The prospect of EUVL (Extreme Ultraviolet Lithography) insertion into HVM (High Volume Manufacturing) has never been this promising. As technology is prepared for "lab to fab" transition, it becomes important to comprehend challenges associated with integrating EUVL infrastructure within existing high volume chip fabrication processes in a foundry fab. The existing 193nm optical lithography process flow for reticle handling and storage in a fab atmosphere is well established and in-fab reticle contamination concerns are mitigated with the reticle pellicle. However EUVL reticle pellicle is still under development and if available, may only provide protection against particles but not molecular contamination. HVM fab atmosphere is known to be contaminated with trace amounts of AMC’s (Atmospheric Molecular Contamination). If such contaminants are organic in nature and get absorbed on the reticle surface, EUV photon cause photo-dissociation resulting into carbon generation which is known to reduce multilayer reflectivity and also degrades exposure uniformity. Chemical diffusion and aggregation of other ions is also reported under the e-beam exposure of a EUV reticle which is known to cause haze issues in optical lithography. Therefore it becomes paramount to mitigate absorbed molecular contaminant concerns on EUVL reticle surface. In this paper, we have studied types of molecular contaminants that are absorbed on an EUVL reticle surface under HVM fab storage and handling conditions. Effect of storage conditions (gas purged vs atmospheric) in different storage pods (Dual pods, Reticle Clamshells) is evaluated. Absorption analysis is done both on ruthenium capping layer as well as TaBN absorber. Ru surface chemistry change as a result of storage is also studied. The efficacy of different reticle cleaning processes to remove absorbed contaminant is evaluated as well.
In the absence of a pellicle, an EUVL reticle is expected to withstand up to 100 cleaning cycles. EUVL reticles
constitute a complex multi-layer structure with extremely sensitive materials which are prone to damage during
cleaning. The 2.5 nm thin Ru capping layer has been reported to be most sensitive to repeated cleaning, especially
when exposed to aggressive dry etch or strip chemicals . Such a Ru film exhibits multiple modes of failure under
wet cleaning processes. In this study we investigated the Ru peeling effect. IR-induced thermo-stress in the multilayer
and photochemical-induced radical attack on the surface are investigated as the two most dominant
contributors to Ru damage in cleaning. Results of this investigation are presented and corrective actions are
Current and future lithography techniques require complex imaging improvement strategies. These imaging
improvement strategies require printing of sub-resolution assist-features (SRAF) on photomasks. The size of SRAF’s has
proven to be the main limiting factor in using high power Megasonic cleaning process on photomasks. These features,
due to high aspect ratio are more prone to damage at low Megasonic frequencies and at high Megasonic powers.
Additionally the non-uniformity of energy dissipated during Megasonic cleaning is a concern for exceeding the damage
threshold of the SRAFs. If the cavitation events during Megasonic cleaning are controlled in way to dissipate uniform
energy, better process control can be achieved to clean without damage. The amount and type of gas dissolved in the
cleaning liquid defines the cavitation behavior. Some of the gases possess favourable solubility and adiabatic properties
for stable and controlled cavitation behaviour. This paper particularly discusses the effects of dissolved Ar gas on
Megasonic characteristics. The effect of Ar Gas is characterized by measuring acoustic energy and Sonoluminscense.
The phenomenon is further verified with pattern damage studies.
Acoustic energy applied through the cleaning media results into two kinds of cavitation effects; namely stable and
transient cavitation. A uniformly pulsating bubble transforms into stable cavitation behavior whereas a bubble implosion
implies transient cavitation. Pattern damage of sensitive features on advanced masks as well as Ru pitting on EUVL
reticles is mostly the result of transient cavitation. Stable cavitation on the other hand produces a very narrowly
controlled energy distribution which allows cleaning without damage. Stable cavitation can be achieved by suitably
tailoring physical, chemical and thermodynamic properties of the liquid and gas media.
