Abstract:
A surface cleaning apparatus comprising a chamber, and a thermal transfer device. The chamber is capable of holding a semiconductor structure therein. The thermal transfer device is connected to the chamber. The thermal transfer device has a surface disposed inside the chamber for contacting the semiconducting structure and controlling a temperature of the semiconductor structure in contact with the surface. The thermal transfer device has a thermal control module connected to the surface for heating and cooling the surface to thermally cycle the surface. The thermal control module effects a substantially immediate thermal response of the surface when thermally recycling the surface.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a workpiece cleaning apparatus and, more particularly, to a workpiece cleaning apparatus using a dense phase fluid. 
     2. Prior Art 
     Conventional solvent based cleaning processes are continuously being looked at for alternative techniques to reduce the ill effects placed on the environment in the form of water and air pollution as well as ozone depletion. Government legislation continues to be introduced to protect the environment. Accordingly, there is an ever increasing desire to find alternative non-polluting methods of solvent based processing. The use of dense phase gases and mixtures of gases and cosolvents or surfactants is being explored. The solvent properties of compressed gases is well known. In the late 1800&#39;s, Hannay and Hogarth found that inorganic salts could be dissolved in supercritical ethanol and ether. Buchner discovered by the early 1900&#39;s, that the solubility of organics such as naphthalene and phenols in supercritical carbon dioxide increased with pressure. Within forty years Francis had established a large solubility database for liquefied carbon dioxide which showed that many organic compounds were completely miscible. 
     One industry that employs extensive conventional solvent based cleaning processes is semiconductor manufacturing. 
     Future restrictive legislation on conventional solvent based processes would thus have a significant adverse impact on the industry, and ultimately on the ability of consumers to have affordable electronic devices which are desired in ever increasing numbers. By way of example, the conventional process to manufacture a semiconductor device or workpiece generally involves a silicon wafer that undergoes numerous processing steps where materials are deposited in and on the wafer. Through this repetitive processing, electrical circuits are created within the wafer. As part of the manufacturing process there are a number of cleaning steps where conventional predominately aqueous based chemistry is used to perform the “cleaning” of the surface and film at each particular step. The chemistry is selected based on its performance on the exposed structures. Conventional wafer cleaning generally employs a batch processing method due in part to the time involved to ensure that the wafer surface is sufficiently cleaned, and the possibility of minimizing the amount of conventional solvent that may be used during the cleaning process. In view of the extensive use of conventional solvents in semiconductor manufacturing, the future restrictions on use of conventional solvents, as well as the uncertainty as to the time frame and extent of those restrictions, has a major adverse impact on the industry. Accordingly, the industry desires alternatives to conventional aqueous based chemistries such as using dense phase fluids or a mixture of dense phase fluids and cosolvents for semiconductor wafer cleaning. 
     There are a number of consideration when seeking alternatives to conventional cleaning methods. A major consideration is the desire for the throughput rate to be as high as possible and at least equal to throughput rates of conventional processes. Generally, this can be accomplished in one of two modes. The first is a batch processing method that would be similar to the existing aqueous based equipment solutions. An alternative would be to insert the processing solution just after the existing film processor. This insertion point becomes advantageous when the typical film processors in the industry operate on a single workpiece throughput integrated. Currently, there is a desire to operate on a single workpiece process and move away from batch processing in the industry. The introduction of dense phase gases processors has the potential to be easily integrated into the desired semiconductor tooling methodology as a single workpiece processor. The challenge in this case is that it is desired that processing rates for pre film deposition cleaning and post film deposition cleaning be on the same order as the film deposition. Conventional cleaning methods using alternative solvents have however fallen well short of this goal. 
     U.S. Pat. No. 5,013,366 describes a cleaning process for removing contamination from a substrate wherein the substrate to be cleaned is contacted with a dense phase gas at a pressure equal to or above the critical pressure of the dense phase gas. The phase of the dense phase gases is then shifted between the liquid state and the supercritical state by varying the temperature of the dense phase fluid in a series of steps between temperatures above and below the critical temperature of the dense fluid. At each step in the temperature change, the dense phase gas possesses different cohesive energy density or solubility properties. Temperature control of the supercritical process is performed through applying the thermal changes to the processing apparatus. In this case it is applied to a large thermal mass (high pressure vessel) and as a result a considerable amount of time is used for the apparatus to achieve the target process parameters. 
