Apparatus and method for evaluating a wafer of semiconductor material

Abstract

An apparatus and method uses diffusive modulation (without generating a wave of carriers) for measuring a material property (such as any one or more of: mobility, doping, and lifetime) that is used in evaluating a semiconductor wafer. The measurements are carried out in a small area, for use on wafers having patterns for integrated circuit dice. The measurements are based on measurement of reflectance, for example as a function of carrier concentration. In one implementation, the semiconductor wafer is illuminated with two beams, one with photon energy above the bandgap energy of the semiconductor, and another with photon energy near or below the bandgap. The diameters of the two beams relative to one another are varied to extract additional information about the semiconductor material, for use in measuring, e.g. lifetime.

What is claimed is:  
     
         1 . An apparatus for evaluating a wafer, said apparatus comprising: 
 a first source of a first beam of photons having a first intensity modulated at a frequency sufficiently low to avoid creation of a wave of charge carriers in a region of said wafer when said first beam is incident on said region;    a second source of a second beam of photons, said photons in said second beam having energy sufficiently lower than said energy of said photons in said first beam to avoid creation of more than a negligible number of charge carriers in said region when said second beam is incident on said region; and    a photosensitive element located in a path of a portion of said second beam, said portion being modulated at said frequency after reflection by said region, said photosensitive element generating a first signal indicative of a first concentration of said charge carriers created in said region by incidence of said first beam.    
     
     
         2 . The apparatus of  claim 1  further comprising: 
 a computer coupled to said photosensitive element and programmed to determine a value of a material property in said region by use of said first signal and a second signal generated by said photosensitive element in response to a change in a parameter related to generation of at least one of said first beam and said second beam.  
 
     
     
         3 . The apparatus of  claim 2  wherein: 
 said parameter is intensity of said first beam; and  
 said computer is programmed to: 
 compute a ratio of (a) difference between said first signal and said second signal and (b) difference between said first intensity and said second intensity; and  
 compare said ratio with a corresponding ratio of a predetermined wafer having a known mobility to determine mobility in said region.  
 
 
     
     
         4 . The apparatus of  claim 2  wherein: 
 said computer determines a value of an attribute, the attribute being a change in said second signal for a unit change in said first signal; and  
 said computer uses the formula  
           μ   unk     =         m   ref       m   unk            μ   ref                       
 to compute mobility in said region, wherein M unk  is said value of said attribute, m ref  is another value of said attribute for a reference wafer, and μ ref  is the mobility of said reference wafer.  
 
     
     
         5 . The apparatus of  claim 2  wherein: 
 said second signal is generated at a distance from said region, said parameter being said distance; and  
 said computer uses a predetermined range of lifetimes of wafers that are acceptable, and a corresponding range of intensity measurements at said distance of wafers having known lifetimes to determine whether said wafer has an acceptable lifetime.  
 
     
     
         6 . The apparatus of  claim 2  wherein: 
 said parameter is the diameter of one of said beams; and  
 said computer uses a predetermined range of lifetimes of wafers that are acceptable, and a corresponding range of intensity measurements for said diameter of said probe beam to determine whether said wafer has an acceptable lifetime.  
 
     
     
         7 . The apparatus of  claim 2  wherein said computer is programmed to: 
 compute a plurality of coefficients of at least a group of said signals when plotted against corresponding values of said parameter; and  
 compare at least one coefficient in said plurality with a corresponding coefficient of a predetermined wafer having a known value of a material property to determine a value of said material property in said region.  
 
     
     
         8 . The apparatus of  claim 7  wherein: 
 said parameter is intensity of said first beam; and  
 said coefficient is a first order coefficient and the material property is junction depth.  
 
     
     
         9 . The apparatus of  claim 7  wherein: 
 said parameter is intensity of said first beam; and  
 the coefficient is a first order coefficient and the material property is mobility.  
 
     
     
         10 . The apparatus of  claim 7  wherein: 
 said parameter is intensity of said first beam; and  
 the coefficient is a zeroth order coefficient and the material property is surface concentration.  
 
