Fine tuning of negative photo-resist sidewall profiles for thick metal lift-off in applications of MEMS and advanced packaging

A lift-off process is used to pattern metals that are nonetchable. The success is dependent on the degree of undercutting of the bottom sacrificial layer and various methods were briefly discussed. It is shown that single-layer negative-tone 7 μm thick MaN-1440 photoresist from Micro Resist Technology produces an excellent re-entrant profile for metal lift-off. The objectives for this paper will be to obtain a thickness of 7 μm for MaN-1440 photoresist and to optimize and fine-tune the sidewall profile for effective lift-off. The results show that the undercut profile can be separately tuned by different photolithography parameters. It was found that the exposure energy determines the top CD while the bottom CD is determined solely by the developing time. The results showed exposure energy of 400 mJ and a developing time of 200 seconds gave an ideal re-entrant profile. CDs ranging from 5μm to 250μm have been successfully patterned. Metal layers ranging from 0.11 to 5.8μm thick were lifted-off with no issues.