Electronic cooling in Superconductor - Insulator - Normal metal (S-I-N) junction is based on the energy selectivity of electron tunneling induced by the superconductor energy gap. Nevertheless, the efficiency of coolers based on such junctions is usually significantly less than theoretically expected. After introducing the principle of superconducting micro-coolers, we present the fundamental limitations to electronic cooling. We focus on the different thermal couplings between electron and phonon thermal baths and the relaxation of hot quasi-particles deposited in the superconductor. We have designed an experiment to monitor independently electron and phonon temperatures. An electronic cooler was studied under out-of-equilibrium conditions, in both the cooling and the heating regimes. The results are interpreted using a thermal model, which takes into account the heat transfers between the electron, phonon and photon baths. In particular, the photonic heat flow related to the thermal noise arising in the circuit resistors can bring an additional heat contribution, depending on the transmission of the biasing circuit. Moreover, we investigate the enhancement of quasi-particles relaxation under magnetic field, leading to an enhanced quasi-particle relaxation. Finally we develop a process enabling to fabricate a S-I-N-I-S cooler with large junctions and a suspended Normal metal island decoupled from the substrate.
