Thickness-dependent Structural Relaxation of Plasma-Enhanced Chemical Vapor Deposited Silicon Oxide Films during Thermal
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Thickness-dependent Structural Relaxation of Plasma-Enhanced Chemical Vapor Deposited Silicon Oxide Films during Thermal Processing Zhiqiang Cao and Xin Zhang Department of Manufacturing Engineering, Boston University, Boston, Massachusetts 02215, USA ABSTRACT This paper presents a microstructure-based mechanism which elucidates seams as a source of density change and voids as a source of plastic deformation, accompanied by a viscous flow. This theory was then applied to explain a series of experimental results that are related to thermal cycling of amorphous dielectric films, such as plasma-enhanced physical vapor deposited (PECVD) silicon oxide (SiOx) films, including stress hysteresis generation and reduction and coefficient of thermalexpansion changes. In particular, the thickness effect was examined; PECVD SiOx films with a thickness varying from 1 to 40 µm were studied, as certain demanding applications in Microelectromechanical Systems (MEMS) require such thick films serving as heat/electrical insulation layers.
INTRODUCTION Plasma enhanced chemical vapor deposition (PECVD) is a special chemical vapor deposition (CVD) process that is facilitated by RF plasma. It enables dielectric films such as oxides, nitrides, and oxynitrides to be deposited at relatively low temperatures (300~400ºC) and with a fast deposition rate, nominally ~1 µm/min [1]. PECVD silicon oxide (SiOx) films have been widely used not only as dielectric layers but also as mechanical components in MEMS [1]. A good example is its application in the so-called Power MEMS [2] (e.g., micro-scale heat engines, coolers, fuel cell chips, and related components), which typically uses PECVD SiOx as electrical and thermal-mechanical insulation layers. To push electrical and mechanical powers to high performance levels, thick (typically in the range from 10 to 40 µm) PECVD SiOx films are being used. However, the ability to utilize such SiOx films with a thickness greater than a few microns is significantly limited by the residual stress generated during deposition and post-deposition thermal processing. It is, therefore, important to understand the thermalmechanical properties of PECVD SiOx films, especially thick films, which are less studied and more problematic. Previous studies have shown that many amorphous materials, including PECVD SiOx, exhibit irreversible property changes when heated to temperatures below what is required for recrystallization [3-6]. This paper shows a complete set of experimental results of stress hysteresis, Coefficient of Thermal Expansion (CTE), as well as density and viscosity changes, all as a function of film thickness. Films with a thickness of 1 to 40µm are studied. In addition, a new theory is developed to relate these stress and material property changes to the microstructure changes during thermal cycling of the films. These results and theory provide insight into the analysis of similar responses of other CVD materials (such as SiNx), physical vapor deposited (PVD) materials (such as AlOx), and low-k silsesq
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