Optics: measuring and testing – By alignment in lateral direction – With registration indicia
Reexamination Certificate
1999-10-26
2002-01-15
Font, Frank G. (Department: 2877)
Optics: measuring and testing
By alignment in lateral direction
With registration indicia
C356S399000, C356S401000, C250S492200
Reexamination Certificate
active
06339471
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a projection exposure apparatus that exposes a pattern formed on a mask to transfer the pattern onto a photosensitive substrate.
In a manufacturing process for semiconductor devices or liquid crystal display devices, a projection exposure apparatus is conventionally used to illuminate a pattern formed on a photomask (or a reticle) with an illumination light and project an image of the pattern onto a photosensitive substrate. Typically, a semiconductor wafer or a glass plate coated with photosensitizer (e.g., photoresist) is used as the photosensitive substrate.
Currently, a step-and-repeat type projection exposure apparatus is widely used. With a step-and-repeat type projection exposure apparatus, a photosensitive substrate is loaded on a substrate stage, which is movable in two dimensions, and the shot areas arranged on the photosensitive substrates are successively exposed into the pattern image of the mask by shifting the photosensitive substrate in a stepwise manner by moving the substrate stage. A scanning-type projection exposure apparatus is also widely used, in which both the mask and the photosensitive substrate are scanned synchronously, thereby exposing the photosensitive substrate into the pattern image of the mask. Two types of scanning type projection exposure apparatus are known, a step-and-scan type apparatus, in which moving from one shot area to the next shot area is performed in a stepwise manner, and a collective-exposure type apparatus, in which the pattern of a large mask is transferred by scanning onto the entire surface of a single photosensitive substrate.
All of these projection exposure systems require highly accurate alignment in order to precisely superpose a pattern of the mask onto the pattern that has already been formed on the photosensitive substrate. The alignment is generally performed by detecting an alignment mark formed on the mask and/or photosensitive substrate using a detection system installed in the projection exposure apparatus.
U.S. patent application Ser. No. 418,260 (filed on Oct. 6, 1989) now U.S. Pat. No. 5,734,478, discloses an LSA (Laser Step Alignment) system that detects the position of an alignment mark formed by several columns of dots by illuminating laser light onto the dot sequence to obtain the light diffracted or scattered by the alignment mark. An FIA (Field Image Alignment) system uses a halogen lamp as a light source to illuminate the alignment mark with a light beam having a broad waveband width. The FIA system picks up the image of the alignment mark and processes the image data of the alignment mark. An LIA (Laser Interferometric Alignment) system illuminates a diffraction-grid alignment mark with laser beams from two different directions and picks up the interference signal of the diffracted light generated from the alignment mark, thereby detecting the position of the alignment mark. The LIA system includes a homodyne system, which is disclosed in U.S. Pat. No. 4,636,077 and uses two laser beams having the same frequency, and a heterodyne system, which is disclosed in U.S. Pat. No. 5,734,478 and uses two laser beams having slightly different frequencies.
Alignment methods are grouped into a TTL (Through-the-Lens) method, a TTR (Through-the-Reticle) method, and an off-axis method. In the TTL method, the position of the photosensitive substrate is measured through the projection optical system. In the TTR method, the positional relation between the reticle and the photosensitive substrate is detected through the reticle. In the latter case, the reticle is used as both a mask and a projection optical system. In the off-axis method, the position of the photosensitive substrate is directly measured without using the projection optical system.
In the LIA system, an alignment mark formed in the vicinity of the pattern field of the mask and an alignment mark formed in the vicinity of a shot area on the photosensitive substrate are simultaneously measured by the alignment system positioned above the mask to directly detect an offset (displacement) between the two marks. Based on the detected offset, the mask or the photosensitive substrate is finely adjusted so that the amount of the offset (displacement) becomes zero.
According to this method, illumination light whose waveband is insensitive to the photoresist layer is used as alignment light. For example, as shown in
FIG. 9
, a laser beam emitted by a semiconductor laser LD and having a wavelength of 690 nm is used. Such a semiconductor laser LD is driven by a driving circuit, which includes an auto output control (APC) circuit
52
to output constant level light. The alignment system installed above the mask guides the alignment light projected onto the mask surface and the alignment light projected onto the photosensitive substrate surface along the same axis from the semiconductor laser to an object lens of the alignment system. The alignment light exiting from the object lens illuminates a diffraction-grid alignment mark formed on the mask (hereinafter, referred to as a reticle mark) and a diffraction-grid alignment mark formed on the photosensitive substrate (hereinafter, referred to as a substrate mark). Taking the advantage of the fact that the two alignment light beams are substantially coaxial, the alignment system photoelectrically detects the light information from the reticle and the light information from the substrate at the same time.
FIG. 10
illustrates the relation between a reticle mark RM formed on the mask
1
and the alignment light. The reticle mark RM is usually formed on the bottom surface
1
B of the mask
1
, which lies on the opposite side of the surface
1
A, which receives the incidence light. The alignment light fluxes fl and f
2
coming from two directions are guided onto the reticle mark RM. The light flux f
1
is reflected and diffracted by the reticle mark RM formed on the bottom surface
1
B of the mask
1
. Then, the 0-th order reflected light f
1
returns to its normal direction. However, a fraction of the 0-th order diffracted light is internally reflected on the top surface
1
A of the mask
1
, and then again reflected and diffracted by the reticle mark RM. As a result, the flux f
1
becomes multiplexed reflected/diffracted light, and returns in the same direction as the 0-th order diffracted light. (
FIG. 10
does not illustrate the light that transmits the bottom surface
1
B of the mask
1
.) The regularly reflected light of the flux f
1
, which exists on the top surface
1
A of the mask
1
, interferes with the multiplexed reflected/diffracted light and the 0-th order diffracted light. Hence, the detection signals generated from the reticle mark RM interfere with each other as they reach the photoelectric detector. The same applies to the light flux f
1
, but f
2
acts from the opposite direction.
In general, laser light used as alignment light is highly coherent, and depending on the kind of semiconductor laser used, the wavelength of the emitted light changes due to a mode hop phenomena, which induces undesirable changes in the interference conditions. As a consequence, the detection signals from the reticle mark are disrupted and the alignment accuracy (reproducibility) worsens.
The mode hop phenomena of a semiconductor laser is considered to be generated by changes in temperature, temporal changes, and the influence of the returning light (the light that is reflected on a plane perpendicular to the optical axis of the optical device comprising the optical system and that returns to the laser light source) and the like. As illustrated in
FIG. 11
, one or more monochromatic wavelengths &lgr;
1
, &lgr;
2
, &lgr;
3
, . . . generated by signal mode oscillation appears at random in a neighborhood of the central wavelength &lgr;
0
. These different wavelengths make the interference among the 0-th order diffracted light, the regularly reflected light, and the multiplexed reflected light unstable. As a result, the alignment between the mask and the photosensitive substra
Font Frank G.
Nikon Corporation
Nixon & Vanderhye P.C.
Punnoose Roy M.
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