In Situ Monitoring of Thin Film Growth by PECVD Using Time Resolved Ellipsometry and Plasma Diagnostics

Although thin film deposition by plasma processes has been investigated for many years and is currently used in many industrial areas, the plasma surface interaction mechanisms are still not fully understood due to the fact that each case of deposition is unique. More recently, nanocomposite thin films consisting of nanoparticles embedded in a solid thin film matrix, have attracted growing interest as multifunctional coatings. Their high tunability have made them great candidates for various applications where innovative simultaneous properties are needed.<br/>In this talk I will focus on experiments carried out in the case of thin oxide films deposition in a low pressure plasma enhanced chemical vapor deposition (PECVD) process based on an inductively coupled RF plasma source (ICP) [1]. This reactor is equipped with a UV-visible spectroscopic ellipsometer in order to monitor <i>in situ</i> the film growth whereas the plasma is investigated by optical emission spectroscopy (OES). I will mainly consider two studies of thin film growth: photocatalytic TiO₂ thin films deposited at low temperature and nanocomposite thin films made of TiO₂ nanoparticles embedded in a silica matrix.<br/>In the case of TiO₂ deposition at low temperature (&lt; 120°C), real time <i>in situ</i> spectroscopic ellipsometry (RTSE) was used to study the growth kinetics and to monitor the film structure as a function of the deposition time, e.g. the film thickness. In the deposition conditions considered (O₂/TTIP plasma, 3 mTorr, 400W) <i>ex situ</i> analyses by Scanning Electron Microscopy (SEM) and transmission electron spectroscopy (TEM) have shown that anatase was obtained. Nevertheless SEM and TEM analyses performed for different film thicknesses have shown that the coalescence of large polycrystalline columns emerging from an assembly of thin columns happened at a critical thickness, designed as coalescence thickness. It was shown that this latter can be determined from RTSE analysis: it corresponds to a slope change in the variation of the film roughness as a function of the film thickness (as measured by RTSE). The coalescence thickness was shown to depend on the deposition conditions and was measured to be about 150 nm in an oxygen rich O2/TTiP 98:2 ICP plasma at a rf power of 400 W. In addition, the formation of large columnar structure was shown to be associated with an important increase in the photocatalytic activity. [1].<br/>The approach retained for nanocomposite deposition was a hybrid deposition process, combining low-temperature plasma deposition and pulsed injection of colloidal solutions. More precisely, a monodisperse TiO₂ nano-colloidal solution was injected in the form of droplets in the low-pressure ICP plasma operated in O₂/HMDSO gas mixtures for the growth of a SiO₂ thin film matrix. The colloidal droplets were used to deliver the nanoparticles to the substrate while protecting them from the reactive plasma. Ideally, the liquid solvent evaporates during transport, leaving nothing but the nanoparticles on the surface of the sample, which will be quickly covered by the continuous deposition of the matrix.<br/>Plasma pressure, time-resolved optical emission spectroscopy and <i>in situ</i> RTSE were used to examine the kinetics driving nanocomposite thin film deposition. It was found that the sharp pressure increase following pulsed liquid injection lowers the electron temperature and density, which mitigates the matrix deposition rate as the nanoparticles are supplied to the film. This effect creates alternating matrix-rich and nanoparticles-rich deposition periods, which can be used as an additional knob for judicious control of the nanoparticle fraction in the film and hence its macroscopic properties [2].<br/><br/>References<br/>[1] W. Ravisy, M. Richard-Plouet, B. Dey, S. Bulou, P. Choquet, A. Granier, A. Goullet, <b>J. Phys. D: Appl. Phys</b> 54, 445303 (2021)<br/>[2] S Chouteau, M Mitronika, A Goullet, M Richard-Plouet, L Stafford and A Granier, J. Phys. D: Appl. Phys 55 505303 (2022)