Evaluating sidewall coverage in a semiconductor wafer

Abstract

A sidewall or other feature in a semiconductor wafer is evaluated by illuminating the wafer with at least one beam of electromagnetic radiation, and measuring intensity of a portion of the beam reflected by the wafer. Change in reflectance between measurements provides a measure of a property of the feature. The change may be either a decrease in reflectance or an increase in reflectance, depending on the embodiment. A single beam may be used if it is polarized in a direction substantially perpendicular to a longitudinal direction of the sidewall. A portion of the energy of the beam is absorbed by the sidewall, thereby to cause a decrease in reflectance when compared to reflectance by a flat region. Alternatively, two beams may be used, of which a first beam applies heat to the feature itself or to a region adjacent to the feature, and a second beam is used to measure an increase in reflectance caused by an elevation in temperature due to heat transfer through the feature. The elevation in temperature that is measured can be either of the feature itself, or of a region adjacent to the feature.

1 . A method of evaluating a feature in a semiconductor wafer, the method comprising: 
 illuminating the wafer with a beam of electromagnetic radiation having a majority of energy polarized in a direction other than parallel to a longitudinal direction of the feature; and    measuring intensity of a portion of the beam reflected by the wafer.    
   
   
       2 . The method of  claim 1  wherein: 
 the feature includes a sidewall of a groove; and    the act of measuring is performed repeatedly at a plurality of locations transverse to the longitudinal direction of the groove.    
   
   
       3 . The method of  claim 2  wherein: 
 the beam has a wavelength greater than thickness of the sidewall.    
   
   
       4 . The method of  claim 1  wherein: 
 the beam has a wavelength greater than a dimension of the feature; and    the beam forms on the wafer a spot of a diameter greater than the dimension.    
   
   
       5 . The method of  claim 1  wherein: 
 the feature includes a trace of reflective material.    
   
   
       6 . The method of  claim 1  wherein: 
 the wafer includes a layer located between a source of the beam and the feature; and    the layer is at least partially transmissive, so that the portion passes through the layer.    
   
   
       7 . The method of  claim 1  wherein: 
 the beam has a majority of energy polarized in a direction at least substantially perpendicular to the longitudinal direction.    
   
   
       8 . The method of  claim 1  wherein: 
 the beam has a predetermined wavelength; and    the method further comprises filtering light of a wavelength other than the predetermined wavelength.    
   
   
       9 . The method of  claim 1  wherein the wafer has a plurality of features including the feature, and the method further comprises: 
 performing the act of measuring for each feature of the plurality; and    comparing measurements of multiple features.    
   
   
       10 . The method of  claim 9  wherein: 
 each feature is a sidewall; and    the act of comparing includes comparing measurements of two sidewalls located opposite to one another in a groove.    
   
   
       11 . The method of  claim 1  wherein the beam is a first beam, and the method further comprises: 
 illuminating the wafer with a second beam of electromagnetic radiation.    
   
   
       12 . The method of  claim 11  wherein: 
 the first beam forms a first spot on the wafer, the second beam forms a second spot;    the act of measuring includes measuring with the first spot and the second spots located on opposite sides of the feature; and    the method further comprises measuring with the first spot and the second spots located on the same side of the feature.    
   
   
       13 . The method of  claim 11  wherein: 
 the second spot at least partially overlaps the first spot.    
   
   
       14 . The method of  claim 13  wherein: 
 the first beam has a first wavelength different from a second wavelength of the second beam;    the second beam is modulated at a predetermined frequency; and    the act of measuring includes measuring intensity of the second beam having the second wavelength and modulated at the predetermined frequency.    
   
   
       15 . The method of  claim 13  wherein: 
 the first beam is polarized substantially perpendicular to the longitudinal direction.    
   
   
       16 . A method of evaluating wafers during fabrication, the method comprising: 
 forming a feature of conductive material in a wafer by using at least one process parameter;    illuminating the wafer with a beam of electromagnetic radiation having a majority of energy polarized in a direction other than parallel to a longitudinal direction of the feature; and    repeatedly measuring intensity of a portion of the beam reflected by the wafer at a plurality of locations transverse to the longitudinal direction; and    changing the process parameter depending on measurements obtained from the act of repeatedly measuring.    
   
