Laser interferometer having a sheath for the laser beam

Apparatus for measuring position of a stage movable along mutually-orthogonal first and second axes, the stage having a first reflector substantially normal to the first-axis, comprising:

• a. a first-axis interferometer for directing a first-axis measurement beam along a first beam path parallel to the first-axis toward the first reflector and for producing a measured-distance signal indicative of an optical path length between the first-axis interferometer and the first reflector;
• b. a first-axis follower system having a first-axis follower which follows movement of the stage along the first-axis while remaining out of contact with the stage;
• and
• c. a first-axis sheath enclosing the first-axis measurement beam along a substantial part of the first beam path wherein the sheath defines an interior volume such that an atmosphere outside the sheath is prevented from entering the sheath, the first-axis sheath including a movable sheath portion carried by the follower such that the sheath has an effective length which varies with movement of the stage along the first-axis while remaining out of contact with the stage.

An exposure apparatus for exposing an image of a reticle onto a substrate mounted on an XY-stage which is movable in a predetermined X-Y plane, the apparatus comprising:

• (a) a base assembly for supporting the XY-stage thereon to move in the X-Y plane;
• (b) a frame structure having a rectangular frame assembly located around the base assembly and an X-follower assembly movable in the X direction on the frame assembly so as to maintain a predetermined space from the XY-stage in the X direction when the XY-stage moves in the X direction;
• (c) a Y-drive actuator partially mounted to the X-follower assembly for generating a force to move the XY-stage in the Y direction relative to the X-follower assembly on the base assembly;
• (d) an X-axis interferometer system fixedly located with respect to the base assembly for directing an X-axis measurement beam along an X-axis beam path toward an X-axis mirror mounted on said XY-stage and for producing an X-axis signal indicative of the position of the XY-stage;
• and
• (e) an X-axis sheath enclosing the X-axis measurement beam along a substantial part of the X-axis beam path, wherein the sheath defines an interior volume such that an atmosphere outside the interior volume is prevented from entering the sheath, the X-axis including a movable sheath portion carried by the X-follower assembly such that the X-axis sheath has an effective length which varies with movement of the XY-stage along the X axis while remaining out of contact with the XY-stage.