U.S. patent application number 15/161666 was filed with the patent office on 2017-11-23 for power semiconductor device with charge balance design.
The applicant listed for this patent is Infineon Technologies AG. Invention is credited to Alice Pei-Shan Hsieh, Hans-Joachim Schulze.
Application Number | 20170338302 15/161666 |
Document ID | / |
Family ID | 60255429 |
Filed Date | 2017-11-23 |
United States Patent
Application |
20170338302 |
Kind Code |
A1 |
Hsieh; Alice Pei-Shan ; et
al. |
November 23, 2017 |
Power Semiconductor Device with Charge Balance Design
Abstract
A semiconductor body having first and second vertically spaced
apart surfaces is formed. A gate trench that vertically extends
from the first surface of the semiconductor body towards the second
surface is formed. A gate electrode and a gate dielectric are
formed in the gate trench. The gate dielectric electrically
insulates the gate electrode from adjacent semiconductor material.
A doped superjunction region vertically extending from a bottom of
the gate trench towards the second surface of the semiconductor
body is formed. The doped superjunction region includes first,
second, and third doped pillars vertically extending from the first
surface of the first semiconductor layer and directly adjoining one
another. The second pillar is laterally centered between the first
and third pillars and has an opposite conductivity type as the
first and third pillars.
Inventors: |
Hsieh; Alice Pei-Shan;
(Unterhaching, DE) ; Schulze; Hans-Joachim;
(Taufkirchen, DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Infineon Technologies AG |
Neubiberg |
|
DE |
|
|
Family ID: |
60255429 |
Appl. No.: |
15/161666 |
Filed: |
May 23, 2016 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01L 29/66068 20130101;
H01L 29/66348 20130101; H01L 21/0465 20130101; H01L 29/0638
20130101; H01L 29/0634 20130101; H01L 29/1095 20130101; H01L
29/66734 20130101; H01L 29/0878 20130101; H01L 29/1608 20130101;
H01L 29/7813 20130101; H01L 29/7397 20130101 |
International
Class: |
H01L 29/06 20060101
H01L029/06; H01L 29/16 20060101 H01L029/16; H01L 21/04 20060101
H01L021/04; H01L 29/10 20060101 H01L029/10; H01L 29/739 20060101
H01L029/739; H01L 29/66 20060101 H01L029/66 |
Claims
1. A method of forming a vertical trenched gate transistor,
comprising: forming a semiconductor body comprising first and
second vertically spaced apart surfaces, a gate trench that
vertically extends from the first surface of the semiconductor body
towards the second surface, a gate electrode disposed in the gate
trench, and a gate dielectric disposed in the gate trench and
electrically insulating the gate electrode from adjacent
semiconductor material; and forming a doped superjunction region
vertically extending from a bottom of the gate trench towards the
second surface of the semiconductor body, the doped superjunction
region comprising first, second, and third doped pillars vertically
extending from the first surface of the first semiconductor layer
and directly adjoining one another, the second pillar laterally
centered between the first and third pillars and having an opposite
conductivity type as the first and third pillars.
2. The method of claim 1, wherein forming the semiconductor body
comprises: providing a first semiconductor layer of a first
conductivity type and having first and second vertically spaced
apart surfaces; and epitaxially depositing a second semiconductor
layer of the first conductivity type on the first semiconductor
layer, wherein the doped superjunction is formed by applying
semiconductor processing to the first surface of the first
semiconductor layer before epitaxially depositing the second
semiconductor layer.
3. The method of claim 2, wherein forming the doped superjunction
region comprises: forming a first trench in the first semiconductor
layer vertically extending from the first surface of the first
semiconductor layer; forming a first doped semiconductor region of
the first conductivity type around a perimeter of the first trench
such that the first doped semiconductor region lines a bottom and
sidewalls of the first trench; forming a second doped semiconductor
region of a second conductivity type that is opposite form the
first conductivity type in the first trench between sections of the
first doped semiconductor region that line the sidewalls of the
first trench; and wherein the second doped semiconductor region
provides the second doped pillar of the doped superjunction region,
and wherein the sections of the first doped semiconductor region
that line the sidewalls of the first trench provide the first and
third doped pillars of the doped superjunction region.
4. The method of claim 3, wherein forming the first doped region
comprises implanting dopant atoms into the perimeter of the first
trench, the dopant atoms penetrating the bottom and sidewalls of
the first trench thereby forming the first doped region within the
first semiconductor layer.
5. The method of claim 3, wherein forming the first doped region
around the perimeter of the first trench comprises epitaxially
depositing a third semiconductor layer that lines the bottom and
sidewalls of the first trench thereby forming the first doped
region within the first trench, wherein a thickness of the third
semiconductor layer is controlled such that a void remains in the
first trench between sections of the third semiconductor layer.
6. The method of claim 3, wherein forming the second doped
semiconductor region comprises, after forming the first doped
region, epitaxially depositing a fourth semiconductor layer that
completely fills the first trench.
7. The method of claim 2, wherein forming the doped superjunction
region comprises: performing masked ion implantation at the first
surface of the first semiconductor layer before epitaxially
depositing the second semiconductor layer thereby forming doped
wells that vertically extend from the first surface of the first
semiconductor layer into the first semiconductor layer.
8. The method of claim 7, wherein performing masked ion
implantation comprises forming first and second doped wells that
vertically extend from the first surface of the first semiconductor
layer into the first semiconductor layer, the first doped well
being wider than the second doped well, the second doped well being
arranged in a lateral center of the first doped well such that
portions of the first well are disposed on both lateral sides of
the second well, wherein the second doped well provides the second
doped pillar of the doped superjunction region, and wherein the
portions of the first well that are disposed on both lateral sides
of the second well provide the first and third doped pillars of the
doped superjunction region.
9. The method of claim 7, wherein performing masked ion
implantation comprises forming first, second, and third wells of
approximately equal width that directly laterally adjoin one
another, wherein the second well is laterally interposed between
the first and third wells, wherein the second well provides the
second doped pillar of the doped superjunction region, and wherein
the first and third wells provide the first and third doped pillars
of the doped superjunction region.
10. The method of claim 1, wherein forming the semiconductor body
comprises providing a first semiconductor layer of a first
conductivity type and having first and second vertically spaced
apart surfaces, wherein the gate trench is formed by etching
semiconductor material from the first surface of the first
semiconductor layer, wherein the doped superjunction region is
formed by implanting first and second conductivity type dopants
into a bottom of the gate trench.
11. The method of claim 1, wherein the second doped pillar has a
second conductivity type that is opposite the first conductivity
type, and wherein the first and third doped pillars have the first
conductivity type and are more highly doped than the first
semiconductor layer.
12. The method of claim 1, wherein the second doped pillar has the
first conductivity type and is more highly doped than the first
semiconductor layer, and wherein the first and third doped pillars
have a second conductivity type that is opposite the first
conductivity type.
13. The method of claim 1, further comprising: forming a second
conductivity type body region vertically extending from the first
surface semiconductor body, the second conductivity type being
opposite the first conductivity type; forming a first conductivity
type source region that is contained within the second conductivity
type body region and directly adjoins the first surface of the
semiconductor body and the gate trench; forming a second
conductivity type collector region that extends from the second
surface of the semiconductor body towards the gate trench; and
forming a first conductivity type field stop region that is
disposed between the collector region and the drift region, wherein
a drift region of the device comprises first conductivity type
semiconductor material disposed between the body region and the
field stop region, and wherein a distance between a bottom of the
doped superjunction region and the field stop region is greater
than 50% of a vertical thickness of the drift region, the vertical
thickness of the drift region being measured as a shortest distance
between the body region and the field stop region.
