Pulling a semiconductor single crystal according to the Czochralski method and silica glass crucible suitable therefor

Abstract

In a known method for pulling a semiconductor single crystal according to the Czochralski method, a semiconductor melt is produced in a silica glass crucible and the semiconductor single crystal is pulled from said melt. The inner wall of the silica glass crucible and the exposed melt surface are in contact with one another and with a respective melt atmosphere in the region of a contact zone running radially around the crucible inner wall, and primary oscillations of the melt are triggered in said contact zone. On this basis, in order to provide a method characterized by reduced melt vibrations and in particular by a simple, short accretion process, according to the invention primary oscillations are triggered which differ from one another in their frequency.

Claims

1. A quartz glass crucible configured for use in for pulling a semiconductor single crystal according to the Czochralski method, said crucible comprising: an inner wall of the crucible having a circumferentially extending contact zone that has a variation in an at least one physical or chemical characteristic thereof, wherein said the characteristic which that varies along the circumferentially extending contact zone is the an internal structure, wherein the internal structure is defined as a bubble content of quartz glass of the inner wall of the crucible, said bubble content being determined over a measurement length of 1 cm, the bubble content varying between a maximum value P.sub.max and a minimum value P.sub.max along the circumferentially extending contact zone; and wherein the characteristic has a first state and a second state, and the variation of the characteristic along the circumferentially extending contact zone is such that the first state thereof and the second state thereof alternate.

2. The quartz glass crucible according to claim 1 wherein the characteristic has a first state and a second state, and the variation of the characteristic along the circumferentially extending contact zone is such that the characteristic changes step by step or gradually from the first state to the second state.

3. The quartz glass crucible according to claim 2, wherein the stepwise or gradual change from the first state to the second state of the characteristic covers at least a tenth of a circumferential length of the contact zone.

4. The quartz glass crucible according to claim 1, wherein the characteristic is in the second state over at least a tenth of a circumferential length of the contact zone.

5. The quartz glass crucible according to claim 1, wherein the internal structure of the inner wall varies within a circumferentially extending variation band that extends from the contact zone toward a crucible bottom and has a width of at least 5 mm.

6. The quartz glass crucible according to claim 5, wherein the stepwise or gradual variation from the first state to the second state of the characteristic covers at least a third of a circumferential length of the contact zone.

7. The quartz glass crucible according to claim 1, wherein the characteristic is in the second state over at least a third of a circumferential length of the circumferentially extending contact zone.

8. The quartz glass crucible according to claim 1, wherein the minimum value P.sub.min is less than 30% of the maximum value P.sub.max.

9. The quartz glass crucible according to claim 1, wherein the minimum value P.sub.min is less than 50% of the maximum value P.sub.max.

10. A quartz glass crucible configured for use for pulling a semiconductor single crystal according to the Czochralski method, said crucible comprising an inner wall of the crucible having a circumferentially extending contact zone that has a variation in an at least one physical or chemical characteristic thereof; wherein said characteristic that varies along the circumferentially extending contact zone is the chemical composition, and wherein the chemical composition includes a hydroxyl group content of quartz glass of the inner wall of the crucible, said hydroxyl group content varying between a maximal concentration C.sub.OH,max and a minimal concentration C.sub.OH,min along the circumferentially extending contact zone.

11. The quartz glass crucible according to claim 10, wherein the minimal concentration C.sub.OH,min is less than 80% of the maximal concentration C.sub.OH,max.

12. The quartz glass crucible according to claim 10, wherein the minimal concentration C.sub.OH,min is less than 60% of the maximal concentration C.sub.OH,max.

13. The quartz glass crucible according to claim 10, wherein the chemical composition of the inner wall varies within a circumferentially extending variation band that runs from the contact zone in the direction of a crucible bottom and has a width of at least 10 mm.

14. The quartz glass crucible according to claim 10, wherein the chemical composition of the inner wall varies within a circumferentially extending variation band that runs from the contact zone in the direction of a crucible bottom and has a width of at least 5 mm.

