Semiconductor device manufacturing: process – Making field effect device having pair of active regions... – Having insulated gate
Reexamination Certificate
2001-12-31
2004-06-15
Cao, Phat X. (Department: 2814)
Semiconductor device manufacturing: process
Making field effect device having pair of active regions...
Having insulated gate
C438S212000, C438S284000
Reexamination Certificate
active
06750104
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to semiconductor devices, and more particularly to power MOSFET devices.
BACKGROUND OF THE INVENTION
Power MOSFET devices are employed in applications such as automobile electrical systems, power supplies, and power management applications. Such devices should sustain high voltage in the off-state while having a low voltage drop and high current flow in the on-state.
FIG. 1
illustrates a typical structure for an N-channel power MOSFET. An N-epitaxial silicon layer
1
formed over an N+ silicon substrate
2
contains p-body regions
5
a
and
6
a
, and N+ source regions
7
and
8
for two MOSFET cells in the device. P-body regions
5
and
6
may also include deep p-body regions
5
b
and
6
b
. A source-body electrode
12
extends across certain surface portions of epitaxial layer
1
to contact the source and body regions. The N-type drain for both cells is formed by the portion of N-epitaxial layer
1
extending to the upper semiconductor surface in
FIG. 1. A
drain electrode is provided at the bottom of N+ substrate
2
. An insulated gate electrode
18
typically of polysilicon lies primarily over the body and portions of the drain of the device, separated from the body and drain by a thin layer of dielectric, often silicon dioxide. A channel is formed between the source and drain at the surface of the body region when the appropriate positive voltage is applied to the gate with respect to the source and body electrode.
The on-resistance of the conventional MOSFET shown in
FIG. 1
is determined largely by the drift zone resistance in epitaxial layer
1
. The drift zone resistance is in turn determined by the doping and the layer thickness of epitaxial layer
1
. However, to increase the breakdown voltage of the device, the doping concentration of epitaxial layer
1
must be reduced while the layer thickness is increased. Curve
20
in
FIG. 2
shows the on-resistance per unit area as a function of the breakdown voltage for a conventional MOSFET. Unfortunately, as curve
20
shows, the on-resistance of the device increases rapidly as its breakdown voltage increases. This rapid increase in resistance presents a problem when the MOSFET is to be operated at higher voltages, particularly at voltages greater than a few hundred volts.
FIG. 3
shows a MOSFET that is designed to operate at higher voltages with a reduced on-resistance. This MOSFET is disclosed in paper No. 26.2 in the Proceedings of the IEDM, 1998, p. 683. This MOSFET is similar to the conventional MOSFET shown in
FIG. 2
except that it includes p-type doped regions
40
and
42
which extend from beneath the body regions
5
and
6
into the drift region of the device. The p-type doped regions
40
and
42
define columns in the drift region that are separated by n-type doped columns, which are defined by the portions of the epitaxial layer
1
adjacent the p-doped regions
40
and
42
. The alternating columns of opposite doping type cause the reverse voltage to be built up not only in the vertical direction, as in a conventional MOSFET, but in the horizontal direction as well. As a result, this device can achieve the same reverse voltage as in the conventional device with a reduced layer thickness of epitaxial layer
1
and with increased doping concentration in the drift zone. Curve
25
in
FIG. 2
shows the on-resistance per unit area as a function of the breakdown voltage of the MOSFET shown in FIG.
3
. Clearly, at higher operating voltages, the on-resistance of this device is substantially reduced relative to the device shown in
FIG. 1
, essentially increasing linearly with the breakdown voltage.
The improved operating characteristics of the device shown in
FIG. 3
are based on charge compensation in the drift region of the transistor. That is, the doping in the drift region is substantially increased, e.g., by an order of magnitude or more, and the additional charge is counterbalanced by the addition of columns of opposite doping type. The blocking voltage of the transistor thus remains unaltered. The charge compensating columns do not contribute to the current conduction when the device is in its on state. These desirable properties of the transistor depend critically on the degree of charge compensation that is achieved between adjacent columns of opposite doping type. Unfortunately, nonuniformities in the dopant gradient of the columns can be difficult to avoid as a result of limitations in the control of process parameters during their fabrication. For example, diffusion across the interface between the columns and the substrate and the interface between the columns and the p-body region will give rise to changes in the dopant concentration of the portions of the columns near those interfaces.
The structure shown in
FIG. 3
can be fabricated with a process sequence that includes multiple epitaxial deposition steps, each followed by the introduction of the appropriate dopant. Unfortunately, epitaxial deposition steps are expensive to perform and thus this structure is expensive to manufacture. Another technique for fabricating these devices is shown in copending U.S. application Ser. No. 09/970,972, in which a trench is successively etched to different depths. A dopant material is implanted and diffused through the bottom of the trench after each etching step to form a series of doped regions (so-called “floating islands”) that collectively function like the p-type doped regions
40
and
42
seen in FIG.
3
. However, the on-resistance of a device that uses the floating island technique is not as low as an identical device that uses continuous columns.
Accordingly, it would be desirable to provide a method of fabricating the MOSFET structure shown in
FIG. 3
that requires a minimum number of epitaxial deposition steps so that it can be produced less expensively while also allowing sufficient control of process parameters so that a high degree of charge compensation can be achieved in adjacent columns of opposite doping type in the drift region of the device.
SUMMARY OF THE INVENTION
In accordance with the present invention, a method is provided for forming a power semiconductor device. The method begins by providing a substrate of a first conductivity type and then forming a voltage sustaining region on the substrate. The voltage sustaining region is formed by depositing an epitaxial layer of a first conductivity type on the substrate and forming at least one trench in the epitaxial layer. At least one doped column having a dopant of a second conductivity type is located in the epitaxial layer, adjacent a sidewall of the trench. The trench is etched using an etchant gas that also serves as a dopant source for the formation of the doped column. For example, if a p-type dopant such as boron is desired, BCl
3
may be used as the etchant gas. Alternatively, if an n-type dopant such as phosphorus is required, PH
3
may be used as the etchant gas. The dopant present in the gas is incorporated into the silicon defining the surfaces of the trench. This dopant is subsequently diffused to form the doped column surrounding the trench. The trench is filled with an insulating material such as silicon dioxide, silicon nitride, polysilicon, or a combination of such materials. The step of filling the trench may be performed before or after the dopant is diffused to form the doped column. Finally, at least one region of the second conductivity type is formed over the voltage sustaining region to define a junction therebetween.
The power semiconductor device formed by the inventive method may be selected from the group consisting of a vertical DMOS, V-groove DMOS, and a trench DMOS MOSFET, an IGBT, a bipolar transistor, and a diode.
In accordance with another aspect of the invention, a power semiconductor device is provided. The device includes a substrate of a first conductivity type and a voltage sustaining region disposed on the substrate. The voltage sustaining region includes an epitaxial layer having a first conductivity type an
Blanchard Richard A.
Hshieh Fwu-Iuan
Cao Phat X.
General Semiconductor Inc.
Mayer Fortkort & Williams PC
Mayer, Esq. Stuart H.
Williams Esq. Karin L.
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