CN103060891B - Device and method for directionally growing fluoride single crystal via lifting small-gradient thermal field - Google Patents

Abstract

The invention discloses a device and a method for directionally growing fluoride single crystal via lifting a small-gradient thermal field, which belongs to the technical field of crystal growth. The prior art has low success rate and bad crystal quality. The device disclosed by the invention is characterized in that a crucible is installed on the upper side of the bottom part of an insulating sleeve via a crucible pedestal; electrode holders which are opposite two by two are connected via connection beams, the middle points of the connection beams are intersected on one point, and the connection beams are connected on the point to form a complete body; the low ends of pulling rods are connected with the connection beams on the point; and electrode rods are dynamically matched with sealing rings. The method disclosed by the invention is characterized in that the pulling rods pull the heating bodies via the connection beams, the electrode holders and the electrode rods, later the thermal field is lifted, and finally the crucible, small crystal tubes, the crucible pedestal and small crystals are static. The success rate of the magnesium fluoride crystals grown according to the invention is over 95%, the dislocation density is reduced to 10-30/cm<2>, the absorption coefficient at the wave band of 0.2-7.5 micrometers is smaller than 2*10E-4/cm, and the quality of the crystals is fully improved.

Description

Device and method for directional growth of fluoride single crystal with small gradient temperature field rise

technical field

The invention relates to a device and method for directional growth of a fluoride single crystal with a small gradient temperature field rising, which grows a fluoride in a specific direction such as a magnesium fluoride single crystal under vacuum conditions, and belongs to the technical field of crystal growth.

Background technique

Fluoride single crystal is a crystal material with anisotropy and birefringence properties, and is mainly used in polarizing elements in the ultraviolet band. Due to the requirements of birefringence characteristics, the cutting of the crystal must be carried out in a specific direction. If the crystal fails to grow in the predetermined direction, the cutting loss will be large and the utilization rate will be low. In order to improve the utilization rate, the crystal must be grown directionally. The existing method for directional growth of fluoride single crystal is a seed drop method. The method grows crystals under vacuum conditions. Taking magnesium fluoride single crystal as an example, magnesium fluoride seed crystals are placed on the bottom of a crucible, and the crucible is connected to a water-cooled descending rod. In the initial stage of crystal growth, the temperature and temperature field at the position of the seed crystal are precisely controlled, so that about half of the seed crystal melts, and then the water-cooled drop rod moves down slowly, and the crucible moves slowly from the hot zone to the cold zone in the furnace, so that the crystal follows The seed crystal grows slowly in its own direction.

The seed drop method has strict requirements on crystal growth equipment and environmental conditions, especially in the early stage of crystal growth, that is, the critical stage of melting about half of the magnesium fluoride seed crystal, which requires very precise temperature control. The temperature field where the seed crystal is located is composed of a heating element, a crucible, and a water-cooled drop rod. The temperature of the temperature field in the middle of the seed crystal must be controlled at 5 to 10 degrees above the melting point of the seed crystal, so that the upper half of the seed crystal is completely melted and the seed crystal is completely melted. The lower half of the crystal does not melt, so that the crystal can grow normally. Magnesium fluoride single crystal is grown by using the existing seed drop method. Since the seed crystal is connected to the water-cooled drop rod at the bottom of the crucible through the crucible seat, the fluctuation of cooling water temperature and flow rate will cause the temperature field of the seed crystal to deviate from the preset temperature field. , resulting in either complete melting or insufficient melting of the seed crystal, both of which will lead to failure of crystal growth.

Another disadvantage of the existing seed drop method is that due to the existence of cooling water, the temperature of the water-cooled drop rod is always less than 100°C, while the melting point of the crystal is much higher than this temperature, such as the melting point of magnesium fluoride crystal is 1255°C, which is This makes the temperature field gradient at the position of the seed crystal very steep, so that when the temperature field in the middle of the seed crystal reaches 5 to 10 degrees above the melting point of the crystal, the temperature in the middle and upper parts of the crucible will be much higher than the melting point of the crystal, resulting in the crystal raw material. Volatilization may even cause a large amount of overflow of raw materials, resulting in failure of crystal growth.

