Resonant body transistors (RBTs) are solid state, actively sensed microelectromechanical systems (MEMS) resonators that can be implemented in commercial CMOS technologies. With small footprint, high- $Q$ , 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 $\sim 4.5$ -GHz frequency. With better energy confinement, the proposed CMOS-RBTs achieve a quality factor $Q$ of 252, which corresponds to $8\times $ improvement over the previous generation RBTs, which did not include PnCs. The presented devices have a footprint of 5 $\mu \text{m}~\times7\mu \text{m}$ . This paper concludes with a discussion of the properties required of a CMOS technology for high performance RBT implementation. [2014-0364]

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

This paper presents a wafer level packaged (WLP) RF-MEMS switch fabricated in a 0.13 μm SiGe BiCMOS process technology for D-Band (110–170 GHz) applications. For the wafer level packaging, thin film encapsulation approach is developed and used during the Back-End-of-Line (BEOL) fabrication process. The fabricated wafer level encapsulated (WLE) RF-MEMS switch provides a high capacitance C on /C off ratio of 11.1. The fabricated WLE RF-MEMS switch shows beyond state of the art RF performances of better than 0.67 dB insertion loss and more than 16 dB isolation in all D-band. The measured maximum isolation of the WLE switch is 51.6 dB at 142.8 GHz with an insertion loss of 0.65 dB.

Thin film wafer level encapsulated RF-MEMS switch for D-Band applications

Frequency sources and high quality filters based on mechanical resonators are essential building blocks for communication systems as well as analog and digital electronics. Driven by the continuous demand for reduction in power, size and overall cost, monolithic integration of mechanical resonators in standard integrated circuit (IC) technology has been the focus of multiple research efforts. Micro-Electro-Mechanical (MEM) resonators offer an ultimate solution, with 100× higher quality factors and 104× smaller footprint, when compared to on-chip LC tank circuits. A new class of truly solid-state, monolithically integrated, GHz-frequencies CMOSMEMS resonators is presented. No post-processing or special packaging of any kind is required beyond the standard CMOS process. Resonant body transistor (RBT) is constructed by using active field-effect-transistor (FET) sensing. A phononic crystal (PnC) implemented in the CMOS back-end-of-line (BEOL) layers along with the bulk wafer are used to create a phononic waveguide. The latter confines acoustic vibrations in the CMOS front-end-of-line (FEOL) layers. Operator-theoretic analysis for these waveguides is presented in explicit analogy to quantum mechanics and photonic waveguides; with a study of perturbation theory, coupled-mode theory and adiabatic theorem. Superior energy confinement is achieved, allowing record high Q ∼ 14, 800 and f◦ ·Q ∼ 4.85× 1013 for CMOS-RBTs at 3GHz. Simulation, modeling, optimization, prototyping and testing of these resonators is presented. RBTs in FinFET technologies are also exploerd, for resonance frequencies up to 33GHz. The thesis also explores the integration of Lamb-mode resonators in standard GaN monolithic-microwave-IC (MMIC) process. The first monolithic 1GHz MEMS-based oscillator in standard GaN MMIC technology is demonstrated, together with monolithic lattice and ladder filters. This allows for complete RF front-ends in GaN MMIC technology. Thesis Supervisor: Dana Weinstein Title: Associate Professor Thesis Supervisor: Luca Daniel Title: Professor

Monolithically integrated MEMS resonators and oscillators in standard IC technology

This paper presents RF-MEMS test vehicles: Single-Pole Single-Throw (SPST), Single-Pole Double-Throw (SPDT) switches and high-voltage generators integrated with SPST switches that are designed and fabricated in IHP's 0.25 μm SiGe BiCMOS technology for K-band (18–27 GHz) space applications. All the fabricated RF-MEMS test vehicles are packaged by a wafer-to-wafer packaging technique. The fabricated RF-MEMS test vehicles are electrically characterized on 8-inch wafer level and the experimental data clearly shows the high yield of the packaged RF-MEMS test vehicles; thus promising for prototyping and volume production.

Packaged BiCMOS embedded RF-MEMS test vehicles for space applications

Packaging poses a critical challenge for commercialization of MEMS (micro-electro-mechanical system) products. The packaging provides the critical structural and environmental protection necessary for fielding repeatable and reliable MEMS devices. For high volume markets such as mobile wireless terminals, where the technical benefits of RF-MEMS are highly attractive, the cost of effective packaging and assembly is a primary barrier to their adoption. RF-MEMS packaging is a nascent engineering field with widely varying approaches being explored to balance the broad range of technological and cost constraints. The cost of the packaging itself and all processing steps used to assemble the final product must be comparable to conventional packaging approaches used throughout the semiconductor industry. At the same time it must provide the necessary protection for the MEMS surfaces throughout the life of the product and not degrade the high RF performance provided by the MEMS. A key related aspect is additional functional integration enabled through MEMS that reduces the number of components required for a given application. In this paper, we describe the breadth of the RF-MEMS technologies and their associated packaging challenges. We follow with a range of implemented and proposed solutions to these challenges from both commercial and academic sources. Detailed comparisons in functionality, size and cost are drawn for a range of proposed metrics. We continue with solution recommendations for various functions and markets based on current technology and conclude with suggestions for future research and development.

S. Cunningham

Papers

Challenges and Solutions for Cost-effective RF-MEMS Packaging