Monocrystal growing method

Abstract: In a method of growing a monocrystalline silicon based on the Czochralski method, the thermal Rossby number is set to at least 30 to develop the convection of a liquid of melted silicon in an axially symmetric form. A tube made of quartz is inserted in the silicon liquid to supply oxygen thereto to control the oxygen density of a resultant crystal. According to this method, there can be grown a silicon monocrystal which contains oxygen in an arbitrary density range from 10¹⁵/cm³ to 10¹⁸/cm³ and which has a uniform oxygen density variation not exceeding 5% in a plane vertical to a direction of the crystal growth.

Description: BACKGROUND OF THE INVENTION The present invention relates to a method of growing a monocrystal of silicon according to a Czochralski method. Description of the Related Art Conventionally, when a monocrystal of silicon is grown in the Czochralski method, in order to control density of oxygen in the crystal and to obtain a uniform distribution of temperature in the liquid melted in a crucible, the crucible is rotated at a high rotary speed. However, when the crucible is turned in such a high rotary speed, convection of the melted liquid becomes to be unstable. Consequently, in the obtained crystal, there appear stripes due to irregular growth thereof. Namely, it is impossible to grow a monocrystalline silicon uniformly containing oxygen therein. In the conventional method of growing a silicon monocrystal according to the Czochralski method, the density of oxygen in the crystal and the uniform temperature distribution in the melted liquid are controlled only by the rotary speed of the crucible. This however cannot control the convection of the liquid and hence there cannot be attained a monocrystal of silicon having a uniform distribution of oxygen therein. A method for regulating concentration and distribution of oxygen in Czochralski grown silicon is known form EP-A-0 055 619. The oxygen concentration and distribution within silicon rods drawn from silicon melt contained in a silica crucible are regulated through variation of both the magnitude and relative sense of direction of seed and crucible rotation rates with uniform distribution of the oxygen being accomplished by increasing crucible rotation rate to preselected values as a function of crystal rod growth and melt consumption. It is therefore an object of the present invention to provide a monochrystal growing method in which the crystal contains oxygen in an arbitrary density range form 10 15 /cm 3 to 10 18 /cm 3 and which has a uniform oxygen density, namely the variation thereof not exceeding 5 % in a plane vertical to a direction of the crystal growth. In accordance with the present invention said object is achieved with the features of independent claim 1. Preferred embodiments are mentioned in the subclaims. The monocrystal growing method according to the present invention particularly comprises the following features. The thermal Rossby number is set to at least 30 when a monocrystal of silicon is grown according to the Czochralski method in which the oxygen density is controlled with a tube made of guartz inserted into the melted silicon. The convection is developed in an axially symmetric contour in the melted liquid to be symmetric with respect to an axis thereof, thereby supplying oxygen uniformly into a plane perpendicular to a direction of crystal growth. For this purpose, the thermal Rossby number is set to at least 30. Moreover, a tube made of quartz in a cylindrical shape is inserted into the liquid to develop an oxygen density in an arbitrary range. According to the crystal growing method, there can be obtained a crystal containing oxygen in an arbitrary density range from 10 15 /cm 3 to 10 18 /cm 3 and having a uniform oxygen density variation not exceeding 5% in a plane vertical to a direction of the crystal growth. When the thermal Rossby number is set to at least 30, the convection of the liquid is developed in an axially symmetric form, which has been confirmed in the X-ray radiography. In the convection formed in such a symmetric contour, oxygen mixed in the liquid is uniformly transported so that the oxygen density variation in the resultant crystal does not exceed 5% in a plane vertical to a direction of the crystal growth. The thermal Rossby number (RoT) is determined according to the following expression. Rot = gd β ΔT/ω 2 r 2 where, letters g, β, ΔT, d, r, and ω respectively stand for