Hoo-Jeong Lee

SOLID-STATE AMORPHIZATION AT TETRAGONAL-TA/CU INTERFACES

This letter describes the formation of a thin amorphous layer at the tetragonal-Ta/Cu interfaces, which appear in copper metallization structures of microelectronic devices. The disordered layer grows up to 4 nm when annealed at between 400 and 600u200a°C. Since Ta and Cu are immiscible according to thermodynamic data, this is an unusual observation. A mechanism for the amorphous phase formation is proposed using both physical and chemical considerations. A high content of Cu is detected in the Ta layer up to 5 nm from the interface when annealed at 600u200a°C. Although the adhesion is promoted by the interface reaction, a sufficiently thick Ta underlayer is recommended for efficient blocking of Cu diffusion. Neither solid-state amorphization nor Cu diffusion into Ta is observed at bcc-Ta/Cu interfaces.

Capacitive micromachined ultrasonic transducers for chemical detection in nitrogen

The authors present the prototype of a chemical sensor using a capacitive micromachined ultrasonic transducer array. Each element in the array consists of a large number of resonating membranes connected in parallel. A five-channel oscillator circuit operates at the resonant frequency around 6MHz in this prototype. The surface of the elements in the array is coated by polymers such as polyallylamine hydrochloride, polyethylene glycol, and polyvinyl alcohol to detect different chemicals. By measuring shift in oscillation frequencies due to the mass-loading effect, analytes, e.g., water and isopropanol, with concentrations around 20ppbv (parts per 109 by volume) range can be detected.

Stress relaxation of free-standing aluminum beams for microelectromechanical systems applications

We investigated the uniaxial tension and stress relaxation properties of micron-scale Al beams for microelectromechanical systems applications in a piezoactuator-driven test apparatus. Pure aluminum and Al-1.5 at.u200a% titanium free-standing beams were fabricated using micromachining procedures. In the tensile tests, we found the yield strength of the Al beams to be approximately 95 MPa. We also observed a significant strengthening effect in the alloyed samples, which had a yield strength approximately 85%u2002higher than the pure Al samples. In stress relaxation tests, we observed a substantial load-drop (about 56% after 10 min) in the Al films, and we propose here that grain boundary sliding is responsible for this relaxation. By comparison, the relaxation of the Ti-alloyed samples was only 15%. We believe that this difference results from the Al3Ti precipitates that form at the Al grain boundaries in the alloy samples.

Tensile failure by grain thinning in micromachined aluminum thin films

This article reports the results of microtensile tests and transmission electron microscopy (TEM) analyses of micron-scale free-standing aluminum thin films. We fabricated the free-standing aluminum beams using micromachining procedures and tested them in a piezo-actuator-driven test apparatus. Microtensile tests revealed that the mechanical characteristics of the free-standing beams are quite different from those of the bulk material. Some unique features of our free-standing Al beams are high yield strength, low ductility, and strain softening. While the high yield strength is attributed to the typically small grain size, the strain softening and the small ductility are explained by locally thinned grains found in TEM analyses. Further TEM analyses suggested that dislocations pass through the free surfaces in the interiors of some large grains and lead to strain softening, which, in turn, results in deformation concentration in such areas and hence localized thinning.

Study of ZrO2 thin films for gate oxide applications

We investigated the microstructures and electrical properties of ZrO2 films deposited by reactive dc magnetron sputtering on Si substrates for gate dielectrics applications. We observed that the refractive index value of the ZrO2 films increased with an increase in deposition powers and annealing temperatures. The ZrO2 films deposited at elevated temperatures are polycrystalline, and both the monoclinic and tetragonal phases exist in the films. Films with higher density and improved crystallinity are obtained at higher deposition temperatures. The interfacial oxide layer between ZrO2 films and Si substrates grew upon annealing in the O2 gas ambient, which is due to the oxidation of Si substrates by the diffusion of oxidizing species from O2 gas ambient. The accumulation capacitance value increased upon annealing in the N2 gas ambient due to the densification of the films, while it decreased in O2 gas ambient due to the growth of the interfacial oxide layer.

A Study on the Bonding Process of Cu Bump/Sn/Cu Bump Bonding Structure for 3D Packaging Applications

This study reports the kinetics of the bonding process of a Cu bump/10 μm thick Sn bonding layer/Cu bump bonding structure, popularly adopted for three-dimensional (3D) packaging, and the mechanical properties of the joints. Characterizing the bonding morphologies of the joints using scanning electron microscopy disclosed that a drastic intermetallic phase transformation occurred around the melting temperature of Sn in a manner similar to the wetting process of SnPb solder on Cu. We also delved into the mechanical behavior of the joints using lap-shear testing to illuminate that a unique feature of the joints is a lack of ductility. In addition, the samples exhibited a sensitive dependence of the joint strength on the bonding conditions (temperature and pressure).

Fabricating capacitive micromachined ultrasonic transducers with direct wafer-bonding and LOCOS technology

We present an improved fabrication method for capacitive micromachined ultrasonic transducers (CMUTs). Recently, a process was developed to fabricate CMUTs using direct wafer-bonding instead of the traditional sacrificial release method. This paper presents a method based on local oxidation of silicon (LOCOS) and direct wafer-bonding to improve the controllability of gap heights and the parasitic capacitance. Critical vertical dimensions are determined by a thermal oxidation process, which allows tight vertical tolerances (< 10 nm) with unmatched uniformity over the entire wafer. Using this process we successfully fabricated CMUTs with gap heights as small as 40 nm with a uniformity of plusmn 2 nm over the entire wafer.

Thermal stability of metal Ohmic contacts in indium gallium zinc oxide transistors using a graphene barrier layer

The excellent impermeability of graphene was exploited to produce stable ohmic contact at the interface between Al metal and a semiconducting indium gallium zinc oxide (IGZO) layer after high-temperature annealing. Thin film transistors (TFTs) were fabricated with and without a graphene interlayer between the Al metal and the IGZO channel region. Metal contact at the interface prepared without a graphene interlayer showed serious instabilities in the IGZO TFT under thermal annealing; however, the insertion of a graphene interlayer between the IGZO channel and the Al metal offered good stability under repeated high-temperature annealing cycles and maintained ohmic contact.

A study on the microstructure and electrical properties of CeO2 thin films for gate dielectric applications

Abstract We investigated the evolution of microstructures and the #electrical properties of CeO 2 thin films deposited by the reactive DC magnetron sputtering method on the (100) silicon substrate upon annealing in the O 2 gas ambient. The CeO 2 thin films deposited at 300°C were polycrystalline. After annealing in ambient O 2 gas, the crystallinity of the CeO 2 film was improved and becomes more dense with annealing time and temperature. The maximum accumulation capacitance of CeO 2 thin film annealed above 700°C in the ambient O 2 gas decreased due to the growth of the interfacial SiO 2 layer between CeO 2 film and silicon substrate by the diffusion of oxidizing species through CeO 2 thin film from the ambient gas.

Optimum design of circular CMUT membranes for high quality factor in air

This paper presents optimum design of circular membrane for high quality factor. A quality factor of CMUT, we designed for resonant chemical sensor, is dominated by Q_air and Q_support. We investigated these two factors independently. We calculate Q_air of circular silicon membrane analytically, and numerically. Calculated Q_air is compared to measured value. In order to find Q_support empirically, we measured quality factor of CMUT in vacuum chamber. Q_air is proportional to (radius/thickness)^-2 and Q_support is proportional to (radius/thickness)³. Thus the optimum aspect ratio of membrane exists for maximum quality factor.