Abstract:
A system, apparatus and method for processing flat articles with acoustical energy. The inventive system, apparatus and method can remove particles from both sides of a wafer more efficiently and effectively. In one aspect, the invention is a system and/or method for processing flat articles wherein a liquid is applied to both major surfaces of the flat article. A first transducer assembly is positioned adjacent to a first of the major surfaces of the flat article and a second member is positioned adjacent to a second of the major surfaces. The first transducer assembly generates and transmits acoustical energy to the first major surface of the flat article while the second member either: (1) reflects the acoustical energy generated by the first transducer assembly back to the second major surface of the flat article; and/or (2) generates and transmits acoustical energy to the second major surface of the flat article.

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATIONS 
       [0001]    The present patent application is a continuation of U.S. patent application Ser. No. 11/625,556, filed Jan. 22, 2007, which in turn claims the benefit of U.S. Provisional Patent Application Ser. No. 60/760,820, filed Jan. 20, 2006; U.S. Provisional Patent Application Ser. No. 60/837,965, filed Aug. 16, 2006; and U.S. Provisional Patent Application Ser. No. 60/850,930, filed Oct. 11, 2006, the entireties of which are hereby incorporated by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates generally to the field of processing flat articles utilizing acoustic energy, and specifically to systems, methods and apparatus that utilize acoustic energy for cleaning flat articles, such as semiconductor wafers. 
       BACKGROUND OF THE INVENTION 
       [0003]    In the field of semiconductor manufacturing, it has been recognized since the beginning of the industry that removing particles from semiconductor wafers during the manufacturing process is a critical requirement to producing quality profitable wafers. While many different systems and methods have been developed over the years to remove particles from semiconductor wafers, many of these systems and methods are undesirable because they damage the wafers. Thus, the removal of particles from wafers, which is often measured in terms of the particle removal efficiency (“PRE”), must be balanced against the amount of damage caused to the wafers by the cleaning method and/or system. It is therefore desirable for a cleaning method or system to be able to break particles free from the delicate semiconductor wafer without resulting in damage to the devices on the wafer surface. 
         [0004]    Existing techniques for freeing the particles from the surface of a semiconductor wafer utilize a combination of chemical and mechanical processes. One typical cleaning chemistry used in the art is standard clean 1 (“SC1”), which is a mixture of ammonium hydroxide, hydrogen peroxide, and water. SC1 oxidizes and etches the surface of the wafer. This etching process, known as undercutting, reduces the physical contact area of the wafer surface to which the particle is bound, thus facilitating ease of removal. However, a mechanical process is still required to actually remove the particle from the wafer surface. 
         [0005]    For larger particles and for larger devices, scrubbers have historically been used to physically brush the particle off the surface of the wafer. However, as devices shrank in size, scrubbers and other forms of physical cleaning became inadequate because their physical contact with the wafers began to cause catastrophic damage to the smaller/miniaturized devices. 
         [0006]    Recently, the application of sonic/acoustical energy to the wafers during chemical processing has replaced physical scrubbing to effectuate particle removal. The terms “acoustical” and “sonic” are used interchangeably throughout this application. The acoustical energy used in substrate processing is generated via a source of acoustical energy, which typically comprises a transducer which is made of piezoelectric crystal. In operation, the transducer is coupled to a power source (i.e. a source of electrical energy). An electrical energy signal (i.e. electricity) is supplied to the transducer. The transducer converts this electrical energy signal into vibrational mechanical energy (i.e. sonic/acoustical energy) which is then transmitted to the substrate(s) being processed. Characteristics of the electrical energy signal, which is typically in a sinusoidal waveform, supplied to the transducer from the power source dictate the characteristics of the acoustical energy generated by the transducer. For example, increasing the frequency and/or power of the electrical energy signal will increase the frequency and/or power of the acoustical energy being generated by the transducer. 
         [0007]    Over time, wafer cleaning utilizing acoustical energy became the most effective method of particle removal in semiconductor wet process applications. Acoustical energy has proven to be an effective way to remove particles, but as with any mechanical process, damage is possible and acoustical cleaning is faced with the same damage issues as traditional physical cleaning methods and apparatus. In the past, cleaning systems utilizing acoustical energy were designed to process semiconductor wafers in batches, typically cleaning twenty-five substrates at once. The benefits of batch cleaning became less important as the size of substrates and the effectiveness of single-wafer cleaning systems increased. The greater value per semiconductor wafer and the more delicate nature of the devices resulted in a transition in the industry toward single-wafer processing equipment. 
         [0008]    An example of a single-wafer cleaning system that utilizes megasonic energy is disclosed in U.S. Pat. No. 6,039,059 (“Bran”), issued Mar. 21, 2000, and U.S. Pat. No. 7,100,304 (“Lauerhaas et al.”), issued Sep. 5, 2006, the entireties of which are hereby incorporated by reference herein. The single-wafer cleaning system that is the subject of U.S. Pat. No. 6,039,059 and U.S. Pat. No. 7,100,304 is commercialized by Akrion, Inc. of Allentown, Pa. under the name GOLDFINGER®. Other examples of single-wafer cleaners that utilize acoustic energy are disclosed in U.S. Pat. No. 7,145,286 (“Beck et al.”), issued Dec. 5, 2006, U.S. Pat. No. 6,539,952 (“Itzkowitz”), issued Apr. 1, 2003, and United States Patent Application Publication 2006/0278253 (“Verhaverbeke et al.”), published Dec. 14, 2006. In single-wafer acoustic cleaning systems, such as the ones mentioned above, a semiconductor wafer is supported and rotated in horizontal orientation while a film of liquid is applied to one or both sides/surfaces of the wafer. A transducer assembly is positioned adjacent to one or the surfaces of the wafer so that a transmitter portion of the transducer assembly is in contact with the film of liquid by a meniscus of the liquid. The transducer assembly is activated during the rotation of the wafer, thereby subjecting the wafer to the acoustic energy generated by the transducer assembly. 
         [0009]    Nonetheless, the industry&#39;s transition to the below 100 nm devices has resulted in additional challenges for manufacturers of semiconductor processing equipment. The cleaning process is no different. As a result of the devices becoming more and more miniaturized, cleanliness requirements have also become increasingly important and stringent. When dealing with reduced size devices, the ratio of the size of a contaminant compared to the size of a device is greater, resulting in an increased likelihood that a contaminated device will not function properly. Thus, increasingly stringent cleanliness and PRE requirements are needed. As a result, improved semiconductor wafer processing techniques that reduce the amount and size of the contaminants present during wafer production are highly desired. 
         [0010]    As a result of these increasingly stringent cleanliness and PRE requirements, the removal of particles from both sides/surfaces of the wafer have been discovered by the present inventors to be playing an increasingly important role in achieving high yields. In existing single-wafer systems, removal of particles from both surfaces of the semiconductor wafer during a cleaning cycle are achieved by providing a single transducer assembly adjacent to one of the surfaces of the wafer. This transducer assembly is operated at a sufficient power level so that the generated acoustic energy passes through the wafer itself to loosen particles on the opposite surface of the wafer. This basic concept is one of the subject inventions of U.S. Pat. No. 6,039,059. This dual-sided cleaning concept is also shown as being utilized and copied in the system disclosed in United States Patent Application Publication 2006/0278253 (“Verhaverbeke et al.”) with the transducer assembly located adjacent the backside of the wafer. 
         [0011]    Despite these advancements in single-wafer systems and methods for cleaning both sides of the wafer, there still remains a need for single-wafer systems that can achieve improved PRE with minimized device damage. Furthermore, the continued miniaturization of devices continues to render existing cleaning systems incapable of achieving an acceptable balance between high PRE and minimized device damage. 
       SUMMARY OF THE INVENTION 
       [0012]    It is therefore an object of the present invention to provide a system, apparatus and method for processing flat articles, such as semiconductor wafers, with acoustical energy. 
         [0013]    Another object of the present invention is to provide a system, apparatus and method for simultaneously cleaning both surfaces of flat articles, such as semiconductor wafers, with acoustical energy. 
         [0014]    Still another object of the present invention is to provide a system, apparatus and method for simultaneously cleaning both surfaces of flat articles, such as semiconductor wafers, with acoustical energy that improves PRE and/or reduces damage to the flat article. 
         [0015]    Yet another object of the present invention is to provide a system, apparatus and method for applying acoustical energy to the bottom surface of a rotating flat article. 
         [0016]    A further object of the present invention is to provide a system, apparatus and method for simultaneously cleaning both surfaces of flat articles, such as semiconductor wafers, that utilize acoustic energy reflection. 
         [0017]    A still further object of the present invention is to provide an apparatus and method that allows existing single-wafer cleaners to be retrofitted to achieve improved cleaning of both surfaces of the wafer. 
         [0018]    A yet further object of the present invention is to provide a system, apparatus and method that achieves increased liquid coupling between a transducer assembly and the bottom surface of a flat article. 
         [0019]    Yet another object of the present invention is to provide a system, apparatus and method that increases the backside particle removal efficiency in a single-wafer cleaning system without increasing damage to devices located on the topside of the wafer. 
         [0020]    Still another object of the present invention is to provide a system, apparatus and method for applying megasonic energy to the backside of a flat article. 
