Abstract:
In an ion implanter, an elastomer seal is used for cryogenic processing and includes an internal cavity defined by a pair of opposing side walls and a bottom wall disposed therebetween. An electrically conductive spring is disposed within the cavity and extends along a length of the seal. The seal is configured to provide a lateral biasing force against the pair of opposing side walls and to conduct an applied current which results in heat being generated that emanates at least along the pair of opposing side walls. In this manner, the heat from the spring maintains the temperature of the seal above its brittle point. This allows the elastomer seal to maintain its pliability and consequently its sealing integrity during processing at cryogenic temperatures.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    Embodiments of the invention relate to the field of semiconductor device fabrication. More particularly, the present invention relates to a reliable seal used in cryogenic processing environments. 
         [0003]    2. Discussion of Related Art 
         [0004]    Ion implantation is a process used to dope impurity ions into a semiconductor substrate to obtain desired device characteristics. An ion beam is directed from an ion source chamber toward a substrate. The depth of implantation into the substrate is based on the ion implant energy and the mass of the ions generated in the source chamber. One or more ion species may be implanted at different energy and dose levels to obtain desired device structures. In addition, the beam dose (the amount of ions implanted in the substrate) and the beam current (the uniformity of the ion beam) can be manipulated to provide a desired doping profile in the substrate. However, throughput or manufacturing of semiconductor devices is highly dependent on the uniformity of the ion beam on the target substrate to produce the desired semiconductor device characteristics. 
         [0005]    It has been discovered that a relatively low target substrate or wafer temperature during ion implantation improves implant performance. In particular, lower wafer temperatures reduce the amount of damage caused when ions hit the substrate (“damage layer”) which improves device leakage currents. This allows manufacturers to create abrupt source-drain extensions and ultra-shallow junctions needed for today&#39;s semiconductor devices. When the temperature of the wafer is decreased, the thickness of the amorphous silicon layer increases because of a reduction in the self-annealing effect. 
         [0006]    Typically, cooling of the target substrate to cryogenic temperatures is done by cooling the platen upon which the substrate is disposed in the range of below room temperature to about −100° C. Almost all existing low-temperature ion implanters cool wafers directly during ion implantation. In order to maintain sufficient contact between the cooling elements within the wafer processing vacuum and to provide for efficient thermo-coupling, a reliable gas seal must be maintained. In particular, seals are disposed between the cooling elements and the platen to prevent gas leakage and to maintain the vacuum chamber housing and the target wafer at the desired temperature. These seals are typically made from, for example, polyvinyl chloride, elastomers such as thermoplastic elastomers and/or other plastic materials. In order for the seals to provide a uniform seal between the cold sealing surfaces, they must be pliable. However, at cryogenic processing temperatures, PVC, TPE and other elastomer materials become brittle at approximately −20° C. and −60° C., respectively. Since wafer cryogenic processing is typically performed at temperatures down to −100° C., these seals may become brittle during processing. This may compromise thermocoupling between components. This may affect the vacuum environment for processing tools which may negatively impact manufacturing and device throughput. Consequently, there is a need to provide seals disposed between components in semiconductor processing equipment that maintain their sealing properties and pliability at cryogenic processing temperatures. 
       SUMMARY OF THE INVENTION 
       [0007]    Exemplary embodiments of the present invention are directed to a seal used in cryogenic processing environments. In an exemplary embodiment, the seal includes an internal cavity defined by a pair of opposing side walls and a bottom wall disposed therebetween; and an electrically conductive spring disposed within the cavity and extending along a length of the seal. The spring is configured to provide a lateral biasing force against the pair of opposing side walls and to conduct an applied current from a first end of the spring to a second end of the spring. The current through the spring results in heat within the spring which emanates toward at least the pair of opposing side walls thereby maintaining their pliability and sealing function. 
         [0008]    In another exemplary embodiment, an apparatus for use in an ion implanter includes a platen configured to receive a target substrate; a pad disposed beneath the platen where the pad has a plurality of channels configured to receive coolant material therethrough to reduce the temperature of the platen to approximately −100° C. The platen is capable of being displaced along a top surface of the pad. A lip seal is disposed between and in contact with a bottom surface of the platen and the top surface of the pad. The seal has an internal cavity defined by a pair of opposing side walls and a bottom wall disposed therebetween and an electrically conductive spring with is disposed within the cavity and extends along a length of the seal. The spring is configured to provide a lateral biasing force against the pair of opposing side walls and conducts an applied current from a first end of the spring to a second end of the spring. The current results in heat generated from the spring and emanates toward the pair of opposing side walls. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  illustrates a block diagram of a representative ion implanter. 
