PULLING METHOD OF SINGLE CRYSTALS

Abstract: Pulling method of single crystals (6) by the Czochralski technique using a single-crystal pulling apparatus having a furnace (2) consisting of separately controlled heaters (7, 8), wherein the temperature at the seed holder (11) is measured and power is supplied to the heaters (7, 8) so that the temperature difference (ΔT) between the crucible (1) and the seed holder (11) follows a predetermined pattern as a function of the pulled-up length (Δx).

Description: Pulling Method of Single Crystals The invention relates to a pulling method of Single Crystals according to the opening part of claim 1. In particular this invention relates to manufacturing of single crystals by such techniques as CZ (Czochralski) method and LEC (Liquid Encapsulated Czochralski) method, and especially to a method of controlling the temperature of separately controlled heaters used in the above methods. An apparatus of a well known method according the LEC method comprises a chamber, wherein a raw material in a crucible is melted under a layer of B203 glass by heating with a heater, a seed crystal is dipped below the surface of the melt, and then a single crystal is pulled up using the seed crystal. In this method, the controlling of the temperature of the heater is essential in order to obtain the desired temperature distribution in the chamber, especially to maintain a small and constant temperature gradient across the solid-liquid interface, that is, the borderline between the solid part and liquid part, during pulling of the single crystal. An example of such a setup can be seen in Toku-ko-sho (Publication No.) 52-39787. In the case that the apparatus has only one heater, the single crystal is cooled rapidly by the convection of highpressure gas in the chamber just after the single crystal comes out of the B203 melt, and the temperature gradient across the solid-liquid interface increases. As a result, the dislocation density of the single crystal increases, especially at the tail end of the crystal. A new method has been developed for alleviating the abovementioned problem. The furnace of this method consists of two heaters a lower heater for heating the material melt and an upper heater for heating the B203 melt and pulled-up single crystal in order to maintain a small and constant temperature gradient across the solid-liquid interface while the crystal is being pulled up. Examples of such a setup can be seen in Toku-ko-sho (Publication No.) 56-28872, Toku-Kai Sho (Laying Open No.) 54-13470 and Toku-Kai-Sho (Laying Open No.) 57-11897. In this case the temperature distribution in the furnace chamber is controlled by measuring the temperatures at certain points by thermocouples. In other words, during pulling, the temperature of the lower heater is decreased gradually according to a temperature program, while the temperature of the upper heater is kept constant or changed only slightly. Therefore, the temperature of the single crystal is not directly controlled, but influenced by radiation heat from the upper heater by thermal conduction from the melt , and by cooling by convection of the gas in the chamber. As mentioned above, the method using separate heaters enables maintaining a small and constant temperature gradient across the solid-liquid interface during pulling, but the control method using thermocouples placed outside the heaters is not sufficient for optimum control of the temperature gradient across the solid-liquid interface. Accordingly, the above method cannot avoid such defects as increasing the dislocation density of the single crystal and causing damage by vaporizing the volatile components (such as As and P). It is the object of this invention to provide a method for solving the above-mentioned problems, that is, a method whereby it is easy to maintain a small and constant temperature gradient across the solid-liquid interface, to. obtain a single crystal with low dislocation density throughout the crystal, and to reduce vaporization of the volatile components. This invention is a method for manufacturing single crystals, using either the CZ or LEC technique, wherein the pulling apparatus has a heating means consisting of more than one heater, being characterized in that the temperature at the seed holder is also monitored so that the power supplied to the heaters can be individually controlled in order to make the temperature difference between the crucible and seed holder follow a predetermined program according to the length of single crystal grown. Although the seed holder may have been considered as a place for monitoring the temperature, it was never put into practice. The seed holder is the most appropriate place for monitoring the temperature of the crystal during pulling because it is located near the pulled crystal and contacts it thermally in a direct way. Monitoring the temperature of the high-pressure gas around the crystal is not suitable because the temperature distribution is large and unstable. The method of this invention is applicable to single crystals composed of m-V compounds, n-VI compounds, mixtures of them, Si semiconductors, Ge semiconductors, oxides, nitrides, and carbides which are pulled by means of the CZ method or LEC method. Brief description of drawings Figure 1 is a cross section of a conventional case wherein a single heater is used. Figure 2 is a cross section of a case wherein two heaters are used. Figure 3 is a cross section of a case wherein this invention is applied. Figure 4 (b) is an illustration of the temperature difference between the crucible and seed holder as a function of pulled distance, in the case that this invention is applied. Figure 4 (a) is a schematic repre sentation of the temperature control technique. Figure 4 (c) shows changes of temperature. Figure 5 is an illustration of etch pit densities at the front part and tail part of the grown single crystal. - 5 (1) and (2) correspond to conventional methods, and 5 (3) corresponds to the method of this invention. 