Abstract:
A spin chuck for holding semiconductor wafers includes one or more damping mechanisms to limit the force with which chuck pins impact the wafer edge following wafer shift. The damping mechanism may be a linear or rotary dashpot. The dashpot or dashpots are mounted on a surface of the chuck body and include a control arm that contacts a common gear ring that in turn drives the chuck pins during radially inward and outward movement.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to devices for holding wafer-shaped articles, such as semiconductor wafers. 
     Semiconductor wafers are subjected to various surface treatment processes such as etching, cleaning, polishing and material deposition. To accommodate such processes, a single wafer may be supported in relation to one or more treatment fluid nozzles by a chuck associated with a rotatable carrier, as is described for example in U.S. Pat. Nos. 4,903,717 and 5,513,668. 
     The patents referenced above operate on the Bernoulli principle, such that the wafer receives subjacent support from a gas cushion rather than by contact with the chuck. Such chucks nevertheless ordinarily include a circular series of pins that are positioned radially outwardly of a wafer positioned on the chuck. Those pins prevent lateral displacement of the wafer relative to the chuck. 
     As described in U.S. Pat. No. 4,903,717, it is also advantageous to construct the pins such that each projects upwardly from a respective pivotal base. The axes of the pin and base are vertical but offset from one another such that pivoting of the base causes the associated pin to travel along a circular arc and hence be adjustable in its radial position. The pivot bases are each provided with gear teeth, which mesh with the teeth of a common gear ring that is coaxial with the axis of rotation of the chuck. Rotation of the gear ring relative to the chuck thus causes all of the pins to move conjointly and to the same extent. 
     That construction permits the pins to be moved radially outwardly for placement or removal of a wafer, and then to be moved radially inwardly to be brought into contact with the peripheral edge of a wafer. Such contact prevents not only lateral displacement of the wafer relative to the chuck, but also relative rotation between the wafer and chuck as the chuck is spun. 
     Rotation of the wafer relative to the chuck is referred to as “wafer shift.” Whereas the wafer and chuck are generally not intended to move relative to one another, a controlled amount of wafer shift is desirable to ensure correct cleaning and/or etching of the wafer bevel in the area where the pins touch wafer. This involves shifting the wafer by several degrees relative to the chuck during the cleaning as well as etching step. The wafer shift is carried out in a specific time interval. Within this time interval the gear ring inside of the chuck opens the pins of the chuck and the wafer is shifted. The gear ring is moved back by spring forces. 
     In spin chucks utilized in process modules for single wafer wet processing, such as those made by Lam Research Corporation, wafer shift can be performed by accelerating or decelerating the chuck during the process steps. That is, the inertia of the gear ring, which drives the chuck pins, is such that there is slight relative rotation between the gear ring and the chuck body during acceleration and deceleration of the chuck, which causes the pins to undergo an opening and closing movement and release the wafer for several milliseconds as the chuck is accelerated or decelerated. During that interval the wafer is not clamped and rotates by several degrees relative to the chuck. Similarly, if the tooth gear is driven instead of the chuck base body the relative movement of the chuck base body against the tooth gear is caused by the inertia of the chuck base body, with the same effect on the chuck pins. 
     SUMMARY OF THE INVENTION 
     The present inventors have discovered that, in chucks of the type described above, the pins close very rapidly, which causes excessive wear and groove formation in the pins over time. This can result in decreasing the working lifetime of the chuck, and impairing the desired controlled wafer shift during wafer processing. 
     According to the present invention, a device for holding wafer-shaped articles, such as semiconductor wafers, is equipped with a damping mechanism that controls the force with which the contact elements of the device impact the workpiece. A preferred embodiment of the present invention is a spin chuck, and especially a spin chuck within a process module for single wafer wet processing, in which the damping mechanism controls and limits the force, with which the chuck pins impact the wafer periphery when the pins resume contact with the wafer following wafer shift. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other objects, features and advantages of the invention will become more apparent after reading the following detailed description of preferred embodiments of the invention, given with reference to the accompanying drawings, in which: 
         FIG. 1  is a schematic perspective view from above of a chuck according to an embodiment of the invention, with a wafer in position; 
         FIG. 2  is a schematic perspective view from below of the chuck of  FIG. 1 , in which the lower base body has been removed, and corresponding to the state, in which a wafer is being gripped by the chuck pins; 
         FIG. 3  is a view similar to that of  FIG. 2 , but corresponding to the state, in which a wafer is not being gripped by the chuck pins; 
         FIG. 4A  is a schematic sectional view of the damper mechanism  51  in the open position of the chuck; 
         FIG. 4B  is a schematic sectional view of the damper mechanism  51  in the closed position of the chuck; 
         FIG. 5A  is a schematic sectional view of an alternative embodiment of the damper mechanism  51  in the open position of the chuck; and 
         FIG. 5B  is a schematic sectional view of an alternative embodiment of the damper mechanism  51  in the closed position of the chuck. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     In  FIG. 1 , a chuck  10  is in a working position, which means that a wafer W is held by the pins  25 . The circular arrow depicts the clockwise rotation of the chuck. Chuck  10  includes an upper base body  11  and a lower base body  12 . The wafer is held peripherally by a series of pin assemblies  23 , each including an eccentrically mounted chuck pin  25 . As discussed above, when the pin assembly  23  is rotated about its axis (parallel to the rotation axis of the chuck) the pin  25  is moved towards and away from the wafer edge. 
