Defect-free silicon oxide layers for the Avogadro project

Challenge  

The kilogram is the last quantity in the SI International System of Units remaining to be defined by a macroscopic body - the primary standard kilogram. All other units have already been reduced to atomic processes, molecular properties or natural constants. If we can, with the highest degree of precision, count the atoms in a silicon crystal with a mass of 1 kg, the material kilogram can in future also be replaced by a physical constant. 

Solution  

On the surface of the silicon spheres there is always a natural layer of SiO₂, which also has an influence on the mass and volume of the spheres. This native layer grows slowly, but sometimes also very unevenly. This makes it very difficult to measure the actual weight of both the oxide layer and the sphere. For the redetermination of the Avogadro constant, an alternative, homogeneous SiO₂ coating is therefore being investigated in order to reduce measurement uncertainty and to be able to precisely determine the volume and mass of the sphere.

With the aid of the atomic layer deposition (ALD) technique available at the Fraunhofer IST, it is possible to deposit stoichiometric SiO₂ layers with a defined roughness and variable layer thickness whose properties meet the high requirements of the project. They are reproducible and can be applied to the sphere as an extremely thin oxide layer of uniform thickness. Potential impurities such as carbon or nitrogen are below the detection limit, and the roughness of the layers remains below one nanometer.

Added value  

The SiO₂ coatings developed at the Fraunhofer IST can be applied not only to spherical systems but also to surfaces of any complexity. Possible future fields of implementation are therefore diverse and range from optical applications, through semiconductors and electronics, and on to medical technology.