Neutral Beam Etching

el Otell, Ziad (2013). Neutral Beam Etching. PhD thesis The Open University.



The aim of this research is to better understand the behaviour of pulsed discharges and electron dynamics for the purpose of tailoring the plasma properties for neutral beam etching (NBE) applications.

A capacitively coupled plasma formed in a research system was used for a study of pulsed tailoring in an electropositive plasma. A combination of high time resolved optical diagnostics, plasma imaging and optical emission spectroscopy, and hairpin probe measurements were used to study the electron density and the energy distribution function during the ignition phase of a repetitively pulsed plasma. Two different waveforms were used to modulate the envelope of the input RF-voltages in order to control the ignition phase, by changing the increase rate of the electron density and evolution of the electron energy distribution function (EEDF). The results of this study indicate that the increase rate of the electron density and the EEDF, during operation, can be influenced and even controlled to some extent by pulse tailoring. Electron densities of the order of 1016 m-3 were obtained, and EEDFs of a highly non-Maxwellian nature were characterised during the ignition phase. Also, the ignition timescales were controlled by applying pulse tailoring from a few microseconds (typically 2 μs) to a few tens of microseconds (80 μs) for the different input waveforms.

An inductively coupled plasma in an industrial plasma etching tool was used to study pulse tailoring in electropositive and electronegative discharges. The same environment was used to create a source to from energetic negative ions which could then be extracted and neutralised. Similar diagnostic techniques, as those used in the research source, in addition to RF-probes were used to characterise the inductive source. Optical emission spectroscopy and electron density measurements showed that the plasmas, almost instantaneously, ignite in the H-mode. The EEDFs were characterised by a Maxwellian distribution with an electron temperature ranging between 1.2 up to 1.6 eV, and electron densities of the order 1018 m-3 were measured, depending on the operating conditions. This source was also used for preliminary NBE studies. Neutralisation efficiencies ranging between 70% and 95% were measured, and etch rates of 25 and 30 nm/min were found.

Finally, a novel technique was developed to monitor in-situ and in real time the plasma-induced damage on thin films, in particular low-k dielectrics. This technique uses wafer tiles to simultaneously measure the plasma properties and plasma-induced damage on the thin films. An analytical model, based on the ion flux probe model, was used to to extract the plasma parameters and mimic the existence of thin on the probe surface, by a resistor and a capacitor connected in parallel. The model managed to extract exact values of a dummy load, while a reasonable fit could not be achieved for thins silicon dioxide films. Initial results showed that, even for a well characterised material of a known permittivity and thickness, the model could not extract absolute resistivity and capacitance. Plasmas surface interactions were deduced to be the main factor for this result, which were not included in the model used in this study. However, plasma-induced damage/changes on low-k thins films by different plasma chemistries, such as argon and hydrogen, were qualitatively measured in real-time. The wafer probe was used to monitor the etch rate of an SF6 plasma for low-k thin films in realtime and in-situ. This technique could also be used to measure the neutralisation efficiency and possibly the etch rates of our NBE source in real time.

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