The lithography challenges posed by the 20 nm and 14 nm nodes continue to place strict minimum feature size
requirements on photomasks. The wide spread adoption of very aggressive Optical Proximity Correction (OPC) and
computational lithography techniques that are needed to maximize the lithographic process window at 20 nm and 14 nm
groundrules has increased the need for sub-resolution assist features (SRAFs) down to 50 nm on the mask. In addition,
the recent industry trend of migrating to use of negative tone develop and other tone inversion techniques on wafer in
order to use bright field masks with better lithography process window is requiring mask makers to reduce the minimum
feature size of opaque features on the reticle such as opaque SRAFs. Due to e-beam write time and pattern fidelity
requirements, the increased use of bright field masks means that mask makers must focus on improving the resolution of
their negative tone chemically amplified resist (NCAR) process.
In this paper we will describe the development and characterization of a high resolution bright field mask process that is
suitable for meeting 20 nm and early 14 nm optical lithography requirements. Work to develop and optimize use of an
improved chrome hard mask material on the thin OMOG binary mask blank1 in order to resolve smaller feature sizes on
the mask will be described. The improved dry etching characteristics of the new chrome hard mask material enabled the
use of a very thin (down to 65 nm) NCAR resist. A comparison of the minimum feature size, linearity, and through pitch
performance of different NCAR resist thicknesses will also be described. It was found that the combination of the
improved mask blank and thinner NCAR could allow achievement of 50 nm opaque SRAFs on the final mask.. In
addition, comparisons of the minimum feature size performance of different NCAR resist materials will be shown. A
description of the optimized cleaning processes and cleaning durability of the 50 nm opaque SRAFs will be provided.
Furthermore, the defect inspection results of the new high resolution mask process and substrate will be shared.
The lithography challenges posed by the 22 nm node continue to place stringent requirements on photomasks.
The dimensions of the mask features continue to shrink more deeply into the sub-wavelength scale. In this
regime residual mask electromagnetic field (EMF) effects due to mask topography can degrade the imaging
performance of critical mask patterns by degrading the common lithography process window and by magnifying
the impact of mask errors or MEEF. Based on this, an effort to reduce the mask topography effect by
decreasing the thickness of the mask absorber was conducted. In this paper, we will describe the results of our
effort to develop and characterize a binary mask substrate with an absorber that is approximately 20-25% thinner
than the absorber on the current Opaque MoSi on Glass (OMOG) binary mask substrate.
For expediency, the thin absorber development effort focused on using existing absorber materials and deposition
methods. It was found that significant changes in film composition and structure were needed to obtain a
substantially thinner blank while maintaining an optical density of 3.0 at 193 nm. Consequently, numerous
studies to assess the mask making performance of the thinner absorber material were required and will be
described. During these studies several significant mask making advantages of the thin absorber were
discovered. The lower film stress and thickness of the new absorber resulted in improved mask flatness and up
to a 60% reduction in process-induced mask pattern placement change. Improved cleaning durability was
another benefit. Furthermore, the improved EMF performance of the thinner absorber [1] was found to have the
potential to relieve mask manufacturing constraints on minimum opaque assist feature size and opaque corner to
corner gap.
Based on the results of evaluations performed to date, the thinner absorber has been found to be suitable for use
for fabricating masks for the 22 nm node and beyond.
In optimizing e-beam resist process conditions for photomask lithography, the primary
performance measurements for optimization are resolution, critical dimension uniformity
(CDU), line edge roughness (LER), and linearity. Through technology nodes, one
parameter that has consistently shown a critical impact on these factors is the post
exposure bake (PEB) condition. With 32nm e-beam resist technologies having reduced
temperature sensitivity, this paper investigates the current impact of PEB conditions. The
PEB assessment will summarize the influence of PEB temperature, duration and
environment flow on 32 nm positive tone resists by reporting and analyzing two of the
primary performance measurements: CDU and LER.
Use of optical photomasks will extend to the 22-nm node and beyond. Mask minimum resolution and critical dimension
(CD) requirements for this node are very challenging to the mask industry. Optimization of resist materials and resist
thickness are key factors for improving CD performance. In general, thinner resists result in better minimum resolution
performance. The minimum useable resist thickness is often linked to the chrome hard mask dry etching performance.
More specifically, improvement of chrome etch rate selectivity to resist while simultaneously maintaining good CD
performance is difficult. In order to use a very thin e-beam resist, the underlying chrome hard mask material thickness
needs to be thin or it needs to be comprised of a material that has a fast etch rate and good dry etch selectivity to resist.
Use of thin and/or fast etch rate hard mask materials that are capable of reducing dry etch induced CD error such as etch
bias, etch bias uniformity, etch bias linearity, and etch global loading effect is required for meeting 22-nm mask
requirements. In this paper, the dry etching effect dependence on hard mask thickness, hard mask material composition
and resist thickness for building advanced binary masks for 22-nm node is studied. The results from this work will show
that dry etch induced CD error such as etch bias, etch bias uniformity, etch bias linearity, and etch global loading effect
are significantly improved by use of an ultra thin or high etch rate hard mask material.
Two key parameters of attenuated phase shift masks are critical dimension uniformity (CDU) and phase uniformity.