1 ... crucible, 2, 12, 22 ... furnaces 3 ... raw material melt, 4 ... B203 melt, 5 ... seed crystal, 6 ... single crystal, 7, 23, 47 ... upper heaters, 8, 24, 43 ... lower heaters, 9, 10, 28, 29 ... thermocouples, 11 ... seed holder, 25, 26, 27 ... temperature monitoring points, 41, 45 ... automatic temperature controllers, 42 ... base program for controlling temperature of lower heater, 46 ... program for controlling temperature of upper heater Description of prior art: Figure 1 shows a schematic structure inside the chamber in the LEC method, wherein a raw material 3 in a crucible 1 is melted under a layer of B203 glass 4 by heating with a heater 12 , a seed crystal 5 is dipped below the surface of the melt 3 , and then a single crystal 6 is pulled up using the seed crystal 5 . In this method, the controlling of the temperature of the heater is essential in order to obtain the desired temperature distribution in the chamber, especially to maintain a small and constant temperature gradient across the solid-liquid interface, that is, the borderline between the solid part and liquid part, during pulling of the single crystal. An example of such a setup can be seen in Toku-ko-sho (Publication No.) 52-39787. In the case that the apparatus has only one heater 12 , as shown in Figure 1, the single crystal 6 is cooled rapidly by the convection of highpressure gas in the chamber just after the single crystal comes out of the B2O3 melt 4 , and the temperature gradient across the solid-liquid interface increases. As a result, the dislocation density of the single crystal increases, especially at the tail end of the crystal. A new method has been developed for alleviating the above-mentioned problem. As shown in Figure 2, the furnace 22. consists of two heaters 23 and 24- a lower heater 24 for heating the material melt 3 and an upper heater 23 for heating the B203 melt 4 and pulled-up single crystal 6 --in order to maintain a small and constant temperature gradient across the solid-liquid interface while the crystal is being pulled up. Example of such a setup can be seen in Toku-ko-sho (Publication No.) 56-28872, Toku-Kai-Sho (Laying Open No.) 54-13470 and Toku Kai-Sho (Laying Open No.) 57-11897. In the case shown in Fig. 2, the temperature distribution in the furnace chamber is controlled by measuring the temperatures at points 25, 26, and 27 by thermocouples 28, 29, and 10, respectively. In other words, during pulling, the temperature of the lower heater 24 is decreased gradually according to a temperature program, while the temperature of the upper heater 23 is kept constant or changed only slightly. Therefore, the temperature of the single crystal 6 is not directly controlled, but influenced by radiation heat from the upper heater 23 , by thermal conduction from the melt 3 , and by cooling by convection of the gas in the chamber. As mentioned above, the method using separate heaters enables maintaining a small and constant temperature gradient across the solid-liquid interface during pulling, but the control method using thermocouples placed outside the heaters is not sufficient for optimum control of the temperature gradient across the solid-liquid interface. Accordingly, the above method cannot avoid such defects as increasing the dislocation density of the single crystal and causing damage by vaporizing the volatile components (such as As and P). Hereinafter, this invention is described in detail by examples and the attached figures. Figure 3 shows a cross-sectional view of an apparatus which is one of the examples of this invention. In Figure 3, the numeral 2 is a furnace consisting of two heaters--an upper one (7) and lower one (8). The upper heater (7) is used mainly for heating the part of the single crystal (6) above the solidliquid interface and the top part of the B203 melt (4). The lower heater (8) is used mainly for heating the raw material melt (3). In this invention, temperature measuring is carried out by a thermocouple (9) at the seed holder (11) and a thermocouple (10) at the bottom of the crucible (1). Such temperatures are controlled as follows. As a condition for maintaining the optimal temperature gradient across the solid-liquid interface during pulling, the temperature difference between the bottom of the crucible (1) and the seed holder (11) is intentionally varied by controlling the power supply to the upper heater (7) as a function of the length of crystal grown. Figure 4 (a) shows the above control technique schematically. The temperature at the bottom of the crucible