Jin-Seok Park

Characterization of undoped and Cu-doped ZnO films for surface acoustic wave applications

Cu-doped ZnO (denoted by ZnO:Cu) films have been prepared by RF magnetron co-sputtering of a ZnO target with some Cu-chips attached. X-Ray diffraction (XRD) spectra of deposited ZnO:Cu films were measured and texture coefficient (TC) values for (002)-orientation were estimated. Optimal ranges of RF powers and substrate temperatures for obtaining high TC values were determined. Effects of Cu-doping conditions (such as Cu-chip sputtering areas and O2/(Ar+O2) mixing ratios) on TC values, electrical resistivities, and relative Cu-compositions of deposited films have been systematically investigated. X-Ray photoelectron spectroscopy (XPS) study suggests that the relative densities of metallic copper (Cu0) atoms and CuO (Cu2+)-phases within deposited films may play an important role in determining their electrical resistivities. Highly resistive (>1010 Ωcm) ZnO films with high TC values (>80%) can be achieved by Cu-doping. Surface acoustic wave (SAW) devices with ZnO:Cu (or ZnO)/interdigital transducer (IDT)/SiO2/Si configuration were also fabricated to estimate the effective electro-mechanical coupling coefficient (keff2) and insertion loss. The devices using Cu-doped ZnO films have higher keff2 and lower insertion loss, compared with those using undoped films.

Experimental and theoretical investigation on the relationship between AlN properties and AlN-based FBAR characteristics

Film bulk acoustic resonators (FBARs) with an Al/AlN/Mo/Si (111) configuration are fabricated. In particular, the effects of deposition conditions on material properties of AlN films grown on Mo/Si substrate as well as the performance of FBARs are studied. Piezoelectric AlN films are deposited using RF magnetron sputtering at RF power=250 W/spl sim/600 W, N/sub 2//Ar ratio=5/25/spl sim/25/5, working pressure=5 mTorr, substrate temperature=250/spl deg/C. For all the deposited AlN films, the X-ray diffraction (XRD) spectra and full width at half maximum (FWHM) of rocking curves are measured in terms of the deposition conditions, to characterize the c-axis preferred orientation and crystal quality. The frequency response characteristics (S/sub 11/) of the fabricated FBARs are also measured. The experimental results indicate that the characteristics of FBARs can be determined by the material properties of the AlN films. Furthermore, the theoretical relation ship between the impedance parameters (R/sub m/) of the BVD model and the AlN properties has been established.

A novel method of fabricating ZnO/diamond/Si multilayers for surface acoustic wave (SAW) device applications

Abstract A novel process for fabricating ZnO/diamond/Si for a surface acoustic wave device is as follows: to form a trench of 10 μm in depth, the Si wafer is chemically etched by employing the SiO2 layer as a mask. Selective growth of polycrystalline diamond film is carried out by a microwave plasma CVD using nominal conditions of 700 W microwave power, 40 torr pressure, 0.5% CH4/H2 ratio, 630 °C temperature, and −200 V bias enhancement for initial nucleation. After removing the SiO2 layer, indirect bonding of a Si handle wafer is performed at low temperatures of approximately 90 °C. Finally, the bottom Si wafer is mechano-chemically polished until the surface (backside) of the diamond is exposed. Raman and field emission SEM observations show that a high quality diamond film is selectively grown only on the trenched Si region. It has also been found from the AFM results that the backside surface roughness of the exposed diamond film is measured to be lower than 10 nm.

Effects of lattice mismatches in ZnO/substrate structures on the orientations of ZnO films and characteristics of SAW devices

Polycrystalline ZnO films are deposited using RF magnetron sputtering on various substrate materials including AlN/Si-(111), sapphire, DLC/Si-(100), SiO2/Si-(100) and Si-(111). The structural parameters of deposited ZnO films, such as texture coefficient (TC) value for (002)-orientation, crystallite size and full-width at half-maximum (FWHM) at ZnO (002)-peak, are compared. Surface acoustic wave (SAW) devices are also fabricated by using a lift-off method, with the configuration of IDT/ZnO/substrate. The frequency response characteristics (including S21) of the fabricated SAW devices are measured and the device parameters, such as insertion losses and side-lobe rejection level, are estimated in terms of the substrate materials. Experimental results indicate that the lattice matching as well as the structural similarity between ZnO and substrate may be essential for determining the SAW device characteristics.

Roughness control of polycrystalline diamond films grown by bias-enhanced microwave plasma-assisted CVD

Abstract A simple growth technique to control the surface roughness of polycrystalline diamond films is proposed. The films are grown using a microwave plasma-assisted chemical vapor deposition method, with varying the methane (CH 4 ) concentration at the stage of bias-enhanced nucleation. It is found from the field-emission scanning electron microscope spectra that nanocrystalline diamond nuclei are formed at a relatively high methane concentration, causing a secondary nucleation at the accompanying growth step. The RMS value of surface roughness for grown films, which is estimated from the atomic force microscope images, monotonically decreases from 165 to 53 nm with methane concentration. It is also observed that the surface roughness is closely related to the nucleation density. In addition, employing a two-step growth method, which consists of first-growth at 500 W and subsequent second-growth at 800 W, enables the as-grown polycrystalline diamond film to have a smaller roughness of approximately 46 nm. This is believed to be due to the secondary nucleation effect induced during the growth step.

Electron-emission from nano- and micro-crystalline diamond films: the effects of nitrogen and oxygen additives

Abstract Diamond films are grown on Si substrate by microwave plasma CVD using CH 4 +H 2 (for undoped) and additive N 2 (for nitrogen-incorporated) with/without O 2 as precursors. Crystal structures for grown films, such as micro- and nano-crystalline and surface morphologies are characterized in terms of growth condition by Raman and field-emission SEM, respectively. Cathodoluminescence (CL) spectra are monitored to identify the nitrogen-incorporation in grown diamond films. Relative intensity ratios of nitrogen-related band to band-A (denoted by I N / I A ) are also estimated from the CL characteristics and the influence of additive N 2 and O 2 precursors on the I N / I A ratio is analyzed. For all-grown films, electron-emission characteristics are measured, from which threshold fields for the emission are also estimated. Observed emission properties are correlated with crystal structures and morphologies obtained from grown films by considering the structural transformation from micro- to nano-crystalline as well as the nitrogen-induced defect states.

Mesenchymal Stem Cells Modulate the Functional Properties of Microglia via TGF‐β Secretion

The regulation of microglial cell phenotype is a potential therapeutic intervention in neurodegenerative disease. Previously, we reported that transforming growth factor‐β (TGF‐β) levels in mesenchymal stromal cells (MSCs) could be used as potential biological markers to predict the effectiveness of autologous MSC therapy in patients with amyotrophic lateral sclerosis. However, the underlying mechanism of TGF‐β in MSCs was not fully elucidated in determining the functional properties of microglia. In this study, we aimed to clarify the role of TGF‐β that is involved in MSC effectiveness, especially focusing on microglia functional properties that play a pivotal role in neuroinflammation. We found that MSC‐conditioned media (MSC‐CM) inhibited proinflammatory cytokine expression, restored alternative activated microglia phenotype markers (fractalkine receptor, mannose receptor, CD200 receptor), and enhanced phagocytosis in lipopolysaccharide (LPS)‐stimulated microglia. In addition, TGF‐β in MSC‐CM played a major role in these effects by inhibiting the nuclear factor‐κB pathway and restoring the TGF‐β pathway in LPS‐stimulated microglia. Recombinant TGF‐β also induced similar effects to MSC‐CM in LPS‐stimulated microglia. Therefore, we propose that MSCs can modulate the functional properties of microglia via TGF‐β secretion, switching them from a classically activated phenotype to an inflammation‐resolving phenotype. The latter role may be associated with the inhibition of neuroinflammatory processes in neurodegenerative disorders.

A Two-Mask Process for Fabrication of Bottom-Gate IGZO-Based TFTs

A simple process is presented with which a bottom-gate-type oxide thin-film transistor (TFT) can be fabricated by using two photomasks. The active channel, the source-drain electrode, and the pixel electrode layers were simultaneously formed via a single photolithography using a gray-tone mask (GTM). In particular, the gray-tone profiles of the photoresist were carefully observed to ensure process feasibility with the GTM. From the transparent-oxide TFTs fabricated in this letter, functional indices, such as threshold voltage <i>V</i>_T = 4.13 V (at <i>V</i>_DS = 10 V), subthreshold swing S = 0.59 V/dec, field-effect mobility μ_FE = 12.41 cm²/V·s, on-off current ratio lesser than 8 ×10⁶, and transmittance higher than 85%, were obtained.

Improved Resonance Characteristics by Thermal Annealing of W/SiO2 Multi-Layers in Film Bulk Acoustic Wave Resonator Devices

In this paper, we, for the first time, present the effects of the thermal annealing of the W/SiO2 multi-layer Bragg reflectors on the resonance characteristics of the ZnO-based film bulk acoustic wave resonator (FBAR) devices. In order to improve the resonance characteristics of the FBAR devices, we employed a thermal annealing process after the Bragg reflectors were formed on a silicon substrate using a radio frequency (RF) magnetron sputtering technique. As a result, the resonance characteristics of the FBAR devices were observed to strongly depend on the annealing conditions applied to the Bragg reflectors. The FBAR devices with the Bragg reflectors annealed at 400°C/30 min showed excellent resonance characteristics as compared to those with the non-annealed (as-deposited) Bragg reflectors. The newly proposed simple thermal annealing process will be very useful to more effectively improve the resonance characteristics of the future FBAR devices with multi-layer Bragg reflectors.

Identification of mutations in Korean patients with amyotrophic lateral sclerosis using multigene panel testing.

Amyotrophic lateral sclerosis (ALS) is a rapidly progressive neurodegenerative disease involving motor neurons. Because a growing number of genes have been identified as the genetic etiology of ALS, simultaneous screening of mutations in multiple genes is likely to be more efficient than gene-by-gene testing. In this study, we performed a multigene panel testing by using targeted capture of 18 ALS-related genes followed by next-generation sequencing. Using this technique, we tried to identify mutations in 4 index patients with familial ALS and 148 sporadic ALS in Korean population and identified 4 known mutations in SOD1, ALS2, MAPT, and SQSTM1 genes, respectively, and 28 variants of uncertain significance in 9 genes. Among the 28 variants of uncertain significance, 6 missense variants were found in highly conserved residues and were consistently predicted to be deleterious by in silico analyses. These results suggest that multigene panel testing is an effective approach for mutation screening in ALS-related genes. Moreover, the relatively low frequency of mutations in known ALS genes implies marked genetic heterogeneity at least in Korean patients with ALS.