Abstract:
A substrate processing pallet can cool a substrate. A substrate processing pallet can include a base member; an interface pad attachable to the base member, the interface pad having substantially the same coefficient of thermal expansion as the base member and adapted to facilitate cooling of the substrate; and a surface of the base member having features for aligning a substrate on the interface pad. A substrate processing pallet can also include a base member; an interface pad attachable to the base member; an electrostatic chuck for gripping the substrate during processing; an energy storage system for storing energy to sustain the electrostatic chuck at sufficient charge to sustain grip the substrate during processing; and a conduit for transporting gas to a backside of the substrate to facilitate cooling of the substrate.

Description:
FIELD OF THE INVENTION 
     The invention relates generally to substrate processing pallets. More particularly, the invention relates to substrate processing pallets adapted to cool substrates and related methods and apparatus for cooling substrates. 
     BACKGROUND OF THE INVENTION 
     Substrate processing, such as semiconductor wafer processing, can raise the temperature of the substrate above an optimal processing value. For example, substrate heating during physical vapor deposition (PVD) can cause residual thermal stress in a deposited film, lead to poor adhesion of the deposited film, and otherwise damage sensitive substrates. 
     Some substrate processing tools use batch processing, whereby semiconductor wafers are transported on pallets through the tool and processed while held by self-contained wafer holders. For example U.S. Pat. Nos. 6,530,733, 6,682,288, and 6,821,912, the disclosures of which are incorporated herein by reference, describe substrate processing pallets for batch processing, related substrate processing machines, and methods in which the pallets are exposed to various temperatures during processing. 
     In general, the temperature of substrates processed on a pallet depends on the processing power and the degree of thermal contact between the substrate and the pallet. Temperature control can be accomplished by reducing the processing power, but this reduces the tool throughput, which is not desirable because it increases the processing cost per substrate. It is preferable to control temperature by maintaining sufficient thermal contact between the substrate and the pallet. 
     Semiconductor processing is often accomplished at low pressures in a vacuum chamber. Films deposited by PVD, for example, are often deposited at gas pressures of a few mTorr. At these low pressures, the thermal contact between surfaces is often low, and may not provide sufficient cooling for high throughput processing. 
     Substrate Cooling Principles: 
     Several general principles can maximize substrate cooling on a pallet. These include using pallet materials with high thermal conductivity, ensuring that the substrate and pallet are parallel and in good physical contact, decreasing the roughness of the surfaces, increasing the pallet heat capacity and increasing the gas pressure at the interface. 
     Materials with good thermal conductivity include various metals and ceramics. Good physical contact can be maintained by having flat, parallel substrate and pallet surfaces and by increasing the pressure of the substrate onto the pallet. Decreasing the surface roughness is accomplished by use of smooth polished surfaces, since smooth surfaces typically have twice the solid spot conductance of rough surfaces for the same pressure of substrate onto pallet. Increasing the heat capacity of the pallet improves cooling, because for the same process heat loading, the equilibrium tray temperature remains cooler. 
     Increasing the gas pressure at the interface increases substrate cooling, because heat transfer across a gap is primarily due to gas conduction, which is essentially a linear function of gas pressure under typical processing conditions. Increasing gas pressure at the interface to facilitate cooling is known as active cooling using backside gas pressure. In contrast, employing smooth surfaces to facilitate thermal conduction and cooling is known as passive cooling. 
     The substrate can be clamped against the pallet by mechanical or electrical means to increase the gas pressure at the interface. Clamping is necessary because otherwise the gas can either escape or possibly lift the substrate off the pallet. The electrostatic chuck (ESC) is an electrical method that permits uniform holding over virtually the entire substrate area and avoids edge exclusion and particles associated with mechanical clamps. 
     Pallets with Passive Cooling: 
     The simplest pallet consists of a tray fabricated from a single block of metal, such as aluminum. An improved surface smoothness can be achieved by bonding ceramic or semiconductor pads to the metal pallets to act as the interface between substrate and pallet. Silicon wafers can be used as interface pads because they are relatively inexpensive and are typically have sub-micron surface roughness resulting from chemical-mechanical polishing (CMP). A surface polished with CMP is much smoother than a metal pallet, with a resulting increase in substrate cooling. Such a tray is available as Part No. K11007815: Aluminum Silicon Tray Assembly, from NEXX Systems, Inc. of Billerica, Mass. 
     There are significant limitations to the substrate cooling ability of passive-cooling pallets made from either single metal block or containing ceramic or semiconductor interface pads. The surface finish and the flatness that can be achieved cost-effectively using standard metal fabrication techniques limit the cooling of simple metal pallets. Pallets containing ceramic or semiconductor pad interfaces are not robust because of thermal expansion mismatch between the metal pallet and the ceramic or semiconductor. As listed in Table 1, silicon has a thermal expansion coefficient approximately eight times smaller than the pallet&#39;s aluminum base metal. When such pallets are heated, the higher thermal expansion of the metal pallet can cause cracking of the pad, creating a rough surface and requiring expensive pad replacement. 
                                   TABLE 1                   Thermal properties of materials used in substrate       processing pallets over the 20° C.-80° C.       pallet temperature use range. Values for aluminum       are for a range of alloys. Values for alumina/PTFE       are for a range of commercially available compositions.                Thermal Conductivity   Thermal Expansion       Interface Pad Material   (W/cm° C.)   Coefficient (ppm/° C.)               Aluminum   1.5-2.4   21-25       Silicon   1.2-1.8   2.6-3.2       Alumina/PTFE    0.01-0.025   20-30       composite                    
Pallets with Active Cooling:
 