14. The method of claim 13, wherein the distance between the bottom
of the doped superjunction region and the field stop region is
greater than 70% of a vertical thickness of the drift region.
15. The method of claim 1, wherein semiconductor body comprises
silicon, and wherein providing the semiconductor body comprises at
least one of: providing a bulk silicon substrate and epitaxially
growing one or more semiconductor layers having the first
conductivity type on the bulk silicon substrate; providing a FZ
(floating zone) silicon wafer with an intrinsic doping of the first
conductivity type; providing a MCZ (magnetic Czochralski) silicon
wafer with an intrinsic doping of the first conductivity type.
16. The method of claim 1, wherein the semiconductor body comprises
silicon-carbide.
17. A method of forming a vertical trenched gate transistor in a
semiconductor body, the semiconductor body having first and second
vertically spaced apart surfaces, the vertical trenched gate
transistor comprising an n-type source region extending form the
first surface into the semiconductor body, a p-type body region
disposed beneath and adjoining the source region, an n-type drift
region disposed beneath and adjoining the body region, an n-type
field stop region that is more highly doped than the drift region
disposed beneath and adjoining the doped n-type drift region, a
gate trench extending from the first surface through the source and
body regions, and a gate electrode disposed in the gate trench and
being configured to control a vertical current flowing between the
first and second surfaces, the method comprising: forming a doped
superjunction region vertically extending from a bottom of the gate
trench towards the second surface of the semiconductor body, the
doped superjunction region comprising first, second, and third
doped pillars vertically extending from the first surface of the
first semiconductor layer and directly adjoining one another, the
second pillar laterally centered between the first and third
pillars and having an opposite conductivity type as the first and
third pillars.
18. The method of claim 17, wherein the semiconductor body is
provided by: providing a first semiconductor layer of a first
conductivity type and having first and second vertically spaced
apart surfaces; and epitaxially depositing a second semiconductor
layer of the first conductivity type on the first semiconductor
layer, wherein the doped superjunction is formed by applying
semiconductor processing to the first surface of the first
semiconductor layer before epitaxially depositing the second
semiconductor layer.
19. The method of claim 18, wherein forming the doped superjunction
region comprises: forming a first trench that vertically extends
beneath the first surface of the first semiconductor layer before
epitaxially depositing the second semiconductor layer; forming a
first doped semiconductor region around a perimeter of the first
trench such that the first doped semiconductor region lines a
bottom and sidewalls of the first trench; and forming a second
doped semiconductor region in the first trench between sections of
the first doped semiconductor region that line the sidewalls of the
first trench, wherein forming the first doped semiconductor region
comprises at least one of: implanting n-type dopant atoms into the
perimeter of the first trench; and epitaxially depositing a third
n-type semiconductor layer that lines the bottom and sidewalls of
the first trench.
20. The method of claim 18, wherein forming the doped superjunction
region comprises: performing masked ion implantation at the first
surface of the first semiconductor layer before epitaxially
depositing the second semiconductor layer thereby forming doped
wells that vertically extend beneath the first surface of the first
semiconductor layer.
21. The method of claim 18, wherein the semiconductor body is
provided by a first semiconductor layer of a first conductivity
type and having first and second vertically spaced apart surfaces,
wherein the gate trench is formed by etching semiconductor material
from the first surface of the first semiconductor layer, wherein
the doped superjunction region is formed by implanting first and
second conductivity type dopants into a bottom of the gate
trench.
22. A vertical trenched gate transistor being formed in a
semiconductor body having first and second vertically spaced apart
surfaces, the vertical trenched gate transistor comprising: an
n-type source region extending form the first surface into the
semiconductor body; a p-type body region disposed beneath and
adjoining the source region; an n-type drift region disposed
beneath the body region; an n-type field stop region that is more
highly doped than the drift region disposed beneath and adjoining
the drift region; a gate trench extending from the first surface
through the source and body regions, and a gate electrode disposed
in the gate trench and being configured to control a vertical
current flowing between the first and second surfaces; a doped
superjunction region directly adjoining and disposed beneath the
gate trench, the doped superjunction region comprising first,
second, and third doped pillars, the second pillar laterally
centered between the first and third pillars and forming a p-n
junction with the first and third pillars, wherein the first and
third doped pillars each comprise a planar uppermost boundary that
is substantially parallel to the first surface.
23. The vertical trenched gate transistor of claim 22, wherein the
distance between a bottom of the doped superjunction region and the
field stop region is greater than 70% of a vertical thickness of
the drift region.
24. The vertical trenched gate transistor of claim 22, wherein the
distance between a bottom of the doped superjunction region and the
field stop region is greater than 90% of a vertical thickness of
the drift region.
25. The vertical trenched gate transistor of claim 22, wherein the
second doped pillar is an n-type region with a higher doping
concentration than the drift region, and wherein the first and
third pillars are p-type regions.
26. The vertical trenched gate transistor of claim 22, wherein the
planar uppermost boundaries of the first and third pillars are
vertically spaced apart from the body region by a portion of the
drift region.
27. The vertical trenched gate transistor of claim 22, wherein the
first, second and third doped pillars each comprise a lowermost
boundary, the lowermost boundary of the first, second and third
doped pillars each being vertically spaced apart from the field
stop region by substantially the same distance.
Description
TECHNICAL FIELD
[0001] The instant application relates to power semiconductor
devices, and more particularly relates to drift region structures
that enhance electrical performance of power semiconductor
devices.
BACKGROUND
[0002] Power semiconductor devices, in particular field-effect
controlled switching devices such as a Metal Oxide Semiconductor
Field Effect Transistor (MOSFET) or an Insulated Gate Bipolar
Transistor (IGBT), have been used for various applications
including but not limited to use as switches in power supplies and
power converters, electric cars, air-conditioners, and even stereo
systems. Particularly with regard to power devices capable of
switching large currents and/or operating at higher voltages, low
on-state resistance Ron, high breakdown voltages U.sub.bd, and/or
high robustness are often desired. A power MOSFET typically
includes a drain region, a drift region adjoining the drain region,
and a source region, each having a first conductivity type, and a
body region arranged between the drift region and source region of
a second conductivity type. A power IGBT has a similar construction
as a power MOSFET, except that the first conductivity type drain
region is replaced with a second conductivity type collector
region, thus forming a bipolar junction transistor with a voltage
controlled switch supplying the base current of the BJT.
[0003] One issue of particular concern in power switching
applications is cosmic ray radiation. Cosmic ray radiation refers
to unwanted particle bombardment from the exterior environment into
the operational regions of the device. Although it is more
prevalent in space environments, cosmic ray radiation can occur in
terrestrial environments. The particle bombardments caused by
cosmic ray radiation can set off a chain reaction of impact
ionization, which causes unwanted current filamentation and can
lead to irreversible device failure. Devices that operate with high
electric field gradients, such as power switching devices, are most
vulnerable to failure from cosmic ray radiation. For this reason,
many power semiconductor switching applications require the device
to be ruggedized against cosmic ray radiation. Mitigating high
electric fields at critical locations within the power device
enables robust device performance against harsh operation
conditions such as cosmic ray radiation.
