Frederic Nabki

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.

A high gain-bandwidth product transimpedance amplifier for MEMS-based oscillators

A variable gain differential transimpedance amplifier (TIA) optimized for MEMS-based oscillator applications is presented. The TIA achieves a variable gain of 17 kOmega to 290 kOmega, i.e. a gain range of 25 dB. The 3-dB bandwidths corresponding to these gains are 256 MHz and 103 MHz, respectively. The suitability of the TIA for the targeted application is demonstrated by combining it with a MEMS resonator to create an oscillator at the frequency of 8.29 MHz, with a phase noise of -89 dBc/Hz at a 1 kHz offset frequency.

Micromachined Resonators: A Review

This paper is a review of the remarkable progress that has been made during the past few decades in design, modeling, and fabrication of micromachined resonators. Although micro-resonators have come a long way since their early days of development, they are yet to fulfill the rightful vision of their pervasive use across a wide variety of applications. This is partially due to the complexities associated with the physics that limit their performance, the intricacies involved in the processes that are used in their manufacturing, and the trade-offs in using different transduction mechanisms for their implementation. This work is intended to offer a brief introduction to all such details with references to the most influential contributions in the field for those interested in a deeper understanding of the material.

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.

Low-Stress CMOS-Compatible Silicon Carbide Surface-Micromachining Technology—Part I: Process Development and Characterization

A low-temperature (<; 300 °C) low-stress microelectromechanical systems fabrication process based on a silicon carbide structural layer is presented. A partially conductive sintered target enables low-temperature dc sputtering of amorphous silicon carbide (SiC) at high deposition rates (75 nm/min). The low stress of the structural film allows for mechanically reliable structures to be fabricated, while the low-temperature deposition allows for pre-SiC metallization. The process is designed for low-cost film deposition and for complementary metal-oxide-semiconductor postintegration, stemming from chemical and thermal compatibility. Process flow, deposition, etching, and stress control are discussed, and a detailed process characterization is reported.

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.

A 10-bit 110 kS/s 1.16 μW SA-ADC With a Hybrid Differential/Single-Ended DAC in 180-nm CMOS for Multichannel Biomedical Applications

A 10-bit 110-kS/s successive-approximation analog-to-digital converter (ADC) for multichannel biomedical applications is presented. In order to achieve low-power operation, the ADC utilizes a reduced-speed dynamic comparator, a low-complexity calibration technique, a hybrid single/differential digital-to-analog converter architecture, and an attenuation capacitor with low sensitivity to mismatch errors. Fabricated in 180-nm CMOS, this ADC consumes a total power of 1.16 μW from 1.5 V/1.2 V analog/digital power supplies. The integral nonlinearity is between -1.23 LSB and 1.19 LSB, whereas the differential nonlinearity is between -0.71 LSB and 0.92 LSB. The ADC signal-to-noise-and-distortion ratio and spurious-free dynamic range are 56.1 and 67 dB with a 39.5-kHz sinusoid input, respectively. The ADC figure-of-merit is of 20 fJ per conversion step, which is very competitive, as compared with state-of-the-art ADCs in similar 180-nm CMOS technologies.

Rotational MEMS mirror with latching arm for silicon photonics

We present an innovative rotational MEMS mirror that can control the direction of propagation of light beams inside of planar waveguides implemented in silicon photonics. Potential applications include but are not limited to optical telecommunications, medical imaging, scan and spectrometry. The mirror has a half-cylinder shape with a radius of 300 μm that provides low and constant optical losses over the full angular displacement range. A circular comb drive structure is anchored such that it allows free or latched rotation experimentally demonstrated over 8.5° (X-Y planar rotational movement) using 290V electrostatic actuation. The entire MEMS structure was implemented using the MEMSCAP SOIMUMPs process. The center of the anchor beam is designed to be the approximate rotation point of the circular comb drive to counter the rotation offset of the mirror displacement. A mechanical characterization of the MEMS mirror is presented. The latching mechanism provides up to 20 different angular locking positions allowing the mirror to counter any resonance or vibration effects and it is actuated with an electrostatic linear comb drive. An innovative gap closing structure was designed to reduce optical propagation losses due to beam divergence in the interstitial space between the mirror and the planar waveguide. The gap closing structure is also electrostatically actuated and includes two side stoppers to prevent stiction.

Frequency tunable silicon carbide resonators for MEMS above IC

Micro-electromechanical beam resonators and arrays are fabricated using a custom low-temperature (< 300degC) CMOS-compatible silicon carbide micro-fabrication process. A special feature of this process is that it allows the integration of heaters directly onto the MEMS devices, thus enabling resonant frequency tuning with constant insertion loss and considerable extension of the tuning range. Characteristics for different devices are measured with quality factors of Q ap 1000 and resonant frequencies of up to 21.8 MHz.

A low-power OOK ultra-wideband receiver with power cycling

This paper presents a 0.13 μm CMOS impulse radio ultra-wideband receiver which supports the on-off keying modulation scheme. The receiver includes a wideband low-noise amplifier, and allows for control of the integration window to accommodate different number of pulses per symbol at bandwidths of up to 10.6 GHz. A power cycling scheme is implemented to reduce the power consumption, and allows the system to operate within stringent power requirements. The receiver was simulated at data rates of 10 Mbps with a maximum simulated power usage of 4.5 mW. Power cycling reduces the power consumption by a factor of 3.3.