The particle removal efficiency (PRE) of cleaning processes diminishes whenever the minimum defect size for a specific
technology node becomes smaller. For the sub-22 nm half-pitch (HP) node, it was demonstrated that exposure to high
power megasonic up to 200 W/cm2 did not damage 60 nm wide TaBN absorber lines corresponding to the 16 nm HP
node on wafer. An ammonium hydroxide mixture and megasonics removes ≥50 nm SiO2 particles with a very high PRE.
A sulfuric acid hydrogen peroxide mixture (SPM) in addition to ammonium hydroxide mixture (APM) and megasonic is
required to remove ≥28 nm SiO2 particles with a high PRE. Time-of-flight secondary ion mass spectroscopy (TOFSIMS)
studies show that the presence of O2 during a vacuum ultraviolet (VUV) (λ=172 nm) surface conditioning step will result
in both surface oxidation and Ru removal, which drastically reduce extreme ultraviolet (EUV) mask life time under
multiple cleanings. New EUV mask cleaning processes show negligible or no EUV reflectivity loss and no increase in
surface roughness after up to 15 cleaning cycles. Reviewing of defect with a high current density scanning electron
microscope (SEM) drastically reduces PRE and deforms SiO2 particles. 28 nm SiO2 particles on EUV masks age very
fast and will deform over time. Care must be taken when reviewing EUV mask defects by SEM. Potentially new
particles should be identified to calibrate short wavelength inspection tools. Based on actinic image review, 50 nm SiO2
particles on top of the EUV mask will be printed on the wafer.
Naturally occurring sub 30 nm defects on quartz and Low Thermal Expansion Material (LTEM) substrates were characterized by using Atomic Force Microscope(AFM). Our data indicates that a majority of defects on the incoming substrate are hard defects including large, flat particles with a height less than 5 nm, tiny particles with a size of 10 nm to 30 nm SEVD and pits with a depth of about 9 nm. All the soft particles added by handling with sizes of >50 nm can be removed with a single cleaning process. At least four cleaning cycles are required to remove all of the remaining embedded particles. However, after particle removal in their initial location a shallow pit remains. Based on detailed characterization of defect and surface by AFM, we propose that these hard particles are added during the glass polishing step and therefore it is important to revisit the glass Chemical Mechanical Polishing (CMP) processes and optimize them for defect reduction. A qualitative value for particle removal efficiency (PRE) of >99% was obtained for 20 nm Poly Styrene Latex Sphere (PSL) deposited particles on surface of glass.
Defect smoothing is a critical need for improving defects. There are different methods such as using a smoothing layer
or multilayer deposition; however, smoothing processes tend to add defects of their own to the surface. This paper
presents a novel pit smoothing method based on an anisotropic substrate etch process. Smoothing power is defined as a
metric for comparing the smoothing capability of different smoothing processes. Defect smoothing by cleaning is a
surface modification technique with a smoothing power <10 that does not add defects to the surface. This is
demonstrated by comparing total defects on the mask blank and mask blank substrate for two processes: a standard
ozone-based cleaning and a smoothing cleaning. The smooth/clean methods led to fewer defects on the blank and
substrate surfaces than the standard clean while still meeting extreme ultraviolet (EUV) blank roughness requirements.
Finally, it is shown that smoothed pits are still printable. Therefore, further improvements to the smoothing power of
smooth/clean processes are needed. SEMATECH is currently working to improve smooth/clean processes for low
thermal expansion material (LTEM) EUV substrates.
Defects are still one of the main challenges of extreme ultraviolet (EUV) mask blanks. In particular, a majority(~75%) of
substrate defects are nanometer size pits. These pits are usually created during final surface polishing of the synthetic,
quartz glass substrates. This study presents data that indicates cleaning may also induce pits in the substrate surface.
These pits are typically 20 nm and larger, and are contained in a circular area on the surface, which is scanned by a
megasonic nozzle during cleaning. Concentrated collapse of cavitation bubbles in the areas scanned by megasonic is
expected to be one of the main mechanisms of pit creation. The data indicates the existence of a hard surface layer with
an estimated thickness of approximately 30 to 60 nm, which is resistive to pit creation. After this layer is removed, the
number of pit defects present on the substrate increases dramatically with megasonic cleaning. It is also demonstrated
that, within the detection limits of the atomic force microscope (AFM), the size of a pit does not change due to cleaning.
