Patent Publication Number: US-2010126410-A1

Title: Apparatus and method for pulling silicon single crystal

Description:
TECHNICAL FIELD 
     The present invention relates to an apparatus for pulling a silicon single crystal ingot from a silicon melt while applying a vertical magnetic field to the silicon melt. The present invention also relates to a method of pulling the silicon melt. 
     BACKGROUND ART 
     Conventionally, the Czochralski method (hereafter referred to as CZ method) has been known as a method of growing a silicon single crystal ingot. In the CZ method, high purity silicon single crystal ingot for a semiconductor is grown from a silicon melt stored in a crucible. In the CZ method, a mirror-etched seed crystal is made to contact the silicon melt. Subsequently, a silicon single crystal ingot is grown by pulling the seed crystal while rotating the seed crystal. In this method of growing a silicon single crystal, after forming a seed drawing portion by pulling the seed crystal, a shoulder portion is formed by gradually increasing the diameter of the crystal to a target diameter of the silicon single crystal ingot, and a straight body portion of the silicon single crystal ingot is formed by further pulling the crystal. 
     On the other hand, a silicon single crystal includes impurities. For example, the impurities may be constituted of dopants such as boron and phosphorus that are intentionally added to the crystal so as to control the electric resistivity, and oxygen which is dissolved from a wall of the quartz crucible into the silicon melt during the pulling process and contaminates in the single crystal ingot. When the silicon single crystal ingot is used for forming silicon wafers, those impurities have an influence on the qualities of the wafers. Therefore, the impurities must be controlled properly. In particular, it is important to control the radial distribution of impurities in the silicon single crystal ingot to have an uniform distribution in order to form a wafer having a uniform in-plane distribution of impurities. In a recently used technique (MCZ method: Magnetic Field Applied Czochraski Method), based on the consideration of the above-described problem, a static magnetic field is applied to a melt in the crucible while pulling a single crystal in accordance with the Czochraski Method so as to control thermal convection of the silicon melt. In general, three types of magnetic fields, i.e., a horizontal magnetic field (e.g., Patent Reference 1: Japanese Unexamined Patent Application, first Publication, No. S61-239605), a vertical magnetic field (e.g., Patent Reference 2: Japanese Unexamined Patent Application, First Publication, No. H10-279380), and a cusp magnetic field (e.g., Patent Reference 3; Japanese Unexamined Patent Application, First Publication No. 2003-2782) are known as the static magnetic field. The MCZ method has proved to be effective in stabilizing the temperature of the silicon met by controlling the convention of the silicon melt, and in decreasing dissolution of the crucible caused by the melt. 
     PROBLEM TO BE SOLVED BY THE INVENTION 
     However, in the case of the horizontal magnetic field, in the surface or the arbitrarily selected horizontal section of the silicon melt, it is impossible to apply a uniform magnetic field in the rotational angle. Where electromagnets are installed so as to apply a transverse magnetic filed on a horizontal plane in one direction, and thereby generating a magnetic field directed from one electromagnet to another electromagnet, distribution of the magnetic field in a direction parallel to the application direction is largely different from the distribution of the magnetic field vertical to the application direction. In addition, the magnetic filed has the highest strength in a central portion of the silicon melt, and the strength is decreased in accordance with increasing distance from the central portion. As a result, it is impossible to apply a axisymmetric magnetic field around the central axis of the silicon melt. 
     A vertical magnetic field is applied from a first coil and a second coil having the same diameter which is larger than the outer diameter of the quartz crucible. The first and second coils are installed with a predetermined spacing in between in the vertical direction, and the center of each coil is adjusted to the rotational axis of the quartz crucible. Therefore, by the vertical magnetic field generated from the first and second coils, it is possible to apply a axisymmetric magnetic field uniformly to the silicon melt. However, in the case of the vertical magnetic field, it is impossible to prevent the impurities (for example, oxygen which is dissolved from the wall of the quartz crucible and is contaminated in the melt) from concentrating at the central portion of the surface of the silicon melt, since the convection of the silicon melt is controlled by Lorentz force generated by the magnetic filed. Therefore, there has been a problem that oxygen as an impurity contaminated the silicon single crystal ingot by being captured from the central portion of the surface of the silicon melt. 
