US3342560A - Apparatus for pulling semiconductor crystals - Google Patents

Description

. ECKARDT mm. mam-mm APPARATUS FOR PULLING SEMICONDUCTOR CRYSTALS Filed on. 22, 1964 4 Sheets-Sheet '1 APPARATUS FOR PULLING SEMICONDUCTOR CRYSTALS Filed Oct. 22, 1964 4 Sheets-Sheet 2 Mm a- MW 4 5 MI 51 1 1 F? i f116 1. 1o swim "-lgij A null -||i@. all/11114 'IIIIA hIA Se t. 19, 1967 D. ECKARDT ETAL 4 Sheets-Sheet 5 Filed Oct.

W- 1957 D. ECKARDT ETAL 3,3425% APPARATUS FOR PULLING SEMICONDUCTOR CRYSTALS Filed Oct. 22, 1964- 4 Sheets-Sheet 4 United States Patent 3,342,560 APPARATUS FOR PULLING SEMICONDUCTOR CRYSTALS Dietrich Eckardt, Starnberg, and Friedrich Adolf Mentzel and Heinz Reinke, Munich, Germany, assignors to Siemens & Halske Aktiengesellschaft, Berlin and Munich, Germany, a corporation of Germany Filed Oct. 22, 1964, Ser. No. 405,892 Claims priority, application Germany, Oct. 28, 1963, S 88,661, S 88,062; July 8, 1964, S 91,935

9 Claims. (Cl. 23273) Our invention relates to the production of semiconductor crystals, preferably monocrystals, having a prescribed, for example uniformly distributed, concentration of impurity atoms.

When producing doped semiconductor crystals, particularly monocrystals, by pulling them out of a melt, it has been difiicult to obtain a uniform dopant concentration over the entire pulled length of the crystals. As a rule, the distribution coefficient of an impurity in semiconductor material differs from unity. Hence, the impurity concentration increases or decreases along a crystal rod in dependence upon the pulled length. This greatly aggravates the production of semiconductor components because the slices or pieces severed from the monocrystalline rods exhibit respectively different specific electrical resistance values.

It is an object of our invention to devise a method and apparatus which reliably afford the pulling of crystalline rods, particularly monocrystals, having an adjustable impurity concentration over a desired pulling length, especially over the entire length of the crystal rod being produced.

Another object of our invention is to afford the pulling of monocrystalline rods which exhibit a particularly high degree of crystalline perfection, particularly in the sense that the dislocation density is considerably reduced and the distribution of remaining dislocations is more uniform over the cross section and length of the rod, as compared with monocrystal rods pulled by the conventional methods.

A further object of the invention is to afford producing a large number of monocrystalline semiconductor bodies or wafers from a pulled crystal rod which, relative to one another, exhibit a lower degree of specimen differences than heretofore usually encountered, thus obtaining a better uniformity of the various semiconductor components produced and reducing the amount of rejects.

To achieve these objects, and in accordance with a feature of our invention, the molten semiconductor material from which the rod-shaped crystals are to be pulled with the aid of a crystal seed, is divided into two different size volumina contained in a relatively large vessel and in a smaller vessel respectively, the vessels being in communication with each other. The crystal is pulled out of the small volume of melt contained in the smaler vessel, and the melt in this vessel is continuously replenished by molten material from the larger vessel which serves as a storage container. An essential feature, in conjunction with the foregoing, is the fact that the volume of the melt contained in the smaller vessel, serving as a crucible, is controlled by applying a constrained relative motion between the two vessels, thus maintaining the volume of melt in the smaller vessel constant or controlling it to change in accordance with a predetermined program. The relative motion is applied by drive means located outside of 'the two vessels. Preferably the two vessels are cylindrical and the smaller crucible vessel is located coaxially Within the larger storage Vessel and kept in rotation relative to the larger vessel while being simultaneously displaced in the vertical direction relative to the level of molten material contained in the larger vessel.