     
     
         11 . The apparatus of  claim 7  wherein: 
 said parameter is intensity of said first beam; and  
 the coefficient is a first order coefficient and the material property is sheet resistance.  
 
     
     
         12 . The apparatus of  claim 2  wherein said computer is programmed to: 
 compute a plurality of coefficients of a plot of said signals against corresponding values of said parameter, said parameter being intensity of said first beam; and  
 compare at least one coefficient in said plurality with a corresponding coefficient of a predetermined wafer having been subjected to a process at a known value of a process condition to determine a value of said process condition for said wafer.  
 
     
     
         13 . The apparatus of  claim 2  wherein said computer is programmed to: 
 determine an intersection point of a first line that approximates a high power portion formed by a first group of said signals at a first extremity in a range of values of said signals and a second line that approximates a low power portion formed by a second group of said signals at a second extremity in said range; and  
 compare a coordinate of said intersection point with a corresponding coordinate of an intersection point of a predetermined wafer having a known concentration of active dopants to determine the concentration of active dopants in said region.  
 
     
     
         14 . The apparatus of  claim 2  wherein: 
 said parameter is a diameter of one of said beams; and  
 the material property is lifetime.  
 
     
     
         15 . The apparatus of  claim 2  wherein: 
 said parameter is a distance between said first beam and said second beam; and  
 the material property is lifetime.  
 
     
     
         16 . The apparatus of  claim 1  further comprising: 
 a wafer processing unit; and  
 a computer coupled to said wafer processing unit and to said photosensitive element, said computer being programmed to control operation of said wafer processing unit based on at least said first signal.  
 
     
     
         17 . The apparatus of  claim 16  further comprising a rapid thermal annealer, wherein: 
 said wafer processing unit includes an ion implanter; and  
 said computer is coupled to said rapid thermal annealer, and is programmed to control operation of at least one of said ion implanter and said rapid thermal annealer based on at least said first signal.  
 
     
     
         18 . The apparatus of  claim 1  further comprising: 
 a rapid thermal annealer; and  
 a computer coupled to said rapid thermal annealer and programmed to control operation of said rapid thermal annealer based on at least said first signal.  
 
     
     
         19 . The apparatus of  claim 1  further comprising: 
 a computer programmed to display on a monitor a message indicating acceptance or rejection of a wafer under measurement, the computer being couled to said photosensitive element.  
 
     
     
         20 . The apparatus of  claim 1  further comprising: 
 means for processing said wafer; and  
 a computer coupled to said means for processing and to said photosensitive element, said computer being programmed to control said means for processing in response to at least said first signal generated by said photosensitive element.  
 
     
     
         21 . The apparatus of  claim 1  further comprising: 
 a stage capable of moving said wafer with respect to said first source and said second source; wherein:  
 said photosensitive element generates a plurality of signals related to a corresponding plurality of regions on said wafer when said wafer is moved by said stage.  
 
     
     
         22 . The apparatus of  claim 21  further comprising: 
 a computer coupled to said photosensitive element and programmed to compute a ratio of a local maximum in said plurality of signals to a local minimum in said plurality of signals, compare the ratio with a predetermined limit, and display a message on a monitor indicating acceptance or rejection of said wafer.  
 
     
     
         23 . The apparatus of  claim 21  further comprising: 
 a computer coupled to said photosensitive element and programmed to check if any of said plurality of signals falls outside a predetermined range, and display a message on a monitor indicating acceptance or rejection of said wafer.  
 
     
     
         24 . The apparatus of  claim 21  further comprising: 
 a computer coupled to said photosensitive element and programmed to determine a value of a material property in said region by use of said first signal and a second signal generated by said photosensitive element for said region after a change in intensity of said first beam.  
 
     
     
         25 . The apparatus of  claim 21  wherein: 
 said stage moves said wafer in two dimensions; and  
 said photosensitive element generates at least one signal in said plurality of signals at each region in said plurality of regions.  
 
     
     
         26 . The apparatus of  claim 21  further comprising: 
 a rapid thermal annealer; and  
 said computer is programmed to control operation of said rapid thermal annealer in response to one of said plurality of signals.  
 