     On the other hand, a cusp magnetic field is generated by installing a first coil and a second coil having the same diameter which is larger than the outer diameter of the quartz crucible such that the first and the second coils are placed with a predetermined spacing in between in the vertical direction, and the center of each coil is adjusted to the rotation axis of the quartz crucible, and energizing electric currents in opposite directions in the first and the second magnetic coils. Therefore, in the cusp magnetic field, a transversal magnetic field is applied to the silicon melt in the vicinity of the inner peripheral surface of the quartz crucible. As a result, it is possible to inhibit impurities, e.g., oxygen which is dissolved from the wall of the crucible and is contaminated in the silicon melt, concentrating to the central portion of the surface of the silicon melt and being captured by the silicon single crystal ingot. However, since the strength of the cusp magnetic filed is zero in the vicinity of the solid-liquid interface beneath the silicon single crystal ingot, there has been a problem that the shape of solid-liquid interface cannot be controlled by the magnetic field. Especially, under the recent trend for controlling a shape of solid-liquid interface to attempt at growing silicon single crystal ingot which is free of aggregates of interstitial silicon type point defects in the interior, importance for controlling the shape of the solid-liquid interface is increasing. 
     An object of the present invention is to provide an apparatus and a method for growing a silicon single crystal which enables the application of a uniform magnetic field to a silicon melt axisymmetrically around the central axis of the silicon melt, preventing concentration of impurities such as oxygen to the central portion of the surface of the silicon melt, and effectively controlling the shape of the solid-liquid interface directly beneath the silicon single crystal ingot. 
     DISCLOSURE OF INVENTION 
     Problem to be Solved by the Invention 
     In an apparatus for pulling a single crystal according to the present invention, during pulling a silicon single crystal from a silicon melt while rotating the single crystal at a predetermined rotation speed and rotating a quartz crucible which is installed in a chamber and stores the silicon melt at a predetermined rotation speed, a first coil and a second coil are installed with a predetermined spacing in between in the vertical direction such that the center of each coil is adjusted to the rotation axis of the quartz crucible. A magnetic field is generated between the first and the second coils by energizing electric current in each of the first coil and the second coil in the same direction. In the present invention, a single crystal pulling apparatus of the above-described configuration is improved by a constitution such that the first coil is installed outside the chamber and the second coil is installed inside the chamber. 
     In the above-described apparatus for pulling a silicon single crystal, when electric currents of the same direction are energized in the first and the second coils, the magnetic line of flux of the magnetic field generated by the currents shows a cone-like shape such that a diameter of a profile of the magnetic line of the flux observed from a transverse direction decreases in a downward direction. In this cone-shaped magnetic field, a magnetic field directed to a central portion of the silicon melt is applied uniformly. As a result, it is possible to apply to the silicon melt, a uniform magnetic field which is axisymmetric about the center axis of the silicon melt. 
     The cone-shaped magnetic filed has properties both of the vertical magnetic field and the transverse magnetic field. By a transverse component of the magnetic field, impurities such as oxygen dissolved from the wall of the crucible and contaminated in the melt are prevented from concentrating at the central portion of the melt surface. Therefore, contamination of the silicon single crystal ingot by the oxygen impurity can be reduced sufficiently. 
     The strength of the cone-shaped magnetic field is not reduced to zero in the vicinity of the solid-liquid interface directly beneath the silicon single crystal ingot. Therefore, the shape of the solid-liquid interface can be controlled by the magnetic field. 
     The above-described apparatus for pulling a silicon single crystal may have a constitution such that: a vertical spacing between the first and second coils is greater than 0 and less than or equal to 10000 mm; the diameter D 1  of the first coil is 100 mm or more and 10000 mm or less; the diameter D 2  of the second coil is 5 mm or more and 5000 mm or less; the proportion of the diameter D 1  of the first coil to the diameter D 2  of the second coil is 1 or more and 2000 or less; and a difference between the diameter D 1  of the first coil and the diameter D 2  of the second coil is 2t or more, where t is the thickness of a circumferential wall of the chamber. 
