Patent Publication Number: US-2005139939-A1

Title: Wafer protection device

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to a wafer protection device, and particularly to a wafer protection device to be applied in etching of the micro-electromechanical system (MEMS).  
      2. Description of the Related Art  
      Semiconductor and MEMS techniques have been widely developed in recent years. In MEMS techniques, etching is generally applied in bulk micro-machining of the silicon substrate. Since etching time is related to the thickness of the substrate, such as Si-wafer, the substrate must be disposed in the etching fluid for a certain period of time. In this case, etching is generally performed at the beginning of the manufacturing process, so that the circuit layout on the wafer is not affected by etching.  
      However, in the above-mentioned etching process, the wafer must be disposed in the etching fluid for a certain period of time to perform bulk etching. In this certain period of time, metal atoms contained in the etching fluid are diffused into the wafer, which may further induce furnace contamination.  
      On the other hand, the bulk etching process produces throughholes or deep canals on the wafer, which significantly reduces the strength and resilience of the wafer and increases wafer fracture possibilities in the subsequent manufacturing processes.  
      In conventional bulk wafer etching, alkali fluids such as KOH or TMAH are generally applied as the etching fluid. In this case, the alkali etching fluid has a relatively high etching rate to the passivation layer material, such as silicon nitride or silicon oxide, generally applied on the wafer. That is, the passivation layer material is unable to prevent circuit layout on the wafer from being affected by etching.  
      Accordingly, a conventional wafer protection method in etching applies a corrosion-resistant wafer protection device to separate a selected area of the circuit plotting on the wafer from the etching fluid by providing a sealed cavity between the wafer protection device and the selected area. Since the conventional wafer protection device is corrosion-resistant, the etching fluid is prevented from contacting the area on the wafer.  
      In the above-mentioned conventional wafer protection device, the selected area on the wafer is optimally protected from the etching fluid at room temperature. However, etching is generally performed at a higher operating temperature, such as 85° C. In this case, gas fills the sealed cavity provided by the wafer protection device. Since the sealed cavity has a fixed volume, the pressure P of the gas in the sealed cavity is in proportion to the temperature T of the gas according to the ideal gas equation PV=nRT. That is, if the gas in the sealed cavity has a pressure of 1 atm at room temperature, the pressure of the gas at the operating temperature (85° C.) of etching becomes 1.2 atm. Since the strength and resilience of the wafer are reduced by etching, fractures due to the increased pressure may occur.  
      Further, in this case, even if fractures do not occur, the gas in the sealed cavity cools when etching ends, and the pressure of the gas is decreased to the ambient pressure, which again reduces the strength and resilience of the wafer. That is, the wafer sustains a pressure pulse in the etching process, which may induce fracture or creeping fatigue in the wafer.  
     SUMMARY OF THE INVENTION  
      In view of this, the present invention relates to a wafer protection device to be applied in the etching process of the micro-electromechanical system (MEMS). The wafer protection device of the present invention not only isolates a specific protected area on the wafer from the etching fluid, but modulates the pressure in the isolated cavity between the wafer protection device and the protected area of the wafer, so that the pressure in the isolated cavity is kept at a certain value regardless of the temperature, protecting the wafer from fracture or creeping fatigue due to the pressure change.  
      The present invention discloses a wafer protection device for a wafer. The wafer has a first surface etched with an etching fluid and a second surface, and the wafer protection device is applied to prevent a specific area on the second surface of the wafer from etching. The wafer protection device has a body and a pressure modulating device. The body covers the specific area of the wafer and provides an isolated cavity between the body and the specific area to prevent the specific area from contacting the etching fluid, and the pressure modulating device is provided on the body and connected to the isolated cavity to modulate pressure in the isolated cavity.  
      In the wafer protection device of the present invention, the body may have a base, an elastic device between the base and the wafer, and a fixing device provided on the base for applying force to the elastic device toward the wafer to deform the elastic device, providing a seal.  
