Abstract:
A method and apparatus for megasonic cleaning of semiconductor wafers. The wafer is positioned so that the surface to be cleansed is parallel to and faces the radiating surface of a quartz or similar resonator which receives sonic waves through a liquid medium from a transducer. The sonic waves striking the wafer are preferably at about a 5° to 30° offset angle from a normally directed wave to the plane of the wafer. The layered medium is gasified and serves to couple the transducer to the resonator. A layer of degasified cleaning fluid is positioned between the resonator and the wafer.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is based on Provisional Application No. 60/835,255, filed Aug. 2, 2006, by the same inventors listed herein, and claims priority as to the common subject matter in the respective applications. 
     
     FEDERALLY FUNDED RESEARCH 
       [0002]    Not applicable. 
       SEQUENCE LISTING, ETC. ON CD 
       [0003]    Not applicable. 
       BACKGROUND OF THE INVENTION 
       [0004]    1. Field of the Invention 
         [0005]    This invention generally relates to the megasonic cleaning of semiconductor wafers or the like; and more particularly to a method and apparatus for cleaning the surface of such a wafer while eliminating, or at least substantially reducing, sonic induced damage to the wafer. 
       DESCRIPTION OF RELATED ART 
       [0006]    In connection with the production of semiconductor wafers, it is necessary to thoroughly clean the substrate in order to remove particulates, predeposited layers or strip resist, or other contamination. 
         [0007]    One well known process utilizes ultrasonic cleaning, i.e., the application of high amplitude ultrasonic energy to the substrate in a liquid bath. When the ultrasonic energy is in the range of about 0.60 to 10.00 MHZ, the process is termed megasonic cleaning. In broad terms, megasonic cleaning typically involves immersing the substrate in a tank filled with one of several well known liquid baths, immersing a megasonic transducer in proximity to the substrate with the acoustic output of the transducer being coupled to the surface or surfaces of the substrate by the liquid bath solution. Such liquid bath solutions may, for example, comprise deionized water, standard cleaning solutions, dilute NH 4  OH:H 2 0 2 , or the like. 
         [0008]    In connection with megasonic cleaning as described above, in one or more prior applications for patent assigned to the assignee of the present application, it has been suggested that the use of an oblique angle about 5° to 30° of megasonic energy impingement to a substrate surface can eliminate structural damage while allowing for adequate cleaning on sub64 nm semiconductor wafers. It has been further suggested that the oblique angle can be accomplished by either refraction of the sound waves through a resonator whose surface completely covers the wafer being cleaned, or by direct application of the sonic waves through a resonator whose surface completely covers the wafer being cleaned, or by direct application of the sonic waves from a relatively small transducer head positioned above the substrate at an oblique angle. It has been suggested that the use of higher frequency sonic waves, such as 2 MHz and greater, can reduce sonic induced damage by reducing the size of cavitation bubbles which reduces the strength of the sonic shock waves. The new invention has similar attributes, but its method and apparatus present an improvement in performance as compared to the prior alternatives. 
       BRIEF SUMMARY OF THE INVENTION 
       [0009]    The present invention utilizes the features above set forth for megasonic cleaning the front, backside and/or edge of a semiconductor wafer, in which such wafer surface is positioned in close proximity to a sonic resonating assembly in which the wafer is uniformly exposed to the megasonic energy and the cleaning solution of the liquid bath. Both front and backside, as well as edge cleaning is accomplished when liquid is exposed to such surfaces in the presence of a sonic wave. As will also be made clear, the wafer is generally held so that the surface to be cleaned is parallel to and faces the radiating surface of the megasonic resonator whether the resonator is positioned above or below the wafer in the liquid bath. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a cross-sectional elevational view of a portion of a wafer cleaning tank showing the relative positions of the transducer, resonator and wafer (positioned above the resonator). This figure also diagrammatically illustrates the direction of the sonic waves. 
           [0011]      FIG. 2  is a view similar to  FIG. 1 , but illustrating a second embodiment of the invention including exterior walls between the transducer and the resonator plate. 
           [0012]      FIG. 3  is a view similar to  FIG. 2 , except that the transducer is positioned non-parallel to the wafer. 
           [0013]      FIG. 4  is a top plan view illustrating the position of the transducer assembly of  FIG. 1  or  FIG. 2 , but with the transducer disposed above the wafer. This figure also diagrammatically illustrates the decaying sonic wave path as it moves within the resonator away from the emitting source. 
