Bichoy Bahr

Oscillator Array Models for Associative Memory and Pattern Recognition

Brain-inspired arrays of parallel processing oscillators represent an intriguing alternative to traditional computational methods for data analysis and recognition. This alternative is now becoming more concrete thanks to the advent of emerging oscillators fabrication technologies providing high density packaging and low power consumption. One challenging issue related to oscillator arrays is the large number of system parameters and the lack of efficient computational techniques for array simulation and performance verification. This paper provides a realistic phase-domain modeling and simulation methodology of oscillator arrays which is able to account for the relevant device nonidealities. The model is employed to investigate the associative memory performance of arrays composed of resonant LC oscillators.

Resonant Body Transistors in IBM's 32 nm SOI CMOS Technology

This paper presents unreleased CMOS-integrated MEMS resonators fabricated at the transistor level of IBMs 32SOI technology and realized without the need for any postprocessing or packaging. In this technology, resonant body transistors (RBTs) are driven capacitively and sensed piezoresistively using an n-channel field effect transistor (FET). Acoustic Bragg Reflectors (ABRs) are used to localize acoustic vibrations in the unreleased resonators completely buried under the CMOS metal stack and surrounded by low-κ dielectric. FET sensing is analytically compared with alternative active and passive sensing mechanisms to benchmark CMOS-MEMS resonator performance with frequency scaling. Experimental results from the first generation hybrid CMOS-MEMS RBTs show RBTs operating above 11 GHz with Qs of 24-30 and footprints of 5 × 3 μm. Comparative behavior of devices with design variations is used to demonstrate the effect of ABRs on spurious mode suppression. In addition, the performance of the RBTs is compared with passive electrostatic resonators, which show no discernible peak. Finally, temperature stability of <;3 ppm/K due to complimentary materials in the CMOS stack is analytically and experimentally verified.

Analysis and Design of Weakly Coupled LC Oscillator Arrays Based on Phase-Domain Macromodels

An array of weakly coupled oscillators can generate multiphase signals, i.e., multiple sinusoidal signals with specific phase separations. Multiphase oscillators are attractive solutions in many electronic applications such as the synchronization of multiple processing units in digital electronics and the frequency synthesis in mixed-signal radio frequency circuits. Due to the complexity of multiphase oscillators and the large number of design parameters, novel simulation techniques are highly desired to efficiently handle such large-scale problems. In this paper, an efficient phase-domain simulation technique is proposed to calculate the phase response of inductance capacitance oscillator array. By some practical examples, it is shown how the proposed method can be exploited to identify the array topologies and parameter settings that guarantee stable phase separations. It is also shown how the proposed technique can be used to evaluate phase-noise performance.

Theory and Design of Phononic Crystals for Unreleased CMOS-MEMS Resonant Body Transistors

Resonant body transistors (RBTs) are solid state, actively sensed microelectromechanical systems (MEMS) resonators that can be implemented in commercial CMOS technologies. With small footprint, highQ, and scalability to gigahertz frequencies, they form basic building blocks for radio frequency (RF) front-ends and timing applications. Toward the goal of seamless CMOS integration, this paper presents the design and implementation of phononic crystals (PnCs) in the back-end-of-line (BEOL) of commercial CMOS technologies with bandgaps in the gigahertz frequencies to be used for enhanced acoustical confinement in CMOS-RBTs. Lithographically defined PnC dimensions allow for bandgap engineering, providing flexibility in resonator design, and allowing for multiple frequencies on a single chip. The theoretical basis for analyzing generic PnCs is presented, with focus on the special case of implementing PnCs in CMOS BEOL layers. The effect of CMOS process variations on the performance of such PnCs is also considered. The analysis presented in this paper establishes a methodology for assessing different CMOS technologies for the integration of unreleased CMOS-MEMS resonators. This paper also discusses the importance of uniformity of the acoustical cavity in the nonresonant dimension and its effect on overall resonator performance. A PnC implementation in IBM 32-nm silicon on insulator (SOI) BEOL layers is demonstrated to achieve 85% fractional-bandgap ~4.5-GHz frequency. With better energy confinement, the proposed CMOS-RBTs achieve a quality factor Q of 252, which corresponds to 8× improvement over the previous generation RBTs, which did not include PnCs. The presented devices have a footprint of 5 μm × 7 μm. This paper concludes with a discussion of the properties required of a CMOS technology for high performance RBT implementation.

