Abstract:
Semiconductor material has a low metal concentration at the surface. The semiconductor material has an iron content and/or chromium content on the surface of less than 6.66×10 −11  g/cm 2 . A method for producing this semiconductor material includes a preliminary cleaning, a main cleaning and hydrophilization. A device for use in this method has a container with pyramid-shaped recesses at the bottom.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to polycrystalline silicon, a method for producing it and a device for producing it. 
     2. The Prior Art 
     High-purity semiconductor material is needed for the production of solar cells or electronic components, for example, memory elements or microprocessors. The dopants introduced in a controlled manner are the sole impurities which should be present in such a material. An attempt is therefore made to minimize the concentrations of harmful impurities. It is frequently observed that even semiconductor material produced under high-purity conditions becomes contaminated again in the course of the further processing to produce the final products. Thus, expensive cleaning steps are repeatedly necessary to reacquire the original purity. Foreign metal atoms which are built into the crystal lattice of the semiconductor material disturb the charge distribution and may reduce the functioning of the subsequent component or result in its failure. Consequently, contamination of the semiconductor material by metallic impurities should be avoided. This applies to silicon which is the semiconductor material most frequently used in the electronics industry. 
     High-purity silicon is obtained, for example, by thermal decomposition of readily volatile silicon compounds. These compounds, such as trichlorosilane, can therefore be purified easily by distillation methods. Under these circumstances, polycrystalline silicon is produced in the form of ingots having typical diameters of 70 to 300 mm and lengths of 50 to 2500 mm. A large proportion of the ingots is used to produce crucible-drawn monocrystals, strips and foils or to produce polycrystalline solar-cell raw material. Since these products are produced from high-purity molten silicon, it is necessary to melt solid silicon in crucibles. In order to make this operation as efficient as possible, large-volume, solid pieces of silicon, such as, the polycrystalline ingots mentioned, have to be comminuted before melting. This is normally always associated with a superficial contamination of the semiconductor material. This is because the comminution is carried out with metallic crushing tools, such as jaw crushers or roll-type crushers, hammers or chisels. 
     During the comminution, care has to be taken to ensure that the surfaces of the fragments are not contaminated with impurities. Contamination by metal atoms is to be regarded as critical since these may alter the electrical properties of the semiconductor material in a harmful way. If the semiconductor material to be comminuted is comminuted with mechanical tools, such as steel crushers, the fragments have to be subjected to a surface cleaning before melting. 
     Mechanically machined polycrystalline silicon or polycrystalline silicon granules which have been produced from mechanically machined products can be used as silicon starting material. This starting material is for the production of monocrystalline silicon. However, it is necessary to reduce the concentration of iron atoms and/or chromium atoms which are present on the surface of the mechanically machined polycrystalline silicon. 
     Mechanically machined particles of polycrystalline silicon can be used as starting material for monocrystalline silicon. In this case, the surface of the mechanically machined polycrystalline silicon is etched with a mixture of nitric acid and hydrofluoric acid. This process is widely used, but it cannot adequately reduce the concentration of iron atoms and/or chromium atoms on the surface of the polysilicon. 
     DE-A1 195 29 518 discloses that polycrystalline silicon is first cleaned with a mixture of aqua regia (a mixture of hydrochloric acid and nitric acid) and is furthermore cleaned with hydrofluoric acid. This process, however, achieves only a mean iron value of 73.32×10 −11  g/cm 2 . 
     JP 051-54466 describes a cleaning process in which hydrofluoric acid and nitric acid are used. A mean iron value of 23.31×10 −11  g/cm 2  was achieved. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide semiconductor material, in particular polycrystalline silicon, which has a very low iron/chromium content. 
     It is a further object of the invention to provide a cleaning device with which the low iron/chromium content can be achieved. 
