Periodic patterns and technique to control misalignment between two layers

Abstract

A method and system to measure misalignment error between two overlying or interlaced periodic structures are proposed. The overlying or interlaced periodic structures are illuminated by incident radiation, and the diffracted radiation of the incident radiation by the overlying or interlaced periodic structures are detected to provide an output signal. The misalignment between the overlying or interlaced periodic structures may then be determined from the output signal.

1 . A method of measuring alignment accuracy between two or more patterned layers formed on a substrate comprising, 
 forming test areas as part of the patterned layers, wherein a first diffraction grating is built into a patterned layer A and a second diffraction grating is built into a patterned layer B, where layers A and B are desired to be aligned with respect to each other, zero or more layers of other materials separating layers A and B, the two gratings substantially overlapping when viewed from a direction that is perpendicular to the surfaces of A and B;    observing the overlaid diffraction gratings using an optical instrument capable of measuring any one or more of transmission, reflectance, or ellipsometric parameters as a function of any one or more of wavelength, polar angle of incidence, azimuthal angle of incidence, or polarization of the illumination and detection; and    determining the offset between the gratings from the measurements from the optical instrument using an optical model, wherein the optical model accounts for the diffraction of the electromagnetic waves by the gratings and the interaction of the gratings with each other's diffracted field;    wherein at least one layer between the grating in layer A and the grating in layer B is opaque in the wavelength range of the optical instrument, and the presence of the grating in layer A causes a grating-shaped topography on the surface of the opaque layer.    
   
   
       2 . A method of measuring alignment accuracy between two or more patterned layers formed on a substrate comprising, 
 forming test areas as part of the patterned layers, wherein a first diffraction grating is built into a patterned layer A and a second diffraction grating is built into a patterned layer B, where layers A and B are desired to be aligned with respect to each other, zero or more layers of other materials separating layers A and B, the two gratings substantially overlapping when viewed from a direction that is perpendicular to the surfaces of A and B;    observing the overlaid diffraction gratings using an optical instrument capable of measuring any one or more of transmission, reflectance, or ellipsometric parameters as a function of any one or more of wavelength, polar angle of incidence, azimuthal angle of incidence, or polarization of the illumination and detection; and    determining the offset between the gratings from the measurements from the optical instrument using an optical model, wherein the optical model accounts for the diffraction of the electromagnetic waves by the gratings and the interaction of the gratings with each other's diffracted field;    wherein the optical model represents the electromagnetic field in the gratings and in the layers between the gratings as a sum of more than one diffracted orders.    
   
   
       3 . A method of measuring alignment accuracy between two or more patterned layers formed on a substrate comprising, 
 forming test areas as part of the patterned layers, wherein a first diffraction grating is built into a patterned layer A and a second diffraction grating is built into a patterned layer B, where layers A and B are desired to be aligned with respect to each other, zero or more layers of other materials separating layers A and B, the two gratings substantially overlapping when viewed from a direction that is perpendicular to the surfaces of A and B;    observing the overlaid diffraction gratings using an optical instrument capable of measuring any one or more of transmission, reflectance, or ellipsometric parameters as a function of any one or more of wavelength, polar angle of incidence, azimuthal angle of incidence, or polarization of the illumination and detection; and    determining the offset between the gratings from the measurements from the optical instrument using an optical model, wherein the optical model accounts for the diffraction of the electromagnetic waves by the gratings and the interaction of the gratings with each other's diffracted field;    wherein offset is determined by; 
 calculating, according to a model of a wafer sample, the optical response of the sample with the said two overlapping gratings, the model of the sample taking into account parameters of the sample including any of the overlay misalignment of layers A and B, the profiles of the grating structures, and asymmetries caused in the grating structures by manufacturing processes;  
 changing the parameters of the sample model to minimize the difference between the calculated and measured optical responses; and  
 repeating the previous two steps until the difference between the calculated and measured optical responses is sufficiently small or cannot be significantly decreased by further iterations.  
   
   
   
       4 . The method of  claim 3  wherein at least a portion of the calculation is done at the measurement time.  
   
   
       5 . The method of  claim 3  wherein at least a portion of the calculated optical response is retrieved from a pre-computed database.  
   
   
       6 . The method of  claim 3  wherein the calculation involves interpolating the optical response from pre-computed entries in a database.  
   
