U.S. patent application number 14/591336 was filed with the patent office on 2015-07-02 for resists for lithography.
The applicant listed for this patent is Pixelligent Technologies, LLC. Invention is credited to Zhiyun CHEN, Gregory D. COOPER, Z. Serpil GONEN WILLIAMS, Larry F. THOMPSON.
Application Number | 20150185616 14/591336 |
Document ID | / |
Family ID | 38923800 |
Filed Date | 2015-07-02 |
United States Patent
Application |
20150185616 |
Kind Code |
A1 |
COOPER; Gregory D. ; et
al. |
July 2, 2015 |
RESISTS FOR LITHOGRAPHY
Abstract
New routes involving multi-step reversible photo-chemical
reactions using two-step techniques to provide non-linear resist
for lithography are described in this disclosure. They may provide
exposure quadratically dependant on the intensity of the light.
Several specific examples, including but not limited to using
nanocrystals, are also described. Combined with double patterning,
these approaches may create sub-diffraction limit feature
density.
Inventors: |
COOPER; Gregory D.; (Fulton,
MD) ; CHEN; Zhiyun; (Rockville, MD) ; GONEN
WILLIAMS; Z. Serpil; (Lanham, MD) ; THOMPSON; Larry
F.; (Henly, TX) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Pixelligent Technologies, LLC |
Baltimore |
MD |
US |
|
|
Family ID: |
38923800 |
Appl. No.: |
14/591336 |
Filed: |
January 7, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13748267 |
Jan 23, 2013 |
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14591336 |
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11774171 |
Jul 6, 2007 |
8383316 |
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13748267 |
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60806877 |
Jul 10, 2006 |
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60870795 |
Dec 19, 2006 |
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Current U.S.
Class: |
430/270.1 ;
430/322 |
Current CPC
Class: |
Y10T 428/31504 20150401;
G03F 7/2002 20130101; G03F 7/20 20130101; G03F 7/0043 20130101;
G03F 7/2053 20130101; G03F 7/2022 20130101; G03F 7/2041 20130101;
G03F 7/0047 20130101; Y10T 428/24479 20150115 |
International
Class: |
G03F 7/20 20060101
G03F007/20 |
Claims
1. A resist comprising: at least one matrix material; a photoactive
material; and an acid generator attached to said photoactive
material, said photoactive material undergoing at least one
plural-step reaction comprising absorption of at least one photon
to generate a further photoactive material, said further
photoactive material at least in part reversing to the
first-mentioned photoactive material unless said further
photoactive material absorbs at least another photon to activate
said acid generator to change the solubility of said resist in
response to light, wherein said solubility change is non-linearly
dependent on the intensity of the light.
2. The resist of claim 1 wherein at least one of the
first-mentioned and further photoactive materials comprises
nanocrystals.
3. The resist of claim 1 wherein at least one of the
first-mentioned and further photoactive materials comprises a
semiconductor.
4. The resist of claim 1 wherein at least one of the
first-mentioned and further photoactive materials comprises at
least one organic molecule.
5. The resist of claim 1 wherein at least one of the
first-mentioned and further photoactive materials comprises at
least one inorganic molecule.
6. The resist of claim 1 wherein the acid generator forms an acid
in a manner that is quadratically dependent on the intensity of the
light.
7. The resist of claim 1 wherein said matrix material comprises a
polymer or a molecular glass.
8. The resist of claim 1 wherein said resist further comprises
additives to improve resolution and line edge roughness.
9. The resist of claim 8 wherein said additives comprise bases to
quench photo-generated acids generated by the acid generator.
10. The resist of claim 1 wherein said solubility changes in the
presence of a developer.
11. The resist of claim 1 further including responding to plural
light illuminations separated in time.
12. The resist of claim 1 wherein said resist is configured for use
in immersion lithography.
13. A method of exposing a resist comprising: illuminating at least
part of said resist with a light source; inducing in said resist in
a manner that is quadratically dependent on the intensity of said
illuminating light, at least one first photoactive material,
wherein an acid generator is attached to said first photoactive
material, said first photoactive material, in use, undergoing at
least one plural-step reaction comprising, absorption of at least
one photon to generate at least one second photoactive material,
said second photoactive material at least in part reversing to the
first photoactive material unless said second photoactive material
absorbs at least another photon to activate said acid generator
which ultimately leads to a change in solubility of said
resist.
14. The method of claim 13 wherein said first photoactive material
comprises nanocrystal photosensitive material.
15. The method of claim 13 wherein said first photoactive material
comprises semiconductor photosensitive material.
16. The method of claim 13 wherein said first photoactive material
comprises at least one organic molecule photosensitive
material.
17. The method of claim 13 wherein said first photoactive material
comprises at least one inorganic molecule photosensitive
material.
18. The method of claim 13 wherein said illuminating comprises
illuminating with at least one of the following light wavelengths:
157 nm, 193 nm, 248 nm, 257 nm, 198 nm, 121 nm and 365 nm.
19. The method of claim 13 wherein said illuminating comprises
illuminating with 13.4 nm light.
20. The method of claim 16 wherein said illuminating comprises
illuminating said resist multiple times separated by waiting times
therebetween.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 13/748,267 filed Jan. 23, 2013; which is
division of U.S. patent application Ser. No. 11/774,171 filed Jul.
6, 2007, now U.S. Pat. No. 8,383,316 issued Feb. 26, 2013; which
claims the benefit of U.S. Provisional Patent Application No.
60/806,877 filed Jul. 10, 2006, and U.S. Provisional Patent
Application No. 60/870,795 filed Dec. 19, 2006. The disclosures of
the prior applications are incorporated herein in their entirety by
reference.
TECHNICAL FIELD
[0002] The technology herein relates to I.sup.2 resists for
lithography processes. More particularly, the technology herein
relates to a new concept regarding multi-step photo reactions to
offer quadratic dependence to the exposure intensity. Still more
particularly, non-limiting aspects of the technology herein relate
to providing two-photon or multi-photon absorption with Auger
recombination processes and to semiconductor nanocrystals.
