Wet Chemical Processing with Megasonics Assist for the Removal of Bumping Process Photomasks

A new generation of negative tone resists by TOK, JSR, and Dow Chemical is gaining momentum in advanced packaging applications. Resist thickness requirements are actually increasing to the 50-100 um range as Cu pillars are adopted to accommodate the tighter pitches required in advanced packaging. In order to form pillars the resist must be thicker to contain the entire bump structure. Thick, negative tone resists are more transparent to exposure light wavelengths than positive tone resists and can be exposed by the lithography process much faster (~1 x 104 cross-linking chemical events are driven by 1 photochemical event vs. 100 – 1000 for positive tone resist).1 Therefore exposure times can be shorter, post-exposure bake steps can be shorter, and delay before or after exposure is not necessary, saving photolithography time and CoO. And, due to more complete cross-linking throughout the resist, the resist mask profiles are truer (from Flack et.al, “A Comparison of New Thick Photoresists for Solder Bumping”, SPIE 2005).1 The major drawback to negative tone resists is that the solvent strip times of the highly cross-linked resist masks are much longer than for positive tone. Flack et.al. noted resist strip times of 5 minutes for two positive resists used in their experiments vs. 50 minutes for the AZ-100nXT negative tone resist, using AZ400T at 80°C (most likely in an immersion tool). Long strip times or special stripping requirements are noted on the data sheets of the other majority suppliers of negative resists as well. Akrion engineers have developed a novel, single-wafer, negative PR strip processes using organic solvents plus Goldfinger megasonics, to provide 30 - 70% reductions in process times and the associated chemical consumption when compared to processes that do not utilize a megasonics assist. The multi-step process can be accomplished in a single process chamber, with each step optimized for time and temperature based on the resist thickness and solvent stripping chemistry used. In the Akrion process, a period of solvent exposure at low wafer spin speed is used initially to begin swelling and dissolving the thick photoresist layer. This is followed by a second solvent exposure step using aggressive frontside megasonic energy to promote polymer chain scission throughout the bulk of the photoresist layer. Following this step A DI water rinse and spin dry step are sufficient to completely remove the solvent, dissolved photoresist and any polymer residues. This paper reviews the development of the Akrion process and several examples of improvement in stripping time and results from work with customers.