JPH07514B2 - Single crystal growth method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial field of application) The present invention relates to a high-quality YAG (Y ₃ Al ₅ O ₁₂ ) single crystal or Nd: YAG.
Controlling the diameter of the straight body part in the growth of a single crystal (Yd doped with Nd ₂ O ₃ ).

(Prior Art) Nd: YAG single crystal is normally grown by the pulling method (Czochralski method). The technical problems in growing a single crystal by the pulling method are firstly to grow a good quality crystal that meets the purpose and secondly to grow a crystal having a predetermined diameter. The first issue is the use of high-purity raw materials, stabilization of the atmosphere and temperature during growth, and pressure control. With respect to the second problem, some automatic growing methods using a weight method or an optical method have been proposed. It is a method of pulling up while irradiating a meniscus line with a laser beam (Japanese Patent Laid-Open No. 59-5494), a method having a reference voltage generating mechanism corresponding to the differential value of the weight reduction amount (Japanese Patent Publication No. 54-4345),
This is a method of keeping the temperature of the load cell constant in order to improve the accuracy of the weight signal (Japanese Patent Publication No. 54-4771). All of these show the configuration of the automatic growing system, and no specific control method for the diameter control is shown. Furthermore, there is no example of automatic diameter control for growing high-quality Nd: YAG single crystal, and of course, no example has been published. That is, Nd: YAG single crystal has a very high temperature and a slow pulling rate (0.5 to 1 mm / h).
r) and because it is used in the length direction of the crystal, it is necessary to develop a highly reliable system that is stable for a long period of time, and it is said that automatic growth is difficult. Therefore, the growth of Nd: YAG single crystal is performed by a trained technologist.

(Problems to be solved by the invention) Nd: YAG single crystal has a high growth temperature (1970 ° C) and a very long growth time (300H), so even if the material or composition of the heat insulating refractory is slightly changed. Involved in temperature history. Moreover, Nd: YAG single crystals are very sensitive to thermal changes. Therefore, in order to keep the stability, the refractory structure becomes more complicated, and the crystal monitoring window becomes very small, making it difficult to see the growth state. In the crystal growth under such a situation, the operator controls the diameter with reference to the weight change based on many years of experience. For this reason, it is difficult for even a skilled person to constantly monitor, and it is often observed that the diameter control fails, the crystal quality is deteriorated, and a crystal with a severe unevenness is grown. Naturally, it was difficult for an inexperienced person to grow an Nd: YAG single crystal, and it was almost impossible to grow a single crystal with high quality.

An object of the present invention is to solve this problem and to provide a method by which anyone can control the diameter with good reproducibility and can grow a high quality single crystal.

(Means for Solving Problems) The present invention has a high-frequency coil having a vacuum thermocouple and controls the power supplied to the high-frequency oscillator so that the electromotive force of the vacuum thermocouple approaches the mV setting value (M ₁ ). In the single crystal growth method by the Czochralski method that enables the weight change during the single crystal growth and the high frequency power detection using the high frequency heating furnace with the attached analog controller to control the diameter of the straight body,
The crystal weight (Wi) during growth is Wx + relative to the target weight (Wx)
_{(0.001~0.02) × Wx <V 0} = V 01 × Wi × k / Wx when Wi (where, V ₀₁ is the change amount of the M ₁ immediately before, k is a is 0 to 3) to calculate the Also, set V ₀ = α only when V ₀ is V ₀ > α. This α is determined by the pulling length (l). When l is 0 <l ≦ l ₁ , V ₀ = 30 to 50 μv / hr, l ₁ <l ≦ l
_{When 2} is V ₀ = 20 to 30 μv / hr, l ₂ <l ≦ l ₃ is V ₀ = 15 to 25
When μv / hr, l ₃ <l, V ₀ = 5 to 20 μv / hr (however, l ₁ , l ₂ , l
₃ is the length of the growing crystal, l ₁ = 0 to 45 mm,
l ₂ = 45 to 75 mm and l ₃ = 75 to 110 mm. ) Is set to.