BACKGROUND AND SUMMARY
Lithography and Resist
[0003] Lithography is used to transfer an image or a pattern from a
mask onto a substrate. One example use of lithography is to
manufacture semiconductor devices such as integrated circuits.
Since 1971, advances in lithography have allowed integrated circuit
(IC) manufacturers to reduce minimum feature sizes from 10-20
microns down to 65 nanometers in 2006. This steady miniaturization
has enabled improvements in IC performance and growth in the
semiconductor industry.
[0004] An example optical lithography system includes a light
source, a mask, a projecting optical system and a resist coated
substrate. Light passed through the mask (e.g., a quartz substrate
with chrome patterns on one surface) is collected by the projecting
optical system to form a reduced image on the resist. The resist
changes its chemical properties when exposed to the light. After
developing, an identical or complementary pattern of the mask is
transferred to the resist. Further processing, such as etching as
one example, translates the pattern onto the substrate underneath.
By repeating this technique several times using different masks,
multi-layered structures (e.g., a silicon or other material based
integrated circuit) can be manufactured.
[0005] Generally, resists of the type used for lithography are thin
film materials that change solubility upon exposure to actinic
radiation. Resists can be used as a mask to create a three
dimensional structure. This process can be used to manufacture
electronic devices. There are, in general, two broad families of
resists: negative and positive. Negative resists become less
soluble on exposure (i.e. the exposed area remains after treatment
with an appropriate solvent, developer). Positive resists become
more soluble after exposure (i.e. the exposed resist is removed by
the developer). Within each of these two resist classifications,
many different resists have been used over time. There are many
chemical mechanisms that are known for both types.
[0006] Commercially available resists generally have several
properties including for example: [0007] Adequate sensitivity to
the actinic radiation--Each exposure technology uses a radiation
source that has a finite energy and/or intensity. The sensitivity
of the resist allows the exposure system to operate at sufficient
throughput. [0008] Resolution--Each exposure technology is
developed to produce features useful to manufacture devices with
defined minimum features (three dimensional structures). The resist
is able to resolve these features with good process latitude.
[0009] Adhesion--The resist is a thin film that is spin coated onto
a device surface. The resist adheres to the surface satisfactorily
to allow subsequent processing of the underling thin film. [0010]
Etch resistance--Most device processes involve the removal of
selected portions of a thin film that is not protected by the
resist. The resist "resists" whatever process is used to create the
final, desired, pattern, viz. liquid etching, plasma etching, ion
etching etc. [0011] Low defect density--The resist preferably
should not introduce additional (within reason) defects in the thin
film. [0012] The ability to use "safe processing chemicals" such as
spinning solvent, developers, etc. [0013] Ease of manufacture.
[0014] Adequate shelf life.
[0015] Multiple chemical mechanisms have been utilized for both
positive and negative resists. Some interesting negative resist
mechanisms include cross-linking and molecular weight increase. For
example, when a polymer is cross-linked, it becomes insoluble in
common organic solvents. If the cross-linking can be induced by
exposure to radiation, the material may be used as a resist to
pattern thin films used in the manufacture of electronic devices.
One non-limiting example is the electron beam resist COP, a
copolymer of glycidyl methacrylate and ethyl acrylate.
Cross-linking occurs through the epoxy moiety. Another negative
resist is based on crosslinking of cyclized poly(cis-isoprene) with
bis(arylazide). In addition, solubility of a polymer is generally
related to the molecular weight of the polymer. As the molecular
weight increases, the solubility decreases. Poly(p-hydroxystyrene)
(PHOST), when formulated with bis(arylazide), undergoes a radiation
induced molecular weight increase, resulting in decreased
solubility. The material can be made sensitive to a wide range of
radiation wavelengths by modifying the structure of the
bis(arylazide).
[0016] Example positive resist mechanisms include mechanisms such
as: [0017] Chain scission--Most polymers crosslink as a result of
irradiation; however, a few undergo chain scission and a reduction
in molecular weight. The lower molecular weight allows the exposed
polymer to be selectively dissolved in an appropriate solvent
(developer). Poly(methyl methacrylate) (PMMA) is a well known
polymer that undergoes chain scission and has been widely used as
an electron beam resist. The sensitivity of PMMA is to low to be
used in manufacture. Another family of polymers, poly(olefin
sulfones) exhibit .about.10.times. greater sensitivity than PMMA
and poly(butene-sulfone) has been used for a long time as an
electron beam resist in the manufacture of photomasks. [0018]
Chemical amplification--Very sensitive positive resist based on
chemical amplification have been developed. Example processes
typically involve photo-generation of an acidic species (some base
catalyzed systems have been described) that catalyzes many
subsequent reactions such as de-blocking of a protective groups
that are chemically bound to a matrix polymer. One such system is
based on a matrix resin, poly(4-t-butoxycarbonylstyrene) (TBS) and
arylsulfonium or iodonium salts. Radiation is used to generate an
acid which in turn removes the t-butoxycarbonyl resulting in the
base soluble poly(vinylalcohol). One acid group causes up to
several hundred de-protection events, thus amplifying the desired
reaction. These materials and derivatives thereof are in wide
spread use as the resist of choice in deep--UV (248 nm & 193
nm) lithography.
[0019] All resists used in the current production are linear
resist, they can not generate patterns smaller than the diffraction
limit allows. A non-linear resist combined with double or multiple
patterning is needed to created sub-diffraction limit patterns.
Two-Photon Resist and Multi-Photon Resist
[0020] In a quantum system with two levels, initial level E.sub.1,
and final level E.sub.2, a photon having energy E.sub.2-E.sub.1 can
be absorbed, promoting an electron from E.sub.1 to E.sub.2, in a
one photon absorption process. Also, a less likely process, called
two-photon absorption, can occur. In this process, two photons with
energy (E.sub.2-E.sub.1)/2 can be absorbed simultaneously. A
two-photon absorption process has smaller probability than a
one-photon process because it requires a simultaneous presence of
two photons at same location. Likewise, three-photon, four-photon,
and multi-photon can be absorbed with decreasing probability.
