Oxidation Characteristics of Al–Ta Thin Alloy Films as a Passivation Layer on Cu

Abstract

The oxidation characteristics of Al and Al–Ta alloy films (500 Å) deposited as a passivation layer on Cu have been examined by depth profiling using Auger electron spectroscopy and X-ray photoelectron spectroscopy. Although alloying between Al and Cu takes place, the Al films prevent the oxidation of Cu up to the oxidation temperature of ∼300° C for 1 h in air, at which the surface-oxidized Al2O3 layer maintains the self-passivating ability. At more elevated oxidation temperatures, the further oxidation of Al and the formation of CuO in the Al–Cu alloy layer just beneath the oxide layer are confirmed to occur. On the contrary, the Al–Ta alloy films completely protect the Cu layer from oxidation even after oxidation at 500° C for 1 h in air. This is achieved due to the formation of the Ta buffer layer which separates the Cu layer from the surface-oxidized one, which is formed by the preferential oxidation of Al and the rejection of Cu from the Al2O3 during the oxidation.

Oxidation of copper pads and its influence on the quality of Au/Cu bonds during thermosonic wire bonding process

“…Cu(OH) 2 copper pad. The underlying layer consisted mainly of CuO after heating under an air atmosphere.…”

“…However, it is well known that copper oxidizes easily in air and humid atmospheres. Unlike aluminum oxidation behavior, the copper oxidation rate is high with no self-passivation layer forming to prevent the underlying copper from further oxidation [2]. Therefore, oxide layer formation on a copper pad could be a serious concern in thermosonic wire bonding processes.…”

“…(1) Thin film deposition onto the copper film to prevent the copper from oxidizing [2,4,[6][7][8]. Thin films such as Cr, TiN, Ta, TaN and Al could be used as passivation layer on copper.…”

Formation processes of the surface oxidized layer on Al-Ti alloy films

“…On the other hand, in order to avoid various problems associated with the low melting point (660 °C) of Al, such as electromigration in conductive materials and coating material applications at high temperature, it is often used as an alloy. In the actual example of applying Al as an oxidation protection film of Cu thin film, its effectiveness as an alloy with transition metal has been experimentally verified [1,2] from the viewpoint of application even at high temperature. However, for the oxidized state with Al alloy, although there is a vague understanding that the oxidation state of the pair metal becomes dominant as the alloy varies to a pair metal-rich composition, currently there is no good understanding of the details such as under what conditions the unique surface protectiveness of Al is maintained, with what mechanism it is lost, and what factors determine the latter.…”

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

“…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.…”