DE102007061704A1 - Process for producing a monocrystalline or polycrystalline material - Google Patents

Abstract

The invention relates to a method for producing a monocrystalline or polycrystalline material according to the vertical gradient freeze method, in which lumpy raw material (20) is introduced into a crucible (2) and melted therein and after the vertical gradient freeze Method directed solidified. In order to suppress impurities and damage by liquid melt spraying, the raw material (20) is melted from the top of the crucible so that molten material seeps down and the unmelted raw material (20) in the crucible (2) gradually collapses , In this case, the additional raw material (20) from above to a zone (21) of not yet melted or not completely molten raw material is refilled into the crucible to at least partially compensate for a volume shrinkage of the raw material and to increase the level of the crucible. The method is suitable for the production of polycrystalline silicon for photovoltaics, but also for the production of germanium and calcium fluoride single crystals.

Description

Field of the invention

The The present invention relates to a process for the preparation of monocrystalline or polycrystalline material according to the so-called vertical gradient freeze method (hereinafter VGF method) and in particular relates to a method for the production of polycrystalline silicon for applications in the photovoltaic.

Background of the invention

As a general rule Solar cells for photovoltaic can be made of single crystal Silicon or polycrystalline silicon are produced. While high-quality solar cells made of silicon single crystals, which is technologically more complex and thus more expensive cheaper solar cells usually made of polycrystalline Silicon made, which is less expensive and therefore less expensive. Especially in the production of polycrystalline silicon play Therefore, features leading to a reduction in costs and technological Expense lead, a significant role.

Usually the crucible is filled with lumpy silicon. The subsequent melting leads to liquid silicon it leads to a significant volume shrinkage, due to the significantly different densities of molten Silicon to the previously existing bed. Thus, can in conventional processes only a small part of the crucible volume be used effectively. From the prior art are various Measures known to compensate for volume shrinkage.

US 6,743,293 B2 discloses a process for the production of polycrystalline silicon, in which an annular cap with a corresponding profile is placed on the upper edge of the crucible in order to form an overall container structure with a larger volume. In the container structure, a silicon bed is introduced. After melting the silicon, the silicon melt fills the entire crucible but not the volume trapped by the annular cap. However, the container construction requires a crystallization unit with a larger volume, in particular a greater height, which is undesirable for energetic reasons. Furthermore, it is difficult to provide a suitably dimensionally stable annular top for reuse.

When Alternative to the above method is in crystallization plants, which work according to the Czochralsky process, a continuous one or discontinuous refilling of lumpy Raw material known to shrink volume due to melting of the raw material in the crucible at least partially compensate.

EP 0 315 156 B1 discloses such a crystallizer in which crystalline material is fed to the crucible via a feed tube. In the feed tube, slowdown means in the form of cross-sectional constrictions or profile bends are provided to reduce the falling rate of the crystalline material.

EP 1 338 682 A2 discloses a crystallization plant according to the Czochralsky process in which crystalline material slips over a sloped pipe into the crucible. JP 01-148780 A discloses a corresponding structure. However, complex precautions must be taken to enable the entry of crystalline raw material into the crucible without spatter. Because the splashing of the hot melt in the crystallization plant leads to the damage of components and to impurities that are difficult to remove again.

US 2004/0226504 A1 discloses a sophisticated flap mechanism to suitably reduce the falling rate of the crystalline material upon filling in the crucible. US 2006/0060133 A1 discloses a crystallizer in which crystalline silicon falls down a vertical tube into the crucible. The lower end of the tube is closed by a conical stopper, which gives the crystalline material a radial component of motion.

An alternative to the aforementioned mechanical solutions is a suitable choice of process parameters to partially solidify the surface of the melt at the time of refilling crystalline material. This is, for example, in JP 11/236290 A or JP 62/260791 A disclosed. However, the solidification of the surface of the melt in the crucible leads to an undesirable slowing down of the process.

EP 1 337 697 B1 discloses a crystallization plant according to the Czochralsky method in which crystalline silicon is only poured onto islands of still solid silicon. These islands must be determined using a video system and a complex image analysis. In order to meet these islands, the conveyor must be moved to convey the crystalline silicon in the crucible suitable, which is expensive.

In all Czochralsky crystallizer plants, the crucible is heated from the bottom. In the production of crystalline materials after the VGF process, the raw material is melted from above.

