Abstract:
A system and method for altering and maintaining the temperature of a workpiece, especially at cryogenic temperatures, is disclosed. The platen on which the workpiece is located contains at least one inner conduit through which fluid can flow. An apparatus, in communication with a fluid source, is brought into contact with the platen. For example, an inlet and outlet on the platen and the ports of the apparatus may mate. Once the platen and the apparatus are successfully mated, fluid is passed through the apparatus and into the platen. Once the platen (and therefore the attached workpiece) has reached the desired temperature, the apparatus stops the flow of fluid through the platen. The apparatus and the platen then disengage. The platen is then free to move and rotated as required by the ion implantation process. When the platen temperature deviates from the desired temperature, the above process is repeated.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    Ion implanters are commonly used in the production of semiconductor wafers. An ion source is used to create an ion beam, which is then directed toward the wafer. As the ions strike the wafer, they dope a particular region of the wafer. The configuration of doped regions defines their functionality, and through the use of conductive interconnects, these wafers can be transformed into complex circuits. 
         [0002]    A block diagram of a representative ion implanter  100  is shown in  FIG. 1 . An ion source  110  generates ions of a desired species. In some embodiments, these species are atomic ions, which may be best suited for high implant energies. In other embodiments, these species are molecular ions, which may be better suited for low implant energies. These ions are formed into a beam, which then passes through a source filter  120 . The source filter is preferably located near the ion source. The ions within the beam are accelerated/decelerated in column  130  to the desired energy level. A mass analyzer magnet  140 , having an aperture  145 , is used to remove unwanted components from the ion beam, resulting in an ion beam  150  having the desired energy and mass characteristics passing through resolving aperture  145 . 
         [0003]    In certain embodiments, the ion beam  150  is a spot beam. In this scenario, the ion beam passes through a scanner  160 , which can be either an electrostatic or magnetic scanner, which deflects the ion beam  150  to produce a scanned beam  155 - 157 . In certain embodiments, the scanner  160  comprises separated scan plates in communication with a scan generator. The scan generator creates a scan voltage waveform, such as a sine, sawtooth or triangle waveform having amplitude and frequency components, which is applied to the scan plates. In a preferred embodiment, the scanning waveform is typically very close to being a triangle wave (constant slope), so as to leave the scanned beam at every position for nearly the same amount of time. Deviations from the triangle are used to make the beam uniform. The resultant electric field causes the ion beam to diverge as shown in  FIG. 1 . 
         [0004]    In an alternate embodiment, the ion beam  150  is a ribbon beam. In such an embodiment, there is no need for a scanner, so the ribbon beam is already properly shaped. 
         [0005]    An angle corrector  170  is adapted to deflect the divergent ion beamlets  155 - 157  into a set of beamlets having substantially parallel trajectories. Preferably, the angle corrector  170  comprises a magnet coil and magnetic pole pieces that are spaced apart to form a gap, through which the ion beamlets pass. The coil is energized so as to create a magnetic field within the gap, which deflects the ion beamlets in accordance with the strength and direction of the applied magnetic field. The magnetic field is adjusted by varying the current through the magnet coil. Alternatively, other structures, such as parallelizing lenses, can also be utilized to perform this function. 
         [0006]    Following the angle corrector  170 , the scanned beam is targeted toward the workpiece  175 . The workpiece is attached to a workpiece support. The workpiece support provides a variety of degrees of movement. 
         [0007]    The workpiece support is used to both hold the wafer in position, and to orient the wafer so as to be properly implanted by the ion beam. To effectively hold the wafer in place, most workpiece supports typically use a circular surface on which the workpiece rests, known as a platen. Often, the platen uses electrostatic force to hold the workpiece in position. By creating a strong electrostatic force on the platen, also known as the electrostatic chuck, the workpiece or wafer can be held in place without any mechanical fastening devices. This minimizes contamination and also improves cycle time, since the wafer does not need to be unfastened after it has been implanted. These chucks typically use one of two types of force to hold the wafer in place: coulombic or Johnson-Rahbeck force. 
         [0008]    The workpiece support typically is capable of moving the workpiece in one or more directions. For example, in ion implantation, the ion beam is typically a scanned or ribbon beam, having a width much greater than its height. Assume that the width of the beam is defined as the x axis, the height of the beam is defined as the y axis, and the path of travel of the beam is defined as the z axis. The width of the beam is typically wider than the workpiece, such that the workpiece does not have to be moved in the x direction. However, it is common to move the workpiece along the y axis to expose the entire workpiece to the beam. 
