Interaction between plasma and low-k dielectric materials

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Bao, Junjing, 1981-

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With the scaling of devices, integration of porous ultra low-κ dielectric materials into Cu interconnect becomes necessary. Low-k dielectric materials usually consist of a certain number of methyl groups and pores incorporated into a SiO₂ backbone structure to reduce the dielectric constant. They are frequently exposed to various plasmas, since plasma is widely used in VLSI semiconductor fabrication such as etching, ashing and deposition. This dissertation is aimed at exploring the interaction between plasma and low-κ dielectric surfaces. First, plasma assisted the atomic layer deposition (ALD) of Ta-based Cu barriers. Atomic layer deposition of Ta barriers is a self-limited surface reaction, determined by the function groups on the low-κ dielectric surface. But it was found TaCl₅ precursor could not nucleate on the organosilicate low-κ surface that was terminated with methyl groups. Radical NH[subscript x] beam, generated by a microwave plasma source, could activate the surface through exchanging with the methyl groups on the low-κ surface and providing active Si-NH[subscript x] nucleation sites for TaCl₅ precursors. Results from Monte Carlo simulation of the atomic layer deposition demonstrated that substrate chemistry was critical in controlling the film morphology. Second, the properties of low-κ dielectric materials tended to degrade under plasma exposure. In this dissertation, plasma damage of low-κ dielectric surface was investigated from a mechanistic point of view. Both carbon depletion and surface densification were observed on the top surface of damaged low-κ materials while the bulk remained largely uninfluenced. Plasma damage was found to be a complicated phenomenon involving both chemical and physical effects, depending on chemical reactivity and the energy and mass of the plasma species. With a downstream plasma source capable of separating ions from the plasma beam and an in-situ x-ray photoelectron spectroscopy (XPS) monitoring of the damage process, it was clear that ions played a more important role in the plasma damage process. Increase of dielectric constant after plasma damage was mainly attributed to moisture uptake and was confirmed with quantum chemistry calculation. Annealing was found to be effective in mitigating moisture uptake and thus restoring κ value. Finally, oxygen plasma damage to blanket and patterned low-κ dielectrics was studied in detail. Energetic ions in oxygen plasma contributed much to the loss of film hydrophobicity and dielectric constant through the formation of C=O and Si-OH. Based on results from residual gas analyses (RGA), three possible reaction paths leading to carbon depletion were proposed. This was followed by analytical solution of the evolution of carbon concentration during O₂ plasma damage. O₂ plasma damage to patterned CDO film was studied by TEM/EELS. And the damage behavior was simulated with Monte Carlo method. It was found that the charging potential distribution induced by plasma was important in determining the carbon loss in patterned low-k films. The charging potential distribution was mainly related to the geometry of low-k trench structures. To recover the dielectric constant, several recovery techniques were tried and briefly discussed.