           [0010]      FIG. 2  is a side view of an exemplary portion of a vacuum processing assembly of the ion implanter of  FIG. 1  in accordance with an embodiment of the present disclosure. 
           [0011]      FIG. 3  is a side view of another exemplary portion of a vacuum processing assembly of the ion implanter of  FIG. 1  in accordance with an embodiment of the present disclosure. 
           [0012]      FIG. 4  is a top view of an exemplary lip seal taken along lines A-A of  FIG. 3  in accordance with an embodiment of the present disclosure. 
           [0013]      FIG. 5  is a cross sectional view of an exemplary seal taken along lines B-B of  FIG. 4  in accordance with an embodiment of the present disclosure. 
           [0014]      FIG. 6  is a cross sectional view of an alternative configuration of an exemplary seal taken along lines B-B of  FIG. 4  in accordance with an embodiment of the present disclosure. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0015]    The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention, however, may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. In the drawings, like numbers refer to like elements throughout. 
         [0016]      FIG. 1  is a block diagram of an ion implanter  100  including an ion source chamber  102 . A power supply  101  supplies the required energy to source  102  which is configured to generate ions of a particular species. The generated ions are extracted from the source through a series of electrodes  104  and formed into a beam  95  which passes through a mass analyzer magnet  106 . The mass analyzer is configured with a particular magnetic field such that only the ions with a desired mass-to-charge ratio are able to travel through the analyzer for maximum transmission through the mass resolving slit  107 . Ions of the desired species pass from mass slit  107  through deceleration stage  108  to corrector magnet  110 . Corrector magnet  110  is energized to deflect ion beamlets in accordance with the strength and direction of the applied magnetic field to provide a ribbon beam targeted toward a work piece or substrate positioned on support (e.g. platen)  114 . In some embodiments, a second deceleration stage  112  may be disposed between corrector magnet  110  and support  114 . The ions lose energy when they collide with electrons and nuclei in the substrate and come to rest at a desired depth within the substrate based on the acceleration energy. 
         [0017]    The ion source chamber  102  typically includes a heated filament which ionizes a feed gas introduced into the chamber to form charged ions and electrons (plasma). The heating element may be, for example, a Bernas source filament, an indirectly heated cathode (IHC) assembly or other thermal electron source. Different feed gases are supplied to the ion source chamber to obtain ion beams having particular dopant characteristics. For example, the introduction of H 2 , BF 3  and AsH 3  at relatively high chamber temperatures are broken down into mono-atoms having high implant energies. High implant energies are usually associated with values greater than 20 keV. For low-energy ion implantation, heavier charged molecules such as decaborane, carborane, etc., are introduced into the source chamber at a lower chamber temperature which preserves the molecular structure of the ionized molecules having lower implant energies. Low implant energies typically have values below 20 keV. 
         [0018]      FIG. 2  is a side view of an exemplary vacuum processing assembly  200  associated with an ion implanter  100  shown in  FIG. 1 . Assembly  200  is disposed downstream of the ion beam  95  within a processing chamber maintained at vacuum and is used to provide target wafers or substrates to platen  114  for implantation. Generally, unprocessed target substrates or wafers are typically stored in loadlock chambers (not shown) and transferred to process chamber  200  for processing/implantation and transferred back to one of the loadlocks after processing. The processing assembly  200  includes shaft  201  which supports platen  114  upon which a target substrate or wafer  216  is disposed for processing. A pair of support arms  210   a  and  210   b  are disposed below platen  114  and are used to support a pair of thermal pads  208   a  and  208   b . Support arms  210   a  and  210   b  may be configured to rotate between an engaged position underneath platen  114  and an open position radially away from the platen  114 . Support arms  210   a  and  210   b  may be made, for example from aluminum. 
         [0019]    The pads  208   a  and  208   b  include a plurality of channels  212  to accommodate the flow of coolant, therethrough. The coolant flowing through the channels  212  cools the pads  208   a  and  208   b  which in turn cools the platen  114  by contact with a gas heat transfer fluid in the contact area. The wafer  216  is cooled to a desired temperature for ion implantation by platen  114  due to contact therewith and the gas heat transfer fluid in the contact area between the wafer  216  and platen  114 . The coolant may be, for example, N2 gas at −180 C. Alternatively, pads  208   a  and  208   b  may be integrally formed with respective support arms  210   a  and  210   b  and configured with the plurality of channels  212  to accommodate coolant flow. The channels  212  connect to one or more pipes  214  through shaft  201  that provide the coolant and the heat transfer gas to the pads  208   a  and  208   b  from a supply source. 
