Copper has higher conductivity and better electromigration resistance property than aluminum. Copper is thus widely used as interconnecting wires in Very Large Scale Integration (VLSI) devices. However, copper tends to diffuse in a dielectric layer and causes leakage current and a breakdown of the dielectric layer. Therefore, a barrier layer is disposed between copper interconnecting wires and the dielectric layer to prevent the diffusion of copper. With the development of VLSI devices, especially with the scaling down of high-performance logic devices, the diffusion of copper into the dielectric layer that is disposed between adjacent interconnecting wires can cause a breakdown of the dielectric layer.
According to the breakdown characteristics of the dielectric layer, the breakdown can be divided into two types: an intrinsic breakdown and a Time Dependent Dielectric Breakdown (TDDB). When a voltage is applied to two adjacent copper interconnecting wires that are separated by a dielectric layer, an electric field is generated in the dielectric layer disposed between the two adjacent wires. If the generated electric field has a strength equal to or greater than a critical field strength of the dielectric layer in a copper interconnecting structure, a current will flow in the dielectric layer, and an intrinsic breakdown may occur. However, when the electric field strength is less than an intrinsic breakdown field strength, the intrinsic breakdown does not immediately occur, but the time dependent dielectric breakdown will be induced over a certain period of time due to a weakness in the isolation in the dielectric layer over time under the bias condition.
Furthermore, the many causes for the time dependent dielectric breakdown are described as follows. As the integration degree of chips increases, interconnecting wires become so thin that current density in the interconnecting wires increases exponentially. Under the influence of the high current density, metal ions in the interconnecting wires migrate in a direction opposite to a direction of the electron motion. This phenomenon is called electromigration. In the electromigration phenomenon, migration of the metal ions may induce pileup in a local area so that hillocks are formed in a metal layer of the interconnecting wires. The electromigration may also induce voids in the interconnect wires because of mass loss, thereby degrading the interconnect performance and causing opens to the interconnecting wires.
Therefore, preventing the loss of copper ions in the metal layer in copper interconnecting wires can improve the time dependent dielectric breakdown. In a process for forming the copper interconnecting wires, the copper interconnecting wires are exposed in an environment including an etching gas having an oxidizing ability so that copper atoms on the surface of the copper interconnecting wires are oxidized to CuO. To solve the problem mentioned above, a method for deoxygenizing copper atoms using N2 or H2 is disclosed in a paper entitled “Avoiding Cu Hillocks during the Plasma Process” published in Journal of The Electrochemical Society co-authored by Tsung-Kuei and Kang. The method for deoxygenizing copper atoms by using N2 or H2 is based on the principle: plasma is ionized to be ions or atoms in high voltage environment, then the ions and atoms can deoxygenize Cu from CuO in a deoxidization reaction on the surface of the copper interconnecting wires. However, the copper atoms deoxygenized are still in an unstable state so that the efficacy of improving the time dependent dielectric breakdown by inhibiting the loss of copper ions is not evident. Furthermore, in a plasma etching process in which a trench is formed in an upper interconnecting layer, the plasma may react with a second dielectric layer on the upper interconnecting layer, in which defect appears on a surface of the second dielectric layer. Although the second dielectric layer generally includes low dielectric constant materials, the defect will make a dielectric constant of the second dielectric layer become greater in a subsequent process of forming the copper interconnecting wires.
In view of analysis above, there is a need to provide a post-etching treatment process for the copper interconnecting wire, which can prevent the loss of the copper ions in the metal layer in the copper interconnecting wires, thereby avoiding the time dependent dielectric breakdown and reducing the defect on the surface of the second dielectric layer.