Abstract:
An electrostatic arm transports a semiconductor wafer between a staging area and a processing chamber and also electrically charges the wafer in order to eliminate the need for using gas plasma in the chamber to chuck and dechuck the wafer. The arm includes a pair of electrically conductive members coupled with an electrical power supply which respectively control the polarity of the charge applied to the wafer and create an electrostatic force which holds the wafer on the arm without the need for mechanical clamping. One of the members comprises a plate spaced from the face of the wafer by an air gap, and when charged with a polarity opposite that of the wafer, creates an electrostatic attraction force which holds the wafer the carrier. A controller is used to selectively reverse the polarity of charge on the plate so as to create an electrostatic repulsion force which positively releases the wafer from the carrier.

Description:
TECHNICAL FIELD 
     The present invention generally relates to apparatus and techniques for processing semiconductor devices, such as semiconductor wafers, and deals more particularly with apparatus for transporting and electrostatically chucking the wafer for processing within a processing chamber. 
     BACKGROUND OF THE INVENTION 
     Semiconductor wafers used to fabricate integrated circuits are processed in vacuum chambers using common semiconductor processing techniques, such as CVD, sputtering and etching. The wafer must be securely held in a fixed position within the chamber, and means must be provided to carefully control the temperature of the wafer. In many cases, a cooling gas, such as helium is supplied to the backside of the wafer which serves as a heat transfer medium to assist in controlling wafer temperature. 
     Various techniques have been used in the past to hold the wafer in a desired position within the chamber. Early techniques involved mechanically clamping the topside of the wafer while providing cooling gasses to the backside; this approach is sometimes referred to as “top side clamping”, but has not proved particularly effective, in large part because it suffers from the disadvantage of causing non-uniformity and particle inconsistencies at the extreme edge of the wafer. 
     More recently, electrostatic clamping has found increasing use. This technique makes use of the electrostatic attraction between objects of different electrical potentials, and commonly employs an electrostatic chuck as the device for providing electrostatic clamping forces. Electrostatic chucks are based on the forces of attraction between the charged plates of a capacitor. As in other types of wafer clamping and chucking, cooling gasses to control wafer temperature are supplied to the backside of the wafer, consequently the clamping force resulting from electrostatic forces must be sufficient to overcome the backside pressure on the wafer resulting from application of the heat transfer medium. The electrostatic chuck can be thought of as a capacitor with a conductive plate (a lower electrode fixed within the processing chamber), an insulating layer (a coating on the electrode), and another conductive plate (the wafer). In effect, the chuck is a parallel plate capacitor with a dielectric spacer. In its simplest form, this type of electrostatic chuck is referred to as a unipolar or monopolar electrostatic chuck. A unipolar electrostatic chuck is simply a conducting electrode with the wafer being employed as one of the conducting plates. In order to establish a complete circuit, an electrical connection to the wafer is required. 
     Currently, most unipolar electrostatic chucks use the gas plasma within the chamber as a conductor which completes the electrical circuit necessary to provide the electrostatic clamping force. A significant drawback of unipolar electrostatic chucks is the fact that the actual clamping force is not applied to the wafer until the wafer has been charged and the plasma has been generated in the chamber, the combination of which results in “chucking” the wafer on the electrode. Although an electrical conductor could be used to contact the wafer in lieu of the electrically conductive plasma, such contact could have serious consequences in terms of process reliability and reproducibility. After the wafer is electrostatically “chucked”, the desired process is carried out in the chamber, following which it is necessary to unclamp the wafer. Wafer unclamping is achieved by removing the plasma from the chamber and bleeding off the electrical charge existing in the wafer; this step is commonly referred to as “de-chucking” the wafer. The speed and effectiveness of de-chucking are highly dependent upon the particular processes that have been previously carried out in the chamber. Simply turning off the voltage source to the chuck electrode with the plasma still present will not always result in instantaneous de-chucking since some residual charge may remain in the wafer. In any event, in addition to less than optimal process repeatability, the time required for chucking and de-chucking the wafer necessarily increases the overall time required to process a particular wafer, and thus reduces wafer throughput to the system. 
