As the sizes of active circuit elements in microelectronic devices have continued to decrease, with corresponding increases in the density of circuits in these devices (such as integrated circuits), the pattern-resolution limitations of “optical” microlithography have become burdensome. In other words, optical microlithography is limited by the diffraction limitations of light. To achieve finer resolution of projected patterns, various “next generation lithography” (NGL) technologies currently are under active development. A key approach in this regard is microlithography performed using so-called “soft X-rays” having substantially shorter wavelengths (in the range of approximately 11 to 14 nm) than the deep ultraviolet wavelengths (in the range of approximately 150 to 200 nm) currently used in optical microlithography. Because of its use of substantially shorter wavelengths than used in optical microlithography, soft X-ray (SXR) microlithography offers prospects of substantially better resolution than obtainable using optical microlithography. Tichenor, et al., Proceedings SPIE 2437:292, 1995. SXR microlithography also is termed “extreme ultraviolet” (EUV) microlithography, and has been shown to resolve pattern elements having widths of 70 nm and smaller.
At EUV wavelengths, the refractive index of known materials is extremely close to unity (1; one). Consequently, it currently is impossible to fabricate refracting lenses for use with EUV wavelengths. Instead, EUV optical systems must utilize reflective optical elements. The reflective optical elements are generally of two types; glancing-incidence mirrors and multilayer-coated mirrors. Glancing-incidence mirrors achieve total reflection but at very high (glancing) angles of incidence. Multilayer-coated mirrors achieve high reflectivity of incident EUV at small angles of incidence (including normal incidence) by means of multiple surficial layer pairs of alternatingly laminated layers of different respective materials. For example, for use at EUV wavelengths of approximately 13.4 nm, Mo/Si multilayer-coated mirrors are used, in which the multilayer coating is formed of many layers of molybdenum (Mo) and silicon (Si) alternatingly laminated on the polished surface of a mirror substrate. Mo/Si multilayer-coated mirrors can achieve a reflectivity of 67.5% of normally incident EUV light. Mo/Be multilayer-coated mirrors, comprising alternating layers of Mo and Be, have a reflectivity of 70.2% of normally incident EUV light. Montcalm, Proceedings SPIE 3331:42, 1998.
A conventional EUV lithography (EUVL) system primarily comprises an EUV source, an illumination-optical system, a reticle stage, a projection/imaging-optical system (simply termed a projection-optical system), and a substrate stage. The EUV source is a laser-plasma source, a discharge-plasma source, or a synchrotron (undulator). The illumination-optical system comprises a glancing-incidence mirror that obliquely reflects EUV radiation incident to the mirror at a high angle of incidence. The illumination-optical system also includes a multilayer-coated mirror and a filter that transmits only EUV radiation of a specified wavelength. As noted above, since there are no known materials that are transparent to EUV light, the pattern-defining reticle is a reflective reticle, rather than a transmissive reticle used in optical microlithography.
The projection-optical system receives EUV light reflected from the reticle (and carrying an aerial image of the pattern illuminated by the illumination-optical system). The projection-optical system normally comprises multiple multilayer-coated mirrors that direct the “patterned beam” from the reticle to a resist-coated lithographic substrate (e.g., resist-coated semiconductor wafer). The resist coating renders the substrate “sensitive” to exposure by the patterned beam. As the patterned beam is focused by the projection-optical system onto the surface of the sensitive substrate, the pattern is imprinted (and thus “transferred”) to the resist. Since EUV radiation is absorbed and attenuated by the atmosphere, the entire optical path from the EUV source to the substrate is maintained under high vacuum (e.g., 1×10−5 Torr or less).
As noted above, the projection-optical system comprises multiple multilayer-coated mirrors. Since the reflectivity of a multilayer-coated mirror is not 100%, it is desirable that the number of such mirrors in the projection-optical system be as low as possible to minimize loss of EUV light. For example, projection-optical systems comprising four multilayer-coated mirrors (e.g., Jewell et al., U.S. Pat. No. 5,315,629; Jewell, U.S. Pat. No. 5,063,586) and six multilayer-coated mirrors (e.g., Williamson, U.S. Pat. No. 5,815,310) are known.
Unlike a refractive optical system through which a light flux usually travels in one direction, reflective optical systems usually require that the light flux propagate back and forth in the system. Hence, with a reflective optical system, it is difficult to have a large numerical aperture (NA) due to restrictions imposed by making sure that the mirrors do not eclipse the light flux. Current four-mirror optical systems have a NA of no more than about 0.15. Current six-mirror systems have a somewhat larger NA. Any of these systems typically has an even number of mirrors so that the reticle stage and substrate stage can be disposed on opposite ends of the projection-optical system.
In these reflective optical systems, aberrations must be corrected using a limited number of reflective surfaces. Consequently, one or more of the multilayer-coated mirrors has an aspherical reflective surface. Also, these optical systems typically have a ring-shaped optical field to facilitate correction of aberrations only at a specified range of image-height, but which prevents the entire pattern from being exposed in a single “shot.” Hence, to transfer an entire pattern from the reticle to the substrate, exposure is performed while scanning the reticle stage and substrate stage at respective velocities that differ from each other by the “reduction” (demagnification) factor of the projection-optical system.
