This paper discusses the two-step fabrication of a novel in-plane Si-air linear variable optical filter (LVOF). LVOF has alternating quarter-wave stack layers of high refractive and low refractive index materials sandwiching a tapered cavity. Different passbands can be observed at various positions along the length of the filter. Challenges of LVOF fabrication include depositing consistent thickness of quarter-wave stacks and precise control of the taper angle to be in the range of milli-degrees. In many instances, due to the limitations of thin film deposition systems, surface roughness and deposition thickness vary across entire wafer surface. Such deviations could result in different LVOFs possessing varying response to input signal. <p> </p>Electron-beam lithography (EBL) was utilized for accurate patterning of Si pillars and taper angle which are difficult to achieve using traditional fabrication methods. In the absence of hardmask, SU-8 was used for pattern transfer with Si:SU-8 etch selectivity as high as 60:1. By optimizing SF<sub>6</sub> and C<sub>4</sub>F<sub>8</sub> gas flow and time parameters, aspect ratio of 10:1 and almost- 90° pillars were deep etched into Si with scallop depth <30 nm. High Bragg contrast mirrors were obtained with [HLH]-wedge-[HLH] configuration. <p> </p>This LVOF operates in free space with continuous tuning from 3.1-3.8 μm. FWHM of 95 nm is observed at 3.3 μm. Simulation and other characterization results are discussed. Finally, the proposed LVOF can be wafer-level packaged with normal incidence detector array, suitable light source and other essential optical elements.
This paper demonstrates an approach to vibration energy harvesting using contact electrification or triboelectric mechanism. The device uses a cantilever to realize the contact electrification process when subjected to external vibrations. The device utilizes stiffening in the cantilever beam introduced by contact between two triboelectric layers to broaden the bandwidth of the vibrational energy harvester. The operating bandwidth of the energy harvester is shown to increase from 4.4 Hz to 17.8 Hz at RMS output voltage level of 60mV. The device was also observed to demonstrate continuous improvement in bandwidth as the acceleration level increased.
We experimentally demonstrate a switchable metamaterial absorber for infrared spectral region using MEMS technology. In order to achieve active tunability; air gap is introduced as the part of dielectric spacer layer and is electrostatically actuated. As the air gap is decreased, the peak absorption wavelength will blue shift accordingly. The tuning range is approximately 700 nm for 300 nm air gap change. Complementary cross is used as the metamaterial unit cell pattern. Owing to the π/2 rotational symmetry of the metamaterial unit cell geometry and out of plane actuation direction of the metamaterial layer, the resultant absorption retains the polarization insensitive characteristics at different actuation states. Additionally high temperature stable materials such as, molybdenum and silicon-di-oxide are used as structural materials for potential use in rugged applications.
The tunable terahertz metamaterial (TTM) has attracted intense research interest, since the electromagnetic response of the metamaterial can be actively controlled through external stimulus, which is of great significance in real time applications. The active control of metamaterial characteristics is crucial in order to provide a flexible and versatile platform for mimicking fundamental physical effects. To realize the electromagnetic tunability, various approaches have been demonstrated to increase the flexibility in applications, such as changing the effective electromagnetic properties. Alternatively, MEMS-based techniques are well developed. The structural reconfiguration is a straightforward way to control the electromagnetic properties. The metamaterial properties can be directly modified by reconfiguring the unit cell which is the fundamental building block of metamaterials. Currently, our research works are focusing on MEMS-based TTM adopting stress-induced curved actuators (SICA) to adjust the resonant frequency of devices. Herein, the proposed TTM designs are double split-ring resonator (DSRR), electric split-ring resonator (eSRR), Omega-ring metamaterial (ORM), symmetric and asymmetric T-shape metamaterial (STM and ATM), respectively. We demonstrated these TTM can be active, continuous, and recoverable control the resonant frequency by using electrostatic or electrothermal actuation mechanism. Therefore, the TTM devices can be effectively used for sensors, optical switches, and filters applications.
A photonics crystal (PhC) waveguide that operates in the slow-light regime is reported in this paper. A second line of circular air holes from the PhC waveguide in a triangular lattice is replaced by a line of elliptical air holes. Based on a three-dimensional plane wave expansion, the lateral shift of elliptical air holes is conducted to enhance the slow-light characteristics. A group index of 166 and a delay-bandwidth product of 0.1812 are derived from an optimized PhC using elliptical air holes with a lateral shift of 160 nm according to the simulation results. A Mach–Zehnder interferometer (MZI) is integrated with the above-mentioned PhC waveguide with a 17 μm length in one of its arms. The measured transmission spectrum of the fabricated MZI embedded with a PhC waveguide shows slow-light interference patterns.
We present the design and the characterization of a polycrystalline silicon (Si)-based photonic crystal (PhC)-suspended membrane, working in the mid-infrared wavelengths. In order to facilitate transmission measurement, the PhC membrane is released by removing the underneath Si substrate. Around 97% reflection and 3% transmission at 3.58-μm wavelength are measured at room temperature. Characterization is also done at 450°C and it reveals that the peak reflection of the PhC membrane shifts by 75 nm to higher wavelengths. This corresponds to a linear wavelength shift of 0.174 nm/°C and the thermo-optic coefficient is calculated to be +1.70×10−4 K−1. By altering the dimension of the PhC air holes, it is also shown that such a thermo-optic effect is compensated.