     A pallet can incorporate an electrostatic chuck (ESC) and active backside gas (BSG) cooling to facilitate cooling. Similar to capacitors, electrostatic chucks can hold a charge for a period of time after being disconnected from their power supply. The ESC self-discharge time depends on its material of fabrication and geometry, which determine the electrical resistivity and the ESC capacitance. Self-discharge times for ceramic or polyimide ESCs are shorter than the several minutes typical of semiconductor processing operations. This requires the ESC to be continuously connected to an energy source during processing. 
     Furthermore, pallets are transported through a tool, which prevents continuous electrical or gas connections. Instead, the connections are made and re-made as the pallet moves through the system. The electrical connections to the pallet can still be protected from plasma and arcing, just as in a fixed system, but in a way that allows for disconnection and re-connection. 
     Pallet Cleaning: 
     Pallets in a plasma processing environment require periodic cleaning. For example, thin films deposited on pallet areas not covered by substrates can be cleaned when the deposits exceed a certain thickness. A pallet can be removed from the tool and cleaned using mechanical or chemical means. Conventional pallets are often cleaned using grit blasting with abrasive media. However, such cleaning can harm delicate pallet features or embed abrasive media in areas from which they are difficult to remove. Chemical cleaning of the entire pallet is a preferable alternative to grit blasting, but pallets often contain adhesives or other polymeric materials which can be harmed by harsh chemicals used for cleaning. 
     SUMMARY OF THE INVENTION 
     In various embodiments, the invention relates to pallets adapted for holding substrates during processing and to substrate processing apparatus and methods adapted to employ the substrate processing pallets. A substrate processing pallet according to the invention provides features for cooling a substrate. 
     A pallet can improve substrate cooling by improving thermal contact between the pallet and the substrate. An interface pad can be used in a pallet to match the coefficient of thermal expansion of the substrate, to mitigate cracking the interface pad during processing. A pallet can also improve substrate cooling by improving backside gas pressure between the pallet and the substrate. An electrostatic chuck, which includes an energy storage system and does not need to be continuously connected to a power source, can be used to grip the substrate, to improve backside gas pressure during processing. The energy storage system can be adapted to increase the self-discharge time of the electrostatic chuck. 
     In one aspect, the invention features a substrate processing pallet having a base member, an interface pad, and a surface of the base member. The interface pad attaches to the base member, has substantially the same coefficient of thermal expansion as the base member, and facilitates cooling of the substrate. The surface of the base member has features for aligning a substrate on the interface pad. 
     In another aspect, the invention features a method for cooling a substrate on a substrate processing pallet. The method includes providing a base member, disposing an interface pad on a surface of the base member, and disposing the substrate on the interface pad. The interface pad has substantially the same coefficient of thermal expansion as the base member, facilitates thermal communication between the substrate and the base member, and facilitates cooling of the substrate. 
     In yet another aspect, the invention features a substrate processing pallet having a base member, an interface pad, an electrostatic chuck, an energy storage system, and a conduit for transporting gas. The interface pad attaches to the base member. The electrostatic chuck grips the substrate during processing. The energy storage system stores energy to sustain the electrostatic chuck at sufficient charge to sustain the grip on the substrate during processing. The conduit transports gas to a backside of the substrate to facilitate cooling of the substrate. 
     In still another aspect, the invention features a method for cooling a substrate on a substrate processing pallet. The method includes providing a base member; disposing an interface pad on a surface of the base member; disposing the substrate on the interface pad; charging an energy storage system; conducting energy from the energy storage system to the electrostatic chuck, to maintain grip of the substrate during processing; gripping the substrate with the electrostatic chuck; and transporting gas to a backside of the substrate to facilitate cooling of the substrate. 
     In other examples, any of the aspects above, or any apparatus or method described herein, can include one or more of the following features. In various embodiments, the base member can include aluminum. In some embodiments, a bonding layer can be used to bond the base member and the interface pad. The interface pad can be a composite material. The composite material can be an alumina particulate and a polymer matrix. 
     In certain embodiments, the substrate processing pallet includes at least one recess adapted to receive a substrate. The recess includes a support structure adapted to contact a portion of the substrate through the composite interface pad. The substrate processing pallet can include a plurality of apertures through each of which a lift pin may extend to initially support the substrate above the recess and to subsequently retract to deposit the substrate onto the composite interface pad. The substrate processing pallet can also feature a plurality of side surfaces. At least one of the side surfaces can have a process positioning feature adapted to engage with a process chamber feature located inside of a process chamber to particularly position the pallet within the process chamber in response to the pallet being placed into the process chamber. At least one of the side surfaces can have a transport positioning feature adapted to engage with a first end effector alignment feature of a first transport mechanism to particularly position the pallet with respect to the end effector. At least one of the side surfaces can have one or more support features. Each support feature can be adapted to engage with a corresponding end effector support feature of the transport mechanism to support the pallet on the end effector during transport. 