The types of projection-optical systems described above for use in EUV microlithography systems are so-called “diffraction-limited” optical systems, in which design-specified performance cannot be achieved unless wavefront aberrations are kept sufficiently small. A standard wavefront error (WFE) of λ/14 or less (using Marechal's root mean square (RMS)) of the exposure wavelength (λ) is a generally accepted tolerance for wavefront-aberration in a diffraction-limited optical system (see Born and Wolf, Principles of Optics, 4th edition, Pergamon Press 1970, p. 469). Under this condition the Strehl intensity (ratio of the maximum intensity of the point-spread functions in an optical system that has aberrations to the same parameter in an aberration-free optical system) is 80% or greater. The projection-optical system in an actual EUVL system usually is manufactured to exhibit an even lower wavefront error.
Wavelengths primarily in the range of 11 to 13 nm are used as the exposure wavelength in current EUVL systems that are the subject of intensive research and development effort. The acceptable form error (FE) for individual mirrors for a given wavefront error in an optical system is given by the following equation:FE=WFE/2/m1/2(RMS)  (1)wherein “m” is the number of mirrors constituting the optical system. The divisor of 2 accounts for the fact that both incident light and reflected light are affected by the form error in a reflective optical system. Hence, the allowable form error for each individual mirror in a diffraction-limited optical system is expressed by the following equation for wavelength λ and mirror count m: FE=λ/28/m1/2(RMS)  (2)For example, at λ=13 nm, FE=0.23 nm RMS in a four-mirror optical system, and FE=0.19 nm RMS in a six-mirror optical system. In view of these specifications, each mirror must be polished and coated with extremely high accuracy and precision.
Under current circumstances, even if the surface of an optical element were precision-finished to within tolerances, a problem can arise during mounting of the element in an optical “column” or other structure that holds the elements relative to each other in the optical system. More specifically, whenever an optical element is being held by a holding device, the holding members of the device press against the optical element. The resulting stress in the optical element causes distortion of the surface of the optical element.
In addition, as noted above, multilayer-coated mirrors are not 100% reflective to incident EUV radiation. The non-reflected radiation typically is absorbed by the mirror and converted into heat. Consequently, multilayer-film mirrors are susceptible to thermal fluctuations (notably thermal expansion) during use. As a mirror being held by a conventional holding device undergoes thermal expansion, the force locally applied to the mirror by the holding members of the device changes due to the different coefficients of thermal expansion of the mirror and holding device. The resulting stress on the mirror causes deformation of the mirror. A deformed mirror in the projection-optical system of an EUVL system causes the entire optical system to exhibit increased aberrations, which decreases the accuracy and precision of microlithography being performed using the system.
Recently, a groundbreaking advance was reported in which a sub-nanometer form error on a multilayer mirror was corrected by polishing or etching away, at the location of the form error, one layer at a time from the surface of the multilayer-coating. Yamamoto, 7 th International Conference on Synchrotron Radiation Instrumentation, Berlin, Germany, Aug. 21-25, 2000, POS2-189). Correcting the surface profile of a multilayer-coated mirror by removing part of the multilayer-coating requires use of equipment capable of controlling the amount of multilayer-coating actually removed to extremely high accuracy and precision. In actual practice, it currently is difficult to control, in a precise manner, the amount of multilayer-coating removed, regardless of whether material is removed by wet-etching, polishing, or application of a directed stream of metal or ceramic powder (including “sandblasting”).
The Yamamoto technique is a “global” technique applied to the entire surface of the multilayer coating. A related technique involves “local” removal of one or more layers from a region (zone) of the surface. When correcting wavefront error using a local technique, it is necessary that the edges of zones in which one or more layers have been removed not be abrupt, but rather have a gradation with respect to amount of material actually removed. This is because corrections to the phase of reflected light vary with the amount of multilayer-coating actually removed, and because the degree of phase-correction varies within the surface of the multilayer-coated mirror. Since the amount of surficial processing applied for making these types of corrections is on the order of a few nanometers to several tens of nanometers, the distribution of area over which processing is performed also must be controlled with extremely high accuracy and precision.
Conventionally, during polishing, an optical element is rigidly held using an adhesive. Unfortunately, adhesives tend to soften and/or dissolve when contacted by a polishing fluid, which results in an inaccuracy with which the optical element is being held during polishing. An optical element being held inaccurately or held with inadequate stability during polishing tends to result in poor polishing results. An optical element also conventionally is held rigidly by an adhesive during formation of a multilayer coating on the surface of the optical element, and multilayer-coating procedures typically are performed inside a vacuum chamber at high vacuum. Since adhesives tend to outgas under high vacuum, attainment of the requisite high vacuum can be difficult. In addition, conventional multilayer-coating processes typically are preceded by solvent-washing of the optical element. Contact of the adhesive by the solvent can dissolve some of the solvent and form a solvent deposit on the surface of the optical element. Such a deposit can prevent the multilayer coating from adhering properly to the surface.