         [0020]    Seals  220   a ,  220   b  are disposed between pads  208   a ,  208   b  (respectively) and platen  114  to form seals there between. Seals  220   a ,  220   b  may be formed from an elastomer or other pliable plastic material to form a deformable seal between these components within the vacuum environment. Seals  220   a ,  220   b  may be disposed within a channel in either the underside of platen  114  or pads  208   a ,  208   b . The seals  220   a ,  220   b  may be referred to as “lip seals” since, as is typical in wafer processing, platen  114  and consequently wafer  216  are displaced in any one of several directions to allow for scanning of beam  95  toward wafer or substrate  216 . Seals  220   a ,  220   b  may be lip seals which accommodate rotational movement of platen  114  with respect to thermal pads  208   a ,  208   b.    
         [0021]    Although seals  220   a ,  220   b  are described herein with reference to positioning between platen  114  and pads  208   a ,  208   b , seals  220   a  and  220   b  may be disposed between any of the components within a cryogenic processing environment. For example, seals  220   a  and/or  220   b  may be used to provide a rotary seal between a shaft, such as shaft  201 , and a rotating assembly such as, platen  114 . In this configuration, the seal ( 220   a ,  220   b ) is disposed around the shaft  201  between the shaft  201  and the rotating assembly. The shaft  201  typically has passages or channels similar to channels  212  there through to carry cryogenic processing media (e.g. gas) from the stationary shaft  201  to the rotating assembly (e.g. platen  114 ). 
         [0022]    Each of the seals  220   a ,  220   b  includes a spring (see  FIG. 4 ) disposed between walls of the seal to provide a lateral or outward biasing force. This lateral force allows the seal to provide relatively tight contact between the platen  114  and respective pads  208   a ,  208   b . Because seals  220   a ,  220   b  are in contact with pads  208   a ,  208   b , it is subject to cryogenic processing temperatures in the range from below room temperature to about −100° C. At these low temperatures, the material used to form the seals  220   a ,  220   b  would typically become brittle thereby compromising the seal formed between the platen  114  and corresponding pads  208   a ,  208   b . However, a low voltage or current is supplied to the spring within each of the seals  220   a ,  220   b  which forms a heater within each seal. The amount of heat supplied from each of the springs to the walls of each seal  220   a ,  220   b  prevents the material from becoming brittle thereby maintaining its pliable sealing properties while not being large enough to negatively impact the thermocoupling of cryogenic temperatures from pads  208   a ,  208   b  to platen  114 . 
         [0023]      FIG. 3  is a side view of another exemplary vacuum processing assembly  300  associated with an ion implanter  100  shown in  FIG. 1 . Similar to assembly  200  shown in  FIG. 2 , assembly  300  is disposed downstream of the ion beam  95  within a processing chamber maintained at vacuum and is used to provide target wafers or substrates to platen  114  for implantation. The processing assembly  300  includes platen  114  used to support a target substrate or wafer  216 . In contrast to the assembly  200  shown in  FIG. 2  which utilizes a pair of support arms  210   a ,  210   b  and a pair of thermal pads  208   a ,  208   b , assembly  300  includes one support arm  310  disposed below platen  114  used to support thermal pad  308  which extends substantially the length of platen  114 . Support arm  310  may be configured to move vertically upward to position thermal pad  308  in contact with the underside of platen  114  during wafer processing and to move vertically downward to position thermal pad  308  away from the underside of platen  114  when cryogenic processing is not employed. A thermal pad  308  extends the length of platen  114  and includes a plurality of channels  312  to accommodate the flow of coolant, therethrough. The channels  312  connect to one or more pipes  314  that provide the coolant and the heat transfer gas to the pad  308  from a supply source (not shown). The coolant flowing through the channels  312  cools the pad  308  which in turn cools the platen  114  by contact with a gas heat transfer fluid in the contact area. 
         [0024]    In this configuration, a single lip seal  320  is disposed between thermal pad  308  and platen  114 . Seal  320  includes a spring  330  (see  FIG. 4 ) disposed between walls of the seal to provide an outward biasing force. A low voltage or current is supplied to the spring within seal  320  which forms a heater within the seal. The amount of heat supplied from the spring  330  to the walls of seal  320  prevents the material from becoming brittle thereby maintaining its pliable sealing properties while not being large enough to negatively impact the thermocoupling of cryogenic temperatures from pad  308  to platen  114 . 
