Ashing of photoresists using dielectric barrier discharge cryoplasmas

Abstract

Plasma ashing of photoresists is a critical step in advanced microelectronics manufacturing as it often leads to extensive damage in porous organosilicate low-j 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 ¼ 0:8 À 1:6 kV, and a frequency of f ¼ 20 kHz in both Ar=O 2 and Ar=O 2 /N 2 gas mixtures. While the ashing rates at 170 K in Ar=O 2 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=O 2 =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-j dielectric materials in interconnects, which will allow nanoelectronic devices to fully benefit from the introduction of such porous materials.

Prospects for dielectric constant reduction in integrated circuits interconnects

“…Even if the preliminary results were promising, there has been very little investigations of such processes in the literature. [21,22] …”

Review on plasmas in extraordinary media: plasmas in cryogenic conditions and plasmas in supercritical fluids

“…A more detailed analysis of the effect of cryoplasma ashing on photoresists revealed that depending on the temperature and gas composition, different types of molecular groups are removed or modified [48]. So far, there have been no specific applications, but we believe the inherent ability of cryoplasmas to actively control the plasma chemistry and gas temperature with modest local heating can enable their use in emerging fields of plasma science and technology, namely plasma medicine and biotechnology.…”

Experimental and numerical investigation of time evolution of discharge current and optical emission in helium–nitrogen cryoplasmas

“…These studies reported that certain reaction rates strongly depend on T g . For example, the reaction rate constant of the metastable He (He m ) conversion reaction (R6 in table 1) strongly depends on the gas temperature [12] and was too low to be measured below 65 K, due to a small hump in the potential energy for the combination of He atoms in the 2 3 S and 1 1 S states [21], which results in the existence of the activation energy (∼0.067 eV) [11]. Also, the slow decay of plasma afterglow due to long ambipolar diffusion lengths [22] and a long relaxation time for the rotational distribution of He 2 molecules [23,24] have been reported.…”

“…for the treatment of highly heat sensitive materials such as nano-porous structures and frozen materials. For example, it has been reported that the effective reduction of T g reduces damage in nano-porous low-k materials during ashing [2,3].…”