Abstract:
An apparatus and an associated method. The apparatus includes a chuck in a process chamber, an array of three or more ultrasonic sensors in the process chary a ceramic ring surrounding the chuck, and a controller connected to the ultrasonic sensors. The chuck is configured to removeably hold a substrate for processing. Each ultrasonic sensor may send a respective ultrasonic sound wave to a respective preselected peripheral region of the substrate and receive a respective return ultrasonic sound wave from the preselected peripheral region. The controller may compare a measured position of the substrate on the chuck to a specified placement of the substrate on the chuck based on a measured elapsed time between sending the ultrasonic sound wave and receiving the return ultrasonic sound wave for each ultrasonic sensor. The method compares a measured position of the substrate on the chuck to a specified position on the chuck.

Description:
[0001]    This application is a continuation application claiming priority to Ser. No. 14/183,631 filed Feb. 19, 2014. 
     
    
     BACKGROUND 
       [0002]    The present invention relates to the field of semiconductor processing systems; more specifically, it relates to a method and apparatus for determining the location of semiconductor substrates on chucks used in semiconductor processing. 
         [0003]    If a substrate is not properly positioned on a chuck during processing, material can be deposited on the chuck that lead to processing defects on substrates subsequently placed on the chuck, If the mispositioned material is not immediately detected, many defective substrates can be produced before the problem can be corrected. Accordingly, there exists a need in the art to mitigate the deficiencies and limitations described hereinabove. 
       BRIEF SUMMARY 
       [0004]    A first aspect of the present invention is an apparatus, comprising: a chuck in a process chamber, the chuck configured to removeably hold a substrate for processing; an array of two or more ultrasonic sensors arranged in the process chamber, each ultrasonic sensor of the two or more ultrasonic sensors arranged relative to the chuck so as to send a respective ultrasonic sound wave to a respective preselected region of the substrate and receive a respective return ultrasonic sound wave from the preselected region of the substrate; and a controller connected to the array of two or more ultrasonic sensors and configured to compare a measured position of the substrate on the chuck to a specified placement of the substrate on the chuck based on a measured elapsed time between sending the ultrasonic sound wave and receiving the return ultrasonic sound wave for each ultrasonic sensor of the array of two or more ultrasonic sensors. 
         [0005]    A second aspect of the present invention is a method, comprising: providing an apparatus comprising: a chuck in a process chamber, the chuck configured to removeably hold a substrate for processing; an array of two or more ultrasonic sensors arranged in the process chamber, each ultrasonic sensor of the two or more ultrasonic sensors arranged relative to the chuck so as to send a respective ultrasonic sound wave to a respective preselected region of the substrate and receive a respective return ultrasonic sound wave from the preselected region of the substrate; and a controller connected to the array of two or more ultrasonic sensors and configured to compare a measured position of the substrate on the chuck to a specified placement of the substrate on the chuck based on a measured elapsed time between sending the ultrasonic sound wave and receiving the return ultrasonic sound wave for each ultrasonic sensor of the array of two or more ultrasonic sensors; placing the substrate on the substrate chuck; and measuring a position of the substrate on the chuck using the array of two or more ultrasonic sensors and comparing the measured position to a specified position on the chuck. 
