Nucleation Kinetics of Electroless Cu Deposition on Ruthenium Using Glyoxylic Acid as a Reducing Agent

Abstract

Glyoxylic acid is seen as a promising candidate to replace formaldehyde as reducing agent in electroless Cu baths. For deposition on ruthenium, the anodic reaction of glyoxylic acid has been evaluated and compared to formaldehyde using linear sweep voltammetry. Significant differences were observed for the deposition of copper on ruthenium. First of all, a faster nucleation was inferred from open-circuit potential measurements, which is beneficial as it reduces the total process time. Secondary, we found 2,2 bipyridyl worked as stabilizer and brightener in this glyoxylic acid-based electroless bath. Thirdly, the purity of the copper films improved when 2,2 bipyridyl was present in the solution. Using the optimized composition, we demonstrate a conformal Cu seed layer deposition (∼100 nm) inside high aspect ratio (16.7) through-Si vias. This work shows the feasibility for electroless Cu seeding in a through-Si via metallization sequence.

Role of Bath Composition in Electroless Cu Seeding on Co Liner for through-Si Vias

“…22 In addition, in our previous study, it was clarified that glyoxylic acid can show anodic oxidation on both Cu and Ru. 26 Figure 1 shows linear sweep voltammograms of glyoxylic acid and formaldehyde for a 30 nm Co liner. For the case of formaldehyde, anodic oxidation was absent in the range from −0.65 to −0.45 V, which is the potential range in which the reducing agent should be active to lead to reduction of the Cu-EDTA complex.…”

“…In recent work, we found that the glyoxylic acid oxidation on Ru is impacted by dissolved oxygen. 26 Therefore, removing the oxygen by N 2 bubbling and/or deposition in an inert atmosphere could increase the reactivity of the reducing agent for the ELD process. 29 Cathodic reaction of Cu complex on Co.-For a self-catalyst electroless process, both reducing agent and complexed Cu ions should be present.…”

Cu seeding using electroless deposition on Ru liner for high aspect ratio through-Si vias

“…Furthermore, the driving force of the ELD reaction is the oxidation of the reducing agent at the surface, which can be strongly impacted if the surface has native oxide or other foreign contamination. In this work, glyoxylic acid is used as a reducing agent [2][3][4][5]. Fig.…”

“…Therefore, the nucleation of Cu deposition can take place without a Ru surface activation step. However, such an in-situ cleaning process likely causes insufficient cleaning, which could result in an increased Cu/Ru interfacial impurity levels [4]. Furthermore, the increase of the temperature shortens the bath life time and therefore increases the total process cost.…”

“…In order to overcome these challenges, developing a more conformal Cu seeding to replace PVD is an alternative route. Electroless deposition (ELD) is a wet chemical deposition process that can react with a catalytic surface [2][3][4][5]. The combination of atomic layer deposition (ALD) liner/barrier and the ELD seed is attractive approach for replacing the PVD based metallization, because of its compatibility with high aspect ratio features.…”