U.S. patent application number 10/307160 was filed with the patent office on 2003-06-26 for enhanced n-type silicon material for epitaxial wafer substrate and method of making same.
This patent application is currently assigned to SUMCO Oregon corporation. Invention is credited to Fukuto, Nobuo, Kim, Seung-Bae, Kirscht, Fritz G., Snegirev, Boris A., Todt, Volker R., Wildes, Peter D..
Application Number | 20030116083 10/307160 |
Document ID | / |
Family ID | 23395820 |
Filed Date | 2003-06-26 |
United States Patent
Application |
20030116083 |
Kind Code |
A1 |
Kirscht, Fritz G. ; et
al. |
June 26, 2003 |
Enhanced n-type silicon material for epitaxial wafer substrate and
method of making same
Abstract
An enhanced n.sup.+ silicon material for epitaxial substrates
and a method for producing it are described. The enhanced material
leads to improved gettering characteristics of n/n.sup.+ epitaxial
wafers based on these substrates. The method for preparing such
n.sup.+ silicon material includes applying a co-doping of carbon to
the usual n dopant in the silicon melt, before growing respective
CZ crystals. This improves yield of enhanced n.sup.+ silicon
material in crystal growing and ultimately leads to device yield
stabilization or improvement when such n/n.sup.+ epitaxial wafers
are applied in device manufacturing.
Inventors: |
Kirscht, Fritz G.; (Salem,
OR) ; Wildes, Peter D.; (West Linn, OR) ;
Todt, Volker R.; (Corvallis, OR) ; Fukuto, Nobuo;
(Salem, OR) ; Snegirev, Boris A.; (Woodburn,
OR) ; Kim, Seung-Bae; (Salem, OR) |
Correspondence
Address: |
MARGER JOHNSON & MCCOLLOM PC
1030 SW MORRISON STREET
PORTLAND
OR
97205
US
|
Assignee: |
SUMCO Oregon corporation
1351 Tandem Ave. N.E.
Salem
OR
97303
|
Family ID: |
23395820 |
Appl. No.: |
10/307160 |
Filed: |
November 27, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10307160 |
Nov 27, 2002 |
|
|
|
09354994 |
Jul 16, 1999 |
|
|
|
6491752 |
|
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Current U.S.
Class: |
117/21 |
Current CPC
Class: |
C30B 29/06 20130101;
C30B 15/04 20130101 |
Class at
Publication: |
117/21 |
International
Class: |
C30B 015/00; C30B
021/06; C30B 027/02; C30B 028/10; C30B 030/04 |
Claims
1. An enhanced n.sup.+ silicon crystal material for epitaxial
substances having: a dopant of n-type material at a concentration
of at least 10.sup.18 atoms cm.sup.-3; and a co-dopant of carbon at
a concentration of at least 10.sup.16 atoms cm.sup.-3.
2. An enhanced n.sup.+ silicon crystal material for epitaxial
substances as defined in claim 1 wherein said dopant of n-type
material is arsenic at a concentration of at least 10.sup.19 atoms
cm.sup.-3.
3. An enhanced n.sup.+ silicon crystal material for epitaxial
substances as defined in claim 1 wherein said dopant of n-type
material is antimony.
4. An enhanced n.sup.+ silicon crystal material for epitaxial
substances as defined in claim 1 wherein said dopant on n-type
material is phosphorus at a concentration of at least 10.sup.19
atoms cm.sup.-3.
5. An enhanced n.sup.+ silicon crystal material for epitaxial
substances as defined in claim 1 in which the co-dopant of carbon
is present in a concentration of at least 1.9.times.10.sup.16 atoms
cm.sup.-3.
6. An enhanced n.sup.+ silicon crystal material for epitaxial
substances as defined in claim 1 in which the co-dopant of carbon
is present in a concentration sufficient in the presence of said
n.sup.+ doping concentration to produce
oxygen-precipitation-related bulk defects in the material of at
least 10.sup.9 defects cm.sup.-3.
7. A wafer of an enhanced n.sup.+ silicon crystal material
according to claim 1, including: an epitaxial layer on a major
surface of the substrate wafer, thereby providing an active device
layer for active device layer formation decoupled from defects in
the substrate wafer, the epitaxial layer having an n-type dopant
concentration at least 3 orders of magnitude less than the n.sup.+
doping concentration.
