Ion beam milling

Ion beam milling is very similar to sputtering, but the sample is now the target. So the top layers of the sample will be etched down by Ar$^+$-ions that are accelerated towards the sample. Ion beam milling is used for several different process steps during the creation of the MTJs (confer chapter 6) or to embed the conducting lines into the Si-wafer (chapters 3 - 5).

Figure 2.2: Full recorded spectrum of the layer stack TMR-DP15 from the quadrupole mass spectrometer. Because the channels for different masses are not fully separated, some artefacts occur (e.g. the rise of Al at the end of the spectrum is only related to the Si peak).

The ion beam milling is also done in a apparatus built in Bielefeld. It has a base pressure of at least $2\cdot 10^{-6}$mbar and an argon pressure during the etching of $8\cdot 10^{-4}$mbar. The ion source is operated at a discharge voltage of 50V, a beam voltage of 400V, an accelerator voltage of 30V and with a beam current of 6mA. The sample current is measured and kept at about 400$\mu$A to keep the etching rate constant. To ensure homogeneous etching and to avoid short-circuited TMR elements, the sample holder is tilted by 30° and rotates slowly. A comprehensive description of the apparatus can be found in [101].

The etching is monitored with a quadrupole mass spectrometer, which allows to stop the process exactly in the desired layer of the stack. Figure 2.2 shows a recorded layer stack as it is used in chapter 6 of this thesis. The Al$_2$O$_3$ can easily be seen at around 1600sec. After etching the complete layer stack, the process time for the TMR-elements and the bottom electrodes can easily be calculated.