DOE OSTI2012
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 (Si{sub 3}N{sub 4}). Accordingly, an objective of this work is to characterize the photoresist strip rate under varying conditions of H{sub 2} plasma and the effects of these conditions on Si and Si{sub 3}N{sub 4} 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 Si{sub 3}N{sub 4} films were exposed to downstream pure-H{sub 2} discharges and their removal rates were characterized by ellipsometry as a function of the following parameters: substrate temperature, reactor pressure, H{sub 2} flow rate, and source power. The authors found that the H{sub 2}-based dry ash and Si{sub 3}N{sub 4} etch are both thermally activated reactions, evidenced by the steady increase in etch rate as a function of temperature, with activation energies of {approx}5.0 and {approx}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 {approx}40 deg. 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 Si{sub 3}N{sub 4} removal rates increase initially with increasing pressure, reaching maxima at {approx}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 H{sub 2} 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 Si{sub 3}N{sub 4} with Si:Si{sub 3}N{sub 4} 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 Si{sub 3}N{sub 4} etch rates.