Method for controlling a distance between two objects, inspection apparatus and method
A broadband spectroscopic analysis is used for controlling a distance (d) between a miniature solid immersion lens (SIL, 60 ) and a metrology target ( 30 ′). An objective lens arrangement ( 15, 60 ) including the SIL illuminates the metrology target with a beam of radiation with different wavelengths and collects a radiation ( 709 ) reflected or diffracted by the metrology target. A mounting ( 64 ) holds the SIL within a distance from the metrology target that is less than the coherence length of the illuminating radiation ( 703 ). A detection arrangement ( 812, 818 ) produces a spectrum of the radiation reflected or diffracted by the metrology target. The distance between the SIL and the metrology target or other target surface can be inferred from spectral shifts observed in the detected spectrum. Servo control of the distance is implemented based on these shifts, using an actuator ( 66 ).
1. A method for monitoring a gap between an optical element and a target surface, the method comprising:
positioning the target surface at a distance relative to the optical element;
using the optical element to illuminate a part of the target surface with radiation comprising different wavelengths;
collecting radiation reflected by the target surface back into the optical element;
measuring an electromagnetic spectrum of the collected radiation; and
deriving information about the distance between the optical element and the target surface based on a shift in wavelength of one or more features in the spectrum, wherein the distance is less than a coherence length of the radiation.
2. The method as claimed in claim 1 , wherein the radiation comprising different wavelengths comprises a continuous spectrum over a range of wavelengths and the deriving is performed based on a shift in the wavelength of the one or more features in the spectrum compared to one or more features in a previously measured spectrum or one or more features in a given spectral distribution.
3. The method as claimed in claim 1 , wherein the optical element and the target surface are illuminated with the radiation at normal incidence.
4. The method as claimed in claim 1 , wherein the optical element and the target surface are illuminated with the radiation at oblique incidence.
5. The method as claimed in claim 1 , wherein the deriving comprises performing a Fourier transform on at least a portion of the measured spectrum, the Fourier transform being used to determine the shift in the wavelength of the one or more features in the measured spectrum.
6. The method as claimed in claim 1 , wherein the deriving comprises identifying one or more local extrema in the measured spectrum and tracking changes in wavelength of the local extrema.
7. The method as claimed in claim 1 , further comprising adjusting the distance between the optical element and the target surface in response to the distance information derived in the deriving.
8. The method as claimed in claim 7 , wherein the using, the collecting, the measuring, the deriving, and the adjusting are repeated continuously to implement servo control of the distance between the optical element and the target surface.
9. The method as claimed in claim 1 , wherein the optical element is all or part of an objective lens system of an inspection apparatus.
10. The method as claimed in claim 9 , wherein the optical element comprises a solid immersion lens element associated with the objective lens system of the inspection apparatus.
11. An apparatus for monitoring a gap between a first optical element and a target surface, the apparatus comprising:
one or more optical components configured to deliver, through the first optical element, radiation to illuminate a part of the target surface, the radiation comprising different wavelengths, wherein the first optical element is positioned at a distance relative to the target surface;
a second optical element configured to collect radiation reflected by the target surface back into the first optical element;
a spectrometer configured to measure an electromagnetic spectrum of the collected radiation; and
a processor configured to identify a shift in wavelength of one or more features in the measured spectrum compared to one or more features in a previously measured spectrum or one or more features in a given spectral distribution and to derive from the identified shift information about the distance between the first optical element and the target surface.
12. The apparatus as claimed in claim 11 , wherein the radiation comprising different wavelengths comprises a continuous spectrum over a range of wavelengths and the support apparatus is arranged to operate with the distance less than a coherence length of the radiation.
13. The apparatus as claimed in claim 11 , wherein the first optical element and the target surface are arranged to be illuminated with the radiation at normal incidence.
14. The apparatus as claimed in claim 11 , wherein the first optical element and the target surface are arranged to be illuminated with the radiation at oblique incidence.
15. The apparatus as claimed in claim 11 , wherein the processor is further configured to perform a Fourier transform on at least a portion of the measured spectrum, and to use the Fourier transform to determine the shift in the wavelength of the one or more features in the measured spectrum.
16. The apparatus as claimed in claim 11 , wherein the processor is further configured to identify one or more local extrema in the measured spectrum and to track changes in wavelength of the local extrema.
17. The apparatus as claimed in claim 11 , wherein the processor and the support apparatus are configured to adjust the distance between the first optical element and the target surface in response to the distance information derived by the processor.
18. The apparatus as claimed in claim 17 , wherein the processor and support apparatus are configured to implement servo control of the distance between the first optical element and the target surface.
19. A system, comprising:
an inspection apparatus configured to optically inspect target structures on a target surface of a substrate; and
an apparatus configured to monitor a gap between a solid immersion lens element associated with an objective lens of the inspection apparatus and the target surface, the apparatus comprising:
one or more optical components configured to deliver through the solid immersion lens element, radiation to illuminate a part of the target surface, the radiation comprising different wavelengths;
the objective lens configured to collect radiation reflected by the target surface back into the solid immersion lens element;
a spectrometer configured to measure an electromagnetic spectrum of the collected radiation; and
a processor configured to identify a shift in wavelength of one or more features in the measured spectrum and for deriving, from the measured shift, information about the distance between the solid immersion lens element and the target surface.
20. A method, comprising:
positioning a target surface at a distance relative to an optical element;
using the optical element to illuminate a part of the target surface with radiation comprising different wavelengths;
collecting radiation reflected by the target surface back into the optical element;
measuring an electromagnetic spectrum of the collected radiation; and
using a non-transient computer readable medium comprising machine-readable instructions to cause one or more processors to identify a shift in wavelength of one or more features in a measured spectrum and to derive, from the measured shift, information about the distance between the optical element and the target surface,
wherein the radiation comprising different wavelengths comprises a continuous spectrum over a range of wavelengths, and
wherein the deriving is performed to derive information about the distance at least when the distance is less than a coherence length of the radiation.