A lithographic system for patterned exposure of radiation-sensitive resist comprises a radiation source, a mask, a converging optical element having a focal plane, and a phase-shifting optical element disposed at the focal plane of the converging optical element. The lithographic system produces enhanced images.

TECHNICAL FIELD 
This invention generally relates to a lithography imaging system and, more 
particularly, to an optical focal plane phase-shifting element used in 
conjunction with a conventional lithographic transmission mask to produce 
enhanced images. 
BACKGROUND OF THE INVENTION 
There is a need for lithographic technologies that can produce features on 
semiconductors that are below half a micron in dimension. Currently, there 
are three competing technologies that may be used to accomplish this: 
X-ray, deep-uv (ultraviolet), and phase-shifting masks. Compared to the 
conventional lithography for semiconductor processing, the X-ray and 
deep-uv technologies are conceptually distinct and practically in need of 
new steppers, elements, resists, etc. In addition to escalating costs, 
these technologies are often plagued with reliability issues and, in 
particular, with respect to lasers and synchrotrons as the light source. 
Phase-shifting techniques show considerable promise for extending the 
submicron performance of the state-of-the-art optical lithographic tools. 
The prospect of avoiding the costs and complexity of developing new 
lithographic processes and new capital equipment has in recent years 
provided great incentives for investigation into phase-shifting 
techniques. In phase-shifting technology, a layer of appropriate width, 
thickness, and shape is added to each element of a conventional 
transmission mask. The phase shift caused by this layer, as well as the 
subsequent interference between the phase-shifted light and the 
nonphase-shifted light transmitted through the respective parts of the 
mask for each element, greatly improves the contrast of the projected 
image of that element. Phase shifting results in enhanced optical 
resolution which allows smaller elements to be projected. However, one or 
more appropriately shaped phase-shifting components has to be added for 
every element to be projected. For example, to produce a 64-megabit DRAM, 
tens of millions of phase-shifting elements have to be incorporated in the 
phase-shifting masks used to produce this device. Being extremely complex, 
these masks are difficult to design, fabricate, inspect, and repair. For 
general references on conventional phase-shifting techniques see, for 
example, Levenson et al., "Improving Resolution in Photolithography with a 
Phase-shifting Mask", IEEE Transactions on Electron Devices, Vol. ED-29, 
No. 12, pp. 1828-1836, December 1982; and Levenson, "Phase-shifting Mask 
Strategies: Line-space Patterns", Microlithographic World, Vol. 1, No. 4, 
pp. 6-12, September/October 1992, which references are incorporated herein 
by reference. 
An example of a prior art lithography system is disclosed in U.S. Pat. No. 
4,947,413. This patent discloses a lithography system capable of doubling 
the spatial frequency resolution associated with conventional systems. An 
aperture filter is positioned to intercept the Fourier transform of the 
mask being exposed. The filter is configured to block certain orders of 
the diffraction pattern from reaching the wafer. The remaining orders 
reaching the wafer will produce a cosine-type interference pattern with a 
period half of the period. There still is a need in the art for a 
lithographic imaging system which produces enhanced images of intricate 
patterns. 
It is an object of the present invention to provide a lithographic imaging 
system which produces enhanced images. 
Other objects and advantages will be apparent from the following 
disclosure. 
SUMMARY OF THE INVENTION 
The present invention relates to a lithographic system for patterned 
exposure of radiation-sensitive resist. The system comprises a radiation 
source, a mask, a converging optical element having a focal plane, and a 
phase-shifting optical element disposed at the focal plane of the 
converging optical element. The lithographic system produces enhanced 
images.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1A, there is shown an embodiment of the lithographic 
imaging system of the present invention. The imaging system generally 
comprises a conventional imaging system having a phase-shifting optical 
element disposed at the focal plane of the converging optical element. In 
FIG. 1A, a source of radiation (e.g., a light source) 102 is shown 
illuminating mask 104. Suitable radiation includes electromagnetic or 
electron beam radiation, preferably electromagnetic, preferably 
ultraviolet or X-ray, and more preferably ultraviolet radiation at a 
wavelength of about 248-365 nm. Suitable radiation sources include 
mercury, mercury/xenon, and xenon lamps, excimer laser, electron beam, or 
X-ray. Suitable masks include transmission masks comprising patterned 
opaque and transmissive regions optionally provided with phase-shifting 
regions. Suitable masks for use in lithography are well known to those 
skilled in the art. 
The mask 104 is imaged through a converging optical element 106 to produce 
a lithographic image 108 on a substrate 110. Suitable converging optical 
elements include transparent converging lenses and reflective optics, 
e.g., parabolic mirrors which deviate the direction of incident light to a 
Fourier plane. Suitable converging optical elements are known to those 
skilled in the art such as those disclosed in Meyer, Encyclopedia of 
Physical Science and Technology, Vol. 9, 1987, the disclosure of which is 
incorporated herein by reference. 
