Room temperature thermally evaporated thin Au film on Si suitable for application of thiol self-assembled monolayers in micro/nano-electro-mechanical-systems sensors

Abstract

Gold is a standard surface for attachment of thiol-based self-assembled monolayers (SAMs). To achieve uniform defect free SAM coatings, which are essential for bio/chemical sensing applications, the gold surface must have low roughness, and be highly orientated. These requirements are normally achieved by either heating during Au deposition or post deposition Au surface annealing. This paper shows that room temperature deposited gold, can afford equivalent gold surfaces, if the gold deposition parameters are carefully controlled. This observation is an important result as heating (or annealing) of the deposited gold can have a detrimental effect on the mechanical properties of the silicon on which the gold is deposited used in microsensors. The paper presents the investigation of the morphology and crystalline structure of Au film prepared by thermal evaporation at room temperature on silicon. The effect of gold deposition rate is studied, and it is shown that by increasing the deposition rate from 0.02 nm s -1 to 0.14 nm s -1 the gold surface RMS roughness decreases, whereas the grain size of the deposited gold is seen to follow a step function decreasing suddenly between 0.06 and 0.10 nm s -1 . The XRD intensity of the preferentially [111] orientated gold crystallites is also seen to increase as the deposition rate increases up to a deposition rate of 0.14 nm s -1 . Formation and characterization of 1-dodecanethiol on these Au coated samples is also studied using contact angle. It is shown that by increasing the Au deposition rate the contact angle hysteresis (CAH) decreases until it plateaus, for a deposition rate greater than 0.14 nm s -1 , where the CAH is smaller than 9 degrees which is an indication of homogeneous SAM formation, on a smooth surface. 2

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