SPIE Advanced Lithography 2014, 9052-15 Opt. Microlithography XXVII, Voelkel et. al., February 25, 2014
Lithographic process window optimization for mask aligner proximity
lithography
Reinhard Voelkel*a, Uwe Voglera, Arianna Bramatia, Andreas Erdmannb, Nezih Ünalc,
Ulrich Hofmannc, Marc Hennemeyerd, Ralph Zoberbierd, David Nguyene, Juergen Bruggere
a
SUSS MicroOptics SA, Rouges-Terres 61, CH-2068 Hauterive, Switzerland
b
Fraunhofer Institut IISB, Schottkystr. 10, D-91058 Erlangen, Germany
GenISys GmbH, Eschenstr. 66, D-82024 Taufkirchen, Germany
d
SÜSS MicroTec Lithography GmbH, Schleissheimerstrasse 90, D-85748 Garching, Germany
École Polytechnique Fédérale de Lausanne, EPFL STI IMT, CH-1015 Lausanne, Switzerland
ABSTRACT
We introduce a complete methodology for process window optimization in proximity mask aligner lithography. The
commercially available lithography simulation software LAB from GenISys GmbH was used for simulation of light
propagation and 3D resist development. The methodology was tested for the practical example of lines & spaces, 5
micron half-pitch, printed in a 1 micron thick layer of AZ® 1512HS1 positive photoresist on a silicon wafer. A SUSS
MicroTec MA8 mask aligner, equipped with MO Exposure Optics® was used in simulation and experiment. MO
Exposure Optics® is the latest generation of illumination systems for mask aligners. MO Exposure Optics® provides
telecentric illumination and excellent light uniformity over the full mask field. MO Exposure Optics® allows the
lithography engineer to freely shape the angular spectrum of the illumination light (customized illumination), which is a
mandatory requirement for process window optimization. Three different illumination settings have been tested for 0 to
100 micron proximity gap. The results obtained prove, that the introduced process window methodology is a major step
forward to obtain more robust processes in mask aligner lithography. The most remarkable outcome of the presented
study is that a smaller exposure gap does not automatically lead to better print results in proximity lithography - what the
“good instinct” of a lithographer would expect. With more than 5'000 mask aligners installed in research and industry
worldwide, the proposed process window methodology might have significant impact on yield improvement and cost
saving in industry.
Keywords: mask aligner, proximity lithography, customized illumination, source-mask optimization, process window
optimization, lithography simulation, advanced mask aligner lithography, AMALITH
1. INTRODUCTION
The concept of lithographic process window optimization 2,3 goes back to the late 80s. A process window is a collection
of values of process parameters that allow manufacturing within desired specifications. The lithographic process window
in projection lithography is typically defined as the set of values for focus and exposure to control critical dimension
(CD). The maximal inscribed rectangle or ellipses in this plot then represent the process window. The overall process
window for a specific lithography task is obtained from intersection or overlap of process windows for all different
layout patterns existing in a mask design. Process window analysis was developed for projection lithography, where it is
well established today4. Process window optimization has helped much to reduce the minimum feature size and to
improve the yield in production by giving a qualitative representation of the processes stability.
To the best knowledge of the authors, process window analysis and optimization has never been investigated in a
systematic way for mask aligner lithography. The reasons were manifold: no suitable lithography and resist simulation
tools, the immutable exposure light settings in most mask aligners, and the significant variance of uniformity and angular
spectrum over the mask field in standard mask aligner tools. The situation changed during the last decade. Projection
*
[email protected], +41-32-5664444, www.suss.ch, www.suss.com
SPIE Advanced Lithography 2014, 9052-15 Opt. Microlithography XXVII, Voelkel et. al., February 25, 2014
lithography software tools like Dr.LiTHO5 from Fraunhofer IISB (www.drlitho.com) have been adapted for the
simulation of mask aligner lithography. GenISys GmbH (www.genisys-gmbh.com) released LAB6, a lithography
simulation software with special focus on proximity lithography and 3D resist development modeling. Research
teams7,8,9,10 profited from these simulation tools to further investigate simulation and lithography enhancement
techniques for mask aligner lithography.
On the hardware side, the introduction of MO Exposure Optics® fundamentally changed the illumination system11,12 of
mask aligners. MO Exposure Optics® is based on a relay system of two microlens-based Köhler integrators and ensures
excellent light uniformity and identical angular spectrum over the full mask field. Telecentric illumination and full
control of the illumination settings (customized illumination) now enable simulation and optimization of mask aligner
lithography from the light source to the resist pattern (source-mask optimization). Lithography enhancement methods
from projection lithography can now be applied to mask aligner lithography.
