Signature of growth-deposition technique on the properties of PECVD and thermal SiO2

Signature of growth-deposition technique on the properties of PECVD and thermal SiO2

Subimal Majee1*, Devesh Barshilia1, Sanjeev Kumar1, Prabhash Mishra2 and Jamil Akhtar1 1CSIR-Central Electronics Engineering Research Institute (CEERI), Pilani 333031, Rajasthan, India 2Nano-Science Center, Jamia Millia Islamia, New Delhi, 110025, India *E-mail: subimal.majee@polytechnique.edu

Abstract. In this article, we report the process induced variation in the characteristics of PECVD deposited and thermally grown silicon dioxide (SiO2) thin film. We find key differences in the porosity, arrangement of the nano-pores, surface roughness, refractive index and electrical resistivity of the SiO2 thin films obtained by the two methods. While the occurrence of the nanoporous structure is an inherent property of the material and independent of the process of film growth or deposition, the arrangements of these nano-pores in the oxide film is process dependent. The distinct arrangements of the nano-pores are signatures of the deposition/growth processes. Morphological analysis has been carried out to demonstrate the difference between oxides either grown by thermal oxidation or through PECVD deposition. The tunable conductive behavior of the metal filled nano-porous oxides is also demonstrated, which has potential to be used as conductive oxides in various applications.

Keywords: Silicon dioxide; PECVD; Thermal oxidation; Porosity

1. Introduction

Nanoporosity in thin membranes and thin films is either an inherent property of the materials or sometimes process dependent. Although, porosity is mostly undesirable in active devices, there are several areas where nano-porous membranes or thin films are absolutely necessary. As a matter of fact, each of the nano-pores inside a porous material has the potential to be an active device if properly tuned by some means. It is still an open field of research the communications between each of the nano-pores in nano-scale dimensions and their effect as a whole in the bulk material. In recent times, nano-porous thin films have attracted much research interests owing to their potential applications in fuel cells [1-6]; pressure and gas sensors [7-8]. The utilization of nanoporous membranes in such devices require geometrically controllable nanopores with controlled pore size and pore-density arrangements [6].

In the flexible electronics research, the most commonly used insulators are low temperature plasma enhanced chemical vapor deposited (PECVD) silicon dioxide (SiO2) and silicon nitrides (SiNx) [9]. There is a huge ongoing research on fabrication of low cost transparent conductive oxide (TCO) films [10], which has applications in flexible displays, organic solar cells, and organic light emitting diodes and so on [10-13]. Indium tin oxide (ITO) being one of the major TCOs has drawback, like, high processing cost. Therefore, alternative solutions are 2

being researched to find out novel conductive oxide films. Silicon dioxide (SiO2) thin film is one of the widely researched materials for several decades which have applications as the dielectric interlayer in the microelectronic industries [14]; moisture permeation barriers [15]; optical coatings [16] and so on. Although this material has a huge prospect in the above mentioned areas, the insulating nature of the oxide films does not allow their applications as TCOs. Expensive chemical doping is required to obtain electrical conductivity in the SiO2 thin films.

The aim of this present study is to characterize the nano-porosity of thermally grown and PECVD deposited SiO2 thin films. The distinct arrangements of the nano-porous matrices in those two processes show the signatures of growth/deposition methods. We also demonstrate a simple low cost method to fabricate conductive nano-porous SiO2 thin films without any intentional chemical doping. The conductive nanoporous oxide films have electrical resistivity 6 orders of magnitude lower compared to the insulating SiO2 films.

2. Experimental details 2.1. Oxide growth in oxidation furnace and RF PECVD deposition
   Thermally grown SiO2 thin films with thickness around 1 µm have been grown in oxidation furnace at CEERI-Pilani with temperature ~1100 C. Dry-wet-dry oxidation cycles have been used for total duration of ~3 hr on cleaned Si wafer substrates. For the RF PECVD deposition of SiO2 thin films (thickness ~1µm) on cleaned Si wafer substrates, the working pressure and substrate temperature have been fixed at 0.3 Torr and 300 C. We used an optimized recipe for oxide deposition where the flow rates for the source gases were: 5 sccm (SiH4); 350 sccm (N2O); 300 sccm (N2) and 170 sccm (Ar). The RF (13.56 MHz) power density has been fixed at 155 mW/cm2. The optimized deposition rate (rd) was 50 nm/min. 2.2. Filling of nano-pores with metal In order to obtain conducting oxide films, the

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