Diamond is an interesting candidate for power applications. However technological improvements have to be pursued for the fabrication of high-performance power devices. For example, ohmic contacts on n-type diamond have not yet been demonstrated, especially due to the difficulty to achieve high n-doping. We study in this work the fabrication of ohmic contacts on n-layers. We use 4 identical phosphorus doped epilayers ([P] = 6x1017 at.cm-3 over 1.3 µm thick): one was kept as reference and, for the 3 others, we perform phosphorus ion implantations by high temperature beam line or plasma immersion implantation PIII. The implantations were done using tools developed and manufactured by IBS, IMC200 and Pulsion®. A fifth sample (N5) is used for comparison: it is a highly phosphorus doped epilayer ([P] = 1.6x1020 at.cm-3 over 0.2 µm thick). The Ti/Pt/Au contacts are realized using a SiO2 passivation layer. They are characterized using linear and circular Transmission Line Method (TLM and cTLM). A specific contact resistance of 4.9 x 10-1 .cm2 is calculated on the highly phosphorus doped epilayer (N5). Such high value shows the difficulty of making ohmic contacts on n-type diamond. For the samples that have been ion implanted by PIII, the linear TLM measurements show an ohmic behavior whereas cTLM measurements indicate a Schottky behavior. In order to understand this difference, TEM analysis is performed in the region of the contacts. For the linear TLM, the TEM images highlights the presence of a graphite layer localized between the metallic pads of the TLM pattern, under the SiO2 passivation layer. Such graphite layer is responsible of the conduction observed. For the cTLM, no graphite layer is observed. Further studies are under investigations to explain the formation of the graphite layer.