     These objects are achieved by the present invention, which relates to a composition comprising a semiconductor material which has a low metal concentration on the surface. This semiconductor material has an iron content and/or chromium content on the surface of less than 6.66×10 −11  g/cm 2 . This semiconductor material has a particle diameter of from 0.1 to 350 mm, preferably from 20 to 150 mm. 
     The semiconductor material includes silicon, indium phosphide, germanium or gallium arsenide. Silicon, in particular polycrystalline silicon, is preferred. 
     A polycrystalline silicon is comminuted by means of metallic crushing tools. These are made of steel which has been hardened with iron carbide and/or chromium carbide. 
     During the comminution of the polycrystalline silicon with these metals, the metal particles are pressed into the oxide layer and the uppermost layer of the silicon lattice. For this reason, cleaning mixtures which do not remove silicon, for example HCl/H 2 O 2 , cannot completely remove the abraded metal material. A complete cleaning is consequently necessary. 
     The invention furthermore relates to a method for producing semiconductor material which has a low metal concentration, which method comprises washing polycrystalline silicon in a preliminary cleaning in at least one stage with an oxidizing cleaning solution, washing it in a main cleaning in a further stage with a cleaning solution which contains nitric acid and hydrofluoric acid, and, during hydrophilization, washing it in yet a further stage with an oxidizing cleaning liquid. 
     In a preliminary cleaning step, the semiconductor material, in particular the polycrystalline silicon fragment, is preferably first rinsed with fully demineralized water having a resistivity of 15-18 Mohm and then washed with an oxidizing cleaning solution. This oxidizing cleaning solution is preferably hydrogen peroxide, mineral acids, such as hydrochloric acid, nitric acid, sulfuric acid, phosphoric acid, perchloric acid, hydrofluoric acid, liquid, linear or branched organic acids containing 1 to 8 carbon atoms, such as acetic acid, formic acid, butyric acid, etc. 
     All these oxidizing substances can also be used, insofar as it is technically possible, in any desired mixtures, which may also contain hydrogen peroxide in addition. Furthermore, surfactants may also be present, such as, anionic surfactants preferably, ammonium lauryl sulfate or sodium alkyl sulfate. 
     An aqueous oxidizing cleaning solution is preferred which contains 4 to 20% w/w hydrochloric acid, 1 to 10% w/w hydrofluoric acid and 1 to 5% w/w hydrogen peroxide as a mixture. The balance is 65 to 94% w/w of water. The percentages by weight are based upon the total weight of the cleaning solution. This preliminary cleaning is preferably followed by a rinsing step using fully demineralized water having a resistivity of 15-18 Mohm. 
     This is followed by the main cleaning step, in which an aqueous mixture of nitric acid and hydrofluoric acid is used. This aqueous mixture preferably contains 60 to 70% w/w nitric acid, 1 to 5% w/w hydrofluoric acid and 25 to 39% by weight of water. The percentages by weight are based upon the total weight of the cleaning solution. 
     The main cleaning step may also preferably be divided up by first washing with an aqueous cleaning solution containing hydrofluoric acid and nitric acid, in which the content of nitric acid is less than 70% w/w, said mixture preferably containing 60 to 70% w/w nitric acid, 1 to 5% w/w hydrofluoric acid and 25% to 39% by weight of water. Then washing is carried out in a further step with a cleaning solution which contains hydrofluoric acid and nitric acid. Here the content of nitric acid is greater than 70% w/w, and the mixture preferably contains 70 to 95% w/w nitric acid, 1 to 5% w/w hydrofluoric acid and 0% to 29% by weight of water. The percentages by weight are based upon the total weight of the cleaning solution. The main cleaning in two steps makes possible a silicon removal by etching of less than 5 μm. 
     Furthermore, the mixtures of nitric acid and hydrofluoric acid may furthermore contain the abovementioned mineral acids, along with liquid, linear or branched organic acids containing 1 to 8 carbon atoms, hydrogen peroxide and surfactants. All the substances may be contained in any concentrations and mixing ratios, provided this is technically possible. 
     This preliminary cleaning step is preferably followed by a rinsing step using fully demineralized water having a resistivity of 15-18 Mohm. 
     This is followed by hydrophilizing with an oxidizing cleaning liquid. This cleaning liquid is preferably composed of mineral acids, as defined above, and liquid, linear or branched organic acids containing 1 to 8 carbon atoms, as defined above. 
