Apparatus for crucible-free zone melting of semiconductor crystal rods

Abstract

Polycrystalline semiconductor rods are converted to dislocation-free monocrystal rods by positioning a polycrystalline rod within a crucible-free zone melt environment with a seed crystal attached to one end thereof, generating a melt zone at the juncture of the seed crystal and the polycrystalline rod, controllably moving the melt zone away from the juncture and through the polycrystalline rod to a select point thereon, uniformly supporting the cone-shaped lower portion of the rod being processed so as to prevent oscillations and the like at the juncture of the rod in the seed crystal and controllably moving the melt zone from the select point to the remainder of the rod. A uniform support is preferably provided by an axially movable funnel-shaped casing which is attached to the seed crystal holding member and which, when moved into its operating position, is filled with a self-adjusting oscillation or vibration dampening means, such as molten metal, metal spheroids, quartz particles, sand, etc.

Description

while which may also include liquified liquefied or molten metal, is essentially self-adjusting to any irregularities in the rod circumference and provides a uniform and complete support for the enclosed area thereof and prevents oscillations or the like so that dislocation-free crystals are readily and reliably produced by following the principles of the invention.

In one preferred embodiment of the invention, the dampening means comprises rigid inserts composed of a very pure metal having a relatively low melting point, such as aluminum, which are positioned at the upper periphery of the funnel-shaped casing so that upon the upward movement of the casing, the inserts contact adjacent points of the rod and the ends of the inserts melt to form a eutectic mixture with the rod material and then solidify to provide positive anti-oscillation support. In comparison with the arrangement suggested by German Offenlegungsschrift No. 1,519,901, the instant invention, including the above preferred embodiments thereof, provides a uniform support for any and all uneven points or the like at the rod circumference thereby avoiding the drawbacks noted earlier and providing positive and complete support for the growing monocrystalline rods.

In accordance with the principles of the invention, the uniform support for the bottom of the rod being processed is provided when the melt zone has traveled through the rod to a select point thereon, preferably at least about 10 cm above the highest reach of the funnel-shaped support means.

A preferred embodiment of an apparatus constructed and operable in accordance with the principles of the invention comprises two vertically opposing and approximately coaxially aligned rod mounting members in a crucible-free zone melt apparatus which may typically include an induction heating coil and an enclosed chamber along with means for supplying energy to the coil and for supplying a select atmosphere to the chamber. At least one of the rod mounting members is provided with a seed crystal which is melt-connected to a rod to be treated (i.e. a melt zone is generated at adjacent surfaces of the seed crystal and the rod and the molten surfaces are brought together to form a unified molten mass which is allowed to solidify and form a solid juncture. The rod mounting member having the seed crystal therein is also coupled with a funnel-shaped hollow casing which is axially and rotatably movable with the mounting member and is independently axially movable relative to the mounting member. This funnel-shaped casing is axially movable a distance sufficient to encompass the cone area of a rod above a seed crystal and may be associated with means for providing a self-adjusting vibration or oscillation dampening means to the casing. The self-adjusting oscillation dampening means is selected from the group consisting of particulate silicon, quartz, sand, metal (metal spheroids) liquified liquefied and re-solidifiable metal having a relatively low melting point, i.e. below the melting point of the rod being treated, such as aluminum, lead or indium, which may be provided in initially molten form to simply fill any space between the hollow funnel-shaped casing or be provided as solid inserts, the ends of which melt upon contact with the hot cone area of the rod being processed so as to form a eutectic mixture and upon solidification form a solid support for the rod. The flowable dampening means may be added to the funnel-shaped casing with the aid of an appropriate filling device, such as a conduit and the non-flowable dampening means may be added to the funnel-shaped casing during assembly thereof.

In the drawings, only essential features of the crucible-free zone melt apparatus have been illustrated in order to simplify the understanding of the invention and workers in the art will readily recognize and supply other elements and/or features required to obtain an operable zone melt apparatus and such operable apparatus will hereinafter and in the claims be referred to as a crucible-free zone melt apparatus and/or environment.

As shown at FIG. 1, a monocrystalline seed 2 is mounted in a mounting member 3 within a crucible-free zone melt apparatus and/or environment ZM. The seed crystal 2 is melt-connected with the lower end of a semiconductor crystalline rod 4 so as to form a juncture 2a therebetween. The rod 4 and the seed crystal 2 may, for example, be composed of silicon. A melt zone 6 is generated, as with the aid of induction heating coil 5, and controllably moved in an axial direction through the rod 4 from the juncture 2a upward to a select point P thereon. Movement of the melt zone is achieved by relative movement between the rod 4 and the heat coil 5.

A hollow funnel-shaped casing 7, for example, composed of titanium, steel, silicon or graphite, is mounted to encase the mounting member 3 and since the drive means (schematically illustrated as the vertically double-headed arrow and the horizontally curved double-headed arrow) for the mounting member is outside the bottom wall 11 of the crucible-free zone melt apparatus ZM, the casing 7 is axially and rotatably movable with the mounting member 3. In addition, casing 7 is independently axially movable relative to the mounting member 3. Thus, when the mounting member 3 is axially or rotatably moved, the casing 7 is likewise moved but the casing 7 may be axially moved independent of the mounting member 3. The funnel casing 7 is supported on a stud 8 which is attached to a pin 9 arranged within the mounting member 3 and which is axially movable. The stud 8 may be positioned within a guide slot 10 within the mounting member 3. Workers in the art will readily appreciate that other coupling means may also be utilized in accordance with the principles of the invention. Reference numeral 11 designates a portion of the bottom wall of an enclosed chamber typically utilized in a crucible-free zone melt apparatus and various seal means, drive means, atmosphere control means, energy supply means, etc. have been omitted from the drawings in order to simplify the understanding of the invention and to provide a better overall view of the essential features of the invention.

