Patent Document

BACKGROUND OF THE INVENTION 
       [0001]    The invention relates to a new methods for post-texturing cleaning, surface conditioning, and rinsing silicon wafers or similar surfaces, with particular, although not exclusive, applicability in photovoltaic applications. 
         [0002]    Texturing of the wafer surface is usually the first step of the single emitter photovoltaic (PV) manufacturing process for both mono- and multi-crystalline silicon wafers. The texturing process roughens the surface and reduces the reflection of the silicon surface by etching along crystal planes and grain boundaries to increase the surface area to provide more light trapping. In addition to texturing, the initial wet chemical process bath or baths also must remove saw-damage, undesirable contamination, and condition the silicon surface to be hydrophilic so as to allow uniform doping for the emitter formation. 
         [0003]    Two of the most widely used processes, developed at the Energy Research Centre of the Netherlands (“ECN”) and the University of Konstanz (“UKN”) each begin with a concentrated HF/HNO 3  texturing bath that also removes any saw damage. KOH with IPA or a surfactant or NaOH can also be used for texturing, either alone or after the HF/HNO 3 . The ECN and UKN processes are disclosed in A. W. Weeber, A. R. Burgers, M. J. A. A. Goris, M. Koppes, E. J. Kossen, N. C. Rieffe, W. J. Soppe, C. J. J. Tool, and J. H. Bultman, 19th European Photovoltaic Solar Energy Conference (2004), and Hauser, et al., U.S. Pat. No. 7,192,885, respectively. 
         [0004]    The texturing process composition, used for multi-crystalline wafers, is comprised of 20% to 55% H 2 O, 10% to 40% concentrated HF (49 wt %) and 20% to 60% concentrated HNO 3  (approximately 65 wt %), with additives such as acetic acid (H-AC) and a surfactant. The texturing process is carried out at ambient or lower temperature with a controllable etch rate. Other texturing composition ranges can also be used. Typically, the wafers are exposed to the chemical between 1 and 5 minutes. 
         [0005]    For mono-crystalline silicon, the preferred texturing solution is alkaline-based. Dilute NaOH solutions or KOH solutions, with and without additives such as isopropyl alcohol (IPA) or ethylene glycol, are suitable for texturing, particularly for mono-crystalline silicon. Although acid texturing baths, including those described above for multi-crystalline wafers can be used, a KOH or NaOH bath is used subsequently to remove the porous silicon that remains on the surface. An HF/HCl bath is typically performed after the alkaline hydroxide step to facilitate removal of the mobile ions, metallic contamination, and to strip the chemical oxide. The final oxidation step creates a homogeneous hydrophilic surface allowing uniform phosphorus doping to create the emitter. 
         [0006]    The use of dilute chemical baths in processing silicon wafers is known in the art as exemplified by Olesen, et al., U.S. Pat. No. 6,158,445, the techniques outlined by researchers at IMEC published in Proceedings of the 7th International Symposium on Cleaning, SCP Global Technologies, Boise, Id., May 1-3, 2000; Kashdoush, et al., U.S. Pat. No. 6,837,944; Tsao, et al., U.S. Pat. No. 6,165,279; and Puri, et al., U.S. Pat. No. 6,681,781. Moreover, it is well-known in the art to use a dilute HCl rinse after SC-1 baths in semiconductor manufacturing. 
         [0007]    However, most such methods contemplate a separate water rinse or other neutralizing method performed after the dilute chemical bath. De-ionized water (DIW) rinsing or an equivalent is generally required because these methods are intended for use in semiconductor manufacturing where contamination levels on the wafer surface must be lower than those for solar cell manufacturing. There is no suggestion of how rinse water or the dilute chemical baths could be recycled and/or reused. 
         [0008]    The objective of the current invention to provide a method for post-texturing and cleaning baths for silicon wafers and the like that reduces the amount of chemicals and water used in the process while retaining an acceptable level of cleansing and surface conditioning. 
       BRIEF SUMMARY OF THE INVENTION 
       [0009]    The present invention discloses an improved rising process for the post-texturing and cleaning baths for silicon wafers and similar surfaces that is especially well-suited to silicon wafers intended to be used for photovoltaic applications. The inventors recognized that for photovoltaic and similar applications a dilute bath used for one step in the process can be reused in other steps because the purity and surface conditioning requirements are not as stringent as with semiconductor wafers. Thus, after the texturing step, rather than using concentrated chemical baths for cleaning and water rinses with new rinse water after these post-texturing baths, the present invention replaces the concentrated chemical baths and subsequent water rinses with combined rinsing and cleaning steps using dilute chemical baths. This replacement also allows for the reuse of chemicals and water recycled from other steps, with or without the addition, or “spiking”, of relatively small amounts of additional chemicals. Furthermore, by using appropriately selected chemicals in a particular order, the current invention allows the number of chemical baths to be reduced. The invention can generally be applied to wet cleaning processes that involve a plurality of chemicals, in individual tanks, with a rinsing step after each chemical step. 