     All these substances can also be used, as far as technically possible, in any mixtures, and hydrogen peroxide may also be present in the mixture. Furthermore, surfactants, as defined above may also be present in the mixture. 
     A preferred cleaning solution contains 4 to 20% w/w hydrochloric acid and 1 to 3% w/w hydrogen peroxide, 8 to 70% w/w nitric acid and 7 to 87% w/w water. The percentages by weight are based upon the total weight of the aqueous cleaning solution. The hydrophilization step is preferably followed by a rinsing step using fully demineralized water having a resistivity of 15-18 Mohm. 
     Preferably, all the cleaning steps are carried out in such a way that the semiconductor material to be cleaned is immersed in a container having openings in a cleaning liquid, and cleaning liquid flows in through the openings and wets the polycrystalline silicon, then the container is raised far enough out of the liquid for the cleaning liquid to be able to flow completely out through the openings in the container. Associated with these cleaning liquids is also the wash with fully demineralized water having a resistivity of 15-18 Mohm. 
     In the method according to the invention, the semiconductor material in a container is immersed at least twice in the cleaning liquid by means of a lifting and lowering movement. Preferably, these lifting and lowering movements are carried out in such a way that the containers can drain completely while they are lifted out of the cleaning solution, the cleaning solution flowing out completely. Preferably, this lifting and lowering movement is carried out 10 times per minute for up to 4 minutes. 
     The cleaning process is preferably carried out at normal pressure and room temperature (25° C.). The main cleaning process step is preferably carried out at 5° C. to 12° C. and particularly preferably at 8° C. to 10° C. Because of the low temperature, the residence time can be prolonged (for example, 3 to 5 seconds to 7 to 10 seconds), which effects a reduction in stain formation. 
     Cleaning devices for cleaning semiconductor material are known in which the semiconductor material, in particular the polycrystalline silicon fragments, is rotated in a roller or is moved up and down in the acid by lifting and lowering movements. In the roller process, appreciable plastics material abrasion which contaminates the baths and, consequently the surface of the polysilicon fragments occurs during the circulation of the sharp-edged polysilicon fragments. The process of lifting and lowering movements is notable for the fact that too low a flow through the polysilicon bulk material reduces the cleaning action (see also Comparison Experiment in Table 10). 
     The invention furthermore relates to a cleaning device which has a container having openings, the cleaning device moving the container by means of lifting and lowering movement so that the container is completely removed from the cleaning liquid. The cleaning device is constructed such that a motor lifts a preferably rectangular container with lifting and lowering movements. This container contains the semiconductor material and has openings at least in the bottom. The motor lifts it out of the cleaning liquid with lifting and lowering movements so that the container is completely removed from the cleaning liquid and also drains completely during each of these lifting and lowering movements. 
     Preferably, the container has pyramid-shaped recesses at the bottom of the container. Each of the apices of the pyramids preferably points upward from the bottom of the container. Thus the bottom of the container is covered with pyramid-shaped recesses whose apices point upward from the bottom preferably perpendicularly or, alternatively, at a slight angle. 
     These pyramid-shaped recesses have openings which are situated preferably in the triangular sides of the pyramid-shaped recesses. In the process according to the invention, the cleaning liquid can drain through these openings. These openings have a size of preferably 2 mm to 8 mm, particularly preferably of 2 mm to 4 mm. 
     An advantage of the invention is that a semiconductor material can be obtained which has an iron content and/or chromium content on the surface of less than 6.66×10 −11  g/cm 2 , preferably an iron content and/or chromium content on the surface of less than 2×10 −11  g/cm 2 , particularly preferably of 1×10 −11  g/cm 2 . Furthermore, the semiconductor material cleaned in this way no longer exhibits any stains, such as those which otherwise occur with standard cleaning devices. 
     Comparison of Various Cleaning Methods According to the Prior Art 
     The iron values are specified in 10 −11  g/cm 2 . 
     
