Abstract:
A method and apparatus for the micro- or nano-machining of a material using the controlled formation of individual cavitation bubbles, by immersing a work piece having a work surface in a liquid, generating a cavitation bubble proximate to the work whereby a re-entrant micro-jet formed during the collapse of the cavitation bubble is directed toward the work surface to effect micro- or nano-machining.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]    This application claims priority to application Ser. No. 60/350849, filed on Jan. 18, 2002, entitled “Method And Apparatus For The Controlled Formation Of Cavitation Bubbles.” 
     
    
     
       BACKGROUND  
         [0002]    1. Field of the Invention  
           [0003]    This invention relates generally to the formation and control of individual micron size and submicron size cavitation bubbles for use in nanofabrication operations. More particularly, embodiments of the invention teach methods and apparatus for control of a re-entrant micro-jet formed upon collapse of an individual or array of cavitation bubbles and directing the impact of the micro-jet toward a work surface with a high degree of precision.  
           [0004]    2. Description of the Related Art  
           [0005]    In general, the production of cavitation has been a phenomena many have tried to avoid. Cavitation in a liquid is the formation, growth, and collapse of gaseous and vapor bubbles due to the reduction of pressure below the vapor pressure of the liquid at the working temperature. Pump impellers, boat props, and similar applications experience cavitation which can produce rapid damage and erosion of surfaces. It has been well known for many years that ultrasonic cleaning devices, which function by the creation of cavitation bubbles, can produce significant surface damage to even the hardest of materials. Studies by a number of authors have revealed that one significant element in producing the damage caused by cavitation occurs when a cavitation bubble collapses in the vicinity of a surface, launching what is called a re-entrant micro-jet toward the surface. This liquid jet can produce velocities as high as 1500 m/s, and is capable of damaging the hardest materials known.  
           [0006]    Recently, a number of applications have been developed utilizing the formation of cavitation bubbles through the use of laser light or electrical discharge. Esch et al. (U.S. Pat. No. 6,139,543) and Herbert et al. (U.S. Pat. No. 6,210,400) disclose the use of laser light introduced into a catheter device for the purpose of creating cavitation bubbles, whose expansion and collapse are utilized to pump fluids in and out of the catheter. Hammer et al. (U.S. Pat. No. 5,738,676) discloses a laser surgical probe with a special lens designed to produce the cavitation bubbles further from the end of the fiber optics, to reduce the damage formed (presumably by the re-entrant micro-jets launching into the lens on the end of the cable). Such damage was also reported by Rol et al. in “Q Switched Pulses and Optical Breakdown Generation Through Optical Fibers”,  Laser and Light in Ophthalmology , Vol. 3, No. 3, 1990. Palanker (U.S. Pat. No. 6,135,998) describes a method for performing electrosurgery using sub-microsecond, high power electrical pulses applied to an electrosurgical probe interface. The tool described by Palanker provides a cutting force by both the plasma generated by the electrical arc and shock waves produced by collapsing cavitation bubbles.  
           [0007]    In each of the references cited above, there has been no attempt to control the direction and impact of the powerful micro-jets formed upon the collapse of the cavitation bubbles created when highly focused energy is introduced into a liquid. Without such control, concern of collateral damage cannot be avoided, especially when such tools are used in the human body in a medical application.  
           [0008]    Recently as well, there has been a significant interest generated in the field of nanotechnology, for methods needed to fabricate micron and submicron devices and nanomachines. There are very few fabrication tools available that can cut, drill, peen, deform, or otherwise modify features of a surface on a submicron to nanometer scale. Much of the technology developed by the semiconductor industry requires the fabrication of structures utilizing photolithographic processing. This technology is not as flexible as may be required, and will have certain difficulties when applied to biological nanotechnology systems. Advancing the state of the art required by nanotechnology applications will require fabrication technologies operating at least 1 to 2 orders of magnitude below that of current capabilities in the semiconductor process arena.  
           [0009]    The prior state of the art therefore has yet to provide a fabrication technology capable of operating in the nanometer region by harnessing the powerful phenomena of the re-entrant micro-jet formed during the collapse of a precisely located cavitation bubble.  
         SUMMARY  
         [0010]    The present invention provides a method for the micro-machining or nano-machining of a material using the controlled formation of individual cavitation bubbles comprising by immersing a work piece having a work surface in a liquid, generating a cavitation bubble in a pre-determined location proximate to said work piece, whereby a re-entrant micro-jet formed during the collapse of said cavitation bubble is directed toward said work surface, and micro- or submicro-machining said work piece.  
