On the photoresist stripping and damage of ultralow k dielectric materials using remote H2- and D2-based discharges

Abstract

Blanket films of ultralow dielectric constant (ULK) materials and 193nm photoresist films have been processed downstream from hydrogen and deuterium-based discharges produced using an inductively coupled plasma reactor. Photoresist ashing rates and ULK modifications have been determined as a function of process parameters. The explored ULK materials differed widely in porosity and carbon content. The effect of processing time, substrate temperature (200–300°C), and gas composition on the surface and bulk chemical composition of ULK materials was monitored and quantified by ex situ ellipsometry and time-of-flight secondary ion mass spectrometry (SIMS). The stripping rates of 193nm photoresist films were found to strongly depend on processing temperature and only weakly on the nature of the H2/additive gas mixture. The authors found that hydrogen (or deuterium) fully penetrates the high porosity ULK layer, whereas for low porosity materials, such penetration is limited to a 50nm near-surface region. SIMS measurements also reveal that H2 (D2) diffusion into carbon-rich ULK layers can cause substantial carbon depletion throughout the penetration region. ULK damage values increase with temperature and injection of gas additives such as argon, helium, and nitrogen to H2 or D2 process gases. For each ULK material, the amount of damage depends on the gas mixing ratio; in general, high percentages of nitrogen in H2∕N2 (or D2∕N2) mixtures cause the most damage. Overall, the results demonstrate that ULK ashing damage depends strongly on both ULK material properties and H2-based plasma process parameters. In addition, the authors observed in this work a kinetic isotope effect for stripping of 193nm photoresist films in H2∕D2∕N2-based discharges. For given ashing process conditions, the photoresist ashing rate decreases by a factor of 1.414 (or square root of 2) in D2 plasma compared to H2 plasma. This can be explained by the influence of the H or D mass on the chemical reaction rate through a change in the frequency of nuclear vibrations of the reacting atoms. The presence of the kinetic isotope effect for gas mixtures provides unambiguous evidence of the rate-limiting role of atomic hydrogen in the fundamental etching reaction. Simultaneously processed ULK materials showed minor film thickness changes (<10nm) in H2 or D2 discharges, and the ULK damage level does not reflect a kinetic isotope effect. Therefore the H∕D isotope effect can be used to separate H2∕D2 associated ashing and etching processes from other chemistries or mechanisms.