     Another type of electrostatic chuck referred to as a bipolar chuck is sometimes used, which is constructed from a pair of electrodes separated from the wafer by an insulating layer. The bipolar configuration has the disadvantage of being considerably more complex in construction than the unipolar configuration and typically generates a clamping force which is considerably less that of a comparably sized unipolar chuck. 
     It would therefore be desirable to provide a unipolar type electrostatic chuck which is capable of chucking and de-chucking the wafer without the need for using the plasma within the chamber to complete the circuit necessary for creating the electrostatic chucking force. The present invention is directed to satisfying this requirement. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the invention, an electrostatic chuck and transport system is provided for holding an electrically conductive workpiece, such as a semiconductor wafer or the like, within a processing chamber. The system includes an electrode within the chamber upon which the wafer may be electrostatically clamped. An electrostatic arm is provided for transporting the wafer from a staging position, such as a cassette or loadlock chamber, to a position within the chamber in proximity to the electrode. The electrostatic arm includes an electrically conductive contacting member for engaging the wafer and applying an electric charge to the wafer having a polarity opposite of that of the polarity of the electrode. The arm further includes an electrically conductive plate member that may be charged with an electrical potential which either electrostatically attracts, and therefore clamps the wafer to the arm, or repels the wafer and thereby releases the wafer from the arm. Electrical power supply means, as well as control means are provided for selectively charging the electrode, contacting member and plate member with the desired polarities so as to electrostatically clamp and unclamp the wafer to the arm, while at the same time pre-charging the wafer before the latter is deposited and electrostatically clamped onto the electrode. The contacting member and plate member are separated by a layer of insulation and are concentrically disposed within an outer housing formed of electrically insulated material. 
     According to another aspect of the invention, apparatus for transporting and charging the wafer is provided for use in an electrostatic chuck. The apparatus includes a carrier moveable between stand by and staged positions, electrical power supply means, an electrically conductive contact member on the carrier which is coupled with the power supply for electrically contacting the wafer and for applying an electrical charge to the wafer; an electrical conductive plate member on the carrier which is coupled with the power supply for selectively electrostatically attracting or repelling the wafer; means on the wafer for electrically insulating the contact member from the plate member; and, means for controlling the power and the polarity of the power supplied from the power supply to the plate member and the contacting member. 
     Accordingly, it is the primary object of the present invention to provide an electrostatic chucking system which substantially increases wafer throughput by eliminating the need for employing a gas plasma as an electrically conductive medium. 
     Another object of the invention is to provide a system as described above for transporting wafers from a staging area to a chamber while simultaneously charging the wafer. 
     A further object of the present invention is to provide a system of type mentioned above which utilizes an electrostatic clamping force for holding the wafer during transport from a staging area to chamber. 
     A further object of the present invention is to provide an electrostatic arm which is particularly simple in construction and can be used to both transport and pre-charge a wafer, without the need for mechanically clamping the wafer. 
     These, and further objects and advantages of the present invention will be made clear or will become apparent during the course of the following description of a preferred embodiment of the present invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a combined block and top plan view of an electrostatic arm forming one embodiment of the present invention; 
     FIG. 2 is a combined schematic and cross-sectional view of the arm of FIG. 1, with a semiconductor wafer loaded thereon. 
