Abstract:
A method and system for cooling an electrostatically shielded radio frequency (ESRF) plasma source. The method and system utilize an electrostatic shield, having plural ribs, which vaporizes a coolant and sprays the vapor against a process tube or a bias shield. The vapor is either sprayed underneath the ribs or between adjacent ribs. This design avoids using baths of liquid coolants that can absorb gases which lead to arcing between induction coils.

Description:
CROSS-REFERENCE TO CO-PENDING APPLICATIONS 
     The present application is related to and claims priority to U.S. provisional application Ser. No. 60/095,036, filed Aug. 3, 1998, and entitled “ESRF CHAMBER COOLING SYSTEM AND PROCESS.” The contents of that application are incorporated herein by reference. 
     The present application is related to the following co-pending applications: Ser. No. 60/059,173, entitled “Device and Method for Detecting and Preventing Arcing in RF Plasma Systems,” Ser. No. 60/059,151, entitled “System and Method for Monitoring and Controlling Gas Plasma Processes,” and Ser. No. 60/065,794, entitled “All-Surface Biasable and/or Temperature-Controlled Electrostatically-Shielded RF Plasma Source.” The present application is also related to co-pending application Ser. No. 60/095,035, entitled “ESRF COOLANT DEGASSING PROCESS,” filed Aug. 3, 1998, and application Ser. No. PCT/US99 /17520 filed Aug. 3, 1999 entitled “ESRF COOLANT DEGASSING PROCESS,” filed concurrently with the present application, also naming Wayne L. Johnson as an inventor. Each of those co-pending applications is incorporated herein in its entirety by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention is directed to a method and system for cooling a plasma processing system, and in particular to a method and system for utilizing: (1) coolants that are expanded through an orifice and converted to gas that is applied to an exterior of a process tube and (2) an electrostatic shield that cools the processing tube by evaporating coolant using evaporation orifices in the electrostatic shield and directing the cooled vapor onto the process tube. 
     2. Description of the Background 
     In order to fabricate semiconductor wafers with submicron features using etch and deposition processes, modem semiconductor processing systems utilize plasma assisted techniques such as reactive ion etching (RIE), plasma enhanced chemical vapor deposition (PECVD), sputtering, reactive sputtering, and ion assisted vapor deposition (PVD). In addition to the above-referenced co-pending applications, another example of a gas plasma processing system is described in U.S. Pat. No. 5,156,345, to Wayne L. Johnson, the inventor of the present application. In such known systems, a gas is introduced to a processing environment wherein a gas plasma is formed and maintained through the application of radio frequency (RF) power. Typically, RF power is inductively coupled to the plasma using a helical coil. 
     Normally, the generation of a gas plasma also produces a substantial amount of heat that must be removed in order to maintain the processing system at a process-specific temperature. The removal of this heat has heretofore been inefficient and based on a cumbersome design. Known ESRF plasma sources have been cooled using baths of liquid coolants, such as FLUORINERT, which also act as dielectrics. The definition of a good dielectric at radio frequencies is that the fluid must have a low power loss per unit volume when exposed to an intense electric field. However, these particular fluids disadvantageously adsorb large quantities of gas, such as air. The four main sources of adsorbed gas are: (1) gas already trapped in the liquid prior to shipment (i.e., gas that was adsorbed prior to receipt by the plasma processing system user), (2) gas adsorbed into the liquid when the liquid is exposed to air, e.g., during pouring between containers before using the liquid, (3) gas adsorbed when air in the chamber is replaced by fluid during an initial filling cycle, and (4) the presence of air in any part of the system when fluid is being pumped. Furthermore, gas may be adsorbed into the coolant after the coolant pumps are stopped. When the pumps stop, if coolant in the high parts of the system drains to lower parts, then air replaces the drained coolant. When the pumps are restarted, the air may be broken down into bubbles which become another source of adsorbable gas. 
     In the high field regions, strong dissipation can occur leading to high local heating, hence, raising the local temperature of the coolant fluid. In so doing, the rate of gas evolution is increased permitting more gas to come out of solution, and generate bubbles that coalesce on the coil surface by dielectro-fluoretic attraction. The attached bubbles generate a dielectric difference at the coil surface which leads to non-uniform electric fields, localized heating, and arcing. This arcing can occur at voltages well below the measured dielectric strength of the fluid if the gas is not evolved from the liquid coolant before use in the resonator cavity. For example, FLUORINERT adsorbs a volume of gas equivalent to its own liquid volume and must be treated to remove the trapped gas. 
