DISPOSITIVO E PROCEDIMENTO PER TIRARE DALLA MASSA FUSA ASTE DI SEMICONDUTTORI MOLTO PURI

Description

ι Wa~Ch 7806 DESCRIPTION of the industrial invention entitled: "A DEVICE FOR PULLING SEMICONDUCTOR ELEMENTS FROM THE MELTED MASS" of the German company WACKSR-CHEMTRQUIC GESELLSCHAFT-EI^KTROJJI^-GRUKDSTOFFE mbH, with headquarters in .BDRGHAUSEN - (FEDERAL REPUBLIC OF GERMANY) - and / <* f ** / * / * / o / e / o / ¢ / ^ / o / <> / · The invention relates to a device and a process for partially covering the crucible and the semiconductor melt located in the crucible, when pulling crystalline semiconductor rods by means of the crucible pulling process according to Czochralski" When pulling silicon crystals from the melt .by means of the well-known process of drawing from the crucible according to 0 za dirai ski., from the reaction of the silicon melt with the quartz crucible containing the melt, volatile silicon oxide is formed which is deposited on the colder edge of the crucible,on the silicon crystal and on the draft shaft as well as on the inner wall of the draft reactor. If such condensates or agglomerates of condensed silicon oxide fall into the melt during the crystal draft, they lead to defects in the growing silicon single crystal or even cause purely polycrystalline growth. By means of a method according to the patent application filed 2054 B*04 6, in which shielding gas is introduced through a concentric tube enclosing the draft shaft during the draft, the condensation of silicon monoxide on the crystal rod as well as on the upper draft shaft can be prevented and it can be reduced on the crucible core, but not in the draft chamber and in particular on the subsequent charging devices located in the draft chamber. Also by means of an apparatus,as described in German patent application No. 1,619,966 and which consists in a covering being placed on the semiconductor melt in such a way that condensed solid particles from the evaporated melt can fall onto the covering and not into the melt, the problem of silicon monoxide agglomerates condensing on the walls of the vessel and in particular on the recharging devices is not solved. If a draft system having such a covering is operated with a recharging device, then in the case of recharging the covering above the surface of the melt must be pushed to one side. During the remelting of the solid silicon or during the addition of liquid silicon, the silicon monoxide agglomerates adhering to the recharging device fall onto the surface of the melt and,in the continuation of the drawing process, they can lead to growth defects or even to polycrystalline growth of the silicon rod pulled from the melt", A further problem in the crucible draft lies in the reflux of carbon monoxide onto the silicon melt. Carbon monoxide is formed during crystal drafting by the reaction with silicon monoxide of carbon from the graphite parts of the heating device or from the graphite crucible surrounding the quartz crucible. Although these gases are usually constantly pumped down from the draft reactor, this alone cannot prevent some of them from reacting with the silicon melt. During crystal drafting, the carbon dissolved in the melt, due to its egro-reaction coefficient of 0.07, is then incorporated into the growing silicon rod. However, carbon leads to crystallization defects in solid silicon, such as carbon vortices in particular, which have a negative effect on the properties of structural elements produced from this material. Due to the poor heat dissipation through the rod, the draw rates typical of crucibleless zone melting cannot even be achieved when drawing from the crucible. The reason for this lies in the heat radiation from the crucible wall toward the growing silicon rod. This heat radiation, which originates less from the surface of the melt than from the crucible wall, increases during the drawing process as the melt surface decreases, thus exposing the heat-radiating crucible surface. This heat radiation and the resulting high temperatures promote additional vortex defects on the rod.The basis of the invention was therefore the attempt to attenuate or, by means of suitable measures, possibly eliminate altogether the factors mentioned which act negatively on the process of drawing from the crucible according to Caohralski.XI This problem is solved by covering the molten mass, the crucible and the space bordering the crucible laterally, during the drawing of the crystal, by means of a device consisting of a flat upper annular edge, projecting beyond the edge of the crucible, and of a projection, connected to it and extending from the internal edge downwards, with a