DE102012100147A1 - Process for the preparation of mono-, quasi-mono- or multicrystalline metal or semimetal bodies - Google Patents

Abstract

The invention relates to a method for producing a mono-, quasimono- or multicrystalline metal or semi-metal body by directional solidification from a melt 3, in particular mono-, quasimono- or multi-crystalline silicon bodies using the vertical gradient freeze method (VGF method ). In the process, the melt solidifies in a crucible 2 under the action of a temperature gradient which runs in a vertical direction and from the upper end of the crucible 2 to its lower end, directed towards the mono-, quasimono- or multicrystalline metal or semimetal body. Before the melt 3 is introduced into the crucible 2 or before it is generated, the bottom of the crucible is covered with a thin separating layer 35, 36. According to the invention, the thin separating layer 35, 36 is formed from quartz glass, in particular from a highly pure, also synthetically produced quartz glass. Ingots produced in this way show very homogeneous properties over their entire volume and also have only a very low level of contamination in the edge areas.

Description

Field of the invention

The present invention generally relates to the production of comparatively large mono-, quasi-mono- and multicrystalline material blanks according to the vertical-gradiant-freeze method (also referred to below as VGF method), in particular of mono-, quasimono- and multicrystalline metal or semimetal bodies. preferably of mono-, quasi-mono- and multicrystalline silicon for applications in photovoltaics or mono-, quasimono- and multicrystalline germanium crystals.

Background of the invention

Solar cells should have the highest possible efficiency in the conversion of solar radiation into electricity. This depends on several factors, among others, the purity of the starting material, the penetration of impurities during the crystallization from the contact surfaces of the crystal with the crucible into the interior of the crystal, the penetration of oxygen and carbon from the surrounding atmosphere into the interior of the crystal and also from the growth direction of the individual crystal grains.

In addition to the production of monocrystalline silicon by the Czochralski process, the prior art also discloses the production of large-volume mono-, quasi-mono- and multicrystalline silicon ingots by directed solidification of molten silicon in a crucible. The crystal melt is deprived of heat at its bottom, so that a crystal grows from bottom to top. The result is a block of mono-, quasi-mono- or multicrystalline silicon.

In this case, an orientation of the isotherms of the temperature field is flat and parallel to the base of the crucible, d. H. horizontally, which results in a flat phase boundary so that the individual crystals then grow vertically from bottom to top.

The German patent application DE 10 2007 038 851 A1 The assignee, the contents of which are expressly incorporated herein by reference, discloses a method of producing monocrystalline silicon by the VGF method. The bottom of a crucible made of quartz glass is covered with a thin plate of monocrystalline silicon to achieve monocrystalline crystal growth. This plate can also consist of several smaller plates of monocrystalline silicon and becomes part of the monocrystalline silicon to be produced. The crucibles are usually purchased from the manufacturer and usually consist of a quartz glass having a degree of contamination of> 50 ppm. These impurities diffuse into the Si ingot during crystallization and thus make relatively large edge regions of the Si ingot unusable for later use for high-purity Si solar cells.

WO 2010/088046 A1 discloses a comparable method.

In the production of multicrystalline silicon according to the above method, although the seed crystal plates are not necessary at the bottom of the crucible. However, here too the soil of the multicrystalline Si ingot in particular is permeated with impurities, which results in an unacceptably high level of contamination.

EP 0 748 885 A1 discloses the use of a crucible formed from fused quartz to produce monocrystalline silicon by the Czochralski process. To avoid the formation of cristobalite layers near the surface of the crucible whose surface is provided with at least one devitrification-promoting coating.

EP 1 304 399 B1 discloses the coating of the inner surfaces of a quartz glass crucible with a coating for promoting crystallization at the surface.

DE 1 138 514 discloses a crucible formed of a refractory metal, such as tantalum, tungsten or molybdenum, whose inner surface is fixedly bonded to a quartz glass surface to reduce the release of contaminants.

Summary of the invention

It is therefore an object of the present invention to provide a more efficient process for producing a mono-, quasi-mono- or multicrystalline metal or semimetal body by directional solidification from a high purity melt.

These and other objects are achieved according to the present invention by a method having the features of claim 1. Further advantageous embodiments are the subject of the dependent claims.