According to another feature of our invention it is preferable to adjust the concentration of the impurity substance in the two vessels in accordance with the distribution coefficient of the impurity substance in the semiconductor material. This distribution coefficient is defined as the ratio of the impurity concentration. in the solid semiconductor crystal to the impurity concentration in the molten semiconductor material. The distribution coeflicient depends not only upon the properties of the material but also upon the pulling speed at which the crystal is being pulled out of the melt. This so-called effective distribution coefficient is to be taken into account when selecting the impurity concentration in the semiconductor material being used in the process. It is particularly advantageous to select the impurity concentration in the two vessels so that the ratio of the impurity concentration in the smaller vessel to that in the larger vessel corresponds to the reciprocal value of the effective distribution coefiicient.

According to one mode of performing the method of the invention, the larger vessel is supplied with a melt Whose impurity concentration is equal to the concentration of the donors or acceptors built into the resulting crystal.

In apparatus according to the invention, the two vessels are mounted preferably in concentric relation to the pulling axis. According to another feature of the invention, the relatively small crucible vessel is connected with a vertical shaft which passes through the bottom of the larger storage vessel, both vessels having essentially the shape of an upwardly open cup. The shaft is coupled with a drive for imparting rotational movement to the crucible vessel, and with another drive for displacing this vessel in the vertical direction. The surrounding larger vessel may remain immovable during operation of the apparatus. Suitable coupling means are preferably provided in order to permit separately or simultaneously rotating and vertically displacing the crucible vessel.

The movements, particularly the rotation, are preferably controlled to maintain in the smaller vessel a temperature distribution favorable to the formation of a monocrystal. This is done especially by maintaining the thermal center point substantially coincident with the geometrical center of the melt in the smaller vessel.

According to another, more specific feature of our invention, the pulling operation is controlled by continuously sensing or measuring the change in weight of the small crucible vessel resulting from the change in volume of melt contained in the crucible vessel. The resulting measuring signals are used, through suitable control or regulating devices, preferably of electrical type, to correspondingly control the pulling operation by the amount and in the sense required for adjusting or substantially preserving a predetermined volume of melt in the crucible vessel.

According to another, more specific feature of the invention, the volumetric change of the melt contained in the crucible vessel, as manifested by the resulting change in weight, is sensed by a force gauge, for example a spring scale or similar device.

The control may be effected by providing the springscale device or other force gauge with an electric control contact which acts through an amplifying control system upon the above-mentioned drive for changing the vertical position of the crucible vessel. The control contact essentially functions to define a fixed position for the upper rim of the crucible vessel above the level of the melt contained in the amount of melt in the crucible vessel. is regulated to a desired constant value. The control contact is preferably made adjustable. It then depends upon the adjustment of the control contact being kept fixed or varied the larger storage vessel, so that whether the volume of melt is regulated to remain constant or is varied in accordance with a given control program. It is particularly advantageous that the resulting control or equalization in volume can thus be effected continuously.

As mentioned, the two vessels communicate with each other for permitting molten material from the larger storage vessel to continuously replenish the material being consumed in the smaller crucible vessel by the crystalpulling operation. With the crucible vessel located in the storage vessel and at least partially submerged in molten material, the communication preferably consists of at least one bore traversing the vertical side wall of the smaller vessel. As a rule, a single capillary bore suffices in a crucible vessel of graphite, this bore being preferably tangential to the interior cup space of the crucible vessel so that the external opening of the bore leads the internal opening during the above-mentioned rotational movement of the crucible vessel.

The above-mentioned and further objects, advantages and features of our invention, said features being set forth with particularity in the claims annexed hereto, will be apparent from, and will be described in, the following with reference to examples of the method and embodiments of apparatus according to the invention illustrated on the accompanying drawings in which:

FIG. 1 shows in vertical section a crystal pulling apparatus together with a schematically illustrated system of drive means for the appertaining crucible vessel.

FIG. 2 is a vertical section of the same apparatus and shows schematically a different system of drive and control means.

FIG. 3 is a vertical section through the crucible vessel in apparatus according to FIGS. 1 and 2, the section being taken along the line III-III in FIG. 4.

FIG. 4 is a plan view corresponding to FIG. 3.