Shape optimization of solidair porous phononic crystal slabs with widest full 3D bandgap for in-plane acoustic waves

The use of Phononic Crystals (PnCs) as smart materials in structures and microstructures is growing due to their tunable dynamical properties and to the wide range of possible applications. PnCs are periodic structures that exhibit elastic wave scattering for a certain band of frequencies (called bandgap), depending on the geometric and material properties of the fundamental unit cell of the crystal. PnCs slabs can be represented by plane-extruded structures composed of a single material with periodic perforations. Such a configuration is very interesting, especially in Micro Electro-Mechanical Systems industry, due to the easy fabrication procedure. A lot of topologies can be found in the literature for PnCs with square-symmetric unit cell that exhibit complete 2D bandgaps; however, due to the application demand, it is desirable to find the best topologies in order to guarantee full bandgaps referred to in-plane wave propagation in the complete 3D structure. In this work, by means of a novel and fast implementation of the Bidirectional Evolutionary Structural Optimization technique, shape optimization is conducted on the hole shape obtaining several topologies, also with non-square-symmetric unit cell, endowed with complete 3D full bandgaps for in-plane waves. Model order reduction technique is adopted to reduce the computational time in the wave dispersion analysis. The 3D features of the PnC unit cell endowed with the widest full bandgap are then completely analyzed, paying attention to engineering design issues.

Temperature coefficient of frequency modeling for CMOS-MEMS bulk mode composite resonators

CMOS-MEMS resonators, which are promising building blocks for achieving monolithic integration of MEMS structure, can be used for timing and filtering applications, and control circuitry. SiO2 has been used to make MEMS resonators with quality factor Q > 104, but temperature instability remains a major challenge. In this paper, a design that uses an embedded metal block for temperature compensation is proposed and shows sub-ppm temperature stability (-0.21 ppm/K). A comprehensive analytical model is derived and applied to analyze and optimize the temperature coefficient of frequency (TCF) of the CMOS-MEMS composite material resonator. Comparison with finite element method simulation demonstrates good accuracy. The model can also be applied to predict and analyze the TCF of MEMS resonators with arbitrary mode shape, and its integration with simulation packages enables interactive and efficient design process.

Phononic crystals for acoustic confinement in CMOS-MEMS resonators

This work presents the first implementation of phononic crystals (PnCs) in a standard CMOS process to realize high-Q RF MEMS resonators at GHz frequencies without the need for any post-processing or packaging. An unreleased acoustic resonant cavity is defined using a PnC comprising back-end of line (BEOL) materials such as routing metals and low-k intermetal dielectric. A CMOS-MEMS resonant body transistor (RBT) with electrostatic driving and piezoresistive sensing is implemented within this cavity. This results in a 10× enhancement in Q at resonance and improved suppression of spurious modes off-resonance as compared to first-generation CMOS-integrated devices. The first PnC-confined RBT is demonstrated in IBMs 32 nm SOI process at 2.8 GHz with Q of 252, spanning a footprint of 5 μm × 7 μm.

Analysis and Design of Boolean Associative Memories Made of Resonant Oscillator Arrays

This paper investigates some relevant open issues related to implementing Boolean associative memories using oscillator arrays. At the circuit level, the employment of a class of MEMS-based oscillators which is ideal for large arrays realizations is herein considered. At the system level, the crucial problems of array connectivity and spurious patterns generation are explored in detail. As a result, an enhanced training rule is proposed which is able to simplify the array architecture while improving memory association capability.

16.8 1GHz GaN-MMIC monolithically integrated MEMS-based oscillators

Low phase noise oscillators are essential building blocks in the front end of all communication systems. With the continuous demand for higher data rates and reduced size, weight, and power consumption, significant research in the past decade has been geared toward integrating high-Q GHz frequency MEMS resonators with standard circuit technologies. This paper presents monolithic integration of GaN MEMS resonators in GaN MMIC technology, which has become mainstream in RF LNA and PA design. Groups at MIT, Michigan and IEMN have previously demonstrated GaN MEMS resonators co-fabricated with HEMTs. This work is the demonstration of a single-chip closed-loop oscillator circuit implementing both passive and active devices in this growing technology platform.

Reducing Phase Noise in Multi-Phase Oscillators

This paper investigates phase noise mechanism in arrays of resonant LC oscillators. Such arrays represent today a promising solution for the generation of multi-phase signals needed in several advanced applications. The analysis presented in this paper relies on consolidated phase-domain macromodels as well as on the original concept of noise transfer function illustrated herein. The proposed analysis sheds new light on noise generation in oscillator arrays and is able to explain certain noise degradation effects observed in nonreciprocal coupling networks. Phase-domain simulation together with noise transfer function concept provide a very efficient computational tool for rapid calculations of phase response and output noise. Thanks to this efficient tool and to the gained qualitative understanding, we are able to propose a chain array configuration enhanced by the injection of a clean, low-noise signal. In this paper, it is shown how the injected chain array can provide the prescribed phase separation while significantly reducing output phase noise.