     FIG. 3 is a view similar to FIG. 2 but further showing an electrode and a lifting mechanism for transporting the wafer between the electrode and the arm; 
     FIG. 4 is a view similar to FIG. 3 but showing the wafer lifted off of the arm; 
     FIG. 5 is a combined block and diagrammatic view of a semiconductor processing chamber, and showing the wafer electrostatically clamped on the electrode within the chamber; 
     FIG. 6 is a diagrammatic view showing the wafer raised by the lifting mechanism above the electrode; 
     FIG. 7 is a view similar to FIG. 3 but showing a subsequent step; 
     FIG. 8 is a combined block and diagrammatic view of an alternate form of the electrostatic arm of the present invention; 
     FIG. 9 is a combined schematic and cross sectional view of the arm of FIG. 8 showing the wafer electrostatically clamped on the arm; 
     FIG. 10 is a view similar to FIG. 9 but showing the arm having transported the wafer to a staging position, in proximity to the electrode; 
     FIG. 11 is a view similar to FIG. 10 but showing a subsequent processing step in which the wafer is released from the arm for deposit onto the electrode; 
     FIG. 12 is a combined block and diagrammatic view of a semiconductor processing chamber; 
     FIG. 13 is a view similar to FIG. 11 but showing the reversal of the charge on the electrode and the plate, in preparation for chucking the wafer onto the arm; 
     FIG. 14 is a view similar to FIG. 13 but showing a subsequent step in which the wafer has been chucked onto the arm in preparation for return transport to the staging area. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring first to FIGS. 1-7, inclusive, the present invention generally relates to an electrostatic chucking system employing an inventive electrostatic arm, generally indicated by the numeral  20 , for transporting an electrically conductive workpiece, such as the semiconductor wafer  26  between a staging position and a work area within a semiconductor processing chamber  54  (FIG.  5 ). The staging position may comprise, for example, a cassette or other holding area within a load lock chamber, and the work area within the processing chamber may comprise an electrically conductive substrate, such as an electrode  42  which is employed to electrostatically clamp the wafer  26  in a fixed position within the chamber during processing. The process carried out on the wafer  26  within the chamber may be one of several common processes such as CVD, etching, or sputtering. As will become later apparent, the arm  20  is utilized not only to transport the wafer  26 , but also controls the electrostatic charge applied to the wafer  26 . 
     Although not specifically shown herein, it is to be understood that the arm  20  will include any various types of conventional mechanisms to move the arm vertically and horizontally in a rectilinear or curvilinear fashion. The details of such mechanisms depend on the particular application and are well known in the art, consequently they need not be described in detail herein. 
     The arm  20  includes a elongate arm section  22  connected with a generally cylindrical outer housing  24  provided with an open top and a closed bottom. The arm section  22  and housing  24  are formed of an electrically insulative material, such as plastic for example. Within the outer housing  24 , there is sleeved an electrically conductive contact member  34  shaped in the form of a cup having a cylindrical sidewall and a bottom concentrically disposed within the outer housing  24 . The upper edge of the contacting member  34  forms a face to face ring contact with the backside face of the wafer  24 . In the embodiment shown in FIGS. 1-7, the contact member  34  forms a ring shaped support upon which the circumferential periphery of the wafer  26  rests. As shown in FIG. 2, the upper circumferential edge of the housing  24  is beveled outwardly and extends slightly above the upper edge of the contact member  34  and thus aids in guiding the outer periphery of the wafer  24  into registered engagement with the contact member  34 , thus assuring good electrical continuity therebetween. 
     Concentrically disposed within the contact member  34  is an electrically conductive plate  38  which is separated and electrically insulated from contact member  34  by means of a layer  36  of insulation. The plate  38  is spaced slightly below the upper edge of contact member  34  to form an air gap  40  between such upper face and the bottom or backside face of the wafer  26 . The upper surface of plate  38  is also coated with a layer of insulating material. 
     As best seen in FIGS. 3-6, an electrically conductive substrate in the form of an electrode  42  is disposed within the processing chamber  54  and acts as a chuck body for holding the wafer  26  during processing. The electrode  42  is provided with a layer  44  of electrically insulated material which forms an insulative barrier between the electrode  42  and the wafer  26 . 
     The system includes electrical power supply means comprising a first voltage source  28  coupled with the electrostatic arm  20  by means of a pair of lead wires  32 , and a second voltage source  48  coupled with the electrode  42 . One of the electrical leads  32  is connected with the contacting member  34 , while the other of such leads is coupled with the plate member  38 . The system further includes a controller  30  for controlling the power supply  28 . The controller  30  includes a series connected switch  52  (FIG. 4) connected between the voltage source  28  and the contact member  34 . The controller  30  is operative to selectively connect or disconnect the voltage source from the arm  20 , as well as to change the polarity of the leads, and thus the polarity of the electrical charge applied to the contact member  34  and plate  38 . 