Thus, the present invention relates to a process for producing a mono-, quasi-mono- or multicrystalline metal or Semi-metal body by directed solidification of a melt, in particular of monocrystalline silicon bodies according to the vertical gradient freeze method (VGF method), in which method the melt in a crucible under the action of a temperature gradient, in a vertical direction and from the top of the crucible extends to the lower end, directionally solidified to the mono-, quasi-mono- or multicrystalline metal or semimetal body, wherein prior to introduction or before the production of the melt in the crucible, the bottom of the crucible is covered with a thin separation layer.

According to the invention, the thin separating layer is made of quartz glass. Quartz glass is inexpensive to obtain in high purity and low impurity content, especially in standard dimensions corresponding to the bottom area of the crucible or an integral fraction ratio thereof. Thus, an efficient separation layer can be provided inexpensively. Due to the properties of quartz glass, in particular the high melting point, the quartz glass separating layer can in particular be provided very thinly and can still sufficiently withstand the high temperatures and the melt received in the crucible, so that overall a very high yield of mono-, quasimono- or multicrystalline metal or semimetal body. After solidification and dissolution of the ingot from the crucible just needs to be separated only the very thin separation layer at the bottom of the ingot.

The layer immediately above it already has the desired almost vanishing impurity content. In this case, the crucible can in particular also be formed from a quartz glass with a higher impurity content. This can be added, for example, for the purpose of mechanical support at the high process temperatures in a surrounding graphite crucible, so that the crucible is constructed overall comparatively inexpensive and simple. Nevertheless, according to the invention, a very high percentage of recoverable mono-, quasi-mono- or multicrystalline metal or metallo-metal ingot with a low impurity content can be achieved.

Surprisingly, it has been found that the thin (n) quartz glass plate (s) do not, at least not appreciably, float in the melt received in the crucible.

Unlike the prior art, according to the present invention, the thin separation layer of quartz glass is not used as a seed plate for inducing crystal orientation of the ingot in a direction parallel to the vertical direction of the crucible. For the purposes of the present invention, the quartz glass does not serve to induce a preferred crystallization direction, but merely serves to separate the melt from the bottom, if appropriate also from side walls, of the crucible. An ingot with homogeneous properties over its entire volume and in particular also over its edge regions can thus be provided more cheaply according to the invention, in particular with a lower impurity content.

According to a further embodiment, the quartz glass is a high-purity synthetic quartz glass which can be obtained cost-effectively with a virtually vanishing impurity content or with a maximum permissible impurity content, which is matched to the respective desired requirement, or can be produced inexpensively.

According to another embodiment, an impurity content of the crucible of iron (Fe) and other elements is at most 30 ppmw (parts per million by weight). The impurity content of the quartz glass on iron (Fe) and other elements is less than 30 ppbw (parts per billion by weight). Thus, already directly adjacent to the thin separation layer portions of the mono-, quasi-mono- or multicrystalline metal or Halbmetallingots have a nearly vanishing impurity content.

According to a further embodiment, the temperature of the bottom of the crucible is maintained at a temperature below the melting temperature of the metal or semi-metal in order to prevent melting of the thin separating layer at least down to the bottom of the crucible. More preferably, the temperature of the bottom of the crucible is controlled or controlled so that melting of the thin separating layer is prevented, at least down to a residual thickness of the thin separating layer of at least 30 microns.

According to another embodiment, the thin release layer completely covers the bottom of the crucible. In this case, the thin separating layer can be formed in one piece. Thin quartz glass panes having dimensions corresponding to the internal dimensions of commercial quartz glass crucibles used to make standard sized metal or semi-metal ingots are commercially available inexpensively. Surprisingly, it has been found that such quartz glass panes in the directional solidification of metal or Halbmetallingots from a melt to achieve a higher Allow yield in a simple and cost-effective manner.

According to another embodiment, the thin separation layer is formed by a plurality of thin fused quartz plates which are placed immediately adjacent to each other to completely cover the bottom of the crucible. This may be advantageous for cost reasons, for example, if quartz glass plates are available only with smaller dimensions than the internal dimensions of the crucible used for directional solidification. For such an arrangement of quartz glass plates directly adjacent to each other on the bottom of the crucible are simple geometric shapes that complement each other well to closed surfaces, such as in particular rectangular, square or equilateral triangular or hexagonal bases of the germ plates.