     In various embodiments, a removable, chemically resistant cover can facilitate cleaning. The chemically resistant cover can include a feature adapted for positioning and/or gripping the substrate. 
     In some embodiments, an electrostatic chuck can include a polyimide. 
     In certain embodiments, the pallet can be cooled. The means for cooling the pallet can include a water cooled plate in thermal communication with the pallet. The means for cooling the pallet can also include an interface layer disposed between the pallet and the means for cooling the pallet, to facilitate thermal conductance between the pallet and the means for cooling the pallet. 
     In various embodiments, a process alignment feature can match corresponding tray alignment features, to facilitate the use of a close tolerance process shield. The shield can be a metal cover adapted to lift during charging and covers a hole during processing. 
     In some embodiments, a charger can charge the energy storage system, a conductor can conduct energy from the energy storage system, and a shield can prevent charged particles from contacting the conductor. The energy storage system can include a high voltage capacitor. The high voltage capacitor can include polypropylene film. The charger can include a spring loaded contact providing power. 
     In certain embodiments, gas can be provided to the pallet. The means for providing gas to the pallet can be a transport finger that engages with a suction cup. 
     In various embodiments, the conduit includes a gas channel, a gas channel seal, a gas distribution system within the pallet, and a gas delivery outlet for uniform gas delivery. 
     In some embodiments, a sensor can measure a gas pressure at the backside of the substrate. 
     In certain embodiments, the electrostatic chuck also includes a discharge circuit for automatically discharging the energy storage system. 
     Other aspects and advantages of the invention will become apparent from the following drawings and description, all of which illustrate principles of the invention, by way of example only. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The advantages of the invention described above, together with further advantages, may be better understood by referring to the following description taken in conjunction with the accompanying drawings. The drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating the principles of the invention. 
         FIG. 1  is a cross-section view of an inline substrate processing machine having illustrative features of the invention. 
         FIG. 2A  is a perspective view of a prior art active cooling pallet. 
         FIG. 2B  is a cross-section detail of features of a prior art active cooling pallet. 
         FIG. 3A  is a perspective view of a portion of an inline substrate processing machine having illustrative features of the invention. 
         FIG. 3B  is a perspective view of the cooled lift-plate portion of the inline substrate processing machine shown in  FIG. 3A . 
         FIG. 3C  is an exploded view of the cooled lift-plate portion of the inline substrate processing machine shown in  FIG. 3A  showing the thermal substrate material. 
         FIG. 4  is a perspective view of an active cooling substrate processing pallet. 
         FIG. 5  is an exploded view of the substrate processing pallet of  FIG. 4  having illustrative features of the invention. 
         FIGS. 6A-6C  are perspective views of various active cooling substrate processing pallets. 
         FIG. 7A  is an exploded view of an active cooling substrate processing pallet. 
         FIG. 7B  is an exploded view of the electrostatic chuck portion of the active cooling substrate processing pallet. 
         FIG. 8A  is a view of an active cooling substrate processing pallet. 
         FIG. 8B  is a detail view of the modular charging system of the active cooling substrate processing pallet. 
         FIG. 8C  is a perspective view of the modular charging system of the active cooling substrate processing pallet. 
         FIG. 9  is a circuit schematic of the modular charging system of  FIG. 8C . 
         FIG. 10A  is a perspective view showing the transport fingers and lift pin-plate assembly portions of the processing machine shown in  FIG. 1  and the substrate processing pallet of  FIG. 6C . 
         FIG. 10B  is a cross-section view of the charging system portion of the pin plate lift assembly. 
         FIG. 10C  is a cross-section view showing the gas distribution system, transport fingers and lift pin-plate assemblies. 
         FIG. 11  is a perspective view of a processing chamber portion of an inline substrate processing machine having illustrative features of the invention. 
         FIG. 12A  is a top view of a substrate processing pallet after removal of a cleanable rim. 
         FIG. 12B  is a top view of the cleanable rim removed from the substrate processing pallet of  FIG. 12A . 
         FIG. 12C  is a cross-sectional view of the substrate processing pallet of  FIG. 12A . 
         FIG. 12D  is a perspective view of a cleanable cover for a substrate processing pallet. 
         FIG. 13A  is a top view of a processing shield. 
         FIG. 13B  is a cross-section view of a section of the processing shield of  FIG. 13A . 
     
