Writing up your PhD thesis

Control of Microstructure in Iron–Carbon Thin Films by Means of Electromigration

“…In fact, some preliminary studies on the effect of externally applied magnetic fields on the microstructure of the Fe(C) films show no detectable change in grain or precipitate size under temperatures, times, and magnetic fields similar to those experienced during electromigration. [22] However, just as in bulk steel samples, a slight strengthening of some texture components was observed due to the magnetic field. [34] Since the electromigration treatment basically replaces precipitates with elongated grains, the first step in optimizing the microstructure is controlling the precipitate distributions.…”

“…Qualitative energy-dispersive X-ray spectroscopy (EDS) measurements in this region (overlaid over the CBS image in Figure 5a and recorded and estimated as discussed in detail in refs. [16,22]) show an increased C concentration that extends into the unaltered, nanocrystalline regions upstream of the elongated grains. The accumulation of C upstream of the large grains shows that the large grains block the C flux and, as expected, that grain boundaries are the dominant transport paths for C.…”

“…The results of these simulations can be found in Supporting Information or in detail elsewhere. [22] α is a measure of the electromigration driving force. [25] Figure 6a shows that there is an apparent threshold for elongated grain formation: at low drift velocities αD (or fluxes, cαD) and low initial concentrations c o , elongated grains do not form, even after electromigration test times that have been scaled to reach and exceed the conditions needed for elongated grain formation at larger drift velocities.…”

“…It is difficult to obtain quantitative estimates of C concentration using EDS, but a variety of methods can be used to estimate absolute values of the C concentration in the ferrite just upstream of flux divergences as between 4 and 5 at%. [22] Since these values were recorded as an average over the width of the complete conductor track and thus also included ferrite grains, it can be assumed that the concentration is many times higher in the grain boundaries in this area. Therefore, values of 19 at% as postulated by Zhang [21] for the maximum stability range of ferrite or a concentration of 16.6 at% as observed by Weck [32] in supersaturated Fe(C) films seem to be reasonable in these areas.…”

“…The geometry is outlined and described in detail elsewhere. [16,22] All patterned chips were heat-treated in a vacuum oven at 465 °C for 4 h to stabilize the grain structure. The pressure was always below 5 Â 10 À6 mbar to avoid oxidation.…”