Krasnow, Nicholas Riordan (The University of Arizona., 2016)
Environmental microorganisms are implicated as the causative agents in a significant portion of healthcare associated infections (HAI) and antimicrobial resistant infections (AMR), which result in increased costs and suffering around the world. Furthermore, common environmental microorganisms participate in microbiological degradation of materials and the bio-fouling of various systems. This also results in a tremendous amount of damage in many different materials and many different sectors. The focus of this dissertation was the development of an additive that could be easily added to common materials to make them self-disinfecting and to protect them from microbial damage. The ultimate goal was to develop an additive that could be added using standard techniques and without adversely affecting the final material. Cuprous iodide (CuI) was determined to be an ideal starting material for the development of improved antimicrobial materials because of its neutral appearance and high antimicrobial activity as compared to other silver and copper materials. It was found that the antimicrobial efficacy of CuI could be amplified if prepared as a small particle and especially in the presence of vinylpyrrolidone polymers. A comminution process was then developed to produce these small particles. By using select copolymers, various CuI small particles formulation were developed to be compatible with a variety of different matrices. The efficacy of these CuI containing matrices was dependent on the compatibility of the CuI formulation with the matrix. A variety of applications were demonstrated with good antimicrobial efficacy where the particles were easily added to the finished material with minimal or no change in appearance.
Padmanabhan Ramalekshmi Thanu, Dinesh (The University of Arizona., 2011)
Fabrication of current generation integrated circuits involves the creation of multilevel copper/low-k dielectric structures during the back end of line processing. This is done by plasma etching of low-k dielectric layers to form vias and trenches, and this process typically leaves behind polymer-like post etch residues (PER) containing copper oxides, copper fluorides and fluoro carbons, on underlying copper and sidewalls of low-k dielectrics. Effective removal of PER is crucial for achieving good adhesion and low contact resistance in the interconnect structure, and this is accomplished using wet cleaning and rinsing steps. Currently, the removal of PER is carried out using semi-aqueous fluoride based formulations. To reduce the environmental burden and meet the semiconductor industry's environmental health and safety requirements, there is a desire to completely eliminate solvents in the cleaning formulations and explore the use of organic solvent-free formulations.The main objective of this work is to investigate the selective removal of PER over copper and low-k (Coral and Black Diamond®) dielectrics using all-aqueous dilute HF (DHF) solutions and choline chloride (CC) - urea (U) based deep eutectic solvent (DES) system. Initial investigations were performed on plasma oxidized copper films. Copper oxide and copper fluoride based PER films representative of etch products were prepared by ashing g-line and deep UV photoresist films coated on copper in CF4/O2 plasma. PER removal process was characterized using scanning electron microscopy and X-ray photoelectron spectroscopy and verified using electrochemical impedance spectroscopy measurements.A PER removal rate of ~60 Å/min was obtained using a 0.2 vol% HF (pH 2.8). Deaeration of DHF solutions improved the selectivity of PER over Cu mainly due to reduced Cu removal rate. A PER/Cu selectivity of ~20:1 was observed in a 0.05 vol% deaerated HF (pH 3). DES systems containing 2:1 U/CC removed PER at a rate of ~10 and ~20 Å/min at 40 and 70oC respectively. A mixture of 10-90 vol% de-ionized water (W) with 2:1 U/CC in the temperature range of 20 to 40oC also effectively removed PER. Importantly, etch rate of copper and low-k dielectric in DES formulations were lower than that in conventional DHF cleaning solutions.