    
    
     DESCRIPTION OF THE INVENTION 
       FIG. 1  is a side, schematic view of an inline substrate processing machine  100  including of a load-lock module  101 , a first processing chamber  102  and a second processing chamber  103 . A transport mechanism, such as that described in U.S. Pat. No. 6,530,733, the disclosure of which is incorporated herein by reference, moves pallets holding substrates between each chamber. Substrates are placed onto pallets in the load-lock  101  by a robotic end-effector (not shown). In operation, pallets are located in each chamber. After the pallet is filled with substrates and air is evacuated from the load-lock  101 , the pallet is transported to the first processing chamber  102 , then to the second processing chamber  103 , and then back to the load-lock  101  for recovery of the processed substrates. The substrate processing machine  100  also includes mass flow controller (MFC)  110  and a capacitance manometer  111  to facilitate processing in the second processing chamber  103 . 
       FIG. 2A  shows a pallet  120  that is fabricated by milling of a single block of aluminum. Recess pocket features  122  locate the substrate and maintain proper substrate positioning during processing. 
       FIG. 2B  shows details of the recess pocket, including the pocket floor  123  and the restraining lip  124 . When located in the pocket, the substrate rests directly on the aluminum base metal  125  without an interface pad. 
       FIG. 3A  shows a load-lock module  101  of an inline substrate processing machine  100  adapted to employ a substrate processing pallet  300 , as well as provide the necessary transport, electrical, and gas interfaces. The substrate processing pallet  300  is shown prior to substrate loading. The water-cooled lift plate  303  is capable of vertical motion, and the transport fingers  301  and  302  are capable of horizontal motion. In some embodiments, the water-cooled lift plate  303  is a cooling plate that is not water cooled. The pallet is shown resting on the water-cooled lift plate  303 , and the fingers  301  and  302  are in recessed slots in the cooling plate. When in a downward, retracted position, the pallet rests on transport fingers  301  and  302 . 
       FIG. 3B  shows the water-cooled lift plate  303  of  FIG. 3A , including recessed slots  409 , water cooling lines  410 , an electrostatic chuck charging pin  415 , a suction cup  408  and a substrate lift pin  411 . During processing, the substrate processing pallet  300  acts as a thermal mass conducting heat away from the substrate. When the pallet is returned to the load-lock  101 , this heat is transferred to the water-cooled lift plate  303 . Suction cups  408  pull the pallet  300  toward the water-cooled lift plate  303 , increasing the contact force and thus facilitating cooling of the pallet  300 . 
       FIG. 3C  shows an interface material  414 , which can be disposed between the pallet  300  and the water-cooled lift plate  303  to facilitate cooling of the pallet  300 . In one embodiment, the interface material  414  is a 0.005″ thick layer of aluminum bonded to the lift-plate using a 0.005″ thick layer of THERMATTACH® T405 material, available from the Chomerics Division of the Parker-Hannifin Corporation, Woburn, Mass. 
       FIG. 4  shows a passive cooling substrate processing pallet  130  designed to transport and cool three 200 mm diameter substrates. The system can be adapted for 100 mm, 150 mm, and 300 mm substrates, as well as other sizes depending on the application. The base  131  is manufactured from aluminum. The interface  132  includes a polymer-ceramic composite with a thermal expansion coefficient closely matched to aluminum, bonded to an aluminum pad. The holes  133  allow lift pins inserted in the cooled lift plate to raise and lower the substrate from the pallet  130 . 
       FIG. 5  shows the components of the substrate processing pallet  130 , including interface pads  132 , bonding layers  135 , and the base  131 . In various embodiments, the interface pads  132  include a composite material having alumina particulate reinforcement in a matrix of PTFE Teflon laminated to an aluminum base. Such a composite material is available in various reinforcement concentrations as the THERMAL CLAD® product line from the Bergquist Company of Chanhassen, Minn. In one embodiment, the composite material is THERMAL CLAD® LT1. In one embodiment, the interface pads  132  are LT1 pads with a dielectric thickness of 0.003″ laminated to a 0.040″ thick aluminum base plate. In various embodiments, the composite material creates a hard, flat, scratch-resistant surface with a thermal conductivity nearly equal to the aluminum base  131 . 
