Abstract:
An apparatus and method for detecting in chamber wafer position and process status are disclosed. A chamber includes a processing pedestal and plurality of lift pins. Each lift pin has an associated load cell for measuring the load exerted by the wafer on the lift pins. Mispositioned wafers or broken wafers will result in load measurements outside of expected ranges. Position of the wafer may be determined from the load distribution sensed on the lift pins.

Description:
TECHNICAL FIELD  
         [0001]    The present invention is generally related to microchip fabrication. More particularly, the invention relates to process controls and equipment related to certain chamber dependant fabrication steps.  
         BACKGROUND OF THE INVENTION  
         [0002]    In microchip fabrication, processing chambers are used in accomplishing various fabrication steps performed on a wafer. For example, among the numerous varieties of steps are the non-exhaustive examples of annealing, cleaning, chemical vapor deposition, oxidation, and nitridation.  
           [0003]    Chambers generally comprise a substantially planar and horizontal pedestal upon which the wafer rests during the processing within the chamber. The pedestal may also be known as a platen, stage or susceptor. A lift mechanism comprising pins which breach the pedestal from underneath provides a controlled disengagement from, and engagement of the wafer to, the pedestal surface. Wafers are transferred into and out of the chamber, and onto and off of the lift mechanism pins, by way of a robotic mechanism including a robot blade or fork at the end of a robotic arm.  
           [0004]    Retention of the wafer to the pedestal while in the chamber is referred to as chucking. Chucking may be accomplished mechanically such as by clamping at the periphery or by a vacuum holding force. Commonly, retention of the wafer to the pedestal is accomplished electrostatically by an electrostatic chuck.  
           [0005]    Wafers tend to be relatively brittle and susceptible to damage anytime one is handled or manipulated. For example, late release of a mechanical chuck or residual charge in an electrostatic chucking mechanism may result in damaged wafers if the pins attempt to separate the wafer from the pedestal prior to its complete release therefrom. If the retention force is great enough the wafer may break before release. The wafer may also break if it releases with a spring action and bounces. A wafer may also become mispositioned with respect to the pins during such a release. A wafer may also become mispositioned with respect to the pins when engagement therewith is undesirably harsh. A wafer mispositioned with respect to the pins will also be mispositioned when it is brought into contact with the pedestal. Apart from late or incomplete release of the wafer from the pedestal, contact of the pins with the wafer may be undesirably harsh during unimpeded separation of the wafer from the pedestal thus damaging the wafer. A wafer may also become mispositioned with respect to the pins during such a harsh contacting. Pin travel during separation of the wafer from the pedestal in preparation for transfer to the robot blade may reach the travel limit undesirably abruptly resulting in breakage of the wafer or mispositioning upon the pins as the wafer bounces upon the pins. Mispositioning upon the pins may result in mispositioning upon the robot blade which may in turn result in damage of the wafer as it is removed from the chamber through the slit valve or access slot or thereafter in a subsequent transfer from the robot blade. Mispositioning upon the pins wherein the wafer is partially supported by the pins and the pedestal might also result in damage or breakage of the wafer when the robot blade enters the chamber to retrieve the wafer.  
           [0006]    An operator may never be alerted to the various situations outlined above in a conventionally automated process resulting in wasteful utilization of chamber processing time on a broken or damaged wafer or on a wafer that is mispositioned on the pedestal. A wafer broken upon its loading into the chamber would desirably not be processed. Unbroken but mispositioned wafers may be subject to much lower yields due to edge effects, for example, in a sputtering process that could be avoided if the wafer were removed from the chamber and properly loaded. A wafer that is broken into several pieces, whether broken pre or post chamber processing, might not be removed from the chamber at all thus resulting in wafer debris within the chamber during further attempted wafer processing.  
         SUMMARY OF THE INVENTION  
         [0007]    Therefore, it is one object of the present invention to provide for an apparatus and method used for detecting a variety of wafer process conditions including wafer position, wafer breakage and wafer chucking.  
           [0008]    It is a further object of the present invention to provide for an apparatus and method used for detecting a variety of wafer process conditions which requires minimal modifications to general purpose semiconductor fabrication processing chambers.  
           [0009]    It is a further object of the present invention to sense wafer position by detecting the load impressed at each of the lift pins of a general purpose semiconductor fabrication processing chambers by a wafer residing thereon.  
