High sensitivity focal sensor for electron beam and high resolution optical lithographic printers

Claim 1

A three slit interferometric focusing device for use in combination with an optical projection system having a desired plane of best focus, said focusing device comprising: A. a substantially coherent source of illumination; B. an optical beamsplitter positioned in the path of the beam of said source of illumination, said beamsplitter operative to produce a first beam in a first direction and a second beam in a second direction; C. a first movable surface coupled for movement to a position of best focus which is at a first distance from said beamsplitter and which is oriented to receive said first beam, said position corresponding to the plane of best focus of said optical projection system; D. means for moving said first surface to said position of best focus; E. a substantially reflective second surface having an opening which is substantially at the center of said second surface, said second surface located a distance from said beamsplitter substantially equal to said first distance and said second surface oriented to receive said second beam; F. an energy detection system capable of measurement at the wavelength of said source of illumination; G. an optically absorbing third surface having an opening which is substantially at the center of said third surface, said third surface placed between said optical beamsplitter and said first movable surface in the path of said first beam so as to substantially absorb a portion of said first beam and so as to enable the remaining portion of said first beam to be received by said third surface; and H. an optically opaque fourth surface with three substantially parallel slits, said fourth surface positioned between said optical beamsplitter and said energy detection system in order to pass a portion of said beam from said source to said detection system.