A Novel Method for Enabling the Thermosonic Wire Bonding of Gold Wire onto Chips with Copper Interconnects

Abstract

The wire bonding of chips with copper interconnects is a challenging issue. The present study develops a method in which the substrates are stored in a temperature and humidity controlled environment such that copper oxide films of appropriate thickness are formed on the substrate prior to the gold-copper thermosonic bonding process. It is shown that this film can be removed during the bonding process, hence rendering the wire bonding process feasible without the requirement for an inert gas environment or the use of metallic cap layers.The miniaturization of electronic components causes the signal delay phenomenon to become increasingly pronounced, and hence limits the ability of these devices to satisfy the requirements of high frequency and high-speed transmission. An effective means of overcoming this problem is to utilize copper rather than aluminum as the interconnect material in integrated circuits, ultralarge scale integration devices, and other electronic components. Compared to aluminum, copper has a lower electrical resistivity and a higher electromigration resistance. 1-4 However, unlike aluminum, copper is easily oxidized at high temperatures and fails to form a passivation oxide layer on its surface, which subsequently protects the sub-layers beneath from excess oxidization. This oxidization phenomenon creates significant problems in the wire bonding process of copper chips. [5][6][7] To overcome this oxidization problem, some researchers have used an inert gas to provide a shielding gas atmosphere between the copper and the air when welding gold wires onto copper pads. [8][9][10] Kajiwara et al. 9 studied the effects of the ambient atmosphere on the ultrasonic ball bondability of gold wires onto copper, aluminum, and gold pads, and showed that the bondability deteriorates when the argon gas contains water, acetone and oxygen, but is enhanced when welding is performed in a vacuum. Meanwhile, Jeng et al. 10 used argon as a shielding gas to facilitate the thermosonic wire bonding of gold wires onto copper pads. The resulting welding quality was analyzed in terms of the observed interfacial microscopic phenomena. The results demonstrated that the use of an appropriate shielding gas resulted in a significant reduction in the copper oxidization induced by the high working temperature of the bonding process. Aoh and Chuang 11 employed an argon shielding technique in the thermosonic wire bonding process to attach gold wires to copper pads at bonding temperatures of 180°C and above. They also conducted high-temperature storage ͑HTS͒ testing to understand the reliability of the Au/Cu ball bond at elevated temperature after prolonged storage, the results shows that the volume of Au-Cu intermetallic compounds formed at the interface is too small to cause degradation of bonding strength after the HTS tests. From the perspective of thermal activation energy, it also indicates that formation of Au-Cu intermetallic compounds at the interface during HTS is more difficult than those between the gold ball and aluminu...

Numerical and experimental analysis of thermosonic bond strength considering interfacial contact phenomena

“…preparation of an oxide film for a copper substrate proposed by Jeng et al [4,5], by which a sound thermosonic bond can be formed. But the process control and optimization still remain significant technological challenges because the thermosonic bonding mechanism has not been well understood [6].…”

Benefits of Flat Polymer Dielectric Surface Loading Organic Semiconductors in Field-Effect Transistors Prepared by Electrode-Peeling Transfer

“…There are also some innovative methods, such as the 1 Author to whom any correspondence should be addressed. preparation of an oxide film for a copper substrate proposed by Jeng et al [4,5], by which a sound thermosonic bond can be formed. But the process control and optimization still remain significant technological challenges because the thermosonic bonding mechanism has not been well understood [6].…”