[0004] Techniques used to tailor the electric field profile and
peak intensity of power switching devices to improve cosmic ray
robustness include (i) increasing the wafer/drift region thickness;
(ii) introducing a thicker graded/diffused base material profile;
(iii) reducing the n-type drift region/intrinsic layer doping
concentration; (iv) optimizing the field-stop (buffer) layer
profile to reduce the peak electric field in the back side of the
device; (V) using deeper p-type junctions at the surfaces to move
the high electric field away from the electrodes; and (VI)
thickening the gate trench oxide to alleviate the electric field
strength at the trench bottom and the top side of the drift region.
However, these approaches often lead to worse electrical
performance trade-offs, e.g., poorer diode reverse recovery
softness and higher on-state losses and hence worse V.sub.ce,sat
(collector-emitter saturation voltage) and E.sub.off (turn-off
loss).
SUMMARY
[0005] A method of forming a vertical trenched gate transistor is
disclosed. According to an embodiment, a semiconductor body having
first and second vertically spaced apart surfaces is formed. A gate
trench that vertically extends from the first surface of the
semiconductor body towards the second surface is formed. A gate
electrode that is disposed in the gate trench is formed and a gate
dielectric that is disposed in the gate trench is formed. The gate
dialectic electrically insulates the gate electrode from adjacent
semiconductor material. A doped superjunction region vertically
extending from a bottom of the gate trench towards the second
surface of the semiconductor body is formed. The doped
superjunction region includes first, second, and third doped
pillars vertically extending from the first surface of the first
semiconductor layer and directly adjoining one another. The second
pillar is laterally centered between the first and third pillars
and has an opposite conductivity type as the first and third
pillars.
[0006] A method of forming a vertical trenched gate transistor in a
semiconductor body having first and second vertically spaced apart
surfaces, the vertical trenched gate transistor having an n-type
source region extending form the first surface into the
semiconductor body, a p-type body region disposed beneath and
adjoining the source region, an n-type drift region disposed
beneath and adjoining the body region, an n-type field stop region
that is more highly doped than the drift region disposed beneath
and adjoining the doped n-type drift region, a gate trench
extending from the first surface through the source and body
regions, and a gate electrode disposed in the gate trench and being
configured to control a vertical current flowing between the first
and second surfaces is disclosed. According to an embodiment of the
method, a doped superjunction region vertically extending from a
bottom of the gate trench towards the second surface of the
semiconductor body is formed. The doped superjunction region
includes first, second, and third doped pillars vertically
extending from the first surface of the first semiconductor layer
and directly adjoining one another. The second pillar is laterally
centered between the first and third pillars and has an opposite
conductivity type as the first and third pillars.
[0007] A vertical trenched gate transistor that is formed in a
semiconductor body having first and second vertically spaced apart
surfaces is disclosed. According to an embodiment, the vertical
trenched gate transistor includes an n-type source region extending
form the first surface into the semiconductor body, a p-type body
region disposed beneath and adjoining the source region, an n-type
drift region disposed beneath and adjoining the body region, and an
n-type field stop region that is more highly doped than the drift
region disposed beneath and adjoining the drift region. The
vertical trenched gate transistor further includes a gate trench
extending from the first surface through the source and body
regions, and a gate electrode and a gate dielectric in the gate
trench, the gate dielectric electrically insulating the gate
electrode from adjacent semiconductor material. The gate electrode
is configured to control a vertical current flowing between the
first and second surfaces. The vertical trenched gate transistor
further includes a doped superjunction region directly adjoining
and disposed beneath the gate trench. The doped superjunction
region includes first, second, and third doped pillars vertically
extending from a bottom of the gate trench. The second pillar is
laterally centered between the first and third pillars and forms a
p-n junction with the first and third pillars. A distance between a
bottom of the doped superjunction region and the field stop region
is greater than 50% of a vertical thickness of the drift region.
The vertical thickness of the drift region is measured between the
body region and the field stop region.
[0008] Those skilled in the art will recognize additional features
and advantages upon reading the following detailed description, and
upon viewing the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] The elements of the drawings are not necessarily to scale
relative to each other. Like reference numerals designate
corresponding similar parts. The features of the various
illustrated embodiments can be combined unless they exclude each
other. Embodiments are depicted in the drawings and are detailed in
the description which follows.
[0010] FIG. 1 illustrates an insulated gate bipolar transistor
having a doped superjunction region disposed at a bottom of the
gate trench, according to an embodiment.
[0011] FIG. 2, which includes FIGS. 2A and 2B, illustrates first
semiconductor layers that can be used to form a power semiconductor
device, according to an embodiment.
[0012] FIG. 3 illustrates forming a trench in a first semiconductor
layer, according to an embodiment.
[0013] FIG. 4 illustrates forming a first doped region that lines
the perimeter of the trench in the first semiconductor layer using
ion implantation, according to an embodiment.
[0014] FIG. 5 illustrates forming a first doped region that lines
the perimeter of the trench using epitaxial deposition, according
to an embodiment.
[0015] FIG. 6, which includes FIGS. 6A and 6B, illustrates forming
a second opposite conductivity type doped region on the first doped
region using epitaxial deposition, according to an embodiment. FIG.
6A illustrates the process being applied to the device of FIG. 4.
FIG. 6B illustrates the process being applied to the device of FIG.
5.
[0016] FIG. 7, which includes FIGS. 7A and 7B, illustrates
epitaxially forming a second semiconductor layer that covers the
first and second doped regions according to an embodiment. FIG. 7A
illustrates the process being applied to the device of FIG. 6A.
FIG. 7B illustrates the process being applied to the device of FIG.
6B.
[0017] FIG. 8, which includes FIGS. 8A and 8B, illustrates forming
a gate trench in the second semiconductor layer. FIG. 8A
illustrates the process being applied to the device of FIG. 7A.
FIG. 8B illustrates the process being applied to the device of FIG.
7B.
[0018] FIG. 9, which includes FIGS. 9A and 9B, illustrates forming
a gate electrode and gate dielectric in the gate trench. FIG. 9A
illustrates the process being applied to the device of FIG. 8A.
FIG. 9B illustrates the process being applied to the device of FIG.
8B.
[0019] FIG. 10, which includes FIGS. 10A and 10B, illustrates two
different embodiments of an insulated gate bipolar transistor that
may be formed according to the techniques described with reference
to FIGS. 2-9, according to an embodiment.
[0020] FIG. 11 illustrates a method of forming a doped
superjunction region by implanting dopant atoms into the first
semiconductor layer to form a first doped well, according to an
embodiment.
[0021] FIG. 12 illustrates implanting dopant atoms into the first
semiconductor layer o form a second doped well, according to an
embodiment.
[0022] FIG. 13, which includes FIGS. 13A, 13B, and 13C, illustrates
three different embodiments of an insulated gate bipolar transistor
with a doped superjunction region that is formed according to the
techniques described with reference to FIGS. 11-12, according to an
embodiment.
[0023] FIG. 14 illustrates a method of forming a doped
superjunction region by implanting dopant atoms into the first
semiconductor layer to form first and third doped wells, according
to an embodiment.
[0024] FIG. 15 illustrates implanting dopant atoms into first
semiconductor layer to form a second doped well of the doped
superjunction region, according to an embodiment.