         [0010]    In one embodiment, post-texturing chemical baths of the current invention are composed of dilute HF/HCl, followed by dilute SC-1 or O 3 /DIW (deionized water) or dilute H 2 O 2  or similar dilute formulations. Thus, the concentrated HF/HCl bath and DIW rinse of conventional cleaning processes may be replaced with a dilute HF/HCl rinse, and the final alkaline bath and DIW rinse may be replaced with a dilute SC-1, H 2 O 2 , or O 3  deionized water (O 3 /DIW) rinse or other combination including HF/O 3 /DIW. 
         [0011]    The use of these dilute rinses allow for the rinsing water to be reused in a controlled manner by cycling the water “upstream” from each rinsing bath. The apparatus for the baths are designed to use the rinsing chemical baths starting with the oxidizing bath (O 3 , SC-1 or H 2 O 2 ) going upstream and reusing the water by spiking with HF and/or HCl, and then upstream for rinsing after the KOH, NaOH bath, and finally using this rinsing water with chemicals after the HF/HNO 3  bath, or any combination of KOH and NaOH baths. For certain steps, temperature or time in the bath can be increased to counteract the decrease in reactivity when using the dilute chemicals. 
         [0012]    A pre-cleaning and/or post-cleaning process can also be included to remove contamination, as well as metallic, mobile ions and particles, from the surface and to create an oxidized or a oxide-passivated} state. The pre-cleaning step can include a dilute SC-1 rinse followed by a dilute HCl rinse, with or without HF added. The post cleaning step may follow the alkaline or O 3 /DIW step with a dilute HF bath with or without HCl added, and then another dilute SC-1 or other alkaline or O 3 /DIW bath. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  shows a schematic representation of an apparatus to implement a prior art cleaning process after the use of a HF/HCl texturing bath and KOH bath for porous silicon removal. 
           [0014]      FIG. 2  shows a schematic representation of an apparatus to implement a prior art cleaning process after the use of a KOH texturing bath. 
           [0015]      FIG. 3  is a schematic representation of an embodiment of the invention employing a HF/HNO 3  texturing bath followed by a KOH bath for porous silicon removal. 
           [0016]      FIG. 4  is a schematic representation of an embodiment of the invention employing a HF/HNO 3  texturing bath. 
           [0017]      FIG. 5  shows the embodiment of  FIG. 3  with the addition of paths for reuse of select rinses. 
           [0018]      FIG. 6  shows the embodiment of  FIG. 3  with the serial use of multiple baths of similar chemical composition and paths for reuse of select rinses. 
           [0019]      FIG. 7  is a schematic representation of an embodiment of the invention employing a KOH texturing bath and a path for reuse of the final rinse. 
           [0020]      FIG. 8  shows the embodiment of  FIG. 7  with the serial use of multiple baths of similar chemical composition and paths for reuse of select rinses. 
           [0021]      FIG. 9  shows the embodiment of  FIG. 3  with the inclusion of a pre-cleaning step and a first alternative set of paths for the reuse of select rinses. 
           [0022]      FIG. 10  shows the embodiment of  FIG. 3  with the inclusion of a pre-cleaning step and a second alternative set of paths for the reuse of select rinses. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0023]      FIGS. 1 and 2  are a schematic representations of a typical apparatus  100 ,  200  that might be used to implement the texturing and cleaning methods shown in the prior art, and with the modifications described below, for the current invention. 
         [0024]    The apparatus include an input conveyer  110 ,  210 , or similar which bring the silicon  120 ,  220  or other materials to a linear transfer robot  125 ,  225  for conveying the silicon  120 ,  220  to the various baths and rinses. An output conveyer  130 ,  230  can be used to remove the silicon  120 ,  220  from the apparatus  100 ,  200  when the processing is complete. It will be understood by those of ordinary skill in the art that all the apparatus discussed would include exhaust means  170  at appropriate locations as well as means (not shown) conventionally used to introduce and remove the chemical baths and DIW rinses all of which could use cooled, ambient, or heated water, depending on the needs of the process. 
