Method and apparatus for determining thickness of a semiconductor substrate at the floor of a trench

Abstract

Apparatus and method for exposing a selected feature of an integrated circuit device such as a selected portion of the metallization layer, from the backside of the integrated circuit substrate without disturbing adjacent features of the device such as the active semiconductor regions. This is performed using a FIB (focused ion beam) etching process in conjunction with observation by an optical microscope to form a trench through the substrate. The process includes a precise optical endpointing technique to monitor the remaining thickness of the semiconductor substrate at the floor of the trench. It is important to terminate etching of the trench so that the trench floor extends as close to the active semiconductor structures as desired and yet is not detrimental to device operation. This is done without introducing a need for any additional tool. This is carried out using an infra-red optical technique which observes the interference fringes generated by the reflections from the silicon substrate surface and from semiconductor device circuitry layers to quantify the remaining semiconductor substrate thickness in the trench.

1 . Apparatus comprising: 
 a stage for holding a Device Under Test (DUT), said DUT comprising a substrate;    a source of a charged particle beam arranged to direct said beam onto said substrate, for etching a cavity in a first surface of said substrate, said cavity having a floor;    a source of a light beam arranged to direct said light beam into said cavity in the substrate so as to be incident on said floor of the cavity;    an optical system arranged to image a portion of said cavity in the substrate, said optical system including an optical objective lens and further including;    a sensor arranged to detect optical interference fringes resulting from said light beam incident on said floor of the cavity, said fringes having low intensity regions and high intensity regions; and    a display coupled to said sensor configured to display an image of said interference fringes, thereby providing indication of a remaining thickness of said substrate between said floor of the cavity and a second surface of said substrate opposite said first surface of said substrate.    
   
   
       2 . The apparatus of  claim 1 , wherein the light beam is infra-red light.  
   
   
       3 . The apparatus of  claim 1 , wherein the charged particle beam is a focused ion beam.  
   
   
       4 . The apparatus of  claim 1 , wherein said first surface of said substrate is a backside surface of an integrated circuit wafer portion.  
   
   
       5 . The apparatus of  claim 1 , further comprising a focusing adjust arranged to adjust a distance between said optical objective lens and said floor of the cavity.  
   
   
       6 . The apparatus of  claim 1 , further including a computer coupled to the sensor, said computer configured to: 
 determine the intensities of said low-intensity regions and said high-intensity regions of said interference fringes;    calculate the relative contrast of said interference fringes; and    determine the remaining thickness of said substrate between said floor of the cavity and said second surface of said substrate opposite said first surface of said substrate from said relative contrast of said interference fringes.    
   
   
       7 . The apparatus of  claim 6 , wherein said computer is further configured to determine an endpoint for said etching of said cavity when a predetermined remaining thickness of said substrate between said floor of the cavity and said second surface of said substrate opposite said first surface of said substrate is reached.