Abstract:
An apparatus for processing substrates includes a chamber, a substrate transfer element for transferring a substrate to and from the chamber, and a substrate support for receiving and holding a substrate within the chamber. The apparatus also includes multiple pins positioned and configured to be received by respective holes in the chamber bottom and moveable between a retracted position and an extended position. A pin actuation system is provided for moving the pins between the retracted position and the extended position. The pin actuation system controls the velocity at which the pins move and varies the speed of the pins by accelerating or decelerating at particular points during the pin cycle. A reduction in the cycle time is facilitated by accelerating the lift pins to relatively high speeds and then slowing the pins down prior to their arrival at locations where the substrate or wafer may be damaged. The throughput of the chamber can be increased, the likelihood of damage to the substrate can be reduced, and bouncing of the substrate while supported by the pins can be reduced.

Description:
BACKGROUND 
     The present invention relates to wafer or substrate processing chambers, and more particularly to the control of wafer lift pins in a rapid thermal processing (RTP) chamber used for processing semiconductor wafers or other substrates. 
     In the semiconductor processing field, various processing chambers are used to perform a variety of processes. These processes can include annealing, cleaning, chemical vapor deposition, oxidation, and nitridation. The processes may be applied under vacuum, under gas pressure and with the application of heat. 
     In one exemplary system for the thermal processing of semiconductor wafers, the wafer is carried by an edge ring which supports it. The wafer substrate is rotated so that the processes occur evenly over the wafer&#39;s surface. Loading and unloading of the wafer is automated. A number of lift pins are accommodated by holes or bores in the chamber bottom located below the wafer and edge ring. The lift pins are movable between retracted and extended positions. In the retracted position, the upper ends or tips of the pins are accommodated within the holes in the chamber bottom so as to be relatively shielded from the processes occurring in the chamber. 
     After a given wafer has been processed, the rotation of the wafer is stopped, and the lift pins are raised from the retracted position to the extended position. During transit from the retracted position to the extended position, the pins contact the lower surface of the wafer, lift it off the edge ring, and finally elevate it well above the edge ring. With the pins in the extended position, a transfer element can be inserted below the wafer. A typical transfer element is an end effector such as a fork or a blade of a robot external to the chamber. The end effector can be inserted into the chamber through a slit valve and is accommodated by the lift pins by being configured to either go around or between the lift pins. Once the end effector is below the wafer, the pins are lowered from the extended position to a retracted position. During the transit between the extended and retracted positions, the pins deposit the wafer on the end effector and then continue downward to the retracted position. The end effector then can be withdrawn from the chamber where it exchanges the wafer for a new one to be processed. The end effector carrying the second wafer then is inserted into the chamber. The pins are raised from the retracted position to the extended position. When the pins reach the level of the end effector, they contact the underside of the wafer and raise the wafer above the end effector until the pins reach the extended position. The end effector then is withdrawn and the pins again lowered from the extended position to the retracted position. When the pins reach the level of the edge ring, the edge ring contacts the underside of the wafer to acquire it from the pins. When the pins reach the retracted position, the edge ring is rotated, and the processing can be commenced. The time required to process each wafer, from its introduction to the chamber to the introduction of the next wafer, is designated the “cycle” time. 
     According to one exemplary system, the lift pins are driven pneumatically. The pins are coupled to the piston of a pneumatic cylinder. Upper and lower chambers above and below the piston are connected by an associated valve to atmosphere and to a compressed air source. The piston is raised by actuating the lower valve so that the lower chamber is connected to the compressed air source while actuating the upper valve so that the upper chamber is connected or vented to atmosphere. To lower the piston, the valve states are reversed. 
     For a given lift pin construction, the speed with which the pins move between the retracted and extended positions (the upstroke) and vice versa (the downstroke) is influenced by the pressure of the compressed air source (typically house compressed air at 60-80 pounds per square inch (psi)) and the particular throttling of the valves which can damp movement of the piston. Because of the throttling and other damping factors, the strokes occur at nearly constant velocity. Since the high cost of the chamber makes time-efficient use desirable, the valve throttling and other parameters should be selected to provide the fastest travel possible without damaging the wafer being handled. 
     The wafer can be damaged by impact of the pins if the pins move upward at an excessive speed. This can occur when the wafer is held either by the edge ring or by the end effector blade. Additionally, if the wafer and pins are moving downward at an excessive speed, the substrate may be damaged by contact with the blade or edge ring upon transfer. Also, if the pins are moving too quickly prior to reaching the upward extreme of their stroke, the wafer will continue moving upward after the pins have stopped and will then fall back down atop the pins and may be damaged. In one exemplary system, for example, the lift pin stroke is approximately 1.18 inches, and it takes approximately two seconds to get from the retracted to the extended position and vice versa. 
