Abstract:
An apparatus and method for leak detection of coolant gas from a chuck. The apparatus includes a chuck having a top surface and configured to clamp a substrate to the top surface, the chuck having one or more recessed regions in the top surface, the recessed regions configured to allow a cooling gas to contact a backside of the substrate; a cooling gas inlet and a cooling gas outlet connected to the one or more recessed regions; a first measurement device connected to the cooling gas inlet and configured to measure a first amount of cooling gas entering the cooling gas inlet and a second measurement device connected to the cooling gas outlet and configured to measure a second amount of cooling gas exiting from the cooling gas outlet; and a controller configured to determine a difference between the first amount of cooling gas and the second amount of cooling gas.

Description:
BACKGROUND 
       [0001]    The present invention relates to the field of semiconductor wafer processing systems; more specifically, it relates to a method and apparatus for detecting foreign material on wafer chucks used in semiconductor processing. 
         [0002]    Foreign material on chucks can lead to defective wafers. If the foreign material is not immediately detected, many defective wafers 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 
       [0003]    A first aspect of the present invention is an apparatus, comprising: a chuck having a top surface and configured to clamp a substrate to the top surface, the chuck having one or more recessed regions in the top surface, the recessed regions configured to allow a cooling gas to contact a backside of the substrate; a cooling gas inlet and a cooling gas outlet connected to the one or more recessed regions; a first measurement device connected to the cooling gas inlet and configured to measure a first amount of cooling gas entering the cooling gas inlet and second measurement device connected to the cooling gas outlet and configured to measure a second amount of cooling gas exiting from the cooling gas outlet; and a controller configured to determine a difference between the first amount of cooling gas and the second amount of cooling gas. 
         [0004]    A second aspect of the present invention is a method, comprising: providing a chuck having a top surface and configured to clamp a substrate to the top surface, the chuck having one or more recessed regions in the top surface, the recessed regions configured to allow a cooling gas to contact a backside of the substrate, the chuck including a cooling gas inlet and a cooling gas outlet connected to the one or more recessed regions; supplying cooling gas to the cooling gas inlet and exhausting the cooling gas from the cooling gas outlet; measuring a first amount of cooling gas entering the cooling gas inlet and measuring a second amount of cooling gas exiting from the cooling gas outlet; and determining a difference between the first amount of cooling gas and the second amount of cooling gas. 
         [0005]    These and other aspects of the invention are described below. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    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 when read in conjunction with the accompanying drawings, wherein: 
           [0007]      FIG. 1  is a schematic cross-section of an exemplary semiconductor process apparatus to which the present invention may be applied; 
           [0008]      FIGS. 2A ,  2 B and  2 C illustrate a mechanism observed to cause defects on semiconductor wafers; 
           [0009]      FIG. 3A  is a schematic top view and  FIG. 3B  is a schematic cross-section of a chuck according to an embodiment of the present invention; 
           [0010]      FIG. 4A  is a schematic top view and  FIG. 4B  is a schematic cross-section of a chuck according to an embodiment of the present invention; 
           [0011]      FIG. 5  is a detailed schematic diagram of a chuck according an embodiment of the present invention; 
           [0012]      FIG. 6  is a schematic diagram of a semiconductor apparatus according to an embodiment of the present invention; and 
           [0013]      FIG. 7  is a flowchart illustrating a method of using the semiconductor apparatus of  FIG. 6 . 
       
    
    
     DETAILED DESCRIPTION 
       [0014]    Embodiments of the present invention detect foreign material (FM) on a semiconductor processing chuck by measuring, with a wafer on the chuck, the amount of coolant gas supplied to the inlet of the chuck and the amount of coolant gas exiting the outlet of the chuck. The difference between these two measurements is used to determine if there is foreign material on the chuck preventing the wafer from sitting flush on the chuck thereby causing coolant gas leakage. 
         [0015]      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 chuck  110  for holding a semiconductor wafer  115 , a set of RF coils  120 , means  125  for introducing reactant gas into the chamber and a vacuum pump port  130 . 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 . 
         [0016]      FIGS. 2A ,  2 B and  2 C illustrate a mechanism observed to cause defects on semiconductor wafers. In  FIG. 2A , a semiconductor 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 , the chuck  110 A (having a center  160 ) that was used to process semiconductor wafer  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 semiconductor wafer  115 A. The cause of the problem was determined to be off-center placement of a previous semiconductor wafer  115 B on chuck  160  allowing deposition of HDP oxide on the chuck in a position normally protected by the semiconductor wafers as illustrated in  FIG. 2C . The deposited HDP oxide prevented the edge of semiconductor wafer  115 A from contacting the chuck in region  155  thereby preventing full cooling of the wafer in the region and causing metal line voids. 
