JPH0383889A - Liquid surface temperature control type single crystal rearing method and apparatus therefor - Google Patents

Abstract

PURPOSE:To prevent occurrence of defect of crystal by carrying out heating and cooling control in the vicinity of interface between solid and liquid and controlling temperature of definite point of melt surface of crystallization part and crucible part to constant when a single crystal is grown while rotating pullingup crystal using high-frequency wave. CONSTITUTION:A heating and cooling substrate 15 is provided between a main heating element 1 for heating melt 5 in a crucible 2 and afterheater 6 for controlling temperature of single crystal after rearing. Then the single crystal is reared by a seed crystal 7 provided in crystal pulling-up rotation shaft 9 from the melt 5 and simultaneously temperature of definite point of melt 5 near the crucible 2 is measured by light thermometer 19 and temperature of heating and cooling substrate 15 is controlled by liquid face temperature control part 20 based on the measured value and surface temperature of melt liquid 5 is retained to constant. Since the surface of melt liquid 5 is lowered as the growth of crystal proceed, main heating element 1 and heating and cooling substrate 15 and after heater 6 are moved at same speed as lowering speed of the surface of melt liquid 5.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention relates to a single crystal growth method using the so-called Czochralski method, in which a single crystal is grown while rotating a pulled crystal using high frequency, and in particular, It is suitable for growing optically homogeneous oxide single crystals with no oxidation.

(Prior Art) The so-called Czochralski method is known as one of the typical single crystal growth methods. This single crystal growth method is
This is a method of growing a pulled crystal while rotating it on the surface of a melt in a crucible using high frequency waves.

Currently, many oxide single crystals for optical and surface acoustic wave applications are
Although crystal defects are grown using this pulling method, it is well known that many of the crystal defects that occur during growth are greatly influenced by the state of the solid-liquid interface during growth, and the shape of the solid-liquid interface is It has been pointed out that high-quality crystals can be obtained if they are made to have a gentle convex shape. Furthermore, it has been recently reported that the shape of the solid-liquid interface is closely related to the convection mode within the crucible melt.

Considering the convection of the melt in the crucible during the growth of oxide single crystals by the pulling method, there is generally a flow of the melt from the crucible wall toward the center of the crystal, and this includes natural convection and Marangoni convection. has been done. On the other hand, the flow from the center of the crystal toward the crucible wall is forced convection due to crystal rotation, and although there are many other types of flow, the above three types of flow govern the solid-liquid interface shape. It is thought that the shape of the solid-liquid interface is determined by a combination of these convection flows.

(Problem to be solved by the invention) Conventionally, by changing the rotational speed of the pulled crystal and controlling the amount of forced convection to some extent, the interface shape of the single crystal with respect to the melt becomes gently convex. A common method is to In other words, as can be seen by examining the solid-liquid interface shape by changing the rotation speed of the bow-raised crystal, there is a crystal rotation speed at which the crystal reverses from white to concave with respect to the melt, but the rotation speed is slightly lower than that rotation speed. This is the method to do so.

However, in reality, it is extremely difficult to stably obtain the desired solid-liquid interface shape using only this method, and there is a problem in that the solid-liquid interface exhibits a complicated shape with a mixture of irregularities.

SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned problems of the prior art and to provide a method and apparatus capable of producing high-quality single crystals free of crystal defects with a high yield.

(Means for Solving the Problems) In view of the above-mentioned circumstances, the present inventors have developed PbMo0. Taking as an example, we examined the shape of the solid-liquid interface by changing the crystal rotation speed when growing a single crystal with a constant diameter, and then conducted experiments to grow single crystals under various conditions in order to make the solid-liquid interface shape as close to flat as possible. . In addition, FIG. 1 is a diagram showing the equipment used for this purpose, in which 1 is a high-frequency coil, 2 is a crucible, 3 is a heat insulator, and 4 is a crucible stand.The raw material in the crucible is heated by the high-frequency coil 1. A melt 5 is formed. , 6 are after heaters which are heated by the high frequency coil 1. 7 is a seed crystal, 8 is a bow-raised crystal (grown crystal), 9 is a pulling rotation axis, 10 is a thermocouple below the crucible, 1
1 is a crystal diameter control device (ADC device). The crystal diameter control device 11 controls the diameter of the pulled crystal by feeding back the deviation between the growing crystal weight signal and the set diameter signal to high frequency power.

