Spectrometer for Grazing Incidence XRF
To avoid scattering of the primary X-rays in air and to minimize the absorption of low energetic fluorescence X-rays (e.g. Si, as Si emits 1.7 keV radiation and Si is used to normalize for quantification and dose determination) on their way to the detector, the whole setup is placed in a vacuum chamber with an overall size of 300 x 300 x 340 mm3. The primary source is a 3kW Mo‑anode X-ray tube with a focal spot of 0.04 x 12 mm2 (long fine focus tube). In order to achieve low radiation background in the fluorescence spectra along with suitable photon flux for thin-film analysis, the primary X-ray beam is monochromatized to 17.44 keV (Mo-Ka) by means of a multilayer monochromator. To avoid unwanted effects due to totally reflected X‑rays at the multilayer surface a 50µm diaphragm is placed between multilayer and sample. The proper Bragg angle between incident radiation and multilayer surface is adjusted by positioning the X‑ray tube using a stepper motor providing a linear translation of the tube versus a fixed slit. The beam intensity after the multilayer is monitored by an ionization chamber. To process higher count rates a silicon drift detector (SDD, Vortex, from Radiant) with an active area of 50 mm2. The detector offers an energy resolution of 140eV with 12µs shaping time at 5.9keV (Mn-Ka) and a maximum processable count rate of 100 kcps. In order to perform all movements necessary for adjustment and measurement of the sample, the sample stage consists of 3 linear stages with a travel range of 25mm each and one rotation stage allowing to perform angle scans in the range of ±0.28 rad with a minimum increment of 1.35µrad. The stage can be tilted around the beam-axis to align the sample parallel to the beam. A sketch of the prototype spectrometer is shown in figure 1.
- X-ray source: 3kW Mo-tube LFF operated at 50kV/40mA and 50kV/55mA
- Monochromator: Multilayer from a commercial TXRF wafer analyzer (Atomika 8010W)
- Detector: Vortex 50 mm2 SDD with provided Electronics
- Measurement (life) time: variable (20 – 120 seconds per point)
- Angular resolution of each scans: variable (23 - 115µrad)
- Measurements were carried out in air and in vacuum (typical <1 mbar)