Roy H. Olsson

Reduction in the thermal conductivity of single crystalline silicon by phononic crystal patterning.

Phononic crystals (PnCs) are the acoustic wave equivalent of photonic crystals, where a periodic array of scattering inclusions located in a homogeneous host material causes certain frequencies to be completely reflected by the structure. In conjunction with creating a phononic band gap, anomalous dispersion accompanied by a large reduction in phonon group velocities can lead to a massive reduction in silicon thermal conductivity. We measured the cross plane thermal conductivity of a series of single crystalline silicon PnCs using time domain thermoreflectance. The measured values are over an order of magnitude lower than those obtained for bulk Si (from 148 W m(-1) K(-1) to as low as 6.8 W m(-1) K(-1)). The measured thermal conductivity is much smaller than that predicted by only accounting for boundary scattering at the interfaces of the PnC lattice, indicating that coherent phononic effects are causing an additional reduction to the cross plane thermal conductivity.

A three-dimensional neural recording microsystem with implantable data compression circuitry

A 256-site, fully implantable, 3-D neural recording microsystem has been developed. The microsystem incorporates four active neural probes with integrated circuitry for site selection, amplification, and multiplexing. The probes drive an embedded data-compression ASIC that successfully detects neural spikes in the presence of neural and circuit noise. The spike detection ASIC achieves a factor of 12 bandwidth reduction while preserving the key features of the action potential waveshape necessary for spike discrimination. This work extends the total number of neural channels that can be recorded across a transcutaneous inductively coupled wireless link from 25 to 312. When a spike is detected, this ASIC serially shifts the 5-bit amplitude and 5-bit address of the spike off of the chip over a single 2.5 Mb/s wired or wireless line. The spike detection ASIC occupies 6 mm/sup 2/ in 0.5 /spl mu/m features and consumes 2.6 mW while the entire microsystem consumes 5.4 mW of power from a 3-V supply.

Band-tunable and multiplexed integrated circuits for simultaneous recording and stimulation with microelectrode arrays

Two thin-film microelectrode arrays with integrated circuitry have been developed for extracellular neural recording in behaving animals. An eight-site probe for simultaneous neural recording and stimulation has been designed that includes on-chip amplifiers that can be individually bypassed, allowing direct access to the iridium sites for electrical stimulation. The on-probe amplifiers have a gain of 38.9 dB, an upper-cutoff frequency of 9.9 kHz, and an input-referred noise of 9.2 /spl mu/V /sub rms/ integrated from 100 Hz to 10 kHz. The low-frequency cutoff of the amplifier is tunable to allow the recording of field potentials and minimize stimulus artifact. The amplifier consumes 68 /spl mu/W from /spl plusmn/1.5 V supplies and occupies 0.177 mm/sup 2/ in 3 /spl mu/m features. In vivo recordings have shown that the preamplifiers can record single-unit activity 1 ms after the onset of stimulation on sites as close as 20 /spl mu/m to the stimulating electrode. A second neural recording array has been developed which multiplexes 32 neural signals onto four output data leads. Providing gain on this array eliminates the need for bulky head-mounted circuitry and reduces motion artifacts. The time-division multiplexing circuitry has crosstalk between consecutive channels of less than 6% at a sample rate of 20 kHz per channel. Amplified, time-division-multiplexed multichannel neural recording allows the large-scale recording of neuronal activity in freely behaving small animals with minimum number of interconnect leads.

Phononic band-gap crystals for radio frequency communications

We report on the experimental and theoretical observation of a phononic band-gap crystal operating in the megahertz regime. Our experimental data show over 25dB suppression of bulk acoustic waves, and our theoretical models predict almost linear scaling to the gigahertz frequencies, thus laying the foundation for the implementation of such devices in radio frequency communications. We further argue that cavities in such systems offer a unique opportunity to couple acoustic energy into a resonator utilizing piezoelectric materials, while at the same time allowing the realization of a resonance cavity in high-Q materials such as silicon oxide, silicon, and tungsten.

