An Elusive Open-Circuit Failure Mode in Thin-Film Chip Resistors

A unique open-circuit failure mode was observed in thin-film discrete chip resistors used in hybrid microelectronic devices. Their Nichrome resistance element is deposited over silicon dioxide (SiO2) on a silicon chip (30 by 30 mils) (0. 769 mm square). Termination pads of aluminum are provided to facilitate wire bonding. To enhance contact, a layer of titanium is interposed between the Nichroome and the aluminum. The resistor failures are caused by two factors: (1) excessive thickness of the Nichrome element, (2) use of excessively high processing temperatures to oxidize the Nichrome to the proper sheet resistance during wafer processing, which results in excessive alloying of the titanium and Nichrome materials. The latter produces an oxidized stress-sensitive interface between the Nichrome and titanium and results in time- and stress-dependent failures. Evidence of excessive reprocessing of the silicon wafers was also found, which may be a good indicator that the supplier was having process-control difficulties. Processes contributing to this failure mode and the controls that have been imposed to prevent recurrence are described.