8. A method for preparing n.sup.+ silicon material comprising the
steps of: applying a dopant of phosphorus to silicon melt in an
amount sufficient to produce an n.sup.+ doping concentration of at
least 10.sup.18 atoms cm.sup.-3; applying a co-dopant of carbon to
the silicon melt in an amount effective to promote oxygen
precipitation in the silicon material in the presence of said
n.sup.+ doping concentration; and applying a seed crystal to said
melt and growing a crystal therefrom by withdrawing the seed in the
Czochralski technique, wherein said co-dopant of carbon is at a
concentration of at least 10.sup.16 atoms cm.sup.-3.
9. A method for preparing n.sup.+ silicon material as defined in
claim 1 wherein said dopant of phosphorus produces an n-type doping
concentration of at least 10.sup.19 atoms cm.sup.-3.
10. A method for preparing n.sup.+ silicon material as defined in
claim 1 wherein said co-dopant of carbon is at a concentration of
at least 10.sup.16 atoms cm.sup.-3.
11. A method for preparing n.sup.+ silicon material comprising the
steps of: applying a dopant of phosphorus to silicon melt in an
amount sufficient to produce an n.sup.+ doping concentration of at
least 10.sup.19 atoms cm.sup.-3; applying a co-dopant of carbon to
the silicon melt; and applying a seed crystal to said melt and
growing a crystal therefrom by withdrawing the seed in the
Czochralski technique; the co-dopant of carbon being applied in an
amount sufficient to produce a carbon concentration of at least
1.9.times.10.sup.16 atoms cm.sup.-3.
12. A method for preparing n.sup.+ silicon material comprising the
steps of: applying a dopant of phosphorus to silicon melt in an
amount sufficient to produce an n.sup.+ doping concentration of at
least 10.sup.19 atoms cm.sup.-3; applying a co-dopant of carbon to
the silicon melt; and applying a seed crystal to said melt and
growing a crystal therefrom by withdrawing the seed in the
Czochralski technique; the co-dopant of carbon being applied in a
concentration sufficient in the presence of said n.sup.+ doping
concentration to produce oxygen-precipitation-related bulk defects
in the material of at least 10.sup.9 defects cm.sup.-3.
13. A method for preparing n.sup.+ silicon material comprising the
steps of: applying a dopant of phosphorus to silicon melt in an
amount sufficient to produce an n.sup.+ doping concentration of at
least 10.sup.19 atoms cm.sup.-3; applying a co-dopant of carbon to
the silicon melt in an amount sufficient to produce a carbon
concentration in the n.sup.+ silicon material of at least
1.9.times.10.sup.6 atoms cm.sup.-3; and applying a seed crystal to
said melt and growing a crystal therefrom by withdrawing the seed
in the Czochralski technique; slicing the crystal into wafers and
manufacturing substrate wafers therefrom; and forming an epitaxial
layer on a major surface of the substrate wafers, thereby providing
an active device layer for active device layer formation decoupled
from defects in the substrate wafer, the epitaxial layer having an
n-type dopant concentration at least 3 orders of magnitude less
than the n.sup.+ doping concentration.
Description
RELATED APPLICATION DATA
[0001] This application is a division of copending U.S. patent
application Ser. No. 09/354,994, filed Jul. 16, 1999, now U.S. Pat.
No. ______, to issue Dec. 10, 2009.
FIELD OFF THE INVENTION
[0002] This invention relates generally to the field of preparing
silicon substrate wafers for use in the formation of semiconductor
devices such as power discrete or power integrated circuits.
BACKGROUND OF THE INVENTION
[0003] Semiconductor devices are built either into polished or
epitaxial silicon wafers. The latter consists of an epitaxial (epi)
layer on top of a polished wafer substrate. Epi layers typically
contain low concentrations of electrically active dopants, usually
phosphorous (n-type conductivity), or boron (p-type conductivity),
typically close to 10.sup.15 atoms cm.sup.-3. Substrates in many
cases contain high concentrations of dopant atoms, which may be
phosphorous, antimony, or arsenic (n-type) or boron (p-type),
typically in the range 10.sup.18-10.sup.19 atoms cm.sup.-3. Dopant
levels close to the solubility limit for respective dopant species
are needed to lower the resistivity of epi wafer substrates, an
important requirement for state-of-the-art power device
applications. Silicon material containing such high levels of
n-dopant is generally called n.sup.+ material. Such material, cut
in slices from respective n.sup.+ crystals, is used for preparing
n.sup.+ substrates for ultimate n/n.sup.+ epi wafers.
