High vacuum chemical vapor deposition (HV-CVD) of alumina thin films

We analyzed along this work the feasibility to produce high quality alumina thin films by High Vacuum Chemical Vapor Deposition (HV-CVD). We study the influence of various parameters on the growth process and on the film quality, such as substrate temperature, gas flow or pressure. We aim to highlight the advantages and the limitations of the HV-CVD technique. A high vacuum CVD reactor has been designed, built and optimized. A novel effusing source has been designed to guarantee a molecules distribution uniformity of 95% on the substrate surface. The main technical challenges faced were to reach the maximum substrate temperature in vacuum, to control the precursor flux and to control the temperature of the gas lines to avoid condensation. For the first time, films of Al2O3 are obtained by HV-CVD from aluminum isopropoxide as precursor in a novel HV-CVD reactor. The HV-CVD approach stands out of all the other CVD methods as it allows excellent prediction of growth rates and film thickness homogeneities if the decomposition probability of the precursor is once determined. In this work, the latter was determined to increase with increasing substrate temperature to 0.2, further increase of temperature reduces the decomposition probability due to increased desorption rate. Pure alumina films are obtained in a reliable and reproducible way. The presence of oxidizing agents, as decomposition partners, reduces the activation energy of deposition process from 33.1 ± 8.2 kJ/mol to 11.4 ± 5.3 kJ/mol in presence of sufficient oxidizing agent. The Al2O3 films are growing at deposition rate from 1 to 50 nm/min. It has been found that, the lower the deposition temperature, the higher the density of the deposited films therefore the higher refractive index. The index of refraction varies from 1.65 ± 0.01 to 1.35 ± 0.01 at 632 nm wavelength from lowest to highest deposition temperature. At low deposition temperature, Al2O3 can contain residual OH-groups and Al=O entities. This content is independent of the total flow rates of the precursor in the range of 5·1015 molecules/cm2·s to 3·1016 molecules/cm2·s impinging on the substrate. Substrate effects have not been observed in the limited range of tests on natural oxidized silicon, 3 µm SiO2 on silicon, quartz, stainless steel, fused silica and 100 nm silicon nitride on top of silicon. The chemical composition of the deposited alumina films measured by EDX and XPS is nearly stoichiometric with 35 ± 5 at% Al and 65 ± 5 at% O, without carbon contamination. Remaining hydrogen in the films has not been studied in detail, but the difference of OH- absorption peaks by FTIR indicates that, a low temperature deposition, hydrogen incorporation is possible. Concerning the optical properties of the HV-CVD deposited films, absorption is very low from 250 nm to 1800 nm wavelength, but due to the porosity and granular structure of the films, light scattering can take place. But propagation loss smaller than 2 dB/cm were measured in channel waveguides fabricated from the HV-CVD alumina films. Planar and channel waveguides demonstrate good guiding properties at 670 nm and 1.55 µm, as well. This work also opens new possibilities to deposit in situ local structures of or in transparent alumina on large surface. Light induced high vacuum chemical vapor deposition of alumina microstructures by 248 nm excimer laser is presented and shows an activation energy of 5.2 ± 0.4 kJ/mol, much smaller than for thermal deposition. The influence of the fluence and the repetition rate is discussed. Electron beam assisted HV-CVD of alumina is demonstrated and proves the feasibility of in situ structuration under HV-CVD conditions.

On the Powder Formation in the Plasma Enhanced Chemical Vapor Deposition Process for the Deposition of SiOx Barrier Coatings

Marina Ricci

Nowadays thin films play an important role in everyday life and in industries. Thin film technology has been developed primarily for the growing demand for development of smaller and smaller devices that require advanced materials and new processing techniques suitable for micro and nano technology. One of the methods of producing thin films is by means of plasma technology. This is a technique by which plasma, also known as 'the fourth state of matter', is used to generate ions, electrons and radicals which deposit at a surface where they will form a thin film. The source of these depositing species could be either gaseous, liquid or solid in origin. Most of the time a plasma is generated by means of an electrical discharge. Plasma technologies are used for various applications. One of these is Plasma Enhanced Chemical Vapor Deposition (PECVD). PECVD systems have widespread applications in the electronic sector, because of their exibility in depositing different films such as silicon oxide thin films (SiOx). These films, currently deposited by organosilicon compounds, such as hexamethyldisiloxane (HMDSO), have dielectric and good barrier properties. The aim of the work at CRPP consists in optimizing PECVD processes for the deposition of these films on polymers. In this study, the species in gaseous phase and the powder produced in oxygen-diluted HMDSO plasmas were experimentally characterized in a radiofrequency (RF) capacitively-coupled reactor at 13.56 MHz. The gaseous phase of these plasmas and the particle formation were studied by in-situ infrared absorption spectroscopy and optical emission spectroscopy. The study of the powder formation is important because the particle deposition imposes an upper limit on the deposition rate. Particle formation was firstly studied as a function of HMDSO/O2 ratio. Analysis of the time evolution of the infrared absorption spectra gives the development of the size, the number density, the structure and the particle composition. At high oxygen dilution the formation of large particles is observed. The analysis of HMDSO/O2 plasma at low O2 content suggests that at the beginning of the process very small particles are formed by polymerization; the same particles do not grow much and keep sizes around 50 nm. The admixture of inert or non-reactant gases, such as Ar or N2, into the plasma promotes polymerization of the monomer; a high value of one of these inert or non-reactant gases leads to small powder particle size, since only polymerization takes place in this case. Particle size around 50 nm is also found in this case. At the same time we see a decrease of the carbon content and a weak reduction of the stoichiometry in SiOx indicated by the shift of the SiOSi asymmetric stretching vibration. This reduction also influences the deposited layers, less oxidized and, thus, quite far from a stoichiometric character. Hexamethyldisilazane (HMDSN) and tetramethylsilane (TMS) are other reagents that have been identified as alternatives to HMDSO. The in-situ infrared absorption spectra show that HMDSO/O2 and HMDSN/O2 plasmas are quite similar, but in the second case a lower SiOSi stretching peak intensity, due to NH and SiN peaks, is seen. TMS/O2 and HMDSN/O2 plasma spectra show a low SiOSi bending and stretching peak intensity, due also to the oxygen atom absence in their chemical structure. Two separate chapters are respectively devoted to the silicon nitride thin films (SiNx) and plasma crystal formation. SiNx layers are deposited by three different organosilicon compounds, such as HMDSN, TMS and bis(dimethylamino)dimethylsilane (BDMADMS). No powder formation is visualized in this contest. The ex-situ infrared absorption spectra show the difficulty to produce stable SiNx films; they present a low SiN peak intensity and are quite hygroscopic. Furthermore, in HMDSO/O2 plasmas at high oxygen dilution and high pressure, later in time into the discharge, accretion leads to larger and larger particles which can finish up in particles forming a plasma crystal. The study of RF harmonics shows the presence of instabilities, clear indications of the passage from small particulate to plasma crystal structure.

