Abstract
High Q-factor inductors are critical in designing high performance RF/microwave circuits on SOI technology. Substrate losses is a key limiting factor when designing inductors with high Q-factors. In this context, we report a substrate engineering method that enables improvement of quality factors of already fabricated inductors on SOI. A novel femtosecond laser milling process is utilized for the fabrication of locally suspended membranes of inductors with handler silicon completely etched. Such flexible membranes suspended freely on the BOX show up to 92 % improvement in Q-factor for single turn inductor. The improvement in Q-factor is reported on large sized inductors due to reduced parallel capacitance which allows enhanced operation of inductors at high frequencies. A compact model extraction methodology has been developed to model inductor membranes. These membranes have been utilized for the improvement of noise performance of LNA working in the 4.9 - 5.9 GHz range. A 0.1 dB improvement in noise figure has been reported by taking an existing design and suspending the input side inductors of the LNA circuit. The substrate engineering method reported in this work is not only applicable to inductors but also to active circuits, making it a powerful tool for enhancement of RF devices.
Highlights
Integrated inductors on Silicon-on-Insulator (SOI) technology are critical components for the proper design of monolithic RF/microwave integrated circuits
The realization of high quality factor (Q-factor) integrated inductors is important for several RF design cases like input impedance matching for LNA [1], high quality LC resonators for voltage controlled oscillators (VCO) [2], filters [3], mixers [4], power amplifiers [5]
We have developed the Femtosecond Laser Assisted Micromachining and Etch (FLAME) process to fabricate membranes of RF circuits [16], [17]
Summary
Integrated inductors on Silicon-on-Insulator (SOI) technology are critical components for the proper design of monolithic RF/microwave integrated circuits. Substrate is one of the most important limiting factors in determining the maximum Q-factor This is especially true of process technologies which use parallel stacking of all metal layers for lowering ohmic resistance of inductor [6], [7]. Improvement of electrical performance of RF devices has been reported by complete removal of handler substrate on SOI wafer and transfer onto thin flexible substrate [10]–[15]. While this approach has been shown to work efficiently, some of the drawbacks are higher number of steps in fabrication, possible RF losses in bonding material, mechanical weakening and overall cost of treatment.
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