Abstract:
The present disclosure presents a chemical vapor deposition reactor having improved chemical utilization and cost efficiency. The wafer susceptors of the present disclosure may be used in a stackable configuration for processing many wafers simultaneously. The reactors of the present disclosure may be reverse-flow depletion mode reactors, which tends to provide uniform film thickness and a high degree of chemical utilization.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Patent Application Ser. No. 61/169,139, which is hereby incorporated by reference in its entirety. 
    
    
     FIELD 
     The present disclosure relates to epitaxial deposition. More particularly, the present disclosure relates to epitaxial deposition of silicon or other semiconducting materials. 
     BACKGROUND 
     Monocrystalline silicon is the most dominant material for photovoltaic applications. The high efficiencies associated with monocrystalline solar cells, combined with the abundance of material, garner appeal for continued use and advancement. But the high processing cost of crystalline silicon material limits the widespread use of these solar modules. At present, the cost of “wafering,” or crystallizing silicon and cutting a wafer, accounts for about 40% finished solar cell cost. If a more direct way of making wafers were possible, great headway could be made in lowering the cost of solar cells. 
     There are different known methods of growing monocrystalline silicon and releasing or transferring the grown wafer. Regardless of the method, a low cost epitaxial deposition process is assumed. 
     Silicon epitaxial (epi) deposition was originally developed for the semiconductor industry. The requirements for the semiconductor industry, in both film properties and cost, are nearly polar opposites of requirements in the solar field. For example, semiconductor epi films are typically less than 5 μm (1 μm=10 −6  meter) thick, while solar requires 60-80 μm of silicon. In order to achieve economies in the solar industry, the silicon cost per watt must reside in the $0.25/watt or approximately $1.00/wafer (assuming a 4 watt cell). 
     The precursor chemistry for epi is predominantly trichlorosilane (TCS), although for thinner films silane (SiH 4 ) may also be used. Epitaxial deposition for each chemical poses unique requirements and challenges in both equipment architecture and process conditions. Based on low cost and abundance, TCS is the chemistry of choice for the solar industry. The present disclosure will generally be described with regard to TCS, but one of ordinary skill in the art will recognize its applications to silane and other precursor chemicals. 
     In order to achieve the necessary economy for solar applications, process cost modeling is studied to identify and optimize equipment performance. Three categories of cost make up the total cost picture: fixed cost (FC), recurring cost (RC) and yield cost (YC). FC is made up of items such as equipment purchase price, installation cost, and robotics or automation cost. RC is largely made up of electricity, gases, chemicals, operator salaries, and maintenance technician support. YC may be interpreted as the total value of parts lost during production. 
     To achieve the cost of ownership (CoO) numbers required by the solar field, all aspects of the cost picture must be optimized. The qualities of a low-cost process are (in order of priority): 1) high productivity, 2) high yield, 3) low RC, and 4) low FC. 
     Designing highly productive equipment requires a good understanding of the process requirements and reflecting those requirements in the architecture. High yield requires a robust process and reliable equipment. Low RC turns out to be the dominant component in the cost model. RC can impact plant site selection based on, for example, cost of local power or availability of bulk chemicals. FC, although important, is diluted by equipment productivity. 
     With the above said, in summary, a highly productive, reliable, efficient reactor may be essential for the production of low cost solar cells. 
     SUMMARY 
     Therefore, it is an object of the present disclosure to provide a CVD reactor having improved chemical utilization and high yield, and thereby reduce costs and improve efficiency. 
     Achieving low RC requires efficient use of chemicals. In an effort to make use of a greater portion of TCS, a longer deposition zone will be described. Providing a longer deposition zone may allow a higher percentage of the TCS to be deposited. In this way, greater chemicals utilization may be realized. By choosing the optimal parameters (chemical concentration, gas speed, reaction dimensions, etc.), productivity and chemical utilization may be optimized. A further object of the present disclosure is minimization of deposition on surfaces other than the wafers, which is also important for effective chemical utilization. The disclosed reactor has a high wafer area to reactor surface area ratio, which tends to minimize parasitic losses. 
