US 20030116176 A1
An apparatus and process for cleaning residual matter, photoresist and other foreign materials off wafers, substrates and semiconductor work pieces including photomasks, compact discs, flat panel displays using megasonic acoustic wave action techniques in conjunction with supercritical fluid cleaning processes, and in particular, for coupling megasonics techniques with supercritical carbon dioxide processing with co-solvents and surfactants, using cycles of soak, rapid decompression and flushing, to improve cleaning capability and to remove submicron particles from the surfaces of wafers.
1. A process for cleaning semiconductor wafers comprising the steps of:
soaking a wafer in a supercritical phase cleaning fluid mixture at an elevated pressure,
applying a megasonic acoustical wave action to the cleaning fluid mixture,
rapidly reducing the elevated pressure to a substantially lower pressure, and
flowing a supercritical flushing fluid across the wafer.
2. The process of
3. The process of
4. The process of
5. The process of
6. The process of
7. The process of
8. The process of
suspending said wafer in a substantially horizontal plane with the preferred side down.
9. The process of
using a process chamber connected to a source of supercritical phase carbon dioxide,
placing a wafer within and closing the process chamber,
filling the process chamber with said supercritical phase carbon dioxide,
pressurizing the process chamber to an elevated supercritical pressure,
adding co-solvents and surfactants to said supercritical phase carbon dioxide thus forming a supercritical phase cleaning fluid mixture,
said soaking a wafer comprising soaking said wafer in said process chamber in said fluid mixture at the elevated pressure.
10. The process of
repeating said steps of pressurizing, adding, soaking, applying, rapidly reducing, and flushing.
11. The process of
12. The process of
13. The process of
14. The process of
15. The process of
16. An apparatus for cleaning semiconductor wafers comprising
a closable cleaning vessel connected to a source of cleaning fluid and having a fluid outlet, said vessel being capable of sustaining the cleaning fluid at supercritical phase temperature and pressure,
said vessel configured with at least one megasonic transducer.
17. The apparatus of
18. The apparatus of
19. The apparatus of
20. The apparatus of
21. The apparatus of
22. The apparatus of
23. The apparatus of
24. The apparatus of
25. The apparatus of
26. The apparatus of
27. An apparatus for cleaning semiconductor wafers comprising
a closable cleaning vessel connected to a source of cleaning fluid components and having at least one exhaust port, said vessel being capable of sustaining the cleaning fluid at supercritical phase temperature and pressure,
said vessel configured with at least one megasonic transducer on the lower platen,
an inverted wafer holder pedestal
apparatus for wafer rotation.
28. The apparatus of
29. The apparatus of
30. The apparatus of
31. An apparatus for cleaning both sides of a semiconductor wafer comprising
a closable cleaning vessel consisting of a base, a lid, and a wafer holder for holding a wafer therein between when said vessel is closed, said vessel having at least one exhaust outlet, said vessel being capable of sustaining the cleaning fluid at supercritical phase temperature and pressure,
each of said base and said lid configured with at least one large surface area megasonic transducer and at least one fluid inlet,
apparatus for wafer rotation.
32. The apparatus of
 This application relates and claims priority for all purposes to pending U.S. application ser. No. 60/351,524, filed Jan. 24, 2002, and is a continuation in part to pending U.S. application Ser. No. 09/837,507 filed Apr. 18, 2001, and Ser. No. 09/861,298 filed May 18, 2001.
 The Applicant has amply described its cleaning process using supercritical fluid, particularly carbon dioxide, in its parent applications, which are hereby incorporated by reference. The instant invention is susceptible of many embodiments. Below is a general description of the steps of a preferred embodiment of the invention, and a description of the relevant aspects of the apparatus.
 The wafer or substrate is immersed in a mixture of supercritical fluid and appropriate co-solvent. The choice of co-solvent will depend upon the materials to be cleaned from the surfaced of the substrate. A suitable surfactant can also be added. A high pressure soaking period will allow the supercritical carbon dioxide and co-solvent to penetrate the materials to be removed. Swelling of these materials will occur along with a debonding of them from the surface of the substrate. The high pressure will permeate the material as well, setting the stage for the rapid decompression pulse. At this point the megasonic transducers will be activated and the controlled rapid depressurization of the system to a lower supercritical phase pressure.
 During the depressurization or decompression pulse there is a rapid expansion of carbon dioxide within the photoresist polymer matrix. At the same time, the continuous pattern of shock waves and acoustic streaming pattern generated by the large surface area megasonics transducer array enhances the breakup and delaminating of the photoresist from the substrate. The acoustic streaming also transfers momentum to fine particles and facilitates their transport off the substrate surface. Coupled with the acoustic streaming action is the rapid outflow of fluid over the substrate surface, applying further removing force to the loose particles.
 The combination of the megasonics and the rapid decompression and fluid outflow mechanisms applied in a medium of supercritical carbon dioxide and co solvent provides a significant advantage over other cleaning methods for separating the unwanted films from the surface of the substrate along with submicron particles. After the decompression step, clean supercritical phase carbon dioxide is then flowed over the surface and the chamber flushed to carry away the unwanted debris. After completion of the cleaning cycle, the megasonics transducer array is deactivated and the process vessel is returned to atmospheric pressure and the substrate unloaded.
 To highlight the key aspects of the process, we are in effect applying five mechanisms jointly to overcome the five forces described in our background section, in order to breakup, loosen and remove the submicron particles without undue damage to the microstructures present on the wafer.
 1. Supercritical carbon dioxide is itself an excellent solvent for non-polar materials.
 2. The use of a suitable co-solvent or mixture of co-solvents to aid in the solubility of the organic materials by diffusing into the polymer matrix with the carbon dioxide.
