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.
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