Siddiqui, Shariq (The University of Arizona., 2011)
Ammonia-peroxide mixture (APM) is a widely used wet chemical system for particle removal from silicon surfaces. The conventional APM solution in a volume ratio of 1:1:5 (NH4OH:H2O2:H2O) is employed at elevated temperatures of 70-80 °C. At these temperatures, APM solution etches silicon at a rate of ~3 Å/min, which is unacceptable for current technology node. Additionally, APM solutions are unstable due to the decomposition of hydrogen peroxide and evaporative loss of ammonium hydroxide resulting in the change in APM solution composition. This has generated interest in the use of dilute APM solutions. However, dilution ratios are chosen without any established fundamental relationship between particle-wafer interactions and APM solutions.Atomic force microscopy has been used to measure interaction forces between H-terminated Si surface and Si tip in APM solutions of different compositions. The approach force curves results show attractive forces in DI-water, NH4OH:H2O (1:100) and H2O2:H2O (1:100) solutions at separation distances of less than 10 nm for all immersion times (2, 10 and 60 min) investigated. In the case of dilute APM solutions, the forces are purely repulsive within 2 min of immersion time. During retraction, the adhesion force between Si surface and Si tip was in the range of 0.8 nN to 10.0 nN. In dilute APM solutions, no adhesion force is measured between Si surfaces and repulsive forces dominated at all distances. These results show that even in very dilute APM solutions, repulsive forces exist between Si surface and particle re-deposition can be prevented.The stability of APM solutions has been investigated as a function of temperature (24 - 65 °C), dilution ratio (1:1:5 - 1:2:100), solution pH (8.0 - 9.7) and Fe2+ concentration (0 - 10 ppb) using an optical concentration monitor. The results show that the rate of H2O2 decomposition increased with an increase in temperature, solution pH and Fe2+ concentration. The kinetic analysis showed that the H2O2 decomposition follows a first order kinetics with respect to both H2O2 and OH- concentrations. In the presence of Fe2+, hydrogen peroxide decomposition follows a first order reaction kinetics with respect to H2O2 concentration.
Balachandran, Rajesh (The University of Arizona., 2015)
Over the years, megasonic energy has been widely used in the semiconductor industry for effective particle removal from surfaces after chemical mechanical planarization (CMP) processes. As a sound wave propagates through a liquid medium, it generates two effects, namely, acoustic streaming and acoustic cavitation. Acoustic streaming refers to time independent motion of liquid due to viscous attenuation, while cavitation arises from the bubble activity generated due to the difference in the pressure field of the propagating wave. Cavitation can be classified into two categories, (1) stable and (2) transient cavitation. When a bubble undergoes continuous oscillations over repeated cycles it is known to exhibit stable cavitation, while a sudden collapse is referred to as transient cavitation. Due to the rapid implosion of the transient cavity, drastic conditions of temperature (5,000-10,000 K) and pressure (hundreds of bars) are generated within and surrounding the bubble. If this phenomenon occurs close to the substrate, it causes damage to the sub-micron features. In this study, emphasis has been laid on understanding acoustic cavitation as it is critical to achieving high cleaning efficiency without any feature damage. The research work described in this dissertation has been divided into three sections. In the first part of the dissertation, the development of a novel sono-electrochemical technique for removal of sub-micron (300 nm) silica particles from conductive surfaces (Ta) has been discussed. The technique employs megasonic field at low pulse time and duty cycle in conjunction with an applied electrical field for achieving superior particle removal efficiency (PRE). In order to demonstrate the effectiveness of the sono-electrochemical technique, cleaning studies were conducted using 300 nm silica particles both in the presence and absence of an applied electrical field in air and argon saturated solutions. In the presence of the megasonic field (0.5 W/cm², 10% duty cycle, 5ms pulse time) alone, about 55% PRE was observed in Ar saturated DI water, while in the presence of the sono-electrochemical field (-1.5V vs Ag/AgCl (sat. KCl)), about 80% PRE was measured. The enhancement in particle removal efficiency was attributed to oscillating hydrogen bubbles formed from water reduction in close vicinity of the tantalum surface, that grow to a resonant size under suitable acoustic conditions and likely cause removal of particles. Interestingly, increasing the applied potential to -2V (vs Ag/AgCl (sat. KCl)) enhanced the particle removal efficiency to about 100%. Investigations were also performed in solutions containing 10 mM potassium chloride (KCl). The results revealed that even at low applied potentials of -1.5V, almost complete particle removal was achieved. This improvement in PRE was attributed to a combined effect of microstreaming and electro-acoustic forces. The results revealed that almost complete removal of particles could be achieved at low power density and duty cycle when a sound field at 1 MHz is used in conjunction with electrochemistry. The second study focuses on the effect of acoustic frequency and transducer power density for the development of a damage-free megasonic cleaning process. Here, an effort was made to characterize cavitation activity at acoustic frequencies of 1, 2 and 3 MHz by means of electrochemical, acoustic