         [0021]    These and other objects are met by the present invention, which in one embodiment of the invention can be a system for processing flat articles comprising: a rotatable support for supporting a flat article; a first dispenser for applying liquid to a first surface of a flat article on the rotatable support; a second dispenser for applying liquid to a second surface of a flat article on the rotatable support; a first transducer assembly comprising a first transducer for generating acoustic energy and a first transmitter acoustically coupled to the first transducer, the first transducer assembly positioned so that when the first dispenser applies liquid to the first surface of a flat article on the rotatable support, a first meniscus of liquid is formed between a portion of the first transmitter and the first surface of the flat article; and a second transducer assembly comprising a second transducer for generating acoustic energy and a second transmitter acoustically coupled to the second transducer, the second transducer assembly positioned so that when the second dispenser applies liquid to the second surface of the flat article on the rotatable support, a second meniscus of liquid is formed between a portion of the second transmitter and the second surface of the flat article. 
         [0022]    In another embodiment, the invention can be a system for cleaning flat articles comprising: a rotatable support for supporting a flat article; a first transducer assembly comprising a first transducer and a first transmitter acoustically coupled to the first transducer, the first transducer assembly positioned so that a first small gap exists between a portion of the first transmitter and a first surface of a flat article on the rotatable support, a first meniscus of liquid being formed between the portion of the first transmitter and the first surface of the flat article when liquid is applied to the first surface; and a second transducer assembly comprising a second transducer and a second transmitter acoustically coupled to the second transducer, the second transducer assembly positioned so that a second small gap exists between a portion of the second transmitter and a second surface of the flat article on the rotatable support, a second meniscus of liquid being formed between the portion of the second transmitter and the second surface when liquid is applied to the second surface. 
         [0023]    In yet another embodiment, the invention can be a system for processing flat articles comprising: a rotatable support for supporting and rotating a flat article in a substantially horizontal orientation; a transducer assembly comprising a transducer for generating acoustic energy, a transmitter acoustically coupled to the first transducer and a dam surrounding at least a portion of a perimeter of the transmitter so as to form a liquid retaining channel between the transmitter and the dam; and the transducer assembly positioned so that apportion of the transmitter is adjacent a bottom surface of a flat article on the rotatable support so that when liquid is applied to the bottom surface of the flat article, a meniscus of liquid is formed between the portion of the transmitter and the bottom surface of the flat article. 
         [0024]    In still another embodiment, the invention can be a transducer assembly for mounting beneath a bottom surface of a flat article comprising: a transducer for generating acoustic energy; a transmitter acoustically coupled to the first transducer; and a dam surrounding at least a portion of a perimeter of the transmitter so as to form a liquid retaining channel between the second transmitter and the dam. 
         [0025]    In a further embodiment, the invention can be a method of manufacturing a transducer assembly comprising: providing a par-cylindrical transmitter plate; bonding one or more transducers to a convex inner surface of the transmitter plate; connecting a housing to the transmitter to create an assembly having a substantially enclosed cavity in which the one or more transducers are located; and encapsulating the assembly with an inert non-reactive plastic. 
         [0026]    In a yet further embodiment, the invention can be a method of processing a flat article comprising: a) supporting a flat article in a substantially horizontal orientation within a gaseous atmosphere, the flat article having a bottom surface and a top surface; b) rotating the flat article while maintaining the substantially horizontal orientation; c) applying a film of liquid to the top surface of the flat article; d) applying a film of liquid on the bottom surface of the flat article; e) applying acoustic energy to the top surface of the flat article via a first transducer assembly comprising a first transducer and a first transmitter, a portion of the first transmitter in contact with the film of liquid on the top surface of the flat article; and f) applying acoustic energy to the bottom surface of the flat article via a second transducer assembly comprising a second transducer and a second transmitter, a portion of the second transmitter in contact with the film of liquid on the bottom surface of the flat article. 
         [0027]    In a still further embodiment, the invention can be a system for processing flat articles comprising: a rotatable support for supporting a flat article in a substantially horizontal orientation; a transducer assembly comprising a transducer for generating acoustic energy and a transmitter acoustically coupled to the transducer, the transducer assembly positioned so that a portion of the transmitter is adjacent a top surface of a flat article on the support so that a first meniscus of liquid is formed between the portion of the transmitter and the top surface when liquid is applied to the top surface; and a reflective member positioned so that a portion of the reflective member is adjacent a bottom surface of a flat article on the support so that a second meniscus of liquid is formed between a portion of the reflective member and the bottom surface when liquid is applied to the bottom surface; and the reflective member positioned so that at least a fraction of the acoustic energy that is generated by the first transducer assembly that passes through the flat article is reflected back toward the bottom surface of the flat article. 
         [0028]    In yet another embodiment, the invention can be a system for processing flat articles comprising: a rotatable support for supporting a flat article in a gaseous atmosphere; a transducer assembly comprising a transducer and a transmitter bonded to the transducer, the transducer assembly positioned so that a first small gap exists between a portion of the transmitter and a first surface of a flat article on the support so that when liquid is applied to the first surface of the flat article, a first meniscus of liquid is formed between the portion of the transmitter and the first surface of the flat article; a reflective member positioned so that a second small gap exists between a portion of the reflective member and a second surface of a flat article on the support so that when liquid is applied to the second surface of the flat article, a second meniscus of liquid is formed between the portion of the reflective member and the second surface of the flat article; and the reflective member positioned so that at least a fraction of the acoustic energy generated by the first transducer assembly that passes through the flat article is reflected back toward the second surface of the flat article by the reflector member. 
         [0029]    In another embodiment, the invention can be a method of processing flat articles comprising: a) supporting a flat article in a substantially horizontal orientation within a gaseous atmosphere, the flat article having a bottom surface and a top surface; b) rotating the flat article while maintaining the substantially horizontal orientation; c) applying a film of liquid to the top surface of the flat article; d) applying a film of liquid on the bottom surface of the flat article; e) applying acoustic energy to the top surface of the flat article via a transducer assembly comprising a transducer and a transmitter, a portion of the transmitter in contact with the film of liquid on the top surface of the flat article; and f) reflecting the acoustic energy generated by the first transducer assembly that passes through the flat article back toward the bottom surface of the flat article via a reflective member that is in contact with the film of liquid on the bottom surface of the flat article. 
         [0030]    These and various other advantages and features of novelty that characterize the invention are pointed out with particularity in the claims annexed hereto and forming a part hereof. However, for a better understanding of the general technology, its advantages, and the objects obtained by its use, reference should be made to the drawings which form a further part hereof, and to the accompanying descriptive matter, in which there is illustrated and described a preferred embodiment of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0031]      FIG. 1  is schematic of an acoustic energy cleaning system according to one embodiment of the present invention. 
           [0032]      FIG. 2  is a perspective view of one structural embodiment of the acoustic energy cleaning system of  FIG. 1 . 
           [0033]      FIG. 3  is a cross-sectional side view of the acoustic energy cleaning system of  FIG. 2 . 
           [0034]      FIG. 4  is a transducer assembly according to one embodiment of the present invention that is utilized in the acoustic energy cleaning system of  FIG. 2  as the bottom-side transducer assembly. 
           [0035]      FIG. 5  is a cross-sectional view of the transducer assembly of  FIG. 4  along cross-section cut A-A of  FIG. 4 . 
           [0036]      FIG. 6  is an exploded view of the transducer assembly of  FIG. 4 . 
           [0037]      FIG. 7  is a schematic of the transducer assembly of  FIG. 4  positioned adjacent a bottom surface of a semiconductor wafer according to an embodiment of the present invention wherein the transducer assembly of  FIG. 4  is shown in cross-section. 
           [0038]      FIG. 8  is a schematic representation of one arrangement of the topside transducer assembly relative to the bottom-side transducer assembly for the acoustic energy cleaning system of  FIG. 2 . 
           [0039]      FIG. 9  is a cross-sectional view of the schematic representation of the transducer assembly arrangement of  FIG. 8  along the cross-section cut B-B of  FIG. 8 . 
           [0040]      FIG. 10  is a schematic representation of an alternative arrangement of the topside transducer assembly relative to the bottom-side transducer assembly for the acoustic energy cleaning system of  FIG. 2 . 
           [0041]      FIG. 11  is a cross-sectional view of the schematic representation of the alternative transducer assembly arrangement of  FIG. 10  along the cross-section cut C-C of  FIG. 8 . 
           [0042]      FIG. 12  is schematic of an acoustic energy cleaning system utilizing a reflective member according to one embodiment of the present invention. 
           [0043]      FIG. 13  is schematic of an acoustic energy cleaning system utilizing a reflective member according to an alternative embodiment of the present invention. 
           [0044]      FIG. 14  shows five alternative embodiments of a transducer assembly that can also act as a reflective member for use in the acoustic energy cleaning system of  FIG. 12 . 
       
