Srinivas Merugu

A High Coupling Coefficient 2.3-GHz AlN Resonator for High Band LTE Filtering Application

This letter reports an aluminium nitride (AlN)based micromechanical resonator with high-effective coupling coefficient (k2eff) and low insertion loss (IL), which are comparable with those of Film Bulk Acoustic Resonators (FBARs). The in-house-fabricated resonator comprises of lithographically patterned top and bottom molybdenum interdigitated electrode fingers and a layer of 1-μm-thick AlN sandwiched in between. Synergetic inter-mode coupling between the constituent thickness mode and the lateral mode can be realized within a wide frequency range, which can be treated as a subcategory of degenerated cross-sectional Lamé mode, enabling the capability of lithographic tuning of resonant frequency yet not compromising k2eff. Measurement results show that the designed 2.3-GHz resonator achieves a k2eff of 6.34% and an IL of 0.26 dB upon direct connection to a network with 50-Ω termination, making it a promising candidate for Wireless Local Area Network (WLAN) and high band Long Term Evolution (LTE) band selection filtering applications.

Improving aluminum nitride plasma etch process for MEMS applications

We present a new plasma etch process optimized for etching piezoelectric aluminum nitride (AlN) film deposited on thin molybdenum (Mo) metal electrode. Such film stack finds application in the integration of AlN-based RF microelectromechanical systems devices. The process is based on Cl2/BCl3/Ar gas chemistry with added buffer gas in inductively coupled plasma reactive ion etching system. The new gas mixture overcomes a generic problem of etched surface roughness without significant drop in AlN etch rate. Using design of experiment, the process window is optimized for improving selectivity to Mo and reducing microtrenching while maintaining smooth etched surface. Finally, an etching rate of 280 nm min−1 with reliable etch stop on Mo electrode and smooth bottom surface is reported. The integration suitability of the developed etch process is tested by etching 2.0 to 5.0 µm size square shaped via holes in 1.0 µm thick (0 0 2) oriented piezoelectric AlN on 0.2 µm thick Mo electrode while integrating contour mode resonators.

GHz spurious mode free AlN lamb wave resonator with high figure of merit using one dimensional phononic crystal tethers

This letter reports a spurious mode free GHz aluminum nitride (AlN) lamb wave resonator (LWR) towards high figure of merit (FOM). One dimensional gourd-shape phononic crystal (PnC) tether with large phononic bandgaps is employed to reduce the acoustic energy dissipation into the substrate. The periodic PnC tethers are based on a 1 μm-thick AlN layer with 0.26 μm-thick Mo layer on top. A clean spectrum over a wide frequency range is obtained from the measurement, which indicates a wide-band suppression of spurious modes. Experimental results demonstrate that the fabricated AlN LWR has an insertion loss of 5.2 dB and a loaded quality factor (Q) of 1893 at 1.02 GHz measured in air. An impressive ratio of the resistance at parallel resonance (Rp) to the resistance at series resonance (Rs) of 49.8 dB is obtained, which is an indication of high FOM for LWR. The high Rp to Rs ratio is one of the most important parameters to design a radio frequency filter with steep roll-off.

Dual MEMS Resonator Structure for Temperature Sensor Applications

This paper reports an acoustic microelectromechanical system (MEMS) resonator structure that features dual-resonant response at 180 and 500 MHz. The MEMS structure uses aluminum nitride as acoustic layer and electrodes with dual design that provide dual-resonance behavior. Each resonant mode operates at the first symmetrical Lamb-wave mode (S0). Due to the large frequency separation between modes, device exhibits differentiated temperature coefficient of frequency) for each mode, which makes this structure suitable for thermometric beat frequency sensing. Reported devices are thus capable to multiply the 20-ppm/°C thermal sensitivity of the individual sensors by one order of magnitude, up to −334 ppm/°C for the thermometric beat frequency sensor.

Cavity-enhanced sacrificial layer micromachining for faster release of thin film encapsulated MEMS

This paper reports a method for fast-release and safe-sealing of thin film encapsulation (TFE) for packaging of piezoelectric MEMS devices fabricated on cavity-SOI wafers. For fast releasing of the TFE, MEMS device trenches and the cavity below them were utilized and this combination acted as etch channels. Etchant can attack the sacrificial layer from the bottom of the MEMS device as well as side-located channels. A side-located etch channel scheme was chosen to ensure safe-sealing without mass-loading. 120 µm  ×  120 µm sized encapsulation on top of the MEMS device with eight isolation trenches connected to the cavity was released in 25 min. This is twice as fast as the TFE fabricated on bulk wafer using a similar encapsulation scheme. This reduction in release time is a consequence of a prefabricated cavity underneath the device which allows the etchant to attack the sacrificial layer at multiple locations as etchant can pass from one isolation trench to another.

A Robust Bilayer Cap in Thin Film Encapsulation for MEMS Device Application

Thin-film encapsulation (TFE) is one of the promising wafer-level packaging techniques for microelectromechanical systems (MEMS) devices. One of the drawbacks of TFE is that it is difficult to encapsulate large MEMS devices due to the downward deformation of the cap layer during the release of the TFE. This paper presents a robust bilayer cap made out of aluminum nitride (AlN)/nickel (Ni) layers, which can solve the problem of downward deformation of the cap layer. This bilayer cap provides both strength and flexibility to the TFE. This bilayer cap is simulated using ANSYS and experimentally demonstrated by encapsulating cavities as large as 1200 μm × 1200 μm with 0.5 μm AlN/0.7 μm Ni, whereas even 400 μm × 400 μm-sized TFE cannot be fabricated without cracks with a solitary 1-μm-thick AlN cap layer. Standard microfabrication processes are used for the fabrication of this bilayer cap.

Reconfigurable AlN resonator filter design based on extended statistical element selection

This paper reports on a novel reconfigurable design for an aluminum nitride (AlN) MEMS resonator filter with a GHz level center frequency (f<inf>0</inf>). Measurement of resonators fabricated by an AlN foundry indicates roughly linear dependence of f<inf>0</inf> on filter location due to process variation over a 4 mm² die area. To improve f<inf>0</inf> performance yield, an Extended Statistical Element Selection (ESES) approach is implemented. The measured ESES 50 Ω matched filter demonstrates a 10× improvement in success rate for meeting the f<inf>0</inf> target of 1.16 GHz within a ±20 kHz tolerance.

Self-healing narrowband filters via 3D heterogeneous integration of AlN MEMS and CMOS chips

We demonstrated the statistical element selection (SES) technique with an array of 1.15 GHz AlN MEMS subfilters fabricated in an 8” Si fabrication facility by A∗STAR, Institute of Microelectronics (IME), Singapore. The chip integration is achieved through a split-fab process where the hermetically thin-film-encapsulated (TFE) MEMS resonators are solder-bump bonded to a CMOS chip fabricated in a Samsung 28 nm process. The SES offers 495 unique filter frequency responses with a tuning range of 150 kHz for the center frequency (f0) and 250 kHz for the bandwidth (BW). An insertion loss (IL) < 3.4 dB and an out-of-band rejection (OBR) > 15 dB were also achieved for the same self-healing filter. Compared to RF signal routing on CMOS, the use of redistribution layers (RDL) on the MEMS chip reduced the routing parasitic capacitance by ∼20x and the resistance by ∼5x, which, in turn, led to a direct 50 Ω matching.

Evaluation of thin film encapsulation strength for commercial packaging

In this study, thin film encapsulation (TFE) strength is evaluated, whether it can withstand the force required for basic system level integration such as compression molding process, flip-chip bonding process, drop test etc. TFE withstood 100 MPa pressure, while TFE started cracking on the application of 200 MPa pressure. The following tests were performed on the TFE to qualify its suitability for normal packaging: (i) Vacuum sucking test, (ii) drop test, and (iii) compression molding.

High-Band AlN Based RF-MEMS Resonator for TSV Integration

This paper reports two types of in-house fabricated aluminium nitride (AlN) based piezoelectric resonators, namely the thickness mode resonator and the Lamb-wave mode resonator, which are capable to be integrated with Through Silicon Via (TSV) technology, forming the basis of advanced filters, duplexers and multiplexers. Both types of the resonators, which are fabricated using a CMOS compatible platform, consist of a layer of 1 µm thick piezoelectric layer and two layers of molybdenum (Mo) electrodes covering the top and the bottom surface of the AlN layer. Resonant frequencies above 2GHz, as well as motional impedance less than 10Ω, are obtained when the fabricated resonators are connected directly to the 50Ω terminations of a network analyzer, making both types of resonators suitable for high-band LTE applications. Furthermore, negligible performance drift was observed for both types of resonators fabricated upon undergoing accelerated thermal cycling test, indicating the superior reliability and long-term stability of the fabricated AlN based MEMS resonators and showing their great potential for communications applications in the automotive industry, where reliability and long-term stability is a key requirement for device performance.