         [0025]    It should be noted that although seals  220   a ,  220   b , and  320  are being described herein with respect to cryogenic processing in an ion implanter, alternative uses of such a heated lip seal configuration may also be employed where pliability of a seal is required to maintain a gas seal between cryogenic sealing surfaces. 
         [0026]      FIG. 4  is a top view of a lip seal  320  taken along lines A-A in  FIG. 3 . The following description of seal  320  is equally applicable to each of seals  220   a ,  220   b  shown in  FIG. 2 . Seal  320  is shown as having a generally circular shape, however alternative configurations may be employed depending on the geometry of the components disposed on either side of the seal. Seal  320  includes a pair of opposing side walls  325   a ,  325   b  and a bottom wall  327  (shown in  FIG. 5 ). These walls may be extruded from an elastomer or other pliable plastic material. Spring  330  is radially disposed toward bottom wall  327  between opposing side walls  325   a ,  325   b . Spring  330  provides a biasing force outward or laterally toward each of side walls  325   a  and  325   b . Spring  330  is made from, for example, a conductive material such as metal and may have a generally circular, helical or other configuration consistent with the requirements of providing an outward biasing force against side walls  325   a ,  325   b  and conducting an applied current or voltage. 
         [0027]    Spring  330  includes a first end  331  and a second end  332  each connected to a respective electrical wire  335 ,  336 . One of the wires  335 ,  336  is connected to a power source and the other is connected to neutral to complete a closed loop circuit. When a voltage or current is applied to spring  330  via wires  335 ,  336 , heat is generated through the spring which radiates along the walls  325   a ,  325   b  and  327  of seal  320  thereby heating the seal and preventing the seal material from becoming brittle. This heat allows seal  320  to remain pliable during low temperature cryogenic processing. The amount of current applied to spring  330  may be regulated based on the amount needed to generate sufficient heat to keep seal  320  pliable at particular cryogenic processing temperatures. This provides the necessary gas seal required by maintaining the temperature of the seal  320  to be compliant with cryogenic sealing surfaces within the vacuum processing chamber.  FIG. 5  is a cross sectional view of lip seal  320  (as well as seals  220   a ,  220   b  in  FIG. 2 ) taken along lines B-B of  FIG. 4 . Seal  320  is defined by opposing side walls  325   a ,  325   b  and a bottom wall  327  all of which forms an inner cavity  321 . Spring  330  may be disposed toward bottom wall  327  between opposing side walls  325   a ,  325   b  and provides a biasing force outward in the direction of arrows  340  toward each of the side walls. This biasing force allows seal  320  to contact cryogenic component surfaces. Side walls  325   a ,  325   b  may be angled inward toward each other or a center point of cavity  321  to provide an appropriate surface area to contact the cold sealing surfaces of either platen  114  or pad  308 . Spring  330  may be a coil spring that defines a continuous electrical path from the first end  331  to the second end  332 . Thus, spring  330  provides a bias force against the side walls  325   a ,  325   b  as well as providing a heating element to heat seal  320  to maintain its pliability and consequently its sealing properties. 
         [0028]      FIG. 6  is a cross sectional view of an alternative configuration of lip seal  420  (as well as seals  220   a ,  220   b  in  FIG. 2 ) taken along lines B-B of  FIG. 4 . Seal  420  is defined by opposing side walls  425   a ,  425   b  and a bottom wall  427  which forms an inner cavity  421 . As can be seen, the walls of seal  420  have a radius of curvature as compared to the walls of seal  320 . This alternative configuration may be adapted for various uses depending on the shape of the cryogenic sealing surfaces. Spring  430  is disposed within cavity  421  between opposing side walls  425   a ,  425   b  and provides a biasing force outward in the direction of arrows  440  toward each of the side walls. This biasing force allows lip seal  420  to contact cryogenic component surfaces. Spring  430  also provides a continuous electrical path to accommodate an applied voltage and current flow to heat spring  430  and consequently maintain the pliability and sealing properties of lip seal  420 . 
         [0029]    While the present invention has been disclosed with reference to certain embodiments, numerous modifications, alterations and changes to the described embodiments are possible without departing from the sphere and scope of the present invention, as defined in the appended claims. Accordingly, it is intended that the present invention not be limited to the described embodiments, but that it has the full scope defined by the language of the following claims, and equivalents thereof.