Measurement of Stress-strain Curves of PECVD Silicon Oxide Thin Films by Means of Nanoindentation

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Measurement of Stress-strain Curves of PECVD Silicon Oxide Thin Films by Means of Nanoindentation Zhiqiang Cao and Xin Zhang Dept. of Manufacturing Engineering, Boston University, 15 Saint Mary's Street, Brookline, MA, 02446 ABSTRACT In this paper, we explore the use of nanoindentation techniques as a method of measuring equivalent stress-strain curves of the PECVD SiOx thin films. Three indenter tips with different geometries were adopted in our experiments, enabling us to probe different regimes of plastic deformation in the PECVD SiOx thin films. A shear transformation zone (STZ) based amorphous plasticity theory is applied to depict the underlying plastic deformation mechanism. INTRODUCTION Plasma-enhanced chemical vapor deposited (PECVD) silicon oxide (SiOx) thin films have been widely used in MEMS/NEMS to form both electrical and mechanical components [1]. Specifically, in Power MEMS (micro energy-harvesting devices such as micro heat engines and related components) [2], PECVD SiOx serves as the insulation layers, and endures a high level of stress even at low temperatures [3]. Severe plastic deformations in the PECVD SiOx thin films often occur which cause device degradation or even prohibit the device process integration [3]. So far, however, the plastic properties of the PECVD SiOx, especially at lower temperatures, are not well understood. In this paper, we try to explore the use of nanoindentation [4-8] as a method of measuring equivalent stress-strain curves of the thin film materials for MEMS applications. The asdeposited PECVD SiOx thin films and a reference standard fused quartz were both tested. The theoretical analysis of nanoindentation strain is based on the previous work of Tabor, Johnson, Hill, Bower, and others [9-13], taking into account of indenter tip shape changes during the indentation process. The indentation stress is taken as the instant hardness, or mean pressure underneath the indenter. Different regimes of plastic deformation were probed by the 1) conical tip with 1 µm radius; 2) Berkovich tip with ~150 nm radius; and 3) Cube corner tip with ~50 nm radius. Based on the knowledge of these nanoindentation load-displacement and stress-strain curves, the plastic deformation mechanism of the PECVD SiOx is depicted by the shear transformation zone (STZ) based amorphous plasticity theory [14-17]. The physical origin of the STZ is elucidated and linked with the plastic deformation dynamics. EXPERIMENTAL DETAILS A 2µm-thick, silane-based PECVD SiOx film was deposited using the same conditions specified in a previous publication [3]. The wafer was subsequently diced into approximately 10 mm × 10 mm squares. The nanoindentation tests were conducted at room temperature on a TriboIndenterTM system (from Hysitron Inc., Minneapolis, MN). Constant rate of loading (CRL) tests were performed on both the PECVD SiOx thin film samples and the standard fused quartz sample in load-control mode, using constant loading rates and the same rate during unloading.

The instant hardness H, defined