Silicon block, method for producing the same, crucible of transparent or opaque fused silica suited for performing the method, and method for the production thereof

Abstract

A method for producing a solar crucible includes providing a crucible base body of transparent or opaque fused silica having an inner wall, providing a dispersion containing amorphous SiO.sub.2 particles, applying a SiO.sub.2-containing slip layer to at least a part of the inner wall by using the dispersion, drying the slip layer to form a SiO.sub.2-containing grain layer and thermally densifying the SiO.sub.2-containing grain layer to form a diffusion barrier layer. The dispersion contains a dispersion liquid and amorphous SiO.sub.2 particles that form a coarse fraction and a fine fraction with SiO.sub.2 nanoparticles. The weight percentage of the SiO.sub.2 nanoparticles based on the solids content of the dispersion is in the range between 2 and 15% by weight. The SiO.sub.2-containing grain layer is thermally densified into the diffusion barrier layer through the heating up of the silicon in the crystal growing process.

Claims

1. A method for producing a solar crucible with a rectangular shape for use in a crystal growing process for silicon, the method comprising: providing a crucible base body of transparent or opaque fused silica comprising an inner wall; providing a dispersion containing amorphous SiO.sub.2 particles; applying a SiO.sub.2-containing slip layer with a layer thickness of at least 0.1 mm to at least a part of the inner wall by using the dispersion; drying the slip layer so as to form a SiO.sub.2-containing grain layer having a casting skin; and thermally densifying the SiO.sub.2-containing grain layer so as to form a diffusion barrier layer, wherein the dispersion contains a dispersion liquid and the amorphous SiO.sub.2 particles that form a coarse fraction with particle sizes in the range between 1 μm and 50 μm and a fine fraction with SiO.sub.2 nanoparticles with particle sizes of less than 100 nm, the fine fraction with the SiO.sub.2 nanoparticles with particle sizes of less than 100 nm accounting for a volume proportion of the casting skin of more than 70%, wherein a weight percentage of the SiO.sub.2 nanoparticles based on a solids content of the dispersion is in the range between 2 and 15% by wt., and wherein the SiO.sub.2-containing grain layer is thermally densified into the diffusion barrier layer through heating up of the silicon in the crystal growing process.

2. The method according to claim 1, wherein the solids content of the dispersion is less than 80% by wt.

3. The method according to claim 1, wherein the dispersion is free of binders, wherein the SiO.sub.2 content of the amorphous SiO.sub.2 particles is at least 99.99% by wt., and wherein a total content of metallic impurities of transition elements is less than 5 wt. ppm.

4. The method according to claim 1, wherein the slip layer is applied by casting the dispersion onto the inner wall.

5. The method according to claim 1, wherein the inner wall is moistened prior to the application of the slip layer, and wherein the inner wall of the crucible base body is a porous inner wall.

6. The method according to claim 1, wherein a green layer obtained after drying of the slip layer has a layer thickness in the range of 0.1-1.5 mm.

7. A method for producing a silicon block in a crystal growing process comprising: providing a solar crucible with a crucible base body of transparent or opaque fused silica comprising an inner wall, of which at least a part is covered by a SiO.sub.2-containing grain layer having a casting skin; and filling the solar crucible with silicon, the silicon being heated so as to form a silicon melt, the silicon melt being cooled down with crystallization and formation of the silicon block, wherein the SiO.sub.2-containing grain layer contains amorphous SiO.sub.2 particles that form a coarse fraction with particle sizes in the range between 1 μm and 50 μm and a fine fraction of SiO.sub.2 nanoparticles with particle sizes of less than 100 nm, the fine fraction of SiO.sub.2 nanoparticles with particle sizes of less than 100 nm accounting for a volume proportion of the casting skin of more than 70%, wherein the weight percentage of the SiO.sub.2 nanoparticles of the SiO.sub.2-containing grain layer is in the range between 2 and 15% by wt., and wherein the SiO.sub.2-containing grain layer is thermally densified during heating up of the silicon.

8. The method according to claim 7, wherein before the formation of the silicon melt, the SiO.sub.2-containing grain layer has reached a density of more than 90% of its theoretical density.

