Kenneth E. Wojciechowski

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.

Ovenized and thermally tunable aluminum nitride microresonators

Frequency tuning of aluminum nitride (AlN) microre-sonators has been demonstrated via localized heating (ovenization) of the resonator. Specifically, piezoelectically driven 625 MHz microresonators were heated by embedded joule heaters in vacuum. A temperature increase of 135°C was achieved with only 2.8 mW of power consumption. This increase corresponds to ∼4500ppm of frequency shift. To minimize heat loss, the devices were suspended from the substrate by high thermal isolation beam type supports. The beams exhibit very high thermal resistance, not only due to their high length to cross-sectional area ratio, but also because they are made of thin-film deposited polycrystalline aluminum nitride. Thin AlN films have been shown to have thermal conductivities that are much lower than that measured in bulk materials. The availability of a power efficient frequency tuning method in aluminum nitride microresonators enables low power ovenization of AlN MEMS-based timing devices and tunable filtering for communication systems.

Fully integrated switchable filter banks

Fully integrated switchable filter have been successfully demonstrated using a ra CMOS SOI process in conjunction with an a (AlN) microresonator process. Single pole-mul were developed in the CMOS SOI process th multi-project wafer runs while the filters were aluminum nitride based microresonators. Each concurrent design cycles and was demonstrated to integration. After design improvements to bo full monolithic integration was implem microresonator filters with the CMOS switc compatibility of the two technologies. A four ch switchable bank of ∼7MHz bandwidth filters demonstrated exhibiting approximately 8 dB of 60dB of stop band rejection.

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.

Oven-Based Thermally Tunable Aluminum Nitride Microresonators

Frequency tuning of aluminum nitride (AlN) microresonators has been demonstrated via localized heating (ovenization) of the resonator. Specifically, piezoelectrically driven ~ 100 MHz microresonators were heated by embedded joule heaters in vacuum. Three different designs with three different film stacks were tested, and among the tested devices, thermal resistances as large as 92 K/mW have been demonstrated, which corresponds to 1-mW power consumption to yield a temperature increase of 92°C. To minimize heat loss, the devices were suspended from the substrate by high thermal isolation beam-type supports. The beams exhibit very high thermal resistance not only due to their high length to cross-sectional area ratio but also because they are made of thin-film-deposited polycrystalline aluminum nitride. Film-deposited AlN has been shown to have thermal conductivity much lower than that measured in bulk materials. Thermal time constants for these devices were measured ranging from submilliseconds to 10 ms depending on the design and film stacks, and frequency tunability was measured as high as 2548 parts per million/mW. The availability of a power-efficient frequency tuning method, coupled with all other performance benefits, makes AlN microresonators a promising candidate for the next-generation timing devices and tunable filters for multiband communication systems.

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.

Elucidating the origin of spurious modes in aluminum nitride microresonators using a 2-D finite-element model

In this work, an approach has been developed to predict the location of large spurious modes in the resonant response of aluminum nitride (AlN) microelectromechanical systems (MEMS) resonators over a wide range of desired operating frequencies. This addresses significant challenges in the design of more complex AlN devices, namely the prediction and elimination of spurious modes in the resonance response. Using the finite element method (FEM), the dispersion curves at wavelengths ranging from 8 to 20 μm were computed. It was determined that the velocities of symmetric Lamb (S0) and high-order antisymmetric (A) modes overlap at specific wavelengths. A 2-D FEM analysis showed that both the S0 and higher order A modes are mutually excited at a common operating wavelength. From this analysis, the coupling-of-modes (COM) parameters were extracted and used to compute the P-matrix and S-parameters using a 6-port transmission matrix. The P-matrix simulation was able to predict the electrical response of the S0 and nearby spurious modes. This work identified specific wavelength regions where COM has limited accuracy because of mode conversion. In these regions, the reflection (κp) and transduction (ζp) parameters change rapidly.

Origins and mitigation of spurious modes in aluminum nitride microresonators

Recently reported narrow bandwidth, < 2%, aluminum nitride microresonator filters in the 100–500 MHz range offer lower insertion loss, 100× smaller size, and elimination of large external matching networks, when compared to similar surface acoustic wave filters. While the initial results are promising, many microresonators exhibit spurious responses both close and far from the pass band which degrade the out of band rejection and prevent the synthesis of useful filters. This paper identifies the origins of several unwanted modes in overtone width extensional aluminum nitride microresonators and presents techniques for mitigating the spurious responses.