Vertical type power mosfet having trenched gate structure

Details Vertical type power mosfet having trenched gate structure Vertical type power mosfet having trenched gate structure

2002-07-01

2004-09-07

Flynn, Nathan J. (Department: 2826)

Active solid-state devices (e.g., transistors, solid-state diode

Field effect device

Having insulated electrode

C257S330000, C257S341000, C257S342000, C438S212000, C438S268000

Reexamination Certificate

active

06787848

ABSTRACT:

CROSS-REFERENCE TO RELATED APPLICATIONS
This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2001-198552, filed Jun. 29, 2001, the entire contents of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor device, and particularly to a vertical type power MOSFET having a trenched gate structure.
2. Description of the Related Art
A trench is formed in a semiconductor substrate, and this trench is used to form a trenched gate structure. These trenched gate structures are used in semiconductor devices such as an IGBT (Insulated Gate Bipolar Transistor) and a MOSFET (MOS-type Field Effect Transistor), and this is particularly advantageous for electrical power applications. For example, because a power MOSFET having a trenched gate structure can attain high switching speeds, high current capacities and a breakdown voltage of about several tens volts to 100 volts, they are widely used for switching a power source in portable devices or personal computers.
An n channel type power MOSFET having a trenched gate structure will be described in the following as an example of such a power MOSFET.
It is to be noted that
FIG. 20
does not represent a prior art. It is a schematic sectional view of the main portions of an n-channel type trench gate structure power MOSFET which was experimentally designed by the present inventors in the process of achieving the present invention.
That is to say,
FIG. 20
illustrates, as an example of a power MOSFET, a cross sectional of one half (half pitch) of a paired vertical type MOSFETS in the plurality of units serially formed on a semiconductor substrate.
The schematic structure is such that an n− type drift layer
108
and a p type base layer
110
are formed as a laminated body in sequence on the semiconductor substrate used as a drain layer
112
, and a trench T is formed on the laminated body. A gate electrode
104
is formed on the surface of the inner wall of the trench T with a gate insulating film
102
interposed therebetween.
The drain electrode
114
is formed at the bottom surface side of the n+ type drain layer
112
which is the semiconductor substrate. An n+ type source region
116
which is adjacent to the gate insulating film
102
, and the p+ type region
118
which is formed adjacent thereto are provided on the p+ type base layer
110
, and a source electrode
120
is formed so as to extend across these regions
116
and
118
.
In this type of power MOSFET, when a predetermined voltage is applied to the gate electrode
104
, an inverse layer is formed on the region adjacent to the gate insulating film
102
of the p-type base layer
110
, and the power MOSFET turns on and current flows between the source electrode
120
and the drain electrode
114
.
However, in the type of power MOSFET shown in
FIG. 20
, there is a problem that even if the devices are made small, the turn-on resistance or on resistance thereof cannot be effectively reduced.
That is to say, in the case of the type of power MOSFET shown in
FIG. 20
, the resistance of the device in the on state, that is the on resistance, is determined mainly by the channel resistance component and the drift resistance component. The channel resistance component is the resistance component of the channel region formed on the inverse layer of the p− type base layer
110
in the on state. On the other hand, the drift resistance component is the resistance component which appears for the on current in the n− type drift layer
108
.
In order to reduce the on resistance of the device, the pitch P of the device unit in
FIG. 20
was reduced to thereby increase device density on the semiconductor substrate. That is, the channel density was increased and thus the on resistance of the device was decreased.
Due to the quick advances in semiconductor size reduction processing technology in recent years, the channel density is being rapidly increased, and the channel resistance component is being greatly reduced. Specifically, size reduction has advanced to the extent that the device pitch P is below 0.5 &mgr;m.
FIG. 20
shows the half pitch structure which is one half the paired device unit. However, in the actual device in which the structure shown in the figure is juxtaposed in both sides thereof, the width of the p-type base layer
110
which is sandwiched between the two adjacent trenched gate structures has been made so small that it is substantially equal to the pitch P, and is less than 0.5 &mgr;m.
Further, under these conditions, the on resistance of the recent power MOSFET is such that the above-mentioned drift resistance component has come to account for approximately two-thirds of the total resistance.
That is to say, even when the manufacturing process is further improved and the device pitch P becomes even smaller, there is the problem that significant reduction in the on resistance of the device can not be expected.
For example, in the case of a power MOSFET of the type having a breakdown voltage of 30 volts, it is extremely difficult to reduce the on resistance to 20 m&OHgr;mm
2
or less.
In order to solve this problem, it is necessary to reduce the thickness t of the drift layer
108
, thereby reducing the drift resistance component. In order to do this, a method can be considered in which the gate insulating film
102
is made thicker and when voltage is applied between the gate electrode
104
(the source electrode
120
) and the drain electrode
114
, the gate insulating film
102
is caused to receive a portion of the applied voltage and thus the thickness of the drift layer
108
can be reduced.
FIG. 21
is a schematic view showing the cross-section structure of the power MOSFET formed based on this concept. The device shown in
FIG. 21
is the same as that shown in
FIG. 20
except that the thickness of the gate insulating film
102
is greater than that in FIG.
20
. Thus the components are indicated by the same reference numerals. That is to say, in the power MOSFET shown in
FIG. 21
, by making the gate insulating film
102
thicker, the portion by which the thickness is increased receives a portion of the applied voltage, and thus the thickness t of the drift layer
108
is reduced.
However, when the thickness of the gate insulating film
102
is increased in this manner, the threshold voltage of the power MOSFET is increased. As a result, the on resistance is increased by the amount by which the channel resistance is increased when the same gate voltage is applied, and a problem is caused that on resistance of the device can not be effectively reduced.
As described above, in the power MOSFET having this structure, because the on resistance is determined by the drift resistance component, there is the problem that even if the device is made smaller, the on resistance thereof can not be efficiently reduced.
BRIEF SUMMARY OF THE INVENTION
A power MOSFET of one aspect of the present invention comprises: a drain layer having a first conductivity type; a drift layer having the first conductivity type provided on the drain layer; a base layer having a second conductivity type provided on the drift layer; a source region having the first conductivity type provided on the base layer; a gate insulating film formed on the inner wall surface of a trench formed through the base layer and reaching at the drift layer; a gate electrode provided inside the gate insulating film provided in the trench, wherein the gate insulating film has a portion adjacent to the drift layer thicker than the portion adjacent to the base layer, and the drift layer has an impurity concentration gradient higher in the vicinity of the drain layer and lower in the vicinity of the source region along a depth direction of the trench.
According to another aspect of the present invention, a power MOSFET comprises: a drain layer having a first conductivity type; a drift layer having the first conductivity type provided on th

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