Patent Publication Number: US-2023141501-A1

Title: Method for forming polycrystalline silicon film

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This patent application claims priority to Japanese Patent Application No. 2021-184177, filed Nov. 11, 2021, the contents of which are incorporated herein by reference in their entirety. 
     TECHNICAL FIELD 
     The present disclosure relates to a method for forming a polycrystalline silicon film. 
     BACKGROUND 
     A technique is known in which an amorphous silicon film, which is doped with impurities that suppress the progression of crystallization, as well as a non-doped amorphous silicon film, are laminated in this order so as to be situated on and above an insulating film, and then the laminated amorphous silicon films are crystallized (see, for example, Patent Document 1). 
     RELATED-ART DOCUMENT 
     Patent Document 
     [Patent Document 1] Japanese Unexamined Patent Application Publication No. 2015-115435 
     SUMMARY 
     A method for forming a polycrystalline silicon film includes forming a first amorphous silicon film having an island shape on a substrate. The method includes forming a second amorphous silicon film, the second amorphous silicon film covering the first amorphous silicon film. The method includes forming a third amorphous silicon film on the second amorphous silicon film. The method includes heating the substrate to a first temperature at which the first amorphous silicon film crystallizes more easily than the second amorphous silicon film. The first amorphous silicon film crystallizes at a temperature lower than that the second amorphous silicon film. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a flowchart illustrating a method for forming a polycrystalline silicon film according to an embodiment; 
         FIG.  2    is a cross-sectional view (1) of the polycrystalline silicon film according to the embodiment, the cross-sectional view being describing for the method for forming the polycrystalline silicon film; 
         FIG.  3    is a cross-sectional view (2) of the polycrystalline silicon film according to the embodiment, the cross-sectional view being describing for the method for forming the polycrystalline silicon film; 
         FIG.  4    is a cross-sectional view (3) of the polycrystalline silicon film according to the embodiment, the cross-sectional view being describing for the method for forming the polycrystalline silicon film; 
         FIG.  5    is a cross-sectional view (4) of the polycrystalline silicon film according to the embodiment, the cross-sectional view being described for the method for forming the polycrystalline silicon film; and 
         FIG.  6    is a schematic diagram of an example of a processing apparatus that performs the method for forming the polycrystalline silicon film according to the embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Non-limiting embodiments of the present disclosure will be described below with reference to the drawings. In the drawings, the same or corresponding members or components are denoted by the same or corresponding numerals, and accordingly, duplicate description will be omitted. 
     Method for Forming Polycrystalline Silicon Film 
     A method for forming a polycrystalline silicon film according to an embodiment will be described with reference to  FIGS.  1  to  5   . In the following description, a case where the polycrystalline silicon film is formed on an insulating film that is disposed in a substrate will be described as an example. 
     As illustrated in  FIG.  1   , the method for forming a polycrystalline silicon film according to the embodiment includes performing a first deposition step S 1 , a second deposition step S 2 , a third deposition step S 3 , and a crystallization step S 4  in this order. 
     First Deposition Step S 1   
     As illustrated in  FIG.  2   , the first deposition step S 1  includes forming a first amorphous silicon film  121  having an island shape, on an insulating film  110  that is formed on the surface of a substrate  100 . The first amorphous silicon film  121  is a film that crystallizes at a temperature lower than that of a second amorphous silicon film  122  described below. The first amorphous silicon film  121  is formed by supplying a first silicon-containing gas onto the substrate  100 . As the first silicon-containing gas, a monosilane (SiH 4 )-containing gas can be used suitably. The first deposition step S 1  may be performed in a processing chamber that is maintained at a pressure greater than a pressure used in the second deposition step S 2 . With this approach, a thermal decomposition temperature of the first silicon-containing gas can be reduced. For example, in a generally used range of pressures (for example, greater than or equal to 1 Pa and less than or equal to 1000 Pa), a thermal decomposition temperature of monosilane is approximately 450° C. to 530° C., and a thermal decomposition temperature of disilane, which is an example of higher order silane described below, is appropriately 350° C. to 420° C. In such a case, when the first deposition step S 1  is performed in the processing chamber maintained at the same pressure as a pressure used in the second deposition step S 2 , a deposition temperature used in the first film forming process S 1  is set to be greater than a deposition temperature in the second deposition step S 2 . In contrast, when the first deposition step S 1  is performed in the processing chamber that is maintained at the pressure greater than the pressure used in the second deposition step S 2 , the thermal decomposition temperature of the monosilane can be reduced. For example, when the first deposition step S 1  is performed in the processing chamber that is maintained at a pressure in the range of greater than or equal to 1.3 kPa and less than or equal to 13 kPa (i.e., greater than or equal to 10 Torr and less than or equal to 100 Torr), monosilane can be thermally decomposed at a temperature in the range of greater than or equal to 350° C. and less than or equal to 420° C. As a result, the first deposition step S 1  and the second deposition step S 2  can be performed at the same temperature. A suitable process condition used in the first deposition step S 1  is as follows. 
     First Silicon-Containing Gas: Monosilane 
     
         
         
           
             Substrate temperature: greater than or equal to 350° C. and less than or equal to 420° C. 
             Pressure of processing chamber: greater than or equal to 1.3 kPa and less than or equal to 13 kPa 
           
         
       
    
     Second Deposition Step S 2   
     As illustrated in  FIG.  3   , the second deposition step S 2  includes forming a second amorphous silicon film  122  that covers the first amorphous silicon film  121 . The second amorphous silicon film  122  is formed by supplying a second silicon-containing gas onto the substrate  100 . As the second silicon-containing gas, a high order silane gas that contains two or more silicon (Si) atoms in one molecule can be suitably used. The high order silane gas has an increased number of silicon atoms in one molecule, and allows a given film to be formed at a low temperature. With this approach, with use of the high order silane gas, the second amorphous silicon film  122  is formed such that the second amorphous silicon film  122  covers the insulating film  110  and the first amorphous silicon film  121 . An example of the high order silane gas includes disilane (Si 2 H 6 ), trisilane (Si 3 H 8 ), tetrasilane (Si 4 H 10 ), or a mixture gas of two or more of the gases described above. A suitable process condition used in the second deposition step S 2  is as follows. 
     Second Silicon-Containing Gas: Disilane 
     
         
         
           
             Substrate temperature: greater than or equal to 350° C. and less than or equal to 420° C. 
             Pressure of processing chamber: greater than or equal to 1 Pa and less than or equal to 1000 Pa 
           
         
       
    
     Third Deposition Step S 3   
     As illustrated in  FIG.  4   , the third deposition step S 3  includes forming a third amorphous silicon film  123  on the second amorphous silicon film  122 . The third amorphous silicon film  123  is formed by supplying a third silicon-containing gas onto the substrate  100 . As the third silicon-containing gas, for example, monosilane, a high order silane gas, a halogen-containing silicon gas, or a mixed gas of two or more of the gases described above can be used. Examples of the halogen-containing silicon gas can include (i) a fluorine-containing silicon gas such as SiF 4 , SiHF 3 , SiH 2 F 2 , or SiH 3 F, (ii) a chlorine-containing silicon gas such as SiCl 4 , SiHCl 3 , SiH 2 Cl 2  (DCS), or SiH 3 Cl, and (iii) a bromine-containing gas such as SiBr 4 , SiHBr 3 , SiH 2 Br 2 , or SiH 3 Br. As the third silicon-containing gas, monosilane can be suitably used from the viewpoint of an increased deposition rate and low costs. The third deposition step S 3  may be performed at the same temperature as the temperature used in the second deposition step S 2 , or may be performed at a temperature different from the temperature used in the second deposition step S 2 . A suitable process condition used in the third deposition step S 3  is as follows. 
     Third Silicon-Containing Gas: Monosilane 
     
         
         
           
             Substrate temperature: greater than or equal to 450° C. and less than or equal to 530° C. 
             Pressure of processing chamber: greater than or equal to 1 Pa and less than or equal to 1000 Pa 
           
         
       
