Process optimization and reliability study for Cu wire bonding advanced nodes

Cu wire bonding has taken over Au wire bonding due to its cost savings and other performance advantages such as higher mechanical strength for complex looping and better electrical performance. Currently, Cu wire bonding of 28nm node devices has been realized in high volume production [1]. To meet the challenges of device reliability for these advanced node devices, we need to understand how to achieve the optimal wire bonding results for the best condition for reliability tests. Previous reliability study by the authors identified one of the key factors in achieving good wire bonding reliability outcome is the wire type [2]. Pd coated Cu wire (PdCu) wire showed much better performance than Bare Cu wire. In this paper, traditional bare Cu wire, AuPd coated Cu wire and Pd doped Cu wire are compared. Biased HAST testing is one of the most challenging reliability tests for Cu wire bonded packages. Biased HAST test was performed for up to 336 hours. A medium grade molding compound was used. Coated AuPdCu wire shows the best performing wire in our study. The Pd doped Cu wire showed much better reliability than traditional bare Cu wire, indicating that the Pd doped Cu wire is a lower cost alternative to Pd coated Cu wire. In advanced node devices such as 28nm and 20nm, the pitch is further reduced to as low as 45um with the bond pad opening of around 40um. Two different wire diameters (15um and 18um) and 6 different ball diameters are tested to understand the correlation between bonded ball diameter, ball to pad contact area and reliability. ProCu5 process was used and all cells achieved over 90% IMC. Bonded ball diameters of 34um and larger passed the reliability tests for both 96 hours and 168 hours. The two smallest bonded ball diameters (30 and 32um) saw low level of failures at 96 hours. This confirms that contact area is an important factor in reliability testing. A 34um bonded ball is suitable for 45um pitch application, which means that reliability can be achieved with a medium grade molding compound for a 45um pitch application. Finer than 45um pitch devices may need higher grade molding compound to ensure reliability. Another objective of this paper is to understand the critical wire bonding responses for good reliability outcome. The wire bonding responses studied here include contact area between ball and pad, intermetallic coverage percentage (IMC%), and Al remain thickness. We found that higher values for contact area and IMC% result in increased reliability. An IMC% value of 85% is generally recommended to ensure an acceptable reliability outcome. The Al remain % is dependent on the application and the process being used. With an optimal process, Al remain around 50% of the initial Al thickness shows good reliability outcome. A less optimal process tends to require less Al remain % to pass reliability.