As a result of experiments, no conditions were found for the shape of the solid-liquid interface to be smooth over the entire single crystal, and it was not possible to eliminate crystal defects such as bubbles and dislocations.

Specifically, when the crystal rotation speed was varied in the range of 20 to 4 Qrpm, a solid-liquid interface shape as shown in FIG. 2 was obtained. In addition, FIG. 2 shows how a molybdate single crystal (P b M o ), which is an oxide single crystal, is prepared using the apparatus shown in FIG.
This figure shows the results obtained by growing O4) and observing striations, bubbles, and dislocations using a stereomicroscope.

As shown in Figure 2, when the crystal rotation speed is 25 rpm, there is a small area in the center of the crystal that exhibits a gentle solid-liquid interface 14, and this area is a high-quality area with no bubbles or subgrains. However, agglomeration of defects was observed before the shape of the solid-liquid interface complexly changed at the boundary between the center and edge of the crystal.

If the crystal rotation speed is lower than 25 rpm, the solid-liquid interface shape becomes quite convex, and there is a lot of distortion on the growth stripes within the grown crystal, causing light scattering along the growth stripes when light is passed through. There were so many that it could not be put to practical use.

In addition, when the crystal rotation speed increases to 25 rpm or more, the solid-liquid interface shape becomes concave with respect to the melt, and although the center of the crystal appears to be wider at first glance, there is a resubgrain in which lattice defects are dispersed in the center. It was not a high quality crystal.

The reason for this is that in the conventional single crystal growth method using high-frequency heating, the crucible wall surface is not heated uniformly, and the upper end of the crucible is ignited compared to the middle part, resulting in a higher temperature at the melt interface. As shown in FIG. 3, a large temperature difference occurs from both wall surfaces of the crucible 2 to the center of the crystal, and as a result, a large natural convection (12 in FIG. 1) occurs toward the center of the crystal. This is caused by forced convection in the reverse direction due to crystal rotation (13 in Figure 1).
) to eliminate or buffer the flow, but it is presumed that this is because the collision between the two flows actually creates a complex interface shape.

In order to suppress natural convection, there is a method that attempts to uniformly heat the crucible wall, and for this purpose it is necessary to reduce the temperature difference between the top, middle, and bottom of the crucible. This method involves changing the pitch and diameter of the high-frequency coil, but even if this allows uniform heating of the crucible wall,
In this pulling method, the melt surface decreases during single crystal growth, so the high frequency coil 1 must be moved in accordance with this rate of decrease, which causes temperature fluctuation factors to change in a complicated manner. Not getting good results.

Alternatively, there is a method in which the heating section of the growing crystal 14 (i.e., the after heater 6) and the heating element for the melt heating section (the high frequency coil 1) are separated and the heating is controlled using separate methods, but there is a certain amount of melt convection. Although control is possible, in reality, the upper part of the crucible is overheated by the high-frequency coil 1 and the temperature of the upper part of the crucible becomes quite high, making it impossible to obtain good results.

Based on the above considerations, the inventor of the present invention has conducted intensive research on the pulling method using high circumferential fatigue from various aspects, and has determined that the melt interface temperature in the radial direction of the crucible can be controlled by heating and cooling the vicinity of the solid-liquid interface. It has been discovered that by controlling the temperature difference from the crucible wall to the center to a constant temperature, melt convection can be effectively suppressed and a high-quality large oxide single crystal can be obtained. This is what we have come to.

That is, in the present invention, when growing a single crystal while rotating a pulled crystal using high frequency, the temperature at a fixed point on the surface of the melt in the crystallization part and the crucible part is controlled by controlling the heating and cooling of the vicinity of the solid-liquid interface. The gist of this invention is a liquid surface temperature controlled single crystal growth method, which is characterized by controlling the liquid surface temperature to a constant level.

In addition, in another aspect of the present invention, in an apparatus for growing a single crystal while rotating a pulled crystal using high frequency, a cooling part and a heating element are provided between a heating element for heating the crucible and an after-heater for heating the pulled crystal. The gist of the present invention is a liquid surface temperature controlled single crystal growth apparatus, which is characterized by being provided with a substrate made up of parts.

The present invention will be explained in further detail below.

(Function) As mentioned above, in the past, since melt heating and growing crystal heating were both performed using high frequency waves, overheating of the crucible wall near the solid-liquid interface could not be prevented, and therefore, large natural convection at the solid-liquid interface could not be prevented. Various countermeasures have been attempted on the premise of allowing this natural convection, which naturally has its limits.

