AMR MEMS sensors
Anisotropic Magneto Resistance
The use of digital compass, which are magnetic field sensors able to detect the angle (i.e. azimuth) relative to the Earth’s magnetic field, is relatively popular and in recent years it has been adopted in consumer applications such as augmented positioning systems (to facilitate the dead reckoning navigation) and mobile phones. The goal being the real-time monitoring of move direction, under no-GPS coverage or rough/harsh conditions.
One of the easiest ways to incorporate magnetic field sensor on silicon is to exploit the properties of “Magneto-resistance anisotropy” (Anisotropic Magneto Resistance – AMR) of some materials. AMR is commonly found in both magnetic and ferromagnetic materials and is particularly enhanced when the geometry of the magnetic element is such that a net residual magnetization is induced by the balance among energetic terms (magnetostatic, magnetoelastic, magnetocrystalline). Anisometric thin films exhibit a strong effect and enable the realization of simple, cost-effective sensors where the electrical resistance is a function of the angle between magnetization (function of the external magnetic field) and current density.
The AMR effect is very remarkable and easily detectable by an electronic control circuit. More specifically, in the case of “permalloy”, an alloy of NiFe with peculiar magnetic properties (e.g. very low coercivity), it shows a remarkable sensitivity variation of electric resistance of up to 5%.
Of course, for the detection in the three dimensional space, three sensors on X, Y, Z axes are suitably and precisely bundled together, thus achieving the full sensitivity along each of the three dimensional axes. The deposition of the NiFe, the definition and stabilization of its magnetic properties and its shaping by means of lithography and selective removal processes are well applicable steps by a semiconductor line, yet under development and not mature for high volume manufacturing. As a matter of fact, the introduction of these materials will require the adoption of a number of innovative process steps and unconventional tools to enable:
- Deposition of NiFe system by sputtering
- Selective removal of the NiFe system by high energy ion beams
- Annealing of NiFe at high temperature and under magnetic field.
At the end of the Lab4MEMS project a pilot line for such AMR materials incorporated into a MEMS process will be available. A demonstration AMR magnetic sensor will be addressed to show the viability of the process. In such a sensor the strip of Permalloy is spontaneously magnetized in the longitudinal direction. When immersed in a magnetic field transversal to this axis, the spontaneous polarization forces the rotation and the change of resistivity. The metal strips placed on top of the permalloy layer, with 45 degrees orientation (barber-pole) drive the current’s flow to be angled at 45 degrees: thus enhancing the linearization of the sensor’s response.
Heterogeneous integration of pressure sensor, microcontroller, tranceiver unit and bulk acoustic resonator
A demonstration AMR magnetic sensor is depicted hereafter, where the strip of permalloy is spontaneously magnetized in the longitudinal direction.
When immersed in a magnetic field transversal to this axis, the spontaneous polarization forces the rotation and the change of resistivity. The metal strips placed on top of the permalloy layer, with 45 degrees orientation (barber-pole) drive the current’s flow to be angled 45 degrees. Thus enhancing the linearization of the sensor’s response.