Spray Pyrolysis Synthesized Self-biased Hexagonal Ferrite Thin Film on GaN, AlN, Si Substrates for Millimeter-wave Devices

随着先进无线通信的出现，较低频率的频谱变得拥塞。为了克服这种拥塞，另一种方法是使用工作在毫米波频段的无源器件来开发紧凑而高效的通信系统。在这些频率下使用的材料是Ba六方铁氧体和锶六方铁氧体。这些铁氧体的各向异性表现出高达60 GHz(波长为5 mm)的强铁磁共振。大气中的氧分子在60 GHz处对电磁波有很强的共振吸收。因此，对于短距离通信系统来说，它是一个非常有吸引力的频段。在目前的方案中，建议在毫米波频率使用自偏置六方铁氧体器件。这导致了从硅窄带隙衬底技术向GaN宽带隙衬底技术的转变，以实现通信系统的高效性能。现在，许多制造商都可以很容易地买到钡和锶铁氧体纳米粉末。最近发现，纳米六方铁氧体粉末可以与商用光刻胶混合，然后作为一层喷涂在GaN衬底上。提出了采用一个超声波发生器、两个超声喷嘴和压缩空气成形器进行喷雾的方案。下一步是在高达600摄氏度的烤箱中固化薄膜，以缩小铁磁共振峰值。这也会导致共振强度加倍。利用自制的准光学毫米波光谱仪、扫描电子显微镜、X射线衍射仪和振动样品磁强计对沉积的纳米铁氧体薄膜进行了全面的表征研究。通信系统需要一个高效功率放大器、一个低噪声放大器和一个纳米铁氧体循环器。然而，制备具有良好晶粒度的高质量纳米铁氧体材料的工艺面临着巨大的挑战。其中一个问题是如何制备出质量好、成本低的自偏压六方铁氧体薄膜。目前可用于MMIC设计的晶片直径约为100 mm，厚度约为100米。典型的厚度是98米的碳化硅和2米的氮化镓。在碳化硅衬底上制备高纯度GaN厚膜是近期的一大挑战。还可以在碳化硅上沉积氮化铝等高纯宽禁带半导体薄膜。这是一项新技术，将在拟议的研究期间进行探索。最初，这项工作是在IBM硅cmos衬底上进行的。较低的温度限制了纳米铁氧体钡铁氧体薄膜的沉积，使其具有孔洞和不均匀的平整度。使用硅cmos衬底的工作也将继续进行。与Qorvo的合作伙伴关系导致了在碳化硅晶片上常规使用GaN。最终目标是将钡铁氧体环行器、功率放大器和低噪声放大器构建在碳化硅晶片上的单个GaN上，以在60 GHz下运行。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

With the advent of advanced wireless communications the spectrum at lower frequencies are congested. To overcome this congestion an alternative approach is to develop compact and high efficient communication system using passive devices operating in millimeter-wave frequency band. The materials used at these frequencies are Ba- and Sr hexaferrites. The anisotropy of these ferrites exhibit strong ferromagnetic resonance up to 60 GHz (5 mm in wavelength). The oxygen molecule in the atmosphere exhibit a strong resonance absorption of electromagnetic waves at 60 GHz. Thus makes it a very attractive frequency band for shorter distance communication system. In current proposal, the use of self-biased hexagonal ferrite devices is suggested in millimeter wave frequencies. This led to shift from silicon, narrowband gap substrate technology to GaN, wideband gap substrate technology for efficient performance of communication system. The Ba- and Sr- ferrite nano size powders are now readily available from a number of manufacturers. Recently it has been found that nano-sized hexagonal ferrite powders can be mixed with commercially available photoresist and then sprayed as a layer on to the GaN substrate. It is proposed to use one ultrasonic generator with two ultrasonic spray nozzles with compressed air shaper for spraying. The next step turns out to be curing of the film in an oven up to 600 degrees centigrade for narrowing of the ferromagnetic resonance peak. It also leads to doubling of the resonance intensity. The comprehensive characterization study of deposited nano-ferrite film has been performed by employing a custom made quasi-optical millimeter wave spectrometer, scanning electron microscope, X-ray diffraction analysis and vibrating sample magnetometer.For a communication system, one would need a high efficient power amplifier, a low noise amplifier and a nano-ferrite circulator. However, the process to manufacture high quality nano-ferrite material with good grain size offer significant challenges. One of the problem is how to deposit self-bias hexagonal ferrite thin film with both good quality and acceptable cost has not been solved yet. Currently available wafers for MMIC design are about 100 mm in diameter with a thickness of about 100m. Typical thicknesses are 98 m of SiC and 2 m of GaN. The development of high purity GaN thick film film on SiC turns out to be big challenge in the near future. A thin film of highly pure wide bandgap semiconductor such as Aluminum Nitride can also be deposited on SiC. It is a new technology and it will be explored during the course of the proposed research period. Originally this work was explored on IBM silicon CMOS substrates. The lower temperature operation restricted the deposition of nano ferrite Ba- ferrite film to porosity and uneven flatness. The work with silicon CMOS substrate will also be continued. The collaborative partnership with QORVO has led to the use of GaN on SiC wafers routinely. The ultimate goal is to have Ba-ferrite circulator, power amplifier and low noise amplifier build on a single GaN on SiC wafer for operation at 60 GHz.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.