    
    
     DESCRIPTION OF THE INVENTION 
       [0045]    Referring first to  FIG. 1 , a schematic of an acoustic energy cleaning system  1000  (hereinafter referred to as “cleaning system  1000 ”) is illustrated according to one embodiment of the present invention. For ease of discussion the inventive system and methods of the drawings will be discussed in relation to the cleaning of semiconductor wafers. However, the invention is not so limited and can be utilized for any desired wet processing of any flat article. 
         [0046]    The cleaning system  1000  generally comprises a top transducer assembly  200 , bottom transducer assembly  300  and a rotatable support  10  for supporting a semiconductor wafer  50  in a substantially horizontal orientation. Preferably, the semiconductor wafer  50  is supported so its top surface  51  is the device side of the wafer  50  while the bottom surface  52  is the non-device side. Of course, the wafer can be supported so that its top surface  51  is the non-device side while the bottom surface  52  is the device side if desired. 
         [0047]    The rotatable support  10  is designed to contact and engage only a perimeter of the substrate  50  in performing its support function. However, the exact details of the structure of the rotatable support  10  are not limiting of the present invention and a wide variety of other support structures can be used, such as chucks, support plates, etc. Additionally, while it is preferred that the support structure support and rotate the semiconductor wafer in a substantially horizontal orientation, in other embodiments of the invention, the system may be configured so that the semiconductor wafer is supported in other orientations, such as vertical or at an angle. In such embodiments, the remaining components of the cleaning system  1000 , including the transducer assemblies  200 ,  300 , can be correspondingly repositioned in the system so as to be capable of performing the desired functions and/or the necessary relative positioning with respect to other components of the system as discussed below. 
         [0048]    The rotary support  10  is operably coupled to a motor  11  to facilitate rotation of the wafer  50  within the horizontal plane of support. The motor  11  is preferably a variable speed motor that can rotate the support  10  at any desired rotational speed ω. The motor  11  is electrically and operably coupled to the controller  12 . The controller  12  controls the operation of the motor  11 , ensuring that the desired rotational speed ω and desired duration of rotation are achieved. 
         [0049]    The cleaning system  1000  further comprises a top dispenser  13  and a bottom dispenser  14 . Both the top dispenser  13  and the bottom dispenser  14  are operably and fluidly coupled to a liquid supply subsystem  16  via liquid supply lines  17 ,  18 . The liquid supply subsystem  16  is in turn fluidly connected to the liquid reservoir  15 . The liquid supply subsystem  16  controls the supply of liquid to both the top dispenser  13  and the bottom dispenser  14 . 
         [0050]    The liquid supply subsystem  16 , which is schematically illustrated as a box for purposes of simplicity, comprises the desired arrangement of all of the necessary pumps, valves, ducts, connectors and sensors for controlling the flow and transmission of the liquid throughout the cleaning system  1000 . The direction of the liquid flow is represented by the arrows on the supply lines  17 ,  18 . Those skilled in the art will recognize that the existence, placement and functioning of the various components of the liquid supply subsystem  16  will vary depending upon the needs of the cleaning system  1000  and the processes desired to be carried out thereon, and can be adjusted accordingly. The components of the liquid supply subsystem  16  are operably connected to and controlled by the controller  12 . 
         [0051]    The liquid reservoir  15  holds the desired liquid to be supplied to the wafer  50  for the processing that is to be carried out. For cleaning system  1000 , the liquid reservoir  15  will hold a cleaning liquid, such as for example deionized water (“DIW”), standard clean 1 (“SC1”), standard clean 2 (“SC2”), ozonated deionized water (“DIO 3 ”), dilute or ultra-dilute chemicals, and/or combinations thereof. As used herein, the term “liquid” includes at least liquids, liquid-liquid mixtures and liquid-gas mixtures. It is also possible for certain other supercritical and/or dense fluids to qualify as liquids in certain situations. 
         [0052]    Furthermore, it is possible to have multiple liquid reservoirs. For example, in some embodiments of the invention, the top dispenser  13  and the bottom dispenser  14  can be operably and fluidly coupled to different liquid reservoirs. This would allow the application of different liquids to the bottom surface  52  and the top surface  51  of the wafer  50  if desired. 
         [0053]    The cleaning system  1000  further comprises a gas supply subsystem  19  that is operably and fluidly coupled to a gas source  20 . The gas supply subsystem  19  is operably and fluidly connected to the top transducer assembly  200  via the gas supply line  21  and to the bottom transducer assembly  300  via the gas supply line  22 . The gas supply subsystem  19 , which is schematically illustrated as a box for purposes of simplicity, comprises the desired arrangement of all of the necessary pumps, valves, ducts, connectors and sensors for controlling the flow and transmission of the gas throughout the cleaning system  1000 . The direction of the gas flow is represented by the arrows on the supply lines  21 ,  22 . Those skilled in the art will recognize that the existence, placement and functioning of the various components of the gas supply subsystem  19  will vary depending upon the needs of the cleaning system  1000  and the processes desired to be carried out thereon, and can be adjusted accordingly. The components of the gas supply subsystem  19  are operably connected to and controlled by the controller  12 . Thus, the transmission of gas from the gas supply subsystem  19  is based upon signals received from the controller  12 . 
         [0054]    As will be described in greater detail below, the gas is supplied to the top and bottom transducer assemblies  200 ,  300  to provide cooling and/or purging to the transducers in the assemblies  200 ,  300  that convert the electrical energy into the acoustic energy. The gas source  20  preferably holds an inert gas, such as nitrogen, helium, carbon dioxide, etc. However, the invention is not limited to the use of any specific gas. Furthermore, as with the liquids, it is possible to have multiple gas sources. For example, in some embodiments of the invention, the top transducer assembly  200  and the bottom transducer assembly  300  can be operably and fluidly coupled to different gas reservoirs. This would allow the application of different gases as desired. 
         [0055]    The cleaning system  1000  further comprises a horizontal actuator  250  that is operably coupled to the top transducer assembly  200  and a vertical actuator  350  that is operably coupled to the bottom transducer assembly  300 . The actuators  250 ,  350  are operably coupled to and controlled by the controller  12 . The actuators  250 ,  350  can be pneumatic actuators, drive-assembly actuators, or any other style desired to effectuate the necessary movement. 
         [0056]    The horizontal actuator  250  can horizontally translate the top transducer assembly  200  between a retracted position and a processing position. When in the retracted position, the top transducer assembly  200  is withdrawn sufficiently away from the rotatable support  10  so that the wafer  50  can be loaded and unloaded without obstruction onto and from the support  10 . When in the processing position, at least a portion of the top transducer assembly  200  is spaced from but sufficiently close to the top surface  51  of the wafer so that when liquid is supplied to the top surface  51  of the wafer  50 , a meniscus of liquid is formed between the top surface  51  or the wafer  50  and that portion of the top transducer assembly  200 . In  FIG. 1 , the top transducer assembly  200  is in the processing position. 