         [0006]    These and other aspects of the invention are described below. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    The features of the invention are set forth in the appended claims. The invention itself, however, will be best understood by reference to the following detailed description of illustrative embodiments then read in conjunction with the accompanying drawings, wherein: 
           [0008]      FIG. 1  is a schematic cross-section of an exemplary semiconductor process apparatus to which the present invention may be applied; 
           [0009]      FIGS. 2A, 2B and 2C  illustrate a mechanism observed to cause defects on wafers; 
           [0010]      FIG. 3A  is a top view,  FIG. 3B  is a side view and  FIG. 3C  is a detailed view illustrating an embodiment of the present invention using horizontal sensors; 
           [0011]      FIG. 4A  is a top view and  FIG. 4B  is a side view illustrating detecting horizontal wafer misplacement according to the embodiment of the present invention of  FIGS. 3A and 3B ; 
           [0012]      FIG. 5A  is a top view and  FIG. 5B  is a side view illustrating an embodiment of the present invention using vertical sensors; 
           [0013]      FIG. 6A  is a top view and  FIG. 6B  is a side view illustrating detecting horizontal wafer misplacement according to the embodiment of the present invention of  FIGS. 5A and 5B ; 
           [0014]      FIG. 7A  is a top view and  FIG. 7B  is a side view illustrating detecting vertical wafer misplacement caused by FM according to the embodiment of the present invention of  FIGS. 5A and 5B ; 
           [0015]      FIG. 8A  is a top view and  FIG. 8B  is a side view illustrating an embodiment of the present invention using both horizontal and vertical sensors; 
           [0016]      FIGS. 9A through 9E  illustrate operation of the ultrasonic sensors according to embodiments of the present invention; 
           [0017]      FIG. 10  is a side view of a wafer position calibration fixture; 
           [0018]      FIG. 11  is a schematic diagram of the tool controller link to the ultrasonic sensors of embodiments of the present invention; 
           [0019]      FIG. 12A  is a flow diagram of an exemplary method of calibrating the apparatus of embodiments of the present invention; and 
           [0020]      FIG. 12B  is a flowchart illustrating a method of using the apparatus of embodiments of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0021]    Circular (disk-like) semiconductor substrates are commonly referred to as semiconductor wafers and the chucks that hold these semiconductor wafers are commonly referred to as wafer chucks. While embodiments of the invention are described using semiconductor wafers (hereinafter wafers), embodiments of the present invention may be applied to other types of substrates that are held in chucks such as metallic and ceramic substrates and to substrates that are square or rectangular or otherwise non-circular. 
         [0022]    Embodiments of the present invention detect misalignment of wafers on a wafer chuck and/or the presence of foreign material (FM) on a wafer chuck by means of an array of three or more ultrasonic detectors that detect the edges of the wafer chuck relative to a calibration position and/or array of three or more ultrasonic detectors that detect the top surface of the wafer relative to a calibration position. Any difference between the measured position and the calibration positions indicate wafer mispositioning and/or FM on the wafer chuck. The positions are measured by the time difference between sending an ultrasonic sound wave to the wafer and receiving a reflected sound wave back. 
         [0023]      FIG. 1  is a schematic cross-section of an exemplary semiconductor process apparatus to which the present invention may be applied. In  FIG. 1 , a semiconductor process apparatus  100  includes a vacuum chamber  105 , a wafer chuck  110  for holding a substrate  115 , a set of radio frequency (RE) coils  120 , means  125  for introducing reactant gas into the chamber and a vacuum pump port  130 . Wafer chuck  110  includes a coolant fluid in line  135  and a coolant fluid out line  140  and a chamber  145  allowing coolant fluid to contact the backside of wafer  115 . 
         [0024]      FIGS. 2A, 2B and 2C  illustrate a mechanism observed to cause defects on wafers. In  FIG. 2A , a wafer  115 A having a center  150 A was processed through a high density plasma (HDP) oxide apparatus and was observed to have a region  155  where the integrated circuit chips were defective due to metal line voiding. In  FIG. 2B , wafer chuck  110 A (having a center  160 ) that was used to process substrate  115 A was examined and found to have a region  165  where HDP oxide had been deposited. The shape and location of region  165  corresponded to that of region  155  of substrate  115 A. The cause of the problem was determined to be off-center placement of a previous wafer  115 B (having a center  150 B)on wafer chuck  110 A allowing deposition HDP oxide on the chuck in a position normally protected by the substrates as illustrated in  FIG. 2C . The deposited HDP oxide prevented the edge of substrate  115 A from contacting the wafer chuck in region  155  thereby preventing full cooling of the substrate in the region and causing metal line voids. 
         [0025]    While the observations were made in an HDP deposition apparatus, embodiments of the present invention are applicable to apparatus that perform plasma depositions of other dielectric materials in addition to HDP oxide, examples of which plasma enhanced chemical vapor deposition (PECVD) of silicon nitride, silicon-oxy-nitride and deposition of silicon oxide using tetraethylorthosilicate (TEOS). The invention is also useful in plasma etch and reactive ion etch (RIE) apparatus. Embodiments of the present invention are useful for detecting FM on wafer chucks from other sources as well. 
