Plasma deposition of Si-N and Si-O passivation layers on three-dimensional sensor devices

Abstract

Silicon oxide (SiO) and nitride (SiN) layers show excellent barrier properties and have been commonly used to protect integrated circuit devices (IC) from moisture and other contamination. In sensor fabrication particularly high demands are made on the passivation layers, since the active sensor area is exposed directly to the environment. Typical requirements on passivation layers are high electrical resistance, high density against moisture penetration, good adhesion and low mechanical stress. In earlier works planar sensors have been successfully passivated by plasma-enhanced chemical vapour-deposited (PECVD) SiO and SiN layers. However, many sensor devices are not available in planar techniques, but as three-dimensional (3D) devices. The object of this work was to develop a PECVD passivation technique for such 3D sensor devices. For the experimental work an electron cyclotron resonance (ECR) plasma reactor was used to deposit passivation layers on model substrates. Deposition rate and layer quality were measured at various substrate locations, orientations and temperatures. The layer quality was determined by ellipsometer data, IR spectra, scanning electron microscopy and moisture diffusion experiments. As result of these investigations the following tendencies could be established. The deposition rate increases in the z-direction (height) by 30% cm −1. At low deposition pressure (0.66–1.33 Pa) the deposition rate depends strongly on the substrate orientation, i.e. it decreases from top to side by ~50% and is even lower on the bottom side. Narrow structures with line widths of 1.3 mm and aspect ratios < 1 could be well passivated. However, narrow undercuts with aspect ratios ≫ 1 could not be passivated sufficiently. At higher deposition pressures (20.35 Pa), more homogeneous film deposition in gaps and a significantly better coating of bond wires could be achieved.