Active solid-state devices (e.g. – transistors – solid-state diode – With means to increase breakdown voltage threshold
Reexamination Certificate
2002-03-12
2004-05-25
Zarabian, Amir (Department: 2822)
Active solid-state devices (e.g., transistors, solid-state diode
With means to increase breakdown voltage threshold
C257S496000
Reexamination Certificate
active
06740952
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a high withstand voltage power semiconductor device to be used by a switching power source, an AC adapter, or to drive a motor or a fluorescent bulb inverter, or the like.
BACKGROUND OF THE INVENTION
Power ICs for switching power sources in commercial use that drive 100 to 200V, for example, require a device withstand voltage of 700 volts or more to drive a transformer. There is a requirement for a control circuit section and, for the straightforward integration thereof, a high withstand voltage lateral MISFET device (RESURF LDMOS) of the kind shown in
FIG. 25
, on the same chip.
This high withstand voltage lateral MISFET device is a device with a design withstand voltage of 700V and has: a p-type channel region (a Pwell and MISFET device body region)
2
, which is formed on the main face side of a p-type semiconductor substrate
1
with a high resistance of 120 &OHgr;cm; an n
+
source region
3
and a p
+
substrate contact
4
, which are formed on the main face side within the channel region
2
; an n
+
drain region
6
, which is separated from the channel region
2
on the main face side of the p-type semiconductor substrate
1
by an n-type drain drift region
5
of relatively low concentration therebetween; a gate electrode layer
9
, which backgates the channel region
2
via a gate insulating film
7
, and which extends toward the drain side on a thermal oxidation film (field oxide film)
8
selectively formed on the main face of the drain drift region
5
; a source electrode layer
11
, which is in conductive contact with the substrate contact
4
and source region
3
via an interlayer insulating film
10
formed on the gate electrode layer
9
; a drain electrode layer
12
, which is in conductive contact with the drain region
6
, and which extends toward the source side on the interlayer insulating film
10
; a field plate
13
, which extends, further than the gate electrode layer
9
, toward the drain side on the interlayer insulating film
10
, and which is in conductive contact with the gate electrode layer
9
via a contact hole
13
a
; a passivation film (protective film, nitride film)
14
, which is formed on the source electrode layer
11
, the drain electrode layer
12
and the field plate
13
; and an enclosing mold resin (epoxy resin or the like)
15
, which covers the passivation film
14
.
The portion of the gate electrode layer
9
which extends toward the drain side on the thermal oxidation film
8
functions as a field plate for alleviating the field concentration of the well end of the channel region
2
. Further, the field plate
13
alleviates the field concentration of the extending tip of the gate electrode layer
9
. In addition, the extending portion of the drain electrode layer
12
on the interlayer insulating film
10
functions as a field plate for alleviating the field concentration of the drain region
6
. For example, the length of extension of the field plate
13
must be sufficiently long to alleviate the field concentration of the extending tip of the gate electrode layer
9
; however, at the same time, a field concentration is produced at the extending tip of the field plate
13
. For this reason, it may be said that, ordinarily, an optimized design is such that a 700V voltage device, whose drain drift length is approximately 60 &mgr;m, has a field plate extension length on the order of 5 &mgr;m. Naturally, a design is required such that the length of extension of the gate electrode layer
9
, or the like, is also not too long.
FIG. 26
is a two-dimensional device simulation figure to show the distribution of equipotential lines for a case in which a drain voltage of 700V is applied to the drain electrode layer
12
in an OFF state, in a high withstand voltage lateral MISFET device with a design withstand voltage of 700V and in which the length of the drain drift region
5
is 60 &mgr;m. Further, in
FIG. 26
, the field plate
13
is shown formed as a layer common also to the source electrode layer
11
, and, unit length in a vertical direction is shown considerably exaggerated in comparison with unit length in a crosswise direction on the figure. The interval between equipotential lines is 100V.
The length of extension Mc from the source side end (bars peak) of the thermal oxidation film
8
of the field plate
13
is longer than the norm and set at 10 &mgr;m, and the total insulating film thickness Tox (oxide film) directly below the extending tip of the field plate
13
is formed with a thickness of 2 &mgr;m. In an attempt to provide a stable and reliable device voltage, this high withstand voltage lateral MISFET device is designed such that, at a PN junction face A of the p-type semiconductor substrate
1
and the drain drift region
5
, directly below the drain region
6
, sacrificial voltage breakdown is forcedly produced. Among the equipotential lines, the low equipotential lines are not oriented toward the extending tip of the gate electrode layer
9
as a result of the effect of the field plate
13
, and bend directly below the gate electrode layer
9
within the drain drift region
5
so as to wrap around the outside of the tip of the field plate
13
. When Mc is long, at a position B, which is in the main face of the drift region and directly below the tip of the field plate
13
, the low equipotential lines are pushed out toward the drain side, and, in the vicinity of this position B, the upper ends of the low equipotential lines are squeezed in the unoccupied interval between the field plate
13
and the extending portion of the drain electrode layer
12
. The potential at position B directly below the tip of the field plate
13
is approximately 100V. The field strength (critical field strength) at the PN junction face A is approximately 3×10
5
V/cm, and makes it possible to obtain a device with a design withstand voltage of 700V, and voltage rate limitation is primarily produced at the PN junction face A. However, since Mc is long in comparison with the norm, it is only natural that the field strength at position B directly below the tip of the field plate
13
already reaches approximately 1×10
5
V/cm even if an unstable voltage breakdown is generated to an extent owing to structural dispersion, or the like, in the vicinity of position B directly below the tip of the field plate
13
.
FIG. 27
is a two-dimensional device simulation figure to show the distribution of equipotential lines for a drain voltage of 500V in a case in which Tox remains as 2 &mgr;m as in
FIG. 26
, and the length of extension Mc of the field plate
13
is 25 &mgr;m. Since Mc is now too long, during the application of the drain voltage of 500V, at position B, which is in the main face of the drift region
5
and directly below the tip of the field plate
13
, since the low equipotential lines are pushed out toward the drift side, the interval between the equipotential lines contracts, and, in the vicinity of position B, the low equipotential lines are squeezed in the unoccupied interval between the field plate
13
and the extending portion of the drain electrode layer
12
such that the bend of these low equipotential lines changes, which leads to further contraction of the interval between the equipotential lines. For a drain voltage of 500V, the potential at position B directly below the tip of the field plate
13
is approximately 100 V, but the field strength is already higher than the approximate 1×10
5
V/cm of the PN junction face A and reaches the critical field strength 3×10
5
V/cm, such that voltage breakdown is produced more at position B directly below the tip of field plate
13
that at PN junction face A. As a result, the realization of a device with a design withstand voltage of 700V is inconceivable, and it is only possible to obtain a device with a voltage of 500V at the most.
Therefore, when the field plate
13
is made long, the interval between equipotential lines contracts synergistically at position B directly below the ti
Fujishima Naoto
Kitamura Akio
Saito Masaru
Tada Gen
Fuji Electric & Co., Ltd.
Rose Kiesha
Rossi & Associates
Zarabian Amir
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