Manish Keswani

Effect of Dissolved CO2 in De-Ionized Water in Reducing Wafer Damage During Megasonic Cleaning in MegPie

Particle removal from wafer surfaces can be accomplished by irradiation of cleaning fluid by sound waves in the MHz frequency range. Unfortunately, unless proper cleaning conditions are chosen, megasonic irradiation may also result in damage to fragile wafer features. Here, we demonstrate a strong effect of dissolved CO2 levels on the reduction of wafer damage during megasonic cleaning. Test structures with L/S patterns were irradiated with 0.93 MHz sound waves at varying power densities and dissolved CO2 levels, in a single wafer spin cleaning tool, MegPie®. Dissolution of increasing amounts of CO2 in air saturated DI water caused a significant decrease in the number of breakages to line structures and also decreased the lengths of the line breakages, at all power densities up to 2.94 W/cm2. This ability of dissolved CO2 to protect against feature damage correlates well with its ability to suppress sonoluminescence in sound irradiated DI water.

Effect of sound frequency and initial concentration on the sonochemical degradation of perfluorooctane sulfonate (PFOS)

Perfluoooctanesulfonic acid (PFOS) is a perfluorinated compound (PFC) highly resistant to conventional advance oxidation processes, which was widely used in industrial activities due to its surfactant nature, olephobic-hydrophobic properties, and chemical inertness. Sonochemical treatment has been suggested as an effective approach to treat aqueous solutions containing minimal levels of PFCs. This study investigates PFOS sonochemical degradation and its dependency on the initial concentration (10-460 μM), and the applied sound frequency (25 and 500 kHz, and 1 MHz). PFOS was degraded by sonochemical treatment at concentrations as high as 460 μM, as demonstrated by fluoride release and total organic content data. PFOS degradation rate was higher at megasonic frequencies (1MHz) compared to ultrasonic frequencies (25-500 kHz). PFOS degradation was controlled by saturation kinetics as indicated by an increase in PFOS degradation rate with increasing PFOS concentration until a maximum, after which the degradation rate was independent of the concentration. The saturation conditions were dependent on the sound frequency, and they were reached at a lower concentration under 1 MHz (100 μM) compared to the 500 kHz frequency (>460 μM). Overall, the results of this study demonstrate that high PFOS concentration can be effectively sonochemically treated using megasonic frequencies.

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.

Megasonic Cleaning of Blanket and Patterned Samples in Carbonated Ammonia Solutions for Enhanced Particle Removal and Reduced Feature Damage

An investigation of particle removal efficiency and feature damage has been conducted in NH₄OH/NH₄HCO₃ cleaning solutions irradiated with megasonic energy. By adjusting the pH of the solution in the range of 8.2 - 8.5, high particle removal efficiency (PRE) was achieved while feature damage was reduced significantly. The sonoluminescence data collected from NH₄OH and NH₄OH/NH₄HCO₃ solutions indicate significant suppression of transient cavitation in alkaline solutions containing aqueous CO₂.

Understanding acoustic cavitation for sonolytic degradation of p-cresol as a model contaminant

Many modern techniques exist for the degradation of organic pollutants in water. Numerous treatment processes which utilize the formation of hydroxyl radicals for oxidation of pollutants have been studied thoroughly. In this study, a three pronged approach has been used to characterize and understand the effect of two distinct acoustic frequencies (37 kHz and 1 MHz) on cavitation behavior. Correlation of this behavior with sonolysis of a target phenol pollutant is described. Hydroxyl radical capture, hydrophone, and microelectrode studies in this work show that megasonic frequencies are more effective for generation of hydroxyl radicals and stable cavitation events than ultrasonic frequencies. UV absorption and fluorescence measurements confirm that the combination of ultrasonic sonolysis with a Fenton reagent achieved complete degradation of p-cresol at 50 mg/L in about 30 min. Cost estimates have been made for different sonication processes and compared with traditional advanced oxidation processes.

Effect of chemical structure on the sonochemical degradation of perfluoroalkyl and polyfluoroalkyl substances (PFASs)

