X-ray or gamma ray systems or devices – Specific application – Lithography
Reexamination Certificate
2000-08-08
2002-03-12
Porta, David P. (Department: 2876)
X-ray or gamma ray systems or devices
Specific application
Lithography
C378S206000
Reexamination Certificate
active
06356616
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The invention pertains to methods and apparatus for transferring, by projection, a pattern defined by a reticle or mask onto a sensitized substrate using a mirror-projection system, such as an X-ray optical system.
BACKGROUND OF THE INVENTION
A projection-exposure apparatus for fabrication of integrated circuits projects and transfers a circuit pattern defined by a reticle or mask (these terms are used interchangeably herein) onto a sensitized substrate (e.g., a semiconductor wafer) through an image forming-apparatus. Such exposure apparatus conventionally use an illumination source (light source), such as an i-line light source. Upon illumination of the reticle with the light source, the illuminated pattern defined by the reticle is projected and inscribed on the substrate. The pattern defined by the reticle is typically either larger than or equal in size to the pattern to be inscribed on the substrate.
Conventional exposure apparatus typically include an illumination-optical system and a projection-optical system. A substrate stage adjusts the substrate's axial position (height) and inclination. The substrate stage also adjusts the substrate's position on the XY plane. When projection-exposing a reticle pattern onto a substrate, the patterned surface of the reticle is first aligned with the substrate surface; that is, the focal points are aligned. For projection exposure using light (e.g., UV light), the projection-optical system ordinarily comprises a plurality of lenses and is arranged so that the pattern formed on the reticle can be focused on the surface of the substrate and transferred in a single shot. Since most projection-optical systems have a field of view of approximately 20 mm
2
, a desired region (e. g., typically a region equivalent to two semiconductor chips) can be exposed at one time.
In recent years, as progress has been made in terms of higher performance and a higher degree of integration in semiconductor integrated circuits, there has been continual demand for increased accuracy and resolution in pattern transfer. Accordingly, errors with respect to imageformation characteristics that accompany inaccurate measurement of the substrate surface height (often caused by insufficient flatness of the substrate surface and/or substrate stage) cannot be ignored.
A focal-point (axial-position) detection system is typically used to determine the substrate surface height in order to achieve alignment of the focal point across a wide exposure field. A slit image is first projected obliquely relative to the optical axis AX (i.e., the slit image is not projected through the projection-optical system) onto each of multiple measurement points located inside the shot region (exposure field) of a substrate. The axial-position detection system (based on the oblique-incidence method) is used to receive light from the reflected image by a two-dimensional array sensor.
Generally, the resolution W of an exposure apparatus is determined by the exposure wavelength &lgr; and the numerical aperture NA of the projection-optical system. The resolution W is calculated as follows:
W=k
1
&lgr;/NA
wherein k
1
is a constant. In order to increase the resolution, it is necessary to shorten the wavelength of the illumination source or to increase the numerical aperture. For example, when an i-line light source having a wavelength of 365 nm is used for illumination, a resolution of 0.5 &mgr;m is obtained at a numerical aperture of approximately 0.5. It is difficult, however, to increase the numerical aperture in such a system. Accordingly, it has been necessary to further shorten the wavelength of the illumination source.
Excimer lasers have begun to be used as sources for illumination as excimer lasers produce light with wavelengths that are shorter than i-line (e.g., 248 nm for KrF and 193 nm for ArF excimer lasers). A resolution of 0.25 &mgr;m may be obtained when using a KrF illumination source, and a resolution of 0.18 &mgr;m may be obtained when using an ArF exposure-illumination source. If X-rays (having a wavelength of about 13 nm) are used as the illumination source, a resolution of 0.1 &mgr;m or better may be obtained.
When such exposure apparatus use X-rays as the illumination source, the projection-optical system must be constructed entirely from reflective mirrors. Unfortunately, it is difficult to design such a projectionoptical system having a broad exposure field. In addition, when attempting to design such a projection-optical system having a higher resolution, the exposure field is even further reduced. A smaller exposure field has the necessary result that the desired pattern region cannot be exposed in a single shot. Accordingly, integrated circuit throughput is decreased and manufacturing costs are increased.
If the exposure field of the projection-optical system is formed in the shape of an annular band, a higher resolution may be obtained in a long, narrow exposure field. Semiconductor chips that are 20 mm
2
or larger may be exposed, even with a projection-optical system having a small exposure field, by scanning the reticle and substrate during the exposure process.
A conventional X-ray-projection-exposure apparatus is illustrated in FIG.
6
. The X-ray-projection-exposure apparatus shown in
FIG. 6
includes an X-ray source
61
, an illumination-optical system
62
, a projection-optical system
51
, a reticle stage
53
that secures a reticle
52
, and a substrate stage
55
to which a substrate
54
is mounted. A vacuum chamber
56
encloses the X-ray source
61
, illumination-optical system
62
, projection-optical system
51
, reticle
52
, reticle stage
53
, substrate
54
, and substrate stage
55
. The exposure apparatus further includes a axial-position detection system
57
a
,
57
b
. The axial-position detection system
57
a
,
57
b
is positioned outside the vacuum chamber
56
.
The illumination-optical system directs X-rays to irradiate a portion of the pattern defined by the reticle. The projection-optical system
51
, typically comprising a plurality of mirrors, is arranged so that the pattern on the reticle
52
is reduced, projected, and focused onto the surface of the substrate
54
. Multi-layer films are formed on the surfaces of the projection-optical system
51
reflective mirrors to heighten the X-ray reflectivity. The projection-optical system
51
has an annular-band shaped exposure field. The projection-optical system
51
projects a portion of the reticle pattern in an annular-band shape onto the surface of the substrate
54
. During exposure, the reticle
52
and substrate
54
are synchronously scanned at a constant speed, so that the desired pattern region on the reticle (e. g., a region corresponding to a single semiconductor chip) can be projected onto the substrate
54
.
Soft X-rays having a wavelength of approximately 13 nm, for which a high reflectivity is obtained from the multi-layer films of the projection-optical system
51
, may be used as the illumination source. However, such soft X-rays are extensively absorbed by air. Accordingly, at least the reticle
52
, substrate
54
and projection-optical system
51
must be disposed inside the vacuum chamber
56
so that the light path of the X-rays is maintained in a vacuum. The interior of the vacuum chamber
56
is evacuated by means of a vacuum pump (not shown).
In such an X-ray-projection-exposure apparatus, since the substrate
54
must be disposed in the vacuum chamber, at least a portion of the light beam of the axial-position detection system
57
a
,
57
b
passes through the vacuum. However, a conventional axial-position detection system cannot be disposed in a vacuum. If the entire axial-position detection system is disposed in a vacuum, radiation of heat generated by the axial-position detection system light source (e.g., a halogen lamp) becomes a problem. Conventionally, when such a light source is operated in an air atmosphere, heat generated by the lamp is dissipated into the surrounding atmosphere. However, if such a li
Klarquist & Sparkman, LLP
Nikon Corporation
Porta David P.
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