Effect of ion angular distributions on microloading in oxygen reactive-ion etching of submicrometer polymer trenches

The etch rate of the resist by energetic reactive ions (O+, O+2) from a 13.56 MHz plasma was studied as a function of the aspect ratio. Using a trilevel sequence, submicrometer grooves with widths of 500, 200, 100, and 50 nm are etched with an oxygen rf plasma in a parallel-plate reactor. Pattern transfer into the polymer photoresist proceeds via a thin silicon nitride mask. With increasing aspect ratio a decrease in etch rate is observed for increasing etch time resulting in a diminished etch depth for small width grooves. This effect is commonly called microloading and its origin is subject to many discussions. With the assumption that lower local etch rates are caused by the characteristics of particle fluxes entering the structures it is necessary to consider the ion energy distributions and the ion angular distributions of O+ and O+2 emerging from the oxygen rf plasma bombarding the surface to be structured. The measurement of ion angular impact energy distributions of O+ and O+2 is performed in an apparatus simulating a parallel-plate rf reactor. Through a small orifice in the rf-powered electrode ions are sampled and are analyzed in a quadrupole mass spectrometer equipped with an energy filter. Tilting the detection system with the vertex lying in the orifice, mass selected ion energy distributions are measured at each angle. O+2 energy distributions consist of the well-known peaked structures caused by charge-exchange collisions in the plasma sheath. The angular distributions are characterized by mace-shaped structures with an angular width of about 2° accompanied by wings with intensity maxima at about ±2° for most of the experimental conditions considered. O+ energy distributions consist of bimodal split high-energy peaks and continuous intensities between 50 eV and the maximal energy defined by the potential drop across the sheath. The angular distributions of the high-energy O+ ions have widths between 2° and 6° while the continuous part of the ion energy distributions exhibits angular widths between 7° and 11°. The phenomena observed in the ion energy distributions and ion angular distributions are a result of scattering mechanisms of ions with O2 parent gas molecules in the plasma sheath. The experimental ion angular distributions are used to model the effect of lower local etch rates. Assuming that direct impingement of particles onto the bottom surface of a structure is the dominant ablation mechanism it is possible to correlate the effect of lower local etch rates with the angularly broadened flux of O+ ions.