[0021] In a two-photon absorption
I x = - .beta. I 2 ( 1 ) ##EQU00001##
where I is the intensity of the beam and .beta. is defined as the
two-photon absorption coefficient to parallel the one photon, or
linear, absorption regime:
I x = - .alpha. I ( 2 ) ##EQU00002##
where .alpha. is the one photon absorption coefficient.
[0022] The two-photon absorption cross section is defined through
the absorption rate:
R=.delta.I.sup.2 (3)
Note here I is the number density of photon (number of photons per
second per unit area) and .delta. the two-photon absorption cross
section.
[0023] Wu et al. proposed a two-photon resist used in optical
lithography. See E. S. Wu, J. H. Strickler, W. R. Harrell, and W.
Webb, Proc. SPIE 1674, 776(1992). In a two-photon resist, the photo
sensitizer in the resist will only be exposed through a two photon
absorption process. Due to the quadratic dependence to the
intensity, the two-photon resist is capable of creating sharpened
features in the resist. As evidenced by the normalized exposure
profile shown by in FIG. 1. A standard testing pattern in
lithography is lines and spaces created by two interference plane
waves. At the diffraction limit, the light intensity distribution
at the resist can be expressed as:
I = 1 + cos ( 4 .pi. NA .lamda. x ) ( 4 ) ##EQU00003##
where NA is the numerical aperture of the optical system and
.lamda. is the wavelength of light.
[0024] In FIG. 1, an aerial pattern is transformed into a sharper
resist profile (P2) compared to a linear resist (P1). P1, P1.5, P2
and P4 are 1, 1.5, 2 and 4 photon absorption profile, respectively.
Combined with double patterning or multi-exposure, the two-photon
resist is capable of producing sub-diffraction limit image and is a
promising technique to extend optical lithography beyond its
current limit. See e.g., Ch. J. Schwarz, A. V. V. Nampoothiri, J.
C. Jasapara, W. Rudolph, and S. R. J. Brueck, J. Vac. Sci. &
Tech. B 19 (6): 2362-2365 (2001). FIGS. 2A-B demonstrates how a
two-photon resist enables double patterning. With two exposures
(P1A and P1B) shifted by a quarter of the spatial period will
result in a uniform exposure (PF1) in a linear resist, as shown in
FIG. 2A, a linear resist sums up the two exposures and results in a
constant exposure, all contrast is lost. A two photon resist is a
non-linear resist. A nonlinear resist has a nonlinear response rate
to either exposure intensity or time, or both. In an ideal
two-photon resist, the two exposures (P2A, P2B) will result in an
exposure profile (PF2) with doubled spatial frequency, as shown in
FIG. 2B. If the spatial frequency of the light pattern of each
exposure is at the diffraction limit then this double patterning
process enables sub-diffraction limit lithography.
[0025] In fact, similar to the above argument a multi-photon
absorption process can be used to produce a multi-photon resist. In
a multi-photon process, the absorption rate, R:
R=.delta.I.sup.P (5)
where P equals to the number of photons involved in one absorption
event. Multi-photon resist is capable of achieving even higher
resolution, as shown in FIGS. 2A-B for an example of P=4 (P4).
[0026] Further, in equation (5), the resolution will still be
improved even if 1<P<2. As shown in FIG. 1. for P=1.5
(P1.5).
[0027] Current two-photon resists, however, are mainly used to
create 3-D patterns, not in planary pattern creation. The main
reason is the extremely high light intensity involved. The
conventional two-photon absorption process is after all a second
order process. It requires absolute coincidence of two photons on
the absorbing molecule. The absorption cross-section is extremely
small, .about.10.sup.-50 cm.sup.4 s. See E. S. Wu, J. H. Strickler,
W. R. Harrell, and W. Webb, Proc. SPIE 1674, 776 (1992). To achieve
a practical intensity, a pico-second or femto-second laser has to
be used. The DUV lasers used in current lithography industry has
pulse width .about.10 ns. We describe a new type of two-photon
resist based on a mechanism other than the traditional two-photon
absorption. The exposure in this resist may have a quadratic or
higher order dependence on the light intensity yet it may not
involve a traditional two-photon absorption, therefore we refer to
it as I.sup.2 resist. A two-photon resist, by our definition, is a
special case of I.sup.2 resist.
BRIEF DESCRIPTION OF THE DRAWINGS
[0028] These and other exemplary illustrative non-limiting features
and advantages of exemplary illustrative non-limiting
implementations will be better and more completely understood by
referring to the following detailed description of presently
preferred illustrative implementations in conjunction with the
drawings, of which:
[0029] FIG. 1 is an exemplary illustrative prior art of a two
photon absorption process;
[0030] FIGS. 2A, 2B show an exemplary illustrative prior art of
using double patterning combined with an I.sup.2 resist to improve
the lithography resolution;
[0031] FIGS. 3A, 3B, 3C demonstrate an exemplary illustrative prior
art of processes in a semiconductor nanocrystal; and FIG. 3D shows
an exemplary illustrative implementation of utilizing the Auger
electron to generate acid with acid generators;
[0032] FIGS. 4A-4G show an exemplary illustrative implementation of
a double patterning process, using a nonlinear resist;
[0033] FIGS. 5A-5F show an exemplary illustrative implementation of
a patterning process, using an I.sup.2 resist;
[0034] FIGS. 6A-6F show an illustrative non-limiting example of how
the reset time of the nanocrystal based I.sup.2 resist in a double
patterning process affects the resolution; and
[0035] FIGS. 7A, 7B and 7C show an illustrative non-limiting
example of achieving uniform exposure across the entire thickness
using the I.sup.2 resist in a multiple exposure and double
patterning process.
DETAILED DESCRIPTION OF EXEMPLARY NON-LIMITING ILLUSTRATIVE
IMPLEMENTATIONS
[0036] A preferred non-limiting illustrative implementation
provides a non-linear resist for lithography. An exemplary
illustrative non-limiting resist implementation comprises for
example a non-linear acid generator and polymeric resins.