Summary of the invention

task The present invention is a process for the preparation a monocrystalline or polycrystalline material according to the VGF method provide, with lumpy raw material as much as possible can be refilled into the crucible without any spatter, to the volume shrinkage during melting of the raw material in At least partially counteract the crucible and a high To reach the level of the crucible.

These Task is solved by a method with the features according to claim 1. Further advantages on the embodiment are the subject of the dependent claims.

According to the invention in a VGF process additional raw material in the Crucible from above on a zone of not yet melted or not completely melted raw material refilled, the volume shrinkage of the raw material in the crucible at least partially compensate. In the inventive Method, the raw material in the crucible from above heated, for example, by a substantially above the entire cross-section of the crucible extending and above arranged by this ceiling heater. Thus, the raw material melts in the crucible from above, so that island formation in the upper Crucible area is not favored. Rather, the seeps melt formed at the upper edge of the crucible, where these underlying gaps in the raw material complies or the structure of the underlying raw material changed, in particular, its surface melts. All in all is the surface of the raw material in the crucible even after reaching the melting temperature rather solid than liquid, so that the additional registered lumpy or crystalline raw material to no or negligible splashes in the crucible leads. This zone preferably extends over the entire cross section of the crucible.

LIST OF FIGURES

following the invention will be described by way of example and with reference to be described on the accompanying drawings. It demonstrate:

1 in a schematic cross section of a crystallization plant according to the present invention; and

2a to 2c three different phases in the melting of the crystalline raw material in the crucible according to the 1 ,

Detailed description of preferred embodiments

According to the 1 includes the whole with the reference numeral 1 designated crystallization plant a quartz crucible 3 the complete and close fitting in a box-like and open-topped support system 4 is absorbed to the softened at the melting temperature of the silicon quartz crucible 3 sufficiently mechanically supported. The quartz crucible 3 extends to the top of the support system 4 so that direct contact of the silicon melt with graphite or other contaminating materials is excluded. The quartz crucible 3 is a commercially available quartz crucible with a footprint of, for example 550 × 550 mm ² , 720 × 720 mm ² or 880 × 880 mm ² and has an inner coating as a separating layer between SiO _{2 of} the crucible and silicon. Above the crucible is a ceiling heater 5 provided, whose base area is greater than or equal to the base of the crucible. On the side surfaces of the crucible is a surrounding coat heater 6 arranged. Here is the distance between the jacket heater 6 and the crucible wall constant over the entire circumference of the crucible.

Below the crucible is a cooling plate 8th arranged, which can be flowed through by a coolant. Between the crucible and the cooling plate 8th is an insulation plate or crucible mounting plate 7 arranged. In this case, the actual holder of the aforementioned crucible is formed so that between the crucible mounting plate supporting the crucible 7 and the cooling plate 8th a gap is formed. In the VGF crystallization process, all are heaters 5 . 6 temperature-controlled. For this purpose, the surface temperatures of the heaters 5 . 6 captured by pyrometer at a suitable location and entered into a control unit, which is connected to the heaters 5 . 6 applied voltage controls or regulates suitable. More specifically, in the fixed crucible VGF method, an axial temperature gradient is established. The temperature profile is shifted by electronically varying the heater temperature so that the phase boundary separating the liquid phase from the crystallized silicon gradually migrates from the bottom of the crucible to the top of the crucible. This leads to a directional solidification of the liquid silicon to polycrystalline silicon. The temperature control takes place in such a way that as even as possible isotherms are formed in the crucible.

In this case, the jacket heater can be designed to build a temperature gradient from the upper edge to the lower edge of the crucible. For this purpose, the jacket heater 6 Also be divided into two or more vertically stacked segments having a decreasing from the upper edge to the lower end of the crucible heating power. The arranged at the same height level segments lead to the formation of planar, horizontal isotherms and thus to form a flat, horizontal phase boundary.

Of the Crucible preferably has a polygonal cross-section, in particular a rectangular or square cross section, on. In this way, the waste for the production of the usually polygonal, in particular rectangular or square solar cells be minimized for the photovoltaic.

The entire crystallization plant 1 is of a preferably pressure-resistant or gas-tight enclosure 9 surrounded, so that inside an inert or reducing inert gas atmosphere can be built.

