US3251658A

US3251658A - Zone refining start-up

Info

Publication number: US3251658A
Application number: US261090A
Authority: US
Inventors: Hambach Anthony; Paul J Schauer
Original assignee: Monsanto Co
Current assignee: Monsanto Co
Priority date: 1963-02-26
Filing date: 1963-02-26
Publication date: 1966-05-17
Anticipated expiration: 1983-05-17
Also published as: GB1055127A

Description

May 17, 1966 A. HAMBACH ETAL ZONE REFINING START-UP 3 Sheets-Sheet 1 Filed Feb. 26, 1963 INVENTOR PAUL J'. SCHAUER ANTHONY HAMBACH TTO R N EY y 7, 1966 A. HAMBACH ETAL 3,251,658

ZONE REFINING START 3 Sheets-Sheet 2 Filed Feb. 26, 1963v R E mm OH WC Wm N L U m ANTHONY HA MB ACH BY ATTQRNEY FIG.4

May 17, 1966 A. HAMBACH ETAL 3,

ZONE REFINING' START-UP Filed Feb. 26, 1963 3 Sheets-Sheet 5 ill. Illllllllllllllllllllllllllllllllllllllmllll lll FIG. 5

INVENTORS PAUL J. SCHAUER ANTHONY HAMBACH ATTORNEY United States Patent 3,251,658 ZONE REFINING START-UP Anthony Hambach, OFallon, and Paul I. Schauer, Creve Coeur, Mm, assignors to Monsanto Company, a corporation of Delaware Filed Feb. 26, 1963, Ser. No. 261,090 16 Claims. (Cl. 23301) This invention relates to methods and apparatus for float zone refining semiconductor materials andmore specifically the invention relates to zone refining apparatus embodying improved means for heating to a conductive temperature a transverse segment of a rodof semiconductor material.

Float zone refining, sometimes referred to as float zone melting, is widely practiced in the semiconductor materials industry for a number of purposes. A primary such purpose comprises preparing a single crystal of semiconductor material from a polycrystalline rod. Other purposes include semiconductor purification and distribu tion'of impurities in semiconductor materials. In most instances a float zone refining operation comprises vertically suspending a rod of semiconductor material such as silicon, germanium or gallium arsenide in a chamber which may be'filled with an inert gas but which in some instances is placed under a very high vacuum. A transverse segment of the rod at an unsupported end thereof,

usually the lower end, is then brought to a molten state by means comprising a high frequency alternating current heater, and the molten terminus of the rod is contacted with a seed crystal of the semiconductor material. The molten zone is then moved'to the opposite end of the rod by relative movement of the rod with respect to the heater employed in creating the molten'zone to thereby complete what is usually the first of a plurality of zone passages.

A preferred means for heating a transverse section of a rod of semiconductor material being zone refined comprises a RF (radio frequency) heating coil surrounding the rod of semiconductor material. One difficulty experienced with this means of heating, however, is that the rod of semiconductor material is not sufiiciently conductive at room temperature that it can be heated by inductive means and at least a segment of the rod must be initially raised to a relatively high temperature before it becomes sufiiciently conductive that a direct coupling with an inductive heating coil can be established. The conventional procedure for accomplishing this comprises surrounding the rod of semiconductor material with a ring or tube of highly conductive material at one end, normally the upper end, which will couple With a RF coil and when heated to a high temperature will result in a transverse segment of the rod of semiconductor material being heated by radiation to a temperature such that it becomes conductive. The transverse heated zone is then moved from under the ring of conductive material by relative movement of the heater coil and rod of semiconductor material and in similar manner is moved to the opposite end of the rod. The current being passed through the heating coil is then incr ased to result in this end of the rod being melted so that it can be fused with a seed crystal.