Description

PREFERRED EMBODIMENTS

(1) The invention will now be explained in more detail with reference to embodiments and a drawing. In detail,

(2) FIG. 1 shows an SEM image of a sintering sample of a fused silica plate with an SiO.sub.2 barrier layer applied thereto as a slip, after a sintering period of 3 h at 1,300° C.,

(3) FIG. 2 shows an SEM image of a sintering sample of a fused silica plate with an SiO.sub.2 barrier layer applied thereto as a slip, after a sintering period of 3 h at 1,500° C.,

(4) FIG. 3 is a photo of a green layer in a reference sample,

(5) FIG. 4 is a schematic representation showing a fused silica crucible provided with a grain layer according to the invention, in a side view,

(6) FIG. 5 is an MDP (microwave detected photoconductivity) image of the charge carrier lifetime in a typical silicon block, cut vertically,

(7) FIG. 6 shows a comparison of the thicknesses of lateral in-diffusion layers determined by means of MDP measurements, in silicon blocks produced by means of solar crucibles with and without diffusion barrier layer,

(8) FIG. 7 shows results of photoluminescence measurements of silicon cells from the corner portion, produced with and without diffusion barrier layer,

(9) FIG. 8 is a diagram with concentration curves of the total iron concentration, measured on several polished structural samples in a direction transverse to the layer, and

(10) FIG. 9 is a diagram with results of GDMS measurements on silicon blocks.

PREPARATION OF A SIO2SLIP—FIRST ALTERNATIVE

(11) Amorphous transparent fused silica grains are mixed into a dispersion liquid in a drum mill lined with transparent fused silica. The transparent fused silica grains consist of transparent fused silica that has been produced from naturally occurring raw material and have grain sizes in the range between 250 μm and 650 μm. This mixture is ground by means of grinding balls of transparent fused silica on a roller block at 23 rpm for a period of 3 days such that a homogeneous slip is formed. In the course of the grinding process the pH is lowered to about 4 due to the dissolving SiO.sub.2.

(12) The SiO.sub.2 grain particles obtained after the grinding of the transparent fused silica grains are of a splintery type and show a particle size distribution that is distinguished by a D.sub.50 value of about 8 μm and by a D.sub.90 value of about 40 μm. SiO.sub.2 nanoparticles with diameters of about 40 nm (“fumed silica”) with a weight percentage of 10% by wt. (based on the solids content of the dispersion) are added to the homogeneous slip. After further homogenization a binder-free SiO.sub.2 slip is obtained. The solids content of the dispersion is 75% by wt.; the SiO.sub.2 content of the amorphous SiO.sub.2 particles is at least 99.99% by wt., and the total content of metallic impurities of the transition elements is less than 2.5 wt. ppm. The content of iron is below 0.5 wt. ppm.

PREPARATION OF A SIO2 SLIP—SECOND ALTERNATIVE

(13) Instead of the transparent fused silica grains of naturally occurring raw material, use is made of SiO.sub.2 grains of synthetically produced transparent fused silica that has a hydroxyl group content of about 800 wt. ppm. These SiO.sub.2 grains are commercially available in a high-purity form and in different grain sizes. The SiO.sub.2 content of the amorphous SiO.sub.2 particles is at least 99.99% by wt. and the total content of metallic impurities of the transition elements is less than 1 wt. ppm. The content of iron is below 0.1 wt. ppm.

(14) The dispersion of deionized water and, amorphous SiO.sub.2 grains with a mean particle size of about 15 μm (D.sub.50 value) is homogenized without grinding balls. SiO.sub.2 nanoparticles with diameters of about 40 nm (“fumed silica”) are added to the homogeneous slip. After further homogenization a binder-free SiO.sub.2 slip is obtained, in which the SiO.sub.2 nanoparticles have a weight percentage of 8% by wt. (based on the solids content of the dispersion), the total solids content of the dispersion being 75% by wt.

Preliminary Test—Sample 1

(15) The crucible material of opaque fused silica has an open porosity and forms an absorbent substrate for slip casting. On this material (body), a slip layer is produced from the binder-free slip by application with a doctor blade (also called “casting”). In this process a SiO.sub.2 slip layer with a thickness of about 1 mm is applied with a doctor blade onto the horizontally supported plate, and directly thereafter a mechanical pressure is exerted on the slip layer by means of the doctor blade device.

