Patent Publication Number: US-7914657-B2

Title: Controlling the thickness of wafers during the electroplating process

Description:
TECHNICAL FIELD 
     Embodiments of the present invention relate to electroplating wafers. More specifically, embodiments of the present invention relate to controlling the thickness of a wafer while electroplating the wafer. 
     BACKGROUND 
     As a part of manufacturing electronic components, such as read write heads, for electronic devices, wafers that the electronic components are made out of are electroplated. In order for a electronic component to function properly, the portion of the wafer that the electronic component is made out of must have an appropriate thickness. Manufacturing uses a “specification” to determine what portions of the wafer have the appropriate thickness. Only the portions of the wafer that have the appropriate thickness can be used to make a electronic component out of. 
     In the conventional process, a wafer is placed in a vat and electric current is passed through the wafer using deflectors associated with a cathode.  FIG. 1  depicts a cathode with a wafer associated with on the cathode. The cathode  110  has 8 deflectors d 1 -d 8  for putting current through the wafer  100 . A separate current is put through each deflector d 1 -d 8 . The 8 regions r 1 -r 8  correspond to the 8 deflectors d 1 -d 8 . The wafer  100  has 8 regions r 1 -r 8  for which current for the corresponding deflector d 1 -d 8  affects the thickness of. 
     A series of wafers are electroplated in a vat one after another. For example, wafer  1  is electroplated, then wafer  2  is electroplated, then wafer  3  is electroplated and so on. The current that is passed through each of the deflectors d 1 -d 8  is adjusted for each wafer that is electroplated in a vat. 
     The current that is applied to a deflector, such as deflector d 3 , associated with a new wafer, such as wafer  4 , is calculated based on the current that was applied to the same deflector d 3  of the previous wafer, e.g., wafer  3 , and the thicknesses at the edge of region r 3  and the center of the previous wafer. For the sake of illustration, “i” is a variable that designates the deflector. In the case of a wafer that has 8 deflectors, “i” will vary from 1 to 8. Ri is the thickness of the wafer  100 &#39;s outer edge for the ith region. Rc is the thickness of the wafer  100  at the center  120 . 
     “Old current i” is the current that was applied to the ith deflector for the previous wafer that was electroplated in a vat. The “old current i” is determined based on the current density and the area of the wafer  100  that is plated (also known as “platting area”). A pattern can be used to specify what area of the wafer  100  is plated and what area of the wafer  100  is not platted. 
     Ri is the thickness of the previous wafer&#39;s outer edge at the ith region. Rc is the thickness of the previous wafer&#39;s center. The new current i is the current that will be applied to the ith deflector for the next wafer that will be electroplated in the same vat. Therefore, the new current i for the next wafer is equal to a ratio of the thickness of the outer edge (Ri) and the center (Rc) times the old current i of the previous wafer, where i specifies a particular deflector. 
     The previous wafer is removed from the vat and the thicknesses at the center (Rc) and at the edges (Ri) that correspond to each of the regions r 1 -r 8  are measured manually. An engineer decides whether to change the current that will be applied to the deflectors for the next wafer using the manually measured thicknesses (Rc and Ris) and the currents “i” for the deflectors of the previous wafer. 
       FIG. 2  depicts the thickness of various portions of a wafer that results from using the conventional process. As can be seen, the thicknesses  210  range from 2.000 to 2.350 for wafer  100 . The wafer  100  is thinnest at the center and thickest at the outer edge. The mean of the thicknesses is 2.2328571 and the standard deviation is 0.1470414. 
     Therefore, there is a need for a way to increase the amount of the wafer that can be used to manufacture electronic components, among other things. 
     SUMMARY OF THE INVENTION 
     Embodiments of the present invention pertain to controlling thickness of wafers during electroplating process. Information pertaining to an old current used during an electroplating process of a previous wafer is received. Information pertaining to the thickness of the previous wafer is received. A new current is automatically determined. The new current is to be used during an electroplating process for a new wafer. The new current is determined based on the information pertaining to the old current and the information pertaining to the thickness of the previous wafer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in and form a part of this specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention: 
         FIG. 1  depicts a cathode with a wafer associated with on the cathode. 
         FIG. 2  depicts the thickness of various portions of a wafer that results from using the conventional process. 
         FIG. 3  depicts a block diagram of a device for controlling thickness of wafers during the electroplating process, according to one embodiments of the present invention. 
         FIG. 4  depicts a flowchart  400  for a method of controlling thickness of wafers during the electroplating process, according to one embodiment. 
         FIG. 5  depicts a flowchart  500  for another method of controlling thickness of wafers during the electroplating process, according to another embodiment. 
