The effects of post-ash cleaning and chemical treatments on the dielectric properties and reliability of Cu/low-k interconnect structures

Abstract

As IC devices continue to shrink, the interconnect delay dominates over the gate delay in the circuit. This has been the primary motivating factor for the industry’s move towards copper and low-κ interconnects. One of the obstacles in implementing a porous low-κ material in a dual damascene structure is the degradation in its dielectric properties due to etching and ashing processes. These processes deplete the carbon from the trench interfaces and increase the hydroxyl content in the dielectric. As a result, the dielectric constant increases and the leakage and reliability characteristics degrade. The electrical characteristics and reliability of a low-κ dielectric with different pore-size and distribution was studied. Due to the difference in pore size and distribution, the interfacial roughness is higher for the material with larger pore-size. This leads to higher defect density which causes higher leakage and degraded reliability characteristics. A low-κ material with smaller pore size and tighter distribution of pores will give better reliability characteristics. FTIR analysis of blanket MSQtype low-κ films shows that etching and ashing processes change the molecular structure by breaking the cage configuration of the Si-O molecules and forming more network Si-O bonds. The etching and ashing processes also increase the hydroxyl content of the dielectric and consequently make the surface more hydrophilic. The analysis shows that a chemical treatment with hexamethyldisilazane (HMDS) vapor restores the carbon content of the film to an extent and reduces the hydroxyl content. This repair of the damage and partial restoration of the κ value leads to better reliability characteristics. Post-ash thermo-chemical treatments were performed in Damascene Cu/low-κ structures. An HMDS vapor treatment, followed by an annealing, was found to recover the carbon content and reduce the hydroxyl content. Such a post-ash thermo-chemical treatment also improves the breakdown and reliability characteristics. HMDS followed by an anneal treatment successfully reduces the capacitance, and as a result the effective κ, by about 3%. This decrease will become more significant as κ values continue to be lowered. The trench side-wall interfacial defect density is reduced by a factor of about 5 and the reliability lifetime is improved by a factor of about 9. Post ash chemical treatments are a useful process step that can improve the reliability of the Cu/low-κ structure and also help in achieving the effective κ target.