Targets for mask aligner lithography optimization8 are manifold: compensating for errors and irregularities like corner
rounding, line width narrowing and edge shortening, elimination of remaining diffraction effects, increasing the gap
range of operation (minimum to maximum gap) and a larger free working distance (proximity gap), as well as resolution
enhancement.
Despite this encouraging progress in simulation, it remains difficult to assess proximity lithography enhancement
measures in practice. The interpretation of resist prints remains difficult. Sidewall angle measurements are laborious and
no suitable CD uniformity measurement tool is available for thick photoresists. Slight variations of the process
parameters might adulterate the results in exposure series. The presented methodology for process window optimization
is a first step to better compare the impact of lithography enhancement techniques in mask aligners. Process related
factors, like temperature, humidity and uncertainties in the wet process might still influence the individual exposure, but
the exposure latitude of the process window now allows the lithography engineer to rate process robustness and to
choose sweet spots.
2. DEFINITIONS AND TERMINOLOGY
13
For this study a 1 micron thick layer of AZ® 1512HS1 positive photoresist on silicon wafer was examined. Resist
simulation was based on the Mack 4 model3 with a development time of 30 seconds and a diffusion length of 25 nm. The
Mack 4 model parameters were set to Rmin = 0.61 nm·s-1, Rmax = 74.05 nm·s-1, Slope = 10, Mth = 0.473233. A photomask
with lines & spaces, 5 micron half-pitch, was used for exposure. The effects of dark erosion were not taken into account.
The raw data from LAB simulation was transferred to Matlab for data analysis and visualization 13.
photomask
height
gap
half-pitch
resist pit
80%
resist
Θ
20%
CD
200nm
sidewall
angle
wafer
Figure 1. (Left) photograph (SEM) of 1 micron thick layer of AZ® 1512HS1 resist on silicon wafer after development, for
lines & spaces, 5 micron half-pitch, (right) scheme of photoresist structure with definitions. The sidewall angle is derived
from 20% and 80% of the resist height. CD is derived from the line width at 200 nm height to exclude any influence of resist
bottom effects and artifacts. Due to secondary diffraction orders unwanted side lobes and corresponding pits in the resist
might appear.
SPIE Advanced Lithography 2014, 9052-15 Opt. Microlithography XXVII, Voelkel et. al., February 25, 2014
Figure 1 shows (left) a photograph (SEM) and (right) a scheme of photoresist after development for a regular lines &
spaces pattern as described above. For this report the critical dimension CD was defined as line width at 200nm height of
the resist layer remaining after development. 200nm height instead of bottom CD was chosen to exclude any influence of
resist bottom effects. In some cases, e.g. for very thick photoresist layers, it might make more sense to use top CD at
80% or 100%. The sidewall angle Θ is derived from 20% and 80% width as shown in Figure 1. Secondary diffraction
orders might generate unwanted resist pits in-between two lines, as shown in the SEM picture Figure 1 (left).
3. SIMULATION OF SHADOW PRINTING LITHOGRAPHY IN A MASK ALIGNER
Lithography simulation for projection is typically based on far-field (Fraunhofer) diffraction theory7. Proximity
lithography is described by near-field (Fresnel-Kirchhoff) diffraction14, which represents a more difficult problem for
theoretical analysis. In the general case, near-field diffraction can be investigated by using so-called rigorous numerical
methods solving the Maxwell equations. For mask feature sizes significantly larger than the wavelength of the
illuminating light and for sufficiently large proximity gaps, approximate methods such as scalar diffraction theory yield
satisfactory results7. The achievable resolution for lines and spaces, half-pitch, for proximity lithography is deduced from
the Fresnel integral formula and given by the following expression7
𝑙𝑖𝑛𝑒 𝑤𝑖𝑑𝑡ℎ (ℎ𝑎𝑙𝑓 − 𝑝𝑖𝑡𝑐ℎ) =
3
𝑑
√𝜆 (𝑔 + ) ≈ √𝜆𝑔.