     U.S. Pat. No. 5,261,965 describes a method and system which is based on first cooling the semiconductor wafer to a predetermined temperature in order to condense a liquid film on the semiconductor wafer surface from a condensable process gas or gas mixture. Then, the method and system promote thermally activated surface reactions and rapidly evaporate liquid film from the semiconductor wafer surface using a high peak power, short pulse duration energy source such as a pulsed microwave source to dissolve surface contaminates and produce drag forces sufficiently large to remove particulates and other surface contaminates from the surface of the semiconductor wafer. Although, the system in this case attempts to improve process times, and hence increase system throughput, the resulting system is highly complex and costly. The present invention overcomes the problems of the prior art as will be described in further detail below. 
     SUMMARY OF THE INVENTION 
     A surface cleaning apparatus comprising a chamber, and a thermal transfer device. The chamber is sized to hold a semiconductor structure therein. The thermal transfer device is connected to the chamber. The thermal transfer device has a surface disposed inside the chamber for contacting the semiconductor structure. The surface of the thermal transfer device is used for controlling a temperature of the semiconductor structure in contact with the surface. The thermal transfer device has a thermal control module connected to the surface for heating and cooling the surface to thermally cycle the surface. The thermal control module effects a substantially immediate thermal response of the surface when thermally cycling the surface. 
     In accordance with a method of the present invention, a method for cleaning a semiconductor structure is provided. The method comprises providing a chamber for holding the semiconductor structure, providing a thermal transfer device, connecting a thermal transfer device to the chamber, placing the semiconductor structure in the chamber, and thermally cycling part of the chamber. The chamber is provided for holding the semiconductor structure and a dense phase fluid. The thermal transfer device is provided with a thermal transfer surface, and a thermal control module connected to the thermal surface. The thermal control module heats and cools the thermal transfer surface. The thermal transfer device is connected to the chamber with the thermal transfer surface inside the chamber. The semiconductor structure is placed in the chamber in proximity to the thermal transfer surface. The thermal transfer surface is thermally cycled with the thermal control module for thermally cycling at least part of the semiconductor structure through a predetermined temperature range. The thermal control module causes a substantially immediate thermal response of the thermal transfer surface during thermal cycling. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing aspects and other features of the present invention are explained in the following description, taken in connection with the accompanying drawings, wherein: 
         FIG. 1  is a schematic diagram of a workpiece cleaning apparatus incorporating features of the present invention; 
         FIGS. 1A-1B  are two other schematic diagrams of the workpiece cleaning apparatus in  FIG. 1 , and a workpiece S processed in the apparatus;  FIGS. 1A-1B  showing the apparatus at respective times during the processing of the workpiece S; 
         FIG. 2  is a phase diagram, according to prior art, for a fluid used in the apparatus shown in  FIG. 1 ; and 
         FIG. 3  is a flow chart graphically depicting a cleaning method used with the cleaning apparatus shown in  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to  FIG. 1 , there is shown an exploded perspective view of a cleaning apparatus  10  incorporating features of the present invention. Although the present invention will be described with reference to the single embodiment shown in the drawings, it should be understood that the present invention can be embodied in many alternate forms of embodiments. In addition, any suitable size, shape or type of elements or materials could be used. 
     The cleaning apparatus  10  may be integrated, or otherwise connected to a fabrication or processing apparatus  1  (only a portion of which is shown for example purposes in  FIG. 1 ) for performing various desired processes on a workpiece. The present invention will be described below for example purposes only, with particular reference to an embodiment where the processing apparatus  1  is a fabrication apparatus for fabricating structures or devices on semiconductor wafers. The present invention however is equally applicable to any other suitable processing apparatus used for processing any other suitable workpiece such as for example flat panel displays. As can be realized, the processing apparatus  1  may include a number of processing chambers (not shown) for processing semiconductor wafers, such as for example chambers for film deposition, masking and etching. The apparatus  1  may also include suitable transporters (not shown) for moving the wafers between the processing chambers as well as to and from the cleaning apparatus  10 . The processing apparatus  1  may incorporate or be connected to more than one cleaning apparatus  10 , and has a controller  100  for controlling the operation of process chambers, transporters, and cleaning apparatus  10 . Wafers are placed in the cleaning apparatus when desired such as before or after film deposition. The cleaning apparatus  10  generally comprises chamber  12 , fluid system  14 , and thermal transfer device  16 . The fluid system  14  is connected to the chamber  12  to deliver and remove cleaning fluid from the chamber. The cleaning fluid is a dense phase or supercritical fluid. The thermal transfer device  16  is also connected to the chamber  12  and helps maintain the chamber at a desired temperature while thermally cycling a wafer therein to effect cleaning of the wafer as will be described in greater detail below. 
