Florence Eschbach

Megasonic cleaning, cavitation, and substrate damage: an atomistic approach

Megasonic cleaning has been a traditional approach for the cleaning of photomasks. Its feasibility as a damage free approach to sub 50 nm particulate removal is under investigation for the cleaning of optical and EUV photomasks. Two major mechanisms are active in a megasonic system, namely, acoustic streaming and acoustic cavitation. Acoustic streaming is instrumental in contaminant removal via application of drag force and rolling of particles, while cavitation may dislodge particles by the release of large energy during cavity implosion or by acting as a secondary source of microstreaming. Often times, the structures (substrates with or without patterns) subjected to megasonic cleaning show evidence of damage. This is one of the impediments in the implementation of megasonic technology for 45 nm and future technology nodes. Prior work suggests that acoustic streaming does not lead to sufficiently strong forces to cause damage to the substrates or patterns. However, current knowledge of the effects of cavitation on cleaning and damage can be described, at best, as speculative. Recent experiments suggest existence of a cavity size and energy distributions in megasonic systems that may be responsible for cleaning and damage. In the current work, we develop a two-dimensional atomistic model to study such multibubble cavitation phenomena. The model consists of a Lennard-Jones liquid which is subjected to sinusoidal pressure changes leading to the formation of cavitation bubbles. The current work reports on the effects of pressure amplitude (megasonic power) and frequency on cavity size distributions in vaporous and gaseous cavitation. The findings of the work highlight the role of multibubble cavitation as cleaning and damage mechanism in megasonic cleaning.

Effects of soft pellicle frame curvature and mounting process on pellicle-induced distortions in advanced photomasks

Lithography registration errors induced by the attachment of soft pellicles on reticles can significantly affect wafer overlay performance for sub-90 nm lithography chip manufacturing. Intel Corporation, Mitsui Chemicals, and the University of Wisconsin Computational Mechanics Center (UW-CMC) have conducted an extensive experimental study to quantify and minimize the pellicle-induced distortions in order to meet advanced mask manufacturing requirements. A comprehensive design of experiment was elaborated to evaluate the effects of frame curvature, adhesive gasket compliance, and mounting load on pellicle-induced distortions for soft pellicle systems. A frame curvature measurement tool was custom-made at the UW-CMC, employing an MTI Instruments capacitive sensor. A TA Instruments dynamic mechanical analyzer was used to determine the elastic modulus of the adhesive gasket materials. Registration measurements were conducted by Intel on test reticles on a 21 × 21 array of grid points, before and after pellicle attachment, to obtain pellicle-induced distortion results. Results characterize the influence of attachment process, type of adhesive gasket, frame curvature, reticle guiding plate configuration, and attachment load on pellicle-induced distortions.

Improving photomask surface properties through a combination of dry and wet cleaning steps

No Abstract Available.

Experimental and simulation investigations of acoustic cavitation in megasonic cleaning

Extreme ultra-violet (EUV) lithography has become the technique of choice to print the ever-shrinking nanoscale features on the silicon wafer. For successful transfer of patterns on to the wafer, the EUV photomask cannot contain defects greater than 30 nm. Megasonic cleaning is a very successful cleaning technique for removal of particles on photomasks, but also causes a relatively high amount of damage to the fragile EUV photomasks thin film structures. Though it is believed that acoustic cavitation is the primary phenomenon responsible for cleaning as well as pattern damage, a fundamental picture of the acoustic cavitation mechanisms in play during megasonic cleaning has not yet clearly emerged. In this study, we characterize the role of acoustic cavitation in megasonic cleaning by examining the effects of acoustic power densities, cleaning solution properties, and dissolved gas content on cavitation via experiments and molecular dynamics (MD) simulations. MD is an atomistic computation technique capable of modeling atomic-level and nanoscale processes accurately making it well suited to study the effect of cavitation on nano-sized particles and patterns.

