Monocrystal growing apparatus

Abstract: The apparatus has: a chamber-defining housing (11); upper vertical shaft (11a) supporting either of a polycrystal rod (12a) and seed crystal (12b); lower vertical shaft (11b) supporting the other; high-frequency coil (13) melting the polycrystal rod (12a) to produce a melt between the poly­ crystal rod and (12a) seed crystal (12b); device introducing a doping gas into the housing (11); separator (14) provided in the housing (11) and separating the whole part of the poly­ crystal rod (12a) other than the melt from the atmosphere of the doping gas, the separator (14) being open to the chamber near the melt; and device passing an inert gas into the separa­ tor (14) so as to prevent the doping gas from invading the chamber. The apparatus produces a monocrystal rod of a resis­ tivity constant along the axis of the monocrystal rod.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a monocrystal growing apparatus and more particularly to a monocrystal growing apparatus building up a monocrystal rod by floating zone method. 2. Description of the Related Art In accordance with floating zone method, an upper vertical shaft supports a polycrystal rod, a lower vertical shaft supports a seed crystal and a high-frequency coil connects both contacting tips of the polycrystal rod and the seed crystal by melting them. Then, the polycrystal rod and the seed crystal are rotated while the upper and lower vertical shafts are together descended, and a melting zone (i.e., melt) produced by the high-frequency coil is progressively transferred toward the polycrystal rod adjacent and above the melt, and thereby a monocrystal is grown on the seed crystal. FIG.4 illustrates this prior-art monocrystal growing apparatus. A chamber-defining housing is indicated at 1. The housing 1 has an upper vertical shaft 1a and a lower vertical shaft 1b which coaxially extend. The upper vertical shaft 1a coaxially supports a polycrystal rod 2a and the lower vertical shaft 1b coaxially supports a seed crystal 2b so that the polycrystal rod 2a and the seed crystal 2b are coaxially aligned. The housing 1 contains a dished single-turn high-frequency coil 3 placed essentially at the center of the chamber of the housing 1. The respective upper and lower vertical shafts 1a and 1b of the prior-art monocrystal growing apparatus rotate the polycrystal rod 2a and the seed crystal 2b at equal or different rotational speeds and concurrently descend at an equal speed to progressively axially transfer the melt toward the polycrystal rod 2a adjacent and above the melt so as to build up a monocrystal rod 2c on the seed crystal 2b. The housing 1 of the monocrystal growing apparatus receives a doping gas (e.g., B₂H₆ when the monocrystal rod 2c is p-type and on the other hand, PH₃ when the monocrystal rod 2c is n-type) introduced through the bottom end of the housing 1 in order to build up the p-or n-type monocrystal rod 2c of a predetermined resistivity. The monocrystal growing apparatus discharges a waste doping gas through the top of the housing 1. The prior-art monocrystal growing apparatus has the following drawbacks in producing the p-or n-type monocrystal rod 2c: That is, since the polycrystal rod 2a and monocrystal rod 2c (hereinafter, the two rods are commonly referred to as mere a semiconductor rod or semiconductor rods) are exposed to the atmosphere of the doping gas introduced into the housing 1, the impurity of the doping gas is not only dissolved directly into the melt but also decomposed to deposit on the high-temperature cylindrical surfaces of the semiconductor rods 2a adjacent the melt, so that the impurity is mixed and diffuses into the semiconductor rods in the course of melting of the semiconductor rods in a monocrystal growth step. Thus, the resistivity of the monocrystal rod 2c must depend on not only the amount of the impurity taken directly into the melt but also the amount of the impurity deposited on a part of the polycrystal rod 2a adjacent the melt and then directly or with a precedent diffusion dissolved in the melt, so that the resistivity of the monocrystal rod 2c is difficult to control. That is, since the polycrystal rod 2a is progressively melted from the bottom towards the top, the polycrystal rod 2a receives the doping impurity at a progressively greater rate as the polycrystal rod 2a descends to melt because an upper part of the polycrystal rod 2a is longer exposed to the doping gas over the surface than a lower part of the same. The prior-art monocrystal growing apparatus entails a problem in that the product resistivity of the monocrystal rod 2c progressively decreases towards the tail of the