Optics: measuring and testing – By alignment in lateral direction – With light detector
Reexamination Certificate
2003-03-18
2004-12-14
Smith, Zandra V. (Department: 2877)
Optics: measuring and testing
By alignment in lateral direction
With light detector
C355S030000, C355S053000, C438S007000, C438S010000, C359S845000
Reexamination Certificate
active
06831744
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a mirror device for transferring a fine circuit pattern, a mirror adjustment method, an exposure apparatus, an exposure method, a semiconductor device manufacturing method using the exposure apparatus, and the like.
BACKGROUND OF THE INVENTION
Reduction projection exposure using ultraviolet light has conventionally been performed as a printing (lithography) method of manufacturing a fine semiconductor element such as a semiconductor memory or logic circuit. The minimum size capable of transfer by reduction projection exposure is proportional to the wavelength of light used for transfer and inversely proportional to the numerical aperture of the projection optical system. To transfer a fine circuit pattern, the wavelength of light for use is being shortened. The wavelength of ultraviolet rays for use is becoming shorter to a mercury-vapor lamp i-line (wavelength: 365 nm), a KrF excimer laser beam (wavelength: 248 nm), and an ArF excimer laser beam (wavelength: 193 nm).
However, semiconductor elements rapidly shrink in feature size, and lithography using ultraviolet light reaches its limit in micropatterning of manufacturing a semiconductor element. To efficiently print a very fine circuit pattern with a semiconductor element line width smaller than 0.1 &mgr;m, a reduction projection exposure apparatus using extreme ultraviolet light (to be referred to as “EUV light” hereinafter) whose wavelength is as short as about 10 to 15 nm is being developed.
In the EUV light region, a substance greatly absorbs light. The use of a lens optical system which utilizes light refraction used for visible light or ultraviolet light is not practical. An exposure apparatus using EUV light adopts a reflecting optical system. In this case, a reticle as a master is a reflecting reticle on which a pattern to be transferred is formed by a light absorber.
Reflecting optical elements employed in the exposure apparatus using EUV light include a multilayered mirror and a grazing incidence total reflection mirror. The real part of the refractive index is slightly smaller than 1 in the EUV region, and total reflection occurs in the grazing incidence state in which EUV light is incident almost parallel to the plane. In general, grazing incidence at several degrees from an irradiation plane provides a high reflectance of several ten % or more. However, the degree of freedom in optical design is low, and it is difficult to use the total reflection mirror in the projection optical system.
An EUV light mirror used for an incident angle almost close to normal incidence is a multilayer mirror obtained by alternately stacking two types of substances having different optical constants. Molybdenum and silicon are alternately stacked on the surface of a glass substrate polished into a precise figure. The layer thickness is, e.g., 2 nm for the molybdenum layer and about 5 nm for the silicon layer. The number of stacked layers is about 20 pairs. A value as the sum of the thicknesses of layers of two types of substances will be called a film period. In this example, the film period is the molybdenum layer thickness (2 nm)+silicon layer thickness (5 nm)=film period (7 nm).
When EUV light is incident on the multilayer mirror, EUV light having a specific wavelength is reflected. Letting &thgr; be the incident angle of EUV light on the multilayer mirror, &lgr; be the wavelength of EUV light, and d be the film period, only EUV light with a narrow bandwidth centered on &lgr; which approximately satisfies Bragg's equation (1):
2
×d
×sin &thgr;=&lgr; (1)
is efficiently reflected. At this time, the bandwidth is about 0.6 to 1 nm.
The reflectance of reflected EUV light is about 0.7 at maximum. Non-reflected EUV light is absorbed in the multilayer film or substrate, and most of the energy is converted into heat. The multilayer mirror exhibits a large optical loss in comparison with a visible light mirror, and the number of mirrors must be minimized. To realize a wide exposure region with a small number of mirrors, a reticle and wafer are simultaneously scanned (scanning exposure) to transfer a pattern in a large area by using only a narrow arcuate region (ring field) spaced apart from the optical axis by a predetermined distance.
A reduction projection exposure apparatus using EUV light in scanning exposure is mainly constituted by an EUV light source, an illumination optical system, a reflecting reticle, a projection optical system, a reticle stage, a wafer stage, an alignment optical system, and a vacuum system.
As th EUV light source, e.g., a laser plasma source is used. A target material in a vacuum vessel
130
is irradiated with a high-intensity pulse laser beam to generate a high-temperature plasma, and EUV light which is emitted by the plasma and has a wavelength of, e.g., about 13 nm is utilized. The target material is a metal thin film, inter gas, droplets, or the like, and is supplied into the vacuum vessel by means such as a gas jet. To increase the average intensity of emitted EUV light, the repetition frequency of the pulse laser is preferably high. The pulse laser is generally operated at a repetition frequency of several kHz.
The illumination optical system is comprised of a plurality of multilayer mirrors or grazing incidence mirrors, an optical integrator, and the like. A collection mirror on the first stage collects EUV light almost isotropically emitted from a laser plasma. The optical integrator uniformly illuminates a mask at a predetermined numerical aperture. An aperture for limiting a region illuminated on the reticle plane to an arcuate figure is formed at a position conjugate to the reticle of the illumination optical system.
The projection optical system includes a plurality of mirrors. A smaller number of mirrors which constitute the projection optical system provide a higher EUV light utilization efficiency, but make aberration correction difficult. The number of mirrors necessary for aberration correction is about four to six. The reflecting surface of the mirror has a convex or concave spherical or aspherical figure. The numerical aperture NA in this case is about 0.1 to 0.2 (the NA is restricted by a numerical restriction aperture
117
shown by FIGS.
1
and
13
). The mirror is fabricated by grinding and polishing a substrate made of a material with a high rigidity, high hardness, and small thermal expansion coefficient, such as low-expansion-coefficient glass or silicon carbide, into a predetermined reflecting surface figure, and forming multilayer films of molybdenum and silicon on the reflecting surface. If the incident angle is not constant depending on the position within the mirror surface, the reflectance of a multilayer film with a constant film period increases depending on the position, shifting the wavelength of EUV light, as is apparent from Bragg's equation. To prevent this, the film period distribution must be set such that EUV light having the same wavelength is efficiently reflected within the mirror surface.
The reticle and wafer stages have systems of scanning these stages in synchronism with each other at a velocity ratio proportional to the reduction magnification. The scanning direction within the reticle or wafer plane is the X-axis, an in-plane direction perpendicular to the scanning direction is the Y-axis, and a direction perpendicular to the reticle or wafer plane is the Z-axis.
A reticle is held by a reticle chuck
116
on the reticle stage. The reticle stage has a driving system of moving the reticle stage along the X-axis at a high speed. The reticle stage also has fine moving systems in the X-axis direction, Y-axis direction, Z-axis direction, and rotational directions around these axes, and can align a reticle. The position and posture of the reticle stage are measured by a laser interferometer, and controlled on the basis of the measurement results.
A wafer is held on the wafer stage by a wafer chuck
120
. The wafer stage has a system of moving the wafer stage
Masaki Fumitaro
Miyake Akira
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
Smith Zandra V.
Stock, Jr. Gordon J.
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