Multiphysics devices

Our research group is dedicated to explore the potential of Scandium Aluminum Nitride (ScAlN) in ferroelectric memory (FeRAM) and piezoelectric RFMEMS applications. In FeRAM application, this recently found ferroelectric material offers several advantages over other materials, including the high Curie temperature, high remnant polarization and steep polarization switching loop. These properties make it a promising candidate to replace current flash memory with large memory window and long reliability, especially in harsh environments such as aerospace and military applications. As a mainstream piezoelectric material, ScAlN's high acoustic velocity, piezoelectric coefficients and low acoustic loss make it attractive for use in radio frequency (RF) resonators & filters, actuators, and various sensors. Besides ScAlN, we are also investigating the LiNbO₃-based RFMEMS devices and the HfZrO₂-based FeRAM and RRAM devices.

With the capability of design, simulation, fabrication, characterization and prototyping of memory and MEMS devices, we are able to provide customized solutions for various applications upon request, including synaptic devices for in-memory computing, RF filters for mobile communication, timing devices, particle and mass sensors, pressure and flow sensors and so on.

Advanced RFMEMS integration platform

• With the state-of-the-art physical vapor deposition (PVD) tools, IME is capable to deposit 8-inch AlN and ScAlN thin films with thickness from tens nm to several μm, arbitrary Sc concentration, high crystallinity and excellent stress & defects control. Along with the deposition, IME has established 8-inch AlN and ScAlN integration platform with complete module technologies including lithography, PVD, CVD, etching, passivation, cavity release and thin film capsulation processes. Leveraging this platform, we are able to fabricate RFMEMS resonators with frequency from 2GHz to 5GHz, coupling coefficient around 15%, quality factor over 1000 and power handling over 32 dBm [1]. Ion beam trimming can be implemented to further improve the device uniformity across the 8-inch wafer.


Figure 1. Left: Impedance plot of the fabricated FBAR resonator; right: insertion loss of the fabricated ladder-type filter.

• Monolithic integration enables the integration of multiple components on a single chip, reducing the chip area and the complexity of assembly and packaging. By integrating multiple components on a single chip, monolithic integration reduces the parasitic effects and interconnect losses associated with traditional packaging methods, leading to improved device performance. IME has established a 3D monolithic integration platform for MEMS devices integrated directly on CMOS wafers. The monolithic integration of RFMEMS filters and RFSOI switches have been demonstrated and reported on the IEDM 2022 [2]. Further implementation of this technology on PMUT arrays, mass sensors and FeRAM devices is under development.

• IME is also developing an 8-inch process platform for LiNbO3 POI wafers. Target applications include GHz resonators and piezoelectric micromachined ultrasonic transducer (PMUT) devices.

References:

[1] Y. Zhu et al., “A high coupling coefficient 2.3-GHz AlN resonator for high band LTE filtering application,” IEEE Electron Device Lett., vol. 37, no. 10, pp. 1344–1346, Oct. 2016.

[2] Y. Zhang et al., “3D Monolithic Integration of ScAlN-based GHz MEMS Acoustic Filters on 200mm RFSOI Wafer”, IEEE International Electron Devices Meeting (IEDM) 2022, San Francisco.

Advanced design, testing and modelling capabilities

• Various FEM and circuit models have been developed to design the devices and investigate the mechanisms of some behaviors. The highly accurate and reliable device models allow us to optimize device performance, reduce development time, and minimize costs. Our design capabilities cover a wide range of RFMEMS resonators and filters, including the film bulk acoustic wave resonators (FBAR), the Lamb wave mode resonators (LWR), the laterally coupled alternative thickness (LCAT) mode resonators and coupled bulk acoustic wave resonators (CBAR) [3-4]. Our special LCAT and CBAR design can achieve both lithographic frequency tunability and high coupling coefficient comparable to FBAR, providing a single-chip solution for integrating filters with different frequencies. A procedure to extract various material parameters by device modelling and fitting with measurements has been established, allowing us to tailor our designs for unique requirements.

