Blazed grating based reflective spectral purity filter for euv imaging systems

An imaging system comprising:

• an extreme ultraviolet (EUV) light source configured to generate a beam of EUV light containing non-negligible out-of-band (OOB) light;
• a reflective spectral purity filter (SPF) for EUV light comprising:
• a substrate including a reflective surface having an angle of incidence of at least 60 degrees;
• and
• a blazed diffraction grating formed on the substrate, the blazed diffraction grating having a predetermined grating pitch and blaze angle configured to simultaneously diffract incident EUV light into a higher-order diffraction angle and confine the non-negligible OOB light to a 0th order reflection angle to angularly separate the EUV light and the non-negligible OOB light;
• a stage configured to hold a substrate to be inspected using the spatially filtered EUV light;
• an imaging system including collection optics having a field of view (FOV) configured to collect the spatially filtered EUV light reflected from the substrate;
• one or more detectors configured to detect the spatially filtered EUV light to inspect the substrate;
• and
• a system controller including one or more processors.

A method for imaging a substrate, the method comprising:

• generating, by an EUV light source, a beam of EUV light containing non-negligible out-of-band (OOB) light;
• spatially filtering, by a reflective spectral purity filter (SPF) for EUV light comprising a substrate including a reflective surface having an angle of incidence of at least 60 degrees and a blazed diffraction grating formed on the substrate configured to simultaneously diffract incident EUV light into a higher-order diffraction angle and confine the non-negligible OOB light to a 0th order reflection angle to angularly separate the EUV light and the non-negligible OOB light, the EUV light;
• directing, by the reflective SPF, the spatially filtered EUV light toward a substrate held on a stage;
• collecting, by an imaging system having a field of view (FOV), the spatially filtered EUV light reflected from the substrate;
• and
• detecting, by at least one detector, the collected spatially filtered EUV light.