 3. A high pressure soak to swell and weaken the polymer materials at high pressure, followed by a rapid depressurization to debond and delaminate the swollen and weakened polymer from the surface.
 4. Megasonics action in the supercritical fluid medium to provide turbulence and energy to enhance the soaking and swelling process; the rendering process during decompression and flushing; and the moving of the loose and broken polymer particles off of the surface of the substrate.
 5. Surfactants to change the zeta potential of the substrate/particle in order to aid in removing very small particles from the surface.
 Unique aspects of the apparatus include the incorporation of the megasonics transducers or multi-segment transducer arrays of significant surface area within the pressure vessel for enhancing the other removal forces or mechanisms; optional rotation or alternate orientation of the wafer or substrate with respect to the transducers for a more distributed megasonics effect on the wafer surface, and through-chamber flow control of the supercritical carbon dioxide for using flow velocity to reinforce the other removal forces. Using megasonics action in combination with supercritical fluid cleaning techniques including high pressure soak and very rapid decompressive pulse cycles allows for faster process times and removal of smaller particles from surfaces with less damage to microstructures.
 A preferred embodiment of the invention utilizes the principle steps:
 1. Fill the process chamber holding the substrate or wafer, with supercritical phase carbon dioxide and pressurize to a higher pressure within the supercritical phase working range of the process chamber.
 2. Add co-solvents and/or surfactants to the chamber, suitable for attacking the residue to be removed.
 3. Soak the wafer in the SCCO2 mixture at the high pressure for a period of time, allowing the fluid mixture and pressure to permeate the residue
 4. Initiate megasonics action within the chamber with a large surface area transducer or segmented array, in proximity to the surface to be cleaned.
 5. Apply a very rapid decompression pulse to the chamber to break up and loosen the residue, continuing the megasonics action through the decompression pulse and associated outflow of supercritical mixture and loose residue to a new equilibrium point at a lower supercritical pressure.
 6. Flush the chamber with clean supercritical fluid, continuing the megasonics action, to further loosen and remove remaining residue.
 7. Raise the chamber pressure again and repeat steps 4-6 as often as needed.
 8. Terminate the megasonics action and fluid flow, depressurize the chamber to atmosphere and remove the wafer.
 Variations on these steps and/or additional process steps, including positive pressure pulsing, liquid phase processing, temperature variations, density variations, wafer spinning, fixed or moving fluid spray bars and nozzles and mechanical agitation are within the scope of the invention, as are configurations providing for multiple wafer batch processing and alternate orientation processing.
 Process conditions for cleaning 200-300 mm wafers include the following preferences:
 1. Carbon dioxide (CO2) is the preferred process gas for reasons well understood in the industry, although the invention is inclusive of other suitable fluids.
 2. Co-solvents are chosen based on the selection of process gas and the chemistry of the material(s) to be removed/cleaned.
 3. Surfactants are selected on the basis of the prior selections.
 4. Initial chamber pressure is at least 5000 psi; temperature is 80 C degree.
 5. A sufficient soak period is required, generally the longer the better up to the limits of an acceptable total cycle time. A two minute soak is used in the preferred embodiment.
 6. Apparatus capability for conducting very rapid depressurization to 1500 psi, to be followed by repressurization to at least 5000 psi; the large differential being significant in the effect of the decompressive pulse.
 7. Megasonics transducer array power inputs in the order of 5-10 Watts/cm2, with transducers capable of megasonic action in a supercritical CO2 medium at the working pressures and temperatures.
 8. System capacity for flushing the chamber with clean supercritical CO2.
 9. Depressurization of the chamber to atmosphere for removal of the wafer(s).
 A useful alternative is to conduct the depressurization pulse step so as to bring the chamber to a condition of higher density of the supercritical mixture, at relatively lower temperature, where megasonics transducer action is more pronounced due to the higher density and acoustic streaming velocity For example:
 1. Pressurize to at least 5000 psi in CO2 at 40 degrees C.
 2. Add co-solvent and soak.
 3. Pulse (very rapidly depressurize) to 2200 psi.
 4. Flow clean supercritical CO2 through the chamber and concurrently apply megasonics agitation.
 5. Depressurize to atmosphere.
 The actual process will depend upon the material(s) being removed. The first preferred embodiment described is representative for photoresist removal. The alternative described above is more representative for residue removal. A third alternative described below would be used for submicron particle removal.
 1. Pressurize to at least 5000 psi in CO2 at 40C.
 2. Add surfactant and soak.
 3. Apply megasonics agitation.
 4. Pulse (very rapidly depressurize) to 2200 psi while maintaining megasonics agitation.
 5. Flow clean supercritical CO2 through the chamber while maintaining megasonics agitation.
 6. Terminate megasonics agitation and depressurize to atmosphere.
 There are three considerations in establishing the maximum limit of pressure, as well as rate and range of pressure changes: materials and hardware design and associated regulatory limits; the type of material and structural aspects of the wafer under process; and the effect of the high pressure, and rate and range of pressure change on the cleaning process itself. As a practical matter, the first two considerations are the limiting factors. The present preferred higher pressure of about 5000 psi should be understood to be merely a practical consideration of pressure vessel regulations and contemporary pressure vessel construction common to the industry, rather than a process-based preferred pressure limit. Yet higher pressures, for example 7500 to 10,000 psi and even much higher pressures will generally provide greater effectiveness in the application of this supercritical fluid cleaning process, but higher pressures are accompanied by attendant issues of equipment design, contamination of the pressure vessel, safety, cost, impact on the wafer material and structure, and process cycle time to range the pressure from ambient to full pressurization.
