Reticle-focus detector, and charged-particle-beam microlithography apparatus and methods comprising same

In a method for performing projection-transfer of a pattern, defined on a reticle, to a sensitive substrate using a charged particle beam, wherein the reticle is illuminated with a charged-particle illumination beam to produce an imaging beam, and the imaging beam is passed through a projection system to the substrate, a method for detecting a focus condition of the reticle, the method comprising:

• (a) providing a focus-detection beam of light directed at an oblique angle of incidence to a surface of the reticle to produce a reflected beam;
• (b) detecting the reflected beam using a height detector configured to produce a corresponding detection signal from the detected light;
• and
• (c) processing the detection signal to produce data concerning an axial height position of the reticle relative to the projection system.

A charged-particle-beam (CPB) microlithography apparatus, comprising:

• an illumination system situated and configured to illuminate a region of a pattern-defining reticle with a charged-particle illumination beam passing through the illumination system;
• a projection system situated and configured to projection-transfer an image of the illuminated region of the reticle onto a corresponding region of a sensitive substrate using an imaging beam passing through the projection system;
• and
• a reticle-focus-detection device situated and configured to detect an axial height position of the reticle relative to the projection system, the reticle-focus-detection device comprising (i) a focus-detection-beam source situated and configured to produce a focus-detection light beam and to direct the focus-detection beam onto a surface of the reticle such that the focus-detection beam is incident on the reticle at an oblique angle of incidence, and (ii) a height detector situated and configured to detect light, of the focus-detection beam, reflected from the reticle surface and to produce a corresponding focus-detection signal, the height detector comprising a light-receiving surface and being configured to measure a lateral displacement of the focus-detection light beam on the light-receiving surface.

In a method for performing projection-transfer of a pattern, defined on a reticle, to a sensitive substrate using a charged particle beam passing through a charged-particle-beam (CPB) optical system, wherein a region of the reticle is illuminated with a charged-particle illumination beam to produce an imaging beam that forms an image of the illuminated region on the substrate, a method for detecting a focus condition of the reticle, the method comprising:

• (a) providing a reticle segmented into multiple subfields arrayed in a two-dimensional array and separated from one another by support struts;
• (b) mounting the reticle on a reticle stage movable at least in a stage-scanning direction;
• (c) detecting a position of the reticle stage;
• (d) while the reticle stage is at a detected position, providing a focus-detection beam of light directed at an oblique angle of incidence to a surface of the reticle to produce a reflected beam;
• (e) detecting light of the reflected focus-detection beam using a height detector configured to produce a corresponding detection signal from the detected light;
• (f) processing the detection signal to produce data concerning an axial height position of the reticle relative to the CPB optical system;
• and
• (g) if the axial height position of the reticle is outside pre-set tolerance limits, then applying a correction to at least one of the axial height position and the CPB optical system until the axial height position is within the tolerance limits.