Concentration-depth calibration and bombardment-induced impurity relocation in SIMS depth profiling of shallow through-oxide implantation distributions: a procedure for eliminating the matrix effect
✍ Scribed by Wittmaack, K.
- Publisher
- John Wiley and Sons
- Year
- 1998
- Tongue
- English
- Weight
- 429 KB
- Volume
- 26
- Category
- Article
- ISSN
- 0142-2421
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✦ Synopsis
Secondary ion yields are known to be strongly enhanced by the presence of oxygen in the analysed sample. The magnitude of the yield enhancement is often signiÐcantly di †erent for impurity and matrix ion species. This kind of SIMS matrix e †ect severely aggravates concentration calibration in depth proÐling through regions of transiently varying oxygen concentration. To eliminate the matrix e †ect, a procedure has been developed that allows the di †erences in yield enhancement to be corrected in a quantitative manner. The procedure will ultimately be required to calibrate proÐles extending through native surface oxide layers. The calibration exercise was carried out for boron in silicon. The dependence of the B'/Si' sensitivity ratio, on the oxygen content of the sample R B, Si , was explored in situ by implanting 1.9 keV ions at OÄ (normal incidence) into a uniformly B-doped reference O 2 ' sample, followed by sputter proÐling through the synthesized oxide with the same beam incident at 75Ä. All measurements were performed at base pressure. During oxygen build-up after initial sputter cleaning the Si' and SiO' yields increased by factors of 200 and 500, respectively, whereas for B' the yield increased only 40 times. Almost inverse yield changes were observed during oxide removal. Bombardment-induced mixing caused a broadening of the oxide/Si interface and some relocation of B atoms. Under internally consistent assumptions the relatively small boron mixing e †ect could be separated from the oxygen-induced B' yield enhancement e †ect. The normalized SiO' signal was used as a measure of the oxygen content of the samples bombarded at the two di †erent I3 SiO ' , impact angles. The B' yields and the sensitivity ratios could be Ðtted very well by polynomial functions.
R B,Si
(I3 SiO ' ) The polynomials were employed to quantify the depth proÐles of 0.5 and 2 keV 11B implanted in Si test samples covered with 6 nm layers of thermal (i.e. thinner than the synthesized oxide layer that can be produced by the SiO 2 1.9 keV beam at OÄ). The compositional changes encountered in passing from the thermal oxide into the Si O 2 ' substrate had be taken into account, not only for time-to-depth conversion but also for concentration calibration based on the measured sensitivity ratios. The changes in erosion rate and Si density around the interface were modelled by error functions. Direct evidence is presented that, for accurate calibration, density and sensitivity changes must be treated separately. Even though the through-oxide variations of are quite di †erent for OÄ R B,Si and 75Ä, the calibrated 2 keV 11B proÐles derived from measurements at these two vastly di †erent impact angles agree very well, even at the interface. This implies that the large matrix e †ect occurring in through-oxide proÐling at 75Ä can be eliminated using the new calibration procedure. Minor di †erences (AE10% ) between the calibrated 2 keV 11B proÐles from measurements at 0Ä and 75Ä can be attributed to di †erences in bombardment-induced relocation. The mixing e †ect is particularly severe for the proÐles of the very narrow and shallow 0.5 keV 11B implantation distributions, which turned out to be heavily distorted at depths below 10 nm, both at 0Ä and 75Ä. Hence it is mandatory, for reasonably accurate proÐle measurements, to use energies that are signiÐcantly O 2 ' (¿50% ) lower than the implantation energy, both for normal and oblique incidence of the probing beam. 1998