 Referring now to FIGS. 1 and 2, there is shown the inverted pressure vessel 10 and process chamber of a first embodiment apparatus, shown in an open condition with a stationary inverted chamber section 12 above a vertically movable lid section 14. The lid section is vertically adjustable between a lower open position and a raised, closed position. The lid section 14 includes a load/unload pedestal 16 piercing a lower heated platen 18, where individual wafers 1 are placed and removed sequentially for processing, robotically for example. There are peripheral CO2 fluid return nozzles 24 outboard of the platen 18, connected to fluid return lines 26, for removing fluid from the chamber. The pedestal 16 is vertically movable within the lid section 14, and may be moved concurrently with the lid section 14 to correctly position the wafer on the lid platen as the lid section 14 is closing to the chamber section 12. A pedestal rotating mechanism 17 provides means for rotating the wafer in either or alternately in both directions for enhanced and more uniform cleaning effect.
 Although the preferred embodiments provide for a fixed inverted chamber, it will be appreciated and is within the scope of the claims that lid, wafer pedestal, and chamber movement is relative; that the inverted chamber may be fixed and the lid moved vertically, or vice versa, or both be vertically movable, in order to achieve closure.
 Referring particularly to FIG. 2, looking upward into the inverted chamber section 12 in this embodiment, it is seen to be configured with a ceiling mounted, downward directed, full disc megasonics transducer 30, with center hole 32 which accommodates a fluid supply nozzle 20, which is supplied by CO2 fluid supply feedthrough 22 from an external source. The chamber section is also configured with sealed electrical supply feedthroughs 34 to power the transducer 30. The chamber has sufficient head space below the transducer to accommodate a wafer when the underside cover section 14 is raised to a closed position, plus further spacing above the wafer sufficient for the radial flow of CO2 over the wafer surface through the chamber volume from center to edge, to the CO2 fluid return outlets 24.
 The lower heating platen 18 of the lid section is a heat exchanger connected to an external source through connections 19 for providing heating and cooling capability to the wafer and the chamber in general. The inverted chamber section 12 may be similarly equipped, for additional chamber general heating/cooling control and/or for cooling the transducer. Other or additional means of heating the platen, such as electrical, are within the scope of the invention.
 Liquid and supercritical CO2, as well as co-solvent and surfactant, are selectively available to the chamber as required from a supply/support system such as previously described by this Applicant in prior applications. Inflow of the CO2 mixture through the chamber is downward and then radial, over the wafer surface. The megasonics action is applied from just above the surface of the wafer. The very rapid decompression and flow of CO2 onto and over the wafer, coupled with the megasonics action, loosens and pushes debris and particles off the wafer surface and out of the process chamber. Separator vessels catch particles and co-solvent. The CO2 is either exhausted or recycled.
 Referring to FIG. 3, there is shown an alternative embodiment to the FIG. 2 ceiling mounted transducer 30. An inverted chamber section 12 is in this embodiment configured with four semicircular, ceiling mounted, downward directed, transducer array sections 40 arranged about a multi-port, heated fluid nozzle 42. The array sections 40 and heated nozzle 42 are supplied by electrical supply feedthroughs, a CO2 fluid supply, and heating fluid feedthroughs (not shown in this view) similar those of the FIGS. 1 & 2 embodiment. The chamber has sufficient head space below the transducer to accommodate a wafer when the underside cover is raised to a closed position, plus further spacing above the wafer sufficient for the radial flow of CO2 over the wafer through the chamber from center to edge, to the CO2 fluid return outlets.
 Each megasonics array section 40 is separately powered, and all are controlled by a common controller such that sequencing of power levels, alternating current phase, and on-off switching can be accomplished for heat control of the array segments or sections, and/or any desired process effects.
 Referring to FIG. 4, there is shown yet another alternative to the transducer array of the embodiment of FIGS. 1 and 2. The inverted chamber section 12 in this embodiment is configured with four pie-shaped, ceiling mounted, downward directed, transducer array sections 50 arranged about a fluid supply center nozzle 52. The vessel is also configured with a CO2 fluid supply feedthrough for the nozzle and sealed electrical supply feedthroughs to power the transducer (not shown in this view) similar to FIG. 1. As in prior embodiments, the chamber has sufficient head space below the transducer to accommodate a wafer when the underside cover is raised to a closed position, plus further spacing above the wafer sufficient for the radial flow of CO2 over the wafer through the chamber from center to edge, to the CO2 fluid return outlets. The array sections 50 are separately wired and controllable for coordinated operation similarly to the array sections 40 of FIG. 3.
 In another enhancement of many embodiments, there is added to the pressure vessel or directly to the wafer pedestal the additional capability to rotate the wafer during the cleaning process, similar to the pedestal rotating mechanism 17 of FIG. 1. Means for rotating the wafer may be other than mechanical. A portion of the through-flow of fluid through the chamber, for example, may be directed to rotating the wafer and/or wafer support. The rotation of the wafer enhances the uniformity of the process and applies additional forces to help remove residue and particles from the surface. The wafer rotation capability can be incorporated with any of the embodiments described herein.
