Patent Publication Number: US-8992692-B2

Title: Adjustable brush cleaning apparatus for semiconductor wafers and associated methods

Description:
FIELD OF THE INVENTION 
     The present invention relates to the field of semiconductor wafers, and more particularly, to an apparatus and method for brush cleaning semiconductor wafers. 
     BACKGROUND OF THE INVENTION 
     Chemical-mechanical polishing (CMP) is performed in the processing of semiconductor wafers. A standard CMP apparatus has a circular polishing pad and a rotating carrier for holding a semiconductor wafer. An abrasive slurry is used on the polishing pad. After a CMP operation, residual particles are left on the surface of the semiconductor wafer. These residual particles need to be removed. 
     Semiconductor wafers are typically cleaned in a cleaning apparatus which includes one or more brush stations each having a pair of rotary brushes for cleaning the major surfaces of the wafers. A conventional cleaning apparatus  10  is illustrated in  FIG. 1 . The cleaning apparatus  10  includes a pair of brushes  12  that clean the major surfaces of a semiconductor wafer  14  placed therebetween. The semiconductor wafer  14  is supported by rollers  16 , which are also used to rotate the wafer. The cleaning apparatus  10  includes at least one spray bar  18  to direct a spray of fluid towards the semiconductor wafer  14 . 
     As semiconductor processes continue to achieve smaller line widths to create semiconductor wafers with greater capacity, post CMP defects control becomes more important for improving wafer yield and reliability. A key factor in brush cleaning is to precisely control the distance between the brush and the semiconductor wafer. If the separation distance is too tight, the residual particles from the CMP operation will scratch  20  the semiconductor wafer  14  as they are brushed off, as illustrated in a highlighted section  21  of the wafer as provided in  FIG. 2 . If the separation distance is to loose, some of the residual particles will not be removed. For example, a residual particle  22  may cause a short between two lines  24  and  26 , as illustrated in a highlighted section  27  of the wafer as also provided in  FIG. 2 . Since the brushes  12  directly contact the semiconductor wafer  14  during cleaning, the separation distance is with respect to a reference position that is inward from a contact surface on the wafer. 
     Position of the brushes relative to the semiconductor wafer may be adjusted based on pressure sensors that detect pressure each brush applies to the wafer. For example, U.S. Pat. No. 5,475,889 discloses a brush assembly that includes a first rotary brush, a brush carriage having first and second arms, a second rotary brush, and a pressure adjustment assembly positioned to engage at least one of the arms of the brush carriage. The pressure adjustment assembly is configured to adjust the pressure applied to the wafer surfaces by the first and second rotary brushes. The brush assembly further includes a control system coupled to the pressure adjustment assembly for controlling operation of the pressure adjustment assembly to selectively increase and decrease the pressure applied to the wafer by the first and second rotary brushes. 
     As an alternative to monitoring pressure, the torque of a brush rotation motor may be monitored. U.S. Pat. No. 7,507,296 discloses monitoring the torque of a brush rotation motor while a brush is in contact with the semiconductor wafer and is being rotated by the motor. The position of the brush relative to the semiconductor wafer may be adjusted based on the monitored torque to regulate the pressure applied to the wafer by the brush. 
     Further developments on monitoring position of the brushes relative to the semiconductor wafer are still desired. This is particularly so as semiconductor processes continue to achieve smaller line widths to create semiconductor wafers with greater capacity. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing background, it is therefore an object of the present invention to improve how brushes for a cleaning apparatus are positioned relative to the semiconductor wafer. 
     This and other objects, features, and advantages in accordance with the present invention are provided by a cleaning apparatus for cleaning a semiconductor wafer comprising at least one rotary brush configured to be positioned to clean the semiconductor wafer, and at least one optical sensing device associated with the at least one rotary brush to sense a distance between a reference position thereon and the semiconductor wafer. At least one actuator may be coupled to the at least one optical sensing device and be configured to position the at least one rotary brush based upon the distance between the reference position on the at least one rotary brush and the semiconductor wafer. 
     By precisely controlling the separation distance between the reference position and the semiconductor wafer, the rotary brushes may be positioned so that wafer yield and efficiency may advantageously be improved. 
     The at least one optical sensing device may be carried by the at least one rotary brush. The at least one rotary brush may comprise at least one optical window, with the at least one optical sensing device being aligned with the at least one optical window. Alternatively, the at least one optical sensing device may be external from the at least one rotary brush. 
