reverse-tone bilayer etch in ultraviolet nanoimprint lithography
V. Sreenivasan, Ian McMackin, Frank Xu, David Wang, and Nick Stacey,
Molecular Imprints; and Doug Resnick, Motorola Research
nanoscale feature replication using imprinting or micromolding techniques
has existed for several years, it was first suggested as a potential
patterning approach nearly 50 years ago. Richard Feynmann, in his famous
1959 lecture "There's Plenty of Room at the Bottom: An Invitation to
Enter a New Field of Physics," discussed the creation and mold-based
replication of nanoscale features.1 He correctly predicted
that the original mold (template) can be written using a version of
electron-beam lithography and discussed the possibility of taking a
metal mold (template) and making multiple copies: "We would just need
to press the same metal plate again into plastic and we would have another
first known large-scale attempt to use this technique involved a template
with recessed structures that was impressed onto a thermoplastic material.
With the combination of heat and pressure, the pattern in the template
was transferred to the thermoplastic material. Compact discs were an
early application of this technology. In the mid-1990s, research in
the area of nanoscale replication created features as small as 10 nm.2
to be extended to a broad set of applications, nanoscale replication
must overcome several practical challenges:
must be able to print fields with nonuniform pattern density at adequate
It must etch nanostructures with appropriate critical dimension control.
must demonstrate precise alignment and overlay capability.
must minimize process-induced defects.
meet these challenges, step-and-flash imprint lithography (S-FIL) was
introduced.3 S-FIL is a step-and-repeat nano-replication
technique based on low-viscosity ultraviolet (UV)- curable liquids.
The use of low-viscosity monomers (which have viscosities of <5 cps)
enables a low-imprint-pressure (<0.25 psi) process, resulting in
low process defect levels. Furthermore, in situ nanoscale alignment
corrections can be made in low-viscosity liquids prior to UV curing,
resulting in <10 nm (3σ) alignment capability over 25 X 25-mm fields.4
bilayer approach, the S-FIL process imprints features into a silicon-containing
material that has been deposited onto an underlying organic layer. This
approach enables the patterning of features with relatively low aspect
ratios. The aspect ratios in the patterned material can then be amplified
using an O2 reactive-ion etch (RIE) step to etch
the underlying organic material. S-FIL's ability to pattern low aspect
ratios is key to minimizing defects, particularly when the template
is separated from the UV-cured material.
to S-FIL and Other Imprint Techniques
sub-50-nm imprint lithography, the etch process requires an increasingly
thin residual film and increasingly high aspect ratios in the smallest
features.5 The reason for this requirement is illustrated
in Figure 1a. A thick residual layer with low-aspect-ratio features
causes significant faceting in small isolated structures during the
breakthrough etch step, potentially resulting in undesirable etch bias
or even feature loss.
1: Schematic diagram showing (a) low-aspect-ratio features with
thick residual layer, and (b) higher-aspect-ratio features. Minimizing
the etch bias in small, isolated features in the S-FIL process requires
the patterning of thin residual layers, the patterning of tall imprint
structures, or both.
way to avoid this problem is to print thin residual layers and/or tall
structures. For example, if 30-nm isolated lines must be etched into
the substrate and aspect ratios no larger than 3:1 are required to minimize
defect propagation, the 30-nm features will be <90 nm tall. To minimize
the undesirable effects of faceting in isolated features, the thickness
of the residual layer must be less than one-third the feature height.
However, this approach requires an etch transfer process that results
in inferior in situ (in-liquid) alignment, defect levels, and throughput.
on the literature and on experiments performed by Molecular Imprints
(Austin, TX) and Motorola Research Laboratories (Tempe, AZ), the four
areas that affect the replication process can be summarized as follows.
with Good Pattern Fidelity. All nanoimprint lithography techniques
require a breakthrough etch step to eliminate the residual layer during
pattern transfer. Good etching requires a thin residual layer and features
with high aspect ratios. As illustrated in Figure 1b, the presence of
a thin residual layer and high aspect ratios leads to minimal faceting
during breakthrough etch.