 Referring to FIG. 5, there is shown still yet another alternative to the transducer array of the prior embodiments. The inverted chamber section 12 in this embodiment is configured with a dual checkerboard array of square, multi-segment, ceiling mounted, downward directed, transducer array sections 60 arranged diagonally so as to occupy two opposing quadrants of the ceiling surface area, spanning the chamber corner to corner. In the remaining pair of opposing quadrants of the ceiling, there are disposed two fluid supply nozzles 62, directed at an angle so as to strike and sweep the wafer surface beneath an adjacent array section 60. This chamber embodiment is coupled with wafer rotation capability such that the entire wafer surface is exposed to the dual effects of the fluid spray and the megasonics action, in addition to the other mechanisms of the cleaning process. The wafer rotation may be accomplished by the pressure of the fluid spray directed from nozzles 62 against the wafer or wafer support assembly, or by other fluid flow dynamics within the vessel such as the through flow of FIGS. 5 and 6.
 The vessel of this embodiment is also configured with CO2 fluid supply feedthroughs for the nozzles and sealed electrical supply feedthroughs to power the transducers. As in prior embodiments, the chamber has sufficient head space below the transducers to accommodate a wafer when the underside cover is raised to a closed position, plus further spacing above the wafer sufficient for the sprayed fluid and subsequent radial flow of CO2 over the wafer through the chamber from center to edge, to the CO2 fluid return outlets. The segments of array sections 60 are wired and controllable similarly to the array sections of prior embodiments. The size of the segments can be optimized for cost and performance.
 A variation of this embodiment is having each of the two checkerboard arrangements of array segments divided into the equivalent of a set of “red” segments and a set of “black” segments such that each set consists of only diagonally adjacent segments. This permits switching of power between the two sets for better heat and power management of the array. Alternatively, the two arrays 60 can be alternated in operation, wafer rotation providing for a uniform effect on the wafer in either case.
 A further variation of this and other multi segment transducer array embodiments provides that megasonic transducer segments and other than megasonic transducer segments such as ultrasonic are interspersed in the array, providing a dual-frequency range sonic action capability to the pressure vessel.
 Referring to FIGS. 6 and 7, there is shown the inverted pressure vessel and process chamber 70 of a fourth embodiment apparatus, shown in an open condition with a stationary inverted chamber section 72 above a vertically movable lid section 74. The lid section is vertically adjustable between a lower open position and a raised, closed position. The lid section includes a load/unload pedestal 76, where individual wafers 1 are placed and removed sequentially for processing, robotically for example. The pedestal shaft pierces a lower heated platen 78. Platen 78 is supported by heating liquid feedthrough lines 79 for controlling heating and cooling within the chamber. The pedestal is vertically movable within the lid section, and may be moved concurrently with the lid section to correctly position the wafer on the lid platen as the lid is closing to the chamber.
 The inverted chamber in this embodiment is configured with a ceiling mounted upper heated platen 80 supported by heating liquid feedthrough lines 82, and with a CO2 process fluid inlet 84 on one side and an opposing side process fluid outlet 86. The opposing inlet and outlet provide for a directional flow of process fluid over the top of the wafer and across the diameter of chamber. Supplemental outlets 87 in lid section 74 provide for low side full drainage, including underside and heavier particles.
 There are shown two pairs of semicircular, wall mounted, radially inward directed, sonic transducer array sections 90 arranged as to have a first pair flanking the CO2 inlet 84. If the process fluid flow is to be limited to one direction through the chamber, referred to here as the fourth embodiment, then this first pair of transducers provides for the propagation of megasonic streaming action generated by the first pair of transducers to be aligned with the fluid flow so as to attain the highest combined removal force from these two mechanisms in one direction across the wafer surface. Wafer rotation is preferably incorporated into this embodiment.
 If the process fluid supply/support system is configured for bi-directional, alternating fluid flows between inlet 84 and outlet 86, referred to here as the fifth embodiment, then the second pair of transducers 90 is disposed around outlet 86, for supplying a megasonic streaming action in the reverse direction coinciding with the reverse direction fluid flow.
 The vessel is also configured with sealed electrical supply feedthroughs 92 to power and control transducer sections 90 as in other embodiments. The chamber has sufficient head space to accommodate a wafer between the upper and lower heated platens when the underside cover is raised to a closed position, plus further spacing above the wafer sufficient for the cross chamber flow of the CO2 mixture over the wafer.
 Supercritical CO2, as well as co-solvent and surfactant, are selectively available to the chamber as required from a supply/support system such as previously described by this Applicant. Forward flow of the CO2 mixture through the chamber is through the side inlet 84, over the wafer surface and out the outlet 86. There may be vanes or other flow control mechanisms incorporated into the chamber designs, as have been described in other of the Applicant's patent disclosures, to provide for improved distribution of the fluid flow across the wafer surface. The megasonics action is applied from both sides of the inlet 84 source of process fluid for forward flow, or the outlet 86 source of process fluid if in a reverse flow, or in an alternating basis in a bi-directional flow pattern, as to align the megasonics streaming action with the fluid flow. The very rapid decompression and flow of CO2 onto and over the wafer, coupled with the megasonics action, loosens and pushes particles off the wafer surface and out of the process chamber. Separator vessels catch particles and co-solvent. The CO2 is either exhausted or recycled.
 In a variation combining features of the embodiments of FIGS. 1 and 2, and FIGS. 6 and 7, the inflow of the CO2 mixture through the chamber is downward through a ceiling mounted nozzle or nozzle array and then radially outward, over the wafer surface to outlets on the side of the vessel similar to the embodiment of FIGS. 1 and 2. The megasonic transducers are two pairs of semicircular, wall mounted, radially inward directed megasonics transducers positioned on each side of the two CO2 fluid outlets, similar to the embodiment of FIGS. 6 and 7.