The above procedure has a number of disadvantages a primary one of which is that transporting a heated but non-molten zone from one end of the rod to the other frequently results in separation of the rod due to small fractures created. incident to the formation of the rod. Breakage of'the rod not only results in material loss but also may result in considerable loss of time since setting up a zone refiner and evacuation of the Zone refining chamber are time consuming operations. To avoid this disadvantage it has been suggested that a tube of Patented May 17, 1966 electrically conductive foil or the like be wrapped around the seed crystal such that it extends beyond the chuck in which the seed is held and using this tube of foil as a preheater, heating a transverse segment of the seed to a conductive temperature. In accordance with this suggestion the heated transverse segment of the seed would then be moved upwardly toward the rod of semiconductor material to be zone refined and by placing the seed very close to the end of the rod of semiconductor material it might be possible to h at the end of the rod to a conductive temperature by radiation from the seed. This suggestion has not been followed in commercial practice, however, because transporting the hot zone through the seed severely damages it and thereby reduces the quality of the product obtained from the zone refiner. A further reason that the suggestion has not been followed is that it is now known to be highly advantageous to use very small diameter seeds and heating a large diameter rod of semiconductor material to a conductive temperature by radiation from the end of a small diameter seed would be difiiicult if not impossible.

A further severe disadvantage of the prior art procedure of preheating as outlined above is that heating to a very high temperature a transverse segment of a semiconductor rod adjacent the chuck in which it is being held by one end frequently results in minor shifts of the rod in the chuck due to thermal expansion of .the chuck, thermal expansion of the rod or possibly through the release of strains present in the rod of semiconductor material. Shifting of the rod of semiconductor material can result in the heater coil dragging on the surface of the rod during the zone refining operation and this in turn results in coil damage and/or misalignment of the coil and additionally may result in contamination of the rod of semiconductor material in the area where it is contacted by the heater. coil.

It is a primary object of this invention to provide methods and apparatus for creating a conductive zone proximate one end of a rod of semiconductor material at which a zone melting operation is to be initiated.

It is another object of the invention to provide methods and apparatus for creating in a rod of semiconductor material suspended at one end in a zone refiner a conductive segment remote from the secured end of the rod.

It is still another object of the invention to provide fioat zone refining apparatus including a preheating means which does not interfere with movement of a rod of semiconductor material being zone refined relative to an inductive heating means for heating the rod of semiconductor material, and does not interfere with relative movement between the rod and a seed crystal disposed for clontact with one end of the rod of semiconductor materia It is a further object of the invention to provide zone refining apparatus including a preheater means moveable to operative and inoperative positions and which does not require additional moving parts extending through the wall of the chamber in which the zone refining operation is to be conducted.

The above as well as other objects of the invention are accomplished by the provision of apparatus comprising an enclosed chamber, means for supporting a rod of semiconductor material, such as silicon or germanium, from one end so that it extends in a generally vertical direction within the chamber, and an electrically conductive member operatively disposed near the end of the rod which is to be contacted with a seed crystal. The apparatus also includes an inductive heating coil which may and normally is disposed within the chamber, and means for traversing the heater longitudinally of the rod of semiconductor material. The conductive member is so constructed that it can be disposed close to but predomremoving the conductive member from operative position such that it does not restrict movement of the inductive heating coil nor movement of the seed crystal once the conductive member has served its intended purpose of heating by radiation a' segment of the semiconductor rod proximate the end thereof that is to be contacted with the seed crystal.

A preferred embodiment of the invention will now be specifically described with reference to the accompanying drawings in which:

FIGURE 1 is a front plan view of one type of zone refining apparatus embodying a preheater in accordance with this invention.

FIGURE 2 is a front plan view of the zone refining apparatus of FIGURE 1, with parts broken away, showing the preheater moved to inoperative position and a zone refining operation in progress.

FIGURE 3 is a cross sectional view taken substantially along the line 3-3' in FIGURE 1.

FIGURE 4 is an enlarged view of a means for positioning the preheater of the apparatus of FIGURE 1.

FIGURE 5 is a view of a portion of the apparatus of FIGURE 1, partially in section and with parts broken away, showing the drive and control means for shaft 38.

With particular reference to FIGURE 1 of the drawings the reference numeral represents a support panel for a zone refiner chamber, generally indicated by the reference numeral 11, designed to be evacuated to a high vacuum by means of a suitable vacuum pump not illustrated. The chamber 11 comprises a bottom wall 12, a top wall 14, a back wall 16 and

side walls

17 and 18. The chamber also comprises a front plate 19 containing a glass window 20 (see FIGURE 2) through which the entire zone refining operation can be observed. The front plate 19 is hinged to the support panel 10 and is provided with a latch 21 so thatit can be closed over the front of the zone refining chamber and secured in that position when the apparatus is in use.