           [0014]      FIG. 5  is a cross-sectional elevational view of the structure shown in  FIG. 4 . 
           [0015]      FIG. 6  is a diagrammatic top view illustrating wafer rotation and nature of transducer movement. 
           [0016]      FIG. 7  is a plan view diagrammatically illustrating a full plate embodiment of the invention with a piezo transducer array and the positioning of external blanket heaters to assist in thermal control. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0017]    The method and apparatus of the present invention is best illustrated and explained by reference to the accompanying drawings. It has already been explained that the purpose of this invention is to provide a damage-free megasonic cleaning of semiconductor wafers. In accordance with this invention, as shown in  FIG. 1 , a wafer  12  is showing having a front surface  13  which is to be cleaned. However, as discussed above, the opposed surface, as well as the wafer edges are also subject to cleaning. In  FIG. 1 , the wafer is illustrated above and parallel to a transducer assembly  14  which includes a generally planar piezo mounted on and depending from a stainless steel plate  18  positioned below and spaced from a resonator plate  20 , preferably formed of quartz. 
         [0018]    Extension walls  22  formed of PFA, aluminum, or the like extend generally normal to the transducer  14 , mounting plate  18  and resonator plate  20 , with appropriate seals  24  and  26  sealing the wall  22  to the resonator plate and the transducer mounting plate respectively. The cross-sectional area defined by the wall  22  is less than the surface  13  of the wafer, but rotation of the wafer and/or reciprocating lateral movement of the transducer and resonator assures all portions of such surface will be exposed to the sonic energy transmitted to such surface. 
         [0019]    The lower surface of resonator plate  20  and extending for the diameter of the wall  22  has a wedge-shaped cut-away portion defining an angle of about 6° to 8° relative to the plane of wafer  12 . This is indicated in the drawings as 7°, which is believed the optimum, but slight variations are also believed to be appropriate. This cut-away portion defines a cavity  32  in the lower surface of the resonator plate  20 . It is understood that all of the parts described above are positioned in a tank (not shown) filled with the cleaning liquid above described. Gasified cleaning liquid is disposed as a layer  50  between the lower surface  13  of wafer  12  and the upper surface of the resonator plate  20 . Degasified coupling liquid  52  is disposed within the chamber defined by the transducer plate  18 , the walls  22  and cavity  32 . 
         [0020]    Using the foregoing arrangement, the cleaning of the wafer utilizes at least one acoustic transducer  14  positioned to cover and seal cavity  32  in resonator plate  20  and transmit sonic energy through a liquid boundary layer contained in the cavity into the resonator plate. The resonator plate has at least one obliquely angled surface, wetted by said liquid and conducts sonic wave energy through the plate to emerge from the opposing planar surface of said plate and impinges at any oblique angle from about 5° to 30° to the surface of the substrate  12  being cleaned. 
         [0021]    The primary attribute of this invention is the use of a liquid boundary layer to acoustically couple the megasonic energy of the piezo electric device, transducer  14  to the sonic resonating assembly plate  20  in such manner that the energy emerges from the opposite surface of the plate at an off-normal angle advantageous to the cleaning of the surface of an opposing wafer&#39;s surface  13  without damage thereto. The transducer is coupled acoustically to the plate through a degasified liquid boundary layer, through the plate to the opposite side which, in turn, acoustically couples to the wafer through a gasified cleaning fluid. At least one megasonic transducer is secured to the resonator assembly, which is tuned, in terms of mass, size and thickness for a given frequency, to pass the sonic energy of the transducer. The resonator assembly is so shaped as to accept the transducer&#39;s acoustic signal at the transducer-to-resonator assembly mounting surface and bend it at the resonator plate&#39;s planar-to-wafer distal surface so that the sonic signal exits at an optimal angle off-normal to the surface of the wafer while the resonator plate&#39;s facing surface remains parallel thereto. Those versed in the art will recognize that the angle of mounting of the transducer to the resonator plate, given a desired off-normal angle of exit, is determined by the materials of construction of the transducer, the resonator assembly, the bonding materials between the two faying surfaces, the sonic density of the fluid interface and the wavelength of the sonic signal as the physical phenomena reasonably follow Snell&#39;s Law and refraction. The following equation describes this phenomenon per Snell&#39;s Law: 
         [0000]    
       