     These and other advantages of the disclosed subject matter, as well as additional novel features, will be apparent from the description provided herein. The intent of this summary is not to be a comprehensive description of the subject matter, but rather to provide a short overview of some of the subject matter&#39;s functionality. Other systems, methods, features and advantages here provided will become apparent to one with skill in the art upon examination of the following FIGURES and detailed description. It is intended that all such additional systems, methods, features and advantages included within this description be within the scope of the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The features, nature, and advantages of the disclosed subject matter may become more apparent from the detailed description set forth below when taken in conjunction with the drawings in which like reference numerals indicate like features and wherein: 
         FIG. 1  shows a top view of an embodiment of a wafer susceptor; 
         FIGS. 2A and 2B  show a side view and an enlarged side view, respectively, of an embodiment of a wafer susceptor; 
         FIG. 3  shows a side view of an embodiment of a reactor with two sets of susceptor plates; 
         FIG. 4  shows a top view of a batch stack reactor (BSR) embodiment; 
         FIGS. 5A and 5B  show a side view and an enlarged side view, respectively, of an embodiment of a double-sided deposition (DSD) susceptor arrangement; 
         FIG. 6  shows a top view of an embodiment comprising an array of susceptors; and 
         FIG. 7  shows a side view of an embodiment of a double-sided deposition reactor. 
     
    
    
     DETAILED DESCRIPTION 
     Although the present disclosure is described with reference to specific embodiments, one skilled in the art could apply the principles discussed herein to other areas and/or embodiments without undue experimentation. 
     The present disclosure references a “template,” which may be viewed as equivalent to a “wafer.” In one embodiment of the present disclosure, the template, after epitaxy, may be used repeatedly to grow and release crystalline wafers. But the use to which the template or wafer is put after epitaxial deposition is beyond the scope of the present disclosure: one of ordinary skill will recognize the myriad uses to which the template might be put without departing from the spirit of the present disclosure. 
     One novel aspect of the reactor of the present disclosure lies in the arrangement of the wafer susceptors (a susceptor is a material used for its ability to absorb electromagnetic energy and impart that energy, in the form of heat, to the wafers). Although the susceptors may be heated electromagnetically, lamps or resistive heating may also be effective. 
     The susceptors of the present disclosure may be stackable, yet they do not rely on stacking for providing the “building blocks” of the overall reactor. The reactors of the present disclosure may or may not be depletion mode reactors (DMRs). “Depletion mode” refers to the depletion or utilization of chemical along the direction of gas flow. As shown in  FIG. 1 , that direction may be reversed to even out film thickness from one end to the other. In embodiments where the direction is not reversed, a tendency to deposit more chemicals in the region closest to the source port may be exhibited. In a forward-flow (i.e. left-to-right) mode, port  10  comprises a source port, and port  12  comprises an exhaust port; in a reverse-flow mode, the opposite is true. For that reason, port  10  may be referred to as “source/exhaust port  10 ,” and port  12  may be referred to as “exhaust/source port  12 .”  FIGS. 1 ,  2 A, and  2 B show different views of the same susceptor arrangement: a top view, a side view, and a detail side view, respectively. As shown in  FIGS. 2A and 2B , the design of ports  10  and  12  lends itself to the stackable nature of the wafer susceptors of the present disclosure. 
     Baffle channels  15  are shown in  FIGS. 1 ,  2 A, and  2 B. These baffle channels comprise a part of the path through which the TCS or other chemical species flows. Pin holes  16 , shown in  FIG. 1  only, provide template lift during the epitaxial deposition process. 
     In these views, template  20  (shown in  FIG. 2B ) is shown inserted into insert pocket  18  (shown in  FIG. 1 ). 
     The various dimensions of the reactor shown may be varied by one of ordinary skill without departing from the spirit of the present invention. 