     In order to avoid arcing due to the rapid evolution of adsorbed gas, known systems gradually increase power to the plasma source while continuously pumping coolant through the ESRF plasma chamber. The gradual increase in RF power takes place over a period of time sufficient to slowly evolve adsorbed gas from the coolant. Although running the coolant in this way evolves trapped gases, a considerable amount of time is required. Often this process will take hours, thereby delaying the use of the plasma system in processing wafers. 
     In addition to the lengthy time period required by known systems to evolve adsorbed gas, the cooling systems coupled to a plasma source may be very cumbersome due to the large cooling lines used in large wafer (i.e., 300 mm) processing systems. Consequently, significant amounts of air are generally adsorbed when the processing chamber has been opened with the coolant lines remaining attached. The lines have typically remained attached since the coolant lines may contain hundreds of pounds of coolant. As a result, lifting the attached lines to open the chamber has been difficult, but not impossible. 
     Previously, it was not known how to replace the large lines with an alternate cooling mechanism. The large lines were required in order to provide the large coolant exchange (e.g., approximately 50-75 gallons/minute) needed to remove the heat from the process tube. Also, flexible lines were difficult to use because of the weight and pressure of the coolant required. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an improved method and system for cooling an ESRF source. 
     It is another object of the present invention to provide an improved method and system for cooling an ESRF source using a vapor coolant instead of a bath of liquid coolant. 
     It is a further object of the present invention to provide an ESRF cavity that can be tuned in atmospheric conditions instead of tuning using elements submerged in a temperature controlled fluid. 
     These and other objects of the present invention are achieved by a method and system utilizing coolant that is evaporated as it passes through a shield (prior to being applied to the exterior of a process tube) to remove the heat generated in an RF powered plasma source. Using a series of nozzles to apply low pressure coolant to the process tube, the present invention removes heat by vaporizing the liquid coolant and then pumping away the vapor. This method avoids the arcing that occurs in systems using baths of liquid coolant. Further, since the dielectric constant of the material around the coil remains close to the same between air and the dielectric fluid, the ESRF cavity can be tuned in air and will remain tuned over a wide range of temperatures. (That is, it is possible to reduce the shift in tuning that would otherwise result from a temperature-based change in the dielectric constant.) 
     More specifically, these and other objects of the present invention are achieved by a method and system utilizing a process tube that is cooled using a shield which surrounds the process tube. By expanding the coolant through a series of expansion orifices (e.g., disposed along the ribs of the shield) to a pressure lower than the coolant&#39;s vapor pressure, the coolant is vaporized as it exits the orifices. That vapor is then impinged upon the process tube to remove heat from the shield and the process tube. Heat is removed through one or more of (1) forced convection of cool vapor over the surface of the process tube and (2) conduction. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete appreciation of the invention and many of the attendant advantages thereof will become readily apparent with reference to the following detailed description, particularly when considered in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a schematic illustration of fluid flow according to the present invention; 
     FIG. 2 is an illustration of a multiple spray nozzle cooling shield; 
     FIG. 3A is an expanded view of the interface between the manifold and a rib of the shield of FIG. 2; 
     FIG. 3B is an expanded view of openings (either orifices or gas holes) in the multiple spray nozzle cooling shield shown in FIG. 2; 
     FIG. 4 is an illustration of a multiple orifice cooling rib shield; 
     FIG. 5 is an expanded view of orifices in the multiple orifice cooling rib shield shown in FIG. 4; 
     FIG. 6 is an expanded view of orifices in the multiple orifice cooling rib shield shown in FIG. 4 which apply the cooled vapor to a bias shield; and 
     FIGS. 7A and 7B are a chart showing the cooling characteristics of various commercially available coolants. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the present invention, ESRF systems are cooled without encountering problems previously associated with the use of untreated liquid coolants, namely arcing around helical coils. One embodiment of the invention uses a liquid dielectric that is evaporated and applied to hot surfaces, rather than submerging the hot surfaces in a liquid coolant. 