Acoustic streaming effects in megasonic cleaning of EUV photomasks : A continuum model

Removal of nano-scale contaminant particles from the photomasks is of critical importance to the implementation of EUV lithography for 32nm node. Megasonic cleaning has traditionally been used for photomask cleaning and extensions to sub 50nm particulates removal is being considered as a pattern damage free cleaning approach. Several mechanisms for removal are believed to be active in megasonic cleaning systems, e.g., cavitation, and acoustic streaming (Eckart, Schlichting, and microstreaming). It is often difficult to separate the effects of these individual mechanisms on contamination removal in a conventional experimental setup. Therefore, a theoretical approach is undertaken in this work with a focus on determining the contribution of acoustic streaming in cleaning process. A continuum model is used to describe the interaction between megasonic waves and a substrate (fused silica) immersed in a fluid (water). The model accounts for the viscous nature of the fluid. We calculate the acoustic vibrational modes of the system. These in turn are used to determine the acoustic streaming forces that lead to Schlichting streaming in a narrow acoustic boundary layer at the substrate/fluid interface. These forces are subsequently used to estimate the streaming velocities that may in turn apply a pressure and drag force on the contaminant particles adhering to the substrate. These effects are calculated as a function of angle of incidence, frequency and intensity of the megasonic wave. The relevance of this study is then discussed in the context of the cleaning efficiency and pattern damage in competing megasonic cleaning technologies, such as immersion, and nozzle-based systems.

ArF lithography reticle crystal growth contributing factors

The formation of photoinduced crystals and haze has become a challenge for 193nm photolithography high volume manufacturing (1-6). Extensive work has been performed to develop alternative to piranha chemistry for photomask cleaning processes in an attempt to eliminate the incidence of clean induced ammonium sulfate crystal formation (9-13). However, additional factors are impacting 193nm reticle optical quality. Sources of molecular contaminants such as environmental factors, outgasing from pellicle and reticle storage material can generate varieties of photoinduced crystals over the reticle useable lifetime (5-6). This paper will quantify and rank contributing factors for crystals generated under high energy UV exposure. A broad range of analytical and metrology techniques (FTIR, IC, TD-GC/MS, Inorganics impinger, AIMSTM, KLA Starlight, UV 172nm) and improvements in technique sensitivity were developed in order to identify crystal structure, quantify photogenerated contaminants levels and assess wafer printability impact. Engineering systems aimed at minimizing organic and inorganic molecular contaminants levels will be suggested.

Numerical and experimental studies of pellicle-induced photomask distortions

Meeting the stringent error budget of 157-nm lithography for manufacturing devices in the sub-100 nm regime requires that all mask-related distortions be minimized, corrected, or eliminated. Sources include the pellicle system, which has been previously identified as a potential cause of image placement error. To characterize pellicle-induced distortions, finite element (FE) models have been developed to simulate system fabrication, including soft pellicles as well as prototype fused silica (hard) pellicles. The main sources of distortions are: (a) temperature variations, (b) initially distorted components, and (c) sag-induced refraction. Temperature variations are an issue if pellicle mounting and exposure take place at different temperatures. Sources of attachment-induced distortions include the initial frame curvature, initial reticle shape, attachment method (mounting tools-induced), frame and gasket materials, and the hard pellicle bow. These attachment-induced distortions were modeled using experimentally measured values of Youngs modulus for adhesive gaskets. Refraction aberration is an issue with bowed hard pellicles which act as optical elements and induce image degradation. These effects were assessed and found to be significant. Results from the experiments and comprehensive FE simulations have characterized the relative importance of the principal sources of pellicle-induced photomask distortions for 157-nm lithography.