Post-CMOS-Compatible Aluminum Nitride Resonant MEMS Accelerometers

This paper describes the development of aluminum nitride (AlN) resonant accelerometers that can be integrated directly over foundry CMOS circuitry. Acceleration is measured by a change in resonant frequency of AlN double-ended tuning-fork (DETF) resonators. The DETF resonators and an attached proof mass are composed of a 1-mum-thick piezoelectric AlN layer. Utilizing piezoelectric coupling for the resonator drive and sense, DETFs at 890 kHz have been realized with quality factors (Q) of 5090 and a maximum power handling of 1 muW. The linear drive of the piezoelectric coupling reduces upconversion of 1/f amplifier noise into 1/f 3 phase noise close to the oscillator carrier. This results in lower oscillator phase noise, -96 dBc/Hz at 100-Hz offset from the carrier, and improved sensor resolution when the DETF resonators are oscillated by the readout electronics. Attached to a 110-ng proof mass, the accelerometer microsystem has a measured sensitivity of 3.4 Hz/G and a resolution of 0.9 mG/radicHz from 10 to 200 Hz, where the accelerometer bandwidth is limited by the measurement setup. Theoretical calculations predict an upper limit on the accelerometer bandwidth of 1.4 kHz.

Realization of a phononic crystal operating at gigahertz frequencies

We report on the experimental realization of a phononic crystal, designed to operate at gigahertz frequencies. Detailed studies of the structure have been performed using finite difference time domain method to determine effects of slab modes in finite-thickness slabs, thus enabling precise guidance of experimental efforts. In particular, we find the slab mode effects mitigated in ultrathin (thickness less than lattice periodicity) and ultrathick (thickness more than ten times lattice periodicity) slabs. Gigahertz-frequency phononic crystals are well poised to find usage as high-Q resonators, waveguides, and coupling elements in a variety of application areas including RF communications.

Phononic crystals operating in the gigahertz range with extremely wide band gaps

Phononic crystals have numerous potential applications including use as filters and oscillators in communications systems and as acoustic isolators for resonant sensors such as gyroscopes. These applications are based on the ability of phononic crystals to exhibit elastic band gaps, frequency bands where the propagation of acoustic waves is forbidden. Here, we focus on solid-solid phononic crystals (solid inclusions in a solid matrix), since they typically exhibit wider band gaps than those observed with air-solid phononic crystals (air inclusions in a solid matrix). We present a micromachined solid-solid phononic crystal operating at 1.4 GHz center frequency with an ultrawide 800 MHz band gap.

Post-CMOS Compatible Aluminum Nitride MEMS Filters and Resonant Sensors

This paper reports post-CMOS compatible aluminum nitride (AlN) MEMS resonators, filters, and resonant sensors for the miniaturization of radio-frequency transceivers and sensor systems. Utilizing a resonator with two closely spaced modes, 2nd order MEMS filters occupying 0.06 mm2 have been realized in a single device. Methods for tuning the bandwidth and center frequency of these filters lithographically have been demonstrated. A 0.5% bandwidth, 108.4 MHz dual mode filter has a measured insertion loss of 9.4 dB with 50 Omega termination which can be reduced to 4.7 dB by terminating the filter with 75 Omega. In order to scale MEMS resonators to higher frequencies without increasing the size or impedance, resonators selectively driven at a harmonic determined by interdigitated drive and sense electrodes have been demonstrated reaching frequencies of 796 MHz with impedances of approximately 100 Omega and quality factors in excess of 750 in air. In the same process resonant sensors based on AlN double-ended tuning fork (DETF) sensing beams have been demonstrated at 727 kHz with quality factors of 2160. An oscillator based on the DETF sensing beams achieves a phase noise of -81 dBc/Hz at 275 Hz offset from the carrier. A 100 ng mass coupled to a pair of DETF sensors achieves an acceleration sensitivity of 565 mG/radicHz for accelerations from 275 to 1100 Hz.

Origin of reduction in phonon thermal conductivity of microporous solids

Porous structures have strong tunable size effects due to increased surface area. Size effects on phonon thermal conductivity have been observed in porous materials with periodic voids on the order of microns. This letter explores the origin of this size effect on phonon thermal conductivity observed in periodic microporous membranes. Pore-edge boundary scattering of low frequency phonons explains the temperature trends in the thermal conductivity; further reduction in thermal conductivity is explained by the porosity.

Single-chip precision oscillators based on multi-frequency, high-Q aluminum nitride MEMS resonators

Aluminum nitride (AlN) contour mode resonators have been of interest because of their high quality factor, low impedance, large number of frequencies on a single chip and compatibility with CMOS processes [1–3]. While AlN low insertion loss filters [1–3] and oscillators [4–7] have been demonstrated, CMOS integration has yet to be accomplished. This work represents the first time fully-released contour mode AlN microresonators have been integrated with CMOS circuitry to obtain completely monolithic frequency references.