NDLI: Suppression of Stress Relaxation in MEMS Multilayer Film Microstructures by Use of ALD Nanocoatings

Content Provider	The American Society of Mechanical Engineers (ASME) Digital Collection
Author	Zhang, Yanhang Martin, L. Dunn Jeffrey, W. Elam Steven, M. George
Copyright Year	2002
Abstract	We study the suppression of stress relaxation in MEMS multilayer film microstructures by the use of alumina nanocoatings realized by atomic layer deposition (ALD). Gold (0.5 μm thick) / polysilicon (1.5 or 3.5 μm thick) beam and plate microstructures were fabricated by the MUMPs surface micromachining process. The microstructures were then coated on both sides with a 40 nm thick amorphous Al2O3 layer by ALD. The beam and plate microstructures were initially thermal cycled between room temperature and 190°C tostabilize the gold microstructure. After the initial thermal cycles, the microstructures were cooled from 190°C to 120°C and held at 120°C for about 2000 hours (three months). We measured, using an interferometric microscope with a custom-built temperature chamber, full-field deformed shapes (and from these determined the average curvatures in x- and y- directions) of the microstructures during the initial thermal cycles, during the cooling process from 190 °C to 120 °C, and during the isothermal hold. Measurements were made on both coated and uncoated microstructures to assess the influence of the coating. We find that while the 40 nm thick coating has a small effect on the thermoelastic response of the microstructure, it significantly reduces the extent of stress relaxation during the isothermal hold. We modeled the curvature evolution with time assuming the stress relaxation mechanism is power-law creep in the gold, ε˙ = Aσn, and that the polysilicon and alumina deform elastically. The simple model describes the observed behavior reasonably well for the uncoated microstructures (when the power-law parameters are fit using the measured curvature), however, for the coated microstructures, the model predicts a decrease in the stress relaxation, but nowhere near the magnitude observed. This suggests that not only is the stress state in the gold film altered by the nanoscale coating, but also the fundamental deformation mechanisms are altered.
Sponsorship	Microelectromechanical Systems
Starting Page	179
Ending Page	187
Page Count	9
File Format	PDF
ISBN	0791836428
DOI	10.1115/IMECE2002-39298
Volume Number	Microelectromechanical Systems
Conference Proceedings	ASME 2002 International Mechanical Engineering Congress and Exposition
Language	English
Publisher Date	2002-11-17
Publisher Place	New Orleans, Louisiana, USA
Access Restriction	Subscribed
Subject Keyword	Microelectromechanical systems Relaxation (physics) Cycles Temperature Cooling Deformation Creep Micromachining Nanoscale phenomena Coatings Stress Thermoelasticity Polysilicon Coating processes Shapes Microscopes
Content Type	Text
Resource Type	Article

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