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                   
                 Mean 
                   
                 Number 
               
               
                   
                   
                 Literature 
                 iron 
                 Standard 
                 of 
               
               
                   
                 Method 
                 reference 
                 value 
                 deviation 
                 values 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Etch 
                 5/3 
                 23.31 
                 19.98 
                 15 
               
               
                   
                 HF/HNO 3   
               
               
                   
                 20 min 
               
               
                   
                 50 μm 
               
               
                   
                 HCl/H 2 O 2   
                 1,2 
                 59.94 
                 99.9 
                 12 
               
               
                   
                 HF 5% w/w 
                 1,2 
                 13.32 
                 13.32 
                 1,000 
               
               
                   
                 HCl 8% w/w 
               
               
                   
                 H 2 O 2   
               
               
                   
                 2% w/w 
               
               
                   
                 RCA cleaning 
                 1,2 
                 9.96 
                 39.96 
                 14 
               
               
                   
                 HF/H 2 O 2   
                 4 
                 1.98 
                 13.32 
                 15 
               
               
                   
                 HCl/HNO 3   
                 3 
                 73.32 
                 53.32 
                 10 
               
               
                   
                 15 min 
               
               
                   
                 HF 5% w/w 
               
               
                   
                 5 min 
               
               
                   
                 twice 
               
               
                   
                   
               
             
          
         
       
     
     Literature References: 
     1.) RCA cleaning, RCA Review, June 1970, W. Kern. 
     2.) Semiconductor International, April 1984, Cleaning Si and SiO 2  Surfaces with Hydrogen Peroxide. 
     3.) Offenlegungschrift DE 195 29518 A1 Tokuyama Corp., Polykristallines Silicium und Verfahren zur Herstellung (Polycrystalline silicon and method of production). 
     4.) Czochralski method for cleaning polysilicon by using a mixture of hydrogen fluoride, hydrogen peroxide and water, JP 05004811 A, Shinetsu Handotai Co. Ltd. 
     5.) Hi-silicon Co. Ltd. Kojundo Silicon KK, JP 05154466 A, Cleaning polysilicon by washing with a mixture of nitric and HF acid. 
     Performance of the Metal Analyses on the Cleaned Polysilicon Fragments 
     Polysilicon weighing 100 g is sprayed with 40 ml of 1:4 HF/HNO 3  in a Teflon beaker. The etching acid is collected in a Teflon beaker. The acid is then evaporated off and the residue is taken up in 5 ml of water. The metal content of the aqueous solution is measured using the ICP-AES (inductively coupled ion plasma atomic emission spectroscope supplied by the Spectro Company. The metal content of the polysilicon surface is calculated from the measured values. The data are in g/cm 2 . 
     Comparison of Various Cleaning Methods for Removing Stainless Steel Abrasion from the Silicon Surface 
     The iron values are specified in 10 −11  g/cm 2 . 
     
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                   
                   
                 Stand- 
                 Mean 
                 Stand- 
                   
               
               
                   
                 Mean 
                 ard 
                 chrom- 
                 ard 
                 Number 
               
               
                   
                 iron 
                 devi- 
                 ium 
                 devi- 
                 of 
               
               
                 Method 
                 value 
                 ation 
                 value 
                 ation 
                 values 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Etch 
                 23.31 
                 19.98 
                 13.32 
                 1.33 
                 15 
               
               
                 HF/HNO 3   
               
               
                 20 min 
               
               
                 50 μm 
               
               
                 HCl/H 2 O 2   
                 59.94 
                 99.9 
                 19.98 
                 2.22 
                 12 
               
               
                 Pickle 
                 39.96 
                 26.64 
                 19.98 
                 1.66 
                 12 
               
               
                 HF 5% w/w 
               
               
                 HCl 8% w/w 
               
               
                 H 2 O 2  3% w/w 
               
               
                 Pickle/ 
                 &lt;6.66 
                   
                 &lt;6.66 
                   
                 13 
               
               
                 etch 
               
               
                 12 μm 
               
               
                 Etch 
                 19.98 
                 16.65 
                 13.32 
                 1.33 
                 10 
               
               
                 HF/HNO 3   
               
               
                 60 min 
               
               
                 400 μm 
               
               
                   
               
             
          
         
       
     
    
    
     The conditions, according to the invention, of the combined method are explained in greater detail in the following examples: 
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     EXAMPLE 1 
     A polysilicon ingot is contaminated during contact with metal tools on average with 1,332×10 −11  g/cm 2  iron on the polysilicon surface. The subsequent cleaning is carried out as described in Table 3. The necessary acid concentrations, the material removed by etching and the cleaning times for the production of polysilicon fragments having an iron content on the polysilicon surface of &lt;6.66×10 −11  g/cm 2  can be found in Table 3. 
     Conditions During the Performance of the Pickling/Etching Process 
     
       
         
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                   
                   
                   
                   
                 Material 
                   
               
               
                   
                   
                 Reaction 
                   
                 Temperature 
                 removed by 
               
               
                   
                 Substep 
                 time 
                   
                 degrees C. 
                 etching 
               
               
                   
