FR2569430A1 - APPARATUS FOR EXTRACTING SINGLE CRYSTALS FROM A BATH OF MOLTEN SEMICONDUCTOR MATERIAL CONTAINED IN A CRUCIBLE - Google Patents

Abstract

TO INCREASE THE EXTRACTION SPEED OF THE MONOCRISTAL 6, IT IS NECESSARY TO INCREASE THE TEMPERATURE GRADIENT (DTDX) IN THE SOLID PHASE OF THE MONOCRISTAL AT THE SOLID-LIQUID INTERFACE. SINCE THE SINGLE CRYSTAL IS ALWAYS HEATED BY THE HEAT RADIATION FROM HEATING DEVICE 4, IT IS PREFERABLE TO REDUCE THE TEMPERATURE OF IT. THE LOWERING OF THE TEMPERATURE OF THE HEATING DEVICE, HOWEVER, CREATES THE PROBLEM THAT THE SURFACE OF THE BATH 3 IS EASILY SOLIDIFIED IN THE SURFACE ZONE 3A CONTIGATED TO THE INTERNAL WALL OF THE CRUCIBLE 2. TO SOLVE THIS PROBLEM, THE HEATING DEVICE 4 IS CONSTRUCTED. SO THAT THE UPPER PART OF THE BATH 3 IS TAKEN TO A HIGHER TEMPERATURE THAN THE LOWER PART OF THE BATH.

Description

The present invention relates to an apparatus for producing single crystals,

  comprising a crucible intended to contain

  a bath of molten material from which one can form a crystal, a dispo-

  heating sitive to heat the bath and a suitable device for extracting the single crystal in the form of a bar or ingot from

molten material bath.

  Generally, the Czochralski method (CZ method) has hitherto been used to extract or draw a silicon single crystal. This method is illustrated in Figure 13. A bath of molten material, from which a single crystal can be drawn, is contained in a quartz crucible 2, itself housed in a graphite crucible 1

  and heated by a heat generator 4 which surrounds the crucible 1.

  A single crystal 6 in the form of a bar is pulled from the bath 3 using a mandrel 7 and starting with a seed 5. During the drawing, the crucibles 1 and 2 and the single crystal 6 are rotated at constant speeds in direction reverse by a shaft 8 on the one hand and the mandrel 7 on the other hand. The shaft 8 also raises the crucible 1 so that the surface of the bath 3 remains at a predetermined position by

  relation to the heat generator 4.

  Suppose that the solid-liquid interface between the

  single crystal 6 and the bath 3 is flat and there is no graph

  dient of temperature in the radial direction of the single crystal 6, the maximum extraction speed Vmax of the single crystal according to this method max CZ can be defined as follows: k dT max h '* dX ok designates the thermal conductivity of the single crystal 6, h the heat of fusion, p the density and dT / dX the temperature gradient

  in the solid phase (single crystal 6) at the solid-liquid interface.

  X denotes a coordinate in the longitudinal direction of the mono-

  crystal. As k, h and f are factors inherent in the material in the expression given above, it is necessary to increase dT / dX if one wants to obtain a higher extraction speed Vmax. However, with the CZ method mentioned in the above, because the drawn single crystal 6 is heated by the heat radiation from the surface of the bath 3, the internal wall of the crucible 2 and the heat generator 4, the value dT / dX is inevitably reduced, so the speed

  in practice is relatively low.

  This extraction speed can be increased by reducing

  overall the temperature of the heat generator 4. However

  dant, as also emerges from Figures 10A and 10B, since the temperature on the surface of the bath 3 is significantly lower than in the central part of the mass of liquid, if the temperature of the heat generator 4 is reduced, the material of the bath 3 in the surface area 3a contiguous to the crucible 2 solidifies. The resulting disturbances make it impossible to pull the single crystal 6 continuously. Therefore, with the CZ method, using an extraction machine like the one shown in Figure 13, the maximum extraction speed that can be obtained is about 1 mm / min. In FIGS. 10A, 11A and 12A, the temperature gradient is T1 <T2 <T3; moreover, the temperature of the isotherm or curve connecting points of the same temperature T2 is approximately equal to that of T4 and the temperature of the isotherm

is approximately equal to that of T5.

