PhD Thesis Arne Lüker final version V4 - Cranfield University

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70 Characterisation of Ferroelectric Thin Films Characterisation of Ferroelectric Thin Films (Ba,Sr)TiO3 and (Pb,Sr)TiO3 thin films were deposited on pieces of prime-grade 4” Si- wafers with 200 nm thermal oxide. For the characterisation of electrical and physical properties of the ferroelectric thin films a standard procedure, which was developed for Pb(Zr,Ti)O3 (PZT), was adopted. In this scheme a Ti/Pt, 8nm/100nm, bottom electrode was deposited on top of the oxide layer. It is common knowledge that this metallisation layer enhances the crystallisation of PZT and nearly all other ferroelectrics. In a later chapter we will see that this is not necessary for PST. Fig. 3.1: The Wayne Kerr precision component analyser 6425 with probe station and power supply for the measurement of the tunability However, after the sol-gel deposition and XRD- , AFM-, and SEM-characterisation of either BST or PST Cr/Au dots were evaporated through a shadow mask. Electrical properties, e.g. permittivity, loss and tunability, were measured on a probe station with a Wayne Kerr precision component analyser 6425, the hysteresis was measured using a RT66 ferroelectrics test system (Radiant Technologies, USA), and the permittivity-temperature dependence (Curie- temperature) was characterised with the help of a poling-rig, which could operate in a temperature range of –40°C to +60°C while keeping the sample under constant vacuum to avoid condensation of water. Auger analysis of the PST-, PZT-SiO2 interface was the last experiment with remarkable results in this study. In this chapter some of these methods and techniques are described in more detail.
Sol-Gel derived Ferroelectric Thin Films for Voltage Tunable Applications 3.1 Sputter deposition and Thermal Evaporation 3.1.1 Sputter Deposition Sputter deposition involves the bombardment of a target with energetic ions in order to generate a vapour that subsequently deposits onto a substrate. In this study two types of sputtering were used: DC magnetron sputtering for the deposition of Pt (target: Pi-KEM, UK 99.9% purity) and RF magnetron sputtering for the deposition of Ti (target: Pi-KEM, UK 99,7% purity). The basic principle behind both techniques is the same and begins with the evacuation of the process chamber below 1.3·10 -3 Pa, into which Ar gas is introduced at low pressure of 1.3 Pa. A glow-discharge plasma is struck between the target (cathode) and an adjacent shield (anode) by applying either a steady voltage (DC sputtering) or an AC voltage (RF sputtering). Positive Ar ions are accelerated towards the target and “sputter” atoms, through momentum exchange, out into the chamber. Those that hit the substrate condense to form a thin film. Secondary electrons are also emitted through the bombardment of the target and these maintain the plasma by ionising further Ar atoms. Permanent magnets behind the target form a toroidal field that confines the plasma to a ring close to the target. Because the path length of electrons within the plasma is increased, the minimum gas pressure to sustain the plasma is reduced. This helps to boost sputtering rates since the sputtered atoms are subjected to fewer collisions on their way to the substrate. RF sputtering is advantageous for insulating targets or targets with reduced conductivity as it prevents charge build up and also reduces the drive voltage necessary to maintain the plasma. A Nordiko 2000 sputtering system was used during this investigation. This machine was equipped with three magnetrons (two for RF sputtering and one for DC sputtering). Samples were loaded onto one of six substrate holders, which, in turn were fitted to a motorised central spindle. A computer was used to rotate the samples into the correct position above the relevant target. A 10 min pre-sputter was employed to remove surface oxide and/or contaminations from the Ti target. In case of Pt, a shorter, 2 min pre-sputter was used that took into account the reduced reactivity of the material and its significant monetary value. The standard procedure involved depositing an 8 nm Ti adhesion layer, followed by 100 nm of Pt, without breaking the vacuum. Adhesion layers are used to join 71
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PhD Thesis Arne Lüker final version V4 - Cranfield University

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