monocrystal rod 2c as seen in FIG.5. The present invention was made in view of the above problem. SUMMARY OF THE INVENTION An object of the present invention is to provide a monocrystal growing apparatus producing a monocrystal rod with the resistivity essentially constant along the axis of the monocrystal rod. In order to achieve the object, the monocrystal growing apparatus of the present invention comprises: a chamber-defining housing; an upper vertical rod extending into the chamber-defining housing, the upper vertical shaft supporting either of a polycrystal rod and a seed crystal; a lower vertical shaft extending into the chamber-defining housing coaxially with the upper vertical shaft, the lower vertical shaft supporting the other of the polycrystal rod and the seed crystal; a high-frequency coil provided within the chamber-defining housing and melting a part of the polycrystal rod to produce a melting zone between the polycrystal rod and the seed crystal, the polycrystal rod and the seed crystal being rotated while the melting zone is progressively transferred toward the polycrystal rod provided adjacent and above the melting zone; means for introducing a doping gas into the chamber-defining housing; means, provided in the chamber-defining housing, for separating the whole part of the residual polycrystal rod other than the melting zone from the atmosphere including the doping gas, the separation means being open to the chamber near the melting zone; and means for passing an inert gas into the separation means so as to prevent the doping gas from invading the interior of the separation means, the monocrystal growing apparatus building up an impurity doped monocrystal. In accordance with the present invention, the separation means and the inert gas passing means together protect the whole part of the polycrystal rod other than melting zone from taking in the doping impurity of the doping gas during monocrystal growing-up. The resistivity of a grown monocrystal rod essentially depends on only the rate of the doping impurity taken into the monocrystal rod directly through the melting zone. Thus, the present invention achieves a high-accurate resistivity control of the monocrystal rod and provides a monocrystal rod of a resistivity uniform across the full length of the monocrystal rod. BRIEF DESCRIPTION OF THE DRAWINGS FIG.1 is a longitudinal section through a monocrystal growing apparatus of a first embodiment of the present invention; FIG.2 is a graph representing the resistivity of and in the axis of a monocrystal rod built by the monocrystal growing apparatus of FIG. 1; FIG.3 is a longitudinal section through a monocrystal growing apparatus of a second embodiment of the present invention; FIG.4 is a longitudinal section through a prior-art monocrystal growing apparatus; and FIG.5 is a graph representing the resistivity of and in the axis of a monocrystal rod built by the prior-art monocrystal growing apparatus of FIG. 4. DESCRIPTION OF THE PREFERRED EMBODIMENTS The preferred embodiments of the present invention will be described with reference to FIGS. 1-3 and 5 hereinafter. FIG.1 illustrates a monocrystal growing apparatus of a first embodiment of the present invention. As shown in FIG.1, the monocrystal growing apparatus comprises a chamber-defining housing 11, an upper vertical shaft 11a extending into the housing 11, a lower vertical shaft 11b extending into the housing 11 coaxially with the upper vertical shaft 11a, and a dished single-turn high-frequency coil 13 placed essentially at the center of the chamber. Both the upper and lower vertical shafts 11a and 11b are rotatable and vertically movable. The upper vertical shaft 11a coaxially supports a polycrystal rod 12a so that the top of the polycrystal rod 12a is fastened to the bottom of the upper vertical shaft 11a. The lower vertical shaft 11b coaxially supports a rod-shaped seed crystal 12b so that the bottom of the seed crystal 12b is fastened to the top of the lower vertical shaft 11b. The housing 11 has a hood 14 fastened to the top wall thereof directly above the high-frequency coil 13 and covering or largely separating the whole part of the polycrystal rod 12a provided adjacent a melt in the polycrystal rod 12a from the atmosphere of a doping gas filling the chamber of the housing 11 and introduced through a bottom center projection of the housing 11. The bottom of the hood 14 is open to the chamber near the melt. The inner wall of the hood 14 is tapered to the narrower open bottom of the hood 14 depending towards the high-frequency coil 13. Inert gas, e.g., argon gas is passed into the hood 14 through a top center projection of the housing 1. The