2
2
(1)
Here λ is the wavelength, 𝑔 the proximity gap and 𝑑 the resist thickness15. The resolution degrades with the square root
of the proximity gap. For this study13, the lithography simulation software LAB6 v4.1.0 from GenISys GmbH was used
for simulation. LAB provides full 3D simulation of shadow printing lithography in mask aligners for multiple
wavelengths and different illumination settings. The calculation of the aerial image is based on Kirchhoff scalar
diffraction theory solving the Rayleigh-Sommerfeld integral. Propagation in the resist is simulated by transfer matrix
model (thin film algorithm) including bleaching effects 7. The light-induced modification and the development of the
photoresist material are described by the Dill parameters (extinction in the unbleached/bleached state and
photosensitivity of resist) and by the Mack 4 (development rate) parameters. The bulk image intensities are transferred
into inhibitor concentrations which define the dissolution rate and the resulting resist profile after development. The
resist data is transferred to Matlab® for systematic analysis and visualization13.
The angular spectrum of the mask illumination light in a mask aligner is product- and application-specific. An angular
divergence of typically ±3° to ±4° is preferred for contact, and ±1° to ±2° for proximity lithography. For SUSS MicroTec
mask aligners, the illumination optics for contact and small proximity gaps is referred as HR Optics (HR: high
resolution) and LGO Optics (LGO: large gap) for proximity.
HR(±3°) Optics
LGO(±1.4°) Optics
LGO(±0.7°) Optics
±0.7°
Figure 2. Different illumination settings for SUSS mask aligners: (left) HR(±3°) Optics, (center) LGO(±1.4°) Optics and
(right) LGO(±0.7°) Optics.
Figure 2 shows the three different illumination settings examined within this study.
SPIE Advanced Lithography 2014, 9052-15 Opt. Microlithography XXVII, Voelkel et. al., February 25, 2014
4. DIFFRACTION EFFECTS FOR LINES & SPACES, 5 MICRON HALF-PITCH
As discussed above, proximity lithography in mask aligners is dominated by diffraction effects at the photomask pattern.
Figure 3 shows the intensity profiles obtained by simulation in LAB6. A regular pattern with lines & spaces, 5 micron
half-pitch, was illuminated with different illumination settings. Only one opening of the mask is shown in Figure 3. The
contour lines represent the intensity threshold levels obtained from simulation. The intensity coefficient is normalized to
1.0 for the dose-to-clear exposure dose.
HR(±3°) Optics
LGO (±1.4°) Optics
(contact lithography settings)
(proximity lithography settings)
Figure 3. Intensity profiles in air obtained by simulation in LAB lithography software for a photomask with lines & spaces,
5 micron half-pitch, illuminated with 365nm wavelength. Two different angular spectra are shown: (left) HR(±3°)
illumination and (right) LGO(±1.4°). The contour plot isobars correspond to equal exposure for the same exposure time. The
value 1.0 is equivalent to the dose-to-clear for a 1 micron thick layer of AZ® 1512HS positive photoresist on silicon.
4.1 Illumination coefficient contour plots
In both plots a central “red” spot with more light for 25 to 40 micron exposure gap is observed. This spot is generated by
diffraction at the edges of the 5 micron wide window in the mask layer. The intensity distribution behind the 5 micron
wide window looks very similar to the intensity distribution observed behind a 5 micron microlens with low optical
power16. As light is funneled towards the center, the light intensity is getting lower in outer regions. As result, the
contour lines show a remarkable narrowing at 30 micron gap. At 50 to 60 micron gap, the green area in Figure 3 is back
to 5 microns width, however, unwanted intensity peaks from secondary diffraction orders appear left and right.
Simulation was performed for an extended lines & spaces pattern, thus light from adjacent mask apertures overlap. As
shown in Figure 1, these peaks might lead to an unwanted resist structure (pits).
SPIE Advanced Lithography 2014, 9052-15 Opt. Microlithography XXVII, Voelkel et. al., February 25, 2014
Figure 4 shows the contour plot of Figure 3 (right), corresponding to LGO(±1.4°) illumination, in more detail. A
coefficient of 0.2 corresponds to an exposure dose five times higher than the dose-to-clear for a 1 micron thick layer of
AZ® 1512HS positive photoresist on silicon. The contour plots indicate, that it is almost impossible to obtain a 5 micron
line width at a proximity gap of 30 microns by using 5 micron line width on mask level. For a value of 0.2 the contour
line width is 7 microns, for coefficient values > 0.4 the contour line is always < 3 microns. The process latitude for 30
micron gap is rather narrow. At a larger proximity distance > 40 micron a much larger exposure latitude is observed.
