Apparatus and method for image optimization of samples in a scanning electron microscope

Abstract

A system and method for identifying an optimal landing energy of a probe current in a scanning electron microscope system. A probe current having a known landing energy is directed at a sample for producing a signal electron beam. The current of the signal electron beam is measured by directing the beam to a current detector for calculating a current yield, which is the ratio of the signal current to the probe current. The landing energy can then be changed for subsequent measurements of the signal current to identify the landing energy which produces a desired current yield. Once identified, the landing energy value can be used to produce a signal electron beam directed towards an imaging detector to generate topographic images of samples.

What is claimed is:  
     
         1 . A method of identifying an optimal landing energy of a probe current in a scanning electronic microscope for use in obtaining an optimal topographic image of a sample, comprising the steps of: 
 (a) directing the probe current to the sample at a selected landing energy to produce a signal electron beam from the sample;    (b) measuring the probe current;    (c) providing a signal electron beam current detector;    (d) receiving the signal electron beam at the signal electron beam current detector to obtain a signal electron beam current measurement;    (e) calculating a ratio of the signal electron beam current measurement to the measured beam energy of the probe current;    (f) repeating steps (a) through (e) at a landing energy other than the selected landing energy; and    (g) comparing the calculated ratios at each landing energy to identify the optimal landing energy.    
     
     
         2 . The method of  claim 1 , wherein said providing step comprises applying to the sample a voltage having a value proximate a value of the voltage of the probe current, and using the signal electron beam to locate a desired position for the signal electron beam current detector.  
     
     
         3 . The method of  claim 1 , wherein said measuring step (b) comprises disposing a probe current detector in a path of the probe current.  
     
     
         4 . The method of  claim 1 , wherein said receiving step comprises directing the signal electron beam to said signal electron beam current detector.  
     
     
         5 . The method of  claim 4 , wherein said step of directing the signal electron beam further comprises employing a Wein filter disposed in the path of the signal electron beam.  
     
     
         6 . The method of  claim 1 , wherein said step (g) further comprises identifying the optimal landing energy as a landing energy corresponding to a ratio having a value proximate “1”.  
     
     
         7 . The method of  claim 1 , further comprising a step (h) of selecting the identified optimal landing energy, directing the probe current to the sample at the optimal landing energy, and directing a resulting signal electron beam to an imaging detector for obtaining a typographic image of the sample.  
     
     
         8 . The method of  claim 7 , wherein said steps (d) and (h) are performed by employing a Wien filter disposed in the path of the signal electron beam for selectively directing the signal electron beam to one of said imaging detector and said signal electron beam current detector.  
     
     
         9 . The method of  claim 7 , further comprising the step of selectively positioning said signal electron beam current detector within the path of the signal electron beam.  
     
     
         10 . The method of  claim 9 , wherein said step (c) of providing a signal electron beam detector comprises the step of positioning the signal electron beam detector coplanar with, and at a 180° angle from, said imaging detector.  
     
     
         11 . The method of  claim 3 , wherein said measuring step (b) further comprises using a Faraday cup as the probe current detector.  
     
     
         12 . The method of  claim 1 , wherein said providing step (c) further comprises providing a Faraday cup as the signal electron current detector.  
     
     
         13 . The method of  claim 9 , wherein said selectively positioning step further comprises applying a voltage to one of said signal electron beam and said imaging detector.  
     
     
         14 . A scanning electron microscope system for identifying an optimal landing energy of a probe current for obtaining topographic images of samples positioned on a microscope sample plane, comprising: 
 an electron source for generating the probe current along a probe current path in a direction toward the sample plane, for producing a signal electron beam when said probe current irradiates a sample positioned on the sample plane;    a probe current detector positioned for receiving at least a portion of the probe current for measuring the probe current;    an imaging detector positioned for receiving at least a portion of the signal electron beam;    a controller for adjusting a landing energy of the probe current;    a current detector positioned for receiving at least a portion of the signal electron beam; and    means for selectively directing said at least a portion of said signal electron beam to one of said imaging detector and current detector.    
     
     
         15 . The system of  claim 14 , wherein said selectively directing means comprises a Wien filter position in a path of the signal electron beam.  
     
     
         16 . The system of  claim 14 , wherein said selectively directing means comprises an actuator for moving one of said current detector and imaging detector to receive said portion of said signal electron beam.  
     
     
         17 . The system of  claim 14 , wherein said probe current detector comprises a Faraday cup.  
     
     
         18 . The system of  claim 14 , wherein said current detector comprises a Faraday cup.  
     
     
         19 . The system of  claim 14 , further comprising a scanner positioned in the probe current path for generating a raster pattern from the probe current.  
     
     
         20  A system for identifying an optimal landing energy of a probe current in a scanning electronic microscope for use in obtaining an optimal topographic image of a sample, comprising: 
 means for directing the probe current to the sample at a selected landing energy to produce a signal electron beam from the sample;  
 a probe current detector for measuring the probe current;  
 a signal electron beam current detector for obtaining a measurement of the signal electron beam;  
 means for calculating a ratio of the signal electron beam current measurement to the measured beam energy of the probe current;  
 means for obtaining a plurality of signal currents at different landing energies; and  
 means for comparing the calculated ratios at each landing energy to identify the optimal landing energy.