US3660179A

US3660179A - Gaseous diffusion technique

Info

Publication number: US3660179A
Application number: US64381A
Authority: US
Inventors: Timothy J Desmond; Berton P Krumanacker
Original assignee: Westinghouse Electric Corp
Current assignee: CBS Corp
Priority date: 1970-08-17
Filing date: 1970-08-17
Publication date: 1972-05-02
Anticipated expiration: 1989-05-02
Also published as: JPS4933222B1

Abstract

A counter-flow of an inert, or a predominantly inert, gas is provided in a diffusion furnace to provide means for creating turbulence within the diffusion system thereby improving the uniformity of dopant along the length of the diffusion carrier contained therein.

Description

O United States Patent 1151 3,660,179 Desmond et al. 1 May 2, 1972- 54 GASEOUS DIFFUSION TECHNIQUE 3,314,393 4 1967 Haneta ..118/48 3,354,004 11/1967 Reisman et a1.. 17/106 A X [721 g z' 'z i' zfg gg i'ifl fi g fi g} 3,361,600 1/1968 Reisman et a1 ..117/106 A x f n e S 3,517,643 6/1970 GOlClSlell'l et a1 ..118 48 3,594,242 7/1971 Burd et a1 ..252/6236 A X [731 Assignec: Westinghouse Electric Corporation, Pitt- Sburghv Primary Examiner-Tobias E. Levow 2 Filed: 17 970 Assistant Examiner-J. Cooper Attorney-F. Shapoe and C. L. Menzemer 211 Appl. No.: 64,381

I [57] ABSTRACT [52] g A counter-flow of an inert, or a predominantly inert, gas is l t Hon M4 provided in a diffusion furnace to provide means for creating [58] g i 1 18/48 turbulence within the diffusion system thereby improving the l 49 252/62 3 i 62 3 uniformity of dopant along the length of the diffusion carrier contained therein.

[56] References Cited 8 Claims, 1 Drawing Figure UNITED STATES PATENTS 3,021,198 2/1962 Rummel ..117/106 A X S ECOND GAS EOUS STREAM PATENTEDmz I972 3,660,179

FIRST GASEOUS SECOND STREAM Q GASEOUS STREAM WITNESSES n INVENTORS V Timothy J. Desmond 0nd I BEerton P. Krumonucker GASEOUS DIFFUSION TECHNIQUE BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to the processing of semiconductor materials and in particular to the diffusion of a dopant materialinto a body of semiconductor material.

2. Description of the Prior Art Heretofore the process of diffusing a dopant material into semiconductor material comprised a flow of gas including a specified: proportion of dopant material in a gaseous phase was continuously passed through a furnace over and about a series of heated bodies of semiconductor material, the gaseous dopant material being introduced through one end of the furnace and exiting from the other end of the furnace. Invariably, the flow of the gas was relatively smooth. Unfortunately, it has been found that this results in non-uniform doping of the bodies not only from one end of the furnace to the other but often on different portions of the exposed faces of any one individual body.

SUMMARY OF THE INVENTION In accordance with the teachings of this invention, there is provided a process for the uniform gaseous diffusion of a doping material into exposed surfaces of a body of semiconductor material. Briefly, the process comprises supporting within a generally elongated furnace the body of semiconductor material spaced from the bottom of the furnace. The body is heated to an elevated temperature at which diffusion of dopant can occur, and a first gaseous stream comprising, in part, a dopant material is introduced into the furnace through an inlet tube at one end of the furnace. A second gaseous stream having a flow rate greater than the first stream is introduced at the other end of the furnace at a distance from said body and its support. The second gaseous stream is directed initially to flow between the body and the bottom of the furnace, striking the end of the furnace containing the inlet tube thereby reversing its flow direction and causing the two gaseous streams to interrnix in a turbulent manner, thereby depositing dopant material on, and diffusing into the exposed surfaces of the heated body of semiconductor material and exhausting the remainder of the intermixed gaseous streams through the other end of the furnace. A plurality of bodies, such as wafers of silicon can be treated in the furnace. A much higher degree of uniformity of doping of the exposed surfaces of all the bodies is thereby secured.

DRAWING The FIGURE shows, in part, a cross-section of a furnace constructed and operating in accordance with the teachings of this invention.

DESCRIPTION OF THE INVENTION With reference to the FIGURE there is shown furnace apparatus l0 suitable for diffusing semiconductor materials with a suitable dopant material in accordance with the teachings of this invention. The furnace 10 comprises a tube 12 having an inlet port 14 located centrally at one end thereof for introducing a stream of gas which contains a suitable dopant material in vapor form into the tube 12.

