CA1065498A - Open tube gallium diffusion process for semiconductor devices - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A process for diffusing gallium into a semiconductor material is disclosed in which a gaseous mixture of argon and carbon monoxide is used to transport the gallium in an open tube diffusion furnace to the semiconductor material contained in the tube where the gallium diffusion takes place.

Description

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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45,545 ' ~065~98 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-45,545 1~65498 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 .. , ~ .
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 ~o65498 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.

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.

~065498 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 :..
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 45,545 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-45,545 .

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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Claims (5)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. An open tube process for diffusing gallium into silicon comprising the steps:
A. Establishing a first temperature in a first zone of a diffusion furnace tube and a second temperature in a second zone of the diffusion furnace tube, said second temperature exceeding said first temperature;
B. Passing an inert gas through the tube;
C. Inserting a boat containing gallium source material in the first zone of the tube;
D. Inserting a boat containing silicon wafers in the second zone of the tube;
E. Introducing carbon monoxide gas in the inert gas flowing through the tube, which carbon monoxide gas reacts with the gallium source material producing a gas containing gallium, which gas containing gallium flows into the second zone of the tube and deposits gallium on the silicon wafers;
F. Controlling the flow rate of carbon monoxide gas thereby controlling the rate of transport of gallium to the silicon wafers to prevent pitting of the silicon surface;
G. Stopping the flow of carbon monoxide gas through the tube while maintaining the inert gas flow; and, H. Cooling the tube containing the silicon wafers.

2. The process of claim 1 wherein the inert gas is argon and the gallium source material is Ga203.

3. The process of claim 1 wherein the temperature in the first zone is in the range from about 650 to 950°C

and the temperature in the second zone is in the range from about 1250 to 1280°C.

4. The process of claim 1 wherein the flow rate of the inert gas is about 3 to 5 liters per minute.

5. me process of claim 1 wherein at step E, the flow rate of the inert gas is increased to about 7 to 10 liters per minute and the carbon monoxide gas is introduced into the inert gas at a rate of about 50 to 200 cc's per minute