High Resolution in 3 dimensions – TOF-SIMS in the Helium Ion Microscope

Ongoing miniaturization in semiconductor industry, nanotechnology and life science requirement further improvements for high-resolution imaging, fabrication and analysis of the produced nanostructures. Continuously shrinking object dimensions lead to an enhanced demand on spatial resolution and surface sensitivity of modern analysis techniques. Secondary ion mass spectrometry (SIMS), as one of the most powerful techniques for surface analysis, performed on the nanometer scale may comply with this demands. The direct determination of the sputtered ions mass provides elemental and molecular information and even allows to measure isotope concentrations.

During the last decades, primary ion species used in SIMS have been optimized in terms of best ionization probabilities and less molecular fragmentation. Thereby, highest mass-resolution has been one of the biggest design goals in the development of new SIMS spectrometers. In contrast to former developments, our approach aims for ultimate lateral resolution.

In recent years helium ion microscopy has been developed as a valuable tool for nanofabrication and high-resolution imaging. Helium ion microscopy (HIM) utilizes a gas field ion source to form a helium or neon ion beam with a diameter of less than 0.5 nm and 1.8 nm, respectively. This is not only possible for conducting but also for insulating samples without the need for a conductive coating. However, the existing tools suffer from the lack of a well integrated analytic method that can enrich the highly detailed morphological images with materials contrast. While the technology is relatively young several efforts have been made to add such an analytic capability. Past and ongoing activities of various labs for in situ analysis will be summarized.

Recently, we implemented time-of-flight (TOF) spectrometry to measure the energy of backscattered particles, the mass of sputtered ions [1, 2]. In future activities we intent to determine the energy loss of transmitted particles as well. Based on the findings obtained with this first approachof integrating a TOF SIMS setup, a dedicated extraction optics for secondary ions has been designed and tested (see figure 1).

The focus of this presentation will be on the technical realization of the significantly improved setup. The setup can be operated in spot mode to obtain local mass spectra or in imaging mode to obtain element maps of the specimen surface (see figure 2).

New results, drawbacks and derived conclusions for the practical use of this promising technique will be presented [4]. Similarities and differences to the also recently developed system using a sophisticated magnetic sector field analyzer will be shown [5]. We will reveal that SIMS can be performed with unprecedented lateral resolutions.

First experiments revealed a very high relative transmission which is crucial to collect enough signal from nanoparticles prior to their complete removal by ion sputtering. For m / q <= 80 u a mass resolution of delta m <= 0.3 u has been achieved. This is sufficient for many life science applications that rely on the isotope identification of light elements (e.g.: C, N). The lateral resolution of 8 nm has been evaluated using the knife edge method and a 75 % / 25 % criterion and represents a world record for spatially resolved secondary ion mass spectrometry.

The results will be compared to the theoretical limit of achievable lateral and depth resolution and the experimental and physical constraints of this approach will be reviewed.