Micromachining of AFM Cantilevers for Scanning Magnetoresistance Microscopy Applications

M. Costa a,b, J. Gaspar a, R. Ferreira a, M. Martins a, S. Cardoso b, c, and P. P. Freitas a, c

a International Iberian Nanotechnology Laboratory, Braga, 4715-330, Portugal

b Physics Department, Instituto Superior Técnico (IST), Lisbon, 1049-001, Portugal

c INESC-MN/Institute for Nanosciences and Nanotechnologies, Lisbon, 1000-029, Portugal

Various techniques of scanning magnetoresistance microscopy (SMRM) have been previously developed to enable the simultaneous imaging of surface topography and stray magnetic field distributions in order to overcome limitations of magnetic force microscopy (MFM) technique. GMR read-heads [1], micro-hall devices [2] and TMR sensors integrated on piezoeletric stage [3] have been used but lack of acceptable spatial resolution for imaging. To overcome this, magnetoresistive sensors are here integrated into standard atomic force microscopy (AFM) cantilevers and used to simultaneously map both topography and magnetic fields.

This novel device consists of a 400-µm-long, 60-µm-wide, 25-µm-thick tipless cantilever with 2 spin valve (SV) sensors at its end that can be used individually or in differential mode, as illustrated in Fig. 1. The cantilever chip is mounted with the sensor pads wirebonded to a support PCB for connection to electronic instrumentation and readout. The fabrication process depicted in Fig. 2 consists of defining SV sensors by optical lithography and reactive ion etching followed by a lift-off technique to pattern metal contacts on top of a silicon-on-insulator substrate. The sensors are 25-µm-long and 2.5-µm-wide and are passivated by physical vapor deposition of 250 nm of aluminum oxide (Al2O3). The cantilevers are micromachined by deep reactive ion etching and the handle is machined. The process is concluded by an HF vapor release step. Figure 3 shows an SEM graph of a fabricated cantilever with a close-up of the magnetic sensors.

The finished set of devices has been characterized electrically and mechanically. Measured values for the cantilevers stiffness, k, and resonance frequency, fres, are 620 N/m and 250 kHz, respectively. In terms of magnetic response, the SV sensors with a resistance, R, of 279 Ω, achieve a magnetoresistance ratio, MR, and sensitivity, dV/dH, of 3.8 % and 61.33 µV/Oe, respectively, for a bias current, ibias, of 1 mA. For magnetic imaging measurements, the SV devices are connected in a quarter-bridge configuration, whose offset-free output is then amplified and measured. The quarter-bridge incorporating the SV sensor is then calibrated under a known uniform magnetic field being therefore possible to accurately quantify the magnitude of the stray fields of a given sample averaged over the sensor area.

The enhanced capability of the fabricated devices is illustrated in Figure 4, with a 100x100 µm2 scan over a region with 1x20µm2 patterned CoFe structures. The cantilever deflection map containing the topographic information is show in Fig. 4.c and the magnetic information, synchronously obtained from the integrated sensor output, is given in Fig. 4.d. By comparing the two, one can notice the match between the magnetic and the topographic information, apart from a shift in the patterns fingerprint as a result of the offset between contact point and position of the sensor in the cantilever.

This work demonstrates the capability of the fabricated cantilever to be used for SMRM purposes. Besides topographic data, the SV sensor detects magnetic fields with a sensitivity of 61.33 µV/Oe and spatial resolution better than 1 µm.

[1] L. Chang et al., IEEE TRANSACTIONS ON MAGNETICS, VOL. 47, p2548 2011.

[2] M. Chan, et al., IEEE TRANSACTIONS ON MAGNETICS, VOL. 45, p4816 2009.

[3] G. Boero et al., SENSORS and ACTUATORS:A VOL 106, p314 2003.

Figure 1. Top view of cantilever geometry / Figure 2. Microfabrication process schematics
Figure 3. SEM graph of fabricated cantilever with integrated SV sensors / Figure 4. Topography and magnetic imaging data from a scan over patterned CoFe features