A Fourier transform of the spatial frequencies of the radiation is produced 
at the focal plane 111 of the converging optical element 106. To generate 
phase-shifting characteristics, a phase-shifting optical element 112 is 
disposed in the focal plane of the converging optical element 106. The 
phase-shifting optical element 112 is designed to shift spatial frequency 
components at the focal plane such that the image 108 will exhibit 
enhanced resolution, greater contrast, and a larger depth of focus. 
Generally, a transmission mask 104 used in semiconductor manufacture has a 
large number of features with repetitive elements such as parallel lines, 
and therefore the number of spatial frequency components in the radiation 
is small in comparison with the total number of features in the 
transmission mask 104. 
The focal plane phase-shifting optical element 112 produces phase shifts 
for spatial frequency components of the Fourier transform of the image 
associated with the mask 104 to enhance the image 108. For example, the 
optical element could shift the frequency components of all uniformly 
spaced-apart parallel lines in the Fourier plane to enhance the image 
resolution of all of these parallel lines. Suitably, the imaging system 
will have a converging optical element after the focal plane to back 
transformation at the plane of the image. 
The phase-shifting element 112 may also conveniently incorporate nonphase 
elements to block or attenuate selected frequency components to further 
improve the quality of the final image 108 such as disclosed in U.S. Pat. 
No. 4,947,413, the disclosure of which is incorporated herein by 
reference. For example, the frequency component corresponding to the DC 
component could be blocked to enhance contrast; or the frequency 
component, due to unwanted scattering of light or optical aberrations, 
could be blocked to improve the image quality or depth of focus. 
Another embodiment of the lithographic imaging system of the present 
invention is depicted in FIG. 1B. A light source 122 is shown illuminating 
a standard transmission mask 124. The mask 124 is then imaged through 
converging optical element 126 to produce a lithographic image 128 on a 
substrate 130. A Fourier transform of the spatial frequencies of mask 124 
is produced in the focal plane of the optical element 126. A symmetrical 
(45.degree. slope) wedge-shaped focal plane phase-shifting optical element 
132 is positioned at the focal plane of the optical element 126, creating 
a gradual phase shift as a function of frequency. 
The phase-shifting optical element can be formed in other shapes having 
continuous or discrete feature changes. Suitable optical surfaces which 
can be readily made as a phase-shifting optical element are spherical and 
aspherical shapes such as ellipsoids, paraboloids, and hyperboloids of 
revolution. These shapes could be used as a single surface with the second 
surface being an optical flat or in combination on two or more surfaces of 
the phase-shifting element. Suitable symmetrically-shaped elements having 
continuous feature changes include pyramid parabolic or cone-shaped 
elements. Discrete feature changes include steps and the like. 
Phase shifting can also be introduced into the phase-shifting optical 
element by changing the refractive index of the material (e.g., ion 
implantation) or addition of another material with a different refractive 
index. Phase-shifting optical elements are well known to those skilled in 
the art, and suitable phase-shifting optical elements for use in the 
present invention will be known to those skilled in the art. A 
phase-shifting element may conveniently be made of a high optical index 
material, such as quartz, sapphire, or glasses such as BK7 and BaK2. The 
phase-shifting optical element (e.g., 45.degree. wedge) will result in 
enhancement of the image quality (e.g., parallel spaced-apart lines). If 
further enhancement of the image is desired, it can be achieved by an 
iterative process of (i) determining Fourier transform (FT) of the mask 
and the FT of the desired image, and (ii) analyzing (deconvolving) the two 
FTs to determine an FT phase shift which would be a first approximation to 
the phase shift desired to achieve the additional enhancement. The Fourier 
transform of mask features can be calculated by standard Fourier transform 
algorithms such as the fast Fourier transform, the discrete Fourier 
transform, or full numeric evaluation of the appropriate Fourier transform 
integrals. Such calculations are disclosed in Levenson et al., IEEE 
Transactions on Electron Devices, Vol. 29, December 1982, the disclosure 
of which is incorporated herein by reference. This first approximation FT 
phase-shifting optical element can then be iteratively modified and 
evaluated by successive approximations using computer simulation to 
produce an optical element having the desired image enhancement. For 
example, using a parabolic-shaped mask enhances the image, but the 
high-frequency components cause some accompanying image distortion. The FT 
parabolic-shaped mask could then be modified (e.g., by the addition of 
linear components) to achieve less shift in the high frequency. 
