Low-temperature photosensitive polyimide processing for use in 3D integration technologies

3D Integration technologies have to deal with a complex combination of materials and substrates. Downstream processing in this technology field is limited to a temperature range of <250°C, mainly caused by temporary bonding glues or the CTE-mismatch (Coefficient of Thermal Expansion) between the used materials. Therefore it is desirable to reduce process temperatures of polymer passivation processes to a minimum. One option is to use new curing concepts like microwave curing. Another option is to employ newer materials with lower cure temperatures. Polyimides (PI) are one of the most important classes of high performance polymers in this area of packaging technology, but require curing temperatures of 300°C and higher. Recently, low-temperature cure polyimide formulations are available, which have the possibility to reduce the cure temperature to less than 250°C. Less is known about the impact of lithographic process conditions on the imidization kinetics. A photosensitive low temperature cure ester-type polyimide was studied in this work. Firstly, imidization was monitored in the temperature range of 160-250 °C using in-situ transmission FT-IR spectroscopy using spin coated films on silicon substrates. The imidization reaction shows a chemically-controlled and a diffusion controlled region with significant different reaction rates. To achieve high imide contents at low process times a minimum remaining proportion of chemically-controlled reaction rate is necessary, which requires a certain temperature. As photosensitive polyimide films require UV exposure and development processes, the imidization kinetics of the exposed PI pre-cursor film was investigated and the experimental data was described mathematically. It was found that with increased exposure dose the reaction rates in both regions decrease significantly. This is caused by polymer network formation during the UV exposure process, which reduces the molecular mobility within the PI pre-cursor and limits the imidization rate. The FT-IR results show that >99% degree of imidization can be achieved with a curing process of 3h at 230 - 250°C for unexposed films. If the crosslinking degree, caused by high exposure doses, is too high a further increase in cure temperature above 250°C is necessary to yield >99% polyimide. At lower exposure doses the overall decrease of reaction rate may already be compensated by an increase in reaction time. Secondly, thermo gravimetric analysis (TGA) and TGA/FT-IR spectroscopy was used to investigate the imidization process up to 350°C. It was found that most of the imidization reaction occurs in the temperature range of <250°C. At temperatures >300°C further weight losses are detected. In this temperature range volatile components are eliminated from the PI film, which has impact on film stress, dielectric constant and film shrinkage. This effect has to be taken into account when doing further downstream processing of low temperature cured polyimide films.