FIG. 5 shows a different crystal pulling apparatus in vertical section together with a drive and control system identical with the one according to FIG. 2.

FIG. 6 shows schematically and in section an example of drive and control means applicable for those schematically shown in FIGS. 1, 2=and 5.

The same reference characters are used in the various illustrations for corresponding components respectively.

Referring to FIG. 1, the illustrated apparatus is equipped with an axially elongated, tubular cylinder 1 of quartz whose interior constitutes a crystal pulling chamber 2 closed at both ends by cover plates 3 and 4 which are provided with respective valve-controlled inlet and outlet nipples 5 and 6 for protective gas, such as argon, nitrogen or hydrogen. Preferably used is a mixture of hydrogen with argon or nitrogen. A pull spindle 7 passes from above through a central opening in the cover plate 3 and carries a crystal seed 17. A closable tube 8, also mounted in the cover plate 3 has a laterally curved lower end and can be turned about its own axis so that dopants or other impurity substance can be selectively supplied into a cup-shaped storage vessel 9 or into a smaller vessel 10- serving as a crucible for the crystal pulling operation proper. The two vessels consist of graphite for example. The storage vessel 9 is supported on the plate 4 and remains stationary during operation of the apparatus. The crucible vessel 10 is mounted on a centrally located vertical shaft 11 by means of which the crucible vessel can be kept in rotation and also vertically displaced relative to the larger vessel 9. The shaft 11 is connected with a vertical drive 1 2. and with a rotationa drive 13 through suitable transmission means schematically shown at 14 in FIG. 1. The drive 13 alfords maintaining the crucible vessel 10 in continuous rotation. The drive 12 permits raising the crucible vessel 10 to a top position at the beginning of the crystal pulling operation and thereafter gradually lowering the vessel as the pulling operation proceeds and the molten semiconductor material becomes progressively depleted. For sealing the pulling chamber 2, respective gasket rings 15 are disposed between the quartz tube 1 and the cover plates 3 and 4. Suitable as gasket material are heat-resistant substances such as silicone rubber.

As mentioned, the melt located in the small crucible vessel 10 communicates with the molten material contained in the larger vessel 9. For this purpose, the crucible vessel 10 is provided with a capillary bore 16 (FIGS. 1, 3, 4) which extends in a substantialy tangential direction to the inner peripheral wall of the crucible vessel close to the inner bottom thereof. The direction of the bore 16 is such that the flow of semiconductor material from the larger into the smaller vessel 10 is aided by the rotation of the vessel 10. This is achieved for example by having the direction of the bore from the inner wall of the crucible vessel to the outer wall extend substantially in the direction of rotation imparted to the vessel 10. In this manner, the desired volume equalization of the melt in the crucible vessel 10 with the molten material contained in the storage vessel 9 is achieved during rotation of the crucible vessel in a particularly favorable manner. Furthermore, the continuous replenishment of semiconductor material is secured until the pulling process is terminated when the crucible vessel 10, at the end of its downward travel, becomes seated upon the bottom of the storage vessel 9.

The heating of the semiconductor material for melting it and maitaining it in molten condition may be effected in the conventional manner, such as my an electric resistance heater winding mounted on the peripheral surface of the storage vessel 9 or preferably by an electric induction heater winding which may surround either the vessel 9 within the quartz cylinder 1 or may be mounted around the quartz cylinder in coaxial relation to the vessels 9 and 10.

As a rule, the impurity concentration in the monocrystal pulled out of a melt increases near the end of the pulling process when the melt approaches the depletion minimum. Hence, a relatively large portion of the monocrystalline rod may become unsuitable for further use in the production of electronic semiconductor components. For improving the yield, the volume of the storage vessel 9 should be much larger than that of the crucible vessel 10, namely so that the volumetric ratio is larger than 5:1. For example, in the illustrated embodiments, this volumetric ratio is approximately 20:1. Since the yield increases with the volumetric ratio of the storage container 9 to the crucible vessel 10, such choice of a large volumetric ratio affords attaining more than yield of monocrystalline material having a controlled impurity content.