     Thus, it is desirable to facilitate a more efficient use of the chamber by reducing cycle time without unnecessarily risking damage to wafers. 
     SUMMARY 
     In general, according to one aspect, an apparatus for processing substrates includes a chamber, a substrate transfer element for transferring a substrate to and from the chamber, and a substrate support for receiving and holding a substrate within the chamber. The apparatus also includes multiple pins positioned and configured to be received by respective holes in the chamber bottom and moveable between a retracted position and an extended position. 
     A pin actuation system is provided for moving the pins between the retracted position and the extended position. The pin actuation system controls the velocity at which the pins move and varies the speed of the pins by accelerating or decelerating at particular points during the pin cycle. A reduction in the cycle time is facilitated by accelerating the lift pins to relatively high speeds and then slowing the pins down prior to their arrival at locations where the substrate or wafer may be damaged. Such locations or danger points include those in the following non-exhaustive list: (1) during a pin upstroke, the point at which the pins contact a wafer held by a substrate support such as an edge ring; (2) during a pin upstroke, the point at which the pins contact a wafer held by a transfer element such as a robot blade; (3) during a pin upstroke, the peak in pin travel when the pins carry a substrate from the substrate support; (4) during a pin upstroke, the peak in pin travel when the pins carry a substrate from the transfer element; (5) during a pin downstroke, the point at which a substrate held by the pins contacts the transfer element; and (6) during a pin downstroke, the point at which a substrate held by the pins contacts the substrate support. 
     Thus, according to one aspect, the pin actuation system is configured to bring the pins into engagement with a substrate held by the substrate support by (1) raising the pins from below the substrate at a velocity in a first upward velocity range and (2) then slowing the pins to a velocity in a second upward velocity range so that the pins contact a lower surface of the substrate while travelling at a velocity in the second upward velocity range, wherein the second upward velocity range is less than the first upward velocity range. The pin actuation system can further be configured to raise the substrate out of engagement with the substrate support to a height at which the substrate transfer element can be positioned beneath the substrate, by (1) raising the pins at a velocity in the first upward velocity range while the pins support the substrate; (2) subsequently slowing the velocity of the pins to a velocity in the second upward velocity range; and (3) bringing the pins to a stop at the height at which the substrate transfer element can be positioned beneath the substrate. The substrate then can be transferred to the transfer element by lowering the pins from the extended position. 
     According to another aspect, the pin actuation system is configured to transfer a substrate from the pins in their extended position to the substrate support by (1) lowering the pins at a velocity in a first downward velocity range, and (2) subsequently lowering the pins at a velocity in a second downward velocity range so that a lower surface of the substrate contacts the substrate support while travelling at the velocity in the second downward velocity range, wherein the second downward velocity range is less than the first downward velocity range. The pin actuation system can further be configured to lower the pins to the retracted position after the substrate has been transferred to the substrate support, wherein the pins are lowered to the retracted position at a velocity in the first downward velocity range. Additionally, the pin actuation system can be configured to cause the pins to acquire the substrate from the substrate transfer element by (1) raising the pins from a height beneath the substrate transfer element at a velocity in a first upward velocity range, and (2) then slowing the pins to a velocity in a second upward velocity range so that the pins contact the lower surface of the substrate while travelling at a velocity in the second upward velocity range, wherein the second upward velocity range is less than the first upward velocity range. Additionally, the substrate can be raised out of engagement with the substrate transfer element to a height at which the substrate transfer element can be withdrawn from beneath the substrate. 
     In various embodiments, the pin actuation system can be pneumatically-controlled. Alternatively, the pin actuation system can include a threaded rod, such as a lead screw, a stepper motor for driving the threaded rod, and a controller to control movement of the stepper motor. Further details of exemplary implementations of a pneumatically-controlled system and a motorized lead screw system are described below. 
     In some implementations, the first upward velocity range is at least about four times greater than the second upward velocity range. For example, in one particular implementation, the first upward velocity range is about 2.0 inches per second to about 4.0 inches per second, and the second upward velocity range is about 0.25 inches per second to about 0.5 inches per second. Similarly, in some implementations, the first downward velocity range is at least about four times greater than the second downward velocity range. For example, in one specific implementation, the first downward velocity range is about 2.0 inches per second to about 4.0 inches per second, and the second downward velocity range is about 0.25 inches per second to about 0.5 inches per second. 
     Methods of processing a substrate in a chamber also are disclosed and are described in greater detail below. 