[0004] Oxygen is incorporated into crowing crystals applying the
Czochralski (CZ) technique through dissolution of the fused silica
or quartz (SiO.sub.2) crucibles used for holding the silicon melt.
The molten silicon reacts with the SiO.sub.2 crucible wall to form
SiO. Some of the SiO evaporates from the melt at the temperature
and pressure commonly used for silicon crystal growth. However,
some remains in the melt and may be incorporated into the growing
crystal. As the melt is solidified, the contact area between the
melt and the crucible wall decreases while the area of melt surface
available for evaporation of SiO remains substantially constant
until near the end of the crystal growth. Consequently, the
concentration of oxygen in the melt and therefore the concentration
incorporated into the crystal tends to decrease with increasing
distance from the seed end of the crystal. Without any
countermeasures, this leads to an axial oxygen profile which
typically displays decreasing oxygen concentration toward the
tail-end of the crystal. The presence of high concentrations of
n-type dopants in the silicon melt enhances evaporation of SiO
during crystal growing and thereby further reduces the amount of
oxygen incorporated into a growing n.sup.+ crystal, leading to an
axial oxygen profile decreasing heavily toward the tail-end of such
a crystal. Without any state-of-the-art countermeasures, after
reaching a certain percentage of the total length of such a CZ
n.sup.+ crystal, the oxygen concentration typically drops below the
level required to generate adequate oxygen precipitation when such
material is later processed in thermal device manufacturing steps.
The length of the crystal at which the oxygen level drops below
that required for adequate oxygen precipitation is called the
critical crystal length abbreviated L.sub.c.
[0005] Oxygen precipitation in epi wafer substrates is the
prerequisite for internal gettering (IG) typically applied for
controlling the degradation of device yield by way of heavy metal
contamination during the thermal device manufacturing steps. Such
degradation is described in an article by A. Borghesi, B. Pivac, A.
Sassella and A. Stella entitled Oxygen Precipitation in Silicon,
published in the Journal of Applied Physics, Vol. 77, No. 9, May 1,
1995, pp.4169-4244, at 4206-07. Effective IG has been observed at
oxygen precipitation related bulk defect densities in the order of
10.sup.9 atoms cm.sup.-3. This bulk defect density is critical for
effective IG and is referred to hereinafter as the critical defect
number N.sub.c. Epitaxial n/n.sup.+ wafers based on such high
defect density n.sup.+ substrates exhibit superior IG related
leakage resistance and thereby potentially improved device yield.
Thermally induced oxygen precipitation during device processing is
suppressed in the case of n-type dopant atoms in epi wafer
substrates which creates the necessity to introduce large
quantities of oxygen into a crystal. It has been determined
experimentally by the inventors hereof that CZ crystals with
arsenic concentrations in the order to 10.sup.19 atoms cm.sup.-3
need approximately 8.times.10.sup.17 atoms cm.sup.-3 oxygen (ASTM
121-83 calibration) in order to reach the N.sub.c necessary for
effective IG. Without any state of the art countermeasures, L.sub.c
is less than 10% of the total crystal length in this case. In order
to essentially increase L.sub.c, effort heretofore has been
generally directed at reducing the axial variation of oxygen
incorporation. Currently used techniques aiming at axially
homogenizing the oxygen level include adjusting crystal pull speed
and utilizing crystal and crucible rotation, all in conjunction
with controlling gas flow and pressure in the puller chamber.
Another technique is the application of defined magnetic fields
during crystal growth. These countermeasures are technically
sophisticated and/or associated with high cost.
[0006] The presence of carbon in silicon wafers has long been known
to enhance the precipitation of oxygen. For example, Ahlgren et al.
European Application No. 84109528.4 at page 7, lines 26 to 33
teaches that silicon with carbon concentration in excess of 4 ppma
(2.times.10.sup.17 atoms cm.sup.-3) (ASTM 123-76 calibration) can
induce substantial oxygen precipitation in silicon containing less
than 28 ppma(1.4.times.10.sup.18 atoms cm.sup.-3) oxygen (ASTM
121-79 calibration) after a thermal treatment that would induce
negligible oxygen precipitation at lower concentrations of carbon.
