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Particles in C2F6-CHF3 and CF4-CHF3 etching plasmas

Particulate generation has been studied during reactive-ion etching of oxide wafers in C2F6-CHF3 and CF4-CHF3 plasmas using both a commercial etch tool and the GEC reference cell modified to resemble the commercial tool. Under certain discharge process conditions, copious amounts of submicrometre-sized particles are shown to form due to plasma interactions with the oxide substrate. In the commercial tool, particles were detected only by a downstream particle flux monitor, whereas in the reference cell, particles were observed by both in situ laser light scattering and downstream monitoring. In the commercial tool, wafers etched to end-point were shown by post-process surface analysis to be contaminated by submicrometre-sized columnar structures. Previous reports of similar such columnar structures formed during reactive-ion etching of oxide films have attributed the phenomenon to polymer micromasking. However, the results of this study clearly contradict this conclusion and suggest that the presence of columnar oxide etch residues is linked to process-induced particulate contamination. Laser light scattering measurements were made in the reference cell during reactive-ion etching of blanket oxide wafers and used to help clarify the complex processes of particulate nucleation, growth and deposition during oxide etching. Polarization coagulation of spherical particles formed in the reference cell is shown to occur, presumably in the high-field regions of the sheath, forming filamentous rod-like particle aggregates. The implications of this observation for wafer contamination are explored.