     The bonding layer  135  can be, for example, a double-sided pressure-sensitive thermal adhesive tape with an aluminum foil carrier. In various embodiments, the bonding layer  135  is a low-outgassing vacuum-compatible material with total material loss of &lt;1% and a maximum collected volatile condensable material rating of &lt;0.1% as tested according to ASTM Standard E 595-77/84/90. In one embodiment, the bonding later  135  is THERMATTACH® T405 available from the Chomerics Division of the Parker-Hannifin Corporation, Woburn, Mass. 
       FIG. 6A  shows an active cooling substrate processing pallet  350  having two integrated electrostatic chucks  326 , each designed to hold and cool a 300 mm diameter substrate. Each chuck  326  consists of an outer electrode region  327  and an inner electrode region  328 . Each chuck  326  has a plurality of gas cooling holes  329  and gas flow channels  330  designed to provide uniform backside gas pressure. Each chuck  326  also has lift pin holes  331 , which are analogous to holes  133  in  FIG. 4 . 
       FIG. 6B  shows an active cooling substrate processing pallet  325 , which is designed to hold three 200 mm diameter substrates and include the features of the active cooling substrate processing pallet  350  described in  FIG. 6A . 
       FIG. 6C  shows an active cooling substrate processing pallet  300 , which is designed to hold four 150 mm diameter substrates and include the features of the active cooling substrate processing pallet  350  described in  FIG. 6A . 
       FIG. 7A  shows the components of the substrate-processing pallet  325  shown in  FIG. 6B . The pallet  325  has an aluminum base  360  into which gas distribution channels  362  and  363  are milled. However, the base can also be manufactured from another metal or material with similar thermal and mechanical properties as aluminum. Electrostatic chucks  375  are bonded to the base  360  using the bonding layer  135 . The bonding layer  135  has a plurality of holes  361 , which align with cooling holes  329  in the electrostatic chucks  375 , to transport gas to cooling holes  329  and gas flow channels  330 . The bonding layer  135  also includes cutout regions  366 . The gas flow channels  330  can be formed by lithographic etching. 
       FIG. 7B  shows the components of the electrostatic chuck  375 . The base layer  132  can include a dielectric layer to provide electrical insulation between the base layer  132  and an electrode layer  377 . The electrode layer  377  can include copper. The electrode pattern in the electrode layer  377  can be manufactured, for example, using a lithographic technique, such as those commonly used to produce flexible circuits. The electrode layer  377  includes cutout regions  366  that allow electrical connection to be made. In various embodiments, the bonding layer  376  is a heat-curable adhesive layer. In various embodiments, the outer layer  378  is a polyimide film laminated to the electrode layer  377 . In one embodiment the polyimide film bows in the area etched into the electrode layer  377 , to form the gas flow channels  330 . 
     In one embodiment, the bonding layer  376  is a 0.001″ heat curable acrylic film adhesive. One preferred adhesive is PYRALUX® LF0100 Sheet adhesive. In one embodiment, the electrode layer  377  and outer layer  378  are PYRALUX® FR9120 copper clad laminate with a 1 oz/ft 2  copper layer, 0.001″ thick adhesive and 0.002″ thick Kapton polyimide film. Both PYRALUX® materials are available from the DuPont Electronic Materials Company of Research Triangle Park, N.C. 
       FIG. 8A  shows the bottom of the substrate-processing pallet  325 , which includes transport and alignment features that are specific to each processing chamber. One transport feature is the parallel slot recesses  383 , which are each designed to accept transport fingers  301  and  302  shown in  FIG. 3 . The gas inlet  380  and gas outlet  381  are aligned with gas holes in transport fingers  301  and  302 . Suction cups  385  attach to the gas inlet  380  and gas outlet  381 , to provide a seal when the substrate-processing pallet rests on the transport fingers  301  and  302 . Pallet alignment features  388 ,  389 ,  386 , and  387  facilitate positioning the pallet. 
