Ultra-precise ion beam milling of TMR sensors

The tunnel-magneto-resistance effect (TMR) is used in modern high-precision sensors ranging from angular position sensors in automotive industry to read-out sensors in hard-disk-drive industry. 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 lead to a rapidly growing market for TMR sensors. However, the multilayer composition of the TMR sensor (Figure 1) 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.

Ion beam milling with the scia Mill 200 uses argon ions to physically remove the magnetic layers. The ion beam source fabricated by scia Systems allows a precise tuning of the ion density and ion energy. Due to the ion bombardment, the argon ion beam milling allows the removal of all materials used in the TMR stack in contrast to chemical etching. Additionally, the ion beam milling with inert gas like Argon suppresses any after-corrosion effects, which lead to an increased metal resistivity when dry-etching. Operating with a helium-backside wafer cooling, the wafer temperature will be kept low to allow the processing of photoresist.

In Figure 2, another key advantage of ion beam milling is shown: by operating with a secondary-ion-mass spectrometer one can precisely measure the sputtered atoms. With this technique, an exact endpoint detection of the etching is possible. Even the measurement of sub-nm thick Ru-layers is achievable.

Related products: scia Mill 150 & scia Mill 200

  • Full substrate ion beam milling with superior homogeneity
  • Etching with inert gases to avoid after-corrosion
  • Helium cooling of substrates for use of photoresist
  • Reactive gas compatibility in RIBE and CAIBE processing
  • Ion beam source with high stability, adjustable ion energy and ion current density
  • In-situ measurement for exact endpoint detection with SIMS
  • Full software integration and automated processes via recipe

For additional coating processes the scia Coat 200 is recommended.

Fig. 1: Typical TMR stack

Typical TMR stack

Fig. 2: SIMS spectrum

SIMS spectrum during the ion-beam-milling of a TMR stack with the scia Mill 200