Experimental and numerical studies of the effects of materials and attachment conditions on pellicle induced distortions in advanced photomasks

Lithography registration errors induced by the attachment of soft pellicles on reticles can significantly affect wafer overlay performance for sub-100 nm lithography chip manufacturing. Intel Corporation and the University of Wisconsin have conducted an extensive study to identify the various sources of pellicle-induced distortions and methods for error reduction in order to meet advanced mask manufacturing requirements. In this study, pellicle attachment processes and system materials were evaluated to determine the effects on image placement accuracy. In particular, the in-plane distortions due to the pellicle attachment technique, pellicle frame flatness, frame adhesive, and environmental temperature were characterized. At Intel, pellicles were attached to a test reticle with a 21 X 21 array of grid points. Registration measurements were conducted before and after pellicle attachment using an optical distance metrology system. A comprehensive finite element model was developed at the University of Wisconsin to assess the contributions to pellicle-induced distortions from individual components of the pellicle system. Pellicle frame flatness, frame adhesive, and temperatures were measured and used as input to the finite element model. The correlation between simulation results and experimental data was excellent. Analyses were also performed to study pellicle mounting mechanisms and pellicle frame flatness.

Development of polymer membranes for 157-nm lithography

Fluoropolymers were/are successfully used for pellicle manufacturing in 248 and 193 nm lithography. However, all known fluoropolymers rapidly degrade when exposed to high-energy 157 nm irradiation. Lack of suitable polymer “soft” pellicle has become one of the major obstacles for implementing 157 nm lithography. The goal of this research was to investigate the photodegradation mechanisms in fluoropolymers under 157 nm irradiation using various analytical techniques, and establish correlation between polymer structure and transparency/durability. Various polymer platforms, developed by Asahi Glass Corporation, as well as model polymer based on industrially available materials, have been employed in this study. Polymer structures have been analyzed using solution NMR, FTIR, Raman spectroscopy, TOF-SIMS, nanoindentation, outgassing, contact angle, ellipsometry, refractometry, n and k measurements. Transparency and durability of polymer membranes under 157 nm irradiation were established using an F2 157 nm laser as a source of irradiation, and an environmentally controlled chamber. As the result of this study, photodegradation mechanism for some of the tested polymers was tentatively suggested as cleavage of carbonyl, CO, and/or CFO bonds. Additionally, the following general conclusions have been made: environmental moisture, gas environment, and polymer/adhesive solvents affect structure and durability of the exposed polymers; “skin” surface layer can be formed on the surface of the irradiated polymer; polymer membranes are thinning under 157 nm irradiation; polar groups are formed on the irradiated surface. Effects of gas environment, exposure conditions, technology of the sample preparation on the photodegradation mechanism and kinetics were studied. Possible photodegradation pathways have been derived and assessed. Dependence of polymer durability and transparency on such structural features as number of carbon atoms within the ring, oxygen content, type and number of substituents in the Oxygen containing perfluorinated rings, number and location of carbon-oxygen bonds, structure symmetry, relative ratio of cyclic and linear chains, content and type of the hydrogen bonds, were analyzed. Semi-empirical rules to optimize transparency, durability, and mechanical properties of polymer membranes for 157nm exposure, will be discussed.

Pellicle-induced distortions in advanced photomasks

A comprehensive design of experiment was elaborated to evaluate the effects of frame flatness, mask adhesive compliance, and mounting load on pellicle-induced distortions for soft pellicle systems. A dynamic mechanical analyzer was used to determine the elastic modulus of the adhesives materials, and a capacitive sensor-based tool was employed to measure the pellicle frame bow prior to mounting. Registration measurements were conducted on test reticles on a 21 X 21 array of grid points, before and after pellicle attachment. Statistical analysis (Anova test) was conducted to determine if the means for each sample group were statistically discernable. Overall, the magnitude of the distortions was very low for the pellicle mounting mechanism selected. Nevertheless the results indicated that the sample group with the flexible (softer) mask adhesive material exhibited lower distortions than that with conventional (stiffer) mask adhesive. Either larger sample size and/or wider variations in initial frame bow and mounting pressure will be required to assess the impact of these parameters on pellicle-induced distortions. Flexible (softer) mask adhesives are believed to minimize photomask deformation during the mounting process, resulting in lower pellicle-induced distortions.