Bondability Problems Associated with the Ti-Pt-Au Metallization of Hybrid Microwave Thin Film Circuits

The cause for unacceptable bonding behavior of a new high-resolution, close-tolerance hybrid thin film circuit metals system was investigated. The conductor/resistor technology was developed at Hewlett-Packard for microwave applications. It consisted of Ti-PtAu/Ta
2
N where conductors were defined by a sputter etching process with the aid of an auxiliary Ti mask at the Au surface. The cause of the bondability problem was determined by Auger electron spectroscopy (AES) to be residual Ti-O compound at the conductor surface. An evaluation scheme which specifically addressed bond acceptability and integrity with respect to mechanical, visual, and reliability testing was developed. Types of bonds and the methods of testing associated with each bond type were investigated in terms of the apparent bond failure mechanism. Various wire and ribbon bonding processes were examined. A series of bond evaluation techniques which include both qualitative and quantitative ball shear and wire pull tests were performed. A qualitative shear test of the thermocompression Au ball bond was shown to be the most reliable indicator of bond integrity and acceptability. Bond stick rates and wire pull test results were found to be, in themselves, unreliable yardsticks for determining bond quality. Additional information was obtained from AES samples with excellent (close to 100 percent) stick rates but poor shear test characteristics. Various surface treatments to enhance bondability were investigated. Three different concentrations of H
2
O:HF:HNO
3
solutions as well as a Au etch were examined. A Ti etch consisting of H
2
O, HF, and HNO
3
, (10:1:1, respectively) was selected and easily implemented as a final processing step before bonding. Results comparing as-processed with Ti etched (10:1:1) circuits revealed a marked increase in stick rate (from ~50 to ~100 percent), ball bond shear strength and wedge bond pull strength, without altering existing bonding processes currently employed in production. Furthermore, environmental testing of exposed bonds on Ti etched (10:1:1) conductor surfaces confirmed bond integrity.