JP5359845B2 - Single crystal growth equipment - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an apparatus for growing a single crystal, which prevents cracks from being caused in a crystal in the process of producing a single crystal to be used as a substrate for an optical device or the like. <P>SOLUTION: A single crystal free of cracks can be obtained with high reproducibility regardless of a large difference in thermal expansion between a platinum crucible and an LN (lithium niobate) single crystal in the process of growing an LN single crystal by VB (vertical Bridgeman) method, by employing a crucible structure of the present invention having a foil type platinum plate inserted inside the platinum crucible for growing a crystal. <P>COPYRIGHT: (C)2011,JPO&amp;INPIT

Description

  The present invention relates to an apparatus for producing a single crystal used as a substrate for an optical device or the like.

  Ferroelectric nonlinear optical crystals convert wavelength of laser light by using quasi-phase-matching (quasi-phase-matching) that quasi-phase-matches by periodically reversing the dielectric polarization direction by 180 degrees (polarization inversion). It is attracting attention as a device. Among them, LiNbO3 (hereinafter referred to as LN) is used as a main substrate material for quasi-phase matching (hereinafter referred to as QPM) devices because of its large nonlinear optical constant and high conversion efficiency.

The LN single crystal of the substrate material is generally manufactured by a melt solidification method called the Czochralski method (hereinafter referred to as the CZ method).
In the QPM device, since laser light having a high optical density is incident on the LN single crystal wafer, it is desired that the number of defect levels existing in the LN single crystal wafer that absorbs the laser light is as small as possible. In other words, in the LN single crystal for QPM devices, it is important to reduce crystal defects that cause defect levels as much as possible.

  For the production of LN single crystals, there is also a melting and solidification method called the Vertical Bridgman method (hereinafter referred to as VB method). The VB method has the disadvantages that it is difficult to increase the diameter and the crystal growth rate is slow compared to the CZ method, but it is possible to grow crystals under conditions closer to thermal equilibrium, so it has higher quality with fewer crystal defects than the CZ method. It is said that a simple LN single crystal can be obtained.

However, the production of LN single crystals by the VB method is at the research and development stage and has not yet been commercialized.
In the VB method, unlike the CZ method, an LN single crystal is solidified while being in contact with the crucible material in a crucible made of a refractory metal such as Pt, Rh, Ir, and cooled to room temperature as it is.

  LN single crystals are oxides and are generally grown in an oxygen-containing atmosphere. However, when the oxygen partial pressure is low, oxygen vacancies (oxygen vacancies), which are crystal defects, are generated. Therefore, it is preferable to grow the crystal at a high oxygen partial pressure.

  In the growth of LN single crystal, it melts at a high temperature of about 1250 ° C and solidifies by lowering the temperature to grow the crystal. However, when the crystal is chemically bonded to a crucible material such as Pt, Rh, Ir, etc. There is.

The joining of a metal such as Pt, Rh, or Ir and an oxide such as LN is related to the formation of an oxidation phase on the surface of the metal such as Pt, Rh, or Ir. In other words, it is considered that oxygen in the oxidized phase and LN which is an oxide have good compatibility and are chemically bonded through the oxygen. Platinum has the highest free energy of oxide formation among Pt (platinum), Rh, and Ir, so it is most difficult to produce oxide and has a melting point of 1600 ° C or higher. Is considered the most suitable material.
Therefore, in research and development, there are many examples of using a platinum crucible. However, when a crystal crucible is actually grown by the VB method using a platinum crucible, the LN single crystal is bonded to the platinum crucible. At the time of taking out the crystal, the platinum crucible melt-bonded to the LN single crystal is forcibly stripped off.

  In addition, since the LN single crystal and the platinum crucible are fused and bonded, the crystal growth is carried out while lowering the temperature from about 1250 ° C to solidify, and when cooling to room temperature, the difference in thermal expansion coefficient between LN and platinum As a result, a large stress is generated at the joint.

  FIG. 7 shows the amount of shrinkage between the LN single crystal and platinum that occurs when cooled from 1250 ° C. to room temperature, based on the thermal expansion coefficients of the LN single crystal and platinum. The thermal expansion coefficient of the LN single crystal has a large anisotropy, and the thermal expansion coefficient in the a-axis direction of the single crystal is very large. Therefore, in the C-axis (Z-plane) direction of the crystal, as shown in FIG. 8, there is no stress in the molten state of 1250 ° C. or higher, but when cooled to room temperature after solidification, there is a thermal expansion coefficient between the LN single crystal and the platinum crucible. A large stress corresponding to the difference occurs.