15. The quartz glass crucible according to claim 10, wherein the characteristic has a first state and a second state, and the variation of the characteristic along the circumferentially extending contact zone is such that the characteristic changes step by step or gradually from the first state to the second state.

16. The quartz glass crucible according to claim 15 wherein the stepwise or gradual change from the first state to the second state of the characteristic covers at least a tenth of a circumferential length of the contact zone.

17. The quartz glass crucible according to claim 10, wherein the characteristic has a first state and a second state, and the variation of the characteristic along the circumferentially extending contact zone is such that the first state thereof and the second state thereof alternate.

18. The quartz glass crucible according to claim 17, wherein the characteristic is in the second state over at least a tenth of a circumferential length of the contact zone.

19. A quartz glass crucible configured for use for pulling a semiconductor single crystal according to the Czochralski method, said crucible comprising an inner wall of the crucible having a circumferentially extending contact zone that has a variation in an at least one physical or chemical characteristic thereof, wherein said the characteristic which that varies along the circumferentially extending contact zone is the chemical composition and wherein the chemical composition of the inner wall of the crucible is that of synthetically produced quartz glass, quartz glass produced from naturally occurring raw material or a mixture of synthetically produced quartz glass and quartz glass produced from naturally occurring raw material, and variations in proportions of synthetically produced quartz glass and quartz glass produced from naturally occurring raw material in the inner wall result in the chemical composition of the quartz glass along the circumferentially extending contact zone varying at least once.

20. The quartz glass crucible according to claim 19, wherein the chemical composition of the inner wall varies within a circumferentially extending variation band that runs from the contact zone in the direction of a crucible bottom and has a width of at least 5 mm.

21. The quartz glass crucible according to claim 19, wherein the chemical composition of the inner wall varies within a circumferentially extending variation band that runs from the contact zone in the direction of a crucible bottom and has a width of at least 10 mm.

22. The quartz glass crucible according to claim 19, wherein the characteristic has a first state and a second state, and the variation of the characteristic along the circumferentially extending contact zone is such that the characteristic changes step by step or gradually from the first state to the second state.

23. The quartz glass crucible according to claim 22, wherein the stepwise or gradual change from the first state to the second state of the characteristic covers at least a tenth of a circumferential length of the contact zone.

24. The quartz glass crucible according to claim 19, wherein the characteristic has a first state and a second state, and the variation of the characteristic along the circumferentially extending contact zone is such that the first state thereof and the second state thereof alternate.

25. The quartz glass crucible according to claim 24, wherein the characteristic is in the second state over at least a tenth of a circumferential length of the contact zone.

26. The quartz glass crucible according to claim 1, wherein the characteristic is bubble content.

Description

EMBODIMENT

(1) The invention will now be explained in more detail with reference to embodiments and a drawing. In a schematic illustration,

(2) FIG. 1 shows a crystal pulling system for performing the single-crystal pulling method according to the invention;

(3) FIG. 2 shows a first embodiment of the quartz glass crucible according to the invention in section in a view on the inner wall, which shows an annular contact zone with a surface characteristic of a high-frequency variation in circumferential direction;

(4) FIG. 3 shows a second embodiment of the quartz glass crucible according to the invention in section in a view on the inner wall, which shows an annular contact zone with a surface characteristic of a low-frequency variation in circumferential direction;

(5) FIG. 4 shows a first embodiment of the quartz glass crucible according to the invention in section in a view on the inner wall, over the whole height of which a surface characteristic varies and which has several maxima and minima, viewed in circumferential direction;

(6) FIG. 5 shows a further embodiment of the quartz glass crucible according to the invention in section in a view on the inner wall, over the whole height of which a surface characteristic varies and which has a maximum and a minimum, viewed in circumferential direction; and

(7) FIG. 6 shows an apparatus for producing a quartz glass crucible according to the invention.

(8) FIG. 1 schematically shows a single-crystal pulling device. It comprises a quartz glass crucible 1 which is stabilized by a support crucible 2 and which contains a silicon melt 3 which is kept at melt temperature by a heater 4 provided laterally on the crucible wall.