           [0010]    These objects and advantages of the present invention are realized in a process chamber lift pin mechanism adapted to sense loads upon each lift pin. Determination of wafer position is accomplished from the sensed loads. Such mechanism is adapted to detect wafer position, wafer breakage and incomplete dechucking of the wafer. Preferably, the lift pins number three and are distributed in an equilateral triangular pattern. Proper wafer positioning is assumed to be correspondence of the wafer geometric center with the center of the lift pin pattern. Load cells are associated with each lift pin and provide a measure of the load at each lift pin. Load magnitudes and load distribution among the pins are used to detect such process conditions as wafer position, wafer breakage and wafer chucking. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    The present invention will now be described, by way of example, with reference to the accompanying drawings, in which:  
         [0012]    [0012]FIG. 1 is a schematic partial sectional representation of a general purpose semiconductor fabrication processing chamber including lift pin mechanism in accordance with a preferred embodiment of the present invention;  
         [0013]    [0013]FIG. 2 is a plan view of properly and improperly located wafers residing on top of the lift pins of a chamber as shown in FIG. 1 in accordance with a preferred embodiment of the present invention;  
         [0014]    [0014]FIG. 3 is a detailed schematic partial sectional view of a preferred lift pin mechanism in accordance with the present invention;  
         [0015]    [0015]FIG. 4 is a representation of various steps carried out in practicing the apparatus of the present invention and various wafer loading related method steps of the present invention; and,  
         [0016]    [0016]FIG. 5 is a representation of various steps carried out in practicing the apparatus of the present invention and various wafer removal related method steps of the present invention.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0017]    With reference first to FIG. 1, numeral  10  refers to portions of a general purpose semiconductor fabrication processing chamber shown as a partial sectional schematic. Pedestal  20  has an embedded electrode  21  and provides a support surface  5  for wafer  8  during processing thereof. Wafer  8  is shown in phantom for clarity from the remaining portions of FIG. 1. Electrode  21  is charge conductive and provides, in conjunction with application of voltage from power supply  25 , electrostatic retention forces upon wafer  8 . While an electrostatic chuck is illustrated in FIG. 1, other chucking mechanizations and implementations may be utilized without departing from the present invention.  
         [0018]    Lift pin mechanism  13  includes base  15  which carries a plurality of lift pins  18   a ,  18   b , and  8   c . At least, and preferably, three such lift pins  18  are provided though more may be implemented in accordance with the present invention. Each lift pin  18  breaches the pedestal  20  unimpeded. Lift pin mechanism  13  is controlled between lift and lower positions and modes. In lift mode the lift pin mechanism  13  is controlled to an extended position wherein the lift pins  18  extend above the surface  5  of pedestal  20 . In the retracted mode the lift pin mechanism  13  is controlled to a retracted position wherein the lift pins  18  are recessed below the surface  5  of pedestal  20 . Lift pin mechanism  13  is extended and retracted such as by pneumatic means  17  or any wide range of equivalent means including motors, servos, hydraulics, etc.  
         [0019]    In conventional operation, a wafer is transferred into the chamber through a slot valve (not shown) in the sidewall of the chamber by way of a fork or blade of a robotic transfer. Cantilevered wafer transfer mechanisms, vacuum transfer mechanisms and other mechanisms not mentioned may be utilized in the transfer of wafers to the processing chamber. The wafer is placed upon the set of extended lift pins  18 . The lift mechanism is then retracted and the wafer placed in proximity to and supported at its backside by surface  5  of pedestal  20 .  
         [0020]    Computer  29  may take the form of any properly configured general purpose computer, fabrication enterprise controller, or other programmable controller designed or capable of being adapted for implementing the control and sensing functions to be described. It is shown in the present exemplary embodiment being interfaced with power supply  25  and pneumatic means  17  for monitoring and controlling the operation of each of these systems. Computer  29  may include such conventional elements as a central processing unit (CPU), read only memory (ROM) (including removable media drive and mass storage drives such as a hard drive), random access memory (RAM), clock, input/output (I/O) circuitry including buffering and conditioning circuitry including A/D and D/A converters.  
         [0021]    In addition to the previously describe conventional components, sensors  19   a ,  19   b ,  19   c  comprising individual load cells are associated with the apparatus. While only one of the sensors is actually illustrated as interfaced with the computer  29 , each sensor is interfaced with computer  29  such that the sensors&#39; outputs may be monitored and possibly controlled or conditioned as required. The load cells  19  are configured to sense and respond to load or force that is along the major axis of the lift pins. Each sensor  19  is located preferably between the base  15  and each respective lift pin  18 . An exemplary embodiment of a sensor, base and lift pin configuration is described later with respect to FIG. 3.  