     In various implementations, advantages of the invention include the capability of increasing the throughput of the chamber, reducing the likelihood of damage to the substrate, and reducing bouncing of the substrate while supported by the pins. 
     Other features and advantages of the invention will be apparent from the following description, drawings and claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic view of a wafer processing system according to the invention. 
     FIG. 2 is a partial view of a wafer lift system according to the invention. 
     FIG. 3 is another view of the wafer lift system of FIG.  2 . 
     FIGS. 4-8 are schematic illustrations of a pneumatically-controlled substrate lift system according to the invention. 
     FIGS. 9-21 are partial cross-sectional views of the chamber of FIG. 1 in various stages of operation according to the pneumatically-controlled system. 
     FIGS. 22-23 are schematic illustrations of a substrate lift system incorporating a motorized lead screw according to the system. 
     FIGS. 24-35 are partial cross-sectional views of the chamber of FIG. 1 in various stages of operation according to the motorized lead screw system. 
    
    
     DETAILED DESCRIPTION 
     As shown in FIG. 1, a substrate or wafer processing system  20  includes a chamber  22  in which the processing occurs. In the interior of the chamber, a wafer  24  having an upper surface  26 , a lower surface  28 , and a perimeter  30 , can be secured within a pocket  32  of an edge ring  34  or other substrate support. A wafer lift mechanism  36  depends from the chamber bottom  38 . A slit valve  40  located in the chamber wall facilitates the introduction and removal of wafers to and from the chamber  22 . A robot  42  with an end effector formed as a blade  44  at the distal end of the robot&#39;s arm facilitates the transfer of wafers. 
     As shown in FIG. 2, the wafer lifting system includes a fixed portion  50  secured to the chamber bottom  38 . A movable portion  52  is coupled to the fixed portion to permit vertical reciprocation. The movable portion  52  includes three lift pins  54  on an elevator or “spider plate”  56 . The lift pins extend into and are received by holes  58  in the chamber bottom (see, e.g., FIG.  9 ). To maintain a leak tight or vacuum condition, each pin  54  is carried within a bellows  60  (FIG. 2) sealed to the chamber bottom and to the elevator  56 . The movable portion  52  can be raised between a retracted position to an ex 25  tended position as described in further detail below. 
     The fixed portion  50  includes a vertically oriented array of sensors  64 A,  64 B,  64 C and  64 D, mounted on a printed circuit board  65 . Each sensor  64 A,  64 B,  64 C,  64 D includes a photoemitter and a detector for detecting light emitted by the respective photoemitter. A vertically oriented plate  66  is fixed to the movable portion  52  and positioned between the photoemitter and the detector of each sensor  64 A,  64 B,  64 C,  64 D. The plate bears a vertically elongated triggering aperture or slit  68  located to pass sequentially between the photoemitter and detector of each sensor  64 A,  64 B,  64 C,  64 D during vertical movement of the portion  52 . 
     When the aperture  68  is aligned with a sensor  64 A- 64 D, the aperture permits transmission of light from the photoemitter of the sensor to the detector associated with that sensor. This transmission places the sensor in a positive state and causes the sensor to supply a specific input signal to a control system  100  (FIG.  1 ). The control system  100  can be a computer programmed with appropriate control software and coupled to the chamber  22  and robot  42  to control their respective operations. When no light is transmitted between the photoemitter and detector of a given sensor, the sensor is in a negative state and the specific signal is not present. The aperture  68  need not be exactly aligned with a sensor for the sensor to be in a positive state. A range of aperture positions are associated with each sensor and permit the transmission of sufficient light to place the sensor in the positive state. The aperture positions are associated with a range of heights of the pins, or, more precisely, the tips of the pins. The range of pin heights extends between a lower height H 1  and an upper height H 2  which correspond, respectively, to lower and upper aperture positions. The difference between these heights will depend upon the length of the aperture and the properties of the sensor. Thus, there exist respective pairs of heights H A1  and H A2 ; H B1  and H B2 ; H C1  and H C2 ; and H D1  and H D2  for sensors  64 A,  64 B,  64 C,  64 D (see FIG.  2 ). The change in a sensor&#39;s state caused by the lifting system moving the pins through any of these heights can be used to control the speed at which the lifting pins  54  are moved. 
     In any given implementation, fewer or more sensors can be provided and can be utilized in different combinations to initiate and terminate various stages in the operation of the lift system  36 . For example, a single encoder. can be used with its output summed to provide position data or otherwise processed to provide position or velocity data. 