         [0025]    Further, it will be understood that rinsing can be accomplished by any of a variety of well-known methods, including cascade overflow, quick dump rinsing, and spray rinsing or any combination of these methods. Multiple cycles of these rinsing methods may be used, including full or partial dump rinsing cycles, all of which are well-known in the art. Processing can be dual sided; front and back surface or single sided; front or back surface only. The equipment used for this process can be in-line rollers, immersion, or any applicable equipment that allows dilute processing or can be modified for dilute processing or similar means. 
         [0026]    Additions to the process are possible. For example, the rinse water may be subject to gasification or degasification; the rinse may include megasonic cleaning, possibly with the addition of CO 2  to acidify the water and/or N 2  or O 2 ; additives that gives water unique properties at low concentrations or chelators may be used. Further, filtering of the rinsing water can be performed to remove particles and other undesirable contamination 
         [0027]    In  FIG. 1 , which implements a process suitable for both mono- and multi-crystalline silicon, the first bath is a concentrated HF/HNO 3  texturing bath  132  which can be comprised of 20% to 55% H 2 O, 10% to 40% concentrated HF (49 wt %) and 20% to 60% concentrated HNO 3  (approximately 65 wt %), with additives such as acetic acid (H-AC) and a surfactant. The first bath is followed by a DIW rinse  133  using non-recycled water. The second bath is concentrated KOH with IPA, a surfactant  135  to remove any porous silicon. It will be understood that NaOH could replace KOH. The second bath is also followed by a DIW rinse  136  using non-recycled water. The third bath is HF/HCl  140  which removes mobile ions, metallic contamination and the remaining chemical oxide. It, too, is followed by a DIW rinse  141 . The final bath is surface conditioning bath of SC-1  150 , widely used for removing contaminants, such as particles, in silicon substrate processing, is mixture of ammonium hydroxide (NH 40 H), hydrogen peroxide (H 2 O 2 ) and de-ionized water in a volume ratio of 1:4:20. The SC-1 bath is also followed by a DIW rinse  151 . The silicon  120  or other material then proceeds to a drying step  160  before leaving the apparatus  100 . 
         [0028]    In  FIG. 2 , which implements a process suitable for mono-crystalline silicon, the first bath is concentrated KOH with IPA, a surfactant  235  to remove any porous silicon. It will be noted that or with NaOH could replace KOH. The bath is followed by a DIW rinse  236  using non-recycled water. The second bath is HF/HCl  240  and is followed by a DIW rinse  241 . The final bath is surface conditioning bath of SC-1  250  and is also followed by a DIW rinse  251 . As in the embodiment shown in  FIG. 1 , the silicon  120  or other material proceeds to a drying step  260  before leaving the apparatus  200 . 
         [0029]      FIG. 3  shows an apparatus  300  suitable for implementing an embodiment of the current invention. This embodiment is suitable for both mono- and multi-crystalline silicon. The apparatus is similar to those in  FIGS. 1 and 2 , in that it includes an input  310  and output  330  conveyer, a linear transfer robot  325  and exhaust systems  370  at appropriate locations. As in the prior art embodiments, the apparatus of  FIG. 3  also includes a first concentrated HF/HNO 3  texturing bath  332  immediately followed by a DIW rinse  333 . A second bath of concentrated KOH  335  for porous silicon removal, or, alternatively, a bath of NaOH, follows the DIW rinse. However, the current invention replaces the following DIW rinse and subsequent steps of concentrated HF/HCl bath, DIW rinse, SC-1 bath and DIW rinse with a dilute HF/HCl rinse  342  composed of HF at a concentration of 0.1 to 4.0%, extendible to 0.001 to 15% and HCl at a concentration of 0.01 to 1%, extendible to 0.001 to 15% and a final bath of O 3 /DIW  352  at a concentration of 6-20 ppm, extendible to 1-70 ppm. As a preferred alternative, the final bath could also be dilute SC-1 at 100:1:1 or similar, or dilute H 2 O 2  at 10:1 or similar, although other concentrations, particularly higher concentrations, could also be used. In combination, these dilute baths simultaneously perform the necessary surface conditioning and rinsing of the silicon. A conventional drying step  360  follows the final bath. It will be understood by those of ordinary skill in the art that appropriate concentrations and ratios of chemicals, as well as the time and temperature of the baths must be selected to achieve the desired surface conditioning result for the particular application in which the silicon  120  or other materials are to be employed. It will be further understood that appropriate conventional means will be used to monitor and adjust the chemical concentrations. 