cylindrical to conically tapered profile, whose free height corresponds to 0.2 to 1.2 times the height of the crucible, and whose lower opening corresponds to 1.2 to 8 times the cross-sectional area of ​​the semiconductor rod to be drawn. The invention will be better illustrated with reference to the single figure of the attached drawing. * The figure schematically represents the essential part of a drawing system for drawing semiconductor rods from the crucible according to Ozochralski,cor. the preferred form of execution of the device 6 according to ΙΗηνοηκίοηβ.» I).The device according to the invention comprises a vessel-like structure open at the bottom, by means of which the semiconductor melt, the crucible and the chamber to the side of the crucible can be covered during the crystal drawing and a backflow of gases emerging from the melt or forming in the furnace chamber can be effectively introduced into the melt when a suitable flow rate and pressure of the protective gas are set. The device according to the invention is suitably made of a metal or a material with a metallic surface for the reflection of infrared radiation. Suitable materials are, for example, tungsten, niobium, tantalum or, in the drawing of germanium rods, also steel or nickel. In general, however, molybdenum sheet is preferred due to its favorable properties and its relatively low availability. According to the preferred embodiment of the device of the invention, the flat upper annular edge, protruding from the edge of the crucible, has a diameter around it which is smaller than the diameter in- - ? - inside of the melting crucible, and an external diameter which is larger than the external diameter of the melting crucible, consisting of a quartz crucible and a susceptor surrounding it, and is therefore so large that it covers at least the free opening of the furnace chamber, but of course is at most so large that it still fits into the vessel, i.e. the external diameter of the said rim must not be larger than the internal diameter of the vessel. *1* The acute angle between the shell line and the shell line of the projection, which connects to the said annular rim and which extends from the internal edge downwards with a cylindrical profile to preferably tapering conically, amounts to approximately 0 to 150°, preferably 10 to 120°. *On the outside, the projection 2 is surrounded,preferably in part, by one or more cylindrical tubular sections 3, connected to the lower side of the upper annular edge 1, in which case the internal diameter of the cylindrical tubular section or sections is greater than the external diameter of the melting crucible, while by melting crucible in this respect is to be understood the combination formed by the quartz crucible 4 containing the mass- ·} > 3¾ ΐ U 3 3. called οof the transferred and by the graphite crucible 5 surrounding the quartz crucible. The height of the cylindrical tubular section 2 in the pre-execution form corresponds to approximately 0.2 to 0.8 times the free height of the projection 20. In the case of germanium rods, a cylindrical tubular section can also be omitted since in this case volatile germanium oxides cannot form and therefore do not occur. it doesn't even raise the issue of carbon monoxide,', According to the preferred embodiment, at the lower end of the projection 2 there is a flat annular projection 6, facing towards the lanterns, parallel to the upper annular edge 1, in which case the width of said projection 6 or draft diaphragm corresponds to approximately 0.05 up to 0.2.5 times the internal diameter of the melting crucible 1. The remaining structure of the furnace of the draft plant corresponds substantially to the plants commonly used today, around a graphite heating element ?, which melts silicon in pieces, arranged in a melting crucible consisting of a quartz crucible 4 and a graphite layer 5 enclosing this, and maintains the resulting molten mass S at the necessary temperature during the draft process. The melting crucible rests here on a graphite crucible shaft 9, by means of which it can be rotated or moved vertically. The furnace chamber is closed at the bottom by thermal insulation, consisting of several layers of graphite and carbon felt 10, and in which openings 11 and 12 are made for the heating element ? and for .1 crucible shaft 9, which simultaneously allow a removal of gases. On the sides, 13.the furnace chamber is delimited by a further graphite tube 14, insulated externally with carbon felt 15, on which tube 14 rests above;), on a graphite ring 16, hermetically sealed with carbon felt 3.5*. The device according to the invention is adapted, in its dimensions, to the furnace part and to the crucible of a given draft system in such a way that the crucible can be raised between the elements 2 and 3 during the draft of the rod and, at the end of the draft process, comes to rest with its upper end far away, for example". 