET-Praxis” [“power MOSFETs in practice”], Pflaum Verlag, Munich, 1992, pages 33, 34.
An n-conducting power MOSFET has an n-doped semiconductor body into which, proceeding from a front side, p-doped channel zones are introduced. In turn, n-doped source zones are formed in the p-doped channel zones. The rear side of the semiconductor body usually forms the drain zone of the transistor. The semiconductor body usually has a heavily doped semiconductor substrate and a more weakly doped epitaxial layer in which the channel zones are formed and which forms the drift path of the components. A gate electrode is formed in a manner insulated from the semiconductor body. This gate electrode runs adjacent to the source zone, the channel zone and the drift zone. The doping of the abovementioned zones is complementary to the abovementioned dopings in the case of a p-conducting MOSFET.
The sequence of the source zone, the channel zone doped complementary with respect to the source zone and the drift zone doped complementary with respect to the channel zone results in the formation of a parasitic bipolar transistor which, in the case of an n-conducting MOSFET, is an npn bipolar transistor whose base is formed by the channel zone. If, during the operation of the component, p-type charge carriers (holes) reach, for example, in the event of breakdown or commutation, into the channel zone, often also referred to as body zone, then a voltage drop may arise in the channel zone and drive the parasitic bipolar transistor.
The driven parasitic bipolar transistor emits n-type charge carriers (electrons). These electrons inevitably pass into regions with a high field strength, where they bring about, via impact ionization, charge carrier multiplication and thus a second breakdown (second breakthrough). P-type charge carriers (holes) produced as a result of this are conducted into the channel zone. That is to say, into the base of the parasitic bipolar transistor, and thereby amplify the bipolar effect. This process starts to escalate in an uncontrolled manner, and the component is ultimately destroyed.
Edge cells, that is to say, transistor cells at the edge of the transistor cell array, are especially affected by this phenomenon since, for the edge cells, the entire edge substructure acts as an entry region for charge carriers. All holes formed in the edge region traverse the edge cells, resulting in a high voltage drop in the p-doped channel zone. Voltage breakdowns of the field-effect transistor thus occur in an amplified fashion in the region of the edges of the cell array. In order to prevent this, or in order to bring the dielectric strength in the edge region at least to the value of the dielectric strength of transistor cells in the center of the cell array, it is known for the edge cells to be formed differently than the rest of the transistor cells. The essential difference between the edge cells and the transistor cells is that the edge cells have no source zone, so that, rather than a parasitic bipolar transistor, merely a diode is formed by the edge cells, in which case, in the case of an n-conducting MOSFET, a p-doped region of the edge cell, which corresponds to the channel zone in the case of the transistor cells, forms the anode of the diode.
In customary methods for fabricating such vertical power MOSFETS, first a semiconductor body of a specific conduction type (n-conducting in the case of an n-conducting MOSFET) is provided, and channel zones of a complementary conduction type are subsequently introduced into the semiconductor body in a manner proceeding from the front side. In order to form the later gate electrode, first an insulation layer, usually a semiconductor oxide, is deposited onto the semiconductor body, and an electrode layer is subsequently applied to the insulation layer. Cutouts are subsequently produced above the channel zones in the insulation layer and the electrode layer, and the channel zones are subjected to doping reversal in the regions uncovered by virtue of the cutouts in order to form the source zones. A further insulation layer is subsequently deposited onto this configuration, and cutouts are produced in the insulation layer above the channel zones or the source zones in order to make contact with the source zones. In this case, the further insulation layer serves for insulation between the gate electrodes and a source electrode that makes contact with the source zones. In this case, the source electrode is also intended to make contact with the channel zone or anode zone of the edge cells, so that contact holes likewise have to be produced there in the further insulation layer. In order to prevent source zones from being fabricated in the region of the edge cells, the known fabrication method requires an additional method step which follows the fabrication of the cutout in the insulation layer applied to the semiconductor body and the electrode layer. During this method step, a protective layer, for example, a photoresist, is deposited in the region of the edge cells, which covers the surface regions of the semiconductor body that are uncovered in the contact holes, in order thus to prevent a doping reversal.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a method for fabricating a semiconductor component having at least one transistor cell and an edge cell configured adjacent to the transistor cell, which overcomes the above-mentioned disadvantages of the prior art methods of this general type.
In particular, it is an object of the invention to provide a method for fabricating a semiconductor component having at least one transistor cell and an edge cell arranged adjacent to the transistor cell in which an additional method step for preventing a doping in the region of the edge cell can be dispensed with.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for fabricating a semiconductor component. The method includes steps of:
providing a semiconductor body having a front side, a rear side, at least one transistor cell with at least one channel zone extending into the semiconductor body from the front side, at least one edge cell having at least one first terminal zone configured at a distance from the channel zone, an insulation layer applied to the front side of the semiconductor body, an electrode layer applied to the insulation layer, a cutout formed above the channel zone in the insulation layer and in the electrode layer, and a cutout formed above first terminal zone in the insulation layer and in the electrode layer, the channel zone and the first terminal zone being doped by the same conduction type, which is complementary with respect to surrounding regions of the semiconductor body, the edge cell being adjacent the transistor cell, and the first terminal zone extending into the semiconductor body from the front side;
doping a region of the channel zone that is uncovered by the cutout formed above the channel zone with a dopant of a complementary conduction type with respect to the channel zone to form a first complementary doped region, and doping a region of the first terminal zone that is uncovered by the cutout formed above the first terminal zone with a dopant of a complementary conduction type wit
Elms Richard
Wilson Christian D.
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