New low-stress PECVD poly-SiGe layers for MEMS

Abstract

Thick poly-SiGe layers, deposited by plasma-enhanced chemical vapor deposition (PECVD), are very promising structural layers for use in microaccelerometers, microgyroscopes or for thin-film encapsulation, especially for applications where the thermal budget is limited. In this work it is shown for the first time that these layers are an attractive alternative to low-pressure CVD (LPCVD) poly-Si or poly-SiGe because of their high growth rate (100-200 nm/min) and low deposition temperature (520 C-590 C). The combination of both of these features is impossible to achieve with either LPCVD SiGe (2-30 nm/min growth rate) or LPCVD poly-Si (annealing temperature higher than 900 C to achieve structural layer having low tensile stress). Additional advantages are that no nucleation layer is needed (deposition directly on SiO 2 is possible) and that the as-deposited layers are polycrystalline. No stress or dopant activation anneal of the structural layer is needed since in situ phosphorus doping gives an as-deposited tensile stress down to 20 MPa, and a resistivity of 10 m-cm to 30 m-cm. With in situ boron doping, resistivities down to 0.6 m-cm are possible. The use of these films as an encapsulation layer above an accelerometer is shown.[958]

A 10 μm thick poly-SiGe gyroscope processed above 0.35 μm CMOS

“…resonators, accelerometers,…) can be processed together on top of standard CMOS. Moreover, similar processing can be used to fabricate thin film caps above MEMS devices forming an area-saving integrated 0-level package [3]. In conclusion, this process ultimately enables the creation of highly integrated, strongly miniaturized systems with improved performance.…”

PECVD growth of Six:Ge1−x films for high speed devices and MEMS

“…Adding germanium reduces the deposition temperatures as low as 450°C enabling applications of the material in optical bolometers and post-processing of MEMS structures on CMOS devices. For bolometry, the thermal budget of the material should be small i.e., the material being deposited should be exposed for a shorter time to the deposition temperature [3].…”

MEMS Packaging Materials

“…It is also a biocompatible material and is often used in bio-MEMS. Other examples include the use of epitaxially deposited polysilicon as an encapsulation structure for piezoresistive accelerometers [13]; plasma enhanced chemical vapor deposition of polysilicon [14]; and the use of permeable polysilicon to fabricate a vacuum shell over movable elements of a MEMS resonator [15]; and the use of a silicon cap bonded with glass frit over an accelerometer as shown in the case studies in Section 12.11.1. With such methods, use of an outer encapsulant should be considered to strengthen the initial encapsulation structure to withstand the subsequent transfer molding process, and care should be taken to assure that using any additional glob top does not further induce stress, especially during the hardening and curing steps.…”