10 - a few millimeters from the upper edge 1 of the device of the invention to allow a passage of gas through the gap delimited thereby, for which approximately 5 mm should be sufficient to avoid a gas blockage. The distance between the melt and the lower projection of the device according to the invention, i.e. the so-called draft diaphragm 6, is suitably approximately 2 to 4 mm, preferably approximately 5 to 15 mm. This depends substantially on the desired temperature distribution in the rod and in the melt as well as on the flow rate and pressure of the protective gas in the furnace chamber.and is preferably kept constant after the cone has been drawn, that is, during the drawing of the cylindrical part of the rod. Sometimes, however, it can also be advantageous to vary this distance during the drawing, in particular if it is important to compensate for variable conditions caused by the increasing length of the rod and the decreasing height of the molten mass. The tubular section 3 of the device of the invention, by means of which it is placed on the graphite ring 16 of the furnace element, is not necessary, even in the silicon draft, if the melting pot, in the highest possible position, does not protrude above the graphite ring 16. This occurs, for example, when the lateral boundary 14, i.e. the graphite tube surrounding the furnace element, is designed correspondingly higher, or when a correspondingly larger graphite ring 16 is placed on it as a higher closure. The actual procedure is outlined here as follows: after the quartz crucible 4 filled with polycrystalline silicon has been introduced into the graphite crucible 5, the cover device according to the invention is placed on the graphite ring 16, and the crucible for melting the polycrystalline silicon is moved from below towards the cover device until the silicon granulate is only a few millimetres away from the lower edge of the projection 2, at which point the draft diaphragm 6 is preferably located. During the melting of the silicon granulate, the crucible is now raised to the extent that the polycrystalline silicon sinks into the crucible as a result of melting. It is advisable to avoid rotating the crucible during melting in order to avoid the covering device being moved out of its position due to possibly protruding silicon fragments. As soon as the melting is completed, by further raising the quartz crucible 4, the surface of the silicon melt 8 is brought to the desired distance from the lower element 6 of the covering device according to the invention, i.e. to a distance of 2 to 40°, preferably about 5 to 15°. After reaching the etching temperature, the seed crystal 19, connected via the support 17 to the upper drawing shaft 18, is immersed in the melt and the drawing process is started. The etching operation with drawing from a thin conical neck does not differ in any way from the usual drawing from the crucible according to Czochralski. In contrast to the normal drawing from the crucible, the rod can however be drawn along the entire length of the rod at a constant speed by setting a suitable speed of the drawing, since the temperature conditions during the drawing of the rod only slightly build up around the solidification fronts due to the device of the invention. In order to exploit the maximum possible draft speed, however, the extraction of the rod must be slowed down somewhat towards the end, as is generally the case. While when drawing from the crucible without the covering device according to the invention, the draft speed must be reduced considerably during the drawing of the rod, because, as the surface of the melt decreases, more and more radiant heat is transmitted from the exposed crucible walls to the rod and thus the inflation is slowed down, while at the same time the power of the heater must be constantly increased because the crucible is constantly being drawn out of the heater. As the quantity of melt decreases, this is effectively prevented according to the method.el-13 invention by means of the use of the covering device which shields the rod, by reflection, from the radiant heat of the exposed walls of the jewels. During the melting of the silicon granulate and during the actual drawing process, a protective gas, for example argon or helium, is introduced into the reactor above the cover device at a pressure of approximately 0.5 to 1000 mbar, preferably between 5 