A large variety of applications include a wet-chemical etching step which very often involves highly alkaline etching baths. While for example negative resists of the CAR44 series can easily be stabilised by a subsequent flood exposure and hard bake against alkaline conditions, a simple hard bake of positive resists is not sufficient to obtain the desired effect since in this case no stabilising crosslinking takes place. A stabilisation can however be achieved by blocking the reactive alkali-soluble groups in the resist layer. The example of AR-P 3210 successfully showed that (already developed) resist structures can sufficiently be stabilised against alkaline solutions such as IPA, PMA and acetone by a subsequent silylation with HMDS.
Resist structures have been stable in the solvents for several days. For this purpose, it was initially necessary to activate the patterned resist layer by flood exposure. Due to the intense exposure, the photosensitive component (DNQ) is at first converted into indene carboxylic acid. At temperatures in a range between 110 °C – 120 °C, HMDS (gas phase) silylates both the OH-groups of the novolac as well as the indene carboxylic acid, thus making them insoluble in these solvents.
Likewise, this treatment increased the alkali stability. Individual samples could remain in 0,5 n NaOH for several hours without impact, but will nevertheless still not withstand a silicon etch (40 % KOH at 85 °C).
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