An overview of microelectromechanical systems (MEMS) resonators for frequency reference and timing applications is presented. The progress made in the past few decades in design, modeling, fabrication and packaging of MEMS resonators is summarized. In particular, the state-of-the-art technologies for improving the overall performance of MEMS resonators, such as quality factor ( $Q$ ), motional impedance, temperature sensitivity, and initial frequency uniformity, are reviewed in detail. The challenges and opportunities during the commercialization of MEMS resonators are also stated, and future development trends driven either by technology or market are outlined. This paper intends to provide an outlook for possible research directions of MEMS resonators in frequency reference and timing applications. With outstanding reliability, unique multi-frequency functionality on a single chip, and high accuracy, MEMS resonators show great potential for replacing the quartz crystal resonators which have been dominating the timing market since 1920s. [2020-0106]

MEMS Resonators for Frequency Reference and Timing Applications

Since two decades piezoelectric MEMS resonator research has been making a headway by leaps and bounds in multiple fronts such as micro/nanofabrication techniques, device and system design, monolithic integration, packaging, material engineering, etc. Piezoelectricity based frequency selective resonant tanks are an integral part of radio frequency communication, inertial sensors, infrared sensors, environmental monitoring modules, energy harvesters, etc. To improve the functionality of the aforementioned modules it is cardinal to understand the underlying concepts of resonator design, material selection, and performance enhancement techniques. Here in this review paper, a comprehensive overview of various piezoelectric thin-film material properties along with different commercial microfabrication platforms with CMOS integration facility is presented. The device characteristics greatly depend on the resonator mode shape and thus suitable modes and thin-film material configurations can be chosen depending on the target frequency, application, and power budget requirements. In addition to device modeling, a synopsis of the several acoustic and material engineering approaches to enhance the figure of merit of the resonator has been presented. The future avenues of piezoelectric MEMS cover a wide spectrum of possibilities such as 5G communication, quantum processing, frequency combs, photonics integration, gesture sensors, etc.

Piezoelectric MEMS Resonators: A Review

In this paper we report on the development of an aerosol jet printed sensing platform integrating elements of silicon and printed electronics. To demonstrate the technology, thin film humidity sensors have been fabricated over the top surface and sides of pre-packaged integrated circuits using a combination of direct-write aerosol jet deposition and drop-casting. The resistive based sensor consists of an aerosol jet deposited interdigitated nano-particle silver electrode structure overlaid with a thin film of Nafion® acting as a humidity sensitive layer. The fabricated sensor displayed a strong response to changes in relative humidity over the tested range (40% RH–80% RH) and showed a low level of hysteresis whilst undergoing cyclic testing. The successful fabrication of relative humidity sensors over the surface and pins of a packaged integrated circuit demonstrates a new level of integration between printed and silicon based electronics – leading to Printed-Sensor-on-Chip devices. Whilst demonstrated for humidity, the proposed concept is envisaged to work as a platform for a wide range of applications, from bio-sensing to temperature or gas monitoring.

https://cronfa.swan.ac.uk/Record/cronfa35009/Download/0035009-22112017101635.pdf

Printed-Sensor-on-Chip devices – Aerosol jet deposition of thin film relative humidity sensors onto packaged integrated circuits

RF circuit design in deep-submicrometer CMOS technologies relies heavily on accurate modeling of thermal noise. Based on Nyquist's law, predictive modeling of thermal noise in MOSFETs was possible for a long time, provided that parasitic resistances and short-channel effects were properly accounted for. In sub-100-nm technologies, however, microscopic excess noise starts to play a significant role and its incorporation in thermal noise models is unavoidable. Here, we will review several crucial ingredients for accurate RF noise modeling, with emphasis on sub-100-nm technologies. In particular, a detailed derivation and discussion are presented of our microscopic excess noise model. It is shown to qualitatively explain the observed noise (across bias and geometry) in a wide range of commercially available sub-100-nm foundry processes. Besides, the impact of excess noise on the minimum noise figure is discussed.

