# Improving the Lot Fabrication Stability and Performance of Silica Optical Films during PECVD
## 1 Introduction  
Following recent developments in planar lightwave circuits (PLC), silica optical waveguides have come to play an important role in optical fiber communications and optical fiber sensor systems. They offer a number of notable advantages, including low propagation loss, low fiber-to-chip loss, an adjustable refractive index, good integration capability and high mechanical stability [1; 2; 3]. There are various silica optical waveguide devices, including 1 \(\times\)\(N\) (\(N\) = 2, 4, 8, 16, 32, 64...... ) power splitters, arrayed waveguide gratings (AWG), variable optical attenuators (VOA), ring resonators and star couplers, all of which have been successfully fabricated using PLCs [4; 5]. A silica optical waveguide consists of a core layer, an upper cladding layer and a lower cladding layer. The core layer has a high refractive index (RI), whilst the cladding layers have a lower RI. Their low propagation loss is a particularly important property of silica optical waveguides. Typically, this is lower than 0.1 dB/cm. Similarly, they have a low fiber-to-chip coupling loss of less than 0.1 dB/point and a low RI contrast (\(\Delta\) = 0.25-1.5%) [6; 7; 8]. Usually, silica optical waveguides are fabricated using Si or a fused silica substrate. The fabrication process includes the lower cladding layer deposition, the core layer deposition, a masking deposition, photolithography, the core etching and the upper layer deposition. Silica optical films can be used for the core layer, lower cladding layer and upper cladding layer [7; 8]. By adding materials such as germanium (GeO\({}_{2}\)-SiO\({}_{2}\)), phosphorous (P\({}_{2}\)O\({}_{5}\)-SiO\({}_{2}\)), nitrogen (SiON-SiO\({}_{2}\)) and boron (B\({}_{2}\)O\({}_{3}\)-SiO\({}_{2}\)), or a combination of these materials, silica optical films can be used to precisely adjust the RI. Their thickness and residual stress can be controlled during the processing phase. They can be fabricated by means of thermal oxidation and chemical vapor deposition (CVD). The latter can include atmospheric pressure CVD (APCVD), low pressure CVD(LPCVD), plasma enhanced CVD (PECVD) and electron cyclotron resonance CVD (ECRCVD), or a combination of these methods [9; 10; 11; 12]. Flame hydrolysis deposition (FHD) and a sol gel method can also be used [12; 13]. Out of all of these methods, PECVD offers particular advantages. This is because of its ability to control the RI, thickness, residual stress and surface roughness [14; 15]. A lot of research has been devoted to studying the relationship between various process parameters (Radio frequency power, gas flow, chamber temperature, chamber pressure, etc.) and the quality of the film (RI, film thickness, etc.) [14; 15]. However, very few papers have examined the relationship and process stability between them in lot fabricating.  
In this paper, we focus on lot fabrication stability using PECVD for GeO\({}_{2}\)-SiO\({}_{2}\)-based silica optical films in relation to certain key parameters, such as the RI and film thickness. This can be used to determine how well the fabrication process will cope with the required tolerances, for the selection or modification of the fabrication process during optical waveguide design, for the specification of the processing requirements for PECVD machines and, above all, to reduce variability in the fabrication process.