an acceleration of gravity, a coefficient of thermal expansion, a temperature difference, a height of the melt a radius of the melt and a rotary speed of the crucible. The thermal Rossby number can be considered a number without dimension representing an effect of the rotation of the crucible on the natural convection due to buoyancy. It can be considered that the greater the thermal Rossby number is, the more dominant the natural convection becomes, which hence facilitates the convection to be developed in an axially symmetric shape. As above, in the convection of the melt forming the axially symmetric flow, a portion of the quartz tube is melted as above in the melt running in the symmetric flow, thereby growing a monocrystal of silicon having an arbitrary oxygen density in an arbitrary range from 10 15 /cm 3 to 10 18 /cm 3 and an oxygen density variation not exceeding 5% in a plane orthogonal to a direction of the crystal growth. BRIEF DESCRIPTION OF THE DRAWINGS The objects and features of the present invention will become more apparent from the consideration of the following detailed description taken in conjunction with the accompanying drawings in which: Fig. 1 is a conceptual diagram showing the constitution of an apparatus for growing a crystal in accordance with the present invention; and Fig. 2 is a graph for explaining a method of growing a crystal in accordance with the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawings, description will be given of embodiments in accordance with the present invention. Fig. 1 shows an apparatus for growing a crystal in an embodiment in accordance with the present invention. The system of Fig. 1 includes a quartz crucible 1 for keeping therein a solution of melted silicon, a silicon melt 2, a silicon monocrystal 3, a heater 4, a carbon retainer 5 for fixedly retaining the quartz crucible 1, a shaft 6 for rotating the crucible 1, and a quartz tube 7 for supplying oxygen to the silicon liquid. In this embodiment of the method of growing a monocrystal, there is employed a three-inch crucible 1 having a diameter of three inches and the silicon melt 2 is prepared such that the proportion of the radius of the melt to the height or depth thereof is substantially 1 : 1 and the resultant silicon monocrystal 3 has a diameter of two inches. Under this condition, the rotary speed of the crucible 1 is varied from 0 rotations per minute (rpm) to 8 rpm in two cases of temperature discrepancy, namely, 55 K and 80 K between a highest temperature of the liquid 2 and a lowest one thereof. Furthermore, to obtain an oxygen density 10 16 /cm 3 , the quartz tube 7 is positioned to set a top thereof to be three meters apart from a surface of the melt 2. Fig. 2 is a graph showing results of crystal growth in association with relationships between the conditions of growing the crystal represented in terms of the thermal Rossby number and the Taylor number. Taylor number (Ta) is expressed as follows. Ta = (r/d)4 ω 2 r 4 /ν 2 where, ν designates a coefficient of kinematic viscosity of the silicon melt. For the melted silicon, the coefficient of viscosity is almost unchanged with respect to a variation in the temperature and hence can be considered to be fixed. Consequently, the Tailor number is regarded as a number without dimension representing the shape and rotation speed of the solution. In this graph, each of lines drawn from the upper-left corner to the lower-right corner is associated with a fixed temperature difference. According to this method, as embodiments of the present invention, monocrystals are grown under conditions indicated by points 1 to 4 denoted by small circles on the lines where the thermal Rossby number is at least 30. For each of the resultant crystals, measurements have been conducted for the variation in the oxygen density thereof in a plane vertical to the direction of crystal growth by a Fourier transform infrared absorption spectrometer (FT-IR) of a scanning type. Table 1 shows the measured values. Embodiment Rotary speed of crucible (rpm) Temperature difference ( K) Thermal Rossby No. Variation density(%) 1 1 55 230.0 2.5 2 2 55 56.1 3.8 3 1 80 330.3 2.0 4 2 80 82.4 3.5 Comparison example 1 6 55 6.3 13.5 2 8 55 3.5 20.8 3 8 80 5.2 17.9 Moreover, Fig. 2 shows results, denoted by small triangles, of silicon crystal growth under conditions where the thermal Rossby number is at most 30 and where the convection is formed in other than an axially symmetric contour. As can be seen