FIGS. 8 and 9 describe important variations to the above embodiments, applicable where the wafer has a preferred side, normally considered to be the top side, to which the maximum cleaning effort is to be directed. The chamber is arranged right side up, as is distinguished from previous embodiments, and has a topside lid, but the wafer is turned upside down by a wafer transport device or robot and placed in the chamber with the preferred side down for cleaning, thus taking advantage of the force of gravity in addition to the other process mechanisms described herein for removing unwanted material from the wafer surface.
 Any of the side or full disk surface area or partial surface area sonic array designs previously described can be incorporated in the inverted wafer embodiment. The large area arrays would, of course be configured on the floor of the chamber, directed upward towards the wafer surface. The supercritical soak, very rapid decompression pulse, and flush cycle previously described is fully applicable to these sonic action embodiments. Megasonics is the preferred sonic frequency range for the supercritical fluid processes, although ultrasonics with liquid phase processing is within the capability of the apparatus, as well.
 Referring to FIG. 8, vessel 100 has a lower chamber section 102 with heating platen 104 disposed across the bottom surface area, a perimeter fluid inlet 106 disposed at 180 degrees from a perimeter fluid outlet 108. Heating liquid through feed lines 105 support platen 104. A pair of perimeter transducers 112 are disposed around each of inlet 106 and outlet 108. Electrical throughfeeds 113 support each transducer 112. Perimeter wafer supports 110, preferably at least three, are distributed around the chamber circumference so as to support an upside down wafer 1 when deposited thereon by a wafer transport mechanism.
 Lid section 120 is configured with edge seal 122, and is configured with heating platen 124 connected by heating liquid throughfeed lines 125 to a source of heating liquid, for heating the chamber and the wafer. Fluid flow of the CO2 fluid is horizontal across the underside of the wafer from the inlet to the outlet, with bi-directional flow being available as described in prior embodiments. Sonic action is applied with the appropriate pair of transducers 112, to align streaming action with fluid flow as previously described.
 Referring to FIG. 9, vessel 200 has a lower chamber section 202 with multiple perimeter fluid outlets 206 disposed about the circumference of the chamber. A large surface area megasonics transducer array 212 is disposed on the bottom of the chamber, directed upward, and supported by electric feedthrough lines 213. A centerpoint fluid nozzle 208 pierces the transducer array, fed by fluid throughfeed line 209. Upper shaft mounted rotatable wafer support system 210, having radially extending arms 211, preferably at least three, reaching to the chamber circumference so as to grip and support an upside down wafer over the transducer array when deposited therein by a wafer transport mechanism.
 Lid section 220 is configured with edge seal 222, and is further configured with heating platen 224 connected by heating liquid throughfeed lines 225 to a source of heating liquid, for heating the chamber and the wafer. Fluid flow of the CO2 fluid is directed through centerpoint nozzle 208 and radially across the underside of the wafer from the center to the perimeter. Sonic action is applied as previously described.
 Wafer support in the inverted wafer embodiments may be structurally connected to the lid or chamber sections via a perimeter based wafer support system similar to FIG. 8, rather than the shaft mounted system illustrated in FIG. 9. In either case, means for wafer rotation can be incorporated for the same reasons as previously described, such as by a rotable wafer edge support ring driven by fluid flow or other mechanical means.
 It will be readily apparent that features of the various figures may be incorporated in other useful combinations, all within the scope of the invention. For example, the invention extends to a two sided embodiment of the invention configured for applying megasonics action and fluid flow to both sides of a wafer. Upper and lower components of the vessel may be configured with nozzles and large area sonic arrays similar to any of FIGS. 2, 3, 4, 5, while incorporating perimeter outlets similar to FIG. 9. A wafer perimeter support assembly, fixed or rotable as is preferred, may extend from either the upper or lower component of the vessel. Rotational capability may be induced by direct fluid flow as in FIG. 5 or by upper or lower side mechanical means similar to FIG. 1. An axial mounted wafer support system for cleaning both sides may have radial vanes extending from the center shaft and terminating in wafer perimeter support perches so as to permit effective cleaning action to the near side of the wafer.
 These and various other embodiments of the apparatus within the scope of the invention, provide for conducting the below listed embodiments and other variants of the process:
 The initial process will be substantially the same for each: i.e. pressurize the chamber to the desired supercritical CO2 pressure and temperature; add appropriate co-solvent and/or surfactant if desired, and soak the substrate or wafer in this high pressure supercritical environment, then conduct one or more of the following combinations of sequences:
 Surfactants are known to help modify the zeta potential (charge) of the particle and/or substrate surface. Other embodiments of the invention process introduce a surfactant with the CO2 after the initial cleaning for the purpose of removing loose particles. For instance, for stripping photoresist one may use a co-solvent such as propylene carbonate to help swell and debond the resist. Pulsing and megasonics will strip the resist but there may remain small particles that need to be removed. In this case a surfactant can be added to the CO2 mixture.
 It should be noted that the apparatus is intended to be readily adapted to an automated production line, such as for robotic loading and unloading off the extended wafer pedestal when the vessel is open. There is also a notable variant of the invention that eliminates one of the forces of concern; gravity. The top platen assembly may incorporate means for holding the wafer upside down and with the capability for rotation in accordance with the figures and description above.
 It is within the scope of the invention, as will be appreciated by those skilled in the art from the description and drawings provided, that a wafer edge support system can be configured by the principles described and illustrated to support a wafer between two megasonic transducer arrays for cleaning both sides in a single cleaning cycle.
 These and other embodiments within the scope of the invention and the claims that follow will be readily apparent to those skilled in the art from the above description and attached figures. For example, there is a process for cleaning semiconductor wafers comprising the steps of soaking a wafer in a supercritical phase cleaning fluid mixture at an elevated pressure, rapidly reducing the elevated pressure to a substantially lower pressure, applying a megasonic acoustical wave action to the cleaning fluid mixture, and flowing a flushing fluid mixture across the wafer while draining the cleaning fluid mixture. The cleaning fluid mixture may remain in supercritical phase at the lower pressure.