     Chuck  10  is preferably a Bernoulli chuck, where the wafer floats on a gas cushion provided by a number of gas nozzles (not shown) and is also supported from below by the gas cushion due to the Bernoulli effect. 
     In  FIG. 2  the circular arrow again depicts the clockwise rotation of the chuck, although the arrow itself is directed counterclockwise due to the chuck being shown from below in this figure. For ease of illustration the lower base body  12  is not shown. Thus, in  FIG. 2  there is visible not only the upper base body  11  and the pin assemblies  23 , but also the ring gear  30 , whose teeth  37  are in meshing engagement with the teeth  27  formed on the lower portion of each pin assembly  23 . 
     Coil springs  43  are connected at one end to respective spokes  32  of the ring gear  30 , and at their opposite ends to the upper base body  11 . Springs  43  urge ring gear  30  to rotate relative to the upper base body such that the pin assemblies  23  are pivoted so as to bring pins  25  to their radially inner position, in which they contact the wafer W, the “closed position” as shown in  FIGS. 1 and 2 . 
     The chuck base body is connected to a drive shaft (not shown), and when the chuck is driven in the depicted clockwise direction the inertia of the ring gear  30  during acceleration of the chuck will cause the pins  25  to momentarily open, as described above. Conversely, when the chuck is rotated in the counterclockwise direction the chuck must be decelerated in order to momentarily open the pins. 
     A typical angular acceleration for controlled wafer shift would be 3200 deg/s 2 . 
     As discussed above, the present inventors have discovered that, as the gear ring returns from the open position to the closed position during acceleration or deceleration of the chuck, particularly as assisted by the springs  43 , the uncontrolled impact of the chuck pins  25  with the peripheral edge of wafer W gradually damages the pins. 
     To extend the lifetime of the pins and/or get more stable shift performance, a damper system is installed in the chuck. In particular, this embodiment includes a damper  51  mounted to the upper base body  11 . The damper comprises a rod  53  that is urged toward a projecting (“armed out”) position by a coil spring within the damper. When the chuck is closed, with pins  25  contacting the wafer edge, rod  53  of the damper is in contact with an adjacent spoke  32  of gear ring  30 ; however, rod  53  is preferably not fixed to spoke  32 . 
     In  FIG. 3  the chuck is in an open position, which means that a wafer is not held by the pins  25  and thus can freely rotate relative to the chuck. The circular arrow in  FIG. 3  again depicts an accelerated clockwise rotation of the chuck. Due to the inertia of the ring gear  30  the ring gear  30  is (although still rotating clockwise) rotated counterclockwise with respect to the base body  12  of the chuck  10 . This counterclockwise relative motion is carried out against the force of the coil springs  43 , which normally urge the ring gear to pivot towards the closed position. Therefore the pin assemblies  23  rotate clockwise for a fraction of a second and the pins  25  release the wafer.  FIG. 3  shows the chuck in a position where the chuck has just been accelerated and the tooth gear has not yet followed that acceleration. As can be seen in  FIG. 3 , the rod  53  of the damper  51  has not yet armed out and thus does not yet contact the spoke  32 . 
     When the tooth gear  30  is drawn back toward the closed position the spoke  32  will touch the rod  53 , which will be armed out a little bit already. Consequently the fast movement of the tooth gear following the chuck is then prevented by the rod  53 , which will slowly draw in, such that the damper  51  thus limits the velocity of the movement of the tooth gear toward the closed position. 
     The pin closing step during wafer shift is driven by the inertia of the gear ring and the spring force. In a conventional spin chuck having six chuck pins, the total closing energy, which must be dissipated by the six chuck pins has been determined to be 0.246 Nm. Due to technical considerations it is assumed that this energy is converted to elastic energy, plastic energy and “wear energy” (abrasion of the polymer material of the pins and crack formation). 