[0037] A non-linear resist possessing high absorption cross-section
may be integrated into existing optical lithography to create
sub-diffraction limit patterns in production. Consider a series of
exemplary non-limiting reactions:
A + hv .revreaction. k 1 [ B ] [ A ] .sigma. 1 I B B + hv
.revreaction. k 2 [ C ] [ B ] .sigma. 2 I C -> k 3 [ C ] D ( 6 a
& 6 b ) ##EQU00004##
In this patent, "" represents a reversible reaction, hv stands for
a photon with frequency, v. A may be a photo sensitizer in the
ground state. It may include an atom, a group of atoms, a molecule,
a group of molecules, a nanocrystal or a group of nanocrystals. B
may be the same photo sensitizer A at an excited state, i.e.
different electronic configuration, spatial arrangement, ionic
state, etc., or include a different molecule, nanocrystal, group of
atoms, molecules or nanocrystals. C may be the same entity as B at
an excited state, i.e. different electronic configuration, spatial
arrangement, ionic state, etc., or include a different molecule,
nanocrystal, group of atoms, molecules or nanocrystals. And D may
include an atom, a molecule, a nanocrystal, a group of molecules, a
group of nanocrystals, an ion, an electron, a proton, a photon at
different wavelength, a chain scission event, a cross-linking
event, or a series of reactions which may eventually result in the
exposure of a resist.
[0038] I is the intensity of light, [A], [B] and [C] are the
concentrations of A, B, and C respectively. Note that
[A]+[B]+[C]=C.sub.0, which is the initial concentration of A.
.sigma..sub.1 and .sigma..sub.2 are the molar absorptivity of the
reactant A and B at the actinic wavelength, k.sub.1, k.sub.2 are
the reaction rates of the respective reverse reactions. And k.sub.3
is the reaction rate of C.fwdarw.D.
[0039] The reactions described in equations 6a and 6b lead to a
non-linear response of the resist to the light intensity. One
exemplary illustrative non-limiting implementation comprises for
example the reactions in equations (6a&b) are under steady
state, which means the forward and reverse reactions are balanced.
We can derive
R = .sigma. 1 .sigma. 2 k 3 k 1 k 2 + k 2 .sigma. 1 I + .sigma. 1
.sigma. 2 I 2 C 0 I 2 ( 7 ) ##EQU00005##
where R is the generation rate of reaction product D.
[0040] The generation rate of D apparently has a non-linear
relationship to the light intensity I. Under certain circumstances,
for example, if the reaction can be controlled such that, [B],
[C]<<C.sub.0, we can get:
R = .sigma. 1 .sigma. 2 k 3 C 0 k 1 k 2 I 2 ( 8 ) ##EQU00006##
[0041] And if the product D is proportional to the total exposure
in the resist, equation (8) essentially provides an I.sup.2 resist,
which enables double patterning as shown in FIGS. 2A-B. The
reaction described in equation (8), however, employs only the
one-photon molar absorptivity therefore it may be realized with the
intensity achievable in the current lithography infrastructure.
[0042] Other variations and combination of variations of the
equations (6a&b) may also result in the same I.sup.2 dependent
relationship. A non-limiting example may be described as follows
with four co-existing reactions:
A + hv -> [ A ] .sigma. 1 I B B -> k 1 [ B ] B 1 B + hv ->
[ B ] .sigma. 2 I C -> k 3 [ C ] D C -> k 2 [ C ] C 1 ( 9 a ,
9 b , 9 c , and 9 d ) ##EQU00007##
where B.sub.1 and C.sub.1 are the reaction by-products. They may be
the same photo predecessor sensitizers at a different state, i.e.
different electronic configuration, spatial arrangement, ionic
state, etc., or include different atoms, molecules, nanocrystals,
groups of atoms, molecules or nanocrystals, ions, protons, photons
at different wavelength, chain-scission event or events,
cross-linking event or events, or series of reactions.
[0043] Consider another series of exemplary non-limiting
reactions:
A 1 + hv .revreaction. k 1 [ B ] [ A 1 ] .sigma. 1 I B A 2 + hv
.revreaction. k 2 [ C ] [ A 2 ] .sigma. 2 I C B + C -> k 3 [ B ]
[ C ] D ( 10 a , 10 b , and 10 c ) ##EQU00008##
[0044] A.sub.1 and A.sub.2 may be two different photo-sensitizers,
again they can be atoms, group of atoms, molecules, groups of
molecules, nanocrystals or groups of nanocrystals. B may be the
same photo sensitizer A.sub.1 at an excited state, i.e. different
electronic configuration, spatial arrangement, etc., or a different
molecule, nanocrystal, group of atoms, molecules or nanocrystals. C
may be the same photo sensitizer A.sub.2 at an excited state, i.e.
different electronic configuration, spatial arrangement, etc., or a
different atom, molecule, nanocrystal, group of atoms, molecules or
nanocrystals. And D may include an atom, a molecule, a nanocrystal,
a group of molecules, a nanocrystal, an electron, a proton, a
photon at different wavelength, chain scission event or events, a
cross-linking event or events, or a series of reactions which may
eventually result in the exposure of a resist.
[0045] The reactions described in equations 10a, 10b and 10c lead
to a non-linear response of the resist to the light intensity. One
exemplary illustrative non-limiting implementation comprises for
example the reactions in the equations (10a, 10b, and 10c) are
under steady state. Assuming the last reaction is a first order
reaction, then the generation rate of D may be shown as:
R = k 3 [ A 1 ] [ A 2 ] .sigma. 1 .sigma. 2 I 2 k 1 k 2 ( 11 )
##EQU00009##
[0046] The generation rate for D is an I.sup.2 relationship and at
the same time employs only the one-photon molar absorptivity, as
shown in the equation (11), therefore it may be realized with the
intensity achievable in the current optical lithography
infrastructure.
[0047] An exemplary illustrative non-limiting resist implementation
comprises for example semiconductor nanocrystals, acid generators
and polymeric resins. Such non-limiting exemplary nanocrystals may
have bandgaps that are smaller or equal to the lithographic
wavelength. Nanocrystals are loosely defined as particles with
diameter ranging from 1 nm to 100 nm which retain the stoichiometry
and crystal structure of their bulk counterpart. They assume
different names, such as quantum dot, quantum sphere, quantum
crystallite, micro-crystal, colloidal particle, nano-particle,
nano-cluster, Q-particle or artificial atom. They also assume
different shapes, such as spherical, cubical, rod-like, tetragonal,
single or multiple-walled nano-tubes, etc.