     
     
         27 . The apparatus of  claim 1  wherein: 
 said first source generates first photons having energy greater than bandgap energy of a semiconductor material in said region; and  
 said second source generates second photons having energy lesser than said bandgap energy.  
 
     
     
         28 . The apparatus of  claim 27  wherein: 
 said first photons have a first wavelength smaller than 950 nm; and  
 said second photons have a second wavelength larger than 950 nm.  
 
     
     
         29 . The apparatus of  claim 1  wherein: 
 said first beam has a first diameter at a surface of said wafer;  
 said second beam has a second diameter at said surface; and  
 said first diameter is greater than or equal to said second diameter.  
 
     
     
         30 . An apparatus for generating an electrical signal indicative of a property of a region in a wafer, said wafer including a semiconductor material at said region, said apparatus comprising: 
 an oscillator oscillating at a frequency lower than 1000 KHz during operation;    a first source of a first beam, said first source being coupled to said oscillator to generate said first beam at an intensity modulated at said frequency, said first beam containing a plurality of first photons having energy greater than the bandgap energy of said semiconductor material thereby to create a plurality of charge carriers when incident on said region, the number of said plurality of charge carriers being modulated at said frequency without the creation of a wave;    a second source of a second beam, said second beam containing a plurality of second photons having energy lower than said bandgap energy;    a partially transmissive mirror located in the path of each of said first beam and said second beam, said partially transmissive mirror being positioned to reflect one of said first beam and said second beam along a path coincident with the path of the other of said first beam and said second beam thereby to create a combined beam;    a beam splitter positioned in said coincident path;    a sensor capable of sensing said second photons, said sensor being coupled to said beam splitter to receive a group of second photons reflected by said region;    a lock-in amplifier coupled to said oscillator and to said sensor, said lock-in amplifier having an output line;    wherein said lock-in amplifier generates on said output line a signal indicative of an average number of said second photons modulated at said frequency and reflected by said region.    
     
     
         31 . The apparatus of  claim 30  wherein: 
 said sensor includes germanium.  
 
     
     
         32 . The apparatus of  claim 30  wherein said first beam has a power that is adjustable by said first source, and the apparatus further comprises: 
 a computer coupled to said output line and programmed to determine a value of a material property in said region in response to a plurality of values of said second signal generated by adjusting said power of said first beam to a corresponding plurality of levels.  
 
     
     
         33 . The apparatus of  claim 30  wherein: 
 said frequency of modulation by said first source is smaller than 10 Khz.  
 
     
     
         34 . An apparatus for measuring a property of a wafer, said wafer including a semiconductor material, said apparatus comprising: 
 an oscillator capable oscillating at a frequency smaller than 1000 KHz;    a first source of a first beam, said first source being coupled to said oscillator to generate said first beam at an intensity modulated at said frequency, said first beam containing a plurality of first photons having energy greater than the bandgap energy of said semiconductor material thereby to create a plurality of charge carriers when incident on a region of the semiconductor material, the number of said plurality of charge carriers being modulated at said frequency;    a second source of a second beam, said second beam containing a plurality of second photons having energy lower than said bandgap energy, said second beam being polarized;    a partially transmissive mirror located in the path of each of said first beam and said second beam, said partially transmissive mirror being positioned to reflect one of said first beam and said second beam along a path coincident with the path of the other of said first beam and said second beam;    a polarizing beam splitter located in the path of reflection of said second beam from said wafer;    a first sensor coupled to said polarizing beam splitter to receive a first portion of electromagnetic radiation from said polarizing beam splitter; and    a second sensor coupled to said polarizing beam splitter to receive a second portion of electromagnetic radiation from said polarizing beam splitter;    wherein said first portion and said second portion are respectively the sum and difference components of interference of: 
 a portion of said second beam prior to said reflection by said wafer; and  
 another portion of said second beam subsequent to said reflection by said wafer.  
   