         [0026]    It is a feature of the present invention that no machined or ground locating marks such as notches or flats are required to be formed on the wafer surface or edges, thus the sensors can be placed to detect any edge or surface of the wafer. 
         [0027]      FIG. 3A  is a top view,  FIG. 3B  is a side view and  FIG. 3C  is a detailed view illustrating an embodiment of the present invention using horizontal sensors. In  FIGS. 3A and 3B , wafer  115  is positioned on the top surface  112  of wafer chuck  110  so the edges of wafer  115  and chuck  110  are aligned. In one example, wafer chuck  115  is an electrostatic chuck. No portion of top surface  112  of wafer chuck is exposed. Wafer chuck  110  is surrounded by a circular ring  170 . Ring  170  includes ultrasonic sensors  175 A,  175 B,  175 C and  175 D. Ultrasonic sensors  175 A,  175 B,  175 C and  175 D both emit and detect ultrasonic sound waves. While ring  170  is shown spaced away from chuck  110  for clarity, ring  110  may be sized and positioned to just fit around chuck  110  with minimal clearance (e.g., a tenth of an inch or less) as illustrated in  FIG. 3C . In  FIG. 3C , the top of ultrasonic sensor  175 D is coplanar with the top surface of wafer  115  because it is notched into ring  170 . The top surface of ring  170  is coplanar with the top surface of wafer  115  wherever there is no ultrasonic sensor. While ultrasonic sensor  175 D is illustrated as directly facing the edge of wafer  115 , it may be positioned tilted so that the ultrasonic points upward to the edge wafer  115 . In the tilted orientation, if wafer  115  is mispositioned away from ultrasonic sensor  175 D, there will be little to no return signal which may be accounted for in calculating return times with a default value. The variation of  FIG. 3C  may be used in all embodiments of the present invention. Ultrasonic sensors  175 A and  175 C are positioned opposite each other and ultrasonic sensors  175 B and  175 D are positioned opposite each other. In  FIGS. 3A and 3B , the edge  116  of wafer  115  is aligned to the edge  111  of chuck  110  which is the specified and nominal placement position of wafer  115  on chuck  110 . No portion of the top surface of  112  of chuck  110  is exposed when the wafer is placed the specified and nominal placement position. While four ultrasonic sensors are illustrated, a minimum of two ultrasonic sensors placed at 90° relative to the circumference of the wafer may be used with three or more sensors are preferred in this embodiment. 
         [0028]      FIG. 4A  is a top view and  FIG. 4B  is a side view illustrating detecting horizontal wafer misplacement according to the embodiment of the present invention of  FIGS. 3A and 3B . The horizontal direction is defined as any direction parallel to top surface  112  of wafer chuck  110 . The vertical direction is defined as any direction perpendicular to top surface  112  of wafer chuck  110 . In  FIGS. 4A and 4B , wafer  115  is mispositioned in the horizontal direction because a portion  114  of top surface of  112  of wafer chuck  110  is exposed. It will take longer for ultrasonic sound waves to return to ultrasonic sensor  175 D then to ultrasonic sensor  175 B. However, it is more accurate to compare the measured return time (MRT) to a calibrated return time (CRT) for each ultrasonic sensor as described infra. 
         [0029]      FIG. 5A  is a top view and  FIG. 5B  is a side view illustrating an embodiment of the present invention using vertical sensors. In  FIGS. 3A and 3B , wafer  115  is positioned on the top surface  112  of wafer chuck  110  so the edges of water  115  and chuck  110  are aligned. No portion of top surface  112  of wafer chuck is exposed. Ultrasonic sensors  180 A,  180 B,  180 C and  180 D are positioned above the top surface  117  of wafer  115 . By way of example, in  FIG. 5B , ultrasonic sensors  180 A,  180 B,  180 C and  180 D are positioned proximate to or attached to a dome  185  positioned over water chuck  110 . Ultrasonic sensors  180 A,  180 B,  18 C and  180 D are positioned over the periphery of wafer chuck  110  and are arranged at 90° angles to each other relative to center  160  of wafer chuck  110 . Ultrasonic sensors  180 A,  180 B,  180 C and  180 D both emit and detect ultrasonic sound waves. While four ultrasonic sensors are illustrated, a minimum of two ultrasonic sensors placed at 90° relative to the circumference of the wafer may be used with three or more sensors are preferred in this embodiment. 
