Paper PS2-ThA1
Absorption Spectroscopy Diagnostics of a Dual-Frequency Capacitive Dielectric Etch Tool using Ultraviolet Light-Emitting Diodes

Thursday, November 12, 2009, 2:00 pm, Room B2

Dual–frequency capacitively-coupled etch reactors using Ar/fluorocarbon/O₂ mixtures are widely employed for etching of dielectric films for integrated circuit manufacture. CF_x free radicals play an important role in the gas-phase and surface chemistry controlling etching and polymer deposition. The CF₂ radical is the most abundant, and its density can be measured by UV absorption via the A-X band (230-270 nm). Previously Xe arc lamps have been used for the absorption light source, but these sources are rather unstable, limiting the sensitivity of the technique, as well as being cumbersome and relatively expensive. We have successfully replaced the Xe arc with UV light-emitting diodes. The baseline stability is of the order 2x10^-4, compared to 1x10^-3 with an arc lamp. We determined the variation of the CF₂ density as a function of gas composition and power in a modified 2 + 27MHz commercial etch reactor operating in Ar/C₄F₈/O₂. As expected, the CF₂ density decreases rapidly as the O2/C4F8 ratio is increased. The CF₂ density increases with RF power at both frequencies, but is most affected by 27 MHz power. There is speculation that CF₂ may play an important role in either or both the creation and destruction of F- negative ions. However, we did not find any simple correlation between CF₂ density and electronegativity (as determined from electron density and ion flux measurements).

We also attempted to measure the F^- negative ion density by the continuum absorption below 365 nm. However in this case we observed a broad but structured absorption when the O2/C4F8 ratio is small. This absorption is too intense to be attributed to F^-, and we speculate that it is due to unsaturated CxFy oligomerisation products.

We wish to thank the Lam Foundation for financial support.