Multiple Levels. In-liquid alignment has been shown to be promising
for nanoresolution alignment. The low-viscosity monomer material that
makes low-pressure printing possible also enables the template, while
it is in close proximity to the wafer, to make nanoscale gliding movements,
allowing precise in situ alignment corrections. The S-FIL process uses
field-by-field, in-liquid, through-the-template alignment, which results
in nanometer-scale alignment correction capability and in situ correction
prior to UV exposure, where the alignment is locked in. In-liquid alignment
at the sub-10-nm level before and after exposure is shown in Figure
2. However, it has been found that the in-liquid alignment approach
can be disrupted if the template makes direct contact with the substrate
or if a particle is trapped between the template and substrate, a situation
that is aggravated by thin residual films. Hence, a residual layer of
100 nm or more is desirable to achieve robust in-liquid alignment.
2: In-liquid alignment before and after exposure. The average preexposure
alignment is 7.8 nm (3σ), and the average postexposure alignment
is 9.1 nm (3σ).
Defectivity Levels. Low defectivity requires thick residual
film and features with low aspect ratios. Two classes of defects have
the greatest impact on imprint lithography. The first involves the cohesive
failure of film material when the template is separated from the cured
material. Data indicate that this problem is aggravated in the regions
where the highest-aspect-ratio structures exist. The schematic diagrams
in Figures 3a and 3b compare features with low and high aspect ratios.
In Figure 3b, the high-aspect-ratio, high-resolution feature on the
left is most likely to fail during the separation of the template from
the cured material.
3: Comparison between features with (a) low and (b) high height.
Defects are caused by material failure in small features with high
feature height. Hence, the circled feature is most likely to fail
when the template is separated from the cured material.
second class of defects that affects imprint lithography is caused by
template damage. In the S-FIL process, a liquid film separates the template
from the wafer, preventing direct contact and template defect creation
during the printing process. However, when undesirable particles on
the wafer (hard particles that are larger than the residual layer thickness)
go undetected before imprinting, the template can make direct contact
with them and suffer damage. The use of thick residual films (e.g.,
>0.25 µm) and lower-aspect-ratio structures makes it easier
to detect undesirable particles and lower defect levels.
Throughput. High throughput requires thick residual film and
features with low aspect ratio. An important component of the throughput
budget in imprint lithography is the time required for the UV-curable
liquid to fill all the features fully. The use of thick residual layers
leads to a wider channel for liquid to complete the filling process.
It has been reported in the literature that filling speed is a cubic
function of the width of the channel.6 Therefore, thicker
residual layers allow liquid to redistribute significantly faster than
do thinner layers. In addition, low-aspect-ratio features require less
liquid redistribution than high-aspect-ratio ones, since the regions
that contain the features on the template (recessed structures) require
matrix in Table I shows the performance of both thin and thick residual
layers with either low-aspect-ratio or high-aspect-ratio features. From
the standpoint of alignment, defectivity, and throughput, thick residual
layers and low aspect ratios are optimal. However, the opposite is true
for pattern transfer in the S-FIL process, since thin residual layers
and high-aspect-ratio structures are needed to maintain the fidelity
of isolated small structures. Consequently, a process that is significantly
better in every aspect can be achieved by establishing an effective
etch process for thick residual layers and low-aspect-ratio structures.
That process is known as reverse-tone S-FIL (S-FIL/R).
T, D, A
I: Imprint lithography performance matrix for etch (E), throughput
(T), defectivity (D), and alignment (A). (Green = good, blue = fair,
red = poor.)
and the Reverse-Tone Etch Process
variant of the S-FIL process, S-FIL/R uses a reverse-tone bilayer etch
technique. As shown in Figure 4a, step 1 of the process employs a planarizing
topcoat (a spin-on or imprint-planarized silicon-containing organic
material) on an organic imprint material. An optional organic resist
material underneath the imprint layer may be used to provide better
etch selectivity with the underlying substrate and/or better adhesion
with the imprinted material. In step 2 (Figure 4b), the silicon topcoat
is removed using either a dry or wet etch-back process or CMP to expose
the organic imprint material below. In step 3 (Figure 4c), a selective
O2 RIE etch step is typically used to create a
hard mask in the silicon-containing material. The hard mask enables
the etching of thick organic films to create tall resist structures
for pattern transfer.
