dry 193-nm lithography technology to the limit
H. Magoon, Nikon Precision
the last 25 years, there has been dramatic progress in the
pursuit of enhanced lithographic resolution, enabling continuous improvements
in IC production. These advancements have been driven by reducing the
exposure wavelength and increasing the projection lens numerical aperture
(NA). The current generation of photo engineers has extended optical
lithography far beyond what previous generations considered feasible,
and many in the industry believe that the next step forward will be
the implementation of immersion lithography. While the use of immersion
tools is rapidly approaching the production stage, continuing efforts
to extend the capabilities of affordable dry 193-nm lithography will
be critical to maintaining cost-effective semiconductor manufacturing
for some time.
defined in Rayleigh's resolution equation, R = k1
X (λ/NA), the primary method for lowering the minimum resolution
has been through reduction of the exposure source wavelength (λ)
in conjunction with increasing the lithography system's projection lens
numerical aperture. A 193-nm exposure wavelength has been the industry
standard for several years, and today's critical scanners feature lens
NAs >0.90, very close to the dry numerical aperture limit of 1.0.
Thus, the remaining means to extend dry technology is through optimization
of k1, where k1 represents the
process-dependent factors affecting resolution. Previous process-related
resolution enhancement efforts have focused on reticle design, using
methodologies such as phase shifting or pattern splitting on dual masks.
While these techniques improve imaging, they also have significant drawbacks,
including throughput loss and the high cost of reticle generation. When
investigating k1 optimization, it is vital to take
into account all of the factors that influence resolution (including
many aspects of the lithography scanner itself).1
article discusses scanner factors and characteristics vital to the extension
of dry lithography. It also examines ways to manipulate scanner capabilities
to meet the aggressive imaging challenges of the future. In addition,
it focuses on advanced polarization control, which contributes significantly
to enhancing image contrast and providing the process latitude required
for sub-90-nm applications. Also considered are scanner lens design
and manufacturing methodologies, scanner functions useful in optimizing
imaging performance beyond initially specified levels, and ways to minimize
exposure to environmental contamination through scanner design.
modern ultrahigh-NA lenses provide imaging capabilities far exceeding
those of previous-generation lithography systems, industry experts agree
that advanced polarization control will be required to successfully
achieve sub-90-nm critical dimensions (CDs) with sufficient process
latitude. Polarization control enhances image contrast, resulting in
improved resolution and translating into k1 benefits
for the user. Heightened contrast also reduces the lithography process's
sensitivity to deviations in dose or focus errors, extending process
scanner illumination systems are designed to provide a random mixture
of s-polarized light, which enhances image contrast at higher NAs, and
p-polarized light, which degrades image contrast at higher NAs. This
random light was sufficient for yesterday's lower-NA systems with far
less stringent imaging requirements, but it is no longer adequate. To
achieve the critical imaging objectives of sub-90-nm applications, it
will be necessary to control the type of polarized light used; that
is, the s-polarized light must be maximized and the p-polarized light
are two main methods of polarization control. One technique is to incorporate
a polarization filter that separates the s- and p-polarized light components.
The use of filters can cause at least 50% of the light to be lost and
thus result in lower throughput. If laser power is increased in an effort
to compensate for lost light, the usable lifetime of both the laser
and the optics is reduced, which has a negative impact on the tool's
cost of ownership.2
1: Effect of a traditional polarization filter, which leads to reduced
light intensity (top), and advanced polarization control without
loss of intensity (bottom).
alternative technology, developed by Nikon (Tokyo) and known as Polano,
has recently been made available to lithographers.3 This
technology enhances image contrast, without loss of illumination power,
as seen in Figure 1. It is designed to utilize the polarized laser beam
efficiently, controlling the polarization direction while maintaining
the desired degree of polarization in the illumination homogenizer.4
In addition, the control provides significant flexibility, allowing
user selection of the delivered polarization distribution via independent
settings for each different mask exposure file.
2: Effects of advanced polarization control on depth of focus for
a 60-nm line/space pattern: (a) without polarization control, (b)
with polarization control.
depth of focus (DOF) enhancement that was achieved using advanced polarization
control is demonstrated in Figure 2. These results were obtained at
Nikon using the company's NSR-S308F deep-ultraviolet (DUV) lithography
tool with an NA of 0.92, a 6% attenuated phase-shift mask, and standard
dipole illumination. A separate evaluation by Infineon Technologies
(Dresden, Germany) using an NSR-S307E DUV lithography tool with an NA
of 0.85 and an alternating phase-shift mask for a 62-nm line/space pattern
also confirmed the benefits of polarization control. Exposure latitude,
depth of focus, and the mask error enhancement factor all improved using
polarization control, as shown in Figure 3.6
3: Improvements in exposure latitude, depth of focus, and mask error
factor achieved using advanced polarization control for a 62-nm
Design and Manufacturing
considering a scanner's effect on k1, it is necessary
to look at the earliest phases of the lens design and manufacturing
process, as both have a significant impact on the tool's imaging capabilities.