         [0009]    In some applications, it is necessary to perform the desired ion implantation at certain temperatures. As an example, for cryogenic ion implantation, it is necessary to maintain the temperature of workpiece at very low temperatures, despite the fact that constant ion bombardment tends to increase its temperature. One method of achieving this is to pass low temperature fluids through conduits in the platen. These fluids can be gasses, such as helium, nitrogen, or carbon dioxide, or liquids, such as Fluorinert or liquid nitrogen. By keeping the platen extremely cold, the workpiece, by virtue of its physical contact with the platen, preserves its low operating temperature. 
         [0010]    However, as described above, it is also typical for the workpiece support to move along the y axis to irradiate the entire surface of the workpiece. The workpiece, in certain situations, can also be rotated. These movements of the platen typically suggest the use of flexible tubing or some other pliable conduit to supply the fluid to the conduit in the platen. However, at cryogenic temperatures, the flexible tubing is susceptible to fatigue due to the bending stresses from the cyclic movement, and therefore cannot be used. 
         [0011]    Thus, a system and method for maintaining the temperature of a workpiece, while not restricting its freedom of motion, is very desirable. Furthermore, the system and method is preferably immune to fatigue caused by constant bending. 
       SUMMARY OF THE INVENTION 
       [0012]    The problems of the prior art are overcome by the temperature control system described in the present disclosure. The disclosure provides a system and method for altering and maintaining the temperature of a workpiece, especially at cryogenic temperatures. The platen on which the workpiece is located contains at least one inner conduit through which fluid can flow. 
         [0013]    In one embodiment, this inner conduit has an inlet and outlet on the side of the platen. The platen is brought into contact with an apparatus that has two corresponding ports. The inlet and outlet of the platen and the ports on the apparatus then mate together. Once the connection is made, the apparatus then begins the flow of fluid, such as nitrogen, through the conduit in the platen. Once the platen (and therefore the attached workpiece) has reached the desired temperature, the apparatus stops the flow of fluid through the platen. The apparatus and the platen then disengage. The platen is then free to move and rotate as required by the ion implantation process. When the platen temperature deviates from the desired temperature, the above process is repeated. 
         [0014]    In a second embodiment, the inlet and outlet are on the underside of the platen. The apparatus connects the inlet and outlet of the platen via ports located on the top side of the apparatus. In some particular embodiments, the apparatus includes a movable portion that is brought into contact with the underside of the platen. 
     
    
     
       BRIEF DESCRIPTION OF FIGURES 
         [0015]      FIG. 1  represents a traditional ion implanter; 
           [0016]      FIG. 2  represents a perspective view of a first embodiment of the apparatus to modify the temperature of the platen; 
           [0017]      FIG. 3  represents a view of the apparatus of  FIG. 2  in the operative, or docked, position; 
           [0018]      FIG. 4  represents a view of the apparatus of  FIG. 2  in the inoperative, or undocked, position; 
           [0019]      FIG. 5   a  represents an isolated view of the apparatus; 
           [0020]      FIG. 5   b  represents an expanded view of the mating portion of the apparatus shown in  FIGS. 2-4 ; 
           [0021]      FIG. 6  represents a perspective view of a second embodiment of the apparatus; 
           [0022]      FIG. 7  represents a cross-section of the apparatus shown in  FIG. 5  in the operative, or docked, position; 
           [0023]      FIG. 8  represents a top view of the apparatus shown in  FIG. 6  in the inoperative, or undocked, position; and 
           [0024]      FIG. 9  represents a bottom view of the apparatus of  FIG. 6  in the operative, or docked position. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0025]    Temperature plays an important role in ion implantation. While many ion implants are done at room temperature, there are benefits to performing implantation at high temperature or low temperature. For example, cryogenic ion implantation is beneficial in a number of applications, for example, in creating ultrashallow junctions in a crystalline silicon wafer. 
         [0026]      FIG. 2  illustrates a system for changing and regulating the temperature of workpiece in an ion implanter. The system affects the temperature of the workpiece by regulating the temperature of the supporting platen.  FIG. 2  shows a perspective view of the apparatus  220  and also includes a roplat  200 , comprising a base  202 , a platen  204 , and a motor (not shown). The base  202  of the roplat  200  is designed such that it can move the platen  204  in various directions, including axially and rotationally. 
         [0027]    As best seen in  FIG. 5   a,  the apparatus  220  preferably has an alignment mechanism  230 , to insure that the platen  204  and apparatus  220  are properly positioned relative to one another. The apparatus  220  also includes a connection block  260 . The connection block  260  attaches to two or more fluid conduits  240 . The connection block  260  also includes the connectors  245 , which mate with the ports  208  in the platen. The apparatus  220  may also include a mounting frame  250 , which holds the alignment mechanism  230 , the fluid conduits  240  and the connectors  245 . Additionally, the apparatus  220  may include means to move toward the platen when connecting thereto. Such means may include an electric motor or other suitable mechanism. 