This study examines the important role that plasma etch plays in determining these parameters. For optimal results, the
impact which Cr and MoSi etch have on uniformity must be understood not only individually, but also as a
complementary pair. A two-step MoSi etch was developed; the first step was tuned to have a higher etch bias at the edge
than at the center, while the second step had a very uniform etch bias. By controlling the fraction of the MoSi consumed
by each step, the MoSi etch was adapted to complement the Cr etch and thus optimize overall CDU and phase
uniformity.
During the development of optical lithography extensions for 32nm, both binary and attenuated phase shift Reticle
Enhancement Technologies (RETs) were evaluated. The mask blank has a very strong influence on the minimum feature
size and critical dimension (CD) performance that can be achieved on the finished reticle and can have a significant
impact on the ultimate wafer lithographic performance. Development of a suitable high resolution binary mask making
process was particularly challenging. Standard chrome on glass (COG) binary blanks with 70 nm thick chrome films
were unable to support the required minimum feature size, linearity, and through pitch requirements. Two alternative
mask blank configurations were evaluated for use in building high resolution binary masks: a binary (BIN) mask blank
based on the standard attenuated PSM blank and an Opaque MoSi on Glass (OMOG) mask blank consisting of a newly-
developed opaque MoSi [1]. Data comparing the total process bias, minimum feature size, CD uniformity, linearity,
through pitch, etch loading effects, flatness, film stress, cleaning durability and radiation durability performance of the
different binary and attenuated PSM mask blanks are reported. The results show that the new OMOG binary blank offers
significant mask performance benefits relative to the other binary and attenuated PSM mask blanks. The new OMOG
blank was the opaque mask blank candidate most capable of meeting 32nm binary mask fabrication requirements..
As optical lithography is extended for use in manufacturing 45 nm devices, it becomes increasingly important to
maximize the lithography process window and enable the largest depth of focus possible at the wafer stepper.
Consequently it is very important that the reticles used in the wafer stepper be as flat as possible. The ITRS roadmap
requirement for mask flatness for 45 nm node is 250 nm. To achieve this very tight reticle flatness requirement, the stress
of each film present on the mask substrate must be minimized. Another key reticle specification influenced by film stress
on the mask blank is image placement. In this paper, we will describe the development and detailed characterization of a
new low stress Molybdenum Silicide (MoSi) film for use in manufacturing 45 nm node critical level attenuated phase
shift masks to be used in 193 nm immersion lithography. Data assessing and comparing the cleaning durability, mask
flatness, image placement, Critical Dimension (CD) performance, dry etch properties, phase performance, and defect
performance of the new low stress MoSi film versus the previous industry standard A61A higher stress MoSi attenuator
film will be described. The results of our studies indicate that the new low stress MoSi film is suitable for 45 nm mask
manufacturing and can be introduced with minimal changes to the mask manufacturing process.
The properties of phase shifting attenuator films are quantified in a variety of ways. Transverse dimensions are
measured by optical microscopes or scanning electron microscopes. Vertical dimension and profiles are measured by
atomic force microscopes or indirectly by optical scatterometry. The complex refractive index of an attenuator film can
be characterized by ellipsometry or by spectroscopic analysis of reflected and transmitted light. Transmission and phase
measurements can be made with optical interferometric techniques. Data acquired in these ways can be used as inputs
to simulation programs to model the image forming characteristics of the films. For simplicity and speed of calculation,
the simulation programs typically use a thin-mask approximation, in which the vertical absorber geometry is ignored
and the phase shifting attenuator regions are characterized only by their transmission, phase shift, and two-dimensional
geometric shapes. Inclusion of the full three-dimensional profile and complex refractive index of the absorber can be
done, but at the cost of greatly increased calculation time and a loss of the simplicity of understanding afforded by the
thin-mask model. For example, the thin-mask model assumes that every geometrical feature etched into a given
attenuator film will have the same phase and transmission properties. Comparison of thin-mask modeling results with
the full three dimensional model shows that this assumption is not true. The effective dimensional bias, phase,
transmission, and defocus are strong functions of the feature size, pitch, and complex refractive index of the film. Three
dimensional simulations were run for several commercial and developmental high-transmission phase-shifting
attenuator films. The effective phase and dimensional printing bias were calculated as a function of pitch for each film.
Surprising differences were found in the results for the various film types.
Three types of high transmission attenuated phase shift masks were evaluated. The attenuating materials were obtained from commercial and non-commercial sources. Various key performance metrics were investigated. Blanket film transmission and reflection was measured at various wavelengths. Laser durability and cleaning durability were
measured. Standard dry etch processes were used for each film and the profile and surface properties were compared. Final mask transmission and phase were also measured. The summarized results show clear benefits of using some high transmission materials relative to others.
The phase shift mask (PSM) is one of the most effective approaches to improve ArF lithography performance. Recently, the quartz dry etching technology plays an important role to fabricate the PSM, such as space bias type Alternating (Alt.) PSM and chrome-less phase lithography (CPL) mask. The quartz etching profiles seems to be affected the lithography performance. In this paper, preliminary, we evaluate the nominal influences of quartz profile by rigorous electromagnetic field simulation. Then influence of the quartz profile is investigated by measuring the real masks. In this experiment, we intentionally fabricate Alt. PSM and CPL masks with the tapered side-wall and deeper micro-trench. Lithography performances of the real masks are measured by the aerial image measurement system (AIMS fab193). We compare the result of AIMS with simulation. We investigate the AIMS measurement well corresponds to the simulation. Side-wall angle and corner rounding strongly affect the lithography performance. However, micro-trench doesn’t affect a lot.
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