  The prior art documents relating to the present invention are Japanese Patents Nos. 51-47153 and No. 58-1080. The last document in particular describes an embodiment in which a screen against radiation is placed above the bath of molten material in order to allow the extraction of the

  single crystal at a speed of 2 mm / min.

  In view of the above, the invention aims in the first place to provide an extraction device capable of eliminating the drawbacks mentioned above of known extraction devices.

  According to the invention, an apparatus for extracting mono-

  crystals, comprising a crucible intended to contain a bath of material

  molten crystallizable line, a device for drawing a single crystal from this bath and a heating device for heating the bath, is characterized in that the heating device is constructed so that a greater amount of heat can be supplied to the part crucible whose internal wall is contiguous to the surface of the molten crystallizable material bath than to the other parts of the crucible. Because the crucible portion at the height of the surface of the bath is heated more than the rest of the crucible, the overall temperature of the heater or heat generator can be lowered without the liquid from the surface area contiguous to the wall of the crucible may solidify. It thus becomes possible to extract the single crystal at a higher speed than in a conventional device of this type,

  without this causing defects in the single crystal and without affecting

  ter the quality of it in another way.

  According to another characteristic of the invention, the ap-

  similar also includes a thermal covering arranged around the drawn single crystal to thermally insulate it from heat

of the heat generator.

  According to a particularly advantageous embodiment

  geux, the heat generator is a hollow body of generally cylindrical shape, of electrically conductive material, in the wall of which are formed upper and lower slots, all extending

  in the direction of the axis of the heat generator and succeeding one another

  natively at regular angular intervals in the direction of the circumference of the heat generator, so as to define in the wall of the heat generator a meandering path for an electric heating current. With a heat generator thus produced, various possibilities are offered to increase the resistance and

  therefore the temperature of the part of the heat generator

  especially on the part of the crucible where the

bath surface.

  One of these possibilities consists in providing the upper part of the heat generator with a regular upward thinning. It is also possible, or in addition, to extend each of the lower slots at its top end by two short V-shaped slots. According to another variant, the wall of the generator thins out

  bottom up so that the area of its cross section decreases linearly

rement in the upper part.

  The upper part of the heat generator wall

  their can also have at least one constriction which extends around the entire periphery of the generator and is defined by at least one curved recess, which also reduces the cross-sectional area of the heat generator in the upper part. Yet another possibility is to provide the upper part of the wall of the heat generator with several circumferential grooves of rectangular section, which are also intended to reduce the area

  of the straight section - that is to say the section useful for circulation -

  tion of the electric heating current - of the heat generator, so as to again produce the increase in temperature at the top of the heat generator and of the crucible part situated at the level of the surface of the bath. It is also possible, for the same purpose, to reduce the width of the wall sections located between the slots in the upper part of the generator.

  heat, compared to the bottom of it.

  Another embodiment provides for the division of the heat generator into two elements of generally cylindrical shape, also made of electrically conductive material, an upper element and a lower element. The desired effect is obtained in this case by the fact that the upper element is brought to a higher temperature.

  higher than the lower element.

  According to another characteristic of the invention, a

  electric field is applied to the heat generator.

  Other characteristics and advantages of the invention

  will emerge more clearly from the description which follows from

  several non-limiting exemplary embodiments, as well as the accompanying drawings, in which:

  - Figure 1 is an axial section of an embodiment

  sation of the extraction apparatus according to the invention; - Figure 2 is a partial perspective view on a larger scale of a heat generator usable in the apparatus of Figure 1; - Figure 3 is a graphical representation showing the relationship between the density of texture defects and the concentration

  oxygen in the drawn single crystal; -

  - Figures 4 to 6 are vertical sections of the

  wall of the heat generator according to different variants of

  station; - Figure 7 is a side elevational view of part of the wall of a heat generator according to yet another alternative embodiment; - Figures 8 and 9 are axial sections showing other embodiments of the monocrystalline extraction apparatus according to the invention; - Figures 10A and LOB are respectively a schematic axial section of an extraction apparatus of the prior art, showing by isotherms the temperature distribution in the

  bath of crystallizable material contained in the crucible and a representation

  corresponding graphical feeling of the temperature distribution T in the direction of the central axis of the bath; - Figures 11A and 11B are representations

  similar but relating to an extraction device according to the invention

tion;

  - Figures 12A and 12B are also representations

  similar tations, but with the application of a magnetic field to the heat generator of an apparatus according to another embodiment of the invention; and

  - Figure 13 is an axial section showing an apparatus

  reil of extraction of the prior art according to the CZ method.