In short, the success rate of oriented growth of fluoride single crystals using the existing seed drop method is only 50-60%. Since the melting of the seed crystal cannot be observed in real time during the crystal growth process, it is impossible to detect whether the seed crystal melts according to the predetermined requirements in time, and the crystal growth result cannot be known until the crystal growth process is completely completed. If it fails, it will inevitably cause serious waste of raw materials and electric energy. . Such a low success rate and such serious waste have resulted in the price of magnesium fluoride single crystal blanks with specific directions on the market as high as more than 10,000 yuan/kg, which has affected the application of this material in a wider range of fields.

Although there is still a success rate of 50-60% in the directional growth of fluoride crystals by the existing seed drop method, there are still problems in crystal quality. Due to the existence of water-cooled drop rods, there are phenomena such as temperature fluctuations, unstable temperature fields, and excessive temperature gradients. The negative consequences of these phenomena are that the shape of the solid-liquid interface is not an ideal flat and slightly convex interface, which in turn leads to the formation of a large number of internal defects in the crystal. , the dislocation density reaches 60~100/cm ² , and the absorption coefficient reaches 5×10E-4/cm, which is difficult to meet the requirements of high-end polarizing prisms.

Contents of the invention

The purpose of this invention is to increase the success rate of directional growth of fluoride single crystals and improve the quality of crystals. Therefore, we have invented a device and method for directional growth of fluoride single crystals with a small gradient temperature field. The core of its technical solution is to cancel the water-cooling drop rod, control the rise of the heating element, lower the seed crystal in disguise, stabilize the temperature field, reduce the temperature gradient, and completely overcome the problems existing in the existing technology.

In the device for directional growth of fluoride single crystal with small gradient temperature field rising in the present invention, the inner periphery of the vacuum chamber 1 is a thermal insulation cover 2, and the crucible 3 is located in the center of the thermal insulation cover 2, as shown in Figure 1; at the center of the bottom of the crucible 3 There is a crystal cylinder 4, and the crucible base 5 is below the crystal cylinder 4; several heating bodies 6 are suspended around the side wall of the crucible 3 at the same height, and the upper end of the heating body 6 is connected to a water-cooled electrode rod 7, and the electrode rod 7 first passes through the insulation cover 2 The top passes through the sealing ring 8 on the top of the vacuum chamber 2 and is connected to the electrode holder 9; the temperature control thermocouple 10 is located at the height of 1/2~2/3 of the crystal tube 4 on the side of the crystal tube; it is characterized in that the crucible 3 passes through the crucible The seat 5 is placed on the bottom of the insulation cover 2, as shown in Figure 2; each connecting beam 11 connects two opposite electrode seats 9, and the midpoints of each connecting beam 11 intersect at one point, and each connecting beam 11 is at this point The lower ends of the pull rods 12 are connected with each connecting beam 11 at this point;

According to the method for directional growth of fluoride single crystal with small gradient temperature field rise of the present invention, as shown in Fig. 3, the seed crystal 13 is inserted into the seed crystal tube 4, the upper end of the seed crystal 13 is probed into the crucible 3, and the growth material 14 is loaded The crucible 3 generates a growth temperature field by the heating body 6, and the temperature field temperature of the upper half of the seed crystal 13 is controlled by the temperature control thermocouple 10 to be 5-10°C higher than the melting point of the growing fluoride single crystal. After the upper half of the crystal 13 melts, the child crystal 13 moves relative to the temperature field up and down, and the solid-liquid interface moves from the hot zone of the temperature field to the cold zone until the crystal growth is completed; it is characterized in that the lifting rod 12 passes through the connecting beam 11, The electrode holder 9 and the electrode rod 7 pull the heating body 6 upwards, and the temperature field rises accordingly, and the crucible 3, the seed crystal tube 4, the crucible seat 5, and the seed crystal 13 are stationary.