(16) A thin liquid film forms on the densified slip layer applied in this way and a homogeneous and closed surface layer is formed during subsequent surface drying in air.

(17) This creates a body structure which macroscopically leads to the formation of a dense layer of uniform thickness which as a green layer and also as a sinter layer in the sintered state forms an intimate adhesive bond with the crucible substrate. A high segregated fine fraction in the upper portion of the layer (casting skin) can be seen under the microscope. Within, the casting skin, the fraction of fine SiO.sub.2 particles and particularly of SiO.sub.2 nanoparticles is much higher than in the remaining slip layer.

(18) The manner of applying the complete layer thickness in one operation provides, on the one hand, a sufficiently large reservoir of SiO.sub.2 nanoparticles, which is suited for segregation on the surface, and an excessively fast drying of the layer in air is prevented on the other hand, which otherwise would counteract segregation and formation of the casting skin. Consequently, this leads to a slower drying of about 3-5 min and a solidification of the slip layer into the supporting layer which permits the formation of a substantially smooth casting skin.

(19) During casting the slip layer is given its final shape by the action of a tool, such as a doctor blade, a brush, a spatula, or an outlet nozzle from which during application a continuous slip jet exits. Owing to the spreading action of the processing tool the layer surface becomes slightly more liquid, which facilitates the enrichment of SiO.sub.2 nanoparticles also at a comparatively low liquid content. This outcome, i.e., no significant reduction of the liquid content of the slip, can also be expected from other application techniques (such as spraying on (“Aufspritzen”)), in which the slip layer is produced with its whole thickness at once and without division into fine drops of less than 1 mm.

(20) The slip layer produced in this way is dried within 3 minutes into a supporting layer and subsequently dried—still at a slow pace—by being allowed to stand in air for 1 hour. The casting skin is here given a wax-like appearance. The complete drying is carried out in air for 4 to 8 hours by using an IR radiator.

(21) The dried slip layer has a mean thickness of about 0.8 mm. It is also called “green layer”. The SiO.sub.2 nanoparticles enriched in the surface region of the green layer show a high sinter activity and improve the densification of the layer. FIG. 1 shows the layer produced in this manner after sintering for three hours in a sintering furnace at a temperature of around 1,300° C. Complete densification is not yet achieved in this process. The rough rugged fracture surface with many finely distributed pores can clearly be seen.

(22) By comparison, FIG. 2 shows the appearance of a similar sinter layer after sintering for three hours at an elevated temperature of 1,500° C. A crack-free and substantially smooth, strongly densified and vitrified fracture surface of opaque fused silica with a density of about 2.1 g/cm.sup.3 is obtained. The surface layer shows a closed porosity of less than 5% and a thickness of about 750 μm. (Note: The transition from open to closed porosity is in the range of 5-10% according to the literature).

(23) Neither the layer of FIG. 1 nor the layer of FIG. 2 hints at possible crystallization products, neither in the inside nor on the surface.

(24) The high sinter activity of the grain layer produced according to the invention is apparent from the details given in Table 1. Here, sinter duration and sinter temperature and the respective sinter result are summarized in a crosstab.

(25) TABLE-US-00001 TABLE 1 0.5 h 1 h 3 h 5 h 1,300° C. opaque/porous opaque/porous opaque/ opaque/ porous porous 1,350° C. opaque/porous opaque/porous opaque/ opaque/ porous porous 1,375° C. opaque/porous opaque/porous translucent/ translucent/ dense dense 1,400° C. translucent/ translucent/ translucent/ dense dense dense 1,450° C. translucent/ dense

(26) “Opaque/porous” means that the necessary density of the layer is missing and that this layer is not suited as a diffusion barrier layer within the meaning of the invention. The porosity is more than 10% and the density is less than 90% of the theoretical density of transparent fused silica (about 2.2 g/cm.sup.3).

(27) “Translucent/dense” means that the densified sinter layer has a density of at least 90%, preferably at least 95%, of the theoretical density, so that it is suited as a diffusion barrier layer within the meaning of the invention.