emission and fluorescence spectroscopy techniques. Studies conducted with a microelectrode using ferricyanide as an electroactive species showed that at 1 MHz and 2 W/cm², current peaks with a rise and fall time of about 30-50 ms and 80-120 ms were observed, respectively, which were indicative of transient cavitation behavior. Interestingly at higher frequencies (3 MHz), symmetric and oscillatory behavior in the current was observed. The rise and fall times were about 3 orders of magnitude lower at about 50 µs. This oscillatory behavior in the current at 3 MHz was attributed to the presence of stable cavities. Furthermore, hydrophone studies supported the microelectrode studies as they showed a reduction of about two orders of magnitude in the intensity of transient cavitation as frequency was increased from 1 to 3 MHz. Hydroxyl radical (OH*) capture measurements using terephthalate dosimetry corroborated the above results as they illustrated an order of magnitude decrease in OH* generation rate at 3 MHz compared to 1 MHz. These studies suggest that the use of higher megasonic frequencies may be more suitable for damage-free and effective cleaning of patterned surfaces in the semiconductor industry. In the last part of the dissertation, we investigate the effect of solution parameters on cavitation characteristics using a bicarbonate based alkaline chemical cleaning formulation that has been previously demonstrated to be beneficial in achieving effective megasonic cleaning and low damage. The results of this study revealed that in the presence of ammonia (NH₃) or carbonate/bicarbonate ions at concentrations greater than 75 mM or 200 mM respectively, the measured rate of generation of hydroxyl radicals at 1 MHz and 2 W/cm² was significantly reduced. The lower rate of OH· was attributed to scavenging of radicals in these solutions and additionally due to reduced transient cavitation in ammonia solutions. Hydroxyl radical measurements at higher power density of 8 W/cm² showed that carbonate ions were better scavengers of hydroxyl radicals than bicarbonate ions. The study on the effect of bulk solution temperature illustrated that the rate of generation of OH· increased with increase in temperature from 10 to 30 °C suggesting enhanced transient cavitation at higher temperatures (in the investigated range). The use of optimum concentration of ammonia or carbonates ions in cleaning formulation and bulk solution temperature would likely provide desired cleaning with minimum damage.
In integrated circuit (IC) manufacturing, particulate contamination from hundreds of processe steps is a major cause of yield loss. The removal of particles is typically achieved through liquid chemical formulations aided by a sound field in the MHz frequency range. When liquid is irradiated with megasonic waves, dissolved gases play an important role in particle removal and feature damage. To take the advantage of the beneficial effect of CO₂ (aq.), this thesis describes the development and optimization of a megasonic cleaning process using a chemical system containing NH₄OH and NH₄HCO₃ at an alkaline pH in which a specific amount of aqueous CO₂ can be maintained to minimize feature damage. In addition, certain etching effects at a slightly alkaline pH were supported for achieving high particle removal. Sonoluminescence (SL) data were collected from these cleaning solutions and correlated with the cleaning performance. The intensity of SL is believed to be a sensitive indicator of transient cavitation during megasonic irradiation, which is thought to be responsible for fragile feature damage. To further analyze the SL signal with respect to the emission from hydroxyl radicals, single-band filters were used to collect the SL signal in different wavelength ranges. The study of particle removal and feature damage was performed using a single-wafer cleaning tool, MegPie® (ProSys, Inc.), which provided acoustic irradiation at a frequency of 0.925 MHz. Commercially available SiO₂ slurry with 200 ± 20 nm particles was used for particle contamination. Particle removal was investigated on both blanket SiO₂ samples and patterned samples. Feature damage studies were conducted on patterned samples by examining the number of line breakages per unit area. By adjusting the pH in NH₄OH/NH₄HCO₃ solutions from 7.8 to 8.5, the amount of CO₂ (aq.) was varied. At a pH of 8.2 with ~ 320 ppm CO₂ (aq.) in the cleaning solution, a high particle removal efficiency was achieved (> 90%) at an acoustic power intensity of 1 W/cm² for an exposure time of 60 s, and the feature damage was reduced by > 50%. For SL signal analysis, band filters in the wavelength range of (i) 280 – 305.5 nm, (ii) 300 – 340 nm, (iii) 335 – 375 nm, and (iv) 374.5 – 397.5 nm were used to resolve the SL spectrum in these wavelength ranges. The filters were sandwiched, one at a time, between the optical window and the photomultiplier tube (PMT) in the Cavitation Threshold (CT) cell. Air-, Ar-, and CO₂-containing DI water (at pH 4.53 with ~ 90 ppm aqueous CO₂) was pumped through the cell at a flow rate of 130 ml/min. The acoustic power was ramped from 0.1 to 4 W/cm² at an acoustic frequency of 0.925 MHz. The SL signal intensity showed the highest value in the ranges of 300 – 340 and 335 – 375 nm in air- and Ar-saturated DI water, which is due to the emission from excited hydroxyl radicals. These results are consistent with an SL spectrum analysis performed using expensive optical set-ups. In CO₂-containing DI water, the SL signal intensity was suppressed by a factor of 100. The methodology reported in this work is simple, inexpensive, and capable of capturing SL spectral features due to hydroxyl radicals.
