Bipolar extraction for bio-imaging SIMS

Jung-Hwan Kim^*1,2, Myoung Choul Choi², Nicholas Winograd¹

¹Department of Chemistry, The Pennsylvania State University, 104 Chemistry Building, University Park, Pennsylvania 16802, USA

²Mass Spectrometry & Advanced instrumentation Group, Korea Basic Science Institute, 161 Yeongu dangi-ro, Ochang-eup, Cheongju-si, Chungbuk, 28119, South Korea

Email: junghwankim74@gmail.com [mailto:junghwankim74@gmail.com]

Cluster ion projectiles such as C₆₀⁺ and Ar_n⁺ in SIMS imaging are able to provide an enhanced ion yield, low damage cross section and a fast erosion rate for molecular depth profiling experiments.[1] A new TOF-SIMS instrument by decoupling the mass spectrometry from secondary ion generation, the J105 chemical imager from Ionoptika, utilizes continuous cluster ion sources(C₆₀⁺ and Ar_n⁺) and is capable of providing maximum spatial resolution and maximum mass resolution at the same time.[2, 3]

However, even the cluster ion sources exhibit surface charging due to the higher SIMS yield and higher secondary ion extraction field. To compensate for surface charging, we employ a bipolar extraction field for large bio-samples such as tissue sections and molecular films. The bipolar extraction field provides a stable SIMS signal for both C₆₀⁺ and Ar_n⁺ cluster ion sources. It can lead to improved molecular depth profiling and sub-micron imaging for bio-imaging applications. The mechanism behind this unique charge compensation approach will be discussed in detail.

[1] N. Winograd, Analytical Chemistry 77 (2005) 143A-149A

[2] R. Hill, P.W.M. Blenkinsopp, Applied Surface Science 231-232 (2004) 936-939

[3] R. Hill, P. Blenkinsopp, S. Thompson, J. Vickerman, J. Fletcher 43 (2011) 506-509

A Time-of-Flight Secondary Ion Mass Spectrometer using MeV energy heavy ions (MeV ToF-SIMS) has been recently commissioned at the Ruđer Bošković Institute (RBI) in Zagreb. Secondary molecular ions are extracted from the sample surface after the impact of heavy MeV Ions (primary ions) using an acceleration potential difference between the sample surface and a grounded extractor. Using this technique imaging of high mass molecules (up to 1000 Da) is possible due to the increased yield produced in SIMS with MeV primary Ions.

The first experiments have indicated a strong dependence of the secondary ion yield on experimental parameters such as primary ion type and energy as well as setup characteristics (sample orientation and distance from the extractor). In order to explore in detail these dependencies a stable geometry and reliable measurement of the ion beam current is necessary. This was a challenging task in a MeV SIMS setup since heavy ions produce a high number of secondary ions and low microbeam currents (often lower than 1 pA) cannot be reliably measured directly. Under these circumstances, especially as the sample holder for the MeV ToF SIMS setup is on a HV potential (5 kV at the RBI setup), alternative techniques were considered.

The obvious alternative for current measurement was indirect determination using the RBS, which is only effective for beam currents that are relatively high for MeV ToF SIMS. Therefore a more reliable and reproducible method has been selected which involves direct measurement using a PIN-diode detector that occasionally intercepts the ion beam. This method has been realized at the RBI microprobe beamline using a stepper motor driven PIN diode holder. In addition to the PIN diode, a tantalum foil intercepts the beam as well to enable measurements of conventional RBS signal in cases of higher currents that have to be monitored during damage cross section measurements.

In this work we present the design of the constructed system, the calibration method for the beam current measurement, its performance in terms of reproducibility, yield measurements for different sample types and a first measurements of the damage cross sections. The measurement of the damage cross section is a value independent of the setup used and may lead to a better understanding of the processes occurring during the electronic stopping and MeV SIMS.