Chong-Ook Park

Ceramics for chemical sensing

Many types of sensors have been developed to detect chemical species in the gas phase. These include optical based on color change or fluoresence, surface acoustic wave (SAW) devices, electrochemical, chemoresistive/semiconductive, field effect transistors (FET), metal-insulator-semiconductor (MIS) diode devices, and many other. Among these, resistive type sensors based on ceramic oxides are particularly attractive because of their low cost, wide range of applications and potential for use in electronic nose. This article focuses mainly on the resistive/semiconductive, especially the surface conductive ceramic oxide type gas sensors. The main emphasis is on the basic principles involving gas-solid reactions. Also discussed are selected applications with an emphasis on sensor design issues. Since SnO2 can be used as a model system for oxide-based sensors, most of the discussions focuses on this system, though other systems are occasionally highlighted illustrating recent developments.

A New Route toward Ultrasensitive, Flexible Chemical Sensors: Metal Nanotubes by Wet-Chemical Synthesis along Sacrificial Nanowire Templates

We developed a novel low-temperature, wet-chemical process for the facile synthesis of metal nanotube arrays through the reduction of metal precursors along sacrificial metal oxide nanowire templates and demonstrated its applications to the ultrasensitive, low-power, mechanically robust, and flexible chemical sensors. The in situ dissolution of ZnO nanowire templates, which were hydrothermally grown on electrode surfaces, during the reaction allows the direct formation of tubular Pd nanostructures on the sensor devices without the need of complex processes for device integration or template removal. Moreover, this simple synthesis was carried out at low-temperature with mild chemical conditions; therefore we could make Pd nanotube devices not only on silicon substrates but also on flexible polymer substrates. The H(2) sensing of such Pd nanotube devices was investigated under various mechanical loading and showed excellent reliability and robustness. The sensitivity of our devices was found to be at least 2 orders of magnitude higher than literature values for H(2) sensors, which can be attributed to the high surface area and the well-formed interconnect of Pd tubular nanostructures in our devices.

Ceramic electrolytes and electrochemical sensors

The electrochemical method involving solid electrolytes has been known as a selective and an accurate way of sensing chemical species in the environment and even in liquid metal for some time. The most successful among the electrochemical sensors are the emission control sensor (λ-sensor) for the automobile engine and the oxygen sensor used in steelmaking, both made of stabilized zirconia. This article presents an overview of basic principles of various types of electrochemical sensors including active (potentiometric) and passive (amperometric) sensors. Recent advances in oxygen (O2), carbon dioxide (CO2) and hydrogen (H2) sensors are also presented.

Barrier properties and failure mechanism of Ta-Si-N thin films for Cu interconnection

Cosputtered Ta–Si–N amorphous films of ten different compositions were investigated as a barrier material for Cu interconnection. The films of relatively low nitrogen content (<47 at. %) undergo an abrupt failure with the formation of tantalum silicides and copper silicide between Si and Cu during annealing. Ta43Si4N53 thin film is readily crystallized into TaNx in spite of a remarkable chemical stability with Cu. The films containing nitrogen more than 51 at. % are sacrificial barriers which show the formation of Cu3Si phase at Ta–Si–N/Cu interface even before the films crystallize to form tantalum silicide. According to electrical tests, the barriers which show the sacrificial characteristics are most effective and show no electrical degradation even after annealing at 500 °C for an hour in Si/Cu and 525 °C for an hour in SiO2/Cu metallization.

Structural and chemical stability of Ta–Si–N thin film between Si and Cu

Thermal stability and barrier performance of reactively sputter deposited Ta-Si-N thin films between Si and Cu were investigated. RF powers of Ta and Si targets were fixed and various N 2 /Ar flow ratios were adopted to change the amount of nitrogen in Ta-Si-N thin films. The structure of the films are amorphous and the resistivity increases with nitrogen content. After annealing of Si/Ta-Si-N(300 A)/Cu(1000 A) structures in Ar-H 2 (10%) ambient. sheet resistance measurement. X-ray diffraction (XRD). scanning electron microscopy (SEM). energy dispersive spectroscopy (EDS) and Auger electron spectroscopy (AES) were employed to characterize barrier performance. Cu 3 Si and tantalum silicide phase are formed at the same temperature. and the interdiffusion of Si and Cu occurs through the local defect sites. In all characterization techniques. nitrogen in the film appears to play an important role in thermal stability and resistance against Cu diffusion. A 300 A thick Ta 43 Si 4 N 53 barrier shows the excellent barrier property to suppress the formation of Cu 3 Si phase up to 800°C.

Improved TiN film as a diffusion barrier between copper and silicon

Abstract Reactively-sputtered TiN films were studied as a copper diffusion barrier in the Cu/TiN/Ti/Si and Cu/TiN/Ti/SiO2/Si multi-layer structures. From the viewpoint of the microstructure of TiN, the diffusion barrier property of TiN against copper improved when the grain boundary of TiN (as the diffusion path of copper) was extended and densified, which was confirmed by the increase of the breakdown temperature of the TiN diffusion barrier detected by various characterization methods. The 40-nm thick TiN with double deposition (extension of the grain boundary of TiN) and stuffing (the densification of the grain boundary of TiN) by RTP treatment (NH3, 600°C, 1 min) was found to be stable up to 575°C for 2 h by the C–V method.

Potentiometric CO2 gas sensor with lithium phosphorous oxynitride electrolyte

This work was supported by a grant from the National Science Foundation (EEC-9523358) with matching support from the State of Ohio and an Industrial Consortium.

High-resolution nanotransfer printing applicable to diverse surfaces via interface-targeted adhesion switching.

Nanotransfer printing technology offers outstanding simplicity and throughput in the fabrication of transistors, metamaterials, epidermal sensors and other emerging devices. Nevertheless, the development of a large-area sub-50 nm nanotransfer printing process has been hindered by fundamental reliability issues in the replication of high-resolution templates and in the release of generated nanostructures. Here we present a solvent-assisted nanotransfer printing technique based on high-fidelity replication of sub-20 nm patterns using a dual-functional bilayer polymer thin film. For uniform and fast release of nanostructures on diverse receiver surfaces, interface-specific adhesion control is realized by employing a polydimethylsiloxane gel pad as a solvent-emitting transfer medium, providing unusual printing capability even on biological surfaces such as human skin and fruit peels. Based on this principle, we also demonstrate reliable printing of high-density metallic nanostructures for non-destructive and rapid surface-enhanced Raman spectroscopy analyses and for hydrogen detection sensors with excellent responsiveness.

Effects of annealing conditions on the properties of tantalum oxide films on silicon substrates

The formation of a SiO2 layer at the Ta2O5/Si interface is observed by annealing in dry O2 or N2 and the thickness of this layer increases with an increase in annealing temperature. Leakage current of thin (less than 40 nm thick) Ta2O5 films decreases as the annealing temperature increases when annealed in dry O2 or N2. The dielectric constant vs annealing temperature curve shows a maximum peak at 750 or 800° C resulting from the crystallization of Ta2O5. The effect is larger in thicker Ta2O5 films. But the dielectric constant decreases when annealed at higher temperature due to the formation and growth of a SiO2 layer at the interface. The flat band voltage and gate voltage instability as a function of annealing temperature can be explained in terms of the growth of interfacial SiO2. The electrical properties of Ta2O5 as a function of annealing conditions do not depend on the fabrication method of Ta2O5 but strongly depend on the thickness of Ta2O5 layer.

Characterization of TiN barriers against Cu diffusion by capacitance–voltage measurement

Sputtered TiN was studied as a diffusion barrier in Cu/TiN/Ti/Si and Cu/TiN/Ti/SiO2/Si multilayer structures using various characterization methods, and their sensitivities for detecting breakdown of the barrier were compared. It was confirmed by scanning electron microscopy and Auger electron spectroscopy that breakdown of the TiN barrier occurred through out-diffusion of Si in addition to in-diffusion of Cu. Breakdown temperatures varied by more than 100 °C depending on characterization methods, and capacitance–voltage (C–V) measurement was most sensitive for detecting the failure of the TiN barrier. The effects of rapid thermal annealing (RTA) on barrier properties of TiN were investigated, and it was found by C–V measurement that the TiN(400 nm) RTA treated at 700 °C in a NH3 ambient was stable up to 590 °C for 2 h, while the reference TiN (400 nm) was stable up to 450 °C for 2 h.