Abstract:
The Siemens process for deposition of polycrystalline silicon in the form of rods in a sealed reactor is improved by, after introduction of deposition gas has ceased, introducing a ventilating gas into the partially sealed reactor, withdrawing a gas stream from the reactor, and monitoring the components in the gas stream withdrawn until a desired concentration of one or more components is reached, and opening the reactor to remove the rods.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is the U.S. National Phase of PCT Appln. No. PCT/EP2014/050437 filed Jan. 13, 2014, which claims priority to German Application No. 10 2013 200 660.8 filed Jan. 17, 2013, the disclosures of which are incorporated in their entirety by reference herein. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a process for depositing polycrystalline silicon. 
     2. Description of the Related Art 
     Polycrystalline silicon (polysilicon) serves as a starting material for production of monocrystalline silicon for semiconductors by the Czochralski (CZ) or zone melting (FZ) processes, and for production of mono- or polycrystalline silicon by various pulling and casting processes for production of solar cells for photovoltaics. 
     Polycrystalline silicon is generally produced batchwise in the Siemens process. This involves thermally decomposing a silicon-containing reaction gas or reducing it by means of hydrogen, and depositing it as high-purity silicon on thin filament rods of silicon, called “thin rods” or “cores”. 
     The silicon-containing component of the reaction gas is generally monosilane or a halosilane of the general composition SiH n X 4-n  (n=0, 1, 2, 3; X=Cl, Br, I). It is preferably a chlorosilane or chlorosilane mixture, more preferably trichlorosilane. Predominantly SiH 4  or SiHCl 3  (trichlorosilane, TCS) is used in a mixture with hydrogen. 
     The Siemens process is performed in a deposition reactor. EP 2 077 252 A2 describes the typical construction of a reactor type used in the production of polysilicon. In the most commonly used embodiment, the deposition reactor comprises a metallic base plate and a coolable bell jar placed onto the base plate so as to form a reaction space within the bell jar. The usually bell-shaped deposition reactor has to be closeable in a gas-tight manner since the reaction gases are corrosive and, in a mixture with air, have a tendency to self-ignition or to explosion. The base plate is provided with one or more feed orifices and one or more removal orifices for the gaseous reaction gases, and with holders which help to hold the thin rods in the reaction space. In general, two adjacent rods are connected by a bridge at their free ends, opposite the held foot ends, to form a U-shaped support body. The U-shaped support bodies are heated up to the deposition temperature by direct passage of current and the reaction gas is supplied. 
     A typical procedure for deposition of polysilicon involves opening a shutoff valve for the reaction gas flowing to the reactor and a shutoff valve for the offgas flowing out of the reactor. The reaction gas flows through a feed orifice in the base plate into the closed deposition reactor. The silicon is deposited therein on the thin rods heated by direct passage of current. The hot offgas formed in the reactor leaves the reactor through a removal orifice in the base plate and can then be subjected to a processing operation, for example a condensation, or can be sent to a scrubber. 
     In the deposition of silicon, the halogen-containing silicon compounds, for example trichlorosilane, decompose from the gas phase on the surface of the heated thin rods. In the course of this, the diameter of the thin rods grows. After the attainment of a desired diameter, the deposition is ended and the polysilicon rods formed are cooled to room temperature. 
     After cooling the rods, the bell jar is opened and the rods are withdrawn with deinstallation aids for further processing. Subsequently, the bell jar and base plate of the reactor are cleaned and provided with new electrodes and thin rods for the next deposition batch. After the bell jar has been closed, the process for depositing the next batch of polysilicon is again performed as described. 
     From the time of opening of the reactor until the deinstallation of the batch deposited, the polysilicon rods are in contact with ambient media such as room air with the corresponding constituents of nitrogen, oxygen, moisture, but also impurities in the form of extraneous constituents present in the air (metals, nonmetals, gases). The opening also causes a possible exchange of gas between the reactor interior and the deposition room. In this case, reactants, products or even constituents which have already reacted fully or partly (e.g. HCl (g) ) which remain in the reaction space after the deposition can enter into the ambient air through exchange of gas. 
     More particularly, this is the case for the penetration of moisture from the ambient air into the reactor. When moisture penetrates specifically into the feed and removal lines of the reactor, and when bell jar deposits are present (solids of main constituents remaining on the inside of the reactor after the deposition, containing the elements Si, Cl and O), halosilane residues, for example unconverted reaction gas, or halosilanes or polysilanes formed in the process, result in the formation of corrosive hydrogen halides, for example hydrogen chloride. These corrosive gases can escape from the deposition reactor into the production room and lead, for example, to corrosion on lines, fittings, and technical components therein. 