     In greater detail now, and referring still to  FIG. 1 , the chamber  12  defines a closed plenum  18  and is constructed to withstand the conditions of the cleaning environment in the apparatus. As noted before, the cleaning apparatus uses a dense phase or supercritical fluid. In this embodiment, the fluid may be CO 2  or a mixture of CO 2  and other fluids.  FIG. 2  is a phase diagram for CO 2  according to the prior art. The diagram in  FIG. 2  shows that the critical temperature of CO 2  is 305° K. (32° C.), and the critical pressure is 72.9 atmospheres (1073 PSI). In alternate embodiments, the fluid may be any suitable gases which may be converted to supercritical fluids, which will not degrade the structural or chemical properties of the wafers or film deposition on the wafers cleaned in the apparatus. The fluid may also be a mixture of two or more dense phase fluids selected to provide the optimal solvent properties for anticipated containments of the wafers being cleaned. 
     To withstand the parameters of the environment to which it will be subjected, as noted above, the vessel or chamber  12  may be made of corrosion resistant metal, such as for example stainless steel. The chamber  12  is sized to hold 200 mm or 300 mm wafers. Referring now also to  FIGS. 1A-1B , the apparatus  10  in the embodiment is shown with one wafer S in the chamber  12 . In alternate embodiments, the chamber of the cleaning apparatus may be sized to hold any desired number of wafers, such as for batch processing. As seen in  FIG. 1 , the chamber  12  may include one or more inlet ports  20  (only one port  20  is shown for example purposes) for introducing the cleaning fluid into plenum  18 . The chamber  12  may include one or more outlet ports  22  (one port  22  is shown for example purposes) for drawing the cleaning fluid from the plenum  18 . The chamber  12  may have a wafer opening or aperture (not shown) which allows the wafer S (see  FIG. 1 ) to be transported into and out of the chamber. 
     The fluid system  14  may include a fluid supply (not shown), one or more pumps (not shown) and suitable piping  24  to connect the fluid supply to the chamber  12 . The fluid system  14  may also include suitable valves  26 ,  28  to open and close the supply and discharge ports  20 ,  22  of the chamber. The valves  26 ,  28  may be electronically connected to the controller  100 , as shown in  FIG. 1 , to allow the controller to open and shut the valves according to a desired processing program in the controller. The fluid supply of the fluid system  14  may hold the cleaning fluid used in apparatus  10  with the fluid in a supercritical state. Otherwise, the fluid may be stored in the fluid supply, in a liquid, or gaseous state as desired. In this case, the fluid system  14  could include a converter to convert the fluid from its stored state to the dense phase state in which the fluid is fed into the chamber  12 . The fluid system  14  may be an open or closed system as desired. In the case of an open system, fluid system  14  has an exhaust section  30  (see  FIG. 1 ) which connects the discharge port  22  of the chamber  12  to a discharge to atmosphere. This is possible when using CO 2  because CO 2  is non-toxic. When using other fluids, it may be desired to have a closed system with the discharge ports  22  of the chamber connected to the fluid supply by a return line (not shown). The return line may include appropriate scrubbers and as filters or any other suitable means for removing contaminants from the used fluid removed from the chamber  12 , before returning the fluid to the supply. 
