Abstract:
A hot wire device and method for depositing semiconductor material onto a substrate in a deposition chamber in which the ends of at least two filaments are clamped into a filament holder and heated by supplying current, wherein a voltage for generating an electrical current is applied in temporal succession to filaments made of differing materials so that a number of differing semiconductors corresponding to the number of consecutively heated filament materials can be consecutively deposited onto the substrate without opening the chamber.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a hot wire method for depositing semiconductor material onto a substrate, and to a device for carrying out the method. 
     In the hot wire chemical vapor deposition (HWCVD) method, a gas mixture is decomposed into fragments in the vicinity of the substrate using a heated filament. A decomposition product is deposited onto the substrate. The chemical composition of the layers can be adjusted very well by way of the selection and the mixing ratio of the types of gas that are employed. 
     When producing semiconductor layers, such as microcrystalline silicon (μc-Si:H) and microcrystalline silicon carbide (μc-SiC:H), using the hot wire method, the selection of the filament material is of decisive importance for the quality of the layers and for the stability of the filaments during the process, and over an extended operating time. It was found that tantalum (Ta) is the material best suited for producing μc-Si:H. Rhenium (Re) is found to be best material for producing μc-SiC:H. While rhenium may also be used to produce μc-Si:H, alternating deposition of μc-Si:H and μc-SiC:H results in destruction of the rhenium filaments. 
     The drawback is that it is not possible to deposit multiple semiconductor layers in a hot wire (HWCVD) chamber with a fast method. 
     SUMMARY OF THE INVENTION 
     It is the object of the invention to provide a hot wire (HWCVD) method by way of which semiconductor layers can be quickly deposited, and to provide a device for carrying out the method. 
     The hot wire method is carried out so as to deposit semiconductor materials onto a substrate in a deposition chamber. For this purpose, the ends of at least two filaments are clamped into a filament holder in the chamber. A voltage is applied so as to heat the filaments and thus thermally or catalytically decompose precursor compounds and deposit the desired semiconductors onto the substrate. Filaments made of differing materials are advantageously connected to a voltage source and energized in temporal succession, so that a number of differing semiconductors corresponding to the number of consecutively heated filament materials are consecutively deposited onto the substrate without opening the chamber. 
     It was recognized as part of the invention that the deposition of differing layers is possible in the prior art only after the filament material in the deposition chamber has been changed by way of a complex process. Changing the filament material requires the deposition chamber to be opened and the expensive filaments to be replaced, and thus entails additional time for introducing the new filament. As an alternative, individual chambers fitted with differing filaments are required for each deposition process, however the drawback here is that the coated substrate has to be removed from one chamber and transferred into another chamber for further coating. This measure thus also results in lost time. 
     According to the invention, the ends of at least two filaments made of differing materials are fixed and clamped into filament holders. The filaments are energized in temporal succession by supplying current and connection to one or more voltage sources. This advantageously causes only the filament material that is required for depositing a semiconductor to be heated, and the other filament to be electrically activated after the material has been deposited, so that at least two semiconductor materials are deposited in temporal succession without opening the chamber. The voltage is applied to decompose what are known as precursor compounds, which pass over the filaments in gaseous form. The compounds dissociate into fragments. 
     The fragments optionally react with additional fragments or precursor compounds before they are deposited onto the substrate. 
     Thus, fast and cost-effective consecutive deposition of differing semiconductor materials onto the substrate is advantageously achieved. 
     In one embodiment of the invention, a filament holder is selected which comprises at least two power supply elements that are electrically insulated from each other, for at least two different filament materials. Each power supply element advantageously causes heating of a filament, or of multiple filaments, made of one material, and thus the decomposition of a precursor compound and the deposition of the desired semiconductor aligned with the heated filaments. 
     It is particularly advantageous for multiple filaments that are made of the same material to be simultaneously heated by each power supply element. The supply of current for heating filaments of one filament type is distributed. 
     This advantageously allows a greater amount of semiconductor material to be deposited onto a larger substrate surface aligned with the simultaneously heated filaments of a particular type. 
