Radiation imagery chemistry: process – composition – or product th – Imaging affecting physical property of radiation sensitive... – Radiation sensitive composition or product or process of making
Reexamination Certificate
1998-07-16
2001-08-14
McPherson, John A. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Imaging affecting physical property of radiation sensitive...
Radiation sensitive composition or product or process of making
C430S290000, C430S321000, C430S322000, C430S325000, C430S396000, C430S945000
Reexamination Certificate
active
06274288
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to changing optical characteristics by application of radiation in a way which tends to contain the radiation. More specifically, an aspect of the present invention relates to using radiation to change characteristics of a polymer in a way that induces the polymer to better contain the radiation, e.g., using self-trapping and self-focusing.
BACKGROUND AND SUMMARY
Many applications are known for carriers and modifiers of radiation. Optical radiation, for example, can be contained by optical waveguides (e.g., optical fibers) and guided from one source to another. Lenses are often used for focusing optical radiation. One desirable characteristic of many of such devices is their ability to contain the radiation, i.e., to minimize the amount of the radiation that escapes from the desired location to undesired locations. A medium that better contains the radiation is more efficient.
Many polymers are physically changed when radiation is applied thereto. For example, the optical index of refraction of a photopolymer changes when it is exposed to optical radiation, i.e., light. However, usual changes in index of refraction based on this kind of “photopolymerization” exhibits a relatively slow response to optical intensity changes. For example, light-induced changes in index of refraction in a polymer may take on the order of milliseconds to complete.
Throughput, however, is often very important in production of these devices. The prior art has sometimes illuminated the photopolymer with intense light for a short time, to increase the fabrication throughput. The illumination is often turned off before the index of refraction change or any nonlinear optical effects appear. This allows the devices to be made at the maximum speed possible.
One aspect of the present invention goes against this established teaching by using relatively long exposures and inducing an index change in the material of interest during the exposure. Unexpectedly, the materials and operations described herein enable the index change to change the radiation passing properties of the material in a way that tends to contain the radiation that causes the index change. Two different mechanisms are described herein: self-trapping and self-focusing.
The field of high resolution projection photolithography has been limited by the resolution and depth-of-focus during the stage where patterns are formed. This in turn limits the integrated circuit density that can be produced using these patterns.
The smallest feature size x
min
that can be projected by a coherent imaging system is
x
min
=
λ
2
N
.
A
.
(
1
)
and the depth-of-focus (DOF) is
DOF
=
λ
2
·
(
N
.
A
.
)
2
(
2
)
where &lgr; is the wavelength of the illumination and N.A. is the numerical aperture of the optical projection system.
A direct route to patterning smaller features photolithographically is to reduce the wavelength from the i-line standard of today (365 nm) to excimer laser wavelengths (248 or 193 nm). The N.A. is typically 0.5, so the minimum feature size is on the order of the exposure wavelength.
However, many have desired to use even smaller features, e.g., a 0.1 &mgr;m (=100 nm) process. At this point, the resolution and depth-of-focus constraints of conventional optical lithography become severe. Therefore, techniques to extend the performance of optical lithography into this regime are of particular technological and economic significance.
It is hence desirable to reduce the wavelength of the optical beam, for example, to less than 248 or 193 nm. Light sources capable of emitting such short wavelengths and compatible photoresists are uncommon and costly. Therefore, conventional wisdom has implied that the resolution and depth-of-focus determined by the shortest practical optical wavelength (193 nm) has placed a fundamental limit on further miniaturization of integrated circuits using conventional optical lithography. There is a need for techniques that will enable making smaller features.
In view of this need, the present invention describes new techniques to extend the performance of photolithography by properly tailoring the nonlinear optical response of the photoresist in tandem with the optical exposure. The present invention shows exposure techniques exploiting self-focusing and self-trapping. These techniques provide significant improvement in the resolution and depth-of-focus above the inherent values of the optical projection system, even without replacing the existing exposure tools. These techniques could be used in conjunction with improved exposure tools, e.g., lasers having wavelengths less than 200 nm, or x-ray exposure tools, for even better response.
Self-focusing and self-trapping are two examples of nonlinear optical effects which may arise from one of many physical mechanisms. Self-focusing describes the formation of a light induced channel in an illuminated material which confines the optical beam. This channel serves as a lens. Self-trapping occurs when self-focusing substantially exactly counteracts beam spreading due to diffraction. When this happens, the cross section of the light induced channel remains substantially constant with propagation distance over the distance of the self-trapping. Other similar mechanisms also exist. For example, a modified self-trapping effect occurs when self-focusing is somewhat less than beam spreading due to diffraction. In that case, the change slows the rate with which diffraction occurs.
A material exhibits self-trapping or self-focusing when the index of refraction changes in the presence of optical radiation in a way to induce waveguiding of that same optical beam which causes the index of refraction to change.
The conditions under which a beam is self-trapped may be quantified. If, for instance, the Gaussian beam waist is &ohgr;
0
the beam expands by diffraction in a homogeneous medium having an index of refraction n
0
with an angular divergence given by:
θ
1
≈
tan
-
1
(
λ
π
n
o
ω
o
)
(
3
)
where &lgr; is the wavelength of light in vacuum, and &thgr;
1
is an angle describing the divergence of the beam relative to normal. Beam spreading by diffraction is avoided if the angular envelope given by Eq. 3 is trapped within the waveguide by total internal reflection. That is, light of a certain optical mode will be guided through a medium along a waveguide distinguished by a region of larger index of refraction relative to the surroundings.
In an optical material of this type, the waveguide is called the core, and the surroundings is called the cladding. The guidance condition for an optical waveguide of radius r and index n
0
depends on the index difference An between the core and cladding, as quantified by the following relation:
k r
(2
n
0
&Dgr;n
)
½
≧2 (4)
Self-trapping occurs when this waveguide is formed by the same beam which is trapped by the waveguide. Upon trapping, the beam diameter remains nominally constant, independent of the propagation distance.
In general, the diameter of a trapped beam may be slightly modulated along the propagation direction, as if waveguiding by the medium were due to a periodic sequence of convex lenses. This results in a channel with diameter variations. In this case, self-focusing does not exactly balance diffraction point-by-point along the longitudinal direction. Nevertheless, on average, the beam is trapped.
In the prior art, self-trapping and self-focusing have only been observed for a highly restrictive set of material and exposure conditions which many of ordinary skill in the art have believed to be impractical. Waveguiding based on the Kerr effect vanishes when the beam intensity is reduced below a critical value on the order of megawatts per square cm, as reported in Chiao et al., Phys. Rev. Lett., 13, 479-482 (1964). This critical value is typically so large (MW cm
−2
) that self-focusing and self-trapping are difficult to obs
Kewitsch Anthony S.
Yariv Amnon
California Institute of Technology
Fish & Richardson P.C.
McPherson John A.
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