   
       7 . A method of measuring alignment accuracy between two or more patterned layers formed on a substrate comprising, 
 forming test areas as part of the patterned layers, wherein a first diffraction grating is built into a patterned layer A and a second diffraction grating is built into a patterned layer B, where layers A and B are desired to be aligned with respect to each other, zero or more layers of other materials separating layers A and B, the two gratings substantially overlapping when viewed from a direction that is perpendicular to the surfaces of A and B;    observing the overlaid diffraction gratings using an optical instrument capable of measuring any one or more of transmission, reflectance, or ellipsometric parameters as a function of any one or more of wavelength, polar angle of incidence, azimuthal angle of incidence, or polarization of the illumination and detection; and    determining the offset between the gratings from the measurements from the optical instrument using an optical model, wherein the optical model accounts for the diffraction of the electromagnetic waves by the gratings and the interaction of the gratings with each other's diffracted field;    wherein the first and second diffraction gratings have different pitches.    
   
   
       8 . A method of measuring alignment accuracy between two or more patterned layers formed on a substrate comprising: 
 forming test areas as part of the patterned layers, wherein a first diffraction grating is built into a patterned layer A and a second diffraction grating is built into a patterned layer B, where layers A and B are desired to be aligned with respect to each other, zero or more layers of other materials separating layers A and B, the two gratings substantially overlapping when viewed from a direction that is perpendicular to the surfaces of A and B,    observing the overlaid diffraction gratings using an optical instrument capable of measuring any one or more of transmission, reflectance, or ellipsometric parameters as a function of any one or more of wavelength, polar angle of incidence, azimuthal angle of incidence, or polarization of the illumination and detection; and    determining the offset between the gratings from the measurements from the optical instrument using an optical model, wherein the optical model accounts for the diffraction of the electromagnetic waves by the gratings and the interaction of the gratings with each other's diffracted field;    wherein at least one of the two gratings contains more than one line per pitch, the widths of the at least two lines in each pitch being substantially different from each other.    
   
   
       9 . A method of determining a degree of registration between an upper layer and a lower layer formed on a substrate, each of said layers including a periodic structure formed thereon and arranged to at least partially overlap, said method comprising the steps of: 
 illuminating the layers with a probe beam of radiation;    monitoring the zeroth order light diffracted from the layers;    generating a parameterized model representing the geometry and registration of parameters of the model; and    comparing the predicted optical response with the monitored zeroth order light to determine the registration of the structures.    
   
   
       10 . A method as recited in  claim 9  wherein said predicting step is at least partially carried out in advance for a number of different parameters and wherein the corresponding responses are stored in a database for later comparison with the monitored response.  
   
   
       11 . A method as recited in  claim 9  wherein the predicting and comparing steps are repeated while changing the parameters used in the predicting step in order to cause the predicted optical response to converge with the monitored response.  
   
   
       12 . A method as recited in  claim 9  wherein said probe beam is generated from a broadband source and said monitoring step is carried out as function of wavelength.  
   
   
       13 . An apparatus for determining overlay error between two or more patterned layers of a sample, comprising, 
 a metrology target comprising a first diffraction grating built into a patterned layer A and a second diffraction grating built into a patterned layer B, where layers A and B are part of the sample under test and layers A and B are desired to be aligned with respect to each other, the two gratings substantially overlapping when viewed from a direction that is perpendicular to the layers A and B;    an optical instrument that illuminates part or all of the metrology target and that measures properties of light that has interacted with the metrology target as a function of any one or more of polar angle of incidence, azimuthal angle of incidence, and polarization of the illumination and detection; and    a processor which estimates the offset of the grating pair from the measured properties;    wherein the first and second diffraction gratings have different pitches.    
   
   
       14 . An apparatus for determining overlay error between two or more patterned layers of a sample comprising: 
 a metrology target comprising a first diffraction grating built into a patterned layer A and second diffraction grating built into a patterned layer B, where layers A and B are part of the sample under test and layers A and B are desired to be aligned with respect to each other, the two gratings substantially overlapping when viewed from a direction that is perpendicular to the layers A and B;    an optical instrument that illuminates part or all of the metrology target and that measures properties of light that has interacted with the metrology target as a function of any one or more of polar angle of incidence, azimuthal angle of incidence, and polarization of the illumination and detection; and    a processor which estimates the offset of the grating pair form the measured properties;    wherein at least one of the two gratings contains more than one line per pitch, the widths of the at least two lines in each pitch being substantially different from each other.    
   