4: The S-FIL/R process improves the ability of imprint lithography
to align multiple layers, reduce defects, maintain etch CD control,
and maximize throughput.
approach is different from other bilayer approaches since the etch process
reverses the tone of the lithographic structure created in the original
patterned material. Moreover, blanket material removal in step 2 does
not have a negative effect on the shape of the imprinted feature and,
therefore, critical dimension (CD) control, while blanket breakthrough
etch in the S-FIL process causes faceting of the imprinted features
and CD control issues.
S-FIL/R approach not only enables the printing of significantly thicker
imprints without adversely affecting etch transfer, but it also enables
the printing of low-aspect-ratio structures. The topcoat material can
be readily designed to incorporate ultrahigh (>20%) silicon by weight,
which is difficult to incorporate into imprint materials, given all
the other process constraints. Therefore, the feature height required
to achieve an acceptable etch mask in the bilayer resist can be smaller
than half that of a traditional imprint process. These two key features—thicker
residual layers and lower-aspect-ratio imprinted structures—result
in better in-liquid alignment capability, lower susceptibility to defects,
better CD control through etch, and the potential for higher throughput.
A scanning electron microscope (SEM) image showing 60-nm dense lines
after bilayer S-FIL/R resist etch is presented in Figure 5.
5: SEM image of 60-nm 1:1 features obtained using the S-FIL/R process.
toning has general applicability. It can be utilized for any imprint
lithography technique, not just S-FIL. It is also applicable to lithographic
structures created by E-beam lithography or photolithography. For example,
if a negative- tone E-beam resist is desirable from a process and resolution
standpoint while a dark-field photomask must be written with the process,
the bright-field mask (opposite tone) can be patterned using an E-beam
and the reverse-toning etch process can be used to obtain the desired
and Etching Over Preexisting Topography
major benefit of the S-FIL/R process is that it is more tolerant of
preexisting topography than the S-FIL process.
6: Pattern transfer over preexisting topography using the S-FIL
process: (a) plane of optical flat (dashed line) during imprint
planarization; (b) breakthrough etch, after which region A requires
additional etching to open up the underlying organic material and
region B does not; and (c) postbreakthrough etch stage, where the
residual layer in region A has been removed after additional etching
and the residual layer in region B has been eliminated.
Transfer over Preexisting Topography Using S-FIL. Figure 6
shows a process flow for pattern transfer over preexisting topography.
A similar etch transfer process has been described in the literature.3
The preexisting topography is first planarized using an imprint planarization
step, in which imprinting is performed using an organic UV-curable liquid
and an optical flat. This step is followed by imprinting in which patterned
structures are created in the silicon-containing material. During breakthrough
etch, minimal residual-layer variation is mandatory. More specifically,
if the maximum and minimum residual-layer thickness of the imprinted
material is tmax and tmin, the
height of the imprinted feature is h, and the parameter s is a safety
factor >1, the following inequality constraint must be satisfied:
equation accounts for the faster deterioration of small isolated structures
because of faceting. The variation in the residual layer (Δt)
is affected by various factors, including shrinkage of the imprint planarization
organic material during UV curing, flatness errors in the optical flat,
and the mismatch between the optical-flat surface and the average substrate
faster deterioration of small isolated structures can be seen in a pattern-transfer
case involving the creation of 50-nm structures over topography with
200-nm variations. Since the imprinting of features with aspect ratios
greater than 2:1 causes defect problems, the feature height h in that
case is limited to 100 nm. There are three types of variations:
Δt1: The total residual-layer variation resulting
from the mismatch between template and substrate is ~25 nm TIR for double-sided
polished silicon wafers. (That variation can be substantially greater
for single-sided polished wafers and other substrates, such as smaller
silicon, gallium arsenide [GaAs], and indium phosphide [InP] wafers.)
Shrinkage of ~10% during UV curing causes planarization film nonuniformity
of 20 nm.
The best optical flats over a field can be of λ/20 quality, leading
to an additional error of ~30 nm.
the safety factor for maintaining the CD of isolated features is assumed
to be 2, the total Δt must be <50 nm. However, Δt
= (Δt1 + Δt2 + Δt3)
= 75 nm, an unacceptable error level that leads to the situation depicted
in Figure 6c, in which the feature in region B has essentially disappeared
by the time the breakthrough etch in region A has been completed and
is ready for pattern transfer. That situation can be worse in the case
of GaAs and InP substrates.