     It is particularly advantageous for the materials of the at least two different filaments that are clamped into the device to be rhenium and tantalum. This advantageously causes reproducible and lasting μc-Si:H and μc-SiC:H semiconductor layers to be produced in many deposition processes, without the drawback of the filament materials becoming damaged, and without opening the chamber. 
     It is particularly advantageous that, during the process, μc-Si:H and μc-SiC:H can be alternately deposited using the tantalum filament and rhenium filament without having to open the hot wire chamber to replace the filaments. The starting substances used for producing μc-Si:H are the gases silane (SiH 4 ) and hydrogen (H 2 ) and the gases for producing μc-SiC:H are monomethyl silane (H 3 Si—CH 3 ) and H 2 . The gases phosphine (PH 3 ), diborane (B 2 H 6 ), trimethyl boron (B(CH 3 ) 3 ) and trimethyl aluminum (Al 2 (CH 3 ) 6 ) are employed for doping. 
     The device for carrying out the hot wire (HWCVD) method is characterized by a filament holder for receiving at least two different filaments which simultaneously connects the filaments made of a first material to a different circuit than the filaments made of a second or third material and so forth. 
     The filament, or the filaments, of one material type are connected to a particular circuit by way of the filament holder. 
     Thus, “at least two filaments” shall mean that the holder can hold at least two filaments made of differing materials. It is also possible to dispose two, three or more filaments made of the same material next to, or between, two, three or more filaments made of another material, for example in an alternating fashion. It is also possible to hold even more filaments made of differing materials in the filament holder in alternating fashion, wherein the filaments made of one material are always simultaneously connected to a particular circuit or to a power source. This advantageously causes only the filaments made of one particular material to be simultaneously heated, while the filaments made of the remaining materials are not heated at that time. When the semiconductor has been deposited, the filament made of a second material is, or the filaments made of a second material are, heated, and the semiconductor intended for this deposition is deposited. This advantageously causes only one particular semiconductor to be deposited at a time onto the substrate by heating of the corresponding filament material for the dissociation of the precursor compound for this semiconductor. 
     The device advantageously comprises a filament holder having two holding blocks for receiving the ends of filaments oriented parallel to each other. The holding blocks are advantageously made of ceramic material or another electrically insulating, current-resistant and vacuum-proof material. This is advantageously achieved by being able to clamp the filaments into the mutually parallel end faces of the ceramic holders, without the power supply elements and terminals influencing each other electrically, because the ceramic material by nature acts as an insulator. The material can preferably be made of Macor™. MACOR comprises approximately: 46% SiO 2 , 17% MgO, 16% Al 2 O 3 , 10% K 2 O, 7% B 2 O 3  and 4% F. 
     The holding blocks comprise at least two current-conducting rails as power supply elements. The conductor rails are mounted on a holding block, for example, and form part of differing circuits, or alternatively a power source, which may a single power source, may make electrical contact with the rails consecutively. Each conductor rail is in electrical contact with the filaments made of one particular material type and distributes the current from the power source to these filaments. Each rail is thus intended for heating the filaments made of one particular material, and thus for heating the filament, or the filaments, so as to deposit a particular semiconductor. Each conductor rail is in electrical contact with a power source and the filaments, or distributes the current from this power source to the filaments made of the same material. 
     A current-conducting rail thus makes contact with the filaments made of identical material, so that advantageously a particular semiconductor from the precursor gas that is introduced into the chamber dissociates and can be deposited over a larger area, in keeping with the fixation of the filaments over the substrate. It is also possible that contact is made by a conductor rail with only a single filament made of a particular material. 
     The filament holder particularly advantageously also comprises gas supply lines aligned with the filaments. For this purpose, in addition to the conductor rails and the clamps for the filaments, the holding blocks comprise clamps for the gas supply lines disposed parallel to the clamps for the filaments. The gas supply lines are, for example, arranged in tubular form, having outlet holes beneath the filaments in the holding blocks. This advantageously causes the gas to be conducted to the filaments in a targeted manner, resulting in savings in terms of consumption. 
     It is thus possible for the holding blocks to advantageously comprise, for example, a total of six clamps for alternately clamping, in each case, three filaments made of the same material. For example, the ends of the first filament of the first type are first clamped into the two holding blocks and thus are fixed between the holding blocks. Thereafter, a filament of the second type follows. The first type may be a filament made of rhenium, and the second type may be a filament made of tantalum. Then, another filament of the first type follows, and a still further filament of the second type follows. This alternating positioning of filaments of the at least two types particularly advantageously results in uniform heating of the gas volume of a hot wire chamber over the substrate. 
     Of course, it is possible to alternately dispose more than two types of filaments between the holding blocks of the device. The number of power supply elements that are required for the holding blocks must then at least correspond to the number of the filament types that are used. 
     Conductor rails serving as power supply elements may be disposed on the holding blocks for this purpose. These are connected to the power supply of the hot wire chamber so that the filaments made of one material (filament type 1) are consecutively activated separately from the filaments made of a second material (filament type 2). This advantageously causes only filaments of one type to be energized and heated, while the other filament type, or types, is not, or are not, in operation. 
     A hot wire chamber according to the invention for carrying out the method according to any one of the preceding claims is characterized by filament holders for receiving the ends of the two filaments, wherein the filament holder can establish at least two separate circuits for connection to the at least two circuits of the hot wire chamber. 
     The invention will be described in greater detail hereafter based on one exemplary embodiment, without thereby limiting the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a cross-sectional view of the hot wire (HWCVD) deposition chamber according to the invention; 
         FIG. 2  shows an exploded view of the design according to the invention of a device for carrying out the hot wire (HWCVD) method; and 
         FIG. 3  shows  FIG. 2  as a design according to the invention, including the filament holder and the gas supply lines as well as the connections to the circuits by way of the two conductor rails for each holding block. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  shows the deposition chamber according to the invention. Gas is conducted via the main gas valve  1  into the chamber by way of the gas supply line  2 . The gas flows along the filaments  6  into the chamber beneath the filament holder  5  comprising two holding blocks. The substrate  7  is disposed parallel to the filaments  6  in the chamber. The heater  4  for the substrate  7  heats the same. Heat shields  3  shield the chamber from the heater  4 . The pump system  9  and the robot chamber  8  complete the chamber. 
       FIG. 2  shows an exploded view of the device according to the invention for carrying out the method according to the invention. The filament holder according to the invention comprises two holding blocks  3 ,  4 , in addition to conductor rails  1 . 1  and  1 . 2  and filament holding sleeves  2 . 
     Each of the two holding blocks  3 ,  4  is composed of an upper ceramic base body  3  and a lower ceramic flat body  4 . The upper and lower bodies are screwed together by way of screws and the holes  4   k  on  3   k  and  4   l  on  3   l . When assembled, the holding body  3  and the flat body  4  are pressed against each other, so that the semicircular openings  3   a  on  4   a ,  3   b  on  4   b  and  3   c  on  4   c  in each case join to form a circular clamping mount for a respective gas supply line (see  FIG. 3 ). 
     The filaments are held as follows: The conductor rails  1 . 1  and  1 . 2  are placed onto the filament holder  3  in a mutually engaging way, without coming in contact with each other, so that the holes  1   e ,  1   g  and  1   i  of conductor rail  1 . 1  are seated on the holes  3   e *,  3   g * and  3   i * of the base body  3 . The filament holding sleeves  2  (only the sleeve on the far right is denoted by reference numeral  2 ) are thus introduced into the holes  3   d  to  3   i  of the filament holder so that the holes  2   d ,  2   e ,  2   f .  2   g ,  2   h  and  2   i  of the filament holding sleeves  2 , as well as the holes  1   e ,  1   g  and  1   i  of the conductor rail  1 . 1  and the holes  1   d ,  1   f  and  1   h  of the conductor rail  1 . 2 , as well as the holes  3   d * to  3   i * are positioned on top of each other so as to screw the parts to the filaments. 
     Screws are introduced for this purpose into the holes  1   d  to  1   i  of the conductor rails  1 . 1  and  1 . 2  and clamp the conductor rails against the holder  3 . The screws also fasten the sleeves  2  to the holder  3 . A filament that was introduced into the interior cavity of the sleeve  2  is thus clamped against the sleeve. The small holes at the end faces of the sleeves  2  show the passages for the total of six filaments that are fixed between the holding blocks, refer to  FIG. 3 . 
     These processes are described by way of example for the right part of the device according to the invention. Of course, when joined, three filaments made of two material types are alternately introduced into the right and left filament holders, respectively, and clamped, as described above. 
     The procedure is shown in  FIG. 3 . Reference numerals  31 . 1 ,  31 . 2 ,  33  and  34  correspond to reference numerals  1 . 1 ,  1 . 2 ,  3  and  4  of  FIG. 2 , and reference is made to  FIG. 2  for further description. 
     The filaments  31   d ,  31   f  and  31   h  are made of rhenium. They are clamped into the sleeves  2   d ,  2   f  and  2   h , as is shown in  FIG. 2 , by way of the holes  3   d ,  3   f  and  3   h.    
     The filaments  31   e ,  31   g  and  31   i  are made of tantalum. They are clamped into the sleeves  2   e ,  2   g  and  2   i , as is shown in  FIG. 2 , by way of the holes  3   e ,  3   g  and  3   i.    
     The filaments are fixed between the right and left holders  33 ,  34  and as described for  FIG. 2 . The filaments are pushed into the sleeves corresponding to the filaments for this purpose, and are screwed to the holding blocks  33 ,  34 . Conductor rails  31 . 1  and  31 . 2  are disposed on the holding blocks for this purpose. The filament is clamped via the holes in the conductor rails, as is described for  FIG. 2 , and fixed between the blocks  33 ,  34 . 
     During operation, either gas types 1 or 2 are consecutively introduced via holes  35  into the chamber by way of the gas supply lines  32   a ,  32   b  and  32   c . Each gas supply line  32   a - c  supplies two filaments with precursors or is directed to two filaments, refer to  FIG. 3 . 
     Clamp A is connected to a direct current source (not shown). When clamp A on the conductor rail  31 . 2  is energized via the power supply element, the filaments  31   i ,  31   g  and  31   e , which are conductively connected to the conductor rail  31 . 2 , are energized. The first circuit is closed via clamp A*. 
     The gas mixture that is conducted through the hollow pipe  32   a - c  is fragmented by the heated rhenium filaments. 
     The power supply on clamp A is interrupted and the gas supply is stopped. 
     Clamp B is then energized. When clamp B on conductor rail  31 . 1  is energized via the power supply element, the filaments  31   d ,  31   f  and  31   h , which are conductively connected to the conductor rail  31 . 1 , are energized. The second circuit is closed via clamp B*. 
     The gas mixture that is conducted through the hollow pipe  32   a - c  is fragmented by the tantalum filaments. 
     During operation of the rhenium filaments, the tantalum filaments thus are dormant, or are not heated, and vice versa. 
     The assembled filament holder is screwed onto flanges  10  of the chamber, as is shown in  FIG. 1 , using the holes  4   m  and  4   n , as shown in  FIG. 2 . 
     One exemplary embodiment for producing μc-Si:H and μc-SiC:H layers using the novel HWCVD filament holder will be described hereafter. 
     The depositions for testing the filament holder for the alternating production of μc-Si:H and μc-SiC:H using the HWCVD method, without changing the filament materials between depositions, take place in a HWCVD deposition system comprising a load-lock chamber. The glass substrates, each measuring 10×10 cm 2 , are inwardly transferred through the load lock. For further processing of the μc-SiC:H layers to obtain solar cells, glass substrates that are partially coated with etched ZnO are utilized. Si wafers are also used as substrates for analyzing individual layers by way of infrared spectroscopy. 
     Three rhenium filaments (for μc-SiC:H) and three tantalum filaments (for μc-Si:H) deposition, which are connected in parallel, are located in each filament holder. The differing filament materials can be activated separately, which is to say supplied with voltage. 
     The following process conditions are established for μc-Si:H deposition: 
     Substrate temperature: 180° C.; temperature of the tantalum filaments: 1900 to 2000° C.; process gases: silane diluted to 2 to 10% in hydrogen; total gas flow rate: 100 sccm; process pressure: 0.06 hPa. 
     The following process conditions are established for μc-SiC:H deposition: 
     Substrate temperature: 220° C.; temperature of the rhenium filaments: 1700 to 1800° C.; process gases: monomethyl silane diluted to 0.3% in hydrogen; total gas flow rate: 100 sccm; process pressure: 0.75 hPa. 
     The deposition sequences and times are shown in the table: 
     
       
         
               
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Material 
               
             
          
           
               
                   
                 1) Re 
                 2) Ta 
                 3) Re 
                 4) Ta 
                 5) Re 
               
               
                   
                   
               
             
          
           
               
                   
                 Deposition 
                 19 
                 6 
                 24 
                 12 
                 3 
               
               
                   
                 times, total 
               
               
                   
                 (hours 
               
               
                   
                   
               
             
          
         
       
     
     The indicated multiple filament material switchovers did not result in any discernible damage to the filament materials. The accumulated layer thicknesses are approximately 5 μm for μc-Si:H and approximately 3 μm for μc-SiC:H. 
     The quality of the μc-Si:H layers was analyzed by way of electrical photoconductivity and dark conductivity measurements, and measurements of the hydrogen content and the hydrogen bond structure by way of infrared spectroscopy and optical absorption. The layers exhibit features that are typical of good electro-optical quality, in keeping with those that are produced with filament holders using only one filament material. 
     The quality of the μc-SiC:H layers was analyzed by installing these layers in solar cells with μc-Si:H absorber layers. The μc-SiC:H window layers produced with the double filament holder result in solar cells of as high a quality as solar cells produced using a filament holder having only one filament material.