   
       15 . An apparatus for determining overlay error between two or more patterned layers of a sample, comprising, 
 a metrology target comprising a first diffraction grating built into a patterned layer A and a second diffraction grating built into a patterned layer B, where layers A and B are part of the sample under test and layers A and B are desired to be aligned with respect to each other, the two gratings substantially overlapping when viewed from a direction that is perpendicular to the layers A and B;    an optical instrument that illuminates part or all of the metrology target and that measures properties of light that has interacted with the metrology target as a function of any one or more of polar angle of incidence, azimuthal angle of incidence, and polarization of the illumination and detection; and    a processor which estimates the offset of the grating pair from the measured properties;    wherein at least one other layer of material separates layers A and B at the metrology target.    
   
   
       16 . An apparatus for determining overlay error between two or more patterned layers of a sample, comprising, 
 a metrology target comprising a first diffraction grating built into a patterned layer A and a second diffraction grating built into a patterned layer B, where layers A and B are part of the sample under test and layers A and B are desired to be aligned with respect to each other, the two gratings substantially overlapping when viewed from a direction that is perpendicular to the layers A and B;    an optical instrument that illuminates part or all of the metrology target and that measures properties of light that has interacted with the metrology target as a function of any one or more of polar angle of incidence, azimuthal angle of incidence, and polarization of the illumination and detection; and    a processor which estimates the offset of the grating pair from the measured properties:    wherein the processor has been programmed to iteratively (i) calculate an optical response for a set of sample parameters, including overlay misalignment, (ii) compare the measured properties with the calculated optical response, and (iii) change one or more sample parameters so as to minimize the difference between the measured properties and the calculated optical response,    wherein the calculation of an optical response is according to an optical model of the sample that accounts for the diffraction of electromagnetic waves by the pair of gratings of the metrology target and the interaction of the gratings with each other's diffracted field.    
   
   
       17 . The apparatus of  claim 16  wherein the processor has access to a pre-computed database from which at least a portion of the calculated optical response can be retrieved.  
   
   
       18 . The apparatus of  claim 17  wherein the calculation performed by the programmed processor involves interpolating the optical response from pre-computed entries in said database.  
   
   
       19 . An apparatus for determining the overlay error comprising, 
 a metrology target comprising a first diffraction grating built into a patterned layer A and a second diffraction grating is built into a patterned layer B, where layers A and B are desired to be aligned with respect to each other, the two gratings substantially overlapping when viewed from a direction that is perpendicular to the layers A and B;    an ellipsometer that illuminates part or all of the metrology target and that measures properties of light that has interacted with the metrology target; and    a processor which estimates the offset of the grating pair from the pair's measured optical characteristics.    
   
   
       20 . The method of  claim 19  wherein first and second diffraction gratings have different pitches.  
   
   
       21 . The apparatus of  claim 19  wherein at least one of the two gratings contains more than one line per pitch, the widths of the at least two lines in each pitch (unit cell) being substantially different from each other.  
   
   
       22 . The apparatus of  claim 19  wherein at least one other layer of material separates layers A and B at the metrology target.  
   
   
       23 . The apparatus of  claim 19  wherein the ellipsometer measures properties of light that has interacted with the metrology target as a function of wavelength.  
   
   
       24 . The apparatus of  claim 19  wherein the processor has been programmed to iteratively (i) calculate an optical response for a set of sample parameters, including overlay misalignment, (ii) compare the measured properties with the calculated optical response, and (iii) change one or more sample parameters so as to minimize the difference between the measured properties and the calculated optical response, 
 wherein the calculation of an optical response is according to an optical model of the sample that accounts for the diffraction of electromagnetic waves by the pair of gratings of the metrology target and the interaction of the gratings with each other's diffracted field.    
   
   
       25 . The apparatus of  claim 24  wherein the processor has access to a pre-computed database from which at least a portion of the calculated optical response can be retrieved.  
   
   
       26 . The apparatus of  claim 25  wherein the calculation performed by the programmed processor involves interpolating the optical response from pre-computed entries in said database.  
   
   
       27 . The apparatus of  claim 19 , wherein the metrology target further includes a second pattern built into layers A and B, and wherein the optical instrument further includes a camera disposed to observe said second pattern and obtain measurements therefrom of any gross overlay errors, said processor connected to also receive said measurements from said camera.  
   