         [0030]      FIG. 6A  is a top view and.  FIG. 6B  is a side view illustrating detecting horizontal wafer misplacement according to the embodiment of the present invention of  FIGS. 5A and 5B . In  FIGS. 6A and 6B , wafer  115  is mispositioned the same as in  FIG. 4B . Thus, the return time for ultrasonic sensor  180 D in  FIG. 6B  will be greater when compared to the return time of ultrasonic sensor  180 D in  FIG. 5B . 
         [0031]      FIG. 7A  is a top view and  FIG. 7B  is a side view illustrating detecting vertical wafer misplacement caused by FM according to the embodiment of the present invention of  FIGS. 5A and 5B . In  FIGS. 7A and 7B , FM  190  on the top surface of  112  of wafer chuck  110  causes wafer  115  to not fully contact wafer chuck  110 . Thus, the return time for ultrasonic sensor  180 D in  FIG. 7B  will be less when compared to the return time for ultrasonic sensor  180 D in  FIG. 5B . 
         [0032]      FIG. 8A  is a top view and  FIG. 8B  is a side view illustrating an embodiment of the present invention using both horizontal and vertical sensors. In  FIGS. 8A and 8B , ultrasonic sensors  175 A,  175 B,  175 C and  175 D of circular ring  170  and ultrasonic sensors  180 A,  180 B,  180 C and  180 D positioned proximate to or attached to dome  185  are present. 
         [0033]      FIGS. 9A through 9E  illustrate operation of the ultrasonic sensors according to embodiments of the present invention. In  FIG. 9A , an ultrasonic sensor  195  emits a high-frequency sound wave  200  at time  205  which bounces off a target  210  and generates a return signal (again a high-frequency sound wave)  215  that is detected by ultrasonic sensor  195  at time  220 . The difference in time  205  and time  220  is the return time.  FIG. 9B  illustrates that the concept of the usable sensing range of ultrasonic sensor  195 . SD min  is the minimum distance from ultrasonic sensor  195  that an object can be detected. Inside this distance no usable signal will be generated by the sensor. SD max  is maximum distance from ultrasonic sensor  195  that an object can be detected. Past this distance no usable signal will be generated by the sensor. Thus, there is a sensing range S R  that an object must be in for its distance from the sensor to be measured. Ultrasonic sensors can be operated in two modes, unfocused and focused.  FIG. 9C  illustrates the unfocused mode of ultrasonic sensor  195  in which there is a Fresnel zone Z 1  and a Fraunhofer zone Z 2 .  FIG. 9D  illustrates the focused mode of ultrasonic sensor  195  in which there is a focal depth Z 3  and a focal zone Z 4 . It is preferred that the ultrasonic sensors of embodiments of the present invention be run in focused mode.  FIG. 9E  plots sensor output vs. distance. S R  is obtained from the data sheet of the ultrasonic sensor. Lower set limit (LSL) and upper set limit (USL) are the detection distances set by the user via. DIP switches or software commands to the sensor. The output of the sensor emits and detects thus falls between a minimum value V min  and a maximum value V max  that corresponds to the LSL and USL. The output may be milliamps, volts, digital output or sensor cycles. The cross-hatched area defines the parametric limits of the system. 
         [0034]      FIG. 10  is a side view of an exemplary wafer position calibration fixture. In  FIG. 10 , wafer  115  is aligned to the edges of chuck  110  using a cylindrical guide ring  225  positioned between ring  170  and wafer chuck  110 . This is the position which every water processed through the tool is supposed to be placed by the wafer handling system. This sets the distances between the ultrasonic sensors (not shown) and the edge of wafer  115 . Other calibrations fixtures may be used. 