(9) The quartz glass crucible 1 is rotatable about a rotation axis 5. The silicon single crystal 6 is pulled upwards out of the melt 3 and is rotated in this process in opposite direction with respect to the crucible 1, as indicated by the directional arrow 7.

(10) The single crystal 6 which is pulled upwards is surrounded by a heat shield 8. Argon is continuously supplied through the gap between heat shield 8 and single crystal 6, the argon forming the melt atmosphere 11 within the pulling chamber (not shown in the figure) and serving gas-flushing purposes.

(11) The melt surface 9 in the quartz glass crucible 1 is kept at a constant level in the course of the pulling process. For this purpose the quartz glass crucible 1 follows in upward direction, as shown by the directional arrow 10. At this position, which is here called contact zone 13, the inner wall 12 of the quartz glass crucible 1, the silicon melt 3 and the melt atmosphere 11 are thus in direct contact with one another.

(12) The invention aims at varyingat least in the region of the contact zone 13a characteristic of the surface of the inner wall 12 of the quartz glass crucible in radially circumferentially extending direction. The radially varying surface characteristic is e.g. the hydroxyl group, the surface roughness, the bubble content, or quartz glass quality in the sense that this is quartz glass of naturally occurring or of synthetically produced start material.

(13) FIGS. 2 to 5 schematically show suitable quartz glass crucibles with radially circumferentially extending profiles of a surface characteristic. The characteristic is varied at the height of the radially circumferentially extending line of the contact zone 13 which in this case corresponds to the height of the starting zone.

(14) A coordinate plane in which the extent of the formation or the concentration K of the respective surface characteristic is plotted against the circumferential length L of the contact zone 13 is respectively laid over the view on the inner wall 12 of the crucible in the figures, with the figures only showing half of the total circumference.

(15) The ordinate value 100 of K corresponds to the useful or technologically feasible maximum value of the characteristic in question in its formation A; and the ordinate value 0 of K symbolizes the useful or technologically feasible minimum value of the characteristic in question in its formation A or the technologically feasible or useful value of the characteristic in question in its formation B.

(16) If the surface characteristic is the hydroxyl group content of the quartz glass, this content varies expediently between 80 wt. ppm (minimum value) and 150 wt. ppm (maximum value).

(17) If the surface characteristic is the surface roughness R.sub.a of the inner value, this value varies between 5 m (minimum value) and 200 m (maximum value). The value for the surface roughness is determined according to DIN 4768 as a mean roughness depth R.sub.a.

(18) If the surface characteristic is the bubble content of the quartz glass within the crucible wall in the region of the contact zone 13, it will vary between 0.01% (minimum value) and 0.03% (maximum value), namely as a mean value, measured over a layer thickness of 2 mm.

(19) In the case of the quartz glass quality the surface characteristic varies between quartz glass of naturally occurring start material and quartz glass of synthetically produced start material.

(20) In the embodiment shown in FIG. 2, the surface characteristic varies around the contact zone 13, as indicated by the profile. The same profile or at least a profile similar to the illustrated profile is also found in a certain surface area of the inner wall 12 of the crucible underneath the contact zone 13. This surface area, which is called variation band 14, is visible in the diagram as an area with a gray background. In the embodiment the variation band 14 extends from the contact zone 13 approximately 30 mm downwards in the direction of the crucible bottom.

(21) The formation/concentration K of the surface characteristic varies within the contact zone 13 (or within the radial circumferential course of the variation band 14) irregularly, but constantly. The variation width of the change only corresponds to a small range of the total possible scale of K. The radially circumferentially extending profile of K shows a plurality of relative maxima and minima which define a mean variation frequency (distance of maximum to maximum) of about

(22) 0.04 cm.sup.1.

(23) In contrast to FIG. 2, in the embodiment shown in FIG. 3, the surface characteristic within the contact zone 13 or within the variation band 14 having a width of 50 mm varies almost regularly sinusoidally and at a much lower frequency of about 0.014 cm.sup.1, but also only in a small range of the total scale of K.