Transfer over Preexisting Topography Using S-FIL/R.
Figure 7 shows a process flow for pattern transfer over preexisting
topography using the S-FIL/R process. In this process, the silicon-containing
planarizing topcoat is best applied using a planarizing spin-on film.
This planarization process has two major advantages in conjunction with
S-FIL/R: First, the faceting of features during blanket breakthrough
etch is not a problem, as illustrated in Figure 7b; and second, the
degree of planarization represented by Δt is not affected
by substrate flatness variations that are of low spatial frequency (more
than the order of a millimeter or greater).
7: Pattern transfer over preexisting topography using the S-FIL/R
process (a) before and (b) after breakthrough etch. No faceting
of features during etch has occurred.
the S-FIL/R process must satisfy the inequality constraint equation,
the safety factor can be smaller than in the S-FIL process. In the case
of S-FIL/R, Δt is largely affected by the height, size,
and distribution of the features, factors that are well understood in
S-FIL/R is used for pattern transfer of 50-nm structures over topography
with 200-nm variations, the presence of silicon-containing materials
makes it possible to imprint features in the organic material with aspect
ratios of 1.5:1. The feature height is then about 75 nm.
many nanoimprint lithography applications, including photonics, magnetic
storage, and contact-hole creation in CMOS devices, printed features
tend to be gratings, pillars, and contact-holes, which at any given
lithographic level are substantially smaller than 1 µm. In addition,
in CMOS applications, the data for a given lithographic level can be
manipulated to include dummy features, which further assist the spin-on
planarization process. Hence, in the S-FIL/R process, the planarization
process is largely unaffected by preexisting topography, as depicted
in Figure 7a, which shows pattern transfer before and after breakthrough
etch. For 75-nm-tall features, a degree of planarization (Δt)
that is approximately 10% of the height (~8 nm) has been obtained. In
addition, it has been demonstrated that the S-FIL/R process is significantly
more robust than the S/FIL process in the case of GaAs and InP substrates.
and Etching Contacts
patterning and etching high-resolution contacts is considered a major
challenge for photolithography, these processes often necessitate the
use of new lithographic technology. In S-FIL/R, these processes are
performed by patterning pillars in the organic material and then conducting
spin-on planarization using a silicon-containing material. This approach
has two advantages over the standard S-FIL process:
template-fabrication process in S-FIL/R is quite similar to the patterning
of binary contact masks, since the total area exposed to an E-beam in
a positive-tone resist is about 2% or less. In contrast, patterning
an S-FIL contact template using a positive-tone resist is very challenging.
the S-FIL process, holes are created directly in a silicon-containing
material, and the height of the holes must be greater than that of the
pillars because of faceting issues and the lower silicon content in
the imprint material. Consequently, a high-resolution contact template
and long features (sub-100-nm pins with aspect ratios of 2:1 or greater)
protruding out from the template are required. As a result, the features
are very fragile and likely to fail, causing repeating defects. In contrast,
the S-FIL/R process uses robust templates with low-aspect-ratio holes.
8: Patterning using the S-FIL/R process: (a) imprinted organic pillars,
and (b) contacts after reverse toning.
8 presents SEMs of patterned pillars that were reversed into contacts.
Figure 9 shows 60-nm contact holes that have been patterned using the
9: Patterning of 60-nm contacts using the S-FIL/R process.
tools are sometimes referred to as the milling machines of the 21st
century. They have revolutionized the electronics industry and are continuing
to enable many applications at the micro- and nanoscale.
has been shown previously that the S-FIL process can fabricate sub-50-nm
structures, complicated patterns, and 3-D structures, while providing
precise overlay capability and low process defectivity.3–5
The S-FIL/R process represents an improvement over the S-FIL process
by enabling the printing of lower-aspect-ratio structures and thicker
residual films during imprinting. It also has significant advantages
over its predecessor in its ability to perform transfer patterning over
and S-FIL/R technology will likely be used to manufacture optical devices,
microdisplays, and nanoscale electronics. Their impact on mainstream
silicon fabrication will probably be a direct function of how well they
can overcome a key manufacturing challenge: minimizing long-term defects
to maximize yields. The challenge facing silicon fabrication is to develop
and demonstrate an S-FIL process that can approach the long-term yield
and productivity of photolithography.
work described in this article was partially supported by grants from
the DARPA Advanced Lithography program (Grants No. N66001-01-1-8964
and N66001-02-C-8011) and the NIST Advanced Technology Program (Grant
Feynmann, "There's Plenty of Room at the Bottom: An Invitation to Enter
a New Field of Physics," presented at The American Physical Society
Meeting, California Institute of Technology, December 29, 1959, published
in Caltech's Engineering and Science, February 1960.
