  Gas Electron Multiplier (GEM) detectors are crucial for enabling
high-resolution X-ray polarisation of astrophysical sources when coupled to
custom pixel readout ASIC in Gas Pixel Detectors (GPD), as in the Imaging X-ray
Polarimetry Explorer (IXPE), the Polarlight cubesat pathfinder and the PFA
telescope onboard the future large enhanced X-ray Timing and Polarimetry (eXTP)
Chinese mission. The R&D efforts of the IXPE collaboration have resulted in
mature GPD technology. However, limitations in the classical wet-etch or
laser-drilled fabrication process of GEMs motivated our exploration of
alternative methods. This work focuses on investigating a plasma-based etching
approach for fabricating GEM patterns at Fondazione Bruno Kessler (FBK). The
objective is to improve the aspect ratio of the GEM holes, to mitigate the
charging of the GEM dielectric which generates rate-dependent gain changes.
Unlike the traditional wet-etch process, Reactive Ion Etching (RIE) enables
more vertical etching profiles and thus better aspect ratios. Moreover, the RIE
process promises to overcome non-uniformities in the GEM hole patterns which
are believed to cause systemic effects in the azimuthal response of GPDs
equipped with either laser-drilled or wet-etch GEMs. We present a GEM geometry
with 20 micrometers in diameter and 50 micrometers pitch, accompanied by
extensive characterisation (SEM and PFIB) of the structural features and aspect
ratios. The collaboration with INFN Pisa and Turin enabled us to compare the
electrical properties of these detectors and test their performance in their
use as electron multipliers in GPDs. Although this R&D work is in its initial
stages, it holds promise for enhancing the sensitivity of the IXPE mission in
X-ray polarimetry measurements through GEM pattern with more vertical hole
profiles.