         FIG. 6  depicts results using various embodiments of the present invention. 
     
    
    
     The drawings referred to in this description should not be understood as being drawn to scale except if specifically noted. 
     DETAILED DESCRIPTION 
     Reference will now be made in detail to various embodiments of the invention, examples of which are illustrated in the accompanying drawings. While the invention will be described in conjunction with these embodiments, it will be understood that they are not intended to limit the invention to these embodiments. On the contrary, the invention is intended to cover alternatives, modifications and equivalents, which may be included within the spirit and scope of the invention as defined by the appended claims. Furthermore, in the following description of the present invention, numerous specific details are set forth in order to provide a thorough understanding of the present invention. In other instances, well-known methods, procedures, components, and circuits have not been described in detail as not to unnecessarily obscure aspects of the present invention. 
     OVERVIEW 
     With the conventional, a new current is determined manually. According to one embodiment of the present invention, the new current is determined automatically. By determining the new current automatically, manufacturing wafers becomes more efficient and therefore can result in less expensive electronic components. 
     With the conventional way, a new current is determined based only on data from one previous wafer electroplated in that vat. For example, if wafers  1 ,  2 ,  3  are electroplated in a vat one after another, then the new current that is to be applied to a deflector while electroplating wafer  2  is based solely on data from wafer  1 . Similarly, the new current that is applied to a deflector while electroplating wafer  3  is based solely on data from wafer  2 . 
     According to another embodiment of the present invention, data from more than one previous wafer that was previously electroplated is used for determining the new current that will be used for electroplating the next wafer that will be electroplated. Continuing the example, the new currents that will be used in electroplating wafer  3  can be based on the thicknesses (at the centers and at each of the regions) and the old currents (applied at each of the deflectors) that were used for electroplating the previous  2  wafers (wafers  1  and  2 ). By using data from more than one previous wafer that was previously electroplated in a vat, the new current that is to be applied is less likely to overshoot or undershoot a current setting that would result in the best thickness. Therefore, the variability of the thicknesses associated with the wafers made using various embodiments of the present invention in comparison to the variability of thicknesses  210  associated with a conventional wafer  100 , as will become more evident. Reducing the variability in the thicknesses results in a higher yield of electronic components for a given number of wafers. A higher yield of electronic components in turns results in lower the cost of manufacturing electronic components. 
     Device for Controlling Thickness of Wafers During Electroplating Process 
       FIG. 3  depicts a block diagram of a device for controlling thickness of wafers during the electroplating process, according to one embodiments of the present invention. The blocks that represent features in  FIG. 3  can be arranged differently than as illustrated, and can implement additional or fewer features than what are described herein. Further, the features represented by the blocks in  FIG. 3  can be combined in various ways. The device  300  can be implemented using software, hardware, firmware, or a combination thereof. 
     Device  300  includes an old current receiver  310 , a thickness receiver  320 , and a new current determiner  330 . A vat is used for electroplating a series of wafers. For example, one wafer is electroplated in a vat, then a second wafer is electroplated in the same vat, then a third wafer and so on. Each electroplating process that is performed on the series of wafers is commonly referred to as a “run.” For the purpose of illustration, a wafer that is electroplated prior to another wafer shall be referred to as a “previous wafer.” The wafer that is electroplated after the “previous wafer” shall be referred to as a “new wafer” or the “next wafer.” Two or more wafers that are electroplated before another wafer shall be referred to as “previous wafers.” For example, the first, second, and third wafers shall be the “previous wafers” with respect to the fourth wafer. 
     The old current receiver  310  receives information pertaining to an old current used during an electroplating process of a previous wafer. The thickness receiver  320  receives information pertaining to the thickness of the previous wafer. The new current determiner  330  automatically determines a new current to be used during an electroplating process for a new wafer. The new current determine  330  can determine the new current based on the information pertaining to the old current used for the previous wafer and the thickness of the previous wafer. 
     According to one embodiment, a new current is determined on a per deflector basis. According to one embodiment, the equation for determining the new currents “i” that are applied to each of the deflectors “i” is
 
new current  i =lambda×(( Ri/Rc )×old current  i )+(1−lambda)×old current  i )  (1)
 
     where “i” is a variable that specifies a particular deflector d 1 -d 8  or a region r 1 -r 8  that corresponds to that particular deflector, lambda is a variable that takes into account data for more than one previous wafer as will become more evident, Rc is the thickness of the previous wafer at the center, and Ri is the thickness of the previous wafer at the edge of the ith region. 