         [0017]    While the observations were made in an HDP deposition apparatus, the 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. The embodiments of the present invention are useful for detecting other sources of foreign material on chucks as well. 
         [0018]    While the embodiments of the invention are described using wafers which are circular disks of semiconductor material, a wafer is an example of a substrate to which the embodiments of the present invention may be applied. For example, the embodiments of the present invention may be applied to metallic and ceramic substrates and to substrates that are square or rectangular. 
         [0019]      FIG. 3A  is a schematic top view and  FIG. 3B  is a schematic cross-section of a chuck according to an embodiment of the present invention.  FIGS. 3A and 3B  are intended to show the coolant gas grooves in the top surface of the chuck and inlet and outlet routes to and from the grooves. In  FIGS. 3A and 3B , chuck  165  has circular coolant gas inlet grooves  170 A and  170 B and circular coolant gas outlet grooves  175 A and  175 B recessed into the top surface of the chuck. Coolant gas inlet grooves  170 A and  170 B are connected to a coolant gas inlet  180  by galleries  185  in the body of chuck  165 . Coolant gas outlet grooves  175 A and  1750 B are connected to a coolant gas outlet  190  by galleries  195  in the body of chuck  165 . From  FIG. 3B  it is apparent that the entire interior surface  196  of chuck  165  (containing grooves  170 A,  170 B,  175 A and  175 B) is recessed below an outer rim  197  of the chuck. This geometry allows the periphery of wafer  115  to be supported while leaving a gap  198  between the backside of the wafer and interior surface  196  allowing direct contact of the coolant gas with the wafer. Other structures, not shown in  FIGS. 3A and 3B  are contained within chuck  165  and are illustrated in  FIG. 5  and described infra. 
         [0020]      FIG. 4A  is a schematic top view and  FIG. 4B  is a schematic cross-section of a chuck according to an embodiment of the present invention.  FIGS. 4A and 4B  are intended to show the coolant gas grooves in the top surface of the chuck and inlet and outlet routes to and from the grooves. In  FIGS. 4A and 4B , chuck  200  has a circular coolant gas inlet groove  205  and circular coolant gas outlet groove  210  recessed into the top surface of the chuck. Coolant gas inlet groove  205  is connected to a coolant gas inlet  215  by a gallery  220  and coolant gas outlet groove  215  is connected to a coolant gas outlet  225  by a gallery  230 . From  FIG. 4B  it is apparent that the entire interior surface  232  of chuck  200  (containing grooves  205  and  210 ) is recessed below an outer rim  233  of the chuck. This geometry allows the periphery of wafer  115  to be supported while leaving a gap  234  between the backside of wafer  115  and interior surface  232  allowing direct contact of the coolant gas with the wafer. Other structures, not shown in  FIGS. 4A and 4B  are contained within chuck  200  and are illustrated in  FIG. 5  and described infra. 
         [0021]      FIG. 5  is a detailed schematic diagram of a chuck according an embodiment of the present invention.  FIG. 5  is intended to show the various components of a chuck according to the embodiments of the present invention. In  FIG. 5 , a chuck  240  includes a coolant inlet groove  245  connected to a coolant gas inlet  250  and a coolant gas outlet groove  255  connected to a coolant gas outlet  260  in a top surface  265  of chuck  240  which is recessed below a top surface  270  of a peripheral region of chuck  240 . This allows direct contact between the backside of wafer  115  and the coolant gas which is critical to properly cooling wafer  115  during processing and to the operation of the embodiments of the present invention. The fact that the coolant supplied to grooves  245  and  255  is a gas and not a liquid is also critical to the operation of the embodiments of the present invention. Chuck  240  also includes an optional coolant liquid chamber  275  connected between a coolant liquid inlet  275  and a coolant liquid outlet  280 . Chuck  240  is an electrostatic chuck and therefore contains electrodes  285  embedded in (shown) or near top surface  265  of chuck  240 . The top surface  265  may include a dielectric layer so an electrostatic attraction force is generated to attract and hold wafer  115  to chuck  240  when a DC voltage is applied to electrodes  285 . While an electrostatic chuck is preferred, the embodiments of the present invention may be applied to chucks using a mechanical clamp of the wafer. Chuck  240  also includes a temperature sensor  290  connected to a signal line  295  that may be used to control the flow of coolant gas. Temperature sensor  290  may be embedded in the body of chuck  240  as illustrated or placed in the coolant gas outlet stream. 