On the other hand, in the present invention, since natural convection at the solid-liquid interface is actively suppressed, it is possible to control the temperature in the crucible radial direction.

The means for this is to control the heating and cooling of the vicinity of the solid-liquid interface, thereby controlling the temperature at fixed points on the surface of the melt in the crystallization part and the crucible part to be constant.

Specifically, as will be described later, an apparatus having a configuration as shown in FIG. 4 is used, but as shown in FIG. 5(a),
By providing the heating and cooling means 15 on the outside of the crucible wall corresponding to the vicinity of the solid-liquid interface 14, it is possible to control the temperature at a fixed point on the crystallization part and the melt surface of the crucible part to be constant. The liquid interface 14 can be made into a gentle convex shape, and in some cases, a water cooling body 16 or a heating body 17 can be added on the solid-liquid interface as shown in FIG. 5(b). be able to.

According to this method, as shown in FIG. 6, a gently convex solid-liquid interface can be formed even in a wide range of crystal rotation speeds.

In addition, FIG. 6 shows how a molybdate single crystal (P b M
o O4) was grown, the obtained single crystal was cut parallel to the growth direction, both sides were mirror polished, and then 5% Na
This shows the results of etching with OH water poison and observing striations, bubbles, and dislocations using a stereomicroscope.The high-quality part at the center of the crystal was widely spread, and no bubbles or dislocations were observed. In particular, the crystal rotation speed is 5 rpm.
At the time, high-quality parts accounted for 75% of the total.

Specifically, the single crystal growth method according to the present invention can have the following various embodiments.

FIG. 4 shows an example of a single crystal growth apparatus. In the figure, l' is the main heating element (high frequency coil) for melting the raw material in the crucible 2 and heating the melt 5, 7 is the seed crystal, 9 is the pulling shaft, 10
1 is a thermocouple under the crucible, and 1 is a crystal diameter control device. Further, 15 is a heating/cooling board, 16 is a cooling body, 17 is a heating body, 6' is an after heater, 18 is a thermocouple for after heater, and 19 is a light thermometer.

In such an apparatus, for example, a heating and cooling substrate IS is provided between the main heating element 1' that melts the raw material in the crucible and the after-heater 6' that heats the single crystal after growth, and the surface of the melt near the crucible is heated. The temperature at a fixed point is measured by an optical thermometer 19, and the liquid surface temperature controller 2 is controlled so that the deviation between this temperature and the temperature signal used to control the crystal diameter during crystal growth becomes zero by the controller 11.
By controlling the temperature of the heating and cooling substrate 15 at 0,
The temperature of the surface of the melt can be kept constant.

In addition, as the crystal growth progresses, the surface of the melt decreases, so the main heating element 1', the heating substrate 15, the after heater 6'
is moved at the same speed as the rate of decline of the melt surface.

In order to keep the temperature of the melt surface stable and constant, a cooling body 16 is placed near the crucible above the melt for cooling, or a heating element 17 is placed near the grown crystal on the melt surface. By doing so, similar effects can be brought about. The cooling and heating elements are lowered at the same rate as the liquid level.

In this way, it becomes possible to control the temperature in the radial direction of the crucible, which was previously impossible, and therefore it is possible to control the strong natural convection toward the center of the crucible due to overheating at the ends of the crucible. In other words, convection control at the solid-liquid interface is significantly improved, so that the solid-liquid interface can be formed into a gentle convex shape relative to the melt.

Furthermore, in order to permanently maintain this solid-liquid interface shape,
Considering that there is a close relationship between the shape of the solid-liquid interface and the crystal rotation speed, it is sufficient to perform the crystal rotation at a low speed, preferably about 15 rpm or less, more preferably 5 rpm or less, as shown in Figure 6. It is.

Note that the device shown in FIG. 4 can be modified in various ways.

For example, in the past, the high-frequency coil 1 (FIG. 1) for the main heating element was also used as the after-heater 6', but with the present invention, although such a configuration is also possible,
In order to improve the temperature control of the afterheater, it is preferable to stack substrates of the same or similar type as the heating/cooling substrate 15 in multiple stages to form the afterheater.