     In the above-described apparatus for pulling a silicon single crystal, it is possible to effectively control the strength of the magnetic field in the vicinity of the solid-liquid interface directly beneath the silicon single crystal ingot. By this magnetic field, the shape of the solid-liquid interface is effectively controlled. Preferably, T may be greater than 0 and less than or equal to 8000 mm, the diameter D 1  of the first coil may be 500 mm or more and 5000 mm or less, and the diameter D 2  of the second coil may be 50 mm or more and 500 mm or less. 
     In a method of pulling a silicon single crystal according to the present invention, during pulling a silicon single crystal from a silicon melt while rotating the single crystal at a predetermined rotation speed and rotating a quartz crucible which is installed in a chamber and stores the silicon melt at a predetermined rotation speed, a first coil having a coil diameter which is larger than a diameter of the chamber is installed outside the chamber such that a center of the coil corresponds to a rotation axis of the quartz crucible, and a second coil is installed inside the chamber with a predetermined spacing in the vertical direction from the first coil such that a center of the second coil corresponds to the rotation axis of the quartz crucible, and a magnetic field is generated between the first and the second coils by energizing the electric current in each of the first coil and the second coil in the same direction. 
     In the above-described single crystal pulling method, a center (half) position of the predetermined spacing T between the first coil and the second coil is controlled to be at the same or a lower level as the surface of the silicon melt such that 0 mm≦|H|≦10000 mm is satisfied, where H is the distance of the center position from the surface of the silicon melt. 
     In the above-described method of pulling a silicon single crystal, by pulling a silicon single crystal while controlling the center position to at the same or a lower level as the surface of the silicon melt such that the distance of the center position from the surface of the silicon melt satisfies 0 mm≦|H|≦10000 mm, a predetermined convection is generated in the silicon melt. By this convection, the solid-liquid interface which has largely had a downwardly convex shape in the prior art takes a nearly flat shape at the same level as the melt surface. As a result, axial thermal gradient in the silicon single crystal shows nearly constant values in the radial distribution, and it is possible to grow relatively easily a silicon single crystal which is of high quality and is defect free throughout nearly the whole length. Where |H| exceeds 10000 mm, it is difficult to control the oxygen and the shape of the solid-liquid interface because of insufficient strength (magnetic flux density) of the magnetic field in the melt. As a preferable range, 0 mm≦|H|≦500 mm may be satisfied. 
     In the above-described method of pulling a single crystal, where I 1  denotes the current energized in the first coil and I 2  denotes the current energized in the second coil, magnetic flux density may be controlled to be 0.001 to 0.1 T (Wb/m 2 ) at a position within the inner diameter of the quartz crucible at the same level as the center position by energizing electric currents in the first coil and the second coil such that I 1  and I 2  are controlled in the range from 0.1 to 10 30  A and satisfy 0.001≦(I 1 /I 2 )≦1. 
     In the above-described method of pulling a silicon single crystal, it is possible to control the strength of the magnetic field in the vicinity of the solid-liquid interface directly beneath the silicon single crystal ingot effectively, thereby producing a silicon single crystal ingot that does not include aggregates of interstitial silicon type point defects in the interior. Where the magnetic flux density at the position within the inner diameter of the quartz crucible at the same level as the center position is lower than 0.001 T (Wb/m 2 ), oxygen cannot be controlled sufficiently because of insufficient strength (magnetic flux density) of the magnetic field in the melt. Where the magnetic flux density at the position within the inner diameter of the quartz crucible at the same level as the center position exceeds 1.0 T (Wb/m 2 ), the shape of the solid-liquid interface cannot be controlled sufficiently because of insufficient development of the shape of the solid-liquid interface. Preferably, I 1  and I 2  may be within the range from 100 to 10 10  A, and the magnetic flux density at the position within the inner diameter of the quartz crucible at the same level as the center position may be in the range from 0.01 to 0.5 T (Wb/m 2 ). 
     EFFECT OF THE INVENTION 
     In the apparatus for pulling a silicon single crystal according to the present invention, since the first coil is installed outside the chamber and the second coil is installed inside the chamber, by energizing electric currents in the first and the second coils in the same direction, it is possible to apply a pone-shaped magnetic field such that a diameter of a profile of the magnetic line of the flux observed from a transverse direction decreases towards a downward direction. In the cone-shaped magnetic field, the magnetic field directed to the center portion of the silicon melt is applied uniformly. As a result, it is possible to apply a uniform magnetic field that is axisymmetric about the central axis of the silicon melt. The cone-shaped magnetic field has properties of a vertical magnetic field and a transverse magnetic field. By the horizontal component of the magnetic field, impurities such as oxygen dissolved from the wall of the crucible and contaminated in the melt are prevented from concentrating to the central portion of the melt surface. Moreover, since the strength of the cone-shaped magnetic field is not reduced to zero even in the vicinity of solid-liquid interface directly beneath the silicon single crystal ingot, it is possible to use the cone-shaped magnetic field for controlling the shape of the solid-liquid interface. 
     In the method of pulling a silicon single crystal according to the present invention, a center (half) position of the predetermined spacing T between the first coil and the second coil is controlled to be at the same or a lower level as the surface of the silicon melt such that 0 mm≦|H|≦10000 mm is satisfied, where H is the distance of the center position from the surface of the silicon melt. Therefore, a predetermined convection is generated in the silicon melt. By this convection, the solid-liquid interface which has largely had a downwardly convex shape in the prior art takes a nearly flat shape at a same level as the melt surface. As a result, axial thermal gradient in the silicon single crystal shows nearly constant values in radial distribution, and it is possible to relatively easily grow a silicon single crystal which is of high quality and is defect free throughout nearly the whole length. At that time, where I 1  denotes the current energized in the first coil and I 2  denotes the current energized in the second coil, by controlling magnetic flux density to be 0.001 to 0.1 T (Wb/m 2 ) at the position within the inner diameter of the quartz crucible at the same level as the center position by energizing electric currents in the first coil and the second coil such that I 1  and I 2  are controlled in the range from 0.1 to 10 30  A and satisfy 0.001≦(I 1 /I 2 )≦1, it is possible to effectively control the strength of the magnetic field in the vicinity of the solid-liquid interface directly beneath the silicon single crystal ingot effectively, thereby producing a silicon single crystal ingot having interior structure that is free of aggregates of interstitial silicon type point defects. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram showing a constitution of an apparatus for pulling a silicon single crystal according to the present embodiment. 
         FIG. 2  is a enlarged view showing a relation between coils and a crucible in the apparatus of  FIG. 1 . 
         FIG. 3  is a sectional view along the line A-A in  FIG. 2  and shows directions of the magnetic field applied to the silicon melt in the crucible. 
         FIG. 4  is a graph based on Voronkov&#39;s theory showing that vacancy type point defects are dominant in a ingot formed where V/G ratio is not lower than a critical point, and that interstitial silicon type point defects are dominant in an ingot formed where V/G ratio is not higher than a critical point. 
     
    
    
     EXPLANATION OF SYMBOLS 
     
         
           11  chamber 
           12  silicon melt 
           13  quartz crucible 
           25  silicon single crystal ingot 
           41  first coil 
           42  second coil 
           43  magnetic field 
           43   a  center position between the first coil and the second coil 
         H distance of the center position of the predetermined spacing T from the surface of the silicon melt 
         D 1  diameter of the first coil 
         D 2  diameter of the second coil 
         T predetermined spacing 
         t thickness of the circumferential wall of the chamber 
       
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     Next the best mode for carrying out the invention is explained with reference to the drawings. However, it should be noted that the present invention is not limited to the below-described embodiments. 
     An apparatus  10  for pulling a silicon single crystal according to the present invention is shown in  FIG. 1 . A quartz crucible  13  that stores a silicon melt  12  is installed in a chamber  11  of the apparatus  10 . The outer periphery of the quartz crucible  13  is covered by a graphite susceptor  14 . A bottom face of the quartz crucible  13  is fixed on the upper end of the support shaft  16  via the graphite susceptor in between, and the lower end of the support shaft  16  is connected to a crucible, driving unit  17 . Although they are not illustrated in the drawing, a first rotating motor for rotating the quartz crucible  13  and an elevating motor for driving the quartz crucible up and down are equipped to the crucible driving unit  17 . By these motors, the quartz crucible can be rotated in a predetermined direction, and can be driven upward and downward. The outer periphery of the quartz crucible  13  is surrounded by a heater with a predetermined spacing in between. The heater  18  is surrounded by a heat insulating cylinder  19 . High purity polycrystalline silicon materials installed in the quartz crucible  13  are heated and molten by the heater  18  into a silicon melt  12 . 
     A cylindrical casing  21  is connected to the upper end of the chamber  11 . A pulling unit  22  is provided to the casing  21 . The pulling unit  22  comprises a pulling head (not shown) which is provided to the upper end of the casing  21  and is tunable at a horizontal state, a second rotating motor (not shown) for rotating the pulling head, a wire cable  23  which is suspended from the head towards the rotation center of the quartz crucible  13 , and a pulling motor (not shown) for winding and unwinding the wire cable  23 . A seed crystal  24  which is to be dipped in the silicon melt  12  for pulling the silicon single crystal ingot  25  is attached to the lower end of the wire cable  23 . 
     A gas supply/exhaustion unit  28  for supplying an inert gas to the silicon single crystal side in the chamber  11  and exhausting the inert gas from the crucible inner periphery side in the chamber  11  is connected to the chamber  11 . The gas supply/exhaustion unit  28  comprises a supply pipe  29  one end of which is connected to the circumferential wall of the casing  21  and another end of which is connected to a tank (not shown) that stores the above-described inert gas, and an exhaustion pipe  30  one end of which is connected to a bottom wall of the chamber  11  and another end of which is connected to a vacuum pump (not shown). A first flow control valve  31  and a second flow control valve are respectively provided to the supply pipe  29  and the exhaustion pipe  30  so as to control flow rates of the inert gas flowing in the pipes  28 ,  30 . 
     An output axis of the pulling motor (not shown) is provided with an encoder (not shown). A crucible driving unit  17  is provided with an encoder (not shown) for detecting the elevated position of the support shaft. Bach detection output of the two encoders is connected to a controlling input of a controller (not shown) and controlling outputs of the controller are respectively connected to the pulling motor of the pulling unit  22  and the elevating motor of the crucible driving unit  17 . The controller is provided with a memory (not shown). A winding length of the wire cable  23  corresponding to the detection out put of the encoder, that is a pulling length of the silicon single crystal ingot  25  is stored as a first map in the memory. A surface level of the silicon melt  12  in the crucible  13 , corresponding to the pulling length of the silicon single crystal ingot  25 , is recorded as a second map in the memory. Based on the detection output of the encoder in the pulling motor, the controller controls the elevating motor of the crucible driving unit  17  such that the melt surface of the silicon melt  12  in the quartz crucible  13  is maintained at a constant level. 
     A heat shielding member  36  surrounding the outer periphery of the silicon single crystal ingot  25  is provided between the outer periphery of the silicon single crystal ingot  25  and the inner periphery of the quartz crucible  13 . The heat insulating member is constituted to have a cylindrical shape, and comprises a cylinder portion  37  that blocks the heat radiation from the heater  18 , and a flange portion  38  that is connected to an upper edge of the cylinder portion  37  and nearly horizontally extends to the outward direction. By placing the above-described flange portion  38  on the heat insulating cylinder  19 , the heat shielding member  36  is fixed inside the chamber  11  such that the bottom edge of the cylinder portion  37  is positioned above the surface of the silicon melt  12  with a predetermined distance in between. A protruding portion  39  protruding towards the inside on the cylinder is provided to the lower part of the cylinder portion  37 . 
     In the pulling apparatus, a first coil  41  and a second coil  42  each having a center corresponding to the rotational axis of the quartz crucible  13  are installed with a predetermined spacing T in between in the vertical direction. The first coil  41  is installed outside the chamber  11  and the second coil  42  is placed inside the chamber  11 . As shown in  FIG. 2 , the vertical spacing T between the first coil  41  and the second coil  42  is 0 or more and 10000 mm or less. A diameter D 1  of the first coil  41  is 100 mm or more and 10000 mm or less. A diameter D 2  of the second coil  42  is 5 mm or more and 5000 mm or less. The first coil  41  and the second coil  42  are installed such that a proportion of the diameter D 1  of the first coil  41  to the diameter D 2  of the second coil  42  is 1 or more and 2000 or less, and a difference between the diameter D 1  of the first coil  41  and the diameter D 2  of the second coil  42  is 2t or more, where t denotes a thickness of the circumferential wall of the chamber  11  ( FIG. 1 ). The pulling apparatus  10  is constituted such that the single crystal ingot  25  is pulled while generating a magnetic field  43  between the first coil  41  and the second coil  42  by energizing electric currents in the first and the second coils  41 ,  42  in the same direction. 
