Mikko Utriainen

Studies of metallic thin film growth in an atomic layer epitaxy reactor using M(acac)2 (M = Ni, cu, Pt) precursors

Abstract Feasibility of metallic thin film growth by atomic layer epitaxy (ALE) was studied in the case of divalent metal β-diketonate-type precursor M(acac)2 (M=Ni, Cu and Pt) at 250°C and 1 mbar. Metallic films were obtained by two different approaches: (i) direct layer-by-layer reduction of the adsorbed precursor by H2, and (ii) by first depositing a metal oxide thin film by ALE and then converting it to the metallic form by a separate reduction step. The latter case was demonstrated by converting a NiO thin film, deposited at 250°C on glass substrate with O3 as an oxygen source, to metallic Ni at 260°C and 1 atm by 5% H2. AFM study indicated, however, pinhole formation and, thus, deformation of the initial dense structure of NiO. In the case of direct layer-by-layer reduction by H2, attention was focused on the effects of the substrate on the film growth by applying Ti, Al, Si(100) and glass substrates. X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) studies indicated that metallic Ni, Cu and Pt film growth proceeded preferably on Ti and Al substrates. On Si substrate, the film growth was restricted by interdiffusion and silicide formation; whereas on the glass substrate, the film growth was related to the gas phase stability of the precursors and proceeded mainly by pyrolysis.

Effects of intermediate zinc pulses on properties of TiN and NbN films deposited by atomic layer epitaxy

The reasons for the improvements gained by using intermediate zinc pulses in atomic layer epitaxy growth of TiN and NbN films were examined by a comprehensive characterization and comparison of films prepared from TiCl4 or NbCl5 and NH3 with and without zinc. The characterization techniques used comprise time-of-flight elastic recoil detection analysis, secondary ion mass spectrometry, Rutherford backscattering spectrometry, nuclear resonance broadening, proton backscattering spectrometry, deuteron induced reactions, proton induced X-ray emission, atomic force microscopy, scanning electron microscopy, X-ray diffraction, and Hall effect and reflectance measurements. The effect of zinc was found to be manifold: both compositional and structural changes were observed. In the case of TiN the major improvement gained by using zinc was significantly decreased oxygen contamination whereas a marked increase of grain size was the dominant effect observed with NbN. A clear correlation between the compositional and structural changes and the improvements of the electrical properties was established.

Studies of NiO thin film formation by atomic layer epitaxy

Abstract Nickel oxide thin film formation by atomic layer epitaxy (ALE) was studied including aspects of surface chemistry and structure of the prepared films. The applied source materials were Ni(acac)2, Ni(apo)2, Ni(dmg)2 and O2, H2O, N2O, O3, CH3COOH for Ni and O, respectively. The thermal behaviour of the Ni-containing precursors was studied by thermogravimetry and by experiments in the ALE reactor. The reactivity of the Ni-precursors towards various oxygen sources was studied at the deposition temperature of 250°C using soda-lime glass substrates. The crystalline rocksalt-type NiO was obtained only when O3 was used as an oxygen source. The film was in this case (100) oriented NiO independent on the applied Ni-precursor. The NiO ALE process was optimized for the reaction between Ni(acac)2 and O3. The surface-saturated growth rate was limited by the steric hindrance of the ligand and was 0.62 A cycle−1. A slightly increased growth rate (0.72 A cycle−1) was observed, when H2O was supplied together with O3 as an oxygen source. Structure of the deposited films was studied by atomic force microscopy and X-ray diffraction techniques and some structural variations related to high ozone doses were observed.

SnO2 sol–gel derived thin films for integrated gas sensors

Abstract In this paper, we present for the first time the compatibility of sol–gel method for SnO2 thin film preparation with the silicon technology for integrated gas sensor microfabrication. An integrated circuit (IC) compatible test structure of medium power consumption equipped with boron-doped silicon heater and Au/W metallization is developed. The acid composition of the (liquid) sol phase, the thermal budget of sensing layer structuring, selective wet etching of SnO2 sensing film, thickness uniformity and step coverage of SnO2 sol–gel films are fitted with the requirement of above test structure where metal layer is deposited before SnO2 film. Nanometric grain sizes of undoped and antimony doped polycrystalline SnO2 films are obtained, as revealed by XRD investigations. The AFM measurements of SnO2 thin films deposited on existing Au/W metallization shown the excellent step coverage and morphology of SnO2 films used for gas sensing applications. Low temperature gas sensing properties of our SnO2 sol–gel derived thin films in reducing (CH4, CH3COOH) and oxidizing (NO2) are preliminary reported by using our integrated test structure.

The effect of growth parameters on the deposition of CaS thin films by atomic layer epitaxy

Ca(thd)2 (thd = 2,2,6,6-tetramethyl-3,5-heptanedione) and H2S were used as precursors for the deposition of CaS thin films by atomic layer epitaxy (ALE). The growth on soda lime glass with and without alumina coating was studied by varying source furnace and substrate temperatures. In addition, the Ca(thd)2 and H2S pulse and purge times as well as the total number of ALE cycles were varied to observe their influence on the growth. A narrow processing window for the ALE growth was found when the source and substrate temperatures were 180–200°C and 325–400°C, respectively. The CaS thin films processed under optimized conditions were crystalline and smooth.

Negative admittance in resistive metal oxide gas sensors

The negative admittance effect is observed in a WO3-based resistive gas sensor MOS1 from Environics Oy. The effect is caused by electron trapping (i.e. oxygen ionization) at the grain boundary. The results show that the current component related to the modulation of the grain-boundary barrier dominates in dry clean air and the charging or discharging current dominates in humid air conditions. An equivalent electrical circuit model for the sensor response is presented.

Structural and functional properties of SnO/sub 2/ sol-gel derived thin films for integrated gas sensors

We present the structural and gas sensing properties of the sol-gel derived SnO/sub 2/ thin films exposed to small concentrations of reducing and oxidant ambient. XRD investigations show the structural evolution of the film as a function of sol-gel processing parameters. Nanometric grain size relation to metal atom doping has been obtained. For the functional characterisation of our SnO/sub 2/ sensing films, we propose an integrated technology for a simple test microstructure, based on resistive heater made of boron doped silicon and W/Au metallization for both sensor electrodes and heater contact. AFM investigations of SnO/sub 2/ thin films revealed the morphology of the films used for gas sensing applications. A test structure having medium electrical power consumption (below 900 mW) was used for the detection of the controlled concentrations of CH/sub 4/, CH/sub 3/COOH, and NO/sub 2/ at low temperatures.