  The following description of the first embodiment

  sation of the invention uses the same references as Figure 13 for identical or similar elements whose various functions will not be described in more detail When this is not

not necessary.

  In this first embodiment, represented in FIG. 1, as in the known apparatus of FIG. 13, a graphite crucible 1 contains a quartz crucible 2 with a bath 3, in this case of molten silicon. A graphite heating device or heat generator 4 and a thermal insulation 9 are

  arranged around the crucible 1 and the insulation 9 is itself surrounded

  d of cooling jackets 10a, o10b and 10c, The cooling jacket 1Gb has a sight 12 for the observation of the drawn single crystal 6. The bottom of the cooling jacket 10a contains an exhaust pipe 13 for the evacuation of a neutral gas (atmospheric gas) introduced from above into the enclosure delimited by the cooling shirts 10a, 10b and 10c. The crucible 1 is supported at the bottom by a shaft 8 which serves to rotate and raise / lower this crucible and which crosses with clearance an opening 10d formed in the bottom of the cooling jacket 10a. The heat generator 4 is fixed by its lower end to a tray

  annular 14, itself supported from below by two rods 15 which

  put to raise and lower the heat generator 4 and which cross with play two openings 10e and 10f in the bottom of the jacket 10a. Above the bath 3 of molten silicon, there is also a thermal covering 16, of cylindrical shape, which is made of molybdenum and whose internal diameter is slightly larger.

  than the outside diameter of the single crystal 6 drawn. Above this overlap

  strictly thermal 16, a seed 5 is held by a mandrel 7 attached to the lower end of an extraction bar 17, which makes it possible to form a single crystal 6 in the form of a bar from the base

germ 5.

  Figure 2 shows a possible embodiment

  of the heat generator 4. It is a hollow body of general shape

  cylindrical spleen of a conductive material, for example graphite, the upper part 4a tapers upward conically. The

  wall of the heat generator 4 has a series of upper slots

  4b and a series of lower slots 4c, which are all

  oriented in the direction of the axis of the heat generator and succeed one another

  tooth alternately at regular angular intervals in the direction of the circumference of the heat generator. In addition, your upper ends of the slots 4c starting from the bottom are bifurcated or extended by two short slots 4d and 4e in V, each forming an angle of 45 for example with the slot 4c. The axial slots 4b and 4c thus delimit in the wall of the heat generator a meandering path, by which an electric current is circulated

  which generates heat by ohmic losses.

  To draw the single crystal from the bath 3 of molten silicon using the seed 5 and using the extraction apparatus thus constructed, the two crucibles 1 and 2 are rotated by means of the shaft 8, in the clockwise for example,

  and the seed 5 is turned in the opposite direction by the extraction bar

  tion 17. Of course, the directions of rotation can also be reversed. The extraction bar 17 is also gradually raised

  by a mechanism (not shown) for drawing the single crystal 6.

  In addition, the two crucibles 1 and 2 are gradually raised so that the surface of the bath 3 retains a predetermined position.

  relative to the heat generator 4.

  Thanks to the conical upper part 4a of the heat generator 4 and to the additional V-shaped slots 4d and 4e formed at the upper end of each of the slots 4c starting from the lower edge of the heat generator, a cross-sectional area in each of the portions wall delimited by two slots 4b starting from the upper edge of the generator is small compared to the cross-sectional area in the rest of the generator. Especially the sections

  at the height of the V slots 4d and 4e are relatively small.

  Therefore, when the generator is traversed by an electric current, the conical portions defining the upper part 4a of the wall of the generator are brought to a higher temperature than the rest of the generator 4. Consequently, like the upper part 4a of the wall of the heat generator is situated at the height of the surface of the bath 3, and the part of the crucible 2 contiguous to this surface and the corresponding part of the crucible 1 are

  heated more than the rest of the two crucibles, your different

  rence of temperature between the surface part 3a of the bath, which is contiguous to the internal wall of the crucible 2, and the maximum temperature in the bath 3 is relatively low, as can be seen in FIGS. 11A and 11B, compared to the difference between these two temperatures in an apparatus of the prior art, see FIGS. 10A and 10B. In addition, due to the conical upper part 4a, the total electrical resistance of the heat generator 4 is higher than that of a conventional heat generator of similar type, which makes it possible to obtain a higher temperature, on generator 4, with a current of the same intensity. At equal temperature, the invention therefore makes it possible to reduce the intensity of the current

  in comparison with a heat generator of the prior art.

