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Michelle Chen1 Feifei Lian1 Miguel Muñoz Rojo1 Aditya Sood1 Kenneth Goodson1 Eric Pop1

1, Stanford University, Stanford, California, United States

Increases in nanoscale device packing density and energy consumption have led to a growing need for improved thermal management technologies. Thermal switches or “thermal transistors” offer a way to dampen sudden thermal transients in order to reduce thermal cycling loads and improve device and system reliability. For example, a 10x thermal switching ratio could reduce the temperature swing of a device by up to 10x and increase its lifetime by 3000x [1].
In this work, we demonstrate the first scalable, CMOS-compatible thermal switches based on suspended graphene. Graphene is selected as the switch membrane due to its high Young’s modulus, and high thermal and electrical conductivity [2-4]. We show reversible switching cycles at low (< 2 V) pull-in voltages. Using voltage pulses, we modulate the electrostatic deflection of suspended graphene to contact it to the underlying substrate. The controlled, partial collapse of graphene micro-ribbons and graphene-supported metal beams directionally channels heat for thermal management applications.
Large area graphene is grown on copper foil via chemical vapor deposition. We fabricate bilayer graphene stacks on 540 nm thick SiO2 on highly doped silicon using sequential PMMA-assisted wet transfers, and pattern devices using optical photolithography. We define graphene ribbon arrays using a copper sacrificial layer and oxygen plasma etching, and deposit 3 nm thick Cr channels on top of the graphene. 5 μm wide Cr/Au or Pt contacts are deposited, clamping the graphene and chrome channels to the oxide, and serving as the etch mask. We use buffered oxide etch to remove approximately 520 nm of underlying oxide, and critical point drying is used to suspend the graphene over oxide pillars.
We electrostatically collapse the graphene by applying a voltage between the metal contact and the highly doped silicon substrate. We observe a graphene pull-in voltage of ~1.8 V, and demonstrate thermal switching for ~10 cycles without showing signs of irreversible collapse. (More cycles could be possible, as devices were not tested to the point of failure.) Using optical pump-probe reflectance techniques., we measure the changes in thermal conductance between the suspended (OFF) and collapsed (ON) state of the device. These represent the first demonstration of nanoelectromechanical thermal switches based on graphene, with potential applications for management of thermal transients and of electronics reliability.

[1] Bayerer et al., Conf. Int. Power Sys. (CIPS), 2008
[2] Lee et al., Science, 2008, 18, 385
[3] Pop et al., MRS Bulletin, 2012, 7, 1273
[4] Milaninia et al., Appl. Phys. Lett., 2009, 95, 183105

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