      Further, the present invention also discloses a method of etching a wafer, which comprises the steps of: providing a wafer protection device for the wafer; positioning the wafer on the wafer protection device; disposing the wafer protection device into etching fluid, wherein the wafer protection device protects a specific area on the wafer from etching; and modulating the interior pressure of the wafer protection device. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The present invention is more fully understood by reading the subsequent detailed description in conjunction with the examples and references made to the accompanying drawings, wherein:  
       FIG. 1  is a schematic view showing the first embodiment of the wafer protection device of the present invention;  
       FIG. 2   a  is a schematic view showing the second embodiment of the wafer protection device of the present invention;  
       FIG. 2   b  is a schematic view showing the third embodiment of the wafer protection device of the present invention;  
       FIG. 2   c  is a schematic view showing the fourth embodiment of the wafer protection device of the present invention;  
       FIG. 3  is a schematic view showing the fifth embodiment of the wafer protection device of the present invention;  
       FIG. 4   a  is a schematic view showing the sixth embodiment of the wafer protection device of the present invention;  
       FIG. 4   b  is a schematic view showing an embodiment of the outlet valve in  FIG. 4   a;    
       FIG. 4   c  is a schematic view showing an embodiment of the inlet valve in  FIG. 4   a ; and  
       FIG. 5  is a schematic view showing the seventh embodiment of the wafer protection device of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     THE FIRST EMBODIMENT  
      An embodiment of the wafer protection device of the present invention is hereinafter described in detail with reference to  FIG. 1 .  
      The wafer protection device of the present invention is applied to a wafer  10 , in which the wafer  10  has a first surface  110  etched with an etching fluid  60  and a second surface  120 , so that the wafer protection device prevents a specific area on the second surface  120  of the wafer  10  from etching.  
      The wafer protection device of the present invention has a body and a pressure modulating device. The body covers the specific area on the second surface  120  of the wafer  10  and provides an isolated cavity  50  between the body and the specific area to prevent the specific area from contacting the etching fluid  60 . The specific area of the wafer  10  is generally provided with circuit layout that requires protection from etching. Further, the pressure modulating device is provided on the body and connected to the isolated cavity  50  to modulate pressure in the isolated cavity  50 .  
      In this embodiment, the body has a base  22 , an elastic device  24 , and a fixing device  26 , as shown in  FIG. 1 . The base  22  is a plate made of corrosion-resistant material. The elastic device  24  is a corrosion-resistant elastic ring, such as an O-ring, provided between the base  22  and the wafer  10 . The fixing device  26  has a plurality of springs, provided on the base  22  for applying force to the elastic device  24  toward the wafer  10 , so that the elastic device  24  is compressed and deformed, providing a seal between the etching fluid  60  and the specific area. The corrosion-resistant material of the base  22  and the elastic device  24  should be compatible with the etching fluid  60 . For example, the base  22  is made of Teflon when alkali fluids such as KOH or TMAH are applied as the etching fluid  60 .  
      Further, the pressure modulating device in this embodiment is a connecting pipe  30 , provided on the body and connecting the isolated cavity  50  to the external environment to modulate the pressure in the isolated cavity  50 .  
      It should be noted that the fixing device  26  of the wafer protection device of the present invention applies force to the elastic device  24  toward the wafer  10 , providing a seal. Further, the pressure modulating device is connected to the isolated cavity  50  to modulate the pressure in the isolated cavity  50 . That is, neither the fixing device  26  nor the pressure modulating device of the present invention are limited to the device as disclosed in this embodiment.  
     THE SECOND EMBODIMENT  
      A further embodiment of the wafer protection device of the present invention is hereinafter described in detail with reference to  FIG. 2   a , in which the fixing device  26  differs from that of the first embodiment. The other components of the wafer protection device in this embodiment are the same as the first embodiment and description thereof is thus omitted.  