The first crucial element is the lens design. The use of kinematic lens
mountings, for example, eliminates stress-induced birefringence. Another
vital aspect is the quality of the materials used. Impurities in calcium
fluoride (CaF2), which is used in some lens elements,
are known to increase short-range flare, which can degrade image contrast
and, if varied across the field, can contribute to across-field CD variation.
Therefore, only materials with the highest purity and lowest possible
birefringence should be used.6
addition, because surface roughness also affects short-range flare,
lens polishing, annealing, and assembly techniques have a significant
influence on a scanner's imaging performance. Effective lens-manufacturing
methodologies incorporate sophisticated polishing techniques, including
the repolishing of aspherics, which can be extremely effective in compensating
for higher-order aberrations. In addition, a power spectral density
analysis should be performed for each lens element surface to ensure
that superior quality control is achieved.
important aspect of successful lens-manufacturing processes is the ability
to measure the wave-front aberration content accurately. This capability
is achieved by using a phase-measurement interferometry (PMI) system,
which determines Zernike coefficients up to F81. For optimal
results, PMI data measurements should be performed at several stages
throughout the lens-manufacturing process. As indicated in Figure 4,
progress in these technologies has enabled a continued decrease in projection
lens aberration content over several generations of scanners.
4: Residual lens aberration versus NA for four generations of Nikon
scanner's imaging performance can also be enhanced by fine-tuning the
lens aberration content for user-specific applications and imaging configurations
(i.e., NA and illuminator settings). Such tuning can have a significant
impact on the tool's full-field resolution capabilities. As discussed
elsewhere, it is understood that aberrations in areas of a lens not
being used to image a particular pattern (or patterns) do not influence
ultimate imaging capabilities.7 This combination can therefore
provide a pathway for application-specific lens tuning.
a scanner can be optimized for specific pattern and imaging conditions,
however, a variety of information is required. It is necessary to understand
the present aberration state of the lens, which is obtained via in situ
PMI tools such as the ISI aberration measurement instrument from Litel
Instruments (San Diego).8 In addition, the present adjustment
state of the lens, which is read from the scanner itself, must be understood.
Knowledge of the impact of various adjustments on the aberration state,
which is inherent to the lens design, is also required. Beyond these
prerequisites, a vehicle to convert the scanner's present aberration
state, specific pattern details, and imaging goals into a usable lens
prescription—paired with a lens capable of field tuning—is
technology for aberration optimization (TAO) is one such vehicle. After
the user defines the specific pattern requirements
and imaging performance criteria (e.g., allowable
limits and assigned priorities), software determines the lens prescription
for that scanner that best satisfies the user's objectives.
evaluate the system's capabilities, tests were run using five-bar patterns
of 150-nm lines on a 300-nm pitch at four different orientations (0°,
90°, 45°, and 135°). The scanner used in the tests had not
been adjusted previously and had a considerable amount of linewidth
abnormality (LWA), as seen in Figures 5a and 5c. However, its spherical
aberration (SA) and total focus deviation (TFD) were considered acceptable.
Therefore, the goal was to reduce LWA without negatively affecting TFD
or SA. As shown in Figures 5b and 5d, results improved markedly following
implementation of the TAO-recommended prescription, with LWA well below
the 0.03 target. In addition, SA was reduced and TFD was successfully
maintained. After the implementation, measured results mirrored the
software's prediction, confirming its simulation capabilities.
5: Measured LWA for five-bar patterns of 150-nm lines: (a) 0°
and 90° before lens prescription, (b) 0° and 90° after
prescription, (c) 45° and 135° before prescription, and
(d) 45° and 135° after prescription. The target LWA was
0.030. LWA values are shown as vector components. (→ 0.020.)
should be noted that although the level of improvement for a particular
scanner depends on such factors as the starting lens adjustment condition
and illumination distribution, technologies such as TAO enable lithography
engineers to significantly enhance imaging capabilities and provide
a means to extend the usefulness of existing systems to next-generation
applications. Such technologies also allow a single lens to be optimized
and used for multiple layers, reducing the need for dedicated scanners
and increasing flexibility in the fab. With the increasing price of
each generation of lithography systems, such benefits are valuable to
semiconductor manufacturers seeking to control costs.
scanner features can also be employed to achieve imaging objectives,
such as expanding the depth of focus. Methods such as the continuous
DOF expansion procedure (CDP) have been shown to be effective in increasing
the DOF for contact holes without affecting throughput. With CDP, the
wafer is tilted along the scanning direction, while the wafer stage
continuously moves upward or downward during exposure. Recent evaluations
of the technology using an S307E scanner with an NA of 0.85 indicated
that DOF for a 90-nm contact hole improved from 0.2 to 0.3 Ám. This
finding supported earlier data showing a 35% improvement in DOF for
250-nm contact holes (Figure 6). Such technologies provide an enhanced
process window, eliminating the need for double exposures and thereby
enabling throughputs comparable to those for a standard wafer exposure.
