Unique properties of monocrystalline diamond make it a highly used material for power electronics or detectors applications. One of the major issues remains the upscaling of monocrystalline substrates limited to a few mm². Heteroepitaxial diamond appears to be a credible alternative [1]. However, the crystalline quality of heteroepitaxial diamond films is still lower than monocrystalline substrates. The key step is the Bias Enhanced Nucleation (BEN) during which “domains” are created [2]. During BEN, a kinetic occurs associated to nucleation and 2D growth of domains coupled with an iridium roughening. Nucleation mechanisms implied are not fully understood [3]. In some conditions, domains evolves toward diamond foam [4]. An in situ characterization would be beneficial to identify the mechanisms and to control the diamond/iridium interface. We succeeded in getting an in situ signature of the beginning of diamond foam formation by laser reflectometry. However, this technique was unsensitive to domains formation. We investigated here the Ellipsometric Spectroscopy (ES) which is a promising technique because of its sensitivity to optical properties and surface roughness. To first make ex situ ES analysis, we analyzed the morphology, the roughness and the surface chemistry by combined measurements done before and after BEN: FE-SEM on the same area of the substrate, AFM and XPS. This allowed us to create an ES model of domains which reports on the influence of domains and/or the iridium roughening. To decouple their influence in ex situ ES analysis, we succeeded to develop an etching technique to selectively remove domains without changing the iridium morphology. Such ES characterization could be further implemented for in situ nucleation monitoring. Acknowledgments to ANR & DGA for funding DIAMWAFEL (ANR-15-CE08-0034). [1] Schreck, MRS Bulletin, 2014, 39, 504-510 [2] Vaissiere, DRM, 2013, 36, 16-25 [3] Schreck, Scientific Reports, 2017, 7, 44462 [4] Vaissière, DRM, 2016, 68, 23-27