At Fraunhofer IPMS Dresden micromechanical mirror arrays are developed and fabricated using a high-voltage CMOS
process for applications such as lithographic mask writers and adaptive optics. Different approaches for the fabrication of
micromechanical mirror arrays with up to 1 million analogue addressable pixels in a MEMS-on-CMOS technology are
discussed: sacrificial layer technologies of 1-level actuators made from a single Al-TiAl-Al structural multilayer or 2-level actuators with an additional TiAl hinge layer respectively. Also the fabrication of single crystalline Si micro-mirrors
using layer-transfer bonding is discussed.
Large scale arrays of more than 67k micromirrors of monocrystalline silicon with underlying planar actuation electrodes have been fabricated. The mirrors were fabricated by transferring a 300nm thick silicon layer from a silicon-on-insulator (SOI) wafer to a wafer containing metal electrodes by adhesive wafer bonding in a thermo-compression bonding tool. The bonding was followed by grinding and spin-etching of the handle silicon and the buried oxide, which leaves only the thin device silicon on the electrode wafer. Mirrors and metal plugs were formed using standard micromachining techniques such as sputtering and dry etching. The arrays consist of 16μm×16μm mirrors with 0.7μm wide and 2μm long torsional hinges. Deflection is achieved by applying a voltage between the mirrors and one of two underlying electrodes. It was found that 15V is enough to deflect the mirrors 48nm, which is sufficient to create a black pixel in a diffractive deep UV application that involves modulation of 193nm light. Furthermore, no measurable instability due to plastic hinge deformation or charging could be determined by static deflection for more than one hour. The developed fabrication process is fully CMOS ompatible and can be directly applied to fabricate spatial light modulators (SLM) with mirror arrays in excess of one megapixels with individually addressable analog mirrors that are truly drift free. Application areas are photolithographic mask writers or systems for maskless lithography.
Spatial light modulators (SLMs) based on micromirrors for use in DUV lithography and adaptive optics require very high mirror planarity as well as mirror stability. The ideal mechanical properties of monocrystalline silicon make this material ideally suited for use in high precision optical MEMS devices. However, the integration of MEMS with CMOS poses certain restrictions on processing temperatures as well as on the compatibility of materials. The key to the successful fabrication of monocrystalline silicon micromirrors on CMOS is the silicon layer transfer process. Here, we discuss two carefully adapted wafer bonding processes that are CMOS compatible and that allow the transfer of a 300nm thick monocrystalline silicon thin film from a SOI donor wafer. One process is based on adhesive bonding using a patterned polymer layer, while the other process is based on direct bonding to a planarization layer of polished glass.