Depending from the top 14 of the zone refining chamher is a rod holder means 22 which can be conventional in construction and which is designed to hold a rod of semiconductor material 24 proximate the upper end thereof such that it extends in a generally vertical direction within the chamber 11. While the rod of semiconductor material can deviate from a strictly vertical position, any substantial deviation is generally undesirable since it promotes the falling out of the molten zone of semiconductor material and may necessitate the use of levitation means to prevent loss of the molten zone.

Surrounding the rod 24 of semiconductor'material there is illustrated a RF heater coil 26 which can suitably be formed from tubular silver stock or the like and which is designed to conduct RF current produced by a conventional RF generator, not illustrated. The heater 26 is carried by

support arms

27 and 28 which extend from a transverse member 30 moveable upon a traverse rod 32. The traverse rod 32 extends parallel to the rod of semiconductor material 24 and is secured to the top and bottom walls of the zone refiner chamber 11. Means, not illustrated, are provided for rotation of the traverse rod 32 in either direction and at any selected rate so that the heater 26 can be moved the length of the rod of semiconductor material 24 as desired.

Disposed immediately below the bottom terminus of the semiconductor rod 24 is a seed holder chuck 34 designed to hold a seed 36 formed from a single crystal of the same semiconductor material as rod 24. Seed 36 can be conventional in configuration but is preferably of much smaller diameter than the rod of semiconductor material 24 and preferably is of sufl'lcient length that portions of it can be melted upon successive zone passes without the heater coil inductively heating the chuck 34. The chuck 34 is mounted upon a vertically extending rotatable shaft 38 which extends through the bottom wall 12 of the zone refining chamber 11. The shaft 38 sup porting chuck 34 is also longitudinally moveable so that the seed crystal 36, which is spaced from the rod of semiconductor material 24 at the start of the zone refining operation, as shown in FIGURE 1 of the drawings, can be fused tothe lower end of the rod- 24 as shown in FIGURE 2. Means comprising, for example, an electric motor 35 and a gear reduction train 37 (see FIGURE 5 of the drawings), are provided for rotation and longitudinal movement of shaft 38. Control means of conventional design and comprising, for example, a combined electrical switch and rheostat 39 is also provided for initiating rotation of shaft 38 and seed 36 andfor controlling their rate of rotation.

Secured to the bottom wall 12 of chamber 11 by any suitable means, such as welding, is an upstanding tubular member 40 open at the upper end and having a plurality of evenly spaced openings 42 through the wall thereof. Member 40 extends generally parallel to the longitudinal axis of semiconductor rod 24 but is spaced from the semiconductor rod a distance such that it does not interfere with the movement of the heater coil nor with the loading and unloading of semiconductor material into and out of the chamber 11. Extending into the open upper end of tubular member 40 and freely pivotable therein is an inverted U-shaped member 44 carrying an arm 46 adjustably secured to its upper extremity by any suitable means such as

bolts

48 and 50. The extent to which member 44 extends into tubular member 40 is limited by a pin 51 inserted into a selected one of the openings 42 (see FIGURE 3).

Secured to the unsupported end of arm 46 is a semicylindrical conductive member 52 which has a concave surface 54 facing the rod 24 of semiconductor material, and which serves as a displaceable preheater for a thin transverse segment of the rod 24 in a manner which will subsequently be made apparent. Member 52 can be formed of any suitable material capable of readily conducting electrical currents but is preferably formed of a material which does'not objectionably contaminate the material being zone refined. Examples of suitable conductive materials from which semi-cylindrical member 52 can suitably be formed are molybdenum, silver and carbon. Molybdenum is preferred because of its good ductility characteristics and high temperature rigidity. The length of arm is adjusted such that the semi-cylindrical member 52 can be positioned to closely conform to the peripheral surface of the rod 24 of semiconductor material, the radius of curvature of the concave face of the semi-cylindrical member being such that the face conforms to the peripheral surface of the rod.