[0025] FIG. 16 illustrates three different embodiments of an
insulated gate bipolar transistor that may be formed according to
the techniques described with reference to FIGS. 14-15, according
to an embodiment.
[0026] FIG. 17 illustrates a method of forming a doped
superjunction region by forming a gate trench in a semiconductor
body and implanting dopants into a bottom of the gate trench,
according to an embodiment.
DETAILED DESCRIPTION
[0027] Embodiments disclosed herein include a power semiconductor
device. According to an embodiment, the power semiconductor device
is an IGBT with a vertical trenched-gate electrode structure. The
power semiconductor device includes a doped superjunction region
that is disposed at the bottom of the gate trench and vertically
extends into the drift region of the device. The doped
superjunction region includes three doped pillars or stripes of
alternating conductivity type (i.e., p-n-p or n-p-n). The doped
superjunction region vertically extends no more than halfway into
the drift region.
[0028] Various methods for forming the power semiconductor device
are disclosed. Embodiments of these methods include providing a
lightly doped first conductivity type first semiconductor layer.
The doped superjunction region is formed in the first semiconductor
layer. A variety of different techniques are disclosed for forming
the doped superjunction region in the first semiconductor layer.
One technique involves forming a trench in the first semiconductor
layer, lining the sidewalls of the trench with first conductivity
type semiconductor material, followed by filling the trench with
second conductivity semiconductor material. Another technique
involves performing masked ion implantations at the first surface
of the first semiconductor layer to form doped wells that extend
into the first semiconductor layer. These doped wells provide the
first, second, and third pillars of the doped superjunction region.
After the doped superjunction region is formed, a semiconductor
second layer is epitaxially grown on the first semiconductor layer
such that the second semiconductor layer covers the doped
superjunction region. The gate trench is formed in the second
semiconductor layer, with the bottom of the gate trench extending
to the doped superjunction region. Active device regions, e.g.,
source, body, collector, emitter, etc., are formed in the second
semiconductor layer.
[0029] The disclosed power semiconductor device and corresponding
methods for forming the power semiconductor device have several
notable advantages. For example, the structure of the doped
superjunction region at the bottom of the gate trench avoids very
high electric fields at the vicinity of bottom of the gate trench
and thus advantageously improves the robustness of the device with
respect to cosmic ray radiation. Moreover, the structure of the
doped superjunction region beneficially improves the electrical
performance of the device including switching losses and switching
speed, while maintaining desirable breakdown voltage and
on-resistance. Furthermore, the processes used to form the doped
superjunction substantially less expensive, more reliable, and more
controllable in comparison to prior art techniques for forming
superjunction structures.
[0030] Referring to FIG. 1, an insulated gate bipolar transistor
100 is depicted. The insulated gate bipolar transistor 100 is
formed in a semiconductor body 102 that has a first surface 104 and
a second surface 106 that is vertically spaced apart from the first
surface 104. The insulated gate bipolar transistor 100 includes, in
successive order from the first surface 104 to the second surface
106, a first conductivity type (e.g., n-type) source region 108, a
second conductivity type (e.g., p-type) body region 110, a first
conductivity type drift region 112, and a second conductivity type
collector region 114. The majority carrier concentration of the
drift region 112 can be in a range between 10.sup.12 cm.sup.-3 and
5.times.10.sup.14 cm.sup.-3, for example, 8.times.10.sup.13
cm.sup.-3. The majority carrier concentration of the source region
108, the body region 110, and the collector region 114 can be in a
range between 10.sup.16 cm.sup.-3 and 10.sup.21 cm.sup.-3, for
example, 1.times.10.sup.16 cm.sup.-3 for the body region 110,
1.times.10.sup.17 cm.sup.-3 for the collector region 114,
1.times.10.sup.19 cm.sup.-3 for the source region 108. The source
region 108 is in ohmic contact with an emitter electrode 116 that
is disposed on the first surface 104 of the semiconductor body 102,
and the collector region 114 is in ohmic contact with a collector
electrode 118 that is disposed on the second surface 106 of the
semiconductor body 102. A gate trench 120 vertically extends from
the first surface 104 of the semiconductor body 102 through the
source and body regions 108, 110 and into the drift region 112. A
vertical length of the gate trench 120 can be in the range of 1-7
.mu.m. An electrically conductive gate electrode 124 and an
electrically insulating gate dielectric 122 are disposed in the
gate trench 120. In a conventionally known manner, the insulated
gate bipolar transistor 100 is configured to control a current
between the emitter/source terminal and the collector terminal
responsive to a voltage applied to the gate electrode 124. When the
emitter and collector electrodes 116, 118 are forward biased and a
voltage is applied to the gate electrode 124, a conductive channel
arises in the body region 110. This conductive channel provides the
base current for a vertical bipolar transistor that is formed by
the body region 110, the drift region 112, and the collector region
114. In a forward blocking condition, i.e., when the emitter and
collector electrodes 116, 118 are reverse biased, the p-n junction
between the body region 110 and the drift region 112 becomes
reverse biased and a space charge region expands across the drift
region 112. At high blocking voltages, e.g., 200V, 400V or more,
significant electric field arises in the device, and in particular
near the bottom of the gate trench.
[0031] Optionally, the insulated gate bipolar transistor 100 may
include a first conductivity type field stop region 126 that is
more highly doped than the drift region 112, and is interposed
between the drift region 112 and the collector region 114. The
field stop region 126 is configured to reduce the peak electric
field at the collector side of the device and thereby improve the
breakdown characteristics. In the case of a short circuit condition
or a cosmic ray radiation event, a high electric field may arise in
the vicinity of the field-stop region 126 due to high electron
current density in this region. The field stop region 126 may have
a doping concentration in a range between 5.times.10.sup.14
cm.sup.-3 and 1.times.10.sup.17 cm.sup.-3. In the depicted
embodiment, the field stop region 126 is disposed only at the
interface with the collector region 114. Alternatively, multiple
field stop regions can be vertically throughout the lower half of
the drift region 112.
[0032] As a further option, the insulated gate bipolar transistor
100 may include a first conductivity type injection region 127 that
is more highly doped than the drift region 112, and is interposed
between the drift region 112 and the body region 110. The injection
region 127 enhances on-state conduction performance by injecting
majority carriers into the drift region 112. The first conductivity
type injection region 127 may have a doping concentration in a
range between 5.times.10.sup.14cm.sup.-3and 1.times.10.sup.18
cm.sup.-3.
[0033] The insulated gate bipolar transistor 100 further includes a
doped superjunction region 128. The doped superjunction region 128
is disposed within the drift region 112 beneath the gate trench
120. According to an embodiment, the doped superjunction region 128
directly adjoins the bottom of the gate trench 120. The doped
superjunction region 128 extends from the bottom of the gate trench
120 in a vertical direction towards the second surface 106 of the
semiconductor body 102. The doped superjunction region 128 contains
at least two discrete regions with a different doping type or
concentration than the surrounding drift region 112. For example,
these various doped regions of the many each have a majority
carrier concentration in the range of 5.times.10.sup.14 cm.sup.-3
and 10.sup.17 cm.sup.-3.
[0034] According to an embodiment, the doped superjunction region
128 includes first, second and third doped pillars 130, 132, 134.
