Ashing of photoresists using dielectric barrier discharge cryoplasmas

Plasma ashing of photoresists is a critical step in advanced microelectronics manufacturing as it often leads to extensive damage in porous organosilicate low - κ dielectrics and hinders the use of highly porous films in interconnects. To reduce plasma damage, the authors investigated the feasibility of ashing a 248-nm photoresist with cryoplasma. The authors ashed photoresist-coated silicon wafers with dielectric barrier discharge microplasma generated at temperatures of 170-291 K, a pressure of 100 Torr, applied voltages of V appl = 0.8 - 1.6 kV, and a frequency of f = 20 kHz in both Ar / O ₂ and Ar / O₂/ N 2 gas mixtures. While the ashing rates at 170 K in Ar / O₂ decreased to about 20% of the ashing rates achieved at room temperature and 240 K, the addition of N 2 to the plasma gas enhanced the ashing rates by a factor of 1.5-2. Optical emission spectroscopy measurements of the plasmas showed that, in the Ar / O₂ / N 2 mixture, the main reactive species are N 2 radicals; x-ray photoelectron spectra of the ashed photoresists indicated that ashing is initiated from oxygen-containing functional groups of the photoresist. This study showed that decreased ashing rates at low plasma gas temperatures can be significantly enhanced by adjusting the plasma chemistry and that cryoplasma offers a viable process to minimize the damage from ashing of low - κ dielectric materials in interconnects, which will allow nanoelectronic devices to fully benefit from the introduction of such porous materials.