       FIG. 8A  also shows six integrated energy storage modules: three energy storage units  399  that have capacitors to be charged to one voltage polarity and three energy storage units  400  that have capacitors to be charged to the opposite polarity. The substrate-processing pallet  325  can be a bi-polar electrostatic chuck requiring access to at least two such modules, one for each voltage polarity. Various other embodiments can have a plurality of chucks connected in parallel to an even number of energy storage modules. In one embodiment, the energy storage unit  399  or  400  is chargeable, removable, and can be separated into a plurality of modules. A preferred energy storage unit or module is a high-voltage capacitor with sufficiently low leakage current to maintain an approximately full electrostatic chuck charge during processing. 
       FIG. 8B  shows the bottom of the energy storage unit  400  including a high-voltage storage capacitor  401 , discharge resistor  402 , and high-voltage contact  403 . An insulated wire (not shown) connects electrostatic chuck electrode to the high-voltage contact  403  by means of the ground terminal  405 . 
       FIG. 8C  shows the top of the energy storage unit  400 . The charging pill  404  and high-voltage contact  403  together constitute a normally-open switch which is closed when the pill is pushed up against the contact during charging. An insulating support structure  406  constrains and guides charging pill  404 . 
     In various embodiments, the high-voltage capacitor is a polypropylene film type capacitor that exhibits relatively low leakage current over an about 20° C. to about 80° C. substrate processing temperature range. In one embodiment, the capacitor is the 630V, 0.43 uF model ECWF6434 manufactured by the Panasonic Corporation. 
       FIG. 9  shows a circuit schematic for the energy storage unit  400 , including an external high-voltage power supply  425 , the charging pill  404  and high-voltage contact  403 , storage capacitor  401 , discharge resistor  402 , electrostatic chuck electrode  430 , and ground terminal  405 . 
     An active cooling pallet may have means to automatically discharge the storage system to ensure safe handling. The means to automatically discharge the storage system can be a discharge circuit that includes a resistor in series with the storage capacitor, the value of which results in the minimum stored charge when the pallet, charged in the vacuum chamber, is vented to atmosphere. In various embodiments, the energy storage module automatically discharges when vented to atmosphere, which can facilitate system reproducibility and safe handling. In one embodiment, a storage capacitor that is charged to high voltage in vacuum can automatically discharge to a value of less than about 50 V. 
     The value of discharge resistor  402  can be optimized by charging the storage capacitor  401  to full voltage in the load-lock  101  under high vacuum conditions, venting the load-lock to the atmosphere, which creates an arc discharge inside the module, and then measuring the remaining capacitor voltage. An optimal resistor value results in the minimum residual voltage and generally depends on the choice of storage capacitor. In one embodiment, the discharge resistor  402  is a 1000 Ohm, ½ W carbon composition resistor. 
       FIG. 10A  shows an assembly  333  including the processing-pallet  300  positioned on transport fingers  301  and  302  and the water-cooled lift plate  303 , which can be inside the load-lock  101 . 
       FIG. 10B  shows the charging system portion of the assembly  333 . During charging, the water-cooled lift plate  303  is in a raised position, so that charging pill  404  is in contact with the high-voltage contact  403 . Voltage from external high-voltage power supply  425  is conducted to the modular energy storage modules  399  or  400  by the charging pin  415 , which is insulated by the insulating sleeve  416 . After charging, the water-cooled lift plate  303  is lowered to a retracted position, so that pallet  300  rests on transport fingers  301  and  302 , and charging pill  404  rests on the grounded lower cover of module  400 . In this lowered position, charging pill  404  prevents charged particles from entering charging module  400 , thus preventing arcing or discharge of capacitor  401 . 
       FIG. 10C  is a side, cross-section view of the processing pallet  300  and transport finger  301 . A tube  407  inside the transport finger  301  transports gas to the underside of processing pallet  300 . Suction cup  385  provides a seal between the transport finger  301  and gas inlet  380 . During processing, gas flow can be provided to the transport fingers by the MFC  110 . Gas flow can be controlled by a feedback loop which uses the backside gas pressure as the controlling variable. Gas pressure can be monitored by the capacitance manometer  111 . A preferred instrument, which contains both the MFC and manometer, is the Model 649 Controller available from MKS Instruments of Wilmington, Mass. 