According to the 1 is laterally to the crystallization plant a refill container 14 for solid silicon with the crystallization plant 1 connected. The solid silicon is pourable, particulate silicon that has a suitable shape and bulk density. Preferably, this silicon is crystalline silicon. At the bottom of the container 14 is a refill funnel 13 provided, which is directed to a second conveyor, so that silicon material from the container 14 on the second conveyor 12 slips. At the bottom of the funnel 13 a metering mechanism is provided, for example. A flap or a valve. The second conveyor is preferably located completely outside the crystallization plant 1 , in particular outside the heated area of the crystallization plant. According to the 1 promotes the second conveyor 12 the raw material parallel to the drawing plane of the 1 , The second conveyor 12 downstream is a first conveyor 11 , The first conveyor 11 protrudes into the heated area of the crystallization plant 1 in, for example, about 1/3 of its total length, and protrudes with its front end approximately to the middle of the crucible. For the conveyors 11 . 12 These are conventional vibratory conveyors which convey the raw material via temperature-resistant conveyor rails, for example of silicon carbide. The crystallization plant 1 thus has two independent conveyors 11 . 12 on, which are arranged one above the other, so that of the first conveyor 11 subsidized raw material can be completely emptied into the crucible. Thus, the risk of contamination or even a setting of the first conveyor 11 prevented.

As the skilled person will be readily apparent, can in the crystallization plant according to the invention also any other conveyors can be used which are sufficiently stable in temperature and pourable Can transport raw material into the crucible.

The refill container 14 is a sensor 16 assigned, which can detect the amount of raw material output. This detection can in particular mechanically, preferably by detecting the current weight of the second conveyor 12 , acoustically, visually or otherwise done without contact. Above the crucible is still a temperature sensor 17 for recording the surface temperature of the crucible filling 10 arranged. At the sensor 17 it can be a pyrometer. Above the crucible is also a visual inspection system 18 covering the entire surface of the crucible filling 10 detected, in particular by means of a video camera, not shown, whose images are read out and evaluated in the central control unit CPU. For this purpose, suitable image evaluation algorithms can be used, as described in more detail below. Above the crucible is according to the 1 also a distance sensor 19 arranged the distance of the surface of the crucible filling 10 to the sensor 19 measures. Preferably, a laser distance measuring device is used for this purpose. In knowledge of the height of the distance sensor 19 above the bottom of the crucible thus the current level in the crucible can be continuously detected.

The entire crystallization plant 1 is operated under the control of a central control unit CPU. This is not just for a suitable control of the heater 5 . 6 and the cooling plate 8th responsible but also for controlling the replenishment of silicon raw material by metered output from the container 14 and control of the conveyor 11 . 12 as well as for the evaluation of the sensors 16 to 19 ,

Based on 2a to 2c the principle of the VGF process according to the invention for the production of polycrystalline silicon is first described below. According to the 2a is the crucible at the beginning of the process 2 up to its upper edge with a suitable silicon charge 20 filled. To melt the silicon heats the ceiling heater of the crystallization system, the silicon charge from above to a temperature above the melting temperature of the silicon. The energy supply can additionally via the lateral jacket heater 6 (see. 1 ) and possibly via a bottom heater. The silicon bed 20 So first, it will be at the top of the crucible 2 melted. As indicated by the arrows, this trickles or seeps up molten, liquid silicon now through the underlying silicon bulk down. In the case of seepage, the silicon bed underneath is partially melted, so that its shape and bulk density also change as a result of partial re-solidification. Altogether it happens like in the 2 B shown, to form a so-called "mud zone" 21 at the top of the crucible filling. This zone 21 extends in the form of a more or less thin band over the entire cross section of the crucible 2 and consists of raw material that has not yet been melted or completely melted. In this condition, the crucible filling is in the crucible 2 slumped or shrunk together by a certain distance, resulting in the distance sensor 19 is detected. The collapse can also with the help of the visual inspection system 18 and appropriate image evaluation can be determined. In the process, the temperature sensor detects 17 continuously the temperature of the surface of the crucible filling. In particular, with the help of the temperature sensor 17 detects that and at what time the surface temperature of the crucible filling reaches or exceeds the melting temperature of the raw material. As described in more detail below, the central control device triggers with suitable design of the mud zone 21 as through the sensors 17 to 19 detected, the refilling of silicon raw material 20 out. To this end, as described above, the output of silicon raw material from the container 14 (see 1 ) and the operation of the conveyor 12 . 11 triggered. The actually in the crucible 2 introduced amount of silicon raw material 20 is done with the help of the refill container 14 assigned conveyor sensor 16 detected. The central controller ensures that not too much silicon raw material 20 is refilled, this particular not on the upper edge of the crucible 2 protrudes. Refilling of silicon raw material 20 can be done continuously or in multiple, time-delayed process steps, as described in more detail below.

