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Lithium Niobate Acoustoelectric Platforms for Integrated Non-Reciprocal RF MEMS Devices
thesisposted on 16.03.2021, 18:24 by Matthew J Storey
Some of the biggest challenges with analog signal processing at radio frequencies (RF) are: RF loss at the frequency of interest, large enough fractional bandwidth, and sufficient delay. It is difficult to achieve enough delay in radio front ends using a purely electromagnetic approach since it is limited to a fraction of the speed of light. A solution has been the use of acoustic RF devices, such as surface acoustic wave (SAW) delaylines and MEMS filters. For some acoustic RF devices, like high performance Transmit and Receive SAW correlators, the long delays introduce significant propagation losses. These propagation losses can be compensated within the device by integrating a low noise amplifier into the acoustic correlator architecture. This can be accomplished by designing the SAW correlator on a high performance acoustoelectric (AE) platform. The AE effect is a phenomenon where nearby free carriers can interact with a travelling acoustic wave. Free carriers in close proximity to a piezoelectric material can interact with a travelling acoustic wave through its periodic potential. When a drift field is applied, depending on the relative velocity difference between the free carriers and acoustic wave, energy can either be transferred into (amplification) or out of (attenuation) the acoustic wave.
This thesis investigates the design and feasibility of AE MEMS devices on several Lithium Niobate (LN) platforms. First, the key acoustic and free carrier parameters are discussed and optimized for an ideal high performance AE material stack. In order to debug and analyze the performance of intermediate steps in the process of making high performance AE MEMS devices, three LN-based platforms are used throughout this work. These platforms help further examine some of the key challenges associated with making a high performance AE platform, like wafer bonding, fabrication, device design, and device operating conditions. These material stacks consist of: thin film LN bonded to a silicon wafer (LNOSi), thin film LN bonded to a silicon on insulator wafer (LNOSOI), and epitaxial indium gallium arsenide bonded to a LN wafer (InGaAs-LN).
The acoustic and piezoelectric performance of SAW devices on the LNOSi and LNOSOI platforms are modeled using COMSOL Multiphysics. A full study is performed to determine the piezoelectric coupling coefficient variation vs. device wavelength, propagation angle, transducer metal, and acoustic mode. A lumped element cross-field Mason model is modified to include substrate conductivity and simulated in Advanced Design System (ADS) software. SAW delaylines are then fabricated with both aluminum (Al) and gold (Au) Interdigital Transducers (IDT) and measured to compare to the simulated results. The analytical AE theory is then presented and calculations are performed to determine the desired (optimum) carrier concentration for AE devices. In addition to the 1D analytical AE model, initial work is done on developing a generalized 2D Finite Element Analysis (FEA) AE modeling scheme in COMSOL. The results for a piezoelectric semiconductor bulk acoustic wave (BAW) resonator and SAW delayline amplifier are presented.
On the LNOSi platform, gate controlled passive AE delaylines are fabricated and measured to examine the effects of LN bonding on Silicon free carrier concentrations and interface charges. Then, the fabrication and initial measurement results for doped Silicon AE delayline amplifiers are outlined. Based on the device design, the non-reciprocal nature of the AE effect can be used for more than just amplification and loss compensation. Using the InGaAs-LN platform, several classes of AE devices are designed and tested in pulsed mode operation. First, a series of segmented AE delayline amplifiers are measured to look at how the relative AE gain performance and input DC power scale with acoustic frequency, segment unit length, and number of segments. By taking advantage of the non-reciprocal shift in acoustic velocity, a dual-voltage AE delayline phase shifter is designed and tested. Routing of the acoustic waves between parallel delaylines can be accomplished through multistrip couplers (MSC) and can increase the library of possible AE device designs. The simplest example is a 3-port AE switch, which is designed and tested. The demonstration of these AE MEMS devices opens the door to a larger library of non-reciprocal acoustic devices utilizing the AE effect in high performance integrated material platforms.