The foregoing description of the apparatus shown in FIG. 1 may be identical with the correspondingly denoted components in FIGS. 2 and 5, and will be further described in a later place.

The foregoing description of the apparatus shown in FIG. 1 also applies to the embodiment according to FIG. 2 which differs only with respect to the following components. A ballast weight 18 is attached to the crucible shaft 11 and is rated to cause the crucible vessel 10 to be lowered and immersed in the molten material contained in the storage vessel 9. A force gauge 19, such as a springscale device, senses or measures the force required for lifting the weighted crucible and shaft assembly to a height at which the upper rim of the crucible vessel 10 protrudes out of the level of molten material contained in the storage vessel 9 while the bore 16 remains fully submerged. The force gauge or spring-scale device 19 is provided with control means, such as an electric contact, which controls an amplifying controller 20 to actuate the vertical drive 12 in the direction and by the amount required to maintain the crucible vessel 10 thus immersed in the molten material of the storage vessel relative to the level of the molten material. As a result, the crucible vessel 10 is gradually lowered as the crystal pulling operation proceeds. This results in obtaining monocrystalliue rod continuously growing in length, until the crucible vessel becomes seated upon the bottom of the storage vessel 9. During pulling operation, the crucible vessel 10 is kept in rotation by the rotational drive 13 acting through the transmission means 14 upon the shaft 11.

Following is an example of performing the method of the invention for producing a rod-shaped monocrystal of germanium with the aid of apparatus as shown in FIG. 1.

The two vessels 9 and 10, consisting of graphite, are provided with germanium substantially free of doping substances. The germanium is heated, preferably inductively in the above-mentioned manner, to a temperature of about 950 C. until it is molten in both vessels 9 and 10. Thereafter the vertical drive 12 is operated and the shaft 11 with the crucible vessel 10 is lifted until the upper rim of vessel 10 is located above the level of the melt in storage vessel 9 a distance corresponding to the desired volume of molten material in the crucible 10. Thereafter the rotational drive 13 switched on so that during the following crystal pulling operation the crucible vessel 10 remains rotating at a speed between 10 and 100 rotations per minute. At this stage, the deisred doping substance is supplied through tube 8 to the molten germanium in the crucible vessel 10, the quantity of dopant being in accordance with the desired specific electrical resistance of the monocrystalline material.

For producing crystals of n-type germanium, antimony may thus be employed as doping substance. For producing p-type germanium, indium may be employed. Both doping substances have a small effective distribution coefficient in germanium. This has the advantage that relatively large quantities of the impurity substance can be introduced, so that any inaccuracies resulting from the weighing of the amount of impurity substance are largely prevented, or the production of a pre-alloy becomes unnecessary. The same advantage is promoted by the above-mentioned choice of a large volumetric ratio of vessels 9' and 10. The larger this ratio, the less will any inaccuracy with respect to the quantity of impurity substance have any detrimental effect upon the product.

Now the apparatus is ready for the pulling operation proper. The pulling spindle 7 is lowered until the crystal seed 17 dips into the melt contained in the crucible vessel 10. After the optimal temperature is regulated in the conventional manner, a monocrystal commences to grow from the seed. Simultaneously, the pulling spindle 7 is pulled upwardly at the growing rate of the crystal. Simultaneously, the volume of molten material in the crucible vessel 10 tends to decrease. However, the crucible vessel is lowered by the drive 12 as continuously as feasible so that the volume of the crucible melt is continuously replenished by semiconductor material flowing from the storage vessel 9 through the capillary bore 16. Thus the volume of melt in the crucible vessel 10 is kept constant or, if desired, is varied in accordance with a predetermined program. As mentioned, the supply of the replenishing material is promoted by placing the bore 16 so that it extends tangentially to the inner peripheral wall of the "crucible vessel 10 and has its direction substantially coincide with the direction of crucible rotation.

