Reticle Inspection Systems and Method

A reticle inspection system, comprising:

• a coherent illumination source configured to illuminate respective portions of an inspection reticle and a reference reticle;
• a first reflective element configured to direct illumination from the illumination source to the inspection reticle;
• a second reflective element configured to direct illumination from the illumination source to the reference reticle;
• an inspection interferometer branch having a first microscope objective system configured to apply a first Fourier transform to a first light beam propagated from the illuminated portion of the inspection reticle;
• a reference interferometer branch having a second microscope objective system configured to apply a second Fourier transform to a second light beam propagated from the illuminated portion of the reference reticle;
• a beam splitter configured to receive and combine the first and second light beams;
• a first interferometer element configured to induce a 180-degree phase shift between the inspection interferometer branch and the reference interferometer branch and to direct the first light beam from the first microscope objective system to the beam splitter;
• a second interferometer element configured to direct the second light beam from the second microscope objective system to the beam splitter;
• a Fourier lens configured to provide an inverse Fourier transform to the combined first and second light beams whereby an image is formed;
• and
• a detector configured to detect the image, the image representing a difference in amplitude and phase distributions of the inspection reticle and the reference reticle,
• wherein the inspection interferometer branch has an optical path between the illumination source and the detector and the reference interferometer branch has an optical path between the illumination source and the detector, and
• wherein an optical path length difference between the inspection interferometer branch and the reference interferometer branch is less than a coherence length of the illumination source.

A lithography system having a reticle inspection system, the reticle inspection system comprising:

• a coherent illumination source configured to illuminate respective portions of an inspection reticle and a reference reticle;
• an inspection interferometer branch having a first microscope objective system configured to apply a first Fourier transform to a first light beam propagated from the illuminated portion of the inspection reticle;
• a reference interferometer branch having a second microscope objective system configured to apply a second Fourier transform to a second light beam propagated from the illuminated portion of the reference reticle;
• a beam splitter configured to receive and combine the first and second light beams;
• an interferometer element in the inspection interferometer branch, the interferometer element configured to induce a 180-degree phase shift between the inspection interferometer branch and the reference interferometer branch and to direct the first light beam from the first microscope objective system to the beam splitter;
• a Fourier lens configured to provide an inverse Fourier transform to the combined first and second light beams whereby an image is formed;
• and
• a detector configured to detect the image, the image representing a difference in amplitude and phase distributions of the inspection reticle and the reference reticle,
• wherein the inspection interferometer branch has an optical path between the illumination source and the detector and the reference interferometer branch has an optical path between the illumination source and the detector, and
• wherein an optical path length difference between the inspection interferometer branch and the reference interferometer branch is less than a coherence length of the illumination source.

A method of inspecting a reticle, comprising:

• illuminating with a coherent illumination source respective portions of a surface of an inspection reticle and a surface of a reference reticle;
• applying a Fourier transform to scattered light from the illuminated portions;
• shifting the phase of the transformed light from one of the inspection reticle and the reference reticle such that a phase difference between the transformed light from the inspection reticle and the transformed light from the reference reticle is 180 degrees;
• combining the transformed light from the illuminated portions;
• applying an inverse Fourier transform to the combined light;
• and
• detecting the combined light at a detector,
• wherein an optical path length difference between a first optical path between the illumination source and the detector via the inspection reticle and a second optical path between the illumination source and the detector via the reference reticle is less than a coherence length of the illumination source;
• and
• wherein the combined light detected by the detector is in the form of an image representing a difference in amplitude distributions of the inspection reticle and the reference reticle.