Therefore, since a large tensile stress is generated on the surface of the LN single crystal, there is a problem that a crack occurs in the LN single crystal.
The platinum crucible needs to have the function of holding the LN melt at 1250 ° C. If the solidified platinum crucible deforms and breaks, the upper LN melt leaks from the crucible, so the platinum crucible itself is stressed. The thickness of the platinum crucible itself cannot be reduced so that it deforms as it occurs.

  For such a problem in the VB method, as in Patent Document 1, for example, when a lithium tetraborate single crystal is grown, it is placed inside a heat-resistant outer container (crucible) such as alumina, quartz glass, or graphite. A manufacturing apparatus is disclosed in which a thin platinum crucible is provided and crystals are grown therein to reduce strain applied to the crystals and suppress the occurrence of cracks.

  Patent Document 2 discloses that a single crystal growing crucible made of a high melting point ceramic material such as alumina is formed by coating platinum on the inner cylinder of platinum or the inner part of the inner cylinder made of a refractory metal material. A configuration is described in which the thickness can be reduced and the amount of expensive noble metals such as platinum used can be reduced.

  Patent Document 3 describes that the inner surface of the crucible ranging from the upper end of the graphite crucible to 1 to 5 cm below the upper surface of the raw material melt is covered with a noble metal foil such as platinum. It is described that a single crystal is obtained by lifting and removing the metal foil together from the crucible and removing the metal foil. Even if the depth range covered with this metal foil exceeds 5 cm, the anti-adhesion effect can be obtained, and when the grown crystal is extracted from the crucible, the platinum foil and the crystal were fixed to each other, Since the crucible and the platinum foil are not fixed, it is described that the crystal could be easily extracted without breaking the crucible, and that the obtained crystal was a high-quality lithium tetraborate single crystal without cracks. .

Japanese Patent Laid-Open No. 9-20596 Japanese Utility Model Publication No. 58-160273 JP-A-10-287492

Cracks generated in the single crystal have a problem that the single crystal is melt-bonded with a crucible such as platinum and is generated during growth or when the single crystal is extracted from the crucible.
For this reason, as in Patent Document 1 and Patent Document 2, a double crucible method in which a thin platinum crucible is provided and a method of coating the inner surface of the crucible with platinum have been studied. Because of the high melting point, cracks occurred in the crystal.
Further, in the method of Patent Document 3, since there is no formation of a platinum foil at the lower part of the crucible where the crystal has grown (when the direction of crystal growth proceeds from the lower part to the upper part), there is a problem that fusion bonding between the crystal and the platinum crucible occurs. was there.

The present invention has been made to solve the above-described problem, and an object of the present invention is to stably obtain a high-quality single crystal by eliminating the generation of cracks during crystal cooling in single crystal growth.

In order to solve the above problems, a single crystal growth apparatus according to the present invention comprises:
A crucible body made of a heat-resistant material holding a raw material melt of a crystalline substance and a side heater arranged around the crucible body, and growing a single crystal while changing the temperature distribution of the crucible body up and down In the crystal growth apparatus to be made,
On the inner surface of the crucible body in contact with the raw material melt, it has a crucible inner surface covering member made of platinum foil ,
The platinum foil used for the crucible inner surface covering member has a thickness of 0.03 mm to 0.1 mm,
The inner surface of the crucible body and the platinum foil are spot-welded at intervals of 5 mm to 30 mm .

  The crucible inner surface covering member is configured to cover the entire inner surface of the crucible main body with which the raw material melt contacts so that the raw material melt supplied to the crucible inner surface does not leak into the inner surface of the crucible main body. Yes.

  With such a configuration, the LN single crystal is melt-bonded to platinum, but the crystal to be grown is melt-bonded only to the platinum foil covering the inner surface of the crucible. Therefore, the generated stress causes the crucible inner surface covering member made of platinum foil to be plastically deformed, so that no crack is generated in the crystal.