  As indicated at the beginning, to increase the maximum extraction speed Vmax, the temperature gradient (dT / dX) must be increased in the solid phase, that is to say in single crystal 6, at the solid-liquid interface. In other words, it is preferable to reduce the temperature of the heat generator 4 because the single crystal drawn from the bath is heated by

  heat radiation from the heat generator 4.

  With the apparatus according to the invention, the lowering of

  The temperature of generator 5 to increase the temperature gradient

  erection (dT / dX) is possible because, as indicated above, the temperature difference between the surface area 3a of the bath and the maximum temperature more at the heart of the bath 3 is small. Therefore, without risk of solidification in the superficial zone 3a contiguous to the internal wall of the crucible 2, the temperature of the generator 4

  can be lowered, which results in a decrease in the radius-

  heat on the ingot extracted, that is to say by an element

  temperature gradient (dT / dX). The extraction speed can therefore be increased by up to 0.2 mm /

  min for example compared to the speed achievable in the device

  conventional eye. The single crystal 6 can additionally be drawn continuously, which increases the productivity and reduces the costs of

manufacture of the single crystal 6.

  Figure 3 is a graph showing the density of texture defects in the ingot drawn. It shows that the density of the defects in the single crystal 6 is very high at the conventional extraction speed of approximately 1 mm / min and that it is very low at an extraction speed of approximately 2 mm / min. The invention

  consequently makes it possible to improve the quality of the single crystal drawn.

  The heating of the single crystal 6 by the heat radiation from the generator 4 is also reduced by the arrangement of the covering 16 above the bath 3. This results in a further increase in the temperature gradient (dT / dX) and by the possibility

  increase the extraction speed accordingly.

  Yet another advantage is that, because the resistance

  total electrical tance of the heat generator 4 is higher than that of a conventional generator and the fact that the heat is better distributed in the bath 3 of molten material, this bath can be brought to the same temperature with a lower power electric, so that the consumption of electric energy by a heat generator according to the invention is lower than that

of a conventional generator.

  Of course, numerous modifications within the framework of the technical concept of the invention are possible in the embodiment which has just been described. For example, it might suffice to provide only the conical upper part 4a, without the V-shaped slots 4d, 4e. Conversely, we could be satisfied with

  form the V-shaped slots 4d, 4e without thinning the upper part 4a.

  In addition, as shown in Figure 4, it is possible to give a conical shape, in cross section, to the entire wall of the heat generator 4, from bottom to top. Figure 5 shows that it is possible

  also to form curved recesses 4f to create a narrowing

  sement in the upper part of the wall of the heat generator 4. Another possibility is illustrated in Figure 6, where a number of circumferential grooves 4g of different depths are dug in the upper part of the wall of the generator 4. The figure 7 shows that it is still possible to vary the width of the wall sections which remain between

  the slots extending from the upper edge and the lower edge of the

  rator. More precisely, the width t of these sections is smaller 1 * at the top of the heat generator 4 than the

  width t2 of said sections in the lower part of the generator.

  According to yet another variant, the upper part of the generator can be made of a material with higher resistivity than the

lower part.

  FIG. 8 represents a second embodiment of the apparatus according to the invention, o the heat generator 4 is divided into two superimposed elements, 18 and 19e both having a generally cylindrical shape. In this embodiment, the upper element 18 of the heat generator is brought to a temperature

  higher than the lower element 19.

  If the bath 3 is heated by induction by means of one or more high frequency coils, it is possible to increase the number of turns per unit of length in the upper part of the high frequency coil, compared to the number of turns in the lower part of the coil.