   
       28 . The apparatus of  claim 27 , wherein said second pattern comprises a box-in-box pattern.  
   
   
       29 . The apparatus of  claim 27 , wherein said second pattern comprises a bar-in-bar pattern.  
   
   
       30 . A method of measuring alignment accuracy between two or more patterned layers formed on a substrate comprising, 
 forming test areas as part of the patterned layers, wherein a first diffraction grating is built into a patterned layer A and a second diffraction grating is built into a patterned layer B, where layers A and B are desired to be aligned with respect to each other, zero or more layers of other materials separating layers A and B, the two gratings substantially overlapping when viewed from a direction that is perpendicular to the surfaces of A and B;    observing the overlaid diffraction gratings using an optical instrument capable of measuring any one or more of transmission, reflectance, or ellipsometric parameters as a function of any one or more of wavelength, polar angle of incidence, azimuthal angle of incidence, or polarization of the illumination and detection;    determining the offset between the gratings from the measurements from the optical instrument using an optical model, wherein the optical model accounts for the diffraction of the electromagnetic waves by the gratings and the interaction of the gratings with each other's diffracted field; and    observing at least one second test area on said substrate using a camera, the second test area having a pattern built into layers A and B for measuring any gross overlay errors, and wherein determining the offset includes using gross overlay measurements obtained from the camera;    wherein said pattern in said second area comprises a bar-in-bar pattern.    
   
   
       31 . An apparatus for determining overlay error between two or more patterned layers of a sample, comprising. 
 a metrology target comprising a first diffraction grating built into a patterned layer A and a second diffraction grating built into a patterned layer B, where layers A and B are part of the sample under test and layers A and B are desired to be aligned with respect to each other, the two gratings substantially overlapping when viewed from a direction that is perpendicular to the layers A and B;    an optical instrument that illuminates part or all of the metrology target and that measures properties of light that has interacted with the metrology target as a function of any one or more of polar angle of incidence, azimuthal angle of incidence, and polarization of the illumination and detection; and    a processor which estimates the offset of the grating pair from the measured properties;    wherein the metrology target further includes a second pattern built into layers A and B, and wherein the optical instrument further includes a camera disposed to observe said second pattern and obtain measurements therefrom of any gross overlay errors, said processor connected to also receive said measurements from said camera; and    wherein said second pattern comprises a bar-in-bar pattern.    
   
   
       32 . A method of measuring misalignment between two or more patterned layers formed on a substrate comprising, 
 forming target areas as part of the patterned layers, wherein a first diffraction grating is built into a patterned layer A and a second diffraction grating is built into a patterned layer B, where layers A and B are desired to be aligned with respect to each other, zero or more layers of other materials separating layers A and B, the two gratings substantially overlapping when viewed from a direction that is perpendicular to the surfaces of A and B;    observing the overlaid diffraction gratings using an optical signal processor capable of measuring any one or more of transmission, reflectance, or ellipsometric parameters as a function of any one or more of wavelength, polar angle of incidence, azimuthal angle of incidence, or polarization of the illumination and detection; and    determining the misalignment between the gratings from the measurements from the optical signal processor using a reference signal, wherein the reference signal accounts for the diffraction of the radiation waves by the gratings and the interaction of the gratings with each other's diffracted field;    wherein at least one layer between the grating in layer A and the grating in layer B is opaque in the wavelength range of the signal processor, and the presence of the grating in layer A causes a grating-shaped pattern on the surface of the opaque layer.    
   
   
       33 . A method of measuring misalignment between two or more patterned layers formed on a substrate comprising, 
 forming target areas as part of the patterned layers, wherein a first diffraction grating is built into a patterned layer A and a second diffraction grating is built into a patterned layer B, where layers A and B are desired to be aligned with respect to each other, zero or more layers of other materials separating layers A and B, the two gratings substantially overlapping when viewed from a direction that is perpendicular to the surfaces of A and B;    observing the overlaid diffraction gratings using an optical signal processor capable of measuring any one or more of transmission, reflectance, or ellipsometric parameters as a function of any one or more of wavelength, polar angle of incidence, azimuthal angle of incidence, or polarization of the illumination and detection; and    determining the misalignment between the gratings from the measurements from the optical signal processor using a reference signal, wherein the reference signal accounts for the diffraction of the radiation waves by the gratings and the interaction of the gratings with each other's diffracted field;    wherein the reference signal represents the radiation field in the gratings and in the layers between the gratings as a sum of more than one diffracted orders.    
   
