FR2806743A1 - METHOD AND DEVICE FOR GROWING SINGLE CRYSTALS, IN PARTICULAR OF CaF2 - Google Patents

Abstract

The subject of the present invention is a method and a device for growing single crystals, in particular CaF2. As part of said method, a stack of crucibles (100, 101,..., 106,...) containing the raw material is translated continuously, without stopping, without jolts, successively through a chamber of melting (C1) and an annealing chamber (C2).

Description

The subject of the present invention is a method for growing

  single crystals and an associated device, suitable for implementing said method. Said method and device are particularly suitable for putting

  in the growth of single crystals of calcium fluoride (CaF2).

  To increase the integration density of electronic components on a semiconductor wafer and insofar as insolation light of very short wavelength (less than 248 nm) is necessary to improve

  resolution, ultra-high performance optical systems are required.

  To obtain such optical systems, the most widely used technique to date uses fused silica. According to another technique, already used, in particular by the companies Bicron and Schott, we use

  single crystals of calcium fluoride (CaF2).

  To obtain such single crystals or single crystals of the same type (single crystals of alkaline-earth fluoride, generally, or even single crystals of silica), the procedure is known as the Stockbarger or Bridgman method. According to said method, the crystals are generated in an oven, inside which a crucible containing the molten material is moved, along a vertical axis, from top to bottom, from a hot zone to a cold zone. The temperature of said hot zone is kept higher than the melting temperature of the material in question (it is higher than 1525 C, for CaF2). Speed

  progression of the crucible is about 0.3 to 5 mm / h.

  By passing from the hot zone to the cold zone, the material crosses an zone of strong thermal gradient. Crystallization takes place in the crucible when the material reaches the zone where the temperature is below its melting temperature. The crystallization front, fixed, propagates inside the crucible, within the material, from bottom to top, insofar as said crucible is animated

in a downward movement.

  To avoid any oxidation of the material and the components of the furnace, said furnace is generally maintained under vacuum. The crucible is made of a material which resists chemical attack on the material it contains. It's about

  usually a graphite crucible.

  In fact, said method is mainly implemented according to two variants. * According to the first, implemented batchwise, a crucible stack (each crucible of said stack containing the material in question) is cold loaded in the upper zone of an oven, with a vertical axis, upper zone intended to become the hot zone of said oven. The oven, loaded in its upper zone, is heated. The pile of crucibles is, initially, kept heated, at a temperature higher than the melting temperature of the material in question, in said upper zone or hot zone of said furnace. In a second step, said stack is lowered and maintained in the lower zone of said oven, known as the cold zone, maintained at a temperature lower than that of the hot zone,

  lower than the melting temperature of said material in question.

  The main drawbacks of this variant of implementation are its low productivity and its poor yield. Productivity is hampered by dead times, before and after the growth of the crystals (dead times for loading the oven, vacuuming said oven, heating the two zones of said oven, cooling said two zones, unloading said oven) . The yield is affected by the fact that each crucible of the stack is not exposed to the same thermal conditions, during a complete cycle of said stack. The crucible at the bottom of said stack is in particular more rapidly brought into contact with the cold zone. It remains longer in said cold zone. In addition, such discontinuous thermal conditions (hot zone / zone

  cold), inside the same oven, are difficult to control.

  According to the second variant, described in the Russian author's certificate No. 2161891 filed on August 8, 1975, the aim is to implement a continuous heat treatment. A stack of crucibles occupies the entire height of an oven, with a vertical axis, which has two superimposed heat treatment chambers. Each crucible of the stack passes successively, during a translational movement of said stack, from top to bottom, the upper chamber and then the lower chamber. For each evacuation of the crucible from the bottom of the pile (crucible which has therefore successively passed through said two heat treatment chambers), said pile is stopped, in its movement of vertical translation (the crucible surmounting said crucible from the bottom of the pile being sandwiched between jaws, for maintaining and stabilizing said stack). This implies a certain discontinuity in the heat treatment; this implies, in any event, decelerations and accelerations of the movement of the pile, jolts, responsible for vibrations within the treated material. This is strongly

  detrimental to an optimized growth of the sought single crystals.

  In such a context, the inventors have designed and developed an optimized process for growing single crystals. Said method is also based on the so-called Stockbarger or Bridgman method, described above. It is optimized in that it is really a continuous heat treatment process, which ensures that each crucible of the stack in translation in the furnace has the same history (all

  particularly with regard to thermal conditions), and this without

blows, without any vibration.