According to another embodiment, the metal or semimetal body is a monocrystalline metal or semimetal body which is divided into a plurality of monocrystalline metal or semimetal bodies by sawing along saw lines extending in the vertical direction, the plurality of quartz glass plates thus being on the ground be arranged of the crucible that edges of the quartz glass plates set the beginning of the later sawing lines. These can in particular be matched to the typical dimensions of components, for example, the dimensions of solar cells for use in photovoltaics, so that they can be specified by simply sawing along the predetermined by the edges of the quartz glass saw lines in the desired dimensions, possibly after further Processing at the edges According to a further embodiment, the plurality of thin quartz glass plates has a constant thickness, so that when separating the lower end of the ingot, it is simply necessary to separate only one layer with the maximum thickness of the quartz glass plates used.

According to a further embodiment, the thin separating layer has a thickness of at least 50 μm, more preferably of at least 1000 μm. Experiments by the inventors have shown that, in particular in the directional solidification of Si ingots, the aforementioned thickness is sufficient for sufficient mechanical stability of the separating layer of quartz glass, in particular complete softening of the quartz glass layer is prevented, at least up to a residual thickness of 30 μm considered to be minimal , Upwards, the thickness is limited only by the availability of thick glass plates, yet it makes sense to make the thickness as small as possible.

According to a further embodiment, the quartz glass is coated with a release agent, which allows easy separation or detachment of the ingot from the release layer after directional solidification. Release agents with a low impurity content are available and known in the art.

According to a further embodiment, prior to the introduction or before the production of the melt in the crucible, side walls of the crucible are additionally covered with a thin separating layer of the quartz glass in the above-described manner in order to prevent contamination of the sidewall regions of the ingot.

Another preferred aspect of the present invention relates to the use of the method described above for producing a monocrystalline silicon ingot by means of a vertical gradient freeze crystal pulling (VGF) process as a starting material for photovoltaic devices, such as photovoltaic solar cells modules.

In particular, a device having a stationary crucible and a heating device for melting the silicon present in the crucible can be used to carry out the aforementioned method. In this case, the heating device and / or a thermal insulation of the device are / is designed so as to form a temperature gradient in the longitudinal direction or vertical direction in the crucible. This is normally done by keeping the bottom of the crucible at a lower temperature than the top of the same. Furthermore, in such a device, the heating device has a jacket heater for suppressing a heat flow perpendicular to the longitudinal direction, that is directed horizontally outward on.

In this case, the jacket heater is preferably a single-zone heater which is designed so that its heating power in the longitudinal direction or vertical direction decreases from the upper end to the lower end of the crucible in order to at least contribute to the maintenance of the temperature gradient formed in the crucible. In other words, by varying the heat output of the jacket heater in the longitudinal direction of the crucible in a continuous or discrete manner, the formation of a predetermined temperature gradient in the crucible is at least supported. This temperature gradient is predetermined in the crucible by different temperatures of a lid heater and a bottom heater in a conventional manner. The temperature of the bottom heater at the bottom of the crucible is lower, in particular below the melting temperature of the silicon to be processed. Suitably, the bottom heater does not necessarily extend over the entire base of the crucible. The formation of a planar phase boundaries in the material to be crystallized, for example silicon, with a bottom heater extending over the bottom surface of the crucible, although the most accurate feasible, but in practice sufficiently flat phase boundary can also be achieved with an annular bottom heater, the in the crystallization phase with respect to its temperature drop over the process time is very well matched to the temperature profile of the jacket heater.

LIST OF FIGURES

The invention will now be described by way of example and with reference to the accompanying drawings, from which further features, advantages and objects to be achieved will result. Show it:

1 in a schematic sectional view of an apparatus for producing monocrystalline silicon according to the present invention;

2a in a schematic plan view according to a first embodiment, the arrangement of a total of four quartz glass plates to form a thin separation layer on the bottom of a crucible and the orientation of sawing lines along which the ingot is divided into smaller blocks after directional solidification;

2 B in a schematic plan view according to a second embodiment, the arrangement of a total of two quartz glass plates to form a thin separation layer on the bottom of a crucible and the orientation of a sawing line along which the ingot is divided into two smaller blocks after directional solidification; and

2c a schematic plan view according to a third embodiment, the covering of the entire bottom of the crucible with a single, integrally formed quartz glass plate to form a thin separating layer according to the present invention.

