Intrinsic stress-induced bending as a platform technology for controlled self-assembly of high-Q on-chip RF inductors

Abstract

This work reports on the modelling and process technology development for the design and fabrication of vertical, 3D, monolithic RF-MEMS inductors based on self-assembly via intrinsic stresses otherwise referred to as residual or internal stresses in thin films. Stressinduced bending in different cantilever designs were modelled at various film thicknesses using finite element analysis method and bending conditions were optimized. Intrinsic stress-induced bending mechanism is verified by fabrication of bi-layer metallic micro cantilever structures with varying stress conditions which reach bending angles of up to 137° and possibly more upon release. By modulating the loading mode (tensile or compressive) along the beam length, complex out-of-plane wavy cantilevers with multiple upward and/ or downward bends were realized. The fabrication and modelling results display large overlap which further demonstrates the applicability of intrinsic stress-induced bending as a controllable technology towards fabrication of out-of-plane 3D micro components. Additionally, as a potential application to RF-MEMS inductors, stress-induced self-assembly of patterned thin film stacks into out-of-plane inductor topologies of varying geometries was investigated. Electromagnetic modelling tools were used to study the effect of bending on inductor performance (primarily the Q factor and self-resonance frequency-f SR ), and results were evaluated by comparison to planar inductors of the same number of turns and dimensions, which revealed Q factor and f SR improvement of more than 100% upon bending away from substrate surface.

Nonlinear restructuring of patterned thin films by residual stress engineering into out-of-plane wavy-shaped electrostatic microactuators for high-performance radio-frequency switches

“…Metallic thin films, when deposited on a substrate, often develop residual stresses that depend on the type of deposition technique as well as the deposition parameters 22 . By stacking multiple thin films with different residual stresses, a stress gradient can be created along the thickness, through which out-of-plane bending can be induced in thin films upon their release from the substrate.…”

“…To achieve controllable bending, we first characterized the stresses in Cr and Cu films based on Stoney’s equation (detailed in Supplementary Information Section S1 ). Evaporated chromium has been shown to possess a large tensile stress at low thicknesses (below 50 nm), while the tensile stress in Cu is generally low over a broad range of thicknesses 22 , 23 . Consistent with the literature 23 , we observed a large stress variation in the Cr thin film with the thickness (Fig.…”

Integrated On-Chip Transformers: Recent Progress in the Design, Layout, Modeling and Fabrication

“…Likewise, on-chip transformer with three-solenoid windings functioning as a balun with improved performance was fabricated based on a CMOS compatible post-process called “concave-suspending micromachining” [14,78]. A more recent effort to realize on-chip transformers with CMOS-compatible processes employed the concept of residual stresses [79] to roll-up planar membranes into a three-dimensional coil geometry [76] (Figure 6e). To improve the performance of the transformer, attempts were also made to incorporate magnetic cores to the on-chip coil [80,81,82], where the magnetic layer was built over the spiral transformer in the form of a bridge structure.…”

Machine Learning-Based Modeling and Generic Design Optimization Methodology for Radio-Frequency Microelectromechanical Devices

“…Therefore, along with better performing active circuitry, high-performance RF passive components have also become a key to realize any modern-day RFIC transceiver. Unfortunately, owing to inherent technological limitations, RF passive devices based on standard IC fabrication techniques face a large number of losses (e.g., substrate proximity loss), and hence restrict the overall performance of RFICs [1][2][3]. On the other hand, the field of MEMSs (microelectromechanical systems) has surfaced as a potential substitute for fabricating integrated high-performance on-chip RF passive components using advanced micromachining techniques [1][2][3][4].…”

“…Unfortunately, owing to inherent technological limitations, RF passive devices based on standard IC fabrication techniques face a large number of losses (e.g., substrate proximity loss), and hence restrict the overall performance of RFICs [1][2][3]. On the other hand, the field of MEMSs (microelectromechanical systems) has surfaced as a potential substitute for fabricating integrated high-performance on-chip RF passive components using advanced micromachining techniques [1][2][3][4]. MEMS-based RF passives (i.e., inductors, tunable capacitors, switches, transformers, phase shifters, etc.…”