      In this embodiment, a plurality of screw bolts  262  is applied as the fixing device  26 . When the screw bolts  262  are screwed onto the body  22 , a compressing force is applied on the elastic device (that is, the O-ring)  24 , providing a seal.  
      Further, in this embodiment, the pressure modulating device is a connecting pipe  30 , similar to the pressure modulating device in the first embodiment; however, the connecting pipe  30  in this embodiment has a bent portion  302  therein so that the wafer protection device is arranged side by side in a stack to reduce the stack volume.  
     THE THIRD EMBODIMENT  
      A further embodiment of the wafer protection device of the present invention is hereinafter described in detail with reference to  FIG. 2   b , in which the fixing device  26  differs from that of the first embodiment. Description of other components in this embodiment is thus omitted.  
      In this embodiment, a plurality of U-shaped clamps  264  is applied as the fixing device  26 . When the U-shaped clamps  264  clamp the body  22 , a compressing force is applied on the elastic device (that is, the O-ring)  24 , providing a seal.  
      Further, in this embodiment, the pressure modulating device is a connecting pipe  30  with a bent portion  302  therein similar to the connecting pipe  30  in  FIG. 2   a.    
     THE FOURTH EMBODIMENT  
      A further embodiment of the wafer protection device of the present invention is hereinafter described in detail with reference to  FIG. 2   c , in which the fixing device  26  differs from that of the first embodiment. Description of the other components of the wafer protection device in this embodiment is omitted.  
      In this embodiment, a plurality of retaining rings  266  is applied as the fixing device  26 . When the retaining rings  266  retain the body  22  to the wafer  10 , a compressing force is applied on the elastic device (that is, the O-ring)  24 , providing a seal.  
      Further, in this embodiment, the pressure modulating device is a connecting pipe  30  with a bent portion  302  therein similar to the connecting pipe  30  in  FIG. 2   a.    
     THE FIFTH EMBODIMENT  
      A further embodiment of the wafer protection device of the present invention is hereinafter described in detail with reference to  FIG. 3 , in which the pressure modulating device in this embodiment differs from the connecting pipe  30  in the first to fourth embodiments. The other components of the wafer protection device in this embodiment are the same as the first embodiment as shown in  FIG. 1 , and description thereof is thus omitted.  
      In this embodiment, the pressure modulating device is a sealed pipe  32  with an elastic membrane  322  as a seal. The sealed pipe  32  is connected to the isolated cavity  50 , so that the isolated cavity  50  and the interior space of the sealed pipe  32  become airtight, and the elastic membrane  322  of the sealed pipe  32  is inflated or deflated to modulate the pressure in the isolated cavity  50 .  
     THE SIXTH EMBODIMENT  
      A further embodiment of the wafer protection device of the present invention is hereinafter described in detail with reference to  FIG. 4   a , in which the pressure modulating device in this embodiment differs from the connecting pipe  30  in the first to fourth embodiments. The other components of the wafer protection device in this embodiment are the same as the first embodiment as shown in  FIG. 1 , and description thereof is thus omitted.  
      In this embodiment, the pressure modulating device has an outlet valve  34  and an inlet valve  36 . An example of the outlet valve  34  is shown in  FIG. 4   b , and an example of the inlet valve  36  is shown in  FIG. 4   c.    
      The outlet valve  34  in  FIG. 4   b  has a spring  342  and a one-way outward valve  344  that unidirectionally connects the isolated cavity  50  to the external environment. Further, the inlet valve  36  in  FIG. 4   c  has a spring  362  and a one-way inward valve  364  that unidirectionally connects the external environment to the isolated cavity  50 .  