                 
         
             
             
         
       
     
         [0022]    It is believed that an approximate 6° to 8°, and preferably 7° angle of incidence of sonic energy to the surface of the wafer is optimal for cleaning particulate from the surface of the wafer while avoiding inducing damage thereto. Other angles between 5° to 30° have been tested and shown to work, however with less cleaning efficiency. A range of suitable materials may be used for the resonator assembly depending in part upon wafer processing materials compatibility. One such suitable material frequently chosen for its purity in semiconductor wafer processing is quartz. Using quartz, as shown in  FIG. 1 , with an acoustically coupled liquid boundary layer of degasified water having an approximate impingement angle of 7° off-normal to the first surface of the quartz will create a refracted angle within the quartz of approximately 27°, which is convenient to produce the desired 7° off-normal emission angle in water as it exits the plate and continues on through the gasified cleaning fluid to strike the wafer. 
         [0023]    The mounted transducer-to-resonator angle of about 6° to 8°, and preferably about 7° would be the same for other suitable sonic refracting materials, such as stainless steel, aluminum, silicon carbide coated graphite, etc. However, the internal refraction angle within the material would vary as a result of the velocity of sound for the given material to achieve the desired 7° optimum emission angle in the cleaning fluid  50 . 
         [0024]      FIG. 2  illustrates a slightly modified arrangement in which the transducer is spaced from the quartz resonator plate  20  by PFA Teflon or aluminum extension walls  22 . These walls are designed to eliminate reflected waves  40  from interfering with the frequency waves  42  emitted from the transducer  14 . Here, an acoustically damping tungsten carbide or similar epoxy  44  is applied to walls  22  to eliminate sonic reflections from the walls going back to the transducer. The degasified liquid boundary is indicated at  52  and the gasified cleaning liquid at  50 . 
         [0025]    With reference to  FIG. 3 , it is noted that the transducer  14  and its mounting plate  18  are disposed at about a 6° to 8° offset angle from the horizontal and from the plane of wafer  12 . The resonator plate  20 , however, has parallel upper and lower surfaces and does not have the cut-out portion  32 . A 7° angle is believed to give the best results. The sonic effect on the lower surface of the wafer will accordingly remain the same as that illustrated in  FIGS. 1 and 2  of the drawings. 
         [0026]      FIGS. 4 and 5  represent the transducer  14  disposed above the wafer  12  and diagrammatically illustrates the sonic path  60  as it moves within the quartz resonator  20  and away from the transducer  14 . The energy dissipates as it moves from left to right as illustrated in the drawing. 
         [0027]    In  FIG. 6 , rotation of the wafer  12  is shown, along with the linear movement  71  of transducer  14 . The degassed boundary layer  50  is also indicated, along with a manifold  72  for gasified liquid distribution. 
         [0028]    The foregoing description is not an attempt to explain or illustrate all possible modifications or variations of the present invention, but rather to illustrate that the plane of the radiating surface of the refracting resonator plate  20  is disposed parallel to the confronting surface of the wafer  12 , thereby comprising the uniformity of the resulting oblique angle sonic wave bounce that occurs. 
         [0029]    The invention also lends itself to temperature control of a boundary layer fluid so as to permit control of the transducer temperature, particularly during the exothermic active-megasonic cycle. This thermal control of the transducer both protects and extends the life of the transducer and permits higher temperature wafer processes up to about 95° C. 
         [0030]    If additional heat is required by the process, it is easy to heat the plate  20  by external blanket heaters mounted to the back surface of the quartz resonator as has been disclosed in a prior application owned by the assignee of this invention, and shown in  FIG. 7  of the drawings. 
         [0031]    The sonic resonator plate assembly can be much thinner than the previously suggested 30° angle direct bonded approach. This reduces cost and allows for faster heating when a heated assembly is required (especially when quartz is used). 
         [0032]    The manufacturing of the boundary-layer-based assembly is much simpler and less costly to produce. The piezo crystals are mounted to standard plate materials to form the transducer assembly, which in turn is mounted to the resonator plate by means that permit its rapid removal, rather than less expedient, more costly, and lower dependable method of directly bonding the piezo to large, relatively exotic, unwieldy structures. 
         [0033]    Other features of the present invention are set forth below. 
         [0034]    Replacement of the transducer subassembly on the resonator plate without destruction of the entire assembly becomes possible with a boundary-coupled transducer, as compared to the directly bonded techniques more usually used. 
         [0035]    The liquid coupling of the sonic energy from transducer to resonator plate is typically far more accepting of a range of sonic frequencies than are solid transducer-to-plate interfaces. This at least provides the opportunity for future in situ swap-outs to different frequency transducers (with associated changes in emission angles corresponding to the rules of Snell&#39;s Law). 
         [0036]    The design allows the sonic resonator assembly to be mounted facing the front surface of the wafer from above or below. 
         [0037]    This invention can incorporate a full surface resonator plate, of a size larger than that of the wafer, thereby completely covering the wafer surface as illustrated in  FIG. 7 , or it can be a smaller, above-the-wafer, head-type assembly, as shown in  FIG. 5 . 
         [0038]    Relative to the latter, the transducer resonating plate  50  is supported on a translating mechanism (not illustrated herein) that enables the plate to be moved radially across and above the wafer. This radial motion can be accomplished by using a swinging action where the apparatus swings from the side to an over-center position above the wafer. 
         [0039]    In another embodiment of the invention, the radial motion can be programmed for entrance rate, extension distance, and number of moves per cleaning cycle. This, across the wafer by normalizing the dwell time at any one point from center to edge of the wafer. Radial non-uniformity is a typical problem of most prior art rotating single wafer megasonic systems. 
         [0040]    In yet another embodiment of the invention, the transducer assembly can include the distribution of its own cleaning and rinsing chemistries to impinge on the leading surface to the transducer yielding uniform and efficient distribution of fluids, especially with the movements disclosed in  FIG. 6 .