A comparison of pad metallization in miniaturized microfabricated silicon microcantilever-based wafer probes for low contact force low skate on-wafer measurements

Abstract

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.

On overtravel and skate in cantilever-based probes for on-wafer measurements

“…Although evidently feasible, the characterization of the skate of a microcantilever appears to be challenging [9]. As the model developed here is-at least in principle-valid at different length scales, it is simpler first to characterize the skate of a macroscopic cantilever and compare these results with the predictions of the model.…”

Tilt-related alignment issues in miniature micromechanical on-wafer electrical probes composed of multiple flexible microcantilevers

“…This may be necessary to achieve a desired contact force on the n th cantilever. In other words, the overtravel required for planarity on the n th cantilever may not result in enough contact force for a good electrical contact [6].…”

“…One of these challenges is an understanding of the link between the positioning of such probes and their inherent mechanical flexibility. This is particularly important in terms of the impact of positional errors on optimum electrical contact [6]; something that has been studied in rigid commercial probes [7][8][9][10][11][12][13][14]. In contrast, the micromechanical behaviour (bending and twisting) of miniature microcantilever-based probes must be taken into account for optimum probe contacting [15][16][17].…”

Quantifying and correcting tilt-related positioning errors in microcantilever-based microelectromechanical systems probes

“…However, the latter studies did not take into account the mechanical flexibility of miniature, potentially microelectromechanical systems (MEMS)-based, probes. Microcantilevers can be used to make all manners of miniaturized electrical probes, including RF probes [18][19][20][21]. Microcantilevers are fabricated using MEMS fabrication techniques-this enables probes to be small, mechanically flexible, and compatible with the incorporation, at the fabrication stage, of microelectronics' materials and devices.…”