The crucible inner surface covering member is formed so as to horizontally divide the inner surface of the crucible body into an upper part and a lower part .

The crucible inner surface covering member may be formed by pressing a platinum foil into the inner surface of the crucible main body with a mold, forming a peripheral cylindrical portion and a bottom portion, and welding or caulking the joint portion.
The crucible body is preferably platinum.

Furthermore, the crucible body is preferably made of platinum, and at least a part of the crucible inner surface covering member is preferably welded and fixed to the crucible body.

Due to the crucible structure of the present invention in which a foil-like platinum plate is inserted inside a platinum crucible for crystal growth, reproducibility is observed despite the large thermal expansion difference between the platinum crucible and the LN single crystal in the LN single crystal growth by the VB method. A single crystal with no cracks can be obtained.

It is a basic lineblock diagram of the single crystal growth device by VB method. It is a schematic block diagram of the crucible of this invention. Cross-sectional view of crucible inner surface covering member composed of foil-shaped platinum plate It is a schematic block diagram which shows the raw material melt input initial state of a crystalline substance with the structure of this invention. It is a schematic block diagram which shows the single crystal growth middle state by the structure of this invention. It is a schematic block diagram which shows the cooling state after completion | finish of a single crystal growth by the structure of this invention. FIG. 4 is a relationship diagram between a thermal expansion coefficient and temperature of an LN single crystal and platinum. It is explanatory drawing of the generated stress to the crucible accompanying LN single crystal growth.

A single crystal growth apparatus showing an embodiment of the present invention will be described with reference to the drawings.
FIG. 1 shows a basic configuration diagram of an apparatus for growing a single crystal by a vertical fridge man (VB) method. FIG. 2 shows a configuration diagram of a crystal crucible platinum crucible which is a main part of the present invention.

  The cup-type crucible body 1 shown in FIGS. 1 and 2 is made of platinum, and has a thickness of 0.5 mm, an inner diameter of 53 mm, and a height of 150 mm, for example, a thickness of 0.3 to 0.5 mm is often used. The crucible body 1 is not made of platinum, but may have any heat resistance that can withstand the temperature used in the crystal growth apparatus of the present invention. Alumina, cordierite, mullite, quartz glass, boron nitride In addition, one having at least one of graphite as a main component can be used.

And the inner surface of the crucible main body 1 forms the crucible inner surface covering member covered with the foil-like platinum plate 2 processed into a cylindrical shape having a thickness of 0.03 to 0.1 mm, for example.
The range covered with the foil-like platinum plate 2 may be the entire inner surface of the crucible body 1 or the liquid surface at the upper part of the crucible body 1 and filled with the raw material melt.

  Although the method and shape of covering the inner surface of the platinum crucible 1 with the foil-shaped platinum plate 4 are not particularly limited, the crucible inner surface covering member that covers the inner surface with which the raw material melt of the crucible body 1 is in contact is, for example, foil-shaped platinum. The plate 4 may be an earth-like shape and fit into the inner periphery of the platinum crucible, or the foil-like platinum plate 4 having a shape similar to that of the platinum crucible 1 may be made by a pressing die. For example, as shown in FIG. 3, the crucible inner surface covering member made of the foil-like platinum plate 4 divides the inner periphery and the bottom surface of the platinum crucible 1 and the foil-like platinum member a 21a and the foil-like platinum shown in FIG. The member b 21b may be made, or the crucible inner surface covering member may be divided vertically by the foil-like platinum member c 21c and the foil-like platinum member d 21d as shown in FIG. 3b or FIG. You may form in the horizontal division | segmentation by the member e21e and the foil-shaped platinum member f21f. And about a division | segmentation location, what is necessary is just to comprise the junction part 25 liquid-tightly by methods, such as welding, welding, melt | fusion, and caulking, so that a raw material melt may not leak.

  In this example, in order to fix the foil-like platinum plate 2 to the inner surface of the platinum crucible body 1, spot welding (spot welding diameter φ1 mm or less) (not shown) was performed vertically and horizontally at intervals of 10 mm. Spot welding is not always necessary, but by performing spot welding, the crucible and the platinum foil are fixed, so that the workability of placing seed crystals and introducing crystal raw materials is improved, and spot welding is peeled off during crystal cooling. Therefore, no cracks occur in the crystal.

