Method for the preparation of a silicon single crystal rod with uniform distribution of crystal defects and use of an apparatus therefor

Abstract: Proposed is an improvement in the method and single crystal growing chamber for the preparation of a single crystal rod of silicon by the Czochralski method, according to which the distance between the surface of the melt of silicon contained in a crucible and the lower surface of the top wall of the crystal growing chamber is equal to or larger than the diameter of the crucible and the heat-insulating cylinder surrounding the crucible containing the melt of silicon and the heater has such a height as to reach the lower surface of the top wall of the chamber so as to keep the single crystal rod under growing is kept at a temperature not lower than 700 °C until reaching the lower surface of the top wall of the chamber thereby decreasing the density of the crystal defects of BMD type in the seed end of the single crystal rod as grown so that the uniformity in the distribution of BMD density is increased throughout the single crystal rod.

Description: BACKGROUND OF THE INVENTION The present invention relates to a method for the preparation of a silicon single crystal rod or, more particularly, to a Czochralski method for the preparation of a silicon single crystal rod with uniform distribution of crystal defects along the direction of the crystal growth as well as the use of an apparatus therefor. As is known, it is unavoidable that microscopic crystal defects are introduced into a silicon single crystal rod grown by the Czochralski method in the course of the crystal growth. In many cases, these microscopic defects are those derived mainly from oxygen precipitation and can be microscopically detected when a wafer taken from a silicon single crystal rod by slicing is subjected to a heat treatment for the oxygen precipitation followed by cleavage and the thus obtained cleaved surface is examined after preferential etching. These microscopic defects are called BMDs (bulk microdefects). These BMDs act as a gettering site in the manufacturing process of LSIs and play a role to take up various impurities introduced in the manufacturing process of the devices so as to keep cleanness of the surface areas for the formation of the device resulting in an increased yield of acceptable devices. When the density of the BMDs is too large, on the other hand, adverse influences are caused also in the areas for the formation of the device resulting in the degradation of the performance of the device such as generation of a leakage current. When the density of the BMDs is too small, the above mentioned gettering effect cannot be high enough as a matter of course. It is important, accordingly, that the density of the BMDs introduced into a single crystal rod of silicon in the course of crystal growth is adequately controlled to be not too high nor too low. The optimum density of the BMDs naturally depends on the types of the manufacturing procedures of the device, conditions of the heat treatment, degree of in-process contamination and so on so that the yield of acceptable silicon single crystal rods largely depends on whether or not the actually obtained density of the BMDs falls within the target range of the BMD density. Nevertheless, the density of BMDs in a silicon single crystal rod grown by the Czochralski method widely differs between the head portion directly connected to the seed crystal and the tail portion even in one and the same silicon single crystal rod. For example, the density of BMDs in the head portion of a silicon single crystal rod is larger than in the tail portion usually by at least 100 times or, sometimes, by several thousands times. This fact leads to a problem that only a very small portion of a single crystal rod can meet the specific requirement relative to the density of BMDs greatly limiting the utilizability of a silicon single crystal rod. Various proposals and attempts have been made heretofore to solve this problem but no satisfactory solution of the problem has been obtained. DE 3 905 626 A1 discloses a method and an apparatus for producing silicon single crystals. The teaching of this document is to hold a silicon single crystal rod at least two hours in a temperature region of 600 to 800°C and not longer than 140 minutes in a temperature region between 850 and 1050°C. EP 0 308 308 A1 and EP 0 747 515 A2 disclose an apparatus for growing silicon single crystal rods according to the Czochralski method provided with a heater and heat insulating members surrounding the crucible. The DE 4 442 239 A1 and the EP 0 504 837 A2 also disclose methods and apparatus to produce silicon single crystals according to the Czochralski method but are silent as regards the special process conditions according to the present invention and the effects obtained by the method according to the present invention i. e. a very uniform distribution of the crystal defects or BMDs in the silicon single crystal rod. SUMMARY OF THE INVENTION The present invention accordingly has an object to provide a