Apparatus and process for bulk wet etch with leakage protection

Abstract

When using hot alkaline etchants such as KOH, the wafer front side, where various devices and/or circuits are located, must be isolated from any contact with the etchant. This has been achieved by using two chambers that are separated from each other by the wafer that is to be etched. Etching solution in one chamber is in contact with the wafer's back surface while deionized water in the other chamber contacts the front surface. The relative liquid pressures in the chambers is arranged to be slightly higher in the chamber of the front surface so that leakage of etchant through a pin hole from back surface to front surface does not occur. As a further precaution, a monitor to detect the etchant is located in the DI water so that, if need be, etching can be terminated before irreparable damage is done.

1 . An apparatus for wet etching a wafer having front and back surfaces, comprising: 
 first and second chambers separated one from another by said wafer whereby said back surface is part of said first chamber and said front surface is part of said second chamber;    in said first chamber, contacting said back surface at a first pressure, a first liquid that etches said wafer; and    in said second chamber, a second liquid located at a height, that does not etch said wafer, contacting said front surface at a second pressure that equals said first pressure, whereby leakage of said first liquid to said front surface, through a pin hole in said wafer, cannot occur.    
   
   
       2 . The apparatus described in  claim 1  wherein said first pressure can be balanced by adjusting said height of the second liquid.  
   
   
       3 . The apparatus described in  claim 1  wherein said wafer has a diameter between about 10 and 20 cm.  
   
   
       4 . The apparatus described in  claim 1  wherein said wafer has a thickness between about 0.5 and 0.725 mm.  
   
   
       5 . An apparatus for wet etching a silicon wafer having front and back surfaces, comprising: 
 first and second chambers that are separated from each other by said silicon wafer whereby said back surface is part of said first chamber and said front surface is part of said second chamber;    seals to prevent liquid from flowing between said chambers;    a first tank containing a solution of potassium hydroxide maintained at a first temperature;    said first tank being connected to said first chamber through a first pumping path and directly connected to said first chamber through an etchant return path;    a second tank containing deionized water maintained at a second temperature;    said second tank being connected to said second chamber through a second pumping path;    said second chamber being directly connected to a water reflow tank which is connected and to said second tank through a water return path;    means for introducing a balance in pressure between liquid contacting said back surface and liquid contacting said front surface; and    in said second chamber, a detector of potassium hydroxide.    
   
   
       6 . The apparatus described in  claim 5  wherein said means for introducing a difference in pressure between liquid contacting said back surface and liquid contacting said front surface further comprises means to control water height in said water reflow tank relative to water height in said second chamber.  
   
   
       7 . The apparatus described in  claim 5  wherein said detector of potassium hydroxide is a pH meter.  
   
   
       8 . The apparatus described in  claim 5  wherein said first pumping path terminates inside said first chamber at a shower head that emits potassium hydroxide solution in a direction that is parallel to said back surface.  
   
   
       9 . The apparatus described in  claim 8  wherein said shower head rotates while it emits potassium hydroxide solution into said first chamber, thereby inducing turbulence.  
   
   
       10 . The apparatus described in  claim 5  wherein said second pumping path terminates inside said second chamber at a shower head that emits deionized water in a direction that is parallel to said front surface.  
   
   
       11 . The apparatus described in  claim 10  wherein said shower head rotates while it emits deionized water into said second chamber, thereby inducing turbulence.  
   
   
       12 . The apparatus described in  claim 5  further comprising a section having high flow impedance in said water reflow path to reduce deionized water flow rate.  
   
   
       13 . The apparatus described in  claim 5  wherein said first temperature is between about 60 and 90° C.  
   
   
       14 . The apparatus described in  claim 5  wherein said second temperature is between about 60 and 90° C.  
   
   
       15 . The apparatus described in  claim 5  wherein deionized water flows into and out of said first chamber at a flow rate of between about 10 and 100 sccm.  
   
   
       16 - 26 . (canceled)