CAREER:Doped Aluminum Nitride Ferroelectric Microelectromechanical Systems

在蜂窝电话中，压电材料将射频信号转换成机械振动，形成微型、高选择性的前端滤波器，对有效利用射频（RF）频谱至关重要。虽然在现有的蜂窝频段中声学滤波技术已经得到了很好的发展，但在为宽带第五代（5G）蜂窝网络技术分配的毫米波频段中，缺乏用于减轻干扰的微型滤波技术。拟议的研究将研究将声学前端滤波技术的频率缩放到毫米波频段的新方法，而不会降低现有频率缩放方法的性能。这项研究的成功将使可靠的第五代无线网络具有更强的抗干扰能力。这些应用将产生重大的社会影响。声学滤波技术要满足无线通信系统（如第五代（5G）移动网络）的需求，需要克服两大挑战。首先，当声学谐振器和滤波器的频率超过6千兆赫（GHz）时，尺寸迅速缩小，由于一连串的寄生效应而显著降低了性能。其中包括谐振器质量因子的退化，这是由谐振器表面的散射和薄金属电极中的电阻损耗引起的，以及谐振器机电耦合的退化，因为器件电容的比率与与片上路由相关的电容相比下降了。其次，现有声学谐振器的固定频率操作限制了在路由和开关引入的损耗、面积和成本变得不可行的之前可以添加的频带数量。本研究旨在研究和利用最近发现的氮化铝钪（AlScN）薄膜中的铁电性来解决这些基本挑战。提出的研究将开发材料沉积技术，允许通过钪掺杂的变化，通过铝钪氮化膜堆栈的厚度来系统地定制铁电性能。这些技术将用于形成薄膜堆，其中矫顽力场已被设计成随器件厚度变化，允许在可控深度下选择性地反转氮化铝钪薄膜中特定区域的铁电极化。通过薄膜厚度的周期性极化层将实现选择性地激发高阶泛音声学共振，从而实现戏剧性的频率缩放，而不会大幅降低谐振器尺寸，从而降低先前频率缩放尝试的质量因子和机电耦合。选择性极化技术将扩展到声学谐振器，可以动态地在多个八度的频率上重新配置。最后，将探讨有关氮化铝钪薄膜特性的基本问题，这些特性最终定义了所提出的射频（RF）器件的性能限制。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Proposal Title:CAREER: Doped Aluminum Nitride Ferroelectric Microelectromechanical SystemsNontechnical AbstractIn cellular phones, piezoelectric materials convert radio frequency signals into mechanical vibrations that form the miniature, highly selective, front-end filters critical to efficient utilization of the radio-frequency (RF) spectrum. While acoustic filter technologies are well developed in the existing cellular bands, miniature filtering technologies for mitigating interference are lacking in the Millimeter Wave bands that have been allocated for broadband fifth-generation (5G) cellular network technology. The proposed research will study new methods for scaling the frequency of acoustic front-end filtering technologies to the Millimeter Wave bands without the degradation in performance that plaques existing frequency scaling approaches. Success of the proposed research will enable reliable fifth-generation wireless networks that are more robust to interference. The applications will have major societal impacts.Technical AbstractThere are two grand challenges to overcome in acoustic filter technologies as they are scaled to meet the needs of wireless communication systems, such as fifth-generation (5G) mobile networks. First, as the frequency of acoustic resonators and filters are scaled beyond 6 giga Hertz (GHz), the dimensions rapidly shrink, significantly degrading the performance due to a litany of parasitic effects. These including degradation of the resonator quality factor arising from scattering at the resonator surfaces and from resistive losses in the thinned metal electrodes, and degradation of the resonator electromechanical coupling as the ratio of the device capacitance falls in comparison to the capacitance associated with on-chip routing to the tiny acoustic devices. Second, the fixed frequency operation of existing acoustic resonators limits the number of bands that can be added before the losses, area, and cost introduced by the routing and switches becomes unworkable. This research seeks to study and exploit the recently discovered ferroelectricity in aluminum scandium nitride (AlScN) thin films to address these fundamental challenges. The proposed research will develop material deposition techniques allowing for the systematic tailoring of the ferroelectric properties through the thickness of an aluminum scandium nitride film stack via variations in scandium doping. These techniques will be utilized to form film stacks where the coercive fields have been engineered to vary through the device thickness, allowing for the selective inversion of the ferroelectric polarization of specific regions in the aluminum scandium nitride films at controllable depths. Periodically poled layers through the film thickness will be realized to selectively excite high order overtone acoustic resonances that enable dramatic frequency scaling without the steep reductions in resonator dimensions that degraded both the quality factor and electromechanical coupling of prior frequency scaling attempts. The selective poling techniques will be extended to demonstrate acoustic resonators that can be dynamically reconfigured over many octaves in frequency. Finally, fundamental questions pertaining to the aluminum scandium nitride film properties that ultimately define the performance limits of the proposed radio-frequency (RF) devices will be explored.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.