from this table, in each silicon crystal grown when the thermal Rossby number is 30 or more, the variation in the oxygen density is restricted to be at most 5%. In addition, description will be given of embodiments 5, 6, and 7. Setting the temperature of the melt to 55 K and fixing the rotary speed of the crucible to one rpm, the operation to grow monocrystalline silicon is accomplished by changing the position of the top of the quartz tube 7 relative to the surface of the melt to adjust the amount of oxygen to be mixed therein. The distance between the top of the tube 7 and the liquid surface is selected as one, five, and ten millimeters. Resultant monocrystals are investigated by a Fourier transform infrared absorption spectrometer (FT-IR) of a scanning type to measure the oxygen density in the silicon monocrystal 3 and the variation in the oxygen density in a plane vertical to the direction of crystal growth. Table 2 shows the measured values. Embodiment Position of quartz tube (mm) Oxygen density (/cm 3 ) 5 1 10 15 6 5 10 16 7 10 10 18 As can be seen from Table 2, by moving the position of the top of the quartz tube 7 to alter the volume of oxygen mixed in the melt, the oxygen density of the monocrystalline silicon 3 can be arbitrarily set in a range from about 10 15 /cm 3 to about 10 18 /cm 3 . Moreover, using a crucible 1 having a diameter of 15 inches and a crucible having a diameter of 20 inches, the monocrystal growth is conducted. In this case, the resultant crystals have diameters of eight and ten inches, respectively. In addition, to set the oxygen density to 10 17 /cm 3 , the top of the quartz tube 7 is situated at a position seven millimeters apart from the liquid surface. As embodiments 7, 8, and 9, the crystal growth is achieved with the temperature discrepancy fixed to 55 K and the thermal Rossby number selected to be at least 30. Table 3 shows measured results of the embodiments. Embodiment Diameter of crucible (inches) Rotary speed of crucible (rpm) Thermal Rossbynumber Change in oxygen density(%) 8 15 1 40.8 4.2 9 20 1 31.6 4.5 10 20 0.5 126.4 3.2 As shown in this table, also in the case where large-sized crucibles are used, it has been confirmed that when the thermal Rossby number is 30 or more, there can be obtained a monocrystal of silicon having a uniform oxygen density, namely, the variation in the oxygen density is at most 5%. As above, it is confirmed that when the convection of the melt is developed in an axially symmetric form, there can be grown a monocrystalline silicon having an oxygen density in an arbitrary range from 10 15 /cm 3 to 10 18 /cm 3 and a uniform oxygen distribution, namely, the variation in the oxygen density is at most 5% in a plane orthogonal to the direction of crystal growth. In accordance with the present invention as described above, there is advantageously grown a uniform monocrystal of silicon according to the Czochralski method in which the variation in the oxygen density is at most 5% in a plane vertical to the direction of crystal growth.

Claims: A monocrystal growing method of growing a monocrystal of silicon based on a Czochralski method comprising the steps of: preparing a silicon melt (2) in a crucible (1) such that the proportion of the radius of said melt (2) to the depth of said melt (2) is substantially 1:1; setting the Thermal Rossby number to 30 or more; inserting a tube (7) made of quartz in said silicon melt (2) and supplying oxygen from said quartz tube (7) to said solution, thereby controlling the density of oxygen in the monocrystal thus grown; and varying the difference between the highest temperature and the lowest temperature of the melt from 55 K to 80 K. A method according to claim 1 characterized in that a position of a bottom of said quartz tube (7) is about one to ten millimeters apart from the surface of said melt (2). A method according to claim 1 or 2 characterized in that the rotary speed of said crucible (1) is varied from zero rotations per minute to eight rotations per minute. A monocrystal of silicon (3) grown through a method according to claim 1, 2 or 3, characterized in that the variation of the oxygen density in a plane orthogonal to the direction of the crystal growth is at most 5 %.

Inventors: EGUCHI MINORU, WATANABE MASAHITO, EGUCHI, MINORU, WATANABE, MASAHITO, EGUCHI, MINORU, NEC CORPORATION, WATANABE, MASAHITO, NEC CORPORATION
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1993
Main classification: C30B15
IPC codes: C30B15