Semiconductor device manufacturing: process – Forming schottky junction – Compound semiconductor
Reexamination Certificate
2003-05-05
2004-08-03
Guerrero, Maria (Department: 2822)
Semiconductor device manufacturing: process
Forming schottky junction
Compound semiconductor
C438S570000, C438S571000, C438S572000, C438S092000, C438S167000
Reexamination Certificate
active
06770548
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to rectifiers and more particularly to Schottky barrier rectifying devices, and methods of forming these devices.
BACKGROUND 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. 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 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.
As the voltages of modern power supplies continue to decrease in response to the need for reduced power consumption and increased energy efficiency, it becomes more advantageous to decrease the on-state voltage drop across a power rectifier, while still maintaining high forward-biased current density levels. As well known to those skilled in the art, the on-state voltage drop is generally dependent on the forward voltage drop across the metal/semiconductor junction and the series resistance of the semiconductor region and cathode contact.
The need for reduced power consumption also generally requires minimizing the reverse-biased leakage current. The reverse-biased leakage current is the current in the rectifier during a reverse-biased blocking mode of operation. To sustain high reverse-biased blocking voltages and minimize reverse-biased leakage currents, the semiconductor portion of the rectifier is typically lightly doped and made relatively thick so that the reverse-biased electric field at the metal/semiconductor interface does not become excessive. The magnitude of the reverse-biased leakage current for a given reverse-biased voltage is also inversely dependent on the Schottky barrier height (potential barrier) between the metal and semiconductor regions. Accordingly, to achieve reduced power consumption, both the forward-biased voltage drop and reverse-biased leakage current should be minimized and the reverse blocking voltage should be maximized.
According to U.S. Pat. No. 5,612,567, desirable effects are achieved with the device of
FIG. 1
, due to the occurrence of charge coupling between the majority charge carriers in the mesa-shaped portion of the drift region
14
and the portion of the metal anode
18
opposite the insulated sidewalls
16
a
,
16
b
of the trenches. Specifically, the electric field at the center of the metal-semiconductor contact (Schottky contact) is reduced significantly relative to an ideal plane-parallel rectifier. The reduction in electric field at the center of the Schottky contact causes a significant decrease in the reverse-biased leakage current through a reduction in Schottky barrier height lowering. Reverse-biased leakage current is the current in the rectifier during a reverse-biased (blocking) mode of operation. Moreover, the peak in the electric field profile shifts away from the metal-semiconductor contact and into the drift region. As the peak of the electric field moves away from the Schottky contact, the mesa is able to support more voltage, and thus provides higher breakdown voltages (reverse blocking voltages) than those of an ideal parallel-plane rectifier.
A graphical illustration of the breakdown voltage versus trench oxide thickness for the Schottky rectifier shown in
FIG. 1
is illustrated in
FIG. 2
, which is a reproduction of FIG. 12 of the aforementioned patent. In particular, the breakdown voltage is shown as monotonically increasing with oxide thickness up to at least 2200 Angstroms. The graphical illustration of
FIG. 2
was obtained for a Schottky rectifier having a mesa width and cell pitch of 0.5 microns and 1 micron, respectively, and a trench depth and drift region thickness of 3 microns and 4 microns, respectively.
As
FIG. 2
indicates, Schottky rectifiers to be used in high voltage applications require a relatively thick trench oxide layer. The oxide layer is typically grown by a thermal technique, which is advantageous employed because it provides good epitaxy with a reduced defect density at the oxide-semiconductor interface. Unfortunately, the slow growth rates associated with thermally grown oxide layers make it difficult to achieve a trench oxide layer with a thickness upwards of 2000 Angstroms. Moreover, alternative growth techniques such as chemical vapor deposition (CVD), while having greater deposition rates, produce a greater defect density and hence a higher charge at the oxide-semiconductor interface.
Accordingly, there remains a need within the art to provide a trench Schottky rectifier device that can be operated at high voltages and which is relatively easy to fabricate.
SUMMARY OF THE INVENTION
The above and other needs are met by the present invention. In particular, a Schottky rectifier is provided which comprises: (a) a semiconductor region having first and second opposing faces, with the semiconductor region comprising a cathode region of first conductivity type adjacent the first face and a drift region of the first conductivity type adjacent the second face, and with the drift region having a lower net doping concentration than that of the cathode region; (b) one or more trenches extending from the second face into the semiconductor region and defining one or more mesas within the semiconductor region; (c) an insulating region adjacent the semiconductor region in lower portions of the trench; (d) and an anode electrode that is (i) adjacent to and forms a Schottky rectifying contact with the semiconductor region at the second face, (ii) adjacent to and forms a Schottky rectifying contact with the semiconductor region within upper portions of the trench and (iii) adjacent to the insulating region within the lower portions of the trench.
Preferably, the semiconductor is silicon, the first conductivity type is n-type conductivity, and a cathode electrode is provided on the first face.
The lower portions of the trenches preferably correspond to approximately 25 to 40% of the depth of the trenches. In some embodiments, the trench extends into the cathode region, with t
Hshieh Fwu-Iuan
So Koon Chong
General Semiconductor Inc.
Guerrero Maria
Mayer Fortkort & Williams PC
Mayer, Esq. Stuart H.
Williams Esq. Karin L.
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