Inserted itno the concave face 54 of member 52 is an insert 56 having a conical protruding face 58 (see FIG- URE 3). The apex of the conical face 58 is removed from the concave face 54 of member 52 a distance equal at least to the normal surface variations in the rod of semiconductor material 24 so that when semi-cylindrical member 52 is urged toward the rod of semiconductor material, the

apex of face 58 touches the rod of semiconductor material making substantially point contact. Thus, concave surface 54 is at all times in predominantly spaced apart relationship to the surface of the rod of semiconductor material. This arrangement minimizes contamination of the rod of semiconductor material by contact with conductive member 5 2 and provides a means for laterally positioning the member 52 so that it is very close to but a selected mean distance from the peripheral surface of the semiconductor rod.

Extending from chuck 34 is a rod 60 which constitutes a first one of a pair of contact members with the depending unsupported end 62 of member 44 constituting the second. Rod 60 extends such that when chuck 34 is rotated with member 52 in operative position, end 62 of member 44 is struck by rod 60 and member 44 is pivoted about the axis of upstanding tubular member 40 thereby removing conductive member 52 from operation position as illustrated in FIGURE 2 of the drawings. With member 52 in inoperative position and disposed adjacent wall 17 of chamber 11, chuck 34 is freely rotatable and seed 36 can readily be moved into contact with the lower end of rod 24. It will further be noticed then with the conductive member 52 in inoperative position as shown in FIGURE 2, heater coil 26 can be lowered below the level of the lower extremity of rod 24 so that a zone refining passage can be started by first melting a portion of the seed crystal itself. This is usually desirable in at least the last zone passage when a zone refining operation comprises a plurality of passages.

In operation, the front plate 19 is pivoted to permit ac cess to the zone refining chamber and a rod 24 of semiconductor material to be zone refined, such as a polycrystalline silicon rod, is inserted into rod holder 22 so that it depends from its upper end vertically within the chamber 11 with its lower end unsupported. Shaft 38 is then partially retracted from the chamber 11 to provide adequate clearance between the lower end of rod 24 and the chuck 34 so that'seed 36 can be inserted.

With the rod of semiconductor material 24 and seed 36 in operative positions heater coil 26 is lowered so that it is in a plane immediately above the lower end of rod 24 and U-shaped member 44 is pivoted such that semicylindrical member 52 is disposed immediately below or above the heater coil in closely confining relationship to the periphery of the extreme lower portion of the rod 24. For positioning of member 52 axially of rod 24 U-sh-aped member 44 can be raised or lowered by removing pin 51 and inserting it in a higher or lower one of openings 42 and for the lateral positioning of member 52, if necessary, the length of arm 46 can be varied by loosening of

bolts

48 and 50. When member 42 is pivoted about the longitudinal axis of tubular member 40 until the point of insert 56 touches the peripheral surface of the semiconductor rod, member 52 normally will spring back very slightly when the pressure urging it toward the rod of semiconductor material is released so that even the apex of insert 52 is slightly spaced from the rod of semiconductor material. If, however, contact continues the insert 52 insures that the area of contact will be minimal.

With the member 52 and the heater coil 26 in proper positions, plate 20 is pivoted to close chamber 11 and the chamber is evacuated. The source of RF current is then placed in operation and current is fed through heater coil 26 in a conventional manner thereby heating member 52 to an elevated temperature by induced currents. Since member 52 is disposed closely adjacent to the end of rod 24, the end of the semiconductor rod is simultaneously heated by radiation from member 52 and once the end of the rod has reached a conductive temperature, the motor for rotating shaft 38 is activated by switch 39 and chuck 34 is rotated through at least about one turn. This results in rod 60 contacting the end 44 of member 62 pivoting the member 62 about the axis of tubu'lar member 43 and thereby removing conductive member 52 from operation position. The current through heater 26 is then increased to effect the melting of the lower extremity of rod 24 and seed chuck 34 is raised to effect fusing of the seed crystal 36 with the molten end of rod 24. The zone refining operation is then continued in a conventional man ner.

Having thus described our invention and one preferred embodiment thereof what we desire to claim and secure by Letters Patent is: i

What is claimed is:

1. In an apparatus for zone melting a rod of semiconductor material and including an enclosed chamber, means for supporting a rod of semiconductor material proximate the upper end thereof such that it extends 6. vertically within said chamber and such that the lower end of said rod is unsupported, means for securing a seed crystal below and in spaced relationship to said rod supporting means, means for moving said seed-securing means relative to said rod supporting means such that a seed crystal secured by said seed securing means can be brought into contact with the lower extremity of a rod of semiconductor material held by said rod supporting means, inductive heating means for heating a thin transverse segment of said rod, and traverse means for moving said heating means longitudinally of said rod; an electrically conductive member operatively disposed proximate said seed securing means such that when heated to an elevated temperature the lower extremity of a rod of semiconductor material held by said rod supporting means is heated by radiation from said conductive member, displacement means for'removing said conductive member from operative position such that it does not restrict movement of said seed securing means not of said heating means relative to said rod supporting means, and control means disposed outside said chamber for initiating operation of said displacement means.