     The portion  52  of the wafer lifting system  36  is moved relative to the fixed portion  50  by means of a slide  62  which, according to one embodiment, is controlled pneumatically. In the pneumatically controlled emodiment, the fixed portion  50  includes a pneumatic cylinder  72  (FIGS.  4 - 8 ). A piston  74  within the pneumatic cylinder  72  is linked to the movable portion  52  so that vertical movement of the piston  74  produces an associated vertical movement of the movable portion  52 . The piston  74  divides the pneumatic cylinder  72  into an upper chamber  72 A and a lower chamber  72 B. A group of four 3-way direct control valves  76 A,  76 B,  76 C,  76 D establishes selective communication between the cylinder chambers  72 A and  72 B on the one hand, and a pneumatic source  200  and the atmosphere (ATM) on the other hand. Exemplary valves can be direct-acting solenoid valves such as those available from Precision Dynamics, Inc., New Britain, Conn. Each valve  76 A through  76 D has a respective primary port  78 A,  78 B,  78 C,  78 D, a normally closed port  80 A,  80 B,  80 C,  80 D, and a normally open port  82 A,  82 B,  82 C,  82 D. When a valve is in an unenergized state, communication is between the primary port and the normally open port. When the valve is in an energized state, communication is between the primary port and the normally closed port. Optionally, some of the normally closed and normally open ports can have a throttle  84  for further restricting flow through the associated port. The setting of each throttle can be used to affect the speed at which the portion  52  moves. An exemplary throttle can be provided by a speed controller such as Series AS of SMC Pneumatics Inc. 
     In a first mode of operation (FIG. 4) referred to as an “up fast” mode, the valve  76 B is not energized and the remaining valves are energized. Accordingly, the lower cylinder chamber  72 B is exposed to the pneumatic source  200  through the valves  76 C and  76 D in series. The upper cylinder chamber  72 A is vented to atmosphere through the valves  76 A and  76 B in series. When operated in the first mode, the pins  54  move upward at a velocity in a first velocity range. 
     In a second mode (FIG. 5) designated the “up slow”  10  mode, flow is restricted compared to flow in the “up fast” mode. The valve  76 A is de-energizing so that the upper cylinder chamber  72 A communicates with the atmosphere through the valve  76 A and a throttle  84 A in series. The valves  76 C and  76 D remain energized, and the state of the valve  76 B is irrelevant. When operated in the second mode, the pins  54  move upward at a velocity in a second upward velocity range lower than the first upward velocity range. 
     In a third or “down fast” mode (FIG.  6 ), the valves  76 A,  76 B,  76 C are energized while the valve  76 D is de-energized. The lower cylinder chamber  72 B communicates with the atmosphere through the valves  76 C and  76 D in series. The upper cylinder chamber  72 A communicates with the source  200  via the valves  76 A and  76 B and, optionally, a throttle  84 B in series. The throttle  84 B can be provided to compensate for acceleration of the movable portion  52  due to gravity. When operated in the third mode, the pins  54  move downward at a velocity in a first downward velocity range. 
     In a fourth or “down slow” mode (FIG.  7 ), the upper cylinder chamber is exposed to the source  200  as in the “down fast” mode. The valve  76 C is de-energized so that the lower cylinder  72 B communicates with the atmosphere via the valve  76 C and a throttle  84 C. The state of the valve  76 D is immaterial. When operated in the fourth mode, the pins  54  move downward at a velocity in a second downward velocity range lower than the first downward velocity range. 
     In a fifth or “down unpowered” mode (FIG.  8 ), both the upper and lower cylinder chambers  72 A and  72 B are vented to atmosphere through the pairs of valves  76 A,  76 B,  76 C,  76 D, respectively. In this mode, acceleration is caused by gravitational acceleration along with any stored energy such as from compression of the bellows  60 . 
     In the following description, it is convenient to refer to various pin positions as follows. A fully retracted pin height (H R ) is defined as a zero or reference height. In the fully retracted position H R , the upper ends of the pins  54  are accommodated within the holes in the chamber bottom so as to be shielded from the processes occurring in the chamber. In addition, H F  is the height at which the pins engage a wafer held by the fixture or edge ring, H T  is the height at which the pins engage a wafer held by the blade or other transfer element, and the fully extended height is H E . 
