Resolution limits of secondary electron dopant contrast in helium ion and scanning electron microscopy

Towards quantification of doping in gallium arsenide nanostructures by low-energy scanning electron microscopy and conductive atomic force microscopy

We calculate a universal shift in work function of 59.4 meV per decade of dopant concentration change that applies to all doped semiconductors and from this use Monte Carlo simulations to simulate the resulting change in secondary electron yield for doped GaAs. We then compare experimental images of doped GaAs layers from scanning electron microscopy and conductive atomic force microscopy. Kelvin probe force microscopy allows to directly measure and map local work function changes, but values measured are often smaller, typically only around half, of what theory predicts for perfectly clean surfaces.

Comparative study of image contrast in scanning electron microscope and helium ion microscope

Images of Ga⁺ -implanted amorphous silicon layers in a 110 n-type silicon substrate have been collected by a range of detectors in a scanning electron microscope and a helium ion microscope. The effects of the implantation dose and imaging parameters (beam energy, dwell time, etc.) on the image contrast were investigated. We demonstrate a similar relationship for both the helium ion microscope Everhart-Thornley and scanning electron microscope Inlens detectors between the contrast of the images and the Ga⁺ density and imaging parameters. These results also show that dynamic charging effects have a significant impact on the quantification of the helium ion microscope and scanning electron microscope contrast.

Toward site-specific dopant contrast in scanning electron microscopy

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.

Heating-induced variations of secondary electron emission from ion-cleaned copper samples

Secondary electron (SE) emission due to electron impact depends strongly on surface conditions. The variations of SE yield and spectrum with the heating temperature of Ar-ion-cleaned oxygen-free copper samples are therefore measured in situ in a multifunctional ultrahigh vacuum system. The SE yield and the SE spectrum are observed to increase and to narrow, respectively, after sample heating. The maximum SE yield increases from 0.97 before heating to 1.25 after heating at ∼313 °C, and the corresponding full width at half maximum of SE spectrum decreases considerably from 9.3 to 5.5 eV. More CO2 and Ar ions are shown to desorb at a higher heating temperature by residual gas analysis, indicating their contribution to the reduction in work function and surface potential barrier. Ar-ion desorption appears to affect the SE spectrum more than the SE yield. The obtained results provide a new insight into complicated surface influences on SE emission in thermal applications of scanning electron microscopy.

In situ thickness assessment during ion milling of a free-standing membrane using transmission helium ion microscopy

We describe a novel method for in situ measurement of the local thickness of a freely suspended solid-state membrane after thinning with a focused helium ion beam. The technique utilizes a custom stage for the helium ion microscope that allows the secondary electron detector used for normal imaging to collect information from ions transmitted through the sample. We find that relative brightness in the transmission image scales directly with the membrane thickness as determined by atomic force microscopy measurements.

Nano-engineering with a focused helium ion beam

Although Helium Ion Microscopy (HIM) was introduced only a few years ago, many new application fields are budding. The connecting factor between these novel applications is the unique interaction of the primary helium ion beam with the sample material at and just below its surface. In particular, the HIM secondary electron (SE) signal stems from an area that is very well localized around the point of incidence of the primary beam. This makes the HIM well-suited for both high-resolution imaging as well as high resolution nanofabrication. Another advantage in nanofabrication is the low ion backscattering fraction, leading to a weak proximity effect. The lack of a quantitative materials analysis mode (like EDX in Scanning Electron Microscopy, SEM) and a relatively low beam current as compared to the SEM and the Gallium Focused Ion Beam are the present drawbacks of the HIM.