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Exploiting Phase-change Material for Millimeter Wave Applications

Chen, Shangyi
http://orcid.org/0000-0003-0103-2701
http://rave.ohiolink.edu/etdc/view?acc_num=osu1626953435684434

Abstract Details

2021, Doctor of Philosophy, Ohio State University, Mechanical Engineering.
With the advantages of high bandwidth and abilities to see through opaque materials, millimeter wave (mmW) band (30 to 300 GHz) has been intensively explored in recent years. Although there are increasing demands for reconfigurable mmW systems for their potential applications in defense, switching, imaging, and sensing, overcoming the limitations such as high losses and large power consumption in mmW systems is still a challenge. Phase change materials (PCM) like vanadium dioxide (VO2), which have novel and tunable physical properties such as electrical resistivity and optical transmittance, are appealing choices for mmW reconfiguration to provide faster operation speed and lower loss microsystems. One aspect of VO2 thin film that is not fully exploited is the metal-insulator transition (MIT) region, where the electrical resistivity changes about four orders of magnitude with external stimuli. In this work, we present a highly sensitive antenna-coupled VO2 microbolometer for mmW imaging. The proposed microbolometer takes advantage of the large thermal coefficient of resistance (TCR) of VO2 at the non-linear region. The thermal resistance of the device is significantly improved by micro-electro-mechanical systems (MEMS) techniques to suspend the device above the substrate, compared with non-suspended microbolometers. The finite element method is employed to analyze the electrothermal and electromagnetic performance of the device. The frequency range of operation is 65 to 85 GHz, and the realized gain at broadside is > 1.0 dB. Simulation results indicate a high responsivity of 1.72x10^3 V/W and a low noise equivalent power (NEP) of 33 pW/√Hz. Targeting for broader applications, it is highly desired to deposit VO2 thin films on silicon (Si) substrate. Here, we employ the annealed alumina (Al2O3) buffer layers to obtain high-contrast VO2 thin films. The fabrication details for the Al2O3 buffer layers using atomic layer deposition (ALD) and VO2 thin films using DC sputtering are presented. By four-point probe measurements, the electrical resistivity contrasts of VO2 thin films on amorphous ALD Al2O3 and annealed ALD Al2O3 are 3.66x10^3 and 1.46x10^4, respectively. This improvement by a factor of 4 is the direct result of employing the rapid thermal annealing process (RTA) to crystallize the ALD Al2O3 buffer layer. The dry etching properties of VO2 and annealed ALD Al2O3 thin films are also analyzed. We then present the fabrication and characterization of the VO2 microbolometer. An integrated coplanar waveguide (CPW) feed structure is utilized to directly apply the power to the microbolometer. The three-mask photolithography microfabrication process is demonstrated. The deposition technique of VO2 thin films on annealed ALD Al2O3 on Si is employed in the fabrication process. After patterning VO2 and the deposition of the antenna by the lift-off process, the deep reactive ion etching (DRIE) is performed to suspend the microbolometer in the air. The large TCR of VO2 is utilized by biasing the device in the MIT. The antenna characterization and electrical performance of the VO2 microbolometer are carried out using the probe station with the network analyzer. The current-voltage plot is obtained by the source meter. The DC responsivity (6.55x10^4 V/W) is then estimated from the ratio of threshold voltage to trigger the MIT over input power changes. The frequency band of the designed antenna is measured to be 36.5 – 42.5 GHz. In addition, the concept of utilizing the proposed VO2 microbolometer for a mmW imager is demonstrated by the radiation pattern measurement of a 4x4 VO2 microbolometer array. Besides, it is highly desired to analyze the lifetime or reliability of VO2 thin films that are utilized in practical devices. We report the lifetime investigations of the reversible phase transition of VO2 thin film integrated into CPWs under fast and direct thermal cycling. The Joule heaters nearby the patterned VO2 thin film are employed to provide stable and fast thermal excitation across the transition region. Meanwhile, the millimeter wave signal is modulated by the integrated CPW when VO2 is thermally actuated. The finite element simulation is conducted to analyze the feasibility of the CPW design and the required power for the Joule heaters to actuate VO2 thin film. A test controller board is utilized to generate configurable pulse width modulation (PWM) waveforms for the Joule heaters. The heating/cooling cycling is tested with a VO2 DC resistor integrated with the designed Joule heaters. The on-wafer probe measurements are carried out to track different states of VO2 and provide an indication of VO2 thin film degradation. The presented results reveal the potential of VO2 for future applications that need multiple and reversable cycles.
Nima Ghalichechian (Advisor)
Hanna Cho (Committee Member)
Renee Zhao (Committee Member)
164 p.
Electrical Engineering; Materials Science; Mechanical Engineering; Nanotechnology
Vanadium dioxide, phase change materials, metal-insulator transition, microbolometer, MEMS, millimeter wave, dipole antenna, reliability, thermal cycling, coplanar waveguide

Recommended Citations

Citations

  • Chen, S. (2021). Exploiting Phase-change Material for Millimeter Wave Applications [Doctoral dissertation, Ohio State University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=osu1626953435684434

    APA Style (7th edition)

  • Chen, Shangyi. Exploiting Phase-change Material for Millimeter Wave Applications. 2021. Ohio State University, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=osu1626953435684434.

    MLA Style (8th edition)

  • Chen, Shangyi. "Exploiting Phase-change Material for Millimeter Wave Applications." Doctoral dissertation, Ohio State University, 2021. http://rave.ohiolink.edu/etdc/view?acc_num=osu1626953435684434

    Chicago Manual of Style (17th edition)

osu1626953435684434
168
© 2021, all rights reserved.
This open access ETD is published by The Ohio State University and OhioLINK.
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