XANES fingerprinting: a technique for investigating CCD surface structures and measuring dead layer thicknesses

We describe a method for determining the compositions, abundances and thicknesses of dead layers above a silicon solid-state soft X-ray detector using the structural information embedded in X-ray Absorption Near Edge Structure (XANES). We demonstrate the feasibility of the technique and show that precisions of ∼1% are possible. In contrast to scanning electron microscopy (SEM), which yields information on the linear extent of surface features, XANES can also provide elemental and bonding information—potentially to a precision of 1 atom in 1010. Using total photocurrent measurements of crystalline silicon, amorphous SiO2 and amorphous Si3N4 in conjunction with the measured quantum efficiency across the silicon K edge, we have derived the relative contributions and thicknesses of the various overlying layers in an X-ray sensitive front-illuminated charge coupled device (CCD) by modelling. The results are found to be in good agreement with the actual thicknesses provided by the manufacturer. Typical accuracies of 10% were obtained which are almost a factor of 2 better than those obtained by process control. Higher precisions should be readily achieved by exploiting the structural information contained in the Extended X-ray Absorption Fine Structure (EXAFS) region of the spectrum. Lastly, the results from this work suggest that if the structural information contained in K-edge quantum efficiency measurements can be combined with X-ray Photoelectron Spectroscopy (XPS), it should be possible to tomographically isolate and image particular surface features using spatially resolved XANES spectroscopy.