inert gas passes down through the interior of the hood 14 and a clearance between the polycrystal rod 12a and the wall edge of the open bottom of the hood 14 into the atmosphere of the doping gas. The monocrystal growing apparatus of the first embodiment with the above arrangement builds up a monocrystal rod 12c. The hood 14 and the inert gas passing down through the interior of the hood 14 out of the bottom of the hood 14 cowork essentially to separate the part of the polycrystal rod 12a other than, i.e., adjacent the melt in the polycrystal rod 12a from the atmosphere of the doping gas, so that the doping gas cannot invade to the part adjacent the melt of the polycrystal rod 12a. That is, the part adjacent the melt of the polycrystal rod 12a cannot catch the impurity of the doping gas. Thus, the resistivity of the monocrystal rod 12c essentially depends on only the rate of the impurity taken directly into the melt. As shown in FIG.2, the monocrystal growing apparatus of the first embodiment provides the monocrystal rod 12c of a resistivity constant across the axis of the monocrystal rod 12c. FIG.3 illustrates a monocrystal growing apparatus of a second embodiment of the present invention. The second embodiment differs from the first embodiment in that a hood 14 of the second embodiment comprises a hollow cylindrical bottom portion 14a surrounding and coaxially with the polycrystal rod 12a with a predetermined narrow spacing between the inner wall of the hood bottom portion 14a and the polycrystal rod 12a. Since the portions of the monocrystal growing apparatus other than the hood bottom portion 14a have the same arrangements as those of the apparatus of the first embodiment, these portions have the same labels and the descriptions of these portions will not be repeated. The second embodiment has essentially the same advantages as the first embodiment. In accordance with the second embodiment, the narrow spacing extending between the hood bottom portion 14a and the polycrystal rod 12a reinforces the sealing operation of the inert gas so that the doping gas will not invade the interior of the hood 14. Thus, the second embodiment better facilitates the resistivity control in building up the monocrystal rod 12c. In other embodiments of the present invention not illustrated in FIGS., a monocrystal growing apparatus may comprise essentially the same hood covering or largely separating a seed crystal 12b side adjacent the melt from the atmosphere of the doping gas. In addition, alternatively an upper vertical shaft 1 may support a seed crystal and a lower vertical shaft may support a polycrystal rod.

Claims: Apparatus suitable for growing an impurity-doped monocrystal, comprising a housing (11) defining a chamber; vertical shafts (11a,11b) extending respectively into the upper and lower ends of the chamber, one shaft being adapted to support a polycrystal line rod (12a) and the other a seed crystal (12b); a high-frequency coil (13) adapted to melt a part of the rod and produce a melting zone between the rod and the crystal; and means for relatively rotating the rod and the crystal while the melting zone progresses toward the rod; and an inlet for a doping gas into the chamber, characterised by means (14) for separating the rod, other than the melting zone, from the doping gas, wherein the separation means is open to the chamber near the melting zone, and by means for passing an inert gas into the separation means so as to prevent the doping gas from invading the interior of the separation means. Apparatus as claimed in claim 1, wherein the separation means comprises a hood (14). Apparatus as claimed in claim 2, wherein the hood includes a hollow cylindrical portion surrounding a part of the rod adjacent the melting zone. Apparatus as claimed in claim 2 or claim 3, wherein the interior surface of the hood tapers towards its bottom. Apparatus as claimed in any preceding claim, which further comprises second means for separating a seed crystal side adjacent the melting zone from the doping gas, wherein the separation means is open to the chamber near the melting zone, and second means for introducing an inert gas into the second separation means. A method for monocrystal growth, which comprises operating apparatus as claimed in any preceding claim, in which the rod and crystal are present, with a melting zone therebetween. A method according to claim 6, wherein the rod is above the melting zone.

Inventors: MACHIDA EIICHI, WATANABE MASATAKA, MACHIDA, EIICHI, WATANABE, MASATAKA
Applicants: SHINETSU HANDOTAI KK, SHIN-ETSU HANDOTAI COMPANY LIMITED
Application year: 1990
Main classification: C30B13
IPC codes: C30B13, H01L21