Extreme ultraviolet lithography (EUVL) is a strong contender for the 32 nm generation and beyond. A defect-free mask
substrate is an absolute necessity for manufacturing EUV mask blanks. The mask blank substrates are, therefore,
cleaned with different cleaning processes to remove all defects down to 30 nm. However, cleaning suffers from the
defects added by various sources such as the fab environment, chemicals, ultra pure water, and the cleaning process
itself. The charge state of the substrate during and after cleaning also contributes to the number of adder defects on the
substrate. The zeta potentials on the substrate surface and the defect particles generated during the cleaning process
determine whether the particles get deposited on the surface. The zeta potential of particle or substrate surfaces depends
on the pH of the cleaning fluids. Therefore, in this work, pH-zeta potential maps are generated for quartz substrates
during the various steps of mask cleaning processes. The pH-zeta potential maps for defect particles commonly seen on
mask substrates are measured separately. The zeta potential maps of substrate and contaminant particle surfaces are
used to determine whether particles are attracted to or repulsed from the substrate. In practice, this technique is
especially powerful for deriving information about the origin of particles added during a cleaning process. For example,
for a known adder with a negative zeta potential, all cleaning steps with a positive zeta potential substrate could be the
source of added particles.
The capability of SEMATECH's Lasertec M7360 inspection tool to detect particles of different sizes and composition was studied on the surface of fused silica and MoSi multilayers (MLs) with a Si cap layer. Particles of Au, Ag, SnO2, Fe2O3, and Al2O3 were deposited and inspected 10 times with the M7360. Tool pixel size histograms were used to calculate the average pixel size per particle category. The calibration curves of pixel size for polystyrene latex (PSL) spheres were used to convert the average pixel size to the optical size of the defects as detected by the M7360. Selective sets of each category of particles then were reviewed by atomic force microscope (AFM) to calculate the sphere equivalent volume diameter (SEVD) of the particles. The contribution of the surface on which particles were deposited and defect composition and shape were studied. Our results indicate that for Fe2 O3 and SnO particles, size distribution on the surface of fused silica and MLs is similar and no effect of the substrate was observed. The AFM-measured SEVD size of particles were close to the nominal size of particles specified by the particle supplier. Optical size of particles were found to be larger or smaller than SEVD size for the different particles. In the case of the Au particles, the PSL equivalent optical size was found to be larger than the SEVD in good agreement with the modeling. By using prefabricated rectangular defects on a fused silica surface, we showed that the M7360 differentiates between the PSL and SEVD size of prefabricated defects. The PSL size is smaller than the SEVD size of prefabricated defects for particle sizes below 100 nm.
The capability of hydrogenated water to clean EUV blank substrates was examined. The hydrogenated water cleaning
process was compared with an H2O2/NH4OH/H2O mixture (SC1) and ozonated water cleaning processes. A small
amount ammonia added to the hydrogenated water improved the particle removal efficiency. The concentration of
hydrogen and the method used to dispense the water had little effect. The use of ozonated and hydrogenated water
together gave high particle removal efficiencies, which were similar to those obtained using SC1. Additionally, the use
of ozonated water with hydrogenated water further reduced the amount ammonia required to achieve high particle
removal efficiencies. With further process optimization hydrogenated and ozonated water has the potential to replace
SC1 in cleaning EUV substrates.
Extreme ultraviolet (EUV) substrates have stringent defect requirements. For the 32 nm node, all particles larger than 26
nm must be removed from the substrate. However, real defects are irregularly shaped and there is no clear dimension for
an irregular particle corresponding to 26 nm. Therefore, the sphere equivalent volume diameter (SEVD) for a native
defect is used. Using this definition and defect detection measurements, all particles larger than 20 nm must be removed
from the substrate. Atomic force microscopy (AFM) imaging and multiple cleaning cycles were used to examine the
removal of particles smaller than 50 nm SEVD. Removal of all particles larger than 30 nm was demonstrated. Particles
that required multiple cleaning processes for removal were found to be partially embedded. The best cleaning yield can
be obtained if the cleaning history of the substrate is known and one can choose the proper cleaning processes that will
remove the remaining particles without adding particles. Ag, Au, Al2O3, Fe2O3, and CuO particles from 30 nm to 200 nm
were deposited on quartz surface. It was shown that these deposited defects are much easier to remove than native
Due to the increasing impact of smaller particles, mask cleaning continues to become more and more challenging in EUV lithography. To improve mask cleaning efficiency, advances in the fundamental understanding of the interaction between defect particles and mask surfaces are necessary. For this reason, surface force measurements were performed with an atomic force microscope on various mask surfaces relevant to EUV lithography. Experiments in air were carried out to illustrate particle interaction during mask transport and storage, while measurements in deionized ultrapure water were undertaken to investigate the influence of a basic cleaning chemistry. The effects of particle size were studied using SiNx tips with a nominal radius of 10 nm and spherical SiO2 probes with a radius of 500 nm. Particle interactions with mask surfaces in air were characterized by adhesion. Due to comparable surface roughness and surface chemistry, adhesion forces of a quartz mask substrate and a mask blank were similar. However, for a SiO2 sphere, the absolute values of the measured adhesive forces were greater than for a conventionally fabricated SiNx tip consistent with the probes' relative radii. Using a quartz mask substrate and deionized water as the intervening medium, the probe-substrate interaction observed was no longer characterized by attraction, but dominated by repulsive forces and hence potentially advantageous for cleaning purposes.