It appears that that work refers to the addition of carbon by the
usual means as set forth above. Thus, the work accepts the carbon
which is introduced as a necessary "evil" in consequence of the
available equipment used in 1984 and sampling the carbon content
along the length of the grown crystal to determine what portion can
be advantageously used. Such carbon introduction is uncontrolled
and mainly due to the graphite parts used in the puller
construction. In current state of the art crystal pullers it is
possible to maintain carbon at levels below 5.times.10.sup.15 atoms
cm.sup.-3 in spite of the use of graphite heaters and insulation.
Moreover, the European application makes no mention of the presence
of n-type or p-type doping materials and it is directed to lightly
doped silicon crystals for substrates.
[0007] Developments aimed at reducing carbon contamination in
crystal growth were originally driven by experimental evidence of
detrimental device impact of carbon if present in certain
concentration levels within critical device regions of wafers. In
the case of epi wafer substrates it is highly unlikely that carbon
would enter a critical device regions (typically located in epi
layers deposited on top of a substrate) because carbon is a slow
diffuser in silicon. Even so, current epi wafer specifications
typically still call for carbon concentrations below 10.sup.16
atoms cm.sup.-3.
SUMMARY OF THE INVENTION
[0008] The present invention is directed to a process for growing
silicon crystals wherein predetermined amounts of carbon are added
in a controlled fashion to produce the level of oxygen
precipitation desired. This process can be effective in n.sup.+
doped silicon epi substrates at carbon levels significantly lower
than 2.times.10.sup.17 atoms cm.sup.-3. Rapidly increasing carbon
concentration is observed only toward the tail-end of carbon
co-doped crystals because its incorporation into the crystal is
controlled by the segregation behavior.
[0009] Such carbon doping of CZ silicon at a very low concentration
can strongly increase the oxygen precipitation in heavily n-doped
materials. Moreover, there is a relationship between co-doped
carbon, oxygen concentration and bulk defect density after
annealing, enabling predetermination of the amount of carbon to be
added to achieve the bulk defect level necessary for effective
internal gettering. The established methodology allows development
of simple and low-cost crystal growing processes leading to
enhanced n-type silicon material for epitaxial wafer
substrates.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] Additional objects and features of the invention will be
more readily apparent from the following detailed description and
appended claims when taken in conjunction with the drawing, in
which:
[0011] FIG. 1 is a lateral sectional view of a pulling apparatus
using the Czolchralski technique suitable for use with the present
invention;
[0012] FIG. 2 is a graph showing the axial distribution of oxygen
concentration along the length of a heavily arsenic doped crystal
co-doped with carbon in accordance with the invention;
[0013] FIG. 3 is a graph similar to FIG. 2 but showing the axial
distribution of carbon concentration along the length of a heavily
arsenic doped crystal co-doped with carbon in accordance with the
invention;
[0014] FIG. 4 is a graph showing percentage of the grown crystal
suitable for internal gettering as a function of the amount of
carbon added to the melt; and
[0015] FIG. 5 is a graph showing the required oxygen concentration
for internal gettering as a function the amount of carbon added to
the melt.
DESCRIPTION OF THE PREFERRED EMBODIMENT
[0016] FIG. 1 shows one of several forms of a pulling apparatus
using the Czolchralski technique suitable for use with the
invention. A quartz crucible 11 is positioned inside a pulling
chamber 13. The quartz crucible 11 is attached to a rotatable
bottom shaft 15. A heater 17 is provided around the quartz crucible
11 for heating and controlling the temperature of a melt M in the
quartz crucible 11. The melt is primarily silicon but includes
dopants. A heat retaining tube 19 is provided between the heater 17
and the pulling chamber 13. An annular supporting member 21 is
attached at the top surface of the heat retaining tube 19.
[0017] To produce a silicon monocrystal by the CZ technique,
polycrystalline silicon and the desired dopant(s), for example, P,
B, Sb or As, are placed in the quartz crucible 11. A seed crystal
is attached to and supported by a bracket 29 on a pulling shaft.
The pulling chamber 13 is evacuated to a vacuum, and the heater 17
melts the polycrystalline silicon and the dopant(s). An inert
carrier gas, such as argon, is passed through an inlet 31 into the
pulling chamber 13 around the quartz crucible 11 and out the
discharge 33. At the same time, the seed crystal is immersed in the
melt in the quartz crucible 11. The pulling shaft then withdraws
the seed crystal at a predetermined speed while rotating relative
to the quartz crucible 11.
