X-Band Fin Resonant Body Transistors in 14nm CMOS Technology

Here we present the first demonstration and in-depth study of unreleased acoustic resonators in 14nm FinFET technology in the IEEE X band, which offer a zero-barrier-to-entry solution for high Q, small footprint, resonant tanks integrated seamlessly in advanced CMOS nodes. These devices leverage phononic waveguides for acoustic confinement, and exploit MOS capacitors and transistors inherent to the technology to electromechanically drive and sense acoustic vibrations. Sixteen device variations are analyzed across thirty bias points to discern the impact of phononic confinement, gate length, and termination scheme on resonator properties. The limiting factor in FinFET resonator performance among design variations tested is shown to be Back End of Line (BEOL) confinement, with devices with acoustic waveguides incorporating Mx and Cx metal layers exhibiting 2.2x higher average quality factor (Q) and peak amplitude, with maximum Q increasing from 115 to 181 and maximum amplitude scaling from 0.8 to 4.5 μS. A detailed analysis of biasing in the highest performing device shows good fit with a derived model, which addresses the velocity saturated piezoresistive effect for the first time in active resonant transistors. Peak differential transconductance that is dominated by changes in the silicon band-structure, as expected from an analysis that includes contributions from the piezoresistive effect, electrostatic modulation, and silicon bandgap modulation. As demand for wireless services grows, so too does the demand for higher frequency radio communication where bandwidth is more plentiful. With 5G FR2 bands extending beyond 10 GHz and FR1 intra-band congestion growing, novel solutions are required to enable high performance, adaptable radios. As a key element of both the filters and voltage controlled oscillators used in radios, high frequency resonators represent an active area of research, with recent developments including thin film bulk acoustic resonators with novel ferroelectric materials, as well as high frequency Lamb and Lamé wave resonators using piezoelectric lithium niobate or aluminum nitride thin films [1–4]. To address band congestion, multiple-input-multiple-output (MIMO) arrays are becoming increasingly common as a way to multiplex transmissions between users in the same band, with large arrays allowing for increased performance across end-users sharing spectrum [5]. The beam-forming enabled by such an antenna array is particularly promising for higher frequency bands, where propagation conditions are less stable due to increased RF absorption. These RF front-ends of increasing complexity require many more analog components to be integrated in a compact form factor, leading to active research in RF integration into traditional digital processes. Many high performance technologies require integration in MEMS-first or MEMS-last process schemes [6–8], which provides space and power savings over off-chip RF integration. The use of phononic crystal confinement, however, opens the door for resonator integration directly into a standard CMOS process, potentially enabling larger MIMO arrays in a smaller footprint at lower power [9, 10]. In addition to reducing parasitics from routing high frequency signals over long distances, this also provides the opportunity to use high performance transistors for active transduction, achieving potentially record-breaking performance [11]. Such an approach also opens the door for acoustic coupling between devices co-located on the same chip, reducing the need for power hungry phase locked loops and opening the door for exploration of highly scaled coupled oscillator systems for neuromorphic computing [12, 13]. This paper continues the evolution of such technology by introducing the first CMOS FinFET-based resonators utilizing acoustic waveguiding confinement operating in the IEEE X band (8-12 GHz). ∗Jackson Anderson (ander906@purdue.edu), Yanbo He, and Dana Weinstein are with the Department of Electrical and Computer Engineering, Purdue University, West Lafayette, Indiana, USA †Bichoy Bahr is with Texas Instruments Inc. Kilby Labs, Dallas, Texas, USA 1 ar X iv :2 10 7. 00 60 8v 1 [ ph ys ic s. ap pph ] 1 J ul 2 02 1