  The prior art set out above as well as the invention set out below are only under the mechanical aspect of the process (and its associated device) in question; the chemical aspect, and in particular the advantageous presence of a fluorinating agent, not being recalled here, having been described, in general, in the literature. The process and device of the invention are referred to in the present text as the process and device for growing single crystals. This qualification cannot be limiting. They can more generally be described as a method and device for growing crystals (poy- and single crystals) insofar as they are obviously suitable for generating polycrystals. Their use for generating only such polycrystals is not excluded from the scope of the present invention (but it however appears hardly pragmatic insofar as polycrystals can be obtained much more simply). The method and device of the invention can more precisely be qualified as a crystal growth method and device, optimized to obtain a

  interesting yield in single crystals.

  The process of the invention is therefore a process for growing single crystals (of the CaF2, BaF2 .... SiO2 ... type), implemented away from impurities (generally under vacuum and / or in an atmosphere controlled) in a vertical axis oven, inside which two superimposed heat treatment chambers are arranged: a first chamber called the melting chamber and a second chamber called the annealing chamber, a significant thermal gradient being

  formed between said first and second chambers.

  Said method of the invention comprises: - maintaining and translating along a vertical axis a stack of crucibles containing the raw material (said raw material, within each crucible, generally occurring in the form of a powder or 'a pre-melted disc), inside said oven; said stack of crucibles having, in regime, a height greater than the sum of the heights of said first and second superimposed chambers and being maintained and translated, in a direction such that each of the crucibles constituting it successively crosses said first chamber then said second chamber, under the action of means, acting on at least the crucible at the bottom of said stack and arranged in a third chamber called translation chamber, positioned under all of said first and second chambers; - Loading a new crucible, upstream of said first chamber, at one of the ends of said stack and unloading the crucible having successively passed through said first and second chambers, at the other of the ends of said stack; the loading and unloading operations being carried out at the same frequency so that the height of said stack, in

  regime, is kept substantially constant.

  In this, the method of the invention is a method of the type described in the Russian author's certificate mentioned above. It can however already be noted that, according to the method of the invention, the pile of crucibles can go up or down, the superimposed melting and annealing chambers being positioned

adequately.

  According to a first variant, the stack of crucibles is translated from the top to the bottom, the first (melting) chamber then surmounting the second chamber

  (annealing) which itself overcomes the translation chamber.

  According to a second variant, the pile of crucibles is translated from the bottom to the top, the first (melting) chamber then surmounting the

  translation and located under the second (annealing) chamber.

  Said first variant is a priori preferred.

  The thermal conditions ensured in each of the two melting and annealing chambers as well as the resulting thermal gradient are those necessary to obtain the expected effect: the formation and growth of

single crystals within the crucibles.

  Characteristically, within the framework of the process of the invention as specified above, the loading operations (crucibles loaded with raw material) and unloading operations (said loaded crucibles, having successively passed through the two superimposed heat treatment chambers ) are implemented without stopping the translation of the stack of crucibles, within the furnace. Said translation is implemented in a perfectly continuous manner. Said battery is, according to the invention, driven continuously, without jerks. The heat treatment thus implemented is carried out in a perfectly continuous, perfectly

  identical for all crucibles, without any jerks.

  To start the process of the invention, as described above, use is generally made of a first stack of crucibles, called priming. It can be a stack of empty crucibles or directly a stack of loaded crucibles,

  the content of said crucibles of said priming stack then being intended for scrap .....

  In the context of the implementation of the method of the invention, the stack of loaded crucibles can only be driven by the continuous translational movement which transfers each crucible, successively, through the melting chamber, then through the chamber annealing. It is advantageously driven both by said continuous translational movement and by a rotational movement on itself. This optimizes the homogeneity of the heat treatment in the

  mass of material, within each crucible of said pile.

  For maintaining and setting in motion of said stack - movement of simple translation or combined movement of translation and rotation; movement, in any event, perfectly continuous - adequate means, arranged in the translation chamber, act, as indicated above, on at least the crucible at the bottom of the pile. For obvious reasons of stability of said

  stack and perfect drive thereof, said means act advantageously

  ment on at least the two crucibles (2, even 3, even 4) at the bottom of said pile.

