Toward Site-Specific Dopant Contrast in Scanning Electron Microscopy

Abstract

Since semiconductor devices are being scaled down to dimensions of several nanometers there is a growing need for techniques capable of quantitative analysis of dopant concentrations at the nanometer scale in all three dimensions. Imaging dopant contrast by scanning electron microscopy (SEM) is a very promising method, but many unresolved issues hinder its routine application for device analysis, especially in cases of buried layers where site-specific sample preparation is challenging. Here, we report on optimization of site-specific sample preparation by the focused Ga ion beam (FIB) technique that provides improved dopant contrast in SEM. Similar to FIB lamella preparation for transmission electron microscopy, a polishing sequence with decreasing ion energy is necessary to minimize the thickness of the electronically dead layer. We have achieved contrast values comparable to the cleaved sample, being able to detect dopant concentrations down to 1×10(16) cm-3. A theoretical model shows that the electronically dead layer corresponds to an amorphized Si layer formed during ion beam polishing. Our results also demonstrate that contamination issues are significantly suppressed for FIB-treated samples compared with cleaved ones.

The Mechanistic Determination of Doping Contrast from Fermi Level Pinned Surfaces in the Scanning Electron Microscope Using Energy-Filtered Imaging and Calculated Potential Distributions

“…For the freshly cleaved specimen, p was brighter than n under standard imaging conditions (Fig. 1b), as expected (Oatley & Everhart, 1957; Perovic et al, 1995, 1998; Venables et al, 1998; Elliott et al, 2000; Sealy et al, 2000; Elliott, 2001; Elliott et al, 2002; Lin & Lee, 2003; El-Gomati et al, 2004; Buzzo et al, 2006, 2007; Chee et al, 2008 a , 2008 b ; Chee et al, 2010, 2011; Liu & Lee, 2011 a , 2011 b ; Chung et al, 2011; Farrell et al, 2013; Xu et al, 2013; Druckmüllerová et al, 2014; Chee, 2016 a ; Alugubelli et al, 2019; Chee, 2019, 2020). Moreover, surface treatment using fresh (degraded) NH 4 F (aq.)…”

“…6a). Hence, other than the radically curtailed oxidation-and cleavage-related artifacts (Druckmüllerová et al, 2014), that the patch fields are abolished may explain the diminished scatter in the doping contrast data (Fig. 3b).…”

“…However, their metal–semiconductor contact model, if physically relevant, might be relatable only to a limited low doping concentration range (for it does not account for the effects of both barrier height and width, viz., at least thermionic-field emission at moderate doping, for instance), and is incongruous with our quantification approach spanning the lightly and moderately doped to degenerate semiconductors, particularly for n -type doping (Chee et al, 2011; Chee, 2016 a ). We demonstrate strong doping contrast, not from adventitious metal–semiconductor contacts, but from Si surfaces subjected to wet (Angermann, 2008) or dry (Druckmüllerová et al, 2014) etch procedures that dissociate and further inhibit any contaminating hydrocarbon or oxygen adsorption. Specifically, we illustrate the surface band-bending effects prevailing after salient process features pertinent to Si ultra-large-scale integration (ULSI) and micro-/nano-electro-mechanical systems (MEMS/NEMS) manufacturing: e.g.…”

Optimized dopant imaging for GaN by a scanning electron microscopy

“…[23][24][25] However, the doping contrast formation is also dependent on the local external electric field, 26,27 surface band-bending field, 28,29 metal-semiconductor contacts, [30][31][32][33] refraction effects at the semiconductor-vacuum interface 34,35 and the collection solid angle of the SE detector. 24,36,37 Gallium nitride (GaN), one of the third-generation wide bandgap semiconductors, has received much attention owing to its high breakdown electric fields, high thermal conductivity, good chemical stability and strong irradiation resistance (related to its high internal and external quantum efficiencies). 38,39 Therefore, GaN has broad application prospects in the fields of optoelectronics, highpower devices and high-frequency microwave devices.…”

“…Numerical results have shown that the doping contrast is mainly a function of bulk built‐in voltages of the p‐n junction 23–25 . However, the doping contrast formation is also dependent on the local external electric field, 26,27 surface band‐bending field, 28,29 metal–semiconductor contacts, 30–33 refraction effects at the semiconductor–vacuum interface 34,35 and the collection solid angle of the SE detector 24,36,37 …”