Copper wire bond pad/IMC interfacial layer crack study during HTSL (high temperature storage life) test

Abstract

Copper wire has been popular in these years when facing ever-increasing gold prices. The success of large scale conversion of gold to copper wire in microelectronics could achieve successfully when all the failure mechanisms that can be discerned during reliability testing. One of these mechanisms is corrosion of the contact between the copper (Cu) ball and the aluminum (Al) bond pad, consisting of various intermetallic compounds (IMCs), which are more sensitive to corrosion compared to gold (Au)-Al IMC. The occurrence of the related open circuit failures are often reported as a consequence of humidity related stress test, HAST or PCT. Most of paper has been published focusing on the mechanism of corrosion during humidity related stress test. Recently, wire bond pad/IMC interfacial layer crack issue was also found during high temperature storage test. The occurrence of the open circuit failures are also reported related to high temperature storage test. In this paper, a CMOS 90nm low k device with 66um fine pitch and 1.2um bond pad metal thickness was test vehicle to study the mechanism of IMC layer crack during high temperature storage test. First, failure analysis and control experiment were conducted on failed unit to dig out root cause. Failure analysis showed element sulphur was found on the periphery of bonded ball and under the bonded ball. The Sulphur was suspected to be root cause to the IMC layer crack. Next, an experiment was conducted to investigate the root cause. The factors cover different IMC coverage, different type of epoxy molding compound with different level of sulfur content. Wire pull, ball shear and IMC data were collected. Strip level thermal aging test at 225 degree C was also performed with recording pull strength and its failure mode. Molded units were subject to high temperature storage at 175 degree C. The cells with higher IMC coverage and molded by low sulfur content compound could pass electronic test and wire pull test post decamping without ball lifted issue. Last, confirmation run was performed with low Sulphur content reformulated molding compound. The units were subject to temperature cycle, HAST and HTSL. The test result showed the lot with IMC coverage over control limit could pass HTSL and T/C. No delamination was found post temperature cycle by SCAM check. Regarding copper wire bond pad/IMC interfacial layer crack, IMC coverage and sulfur content of epoxy molding compound should be considered seriously.