     Initially, the lift pins  54  may be in a retracted position at the reference height H R  within the chamber bottom (FIG.  9 ). The wafer  24  is supported by the edge ring  34 . A process has been performed on the wafer, and the wafer must be exchanged for a fresh wafer. In a first stroke, the pins  54  are raised beyond the position shown in FIG. 10 wherein the pins  54  initially engage the lower surface  28  of the wafer  24  at height H F . In a first stage of movement, the lifting system  36  raises the pins  54  in the up fast mode. During this stage, the aperture  68  permits light to pass between the photoemitter and detector of the lowermost sensor  64 A only. Eventually, the pins  54  reach the intermediate height H B1  at which the aperture  68  permits light to pass between the photoemitter and detector of the lower middle sensor  64 B. The transmission of such light produces a signal from the sensor  64 B to the control system  100  causing the lifting system to be switched to the up slow mode during a second stage of movement. During the second stage, the pins  54  pass through the position shown in FIG. 10 at the reduced speed to acquire the wafer  24  from the edge ring  34 . Shortly thereafter, when the pins  54  reach the intermediate height H B2 , the aperture  68  passes beyond the sensor  64 B, no longer permitting the transmission of light between the photoemitter and detector of that sensor and terminating the input signal from that sensor. The termination of the input signal provided by the sensor  64 B to the control system  100 , causes the control system to return the lifting system to the up fast mode during a third stage of movement. 
     Prior to reaching the fully extended position or the peak in their travel at height H E , the pins  54  reach the intermediate height H D1  at which the aperture  68  first permits the transmission of light between the photoemitter and detector of the uppermost sensor  64 D to produce an input signal from that sensor to the control system  100 . The input from the uppermost sensor  640  causes the control system to return the lifting system to the up slow mode in a fourth stage of movement during the upstroke. This return to the up slow mode reduces any upward propulsion of the wafer when the pins reach their extended position at height H E  (FIG.  11 ). With the pins  54  in the extended position, the blade  44  can be inserted below the wafer (FIG.  12 ). 
     With the blade  44  in place beneath the wafer, the control system  100  initiates a downstroke of the lifting system. In a first stage of the downstroke, the lifting system is placed in the down unpowered mode. In that first stage, the pins  54  descend, depositing the wafer on the blade  44  at height H T  (FIG.  13 ). After depositing the wafer on the blade  44 , the wafer lifting system and the pins  54  reach a height H C2  at which the aperture  68  permits light to pass between the photoemitter and detector of the upper middle sensor  64 C. The transmission of the light produces a signal from the sensor  64 C to the control system  100  causing the lifting system to be switched to the down fast mode in a second stage. Shortly thereafter, the aperture  68  passes beyond the sensor  64 C with the pins at height H C1  (FIG.  14 ). Light no longer passes between the photoemitter and the detector, and the input signal from the sensor  64 C is terminated. Termination of the input signal causes the control system  100  to withdraw the robot end effector  44  and wafer from the chamber (FIG.  15 ), whereupon the wafer can be exchanged for a second, fresh wafer. 
     In one implementation, the pins  54  and lifting system  36  continue to proceed downward to the retracted position. Optionally, a brake (not shown) can be provided to hold the lifting system  36  in an intermediate position such as that shown in FIGS. 15 and 16. Use of the brake to hold the lifting system in the intermediate position can reduce the time required to return the pins  54  from the retracted position. 
     At this point, the end effector  44  carrying a fresh wafer  24 ′ is introduced to the chamber  22  (FIG. 16) in substantially the same position as the wafer  24  in FIG.  14 . With the end effector  44  and second wafer  24 ′ in position, a second upstroke is initiated. If the pins  54  have been lowered all the way to the reference height H R , then the lifting system  36  and pins are moved in the up fast mode. Eventually, the pins  54  reach the intermediate height H C1  where the aperture  68  permits light to pass between the photoemitter and detector of the upper middle sensor  64 C. The transmission of such light produces a signal from the sensor  64 C to the control system  100  causing the lifting system  36  to be switched to the up slow mode in a second stage of movement. During the second stage, at the reduced speed, the pins  54  pass through the position shown in FIG. 17 to acquire the substrate  24 ′ from the blade  44 . When the pins  54  and lifting system  36  reach the height H D1  (FIG.  18 ), an input signal from the uppermost sensor  64 D to the control system  100  causes the control system to withdraw the blade (FIG.  19 ), and a second downstroke is initiated. 