     For example, assume that the variable “i” specifies a deflector or region that corresponds to that deflector. The thickness of the previous wafer can be measured at the center of the previous wafer and at the edge of the previous wafer for each region i of the previous wafer. 
     Further, the old current that was applied to the previous wafer can be on a per deflector basis. Therefore, the old current receiver  310  can receive an old current “i” for each deflector i used for the previous wafer and the thickness receiver  320  can receive a thickness for the center of the previous wafer and the thicknesses at the edge of the previous wafer for each region “i” of the previous wafer. Then the new current determiner  330  can automatically determine the new currents “i” that will be applied to the deflectors “i” while electroplating a new wafer based on the old currents “i” for each of the deflectors “i” and the thicknesses measured at the center (Rc) and the edges (Ris). “Ris” refers to all of the thicknesses at the edges for all of the regions “i” of a wafer. 
     Lambda 
     As already stated, according to one embodiment, data from more than one wafer that was previously electroplated is used for determining the new current that will be used for electroplating the next wafer that will be electroplated. Lambda is a weighting factor used in equation 1 to calculate exponentially weighted moving average (EWMA) based on thickness data and currents from more than one wafer that was previously electroplated. According to one embodiment, lambda represents weighting factor for calculation of an exponentially weighted moving average (EWMA) of the currents. As is well known in the state of the art of statistics, the value of Lambda is range from 0 to 1. 
     Deflectors 
     According to one embodiment, the currents that are applied to deflectors are determined independently. As already described herein, the new current “i” that is applied to a particular deflector “i” for a new wafer can be based on data, such as the old current “i” and thickness Ri, for a deflector “i” for a previous wafer, among other things. For example, the new current that is applied to deflector d 1  for a new wafer can be based on the old current that was applied to deflector d 1  for a previous wafer and the thickness that was measured at the edge of the previous wafer at region r 1 . 
     However, there may be interactions between two adjacent regions, such as region r 1  and r 2  or region r 1  and r 8 . More specifically, the data gathered for the thickness of the edges of regions r 1  and r 2  for previous wafers indicate that historically the edge for region r 1  tends to be relatively thick and the edge for region r 2  tends to be relatively thin. Therefore, the currents that are applied to the deflectors, according to one embodiment, take into account interactions between the regions of a wafer that correspond to those deflectors. For example, the currents that are applied to deflectors d 1  and d 2  for a new wafer can be adjusted based on the gathered historical data to cause the relative thickness of regions r 1  and r 2  to even out. 
     In another example, the relative thickness of a first group of adjoining regions may be relatively thick and the thickness of a second group of adjoining regions may be relatively thin. Assume that the first and second groups may adjoin each other. Therefore, according to one embodiment, the currents that are applied to the deflectors that correspond to the first group of adjoining regions and second group of adjoining regions can be adjusted to cause the relative thickness of the regions to even out. For example, a current A may be applied to all of the deflectors that correspond to the first group of regions and a current B may be applied to all of the deflectors that correspond to the second group of regions. By making current B higher than current A, the thickness of the first group of regions may be decreased and the thickness of the second group of regions may be increased. 
     Although, the currents that are applied to deflectors are determined independently, two or more of the currents may be the same. 
     Methods of Controlling Thickness of Wafers During the Electroplating Process 
       FIG. 4  depicts a flowchart  400  for a method of controlling thickness of wafers during the electroplating process, according to one embodiment, and  FIG. 5  depicts a flowchart  500  for another method of controlling thickness of wafers during the electroplating process, according to another embodiment. Although specific steps are disclosed in flowcharts  400 ,  500 , such steps are exemplary. That is, embodiments of the present invention are well suited to performing various other steps or variations of the steps recited in flowcharts  400 ,  500 . It is appreciated that the steps in flowcharts  400 ,  500  may be performed in an order different than presented, and that not all of the steps in flowchart  400 ,  500  may be performed. 
     For the purposes of illustration, the description of flowcharts  400  and  500  shall refer to  FIG. 3  and equation 1. Further, for the purpose of illustration, assume that a series of previous wafers  1 - 10  have been electroplated in the same vat and wafer  10  has just been taken out of the vat. Preparations are being made to electroplate a new wafer  11  in the same vat. Further, for the purposes of illustration, assume that EWMAs of the old currents that were applied to the deflectors d 1 -d 8  for wafers  1 - 9  have been calculated. Further, for the purposes of illustration, assume that 8 deflectors are used for electroplating the wafers, thus, the wafers have 8 regions. The variable “i” shall be used for referring to each of the deflectors d 1 -d 8 . For the sake of simplicity, flowcharts  400  and  500  shall refer to deflector d 1 . 