In this paper we investigate a new cleaning chemistry that has favorable physical and thermodynamic properties to
produce stable MegaSonic cavitation. The cavitation created in this chemistry is characterized by measuring acoustic
energy as well as by pattern damage and particle removal efficiency on mask level. The chemical properties (pH and zeta
potential) of this chemistry are compared with conventional cleaning chemistries. Its effects on CD shift as well as phase
and transmission loss are also studied.
Physical force wet cleaning technologies (MegaSonic & Droplet Spray) are considered the supreme challenge in 193i reticle cleaning due to smaller critical dimension, high feature aspect ratio, and sensitive interfaced fragile features. However this was not considered equally challenging in EUV masks. Recently, MegaSonic cavitation has been linked to Ru (capping layer) pitting issues; making the use of acoustic based cleaning questionable for EUVL reticles. Nevertheless, acoustic energy is necessary to remove particles trapped in deep and congested trenches. This strengthens the need to control the physical force energy within the damage free regime even further. In this study we have investigated method to control MegaSonic cavitation as well as established a link between pattern damage observed in optical mask and Ru pitting in EUV masks. Effect of different cleaning chemistries typically used in mask cleaning is compared with a new cleaning chemistry (referred to as chemical A). A new POR (Process of Record) based on chemical A is qualified and its effects on PRE, absorber CD and EUV-reflectivity are discussed.
As the feature size of the mask shrinks, the feature becomes more fragile and the potential for physical force damage during cleaning increases. At the same time, increased feature density of the mask makes it difficult to remove particles from congested trenches without physical force cleaning. Acoustic energy has the ability to suppress the hydro-dynamic boundary layer thereby transferring the physical force impact closer to particles trapped in the deep trenches of the mask. MegaSonic, which employs acoustic energy, is a preferred physical force cleaning technology for advanced masks. However MegaSonic can be extremely aggressive if the energy distribution is not contained within the narrowest process window available. In this paper, liquid media properties and their effect in controlling MegaSonic energy is evaluated. A chemistry is identified which provides favorable gaseous properties for controlling MegaSonic cavitation. The effect of this chemistry is characterized by measuring acoustic energy and Sonoluminscense. The phenomenon is further verified with pattern damage studies.
To overcome the challenge of particle removal without pattern damage, it is important to control and optimize the
physical force impact into a narrow energy distribution regime. This can be achieved by gaining in-depth knowledge into the fundamentals of physical cleaning methods as well as the process parameters affecting the performance of these systems. Droplet Spray is considered to be a gentle physical force technique. The droplet size and droplet velocity defines the kinetic energy of a droplet which transfers momentum and pressure onto the substrate surface resulting into particle removal or pattern damage. Droplet pressure is directly influenced by gas and liquid flow rate as well as nozzle design (orifice size, etc.) and process parameters like nozzle distance and scan speed across the substrate surface. In this paper, effect of process and hardware parameters on droplet pressure transfer are presented and related to kinetic energy and momentum as calculated from droplet velocity and size. Effect of these process parameters is also studied on pattern damage and particle removal efficiency. Distance, nozzle hardware, gas and liquid flow rate are found to be independent process parameters which show specific effects on nozzle performance; therefore they all are optimized individually.
We studied the erosion and feature stability of fused silica patterns under different template cleaning conditions. The conventional sulfuric acid and hydrogen peroxide mixture (SPM) cleaning is compared with an advanced nonacid process. Spectroscopic ellipsometry optical critical dimension measurements were used to characterize the changes in pattern profile with good sensitivity. This study confirmed the erosion of the silica patterns in the traditional acid-based SPM cleaning mixture (H 2 SO 4 +H 2 O 2 ) at a rate of ∼0.1 nm per cleaning cycle. However, the advanced nonacid cleaning process only showed critical dimension shift of ∼0.01 nm per cleaning. Contamination removal and pattern integrity of sensitive 20-nm features under MegaSonic assisted cleaning was also demonstrated.
