Note: Descriptions are shown in the official language in which they were submitted.
BACKGROUND OF THE INVENTION
Field of the Invention:
The present invention relates to a di^fusion pro-
cess and more specifically to open tube diffusicn of gallium
into silicon semiconductor material.
Description of the Prior Art:
Both sealed tube and open tube processes for dif--
fusing gallium into silicon wafers are known in the prior
art.
In the sealed tube process ultra high purity ele-
mental gallium is sealed together with silicon wafers in a
high purity quartz tube which is either evacuated or back-
filled with an inert gas. The sealed tube diffusion thentakes place in a single zone diffusion furnace.
In the open tube diffusion process a moist hydro-
gen gas is used to transport gallium from a low temperature
zone of the tube to silicon wafers located in a high tem-
perature zone of the tube. For example, the gallium may
exist in the low temperature zone in the form of Ga203
- which reacts with the moist hydrogen gas in a known manner
to deposit the gallium on the surfaces of the silicon wafers
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Both Or the above-descrlbed processes contaln cer-
taln lnherent dlsadvantages known to those practlclng ln the
art.
m e most serlous dlsadvantage o~ the prlor art
open tube process ls the danger involved in the use o~
hydrogen gas ln a diffuslon furnace. The present lnvention
describes a novel open tube gallium di~usion process that
eliminates the dangers of hydrogen while achieving the
superlor device prQperties of the more expensive sealed tube
process.
SUMMARY OF THE INVENTION ;
m e present invention is an open tube process for
di~fusing gallium into silicon comprising the steps: estab- -
lishing a first temperature in a first zone of a dirfusion
furnace tube and a second temperature in a second zone of
; the diffusion furnace tube, said second temperature exceeding
- said first temperature; passing an inert gas through the -
tube; inserting a boat containing gallium source materlal ln
the flrst zone of the tube; inserting a boat containlng
slllcon wafers in the second zone of the tube; introducing
carbon monoxide gas in the inert gas flowing through the
tube, whlch carbon monoxide gas reacts with the gallium
source material produclng a gas contalnlng galllum, whlch
gas containing gallium flows lnto the second zone of the
tube and deposlts galllum on the silicon wafers; stopping
the rlow of carbon monoxide gas through the tube whlle
maintaining the inert gas flow; and cooling the tube contalning
the sllicon wa~ers.
BRIEF DESCRIPTION OF THE DRAWINGS
Flgure 1 is a schematic cross-section taken length-
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wlse through the:center Or an open tube dirruslon rurnace;
Flgure 2 is a rlow chart deplctlng the steps of
the lnventlve process; and,
Flgure 3 ls a semllog graph showlng a dlrfuslon
profile of a sillcon wa~er doped wlth galllum ln accordance
wlth the present lnventlon.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Flgure 1 schematlcally lllustrates an open tube
dlrfusion furnace 8 havlng a cyllndrlcal shaped tube 10,
heating coils 12 and 14, and gas source means 16 and 18.
The furnace 8 has two separately controllable
heatlng systems comprising heating coils 12 and 14 whlch
produce two dlfferent temperature zones in the tube 10.
Associated with heating coils 12 is temperature zone 1 which
is set at a temperature sultable for the chemlcal reactlon: :
Ga203(S) + 2CO(g) ~ Ga2(g) + 2C2(g)
Associate~ wlth heatlng colls 14 is temperature
zone 2 whlch is set at a temperature sultable for galllum
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deposition on silicon and dlfruslon into silicon, the deposi-
tion being achieved by the chemical reaction:
Ga20(g) + Si(s) ~ ~ 2Ga(g) + SiO(g)~
Located in zone 1 Or the tube 10 is a quartz boat
20 containing gallium source material 22 which, in accordance
with the first of the above reactions, is the powdered solid
~a23
Located in zone 2 of the tube 10 is a quartz boat
24 containing silicon wafers 26 which are doped with gallium
in accordance with the second Or the above reactlons.
Now re~erring to Figure 2, a flow chart ls shown
30 whlch deplcts the-crltical steps ln the inventlve process
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using the diffusion furnace 8 of Figure 1.
In step l of Figure 2, the diffusion furnace 8 is
temperature profiled to provide two different temperature
zones. Zone 1 is preferably set in a range of 650C to
950 C and zone 2 is preferably set in a temperature range of
1250C to 1280C. The two zones are typically separated in
the tube 10 by about 20 to 30 inches. Both zone 1 and zone
2 are maintained at + 0.5C over a length sufficient to
encompass the source boat 20 and silicon containing boat 24
respectively. Such length is typically about 6 to 8 inches
in the case of zone 1 and lO to 14 inches in the case of
` zone 2.
In step 2, an inert gas, as shown by gas source
16, is passed through the tube at a flow rate of about 3 to
5 liters per minute. Argon and helium are suitable inert
gases, argon being presently preferred. The inert gas
provides a controlled environment in the tube 10 for producing
the above-described chemical reactions without interference
from unwanted impurities. The inert gas continues to flow
through the tube lO during the balance of the diffusion pro-
cess (typically about ll to 24 hours) and cooling phase
(typically about 5 hours).
In step 3, the gallium source material 22 contained
in the quartz boat 20 is placed in zone l of the tube lO.
