Nima Ghalichechian

Design, Fabrication, and Characterization of a Rotary Micromotor Supported on Microball Bearings

We report the design, fabrication, and characterization of a rotary micromotor supported on microball bearings. This is the first demonstration of a rotary micromachine with a robust mechanical support provided by microball-bearing technology. A six-phase bottom-drive variable-capacitance micromotor (Phi = 14 mm) is designed and simulated using the finite-element (FE) method. The stator and the rotor are fabricated separately on silicon substrates and assembled with the microballs. Three layers of low-k benzocyclobutene polymer, two layers of gold, and a silicon microball housing are fabricated on the stator. Microball housing and salient structures (poles) are etched in the rotor and are coated with a silicon carbide film that reduces the friction without which the operation was not possible. A top angular velocity of 517 r/min, corresponding to the linear tip velocity of 324 mm/s, is measured at plusmn150-V and 800-Hz excitation. This is 44 times higher than the velocity previously demonstrated for linear micromotors supported on the microball bearings. A noncontact method is developed to extract the torque and the bearing coefficient of friction through dynamic response measurements. The torque is indirectly measured to be -5.62 plusmn 0.5 muN ldr m at plusmn150-V excitation which is comparable with the FE simulation results predicting -6.75 muN ldr m. The maximum output mechanical power at plusmn150 V and 517 r/min was calculated to be 307 muW. The bearing coefficient of friction is measured to be 0.02 plusmn 0.002 which is in good agreement with the previously reported values. The rotary micromotor developed in this paper is a platform technology for centrifugal micropumps used for fuel-delivery and cooling applications.

Microball-bearing-supported electrostatic micromachines with polymer dielectric films for electromechanical power conversion

This paper presents our latest research activities toward the development of electrostatic micromotors/microgenerators with a microball-bearing support mechanism and benzocyclobutene (BCB) low-k polymer insulating layers. The primary applications of these devices are long-range, high-speed micropositioning, high-speed micro pumping and micro power generation. In this paper, we present the development of the first generation of microball-bearing-supported micromachines. This device is a 6-phase, bottom-drive, linear, variable-capacitance micromotor. The design and fabrication of the linear micromotor, and characterization of the motor capacitance, force and motion in 3-phase and 6-phase excitation modes are presented. The micromotor consists of a silicon stator, a silicon slider and four stainless-steel microballs. The aligning force profile of the micromotor was extracted from simulated and measured capacitances of all phases. An average total aligning force of 0.27 mN with a maximum of 0.41 mN at 100 V dc was measured. The ac operation of the micromotor was verified by applying square-wave voltages and characterizing the slider motion. An average slider speed of 7.32 mm s −1 at 40 Hz and 120 V P-P was reached without losing the synchronization. The design, fabrication and characterization methods presented in this paper can be used as a technology platform for developing rotary micromachines. (Some figures in this article are in colour only in the electronic version)

Integration of benzocyclobutene polymers and silicon micromachined structures using anisotropic wet etching

Integration of thick, low-k dielectric benzocyclobutene (BCB) film with deep etched structures in silicon allows the fabrication of microelectromechanical systems (MEMS) devices with low parasitic loss. A fabrication process is developed for integration of 1-μm-thick BCB low-k dielectric film and 200-μm-deep anisotropically etched grooves in silicon with potassium hydroxide (KOH). In order to protect the low-k film during the highly corrosive, long, high-temperature KOH etching process, gold (Au) is used as an etch mask. Chromium (Cr) is used to improve the adhesion of Au to the underlying BCB layer. Metal-BCB adhesion is the key parameter in this masking design. Partial cure of BCB at 210 °C for 40 min with appropriate surface treatment (adhesion promoter) prior to metallization and full cure at 250 °C for 1 h after metallization, together with Cr/Au sputtering at 200 °C, improves the adhesion dramatically. The adhesion strength of metal films to BCB was experimentally verified in a qualitative manner. V...

Permittivity and Loss Characterization of SU-8 Films for mmW and Terahertz Applications

The dielectric permittivity measurement of thick SU-8 film is presented for the entire frequency band of 1 GHz to 1 THz. SU-8 is a high-resolution UV-patternable photoresist that can be used for fabrication of high-aspect-ratio 3-D structures for millimeter-wave and terahertz devices. Here, we report the measured dielectric constant and loss tangent of SU-8 films using terahertz time-domain spectroscopy. A quadratic polynomial model is established for the accurate calculation of complex permittivity up to 1 THz. The loss tangent of fully cross-linked 430- μm-thick SU-8 film was measured to be 0.015, 0.027, and 0.055 at 1, 200, and 1000 GHz, respectively. Similarly, relative permittivity was found to be 3.24, 3.23, and 2.92. The fabrication process and level of cross-linking were demonstrated to have significant impact on the loss behavior of this material and the impact of cross-linking on dielectric permittivity is quantified across a wide frequency band. The characterization results reported in this letter are a platform for developing next-generation millimeter-wave and terahertz devices.

