Systems and methods for evaluating semiconductor layers

Abstract

An apparatus for characterizing multilayer samples is disclosed. An intensity modulated pump beam is focused onto the sample surface to periodically excite the sample. A probe beam is focused onto the sample surface within the periodically excited area. The power of the reflected probe beam is measured by a photodetector. The output of the photodetector is filtered and processed to derive the modulated optical reflectivity of the sample. Measurements are taken at a plurality of pump beam modulation frequencies. In addition, measurements are taken as the lateral separation between the pump and probe beam spots on the sample surface is varied. The measurements at multiple modulation frequencies and at different lateral beam spot spacings are used to help characterize complex multilayer samples. In the preferred embodiment, a spectrometer is also included to provide additional data for characterizing the sample.

1 . An apparatus for evaluating the characteristics of a sample, comprising: 
 means for adjusting a lateral displacement of a first spot and a second spot on a surface of the sample, the first spot being formed by an intensity modulated pump beam for periodically exciting the sample and the second spot being formed by a probe laser beam;    means for varying the modulation frequency of the pump laser beam at each lateral displacement of the first spot and second spot;    means for measuring a power of a portion of the probe beam reflected from the second spot; and    means for determining a measure of modulated optical reflectivity of the sample at each lateral separation and each modulation frequency, and evaluating the characteristics of the sample using each measure of modulated optical reflectivity.    
   
   
       2 . An apparatus as recited in  claim 1 , wherein: 
 the lateral displacement is adjusted such that sample measurements are taken as the lateral displacement between the pump and probe beam spots is continuously changed.    
   
   
       3 . An apparatus as recited in  claim 1 , wherein: 
 the lateral displacement is adjusted such that sample measurements are taken at discrete separation intervals.    
   
   
       4 . An apparatus as recited in  claim 1 , wherein: 
 the lateral displacement is adjusted such that the spacing between the pump and probe beam spots is varied over a range from an aligned overlapping position to a spacing of at least 10 microns.    
   
   
       5 . An apparatus as recited in  claim 1 , wherein: 
 a modulation frequency of the pump laser beam is operable to be varied from 100 KHz to 100 MHz.    
   
   
       6 . An apparatus as recited in  claim 1 , wherein: 
 the lateral displacement is adjusted by altering a direction of one of the probe beam and the pump beam.    
   
   
       7 . An apparatus according to  claim 1 , further comprising: 
 means for combining the pump and probe beams while adjusting the lateral displacement.    
   
   
       8 . An apparatus according to  claim 1 , wherein: 
 at least one lateral displacement is a lateral displacement of zero distance, such that the first and second spots at least partially overlap.    
   
   
       9 . An apparatus for evaluating the characteristics of a sample, comprising: 
 means for adjusting a lateral displacement of a first spot and a second spot on a surface of the sample, the first spot being formed by an intensity modulated pump beam for periodically exciting the sample and the second spot being formed by a probe laser beam;    means for measuring a modulated power of a portion of the probe beam reflected from the second spot, at each lateral separation, and generating a first output signal in response thereto;    means for measuring an intensity of the portion of the probe beam reflected from the second spot, at each lateral separation, and generating a second output signal as a function of the angle of incidence on the sample; and    means for evaluating the characteristics of the sample based on a combination of the first and second output signals.    
   
   
       10 . An apparatus as recited in  claim 9 , wherein: 
 the lateral displacement is adjusted by altering a direction of one of the probe beam and the pump beam.    
   
   
       11 . An apparatus according to  claim 9 , wherein: 
 at least one lateral displacement is a lateral displacement of zero distance, such that the first and second spots at least partially overlap.    
   
   
       12 . An apparatus for evaluating the characteristics of a sample, comprising: 
 means for adjusting a lateral displacement of a first spot and a second spot on a surface of the sample, the first spot being formed by an intensity modulated pump beam for periodically exciting the sample and the second spot being formed by a probe laser beam;    means for measuring a modulated power of a portion of the probe beam reflected from the second spot, at each lateral separation, and generating a first output signal in response thereto;    means for measuring a change in polarization state of the portion of the probe beam reflected from the second spot, at each lateral separation, and generating a second output signal that represents an integration rays within in the probe beam having multiple angles of incidence; and    means for evaluating the characteristics of the sample based on a combination of the first and second output signals.    
   
   
       13 . An apparatus according to  claim 12 , wherein: 
 each of the pump beam and the probe beam is linearly polarized.    
   
   
       14 . An apparatus as recited in  claim 12 , wherein: 
 the lateral displacement is adjusted by altering a direction of one of the probe beam and the pump beam.    
   
   
       15 . An apparatus according to  claim 12 , wherein: 
 at least one lateral displacement is a lateral displacement of zero distance, such that the first and second spots at least partially overlap.    
   
   
       16 . An apparatus for evaluating the characteristics of a sample, comprising: 
 means for adjusting a lateral displacement of a first spot and a second spot on a surface of the sample, the first spot being formed by an intensity modulated pump beam for periodically exciting the sample and the second spot being formed by a probe laser beam;    means for varying a spot size of at least one of the first and second spots at each lateral displacement of the first spot and second spot;    means for measuring a modulated power of a portion of the probe beam reflected from the second spot, at each lateral separation and each spot size; and    means for evaluating the characteristics of the sample based on the modulated power at each lateral separation and each spot size.    
   
   
       17 . An apparatus as recited in  claim 16 , wherein: 
 the lateral displacement is adjusted by altering a direction of one of the probe beam and the pump beam.    
   
   
       18 . An apparatus according to  claim 16 , further comprising: 
 means for combining the pump and probe beams while adjusting the lateral displacement.    
   
   
       19 . An apparatus according to  claim 16 , wherein: 
 at least one lateral displacement is a lateral displacement of zero distance, such that the first and second spots at least partially overlap.