The rotation of the crucible vessel 10 also provides for good mixing of the molten semiconductor material so that the impurity substance is uniformly distributed over the entire cross section and the entire length of the crystal being pulled out of the melt. Furthermore, the rotation or the control of the rotational speed affords an advantageous control of the temperature distribution in the melt contained in the crucible vessel. This results in a considerable reduction of the dislocation density.

The following example of performing the method of the invention requires the use of apparatus as shown in FIG. 2. Forthe production of a rod-shaped monocrystal of germanium, substantially non-doped germanium is sup- .plied to the two graphite vessels 9 and 10 and is molten in both vessels by inductive heating at a temperature of about 950 C. Thereafter, the crucible vessel 10 is weighted by attaching to the shaft 11 the amount of Weight 18 required for immersing the crucible vessel 10 in the molten material contained in the storage vessel 9. To adjust a desired volume of germanium in the crucible vessel 10, this vessel is then lifted by the vertical drive 10 with a given force determined by the force gauge 19. The control system described above then operates to maintain the upper rim of crucible vessel 10 positioned at a given distance above the level of molten material in the storage vessel 9. After the crucible vessel l l) has reached its proper starting position just described, the rotational drive 13 is started and, during the following stages of the crystal pulling operation, the crucible vessel 10 is kept in rotation, preferably at a speed between 10 and r.p.m. After starting the rotation of the crucible vessel, a desired quantity of impurity substance is supplied into the crucible vessel with the aid of the tube 8 as described in the foregoing. The impurity substance may consist of antimony for producing n-type germanium crystals, or of indium for producing p-type crystals, also as already explained.

The crystal pulling proper is then commenced and continued in the above-described manner. As the crystal c0ntinues growing, the level of molten material in the crucible vessel 10 tends to become lower. The resulting change in weight, sensed by force gauge 19, acts through the correspondingly adjusted control contact, or in some other suitable manner, upon the amplifying control systom 20 which controls the vertical drive 12 to lower the crucible vessel 10 the amount required for reestablishing the desired volumetric adjustment. By adjusting the force gauge, or the above-mentioned control contact of the gauge, the, volume of melt in the crucible vessel 10 is kept constant or otherwise controlled in accordance with a predetermined program, depending upon whether it is desired to produce monocrystals having a constant specific electrical resistance throughout, or monocrystals whose specific resistance changes along the crystal length in accordance with a given program. It will be apparent that in the described manner, the change of molten volume in the crucible vessel 10 is, in eifect, utilized for a feedback control of the vertical crucible displacement.

As mentioned above, the yield of useful material from a monocrystalline rod pulled in accordance with the invention is increased by increasing the ratio of storagevessel volume to crucible-vessel volume. A further increase in yield is obtainable by greatly delaying the increase in impurity concentration toward the end of the pulling process. This can be done, for example, by employing a storage vessel whose lower portion has a considerably smaller diameter than the upper portion, the height of the lower portion being approximately equal to that of the crucible vessel, and the inner width of the lower portion of smaller diameter being slightly larger than the outer width of the crucible vessel.

Preferably, the annular clearance between the outer periphery of the crucible vessel and the inner periphery of the storage vessel in its lower portion of reduced diameter is made so small that the molten material just remains capable of flowing through the clearance and through the above-mentioned bore into the crucible vessel when the latter approaches the bottom of the storage vessel. We have found it particularly favorable to make this clearance only slightly larger than 1 mm.

To avoid cracking of the vessels during freezing of the molten material, we prefer using a crucible vessel of frusto-conical shape tapering in the downward direction, and to also give the inner wall surface of the storage vessel a downwardly and frusto-conically tapering shape adapted to that of the crucible vessel. Then, when the molten material freezes, the crucible can glide upwardly in the direction of the pulling axis. We have also found it advisable to make the wallsof the storage vessel as thin as feasible in the upper vessel portion, preferably by giving the vessel an upwardly tapering wall thickness.

A particularly favorable yield is obtained with the aid of apparatus as shown in FIG. 5, which incorporates the various improvement features mentioned in the foregoing. Generally, this apparatus corresponds to the one shown in FIGS. 1 and 2 and is equipped with a control drive system of the type shown in FIG. 2.

In the apparatus according to FIG. 5, the storage vessel 9 has a lower portion whose diameter is much smaller than that of the upper portion and also has considerably smaller axial height. The diameter of the lower portion is only slightly larger than the outer diameter of the crucible vessel 10 so that a slight clearance gap 22 remains around the outer circumferential surface of the crucible vessel 10 when the latter has reached its lowermost position shown in FIG. 5. The lateral capillary bore 16, being located slightly above the inner bottom of the crucible vessel 10, thus opens into the annular clearance gap 22, and the latter is made just large enough to permit a continuous flow of material from the upper vessel through the clearance 22 and the bore 16 into the crucible vessel 10 during the downward travel of the latter until it reaches the illustrated lowermost position. Thus the amount of melt contained in the storage vessel 9 can be almost fully consumed in the crucible vessel 10 by the crystal pulling operation, thus almost completely converting the molten material originally contained in the storage vessel 9 to monocrystalline constitution, and controlling the impurity concentration along the entire length of the resulting monocrystalline rod to remain constant or correspond to a predetermined program. Despite the fact that the much wider upper portion of the storage vessel 9 permits giving it any desired large volume in comparison with the small volume of the crucible vessel 10, the increase in impurity concentration occurs only in the terminating interval of the pulling time and at a rather steep rate of change, so that only a slight portion of the resulting rod, such as to becomes unsuitable for the subsequent production of semiconductor components.

As in the embodiments described with reference to FIGS. 1 to 4, the rotation of the crucible vessel provides for good mixing of the material contained therein and consequently secures a uniform distribution of the impurity atoms over the entire length and cross section of the crystal being pulled. The rotational motion also provides for a favorable temperature distribution of the melt contained in the crucible vessel with the result of greatly reducing the dislocation density.

Thus the monocrystalline rods produced in this manner by the method of the invention are distinguished not only by a particularly uniform distribution of the donors or acceptors in the semiconductor material and hence by a substantially constant specific resistance over the pulled length, but they also excel by a particularly high degree of crystalline perfection due to the just-mentioned favorable temperature distribution in the crucible. We have found that the density of any remaining dislocations is very greatly reduced in comparison with monocrystalline rods pulled from a melt in accordance with the conventional methods, and that the distribution of such dislocations over the cross section is also more uniform.

For example, by applying the method of the invention, we have produced monocrystalline germanium rods of 500 mm. length and about 30 mm. diameter in which the variations of the specific resistance along 90% of the pulled length amounted to only +-l0% relative to an accurately adjustable median value, and in which the dislocation density was 1000 to 3000 per cm.

Corresponding improvements are obtained with silicon or other semiconductor materials in which case the operating conditions, such as the impurity substances used as dopants, may have to be chosen accordingly. Semiconductor monocrystals and slices or wafers severed from rods made according to the invention are particularly well suitable for the production of electronic semiconductor devices such as transistors, rectifiers, thyristors and the like. By virtue of the high uniformity of the electrical and crystallographic properties, particularly the possibility of maintaining a substantially constant specific resistance over the entire pulled length and the entire cross section of the monocrystalline rods, the occurrence of specimen differences in the transistors, rectifiers and other semiconductor components made in this manner is greatly reduced.

It will be understood from the foregoing that the particular design and other details of the control and drive components 12 to 14 and 18 to 20 are not essential to the invention proper and that these components may be given any of a great variety of configurations and types well known and available in the art. The example of these components schematically shown in FIG. 6 is presented only as illustrative of one of these available possibilities, the details of this example being not essential to the invention proper with the exception of those already mentioned and described in the foregoing.

The drive and control system according to FIG. 6 may correspond to the one shown in FIG. 2 (or FIG. 5). The shaft 11 of the crucible vessel 10 carries a fixedly mounted shoulder ring 181 upon which a number of slotted weighting rings 182, 183 are stacked. These rings jointly constitute the ballast weight denoted as a whole by 18. The upper portion of shaft 11 is displaceable vertically with respect to its lower portion 11a whose vertical position is fixed. The displaceable upper portion of shaft 11 rests upon a helical spring 192 of a force gauge 19 of the spring-scale type and is non-rotatably guided in a sleeve 191 in which the spring 192 is mounted. The sleeve 191 rotatable together with the lower shaft portion 11a and is in rigid connection with the lower portion 11a of the shaft thus being prevented from moving upwardly or downwardly. The shaft 11 carries a fixedly mounted, grooved ring 193 straddled by a lever 194 which is pivoted at 195 to the stationary frame structure of the apparatus and carries an insulating pin 196. When the weight of the crucible vessel 10 with the melt contained therein is above an adjusted value, the pin 196 moves downward and closes the lower contact of a control switch 197. When the crucible weight is too low, the pin 196 moves upward and closes the upper contact of switch 197. Depending upon which contact is closed at a time, the amplifying control system 20, shown schematically in FIG. 6 as a relay-type network, energizes one or the other of the field windings 122, 123 of the vertical drive motor 12 which shifts the shaft of the crucible vessel vertically in the direction required to vary the force of spring 192 toward the value at which both contacts of switch 197 are open and the vertical drive motor 12 is stopped.

The control switch 197 can be vertically adjusted to a desired position by means of a spindle 198 and a wheel or gear 199. When the adjustment is kept constant during the pulling operation, the control system operates to regulate the process for constant volume of melt in the crucible vessel and thus for constant specific resistance throughout the useful length of the resulting mono-crystalline rod. However, when the gear 199 is operated during the pulling operation in accordance with a given program, the volume of melt in the crucible vessel and consequently the resistance properties of the resulting rod are changed accordingly.

The lower shaft portion 11a has a fixed shoulder ring 141 which is supported on a bearing mounted on a sleeve 142. The sleeve 142 is in threaded engagement with a gear 144 which is rotatable but not axially displaceable. Consequently, rotation of gear 144 causes lifting or lowering of the sleeve 142 which is prevented from rotating by having a lateral pin 143 engaged in a vertical guide of the stationary supporting structure. Rotation is imparted to the gear 144 by a pinion and a gear 146 driven from the rotor 121 of the above-mentioned motor 12.

The lower portion 11a of the shaft is in sliding engagement with a gear 131 which is rotatable but not axially displaceable and is driven through a pinion 134 from the motor of the rotational drive 13. The gear 131 has a key engaging a longitudinal slot 132 of the shaft 11. During pulling operation, the motor 13 and gear 131 continuously rotate the shaft Ila-11 as the latter is being lifted and subsequently lowered by the operation of the vertical drive.

To those skilled in the art it will be obvious upon a study of this disclosure, that our invention permits of various modifications and can be given embodiments other than particularly illustrated and described herein, without departing from the essential features of the invention and within the scope of the claims annexed hereto.

We claim:

1. Apparatus for pulling semiconductor crystals, comprising crystal pulling means defining a vertical axis, two upwardly open cup-shaped vessels of different volumetric sizes respectively for containing different amounts of molten semiconductor material, said two vessels being mounted coaxially beneath said pulling means and said smaller vessel being located inside said larger vessel, said larger vessel being stationary and said smaller vessel being rotatable, rotational drive means connected to said smaller vessel for maintaining it in rotation during crystal pulling operation, said larger vessel having a bottom recess of smaller diameter than in the upper portion of said vessel, said recess having a depth approximately equal to the height of said smaller vessel and a diameter adapted with clearance to the diameter of said smaller vessel so as to permit said smaller vessel to be lowered into said recess.

2. In crystal pulling apparatus according to claim 1, said smaller vessel having a shaft extending downwardly through the bottom of said larger vessel, said vertical displacement control means and said rotational drive means being coupled to said shaft.