         [0057]    Similarly, the vertical actuator  350  can vertically translate the bottom transducer assembly  300  between a retracted position and a processing position. For the bottom transducer assembly  300 , the retracted position is a lowered position where the wafer  50  can be safely loaded onto the support  50  without contacting the bottom transducer assembly  300  and/or interfering with other processes that may be carried out on the bottom surface  52  of the wafer  50  that require additional space. When the bottom transducer assembly  300  is in its processing position, at least a portion of the bottom transducer assembly  300  is spaced from but sufficiently close to the bottom surface  52  of the wafer  50  so that when liquid is supplied to the bottom surface  52  of the wafer  50 , a meniscus of liquid is formed between the bottom surface  52  of the wafer  50  and that portion of the top transducer assembly  200 . In  FIG. 1 , the bottom transducer assembly  300  is in the processing position. 
         [0058]    While the actuators  250 ,  350  are exemplified in system  1000  as being horizontal and vertical actuators respectively, in other embodiments of the invention, different styles of actuators can be used in the place of each. For example the actuator operably coupled to the bottom transducer assembly  300  can be a horizontal, vertical, angled translation actuator or a pivotable actuator. The same options exist for the actuator operably coupled to the top transducer assembly  200 . 
         [0059]    A position sensor  329  is provided in the cleaning system  1000  so that the position of the bottom transducer assembly  300  can be monitored and controlled effectively. The position sensor  329  measures the distance between the bottom transducer assembly  300  and the bottom surface  52  of the wafer  50  so that the proper distance between the two can be achieved to effectuate the proper processing gap for formation of the liquid meniscus. The position sensor  329  is operably and communicably coupled to the controller  12 . More specifically, the position sensor  329  generates a signal indicative of the measured distance and transmits this signal to the controller  12  for processing. While the sensor  329  is illustrated as being connected to the bottom transducer assembly  300 , it can be mounted almost anywhere in the cleaning system  1000  so long as it can perform its position indicating function. 
         [0060]    The cleaning system  1000  also comprises an electrical energy signal source  23  that is operably coupled to the top transducer assembly  200  and the bottom transducer assembly  300 . The electrical energy signal source  23  creates the electrical signal that is transmitted to the transducers (discussed later) in the top transducer assembly  200  and the bottom transducer assembly  300  for conversion into corresponding acoustic energy. The desired electrical signals can be sent to the top and bottom transducer assemblies  200 ,  300  concurrently, consecutively and/or in an alternating fashion, depending on the process needs. The electrical energy signal source  23  is operably coupled to and controlled by the controller  12 . As a result, the controller  12  will dictate the frequency, power level, and duration of the acoustic energy generated by the top transducer assembly  200  and the bottom transducer assembly  300 . Preferably, the electrical energy signal source  23  is controlled so that the acoustic energy generated by the top transducer assembly  200  and the bottom transducer assembly  300  has a frequency in the megasonic range. 
         [0061]    Depending on system requirements, it may not be desirable to use a single electrical energy signal source to control both the top transducer assembly  200  and the bottom transducer assembly  300 . Thus, in other embodiments of the invention, multiple electrical energy signal sources may be used, one for each transducer assembly. 
         [0062]    The controller  12  may be a processor, which can be a suitable microprocessor based programmable logic controller, personal computer, or the like for process control. The controller  12  preferably includes various input/output ports used to provide connections to the various components of the cleaning system  1000  that need to be controlled and/or communicated with. The electrical and/or communication connections are indicated in dotted line in  FIG. 1 . The controller  12  also preferably comprises sufficient memory to store process recipes and other data, such as thresholds inputted by an operator, processing times, rotational speeds, processing conditions, processing temperatures, flow rates, desired concentrations, sequence operations, and the like. The controller  12  can communicate with the various components of the cleaning system  1000  to automatically adjust process conditions, such as flow rates, rotational speed, movement of the components of the cleaning system  1000 , etc. as necessary. The type of system controller used for any given system will depend on the exact needs of the system in which it is incorporated. 
         [0063]    The top dispenser  13  is positioned and oriented so that when a liquid is flowed therethough, the liquid is applied to the top surface  51  of the substrate  50 . When the substrate  50  is rotating, this liquid forms a layer or film of the liquid across the entirety of the top surface  51  of the substrate  50 . Similarly, the bottom dispenser  14  is positioned and oriented so that when a liquid is flowed therethough, the liquid is applied to the bottom surface  52  of the substrate  50 . When the substrate  50  is rotating, this liquid forms a layer or film of the liquid across the entirety of the bottom surface  52  of the substrate  50 . 
         [0064]    The top transducer assembly  200  is positioned so that a small gap exists between a portion of the top transducer assembly  200  and the top surface of the water  50 . This gap is sufficiently small so that when the liquid is applied to the top surface  51  of the wafer  50 , a meniscus of liquid is formed between the top surface  51  of the wafer  50  and the portion of the top transducer assembly  200 . Similarly, the bottom transducer assembly  300  is positioned so that a small gap exists between a portion of the bottom transducer assembly  300  and the bottom surface  52  of the wafer  50 . This gap is sufficiently small so that when the liquid is applied to the bottom surface  52  of the wafer  50 , a meniscus of liquid is formed between the bottom surface  52  of the wafer  50  and the portion of the bottom transducer assembly  300 . The meniscus is not limited to any specific shape. 
         [0065]    As will be noted, the top and bottom transducer assemblies  200 ,  300  are generically illustrated as boxes. This is done because, in its broadest sense, the invention is not limited to any particular structure, shape and/or assembly arrangement for the transducer assemblies  200 ,  300 . For example, any of the transducer assemblies disclosed in U.S. Pat. No. 6,039,059 (“Bran”), issued Mar. 21, 2000, U.S. Pat. No. 7,145,286 (“Beck et al.”), issued Dec. 5, 2006, U.S. Pat. No. 6,539,952 (“Itzkowitz”), issued Apr. 1, 2003, and United States Patent Application Publication 2006/0278253 (“Verhaverbeke et al.”), published Dec. 14, 2006, can be used as the top and/or bottom transducer assembly  200 ,  300 . Of course, other styles of transducer assemblies can be used, such as those having an elongated transmitter rod supported at an angle to the surface of the wafer. 
         [0066]    Referring now to  FIG. 2 , a preferred structural embodiment of the cleaning system  1000  is illustrated. Like numbers are used in  FIGS. 2-14  to indicate the corresponding structural manifestation of the schematically illustrated components of  FIG. 1 . 