and 20 mbar. This protective gas flows through the lower opening 15 of the cover or through the gap 21 formed by the silicon rod 20 and the drawing diaphragm 6, reaching the surface of the melt, in which case it carries silicon oxide and, out of the crucible, also carbon monoxide and, in accordance with the pressure gradient generated by the constant removal of the gases by pumping through the bottom plate of the reactor, is removed through the gaps 11 and 12 out of the furnace element of the reactor. draft and is removed from it through the bottom plate underneath the reactor.The inert gas flow rate here naturally depends on the geometric conditions in the draft reactor in question and on the quantity of melt, and is approximately between 100 and 1000 kJ. At 15 and 2QQG KΚΙittil' / òra, in which case it must be ensured that the set one-way flow direction is maintained and that backflow or backward diffusion of silicon monoxide and carbon monoxide into the melt or into the space above the annular gap is excluded. During the pulling of the rod, silicon can be supplied, for example, through a heated quartz glass tube 22, which is immersed in the melt 3 of aluminum, passing from the outside through the wall 2 of the vessel and the upper annular edge 1 of the device of the invention. According to the process of the invention, it is already possible to achieve drawing speeds of between 5 mm / minute and more, for example with rods with a diameter of 80 mm. By optimising the geometric conditions and the drawing parameters, even higher drawing speeds can be reliably achieved. By reducing or avoiding the condensation of silicon monoxide in the chamber above the melt, it is possible to drastically reduce the decomposition rates in the single crystal draft. This also allows the incorporation of all types of devices, for example, devices for subsequent charging of silicon granulate, devices for melting pellets or for the supply of additional liquid silicon, or for the addition of dopant, without disturbing the single crystal draft by the effect of falling particles of silicon monoxide condensate. Since the reflux of carbon monoxide into the melt is effectively suppressed according to the invention, the silicon carbon content released from the crucible can be drastically reduced. A further advantage lies in the fact that high-temperature separations, which lead to point defects, can be reduced by lowering the rod temperature due to a steeper temperature drop behind the solidification front, caused by the inventive covering. * Due to the more constant temperature drop in the rod over time and the more constant draft rate during the drafting process, a more uniform crystal quality can also be achieved over the entire length of the rod, compared to what was previously possible. » 17 - if possible* 11 With the inventive covering, the heat radiation in the cooled reactor wall above the draft chamber is significantly reduced. This allows the heating power to be reduced by approximately 30 to 40°C, which, in addition to the increased draft rates, contributes to a more economical process.The invention is suitable for the production of monocrystalline materials, in particular silicon monocrystalline materials, and polycrystalline semiconductor materials for photovoltaic solar cells, in particular silicon. Although the carbon monoxide problem or difficulties similar to silicon monoxide depositions do not arise when drawing germanium rods, the present process can also be applied advantageously here, since, due to the improved thermal shielding, it allows much higher drawing speeds and therefore increases the production per single drawing system and thus the cost-effectiveness of the drawing process. ...of. application. In a draft furnace with a furnace structure and a cover according to the invention, as shown schematically in the accompanying drawing, a quartz glass crucible 22 cm in diameter and 18 cm high is filled with 11 kg of polycrystalline silicon. A device according to the invention is then centrally mounted on the graphite cover plate of the draft furnace by the crucible. The dimensions of the device according to the invention are as follows: width of the upper edge 5 cm, outer diameter of the upper edge 32 Ω 5 cm, height of the projection 16 cm, which tapers conically downwards at an acute angle of 16° between the line of mantle and mantle line, width of the lower edge (draft diaphragm) 3 era, height of the outer tubular section 10 ctiu Material: molybdenum sheet, thickness 1 mm The crucible filled with polycrystalline silicon is brought close to the covering device from below until there is only a distance of approximately 