Plasma sheath tailoring for advanced 3D plasma etching: the effects of mask geometry, etching materials, and ion energy.

Three-dimensional (3D) etching of materials by plasmas is a key challenge for microstructuring applications to produce advanced sensors, optics, and microfluidics. We previously demonstrated that a local magnetic field combined with a metallic mask can manipulate the plasma dynamics above the substrate, resulting in asymmetric etch profiles in Si and a-C:H films in an Ar/CF₄/O₂ plasma [1]. The experiments were explained by E × B drifts during the local sheath expansion in the RF plasma, as seen in a particle-in-cell simulation. This tailors the plasma density distribution inside the mask, thus affecting the transport to the surface.

However, it remains unclear which parameters can be used to control the shape of the etch profile to ultimately enable complex 3D structures. To identify these parameters, this work investigates the influence of mask geometries, substrate material, and energy of the incident ions.

The mask shape influences the redeposition of sputtered CF-containing polymers from the mask surface onto the substrate and the etching profiles (measured by stylus profilometry). SiO₂ and various glasses show a significantly more pronounced effect of spatially different etching rates and a different dependence on the ion energy compared to pure Si, making them especially promising. This is presumed to be due to differences in the etching chemistry of Si and SiO₂.

References: [1] Jüngling et al. Appl. Phys. Lett. 12 February 2024; 124 (7): 074101.