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Interferometry is one of the most accurate methods for measuring distances and positions and has become indispensable as an important component of precision machines in almost all technology sectors. The basic principle is based on the comparison of a light beam from the sample with a reference steel, whereby accuracies in the sub-nanometer range can be achieved. Previous solutions based on the concept of a Michelson interferometer required the splitting and precise adjustment of the two partial beams, which increases the complexity and cost of such systems. A significant simplification of the Michelson interferometer is the so-called standing wave interferometer (SWIF). The excitation, reflection and detection take place along one optical axis, so that the reference beam path and the associated precision requirements for the alignment are not necessary. However, this much simpler concept only works if the detectors themselves are designed as semi-transparent components and are aligned with a distance to each other that is precisely defined to within a few nanometers.
For this purpose, the CiS Research Institute is developing transparent detectors whose active layers consist of doped silicon or polysilicon. To realize a defined semi-transparent detector for light wavelengths in the visible or near-infrared range required new technologies and highest accuracy in the fabrication of active layers only a few 10 nm thin. In a double configuration (tandem) consisting of two such sensors, it is possible to realize standing-wave interferometers with direct resolution of the direction of motion and to measure distances with high precision. The required technologies were developed by CiS within the project "Tandemdiode". First demonstrators are available in the meantime.

The following performance parameters are aimed at for the transparent detectors:
• Very good diode behavior (reverse current < 1 nA/mm2, series resistance < 5kΩ).
• Sufficient photosensitivity of about 2 µA / mW @633 nm
• Cut-off frequency: > 10 MHz
• Good transparency: >80 % per single diode)
• Low reflection: < 5 %
• Low scattering of device parameters
• Low unevenness of the layers
• High accuracy / reproducibility of layer thicknesses in the process

At the symposium of the World Interferometry Days on 11.04.2022 in Ilmenau, the previous research and development results were presented and explained.

The described research and development work was funded in the research project "Tandemdiode", by the German Federal Ministry of Economic Affairs and Climate Action (BMWK).
Funding code: 49VF200011