Finally, the state according to the 2c reached, in which the crucible 2 completely up to its upper edge with a silicon melt 22 is filled. In this state, then the further cooling and solidification of the silicon melt 22 to polycrystalline silicon according to the known VGF process. After the process remains a silicon ingot whose cross-section that of the crucible 2 equivalent. To minimize wastage in the manufacture of photovoltaic elements, the crucible is 2 polygonal according to the invention, in particular rectangular or square.

Hereinafter, the operation of the crystallization plant according to the 1 will be described in more detail with reference to preferred embodiments.

Embodiment 1

With the help of the temperature sensor 17 For example, the surface temperature of the silicon bed in the crucible is continuously detected. Thus it can be stated that and when the melting temperature of silicon is reached. Depending on the heat output for heating the crucible, the silicon bed suffers more or less rapidly. The silicon bed melts from the surface. A predetermined period of time after reaching the melting temperature of silicon additional silicon raw material is introduced into the crucible with the aid of the conveyors. The delivery rate is suitably adjusted depending on the actual heating power for heating the crucible. The amount of silicon raw material actually introduced into the crucible is determined by means of the sensor 16 detected. The silicon bed continuously collapses in the crucible. The addition of the additional silicon raw material can be carried out continuously or at predetermined time intervals and metering amounts, in each case corresponding to the actually used heating power for heating the crucible.

Embodiment 2

With the help of the temperature sensor 17 For example, the surface temperature of the silicon bed in the crucible is continuously detected. The central controller has previously detected what amount of silicon charge has been introduced in the crucible. Or this amount may be entered in advance in the central controller. Depending on the actual heat output for heating the crucible and the amount of raw material currently in the crucible, a predetermined amount of additional raw material is replenished into the crucible. This refilling can be carried out continuously or in several, time-delayed steps, to each of which a predetermined amount of additional raw material is introduced.

Embodiment 3

With the help of the sensor 17 the surface temperature of the crucible filling is detected continuously and thus determines the time at which the melting temperature of silicon is reached. A predetermined time after reaching the melting point, depending on the actual heating power of the crucible, a predetermined amount of additional raw material in the Refilled crucible. This step is repeated after predetermined time intervals corresponding to the actual heating power of the crucible, until the condition according to the 2c is reached.

Embodiment 4

With the help of the temperature sensor 17 the surface temperature of the crucible filling is continuously monitored. Further, using the visual inspection system 18 and / or the distance sensor 19 the level of the crucible continuously monitored. After dropping the level by a predetermined amount, caused by the volume shrinkage of the silicon bed, a predetermined amount of additional raw material is replenished in the crucible. This step is repeated when the level of the crucible after refilling is again bagged by a second predetermined height. The height by which the level drops between the refilling steps is reduced due to the increasing filling of the crucible. Alternatively, instead of operating in discrete predetermined steps, the replenishment of raw material may also be initiated whenever a predetermined level of the crucible is exceeded.

to Process control can be the central control device based on empirical values To fall back on. This concerns in particular the required Time to melt the bed at known Heating power, which includes, among other things, the thermal conductivity of the crucible is dependent. Such experience can be based on previous processes or numerical Simulations are determined. The empirical values can also continuously updated by monitoring other processes become.

The melting point of silicon is given in a very narrow temperature range. However, the phase diagrams of other materials may vary significantly in the melting point range. Therefore, the visual inspection system can provide further conclusions on the condition of the crucible filling and the existence of a so-called "mud zone". In particular, the image analysis of the visual inspection system can be carried out in a similar manner as in the EP 1 337 697 B1 the entire contents of which are hereby expressly included for purposes of disclosure by reference. Such an image evaluation can be used in particular for the determination of still unmelted surface areas of the crucible filling.

The Position at which the first conveyor introduced raw material enters the crucible can, according to a another embodiment also by adjusting the front End of the first conveyor can be varied, in particular also in adaptation to the evaluation of the information of the visual Inspection system. According to another embodiment may the front end of the first conveyor also out and moved to the entry of the raw material in the crucible over the entire surface of the crucible filling to uniform. As the skilled person will be readily apparent that is suitable inventive method not only for the production polycrystalline silicon according to the VGF method but also for the production of any single crystals, in particular of germanium and calcium fluoride single crystals.