                   
               
             
             
               
                   
                 Rinsing 
                 5 minutes 
                   
                 25 ± 5 
                 — 
                   
               
               
                   
                 with water 
                   
                   
                 degrees 
               
               
                   
                 Preliminary 
                 20 minutes 
                   
                 22 degrees 
                 0.02 μm 
               
               
                   
                 cleaning: 
               
               
                   
                 Pickle 
               
               
                   
                 HF 5% w/w 
               
               
                   
                 HCl 8% w/w 
               
               
                   
                 H 2 O 2  3% w/w 
               
               
                   
                 Rinsing 
                 5 minutes 
                   
                 22 degrees 
                 — 
               
               
                   
                 Main clean- 
                 5 minutes 
                   
                  8 degrees 
                 12 μm 
               
               
                   
                 ing: 
               
               
                   
                 Etching 
               
               
                   
                 HF/HNO 3   
               
               
                   
                 HF 3% w/w 
               
               
                   
                 HNO 3  65% w/w 
               
               
                   
                 Rinsing 
                 5 minutes 
                   
                 22 degrees 
                 — 
               
               
                   
                 Hydro- 
                 5 minutes 
                   
                 22 degrees 
                 — 
               
               
                   
                 philization 
               
               
                   
                 HCl 8% w/w 
               
               
                   
                 H 2 O 2  2% w/w 
               
               
                   
                 Rinsing 
                 5 minutes 
                   
                 22 degrees 
                 — 
               
               
                   
                 Drying with 
                 20 minutes 
                   
                 80 degrees 
                 — 
               
               
                   
                 high-purity 
               
               
                   
                 class 100 
               
               
                   
                 air 
               
               
                   
                   
               
             
          
         
       
     
     EXAMPLE 2 
     100 g of polysilicon is abraded for 10 seconds with a V4A stainless steel screw. The uncleaned polysilicon contains on average 666,000×10 −11  g/cm 2  iron and 133,200×10 −11  g/cm 2  chromium on the surface. The subsequent cleaning is carried out as described in Table 3. 
     EXAMPLE 3 
     A polysilicon ingot is comminuted in a crusher having metal jaws. The uncleaned material contains on average 5,328×10 −11  g/cm 2  iron on the polysilicon surface. The subsequent cleaning is carried out as described in Table 4. The required acid concentrations, the material removed by etching and the cleaning times for the production of polysilicon fragments having an iron content on the surface of &lt;6.66×10 −11  g/cm 2  can be found in the table. 
     Conditions During the Performance of the Pickling/Etching Process for Material from Example 3 
     
       
         
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 4 
               
               
                   
                   
               
               
                   
                   
                   
                   
                   
                 Material 
                   
               
               
                   
                   
                 Reaction 
                   
                 Temperature 
                 removed by 
               
               
                   
                 Substep 
                 time 
                   
                 degrees C. 
                 etching 
               
               
                   
                   
               
             
             
               
                   
                 Rinsing 
                 6 minutes 
                   
                 25 ± 5 
                 — 
                   
               
               
                   
                 with water 
                   
                   
                 degrees 
               
               
                   
                 Preliminary 
                 48 minutes 
                   
                 22 degrees 
                 0.05 μm 
               
               
                   
                 cleaning: 
               
               
                   
                 Pickle 
               
               
                   
                 HF 5% w/w 
               
               
                   
                 HCl 8% w/w 
               
               
                   
                 H 2 O 2  3% w/w 
               
               
                   
                 Rinsing 
                 6 minutes 
                   
                 22 degrees 
                 — 
               
               
                   
                 Main clean- 
                 12 minutes 
                   
                  8 degrees 
                 40 μm 
               
               
                   
                 ing: 
               
               
                   
                 Etching 
               
               
                   
                 HF/HNO 3   
               
               
                   
                 HF 3% w/w 
               
               
                   
                 HNO 3  65% w/w 
               
               
                   
                 Rinsing 
                 6 minutes 
                   
                 22 degrees 
                 — 
               
               
                   
                 Hydro- 
                 12 minutes 
                   
                 22 degrees 
                 — 
               
               
                   
                 philization 
               
               
                   
                 HCl 8% w/w 
               
               
                   
                 H 2 O 2  2% w/w 
               
               
                   