     In this exemplary embodiment, the thickness of insert pocket  18  is approximately 6 mm, and the length of the whole assembly is approximately 50 cm. The diameter of ports  10  and  12  may be approximately 15 mm. 
       FIG. 3  shows reactor  30 , which includes two sets of stacked susceptor plates, similar to the susceptor plates shown in the preceding three FIGURES. The reactor of  FIG. 3  is a depletion mode reactor. 
     Reactor  30  includes source/exhaust port  40  and exhaust/source port  42 . The maid body of reactor  30  is housed in quartz muffle  35 . As shown, reactor  30  uses lamps  36  for heating the susceptor plates. 
     During the reaction (or reduction) of TCS with hydrogen gas, hydrochloric acid (HCl gas) is produced. In fact, if the reaction were fed with additional H 2  and allowed to extend over a longer zone or time, the concentration of HCl could continue to rise past the point of reaction inhibition and begin to etch the silicon template. While this is generally a state to be avoided, etching of silicon may be employed to clean the downstream exhaust passages. In effect, by allowing a sufficient level of HCl to build up, one could operate the reactor of the present disclosure in a self-maintaining mode by having the produced HCl gas etch away unwanted deposited silicon. 
       FIG. 4  shows reactor  50 , an embodiment of the present disclosure known as a batch stack reactor (BSR). In this configuration, the susceptor plates are stacked to increase the batch load to, in some embodiments, several hundred wafers. By purging the exterior of the susceptors with H 2  gas, the quartz bell jar is protected from silicon deposition. Most known bell jar reactors are not protected from TCS and require periodic HCl cleaning to remove unwanted deposited silicon. This process may interrupt production, thereby adversely affecting the cost per wafer (i.e. CoO). 
     Reactor  50  is housed in quartz bell jar  52 . In the embodiment shown, reactor  50  includes separate ports for TCS and H 2 , although this is not a necessary feature of the present disclosure; in other embodiments, TCS and H 2  may be premixed and fed through the same ports. As shown, H 2  source/exhaust ports  54  and TCS source/exhaust ports  55  are at one end of the reactor; H 2  exhaust/source ports  56  and TCS exhaust/source ports  57  are at the other end. These ports may be differentiated only when acting as source ports. When a given port is being used in an exhaust capacity, it will be exhausting gas that has already been mixed inside the reactor. 
     TCS reduction with H 2  may result when the gases are mixed at the appropriate temperature.  FIG. 4  shows an arrangement of separating the precursors until the point of use at each susceptor. This method may further extend chemical utilization and runtime favoring further improved CoO. 
     In the arrangement shown in  FIGS. 5A and 5B , each template is exposed to process gases on both sides. This feature enables dual side deposition, which has a compounding effect of both increased chemical utilization and lower epi cost per wafer. 
     The susceptors shown in  FIGS. 5A and 5B  are generally similar in use to the ones shown in  FIGS. 2A and 2B , and may be incorporated into various types of reactor configurations. 
     The dual sided susceptors may be stackable (as shown in the embodiment of  FIG. 3 ), yet they may also be arranged in a matrix as shown in  FIG. 6 . 
       FIG. 7  shows a side view of a depletion mode reactor using the dual sided susceptors of  FIGS. 5A and 5B . It is generally similar in structure to the reactor shown in  FIG. 3 , but with a dual sided susceptor in place of the stacked susceptors. 
     Those with ordinary skill in the art will recognize that the disclosed embodiments have relevance to a wide variety of areas in addition to those specific examples described above. In particular, any of the disclosed susceptors could be placed into any of the disclosed reactor arrangements without undue experimentation by one of ordinary skill in the art. 
     The foregoing description of the exemplary embodiments is provided to enable any person skilled in the art to make or use the claimed subject matter. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without the use of the innovative faculty. Thus, the claimed subject matter is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein. 
     It is intended that all such additional systems, methods, features, and advantages that are included within this description be within the scope of the claims.