TOF‐SIMS Spectra Introduction - mikee9265/SIMS-Wiki GitHub Wiki
A first glance at a mass spectrum from a TOF-SIMS instrument can be bewildering. The Y-axis of the raw data is simply secondary ion counts. The X-axis is, in its original form, the time to digital converter (TDC) channel number, each channel representing a quantum of time set by the TDC bin size (e.g., 156 ps). The X axis is rarely plotted in units of time, however. Instead the channels are converted to m/e (mass to charge ratio) with a proper calibration. Secondary ions with the same charge (usually 1) have been given essentially the same energy; so given the fact that E = (1/2)m(x/t)2, where E is the energy, m is the mass, x is the flight distance, and t is the time, secondary ions with different masses will have different arrival times and, therefore, will fall into different TDC channels. Solving for t one finds t = x(m/2E)0.5. E and x are constant. The flight time t is therefore proportional to the square root of the mass. The higher the mass, the longer the flight times, but the relationship is not linear. The larger the mass, the fewer channels there are in each mass interval. Thus a restriction of the modern TDC with a set channel size is decreasing mass resolution with mass. Fortunately, in most situations the channel duration is small enough so that this has little practical impact on the results.
Unlike many other mass spectrometric methods, TOF-SIMS has as much as six orders of magnitude of dynamic range, so to see all the peaks in the spectrum, it is convenient to set the Y-axis to be the log of the secondary ion counts.
This plot reveals a peak at almost every integral mass. Blown up, a high-resolution spectrum will have many peaks at each integral mass. The most significant peaks are often not the most intense. Clearly there is a wealth of data here, but the task of interpreting it may seem daunting.
The reason there can be many peaks at each integral mass is that the elements of the periodic table have masses that are nearly but never quite integral (except for carbon, C), and the mass differences from the integral “nominal” mass varies from element to element. Therefore, species with different combinations of elements or different numbers of the same elements that turn out to have the same nominal mass are nonetheless distinguishable from each other when the analysis is performed with sufficient mass resolution. A high mass resolution measurement allows the determination of the empirical formulae for many of the peaks in the spectrum. This next figure shows an example of how a high mass resolution TOF-SIMS spectrum can reveal many peaks at a single nominal mass.
The sputter event can produce ions of either polarity. That is, ions will be produced with (usually a single) positive charge or with (again, usually a single) negative charge. In the commercial TOF-SIMS instruments currently available, you cannot detect ions of both polarities at once. The extraction field that gives the secondary ions most of their energy must be set at the start of each primary ion pulse, and depending on its polarity, will either accelerate positive ions away from the sample into the spectrometer while simultaneously driving any negative ions formed back into the sample or it will accelerate negative ions into the spectrometer and drive the positive ions back into the sample. In fact, the whole instrument is placed into either positive or negative ion mode, with most of the voltages on the ion optics of the spectrometer almost exactly reversed (switching from pulse to pulse puts an enormous strain off . system's electronics, and so such switching is rarely done within a given acquisition).
Neutral species, materials sputtered from the surface that are not ionized, comprise the majority of the sputtered material but they are not detected in TOF-SIMS. Many methods have been tried for post-ionization. For various reasons, none of these methods have achieved the success of the TOF-SIMS method itself.
The surfaces that have been and will be analyzed by TOF-SIMS are diverse, and no single work can capture all the spectral features one may find. Much interpretation is made possible by the fact that much is known about these surfaces before the analysis. Labs that specialize in the analysis of one type of sample or another will have expertise suited to the interpretation of the spectra from those types of samples. Industrial labs maintain private collections of standard spectra suited to their work. For any given sample set, the general guidelines given that follow will have to be supplemented with specialized knowledge.