US 3615261 A
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States Patent Inventors Donald R. Causey;
John R. Lenzing, both of Scottsdale, Ariz. Appl. No. 812,897 Filed Apr, 2, 1969 Patented Oct. 26, 1971 Assignee Motorola, Inc.
Franklin Parlt, Ill.
METHOD OF PRODUCING SINGLE SEMICONDUCTOR CRYSTALS 10 Claims, No Drawings US. Cl 23/301 SP, 148/172, 252/62.3 E, 252/512 Int. Cl B0lj 17/00, I-IO ll 7/40 Field of Search 252/512,
 References Cited UNITED STATES PATENTS 3,501,406 3/1970 Kappelmeyer et al. 23/30] SP Primary Examiner-Douglas J. Drummond Att0rney-Mueller and Aichele ABSTRACT: A method of making an ingot length of a single semiconductive crystal having substantially uniform resistivity throughout most of the length of the ingot is disclosed. An inert helium atmosphere is placed about a mass of silicon material and then the silicon mass is melted. A seed of silicon is inserted into the melt and withdrawn at a rate which provides a substantially uniform ingot diameter. As the seed is withdrawn from the melt, the pressure in the system is slowly reduced to vaporize the impurity from the melt so as to maintain a substantially constant impurity concentration in the melt during the formation ofthe ingot METHOD OF PRODUCING SINGLE SEMICONDUCTOR CRYSTALS BACKGROUND OF THE INVENTION This invention relates to a method of making a single semiconductor crystal and more particularly to a method of making an ingot of single semiconductive crystal having a uniform resistivity throughout a substantial portion of the length of the ingot.
In the manufacture of many semiconductor devices, it is desirable to obtain single crystals of semiconductor material which exhibit substantially uniform resistivity or conductivity throughout their lengths. Inasmuch as the resistivity is a function of certain active impurities present in the semiconductor crystal, this makes fabrication of single crystals having a substantially uniform impurity content highly desirable. The usual semiconductor materials are silicon and germanium, and the impurities most widely used are phosphorus, arsenic, alud minum, boron, gallium, indium and antimony.
ln processing silicon or germanium for use in semiconductor crystal devices, it is usually necessary to melt and recrystallize the semiconductor in order to control its impurity content. In the Czochralski process, a widely used process for the commercial production of silicon, the semiconductor material is melted completely; after which, a seed crystal is brought into contact with the melt and slowly withdrawn, forming a single crystal by continuing accretion to the seed. In this growing method, the concentration of impurities in the recrystallized material tends to increase with continuing crystallization. As a result, the recrystallized material at the bottom of the ingot has a higher impurity concentration and the ingot does not exhibit the desired uniform resistivity or conductivity profile over its length. The typical resistivity profile of a silicon ingot containing phosphorus as an impurity obtained by the standard Czochralski process indicates about a 60 percent change in resistivity over 80 percent of the ingot. With a resistivity profile such as this showing such a large change in resistivity, the yield for a zener diode crystal doped with phosphorus, for example, is low-about to percent.
It is an object of this invention to provide a method of making ingot lengths of single semiconductor crystals having a substantially uniform resistivity throughout most of its length. It is another object of this invention to provide a method of making single semiconductor crystals in an ingot having a sub stantially uniform resistivity profile.
SUMMARY OF THE INVENTION These and other objects of this invention are accomplished by a method in which a semiconductive material such as silicon containing a conductivity-type determining impurity is placed in a receptacle surrounded by an inert gas atmosphere, such as helium. The silicon mass is then heated to a temperature in the vicinity of 1,420 C. and melted. A seed of single crystal silicon is then inserted into the melted silicon and then withdrawn at a rate which provides a substantially uniform diameter for the single crystal ingot. In accordance with this invention, as the seed is withdrawn from the melt, the pressure of the inert gas atmosphere in the system about the melted silicon is slowly reduced so as to maintain a substantially constant impurity concentration in the melted silicon. After the single crystal ingot has been grown to the desired length, the ingot is cooled to room temperature. The resultant ingot has a substantially uniform impurity concentration throughout most of the length thereof. Since the impurity concentration is substantially uniform, the resistivity profile is also substantially uniform throughout most of the length thereof.
DESCRIPTION OF THE PREFERRED EMBODIMENTS The invention will now be described in more detail. The semiconductive material employed is either silicon or germanium. Phosphorus, arsenic and antimony are the impurities most frequently used for the production of N-type silicon. Antimony and indium are the preferred impurities used with germanium in the practice of this invention. The concentration range of the impurity may vary over a broad range from a low of 1X10 atoms/cc. (2 parts per billion) to 2.2x 1 0 atoms/cc. (1 million parts per billion) depending upon the desired resistivity of the resultant single crystal.