  In the crystal growth apparatus comprising the VB method, the seed crystal 3 is placed at the bottom of the crucible 1 and the raw material melt 4 is introduced thereon to grow the single crystal 5 in the crucible 1. The crystal solid-liquid interface 41 exists.

  The crucible 1 is placed and fixed on a stage 10 movable up and down, and a furnace core tube 11 is provided so as to cover the periphery of the crucible 1, and a side heater 12 capable of arbitrarily setting the temperature in the vertical direction around the periphery. Place. These are disposed inside a housing (not shown) having a heat insulating material on the inner surface, and prevent heat from escaping to the outside.

In this example, a non-stoichiometric composition LN (Li 2 O / Nb 2 O 5 = 48.5 / 51.5) having a diameter of 2 inches and a height of 30 mm was arranged as a seed crystal 3 at the bottom of the platinum crucible 1. The surface of the seed crystal 3 is a Z plane.
After that, the raw material melt 4 (sintered body raw material) of non-stoichiometric composition LN synthesized in advance was filled while flowing a volume of 100 mm in height of φ2 inches, and a temperature above the LN melting point (above 1250 ° C.) ).

  Since the temperature on the seed crystal 3 side is controlled to be equal to or lower than the melting point of LN (1250 ° C. or lower), the temperature distribution in the furnace is a temperature gradient shown in FIG. For this reason, the surface on the sintered compact raw material side of the seed crystal 3 that is the solid-liquid interface 41 slightly melts back (melts back), but most of the seed crystal does not melt.

Then, the platinum crucible 1 is gradually lowered at a speed of 0.3 mm / hr or less by the stage 10 without changing the temperature of the growth furnace to solidify the LN melt.
Since the temperature distribution of the growth furnace hardly changes, the solid-liquid interface 41 in the platinum crucible 1 gradually rises as the platinum crucible 1 is lowered by the stage 10.

  The non-stoichiometric composition LN (Li2O / Nb2O5 = 48.5 / 51.5) is a congruent melting composition (congruent composition), and the melting point is the stoichiometric composition (stoichiometric composition) at 1250 ° C or less as the binary equilibrium state of Li2O-Nb2O5. The LN single crystal shifted to is deposited on the seed crystal.

After solidifying the whole melt 4 by the above operation, it is cooled to room temperature over about 20 hours, and the LN single crystal 5 is taken out.
In the above description, the apparatus using the method of gradually lowering the platinum crucible 1 by the stage 10 and solidifying the LN melt without changing the temperature of the growth furnace has been described. However, the temperature distribution of the crucible body is changed. However, as a method of growing a single crystal, the platinum crucible 1 may be fixed at a position and the heater 12 having a temperature distribution may be moved up and the position of the heater 12 is fixed, and the temperature distribution of the heater 12 is Control may be performed in accordance with crystal growth.

  In the filling and melting state of the raw material melt 4 (sintered body raw material) shown in FIG. 4, the inner surface of the platinum crucible 1 of the present invention is covered with the foil-like platinum plate 2. The liquid is in contact with only the foil-like platinum plate 2 and is not in direct contact with the platinum crucible 1.

  As crystal growth proceeds, as shown in FIG. 5, the raw material melt 4 (LN melt) is all solidified to become an LN solid phase 45 immediately after the end of single crystal growth (1200 to 1230 ° C.) because of high temperature. Since the crystals have not yet contracted, the LN solid phase 45 is in close contact with the foil-like platinum plate 2, and the foil-like platinum plate 2 is in close contact with the platinum crucible 1, and the foil-like platinum plate 2 is in contact with the LN solid-state 45. The platinum crucible 1 is not in direct contact.

  In the state of FIG. 5, the foil-like platinum plate 2 is estimated to be melt-bonded to the surface of the LN solid phase 45, but the temperature of the LN solid phase 45 is close to the solidification temperature, and the difference in thermal expansion coefficient has little effect Therefore, the state of FIG. 5 is not significantly different from FIG. 4 showing the state of the raw material melt 4 before solidification, and the positional relationship between the platinum crucible 1 and the foil-like platinum plate 2.

