DE2140070A1 - Device for pulling crystals - Google Patents

Description

DR. BERG DIPL.-ING. STAPF 214QQ7Q

PATENT LAWYERS

8 MUNICH 8O, MAUERK1RCHERSTR.

Dr. Berg Dipl.-Ing. Stapf, 8 Munich 80, Mauerkircherstraße 45

Your reference your letter Our reference date

21 448 August 10, 1971 i

Attorney file 21 448

MONSANTO COMPANY St. Louis ₉ Missouri / USA

Device for pulling crystals

The invention relates to apparatus for pulling crystals, and more particularly relates to one Crucibles for use in pulling semiconductor crystals. In particular, the invention is directed

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a new type of crucible greening for the recording of molten semiconductor material, in which improved thermal insulation is provided.

Refractory crucibles are part of conventional devices for pulling semiconductor crystals well known (see Fig. 1 of the drawing). These crucibles are used to hold a batch molten semiconductor material, from which semiconductor rods are drawn, and can be found, for example, in the well-known crystal pulling devices according to Czochralski application. These crucibles are common made of a suitable refractory material such as graphite, in order to protect the during drawing of the Crystals by either resistance or high frequency induction heating produced very high levels of crystal wax turns to withstand temperatures. Because graphite with certain molten semiconductor materials, for example Silicon, which reacts in a disadvantageous way, is the cavity of the crucible mostly with a quartz lining provided with which alone dae melted Semiconductor material comes into contact when pulling crystals.

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While pulling crystals, the temperature should of the crucible are kept at a predetermined high value, so that a sufficiently large amount of heat. from the crucible via the quartz lining through thermal conduction can be transferred to the semiconductor material contained therein. For the proper execution of the pull the temperature of the molten semiconductor material should be substantially uniform throughout the batch and kept just above the melting point of the material. Because of this, losses should be due Thermal conduction during crystal pulling from the crucible and the melt of the semiconductor material contained therein be kept away as low as possible. By reducing the flow of heat from the interior of the crucible and the The melt of the semiconductor material contained therein can be used to reduce the operating behavior when pulling crystals and reduce the occurrence of undesirable temperature gradients in the crucible.

To reduce the heat losses of the crucible of a device for pulling semiconductor crystals by conduction, a known arrangement has an elongated, thin holder, by means of which the Crucible is held at a predetermined location in a crystal growth chamber (see the bracket 28 in Fi, g, 1). The bracket is usually made of one

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Refractory material made in order to be able to withstand the high temperatures prevailing in the crystal growth chamber. Such a cantilevered holder of the crucible limits the heat losses of the crucible based on heat conduction to the amount of heat dissipated by the holder. The heat dissipation from the crucible via the bracket depends on the
Size of the cross-sectional area and the thermal conductivity of the holder and the location of the attachment point of the holder on the bottom of the crucible.

The above-described conventional arrangement of the
Although the crucible largely fulfills the intended purpose, the geometry and the spatial arrangement of the holder still result in considerable, disruptive losses due to heat conduction away from the crucible. By attaching the long, thin holder in full contact with the geometric center of the bottom of the crucible, there is a considerable heat dissipation from the molten semiconductor charge inside the crucible via the quartz lining and the bottom of the crucible to the cylindrical holder. As can be seen from Fig. 1, the cylindrical holder 28 is fixed in the center of the underside of the crucible. When attached at this point, the bracket receives more heat from inside the crucible

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fed, and discharged, than with any eccentric arrangement on the underside of the crucible. On the other hand, however, it is for the sake of secure attachment and balance of the crucible more expedient when using such a long, thin holder this in or near the geometric To be arranged in the center of the crucible bottom.

In the event of an excessive flow of heat from the crucible to its holder, the bottom of the crucible is supercooled, as a result of which an undesirable temperature gradient is established between the heated side walls of the crucible and the bottom closing off the interior space. The undercooling of the crucible bottom often causes the temperature of the semiconductor melt contained in the crucible to drop to a value at which dendrites form on the semiconductor crystal pulled from the melt. To avoid the formation of dendrites, the semiconductor crystal must be drawn out of the melt very quickly. With such rapid, sudden Draw River, however, remain on average 10% of the semiconductor melt in the crucible, resulting in a waste of material and deterioration of the V / irkungsgrades when pulling the crystals results in "high serious, however, the occurring therein during rapid pulling of the crystal from the melt, heat shock _s through which the desired

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Crystal structure is disturbed in certain areas. This results in further losses of material from the batch used for drawing. Using the methods described above and in Pig. I also shown the known crucible arrangement for pulling silicon crystals mixed with antimony, that the antimony exceeds its solid solubility in silicon after pulling about 55% of the charge due to the heat conduction resulting from the melt. At this point the antimony begins to precipitate, so that the remaining 45 % of the charge can no longer be used for pulling crystals.

The invention essentially solves the problem of providing a new and improved crucible assembly which has all of the advantages of the crucible assemblies for corresponding purposes without the aforementioned Disadvantages of known arrangements occur there. For this purpose is between the crucible and one A new type of crucible holder is used in the elongated, thin crucible holder. This very weakly thermally conductive Crucible mount is arranged in the Ilochfrequenzfeld heating the crucible, so that its inductive heating Reduced heat loss of the crucible due to thermal conduction. Since the crucible holder encloses the lower part of the crucible, it also has the effect of reducing the heat loss of the crucible due to radiation. In this way, the

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invention of a novel and improved crucible arrangement for a device for pulling semiconductor crystals. The crucible assembly has improved heat transfer properties, works with a high degree of efficiency and promotes the maintenance of the desired high temperature of a melting crucible • and the molten semiconductor charge contained therein.

Lin essential advantage when using the invention Crucible arrangement when pulling crystals consists in avoiding the formation of dendrites on the pulled semiconductor crystal.

The device according to the invention offers the further advantage that when they are used, the semiconductor melt in the v; e s ent borrowed are completely drawn out of the crucible so that there is no waste of material due to material remaining in the crucible. When using the crucible assembly according to the invention for pulling Semiconductor crystals can prevent viarmes shocks and the resulting structural damage Avoid the crystal in an advantageous manner.

Furthermore, the arrangement according to the invention offers the advantage that the time of precipitation can thus be delayed,

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when silicon crystals that are heavily mixed with other substances are to be pulled.

Essentially, the invention provides a crucible arrangement for an apparatus for pulling semiconductor crystals with a melting crucible, which has a cavity for receiving a molten semiconductor material to be drawn in having. In a certain area there is a fit on the lower part of the crucible, with one below it of the crucible extending around an opening wall. The crucible mount limits the heat dissipation from the crucible to a minimum and also restricts heat radiation from the interior of the crucible.

According to one feature of the invention, several elongated openings are formed in a wall of the crucible mount. The openings cause a considerable reduction in the size of the heat conduction path and in the socket thus the heat loss of the crucible due to thermal conduction.

According to a further feature of the invention, both the crucible and its fit are within the effective range an associated cooking frequency induction heating coil arranged. The simultaneous induction heating of the two Parts causes a further reduction in the heat loss of the crucible through heat conduction.

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Further features, details and advantages of the invention result from the following description of an exemplary embodiment based on the drawing. It shows:

Fig. 1 shows a partially isometric and partially in section shown embodiment of a Device for pulling crystals,

FIG. 2 shows a disassembled and exploded view Oblique view of a preferred embodiment the crucible arrangement according to the invention and

FIG. 3 is a sectional view of the parts of FIG assembled crucible arrangement according to the line 3-3 in FIG. 2.

The known device for pulling crystals, shown in FIG. 1 and denoted as a whole by 7, has a crystal growth chamber 8 in which the crucible 10 made of graphite is arranged of semiconductor crystal pulling is often referred to as "core crucible". Accordingly, the crucible 2 and 3 can in the following with reference to Figures, the illustrated embodiment Lrflndung and are referred to in the known embodiment according to B ¹ I ^ ₉ 1 as the "core crucible". The term "crucible assembly" includes

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In the meaning used here, among other things, the actual crucible, all parts holding the crucible and the associated induction heating devices for the crucible. The cavity of the araphite Tiegola la can be provided with an exchangeable quartz lining 12 which does not chemically react with a molten silicon semiconductor charge 1'4 which is pulled out of the cavity at relatively high temperatures.

The monocrystalline semiconductor rod pulled from the polycrystalline semiconductor charge 14 forms an attachment 16 when pulled from the charge 14 by adhering to a nonocrystalline crystal seed 17. A pigeon-hole frequency induction or "work" coil 22 for heating the semiconductor charge Ik is expediently arranged in a coolant chamber 26 containing a suitable coolant 24, which in turn runs essentially concentrically to the crystal growth chamber 0.

The coolant chamber 26 and the crystal ^T .7achstumskammer 3 are arranged on a base 30 over which a ο the crucible 10 at a predetermined height within the growth chamber and protrudes with respect to the associated induction heating coil supporting holder 28 22nd

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When the coil 22 is affected by a high-frequency current, eddy currents arise in the graphite crucible 10, under the effect of which the molten charge 1H is heated and kept at an elevated temperature.

The elongated, cylindrical holder 28 engages in the central region 32 of the bottom of the crucible 10. the Sum of the lengths of all heat-Leltwege between the Area 32 and all parts of the molten semiconductor charge l'l is smaller than the sums of the corresponding Routes to any other locations on the bottom of the crucible 10. This geometric design of the known Crucible arrangement according to FIG. 1 causes considerable heat loss of the crucible 10 by heat dissipation along it the holder 28. This undesirable heat dissipation from the crucible 10 causes the temperature of the molten to drop Semiconductor charge 14 and thus exerts the adverse effects described below on the Pulling the crystals out.

For a better understanding of the remarkable advantages achieved by the invention over the prior art the high-frequency induction heating of the crucible 10 in FIG. 1 must first be considered. The through the Induction heating generated coil 25 comes essentially

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completely on the cylindrical outer wall 29 of the crucible 10 to the effect. Their heating is based on induced eddy currents which are essentially limited to the vertical outer regions of the crucible 10. The heating of the surface is transferred to other areas of the crucible 10 by conduction. The vibrating area 32 of the bottom of the crucible 10 with the bore provided therein is almost completely excluded from any induction heating of the crucible 10. Therefore - initially neglecting the heat dissipation through the holder 28 - there is a temperature gradient between the outer wall 29 of the crucible 10 and its bottom 31, which is based on the fact that the induction heating of the crucible 10 is essentially effective in the surface area of the vertical wall 29. Even without the holder 28 shown in FIG. 1, the temperature of the bottom 31 of the crucible 10 would therefore be lower than. that of the vertical side wall 29. This temperature difference is also communicated to the melt Ik from which the semiconductor rod is drawn. However, since the temperature of the melt 14 should be kept slightly above the melting point of the semiconductor material to be drawn, there is an upper limit for the heating of the melt Ik. Subcooling of the bottom 31 cannot therefore be compensated for simply by increasing the energy supply to the coil 25, since this would overheat the melt Ik in the upper area.

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Now, however, there is still the thermally conductive contact between the holder 23 and the recess 32 in the bottom 31 of the Crucible 10 to consider. Because of its thermal conductivity, the graphite holder is reinforced the temperature gradient existing between the vertical side wall 29 of the crucible 10 and the bottom 31, this temperature gradient also being transferred to the semiconductor melt 14. When the melt is removed as the crystal grows, its level continues to sink into the hemispherical bottom of the lining 12, as a result of which the melt 14 continues to cool down in the lower area. Because of this appearance and the heat loss of the crucible 10 by dissipation via the holder 28 occurs at the lower region of the Melt 14 in FIG. 1 in the final phase of crystal growth hypothermia and, as a result, undesirable crystal nucleation. The bracket 28 thus causes a relatively large temperature gradient between the upper and lower areas of the melt 14, which favors the formation of dendrites in the final phase of the crystal pulling process. Through education Drawbacks and waste of material caused by dendrites have already been discussed above.

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In the preferred embodiment of the invention shown in FIGS. 2 and 3, a refractory crucible 3 ^ made of graphite has a cavity 36 delimited by a cylindrical sixth wall 39, which merges into a hemispherical bottom 43 in the lower part. The cylindrical Außenv / anclung Jo ces crucible 3 ^ can in the same way as with the arrangement nacii Flg. be inductively heated by means of a high-frequency induction coil 45. An inwardly directed shoulder 40 is formed along the lower edge of the crucible ~ $ k , by means of which the crucible can be inserted into a holder 42 described below with a frictional connection.

The crucible holder 42 has a cylindrical wall 44 which delimits a recess 46 for the fitting insertion of the shoulder 40 of the crucible J> k . Several elongated openings or breakthroughs 50, 52 are formed in the fit, and very narrow heat conductor bridges 5 ^ 56,58 extend between polyhedra. The V / arm conductor bridges have small cross-sectional areas compared to the length of the openings 50, 52. The effluent in the cylindrical wall 44 from the crucible 34 is limited in its heat flux to the narrow bridges "The crucible replaced pk has a flat bottom OQ, which integrally

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is connected to the cylindrical wall 44. the cylindrical Jandung 4 4 and the bottom 60 of the crucible mount 42 form a shield against heat losses of the crucible 34 through radiation.

An elongated, hollow, cylindrical holder 62 is fitted snugly into an opening 37 in the flat bottom 60 of the crucible holder. The base 60 of the holder 42 rests on a step 66 on the cylindrical outer wall 64 of the holder 62.

Heat losses occur in the arrangement according to the invention of the crucible 34 by conduction only via a conduction path instead, which in each case by a lateral segment of the electrode part of the crucible 34 and the cylindrical Wall 44 of the crucible holder 42 is formed. The heat I flowing away through the wall 44 of the fitting 42 must thereby one of the between the elongated openings 50, formed bridges 54, 56 or 58 pass. The heat flow from the crucible 34 is thus before the transition to the cylindrical upper part 68 of the holder 62 by the novel Formation of the cylindrical wall 44 braked.

The crucible mount 42 and the reduction it brings about of the temperature gradient between the vertical

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Sidewall 38 and the bottom 40 of the crucible 34 represents an important feature of the invention. By this arrangement, the bottom 40 of the crucible 34 in the middle area no heat withdrawn by dissipation directly via a bracket, so that this area, .like explained below, is heated to substantially the same temperature as the vertical side wall of the crucible. The heat loss based on heat conduction now runs from all areas of the crucible 34 to the vertical Wall 44 of the crucible holder 42, in the lower region of the crucible that is in the horizontal direction, which leads to Consequence has that the total heat loss of the crucible 34 in relatively uniform distribution occurs in all parts of the same. In known arrangements between the upper and lower regions of the semiconductor melt 47 occurring As a result, the temperature gradient is considerably reduced or even eliminated. In addition, since the cylindrical Inside wall of the wall 44 of the crucible holder 42 the heat radiated thereon from the lower part of the crucible again reflects back, a decrease between the bottom 40 and the vertical side wall 38 of the crucible 34 possibly still existing temperature gradient still present * From this novel feature of the crucible mount 42

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the result is a relatively uniform temperature distribution in all parts of the crucible 34 and those contained therein Semiconductor melt.

The graphite-made cylindrical holder 62 has a smaller diameter than the crucible holder 42, and the cross-sectional area of the wall 64 of the holder 62 is considerably smaller than the total cross-sectional area of the upper part of the cylindrical wall 44 of the holder 42. The holder 62 is coaxial with the Crucible 34 and the crucible holder 42 arranged and has sufficient strength to hold these two parts in a position shown in FIG. 3 with respect to the high-frequency induction heating coil 45. The relatively thin cylindrical wall 6% of the holder 62 results in a further reduction in the dissipation of heat from the mount 42 via the holder 62.

The length of the heat conduction path from the crucible 34 over the Socket 42 and the cylindrical wall 64 of the holder 62 is considerably larger than that of the i / ärmeleitungsweges vom Central region 32 of the crucible bottom over the holder 28 to the base 30 in the known arrangement according to FIG. 1. The elongated openings 50, 52 cause a further

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Reduction of heat dissipation, ao dajo die-combination these two novel characteristics of the Lrfinaung one compared to the known crucible structure according to FIG. 1 considerably allows more advantageous melting crucible arrangement. With this arrangement, the high frequency field generated by the high frequency current supplied to the coil 45 has an effect in addition to the actual crucible 34 also the fit 42, as a result of which heat losses of the crucible 34 further reduce by derivation via the socket 42.