     Turning now to the drawings in which like reference numerals designate identical or corresponding parts throughout the several views, FIG. 1 is a schematic illustration of a cooled ESRF processing system that utilizes vaporized coolant as a replacement for baths of liquid coolants. According to the illustrated embodiment, an ESRF plasma source  100  is cooled by a source cooling chamber  105  using a vapor to remove heat from heated surfaces. A coolant is pumped from a fluid mass storage chamber  110  by a high pressure pump  115  through a length of tubing  117  and subsequently through a valve  120   a.  The pressure of the coolant is measured by a high pressure manometer  125   a.  The coolant then passes through a remotely controllable valve  130 , through which the coolant mass flow rate is controlled. Upon exiting the mass flow controller  130 , the coolant enters a manifold  305  (atop the shield  300 ; see FIG. 2) that distributes coolant to the ribs  303  of the shield. As described, the coolant enters the ribs at a pressure (1) greater than the pressure maintained within the source cooling chamber  105  and (2) less than the coolant vapor pressure. The pressure difference between the coolant entering the ribs of the shield and the vapor environment of the source cooling chamber  105  is such that, as the coolant emerges from an expansion orifice, it expands, hence, reducing the pressure to below its vapor pressure. At this point the coolant changes phase from a liquid to a gas. As the gas continues to expand to the ambient pressure within the source cooling chamber  105 , it cools further (converting thermal energy to kinetic energy). Therefore, heat is transported away from the process tube by two mechanisms, namely: (1) conductive-convective heat transfer between the coolant (liquid state) and the process tube, and (ii) conductive-convective heat transfer between the process tube and the cool vapor impinging upon its surface. The shield ribs are, in turn, cooled by the first two mechanisms. Heat is transported to the coolant (in liquid state) as it flows through the manifold and ribs of the shield prior to passage through the expansion orifices  135 . The heat transfer rate is proportional to (1) the surface area of the coolant conduits (within the distribution manifold and the shield ribs), (2) the temperature difference between the process tube (that is loosely thermally coupled to the ribs) and the shield ribs, and (3) the heat transfer coefficient. 
     Moreover, the heat transfer coefficient is primarily dependent upon the coolant flow rate and the coolant thermal properties (i.e., thermal conductivity, viscosity, specific heat at constant pressure, density, etc.). Secondly, heat is expelled from the shield ribs as the latent heat for vaporization of the coolant (necessary for the coolant to change phase from a liquid state to a gas state) is provided by both the hot shield ribs and contact of the coolant with the process tube. For an evaporating liquid, the heat transfer rate is proportional to the latent heat of vaporization and the coolant mass flow rate. 
     Lastly, the third mechanism represents direct heat transfer between the process tube and the cool vapor as it impinges upon the process tube surface. Similarly, the heat transfer rate is proportional to the process tube surface area, the temperature difference between the process tube and the impinging gas, and the heat transfer coefficient. For an array of impinging jets, the heat transfer coefficient is dependent upon the total area of expansion orifices, the distance between the orifice and the process tube, the gas velocity, and several gas properties (including the thermal conductivity, viscosity, specific heat at constant pressure, density, etc.). 
     The previously cooled gas absorbs heat in the source cooling chamber  105  and is eventually pumped through another length of pipe and a valve  120   b  before entering a condenser  140 . In the condenser, the vapor goes through a reverse phase change, being converted back to a liquid. During the conversion, heat is removed from the vapor as it becomes a liquid. The liquid then is pumped again by the high pressure pump  115 . However, since the process of pressurizing the liquid in pump  115  typically adds thermal energy to the liquid, the liquid is first passed through a heat exchanger  145  in which heat can be given off before returning the liquid to the fluid mass storage chamber  110  for another cycle. 
     In order to control the cooling cycle, the high pressure manometer  125   a  and a second manometer  125   b  measure pressures at two locations in the cooling cycle—one before expansion and one after expansion. From the two pressures, a flowrate of the liquid and vapor can be determined. Using the flowrate, the cooling cycle can be controlled by controlling the flow control valve  130  to maintain a flow rate which provides a sufficient amount of vaporized coolant without creating a pool of coolant within the source cooling chamber  105 . 
     The system also includes a vacuum pump  160  connected on the vapor side of the expansion orifice  135 . This pump is used to evacuate the system of air before operation begins. The amount of vacuum created by the vacuum pump  160  is (1) controlled by valves  120   c  and  120   d  and (2) measured by the vacuum pressure manometer  155 . The vacuum pump  160  can likewise be controlled to pump more or less based on the reading of the manometer  155 . After evacuating the system, the system is checked for leaks and then back-filled with a predetermined charge of coolant. 
     In addition to having an inlet and outlet used during normal fluid flow, the fluid mass storage chamber  110  also includes a residual contaminant relief valve. This valve can be used to bleed off gaseous contaminants that rise to the top of the fluid mass storage chamber. The relief valve is either pressure activated or manually controlled. 