         [0067]    In the cleaning system  1000  of  FIG. 2 , the top transducer assembly  200  comprises an elongate rod-like transmitter  201  that is acoustically coupled to a transducer  203  (visible in  FIG. 3 ) that is located within housing  202 . Many of the details of this style of elongate rod-like transmitter  201  are disclosed in U.S. Pat. No. 6,684,891 (“Bran”), issued Feb. 3, 2004 and United States 6,892,738 (“Bran et al.”), issued May 17, 2005, the entireties of which are hereby incorporated by reference. The top transducer assembly  200  is operably coupled to drive assembly/actuator  250  that can move the rod-like transmitter  201  between a retracted position and a processing position. When the rod-like transmitter  201  is in the retracted position, the rod-like transmitter  201  is located outside of the process bowl  204  so that a wafer  50  can be placed on the rotatable support  10  without obstruction. More specifically, the drive assembly  250  withdraws the rod-like transmitter  201  through an opening in a side wall of the process bowl  204 . When in the processing position, the rod-like transmitter  201  is position directly above the top surface  51  of a wafer  50  on the rotatable support  10 . The rod-like transmitter  201  is in the processing position in  FIG. 2 . 
         [0068]    The bottom transducer assembly  300  is located at the bottom of the process bowl  204 , at a position below the rotatable support  10 . The bottom transducer assembly  300  comprises a dam  301 , a transmitter  302  and a base  303 . The bottom dispenser  14  is in the form of a plurality of sprayers located within the base  303  itself, rather than a single nozzle dispenser. 
         [0069]    Referring now to  FIG. 3 , it can be seen that the rotatable support  10  is located within the process bowl  204 . The rotatable support  10  supports a wafer  50  in a substantially horizontal orientation in the gaseous atmosphere of the process bowl  204 , which surrounds the periphery of the wafer  50 . The rotatable support  10  is operably connected to the motor assembly  11 . The motor assembly rotates the wafer about the central axis. The motor assembly  11  can be a direct drive motor or a bearing with offset belt/pulley drive. 
         [0070]    The rotatable support  10  supports the wafer  50  at an elevation and position between the elongate rod-like transmitter  201  of the top transducer assembly  200  and the transmitter  302  of the bottom transducer assembly  300 . When the wafer  50  is so supported, the transmitter  201  of the top transducer assembly  200  extends in a substantially parallel orientation over the top surface  51  of the wafer  50  in a close spaced relation. Similarly, the transmitter  302  of the bottom transducer assembly  300  extends in a substantially parallel orientation below the bottom surface  52  of the wafer  50  in a close spaced relation. These close spaced relations are such that when liquid is applied to the top and bottom surfaces  51 ,  52  from the dispensers  13 ,  14  respectively, meniscuses of liquid are respectively formed between a portion of the transmitter  201  and the top surface  51  of the wafer  50  and between a portion of the transmitter  302  and the bottom surface  52  of the wafer  50 . 
         [0071]    The bottom transducer assembly  300  is operably connected to the lifter/actuator  350 . The lifter/actuator  350  can be a pneumatic lifter and can also comprise brackets. The lifter  350  can move the bottom transducer  300  assembly between a processing position and a retracted position. In  FIG. 3 , the bottom transducer assembly  300  is in the processing position, which is a raised position in which the transmitter  302  is in the close spaced relation discussed above. When in the retracted position, the bottom transducer assembly  300  is in a lowered position to ensure that the wafer  50  is not damaged during insertion onto the rotatable support  10 . 
         [0072]    The transducers  203 ,  305  of the top and bottom transducer assemblies  200 ,  300  are acoustically coupled to the transmitter  201 ,  302  respectively. This can be done through a direct bonding or an indirect bonding that utilizes intermediary transmission layers. The transducers  230 ,  305  are operably coupled a source of an electrical energy signal. The transducers  203 ,  305  can be a piezoelectric ceramic or crystal, as is well known in the art. 
         [0073]    Referring now to  FIGS. 4-7  concurrently, the bottom transducer assembly  300  is illustrated removed from the cleaning system  1000  so that its details are visible. It should be understood that the bottom transducer assembly  300 , in of itself, is a novel device that can constitute an embodiment of the invention. 
         [0074]    The bottom transducer assembly  300  comprises a base structure  303 , a housing  304 , a transmitter  302 , a transducer  305  and a dam  301 . The base structure  303  is preferably made of PTFE or other non-contaminating material that is suitable rigid. The base structure  303  has a top convex surface that is a generally par-spherical shaped. The base structure  303  connects to and supports the remaining components of the bottom transducer assembly  300 . The base structure  303  also comprises a plurality of liquid dispensing holes/nozzles  14  that are adapted to supply a film of liquid to the bottom surface of a wafer during processing. The holes/nozzles  14  are located on both sides of the transmitter  302  in two separate rows that extend along the length of the transmitter  302 . 
         [0075]    The transmitter  302  is a generally par-cylindrical shaped plate having a convex outer surface  306  and a concave inner surface  307 . The transmitter  302 , however, can take on a wide variety of other shapes and sizes. The transmitter  302  can be constructed of any material that transmits acoustic energy generated by the transducer  305 , including without limitation quartz, sapphire, boron nitride, plastic, and metals. One suitable metal is aluminum. 
         [0076]    The outer convex surface of the transmitter  302  terminates in an apex  313 . Because the transmitter  302  is a par-cylindrical shape, this apex  313  ( FIG. 7 ) forms an elongate edge along  314  along the length of the transmitter. Of course, as used herein, the term elongate edge is not limited to the apex of an elongated curved surface but also includes, among other things, the meeting of two surfaces. Furthermore, in other embodiments, the transmitter  302  may be spherical in nature, thus, the apex could be a point. 
         [0077]    The transducer  305  is a curved plate having a convex upper surface  308  and concave lower surface  309 . The construction of transducers that convert electrical energy into acoustical energy is very well known in the art. The convex surface  308  of the transducer has a curvature that generally corresponds to the curvature of the inner concave surface  307 . The transducer  305  is acoustically coupled to the transmitter  302  so that acoustic energy generated by the transducer  305  propagates through the transmitter  302  and to the wafer  50 . More specifically, the convex upper surface  308  of the transducer  305  is bonded to the concave inner surface  307  of the transmitter. This bonding can be a direct bonding between the surfaces  307 ,  308  or can be an indirect bonding utilizing intermediary transmission layers. In other embodiments, the transducers may be flat plates or other shapes. Moreover, while the bottom transducer assembly  300  is illustrated as utilizing a single transducer  305 , a plurality of transducers can be used if desired to create the acoustic energy. Preferably, the transducer  305  is adapted to generate megasonic energy. 
         [0078]    The transmitter  302  is connected to the housing  304  so as to form a substantially enclosed space  310  in which the transducer  305  is located. Any suitable means can be used to connect the housing  304  to the transmitter  302 , including adhesion, heat welding, fasteners or a tight-fit assembly. A plurality of openings  311  are provided in the bottom portion of the housing  304 . The openings  311  are provided to allow a gas to be introduced into and/or out of the space  310  so that the transducer  305  can be cooled and/or purged. The openings  311  are operably connected to the gas source  20  as described in  FIG. 1 . The housing  304  also comprises an opening  312  for allowing the electrical connections (i.e., wires) that are necessary to power the transducer  305  to pass into the space  310 . This opening  312  can also be used to allow the gas to escape the space  310 . The housing  304  can take on a wide variety of shapes and structures and is not limiting of the present invention. In some embodiments, the housing may be merely a plate or other simple structure. 
