Abstract:
A lift pin assembly for use in a reactor for processing a workpiece includes plural lift pins extending generally parallel with a lift direction, each of the plural lift pins having a top end for supporting a workpiece and a bottom end. A lift table faces the bottom ends of the pins and is translatable in a direction generally parallel with the lift direction. A small force detector senses a force exerted by the lift pins that is sufficiently large to indicate a chucked wafer and sufficiently small to avoid dechucking a wafer. A large force detector senses a force exerted by the lift pins in a range sufficient to de-chuck the wafer.

Description:
BACKGROUND OF THE INVENTION 
   An electrostatic chuck can be employed in a plasma reactor chamber to hold a semiconductor wafer onto a wafer support surface within the chamber during plasma processing of the wafer. The electrostatic chuck consists of a flat electrode or conductive grid insulated by a flat insulating or semi-insulating layer over which the wafer is placed. Typically, thin lift pins extend upwardly through the electrostatic chuck to receive the wafer above the electrostatic chuck from a robotic device which is then retracted. The lift pins are then retracted downwardly until the wafer rests on the wafer support surface. A large D.C. chucking voltage is applied to the electrode, typically with respect to a chamber wall ground. The wafer is typically referenced to ground indirectly through the plasma. The application of the large DC voltage to the electrostatic chuck electrode with the plasma “on” produces a large electrostatic force that holds the wafer in place. Plasma processing of the wafer is then performed, after which the lift pins extend upwardly to lift the wafer from the electrostatic chuck to the robot device for removal from the chamber. For high throughput, the lift pins are moved at a relatively high speed during their downward retraction and upward extension motions. The removal of the applied D.C. chucking voltage does not necessarily eliminate the potential difference between the wafer being chucked and the surface of the electrostatic chuck. Residual charge can remain, resulting in an residual attractive force between wafer and chuck. Depending on the magnitude of the residual force and the velocity of the lift pins during their upward extension, the wafer may be broken. 
   Lifting the pins when the plasma is “on” provides a discharge path for the current caused by the separation of the wafer from the chuck, but if the residual force is large, the wafer may still break. 
   Some prior art methods use the gas flow rate of a heat transfer gas (such as helium) delivered to the surface of the electrostatic chuck at constant pressure as a measure of residual chucking force. This method generally fails when some portion of the wafer “dechucks”, allowing the heat transfer gas to leak out at high flow rate, while another portion of the wafer remains chucked and may be broken in a subsequent upward motion of the lift pins. 
   Currently, there is no way to detect occurrence of such an error prior to destruction of the wafer. 
   A related problem is that process failure (due to excessive heating or poor temperature control) may occur if the wafer is not securely chucked to the wafer support surface prior to the start of plasma processing in the chamber. Currently, there is no way of confirming the adequacy of the chucking force prior to the start of plasma processing of the wafer without supplying a heat transfer gas (such as helium) to the surface of the electrostatic chuck at constant pressure and monitoring the gas flow rate as a measure of residual chucking force. However for some applications, in particular at high-bias-voltage, heat transfer gases may break down electrically, causing the wafer to dechuck and potentially destroying the wafer and electrostatic chuck. A method of confirming the adequacy of the chucking force prior to processing is required. 
   SUMMARY OF THE INVENTION 
   A lift pin assembly for use in a reactor for processing a workpiece includes plural lift pins extending generally parallel with a lift direction, each of the plural lift pins having a top end for supporting a workpiece and a bottom end. A lift table faces the bottom ends of the pins and is translatable in a direction generally parallel with the lift direction. A small force detector senses a force exerted by the lift pins that is sufficiently large to indicate a chucked wafer and sufficiently small to avoid dechucking a wafer A large force detector senses a force exerted by the lift pins in a range sufficient to de-chuck the wafer. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  depicts a lift pin assembly embodying the invention. 
       FIG. 2  depicts a plasma reactor incorporating the lift pin assembly of  FIG. 1 . 
       FIG. 3  depicts a wafer chucking and de-chucking process for the plasma reactor of  FIG. 2 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring to  FIG. 1 , an electrostatic chuck (ESC)  8  consists of an insulating or semi-insulating layer  10  encapsulating a thin planar chucking electrode or conductive grid  12 , the top surface  10   a  of the insulating surface forming a wafer support surface. The insulating layer  10  may be aluminum nitride, for example, and is supported on an ESC base  14 . A D.C. chucking voltage supply and controller  16  is coupled to the chucking electrode  12 . At least three lift pins  18  extending through holes  20  in the ESC  8  are supported on an axially translatable lift table  22 . For each lift pin  18 , the lift table  22  has a recess  24  covered by an elastic spring plate  26  that is held onto the lift table at one end  26   a  by a fastener  26   b . The other end  26   c  of the spring plate  26  is free to deflect axially. Optionally, an insulating (e.g., ceramic) fitting  28  on the bottom end of the lift pin  18  supports the lift pin  18  on the lift table  22 . The lift table  22  is cantilevered from the top end of an axial piston  30  that is axially translated by a lift servo motor assembly  32 . Bellows  34  maintain a vacuum seal of the chamber interior while allowing up and down motion by the lift table  22 . 
