Projection-microlithography apparatus, masks, and related methods incorporating reticle-distortion measurement and correction

A reticle for exposing a pattern onto a sensitized substrate using a charged particle beam, the reticle comprising:

• (a) a thin film defining a mask pattern divided into multiple mask subfields each comprising a membrane defining a respective portion of the mask pattern through which a charged particle beam may pass;
• (b) a respective boundary region situated between adjacent reticle membranes;
• (c) a support grid, having a thin-film-facing portion and an opposing surface, of intersecting struts extending along respective boundary regions so as to support the thin film with the membranes extending between respective adjacent struts;
• and
• (d) an alignment mark, situated on or in the support grid, for measuring and correcting distortion of the reticle before exposing a mask subfield to the charged particle beam.

A charged-particle-beam projection-microlithography apparatus, comprising:

• (a) a beam source that emits a charged particle beam along an optical axis;
• (b) an aperture located on the optical axis downstream of the beam source, the aperture limiting a transverse area of the charged particle beam passing through the aperture;
• (c) a reticle stage situated downstream of the aperture, the reticle stage being operable to hold a segmented reticle comprising a pattern-defining thin film and a support grid, the reticle defining an alignment mark;
• (d) a first deflector for selectively deflecting the charged particle beam, passing through the aperture, to a selected mask subfield on the reticle mounted to the reticle stage so as to irradiate the selected mask subfield;
• (e) a first detector situated relative to the reticle so as to detect a position of the alignment mark, the first detector producing data corresponding to the position of the alignment mark indicative of a possible distortion of the reticle;
• (f) a controller to which the first detector is operably connected;
• (g) a projection system located downstream of the reticle for refracting the charged-particle beam passing through the reticle so as to form an image of the irradiated mask subfield on a corresponding location on a surface of a sensitized substrate;
• and
• (h) a correction-optical system situated between the reticle and the substrate and operably connected to the controller, the correction-optical system being operable to selectively alter, as commanded by the controller, a parameter of the charged particle beam so as to correct the reticle distortion based on data produced by the first detector and processed by the controller.

A method for correcting an error in an image of a portion of a reticle and for projecting a corrected image of the reticle portion onto a sensitized surface of a substrate using a charged particle beam, the method comprising the steps:

• (a) providing a reticle comprising a thin film defining a pattern segmented into multiple mask subfields, each mask subfield comprising a membrane defining a respective portion of the mask pattern through which a charged particle beam may pass, the reticle membranes being separated from one another by boundary regions, the reticle further comprising a support grid of intersecting struts coextensive with the boundary regions and alignment marks individually situated on or in either a downstream- or upstream-facing surface of the support grid at respective prescribed locations around a respective reticle membrane;
• (b) before exposing a selected mask subfield to the charged particle beam to projection-expose the pattern portion defined by the respective reticle membrane onto the substrate, (i) determining an initial position of the respective reticle membrane at a first moment in time;
• (ii) storing data, corresponding to the determined initial position, in a memory;
• and (iii) determining an actual position of the respective reticle membrane by detecting, at a second moment in time after the first moment, positions of the alignment marks situated around the respective reticle membrane, thereby producing actual-position data for the reticle membrane;
• (c) recalling the initial-position data and comparing the actual-position data with the recalled initial-position data to obtain data concerning a type and amount of correction required to make the actual position coincide with the initial position, the correction being to at least one of scaling, rotation, and astigmatism errors resulting from distortion of the reticle arising between the first and second moments in time;
• (d) based on the data determined in step (c), altering a characteristic of the charged particle beam to correct the at least one of scaling, rotation, and astigmatism errors by the determined correction amount;
• and
• (e) exposing the selected mask subfield using the corrected charged particle beam to form an image of the respective reticle membrane on the substrate.

In a projection-exposure method in which a charged particle beam is used to project a reticle pattern onto a sensitive substrate, the reticle pattern being defined by a segmented mask comprising multiple mask subfields separated from one another by struts of a support grid, an improvement comprising:

• (a) providing multiple distortion-measurement marks on the reticle;
• (b) placing the reticle on a reticle stage that holds the reticle for projection-exposure of the mask pattern;
• (c) directing an energy beam at the distortion-measurement marks on the reticle to allow the energy beam to interact with the individual distortion-measurement marks;
• (d) detecting a component of the energy beam propagating from the individual distortion-measurement marks as a result of interaction of the energy beam with the respective distortion-measurement marks so as to measure an actual position of the respective distortion measurement marks;
• (e) comparing the actual positions with ideal positions of the distortion-measurement marks;
• and
• (f) calculating a reticle distortion from the comparisons made in step (e).

In a method for projection-exposing a substrate with a mask pattern using a charged-particle-beam projection-exposure apparatus, a reticle-distortion measuring method, comprising:

• (a) directing the charged particle beam in sequence at multiple distortion-measurement marks on a reticle mounted on a reticle stage of the projection-exposure apparatus;
• (b) detecting particles from the charged particle beam reflected from the distortion-measurement marks;
• (c) from data produced in step (b), measuring the respective actual positions of the distortion-measurement marks and comparing data on the respective actual positions with corresponding data on respective ideal positions of the distortion-measurement marks;
• and
• (d) from the comparison performed in step (c), calculating a distortion of the reticle.

A reticle-distortion-measurement method used in a charged-particle-beam projection-exposure apparatus that projects a pattern defined by a reticle onto a sensitive substrate using a charged particle beam, the method comprising:

• (a) directing a charged particle beam as used for projection-exposure of the reticle at a plurality of distortion-measurement marks situated on a reticle mounted on a reticle stage of the projection-exposure apparatus, the distortion-measurement marks being permeable to the charged particle beam such that a portion of the charged particle beam passes through the distortion-measurement marks and strikes corresponding reference marks situated on a substrate stage on which the substrate is mounted during projection-exposure;
• (b) detecting particles of the charged particle beam propagating from the reference marks due to interaction of charged particles from the distortion-measurement marks with the reference marks, and, from such detections, measuring actual positions of each of the distortion-measurement marks;
• and
• (c) from data concerning the actual positions obtained in step (b), and from data concerning ideal positions of the distortion-measurement marks with which the data obtained in step (b) is compared, calculating reticle distortion.