Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS)


[Description] [Instrument] [Sample Requirement] [Application

Description

Principle of SIMS: In a SIMS experiment, a sample is bombarded with a primary ion beam of kinetic energy of 1-25 keV. Upon bombardment, the kinetic energy and momentum of the impinging ions are transferred to the sample via a collision cascade process.  When the transferred recoil energy is higher than the surface binding energy, secondary particles are emitted from the surface. A very small fraction (<1%) of these particles consists of both positive and negative ions. The collection of these positive or negative ions by a mass spectrometer constitutes the basis of SIMS. A SIMS experiment can be carried out in the dynamic or static mode. In the dynamic mode, the incident ion current is high and consequently the surface is rapidly damaged and eroded away. Therefore, dynamic SIMS is used to provide in-depth distribution of elements (depth profiling). In static mode, the total incident ion density is kept below 1012 ions/cm2 (static conditions). In this case, only one per thousand of the surface atoms is directly bombarded by the primary ion (assuming a surface atom density of 1015 atoms/cm2 1 monolayer). The surface damage is negligible and the emitted secondary ions would contain not only atomic species but also molecular species (Fig. 1). Therefore, static SIMS is used to provide information about the molecular surface structure of materials.

 
 


 
Figure 1: Schematic diagram showing the basic principle of Static SIMS [1]

 ToF–SIMS: ToF-SIMS uses the state-of-the art ToF mass spectrometer to detect the secondary ions. It is optimized for static SIMS analysis. In the ToF-SIMS experiment, a pulsed beam strikes a sample surface and the secondary ions are extracted from the sample and accelerated into a field-free ToF mass spectrometer with a common energy. If the initial kinetic energy is neglected, the secondary ions will have the same kinetic energy E and their velocity in the filed-free drift depends only on their mass:

                                                             (1)

where q, m and v are respectively the charge, mass and velocity of the secondary ions and V is accelerated voltage. If these ions travel to a flight path (L) in the drift before reaching the detector, their time-of-flight (t) is proportional to the square root of their mass:

                                                                                    (2)

Thus the light ions will have higher velocity and will reach the detector sooner than the heavier ones. In this way, secondary ions differing in mass are separated according to their flight time. The time difference between the primary ion pulse (start pulse) and the pulse produced by secondary ion at the detector (stop pulse) is measured and converted to an amplitude signal via a time-to-digital converter (TDC). The pulses are then sorted to produce mass histogram.

ToF-SIMS experiments can be operated in three different modes and provide different information:

          Surface spectroscopy mode: In this case, ToF-SIMS analysis is carried out under static conditions. The secondary ions coming from the sample surface are very characteristic of the original surface structure and provide a “fingerprint” of the molecular structure of the material. Static ToF-SIMS is very surface sensitive and only probes the top one or two atomic layers (< 1 nm) of the material.

          Surface imaging mode: By scanning a finely focused primary ion beam over the surface, the mass resolved ion images can be obtained. These ion images provide a two dimensional distribution of different chemical species on surface.

          Depth profiling mode: In contrast to static mode, a high ion current is used to sputter the material. The in-depth distribution of elements can be obtained in this case.  

ToF-SIMS is extremely sensitive, with detection limit down to ppb level. However, it has a difficulty in providing quantitative data.

 Reference

[1]  C.-M. Chan and L.T. Weng, Reviews in Chemical Engineering, 16 (2000) 341-408

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Instrument at MCPF

MCPF has a Physical Electronics 7200 ToF-SIMS spectrometer. This instrument has the full capabilities of high mass-resolution surface spectroscopy, high spatial resolution imaging and depth profiling. Some important features of the instrument are:

      Cs+ primary source for high mass range surface spectroscopy analysis

      Ga+ gun for high spatial resolution chemical mapping (minimum beam size ca. 0.25 m)

      A two-stage reflectron ToF mass analyzer

      A 256 stops time-to-digital converter (TDC) with 156 ps time resolution combined with multichannel plates and a post-acceleration voltage of up to 15 kV for ion detection

      Mass resolution ca. 10 000

      A low energy (0 – 70 eV) electron flood gun for insulating materials

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Sample requirement

As ToF-SIMS experiments are carried out under ultra-high vacuum, the samples to be analysed must be vacuum compatible. Both conductive and insulating materials can be analysed.


Application examples

*           ToF-SIMS analysis on organic silicone

*           ToF-SIMS analysis on polymer blend

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