US2904663A - Apparatus for zone melting of semiconductor material - Google Patents

Description

Sept. 15, 1959 R. EMEls ETAL APPARATUS FOR ZONE MELTING OF SEMICONDUCTOR MATERIAL Filed 0G12. 31, 1958 f f z z vW///fl V/HV/r 8 /A ,/l /f/f/ f//f//l/f g @www 7 United States Patent APPARATUS FOR'ZONE 'MELTING OF SEMICONDUCTOR MATERIAL Reimer Emeis and Joachim Haus, Pretzfeld, Ofr.,-Ger

many, assignors to Siemens-Schuckertwerke Aktiengesellschaft, Berlin-Siemensstadt, and Erlangen, Germany, a `German corporation ApplicationOctober 31, 1958, Serial No. 771,037

Claims priority, application Germany November 15, 1957 9 Claims. (Cl. 219-10.43)

Our linvention relates generally-to the`processing `of electric` semiconductor bodies by melting a narrow zone of the body and progressively shifting the molten Zone along the body. This method, generally known' as zone melting or zone pulling, serves to purify the material, to dope it with donor or acceptor substance, or to convert it from polycrystalline to monocrystalline condition.

For electrical purposes, such as the production of semiconductor devices with one or morep-n junctions as required, for example, in rectifiers, transistors and photodiodes, a semiconductor raw material. of an extremely. high degree of purity is required. vThis applies particularly to semiconductor devices having a monocrystalline semiconductor body of silicon, or such substances as germanium, indiumantirnonide, indiumarsenide or other inter-metallic compounds in the groupof AIHBIV semiconductors.

Special processes and devices have `been developed for obtaining the desiredhigh purity. One of these methods is the Crucible-free Zonemelting suitableffor example, for purification of the semiconductor material or the pulling of monocrystals. According to` this known method, a rod of the semiconductor material isvertically `clamped at both ends and is then melted along afnarrow vertical zone, for example, by meansof an induction-'coil energized by medium-frequency or high-frequency current. The melting may also be effected by radiating heat supplied with the aid of a ring-shaped-heater energized b-y direct current or low-frequency alternating current such as the generally available utility-line current of 50 'o1-.6() c.p.s.

Theinduction coil or ring-shaped heater, whose axial extent is small in comparison with the length of the semiconductor rod, causes melting only within the narrow zone of the rod located within the heater and is slowly moved in the longitudinal direction of the rod. The process is preferably performed in high vacuum-so that aside from the purification effected by the Zone melting proper, certain impurities are also removed from the semiconductor material by evaporation. However,-the process may also be performed in a protective gas atmosphere.

Such Crucible-free zone melting methods encounter considerable difficulties due to the necessity of supplying electric current from the outside of the hermetically sealed processing space to the heating device movable within that space. The ltwo current supply 'leads' for the heater current, havingk frequency vof ione toi-five niegacycles per second, should be as close as possible to each other and should be virtually free of inductivity. `One end of the current supply structure must pass through `the bottom wall of the processing vessel to receive-heating power from the outside, whereas the other end must be connected with the heater which, as mentioned, is m'ovable in the vertical direction, The supply leads must not touch other parts or the vessel wall of the processing equipment during the operational displacement of the heater. The current supply means, furthermore, should ICC notintei-fereWith,-or lobstruct, the necessary manipula- 'tionswhen=theprocessing'vessel is opened. It is-further necessary to provide for sufficient cooling of the heater becausel it closely surrounds themelting rzone and would otherwisebe "excessively heated by radiation fromthe molten Zone of the semiconductor body. `Such cooling is-necessaryto: preventundesired impurities of the heater material from'evap'orating vduring operation'in Vacuum thus reaching and contaminating the semiconductor material'being processed.

`It is object of 'our invention to provide a device 'whichisatises -all of the above mentioned requirements and desiderata,'and^which Icombines a relatively simple design with a-reduction in 4space requirements and minimized electric losses -in comparison with the zone-melting devices' heretoforeavailable.