FIG. 1 illustrates the relative position of the elements before the rod end 4a is provided with support. The end 4a at this stage has grown as a monocrystal without dislocations at the relatively thin connecting or bridging piece 12 during a so-called bottle-neck-shaped constriction or necking process whereby a melt connection was formed between the seed crystal and the rod 4 and the melt zone moved therefrom through the rod one or more times. Since during such a process, both the rod 4 and the seed crystal 2 are rotated about their respective axes, there is a danger that the end 4a of the rod 4 which grows at the bottle-neck-shaped constriction 12 may begin to oscillate or vibrate when the melt zone 6 moves sufficiently from the juncture 2a between the seed crystal 2 and the rod 4.

In instances where a silicon rod of about 40 mm diameter is being so processed, the distance between the bridging piece 12 and the melt zone at which such oscillations are noted is about 70 cm. The amplitudes of such oscillations are often so great that the growth process must be interrupted and corrective measures taken.

In accordance with the principles of the invention, before the melt zone 6 reaches the critical distance or point from the bottle-neck-shaped constriction 12, the funnel-shaped casing 7 moved upwardly with the aid of the pin 9 so as to encase the cone area 4a of the rod 4 positioned above the seed crystal 2, such as shown in FIGS. 2 and 3. In order to insure a positive support and oscillation arrest, a self-adjusting dampening or stabilizing means 13 is provided between the interior of the funnel-shaped casing 7 and the periphery of the lower end 4a of the rod 4.

FIG. 2 illustrates a funnel-shaped casing 7 being filled with the dampening means 13, such as particulate quartz or sand, via a filling conduit 14. Granular silicon, metal spheroids, sand or a compatible (i.e. substantially inert and non-contaminating) metal, such as molten lead or indium, which solidifies within the casing 7 upon coming in contact with the relatively hot lower rod portion may also be used in place of the particulate quartz or sand. As is apparent from FIG. 2, once the casing 7 has been moved to its operative or supporting position, the rod 4 can no longer oscillate on the bottle-neck-shaped constriction. The lower rod portion 4a follows the seed crystal and the narrow constriction or bridging piece 12 is already sufficiently cold at this time so that no dislocations or the like can form. The remaining reference numerals shown in FIG. 2 are identical to those utilized in FIG. 1 and refer to identical elements discussed in conjunction with FIG. 1.

FIG. 3 illustrates another embodiment of the invention for supporting a growing monocrystal so as to avoid formation of dislocations or other irregularities therein. In this embodiment, inserts 15 are attached via mountings 16 along the upper periphery of a funnel-shaped casing 7 so that such inserts gently touch the periphery of the cone area 4a when the casing is moved to its upward position. The inserts 15 are composed of a compatible, relatively low melting, extremely pure metal, for example, aluminum, which melts upon contact with the still-hot rod material and alloys with or forms a eutectic mixture therewith. While the melt zone continues its passage upwardly through the remaining portions of the rod 4, the alloyed area of the rod cone circumference cools and a solid connection forms which prevents any oscillations or vibrations and provides a uniform support for any and all uneven points on the cone circumference. In this embodiment, it is important to select a metal for the inserts which has an appropriate melting point. The remaining reference numerals in this Figure are identical with those utilized in FIGS. 1 and 2 and refer to similar elements as discussed in conjunction with FIGS. 1 and 2.

As is apparent from the foregoing specification, the present invention is susceptible of being embodied with various alterations and modifications which may differ particularly from those that have been described in the preceding specification and description. For this reason, it is to be fully understood that all of the foregoing is intended to be merely illustrative and is not to be construed or interpreted as being restrictive or otherwise limiting of the present invention, excepting as it is set forth and defined in the hereto-appendant claims.

Claims

1. In a device for crucible-free zone melt processing of a semiconductor crystal rod comprising:

an operative crucible-free zone melt apparatus including two vertically opposing approximately coaxially mounted members for supporting a semiconductor rod within said apparatus, a means for generating an annular melt zone on a portion of said semiconductor rod and a means for moving said melt zone along the length of said rod; the improvement comprising

a funnel-shaped casing coupled to one of said mounting members in an axially movable manner in relation to said one mounting member, said casing being movable to an uppermost position for encompassing a cone area of the rod supported between said mounting members, and having relatively low melting pure metal inserts along the upper periphery thereof for contacting adjacent rod portions when said casing is moved to its uppermost position, said metal inserts melting upon contact with the still-hot rod as such rod is traversed by the melt zone so as to form a eutectic mixture with the material of the rod.

2. A device as defined in claim 1 wherein said metal inserts are composed of a metal selected from the group consisting of lead, indium and aluminum.

3. A device as defined in claim 1 wherein said metal inserts are composed of aluminum.