Miniaturized, microfabricated microelectromechanical systems (MEMS)-based wafer probes are used here to evaluate different contact pad metallization at low tip forces (<mN) and low skate on the on-wafer pads. The target application is low force RF probes for on-wafer measurements which cause minimal damage to both probes and pads. Low force enables the use of softer, more conductive metallisation. We have studied four different thin film contact pad metals based on their thin film electrical resistivity and micro-hardness: gold, nickel, molybdenum, and chromium. The contact pads sizes were micrometre (1.9×1.9 µm2) and sub-micrometre (0.6×0.6 µm2). The contact resistance of Au-Au, Ni-Au, Mo-Au, and Cr-Au was measured as a function of tip deflection. The tip force (loading) of the contacts was evaluated from the deflection of the cantilever. It was observed that an overtravel of 300 nm resulting in a contact force of ~400 µN was sufficient to achieve a contact resistance <1 Ω for a sub-micrometre gold contact pad. Our results are compared with an analytical model of contact resistance in loaded metal-metal contacts—a reasonable fit was found. A larger contact resistance was observed for the other metals—but their hardness may be advantageous when probing other materials. Using a combination of a rigid silicon cantilever (>1000 Nm-1) and small contact pads enabled us to show that it is the length of the pad (in contact with the surface) which determines the contact resistivity rather than the total contact pad area.
This paper presents an improved technique for monitoring and controlling the contact condition of on-wafer RF probes with nanometer accuracy to enhance the measurement repeatability. The setup consists of a vector network analyzer, a modified probe station with a planar calibration substrate aligned under microwave GSG probe through a closed-loop nanopositioner and a camera system. A fully one-port SOL calibration is performed in the frequency range 0.05-50 GHz. A repeatability study based on standard deviations of the measured data considering both conventional and proposed approaches is described. From these experimental results, the improvement of the technique proposed is achieved by accurately controlling the probe contacts.
A microwave nano-probing station incorporating home-made MEMS coplanar waveguide (CPW) probes was built inside a scanning electron microscope. The instrumentation proposed is able to measure accurately the guided complex reflection of 1D devices embedded in dedicated CPW microstructures. As a demonstration, RF impedance characterization of an Indium Arsenide nanowire is exemplary shown up to 6 GHz. Next, optimization of the MEMS probe assembly is experimentally verified by establishing the measurement uncertainty up to 18 GHz.
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