Abstract:
A method of cleaning a substrate comprises placing the substrate on a rotating fixture, placing a liquid on at least one side of the substrate, and creating a standing wave of megasonic energy oriented generally parallel to the substrate. The standing wave generates an associated pattern of high-agitation regions in the liquid. The method further comprises moving the standing wave back-and-forth so as to move the pattern of high-agitation regions about with respect to the substrate. An apparatus for cleaning substrates comprises a support to rotate the substrate about a first axis, and a transmitter extending generally parallel to a surface of the substrate. The apparatus further comprises a megasonic transducer in acoustically coupled relation to the transmitter, and a reciprocation drive in fixed relation to the transmitter. The reciprocation drive moves the transmitter back-and-forth within a plane generally parallel to the surface of the substrate.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     The present application is a divisional application of U.S. patent application Ser. No. 10/140,029, filed on May 6, 2002, the entirety of which is hereby incorporated by reference. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates to an apparatus and method for cleaning substrates, including semiconductor wafers or other such items requiring extremely high levels of cleanliness.  
       BACKGROUND OF THE INVENTION  
       [0003]     Substrates such as semiconductor wafers are frequently cleaned in a cleaning solution into which megasonic energy is propagated. Megasonic cleaning systems, which operate at a frequency over twenty times higher than ultrasonic, safely and effectively remove particles from materials without the negative side effects associated with ultrasonic cleaning.  
         [0004]     One type of megasonic cleaning apparatus shown in U.S. Pat. No. 6,140,744 comprises a piezoelectric transducer coupled to a transmitter in the form of a probe. The transducer is electrically excited such that it vibrates, and the probe transmits high frequency energy into liquid sprayed onto an adjacent, rotating semiconductor wafer. The agitation of the cleaning fluid produced by the megasonic energy loosens particles on the wafer. Contaminants are thus vibrated away from the surfaces of the wafer and removed through the continuous flow of the cleaning fluid.  
         [0005]     In U.S. Pat. No. 4,537,511, issued to Frei, an elongated metal tube in a tank of cleaning fluid is energized in the longitudinal wave mode by a transducer that extends through a wall of the tank and is attached to the end of the tube. In order to compensate for relatively high internal losses, the radiating arrangement uses a relatively thin-walled tubular member.  
         [0006]     A need exists for an improved apparatus and method which can be used to clean semiconductor wafers and other substrates.  
       SUMMARY OF THE INVENTION  
       [0007]     In accordance with one embodiment of the invention, a method of cleaning a substrate comprises placing the substrate on a rotating fixture, applying a liquid to at least one side of the substrate, and creating a standing wave of megasonic energy oriented generally parallel to the substrate. The standing wave generates an associated pattern of high-agitation regions in the liquid. The method further comprises creating relative back-and-forth movement between the standing wave and the substrate so as to move the pattern of high-agitation regions with respect to the substrate.  
         [0008]     In accordance with another embodiment of the invention, an apparatus for cleaning substrates comprises a rotary support which is adapted to support the substrate and rotate it about a first axis, and a megasonic energy transmitter extending generally parallel to a surface of the substrate. The apparatus further comprises a megasonic transducer in acoustically coupled relation to the transmitter, and a reciprocation drive in fixed relation to the transmitter. The reciprocation drive moves the transmitter back-and-forth within a plane generally parallel to the surface of the substrate. The megasonic transmitter agitates a liquid on a surface of the substrate, which results in a pattern of high-agitation regions formed in the liquid by the transmitter.  
         [0009]     In still another embodiment a method of cleaning a substrate comprises placing the substrate on a rotating fixture, placing a liquid on at least one side of the substrate, and creating a standing wave of megasonic energy oriented generally parallel to the substrate. The standing wave generates an associated pattern of high-agitation regions in the liquid. The method further comprises causing back-and-forth movement of the standing wave relative to the substrate or vice versa so as to move the pattern of high-agitation regions about with respect to the substrate.  
         [0010]     In still another embodiment a method of cleaning a substrate comprises placing the substrate on a rotating fixture, applying a liquid to at least one side of the substrate, and creating a wave of megasonic energy oriented generally parallel to the substrate, the wave generating high-agitation regions in the liquid. The method further comprises moving the high-agitation regions about with respect to the substrate.  