     The following process steps illustrate the operation of the load-lock hardware shown in  FIGS. 3 and 10  and the features of the active cooling pallet  300 . 
     1. Load one or more substrates onto the pallet  300  by robotic means. 
     2. Evacuate load-lock  101  to a pressure of about 10 −5  Torr or less. 
     3. With water-cooled lift plate  303  in the raised position, supply high-voltage to charge the energy storage modules  399  or  400 . Optionally, test the self-leakage rate of storage module  399  or  400  and electrostatic chuck  375 . 
     4. With the water-cooled lift plate  303  in a lowered position, rest the processing pallet  300  on the transport fingers  301  and  302  to establish a seal. 
     5. Direct gas through the transport fingers  301  and  302  to the backside of the substrate using the MFC  110 . 
     6. Using the control loop, set the flow to achieve the desired backside pressure. 
     7. If the equilibrium flow is within a pre-determined range, then the system is functioning properly, and the pallet is ready for further processing. Otherwise various corrective actions are taken to handle the fault, which can be due to either substrate or pallet problems. 
       FIG. 11  shows the second processing chamber  103 , which can provide the necessary environment for processing using either a passive cooling pallet  130  or an active cooling pallet  300 . The passive cooling pallet  130  or active cooling pallet  300  rests on the transport fingers  113  and  114 . The transport fingers  113  and  114  are attached to a transport crossbar  117 , which can scan the pallet back-and-forth under a linear deposition curtain to deposit a uniform film on the substrate. 
     When processing with an active cooling pallet  300 , the mass flow controller  110  directs gas into the processing chamber  103  through rigid tubing  115  and flexible tubing  118  to the transport finger  113 . An active cooling pallet  300  (not shown) receives gas through inlet  380 . The gas outlet  381  can be connected to a capacitance manometer  111  by flexible tubing  119  and rigid tubing  116 . During processing, a feedback control loop can vary the gas flow output of the MFC  110  to maintain a constant value of backside gas pressure as measured by the capacitance manometer  111 . 
       FIG. 12A  shows a substrate processing pallet  420  adapted to receive a cleanable rim (not shown). The pallet  420  includes a pallet body  422  and a mount  423  for a cover (not shown). Both passive cooling pallets and active cooling pallets can be adapted to receive a cleanable rim. 
       FIG. 12B  shows a cleanable rim  421  adapted to attach to the pallet  420 . The rim edge  424  can contain substrate alignment and/or containment features. Such features can be used to prevent rotation of substrates and/or to prevent substrate loss during transport. The rim  421  is fabricated from a metal that is resistant to attack by chemical cleaning agents. 
       FIG. 12C  shows a pallet assembly  426  with a cleanable rim  421  attached to the pallet body  422  by a bolt and the mount  423 . 
       FIG. 12D  shows a cleanable cover  444 , which can attach to the top of pallet  420  and can be removed for cleaning. 
     In various embodiments, combinations of cleanable rims and cleanable covers can mask a substrate processing pallet. For example, combinations of cleanable rims and cleanable covers can mask the exposed surfaces of substrate processing pallets such as those shown in  FIGS. 4-7 , to protect the pallets from processing and mitigate the need for cleaning the pallets. Rather, the cleanable rims and cleanable covers can be removed and cleaned. In various embodiments, cleanable rims and cleanable covers can be made from stainless steel. 
       FIG. 13A  shows a processing shield  450  adapted to fit on top of a passive cooling pallet or an active cooling pallet. The shield includes a lip  451  defining a window  421 . The shield  450  can allow plasma and/or reactants to interact with the substrate, but prevent plasma and/or reactants from interacting with the pallet. 