Further, as the heating/cooling substrate 15, one having the form shown in FIG. 7 is preferable. This heating/cooling substrate 15 has a disk-shaped or disc-shaped substrate 30 with a space in the center, and a concave heat reflecting surface 31 of an arbitrary curvature is formed in the space of the substrate, and a concave heat reflecting surface 31 of an arbitrary curvature is formed on the back side of the substrate. Water cooling section 32 that cools the heat reflecting surface 31
A heating element 33 such as a spiral wire, a round bar, a square plate, or a thin film is arranged on the heat reflecting surface 31. 34 is a cooling water inlet/outlet. The heat reflecting surface 31 is subjected to appropriate surface treatment or processing such as metal plating. The heating and cooling plates 15 having such a configuration are arranged on the outside of the crucible wall in one stage or in a plurality of stages stacked in the furnace axis direction.

When arranging in multiple stages, the recesses 31 forming the heat reflecting surface are shaped in half on the upper or lower side of the cross section of each board 30, and one recess 31 is formed by a pair of adjacent boards. Alternatively, an intermediate body (eg, a water-cooled copper plate, a heat insulating material, etc.) without a recess may be arranged between each substrate.

Since the heating/cooling board 15 has the above configuration, it is possible to efficiently control the temperature in a narrow range due to the heat insulating effect of the water cooling section and the heat collecting effect of the heat reflecting surface.

Since the thickness can be reduced, interaction between heating and cooling substrates can be extremely reduced, especially when stacked in multiple stages.

The embodiment shown in FIG. 8 is preferable for the cooling body 16. This cooling body 16 consists of a water cooling ring 40, to which hanging rods 41 for cooling water in and out are connected by welding or the like, and as shown in FIG. It can be suspended above the liquid surface.

As the heating body 17, the embodiment shown in FIG. 9 is preferable. This heating element 17 has a heating element 51 held on a holding disc 5o, and is connected to an insulating hanging rod 52 with a pin 53, so that it can be suspended on the surface of the melt as shown in FIG. . Note that the heating element 17 may be the same as the heating element 33 used in the heating/cooling board 15.

Furthermore, in order to integrate the cooling body 16 and the heating body 17,
The embodiment shown in FIG. 10 is preferable, in which a heating element 51 is wrapped around the water cooling ring 40 via an insulating heat-insulating layer 60, and an insulating hanging rod 52 and a cooling water inlet/outlet hanging rod 41 are attached to these. , as shown in FIG. 4, can be suspended on the surface of the melt.

Next, examples of the present invention will be shown.

(Example) A single crystal was grown using the apparatus shown in FIG. 4 under the following growth conditions.

In addition, to provide a supplementary explanation of the device shown in FIG. 4, 1'
is the high-frequency coil which is the main heating element, and there are 10 thermocouples under the crucible.
Temperature program controlled by

The temperature of the afterheater 6' is controlled by a dedicated thermocouple 18. The temperature of the heating/cooling substrate 15 provided between the main heating element 1' and the after-heater 6' is determined by the temperature of the melt surface near the crucible measured by the optical thermometer 19 and the crystal diameter control bag. The temperature is controlled to be equal to the temperature at t17. Since crystal growth slows down and the liquid level drops, the main heating element 1', heating/cooling board 15, after heater 6', and optical thermometer 19 are lowered at the same speed.

However, platinum crucible 2 (150φX150hX2゜5t
) is fixed on the crucible stand 4 and remains stationary. The melt in the crucible is about 80% melted. There is a pulling shaft 9 at the tip of the crystal diameter control device 17, a seed crystal 7 is fixed at the tip, and a single crystal will grow from this tip.

As an example of an oxide single crystal, a lead molybdate single crystal was grown using the apparatus described above. The crystals were grown by varying the crystal rotation speed while keeping the crystal diameter at 70 mm.

Growth conditions〉 Growth crystal: MoPbO* Temperature gradient (vertical direction): 30°C/direct growth <100> Growth speed: 4mm/hr Coil movement speed: 1.4B/hr Heating substrate movement speed: 1.4mm /hr Radial temperature gradient: 5℃/C11 Innermost temperature: 1083℃ Crystal rotation speed: 3rpm, 5rpm, 10rp+a, 1
5 rpm, 2 Orpm The obtained single crystal was sliced to a thickness of 2 mm in parallel to the growth direction, mirror polished on both sides, etched with a 5% NaOH aqueous solution, and WA observed for striations, bubbles, and dislocations using a stereomicroscope. the result.

A gently convex solid-liquid interface shape with respect to the melt was obtained, and there were no bubbles or dislocations, resulting in a large, high-quality single crystal. The yield was 75% when the crystal rotation speed was 5 rpm.