         [0011]     All of these embodiments are intended to be within the scope of the invention herein disclosed. These and other embodiments of the present invention will become readily apparent to those skilled in the art from the following detailed description of the preferred embodiments having reference to the attached figures, the invention not being limited to any particular preferred embodiment(s) disclosed. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]     Having thus summarized the general nature of the invention and its essential features and advantages, certain preferred embodiments and modifications thereof will become apparent to those skilled in the art from the detailed description herein having reference to the figures that follow, of which:  
         [0013]      FIG. 1  is a schematic side elevation view of a known megasonic wafer cleaner;  
         [0014]      FIG. 2  is a schematic side elevation view of a transmitter-wafer junction of the prior-art cleaner of  FIG. 1 ;  
         [0015]      FIG. 3  is a schematic side elevation view of the inventive megasonic wafer cleaner; and  
         [0016]      FIG. 4  is a schematic side elevation view of a transmitter-wafer junction of the inventive cleaner of  FIG. 3 .  
         [0017]      FIG. 5  is a perspective view of one embodiment of a reciprocation drive for use with the cleaner of  FIG. 3 .  
         [0018]      FIG. 6  is a partial perspective view of the reciprocation drive of  FIG. 5 .  
         [0019]      FIG. 7A  is a graph of one control methodology for the reciprocation drive.  
         [0020]      FIG. 7B  is a graph of another control methodology for the reciprocation drive.  
         [0021]      FIG. 8  is a graph of another control methodology for the reciprocation drive.  
         [0022]      FIG. 9  is a schematic view of a modulation system for the cleaner. 
     
    
     DETAILED DESCRIPTION  
       [0023]      FIGS. 1-2  depict a known megasonic cleaning apparatus  10 , generally comprising a tank-and-fixture assembly  12  and a transmitter assembly  14 . The tank-and-fixture assembly  12  is made up of a tank  16  inside of which is disposed a fixture  18  supporting a substrate  20  (such as a semiconductor wafer, photomask, flat-panel display, magnetic heads, or any other similar item requiring a high level of cleanliness). The fixture  18  generally comprises a motor  22 , shaft  24 , hub  26 , spokes  28 , and an annular rim  30 . The rim  30  supports the substrate  20  as it is rotated about a generally vertical axis by the motor  22 , in cooperation with the shaft, hub, spokes, etc. Upper and/or lower nozzles  32 ,  34  dispense a liquid, typically deionized water or other cleaning solution, onto the upper and/or lower surfaces of the substrate  20 . A drain line  36  in the lower end of the tank  16  permits accumulated cleaning solution to exit therefrom.  
         [0024]     In the form illustrated herein, the transmitter assembly  14  comprises an elongated element  38 , which can be termed a probe, acoustically coupled to a megasonic transducer (not shown) inside of a housing  40 . The housing  40  is mounted to a support  42  so that the shaft of the probe  38  extends generally parallel to the surface of the substrate  20  and is separated therefrom by a narrow gap  44 . The support  42 , along with the transmitter assembly  14 , is movable upwardly or is retractable to allow insertion/removal of substrates to the fixture  18 . A slot  46  may be included in the tank  16  to permit the probe  38  to be pivoted in and out of the tank; alternatively, any other suitable method or structure may be employed to facilitate movement of the probe  38  in and out of the tank where necessary.  
         [0025]     In operation, high-frequency electrical power is supplied to the megasonic transducer, which vibrates at a high, megasonic frequency. This vibration is transmitted to the probe  38 , which also vibrates at a megasonic frequency. The megasonic vibration of the probe  38  agitates the meniscus of liquid on the substrate near the probe, creating a cleaning action on the surface of the substrate. Where the lower nozzle  34  is employed to provide cleaning liquid on the lower surface of the substrate  20 , this lower-surface liquid is also agitated in the areas nearest the probe. As the substrate rotates under the probe, substantially the entire surface of the substrate is exposed to the cleaning action generated by the probe and agitated liquid.  