         [0079]    In order to further protect the wafer  50  from possible contamination, once the transmitter  302  is connected to the housing  304 , the combined assembly may be fully encapsulated with an inert non-contaminating plastic, such as TEFLON® or the like. This also serves to protect the transmitter  302  from chemical attack. When the transmitter  302  is so encapsulated and/or coated, the encapsulation and/or coating is considered part of the transmitter  302 . 
         [0080]    Referring exclusively to  FIGS. 4 and 7 , the bottom transmitter assembly  300  further comprises a dam  301  that surrounds the periphery/perimeter of the transmitter  302 . The dam  301  forms an upwardly protruding ridge  316  having an angled inner surface  317 , an outer surface  318  and a top edge  319 . The dam  301  forms a liquid retaining channel  315  on both sides of the transmitter  302 . More specifically, the inner surface  317  of the ridge  316  forms a channel/groove with the transmitter  302 . Of course, in some embodiments, the dam  301  could be used to form the channel  315  in other ways and/or through cooperation with other structures. 
         [0081]    The dam  301  is a rectangular frame-like structure but can take on other shapes. The dam  301  also does not have to surround the entire periphery of the transmitter  302  but can surround only a small portion if desired. The dam  301  can be constructed of HDPE, PVDF, NPP or any other material. Preferably, the material chosen is chemically resistant and mechanically stable. 
         [0082]    The dam  301  is implemented into the bottom transducer assembly  300  to increase the size of the meniscus that couples the transmitter  302  to the bottom surface  52  of the wafer  50 . This facilitates an increased amount of acoustic energy being transmitted to the wafer  50  for improved cleaning. As illustrated in  FIG. 7 , without the dam  301 , the meniscus couples only area A of the transmitter  302  to the wafer  50 . However, with the dam  301 , the meniscus coupling area is increased to area B. 
         [0083]    Referring now to  FIGS. 8-12 , the possibilities for the relative arrangement of the bottom transducer assembly  300  and the top transducer assembly  200  with respect to one another in the cleaning system  1000  will be discussed. 
         [0084]    Referring first to  FIGS. 8 and 9 , an arrangement is illustrated wherein the transmitter  201  of the top transducer assembly  200  is aligned with and opposes the transmitter  302  of the bottom transducer assembly  300 . A wafer  50  is illustrated as being in between the assemblies  200 ,  300 . As liquid  70  is applied to the top surface  51  of the wafer  50 , a meniscus of liquid  72  is formed between a bottom portion  207  of the transmitter  201  of the top transducer assembly  200  and the top surface  51  of the wafer  50 . Similarly, as liquid  70  is applied to the bottom surface  52  of the wafer  50 , a meniscus of liquid  71  is formed between the transmitter  302  of the bottom transducer assembly  300  and the bottom surface  52  of the wafer  50 . As can be seen, the coupled portions of the top transmitter  201  and the bottom transmitter  302  oppose one another in an aligned manner. As a result, it is possible that the acoustic energy is generated by the top and bottom transducer assemblies  200 ,  300  and transmitted to the wafer via the meniscuses  71 ,  72  can interfere with and/or cancel one another out. 
         [0085]    Thus, it may be desirable, in certain instances, to operate the top and bottom transducer assemblies  200 ,  300  in an alternating and/or consecutive manner during a wafer cleaning cycle. In other embodiments, one may want to activate the operate the top and bottom transducer assemblies  200 ,  300  concurrently if interference is not an issue. 
         [0086]    Referring now to  FIGS. 10 and 11  concurrently, an alternative relative arrangement of the bottom transducer assembly  300  and the top transducer assembly  200  with respect to one another in the cleaning system  1000  is illustrated. In this embodiment, the transmitters  201 ,  302  of the top and bottom transducer assemblies  200 ,  300  are not aligned and do not oppose one another. Thus, interference should not be a problem during simultaneous generation and transmission of acoustic energy to the wafer. While the horizontal angle of separation between the top and bottom transmitters  201 ,  302  is 90 degrees in the illustration, any other angle can be used, including without limitation 180 degrees, 45 degrees, etc. 
         [0087]    It was discovered during the creation of the above described system that improved cleaning results were achieved by just having the bottom transducer assembly  300  present in the cleaning system  1000  and arranged as shown in  FIG. 8 , even when not activated (i.e., passive). It was discovered that the transmitter  302  of the bottom transducer assembly  300  was reflecting at least a fraction of the acoustic energy that was generated by the top transducer assembly  200  back toward the bottom surface  51  of the wafer  50 . Therefore, in another aspect, the invention is a novel system that utilizes a passive reflective member coupled to the opposite surface of the wafer than the active transducer assembly. 
         [0088]    Referring now to  FIG. 12 , a cleaning system  2000  that utilizes a passive backside reflective member  400  is schematically illustrated. The cleaning system  2000  is identical to that of cleaning system  1000  except that the bottom transducer assembly is replaced by a reflective member  400 . In fact, in some embodiments, the reflective member  400  could be a transducer assembly, such as the one described above, that is not activated. However, the reflective member  400  is not so limited and can take on a much broader variety of structures. Thus, a detailed explanation of the cleaning system  2000  will be omitted with the understanding that the description of cleaning system  1000  above will suffice for like parts. Like numbers are used to reference like parts. 
         [0089]    The reflective member  400  could be a mere plate or other structure. Preferably, the reflective member  400  is made of a material that has an acoustical impedance value (Za) that is much greater than that of water. In one embodiment, it is preferred that the acoustical impedance value be at least greater than 5.0 Mrayl, such as quartz. It may also be preferred that the reflective member  400  be spaced from the surface of the wafer  50  to which it is fluidly coupled by a distance that is a one-fourth interval of the wavelength of the acoustic energy being generated by the top transducer assembly  200 . In some alternative embodiments the reflective member  400  may be used to absorb the acoustical energy instead of reflecting it. 
         [0090]    The reflective member  400  may be made of a variety of materials the selection of which is dependent upon whether or not it is intended to be used as a reflector or an absorber of the acoustical energy. In the embodiment shown in  FIGS. 12 and 13  the reflective member  400  is designed to reflect acoustical energy. The reflective member  400  may be made of materials such as quartz, sapphire, silicone carbine, or boron nitride. Should acoustical energy wish to be absorbed the member  400  can be constructed out of PolyVinylidine DiFluoride (PVDF) or polytetrafluoroethylene (PTFE) (Also commonly sold under the trade name TEFLON®). The materials chosen are based upon their respective acoustical impedance (Za). Table 1 (below) provides a list of materials and the Zas associated with them. 
         [0000]    
       
