Characterization of hydrogen–plasma interactions with photoresist, silicon, and silicon nitride surfaces

For the 45 nm technology node and beyond, a major challenge is to achieve reasonably high photoresist ash rates while minimizing the loss of the silicon (Si) substrate and its nitride (Si3N4). Accordingly, an objective of this work is to characterize the photoresist strip rate under varying conditions of H2 plasma and the effects of these conditions on Si and Si3N4 etch rates. In addition, we discuss in detail the fundamental mechanisms of the reactions between H atoms and the above substrates and successfully reconcile the process trends obtained with the reaction mechanisms. In this work, photoresist, Si, and Si3N4 films were exposed to downstream pure-H2 discharges and their removal rates were characterized by ellipsometry as a function of the following parameters: substrate temperature, reactor pressure, H2 flow rate, and source power. The authors found that the H2-based dry ash and Si3N4 etch are both thermally activated reactions, evidenced by the steady increase in etch rate as a function of temperature, with activation energies of ∼5.0 and ∼2.7 kcal/mol, respectively. The Si substrate exhibits a rather unique behavior where the etch rate increases initially to a maximum, which occurs at ∼40 °C, and then decreases upon a further increase in temperature. The decrease in the Si etch rate at higher temperatures is attributed to the activation of competing side reactions that consume the chemisorbed H atoms on the Si surface, which then suppresses the Si-etch step. The photoresist and Si3N4 removal rates increase initially with increasing pressure, reaching maxima at ∼800 and 2000 mTorr, respectively, beyond which the removal rates drop with increasing pressure. The initial increase in removal rate at the low-pressure regime is attributed to the increased atomic-hydrogen density, whereas the decrease in ash rate at the high-pressure regime could be attributed to the recombination of H atoms that could occur by various mechanisms. At temperatures where the reaction rates are relatively fast, the photoresist and Si removal rates both increase continuously with the H2 flow rate, indicating that both reactions are in the supply-limited regime. For the range of process conditions explored here, we find that the etch rates of Si are generally much higher than those for Si3N4 with Si:Si3N4 etch-rate ratios that vary from 25 to ≫520. Based on the process trends obtained here, we have identified a process window—high temperature and intermediate pressure—that attains relatively high photoresist ash rates and low Si and Si3N4 etch rates.