VGF and VB linked indium phosphide single crystal growth method

Abstract

The invention discloses a VGF and VB linked indium phosphide single crystal growth method, which comprises the following steps of: calibrating a thermal field, and loading materials into a furnace after the thermal field is calibrated; under the rotation of the PBN crucible, the cooling gradient of each temperature zone is adjusted through a VGF method, and crystal growth in the seeding-shouldering stage is completed; a constant thermal field is adopted, a VB method is adopted, and equal-diameter growth of crystals is completed by adjusting the descending speed of a PBN crucible; and discharging, demolding and wafer processing. The indium phosphide single crystal produced by the VGF and VB linked indium phosphide single crystal growth method is low in dislocation density, the electrical parameter nonuniformity of the crystal is smaller, and the method not only provides a new method for the growth of the indium phosphide single crystal with low dislocation, low stress and high uniformity, but also establishes a thermal field reference calibration method in an empty furnace state.

Description

Technical Field

[0001] The present invention relates to the field of semiconductor materials, and in particular to a VGF and VB linked indium phosphide single crystal growth method. Background Art

[0002] Indium phosphide (InP) is an important III-V compound semiconductor material with high saturated electron drift velocity, luminescence wavelength suitable for low-loss optical fiber communication, strong radiation resistance, good thermal conductivity, high photoelectric conversion efficiency, and a wide bandgap. Therefore, indium phosphide wafers can be widely used in the manufacture of optical module devices, sensor devices, high-end RF devices, etc.

[0003] The process of growing an InP single crystal is actually a temperature-controlled phase change process. First, the InP polycrystal is heated to melt it, transforming from a solid phase to a liquid phase, and then the temperature is lowered to solidify it into an InP single crystal, transforming from a liquid phase to a solid phase. Internationally, Japan's Sumitomo Electric and the United States' AXT Company use VB and VGF technologies to grow low-dislocation single crystals, respectively, and occupy more than 80% of the InP substrate supply market.

[0004] The VGF method of InP single crystal growth technology is a single crystal growth method that designs a specific temperature field distribution and temperature gradient, so that the solution solid-liquid interface moves from bottom to top at a certain speed while the crucible is fixed, and the single crystal slowly grows upward from the seed crystal end. By controlling the axial and radial temperature gradients in the furnace to maintain the optimal shape of the solid-liquid interface, it has the advantages of low dislocation and low stress.

[0005] The VB method InP single crystal growth technology is to fix the thermal field of the single crystal furnace, move the crucible or furnace body through mechanical action, and crystallize and nucleate the solid-liquid interface through the temperature gradient zone to achieve crystal growth. Because the VB technology uses the crucible to rotate and move gently downward, it has a centrally symmetrical temperature field and flow field, which has advantages in controlling the uniformity of crystal quality.

[0006] At present, there is a significant volatility problem in the thermal field of the indium phosphide single crystal furnace. The existing heating element insulation layer is mainly composed of a multi-layer filling and coating molding consisting of an inner layer of wet felt, a middle layer of zirconium-containing fiber felt, and an outer layer of zirconium-containing fiber felt. In the continuous cycle of high temperature growth and cooling to room temperature of the indium phosphide single crystal, the inner layer of wet felt and the middle layer of zirconium-containing fiber felt are non-uniform materials, and there are differences in their material composition, bulk density, and water content. Due to thermal expansion and contraction, they shrink and deform, forming gaps, and the insulation sand leaks from the gaps, causing thermal field fluctuations, which has an adverse effect on stable temperature control in production. Summary of the invention

[0007] The purpose of the present invention is to fill the technical gap of VGF-VB linkage crystal growth technology in the growth of indium phosphide single crystals based on the technical advantages of VGF method in low dislocation and low stress and VB method in high uniformity, and solve the key technology of indium phosphide single crystal with low dislocation, low stress and high uniformity. At the same time, in view of the problem of thermal field fluctuation of indium phosphide single crystal furnace, an integrated cast heating element technology is proposed to solve the problem of thermal field stability and thermal field reference calibration.

