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Hydrogen silsesquioxane (HSQ) bilayer resist (BLR) processes are attractive to obtain nano-sized features with high aspect ratio by dry-transferring thin e-beam pattern to thick underlayer to strengthen the etch resistance. However, there are drawbacks of high e-beam dosage for HSQ patterning and difficulty in controlling the underlayer resist profile by O2 plasma with anisotropic etching. In this study gap-fill type HSQ/ZEP520A BLR processes were studied to overcome these problems. The advantage of gap-fill type BLR processes is that the dosage for patterning on thick ZEP520A e-beam positive resist is not as high as that for HSQ and the resist profile can be tuned by exposure and development processes without depending on O2 plasma. By gap-filling of HSQ in ZEP520A trench patterns and then stripping ZEP520A by O2 plasma the tone is conversed from trench to line. The gap filling quality attributes include (1) the void size and number of HSQ lines and (2) spacer adhesion on HSQ line edge. Only the non-diluted HSQ solution could completely fill the trench and the HSQ line formed after stripping of ZEP520A. The spacer formed by diluted HSQ is found to be composed of oxide without any ZEP520A-related elements by FTIR analysis. The ZEP520A trench CD monotonically increases with decrease of W/L ratio. The HSQ line CD also follows the same trend. The extension of HSQ in ZEP520A, i.e. HSQ line CD minus ZEP520A trench CD, basically follows the reverse trend. It is therefore concluded that extension of HSQ lines in ZEP520A and HSQ spacers are formed from the diffused HSQ in trench sidewall without any reaction with ZEP520A. Voids were generally observed at the bottom of the HSQ line. Size and quantity of voids are larger for lower W/L ratios, indicating that the voids were formed due to insufficient HSQ volume for gap-filling. Increasing e-beam dose, baking or reflow temperature, and reflow of ZEP520A before HSQ coating could reduce the void formation. Multiple gap-filling with 1:14 diluted HSQ can lead to void-free lines. The HSQ spacer becomes thicker with less diluted HSQ, slower spin speed, reduced ZEP520A development time and HSQ PCB temperature. The smallest HSQ island of 46.3 nm was obtained by two reflows plus HSQ gap filling and baking processes, a significant size for the hardmask of metal island etching or mold of contact-hole nano-imprint for 45 nm node. The width of HSQ spacers is generally within 10-25 nm, potentially applicable to transistor gate patterning in 22 nm node and beyond.
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