Copper has been used as interconnects in ultra large scale integrated circuits (ULSI) because Cu has a high electrical conductivity and high resistance to electromigration. Typical Cu interconnection requires a barrier to prevent Cu diffusion because detrimental reactions occur between Cu and silicon (Si) at a temperature as low as 200 °C [1] and [2]. The barrier layer typically has a high electrical resistivity, which suggests the use of Cu alloy film free from the barrier. Cu-alloy films, which dope Cu with soluble or insoluble elements, can retard the intermixing between Cu and Si and further improve resistance to electromigration.
As the feature sizes used in the ULSI decrease toward 16 nm, a process that deposits a conformal Cu film is urgently needed. Many researchers have extensively studied Cu and Cu-alloy films prepared by sputtering [3], [4] and [5]. However, a damascene process is used to produce copper interconnections during the manufacturing of ULSI. A wet electrochemical process is employed to deposit the Cu film. Therefore, an electrochemical method to prepare high quality Cu films is essential for ULSI at technology nodes that are a few tens of nanometers in size. Currently, atomic layer deposition (ALD) is typically employed because it is capable of forming a conformal film to deposit these layers, especially for deep trenches and vias. In this study, the electrochemical form of ALD (electrochemical atomic layer deposition, ECALD) enables for the formation of Cu interconnection to be an entirely wet process [6], [7] and [8].
To maintain the lowest possible electrical resistivity of the Cu alloy film, a Cu(Ag) film may be a potential material for interconnects in microelectronics. In this study, Cu and Cu(Ag) were deposited on Ru/Si substrates using ECALD. According to the fabrication of copper interconnects in microelectronics, electroplating was employed to deposit the Cu(Ag) alloy films from a sulfuric acid electrolyte [9] and [10]. The underpotential co-deposition of Au–Cu alloys has been performed [11], indicating that the copper alloy film can be co-deposited by selecting the appropriate deposition potential of a mixing solution. By adjusting the concentration of the solution, precise alloy composition control using underpotential deposition (UPD) can be achieved. Cu(Ag) deposition using UPD by ECALD is reported for the first time. The UPD involves the deposition of an atomic layer of one element on a second layer of an element at a potential prior to that needed to deposit the element on itself. The process of layer-by-layer atomic deposition and the electrical properties of the resulting Cu and Cu(Ag) films will be discussed.