    
     Crystallization Step S 4   
     The crystallization step S 4  includes heating the substrate  100  to a first temperature to crystallize the first amorphous silicon film  121 , the second amorphous silicon film  122 , and the third amorphous silicon film  123 , thereby forming a polycrystalline silicon film. The first temperature is a temperature at which the first amorphous silicon film  121  crystallizes more easily than the second amorphous silicon film  122 . In this case, as illustrated in  FIG.  5   , crystallization of the first amorphous silicon film  121 , the second amorphous silicon film  122 , and the third amorphous silicon film  123  progresses by nucleating the first amorphous silicon film  121  having the island shape. As a result, the polycrystalline silicon film having a large grain size can be formed. The numeral  124  in  FIG.  5    schematically indicates a crystal grain boundary between adjacent mono-crystalline regions. For example, when monosilane is used in the first deposition step S 1  and disilane gas is used in the second deposition step S 2 , the first temperature may be greater than or equal to 550° C. and less than or equal to 600° C. The crystallizing step S 4  is performed under an inert gas atmosphere such as a nitrogen atmosphere or an argon atmosphere, or under a reducing gas atmosphere such as a hydrogen atmosphere. 
     In the above method for forming a polycrystalline silicon film according to the embodiment, the first amorphous silicon film  121  having an island shape is first formed on the substrate  100 . Next, the second amorphous silicon film  122  that covers the first amorphous silicon film  121  is formed. Then, the third amorphous silicon film  123  is formed on the second amorphous silicon film  122 . Subsequently, the substrate  100  is heated to a first temperature. In this case, the first amorphous silicon film  121  is a film that crystallizes at a temperature lower than that of the second amorphous silicon film  122 , and the first temperature is a temperature at which the first amorphous silicon film  121  crystallizes more easily than the second amorphous silicon film  122 . Thus, crystallization of the first amorphous silicon film  121 , the second amorphous silicon film  122 , and the third amorphous silicon film  123  progresses by nucleating the first amorphous silicon film  121  having the island shape. As a result, the polycrystalline silicon film having a large grain size can be formed. 
     Processing Apparatus 
     An example of a processing apparatus that performs a method for forming the polycrystalline silicon film according to the embodiment will be described with reference to  FIG.  6   . A processing apparatus  1  is a batch-type apparatus that performs a process in which substrates are processed at the same time. Each substrate may be, for example, a semiconductor wafer (hereinafter simply referred to as a “wafer W”). 
     The processing apparatus  1  includes a processing chamber  10 , a gas supply  30 , an exhaust device  40 , a heater  50 , a controller  80 , and the like. 
     An interior of the processing chamber  10  can be depressurized, and the processing chamber  10  accommodates wafers W. The processing chamber  10  includes a cylindrical inner tube  11  having a lower end that is open and having a ceiling, and also includes a cylindrical outer tube  12  that covers the outer side of the inner tube  11 . The lower end of the outer tube  12  is open and the outer tube  12  has a ceiling. The inner tube  11  and the outer tube  12  are each formed of a heat-resistant material such as quartz, and are coaxially arranged to form a double tube structure. 
     The ceiling of the inner tube  11  is flat, for example. An accommodation portion  13  that accommodates a gas nozzle along the longitudinal direction (vertical direction) of the inner tube  11  is formed at one side of the inner tube  11 . A portion of the sidewall of the inner tube  11  protrudes outward to form a protruding portion  14 , and the inside of the protruding portion  14  is formed as the accommodation portion  13 . 
     A rectangular opening  15  is formed in the sidewall of the inner tube  11  on the other side of the inner tube  11  along the longitudinal direction (vertical direction) of the inner tube  11  so as to face the accommodation portion  13 . 
     The opening  15  is a gas exhaust port formed so as to be capable to exhaust the gas in the inner tube  11 . The opening  15  has the same as a length of a wafer boat  16 , or extends vertically, i.e., both upwards and downwards, to be longer than the length of the wafer boat  16 . 
     A lower end of the processing chamber  10  is supported by a cylindrical manifold  17  made of, for example, stainless steel. A flange  18  is formed on an upper end of the manifold  17 , and a lower end of the outer tube  12  is provided to be supported on the flange  18 . A sealing member  19 , such as an O-ring, is interposed between the flange  18  and the lower end of the outer tube  12  so that an interior of the outer tube  12  is hermetically sealed. 
     An annular support  20  is provided at an inner wall of the upper portion of the manifold  17 , and the lower end of the inner tube  11  is provided to be supported on the support  20 . A cover  21  is hermetically attached to an opening at the lower end of the manifold  17  through the sealing member  22  such as an O-ring, so as to hermetically close the opening at the lower end of the processing chamber  10 , that is, the opening of the manifold  17 . The cover  21  is made of stainless steel, for example. 