         [0028]    One or more conduits are formed within the platen  204 , which are used to circulate fluid, either gas or liquid, through the interior of the platen. The use of conduits to modify and/or regulate the temperature of the platen is highly effective, since the fluid passes through the interior of the platen, and therefore directly contacts the platen. 
         [0029]    In one embodiment, shown in  FIGS. 2-4 , the conduit or conduits within platen  204  terminate in two or more ports  208  on the side of the platen  204 . In certain embodiments, these termination ports  208  are simply openings in the side of the platen  204 , similar to a female connector port. In other embodiments, these termination points  208  extend from the side of the platen  204  and include a connection mechanism, such as a male connector or tapered extending nozzle. 
         [0030]    As described above, the ports  208  on the platen  204  are adapted to mate with the connectors  245  of the apparatus  220 . The connectors  245  of the apparatus  220  may be tapered extending nozzles, or other suitable attachment mechanisms that can be press fit together with the ports  208  of the platen. Alternatively, the male portion of the connector can be on the platen, with the female portion on the apparatus. In some embodiments, the simpler piece of the mating mechanism is on the platen  204  to simplify packaging. In this scenario, the active component will be on the apparatus  220 . 
         [0031]      FIG. 4  shows a top view of the temperature regulating apparatus  220  retracted from the roplat  200  and platen  204 . In this undocked position, the platen  204  is ready for ion implantation, as the temperature regulating apparatus  220  is not in the ion beam path and does not affect the ability of the roplat  200  to move and rotate the platen  204 . 
         [0032]    To regulate the temperature of the platen  204 , the connectors  245  of the apparatus  220  must be brought into fluid communication with the conduit in the platen  204 .  FIG. 3  shows the apparatus  220  and roplat  200  in this operative docked position. The connector  245  of the apparatus preferably comprises tapered nozzles, so as to extend into the ports  208  on the platen  204 . The tapered nozzles and ports form a fluid-tight seal. This eliminates the need for any manual intervention during the coupling and decoupling process. While not shown in the Figures, in some embodiments, coupling pressure alone may be insufficient. The apparatus may include local clamping force generated by a CAM or solenoid. Alternatively, the pressure of the coolant itself may be used to force a seal. 
         [0033]    To initiate the process, the platen, if rotated, moves into its docked position, as shown in  FIG. 3 . In one embodiment, the temperature regulating apparatus  220  is able to move in an axial direction toward the roplat and specifically toward the platen  204 , by virtue of a motor or other suitable mechanism, such that the connection nozzles  245  on the apparatus  220  properly mate with the corresponding ports  208  on the platen. This embodiment requires the use of flexible conduits between the wall enclosing the vacuum environment and the movable apparatus  220 . As stated above, a fluid tight seal is achieved, preferably through a press fit technique, or a self-clamping mechanism. Alternatively, a locking mechanism to force a vacuum tight seal is activated via pressure, electrical current, a screw or other mechanisms to make the seal. As noted above, if such a locking mechanism is utilized, it is preferably mounted on the apparatus  220 . 
         [0034]    Once an integral seal has been made, fluid, in the form of gas or liquid, is circulated through the fluid conduits, the nozzles  245  and into the platen  204 . After the platen reaches the desired temperature, the flow of fluid stops. In some embodiments, control valves are used to regulate the flow of fluid through the apparatus  220 . The apparatus is then disengaged from the platen  204 . In some embodiments, the residual fluid within the conduits and platen is removed via a low pressure return to insure that excess fluid does not contaminate the environment. In other embodiments, a gas, such as nitrogen, is passed through the conduits to purge any residual fluid. 
         [0035]    In an alternate embodiment, the temperature regulating apparatus  220  is held in place and the roplat  200  is moved toward the apparatus  220  to engage with the alignment mechanism and the apparatus connectors  245 . 
         [0036]      FIG. 5   b  represents an expanded view of the connection block  260  of the apparatus  220 . On one end of the block  260  is one or more couplers  265  to connect to fluid conduits which deliver the fluid to and from the platen. These couplers  265  may be traditional couplers used to connect conduits. Industry standard fittings, such as SwageLok®, can be used. Alternatively, a threaded pipe style fitting could be used. On the opposite end of the block are the connectors  245 , used to connect to the platen  204 . As shown in  FIGS. 2 and 5   a,  the connection block  260  is mounted on mounting frame  250 . 