0.2
0.4
0.6
0.8
1.0
1.2
Figure 4. Intensity profiles from Figure 3 (right), corresponding to LGO (±1.4°), as contour plot. The contour lines
correspond to the equal dose levels for similar exposure time. The value 1.0 is equivalent to the dose-to-clear. (Left)
intensity coefficients indicated by different colors. (Right) Separate plots for intensity coefficients of 0.2, 0.4, 0.6, 0.8, 1.0
and 1.2 from (left). A coefficient of 0.2 corresponds to a dose of five times the dose-to-clear for a 1 micron thick layer of
AZ® 1512HS positive photoresist on silicon. A coefficient of 2.0 corresponds to a dose of 50% of dose-to-clear.
5. PROCESS WINDOW FOR LGO(±1.4°) ILLUMINATION SETTINGS
5.1 Process window for CD of 5 ±1 micron
As discussed, the process windows is application-specific. For some applications it is mandatory to achieve sidewall
angles Θ > 85°, but the actual dimensions of the resist structure are less relevant. For other applications, it is mandatory
to print resist structures with very small tolerances for CD, but the sidewall angles are not critical.
B
A
C
Figure 5: Process windows for critical dimension, 5 micron CD with ±1 micron tolerance, for LGO(±1.4°) illumination at
365nm wavelength, referring to an illumination light intensity distribution shown in Figure 3 (right) and Figure 4. The dark
SPIE Advanced Lithography 2014, 9052-15 Opt. Microlithography XXVII, Voelkel et. al., February 25, 2014
areas show combinations of proximity gaps and dose values, which produce CD values in the range of 5 ±1 micron. The
exposure latitude is indicated by rectangles fitted into the process window.
Figure 5 shows the obtained process windows for critical dimension 5 micron CD with ±1 micron tolerance. Illumination
using LGO(±1.4°) and 365nm wavelength referring to the intensity profiles shown in Figure 3 (right) and Figure 4.
Simulation was performed with LAB software from GenISys for a 1 micron thick layer of AZ® 1512HS positive
photoresist on silicon. The process windows shown in Figure 5 confirm the prediction made in the previous chapter, that
it is impossible to print a 5 micron line width at a proximity gap of 30 micron with a mask pattern of 5 micron width
using current illumination settings. The exposure latitude is indicated by rectangles fitted into the process window. From
contact to 25 microns (A) an exposure latitude of ±25mJ/cm2 is obtained. For proximity lithography there are two
options (B) and (C). For a proximity gap of 50 micron and a dose of 130 mJ/cm2 (B) the exposure latitude is ±25mJ/cm2
for a gap latitude of ±12 microns. In production environment a process window (C) with less critical gap latitude is
preferred. Exposure dose and development is usually very well controlled, whereas the gap latitude comprises different
factors like gap setting accuracy, mask bending, wafer flatness and all resist related surface defects like resist bubbles
and resist edge bead. In case (C) an exposure latitude of ±10 mJ/cm2 corresponds to a gap latitude of ±20 microns.
5.2 Process window for sidewall angle
In the process window shown in Figure 5 the CD was derived from 200nm resist height value as defined in Figure 1 with
no restriction to sidewall angle. This is not very realistic for most practical applications and in production.
Figure 6. Process window for sidewall angle and no restriction to critical dimension for lines & spaces, 5 micron half-pitch,
printed with LGO(±1.4°) illumination. (Left) sidewall angle requirement set Θ > 60° and (right) Θ > 80°. The green areas
show combinations of proximity gaps and dose values, which produce sidewall angles in the specified ranges.
Figure 6 shows the process windows for different sidewall angle requirements, but no restrictions to CD. For a sidewall
angle requirement of Θ > 80° it is very difficult to find a sweet spot for a robust lithography process. This is not
surprising. The used photoresist AZ® 1512HS works best as thin photoresist for higher resolution, but is not optimized
for obtaining steep sidewall angles.
5.3 Process window for side lobe printing
In Figure 4 (left), at (x, gap) positions (-5, 30) and (+5, 30), light from secondary diffraction orders is found. This light
might create unwanted pits in the resist as observed in Figure 1 (left). This effect is monitored by the side lobe printing
process window shown in Figure 7 for the exposure conditions described in previous chapters. The lower borderline
corresponds to the dose-to-clear exposure condition. Below this line the resist is not fully dissolved. Opaque areas inside
the window and above the higher borderline are – unwantedly – cleared. For an exposure gap of 50 microns the
secondary diffraction orders, viewable as “islands” Figure 1 (left), fully expose the resist and produce unwanted pits in
the resist. Similar effects are observable for a 30 micron proximity gap.