The semiconductive material, that is, the silicon or germanium, is placed in a crucible which is made of carbon in the case of germanium and quartz or glassy carbon in the case of silicon, as is the practice in the art.
An inert atmosphere is provided in the system by passing an inert gas, preferably helium or argon, over the silicon at a flow rate of 20 to 30 cubic feet per hour with a preferred flow rate being 25 cubic feet per hour. Other gases such as neon or xenon may be employed.
The semiconductive material is then melted. Silicon is heated to a temperature in the range of l,420 C., whereas germanium is heated to a temperature in the vicinity of 946 C. After the semiconductive material mass has been melted, a seed of the same semiconductive material is inserted into the melted mass; for example, a silicon seed is inserted into the melted silicon.
As soon as a crystal commences to form about the seed, the seed crystal is slowly withdrawn at a rate such that a single crystal ingot having a relatively uniform diameter is obtained.
In accordance with the practice of this invention, a single crystal ingot having a substantially uniform resistivity profile over a major portion of its length is obtained by vaporizing the excess impurity from the melt as the seed or ingot is withdrawn thereby maintaining the impurity concentration of the melt at a substantially uniform level. In order to effectively maintain a substantially uniform or constant impurity concentration in the melt, it is necessary to gradually reduce the pressure in the system thereby vaporizing more impurity from the melt and preventing the concentration of the impurity in the melt from increasing.
The pressure in the system surrounding the semiconductive material mass is regulated by a series of vacuum pumps and by a valve attached to the inert gas source. The pressure of the system at the beginning of the process varies from about 50 to 760 mm. of Hg with the preferred pressure being around 100 mm. Hg. As soon as the withdrawal of the seed from the melt is started, the inert gas flow, preferably helium or argon, is gradually decreased by slowly closing or turning off the valve leading to the system from the inert gas supply. At the same time that the helium gas flow is being decreased, the vacuum system slowly reduces the pressure in the system. As the pressure is reduced, more of the impurity from the melt is vaporized thereby removing impurities from the melt which would normally increase impurity concentration level of the melt. By the proper programming or scheduling of the pressure reduction in the system, the impurity level in the molten mass or melt is maintained at a substantially uniform level. By maintaining the impurity concentration at a uniform level, the resultant resistivity of the semiconductor ingot is maintained at a uniform level thereby resulting in a uniform resistivity profile over about to percent of the length of the ingot.
The pressure is reduced from 50 mm. to 760 mm. Hg down to a level of from 10 mm. Hg to 1x110 mm. Hg. The final pressure depends upon the resistivity desired in the ingot; that is, a pressure of the order of 1X10 mm. Hg is used to obtain an ingot having a resistivity of 45 ohm centimeters, whereas a pressure of the order of 1 mm. Hg is used to obtain a resistivity of 0.005 ohm centimeters. The time required to reduce the pressure varies from k to 4%, the time required being less for ingots having higher resistivity. The pressure reduction can be carried out by the use of automatic instrumentation thereby lending itself readily to the commercial manufacturing of semiconductor ingots.
The complete growth cycle time of the ingot may vary from about 3 hours to 10 hours, depending to a large extent on the size of the silicon mass. Upon completion of the growing of the ingot, the ingot is then cooled and the pressure increased (example l) to atmospheric pressure.
Silicon containing 3.6Xl atoms/cc. of phosphorus was placed in the crucible. Helium, at a rate of 25 cubic feet per hour, was passed around the silicon mass. The silicon was heated to a temperature of about 1,420" C. and melted. A sina. providing a mass of semiconductive material selected from the group consisting of germanium and silicon and containing conductivity-type impurity having a vapor pressure above that of said material,
gle crystal silicon seed was inserted into the melt and when the 5 b. passing a gas taken from the group consisting of helium seed crystal began to grow, the seed was slowly withd and argon about said mass at a flow rate of about to 30 from the melt. The helium flow and the pressure in the system cubic feet P hour. was slowly reduced over a period of 1.75 hours from 100 mm. c. melting said mass, Hg to 0.04 mm. Hg. The pressure was maintained at 0.04 mm. d. inserting a seed of said mass material into said melted Hg for 3.75 hours to complete the growth of the ingot. The 10 mass, ingot was cooled and the pressure was increased to ate. withdrawing said seed at a rate such that the semiconducmospheric pressure. The ingot had a tapered tip at both ends tive material from the melt forms a single crystal which was removed. Each tip amounted to 10 percent of the propagated from said seed crystal, ingot length. The resistivity profile for the 80 percent of the f. reducing the flow rate of said gas during said withdrawing ingot which remained after the top and bottom tapered porstep, and tions of the ingot were removed was substantially uniform, the g. (reducing the pressure) during said withdrawing step, resistivity of the top end of the ingot being 0.0182 0. cm. and reducing the pressure of said inert gas atmosphere at a the resistivity of the bottom end being 0.0180 0 cm. rate and to a value determined by said predetermined re- EXAMPLES 2 THROUGH 5 Time, hours Ingot resistivity P cone. Pressure, mm. Hg 9 cm. in melt, Changing Constant atoms/cc. pressure pressure Original Final Top Bottom 2. 6X10" 2. 0 3. 5 100 1.0 .005 .0062 8. 4X107 .75 4. 75 100 018 .041 .035 1. 5x10" .5 5. 0 100 .0001 .113 .100 2. X10 5 4. 5 50 0001 49. 4 43. 8
a 80% of ingot 4 and 5; 70% of ingot 2; 60% of ingot 3.