         [0035]      FIG. 11  is a schematic diagram of the tool controller link to the ultrasonic sensors of embodiments of the present invention. In  FIG. 11  an array  230  of ultrasonic sensors are connected to a tool controller  240 . Tool controller controls both the operation of the ultrasonic sensors and of the loading and unload and processing of the wafers. Tool controller  240  includes a microprocessor  245 , a memory  250 , an input device  255  (which may conation such devices as a keyboard, a mouse, switches, buttons, etc), and a display unit  260 . Microprocessor  245  sends control signals (which include on/off signals and may include software instructions to program the ultrasonic sensors) to the ultrasonic sensors of array  230 . Microprocessor  245  also calculates return times from the ultrasonic sensors of array  230  and then stores the return memory unit  250 . 
         [0036]      FIG. 12A  is a flow diagram of a method of calibrating the apparatus of embodiments of the present invention. In step  265 , a guide ring is placed around the wafer chuck. In step  270 , a wafer is placed inside the guide ring on the chuck. In step  275 , the guide ring is removed, leaving the wafer in place. In step  280 , the ultrasonic sensors are activated and the sensor return times are measured. In step  285 , the sensor return times are recorded as calibration return times (CRTs). There is a CRT for each sensor. The wafer is then removed from the chuck in step  290 . 
         [0037]      FIG. 12B  is a flowchart illustrating a method of using the apparatus of embodiments of the present invention. Prior to this operation, the ultrasonic sensors will have been calibrated as described supra. In step  300 , a wafer is placed on the chuck by the handling system. The tool is at room pressure. In step  305 , the ultrasonic sensors are turned on and the response time (MRT) of each of the ultrasonic sensors is measured. The ultrasonic sensors are then turned off. In step  310 , the MRTs are compared to the CRTs of the respective ultrasonic sensors. If all the MRTs match the CRTs (within pre-defined limits) then the method proceeds to step  315 . In step  315 , the process chamber is closed and evacuated. In step  320 , the wafers are processed. Exemplary processes include, but are not limited to depositions of dielectric materials, deposition of HDP oxide, PECVD silicon nitride, PECVD silicon-oxy-nitride deposition of TEOS, plasma etch and RIE. In step  325 , the process chamber is purged and in step  330  the wafer is removed from the chuck. In step  335 , if there is another wafer to be processed, the method loops back to step  300 , otherwise the method terminates. 
         [0038]    Returning to step  3110 . If, in step  310 , for at least one ultrasonic sensor its MRT does not match the CRT then the method proceeds to step  340 . In step  340 , it is determined whether the tool is being operated in control mode or data mode. If the tool is being operated in data mode, logistical data as to date/time, wafer ID and which ultrasonic sensor had the MRT to CRT mismatch is recorded in step  345  and the method proceeds to step  320 . This data can then be correlated with post processing inspection/test data for this wafer and subsequently processed wafers to determine if there is a correlation between the MRT to CRT mismatch and the inspection/test data. The chuck can also be inspected after processing of subsequent wafer processing is complete. It should be noted that the MRT and the CRT can be converted to distance using the formula Distance to object=½ (speed of sound) X the MRT or CRT. Thus measuring return time also measures distance. Since the sensor positions are known, the exact region of the wafer chuck can be identified for inspection. 
         [0039]    Returning to step  340 , if the tool is being operated in control mode then the method proceeds to step  350 . In step  350 , processing is aborted and the wafer removed from the chuck. Next the method proceeds to step  355 . Step  355  is similar to step  345 . Next the method proceeds to step  360  where the chuck is inspected and cleaned if necessary. The method may then start over at step  300 . 
         [0040]    Thus, embodiments of the present invention provide an apparatus and method for determining the location of wafer on wafer chucks used in semiconductor processing and for detecting miss-positioning thereof and are applicable to non-semiconductor substrates or semiconductor substrates that have a shape different from a semiconductor wafer. 
         [0041]    The descriptions of the various embodiments of the present invention have been presented for purposes of illustration, but are not intended to be exhaustive or limited to embodiments disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the described embodiments. The terminology used herein was chosen to best explain the principles of embodiments, the practical application or technical improvement over technologies found in the marketplace, or to enable others of ordinary skill in the art to understand embodiments disclosed herein.