 Further, the steps of reducing pressure and applying megasonic action and flowing the flushing fluid mixture may be undertaken at substantially concurrently. The cleaning fluid mixture may comprise carbon dioxide and a co-solvent. The flushing fluid mixture may comprise carbon dioxide and a surfactant.
 As another example, there is an apparatus for cleaning semiconductor wafers comprising a closable cleaning vessel connected to a source of cleaning fluid components and having an exhaust port, where the vessel is capable of sustaining the cleaning fluid at supercritical phase temperature and pressure, and the vessel is configured with a megasonic transducer. The cleaning fluid components may comprise carbon dioxide, and may further comprise supercritical phase carbon dioxide, co-solvent, and surfactant.
 The vessel may comprise an inverted cleaning chamber, a vertically movable underside lid, where the lid is configured with a vertically movable wafer pedestal. The transducer may be at least one ceiling mounted, downward directed transducer and/or be at least one lower platen mounted, upward directed transducer and/or be at least one side mounted, horizontally directed transducer.
 There are numerous other examples of the invention. For example, there is a process for cleaning semiconductor wafers consisting of the steps of soaking a wafer in a supercritical phase cleaning fluid mixture at an elevated pressure, applying a megasonic acoustical wave action to the cleaning fluid mixture, rapidly reducing the elevated pressure to a substantially lower pressure, and flowing a supercritical cleaning fluid across the wafer. The cleaning fluid mixture preferably remains in supercritical phase at the lower pressure. The step of applying megasonic acoustical wave action may be conducted concurrently with the step of soaking, and/or with the steps of rapidly reducing pressure and flushing. The cleaning fluid mixture may be carbon dioxide and a co-solvent. The flushing fluid may be carbon dioxide and a surfactant.
 Also, the wafer may have a preferred side to which the cleaning is directed, and where the process further consists of the initial step of suspending the wafer in a substantially horizontal plane with the preferred side down.
 Further, the process may include the preliminary steps of using a process chamber connected to a source of supercritical phase carbon dioxide, placing a wafer within and closing the process chamber, filling the process chamber with the supercritical phase carbon dioxide, and pressurizing the process chamber to an elevated supercritical pressure. Co-solvents and surfactants are added to the supercritical phase carbon dioxide forming a supercritical phase cleaning fluid mixture, either before or after it is pumped into the process chamber. Then soaking the wafer in the process chamber in the fluid mixture at the elevated pressure. And the steps of pressurizing, adding, soaking, applying, rapidly reducing, and flushing may be repeated as often as needed.
 The elevated pressure may be at least 5000 psi. The substantially lower pressure may be about 1500 psi. The soaking step may have a period of not more than about two minutes. The temperature within the process chamber may be maintained at about 80 degrees Centigrade. The megasonic acoustical wave action being applied to the surface of the wafer may be done with a transducer array having power input in the range of 5-10 watts/cm2.
 As another example of the invention, there may be an apparatus for cleaning semiconductor wafers consisting of a closable cleaning vessel connected to a source of cleaning fluid, having a fluid outlet, and being capable of sustaining the cleaning fluid at supercritical phase temperature and pressure, where the vessel is configured with at least one megasonic transducer. The cleaning fluid may be supercritical carbon dioxide and may be a mixture of supercritical carbon dioxide and suitable co-solvents, and/or surfactants.
 The vessel may have an inverted cleaning chamber, and a vertically movable underside lid, there the lid is configured with a vertically movable wafer support system. And the wafer support system may be configured for supporting a wafer upside down in the chamber. Or the vessel may have an upright cleaning chamber and an inverted wafer support system. Further, the vessel may have an inverted cleaning chamber, and a vertically movable underside lid, where the lid is configured with a rotable wafer holding mechanism.
 The transducer may be at least one ceiling mounted, downward directed transducer. The transducer may be a multi-segment transducer array. It may be configured for inter-segmentally variability in operational parameters. The transducer may be one or more side mounted, horizontally directed transducers.
 As yet another example, there is an apparatus for cleaning semiconductor wafers consisting of a closable cleaning vessel connected to a source of cleaning fluid and having at least one exhaust port, capable of sustaining the cleaning fluid at supercritical phase temperature and pressure, and configured with at least one megasonic transducer on the lower platen, and having an inverted wafer holder pedestal apparatus mechanized for providing wafer rotation. The transducer may be at least one, and preferably at least two side-mounted, horizontally directed transducers. Or the transducer may be a multi-segment, large area transducer array.
 A further example is an apparatus for cleaning both sides of a semiconductor wafer, having a closable cleaning vessel consisting of a base, a lid, and a wafer holder for holding a wafer in the chamber formed between the base and lid when the vessel is closed. The vessel has at least one exhaust outlet, and is capable of sustaining the cleaning fluid at supercritical phase temperature and pressure. Further, the base and lid are configured with at least one large surface area megasonic transducer and at least one fluid inlet, and an apparatus for rotating the wafer holder.
 Other and various embodiments within the scope of the invention and the appended claims will be apparent to those skilled in the art from the description and figures included.
FIG. 1 is a cross section view of the inverted process vessel of the first embodiment having a wafer pedestal in the underside lid and a large surface area transducer array mounted in the ceiling of the process chamber, configured around a centerpoint fluid spray nozzle.
