Louis C. Frees

MASS SPECTROMETRIC MEASUREMENTS ON INDUCTIVELY COUPLED FLUOROCARBON PLASMAS : POSITIVE IONS, RADICALS AND ENDPOINT DETECTION

Positive ions and radicals in C2F6 and CHF3 high density discharges were measured using a direct-line-of-sight mass spectrometer. The ion energy distributions of the dominant ions were measured as a function of process conditions. Appearance potential mass spectrometry was performed to measure trends of the radical densities. For C2F6 plasmas CF3 and CF3+are the most abundant neutral and ionic species, respectively. CF3 is the most abundant neutral species for a CHF3 plasma, whereas CHF2+ and CF+ are the most abundant ionic species at 600–1000 and 1400 W, respectively. Erosion of the quartz coupling window is an important contaminant source for our inductively coupled plasma system. For comparison, downstream mass spectrometry was also applied using a closed ion source system since this approach is of interest for real-time monitoring and control. Endpoint detection for Si and SiO2 film etching in a CHF3 plasma was investigated using the downstream mass spectrometer system and compared with data obtained ...

Miniature quadrupole residual gas analyzer for process monitoring at milliTorr pressures

Monitoring of gas composition for processes in the mTorr range is not possible with a conventional residual gas analyzer (RGA) unless a pressure reduction pumping system is used. A miniature sensor, having dimensions compatible with the mean-free path of gas molecules can operate directly at mTorr pressures while being less expensive and more compact than a conventional apparatus. This article describes a quadrupole RGA which is scaled down by a factor of 7 to operate at pressures up to 10 mTorr. This sensor requires different construction methods and drive electronics than a traditional RGA. A unique ion source with high output has been developed. The small sensor has resolution characteristics very similar to standard size models but sensitivity is reduced in proportion to the ratio of cross-sectional area for each quadrupole r0. Response is linear to the mTorr range and a correction method for gas scattering, based on pressure measured by a total pressure gauge included in the ion source design, extend...

Surface etching mechanism of silicon nitride in fluorine and nitric oxide containing plasmas

The etch rate of silicon nitride (Si3N4) in the afterglow of fluorine-containing plasmas is strongly enhanced when both nitrogen and oxygen are added to the remote discharge. This effect is attributed to the formation of nitric oxide (NO), which we identify as a highly reactive precursor for the etching of Si3N4. The Si3N4 etch rate, surface oxidation, and the depletion of the surface of N atoms show a linear dependence on the NO density. In order to determine the products of the NO reaction at the Si3N4 surface, mass spectrometry was performed in immediate proximity to the surface with a specially designed movable sampling orifice. Both SiF4 and N2 are identified as primary etch products, but a smaller amount of N2O was also detected. Based on our results, we suggest that NO enhances the removal of N from the Si3N4 surface by the formation of gaseous N2, and leaving behind an O atom, while the overall surface oxidation remains very low, and the reactive layers are very thin. This modified surface reacts ...

Mass‐dependent effects of channel electron multipliers in residual gas analyzers

Residual gas analyzers (RGA’s) are increasingly being used to obtain quantitative information about vacuum processes. The gain for an ion [electron multiplier (EM) output current divided by the Faraday cup (FC) output current] determined by experiment for a combination FC/EM sensor was not constant for increasing mass. The gain for ions of different masses was also found to be dependent on the chemical nature of the ion, reflecting the conversion efficiency of incident ions to secondary electrons [U. Fehn, Int. J. Mass Spectrom. Ion Phys. 15, 391 (1974); 21, 1 (1976)]. Allowance for these effects makes quantitative RGA measurements in the EM mode of the sensor more reliable. Methods for determining a correction factor are discussed.

Spatially resolved mass spectrometric sampling of inductively coupled plasmas using a movable sampling orifice

A quadrupole mass spectrometer equipped with a special sampling tube and positioned on a linear-motion table is shown to be capable of obtaining important information on the variation of the discharge properties with position. We measured signals obtained by post-ionization of species sampled from Ar and C2F6 high-density plasmas using electron ionization energies of 30 and 16 eV. The variation of the ion signal with position of the sampling orifice inside the reactor provides evidence of the spatial nonuniformity of the discharge and the processes that control these variations. For Ar discharges we show evidence of gas heating which produces a nonuniform radial distribution of the neutral species. The neutral species are depleted in the center of the discharge due to plasma heating. The neutral gas temperature that can be extracted from the variation in density is consistent with temperature measurements using spectroscopic probes. We also monitored the intensity of different fluorocarbon ions as a funct...