[0048] Due to their small size, nanocrystals often demonstrate
dramatically different physical properties from their bulk
counterparts. Most prominent are the size-quantization and the
tunability of the bandgap. For example in one of the model
semiconductor nanocrystal material, CdSe, the optical absorption
can be shifted from .about.700 nm to .about.400 nm by simply
changing the size. See C. B. Murray, D. J. Norris, M. G. Bawendi,
J. Am. Chem. Soc. 115, 8706(1993).
[0049] As shown in FIG. 3A, one nanocrystal (NC) with quantized
energy level is struck with a photon with an energy equal to or
larger than the bandgap. This photon promotes an electron (Q1) from
an energy level in the valence band (E1) to a level in the
conduction band (E2), leaving a hole (H1) in E1. The electron and
hole quickly thermalize to the lattice and relax their energy to
their respective lowest energy levels (EC and EV) and form an
exciton, allowing a second absorption of photon with the same
energy. This process usually happens in less than 1 pico-second.
See V. I. Klimov, D. W. McBranch, C. A. Leatherdale, and M. G.
Bawendi, Phys. Rev B 60, 13740(1999). If a second photon comes in
before the first exciton recombines, as shown in FIG. 3B, then a
second electron-hole pair (Q2 and H2) is created, which
subsequently relaxes its energy and forms another exciton, as shown
in FIG. 3B. Note that in FIG. 3B, although there are two electrons
and holes at the same energy levels, it does not violate the Pauli
exclusion principle. Said electron-hole pairs are in the form of
excitons, which are bosons and do not obey the exclusion principle.
In the particular examples in FIG. 3C, the energy released by the
recombination of Q1 and H1 is transferred to Q2 through an Auger
process. The electron Q2 gains enough energy to overcome the
interface barrier between the nanocrystal and the surrounding
medium to be ejected out of the nanocrystal and forms an Auger
electron (QA), leaving a positive hole in the said nanocrystal, as
shown in FIG. 3C. In said nonlinear resist, ES can be provided by a
surface level, interface level, defect level in the surrounding
medium, or a surfactant or an electron scavenger, a photo-acid
generator or other functional chemicals in the resist.
[0050] The process described here may be described in light of
equations (5). We only have to replace A with the semiconductor
nanocrystal, replace B with the same nanocrystal and one exciton,
replace C with the same nanocrystal and two excitons, and replace D
with a charged nanocrystal and an ejected electron. As follows:
NC + hv .revreaction. k 1 [ C 2 ] .sigma. 1 [ C 1 ] I NC ( e - h )
NC ( e - h ) + hv .revreaction. k 2 [ C 3 ] .sigma. 2 [ C 2 ] I NC
( e - h ) 2 -> k 3 [ C 3 ] NC ( h ) + e ( 12 a & 12 b )
##EQU00010##
where e represents an electron, h a hole, and (e-h) an exciton;
[C1], [C2], [C3] are the concentration of nanocrystal with no
exciton, one exciton and two excitons respectively; and again
[C1]+[C2]+[C3]=C.sub.0, the initial nanocrystal concentration.
[0051] As has been demonstrated by V. I. Klimov, A. A.
Mikhailovsky, D. W. McBranch, C. A. Leatherdale, M. G. Bawendi,
Science, 287, 1011(2000), the life time of two excitons in a
nanocrystal is at least an order of magnitude shorter than one
exciton. This means that we can assume k.sub.2>>k.sub.1, and
[C.sub.2]<<[C.sub.1]. Also, since the life time of the
excitons (<ps) are usually much smaller than the duration of a
pulsed used in optical lithography (.about.10 ns), the reaction may
be considered at steady state. Hence the electron generation rate
is quadratically dependent on the light intensity, as predicted by
(8).
[0052] The equivalent two-photon absorption cross section was found
to be 10.sup.-40 cm.sup.4 s, much larger than that of the
conventional two photon resist. See M. Haase, H. Weller, A.
Henglein, J. Phys. Chem, 92, 4706(1988). This large absorption
cross section allows this resist to be exposed at a much lower
light intensity, namely, a level achievable by the laser used in
the lithography industry.
[0053] An aspect of preferred non-limiting illustrative
implementation provides a nanocrystal based non-linear resist for
lithography. As shown in FIG. 3D, the Auger electron QA, can be
further transferred to an acid generator (AG), either attached to
the surface or in the close proximity of the nanocrystal NC, the
acid generator AG accepts the Auger electron QA and releases an
acid (AC) (most conventional photo-acid generators used in
lithography have been shown to be able to react with low energy
electrons to release acids. See Atsuro Nakano, Takahiro Kozawa,
Seiichi Tagawa, Tomasz Szreder, James F. Wishart, Toshiyuki Kai,
and Tsutomu Shimokawa, Jpn. J. Appl. Phys., 45, L197-L200 (2006).),
said acid AC subsequently reacts with polymeric resin in said
non-linear resist to change the solubility of said polymeric resin
in an appropriate development solvent in the areas exposed. The
hole left behind, H2, may eventually be trapped in a defect level,
surface level, interface level, a hole scavenger, or any functional
chemicals in the resist.
[0054] Another aspect of the preferred non-limiting illustrative
implementation of providing a nanocrystal based resist is that the
Auger process can eject a hole instead of an electron, in this case
the acid generator AG donates an electron to the ES level or the
said nanocrystal directly, recombine with the hole generated by the
Auger process and release an acid, this acid reacts with polymeric
resin in said non-linear resist to change the solubility of said
polymeric resin in an appropriate development solvent in the areas
exposed.
[0055] Another aspect of the preferred non-limiting illustrative
implementation of providing a nanocrystal based resist is that the
non-linearity is a result of two-photon absorption, as described in
equation (1), between two energy levels inside said nanocrystal.