   
       34 . A method of measuring misalignment between two or more patterned layers formed on a substrate comprising, 
 forming target areas as part of the patterned layers, wherein a first diffraction grating is built into a patterned layer A and a second diffraction grating is built into a patterned layer B, where layers A and B are desired to be aligned with respect to each other, zero or more layers of other materials separating layers A and B, the two gratings substantially overlapping when viewed from a direction that is perpendicular to the surfaces of A and B;    observing the overlaid diffraction gratings using an optical signal processor capable of measuring any one or more of transmission, reflectance, or ellipsometric parameters as a function of any one or more of wavelength, polar angle of incidence, azimuthal angle of incidence, or polarization of the illumination and detection; and    determining the misalignment between the gratings from the measurements from the optical signal processor using a reference signal, wherein the reference signal accounts for the diffraction of the radiation waves by the gratings and the interaction of the gratings with each other's diffracted field;    wherein misalignment is determined by; 
 calculating, according to a reference signal of a wafer sample, the derived signal of the sample with the said two overlapping gratings, the reference signal of the sample taking into account parameters of the sample including any of the overlay misalignment of layers A and B, the profiles of the grating structures, and asymmetries caused in the grating structures by manufacturing processes;  
 changing the parameters of the sample reference signal to minimize the difference between the calculated and measured derived signals; and  
 repeating the previous two steps until the difference between the calculated and measured derived signals is sufficiently small or cannot be significantly decreased by further iterations.  
   
   
   
       35 . The method of  claim 34  wherein at least a portion of the calculation is done at the measurement time.  
   
   
       36 . The method of  claim 34  wherein at least a portion of the calculated derived signal is retrieved from a pre-computed database.  
   
   
       37 . The method of  claim 34  wherein the calculation involves interpolating the derived signal from pre-computed entries in a database.  
   
   
       38 . A method of measuring misalignment between two or more patterned layers formed on a substrate comprising, 
 forming target areas as part of the patterned layers, wherein a first diffraction grating is built into a patterned layer A and a second diffraction grating is built into a patterned layer B, where layers A and B are desired to be aligned with respect to each other, zero or more layers of other materials separating layers A and B, the two gratings substantially overlapping when viewed from a direction that is perpendicular to the surfaces of A and B;    observing the overlaid diffraction gratings using an optical signal processor capable of measuring any one or more of transmission, reflectance, or ellipsometric parameters as a function of any one or more of wavelength, polar angle of incidence, azimuthal angle of incidence, or polarization of the illumination and detection; and    determining the misalignment between the gratings from the measurements from the optical signal processor using a reference signal, wherein the reference signal accounts for the diffraction of the radiation waves by the gratings and the interaction of the gratings with each other's diffracted field;    wherein the first and second diffraction gratings have different pitches.    
   
   
       39 . A method of measuring misalignment between two or more patterned layers formed on a substrate comprising: 
 forming target areas as part of the patterned layers, wherein a first diffraction grating is built into a patterned layer A and a second diffraction grating is built into a patterned layer B, where layers A and B are desired to be aligned with respect to each other, zero or more layers of other materials separating layers A and B, the two gratings substantially overlapping when viewed from a direction that is perpendicular to the surfaces of A and B,    observing the overlaid diffraction gratings using an optical signal processor capable of measuring any one or more of transmission, reflectance, or ellipsometric parameters as a function of any one or more of wavelength, polar angle of incidence, azimuthal angle of incidence, or polarization of the illumination and detection; and    determining the misalignment between the gratings from the measurements from the optical signal processor using a reference signal, wherein the reference signal accounts for the diffraction of the radiation waves by the gratings and the interaction of the gratings with each other's diffracted field;    wherein at least one of the two gratings contains more than one line per pitch, the widths of the at least two lines in each pitch being substantially different from each other.    
   
   
       40 . A method of determining a degree of registration between an upper layer and a lower layer formed on a substrate, each of said layers including a periodic structure formed thereon and arranged to at least partially overlap, said method comprising the steps of: 
 illuminating the layers with a probe beam of radiation;    monitoring the zeroth order light diffracted from the layers;    generating a parameterized reference signal representing the geometry and registration of parameters of the reference signal; and    comparing the predicted derived signal with the monitored zeroth order light to determine the registration of the structures.    
   
