U.S. patent application number 09/815672 was filed with the patent office on 2001-12-13 for edge termination for silicon power devices.
This patent application is currently assigned to INTERSIL CORPORATION. Invention is credited to Dolry, Gary Mark, MurAleedharan, Praveen, Zeng, Jun.
Application Number | 20010050369 09/815672 |
Document ID | / |
Family ID | 23352402 |
Filed Date | 2001-12-13 |
United States Patent
Application |
20010050369 |
Kind Code |
A1 |
Zeng, Jun ; et al. |
December 13, 2001 |
Edge termination for silicon power devices
Abstract
A silicon semiconductor die comprises a heavily doped silicon
substrate and an upper layer comprising doped silicon of a first
conduction type disposed on the substrate. The upper layer
comprises a well region of a second, opposite conduction type
adjacent an edge termination zone that comprises a layer of a
material having a higher critical electric field than silicon. Both
the well region and adjacent edge termination zone are disposed at
an upper surface of the upper layer, and an oxide layer overlies
the upper layer and the edge termination zone. A process for
forming a silicon die having improved edge termination. The process
comprises forming an upper layer comprising doped silicon of a
first conduction type on a heavily doped silicon substrate, and
forming an edge termination zone that comprises a layer of a
material having a higher critical electric field than silicon at an
upper surface of the upper layer. A well region of a second,
opposite conduction type is formed at the upper surface of the
upper layer adjacent the edge termination zone, and an oxide layer
is formed over the upper layer and edge termination zone.
Inventors: |
Zeng, Jun; (Mountaintop,
PA) ; Dolry, Gary Mark; (Mountaintop, PA) ;
MurAleedharan, Praveen; (Wilkes-Barre, PA) |
Correspondence
Address: |
JAECKLE FLEISCHMANN & MUGEL, LLP
39 State Street
Rochester
NY
14614-1310
US
|
Assignee: |
INTERSIL CORPORATION
|
Family ID: |
23352402 |
Appl. No.: |
09/815672 |
Filed: |
March 23, 2001 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
09815672 |
Mar 23, 2001 |
|
|
|
09344868 |
Jun 28, 1999 |
|
|
|
6242784 |
|
|
|
|
Current U.S.
Class: |
257/77 ;
257/E29.012; 257/E29.081; 257/E29.104; 438/268 |
Current CPC
Class: |
H01L 29/1608 20130101;
Y10S 438/931 20130101; H01L 29/0615 20130101; H01L 29/267
20130101 |
Class at
Publication: |
257/77 ;
438/268 |
International
Class: |
H01L 021/336 |
Claims
What is claimed:
1. A silicon semiconductor die comprising: a heavily doped silicon
substrate, an upper layer comprising doped silicon of a first
conduction type disposed on said substrate, said upper layer
comprising a well region of a second, opposite conduction type
adjacent an edge termination zone, said well region and said
adjacent edge termination zone both being disposed at an upper
surface of said upper layer; and an oxide layer overlying said
upper layer and said edge termination zone; wherein said edge
termination zone comprises a layer of a material having a higher
critical electric field than silicon.
2. The die of claim 1 wherein said upper layer is an epitaxial
layer.
3. The die of claim 1 wherein said first conduction type is N and
said second conduction type is P.
4. The die of claim 1 wherein said first conduction type is P and
said second conduction type is N.
5. The die of claim 1 wherein said edge termination zone comprises
a layer of silicon carbide.
6. The die of claim 5 wherein said layer of silicon carbide is
formed by implantation, activation, and diffusion of carbon into
said upper silicon layer.
7. The die of claim 5 wherein said layer of silicon carbide is
formed by deposition .
8. The die of claim 5 wherein said layer of silicon carbide is
formed by 103709 -8heteroepitaxial growth.
9. The die of claim 1 further comprising a front metal layer
overlying and in electrical contact with said well region and a
back metal layer disposed on a bottom surface of said
substrate.
10. The die of claim 9 further comprising a field plate in
electrical contact with said front metal layer.
11. The die of claim 1 wherein said edge termination zone has a
selected thickness.
12. The die of claim 11 wherein said edge termination zone is
recessed in said upper layer and extends into said upper layer to a
depth exceeding the depth of the adjacent well region.