         [0022]    In one example, the coolant gas comprises nitrogen, helium, neon or argon. In one example, the coolant gas is helium. 
         [0023]      FIG. 6  is a schematic diagram of a semiconductor apparatus according to an embodiment of the present invention. In  FIG. 6 , a semiconductor process apparatus  300  includes a chuck  305  (as illustrated in  FIGS. 3A ,  3 B,  4 A,  4 B and  5  and described supra) in a vacuum process chamber  310  (as illustrated in  FIG. 1  and described supra). Semiconductor process apparatus also includes a first mass flow meter  315  connected between a coolant gas inlet and chuck  305 , a second mass flow meter  320  connected between chuck  305  and a coolant gas outlet, and a calculation unit  325  that receives a first flow rate signal  330  from first mass flow meter  315  and a second flow rate signal  335  from second flow rate meter  320 . Calculation unit  325  generates a control signal  340  based on the values of first flow rate signal  330  and second flow rate signal  335 . 
         [0024]    Apparatus  300  includes a wafer load/unload system that places wafers on chuck  305  and removes wafer from chuck  305  after processing (e.g., deposition, plasma etch, RIE). 
         [0025]    First mass flow meter  315  performs a first measurement of the mass of gas per unit of time going into chuck  305  and second mass flow meter  320  performs a second measurement of the mass of gas per unit of time exiting chuck  305 . With a perfect seal between wafer  115  and chuck  305  and no foreign material between wafer  115  and chuck  305  the two measurements should be the same (the values of signals  330  and  340 ) and the difference between them being zero. However, a perfect seal is not achievable (particularly when the coolant gas is helium) even with no foreign material between wafer  115  and chuck  305  so there will be a loss of coolant gas between first flow meter  315  and second flow meter  320  resulting in a difference between the two measurements even when the chuck is free of foreign material. That is, the second flow meter can be expected to read lower than the first flow meter under nominal conditions. This expected difference is determined experimentally and an acceptable maximum loss (e.g., first measurement minus second measurement) is determined. This acceptable maximum loss becomes a specification limit that is programmed into calculation unit  325 . Volumetric flow meters or other means for measuring the amount of coolant gas flow may be substituted for mass flow meters  315  and  320 . Apparatus  300  also includes a wafer handling system that loads wafers onto and unloads wafers from the chuck. This system is also responsible for placing the wafer properly aligned (e.g., centered) to the chuck. 
         [0026]    In a first example, calculation unit  325  will generate a control signal when the measured loss is greater than the specification (e.g., input meter=8 Torr, output meter=7 Torr, measured loss=1 Torr). This signal may trigger an alarm to alert the operator. In a second example, calculation unit  325  will generate a control signal when the measured loss is greater than the specification. This signal may trigger an alarm to alert the operator and stop the wafer from being processed. Optionally, calculation unit may store the measured loss (which may be zero) by wafer or by time, to be displayed in a trend report that may be used to perform maintenance to clean the chuck or to adjust the placement of wafers on the chuck (see  FIG. 2C ). 
         [0027]      FIG. 7  is a flowchart illustrating a method of using the semiconductor apparatus of  FIG. 6 . In step  350 , a wafer is placed on a chuck and coolant gas flow is commenced. In step  355 , the difference in flow rate between the coolant gas inlet and the coolant gas outlet is measured. In step  360 , it is determined if the measured difference (coolant gas flow in minus coolant gas flow out) exceed a specified limit. If the specified limit is exceeded then, in step  365 , the apparatus is stopped and the operator notified to check for foreign material on the chuck and/or check the alignment of the wafer load/unload system. It is a key feature of the present invention that the tool performs the flow rate measurements before wafer processing (e.g., before deposition or etching) to prevent a defective wafer from being manufactured. 
         [0028]    If in step  360 , if the specified limit is not exceeded then in step  370 , the wafer is processed (e.g., a deposition is performed) and the wafer is removed from the chuck. Next, in step  375 , it is determined if another wafer is to be processed. If another wafer is to be processed then the method loops back to step  350 , otherwise the method terminates. 
         [0029]    Thus, embodiments of the present invention provide an apparatus and method for detecting foreign material on a semiconductor processing chuck by determining if there is a higher than specified amount of coolant gas lost between the inlet of the chuck and the outlet of the chuck. 
         [0030]    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 the 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 the 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 the embodiments disclosed herein.