Sanchitha Fernando

Low-Loss Broadband Package Platform With Surface Passivation and TSV for Wafer-Level Packaging of RF-MEMS Devices

Packaging of radio frequency (RF) microelectromechanical system (MEMS) devices requires a good electrical performance, and thus requires the parasitic effects of packaging to be minimal. This paper presents the design, fabrication, and characterization of improved low-loss wafer-level packaging (WLP) platform with through-silicon-via (TSV) interposer for RF-MEMS packaging. The insertion loss because of parasitic effects is reduced using optimized grounding configuration and surface passivation. The coplanar waveguide (CPW) test vehicle, high-frequency antenna, RF tuner, and film bulk acoustic resonator (FBAR) filter are fabricated and characterized using the developed WLP platform with TSV. To determine the RF performance of the package, the CPW transmission lines are fabricated on high-resistivity Si substrates, and the grounding configuration is optimized. The fabrication of the RF-MEMS WLP involves the process of a TSV cap wafer and CPW transmission lines or RF MEMS on the substrate wafer. TSV cap process includes TSV etching, void-free TSV plating, and redistribution layer postprocess on thin TSV wafer. The electrical characterization of the fabricated devices is performed. The optimized model has a wide bandwidth of 26.5 GHz and a packaging loss of 0.1 dB at 10 GHz. This implies that the effect of packaging on the performance of RF device is expected to be minor. The developed WLP platform is used for the 94-GHz antenna, RF tuner, and FBAR filter packaging and the characterization of RF-MEMS devices is presented.

A monolithic 9 degree of freedom (DOF) capacitive inertial MEMS platform

A monolithic 9 degree of freedom capacitive inertial MEMS platform is presented in this paper. This platform for the first time integrates 3 axis gyroscopes, accelerometers, and Lorentz Force magnetometers together on the same chip without using any magnetic materials. This reduces the assembly cost, and fully eliminates the need of magnetic material processing and axis misalignment calibration. The fabricated sensors, vacuum packaged (vacuum ~100mTorr) at wafer level with epi-polysilicon through silicon interposer (TSI) wafer using eutectic bonding, performed within 10% of the simulation results.

AlN-on-SOI platform-based micro-machined hydrophone

This paper reports a piezoelectric aluminum nitride (AlN) based micro-machined infrasonic hydrophone. We have conducted a systematic design study for the hydrophone sensor to meet the stringent requirements of underwater applications. The hydrophone sensor was fabricated on a cavity silicon-on-insulator (SOI) substrate using an in-house CMOS-compatible AlN-on-SOI process platform. A 5 × 5 arrayed hydrophone sensor was characterized thoroughly using an industry-standard hydrophone calibration instrument. The results show that the hydrophone achieved a sound sensitivity of −182.5 dB ± 0.3 dB (ref. to 1 V rms/μPa) and an eligible acceleration sensitivity of only −196.5 dB (ref. to 1 V rms/μg), respectively, a non-linearity of 0.11%, a noise resolution of 57.5 dB referenced to 1 μPa/√Hz within an ultra-low operation bandwidth of 10 Hz∼100 Hz, the highest noise resolution of micro-machined hydrophones reported to date, and better than traditional bulky hydrophones in terms of the same application. The size of the 5 × 5 arrayed hydrophone sensor is about 2 mm × 2 mm.

Integration of AlN with molybdenum electrodes and sacrificial amorphous silicon release using XeF2

This paper presents a new post-CMOS-compatible integration scheme for AlN-based MEMS devices. The proposed scheme integrates molybdenum (Mo) bottom electrodes with an amorphous silicon (a-Si) sacrificial layer, which is etched using XeF2 to release the MEMS structures. This integration approach faces two potential issues, which are solved in this work: (i) poor adhesion of AlN with a-Si, and (ii) XeF2 attacking the Mo electrode during the removal of the a-Si sacrificial layer. The adhesion problem was solved by introducing a thin oxide layer between a-Si and AlN. The sidewalls of the Mo electrodes were protected by a 0.2 µm thick SiN spacer layer from the XeF2 attack. The robustness of the integration scheme was verified by fabricating an FBAR band pass filter. RF measurements on the FBAR band pass filter show that the proposed integration works well and can be utilized for other AlN-based MEMS devices in post-CMOS applications.

Wafer level packaging of RF MEMS devices using TSV interposer technology

This paper presents the design, fabrication and characterization of MEMS wafer level packaging (WLP) with TSV based silicon interposer as cap wafer. High resistivity Si wafers have been used for TSV interposer fabrication mainly to minimize the intrinsic loss of RF MEMS device due to packaging. During development of this RF MEMS WLP, many key challenging processes have been developed such as, high aspect ratio TSV fabrication, double side RDL fabrication, thin wafer handling of TSV interposer and optimization of Au-Sn based TLP bonding. There are several fabrication steps involved in the actual process flow as, a) TSV fabrication and front side RDL patterning and passivation, b) Wafer thinning and backside RDL patterning and passivation c) UBM/ seal ring solder deposition and cavity formation, and d) TLP based wafer bonding of cap TSV interposer wafer with MEMS CPW wafer. Different CPW designs with three passivation schemes have been fabricated mainly to study the effect of passivation on insertion loss and ultimately quantify the packaging insertion loss. In pre-bonding testing, effect of passivation on insertion loss is thoroughly studied. After successful fabrication of the WLP, loss of RF device characteristics due to packaging has been studied. Before and after packaging, S-parameter measurements performed on coplanar waveguides (CPW). Amongst different passivation schemes, CPW structures with poly-silicon passivation have shown better performance.

AlN Actuator for Tunable RFMEMS Capacitor

This paper presents a novel piezoelectric actuator design that achieves low curling due to residual film stress. The proposed actuator maintains the gap between the movable electrode and the fixed electrode nearly constant independent of the residual stress level, improving the reproducibility and reliability of piezoelectric devices. At 20V excitation, the actuator deflects more than 5 µm. The design also achieves a capacitor electrode around 6% of the total actuator area, which is 2.5 times greater than other reported designs. This paper demonstrates the novel actuator in a tunable capacitor, but the actuator may be used in many other applications, such as MEMS switches and micro-mirrors.

Aln-on-SOI platform-based MEMS hydrophone with ultra-low operation frequency and ultra-high noise resolution

This paper reports a piezoelectric aluminum nitride (AlN) based MEMS hydrophone with ultra-low operation frequency and ultra-high noise resolution. The hydrophone was fabricated on a cavity SOI substrate using a 7-mask AlN-on-SOI process platform. The fabricated hydrophone achieved a non-linearity of 0.11%, a sensitivity of -182.5dB±0.3dB (ref. to 1Vrms/μPa) and a noise floor of -125dBV/√Hz, i.e. a noise resolution 57.5dB referenced to 1 μPa/√Hz within the ultra-low operation frequency of 10Hz~100Hz, the highest noise resolution of MEMS hydrophones and better than traditional bulky hydrophones[1].