  FIG. 6 shows a state where the LN solid phase 45 shown in FIG. 5 is obtained and then cooled to room temperature. Due to the difference in thermal expansion coefficient, the LN single crystal 5 is greatly contracted in the radial direction, and the foil-like platinum plate 2 adhered to the surface is pulled along with the contraction of the LN single crystal 5.

  That is, since the LN single crystal 5 contracts from platinum, a gap is generated between the LN single crystal 5 and the platinum crucible 1. The portion of the foil-like platinum plate 2 that is in contact with the LN single crystal 5 is peeled off from the platinum crucible 1 and moved to the side surface of the LN single crystal 5. Thus, when the foil-like platinum plate 2 is used in the present invention, the stress is relieved by the large tension generated on the side surface of the LN single crystal 5 by peeling / breaking the foil-like platinum plate 2. Since it can be cooled in such a state, cracks do not occur in the LN single crystal.

  However, when the foil-like platinum plate 2 is not disposed on the inner surface of the platinum crucible 1 as in the prior art, the LN single crystal 5 is in direct contact with the platinum crucible 1, so that a large tensile stress is applied to the side surface of the LN single crystal 5. This causes cracks in the LN single crystal 5 from the side surface.

  In this embodiment, the foil-like platinum plate 2 is only partially welded to the platinum crucible 1 with a small spot. Since this portion is thin and low in strength, it breaks with the shrinkage of the LN single crystal 5. And peel from the platinum crucible 1. Therefore, in the cooling process, the LN single crystal 5 that contracts greatly is cooled to room temperature without any tensile stress that causes cracks.

No cracks were observed in the LN single crystal 5 grown using the platinum crucible 1 of the present invention.
On the other hand, when a conventional platinum crucible in which the inner surface of the platinum crucible 1 was not coated with the foil-like platinum plate 2 was used as a comparative example, large cracks were observed in the LN single crystal in almost all cases.

  When the foil-like platinum plate 2 in the present example has a thickness of 0.1 mm or more, cracks may occur in the LN single crystal. Moreover, when the thickness of the foil-like platinum plate 2 is less than 0.03 mm, the strength of the foil-like platinum plate 2 is so low that it breaks during the insertion of the raw material melt 4 and the heating process. Since the LN melt 4 leaking from a typical breakage point is in direct contact with the platinum crucible 1, cracks may occur in the LN single crystal 5.

Therefore, the foil-like platinum plate 2 used in the present invention preferably has a thickness of 0.03 to 0.1 mm.
When the foil-like platinum plate 2 is formed in a cylindrical shape and spot welding is performed on the inner side of the platinum crucible 1,
When the interval of spot welding was closer than 5 mm, the LN single crystal 5 sometimes cracked.

Therefore, the interval of spot welding is preferably 5 mm or more, and if it is performed at an interval of 30 mm or less, cracks do not occur. In this way, if spot welding is performed at intervals of 5 to 30 mm, the crucible and the platinum foil are fixed, and the workability of setting the seed crystal and introducing the crystal raw material is improved. In addition, when the crystal is cooled, spot welding is peeled off, so that no crack occurs in the crystal.
However, if spot welding is performed at a denser interval than this, welding is difficult to peel off, stress is applied to the crystal, and cracks may occur in the crystal.

Further, the spot-welded portion of the foil-like platinum plate 2 to the platinum crucible 1 may be only the upper end portion of the platinum crucible 1.

1 Crucible body 2 Foil-shaped platinum plate 21a Foil-shaped platinum member a
22a Crucible inner surface covering member a
3 Seed crystal 4 LN raw material melt 45 LN solid phase 5 LN single crystal

Claims (2)

A crucible body made of a heat-resistant material holding a raw material melt of a crystalline substance and a side heater arranged around the crucible body, and growing a single crystal while changing the temperature distribution of the crucible body up and down In the crystal growth apparatus to be made,
On the inner surface of the crucible body in contact with the raw material melt, it has a crucible inner surface covering member made of platinum foil ,
The platinum foil used for the crucible inner surface covering member has a thickness of 0.03 mm to 0.1 mm,
The crystal growth apparatus, wherein the inner surface of the crucible body and the platinum foil are spot-welded at intervals of 5 mm to 30 mm. The crystal growth apparatus according to claim 1, wherein the crucible inner surface covering member is formed so as to horizontally divide the inner surface of the crucible main body into an upper part and a lower part .