One or more bodies 16 of semiconductor material, for example, wafers of silicon are supported in a boat 18 supported by legs 19 or if wide enough by the walls of the tube 12, so that the boat is spaced a distance above the tube bottom. The bodies 16 are heated to the diffusing temperature by a resistance coil heater 20 encircling the tube 12. A gas inlet 22 is inserted into the other end of the tube 12 and its open end is disposed on the bottom surface of the tube 12. Through the inlet 22 is introduced a counterflow of an inert, or a predominantly inert, gas to create a desired high turbulence in the stream of dopant gas introduced by port 14 within the tube 12. The flow rate of the gas injected into the furnace through the inlet 22- must be greater than the flow rate of the dopant gas from port 14. Preferably the ratio of the flow rate of counterflow gas to dopant gas is of the order of 2: I. If the open end of inlet 22 is placed within several inches from the boat 18 the best results are obtained. It has been found that less pronounced benefits are secured obtaining a more uniform distribution of dopant materials in the semiconductor materials is achieved if the end of inlet 22 is at the end of the boat or if it is more than 6 to 8 inches from the end of the boat.

When no counterflow gas flows from inlet 22, it appears that when the dopant gas enters through the inlet port 14 into the near end of the furnace 12, some turbulence may occur immediately around the port 14 but the dopant gas achieves essentially a laminar flow within a few inches from the inlet port 14 and retains such laminar flow condition by the time it reaches the end of the boat 18 closest to the port 14. Upon reaching and striking the end of the boat 18 closest to the port 14 the laminar pattern of the dopant gas stream is broken up partly, with that portion of the dopant gas stream directly striking the boat 18 being deflected and becoming turbulent thereby tumbling over and about the bodies 16. Some disturbance of the portion of the laminar flowing dopant stream nearer the boat also occurs. The outermost portions of the gas stream appear to be less disturbed and retain most of their laminar flow pattern. After passing the boat 18 the dopant gas flow then assumes a relatively smooth laminar flow pattern and exits at a large outlet 24 the far end of the tube 12.

When the inert gas is introduced into the tube 12 through the inlet 22 inserted through the far end of the furnace, it appears that the inert gas, flowing not only at a greater flow rate than the dopant gas but countercurrent thereto rapidly breaks up the laminar flow pattern of the dopant gas in the bottom of the tube 12, particularly at the rear of and beneath the boat 18 thereby causing a turbulent gas flow pattern to occur. For this reason the inlet 22 is designated hereinafter as a tubular tube. Some of this turbulent gas stream mixes with the turbulent dopant gas stream flow immediately around the top of the boat 18 to provide a better bathing of the exposed surfaces of each individual body 16 to the gaseous dopant material. Additionally, since the gas flow rate of the gas introduced through the turbulator exceeds that of the dopant gas flow rate, the flow beneath the boat 18 towards the end of the tube 12 where port 14 is located creates a venturic effect which tends to force more of the dopant gas stream in turbulent fashion down past and about the bodies 16 from the top of the boat 18 to the bottom. The main stream of gas from the turbulator tube 22, in general, then continues to the end of the tube 12 at port 14, striking the wall and flowing upwardly so that it mixes with the dopant gas entering the tube 12 through inlet port 14, creating I considerable turbulence and mixing thereof, and assumes only partial laminar flow configuration before stroking the boat 18 and the bodies 16, becoming turbulent again and flowing about the bodies 16 and the boat 18, some of which is captured into fresh inert gas continually flowing beneath the boat 18, resumes laminar flow configuration after passing the boat 18, and exits from the tube 12 through the far end at outlet 24. Additionally, the gas introduced from the turbulator tube 22 may create a pressure condition which enables the dopant gas to dwell longer around the bodies 16 longer than previously obtainable. Regardless of our explanation, the net effect of the apparatus and process of this invention is the achievement of better uniformity and reproducibility of the diffusion thereby essentially duplicating the optimum results obtained with the sealed tube diffusion process, the latter process being more expensive and difficult to operate.

The same effect may be achieved by using a gas emerging from the tube 22 at a greater velocity than the initial dopant gaseous stream. The velocity of this stream of gas must be of such magnitude as to carry essentially completely past the underside of the boat 18 to accomplish the venturi effect on the gas bathing the wafers 16 from the top of boat 18 before this stream which has entered from tube 22 is intermixed with the gas stream from tube 14.

The inert gas is a gas which is completely or relatively chemical unreactive with the dopant gases and the bodies used in the process practiced with apparatus 10. However, a portion of the dopant gas required for the overall diffusion process could be introduced into the furnace in the counterflow gas stream through the turbulator tube 22.