     Furthermore due to the pin bending as well as the high contact force between the wafer and the pins it is possible that multiple wafer shifts during one process are not feasible with conventional spin chucks. The root causes for these multiple wafer shift malfunctions are considered to be deadlocking due to pin bending as well as the high friction force which is created by the high clamping force. 
     The damper system according to the invention decreases the amount of energy which must be dissipated by the chuck pins. The damper system, which is similar in some respects to a shock absorber, and/or a rotation-brake is installed to decrease or eliminate the closing energy during the wafer shift. The damper is moreover preferably adjustable, and the inventors have determined that the energy dissipated by the damper is more or less a linear function of the “damper stroke.” The “damper stroke” and thus the energy which is dissipated by the damper mechanism can be controlled. The presently preferred energy dissipation target range is between 20 and 80% of the theoretical closing energy. 
     The damping element is designed so that the clamping element closing velocity does not exceed approximately 1 cm/s (0.01 m/s). 
     A further possibility to decrease as well as to control the wafer shift involves use of a rotation brake. A rotation brake is able to act in both directions. This means that during the opening movement and the closing movement of the ring gear the brake generates a force that opposes movement of the ring gear. By using different rotation brakes as well as different adjustments inside the chuck the brake torque can be adjusted. 
     Any axial movement of the wafer during wafer-shift can be avoided by using a Bernoulli chuck where the wafer floats on a gas cushion and simultaneously is held by said gas cushion due to the Bernoulli effect. Alternatively any axial movement of the wafer during wafer-shift can be avoided by using scalloped or mushroom-shaped pins. 
       FIGS. 4A and 4B  depict the damper mechanism  51  in the open ( FIG. 4A ) and closed ( FIG. 4B ) positions of the chuck. Rod  53  is provided with an enlarged head  55  for contacting an adjacent spoke  32  of ring gear  30 . Within the housing of damper mechanism  51 , the rod  53  bears via an attached plate on a coil spring  57 . The spring force of coil spring  57  is less than that of the coil springs  43 , so that once the inertia of the gear ring is overcome, the coil spring  57  moves from the relaxed state shown in  FIG. 4A  to the compressed state as shown in  FIG. 4B . 
       FIGS. 4A and 4B  depict a spring acting in compression, but the damper may also include a spring acting in tension, e.g., a spring captive on the rod  53  within the housing of damper mechanism  51 , attached at one end to the housing and at the other to the distal end of the rod  53 . 
     Damping mechanisms other than spring-based devices may also be used.  FIGS. 5A and 5B  depict an alternative damper mechanism in which a strip or membrane  59  formed from elastic material is used in place of a spring. 
     Preferably a dashpot (hydraulic damper) is used as the damping mechanism. 
     Dashpots utilize viscous friction for resisting motion. Dashpots consist of a piston that moves through a viscous fluid in conjunction with a spring, e.g. in shock absorbers. The damping force is proportional to the velocity of the movement. During motion this damping force reacts in the opposite direction. This oppositely directed damping force opposes the motion and absorbs energy. 
     The two parameters that mainly characterize dashpots are the stroke and the damping coefficient. Linear displacement of the dashpot is measured by the stroke whereas damping force per unit velocity is measured by the damping coefficient. 
     If a linear dashpot is used the preferred range of the damping coefficient is 2-200 Ns/m, and preferably 6-60 Ns/m (for example 20 Ns/m). 
     If a rotary dashpot is used the preferred range of the damping coefficient is 0.07-7 Nms/rad (torque per angular velocity), and preferably 0.2-2 Nms/rad (for example 0.7 Nms/rad)—this is the actual damping coefficient at which the tooth gear shall be damped. 
       FIGS. 2 and 3  depict a single damper mechanism but it is to be understood that plural damper mechanisms may also be used, so as to distribute the braking force more evenly over the periphery of the chuck as well as to permit the use of smaller individual mechanisms and lighter springs. For example, a spin chuck having six pin assemblies could be equipped with one, three or six damper mechanisms. 
     Although the present invention has been described in connection with spin chucks it may also be used in a non-rotating chuck. Furthermore, although the invention has described in connection with a chuck used for wet chemical processing, it could also be used for dry processes. 
     While the present invention has been described in connection with various preferred embodiments thereof, it is to be understood that those embodiments are provided merely to illustrate the invention, and should not be used as a pretext to limit the scope of protection conferred by the true scope and spirit of the appended claims.