     A rotation shaft  24 , which rotatably supports the wafer boat  16  through a magnetic fluid sealing portion  23 , is provided at the central portion of the cover  21  to pass through the cover  21 . A lower portion of the rotation shaft  24  is rotatably supported by an arm  25 A of an elevation mechanism  25  that includes a boat elevator. 
     A rotation plate  26  is provided at an upper end of the rotation shaft  24 , and the wafer boat  16  that holds the wafers W is provided above the rotation plate  26 , via a heated platform  27  made of quartz. With this arrangement, the cover  21  and the wafer boat  16  are integrally moved up and down by raising and lowering the elevation mechanism  25 . Thus, the wafer boat  16  can be inserted into or removed from the processing chamber  10 . The wafer boat  16  can be accommodated by the processing chamber  10  and substantially horizontally holds a plurality of (for example, 50 to 150) wafers W, such that the wafers are spaced apart from one another when viewed in the vertical direction. 
     The gas supply  30  supplies, into the inner tube  11 , process gas used in each of the first deposition step S 1 , the second deposition step S 2 , and the third deposition step S 3 . The process gas includes a first silicon-containing gas, a second silicon-containing gas, a third silicon-containing gas, and purge gas, and the like. The gas supply  30  includes a gas nozzle  31 . 
     The gas nozzle  31  is made of, for example, quartz, and is provided in the inner tube  11  along a longitudinal direction of the inner tube  11 . Also, a base end of the gas nozzle  31  is bent in an L-shape, and is supported so as to pass through the manifold  17 . Gas holes  32  are formed at the gas nozzle  31  along the longitudinal direction of the gas nozzle, and the process gas is horizontally discharged via the gas holes  32 . The gas holes  32  are arranged at the same intervals as intervals of the wafers W that are supported by the wafer boat  16 , for example. The process gas of which a flow rate is controlled is introduced into the gas nozzle  31 . 
     Although  FIG.  6    illustrates a case where the gas supply  30  includes one gas nozzle  31 , the gas supply  30  is not limited to the manner described above. For example, the gas supply  30  may include multiple gas nozzles. In this case, the first silicon-containing gas, the second silicon-containing gas, the third silicon-containing gas, and the purge gas may be supplied from the same gas nozzle into the inner tube  11 , or may be respectively supplied from different gas nozzles into the inner tube  11 . 
     The exhaust device  40  exhausts the gas discharged from the interior of the inner tube  11 , through the opening  15 . Also, the exhaust device  40  exhausts the gas discharged from a gas outlet  41 , through a space P 1 , which is between the inner tube  11  and the outer tube  12 . The gas outlet  41  is formed at the sidewall of the upper portion of the manifold  17  so as to be situated above the support  20 . An exhaust passage  42  is connected to the gas outlet  41 . A pressure regulating valve  43  and a vacuum pump  44  are separately provided in the exhaust passage  42  to allow the gas in the processing chamber  10  to be exhausted. 
     The heater  50  is provided around the outer tube  12 . The heater  50  is provided, for example, above the base plate  28 . The heater  50  has a cylindrical shape so as to surround the outer tube  12 . The heater  50  includes, for example, a heating element, and heats the wafers W in the processing chamber  10 . 
     The controller  80  controls the operation of each component of the processing apparatus  1 . The controller  80  may be implemented, for example, by a computer. A program used for a computer that performs the operation of each component of the processing apparatus  1  is stored in a storage medium  90 . The storage medium  90  may be, for example, a flexible disk, a compact disk, a hard disk, a flash memory, a digital versatile disc (DVD), or the like. 
     Operation of Processing Apparatus 
     A case where the processing apparatus  1  performs the method for forming a polycrystalline silicon film according to the embodiment will be described below. The controller  80  controls the operation of each component of the processing apparatus  1  to perform the method as follows. 
     First, the wafer boat  16  on which the wafers W are mounted is transferred to the processing chamber  10 . Next, the opening at the lower end of the manifold  17  is closed by the cover  21 , and thus the interior of the processing chamber  10  forms a sealed space. 
     Then, the first deposition step S 1  is performed. In the first deposition step S 1 , the interior of the processing chamber  10  is vacuum-evacuated to be maintained at a first process pressure, and power supplied to the heater  50  is adjusted to increase the temperature of each wafer to a first process temperature. The first process pressure is set to, for example, be greater than or equal to 1.3 kPa and less than or equal to 13 kPa. The first process temperature is set to, for example, be greater than or equal to 350° C. and less than or equal to 420° C. After the interior of the processing chamber  10  is stabilized at the first process pressure and the temperature of each wafer is stabilized at the first process temperature, monosilane as a first silicon-containing gas is supplied into the processing chamber  10 , while the controller  80  rotates the wafer boat  16 . With this approach, the first amorphous silicon film  121  having an island shape is formed on each wafer W. After the first amorphous silicon film  121  having the island shape is formed on each wafer W, supplying of the first silicon-containing gas into the processing chamber  10  is stopped. The rotation of the wafer boat  16  is continued. 