     The second downstroke delivers the fresh substrate  24 ′ to the edge ring  34  for processing. In a first stage of movement, the lifting system  36  lowers the pins  54  in the down unpowered mode. As with the first downstroke, upon reaching the height H C2 , an input signal from the upper middle sensor  64 C to the control system  100  causes the control system to return the lifting system  36  to the down fast mode in a second stage of movement. Prior to the wafer reaching the edge ring  34 , the pins reach the height HB 2 . At that height, the signal provided by the lower middle sensor  64 B to the control system  100  causes the control system to return the lifting system  36  to the down slow mode in a third stage of movement. During this third stage of movement, the pins  54  pass through the position shown in FIG. 20 at the reduced speed to deposit the wafer  24 ′ onto the edge ring  34  at height H F . When the pins descend to the height H B1 , termination of the signal provided by the sensor  64 B to the control system  100  causes the control system to return the lifting system  36  to the down fast mode and also causes the control system to initiate rotation of the edge ring  34 . At the conclusion of this stage of movement, the pins reach the retracted position at the zero height H R  (FIG. 21) and processing of the wafer is commenced. At the end of processing, rotation of the edge ring is stopped, and the wafer  24 ′ can be exchanged for yet another wafer by repetition of the process described above. 
     In other implementations, the pneumatic slide and associated pneumatic hardware can be replaced by one or more position transducers for raising and lowering the pins. A single position transducer can be provided to lift all the lift pins or an individual position transducer can be associated with each lift pin. The position transducers may be used to provide a more precise control over motion of the lift pins than does the pneumatic system. Use of position transducers can eliminate the need for a brake to hold the lift pins in a waiting position just below the height of the transfer element while one substrate is being removed from the chamber and replaced with another. 
     In an alternative embodiment illustrated in FIGS. 22-23, a motorized lead screw  114  can be used to drive the wafer lifting system  36  instead of the pneumatic system described above. The lead screw or other threaded rod  114  is inserted through a hole in the slide  62  and is driven by a stepper motor  102  with its own programmable driver  104 . The lead screw  114  is attached to the motor  102  by a flexible coupling  118  which can include torsion springs to improve alignment. A threaded nut  116 , which is attached to the slide  62 , is disposed about the lead screw  114 . A controller  130 , which is coupled to the control system  100 , can be connected by a cable  132  to a terminal strip  106  to control movement of the motor  102 . Depending on the direction of rotation of the motor  102 , the lead screw  114  moves either upward or downward so as to move the lift pins  54  (not shown in FIGS. 22-23) vertically up or down. The control system  130 , thus, controls the speed of the motor  102  to control the movement of the pins  54 . 
     Although the lift pins  54  are not shown in the motorized lead screw embodiment of FIGS. 22-23, the pins are movably inserted and extend through the elevator or “spider plate”  56  in the same manner as described above with respect to FIG.  2 . 
     In one particular embodiment, a PK264M-02B stepper motor, a CSD2120-T stepper driver, and an SC 8800 controller, all of which are manufactured by Oriental Motors Co. of Japan, can be used as the motor  102 , the driver  104  and the controller  130 , respectively. In the illustrated implementation, each step represents a rotation of 0.9 degrees of the stepper motor  102 , and the pins  54  move vertically about 0.2 inches for each complete revolution of the motor, in other words, for every 400 steps. Additionally, an inertia damper  108 , such as a metal disk, can be provided below the motor  102  to improve the smoothness of acceleration and deceleration. 
     A vertically oriented array of sensors  110 A,  110 B,  110 C,  110 D and  110 E is mounted on a printed circuit board  120  which is attached to the fixed portion  50  by a bracket et  134 . Each sensor  110 A through  110 E includes a photoemitter and a detector for detecting light emitted by the respective photoemitter. In the implementation of FIGS. 22-23, the sensor array is formed as two columns of sensors, with the lowermost sensor  110 A and the uppermost sensor  110 E offset horizontally somewhat from the other sensors  110 B,  110 C and  110 D. 
     A shutter or plate  112  with two vanes  124 A,  124 B is fixed to the movable portion  52  so that one or the other of the vanes can be positioned between the photoemitter and the detector of each sensor  110 A through  110 E as the movable portion moves vertically up or down. The vanes  124 A,  124 B can prevent the transmission of light between the photoemitter-detector pairs of the sensors  110 A through  112 E. Signals from the sensors  110 A through  110 E are provided to the controller  130  and/or the control system  100  to determine or confirm the vertical position of the pins  54 . Specifically, outputs from the sensors  110 A,  110 C and  110 E are coupled to the controller  130 , and outputs from the sensors  110 B,  110 C, and  110 D are coupled to the control system  100 . 
     Occlusion of the sensor  110 C represents a “home” position in which the pins extend above the edge ring  34  to a reference height H H  (see, e.g., FIG.  24 ). Occlusion of the sensor  110 B indicates that the pins  54  are in their fully retracted position within the holes  58  in the chamber bottom (see, e.g., FIG.  30 ). In the fully retracted position, the pins  54  are at a height H R . Similarly, occlusion of the sensor  110 D indicates that the pins  54  are in their fully extended position above the blade  44  of the robot  42  (see, e.g., FIG.  27 ). In the fully extended position, the pins  54  are at a height H E . In the implementation of FIGS. 22-23, the distance between the sensors  110 B and  110 C is approximately 2,700 steps, in other words, about 1.35 inches. Similarly, the distance between the sensors  110 C and  110 D is approximately 2,100 steps, or about 1.05 inches. 