2. Apparatus according to claim 1 including means for adjustably positioning said conductive member longitudinally of a rod of semiconductor material operatively held by said rod supporting means.

3. Apparatus according to claim 2 including means for laterally positioning said conductive member with respect to the periphery of said rod of semiconductive material such that it is adjacent to but in predominantly spaced apart relationship to the surface of said rod.

4. Apparatus according to claim 1 wherein said conductive member has a concave generally semi-cylindrical surface.

5. Apparatus according to claim 4 including a protruding member carried by said conductive member and having an unsupported pointed end extending outwardly beyond said concave surface of said conductive member so that when said surface is biased toward the external surface of a rod of semiconductor material held by said rod supporting means such that the curvature of said surface generally matches the external surface of said rod, contact of said conductive member with. said semiconductor rod is limited substantially to the pointed end of said protruding member.

6. Apparatus according to claim 4 wherein said displacement means comprises an arm carrying said conductive member and pivotable about an axis removed from said conductive member.

-7. Apparatus according to claim 4 wherein said displacement means comprises a first contact means carried by and movable with said conductive member and a second contact means carried by and rotatable with said seed securing means to contact said first named contact means upon rotation of said seed securing means, whereby upon rotation of'said seed securing means said second contact means engages said first contact means to result in said conductive member being removed from operation position.

8. Apparatus according to claim 1 wherein said conductive member is formed of molybdenum.

9. Apparatus according to claim 1 wherein said conductive member is formed of silver.

10. Apparatus according to claim 1 wherein said conductive member is formed of carbon.

11. In an apparatus for zone melting a rod of semiconductor material and including an enclosed chamber,

means for vertically suspending a rod of semiconductor rod by a seed held by said chuck, an inductive heating coil for heating a thin transverse segment of said rod, and traverse means for moving said heating means longitudinally of said rod; an electrically conductive member operatively disposed near said chuck and laterally displaceable from operative position to an inoperative position such that it does not restrict movement, relative to said rod suspending means of said seed holder chuck nor of said heating means, means moveable with said chuck for displacing said conductive member from operative to inoperativeposition upon rotation of said chuck, and control means disposed outside of said chamber for initiating rotation of said chuck.

12. Apparatus according to claim 11 wherein said conductive member is supported by an arm pivotable about a vertical axis remote from but generally parallel to a line extending vertically from said rod suspending means to said seed holder chuck, and including means for adjustably positioning said arm vertically along the extended axis about which it is pivotable.

13. Apparatus according to claim 12 including a depending member pivotable with said arm and extending downwardtherefrom, and rod means moveable with said chuck to engage said depending member upon rotation of said chuck.

14. Apparatus according to claim 13 wherein said conductive member is formed of molybdenum.

15. In a method for zone melting a rod of semiconductor material wherein said rod is suspended vertically from its upper extremity .within an enclosed .chamber, the steps comprising positioning an electrically conductive member near the lower extremity of said rod in surrounding and proximate relationship thereto, positioning a seed crystal in spaced apart relationship to and below the lower extremity of said rod, said seed being of smaller diameter than said rod of semiconductor material, positioning an inductive heating coil adjacent said electrically conductive member and in inductive coupling relationship therewith, passing a high frequency alternating current through said coil to thereby result in the inductive heating of said electrically conductive member and the heating of a transverse segment of the lower extremity of said rod by radiation from said electrically conductive member, said electrically conductive member being formedv of a material which does not result in contamination of said semiconductor material to the extent that it is thereby rendered unsuitable for use and being formed of a material that does not melt at a temperature below that at which said semiconductor material becomes electrically conductive, removing said electrically conductive member from operative position subsequent to said segment of said rod being heated to a temperature such that it is electrically conductive, increasing the magnitude of the current being passed through said coil to inductively heat the lower extremity of the rod and to effect the melting of the lower extremity of said rod, the positional relationship of said seed to said lower extremity of said rod and said conductive member being such that said seed remains in a solid state during the melting of said lower extremity of said rod, thereafter moving said seed relative to said rod to effect contact of said seed with the molten lower extremity of said rod, and traversing the rod with the inductive heater.