     Still referring to  FIG. 1 , the thermal transfer device  16  of the cleaning apparatus  10  generally comprises wafer contact surface  40 , thermal control system  42 , and thermal regulator  50 . Contact surface  40  is located on side of the thermal transfer device  16  and may be formed by a casing or cover of the device. In alternate embodiments the wafer contact surface may be integrally formed with the vessel of the chamber. The wafer contact surface  40  may extend across an entire side of the plenum  18 , or may be localized in a desired region. In the embodiment shown in  FIG. 1 , the thermal transfer device  16  is located at the bottom of chamber  12 , with the contact surface  40  oriented horizontally forming the bottom surface of plenum  18 . In alternate embodiments however, the thermal transfer device and its wafer contact surface, or the wafer contact surface separate from the rest of the thermal transfer device may be positioned in any other suitable position and orientation. For example, the contact surface may be vertically oriented and located along one of the lateral sides of the plenum. The wafer contact surface may also be disposed along the top of the plenum if desired. The thermal control system  42  generally comprises one or more thermo-electric module(s)  42  T, power circuit  44  and power supply  46 . Only one thermo-electric module  42  T is shown in  FIG. 1  for example purposes. Any suitable number of thermo-electric modules may be used depending on the size of available modules and desired area of the contact surface to be thermally controlled. Examples of suitable thermo-electric modules are available from Ferrotec America, Corp. The thermo-electric module(s)  42  T is positioned so that the outer surface of the module forms good thermal transfer contact with the contact surface  40  of the device. It is desirable, that the contact surface  40  be provided by a thermal conductive layer made from a suitable material with good thermal conductivity properties such as cooper alloys for example. The outer surface of the thermo-electric module(s)  42  T is located sufficiently close to the wafer contact surface  40  so that there is substantially no thermal lag between thermal changes of the thermo-electric module(s) surface and surface  40 . In alternate embodiments, the outer surface of the thermo-electric module may itself form the wafer contact surface. In the case where more than one thermo-electric module is used in the thermal control system, the modules are mounted thermally parallel relative to the contact surface. As seen in  FIG. 1 , the module(s)  42  T are connected by DC circuit  44  to DC power supply  46 . The power supply  46  is connected to controller  100  as shown in  FIG. 1  to operate the module (s) as will be described further below. 
     As seen in  FIG. 1  and noted above, one side of the thermo-electric module(s)  42  T contact or form the wafer contact surface  40 , and the opposite surface is disposed to form good thermal contact with the casing or jacket  52  of the thermal regulator  50 . The thermal regulator  50  has jacket  52  which defines thermal transfer fluid passages  54 . The thermal regulator  50  also includes thermal fluid supply  56  and suitable piping or conduits  58  connecting the fluid passages  54  in jacket  52  to the supply  56 . In this embodiment, the fluid jacket  52  is located within the vessel, and accordingly apertures may be formed through the vessel wall to allow passage of the supply and return sections of conduit  58  through the vessel wall to the jacket  52 . In alternate embodiments however, the jacket of the thermal regulator may be included in or outside the vessel wall if desired. The thermal fluid supply  56  may have a suitable heating source (not known), such as for example a heating coil, and a suitable cooling source (not shown), such as for example a radiator or a heat sink, which are used to maintain the thermal fluid, and hence the jacket  52  at a desired temperature. The jacket  52  and fluid supply may include appropriate instrumentation (not shown) connected to the controller  100  to register thermal fluid temperatures and enable the controller to regulate the heating and cooling sources to maintain the thermal fluid at the desired temperature. 
     The thermal regulator  50  maintains the chamber pressure vessel, and hence the environment within plenum  18  at the desired temperature during the cleaning process. The thermo-electric module(s)  42  T are used to locally cycle the temperature of the wafer contact surface  40  by moving heat from or to the surface  40 . The thermal regulator  50  operating at the steady state desired temperature complements the cyclic operation of the thermo-electric module(s)  42  T. The thermal regulator  50  removes heat drawn from the wafer contact surface  40  by the module(s)  42  T, and supplies heat when the module(s)  42  T heat the surface  40 . By way of example, the desired temperature of the chamber  12  and its contents may be a temperature at or above 305° K. (32° C.), the critical temperature of the CO 2  cleaning fluid (see  FIG. 2 ). Accordingly, the temperature of the thermal regulator  50  may be set in a range about the chamber desired temperature (accounting for outer heat losses/gains of chamber  12 ) to maintain the chamber at its desired temperature. During the cleaning process, the