The technical effect of the present invention is that, since the crucible 3 is placed on the bottom of the insulation cover 2 through the crucible seat 5, the water-cooled descending rod under the crucible seat 5 in the prior art is cancelled, and the crucible 3 and crystal tube 4 are in the process of crystal growth. , crucible seat 5, child crystal 13 remain static, and the up-and-down relative motion between child crystal 13 and temperature field, or the solid-liquid interface moves to cold zone from the hot area of temperature field is realized by temperature field rise, and these technical measures can Bring various technical effects. For example, the gradient of the temperature field around the seed crystal 13 becomes smaller, and the temperature of the upper part of the seed crystal 13 is easy to control, which largely avoids the situation that the upper part of the seed crystal 13 does not melt or the seed crystal 13 completely melts; It is possible to normally keep the temperature field temperature of the upper half of the seed crystal 13 at a temperature 5-10°C higher than the melting point of the grown fluoride single crystal, without raising the temperature of the growth material melt much above the melting point of the fluoride single crystal, thus It can reduce the volatilization of crystal raw materials and avoid a large amount of raw materials from overflowing. The success rate of final crystal growth can thus be increased to more than 95%. For another example, the growth temperature is stable, the temperature field gradient is almost constant, the shape of the solid-liquid interface basically presents an ideal flat and slightly convex interface, which greatly reduces the internal defects of the crystal, and the X-ray orientation instrument is used for orientation, with an orientation accuracy of 30″ and an orientation accuracy of When it reaches 2°, the maximum diameter of the magnesium fluoride single crystal ingot grown in the C direction is 100mm, and the length is ^200mm . The absorption coefficient of the wave band is less than 2×10E-4/cm, and the crystal quality is improved in an all-round way. The invention also has a side effect, that is, the cooling structure is simplified along with the cancellation of the original water-cooling drop rod.

Description of drawings

FIG. 1 is a schematic diagram of the overall structure of an apparatus for directional growth of fluoride single crystals with a small gradient temperature field of the present invention. Fig. 2 is an enlarged schematic diagram of the structure of the lower half of the device for directional growth of fluoride single crystals with a small gradient temperature field of the present invention. Fig. 3 is a schematic diagram of the method for directional growth of a fluoride single crystal with a small gradient temperature field increase of the present invention, and this figure is also used as an abstract drawing.

Detailed ways

In the device for directional growth of fluoride single crystal with small gradient temperature field rising in the present invention, the inner periphery of the vacuum chamber 1 is a thermal insulation cover 2, the material of the thermal insulation cover 2 is composite carbon fiber, the crucible 3 is located in the center of the thermal insulation cover 2, and the crucible 3 Add a crucible cover 15 made of graphite to prevent the heat loss of the growth material melt and prevent the growth material from volatilizing, as shown in Figure 1; there is a seed tube 4 in the center of the bottom of the crucible 3, and below the seed tube 4 is a crucible seat 5 and a crucible seat 5 It is made of graphite; several heating bodies 6 are suspended at the same height around the side wall of the crucible 3, and the upper end of the heating body 6 is connected to a water-cooled electrode rod 7, and the electrode rod 7 first passes through the top of the insulation cover 2 and then passes through the sealing ring 8 on the top of the vacuum chamber 2 It is connected with the electrode holder 9; the temperature control thermocouple 10 is located at the height of 1/2~2/3 of the crystal tube 4 on the side of the crystal tube 4; the crucible 3 is placed on the bottom of the insulation cover 2 through the crucible seat 5, as shown in Figure 2; Each connecting beam 11 connects two opposite electrode holders 9, the connecting beam 11 is insulated from the electrode holders 9, and the power line is connected to the electrode holders 9; the midpoints of each connecting beam 11 intersect at one point, and each connecting beam 11 is at this point The lower end of the lifting rod 12 is connected with each connecting beam 11 at this point; the electrode rod 7 is in motion with the sealing ring 8, and the electrode rod 7 is made of red copper, which is hollow and cooled by water, and the vacuum chamber 1 is maintained by the sealing ring 8. the vacuum inside. The electrode rod 7 is connected with the graphite terminal of the heating body 6 through bolts and nuts.