(24) The profiles shown in FIGS. 2 and 3 are particularly suited for radially circumferentially extending variations of the hydroxyl group content of the quartz glass and the surface roughness and the bubble content of the inner wall of the crucible.

(25) In the embodiment shown in FIG. 4, the surface characteristic of the inner wall of the crucible varies not only circumferentially around the contact zone 13, but simultaneously over almost the whole height of the inner wall of the crucible in a similar way. The outlined contact zone 13 also corresponds here to the maximal height of the melt level (=height of the starting zone) at the beginning of the single-crystal pulling method. The variation width corresponds here to almost 100% of the whole scale of K, which means that the characteristic in question varies almost completely between its two formations A and B or between the above-defined minimum and maximum values.

(26) Similar to the profile of FIG. 4, in the embodiment shown in FIG. 5 the surface characteristic also varies between two formations A and B of the characteristic. Here, however, a continuous gradual transition from the one to the other formation takes place over the whole radial circumference, the concentration profile K having only one maximum and only one minimum in each formation. Hence, only two gradual changes take place over the circumference of the inner wall, namely a gradual change from formation A to B over a half of the circumferential length and a gradual change from formation B to A over the other half of the circumferential length.

(27) The change profiles of FIGS. 4 and 5 are particularly useful for radially circumferentially changing the composition of the inner wall of the crucible between sections of quartz glass of naturally occurring start material and sections of quartz glass of synthetically produced start material. They are however equally suited for radially circumferentially extending variations of the hydroxyl group content of the quartz glass and the surface roughness and the bubble content of the inner wall of the crucible.

(28) The manufacture of a quartz glass crucible according to the invention shall now be explained in more detail with reference to an embodiment and with reference to the melt apparatus shown in FIG. 6. The hydroxyl group content of the quartz glass is here varied along a radially circumferentially extending contact zone of the inner wall of the crucible.

(29) The crucible type melt apparatus which is diagrammatically shown in FIG. 6 comprises a melt mold 61 of metal with an inner diameter of 78 cm, a curved bottom and a sidewall with a height of 50 cm. The melt mold 61 is supported to rotate about its central axis 62. Electrodes 64 of graphite which are movable inside the interior 63 in all spatial directions, as shown by the block arrows 78, project into the interior 53 of the melt mold 61.

(30) A plurality of passages 66 through which a vacuum applied to the outside of the melt mold 61 can become operative in the interior 63 are provided in the bottom portion 73 and in the area of the lower wall half 75 of the melt mold 61. Further passages 68 through which a gas can be passed towards the melt mold interior 63 are provided in the upper wall third 74 of the melt mold 61. The passages 68 terminate in a joint groove 69 which is pierced from above into the one half of the upper side of the melt mold wall up to the height of the starting zone Z (corresponds to the height of the contact zone 13 during the intended use). The passages 66; 68 are each sealed with a plug of porous graphite which prevents SiO.sub.2 granules from exiting out of the interior 63.

(31) In a first method step, crystalline granules of natural quartz sand cleaned by hot chlorination are introduced into the melt mold 61. The quartz sand has a grain size ranging from 90 m to 315 m. Under the action of the centrifugal force and by using a template, a rotation-symmetrical, crucible-like grain layer 72 of mechanically compacted quartz sand is formed on the inner wall of the melt mold 61 rotating about the longitudinal axis 62. The layer thickness of the grain layer 72 is about the same in the bottom portion 73 and in the lower side portion 75 and in the upper side portion 74 and is about 25 mm. The height of the grain layer 72 in the sidewall portion corresponds to the height of the melt mold, i.e. 50 cm.

(32) In a second method step, the electrodes 64 are positioned near the grain layer 72 in the melt mold 61 still rotating about its longitudinal axis 62, and an electric arc is ignited between the electrodes 64.

(33) The electrodes 64 are powered with 600 kW (300 V, 2000 A) so that a high-temperature atmosphere is obtained in the melt mold interior 63. A skin layer 77 of dense transparent quartz glass with a thickness of about 0.5 mm is thereby produced on the quartz grain layer 72. The free upper side 65 of the grain layer 72 is thereby also densified.