[0008] In order to solve the above problems, the present invention provides a method for growing an indium phosphide single crystal by linking VGF and VB, comprising the following steps: Step 1: thermal field calibration, after which the furnace is loaded with materials; Step 2, while the PBN crucible is rotating, the temperature gradient of each temperature zone is adjusted by the VGF method to complete the crystal growth in the seeding-shoulder release stage; Step 3, in a constant thermal field, using the VB method, the crystal isodiameter growth is completed by adjusting the descending speed of the PBN crucible; Step 4: demoulding and wafer processing.

[0009] Furthermore, for thermal field calibration, when the furnace is empty, the furnace is divided into 6 temperature zones from bottom to top. The temperature controls of the 6 temperature zones are temperature control 1, temperature control 2, temperature control 3, temperature control 4, temperature control 5 and temperature control 6 respectively. The temperatures of temperature control 1 and temperature control 2 are set to 1015±2°C, the temperatures of temperature control 3 and temperature control 4 are set to 1070±3°C, the temperatures of temperature control 5 and temperature control 6 are set to 1080±2 degrees Celsius. After keeping constant for 2 to 6 hours, the center temperature values ​​of the upper and lower thermocouple position points of the thermal field are used as the basis for judgment, among which the center temperature standard of the upper thermocouple is 1072±2°C, and the center temperature standard of the lower thermocouple is 1062±2°C. When the center temperature deviation of the upper and lower thermal position points from the standard value is ≤3°C, it is judged that the thermal field of the heating element is within the standard range.

[0010] Furthermore, the rotation speed of the PBN crucible is 0.05 r / min-25 r / min.

[0011] Furthermore, in the crystal growth during the seeding-shoulder release stage, in the seeding stage: the cooling rate of temperature zone 1 is 8-14.2℃ / h, the cooling rate of temperature zone 2 is 3.7-7.2℃ / h, the cooling rate of temperature zone 3 is 0.5-1℃ / h, the cooling rate of temperature zone 4 is 0.25-1℃ / h, the cooling rate of temperature zone 5 is 0.25-1℃ / h, and the cooling rate of temperature zone 6 is 0.25-1℃ / h; in the shoulder release stage: the cooling rate of temperature zone 1 is 3.6-6.5℃ / h, the cooling rate of temperature zone 2 is 1.8-3.5℃ / h, the cooling rate of temperature zone 3 is 0.18-0.36℃ / h, the cooling rate of temperature zone 4 is 0.1-0.2℃ / h, the cooling rate of temperature zone 5 is 0.1-0.2℃ / h, and the cooling rate of temperature zone 6 is 0.1-0.2℃ / h.

[0012] Furthermore, the PBN crucible descends at a speed of 0.5-5 mm / h.

[0013] The above technical solution of the present invention has the following beneficial technical effects: VGF+VB indium phosphide crystal growth is achieved. Compared with the VB method indium phosphide single crystal technology, the dislocation density is low, and the maximum dislocation at the tail of a 4-inch indium phosphide crystal grown by VGF+VB linkage can be reduced to ≤300cm -2 ; Compared with the VGF method of indium phosphide single crystal technology, the electrical parameter non-uniformity of the crystal is smaller, and the electrical parameter non-uniformity of the tail is ≤±6%, which provides a new method for the growth of low dislocation, low stress and high uniformity indium phosphide single crystals; a thermal field reference calibration method under an empty furnace state is established, and the center temperature values ​​of the upper and lower thermocouple position points (T7 and T2) of the fixed thermal field are used as the basis for judgment. The center temperature standard of the upper thermocouple (T7) is 1072±2℃, and the center temperature standard of the lower thermocouple (T2) is 1062±2℃. When the center temperature deviation of the upper and lower thermal position points is ≤3℃ from the standard value, it is judged that the thermal field of the heating element is within the standard range, which can increase the yield of indium phosphide single crystal by 5-10%. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 It is a flow chart of the method for growing an indium phosphide single crystal by linking VGF and VB; Figure 2 This is a diagram of the structure of a single crystal furnace for the VGF and VB linked indium phosphide single crystal growth method; Figure 3 It is a schematic diagram of the temperature zones of the VGF and VB linked indium phosphide single crystal growth method; Figure 4 It is a 4-inch InP single crystal grown by the InP single crystal growth method of VGF and VB linkage; Figure 5 This is the dislocation diagram of a 4-inch InP single crystal grown by the InP single crystal growth method using VGF and VB linkage; Figure 6 These are the electrical parameters of 4-inch InP single crystals grown by the InP single crystal growth method combining VGF and VB. DETAILED DESCRIPTION

[0015] In order to make the purpose, technical scheme and advantages of the present invention clearer, the present invention is further described in detail below in conjunction with specific embodiments and with reference to the accompanying drawings. It should be understood that these descriptions are only exemplary and are not intended to limit the scope of the present invention. In addition, in the following description, the description of well-known structures and technologies is omitted to avoid unnecessary confusion of the concept of the present invention.

