Ion beam bombardment and etching generated by inductively coupled plasma for functional materials

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2023-05

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Ion beams generated by a pulsed inductively-coupled (ICP) plasma provides the opportunity to improve etch selectivity, etch uniformity and minimize the surface damage, which are crucial to the semiconductor fabrication processes. The pulsed ICP allows flexible control of the ion energy distribution with lower average ion energy during etching. As the semiconductor industry develops sub-5 nm device fabrication strategies, the process requirements become more strict, such as high throughput, high uniformity, highly anisotropic, high selectivity and low plasma damage. In this study, we first investigated the ion energy distribution functions (IEDF) and ion fluxes of the ion beam generated by a pulsed ICP with argon, hydrogen and oxygen gases. The DC bias voltage that is applied to the boundary electrode was studied between 25 to 200 V to tune the ion energy of the beam, and the corresponding IEDF and ion flux is measured by retarding field energy analyzer. This insight is used in studies of atomic layer etching (ALE) of SiN and changes to tin-oxo cage structures. Model Sn-based material films containing tin-oxo cages with organic ligands are expected to provide good photoelectron production under extreme ultra-violet (EUV) radiation. These areas transformed during EUV irradiation ultimately need to be etched away and various dry etch schemes are under development. This research explores H, O, and Ar ion interactions with a model tin-oxo compound to understand reactions that lead to resist material etching. Tin-oxo cages have a bonding order and stoichiometry, SnC [subscript x] O [subscript y] defined by the organic ligands and counter anions that stabilized the cage. X-ray photoelectron spectroscopy is used to follow changes to the tin-oxo cages. H, O and Ar ions open the cages to different degrees and the Sn 3d [subscript 5/2] features show the near surface stoichiometry – the x and y in SnC [subscript x] O [subscript y] – depends on the ion and the ion energy. At energy above 100 eV the resulting SnC [subscript x] O [subscript y] layer can be sputtered with O ions. H ions also can form volatile SnH₄ leading to some chemical etching until the resulting material becomes enriched in carbon. Plasma-assisted ALE processes typically alternate cycles of chemical modification to weaken the surface bonds followed by ion bombardment to remove a limited amount of material. We explore silicon nitride ALE by utilizing 7.8 eV energy argon ions to achieve ion-activated fluorocarbon (CHF₃) adsorption in the chemical modification step followed by 200 eV argon ion bombardment for the removal step. The low energy Ar ions generated in an inductively coupled plasma source lead to partial dissociation of CHF₃ in the gas region above the surface an in a weakly adsorbed state on the surface to form an adsorbed CH [subscript x] F [subscript y] layer in an ALE-like saturation process. This CH [subscript x] F [subscript y] layer was subjected to 200 eV Ar ions to etch into the SiN film. The presence of adsorbed species and changes in the silicon nitride film were monitored with in-situ Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy and spectral ellipsometry.

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