High surface planarity die bonding of large optical chips
Analog Spatial Light Modulators (ASLM) are highly specialised MOEMS devices, which, in our case, are used as programmable mask in extremely high-resolution DUV mask writing systems. Resolution, precision, contrast and optical uniformity of the mask patterns produced by this new type of mask writers depend on the local and global planarity as well as on the optical contrast of the ASLM devices. Definition, measurement and calculation of the global flatness (GF) of the active area of one-mega pixel ASLM devices, developed and fabricated at the Fraunhofer IPMS in Dresden, is described and applied for the investigations of parameters influencing the chip planarity. Parameters of an adhesive chip bonding technique have been varied and optimised with respect to improved GF. In particular, the influence of the die bonding layers printed on the die attach was investigated. Additionally, an approach based on ultraprecision micro-milling of a sacrificial layer deposited on the die attaches to improve its planarity and thus the GF of the die-bonded chip has been successfully performed.