EPFL2011

Selective Growth of Titanium Dioxide by Low-Temperature Chemical Vapor Deposition

Patrik Hoffmann, Yury Kuzminykh, Michael Oliver Reinke

A key factor in engineering integrated optical devices such as electro-optic switches or waveguides is the patterning of thin films into specific geometries. In particular for functional oxides, etching processes are usually developed to a much lower extent than for silicon or silicon dioxide; therefore, selective area deposition techniques are of high interest for these materials. We report the selective area deposition of titanium dioxide using titanium isopropoxide and water in a high-vacuum chemical vapor deposition (HV-CVD) process at a substrate temperature of 225 °C. Here—contrary to conventional thermal CVD processes—only hydrolysis of the precursor on the surface drives the film growth as the thermal energy is not sufficient to thermally decompose the precursor. Local modification of the substrate surface energy by perfluoroalkylsilanization leads to a reduced surface residence time of the precursors and, consequently, to lower reaction rate and a prolonged incubation period before nucleation occurs, hence, enabling selective area growth. We discuss the dependence of the incubation time and the selectivity of the deposition process on the presence of the perfluoroalkylsilanization layer and on the precursor impinging rates—with selectivity, we refer to the difference of desired material deposition, before nucleation occurs in the undesired regions. The highest measured selectivity reached (99 ± 5) nm, a factor of 3 superior than previously reported in an atomic layer deposition process using the same chemistry. Furthermore, resolution of the obtained patterns will be discussed and illustrated.

Amer Chemical Soc2015

Optical Layers for Thin-film Silicon Solar Cells

Peter Cuony

In this work we develop and analyze optical layers for use in Micromorph solar cells, a tandem configuration with an amorphous silicon top cell and a microcrystalline silicon bottom cell. The morphology of the front electrode has a decisive role in maximizing the efficiency of a solar cell. To reach a better understanding of the requirements for the front electrode surface, we present a wide range of morphologies that can be obtained with as-grown rough zinc oxide (ZnO) and post-deposition argon plasma surface treatments. We correlate the morphological parameters to light scattering in transmission and reflection, and identify the inclination angles of ZnO pyramids as the most pertinent parameter for thin-film silicon solar cells. We show that there are no reflection losses at the interface between as-grown rough ZnO and silicon, and we quantify the reflection losses for smoother interfaces. With titanium dioxide, produced by reactive magnetron sputtering, we demonstrate that for flat interfaces reflection losses of up to 7.4% can be canceled at 550 nm wavelength. We show that in microcrystalline silicon cells p-type silicon sub-oxide (SiOx) can also be used to reduce reflection losses, and at the same time can act as field-creating window layer. We develop a wide range of mixed-phase SiOx layers and with energy-filtered transmission electron microscopy we reveal the filamentous nanostructure of films produced with high hydrogen dilution of the precursor gas mixture. A partial decoupling of electrical and optical parameters is achieved for such films, where the transparency and the refractive index are determined mostly by the silicon oxide matrix, while sufficient transverse conductivity for the use in solar cells is maintained by few-nanometer-wide silicon filaments. In thin-film silicon solar cells, p-type SiOx~0.5 shows the best results as a window layer with enhanced transparency, while strongly phosphorous-doped n-type SiOx~1 is best suited for use as an intermediate reflecting layer in tandem cells due to a low refractive index of 1.8 with sufficient transverse conductivity. The tunable resistance of doped SiOx layers can be used at various locations in a Micromorph cell to quench shunts or bad diodes in spatially non-uniform devices. Implementation of SiOx with these three functionalities in Micromorph cells has significantly contributed to increasing the best initial efficiency from 11.8% in 2006 to 13.5% in 2010. While rough ZnO front electrodes and SiOx-based intermediate reflecting layers both help to improve Micromorph cell efficiencies, they render the devices more sensitive to water-vapor-induced degradation. With lock-in thermography measurements we demonstrate that water contamination affects mainly the cell borders resulting in non-uniform distribution of the local open circuit voltage. Finally we show how free-carrier absorption and surface plasmon polariton absorption can reduce the reflectivity of the back reflector, and we present preliminary results aimed toward the realization of an absorption-free back reflector making use of polished ZnO and sputtered silver layers.

EPFL2011

On the Powder Formation in the Plasma Enhanced Chemical Vapor Deposition Process for the Deposition of SiOx Barrier Coatings

Marina Ricci

EPFL2011

Optical Layers for Thin-film Silicon Solar Cells

Peter Cuony

EPFL2011