Ultra-shallow Depth Profiling

In recent years TOF SIMS has also proved to he a very powerful tool for ultra-shallow depth profiling, having the advantage of simultaneously detecting all elements of interest. The dual beam mode [3.41], in particular, (see Sect. 3.2.2.1) enables optimized depth resolution, because sputtering conditions can be independently optimized. 

Today dynamic SIMS is a standard technique for measurement of trace elements in semiconductors, high performance materials, coatings, and minerals. The main advantages of the method are excellent sensitivity (detection limit below 1 pmol mol ) for all elements, the isotopic sensitivity, the inherent possibility of measuring depth profiles, and the capability of fast direct imaging and 3D species distribution. 

The first SIMS instrument was built by Herzog and Viehboeck [3.42] in Vienna in the nineteen-forties. In the early nineteen-sixties, Herzog and Liebl [3.43] built the first sophisticated SIMS instrument at about the same time as Castaing and Slodzian in Paris [3.44]. In 1970, Benninghoven was the first to use the acronym SIMS [3.45]. 

The bombardment of a sample with a dose of high energetic primary ions (1 to 20 keV) results in the destruction of the initial surface and near-surface regions (Sect. 3.1.1). If the primary ion dose is higher than 10 ions mm the assumption of an initial, intact surface is no longer true. A sputter equilibrium is reached at a depth greater than the implantation depth of the primary ions. The permanent bombardment of the sample with primary ions leads to several sputter effects more or less present on any sputtered surface, irrespective of the instrumental method (AES, SIMS, GDOES. ..). 

Compensation of Preferential Sputtering. The species with the lower sputter yield is enriched at the surface. This effect is called preferential sputtering and complicates, e. g.. Auger measurements. The enrichment compensates for the different sputter yields of the compound or alloy elements thus in dynamic SIMS (and other dynamic techniques in which the signal is derived from the sputtered particles, e.g. SNMS, GD-MS, and GD-OES), the flux of sputtered atoms has the same composition as the sample. 

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From Fig. 4.20, it is apparent that the difficulties in data interpretation in the near surface region are considerable. Shallow and ultra shallow depth profiles are important to the semiconductor industry so procedures for their analysis have been extensively studied [14, 44 9]. The ion mixing region or the extent of the collision cascade from the primary beam needs to be less than the profile of the element of interest to have good depth resolution. 

Figure 3.17 depicts an ultra-shallow TOF SIMS depth profile of a 100-eV B-implant in Si, capped with 17.3 nm Si. The measurement was performed with 600-eV SF5-sputtering and with 02-flooding. The original wafer surface, into which the B was implanted, is indicated by the maxima of the alkali- and C-signals. Because of these contaminants, a minimum is observed in the °Si-signal. The dynamic range of the B-profile is more than 3.5 decades and the depth resolution is <0.5 nm. 

Depth profiling studies of ultra shallow implanted P using NRA has been made by Kobayashi and Gibson (1999) in the energy range of 5-80 keV using P((x, p) Si reaction. 

Kongo, C., Tomita, M., Takenaka, M. (2004) Accurate depth profiling for ultra-shallow implants using backside-SIM. Applied Surface Science, 231-232, 613-611. 

Xuhel, M., Laugier,E, DehUe, D.,Wyon, C, Kwakman, L., Hopstaken, M. (2006) SIMS depth profiling of boron ultra shallow junctions using obliqne 02 beams down to 150 eV. Applied Surface Science, 252,7211-7213. 

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