  In the context of the preferred variant implementation of the method of the invention, said means act on the side wall of at least the crucible at the bottom of the stack, advantageously on the side walls of at least the two crucibles ( 2, or even 3, even 4) from the bottom of said stack. Such a lateral action is particularly suitable for ensuring continuous training without interruption. We talked about the side wall, in the singular, crucibles, insofar as they are generally cylindrical crucibles (therefore having a single side wall). This singular is in no way limiting. In the hypothesis of the intervention - unlikely, a priori - of rectangular crucibles, the means of holding and setting in motion of the pile obviously intervene, advantageously, on at least two of the side walls

  (advantageously opposite) of at least the crucible at the bottom of the pile.

  For the loading of a new crucible and the unloading of a crucible having successively passed through the two superimposed heat treatment chambers, it is possible to operate in different ways. We can in particular be inspired

  of the technique described in the Russian author's certificate identified above.

  Advantageously, in the context of the implementation of the method of the invention, the loading operation and / or (advantageously and) the unloading operation is carried out along the axis of the pile of crucibles. Said axis of the crucible stack

  preferably corresponds to the axis of the oven inside which said stack progresses.

  The process of the invention, as described above in general terms, is perfectly suited, as already indicated, for the growth of (mono) crystals of CaF2. Preferred variants of implementation of said method are described, with reference to the appended figures, in a non-limiting manner, further in

this text.

  According to its second object, the present invention relates to a device suitable for implementing said method, that is to say a device suitable for the continuous growth of single crystals, said device comprising, arranged in an enclosure, with vertical axis : - three superimposed chambers, a first thermal treatment chamber known as the melting chamber, a second thermal treatment chamber known as the annealing chamber and a third chamber known as the translation chamber positioned under all of said first and second chambers; - heating means, associated with each of said two heat treatment chambers, to maintain within them adequate heat treatment temperatures with a consequent thermal gradient between said two chambers; - Means for maintaining and translating, along a vertical axis, through said three superimposed chambers, a stack of crucibles; said means, being arranged in said third chamber, acting on at least the crucible at the bottom of said stack and ensuring said translation in a direction such that each of the crucibles of said stack successively passes through said first chamber then said second chamber; means for communicating, respectively: said first chamber with a crucible loading zone, at the top of said pile and said third chamber with a crucible unloading zone, at the bottom of said pile, when said pile of crucibles is translated from top to bottom; or said third chamber with a crucible loading zone, at the bottom of said pile and said second chamber with a crucible unloading zone, at the top of said pile, when said pile of crucibles is translated from the bottom up; - means to ensure loading and unloading operations; - Means for maintaining said three superimposed chambers and said loading and unloading zones, away from impurities; said means for maintaining and translating said stack, said means for establishing communication and said means for ensuring loading and unloading operations cooperating so that said loading and unloading operations are performed without stopping the translation of

said stack.

  Said means cooperate to ensure a continuous translation of

said stack.

  Within said enclosure (of the crystallization furnace), there are therefore, from top to bottom: - according to a first variant: + the loading zone, + the melting chamber, + the annealing chamber, + the translation chamber , + the unloading area, the means, ensuring the maintenance and translation of the stack of crucibles, ensuring, according to said first variant, said translation from the top to the bottom; - according to a second variant: + the unloading zone, + the annealing chamber, + the melting chamber, + the translation chamber, + the loading zone, the means, ensuring the maintenance and translation of the pile of crucibles , ensuring,

  according to said second variant, said translation from the bottom to the top.

  The device of the invention is advantageously arranged according to the

first variant above.

  In the context of one or other of said variants, said three superimposed chambers for melting, annealing and translation advantageously have the same axis, which preferably coincides with the axis of the pile of crucibles and said loading zone or (and, advantageously and) said unloading area

  is (are) arranged along the same axis.

  To obtain the expected result - loading and unloading of crucibles without stopping the continuous translation of the stack of

  crucibles - by cooperation of the appropriate means indicated above, we have before-

  suitably the means for maintaining and translating said stack which act on the side wall of at least the crucible at the bottom of said stack, which advantageously act on the side walls of at least two crucibles at the bottom of said stack . Said means which maintain and continuously translate said cell constitute the key means of the device of the invention. They can exist in different forms. They can in particular be suitable for ensuring only a simple translational movement of said stack, they can also be suitable for ensuring both a translational movement and a movement of

  rotation of the said stack on itself.