     The cleaning apparatus may further comprise a controller configured to determine a difference between the sensed separation distance and a desired separation distance, and to operate the at least one actuator based on the determined difference. 
     The at least one optical sensing device may comprise a transmitter for transmitting an optical signal to a surface of the semiconductor wafer, and a receiver for receiving a reflected optical signal from the surface of the semiconductor wafer. The optical signal and the reflected optical signal may be used to sense the separation distance. 
     In another embodiment, the at least one rotary brush may comprise first and second rotary brushes on opposites sides of the semiconductor wafer, and the at least one optical sensing device may comprise first and second optical sensing devices. The first optical sensing device may be associated with the first rotary brush, and may comprise a transmitter for transmitting an optical signal through the semiconductor wafer. The second optical sensing device may be associated with the second rotary brush, and may comprise a receiver for receiving the optical signal, with the received optical signal being used to sense the separation distance. The transmitted optical signal comprises a desired separation distance so that first and second actuators position the first and second rotary brushes to the desired separation distance. 
     Another aspect is directed to a method for cleaning a semiconductor wafer with a cleaning apparatus as defined above. The method may comprise positioning the at least one rotary brush to clean the semiconductor wafer, operating the at least one optical sensing device associated with the at least one rotary brush to sense a separation distance between a reference position thereon and the semiconductor wafer, and operating the at least one actuator to position the at least one rotary brush based upon the sensed separation distance. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a brush cleaning apparatus in accordance with the prior art. 
         FIG. 2  is a semiconductor wafer with highlighted sections illustrating a scratch and a residual particle not removed during a cleaning operation in accordance with the prior art. 
         FIG. 3  is a schematic diagram of post CMP cleaning stations in accordance with the present invention. 
         FIG. 4  is a schematic diagram of a brush cleaning apparatus with optical sensing devices inside the rotary brushes in accordance with the present invention. 
         FIG. 5  is a perspective view of a rotary brush from the brush cleaning apparatus shown in  FIG. 4  with a single optical window integrated therein. 
         FIG. 6  is a perspective view of a rotary brush from the brush cleaning apparatus shown in  FIG. 4  with multiple optical windows integrated therein. 
         FIG. 7  is a schematic diagram of the brush cleaning apparatus shown in  FIG. 4  with the optical sensing devices outside the rotary brushes. 
         FIG. 8  is a schematic diagram of a different embodiment of a brush cleaning apparatus with coordinated optical sensing devices inside the rotary brushes in accordance with the present invention. 
         FIG. 9  is a schematic diagram of the brush cleaning apparatus shown in  FIG. 8  with the coordinated optical sensing devices outside the rotary brushes. 
         FIG. 10  is a flowchart illustrating a method for cleaning a semiconductor wafer in accordance with the present invention. 
         FIG. 11  is a cross-sectional side view of a drying apparatus with an exhaust control cap in accordance with the present invention. 
         FIGS. 12 ,  13  and  14  are cross-sectional side views of different embodiments of an exhaust control cap in accordance with the present invention. 
         FIG. 15  is a cross-sectional side view of the exhaust control cap illustrated in  FIG. 11  with cap loading and unloading slot doors in the open position. 
         FIG. 16  is a cross-sectional side view of the exhaust control cap illustrated in  FIG. 11  with cap loading and unloading slot doors in the closed position. 
         FIG. 17  is a flowchart illustrating a method for making an exhaust control cap in accordance with the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout, and prime and multiple prime notations are used to indicate similar elements in alternative embodiments. 
     An overview of post chemical-mechanical polishing (CMP) cleaning stations used to clean semiconductor wafers  12  after CMP will initially be discussed in reference to  FIG. 3 . Each clean station represents one or more steps in the post CMP wafer buffing and cleaning process. For maximum throughput, at least one semiconductor wafer  12  is simultaneously processed by each clean station. Control and synchronization of the clean stations are provided via a connected interface  28  coupled to a control system  29 . 
     As readily appreciated by those skilled in the art, an acidic (low pH) cleaning process is used. The cleaning process may include a hydrofluoric (HF) or standard clean 1 (SC2) cleaning process. To withstand the corrosive effects of the acid, plastic components in the different stations may be used. The plastic may include materials such as PET, aceta (DELRIN), teflon, polypropylene (polypro), polyuerethane. Metal components may also be used, such as stainless steel. 