The first, second and third doped pillars 130, 132, 134 can in
general be formed in any shape that is elongated in the vertical
direction of the semiconductor body 102. According to one
embodiment, the first, second and third doped pillars 130, 132, 134
are shapes as vertical stripes. Each of these stripes may have a
substantially identical width. According to other embodiments, the
stripes have differing widths. According to one embodiment, the
first and third doped pillars 130, 134 have a second conductivity
type majority carrier concentration and the second pillar 132 has a
first conductivity type majority carrier concentration.
Alternatively, the first and third doped pillars 130, 134 can have
the first conductivity type majority carrier concentration and the
second doped pillar 132 can have the second conductivity type
majority carrier concentration. Either configuration is possible,
regardless of the conductivity type of the drift region 112. At
least the second doped pillar 132 may directly adjoin the bottom of
the gate trench 120. Optionally, all three of the first, second,
and third doped pillars 130, 132, 134 may directly adjoin the
bottom of the gate trench 120.
[0035] According to an embodiment, the distance (D1) between a
bottom of the doped superjunction region 128 and the field stop
region 126 is greater than 50% of a vertical thickness (D2) of the
drift region of the device. As used herein, the vertical thickness
(D2) of the drift region is measured as the shortest distance
between the body region 110 and the field stop region 126 in a
direction perpendicular to the first and second surfaces 104, 106
of the semiconductor body 102. In embodiments that include the
injection region 127, the vertical thickness (D2) of the drift
region 112 encompasses this region as well. The vertical length of
the doped superjunction region 128 can vary, depending on the
desired electrical attributes of the device. For example, the
distance between the bottom of the doped superjunction region 128
and the field stop region 126 can be greater than 70% of a vertical
thickness of the drift region 112, and can be greater than 90% of a
vertical thickness of the drift region 112. The vertical length of
the doped superjunction region 128 can be in the range of 1 to 20
.mu.m, and can be 5 or 10 .mu.m in some embodiments. The top of the
doped superjunction region 128 can be spaced apart from the
injection region 127 by a portion of the drift region as depicted
in FIG. 1. Alternatively, the top of the doped superjunction region
128 can directly adjoin the injection region 127.
[0036] FIG. 1 shows only 1 insulated gate bipolar transistor 100
for simplicity sake. However, it is to be understood that the
semiconductor body 102 can include multiple ones of the insulated
gate bipolar transistor 100, with each device being configured
according to one or more of the embodiments described herein. These
devices can be connected together to form a single switch, or
alternatively can have separate terminals and be operated
independent from one another.
[0037] Selected method steps for forming various embodiments of the
insulated gate bipolar transistor 100 of FIG. 1 will now be
discussed with reference to FIGS. 2-16.
[0038] Referring to FIG. 2, a first semiconductor layer 136 is
provided. The first semiconductor layer 136 includes first and
second vertically spaced apart surfaces 138, 140. The first
semiconductor layer 136 may consist of or include one or more of a
variety of semiconductor materials that are used to form integrated
circuit devices, such as silicon (Si), silicon carbide (SiC),
germanium (Ge), a silicon germanium crystal (SiGe), gallium nitride
(GaN), gallium arsenide (GaAs), and the like.
[0039] Two different embodiments for providing the first
semiconductor layer 136 are depicted in FIG. 2. FIG. 2A depicts an
embodiment in which the first semiconductor layer 136 is provided
from a bulk wafer. The bulk wafer can be a FZ (floating zone) wafer
or alternatively can be a MCZ (magnetic Czochralski) wafer. In
either case, the bulk wafer can have an intrinsic doping type and
concentration corresponding to that of the drift region 112, i.e.,
a first conductivity type with a majority carrier concentration in
the range of 10.sup.12 cm.sup.-3 and 5.times.10.sup.14 cm.sup.-3.
Referring to FIG. 2B, the first semiconductor layer 136 is a
compound semiconductor layer that is formed from epitaxy. This
process includes providing a bulk semiconductor substrate 142, such
as a silicon or silicon carbide substrate. Subsequently, a first
semiconductor region 144 (which may include a number of epitaxial
layers) is epitaxially grown on the bulk semiconductor substrate
142. The first semiconductor region 144 may have a doping type and
concentration corresponding to that of the field stop region 126.
Subsequently, a second epitaxial region 146 (which may include a
number of epitaxial layers) is grown on the first epitaxial region
144. The second epitaxial region 146 may have a doping type and
concentration corresponding to that of the drift region 112, i.e. a
first conductivity type with a majority carrier concentration in
the range of 10.sup.12 cm.sup.-3 and 5.times.10.sup.14
cm.sup.-3.
[0040] Referring to FIG. 3, a first trench 148 is formed in the
first semiconductor layer 136. The first trench 148 is formed at
the first surface 138 of the first semiconductor layer 136, and
vertically extends from the first surface 138 towards the second
surface 140 of the first semiconductor layer 136. The first trench
148 can be formed according to any of a variety of commonly known
semiconductor processing techniques. For example, the first trench
148 can be formed by a wet or dry masked etching technique. The
etching can be isotropic or anisotropic.
[0041] Referring to FIG. 4, a first doped semiconductor region 150
that lines a perimeter of the first trench 148 is formed, according
to an embodiment. The first doped semiconductor region 150 has a
majority carrier concentration of the first conductivity type. The
first doped semiconductor region 150 directly adjoins the bottom
and sidewalls of the first trench 148. That is, one side of the
first doped semiconductor region 150 conforms to the shape of the
first trench 148.
[0042] According to the technique used in FIG. 4, the first doped
semiconductor region 150 is formed by implanting first conductivity
type dopants from an external source into the sidewalls and bottom
of the first trench 148. The implantation angle may deviate from 90
degrees such that the dopant atoms penetrate deeply into the
sidewalls of the trench. As a result, the dopant atoms penetrate
the bottom and sidewalls of the first trench 148 so as to form the
first doped semiconductor region 150 within the first semiconductor
layer 136. That is, the first doped region extends inward into the
first semiconductor layer 136 from the bottom and sidewalls of the
first trench 148. A mask (not shown) may be used to prevent the
dopant atoms from penetrating other portions of the first
semiconductor layer 136. Alternatively, the doping atoms can be
incorporated into the sidewalls and the bottom of the first trench
140 by a plasma deposition technique.
[0043] Referring to FIG. 5, the first doped semiconductor region
150 is formed, according to another embodiment. According to this
technique, the first doped semiconductor region 150 is formed by
epitaxially depositing a third semiconductor layer 152 on the first
semiconductor layer 136. Different to the embodiment of FIG. 4, in
this embodiment, the first doped semiconductor region 150 is
disposed inside of the first trench 148 and extends inward from the
bottom and sidewalls of the first trench 148. The third
semiconductor layer 152 is formed from the same semiconductor
material as the first semiconductor layer 136, but has a higher
majority carrier concentration than the first layer. The thickness
of the third semiconductor layer 152 is controlled by the epitaxy
process such that a void 154 remains in the first trench 148
between sections of the third semiconductor layer 152. That is, the
epitaxy process is stopped before the third semiconductor layer 152
completely fills the first trench 148.
[0044] In either case of FIGS. 4 and 5, the first doped
semiconductor region 150 is formed with a doping concentration
corresponding to that of the first and third doped pillars 130,
134. For example, the majority carrier concentration of the first
doped semiconductor region 150 may be in the range of
5.times.10.sup.14 cm.sup.-3 and 10.sup.17 cm.sup.-3.
