7 042 569

7,042,569 Title:

Overlay alignment metrology using diffraction gratings

Alignment accuracy between two or more patterned layers is measured using a metrology target comprising substantially overlapping diffraction gratings formed in a test area of the layers being tested. An optical instrument illuminates all or part of the target area and measures the optical response. The instrument can measure transmission, reflectance, and/or ellipsometric parameters as a function of wavelength, polar angle of incidence, azimuthal angle of incidence, and/or polarization of the illumination and detected light. Overlay error or offset between those layers containing the test gratings is determined by a processor programmed to calculate an optical response for a set of parameters that include overlay error, using a model that accounts for diffraction by the gratings and interaction of the gratings with each others' diffracted field. The model parameters might also take account of manufactured asymmetries. The calculation may involve interpolation of pre-computed entries from a database accessible to the processor. The calculated and measured responses are iteratively compared and the model parameters changed to minimize the difference.

We claim:

1. A method of measuring alignment accuracy between two or more patterned layers formed on a substrate comprising: forming a test area as part of the patterned layers, wherein a firstdiffraction grating is built into a patterned layer A and a second diffraction grating is built into a patterned layer B, the two gratings substantially overlapping, said test area further including one or more additional layers which do not havediffraction gratings formed therein; observing the test area 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 ofincidence, azimuthal angle of incidence, or polarization of the illumination and detection; and determining the geometric parameters of the gratings and the additional layers, said geometric parameters including the offset between the gratings, byfitting the measurements from the optical instrument to 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 theeffects of the additional layers, wherein the first and second diffraction gratings have different pitches.

2. A method of measuring alignment accuracy between two or more patterned layers formed on a substrate comprising: forming a test area as part of the patterned layers, wherein a first diffraction grating is built into a patterned layer A and asecond diffraction grating is built into a patterned layer B, the two gratings substantially overlapping, said test area further including one or more additional layers which do not have diffraction gratings formed therein; observing the test area usingan 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 theillumination and detection; and determining the geometric parameters of the gratings and the additional layers, said geometric parameters including the offset between the gratings, by fitting the measurements from the optical instrument to an opticalmodel, 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 the effects of the additional layers, wherein at least a portion of thecalculations related to the optical model are computed before the observations of the test area are made.

3. A method of measuring alignment accuracy between two or more patterned layers formed on a substrate comprising: forming a first test area as part of the patterned layers, wherein a first diffraction grating is built into a patterned layer Aand a second diffraction grating is built into a patterned layer B, the two gratings substantially overlapping; forming a second test area as part of the patterned layers, wherein alignment markings are built into the patterned layers A and B; observing the first test area 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 ofincidence, or polarization of the illumination and detection; observing the second test area using a camera to generate an image thereof; determining the gross offset between the layers using the image information from the camera; and determining thefine offset between the gratings based on 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 witheach other's diffracted field.

4. A method as recited in claim 3, wherein the alignment markings in said second test area are defined by a bar-in-bar pattern.

5. A method as recited in claim 3, wherein the alignment markings in said second test area are defined by a box-in-box pattern.

6. A method as recited in claim 3, wherein the optical instrument includes a broadband source and the measurements are carried out as function of wavelength.

7. A method as recited in claim 3, wherein the first and second diffraction gratings have different pitches.

8. A method of fabricating a semiconductor wafer, said wafer including at least one patterned layer A formed on a substrate, wherein a first diffraction grating is built into a patterned layer A, said method comprising the step of: processingthe wafer to form a pattern on a layer B, said pattern including a second diffraction grating positioned to substantially overlap the first diffraction grating; observing the overlaid diffraction gratings using an optical instrument capable of measuringany 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 offsetbetween the gratings based on 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'sdiffracted field; and modifying the wafer processing steps based on the determined offset to minimize processing errors, wherein the first and second diffraction gratings have different pitches.

9. A method of fabricating a semiconductor wafer, said wafer including at least one patterned layer A formed on a substrate, wherein a first diffraction grating is built into a patterned layer A, said method comprising the step of: processingthe wafer to form a pattern on a layer B, said pattern including a second diffraction grating positioned to substantially overlap the first diffraction grating; observing the overlaid diffraction gratings using an optical instrument capable of measuringany 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 offsetbetween the gratings based on 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'sdiffracted field; and if the determined offset exceeds a predetermined value, removing the second patterned layer from the wafer and repeating the processing step with adjusted process parameters.

10. A method as recited in claim 9, wherein the optical instrument includes a broadband source and the measurements are carried out as function of wavelength.

11. A method as recited in claim 9, wherein the first and second diffraction gratings have different pitches.

12. A method as recited in claim 1, wherein the optical instrument includes a broadband source and the measurements are carried out as function of wavelength.