     Prior to cleaning, the semiconductor wafers  12  are stored in a wet load station  30 . The first clean station is a megasonic tank  32 , which uses acoustic waves to initially clean the semiconductor wafer  12 . Similar to ultrasonic cleaning, megasonics utilizes a transducer, usually composed of piezoelectric crystals to create megasonic energy. Megasonic energy is of a higher frequency (800-2000 kHz) than typical ultrasonic cleaners (&lt;100 kHz). As a result, the cavitation that occurs is gentler and on a much smaller scale. 
     The second and third clean stations are brush cleaning apparatuses  34 . After the initial cleaning of the semiconductor wafer  12  in the megasonic tank  32 , the wafer is subjected to back-to-back brush cleaning. Each brush cleaning apparatus  32  may use dionized (DI) water as part of the cleaning process. In some applications, hydrofluoric (HF) acid may be used as an alternative. Each semiconductor wafer  12  is brushed and sprayed to remove any residual particles thereon. Both sides of a semiconductor wafer  12  are brushed simultaneously. A semiconductor wafer  12  is cleaned by applying equal amounts of pressure to both ends of the brush assembly for a predetermined period of time. Alternatively, the brush cleaning apparatuses  34  may be configured to brush one side at a time. 
     After each semiconductor wafer  12  has been brushed twice, the next clean station is a drying apparatus  36 . The drying apparatus  36  is also known as a spin-rinse/dryer  36  since it rinses, spins, and dries a semiconductor wafer  12 . A spun and dried wafer  12  is then moved to an unload station  38 . A semiconductor wafer  12  in the unload station  38  represents a clean wafer. 
     A brush cleaning apparatus  34  from the second and third stations will now be discussed in greater detail in reference to  FIGS. 4-6 . The brush cleaning apparatus  34  is configured the same for each station, and includes a pair of rotary brushes  40  each configured to be positioned to clean a semiconductor wafer  12 . An optical sensing device  42  is associated with each rotary brush  40  to sense a separation distance between a reference position  44  thereon and the semiconductor wafer  12 . An actuator  47  is coupled to each optical sensing device  42  and is configured to position the rotary brush  40  based upon the separation distance between the reference position  44  on the rotary brush  40  and the semiconductor wafer  12 . 
     By precisely controlling the separation distance between the rotary brushes  40  and the semiconductor wafer  12  with respect to the reference position  44 , wafer yield and efficiency are advantageously improved. As noted above, if the separation distance is too tight, residual particles from a CMP operation may scratch the semiconductor wafer  12  as they are brushed off. If the separation distance is too loose, some of the residual particles will not be removed. Since the semiconductor wafer  12  is a double sided wafer, the same components are installed on the other side of the wafer. 
     Determination and control of the separation distance between the rotary brush  40  and the semiconductor wafer  12  with respect to a reference position is performed before brushing begins. This reduces the chance of having distorted measurements that may be caused by wetting solutions being applied to surfaces of the semiconductor wafer  12  during cleaning. Nonetheless, determination and control of the separation distance may be made in situ (i.e., during cleaning) provided the measurements are not distorted. 
     In the illustrated embodiment, the optical sensing device  42  is positioned inside or within the rotary brush  40 . Consequently, each rotary brush  40  includes an optical window  60  formed therein, as illustrated in  FIG. 5 . The optical sensing device  42  includes an optical transmitter  46  and an optical receiver  48 . The optical transmitter  46  transmits an optical signal  50  through the optical window  60  to the semiconductor wafer  12 . This results in a reflected optical signal  52  being directed back through the optical window  60  to the optical receiver  48 . 
     A controller  70  is coupled between the optical sensing device  42  and the actuator  47 . The controller  70  uses the optical signals  50 ,  52  to determine the separation distance between the reference position  44  on the rotary brush  40  and semiconductor wafer  12 , as readily appreciated by those skilled in the art. 
     The actuator  47  is coupled to an arm  72  that is coupled to at least one side of the rotary brush  40 . Although not illustrated, the other side of the rotary brush  40  may be coupled to a motor that rotates the brush. The illustrated embodiment is not limited to the support mechanisms (e.g., arm  72 ) used to support a rotary brush  40 . Other support mechanism may be used. In addition, the rotary brushes  40  may be enclosed within a chamber, although not illustrated. 
     The actuator  47  is under the control of the controller  70 . In the illustrated embodiment, the controller  70  is separate from the optical sensing device  42 . Alternatively, the controller  70  may be positioned within the optical sensing device  42 . 
