Optical apparatus and lithographic apparatus using the optical apparatus

A metrology apparatus comprising:

• a light source;
• and
• an optical apparatus comprising:
• a substrate having an aperture for passing light;
• a transmissive optical element covering the aperture of the substrate;
• and
• an optical contact bond between the substrate and transmissive optical element, the optical contact bond being spaced from the aperture a sufficient distance such that stress forces in the transmissive optical element from the optical contact bond to the aperture are below an acceptable stress threshold,
• wherein the light passing through the transmissive optical element generates an interference pattern, which is used to extract a measurement of a characteristic of an object.

A lithographic apparatus comprising:

• a light source;
• and
• an optical apparatus comprising:
• a substrate having an aperture for passing light;
• a transmissive optical element covering the aperture of the substrate;
• and
• an optical contact bond between the substrate and transmissive optical element, the optical contact bond being spaced from the aperture a sufficient distance such that stress forces in the transmissive optical element from the optical contact bond to the aperture are below an acceptable stress threshold,
• wherein the light passing through the transmissive optical element generates an interference pattern, which is used to extract a measurement of a characteristic associated with a patterning process.