Arun John Kadaksham

Current status of EUV mask blanks and LTEM substrates defectivity and cleaning of blanks exposed in EUV ADT

The defectivity of EUV mask blanks remains as one of the key challenges in EUV lithography. Mask blank defects are a combination of defects or particles added on the substrate, added during MoSi multilayer deposition, and during subsequent handling. A recent upgrade to the Lasertec M7360 at SEMATECH has enabled us to detect new defects (sub-30 nm SEVD (Sphere Equivalent Volume Diameter)) on the substrate that were not previously detectable. In this paper, we report our recent investigation of defects on low thermal expansion material (LTEM) substrates and their creation and removal. Data obtained with atomic force microscope (AFM) imaging of defect topography, scanning electron microscope/energy-dispersive spectroscopy (SEM/EDS), and Auger characterization of defect composition is also discussed. Cleaning of mask particles which may have been added by handling in a clean room environment with the ASML Alpha Demo Tool (ADT) with and without static EUV exposure is discussed. Particle contamination on the backside of EUV masks can potentially impact overlay or focus during exposure. We have developed cleaning processes capable of removing backside defects without contaminating the front side of the masks. Backside defects are characterized by AFM, SEM/EDS, and auger microscopy and their topography and composition are presented.

Particle removal challenges of EUV patterned masks for the sub-22nm HP node

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.

Ultra-low roughness magneto-rheological finishing for EUV mask substrates

EUV mask substrates, made of titania-doped fused silica, ideally require sub-Angstrom surface roughness, sub-30 nm flatness, and no bumps/pits larger than 1 nm in height/depth. To achieve the above specifications, substrates must undergo iterative global and local polishing processes. Magnetorheological finishing (MRF) is a local polishing technique which can accurately and deterministically correct substrate figure, but typically results in a higher surface roughness than the current requirements for EUV substrates. We describe a new super-fine MRF® polishing fluid whichis able to meet both flatness and roughness specifications for EUV mask blanks. This eases the burden on the subsequent global polishing process by decreasing the polishing time, and hence the defectivity and extent of figure distortion.

Production of EUV mask blanks with low killer defects

For full commercialization, extreme ultraviolet lithography (EUVL) technology requires the availability of EUV mask blanks that are free of defects. This remains one of the main impediments to the implementation of EUV at the 22 nm node and beyond. Consensus is building that a few small defects can be mitigated during mask patterning, but defects over 100 nm (SiO2 equivalent) in size are considered potential “killer” defects or defects large enough that the mask blank would not be usable. The current defect performance of the ion beam sputter deposition (IBD) tool will be discussed and the progress achieved to date in the reduction of large size defects will be summarized, including a description of the main sources of defects and their composition.

Investigation of EUVL reticle capping layer peeling under wet cleaning

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 [1]. 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 proposed.

Dressed-photon nanopolishing for extreme ultraviolet mask substrate defect mitigation

Although the quality of extreme ultraviolet (EUV) mask substrates has improved by continuous refinement of the polishing processes, the yield of defect-free blanks is still very low. Dressed-photon nanopolishing (DPNP) is a novel vapor phase, photo-chemical, non-contact etching process that has been shown to locally smooth bumps and pits to below 1 nm in height/depth while not affecting the surface roughness. DPNP is based on the concept of a dressed photon, which is a quasi-particle in the optical near field of a surface that can couple with lattice phonons in nanometric regions (< 100 nm). When illuminated with light of a suitable wavelength, such coupled states are generated on a nanometrically rough material surface and impart sufficient energy to an etchant gas to enable its dissociation and etching in the rough regions only. DPNP can be the last polishing step on EUV substrates to eliminate any remnant pits and/or embedded particles on the surface to yield potentially defect-free substrates.

Recent advances in SEMATECH's mask blank development program, the remaining technical challenges, and future outlook

The ability of optical lithography to steadily produce images at increasingly smaller dimension while maintaining pattern fidelity of devices with greater complexity has enabled the success of Moore’s Law. Although 193 nm immersion and double patterning techniques have proven successful in extending optical lithography, the strategies proposed for further extension are too costly to support device manufacturing. As a result, greater focus has been shifted to resolving the challenges hindering extreme ultraviolet lithography (EUVL) adoption as the mainstream lithography solution. While similar to conventional optical lithography, there are unique challenges to EUVL, one of which is the change from transmission masks to the reflective masks required for EUVL. The use of reflective reticles greatly increases complexity of EUV reticle structure when compared to the binary masks used with optical lithography. Maximizing the reflectance an EUV mask requires the use of a multilayer Bragg reflector deposited on a finely polished substrate with a thin absorber film on top used to define the device pattern. Although similar in form to the substrates used in optical lithography, the tolerances on figure, surface finish, and defects are significantly more stringent for EUV substrates. Control of aberrations and maintaining pattern fidelity places tight constraints on the flatness and roughness of the EUV substrate; imperfections and particles can result in printable defects. The Bragg reflector of the EUV mask consists of 40 to 50 Si/Mo bi-layers deposited using an ion beam deposition tool. This film stack must be deposited to meet the reflectivity and uniformity requirements of the exposure tool and must be completely free of defects. The absorber film is typically a tantalum-based nitride layer selected for its ability to absorb EUV radiation and maintain thermal stability. The thickness and morphology of this film must be tightly controlled to enable use as the patterning film for the device. In addition to the increase in complexity of the mask, introduction of EUVL requires infrastructure development of new substrate, mask blank, and finished reticle inspection tools and techniques for handling and storage of a mask without a pellicle. This paper will highlight recent advances in the ability to produce pilot line quality EUV mask blanks to meet the near-term requirements and review the existing technology gaps which must be closed to extend the current capability to meet HVM needs. A special focus will be put on substrate and mask blank defect densities; other process and infrastructure challenges will also be discussed.

New requirements for the cleaning of EUV mask blanks

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 defects.

Low thermal expansion material (LTEM) cleaning and optimization for extreme ultraviolet (EUV) blank deposition

With the insertion of extreme ultraviolet lithography (EUVL) for high volume manufacturing (HVM) expected in the next few years, it is necessary to examine the performance of low thermal expansion materials (LTEMs) and assess industry readiness of EUV substrates. Owing to the high cost of LTEM, most of the development work so far has been done on fused silica substrates. Especially in developing cleaning technology prior to multilayer deposition, fused silica substrates have been used extensively, and defect trends and champion blank data have been reported using multilayer deposition data on fused silica substrates. In this paper, the response of LTEMs to cleaning processes prior to multilayer deposition is discussed. Cleaning processes discussed in this paper are developed using fused silica substrates and applied on LTEM substrates. The defectivity and properties of LTEM to fused silica are compared. Using the dense scan feature of the substrate inspection tool capable of detecting defects down to 35 nm SiO2 equivalent size and appropriate defect decoration techniques to decorate small defects on substrates to make them detectable, cleaning technologies that have the potential to meet high demands on LTEM for EUVL are developed and optimized.

Preparation of substrates for EUV blanks using an etch clean process to meet HVM challenges

Achieving mask blanks with defectivity less than 0.03 defects/cm2 at 30 nm SiO2 equivalent and above is one of the key goals for accomplishing high volume manufacturing capability for EUV lithography. Defect free blanks for lithography start from defect free substrates. Currently, defects on both LTEM and quartz substrates are dominated by pits, scratches, particles and residues left by the polishing processes used to achieve the roughness and flatness specifications of the substrates. Normally, such defects are extremely difficult to be removed and particles often leave pits as they are removed by cleaning. Standard cleaning processes relying on megasonic cavitations for particle removal are insufficient for removing such defects from substrates. It is well known that hydrofluoric acid is an etchant of fused silica (quartz) and buffered HF in different concentrations has been used in the past for cleaning quartz and silicon substrates. Ideally, an etch clean process should not increase the roughness of the substrate while cleaning. However, in the process of etching and removing the defects, the roughness of the substrates is invariably increased which is undesirable. The rate of roughness change is directly dependent on the concentration and time of exposure, which also affects the etch rate and defect removal rate. In this paper we report that a post polishing etch clean process has been developed for ULE and quartz substrates which meet the defectivity, roughness and flatness specifications for EUV blanks. We also examine the effects of substrate roughness on blank roughness, and inspection capability of substrates and blanks at different roughness levels using a defect inspection tool capable of inspecting defects down to 35 nm SiO2 equivalent size. Defect smoothing using etch clean processes have been proposed and demonstrated in the past using an anisotropic etch mechanism. This study focuses on complete removal of defects from EUV substrates, and therefore smoothing is not an issue. Multilayer blank deposition process is known to decorate defects on substrates. We use this as a technique to identify any defects that might be left on the substrate surface after etch cleaning. In most cases, we find that the substrates have low defectivity and do not affect the EUV requirements. We demonstrate that the etch clean process can be used to increase the yield of high quality ULE substrates to meet the high volume production requirements of euv masks.