The excited electron, or hole, may gain enough energy to overcome
the barrier between the nanocrystals and is excited out of the
nanocrystal into energy level ES in FIG. 3C. ES can be provided by
a surface level, interface level, defect level in the surrounding
medium, or a surfactant or an electron scavenger, a photo-acid
generator or other functional chemicals in the resist.
[0056] Another aspect of the non-limiting illustrative exemplary
implementation of providing a nanocrystal based resist is that the
Auger process can eject a hole instead of an electron, in this case
the acid generator AG in FIG. 3D donates an electron to the energy
level ES of the said nanocrystal directly, recombine with the hole
generated by the Auger process and release an acid, this acid
reacts with polymeric resin in said nonlinear resist to change the
solubility of said polymeric resin in an appropriate development
solvent in the areas exposed.
[0057] Another aspect of the preferred non-limiting illustrative
implementation of providing a non-linear resist is to use 193 nm
resist polymers such as co-, ter-, tetra-polymers of; methacrylates
comprising terpolymer tetr-butyl methacrylate, methyl methacrylate,
methacrylic acid; norbornenes comprising copolymer
norbornene-maleic anhydride, copolymer norbornene-sulfur dioxide;
copolymer vinyl ether-maleic anhydride; and their derivatives
[0058] Another aspect of the preferred non-limiting illustrative
implementation of providing a non-linear resist is to use 248 nm
resist polymers such as co-, ter-, tetra-polymers of;
tert-butoxycarbonyls comprising poly
4-tert-butoxycarbonyloxystyrene,
poly(styrene-co-(4-hydroxyphenyl)maleimide),
poly(styrene-co-maleimide), poly(4-hydroxystyrene sulfone),
poly(4-hydroxy-a-methylstyrene),
poly(tert-butoxystyrene-co-4-acetoxystyrene),
poly[4-(2-hydroxyhexafluoroisopropyl)styrend copolymers of
tert-butoxystyrene and tert-butyl acrylates; and their derivatives
and molecular glasses, such as
4-[4-[1,1-Bis(4-tert-butoxycarbonyloxybenzyl)-ethyl]]-r,r-dimethyl
benzylphenol.
[0059] Another aspect of the preferred non-limiting illustrative
implementation of providing a non-linear resist is to use 157 nm
resist polymers such as co-, ter-, tetra-polymers comprising
tetrafluoroethylene-norbornene;
2-trifluoromethylacrylate-norbornene;
2-trifluoromethylacrylate-styrene, 2-trifluoromethylacrylate-vinyl
ether; methacrylates; and their derivatives.
[0060] Another aspect of the preferred non-limiting illustrative
implementation of providing a non-linear resist is to use 365 nm
resists such as diazonaphthoquinone/novolac resist.
[0061] Another aspect of the preferred non-limiting illustrative
implementation of providing a non-linear resist is to use acid
quenchers such as aniline derivatives or 1,8
diazabicyclo[5.4.0]undec-7-ene.
[0062] Another aspect of the preferred non-limiting illustrative
implementation of providing a non-linear resist is to use this
resist with a photoacid generator from a non-exhaustive list of
diaryliodonium salts such as bis(4-tert-butylphenyl)iodonium
trifluoromethane sulfonate; triarylsulfonium salts such as
triphenylsulfonium hexafluoroantimonate; and nanionic photoacid
generators such as 1,2,3,-tris(methanesulfonyloxy)benzene.
[0063] Another preferred non-limiting illustrative implementation
is also to provide a non-linear resist for lithography. An
exemplary illustrative non-limiting resist implementation comprises
for example semiconductor nanocrystals and polymeric resins. Said
nanocrystals have bandgaps smaller or equal to the lithographic
wavelength. In such a resist, said semiconductor nanocrystals
generate Auger carriers (either electrons or holes) upon absorbing
the photons, said carriers may cause scissions in the surrounding
polymers and change the solubility of said polymers in developer
under the exposed area.
[0064] Another preferred non-limiting illustrative implementation
is also to provide a non-linear resist for lithography. Said resist
comprises, but not exclusively, of semiconductor nanocrystals and
polymeric resins. Said nanocrystals have bandgap smaller or equal
to the lithographic wavelength. In such a photo-resist, said
semiconductor nanocrystals generate Auger carriers (either
electrons or holes) upon absorbing the photons, said carriers may
cause cross-linking in the surrounding polymers and change the
solubility of said polymers in developer under the exposed
area.
[0065] One aspect of the above preferred non-limiting illustrative
implementations of providing a nanocrystal based resist is that
said resists may provide high refractive index. The semiconductor
nanocrystals usually have much higher refractive indices at DUV
range than polymers. With significant loading of nanocrystals in
the nonlinear resist, it can be used as a high refractive index
resist, which renders extra resolution benefits when used with
immersion lithography.
[0066] Another preferred non-limiting illustrative implementation
provides a resist for EUV lithography. Said resist includes
semiconductor nanocrystals. EUV lithography uses light with a
wavelength of 13.4 nm, in the soft X-ray range. The photon energy
is higher than the bandgap of any material. In such a resist, a
photon excites electrons from the core levels of the constituent
atoms of the nanocrystals. The excited electron may create multiple
excitons through impact ionization. The recombination energy
released by these multiple electron-hole pairs can be transferred
to one or multiple electrons (or holes) through the Auger process.
These Auger electrons (or holes) possess energies to overcome the
energy barrier between the nanocrystal and surrounding medium. Said
Auger electrons (or holes) may escape the nanocrystals and change
the solubility of the polymer under the exposed area.
[0067] Another preferred non-limiting illustrative implementation
provides an N-photon resist, N.gtoreq.2. Said N-photon resist
includes at least one N-step reaction causing nonlinear response
proportional to I.sup.N. Resolution equal to one Nth of the
diffraction limit can be achieved.
[0068] Another preferred non-limiting illustrative implementation
provides an N-photon resist, N.gtoreq.2. Said N-photon resist
includes semiconductor nanocrystals. In a similar fashion as a
two-photon Auger process described in FIGS. 3A-D, N-photon process
can also occur in nanocrystals, generating an Auger electron or
hole. Resolution equal to one Nth of the diffraction limit can be
achieved.