         
               
               
               
             
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Material 
                 Za 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Alumina 
                 40.6 
               
               
                   
                 Aluminum rolled 
                 17.33 
               
               
                   
                 ARALDITE ® 502/956 20 phe 
                 3.52 
               
               
                   
                 ARALDITE ® 502/956 50 phe 
                 4.14 
               
               
                   
                 ARALDITE ® 502/956 90 phe 
                 12.81 
               
               
                   
                 Beryllium 
                 24.10 
               
               
                   
                 Bismuth 
                 21.5 
               
               
                   
                 Brass 70 cu 30 Zn 
                 40.6 
               
               
                   
                 Brick 
                 7.4 
               
               
                   
                 Cadmium 
                 24 
               
               
                   
                 Carbon vitreous, sigradur K 
                 7.38 
               
               
                   
                 Concrete 
                 8.0 
               
               
                   
                 Copper rolled 
                 44.6 
               
               
                   
                 Duraluminum 17S 
                 17.63 
               
               
                   
                 EPOTEK ® 301 
                 2.85 
               
               
                   
                 Fused silica 
                 12.55 
               
               
                   
                 Germanium 
                 29.6 
               
               
                   
                 Glass pyrex 
                 13.1 
               
               
                   
                 Glass quartz 
                 12.1 
               
               
                   
                 Glass silica 
                 13 
               
               
                   
                 Glucose 
                 5.0 
               
               
                   
                 Gold 
                 63.8 
               
               
                   
                 Granite 
                 26.8 
               
               
                   
                 Indium 
                 18.7 
               
               
                   
