Modeling of complex surface interactions in low and high pressure plasmas

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Wang, Mingmei
Major Professor
Mark J. Kushner
Committee Member
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Chemical and Biological Engineering

Plasma etching (or dry etching) is widely used in the fabrication of integrated cir-cuits (IC). Anisotropic features are easily obtained by controlling reactive ion trajectories in plasma. Twisting and bowing are two main issues during high aspect ratio (HAR) fea-ture etching. Twisting is, instead of a feature etching vertically, the feature twists or turns to the side. Mixing damage by ion bombardment of underlying materials is also a critical problem in post-etch processing. One of the explanations for twisting is charge accumu-lation on feature surfaces during ion bombardment. An asymmetric local electric field (E-field) changes ion trajectories incident into the feature to one direction opposite the E-field. This causes twisting. Bowing at the top of the feature near the mask-SiO2 interface mainly results from bombardment of ions reflecting from eroded mask surface. Ions with energies of many keV are required for HAR etching to get a high etch rate and a straight profile. While, if the ion energy is too high, it will penetrate into the under layer material and produce mixing damage during over etch.

Our research goals are to propose mechanisms to help eliminate twisting by ap-plying high energy electron (HEE) beams, to reduce bowing by protecting the pattern-transferring material photoresist (PR), and to minimize post-etching damage by optimiz-ing operating conditions in HAR etching.

A dc augmented capacitively coupled plasma (CCP) reactor is simulated using the Hybrid Plasma Equipment Model (HPEM) to generate energetic ions as well as HEE beams in Ar/C4F8/O2 gas mixture. Energy and angle distributions and fluxes for reactive species obtained from the HPEM are then input into the Monte Carlo Feature Profile Model (MCFPM) to investigate surface interactions in sub-micron to nano-scale feature etching. HEE generated from secondary emission of the top electrode surface due to ion bombardment is shown to have the ability of neutralizing positive charges accumulated deep into the trench and eliminating twisting. Increasing dc power applied on the top electrode produces more HEE beams with higher energy, which results in a decrease in twisting frequency.

Implantation induced mixing damage is also simulated in the MCFPM by incor-porating an IMPLANT model. The depth of implanting and mixing scales with rf power and ion energy. As such, there will be a tradeoff between high etch rate and low post-etch damage during etching at high bias power.

Ion bombardment can degrade the pattern transferring material, usually a polymer photoresist (PR), and, at the same time, it can induce cross-linking of the PR surface. Cross-linked PR surfaces are more resistive to etch than normal PR. More energetic ion sputtering leads to a higher etching selectivity of SiO2/PR while the PR is eventually eroded in HAR etching. A strategy to protect the PR is to generate Si radicals and VUV photons. Si is generated in the same way as HEE by ion sputtering of the top electrode. Si-C rich layer and Si extracting of F atoms from CxFy polymer on the PR surface may produce an etch stop if the Si flux is sufficiently high. VUV photons could be absorbed by PR surface and generates cross-linking. VUV and Si fluxes together have a synergetic effect of protecting the PR and eliminating bowing.

By functionlizing the surface of polymers, we can increase their surface reactivity to favor the adhesion of metals or other compounds on their surface, enhance their wet-tability or modify their biocompatibility by grafting on their surface specific chemical groups. Functionalization of inner pore surfaces in porous membranes is challenging be-cause the strong electric filed (100's kV/cm to 1MV/cm) formed in thin membranes could damage it. Gas breakdown in the inner pores in porous polymeric materials is in-vestigated in air using nonPDPSIM. Since photoionization is one of the dominant ioniza-tion mechanisms in small pores or porous channels in thin membranes, increasing photoionization cross section can produce ionization across sharp angles and promote plasma propagation. Electrode shape also affects plasma propagation direction. A round electrode is more likely to generate horizontal elements for the electric field and produce more ionization horizontally. When fixing the ration of electric field to gas number den-sity, E/N, different gas pressure and bias voltage combinations favor different plasma propagation paths. Higher gas pressure produces higher plasma density which helps the plasma penetrate narrow and thick `necks' in porous channels.

Sat Jan 01 00:00:00 UTC 2011