(28) In crystal growing processes the crucible wall is heated up to temperatures in the range between 1,375° C. and about 1,450° C. On the assumption that when a silicon charge is heated up in a crucible, at least one hour will typically pass until the maximum temperature is reached, the grain layer normally has the status “translucent/dense” and has thus reached a density of at least 95% of its theoretical density. This is only true to some extent for the maximum temperature of 1,375° C. which the crucible bottom has in the so-called “quasi mono process”. In this case the desired density of the grain layer will only be reached after 3 hours.

Reference Example—Sample 2

(29) The above-described, binder-free SiO.sub.2 slip has a low viscosity and can be used as such directly as a spray slip. In a test this slip was used for producing a spray coating on the absorbent, opaque fused silica body with open porosity.

(30) For coating purposes the opaque fused silica plate was introduced in horizontal orientation into a spay chamber and the top side was successively provided by spraying of the slip with a supporting SiO.sub.2 slip layer having a thickness of about 0.7 mm. A spray gun which is continuously supplied with the spray slip was used for this purpose.

(31) A rough and rugged surface layer is formed on the successively applied slip layer during subsequent surface drying in air within one minute. This result is at any rate partly due to the fact that the drying of the slip layer took place so rapidly because of the porous substrate that a segregation of the fine fraction in the upper region of the slip layer was not possible, so that a dense and closed casting skin could not form.

(32) The further drying process then took place at a slow pace in that the slip layer was allowed to stand in air for eight hours. Complete drying is carried out in air for 4 hours by using an IR radiator.

(33) This yields a rough and cracked inhomogeneous surface layer of opaque, porous fused silica which has the appearance shown in FIG. 3.

(34) The dried green layer could subsequently be sintered in a sintering furnace at a temperature of about 1,410° C. into a densified sinter layer of translucent fused silica with a density of about 2.0 g/cm.sup.3. This density is still acceptable for a diffusion barrier layer.

(35) Coating of a Crucible

(36) The coating of a commercially available opaque fused silica crucible 1, produced from transparent fused silica of naturally occurring raw material, took place in a multistage setup, which is schematically shown in FIG. 4.

(37) The SiO.sub.2 grain layer 2 to be formed for the diffusion barrier is applied on all sides (bottom and sidewalls) to a pre-moistened inner wall of the crucible 1 of porous opaque fused silica which is temperature-controlled to room temperature. Layers of different thickness and geometrical design can thereby be produced. In the embodiment the grain layer was produced by using the above-described SiO.sub.2 slip layer—first alternative—and on the basis of the method described in the preliminary test—sample 1—for application (by doctor blade) and drying of the slip layer. The green layer (=grain layer 2) produced in this way shows a largely uniform average thickness of about 0.8 mm after drying. The thickness of the diffusion barrier layer obtained therefrom after sintering (in the crystal growing process) is about 10% smaller and is thus about 0.7 mm. After application of said grain layer 2 the layer was coated with a suspension of silicon nitride, silica and DI water (layer 3).

(38) The coated opaque fused silica crucible was used in a crystal growing process which shall be explained in more detail hereinafter. The grain layer was here sintered and thus thermally densified into a dense diffusion barrier layer with a mean thickness of about 0.6 mm.

(39) Production of a Silicon Block

(40) Crystal growing for producing a silicon block was carried out by using the coated opaque fused silica crucible and otherwise in a standard process as e.g. described in DE 10 2005 013 410 B4.

(41) During heating in the crystal growing process a densified SiO.sub.2 barrier layer with an average thickness of about 0.6 mm and a porosity of less than 10% evolves from the grain layer. This barrier layer shows no open porosity and is efficiently active as a diffusion barrier layer with respect to the fast surface diffusion of impurities, especially of iron.

(42) Measurement Results

(43) FIG. 5 shows a typical cross-section of the silicon block, measured by means of “MDP” (“microwave detected photoconductivity”). With this measurement method the minority charge carrier lifetime (LTLD) is determined in a semiconductor material. The minority charge carrier lifetime is normally illustrated in a false-color representation—in FIG. 5 the illustration is kept in gray scales and the gray values correspond to the actually colored regions supplemented with the details “g” (green), “b” (blue) and “r” (red). A long charge carrier lifetime is assigned to the green and blue measurement points. The false-color representation is scaled such that the red color is assigned to the zones with a short charge carrier lifetime (LTLD<6 μs). Thus, the zones colored in red represent the in-diffusion zones with short charge carrier lifetime. They represent the inferior material region, which is here called “red zone”. The cake thickness to be removed depends on the thickness of the red zone.

