X-ray or gamma ray systems or devices – Specific application – Lithography
Reexamination Certificate
2000-08-09
2001-12-25
Kim, Robert H. (Department: 2882)
X-ray or gamma ray systems or devices
Specific application
Lithography
C378S034000
Reexamination Certificate
active
06333961
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to microlithography (transfer of a pattern, defined by a reticle or mask, to a sensitive substrate). Microlithography is a key technology used in the manufacture of semiconductor integrated circuits, displays, and the like. More specifically, the invention pertains to reflection masks, to microlithography apparatus employing such masks, and to methods for manufacturing integrated circuits and the like using such masks and microlithography apparatus.
BACKGROUND OF THE INVENTION
In recent years the progressive miniaturization of active elements in semiconductor integrated circuits has generated a critical need for microlithography technology that can achieve correspondingly finer resolution. This need has led to the development of projection microlithography in which, instead of using ultraviolet light as an energy beam, even shorter wavelengths are used such as soft X-rays having a wavelength in the range of approximately 10 to 15 nm. This new type of microlithography also is termed “EUV” (extreme ultraviolet) microlithography.
In the EUV microlithography wavelength range, the refractive indices of materials tend to be very close to 1. As a result, conventional refractive and reflective optical elements cannot be used. Rather, grazing incidence optical components or multilayer mirrors typically are used. A grazing incidence mirror exploits total reflection resulting from its refractive index being slightly less than 1, and a multilayer mirror exploits a multilayer film (“multilayer”) that superimposes and phase-aligns weakly reflected light to produce a net high reflectance of the light.
A conventional EUV microlithography apparatus mainly comprises an X-ray source, an illumination-optical system, a mask, an imaging-optical system, a mask stage, and a wafer (substrate) stage. The apparatus “transfers” an image of a circuit pattern, as defined on the mask, to the wafer. So as to be imprinted with the image, the wafer is coated with an appropriate resist. The image is transferred to (projected onto) the resist by the imaging-optical system. The imaging-optical system typically comprises multiple multilayer mirrors.
The mask typically is a reflection-type mask as disclosed in, for example, Murakami,
Hyomen Gijutsu
(
Surface Technology
) 49:849, 1998; and Murakami et al.,
Jpn. J. Appl. Phys.
34:6696-6700, 1995. In such a mask, an absorber layer (comprising a substance highly absorptive to soft X-ray radiation) is formed, in a prescribed circuit pattern, on or in a multilayer that reflects soft X-rays.
As noted above, an EUV microlithography optical system typically comprises multiple multilayer mirrors and grazing-incidence mirrors. Thus, a soft X-ray beam is reflected multiple times as the beam passes through the optical system. Unfortunately, as the number of multilayer mirrors in the optical system increases, the full-width at half maximum (FWHM) of the reflectance spectrum of EUV light passing through the optical system correspondingly decreases.
Whenever there is a significant difference between the center wavelength of EUV light passing (by reflection) through an optical system consisting of multiple multilayer mirrors and the center wavelength of the EUV light reflected from the multilayer of the reflection mask, a decrease is observed in the combined reflectance of the optical system and the mask. As a result, the quantity of EUV light passing through the optical system and actually reaching the wafer is decreased undesirably.
If, over the plane of the reflection mask, there is a non-uniformity of the thickness period of the multilayer, then the reflectance of the reflection mask at the wavelength used in the microlithography apparatus will vary correspondingly according to position on the mask. This reflectance non-uniformity of the reflection mask is manifest as a non-uniformity in illumination of the wafer (located at an optically conjugate position relative to the reflection mask). As a result, exposure undesirably will vary at different locations on the wafer.
Also, whenever the exposure (i.e., total amount of light energy projected onto the resist on the wafer) exceeds a certain desired range, the linewidth of the circuit pattern transferred onto the wafer exhibits an excessive change that tends to degrade resolution.
SUMMARY OF THE INVENTION
In view of the shortcomings of conventional systems as summarized above, an object of the invention is to provide apparatus and methods that perform microlithographic exposures in which the linewidth of the circuit pattern transferred onto the wafer is substantially unaffected by reflectance non-uniformities of the reflection mask.
To such end, and according to a first aspect of the invention, reflection masks are provided for use especially in microlithography using soft X-rays (i.e., extreme ultraviolet or “EUV” microlithography). A representative embodiment of such a reflection mask comprises a multilayer mirror and an absorptive layer. The multilayer mirror reflects incident electromagnetic radiation (e.g., soft X-rays of a prescribed wavelength). The absorptive layer, superposed on the multilayer mirror, defines elements of a pattern defined by the mask. Through the thickness dimension of the multilayer mirror, the laminations have a thickness period that varies with distance through the thickness dimension.
With such a reflection mask, the full-width at half-maximum (FWHM) (in a reflectance spectrum of the electromagnetic radiation from the multilayer mirror) is larger than in conventional reflection masks. As a result, reflectance of the electromagnetic radiation from respective positions on the multilayer mirror exhibits less change with changes in respective center wavelengths of reflected radiation than in conventional reflection masks. This, in turn, produces less change in wafer illumination over the pattern as transferred to the wafer. (As used herein, a “center wavelength” is a wavelength at which a reflective surface exhibits maximum reflectivity.)
In another representative embodiment of a reflection mask according to the invention, the multilayer mirror is formed by laminating, in a first “block,” multiple layers having a first thickness period and, in a second “block” superposed on the first block, multiple layers having a second thickness period different from the first thickness period. A “block” in this context is a group of superposed laminated individual layers.
More generally, the multilayer mirror can be formed of multiple (two or more) blocks each having a respective thickness period. With such a configuration, the FWHM in the reflectance spectrum of the multilayer mirror is larger than in conventional reflection masks. Such a configuration is especially effective whenever differences in the distribution of thicknesses of layers comprising the multilayer mirror vary substantially, such as resulting from the formation of the multilayer mirror.
Desirably, for use in reflecting soft X-rays, each block comprises alternating layers of molybdenum and silicon.
In another representative embodiment, the reflection mask comprises a multilayer mirror is formed by laminating multiple layers superposedly such that the multilayer mirror has a thickness period that progressively varies with distance through the thickness dimension of the multilayer mirror. Again, for reflecting X-rays, the multilayer mirror desirably comprises alternating layers of molybdenum and silicon. This configuration is especially useful whenever differences in the distribution of layer thicknesses in the multilayer mirror are relatively small.
According to another aspect of the invention, methods (to be used in microlithography) are provided for reducing adverse effects on the linewidth of the pattern, as transferred to the substrate, caused by a non-uniformity of reflection of illumination light from the reflection mask. In a first step of a representative embodiment of such a method, a reflection mask according to any of the embodiments summarized above is provided. The reflection mask is illum
Kiknadze Irakli
Kim Robert H.
Klarquist & Sparkman, LLP
Nikon Corporation
LandOfFree
Reflection masks, microlithography apparatus using same, and... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Reflection masks, microlithography apparatus using same, and..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Reflection masks, microlithography apparatus using same, and... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2588450