[0069] Another preferred non-limiting illustrative implementation
provides a nonlinear resist for quantum interferometric lithography
process employing entangled photons. Said nonlinear resist includes
semiconductor nanocrystals. The entangled photons may be generated
by a parametric down conversion process. The quantum entanglement
enables all said entangled photons to be absorbed simultaneously at
the presence of an N-photon resist. The advantage of the said
process is that it achieves .lamda./2N resolution without high
intensity usually required for a nonlinear resist.
[0070] One aspect of all the above preferred non-limiting
illustrative implementations is that they may provide high etch
resistance. The semiconductor nanocrystals usually have much higher
etch-resistance than polymers. With significant loading of
nanocrystals in the nonlinear resist, it can be used as a high etch
resistance resist, which renders extra resolution benefits over
polymer based resist.
[0071] Another preferred non-limiting illustrative implementation
provides a process of producing devices and structures on a
substrate using an I.sup.2 resist. Said I.sup.2 resist may comprise
nanocrystals and polymeric resins. Said nanocrystals have bandgap
smaller than or equal to the lithographic wavelength.
[0072] Another preferred non-limiting illustrative implementation
provides a process of producing devices and structures on a
substrate using an I.sup.2 resist in a double patterning process.
Said I.sup.2 resist may comprise nanocrystals and polymeric resins.
Said nanocrystals have bandgap smaller than or equal to the
lithographic wavelength. The two exposures may be separated by
periods long enough for the said nanocrystals to reset. The two
exposures may have at least some different pre-determined exposure
parameters to ensure the best resolution of the final resist
exposure profile.
[0073] Another aspect of the preferred non-limiting illustrative
implementation of a process of producing devices and structures on
a substrate is that the pattern on said substrate may have a
resolution that is higher than the diffraction limit of the
wavelength of said illuminating light.
[0074] Another aspect of the preferred non-limiting illustrative
implementation of a process of producing devices and structures on
a substrate is that the nanocrystals may generate electrons or
holes upon absorbing photons within said illuminating light. The
wavelength may comprise at least one of 365 nm, 257 nm, 248 nm, 198
nm, 193 nm, 157 nm, and 121 nm.
[0075] Another aspect of the preferred non-limiting illustrative
implementation of a process of producing devices and structures on
a substrate is that the said nanocrystals may be chosen from the
non-exclusive list of materials: C, Si, Ge, MgO, MgF.sub.2, ZnO,
ZnS, ZnSe, CdS, CdSe, CdTe, HgTe, PbS, BN, AlN, AIBGaN, AlP, AlAs,
BP, BAs, GaN, Ga.sub.2O.sub.3, GaP, GaAs, In.sub.2O.sub.3, InP,
InAs, SiC, Si.sub.3N.sub.4, CaF.sub.2, Al.sub.2O.sub.3, SiO.sub.2,
TiO.sub.2, Cu.sub.2O, ZrO.sub.2, SnO.sub.2, Fe.sub.2O.sub.3,
HfO.sub.2, Gd.sub.2O.sub.3, CeO.sub.2, Y.sub.2O.sub.3, Au, Ag, Al,
Cu, and their various polymorphs and alloys; said nanocrystals may
be in spherical, cubic, rod-like, tetragonal, single or multi-wall
nano-tube or other nano-scale geometric shapes; and particles may
be doped by other elements; said nanocrystals may be coated with
one or more shells of other materials; and said shell material may
comprise any known materials.
[0076] Another aspect of the preferred non-limiting illustrative
implementation of a process of producing devices and structures on
a substrate is that the resist may comprise acid generators and
polymeric resin. Each acid generator may create at least one acid
upon accepting at least one electron (or hole). The acid may
further change the solubility of said polymeric resin in a
developer. The resist may comprise polymeric resin that can change
solubility in a developer upon accepting at least one electron (or
hole).
[0077] Another aspect of the preferred non-limiting illustrative
implementation of a process of producing devices and structures on
a substrate is that the nanocrystals may be non-uniformly
distributed depthwise within said resist.
[0078] Another aspect of the preferred non-limiting illustrative
implementation of a process of producing devices and structures on
a substrate is that the nanocrystals may provide high etch
resistance and/or a high refractive index.
[0079] The exemplary illustrative non-limiting technology herein
may further provide a manufacturing line for creating a pattern on
a substrate, comprising a device that applies a resist comprising
nanocrystals to said substrate; a source of illumination that
illuminates said resist with a pattern of light at a predetermined
wavelength, said nanocrystals absorbing said light to at least in
part expose said resist; and at least one further device that
processes said exposed resist to create said pattern on said
substrate. The processing line same or different illuminator may
illuminate said resist plural times to multiply expose said
resist.
[0080] The exemplary illustrative non-limiting technology herein
may further provide device having a pattern thereon created at
least in part by illuminating a resist disposed on a substrate,
said resist comprising nanocrystals that absorb at least a portion
of said illumination to at least in part expose said resist.
[0081] The illustrative non-limiting exemplary technology herein
may also provide product intermediary comprising: a substrate
having at least one surface; and a resist layer that at least in
part covers said at least one substrate surface, said resist layer
comprising nanocrystals that, when exposed by illuminating light,
absorb at least a portion of said illuminating light to change the
solubility of said resist layer.
[0082] The exemplary illustrative non-limiting technology herein
may further provide an integrated circuit comprising a substrate;
and at least one structure disposed on said substrate, wherein said
structure is formed at least in part using a nonlinear resist
comprising nanocrystals that absorb illumination to change the
resist's solubility. The structure may comprise an electrical
circuit component such as a transistor.
[0083] The exemplary illustrative non-limiting technology herein
further provides an optical device comprising a substrate; and at
least one structure disposed on said substrate, wherein said
structure is formed at least in part using a nonlinear resist
comprising nanocrystals that absorb illumination to change the
resist's solubility. The structure may be three-dimensional.