(34) After formation of the skin layer 77 a vacuum (100 mbar absolute pressure) is applied to the grain layer 72 in the bottom portion 73 and in the lower wall portion 75 in a third method step via the passages 66. At the same time, water vapor is introduced via the passages 68 into the one half of the still porous grain layer 72. The respective gas flows during suction and introduction of water vapor are outlined in FIGS. 1 to 3 by way of arrows.

(35) Due to the flow resistance of the grain layer 72 the water vapor introduced at half the side is distributed substantially only in the one half of the grain layer 72 and also substantially only in the upper side portion 74 around the starting zone Z, so that in this portion of the grain layer the SiO.sub.2 granules are relatively heavily loaded with water vapor.

(36) During further vitrification under vacuum a melt front travels from the inside to the outside through the grain layer 72. Due to the stronger water loading in the one half of the grain layer 72 a vitrified zone is formed with a higher hydroxyl group content than in the other half.

(37) As soon as the melt front is at a distance of about 4 cm from the melt front wall, evacuation is terminated. The rear side of the grain layer 72 thereby also vitrifies in the bottom and lower sidewall portion into opaque, bubble-containing quartz glass. Vitrification is stopped before the melt front reaches the melt mold 61.

(38) Viewed over the circumference at the height of the starting zone Z, one achieves the greatest difference in the hydroxyl group content of the quartz glass between the area of the preceding air introduction (90 wt. ppm)namely in the middle of the length of the groove 69and the exactly opposite portion of the sidewall. It is 130 wt. ppm at that place. The OH group concentration profile obtained over the circumference of the starting zone Z is here equal to that of FIG. 5.

(39) During the intended use of the quartz glass crucible the thin skin layer 77 dissolves within a short period of time. The free surface of the inner wall of the crucible that is then exposed is distinguished by hydroxyl groups having a concentration that at the height of the starting zone Z (=contact zone 13) varies in the radially circumferentially extending direction, as explained with reference to FIG. 5. As a consequence, a different surface tension is obtained for each point between silicon melt and crucible wall and thus different excitation conditions for oscillations, so that melt vibrations are suppressed.

(40) As an alternative to the described method, a hydroxyl group content that is inhomogeneous along the contact zone, i.e. a locally different one, is produced by using a hydrogen-containing burner flame, for instance oxyhydrogen flame. The hydroxyl group content can be adjusted in a locally different way through the degree of the action (temperature and duration) of the burner flame. This method also permits the subsequent generation of a chemical variation of the chemical composition in the case of a quartz glass crucible having a homogeneous crucible wall.

(41) The crucible melt apparatus shown in FIG. 6 is also suited for producing a contact zone 13 with a radially circumferential variation in the bubble content within the crucible wall. For this purpose, in the third method step a hardly soluble gas such as nitrogen orin the embodimentair is passed via the passages 68 into the one half of the still porous grain layer 72 instead of water, which is relatively easily soluble in quartz glass.

(42) Due to the flow resistance of the grain layer 72 the air introduced at half the side is distributed substantially only in the one half of the grain layer 72 and also substantially only in the upper side portion 74 around the starting zone Z, so that in this area of the grain layer one obtains a relatively high concentration of hardly soluble nitrogen.

(43) During further vitrification under vacuum a melt front travels from the inside to the outside through the grain layer 72. Due to the stronger nitrogen loading in the one half of the grain layer 72 a vitrified zone with a higher bubble content is formed in the one half of the grain layer 72 than in the other half.

(44) Viewed over the circumference and at the height of the starting zone Z, one obtains the greatest difference in the bubble content between 0.01% in the area of the preceding air introductionnamely in the middle of the length of the groove 69and the exactly opposite portion of the sidewall. It is 0.03% at that place. The bubble concentration profile obtained in this process over the circumference of the starting zone Z within the crucible wall resembles that of FIG. 5.