1

crystallization system

2

melting pot (general name)

3

quartz crucible

4

Pot support system

5

ceiling heaters

6

jacket heater

7

crucible mounting

8th

cooling plate

9

wrapping

10

filling of the crucible

11

First Conveyor

12

Second Conveyor

13

replenishing hopper

14

refill

15

flange

16

Fördergutsensor

17

Pyrometer / temperature sensor

18

visual inspection system

19

distance sensor

20

lumpy Silicon / Raw

21

"Mud zone" / Zone with partially molten material

22

melt

QUOTES INCLUDE IN THE DESCRIPTION

This list The documents listed by the applicant have been automated generated and is solely for better information recorded by the reader. The list is not part of the German Patent or utility model application. The DPMA takes over no liability for any errors or omissions.

Cited patent literature

• - US 6743293 B2 [0004]
• - EP 0315156 B1 [0006]
• - EP 1338682 A2 [0007]
• JP 01-148780 A [0007]
• US 2004/0226504 A1 [0008]
• US 2006/0060133 A1 [0008]
• - JP 11/236290 A [0009]
• - JP 62/260791 A [0009]
• - EP 1337697 B1 [0010, 0035]

Claims (12)

Process for the preparation of a monocrystalline or polycrystalline material according to the vertical gradient freeze process, in which lumpy raw material ( 20 ) in a crucible ( 2 ) is introduced and is melted in this and directionally solidified, wherein from the upper end to the bottom of the crucible ( 2 ) is built up a temperature profile which is axially displaced so that the phase boundary which separates the liquid phase of crystallized material, starting from the bottom of the crucible gradually towards the upper end of the crucible wanders, in which process the raw material ( 20 ) is melted from the top of the crucible, so that molten material seeps down and not yet molten raw material ( 20 ) in the crucible ( 2 ) and additional raw material ( 20 ) into the crucible from above onto a zone ( 21 ) is replenished with raw material that has not yet been melted or not completely melted in order to at least partially compensate for volume shrinkage of the raw material. Process according to claim 1, wherein the raw material in the crucible ( 2 ) is melted from its upper end so that the zone ( 21 ) as a band of raw material not yet melted or not completely melted over the entire cross-section of the crucible ( 2 ). Method according to one of the preceding claims, wherein a surface temperature of the raw material ( 20 ) in the crucible ( 2 ) is continuously detected, and after a predetermined period of time or immediately after reaching the melting temperature of the raw material, the additional raw material is continuously introduced into the crucible at a rate corresponding to the heating power for heating the crucible. A method according to claim 1 or 2, wherein a surface temperature of the raw material ( 20 ) in the crucible ( 2 ) is continuously detected, and a predetermined amount of the additional raw material is replenished into the crucible depending on the heating power for heating the crucible and the amount of the raw material currently in the crucible. A method according to claim 1 or 2, wherein a surface temperature of the raw material ( 20 ) in the crucible ( 2 ) is continuously detected to determine a time at which the melting temperature of the raw material is reached, and after a predetermined period of time after the time depending on the heating power, a predetermined amount of the additional raw material is replenished in the crucible. A method according to claim 1 or 2, wherein a level of the crucible ( 2 ) is continuously monitored and after a decrease in the level by a predetermined amount, which is dependent on the current level, a predetermined amount of the additional raw material is replenished in the crucible. The method of claim 6, wherein the level monitored by distance measurement, in particular laser distance measurement becomes. Method according to one of claims 4 to 7, wherein the step of refilling is repeated until the crucible ( 2 ) is filled to near its upper edge with a melt. Method according to one of the preceding claims, with the additional raw material when refilling over the cross section of the crucible evened becomes. Method according to one of the preceding claims, wherein the additional raw material with the aid of at least two conveyors ( 11 . 12 ) is introduced, of which a conveyor ( 12 ) located upstream and outside a heated area and another conveyor downstream of this and arranged at least partially within the heated area. Method according to one of the preceding claims, wherein a level of the crucible ( 2 ) is continuously monitored and the refilling of the additional raw material is stopped before overfilling of the crucible has occurred. Method according to one of the preceding claims, wherein the raw material is particulate silicon, in particular pourable polycrystalline silicon.