[0045] Referring to FIG. 6, a second doped semiconductor region 156
is formed on the first doped semiconductor region 150, according to
embodiments. FIG. 6A depicts an embodiment in which the second
doped semiconductor region 156 is formed on the device of FIG. 4.
FIG. 6B depicts an embodiment in which the second doped
semiconductor region 156 is formed on the device of FIG. 5. In
either case, the second doped semiconductor region 156 is formed in
the first trench 148 between the sections of the first doped
semiconductor region 150 that line the sidewalls of the first
trench 148. The second doped semiconductor region 156 is formed by
epitaxially depositing a fourth semiconductor layer 158 of the
second conductivity type. The fourth semiconductor layer 158 is
epitaxially grown on exposed surfaces of the first semiconductor
layer 136 in the case of FIG. 6A or the exposed surfaces of the
third semiconductor layer 152 in the case of FIG. 6B. The epitaxy
process is controlled such that the fourth semiconductor layer 158
is sufficiently thick to completely fill the first trench 148 in
the case of FIG. 6A or to completely fill the void 154 that remains
between the third semiconductor layer 152 in the case of FIG.
6B.
[0046] Referring to FIG. 7, a second semiconductor layer 160 is
formed on the first surface 138 of the first semiconductor layer
136, according to embodiments. FIG. 7A depicts an embodiment in
which the second semiconductor layer 160 is formed on the device of
FIG. 6A. FIG. 7B depicts an embodiment in which the second
semiconductor layer 160 is formed on the device of FIG. 6B. The
second semiconductor layer 160 is epitaxially grown on the first
surface 138 of the first semiconductor layer 136. Before forming
the second semiconductor layer 160, a planarization process may be
applied to the fourth and third semiconductor layers 158, 152 (in
the embodiment of FIG. 6B) or the fourth semiconductor layer 158
(in the embodiment of FIG. 6A) so as to expose the first surface
138 of the first semiconductor layer 136. The second semiconductor
layer 160 has the same conductivity type as the first semiconductor
layer 136, and may have a similar or identical majority carrier
concentration as the first semiconductor layer 136. The first and
second semiconductor layers 136, 160 collectively form the
semiconductor body 102 for the insulated gate bipolar transistor
100 of FIG. 1, wherein the second surface 140 of the first
semiconductor layer 136 forms the second surface 106 of the
semiconductor body 102 and a first surface 162 of the second
semiconductor layer 160 that is opposite from the first
semiconductor layer 136 forms the first surface 104 of the
semiconductor body 102.
[0047] Referring to FIG. 8, the gate trench 120 is formed in the
second semiconductor layer 160, according to embodiments. FIG. 8A
depicts an embodiment in which the gate trench 120 is formed on the
device of FIG. 7A. FIG. 8B depicts an embodiment in which the gate
trench 120 is formed on the device of FIG. 7B. The gate trench 120
may be formed by a wet or dry isotropic etch technique. A mask (not
shown) may be provided on the first surface 162 of the second
semiconductor layer 160 and patterned in a desired geometry of
which the gate trench 120. The gate trench 120 is formed in such a
way that the bottom of the gate trench 120 directly adjoins the
doped superjunction region 128. The processes for forming the doped
superjunction region 128 and for forming the gate trench 120 are
aligned such that the gate trench 120 is at least approximately
laterally centered with respect to the doped superjunction region
128. The gate trench 120 may directly contact the second doped
pillar 132. As shown in FIGS. 8A and 8B, the bottom of the gate
trench 120 also contacts the first and third doped pillars 130, 134
of the doped superjunction region 128. However, this is not
necessary, and the width of the gate trench 120, the doped
superjunction region 128, and the first, second and third doped
pillars 130, 132, 134 can be increased or decreased depending on
user requirements or process capabilities.
[0048] Referring to FIG. 9, the gate electrode 124 and the gate
dielectric 122 are formed in the gate trench 120, according to
embodiments. FIG. 9A depicts an embodiment in which the gate
electrode 124 and the gate dielectric 122 are formed in the device
of FIG. 8A. FIG. 9B depicts an embodiment in which the gate
electrode 124 and the gate dielectric 122 are formed in the device
of FIG. 8B. The gate dielectric 122 may be formed by an oxidation
process and the gate electrode 124 may be formed by depositing an
electrically conductive material (e.g. polysilicon, aluminum, etc.)
in the oxidized gate trench 120.
[0049] FIG. 10 depicts two embodiments of the insulated gate
bipolar transistor 100. FIG. 10A depicts an insulated gate bipolar
transistor 100 formed form the device of FIG. 9A. FIG. 10B depicts
an insulated gate bipolar transistor 100 formed form the device of
FIG. 9B. In either case, after the processing steps described with
reference to FIG. 9 are performed, the remaining device regions of
the of the insulated gate bipolar transistor 100, including the
source region 108, the body region 110, the collector region 114,
the field stop region 126, and the injection region 127 can be
formed by conventional front end of the line doping techniques. For
example, ion implantation or diffusion doping processes can be
performed at the first and second surfaces 104, 106 of the
semiconductor body 102 so as to form doped regions within the
semiconductor body 102.
[0050] Referring to FIGS. 11-12, selected method steps for forming
the insulated gate bipolar transistor 100 are depicted, according
to another embodiment. According to this technique, the method
steps for forming the doped superjunction region 128 as described
with reference to FIGS. 3-6 are replaced with the following steps.
Referring to FIG. 11, the first semiconductor layer 136 as
described with reference to FIG. 2 is provided. A first mask 164 is
provided on the first surface 138 of the first semiconductor layer
136. The first mask 164 is pattered so as to expose a desired
region of the first semiconductor layer 136. Subsequently, first
conductivity type dopants are implanted into the first surface 138
of the first semiconductor layer 136. A thermal anneal may be
performed to activate the dopants. As a result, a first doped well
166 that vertically extends from the first surface 138 of the first
semiconductor layer 136 is formed. The first doped well 166 has a
majority carrier concentration corresponding to that of the first
and third doped pillars 130, 134, i.e., in the range of
5.times.10.sup.14 cm.sup.-3 and 10.sup.17 cm.sup.-3.
[0051] Referring to FIG. 12, the first mask 164 has been removed
and a second mask 168 has been provided on the first semiconductor
layer 136. The second mask 168 is patterned with an opening that
partially exposes and partially covers the first doped well 166.
Subsequently, second conductivity type dopants are implanted into
the first surface 138 of the first semiconductor layer 136 in a
similar or identical manner as described above. As a result, a
second doped well 170 that vertically extends from the first
surface 138 of the first semiconductor layer 136. The second doped
well 170 has a majority carrier concentration corresponding to that
of the second pillar, i.e., in the range of 5.times.10.sup.14
cm.sup.-3 and 10.sup.17 cm.sup.-3. The first doped well 166 is
wider than the second doped well 170 and the second doped well 170
is arranged in a lateral center of the first doped well 166. As a
result, portions of the first doped well 166 are disposed on both
lateral sides of the second doped well 170. These portions form the
first and third doped pillars 130, 134 of the doped superjunction
region 128, respectively. Likewise, the second doped well 170 forms
the second doped pillar 132 of the doped superjunction region 128.
