Automatic photomask and reticle inspection method and apparatus including improved defect detector and alignment sub-systems

An Apparatus for inspecting objects such as photomasks and the like for defects, comprising:

• means for inspecting a selected surface area of an object and for generating a first stream of data having signal values representing the image content of each pixel thereof;
• means for generating a second stream of data having signal values representing the image content of each pixel of said second stream of data;
• memory means for storing corresponding portions of said first and second streams of data;
• detector means for detecting with resolution to a fraction of a pixel any misalignment between the stored first and second portions of data;
• alignment means using subpixel interpolation to correct any detected misalignment in the stored first and second portions of data;
• and
• detector means for comparing corresponding subportions of the aligned first and second portions of data to detect any difference therebetween, and upon detecting such difference, for indicating the presence of a defect at a particular pixel location on the inspected object.

A method of detecting defects in objects such as photomasks, reticles, wafers, printed circuit boards, and the like, comprising the steps of:

• inspecting a selected surface area of an object and generating a first stream of data having signal values representing the image content of each pixel thereof;
• generating a second stream of data having signal values representing the image content of each pixel of said second stream of data;
• storing corresponding portions of the first and second streams of data in memory;
• detecting with a resolution to an integer pixel any misalignment between the stored portions of said first and second streams of data;
• aligning the stored first and second portions of data to the closest integer pixel to correct any detected misalignment therebetween;
• and
• comparing corresponding subportions of the stored and aligned first and second portions of data to detect differences therebetween by correlating data corresponding to a particular pxp array of pixel data from said aligned first portion of data to a plurality of p.times.p arrays of shifted data derived from a corresponding p.times.p array of pixel data from said aligned second portion of data, each said shifted pxp array being shifted a different subpixel increment relative to said corresponding p.times.p array and developing corresponding correlation values, and then comparing said correlation values to a predetermined threshold to detect the presence of a defect.

A method of detecting defects in objects such as photomasks, reticles, wafers, printed circuit boards, and the like, comprising the steps of:

• using a first electro-optical system to inspect a first selected surface area of an object and to generate a first stream of data having signals values representing the image content of each pixel thereof;
• using a second electro-optical system to inspect a second surface area of said object and to generate a second stream of data having signal values representing the image content of each pixel of said second stream of data;
• storing corresponding portions of the first and second streams of data in memory;
• detecting with a resolution to a fraction of a pixel any misalignment between the stored portions of said first and second streams of data;
• aligning the stored first and second portions of data using subpixel interpolation to correct any detected misalignment therebetween;
• and
• comparing corresponding subportions of the stored and aligned first and second portions of data to detect differences therebetween by correlating data corresponding to a particular p.times.p array of pixel data from said aligned first portion of data to a plurality of p.times.p arrays of shifted data derived from a corresponding p.times.p array of pixel data from said aligned second portion of data, each said shifted p.times.p array being shifted a different subpixel increment relative to said corresponding p.times.p array and developing corresponding correlation values, and then comparing said correlation values to a predetermined threshold to detect the presence of a defect.