In the figures, identical reference numerals designate identical or essentially identically acting elements or groups of elements.

Detailed description of preferred embodiments

The 1 shows an example of a vertical-degree freeze-crystallization plant, which is used in a method according to the invention, which is described below by way of example by a method for producing a silicon ingot. The total with the reference numeral 1 designated plant has a crucible with a square cross-section. According to the 1 is the crucible of a quartz glass crucible serving as a crucible 2 formed, the mechanical support in a correspondingly formed graphite container 4 is closely attached. The in the quartz glass crucible 2 absorbed melt 3 , in particular a silicon melt, thus does not come into contact with the graphite container 4 , The crucible is placed upright so that the crucible walls run along the direction of gravity. The melting pot 2 is a commercially available quartz glass crucible with a footprint of, for example 550 × 550 mm, 680 × 680 mm or 1000 × 1000 mm and may be coated with a release agent that serves as a release layer between the side walls of the crucible of the above-mentioned quartz glass and the melt, in particular Silicon melt, can serve.

Above and below the crucible formed in this way is a lid heater 6 or a bottom heater 5 , being between the crucible and the bottom heater 5 a crucible mounting plate 40 , For example, graphite, is arranged, which is indicated only schematically in the illustration. Here, the actual holder of the aforementioned crucible is formed so that between the bottom heater 5 and the crucible mounting plate supporting the crucible 40 a narrow gap is formed. The core zone of the crucible is surrounded by a planar heating element, namely a jacket heater 7 which can be formed in particular in the corner regions of the crucible, as described in detail in the German patent DE 10 2006 017 621 B4 the applicant is described, whose content is hereby expressly incorporated by reference for disclosure purposes. The jacket heater extends substantially over the entire height of the crucible. In the VGF crystallization process, all are heaters 5 - 7 temperature-controlled. To do this, the surface temperatures of the heaters are determined by pyrometers 9a - 9c in an appropriate place, as exemplified in the 1 shown, recorded and entered into a control unit, which is controlled by the heater 5 - 7 controls or regulates flowing electricity.

Alternatively or additionally, the reference numeral 5 designated plate may also be formed as a cooling plate, which can be flowed through by a coolant under the action of a suitable control or regulation. The crucible mounting plate 40 can then be formed as an insulation plate, for example made of graphite. In this case, the actual holder of the crucible is formed so that between the crucible supporting crucible mounting 40 and the cooling plate 5 a narrow gap is formed.

After the VGF process, a starting material is first placed in the crucible 2 introduced and then melted in this. In particular, the procedure can be followed, as in the DE 10 2007 038 851 A1 The applicant discloses the entire content of which is hereby expressly included for the purpose of disclosure. Subsequently, the temperature of each of the three heaters shown is lowered in parallel to the other heaters, so that the melt in the crucible can solidify continuously upward, the phase boundary between the already crystallized and the still molten material horizontal, ie perpendicular to the direction of gravity , runs.

For directional cooling and solidification of the liquid silicon in the crucible 2 to a mono- or quasi-monocrystalline silicon ingot, the bottom heater is maintained at a defined temperature below the melting temperature of the silicon, for example at a temperature of at least 10 K below the melting temperature. At the bottom of the crucible, the crystal growth is now initiated. After a short time, an equilibrium temperature profile sets in and the initiated crystal growth comes to a standstill. In this condition, ceiling and floor heaters have the desired temperature difference, which is equal to the temperature difference between the top and bottom of the jacket heater. Now the heating power of the heater 5 - 7 reduced and in each case parallel to each other. There is a columnar growth of a mono- or quasi-monocrystalline Si block. According to the horizontal phase boundary, the growth is parallel and perpendicular from bottom to top. The monocrystalline Si ingot thus obtained is then cooled to room temperature and taken out.

Again 1 can be taken, is the entire bottom of the quartz glass crucible 2 with a separating layer between the crucible and the melt 3 acting thin plate 35 covered by a quartz glass. The quartz glass can also be a high-purity synthetic quartz glass.

