SIMS on smallest scale

Ongoing miniaturization in semiconductor industry, nanotechnology and life science demands 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 technique for surface analysis, performed on nanometer scale may comply with this challenge. The mass of sputtered ions directly serves elemental and molecular information and even allows measuring isotope concentrations.

During last decades, primary ion species used in SIMS have optimized in terms of best ionization probabilities and small molecule 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.

Typically the lateral resolution is limited by the probe size of the primary ion beam. Minimal probe sizes can be achieved using Gas Field Ionization Sources (GFIS) in a Helium Ion Microscope (HIM). Due to the high brightness of up to 5•109 A•cm-2•sr-1 and the sharp primary ion energy of 30000 ± 1 eV spot sizes below 1 nm can be achieved. In recent years Helium Ion Microscopy has been developed as a valuable tool for nanofabrication and high-resolution imaging. However, in terms of analytical capabilities it is still lacking behind electron microscopes. Recently, we implemented Time of Flight (TOF) spectrometry to the HIM and thus enabled the measurement of the energy of backscattered particles and simultaneously the mass of sputtered ions [1, 2]. SIMS has also been achieved by adding a sophisticated magnetic sector field analyzer to the HIM [3]. In this way SIMS could be performed with unprecedented spot sizes.

Using a sufficient small ion probe size in the HIM the achievable lateral resolution for bulk samples is mainly determined by the size of the collision cascade and the sputtering process by impinging primary ions and backscattered particles. Different binary collision codes were utilized to simulate the ion-solid interaction for various beam and sample parameters. Here we discuss the origin of sputtered particles with respect to the primary ion impact. We will show simulation results for pristine samples as well as in dependence of increasing ion beam fluence. The size of the collision cascade influences the intermixing behavior and therefore the achievable depth resolution [3]. Low primary ion energies would be favorable but widen the spot size in the microscope.

Another constrain for the limited minimal resolution is the finite amount of available sample material, which is analyzed. Therefore it is crucial to collect as much information on the sample composition as possible prior the destruction or intermixing of the sample layers. Collection efficiency of the ion extraction optics should be close to one. Ultimately, the detection limit for an element is determined by the ratio of charged emitted particles during sputtering. This so-called useful yield (UY) was studied for Helium and Neon and could be enhanced by Oxygen gas flooding on the sample surface [4]. The detection limit was calculated for various minimal feature sizes for a 10nm thick layer (Fig 1). Unfortunately the charge state of sputtered particles is strongly influenced by the surface chemistry, which makes quantification hard and standards necessary.

Fig 1: Detection Limit for SIMS as a function of the minimum feature size and a layer thickness of 10 nm for various usefull yields (UY).

The available space in the HIM is strongly limited by the arrangement of multiple devices around the focal point making it necessary to extract the secondary ions and guide them to the mass spectrometer. We will discuss benefits and drawbacks of several approaches.

As a common feature for all kind of mass spectrometers, the ions have to be extracted in a narrow beam and accelerated to a sharp ion energy. Simultaneously, the applied extraction fields near the sample surface or extensions should not distort the primary beam focus and influence the optical performance of the microscope.

The TOF spectrometer presented here is minimal invasive to the microscope and therefore the high-resolution capabilities of the device are not derogated when the TOF setup is not in use. The system can be retrofitted and flange-mounted on one of the free ports the HIM offers.

The TOF measurements are triggered by blanking the primary ion beam into an existing Faraday cup and release the beam for short time windows to ensure minimal applied fluence. Using a custom made blanking electronic we could achieve pulse lengths down to 17 ns. The sputtered secondary ions are accelerated by applying a bias voltage up to +/-500 V towards the extraction system on ground potential. We used ion optics simulations and a genetic algorithm to tune the ion optics inside the spectrometer for high mass resolution and high extraction efficiency. A trajectory simulation for the secondary ions flight paths within the ion extraction system is shown in Fig. 2.

Fig 2: Simulated trajectories of 1000 ions through the ion extraction optics. For best extraction efficiency the sample (on the left side) must be oriented towards the extraction system. The optics was designed to use a maximum extraction potential of +/-500V to ensure use of standard sample holder.

The secondary ion mass spectrometry technique can be used in the microscope to measure mass spectra of selected regions of interest, to create depth profiles on small spots or to map the lateral distribution of selected element masses. The measurements are recorded in list mode and allow post evaluation. Preliminary TOF-SIMS spectra obtained from different samples are presented in Fig 3.

Fig 3: Secondary ion mass spectra using preliminary extraction system for different samples. (a) Reference sample containing carbon, silicon, nickel and gold (b) aluminum foil (c) organic glue.

TOF-SIMS in the HIM is perfectly capable of delivering an excellent elemental contrast for imaging purposes. However the lateral resolution could not reach the intrinsic imaging capabilities by just measuring the secondary electron yield. In order to combine the elemental information with the high resolution imaging correlative microscopy represents the best way. However, quantification of elements in mixed layers cannot be done from pure SIMS measurements without comparison to standards. This drawback of SIMS is partly compensated by additional measurement of TOF backscattering spectra.

In the present contribution we intent to present the technical realization of our TOF-SIMS approach and show first results, drawbacks and derived conclusions for the practical use of this promising technique. We will further stress out the benefits of correlative measurements in different modes (SIMS, backscattered particles, secondary electrons), giving a maximum of information on the subject of interest.

[1] N. Klingner, R. Heller, G. Hlawacek, J. von Borany, J.A. Notte, J. Huang, S. Facsko. Ultramicroscopy 162 (2016), 91-97
[2] R. Heller, N. Klingner, G. Hlawacek. Helium Ion Microscopy, Chapter 12, Springer (2016)
[3] T. Wirtz, D. Dowsett, P. Philipp. Helium Ion Microscopy, Chapter 13. Springer (2016)
[4] T. Wirtz, N. Vanhove, L. Pillatsch, D. Dowsett, S. Sijbrandij, J.A. Notte. Applied Physics Letters 101 (2012)