      In the etching process, the pressure in the isolated cavity  50  increases when the temperature increases. When the pressure in the isolated cavity  50  is higher than the external environment pressure, the outlet valve  34  discharges a portion of gas in the isolated cavity  50  to reduce the pressure. On the other hand, when the etching process ends, the pressure in the isolated cavity  50  decreases when the temperature decreases. When the pressure in the isolated cavity is lower than the external environment pressure, the inlet valve  36  takes in air from the external environment to increase the pressure in the isolated cavity  50 . In this case, the pressure in the isolated cavity  50  is modulated by the outlet valve  34  and the inlet valve  36 .  
      It should be mentioned that the spring  342  of the outlet valve  34  and the spring  362  of the inlet valve  36  is adjusted to keep the pressure in the isolated cavity  50  in a range near that of the external environment. For example, the springs  342  and  362  can be adjusted so that the pressure in the isolated cavity  50  has a tolerance of  5 % in relation to the external environment pressure.  
     THE SEVENTH EMBODIMENT  
      A further embodiment of the wafer protection device of the present invention is hereinafter described in detail with reference to  FIG. 5 , in which the pressure modulating device in this embodiment differs from the connecting pipe  30  in the first to fourth embodiments. The other components of the wafer protection device in this embodiment are the same as the first embodiment as shown in  FIG. 1 , and description thereof is thus omitted.  
      In this embodiment, the pressure modulating device is a downwardly provided conduit  38  that connects the isolated cavity  50  to the etching fluid  60 . The opening of the conduit  38  is positioned below the surface of the etching fluid  60  with a depth of H, and a fluid storage portion  382  is provided in the conduit  38 .  
      In the etching process, the pressure in the isolated cavity  50  increases with the temperature. When the pressure in the isolated cavity  50  is higher than a predetermined pressure, gas in the isolated cavity is discharged to the etching fluid through the conduit  38  to reduce the pressure. On the other hand, when the etching process ends, the pressure in the isolated cavity  50  decreases with the temperature. When the pressure in the isolated cavity  50  is lower than the predetermined pressure, the etching fluid  60  is taken into the conduit  38  and stored in the fluid storage portion  382 , so that the gas in the isolated cavity  50  occupies a relatively smaller volume and is kept in the predetermined pressure. Thus, the conduit  38  controls the volume of the isolated cavity  50  by taking in the etching fluid  60  to modulate the pressure in the isolated cavity  50 .  
      It should be noted that the fluid storage portion  382  is provided so that the etching fluid  60  does not accidentally enter the isolated cavity  50  while being taken into the conduit  38 . The volume of the fluid storage portion  382  is determined in relation to the isolated cavity  50 .  
      It should be mentioned that the above-mentioned embodiments of the present invention is modified or preferably combined in application. For example, the wafer protection device may have a plurality of screw bolts as in the second embodiment as the fixing device  26  of the body while applying the outlet valve  34  and the inlet valve  36  from the sixth embodiment as the pressure modulating device at the same time.  
      The wafer protection device of the present invention is applied in a method of etching a wafer. When a wafer  10  is provided to be etched, a wafer protection device, such as one in any of the above-mentioned embodiments, is provided for the wafer  10 . The wafer  10  is then positioned on the wafer protection device, such that a specific area on the wafer  10 , such as the circuit layout on the second surface  120  of the wafer  10 , is protected by the wafer protection device. Then, the wafer protection device with the wafer  10  thereon is disposed into the etching fluid  60  for etching. At this time, the wafer protection device protects the specific area on the wafer  10  from etching. Further, since the wafer protection device has the pressure modulating device, such as the connecting pipe  30  in the above-mentioned first embodiment, the interior pressure of the wafer protection device is optimally modulated. Thus, the pressure in the isolated cavity is kept within a predetermined range regardless of the temperature, and the wafer  10  in etching is prevented from fracture or creeping fatigue due to the pressure change.  
      While the present invention has been described with reference to the preferred embodiments thereof, it is to be understood that the invention is not limited to the described embodiments or constructions. On the contrary, the invention is intended to cover various modifications and similar arrangements as is apparent to those skilled in the art. Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.