3. Apparatus for pulling semiconductor crystals, comprising crystal pulling means defining a vertical axis, two upwardly open cup-shaped vessels of different volumetric sizes respectively for containing different amounts of molten semiconductor material, said two vessels being mounted coaxially beneath said pulling means and said smaller vessel being located inside said larger vessel and being vertically displaceable relative to said larger vessel, rotational drive means connected to said smaller vessel for maintaining it in rotation during crystal pulling operation, force guage means connected to said smaller vessel and responsive to the weight of the melt contained in said smaller vessel, and a vertical displacement drive connected with said smaller vessel and controlled by said force gauge means for varying the vertical portion of said smaller vessel so as to control the volume of melt contained therein during crystal pulling.

4. In crystal pulling apparatus according to claim 3, said force gauge means comprising an electric control contact, and amplifying control means connecting said control with said vertical drive for controlling the latter.

5. In crystal pulling apparatus according to claim 4, said control contact having adjusting means for varying the setting of said contact in accordance with a desired control operation.

6. In crystal pulling apparatus according to claim 1, said smaller vessel having a bore in its lateral wall, and said clearance being substantially the minimum required for replenishing the melt in said smaller vessel through said clearance and bore.

7. In crystal pulling apparatus according to claim 1, said smaller vessel having a downwardly tapering frust0- conical shape, and said larger vessel having a downwardly tapering inner shape generally adapted to that of said smaller vessel.

8. In crystal pulling apparatus according to claim 1, said larger vessel having a downwardly tapering inner space and having an upwardly tapering wall thickness.

9. In crystal pulling apparatus according to claim 1, said smaller vessel having a lateral bore extending near the vessel bottom from the interior to the outside for communication with said larger vessel, said bore being substantially tangential to the inner periphery of said smaller vessel and extending therefrom substantially in the direction of rotation of said smaller vessel.

References Cited UNITED STATES PATENTS 2,727,839 12/1955 Sparks 23-273. 2,809,136 10/ 1957 Mortimer 23-273 2,876,147 3/1959 Kniepkamp et a1 23-273 2,892,739 6/1959 Rusler 23-301 2,908,004 10/1959 Levinson 148-16 2,944,875 7/1960 Leverton 148-16 2,977,258 3/1961 Dunkle 148-16 3,002,824 10/1961 Francois 148-].6 3,033,660 5/1962 Okkerse 148-l.6 3,067,139 12/1962 Goorissen 148-16 3,194,691 7/ 1965 Dikhoff 23-301 3,198,606 8/1965 Lyons 148-16 3,241,925 3/1966 Cakenberghe 148-16 FOREIGN PATENTS 1,302,043 7/ 1962 France. 1,101,775 3/1961 Germany.

DAVID L. RECK, Primary Examiner. N. F. MARKVA, Assistant Examiner.

Claims (1)

1. APPARATUS FOR PULLING SEMICONDUCTOR CRYSTALS, COMPRISING CRYSTAL PULLING MEANS DEFINING A VERTICAL AXIS, TWO UPWARDLY OPEN CUP-SHAPED VESSELS OF DIFFERENT VOLUMETRIC SIZES RESPECTIVELY FORCONTAINING DIFFERENT AMOUNTS OF MOLTEN SEMICONDUCTOR MATERIAL, SAID TWO VESSELS BEING MOUNTED COAXIALLY BENEATH SAID PULLING MEANS AND SAID SMALLER VESSEL BEING LOCATED INSIDE SAID LARGER VESSEL, SAID LARGER VESSEL BEING STATIONARY AND SAID SMALLER VESSEL BEING ROTATABLE, ROTATIONAL DRIVE MEANS CONNECTED TO SAID SMALLER VESSEL FOR MAINTAINING IT IN ROTATION DURING CRYSTAL PULLING OPERATION, SAID LARGER VESSEL HAVING A BOTTOM RECESS OF SMALLER DIAMETER THAN IN THE UPPER PORTION OF SAID VESSEL, SAID RECESS HAVINGA DEPTH APPROXIMATELY EQUAL TO THE HEIGHT OF SAID SMALLER VESSEL AND A DIAMETER ADAPTED WITH CLEARANCE TO THE DIAMETER OF SAID SMALLER VESSEL SO AS TO PERMIT SAID SMALLER VESSEL TO BE LOWERED INTO SAID RECESS.

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