     The controller  70  includes a processor  74  to determine the separation distance between the rotary brush  40  and the semiconductor wafer  12  with respect to the reference position  44 . This determination is made based on the optical signal  50  and the corresponding reflected optical signal  52 , as readily appreciated by those skilled in the art. Even though the reference position  44  is illustrated with respect to a front face of the optical sensing device  42 , the reference position may be picked to be a different location. The processor  74  also determines a difference between the sensed separation distance and a desired separation distance, and operates the actuator  47  based on the determined difference. 
     To uniformly measure the separation distance between the rotary brush  40  and the semiconductor wafer  12  with respect to the reference position  44 , multiple optical sensing devices  42  may be positioned within the rotary brush  40 . The rotary brush  40  would then have an optical window  60  for each optical sensing device  42  positioned therein. For example, if the rotary brush  40  included three optical sensing devices  42  positioned therein, then there would be three optical windows  60 , as illustrated in  FIG. 6 . 
     As readily appreciated by those skilled in the art, each rotary brush  40  may be porous and/or sponge like and may include a resilient material, such as polyvinyl acetate (PVA). The rotary brushes  40  may also include other materials, or may include different materials. The type of brushes  40  have an effect on the desired separation distance between the reference position  44  and the surface of the semiconductor wafer  12  even though the brushes contact the surfaces of the semiconductor wafer  12  being cleaned. 
     Each optical window  60  may be formed as a separate component with respect to a corresponding optical sensing device  42  associated therewith, as illustrated in  FIG. 4 . In this configuration, the optical window  60  is held in place by the rotary brush  40 . That is, the optical window  60  is integrated into the rotary brush  40 . The optical sensing device  42  is positioned within the rotary brush  40  separately from the optical window  60 . Alternatively, each optical window  60  may be formed to directly contact or connect with a corresponding optical sensing device  42 . In this configuration, the optical sensing device  42  and optical window  60  are positioned within the rotary brush  40  at the same time. 
     Each optical window  60  may be formed out of plastic or glass, for example. Other materials that are transparent to the optical signals  50 ,  52  are readily acceptable also. 
     Referring now to  FIG. 7 , another embodiment of the brush cleaning apparatus  34 ′ is based on the optical sensing devices  42 ′ being outside or external from the rotary brushes  40 ′. For example, one or more optical sensing devices  42 ′ are carried by an arm  72 ′ used to support at least one side of the rotary brush  40 ′. In this embodiment, the rotary brushes  40 ′ no longer require an optical window. 
     Yet another embodiment of the brush cleaning apparatus  34 ″ will now be discussed in reference to  FIG. 8 . In this embodiment, the optical sensing devices  42 ″ between the rotary brushes  40 ″ coordinate with one another through the semiconductor wafer  12 ″. For example, a first optical sensing device  42 ( a )″ includes a transmitter  46 ″ for transmitting an optical signal  50 ″ that is transparent to the semiconductor wafer  12 ″. On the opposite side of the semiconductor wafer  12 ″, a second optical sensing device  42 ( b )″ includes a receiver  48 ″ for receiving the optical signal  50 ″ transmitted by the transmitter  46 ″ and through the semiconductor wafer  12 ″. 
     The received optical signal  50 ″ is used to sense the separation distance with respect to the reference position  44 ″. The frequency of the optical signal  50 ″ may be in the infrared range, such as 155 nm, for example. Other frequencies allowing the transmitter  46 ″ and receiver  48 ″ to communicate through the semiconductor wafer  12 ″ are also acceptable, as readily appreciated by those skilled in the art. 
     A first actuator  47 ( a )″ is coupled to the first optical sensing device  42 ( a )″ and is configured to position the first rotary brush  40 ( a )″ based upon the sensed separation distance. A second actuator  47 ( b )″ is coupled to the second optical sensing device  42 ( b )″ and is configured to position the second rotary brush  40 ( b )″ based upon the sensed separation distance. The transmitted optical signal  50 ″ includes a desired separation distance so that the first and second actuators  47 ( a )″,  47 ( b )″ both position the first and second rotary brushes  40 ( a )″,  40 ( b )″ to the desired separation distance. 
     Determination of the desired separation distance is determined ahead of time, and corresponds to the types of semiconductor wafers  12 ″ being cleaned, as well as the types of brushes  40 ″ being used to clean the wafers. The determination may be based on trial and error, for example. Based on a selected separation distance, the semiconductor wafer  12 ″ is cleaned. Afterwards, the semiconductor wafer  12 ″ is tested to determine wafer yield. 
