Direct wafer bonding has been identified as an en-abling technology for microelectromechanical systems (MEMS). As the complexity of devices increase and the bonding of multiple patterned wafers is required, there is a need to understand the factors that lead to bonding failure. Bonding relies on short-ranged surface forces, thus flatness deviations of the wafers may prevent bonding. Bonding success is determined by whether or not the surface forces are sufficient to overcome the flatness deviations and deform the wafers to a common shape. A general bonding criterion based on this fact is developed by comparing the strain energy required to deform the wafers to the surface energy that is dissipated as the bond is formed. The bonding criterion is used to examine the case of bonding bowed wafers with etch patterns on the bonding surface. An analytical expression for the bonding criterion is developed using plate theory for the case of bowed wafers. Then, the criterion is implemented using finite element analysis, to demonstrate its use and to validate the analytical model. The results indicate that wafer thickness and curvature are important in determining bonding success and that the bonding criterion is independent of wafer diameter. Results also demonstrate that shallow etched patterns can make bonding more difficult while deep features, which penetrate through an appreciable thickness of the wafer, may facilitate bonding. Design implications of the model results are discussed in detail. Preliminary results from experiments designed to validate the model, agree with the trends seen in the model, but further work is required to achieve quantitative correlation. / Singapore-MIT Alliance (SMA)

During fabrication of integrated circuits, fluorocarbon plasma etching is used to pattern dielectric layers. As a byproduct of the process, a fluorocarbon residue is deposited on exposed surfaces and must be removed for subsequent processing. Conventional fluorocarbon cleaning processes typically include at least one plasma or liquid treatment that is oxidative in nature. Oxidative chemistries, however, cause material degradation to next generation low-dielectric constant (low-k) materials that are currently being implemented into fabrication processes. This work addresses the need for alternative fluorocarbon-residue removal chemistries that are compatible with next generation low-k materials. Radical anion chemistries are known for their ability to defluorinate fluorocarbon materials by a reductive mechanism. Naphthalene radical anion solutions, generated using sodium metal, are used to establish cleaning effectiveness with planar model residue films. The penetration rate of the defluorination reaction into model fluorocarbon film residues is measured and modeled. Because sodium is incompatible with integrated circuit processing, naphthalene radical anions are alternatively generated using electrochemical techniques. Using electrochemically-generated radical anions, residue removal from industrially patterned etch structures is used to evaluate the process cleaning efficiency. Optimization of the radical anion concentration and exposure time is important for effective residue removal. The efficiency of removal also depends on the feature spacing and the electrochemical solvent chosen. The synergistic combination of radical anion defluorination and wetting or swelling of the residue by the solvent is necessary for complete removal. In order to understand the interaction between the solvent and the residue, the surface and interfacial energy are determined using an Owens/Wendt analysis. These studies reveal chemical similarities between specific solvents and the model residue films. This approach can also be used to predict residue or film swelling by interaction with chemically similar solvents.

Utilizing bulk micromachining technology, this thesis aimed to develop a flexural plate-wave(FPW) device with novel groove reflection microstructure for high-sensitivity and low insertion-loss biomedical microsystem applications. The influences of the amount and depth of the groove and the distance between the groove and the boundary of ZnO piezoelectric thin-film (DGB) on the reduction of insertion-loss and the enhancement of quality factor (Q) and electromechanical coupling coefficient (K2) were investigated. Three critical technology modules established in this thesis are including the development of (1) a sputtering deposition process of high C-axis (002) orientation ZnO piezoelectric thin-film, (2) an electrochemical etch-stop technique of silicon anisotropic etching and (3) an integration process of FPW device. Firstly, under the optimized conditions of the sputtering deposition process (300¢J substrate temperature, 200 W radio-frequency (RF) power and 30/70 Ar/O2 gas flow ratio), a high C-axis (002) orientated ZnO piezoelectric thin-film with a high X-ray diffraction (XRD) intensity (50,799 a.u.) and narrow full width at half maximum (FWHM = 0.383¢X) can be demonstrated. The peak of XRD intensity of the standard ZnO film occurs at diffraction angle 2£c = 34.422¢X, which matches well with our results (2£c = 34.357¢X). Secondary, an electrochemical etch-stop system with three electrode configuration has been established in this research and the etching accuracy can be controlled to less than 1%. Thirdly, this thesis has successfully integrated the main fabrication processes for developing the FPW device which are including six thin-film deposition processes and six photolithography processes. The implemented FPW device with RIE etched groove reflection microstructure presents a low insertion-loss of -12.646 dB, center frequency of 114.7 MHz, Q factor of 12.76 and K2 value of 0.1876%.

