Flash-Lamp Enabled Atomic Layer Deposition of Titanium Oxide

Abstract

Atomic layer deposition (ALD) processes are widely used in research and development as well as in various production environments. Nowadays, a variety of materials can be deposited using thermal and energy enhanced atomic layer deposition but there are limits that are mainly set by the properties of available precursors and the related processes. Besides the necessity of suitable precursors and chemical reactions to deposit a specific material, there are two major limitations: the thermal self-decomposition of precursor molecules which can be in conflict with the required process temperatures enabling desired film properties, and the typical temperature-related reactivity of precursors and co-reactants with substrate materials. These can affect the initial film growth and can lead to adhesion issues and unwanted interface layers, such as oxides formed by oxidizing agents used during the ALD process.The thermal stability of precursors can be improved by optimizing precursor molecules but this is often related to reduced volatility and reactivity. Additionally, increased process temperatures might be incompatible with the tolerable thermal budget. Hence, processes with precursors chemisorbing at sufficiently low temperatures to avoid self-decomposition in combination with a second,preferably short process step at elevated temperatures to enable the required film properties is desirable.We developed a novel flash lamp enabled atomic layer deposition (FLE-ALD) process to address these limitations of common ALD processes. The process combines the self-limiting chemisorption of precursor molecules in the first half-reaction with a millisecond flash lamp anneal (FLA) during the second half-reaction. The applied heat during the second process step can even enable single-source ALD processes by inducing a thermal decomposition of chemisorbed molecules that form the desired film material.This paper demonstrates the FLE-ALD approach for the deposition of titanium oxide in a single-source process combining titanium tetraisopropoxide (TTIP) precursor exposures with flashes of optical light with a duration in the millisecond range to heat a surface for a very short time and to decompose the chemisorbed TTIP molecules. The heat induced by the short FLA step is distributed into the substrate’s bulk and the process chamber during the subsequent purging step, resulting in a constant substrate temperature and maintaining the self-limiting behavior of the TTIP chemisorption.A process was developed to confirm the self-limiting film growth typical for ALD process as well as the constant growth per cycle by varying and characterizing process parameters like precursor dose, purge times, flash lamp energy, substrate temperature, and cycle numbers. The FLA energy variation confirmed the need for the induced heat to decompose the TTIP molecules. Below an applied voltage of 1 kV, the flash lamps weren’t ignited and no film growth could be observed. This behavior was even confirmed for temperatures up to 300 °C as shown in figure 1, which compares the film growth with and without an FLA step for a wide range of process temperatures. Spectroscopic ellipsometry was used to measure the thicknesses and to investigate the optical properties of the deposited films. The obtained data was successfully described by a model using the optical properties of titanium oxide from literature enabling reliable and reproducible film thickness measurements. This result matches the film composition measured by X-ray photoelectron spectroscopy, showing only titanium and oxygen without noteworthy carbon or hydrocarbon impurities with less than 1 at.%. Figure 1