Keswani, Manish (The University of Arizona., 2008)
Megasonic cleaning is routinely used in the semiconductor industry to remove particulate contaminants from wafer and mask surfaces. Cleaning is achieved through proper choice of chemical solutions, power density and frequency of acoustic field. Considerable work has been done to increase understanding of particle removal mechanisms in megasonic cleaning using different solution chemistries with varying ionic strengths. However, to date, the focus of all these studies of particle removal has been either cavitation or acoustic streaming.The propagation of sound waves through a colloidal dispersion containing ions is known to result in the generation of two types of oscillating electric potentials, namely, Ionic Vibration Potential (IVP) and Colloid Vibration Potential (CVP). These potentials and their associated electric fields can exert forces on charged particles adhered to a surface, resulting in their removal. In addition, the pressure amplitude of the sound wave is also altered in solutions of higher ionic strengths, which can affect the cavitation process and further aid in the removal of particles from surfaces. To test the two hypotheses, investigations have been conducted on the feasibility of removal of charged particles from silicon wafers in electrolyte solutions of different ionic strengths irradiated with a megasonic field of different power densities. Cleaning experiments have been performed using potassium chloride (KCl) as a model electrolyte and silica particles as model contaminant particles. The cleaning performance in KCl solution has been compared to that in other electrolytes solutions such as sodium chloride, cesium chloride and lithium chloride. In order to characterize the cavitation events in KCl solutions, acoustic pressure and sonoluminescence measurements have been performed using hydrophone and cavitation probe respectively. The results indicate that particle removal efficiency (PRE) increases with KCl concentration and transducer power density and much lower power densities are required at higher KCl concentration for a comparable level of cleaning. Further, cleaning performances in NaCl and CsCl were found to be superior to those in KCl and LiCl solutions. Theoretical computations show that the removal forces due to CVP are much larger in magnitude than those due to IVP and are comparable to van der Waals forces.
Chiang, Chieh-Chun (The University of Arizona., 2015)
In semiconductor manufacturing, a large amount (50 billion gallons for US semiconductor fabrication plants in 2006) of ultrapure water (UPW) is used to rinse wafers after wet chemical processing to remove ionic contaminants on surfaces. Of great concern are the contaminants left in narrow (tens of nm), high-aspect-ratio (5:1 to 20:1) features (trenches, vias, and contact holes). The International Technology Roadmap for Semiconductors (ITRS) stipulates that ionic contaminant levels be reduced to below ~ 10¹⁰ atoms/cm². Understanding the bottlenecks in the rinsing process would enable conservation of rinse water usage. A comprehensive process model has been developed on the COMSOL platform to predict the dynamics of rinsing of narrow structures on patterned SiO₂ substrates initially cleaned with NH₄OH. The model considers the effect of various mass-transport mechanisms, including convection and diffusion/dispersion, which occur simultaneously with various surface phenomena, such as adsorption and desorption of impurities. The influences of charged species in the bulk and on the surface, and their induced electric field that affect both transport and surface interactions, have been addressed. Modeling results show that the efficacy of rinsing is strongly influenced by the rate of desorption of adsorbed contaminants, mass transfer of contaminants from the mouth of the feature to the bulk liquid, and the trench aspect ratio. Detection of the end point of rinsing is another way to conserve water used for rinsing after wet processing. The applicability of electrochemical impedance spectroscopy (EIS) to monitor rinsing of Si processed in HF with and without copper contaminant was explored. In the first study, the effect of the nature of surface state (flat band, depletion, or accumulation) of silicon on rinsing rate was investigated. The experimental results show that the state of silicon could affect rinsing kinetics through modulation of ion adsorption. In the second study, silicon was intentionally contaminated by spiking HF with copper ions, cleaned in dilute HCl and then rinsed, and the entire process was followed by continuous impedance measurements. The measured impedance values at different stages have been correlated to the nature of the silicon surface, as characterized by scanning electron microscope (SEM) and inductively coupled plasma mass spectrometry (ICP-MS) methods.