                 Rinsing 
                 5 minutes 
                   
                 22 degrees 
                 — 
               
               
                   
                 Drying with 
                 20 minutes 
                   
                 80 degrees 
                 — 
               
               
                   
                 high-purity 
               
               
                   
                 class 100 
               
               
                   
                 air 
               
               
                   
                   
               
             
          
         
       
     
     Polysilicon heavily contaminated with iron and chromium having a particle diameter of 0.1 to 350 mm can be cleaned to an iron content and chromium content of &lt;6.66×10 −11  g/cm 2  on the surface by a combined method involving a main cleaning step comprising two stages (main cleaning with a cleaning solution containing hydrofluoric acid and nitric acid, the content of nitric acid being less than 70% w/w and a further step with a cleaning solution containing hydrofluoric acid and nitric acid, the content of nitric acid being less than 70% w/w). Compared with the three-stage combined 12 μm method, less than 5 μm of the silicon surface is removed in this four-stage process. 
     Comparison of the Two Combined Cleaning Methods 
     The iron values are specified in 10 −11  g/cm 2 . 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 5 
               
               
                   
                   
               
               
                   
                   
                 Mean iron 
                 Standard 
                 Number of 
               
               
                   
                 Method 
                 values 
                 deviation 
                 values 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Three-stage 
                 3.33 
                 3.33 
                 300 
               
               
                   
                 method with 
               
               
                   
                 material 
               
               
                   
                 removal by 
               
               
                   
                 etching of 
               
               
                   
                 12 μm 
               
               
                   
                 Four-stage 
                 3.33 
                 3.33 
                 20 
               
               
                   
                 method with 
               
               
                   
                 a material 
               
               
                   
                 removal by 
               
               
                   
                 etching of 
               
               
                   
                 &lt;5 μm 
               
               
                   
                   
               
             
          
         
       
     
     In contrast to the three-stage method, stained grey fragments are obtained in the first etching step (material removed approximately 2 μm) in the HF/HNO 3  containing 0.5% w/w HF and less than 70% w/w HNO 3 . Even after the first etch, the iron content of 3.33×10 −11  g/cm 2  is reached on the polysilicon surface. The second etching step (approximately 2 μm) takes place with 0.5% w/w HF and more than 70% w/w HNO 3 . This acid treatment removes the grey stains. 
     The reduction of the material removed by etching from 12 to less than 5 μm compared with the three-stage method reduces the acid consumption and the NO x  gas evolution (see Table 6) by more than 50%. 
     Comparison of the NO x  gas evolution and the acid consumption between the three-stage and four-stage combined cleaning methods. 
     Concentration of the Acids Used 
     HNO 3 : 70% w/w 
     HF: 50% w/w 
     
       
         
               
               
               
               
             
           
               
                   
                 TABLE 6 
               
               
                   
                   
               
               
                   
                   
                 HF/HNO 3  consump- 
                   
               
               
                   
                   
                 tion for 480 kg 
                 NO x  content for 
               
               
                   
                   
                 of polysilicon 
                 3000 m 3  of waste 
               
               
                   
                 Method 
                 fragments 
                 air 
               
               
                   
                   
               
             
             
               
                   
                 Three-stage 
                 220 liters HNO 3   
                 1000 mg/m 3   
               
               
                   
                 method, 12 μm 
                  27 liters HF 
               
               
                   
                 Four-stage 
                 100 liters HNO 3   
                  350 mg/m 3   
               
               
                   
                 method, &lt;5 μm 
                  10 liters HF 
               
               
                   
                   
               
             
          
         
       
     
     When the polysilicon is dissolved, HF is consumed and water is simultaneously produced (see Reaction Equation 1). To maintain the bath concentration in relation to the HNO 3  content and HF content, hydrofluoric acid and nitric acid must therefore continuously be made up. Advantages in the acid consumption arise as a result of the use of 100% w/w HNO 3  and 70% w/w HF or HF gas (see Table 7). 
     