The characteristics of biochemical sensors based on photonic crystal (PC) resonators are investigated in this work. The PC structure consists of holes arranged in a hexagonal lattice on a silicon slab. The nanoring resonator is formed by removing certain holes along a hexagonal trace. The hexagonal nanoring resonator is sandwiched by two PC waveguides that are formed by removing two lines of holes. The trapping of biomolecules, e.g., DNAs or proteins, in a functionalized sensing hole introduces a shift in resonant wavelength peak in the output terminal. We demonstrate two resonator designs: single and dual nanorings. The quality factor of the single nanoring resonator is 2400. The dual nanoring resonator reveals two different resonant modes. The propagated directions of dropped light for these two modes are antiparallel. The quality factors for these two resonant modes are 2100 and 1855, respectively. This dual nanoring resonator has a novel sensing mechanism, making it capable of simultaneously sensing two different biomolecules.
A novel hybrid energy harvester integrated with piezoelectric and electromagnetic energy harvesting mechanisms is investigated. It contains a piezoelectric cantilever of multilayer piezoelectric transducer (PZT) ceramics, permanent magnets, and substrate of two-layer coils. The effect of the relative position of coils and magnets on the PZT cantilever end and the poling direction of magnets on the output voltage of the energy harvester is explored. When the poling direction of magnets is normal to the coils plane, the coils yield the maximum output voltage, i.e., the type I and III devices. The maximum output voltage and power from the PZT cantilever of the type III device are 0.84 V and 176 µW under the vibrations of 2.5-g acceleration at 310 Hz, respectively. And the maximum output voltage and power from the coils are 0.78 mV and 0.19 µW under the same conditions, respectively. The power density from the type III device is derived as 790 µW/cm3 from piezoelectric components and 0.85 µW/cm3 from electromagnetic elements.
Optical microelectromechanical systems (MEMS) technology or micro-optoelectromechanical systems (MOEMS) technology has proven to be an enabling technology for many components of optical networking applications. Due to their widespread applications, MEMS variable optical attenuators (VOA) have been one of the most attractive MEMS-based key devices in the optical communication market. Micromachined shutters and refractive mirrors assembled with 2D and 3D optics are the subjects of tremendous research activity. We conduct a comprehensive literature survey with respect to technologies such as fabrication processes, optical designs, actuators, and systems. Apparently, MEMS VOA technology is still evolving into a mature technology step by step. MEMS VOA technology is not only the cornerstone to support future optical communication technology, but also the best example for understanding the evolution of MOEMS technology and the commercialization of MOEMS devices.
Indium-silver as solder materials for low temperature bonding had been introduced earlier. In theory the final bonding
interface composition is determined by the overall materials composition. Wafer bonding based multiple intermediate
layers facilitates precise control of the formed alloy composition and the joint thickness. Thus the bonding temperature
and post-bonding re-melting temperature could be easily designed by controlling the multilayer materials. In this paper, a
more fundamental study of In-Ag solder materials is carried out in chip-to-chip level by using flip-chip based
thermocompression bonding. Bonding at 180°C for various time duration under various bonding pressure is studied.
Approaches of forming Ag<sub>2</sub>In with re-melting temperature higher than 400°C at the bonding interface are proposed and
discussed. Knowledge learned in this process technology can support us to develop sophisticated wafer level packaging
process based wafer bonding for applications of MEMS and IC packages.
This paper presents design, simulation and fabrication of a wafer level packaged Microelectromechanical Systems
(MEMS) scanning mirror. In particular we emphasize on the process development and materials characterization of In-
Ag solder for a new wafer level hermetic/vacuum package using low temperature wafer bonding technology. The
micromirror is actuated with an electrostatic comb actuator and operates in resonant torsional mode. The mirror plate
size is 1.0 mm × 1.0 mm. The dynamic vibration characteristics have been analyzed by using FEM tools. With a single
rectangular torsion bar, the scanning frequency is 20 KHz. Besides, the hermetically sealed packaged is favored by
commercial applications. The wafer level package is successfully carried out at process temperature of 180°C. With
proper process design, we may lead the form a single phase of Ag<sub>2</sub>In at the bonding interface, in which it is an
intermetallic compound of high melting temperature. This new wafer level packaging approach allows us to have high
temperature stability of wafer level packaged scanning mirror devices. The wafer level packaged devices are able to
withstand the peak temperature in SMT (surface mount technology) manufacturing lines. It is a promising technology for
commercializing MEMS devices.
Silicon photonic crystal (PhC) waveguide based resonator is designed by introducing a micro-cavity within the line
defect so as to form the resonant band gap structure for PhC. Free-standing silicon beam comprising this nanophotonic
resonator structure is investigated. The output resonant wavelength is sensitive to the shape of air holes and defect length
of the micro-cavity. The resonant wavelength shift in the output spectrum is a function of force loading at the center of a
suspended beam with PhC waveguide resonator. The sensing capability of this new nanomechanical sensor is derived as
that vertical deformation is about 20nm at center and the smallest strain is 0.005% for defect length.