   A conventional strain gauge  36  is fastened on the surface of the lift table  22  inside the recess  24  facing the bottom side of the spring plate. A foot  38  fastened to the bottom of the spring plate  22  faces and overlies the strain gauge  36 . A conventional optical detector  40  fastened to the surface of the lift table within the recess  24  acts as a proximity detector, sensing the axial deflections of the free end  26   c  of the spring plate  26 . As each lift pin  18  pushes against a wafer  42 , the downward force imparted by the lift pin  18  deflects the spring plate  26 , moving the spring plate free end  26   c  downwardly so that the foot  38  pushes against the strain gauge  36  with ever increasing force. The proximity detector  40  measures the resulting downward deflection of the spring plate free end  26   c . If lift pin  18  pushes against the wafer  42  while a chucking voltage is applied to the electrode  12 , then the wafer will resist the upward motion of each lift pin  18 , causing the wafer  42  to bend upwardly near its periphery, as shown in  FIG. 1 . The force required to deflect the wafer in this manner is measured by the strain gauge  36  from the pressure exerted against it by the ceramic lift pin holder  28 . The magnitude of the resulting downward deflection by the spring plate is measured by the proximity detector  40 . The strain gauge  36  acts as a high force sensor capable of measuring forces sufficient to de-chuck a wafer after removal of the D.C. chucking voltage from the chucking electrode. The combination of the spring plate and proximity sensor acts as a high resolution low force sensor for sensing a “sub-de-chucking” force sufficiently small to avoid de-chucking the wafer (up to several times the weight of a wafer, or several hundred grams equivalent force for a 300 mm diameter Si wafer). Such a sensor is useful for measuring a lifting force imposed by the lift pins that is sufficient to indicate the presence of a chucking force but which avoids de-chucking the wafer. 
   In alternative embodiments, sensors other than the types disclosed above may perform as the low force sensor and the high force sensor. Such a sensor may be provided for each lift pin  18 . Such sensors are in the lift pin  18  or coupled to the lift pin  18 . Moreover, a single sensor may serve as both the high force sensor and the low force sensor for a given lift pin  18 . 
   In another alternative embodiment, at least one or all of the lift pins  18  may be formed of a semiconductor material (e.g., crystalline or polycrystalline silicon or germanium) or of a conductor (e.g., aluminum). In such a case, a switch  19  may be provided with which the conductive or semiconductive lift pin(s)  18  may be connected to any one of the following at selected times: ground potential, a an electrically floating potential, the ESC electrode  12 . The switch  19  may be employed during wafer chucking and wafer dechucking operations to enhance the control of the D.C. wafer voltage. During plasma processing of the wafer, the switch  19  would be set to connect the lift pin(s)  18  to the floating potential. 
     FIG. 2  illustrates a plasma reactor including a system for exploiting the apparatus of  FIG. 1 . The reactor includes a chamber  50 , a process gas supply  52  and gas distributor  54  and an RF power generator  56  coupled through an impedance match circuit  58  to (for example) the ESC electrode  12  through a D.C.-blocking capacitor  60 . A processor  62  uses the force measurements from the strain gauge  36  and the deflection measurements from the proximity sensor  40  to provide actual force data and deflection data. A process controller  64  uses the force data and the deflection data to control the D.C. chuck voltage delivered by the controller  16  and to control the lift table motor assembly  32  through a lift assembly motor controller  66 . 
   Operation of the process controller  64  is illustrated in  FIG. 3 . First the controller  64  causes the lift pins  18  to extend above the wafer support surface to receive the wafer from a robotic arm (not shown) in the step of block  70  of  FIG. 3 . The controller  64  then causes the lift pins  18  to retract so as to lay the wafer on the wafer support surface of the ESC  8  (block  72 ). Gas is introduced and RF power is applied to initiate a plasma, which indirectly references the wafer to a reference ground. Preferably, plasma source power is used for initiating the plasma for wafer chucking, but bias power may also be used. If bias power is used for initiating the plasma for wafer chucking, then preferably a low RF voltage is applied. The controller  64  then causes a D.C. chucking voltage to be applied to the ESC electrode  12  (block  74 ) with respect to the reference ground (typically a chamber wall). (Alternatively, the voltage is first applied and then RF power is applied to initiate a plasma.) After a typically short time delay, the wafer chucking is then tested by extending the lift pins  18  against the chucked wafer and then extending them a small distance (e.g., 0.2- about 1 mm) to deform the wafer by 0.2- about 1 mm (block  76 ). The deflection of the spring plate  26  sensed by the proximity detector  40  is then used by the data processor  62  to determine whether the lift pin force is at least a small threshold force (e.g., a few hundred grams equivalent force). If the force is above the threshold level (YES branch of block  77 ), the wafer has been successfully chucked and the process controller  64  causes the lift pins to be withdrawn from wafer (block  78 ) and plasma processing of the wafer is performed (block  80 ). (Alternatively, the wafer chucking may be continuously tested by maintaining the lift pins in position and continuously monitoring the force.) Otherwise (NO branch of block  77 ), an alarm is displayed and/or the chucking process is repeated. After wafer processing, the controller  64  causes the chucking voltage