The quartz boat 20 should be constructed so that gas will
flow over an open top portion of the boat 20. For example,
the boat 20 is preferably constructed with a rectangular
opening of approximately 2 inches by 6 inches and fits in
the tube lO with the opening at approximately the center line
of the tube lO.
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In step 4, the boat 24 containing the silicon
wafers 26 is placed in the tube 10 in zone 2. Preferably,
the silicon wafers 26 are aligned in the boat 24 in a stacked
fashion to permit gas flow to reach the entire surface area
of each wafer. At this point in the process, a cap (not
shown) may be positioned over the end of the tube 10 for
exhaust purposes.
In step 5, carbon monoxide gas from source 18 is
i introduced into the tube 10 with the argon gas flow while at
the same time the argon gas is increased to a flow rate of
approximately 7 to 10 liters per minute depending on the
tube diameter. For example, a tube with an inside diameter
of 89 mm would preferably use 7 liters per minute of argon.
The flow rate of the carbon monoxide gas is preferably
approximately S0 to 200 cc's per minute.
In an alternative process for achieving low sur-
face concentrations, carbon dioxide may be fed into the gas
flow at a fraction of the carbon monoxide flow rate, or a
small part of the inert gas (10 to 50 cc/m) may be bubbled
: 20 through a quartz water bubbler set at a fixed temperature in
the range of 15 C to 30 C. If carbon dioxide is used, the
amount of carbon dioxide in relation to carbon monoxide
depends on the desired partial pressure of oxygen in the
system which can be determined from available standard
thermochemical data known to those practicing in the material
- sciences art.
In step 6, at the conclusion of the diffusion run,
the carbon monoxide gas is shut off. Then if any carbon
dioxide or waterbubbled inert gas has been used, it is also
shut off.
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In step 7, the diffusion zone of the tube is then
slowly cooled in a known manner. Preferably, the cooling
rate should not exceed 2C per minute and should continue at
least until the wafers have cooled to about 700 C.
In a sample diffusion run using Ga203 as a dif-
fusant source material gallium was diffused into silicon
wafers under the following conditions. An argon flow rate
of 7 liters per minute was passed through a diffusion fur-
nace tube having a diameter of 89 millimeters. The tempera-
ture in zone 1 containing the Ga203 was set at 900C and the
temperature in zone 2 containing the silicon wafers was set
at 1250C. Pure carbon monoxide was introduced in the tube
at a rate of 100 cc's per minute for a 24 hour period after
which the wafers were slowly cooled at a rate of 2 C per
minute with the argon continuing to flow until the wafers
had cooled to about 700C. The wafers were then removed and
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a diffusion profile measured as shown in Figure 3.
- The wafers used in the above sample diffusion run,
which provided the data for Figure 3, had previously been
- 20 prepared with a 5000 A thick thermal oxide on the surface of
- the wafers. After diffusion, the average minority carrier
- lifetime was measured to be 15 microseconds. It is to be
noted that the best surface condition of the diffused sili-
con is obtained with the presently described process when
more than 3000 A of starting thermal oxide covers the silicon
surfaces. In the presence of moderately oxidizing additives
(for example, H20 and C02) a bare wafer of silicon will grow
an etch resistant mixed oxide film on its surface. If the
rate of transport of gallium to the silicon surface is
greater than the rate of diffusion into the silicon, localized
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alloylng and plttlng of the sillcon surrace will occur.
A unlque advantage Or the pre~ent lnventlon over
prlor open tube dlrruslon processes i8 that the actlve gas,
ln this case carbon monoxlde, may be varled to control the
reactlon rate, whereas with the hydrogen gas process of the
prior art, no such control ls posslble. Thus, the rate Or
transportatlon Or galllum to the slllcon surrace can be
optlmlzed ln relatlon to the galllum dlrruslon rate to pro-
vide optlmum surface condltlons. Furthermore, as can readily
be seen rrom Flgure 3, a dlffuslon proflle may be obtalned
which has been round emplrlcally to be advantageous for high
blocking voltage. Also, to the process of the present
lnventlon maintains the highest known lifetime in the sill-
con substrate while the surface concentration remalns below
solid solubility.
It will be evident to those skilled in the art
~i that, in addltlon to appllcabillty to single ~unction devlces,
,
the process Or the present inventlon may be advantageously
employed to produce reverse switching rectiriers, thyristors `-
and other multi-layered sllicon power devices. For example,
a PNPN structure may be produced in a silicon wa~er having
an inltial N-type background doping as follows:
1. Prepare the silicon wafer to have a silicon
dloxide layer on one ma~or surface while the other ma~or
surface remalns exposed.
2. Insert the wafer lnto a diffusion tube and
diffuse galllum lnto the wafer accordlng to the above-
descrlbed inventive process. A PNP structure is thereby
produced slnce galllum readlly penetrates slllcon dloxide.
3. Stop the galllum dirfusion and initiate an N-
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type diffusion without removing the wafer from the tube,
For example, a phosphorus source gas may be lntroduced into
the tube. Phosphorus diffuses only through the bare ~urface
of the wafer while being masked by the oxide. Thus a PNPN
structure is produced by consecutive gallium and phosphorus
diffusions without removing the wafer from the tube.
The inventive process is clearly adaptable to
produce a great variety of other devices and structures
using well-known s111con dloxlde masklng technlques.
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