SIMS introduction and basic principles

SIMS is a surface analytical technique used in a wide range of experiments. This section contains a brief explanation of depth profiling using dynamic SIMS with particular attention to unusual aspects that cause deviations from the ideal profiles, referred to as SIMS artefacts. The technique operates by bombardment of the sample surface with a primary ion beam causing sputtering of particles, followed by mass spectrometry of the emitted secondary ions. Much of the SIMS used in this thesis was carried out at ITC – IRST, Trento, using a Cameca Wf SC Ultra instrument (36, 37).

0 1 2 3 4 5 6 7 8 9 10 11 12 13 0 20 40 60 80 100 yield (cou nts p e r 5 µ C) Depth (nm) Computer program surface approximation 3 keV As as-implanted. 100 keV He 111

Figure 4.21 Example of the difference between depth spectra calibrated using the surface approximation method and the computer program for a 3 keV As as-implanted sample.

SIMS experiments were also carried out at IMEC, Belgium, using an Atomika 4500 instrument in each case conditions for these experiments are described. Unless otherwise stated, the SIMS measurements presented in this thesis were carried out at ITC – IRST.

In a SIMS instrument an ion source is used to produce primary ions that are formed into a beam by an ion optical column. Often instruments have several different sources to allow a range of ions to be used. The most common primary beams to use are O2+, O-, Ar+, and Cs+. Oxygen ion bombardment enhances the secondary ion yields of

electropositive elements and produces more uniform erosion of the sample than inert gas bombardment. O2+ is the general purpose primary beam for semiconductor samples

where sample charging is not a problem. For insulating samples when sample charging is a problem an electron flood gun can be used to neutralise the sample and sample charging can also be reduced using an O- beam. A beam of Cs+ ions is generally used for optimum sensitivity with electronegative elements (6). In the present studies Cs+ has been used to profile As and F, while O2+ was used for profiling B (38, 39).

Ion optics allows the ions to be accelerated and focused into a small beam that can be rastered over an area of the sample. In these experiments a beam energy of around 4 – 6 keV was used, but the sample was biased to create an impact energy of 0.25 – 1 keV. The beam was rastered over an area of typically around 200 µm2 and data recorded over an area of around 70 µm2 _{depending on the particular experiment.}

Incidence angles of 44 – 76° were obtained in the Cameca, which depended on the impact energy. Normal incidence was used in the Atomika. Erosion rates of 0.2 – 0.9 Å/s were obtained (38, 39). Sputtered secondary ions are energy filtered. This is useful for removing interference of molecular species from an atomic species with the same mass. Secondary ions can then be mass analysed in a mass spectrometer before being detected. Either magnetic sector spectrometers or quadrupole analysers are used. Either positive or negative secondary ions can be monitored (6). Negative As and F ions and positive B ions were collected (38, 39).

The number of ions sputtered from the surface of the sample provides a means for measuring the composition of the sample. The sputtering yield is not linearly related to the concentration of an element present in a sample since the secondary ion yield varies with element (e.g. As or B), with changes to the matrix, (such as going from SiO2

to Si) and varies with the primary bombarding species (6). The erosion rate is also not constant during the first couple of nm. Depth scales are often calibrated on the basis of the final crater depth measurement and assuming a constant erosion rate. The use of a

laser profilometer, for a dynamic depth scale assessment, i.e. at every stage of the developing depth profile the instantaneous actual depth is being determined, can in principle improve this situation. The non constant erosion rate leads to SIMS analysis being unreliable in terms of the concentration quantification and the depth scale in the first couple of nm. (6, 38, 39). These effects are evident with samples spectra shown in this thesis.

The collision cascade produces atomic mixing of the species within the subsurface region. Consequently when species are sputtered from the surface they may not have been occupying the sites at depths that they originally occupied in the unsputtered sample. Once an equilibrium situation is established, the implanted primary ions and collision cascade region are always present and move through the sample ahead of the analysed surface (6). This collisional mixing causes a broadening of the profile and limits the depth resolution obtainable. Using a lower impact energy produces a smaller collision cascade and this can improve the resolution. A reasonable agreement of the downslope of SIMS and MEIS profiles of As as-implanted samples has been found using the Cameca instrument for low impact energies (0.25 – 0.5 keV) and impact angles of around 45° suggesting that for these conditions collisional mixing effects were not appreciable (38, 39).

Comparison of SIMS results with MEIS results have been carried out to try and understand the best methods to quantify the SIMS data accounting for SIMS artefacts (40, 42). This took the form of optimising the calibration of the SIMS profiles. Dopant profiles were calibrated to different monitoring matrix species, and it was found that the SIMS profiles which were calibrated to the 29Si2- monitoring species gave profiles with

the best agreement between MEIS and SIMS. Even so, discrepancies occurred in the first 3 nm. It has been suggested that a shift to the depth scale needs to be applied to align the SIMS profiles to the MEIS ones. Work is continuing in this area.

In spite of the weakness of SIMS, the technique has significant strengths and offers useful comparisons with the MEIS results. SIMS can profile light elements for which the MEIS sensitivity is too low. The sensitivity to detect atoms of the order of 1016 at/cm3 is much greater than MEIS. SIMS registers all of the atoms, whereas in MEIS, in the double alignment configuration, atoms in substitutional lattice sites are invisible to the analysing beam. This provides an interesting comparison of the techniques and gives information as to the fraction of dopant atoms that have taken up substitutional positions. The depth range is also much greater with SIMS than that of

MEIS, using the scattering conditions typical in this project, and this again is useful for studying the diffusion of dopant deeper into a sample.

4.4 Other analysis techniques used