Figure 3. Left: fabricated single-chip acoustic duplexer based on the LCAT resonators [3]; right: fabricated single-chip acoustic duplexer based on CBAR designs [4].

Figure 4. Nonlinear Mason Model for high power analysis of the resonators [5].

• IME has an automatic 8-inch wafer-level RFMEMS device testing platform with RF frequency ranging from 10MHz to 40GHz, high input power up to 40 dBm and wide temperature range from -60 to 300 °C. Besides basic s-parameter testing, the output-input power linearity, one-tone harmonic response and two-tone intermodulation response of the resonators and filters can also be characterized.

References:
[3] Y. Zhu et al., “AlN based Dual LCAT Filters on a Single Chip for Duplexing Application”, IEEE International Ultrasonics Symposium (IUS), 2018
[4] C. Liu et al., “Sc0.15Al0.85N-based 4 GHz Coupled Bulk Acoustic Resonators (CBAR) and Filters for the Single-Chip Duplexer Solution”, IEEE International Ultrasonics Symposium (IUS), 2021
[5] Y. Zhang et al., “Experimental and Numerical Study on the Second Order Harmonic (H2) and Third Order Intermodulation Distortion (IMD3) Response of Scandium Aluminum Nitride Based FBAR Devices with Different Scandium Doping Levels”, IEEE International Ultrasonics Symposium (IUS), 2022

Non-volatile memory for next-generation storage and in-memory computing

• Ferroelectric Random Access Memory (FeRAM) is a type of non-volatile memory that stores data by utilizing the ferroelectric properties of certain materials. Unlike traditional Random Access Memory (RAM), FeRAM does not require power to retain data, making it a suitable option for low-power applications. FeRAM has fast read and write speeds, high endurance, and a long lifespan compared to other non-volatile memory technologies such as Flash Memory, which makes it promising to replace or complement traditional RAM in various applications, including automotive, industrial, and Internet of Things (IoT) devices. IME is developing capacitive FeRAM and ferroelectric tunneling junction (FTJ) devices based on ultrathin ferroelectric HfZrO₂ films (3-10nm) and Al1-xScxN films (20-100nm). Novel approach without high thermal budget has been developed for wake-up of the HfZrO₂ ferroelectricity and improvement of the endurance. As for the Al1-xScxN-based FeRAM devices, high remnant polarization up to over 100 μC/cm2 and steep switching of the polarization can be achieved, enabling high memory window and feasibility of the selector-free FeRAM array. Multiscale device models have been established to understand the polarization switching and leakage in this material, and to simulate the array performance [6]. Further scaling down of the film thickness is under development.

Figure 5. Left: hysteresis loops of ultrathin HfZrO2 films from 3nm to 6nm; right: hysteresis loop of 100nm 30% AlScN film with high remnant polarization and steep switching.

• Conventional computer with separated processor and memory units is suffering from a crucial bottleneck for efficient data movement. On the other hand, innovative computing paradigms such as in-memory computing are intensively studied to further enhance computing energy and time efficiencies toward next milestone. Besides, memory elements with capability of multi-level states per cell with the cross-point array integration forms a promising hybrid memory-computation unit for in-memory computing paradigm. Moreover, analog behavior in ReRAM is also one of the few memory technologies to present synaptic-like programming response as in biological observations, which is an essential building block for neuromorphic computing hardware. IME dedicates to the development of analog memories such as ReRAM and FeRAM so as to achieve high performance and high energy efficiency in-memory computing system. We have showcased the Al₂O₃/Ta₂O₅ based bilayer ReRAM with analog switching characteristics for synaptic applications [7].

Figure 6. Performance of the fabricated Al2O3/Ta2O5 based bilayer analog ReRAM devices.
References:
[6] C. Liu et al., “Multiscale Modeling of Al0.7Sc0.3N-based FeRAM: the Steep Switching, Leakage and Selector-free Array”, IEEE International Electron Devices Meeting (IEDM), 2021
[7] W. Song et al., “Analog switching characteristics in TiW/Al2O3/Ta2O5/Ta RRAM devices”, Appl. Phys. Lett. 115, 133501 (2019)