SPIE Advanced Lithography 2014, 9052-15 Opt. Microlithography XXVII, Voelkel et. al., February 25, 2014
Figure 7. Side lobe printing process window for a 1 micron thick layer of AZ® 1512HS positive photoresist on silicon
exposed as described in the previous chapters. The lower borderline corresponds to the dose-to-clear exposure condition.
Below this line the resist is not fully dissolved. Above the higher borderline also opaque areas are – unwantedly – cleared.
The development time was set to 30 seconds.
5.4 Overlapped process window for all parameters
Assuming a tolerance of ±1 micron for 5 micron CD, a sidewall angle Θ > 60° is probably the best compromise for
proximity lithography using 1 micron thick layer of AZ® 1512HS positive photoresist on silicon. For a complete
assessment of a process all relevant process windows are combined.
Figure 8. Process window for proximity lithography using LGO(±1.4°) illumination settings as described in Figure 2
(center). A tolerance of ±1 micron for 5 micron CD and (left) a sidewall angle Θ > 60° and (right) a sidewall angle Θ > 70°
was set for the process window. The red areas show combinations of proximity gaps and dose values, which produce CD
values in the range of 5 ±1 micron and sidewall angles in the specified ranges. The exposure latitude is indicated by
rectangles fitted into the process window.
SPIE Advanced Lithography 2014, 9052-15 Opt. Microlithography XXVII, Voelkel et. al., February 25, 2014
Figure 8 shows the resulting process windows from overlapping Figure 5, Figure 6 and Figure 7 and plotting the
intersections only, thus fulfilling all three conditions. For relaxed requirements regarding the sidewall angle, shown in
Figure 8 (left), the usable process windows range from 0 to 10 microns and 40 to 60 microns proximity gap
corresponding to an exposure dose of 85 to 160 mJ/cm2.
This result is quite surprising, contrary to the “good instinct” of experienced lithography experts and contrary to the
widely accepted equation (1) for proximity lithography. The common expectation is that the robustness of the proximity
process improves with decreasing proximity gap. An experienced lithography expert will always opt to reduce the
proximity gap to a minimum tolerable value dictated by mask and wafer bending, resist height deviation and gap setting
accuracy. The process window in Figure 8 clearly shows that printing lines & spaces, 5 micron half-pitch, at a 30 micron
proximity gap leads to a very unstable process. Whereas for a 50 micron gap and 120 mJ/cm2 exposure dose a latitude of
±20% is tolerable. The process window for harsher requirements to the sidewall angle of Θ > 70°, shown in Figure 8
(right), indicates that AZ® 1512HS is not the right choice of photoresist for steep sidewall angles.
The improvement of the lithography results for 50 micron proximity gap compared to 30 micron proximity gap can be
expected from the simulated intensity distributions on the right of Figure 3. However, the specific values of the
achievable dose latitude and range of favorable proximity gaps depend strongly on the photoresist parameters and
processing conditions. A careful process window optimization is required for different photoresists and processes.
6. PROCESS WINDOW FOR LGO(±0.7°) ILLUMINATION SETTINGS
In the previous chapter we observed, that printing lines & spaces, 5 micron half-pitch, by using proximity lithography
will typically lead to smaller resist structures. This unwanted effect might have its positive side in the case that a target
CD of 4 micron half-pitch is required. The process window for this case is shown in Figure 9 for illumination with
LGO(±0.7°) Optics, shown in Figure 2 (right). In the case of no criteria for sidewall angles, a stable process window,
shown in Figure 9 (left), could be found. For a sidewall condition of Θ > 60°, shown in Figure 9 (right), the exposure
latitude looks quite similar to the latitude for 5 micron CD with ±1 micron tolerance, shown in Figure 8 (right). Printing
4 micron structures with a 5 micron mask, i.e. compensating diffraction effects by adapting the mask pattern, is a very
useful strategy, also referred as optical proximity correction (OPC).
Figure 9: Process window for target critical dimension CD of 4 microns with ±1 micron tolerance. (Left) no restrictions to
sidewall angle and (right) a sidewall angle Θ > 60° as additional criteria. Exposure latitude is indicated by rectangles fitted
into the process window, mask CD of 5 microns.
SPIE Advanced Lithography 2014, 9052-15 Opt. Microlithography XXVII, Voelkel et. al., February 25, 2014
7. PROCESS WINDOW FOR HR(±3°) ILLUMINATION SETTINGS
As discussed in chapter 3, the illumination settings for mask aligners are preset for contact or proximity lithography.
