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TXRF spectrometer for light element analysis

The measurement set-up consisted of a vacuum chamber suitable for 6”-Wafers (rotatable sample carrier), a 1.9 kW Cr anode X-ray tube (long fine focus) as the source. A multilayer monochromator W/C, an adjustable slit-system modifies the primary spectrum to a monoenergetic beam. A Si(Li) detector (Oxford Premiumgrade B35; 30 mm²) with an ultra thin window (Moxtec AP3.1 300 nm on a Si grid support) (4) equipped with an electron trap (Æ 6mm) to suppress noise originating from Auger-, Photo- and Compton electrons collects the photons. The electron trap consists of 2 high field permanent magnets 6x4x1 mounted inside a cylindrical collimator made of Fe with a center hole of 6 mm diameter and attached to the detector snout.  A XIA pulse processor was used. FWHM @5.9 keV was 134 eV.

Scheme of the chamber
picture of the inside chamber
Spectrum of various samples containing Na, Mg and Al

Detection limits (LLD)have been determined using the Cr anode tube at operating conditions of 30kV and 30mA for 100 s live time, which were extrapolated for 1000s in all cases. These detection limits (Table 1) are compared with the old data achieved before the mechanical adaptions to host the 200 mm wafers. The conversion from pg to atoms/cm² assumes a relevant inspected area of 0.5 cm². The main difference between old and new setup was the distance between tube and detector, in the old setup 100 mm, in the new 180 mm.

Comparison of the detection limits of the previous and the new set-up
Element LLD [pg] previous set-up LLD [pg] new set-up 10^12 Atoms/cm2
Na 230 180 9,4
Mg 180 150 7,4
Al 120 140 6,2

Improvements:

  • modified vacuum chamber suitable to accommodate 6”- 8” wafers
  • Rotatable sample carrier for 6” wafers
    free path-length: up/down stream 25mm; left/right 18mm
  • Sample carrier for 8” wafers with tilting mechanism to avoid any contact with surroundings; measurement @ center of wafer
  • Implementation of a digital x-ray processor
  • Excellent Detection Limits though the fact that the distance tube-detector was increased by 80mm
  • Implementation of a x-ray tube motor - vertical adjustable
  • New set-up was adjusted for optimized conditions under controlled and measured procedures