       
         3Si+4HNO 3 +18HF&lt;===&gt;3H 2 SiF 6 +4NO+8H 2 O Reaction Equation No. 1 
       
     
     Comparison of the Acid Consumptions in the Various Metering Methods 
     
       
         
               
               
               
               
             
           
               
                   
                 TABLE 7 
               
               
                   
                   
               
               
                   
                   
                 HF/HNO 3  consump- 
                 HF/HNO 3  consump- 
               
               
                   
                   
                 tion using 70% 
                 tion using 100% 
               
               
                   
                   
                 w/w HNO 3  and 50% 
                 w/w HNO 3  and 70% 
               
               
                   
                   
                 w/w HF for 480 
                 w/w HF for 480 
               
               
                   
                   
                 kg of polysili- 
                 kg of polysili- 
               
               
                   
                 Method 
                 con fragments 
                 con fragments 
               
               
                   
                   
               
             
             
               
                   
                 Three-stage 
                 220 liters HNO 3   
                 50 liters HNO 3   
               
               
                   
                 method, 12 μm 
                  27 liters HF 
                 15 liters HF 
               
               
                   
                 removal 
               
               
                   
                 Four-stage 
                 100 liters HNO 3   
                 20 liters HNO 3   
               
               
                   
                 method, &lt;5 μm 
                  10 liters HF 
                  6 liters HF 
               
               
                   
                 removal 
               
               
                   
                   
               
             
          
         
       
     
     Performance of the Metal Analyses on Cleaned Polysilicon Fragments 
     Polysilicon weighing 100 g is sprayed with 40 ml of 1:4 HF/HNO 3  in a Teflon beaker. The etching acid is collected in a Teflon beaker. The acid is then evaporated off and the residue is taken up in 5 ml of water. The metal content of the aqueous solution is measured using the ICP-AES (inductively coupled ion plasma atomic emission spectroscope supplied by the Spectro Company). The metal content of the polysilicon surface is calculated from the measured values. The data are in g/cm 2 . 
     In the following examples, the experimental conditions of the four-stage combined method are explained in greater detail: 
     EXAMPLE 4 
     A polysilicon ingot is contaminated during contact with metal tools on average with 1,332×10 −11  g/cm 2  iron on the polysilicon surface. The subsequent cleaning is carried out as described in Table 8. The necessary acid concentrations, the material removed by etching and the cleaning times for the production of polysilicon fragments having an iron content on the polysilicon surface of &lt;6.66×10 −11  g/cm 2  can be found in the table. 
     Conditions During the Performance of the Pickling/Etching Process 
     
       
         
               
               
               
               
               
               
             
           
               
                 TABLE 8 
               
               
                   
               
               
                   
                   
                   
                   
                 Material 
                   
               
               
                   
                 Reaction 
                   
                 Temperature 
                 removed by 
               
               
                 Substep 
                 time 
                   
                 degrees C. 
                 etching 
               
               
                   
               
             
             
               
                 Rinsing 
                 5 minutes 
                   
                 25 ± 5 
                 — 
                   
               
               
                 with water 
                   
                   
                 degrees 
               
               
                 Preliminary 
                 20 minutes 
                   
                 22 degrees 
                 0.02 μm 
               
               
                 cleaning: 
               
               
                 Pickle 
               
               
                 HF 5% w/w 
               
               
                 HCl 8% w/w 
               
               
                 H 2 O 2  3% w/w 
               
               
                 Rinsing 
                 5 minutes 
                   
                 22 degrees 
                 — 
               
               
                 Main clean- 
                 2.5 minutes 
                   
                  8 degrees 
                 2 μm 
               
               
                 ing: Etch 
               
               
                 No. 1 
               
               
                 HF/HNO 3   
               
               
                 0.6% w/w 
               
               
                 HF 
               
               
                 63% w/w HNO 3   
               
               
                 Main clean- 
                 2.5 minutes 
                   
                  8 degrees 
                 2 μm 
               
               
                 ing: 
               
               
                 Etch No. 2 
               
               
                 HF/HNO 3   
               
               
                 0.6% w/w 
               
               
                 HF 
               
               
                 &gt;70% w/w 
               
               
                 HNO 3   
               
               
                 Rinsing 
                 5 minutes 
                   
                 22 degrees 
                 — 
               
               
                 Hydrophi- 
                 5 minutes 
                   
                 22 degrees 
                 — 
               
               
                 lization 
               
               
                 HCl 8% w/w 
               
               
                 H 2 O 2  3% w/w 
               
               
                 Rinsing 
                 5 minutes 
                   
                 22 degrees 
                 — 
               
               
                 Drying with 
                 20 minutes 
                   
                 80 degrees 
                 — 
               
               
                 high/purity 
               
               
                 class 100 
               
               
                 air 
               
               
                   
