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Techinical Info

 

Theory of TOF-SIMS

 

Introduction

 

Time-of-Flight-Secondary Ion Mass Spectroscopy: TOF-SIMS is very simple in concept; a packet of ions given the same kinetic energy fly down an evacuated, field-free tube and arrive at an ion detector in order of their mass-to-charge (m/z) values.  The detector output current, measure as function of time, can easily be converted into a mass spectrum.  It has no fundamental limit, in theory, on the range of m/z values analyzable.

 


 

Basic Principle

 

When energetic "primary" ions (typically, several keV) bombard a sample surface in vacuum, various atoms and molecules are 'sputtered' from that surface. A small percentage of those sputtered species are ionized naturally and may be extracted by an electric field and then transferred into a mass spectrometer, where their mass-to-charge ratios are measured.

 

 

Fig.1 – The diagram showing drift tube.

 

Time-of-flight mass spectrometry is conceptually the simplest means of mass separation used in SIMS. In ToF analysis pulses of secondary ions are accelerated to a given potential (2 to 8 keV) such that all ions possess the approximately same kinetic energy; they are then allowed to drift through a field free space before striking the detector.

 

Ionization: Energy from the primary ion is transferred into the sample surface in such a way that, in the immediate vicinity of the impact site, considerable fragmentation of the original surface occurs, resulting in the release primarily of atomic and low mass molecular species. Further out from the impact site the energy imparted to the substrate results in the release of higher mass molecular species.

 

Static Regime: There is a regime of primary ion bombardment, known as the "static" SIMS regime, during which the number of primary ions impacting the surface per unit area is so low that each successive primary ion strikes a virgin area of the surface. This "static" SIMS regime does not end abruptly, but it is generally accepted to be limited to ~1012- 1013 primary ions/cm2.  TOF-SIMS follows the rule of static regime.

 

TRIFT analyzer: The TRIFT analyzer continually re-focuses the secondary ions within the spectrometer. The analyzer has the maximum angular and energy acceptance, which achieves high spatial resolution and mass resolution by using triple focus hemispherical electrostatic analyzer which has excellent ion transmitting.

 

 

Fig. 2 – diagram of TRIFT analyzer

 


 

Application of TOF-SIMS

 

Analysis Alq3 OLED material: the high sensitivity and low background provided by the TRIFT spectrometer combined with a new ion source provide an enhanced capability to detect high mass fragments from materials such as Alq3.

 

 

Fig.3-positive spectrum of Alq3

 

Imaging rough surface: The images at the top of Figure 4 were acquired in the high solid angle of collection mode and show very little topographic contrast in either the Ni+ or the In+ image.it is clearly superior for chemical imaging of rough surface.

 

In+  Ni+   

               

Fig.4: ion image in highest solid angle of collection mode

 


 

Summing-up

 

(TOF-SIMS) has developed into a powerful tool for surface micro-analytical applications. The technique permits applications which are not attainable with conventional dynamic SIMS (DSIMS). TOF-SIMS is capable of:

 

1) Simultaneous detection of ions over a virtually unlimited mass range;

2) Yielding high mass resolution;

3) Yielding accurate mass determination and accurate ion transmission;

4) Analyzing a sample surface for inorganic and organic contaminants simultaneously with micrometer and sub-micrometer lateral resolutions.

 

For more details, please visit our Brochure or Application Notes.