           [0011]    An apparatus for the micro- or nano-machining of a material using the controlled formation of cavitation bubbles having a work piece immersed in a liquid, an energy source having an energy flow in the liquid sufficient to create a cavitation bubble proximate to the workpiece, wherein the energy flow creates a cavitation bubble proximate to the work piece, and wherein the collapse of said cavitation bubble creates a re-entrant micro-jet directed toward the work piece to effect micro- or nano-machining. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    [0012]FIG. 1A is a schematic view of a cavitation initiation volume in accordance with one embodiment of the present invention.  
         [0013]    [0013]FIG. 1B is a schematic view of a fully expanded cavitation bubble in accordance with one embodiment of the present invention.  
         [0014]    [0014]FIG. 1C is a schematic view of a collapsing cavitation bubble in accordance with one embodiment of the present invention.  
         [0015]    [0015]FIG. 1D is a schematic view of the initial formation of a re-entrant micro-jet induced by the collapsing cavitation bubble in accordance with one embodiment of the present invention.  
         [0016]    [0016]FIG. 1E is a schematic view of a re-entrant micro-jet directed through an aperture to a work surface in accordance with one embodiment of the present invention  
         [0017]    [0017]FIG. 2 is a schematic view of a lens focused laser apparatus for producing cavitation induced re-entrant micro-jets in accordance with another embodiment of the present invention.  
         [0018]    [0018]FIG. 3 is a schematic view of a parabolic mirror focused laser apparatus for producing cavitation induced re-entrant micro-jets in accordance with another embodiment of the present invention.  
         [0019]    [0019]FIG. 4 is a schematic view of a lens focused x-ray source apparatus for producing cavitation induced re-entrant micro-jets in accordance with another embodiment of the present invention.  
         [0020]    [0020]FIG. 5 is a schematic view of a parabolic mirror focused x-ray source apparatus for producing cavitation induced re-entrant micro-jets in accordance with another embodiment of the present invention.  
         [0021]    [0021]FIG. 6 is a schematic view of spatial filter added to a leas focused laser apparatus for producing cavitation induced re-entrant micro-jets in accordance with another embodiment of the present invention.  
         [0022]    [0022]FIG. 7 is a schematic view of an electric discharge apparatus for producing cavitation induced re-entrant micro-jets in accordance with another embodiment of the present invention.  
         [0023]    [0023]FIG. 8 is an apparatus for the production of an array of cavitation induced re-entrant micro-,jets in accordance with another embodiment of the present invention.  
         [0024]    [0024]FIG. 9 is a schematic view of an apparatus for the welding of small particles in a cavitation induced re-entrant micro-jet in accordance with another embodiment of the present invention.  
         [0025]    [0025]FIG. 10 is a table of parameters for the application of various pulsed Gaussian TEM00 lasers for a number of embodiments in accordance with the present invention.  
         [0026]    [0026]FIG. 11 is a table of parameters for the application of an electric discharge for one embodiment in accordance with the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0027]    The sequence illustrated in FIGS.  1 A- 1 E illustrates the formation of a re-entrant micro-jet from the formation and collapse of cavitation bubble in accordance with an embodiment of the present invention.  
         [0028]    [0028]FIG. 1A is a schematic view of a cavitation initiation volume in accordance with one embodiment of the present invention. The energy from a cavitation initiation device (not shown) is focused into a volume  2  aligned over aperture  4 , at a nominal distance  3  from aperture mask  6  placed in proximity to a work piece surface  8 . The intense energy focused into the small focus volume  2  is absorbed by the fluid  1 , causing rapid boiling and expansion of vaporized gasses. Arrows  10  represent the rapid movement of the gas liquid boundary of the cavitation bubble formed in volume  2 . Energy sources may include, but are not limited to: lasers, x-ray sources, ultrasound, electrical discharge, and positrons.  
         [0029]    [0029]FIG. 1B is a schematic view of a fully expanded cavitation bubble in accordance with one embodiment of the present invention. Cavitation bubble  12 , formed from the rapid expansion of vaporized fluid in volume  2  and the momentum of liquid moving away from the center of the focus volume  2 , has reached its maximum diameter  5 . Typically, the maximum diameter  5  of the fully expanded cavitation bubble  12  is approximately 10 to 50 times the diameter of the focus volume  2  shown in the previous FIG. 1A. Gas pressure inside fully expanded cavitation bubble  12  may be as low as the vapor pressure of fluid  1  at its bulk temperature. The pressure of the surrounding fluid  1 , typically at 1 atmosphere absolute or higher, creates a pressure differential on the outer surface of the bubble  12 , driving its subsequent collapse. For fluids  1  such as water at 1 atmosphere and 25° C., the pressure differential can exceed 700 torr.  