This thesis presents the development of electrochemical etch-stop techniques (ECES) to achieve high precision 3-dimensional integrated MEMS sensors with wet anisotropic etching by applying proper voltages to various regions in silicon. The anisotropic etchant is selected as tetra methyl ammonium hydroxide, TMAH, considering its high silicon etch rate, selectivity towards SiO2, and CMOS compatibility, especially during front-side etching of the chip/wafer. A number of parameters affecting the etching are investigated, including the effect of temperature, illumination, and concentration of the etchant over the etch rate of silicon, surface roughness, and biasing voltages. The biasing voltages for passivating the n-well and enhancing the etching reactions on p-substrate are determined as -0.5V and -1.6V, respectively, after a series of current-voltage characteristic experiments. The surface roughness due to TMAH etching is prevented with the addition of ammonium peroxodisulfate, AP. A proper etching process is achieved using a 10wt.% TMAH at 85&deg / C with 10gr/lt. AP. Different silicon etch samples are produced in METU-MET facilities to understand and optimize ECES parameters that can be used for CMOS microbolometers. The etch samples are fabricated using various processes, including thermal oxidation, boron and phosphorus diffusions, aluminum and silicon nitride layer deposition processes. Etching with the prepared samples shows the dependency of the depletion layer between p-substrate and n&amp / #8209 / well, explaining the reason of the previous failures during post-CMOS etching of CMOS microbolometers from the front side. Succesfully etched CMOS microbolometers are achieved with back side etching in 6M KOH at 90 &deg / C, where &amp / #8209 / 3.5V and 1.5V are applied to the p-substrate and n-well. In summary, this study provides an extensive understanding of the ECES process for successful implementations of integrated MEMS sensors.

Fluoride-containing solutions are widely used to etch silicon dioxide-based films. A critical issue in integrated circuit (IC) and microelectromechanical systems (MEMS) fabrication is achievement of adequate selectivity during the etching of different film materials when they are present in different areas on a device or in a stack. The use of organic fluoride-based salts in aqueous/organic solvent solutions can yield etch selectivities <1.9 for thermally-grown silicon dioxide relative to borophosphosilicate glass films, and thus may also obviate the need to add surfactants to the etch solutions to realize uniform etching. Etch studies with aqueous-organic fluoride salt-based solutions also offer insight into the etch mechanism of these materials. Specifically, the importance of water content in the solutions and of ion solvation in controlling the etch chemistry is described. With respect to fluoride-containing solutions, etching of SiO₂ films using aqueous HF-based chemistries is widely used in IC and MEMS industries. To precisely control film loss during cleaning or etching processes, good control over the contact time between the liquid (wet) chemistry and the substrate is necessary. An integrated wet etch and dry reactor system has been designed and fabricated by studying various geometrical configurations using computational fluid dynamics (CFD) simulations incorporating reaction kinetics from laboratory data and previously published information. The effect of various process parameters such as HF concentration, flow rate, and flow velocity on the etch rates and uniformity of thermally-grown silicon dioxide and borophosphosilicate glass films was studied. Simulations agree with experiments within experimental error. This reactor can also be used to wet etch/clean and dry other films in addition to SiO₂-based films using aggressive chemistries as well as aqueous HF under widely different process conditions. A spectroscopic reflectometry technique has been implemented in-situ in this custom fabricated reactor to monitor the thickness and etch rate in wet etching environments. The advantages of this technique over spectroscopic ellipsometry in specific situations are discussed. A first principles model has been developed to analyze the reflectometry data. The model has been validated on a large number of previously published studies. The match between experimental and simulated thickness is good, with the difference ~ 5 nm. In-situ thickness and etch rate have been estimated using Recursive Least Squares (RLS), Extended Kalman Filter (EKF) and modified Moving Horizon Estimator (mMHE) analyses applied to spectroscopic reflectometry using multiple wavelengths with ZnO employed as a model film. The initial guess for EKF and mMHE has been obtained from a CFD model. It has been shown that both EKF and mMHE are less oscillatory than RLS/LS in the prediction of thickness and ER and more robust when a smaller number of wavelengths are used, in addition, the computational time for EKF is less than that of mMHE/RLS. For no restrictions on computational requirements, LS should be the method of choice whereas in the case of faster etching systems, with the availability of a better process model, EKF should be starting point. The choice of algorithm is thus based on sampling rate for data collection, process model uncertainty and the number of wavelengths required.

