Apparatus and method for determining the active dopant profile in a semiconductor wafer

Abstract

A region of a semiconductor wafer is stimulated to create excess carriers in the region, and an interferometer is used to obtain a measured value of a signal that is affected by the stimulation. The apparatus and method use a predetermined profile as a measure of profile of active dopants in the region, if the measured value of the signal matches a simulated value obtained from simulation of conditions present during stimulation, with the predetermined profile of concentration of active dopants in the region under stimulation. The measured profile may be used in some embodiments to determine junction depth. Moreover, the junction depth that is so determined may be compared with specifications for acceptability of the wafer.

What is claimed is:  
     
         1 . A method for evaluating a region of a wafer having a plurality of carriers, the method comprising: 
 stimulating the region to create excess carriers in the region;    using an interferometer to obtain a measured value of a signal that is affected by said stimulating; and    using a predetermined profile as a measure of profile of active dopants in the region if the measured value of the signal matches a simulated value obtained from simulation of conditions present during said stimulating with said predetermined profile of concentration of active dopants in the region under stimulation.    
     
     
         2 . The method of  claim 1  further comprising: 
 using the measured profile to determine junction depth; and  
 comparing the junction depth with specifications for acceptability of the wafer.  
 
     
     
         3 . The method of  claim 1  wherein: 
 during stimulating, excess carriers are formed in the region in a concentration that changes in a periodic manner only with respect to time.  
 
     
     
         4 . The method of  claim 1  wherein: 
 the interferometer measures an interference signal obtained by interference between a reference beam and a portion of a probe beam that is reflected by excess carriers.  
 
     
     
         5 . The method of  claim 4  wherein: 
 the reference beam is formed by another portion of the probe beam that is reflected by a front surface of the wafer.  
 
     
     
         6 . The method of  claim 4  wherein: 
 the reference beam has a wavelength different from the probe beam.  
 
     
     
         7 . The method of  claim 4  wherein: 
 the reference beam is modulated at a frequency that is predetermined to be sufficiently low to ensure that phase of variation of concentration of excess carriers is substantially same as phase of the reference beam over a diffusion length; and  
 diffusion length is a length over which excess carrier concentration decays to 1/e.  
 
     
     
         8 . The method of  claim 4  wherein the modulation is at a predetermined frequency in conformance with the formula:  
         f <( ½πτ)    
       where f is the frequency, and τ is the lifetime of a carrier in the wafer.  
     
     
         9 . The method of  claim 4  wherein: 
 the reference beam includes a plurality of photons that have energy greater than the bandgap energy of the wafer.  
 
     
     
         10 . The method of  claim 4  wherein: 
 the reference beam includes a plurality of photons that have energy greater than the bandgap energy of crystalline silicon but less than the bandgap energy of amorphous silicon.  
 
     
     
         11 . The method of  claim 4  wherein: 
 the probe beam includes a plurality of photons that have energy lower than the bandgap energy of the wafer.  
 
     
     
         12 . The method of  claim 4  wherein: 
 the probe beam includes a plurality of photons that have energy approximately equal to the bandgap energy of the wafer.  
 
     
     
         13 . The method of  claim 4  wherein: 
 the probe beam is coherent;  
 the reference beam is coherent with respect to the probe beam; and  
 the reference beam has a phase that is variable independent of the phase of the probe beam.  
 
     
     
         14 . The method of  claim 4  further comprising: 
 changing the phase of the reference beam; and  
 repeating the using of interferometer.  
 
     
     
         15 . The method of  claim 1  wherein the region is henceforth referred to as first region, the method further comprising: 
 repeating the stimulating and the using of interferometer in a second region; and  
 computing a difference between a second measured value in the second region obtained during the repeating and a first measured value in the first region obtained during the stimulating and the using of interferometer.  
 
     
     
         16 . The method of  claim 15  further comprising: 
 comparing the difference with a predetermined limit; and  
 changing a process parameter used in fabricating the wafer if the difference exceeds a predetermined limit.  
 
     
     
         17 . The method of  claim 1  further comprising: 
 annealing the wafer to activate dopants;  
 wherein the stimulating and the using of interferometer are performed after the annealing.  
 
     
     
         18 . The method of  claim 1  wherein: 
 the measured value of the signal matches the simulated value if a difference therebetween is less than a predetermined amount.  
 
     
     
         19 . An apparatus for evaluating a wafer, said apparatus comprising: 
 means for creating, in a region of the wafer, a plurality of excess carriers that move out of the region by non-wave diffusion;    a source of a probe beam of electromagnetic radiation;    an interferometer located in a path of a signal that is affected by said number of excess carriers in the region; and    a computer coupled to the interferometer, the computer being programmed to identify one of a plurality of profiles of active dopants in the region by use of a value of the signal as measured by the interferometer.    
     
     
         20 . The apparatus of  claim 19  wherein the interferometer includes a lock-in amplifier that detects amplitude and phase of a signal obtained by interference between: 
 the portion of a probe beam reflected by carriers in the region; and  
 another portion of the probe beam reflected by a front surface of the wafer.