Analysis of Heterostructure Formation of Abnormally Oriented Grains in Sputter-Deposited AlScN Films and Its Impact on Bulk Acoustic Wave Devices.

The presence of abnormally oriented grains (AOGs) in sputter-deposited aluminum scandium nitride (AlScN) films significantly degrades their physical properties, compromising the performance of bulk acoustic wave (BAW) devices. This study utilizes first-principles calculations to reveal that in tetrahedral wurtzite AlScN film-doped Sc atoms tend to aggregate at the second nearest-neighbor positions, forming dense ScN octahedral structures. The rock-salt (RS) ScN continued to grow due to further Sc aggregation. However, due to inadequate scandium flux, embryonic RS structures cannot be sustained, resulting in the nucleation of AOGs at the (111) faces of the octahedral ScN structure. Electron microscopy studies indicated that AOGs possess wurtzite structures and originate at tilted grain boundaries. These boundaries were characterized as RS ScN with more Sc atoms. This corroborated the theoretical predictions. BAW resonators and filters fabricated from sputter-deposited AlScN films demonstrate that AOGs degraded the piezoelectricity of AlScN, reducing the resonator's electromechanical coupling coefficient (Keff2). Measurements showed that AOG density increased from edge to center of the 8 in. wafer, resulting in a 3% decrease in average Keff2 in the resonators and a 137 MHz decrease in the filter bandwidth at 5 dB.

Design and Fabrication of a Film Bulk Acoustic Wave Filter for 3.0 GHz-3.2 GHz S-Band.

Film bulk acoustic-wave resonators (FBARs) are widely utilized in the field of radio frequency (RF) filters due to their excellent performance, such as high operation frequency and high quality. In this paper, we present the design, fabrication, and characterization of an FBAR filter for the 3.0 GHz-3.2 GHz S-band. Using a scandium-doped aluminum nitride (Sc0.2Al0.8N) film, the filter is designed through a combined acoustic-electromagnetic simulation method, and the FBAR and filter are fabricated using an eight-step lithographic process. The measured FBAR presents an effective electromechanical coupling coefficient (keff2) value up to 13.3%, and the measured filter demonstrates a -3 dB bandwidth of 115 MHz (from 3.013 GHz to 3.128 GHz), a low insertion loss of -2.4 dB, and good out-of-band rejection of -30 dB. The measured 1 dB compression point of the fabricated filter is 30.5 dBm, and the first series resonator burns out first as the input power increases. This work paves the way for research on high-power RF filters in mobile communication.

Preparation, Characterization, and Application of AlN/ScAlN Composite Thin Films.

Piezoelectric aluminum nitride (AlN) thin film, as a commonly used material for high-frequency acoustic resonators, has been a research hotspot in the RF field. Doping Sc elements in AlN is one of most effective methods to improve the piezoelectricity of the material. In this work, the first principal calculation and Mori-Tanaka model are used to obtain the piezoelectric constants of AlN, ScAlN, and AlN/ScAlN composites. Then, five types of AlN/ScAlN thin films are prepared on 8 inch silicon substrates. The crystal quality, roughness, and stress distribution are measured to characterize the film quality. The results show that composite film can effectively solve the problem of abnormal grains and reduce the roughness. Finally, a lamb wave resonator with an AlN/Sc0.2Al0.8N composite working at 2.33 GHz is fabricated. The effective electromechanical coupling coefficient Keff2 is calculated to be 6.19%, which has the potential to design high-frequency broadband filters.

Aluminum Nitride-Based Adjustable Effective Electromechanical Coupling Coefficient Film Bulk Acoustic Resonator.

The arrival of the 5G era has promoted the need for filters of different bandwidths. Thin-film bulk acoustic resonators have become the mainstream product for applications due to their excellent performance. The Keff2 of the FBAR greatly influences the bandwidth of the filter. In this paper, we designed an AlN-based adjustable Keff2 FBAR by designing parallel capacitors around the active area of the resonator. The parallel capacitance is introduced through the support column structure, which is compatible with conventional FBAR processes. The effects of different support column widths on Keff2 were verified by finite element simulation and experimental fabrication. The measured results show that the designed FBAR with support columns can achieve a Keff2 value that is 25.9% adjustable.

Performance improvement-oriented reentry attitude control for reusable launch vehicles with overload constraint.

This paper investigates the issue of reentry attitude control for reusable launch vehicles with overload constraint, and proposes a performance improvement-oriented control scheme. Incorporating the overload constraint into the performance functions, the presented control law proves to guarantee the overload limitation not violating throughout the reentry phase. Compared with traditional performance-guaranteed methods, the presented control introduces a special performance function to achieve better transient performance. Moreover, the attitude control model is reduced to a first-order system with disturbances, leading to a significant reduction of the sophisticated structure by the standard backstepping based approaches. The implementation of the control scheme in the practical engineering is also discussed including the control moment allocation. Simulation results verify the effectiveness of the proposed control approach.