         [0022]    With additional reference to FIG. 2, the exemplary apparatus as shown in FIG. 1 is shown, in part, in plan view. If one could see through the wafer  8 , the preferred arrangement of the three lift pins  18  would appear in the location of the solid large dots. Each of the pins  18  is distributed to a different apex of an equilateral triangle  30 . The center of wafer  8  is designated by the solid crosshair (+)  27  and designates not only the geometric center but also the center of gravity of the wafer  8 . Wafer  8  is shown in a properly oriented position or placement wherein the wafer  8  is placed central or symmetrical to the lift pin layout. An out of position or mispositioned wafer is illustrated in phantom and labeled  8 ′. Similar center marking is designated by the dashed crosshair (+)  27 ′.  
         [0023]    Overlayed for illustration are the X and Y axes of Cartesian coordinate system shown as bold arrowed lines but not otherwise separately labeled other than by the adjacent X and Y designations. The origin ( 0 , 0 ) of the coordinate system is selected to coincide with one of the lift pins  18 . Furthermore, the X axis is arranged to be collinear with one leg of the equilateral triangle  30 . That is to say the X axis is coincident with one of the lift pins at the origin and with another of the lift pins being coincident with the X axis away from the origin.  
         [0024]    A wafer that is positioned upon a set of lift pins will exert a force upon each of the lift pins that it is in contact with. On a triangular patterned layout of lift pins, each of the lift pins will support a portion of the entire mass of the wafer. If the wafers are substantially uniform in their effective density or mass distribution then the mass distribution upon the lift pins of a regularly and repeatably placed wafer is similarly regular and repeatable and it is expected that such an arrangement would yield predictable mass, force or load measurements if so measured, such as by the exemplary sensors.  
         [0025]    In the present preferred arrangement, the lift pins are distributed in an equilateral triangle pattern. Each of the lift pins is therefore an equivalent distance from the other two lift pins. A properly placed wafer is one that is centrally located upon the lift pins so arranged. The geometric center of the round wafer is aligned with the geometric center of the three lift pins. The geometric center of the round wafer also corresponds to the center of gravity thereto. In such a preferred arrangement it is expected that the load seen by the sensors will be equivalent. Deviation from equivalency suggests reliably that the wafer is mispositioned. The precise position of the wafer can be determined from the known layout of the lift pins and the known center of gravity of the wafer.  
         [0026]    Using the preferred layout of lift pins as described with respect to an equilateral triangle, the following mathematical expressions derived from the geometry of the equilateral triangle layout of the lift pins with a Cartesian coordinate system as set forth in the description of FIG. 2 may be utilized to simply and predictably determine the wafer position with respect to the location of its center of gravity relative to the coordinate system. Other coordinate systems, for example a polar coordinate system, may be equally adaptable for the present invention.  
             x   =           M   1     ×   0.5                 t     +       M   3     ×   t           M   1     +     M   2     +     M   3                     y   =         (       3     /   2     )     ×   t   ×     M   1           M   1     +     M   2     +     M   3                                     
 
         [0027]    Where M1=load at lift pin  18   b , M2=load at lift pin  18   a , and M3=load at lift pin  18   c .  
         [0028]    [0028]FIG. 3 illustrates an exemplary arrangement for a lift pin mechanism suitable to carry out the load sensing aspects of the present invention. Lift pin  18 ′ is coupled at the end proximate to the base  15 ′ to load sensor  19 ′ and sensor  19 ′ is coupled to the base  15 ′ such that sensor is located between the lift pin  18 ′ and base  15 ′. Load sensor  19 ′ is of a known variety of miniature or sub-miniature ring style load cells having an hermetic package  31  of stainless steel or similar material which houses a piezoelectric crystal. Mounting studs  32  are located on opposite sides of the package and serve to attach the package to the lift pin  18 ′ and base  15 ′. The studs  32  also allow for the preloading of the sensor package as is known to be desirable. Output  33  from package  31  may be conditioned or unconditioned. Preferably, the output  33  is a conditioned output which requires internal microcircuit conventionally used to convert the charge generated by the piezoelectric element into a low impedance voltage signal, provide amplification or attenuation, and filtering as required. Such sensors are typically referred to as voltage mode sensors whereas sensors without such conditioning are referred to as charge mode sensors. Voltage mode sensors are preferred for many reasons, among which are relative immunity to such influences as cable movement, electromagnetic and radio frequency interference, and compatibility with conventional I/O interfaces.  
