NanoFab SIMS: High Spatial Resolution Imaging and Analysis Using Inert-Gas Ion Beams

Sybren Sijbrandij,* Alexander Lombardi, Alain Sireuil, Fouzia Khanom,

Brett Lewis, Christelle Guillermier, Doug Runt, and John Notte Carl Zeiss SMT, Inc., PCS Integration Center, 1 Corporation Way, Peabody, MA 01960 *

Abstract: By combining a focused inert-gas ion beam instrument and a custom magnetic-sector mass spectrometer, high spatial resolution imaging and chemical analysis are provided within a single instrument. Sub-nanometer image resolution is achieved by secondary electron (SE) imaging, limited only by the probe-size of the primary beam, while the spatial resolution for chemical mapping obtained via secondary ion mass spectrometry (SIMS) is limited mainly by beam-sample inter- actions to about 10 nm. This article introduces the background behind this development, describes the instrument and its various operating modes, and presents examples of its applications.

Keywords: Helium ion microscopy, secondary electron (SE) imaging, neon ion SIMS, high spatial resolution analysis, NanoFab

Introduction Te ZEISS ORION Helium Ion Microscope (HIM) was

commercially introduced in 2006 [1–3] and has rapidly become a widely used tool for high-resolution imaging and nano-fabri- cation. Te HIM offers sub-nanometer resolution, large depth of field, and special image contrast mechanisms. To add a mate- rials analysis capability, a helium backscatter spectrometer was added in 2009 [4], though the applications for that technique were somewhat limited. By 2012 the capability of using neon was added [5,6] and commercialized as the ZEISS ORION NanoFab. Te neon ion beam, with its increased sputter rate compared to helium, expanded the application-space for nanoscale sample modification processes [7] and enabled secondary ion imaging. Analytical capability was added to the NanoFab in the form

of a custom-designed secondary ion mass spectrometer (SIMS) [8–10] developed through collaboration with the Luxembourg Institute of Science and Technology (LIST) and Lion Nano- Systems, starting in 2012. Te well-developed SIMS technique is based on the identification of secondary ions characteristic of surface elements, when these elements are sputtered by bom- bardment with a primary ion beam. Tis is a powerful method for analyzing surfaces because of its excellent sensitivity for ele- ments (detection limits down to the ppb), high dynamic range, and its ability to differentiate isotopes of most elements. Te SIMS designed for the NanoFab was optimized to take advan- tage of the small primary beam probe-size, resulting in the high- est available spatial resolution SIMS capability. A 2017 prototype of the ZEISS NanoFab SIMS is shown in Figure 1. Te remain- der of this article focuses mainly on instrumental aspects of the combined NanoFab SIMS instrument. While a few results are included here, another article focused primarily on applications of the new instrument will appear in an upcoming issue.

Instrumentation Primary ion beam platform and microscope. Te defin- ing characteristic of the NanoFab is its gas field-ionization

22 doi:10.1017/S1551929519000440

source (GFIS), with the following optical properties: high brightness > 5 × 109

A cm−2 sr−1

< 0.1 nm, and low energy spread < 1 eV. Te ion beam can be focused to a probe-size of < 0.5 nm for He+ for Ne+

, small virtual source size and < 1.9 nm

, when matched with a well-designed electrostatic ion

optical column. Te beam current is adjustable from 100 fA to 100 pA. Other crucial parts of the optical system include high-stability high-voltage power supplies and a liquid- nitrogen-based cryogenic system for cooling the source. Te acceleration voltage can be operated in the range of 10–30 kV. Te vacuum system achieves ∼10−10 source region and ∼10−7

Torr base pressure in the Torr in the work-chamber. To avoid

issues with vibration, the system is solidly constructed with a 2-stage anti-vibration system at its base. Te work-chamber is equipped with a 5-axis motorized

stage (X,Y,Z, Rotation, Tilt). Samples are transported onto the stage via a load-lock. Te work-chamber also houses an Everhart-Tornley (ET) detector to collect the SE signal when imaging with the He+

beam and an electron flood-gun for

charge neutralization on insulating samples. Te chamber has numerous extra ports to accommodate additional equipment and accessories. Te system is computer-controlled for all nor- mal operating functions, such as instrument setup and control, scan control, and image acquisition. Spectrometer integration. Te spectrometer is designed

to fit onto the NanoFab without major platform modifications. Te spectrometer comes with its own electronics rack—housing power supplies, motion controllers, and data-acquisition hard- ware—as well as a computer to facilitate instrument control and communications with the NanoFab. Similar to the man- ner in which an energy-dispersive X-ray spectroscopy (EDS) system is typically added to a scanning electron microscope (SEM), the integration between the SIMS system and Nano- Fab platform is straightforward. Te SIMS spectrometer is run with its own dedicated soſtware. It uses the NanoFab’s remote API (application programming interface) via an Ethernet con- nection to access features, functions, and data on the NanoFab. When the spectrometer is activated, the NanoFab’s scanning system is placed in external mode, and the SIMS scanning sys- tem takes over control of the primary ion beam. Te API is also used for the communication of NanoFab settings so that these can be saved within the SIMS data files. Where safety is concerned, hardware interlocks are used. Te SIMS soſtware performs the main instrument control

and data-acquisition functions. Te SIMS instrument control functions include setting and reading instrument voltages, magnetic field, specimen positions, and the reading and display of ion detection rates (for instrument tuning). Data-acquisition • 2019 May

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