(44) The “red zone” can clearly be seen on the bottom and in the lateral block regions; it is here predominantly caused by solid-state diffusion of impurities from the solar crucible.

(45) The diagram of FIG. 6 shows a comparison of the in-diffusion zones made visible by way of MDP measurements for a silicon block from a crucible without diffusion barrier layer (measurement series (a)) and for a silicon block from a crucible with diffusion barrier layer (measurement series (b)). On the ordinate, width d of the lateral in-diffusion zone which is measured at several height positions of the silicon block is here respectively plotted (in mm). The measurement points of about the same height position of both measurement series are respectively represented as triangles, circles and squares.

(46) It is apparent therefrom that in the silicon block from the crucible with diffusion barrier layer (measurement series b) both the lateral in-diffusion zone on the whole and the area of short charge carrier lifetime (red zone) are comparatively thin.

(47) FIG. 7 shows representations of processed corner wafers in which the cell efficiency is made visible by way of photoluminescence. The two corner wafers are taken from silicon blocks of the same block height. The wafer shown in figure (a) derives from a silicon block which was produced in a crucible without diffusion barrier. The corner wafer shows two dark edge regions that have a lower charge carrier concentration due to harmful in-diffusion. The corner wafer according to figure (b) derives from a block with diffusion barrier. In this wafer, the dark edge region is much smaller.

(48) In the iron concentration profiles of FIG. 8, the concentration of iron C.sub.Fe is plotted in wt. ppm against the measurement position P (in μm). The profiles were measured on polished cross sections of the coated crucible after completion of a crystal growing process.

(49) The total concentration of the iron contamination was determined by inductively coupled plasma mass spectrometry (ICP-MS). The samples designated with “SL1” and “SL2” were prepared by means of crucibles with slip-based diffusion barrier layer according to the invention. In the comparative sample designated with “PLA”, the diffusion barrier layer was produced by using insertion plates of transparent fused silica, as described in the above-mentioned DE 10 2011 082 628 A1. The value zero on the x-axis marks the boundary between the crucible wall (substrate S) and the respective diffusion barrier layers (L).

(50) It can be seen that the diffusion barrier layers L after the Si melting process show much lower Fe concentrations than the crucible material (substrate). A diffusion profile is perceivable. The higher concentration level of the slip-based layers (SL1 and SL2), which originally during application were much purer (Fe level about 0.2 wt. ppm, i.e. similar to the concentration level of the plate sample), is noteworthy. This is an indication that a certain enrichment with Fe that penetrates through the whole layer already occurs during the heating up or densification, respectively, of the sample. This leads to an essential technical demand for a densification of the layer that is as fast as possible.

(51) It is noteworthy that the SiO.sub.2 initial slip has a Fe initial concentration that corresponds approximately to the asymptotic level of the SiO.sub.2 plate material (“PLA”). The fact that the Fe concentration level after the melting process has risen slightly above 1 ppm is indicative of an enrichment of the whole layer during the densification process. Since it must be presumed that the layer which is strongly densified at the end probably shows a similar behavior as the plate material that is dense right from the beginning, this is in support of the assumption that the initial porosity promotes this enrichment, and of the conclusion that the material should densify at temperatures that are as low as possible (below 1410° C.).

(52) FIG. 9 shows results of a GDMS measurement (glow discharge mass spectrometry) for a silicon reference block (called “Ref”) produced without a diffusion barrier layer and a silicon block (called “SL”) produced with a diffusion barrier layer according to the invention, by way of comparison. On the ordinate of the diagram, the respective metal content (iron, copper) C.sub.Me is here plotted in wt. ppm on a logarithmic scale against the distance A (in mm) from the crucible wall. It is apparent therefrom that the metal concentration is decreasing with an increasing distance and that a reduction of the iron and copper content in the finished silicon block could be achieved by using the SiO.sub.2 diffusion barrier. It is striking that in the sample according to the invention with a diffusion barrier layer for iron (SL-Fe) it is not only the edge concentration but also the whole profile that is clearly lower than in the reference sample (Ref-Fe) without diffusion barrier layer.