     This process is repeated for other different separation distances, with each of the corresponding semiconductor wafers  12 ″ being tested afterwards. After trial and error, the separation distance allowing for the best wafer yield is selected. 
     When subsequent semiconductor wafers  12 ″ are to be brushed, the first and second optical sensing devices  42 ( a )″,  42 ( b )″ sense a separation distance of the first and second rotary brushes  40 ( a )″,  40 ( b )″ with respect to the next wafer to be cleaned. The information content of the optical signal  50 ″ to be transmitted between the two optical sensing devices  42 ( a )″,  42 ( b )″ corresponds to the selected separation distance allowing for the best wafer yield. One or both controllers  70 ( a )″,  70 ( b )″ receive this information from the first and second optical sensing devices  42 ( a )″,  42 ( b )″ so as to control positioning of the first and second rotary brushes  40 ( a )″,  40 ( b )″. This process is repeated for the next semiconductor wafer  12 ″. 
     Referring now to  FIG. 9 , another embodiment of the brush cleaning apparatus  34 ′″ is based on the optical sensing devices  42 ( a )′″ and  42 ( b )′″ being external the rotary brushes  40 ′″. For example, the optical sensing devices  42 ( a )′″ and  42 ( b )′″ are carried by the arms  72 ′″ used to support at least one side of the rotary brushes  40 ″. In this embodiment, the rotary brushes  40 ′″ no longer require an optical window. 
     A flowchart  100  for illustrating a method for cleaning a semiconductor wafer  12  with an apparatus  34  as described above will now be discussed. From the start (Block  102 ), the method comprises positioning the at least one rotary brush  40  to clean the semiconductor wafer  12  at Block  104 . The at least one optical sensing device  42  associated with the at least one rotary brush is operated at Block  106  to sense a separation distance between a reference position  44  thereon and the semiconductor wafer  12 . The at least one actuator  47  is operated at Block  108  to position the at least one rotary brush  40  based upon the sensed separation distance. The method ends at Block  110 . 
     Referring now to  FIG. 11 , the drying apparatus  36  for drying a semiconductor wafer  12  will now be discussed in greater detail. After each semiconductor wafer  12  has been brushed twice, the next cleaning station is the drying apparatus  36 . The drying apparatus  36  is also known as a spin-rinse/dryer. 
     The drying apparatus  36  includes a processing chamber  122  that comprises a rinsing section  124  and a drying section  126  adjacent thereto. The rinsing section  124  has a chamber loading slot  134  associated therewith for receiving the semiconductor wafer  12 . The drying section  126  has a chamber unloading slot  136  associated therewith for outputting the semiconductor wafer  12 . 
     An exhaust control cap  140  is carried by the processing chamber  122  and includes a bottom wall  142 , a top wall  144 , and at least one intermediate wall  146  between the bottom and top walls. In the illustrative embodiment, there is an additional intermediate wall  148 . A side wall  150  is coupled to the bottom wall  142 , the intermediate walls  146 ,  148  and the top wall  144  to define a plurality of stacked or parallel exhaust sections  180 ,  182 ,  184 . 
     The exhaust control cap  140  has a cap loading slot  164  aligned with the chamber loading slot  134 , a cap unloading slot  166  aligned with the chamber unloading slot  136 , and at least one exhaust port  160  configured to be coupled to a vacuum source  170 . Even though cap loading slot  164  and the cap unloading slot  166  are illustrated as being spaced apart and separate from one another, in an alternate embodiment they may be formed as a single slot that is shared for loading and unloading the semiconductor wafers  12 . 
     Alignment of the cap loading slot  164  with the chamber loading slot  134  may be at an angle, as illustrated in  FIG. 11 . The angle is in a non-vertical direction with respect to an upper surface of the processing chamber  122 . The angle corresponds to a direction of travel of the semiconductor wafer  12  as it enters the rinsing section  124  of the processing chamber  122 . The angle will vary between different embodiments of a drying apparatus  36  to support the actual direction of travel of the wafer  12 . Similarly, alignment of the cap unloading slot  166  with the chamber unloading slot  136  may also be in a non-vertical direction with respect to an upper surface of the processing chamber  122 , as illustrated in  FIG. 11 . The angle corresponds to a direction of travel of the semiconductor wafer  12  as it leaves the drying section  126  of the processing chamber  122 . The angle will vary between different embodiments of the drying apparatus  36 . For example, the angles may both be in a vertical direction with respect to the processing chamber  122 , for example. 
