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DESCRIPTION
1. Technical Field
The present invention is concerned with a lithographic process that
exhibits improved image quality. In particular, the present invention is
concerned with lithographic masks that provide regions that at least
partially compensate for image degradation caused by the lithographic
process itself. The lithographic masks, pursuant to the present invention,
have sub-resolution halftones incorporated within the mask patterns for
controlling the transmittance of the actinic light exposure area.
2. Background Art
In the fabrication of integrated circuits including integrated circuit
chips, one of the most critical and crucial steps is the lithographic
processing for providing the desired circuit patterns.
For instance, photolithographic processes involve employing a beam of
actinic light, such as U.V. radiation, in order to transfer a pattern from
a photolithographic mask onto a photoresist coating through an imaging
lens. The mask includes opaque and transparent regions such that the
shapes match those of the openings in the resist coating in the desired or
predetermined pattern.
In the case of a positive resist coating, the transparent portions of the
mask correspond to the desired pattern or image to be provided in the
resist coating. When the photoresist coating is a negative resist, the
opaque portions or regions of the mask correspond to the subsequent open
regions to be provided in the photoresist coating.
However, as the size of the features of the desired pattern approaches the
resolution of the photolithographic tool being employed, the contours of
the developed regions depart significantly from those in the desired ideal
pattern. These departures from, or errors in the pattern, are often
pattern-dependent and, accordingly, are difficult to correct with overall
changes in the development procedure. Such problems are discussed, for
example, in U.S. Pat. No. 4,456,371 to Lin, disclosure of which is
incorporated herein by reference.
SUMMARY OF INVENTION
The present invention provides for improved photolithography and improved
image quality. In particular, the present invention substantially
compensates for image degradation that is caused by the photolithographic
process. The present invention is concerned with correcting
pattern-dependent errors in photolithographic processes.
In particular, according to the present invention, improved
photolithographic processes are obtained by employing a photolithographic
mask that contains half tones. The half-tone regions of the
photolithographic mask permit partial compensation for the image
degradation that is caused by the photolithographic process itself. The
half-tone regions are formed from attenuating arrays of opaque or
transparent sub-resolution elements.
In particular, the present invention is concerned with a lithographic
process having improved image quality. The process includes positioning a
member that is to receive an image of a photolithographic mask that
defines areas that are to receive actinic light. The member can be
adjacent the mask or separated therefrom by some defined distance such as
about 500 mm or more having a lens located between the mask and the
member. The photolithographic mask includes a plurality of opaque elements
or transparent elements or semi-transparent elements, or combinations,
each of which being smaller than the resolution of the photolithography to
be employed in the imaging. These opaque elements or transparent elements
control the transmittance of the actinic light exposure area. Choice of
the type of half-tone employed (e.g., transparent or opaque) will depend
upon the practical tradeoffs encountered between the write-time for the
mask (i.e., number of half-tones) versus the difficulty of fabrication
(i.e., polarity-negative or positive resist) of minimum dimensions.
SUMMARY OF DRAWINGS
FIG. 1 is a schematic of a conventional mask containing two objects at
about 10.times. magnification.
FIG. 2 is a schematic of a halftone mask in accordance with the present
invention.
FIGS. 3A and 3B represent images provided by the conventional mask of FIG.
1.
FIGS. 3C and 3D illustrate images provided by the halftone mask in
accordance with the present invention illustrated in FIG. 2.
FIGS. 4A-4C illustrate the Fourier domain of halftone-corrected mask
patterns pursuant to the present invention.
FIG. 4A illustrates the spectrum of spacial frequencies contained in a
grating-like array of halftones.
FIG. 4B illustrates the spectrum of an uncorrected mask pattern.
FIG. 4C illustrates the spectrum of the corrected mask.
FIG. 5 is a schematic of a mask containing phase halftones pursuant to the
present invention.
BEST AND VARIOUS MODES FOR CARRYING OUT INVENTION
The present invention is concerned with obtaining improved photolithography
by employing a mask that contains halftones.
The halftone regions that are present in the photolithographic masks
employed pursuant to the present invention provide for substantial
compensation for the image degradation that is caused by the
photolithographic process itself.
The halftone regions provide for a means of correcting the exposure and
patterns containing geometry-dependent degradation.
In particular, the presence of opaque elements or transparent elements in
the photolithographic mask that are smaller than the resolution of the
photolithography to be employed provide for correction of the exposure to
the actinic light by adjusting the transmittance of the corresponding mask
opening or portion of the mask opening. In particular, when using opaque
elements that are smaller than the resolution of the photolithography to
be employed, these opaque elements will not be individually reproduced,
but instead, will merely reduce the feature exposure. Correspondingly,
finite transmittance can be introduced into a nominally opaque mask
pattern by selectively deleting small opaque elements of the mask coating
that are smaller than the photolithographic resolution to be employed.