[0018] FIGS. 2 and 3 show axial distributions of oxygen and carbon
concentration in a 100 mm diameter silicon crystal doped with
arsenic in the order of 10.sup.19 atoms cm.sup.-3. Before growing
this crystal, 150 mg carbon were initially added to the molten
silicon charge of 30 kg. The inverse concentration characteristics
of oxygen and carbon are evident from the two graphs. Carbon
co-doped n.sup.+ crystals, even without employing means to maintain
high oxygen incorporation throughout the crystal length, yield
significantly higher in terms of potentially high-defect-density
material needed for manufacturing n.sup.+ substrates for ultimately
producing leakage-resistant n/n.sup.+ epitaxial wafers.
[0019] FIG. 4 shows the crystal yield increase as a function of
carbon added to the silicon melt. From FIG. 4 it is obvious that
there is established a simple method for determining the amount of
carbon which must be added to the initial molten silicon charge to
achieve a desired minimum level of bulk defect density over a
defined portion or the entire length of the crystal grown from said
charge employing standard growing technique and by applying a
defined wafer annealing procedure. As an example, in laboratory
tests, it was found that substrate material with arsenic
concentrations in the order of 10.sup.19 atoms cm.sup.-3 reaches
the N.sub.c limit (for effective internal gettering) at an oxygen
concentration of approximately 7.5.times.10.sup.17 atoms cm.sup.-3
if only light carbon doping of 1.9.times.10.sup.16 atoms cm.sup.-3
is applied. This is a substantial improvement over the
8.0.times.10.sup.17 atoms cm.sup.-3 oxygen needed with the typical
state of the art carbon concentration level <10.sup.16 atoms
cm.sup.-3. Oxygen can be further reduced to 6.25.times.10.sup.17
atoms cm.sup.-3 if the carbon concentration at the seed end of the
crystal is increased to 4.3.times.10.sup.16 atoms cm.sup.-3.
Consequently, in carbon-doped crystals there is no, or an extremely
reduced, need to increase L.sub.c by reducing the axial oxygen
variation (increasing the oxygen concentration toward the crystal
tail).
[0020] Upon review of the graph shown in FIG. 5 it is seen that
there is established a simple method to estimate the oxygen
concentration needed over a range of carbon co-doping levels. The
data points "a", "b" and "c" in FIG. 4 and corresponding data
points "d", "e" and "f" in FIG. 5 were derived from tests comparing
three heavily arsenic-doped crystals of 100 mm diameter. The
arsenic concentration in these crystals was targeted to increase
from 1.8.times.10.sup.19 atoms cm.sup.-3 (crystal seed) to
3.8.times.10.sup.19 atoms cm.sup.-3 (crystal tail). The
corresponding amount of arsenic dopant was added to 30 kg charges
of poly-silicon after melting the silicon charge. The first crystal
was grown without intentionally adding carbon (data points "a" and
"d" respectively in FIGS. 4 and 5). The second crystal was grown
after adding only 50 mg high-purity carbon to the melt (data points
"b" and "e" respectively in FIGS. 4 and 5), and for the third
crystal 150 mg high-purity carbon was added (data points "c" and
"f" respectively in FIGS. 4 and 5). No additional countermeasures
for homogenizing the axial oxygen profile were applied. As a
result, the three crystals with varying carbon levels have a
comparable axial oxygen profile: the oxygen concentration falls
from 8.3.times.10.sup.17 atoms cm.sup.-3 at the crystal seed to
4.0.times.10.sup.17 atoms cm.sup.-3 at the crystal tail.
[0021] Summarizing, critical bulk defect density levels, needed for
effective internal gettering in substrates for epitaxial wafers,
can be reached at significantly lower oxygen levels in respective
crystal material, as compared to material without carbon doping.
Applying precipitation testing on wafers from these crystals
(evaluation of N.sub.c as a function of crystal location), it was
found that carbon co-doping clearly increases the critical crystal
length with oxygen precipitation characteristics needed for
effective internal gettering (N.sub.c>10.sup.9 atoms cm.sup.-3).
There is a nearly linear increase of the high-precipitation portion
of these crystals with carbon co-doping (FIG. 4). For example, more
than 50% of the total length of a crystal exceeds L.sub.c when 150
mg of carbon is added to the initial 30 kg charge of silicon. The
oxygen concentration necessary to generate effective internal
gettering is coupled with the added carbon in a well-defined manner
(FIG. 5). This means carbon co-doping can be applied for oxygen
precipitation control in n.sup.+ material used for epi wafer
substrates, instead of sophisticated and/or expensive measures to
increase and axially homogenize the oxygen concentration in such
crystals.
* * * * *