         [0030]    [0030]FIG. 1C is a schematic view of a collapsing cavitation bubble in accordance with one embodiment of the present invention. Cavitation bubble  14  has begun a rapid collapse illustrated by rapid inner movement of its outer surface and arrows  16 .  
         [0031]    [0031]FIG. 1D is a schematic view of the initial formation of a re-entrant micro-jet  20  induced by the collapsing cavitation bubble  16  in accordance with one embodiment of the present invention. Re-entrant micro-jet  20  is launched through aperture  4  toward work surface  8 . Aperture mask  6  serves to block subsequent shock waves produced by collapsing cavition bubble  16  from work surface  8 , allowing only the high velocity, focused re-entrant micro-jet to impact the surface.  
         [0032]    [0032]FIG. 1E is a schematic view of a re-entrant micro-jet directed through an aperture to a work surface in accordance with one embodiment of the present invention. The fully formed re-entrant micro-jet  24  impacts the work surface  8  through aperture  4 . The re-entrant micro-jet  24  may impart the work surface with velocities as high as 1500 meters/second, and are capable of removing material from the hardest surfaces known, such as diamond. These jets may be used to exit, machine, drill through, erode or deform features on the work surface  8 . The diameter of the jets are determined by the size of the cavitation bubble  12  formed, which in turn is determined by the dimensions of the focus volume  2  and the level of energy introduced into said focus volume. As will be illustrated in subsequent figures, the diameter of the re-entrant micro-jet  24  may vary from about 1 micron to about 1 nanometer for focused laser and x-ray energy sources. Electric discharge sources may produce re-entrant micro-jet diameters on the order of 10 to 15 microns. The velocity of the re-entrant micro-jet through the aperture is primarily determined by the distance of the focus volume  2  to the aperture mask  6 , and can vary from ½ the expanded bubble diameter 5 to about 6 times the expanded bubble diameter  5 , with the optimum distance being approximately  3  expanded bubble diameters  5 . The impact force of the re-entrant micro-jet  24  on work surface  8  may be adjusted by altering the distance  7  between the aperture mask  6  and the work surface  8 . At a given jet velocity (or fixed distance between the focus volume  2  and aperture mask  6 ), the impact force will vary inversely with the distance  7 , in a range from approximately zero to  6  expanded bubble diameters  12 , but preferably in a range from zero to 4 bubble diameters  12 . The diameter of the aperture  4  can be in a range from about 1% to 30% of the expanded bubble diameter. The re-entrant micro-jet diameter is, on the order of about 0.2% of the expanded bubble diameter  12 .  
         [0033]    The aperture mask  6  and aperture  4  play an essential role in directing and controlling the action of the re-entrant micro-jet  24 . Without the aperture mask, the collapse of the cavitation bubble ( 12 ,  14 ,  16 ) would still launch a re-entrant micro-jet toward the surface  8 , but the location of impact and the force imparted would be unpredictable, especially on a nanometer scale. In addition, the aperture mask tends to keep shock waves created in the expansion and contraction stages from damaging the surface  8 . Accurate placement of the aperture and the focus volume allow nanometer scale precision cutting, punching, peening, drilling, or deforming operations on sub-micron scale features of the work surface. Many prior art applications are capable of accurate placement of the initial focus volume, but do little or nothing to control the shock waves and re-entrant micro-jet formed upon collapse of the cavitation bubble.  