13. The die of claim 1 further including a field plate.
14. The die of claim 1 further including field limiting rings.
15. The die of claim 1 further including variable lateral doping
concentration.
16. The die of claim 1 further including junction termination
extension.
17. A process for forming a silicon die having improved edge
termination, said process comprising: forming an upper layer
comprising doped silicon of a first conduction type on a heavily
doped silicon substrate; forming an edge termination zone at an
upper surface of said upper layer, said edge termination zone
comprising a layer of a material having a higher critical electric
field than silicon; forming a well region of a second, opposite
conduction type in said upper layer adjacent said edge termination
zone; and forming an oxide layer over said upper layer and said
edge termination zone.
18. The process of claim 17 wherein said upper layer is an
epitaxial layer.
19. The process of claim 17 wherein said first conduction type is N
and said second conduction type is P.
20. The process of claim 17 wherein said first conduction type is P
and said second conduction type is N.
21. The process of claim 17 wherein said edge termination zone has
a selected thickness.
22. The process of claim 17 wherein said edge termination zone
comprises a layer of silicon carbide.
23. The process of claim 22 wherein said forming said layer of
silicon carbide comprises implanting, activating, and diffusing
carbon into said upper silicon layer.
24. The process of claim 22 wherein said forming said edge
termination zone comprises etching said upper surface of said upper
layer prior to forming said layer of silicon carbide, thereby
providing an edge termination zone recessed in said upper
layer.
25. The process of claim 24 wherein said edge termination zone
recessed in said upper layer extends into said upper layer to a
depth exceeding the depth of the adjacent well region.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional application of U.S. patent
application Ser. No. 09/344,868, filed Jun. 28, 1999 (Attorney
Docket No. 87552.99R097/SE-1517PD).
[0002] FIELD OF THE INVENTION
[0003] The present invention relates to silicon power semiconductor
devices and, more particularly, to a silicon semiconductor die
having an efficient and reliable edge termination zone.
BACKGROUND OF THE INVENTION
[0004] PN junctions within semiconductor devices are not infinite,
terminating at the edge zones of a die This edge effect limits the
device breakdown voltage below the ideal value, V.sub.brpp, that is
set by the infinite parallel plane junction. Care must be taken to
ensure proper and efficient termination of the junction at the edge
of the die; if the junction is poorly terminated, the device
breakdown voltage can be as low as 10-20% of the ideal case. Such
severe degradation in breakdown voltage can seriously compromise
device design and lead to reduced current rating as well. In
addition, an inefficient edge termination makes a device unstable
and unreliable if the device is operated in a harsh environment or
over a long period of time.
[0005] Various edge termination techniques have been developed,
including, for example, field plate (FP), described in F. Conti and
M. Conti, "Surface breakdown in silicon planar diodes equipped with
field plate," Solid State Electronics, Vol. 15, pp 93-105, the
disclosure of which is incorporated herein by reference. Another
edge termination approach is field limiting rings (FLR), described
in Kao and Wolley, "High voltage planar p-n junctions," Proc. IEEE,
1965, Vol. 55, pp 1409-1414, the disclosure of which is
incorporated herein by reference. Further edge termination
structures utilized variable lateral doping concentration (VLD),
described in R. Stengl et al., "Variation of lateral doping as a
field terminator for high-voltage power devices, IEEE Trans.
Electron Devices, 1986, Vol. ED-33, No. 3, pp 426-428, and junction
termination extension (JTE), described in V. A. K. Temple,
"Junction termination extension, a new technique for increasing
avalanche breakdown voltage and controlling surface electric field
in p-n junction," IEEE International Electron Devices Meeting
Digest, 1977 Abstract 20.4, pp 423-426, the disclosures of which
are incorporated herein by reference.
[0006] The purpose of all these various techniques is to reduce
electron-hole avalanche generation by lowering the peak electric
field strength along the semiconductor surface and thereby shifting
the avalanche breakdown location into the bulk of the device. To
achieve this goal, the width of the edge termination zone
(L.sub.edge) has to be several times higher than the depletion
width (W.sub.pp) of the parallel-plane portion of the PN junction.
