Contact resistance study of noble metals and alloy films using a scanning probe microscope test station

Abstract

The proper selection of electrical contact materials is one of the critical steps in designing a metal contact microelectromechanical system ͑MEMS͒ switch. Ideally, the contact should have both very low contact resistance and high wear resistance. Unfortunately this combination cannot be easily achieved with the contact materials currently used in macroswitches because the available contact force in microswitches is generally insufficient ͑less than 1 mN͒ to break through nonconductive surface layers. As a step in the materials selection process, three noble metals, platinum ͑Pt͒, rhodium ͑Rh͒, ruthenium ͑Ru͒, and their alloys with gold ͑Au͒ were deposited as thin films on silicon ͑Si͒ substrates. The contact resistances of these materials and their evolution with cycling were measured using a specially developed scanning probe microscope test station. These results were then compared to measurements of material hardness and resistivity. The initial contact resistances of the noble metals alloyed with Au are roughly proportional to their resistivities. Measurements of contact resistance during cycling of different metal films were made under a contact force of 200-250 N in a room air environment. It was found that the contact resistance increases with cycling for alloy films with a low concentration of gold due to the buildup of contamination on the contact. However, for alloy films with a high gold content, the contact resistance increase due to contamination is insignificant up to 10 8 cycles. These observations suggest that Rh, Ru, and Pt and their gold alloys of low gold content are prone to contamination failure as contact materials in MEMS switches.

Vapors in the ambient—A complication in tribological studies or an engineering solution of tribological problems?

“…This is especially true for organic contaminants and thermally or electrically activated metals. Noble metal contacts used for electrical switching applications were found to produce a carbonaceous product both when the organic vapor contaminant was controlled [48] and when it was supplied uncontrolled from the ambient [49]. Cyclohexene and 1-hexene were shown to adsorb onto gold surfaces and undergo hydrogenation under friction on the otherwise inert gold surface [50].…”

Separation modes in microcontacts identified by the rate dependence of the pull-off force

“…Both the bumps and substrates were coated with the contact test material. Using this contact tester, we have studied different contact materials 3 and contact adhesion. 4 Microcontacts may separate in brittle or ductile modes.…”

Gold-based thin multilayers for ohmic contacts in RF-MEMS switches

“…Strategies to improve hardness usually make use of gold alloying with other metals like ruthenium, rhodium (Lee et al 2006;Chen et al 2007) or nickel (Yang et al 2009) or with other noble metals like silver, palladium or platinum (Coutu et al 2004;Lee et al 2006;Chen et al 2007). None of these strategies has proven to be totally successful because at low alloying metal concentrations the hardness improvement is not so evident, while at higher alloying metal concentrations hardness strongly increases, but resistivity increases too and typical problems with brittleness (Chen et al 2007) and surface contamination are found (Kwiatkowski et al 2004). Many metals tend to oxidize or react when exposed to ambient atmosphere and form a nonconductive layer on the surface which must be removed for establishing a good electrical contact.…”

“…Many metals tend to oxidize or react when exposed to ambient atmosphere and form a nonconductive layer on the surface which must be removed for establishing a good electrical contact. Silver for instance, tend to form a sulfide layer, while metals of platinum group, being catalytic active, show a marked affinity for frictional polymers (Kwiatkowski et al 2004) which degrade the contact resistance after repeated switch cycling (Chen et al 2007) or cause random switching failure (Kwiatkowski et al 2004). Surface contamination is a serious obstacle in micro-switches, because the typical contact force (in the lN range) cannot penetrate the nonconductive surface layer and therefore the contact resistance strongly increases because of large tunneling resistivity (Chen et al 2007).…”