FIG. 2 is an underside view upward into the inverted process chamber of the first embodiment, looking at the transducer array of FIG. 1 with a single port fluid spray nozzle disposed at the center of the chamber.
FIG. 3 is an underside view upward into the inverted process chamber of the second embodiment, looking at a multi-segment transducer array configured around a multi-port fluid spray nozzle disposed at the centerpoint.
FIG. 4 is an underside view upward into the inverted process chamber of the third embodiment, looking at a multi-segment transducer array configured around the single port fluid spray nozzle
FIG. 5 is an underside view upward into the inverted process chamber of an embodiment similar to that of FIG. 1, illustrating a large area, multi-segment transducer checkerboard array of square segments, occupying less than all the ceiling area of the chamber, where the wafer pedestal has rotational capability for providing full surface area exposure of the wafer to the transducer array.
FIG. 6 is a cross section view of the inverted process vessel of a fourth embodiment having a wafer pedestal in the underside lid and a multi-segment perimeter transducer array symmetrically disposed around perimeter fluid inlets and outlets.
FIG. 7 is an underside view upward into the inverted process chamber of the fourth embodiment, looking at the multi-segment perimeter transducer array, and illustrating the overlapping transmission pattern of the inlet side transducers across the process chamber.
FIG. 8 is a cross section view of an embodiment having a perimeter based wafer support mechanism for holding and exposing the underside of wafers to the side mounted transducer arrays and underside horizontal fluid flow.
FIG. 9 is a cross section view of another embodiment having an underside large area transducer array and center nozzle, and a ceiling side shaft mounted rotatable wafer support mechanism for holding and exposing the underside of a wafer to the transducer array, where fluid flow is vertically upward through the center nozzle and radially out to outlet ports on the side of vessel.
 This invention relates to apparatus and processes for cleaning residual matter, photoresist and other foreign materials off wafers, substrates and other work pieces including photomasks, compact discs, flat panel displays, and in particular, to cleaning semiconductor wafers, using acoustic wave techniques including megasonics in conjunction with supercritical fluid soaking, rapid decompression, flushing, and related process mechanisms to enhance the cleaning capability and remove submicron particles.
 Among the art related to ultrasonics and supercritical fluids, there is: U.S. Pat. No. 5,337,446 “Apparatus for applying ultrasonic energy in precision cleaning”, and U.S. Pat. No. 5,013,366 “Cleaning process using phase shifting of dense phase gases”.
 With respect to the need to remove submicron particles from semiconductor surfaces, ultrasonic acoustical techniques in unpressurized liquid baths have been used. Ultrasonics causes damage to the microstructures on the semiconductor surface due to cavitation. Calculations done by Spall et al. for turbulent flow of supercritical phase carbon dioxide to remove particles from a semiconductor wafer indicate that extremely high velocities are required, ˜200 cm/s for particles 0.1 micron in diameter. High velocities are needed apparently due to the formation of a boundary layer near the surface. Within this boundary layer, there is a shear in the velocity field leading to a stress which rolls particles away from any given position. The wall shear in case of turbulent flow is much greater than in laminar flow. In laminar flow, the boundary layer is relatively thicker, allowing the velocity to change gradually to its stream value. In case of turbulent flow, the viscous sublayer which develops right next to the wall is much thinner, causing a more abrupt change in the velocity field, thereby setting up a larger wall shear.
 Adhesion forces between a particle and a surface vary linearly with the particle diameter. Removal forces vary as the second power of the particle size. Therefore particle removal becomes more difficult as the particle size decreases. The lift force depends inversely on fluid viscosity, favoring supercritical fluid processes. For the drag force a higher viscosity is preferred, which is not favorable for supercritical fluid processes. However the boundary layer thickness would be much thinner.
 The description and table below indicate that there are many forces that keep particles on surfaces and make cleaning difficult.
 Definition of terms in above table:
 FvdW=Van der Waals forces have 3 components—interactions between permanent dipoles (van der Waals-Keesom force), interaction between permanent dipoles and induced dipoles (van der Waals-Debye force) and interactions between induced dipoles (van der Waals-London force).
 Fdbl=electric double layer force—dominates for small particles (<5 microns). A surface contact potential is created between two different materials based upon each material's respective local energy state. Resulting surface charge buildup needed to preserve charge neutrality sets up a double layer charge region which creates the electrostatic attraction.
 F drag=drag force—function of the fluid velocity
 F lift=lift force—the lower flow at the bottom of the particle relative to the velocity of flow at the top of the particle results in a lifting force, tending to apply a force in the normal direction to the surface. The magnitude of the lift force will depend on the nature of the near-surface flow.
 F grav=gravitational force
 Based on above analysis of adhesion forces for immersion in liquid carbon dioxide and supercritical carbon dioxide versus water, it can be deduced that additional removal forces must be generated to achieve comparable particle removal forces.
 In prior art, supercritical fluids processes for cleaning have dealt with removing films, e.g. photoresist, or large particulates, e.g. etch residues, and not submicron particles.
 Both terms “ultrasonics” and “megasonics” refer here to the generation and transmitting of acoustical wave patterns into a medium as a means of providing or enhancing a cleaning process. Transducer arrays used for this purpose are well known in the art. The difference between ultrasonics and megasonics in this context is the frequency at which the acoustic wave pattern is generated. Ultrasonics is understood in the industry to span frequencies of 20-350 KHz, and is associated in cleaning applications with producing random cavitation. Megasonics refers to a higher frequency band, 700-1000 KHz, and is associated for cleaning purposes with offering minimal, controlled cavitation and frontal cleaning action. The ultrasonic induced cavitation during cleaning of semiconductor wafers has caused erosion and surface damage problems. This became more evident as semiconductor device features became smaller, in the submicron range. With megasonics, only the side of the part that is exposed to or facing the transducer is affected. Using megasonics in aqueous fluids, it is speculated that particle removal is accomplished through a high-pressure wave mechanism rather than by cavitation. The effects of megasonics in supercritical fluids have been heretofore unknown.
