A wafer-level poly-sige-based thin film packaging technology demonstrated on a soi-based high-Q MEM resonator

Abstract

This paper reports a O-level or wafer-level thin film packaging technology for MEMS using polycrystalline silicon-germanium (poly-SiGe) as the base material complemented with a metal seal. Hermetic packages with a cavity pressure below 0.3 mbar are demonstrated on a SOl-based torsional-mode Si resonator. Monitoring the quality factor of these resonators revealed that the low pressure is retained for over 6 months while storing at ambient pressure and room temperature. Moreover, the package survived several months under harsh testing conditions with temperatures up to 125°C and 85% relative humidity. This thin film SiGe-based technology has large potential for the on-wafer packaging of a broad range of MEMS devices.

MEMS packaging and reliability: An undividable couple

“…9. The Q is above 200,000 and remains stable for over 4300 h, which points towards a hermetic package and a vacuum seal (<30 Pa) [8]. Fig.…”

“…2) [8,9] or of (nano-sized) pores [10,11] in the membrane layer. The sacrificial layer etchant, e.g., vapour HF in case silicon oxide is used as the sacrificial layer, enters through these channels.…”

“…The sacrificial layer etchant, e.g., vapour HF in case silicon oxide is used as the sacrificial layer, enters through these channels. After removal of the sacrificial layer the etch channels are sealed using for instance a (conformal) coating of (LP) CVD nitride [7,10], a deposited metal [8], or an epitaxial poly-Si layer [9]. The main advantages of thin film capping are twofold: (1) it is a batch process leveraging on the front-end MEMS process and because of that it can contribute to a process cost reduction, and, (2) it leads to the smallest form factor for a packaged device; compared to chip capping (described below), the total area of the packaged device can considerably be reduced (e.g., by a factor of 2).…”

“…Shrinkage can be as high as a few% (which is large), but, low-shrinkage epoxies with total shrinkage levels of a few tenths of % do exist [23]. An example of a successful EMC encapsulation is presented in [8], in which a poly-SiGe based thin film cap (with a MEM resonator inside) was overmolded at a pressure of 12 MPa and a temperature of 180°C. After molding, the cap was not significantly deformed and the resonator was still fully functional (and within spec).…”

“…While this technique with a minimum detectable He leak rate of 5 Â 10 À12 mbar l/s (the sensitivity of the He mass spectrometer) is appropriate for testing of relatively large packages (order of 1 cm 3 ) it is unacceptable for nearly every MEMS application where the volumes are at least three orders of magnitude smaller [25][26][27]. More accurate evaluation of the hermeticity (leak rate) and vacuum level can be performed using MEM resonators (by monitoring the Q-factor in time) [7,8,30], Pirani gauges (used as pressure sensors) [16,30] and ''cap deflection'' combined with white-light interferometer (WLI) for measuring the out-of-plane deflection with nm-accuracy [15,30]. An example of a Q-factor measured over time for a thin film encapsulated resonator is shown in Fig.…”