NanoFab SIMS
is designed to detect mass-to-charge ratios ranging from 1 up to 240 amu/e. Te ratio of simultaneously detectable masses exceeds 100 (max/min). Te mass resolution m/Δm (FWHM) is typically 400, but this can be improved to ∼1,000 by adjusting the size of the entrance aperture in the secondary beam line. Te transmission of the spectrometer is estimated to be about 40%. Operating modes and spatial resolution. Because the
high spatial resolution performance achievable with the Nano- Fab SIMS is not available on other systems, it is worthwhile to examine its performance and limiting factors in various operating modes. Figure 3 and Table 1 are intended to be read together to provide an overview of performance. When using the instrument as a microscope (with the
SIMS extractor retracted) to image the sample surface using the SE signal, the image resolution is < 1 nm (estimated from edge analysis) with a He+
primary beam. Tis represents the
best condition for imaging the topography of the sample. Te resolution is slightly worse (< 2 nm) when a neon beam is used. For both gases in imaging mode, the resolution is limited mainly by the primary optics (Figures 3a and 3b). With the SIMS unit inserted and operated in negative mode, primary beam SE image resolution is ∼4 nm using the
the He+
Figure 3: Semi-quantitative diagram highlighting the effects of the primary optics, secondary ion optics, and beam-sample interactions on the effective spot size for various operating modes of the instrument. Please refer to Table 1 and the article text for details and further explanation.
the magnetic sector, enabling measurement of the total non- mass-filtered secondary positive ion current (in positive mode) or the total negative secondary ion plus SE current (in negative mode). Tis detector, selected by deactivating the electrostatic deflector (Figure 2), allows the total secondary ion or SE signals to be detected and is used either for alignment or for the acqui- sition of SE images when the SIMS unit is inserted. Te secondary ion beam is steered into the magnetic sec-
tor mass separator by activating the electrostatic deflector. Four additional detectors, placed behind the magnetic sector, provide detection of the mass-filtered secondary ions. One of these detectors is at a fixed position, while the other three are moveable across the full length (450 mm) of the magnetic sector focal plane, allowing selection and parallel detection of four separate masses for depth-profiling or mapping. Tere are two options for acquiring mass spectra: scanning the mag- netic field with constant detector positions or scanning the positions of the moveable detectors at constant magnetic field. Te polarity of all components along the secondary ion-
path is switchable, so that the system can switch between posi- tive and negative ion mode within minutes. Te spectrometer
total ion current (TIC) detector to record the SE signal (while simultaneously collecting a small negative secondary ion signal) (Figure 3c). Tis imaging mode is useful for taking a reference image prior to acquiring SIMS data. Te degradation of resolu- tion is caused by the effect of the electric field in the secondary ion extractor/deflector on the primary beam. Tis electric field is used to bend the secondary ion beam from the sample into the secondary ion transport optics. Te Ne+
primary beam SE image
resolution, with the SIMS unit inserted, is about 5 nm. When using the Ne+
ion as the primary beam, with the
SIMS extractor inserted and detecting either the total ion cur- rent or the mass-filtered secondary ion signal, the lateral spatial resolution limit is about 10 nm. In this case the main limiting factor is not the size of the primary beam, but the interaction of the beam with the sample (Figure 3d). Unlike secondary electrons, which originate from the area of the surface directly impacted by the ion beam [11], secondary ions can be gener- ated and escape from areas of the sample surface that are sev- eral nanometers removed from the point of primary beam impact [8,9]. Tis 10 nm spatial resolution of analysis, limited primarily by beam-sample interactions, is unique to this instru- ment. Te large green circles at the center of Figure 3 represent the primary beam size range (50–100 nm in diameter, depend- ing on the ion source used) of current state-of-the-art instru- mentation for high spatial resolution dynamic SIMS analysis [12], where the beam size clearly dominates the contribution of beam-sample interactions.
Table 1: Limits of spatial resolution for various instrument modes. Letters in the first column relate to the internal parts of Figure 3.
Instrument
(a) Microscope (b)
Beam Mode He+ Ne+
(c) Microscope + SIMS He+ (d)
Ne+ Limit
SE Imaging SE Imaging SE Imaging
2019 May •
www.microscopy-today.com Limited mainly by
∼1 nm Primary Optics ∼2 nm
∼4 nm Primary + Secondary Optics SI Imaging or Analysis ∼10 nm Primary + Secondary Optics + Beam-Sample Interactions 25
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