We studied the erosion and feature stability of fused silica patterns under different template cleaning conditions. The
conventional SPM cleaning is compared with an advanced non-acid process. Spectroscopic ellipsometry optical
critical dimension (SE-OCD) measurements were used to characterize the changes in pattern profile with good
sensitivity. This study confirmed the erosion of the silica patterns in the traditional acid-based SPM cleaning mixture
(H2SO4+H2O2) at a rate of ~0.1nm per cleaning cycle. The advanced non-acid clean process however only showed
CD shift of ~0.01nm per clean. Contamination removal & pattern integrity of sensitive 20nm features under
MegaSonic assisted cleaning is also demonstrated.
Nano-Imprint Lithography (NIL) is considered a promising alternative to optical lithography for technology nodes at
22nm hp and beyond. Compared to other advanced and complex lithography methods, NIL processing is simple and
inexpensive making it a widely accepted technology for pattern media and a potential cost effective alternative for
CMOS applications. During the NIL process, the template comes into direct contact with the resist on the substrate and
consequently template cleanliness plays a decisive role in imprinted substrate quality. Furthermore, if the template has
any form of a defect such as resist residue, stains, particles, surface scratches, chipping and bumping etc. it can lead to
poor quality imprints, low yield and throughput decreases.
The latest ITRS roadmap has stringent CD, CD uniformity, surface roughness and defect control requirements for NIL
templates. Any template cleaning process that is adopted must be able to remove defects while maintaining the critical
parameters outlined by the ITRS. Aggressive chemistries (such as NH4OH or SC1 (NH4OH+H2O2+DI) and strong
physical force treatments (such as MegaSonic & Binary Sprays) may cause damage to the template if not optimized.
This paper presents the cleaning chemical effects on template surface roughness and CD at varying concentrations. The
effect of physical force cleaning on fragile and sensitive pattern features is also presented. Particle & imprint resist
removal efficacy at different process conditions is compared.
Megasonic energy transfer to the photomask surface is indirectly controlled by process parameters that provide an
effective handle to physical force distribution on the photomask surface. A better understanding of the influence of these
parameters on the physical force distribution and their effect on pattern damage of fragile mask features can help
optimize megasonic energy transfer as well as assist in extending this cleaning technology beyond the 22nm node. In this
paper we have specifically studied the effect of higher megasonic frequencies (3 & 4MHz) and media gasification on
pattern damage; the effect of cleaning chemistry, media volume flow rate, process time, and nozzle distance to the mask
surface during the dispense is also discussed. Megasonic energy characterization is performed by measuring the acoustic
energy as well as cavitation created by megasonic energy through sonoluminescence measurements.
Extreme Ultraviolet Lithography (EUVL) is considered the leading lithography technology choice for semiconductor
devices at 16nm HP node and beyond. However, before EUV Lithography can enter into High Volume Manufacturing
(HVM) of advanced semiconductor devices, the ability to guarantee mask integrity at point-of-exposure must be
established. Highly efficient, damage free mask cleaning plays a critical role during the mask manufacturing cycle and
throughout the life of the mask, where the absence of a pellicle to protect the EUV mask increases the risk of
contamination during storage, handling and use. In this paper, we will present effective EUVL mask cleaning
technology solutions for mask manufacturing and in-fab mask maintenance, which employs an intelligent, holistic
approach to maximize Mean Time Between Cleans (MBTC) and extend the useful life span of the reticle. The data
presented will demonstrate the protection of the capping and absorber layers, preservation of pattern integrity as well as
optical and mechanical properties to avoid unpredictable CD-linewidth and overlay shifts.
Experiments were performed on EUV blanks and pattern masks using various process conditions. Conditions showing
high particle removal efficiency (PRE) and minimum surface layer impact were then selected for durability studies.
Surface layer impact was evaluated over multiple cleaning cycles by means of UV reflectivity metrology XPS analysis
and wafer prints. Experimental results were compared to computational models. Mask life time predictions where made
using the same computational models.