   
       41 . A method as recited in  claim 40  wherein said predicting step is at least partially carried out in advance for a number of different parameters and wherein the corresponding responses are stored in a database for later comparison with the monitored response.  
   
   
       42 . A method as recited in  claim 40  wherein the predicting and comparing steps are repeated while changing the parameters used in the predicting step in order to cause the predicted derived signal to converge with the monitored response.  
   
   
       43 . A method as recited in  claim 40  wherein said probe beam is generated from a broadband source and said monitoring step is carried out as function of wavelength.  
   
   
       44 . An apparatus for determining misalignment between two or more patterned layers of a wafer, comprising, 
 a metrology target comprising a first diffraction grating built into a patterned layer A and a second diffraction grating built into a patterned layer B, where layers A and B are part of the wafer under test and layers A and B are desired to be aligned with respect to each other, the two gratings substantially overlapping when viewed from a direction that is perpendicular to the layers A and B;    an optical signal processor that illuminates part or all of the metrology target and that measures properties of light diffracted from the metrology target as a function of any one or more of polar angle of incidence, azimuthal angle of incidence, and polarization of the illumination and detection; and    a processor which estimates the misalignment of the grating pair from the measured properties;    wherein the first and second diffraction gratings have different pitches.    
   
   
       45 . An apparatus for determining misalignment between two or more patterned layers of a wafer comprising: 
 a metrology target comprising a first diffraction grating built into a patterned layer A and second diffraction grating built into a patterned layer B, where layers A and B are part of the wafer under test and layers A and B are desired to be aligned with respect to each other, the two gratings substantially overlapping when viewed from a direction that is perpendicular to the layers A and B;    an optical signal processor that illuminates part or all of the metrology target and that measures properties of light diffracted from the metrology target as a function of any one or more of polar angle of incidence, azimuthal angle of incidence, and polarization of the illumination and detection; and    a processor which estimates the misalignment of the grating pair form the measured properties;    wherein at least one of the two gratings contains more than one line per pitch, the widths of the at least two lines in each pitch being substantially different from each other.    
   
   
       46 . An apparatus for determining misalignment between two or more patterned layers of a wafer, comprising, 
 a metrology target comprising a first diffraction grating built into a patterned layer A and a second diffraction grating built into a patterned layer B, where layers A and B are part of the wafer under test and layers A and B are desired to be aligned with respect to each other, the two gratings substantially overlapping when viewed from a direction that is perpendicular to the layers A and B;    an optical signal processor that illuminates part or all of the metrology target and that measures properties of light diffracted from the metrology target as a function of any one or more of polar angle of incidence, azimuthal angle of incidence, and polarization of the illumination and detection; and    a processor which estimates the misalignment of the grating pair from the measured properties;    wherein at least one other layer of material separates layers A and B at the metrology target.    
   
   
       47 . An apparatus for determining misalignment between two or more patterned layers of a wafer, comprising, 
 a metrology target comprising a first diffraction grating built into a patterned layer A and a second diffraction grating built into a patterned layer B, where layers A and B are part of the wafer under test and layers A and B are desired to be aligned with respect to each other, the two gratings substantially overlapping when viewed from a direction that is perpendicular to the layers A and B;    an optical signal processor that illuminates part or all of the metrology target and that measures properties of light diffracted from the metrology target as a function of any one or more of polar angle of incidence, azimuthal angle of incidence, and polarization of the illumination and detection; and    a processor which estimates the misalignment of the grating pair from the measured properties:    wherein the processor has been programmed to repeatedly (i) calculate a derived signal for a set of parameters, including overlay misalignment, (ii) compare the measured properties with the calculated derived signal, and (iii) change one or more parameters so as to minimize the difference between the measured properties and the calculated derived signal,    wherein the calculation of a derived signal is according to a reference signal of the wafer that accounts for the diffraction of radiation waves by the pair of gratings of the metrology target and the interaction of the gratings with each other's diffracted field.    
   
   
       48 . The apparatus of  claim 47  wherein the processor has access to a pre-computed database from which at least a portion of the calculated derived signal can be retrieved.  
   
   
       49 . The apparatus of  claim 48  wherein the calculation performed by the programmed processor involves interpolating the derived signal from pre-computed entries in said database.  
   
