Nanoscale Observation of Dielectric Damage to Low k MSQ Interconnects from Reactive Ion Etching and Ash Treatment
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Nanoscale observation of dielectric damage to low k MSQ interconnects from reactive ion etching and ash treatment Todd S. Gross and Shaoning Yao University Of New Hampshire, Department of Mechanical Engineering, Materials Science Program , Durham, NH Sri Satyanarayana SEMATECH, Austin, TX ABSTRACT Electrostatic force microscopy (EFM) was used to measure the extent of dielectric damage from plasma processing of nanoporous, low k methyl silsesqioxane (MSQ) interconnect structures with approximately 50 nm spatial resolution. Single level patterns were formed in 200 nm thick MSQ films by reactive ion etching (RIE) and were subsequently backfilled with an MSQ layer that was not exposed to plasma to act as a reference. The backfill was performed on as-etched structures with the photoresist intact and on structures in which the photoresist was removed by an oxygen plasma (ash) treatment. The EFM images on cross sections and feather sections show that the damage from the RIE penetrated ~100 nm in from the sidewall and that the redeposited polymer had a higher k than the MSQ (k~2.2). The etched and ashed MSQ exhibits a higher dielectric constant than the reference MSQ if it was exposed to water and has nearly the same dielectric constant as the reference with no water exposure. This suggests that the damage from the ash acts to make the MSQ hydrophilic. INTRODUCTION SEMATECH has demonstrated that 130 nm Cu damascene interconnect structures can fabricated using nanoporous methyl silsesqioxane (MSQ) as the low k dielectric [1]. However, the effective dielectric constant of the structure is higher than the k = 2.2 of blanket films [2, 3]. The purpose of this project is to determine the presence and spatial extent of low k dielectric damage from processing. We explored the following potential sources of damage: •
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Ash processing: The oxygen plasma that oxidizes the hydrocarbon photoresist also oxidizes the CH3 groups leading to formation of Si-OH groups. This converts the inherently hydrophobic MSQ into a hydrophilic film that can lead to moisture adsorption and significantly increase the effective k value of the dielectric. Furthermore, the pore structure can collapse resulting in an increase in k. We have previously measured increases in the dielectric constant of blanket films on exposure to oxidizing plasma. Reactive ion etch: The reactive ion etch process achieves straight sidewalls by redeposition of polymer on the sidewall. This polymer may have different chemical composition resulting in a different dielectric constant and will most likely have lower porosity. Furthermore, the energetic ions may change the structure and therefore the dielectric constant.
We have previously shown that Electrostatic force microscopy (EFM) can be used to visualize dielectric constant gradients with ~50 nm spatial resolution for dielectric films with dielectric
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constants less than k~6 [4, 5]. At this point, the method is semi quantitative and requires a reference dielectric to confirm that a perceived change in dielectr
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