  Three types of such means are specified below, by way of illustration. The first two types are suitable for simple translation, the third for

  a translation combined with a rotation.

  Said means for ensuring the maintenance and the continuous translation of the stack of crucibles within the enclosure (furnace) with a vertical axis can in particular: - comprise at least one set of at least two rollers with horizontal axes, acting on the wall lateral crucible at the bottom of the stack, and advantageously include, in addition to said set, at least one other set of the same type, acting on at least one other crucible at the bottom of the stack (acting

  advantageously on the crucible overhanging said crucible from the bottom of said pile).

  The set (s) of such rollers advantageously comprises (include) four rollers, distributed at 90. It is therefore recommended to use two superimposed sets (one acting on the crucible at the bottom of the pile, the other on the crucible just above) of four rolls each, made of a suitable material (1st type); - Include at least two sets of at least two clamping pads each, each of said games suitable for ensuring the maintenance of said stack by tightening the crucible at the bottom of said stack and being capable of being moved away from said stack and put in translation along a vertical axis, alternating with the other game,

  to ensure the continuous translation of said stack (2nd type).

  Said means for ensuring the maintenance and the continuous translation / rotation of the stack of crucibles within the enclosure (furnace) with a vertical axis may in particular: - comprise at least one set of at least two rollers with an axis slightly inclined relative to vertically, acting on the side wall of the crucible at the bottom of said stack, and advantageously comprising, in addition to said set, at least one other set of the same type, acting on at least one other crucible at the bottom of said stack (acting advantageously on the crucible overhanging said crucible of

  bottom of said pile). The set (s) of such rollers comprises (include) before-

  three rollers, distributed at 120, made of a suitable material (3rd type).

  Each of the rollers of such a set, in order to rotate the crucible stack during its translation, is offset, with respect to the latter by an angle a (. Each of said rollers turns in fact around an axis which is therefore not parallel to the vertical axis

  longitudinal of the device and which is also not concurrent with said axis.

  Preferred variant embodiments of the device of the invention are

  described, in more detail, with reference to the appended figures, below.

  We now propose to describe the invention as well in

  terms of process than in terms of device, with reference to the appended figures.

  Figure 1 is a longitudinal section of a device (single crystal growth oven) of the invention, equipped with a first type of means

  to maintain and (simple) continuous translation of the crucible stack.

  Figure 2 is a section on II-II of said Figure 1 which shows in

detail said means.

  Figure 3 is a partial longitudinal section of a device of the invention, equipped with a second type of means for ensuring the maintenance and

  (simple) continuous translation of the crucible stack.

  Figure 4 is a section on IV-IV of said Figure 3 which shows in

detail said means.

  Figure 5 is a partial longitudinal section of a device of the invention, equipped with a third type of means for ensuring the maintenance and

  translation (associated with a rotation) of the pile of crucibles.

  Figure 6 is a section on VI-VI of said Figure 5 which shows in

detail said means.

  FIG. 7 is a section on VII-VII of said FIG. 6.

  The process and device of the invention are described below in

  reference to FIG. 1. Said FIG. 1 shows an oven of the invention.

  The external structure of said oven is delimited by an external envelope 8, cooled with water, which isolates the atmosphere of said oven from the ambient. Said external structure also includes a heat shield 7, made of fibrous graphite, positioned "at a distance" from the electrical resistors 2 and 4. Said heat shield 7 protects

  the outer envelope 8 of the thermal radiation.

  Inside said furnace, there are the three superimposed chambers, the melting chamber CI, the annealing chamber C2, the translation chamber C3, and, in alignment with said three chambers, above: the loading area B, at below: the unloading zone D. In fact, we could speak, more precisely, of loading with reference to zone A and of packaging with reference to zone B, of unloading with reference to zone E and of unpacking in reference to the

  zone D (see description of the process below).

  The pile 1 of the crucibles (100, 101, ..., 106, ...) is heat treated in the area C = C1 + C2. Said stack 1 is, according to FIG. 1, continuously translated, from top to bottom. It is in fact maintained and translated by a

  translation mechanism which includes two sets of horizontal rollers 5.

