Apparatus for inspecting defects in a periodic pattern

An apparatus for inspecting defects in a periodic pattern comprising:

• a light source for directing a coherent light beam toward the periodic pattern;
• a Fourier transformer positioned to receive and transform the coherent light beam after it strikes the periodic pattern, said Fourier transformer generating a defect information light component and a periodic pattern information light component, the periodic pattern information light component including zeroth order diffraction light and first and higher order diffraction light;
• filtering means positioned at the backward focal point of said Fourier transformer and having a light blocking area for blocking transmission of first and higher order diffraction light and a circular light transmission area for passing zeroth order diffraction light and the defect information;
• an inverse Fourier transformer positioned to receive and transform light which passes through said circular light transmission area of said filtering means;
• photo-electric converting means for sensing the intensity of the light which passes through said inverse Fourier transformer and generating electrical signals;
• and
• defect distinguishing means connected to said photo-electric converting means for distinguishing the kind of defect in the periodic pattern by determining the amplitude of the electrical signals generated by said photo-electric converting means.

In an apparatus for inspecting defects in a periodic pattern, the combination comprising:

• a light source for directing a coherent light beam toward the periodic pattern;
• a Fourier-transformer positioned to receive and transform the coherent light beam after it passes through the periodic pattern, said Fourier transformer generating a defect information light component and a periodic pattern information light component, the periodic pattern information light component including zeroth order diffraction light and first and higher order diffraction light;
• a beam splitter for splitting the light which passes through said Fourier transformer into a first and second light beam;
• first filtering means positioned to receive the first light beam from said beam splitter, said first filtering means having a light blocking area for blocking transmission of first and higher order diffraction light and a circular light transmission area for passing the zeroth order diffraction light and the defect information light component;
• a first inverse Fourier transformer positioned to receive and transform the light which passes through said circular light transmission area of said first filtering means;
• second filtering means positioned to receive the second light beam from said beam splitter, said second filtering means having a light blocking area for blocking the periodic pattern information light component and a ring-like tight transmission area for passing the defect information light component, said light blocking area of said second filtering means including a spot-like light blocking area which prevents transmission of the zeroth order diffraction light and a peripheral light blocking area disposed around said spot-like light blocking area to prevent transmission of the first and higher order diffraction light;
• and
• a second inverse Fourier transformer positioned to receive and transform the light which passes through said ring-like light transmission area of said second filtering means.