As an illustration of how the above-described embodiment generates a 
phase-shifted image, consider a transmission mask 124 comprising a number 
of equally-spaced lines. FIGS. 2A and 2B depict, respectively, the 
electric field and the intensity of the illumination (e.g., light) of the 
image at the mask generated by a conventional lithographic system. Only 
four lines of the features from the transmission mask 124 are shown in 
FIGS. 2A and 2B for the reason of clarity. Note the large background 
intensity in FIG. 2B. FIG. 2C shows the Fourier transform versus arbitrary 
frequency at the focal plane of the image of FIG. 2A as calculated by fast 
Fourier transform. FIG. 2D depicts the Fourier transform phase of the 
electric field at the focal plane. 
The placement of the wedge phase-shifter 132 at the focal plane of the 
imaging optics 126 introduces a frequency-dependent phase shift of 
45.degree. per arbitrary frequency unit, as depicted in FIG. 2E, into the 
Fourier transform image at the focal plane, resulting in the "shifted" 
phase of the electric field as depicted in FIG. 2F. Finally, the 
phase-shifted Fourier image at the focal plane results in the 
phase-shifted image 128 having an electric field at the wafer as depicted 
in FIG. 2G and a light intensity at the wafer as depicted in FIG. 2H. 
A comparison of FIGS. 2B and 2H shows significant improvements in image 
quality resulting from the present invention. The image in FIG. 2H has 
substantially less background intensity and substantially narrower 
linewidths. Using a wedge phase-shifted optical element, it is possible to 
improve contrast of the image even when there is a large background 
intensity due to the use of a standard transmission mask or due to other 
light transmitted or scattered in the optical system. 
As another example illustrating the present invention, in FIG. 3 there is 
shown a phase-shifting optical element which has discrete steps in 
addition to a linear continuous phase shifting (e.g., a wedge which is 
stepped or notched). The dimension of the discrete change will suitably be 
.lambda./k .+-.n.lambda. where k=2,4 or the like and n=0,1,2 . . . The 
placement of the "stepped" or "notched" wedge phase-shifting element at 
the focal plane of the imaging optics 126 introduces the 
frequency-dependent phase shift as depicted in FIG. 3A into the Fourier 
transform image at the focal plane, resulting in the "shifted" phase of 
the electric field depicted in FIG. 3B. Finally, the phase-shifted Fourier 
image at the focal plane results in the phase-shifted image 128 having an 
electric field as depicted in FIG. 3C and an illumination intensity as 
depicted in FIG. 3D. A comparison of FIGS. 2B and 3D indicates that the 
line features corresponding to FIG. 2B can be imaged with greatly enhanced 
resolution. A comparison of FIGS. 2H and 3D readily shows that, by using 
focal plane phase shifters having discrete dimension changes, the 
resultant image can be enhanced. 
As still another example illustrating the present invention, consider again 
the lithography system depicted in FIG. 1B where a field aperture is 
present in the imaging optics. The field aperture has the effect of 
limiting the frequency components of the light passing through the 
transmission mask and accordingly reduces the resultant resolution in the 
image. FIGS. 4A and 4B depict, respectively, the electric field and the 
intensity distribution that would be generated by a conventional 
lithographic system. FIGS. 4C and 4D depict, respectively, the magnitude 
and the phase of the electric field at the focal plane. Notice that while 
FIG. 4D is identical to FIG. 2D, a comparison of FIGS. 4C and 2C clearly 
shows the loss of high-frequency components originally present in the 
transmission mask 124 owing to the presence of the field aperture in the 
imaging optics. Finally, the phase-shifted Fourier image at the focal 
plane results in the phase-shifted image at the wafer having an electric 
field as depicted in FIG. 4E and a light intensity at the wafer as 
depicted in FIG. 4F. The resolution and contrast of the image are 
enhanced. 
In summary, focal plane phase-shifting optical elements can be used to 
improve the contrast and resolution of mask features having a broad 
spectrum of spatial frequency components. Focal plane shifting elements 
having discrete dimension changes superimposed on a continuous dimension 
change can be applied to repetitive mask features having only a few 
spatial frequency components. Both types of focal plane phase shifters can 
be used simultaneously in the same focal plane or in successive focal 
planes of a conventional lithographic imaging system. 
Those skilled in the art will recognize that the foregoing description has 
been presented for the purposes of illustration and description only. It 
is not intended to be exhaustive or to limit the invention to the precise 
forms disclosed, and many modifications and variations are possible in 
light of the above teachings. Thus, the embodiments set forth herein are 
presented in order to explain the principles of the present invention and 
its practical applications to thereby enable others skilled in the art to 
best utilize the present invention in various embodiments, modifications, 
and equivalents as are suited to the particular use contemplated.