     The above method is preferably used in conjunction with either the multiple spray nozzle cooling shield  300  shown in FIG. 2 (see close-up in FIG. 3B) or with the multiple orifice cooling rib shield  360  shown in FIG.  4 . Both of these shields are advantageous since they provide fluid to be vaporized in sufficient quantities to remove the local heat load but without generating micro-bubbles on the surface of the coils. In an alternate embodiment of the shields  300  and  360 , the shields are electrostatic shields rather than just cooling shields. The rate at which the vapor is pumped out of the chamber must be higher than that for a liquid coolant since the vaporized coolant displaces a larger volume than the corresponding liquid before the phase change. (However, the flow rate of the present invention is less than the flow rate of conventional systems. Whereas 7.6 joules per gram of flow allows a 10 degree C fluid temperature change in known liquid coolant systems, the present invention allows a 100 joule pickup for an equivalent flow.) In general, however, the system controls flow of the liquid coolant to ensure that the liquid coolant is substantially completely vaporized prior to being applied to the process to be (or the bias shield as described in further detail below). Although it is not possible to ensure complete vaporization, substantially complete vaporization allows the vapor to be applied without causing arcing in liquid coolant. 
     As shown in FIG. 2, multiple ribs  303  of the shield  300  are arranged to provide uniform cooling of the outside of a process tube  400  that is housed a short distance (0.1250-0.375 inch) inside of the shield  300 . As shown in FIG. 3A, coolant is pumped into the shield  300  through the manifold  305  (via coolant entry hole  320  shown in FIG. 2) and forced through drill holes  308  (located at the bottom of the manifold  305  and at the top of rib  303 ). As shown in FIG. 3B, once the coolant passes through the bottom of the manifold  305  into the rib  303 , the coolant is then expelled through the openings  310 . In a first embodiment of the shield  300 , the openings  310  act as expansion orifices  135  and direct coolant towards an interior of the shield, i.e., towards an outer shell of the process tube  400 . In an alternate embodiment of the shield  300 , the drill hole  308  acts as the expansion orifice  135  for the corresponding rib  303 . In that embodiment, the openings  310  are simply holes large enough to allow the vaporized coolant to pass there through in a quantity sufficient to cool the process tube  400 . In yet another alternate embodiment, the coolant entry hole  320  (shown in FIG. 2) acts as a single orifice for the entire cooling system. Accordingly, the manifold  305  is filled with vaporized coolant. To accommodate the flow of vaporized coolant, the drill holes  308  and the openings  310  are enlarged as compared to when they carried liquid and/or acted as orifices  135 . 
     As shown in FIG. 4, a multiple orifice cooling rib shield  360  contains multiple individual ribs  330 , each with multiple orifices  340 . Each orifice  340  is designed to cool a single rib  330  and a corresponding portion of the shield  360  adjacent to the rib. (As discussed above, the shield  360  may either be a simple cooling shield or an electrostatic shield.) The embodiment provides an additional heat transport mechanism compared to the first embodiment—i.e., conduction heat transfer between the process tube and the shield ribs as the shield ribs are cooled by the coolant latent heat of vaporization. The density of the orifices  340  increases as the area at the bottom of the conical shield  360  increases. This arrangement matches the need for more heat removal at the bottom of the shield  360 . An enlarged section of a rib  330  is shown in FIG. 5 with three orifices  340 . In one embodiment of the present invention, the orifices are shaped and positioned so as to impinge streams of gas semi-perpendicular to the ribs such that the streams collide and disperse toward the wall of the process tube  400 . In a second embodiment, the orifices are shaped and positioned so as to impinge streams of vapor directly onto the wall of the process tube  400 . In the second embodiment, the streams are directed so as to cool their corresponding half of the portion of the process tube between adjacent ribs. As further described above, the orifices  340  can be replaced by larger openings when the coolant has already been vaporized prior to or upon entering the rib  330  through the drill holes  308 . 
     In addition, the shield  360  is made of a thermally very conductive material in order to remove heat more evenly. In the embodiment in which the shield  360  is an electrostatic shield, the ribs  330  are further designed to have narrow cross sections opposing each other to minimize the capacitive coupling between adjacent ribs. Capacitive coupling across the gap in an electrostatic shield increases insertion loss of the electrostatic shield. 
     Similar to the embodiment shown in FIG. 5, FIG. 6 shows another embodiment in which a slotted bias shield  410  is interposed between the process tube  400  and an electrostatic shield  360 . In this embodiment, the bias shield  410  receives heat from the process tube  400  and is cooled by the vapor rather than cooling the process tube directly. In order to prevent direct electrical contact between the bias shield  410  and the electrostatic shield  330 , an electrical insulator is placed there between. Although the insulator depicted in FIG. 6 is simply an air gap, other electrical insulators are possible. 
     As shown in FIGS. 7A and 7B, nine different coolants were tested to determine how changes in boiling point and other characteristics affect heat removal through vaporization. Although the boiling points ranged from 30 degrees Celsius to 215 degrees Celsius, the flow rate necessary to remove the maximum heat varied only from 1.15 liters/min to 1.11 liters/min. Accordingly, a wide range of coolants can be used in the present invention to remove heat through vaporization of coolants. 
     Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically described above.