The width and position of the first, second and third doped pillars
130, 132, 134 may therefore be controlled by adjusting the process
parameters of the doping steps described with reference to FIGS. 11
and 12. To form high aspect ratio (i.e., large depth in relation to
width) first, second, and third doped pillars 130, 132, 134, it is
possible to use the so-called channeling effect, which utilizes
high-energy implant using a very small implant angle, e.g., less
than 0.15%. Instead of the two step masking process as described
with reference to FIGS. 11 and 12, it is possible to use a single
mask to form both of the first and second doped wells 166, 170.
This can be done by using dopant atoms with different diffusion
coefficients, such as Boron (B), Arsenic (As) or Antimony (Sb).
[0052] FIG. 13 depicts three embodiments of an insulated gate
bipolar transistor 100 having a doped superjunction that is formed
according to the techniques described with reference to FIGS.
11-12. After performing the method steps described with reference
to FIGS. 11-12 and removing any remaining mask, the second
semiconductor layer 160 can be epitaxially grown on the first
semiconductor layer 136, e.g., in the manner described with
reference to FIG. 7. A gate trench 120 can be etched in the second
semiconductor layer 160, followed by the formation of the gate
electrode 124 and the gate dielectric 122 in the gate trench 120,
e.g., in the manner described with reference to FIGS. 8-9. The
remaining device regions of the insulated gate bipolar transistor
100 including the source region 108, the body region 110, the
collector region 114, the field stop region 126, and the injection
region 127 can be formed by conventionally known techniques.
[0053] In the embodiment of FIG. 13A, the second doped pillar 132
does not vertically extend as deep into the first semiconductor
layer 136 as the first and third doped pillars 130, 134. In the
embodiment of FIG. 13B, the second doped pillar 132 vertically
extends approximately as deep into the first semiconductor layer
136 as the first and third doped pillars. In the embodiment of FIG.
13C, the second doped pillar 132 vertically extends further into
the first semiconductor layer 136 as the first and third doped
pillars 130, 134. The different vertical lengths of the first and
second doped wells 166, 170 can be achieved by varying the
implantation energy of the dopant atoms and/or selecting dopant
atoms with different diffusion coefficients.
[0054] Referring to FIGS. 14-15, selected method steps for forming
the insulated gate bipolar transistor 100 are depicted, according
to another embodiment. According to this technique, the method
steps for forming the doped superjunction region 128 as described
with reference to FIGS. 3-6 are replaced with the following steps.
Referring to FIG. 14, the first semiconductor layer 136, as
described with reference to FIG. 2, is provided. A first mask 164
is provided on the first surface of the first semiconductor layer
136. The first mask 164 is patterned with two laterally spaced
openings. First conductivity type dopants are implanted into the
first semiconductor layer 136 in the mask openings, activated, and
in-diffused in the manner described above. As a result, first and
third doped wells 166, 172 that are laterally spaced apart from one
another are formed in the first semiconductor layer 136. The first
and third doped wells 166, 172 have the first conductivity type and
have a higher doping concentration than the adjacent portions of
first semiconductor layer 136.
[0055] The first and third doped wells 166, 172 have a majority
carrier concentration corresponding to that of the first and third
doped pillars 130, 134, i.e., in the range of 5.times.10.sup.14
cm.sup.-3 and 10.sup.17 cm.sup.-3.
[0056] Referring to FIG. 15, the first mask 164 has been removed
and a second mask 174 has been provided on the first semiconductor
layer 136. The second mask is 174 patterned so as to expose a
region of the first semiconductor layer 136 that is between the
first and third doped wells 166, 172. Second conductivity type
dopants are implanted into the first semiconductor layer 136 in the
mask openings and activated in the manner described above. As a
result, a second doped well 170 is disposed between and forms a p-n
junction with the first and third doped wells 166, 172. The second
doped well has a majority carrier concentration corresponding to
that of the second doped pillar 132, i.e., in the range of
10.sup.15 cm.sup.-3 and 10.sup.16 cm.sup.-3.
[0057] FIG. 16 depicts three embodiments of an insulated gate
bipolar transistor 100 having a doped superjunction that is formed
according to the techniques described with reference to FIGS.
14-15. After performing these method steps, any remaining mask can
be removed and the second semiconductor layer 160 can be
epitaxially grown on the first semiconductor layer 136, e.g., in
the manner described with reference to FIG. 7. A gate trench 120
can be etched in the second semiconductor layer 160, followed by
the formation of the gate electrode 124, and the gate dielectric
122 in the gate trench 120, e.g., in the manner described with
reference to FIGS. 8-9. The remaining device regions of the of the
insulated gate bipolar transistor 100 including the source region
108, the body region 110, the collector region 114, the field stop
region 126, and the injection region 127 can be formed by
conventionally known techniques.
[0058] In the embodiment of FIG. 16A, the second doped pillar 132
does not vertically extend as deep into the first semiconductor
layer 136 as the first and third doped pillars 130, 134. In the
embodiment of FIG. 15B, the second doped pillar 132 vertically
extends approximately as deep into the first semiconductor layer
136 as the first and third doped pillars 130, 134. In the
embodiment of FIG. 15C, the second doped pillar 132 vertically
extends further into the first semiconductor layer 136 as the first
and third doped pillars 130, 134. These different configurations
can be achieved by varying the implantation energy for forming the
first, second, and third as described with reference to FIGS.
13-14.
[0059] Referring to FIG. 17, selected method steps for forming the
insulated gate bipolar transistor 100 are depicted, according to
another embodiment. According to this technique, the method steps
for forming the doped superjunction region 128 as described with
reference to FIGS. 3-6 are replaced with the following steps. The
first semiconductor layer 136 is provided. In this embodiment, the
first semiconductor layer 136 provides the entire semiconductor
body 102. That is, the step of epitaxially forming the second
semiconductor layer 160 is omitted. The gate trench 120 is formed
in the first surface 138 of the first semiconductor layer 136. This
may be done using a wet or dry anisotropic etching technique, for
example. As shown in FIG. 17, a mask 178 is formed on the first
surface 138 of the first semiconductor layer 136 and is patterned
in a desired geometry of the gate trench 120. Subsequently, ions
are implanted into the bottom of the gate trench 120. The mask 178
prevents these ions from penetrating other regions of the
semiconductor body 102. The ion implantation process includes two
separate processes of implanting first conductivity type dopants
and second conductivity type dopants. In this way, the first doped
well 166 and the second doped well 170 as previously discussed can
be formed at the bottom of the gate trench 120. The first doped
well 166 can be made wider than the second doped well 170 by
adjusting process parameters including dopant type, implantation
energy, implantation angle, activation time, etc., as between the
two ion implantation processes. The remaining device regions of the
of the insulated gate bipolar transistor 100, including the source
region 108, the body region 110, the collector region 114, the
field stop region 126, and the injection region 127, can be formed
by conventional front end of the line doping techniques.
[0060] One advantage of the technique described with reference to
FIG. 17 is that it is a self-aligned technique. That is, the doped
superjunction region 128 is necessarily laterally centered with
respect to the gate trench 120 because the same mask 178 is used to
form both the gate trench 120 and the doped superjunction region
128. This can advantageously improve yield and/or performance, as
the doped superjunction region 128 is always formed in the correct
area.