is conveyed to an automatic EMI8.1 controller temperature (41) and power is supplied to the lower heater (43) according to a base program for pulling, while a detector (44) determines the temperature difference (AT) between the bottom of the crucible and the seed holder and conveys it to another automatic temperature controller (45) for supply of power to the upper heater (47) according to a program for controlling the upper heater temperature. One of the patterns of the temperature difference (AT) is shown in Figure 4 (b), wherein the slope of bT changes at the point where the top of the crystal rises above the top of the B203 layer. Preferably, the slope when the top of the crystal is above the 3203 should be smaller than the slope when the top of the crystal is within the 3203 layer in order to slow down the cooling speed when the crystal is exposed to the convection of highpressure gases after it rises above the layer of 3203. Further, within each of these two stages, the temperature difference (AT) is preferably changed in proportion to the pulled distance (vex). Figure 4 (c) shows the change of temperature difference (AT) as pulling progresses. Although in Figure 3 the furnace (2 ) consists of two heaters, this invention is not limited to the configuration described above and furnaces consisting of more than two heaters can be applied similarly. Embodiment: A single crystal of GaAs compound was pulled by a modified LEC method utilizing this invention with two heaters. About 1.5kg of polycrystalline material and 240g of B203 were melted in a crucible of 4" diameter. During pulling, the temperature at the bottom of the crucible (1) and the temperature at the seed holder (11) were measured, and the power supplied to the upper heater (7) was controlled so that the above-mentioned temperature difference follows the pattern described in Figure 4 (b) with the progress of drawing-up. The 2"-diameter, 12 cm-long GaAs single crystal was grown with a pulling speed of 6 to 7 mm per hour. For comparison, crystal growth by the other two methods was carried out. In the first method, a single heater was used as shown in Figure 1. In the other method, two heaters were used as shown in Figure 2 (conventional method). The front part ana tail part of the single crystals produced as above were cut into wafers. The surfaces of the wafers were polished and etched, and then their EPD (etch pit density) was measured. Figure 5 showes the results of the measurement: (1), (2), and (3) correspond to the case wherein a single heater was used, the conventional case wherein two heaters were used, and the case of this invention, respectively. It can be seen from these figures that, compared with the EPD of wafers obtained by the conventional methods, the EPD of wafers obtained from the tail part of a single crystal produced by the method of this invention was greatly reduced. The EPD of case 3 is about two-thirds to one-half of the EPD of case 1 and about four-fifths to three-quarters of the EPD of case 2. Effect of invention: The crystal growth method of this invention as described above has the following effects. O By means of using a single-crystal pulling apparatus having a furnace consisting of separately controlled heaters, measuring the temperature at the seed holder, and controlling the power supplied to the heaters so that the difference between the temperature at the crucible and the temperature at the seed holder follows a predetermined pattern as a function of the length of single crystal grown, it is possible to precisely control individual heaters so as to maintain the temperature gradient across the solidliquid interface during pulling the single crystal. Therefore, single crystals having low dislocation density throughout the crystal can be obtained. O2 Because the temperature at the seed holder is always monitored and controlled, it is also possible to control it in order to significantly reduce the vaporizing of valatile components (for example, As, P, etc.) from the surface of the single crystal, and thus there are fewer defects in the crystal structure.

Claims: Claims: 1. Pulling method of single crystals (6) by the Czochralski technique using a single-crystal pulling apparatus having a furnace (2) consisting of separatelycontrolled heaters (7, 8), wherein the temperature at the seed holder (11) is measured and power is supplied to the heaters (7, 8) so that the temperature difference (#T) between the crucibel (1) and the seed holder (11) follows a predeter mined pattern as a function of the pulled-up length (#x) 2. Pulling method of single crystals as set forth in claim, 1, wherein (AT) is in proportion to ( x) 3. Pulling method of single crystals (6) as set forth in claim 2, wherein (AT/Ax) is within the range of 30 to 60 C/cm 4. Pulling method of single crystals (6) as set forth in claim 3, wherein the (AT/AX) at the first stage is larger than the (T/Ax) at the successive stage 5. Pulling method of single crystals (6) as set forth in claim 4, wherein the (AT/Ax) at the first stage is about 50 C/cm, and the (AT/Ax) at the successive stage is about 400C/cm

Inventors: KOTANI, TOSHIHIRO
Applicants: SUMITOMO ELECTRIC INDUSTRIES LIMITED
Application year: 1983
Main classification: C30B15
IPC codes: H01L21, C30B15, C30B29, C30B27