novel and improved Czochralski method for the growth of a silicon single crystal rod to accomplish uniformity of the density distribution of the crystal defects or BMDs in a silicon single crystal rod grown by the Czochralski method in the direction of the crystal growth or the axial direction of the single crystal rod. Thus, the method of the present invention for the preparation of a silicon single crystal rod by the Czochralski method comprises, keeping the length of time, during which a silicon single crystal rod under the process of crystal growth passes through a region where the temperature is in the range from 600 °C to 700 °C, not to exceed 80 minutes, keeping the distance between the surface of the melt of silicon contained in the crucible and the inner surface of the top wall of the crystal growing chamber to be not smaller than the diameter of the crucible and keeping the temperature of the single crystal rod under growth leaving the surface of the melt until reaching the inner surface of the top wall of the crystal growing chamber at 700 °C or higher. The apparatus used for the method according to the invention for the growth of a silicon single crystal rod by the Czochralski method comprises a heat-insulating cylinder which surrounds a crucible to contain a melt of silicon and a heater to melt the silicon in the crucible and has such a height as to reach the lower surface of the top wall of the crystal growing chamber. Further, the crystal growing chamber of the apparatus for the growth of a single crystal rod of silicon has such a height that the distance between the melt of silicon contained in a crucible and held inside of the chamber is not smaller than the diameter of the crucible in addition to the above mentioned feature relative to the height of the heat-insulating cylinder. BRIEF DESCRIPTION OF THE DRAWING Figure 1 is a graph showing distribution of the density of BMDs in a silicon single crystal rod along the direction of the single crystal growth. Figure 2A is a schematic illustration of the inside structure of a conventional single crystal growing apparatus by a vertical axial cross sectional view. Figure 2B is a schematic illustration of the inside structure of the single crystal growing apparatus used in the method of the invention by a vertical axial cross sectional view. Figure 3A is a graph showing the temperature of a silicon single crystal rod under growing as a function of the distance from the surface of the silicon melt and Figure 3B is a graph show the length of time taken by a silicon single crystal under growth passing through temperature ranges each of a 50 °C width. Figure 4 is a graph showing the density distribution of BMDs along the direction of the crystal growth in silicon single crystal rods obtained in two crystal growing apparatuses having different ratios of the crucible diameter and the height from the melt surface to the top wall of the chamber. Figure 5 is a graph showing the density distribution of BMDs along the direction of the crystal growth in silicon single crystal rods obtained in an inventive Example and in a Comparative Example. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS As is shown in the graph of Figure 1, the density of BMDs in a silicon single crystal rod grown by the Czochralski method has a distribution to be high at the end directly connected to the seed crystal, referred to as the seed end hereinafter, at the left end of the graph gradually decreasing toward the tail end at the right end of the graph. This phenomenon is presumably due to the difference in the thermal history which the seeding end and the tail end of the single crystal as grown have received in the course of the single crystal growth affecting the way and amount of the oxygen precipitation. This phenomenon, however, is more or less unavoidable in the manufacturing process of a silicon single crystal by the Czochralski method because it is absolutely impossible to have a completely identical thermal history for all portions of a silicon single crystal rod. Therefore, the silicon single crystal grown in a conventional crystal growing apparatus illustrated in Figure 2A usually has a distribution of the BMD density shown by the broken line curve I of Figure 1 rapidly decreasing from the seed end (left end of the graph) to the tail end (right end of the graph) of the single crystal rod so that the utilizability of a single crystal rod relative to the number of wafers having a BMD density falling within a target range to be taken from the single crystal rod is limited. In view of the fact that the thermal history given to a single crystal rod of silicon in the course of crystal growth can never be identical between the seed end and the tail end, the inventors have come to an idea that the silicon single crystal rod under growth is kept at a temperature of 700 °C or higher until the growing single crystal leaving the surface of the silicon melt in the crucible reaches the top wall of the crystal growing chamber so as to be able to avoid the temperature range of 600 to 700 °C where the oxygen