Reference to FIG. 1 illustrates a conventional photolithographic mask
containing two objects. Black areas (1) represent opaque areas of the
mask, and white areas (2) represent transparent portions of the mask. The
mask is about a 10.times. magnification. The dotted circle (3) indicates
the approximate lithographic resolution for the particular example
represented.
FIG. 2 represents a halftone mask pursuant to the present invention wherein
the black portions (1) represent the opaque portions of the mask, the
white portions (2) represent the transparent portions of the mask, and the
black portions (4) represent the opaque portion being smaller than the
resolution of the photolithography to be employed. The dotted circle (3)
indicates the approximate photolithographic resolution in the example.
As illustrated in FIGS. 3A-3D, the quality of the photolithographic images
is significantly improved by adjustment of the total feature exposure.
In particular, for the particular photolithography illustrated, a
three-quarter micron optical photolithography employing a
diffraction-limited lens is employed between the masks and the wafer, the
wafer being at the imaging side of the lens. The lens numerical aperture
(NA.sub.wafer) is about 0.28, the wavelength is about 436 nanometers, the
reduction is about 10.times., and the pupil filling ratio .sigma. is about
0.7. The approximate resolution of the system is r.sub.mask
=10.times.r.sub.wafer which equals 10.times.(0.5.lambda./NA.sub.wafer)=7.8
microns. The desired pattern, as illustrated in FIG. 1, consists of two
objects (i.e., two clear apertures in a binary mask), a 7.5 micron square
opening (contact hole), and a 7.5.times.25 micron rectangular opening
(line). The corresponding wafer-plane dimensions are reduced by a factor
of about 10.
In this example, the lens resolution r.sub.mask (illustrated as a dotted
circle (3) in FIGS. 1 and 2) is comparable to the minimum feature
dimension of 7.5 microns. The limited resolution causes a loss in
sharpness in exposure latitude for both features. Such also causes a
reduction in the exposing intensity actually transmitted to the wafer
underlying the mask. It is further noted that the shape of the contact
hole is such that it approaches the resolution limit in both dimensions.
This, in turn, causes the contact hole exposure to be lower than that of
the line when a conventional mask is employed.
FIGS. 3A and 3B illustrate images printed with the conventional mask from
FIG. 1. The contours of each printed circuit are calculated using a
computer simulation of the image projected by a partially coherent lens
system as discussed by Rosenbluth, et al., "A Critical Examination of
Sub-micron Optical Lithography Using Simulated Projection Images", Journal
Vacuum Science Technology B1(4), p. 1190 (1983), disclosure of which is
incorporated herein by reference. Developing of the resist sufficient to
bring the diameter of the contact hole to the desired 0.75 micron value
results in overdeveloping the line by about 35% in the critical 0.75
micron dimension. On the other hand, using the conventional mask,
development of the line to the correct width results in underexposure or
underprinting of the contact hole because of its lower exposure, as
illustrated in FIG. 3B.
Reference to FIGS. 3C and 3D illustrates simulations of images provided
with the halftone mask of the present invention, as illustrated in FIG. 2.
The apertures in the 10.times. mask are divided into 0.8 micron.times.0.8
micron sub-resolution pixels. Five out of every eight pixels in the line
are opaque, while in the contact hole, four are opaque. FIGS. 3C and 3D
illustrate that development of either feature of the proper width produces
little, if any, error in the other feature. For instance, the smaller
proportion of open halftones in the line aperture reduces the exposure of
the line image to match that of the contact hole. This exposure correction
is obtained by an increase in exposure time.
The opaque halftone can be produced on the mask using standard fabrication
processes wherein the resolution of the photolithography used is greater
than that to be employed in the subsequent photolithography when the mask
is used to develop the desired patterns. However, usually when employing
present day state-of-the art lithography, the resolution needed to make
the mask is not necessarily any finer than that used to print on the wafer
since the mask is demagnified such as about 5-fold to about 10-fold. For
instance, the halftone mask can be provided by placing a layer of, for
instance, chrome of about 500 angstroms on a transparent glass substrate
followed by a layer of photoresist material. The photoresist is then
developed to provide the desired halftone pattern and the chrome beneath
the photoresist is etched. The photoresist remaining is then removed.
