Photocopying – Projection printing and copying cameras – Distortion introducing or rectifying
Reexamination Certificate
2002-01-16
2004-07-13
Mathews, Alan (Department: 2851)
Photocopying
Projection printing and copying cameras
Distortion introducing or rectifying
C355S053000, C355S067000
Reexamination Certificate
active
06762824
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to exposure apparatuses, and more particularly to illumination and exposure apparatuses used to expose an object to be exposed, such as a single crystal substrate for a semiconductor wafer, and a glass substrate for a liquid crystal display (LCD), a device fabricating method using the exposed object, and a device fabricated from the exposed object. The present invention is suitably applicable, for example, to step-and-scan, scanning, or step-and-repeat exposure apparatuses which expose a single crystal substrate for a semiconductor wafer in a photolithography process.
The “step-and-scan” manner, as used herein, is one mode of exposure method which exposes a mask pattern onto a wafer by continuously scanning the wafer relative to the mask or reticle (hereinafter, these terms are used interchangeably in this application), and by moving, after a shot of exposure, the wafer stepwise to the next exposure area to be shot. The “scanning” manner is another mode of exposure method which uses a projection optical system to project part of a mask pattern onto a wafer, and exposes the entire mask pattern to the wafer by synchronously scanning the mask and wafer relative to the projection optical system. The “step-and-repeat” manner is still another mode of exposure method which moves a wafer stepwise to an exposure area for the next shot every shot of cell projection onto the wafer.
Along with the recent demand on smaller and thinner profile electronic devices, minute semiconductor devices to be mounted onto these electronic devices have been increasingly demanded. For example, a design rule for a mask pattern tries to achieve a line and space (L&S) of 130 nm on a mass production line, and predictably it will be increasingly smaller in the future. L&S denotes an image projected to a wafer in exposure with equal line and space widths, and serves as an index of exposure resolution. The exposure has three important factors—resolution, overlay accuracy, and throughput. The resolution is the minimum size for a precise pattern transfer. The overlay accuracy is a precision with which to overlay multiple patterns over an object to be exposed. The throughput is the number of sheets exposed per unit of time.
Basically, there are two exposure methods—a full-size transfer method and a projection method. The full-size transfer includes a contact method that brings a mask into close contact with an object to be exposed, and a proximity method that slightly spaces them from each other. Although the contact method may provide the high resolution, dusts and silicon fragments enters under the mask in a compressed state, causing the mask to be damaged as well as the exposed object to be flawed and defective. The proximity method ameliorates such problems, but still possibly damages the mask if a distance between the mask and the object to be exposed becomes shorter than the maximum size of a dust particle.
Accordingly, a projection method has been suggested which farther spaces the mask from the object to be exposed. Among projection modes, a scanning projection exposure apparatus has been the recent trend for the improved resolution and expanded exposure area, which exposes the entire mask pattern onto the wafer by projecting part of a mask, onto the wafer and synchronizingly scanning the mask and wafer with each other, continuously or intermittently.
In general, a projection exposure apparatus includes an illumination optical system that uses a beam or ray emitted from a light source to illuminate a mask, and a projection optical system that is located between the mask and the object to be exposed. In order to form a uniform illumination area, the illumination optical system introduces the beam from the light source to a light integrator, such as a fly-eye lens, which includes a plurality of rod lenses, and uses a condenser lens to Kohler-illuminate the mask surface with a plane of exit side of the light integrator as a secondary light source plane.
The following equation gives the resolution R of the projection exposure apparatus using a light-source wavelength &lgr; and the numerical aperture of apertures (NA) of the projection optical system:
R
=
k
1
×
λ
NA
(
1
)
Therefore, the shorter the wavelength becomes and the higher the NA increases, the better the resolution becomes.
In the meantime, a focusing range that maintains desired image-forming performance is called a depth of focus (DOF), and the DOF is given in the following equation:
DOF
=
k
2
×
λ
NA
2
(
2
)
Therefore, the shorter the wavelength becomes and the higher the NA increases, the smaller the DOF becomes. The small DOF would make difficult the focus adjustment, as well as require the higher flatness for a substrate and the more precise focusing accuracy, and thus the large DOF is basically desirable.
It can be understood from these equations 1 and 2 that the shortened wavelength is more effective than the increased NA. Therefore, in recent years, a light source is in transition from the conventional ultrahigh pressure mercury lamp to short-wavelength KrF excimer laser (with a wavelength of approximately 248 nm) and ArF excimer laser (with a wavelength of approximately 193 nm). As the number of optical members that transmit light increases, the shortened wavelength of exposure light lowers the transmittance (and thus the luminous intensity at an image plane and throughput due to decreased power), and therefore, recent projection exposure apparatuses attempt to use a smaller number of optical members for high transmittance. In addition, when a comparatively small ultrahigh pressure mercury lamp is used as a light source, a projection exposure apparatus usually can mount the light source in the apparatus body. On the contrary, when a comparatively bulk excimer laser is used as a light source, the projection exposure apparatus cannot mount the light source in the apparatus body, and the light source has to be separated from the apparatus body (for example, such that the apparatus body is located on a second floor and the light source unit is located on the first floor).
Such a conventional configuration that separates the apparatus body from the light source unit has a problem in that optical axes shift due to different installation environments between the apparatus and the light source unit, lowering the resolution for the exposure. For example, when these apparatus and unit are located on different floors, the different installation environments (such as vibration frequency and phase on each floor) would easily result in discordant optical axes and the fluctuant and uneven luminous intensity at the time of exposure, lowering the resolution. This is because the beam incident upon the illumination optical system that has a shifted optical axis causes the vignetting as well as fluctuant luminous intensity, and a beam that asymmetrically enters the illumination optical system causes the uneven luminous intensity. The fluctuant and uneven luminous intensity hamper control of the exposure amount to the desired amount at the time of exposure, thereby deteriorating the resolution. In particular, in light of the recent demand on the minute pattern, it is expected that even a slight degradation in resolution cannot be neglected in the future.
In order to solve these problems, Japanese Laid-Open Patent Applications Nos. 11-145033 and 2000-77315 propose means for correcting an optical axis in real time. Both references propose to correct an angular shift in the optical axis by pivoting a mirror, and a positional shift in the optical axis by tilting a parallel plate of a specified refractive index. Although the rotation of the mirror attains sufficiently responsive correction, the plane parallel plate undesirably lowers the transmittance and thereby throughput. On the other hand, it is conceivable to correct a positional shift in the optical axis by moving the mirror in parallel, but it is difficult to quickly move the mirror in parallel while keeping the mir
Canon Kabushiki Kaisha
Mathews Alan
Morgan & Finnegan L.L.P.
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