Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment
Energy dispersive X-ray spectroscopy with microcalorimeters
Introduction
Energy dispersive X-ray spectroscopy (EDS) is a standard technique for element identification in material analysis. EDS systems are mounted on scanning electron microscopes (SEM) and use the primary beam of the microscope to generate characteristic X-rays. The composition of the sample is found by analyzing the energy of the characteristic X-rays. The spatial (lateral + vertical) resolution of EDS depends on the sample material and the energy of the primary beam of the SEM (Fig. 1).
In semiconductor device fabrication shrinking feature sizes create the need for analysis of small structures, thin films and small particles. This necessitates low acceleration voltages for the primary beam of the SEM. When this is done, only low-energy X-ray lines will be excited. In this case the energy differences between light element K-lines and L- or M-lines of heavy elements are less than the energy resolution of conventional EDS (FWHM ∼90 eV at 1.5 keV for Si(Li) detectors) and the different lines cannot be resolved. EDS based on microcalorimeter technology achieves an energy resolution sufficient to solve the problem of line overlaps.
Section snippets
Cooling system
The cooling system of the spectrometer consists of a low vibration 4 K pulse tube cooler with separate rotary valve and a single stage ADR with a hold time of more than 8 h at 100 mK (Fig. 2) [1]. The recharge time of the ADR is less than 30 min.Cool down and temperature regulation are completely automated. In order to protect the detector from infrared radiation, windows are placed in front of the detector. These windows lower the transmission at low X-ray energies as shown in Fig. 3.
Detector
The detector
Basic parameters of the system
The best achieved energy resolution was for the Al-Kα transition at 1.5 keV with a FWHM of 7 eV (Fig. 5), which is one order of magnitude better than the resolution of a Si(Li) detector in this energy range (∼90 eV).
Due to the good energy resolution the peak to background ratio is much better than for Si(Li) detectors and so the lower limit of detection for elemental concentrations is expected to be better than for conventional detectors. Samples with homogeneously dissolved impurities of low
Summary
The first industrially used EDS system based on a cryogenic detector has been successfully integrated in routine analysis in the Failure Lab 5 at Infineon Technologies AG in Neuperlach (Munich). An energy resolution of FWHM 15 eV at 1.5 eV is reliably reached. With X-ray lens the count rate i.e., the ‘time to spectrum’ is comparable to the performance of conventional EDS. The cooling system of the spectrometer is fully automated and can be controlled via software. In the future the software needs
Acknowledgments
This work has been supported by the European Program IST 1999-20010 MESA. The authors would like to thank the Beschleuniger-Laboratorium Garching (Munich) and also X-ray Optical Systems (XOS) for use of the lens.
References (2)
- G. Thummes, M. Theiß, M. Bühler, J. Höhne, Reaching 96mK by a pulsetube precooled adiabatic demagnetization...Proceedings of the 11th International Cryocooler Conference, Keystone, Colorado, USA,...
- B. Simmnacher, R. Weiland, E. Langer, M. Bühler, J. Höhne, C. Hollerith, Microcalorimeter energy dispersive X-ray...