         [0034]    [0034]FIG. 2 is a schematic view of a lens focused laser apparatus for producing cavitation induced re-entrant micro-jets in accordance with some embodiments of the present invention. The laser may be selected from Spectra-Physics Pulsed Nd:YAG Series models LAB-130, -150, -170, -190, or Pro-230, -250, -270, -290, or -350, for example. Sealed tank  30  contains liquid filled to a level  32 . Various liquids can be used, but high purity water (&gt;100 k ohms resistivity) is preferred. The beam from laser  34  is directed to lenses  40   a  and  40   b  to collimate the beam, which is then focused by lens  48  at a focal distance  50 . The lenses are housed in housing  42 . Beam focus positioner  36  determines the location of the focus volume  2  relative to the aperture mask  6  with thickness  46 , at a distance  52 . Work surface  8  is moved by precision XYZ stage  60 , to adjust the distance from aperture mask  6  to the work surface, as well as locate the specific area on the work surface to be impacted by the jet  24 . Recall from previous FIGS.  1 A- 1 E, that the position of the focus volume determines the location of the subsequent cavitation bubble  44  and re-entrant micro-jet  24 . An XYZ stage  60  determines the distance  54  from the aperture to the work surface, as well as the XY coordinates of the area to be worked on. An example of stage  60  is a Piezomax Technologies Inc. N-XY100/N-Z25. Fluid inlet  56  and outlet  58  are utilized to provide a constant flushing of the fluid in the tank  30 , in part to remove any debris produced by the machining occurring on the work surface  8 . This debris may negatively impact the absorption of subsequent laser light pulses in the focus volume, as well as potentially contaminate the surface with entrained particle matter introduced into the re-entrant micro-jet. For similar reasons, it may be desirable (although not essential) to filter the incoming fluid stream  62  to remove any particulate contamination. Tank  30  is equipped with a pressure transducer  38  to monitor and control the back pressure. For a sealed tank as shown, this may be done simply by raising the inlet pressure  62  with respect to the outlet pressure  64 , by choking the outlet flow until the tank ambient pressure is as desired, then re-equilibrating the flows once again.  
         [0035]    [0035]FIG. 3 is a schematic view of a parabolic mirror focused laser apparatus for producing cavitation induced re-entrant micro-jets in accordance with another embodiment of the present invention. As was shown in FIG. 2, laser  34  directs a beam into collimator lenses  40   a  and  40   b.  The collimated beam is directed onto a parabolic mirror  66 , which also contains the aperture  4 . Parabolic mirror  66  focuses the collimated laser beam to a focus volume at a distance  52  from the aperture. In this embodiment, distance  52  is fixed by the curvature parameters of the parabolic minor  66 , and therefore the velocity of the re-entrant micro-jet  24  is also fixed. An XYZ stage  60  determines the distance  54  from the aperture to the work surface, as well as the XY coordinates of the area to be worked on. All other features are as described in FIG. 2.  
         [0036]    [0036]FIG. 4 is a schematic view of a lens focused x-ray source apparatus for producing cavitation induced re-entrant micro-jets in accordance with another embodiment of the present invention. X-ray source  70  directs a beam into x-ray lens  72 , which focuses and concentrates the x-ray beam into a focus volume at a distance  52  from an aperture mask  6 . Aperture positioner  76  adjusts distance  52  to alter re-entrant micro-jet velocity through the aperture  4 . Dimension  54 , or the distance of the aperture mask to the work surface  8  is adjusted by XYZ stage as has been previously described. All other features are as described in FIG. 2.  
         [0037]    [0037]FIG. 5 is a schematic view of a parabolic mirror focused x-ray source apparatus for producing cavitation induced re-entrant micro-jets in accordance with another embodiment of the present invention. X-ray source  70  directs a beam onto parabolic x-ray mirror  80  containing an aperture  4 . The x-ray beam is focused into a focus volume at a distance  52  from the aperture  4 , The dimension  54  between the aperture mask  6  and work surface  8  is adjusted by the XYZ stage  60 . In this embodiment, distance  52  is fixed by the curvature parameters of the parabolic mirror  80 , and therefore the velocity of the re-entrant micro-jet  24  is also fixed.  
         [0038]    [0038]FIG. 6 is a schematic view of spatial filter added to a lens focused laser apparatus for producing cavitation induced re-entrant micro-jets in accordance with another embodiment of the present invention. Spatial filter  86  can be optionally added to the previously described embodiments to farther clean up the laser beam or x-ray beam to allow smaller focus volumes. The spatial filter  86  comprises a entrance lens  82 , a pinhole  85 , and an exit lens  83 . Exit lens  83  and lens  40  makes up part of the collimator lens pair as shown in previous figures.  