     
     
         35 . The apparatus of  claim 34  further comprising: 
 a lock-in amplifier coupled to said first sensor and to said second sensor;  
 wherein said lock-in amplifier generates a third signal indicative of the difference between a first signal from said first sensor and a second signal from said second sensor on receipt of said first signal and said second signal, said third signal being in phase with oscillations of said oscillator.  
 
     
     
         36 . The apparatus of  claim 34  wherein said power of said first beam generated by said first source is adjustable, and the apparatus further comprises: 
 a computer coupled to said lock-in amplifier and programmed to determine a value of a material property in said region in response to a plurality of values of said third signal generated by adjusting said power of said first beam to a corresponding plurality of levels.  
 
     
     
         37 . A method for evaluating a wafer, said method comprising: 
 creating a plurality of charge carriers in a region of said wafer, the number of said charge carriers being modulated at a frequency that is sufficiently low to avoid creation of a wave of said charge carriers;    focusing on said region a first beam of first photons having energy lower than bandgap energy of a semiconductor material in said region; and    measuring a first intensity of a portion of said first beam modulated at said frequency after reflection by said region.    
     
     
         38 . The method of  claim 37  further comprising: 
 changing a parameter used in said creating;  
 measuring a second intensity after said changing; and  
 using each of said first intensity and said second intensity to determine a value of a material property in said region.  
 
     
     
         39 . The method of  claim 37  wherein: 
 said parameter is a distance between said first beam and said region; and  
 said material property is lifetime.  
 
     
     
         40 . The method of  claim 37  wherein: 
 said parameter is diameter of one of said beams; and  
 said material property is lifetime.  
 
     
     
         41 . The method of  claim 37  wherein: 
 said creating includes focusing on said region a second beam of second photons having a second intensity modulated at said frequency; and  
 said parameter is an average of said second intensity over a cycle at said frequency.  
 
     
     
         42 . The method of  claim 41  further comprising: 
 determining an attribute of a function, said function at least approximately relating said first intensity and said average of said second intensity to the corresponding values of said parameter before and after said changing; and  
 interpolating said attribute with respect to a plurality of attributes of corresponding functions of semiconductor materials having known values of a material property thereby to determine the value of said material property in said region.  
 
     
     
         43 . The method of  claim 42  wherein: 
 said attribute is a coefficient of a straight line that approximates at least a portion of said function; and  
 said property is one of junction depth, surface concentration, sheet resistance and mobility.  
 
     
     
         44 . The method of  claim 43  wherein: 
 said coefficient is slope;  
 said portion approximates a group of said signals at a high end of a range of said plurality of signals; and  
 said material property is mobility.  
 
     
     
         45 . The method of  claim 43  wherein: 
 said coefficient is slope;  
 said portion approximates a group of said signals at a low end of a range of said plurality of signals; and  
 said material property is junction depth.  
 
     
     
         46 . The method of  claim 43  wherein: 
 said coefficient is intercept; and  
 said material property is surface concentration.  
 
     
     
         47 . The method of  claim 42  wherein: 
 said attribute is a coordinate of an intersection point of a first line that approximates a high power portion formed by a first group of said signals at a first extremity in a range of values of said signals and a second line that approximates a low power portion formed by a second group of said signals at a second extremity in said range; and  
 said property is doping concentration.  
 
     
     
         48 . The method of  claim 42  wherein said wafer is a patterned wafer and said method further comprising: 
 annealing said wafer prior to said measuring; and  
 adjusting annealing of another patterned wafer depending on at least said second intensity.  
 
     
     
         49 . The method of  claim 42  further comprising: 
 repeating said creating and said focusing in a plurality of regions of said wafer; and  
 measuring a plurality of intensities corresponding to said plurality of regions.  
 
     
     
         50 . The method of  claim 49  further comprising: 
 comparing each of said intensities with a predetermined limit to determine a number of defects, each defect being indicated by an intensity exceeding said predetermined limit.  
 
