Paul-Vahe Cicek

A Highly Integrated 1.8 GHz Frequency Synthesizer Based on a MEMS Resonator

A highly integrated 1.7-2.0 GHz digitally programmable frequency synthesizer using a MEMS resonator as its reference is presented. Due to the dimensions of the MEMS device (e.g., 25 mum by 114 mum), the entire system with a total area of 6.25 mm2 can be housed in a small standard chip package. This considerably reduces the form factor and cost of the system, compared to using an external crystal as a reference. The MEMS resonators are clamped-clamped beams fabricated using a CMOS-compatible process. The main structural layer is made of silicon carbide, which provides the resonators with higher power handling capabilities and higher operating frequencies, compared to silicon. The resonators are electrostatically and thermally tunable - an 8.4% frequency tuning is demonstrated for a 9 MHz resonator. The 100 nm vertical transducer gaps of the resonators allow the use of electrostatic actuation voltages as low as 2 V. An integrated high gain-bandwidth trans-impedance amplifier (TIA) is combined with a resonator to generate the synthesizers input reference signal. The TIA employs automatic gain control to mitigate the inherent low power handling capabilities and the non-linearities of the MEMS device, thus minimizing their effect on phase noise. The fractional- N synthesizer employs a 3rd-order 20-bit delta-sigma modulator to deliver a theoretical output resolution of ~ 11 Hz, in order to allow for high output frequency stability when used with an appropriate feedback loop. A fully integrated on-chip dual path loop filter is used to maintain a high level of system integration. With a supply voltage of 2 V, the phase noise for a 1.8 GHz output frequency and a ~12 MHz reference signal is -122 dBc/Hz at a 600 kHz offset, and -137 dBc/Hz at a 3 MHz offset.

Low-Stress CMOS-Compatible Silicon Carbide Surface-Micromachining Technology—Part II: Beam Resonators for MEMS Above IC

Microelectromechanical beam resonators and arrays are fabricated using a custom low-temperature complementary-metal-oxide-semiconductor-compatible silicon carbide microfabrication process, detailed in Part I of this paper. Theoretical aspects are presented with modal analysis and numerical methods. Measurements of the resonant frequency, the quality factor, the transmission, and the tuning characteristics are presented for different device types and dimensions. Trends are analyzed, and performance metrics dependences are investigated. A tuning method based on integrated heaters is introduced and tested, yielding a very desirable constant insertion-loss tuning and a wide tuning range. Quality factors of up to 1493 and resonant frequencies of up to 26.2 MHz are demonstrated. Both the Youngs modulus and the residual stress of the SiC film are extracted (261 GPa and <; ±30 MPa, respectively), and favorably compare to values reported for polysilicon.

Surface-Micromachined CMUT Using Low-Temperature Deposited Silicon Carbide Membranes for Above-IC Integration

This paper presents a surface-micromachining technology to fabricate silicon carbide (SiC)-based capacitive micromachined ultrasonic transducers (CMUTs). The use of dc-sputtered amorphous SiC as a structural layer allows the fabrication process to limit the temperature to a thermal budget of 200 °C, which is the lowest reported to date, making this technology ideally suited for above-IC integration. The high Youngs modulus of the deposited SiC film, along with its very low residual stress, results in high strength and resilient CMUT membranes. The placement of the suspended aluminum electrode directly at the bottom side of the membrane reduces the effective size of the electrostatic transduction gap, resulting in superior electro-mechanical coupling. Fabricated transducers are tested in air with both continuous-wave and pulsed signals, using a pitch-and-catch configuration. The transducer pair, composed of 110-μm-diameter membrane arrays, exhibits a resonant frequency of 1.75 MHz, a 3 dB-bandwidth of 0.15 MHz, and a transmission gain of -38 dB. The CMUT prototypes showcase the versatility of low-temperature dc-sputtered SiC films applied in the field of MEMS.