         [0039]    [0039]FIG. 7 is a schematic view of an electric discharge apparatus for producing cavitation induced re-entrant micro-jets in accordance with another embodiment of the present invention. A positive electrode  88  and negative electrode  90  are immersed in fluid  32  and positioned to generate an arc at a position a distance  52  above aperture mask  6 . Actuator  76  adjusts dimension  52  to position the focus volume a known distance from the aperture mask  6 . The arc is created by rapid discharge of capacitor  96  through switch  94 . Full circuit details are not shown in FIG. 7, but are well known to those skilled in the art. Capacitor  96  is a low inductance, high voltage device as is used in pulse lasers and flash tubes. The rapid discharge and subsequent transient arc creates a cavitation bubble  44  as illustrated in FIGS.  1 A- 1 E.  
         [0040]    [0040]FIG. 8 is an apparatus for the production of an array of cavitation induced re-entrant micro-jets in accordance with another embodiment of the present invention. Work surface  8  is placed parallel to an aperture mask  6   a  containing a plurality of apertures. Cavitation bubbles  44   a,    44   b  (only two are shown for clarity) are formed directly over each aperture in the array by any number of techniques, as previously discussed, such that the re-entrant micro-jets  24   a,    24   b  formed following the collapse of the cavitation bubbles are directed through the apertures  4   a,    4   b  normal to the surface  6   a  and impact work surface  8 . Aperture  4   c,  for example, has diameter  104 . The cavitation bubbles may be formed simultaneously or sequentially, or in some other pattern (such as every other aperture, every two apertures, etc.). If the cavitation bubbles  44  are formed over each aperture simultaneously, then the aperture spacing dimensions  100  and  102  must be determined such that they are at least 6 expanded bubble diameters  12  long. These dimensions may be shortened, for example, to 3 expanded bubble diameters  12  if the Cavitation bubbles are formed over every other aperture, as long as there remains at least 6 fully expanded bubble diameters between any two cavitation bubbles in the array being formed simultaneously. For cavitation bubble spacing closer than the 6 expanded bubble diameters, there is some probability (increasing with decreasing bubble spacing) that the re-entrant micro-jets produced on collapse of the adjacent cavitation bubbles will be directed toward each other, as opposed to being directed through the apertures. This is undesirable.  
         [0041]    The array of cavitation bubbles may be produced by a number of techniques in accordance with the present invention. For example, an array of lasers as illustrated in FIGS. 2, 3, and  6  may be employed. Or a single laser having a fiber optic array employing multiple collimators located over each aperture  4   a,    4   b  may also be used. Additionally, a single laser and collimator may be scanned over the aperture array such that each “firing” of the pulse laser produces a focus volume of light energy over the appropriate aperture position. The same process may also be utilized with the x-ray source. Additionally, the aperture location may be moved by XYZ stage  60  while holding the aperture mask  6   a  fixed over the work surface  8 , utilizing a single laser or x-ray&#39; source. For the case of the electrical discharge, a multiple electrode array may be used, or the array may be positioned under a single electrode pair via the XYZ stage. An array of cavitation bubbles may also be produced by ultrasound techniques. It is well known to those skilled in the art that many ultrasound transducers produce a three dimensional array of cavitation bubbles in a tank of fluid corresponding to a standing wave pattern of sound waves in the fluid. By creating and positioning such a standing wave pattern over the aperture mask  6   a,  cavitation bubbles formed due to the ultrasound will collapse, directing the previously described re-entrant micro-jets through the apertures to the work surface. The properties of the ultrasound generated cavitation bubbles should conform to previously determined requirements as discussed in FIG. 1E.  
         [0042]    [0042]FIG. 9 is a schematic view of an apparatus for the welding of small particles in a cavitation introduced re-entrant micro-jet in accordance with another embodiment of the present invention. Introduction of particulate matter  112  into the re-entrant micro-jet may result in the welding of the particles to each other and/or to the work surface  8 . Small particles  108  stored in a container  106  are released into solution via valve  110  in the vicinity of the focus volume  2 , where a cavitation bubble will be nucleated, as previously described. Particles  108  may be stored in a dry form, but preferably are mixed and suspended in a compatible fluid. Once in solution, these particles  112  will accumulate at the gas liquid interface of the cavitation bubble, and may be entrained into the re-entrant micro-jet as the Cavitation bubble collapses. The very high impact forces of the micro-jet hitting the work surface causes the welding of these particles to each other and the work surface  8 .  
         [0043]    [0043]FIG. 10 is a table of parameters for the application of various pulsed Gaussian TEM00 lasers for a number of embodiments in accordance with the present invention.  
         [0044]    [0044]FIG. 11 is a table of parameters for the application of an electric discharge for one embodiment in accordance with the present invention.