Melanie R. Tuck

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.

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.

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.

VHF and UHF mechanically coupled aluminum nitride MEMS filters

This paper reports the development of narrow-bandwidth, post-CMOS compatible aluminum nitride (AlN) MEMS filters operating in the very (VHF) and ultra (UHF) high frequency bands. Percent bandwidths less than 0.1% are achieved utilizing a mechanically coupled filter architecture, where a quarter wavelength beam attached in low velocity coupling locations is used to connect two AlN ring resonators. The filter bandwidth has been successfully varied from 0.09% to 0.2% by moving the attachment of the coupling beam on the ring to locations with different velocity at resonance. Insertion losses of 11 dB are obtained for filters centered at 99.5 MHz with low termination impedances of 200 Omega. Utilizing a passive temperature compensation technique, the temperature coefficient of frequency (TCF) for these filters has been reduced from -21 ppm/C to 2.5 ppm/C. The reduced TCF is critical for narrow bandwidth filters, requiring only 13% of the filter bandwidth to account for military range (-55 to 125 C) temperature variations compared to 100% for uncompensated filters. Filters operating at 557 MHz are realized using overtone operation of the ring resonators and coupling beam where higher insertion losses of 32 dB into 50 Omega are seen due to the finite resonator quality factor and narrow bandwidth design. Overtone operation allows for the implementation of fully differential and balun type filters where the stop-band rejection is as high as 38 dB despite the increased insertion loss.

Short and long loop manufacturing feedback using a multisensor assembly test chip

A family of silicon test chips for use in making diagnostic measurements during electronics assembly has been developed. These assembly test chips (ATCs) contain sensors that measure a number of variables associated with assembled IC degradation, including the degree of integrated circuit (IC) corrosion, handling damage, electrostatic discharge threat, moisture or humidity, mechanical stress, mobile ion density, bond pad cratering, and high-speed logic degradation. The chips in the ATC family are intended to give manufacturing feedback in four ways: direct feedback in evaluation of an assembly manufacturing line in an objective, nonintrusive way; before and after comparisons on an assembly production line when an individual process, material, or piece of equipment has been changed; resident lifetime monitor for system package aging and ongoing reliability projection; and thermal, mechanical, DC electrical, and high-frequency mock-up evaluation of packaging (including multichip) schemes. >

Super high frequency width extensional aluminum nitride (AlN) MEMS resonators

Width extensional (WE) super high frequency (SHF) aluminum nitride (AlN) resonators have been fabricated using optical lithography. Solidly anchored WE resonators were shown to be superior to beam anchored resonators of the same size and it was verified that simply scaling resonator area does not improve insertion loss (IL). Resonators with an IL of −6.3 dB into 50 ohms at 4.1 GHz and −7.2 dB at 6.8 GHz have been demonstrated. This type of performance at 6.8 GHz is unprecedented for contour mode resonators and represents a 12.6 dB improvement over recently reported SHF AlN resonators.

Micromachined Bulk Wave Acoustic Bandgap Devices

A MEMS bulk wave acoustic bandgap has been designed and experimentally verified. The acoustic bandgaps are realized by including tungsten (W) scatterers in a SiO2 matrix. Wide frequency ranges where acoustic waves are forbidden to exist are formed due to the large density and acoustic impedance mismatch between W and SiO2. The acoustic bandgap structures are fabricated in a 7-mask process that features integrated aluminum nitride piezoelectric couplers. Acoustic bandgaps in a square lattice have been measured at 33 and 67 MHz with up to 35 dB of acoustic rejection and bandwidths exceeding 35% of the midgap.

Microresonant impedance transformers

Widely applied to RF filtering, AlN microresonators offer the ability to perform additional functions such as impedance matching and single-ended-to-differential conversion. This paper reports microresonators capable of transforming the characteristic impedance from input to output over a wide range while performing low loss filtering. Microresonant transformer theory of operation and equivalent circuit models are presented and compared with measured 2 and 3-Port devices. Impedance transformation ratios as large as 18:1 are realized with insertion losses less than 5.8 dB, limited by parasitic shunt capacitance. These impedance transformers occupy less than 0.052 mm2, orders of magnitude smaller than competing technologies in the VHF and UHF frequency bands.

Micro heat spreader enhanced heat transfer in MCMs

The peak thermal power generated in microelectronics assemblies has risen from less than 1 W/cm/sup 2/ in 1980 to greater than 40 W/cm/sup 2/ today, due primarily to increasing densities at both the IC and packaging levels. We have demonstrated enhanced heat transfer in a prototype Si substrate with a backside micro heat channel structure. Unlike conventional micro heat pipes, these channels are biaxial with a greater capacity for fluid transfer. Thermal modeling and preliminary experiments have shown an equivalent increase in substrate thermal conductivity to over 500 W/m.K, or a four times improvement. Optimization of the structure and alternative liquids will further increase the thermal conductivity of the micro heat channel substrate with the objective being polycrystalline diamond, or about 1200 W/m.K. The crucial design parameters for the micro heat channel system and the thermal characteristics of the system are covered.

Evaluation of chip passivation and coatings using special purpose assembly test chips and porous silicon moisture detectors

Two devices which can be used to evaluate the ability of chip passivation or postbond coatings to protect a Si device from moisture penetration and resultant Al corrosion are described. The first device is a test chip with a number of Al triple track and other corrosion measurement structures. The authors present HAST (highly accelerated stress test) data to illustrate the use of this chip in measuring failure rates and determining failure modes. The second device is a rugged, moisture-sensitive porous Si capacitor, which is compatible with high-temperature passivation and postbond IC processing. Data are presented showing the stability of the device relative to that of an anodized Al moisture sensor and showing the variation of capacitance with moisture level. Data are also presented showing that the capacitor can respond to a point source of water located over the porous region but remotely from the top electrode.<<ETX>>