CH646402A5 - Method for producing and garnet gadolinium gallium. - Google Patents

Description

646402

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CLAIM

Process for producing practically perfect piriform bodies of non-crystalline garnet of gadolinium and gallium, having a practically circular cross-section, process comprising the following steps:

a) forming a bath (9) by heating a mixture of Gd203 and Ga203, in a molar ratio of 3: 5, in an iridium crucible (8) on which is placed an iridium covering element ( 16) comprising a circular opening (17) placed above the surface of the bath and whose dimensions are only slightly greater than those of the cross section of the piriform body (7) to be produced;

b) introducing into the bath (9) a seed crystallization seed (2 ') of non-crystalline garnet of gadolinium and gallium through said circular opening (17) of the iridium cover (16);

c) create an ambient atmosphere of nitrogen containing 0.5 to 3% by volume of oxygen around said crucible covered with iridium;

d) removing the seed crystallization seed from the bath, so that the gadolinium and gallium garnet solidifies and crystallizes on said stem, forming a practically perfect massive non-crystalline piriform body (7, 25), of increasing length, having a practically circular section whose dimensions are only slightly smaller than those of the circular opening of said iridium covering element, said piriform body passing through said circular opening of this covering element, when its length increases , practically enclosing the surface of the bath inside the iridium crucible in a chamber delimited by the walls of this crucible, the iridium covering element, the circumferential lateral surface of the piriform body and the surface of the bath, process characterized in that it consists in introducing a continuous flow of nitrogen gas containing from 0.5 to 3% by volume of oxygen in said chamber.

The invention relates to a method for producing a massive garnet of gadolinium and non-crystalline gallium. The invention relates more particularly to a method for producing a garnet of this type using a molten oxide placed in an iridium crucible.

The garnet of gadolinium and non-crystalline gallium is produced in mass, by the well-known technique of Czochralski, by drawing a germ of crystallization in rod on a bath of Gd203 and Ga203 in the molten state, in a molar ratio 3: 5. The bath is generally placed in an iridium crucible, because this metal is considered to be the most advantageous for this purpose, because of its known physical and chemical properties. It is also known to place a cover element, or cover, of iridium on the crucible of this same metal, this crucible constituting a screen against thermal radiation. Gadolinium and gallium garnet is produced in the form of an elongated piriform body of circular cross section, which is then sawn into pellets used as substrates of electronic components, for example for the epitaxial growth of iron / garnet films. It is very important that these substrates, and therefore the crystal from which they are formed, contain no impurities such as iridium inclusions. Indeed, these inclusions propagate in the epitaxial layers formed on the crystalline substrates, with the well-known harmful effects.

It has been observed that these mentioned inclusions of iridium appear with a significant frequency in the lower part of the piriform bodies produced by growth according to the Czochralski process, that is to say in the last part of the piriform body formed by growth. .

The subject of the invention is therefore a process for producing piriform bodies of non-crystalline solid crystal of gadolinium and gallium which contain practically no inclusion of iridium.

The process according to the invention for producing virtually perfect piriform bodies of non-crystalline garnet of gadolinium and gallium of practically circular cross section comprises the following stages, in accordance with the invention:

a) forming a bath by heating a mixture of Gd203 and Ga203, in a molar ratio of 3: 5, in an iridium crucible on which is placed an iridium covering element which has an opening circular placed above the surface of the bath and whose dimensions are only slightly larger than the cross section of the piriform body to be produced, the bath being at a temperature in a range from 1700 to 1800 ° C;

b) the introduction into the bath of a seed crystallization germ, formed of a non-crystalline garnet of gadolinium and gallium, by said circular opening of the iridium covering element;

c) the creation of an ambient atmosphere of nitrogen containing from 0.5 to 3% by volume of oxygen;

d) the removal of the seed crystallization rod from the bath, so that the gadolinium and gallium garnet solidifies and crystallizes on this rod so as to form a massive non-crystalline piriform body whose length increases and whose cross section substantially circular is slightly less than the circular opening of said iridium covering member, said piriform body passing through said circular opening of the iridium covering member as the length of said piriform body increases , so that the surface of the bath placed in the iridium crucible is substantially enclosed in a chamber delimited by the walls of the crucible, the iridium covering element and the circumferential surface of said piriform body,

e) introducing a continuous flow of nitrogen containing 0.5 to 3% by volume of oxygen into said chamber, at a rate sufficient to maintain a nitrogen atmosphere containing from 0.5 to 3% by volume of oxygen inside this chamber.

The improvement brought by the present invention compared to the previous techniques of growth using an iridium crucible and a cover also of iridium resides in the continuous maintenance of an atmosphere of nitrogen and 0, 5 to 3%, preferably 2%, of oxygen in the zone located inside the crucible and close to the surface of the bath, the interface of crystal growth and the lower surface close to the lid d iridium, as well as the interior surface of the iridium crucible which is located above the bath.