     The lowermost and uppermost sensors  110 A and  110 E can be used to limit movement of the motor  102  in the clockwise and counter-clockwise directions. The sensors  110 A and  110 E, therefore, can be used as a safety feature to prevent damage to the motor  102 . The sensors  110 A and  110 E also can be used to allow the system to find the “home” position more quickly when the system is turned on in the event that the motor  102  initially is rotated in the wrong direction. 
     In general, the motor  102  is controlled to raise or lower the lift pins  54  at varying velocities to optimize the throughput of the chamber  22  without damaging the wafers. In a first mode, the pins  54  are moved upward at a velocity in a first upward velocity range. In the illustrated implementation, the first upward velocity range is between about 4,000 and about 8,000 steps per second, in other words, about 2.0 inches to about 4.0 inches per second. In a second mode, the pins are moved upward at a velocity in a second upward velocity range. In the illustrated implementation, the second upward velocity range is between about 500 and about 1,000 steps per second, in other words, about 0.25 inches to about 0.5 inches per second. Thus, in the illustrated implementation, the first upward velocity is at least about four times at great as the second upward velocity. 
     In a third mode, the pins  54  are moved downward at a velocity in a first downward velocity range. In the illustrated implementation, the first downward velocity range is between about 4,000 and about 8,000 steps per second, in other words, about 2.0 inches to about 4.0 inches per second. In a fourth mode, the pins are moved downward at a velocity in a second downward velocity range. In the illustrated implementation, the second downward velocity range is between about 500 and about 1,000 steps per second, in other words, about 0.25 inches to about 0.5 inches per second. Thus, in the illustrated implementation, the first downward velocity is at least about four times at great as the second downward velocity. 
     The slower second and fourth modes can be used, for example, just prior to transferring a wafer to or from either the robot blade  44  or the edge ring  34 . Slowing the pins  54  at such critical times can help prevent damage to the wafer. Similarly, the second mode can be used just prior to reaching the fully extended height H H  to prevent the wafer from bouncing as movement of the pins  54  is stopped. The faster first and third modes can be used at other times to increase the throughput of the chamber  22 . 
     Referring to FIGS. 24-35, operation of the motorized lead screw embodiment of the wafer lifting system  36  is explained. When power is provided to the controller  130 , the controller moves the pins  54  to their “home” position so that the upper tips of the pins are at the height H H  (FIG.  24 ). As indicated previously, the output of the sensor  110 C is used to determine when the pins  54  are in the “home” position. In one implementation, once the pins  54  reach the “home” position, the motor  102  is controlled to move the pins downward an additional predetermined number of steps, for example, 200 steps. The robot blade  44  supporting a substrate or wafer  24 ″ to be processed then is inserted into the chamber  22  (FIG.  25 ). The wafer  24 ″ has an upper surface  26 ″ and a lower surface  28 ″. 
     The controller  130  causes the motor  102  to rotate at a velocity in the first upward velocity range so as to move the pins  54  toward the wafer  24 ″ supported by the robot blade  44 . In the illustrated embodiment, the motor  102  is rotated at about 6,000 steps per second so that the pins  54  move upward at about 3.0 inches per second. As the pins  54  near the lower surface  28 ″ of the wafer  24 ″, the motor is momentarily stopped. The controller  130  then causes the motor  102  to rotate at a velocity in the second upward velocity range so that the pins  54  contact the underside of the wafer  24 ″ at a lower velocity. In the illustrated embodiment, the motor is rotated at about 1,000 steps per second so that the pins  54  contact the underside of the wafer  24 ″ at a speed of about 0.5 inched per second (FIG.  26 ). The motor  102  then is accelerated until it rotates at a velocity in the second upward range, for example, about 4,000 steps per second, to move the pins  54  supporting the wafer  24 ″ upward at a speed of about 2.0 inches per second. As the pins  54  approach their fully extended height H E  above the robot blade  44 , the motor  102  is slowed to a velocity in the second upward range, for example, about 500 steps per second, to move the wafer  24 ″ upward at about 0.25 inched per second. When the controller  130  determines that the pins  54  have reached the extended height H E  (FIG. 27) based on the number of steps the motor  102  has rotated, the motor is stopped. The controller  130  then checks the output of the sensor  110 D to confirm that the pins  54  are, in fact, in the fully extended position. The robot blade  44  is removed from the chamber (FIG.  28 ). 