Examples of the use of this apparatus and process of this invention are asfollows:

A 60 mm. ID flow through tubular furnace was prepared for the diffusion of wafers of silicon semiconductor material. Five 1 12inch diameter wafers of silicon semiconductor material were lapped, polished and cleaned and placed in a boat which in turn was disposed in the furnace with a space below the boat. The boat measured 10 inches in length by l inches in width. The wafers were all parallel to the length of the tube. One of the wafers was placed in each of the corners of the boat and the fifth wafer (wafer No. 3) was placed in the center of the boat.

Thewafers were heated to a temperature of 1,000 C. i l C. by a resistance type heater encircling the furnace 12 in the vicinity of the wafers. A gas stream consisting essentially of 4,300 cc./min. of nitrogen, l80 cc./min. of oxygen, and 9 cc./min. of a gaseous mixture of 2 percent diborane in nitrogen was introduced into the furnace. The gas stream was caused to flow through the furnace and over and about the wafers for 30 minutes.

When a quartz turbulator tube of 9 mm. ID was employed, having its opening 3 inches from the end of the boat the inert gas was nitrogen flowing at a specific flow rate indicated below for each run, the inert gas flow being counter to that of the direction of dopant gas mixture. All measurements were measured with a four point probe and a constant electrical current of l milliampere. The sheet resistance is obtained by multiplying the reading in millivolts vby 4.5. Readings were taken on each side of each wafer.

The sheet resistance of the wafers as determined from the test results were erratic and varied from each other quite a bit by a factor offour, as well as showing an unacceptable variance betweeneach side of the same wafer for wafers 2, 3, 4 and 5. The sheet resistance of the wafers varied too greatly to be acceptable for making production runs of semiconductor devices of uniform characteristics. In addition to this nonuniformity of sheet resistance readings, all wafers had a bluish oxide coating formed only onvthe edge area of each 'wafer nearest the dopant inlet port.

EXAMPLE I! With Turbulator Tube-Nitrogen Gas Flow Rate 25 L/min.

Wafter Side Millivolts l a 27.00 b 23.00 2 a 24.90

The ratio of the turbulator tube gas flow to dopant gas flow is about 5:1. These wafers showed a great improvement in the uniformity of the sheet resistance as determined from the test results for each side 'of each wafer as well as from wafter to wafer. None of the wafers were discolored by the bluish oxide coating noted on the wafers of Example 1. However, the turbu- The ratio of turbulator tube to dopant gas flow is about 2.5 to 1. The sheet resistance as determined from the test results for each wafer varies very little from side to side compared to previous results obtained and also each wafer does not differ much from the balance of the wafers in the test boat. Additionally, the overall effect of the lower flow of nitrogen gas introduced through the turbulator tube is to produce wafers having low sheet resistance values, consequently good doping has been obtained. This is very desirable for semiconductor device applications.

To Turbulator Tube-the effect of temperature in the process the following tests were made: I

EXAMPLE IV Temperature of Wafers 950 C ;t l C.

With TurbulatorNitrogen Gas Flow Rate 10.4 l./min.

, The sheet resistance values as determined from the test results vary greatly from side to side on each wafter and from 7 wafer to wafer in the boat. The sheet resistance values have also increased with a decrease in temperature indicating lesser dopant being deposited on the wafer:

EXAMPLE V Temperature of Wafers 1050 C. 1 1 C.

With Turbulator Tube-Nitrogen Gas Flow Rate 10.4 l./min.

Wafer Side Millivolts The sheet resistance values as determined from the test results have dropped to a lower value being of the order of one-eighth the value obtained at 950 C. But the sheet resistance values are very uniform from side to side on each wafer as well as from wafer to wafer.

EXAMPLE VI 2% Diborane Gas Flow-25 cc./min. Temperature of Wafers 1050 C. i 1 C.

With Turbulator Tube-Nitrogen Gas Flow Rate 10.4 l./min.

Wafer Side Millivolts When compared to the results obtained in Example VI, the sheet resistance values obtained in this experimental run are a little more uniform both in side to side comparison for each wafer and from wafer to wafer. However, the increase in dopant concentration has also increased the sheet resistance values of each wafer slightly.

The results of Examples V and V] are similar to the best results obtained from closed tube diffusion systems. From the results obtained from the experimental runs it has been found that the turbulator tube application for introducing a counterflow of an inert gas into the open tube diffusion process system will, under proper temperature conditions, essentially duplicate the closed tube diffusion process, but at a great saving of time and expense. The sheet resistivity values obtained from the test results of the experiments indicate that too great a counterflow of inert gas beyond a 2:1 to 3:1 range is less desirable if uniformity of the sheet resistance values of all wafers in one production run is required. The sheet resistivity values for a given time for diffusion can be increased by lowering the temperature of the wafer or increasing the volume of diborane gas flow or both. An increase in temperature of the wafers being treated produces a lower resistivity in the wafers. The flow rate of nitrogen for the turbulator tube to the dopant gas flow should preferably not exceed about a 4:1 ratio.

