Depth profiling relative sensitivity factors

Secondary ion mass spectrometry (SIMS) was used to characterise the coatings for their Ti, Ru and O stoichiometry on the surface and as a function of depth into the coating. A PHI 6650 Quadrupole mass spectrometer, with Cs+ as the ion source was used in these studies. The conversion of the measured secondary ion counts to concentration was performed using relative sensitivity factors, which were first determined with a standard sample containing known amounts of RuC>2 and TiC>2. All of the SIMS profiles were repeated several times, to determine the measurement precision, which was typically +10%. 

The energy of the primary electron beam was 2.5 keV and the energy of the Ar+ used in the depth profiles was 2 keV. The relative sensitivity factors were determined using pure standard samples and are presented in Table 14.1. Some samples were studied using an Auger spectrometer equipped with a retarding field analyser. The thickness of the films was... 

Mass Spectrometry A Practical Handbook for Depth Pro ling and Bulk Impurity Analysis, by Wilson et al. [69]. This book is focused on profiling and quantitative analysis primarily in semiconductors and contains many tables of relative sensitivity factor (RSF) data, profile illustrations, and SIMS spectra. 

The final stage of the SIMS analysis was to quantify the profiles obtained in the initial trial. For this a standard was prepared by implanting a known dose (atoms cm ) of Li ions into the target. This was analysed by SIMS, tracking both Li and matrix ion species, and the crater depth measured by profilometry. This allows calculation of a relative sensitivity factor (RSF) which converts the Li-to-matrix ion intensity ratio into a Li concentration in atoms cm as a function of depth. When the unknown was measured, with Li and the same matrix ion signals recorded, then the RSF was applied to the signal ratio to give the Li level in the unknown. 

Fig. 16. (a) Depth profile using 3 keV Cs" " projectiles of a focused ion beam implant of 25 keV Ga+ in InP monitoring GaCs" " and InCs+ ions, (b) Correlation of the signals shown in (a) the slope yields the relative sensitivity factor SoaCs/ SinCs- (c) The SIMS data of (a) converted into the Ga concentration distribution using the factors derived in (b) [59]. Reprinted with permission firom H. Gnaser, J. Vac. Sci. Tech. A12, 452 (1994), 1994, The American Institute of Physics. 

The atomic concentration profile shows an O/Si ratio of about 1.5, which is lower than the expected O/Si ratio of 2. Standard relative sensitivity factors (RSFs) were used in the calculation in order to demonstrate a drawback of ion sputtering, where one species is removed at a faster rate as compared to another. This effect referred to as preferential sputtering usually results in the lighter atoms being removed from the material by the ion beam as compared to the heavier ones. When this occurs, the composition of the surface exposed by the ion beam is altered until a composition is reached where all of the atoms present in the material are removed at an equal rate. Such a layer is referred to as an altered layer. Provided that the relative composition of the original film does not vary as a function of depth, once the altered layer is reached, the relative ratios of the elements should then remain constant throughout the profile. When this is the case and it is possible... 

A relative sensitivity factor (RSF) can be obtained from the depth profile of an ion implant standard. 

In principle GD-MS is very well suited for analysis of layers, also, and all concepts developed for SNMS (Sect. 3.3) can be used to calculate the concentration-depth profile from the measured intensity-time profile by use of relative or absolute sensitivity factors [3.199]. So far, however, acceptance of this technique is hesitant compared with GD-OES. The main factors limiting wider acceptance are the greater cost of the instrument and the fact that no commercial ion source has yet been optimized for this purpose. The literature therefore contains only preliminary results from analysis of layers obtained with either modified sources of the commercial instrument [3.200, 3.201] or with homebuilt sources coupled to quadrupole [3.199], sector field [3.202], or time-of-flight instruments [3.203]. To summarize, the future success of GD-MS in this field of application strongly depends on the availability of commercial sources with adequate depth resolution comparable with that of GD-OES. 

Compared with passive radiometric techniques, the fluorosensor methods for phytoplankton have several attractive features which are unique. These include relative insensitivity to daylight or cloudiness, specifity of response to Chi, and the potential for depth profiling using range-gating techniques. On the other hand, they are sensitive to other factors such as energy transfer, water composition, algae species, and sediments [8, 9]. 

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