A Monolithic LTCC-MEMS Microfabrication Process

A novel end-to-end monolithic process for the fabrication and integration of Micro Electro Mechanical Systems (MEMS) on functional Low Temperature Co-fired Ceramics (LTCC) substrates is presented. The first phase is a conventional LTCC process that produces a functional multi-layer LTCC substrate that embeds all necessary passives in the inner layers and provides for surface to MEMS and other surface devices through built-in vias. The second phase consists of an adapted MEMS process carried out on top of the fabricated LTCC functional substrate. The adapted MEMS process addresses two main challenges that come with the sintered LTCC substrate: surface roughness and anisotropic shrinkage. A polishing step is introduced to achieve nm-level surface roughness taking into account the monolithic nature of the process where metallic via bumps and the LTCC surface must be polished together. To address the anisotropic shrinking problem, the MEMS lithography masks are adjusted to follow the post-sintered LTCC vias' positions, which are captured by an optical image, thereby achieving perfect alignment. The proposed monolithic process is applicable to arbitrary MEMS devices. It is demonstrated through the design, fabrication and testing of a capacitive RF-MEMS switch. The fabricated device is miniaturized by burying DC and RF lines in the LTCC substrate. Its measured performance is on par with that of similar non-monolithically fabricated switches. [2023-0088]