The paper will provide a generic overview of the cleaning sequence which yielded best results, but will also provide
recommendations for an efficient in-fab mask maintenance scheme, addressing handling, storage, cleaning and
As the magnetic storage industry roadmap calls for aggressive terabit/in2 densities over the next few years, the shift from
the current planar media to patterned media; grooved surfaces (discrete track media / DTM) and/or individually defined
magnetic dots (bit patterned media / BPM), will be necessary. Both types of patterned media require lithography to
produce the pattern on the disk and the most promising lithography candidate today is nano-imprint lithography (NIL).
During the imprinting process a thin, round, transparent template made of quartz is functioned as a mold to inversely
transfer the features from its surface to the patterning medium on the disks by direct contact. One issue with this
technique is the high probability of defects due to repeated contact of the template with the resist before, during, and
after UV radiation. Defect management through template cleaning, inspection and defect characterization is critical to
preserve integrity of the process.
In this paper, advanced acid-free cleaning combined with MegaSonic treatment for defect elimination is investigated for
effectiveness on discrete track recording (DTR) and BPM patterned templates. For the experiments, templates containing
250KTPI (100nm track pitch) full surface DTR pattern, 450 KTPI (56nm track pitch) with narrow band DTR pattern,
and 250Gdpsi (50nm track pitch) with narrow band BPM pattern are used. The effect of MegaSonic cleaning on the
pattern integrity of fragile features is studied. General characterization of defect attributes is made feasible through a
series of imprinting and template cleaning cycles focused on resist residues and contaminant removal. Imprinted disks
are analyzed using Candela disk inspection and SEM imaging of the pattern. Template cleaning is performed using
HamaTech MaskTrack TeraPure automated template cleaning system.
The fundamentals of droplet-based cleaning of photomasks are investigated and performance regimes that enable the use
of binary spray technologies in advanced mask cleaning are identified. Using phase Doppler anemometry techniques, the
effect of key performance parameters such as liquid and gas flow rates and temperature, nozzle design, and surface
distance on droplet size, velocity, and distributions were studied. The data are correlated to particle removal efficiency
(PRE) and feature damage results obtained on advanced photomasks for 193-nm immersion lithography.
Step-and-flash imprint lithography (S-FIL®) is a promising lithography strategy for semiconductor manufacturing at device nodes below 32 nm. The S-FIL 1:1 pattern transfer technology utilizes a field-by-field ink jet dispense of a low-viscosity liquid resist to fill the relief pattern of the device layer etched into the glass mask. Compared to other sub-40-nm critical dimension (CD) lithography methods, the resulting high resolution, high throughput through clustering, 3-D patterning capability, low process complexity, and low cost of ownership of S-FIL makes it a widely accepted technology for patterned media as well as a promising mainstream option for future CMOS applications. Preservation of mask cleanliness is essential to avoid the risk of repeated printing of defects. The development of mask cleaning processes capable of removing particles adhered to the mask surface without damaging the mask is critical to meet high-volume manufacturing requirements. We present various methods of residual (cross-linked) resist removal and final imprint mask cleaning. Conventional and nonconventional (acid-free) methods of particle removal are compared and the effect of mask cleaning on pattern damage and CD integrity is also studied.
The optical performance stability of a photomask is one of the most critical factors in the photolithography process and
stringent specifications create greater challenges with each advancing technology node. Throughout its lifetime, a
photomask is exposed to a variety of cleaning cycles. It is essential that the integrity of the mask is preserved throughout
each of these processes. Standard mask cleaning treatments include surface preparation with 172nm VUV for better
wetting, organic resist/particle removal with aqueous ozone (DIO3) and residual ion removal for haze control. However,
high energy radiations from 172nm VUV have been reported to cause overlay shift and wet oxidizing chemistries
adversely affect mask CD and optical properties, ultimately influencing lithography performance.