     Next, the controller  130  causes the motor  102  to move the pins  54  supporting the wafer  24 ″ downward toward the edge ring  34 . Initially, the motor rotates slowly, for example, at a speed of about 500 steps per second, to move the pins downward at a velocity of about 0.25 inches per second. The rotation of the motor  102  then is accelerated to a velocity in the first downward range, for example, about 6,000 steps per second to lower the pins  54  and the wafer  24 ″ at about 3.0 inches per second. As the lower surface  28 ″ of the wafer  24 ″ approaches the edge ring  34 , the speed of the motor  102  is slowed to a velocity in the second downward range, for example, about 1,000 steps per second. The wafer  24 ″ is, therefore, transferred to the edge ring  34  (FIG. 29) as the lift pins  54  move at about 0.5 inches per second. 
     Once the wafer  24 ″ is transferred to the edge ring  34 , the motor  102  is controlled to rotate at a velocity in the first downward range, for example, about 8,000 steps per second, to lower the pins  54  to their fully retracted position H R  (FIG. 30) at a speed of about 4.0 inches per second. The motor  102  is stopped, and the position of the lift pins  54  within the holes  58  in the chamber bottom is confirmed by checking the output of the sensor  110 B. The wafer  24 ″ then can be processed. 
     Following processing of the wafer  24 ″, the motor  102  is controlled to rotate at a velocity in the first upward range, for example, about 7,000 steps per second, to raise the pins  54  toward the wafer supported by the edge ring  34  at about 3.5 inches per second. As the pins  54  approach the edge ring  34 , the motor  102  is slowed to a speed in the second upward range, for example, about 1,000 steps per second, to raise the pins into engagement with the lower surface  28 ″ of the wafer  24 ″ (FIG.  31 ). The motor  102  is stopped momentarily. Next, the motor  102  is accelerated to rotate initially at about 500 steps per second and then to a speed in the first upward range, for example, about 5,000 steps per second, to raise the pins  54  and the processed wafer  24 ″ toward the fully extended position H E  at a speed of about 2.5 inches per second. As the lift pins  54  approach the fully extended position H E the motor  102  is decelerated to a velocity in the second upward range, for example, about 500 steps per second, to raise the pins to the position H E  (FIG. 32) at a speed of about 0.25 inches per second. The motor  102  then is stopped, and the controller  130  verifies the position of the pins  54  by checking the output of the sensor  110 D. Next, the robot blade  44  is inserted into the chamber below the processed wafer  24 ″ supported by the lift pins  54  (FIG.  33 ). 
     With the robot blade  44  positioned below the raised pins  54 , the controller  130  accelerates the motor  102  to lower the pins  54  and the processed wafer  24 ″. Initially, the motor  102  is rotated at a speed of about 500 steps per second. Subsequently, the motor  102  is rotated at a speed in the first downward range, for example, about 5,000 steps per second to lower the processed wafer  24 ″ toward the robot blade  44  at a speed of about 2.5 inches per second. As the lower surface  28 ″ of the wafer  24 ″ approaches the robot blade  44 , the motor  102  is slowed to a speed in the second downward range, for example, about 1,000 steps per second, to transfer the processed wafer  24 ″ to the blade  44  at a speed of about 0.5 inches per second (FIG.  34 ). 
     Once the processed wafer  24 ″ is transferred to the robot blade  44 , the motor  102  is momentarily stopped. The controller  130  then causes the motor  102  to rotate at a speed in the first downward velocity range, for example, about 7,000 steps per second, to lower the lift pins  54  to their “home” position at a height H H  (FIG.  35 ). The controller  130  confirms that the pins  54  are in the “home” position by checking the output of the sensor  110 C. With the pins  54  in the “home” position, the processed wafer  24 ″ supported by the robot blade  44  can be removed from the chamber. A new wafer to be processed then can be brought into the chamber and the cycle begun again. 
     As described above, the stepper motor  102  allows the acceleration and deceleration of the lead screw  114  to be controlled precisely so as to obtain a highly repeatable technique for controlling movement of the lift pins  54 . In this manner, movement of a wafer in the process chamber  22  can be optimized to increase the throughput of the chamber, reduce the likelihood of damage to the wafer, and reduce bouncing of the wafer while supported by the lift pins  54 . 
     Some of the details of the foregoing embodiments are particularly suited for particular processing chambers, such as the RTP Centura XE™, manufactured by Applied Materials, Inc. Different dimensions and pin speeds may be suitable for other substrate processing systems and chambers. 
     Other implementations are within the scope of the following claims.