         [0037]      FIGS. 6-9  represent a second embodiment of the apparatus. In this embodiment, the conduit on the platen  204  is accessed via the bottom surface of the platen  204 . As described above, a conduit is located within the platen  204 . In this embodiment, the entrance and exit points on the platen  204  are located on its underside. As explained previously, the entrance and exit points on the platen  204  can be simply openings, or can include a connection mechanism such as a nozzle. 
         [0038]    In one embodiment, the temperature regulating apparatus  270  is “U” shaped, where two protruding arms  280  form the legs of the “U”. The apparatus  270  and roplat  200  are moved toward one another, such that the roplat  200  enters into the open end of the “U” shape, and then the relative heights of the apparatus  270  and roplat  200  are aligned so as to bring the openings on the underside of the platen  204  in contact with the connection mechanism  275  on the upper side of the apparatus  270 , as shown in  FIG. 6 . In some embodiments, an air bearing system is used to lower the platen onto the arms  280  of the apparatus. In certain embodiments, a motor within the roplat is used to lower the platen so that it contacts the arms  280 . In other embodiments, the apparatus  220  includes means to raise the protruding arms  280  so as to contact the platen. 
         [0039]    In a second embodiment, the protruding arms  280  of apparatus  270  are movable. The apparatus includes a motor capable of actuating the arms through a preferably rotary trajectory. One particular embodiment is shown in  FIG. 8 . In this Figure, two movable arms  280   a ,  280   b  are used, whereby one supplies fluid to the platen  204  and the other arm  280   b  provides the return path. The two arms  280  are rotated about one or more axes so as to move them into position below the platen  204 , by virtue of a motor or other suitable mechanism (not shown). The movable arms  280  are then lifted to contact the platen  204 , or alternatively, the platen  204  is lowered to meet the contacts  275  on the arms  280 , as described above. Once connected, fluid can pass through the conduits as described above. To disengage, the platen and the arms are moved apart relative to one another. The arms are then rotated away from the platen, and therefore are not in the ion beam path. 
         [0040]    Alternatively, a single arm  280 , having two or more conduit termination ports  275  for attachment to the platen, can be used. This single arm can be stationary, or movable, as described above. 
         [0041]    The temperature regulating apparatus, once connected, can be used in a number of ways. In one embodiment, this apparatus is the only mechanism used to regulate the temperature of the platen  204 . In other words, the intermittent connections between the apparatus and the platen  204  are sufficient to regulate the platen temperature throughout all phases of the ion implant process. This is applicable in situations where the entire ion implant is performed at a constant temperature. In this situation, the platen is continuously regulated by a single fluid source fed to the platen through the apparatus. In another embodiment, the conduits in the apparatus are in communication with a plurality of fluid sources, such as extremely cold fluids (liquid nitrogen, etc), nitrogen, carbon dioxide or other fluids. In this embodiment, the apparatus selects the appropriate fluid source to pass through the conduit and into the platen to realize the desired temperature. Such a selection may be accomplished using control valves or any suitable mechanism. Alternatively, the apparatus may comprise a plurality of connection ports  245 , each pair associated with a specific fluid path. In one words, one pair may be the supply and return for liquid nitrogen, while another pair may be the supply and return for water. The apparatus then aligns with the platen in one of a plurality of viable docking positions, depending on the selected fluid source. Such a configuration allows a plurality of different fluids to flow through the platen, and therefore a plurality of different operating temperatures can be achieved. 
         [0042]    In another embodiment, the apparatus is only used in certain modes of operation. For example, the apparatus may only be used to deliver cryogenic fluids to the platen during cold ion implant. During other process steps, the temperature of the platen is regulated using conventional methods, such as water cooling. 
         [0043]    Intermittent cooling of the platen using this apparatus is an effective technique to regulate the temperature of the platen. An existing aluminum platen has a thermal capacitance of about 3000 Joules/° C. A 1E15@20 kV, 15 mA implant is approximately 3000 J. Therefore, one such implant increases the temperature of the platen about 1° C. Cold implants can typically be performed at temperatures up to roughly −60° C. Therefore, if the apparatus was able to cool the platen to a temperature of about −100° C., up to 4 implant cycles could be performed before the platen would need to be cooled again. There are two types of cooling cycles that need to be performed. Initial cool down from room temperature (20° C.) to working temperature (−60° C.) is expected to take about 10 minutes. Recooling the platen between implants or sets of implants is much quickly, preferably on the order of seconds. This insures acceptable throughput.