| CPC G02B 7/195 (2013.01) [G02B 7/1815 (2013.01); G03F 7/70891 (2013.01)] | 20 Claims |

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1. An apparatus, comprising:
a mirror comprising a cutout;
a temperature-regulating body in the cutout of the mirror; and
a sensor configured to detect a distance between the temperature-regulating body and an inner surface of the cutout,
wherein:
the temperature-regulating body comprises a cavity configured to allow a fluid to flow therethrough to regulate a temperature of the temperature-regulating body;
the temperature-regulating body does not contact the cutout of the mirror; and
the apparatus is a semiconductor lithography projection exposure apparatus.
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