Fraunhofer Institute for Surface Engineering and Thin Films ISTAtomic Layer Deposition (ALD) is a modified version of chemical vapor deposition (CVD). This process is characterized by two successive, self-limiting surface reactions which permit ultra-thin, pinhole-free and extremely homogeneous coatings to be deposited. At the Fraunhofer IST primarily thermal ALD processes are developed and analyzed.
Silicon sphere for the deposition of a high-precision and stoichiometric SiO2 coating in scope of the redefinition of the Avogadro constant.
Antireflective coating on a glass sphere of 12 cm diameter.
Advantages of the thermal Atomic Layer Deposition
Compared to conventional PVD or (plasma) CVD processes, atomic layer deposition has a number of advantages:
Application example: Development of transparent conductive oxide layers (TCOs)
One of the major problems in thin-film electronics is the loss of efficiency due to the deposition processes, especially with sputtering processes which cause plasma damage on sensitive electronic surfaces. Apart from the loss in efficiency, such sputtering processes are not useful for coating high aspect ratio nanostructures on the electronic devices. At Fraunhofer IST, we have been developing processes for transparent conductive oxides with thermal atomic layer deposition.
Application example: Coating of LEDs
All of these TCOs have been successfully tested in LED test structures. Some of them showed superior properties, such as lower forward voltages as compared with sputtered TCO films. This is attributed to the lack of plasma damage.
Application example: Diffusion barriers on technical textiles
Another field of application is diffusion barriers on technical textiles and nonwovens deposited by ALD. The ALD coating prevents the dye from bleeding and reduces the diffusion of pyrolysis products during combustion. This process results in flame-retardant properties of the textiles without the use of halogen-containing chemicals or wet-chemical treatment steps.
Application example: Optical functional films
In addition to electronics applications, dielectric coatings for optical applications have also been developed. High-quality oxide coatings for single layer and broad band antireflective coatings for the UV-Vis-NIR range and double-sided coatings can be produced.
Application example: Coating of HIT solar cells
Pure and thin ZnO ALD films were also incorporated in HIT solar cell test structures as a protective layer prior to TCO sputter deposition. A decrease in defect density was observed, confirmed by the increased lifetime of photo-generated electron-hole pairs.