(Operation) In order to perform ideal diameter control, the present inventors have analyzed in detail the relationship between the diameter change of the grown single crystal, the high frequency power and the crystal quality, and conducted various studies.
As a result, from the changes in the crystal and the changes in the high-frequency power, it is possible to control the diameter by controlling the high-frequency power according to the difference between the target weight (Wx) and the crystal weight (Wi). It became clear. The relationship between the target result weight (Wx) and the high frequency power is to keep the high frequency power constant or decrease when the weight (Wi) of the grown crystal is smaller than Wx, and to increase the high frequency power when Wi is larger than Wx. There must be. The magnitude of this high frequency power is
It is determined by the difference between Wx and Wi, but if the amount of change in high-frequency power is too large, it becomes clear that the unevenness of the Nd: YAG single crystal becomes severe and at the same time the quality deteriorates significantly. It also became clear that bubbles or cracks occurred. Therefore, in order to control the crystal diameter while maintaining high quality, it is necessary to minimize the amount of change in high-frequency power and to prevent continuous fluctuations such as increase and decrease. Therefore, in order to obtain a method capable of controlling the diameter under the above conditions, we examined what causes the difference between Wx and Wi.
As a result, it was clarified that Wi changes depending on the thermal environment of the pulling device. In this thermal environment,
It was considered that the refractory material deteriorated with long-term use, the heat was released from the crystal due to crystal growth, the amount of solution decreased, the growth atmosphere changed, and the environment of the growth system changed. It was found from many experiments that these changes did not occur rapidly, but depended on the growth time, that is, the pulling length of the crystal. This is because the thermal change in the initial stage of pulling is very large, but the change becomes smaller as the crystal becomes longer. The crystal quality is also related to the amount of change in high-frequency power and the pulling length time, and the longer the pulling length, the more susceptible it is to power changes. Therefore, it became clear that the control method of high frequency power needs to be changed depending on the pulling length in order to maintain high quality.

On the other hand, the setting of the change amount (V ₀ ) of the control voltage of the high frequency power for keeping the crystal diameter constant is determined by the ratio of Wx and Wi. Training is performed at this V ₀ , and Wi is corrected accordingly. However, if the situation where no correction is required continues after that, the fact is that Wi changes due to changes in the thermal environment as described above. Therefore, Wi is Wx + Wx + (0.001 ~
If the condition 0.02) × Wx <Wi occurs continuously, V ₀ will increase at an accelerated rate. As a result, Wi decreases sharply and the fluctuation of crystal diameter becomes very large.
Crystal quality is poor. Therefore, it became necessary to limit the magnitude of V ₀ . Moreover, it was found from the above-mentioned change with time that V ₀ should be limited also by the pulling length of the crystal. The upper limit of V ₀ can be set by investigating the high frequency power (control voltage) at the time of growing and the voltage change. By linearly approximating the voltage change curve, the curves ( _{1) to} ( ₃₎ in FIG. 2 are obtained, and the points at which the slopes of the straight lines intersect with the respective straight lines, that is, the crystal lengths l ₁ to l ₃ are determined. In the range 0-10% of this gradient
In many experiments, it was desirable to set V ₀ respectively. As a result, as shown in FIG. 1, when 0 <l ≦ l ₂ , α = 30 to 50 μv / hr, and when l ₁ <l ≦ l ₂ , α = 20 to 30 μv / h.
When rl ₂ <l ≤ l ₃ α = 15 to 25 μv / hr, when l ₃ <l α
= 5 to 20 μv / hr (where α is the amount of change in the control voltage of the high frequency power, l ₁ , l ₂ and l ₃ are the lengths of the pulled crystals, respectively, l ₁
= 0 to 45 mm, l ₂ = 45 mm to 75 mm, l ₃ = 75 to 110 mm. )
It has also been clarified that these α, l _{1 to} l ₃ change due to the material change of the refractory material or the deformation of the crucible or the like. or,
In the above, it became clear that Wi can easily change due to temperature fluctuations during the day and night and gas pressure fluctuations. When the amount of change in Wi was examined, it was found to be +0.1 to 2%. As a result, when Wi is within +0.1 to 2% of Wx, it is a good idea to leave it as it is. Therefore, changing V ₀
When Wi is +0.1 to 2% or more of Wx. V ₀ = V ₀₁ × Wi
The k of × k / Wx is a constant for determining V _0, which is equal to the difference between Wi and Wx, but when k is larger than 3, V ₀ becomes large, and as a result, the unevenness becomes sharp and the crystal quality remarkably deteriorates. did. Therefore, k is set according to the past history of Wi and V ₀ .

(Example) Next, this invention is demonstrated with an Example.