precipitation is most deeply affected. While it is known and generally accepted that the amount of oxygen precipitation in a silicon single crystal rod grown by the Czochralski method is greatly increased by subjecting the single crystal rod to a heat treatment at a temperature of 600 °C to 700 °C (see, for example, "Silicon" by T. Abe, page 195, published by Baifukan Publishing Co., 1994). Namely, it is natural that the density of BMDs is increased with a large amount of the oxygen precipitation in the seed end of a silicon single crystal rod which has taken a longer time for passing through this temperature range than the tail end. Accordingly, the length of time taken by the seed end of a single crystal rod to pass through the temperature range of 600 °C to 700 °C could be shortened when the single crystal rod under growing in the crystal growing chamber is kept at a temperature not lower than 700 °C and then rapidly cooled after arrival at the top wall of the crystal growing chamber so that the lengths of time taken by the seed end and the tail end to pass through the temperature range of 600 °C to 700 °C can be approximately identical. With this fact kept in mind, the inventors conducted a numerical calculation to make an estimation of the thermal history of a single crystal rod grown in crystal growing chambers of different structures relative to heat insulation leading to a conclusion that the length of time taken by a silicon single crystal rod under growing to pass through the temperature range of 600 °C to 700 °C can be shortened when the crystal growing chamber has a structure as shown in Figure 2B. In this chamber, in contrast to a conventional crystal growing chamber illustrated in Figure 2A, the heat-insulating cylinder 5 surrounding the heater 4 is provided with an extension 6 to reach the top wall 1 of the crystal growing chamber so as to accomplish heat-keeping in the upper zone of the chamber and to decrease dissipation of heat by radiation from the surface of the single crystal rod 2 under growth thereby preventing decrease of the temperature of the single crystal rod 2 leaving the surface of the silicon melt contained in a crucible 3 from being lower than 700 °C until reaching the top wall 1 of the chamber. On the other hand, the heat-insulating cylinder 5 in the conventional chamber illustrated in Figure 2A does not reach the top wall 1 of the crystal growing chamber allowing dissipation of heat from the single crystal rod 2 going out of the heat-insulating cylinder 5 so that cooling of the single crystal rod 2 is promoted from above. Accordingly, the temperature of the upper part of a single crystal rod under pulling up already becomes lower than 700 °C while the single crystal is still within the chamber. As a consequence, the length of time taken by the seed end of the single crystal rod to pass through the temperature range of 600 °C to 700 °C is necessarily longer than the time taken by the tail end of the single crystal rod 2 to cause a non-uniform distribution of the BMD density within a single crystal rod 2. The upward extension of the heat-insulating cylinder 5 can be accomplished by merely extending the conventional heat-insulating cylinder 5 upwardly necessarily to reach the lower surface of the top wall 1 of the crystal growing chamber. It is sufficient to have the upper end of the extension 6 in the vicinity of the lower surface of the top wall 1 leaving a gap of a few centimeters without requiring actual direct contacting thereof with the top wall 1 of the chamber provided that the single crystal rod under growth can be kept at a temperature of 700 °C or higher until the single crystal rod 2 reaches the top wall 1 of the chamber. It is also preferable to have the distance from the surface of the silicon melt in the crucible 3 and the lower surface of the top wall 1 of the crystal growing chamber larger than the diameter of the crucible 3. This is because, when the distance from the surface of the silicon melt in the crucible and the lower surface of the top wall 1 of the chamber is sufficiently long, the region where the temperature of the growing single crystal rod 2 is not decreased to 600 to 700 °C can also be long and the growing single crystal rod 2 is thereafter cooled down rapidly at the top of the chamber so that the length of time taken to pass through the temperature region of 600 to 700 °C can be shortened leading to suppression of increase in the BMD density in the seed end. When the distance from the surface of the silicon melt in the crucible 3 and the lower surface of the top wall 1 of the chamber is smaller than the diameter of the crucible 3, on the other hand, the temperature of the top wall 1 of the chamber is necessarily increased to decrease the effect of rapid cooling at the top of the chamber and the length of time taken by the growing single crystal rod 2 to pass through the temperature region of 600 to 700 °C is increased. Figure 4 shows the density distribution of BMDs along the axial direction of silicon single crystal rods grown in an apparatus in which the distance H between