Sputtering thin films

Sputtering was used in this thesis for the thin films of MTJs, conducting lines, contact pads and protection layers. The first publication about sputtering is from Grove in 1852 [58], but it took a long time until the micro-electronics demanded the sputtering technique for the production of good metal films. While a general description of the sputtering process can be found in [101] and [75], this section will focus on the used apparatus and their settings for the different films.

Figure 2.1: Sputtering systems used in this thesis

[Commercial sputter system CLAB 600, made by LEYBOLD]     [Home made sputter system, used for SiO$_2$, Tantalum and Gold]

All thin films of the MTJs (see chapter 6) were sputtered in the fully automated commercial sputter system CLAB 600, made by LEYBOLD DRESDEN (see figure 2.1(a)). This system is equipped with six 4'' magnetron sputter sources and one 2'' magnetron sputter source. Two of the 4'' sources are special DC-magnetron sources for ferromagnetic materials and one 4'' source is a RF sputter source for MnIr. The other sources are regular DC-magnetron sputter sources. The base pressure in the sputter chamber is at least $3.5\cdot 10^{-7}$mbar. During sputtering, argon is let into the chamber and the process pressure is kept constant at $1.3\cdot 10^{-3}$mbar by adjusting the shutter in front of the main turbo pump. The sputter power is $115$W for all targets except Gold (the 2'' source) with $29$W. Changing the target materials, installing magnetic masks or wobble mask inside the sputter system is easily possible within a few hours. The layer thicknesses of the materials were frequently calibrated through x-ray diffraction methods or atomic force microscopy (AFM) [62].

The insulating barrier of the MTJ is processed inside the CLAB 600 without vacuum breach. The barrier consists of $1.4$nm Aluminium, that is oxidised for $100$sec in an ECR^2.1 oxygen plasma source, made by ROTH & RAU. At the best parameters [124] of $3\cdot 10^{-3}$mbar oxygen pressure, a microwave power of $275$W and a DC bias voltage at the sample of $-10$V relative to the chamber, the aluminium transforms into an amorphous $1.8$nm thick Al$_2$O$_3$ barrier.

In the MTJs which are prepared during this thesis, the ferromagnetic layers are pinned to an antiferromagnetic layer (MnIr) by exchange bias. Normally, the exchange bias is activated by heating the sample over the Neèl temperature and cooling down the sample in a homogeneous magnetic field. But with this technique, it is not possible to pin the top and bottom ferromagnetic layers in different directions. Therefore, the exchange bias is activated by sputtering the layers below and above the barrier in two different magnetic masks.

Figure 2.1(b) shows the home built sputtering system that is used for conducting lines, contact pads (Ta, Au) and protection layers (SiO$_2$). With a base pressure of at least $2\cdot 10^{-6}$mbar, Tantalum and Gold is deposited at $2.1\cdot 10^{-3}$mbar argon pressure with a power of 25W. The protective SiO$_2$ layers are RF-sputtered from a silica glass target in a gas mixture of $2.1\cdot 10^{-3}$mbar argon and $1.1\cdot 10^{-4}$mbar oxygen at a power of 50W.