Radiant energy – Irradiation of objects or material – Irradiation of semiconductor devices
Reexamination Certificate
2000-11-16
2003-08-19
Lee, John R. (Department: 2881)
Radiant energy
Irradiation of objects or material
Irradiation of semiconductor devices
Reexamination Certificate
active
06608317
ABSTRACT:
FIELD OF THE INVENTION
This invention pertains to microlithography (transfer of an image of a pattern, defined by a reticle or mask, to a sensitive substrate using an energy beam). Microlithography is a key technique used in the manufacture of microelectronic devices such as integrated circuits, displays, and the like. More specifically, this invention pertains to microlithography performed using a charged particle beam such as an electron beam or ion beam. Even more specifically, the invention pertains to shielding components of the charged-particle-beam (CPB) optical system, as used in CPB microlithography apparatus, against external magnetic fields such as stray magnetic fields generated from any of various sources such as the linear motors that actuate the stage on which the substrate is mounted during exposure.
BACKGROUND OF THE INVENTION
Charged-particle-beam (CPB) microlithography is an attractive candidate technique for achieving better pattern-transfer resolution than obtainable using current optical microlithography. One type of CPB microlithography apparatus, termed a “reduction” transfer apparatus, forms a demagnified image of a highly dense and extremely fine pattern, defined on a reticle, on a sensitive substrate (e.g., resist-coated wafer). In view of the required extremely high accuracy of pattern transfer performed using a reduction CPB microlithography apparatus, external stray magnetic fields must be prevented from affecting the charged particle beam. Examples of such fields include magnetic fields generated by electrical current flowing through peripheral wires or movement of conductive bodies outside the CPB-optical system of the apparatus. A notable source of such fields is the linear motors used to provide the reticle stage and wafer stage with movability in the X- and Y-directions.
Several conventional techniques have been proposed and implemented to prevent the effects of stray magnetic fields. One technique involves lining the walls of a room, in which the CPB microlithography apparatus is located, with a material having high permeability (e.g., “Supermalloy” or Permalloy C™, which are ferromagnetic materials containing about 75% Ni). The lining magnetically shields the interior of the room from external magnetic fields. Unfortunately, with this technique, it is difficult to achieve a high magnetic-shielding ratio (ratio of internal magnetic field to external magnetic field). For example, in a room having dimensions of length 4 m, width 4 m, and height 2 m, lining the walls with Supermalloy that is 2 mm thick provides a magnetic-shielding ratio of only about ⅓. Also, creating a room having magnetically shielded walls in this manner is prohibitively costly in many instances.
A second technique involves covering the exterior of a column containing the CPB-optical system with a high-permeability material. Unfortunately, this technique provides little protection against stray magnetic fields generated by linear motors and the like situated inside the column and used to drive the reticle stage and substrate stage. These linear motors normally are disposed very close to CPB-optical components located inside the column, so stray magnetic fields generated by them can have a significant effect on magnetic fields generated by the CPB-optical components.
SUMMARY OF THE INVENTION
In view of the shortcomings of the prior art as summarized above, an object of the invention is to provide charged-particle-beam (CPB) microlithography apparatus and methods that achieve improved shielding against external magnetic fields, including stray magnetic fields generated by nearby linear motors and the like used to actuate the reticle and substrate stages.
To such ends, and according to a first aspect of the invention, CPB microlithography apparatus are provided. An embodiment of such an apparatus comprises an illumination-optical system, a reticle stage, an imaging-optical system, and a substrate stage. The illumination-optical system is situated on an optical axis and configured to illuminate a region of a reticle, defining a pattern to be transferred to a sensitive substrate, with a charged particle illumination beam. The reticle stage is situated and configured to hold the reticle as the reticle is being illuminated by the illumination-optical system, wherein charged particles of the illumination beam passing through the illuminated region of the reticle constitute a patterned beam. The imaging-optical system is situated on the optical axis and configured to receive the patterned beam and project a demagnified focused image, on a sensitive substrate, of the illuminated region of the reticle. The substrate stage is situated and configured to hold the substrate as the patterned beam is projecting the image onto the substrate. The imaging-optical system further comprises a reticle-side electromagnetic projection lens and a wafer-side electromagnetic projection lens situated axially downstream of the reticle-side projection lens. The wafer-side projection lens includes a first magnetic shield having a first axis-facing surface that is rotationally symmetrical about the axis. The first magnetic shield is situated between a downstream-facing surface of the wafer-side projection lens and the substrate stage.
Hence, the first magnetic shield in this embodiment is provided between the wafer-side projection lens and the substrate stage so as to shield against stray magnetic fields extending upward from the direction of the substrate stage. For example, if the substrate stage is driven using linear motors, stray magnetic fields generated from such motors are blocked by the first magnetic shield. Desirably, the first magnetic shield comprises a ferromagnetic substance. Such a substance can alter the magnetic field produced by the wafer-side projection lens. However, since the first magnetic shield is axially symmetrical, and since the entire lens magnetic field includes the first magnetic shield, lens aberrations produced by the first magnetic field can be cancelled using a deflector associated with the wafer-side projection lens.
In the foregoing apparatus, a space between the wafer-side projection lens and the substrate stage can be provided that defines a zone in which a beam from a grazing-incidence-type substrate-height measurement device propagates to and from the substrate between the wafer-side projection lens and the substrate. In such a configuration, the first magnetic shield of the wafer-side projection lens defines a downstream-facing surface that is situated adjacent to but outside the zone.
The apparatus embodiment also can include a liner tube extending along the optical axis and having a downstream end. This configuration can include a rotationally symmetrical “lower” first vacuum wall having an inside-diameter edge and a peripheral edge. The inside-diameter edge is attached circumferentially to the downstream end of the liner tube and extends radially between the wafer-side projection lens and the substrate stage. The configuration also can include a rotationally symmetrical “lower” second vacuum wall having a cylindrical body portion extending upstream in an axial direction outside the wafer-side projection lens and a downstream edge attached circumferentially to the peripheral edge of the lower first vacuum wall. The first magnetic shield can extend along a downstream-facing surface of the lower first vacuum wall from the peripheral edge toward the optical axis.
The apparatus embodiment also can include a deflector situated radially inwardly of the wafer-side projection lens. In this configuration, a cylindrical deflector shield can extend in an axial direction between the deflector and the wafer-side projection lens, wherein the first magnetic shield has an inside-diameter edge having a diameter that is larger than an inside diameter of the deflector shield. The deflector shield desirably is a ferrite stack.
The first axis-facing surface of the first magnetic shield can be conical in profile. Also, the first magnetic shield can have a multilayer structure i
Klarquist & Sparkman, LLP
Lee John R.
Leybourne James J
Nikon Corporation
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