- Conference date: 23-27 August 2004
- Location: Bajina Basta (Serbia and Montenegro)
Radiofrequency discharges in low pressure fluorocarbon gases are used for anisotropic and selective etching of dielectric materials (SiO2 and derivatives), a key step in the manufacture of integrated circuits. Plasmas in these gases are capable not only of etching, but also of depositing fluorocarbon films, depending on a number of factors including the ion bombardment energy, the gas composition and the surface temperature: this behavior is indeed responsible for etch selectivity between materials and plays a role in achieving the desired etched feature profiles. Free radical species, such as CFx and fluorine atoms, play important but complex roles in these processes. We have used laser‐induced fluorescence (LIF), with time and space resolution in pulsed plasmas, to elucidate the kinetics of CF and CF2 radicals, elucidating their creation, destruction and transport mechanisms within the reactor. Whereas more complex gas mixtures are used in industrial processes, study of the relatively simple system of a pure CF4 plasma is more appropriate for the study of mechanisms. Previously the technique was applied to the study of single‐frequency capacitively‐coupled “reactive ion etching” reactors, where the substrate (placed on the powered electrode) is always bombarded with high‐energy ions. In this case it was found that the major source of CFx free radicals was neutralization, dissociation and backscattering of these incident ions, rather than direct dissociation of the feedstock gas. Subsequently, an inductively‐coupled plasma (ICP) in pure CF4 was studied. This system has a higher plasma density, leading to higher gas dissociation, whereas the energy of ions striking the reactor surfaces is much lower (in the absence of additional RF biasing). The LIF technique also allows the gas temperature to be measured with good spatial and temporal resolution. This showed large gas temperature gradients within the ICP reactor, which must be taken into account in reactive species transport. In the ICP reactor we saw significant production of CF and CF2 radicals at the reactor top and bottom surfaces, at rates that cannot be explained by the neutralization of incident ions. These two species are also lost at very high rates in the gas phase. We postulate that these two phenomena are caused by electron‐impact excitation of these radicals into low‐lying metastable levels. The metastable molecules produced (that are invisible to LIF) diffuse to the reactor walls where they are quenched back to their ground state. In the afterglow the gas cools rapidly and contracts, causing gas convection. Whereas the density of the more reactive species decays monotonically in the afterglow, the density of CF2 initially increases. This is partly due to the gas contraction, bringing back CF2 (which is a relatively stable species) from the outer regions of the reactor, and partly due to chemical reactions producing CF2, as it is more thermodynamically stable than the other radical species such as CF and CF3.
- Plasma etching
- Plasma materials processing
- Inductively coupled plasma
- Laser induced fluorescence
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