In addition, halftone pixels can be fabricated in masks that are later
de-magnified by the lithographic tool, such as employing a reduction
stepper. In the event the 2-dimensional mask pattern is Fourier
transformed, such an optical reduction system will not reproduce spacial
frequencies having a period finer than r.sub.mask
=0.5M.lambda./NA.sub.wafer where M is the magnification. In the event the
halftone elements are insufficiently small, however, they will generate
significant Fourier components at resolvable spacial frequencies, thereby
resulting in unacceptable noise in the image. For example, if desired, the
halftones can be arrayed in a grating-like pattern. The spacial
frequencies in the grating must be chosen to lie outside the imaging
bandwidth of the photolithographic tool employed. Ideally, the image cast
by the linear or bi-linear optical reduction system would then consist
only of the gray-level DC component. However, the corrected mask consists
of a multiplication of the high frequency grating with the uncorrected
pattern. In particular, noise results from the low frequency harmonics
generated by the non-linear multiplication as demonstrated in FIG. 4A. The
noise will be localized in the vicinity of feature edges, and is, in fact,
equivalent to the intensity change caused by a small shift in the position
of the image. The residual noise or intensity change due to the image
shift can be made small in comparison to uncorrected optical proximity
effects, particularly in cases where the minimum feature size approaches
the resolution of the lens employed in the photolithographic tool. In such
instances, the halftone pixels are made small in comparison with the lens
resolution.
Reference to FIG. 4A illustrates the spectrum of spacial frequencies
contained in a grating-like array of halftones. A band-limited optical
system will capture only the central DC harmonic, and will then produce
the desired uniform image exposure. Reference to FIG. 4B illustrates the
spectrum of an uncorrected mask pattern whereby the binary lithographic
pattern can not be band-limited. On the other hand, FIG. 4C shows the
spectrum of the corrected mask that consists of the convolution of 4A and
4B. A comparison of FIG. 4C with FIG. 4B illustrates the erroneous
components in the pattern at resolvable frequencies.
Not only can the present invention and use of halftones be employed to
equalize the exposure of different patterns, mask structures in accordance
with the present invention can be provided to carry out uniform correction
within a pattern itself. For instance, the effective gray level can be
varied within an individual mask opening such as for providing higher
effective transmission in corners and along edges of an individual mask.
This allows for more general improvement in the quality of the image.
The techniques of the present invention can be used not only for binary
mask-making processes to be used in the fabrication of half-tone masks as
described in detail above, but also in conjunction with more sophisticated
mask-making processes such as Levenson's phase-layer processes to
fabricate an even more general class of mask (e.g., masks with negative or
non-real transmittance). Such phase-layer process is disclosed by
Levenson, et al., "Improved Resolution and Photolithography with a
Phase-Shifting Mask", IEEE Transaction on Electron Devices, ED-29, page
1828 (1982). For instance, see FIG. 5. The technique could provide a
super-position of four basic amplitudes to synthesize an arbitrary
amplitude in a complex plane. For instance, four different kinds of
halftone elements would be sufficient to define an arbitrary net complex
transmittance for each resolution element in the mask. for instance, the
four elements could include a pixel (11) with a transparent coating of
one-third wave optical thickness, a pixel (12) with coating of two-thirds
wave thickness, a pixel (13), and an opaque pixel (14) on the un-coated
substrate. In FIG. 5, numeral 13 represents the uncoated substrate,
regions of which represent a pixel. The uncoated regions are shown as
white. The phase and amplitude transmittances of the resulting masks can
have a spacial variation that is arbitrary to within the lens resolution.
Such masks form the most general class of two-dimensional optical objects.
Accordingly, the mask pattern can be chosen such that it most fully
corrects for degradation in the lithographic process.
FIG. 5 is a schematic of a mask containing halftones wherein the effective
mask transmittance varies in the y direction, but not in the x direction.
The net amplitude transmittance T(y) of each x-oriented strip is indicated
with an x on the accompanying plot. The negative transmissions shown could
not be obtained with opaque halftones alone. Phase halftones make possible
a general complex transmittance.
In addition, according to the present invention, methods for determining
the maximum allowed pixel size and criteria for such have been provided.
A simple scheme for placing halftone elements within mask apertures is one
in which potential sites for halftones are randomly filled with
probability p (so that p is the attenuation factor). Finite pixel size
then causes the image to contain random shot noise. The signal-to-noise
ratio is approximately the square root of the number of halftone features
within one resolution element of the lens. If each chip contains a large
number of resolution elements, a nominally random placement procedure
should be modified so as to preclude rare noise fluctuations of unusually
large magnitude.
Another and more preferable approach is to systematically array the
halftone elements in a two-dimensional grating.
The intensity error in the vicinity of an edge of a pattern will have a
peak of order
##EQU1##
where a.sub.mask =Ma.sub.wafer is the size of a single halftone element, M
is the magnification, r.sub.mask =Mr.sub.wafer is the resolution (defined
here as 0.5.lambda./NA), and I.sub.o is the baseline exposure level
defined as the intensity inside the image of a large object.
In the absence of correction, the degree of interaction between features
having a critical dimension d is of order
##EQU2##
Eq. 2 may be thought of as the residual lens response at a distance d
outside the geometrical boundary of some feature. In contrast, Eq. 1 is
essentially the peak lens response to a sub-resolution feature of width a.
