Precise, in-situ endpoint detection for charged particle beam processing

Abstract

A system and method for determining precisely in-situ the endpoint of halogen-assisted charged particle beam milling of a hole or trench in the backside of the substrate of a flipchip packaged IC. The backside of the IC is mechanically thinned. Optionally, a coarse trench is then milled in the thinned backside of the IC using either laser chemical etching or halogen-assisted charged particle beam milling. A further small trench is milled using a halogen-assisted charged-particle beam (electron or ion beam). The endpoint for milling this small trench is determined precisely by monitoring the power supply leakage current of the IC induced by electron-hole pairs created by the milling process. A precise in-situ endpoint detection signal is generated by modulating the beam at a reference frequency and then amplifying that frequency component in the power supply leakage current with an amplifier, narrow-band amplifier or lock-in amplifier. The precise, in-situ, endpoint signal is processed and displayed for manual or automatic precise in-situ endpoint detection. This approach avoids or minimizes unintentional damage or perturbation of the active diffusion regions in the IC. A range of further operations on the IC can then be performed.

1 . A system for milling a trench with precise in-situ endpoint detection in a backside of a substrate of an IC, the system comprising: 
 a. a charged particle beam generating column subsystem including a secondary particle detector;    b. a holder for holding the IC in the path of a charged particle beam generated by the column subsystem, said holder and said charged particle beam being positioned in a vacuum chamber;    c. an XY stage for providing relative motion between said IC and said column subsystem;    d. a beam modulator for modulating said charged particle beam at a reference frequency;    e. a halogen-based gas injector disposed to deliver halogen-based gas to an area of the backside of the substrate where the trench is milled with the charged particle beam;    f. an amplifier disposed to measure a reference frequency component of a power supply leakage current of the IC, the amplifier having an output connected to an indicator for monitoring a precise in-situ endpoint signal.    
   
   
       2 . The system of  claim 1  wherein said holder is coupled to said XY stage, said XY stage thereby enabling motion of said IC relative to said column subsystem.  
   
   
       3 . The system of  claim 1  wherein said column subsystem is coupled to said XY stage, said XY stage thereby enabling motion of said column subsystem relative to said IC.  
   
   
       4 . The system of  claim 1  wherein the amplifier is a lock-in amplifier.  
   
   
       5 . The system of  claim 1  wherein said beam modulator is a beam blanker for pulsing the charged particle beam at a reference frequency.  
   
   
       6 . The system of  claim 1  wherein said beam modulator is a raster scanner disposed to scan said charged particle beam at a scanning frequency during milling, said reference frequency being derived from the scanning frequency.  
   
   
       7 . The system of  claim 4  wherein the lock-in amplifier is provided with a reference signal at the reference frequency.  
   
   
       8 . The system of  claim 1  further comprising a milling cut-off to stop milling at a predetermined milling endpoint when the precise in-situ endpoint signal reaches a predetermined state.  
   
   
       9 . The system of  claim 1  wherein the column is an ion-beam column.  
   
   
       10 . The system of  claim 1  wherein the column is an electron beam column.  
   
   
       11 . The system of  claim 1  where an output of the lock-in amplifier is rectified and smoothed for display.  
   
   
       12 . The system of  claim 1  wherein said indicator is a display.  
   
   
       13 . The system of  claim 1  wherein said indicator is an audible signal.  
   
   
       14 . The system of  claim 1  wherein said indicator is a voice synthesizer.