     Next, a pulling method using the apparatus for pulling a silicon single crystal is explained. 
     A silicon single crystal ingot  25  is pulled from the silicon melt  12  while rotating the quartz crucible  13  that stores the silicon melt  12  at a predetermined rotation speed R 1 , rotating a silicon single crystal ingot  25  being pulled from the silicon melt at a predetermined rotation speed R 2 , and applying the magnetic filed  43  to the silicon melt  12  using the first and the second coils  41 ,  42 . 
     As shown in  FIG. 2 , the first coil is installed outside the chamber  11  and the second coil is installed inside the chamber  11 . Therefore, when electric currents in the same direction are energized in the first and the second coils  41 ,  42 , the magnetic field  43  generated by the first and the second coils  41 ,  42  shows a cone-like shape such that a diameter of a profile of the magnetic line of the flux observed from a transverse direction decreases in a downward direction. 
     A state of the magnetic field  43  in a horizontal sectional plane of the silicon melt applied with the above-described cone-shaped magnetic field is shown in  FIG. 3 . As it is clear from  FIG. 3 , a magnetic field directed to the central portion of the melt is applied uniformly to the silicon melt. As a result, a uniform magnetic field that is axisymmetric about the central axis of the melt is applied uniformly to the melt. The cone-shaped magnetic field has properties of both a vertical magnetic field and a transverse magnetic field. By the transverse component, it is possible to prevent the central portion of the surface of the silicon melt from concentrating impurities such as oxygen dissolved from the wall of the quartz crucible and contaminated in the silicon melt. As a result, contamination of the silicon single crystal ingot  25  by the oxygen as an impurity can be reduced sufficiently. 
     The strength of the cone-shaped magnetic field is not reduced to zero even in the vicinity of the solid-liquid interface directly beneath the silicon single crystal ingot. Therefore, it is possible to control the shape of the solid-liquid interface. By pulling the silicon single crystal with a pulling rate profile in accordance with Voronkov&#39;s theory, it is possible to produce a silicon single crystal ingot having no interstitial silicon type point defects in the interior portion. In the Voronkov&#39;s theory, where V (mm/minute) denotes the pulling rate of the silicon single crystal ingot, and G (° C./mm) a thermal gradient in the silicon single crystal ingot in the vicinity of the interface between the silicon single crystal ingot and the silicon melt  12 , V/G (mm 2 /minute·° C.) is controlled in order to grow a high purity silicon single crystal ingot  25  having small number of defects. 
     In the Voronkov&#39;s theory, the relationship between V/G and concentration of point defects is graphically represented. For example, in  FIG. 4 , V/G is plotted along the horizontal axis and concentration of vacancy type point defects and concentration of interstitial silicon type point defects are plotted along the same vertical axis. Using such a graph, Vornkov&#39;s theory explains that a boundary position between vacancy region and interstitial silicon region is determined by V/G. Specifically, vacancy type point defect-dominant silicon single crystal ingot is formed where V/G ratio is not lower than the critical point (critical value), and interstitial silicon type point defect-dominant silicon single crystal ingot is formed where V/G ratio is not higher than the critical point. In  FIG. 4 , [I] denotes a region ((V/G) 1  or less) in which interstitial silicon type point defects are dominant and aggregates of the interstitial silicon type point defects exist, and [V] denotes a region ((V/G) 2  or more) in which vacancy type point defects are dominant in the silicon single crystal ingot and aggregates of the vacancy type point defects exist, and [P] denotes a perfect region ((V/G) 1  to (V/G) 2 ) in which aggregates of vacancy type point defects and aggregates of interstitial silicon type point defects do not exist. A region [V] (((V/G)2 to (V/G) 3 ) adjacent to the region [P] is a region for forming OSF nuclei. 