     In the illustrative embodiment, the exhaust control cap  36  comprises a plurality of stacked exhaust sections  180 ,  182 ,  184 . The exhaust control cap  140  is not limited to any particular number of exhaust sections. In addition, the exhaust control cap  140  is not limited to any particular configuration. The different configurations of the exhaust sections will now be discussed. 
     Exhaust section  180  is configured as a hollow section for fast exhaust control. The hollow section is free of any obstruction. 
     Exhaust section  182  is configured as a baffled section to prevent an exhaust backstream. The illustrated baffles  183  are staggered. However, other configurations for providing a baffled section are readily acceptable. 
     Exhaust section  184  is configured as a chemical species trap section for trapping and absorbing exhaust molecules. The illustrated chemical species trap  185  is configured as a plurality of spaced apart circular tabs. Adhesive may be used to secure the trap  185  to the exhaust section  184 . However, other configurations for providing the chemical species trap  185  are readily acceptable. For example, the chemical species trap  185  may be configured as a rectangular strip. 
     In the illustrated embodiment, each exhaust section  180 ,  182 ,  184  has at least one exhaust port  160  associated therewith. The exhaust control cap  140  advantageously contains and exhausts vapor from the processing chamber  122  of the drying apparatus  36 , and in particular, in the drying section  126 . The stacked exhaust sections also help to reduce exhaust contamination of the semiconductor wafer  12  as it is being removed from the drying apparatus  36 . Also, as the semiconductor wafer  12  is removed, the exhaust control cap  140  helps to prevent DI water or an IPA flow rate spike from causing a water mark or IPA to condense on the semiconductor wafer. This helps to improve wafer yield and reliability being decreased. 
     As noted above, the exhaust control cap  140  is not limited to any particular number of exhaust sections, or any particular configuration or order of the exhaust sections. For example, for the exhaust control cap  140 ′ illustrated in  FIG. 12  includes an additional hollow exhaust section  180 ′. For the exhaust control cap  140 ″ illustrated in  FIG. 13 , only the hollow exhaust section  180 ″ and the baffled exhaust section  182 ″ are included. For the exhaust control cap  140 ″ illustrated in  FIG. 14 , only the hollow exhaust section  180 ′″ and the chemical species trap section  184 ′″ are included. 
     Referring now to  FIGS. 15 and 16 , the exhaust control cap  140  may further comprise cap loading slot doors  190  and cap unloading slot doors  194  that are slidabely positioned to provide further exhaust control. The cap loading slot doors  190  are slideably positioned over the cap loading slot  134 . Similarly, the cap unloading slot doors  194  are slideably positioned over the cap unloading slot  136 . 
     A door controller  196  operates the cap loading and unloading slot doors  190 ,  194  based on a position of the semiconductor wafer  12 . In  FIG. 15 , the cap loading and unloading slot doors  190 ,  194  are open. In  FIG. 16 , the cap loading and unloading slot doors  190 ,  194  are closed. 
     Even though each exhaust section has its own corresponding cap loading and unloading slot doors  190 ,  194 , this may not always be the case. Selected exhaust sections may have cap loading and unloading slot doors  190 ,  194 . Alternatively, only the upper or outer most exhaust section may have cap loading and unloading slot doors  190 ,  194 . 
     A flowchart  200  for illustrating a method for making an exhaust control cap  140  for a semiconductor wafer drying apparatus  36  as described above will now be discussed. From the start (Block  202 ) the method comprising forming at Block  204  a plurality of stacked exhaust sections  180 ,  182 ,  184  defined by a bottom wall, a top wall, at least one intermediate wall, and a side wall coupled to the bottom wall, the top wall and the at least one intermediate wall. A cap loading slot  164  is formed at Block  206  to be aligned with a chamber loading slot  134  in the drying apparatus  36 . A cap unloading slot  166  is formed at Block  208  to be aligned with a chamber unloading slot  166  in the drying apparatus  36 . The method further comprises forming at least one exhaust port  160  for the plurality of stacked exhaust sections  180 ,  182 ,  184  to be coupled to a vacuum source  170 . The method ends at Block  212 . 
     Many modifications and other embodiments of the invention will come to the mind of one skilled in the art having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is understood that the invention is not to be limited to the specific embodiments disclosed, and that modifications and embodiments are intended to be included within the scope of the appended claims.