A Nano-Scale Investigation of Material Transfer Phenomena at Make in a MEMS Switch

Abstract

MEMS switches have considerably improved over the last decade, however their lack of reliability remains a weak point for a large scale production. The main limiting factor comes from the electrical contacts. In particular, material transfer at the nano-scale is of significant importance in terms of performance and lifetime, however the existing literature remains rather limited. In this paper we present original experiments carried out in air using a modified atomic force microscope (AFM) equipped with a tipless conductive cantilever representing the mobile contact. The fixed contact is composed of a Si substrate covered with the metal of interest (Au, Ru or Pt). The experimental setup is configured to perform successive commutations at extremely low closing/opening speeds of about 10nm/s. This study focuses on the closing sequence under 5V DC, the current being limited to 1mA. The results show a sudden current increase when the contact gap becomes smaller than a few tens of nanometers. This emission of electrons from the cathode tends to follow the Fowler-Nordheim theory and leads to the damage of the opposite contact member (anode) thus causing, by impact heating, the evaporation of the anode material and its deposition on the opposite contact member (cathode). A material transfer from anode to cathode can then be observed and explained.

Review: Electrostatically actuated nanobeam-based nanoelectromechanical switches – materials solutions and operational conditions

“…This process causes a temperature increase of the other electrode (anode), resulting in thermal evaporation of the electrode material and its transfer to the opposite electrode. For metal electrodes, the material transfer issues are reported for the source–drain bias exceeding 5 V. When the contact electrodes are made of two different materials [ 106 ], material transfer results in an increase of adhesion in the contact and also makes the surfaces rougher, thus increasing resistance in the contact. For self-mated contacts, for example, in an all-molybdenum switch, material transfer was reported to be a possible cause for the observed contact resistance rise after approximately 10 4 switching cycles operating at 1 V drain voltage in a 3T configuration with 100 nm gap between the beam and the drain electrode [ 19 ].…”

Contact degradation due to material transfer in MEM switches

“…Indeed, until recently this phenomenon had not been put ahead, so only a few authors have tried to explain the transfer process [5,6,14]. Electromigration is a well-known transfer mechanism in microelectronics but it cannot explain the observed results: on one hand, during AFM experiments the current is immediately suppressed once the contact is closed.…”

A review of micro-contact physics for microelectromechanical systems (MEMS) metal contact switches

“…7 Distinct from electromigration, field emission is also responsible for material transfer phenomena. 123 Field emission is the transfer or emission of electrons induced by an electrostatic field. Literature in this area is limited, however, Poulain et al conducted an investigation into the phenomena using a modified atomic force microscope.…”

“…123 At that range, the team deduced that the emission of electrons from the cathode followed the Fowler-Nordheim theory and led to damage on the opposite contact member. 123 The damage to the opposite contact consisted of evaporated anode material caused by impact heating (electrons leaving the anode heated the material and caused evaporation of anode contact material to the cathode interface). 123 The reported transfer of material due to field emission occurred with an open-circuit voltage across the two contact members of 5 V and a test current limited to 1 mA when the contact was closed.…”

“…123 The damage to the opposite contact consisted of evaporated anode material caused by impact heating (electrons leaving the anode heated the material and caused evaporation of anode contact material to the cathode interface). 123 The reported transfer of material due to field emission occurred with an open-circuit voltage across the two contact members of 5 V and a test current limited to 1 mA when the contact was closed. These results corroborate those of Yang et al who demonstrated micro-contact degradation under various hot/cold switching conditions.…”