Chisaki Takubo

Laterally-driven deformation-robust MEMS gyroscopes with three sets of symmetrically arranged folded-beam suspensions

Microelectromechanical systems (MEMS) vibratory gyroscopes are used in a wide range of applications. Gyroscopes which have long-term stability and high-reliability even when they are used in harsh environments such as in a wide range of temperatures will be useful for automotive systems and robotics applications. We report a deformation-robust gyroscope, which has three sets of symmetrically arranged folded beams (SAF), which cancel and/or dissipate deformation or internal stress over a wide range of temperatures. In addition, we found that a triangularly supported one-sided open frame by the SAF separated spurious modes from the drive and sensing modes.

New high performance SAW convolvers used in high bit rate and wideband spread spectrum CDMA communications system

New surface acoustic wave (SAW) convolver structures with high conversion efficiency and self-temperature compensation characteristics have been developed. Strong piezoelectric substrates, regardless of temperature coefficients of delay (TCD), can be used in these convolvers. New demodulation techniques using the developed SAW convolver for high bit rate and wideband spread spectrum code division multiple access (CDMA) communications have also been developed. I- and Q-channel demodulation data can be derived directly from binary phase shift keying (BPSK) or quadri-phase shift keying (QPSK) CDMA signals. In an experiment using a 128/spl deg/ YX-LiNbO/sub 3/ substrate, CDMA signals of 9 Mbps (megabits per second) with 60 Mcps (megachips per second) spread by 13-chip Barker code and 11 Mbps with 140 Mcps spread by 25-chip Shibas code were clearly demodulated, demonstrating the effectiveness of these techniques for use in future CDMA communications.

Fabrication of sub-transistor via holes for small and efficient power amplifiers using highly selective GaAs∕InGaP wet etching

We investigated the properties of a citric acid-based GaAs∕InGaP selective wet etchant. We found that the citric acid-based etchant has a much higher selectivity and exhibits less undercutting than those of a conventional sulfuric acid-based etchant that we had used. Then, we applied the citric acid-based wet etchant to the fabrication of via holes with high thermal diffusibility underneath heterojunction bipolar transistors. The citric acid-based etchant exhibited GaAs∕InGaP selectivity of about 9800 at pH 9.0 and enabled reliable etch stopping without pinholes. The citric acid-based etchant also suppressed undercutting and enabled the integration of the via holes. We used the optimized citric acid-based wet etching for finishing the via hole etching after rough high-speed wet etching with the conventional sulfuric acid-based etchant. The two-step wet etching process resulted in a successful fabrication of the sub-transistor via hole structure with collector-up heterojunction bipolar transistors.

An InGaP/GaAs collector-up tunnelling-collector HBT and subtransistor via-hole structure for small and highly efficient power amplifiers

A novel structure of InGaP/GaAs collector-up tunneling-collector heterojunction bipolar transistors (C-up TC-HBTs) with sub-transistor via-holes, for use in small power amplifiers, is presented. Having the via-holes directly under the C-up UBTs is convenient in terms of thermal conduction; power amplifiers composed of multi-finger HBTs in this configuration take up dramatically less area than those with devices in the conventional configuration. The result was the demonstration of thermally stable operation for a 4-finger C-up TC-HBT at up to 0.9 mW//spl mu/m/sup 2/, in spite of the low finger pitch of only 15 /spl mu/m. Moreover, a small 32-finger C-up TC-HBT, with a total area of 0.25 /spl times/ 0.31 mm, was capable of delivering a power-added efficiency of 52% at 24.4 dBm in wide-band CDMA operation. These results show the strong potential for microwave application of high-efficiency power amplifiers composed of C-up TC-HBTs.

New high-performance SAW convolvers and their fundamental experiments for high-bit-rate CDMA communications system

New SAW-convolver structures with high-performance characteristics were proposed. Strong piezoelectric substrates which provide high-conversion efficiencies can be used. Fundamental experiments showed their complete temperature-compensation effects unrelated to TCDs of the substrates and applicabilities of high-bit-rate CDMA transmissions with more than several Mbps.

Out-of-plane axis SOI MEMS gyroscope with initially displaced vertical sensing comb

We developed a capacitive silicon on insulator (SOI) microelectro-mechanical systems (MEMS) tuning-fork gyroscope and describe its structure and performance.

New SAW-convolver demodulation techniques using Costas-Loop synchronization for high-bit-rate CDMA signals

A new synchronous-demodulation method for high-speed CDMA signal, that combines a SAW convolver with a Costas-Loop circuit has been proposed. In the receiver, synchronization of the received CDMA signal and the LO signal can be carried out independent of the transmitter. A QPSK-CDMA signal of 9 Mbps with 60 Mcps has been clearly demodulated using this method.

New SAW-convolver demodulation techniques for very-high-speed CDMA communications

New SAW-convolver demodulation techniques for very-high-speed CDMA communications were proposed. QPSK-CDMA signals of 9 Mbps with 60 Mcps and 11 Mbps with 140 Mcps were clearly demodulated, which showed the techniques would be very powerful for future CDMA communications.

Backside-emitter-structure C-up GaAs HBTs for small power amplifiers

In this paper, we have successfully fabricated ballast-free HBTs with L of 15 /spl mu/m and V/sub ce,.sat/ of 0.01 V by employing a backside-emitter-structure collector-up (C-up) configuration with ohmic collector contacts. Preliminary results on performance of these HBTs show their strong potential for application in small and highly efficient PA MMICs.

Design of C-up GaAs HBTs with backside emitter for small power amplifiers

Poor thermal conduction of a GaAs substrate in GaAs heterojunction bipolar transistor (HBT) power amplifiers (PAs) previously required thermal designs such as ballast resistors and a large finger pitch (/spl sim/ 30 /spl mu/m). This paper describes the successful fabrication of ballast-free PAs with a small finger pitch (15 /spl mu/m) using a collector-up (C-up) configuration with a backside emitter structure. Having via-holes directly under C-up HBTs is convenient in terms of thermal conduction; PAs composed of multi-finger HBTs in this configuration occupy one-third less area than those with HBTs in the conventional configuration. A small 32-finger C-up HBT we fabricated with a total area of 0.25 /spl times/ 0.31 /spl mu/m/sup 2/ delivered a power-added efficiency of 52% at 24.4 dBm in wide-band CDMA operation. The results show that C-up HBTs with a backside emitter structure have strong potential for microwave high-power application.