               
             
          
         
       
     
     EXAMPLE 5 
     100 g of polysilicon is abraded for 10 seconds with a V4A stainless steel screw. The uncleaned polysilicon contains on average 666,000×10 −11  g/cm 2  iron and 133,200×10 −11  g/cm 2  chromium on the surface. The subsequent cleaning is carried out as described in Table 8. 
     EXAMPLE 6 
     A polysilicon ingot is comminuted in a crusher having metal jaws. The uncleaned material contains on average 5,328×10 −11  g/cm 2  iron on the polysilicon surface. The subsequent cleaning is carried out as described in Table 9. The required acid concentrations, the material removed by etching and the cleaning times for the production of polysilicon fragments having an iron content on the polysilicon surface of &lt;6.66×10 −11  g/cm 2  can be found in the table. 
     Conditions During the Performance of the Pickling/Etching Process for Material from Example 6 
     
       
         
               
               
               
               
               
               
             
           
               
                 TABLE 9 
               
               
                   
               
               
                   
                   
                   
                   
                 Material 
                   
               
               
                   
                 Reaction 
                   
                 Temperature 
                 removed by 
               
               
                 Substep 
                 time 
                   
                 degrees C. 
                 etching 
               
               
                   
               
             
             
               
                 Rinsing with 
                 5 minutes 
                   
                 25 ± 5 
                 — 
                   
               
               
                 water 
                   
                   
                 degrees 
               
               
                 Preliminary 
                 48 minutes 
                   
                 22 degrees 
                 0.02 μm 
               
               
                 cleaning: 
               
               
                 Pickle 
               
               
                 HF 5% w/w 
               
               
                 HCl 8% w/w 
               
               
                 H 2 O 2  3% w/w 
               
               
                 Rinsing 
                 6 minutes 
                   
                 22 degrees 
                 — 
               
               
                 Main clean- 
                 6 minutes 
                   
                  8 degrees 
                 5 μm 
               
               
                 ing: Etch 
               
               
                 No. 1 
               
               
                 HF/HNO 3   
               
               
                 HF 0.6% w/w 
               
               
                 HNO 3  63% w/w 
               
               
                 Main clean- 
                 6 minutes 
                   
                  8 degrees 
                 5 μm 
               
               
                 ing: 
               
               
                 Etch No. 2 
               
               
                 HF/HNO 3    
               
               
                 HF 0.6% w/w 
               
               
                 HNO 3  &gt;70% 
               
               
                 w/w 
               
               
                 Rinsing 
                 6 minutes 
                   
                 22 degrees 
                 — 
               
               
                 Hydrophi- 
                 12 minutes 
                   
                 22 degrees 
                 — 
               
               
                 lization 
               
               
                 HCl 8% w/w 
               
               
                 H 2 O 2  3% w/w 
               
               
                 Rinsing 
                 6 minutes 
                   
                 22 degrees 
                 — 
               
               
                 Drying with 
                 20 minutes 
                   
                 80 degrees 
                 — 
               
               
                 high-purity 
               
               
                 class 100 
               
               
                 air 
               
               
                   
               
             
          
         
       
     
     Influence of the Type of Lifting/Lowering Movement on the Pickling/Etching Process 
     The iron values are specified in 10 −11  g/cm 2   
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 10 
               
               
                   
                   
               
               
                   
                 Type of 
                   
                   
                   
               
               
                   
                 lifting/lowering 
                 Mean iron 
                 Standard 
                 Number of 
               
               
                   
                 movement 
                 values 
                 deviation 
                 values 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Lifting and 
                 13.32 
                 13.32 
                 25 
               
               
                   
                 lowering in the 
               
               
                   
                 acid only 
               
               
                   
                 Movement in which 
                 3.33 
                 3.33 
                 200 
               
               
                   
                 the process dish 
               
               
                   
                 is continuously 
               
               
                   
                 filled and 
               
               
                   
                 drained 
               
               
                   
                   
               
             
          
         
       
     
     While a few embodiments of the present invention have been shown and described, it is to be understood that many changes and modifications may be made thereunto without departing from the spirit and scope of the invention as defined in the appended claims.