Active solid-state devices (e.g. – transistors – solid-state diode – Schottky barrier – With means to prevent edge breakdown
Reexamination Certificate
2000-12-15
2002-07-16
Prenty, Mark V. (Department: 2822)
Active solid-state devices (e.g., transistors, solid-state diode
Schottky barrier
With means to prevent edge breakdown
C257S520000, C257S622000
Reexamination Certificate
active
06420768
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to rectifying devices and more particularly to trench Schottky barrier rectifiers as well as methods of forming these devices.
BACKGROUND AND SUMMARY OF THE INVENTION
Rectifiers exhibit relatively low resistance to current flow in a forward direction and a high resistance to current flow in a reverse direction. Trench Schottky barrier rectifiers are a type of rectifier that have found use as output rectifiers in switching-mode power supplies and in other high-speed power switching applications such as motor drives. These devices are capable of carrying large forward currents and supporting large reverse blocking voltages.
U.S. Pat. No. 5,365,102 to Mehrotra et al. and entitled “Schottky Barrier Rectifier with MOS Trench”, the entire disclosure of which is hereby incorporated by reference, discloses trench Schottky barrier rectifiers which have a higher breakdown voltage than is theoretically attainable with an ideal abrupt parallel-plane P-N junction. A cross-sectional representation of one embodiment of the described rectifiers is illustrated in FIG.
1
. In this figure, rectifier
10
includes a semiconductor substrate
12
of first conductivity type, typically N-type conductivity, having a first face
12
a
and a second opposing face
12
b
. The substrate
12
comprises a relatively highly doped cathode region
12
c
(shown as N+) adjacent the first face
12
a
. A drift region
12
d
of first conductivity type (shown as N) extends from the cathode region
12
c
to the second face
12
b
. Accordingly, the doping concentration of the cathode region
12
c
is greater than that of the drift region
12
d
. A mesa
14
having a cross-sectional width “Wm”, defined by opposing sides
14
a
and
14
b
, is formed in the drift region
12
d
. The mesa can be of stripe, rectangular, cylindrical or other similar geometry. Insulating regions
16
a
and
16
b
(described as SiO
2
) are also provided on the mesa sides. The rectifier also includes an anode electrode
18
on the insulating regions
16
a
,
16
b
. The anode electrode
18
forms a Schottky rectifying contact with the mesa
14
at second face
12
b
. The height of the Schottky barrier formed at the anode electrode/mesa interface is dependent on the type of electrode metal and semiconductor (e.g., Si, Ge, GaAs, and SiC) used and is also dependent on the doping concentration in the mesa
14
. Finally, a cathode electrode
20
is provided adjacent the cathode region
12
c
at the first face
12
a
. The cathode electrode
20
ohmically contacts cathode region
12
c.
In a process described in U.S. Pat. No. 5,365,102, drift region
12
d
is provided by epitaxial growth on substrate
12
c
. Trenches are then etched through photoresist-patterned nitride layers, forming discrete mesas
14
having thermal oxidation resistant nitride caps. Insulating regions
16
, preferably silicon dioxide, are formed on the trench sidewalls and bottoms
22
b
, but not on the tops of the mesas
14
(faces
12
b
) because of the presence of the nitride regions. The nitride regions (as well as any stress relief oxide regions, if present) are removed, and anode
18
and cathode
20
metallization provided. For more information, see U.S. Pat. No. 5,365,102.
As is discussed more fully below, the present invention concerns improvements in trench Schottky barrier rectifiers related to those in U.S. Pat. No. 5,365,102 and to processes for making such trench Schottky barrier rectifiers.
SUMMARY OF THE INVENTION
According to an embodiment of the invention, a method of forming a trench Schottky barrier rectifier is provided. The method comprises:
(a) Forming a semiconductor region having first and second opposing faces. The semiconductor region comprises a drift region of first conductivity type adjacent the first face and a cathode region of the first conductivity type adjacent the second face. The drift region has a lower net doping concentration than the net doping concentration associated with the cathode region.
(b) Forming a plurality of trenches that extend into the semiconductor region from the first face. These trenches define a plurality of mesas within the semiconductor region and form trench intersections at a plurality of locations.
(c) Providing an oxide layer that covers the semiconductor region at locations that correspond to trench bottoms and lower portions of the trench sidewalls.
(d) Providing a polysilicon region that is disposed within the trenches over the oxide layer.
(e) Providing insulating regions over the oxide layer at the trench intersections.
(f) Providing an anode electrode that is adjacent to and forms a Schottky rectifying contact with the drift region.
Where desired, the rectifier can be provided with a cathode electrode on the second face of the semiconductor region.
The semiconductor is preferably a silicon semiconductor and has n-type conductivity. Preferred insulating regions are borophosphosilicate glass regions.
The step of forming the semiconductor region preferably includes providing a semiconductor substrate corresponding to the cathode region, and subsequently growing an epitaxial semiconductor layer corresponding to the drift region on the substrate.
The step of forming the trenches preferably comprises: forming a patterned masking layer over the first face of the semiconductor region and etching the trenches through the masking layer. In some embodiments, the trenches are etched into the drift regions, but not into the cathode region. In others, the trenches are etched sufficiently deeply such that they extend through the drift region and into the cathode region.
The steps of forming the oxide layer, the polysilicon region, and the insulating regions preferably further comprise the following: (a) forming an oxide layer on the first face of the semiconductor region and within the trenches, for example, by thermal growth or by oxide deposition processes; (b) forming a polysilicon layer over the oxide layer; (c) etching the polysilicon layer such that that portions of the oxide layer are exposed over the first face, and portions of the oxide layer are exposed over upper portions of the trench sidewalls; (d) forming an insulating layer over the oxide layer and the etched polysilicon layer; (e) forming a patterned etch resistant layer over the insulating layer at the trench intersections; and (f) etching the insulating layer and the oxide layer where not covered by the patterned etch resistant layer.
According to another embodiment of the invention, a trench Schottky barrier rectifier is provided. The rectifier comprises:
(a) A semiconductor region having first and second opposing faces. The semiconductor region comprises a drift region of first conductivity type adjacent the first face and a cathode region of the first conductivity type adjacent the second face. The drift region has a lower net doping concentration than that of the cathode region.
(b) A plurality of trenches extending into the semiconductor region from the first face. The trenches define a plurality of mesas within the semiconductor region, and the trenches form a plurality of trench intersections.
(c) An oxide layer covering the semiconductor region on bottoms of the trenches and on lower portions of sidewalls of the trenches.
(d) A polysilicon region disposed over the oxide layer within the trenches.
(e) Insulating regions at the trench intersections that cover a portion of the polysilicon region and a portion of the oxide layer.
(f) An anode electrode that is adjacent to and forms a Schottky rectifying contact with the drift region.
A number of trench intersection angles are possible. In one preferred case, the trenches intersect at right angles to one another. A number of configurations are possible for the insulating regions at the trench intersections. In one preferred instance, the insulating regions are rectangular when viewed from above the trenches.
One advantage of the present invention is that trench Schottky barrier rectifiers, in which cells are defined by intersecting trenches, can be forme
Amato John E.
Hshieh Fwu-Iuan
So Koon Chong
Bonham David B.
General Semiconductor Inc.
Mayer Fortkort & Williams PC
Prenty Mark V.
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