[0016] like Figure 1 As shown, a method for growing an indium phosphide single crystal by linking VGF and VB is provided, comprising the following steps: Step 1: thermal field calibration, after which the furnace is loaded with materials; Step 2, while the PBN crucible is rotating, the temperature gradient of each temperature zone is adjusted by the VGF method to complete the crystal growth in the seeding-shoulder release stage; Step 3, in a constant thermal field, using the VB method, the crystal isodiameter growth is completed by adjusting the descending speed of the PBN crucible; Step 4: demoulding and wafer processing.

[0017] When calibrating the thermal field, Figure 2 and Figure 3 As shown, in the empty furnace state, the furnace is divided into 6 temperature zones from bottom to top, and the temperature controls of the 6 temperature zones are respectively temperature control 1, temperature control 2, temperature control 3, temperature control 4, temperature control 5 and temperature control 6. The temperatures of temperature control 1 and temperature control 2 are set to 1015±2°C, the temperatures of temperature control 3 and temperature control 4 are set to 1070±3°C, and the temperatures of temperature control 5 and temperature control 6 are set to 1080±2 degrees Celsius. After constant temperature for 2 to 6 hours, the central temperature values ​​of the upper and lower thermocouple position points of the thermal field are used as the basis for judgment. The upper thermocouple temperature is T7, and the lower thermocouple temperature is T2. The central temperature standard of the upper thermocouple is 1072±2°C, and the central temperature standard of the lower thermocouple is 1062±2°C. When the central temperature deviation of the upper and lower thermal position points is ≤3°C from the standard value, it is judged that the thermal field of the heating element is within the standard range.

[0018] The rotation speed of the PBN crucible is 0.05 r / min-25 r / min, preferably 2-20 r / min.

[0019] During the crystal growth in the seeding-shoulder release stage, in the seeding stage: the cooling rate of temperature zone 1 is 8-14.2℃ / h, the cooling rate of temperature zone 2 is 3.7-7.2℃ / h, the cooling rate of temperature zone 3 is 0.5-1℃ / h, the cooling rate of temperature zone 4 is 0.25-1℃ / h, the cooling rate of temperature zone 5 is 0.25-1℃ / h, and the cooling rate of temperature zone 6 is 0.25-1℃ / h; in the shoulder release stage: the cooling rate of temperature zone 1 is 3.6-6.5℃ / h, the cooling rate of temperature zone 2 is 1.8-3.5℃ / h, the cooling rate of temperature zone 3 is 0.18-0.36℃ / h, the cooling rate of temperature zone 4 is 0.1-0.2℃ / h, the cooling rate of temperature zone 5 is 0.1-0.2℃ / h, and the cooling rate of temperature zone 6 is 0.1-0.2℃ / h.

[0020] The PBN crucible descending speed is 0.5-5mm / h.

[0021] In the specific embodiment, embodiment 1: (1) A heating element formed by wet vacuum integral casting of zirconium-containing ceramic fiber wool is used, and the thermal insulation density of the zirconium-containing ceramic fiber wool is 420kg / m³.

[0022] (2) After the heating element is assembled, in the empty furnace state, set the temperature of temperature control 1 and temperature control 2 to 1015°C, the temperature of temperature control 3 and temperature control 4 to 1072°C, and the temperature of temperature control 5 and temperature control 6 to 1080°C. After keeping constant for 4 hours, measure the center temperature values ​​of the upper and lower thermocouple position points (T7 and T2) of the fixed thermal field. The center temperature standard of T7 is 1071.9°C, and the center temperature standard of the lower thermocouple (T2) is 1060.8°C, which meets the thermal field benchmark.

[0023] (3) After the furnace is loaded according to the standard method in the industry, the PBN crucible is started to rotate at a speed of 2r / min.

