Suppression filter, radiation collector and radiation source for a lithographic apparatus; method of determining a separation distance between at least two reflective surface levels of a suppression filter
Disclosed is a suppression filter having a profile defining at least two reflective surface levels, each reflected surface level being separated by a separation distance. The separation distance is such that the reflective suppression filter is operable to substantially prevent specular reflection of radiation at a first wavelength and at a second wavelength incident on said reflective suppression filter. Also disclosed is a radiation collector, radiation source and lithographic apparatus comprising such a suppression filter, and to a method of determining a separation distance between at least two reflective surface levels of a suppression filter.
1. A reflective suppression filter of an EUV radiation source, comprising:
a first area and a second area at respective first and second reflective surface levels,
wherein the first area is adjacent the second area at a separation distance determined in a direction substantially perpendicularly to the first area and to the second area, the separation distance being based in part on a correction factor of an angle of incidence (α) of radiation incident on the reflective suppression filter, wherein the correction factor value≠1 and is equivalent to:
2
cos
α
1
+
cos
(
2
α
)
;
wherein the reflective suppression filter is configured to suppress reflection of a first component of a first wavelength and a second component of a second wavelength of the radiation incident on the reflective suppression filter, the first wavelength being different from the second wavelength,
wherein the separation distance is configured to cause substantially destructive interference between the first component reflected off the first area and the first component reflected off the second area and to cause substantially destructive interference between the second component reflected off the first area and the second component reflected off the second area.
2. The reflective suppression filter of claim 1 , wherein the separation distance is made to be close to proportional to a first odd number of half the first wavelength and close to proportional to a second odd number of half the second wavelength.
3. The reflective suppression filter of claim 1 , further comprising:
a third area at the first reflective surface level and a fourth area at the second reflective surface level; and
wherein the third area is adjacent the fourth area and the second area.
4. The reflection suppression filter of claim 1 , wherein:
the first wavelength lies in a first interval between 10.5 micron and 10.7 micron, and
the second wavelength lies in a second interval between 9.3 micron and 9.5 micron.
5. The reflective suppression filter of claim 1 , wherein:
the first reflective surface level and the second reflective surface level are part of a series of successive reflective surface levels, and
each subsequent reflective surface level in the series of successive reflective surface levels defines an at least part of an ellipsoidal profile of the suppression filter.
6. The reflective suppression filter of claim 1 , wherein the separation distance is not uniform across the first reflective surface level and the second reflective surface level respectively.
7. An EUV radiation source operative to generate EUV radiation by irradiating fuel with laser radiation to generate a plasma, the EUV radiation source comprising a radiation collector comprising a reflection suppression filter comprising:
a first area and a second area at respective first and second reflective surface levels,
wherein the first area is adjacent the second area at a separation distance determined in a direction substantially perpendicularly to the first area and to the second area, the separation distance being based in part on a correction factor of an angle of incidence (α) of radiation incident on the reflective suppression filter, wherein the correction factor value≠1 and is equivalent to:
2
cos
α
1
+
cos
(
2
α
)
;
wherein the reflective suppression filter is configured to suppress reflection of a first component of a first wavelength and a second component of a second wavelength of the radiation incident on the reflective suppression filter, the first wavelength being different from the second wavelength,
wherein the separation distance is configured to cause substantially destructive interference between the first component reflected off the first area and the first component reflected off the second area and to cause substantially destructive interference between the second component reflected off the first area and the second component reflected off the second area.