thermal-electric module(s)  42  T may be energized (by directing current through circuit  44  in a desired direction) to cool the wafer contact surface  40  to a temperature below the critical temperature of the CO 2  fluid. The chamber  12  is shown in this condition in  FIG. 1A . The substantially no thermal lag between the thermal change of the cooling outer surface of the thermo-electric module(s)  42  T and the thermal change of surface  40  ensures that the temperature of the contact surface  40  drops immediately as the thermo-electric module(s) is energized. As seen in  FIG. 1A , operation of the thermo-electric module(s)  42  T to cool surface  40 , causes the thermo-electric module(s) to drain heat (indicated by arrow Q 1  in  FIG. 1A ) away from surface  40 . The thermo-electric module(s) outer surface (opposite the cooling surface) in contact with the thermal regulator jacket  52  is heated, the heat Q 1  is transferred via contact with the jacket  52  to the thermal regulator  50  and is subsequently removed by the thermal transfer fluid of the regulator. To complete the cycle, the thermo-electric module(s)  42 T is energized (by reversing the direction of current through circuit  44 ) to rapidly heat the contact surface  40  back to or above the critical temperature of the cleaning fluid. The chamber  12  is shown in this condition in  FIG. 1B . In this case, heat is moved by the module(s)  42  T (in the direction indicated by arrow Q 2  in  FIG. 1B ) to the contact surface  40  of the chamber  12 . The outer surface of the module(s)  42  T in contact with jacket  52  of the thermal regulator is cooled, while the outer surface in contact with surface  40  is heated. The thermal regulator  50  replenishes the heat moved by the thermo-electric module(s)  42  T to the contact surface  40 . 
       FIG. 3  is a flow chart graphically depicting the cleaning method used with cleaning apparatus  10  for cleaning a wafer S. The sequence shown in  FIG. 3  is merely exemplary, and in alternate embodiments the steps of the cleaning method may be performed in any desired order. As seen in  FIG. 1A , the wafer S to be cleaned, is transported into the chamber  12  from the processing apparatus  1  (block PA in  FIG. 3 ). The wafer S is positioned in the chamber to place the wafer S in contact with contact surface  40 . The chamber temperature may be equal to or higher than the critical temperature of the dense phase fluid. The dense phase fluid is fed into the chamber  12  from fluid system  14 . In block PB of the flow chart in  FIG. 3 , the temperature of the wafer S is lower rapidly to a temperature below the critical temperature of the dense phase fluid. This is done by rapidly cooling the contact surface  40  locally with the thermo-electric module(s)  42  T as described before. As the wafer temperature falls below the critical temperature, the dense phase fluid in contact with the surface of the cooled wafer changes phase to a liquid. The temperature of the wafer S is maintained at the lowered level (block PC) for the liquid in contact with the wafer to wet the surface of wafer S. Capillary action draws liquid into the structure formed on the wafer S. In block PD, the wafer S is rapidly heated back to the temperature above the critical temperature of the cleaning fluid. The wafer S is heated by rapidly heating the contact surface  40  locally with the thermo-electric module(s)  42  T as also described before. Raising the wafer temperature above the critical temperature causes the liquid on the wafer S to change phase back to a dense phase fluid. After transitioning to a dense phase fluid, the temperature of the dense phase fluid in immediate proximity of the wafer S continues to rise with the rising temperature of the wafer. As the fluid temperature rises, the diffusivity of the fluid increases such that the contaminants on the surface of the wafer S are miscible in the fluid. The contaminated cleaning fluid may then be drawn off using system  14  (block PE). The cleaned wafer S may then be removed from the cleaning apparatus for post cleaning operation. 
     The cleaning apparatus  10  and method use thermoelectric technology to provide a means to control dense gas phase changes. The cleaning apparatus  10  allows for rapid thermal changes to be applied directly to a workpiece and the surrounding dense phase gas. This ability to apply a rapid temperature change is used advantageously in the cleaning by directly applying phase shifting energy compared to the conventional mechanical techniques. 
     The cleaning apparatus  10  provides a phase transition from liquid to supercritical and liquid to solid with a minimum amount of energy transfer by avoiding the typical transfer of conditioning a high pressure vessel to the desired operating temperature. In the cleaning apparatus  10 , thermal phase transitions can be applied directly to the work piece within minutes verses hours in the conventional process to bring a large stainless steel pressure vessel to the required processing temperatures. 
     It should be understood that the foregoing description is only illustrative of the invention. Various alternatives and modifications can be devised by those skilled in the art without departing from the invention. Accordingly, the present invention is intended to embrace all such alternatives, modifications and variances which fall within the scope of the appended claims.