4. Buckled Thin Film Cap Transfer Packaging Technology

Thin film packaging caps are generally fabricated through sacrificial etch via access holes after deposition of the material over MEMS devices as depicted in Chap. 1. Access hole sacrificial etching would be a barrier for the thin film encapsulation to realize high throughput packaging technology. As demonstrated in previous section, polymers caps in tensile stress exhibit downward deflection making the housing cavities volume reduced from the designed one. Moreover, the polymer materials used for the packaging have relatively high tensile stress resulting in high wafer bow which is a barrier to achieve high yield transfer packaging. As a solution for the existing issues, buckling of thin film is interesting as it makes naturally out-of-plane deflection due to applied compressive stress. The buckling of thin film is typically considered as a major cause of failure creating delamination or crack etc. However, such a suspended buckled film would be useful for MEMS packaging. Difficulty exists in making buckled thin film as packaging cover. As the developed transfer packaging uses hydrophobicity for the transfer of the packaging cap, film in compressive stress can’t be grown directly on the surface. Therefore, buckling of thin film on compliant substrate is adopted to use the buckled film as a packaging cap. It is known that thin elastic film attached to thick compliant substrate can lead to buckling instability as the compliant substrate is subjected to compressive stress.