Eq. 1 is then considerably smaller than Eq. 2 when a.sub.mask is small
compared to r.sub.mask (if d and r.sub.wafer are comparable).
The increment represented in Eq. 1 will not fluctuate as with randomly
dispersed halftone features. In addition, Eq. 1 represents an upper limit
to the error introduced by a halftone grating.
In order to estimate the error introduced when a halftone mask is used in
place of a mask containing true gray-levels, reference is made to the
simple one-dimensional case in which a half-plane is imaged under
incoherent illumination. The region x<0 is deemed to be opaque, and the
region x>0 to be either a continuous film of 50 percent transmittance, or
an equal line-space grating of pitch 2a.
Under these conditions, as discussed by Goodman, "Introduction to Fourier
Optics", McGray-Hill, Chapter 6, 1968, the image intensity is given by
##EQU3##
where g(X.sub.o) is the mask transmittance, and where, under the
assumption of incoherent one-dimensional imaging, the intensity response
function is given by
##EQU4##
The intensity difference between the true gray-level image and that
generated with halftones is
##EQU5##
which yields Eq. 1 above. Eq. 6 assumes that the halftone pixels are small
enough that h can be approximated by a two-term Taylor expansion within
each pixel.
The above approach forms the basis of a general two-dimensional calculation
in which the illumination can be partially coherent. A general
two-dimensional halftone grating that fills a mask aperture of arbitrary
shape can be analyzed.
The image amplitude cast through the halftone object by a single source
point as discussed by Born, et al., "Principles of Optics", 5th Ed.
(Pergamen, Oxford, (1975), Chapter 10, is
##EQU6##
where S(X.sub.s) is the strength of a source point at X.sub.s, h(X.sub.i)
is the amplitude impulse response, p(X.sub.o ; X.sub.s) is the amplitude
illuminating the object plane due the single source point, and g(X.sub.o)
is, as above, the periodic halftone transmittance function. Let the
transmittance function in the jth period of the halftone grating be
written as
##EQU7##
and where X.sub.oj in Eq. 10 is defined implicitly by
##EQU8##
(i.e., X.sub.oj is the geometrical center of the jth halftone period).
The halftone period is assumed to be sufficiently small to permit a two
term Taylor expansion of the imaging and illuminating fields within each
period. Then changing the variable of integration in Eq. 8 from X.sub.o to
X.sub.o :
##EQU9##
which (using Eqs. 9 and 12 while integrating over the halftone terms, and
approximating the resulting sums of h and p terms by integrals) results in
##EQU10##
If d.sub.s is a differential vector directed along the edge of the
aperture, and z is directed normal to the object plane, then using a
version of Stoke's theorem, as discussed by Gradshtlyn, et al., "Table of
Integrals", Series 1 and Products and Jeffrey, Academic Press, 1980,
entrs. #10.723, p. 1091, Eq. 14 reduces to
##EQU11##
i.e. dm is an infinitesimal vector directed normal to the aperture edge.
Finally, the product VV* is formed and integrated over all source points
X.sub.s. Using the definition of the mutual coherence
##EQU12##
and subtracting the image intensity corresponding to a true
continuous-tone film of transmittance gg* is obtained
##EQU13##
Eq. 19 indicates that the departure from a true continuous-tone film image
may be thought of as an interference between two sources; the
continuous-tone film image itself, and an amplitude equivalent to that
transmitted by a slit-like opening tracing the feature boundary. The width
of this slit is essentially <r>, which is of the order of the width of one
halftone. As in the example discussed above, this suggests that the
residual error is small compared to uncorrected proximity effects.
Equivalently, one may regard the halftone aperture as a continuous-tone
film aperture that has been shifted in position a distance <r> since such
a shift changes the transmitted amplitude by an edge-like contribution
identical to that in Eq. 16. A similar contribution arises when the
aperture does not contain an integral number of periods.
A more general non-periodic halftone object can similarly be regarded as a
variable transmittance object that has undergone small internal
deformations; in the non-periodic case the deformations are inhomogeneous.
In a similar manner, the distribution of the subresolution elements can be
determined in the case when the member to receive the image is positioned
in proximity of the mask without a lens in-between. In this situation,
h(x.sub.i -x.sub.o) is replaced with
##EQU14##
where .lambda. is the wavelength of the actinic light and z is the
distance between the mask and the member. More details on this transfer
function h(x.sub.i -x.sub.o) and better transfer functions can be found in
Lin, Polymer Eng. and Sci., Vol. 14, p. 1317, 1975 and J. Opt. Soc. Am.,
Vol. 62, p. 977, 1972. The subresolution element in the case of proximity
printing has now a dimension smaller than
##EQU15##
* * * * *
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