     Especially the hydrogen halide corrodes reactor components, including the feed and removal lines in the reactor. The corrosion process gives rise to damage in the form of rust formation on steel surfaces in, for example, components of the deposition plants (flanges, connections). The corrosion which has occurred firstly causes a change in the surface properties and, as a result, releases metal particles (for example steel and alloy constituents Fe, Cr, Ni, Mn, Zn, Ti, W), and also releases electrically active dopants such as boron, phosphorus, aluminum and arsenic. These substances are introduced into the silicon deposited to an increased degree in the subsequent deposition, particularly on commencement of deposition, and deposited on the rod surface of polysilicon rods present in the production space. 
     Particularly, corroded steel can lead to unwanted deposits on the rod surfaces when the deposition reactors are opened for batch changeover and deinstallation of the polycrystalline silicon rods. The release of, for example, iron and deposits thereof on the rod surface can lead to a reduction in the lifetime in the resulting product for the semiconductor or solar industry. 
     U.S. Pat. No. 7,927,571 (DE 102006037020 A1) discloses a method for the batch production of high purity polycrystalline silicon, in which an inert gas is fed through the supply line and the discharge line into the open reactor from the time when the deposition reactor is opened in order to extract the first substrate body with deposited silicon until the time when the reactor is closed in order to deposit silicon on the second substrate body. 
     GB 1532649 discloses a method of depositing polycrystalline silicon on a graphite surface, wherein the closed reactor is purged with an inert gas, for example argon, before commencement of the heating of the deposition surface or shortly before the deposition. This inert gas purging for purging of a closed reactor during the course of the process is effected for inertization or for avoidance of explosive gas mixtures (oxygen removal). 
     The feeding of inert gas into the reactor during the process or after the opening of the reactor, which is disclosed in the prior art, does not solve the problem of reactive depletion of the bell jar deposits. Nor is the problem of introduction of extraneous material into the reaction space and onto the rod surfaces in the operation of reactor opening remedied thereby. 
     US 2012/0100302 A1 discloses a method for producing polycrystalline silicon rods by deposition of silicon on at least one thin rod in a reactor, wherein, before the silicon deposition, hydrogen halide at a thin rod temperature of 400-1000° C. is introduced into the reactor containing at least one thin rod and is irradiated by means of UV light, as a result of which halogen and hydrogen radicals arise and the volatile halides and hydrides that form are removed from the reactor. This cleans the thin rod surface before the start of deposition. During the batch changeover or the installation of the thin rods, feed and removal lines and the bell jar are purged with inert gas (nitrogen) in the open state. 
     SUMMARY OF THE INVENTION 
     The problem addressed by the present invention was that of avoiding the disadvantages of the prior art. These and other problems are solved by a process for depositing polycrystalline silicon, comprising introducing a reaction gas comprising a silicon-containing component and hydrogen into a reactor, as a result of which polycrystalline silicon is deposited in the form of rods. In the process, the reactor, after the deposition has ended, is opened and ventilated for a particular period before rod deinstallation begins. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a closed deposition reactor on a base plate. 
         FIG. 2  illustrates a deposition reactor raised from the base plate during ventilation. 
         FIG. 3  illustrates means for ventilating a deposition reactor. 
         FIG. 4  illustrates the relationship between limiting concentration and initial HCl concentration. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The period of ventilation begins with the first opening of the reactor after the deposition of a batch of polycrystalline silicon and includes the period after the deposition has ended until the removal of the bell jar or parts thereof with the purpose of deinstalling the first polycrystalline rod from the reactor. In the case of rod deinstallation too, the system is opened and necessarily ventilated and purged. 
     Ending of the deposition is understood to mean that the rods have reached the desired final diameter and the supply of reaction gas is ended. 
     Typically, in a process for depositing polycrystalline silicon, a U-shaped support body composed of silicon is secured in an opened deposition reactor, the deposition reactor is sealed air-tight, the U-shaped support body is heated by direct passage of current, a silicon-containing reaction gas and hydrogen are introduced through a feed line into the deposition reactor, as a result of which silicon is deposited from the reaction gas on the support body. In the course of this, the diameter of the support body grows. This forms an offgas which is removed from the deposition reactor through a removal line. On attainment of a desired diameter of the support body, the deposition is ended, the support body is cooled to room temperature, the inner surface of the deposition reactor is opened to the environment and the support body is removed from the deposition reactor, with securing of a second U-shaped support body in the deposition reactor. In a departure from this, the invention envisages, from the attainment of the desired target diameter of the support body and ending of the deposition, conditioning of the reactor interior with gaseous, liquid and/or solid constituents present therein and of the bell jar deposits present therein, in a suitable manner under defined conditions, prior to complete opening of the deposition reactor for the purpose of rod deinstallation. 
     Preferably, the reactor is opened by raising the reactor bell jar above the base plate. 
     Preferably, the reactor is opened by opening a sightglass. 
     Preferably, the reactor is opened by opening a flange or input gas or offgas lines. 
     Preferably, a medium is fed into the reactor and then removed again during the opening of the reactor. 
     Preferably, the medium is fed in through a sightglass, the medium being removed again via offgas orifices or a second sightglass. 
     Preferably, the medium is fed in and removed through the same orifice. 
     Preferably, the medium fed in is air, nitrogen, moisture, in each case individually or in combination. 
     Preferably, the gases fed in and removed are monitored for discharged constituents resulting from the reactive depletion of the bell jar deposits. 
     Example 1 
     After ending the chemical reaction (the deposition), the deposition reactor is opened, so that the polysilicon rods can subsequently be deinstalled with low contamination from the deposition reactor. 
     For this purpose, the inner surface of the deposition reactor is opened under defined conditions. The inner surface of the deposition reactor includes the regions shown in  FIG. 1 . 
       FIG. 1  shows a closed deposition reactor  2  standing on a base plate  1 . The inner surface comprises the surface  3  facing the interior of the reactor. 
     The inner surface of the deposition reactor can be opened at various points. 
     For example, the bell jar can be raised from the base plate, such that a medium for reactive depletion of the bell jar deposits can be supplied through the gap which forms. 
     This procedure is shown in  FIG. 2 . 
       FIG. 2  shows a deposition reactor, with the bell jar  2  thereof (body and hood) raised from the base plate  1  in the course of ventilation. 
     Through the gap  5  which forms, a medium suitable for reactive depletion of the bell jar deposits is introduced, which is removed via the suction  4  or the sightglass  6 . 
     This medium is gaseous and may contain different proportions of moisture at a defined temperature. For the reduction of the surface metal contamination of the deposited polysilicon rods, the gap height which forms and the ventilation period are important. 
     It has been found that there is a connection between surface metal contamination of the polysilicon rods and both the gap height and the ventilation time. 
     In this context, the following should be noted: the smaller the gap and the longer the airing time, the higher the surface metal contamination on the deposited polysilicon rods. 
     Between the base plate and bell jar of the deposition reactor, supports may be provided. 
     In this case, the gap which forms and is utilized for airing and media supply is, as a percentage, 0.5-15% of the total height of the deposition reactor (body+hood) above the base plate, preferably 0.9-11%, more preferably 1.2-9%. 
     The period for the process step of ventilation makes up less than one tenth of the process step of deposition, and the volume flow rate of the medium introduced is 50-2000 m 3 /h and preferably 100-500 m 3 /h, more preferably 150-300 m 3 /h. 
     Example 2 
     A second version of the process is shown in  FIG. 3 . 
       FIG. 3  shows means for ventilating the deposition plant before the batch changeover, namely through one or more sightglasses  6  or other orifices, for example flanges or water-cooled flanges and/or exclusively via the offgas orifice at the top  4 . In this case, the bell jar  2  remains on the base plate  1 . 
     In addition to the properties which have been described in the above version, the following changes are made: 
     A further means of opening can be effected in the region of the bell jar, without raising the deposition reactor from the base plate. 
     In this case, the extended means for ventilation of the deposition plant before the batch changeover involve ventilating the plant through one or more sightglasses  6  or other orifices, for example flanges or water-cooled flanges, and/or exclusively via the offgas orifice at the top  4 . In this case, the bell jar  2  remains on the base plate  1 . 
     One advantage of this method is that the bell jar deposits present in the reactor can be depleted by reaction under defined conditions. 
     The defined supply of media which serve for the reactive depletion of the bell jar deposits and discharge of HCl [g]  is possible in a relatively simple manner. 
     Possible media include air, synthetic air, air-conditioning system air, nitrogen, argon, helium, protective gases etc. 
     The media used can, for example, be preconditioned by the controlled supply of moisture. 
     In this way, more exact regulation of the flow rate of the in- and out-flowing medium is also possible. 
     In this way, the period between the end of the deposition process and re-equipping of the deposition reactor with thin rods is minimized. 
     A further advantage is that the surface contamination of the polysilicon rods can be reduced through opening of the reactor and the use of filter units which are provided in the region of the open surface, and through the suitable selection of the medium supplied for ventilation. 
     Table 1 shows surface metal contamination of the polysilicon rods after ventilation of the deposition plant by raising the bell jar (Example 1) and through one of the sightglasses (Example 2). 
     The discharge of HCl [g] and further reaction products and of medium used for airing is performed through offgas orifices into the suction. 
     
