Radiation imagery chemistry: process – composition – or product th – Radiation modifying product or process of making – Radiation mask
Reexamination Certificate
2002-05-31
2004-12-21
Rosasco, S. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Radiation modifying product or process of making
Radiation mask
C359S359000
Reexamination Certificate
active
06833223
ABSTRACT:
FIELD
This disclosure pertains to reflective elements (reflective mirrors) that are especially suitable for use in “X-ray” optical systems. By “X-ray” is meant not only the conventional “hard” X-ray wavelengths of the electromagnetic spectrum but also the so-called “soft X-ray” (also termed “extreme ultraviolet” or EUV) wavelengths. More specifically, the disclosure pertains to multilayer-film-coated mirrors that can be used in any of various X-ray optical systems such as X-ray microscopes, X-ray analysis equipment, and X-ray exposure (microlithography) apparatus.
BACKGROUND
As the density of active-circuit elements in microelectronic devices (e.g., integrated circuits, displays, and the like) has continued to increase with corresponding decreases in the size of active-circuit elements in such devices, the resolution limitations of optical microlithography have become apparent. To obtain better resolution of circuit elements, especially such elements having a width of 0.15 micrometer or less, increasing attention has been directed to the development of a practical “next generation” microlithography technology.
A key candidate for next-generation microlithography exploits the short wavelengths of X-ray radiation. For example, EUV radiation is in the wavelength range of 11 to 14 nm, which is substantially shorter than the 157-nm wavelength representing the shortest achievable wavelength used in the deep UV radiation used in conventional optical microlithography. These shorter wavelengths in the X-ray portion of the electromagnetic spectrum offer tantalizing prospects of substantially improved pattern-element resolution (e.g., 70 nm or less) in microlithography. See, e.g., Tichenor et al.,
Transactions SPIE
2437:292 (1995).
The complex refractive index “n” of substances in the wavelength range of X-rays is expressed as n=1−&dgr;−ik (wherein &dgr; and k are complex numbers). The imaginary part k of the refractive index expresses X-ray absorption. Since &dgr; and k are both considerably less than 1, the refractive index in this wavelength range is extremely close to 1. Consequently, optical elements such as conventional lenses cannot be used. Reflective optical elements, on the other hand, are practical and currently are the subject of substantial research and development effort.
From most surfaces, X-rays exhibit useful reflection only at oblique angles of incidence. In other words, the reflectivity of X-rays is extremely low at angles of incidence less than the critical angle &thgr;
c
of total reflection, which is about 20° at a wavelength of 10 nm. Angles greater than &thgr;
c
exhibit total reflection. Hence, many conventional X-ray optical systems are so-called “oblique-incidence” systems in which the X-radiation is incident at angles greater than &thgr;
c
to the reflective surfaces in the optical systems. (The angle of incidence is the angle formed by the propagation axis of an incident beam relative to a line normal to the surface at which the propagation axis is incident.)
It has been found that multilayer-film mirrors exhibit high (albeit not total) reflectivity to X-radiation. The multilayer coating typically comprises several tens to several hundreds of layers. The layers are of materials exhibiting the highest available boundary-amplitude reflectivity. The thickness of each layer is established based on light-interference theory so as to achieve alignment of the phases of light waves reflected from the various layers. Multilayer-film mirrors are formed by alternately laminating, on a suitable substrate, a first substance of which the difference between its refractive index in the X-ray wavelength band to be used and its refractive index (n=1) in a vacuum is relatively large and a second substance of which this difference is relatively small. Conventional materials satisfying these criteria and exhibiting good performance are tungsten/carbon and molybdenum/carbon composites. These layers are usually formed by thin-film-formation techniques such as sputtering, vacuum deposition, CVD, etc.
Since multilayer-film mirrors also are capable of reflecting X-radiation at low angles of incidence (including perpendicularly incident X-radiation), these mirrors can be incorporated into X-ray optical systems exhibiting lower aberrations than exhibited by conventional oblique-incidence X-ray optical systems.
A multilayer-film mirror exhibits a wavelength dependency in which strong reflection of incident X-radiation is observed whenever Bragg's equation is satisfied. Bragg's equation is expressed as 2d sin(&thgr;′)=n&lgr;, wherein d is the period length of the multilayer coating, &thgr;′ is the angle of incidence measured from the incidence plane (i.e., &pgr;/2−&thgr;), and &lgr; is the X-ray wavelength. Under conditions satisfying Bragg's equation, the phases of the reflected waves are aligned with each other, thereby enhancing reflectivity of the surface. For maximal reflectivity, the variables in the equation are selected so that the equation is fulfilled.
Whenever the multilayer coating of an X-ray mirror comprises alternating layers of molybdenum (Mo) and silicon (Si), the mirror exhibits high reflectivity at the long-wavelength side of the L-absorption end of silicon (i.e., at 12.6 nm). Thus, a multilayer-film mirror exhibiting high reflectivity (over 60% at direct incidence, &thgr;=0°) at &lgr;≈13 nm can be prepared with relative ease. As a result, Mo/Si multilayer-film mirrors are the currently most promising mirror configuration for use in reduction/projection microlithography performed using soft X-ray (EUV) radiation. This type of microlithography is termed extreme ultraviolet lithography (EUVL).
Whereas Mo/Si multilayer-film mirrors exhibit high reflectivity, as discussed above, their performance depends upon the wavelength of incident radiation and upon the angle of incidence, as indicated by Bragg's equation. Especially with curved multilayer-film-coated mirror surfaces, the angle of incidence of an X-ray beam differs at various points on the surface of such a mirror used in an illumination-optical system or a projection-optical system of an EUVL system. The difference in incidence angle over the mirror surface can range from several degrees to several tens of degrees. Consequently, whenever a multilayer film is formed with a uniform thickness over the entire surface of the mirror substrate, differences in reflectivity at the mirror surface will be evident as a result of the differences in the angle of incidence.
FIG. 6
is a graph showing a theoretical relationship of reflectivity to the angle of incidence of a multilayer-film mirror having a period length of 69 Å, a lamina count of 50 layer pairs, and an incident-light wavelength of 13.36 nm. The abscissa is angle of incidence and the ordinate is reflectivity. The solid-line curve denotes reflectivity of s-polarized light and the dotted line denotes reflectivity of non-polarized light. The period length is the total thickness of one pair of layers (i.e., in the case of a Mo/Si multilayer coating, one Mo layer with its adjacent Si layer). The ratio of the thickness of a single Mo layer to the period length is denoted &Ggr;; in this example &Ggr; is constant at 0.35. As can be seen from
FIG. 6
, reflectivity changes with the angle of incidence. Reflectivity is nearly 74% at a 0° angle of incidence, and decreases to less than 60% at a 110° angle of incidence. This represents a greater than 10% drop in reflectivity.
A conventional countermeasure to the reflectivity drop noted above involves providing the thickness of the multilayer coating with a distribution that changes over the mirror surface in a manner serving to offset the change in reflectivity. Thus, light of a specified wavelength is reflected with high reflectivity at the various angles of incidence characteristic of various respective points on the reflective surface.
For example,
FIG. 7
is a graph showing the relationship of the period length and of total film thickness
Klarquist & Sparkman, LLP
Nikon Corporation
Rosasco S.
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