[0084] Another common aspect of the all the previously disclosed
preferred non-limiting illustrative implementations is that they
are intended to be compatible and be used in dry, water immersion,
or high refractive index immersion lithography.
EXAMPLES
[0085] A non-limiting illustrative example of a double patterning
process using I.sup.2 resist to create trenches is shown in FIGS.
4A-G. In FIG. 4A, a substrate (SUB) is cleaned and treated
accordingly. In FIG. 4B, a layer of said I.sup.2 resist (RE) is
applied on top of the said substrate and a pre-bake is carried out
to remove the solvent in the I.sup.2 resist. FIG. 4C, an exposure
using a lithographic process is then provided to expose the first
group of desired areas (A1, A2) of said nonlinear resist. FIG. 4D,
a second exposure is provided to expose the second group of desired
areas (A3, A4) of said I.sup.2 resist. In FIG. 4E, the I.sup.2
resist is developed and the exposed area removed and a post bake is
carried out. In FIG. 4F, an etch process is then carried out to
create trenches (TR1, TR2, TR3, TR4) at the exposed area. And
finally in FIG. 4G, the rest of the resist is stripped off.
[0086] A non-limiting illustrative example of a process to create
two trenches is shown in FIGS. 5A-F. In FIG. 5A, a substrate (601)
is cleaned and treated accordingly. In FIG. 5B, a layer of said
nonlinear resist (602) is applied on top of the substrate (601) and
a pre-bake is carried out to remove the solvent in the nonlinear
resist. In FIG. 5C, an exposure using a lithographic process is
then provided to expose the desired areas (603, 604) of said
nonlinear resist. In FIG. 5D, the nonlinear resist is developed and
the exposed area removed and a post bake is carried out. In FIG.
5E, an etch process is then carried out that create trenches (605,
606) at the exposed area. And finally in FIG. 5F, the rest of the
resist is stripped off. By repeating this process multiple times,
and replacing the etching process with other processes such as ion
implantation, film deposition, oxidation, etc., an electronic
device or other 3-D structures can be fabricated.
[0087] Another non-limiting illustrative example provides a method
of creating sub-diffraction patterns using double patterning with
the disclosed I.sup.2 resist, as shown in the normalized exposure
profile in FIGS. 6A-F. First, a layer comprising said resist is
spin-coated on a silicon wafer. The said nonlinear resist is then
exposed to a diffraction limited fringe pattern. The shaded area
(EX2A) received high enough intensity to undergo two-photo process.
In the area EX1A, only one-photon absorption occurred. After the
first illumination, all the photon-generated electrons and holes in
EX1A will recombine (reset). After EX1A is fully reset, as shown in
FIG. 6B, a second exposure with identical fringe pattern, shifted
half a period relative to the first pattern, is applied, as shown
in FIG. 6C. The same situation repeats, area EX2B undergoes
two-photon process while EX1B only experience one-photon
absorption. The final resist exposure profile of half the
diffraction limit is thus created, as shown in FIG. 6D.
[0088] Note that if the second exposure occurred before EX1A fully
reset, part of or all of EX1A will also be exposed, reducing the
final resolution. FIG. 6E demonstrates the worst case scenario
where the second exposure occurs before EX1A undergoes any reset at
all, the area EXO, which is the overlapped area between EX1A and
EX1B, will also go through two-photon process. The final exposure
profile shown in FIG. 6F has a flat bottom, which reduces the
contrast and changes the ratio between lines and spaces. For an
ideal two-photon resist the profile in FIG. 6F may still provide
sub-diffraction limit resolution.
[0089] Another non-limiting illustrative example provides for
having a linear photo base generator in addition to the I.sup.2
acid generator. The presence of base in the resist serves to
neutralize the acid and can be used to improve the performance of
the resist. The resultant acid concentration will be proportional
to I.sup.2-.alpha.l where .alpha. represents the relative
sensitivity of the photo base generator.
[0090] Another non-limiting illustrative example of creating
patterns using the disclosed resist is to vary exposure focal point
and/or intensity of each exposure in a multi-exposure process to
create a uniform exposure profile throughout the entire thickness
of the said nonlinear resist.
[0091] Since the energy of the photon used in the lithography is
larger than the bandgap of the nanocrystals, the said nonlinear
resist may have relatively large absorbance. The top of the resist
layer will have higher exposure dosage than the one received by the
bottom of the resist. As shown in FIGS. 7A-C, this problem can be
overcome by forming an image with at least two exposures. In the
first exposure, the focal plane of the projection system is
adjusted so that a diffraction limited image (PT1) is focused on
the surface of the resist, the light pattern at the bottom of the
resist (PB1) is blurred and attenuated as shown in FIG. 7A. The
intensity of the exposure will be adjusted so that the light
intensity at the bottom of the resist is low enough so the exposure
of the resist is negligible.
[0092] In the second exposure, the focal plane of the projection
system is adjusted so that a diffraction limited image (PB2) is
focused on the bottom of the resist and the light pattern at the
surface is blurred (PT2) as shown in FIG. 7B. The intensity of the
exposure will be adjusted so that the light intensity at the
surface of the resist is low enough so that the exposure of the
resist is negligible. Or the exposure is almost uniform so it does
not reduce the spatial frequency of the existing exposure. The
final exposure, as shown in FIG. 7C, will have same exposure
profile at both the top (PTF) and the bottom of the resist
(PTB).
[0093] Multiples exposures can also be performed to achieve uniform
exposure by adjusting the pre-calculated focal plane and intensity
of each exposure to predetermined values.
[0094] Another non-limiting illustrative example of providing a
resist layer comprising nanocrystals is that the resist layer
possesses a concentration gradient profile depth-wise. Said
concentration profile may provide higher sensitivity towards the
bottom of the resist layer since the light intensity at the bottom
of the resist is lower; and lower sensitivity towards the top of
resist since the top of the resist receives more light
intensity.
[0095] While the technology herein has been described in connection
with what is presently considered to be the most practical and
preferred implementation, it is to be understood that the invention
is not to be limited to the disclosed implementations, but on the
contrary, is intended to cover various modifications and equivalent
arrangements included within the scope of the appended claims.
* * * * *