Chou, PR Krauss, and PJ Renstrom, "Nanoimprint Lithography," Journal
of Vacuum Science and Technology B 14, no. 6 (1996): 4129–4133.
Colburn et al., "Development and Advantages of Step-and-Flash Imprint
Lithography," Solid State Technology 46, no. 7 (2001): 67–78.
Choi et al., "Distortion and Overlay Performance of UV Step and Repeat
Imprint Lithography" (paper presented at the Micro- and Nano-Engineering
International Conference, Rotterdam, The Netherlands, September 19–22,
Sreenivasan, "Status of the Step and Flash Imprint Lithography Technology"
(paper presented at the Micro- and Nano-Engineering International Conference,
Rotterdam, The Netherlands, September 19–22, 2004).
Colburn, "Step and Flash Imprint Lithography," PhD diss., University
of Texas at Austin, 2001.
V. Sreenivasan, PhD, is cofounder and CTO of Molecular Imprints
(Austin, TX). He is on a leave of absence from the University of Texas
at Austin, where he is an associate professor of mechanical engineering
and Thornton Centennial Fellow in engineering. Sreenivasan specializes
in the design of ultraprecision machine systems for semiconductor processes.
His specific research interests include the study of nanostructure fabrication
using imprint lithography, flexure-based micro- and nanoprecision machines,
precision optical systems, and real-time sensing architectures. He received
a PhD in mechanical engineering from Ohio State University in Columbus.
(Sreenivasan can be reached at 512/334-1210 or email@example.com.)
McMackin, PhD, is manager of the process and applications group
in the advanced lithography division of Molecular Imprints, which he
joined shortly after the company's inception. The group is responsible
for developing the lithography process from imprint through pattern-transfer
etch, concentrating on CMOS-compatible applications. The group's work
encompasses the formulation of imprint resists, development of etch
recipes, and engineering of imprint tool subsystems that increase lithography
performance. He received a BS in physics from Southern Illinois University
in Edwardsville and a PhD from the Institute of Optics, University of
Rochester in New York. (McMackin can be reached at 512/339-7760, ext.
215, or firstname.lastname@example.org.)
Xu, PhD, works on material development at Molecular Imprints,
which he joined in 2003. Before joining the company, he spent nine years
at 3M and Chorum Technologies working on optoelectronic materials. He
received a PhD in polymer science and engineering from the University
of Massachusetts at Amherst. (Xu can be reached at 512/339-7760, ext.
252, or email@example.com.)
Wang is an etch process engineer at Molecular Imprints, which
he joined in 2003. He received a BS in electrical engineering from the
University of Texas at Austin. (Wang can be reached at 512/339-7760,
ext. 287, or firstname.lastname@example.org.)
Stacey is a materials manager at Molecular Imprints, where
he develops new imprint materials for use with customer processes. He
has been with the company since 2003. Stacey has more than 15 years
of experience in the electronics and imprint fields. Previously, he
worked at 3M and the University of Texas at Austin. (Stacey can be reached
at 512/339-7760, ext. 261, or email@example.com.)
Resnick, PhD, is a fellow of the technical staff at Motorola
Labs and a member of Motorola's scientific advisory board. He is responsible
for developing Motorola's step and flash imprint lithography research
program. He joined the company in 1990 and led the development effort
for x-ray mask-pattern transfer. Before joining Motorola Labs, Resnick
worked at AT&T Bell Laboratories. His development projects included
x-ray lithography, GaAs direct-write, and plasma etching of photomasks.
He has authored or coauthored more than 100 technical publications and
holds 16 U.S. patents. He received a PhD in solid-state physics in 1981
from Ohio State University in Columbus. (Resnick can be reached at 480/413-7743