A Combined Process of Formic Acid Pretreatment for Low-Temperature Bonding of Copper Electrodes

Abstract

A new combined process of formic acid pretreatment for low temperature Cu direct bonding was proposed. Cu film and Cu microelectrodes samples were treated by formic acid and successfully bonded together at 200 • C. Cu surface oxide was reduced at 200 • C using formic acid treatment without/with Pt catalyst. Particularly, the treatment with heated Pt catalyst is more effective for Cu surface reduction than that without Pt catalyst or with room temperature Pt catalyst. After formic acid treatment under various conditions, Cu film and Cu micro-electrodes samples were bonded together at 200 • C in N 2 . Among them, the bonded samples treated by formic acid using heated Pt catalyst showed the strongest bonding strength. Very few oxide and carbon was remained at the bonding interface. It also proves that the treatment can reduce surface oxide resulting in strong bonding at low temperature.

Plasma assisted bonding of copper and silver substrates

“…Cu was investigated by previous studies [3,4], according to these, Cu was reduced by formic acid. Therefore, both Ag and Cu surfaces were reduced by formic acid, and it is considered that the reduction of oxidized Ag or Cu layer contributes strong bonding strength.…”

“…Therefore, Cu direct bonding requires a chemical mechanical polishing (CMP) to form a smooth and clean surface, and CMP-Cu can be bonded by ion beam activation process [1], or by a hydroxyl bonding [?]. Furthermore, Cu direct bonding was achieved by a bonding method using formic acid for simultaneously bonding and control of the oxide layer on Cu surface at 200 • C [3,4].…”

“…Further, Cu oxide layer can be controlled by formic acid activation process [3,4], hence we also suggest to append this process to conventional surface activation process using plasma or ion beam in order to achieve lower temperature process than diffusion bonding of Cu and Ag.…”

Direct bonding for dissimilar metals assisted by carboxylic acid vapor

“…Recently, a low-temperature low-vacuum Cu to Cu bonding process via surface modification with formic acid vapors has been proposed by Suga et al [13][14][15] Taking the advantage of the clean surface, the barriers for the diffusion of Cu atoms is lowered and Cu-Cu bonding at 200°C can be achieved. This vapor-assisted bonding technique has a great potential to extend into the joining for different metal substrates.…”

Cu/Cu direct bonding by metal salt generation bonding technique with organic acid and persistence of reformed layer

“…The problem is that an actual bonding surface consists of an oxide film and a machined layer; [3][4][5][6][7] therefore, adhesion between surfaces at the bond interface and removal of the oxide film represent necessary requirements to obtain high bond strength. Recently, ultrasonic vibration, [8][9][10][11][12][13] plasma processing, [14][15][16][17][18][19] and organic acid pretreatment [20][21][22][23] have been investigated as methods for breaking and cleaning a superficial oxide film. Indeed, in a previous study, we showed that modification of an oxide film with formic acid greatly improves the strength of bonding between tin surfaces 22) and between tin and copper.…”