@To-this end, and in accordance with affeature `of our invention, `the devicefor ciucible-freezone melting kof rod-shaped semiconductor material, particularly silicon, for use in` semiconductor devices such as rectiiiers, transistors, orphotodiode, with afprocessing vessel, preferablya high-vacuum vessel, whose width is large in comparison with the space occupied by the material to be processed and by the means *for mounting and heating the material. We further provide 'the device with a current supply assembly formed of a-plurality of conducting tubes of respectivelyy different diameters, the outermost tube, having the largest diameter, passing through an opening in awall, preferably the bottom, of the processing vessel and being axially displaceable in hermetically sealed relation thereto. The individual tubes of the assembly are rigidly joined with each other and'form concentric interspaces, each of which is sealed so that the assembly asa whole constitutes an axially displaceable vacuum-tight plug with respect to the processing vessel. The outer tube consists preferably .of hard chrome steel and has a ground-or polished outer surface so that it ycan slide with relative ease in the gas-tight seal-preferably formed-by an oilcontainingV gasket struct-ure. The hollow space in the innermost tube and the interspace between the outermost and the next inner tube arepreferably used for supplying and discharging a circulating coolant such as water. The space between the current conducting tubes is preferably iilled with insulating material. However, this interspace may also be evacuated, for example, by connecting it with the high-vacuum space within-the processing vessel of the zone melting device. The mutually insulated conductor tubes are electrically connected with the terminals ofthe electric heater within the vessel and serve forsupplying current from the outside to the heater.

A particular advantage ofthe device according to the invention, in addition to'tthose mentioned above, results from the inherently rigid construction of the entire current supply assembly. That is, according to another feature of our invention, the heating coil or other heater is rigidly fastened to the inner ends of the assembly tubes so that the current supply assembly also serves for axially displacing the heater during zone-melting operation. This differs favorably from the crucible-free zone melting devices heretofore `knownl in which the heating coil is fastened to a slider travelling up and down on a screw spindle driven from the outside of the processing vessel, thus requiring a seal between spindle and vessel wall that effects sealing for longitudinal as well as rotational displacement. In a device according to the invention, the displacement of the current supply assembly and of the heating coil fastened thereto is also effected from the outside of the vessel, but requires only a linear displacement-of the assembly thus permitting the use of a more reliableseal'and minimizing the amount of material and space 'required within the processing vessel for imparting displacement to the heater,

The invention will be more fully explained with reference to the embodiment illustrated by way of example on the drawing in which:

Fig. 1 shows schematically ya vertical sectional view of a complete Crucible-free zone melting operator according to the invention, and

Fig. 2 shows on a larger scale a vertical and axial section of the current-supply assembly that forms part of the same device.

According to Fig. 1, the device comprises a vessel composed of bell 1 of relatively great width which is vacuumtightly seated upon a bottom plate 2. The bell 1 may consist of steel or other sheet metal and is preferably provided with an observation window 1a of glass. The processing space within the vessel can be evacuated through a nipple 1b to be connected to a vacuum pump. If desired, an inert gas may also be supplied through the nipple. Mounted on the bottom plate 2 of the vessel is a frame structure 2l which carries a holder 22 vertically above another holder 23 that is mounted on the bottom plate 2 and may be fastened to a shaft 2S passing through a seal in the bottom plate in order to permit imparting rotation to the semiconductor material 24 during the zone melting operation, as is desirable for some purposes.

The semiconductor rod 24, for example of silicon, is held in and between the two holders 22 and 23 and is surrounded by an inductive heating coil 26. During operation of the device, with rod 24 inserted as shown and the processing space evacuated, the coil 26 is energized by electric current and causes a narrow horizontal zone of the rod 24 to melt, while being slowly shifted upwardly or downwardly along the rod thus gradually passing the melted zone through the entire body for the purpose of purifying it and/or converting it to monocrystalline condition.

Mounted in an opening of the bottom plate 2 is a gastight seal 3 which may consist, for example, of an oillled gasket-type sealing structure as shown in Fig. 2. A steel pipe 4 is slidably seated within the annular seal 3. The pipe 4 consists of a hard chrome steel or stainless steel and is ground to a highly polished condition for improving the seal and facilitating vertical displacement of the tube within the seal. Thus the current-supply assembly as a whole, including the heating coil 26 mounted thereon, can readily be moved vertically up or down Without appreciably impairing the vacuum within the processing vessel. Located within the steel tube 4 but coaxially spaced therefrom is a copper tube 5. Another copper tube 6 is coaxially located within tube in spaced relation thereto. The two copper tubes 5 and 6 serve as conductors for the heater current. That is, these two tubes are connected, outside of the processing vessel, with the terminals of the course of heating power.

In the illustrated embodiment the annular interspace between tubes 5 and 6 is filled with insulating materials 7 which may consist of polytetratluorethylene tape commercially available under the trade-mark Teflon, and the ends of the insulating body formed by the tape may be sealed by a casting of parailn. In lieu thereof, the interspace between tubes 5 and 6 may also be connected with the high vacuum obtaining within the vessel of the zone melting device, In both cases, the occurrence of arcing between the two conductor tubes 5 and 6 is reliably prevented even if the spacing between them is only slight. An end seal 8 of polytetralluorethylene (Teflon) is provided at the lower end of the tubes for sealing the interspace toward the outside. A similar sealing body SA is provided at the upper end of the tubes.

