Characterization of a high-photospeed positive thick photoresist for lead-free solder electroplating

Abstract

The introduction of lead free solder (SnAg) electroplating for advanced packaging applications has created new challenges for thick photoresist processes. Dry film laminate photoresist materials do not provide sufficient resolution or process control for the smaller bump diameters used with advanced design rules. The traditional positive tone DNQ-Novolak photoresists require large exposure doses and long processing times that are unacceptable for the total cost of ownership (COO) of the lithography cell in a manufacturing environment. Standard chemically amplified positive photoresists have interactions between the PAG and Cu or Cu 2 O at the soft bake step which makes it impossible to obtain sufficient pattern fidelity for electroplating lead free solder on copper seed layer substrates. This study will characterize a novel positive tone, chemically amplified photoresist (TOK PC-0059B PM) that can be double coated up to 100 μm thick for lead free solder electroplating. The PAG in the photoresist has been formulated to prevent any interaction with Cu or Cu 2 O. This photoresist exhibits extremely high photospeed, excellent pattern resolution and ease of stripability after electroplating. The lithographic performance of the thick positive photoresist will be optimized on 300mm wafers using a broad band, low numerical stepper. Enhanced process latitude and very high photospeeds will be demonstrated for thicknesses up to 100 μm. Cross sectional SEM analysis, process linearity, and electroplating performance are used to establish the lithographic capabilities.