Preferably, the impurity content of the quartz glass of iron (Fe) is at most 30 ppbw and / or the impurity content of the quartz glass of sodium (Na) is at most 30 ppbw and / or the impurity content of the quartz glass is chromium (Cr), nickel (Ni), tungsten (W), molybdenum (Mo) or vanadium maximum 1 ppbw.

As in the 2a and 2 B shown, the thin release layer 35 from a plurality of thin quartz glass plates 36a - 36d be formed, which are arranged immediately adjacent to each other to the bottom of the crucible 2 completely cover. The thin quartz glass plates 36a - 36d preferably have a constant thickness. Preferably, the thin release layer 35 a thickness of at least 50 μm, more preferably at least 500 μm. The quartz glass can be coated with a release agent.

Although in the 1 not shown, may prior to introduction or before the production of the melt 3 in the crucible 2 in addition, side walls of the crucible 2 be covered in a corresponding manner with a thin separation layer of the quartz glass.

The 2a shows the arrangement of a total of four thin quartz glass plates 36a - 36d on the bottom of the crucible in a plan view to form in this area a thin separation layer between the crucible and the melt received therein. It can be seen that the edges of the quartz glass plates 36a - 36d abut one another directly so that the bottom of the crucible is completely covered, even in the corners thereof. The lines 37 and 38 indicate in plan view sawing lines along which the square-section Si ingot after directional solidification is divided into four smaller square blocks having identical bases, for example sawing by a wire saw. Of course, depending on the size of the ingot by appropriate sawing by means of a wire saw also several square blocks can be formed (eg 25 5 '' blocks or 16 6 '' blocks or other numbers according to the base of the ingot). Again 2a can be removed, the saw lines run exactly along the edges of the quartz glass plates 36a - 36d , In this way, the dislocations detectable in a horizontal sectional plane of the ingot or blocks are further reduced

The 2 B shows in a corresponding manner the arrangement of a total of two thin quartz glass plates 36c . 36d on the bottom of the crucible in a plan view to form in this area a thin separation layer between the crucible and the melt received therein. By sawing the solidified Ingos along the saw line 37 and in the vertical direction can be achieved in a corresponding manner two smaller largely dislocation-free ingots of half size.

The 2c Finally, in a plan view shows that the entire bottom of the crucible is covered with a single thin quartz glass plate, as a thin separating layer 35 between the crucible and the melt received therein acts.

embodiment

To produce a multicrystalline silicon ingot, a quartz crucible having a square basic shape measuring 680 × 680 mm and a height of 450 mm was used. The bottom of the crucible was covered with a thin separating layer formed of four individual quartz glass plates. On this separation layer was then a silicon granules fine or medium grain size refilled to the top of the crucible. The Si granules were melted from above through a lid heater. The mantle heaters were also on, while the bottom of the crucible was not heated. It was worked with a Abschmelzrate of 5 cm / h, which could be varied according to other series in the range between 1 cm / h and 10 cm / h. During melting, the phase boundary solid / liquid was first lowered over the crucible from top to bottom. The heat input from the top was relatively large while the heat losses at the bottom of the crucible were relatively low, so as to allow a suitable rapid, energy-efficient melting.

In a second step, the heat removal at the bottom of the crucible was increased and at the same time the heat input from above reduced. As a result, the direction of movement of the phase boundary is solid / liquid reversed again. It was possible to observe the directional solidification of a monocrystalline Si ingot whose crystalline structure was predetermined by that of the seed plates. The ratio between heat input from above and heat removal at the bottom of the crucible determines the rate of solidification.

Even with the use of quartz glass plates having a thickness of only 50 μm, only the lowermost portion of the ingot to a thickness corresponding to the starting thickness of the quartz glass plates used had to be separated from the ingot to produce a homogeneous ingot with a low impurity content even in its periphery , especially at its lower end. Accordingly, the yield was very high.

Comparative example

Under appropriate process conditions, a Si ingot was made without placing quartz glass plates on the bottom of the crucible. This had in the edge areas, but in particular at its lower end, a significant impurity content, which was still significantly higher, especially at intervals of 5 mm from the edge of the Si ingot as in the above embodiment.

