Focused ion beam milled sub-micron capacitive gaps in coplanar transmission lines

By milling sub-micron wide air gaps with a focused ion beam (FIB) into coplanar waveguide (CPW) transmission lines, it was demonstrated that an FIB can be an effective tool for micro- and MM-wave device fabrication. The milled gaps, with widths of about 70 nm, were cut across the center conductor of the CPW lines, as well as across shunt line connections. Straight cuts and angle cuts were investigated. Capacitances of 8.5 to 12 fF were extracted from the equivalent circuit models and they possess nearly ideal thin-film capacitor topology due to the absence of significant inductive effects. The devices were measured and characterized from 1 to 65 GHz. The effects of gallium contamination during milling were also taken into account in the circuit model.