The invention will be described in more detail with reference to the accompanying drawings by way of non-limiting example and in which:

fig. 1 is a schematic axial section of an apparatus suitable for implementing the method of the invention;

fig. 2 shows a non-crystalline piriform body of substantially circular cross section which is produced by the method of the invention;

fig. 3a is a schematic axial section of the crucible of FIG. 1 before the start of crystal growth, and figs. 3b and 3c are a schematic axial section and a schematic cross section of this crucible during the growth of the crystal.

Fig. 1 represents an enclosure 1 which envelops a device for drawing a crystal and the ambient gaseous atmosphere. A bath 9 of Gd203 and Ga203, in a molar ratio of 3: 5, is placed inside the enclosure 1 in a crucible 8 of iridium. A cover 16 of iridium, comprising a circular central opening 17, rests on the top of the crucible 8, its lower surface forming a screen against radiation, in a manner well known in the art, in order to reduce the heat losses from the bath 9. The central circular opening 17 is calculated so as to have a cross section which is only slightly greater than the cross section of the piriform body to be produced, represented at 25 in FIG. 2. The sides and bottom of the crucible 8 are surrounded by insulation 15. This insulation, which preferably consists of zirconia, is intended to reduce the energy required to maintain the bath 9, to reduce

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temperature gradients along the crucible and mitigate temperature fluctuations from fluctuations in the grid voltage, convection cooling by the atmosphere, as well as other disturbances. A tube 11 forms a passage through which the temperature of the bottom of the crucible 8 can be determined, for example using a radiation pyrometer concentrated on the center of this bottom.

A ceramic washer 4, for example of alumina, is supported by a tube 5, preferably of zirconia. The washer 4 constitutes a second screen against radiation and restricts the convection currents of the atmosphere, preventing them from entering the top of the crucible and reaching the crystal 7 in the process of growth. It is therefore intended to reduce the vertical temperature gradients in the vicinity of the crystal being grown, and to improve the effects produced by the cover 16.

A sleeve 6, for example of silica, is intended to retain the insulation 15 and is part of the assembly of insulation surrounding the crucible 8. The tube 5 for supporting the washer 4 is also part of the assembly of insulation.

The crucible 8 and all of the insulation which surrounds it rest on a ceramic base 12, for example of zirconia (Zr02). The whole apparatus is enclosed in a bell 3 sealed on a seat plate 13. The latter is made of a suitable material, for example fiberglass bonded by a silicone. The major part of the ambient gas atmosphere which the bell 3 must contain, that is to say the gaseous atmosphere which does not react with the bath being in the crucible, for example nitrogen mixed with 0, 5 to 3%, preferably 2%, by volume of oxygen, is introduced in a continuous flow into a manhole tube 14 which communicates with the tube 11. The gas introduced into the bell 3 exits through the hole 18 of the latter, hole through which the seed crystallization seed is introduced. The rod 2, for example of alumina, comprises a part 2 ′ which constitutes the germ formed by a crystal of gadolinium and non-crystalline gallium and whose longitudinal axis of symmetry 20 is common with the axis 30 of growth of the crystal 7, the orientation of the non-crystalline substance of the seed 2 'being predetermined and being a function of the industrial use for which the garnet is intended. The preparation of a seed crystallization seed of this type is a matter of routine, this stem ensuring the production of a massive non-crystalline substance.

The temperature of the bath is maintained in the apparatus described above in a range between 1700 and 1800 ° C, and a non-crystalline mass is drawn from the bath so that it has a circular cross section and an increasing length, by example 150 to 450 mm, and a diameter of about 75 mm, this mass being produced by methods known in this technique and described, for example, in the US patent No. 3715194. The resulting non-crystalline piriform body is shown at 25 in FIG. 2, its cross section being uniform and practically circular.

