Enhancing doping contrast and optimising quantification in the scanning electron microscope by surface treatment and Fermi level pinning

Abstract

Recent advances in two-dimensional dopant profiling in the scanning electron microscope have enabled a high throughput, non-contact process diagnostics and failure analysis solution for integrated device manufacturing. The routine (electro)chemical etch processes to obtain contamination-free, hydrogen-terminated silicon surfaces is industrially important in ULSI microfabrication, though doping contrast, which is the basis for quantitative dopant profiling, will be strongly altered. We show herein that ammonium-fluoride treatment not only enabled doping contrast to be differentiated mainly by surface band-bending, but it enhanced the quality of linear quantitative calibration through simple univariate analysis for SE energies as low as 1 eV. Energy-filtering measurements reveal that the linear analytical model broached in the literature (c.f. Kazemian et al., 2006 and Kazemian et al., 2007) is likely to be inadequate to determine the surface potential across semiconductor p-n junctions without suitable deconvolution methods. Nevertheless, quantification trends suggest that energy-filtering may not be crucial if patch fields and contamination are absolutely suppressed by the appropriate edge termination and passivation.

Fermi level pinning characterisation on ammonium fluoride-treated surfaces of silicon by energy-filtered doping contrast in the scanning electron microscope

“…The contrast magnitude is enhanced only from p -regions that are sufficiently highly doped ( e.g. >10 19  acceptors cm −3 )7, or from n -regions so that they can be distinguished from intrinsic regions4. The surface states on the silicon in Fig.…”

“…500 ms), as documented in ref. 7 and detailed in Methods. Since contrast inversion is attributed to the presence of patch fields – which diminishes as surface bend-bending increases with the surface state density9 - the above result is at variance with reports in the literature of a reduced number of surface states after NH 4 F-treatment451819.…”

“…Due to recent developments in SEM instrumentation, the application of doping contrast has evolved not only in physical and failure analysis of integrated circuits, critical dimension measurements on a semiconductor production line, etc. , but mapping of electrically active dopants at high spatial resolution and sensitivity23456789. A resolution up to 1 nm is achievable56 and doping contrast can be measured proportionately from dopant concentrations less than 10 14 up to more than 10 20  atoms cm −3 , at a quantification accuracy of at least ± 3% 78.…”

“…Doping contrast may also be produced due to ad-layer-semiconductor contacts that alter the surface band-bending depending on the doped region14. Hence, sufficient sensitivity and quantification accuracy cannot be reliably achieved unless the near-surface effects, including that of surface band-bending2315 and patch fields416 on doping contrast are understood and well-controlled79. For example, to avoid surface-geometry influence from a non-homogeneous semiconductor sample that may hamper quantification via patch fields78, one approach may be to induce Fermi level pinning on the surface9, thereby resulting in surface band-bending and a surface junction potential that reduces from that in the bulk.…”

“…Hence, sufficient sensitivity and quantification accuracy cannot be reliably achieved unless the near-surface effects, including that of surface band-bending2315 and patch fields416 on doping contrast are understood and well-controlled79. For example, to avoid surface-geometry influence from a non-homogeneous semiconductor sample that may hamper quantification via patch fields78, one approach may be to induce Fermi level pinning on the surface9, thereby resulting in surface band-bending and a surface junction potential that reduces from that in the bulk. Therefore the aim of this study is to examine and demonstrate for the first time, a uniquely innovative, rapid and facile route based on doping contrast to determine a nearly-equipotential surface of a semiconductor after wet-chemical etching, characterised by Fermi level pinning due to a high density of surface states that traps charges and removes patch fields with essentially no surface charge variation across the p - n junction.…”