Performance Improvement of Graphenic Carbon X-ray Transmission Windows

Graphenic carbon (GC) x-ray transmission windows for EDX and XRF applications with a high transparency for x-rays below 2.5 keV have been fabricated on 6 inch wafers with a CMOS-compatible CVD process. GC windows with an open diameter of 7.4 mm and a thickness of 770 nm withstand up to 6.5 bars of differential pressure. A high transmissivity of 40 % for fluorine Kα (0.677 keV) radiation is demonstrated for a GC thickness of 650 nm. The GC membranes outperform beryllium (Be) windows, in terms of higher x-ray transmission and better mechanical stability while avoiding the toxicity of Be. Optical profilometry has been employed to visualize a large deformation of the GC layer during the window fabrication. This seems to limit the thickness of the GC windows that can currently be fabricated. A two-step growth process can overcome these limitations and windows with a thickness of up to 6 µm have been realized.