This disclosure pertains to microlithography, which is a key technique used in the manufacture of microelectronic devices such as integrated circuits, displays, thin-film magnetic pickup heads, and micromachines. Microlithography generally involves the imaging of a pattern, usually defined by a reticle or mask, onto a surface of a substrate having a layer (termed a xe2x80x9cresistxe2x80x9d) imprintable with the image in a manner similar to photography. More specifically, this disclosure pertains to microlithography performed using a charged particle beam as an energy beam, instead of a beam of ultraviolet light as used currently in optical microlithography. Even more specifically, the disclosure pertains to methods and devices for detecting and compensating for stray floating magnetic fields so as to prevent adverse effects of such fields on the charged particle beam during image transfer.
It is well known that the level of integration of microelectronic devices has continued to increase, accompanied by ever greater miniaturization and density of individual components of the microelectronic devices. The level of integration now has reached a level in which minimum linewidths of pattern features as imaged on the substrate are about 100 nm. It is becoming rapidly impossible at this level of integration and miniaturization to use conventional optical microlithography for pattern transfer because the required resolution exceeds the diffraction limit of optical microlithography. As a result, substantial effort is being made to develop a practical xe2x80x9cnext generationxe2x80x9d microlithography technology.
An important candidate next-generation microlithography technology is step-and-repeat microlithography performed using a charged particle beam such as an electron beam or ion beam. Electron-beam microlithography offers prospects of substantially greater pattern-transfer resolution for reasons similar to reasons why electron microscopy provides substantially better imaging resolution than obtained using optical microscopy.
Whenever a charged-particle-beam (CPB) microlithography apparatus is used for transferring a pattern from a reticle to a sensitive substrate, a commonly encountered problem is disturbance of the trajectory of the charged particle beam by external floating magnetic fields. These disturbances adversely affect pattern-transfer accuracy and resolution. A conventional remedy is to employ three Helmholtz coils (one for each of the x-axis, y-axis, and z-axis directions, each being about 50 cm in diameter, and individually having mutually perpendicular respective axes) positioned about 4 m from the CPB microlithography apparatus. The coils are supplied with respective electrical currents with the objective of causing the respective coils to generate respective countervailing magnetic fields that collectively cancel at least a portion of the floating magnetic field.
Unfortunately, experience has shown that use of Helmholtz coils configured as described above interferes with other peripheral power sources, and requires a substantially larger clean-room facility to house the CPB microlithography apparatus. Also, the efficiency with which floating magnetic fields actually are shielded using this approach (i.e., the xe2x80x9cshielding ratioxe2x80x9d) is limited to approximately {fraction (1/10)}, which is insufficient for attaining satisfactory results. Another problem with this approach is that it is ineffective for canceling stray magnetic fields produced by sources (e.g., linear motors used for actuating the reticle and substrate stages) located between the optical axis of the CPB microlithography apparatus and the Helmholtz coils.
In view of the shortcomings of conventional approaches for reducing floating magnetic fields, as summarized above, the present invention provides, inter alia, charged-particle-beam (CPB) microlithography systems exhibiting improved cancellation of floating magnetic fields with an improved shielding ratio, without having to enlarge the system excessively.
To such ends, a first aspect of the invention is directed to CPB microlithography systems that comprise, on an optical axis, a CPB optical system that includes an illumination-optical system and a projection-optical system. The illumination-optical system illuminates a selected region on a reticle with a charged-particle illumination beam to form a patterned beam carrying an aerial image of the illuminated region on the reticle. The projection-optical system causes the patterned beam to form an actual image of the illuminated region on a surface of a substrate. In the context of such systems, according to the first aspect of the invention, devices are provided for detecting and canceling magnetic fields external to the CPB optical system. An embodiment of such a device comprises a magnetic-field sensor situated and configured to detect a magnetic field external to the CPB optical system. The embodiment also comprises a magnetic-field-compensation coil situated between the illumination-optical system and the projection-optical system or between the projection-optical system and the wafer stage. The embodiment also includes a magnetic-field-compensation circuit connected to the magnetic-field-compensation coil and configured to adjust an electrical current delivered to the magnetic-field-compensation coil so as to cause the magnetic-field-compensation coil to produce a magnetic field that cancels at least a portion of the external magnetic field detected by the magnetic-field sensor.
Thus, external magnetic fields (including magnetic fields of a fluctuating or floating nature, are detected using a magnetic-field sensor that is provided within the CPB optical system and thus within the CPB microlithography system. At least one electrical current is delivered to the magnetic-field-compensation coil in a manner that causes the magnetic-field-compensation coil to produce a magnetic field that cancels at least a portion of the external magnetic fields. Hence, the external magnetic fields are locally canceled using relatively small coils, compared to conventional field-canceling devices. Consequently, large-scale equipment is not needed for canceling the external fields. The effect of the magnetic-field-compensation coil desirably is combined with the shielding effects of any ferromagnetic shielding enveloping the CPB optical system.
