Radiant energy – Irradiation of objects or material – Irradiation of semiconductor devices
Reexamination Certificate
2003-07-01
2004-11-02
Lee, John R. (Department: 2881)
Radiant energy
Irradiation of objects or material
Irradiation of semiconductor devices
C250S442110, C250S441110
Reexamination Certificate
active
06812472
ABSTRACT:
FIELD
This disclosure pertains to microlithography, which is a process by which a pattern is transferred from a mask or reticle to a lithographic substrate, such as a semiconductor wafer, using an energy beam. More specifically, the disclosure pertains to microlithography performed using a charged particle beam such as an electron beam or ion beam. Even more specifically, this disclosure pertains to robotic manipulators used in conjunction with charged-particle-beam (CPB) microlithography systems for, e.g., moving reticles and substrates into and out of position for exposure.
BACKGROUND
Conventional CPB microlithography systems typically include at least one robotic manipulator used for conveying pattern-defining reticles or masks (termed “reticles” herein) and/or lithographic substrates into and out of position for exposure. Robotic manipulators are highly desirable over manual manipulation of these objects for many reasons, including rapidity and consistency of operation as well as cleanliness, etc. For example, a robotic manipulator usually is used for moving semiconductor wafers, coated with resist, from a wafer cassette to a substrate stage on which the substrates are exposed individually and for moving exposed substrates to cassette that holds exposed wafers. A robotic manipulator also is used for moving reticles from a reticle cassette to a reticle stage, on which the reticles are held individually for exposure, and for moving reticles, after use for exposure, back to the reticle cassette. In a CPB microlithography system, these conveying motions performed by the respective robotic manipulators include motions into and out of vacuum chambers, which involves motions into and out of load-lock chambers, gate valves between chambers, and the like. Typically, the entire motion sequence for reticles and wafers is completely automated to avoid any direct human contact with the substrates and reticles. A typical robotic manipulator includes a moving member, such as an arm, as well as machine components such as ball screws and bearings. For maximal durability, these various components conventionally are made of metal, more specifically magnetic metal.
For maximal “throughput” (i.e., number of substrates that can be processed microlithographically per unit time), motions of reticles and substrates by the robotic manipulators usually occur while lithographic exposures are being performed simultaneously, at least to some degree. However, as a manipulator made of magnetic material is operated so as to cause movement of a part of the manipulator, the manipulator produces a moving “stray” magnetic field. If the moving part of the manipulator is located near an exposure location or near the trajectory of the charged particle beam, the moving magnetic field can cause a significant perturbation of the beam trajectory and/or exposure fidelity, which typically results in a distortion or other imaging fault of the pattern as actually formed on the resist-coated substrate.
Exemplary stray magnetic fields generated in this manner include direct-current (DC) disturbances originating in the magnetic materials of motors built into the manipulators, direct-current/alternating-current (DC/AC) disturbances generated by electrical currents flowing to and from the manipulator, and DC/AC disturbances generated during actuation of the manipulator. Among these various disturbances, a particularly large disturbance is manifest in the magnetic-field fluctuations caused by movement of magnetic materials such as the arm of a robotic manipulator during actuation of the manipulator.
Microlithographic exposures typically occur at or near the optical axis of the CPB microlithography system. In such systems (e.g., electron-beam microlithography systems), a stray magnetic field (e.g., as generated by a motor or other source in a robotic manipulator) has a magnitude that decreases in inverse proportion to the square of the distance between the source of the field and the optical axis. With robotic manipulators having a wide range of arm motion, the effects of stray magnetic fields conventionally are reduced by installing the “main unit” (containing motors and the like) of the manipulator at a maximal distance from the optical axis. However, if the arm itself is made of a magnetic material, then significant magnetic-field fluctuations affecting the beam trajectory tend to occur regardless of the distance between the main unit and the optical axis. These effects arise due to the rather large operational range of the arm and to the arm closely approaching the optical axis at least during some of its motions.
Plots of magnetic-field intensity (B) resulting from representative movements of a conventional robotic manipulator are shown in
FIG. 3
, in which the ordinate is output in volts and the abscissa is time (sec). The plots were generated by periodically moving the robotic manipulator in the vertical (Z) direction while measuring magnetic-field intensity B at a location that is separated by approximately 300 mm from the center position of the manipulator. Sensor output was in volts. In this example the manipulator was made of a magnetic stainless steel. Measurements of the magnetic-field intensity were performed using a 3-axis DC sensor (DC to approximately 5 Hz, resolution 5 &mgr;Gauss). The solid-line plot denotes magnetic-field intensity (B
x
) in the X-direction at the measurement point; the dot-dashed line denotes magnetic-field intensity (B
y
) in the Y-direction at the measurement point; and the dashed line denotes magnetic-field intensity (B
z
) in the Z-direction at the measurement point.
FIG. 3
indicates that vertical (Z-direction) movement of the robot produces a magnetic-field fluctuation of approximately 6.8 mG in the X-direction, approximately 2.0 mG in the Y-direction, and approximately 0.4 mG in the Z-direction. Conventionally, active magnetic-field cancellers have been used for reducing the magnitude of these magnetic-field disturbances. An “active” magnetic-field canceller comprises an energizable component such as a Helmholtz coil or the like in surrounding relationship to a field-vulnerable portion of the microlithography system. The Helmholtz coil is energized in a controllable manner by a coil-energization circuit. The active canceller also includes a Gauss meter situated and configured to measure the magnitude of a potentially disturbing magnetic field in the vicinity of the field-vulnerable portion and to provide data regarding the detected field to the coil-energization circuit in a feedback manner. Thus, the coil-energization circuit supplies electrical current to the Helmholtz coil based on field measurements obtained by the Gauss meter. In response to a detected magnetic field that potentially could disturb the beam, the Helmholtz coil generates a countervailing magnetic field having a magnitude equal to that of the detected field but a direction opposite the direction of the detected field. As a result, the detected field is canceled.
Due to the high intensity and wide distribution of the magnetic field produced by Helmholtz coils and to limitations on the size of the Gauss meter, it is difficult to install the Gauss meter in a confined space (“target space”) in which magnetic-field cancellation is desired and to achieve ideal field cancellation. To solve this problem one or more small correcting coils conventionally are situated in the vicinity of the Gauss meter. The correcting coils are used for correcting deviations of the magnetic field between the actual installation position of the Gauss meter and the target space. Unfortunately, the correcting coils and Helmholtz coils are fixed in position. Use of a robotic manipulator that generates beam-disturbing magnetic fields requires use of multiple corrective-field-generation sources that are operated sequentially. As a result, the magnetic field at the location of the Gauss meter frequently does not correspond with the magnetic field of the target space on a one-to-one basis, resulting in substantial difficulty in canceling the poten
Klarquist & Sparkman, LLP
Lee John R.
Leybourne James J.
Nikon Corporation
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