The upper ends of tubes 5 and 6 are connected with the tubular turns of the heater coil 26 at 10 and 11.

The tubes 5 and 6 thus also serve to provide channels for the circulation of coolant, such as water, through the turns of the heater coil. The entrance and exit of the flowing coolant are indicated in Fig. 2 by arrows 9.

The connection of the heater coil 26 at i0 and il to pipes 5 and 6 is rigid, whereas the connection of the current supply to the pipes 5 and 6 and the supply of coolant to those tubes outside of the processing vessel are preferably flexible.

It will be obvious to those skilled in the art, upon a study of this disclosure, that our invention permits ot various modifications within respect to details and may be embodied in devices other than the one illustrated and described herein, without departing from the essential features of our invention and within the scope of the claims annexed hereto.

We claim:

l. Apparatus for Crucible-free zone melting of rodshaped semiconductor material, comprising a vacuum Vessel, axially spaced holders in said vessel for securing the rod material between them, an annular electric heating member extending about the axis of said holder and being axially short with respect to the spacing between said holders, said heating member being displaceable along said axis for Zone melting the rod material, a device for supplying electric current to said heating member comprising an assembly of electrically conducing tubes of respectively dilerent diameters extending coaxially one about the other in insulated and mutually spaced relation so as Yto form an interspace, said interspace being sealed, said vessel having a wall portion with an opening, the outermost tube of said assembly passing from within said vessel through said opening to the outside and being axially displaceable in sealed relation to said wall portion, said tubes being electrically connected with said heating member.

2. In Zone-melting appartus according to claim l, said assembly comprising three coaxial tubes of which the two inner ones consist of copper for conducing electric current to said heating member, and the outermost tube consists of chrome-containing steel and has a polished outer surface.

3. In Zone-melting operation according to claim l, said interspaced between said tubes being lled with an insulating material.

4. In Zone-melting operation according to claim l, said interspace between said tubes being evacuated.

5. In Zone-melting operation according to claim l, said interspace between said tubes being sealed toward the outside of said vessel and being in communication with the interior of said Vessel so as to be evacuated when in operation.

6. Zone-melting apparatus according to claim l, comprising a plug of polytetrauorethylene located at the end of said interspace between said tubes and sealing said interspace.

7. Apparatus for Crucible-free zone melting of rodshaped semiconductor material, comprising a vacuum vessel, axially spaced holders in said vessel for securing the rod material between them, an annular electric heating member extending about the axis of said holder and being axially short with respect to the spacing between said holders, said heating member being displaceable along said axis for zone melting the rod material, a device for supplying electric current to said heating member comprising an assembly of electrically conducting tubes of respectively different diameters extending coaxially one about the other in insulated and mutually spaced relation so as to form an interspace, said interspace being sealed, said vessel having a wall portion with an opening, the outermost tube of said assembly passing from within said vessel through said opening to the outside and being axially displaceable in sealed relation to the said wall portion, said heating member being rigidly joined with said assembly to be displaced together therewith and being electrically connected with said tubes to receive electric current therefrom.

8. In zone-melting apparatus according to claim 7, said heating member consisting of a tubular winding to be traversed by coolant, the inner one of said tubes being in communciation with one end of said winding, and the interspace between the outermost and the next inner tube being in cominunciation with the other end of said tubular winding for passing said coolant from the outside of said vessel serially through said inner tube, said winding and said interspace.

9. Apparatus for Crucible-free zone melting of rodl shaped semiconductor material, comprising a vacuum ves sel, axially spaced holders in said vessel for securing the rod material between them, an inductance heater member consisting of a tubular coil extending about the axis of said holder and being axially short with respect to the spacing between said holders, said heating member being displaceable along said axis for zone melting the rod material, a device for supplying electric current to said heating member comprising an assembly of three metal tubes extending coaxially about one another in mutually spaced relation, the inner one of said tubes being electrically insulated from the middle tube and the interspace between them being sealed, the outermost tube and the middle tube forming together a sealed jacket space, said vessel having a wall portion with an opening, the outer most tube of said assembly passing from within said vessel through said opening to the outside and being axially displaceable in sealed relation to said wall portion, said tubular coil being rigidly secured to said tube assembly and having its ends communicating with said inner tube and said jacket space to be supplied with coolant from the outside, said ends of said coil being electrically connected with said inner tube and said middle tube respectively.

References Cited in the tile of this patent UNITED STATES PATENTS 2,686,864 Wroughton et al Aug. 17, 1954 2,743,199 Hull et al. Apr. 24, 1956 20 2,870,309 Capita Jan. 20, 1959

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