  The material, powder, inside the crucibles is melted in the melting zone CI, upper, heated by the electrical resistances in graphite 2. Said material, melted, then cools slightly while passing from said melting zone CI to the annealing zone C2, which is heated by other resistors of the same type 4. The temperature gradient inside the oven, between said zones CI and C2, is obtained by a diaphragm 3, which partially isolates said zone C1 "hot" of said "cold" zone C2. A cold water circulation can be provided inside said diaphragm 3, in order to obtain a more substantial intensity gradient. As this zone passes, the temperature transition inside the

  material, in each crucible, creates a stationary crystallization front.

  The translation chamber C3 is cooler than the annealing chamber C2. A second diaphragm 6, which can also be cooled by circulation of water, intervenes to partially isolate said chamber C3 from said chamber C2. Said diaphragm 6 thus protects the rollers 5 from excessive heat. Said rollers 5 are advantageously coated with silicone. The friction with the crucibles (100, 101, ...., 106, ...) is thereby improved. In the variant shown, there are two sets of four rollers 5: a first set which acts on the crucible at the bottom of the stack and a second set which acts on the crucible 101. This gives good stability to the stack 1 of crucibles, in all directions, as well as a

  perfect drive of the latter, in translation, from top to bottom.

  In Figure 2, we can perfectly see one of these games with four horizontal rollers. Each roller 5 is driven by a shaft 19 which crosses the outer casing 8 of the oven as well as the heat shield 7. The crossings of said casing 8 are made in a perfectly sealed manner, said oven generally operating under vacuum. All the bearings and mechanical drive devices are located outside of said furnace. The two sets of four shafts 19 are coupled by sets of gears and driven in rotation by a single motor (not shown), this in order to ensure simple and reliable synchronization of the

roller rotation 5.

  The device of the invention, as shown in these Figures 1 and 2

works as follows.

  A new crucible 9 is periodically loaded inside. When the loading area B (airlock) is at ambient temperature and pressure, two symmetrical doors 12 are advantageously open and a support 10 rises in the sampling position to take hold of said new crucible 9. Said support 10, loaded with said crucible 9, descends inside zone B. The two doors 12 close. The vacuum is created inside said zone B, by means of a pump (not shown) and the temperature is increased by virtue of the electrical resistors 1 1, similar to those, 2 and 4, located inside the oven. When the pressure and temperature conditions inside zone B are similar to those of the melting chamber C1, the door 13 opens automatically and the new crucible 9 is loaded at the top of the stack 1. The support 10 then withdraws inside the zone B, the door 13 is closed. Said zone B is returned to the

  ambient temperature and pressure ...

  For unloading, at the bottom of stack 1, the process is similar but reversed. When the crucible 100 at the bottom of the stack 1 is ready to leave the rollers 5, the door 14 opens automatically. A support 16 then rises to receive said crucible 100. Said support 16, loaded with said crucible, descends into the unloading zone D (airlock: which has been set to the same pressure and temperature conditions as the translation chamber C3). For the rise in temperature, electrical resistors 17 are used. The door 14 then closes and said zone D can be brought back to ambient temperature and pressure. The crucible thus separated from the pile 1 has been referenced 100 ′ in FIG. 1. When the zone D is at ambient temperature and pressure, the double door 18 opens, the support 16 descends and the crucible 100 ′ can be recovered. The support 16 then rises in zone D, in the standby position of the crucible 100. The double door 18 is closed and said zone D is brought to the same temperature conditions and

  pressure that the translation chamber C3 ...

  The devices according to FIGS. 3 and 5 operate in a similar manner, with however means, to ensure the maintenance and the translation

  continues from stack 1 of different crucibles.

  The means appearing in FIGS. 3 and 4 ensure, just like the means appearing in FIGS. 1 and 2, a simple translation of the stack 1 of crucibles. Said means according to FIGS. 3 and 4 are more complex than those according to FIGS. 1 and 2. However, they have the advantage of acting without exerting a bearing, of friction on the crucibles. Said means, according to Figures 3 and 4, comprise four clamping pads 20, 23, 24 and 25, which are capable of retaining and moving down the stack of crucibles. The movements of these clamping pads 20, 23, 24 and 25 are not rotations but vertical translations and

  successive (horizontal) radials, mechanically synchronized.