       
         
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                 Example 1 
                 Example 2 
               
               
                   
                 Metal 
                 Median [%]/Scatter 3σ 
                 Median [%]/Scatter 3σ 
               
               
                   
                   
               
             
             
               
                   
                 Fe 
                 100 
                 60 
               
               
                   
                 Cr 
                 100 
                 40 
               
               
                   
                 Ni 
                 100 
                 95 
               
               
                   
                 Na 
                 100 
                 32 
               
               
                   
                 Zn 
                 100 
                 22 
               
               
                   
                 Al 
                 100 
                 56 
               
               
                   
                 Cu 
                 100 
                 28 
               
               
                   
                 Mg 
                 100 
                 64 
               
               
                   
                 Ti 
                 100 
                 75 
               
               
                   
                 W 
                 100 
                 91 
               
               
                   
                 K 
                 100 
                 96 
               
               
                   
                 Ca 
                 100 
                 62 
               
               
                   
                 Co 
                 100 
                 78 
               
               
                   
                   
               
             
          
         
       
     
     Example 3 
     In a further version of the process, moistened nitrogen and/or air (including air in the form of air-conditioning air) is introduced into the deposition reactor as the medium for ventilating the deposition plant, and the reactive depletion of the bell jar deposits is monitored via the amount of hydrogen chloride gas formed by online monitoring, and the process is ended depending on a limiting concentration to be achieved. 
     Through the use of moistened nitrogen and/or air, the required process time for attainment of the required limiting concentration can be distinctly shortened up to a shortening of the ventilation time by a factor of 3. 
     The possible saving of process time is indicated in  FIG. 4  by Δt. 
     Moreover, the ending of the process with respect to a target parameter is possible, and so the process step lasts only as long as required for technical purposes. 
     This situation is illustrated in the figure which follows. 
       FIG. 4  shows that different batches require different periods for attainment of the required limiting concentration depending on the initial hydrogen chloride concentration in the suction (as an indicator for the progress of the process step). 
     The time Δt can be saved through the use of an online monitoring system compared to the fixing of a particular fixed process run time. 
     This reduces the deposition-free time. 
     The desired protection comprises the process step of ventilating the deposition plants after the deposition time has ended and before the deinstallation of the polysilicon rods.