Photocopying – Projection printing and copying cameras – Illumination systems or details
Reexamination Certificate
2000-06-29
2003-02-25
Adams, Russell (Department: 2851)
Photocopying
Projection printing and copying cameras
Illumination systems or details
C355S053000, C355S055000, C355S067000, C355S077000, C430S311000, C430S312000
Reexamination Certificate
active
06525806
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method to form microlithographic images using a projection exposure system for fabricating semiconductor devices.
BACKGROUND OF THE INVENTION
Semiconductor lithography involves the creation of small three dimensional features as relief structures in a photopolymeric or photoresist coating. These features are generally on the order of the wavelength of the ultraviolet (UV) radiation used to pattern them. Currently, exposure wavelengths are on the order of 150 to 450 nm and more specifically 157 nm, 293 nm, 248 nm, 365 nm, and 436 nm. The most challenging lithographic features are those which fall near or below sizes corresponding to 0.5&lgr;/NA, where &lgr; is the exposing wavelength and NA is the objective lens numerical aperture of the exposure tool. As an example, for a 248 nm wavelength exposure system incorporating a 0.60NA objective lens, the imaging of features at or below 0.25 micrometers is considered state of the art.
FIG. 1
shows the configuration of a projection exposure system. Such an exposure system can be used in a step-and-repeat mode (referred to a stepper tool) or in a step-and-scan mode (referred to as a scanner tool). A UV or vacuum ultraviolet (VUV) source
1
is used to pass radiation through the illumination system
2
using a condenser lens system
3
and a fly's eye microlens array
4
. An aperture
5
shapes the illumination profile to a defined area and radiation is reflected from a mirror
6
to pass through an illumination lens
7
to illuminate a photolithographic mask
8
. Upon illumination of the photomask
8
, a diffraction field
11
distributed as spatial frequency detail of the photomask
8
is directed through the objective lens system
9
to be imaged onto the photoresist coated semiconductor substrate
10
. Such an exposure system forms an image by collecting at least more than the 0
th
-order of the diffraction field from the photomask
8
with the objective lens
9
. The absolute limitation to the smallest feature that can be imaged in any optical system corresponds to 0.25&lgr;/NA. Furthermore, the depth of focus (DOF) for such an exposure tool can be defined as +/− k
2
&lgr;/NA
2
where k
2
is a process factor that generally takes on a value near 0.5
As geometry sizes continue to shrink further below 0.5&lgr;/NA, methods of resolution enhancement are being required to ensure that intensity images (also known as aerial images) are produced with adequate fidelity and captured within a photoresist material. Such methods of resolution enhancement developed over recent years can allow for improvement in addition to those made possible with shorter exposing wavelengths and larger numerical apertures. These methods in effect work to control the weighting of diffraction energy that is used for imaging. This diffraction energy corresponds to the spatial frequency detail of a photomask. Phase-shift masking (PSM), off-axis illumination (OAI), and optical proximity correction (OPC) all lead to image enhancement through control of the weighting of diffraction energy or spatial frequency that is collected by an objective lens. As an example, an attenuated phase shift photomask accomplishes a phase shift between adjacent features with two or more levels of transmission [see B. W. Smith et al, J. Vac. Sci. Technol. B 14(6), 3719, (1996)]. This type of phase shift mask is described for instance in U.S. Pat. Nos. 4,890,309 and 5,939,227. Radiation that passes through clear regions of the such a mask possess a phase (&phgr;) that is dependant on the refractive index and thickness of the mask substrate. Radiation is also Transmitted through dark features formed in the attenuating material by proper choice of a material that has an extinction coefficient value generally less than 1.0. The radiation that passes though these dark features possesses a phase that depends upon the refractive index and extinction coefficient values of the mask substrate and of the attenuating material. It is chosen so that a 180 degree phase shift (&Dgr;&phgr;) is produced between clear regions and dark regions. Selection of a masking material with appropriate optical properties to allow both a 180 degree phase shift and a transmission of some value greater than 0% will reduce the amplitude of the zero diffraction order produced by illumination of the mask. Comparison of the resulting frequency plane distribution with that of a conventional binary mask can demonstrate this effect. The normalized zero diffraction order amplitude for a binary mask is 0.5 and the first order amplitude is 1/&pgr; as seen from FIG.
2
A. Using a 10% attenuated phase shift mask, the normalized zero diffraction order amplitude is 0.4 and the first order amplitude is 1.1/&pgr; shown in FIG.
2
B. This reduction in the zero diffraction order reduces the amplitude biasing of the higher order frequency components and produces an image amplitude function that has significant negative electric field energy. This leads to air aerial image intensity (which is the square of the amplitude image) that retains zero values at edges of opaque features. This edge sharpening effect leads to higher resolution when imaging into high contrast photoresist materials.
An attenuated phase shift mask requires a complex infrastructure of materials, deposition, etching, inspection, and repair techniques to replace the mature chromium on quartz binary photomask process. This field has been investigated for over ten years and it is not yet certain if suitable materials will exist for 248 nm, 193 nm, or shorter wavelengths. Stronger phase shift masking is difficult because of geometry, materials, and process issues and diffractive and scattering artifacts produced during imaging. As a result, it is questionable how practical phase shift masking will be for use in integrated circuit (IC) manufacturing.
Off-axis or modified illumination of a photomask can produce a similar frequency modifying effect [see B. J. Lin, Proc. SPIE 1927, 89, (1993) and see B. W. Smith,
Microlithography: Science and Technology
, Marcel Dekker: New York, Ch. 3, 235 (1998)].
FIG. 3
shows an example of the prior art, depicting illumination with an annulus and quadrupole illumination profiles. Definition of the shape of illumination can be carried out for instance in the position of the shaping aperture
5
shown in FIG.
1
. Other methods of shaping can include the use of beam splitters, diffractive optical elements, or other optical approaches. Through the us, of such annular or quadrupole illumination, diffraction orders can be distributed in the objective lens
9
of
FIG. 1
with minimal sampling of the central portion of the objective lens pupil. An example, is shown in
FIGS. 4
a
through
4
c
for annular, quadrupole, and weak quadrupole illumination. The impact is similar to the reduction of the zero diffraction order or frequency produced with phase shift masking. In this case, the zero order takes on the shape of the illumination distribution in the condenser lens pupil. If appropriately designed, the central portion of the objective lens pupil can be avoided by diffraction energy.
Modified or off-axis illumination can suffer exposure throughput, orientation, and proximity effect problems. Contact mask features, for example, exhibit little improvement with off-axis illumination. Implementation is therefore limited for many applications, also limiting practicality.
The use of optical proximity correction (OPC) can also result in a reduction of zero order diffraction energy in the frequency plane. Methods of proximity effect reduction have been introduced which are comprised of additional lines, sometimes referred to as OPC assist features, into a mask pattern. This was first disclosed in U.S. Pat. No. 5,242,770. The patterning is such that an isolated line is surrounded by sub-resolution OPC assist line features on either side of the line, better matching edge intensity gradients of isolated features on the mask to more dense features on the mask.
F
Adams Russell
ASML Netherlands B.V.
Brown Khaled
Pillsbury & Winthrop LLP
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