Flash-Lamp-Enhanced Atomic Layer Deposition of Thin Films

Atomic Layer Deposition (ALD) is a particular thin film deposition technology which is based on alternating saturated surface reactions. As a result, the film growth proceeds in a self-limiting manner enabling the deposition of thin films with excellent thickness control, uniformity, and conformity. Although a large number of materials have been deposited by ALD so far for various applications, there are still some challenges in ALD.

The deposition temperatures in ALD are typically lower compared to CVD due to the limited thermal stability of ALD precursors. As a consequence of the lower energy available for film formation the films may not meet the properties needed for application. In these cases a post deposition annealing is required to improve the film properties, e.g. to obtain the desired film structure, density, or purity. However, this high temperature processing is often impracticable due to a restricted thermal budget of the substrate, in particular when coating temperature sensitive substrates. Secondly, the reactants of an ALD process, e.g. oxygen, may react with the substrate itself leading to the formation of a parasitic interfacial layer. In order to avoid this issue, the proper choice of reactants or the use of an alternative deposition technique is essential. Furthermore, many ALD processes suffer from substrate inhibited film growth accompanied by inefficient precursor consumption and the formation of films with unfavorable properties. Finally, there are materials of interest, e.g. titanium, which so far can not be deposited by thermal ALD at all. These limitations may be overcome by the application of flash lamp annealing (FLA) in ALD.

In FLA the substrate is exposed to a light flash with durations typically in the millisecond range. The light energy is absorbed within the top layers of the sample causing a rapid heating of the surface near region. On the contrary, the bulk material experiences no or only moderate heating. Consequently, FLA is a suitable technology to power high temperature processes even on temperature sensitive substrates.

The film growth in flash lamp enhanced ALD is induced by this effect. Thereby, each process cycle consist of both a precursor pulse and the irradiation of the substrate with a light flash. During each single flash the surface temperature exceeds the threshold temperature which is required to achieve the thermal decomposition of adsorbed precursor molecules or to activate chemical reactions between the adsorbed precursor molecules and a second reactant. The film growth proceeds step-by-step and thus the film thickness can be controlled by varying the number of cycles. In addition, FLA in each cycle results in the periodical annealing of the already grown film and hence may lead to an improved film quality. Consequently, flash lamp enhanced ALD has a high potential for the realization of single-source processes, for the reduction of growth delay in the initial phase of film growth, for the deposition of high purity thin films, and for the deposition of new materials.

In this work the principle of flash lamp enhanced ALD will be presented in detail, the technology will be reviewed and classified. Thereafter, we will give an overview about our studies on the flash enhanced ALD of aluminum-, ruthenium-, and tantalum-based thin films. These depositions were realized by flashing periodically on a substrate during the precursor pulses. We will show that the film growth is induced by the flash heating and the processes exhibits typical ALD characteristics. The obtained relations between flash parameters and film growth parameters will be discussed with the use of simulation results illustrating the temperature profile during the FLA treatment. Moreover, this work addresses the potentials of this technology as well as the technical challenges.