Short wavelength metrology imaging system
An extreme ultraviolet (EUV) AIM tool for both the EUV actinic lithography and high-resolution imaging or inspection is described. This tool can be extended to lithography nodes beyond the 32 nanometer (nm) node covering other short wavelength radiation such as soft X-rays. The metrology tool is preferably based on an imaging optic referred to as an Achromatic Fresnel Optic (AFO). The AFO is a transmissive optic that includes a diffractive Fresnel zone plate lens component and a dispersion-correcting refractive lens component. It retains all of the imaging properties of a Fresnel zone plate lens, including a demonstrated resolution capability of better than 25 nanometers and freedom from image distortion. It overcomes the chromatic aberration of the Fresnel zone plate lens and has a larger usable spectral bandwidth. These optical properties and optical system designs enable the development of the AFO-based AIM tool with improved performance that has advantages compared with an AIM tool based on multilayer reflective mirror optics in both performance and cost.
1. An optical system comprising:
an extreme ultraviolet radiation source;
a spectral filter that filters ultraviolet radiation generated by the source;
a reflective condenser that directs the ultraviolet radiation onto a sample at an angle of between normal to the sample and 7 degrees off normal;
an aperture for spatially filtering the ultraviolet radiation;
an objective lens comprising a Fresnel zone plate lens that forms an image of the ultraviolet radiation from the sample; and
a spatially resolved detector for detecting the image of the sample formed by the objective lens.
2. An optical system as claimed in claim 1 , wherein the ultraviolet radiation has a wavelength of 13 to 14 nanometers and the objective comprises a zone plate made from molybdenum (Mo), niobium (Nb), Technetium (Tc), or Ruthenium (Ru).
3. An optical system as claimed in claim 1 , wherein the source is a laser-plasma source.
4. An optical system as claimed in claim 1 , wherein the source is a gas discharge source.
5. An optical system as claimed in claim 1 , wherein the spectral filter is a multilayer notch filter.
6. An optical system as claimed in claim 1 , wherein the condenser is a multilayer coated spherical surface.
7. An optical system as claimed in claim 1 , wherein the detector is a CCD camera.
8. An optical system as claimed in claim 1 , wherein the detector is a CMOS camera.
9. An optical system as claimed in claim 1 , wherein the source uses emission from a copper target.
10. An optical system as claimed in claim 1 , wherein the sample is a lithography mask.
11. An optical system as claimed in claim 1 , wherein the ultraviolet radiation has a wavelength of 13 to 14 nanometers and the objective comprises a zone plate made from molybdenum (Mo).
12. An optical system as claimed in claim 1 , wherein the ultraviolet radiation has a wavelength of 13 to 14 nanometers and the objective comprises a zone plate made from niobium (Nb).
13. An optical system as claimed in claim 1 , wherein the ultraviolet radiation has a wavelength of 13 to 14 nanometers and the objective comprises a zone plate made from technetium (Tc).
14. An optical system as claimed in claim 1 , wherein the ultraviolet radiation has a wavelength of 13 to 14 nanometers and the objective comprises a zone plate made from ruthenium (Ru).
15. An optical system comprising:
an extreme ultraviolet radiation source;
a spectral filter that filters ultraviolet radiation generated by the source;
a reflective condenser that directs the ultraviolet radiation onto a sample at an angle of between normal to the sample and 7 degrees off normal;
an aperture for spatially filtering the ultraviolet radiation;
an objective lens that forms an image of the ultraviolet radiation from the sample; and
a spatially resolved detector for detecting the image of the sample formed by the objective lens; and
wherein a virtual source of the extreme ultraviolet radiation source formed by the condenser and a region of interest of the sample, which is a mask, reside on a Rowland circle determined by the condenser.
16. An optical system comprising:
an extreme ultraviolet radiation source;
a spectral filter that filters ultraviolet radiation generated by the source;
a reflective condenser that directs the ultraviolet radiation onto a sample at an angle of between normal to the sample and 7 degrees off normal;
an aperture for spatially filtering the ultraviolet radiation;
an objective lens that forms an image of the ultraviolet radiation from the sample; and
a spatially resolved detector for detecting the image of the sample formed by the objective lens; and
wherein the objective lens comprises an achromatic Fresnel optic with a silicon refractive lens.
17. An optical system comprising:
an extreme ultraviolet radiation source;
a spectral filter that filters ultraviolet radiation generated by the source;
a reflective condenser that directs the ultraviolet radiation onto a sample at an angle of between normal to the sample and 7 degrees off normal;
an aperture for spatially filtering the ultraviolet radiation;
an objective lens that forms an image of the ultraviolet radiation from the sample; and
a spatially resolved detector for detecting the image of the sample formed by the objective lens; and
wherein the objective lens comprises an achromatic Fresnel optic with a refractive lens made from copper.