Nd: YAG single crystal raw material (high purity Al ₂ O ₃ , Y ₂ O ₃ 0.8 at% Nd) was added to the Ir crucible 1 of 85φ × 100h × 1.7t in the single crystal growth apparatus shown in FIG.
2100 g of the dope was mixed and weighed and mixed in an appropriate amount, and a heat-resistant refractory was installed and provided in the center of the high-frequency coil 3. In response to a command from the personal computer 5, the analog controller 8 and the high frequency oscillator 9 are transmitted via the D / A converter 7.
By this, electric power was applied to the high frequency coil 3 to melt the raw material in the Ir crucible 1. Next, a YAG single crystal (not Nd-doped) was used as a seed crystal <111>, and it was immersed in the above-mentioned melt, and it was confirmed that the temperature was optimum, and pulling was started. The pulling speed was 1 mm / hr and the rotation speed was 20 rpm. Set the program of the target diameter (Dx) so that the thickness of the grown crystal becomes about 30 ° after the pulling is started, and use the signal 4 from the load cell or the signal from the vacuum thermocouple 10 to generate a high frequency with a personal computer. It controls the output of electric power. After 29 hours from the start of pulling up, Dx reached 30φ, so after straightening the shoulder, diameter control of the straight body was started. At this time, the target weight (Wx) was 35 g, the crystal weight (Wi) was also 35 g, and the change amount (V ₀ ) of the control voltage of the high frequency power was 18 μv / hr. 41 hours after the pulling started, Wx was 60g, but Wi was 61.2g. At this time, V ₀ was 53 μv / hr, but V ₀ was set to 35 μv / hr and the growth was continued.

Also, 105 hours after the pulling, Wx was 380 g, but Wi was 377 g. At this time, V ₀ was 26 μv / hr. Therefore V ₀ is 20
The growth was continued by setting to μv / hr. Similarly, while controlling the diameter with the personal computer 5, the crystal was separated after about 160 hours to complete the growth. The grown crystal has a very smooth straight body with no corners, and when a rod (4φ x 63.5 mm) cut out from the crystal is birefringence measured by phase difference, Δn = 1 x 10 ^-7 or less is obtained. , A high quality Nd: YAG single crystal with very little optical distortion was obtained.

(Effects of the Invention) According to the present invention, anyone can make an ideal shoulder and it is effective for automatic growth.

[Brief description of drawings]

1 and 2 are diagrams for explaining the diameter control of the straight body portion according to the present invention, and FIG. 3 is a diagram showing an example of a single crystal growing apparatus. 1 is an Ir crucible, 2 is a Nd: YAG single crystal, 3 is a high frequency coil,
4 is a load cell, 5 is a personal computer, 6 is A /
D converter, 7 D / A converter, 8 analog controller,
9 is a high frequency oscillator, 10 is a vacuum thermocouple.

Claims (2)

[Claims] 1. A high-frequency heating furnace having a vacuum thermocouple in the high-frequency coil, and an analog controller for controlling the power supplied to the high-frequency oscillator so that the electromotive force of the vacuum thermocouple approaches the mV set value (M ₁ ). In the single crystal growth method by the Czochralski method, in which the weight is detected during the growth of the single crystal by using, and the increase rate is changed according to a predetermined program, M ₁ is changed to control the crystal diameter. ₁ of variation (V ₀₎ pulling the length of the value of the upper single crystal by (l) 0 <when _{l ≦ l 1 V 0 = 30~50μV} / hr, when l ₁ <l ≦ l ₂ V ₀ = 20 to 30 μV / hr, when L ₂ <l ≤ l ₃ V ₀ = 15 to 25 μV
When / hr, l ₃ <l, V ₀ = 5 to 20 μV / hr (provided that l ₁ , l ₂ and l ₃ are the lengths of the crystals during growth and l ₁ = 0 to 45 mm, l ₂
= 45 to 75 mm, l ₃ = 75 to 110 mm. ) Is set to a method for growing a single crystal. 2. The crystal weight (Wi) of V ₀ is the target weight (Wx)
On the other hand, if Wi> Wx + (0.001 to 0.02) × Wx, V ₀ = V ₀₁ × Wi
× k / Wx (However, V ₀₁ is the amount of change of M ₁ immediately before, and k is 0.
~ 3. ) The method for growing a single crystal according to claim (1).