         [0026]      FIG. 2  depicts schematically the shaft of the probe  38  and an adjacent portion of the substrate  20 , during operation of the cleaning apparatus. In the probe  38  there is developed a standing longitudinal wave of megasonic energy that acts generally along the longitudinal axis of the probe shaft, shown in  FIG. 2  as the line A-A. This standing wave is characterized by antinodes (i.e., zones exhibiting alternating compression and expansion) occurring at 8/4 and 38/4 as indicated in  FIG. 2 . The nodes of the standing wave thus occur at 8/2 and 8. Due to the mechanics of longitudinal waves, a radial component of the standing longitudinal wave is generated, having a similar wavelength as the longitudinal wave and a series of nodes and antinodes that is phase-shifted by 90.degree. (8/4) with respect to the longitudinal wave. Thus the radial-component antinodes occur at 8/2 and 8 as shown in  FIG. 2 , creating a pattern of high-agitation regions  48  corresponding to the radial antinodes.  
         [0027]     Such megasonic cleaners have proven quite effective, but have suffered from several drawbacks. First, due to the pattern of high-agitation regions created by the probe  38 , uneven cleaning of the substrate  20  may take place, with more cleaning action occurring near the high-agitation regions  48  and reduced cleaning performance in the “gaps” between the high-agitation regions. It has also been observed that the higher-intensity megasonic energy found in the high-agitation regions  48  can damage electronic devices on the substrate, particularly when used to clean substrates forming densely-packed and/or high-aspect devices, as has become increasingly commonplace in recent years. Finally, it has often been found necessary to increase the power supplied to the transducer, and/or expose the substrate to the megasonic energy radiated by the probe for prolonged periods of time, in order to facilitate satisfactory cleaning of the substrate between the high-agitation regions.  
         [0028]     Additional details pertaining to megasonic cleaning systems and not necessary to recite here may be found in Assignee&#39;s above referenced U.S. Pat. No. 6,140,744, issued Oct. 31, 2000 and entitled SUBSTRATE CLEANING SYSTEM, the entirety of which patent is hereby incorporated by reference herein and made a part of this disclosure.  
         [0029]      FIGS. 3 and 4  illustrate a preferred embodiment of the inventive substrate cleaning apparatus  110 . The apparatus  110  may be generally similar to the apparatus  10  disclosed above and depicted in  FIGS. 1-2 , incorporating the transmitter assembly  14 , tank  16 , fixture  18 , etc. However, the present invention is by no means limited to application in the specific cleaning apparatus  10  disclosed above; one of skill in the art will appreciate that the present invention encompasses use in connection with other megasonic cleaners which generate a standing megasonic wave oriented generally parallel to a surface of a substrate being cleaned. Other suitable examples of prior-art megasonic cleaners are disclosed in U.S. Pat. No. 6,140,744, incorporated by reference above. In one preferred embodiment, the probe  38  has a shaft of 1 cm diameter and is driven at a frequency of about 835 kHz.  
         [0030]     The apparatus  110  incorporates a reciprocation drive  112  which is operatively connected to the transmitter assembly  14  (by, for example, installation between the housing  40  and the support  42  as shown) so as to impart a reciprocating motion to the probe  38  and to the standing wave developed therein. In reciprocating the probe  38  and standing wave, the reciprocation drive  112  moves the pattern of high-agitation regions  48  back-and-forth with respect to the substrate surface (see  FIG. 4 ), providing more uniform cleaning of the substrate. As the probe  38  moves into and through the location occupied by the displaced probe  38 ′, the high-agitation regions  48  correspondingly move into and through the location occupied by the displaced high-agitation regions  48 ′, which location now benefits from the cleaning action associated with the regions  48 ′.  
         [0031]     The reciprocation drive  112  may comprise any suitable mechanism needed to achieve the reciprocation performance discussed herein, and the specific components of the drive  112  will be readily ascertainable by one of ordinary skill in the art of substrate processing machine design.  
         [0032]     In one embodiment, the reciprocation drive  112  comprises a linear reciprocation drive and imparts a substantially linear reciprocation motion to the probe/standing wave. It is presently preferred that the reciprocation drive  112  moves the probe back-and-forth substantially linearly, generally along the longitudinal axis A-A of the probe  38 . However, the probe  38  may alternatively be reciprocated along any suitable line generally parallel to the surface of the substrate  20  (such as a laterally oriented line, i.e. one oriented either generally perpendicular to the axis A-A, or a line oriented transverse to the axis A-A). Preferably, the reciprocation drive  112  imparts a substantially linear reciprocation action to the probe  38  (and the standing wave formed therein) in which the probe/wave is alternately displaced in a first direction by a distance preferably equal to about 0.5 to 2.0 times the wavelength of the megasonic energy in the probe, and displaced in the opposite direction by a substantially equal distance. Therefore, where the wavelength of the applied megasonic energy is about 0.3″ (as is commonly employed in the semiconductor industry) the preferred reciprocation distance is about 0.150″ to about 0.6″, although smaller or larger distances may be suitable as well. In one embodiment, the reciprocation rate is about 0.1 to 2.0 cycles per second; of course, any suitable reciprocation rate may be employed.  
         [0033]     In another embodiment, the reciprocation drive  112  comprises an angular reciprocation drive and imparts an angular reciprocation motion to the probe/standing wave. The degree of angular back-and-forth displacement should be sufficient to provide the desired uniform cleaning of the substrate  20 . As with the linear reciprocation disclosed above, in one embodiment, the reciprocation rate is about 0.1 to 2.0 cycles per second; of course, any suitable reciprocation rate may be employed.  
         [0034]     As a further alternative, the reciprocation drive  112  could be operatively connected to the fixture  18  (and thereby to the substrate  20 ), instead of or in addition to the connection of the drive  112  to the transmitter assembly  14 . Such reciprocation of the fixture  18 /substrate  20  may be performed according to the parameters detailed above with respect to reciprocation of the probe. Furthermore, it is contemplated that any suitable method or structure can be employed, so long as relative back-and-forth movement is created between the substrate and the probe/standing wave/high-agitation regions.  
         [0035]      FIGS. 5 and 6  depict one embodiment of a reciprocation drive  112  which may be incorporated in the apparatus  110 . The reciprocation drive  112  includes a motor  150  (which may comprise a servo-motor), a linear bearing  152 , a drivescrew  154  (which may comprise a leadscrew or a ballscrew), and a carrier  155  which engages the drivescrew  154  and the bearing  152 . Thus, as the motor  150  turns the drivescrew  154  via a belt drive, gear train, etc. (not shown) located in a housing  180 , the carrier  155  advances forward or backward on the drivescrew  154  and the bearing  152 . Proximity sensors  156  may be employed for sensing the limit and home positions of the carrier  155 . The housing  40  and probe  38  are mounted to the carrier  155  via a bracket  158 .  
         [0036]     The reciprocation drive  112  shown in  FIGS. 5 and 6  may be driven by a controller (not shown) which positions the probe  38  with respect to the substrate  20  (see  FIG. 3 ). Various control strategies may be implemented to maximize performance of the apparatus  110 . These strategies may be selected depending upon many factors, for example, the size of the substrate, the cleaning solution used, the sensitivity of the structures located on the surface of the substrate, and the degree of cleanliness required, among others. These control strategies can be illustrated graphically, for example on a two-dimensional graph.  
         [0037]      FIG. 7A  depicts the position of the probe  38  over time, in accordance with one suitable control strategy. More specifically,  FIG. 7A  plots the position of the probe  38  on the y-axis and time on the x-axis. Movement toward the center of the substrate corresponds to upward movement on the y-axis, and movement toward the edge of the substrate corresponds to downward movement on the y-axis. The solid line in  FIG. 7  thus represents the position of the probe  38  over time with respect to the substrate  20 . In accordance with the depicted control strategy, the motor  150  extends the probe  38  toward the center of the substrate (preferably in a generally radial orientation with respect to the substrate) at a substantially constant linear velocity with respect to the bearing  152  until the probe tip reaches a limit near, at or just beyond the center of the substrate. The probe is then stopped and reversed at a substantially constant linear velocity toward an opposite limit near, at or beyond the edge of the substrate. The probe may be advanced back and forth in this manner as many times as desired throughout a cleaning cycle, to form a “sawtooth” pattern as depicted in  FIG. 7A .  
         [0038]      FIG. 7B  depicts another suitable control strategy for movement of the probe  38 . This strategy comprises a sinusoidal pattern, in which the motion of the probe is gradually slowed and then reversed at opposing limits near, at or beyond the center and edge of the substrate. In another control strategy, shown in  FIG. 8 , the probe makes a long initial excursion toward the center of the substrate and is then reciprocated in a relatively tight sawtooth pattern in which the limits of travel are narrowly spaced in comparison to the distance over which the probe travels during the initial excursion. The distance between the limits of travel of the sawtooth portion shown in  FIG. 8  may, in one embodiment, be equivalent to between about one-half and about twice the wavelength of the megasonic energy being driven through the probe. In further variations, a “tight” sinusoidal pattern or any other suitable tight control strategy may follow the initial excursion shown in  FIG. 8 .  