         [0029]    In a preferred implementation and practice, computer  29  (FIG. 1) executes certain instruction sets related to process control, sensor measurements, and calculations. Preferably, the instruction sets to be described with respect to the present invention are part of a much larger set of instructions executed as part of the general process controls. FIG. 4 generally sets forth steps related to a variety of functions corresponding to wafer loading prior to processing whereas FIG. 5 generally sets forth steps related to a variety of functions corresponding to wafer removal subsequent to processing.  
         [0030]    First with reference to step  401  of FIG. 4, a wafer is robotically loaded into the processing chamber through a slit valve in the sidewall thereof. The wafer is transferred onto the lift pins from the robot blade. The load sensors are monitored or read at step  402 . A determination is next made at step  403  whether the wafer is broken. No load being sensed at the lift pins would indicate this. A broken wafer would set an alarm and preferably suspend operation as indicated by step  410 . The routine would terminate at that point with appropriate flags set to prevent continued execution of any routine to which the present routine would return. Alternatively, an intact wafer would pass to step  404  whereat determination is made as to wafer position such as by the mathematical expressions previously described. An unacceptable position would be detected by step  405  and a negative response thereat. Remedial action may then be taken as shown in step  409  whereat the wafer position would be adjusted, preferably in automated fashion by provision of the relevant position data to the robotic transfer mechanism. Step  408  may optionally be implemented to cause the recalibration of the robotic transfer mechanism. Such may be desirable in the event that analysis of multiple wafer transfers indicates that the wafer transfers statistically indicate a need for adjustment. Such adjustments may be accomplished through software adjustments or trimming in many cases until appropriate mechanical adjustments can be made such as at scheduled maintenance intervals. This process is repeated beginning at step  402  until an acceptable position is achieved. An acceptable position on the other hand passes control to step  406  which indicates the general execution of the present substantive process control to take place in the chamber including retraction of the lift pins and chucking of the wafer to the pedestal. Step  407  next represents steps to remove the wafer subsequent to processing as detailed in FIG. 5.  
         [0031]    With reference to FIG. 5 the set of exemplary steps for wafer removal subsequent to processing is illustrated. At step  501  a chuck release is commanded. Next at step  502  the lift pins are commanded to rise to separate the wafer from the pedestal. Lift pin loads are monitored t step  503 . At step  504  if the lift pins are not completely extended then the lift is determined to be incomplete and lift pin loads can next be checked at step  505 . Step  505  determines from the monitored loads and predetermined limits whether the loads are excessive. Excessive loads indicate an incomplete dechucking and potential for wafer damage if continued increases of lift pin force is encountered. Step  506  represents remedial process action in the event of lift pin loads being excessive. Incomplete dechucking may merely require a wait state as indicated for charge to dissipate from the chuck and retention force to follow similarly. Other remedial steps may be taken including recommending a dechucking, lowering or relaxing of the lift pin positions or operator interventions. For the present example it is assumed that a wait will rectify the incomplete dechucking and control subsequent to the wait is passed back to step  502 . Where pin loads are indicated as not being too high at step  505 , control is looped back to step  503 . Lift pin completion as indicated by an affirmative response at step  504  causes execution of a determination of whether the wafer is broken. As previously described, a broken wafer would be indicated by no load being sensed at the lift pins. A broken wafer would set an alarm and preferably suspend operation as indicated by step  509 . The routine would terminate at that point with appropriate flags set to prevent continued execution of any routine to which the present routine would return. Alternatively, an intact wafer would pass to step  508  whereat wafer position is determined. A mispositioned wafer at this step may indicate that the release was too harsh and the wafer popped off of the pedestal or was not released smoothly. An acceptable position of the wafer as determined at step  510  would result in control being passed to step  511  whereat wafer removal steps including robotic transfer is controlled. An unacceptable position of the wafer as determined at step  510  would result in control being passed to step  512  whereat the positional information is communicated to the robotic transfer mechanism such that adjustments in the blade positioning can be made to account for the mispositioning. Control would then pass to step  511  whereat wafer removal steps including robotic transfer are controlled.  
         [0032]    The invention has been described with respect to certain preferred embodiments to be taken by way of example and not by way of limitation. Certain alternative implementations and modifications may be apparent to one exercising ordinary skill in the art. Therefore, the scope of invention as disclosed herein is to be limited only with respect to the appended claims.  
         [0033]    The invention in which an exclusive property or privilege is claimed are defined as follows: