Pat Marmillion

A method to determine the origin of remaining particles after mask blank 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.

Cleaning of MoSi multilayer mask blanks for EUVL

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 SEMATECHs 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 SEMATECHs 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.

Dependency of EUV mask defects on substrate defects

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 substrates 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. SEMATECHs 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.

Advanced photomask cleaning

Mask cleaning has been a significant challenge. Advanced PhotoMasks have proven to be even more difficult. The experimental work on 157nm systems uncovered an issue of particle growth under the pellicle. Since the mask blank had a different composition from existing production mask blanks, there was not a concern about current production impact. Investigations were started after a few incidents occurred on 193nm masks. The investigations demonstrated that the masks have a consistent family of contaminants that are on all chrome absorber masks. The initial work provided clues to the nature of the particle growth and some indication of the potential sources. The issues seemed to evolve from the total system and not a single contaminant source. Currently, hard defects due to particle growth under the pellicle occur industry wide. This paper will provide the methodology employed for a recent cleaning evaluation and identify some of the culprits that cause particle growth. The issue has grown to a major problem and needs to be quickly addressed.

A study of damage mechanisms during EUV mask substrate cleaning

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.

Phase defect detection with spatial heterodyne interferometry

Phase shift techniques introduced in photolithography to further improve resolution produce a new set of challenges for inspection. Unlike the high contrast provided by patterned and unpatterned areas on a binary mask, phase errors do not provide significant contrast changes, since the phase change is imparted by a difference in material thickness. Surface topology measurements can be used to identify phase defects, but methods for surface topology inspection are typically slow or can damage the surface to be measured. In this study, Spatial Heterodyne Interferometry (SHI) has been considered as a possible method for high-speed non-contact phase defect detection. SHI is an imaging technique developed at Oak Ridge National Laboratory that acquires both phase and amplitude information from an optical wavefront with a single high-speed image capture. Using a reflective SHI system, testing has been performed with a mask containing programmed phase defects of various sizes and depths. In this paper, we present an overview of the SHI measurement technique, discuss issues such as phase wrapping associated with using SHI for phase defect detection on photolithographic masks, and present phase defect detection results from die-to-die comparisons on a 248 nm alternating aperture phase shift mask with intentional phase defects.

Hydrogenated water application for particle removal on EUV mask blank substrates

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.

Cleaning of low thermal expansion material (LTEM) substrates for mask blanks in EUV lithography

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.

A benchmark investigation on cleaning photomasks using wafer cleaning technologies

As new technologies are developed for smaller linewidths, the specifications for mask cleanliness become much stricter. Not only must the particle removal efficiency increase, but the largest allowable particle size decreases. Specifications for film thickness and surface roughness are becoming tighter and consequently the integrity of these films must be maintained in order to preserve the functionality of the masks. Residual contamination remaining on the surface of the mask after cleaning processes can lead to subpellicle defect growth once the mask is exposed in a stepper environment. Only during the last several years, has an increased focus been put on improving mask cleaning. Over the years, considerably more effort has been put into developing advanced wafer cleaning technologies. However, because of the small market involved with mask cleaning, wafer cleaning equipment vendors have been reluctant to invest time and effort into developing cleaning processes and adapting their toolset to accommodate masks. With the advent of 300 mm processing, wafer cleaning tools are now more easily adapted to processing masks. These wafer cleaning technologies may offer a solution to the difficulties of mask cleaning and need to be investigated to determine whether or not they warrant continued investigation. This paper focuses on benchmarking advanced wafer cleaning technologies applied to mask cleaning. Ozonated water, hydrogenated water, super critical fluids, and cryogenic cleaning have been investigated with regards to stripping resist and cleaning particles from masks. Results that include film thickness changes, surface contamination, and particle removal efficiency will be discussed.