Electrical characterization of benzocyclobutene polymers for electric micromachines

An approach using interdigitated capacitors for electrical characterization of CYCLOTENE, a spin-on low-k benzocyclobutene (BCB)-based polymer is introduced and the effect of moisture uptake is investigated. The dielectric constant of CYCLOTENE is extracted from capacitance measurements with a systematic error less than 0.1%, giving an average value of 2.49 with a standard deviation of 1.5%. The dielectric constant increases by 1.2% after a humidity stress of 85% RH at 85/spl deg/C. The I-V characteristics of CYCLOTENE show a dependency of breakdown strength and leakage current on the geometrical dimensions of the device under test. A breakdown strength of 225V//spl mu/m and 320 V//spl mu/m for 2-/spl mu/m and 3-/spl mu/m finger spacing, respectively, and a leakage current of a few to tens of pA are measured. The I-V characteristics degrade drastically after the humidity stress, showing a breakdown strength of 100 V//spl mu/m and 180 V//spl mu/m for 2-/spl mu/m and 3-/spl mu/m finger spacing, respectively, and a maximum increase in the leakage current as large as one order of magnitude. The maximum performance and long-term reliability of an electric micromachine are adversely affected by the degradation of the breakdown voltage and the leakage current after moisture absorption. It is expected, however, that the electrical efficiency is improved using BCB-based polymers with negligible dependency on moisture absorption.

Ultrawideband Array With 70° Scanning Using FSS Superstrate

We present a wideband tightly coupled dipole array (TCDA) with integrated balun and a novel superstrate consisting of printed frequency selective surface (FSS) for wide angle scanning. Although previous TCDAs have had decent scanning performance up to ±60°, use of dielectric superstrates are usually required, resulting in additional cost and fabrication complexity. In this paper, we replace the bulky dielectric layer(s) with periodic printed elements and yet achieve wide-angle and wideband impedance matching. The proposed approach provides superior performance of 6.1:1 bandwidth (0.5-3.1 GHz) with VSWR <; 3.2 when scanning ±75° in E plane, ±70° in D plane and ±60° in H plane. The FSS, radiating dipoles and feed lines are designed and fabricated on the same vertically oriented printed circuit board, resulting in a low-cost and lightweight structure as compared to other low profile arrays. Measured scanning patterns of a 12 × 12 prototype are presented, showing good agreement with simulations.

Vision-Based Microtribological Characterization of Linear Microball Bearings

Microball bearings can potentially provide robust and low friction support in micromachines such as micromotors and microgenerators. Their microtribological behavior needs to be investigated for design and control of such micromachines. In this paper a vision-based, non-intrusive measurement method is presented for characterization of friction in linear microball bearings. Infrared imaging is used to directly observe the dynamics of microballs and track the motion of bearing components. It is verified that microballs roll most of the time with occasional sliding or bumping resulting from fabrication nonuniformity. The friction-velocity curve demonstrates evident hysteresis. The dependence of frictional behavior on several factors is studied.Copyright

A microball-bearing-supported linear electrostatic micromotor with benzocyclobutene polymer insulating layers

We report for the first time the successful fabrication and operation of a 6-phase, bottom-drive, linear, variable-capacitance micromotor (B-LVCM) using microball bearings and benzocyclobutene (BCB) polymer insulating layers. The device consists of a silicon stator, a silicon slider, and four stainless-steel microballs. The aligning force profiles were extracted from simulated and measured capacitance with a maximum of 0.17 mN per phase at 100 V. The average slider speed at frequencies of 10 Hz and 20 Hz was measured to be 1.82 mm/s and 3.56 mm/s, which is in good agreement with the theoretical value of 1.80 mm/s and 3.60 mm/s, respectively.

A Rotary Micromotor Supported on Microball Bearings

We report the successful development of the first operational bottom-drive, rotary, variable- capacitance micromotor supported on microball bearings. The rotor and stator of the micromotor were fabricated using a 9-level-mask process. Silicon carbide coating was used for reducing friction of silicon microball housing on the rotor. Top angular velocity of 517 rpm corresponding to the linear tip velocity of 324 mm/s was measured at 150 V and 800 Hz excitation. This is 44 times higher than the velocity previously measured for the linear micromotors supported on microball bearings. Characterization of two different designs showed a good agreement between predicted and measured velocities for the rotary machine. The rotary micromotor developed in this study is a platform technology for centrifugal micropumps for fuel delivery applications.

Bandwidth reconfigurable THz filter employing phase-change material

We propose a bandwidth reconfigurable spatial filter using frequency selective surfaces (FSS) combined with phase-change materials (PCMs), vanadium dioxide (VO2) for advanced terahertz (THz) applications. The FSS filter is simulated using sapphire as substrate and employs circular loops FSS with VO2 rings in between. Simulations of this filter show bandwidth difference in the frequency range of 0.4 - 0.75 THz. As a demonstration, we tested a reconfigurable FSS geometry and observed a 15% in bandwidth change as per design.