     Then, the second deposition step S 2  is performed. In the second deposition step S 2 , the pressure of the interior of the processing chamber  10  is changed from the first process pressure to a second process pressure, and the temperature of each wafer is adjusted to the second process temperature by adjusting the power supplied to the heater  50 . The second process pressure is set to, for example, be greater than or equal to 1 Pa and less than or equal to 1000 Pa. The second process temperature is set to be the same as the first process temperature, for example. With this approach, it is not necessary to change the temperature used in the second deposition step S 2 , thereby increasing productivity. However, the second process temperature may be different from the first process temperature. After the interior of the processing chamber  10  is stabilized at the second process pressure and the temperature of each wafer is stabilized at the second process temperature, disilane as a second silicon-containing gas is supplied into the processing chamber  10 , in a state where the rotation of the wafer boat  16  is continued. With this approach, the second amorphous silicon film  122  that covers the first amorphous silicon film  121  is formed. After the first amorphous silicon film  121  is entirely covered with the second amorphous silicon film  122 , supplying of the second silicon-containing gas into the processing chamber  10  is stopped. The rotation of the wafer boat  16  is continued. 
     Then, the third deposition step S 3  is performed. In the third deposition step S 3 , the pressure of the interior of the processing chamber  10  is changed from the second process pressure to a third process pressure, and the temperature of each wafer is adjusted to a third process temperature by adjusting the power supplied to the heater  50 . The third process pressure is set to, for example, be greater than or equal to 1 Pa and less than or equal to 1000 Pa. The third process temperature may be, for example, the same as the second process temperature, or may be different from the second process temperature. After the interior of the processing chamber  10  is stabilized at the third process pressure and the temperature of each wafer is stabilized at the third process temperature, monosilane as a third silicon-containing gas is supplied into the processing chamber  10 , in a state where the rotation of the wafer boat  16  is continued. With this approach, the third amorphous silicon film  123  is formed on the second amorphous silicon film  122 . The third amorphous silicon film  123  is formed to be thicker than, for example, the second amorphous silicon film  122 . After the third amorphous silicon film  123  having a desired film thickness is formed, supplying of the third silicon-containing gas into the processing chamber  10  is stopped. The rotation of the wafer boat  16  is continued. 
     Subsequently, the crystallization step S 4  is performed. In the crystallization step S 4 , the interior of the processing chamber  10  is subject to an inert gas atmosphere, and the temperature of each wafer is adjusted to a fourth process temperature by adjusting the power supplied to the heater  50 . The inert gas atmosphere may be, for example, a nitrogen atmosphere or an argon atmosphere. Instead of the inert gas atmosphere, a reducing gas atmosphere such as a hydrogen atmosphere may be used. The fourth process temperature is set to a temperature at which the first amorphous silicon film  121  crystallizes more easily than the second amorphous silicon film  122 . For example, the fourth process temperature is greater than or equal to 550° C. and less than or equal to 600° C. With this approach, crystallization of the first amorphous silicon film  121 , the second amorphous silicon film  122 , and the third amorphous silicon film  123  progresses by nucleating the first amorphous silicon film  121  having the island shape. As a result, the polycrystalline silicon film having a large grain size can be formed. 
     After the polycrystalline silicon film is formed, the wafers W are transferred out of the processing chamber  10  in reverse order from the order in which the wafers W are transferred into the processing chamber  10 . 
     While certain embodiments are described using the method for forming a polycrystalline silicon film, these embodiments are presented by way of example only, and are not intended to limit the scope of the disclosures. Indeed, the embodiments described herein may be embodied in a variety of other forms. Furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the scope of the disclosures. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope of the disclosures. 
     The above embodiments are described using a case where the processing apparatus is a batch-type apparatus in which wafers are processed at the same time. However, the present disclosure is not limited to the above type. For example, the processing apparatus may be a single-wafer processing apparatus in which wafers are processed one by one. 
     According to the embodiments of the present disclosure, a polycrystalline silicon film having a large grain size is capable of being formed.