As those skilled in studying the above description will be readily apparent, can be made with solar cells, which are made of a Si-ingot produced by the inventive method, due to the much lower impurity content in particular in the edge regions of the ingot and the consequent higher homogeneity , a higher efficiency can be achieved. Since less waste is produced for the production of solar cells and other components, the yield is higher when using the method according to the invention, which helps to save costs. Estimates by the inventors indicate a cost advantage of up to 10%.

LIST OF REFERENCE NUMBERS

1

crystallization system

2

quartz glass crucible

3

melt

4

graphite crucible

5

bottom heater

6

cover heater

7

Sheath heater / flat side heater

9a-9c

temperature sensor

35

Separating layer of quartz glass

36a-36d

high-purity quartz glass pane

37

parting line

38

parting line

40

Insulation layer / crucible mounting

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• DE 102007038851 A1 [0005, 0039]
• WO 2010/088046 A1 [0006]
• EP 0748885 A1 [0008]
• EP 1304399 B1 [0009]
• DE 1138514 [0010]
• DE 102006017621 B4 [0037]

Claims (13)

Process for producing a mono-, quasimono- or multicrystalline metal or semimetal body by directional solidification from a melt ( 3 ), in particular of monocrystalline silicon bodies according to the vertical gradient freeze method (VGF method), in which method the melt is heated in a crucible ( 2 ) under the influence of a temperature gradient, which in a vertical direction and from the upper end of the crucible ( 2 ) extends to the lower end, directionally solidifies the mono- or multicrystalline metal or semi-metal body, wherein prior to introduction or before the production of the melt ( 3 ) in the crucible ( 2 ) the bottom of the crucible with a thin separating layer ( 35 . 36 ), characterized in that the thin release layer ( 35 . 36 ) is formed of quartz glass. The method of claim 1, wherein the quartz glass is a high purity synthetic silica glass. A method according to claim 1 or 2, wherein an impurity content of the fused silica to iron (Fe) is at most 30 ppbw and / or wherein an impurity content of the fused silica to sodium (Na) is at most 30 ppbw and / or wherein an impurity content of the fused silica to chromium (Cr ), Nickel (Ni), tungsten (W), molybdenum (Mo) or vanadium is at most 1 ppbw. Method according to one of the preceding claims, wherein the thin separating layer ( 35 . 36 ) the bottom of the crucible ( 2 completely covered. Method according to one of the preceding claims, wherein the thin separating layer ( 35 . 36 ) of a plurality of thin quartz glass plates ( 36a - 36d ) are arranged, which are arranged directly adjacent to each other to the bottom of the crucible ( 2 ) completely cover. A method according to claim 5, wherein the metal or semimetal body is a monocrystalline metal or semimetal body which is sawed along sawing lines extending in the vertical direction (US Pat. 37 . 38 ) is divided into a plurality of monocrystalline metal or semi-metal bodies and the plurality of quartz glass plates ( 36a - 36d ) so on the bottom of the crucible ( 2 ), that edges of the quartz glass plates ( 36a - 36d ) determine the beginning of the later sawing lines. Method according to claim 5 or 6, wherein the plurality of thin quartz glass plates ( 36a - 36d ) have a constant thickness. Method according to one of the preceding claims, wherein the thin separating layer ( 35 . 36 ) has a thickness of at least 50 μm, more preferably of at least 500 μm. Method according to one of the preceding claims, wherein the quartz glass is coated with a release agent. Method according to one of the preceding claims, wherein the crucible ( 2 ) has a rectangular or square cross-section. Method according to one of the preceding claims, wherein before the introduction or before the production of the melt ( 3 ) in the crucible ( 2 ) in addition, side walls of the crucible are covered with a thin separating layer of quartz glass. A method according to any one of the preceding claims, wherein the semimetal is silicon and the temperature of the bottom of the crucible is maintained below 1400 ° C, more preferably below 1380 ° C. Use of the method according to one of the preceding claims for producing a mono-, quasi-mono- or multicrystalline silicon ingot by means of a vertical gradient freeze crystal pulling method (VGF method) as starting material for photovoltaic components, in particular solar cells for photovoltaic modules.

Images