For example, if L.sub.edge=2.98 W.sub.pp, 98.7% of V.sub.brpp can
be achieved when an "ideal edge termination," as described in Drabe
and Sittig, "Theoretical investigation of plane junction
termination," Solid State Electronics, 1996, Vol. 3, No. 3, pp
323-328, the disclosure of which is incorporated herein by
reference, is used. In practice, a longer Ledge than the
theoretical value should be used to guarantee device reliability.
However, it is very important to point out that, even with very
efficient edge termination, electron-hole impact generation at a
rate of about 1.times.10.sup.18 pairs/cm.sup.3.s, still exists
along the semiconductor surface.
SUMMARY OF THE INVENTION
[0007] A silicon semiconductor die of the present invention
comprises a heavily doped silicon substrate and an upper layer
comprising doped silicon of a first conduction type disposed on the
substrate. The upper layer comprises a well region of a second,
opposite conduction type adjacent an edge termination zone that
comprises a layer of a material having a higher critical electric
field than silicon. Both the well region and adjacent edge
termination zone are disposed at an upper surface of the upper
layer, and an oxide layer overlies the upper layer and the edge
termination zone.
[0008] Further in accordance with the present invention is a
process for forming a silicon die having improved edge termination.
The process comprises forming an upper layer comprising doped
silicon of a first conduction type on a heavily doped silicon
substrate, and forming an edge termination zone that comprises a
layer of a material having a higher critical electric field than
silicon at an upper surface of the upper layer. A well region of a
second, opposite conduction type is formed at the upper surface of
the upper layer adjacent the edge termination zone, and an oxide
layer is formed over the upper layer and edge termination zone.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] FIG. 1 schematically illustrates the formation of an edge
zone in an silicon die by implantation and diffusion.
[0010] FIGS. 2-4 schematically depict the formation of a silicon
die having a silicon carbide edge zone of a selected thickness.
[0011] FIGS. 5 and 6 illustrate the leakage current and
electron-hole impact ionization generation contours at the onset of
edge breakdown for a prior art die having a field plate.
[0012] FIGS. 7 and 8 depict the leakage current and electron-hole
impact ionization generation contours at the onset of edge
breakdown for a die of the present invention that includes a field
plate.
[0013] FIGS. 9 and 10 illustrate the leakage current and
electron-hole impact ionization generation contours at the onset of
edge breakdown for a second prior art die having a field plate, a
thin oxide layer, and a deep P-well.
[0014] FIGS. 11 and 12 depict the leakage current and electron-hole
impact ionization generation contours at the onset of edge
breakdown for a second embodiment of the present invention, wherein
the die does not include a field plate.
[0015] FIG. 13 is a plot comparing electron-hole avalanche
generation rates for silicon dies of the prior art and the present
invention.
[0016] FIGS. 14A and 14B compare surface depletion layer boundaries
in dies of the prior art and the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0017] The present invention provides more efficient and reliable
edge termination for silicon power semiconductor devices compared
with currently known techniques. In accordance with the invention,
the silicon within an edge zone of a silicon die is replaced with a
material having a higher critical electric field (E.sub.crit),
which is the maximum electric field under breakdown condition, and
a lower impact ionization generation rate, which is the number of
electron-hole pairs generated by an electron or a hole per unit
distance traveled.
[0018] Replacement of silicon with a suitable material for this
purpose can be accomplished in several ways, including, for
example, implantation or deposition, or by heteroepitaxial growth,
as described, for example, in Madapura et al., "Heteroepitaxial
Growth of SiC on Si (100) and (111) by Chemical Vapor Deposition
Using Trimethylsilane," Journal of the Electrochemical Society,
1999, Vol. 46, No. 3, pp 1197-1202, the disclosure of which is
incorporated herein by reference. SiC, because of its high
E.sub.crit (.about.12 times higher than Si) and compatible thermal
oxidation process with silicon, is a useful replacement material
for silicon in a die edge zone. Possible processes to produce the
SiC material edge to a controlled depth in a silicon substrate are
illustrated in FIGS. 1-4.
