Mass-selected ion angular impact energy distributions at the powered electrode in CF4 reactive-ion etching

Abstract

The ion energy distributions and ion angular distributions of CF+, CF+2, and CF+3 currents were measured at the powered electrode of a capacitively coupled 13.56 MHz discharge parallel-plate reactor running on CF4. The pressure range is varied between 5 and 50 mTorr, and by adjusting the rf power dissipated in the plasma the dc bias potential at the cathode is varied between −150 and −450 V. Ions are sampled through a small orifice in an aluminum wafer bonded to the cathode. Downstream the orifice ions are selected in a quadrupole mass spectrometer equipped with an energy filter. Tilting this detection system with the vertex lying in the orifice ion angular distributions are measured. The ion energy distributions were found to exhibit structures which appear as continua. The structures result from collision processes of CF+x (x=1–3) with the CF4 parent gas in the sheath. Charge-exchange collisions could be identified neither in the ion energy distributions nor in the ion angular distributions. The ion angular distributions of CF+3 at the powered electrode are characterized by angular widths between 3° and 4° for the entire energy range. Angular distributions of CF+2 have angular widths of about 5° for high-energy ions while low-energy ions show angular widths of about 15°. Similar observations are made for CF+ ions with angular widths of about 6° for high-energy ions and about 16° for low-energy ions. In general, experiments indicate a slight increase in angular width for increasing CF4 pressure. The existence of continua in the energy distributions and the broad angular distributions are interpreted as being caused by the combined effect of elastic scattering and dissociative collisions in the sheath. Additionally, for increasing CF4 pressure multiple scattering becomes important giving rise to further broadening of impact angles and intensifying low-energy continua. Some consequences for the interpretation of profile developments in submicrometer dry etching are discussed.