Q. X. Zhang

Effects of surface roughness on electromagnetic characteristics of capacitive switches

This paper studies the effect of surface roughness on up-state and down-state capacitances of microelectromechanical systems (MEMS) capacitive switches. When the root-mean-square (RMS) roughness is 10 nm, the up-state capacitance is approximately 9% higher than the theoretical value. When the metal bridge is driven down, the normalized contact area between the metal bridge and the surface of the dielectric layer is less than 1% if the RMS roughness is larger than 2 nm. Therefore, the down-state capacitance is actually determined by the non-contact part of the metal bridge. The normalized isolation is only 62% for RMS roughness of 10 nm when the hold-down voltage is 30 V. The analysis also shows that the down-state capacitance and the isolation increase with the hold-down voltage. The normalized isolation increases from 58% to 65% when the hold-down voltage increases from 10 V to 60 V for RMS roughness of 10 nm.

A nanomachined optical logic gate driven by gradient optical force.

In this letter, a nanomachined optical logic gate using optical gradient force is demonstrated. The device consists of a partially free-hanging silicon double-ring resonator developed by the nano-electro-mechanical system technology. The logic NOR gate function is demonstrated at 20 Mb/s with a high extinction ratio of about 21.3 dB. This proposed NOR gate has the advantages of low power consumption (∼0.5 mW), highly compacted size (40 μm × 45 μm), and easy batch fabrication which has potential applications in silicon-photonic integration for digital signal processing.

Characterization and optimization of dry releasing for the fabrication of RF MEMS capacitive switches

This paper discusses fabrication aspects of photoresist sacrificial layers for fabricating metal bridges of capacitive radio frequency (RF) microelectromechanical systems (MEMS) switches. First, reflow of the photoresist layer after lithography is investigated for reducing mechanical fracture of the metal layer by smoothing the edges of the sacrificial layer. Second, the dry-etch releasing process of the structures in an O2 plasma has been investigated by identifying suitable etching parameters. The findings in this paper reveal that the mechanical performance of the released bridges strongly depends on the etch parameters. It is shown that especially the etching power affects the mean stress and the stress gradient in the bridge, which results in buckling and deformed bridge shape for an etching power above 500 W, drastically increasing the actuation voltage and reducing the down-state capacitance. Finally, the paper presents a suitable parameter set for the release etching of capacitive MEMS metal bridges.

High-performance inductors on plastic substrate

Wafer-transfer technology (WTT) has been applied to transfer RF inductors from a silicon wafer to an opaque plastic substrate (FR-4). By completely eliminating silicon substrate, the high performance of integrated inductors (Q-factor > 30 for inductance /spl sim/3 nH with resonant frequency /spl sim/23 GHz) has been achieved. Based on the analysis of a modified /spl pi/-network model, our results suggest that the performance limitation is switched from being a synthetic mechanism of substrate and metal-ohmic losses on low resistivity Si-substrate to merely a metal-ohmic loss on FR-4. Thus, the inductor patterns, which are optimized currently for RFICs on silicon wafer, can be further optimized to take full advantage of the WTT on new substrate from the newly obtained design freedom.

Successful transferring of active transistors, rf-passive components and high density interconnect from bulk si to organic substrates

A thermal bonding based wafer-transfer-technology (WTT) has been developed and successfully applied to transfer active devices, RF-passive components and high density interconnect pre-fabricated by CMOS technology onto a flexible organic substrate. The intrinsic performances of transistors and interconnect are preserved with excellent reliability, and the RF-passive components are clearly demonstrated in much enhanced RF characteristics

A miniature tunable coupled-cavity laser constructed by micromachining technology

This letter presents a miniature tunable coupled-cavity laser by integrating a Fabry-Perot chip, a gain chip and a deep-etched parabolic mirror using micromachining technology. The mirror is to actively adjust the gap between chips, enabling the optimal mode selection. Single-mode operation with a tuning range of 16.55nm and a side-mode-suppression ratio of >25.1dB is demonstrated. The device overcomes phase mismatching and instability problems encountered in conventional fixed-gap coupled-cavity lasers.

A micromachined tunable coupled-cavity laser for wide tuning range and high spectral purity.

This paper presents the design and experimental study of a coupled-cavity laser based on the micromachining technology for wide tuning range and improved spectral purity. The core part of this design utilizes a deep-etched movable parabolic mirror to couple two identical Fabry-Pérot chips and thus allows the active adjustment of the cavity gap so as to optimize the mode selection and to increase the tuning range as well. In experiment, the laser achieves the single longitudinal mode output over 51.3 nm and an average side-mode-suppression ratio of 22 dB when the tuning current varies from 5.7-10.8 mA. The measured wavelength tuning speed is 1.2 micros and the single mode output is stable at any wavelength when the tuning current is varied within +/- 0.06 mA. Compared with the conventional fixed cavity gap coupled-cavity lasers, such design overcomes the phase mismatching and mode instability problems while maintaining the merit of high-speed wavelength tuning using electrical current.

RF MEMS switch integrated on printed circuit board with metallic membrane first sequence and transferring

This letter reports, for the first time, on RF MEMS switches integrated on flexible printed circuit boards (i.e., FR-4) using transfer technology. The devices were first processed on Si-substrate using a modified MEMS sequence and subsequently transferred onto an FR-4 substrate by thermal compressive bonding, mechanical grinding, and wet removal of silicon. The switches were demonstrated with flat metal membrane (top electrode), precisely controlled gap between the membrane and bottom electrode, low insertion loss (/spl les/ 0.15 dB at 20 GHz), and high isolation (/spl sim/ 21 dB at 20 GHz). This technology shows the potential to monolithically integrate RF MEMS components with other RF devices on organic substrate for RF system implementation.

Evaluation of Stresses in Thin Device Wafer using Piezoresistive Stress Sensor

In this work, piezoresistive stress sensors have been used to evaluate the stresses in thin device wafer. For the evaluation, device wafers having piezoresistive stress sensors were fabricated. The stress sensors were then calibrated to determine the piezoresistive coefficients. The evaluation of stresses in device wafer was carried out after thinning the device wafers to three different thicknesses ranging from 400 ¿m to 100 ¿m. The thinning process was performed with the help of commercial wafer back-grinding machine and the complete thinning process included rough grinding, then fine grinding, and finally polishing. It is found that wafer back-grinding of device wafer generates a large amount of compressive stress at the device wafer surface and the amount of stress increases exponentially with the decrease in wafer thickness. The stresses were also evaluated after mounting the thin device wafers on dicing tape. It is found that the mounting on dicing tape generates tensile stress at the device wafer surface. These trends of stresses in the thin device wafers were confirmed with the bending profile of thin device wafers. A detailed explanation for the development of stresses in the thin device wafer is provided in the paper.

Tunable dual-wavelength laser constructed by silicon micromachining

This paper presents a tunable dual-wavelength laser by integration of a semiconductor gain chip with silicon-micromachined grating and mirrors onto a silicon substrate. The wavelength tuning is demonstrated by rotating the micromirror. With one wavelength being tuned and the other fixed, the laser output presents a tunable spectral separation from −28.38to+24.18nm. The laser output reaches 2.9mW with far-field divergences of 37° and 30° in the vertical and horizontal directions, respectively. Besides, line broadening is observed with the reduction of the spectral separation.