Ultra-precise Ion Beam Milling of TMR Sensors

The tunnel-magneto-resistance effect (TMR) is used in modern high-precision spintronic sensors ranging from angular position sensors in automotive industry to read-out sensors in hard-disk-drive industry. The main components of TMR sensors are the magnetic tunnel junctions (MTJs), which exploit the TMR effect. Two ferromagnetic (FM) layers are separated by a non-magnetic and electrically isolating barrier layer. The top layer (free FM layer) is a soft magnetic layer, which easily changes its magnetization direction in an external magnetic field. The bottom layer (pinned FM layer) is hard magnetic, and thus conserves its magnetization direction in an external magnetic field below a certain switching field. Depending on the magnetization direction of the free FM layer relative to the pinned FM layer, the electron tunnel probability changes, and thus, the electrical resistivity of the MTJ. A minimum resistivity is found for a parallel magnetization of free and pinned FM layer.

Comparing to standard giant-magneto-resistance sensors, the TMR has a higher thermal stability and increased signal output while decreasing the sensor’s power consumption. These advantages result in a rapidly growing market for TMR sensors. However, the multilayer composition of the TMR sensor (see Fig. 2) leads to issues regarding the necessary etching, which is essential for electrical contacting of the sensor. Classical dry-etching methods are limited due to the generally poor reactivity of magnetic materials like CoFe, CoPt or NiFe. Additionally, the required reactive gas like chlorine can lead to after-corrosion of the sensor electrodes.

The ion beam milling process applies ion bombardment by argon ions and thereby allows to remove all materials used in the TMR stack in contrast to chemical etching. The ion beam source allows a precise tuning of the ion density and ion energy. Additionally, the dry-etching with inert gas like argon, suppresses any after-corrosion effects, which lead to an increased metal resistivity. Operating with a helium backside wafer cooling, the wafer temperature will be kept low to allow the processing of photoresist.

In Figure 3 another key advantage of ion beam milling is shown: operating with a secondary ion mass spectrometer (SIMS) allows to precisely measure the sputtered atoms, and thus an exact end point detection of the etching is possible. Even the measurement of sub-nm thick Ru-layers is achievable.