     A conventional lifting mechanism  46  formed of an electrically insulative material and driven by a suitable motor source such as stepping motor  50 , is disposed beneath the electrode  42 . Lifter  46  is operative to transport the wafer  26  between the arm  20  and the electrode  42 . When the arm  20  is shifted to its staged position in vertical alignment with the electrode  42 , lifter  46  is raised to contact the wafer  26  and lift the latter above the elevation of the arm  20 , as best seen in FIG.  4 . Stepper motor  50  is then actuated to lower the lifter  46  so that the wafer  26  descends onto the electrode  42 . The lifter mechanism  46  is conventional in design and may be placed immediately below the electrode  42 , in which case certain components of the arm  20  are provided with a cut away  27  (FIG. 1) to accommodate travel of the lifter  46 . 
     As shown in FIG. 5, the processing chamber  54  houses an upper electrode  58  and a lower electrode  42  upon which the wafer  26  rests. Lower and upper electrodes  42 ,  58  are coupled with the power supply  48  which in turn is controlled by a second controller  56 . A high voltage applied to the lower and upper electrodes  42 ,  58  ionizes a gas introduced into the chamber  54  to create a gas plasma  57  within the chamber  54  used in processing the wafer  26 . 
     In use, the first step in chucking a wafer  26  for processing consists of moving the electrostatic arm  20  to a cassette or load station (not shown) where a wafer  26  to be processed is present. Controller  30  then turns on the power supply  28  resulting in the application of a voltage to the arm  20  which produces a positive charge on the plate  38  and a negative charge on the contact member  34 . The arm  20  and wafer  26  are then relatively moved to place the wafer  26  on the arm  20  such that the outer periphery of the backside of the wafer  26  rests upon the upper edge of the contact member  34 . Since contact member  34  is negatively charged, and is in contact with the wafer  26 , wafer  26  also becomes negatively charged. Because the plate  38  is positively charged, electrostatic attractive force is created between the wafer  26  and plate  38  which clamps or “chucks” wafer  26  onto the arm  20 . 
     Arm  20  may then be rapidly moved so as to transport the wafer  26  into the process chamber  54 , to a position vertically aligned with the electrode  42 . At this point, switch  52  (FIG. 4) is opened and the polarity of the voltage supplied by source  28  is reversed, thereby applying a negative charge to the plate member  38 . Since wafer  26  carries a residual negative charge, an electrostatic force of repulsion is created between the plate  38  and wafer  26 . Motor  50  is actuated to raise lifter  46  until the latter contacts and elevates the wafer  26  above the arm  20 , as best seen in FIG.  4 . The arm  20  is then retracted away from the chamber, whereupon lifter  46  descends to place the negatively charged wafer  26  upon the electrode  42 . Then controller  56  operates the power supply  48  to apply a positive charge to the lower electrode  42  thus clamping or “chucking” the wafer  26  in place within the chamber  56 . Backside cooling of the wafer  26  is then commenced, following which the process (CVD, sputtering, etching, etc.) may be started within the chamber  52 . As the process is started, a gas plasma  57  is created within the chamber  52  which acts as a conductor to complete a circuit loop between the power supply  48 , electrode  42  and wafer  26 . The amount of the electrostatic force chucking the wafer  26  onto the electrode  42  may be varied by the controller  56  which operates the voltage to the power supply  48 . 
     After processing of the wafer  26  in the chamber  54  is complete, the plasma within the chamber  54  is no longer present, but the wafer  26  retains a negative residual charge. Controller  56  then reverses the polarity of the power supply  48  (FIG.  6 ), thus changing the polarity of the charged electrode  42  from positive to negative. With both electrode  42  and wafer  26  negatively charged, an electrostatic repulsion force therebetween is created which “de-chucks” the wafer  26 . At this point motor  50  is actuated to cause the lifter  46  to raise the wafer  26  and arm  20  is then again shifted into the chamber and power supply  28  is energized to place a positive charge on the plate  38 , and a negative charge on the contact member  34 . Lifter  46  is lowered so that wafer  26  is deposited on the arm  20 , resulting in the wafer  26  being electrostatically chucked onto the arm  20 . The arm  20  is then shifted to transport the processed wafer to the staging position at a load lock or cassette. 