Another important feature of the invention lies in the formation of the bottom portion of the crucible mount 42, through which its use with existing crucibles and brackets 62 is facilitated. lüan look at the Example of the known crucible arrangement according to FIG. 1, in which a holder 28 is inserted directly into a cylindrical Recess 32 is inserted in the underside of the crucible 10. For the application of the training according to the invention it is therefore not necessary to produce a special crucible and a special holder 62, but one can use existing crucibles and brackets in the manner shown in FIG. The preferred execution the invention is thus simple and economical Structure and nevertheless has the improved heat-insulating properties described above.

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The crucible 34, the fitting 42 and the holder 62 are preferably made of a refractory or heat-resistant material such as graphite, which can withstand the typical high temperatures in the region of 1600 ° C. when pulling crystals. Otherwise, instead of graphite, a heat-resistant metal such as molybdenum, tantalum or tungsten or heat-resistant ceramic materials such as alumina (Al ₂ O ^) can be used without departing from the scope of the invention.

Furthermore, the X ¹ UUmIiClIe shape of the crucible arrangement described can be modified within the scope of the invention. Thus, the individual parts of the arrangement shown in FIGS. 2 and 3 can have a different cylindrical shape than the one shown, with openings or breakthroughs that correspond in terms of their effect to the design described above. The elongated openings 50, 52 are not restricted to the number, size and shape shown in FIG.

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The "crucible arrangement according to the invention is not limited to use with high frequency induction heating, but can also be used with resistance heating Find use, where the well-known carbon heating resistors used x ^ ground. These heating resistors are usually arranged near the vertical wall of the crucible and heat it through Radiation. It can be seen from this that the invention can be used extremely advantageously in resistance heating, since the heating resistors tend to have a temperature gradient corresponding to that explained above with reference to FIG to evoke.

The application of the invention is not limited to the crystal pulling of silicon, but is also suitable for pulling individual crystals from germanium as well as from certain materials suitable for lasers such as garnet and ruby. When pulling individual germanium crystals is a quartz liner 12 of the Pig crucible. I not necessary, since the graphite of the crucible 3 ^ is not undesirable Way reacts with the molten germanium. For this reason, the quartz lining 12 shown in FIG. 1 is the Crucible in Figs. 2 and 3 not shown. However, if the arrangement according to FIGS. 2 and 3 is used for pulling mono-

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If crystalline silicon is used, the crucible 3 ² J can of course also have a quartz lining.

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Claims (6)

2U0070 claims: 1. Apparatus for pulling crystals, marked through a crystal growth chamber (8), through a heating device (25 or 45) for heating a part the growth chamber, through an initially the heating device Crucible assembly located in the growth chamber (10 or 34) and a bracket (28 or /. C2) for the crucible to reduce heat loss through heat conduction. 2. Apparatus according to claim 1, characterized in that the holder of the crucible (34) a along the crucible its circumference enclosing fit (42) and a socket adjustable with respect to the heating device holding holder (62) and that the socket limits the heat loss of the crucible by radiation. 3. Device according to claim 1 or 2, characterized in that that the fitting (42) has walls (44) in contact with the crucible (34), in multiples of a number mutually spaced openings (50, 52) are formed so that the thermal conductivity of the Fit is restricted. ORIGINAL BATHROOM - 23 - 209808/1732 2H0070 4. Apparatus according to claim 1, 2 or 3a, characterized in that that the socket (42) has a bottom (60) with a relative to the wall (44) arranged in the middle has opening therein (37) and that in the opening one for holding the crucible (34) and its Fit (42) in a specific position with respect to an associated induction heating coil (45) serving holder (62) can be used. 5. Device according to one of claims 1 to 3, characterized in that formed in one in the crucible holder (42) Opening (37) an elongated cylindrical holder (62) can be used, which is from a passage is penetrated by a considerably smaller diameter than a passage passing through the fitting (42), wherein the crucible can be held in a certain position by means of the holder and heat losses from the crucible and the Ball socket due to the length of the holder and the relatively small cross-sectional area of the walls (64) of the Bracket are limited to a minimum. 6. Device according to one of claims 1 to 3 »characterized in that the _/ E'nahm (42) to the walls (44) extending and an opening (37) for BAD ORiGfNAL - 24 209808/1732 the receptacle of the holder (62) having the bottom (60) and that the crucible (3 ¹ O _9, the crucible holder (42) and the holder (62) are made of a heat-resistant material which can withstand the high temperatures occurring when pulling semiconductor crystals able. BATH ORIGINAL 209808/1732