   
       17 . The method of  claim 16  further comprising: 
 determining a coefficient of a function that fits the measurements;    comparing the coefficient against a predetermined limit and performing the changing based on an outcome of the comparing.    
   
   
       18 . A method of evaluating a feature in a semiconductor wafer, the method comprising: 
 illuminating the wafer with two beams, each beam forming a spot; and    performing a measurement with the first spot and the second spots located on opposite sides of the feature.    
   
   
       19 . The method of  claim 18  wherein: 
 each of the two beams includes unpolarized light.    
   
   
       20 . The method of  claim 18  wherein: 
 each of the two beams includes light polarized in a direction other than parallel to a longitudinal direction of the feature.    
   
   
       21 . The method of  claim 18  wherein: 
 the method further includes modulating intensity of a first beam at a predetermined frequency, said predetermined frequency being sufficiently small to avoid creation of a wave; and    the act of performing includes measuring intensity of a portion of the second beam reflected by the wafer, the portion being modulated at the predetermined frequency.    
   
   
       22 . The method of  claim 18  further comprising: 
 performing a second measurement with the first spot and the second spots located on the same side of the feature.    
   
   
       23 . A method of evaluating a feature in a semiconductor wafer, the method comprising: 
 illuminating the wafer with two beams, a first beam having a first intensity larger than a second intensity of a second beam, the first beam having a majority of energy polarized in a direction other than parallel to a longitudinal direction of the feature; and    measuring intensity of a portion of the second beam reflected by the wafer    
   
   
       24 . The method of  claim 22  wherein: 
 the method further includes modulating intensity of the first beam at a predetermined frequency, said predetermined frequency being sufficiently small to avoid creation of a wave; and    the act of measuring includes measuring intensity of a portion of the second beam reflected by the wafer, the portion being modulated at the predetermined frequency.    
   
   
       25 . The method of  claim 22  wherein: 
 each of the two beams is a laser beam.    
   
   
       26 . The method of  claim 22  wherein: 
 the first beam is an electron beam; and    the second beam is a laser beam.    
   
   
       27 . The method of  claim 22  wherein: 
 the first beam forms a first spot; and    the second beam forms a second spot that at least partially overlaps the first spot.    
   
   
       28 . The method of  claim 22  further comprising: 
 forming the feature by using at least one process parameter; and    changing the process parameter depending on measurements obtained from the act of measuring.    
   
   
       29 . An apparatus for evaluating a feature in a wafer, the apparatus comprising: 
 a laser source for generating a beam polarized in a direction other than parallel to a longitudinal direction of the feature; and    a photosensitive element located in a path of radiation of electromagnetic energy from the wafer.    
   
   
       30 . The apparatus of  claim 29  further comprising: 
 a circuit coupled to the laser to move the beam along a line across the feature; and    a monitor for displaying a graph of a signal generated by the photosensitive element as a function of distance along the line.    
   
   
       31 . The apparatus of  claim 30  wherein: 
 the line is at least substantially perpendicular to the longitudinal direction of the feature.    
   
   
       32 . The apparatus of  claim 29  further comprising: 
 an oscillator capable of oscillating at a frequency lower than 25000 Hz, the oscillator being coupled to the laser source; and    a lock-in amplifier coupled to said oscillator and to said photosensitive element.    
   
   
       33 . The apparatus of  claim 32  wherein during operation: 
 said oscillator causes said laser source to generate said beam at an intensity modulated at said frequency; and    said lock-in amplifier generates a signal indicative of reflectivity of said wafer.    
   
   
       34 . The apparatus of  claim 29  further comprising: 
 a computer coupled to the photosensitive element and programmed to determine a dimension of the feature.    
   
   
       35 . The apparatus of  claim 34  further comprising: 
 a memory having encoded therein values generated from at least one test wafer having a feature of a known property;    wherein the computer is programmed to use a signal generated by the photosensitive element to look up a value of property for the wafer, based on the values in memory.    
   
   
       36 . An apparatus comprising: 
 means for illuminating a semiconductor wafer with a beam of electromagnetic radiation having a majority of energy polarized in a direction other than parallel to a longitudinal direction of the feature; and    means for measuring intensity of a portion of the beam reflected by the wafer, the means for measuring being coupled to the means for illuminating.    
   
   
       37 . The apparatus of  claim 36  further comprising: 
 means for displaying measurements generated by the means for measuring, as a function of distance.