Apparatus and methods for detecting overlay errors using scatterometry

1. A method for determining an overlay error between at least two layers in a multiple layer sample, the method comprising:

using an imaging optical system to measure a plurality of measured optical signals from a plurality of periodic targets on the sample, wherein the targets each have a first structure in a first layer and a second structure in a second layer, wherein there are predefined offsets between the first and second structures; and

using a scatterometry overlay technique to analyze the measured optical signals of the periodic targets and the predefined offsets of the first and second structures of the periodic targets to thereby determine an overlay error between the first and second structures of the periodic targets,

wherein the scatterometry overlay technique is a phase based technique that includes representing each of the measured optical signals as a set of periodic functions having a plurality of known parameters and a plurality of unknown parameters that include an unknown overlay error parameter and analyzing the set of periodic functions to solve for the unknown overlay error parameter to thereby determine the overlay error, wherein the number of periodic targets equals the number of unknown parameters,

wherein the imaging optical system is configured to have an illumination and/or collection numerical aperture (NA) and/or spectral band selected so that a specific diffraction order is collected and measured for the plurality of measured optical signals.

2. A method as recited in claim 1 , wherein the illumination NA of the imaging optical system equals the collection NA, an incident beam of the optical system is normal to a surface of the sample, and the imaging optical system is configured to meet a following condition:

nλ=d ( NA i +NA c )

wherein n equals 1, λ is a wavelength, d is a pitch of a target's structures, NA i is the illumination numerical aperture, and NA c is the collection numerical aperture.

3. A method as recited in claim 1 , wherein the analysis of the measured optical signals includes deriving spectral information from the measured optical signals using a transform.

4. A method as recited in claim 1 , wherein the overlay error is determined without comparing any of the measured optical signals to a known or reference signal from a sample target having a known overlay error.

5. A method as recited in claim 1 , wherein the overlay error is determined without comparing the measured optical signals to calibration data.

6. A method as recited in claim 1 , wherein the illumination NA of the imaging optical system equals the collection NA, an incident beam of the optical system is normal to a surface of the sample, and the imaging optical system is configured to meet a following condition:

nλ≧ 2 dNA (1+ε)

wherein n equals 1, λ is a wavelength, d is a pitch of a target's structures, NA is the numerical aperture of the optical system, and ε is an approximation factor for structures of the periodic targets which are not infinitely periodic.

7. A method as recited in claim 1 , wherein the measured optical signals are in the form of one or more images.

8. A method as recited in claim 1 , wherein the spectral band of the optical system is selected by adjusting a wavelength selection device or a wavelength modulation device.

9. A method as recited in claim 6 , wherein ε is about less than 0.5.

10. A method as recited in claim 7 , wherein the one or more images include center portions of each target and the image center portion of each target is analyzed.

11. A method as recited in claim 8 , wherein the wavelength selection device is from a group consisting of a set of band pass interference filters, a set of continuously varying bandpass interference filters, a set of grating based spectrometers, a set of Fourier transform interferometers, and a set of acousto-optic tunable filters and the wavelength modulation device is controlled by changing one or more optical path lengths therein.

12. A method as recited in claim 8 , wherein the wavelength selection device or the wavelength modulation device is positioned within the imaging optical system's illumination path.

13. A method as recited in claim 8 , wherein the wavelength selection device or the wavelength modulation device is positioned within the imaging optical system's collection path.

14. A method as recited in claim 8 , wherein the imaging optical system further includes a polarizer in the illumination path and an analyzer in the collection path.

15. A method for determining an overlay error between at least two layers in a multiple layer sample, the method comprising:

(a) using an imaging optical system to measure a plurality of measured optical signals from a plurality of periodic targets on the sample, wherein the targets each have a first structure in a first layer and a second structure in a second layer, wherein there are predefined offsets between the first and second structures; and

(b) using a scatterometry overlay technique to analyze the measured optical signals of the periodic targets and the predefined offsets of the first and second structures of the periodic targets to thereby determine an overlay error between the first and second structures of the periodic targets,

wherein the scatterometry overlay technique is a phase based technique that includes representing each of the measured optical signals as a set of periodic functions having a plurality of known parameters and an unknown overlay error parameter and analyzing the set of periodic functions to solve for the unknown overlay error parameter to thereby determine the overlay error,

wherein the imaging optical system is configured to have an illumination and/or collection numerical aperture (NA) and/or spectral band selected so that a specific diffraction order is collected and measured for the plurality of measured optical signals,

wherein the imaging optical system has a broadband source for generating an optical incident beam having multiple wavelengths, a detector for detecting a measured signal from the sample in response to the incident beam and a filter for selectively passing particular one or more wavelengths of the output signal to the detector, wherein using the imaging optical system includes directing at least one radiation beam towards each target to measure a plurality of measured signals from the periodic targets while adjusting the filter so as to pass a particular one or more wavelengths of the measured signals through the filter towards the detector in the form of a plurality filtered signals,

wherein the analysis of the filtered signals and the predefined offsets includes obtaining an intensity from some or all of the pixels of an image of each target and combining the intensities of each target together to give an intensity value for each target at a particular setting of the filter.

16. A method as recited in claim 15 , wherein the analysis of the filtered signals and the predefined offsets further includes determining a periodic function of the overlay error based on the intensity values for each target and determining the overlay error based on such periodic function.

17. A method as recited in claim 15 , wherein the filter is adjusted so as to give a maximum difference between the target's intensity values.

18. A method as recited in claim 17 , further comprising repeating operations (a) and (b) over multiple wavelengths, wherein the measured optical signals having the largest contrast are used to determine overlay error.