to be turned off (block  82 ) or may be set to a “dechucking” voltage. This “dechucking” voltage may be zero volts, or may be set to a non-zero value which is selected to minimize the residual electric field between wafer and chuck surface caused by residual charge on the surfaces. Preferably, the plasma is on during the dechuck operation to provide a discharge path from the wafer. Preferably, plasma source power is applied for the dechuck operation, but bias power may be used. Preferably the RF voltage on the wafer is low during the dechuck operation. After a time delay, the controller  64  then causes the lift pins  18  to extend upwardly to contact the wafer and then continue further until the lift pins travel a larger distance (e.g., 3 mm) to deflect the wafer by that larger distance (block  84 ). The output of the force sensor is monitored (block  86 ) to ensure that the force sensor output, which increases as the residual charge force is met, finally returns to zero (signifying successful dechucking) by or before the lift pins have traveled the larger (3 mm) distance after contacting the wafer. If the sensed force does reach approximately zero after the lift pins have traveled the larger distance (3 mm), the wafer has been successfully dechucked (YES branch of block  88 ), and the controller  64  causes the lift pins to extend the full distance required (e.g., about 25 mm) to return the wafer to the robot mechanism for removal (block  92 ). If the sensed force has not returned to zero (neglecting the weight of the wafer) at this point (NO branch of block  88 ), then an alarm is sounded to halt the wafer removal process (to avoid wafer breakage) until the chucking force or voltage can be thoroughly removed (block  90 ). 
   In an alternative embodiment, after wafer processing, the controller  64  causes the chucking voltage to be turned off (block  82 ) or may be set to an initial “dechucking” voltage. Preferably, the plasma is on during the dechuck operation to provide a discharge path from the wafer. Preferably, plasma source power is applied for the dechuck operation, but bias power may be used. Preferably the RF voltage on the wafer is low during the dechuck operation. After a time delay, the controller  64  then causes the lift pins  18  to extend upwardly to contact the wafer and then continue further until the lift pins travel a larger distance (e.g., 3 mm) to deflect the wafer by that larger distance (block  84 ). The output of the force sensor is monitored (block  86 ). In accordance with this alternative embodiment, during the monitoring step of block  86 , the “dechucking” voltage is stepped or ramped through a range of “dechucking” voltages in order to find the ideal de-chucking voltage to compensate for any remnant charge that would otherwise prevent the sensed force from decreasing to the predetermined minimum. When the force sensor output finally decreases to the minimum or returns to zero (signifying successful dechucking—YES branch of block  88 ) the dechucking voltage is set to zero volts, and the controller  64  causes the lift pins to extend the full distance required (e.g., about 25 mm) to return the wafer to the robot mechanism for removal (block  92 ). If the sensed force has not returned to zero at this point, then an alarm is sounded to halt the wafer removal process (to avoid wafer breakage) until the chucking force or voltage can be thoroughly removed (block  90 ). However, because the dechucking voltage is ramped during the monitoring step of block  86  in this alternative embodiment, the probability of resorting to the alarm of block  90  is reduced. 
   In a preferred embodiment, the controller  64  acts as a feedback loop continuously controlling the upward motion of the lift pins. If the pin force does not fall to a minimum value (e.g., zero) by the time the lift pins have pushed the wafer the large distance (e.g., 3 mm), then the controller  64  immediately halts the pin movement to avoid wafer breakage. This allows the lift pin upward motion to be continuous provided that the lift pin force falls to the minimum level within the required lift pin travel distance. 
   Optionally, conductive or semi-conductive lift pins may be used to help discharge the wafer-chuck capacitance and facilitate the dechucking operation. The pins may be directly connected to a reference ground or to the isolated electrostatic chuck electrode. Alternatively and preferably, the pins may be switched to be connected to a reference ground or to the isolated electrostatic chuck electrode just when dechucking or chucking, but can remain isolated in normal processing. 
   In the case of an alarm condition, the controller may optionally re-try a dechucking operation. 
   The foregoing thus requires two force sensors, namely one capable of measuring a small force (e.g., a few hundred grams) that can be exerted against the wafer to confirm successful chucking without dechucking the wafer, and one capable of measuring a large force (e.g., about 50 kg) sufficient to overcome residual charge-induced force to de-chuck the wafer. In the embodiment of  FIG. 1 , the low-force sensor is a spring plate and proximity sensor, while the high-force sensor is the strain gauge sensor. However, both sensors may be of the same type, provided both operate with sufficient accuracy within their respective ranges. Moreover, if a single sensor suffices to cover both force levels (200 grams for the low force and 50 kg for the high force), then the foregoing procedure can be carried out using the single sensor rather than two sensors. 
   While the above specification describes an example of a single-electrode (monopolar) chuck, the apparatus and method may also be applied to bipolar or multipolar chucks. 
   While the invention has been described in detail by specific reference to preferred embodiments, it is understood that variations and modifications thereof may be made without departing from the true spirit and scope of the invention.