  FIG. 9 represents a third embodiment.

  tion of the apparatus according to the invention, o an electromagnet 21 is installed near the cooling jacket 10a for drawing the single crystal 6 with application of a magnetic field to the bath 3 of molten material, according to a method called MCZ. In this

  embodiment, such as bath 3 - which is electrically conductive

  stick - is subjected to electromagnetic forces, it is possible to suppress thermal convection. The convection suppressed, because the temperature distribution in the heat generator 4 is well reproduced in the temperature distribution of the bath 3, as can be seen in FIGS. 12A and 12B, it becomes possible to reduce the temperature at the core of the bath, while increasing the temperature at the periphery of the bath, and further reducing the temperature difference between the core and the surface area of the bath 3, compared to the temperature differences in the first and second embodiments. It follows that the extraction speed of the single crystal 6 can be increased more than in a device without the application of a magnetic field. Although in

  the example shown in Figure 9, the magnetic field is applied

  qué in the direction lateral to the device, it is also possible to apply it in the direction of its length, that is to say in the axial direction.

Example I:

  A 20 kg charge of polycrystalline silicon material is introduced into a crucible with a diameter of approximately cm, the material is melted and a single crystal 6 is drawn from the bath thus obtained. According to the conventional method, the liquid in bath 3 solidifies at an extraction speed of 1.2 mm / min in the

  zone 3a. In the method according to the invention, with provision of the overlap

  really thermal 16 over the bath 3, the single crystal 6 can be

  fired at a speed of 1.5 mm / min. When installing the overlay

  11. really 16 and that in addition a heat generator 4 is used whose upper part 4a is conical and has V-shaped slots 4d and 4e, it is possible to draw a single crystal 6 of a diameter

  about 10 cm at an extraction speed of 2 mm / min.

EXAMPLE 2:

  In the same way, a 20 kg load of a polycrystalline silicon-based material is introduced into a crucible with a diameter of about 30 cm, the material is melted and then a single crystal 6 is drawn from the bath thus obtained. With covering arrangement

  thermal 16 over bath 3 and additionally using a

  heat erector 4 whose upper part 4a tapers conically and has the V-shaped slots 4d and 4e, the single crystal can be pulled at a speed of 2 mm / min. When additionally applying a magnetic field as shown in Figure 9, it is possible to wear

  the extraction speed at 2.3 mm / min.

Claims (11)

  1. Apparatus for extracting single crystals, comprising a crucible intended to contain a bath of molten crystallizable material, a device for drawing a single crystal from the bath and a device   heater or heat generator to heat the bath, charac-   characterized in that the heat generator (4) is constructed so that a greater quantity of heat can be supplied to the crucible part (2) whose internal wall is contiguous with the surface of the bath (3) of material crystallizable melted than others parts of the crucible (2).   2. Apparatus according to claim 1, further comprising a thermal covering (16) disposed around the single crystal (6) taken from the bath (3) to thermally isolate the single crystal from heat of the heat generator (4).   3. Apparatus according to claim 1 or 2, the gen-   heat radiator (4) is a hollow body of generally symmetrical shape, made of electrically conductive material, in the wall of which are formed upper (4b) and lower (4c) slots, all extending   in the direction of the axis of the heat generator and succeeding one another   natively at regular angular intervals in the direction of the circumference of the heat generator, so as to define in the wall of the heat generator a meandering path for a current electric heating.   4. Apparatus according to claim 3, o the wall of the heat generator gradually thins upward in its the top part.   5. Apparatus according to claim 3 or 4, o each lower slot (4c) is extended at its upper end by two short slots (4d, 4e) in V. 6. Apparatus according to claim 3 or 5, where the generator wall thins uniformly from bottom to top, so that the area of its cross section decreases linearly in the part higher at least.   7. Apparatus according to claim 3, where the upper part   the wall of the heat generator has at least one narrowing defined by at least one curved recess- (4f) and intended   reduce the cross-sectional area of the heat generator.   8. Apparatus according to claim 3, where the upper part   of the wall of the heat generator has circumferential grooves (4g) of rectangular section, intended to reduce   the cross-sectional area of the heat generator.   9. Apparatus according to claim 3, where the width of the wall portions located between the slots is narrower at the top than at the bottom of the generator. heat.   10. Apparatus according to claim 1 or 2, the gen-   heat radiator is divided into two generally cylindrical elements, of electrically conductive material, an upper element (18) and a lower element (19), the upper element being carried   at a higher temperature than the lower element.   11. Apparatus according to any one of claims 1   at 10, o a magnetic field is applied to the heat generator.