the surface of the silicon melt in the crucible and the chamber top to the diameter D of the crucible, i.e. H:D, was 0.9 (curve I) and in an apparatus in which the H:D ratio was 1.2 (curve II). A silicon single crystal rod was grown by using crystal growing chambers having the above mentioned structures illustrated in Figures 2A and 2B, respectively, and the temperature distribution was measured to give a result as is shown in Figure 3A, in which the horizontal arrow indicates the position of the chamber top, by the curves I and II for the conventional chamber and the chamber used in the method of the invention, respectively, that the temperature could be kept at 700 °C or higher in the region between the surface of the melt in the crucible 3 and the top wall 1 of the chamber so that the length of time taken by the seed end of the growing single crystal rod 2 to pass through the temperature region of 600 to 700 °C could be reduced not to exceed 80 minutes as compared with the corresponding length of time of 100 minutes or longer in a conventional crystal growing chamber as is shown in Figure 3B by the curves I and II for the single crystal rods growing in the conventional chamber and the chamber used in the method of the invention, respectively. The density of BMDs in a silicon single crystal rod had a distribution as shown by the solid line curve I in Figure 1 indicating a substantial decrease in the BMD density in the seed end of the single crystal rod and increased uniformity of the BMD density throughout the single crystal rod from the seed end to the tail end. In the following, the inventive method is illustrated in more detail by way of an Example and a Comparative Example. Example and Comparative Example A fused silica glass crucible of 45,72 cm (18-inch) diameter was charged with 50 kg of polycrystalline silicon blocks as the starting material and a silicon single crystal rod of a crystallographic axial orientation of <100> having a diameter of 15,24 cm (6 inches) was grown by the Czochralski method. Two types of crystal growing chambers were used including the conventional chamber illustrated in Figure 2A and the chamber used in the method of the invention illustrated in Figure 2B in which the region from the surface of the silicon melt in the crucible 3 to the lower surface of the top wall 1 of the chamber could be kept at a temperature of 700 °C or higher. Each of the single crystal rods thus obtained was subjected to the measurement of the density of BMDs from the seed end to the tail end to give the results shown in Figure 5 by the curves I and II for the single crystal rods grown in the conventional chamber and the chamber used in the method of the invention, respectively. As is clear from the graph of Figure 5, the single crystal rod obtained in the crystal growing chamber used in the method of the invention had a decreased BMD density in the seed end to accomplish an approximately uniform distribution of the BMD density in the direction of the crystal growth as indicated by the curve II as compared with the curve I. The measurement of the BMD density was carried out in the following manner. Thus, wafers having a thickness of about 1 mm were prepared by slicing the straightly cylindrical portion of the single crystal rod and the wafers were subjected to a heat treatment at 800 °C for 4 hours and then at 1000 °C for 16 hours in an atmosphere of oxygen and the cleaved surface of the wafer was subjected to a preferential etching treatment followed by counting of BMDs using an optical microscope by scanning in the radial direction.

Claims: A method for the preparation of a silicon single crystal rod by the Czochralski method which comprises keeping the length of time taken by a silicon single crystal rod under the process of growth to pass through a region where the temperature is in the range from 600 °C to 700 °C does not exceed 80 minutes, keeping the distance between the surface of the melt of silicon contained in the crucible and the lower surface of the top wall of the crystal growing chamber to be equal to or larger than the diameter of the crucible and keeping the single crystal rod under growth leaving the surface of the melt until reaching the lower surface of the top wall of the crystal growing chamber at a temperature of 700 °C or higher. Use of a single crystal growing chamber for the preparation of a single crystal rod of silicon according to the method of claim 1 which single crystal growing chamber comprises a heat-insulating cylinder, which surrounds the crucible containing the melt of silicon and the heater to melt the silicon in the crucible, the heat-insulating cylinder having such a height as to reach the lower surface of the top wall of the crystal growing chamber.

Inventors: OHTA TOMOHIKO, SAKURADA MASAHIRO, SATO WATARU, OHTA, TOMOHIKO, SAKURADA, MASAHIRO, SATO, WATARU, Ohta, Tomohiko, 307, 89-2, Aza Haranaka, Sato, Wataru B404 Ryokufu-ryo
Applicants: SHINETSU HANDOTAI KK, SHIN-ETSU HANDOTAI COMPANY LIMITED
Application year: 1996
Main classification: C30B15
IPC codes: C30B29, H01L21, C30B33, C30B15