Mask aligners for contact lithography are typically equipped with an illumination optics providing a larger angular
spectrum of ±3° to ±4°. For proximity lithography, better collimated light with typically ±1° to ±2° angular spectrum is
preferred. For SUSS mask aligners, the illumination optics for contact and small proximity gaps is referred to as HR
Optics (HR: high resolution). For proximity lithography at large gaps, the illumination optics is referred to as LGO
Optics (LGO: large gap). For mask aligners equipped with MO Exposure Optics ® the illumination settings are defined by
illumination filter plates (IFP) and can easily be changed11,12. In the following we examine the process window for a
mask aligner configured for contact lithography, i.e. equipped with HR (±3°) Optics shown in Figure 2 (left), but used
for proximity lithography.
Figure 10: Process window for critical dimension CD 5 micron with ±1 micron tolerance using HR(±3°) illumination optics,
designed for contact or small gap proximity lithography. (Left) process window with no restrictions to sidewall angle and
(right) a sidewall angle Θ > 60° as additional criteria. Exposure latitude is indicated by rectangles fitted into process
window.
Figure 11: Process window for critical dimension CD 5 micron with ±1 micron tolerance using HR(±3°) illumination optics,
designed for contact lithography. (Left) process window for side lobe printing and (right) the intersection of all three process
windows.
SPIE Advanced Lithography 2014, 9052-15 Opt. Microlithography XXVII, Voelkel et. al., February 25, 2014
Figure 10 and Figure 11 show the corresponding process windows for using HR(±3°) illumination optics, designed for
contact and small proximity gap lithography. The comparison of Figure 11 for HR(±3°) illumination optics and Figure 8
for LGO(±1.4°) illumination optics demonstrates the significant influence of the illumination settings on proximity
lithography.
In practice, the choice if a mask aligner is equipped with HR or LGO illumination optics is made when the mask aligner
is purchased. Typically, a changeover from HR to LGO or vice versa requires the investment in new optical parts and a
fine adjustment of the illumination optics after each exchange. MO Exposure Optics®, the latest generation of
illumination optics for SUSS MicroTec mask aligners allows a changeover of the illumination settings within less than a
minute by changing an illumination filter plate (IFP). Lithography simulation software LAB6 allows lithography
engineers to evaluate any kind of illumination settings, including ring, dipole, quadrupole, multipole and even more
sophisticated angular spectra. The introduced process window methodology is a valuable method to assess the impact of
different illumination settings on the robustness of the overall lithographic process.
8. CONCLUSION AND OUTLOOK
The purpose of this report was the introduction of a complete methodology for process window optimization to
proximity mask aligner lithography. Recently, MO Exposure Optics®, a new type of illumination system was introduced
for SUSS MicroTec mask aligners. MO Exposure Optics® provides telecentric illumination, excellent light uniformity
over the full mask field and allows the lithographic engineer to change and optimize the illumination settings in
proximity lithography (customized illumination). Diffraction effects in proximity lithography could now be reduced by
using optical proximity correction (OPC) and source-mask optimization (SMO). It was demonstrated recently, that these
well-known lithography enhancement methods can be applied successfully to mask aligner lithography. However, it
remained difficult for the lithographic engineer to visualize and assess their impact on process robustness. The proposed
process window methodology now closes the gap. Process window optimization proved to be a valuable method to
obtain more robust lithography processes in mask aligner lithography. The most important outcome of this first report is,
that in proximity lithography a smaller exposure gap does not automatically lead to better print results. This is contrary to
what the “good instinct” of a lithography engineer would expect. Although some engineers might know about this, the
majority of the engineers working with mask aligners will not be aware. Regarding an installed base of more than 5’000
SUSS MicroTec mask aligners, the introduced process window methodology might have a significant impact on yield
improvement and cost saving in industry worldwide.
Some of the typical characteristics of the process window, such as achievable resolution, most appropriate ranges of
proximity gaps and qualitative impact of illumination settings can be already derived from computed intensity
distributions in the Fresnel region of the mask. However, the specific values of the achievable dose latitude and range of
favorable proximity gaps depend strongly on the photoresist parameters and processing conditions. A careful process
window optimization is required for different photoresists and processes. The presented results, obtained from
simulation, were verified by exposure tests using a 1 micron thick layer of AZ® 1512HS positive photoresist on a silicon
wafer in a SUSS MA8 mask aligner equipped with the described illumination settings13. The experimental results
corresponded largely to the simulation results and proved the applied methodology.
SPIE Advanced Lithography 2014, 9052-15 Opt. Microlithography XXVII, Voelkel et. al., February 25, 2014
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