5 mm between the polycrystalline silicon granulate and the lower edge of the covering according to the invention. f 19 - In this position, the polycrystalline silicon is melted under a pressure of 10 mbar of argon at a flow rate of 800 liters / hour. During melting, the crucible is raised several times as the polycrystalline silicon sinks. After liquefaction, the melt is brought to the etching position, i.e. the surface of the melt is approximately 10 mm below the lower element of the cover. Once the etching temperature has been reached, the seed crystal is immersed in the melt and the single crystal is generated using the usual technique and formed to a diameter of approximately 30 mm. The crystal is pulled from the cylindrical area of ​​the rod at a speed of 2.4 rpm, with the crystal rotating at 15 rpm and the crucible rotating in the opposite direction at 5 rpm. The diameter of the draft opening of the lower element of the cover, protruding towards the ground, is 120 rem. During the draft in the cylindrical area of ​​the rod, the distance

Claims

1. between 1¾ mm. ilis&'ed π detfco ©iemeiito à.nf©j?3Jor© (3.· and* the covering according to the invention is maintained at a value of approximately 10 Ma* Kel draft of the tip, i.e. - at the end of the draft of the rod eXinùi'iea by continuously reducing the diameter of the Bionocrystal up to 0 m, the crucible is lowered maintaining the draft speed of 2* 4·' mm / minute * the fixed length of the cylindrical zone of the resulting rod of monocrystalline silicon amounts to 820 - HIWlDXOÀZrOlSI - 1.Device for partially covering the crucible and the molten semiconductor mass located in the crucible during the drawing of crystalline semiconductor rods by the process of drawing from the crucible according to C addirai siti » which device consists of a small upper annular edge, as inferred from the edge of the crucible, and a projection with an integrated corner at 0 £>*->0 <> which extends > •rno towards is' 0 the bottom with profile, retracted at the side, the corresponding to 0.2 up to 1.2 · squat x:!.at the height of the crucible and whose lower opening covers 1*2 to 8 times the cross-sectional area of ​​the semiconductor rod to be pulled » 2* Device according to claim 1» characterised in that the external diameter of the flat upper annular edge, protruding from the edge of the crucible, is greater than the external diameter of the melting crucible, but at most equal to the internal diameter of the vessel, and its internal diameter is smaller than the internal diameter of the melting crucible» 3* Device according to claim 1 or 2, characterised in that the acute angle between the shell line and the shell line of the project amounts to approximately 0 to 150°, preferably 10 to 120°.

4. Device according to any of claims 1 to 3, characterised in that the projection is partly surrounded externally by a cylindrical tubular section, connected in a gas-tight manner to the lower side of the annular upper edge, in the.in which case the internal diameter of the said cylindrical tubular section is greater than the external diameter of the melting pot; 5. Device according to claim 4 or any of claims 1 to 3, characterized in that the height of the said cylindrical tubular section corresponds to 0.2 to 0.8 times the free height of the projection; 6. Device according to any of claims 1 to 5, characterized in that the lower end of the projection is made in the form of a flat annular projection, facing inwards, parallel to the upper annular edge.

7. Device according to claim 6 • , . . 1 · ·: ,V and whatever;.'3. of claims 10 to 5, characterized in that the flat annular projection,jn COj-^-uTpcoidouz& of the lower end of the ag- / ar.bo, corresponds in width to 0.05 to 0.55vo.·.be .λ..». internal cavities of the melting crucible. β» P -''recession for the draft from the crucible of...£· co on, „,> eiii:*. cox.'.cLU.ctors crystalline accordingy:vV'.-..!,s.lrji;l( ctraticriz«ftc from this the mass melted in the crucible and the space located laterally to the crucible *.o , during the draft of the crystal, are - jl° by means of a device of some kind - y;' βίT~ oc-'-lo re-fixcazì.ojo.

5. from .1 to '7* - --ro din ente sc-cónào the claim 6,c cu. .4 ul ori a ;; at ò da eie clic, after the. drawing of the cone Hi; * during the drawing of the cylindrical area of ​​the rod * the distance between the cover and the surface of the molten mass is kept unchanged * 10. Process according to claims 8 and 9, characterised in that the surface of the molten mass is maintained at a constant height by successively moving the crucible *pp WÀCKER-CHEKITRONIC GESELLSCHAFT FUR ELEKTRONIX-GRUNDSTOFFE mbH