[0024] (4) Single crystal growth The VGF+VB linkage single crystal growth scheme is shown in Table 1: Growth stage Time / h Temperature Zone 1 Temperature Zone 2 Temperature zone 3 Temperature zone 4 Temperature Zone 5 Temperature Zone 6 Pulling speed mm / h Rotation r / min seeding 3 10 4.67 0.67 0.33 0.33 0.33 0 2 Shoulders 30 4.5 2.43 0.23 0.13 0.13 0.13 0 2 Equal diameter 60 0 0 0 0 0 0 2 2 ending 18 0 0 0 0 0 0 2 2 Table 1 (5) After demolding, the conventional indium phosphide wafer processing process is followed to finally obtain indium phosphide wafers. The crystal quality is characterized by dislocation density, uniformity of electrical parameters of the head and tail wafers, and double crystal swing half-peak width. The test results are shown in Table 4.

[0025] Embodiment 2: (1) A heating element formed by wet vacuum integral casting of zirconium-containing ceramic fiber wool is used, and the thermal insulation density of the zirconium-containing ceramic fiber wool is 480kg / m³ for standby use.

[0026] (2) After the heating element is assembled, in the empty furnace state, set the temperature of temperature control 1 and temperature control 2 to 1017°C, the temperature of temperature control 3 and temperature control 4 to 1073°C, and the temperature of temperature control 5 and temperature control 6 to 1082°C. After keeping constant for 6 hours, measure the center temperature values ​​of the upper and lower thermocouple position points (T7 and T2) of the fixed thermal field. The center temperature standard of T7 is 1073..5°C, and the center temperature standard of the lower thermocouple (T2) is 1063.8°C, which meets the thermal field benchmark.

[0027] (3) After the furnace is loaded according to the industry standard method, the PBN crucible is started to rotate at a speed of 12r / min.

[0028] (4) Single crystal growth The VGF+VB linkage single crystal growth scheme is shown in Table 2: Growth stage Time / h Temperature Zone 1 Temperature Zone 2 Temperature zone 3 Temperature zone 4 Temperature Zone 5 Temperature Zone 6 Pulling speed mm / h Rotation r / min seeding 3 14.2 6.6 0.95 0.95 0.95 0.95 0 12 Shoulders 30 6.5 3.5 0.33 0.18 0.18 0.18 0 12 Equal diameter 60 0 0 0 0 0 0 3 12 ending 18 0 0 0 0 0 0 3 12 Table 2 (5) After demolding, the conventional indium phosphide wafer processing process is followed to finally obtain indium phosphide wafers. The crystal quality is characterized by dislocation density, uniformity of electrical parameters of the head and tail wafers, and double crystal swing half-peak width. The test results are shown in Table 4.

[0029] Embodiment 3: (1) A heating element formed by wet vacuum integral casting of zirconium-containing ceramic fiber wool is used, and the thermal insulation density of the zirconium-containing ceramic fiber wool is 350kg / m³ for standby use.

[0030] (2) After the heating element is assembled, in the empty furnace state, set the temperature of temperature control 1 and temperature control 2 to 1013°C, the temperature of temperature control 3 and temperature control 4 to 1070°C, and the temperature of temperature control 5 and temperature control 6 to 1078°C. After keeping constant for 2 hours, measure the center temperature values ​​of the upper and lower thermocouple position points (T7 and T2) of the fixed thermal field. The center temperature standard of T7 is 1070.1°C, and the center temperature standard of the lower thermocouple (T2) is 1062.3°C, which meets the thermal field benchmark.

[0031] (3) After the furnace is loaded according to the industry standard method, start the PBN crucible rotation at a speed of 8r / min.

[0032] (4) Single crystal growth The VGF+VB linkage single crystal growth scheme is shown in Table 3: Growth stage Time / h Temperature Zone 1 Temperature Zone 2 Temperature zone 3 Temperature zone 4 Temperature Zone 5 Temperature Zone 6 Pulling speed mm / h Rotation r / min seeding 3 11.4 5.3 0.8 0.8 0.8 0.8 0 8 Shoulders 30 5.8 2.8 0.3 0.15 0.15 0.15 0 8 Equal diameter 60 0 0 0 0 0 0 5 8 ending 18 0 0 0 0 0 0 5 8 Table 3 (5) After demolding, the conventional indium phosphide wafer processing process is followed to finally obtain indium phosphide wafers. The crystal quality is characterized by dislocation density, uniformity of electrical parameters of the head and tail wafers, and double crystal swing half-peak width. The test results are shown in Table 4.