  The diametrically opposed clamping pads 20 and 23 operate alternately with the diametrically opposed clamping pads 24 and 25. Clearly seen in Figure 4, the clamping pads 20 and 23 which drive the stack 1 down while the clamping pads 24 and 25 are no longer in contact with said stack 1 and move them up. Before said clamping pads 20 and 23 reach their low position, said clamping pads 24 and 25 are pressed against said stack 1, ensuring, continuously, a maintenance and a progression downwards thereof. Then, said clamping pads 20 and 23 are radially spaced and moved upward. Each bearing is activated independently for its two types of movement (horizontal movements and vertical movements), thanks to shafts and cams. Thus, the cam 21 intervenes for the vertical translation of the bearing 20 and the cam 22 for its radial translation. Advantageously, such cams are used to the extent that, therefore, all of the drive movements are rotations. The crossing of the heat shield 7 and the outer casing 8 by the intervening shafts poses hardly any problem

sealing.

  The synchronization of the movements of all the clamping bearings can be obtained either mechanically with a single motor and suitable gears, or by using brushless motors with a

  electronic synchronization by command.

  The device according to FIGS. 5, 6 and 7 incorporates means, in the translation chamber C3, capable of ensuring the maintenance and the translation with

  continuous rotation of the pile 1 of crucibles.

  Said means consist of three rollers 30, 30 'and 30 ". The axis of each of said rollers 30, 30' and 30" is slightly inclined relative to the vertical. The angle of inclination has been referenced ac in FIG. 7. It develops in the plane (YOZ). Said angle of inclination is generally of the order of a degree. One of said rollers - the roller 30 is rotated by the motor 31 via the transmission means 32, of the belt or chain type. The other two rollers,

o10 30 'and 30 ", are free.

  The invention can further be clarified, purely by way of illustration, by

the following data.

  An oven of the type shown schematically in the appended figures is used

  for the growth of single crystals of CaF2.

  The overall dimension of said furnace is of the order of 2 to 3 meters, its diameter is approximately 1 meter. Graphite crucibles are used. Their diameter is 300 to 400 mm; their height of about 100 mm. Oven flow

  is about a crucible a day or a crucible every other day.

  The temperatures in the various chambers of the said furnace are:> 1525 C for the melting chamber (upper chamber), <1525 C for the annealing chamber (intermediate chamber whose upper part is at 1200-1500 C and the lower part at 200-300 C), and less than 200 C for the translation chamber (lower chamber).

Claims (12)