         [0030]      FIG. 4  shows an alternative embodiment for implementing the current invention. As with conventional systems, the apparatus  400  includes an input  410  and output  430  conveyer, a linear transfer robot  425  and exhaust systems  470  at appropriate locations. The embodiment also includes a concentrated HF/HNO 3  texturing bath  332 . Further, and significantly, as with the embodiment in  FIG. 3 , this embodiment includes the use of the dilute HF/HCl rinse  442  and the final bath of O 3 /DIW  452  as described in connection with  FIG. 3 , to perform the necessary surface conditioning and rinsing of the silicon before the drying step  460 . However, this embodiment replaces the DIW rinse  333  following the texturing bath  332  and the KOH bath  335 , with a single dilute rinse step  434  of HF/O 3  or other oxidizing/Si etching dilute bath. A preferred concentration is HF at a concentration of 0.1 to 4.0%, extendible to 0.001 to 10%, O 3 /DIW  352  at a concentration of 6-20 ppm, extendable to 1-70 ppm. 
         [0031]      FIG. 5  shows an apparatus  500  that is essentially identical to the embodiment of  FIG. 3  but with reuse of the water and chemicals from the final dilute bath  352  in other steps. Incoming high-purity water first is used at the final rinsing cycle  352 ; either as O 3 /DIW or dilute SC-1 at 100:1:1 or similar, or dilute H 2 O 2  at 10:1 or similar are potential rinsing baths, this is performed in a recirculated, filtered, ambient bath. The used rinse water is removed from the rinse cycle  352 , spiked with HF/HCl and moved  580  to be used as the dilute HF/HCl rinse  342  in a recirculated, filtered, ambient bath after the KOH or NaOH cleaning bath  335 . The dilute HF/HCl bath contents are then transferred  590  to a third recirculated, filtered, ambient bath that serves as the rinse  533  after the HF/HNO 3  texturing bath  332 , before finally being discarded. It may be spiked with additional oxidizing fluid; hydrogen chloride; hydrogen fluoride; or combined HF/HCl. 
         [0032]    It will be understood that while there are a variety of chemicals and concentrations that can be used to obtain the desired surface condition effects, to properly implement this invention the chemical baths that can be reused or recycled must be chosen to meet the combined goal. For example, at the time of transfer of the O 3 /DIW bath to the HF/HCl bath, there are residual amounts of O 3  in the bath, but because the concentration of the HF/HCl is more than the O 3  and because O 3  decomposes to molecular oxygen (O 2 ) with no byproduct of the reaction, it is a preferable bath for both functionality, in that it provides a hydrophilic surface, and reactivity, in that it does not interfere with the functionality of the HF/HCl bath. 
         [0033]    As shown in  FIG. 6 , it is also possible to have multiple dilute baths using the same chemicals. As an example, the process in this apparatus  600  is essentially identical to that shown in  FIG. 5 , but includes the use of multiple baths. Incoming high-purity water first is used at the final rinsing cycle  352  as described in  FIG. 5  in a recirculated filtered, ambient bath. The used rinse water is removed from the rinsing cycle  352 , spiked with HF/HCl and moved  680  to another recirculated, filtered, ambient bath  644 . After use in bath  644  the rinse water is removed, spiked with more HCl and/or HF, and moved  685  to a recirculated filtered, ambient bath  643  that is directly after the concentrated KOH or NaOH bath  335 . Again the rinse is removed and moved  690  to its final use in a recirculated, filtered, ambient bath  633  after the HF/HNO 3  texturing bath. It may be spiked with additional oxidizing fluid; hydrogen chloride; hydrogen fluoride; or combined HF/HCl. After this use the contaminated rinsing water is discarded. It will be understood that the example given is the HF/HCl bath; however, any dilute chemical bath has the potential to be reused in multiple baths of essentially the same chemical composition. 
         [0034]      FIG. 7  shows an embodiment of the invention where the initial bath is concentrated KOH  735 , similar to the process shown in  FIG. 2 . As in  FIG. 2 , the apparatus  700  includes an input  210  and output  230  conveyer, a linear transfer robot  225 , appropriately placed exhaust means,  270 , and a final drying stage  260 . Similar to the embodiment shown in  FIG. 3 , but without the texturing bath  332  and rinse  333  that precedes the KOH bath  335  used for porous silicon removal, there is no DIW rinse after the KOH bath  735 . Instead, as with the previously described embodiments of the invention, the silicon is immediately place into a dilute, recirculated, ambient HF/HCl bath  742  at concentrations previously described in connection with  FIG. 3 , and