Fig. 3a illustrates the iridium crucible 8, the iridium cover 16, the bath 9 and the seed crystallization seed 7 as shown in FIG. 1 at the position preceding the drawing of the crystal. At this time, the gaseous atmosphere inside the iridium crucible 8, above the bath 9, is substantially the same as the ambient atmosphere which the bell 1 must contain and which is introduced by the tube. 11 of fig. 1. Fig. 3b illustrates the arrangement of FIG. 3a after the start of crystal growth, the length of the piriform body 25 during production having increased so that this body passes through the opening 17 of the iridium cover 16. At this stage of operations, which remains until 'that the piriform body has been produced to the desired length, for example from 150 to 450 mm, a chamber 35 is formed and delimited by the lower surface of the iridium cover 17, the internal surface 36 of the iridium crucible 8 and the circumferential surface of the piriform body 25, as well as the surface of the bath. This chamber contains practically the surface of the bath 9, and the ambient atmosphere prevailing in the chamber 35 is that to which the bath 9, the crystal growth interface 40 and the neighboring surfaces of iridium are exposed. It has been discovered in the context of the invention that, under the conditions shown in FIG. 3b and prevailing during most of the crystal drawing, the oxygen of the ambient atmosphere contained in the chamber 35 gradually depletes, if it is not replaced, compared to the ambient atmosphere prevailing in the bell 3, because the only possible passage for the desired ambient atmosphere and prevailing in the bell 3 is that which is delimited by the small passage 17 'included between the sides of the piriform body 25 and the opening of the cover 16, this weak passage allowing neither the complete replenishment nor the homogenization of the atmosphere in the chamber 35. In other words, the atmosphere prevailing in this chamber is practically isolated from the ambient atmosphere contained in the bell 3. The progressive impoverishment oxygen in the chamber 35 (if it is not compensated), and therefore on the surface of the bath and at the growth interface 40, causes an increase in the presence of iridium inclusions in the piriform body, with the consequence that the last part of this body produced by growth and indicated at 50 in fig. 2, that is to say the one whose growth interface has undergone the strongest effects of oxygen depletion, contains a large number of iridium inclusions, for example 100 per cubic centimeter, which it is desirable to avoid. The inclusions, which are well known in this technique, consist of small individual metal plates or particles from 1 to 20 µm in diameter. According to the invention, this harmful effect is avoided by introducing into chamber 35 a continuous flow of nitrogen containing from 0.5 to 3%, preferably 2%, by volume of oxygen, for example by a tube. of iridium 47 which, in the example shown, communicates with the chamber 35 through the cover of iridium 16. The flow of gas passing through the tube 47 and entering the chamber 35 is such that the gaseous atmosphere ambient temperature and located in the bell 3 is maintained in the chamber 35 above the surface of the bath and at the growth interface 40 during the entire process of drawing the crystal. As a result, the presence of harmful iridium inclusions in the piriform body 25 is practically avoided. The replenishing gas flow is preferably introduced, as shown in the drawing, through a hole in the iridium cover 16 which is close to the lateral surface of the crucible, so that this gas fills the chamber 35 with practically continuous so as to guarantee the presence of the desired ambient pressure in this chamber. Other arrangements making it possible to introduce the replenishment flow into the chamber 35 can be used, this atmosphere being able for example to be introduced through the lateral partition of the crucible so as to guarantee the same filling.

The proper gas flow rate can be easily determined for the particular device involved. For example, by knowing the volume of the chamber 35, Vc, by measurement or by calculation, a suitable flow rate can for example be equal to 0.2 to 15 Vc / min. Therefore, assuming that the volume of the chamber Vc is equal to 1 dm3, a suitable gas flow range is between 0.2 and 15 l / min. It is possible to use higher flow rates, provided that the surface of the bath is not disturbed. The volume of the chamber Vc necessarily increasing during the lowering of the level of the bath in the crucible during the growth of the crystal, it must be taken into account when adopting the flow rate of the gas. The examples which follow are intended to give a good understanding of the invention, but cannot in any way limit it.

Example 1:

About 11,500 g of Gd203 and Ga203, in a molar ratio of 3: 5 (3.02: 4.98), were placed in an iridium crucible with an inside diameter of 135 mm, a wall with a thickness 2.5 mm and a height of 145 mm. An iridium cover 140 mm in diameter, 2.5 mm thick and a circular central hole of 89 mm was placed on the top of the iridium crucible. This crucible was placed inside an eight-coil induction heating coil having an internal diameter of 190 mm. The crucible was placed on a base containing packed granules of zirconia, and the space between the coil and the crucible was also filled with

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zirconia granules. The entire device was enclosed in an aluminum bell (785 dm3) with an opening at the top. A nitrogen atmosphere containing about 2% by volume of oxygen was placed inside the bell by an inlet placed under the crucible and at a distance from the latter. The gas flow rate was 1.12 m3 / h. An iridium tube (6.25 mm inside diameter) placed 25 mm from the side wall of the crucible communicated with the inside of the crucible through the cover. In this example, no gas flow passed through the iridium tube. The induction heating coil was supplied with energy by a known group of high frequency induction heating, and the power was increased until the current introduced into the iridium crucible heated it to white. The heat coming from the iridium crucible formed by conduction a bath inside this crucible. The height of the wall of the crucible above the bath was at this time about 12.5 mm. A seed crystal formed of a non-crystalline garnet stem of gadolinium and gallium of 9.5 mm in diameter (having an orientation <111>) was lowered by the opening of the cover of iridium and brought into contact with the surface of the bath. The germ was then removed from the bath at a rate of about 7.25 mm / hr for 30 hr. The height of the wall of the crucible above the bath at the end of the growth was 115 mm. A piriform body 230 mm long and 81 mm in diameter, of circular cross section, was produced and contained numerous iridium inclusions (at least 100 per cubic centimeter) at the bottom, i.e. in the last third of the piriform body.

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Example 2:

The same operations as those of Example 1 were practically carried out, except that nitrogen containing 2% oxygen by volume was introduced by continuous jet through the iridium tube into the crucible at a flow rate of 0, 28 m3 / h. A piriform body with a diameter of 80 mm and about 200 mm in length was produced and contained few iridium inclusions (no more than 5 per cubic centimeter) over its entire length.

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Example 3:

The same process as that of Example 1 was practically followed, except that nitrogen containing 2% by volume of oxygen was introduced continuously by the iridium tube into the crucible at a rate

20 of 0.056 m3 / h. A piriform body 80 mm in diameter and approximately 200 mm in length was produced and contained few iridium inclusions (no more than 5 per cubic centimeter) over its entire length.

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2 sheets of drawings