       FIG. 13B  shows a cross-section of the shield  450 , which includes a lip  451  having a shield rim  453  and a plasma acceptance channel  452 . The window  421  can allow relatively unobstructed access to the substrate. To optimize processing, the window  421  should be of the minimum possible diameter that allows relatively unobstructed access to the substrate, but otherwise masks the pallet. For a window  421  diameter only marginally greater than the substrate diameter, relatively unobstructed access to the substrate requires accurate placement of the window  421  relative to the substrate. For example, the shield rim  453  should be aligned with the rim edge  424  to close tolerances. Process alignment features can facilitate close tolerance placement of the substrate relative to the pallet and close tolerance placement of the pallet relative to the shield  450 . 
     Accurate placement of the substrate relative to the pallet requires accurate placement of the pallet in the load-lock, coupled with precise and accurate substrate handling, which can be achieved by robotic means. Pallet alignment features  388  and  389  in  FIG. 8A  align with load-lock tapered pins  412  and  413  in  FIG. 3B . When the pallet  300  is pulled toward water-cooled lift plate  303  by suction cups  408 , the tapered pins engage with the pallet features to accurately align the pallet for substrate loading. Accurate location of the pallet in the processing chamber with respect to the shield  450  is accomplished using alignment features and pins. Tapered alignment features in the processing chamber center the tray align with features  386  and  387  in  FIG. 8A  and center the tray when the features engage. 
     Experimental Example 
     A passive cooling pallet  130  as shown in  FIG. 4  and an active cooling pallet  325  as shown in  FIG. 6B  were fabricated and subjected to identical processing conditions as an Aluminum Silicon Tray Assembly (ASTA) Part No. K11007815, which has silicon interface pads bonded to an aluminum base. The substrates used were three 200 mm diameter 750 micron thick silicon wafers. A 2-micron aluminum metal film was deposited on the substrates using a NIMBUS™ XP magnetron sputtering system available from NEXX Systems, Inc. of Billerica, Mass. The magnetron sputtering power was 23 kW, the deposition time was 212 s, and the background gas pressure in the processing chamber was 2.5 mTorr. Non-reversible temperature indicating labels were attached to each substrate to record the maximum attained temperature. 
     Table 2 shows that substrates on the passive cooling pallet  130  and the active cooling pallet  325  were cooled to lower temperatures than when processed using the ASTA pallet. In addition, the passive cooling pallet  130  did not fracture under the test conditions, whereas a portion of the ASTA pallet did. 
     
       
         
               
             
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Measurements of the maximum temperature achieved by 
               
               
                 silicon wafer substrates when transported by various substrate 
               
               
                 pallets of the passive and active type and subjected to the same 
               
               
                 PVD processing recipe. Substrates were 200 mm diameter 
               
               
                 silicon wafer substrates. 
               
             
          
           
               
                   
                 Backside 
                 Pad 
                 Max. Substrate 
               
               
                   
                 Gas Pressure 
                 Thickness 
                 Temperature 
               
               
                 Pallet Type 
                 (mTorr) 
                 (mm) 
                 (° C.) 
               
               
                   
               
             
          
           
               
                 ASTA pallet 
                 2.5 
                 0.75 
                 250 ± 3 
               
               
                 Passive cooling pallet 130 
                 2.5 
                 1.25 
                 240 ± 9 
               
               
                 Active cooling pallet 325 
                 2500 
                 1.25 
                  60 ± 5 
               
               
                   
               
             
          
         
       
     
     While the invention has been particularly shown and described with reference to specific embodiments, it should be understood by those skilled in the art that various changes in form and detail may be made without departing from the spirit and scope of the invention as defined by the appended claims.