[0033] Test results table Table 4 like Figure 4 Shown is a 4-inch indium phosphide single crystal grown by the indium phosphide single crystal growth method of the present invention in which VGF and VB are linked.

[0034] like Figure 5 As shown in the figure, the InP single crystal growth technology using the VGF-VB combined method has significant advantages in controlling low dislocation density. In the process of InP growth by the VGF method, the temperature of each temperature zone continues to drop, and the cooling of the overall thermal field cannot be completely uniform. There are inevitable local temperature fluctuations, which lead to temperature fluctuations at the growth interface and generate thermal stress, and dislocations are prone to multiplication. In the process of InP growth by the VGF and VB linkage growth, the radial temperature of the crystal is more uniform based on the rotation of the crucible, the radial temperature gradient becomes smaller, and the thermal stress on the growth interface is reduced accordingly. In the process of equal diameter, the thermal field is in a constant state, and the crystal grows only by the descent of the crucible. The constant thermal field reduces the possibility of temperature fluctuations, and the probability of the crystal being affected by temperature fluctuations is reduced, and it is less affected by thermal stress, resulting in a smaller dislocation density.

[0035] like Figure 6 As shown, the resistivity data obtained by Hall detection is Figure 6 (a) Figure 6 As shown in (b): the VGF-4.5S23C87 crystal grown by the VGF method has a head resistivity consistency deviation of -1%~3%, and a tail resistivity radial consistency deviation of -10%~5%; the VGF-VB-4.5S23C51 crystal grown by the VGF-VB combined method has a head resistivity consistency deviation of -1%~2%, and a tail resistivity consistency deviation of -2%~1%. The radial uniformity of the head resistivity of the crystals grown by the two processes is consistent. This is because the shoulder release part of both processes is grown by VGF. Although the shoulder release process under the VGF-VB linked growth increases the crucible rotation, there are few InP crystals in the early stage of growth, and most of them are still in a solution state. There is relatively little heat transfer through the crystal, and the solid-liquid interface is in a relatively flat state. The doping impurities in the early stage of growth are evenly distributed, showing good resistivity consistency at the crystal head. Figure 6 In (b), it is observed that the tail resistivity of VGF-4.5S23C87-T shows a trend of gradually decreasing in the four radial directions. This phenomenon indicates that when the VGF-4.5S23C87 crystal grows to the tail, the center first crystallizes and the edge crystallizes later. Since the segregation coefficient of the dopant S in InP is 0.68, most of the dopant is segregated to the part that crystallizes later, indicating that the radial temperature of the VGF method InP process from the equal diameter to the tail single crystal growth link is uneven, the center is low and the edge is high, and the solid-liquid interface is convex to the melt distribution; compared with the VGF-VB combined method InP process, the equal diameter stage is affected by the rotation of the crucible, and the radial gradient is always evenly distributed, so the crystal is subjected to less radial thermal stress, the solid-liquid interface is flat, and the doping distribution is uniform, showing high resistivity uniformity.

[0036] In the InP single crystal, the carrier concentration tends to increase gradually from the head to the tail, and the closer to the tail, the faster the carrier concentration changes. This is because the solid-liquid interface moves from bottom to top, the volume of the melt gradually decreases, and the segregation phenomenon causes the concentration of doped S impurities in the melt to gradually increase until it is completely crystallized. The S doping concentration gradually increases and contributes to carriers, which increases the carrier concentration. The later the solidified crystal changes, the faster it changes, which is consistent with the change of S doping concentration. The carrier concentration data obtained by Hall detection are shown in Figure 2. Figure 6As shown in (c): the VGF-4.5S23C87 crystal grown by the VGF method has a head carrier concentration consistency deviation of -3%~3%, and a tail carrier concentration radial consistency deviation of -8%~15%; the VGF-VB-4.5S23C51 crystal grown by the VGF-VB linkage method has a head carrier concentration consistency deviation of -3%~3%, and a tail carrier concentration consistency deviation of -6%~6%. The radial uniformity of the head carrier concentration of the crystals grown by the two processes is consistent, which is also based on the fact that both processes use VGF growth in the shoulder area. In the early stage of growth, there are few InP crystals, and most of them are still in a solution state. The heat transfer area of ​​the crystal at the shoulder position is small, and it is less affected by the high temperature fluctuation of the heating resistor wire, and the solid-liquid interface is in a relatively flat state; similarly, from Figure 6 (d) It is observed that the tail carrier concentration of VGF-4.5S23C87-T shows a slightly increasing trend in the four radial directions from the center to the edge, which also confirms that the solid-liquid interface of the VGF-4.5S23C87 crystal using the VGF method InP process in the tail single crystal growth stage is convex to the melt, while the VGF-VB-4.5S23C51 crystal uses the VGF-VB combined method InP process. In the equal-diameter stage, due to the fixed thermal field conditions, the radial gradient distribution is uniform under the rotation and movement of the crucible, the solid-liquid interface is flat, and the doping distribution is uniform, thus showing a high uniformity of carrier concentration distribution.