Although copper (Cu) is the preferred interconnect material due to its lower resistivity than aluminum (Al), Cu subtractive etching processes have not been developed at temperatures less than 180 °C, primarily due to the inability to form volatile etch products at low temperature. The conventional damascene technology avoids the need for subtractive etching of Cu by electroplating Cu into previously etched dielectric trenches/vias, followed by a chemical/mechanical planarization (CMP) process. However, a critical "size effect" limitation has arisen for damascene technology as a result of the continuing efforts to adhere to "Moore's Law". The size effect relates to the fact that the resistivity of damascene-generated lines increases dramatically as the line width approaches the sub-100 nm regime, where feature size is similar to the mean free path of electrons in Cu (40 nm). As a result, an alternative Cu patterning process to that of damascene may offer advantages for device speed and thus operation. This thesis describes investigations into the development of novel, fully-plasma based etch processes for Cu at low temperatures (10 °C). Initially, the investigation of a two-step etch process has been studied. This etch approach was based on a previous thermodynamic analysis of the Cu-Cl-H system by investigators at the University of Florida. In the first step, Cu films are exposed to a Cl₂ plasma to preferentially form CuCl₂, which is believed to be volatilized as Cu₃Cl₃ by subsequent exposure to a hydrogen (H₂) plasma (second step). Patterning of Cu films masked with silicon dioxide (SiO₂) layers in an inductively coupled plasma (ICP) reactor indicates that the H₂ plasma step in the two-step process is the limiting step in the etch process. This discovery led to the investigation of a single step Cu etch process using a pure H₂ plasma. Etching of blanket Cu films and Cu film patterning at 10°C, display an etch rate ~ 13 nm/min; anisotropic etched features are also observed. Comparison of H₂ plasma etching to sputtering of Cu films in argon (Ar) plasmas, indicates that both a chemical component and a physical component are involved in the etching mechanism. Additional studies using helium plasmas and variation of power applied to the plasma and etching surface demonstrate that the etch rate is controlled by reactive hydrogen species, ion bombardment flux and likely photon flux. Optical Emission Spectroscopy (OES) of the H₂ plasma during the Cu etching process detects Cu emission lines, but is unable to identify specific Cu etch products that desorb from the etching surface. Variation of Cu etch rates as a function of temperature suggests a change in mechanism for the removal of Cu over the temperature of -150 °C to 150 °C. OES analyses also suggest that the Cl₂ plasma step in the two-step process can inhibit Cu etching, since the subsequent H₂ (second) plasma step shows a time delay in film removal. Preliminary results of the etching of the SiO₂ mask material in H₂ plasmas with various intentionally introduced contaminants demonstrate the robustness of the H₂ plasma Cu etch process.

Electrowetting is a phenomenon that controls the wettability of liquids on solid surfaces by the application of electric potential. It is an interesting method to handle tiny amounts of liquid on solid surfaces. In recent times, researchers have been investigating this phenomenon and have reported some unexplained behavior and degradation in the Electrowetting system performance. Electrowetting systems include the presence of electric field and different materials from metals to dielectrics and electrolytes that create an environment in which corrosion processes play a very important role. With the small dimensions of the electrodes, corrosion can cause failure quickly when the dielectric fails. In this work, commonly used dielectric films such as silicon dioxide and silicon nitride were deposited using Plasma Enhanced Chemical Vapor Deposition and characterized on the basis of thickness uniformity, etch rate measurements, Dry current – voltage measurements and Wet current – voltage measurements. Sputtered silicon dioxide films were also characterized using the same methods. The correlation between Dry I – V and Wet I – V measurements was studied and a comparison of dielectric quality of films based on these measurements is presented. Also, impact of different liquids on the dielectric quality of films was studied.

In this dissertation, an exosome capturing ciliary micropillar array microfluidics is introduced and applied to evaluate the response of resistant cancer cells under nanoparticle encapsulated chemotherapy. Cancer cells are able to develop different mechanisms to resist therapeutic treatment, frequently causing chemotherapy failure. Active drug expulsion is one of the usual resisting schemes to reduce intracellular drug accumulation to a non-effective level. Evidence has suggested a potential exosomal pathway is used by multi-drug resistant (MDR) cancer cells to expel drugs. Here I study the exosomes derived from MDR cancer cells treated by nanotherapeutics aiming to establish the correlation between nanotherapeutics and exosomal pathway for drug expulsion. The outcome would boost further understanding of cancer MDR, and in turn direct the development of pharmaceutical nanoparticles to overcome MDR cancer. To effectively isolate exosomes for drug expulsion evaluation, a ciliary micropillar structure is fabricated employing microelectromechanical systems (MEMS) and metal assisted chemical etching (MACE) techniques. The ciliary micropillar is fabricated in two major steps: deep silicon etch (DSE) for pillars followed by a MACE process to etch nanowires on the pillars. The concept of using MACE as an alternative to DSE is also explored to reduce fabrication cost. With optimized parameters, it shows a comparable result to DSE. COMSOL simulation revealed that ciliated micropillars exhibited a unique advantage as a unit structure for capturing small particles in fluid flow, according to particle filtration theory. A nanowire layer with high permittivity allows fluid streamlines to pass through, and increases interaction with particle carrying fluid to increase the probability of particle interception. Nanowires on the pillar can trap specific sized particles due to their characteristic dimension. Thanks to the weaker stability of porous silicon nanowires, trapped particles can then be released by dissolving these nanowires without damage to the particles themselves. A microfluidic chip is fabricated with an optimized circular micropillar arrangement for resistance reduction, and its particle filtration performance is demonstrated by processing model cell culture medium. The device is applied to study MDR cells’ response to micelle encapsulated paclitaxel treatment. Cell culture medium from sensitive and MDR variant of MDA-MB-231 cells treated with pure and capsulated drugs are processed through the device for exosome isolation. Drug volume in collected exosomes is determined after measurement. By measuring drug efflux through exosomes, it is determined that MDA-MB-231MDR cells do use an exosomal pathway to expel drugs, but other mechanisms are also at play. Nanoparticle encapsulation results in higher drug concentration in exosomes partly because the origin of exosomes and nanoparticle intake through endocytosis share some similar route. / text

Progress in the microelectronics industry is driven by smaller and faster transistors. As feature sizes in integrated circuits become smaller and liquid chemical waste becomes an even greater environmental concern, gas expanded liquids (GXLs) may provide a possible solution to future device fabrication limitations relative to the use of liquids. The properties of GXLs such as surface tension can be tuned by the inclusion of high pressure gases; thereby, the reduced surface tension will allow penetration of cleaning solutions into small features on the nanometer scale. In addition, the inclusion of the gas decreases the amount of liquid necessary for the photoresist and etch residue removal processes. This thesis explores the role of CO2-based GXLs for photoresist and etch residue removal. The gas used for expansion is CO2 while the liquid used is methanol. The cosolvent serving as the removal agent is tetramethyl ammonium hydroxide (TMAH) which upon reacting with CO2 becomes predominantly tetramethyl ammonium bicarbonate (TMAB).

abstract: Micromachining has seen application growth in a variety of industries requiring a miniaturization of the machining process. Machining at the micro level generates different cutter/workpiece interactions, generating more localized temperature spikes in the part/sample, as suggested by multiple studies. Temper-etch inspection is a non-destructive test used to identify `grind burns' or localized over-heating in steel components. This research investigated the application of temper-etch inspection to micromachined steel. The tests were performed on AISI 4340 steel samples. Finding, indications of localized over-heating was the primary focus of the experiment. In addition, change in condition between the original and post-machining hardness in the machined slot bottom was investigated. The results revealed that, under the conditions of the experiment, no indications of localized over-heating were present. However, there was a change in hardness at the bottom of the machined slot compared to the rest of the sample. Further research is needed to test the applicability of temper-etch inspection to micromilled steel and to identify the source of the change in hardness. / Dissertation/Thesis / M.S.Tech Technology 2010