(Effects of the Invention) As detailed above, conventionally, in growing single crystals such as oxide single crystals by the pulling method using high frequency, overheating occurs at the upper end of the crucible and large natural convection occurs. However, according to the present invention, natural convection can be suppressed by controlling the temperature difference on the melt surface in the radial direction of the crucible. At the same time, the shape of the solid-liquid interface is controlled by combining with forced convection, making it possible to make the solid-liquid interface shape smooth and convex, greatly expanding the optically high-quality area with no crystal defects. 6 The present invention can be applied to the growth of various single crystals, but is particularly applicable to the growth of oxide single crystals such as lead molybdate, bismuth silicate, lithium niobate, lithium tantalate, and YAG. suitable.

[Brief explanation of drawings]

Figure 1 is an explanatory cross-sectional view showing a conventional single crystal growth apparatus using high-frequency heating, and Figures 2 (a) to (e) show the shapes of the solid-liquid interface obtained by the conventional single crystal growth method at different crystal rotation speeds. FIG. 3 is an explanatory diagram showing the temperature gradient of the melt surface in the radial direction of the crucible in a conventional single crystal growth method; FIG. 4 is an explanatory sectional view showing an example of the single crystal growth apparatus according to the present invention; 5 (a) to (c) are explanatory diagrams showing the shape of the solid-liquid interface in the single crystal growth method according to the present invention, (a) is when a heating and cooling substrate is provided, and (b) is when a cooling body is further provided. (c) shows the case where a heating element is further added, and Fig. 6 (a) to (e) show the shape of the solid-liquid interface in the single crystal growth method according to the present invention at different crystal rotation speeds. Figures 7(a) and 7(b) are diagrams showing the heating and cooling board.
) is a sectional view, (b) is a plan view, FIGS. Figures (a) and (b) are views showing the heating body, (a) is a plan view, (b) is a side view, and Figures 10 (a) and (b) are views showing the heating body, (a) is a plan view, and (b) is a sectional view. 1... High frequency coil, 1'... Main heating element (high frequency coil), 2... Crucible, 3... Heat insulating material, 4... Crucible stand, 5... Melt, 6.6 ′... After heater 7... Seed crystal, 8... Growing crystal, 9... Crystal pulling rotating shaft, 10... Thermocouple under crucible, 11...
Crystal diameter control device, 12... Natural convection, 13... Forced convection, 14... Solid-liquid interface, 15... Heating and cooling substrate, 16... Cooling body, 17... Heating body, 18 ...Thermocouple for after-heater, 19...Light thermometer, 20.
...Liquid surface temperature control section, 30...Substrate, 31...Heat reflecting surface, 32...Water cooling section, 33...Heating element, 34...
・Cooling water inlet/outlet, 40... Water cooling ring, 41... Cooling water inlet/outlet hanging rod, 50... Holding disk, 51... Heating element, 52... Insulating hanging rod, 53... Stop Bottle, 60...
・Insulating insulation layer. Figure 2 Figure 6 Figure 6

Claims (8)

[Claims] (1) When growing a single crystal while rotating the pulled crystal using high frequency, the temperature at a fixed point on the melt surface in the crystallization part and the crucible part is kept constant by controlling heating and cooling near the solid-liquid interface. A method for growing a single crystal by controlling liquid surface temperature. (2) The method according to claim 1, wherein the temperature of the melt surface in the radial direction of the crucible is controlled to maintain a constant temperature difference from the crucible part to the central part. (3) The method according to claim 1 or 2, wherein the heating/cooling means is moved at the same speed as the rate at which the melt surface decreases. (4) The method according to claim 1, 2 or 3, wherein the pulled crystal is grown at a low rotational speed. (5) In an apparatus for growing a single crystal while rotating a pulled crystal using high frequency, a cooling section and a heating section are provided between a heating element for heating the crucible and an after-heater for heating the pulled crystal. A liquid surface temperature controlled single crystal growth apparatus characterized by being equipped with a heating and cooling substrate. (6) The apparatus according to claim 5, characterized in that a cooling body having a water-cooled ring-shaped pipe arranged movably up and down is provided above the surface of the melt in the crucible. single crystal growth equipment. (7) The apparatus according to claim 5 or 6, characterized in that a heating element in which a ring-shaped heating element is arranged so as to be movable under pressure is provided above the surface of the melt in the crucible. single crystal growth equipment. (8) Claim 6 or 7, wherein the heating element and the cooling element are integrated.
The device described in.