Patterning of metal traces and contact pads over an array of tip-well-trench structures for graphene vibration-based applications.

Freestanding graphene is a promising candidate for harvesting energy from vibration forces as small as a pico-newton. Our research consists of 4 steps, which are, putting crosshairs on silicon wafers, patterning of the tip, well and trench structures, patterning of metal traces and contact pads, and the transfer of graphene over the wafer to act as a variable capacitor. This study will focus on step 3, following the patterning of the well & trenches and the formation of tips from the use of buffered oxide etchant (BOE). We spin coat the wafer with polymethyl methacrylate (PMMA) and bake it at 180^o for 5 minutes. Afterward, an electron beam lithography (EBL) device is used to pattern the position and size of metal traces and contact pads. These are precisely set up in the trenches using the crosshair marks. The metal contact pad connects to the tip, while the other to graphene and ends in a horseshoe-like design. This horseshoe has a length of 14um on the sides and 20um at the back with a width of 4um and at a distance of 3.5um from each part of the tip. The pattern is designed to ensure that the transferred graphene only makes electrical contact with the metal contact pad and not with the tip. It is then developed using methyl isobutyl ketone (MIBK) and 5nm of chromium and 100nm of gold are deposited on the chip. The tip deposited with gold acts as one-half of the variable capacitor. Then we use remover PG for liftoff, clean the chip, and blow dry it with nitrogen. We use an optical microscope to obtain the outlook of the patterning over the whole wafer. Then an atomic force microscope (AFM) is used to determine the thickness of the metal layer and the distance between the tip and top of the wafer.