[0019] FIG. 1 illustrates the implantation of carbon, C, into the
edge zone 101 of a die 100, shown as an N-epitaxial layer, using an
oxide mask 102. A high temperature process, such as laser-promoted
local annealing, can be used to activate and diffuse the C implant,
resulting in the edge surface layer 103 of silicon die 100 being
converted to SiC.
[0020] FIGS. 2-4 illustrate another possible process to make a SiC
die edge zone. First, a recessed edge zone 201 is etched by either
a dry or a wet etch procedure to a specified depth in a silicon die
200, represented as an N-epitaxial layer, using an oxide mask 202
to prevent silicon removal from the active region 203, as shown in
FIG. 2. A SiC layer 204, which is of the same conduction type as
the epitaxial layer, is formed on recessed edge zone 201 using
heteroepitaxial growth or deposition techniques, as shown in FIG.
3. Oxide mask 202 is then removed, and chemical-mechanical
polishing (CMP) is performed to produce a fully planarized die 205
that includes SiC edge zone 204a having a selected thickness, as
demonstrated in FIG. 4.
[0021] Computer simulations have been performed to verify the
concept for a wide range of breakdown voltages, from 600V to 30V.
Simulated leakage current and electron-hole impact ionization
generation contours, 511 and 508, respectively, at the onset of
edge breakdown for a prior art die 500 with field plate edge
termination are shown in FIGS. 5 and 6, respectively. Die 500
includes a substrate 501 bearing an N-epitaxial layer 502, in which
is implanted a P-well 503. On the surface 504 of epitaxial layer
502 is deposited an oxide layer 505 and, in contact with P-well
503, a front metal layer 506 that is further in contact with a
field plate 507. A back metal layer (not shown) is formed on the
bottom of substrate 501.
[0022] As shown in FIG. 6, the highest electron-hole generation
site 508 is located close to the intersection of the PN junction
509 between P-well 503 and N-epitaxial layer 502 and silicon upper
surface 504. This is due to the termination of PN junction 509 to
form a planar diffused junction having a finite curvature, which
causes electric field crowding near upper surface 504 and leads to
large impact ionization values at the die edges. Consequently, the
breakdown occurs at junction termination edge 508 rather than in
the parallel plane portion 510.
[0023] The breakdown characteristics of a die 700 of the present
invention having a field plate edge and a SiC edge zone formed by C
implantation and diffusion have also been simulated. Die 700,
schematically depicted in FIGS. 7 and 8, includes a substrate 701
bearing an N-epitaxial layer 702, in which is implanted a P-well
703. At the surface 704 of epitaxial layer 702 is formed a SiC edge
zone 705. An oxide layer 706 is formed on SiC edge zone 705, and a
front metal layer 707 interconnects P-well 703 with a field plate
708. A back metal layer (not shown) is formed on the bottom of
substrate 701.
[0024] FIGS. 7 and 8 illustrate the simulated avalanche leakage
current and impact ionization contours, 709 and 710, respectively,
for die 700. The breakdown location 710 is completely screened from
the oxide layer 706 by SiC edge zone 705, and there is very little
electron-hole generation along upper surface 704. The breakdown
voltage for die 700 is higher than that observed for die 500.
[0025] In order to reduce the electron-hole avalanche generation
rate along the Si/oxide interface and improve device reliability, a
prior art die 900 makes use of a deeper PN junction and thinner
oxide to lower the curvature effect. Die 900 with field plate edge
termination includes a substrate 901 bearing an N-epitaxial layer
902, in which is implanted a deep P-well 903. On the surface 904 of
epitaxial layer 902 is deposited a thin oxide layer 905 and, in
contact with P-well 903, a front metal layer 906 that is further in
contact with a field plate 907. A back metal layer (not shown) is
formed on the bottom of substrate 901.
[0026] FIGS. 9 and 10 illustrate the simulated avalanche leakage
current and impact ionization contours, 908 and 911, respectively,
for prior art die 900. By properly choosing the depth of PN
junction 909 and the thickness of oxide layer 905, the breakdown
location is moved to the parallel plane portion 910 of PN junction
909. As a result, the device reliability of die 900 can be
substantially improved. However, although the avalanche breakdown
location is shifted into the bulk silicon, there still exists a
certain level of electron-hole generation along the interface 904
between epitaxial layer 902 and oxide layer 905. The simulation
gives an impact ionization generation rate at breakdown location
911 of about 1.times.10.sup.21 pairs/cm.sup.3.s and a generation
rate at the same voltage of about 1.times.10.sup.18
pairs/cm.sup.3.s at surface 904.
