Development of metallic magnetic micro-calorimeters for ECHo

The Electron Capture ¹⁶³Ho experiment ECHo aims to probe the electron neutrino mass on a sub-eV level via analysis of the calorimetrically measured electron capture spectrum of ¹⁶³Ho. For this metallic magnetic calorimeters (MMC) will be used, which are operated at millikelvin temperature. The performance achieved by first prototypes of MMC detector arrays with embedded ¹⁶³Ho already show that an energy resolution of ΔE_FWHM < 3eV and a signal rise time of τ < 1 μ s for ECHo can be reached. We present the current status of the detector developments for ECHo-1k, which is the first phase of the experiment with an activity of 1 kBq of the source and an expected sensitivity on the neutrino mass of below 10 eV. This includes the design and fabrication of the ECHo-1k detector and the necessary not standard fabrication processes to prepare the detector for the ion implantation of ¹⁶³Ho at RISIKO in Mainz. We also present simulations and experimental studies concerning the implantation process itself and measurements of the expected additional heat capacity of the detector due to the implantation of ¹⁶³Ho which is a crucial parameter for the detector performance.