Measurement system for low force and small displacement contacts

To support the continued miniaturization of electrical contacts in multichip systems, three-dimensional (3-D) systems, wafer probe cards, and MEMS relays, there is a need for combined measurements of electrical and mechanical phenomena during contact formation. We have carried out a study of electrical contacts in the nN-mN force range for future generation probe cards and novel electronic packaging. One critical phenomenon in the contact formation process is nm-scale deformation of the material layers. To directly study this contact displacement, we have designed a measurement system comprised of a piezoresistive cantilever and an optical interferometer. Together, this system simultaneously measures contact resistance (mOhm to kOhm), force (nN to mN), and displacement (nm-mum). These measurements allow the first direct observation of contact mechanical behavior in this important application range. These measurements show that asperities at the contact surface dominate the behavior of the contacts, causing deviations from the Hertzian model of elastic contacts. This paper describes the design and construction of this apparatus, and the operation in a contact mechanics experiment.