     The perfect region [P] is further divided to a region [P I ] and a region [P V ]. In [P I ] region, V/G ratio ranges from the above-described (V/G) 1  to the critical point. In [P V ] region, V/G ratio ranges from the critical point to the above-described (V/G) 2 . That is, [P I ] is a region that is adjacent to the region [I] and has a interstitial silicon type point defect concentration lower than the lowest concentration of interstitial silicon type point defects for forming a interstitial-type dislocation, and [P V ] is a region that is adjacent to the region [V] and has a vacancy type point defect concentration lower than the lowest concentration of vacancy type point defects for forming an OSF. In the crystallization process, micro-defects constituting nuclei of the above-described OSF are introduced in the crystal, and the OSF appears, for example in a thermal oxidation process during a device production process, and causes malfunction such as increase of leak current in the produced device. 
     In  FIG. 2 , the center position  43   a  of the predetermined spacing T between the first coil and the second coil is controlled to be at the same or lower level as the surface of the silicon melt  12  such that 0 mm≦|H|≦10000 mm is satisfied, where H is a distance of the center position  43   a  from the surface of the silicon melt  12 . 
     Where I 1  denotes the current energized in the first coil  41  and I 2  denotes the current  42  energized in the second coil, magnetic flux density at a position at the same level as the center position  43   a  and within the inner diameter of the quartz crucible  13  (a position in a plane which is surrounded by the inner wall of the quartz crucible  13  at the same level as the center position  43   a ) is controlled to be 0001 to 0.1 T (Wb/m 2 ) by energizing electric currents in the first coil  41  and the second coil  42  such that I 1  and I 2  are controlled in the range from 0.1 to 10 30  A and satisfy 0.001≦(I 1 /I 2 )≦1. 
     Here the distance H of the center position  43   a  from the surface of the silicon melt  12  is specified to be in the above-described range since it is difficult to control the oxygen and the shape of the solid-liquid interface because of insufficient strength where |H| exceeds 10000 mm. The electric currents in the first and second coils  41 ,  42  are controlled since the Lorentz force for generating convection in the silicon melt  12  must be increase with increasing diameter  13  of the quartz crucible. Where the currents are set to be outside the above-described ranges, the melt convection does not exhibit an ideal pattern and the solid-liquid interface cannot be controlled. 
     As described above, by controlling the center position  43   a  of the predetermined spacing T between the first coil  41  and the second coil  42 , and by controlling the strength of the magnetic field  43 , as shown in  FIG. 2 , predetermined convections  44 ,  45  are generated in the silicon melt. By these convections, the shape of solid-liquid interface  25   a  which largely had a downwardly convex shape in the prior art takes nearly flat shape at the same level as the melt surface. At that sate, the silicon single crystal ingot  25  is pulled while rotating the ingot at a predetermined rotation speed and rotating the quartz crucible  13  at a predetermined rotation speed. In this process, vertical thermal gradient G in the silicon single crystal ingot  25  shows uniform radial distribution. Thus, it is possible to reduce the variation in V/G along the radial distance. As a result, a silicon single crystal ingot  25  which is defect-free and of high quality throughout nearly the whole length can be produced relatively easily in accordance with the V/G model of Voronkov. 
     INDUSTRIAL APPLICABILITY 
     By pulling a silicon single crystal using the apparatus for pulling a single crystal according to the present invention, it is possible to prevent impurities, for example, oxygen which, is dissolved from the wall of the quartz crucible and contaminates in the melt, from concentrating to the central portion of the surface of the silicon melt. Therefore, the amount of oxygen as an impurity contaminated into the silicon single crystal can be reduced sufficiently. 
     In the apparatus for pulling a single crystal according to the present invention, it is possible to control the shape of solid-liquid interface directly beneath the silicon single crystal ingot. By pulling the silicon single crystal ingot from the silicon melt in accordance with a predetermined pulling rate profile, it is possible to produce a silicon single crystal ingot interior of which is free of aggregates of interstitial silicon type point defects.