Detailed Investgations of Inner Cavity Pressure of MEMS Devices Sealed by Wafer Bonding

Abstract

Hermetic sealing is important regarding functionality and reliability for MEMS components. Typically this sealing is done at the wafer level using wafer bonding, which simultaneously also provides mechanical protective caps. An existing test structure for inner cavity pressure measurement, utilizing the pressure dependence of heat transfer in gasses, is used for detailed investigations of factors, which influence the inner cavity pressure in the wafer pre-processing and wafer bonding step itself. These investigations ultimately allowed optimized wafer processing, improved process control and reliability investigation.

Wafer-Level Vacuum-Packaged Translatory MEMS Actuator with Large Stroke for NIR-FT Spectrometers

“…Compared to alternative WLVPs, the glass-frit bonding process offers long-term hermeticity along with good sealing properties and tolerates the topography of the joining partners by a planarizing effect of the softened glass-frit during the bonding process. In parallel, conductive tracks or metallic lead-throughs for device signaling can be covered within the softened glass, preventing the need for processing complex and obviously more expensive vertical device signaling [ 48 , 49 ], with new questions arising related to hermeticity and device design and compatibility. Additionally, glass-frit bonding offers process flexibility due to a wide variety of suitable substrates including CMOS compatibility [ 50 ].…”

“…For this work, glass-frit bonding [ 47 , 48 , 49 , 50 , 51 , 52 ] is the most reliable bonding method, allowing hermetic sealing over surface topologies several µm high. This bonding approach allows realizing hermetic electrical interconnects using existing metal lines without complex technological changes for the DW.…”

“…Hence, a thin film getter [58][59][60] is needed for this WLVP, using a highly efficient zirconium (Zr)-based thin-film getter from SAES Getters (SAES Getters S.p.A., Lainate (MI), Italy). For this work, glass-frit bonding [47][48][49][50][51][52] is the most reliable bonding method, allowing hermetic sealing over surface topologies several µ m high. This bonding approach allows realizing hermetic electrical interconnects using existing metal lines without complex technological changes for the DW.…”

“…Hence, a hermetic bond technique is required which can tolerate high surface topologies of ≥2 µm. For this NIR-WLVP application, glass-frit bonding (which requires elevated process temperatures of 430–440 °C) was selected to be the best sealing method [ 46 , 47 , 48 , 49 , 50 , 51 , 52 ] in order to remain compatible with the fixed MEMS scanner process (see Table 2 for a comparison of alternative hermetic bonding techniques). Earlier work on glass-frit-bonded WLVP of AME75 processed MEMS mirrors resulted in 500 Pa internal pressure [ 52 ] (without getter) (in this work, <1 Pa was required).…”