Limitations of patterning thin films by shadow mask high vacuum chemical vapor deposition

A key factor in engineering integrated devices such as electro-optic switches or waveguides is the patterning of high quality crystalline thin films into specific geometries. In this contribution high vacuum chemical vapor deposition (HV-CVD) was employed to grow titanium dioxide (TiO2) patterns onto silicon. The directed nature of precursor transport - which originates from the high vacuum environment during the process - allows shading certain regions on the substrate by shadow masks and thus depositing patterned thin films. While the use of such masks is an emerging field in stencil or shadow mask lithography, their use for structuring thin films within HV-CVD has not been reported so far. The advantage of the employed technique is the precise control of lateral spacing and of the distance between shading mask and substrate surface which is achieved by manufacturing them directly on the substrate. As precursor transport takes place in the molecular flow regime, the precursor impinging rates (and therefore the film growth rates) on the surface can be simulated as function of the reactor and shading mask geometry using a comparatively simple mathematical model. In the current contribution such a mathematical model, which predicts impinging rates on plain or shadow mask structured substrates, is presented. Its validity is confirmed by TiO2-deposition on plain silicon substrates (450 degrees C) using titanium tetra isopropoxide as precursor. Limitations of the patterning process are investigated by the deposition of TiO2 on structured substrates and subsequent shadow mask lift-off. The geometry of the deposits is according to the mathematical model. Shading effects due to the growing film enables to fabricate deposits with predetermined variations in topography and non-flat top deposits which are complicated to obtain by classical clean room processes. As a result of the enhanced residual pressure of decomposition products and titanium precursors and the corresponding surface coverage of them, growth conditions differ in proximity of the shadowed areas compared to the case of plain substrates and the obtained film thickness differs significantly from predictions. (C) 2014 Elsevier B.V. All rights reserved.

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

Selective light induced chemical vapour deposition of titanium dioxide thin films

Estelle Wagner

Light Induced Chemical Vapour Deposition (LICVD) of titanium dioxide thin films is studied in this work. It is shown that this technique enables to deposit locally and selectively a chosen crystalline phase with a precise controlled thickness at low substrate temperature, allowing even the use of polymer substrates. A home made LICVD reactor was set up, consisting of a main chamber in which the substrate was placed on a temperature controlled plate and could be irradiated perpendicularly through a window by a 250 ns long pulse XeCl excimer laser (308 nm) with a controlled fluence (energy per surface area per pulse). A liquid precursor (titanium tetra-isopropoxide) was brought by an oxygen carrier gas flow in the chamber and a nitrogen flow was added to prevent deposition on the window. The total pressure in the chamber was maintained at 10 mbar. Projection of a mask image on the substrate was realized. Titanium dioxide films could be deposited locally by this technique with a thickness ranging from few nanometers to several micrometers. Deposits were realized at substrate temperatures as low as room temperature and with fluences between 1 and 400 mJ/cm2. The deposited films were characterized by profilometry, Atomic Force Microscopy, optical microscopy, Scanning Electron Microscopy, Transmission Electron Microscopy, X-ray diffraction, Raman spectroscopy, X-ray Photoelectron Spectroscopy, UV-vis spectroscopy and ellipsometry. A detailed and systematic study of the influence of some selected process parameters (substrate temperature and irradiation dose principally) on the deposition rate and material properties was realized on glass and silicon substrates. An empirical equation of the growth rate as a function of the different experimental parameters was derived and was tested successfully to obtain a precise control of the deposited thickness (a precision better than 10 nm was demonstrated for films below 300 nm), with growth rates up to 300 nm/min. From this equation, a deposition mechanism involving both a photolytic reaction and a thermal contribution of the substrate temperature was proposed. The deposited material was shown to consist of TiO2 with a possible very low additional carbon contamination. Depending on the deposition conditions, amorphous, anatase or rutile material was deposited. One parameter explaining the variation of the crystalline state of the material was the variation of the laser induced temperature rise which was simulated theoretically in parallel. Surface temperature rises in the order of several hundreds of degrees were calculated during the laser pulse, varying with the substrate, the fluence and the film thickness. The optical properties of the deposited material vary in a correlated way with the crystalline state of the material, and high indexes of refraction (up to 2.6 at 633 nm) were measured. Selective deposition in the irradiated area was studied. Provided substrate temperature was kept below 150°C, no thermal deposition was observed, and deposits were strictly located in the irradiated area. The edge resolution (around 10 micrometers) was demonstrated to be due to the optical aberrations of the mask projection set up. No effect of substrate temperature and irradiation conditions on the resolution could be evidenced, but resolution decreased with increasing deposited thickness due to optical effects. Thanks to the low substrate temperature required for the deposition process, deposition on thermo- fragile substrates (such as polymers, in particular PMMA) was demonstrated. However, in the case of polymer substrates, the fluence had to be kept below 15 mJ/cm2 not to damage or ablate the polymers, resulting in very low growth rates (in the order of 10-3 nm/pulse). In these conditions, only amorphous films with about 10% of carbon contamination could be obtained. Additionally, films were shown to crack above a certain deposited thickness, which is attributed to thermal effects. As a matter of fact, calculations showed a high temperature rise up to 70°C of the polymer/oxide interface. Deposition on polymers coated with transparent conductive oxide was also demonstrated.

EPFL2003

Selective light induced chemical vapour deposition of titanium dioxide thin films

Estelle Wagner

EPFL2003