Bulk Mode Disk Resonator With Transverse Piezoelectric Actuation and Electrostatic Tuning

This paper presents a wine-glass bulk mode disk resonator based on a novel transverse piezoelectric actuation technique to achieve bulk mode resonance of the single crystalline silicon disk structure. The device is fabricated in a commercial microelectromechnical systems (MEMS) process, and combines reasonable quality factor and superior motional resistance in a low-cost technology. External capacitive electrodes are used for electrostatic tuning of the resonance frequency, relying on the electrostatic spring softening effect. The resonator was measured to have a resonance frequency of ~15 MHz and a quality factor of ~2000 in atmospheric pressure, increasing to ~5000 in 100-mtorr vacuum. The temperature co-efficient for the frequency of the device was also measured to be about -40 ppm/°C. The resonator requires no dc voltage for operation, but its resonance frequency can be tuned by varying the applied dc voltage on the capacitive electrodes with a factor of 1 ppb/V2.

MEMS wafer-level vacuum packaging with transverse interconnects for CMOS integration

A novel vacuum wafer-level packaging technology for micro-electromechanical systems (MEMS) is presented. It supports monolithic integration with electronics, and is suitable for different MEMS processes. Bulk-etched transverse feedthroughs are used to connect with the encapsulated systems. Silicon carbide is successfully used for membrane stress cancellation and improved hermeticity.

Surface Micromachined Combined Magnetometer/Accelerometer for Above-IC Integration

This paper presents a combined magnetometer/accelerometer sharing a single surface micromachined structure. The device utilizes electrical current switching between two perpendicular directions on the structure to achieve a 2-D in-plane magnetic field measurement based on the Lorentz force. The device can concurrently serve as a 1-D accelerometer for out-of-plane acceleration, when the current is switched off. Accordingly, the proposed design is capable of separating magnetic and inertial force measurements, achieving higher accuracy through a single compact device. The sensor supports static operation at atmospheric pressure, precluding the need for complex vacuum packaging. It can alternatively operate at resonance under vacuum for enhanced sensitivity. The device is fabricated using a low-temperature surface micromachining technology, which is fully adapted for above-IC integration on standard CMOS substrates. The resonance frequency of one of the fabricated structures is measured to be 6.53 kHz with a quality factor of ~30 at a 10-mTorr ambient vacuum level. The magnetic field and acceleration sensitivities of the device are measured using discrete electronics to be 1.57 pF/T and 1.02 fF/g, respectively, under static operation.

A novel prototyping method for die-level monolithic integration of MEMS above-IC

This work presents a convenient and versatile prototyping method for integrating surface-micromachined microelectromechanical systems (MEMS) directly above IC electronics, at the die level. Such localized implementation helps reduce development costs associated with the acquisition of full-sized semiconductor wafers. To demonstrate the validity of this method, variants of an IC-compatible surface-micromachining MEMS process are used to build different MEMS devices above a commercial transimpedance amplifier chip. Subsequent functional assessments for both the electronics and the MEMS indicate that the integration is successful, validating the prototyping methodology presented in this work, as well as the suitability of the selected MEMS technology for above-IC integration.

A MEMS-based temperature-compensated vacuum sensor for low-power monolithic integration

This paper presents a MEMS resonator-based vacuum sensor with a low-power transimpedance amplifier and a mixer-based frequency-to-digital converter. The MEMS resonator is fabricated in a CMOS-compatible process, and a 130 nm CMOS technology is used to design the integrated circuitry. The vacuum sensor operates in the pressure range from 10 to 1200 mbar with a resolution of ~2 mbar. The system is temperature-compensated between −10°C and 60°C. The simulated power consumption of the entire system is less than 495 μW from a 1 V supply.

A MEMS-based vacuum sensor with a PLL frequency-to-voltage converter

This paper presents a MEMS resonator-based vacuum sensor with a low power transimpedance amplifier and PLL-based frequency-to-voltage converter. The MEMS resonator is fabricated in a CMOS-compatible process, and a 90 nm CMOS technology is used to design the integrated circuitry. The vacuum sensor has a range from 10 mbar to 1200 mbar and a resolution of less than 5 mbar. The simulated power consumption of the entire system is less than 520 µW from a 1 V supply.

Low actuation voltage silicon carbide RF switches for MEMS above IC

This paper presents a CMOS-compatible RF MEMS technology to build low actuation voltage switches. SiC increases the stiffness of the switches to improve reliability and durability. A design methodology is introduced to optimize tradeoffs between important system criteria, i.e., voltage levels, signal performance and switching speed. Simulations are used to evaluate devices designed with the technology.