  1. Method for growing single crystals, implemented away from impurities in a vertical axis oven, inside which are arranged two superposed heat treatment chambers (CI and C2), a first chamber (Cl) called melting chamber and a second chamber (C2) said annealing chamber, a consequent thermal gradient being formed between said first (CI) and second (C2) chambers, said method comprising: - maintaining and translating along a vertical axis of a stack (1) of crucibles (100, 101, 106, ...) containing the raw material, inside said oven; said stack (1) of crucibles (100, 101, ..., 106, ...) having, in regime, a height greater than the sum of the heights of said first (CI) and second (C2) superimposed chambers and being maintained and translated, in a direction such that each of the crucibles (100, 101, 106, ...) constituting it successively passes through said first chamber (CI) then said second chamber (C2), under the action of means (5; 20 and 23, 24 and 25; 30, 30 ', 30 "), acting on at least the crucible (100) at the bottom of said stack (1) and arranged in a third chamber (C3) said translation chamber positioned under the set of said first and second chambers (CI and C2); - the loading of a new crucible, upstream of said first chamber (CI), at one of the ends of said stack (1) and the unloading of the crucible having successively passed through said first (CI) and second (C2) chambers, at the other end of said stack (1); the loading and unloading operations ement being implemented at the same frequency so that the height of said stack (1), in regime, is kept substantially constant; said growth process being characterized in that said translation of said stack (1) is continuous, said operations of loading and   unloading being implemented without stopping said translation.   2. Method according to claim 1, characterized in that it comprises   in addition the rotation on itself of said stack (1) of crucibles (100, 101 ,. 106, ...) in translation.   3. Method according to one of claims 1 or 2, characterized in that   said stack (1) of crucibles (100, 101., 106, ...) is maintained and set in motion, by simple translation or by translation and rotation, under the action of means (5; 20 and 23, 24 and 25; 30, 30 ', 30 ") which act on the side wall of at least the crucible (100) at the bottom of said pile (1), advantageously on the walls   lateral of at least the two crucibles (100 and 101) at the bottom of said stack (1).   4. Method according to any one of claims 1 to 3, characterized   in that the loading operation or (and) the unloading operation is (are) carried out in the axis of said stack (1), advantageously in the common axis of said stack (1) and said oven.   5. Method according to any one of claims 1 to 4, characterized   in that it is used for the growth of CaF2 crystals.   6. Device suitable for the continuous growth of single crystals comprising, arranged in an enclosure with a vertical axis: - three superimposed chambers (CI, C2 and C3), a first chamber (C 1) of heat treatment called melting chamber, a second thermal treatment chamber (C2) called annealing chamber and a third chamber (C3) said translation chamber positioned under the assembly (C1 + C2) of said first (CI) and second (C2) chambers; - heating means (2, 4), associated with each of said two heat treatment chambers (C1 and C2), to maintain within them adequate heat treatment temperatures with a consequent thermal gradient between said two chambers (C1 and C2 ); - means (5; 20 and 23, 24 and 25; 30, 30 ', 30 ") for maintaining and translating, along a vertical axis, through said three superimposed chambers (CI, C2 and C3), d 'a stack (1) of crucibles (100, 101 ...., 106, ...); said means (5; 20 and 23, 24 and 25; 30, 30', 30 ") being arranged in said third chamber (C3), acting on at least the crucible (100) at the bottom of said stack (1) and ensuring said translation in a direction such that each of the crucibles (100, 101, ...., 106, ...) of said stack (1) successively passes through said first chamber (CI) then said second chamber (C2); - Means (13, 14) for communicating, respectively: said first chamber (C1) with a loading zone (B) of crucibles, at the top of said pile (1) and said third chamber (C3) with a zone unloading (D) of crucibles, at the bottom of said pile (1), when said pile (1) of crucibles (100, 101, ..., 106, ...) is translated from top to bottom; or said third chamber with a crucible loading zone, at the bottom of said pile and said second chamber with a crucible unloading zone, at the top of said pile, when said pile of crucibles is translated from the bottom up; - means (10, 16) for carrying out the loading and unloading operations; - Means for keeping said three superimposed chambers (CI, C2 and C3) and said loading (B) and unloading (D) zones, protected from impurities; characterized in that said means (5; 20 and 23, 24 and 25; 30, 30 ', ") for maintaining and translating said stack (1), said means (13, 14) for establishing communication and said means (10, 16) for carrying out loading and unloading operations cooperate so that said loading and unloading operations are carried out without stopping the translation of said stack (1).   7. Device according to claim 6, characterized in that said means (5; 20 and 23, 24 and 25; 30, 30 ', 30 ") for maintaining and translating said stack (1) act on the wall side of at least the crucible (100) from the bottom of said pile (1), advantageously on the side walls of at least   the two crucibles (100 and 101) at the bottom of said pile (1).   8. Device according to one of claims 6 or 7, characterized in that   that said three chambers (C1, C2 and C3) have the same axis, which advantageously corresponds to the axis of the stack (1) of crucibles (100, 101, ..., 106, ...); said loading area (B) or (and) said unloading area (D) being arranged (s) along said axis.   9. Device according to any one of claims 6 to 8,   characterized in that said means for maintaining and translating said stack (1) comprise at least one set, of at least two rollers (5) with horizontal axes, acting on the side wall of the crucible (100) from below of the stack (1), and advantageously comprise, in addition to said set, at least one other set of the same   type, acting on at least one other crucible (101) from the bottom of the pile (1).   10. Device according to any one of claims 6 to 8,   characterized in that said means for maintaining and translating said stack (1) comprise at least two sets (20 and 23, 24 and 25) of at least two clamping bearings (20, 23; 24, 25) each, each of said sets (20 and 23, 24 and 25) suitable for maintaining said stack (1) by tightening the crucible (100) at the bottom of said stack (1) and being capable of being moved away from said stack (1) and set in translation along a vertical axis, alternating with the other set,   to translate said stack (1).   11. Device according to any one of claims 6 to 8,   characterized in that said means (30, 30 ', 30 ") which maintain and continuously translate said stack (1) also rotate on itself of said stack (1).   12. Device according to claim 11, characterized in that said means comprise at least one set, of at least two rollers (30, 30 ', 30 ") with axis slightly inclined (ac) relative to the vertical, acting on the side wall of the crucible (100) at the bottom of said stack (1), and advantageously comprise, in addition to said set, at least one other set of the same type, acting on at least one other crucible at the bottom of said pile.