 Improvement in the supercritical fluid cleaning process for cleaning semiconductor wafer surfaces and other articles is needed. The integration of megasonics into this supercritical fluid process is introduced in the description that follows.
 Using a suitable apparatus with a pressurized process chamber, as is further described below, the principle steps of the process of the invention for removing the identified type of residue are as follows:
 1. Place the substrate, wafer, or other article of interest in an environment of supercritical fluid, preferably carbon dioxide, mixed with co-solvents and/or surfactants suitable for the material to be removed, at a higher pressure within a working supercritical pressure range. This requires a suitable heated, pressure vessel and supporting systems as is well understood by those skilled in the art. The higher pressure within the supercritical range is necessary to accomplish the decompression step described below. The pressure may be raised slowly or by any inflow, pump, or pulsation method within the capability of the apparatus.
 2. Soak the wafer in the supercritical fluid mixture under the higher supercritical pressure for a period of time, allowing the mixture and the pressure to permeate the material to be removed. As will be apparent to those skilled in the art upon reviewing this disclosure, the period for soaking depends on variables such as what materials or residues, how much residue, how many repetitions or cycles of this process are expected to be conducted on the wafer, what cleaning materials, and what end result is sought. Part of the intent is to have the supercritical fluid mixture permeate the residue and perform its softening and weakening effect. Part of the intent of the soak is that the high pressure permeate the surface of the residue to a depth that upon rapid decompression of the chamber will provoke a physical rendering of at least a surface layer of the weakened residue, as has been described in related application PCT/US01/15999, published on or about Nov. 18, 2001, which is incorporated herein by reference.
 3. Apply one or more very rapid decompression pulses between the higher and a lower supercritical pressure to the wafer to break up and loosen the residue. Generally speaking, the wider the pressure differential and faster it is applied, the more effective its rendering action.
 4. Flush the wafer with clean supercritical phase carbon dioxide to remove the loose residue debris from the wafer surface and carry it out of the cleaning chamber, thereby restoring the chamber fluid to a clean state. The flush step may incorporate or be followed by an increase in pressure, gradual or pulsed, to the higher supercritical pressure, and above steps repeated, if continued or additional cleaning action is desired.
 5. Apply megasonics action with a large surface area transducer array to the mixture and hence to the surface of the wafer as described herein, at one or more of: (a) during the soak to promote deeper penetration of the mixture and pressure into the residue surface, (b) during the decompression pulse to enhance the rendering of the surface layer of residue, (c) immediately after the decompression pulse to extend and continue the physical stress on the weakened residue, and (d) during the flush to aid in removing and carrying the loose debris off the surface of the wafer for removal with the outgoing fluid flow. The megasonics transducer array is preferably operated continually during the cleaning cycle, and may be operated or modulated intermittently or intersegmentally in any of phase, power level, frequency, and on-off switching modes, with selected or varied proximity to the surface of the substrate, all as may optimize the additional effects of the megasonics action on the cleaning process.
 As is apparent from the above, an important aspect of this invention is the combination of the cycle of high pressure soak, rapid decompression, and flush, and the megasonics action. In the prior art of megasonics there is mention of pulsing but it has to do with pulsing the input power to the transducer, nothing to do with a change in total pressure. The combination of the process mechanisms described here has a dramatic further effect for cleaning and removing particles in the submicron range.
 One prior art specimen on megasonics, U.S. Pat. No. 5,800,626, discloses control of the gas content in the cleaning process liquid for improved megasonic cleaning of semiconductor wafers. It indicates that the effectiveness of particle removal from wafers using megasonics action and dilute chemistry was found to be dependent on the total gas content in the deionized water. This activity was conducted in unpressurized systems without reference to the significance of total system pressure or megasonic action in a supercritical fluid.
 Recent prior art U.S. Pat. Nos. 6,286,231 and 6,357,142 discuss the use of “sonics” with pressurized fluids to enhance cleaning performance. The descriptions provided teach the use of megasonics or “sonics” when the fluid is in the liquid phase, but are unhelpful with respect to the utilization of megasonics in supercritical fluids.
 With the apparatus of the present invention, there is included the capability to alter the chamber environment between the liquid, gas and supercritical states, to apply megasonic action to the wafer in the supercritical fluid phase, and to control the formation of bubbles and pressure wave propagation in the supercritical fluid mixture to greatest advantage for improved cleaning of submicron particles.
 This disclosure describes the process and apparatus for precision cleaning of surfaces, including removal of photoresist and etch residue from semiconductor wafers, post CMP (chemical mechanical polishing) cleaning, photomask cleaning, bare Silicon wafer cleaning, flat panel displays cleaning, ceramic substrate cleaning, and hard disk drives cleaning, etc.
 It is therefore an objective of the invention to provide an apparatus and process for cleaning residual matter, photoresist and other foreign materials off wafers, substrates and other work pieces including photomasks, compact discs, flat panel displays.
 It is in particular an objective to provide for cleaning semiconductor wafers, using acoustic wave techniques including megasonics in conjunction with supercritical fluid soaking, rapid decompression, flushing, and related process mechanisms to enhance the cleaning capability and remove submicron particles.
 Other and various objectives and advantages will be apparent to those skilled in the art from the figures and description of preferred embodiments that follows.