         [0039]     In yet another suitable control strategy, the probe  38  is moved at a slower rate at one or both of the endpoints of its range of travel (near the edge and center of the substrate) than in some or all of the probe&#39;s travel range between these endpoints.  
         [0040]     It should be further noted that in any of the control strategies depicted and/or discussed herein, the megasonic power supplied to the probe  38  may be switched on or off at appropriate points in the movement profile of the probe, so as to provide even cleaning of the substrate with minimal damage to the structures formed thereon. For example, in the strategy shown in  FIG. 12 , the power may be switched off during the initial excursion and then switched on during the subsequent “sawtooth” movement of the probe.  
         [0041]     In another embodiment, the frequency of the megasonic energy imparted to the probe  38  can be modulated over time, instead of or in addition to moving the probe with respect to the substrate. As the frequency is varied over time, the high-agitation regions  48  (see  FIG. 4 ) converge (as the frequency increases) or diverge (as the frequency decreases). Thus, by modulating the frequency the regions  48  can be moved with respect to the substrate  20 , to generate a more even cleaning action as discussed above. In one embodiment, the frequency is varied steadily over time about a center frequency of approximately 835 kHz. In another embodiment, the degree of variation may be about .+−.2 kHz with respect to a chosen center frequency. In another embodiment, the frequency is modulated with a period of about 1 to 2000 milliseconds between successive frequency peaks or troughs. Naturally, any suitable center frequency, degree of variation, or period may be employed in other embodiments.  
         [0042]      FIG. 9  depicts one embodiment of a modulation system  300  which may be employed to modulate the frequency of the megasonic energy imparted to the probe  38 . The modulation system  300  includes a tuner or signal generator  302  and a power supply  304 , both of which are electrically connected to an amplifier  306 . The amplifier  306  drives a megasonic transducer  308 , which is located in the probe housing  40  as discussed above and may be generally similar to the transducer described above and in the incorporated U.S. Pat. No. 6,140,744. The tuner  302  may comprise any suitable commercially available tuner having modulation capability. Alternatively, the tuner  302  may comprise program instructions executable by a computer, control system, processor, etc. in communication with the amplifier  306  and/or power supply  304 . It should be further noted that the tuner  302 , power supply  304  and amplifier  306  may comprise physically separate components; alternatively, two or more of these items may be combined as a single device. By modulating the frequency of the signal generated by the tuner  302  and passed to the amplifier  306 , one can modulate the frequency of the power delivered by the amplifier  306  to the transducer  308 .  
         [0043]     The megasonic cleaning apparatus and method disclosed herein provides numerous performance advantages over known cleaners. As mentioned above, the apparatus  110  facilitates more uniform cleaning of the entire substrate surface, eliminating the reduced cleaning observed “between” the high-agitation regions in a stationary cleaner. In addition, the apparatus  110  reduces the tendency of a cleaner to damage the electronic devices on the surface of the substrate at the high-agitation regions, by avoiding prolonged exposure of any portion of the substrate surface to the higher energy associated with the high-agitation regions. Finally, the apparatus  110  requires less time and/or less power to clean a substrate of a given size, as substantially the entire surface of the substrate gains exposure to the high-energy, high-agitation regions during the cleaning process.  
         [0044]     Although this invention has been disclosed in the context of certain preferred embodiments and examples, it will be understood by those skilled in the art that the present invention extends beyond the specifically disclosed embodiments to other alternative embodiments and/or uses of the invention and obvious modifications and equivalents thereof. In particular, the transmitter element that extends close to the surface of the substrate may have various shapes in addition to the elongated rod element illustrated in the drawings. For example, the element can have a flat lower surface with any desired cross-section including a hollow element. Further, although the element illustrated is supported in cantilever fashion, the transmitter could be supported from above the substrate being cleaned. Thus, it is intended that the scope of the present invention herein disclosed should not be limited by the particular disclosed embodiments described above, but should be determined only by a fair reading of the claims that follow.