Using a device according to this embodiment, it is possible to achieve a shielding ratio of better than {fraction (1/30)} against floating external magnetic fields. The external magnetic fields that are effectively canceled principally include external magnetic fields generated by equipment other than the CPB microlithography system.
The reticle typically is mounted on a reticle stage situated between the illumination-optical system and the projection-optical system. In such a system configuration, the magnetic-field sensor and the magnetic-field-compensation coil desirably are situated between the illumination-optical system and the reticle stage. Generally, the axial space between the illumination-optical system and the reticle is wider than the respective space between the reticle and the projection-optical system. Hence, placing the magnetic-field sensor and magnetic-field-compensation coil between the illumination-optical system and the reticle stage provides efficient utilization of space in the CPB microlithography system.
Further desirably, each of the magnetic-field sensor and magnetic-field-compensation coil comprises a respective set of three coils, one coil for each of an x-axis direction, a y-axis direction, and a z-axis direction, respectively, of a Cartesian coordinate system, wherein the optical axis is parallel to the z-axis direction. The coils of the magnetic-field sensor detect the external magnetic field in the x-axis, y-axis, and z-axis directions, respectively. The coils of the magnetic-field-compensation coil generate respective magnetic fields in the x-axis, y-axis, and z-axis directions, respectively. Thus, external magnetic fields oriented in any direction in three-dimensional space can be detected independently and independently canceled.
The coils of the magnetic-field sensor desirably comprise three coils. In such a configuration, one of the coils is a z-axis coil that is wound about the z-axis and configured to detect a magnetic field in the z-axis direction. A second coil is an x-axis coil wound about an axis parallel to the y-axis and configured to detect a magnetic field in the x-axis direction. The third coil is a y-axis coil wound about an axis parallel to the x-axis and configured to detect a magnetic field in the y-axis direction.
Similarly, the coils of the magnetic-field-compensation coil desirably comprise three coils. In such a configuration, one of the coils is a z-axis coil wound about the z-axis and configured to generate a magnetic field oriented in the z-axis direction. Another of the coils is an x-axis coil wound about an axis parallel to the y-axis and configured to generate a magnetic field oriented in the x-axis direction, and yet another of the coils is a y-axis coil wound about an axis parallel to the x-axis and configured to generate a magnetic field in the y-axis direction.
The magnetic-field sensor desirably is displaced from the optical axis farther than the magnetic-field-compensation coil. By placing the magnetic-field sensor at a position laterally separated from the z axis, external magnetic fields can be detected accurately without detection being influenced by internal magnetic fields. Also, by placing the magnetic-field-compensation coil nearer the z-axis than the magnetic-field sensor, it is possible to effectively compensate for magnetic fields in the vicinity of the optical axis.
The device can be configured such that the magnetic-field sensor comprises a coil configured to serve as both a magnetic-field sensor coil and a magnetic-field-compensation coil. In this configuration, the magnetic-field sensor and the magnetic-field-compensation coil are combined to provide a commensurate reduction in the number of components of the CPB optical system, allowing the system to be made more compact. With such a configuration, rather than regulating the electrical current flowing to the compensation coil in a manner that reduces the magnetic field detected by the coil becomes substantially to zero magnitude, the specific multiple ratio of the detected magnetic field relative to the generated magnetic field top the generated magnetic field can be experimentally determined. The compensating electrical current has a magnitude established according to the ratio. This allows, for the first time, the effects of external magnetic fields to be completely canceled.
In addition, by using a coil a magnetic sensor to measure floating magnetic fields between exposures and using a coil for generating a compensating magnetic field during exposures, passing an electric current through the coil negates the measured floating magnetic field.
According to another aspect of the invention, methods are provided, in the context of CPB microlithography methods, for detecting and canceling magnetic fields external to a CPB optical system. In an embodiment of such a method, a magnetic field external to the CPB optical system is detected. A magnetic-field-compensation coil is placed relative to the CPB optical system. Based on the detected external magnetic field, electric current is supplied to the magnetic-field-compensation coil to produce a corresponding magnetic field that cancels at least a portion of the detected external magnetic field.
The method can further include the step of determining a ratio of the detected external magnetic field and the electric current supplied to the magnetic-field-compensation coil in advance. The electric current supplied to the magnetic-field-compensation coil is determined based on the detected external magnetic field and the ratio.
The foregoing and additional features and advantages of the invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.