X-ray or gamma ray systems or devices – Specific application – Lithography
Reexamination Certificate
2000-10-06
2003-02-18
Porta, David P. (Department: 2882)
X-ray or gamma ray systems or devices
Specific application
Lithography
C378S035000, C378S084000, C378S085000
Reexamination Certificate
active
06522716
ABSTRACT:
FIELD OF THE INVENTION
This invention pertains to microlithography (projection-transfer of a pattern, defined by a reticle or mask, onto a substrate). Microlithography is a key technology used in the manufacture of microelectronic devices such as integrated circuits, displays, and the like. More specifically, this invention pertains to microlithography performed using so-called “soft X-rays” (SXR), also known as “extreme ultraviolet” (EUV) radiation (these two terms are used interchangeably herein). Even more specifically, the invention pertains to multilayer mirrors that are reflective to soft X-rays, as used in microlithography apparatus employing SXR (EUV) radiation, and to image formation in the SXR (EUV) band.
BACKGROUND OF THE INVENTION
In recent years in response to the ever-increasing miniaturization and densification of microelectronic circuit elements as used in, for example, semiconductor integrated circuits, microlithography apparatus and methods have been developed that employ a soft X-ray beam as an energy beam in order to achieve better resolution of circuit elements. Soft X-ray (SXR) radiation has a wavelength generally within the range of 11 nm to 14 nm, which is significantly shorter than the radiation used to date (up to deep ultraviolet) in optical microlithography. In other words, microlithography technology used to date is compromised by diffraction limits, which prevent obtaining ever increasing resolution (e.g., see Tichenor et al.,
Proc. SPIE
2437:292, 1995).
SXR microlithography (also termed herein “extreme ultraviolet,” or EUV microlithography) offers prospects of attaining better resolution of circuit features than current microlithography technology. Specifically, EUV microlithography is hailed as the “microlithography of the future,” capable of achieving resolutions of about 70 nm and smaller, which cannot be achieved using so-called “optical microlithography” (performed using a wavelength of about 190 nm or more).
With EUV wavelengths, the refractive index of substances is extremely close to unity. As a result, conventional optical elements for achieving refraction and/or reflection of optical wavelengths cannot be used. Instead, grazing-incidence mirrors or multilayer-film mirrors typically are used. Grazing-incidence mirrors achieve total reflection with a refractive index of slightly less than unity, and multilayer-film mirrors achieve a high overall reflectivity by passing weakly reflected light through multiple phase-matched convolutions. For example, a reflectivity of 67.5% can be obtained of a normal incident beam having a wavelength of about 13.4 nm using a reflective mirror comprising a Mo/Si multilayer film, in which molybdenum (Mo) layers and silicon (Si) layers are alternately laminated. A reflectivity of 70.2% can be obtained of a directly incident beam having a wavelength of about 11.3 nm using a reflective mirror comprising a Mo/Be multilayer film, in which Mo layers and beryllium (Be) layers are alternately laminated. E.g., see Montcalm,
Proc. SPIE
3331:42, 1998.
Conventional soft X-ray microlithography apparatus comprise a soft X-ray source, an illumination-optical system, a mask stage, an imaging-optical (projection-optical) system, and a substrate stage. The SXR source can be a laser-plasma source, a discharge-plasma source, or a synchrotron-radiation source. The illumination-optical system comprises grazing-incidence mirrors each having a respective reflective surface that reflects SXR radiation that is obliquely incident to the reflective surface, multilayer-film mirrors each having a reflective surface are formed by a multilayer film, and a filter that transmits only SXR radiation of a specified wavelength. Thus, the mask is illuminated by SXR radiation having a desired wavelength.
Since no known substances are transparent to SXR radiation, the mask is a so-called “reflective mask” rather than a conventional transmission-type mask. The imaging-optical system comprises multiple multilayer-film mirrors, and is configured to form an image, of the irradiated region of the mask, on the substrate (e.g., semiconductor wafer) to which a layer of a suitable resist has been applied. Thus, the image is transferred to the layer of resist. Since SXR radiation is absorbed and attenuated by the atmosphere, the SXR light path in the microlithography apparatus normally is maintained at a certain vacuum (e.g., 1×10
−5
Torr or less).
As noted above, the imaging-optical system comprises multiple multilayer-film mirrors. Since the reflectivity of a multilayer-film mirror is not 100 percent, the imaging-optical system desirably consists of as few such mirrors as possible to minimize light loss. Thus far, imaging-optical systems comprising four multilayer-film mirrors (e.g., Jewell and Thompson, U.S. Pat. No. 5,315,629; and Jewell, U.S. Pat. No. 5,063,586) and six multilayer-film mirrors (e.g., Williamson, U.S. Pat. No. 5,815,310) have been reported. Unlike refractive optical systems through which a light flux propagates in one direction, reflective optical systems are characterized by the doubling back of the light flux on itself within the optical system. Hence, it is difficult to obtain a large numerical aperture (NA) due to restrictions such as avoiding eclipsing the light flux with the mirrors.
Whereas a NA of no more than about 0.15 can be obtained in a four-mirror imaging-optical system, it is possible for a six-mirror optical system to have an even greater NA. Normally, an even number of mirrors is used in the imaging-optical system so that the mask stage and the substrate stage can be situated on opposite sides of the optical system. Since the imaging-optical system must correct aberrations using a limited number of surfaces, each of the mirrors typically has an aspherical profile, and a ring-field imaging scheme is used in which aberrations are corrected only in the proximity of a desired lateral displacement from the optical axis. To transfer the entire mask pattern onto the substrate, exposure is performed while scanning the mask stage and the substrate stage at respective velocities that differ from each other according to the magnification ratio of the imaging-optical system.
Imaging-optical systems, as discussed above, for use in SXR microlithography apparatus are so-called “diffraction-limited” optical systems. These optical systems cannot achieve the performance levels for which they were designed unless wavefront aberrations can be minimized adequately. A guideline for tolerances of wavefront aberration in a diffraction-limited optical system is Marechal's standard, in which the root-mean-square (RMS) departure of the wavefront from a reference sphere that is centered on the diffraction focus does not exceed &lgr;/14, wherein &lgr; is wavelength. Born and Wolf,
Principles of Optics,
7
th
edition, Cambridge University Press, 1999, p. 528. These are the conditions for obtaining 80% or more of the Strehl intensity (the ratio of maximum point-image intensities in an optical system with aberrations to maximum point-image intensities in an optical system with no aberrations).
In the EUV microlithography techniques currently under vigorous research and development, exposure “light” is used having a wavelength primarily in the range of 13 nm to 11 nm. With respect to wavefront error (WFE) in an optical system, the form error (FE) allowed for each individual mirror is given by Equation (1):
FE=
(
WFE
)/2
/{square root over (n)}
(
RMS
value) (1)
In Equation (1), “n” is the number of mirrors that make up the optical system, and WFE is divided by 2 because wavefront error is double the form error. This is because both incident light and reflected light in a reflective optical system are subject to the effects of each respective form error.
The form error (FE) allowed for each individual mirror in a diffraction-limited optical system is given by Equation (2), relative to wavelength &lgr; and number of mirrors n:
FE
=&lgr;/28
/{square root over (n)}
(
RMS
value) (2)
In the case of
Ichihara Yutaka
Murakami Katsuhiko
Barber Therese
Klarquist & Sparkman, LLP
Nikon Corporation
Porta David P.
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