Cu electroplating process has been extensively used in the fabrication of on-chip interconnects such as Cu pillar, TSV, micro-bump of microelectronic devices for their further integration [1]. In general, the process is performed in a acid plating bath made up of inorganic components (copper sulfate, chloride ion, and sulfuric acid) and organic additives including suppressor, accelerator, and leveler. The organic additives play very critical roles in changing the shape, uniformity of deposits [2]. In the case of Cu pillar for flip chip packaging, a flat surface on the top of Cu pillar is required for implementation of the stacking materials like Sn-Ag solder [3].

In this study, Cu pillar with flat surface was successfully electroplated onto photoresist(PR) patterned Si wafer by controlling composition of the three organic additives. In order to understand the correlation between characteristics of additives (functional group, molar mass, N density and so on) and deposits,  electrochemical behavior, PR reactivity and shape of Cu pillar have been investigated by galvanostatic potential transient measurements, contact angle measurement and scanning electron microscopy, respectively. As a result of the investigation, we will identify effects of the additives on formation of Cu pillar during electroplating process, and apply electroplating technology to many part of electronic devices from wide pillar to fine-pitch micro-bump.

Fig. 1. SEM images of (a) cross-section view and (b) tilted view of Cu pillar (100 um in diameter and 40um in height) and (c) chronopotentiometry graph (inner image: confocal laser scanning microscopic graph and OM image of top view)

[1] P.C. Andricacos, The Electrochemical Society Interface, spring (1999) 32-37.

[2] W.C. Chen, T.T. Huang, C.H. Lai, S.W. Chou, C.H. Shih, ECS Transactions, 52 (2013) 453-460.

[3] S. Chung, E. Kuo, M. Tseng, IMPACT 2009. 4th International. IEEE, (2009) 432-435.