RF-Noise Modeling in Advanced CMOS Technologies

In this paper, the behavior of radio frequency (RF) CMOS noise up to 24 GHz is analyzed and verified with measurements over a wide range of bias voltages and channel lengths. For the first time, approaches for excess noise factor modeling are validated versus measurements. Furthermore, important RF CMOS figures of merit are examined over many CMOS generations. With the scaling of CMOS technology, optimum RF performance is shown to be shifted from higher moderate toward lower moderate inversion, providing important guidelines for RFIC design. The results are validated with the charge-based EKV3 compact model, which considers short-channel effects such as channel length modulation, velocity saturation, and carrier heating.

CMOS Small-Signal and Thermal Noise Modeling at High Frequencies

A millimeter wave integrated circuit (IC) chip. The IC chip comprises an IC die and a wire bond ball grid array package encapsulating the IC die. The wire bond ball grid array package comprises a solder ball array, a millimeter wave transmit channel, and a millimeter wave receive channel, wherein each millimeter wave transmit and receive channel electrically couples the IC die to a signal ball of the solder ball array and is configured to resonate at an operating frequency band of the millimeter wave IC chip.

Millimeter wave integrated circuit with ball grid array package including transmit and receive channels

Experimental and simulation results of high-frequency channel noise in MOSFETs with 40-, 80-, and 110- nm gate lengths are presented. The measured dc I-V characteristics can be matched using the drift-diffusion (DD) and hydrodynamic (HD) transport models, both incorporating velocity saturation. The DD model grossly underestimates the measured noise, demonstrating the inadequacy of channel-length modulation and impact ionization to explain the excess noise. The HD model generates higher noise but not enough, showing that introduction of carrier heating is still insufficient to explain the experimental results. The underprediction of noise using the HD model can be mitigated by a suitable choice of the energy relaxation time and saturation velocity; however, simultaneous matching of both noise and dc I-V does not produce satisfactory results. Thus, TCAD simulators are unable to simulate this excess-noise mechanism at this time. Experimental data support that, at 40 nm gate lengths, noise can be described by a shot noise like expression.

A Physical Understanding of RF Noise in Bulk nMOSFETs With Channel Lengths in the Nanometer Regime

This work introduces an integrated reference clock system using a 2.5 GHz mirror-encapsulated BAW (bulk acoustic wave) resonator as the frequency source. With acoustic mirrors also placed on top of the resonator, this symmetrical DBAR (dual-Bragg acoustic resonator) structure offers extra design freedoms to alter the resonator’s acoustic properties. A DBAR requires no cavity on either side of the resonator yet still immune to mass loading effect from contamination and assembly. This advantage enables cost-effective system integration by directly marrying a DBAR and a CMOS die in a plastic package. System chips utilizing DBAR as the high-frequency reference clocks are demonstrated here. With compensation algorithm and careful design of the package, these DBAR-based clocking systems can achieve ±25 ppm stability over the entire operation temperature range (including aging, solder shift, etc.), resulting industry’s first crystal-less BLE (Bluetooth Low Energy) radio chip.

Integrated High-frequency Reference Clock Systems Utilizing Mirror-encapsulated BAW Resonators

A frequency reference device that includes a frequency reference generation unit to generate a frequency reference signal based on an absorption line of a gas.

Detection and locking to the absorption spectra of gasses in the millimeter-wave region

Described examples include a millimeter wave atomic clock apparatus, chip scale vapor cell, and fabrication method in which a low pressure dipolar molecule gas is provided in a sealed cavity with a conductive interior surface forming a waveguide. Non-conductive apertures provide electromagnetic entrance to, and exit from, the cavity. Conductive coupling structures formed on an outer surface of the vapor cell near the respective non-conductive apertures couple an electromagnetic field to the interior of the cavity for interrogating the vapor cell using a transceiver circuit at a frequency that maximizes the rotational transition absorption of the dipolar molecule gas in the cavity to provide a reference clock signal for atomic clock or other applications.

Django Trombley

Papers

Millimeter wave integrated circuit with ball grid array package including transmit and receive channels

A Physical Understanding of RF Noise in Bulk nMOSFETs With Channel Lengths in the Nanometer Regime

Integrated High-frequency Reference Clock Systems Utilizing Mirror-encapsulated BAW Resonators

Detection and locking to the absorption spectra of gasses in the millimeter-wave region

Rotational transition based clock, rotational spectroscopy cell, and method of making same