                 Iron 
                 46.4 
               
               
                   
                 Iron cast 
                 33.2 
               
               
                   
                 Lead 
                 24.6 
               
               
                   
                 Lithium 
                 33.0 
               
               
                   
                 Magnesium 
                 10.0 
               
               
                   
                 Marble 
                 10.5 
               
               
                   
                 Molybdenum 
                 63.1 
               
               
                   
                 Nickel 
                 49.5 
               
               
                   
                 Paraffin 
                 1.76 
               
               
                   
                 Polyester casting resin 
                 2.86 
               
               
                   
                 Porcelain 
                 13.5 
               
               
                   
                 PVDF 
                 4.2 
               
               
                   
                 Quartz × cut 
                 15.3 
               
               
                   
                 Rubidium 
                 1.93 
               
               
                   
                 Salt crystalline × direction 
                 10.37 
               
               
                   
                 Sapphire, aluminum oxide 
                 44.3 
               
               
                   
                 SCOTCH ® tape 2.5 mils thick 
                 2.08 
               
               
                   
                 Silicon very anisotropic approx 
                 19.7 
               
               
                   
                 Silicon carbide 
                 91.8 
               
               
                   
                 Silicon nitride 
                 36 
               
               
                   
                 Silver 
                 38.0 
               
               
                   
                 Steel mild 
                 46.0 
               
               
                   
                 Steel stainless 
                 45.7 
               
               
                   
                 STYCAST ® 
                 2.64 
               
               
                   
                 Tantalum 
                 54.8 
               
               
                   
                 TEFLON ® 
                 2.97 
               
               
                   
                 Tin 
                 24.2 
               
               
                   
                 Titanium 
                 27.3 
               
               
                   
                 Tracon 
                 4.82 
               
               
                   
                 Tungsten 
                 101.0 
               
               
                   
                 Uranium 
                 63.0 
               
               
                   
                 Vanadium 
                 36.2 
               
               
                   
                 Wood cork 
                 0.12 
               
               
                   
                 Wood pine 
                 1.57 
               
               
                   
                 Zinc 
                 29.6 
               
               
                   
                 Zinc oxide 
                 36.4 
               
               
                   
                 Zirconium 
                 30.1 
               
               
                   
                   
               
             
          
         
       
     
         [0091]    The acoustical impedance Za of a material is defined as the product of the density of that material times the velocity of sound in that material. The units for Za are Mrayl or (kg/m 2 s×10 6 ). Acoustical energy transmission is affected by the differences in the Za of the materials through which the acoustical energy must pass. More specifically, large differences in the Za between adjacent materials through which the acoustical energy must pass results in increased impedance of the acoustical energy. 
         [0092]    Due to the acoustical impedance values of the various surfaces of the reflective member  400 , the acoustical energy is effectively transmitted back towards the wafer  50 . This effectively cleans the bottom surface  52  without having to provide additional transducers. As discussed above, the reflective member  400  is made of a material with a Za that is greater than the fluid through which the acoustical energy is transmitted. Preferably the Za should be greater than 5 Mrayl, and more preferably greater than 15 Mrayl, such as quartz. The reflective member  400  may be hollow in order to create an additional transitional space that causes the acoustical energy to be reflected again as it passes through the reflective member  400 . During the cleaning process there may be continuous reflection between the wafer  50  and the reflective member  400  and it may continue until the acoustical energy diminishes in the system. 
         [0093]      FIG. 13  shows an alternative embodiment of the passive cleaning system  2000  wherein the reflective member  400  is positioned adjacent the top surface  51  of the wafer  50  rather than the bottom surface  52 . A bottom transducer assembly  300  is used instead of a top transducer assembly  200 . This embodiment operates in much the same fashion as the embodiment shown in  FIG. 12  except with the reflective member  400  and the transducer assembly  300  being reversed. 
         [0094]    Referring now to  FIG. 14 , it has been discovered that it may be preferable to utilize hollow tubular structures as the reflective member  400 . Examples of hollow tubular members  500 A-E are exemplified. The hollow tubular member  500 A-E can be fitted with transducers  305 A-E if desired. The tubular member can be made of quartz, plastic, metals, or other materials. These tubular members  500 A-E will have different effects on the transmission of the acoustical energy. The tubular members  500 A-E modifiers may be cylindrically shaped, triangular shaped, and trapezoidal shaped. It should be understood that other shapes may be used and are not limited to those shown, the selection of the shape may vary depending upon the desired results. 
         [0095]    The rounded or angled tubular members  500 A-E also may be used to direct the reflected acoustical energy at lower angles than that which it is at when it is directed at the wafer  50 . 
         [0096]    Typically these angles are less than 40°. By reflecting the acoustical energy at a shallow angle, a majority of the acoustical energy will be focused on the bottom surface  52  of the wafer  50  from the top transducer assembly  200 . 
         [0097]    It has also been discovered that the placement of the reflective member  400  from the wafer  50  also plays a role in effectively removing particles. The distance, or gaps, between the reflective member  400 , the transducer assembly  200  or  300  and the wafer  50  is determined so as to accommodate the frequency of the wavelength. The equation for the wavelength is: 
         [0000]    
       
         
           
             
               
                 
                   λ 
                   = 
                   
                     
                       v 
                       ω 
                     
                     f 
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
         [0000]    where λ=wavelength of an acoustical wave, v w  is the speed of propagation of the wave, and f=frequency of the wave in 1/s=Hz. Odd ¼ wavelength (e.g. ¼, ¾, 1¼) gaps tend to act as matching layers that permit energy to pass into the next media, and even ¼ wavelengths (e.g. 0.5, 1.0, 1.5, 2.0) gaps between the wafer  50  and the reflective member  400  tend to enhance the reflective property at the media interface. For example, in  FIG. 12 , the gap between the top transducer assembly  200  and the wafer  50  may be set for 1 and ¼ wavelengths in order to enhance the transmission of the acoustical energy through the cleaning liquid and the wafer  50 . On the opposite side, the gap between the reflective member  400  and the wafer  50  may be set at 1.0 wavelength (i.e. even) in order to enhance the reflection property so as to keep the transmission of acoustical energy directed towards the bottom surface  52  of the wafer  50 . In the example provided, when using water and a frequency of 835 kHz, the 1 and ¼ wavelength, the gap between the transducer assembly  200  and the wafer  50  is approximately 0.087″. The gap between the reflective member  400  and the wafer  50 , the 1.0 wavelength, is approximately 0.070″. 
         [0098]    It is to be understood, however, that even though numerous characteristics and advantages of the present invention have been set forth in the foregoing description, together with details of the structure and function of the invention, the disclosure is illustrative only, and changes may be made in detail, especially in matters of shape, size and arrangement of parts within the principles of the invention to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed.