Keekeun Lee

A novel wireless, passive CO2 sensor incorporating a surface acoustic wave reflective delay line

A 440 MHz wireless and passive surface acoustic wave (SAW) chemical sensor was developed for CO2 detection. The developed SAW gas sensor is composed of single phase unidirectional transducers (SPUDTs), three shorted grating reflectors, and CO2-sensitive polymer film on 41° YX LiNbO3 substrate. Coupling of modes (COM) modeling was used to find optimal design parameters. Using the extracted design parameters, the SAW device was fabricated. Teflon AF 2400 was used as the sensitive film because it provides high CO2 solubility, permeability and selectivity. In wireless device testing using a network analyzer, four sharp reflection peaks with high signal-to-noise (S/N) ratio, small signal attenuation, and few spurious peaks were observed in the time domain. The time positions of the reflection peaks were well matched with the predicted values from the simulation. Infusion of CO2 into the chamber induced large phase shifts of the reflection peaks. Good linearity and repeatability were observed for a CO2 concentration of 0–450 ppm. The obtained sensitivity was 1.98° ppm−1. Temperature and humidity effects were also investigated during the sensitivity evaluation process.

A novel 440 MHz wireless SAW microsensor integrated with pressure?temperature sensors and ID tag

This paper presents the development of a 440 MHz range surface acoustic wave (SAW)-based microsensor integrated with pressure–temperature sensors and ID tag. Two piezoelectric substrates were bonded, in which a ~150 µm air gap was structured by metal poles. The pressure sensor was placed on the top substrate, whereas the ID tag and temperature sensor were located on the bottom substrate. Coupling of modes (COM) modeling was used to find optimal design parameters. Using the extracted optimal design parameters, the SAW device was fabricated. In wireless device testing using a network analyzer, sharp reflection peaks with high S/N ratio, small signal attenuation and small spurious peaks were observed in the time domain. All the reflection peaks were well matched with the predicted values from the simulation. With 10 mW RF power from the network analyzer, a ~1 m readout distance was observed. Depending on applied external pressure, the phase shifts of the reflection peaks were linearly varied. The evaluated sensitivity was about ~2.9° kPa−1. Eight sharp ON reflection peaks were observed for the ID tag. The temperature sensor was characterized from 20 °C to 200 °C. A large phase shift per unit temperature change was observed.

Broadband-absorbing hybrid solar cells with efficiency greater than 3% based on a bulk heterojunction of PbS quantum dots and a low-bandgap polymer

Currently existing common conjugated polymer:PbS quantum dot (QD)-based hybrid bulk heterojunction (BHJ) solar cells show efficiencies of less than 1% owing to improper bandgap alignment and poor coupling at the hybrid material interfaces. However, herein we report that PbS QD-based hybrid BHJ solar cells provide greatly increased efficiencies of more than 3%, which is attributed to the employment of a new kind of donor polymer poly[2,6-(4,4′-bis(2-ethylhexyl)dithieno[3,2-b:2′,3′-d]silole)-alt-4,7(2,1,3-benzothiadiazole) (PSBTBT), the optimization of donor–acceptor (D–A) band-offsets by strategically changing the diameter of PbS QDs in the donor matrix, and the modification of hybrid material interfaces via a chemical treatment of ligands around the QD surface. The optimized hybrid nanocomposite device performs well in good combination between the low-bandgap polymer and near-infrared (NIR)-absorbing PbS QDs, and it enables a broad spectral response from UV to NIR under an energetically favorable type-II heterojunction system, leading to high efficiencies of up to 3.48% under an air mass 1.5G illumination. The efficiency is higher than 3.39%, which corresponds to the efficiency value for the purely organic device fabricated in this study by utilizing [6,6]-phenyl-C71-butyric acid methyl ester (PCBM), which is the most widely studied electron acceptor in BHJ systems. These findings suggest that our hybrid BHJ blends are very promising, not only for use as energy conversion platforms to substitute all-organic or all-inorganic systems, but also for optoelectronic devices that require a broad spectral reaction.

Biocompatible benzocyclobutene-based intracortical neural implant with surface modification

This paper presents the fabrication of a benzocyclobutene (BCB) polymer-based intracortical neural implant for reliable and stable long-term implant function. BCB polymer has many attractive features for chronic implant application: flexibility, biocompatibility, low moisture uptake, low dielectric constant and easy surface modification. A 2 µm thick silicon backbone layer was attached underneath a flexible BCB electrode to improve mechanical stiffness. No insertion trauma was observed during penetrating into the dura of a rat. In vitro cytotoxicity tests of the completed BCB electrode revealed no toxic effects on cultured cells. The modified BCB surface with a dextran coating showed a significant reduction in 3T3 cell adhesion and spreading, indicating that this coating has the potential for lowering protein adsorption, minimizing inflammatory cell adhesion and glial scar formation in vivo, and thereby enhancing long-term implant performance.

Development of a high-sensitivity strain measurement system based on a SH SAW sensor

A strain measurement system based on a shear horizontal surface acoustic wave (SH SAW) was developed. The developed system is composed of a SAW microsensor, a printed circuit board (PCB), an adhesive and a strain gauge. When a compression force is applied to the PCB by the strain gauge, the PCB is bent so that external strain energy can be evenly delivered to the microsensor without any detachment of the sensor from the board. When a stretching force is applied to the PCB under the condition that one side of the PCB is fixed and the other side is modulated, the actual length of the SAW delay line between the two interdigital transducers (IDTs) is increased. The increase in the delay line length causes a change in the time for the propagating SAW to reach the output IDT. If strain energy is applied to the piezoelectric substrate, the substrate density is changed, which then changes the propagation velocity of the SAW. Coupling-of-modes modeling was conducted prior to fabrication to determine the optimal device parameters. Depending on the strain, the frequency difference was linearly modulated. The obtained sensitivity for stretching was 17.3 kHz/% for the SH wave mode and split electrode. And the obtained sensitivity for bending was 46.1 kHz/% for the SH wave mode and split electrode. The SH wave showed about 15% higher sensitivity than the Rayleigh wave, and the dog-bone PCB showed about 8% higher sensitivity than the rectangular PCB. The obtained sensitivity was about five times higher than that of existing SAW-based strain sensors.

Enhanced Sensitivity of Novel Surface Acoustic Wave Microelectromechanical System-Interdigital Transducer Gyroscope

In this paper, we present a novel microelectromechanical system-interdigital transducer (MEMS-IDT) surface acoustic wave (SAW) gyroscope with an 80 MHz central frequency on a 128 YX LiNbO3 wafer. The developed MEMS-IDT gyroscope is composed of a two-port SAW resonator, a dual delay line oscillator, and metallic dots. The SAW resonator provides a stable standing wave, and the vibrating metallic dot at an antinode of the standing wave induces the second SAW in the normal direction of its vibrating axis. The dual delay line oscillator detects the Coriolis force by comparing the resonant frequencies between two oscillators through the interference effect. The coupling of mode (COM) modeling was used to extract the optimal design parameters prior to fabrication. In the electrical testing by the network analyzer, the fabricated SAW resonator and delay lines showed low insertion loss and similar operation frequencies between a resonator and delay lines. When the device was rotated, the resonant frequency differences between two oscillators linearly varied owing to the Coriolis force. The obtained sensitivity was approximately 119 Hz deg 1 s 1 in the angular rate range of 0 –1000 deg/s. Satisfactory linearity and superior directivity were also observed in the test. # 2009 The Japan Society of Applied Physics DOI: 10.1143/JJAP.48.06FK09

The development of a wireless Love wave biosensor on 41° YX LiNbO3

This paper presents a novel wireless Love-wave-based biosensor using a polymethyl methacrylate (PMMA) waveguide and protein A receptor layers on a 41? YX LiNbO3 piezoelectric substrate for immunoglobulin G (IgG) detection. A 440?MHz reflective delay line composed of single-phase unidirectional transducers (SPUDTs) and three shorted grating reflectors was fabricated as the sensor element. A theoretical modeling was performed to describe the wave propagation of Love wave devices on a 41? YX LiNbO3 piezoelectric substrate with large piezoelectricity. The fabricated devices were wirelessly characterized by using the network analyzer as the reader unit. The resultant reflection peaks showed large signal/noise ratio, sharp peaks, and few spurious signals. The binding of the IgG to the protein A receptor layer induced large phase shifts of the reflection peaks due to the mass loading effect. Good linearity, reproducibility, and high sensitivity were observed in the IgG concentration range 1?65?nM. Unique advantages such as high sensitivity and a simple wireless measurement method over other currently available biosensors are also presented.

Surface Acoustic Wave Based Pressure Sensor with Ground Shielding over Cavity on 41° YX LiNbO3

A surface acoustic wave (SAW)-based pressure sensor was fabricated for stable mechanical compression force measurement. A single phase unidirectional transducer (SPUDT) and two acoustic tracks were employed to minimize inherent insertion loss and improve reflectivity from the reflectors. The coupling of modes (COM) theory and finite element methods (FEMs) were used to determine optimal design parameters. A LiNbO3 diaphragm was bonded to a heavily doped silicon substrate with a cavity of ~250 µm deep, in which gold was lined all over the inner cavity to reduce the coupling loss of SAW energy to the surrounding atmosphere. As a mechanical compression force was applied to the diaphragm, the diaphragm bent, resulting in phase shifts of the reflected peaks. The phase shifts were modulated depending on the amount of mechanical compression applied. The measured reflection coefficient S11 showed good agreement with simulated results.

MEMS spring probe for non-destructive wafer level chip test

MEMS spring probes were fabricated for non-destructive testing of ultra-small pad-pitch chips. The probes had high suspension from the bottom planar surface, uniformity, high elastic spring constant and low contact resistivity. All contacts took place simultaneously. The contact area at the tip was 70 µm2. Fabricated probes showed great spring properties when compression force was applied from the top glass substrate. No noticeable physical markings were observed on the pads over a known range of compression. At the contact resistivity test, as the compression force was increased, the contact resistivity was decreased due to increase of contact surface area. Low RF signal loss was observed through metal probes due to low resistive loss and capacitive parasitics. Measured scattering parameter S11 was approximately −50 dB and S21 was −0.5 dB up to 30 GHz. Inductance and capacitance of the MEMS probe were negligible.

Development of Hybrid Photovoltaic Cells by Incorporating CuInS2 Quantum Dots into Organic Photoactive Layers

Hybrid photovoltaic (PV) cells containing CuInS2 quantum dots (CIS QDs) incorporated into organic PV layers were developed through a simple solution process. Four types of such cells were fabricated by blending poly(3-hexylthiophene-2,5-diyl):[6,6]-phenyl C61 butyric acid methyl ester (P3HT:PCBM) and CIS QDs in weight ratios of 1:1:0, 1:1:0.2, 1:1:0.5, and 1:1:1. The hybrid PV cell with CIS QDs exhibited increased light absorption and greater quantum efficiency than the purely organic PV cell. In cell performance tests under an air mass (AM) 1.5 solar simulator, the short-circuit current of the hybrid cells with CIS QDs reached up to 10.12 mA/cm2; the corresponding power conversion efficiency was 2.76%, which was 13% higher than that of purely organic PV cells. This indicates that the CIS QDs function as an additional light absorber encouraging device performance improvement.