Extreme ultraviolet lithography (EUVL) is being considered as the enabler technology for the manufacturing of future
technology nodes (30 nm and beyond). EUV mask blanks are Bragg mirrors made of Mo and Si bilayers and tuned for
reflectivity at a wavelength λ ~13 nm. Implementation of EUVL requires that the mask blanks be free of defects at 30
nm or above. However, during the deposition of MoSi multilayers and later during the handling of blanks, defects are
added to the blank. Therefore, the cleaning of EUV mask blanks is a critical step in the manufacturing of future devices.
The particulate defects on the multilayer-coated mask blanks can either be embedded in or under the MoSi layers or
adhered to the top capping layer during the deposition process. The defects can also be added during the handling of
photomasks. Our previous studies have shown successful removal of the handling-related defects at SEMATECH's
Mask Blank Development Center (MBDC) in Albany, NY. However, cleaning embedded and adhered defects presents
new challenges. The cleaning method should not only be able to remove the particles, but also be compatible with the
mask blank materials. This precludes the use of any aggressive chemistry that may change the surface condition leading
to diminished mask blank reflectivity. The present work discusses the recent progress made at SEMATECH's MBDC in
cleaning backside Cr-coated mask blanks with a MoSi multilayer and a Si cap layer on the top surface. Here we present
our data that demonstrates successful removal of sub-100 nm particles added by the deposition process. Surface
morphology and defect composition on the surface of the MoSi multilayer are discussed. EUV reflectivity measurements
and atomic force microscopy (AFM) images of the mask blank before and after cleaning are presented. The present data
shows that no measurable damage to the EUV mask blank is caused by the cleaning processes developed at the MBDC.
The feasibility of removing defects from the surface of extreme ultraviolet (EUV) substrates by nanomachining is being
investigated. A commercially available atomic force microscope (AFM) based photomask repair tool was used. A
specific class of defects which has resisted all other removal techniques was targeted. Three AFM probes of varying
sharpness were evaluated. All of the probes removed the majority of each but fell short of achieving the desired 2006
high spatial frequency roughness specification of 0.2nm. Results reported are preliminary; future work will focus on
optimization of scanning parameters and tip geometry targeting specific residual defects reported in the text.
Extreme ultraviolet (EUV) mask blanks must have nearly zero defects larger than 30 nm. Mask blank defects are an accumulation of defects present on the substrate, defects added during the multilayer (ML) deposition process, and defects added by handling the mask blank. A majority of the detectable defects are already present on the substrate before the ML deposition. However, very few of the defects present on the substrate before the ML deposition are detectable. This raises the question of whether the substrate's surface condition contributes to the total number of defects on the mask blank. Here the results of investigations on the relation between the total number of defects on the multilayer and the substrate surface condition are presented. The final surface condition is determined by the mask cleaning process. Correlation studies between defect maps before and after multilayer deposition are presented, and the relation between final defect size on the multilayer and substrate are discussed. SEMATECH's Mask Blank Development Center (MBDC) has a unique capability to characterize the surface of EUV glass substrates by atomic force microscopy (AFM), scanning electron microscopy (SEM), surface energy measurement, and zeta potential metrology. A series of experiments were performed in which different cleaning processes were used to modify the substrate surface condition before multilayer deposition. The effect of the cleaning process on the number of pits and particles after ML deposition was examined. The results indicate that although there is a direct relationship between the number of defects remaining on the substrate and mask blank defects after multilayer deposition, the variation in the total number of defects on the mask blank mainly corresponds to pits and particles already present on the substrate before cleaning and are not the result of the cleaning processes that were used before multilayer deposition.
Defects on an extreme ultraviolet (EUV) mask blank strongly depend on the defects on the mask blank substrate. Any imperfection on the substrate surface in the form of a particle, pit, and scratch will appear on the EUV mask blank. In this article, we study the effect of the cleaning process on the creation of defects on the EUV substrate and mask blank. Added particles could be removed by improving the cleaning tool and the cleaning process. Pits are generally created when many large defects, particularly glass-like materials, are present on the surface and the substrate is exposed to a high energy cleaning step. Comparison of different high energy steps in a typical cleaning process suggests that the megasonic step most likely creates pits. Current cleaning processes developed in the Mask Blank Development Center (MBDC) have been optimized so that no added pits or particles are observed after using them.
Low thermal expansion material (LTEM) substrates were cleaned with recipes developed to clean blank quartz substrates. These recipes were capable of cleaning the LTEM without damaging the LTEM substrate. No effect of etching doped metals in LTEM was observed in these experiments. However, LTEM substrates currently require multiple cleaning cycles to obtain the same removal or cleaning efficiencies as quartz substrates. In addition, no change in the surface roughness or degradation of the backside choromium layer was observed.