     In step  410 , the process begins. 
     In step  420 , information pertaining to an old current used during an electroplating process of a previous wafer is received. For example, the old current receiver  310  receives the old current that was applied to deflector d 1  when electroplating wafer  10 . The old current may have been stored, for example, by device  300  and the old current receiver  310  can receive the stored old current. According to one embodiment, an EWMA (lambda) is calculated that takes into account the old current that was applied to deflector d 1  for wafer  10 . 
     In step  430 , information pertaining to the thickness of the previous wafer is received. For example, when wafer  10  is removed from the vat, the thickness (Rc) of the center and the thickness (R 1 ) at the edge of region r 1  is measured. The thickness receiver  320  can receive the thicknesses Rc and R 1 . 
     In step  440 , a new current is determined automatically. For example, the new current that will be applied to deflector d 1  while electroplating wafer  11  is calculated using the old current that was received by current receiver  310  in step  420  (e.g., that was applied to deflector  1 ) and the thicknesses (Rc and R 1 ) that were received by the thickness receiver  320  in step  430 , for example, using equation 1. Note, that since a device  300  is being used for determining the new current, the new current can be determined automatically rather than manually as is the case with the conventional process. 
     In step  450 , the process stops. 
     According to one embodiment, steps  410 - 450  determine the new current that is to be applied to a particular deflector  1 . Steps  410 - 450  can be performed in order to determine the new currents that are to be applied to the other deflectors d 2 -d 8  in a manner similar to that described above for deflector d 1 , for example, until a new current has been determined for each of the deflectors d 1 -d 8 . 
     In step  510 , the process begins. 
     In step  520 , information pertaining to old currents used during electroplating process of previous wafers is received. For example, the old current receiver  310  receives the old currents that were applied to deflector d 1  when electroplating wafers  1 - 10 . The old currents may have been stored, for example, by device  300  and the old current receiver  310  can receive the stored old currents. According to one embodiment, an EWMA (lambda) is calculated that takes into account the old current that was applied to deflector d 1  for wafer  10 . Note, that since the EWMA that pertains to deflector d 1  for wafer  10  is a weighted moving average that uses the old currents that were applied to deflectors d 1  for wafers  1 - 10 , information pertaining to old currents used during electroplating process of previous wafers is received by the old current receiver  310 , according to one embodiment. 
     In step  530 , information pertaining to the thickness of a previous wafer is received. For example, when wafer  10  is removed from the vat, the thickness (Rc) of the center and the thickness (R 1 ) at the edge of region r 1  is measured. The thickness receiver  320  can receive the thicknesses Rc and R 1 . 
     In step  540 , a new current is determined. For example, the new current that will be applied to deflector  1  while electroplating wafer  11  is calculated using the old currents that was received by current receiver  310  in step  520  (e.g., that was applied to deflector d 1 ) and the thicknesses (Rc and R 1 ) that were received by the thickness receiver  320  in step  530 , for example, using equation 1. According to one embodiment, the EWMA (lambda) that pertains to deflector d 1  for wafer  10  is used to determine the new current. 
     Note, that since the EWMA that pertains to deflector d 1  for wafer  10  is a weighted moving average that uses the old currents that were applied to deflectors d 1  for wafers  1 - 10 , the new current is determined based on information pertaining to the old currents, according to one embodiment. 
     By using method  500  the variability of the thicknesses  610  associated with the new wafer  600  is reduced in comparison to the variability of thicknesses  210  associated with a conventional wafer  100 . For example,  FIG. 6  depicts results using various embodiments of the present invention. Note that the thickness  610  of various portions of the wafer  600  vary from only 2.025 to 2.125, that the mean is 2.0874643, and the standard deviation is only 0.0272458 in comparison to the results depicted by  FIG. 2  for wafer  100 . Thus, the various embodiments of the present invention result in significant improvements from the conventional process since the thicknesses  610  across wafer  600  are much more even than thicknesses  210  that result from the conventional process used for wafer  100  (e.g., the variability of the thicknesses  610  associated with wafer  600  is reduced in comparison to the variability of the thicknesses  210  associated with conventional wafer  100 ). Further, there has been a long felt need for the improved results provided by various embodiments of the present invention. 
     In step  550 , the process stops. 
     According to one embodiment, steps  510 - 550  determine the new current that is to be applied to a particular deflector d 1 . Steps  510 - 550  can be performed in order to determine the new currents that are to be applied to the other deflectors d 2 -d 8  in a manner similar to that described above for deflector d 1 , for example, until a new current has been determined for each of the deflectors d 1 -d 8 .