Dopant profiling based on scanning electron and helium ion microscopy

Abstract

Highlights• Strong doping contrast from n-type regions in the SHIM without energy-filtering.• Sensitivity limits are established of the SHIM and SEM techniques.• We discuss the impact of SHIM imaging conditions on quantitative dopant profiling.• Doping contrast stems from different surface layer thicknesses in the SHIM and SEM.In this paper, we evaluate and compare doping contrast generated inside the scanning electron microscope (SEM) and scanning helium ion microscope (SHIM). Specialised energy-filtering techniques are often required to produce strong doping contrast to map donor distributions using the secondary electron (SE) signal in the SEM. However, strong doping contrast can be obtained from n-type regions in the SHIM, even without energy-filtering. This SHIM technique is more sensitive than the SEM to donor density changes above its sensitivity threshold, i.e. 10 16 or 10 17 donors cm -3 respectively on specimens with or without a p-n junction; its sensitivity limit is well above 2 × 10 17 acceptors cm -3 on specimens with or without a p-n junction. Good correlation is found between the widths and slopes of experimentally measured doping contrast profiles of thin p-layers and the calculated widths and slopes of the potential energy distributions across these layers, at a depth of 1 -3 nm and 5 -10 nm below the surface in the SHIM and the SEM respectively. This is consistent with the mean escape depth of SEs in Si being about 1.8 nm and 7 nm in the SHIM and SEM respectively, and we conclude that short escape depth, low energy SE signals are most suitable for donor profiling. Keywords: Doping contrast; Secondary electron energy filtering; Sensitivity limit; Electric potentials; Escape depth; p-n junction I. INTRODUCTIONThe future of the semiconductor industry depends critically on the ability to map dopants rapidly at high spatial resolution, and with high sensitivity. New spectroscopic techniques are in considerable demand to cope with the advent of next generation semiconductor devices having ultra-shallow junctions. Hence, dopant profiling at a resolution of sub-10 nm and detection sensitivity over a range of ~10 16 -10 20 dopants cm -3 are important requisites. Using an SEM, it is possible to provide a rapid and contactless technique for the two-dimensional mapping of electrically active dopant profiles based on SE doping contrast (Perovic et al., 1995;Turan et al., 1996;Castell et al., 1999;Elliott et al., 2002). Under standard imaging conditions, the p-type regions appear bright and the n-type regions appear dark, therefore doping contrast can be used to determine the position of electrical p-n junctions. The doping contrast mechanism is due to the built-in electric field across a p-n junction, modified by the effects of surface bandbending and external patch fields as the SEs are scattered by the surface electric potentials (Chee et al., 2011). Oatley et al. (1957) first studied SE doping contrast from p-n junctions where it was shown that reverse biasing enhances contrast by changing the electric ...

Highest resolution chemical imaging based on secondary ion mass spectrometry performed on the helium ion microscope

“…Defect localisation within conductive structures at the nano-scale (sub-10 nm), while minimising the damage applied to the analysed zone, can be performed within the HIM by using the voltage contrast mechanism [22]. Another effect that comes into play for investigations of semiconductors is the static capacitive effect, which is used to reveal dopant concentration changes even if quantification remains challenging [23,24]. An application, in which the electron flood gun has demonstrated its usefulness, has been reported by Baggott et al [25] for imaging ceramic silicon nitride samples.…”

Evaluation of secondary electron intensities for dopant profiling in ion implanted semiconductors: a correlative study combining SE, SIMS and ECV methods

“…in the region where SEs are emitted from. This improves the spatial resolution in HIM compared to scanning electron microscopy (SEM) [10,11]. It was also shown that the SE emission in HIM is more sensitive to the surface electronic variations compared to SEM, which makes HIM more suitable compared to SEM for dopant profiling with improved sensitivity and spatial resolution [12].…”

“…The emergence of helium ion microscopy (HIM) has further improved the spatial resolution and sensitivity of dopant profiling using SE imaging [7][8][9][10][11]. HIM produces a smaller spot size owing to the atomically sharp ion source and lower de Broglie wavelength (due to the heavier ion mass).…”

Co-Registered In Situ Secondary Electron and Mass Spectral Imaging on the Helium Ion Microscope Demonstrated Using Lithium Titanate and Magnesium Oxide Nanoparticles

“…Such domains are no longer visible when sputter coated with 10 nm gold layers to make them conductive for SEM analysis . The HIM has also been used for imaging of dopant profiles in semiconductor materials and visualization of the conductivity of graphene layers . Channelling contrast (both in SE and BSI modes) has been used to determine crystallographic orientation in gold films. , The high resolution milling capabilities of helium have been used to fabricate a wide range of nanoscale structures/devices, including nanopores for biomolecule identification, graphene nanodevices, ,, nanostructured silicon nitride membranes, and nanophotonic structures with smaller feature sizes and improved optical properties. − The addition of neon − has extended the milling/machining capabilities of the tool by providing increased milling rates and lower implantation and subsurface damage.…”

“…Such domains are no longer visible when sputter coated with 10 nm gold layers to make them conductive for SEM analysis. 14 The HIM has also been used for imaging of dopant profiles in semiconductor materials 15 and visualization of the conductivity of graphene layers. 16 Channelling contrast (both in SE and BSI modes) has been used to determine crystallographic orientation in gold films.…”