Effects of Electron Radiation Generated during E-beam Evaporation on a
Photoresist Liftoff Process
Keywords: … Secondary electron, E-beam evaporation, photoresist, residue
In compound semiconductor manufacturing, metallized wafers are put through a wet strip process in Nmethyl pyrrolidone (NMP) to dissolve the photoresist, liftoff the unwanted metals, and to form the electrical circuit. There is a wide selection of photoresist suitable for liftoff process. However, most of the available resist can be cross linked if exposed to excessive heat or when bombarded by electrons with sufficient energies. Cross linked resist will not dissolve completely in the normal wet strip chemicals and a residue will result. Although the resist residue can usually be completely removed with more aggressive wet and dry strip processes, the additional rework steps negatively impact the production flow and ship schedule. One theory that explains the residue issue after metal deposition is energetic electron radiation cross linking the photoresist during the E-beam evaporation process. We have conducted a series of experiments to understand the source of the electrons and to quantify the level of secondary electron emission during the deposition process. Results of our work confirmed that backscattered and secondary electron emission is dependent on the evaporation material type. Further, there is strong evidence that suggests impurities in the source material directly influence electron emission. Based on the results of the investigation work, we have taken measures to prevent the problem from reoccurring.