Kumari, Sangita (The University of Arizona., 2011)
This dissertation describes the finding that dissolved carbon dioxide is a potent inhibitor of sonoluminescence and describes the implications of the finding in the development of improved megasonic cleaning formulations. Megasonic cleaning, or the removal of contaminants particles from wafer surfaces using sound-irradiated cleaning fluids, has been traditionally used in the semiconductor industry for cleaning of wafers. Recently however, advancing technology and miniaturization has made wafer features increasingly susceptible to damage by megasonic energy. International Technology Roadmap for Semiconductors (ITRS) 2011 predicts the critical particle diameter, critical particle count and killer defect numbers to be 22 nm, 113 #/wafer and 4.3 #/mm², respectively, on a 300 mm wafer for 45 nm technology node. A critical challenge in the field, therefore, is to achieve removal of small particles (22 nm to 200 nm) without causing damage to fine wafer features. The work described here addresses this challenge by identifying sonoluminescence and solution pH as two key factors affecting damage and cleaning efficiency, respectively and establishing novel means to control them using CO₂(aq) release compounds in the presence of acids and bases. Sonoluminescence (SL) behavior of the major dissolved gases such as Ar, Air, N₂, O₂ and CO₂ was determined using a newly designed Cavitation Threshold Cell (CT Cell). SL, which is the phenomenon of release of light in sound-irradiated liquids, is a sensitive indicator of cavitation, primarily transient cavitation. It was found that all the tested dissolved gases such as Ar, Air, N₂ and O₂, generated SL signal efficiently. However, dissolved CO₂ was found to be completely incapable of generating SL signal. Based on this interesting result, gradual suppression of SL signal was demonstrated using CO₂(aq). It was further demonstrated that CO₂(aq) is not only incapable but is also a potent inhibitor of SL. The inhibitory role of CO₂(aq) was established using a novel method of controlled in-situ release of CO₂ from NH₄HCO₃. ~130 ppm CO₂(aq) was shown to be necessary and sufficient for complete suppression of SL generation in air saturated DI water. The method however required acidification of solution for significant release of CO₂, making it unsuitable for the design of cleaning solutions at high pH. Analysis of the underlying ionic equilibria revealed that the loss of released CO₂(aq) upon increase in pH can be compensated by moderate increase in added NH₄HCO₃. Using this method, simultaneous control of SL and solution pH was demonstrated in two systems, NH₄HCO₃/HCl and NH₄OH/CO₂, at two nominal pH values; 5.7 and 7.0. Damage studies were performed on wafer samples with line/space patterns donated by IMEC and FSI International bearing Si/metal/a-Si gate stacks of thickness ~36 nm and Si/Poly-Si gate stacks of thickness ~67 nm, respectively. A single wafer spin cleaning tool MegPie® was used for the generation of megasonic energy for inducing damage to the structures. It was demonstrated that CO₂ dissolution in DI water suppresses damage to the gate stacks in a dose-dependent manner. Together, these studies establish a systematic and strong correlation between CO₂(aq) concentration, SL suppression and damage suppression. Significant damage reduction (~50 % to ~90 %) was observed at [CO₂(aq)] > ~300 ppm. It was also demonstrated that CO₂(aq) suppresses damage under alkaline pH condition too. This demonstration was made possible by the successful design of two new cleaning systems NH₄HCO₃/NH₄OH and CO₂/NH4OH that could generate CO₂(aq) under alkaline conditions. Damage suppressing ability of the newly designed cleaning systems were compared to the standard cleaning system NH₄OH at pH 8.2 and it was found that NH₄HCO₃/NH₄OH and CO₂/NH₄OH systems were 80 % more efficient in suppressing damage compared to the standard NH₄OH cleaning system. Finally, megasonic cleaning studies were conducted in the same single wafer spin cleaning tool MegPie®, using SiO₂ particles (size 185 nm) deposited on 200 mm oxide Si wafers, as the contaminant. It was found that the standard cleaning chemical, NH₄OH, pH 8.2, was effective in achieving > 95 % particle removal for 2 min irradiation of megasonic energy at power densities > 0.7 W/cm². Based on these results, a new system, NH₄HCO₃/NH₄OH, was designed with an aim to release ~300 ppm CO₂ at pH 8.2. It was demonstrated that newly designed system NH₄HCO₃/NH₄OH, allowed significant suppression of damage in comparison to NH₄OH while maintaining > 90 % cleaning efficiency that was comparable to NH₄OH solution, at the same acoustic power densities. Taken together, these studies establish a potent and flexible means for the inhibition of SL generation over a wide pH range and acoustic power densities and demonstrate its use in suppression of wafer damage without compromising megasonic cleaning efficiency.
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