[0037] It should be understood that the above specific embodiments of the present invention are only used to illustrate or explain the principles of the present invention, and do not constitute a limitation of the present invention. Therefore, any modifications, equivalent substitutions, improvements, etc. made without departing from the spirit and scope of the present invention should be included in the protection scope of the present invention. In addition, the appended claims of the present invention are intended to cover all changes and modifications that fall within the scope and boundaries of the appended claims, or the equivalent forms of such scope and boundaries.

Claims

1. A method for growing an indium phosphide single crystal by linking VGF and VB, characterized in that: The following steps are involved: Step 1: thermal field calibration, after which the furnace is loaded with materials; Step 2, while the PBN crucible is rotating, the temperature gradient of each temperature zone is adjusted by the VGF method to complete the crystal growth in the seeding-shoulder release stage; Step 3, in a constant thermal field, using the VB method, the crystal isodiameter growth is completed by adjusting the descending speed of the PBN crucible; Step 4: demoulding and wafer processing.

2. The VGF and VB linked indium phosphide single crystal growth method according to claim 1, characterized in that: For the thermal field calibration, in an empty furnace state, the furnace is divided into 6 temperature zones from bottom to top, and the temperature controls of the 6 temperature zones are respectively temperature control 1, temperature control 2, temperature control 3, temperature control 4, temperature control 5 and temperature control 6. The temperatures of temperature control 1 and temperature control 2 are set to 1015±2°C, the temperatures of temperature control 3 and temperature control 4 are set to 1070±3°C, and the temperatures of temperature control 5 and temperature control 6 are set to 1080±2 degrees Celsius. After keeping constant for 2 to 6 hours, the center temperature values ​​of the upper and lower thermocouple position points of the thermal field are used as the basis for judgment, wherein the center temperature standard of the upper thermocouple is 1072±2°C, and the center temperature standard of the lower thermocouple is 1062±2°C. When the center temperature deviation of the upper and lower thermal position points from the standard value is ≤3°C, it is judged that the thermal field of the heating element is within the standard range.

3. The method for growing indium phosphide single crystals by linking VGF and VB according to claim 1, characterized in that: The rotation speed of the PBN crucible is 0.05 r / min-25 r / min.

4. The method for growing indium phosphide single crystals by linking VGF and VB according to claim 1, characterized in that: In the crystal growth in the seeding-shoulder release stage, in the seeding stage: the cooling rate of temperature zone 1 is 8-14.2°C / h, the cooling rate of temperature zone 2 is 3.7-7.2°C / h, the cooling rate of temperature zone 3 is 0.5-1°C / h, the cooling rate of temperature zone 4 is 0.25-1°C / h, the cooling rate of temperature zone 5 is 0.25-1°C / h, and the cooling rate of temperature zone 6 is 0.25-1°C / h; in the shoulder release stage: the cooling rate of temperature zone 1 is 3.6-6.5°C / h, the cooling rate of temperature zone 2 is 1.8-3.5°C / h, the cooling rate of temperature zone 3 is 0.18-0.36°C / h, the cooling rate of temperature zone 4 is 0.1-0.2°C / h, the cooling rate of temperature zone 5 is 0.1-0.2°C / h, and the cooling rate of temperature zone 6 is 0.1-0.2°C / h.

5. The VGF and VB linked indium phosphide single crystal growth method according to claim 1, characterized in that: The PBN crucible descends at a speed of 0.5-5 mm / h.

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