Experimental approach for laser direct imaging of negative tone polyimide resists in semiconductor industry

BackgroundIn semiconductor chip manufacturing, lithographically structurally polyimide resists are used as passivation layers. Mask Aligners are used for proximity lithography but have constructional and technical limitations such as the requirements of chip-specific chrome on glass (COG) masks and show undesired exposure effects as diffraction.AimWe aim to approach the evaluation of laser direct imaging (LDI) for polyimide material structuring. LDI uses a laser to expose photosensitive materials with a digitally guided beam. This research focuses on whether LDI can keep up or even surpass existing methods.ApproachWe take resist-to-substrate adhesion as the main parameter to be evaluated. Shear tests of 22-μm-thick polyimide coated on 300-mm silicon wafers were conducted.ResultsLDI samples show a slightly lower shear strength compared with Mask Aligners, independent of the substrate’s surface condition and exposure density. Based on experimental results, potential root causes that might impact the shear strengths are discussed, such as exposure wavelengths, surface condition et cetera.ConclusionsIt is assumed that the different temporal characteristics of the LDI tool to expose the resist is likely to be a key parameter to explain the observed shear strength results.