[0027] The present invention provides further improvement, without
the need for changing junction depth and oxide thickness, over the
results obtained with prior art die 900. Die 1100, schematically
depicted in FIGS. 11 and 12, includes a substrate 1101 bearing an
N-epitaxial layer 1102, in which is implanted a P-well 1103. At the
surface 1104 of epitaxial layer 1102 is formed a SiC edge zone 1105
that extends into N-epitaxial layer 1102 to a depth below that of
P-well 1103. An oxide layer 1106 is formed on SiC edge zone 1105,
and a front metal layer 1107 interconnects P-well 1103. Unlike
previously described dies, die 1100 includes no field plate. A back
metal layer (not shown) is formed on the bottom of substrate
1101.
[0028] By making the SiC edge layer deeper than the planar PN
junction, edge termination with the ideal breakdown voltage can be
achieved. Furthermore, the field plate can be omitted without
degrading the device breakdown characteristics. FIGS. 11 and 12
illustrate the simulated avalanche leakage current and impact
ionization contours, 1110 and 1111, respectively, for die 1100 of
the present invention. The breakdown location 1108 is optimally
situated at P-N junction parallel plane portion 1109. In addition,
the electron-hole generation at upper surface 1104 is extremely
low.
[0029] FIG. 13, a plot of impact ionization along the Si/SiO.sub.2
interface versus distance from the P-N junction at the interface,
depicts the surface carrier generation characteristics of the field
plate-containing prior art dies 500 (cf FIGS. 5,6) and 900 (cf.
FIGS. 9,10) shown in FIGS. 5 and 9, along with die 1100 (cf. FIGS.
11, 12) of the present invention. The SiC edge termination included
in die 1100 lowers the electron-hole avalanche generation rate more
than 20 orders of magnitude compared with prior art die 500, and
more than 16 orders of magnitude compared with prior art die 900.
Furthermore, the breakdown voltage of die 1100 is desirably
increased as a result of the thicker net epitaxial layer, which is
defined by the distance between the parallel plane portion 1109 of
the PN junction and highly doped substrate 1101.
[0030] Another improvement provided by the present invention is a
reduction in edge termination area, which is controlled by the
width of the surface depletion layer. Edge termination in
accordance with the present invention does not change the curvature
of the edge planar junction and the equal-potential contour
distributions. Therefore the width of the surface depletion layer,
which is less than the depletion width of the parallel plane
portion, does change. According to the analysis described in the
previously mentioned paper of Drabe and Sittig, the area of edge
termination in die 1100 is expected to be about half that of the
theoretical "ideal" Si edge termination.
[0031] In the absence of any termination structures, the width of
the edge zone containing material with a higher critical electrical
field than silicon can be chosen to be equal to the width of the
surface depletion layer of the edge planar junction. To verify
this, the width of the SiC edge zone 1105 in die 1100 (cf. FIG. 12)
is reduced to correspond to the surface depletion layer boundary
1112 of the PN junction 1109. The simulated breakdown
characteristic does not change, and the breakdown voltage also
remains the same. The depletion layer boundary 1112 of die 1100 at
the onset of avalanche breakdown is shown in FIG. 14A. The
depletion layer boundary 912 of field plate-containing die 900 (cf
FIG. 10) is depicted in FIG. 14B. The width of the depletion layer
in prior art die 900 is at least two times greater than that of die
1100 of the present invention.
[0032] In addition to the described field plate (FP), the edge
termination of the present invention can be advantageously applied
in semiconductor dies that include other edge terminating features
such as, for example, field limiting rings (FLR), variable lateral
doping concentration (VLD), and junction termination extension
(JTE).
[0033] The present invention has been described in detail for the
purpose of illustration, but it is understood that such detail is
strictly for that purpose, and variations can be made therein by
those skilled in the art without departing from the spirit and
scope of the invention, which is defined by the following
claims.
* * * * *