Energy-filtered PEEM with nanoESCA : principles, capabilities and application towork-function imaging with high sensitivity and lateral resolution
O. Renault1, N. Barrett2, A. Bailly1, L. Zagonel2, N. Fairley3, B. Krömker4, D. Funnemann4
1
CEA-DRT/Léti, Physical Characterization Laboratory, CEA-Grenoble, France- 2CEADSM/DRECAM/SPCSI – CEA Saclay – 91191 Gif sur Yvette, France- 3Casa Software Ldt,Teignmouth, Devon, TQ14 8NE, United Kingdom- 4Omicron Nanotechnology GmbH, 65232
Taunnustein, Germany.
The importance of energy-filtering in PEEM-based imaging methods has been shown inrecent years with the availability of powerful instruments [1, 2]. However, conventionalenergy-filtering suffers from the spherical aberration (α2-term) of the single hemisphericalanalyzer that blurs the image at low pass energies. A new instrument, the nanoESCA,
combines a fully electrostatic PEEM column and a double hemispherical analyzer as energy
filter that corrects these spherical aberrations [3]. The energy filter can then be operated at
low pass energies (and therefore, high transmission) without detrimental effects on the spatialresolution. It also largely retains the time structure of the electron signal, which is importantfor future time-resolved studies. The first instrument of this kind has recently been installed at
the Nanocharacterization Center of CEA in Grenoble. It allows to perform optimized energyfiltered PEEM using various excitation sources : UV, VUV, soft x-rays using synchrotron
radiation for high resolution XPEEM and x-rays from a focused monochromated AlKα source
for XPEEM at lower spatial resolution. Demonstrated (but not ultimate) spatial edge
resolutions with energy-filtering are 80 and 120 nm in XPEEM with soft x-rays excitation ofthreshold and core-level electrons respectively [3], and 40 nm in UV-PEEM [4]. After a firstsection that will present the instrument and its capabilities as summarized above, we will
focus on results on PEEM imaging of polycrystalline copper using work-function contrast.
We will first show how energy-filtering allows spectromicroscopic information from UV
excitation with a direct relation between local work function changes (due to different grainorientations) and the corresponding energy distribution of the emitted electrons [4];particularly, the local photoemitted intensity does not seem to follow the Fowler law. Second,from a data set of energy-filtered VUV-PEEM images, a treatment using Principal
Component Analysis allows to generate meaningful nanospectra over typical areas of 50x50
nm2 that are used to estimate, within an uncertainty of 20 meV, the local work function fromproper analysis [4] of the secondary electron emission onset. Thus, a work function image canbe reliably reconstructed. Besides three zones previously identified arising from <111> (Φ=4.77 eV), <110> (Φ=4.54 eV), and <100>-oriented grains (Φ =4.62 eV), a faint, additionalzone appears (Φ=4.69 eV). These results are important for the local analysis of defects inmaterials.
[1] T. Schmidt, S. Heun, J. Slezak, J. Diaz, K. C. Prince, G. Lilienkamp, E. Bauer, Surf. Rev. Lett. 5 (1998), 1287. [2]E. Bauer, J. Electron Spectrosc. Related Phenom. 114-116 (2001), 975-987. [3] Escher et al., J. Phys: Condens.
Matter 2005;17:1329. [4] Renault et al., Surface Interface Anal. 2006;38:375.