ALD Enabled Wafer Level Polymer Packaging for MEMS

Wafer level polymer packaging for MEMS is a cost-effective approach that is also compatible with microelectronic packaging technologies. However, polymer packages are not hermetic and cannot be used for MEMS devices, which usually demand vacuum or low moisture environment inside the packages. This problem can be solved by applying atomic layer deposition (ALD) of nano-scaled Al 2O3 or other inorganic materials over the polymer packages. Defects and mechanical cracks in ALD coatings are major concerns for hermetic/vacuum sealing. Several techniques have been developed to inspect such defects and cracks. Assisted by the electroplating copper technique, we have reduced the defect density by 1000 times for an ultra-thin, 2-nm ALD Al2O 3 film. Such an ultra-thin coating is essential to enhance coating's mechanical toughness. The toughness is usually determined by monitoring coating's crack initiation and growth in a bending test. A real-time, non-destructive inspection technique has been developed for in-situ characterization of an ALD film coated on a surface or buried in a multilayer structure. With the knowledge and technology established, we have successfully demonstrated a wafer-level polymer packaging process for MEMS using a Pirani gauge as the vacuum sensor. The leak rate through the polymer package has been reduced by 100 times by the ALD Al2O3 coating. More importantly, we have developed models and identified issues that are critical to ALD-enabled wafer level polymer packaging for MEMS.