According to the method for directional growth of fluoride single crystal with small gradient temperature field rise of the present invention, as shown in Figure 3, a small piece of magnesium fluoride single crystal is processed into a cylindrical shape as seed crystal 13, seed crystal 13 has a diameter of 15 mm and a length of 65 mm. The axial direction of the cylinder is the single crystal C direction, and the utilization rate of the fluoride single crystal ingot grown along this direction is the highest; the seed crystal 13 is inserted into the seed crystal cylinder 4, and the inner diameter of the seed crystal cylinder 13 matches the outer diameter of the seed crystal 13 , keep the vertical state of the seed crystal 13; the upper end of the seed crystal 13 penetrates into the crucible 3. The magnesium fluoride particle growth material 14 is loaded into the crucible 3 . Turn on the rotary vane vacuum pump and the oil diffusion pump in sequence, and evacuate the vacuum chamber 1 to a vacuum of 2×10E-3 Torr. The growth temperature field is generated by the heating body 6, and the furnace temperature rises from room temperature at a rate of 25°C/hour to a temperature between 5°C and 10°C above the melting point of magnesium fluoride crystals, that is, a temperature between 1260°C and 1265°C, such as 1263°C. 14 Slowly melt into a melt, and then control the constant temperature. During the constant temperature period, the melt fully excludes gases and other impurities. The temperature field temperature of the upper half of the seed crystal 13 is controlled at this temperature by the temperature control thermocouple 10 . After 5 hours of constant temperature, the upper half of the seed crystal 13 melts, and the heating body 6 is pulled up by the lifting rod 12 through the connecting beam 11, the electrode holder 9, and the electrode rod 7 at a speed of 1 to 3 mm/h, such as 1 mm/h, and the temperature field As it rises, the crucible 3, seed tube 4, crucible seat 5, and seed crystal 13 are stationary, and the seed crystal 13 moves up and down relative to the temperature field. In the cold zone of the temperature field, the solid-liquid interface also moves from the hot zone to the cold zone at a speed of 1 mm/h. Through the mass transport in the melt, the crystal grown in the crucible 3 will completely follow the inherent characteristics of the seed crystal 13. After 200 hours, the crystal growth is completed, and the temperature is controlled by the temperature control thermocouple 10 to automatically cool down at a rate of 30°C/h. When the temperature drops to 150°C, stop supplying power to the heating body 6, and cool naturally to room temperature to obtain A complete magnesium fluoride single crystal in the same direction as the seed crystal 13, with a crystal ingot diameter of 58 mm and a length of 160 mm, and the axis of the crystal ingot cylinder is parallel to the C direction of the crystal. The fluoride single crystal also includes CaF ₂ , BaF ₂ , and LiF crystals.

Claims (4)

1. A device for directional growth of a fluoride single crystal with a small gradient temperature field rising, the inner periphery of the vacuum chamber (1) is an insulating cover (2), and the crucible (3) is located at the inner center of the insulating cover (2); in the crucible (3) ) bottom center has a seed tube (4), and below the seed tube (4) is a crucible seat (5); several heating bodies (6) are suspended at the same height around the side wall of the crucible (3), and the upper end of the heating body (6) Connect the water-cooled electrode rod (7), the electrode rod (7) first passes through the top of the insulation cover (2) and then passes through the sealing ring (8) on the top of the vacuum chamber (1) to connect with the electrode holder (9); the temperature control thermocouple ( 10) Located at 1/2 to 2/3 of the height of the crystal tube (4) on the side of the crystal tube; it is characterized in that the crucible (3) is placed on the bottom of the insulation cover (2) through the crucible seat (5); each connection Beams (11) connect the electrode holders (9) that are opposite in pairs, and the midpoints of each connecting beam (11) intersect at one point, and each connecting beam (11) is connected to each other at this point. The lower end is connected with each connecting beam (11) at this point; the electrode rod (7) is dynamically matched with the sealing ring (8). 2. The device for directional growth of fluoride single crystal with small gradient temperature field rise according to claim 1, characterized in that the connecting beam (11) is insulated from the electrode holder (9), and the power line is connected to the electrode holder (9). 3. A method for directional growth of fluoride single crystals in a small gradient temperature field, the seed crystal (13) is inserted into the seed crystal tube (4), the upper end of the seed crystal (13) is probed into the crucible (3), and the growth material ( 14) Put it into the crucible (3), generate the growth temperature field by the heating body (6), and control the temperature field temperature of the upper half of the seed crystal (13) by the temperature control thermocouple (10) to be higher than the grown fluoride When the temperature of the single crystal melting point is 5-10°C, when the upper part of the seed crystal (13) melts, the seed crystal (13) moves up and down relative to the temperature field, and the solid-liquid interface moves from the hot area of the temperature field to the cold area until the crystal The growth is completed; it is characterized in that several heating bodies (6) are suspended at the same height around the side wall of the crucible (3), and the lifting rod (12) passes through the connecting beam (11), the electrode seat (9), and the electrode rod (7) When the heating body (6) is pulled upwards, the temperature field rises thereupon, and the crucible (3), seed crystal tube (4), crucible seat (5), and seed crystal (13) are stationary. 4. the method for directional growth of fluoride single crystal with small gradient temperature field rise according to claim 3, is characterized in that, by pulling bar (12) through connecting beam (11), electrode seat (9), electrode bar (7) ) pull the heating body (6) upward at a speed of 1-3 mm/h.