   
       50 . An apparatus for determining misalignment comprising, 
 a metrology target comprising a first diffraction grating built into a patterned layer A and a second diffraction grating is built into a patterned layer B, where layers A and B are desired to be aligned with respect to each other, the two gratings substantially overlapping when viewed from a direction that is perpendicular to the layers A and B;    an optical system that illuminates part or all of the metrology target and that measures properties of light diffracted from the metrology target; and    a processor which estimates the misalignment of the grating pair from the pair's measured optical characteristics.    
   
   
       51 . The method of  claim 50  wherein first and second diffraction gratings have different pitches.  
   
   
       52 . The apparatus of  claim 50  wherein at least one of the two gratings contains more than one line per pitch, the widths of the at least two lines in each pitch (unit cell) being substantially different from each other.  
   
   
       53 . The apparatus of  claim 50  wherein at least one other layer of material separates layers A and B at the metrology target.  
   
   
       54 . The apparatus of  claim 50  wherein the optical system measures properties of light diffracted from the metrology target as a function of wavelength.  
   
   
       55 . The apparatus of  claim 50  wherein the processor has been programmed to repeatedly (i) calculate a derived signal for a set of parameters, including overlay misalignment, (ii) compare the measured properties with the calculated derived signal, and (iii) change one or more parameters so as to minimize the difference between the measured properties and the calculated derived signal, 
 wherein the calculation of a derived signal is according to a reference signal of the wafer that accounts for the diffraction of radiation waves by the pair of gratings of the metrology target and the interaction of the gratings with each other's diffracted field.    
   
   
       56 . The apparatus of  claim 55  wherein the processor has access to a pre-computed database from which at least a portion of the calculated derived signal can be retrieved.  
   
   
       57 . The apparatus of  claim 56  wherein the calculation performed by the programmed processor involves interpolating the derived signal from pre-computed entries in said database.  
   
   
       58 . The apparatus of  claim 50 , wherein the metrology target further includes a second pattern built into layers A and B, and wherein the optical signal processor further includes a camera disposed to observe said second pattern and obtain measurements therefrom of any gross misalignments, said processor connected to also receive said measurements from said camera.  
   
   
       59 . The apparatus of  claim 58 , wherein said second pattern comprises a box-in-box pattern.  
   
   
       60 . The apparatus of  claim 58 , wherein said second pattern comprises a bar-in-bar pattern.  
   
   
       61 . A method of measuring misalignment between two or more patterned layers formed on a substrate comprising, 
 forming target areas as part of the patterned layers, wherein a first diffraction grating is built into a patterned layer A and a second diffraction grating is built into a patterned layer B, where layers A and B are desired to be aligned with respect to each other, zero or more layers of other materials separating layers A and B, the two gratings substantially overlapping when viewed from a direction that is perpendicular to the surfaces of A and B;    observing the overlaid diffraction gratings using an optical signal processor capable of measuring any one or more of transmission, reflectance, or ellipsometric parameters as a function of any one or more of wavelength, polar angle of incidence, azimuthal angle of incidence, or polarization of the illumination and detection;    determining the misalignment between the gratings from the measurements from the optical signal processor using a reference signal, wherein the reference signal accounts for the diffraction of the radiation waves by the gratings and the interaction of the gratings with each other's diffracted field; and    observing at least one second test area on said substrate using a camera, the second test area having a pattern built into layers A and B for measuring any gross misalignments, and wherein determining the misalignment includes using gross overlay measurements obtained from the camera;    wherein said pattern in said second area comprises a bar-in-bar pattern.    
   
   
       62 . An apparatus for determining misalignment between two or more patterned layers of a wafer, comprising. 
 a metrology target comprising a first diffraction grating built into a patterned layer A and a second diffraction grating built into a patterned layer B, where layers A and B are part of the wafer under test and layers A and B are desired to be aligned with respect to each other, the two gratings substantially overlapping when viewed from a direction that is perpendicular to the layers A and B;    an optical signal processor that illuminates part or all of the metrology target and that measures properties of light diffracted from the metrology target as a function of any one or more of polar angle of incidence, azimuthal angle of incidence, and polarization of the illumination and detection; and    a processor Which estimates the misalignment of the grating pair from the measured properties;    wherein the metrology target further includes a second pattern built into layers A and B, and wherein the optical signal processor further includes a camera disposed to observe said second pattern and obtain measurements therefrom of any gross misalignments, said processor connected to also receive said measurements from said camera; and    wherein said second pattern comprises a bar-in-bar pattern.