Spin-on Low-k Etch Causes 193 nm Resist Deformation

Low-k dielectrics face continued challenges, and 193 nm resists face their own challenges. So what do you get when you cross a 193 nm resist with a low-k dielectric etch? More challenges, of course. In fact, during etch of spin-on low-k dielectrics, 193 nm resists tend to deform — with line narrowing, overhangs and sidewall slope.

Researchers from Philips Research (Leuven, Belgium; and Eindhoven, Netherlands), in conjunction with JSR Corp. (Tsukuba, Ibaraki, Japan), set out to test how various process and materials parameters cause resists to deform, seeking to discover optimal conditions for minimal resist deformation. They presented their results at last month's International Interconnect Technology Conference (IITC).

Although 248 nm resists show slight deformation from dry etching, the impact is negligible. In contrast, the researchers observed >20% linewidth shrink caused by 193 nm resist deformation during single-damascene etch. Their studies showed a difference in those results depending on the chemistry, pressure or power used in the experiments.

They used JSR's LKD-5109 low-k material, a porous MSQ-type material with a dielectric constant of ~2.2. The 248 and 193 nm resists were spin-coated onto low-k spin-on hard masks, then developed and exposed. The researchers performed the experiments under various conditions, in one case, for example, varying the fluorocarbon chemistry used while maintaining constant pressure and power settings. In other experiments, pressure or power settings were adjusted.

What they found was that the deformation of 193 nm resists begins at the plasma ignition step, where ion bombardment heats up the resist and causes it to expand rapidly. This ultimately results in >20% linewidth shrink during single-damascene etching for LKD-5109 with dual spin-on hard masks. In general, the polymers in 193 nm resists have a lower Tg (glass transition temperature) and a higher thermal expansion coefficient than the polymers in 248 nm resists, a possible reason 193 nm resists deform more severely, the researchers noted. However, plasma ignition with argon and CH₂F₂ chemistries deforms the 193 nm resists more severely than do N₂ and CF₄ chemistries.

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