The prototype kilogram is the unit of mass upon which the weight of a kilo is based. And it’s becoming increasingly lighter. The reason for this, however, is unknown. Now researchers are seeking alternatives for the platinum-iridium alloy artefact that is stored in a safe in Paris. The basic plan is to redefine the kilogram. In the future, a physical constant will replace the material kilogram.

Thus, a team of scientists from the Physikalisch-Technische Bundesanstalt (PTB) (the national metrology institute of Germany) is conducting experiments with spheres of isotope-enriched silicon, which could be used as a new calibration standard. The experts, therefore, must determine the Avogadro constant, which indicates the number of atoms in one mole. "We calculate the number of atoms in a sphere and use mathematical methods to obtain the number of atoms per mole. In simple terms, we find out how much a silicon atom weighs and, through inverse conclusion, can thus determine how many silicon atoms are needed for a kilogram," explains Dr. Ingo Busch, physicist at the PTB in Braunschweig. "The mole is the mediator between the atomic mass scale and the kilogram."

During the production of these spheres at the PTB, a natural oxide layer of silicon dioxide (SiO2) is formed. This also has an influence on the mass and volume of the silicon spheres. The problem is that the native layer grows slowly and, in part, unevenly. This makes it very difficult to measure the actual weight of both the oxide layer and the sphere. Therefore, an alternative homogeneous coating is required to reduce measurement inaccuracies and precisely determine the volume and mass of the sphere.

Researchers of the neighboring Fraunhofer Institute for Surface Engineering and Thin Films IST have succeeded in coating a silicon sphere with such an SiO2 surface that it meets the highest standards. "With our method, we can apply a layer of SiO2 with a precisely defined roughness and an adjustable layer thickness to the sphere. In addition, the layer is also stoichiometric, which means that the ratio of the individual atoms remains constant among each other or the ratio between silicon and oxygen," says Tobias Graumann, a scientist at IST.

For the coating, the researchers at the IST selected the Atomic Layer Deposition (ALD). The advantage of this method: A reproducible, extremely thin oxide layer with a homogeneous thickness can be applied to the sphere. Potential impurities, such as carbon or nitrogen, are below the limit of detection. The roughness of the layers remains below a nanometer. "The sphere's roughness is not significantly increased by the coating. This is a factor which keeps the measurement inaccuracy below 10 micrograms. Even a fingerprint weighs more," says Graumann. And the time factor also plays an important role. By applying SiO2, the manufacturing process can be accelerated. In contrast, the growth of the native oxide layer can take several months.