As can be seen from the examples, the resistivity varies to a sistivity to maintain a substantially constant impurity con relatively small extent from the top of the ingot to the bottom centration in said melted mass. of the ingot when compared with the standard Czochralski 5. A method as described in claim 4 wherein said gas flow is process which is of the order of 60 percent change in resistivireduced to less than 1 cubic foot per hour in 30 to 90 minutes. ty over 80 percent of the phosphorous-doped s licon ingo 6, A method as described in claim 4 wherein the pressure is The method taught in accordance with this invention provides reduced f between 50 m 760 mm, Hg m 1X 1 14 mm Hg an economical way of increasing the yield of constant resistivi- 7. The method according to claim 1 wherein ty semiconductor crystal in an ingot.
we claim at the initiation of the withdrawing step, the inert gas atmosphere has a predetermined value and l. A method of making a single semiconductive crystal hav- 4O b. the rate of reduction of pressure of said inert atmosphere ing a predetermined resistivity comprising the steps of:
comprises a rate reduction from said predetermined presa. providing a mass of semiconductive material selected sure value to a lower value determined by said predeterfrom the group consisting of germanium and silicon and t d t d f d t mined resistivity and a continuation of the said lower con ain ng a pre e ermine amoun 0 con uc M yyp value of pressure during the remaining withdrawal of said determining impurity having a vapor pressure above that Crystal of said material, I
b. providing an inert gas atmosphere about said mass, Thg t P ficcordmg 1 Claim Wherel" melting Said mass, a. at the initiation of the withdrawing step, the inert gas atd. inserting a seed of said mass material into said melted mosphele has a prFdetermllled p Yaiue and mass b. the rate of reduction of pressure of said inert atmosphere e. withdrawing said seed at a rate such that the semiconduccomprises a raw reducnon from Predeer'mmed P tive material from the mg forms a Single crystal sure value to a lower value determined by said predeterpropagated from Said Seed crystal, mined resistivity and a continuation of said lower presf. (reducing the pressure at a programmed rate of said inert Sure value dunng remammg wnhdrawal of Sam gas atmosphere) during said withdrawing step, reducing Crystal the pressure of said inert gas atmosphere at a rate and to a 9. The method according to claim 6 wherein the reduction value determined by said predetermined resistivity to in Pr s r i in wo Stages Comprising maintain a substantially constant impurity concentration a rfiduCIlOn f between 50 I 760 mm- Hg to between i id l d mass, d lto 1X10 mm. Hg during a portion of the crystal pulling g. cooling said withdrawn material to yield a single crystal process d having a Substantially uniform resistivity Profile 0V6! a b. a continuation of the pressure between 10 to 1X10 mm. major portion of its length. Hg throughout the remaining portion of the crystal -2. A method as described in claim 1 wherein said inert gas withdrawal process. atmosphere taken from the group consistmg of hehum and 10. The method according to claim 6 wherein the reduction argon I of pressure from between 50 to 760 mm. Hg to about 1 mm. A method as descnbed m clalm 1 wherein said lmpfmty Hg is at a linear rate with respect to time and the reduction taken from the group consisting of phosphorus, arsenic, and from about 1 mm Hg to 1X10 mm. Hg is exponemial as annmony' determined by the needle valve construction being closed at a 4. A method of making a single semiconductive crystal havnormal rate ing a predetermined resistivity comprising the steps of:
UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No- 3, 615, 261 Dated October 26 1971 Inventor(s) Donald R. Causey, et a1 It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:
Column 2, lines 60 and 62, "10114" hould read 1O4-- Column lines 36, 59, 61 and 67, "10 should read 10- Signed and sealed this 10th day of October 1972.
EDWARD M.FLE'ICI ER,JR. ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents PC4050 USCOMM-DC 6O376-F'69 .5. GOVERNMENT PRINTING OF'DCEi I". 0-360-831.