Operating high-density plasma sources in a low-density range: Applications to metal etch processes

Abstract

A new way of operating high-density plasma sources is presented which allows the resist/metal selectivity to be significantly increased during a metal etching process while maintaining the anisotropy of the etching profile. The principle of the technique is to operate a high-density plasma source in a low-density range in which the radio-frequency (rf) source power is lower than the chuck power. When the source is operated under these conditions, the ion density is strongly decreased while the source is still operated at low pressures. Optical emission spectroscopy has been used to compare the dissociation efficiency of the source operated in the low- and high-density modes. The photoresist etch rate was measured using light reflectometry. In some cases, after etching, the resist sample was transferred in vacuo into an x-ray photoelectron spectrometer analysis chamber to measure the chlorine coverage on the resist surface. The ion current density (Ji) was determined using a new type of electrostatic probe. The ion flux was determined from the discharging of a RF-biased capacitance in series with the probe. Finally, a simple model is suggested to explain the impact of the plasma operating conditions (source power and chuck power) on the resist etch rate.