Perfluoroalkyl surfactants include chemicals characterized by a fully fluorinated carbon chain (hydrophobic and oleophobic tail) bound to a hydrophilic head (a carboxyl or sulfonic group). These compounds are toxic and highly resistant to chemical/biological attack, and some are known to be bio-accumulative. This study investigates the sonochemical degradation at 500 kHz of different carboxylic and sulfonic perfluoroalkyl and polyfluoroalkyl substances (PFASs, 1.7 mM total organic fluorine) to assess the effect of chain length, functional head group, and substituents (–CH2–CH2– moiety and ether group) on the degradation rate. Under these conditions, the rates of defluorination determined for two widely used perfluoroalkyl substances, perfluorooctanesulfonate (PFOS) and perfluorooctanoic acid (PFOA), were 3.5 to 3.7 μM F− min−1, respectively. The degradation rate of perfluoroalkyl sulfonates decreased with the perfluorocarbon chain length as indicated by the 1.3 and 1.9-fold lower defluorination rates for perfluorohexane- and perfluorobutane sulfonate than that of PFOS. A similar trend was observed during the sonolysis of perfluoroalkyl carboxylate analogs with 6, 5 or 3 carbon atoms which had 1.1-, 1.8-, and 2.3-fold lower defluorination rates, respectively, than that of PFOA. Furthermore, perfluoroalkyl compounds appeared more amenable to sonolysis than the polyfluoroalkyl analogues with the same number of C atoms (defluorination rate of PFOS/6:2 fluorotelomer sulfonate ≈ 2.3). The results demonstrate that sonolysis is a promising approach to treat PFASs in aqueous streams. Furthermore, they underscore that the chemical structure of PFASs has a marked effect on the rate at which they undergo sonochemical degradation.

Megasonic metrology for enhanced process development

Acoustic cavitation is known to be a primary source of both cleaning and damage of wafers during their megasonic processing. Understanding the response of process fluids to variables like acoustic power recipe and dissolved gases is an important first step in achieving damage-free megasonic cleaning of wafers. This paper reports the development of a portable, UV light tight, cavitation threshold (CT) cell to measure sonoluminescence (SL) signal arising from cavitation. The closed cell, integrated with a gas sensor and contactor, allows SL measurements under very controlled conditions. Using the CT cell the effect of the concentration of dissolved O2, CO2 and air on SL signal has been investigated. Results show that SL varies linearly with dissolved O2 concentration while CO2 is found to be incapable of supporting SL. This study also demonstrates a novel method for precise control of SL through addition of an O2 scavenger with fast O2 removal kinetics.

Contactless bottom-up electrodeposition of nickel for 3D integrated circuits

Packaging applications in the semiconductor industry rely on electrodepositing metals into high aspect ratio (HAR) vias without the formation of any defects or voids. The process and economic efficiency of conventional methodologies are limited by the ability to achieve high deposition rates along with uniformity of the deposited metal layer. In this work, a contactless and scalable electrodeposition technique has been developed to deposit metallic nickel onto p-doped silicon wafers. The effect of various process variables such as deposition and etchant solution composition and concentration, solution temperature and stirring on nickel deposition rates have been investigated. The importance of backside silicon oxidation and subsequent oxide etching on the kinetics of nickel deposition on frontside silicon has been highlighted.

Sonochemical degradation of perfluorinated chemicals in aqueous film-forming foams

Aqueous film-forming foams (AFFFs) are complex mixtures containing 1-5% w/w fluorocarbons (FCs). Here, we have investigated degradation of two commercial AFFF formulations, 3M and Ansul, using sound field at 500kHz and 1MHz, with varying initial concentrations ranging from 200 to 930× dilution. The foams were readily degraded by 1MHz, with percentage of defluorination ranging from 11.1±1.4% (200× dilution of 3M) to 47.1±5.8% (500× dilution of Ansul). Removal of total organic carbon (TOC) ranged from 16.0±1.4% (200× dilution Ansul) to 39.0±7.2% (500× dilution Ansul). Degradation of AFFF was affected by sound frequency with rates of defluorination 10-fold greater when the frequency was 1MHz than when it was 500kHz. Mineralization of TOC was 1.5- to 3.0-fold greater under 1MHz than 500kHz. Rate of fluoride release was 60% greater for the greatest initial concentration of FC in Ansul compared to the least initial concentration. While the rate of mineralization of AFFF was directly proportional to the initial concentration of Ansul, that was not the case for 3M, where the rates of mineralization were approximately the same for all three initial concentrations. Results of the study demonstrate that sonolysis is a promising technology to effectively treat AFFFs.

Synthesis of porous silicon through interfacial reactions and measurement of its electrochemical response using cyclic voltammetry

Porous silicon, an excellent material with fascinating physical and chemical properties, is usually formed by anodic polarization of single crystalline silicon in HF based solutions. Here, we show fabrication of porous silicon films ∼0.5–250 μm thick consisting of macropores and mesopores using a contactless electrochemical approach, where the silicon substrate is not under any external bias. Pore dimensions and porosity have been characterized by scanning electron microscopy (SEM) while subsequent cyclic voltammetry (CV) investigations delineate the underlying topographical differences between blanket and porous silicon surfaces. Our work not only offers a new scalable means of fabricating porous silicon structures but also questions the reliability of existing theories that depend on localized collection of electronic hole carriers through anodization of silicon for pore formation. We believe our results will open pathways for development of realistic models for porous silicon formation.