then into a final bath  752  of O 3 /DIW, dilute SC-1, or dilute H 2 O 2  as previously described. This embodiment also employs reuse of the final rinse liquid. After use in the recirculated, filtered, ambient final bath  752 , the water is removed, spiked with HF/HCl and moved  790  to be used as the dilute HF/HCl bath  742  before finally being discarded. The process of  FIG. 8  is essentially identical, but uses multiple dilute HF/HCl baths  853 ,  854  rather than the single bath  752 . The rinse liquid from the final bath  752  is removed after use, spiked with HF and/or HCl, and moved  891  into the second dilute HF/HCl bath  854 . After use in the second dilute HF/HCl bath  854 , the liquid is again removed, preferably spiked with more HF and/or HCl, and moved  892  to be used as the first dilute HF/HCl bath  853  immediately after the concentrated KOH bath  735 . After use in the first HF/HCl bath  853 , the liquid is discarded. 
         [0035]    The invention can also be implemented with a pre-cleaning step before the texturing bath. Precleaning is often desirable since the sawing of the wafers slices exposes the pristine silicon to the saw material and also to lubrication, although in certain circumstances and processes, incoming wafers may have sufficiently low contamination levels so that a pre-clean is not needed. Without precleaning, copper and organic lubricating oils build-up in the texturing bath over time and can be distributed throughout the system, even with optimized rinsing. By cleaning the wafer before the texturing process with a dilute bath or series of dilute baths, the life of the entire process and each individual bath can be increased. 
         [0036]    The pre-cleaning process is typically composed of a SC-1 bath, either concentrated or dilute, followed by a SC-2 or dilute HCl bath or rinse. But, since a pre-clean with concentrated chemicals may affect the final texturing by exposing grain boundaries or etching the silicon that is detrimental to the final roughness desired, dilute chemical rinses are preferable because they do not affect the texture of the surface. Each of the concentrated SC-1 bath and SC-2 bath is followed by a DIW rinse. However, the dilute HCl rinsed wafers can proceed without additional rinsing into the texturing bath. Hydrofluoric acid may also be spiked into the water or used alone in place of HCl. The “pre-cleaning” baths may also be included after the texturing bath and subsequent rinses; however, the texturing bath would then still be subject to increased contamination. Also, additional precleaning and surface conditioning steps can be added if needed; for example, the sequence SC-1 bath to HF/HCl bath to SC-1 bath to render the surface hydrophilic to allow easier wetting of the surface prior to the texturing bath. 
         [0037]    The apparatus structures  900 ,  1000  shown in  FIGS. 9 and 10  are similar to those of  FIG. 3 , however, immediately prior to the HF/HNO 3  texturing bath  332 , the silicon  120  or other material is pre-cleaned in a dilute oxidizing rinse  998  similar in chemical composition to the final rinse  352  described above in connection with  FIG. 3  and is then treated to a dilute HF/HCl bath  999 . It will be noted that other dilute baths could also be used as suggested above.  FIGS. 9 and 10  show different alternatives for reuse of rinses when pre-cleaning is included in the process. In  FIG. 9 , the incoming high-purity water first is used in the dilute HF/HCl post-clean bath  342  and the water is then moved  902  and reused in the HF/HCl pre-clean bath  999  before being discarded. Additionally, new incoming high-purity water is first used in the dilute SC-1 or similar chemical post-clean bath  352 ; the water is then moved  901  for reuse as the SC-1 pre-clean rinse  998  before being discarded. The reused water may first be spiked with appropriate chemicals such as HF, HCl, SC-1 or the like. In  FIG. 10  incoming high-purity water first is used in the dilute SC-1 or similar chemical post-clean bath  352 , the rinse solution is then removed, spiked with HF and/or HCl and moved  1001  for use in the dilute HF/HCl post-clean bath  342 . After this use, the rinse is again removed and, after, optionally, spiking with more HF and/or HCl, moved  1002  for use in the HF/HCl pre-clean bath  999  before being discarded. 
         [0038]    It should be noted that he invention does not preclude additions to the processing, such as additional cleaning baths to remove particles, an additional final bath to render the surface hydrophilic instead of hydrophobic; or the inclusion of sonic agitation or other physical effect. 
         [0039]    As demonstrated by the following Table 1, the current invention is anticipated to result in significant reductions in chemicals and water used in the processing of silicon wafers for solar cell applications from the conventional concentrated chemical method. 
         [0000]    
       
         
               
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Concentrated Process 
                 Simplified Process 
               
             
          
           
               
                   
                 Number 
                 Volume 
                   
                 Number 
                 Volume 
               
               
                   
                 of Rinsing 
                 Used 
                   
                 of Rinsing 
                 Used 
               
               
                 Step 
                 Tanks 
                 (Liters) 
                 Step 
                 Tanks 
                 (Liters) 
               
               
                   
               
             
          
           
               
                 HF/HNO 3   
                 2 
                 100 
                 HF/HNO 3   
                 2 
                 100 
               
               
                 Rinse 
                   
                   
                 Rinse 
               
               
                 Alkaline 
                 2 
                 100 
                 Alkaline 
                 2 
                 100 
               
               
                 Rinse 
                   
                   
                 Rinse 
               
               
                 HF/HCl 
                 2 
                 100 
                 Dilute 
                 1 
                 50 
               
               
                 Rinse 
                   
                   
                 HF/HCl 
               
               
                 Oxidizing 
                 2 
                 100 
                 Proprietary 
                 1 
                 50 
               
               
                 Rinse 
                   
                   
                 Rinse 
               
               
                 Final Rinse 
                 1 
                 50 
                   
                   
                   
               
               
                 TOTAL 
                 9 
                 450 
                 TOTAL 
                 6 
                 300 
               
               
                   
               
             
          
         
       
     
         [0040]    The numbers for the concentrated process are actual bath volumes and rinsing cycles currently performed with concentrated chemicals using three dump rinse cycles on MTS equipment. The simplified process numbers include the anticipated volume reduction resulting from the elimination of one of the rinsing tanks. Additional reductions are achieved through the reuse of rinsing water. 
         [0041]    As might be expected, the reuse of the water, cycling from one rinse tank to another, reduces the amount of water needed. Further, concentrated chemicals require large amounts of water to rinse the chemicals off the wafers to reach the desired low concentration levels, as the carry-over of chemical into the rinse water can be significant. Due to carry-over, the first stage of rinsing required that chemical to be neutralized and diluted to prevent further process, and then more water is used to render the wafer surface free of chemical. Less water is needed to rinse the concentrated chemicals; dilute baths are used that act as the rinsing process. 
         [0042]    Furthermore, apparatus associated with the claimed method use less energy, exhaust, and materials, and have simpler facilitation requirements. The use of fewer baths and processing steps equates to a smaller system footprint and lower processing costs. Thus, an environmentally greener process is obtained when compared with the concentrated chemical process. 
         [0043]    The invention may have applicability in other processes where multiple baths are needed, ultrahigh purity of water is not an issue and there are not high tolerances for surface cleanliness requirements of particles, metallic, or organic contamination. Such applications might include (i) silicon ingot, raw silicon, and poly silicon manufacturing; (ii) medical device cleaning; (iii) manufactured or milled parts cleaning; and (iv) panel cleaning, for LCD screens and solar panels or other similar parts. Therefore, while the present invention has been shown and described with reference to the foregoing preferred embodiments, it will be apparent to those skilled in the art that changes in form, connection, and detail may be made therein without departing from the spirit and scope of the invention as defined in the appended claims.

Technology Category: 4