     
     
         51 . The method of  claim 49  further comprising: 
 computing a ratio of a local maximum in said plurality of intensities to a local minimum in said plurality of intensities; and  
 comparing the ratio with a predetermined limit to determine acceptance or rejection of said wafer  
 
     
     
         52 . The method of  claim 37  further comprising: 
 changing an average concentration of charge carriers in said region;  
 measuring a second intensity after said changing;  
 using each of said first intensity and said second intensity to determine a value of a material property in said region;  
 repeating said creating and said focusing in a plurality of regions of said wafer; and  
 measuring a plurality of intensities corresponding to said plurality of regions.  
 
     
     
         53 . A method for evaluating a wafer, said method comprising: 
 focusing on a region of said wafer a first beam of first photons having energy lower than the bandgap energy of a semiconductor material in said region, said first beam being polarized;    focusing on said region a second beam of second photons having energy greater than said bandgap energy, said second beam having an intensity modulated at a predetermined frequency, said second beam creating a plurality of charge carriers when incident on said region, said predetermined frequency being sufficiently small to avoid the creation of a wave of said charge carriers;    reflecting said first beam at said predetermined frequency by using said charge carriers;    interfering a reflected portion of said first beam with an unreflected portion of said first beam to obtain a sum component and a difference component;    measuring a difference between a first magnitude of said sum component and a second magnitude of said difference component.    
     
     
         54 . The method of  claim 53  further comprising, prior to said interfering: 
 passing said reflected portion and said unreflected portion through a filter, said filter blocking the passage of said second beam.  
 
     
     
         55 . The method of  claim 53  further comprising: 
 annealing said wafer prior to said focusing; and  
 adjusting annealing of another wafer depending on said difference.  
 
     
     
         56 . The method of  claim 53  further comprising: 
 interpolating said difference with respect to a plurality of difference measurements of semiconductor materials having known values of a material property to determine a value of said material property in said region.  
 
     
     
         57 . The method of  claim 53  further comprising: 
 changing a parameter used in said focusing on said region; and  
 measuring another difference after said changing.  
 
     
     
         58 . The method of  claim 53  further comprising: 
 determining a coefficient of a function, said function relating said difference and said another difference to the corresponding values of said parameter before and after said changing; and  
 interpolating said coefficient with respect to a plurality of coefficients of corresponding functions of semiconductor materials having known values of a material property to determine a value of said material property in said region.  
 
     
     
         59 . The method of  claim 53  wherein: 
 said wafer has a plurality of doped regions.  
 
     
     
         60 . A method for evaluating a wafer, said method comprising: 
 creating a plurality of charge carriers in a region of said wafer, the number of said charge carriers being modulated at a frequency that is sufficiently low to avoid creation of a wave of said charge carriers;    focusing on said region a probe beam of photons having energy lower than bandgap energy of a semiconductor material in said region;    measuring a first intensity of a portion of said probe beam modulated at said frequency after reflection by said region;    changing the concentration of said charge carriers at a surface of said wafer;    measuring, after said changing, a second intensity of a portion of said beam modulated at said frequency after reflection by said region;    determining a value of an attribute of a function that relates, at least approximately, said first intensity and said second intensity to the corresponding values of said concentration before and after said changing; and    interpolating said attribute with respect to a plurality of attributes of corresponding functions of semiconductor materials having known values of a property thereby to determine the value of said property in said region.    
     
     
         61 . The method of  claim 60  wherein: 
 said property is mobility μ unk ; and  
 said interpolating includes using the formula  
           μ   unk     =         m   ref       m   unk            μ   ref                       
 to compute mobility, wherein M unk  is said value of said attribute, m ref  is another value of said attribute for a reference wafer, and μ ref  is the mobility of said reference wafer.  
 
     
     
         62 . The method of  claim 60  wherein said creating includes focusing on said region a generation beam having an intensity modulated at said frequency, said method further comprising: 
 moving said probe beam relative to said generation beam;  
 measuring intensity after said moving;  
 repeating said moving and said measuring at least once;  
 determining a value of said distance at which said intensity is the largest; and using said value to maintain said probe beam in alignment with said generation beam.  
 
     
     
         63 . The method of  claim 60  wherein said wafer is formed of prime material.