Radiant energy – Photocells; circuits and apparatus – Photocell controls its own optical systems
Reexamination Certificate
1999-04-06
2001-02-20
Lee, John R. (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Photocell controls its own optical systems
C355S055000, C356S399000
Reexamination Certificate
active
06191429
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to semiconductor fabrication and more particularly to a lithography exposure apparatus (aligner) for transferring a circuit pattern from a mask or a reticle onto a sensitive substrate.
The present invention also relates to a system for detecting a focal point on a workpiece (wafer, substrate or plate etc.) and for detecting a tilt of the workpiece, which is applicable to certain kinds of apparatus such as an apparatus for manufacturing a workpiece or imaging a desired pattern in a surface of a workpiece using a laser or electron beam and an apparatus for optically inspecting the state of a surface of a workpiece.
2. Description of the Related Art
Recently, dynamic random access memory semiconductor chips (DRAMs) having an integration density of 64 Mbits have been mass-produced by semiconductor fabrication techniques. Such chips are manufactured by exposing a semiconductor wafer to images of circuit patterns to form e.g. ten or more layers of circuit patterns in a superposition manner.
Presently, lithography apparatuses used for such chip fabrication are projection aligners in which a circuit pattern drawn in a chromium layer on a reticle (mask plate) is transferred onto a resist layer on a wafer surface through a 1/4 or 1/5 reduction optical imaging system by irradiating the reticle with i-line radiation (wavelength: 365 nm) of a mercury discharge lamp or pulse light having a wavelength of 248 nm from a KrF excimer laser.
Projection exposure apparatuses (projection aligners) used for this purpose are generally grouped, according to the types of imaging optical system, into those using a step-and-repeat system, i.e., so-called steppers, and those using a step-and-scan system which has attracted attention in recent years.
In the step-and-repeat system, a process is repeated in which, each time a wafer is moved to a certain extent in a stepping manner, a pattern image on a reticle is projected on a part of the wafer by using a reduction projection lens system formed only of a refractive optical material (lens element) and having a circular image field or an unit magnification projection lens system formed of a refractive optical material (lens element), a prism mirror and a concave mirror and having a noncircular image field to expose a shot area on the wafer or plate to the pattern image.
In the step-and-scan system, a wafer is exposed to an image of a portion of a circuit pattern on a reticle (for example, in the form of a circular-arc slit) which is projected on the wafer through a projection optical system. Simultaneously, the reticle and the wafer are continuously moved at constant speeds at a speed ratio according to the projection magnification, thus exposing one shot area on the wafer to the image of the entire circuit pattern on the reticle in a scanning manner.
For example, as described on pp 256 to 269 of SPIE Vol. 922 Optical/Laser Microlithography (1988), the step-and-scan system is arranged so that, after one shot area on the wafer has been scanned and exposed, the wafer is moved one step for exposure of an adjacent shot area, and so that the effective image field of the projection optical system is limited to a circular-arc slit. Also, the projection optical system is considered to be a combination of a plurality of refractive optical elements and a plurality of reflecting optical elements, such as one disclosed in U.S. Pat. No. 4,747,678 (to Shafer).
U.S. Pat. No. 5,194,839 (to Nishi) discloses an example of an aligner in which a step-and-scan system is realized by mounting a stepper reduction projection lens having a circular image field. This publication also discloses a method in which a pattern image projected at the time of scanning exposure is transferred onto a wafer by increasing the depth of focus (DOF) by a predetermined amount on the wafer.
In the field of lithography technology, it is now desirable to be able to fabricate semiconductor memory chips having an integration density and fineness of the 1 or 4 Gbit class by light exposure. Since light exposure techniques have a long technological history and are based on a large amount of accumulated know-how, it is convenient to continue use of light exposure techniques. It is also advantageous to use light exposure techniques considering drawbacks of alternative electron beam exposure or X-ray technologies.
It is believed that resolutions in terms of minimum line width (feature width) of about 0.18 &mgr;m and 0.13 &mgr;m are required with respect to 1 Gbit and 4 Gbit memory chips, respectively. To achieve resolution of such a line width, far ultraviolet rays having a wavelength of 200 nm or shorter, e.g., those produced by an ArF excimer laser, are used for illumination for irradiating the reticle pattern.
As optical vitreous materials having a suitable transmittance with respect to far ultraviolet rays (having a wave-length of 400 nm or shorter), quartz (S
i
O
2
), fluorite CaF
s
, lithium fluoride (L
i
F
2
), magnesium fluoride (MgF
2
) and so on are generally known. Quartz and fluorite are optical vitreous materials indispensable for forming a projection optical system having high resolution in the range of far ultraviolet rays.
However, it is necessary to consider the fact that, if the numerical aperture (NA) of a projection optical system is increased to attain high resolution while the field size is increased, the diameter of lens elements made of quartz or fluorite becomes so large that it is difficult to manufacture such lens elements.
Also, if the numerical aperture (NA) of the projection optical system is increased, the depth of focus (DOF) &Dgr;F is inevitably reduced. In general, the depth of focus &Dgr;F is defined by wavelength, numerical aperture NA, a process coefficient Kf (0<Kf<1) as shown below if the Rayleigh's theory of imaging formation is applied:
&Dgr;
F=Kf
·(&lgr;/
NA
2
)
Accordingly, the depth of focus &Dgr;F in the atmosphere (air) is about 0.240 &mgr;m if the wavelength is 193 nm, that is, equal to that of ArF excimer laser light, the numerical aperture NA is set to about 0.75 and the process coefficient Kf is 0.7. In this case, the theoretical resolution (minimum line width) &Dgr;R is expressed by the following equation using process coefficient Kr (0<Kf<1):
&Dgr;
R=Kr
·(&lgr;/
NA
)
Accordingly, under the above-mentioned conditions, the resolution &Dgr;R is about 0.154 &mgr;m if the process coefficient Kr is 0.6.
As described above, while it is necessary to increase the numerical aperture of the projection optical system in order to improve the resolution, it is important to notice that the depth of focus decreases abruptly if the numerical aperture is increased. If the depth of focus is small, there is a need to improve the accuracy, reproducibility and stability with which an automatic focusing system for coincidence between the best imaging plane of the projection optical system and the resist layer surface on the wafer is controlled.
On the other hand, considering the projection optical system from the standpoint of design and manufacturing, a configuration is possible in which the numerical aperture is increased without increasing the field size. However, if the numerical aperture is set to a substantially larger value, the diameter of lens elements is so large that it is difficult to form and work the optical vitreous material (e.g. quartz and fluorite).
Then, as a means for improving the resolution without largely increasing the numerical aperture of the projection optical system, an immersion projection method may be used in which the space between the wafer and the projection optical system is filled with a liquid, see U.S. Pat. No. 4,346,164 (to Tabarelli).
In this immersion projection method, the air space between the wafer and the optical element constituting the projection optical system on the projection end side (image plane side) is filled with a liquid having a refractive index close to the refractive index of the photoresist layer, to increase th
Klivans Norman R.
Lee John R.
Nikon Precision Inc.
Pyo Kevin
Schmidt Mark E.
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