     Attention is now directed to FIGS. 8-14 which depict an alternate, preferred form of the electrostatic chucking system of the present invention. The system broadly includes an electrostatic arm, generally indicated by the numeral  60  which includes essentially the same components as the arm  20  previously described, except that the components are inverted. More specifically, the arm  60  includes an elongated arm section  62 , an outer housing  66  formed of electrically insulated material. A cylindrically shaped contact member  76  is sleeved within the cylindrical body of housing  66 , and includes a closed top or end wall, and an open bottom. A plate member  64  is concentrically disposed within the cylindrical contact member  76 , and has its lower face spaced from the bottom edge of the contact member  76  to form an air gap  74  between the wafer  26  to be transported and the plate  64 . The lower surface of plate  64  is also coated with an insulating material. A layer of electrically insulative material  78  is provided between the plate  64  and the contact member  76 . The lower edge of the housing section  66  extends down past the lower edge of contact member  76  and is slightly beveled to assist in bringing the wafer  26  into registered contact with the contact member  76 . 
     While the electrostatic arm  60  shown in FIGS. 8-14 is similar to arm  20  shown in FIGS. 1-7, the system of FIGS. 8-14 does not employ the previously described lifter mechanism, but rather relies on electrostatic chucking of the wafer  26  on the arm  60  and movement of the arm  60  itself to transport the wafer  26  from the staging position directly onto the electrode, designated in FIGS. 8-14 by the numeral  80 . As in the case of the previous embodiment, electrode  80  is provided with a layer of insulation  82  which electrically insulates the electrode  80  from the wafer  26 . Power supply means are provided comprising first and second voltage sources  68 ,  84  which respectively apply power to the arm  60  and electrode  80 . 
     The wafer transport sequence using the arm  60  may be described as follows: the arm  60  is first moved to the staging position (cassette or load station) and the arm  60  is then energized by the power source  68  which applies a positive charge to the plate  64  and a negative charge to the contact member  76 . The arm  60  is then moved downwardly so the lower edge of the contact member  76  engages the wafer  26 . With the plate  64  positively charged, and the wafer  26  negatively charged, via the negatively charged contact member  76 , the wafer  26  is electrostatically chucked and therefore clamped onto the arm  60 . The arm  60  then moves into the chamber  54  and downwardly to place the negatively charged wafer  26  onto the electrode  80 . Switch  86  is open to remove power from the arm  60 , whereupon the plate  64  reverses charge from positive to negative, thus releasing the negatively charged wafer  26 . Power supply  84  is then actuated to apply a positive charge to the electrode  80  which attracts and therefore electrostatically chucks the negatively charged wafer  26  onto the electrode  80 . 
     The arm  60  is then shifted out of the chamber following which back side cooling is provided to the wafer  26  and the process within the chamber  54  may be commenced,. Again, a gas plasma  57  is created within the chamber  54  and the process (CVD, sputtering, etching, etc.) is carried out during which time the plasma again acts as a conductor closing a circuit loop between the power supply  84 , electrode  80  and wafer  26 . As in the previously described embodiment, the controller  56  controls the voltage output by the power supply which in turn determines the magnitude of the electrostatic chucking force applied to the wafer  26 . Upon completion of the process, the plasma is terminated but the negative charge remains in the wafer  26 . The arm  60  is then moved into the chamber, and down into close proximity to the wafer  26 . Power is applied to the arm  60  so that the plate  64  is positively charged and the contacting member  76  is negatively charged, thereby creating an electrostatic attractive force which chucks the wafer  26  onto the arm  60 . Simultaneously, controller  56  operates the power supply  84  so that the charge on electrode  80  is reversed from positive to negative. With the wafer  26  held on the arm  60 , arm  60  is retracted from the chamber to transport the wafer  26  back to the load lock or cassette. Power supply  84  may then be turned off. 
     From the foregoing, it is apparent that the electrostatic transporting and chucking system described above not only provides for the reliable accomplishment of the objects of the invention, but does so in a particularly simple and economical manner. It is recognized, of course, that those skilled in the art may make various modifications and additions to the preferred embodiment chosen to illustrate the invention without departing from the spirit and scope of the present contribution to the art. Accordingly, it is to be understood that the protection sought and to be afforded hereby should be deemed to extend to the subject matter claimed and all equivalents thereof fairly within the scope of the invention.