6: Depth of focus for 250-nm contact holes using a scanner with
and without the continuous DOF expansion procedure.
manufacturers are well aware that both their processes and processing
equipment can have a significant influence on final imaging. Such process
factors as resist coating nonuniformities, varied topography across
the field or wafer, and edge effects can all cause across-wafer and
shot-to-shot CD variation. Root-cause investigations of variations are
frequently time-consuming, and corrections may involve costly changes.
Therefore, scanner features able to mitigate process-induced variability
will deliver k1 benefits.
resist is dispensed onto the wafer surface, wafers are spun rapidly
to distribute the liquid evenly. Any irregularities in spin-process
rotation control can lead to uneven resist coatings. Deviations in temperature
control during prebake can also contribute to uneven coatings. Fortunately,
scanners that permit flexible dose adjustment across the wafer can mitigate
the impact of processing inconsistencies on incoming wafers. There may
also be cases where certain exposure shots experience repeatable shifts
in optimum dose or abnormal best focus, which cannot be eliminated by
standard engineering methods. Irregular shots may be induced by resist-thickness
variation or extreme topography, which result in abnormal behavior at
the wafer edge. However, regardless of the source of irregularities,
the ability to define individual exposure conditions for each shot can
compensate for them.
DUV lithography systems are far more susceptible to the effects of environmental
molecular contamination than were earlier systems, which patterned significantly
larger features.9 Environmental contaminants can cause film
formation involving the scanner's projection optical components, degrading
lens coatings and negatively affecting transmittance of the optical
elements. The accompanying decrease in illumination power and uniformity
contributes to throughput loss and leads to a need to replace costly
optics. Such contamination-related films also have detrimental imaging
effects, including reduced image contrast and elevated CD nonuniformity.
level of environmental contamination is present in all cleanrooms. However,
it is possible to minimize tool components' exposure to such contaminants
and their effects through meticulous protection of the scanner's main
chamber and projection lens. Various protective measures are implemented
during system design and manufacture:
selection. Only lens-element mountings completely free of contamination
and contamination-causing materials should be used.
Engineering solutions. Comprehensive filtration systems and
optimized chamber airflow should be coupled with nitrogen purging of
Manufacturing solutions. Lens components should undergo UV-light
and chemical cleaning before being integrated into the scanner.
Storage/shipment considerations. Only containers filled with
pure nitrogen should be used to store and ship optical components.
measures such as these have been effective in maintaining optimal equipment
progress has been made in pursuit of pattern resolution enhancements,
enabling continuous advances in IC manufacturing. Historically, these
technological achievements have been driven by reductions in lithography
systems' exposure wavelength paired with increases in lens
NAs. A 193-nm exposure wavelength has been used for several years, and
critical scanners incorporate ultrahigh NAs. Therefore, future
improvements will come from optimizing the process-related factors that
can affect resolution.
aspects of scanner design and manufacturing have a significant impact
on resolution capabilities, including minimization of flare and aberration
content, benefits provided by advanced polarization control, and preventive
measures to protect sensitive scanner components from environmental
contamination. In addition, both dry and immersion systems incorporate
many features that can be used to minimize process influences, enable
imaging beyond specified levels, and extend the useful lifetime of existing
equipment. These factors are critical to the extension of cost-effective
dry lithography, a first step toward enabling IC manufacturers to continue
to provide technology innovations for years to come.
article is a revised version of a poster presentation at the IEEE/SEMI
Advanced Semiconductor Manufacturing Conference, held April 11–12,
2005, in Munich. The author wishes to acknowledge the significant contributions
of Steve Slonaker and Gene Fuller. She would also like to thank Bernie
Wood and members of the Nikon Precision and Nikon Precision Europe technical
departments for their support.
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H. Magoon joined Nikon Precision in 1995, holding both technical
and supervisory positions in the applications department. During that
time, she specialized in methods of productivity optimization
in the lithography sector and coauthored several publications. In 2003,
Magoon joined the company's marketing department as marketing manager,
focusing on the development and launch of new products and
technologies. She received a BS in chemistry from St. Michael's
College in Colchester, VT, in 1995. (Magoon can be reached at 802/879-5027