[0061] The inventors have discovered several advantages to the
methods for forming the power semiconductor device in comparison to
prior art techniques. These advantages include reduced processing
cost and improved repeatability and yield in comparison to prior
art techniques. Conventionally, superjunction structures for power
semiconductor devices are formed from elongated p-type pillars in
an n-type drift region 112 (or vice-versa). These elongated pillars
are typically adjacent and laterally offset form the gate trench
120. Conventional techniques for forming these structures include a
multilayer epitaxial growth process that involves the successive
formation of doped regions in each epitaxial layer. Alternatively,
these elongated pillars can be formed by a deep trench technique
that involves the formation of high aspect ratio trenches. In
either case, these processing steps are costly, time consuming and
difficult to control. In particular, it is difficult to form these
elongated pillars with a substantially homogenous doping
concentrations. Charge imbalance in these structures can
detrimentally impact the blocking capability of the device. By
contrast, the presently disclosed methods used to form the doped
superjunction region 128 involve cost-effective and highly
controllable techniques, including ion implantation, trench
etching, and epitaxial growth. Many fewer epitaxial cycles are
required in comparison to a multilayer epitaxial growth process.
Moreover, the doping concentration of the pillars in the doped
superjunction region 128 is highly uniform as a result of these
techniques. In some cases, process variation may produce first,
second, and third doped pillars 130, 132, 134 having varying
vertical heights, e.g., as depicted in FIGS. 13 and 16. The
inventors have discovered that these variations have a negligible
impact on the electrical performance of the device with respect to
key electrical parameters including turn-off losses, shorter delay
time and shorter turn-off time. Thus, the process window for
forming the doped superjunction region 128 is greatly enhanced in
comparison to the prior art techniques.
[0062] Furthermore, the inventors have discovered several
improvements to the electrical characteristics of the power
semiconductor device in comparison to the prior art devices. These
advantages include a drift region structure that produces the
combined benefits of shielding the bottom of the gate trench 120
from high electric fields while simultaneously introducing
compensating charges in the drift region of the device that improve
the switching performance of the device. Because the doped
superjunction is located at the bottom of the gate trench 120 and
includes a p-n junction, an electrically insulating space charge
region arises that reduces the electric field strength at the
bottom of the gate trench 120 and hardens the device against
failure mechanisms associated with high electric field strength,
including cosmic ray radiation.
[0063] In regards to switching performance, the doped superjunction
utilizes the superjunction principle to lower turn-off losses,
decrease delay time and decrease turn-off time with minimal impact
on on-state resistance and breakdown voltage. Generally speaking,
the switching performance of a power transistor depends upon how
quickly the device can remove free carriers from the drift region
during turn-off so that the device can enter a blocking state.
Although one can improve the switching performance by decreasing
the doping concentration of the drift region, this results in an
unfavorable increase to the on-state resistance of the device and
higher ohmic losses. The superjunction principle beneficially
shifts this tradeoff by introducing compensating charges in the
drift region. By introducing compensating charges in the drift
region, a space charge region will arise more quickly when the
device is turned off. As a result, improved turn-off losses,
shorter delay time and shorter turn-off time can be realized
without compromising on-state performance.
[0064] The inventors have found in particular that the currently
disclosed design of the doped superjunction region 128, which does
not extend more than 50% of a vertical thickness of the drift
region 112, provides favorable electrical characteristics in
comparison to conventional superjunction structures. Generally
speaking, it is desirable to balance charges as much as possible in
the drift region of the device for rapid switching time. To this
end, the conventional superjunction structures for vertical power
semiconductor devices vertically extend completely, or close to
completely, to the bottom of the drift region. In this way, a
complete charge balance or near complete charge balance throughout
the device can be achieved. However, this design may lead to very
fast switching speeds, which can be problematic in some cases. For
example, in certain applications, fast switching times can lead to
higher voltage overshoot and enable higher switching frequency
which then induces a significant amount of electromagnetic
interference (EMI). Some applications place an upper limit for
dV/dt (i.e., switching speed) to 5 kV/.mu.s because anything higher
can lead to reliability issues. The device described herein
exhibits significant reduction in the turn-off losses without much
change in the on-state voltage. Meanwhile, the switching speed
(dV/dt) only marginally increases in comparison to power
semiconductor devices that do not include any superjunction
structures. Put another way, the limited depth doped superjunction
region 128 described herein nearly approximates the beneficial
characteristics of conventional superjunction structures relating
to switching efficiency while avoiding the drawbacks of
conventional superjunction structures associated with ultra-fast
switching times.
[0065] The present specification refers to a "first" and a "second"
conductivity type of dopants that semiconductor portions are doped
with. The first conductivity type may be n-type and the second
conductivity type may be p-type (or vice versa). As is generally
known, depending on the doping type or the polarity of the source
and drain regions, MOSFETs may be n-channel or p-channel MOSFETs.
For example, in an n-channel MOSFET, the source and the drain
region are doped with n-type dopants, and the current direction is
from the drain region to the source region. In a p-channel MOSFET,
the source and the drain region are doped with p-type dopants, and
the current direction is from the source region to the drain
region. Insulated gate bipolar transistors can likewise be
configured with a MOSFET portion that is an n-channel MOSFET or a
p-channel MOSFET. Bipolar transistors can be p-n-p devices or n-p-n
devices. As is to be clearly understood, within the context of the
present specification, the doping types may be reversed. If a
specific current path is described using directional language, this
description is to be merely understood to indicate the path and not
the polarity of the current flow, i.e. whether the transistor is a
p-channel or an n-channel transistor.
[0066] The terms "wafer," "substrate," and "semiconductor
substrate" used in the following description may include any
semiconductor-based structure that has a semiconductor surface.
Wafer and structure are to be understood to include silicon,
silicon-on-insulator (SOI), silicon-on sapphire (SOS), doped and
undoped semiconductors, epitaxial layers of silicon supported by a
base semiconductor foundation, and other semiconductor structures.
The semiconductor need not be silicon-based. The semiconductor
could as well be silicon-germanium, germanium, or gallium arsenide.
According to embodiments of the present application, generally,
silicon carbide (SiC) or gallium nitride (GaN) is a further example
of the semiconductor substrate material.
[0067] The terms "lateral" and "horizontal" as used in this
specification intends to describe an orientation parallel to a
first surface of a semiconductor substrate or semiconductor body.
This can be for instance the surface of a wafer or a die.
[0068] The term "vertical" used in this specification intends to
describe an orientation which is arranged perpendicular to the
first surface of the semiconductor substrate or semiconductor
body.
[0069] Spatially relative terms such as "under," "below," "lower,"
"over," "upper," and the like, are used for ease of description to
explain the positioning of one element relative to a second
element. These terms are intended to encompass different
orientations of the device in addition to different orientations
than those depicted in the figures. Further, terms such as "first,"
"second," and the like, are also used to describe various elements,
regions, sections, etc., and are also not intended to be limiting.
Like terms refer to like elements throughout the description.
[0070] As used herein, the terms "having," "containing,"
"including," "comprising," and the like are open ended terms that
indicate the presence of stated elements or features, but do not
preclude additional elements or features. The articles "an," and
"the" are intended to include the plural as well as the singular,
unless the context clearly indicates otherwise.
[0071] The description of the embodiments is not limiting. In
particular, elements of the embodiments described hereinafter may
be combined with elements of different embodiments.
[0072] With the above range of variations and applications in mind,
it should be understood that the present invention is not limited
by the foregoing description, nor is it limited by the accompanying
drawings. Instead, the present invention is limited only by the
following claims and their legal equivalents.
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