16. A method according to claim 15 in which said rod of semiconductor material is silicon and said electrically conductive member is formed of molybdenum.

References Cited by the Examiner UNITED STATES PATENTS 2,901,325 8/1959 Theurer 23-301 X 2,972,525 2/1961 Emeis. 3,113,841 12/1963 Reuschel 23301 X OTHER REFERENCES Contribution To The Floating Zone, Buehler, June 1957; Refining of Silicon, Review of Scientific Instruments.

NORMAN YUDKOFF, Primary Examiner.

G. HINES, A. J. ADAMCIK, Assistant Examiners.

Claims

15. IN A METHOD FOR ZONE MELTING A ROD OF SEMICONDUCTOR MATERIAL WHEREIN SAID ROD IS SUSPENDED VERTICALLY FROM ITS UPPER EXTREMITY WITHIN AN ENCLOSED CHAMBER, THE STEPS COMPRISING POSITIONING AN ELECTRICALLY CONDUCTIVE MEMBER NEAR THE LOWER EXTREMITY OF SAID ROD IN SURROUNDING AND PROXIMATE RELATIONSHIP THERETO, POSITIONING A SEED CRYSTAL IN SPACED APART RELATIONSHIP TO AND BELOW THE LOWER EXTREMITY OF SAID ROD, SAID SEED BEING OF SMALLER DIAMETER THAN SAID ROD OF SEMICONDUCTOR MATERIAL, POSITIONING AN INDUCTIVE HEATING COIL ADJACENT SAID ELECTRICALLY CONDUCTIVE MEMBER AND IN INDUCTIVE COUPLING RELATIONSHIP THEREWITH, PASSING A HIGH FREQUENCY ALTERNATING CURRENT THROUGH SAID COIL TO THEREBY RESULT IN THE INDUCTIVE HEATING OF SAID ELECTRICALLY CONDUCTIVE MEMBER AND THE HEATING OF A TRANSVERSE SEGMENT OF THE LOWER EXTREMITY OF SAID ROD BY RADIATION FROM SAID ELECTRICALLY CONDUCTIVE MEMBER, SAID ELECTRICALLY CONDUCTIVE MEMBER BEING FORMED OF A MATERIAL WHICH DOES NOT RESULT IN CONTAMINATION OF SAID SEMICONDUCTOR MATERIAL TO THE EXTENT THAT IT IS THEREBY RENDERED UNSUITABLE FOR USE AND BEING FORMED OF A MATERIAL THAT DOES NOT MELT AT A TEMPERATURE BELOW THAT AT WHICH SAID SEMICONDUCTOR MATERIAL BECOMES ELECTRICALLY CONDUCTIVE REMOVING SAID ELECTRICALLY CONDUCTIVE MEMBER FROM OPERATIVE POSITION SUBSEQUENT TO SAID SEGMENT OF SAID ROD BEING HEATED TO A TEMPERATURE SUCH THAT IT IS ELECTRICALLY CONDUCTIVE, INCREASING THE MAGNITUDE OF THE CURRENT BEING PASSED THROUGH SAID COIL TO INDUCTIVELY HEAT THE LOWER EXTREMITY OF THE ROD AND TO EFFECT THE MELTING OF THE LOWER EXTREMITY OF SAID ROD, THE POSITIONAL RELATIONSHIP OF SAID SEED TO SAID LOWER EXTREMITY OF SAID ROD AND SAID CONDUCTIVE MEMBER BEING SUCH THAT SAID SEED REMAINS IN A SOLID STATE DURING THE MELTING OF SAID LOWER EXTREMITY OF SAID ROD, THEREAFTER MOVING SAID SEED RELATIVE TO SAID ROD TO EFFECT CONTACT OF SAID SEED WITH THE MOLTEN LOWER EXTREMITY OF SAID ROD, AND TRAVERSING THE ROD WITH THE INDUCTIVE HEATER.