The invention relates to a process for cleaning polycrystalline silicon chunks.
Polycrystalline silicon, polysilicon for short, is nowadays produced industrially in large amounts and serves, inter alia, as a raw material for applications in photovoltaics and for the production of single crystals at wafer manufacturers. In all applications, a high purity of the raw material is desired.
High-purity silicon is typically obtained by thermal decomposition of volatile silicon compounds which are therefore easy to purify by means of distillation processes, for example trichlorosilane. The silicon is deposited in polycrystalline form, in the form of rods having typical diameters of 70 to 300 mm and lengths of 500 to 2500 mm.
A significant portion of these polycrystalline rods are subsequently processed further by means of crucible pulling (Czochralski or CZ process) to give single crystals, or used for production of polycrystalline base material for photovoltaics. In both cases, high-purity, molten silicon is required. For this purpose, solid silicon is melted in crucibles.
The polycrystalline rods are comminuted prior to melting, typically by means of metallic breaking tools, such as jaw or roller crushers, hammers or chisels.
In the course of comminution, however, the high-purity silicon is contaminated with extraneous atoms. These are especially metal carbide or diamond residues, and metallic impurities.
Therefore, silicon chunks are cleaned prior to further processing and/or packaging for higher-value applications, for example for single-crystal pulling. This is typically done in one or more chemical wet cleaning steps.
This involves using mixtures of different chemicals and/or acids in order to remove adhering extraneous atoms in particular from the surface again.
EP 0 905 796 B1 claims a method for producing silicon which has a low metal concentration, characterized in that the silicon is washed in a preliminary cleaning in at least one stage with an oxidizing cleaning solution, which contains the compounds hydrofluoric acid (HF), hydrochloric acid (HCl) and hydrogen peroxide (H2O2), and is washed in a main cleaning in a further stage with a cleaning solution which comprises nitric acid (HNO3) and hydrofluoric acid (HF) and, for hydrophilization, is washed in a further stage with an oxidizing cleaning solution.
As a result of the entrainment of acid into the rinse water, as a result of the chemical reaction with metal particles and for the dissolution of the silicon in HF/HNO3 etching, acid is consumed.
To maintain a particular acid concentration, there is therefore a constant need to dose further fresh acid.
Unlike the cleaning of silicon wafers, the bulk material to be cleaned, as a result of the different size classes of the polycrystalline silicon chunks, has constantly varying surfaces.
Polysilicon can be classified into chunk sizes, each of which is defined hereinafter as the longest distance between two points on the surface of a silicon chunk (=max. length), as follows:                chunk size 1 (CS 1) in mm: about 3 to 15;        chunk size 2 (CS 2) in mm: about 10 to 40;        chunk size 3 (CS 3) in mm: about 20 to 60;        chunk size 4 (CS 4) in mm: about 40 to 110;        chunk size 5 (CS 5) in mm: about 110 to 170;        chunk size 6 (CS 6) in mm: about 150 to 230.        
The specific surface areas for the different chunk sizes are:                CS 6: about 0.05 cm2/g;        CS 5: about 0.5 cm2/g;        CS 4: about 1 cm2/g;        CS 3: about 2 cm2/g;        CS 2: about 5 cm2/g;        CS 1: about 10 cm2/g.        
The dosage of fresh acids in HF/HNO3 mixtures or HF/HCl/H2O2 solutions (cf. EP 0 905 796 B1) varies between 1 and 2000 liters per hour between CS 6 and chunk size 1.
For equal chunk sizes of the polysilicon too, the specific surface area varies by at least 20% between different batches. Here too, the acid consumption and hence the dosages required vary from batch to batch.
This means that the further dosage, even in the case of polycrystalline silicon chunks of one and the same chunk size, had to be adjusted constantly in order to keep the conditions constant in the cleaning bath.
In order to ensure a stable operating regime in the cleaning of polysilicon envisaged for applications in the semiconductor industry, experience has shown that the dosage system must have an accuracy of 10% or better.
Manual further dosage (operation by hand) is very complex and can barely ensure such a dosage accuracy.
Therefore, the further dosage, for a particular chunk size class, is always attuned to batches with the greatest specific surface area within this chunk size class.
A majority of the batch of a particular chunk size thus always runs with an overdosage, which inevitably leads to a higher acid consumption and makes the process less economically viable.
Alternatively, substantially automated regulation of the further dosage can be effected.
Closed-loop control circuits are used in chemical plants, inter alia, for temperature regulation, for fill level regulation, for flow regulation or for pH regulation.
Customary closed-loop control systems are based on continuous measurements of the parameter to be regulated.
For this purpose, corresponding sensors are used, which give measurements continuously.
However, sensors with which the composition of chemical cleaning solutions comprising several components are determined continuously and which could give the corresponding measurements without time delay are not available in the current state of the art.
The determination of the composition of such solutions requires parallel performance of several different analysis processes to determine the individual components.
For example, ion-selective electrodes are known for potentiometric determination of fluoride, with which the HF content of an HF/HNO3 etch mixture can be determined.
The nitrate content in an HF/HNO3 mixture can be determined, for example, by means of a photometric process.
Alternatively, the composition of such solutions can be determined by employing a titration process based on the DET method (DET=dynamic equivalence point titration).
A corresponding process is known, for example, from DE 198 52 242 A1.
It relates to the determination of concentration of acids in an acid mixture by means of dynamic equivalence point titration, wherein the acid mixture consisting of nitric acid, hydrofluoric acid, hexafluorosilicic acid and optionally further organic and/or inorganic compounds is admixed with a basic titer until an equivalence point between a hydrogen ion concentration of 10−2 to 10−3.5 is attained, then admixing with the titer is continued until an equivalence point between a hydrogen ion concentration of 10−4 to 10−5 is attained, and finally admixing with the titer is continued until an equivalence point between a hydrogen ion concentration of 10−10 to 10−11 is attained.
However, both the titration process just described and the analysis processes conducted in parallel only give a value every 5 to 60 minutes.
For further dosage of acids, membrane pumps or gravimetric systems such as dosage balances can be used.
However, it has been found with such dosage pumps that the desired dosage accuracy of 10% or better is not always attainable.
Typically, compressed air membrane pumps and motorized dosage pumps have a vent valve in the suction line. This is intended to counteract the problem that air is also sucked in the first strokes of a suction cylinder. Only after a few strokes has the air escaped again from the lines.
It has been found, however, that these vent valves do not work reliably when aggressive media such as acids are being sucked in.
Even in the case of conventional gravimetric systems such as dosage balances, the dosage accuracy is at best 10%.
Due to the insufficient accuracy of further dosage, a stable operating regime is impossible.
The problems described gave rise to the objective of the invention.