Previously, HamaTech APE successfully demonstrated an advanced cleaning method using photolyzed DIO3 with
minimal metal layer damage. In this paper, performance of different media under the UV photolysis effect is explored
for various steps in the cleaning process. Photolyzed DI water based surface preparation of photomask under
atmospheric conditions without any overlay shift is demonstrated. Alternative chemicals with higher photolysis rates are
explored for resist stripping applications. Phase/Transmission and CD change on a PSM (Phase Shift Mask) are
compared between regular and modified processes. Potential improvements in residual ion removal using combination of
radiation and hot DI water are also presented.
Mask defectivity is an acknowledged road block for the introduction of EUV lithography (EUVL) for manufacturing.
There are significant challenges to extend the conventional methods of cleaning developed for standard 193nm optical
photomask to meet the specific requirements for EUV mask structure and materials. In this work, the use of UV
activated media for EUV mask surface cleaning is evaluated and the effects on Ru capping layer integrity are compared
against conventional cleaning methods. Ru layer surface is analyzed using roughness measurements (AFM) and
reflectivity changes (EUV-R and optical).
In recent years, photomask resist strip and cleaning technology development was substantially driven by the industry's
need to prevent surface haze formation through the elimination of sulfuric acid from these processes. As a result, ozone
water was introduced to the resist strip and cleaning processes as a promising alternative to a Sulfuric - Peroxide
Mixture (SPM). However, with the introduction of 193i double patterning, EUVL (Extreme Ultraviolet Lithography) and
NanoImprint Lithography (NIL) the demand on CD-linewidth control and surface layer integrity is significantly
expanded and the use of ozone water is questionable. Ozone water has been found to cause significant damage to metal
based mask surface layers, leading to significant changes in optical properties and CD-linewidth shift.
In this paper HamaTech APE demonstrates the use of an alternative acid-free resist strip and cleaning process, which not
only overcomes the named drawbacks of conventional ozone water use, but reduces resist strip time by 50% to 75%. The
surface materials investigated during this study are; chrome absorber layers on binary masks, MoSi based shifters,
chrome hard mask layers on EPSM, and ruthenium capping layers on EUV masks. Surface material integrity and CD-stability
results using this new, acid-free approach are presented in the following pages.
Step-and-Flash Imprint Lithography (S-FIL) is a promising lithography strategy for semiconductor manufacturing at device nodes below 32nm. The S-FIL 1:1 pattern transfer technology utilizes a field-by-field ink jet dispense of a low viscosity liquid resist to fill the relief pattern of the device layer etched into the glass mask. Compared to other sub 40nm CD lithography methods, the resulting high resolution, high throughput through clustering, 3D patterning capability, low process complexity, and low cost of ownership (CoO) of S-FIL makes it a widely accepted technology for patterned media as well as a promising mainstream option for future CMOS applications.
Preservation of mask cleanliness is essential to avoid risk of repeated printing of defects. The development of mask cleaning processes capable of removing particles adhered to the mask surface without damaging the mask is critical to meet high volume manufacturing requirements. In this paper we have presented various methods of residual (cross-linked) resist removal and final imprint mask cleaning demonstrated on the HamaTech MaskTrack automated mask cleaning system. Conventional and non-conventional (acid free) methods of particle removal have been compared and the effect of mask cleaning on pattern damage and CD integrity is also studied.
The use of MegaSonic energy is widely accepted in photomask cleaning. For the advanced technology nodes, beyond
65nm, the problem of damaged sub resolution assist features (SRAF) becomes highly prevalent. Such feature damages
are often related to the application of MegaSonic energy.
We investigated the influence of common cleaning media and MegaSonic parameters for damaging SRAF patterns. A
special option of our cleaning tool was utilized to test a large number of different settings with low resources for test
mask and defect inspections. In this paper we will present the results of our investigations and present conditions for
MegaSonic cleaning which will enable the wide use of this technology beyond the 45nm technology node.