As mentioned previously the location of the turbulator tube in the furnace in respect to the boat containing the wafers is a factor for best results. The inert gas flow exit from the turbulator tube in the order of from 3 to 4 inches from the end of the boat for the dimensions of the furnace, the turbulator tube,

and the boat described in the Examples, has given excellent results.

The gaseous stream introduced through the turbulator tube need not be entirely inert. Part of the gaseous stream may comprise a dopant material, preferably the same as in the first gaseous dopant stream. in the instance of doping silicon semiconductor material with boron from .diborane a small amount of oxygen may also be included in the second gaseous stream.

Although the invention has been taught specifically for a round tubular pass through furnace, the process application is suitable for furnaces of other shapes provided the two gas streams are counterflow to each other and the one stream of greater velocity and/or of greater volume is directed initially below the wafers being doped.

We claim as our invention:

l. A process for the uniform gaseous diffusion of a dopant into a body of semiconductor material comprising:

a. supporting within a tubular furnace said body of semiconductor material spaced from the bottom of the furnace; b. heating said body of semiconductor material to a temperature at which the dopant diffuses into said body; I

c. introducing a first gaseous stream comprising, a dopant material through an inlet port in one end of the furnace into the furnace confines;

d. introducing a second gaseous stream comprising an inert gas through the other end of the furnace into the furnace confines, at a predetermined distance from said body support, said gaseous stream having a flow rate greater than the flow rate of the first gaseous stream;

e. directing the flow of said second gaseous stream to flow beneath the body of semiconductor material between said body and said bottom of the furnace, thence to strike the end of the furnace containing the inlet port therein to change the direction of the flow of said second gaseous stream thereby reversing its flow direction and causing said first and second gaseous stream to intermix in a turbulent manner;

diffusing said dopant material from said intermixed gases in exposed surfaces of said heated body of semiconductor material; and

g. exhausting the remaining portions of said first and said second gaseous streams through said other end of the said furnace.

2. The process as defined in claim 1 in which said inert gas is one selected from the group consisting of nitrogen and argon.

3. The process as defined in claim 2 wherein said second gaseous stream further contains, a dopant material.

4. The process as defined in claim 1 wherein said second gaseous stream has a flow rate of at least twice the flow rate of said first gaseous stream.

5. The process as defined in claim 1 wherein the first gaseous stream comprises oxygen, nitrogen and a doping gas mixture of dibrane and nitrogen and the second gaseous stream comprises nitrogen.

6. The process as defined in claim 5 wherein the second gaseous stream further contains oxygen, and

diborane.

7. The process as defined in claim 5 wherein the flow rate of the oxygen is cc./min., the flow rate of nitrogen of the first gaseous stream is 4,300 cc./min., the flow rate of the diborane doping gas mixture is 9 cc./min., the flow rate of nitrogen in the second gaseous stream is no greater than 15 l./min., the furnace has an inside diameter of 60 mm. ID, said predetermined distance is from 3 to 4 inches, and said semiconductor material is'silicon.

8. The process as defined in claim 7 wherein the flow rate of nitrogen in said second gaseous stream is 10.4 l./min.

Claims

2. The process as defined in claim 1 in which said inert gas is one selected from the group consisting of nitrogen and argon.
3. The process as defined in claim 2 wherein said second gaseous stream further contains, a dopant material.
4. The process as defined in claim 1 wherein said second gaseous stream has a flow rate of at least twice the flow rate of said first gaseous stream.
5. The process as defined in claim 1 wherein the first gaseous stream comprises oxygen, nitrogen and a doping gas mixture of dibrane and nitrogen and the second gaseous stream comprises nitrogen.
6. The process as defined in claim 5 wherein the second gaseous stream further contains oxygen, and diborane.
7. The process as defined in claim 5 wherein the flow rate of the oxygen is 180 cc./min., the flow rate of nitrogen of the first gaseous stream is 4,300 cc./min., the flow rate of the diborane doping gas mixture is 9 cc./min., the flow rate of nitrogen in the second gaseous stream is no greater than 15 l./min., the furnace has an inside diameter of 60 mm. ID, said predetermined distance is from 3 to 4 inches, and said semiconductor material is silicon.
8. The process as defined in claim 7 wherein the flow rate of nitrogen in said second gaseous stream is 10.4 l./min.