Details of Grant 

EPSRC Reference:	GR/H65177/01
Title:	THE CHEMICAL CHARACTERIZATION OF SEMICONDUCTOR LAYERS WITH SUB-NM DEPTH RESOLUTION
Principal Investigator:	Dowsett, Professor MG
Other Investigators:	Lewis, Professor MH Parker, Professor E Kubiac, Dr R
Researcher Co-Investigators:
Project Partners:
Department:	Physics
Organisation:	University of Warwick
Scheme:	Standard Research (Pre-FEC)
Starts:	01 March 1993	Ends:	31 March 1996	Value (£):	140,058
EPSRC Research Topic Classifications:	Materials Characterisation
EPSRC Industrial Sector Classifications:
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Panel History:
Summary on Grant Application Form
To develop a complete quantification algorithm for SIMS depth profiling of MBE grown ultra-thin layers and thin structures generally using delta layers as reference materials. To minimise the inaccuracy in the data due to the use of surface profilometry and other crater depth measuring methods. To explore the complimentary use of SIMS and TEM in achieving true sub-nm depth resolution in SIMS depth profiles. To investigate the use of ultra-low energy ions in depth profiling using a new floating ion gun (FLIG).Progress:We now have a full quantification algorithm for SIMS depth profiling based on a forward model and maximum entropy (Warwick maximum entropy (WME)). This is being applied to delta and other doping structures in silicon and gallium arsenide. At present convolution using an experimentally measured response is used as the model. The process is therefore suitable for quantification in the dilute limit. It may also give an improved estimate of the chemical profile in the non-dilute case, but this is dependent on the net departure from linearity in atomic mixing and ion yields. In the future, we wish to progress to a non-linear forward model which will be more accurate in such cases. WME gives greatly enhanced depth resolution over Fourier transform deconvolution. Quantification of both depth and concentration, with factors such as differential shift accounted for, is carried out formally in the convolution kernel. Recently, we have applied the method to silicon deltas in gallium arsenide, and have observed concentration dependent diffusion effects <1.6nm at a depth of 1-m. (These data were part of an international round robin and had 5 times better depth resolution than the best of the competition.) We have investigated the shape of the SIMS response function, and examined the relationship between both noise (WME uses an empirically determined noise model as part of the quantification process) and commonly used resolution parameters such as decay length and the distinguishability of adjacent features. We find that width and slope parameters give an optimistic estimate of distinguishability. (For example, we can readily achieve decay lengths of 1 nm but under these conditions, deltas with a 2 nm separation are really not distinguishable.) We are developing valley criteria which give a more accurate idea of resolution, and will be reporting these to ISO TC201. We have developed depth standards based on delta layers where layer depths and separations are determined by TEM. These are used to establish the SIMS erosion rates. As with any depth calibration method, the accuracy is dependent on the stability of the SIMS ion column. The column on our EVA 2000 instrument has been progressively modified during the project to give <1% current variation over many hours operation. In addition we are assessing the feasibility of continuous dose measurements to improve the accuracy still further. The FLIG (developed here with Paul Fund support) has made a significant difference to our ability to depth profile at ultra-low energies. No currently available commercial ion column can achieve better than a few 10s nA at sub-keV energies. We can profile at 0.5 -A at I keV and >200 nA at 500 eV. Profiling is practical down to -m depths at energies as low as 250 eV. Even without WME we can achieve what are currently the best routine depth resolutions in the world. Our plans include extending the quantification method to the non-linear (high concentration) regime, and improving the FLIG still further. We will be seeking EPSRC support for this project. Copies of the 10 research papers published or currently in preparation in association with this project are available from the principal investigator.
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