Diagnostics of positive and negative ions in high-density CF/sub 4/ plasmas by time-of-flight mass spectrometry

Summary form only given. High-density plasmas employing carbon-tetrafluoride (CF/sub 4/) gas have widely been used for etching processes of SiO/sub 2//Si in the fabrication of ultralarge-scale integration circuits. In the present paper, we report on the diagnostics of positive and negative ion species in helicon-wave excited CF/sub 4/ plasmas by time-of-flight mass spectrometry. The CF/sub 4/ plasmas were produced in a linear magnetic field of 1 kG by an RF (13.56 MHz) field applied to a m=1 helical antenna wound around a quartz glass tube of 3 cm in diameter. To avoid melting the quartz glass tube, the plasmas were produced periodically with a repetition rate of 5 Hz and a discharge duration of 10 ms. The CF/sub 4/ gas pressure range was 1.7/spl sim/10 mTorr with a fixed flow rate of 10 ccm. High electron densities of 10/sup 11//spl sim/10/sup 13/ cm/sup -3/ were obtained by the helicon-wave discharge for RF powers of 0.2/spl sim/3 kW. The time-of-flight mass spectrometer was attached on the end plate which was located 55 cm away from the edge of the helical antenna. The spectrometer was composed of an extraction electrode with a pinhole of 0.5 mm in diameter, an acceleration electrode, focusing electrodes, a gate electrode two pairs of deflector electrodes, a flight tube (1.2 m in length), and a Faraday cup detector. The positive and negative ions were extracted from the CF/sub 4/ plasmas and were accelerated by 1.8 kV to obtain the ion-mass spectrum. Various positive ion species of C/sup +/, F/sup +/, CF/sup +/, CF/sub 2//sup +/, and CF/sub 3//sup +/ were observed.