Magnetic and Electromagnetic Microsystems 1. Magnetic Sensors 2. Magnetic Actuators 3. Electromagnetic Sensors 4. Example: magnetic read/write head (C) Andrei Sazonov 2005, 2006 1
Magnetic microsystems Electromagnetic microsystems - Application areas: 1. Magnetic Field Measurements (characterization, quality control - ). 2. Proximity and Angle Detectors (vehicle detection, machinery part motion, orientation systems magnetic compass). 3. Power meters (electric current sensors). 4. Medical Systems (brain electrical activity sensors) 5. Magnetic recording and storage (read/write heads, magnetic tapes, hard drives). (C) Andrei Sazonov 2005, 2006 2
Hall effect (Hall plates, magnetodiodes, ) Tunnel diode Magnetic sensors Magnetometers (carrier domain, flux gate, ) SQUIDs (superconducting) (C) Andrei Sazonov 2005, 2006 3
Magnetic sensors: 1. Hall effect based. Hall effect = charge carriers traveling in the presence of a perpendicular magnetic field are deflected by the Lorentz force, thus building up a voltage between two sides of a conductive slab. Hall voltage is proportional to: - current (directly); - magnetic flux density (directly); - plate thickness (inversely); - carrier density (directly). Hall voltage: V Hall = R Hall I x B z /T = R Hall J x WB z R Hall Hall coefficient. Therefore, thin semiconductor membranes are the best for Hall sensors. (C) Andrei Sazonov 2005, 2006 4
Various Hall sensor configurations: Sense contacts Drive contacts L W >> T Applications: External magnetic field sensors. Problem offset voltages (due to mask misalignment, fabrication caused or external stress and thermal drift). Correction is possible through the circuitry: for example, periodically reverse the sensing and driving terminals and to average the output signals. (C) Andrei Sazonov 2005, 2006 5
Hall plates are easy to integrate with any BJT or CMOS process as the fabrication mainly involves the contact fabrication. Hall sensor in BJT process: - p+ pockets for isolation; - n+ contacts; - metal deposition and patterning. Hall sensor MOS process: same as MOSFET fabrication. (C) Andrei Sazonov 2005, 2006 6
Magnetoresistor. Due to deflection, charge carriers in the magnetic field travel longer distances across the materials; thus, the conductivity decreases: σ(b z ) = σ 0 (1-r 2 μ 2 B z ), μ mobility; r constant (0.8 to 2.0). Magnetodiode. Consider thin film wide diode. Under forward bias, recombination of carriers on the interfaces largely exceeds that in the bulk and depends on the interface quality. If such a diode is fabricated, so that one interface is much more defective than the other, then carrier deflection in external magnetic field can modulate diode current depending on the field direction. p+ Si SiO 2 i- Si n+ Si substrate (C) Andrei Sazonov 2005, 2006 7
Substrate is chosen so that recombination rate on its interface with Si is much higher than that of SiO 2 (say, sapphire, a-sin x ). Under forward bias, electrons and holes are injected into intrinsic layer drifting in opposite directions. Therefore, magnetic field will deflect them in the same direction (resulting in high or low recombination and thus modulating the current). Advantage: High sensitivity (about 10 times more sensitive than Hall sensor). Drawbacks: - non-linearity; - temperature dependent. (C) Andrei Sazonov 2005, 2006 8
Magnetotransistor: a BJT with one emitter and several collectors and bases to pick up the current depending on where it is deflected by magnetic field. Example: Dual collector vertical npn device; the current is split between two collectors; by adjusting the base voltages they can be independently controlled. Split-drain MAGFET: a MOSFET with an extra drain; the difference between the two drain currents is induced by external magnetic field. (C) Andrei Sazonov 2005, 2006 9
2. Magnetometers. Carrier domain based magnetometers. Carrier domain a region of semiconductor containing a high density of carriers of equal concentrations. The carriers are confined within the domain by potential gradients. Lorentz force moves the entire domain providing changes in the signal. Example: symmetric BJT structure with body bias. Consists of 2 BJTs: npn and pnp. npn collector is the pnp base. pnp collector is the npn base. Lateral BJT size is large, therefore, we have current crowding in the central region, thus forming carrier domain. External magnetic field slightly moves it resulting in the difference between left and right side contacts. (C) Andrei Sazonov 2005, 2006 10
Flux gate based magnetometers. Recall a B-H diagram for the ferromagnetics. Between P and Q, magnetic flux density linearly increases with proportionality coefficient μ magnetic permeability. Between Q and R, all domains are aligned saturation occurs, i.e., permeability changes. If we have a solenoid with ferroelectric core brought close to Q, then external magnetic field variation will provide changes in μ, therefore, changes in the solenoid inductance: L = μ o μn 2 A/l S -H c B s B r -B r B O -B s P Q H c R H N number of coils, A cross sectional area, l length. Application: solid state compasses. (C) Andrei Sazonov 2005, 2006 11
Fabrication: thin film deposition, electroplating. Example: 30 turns coil, permalloy core (μ r = 100,000), length of 1.5 mm, cross-sectional area of 100μm 2 (1μm x 100μm). Inductance, L = 1.26x10-6 * 10 5 * 30 2 * 1x10-10 / 1.5x10-3 = 7.6x10-6 H. To measure L, RLC circuit is used. AC signal is applied, and external magnetic field changes parameters of the circuit; the change in the voltage or in the current can be measured. Sensitivity: 0.1 nt. (C) Andrei Sazonov 2005, 2006 12
Tunneling magnetometers. Fabricated by bulk micromachining. Tunneling tip is fabricated at the bottom of Si cavity. Silicon nitride membrane (500nm-1μm thick, 2.5mmx2.5mm) with electrodes deposited on the bottom side and thin film planar coil on the top side is excited by oscillating current. The Lorentz force generated: F = NL w IBsinθ, N number of coil loops; L W length of coil segments; I coil current; θ - angle between magnetic field vector and coil segments. 200Hz AC excitation current is used; the voltage to keep tunneling current constant is proportional to F. Advantages:- low power (< 100 mw); - high sensitivity (~ 1 nt). (C) Andrei Sazonov 2005, 2006 13
3. SQUID = superconductive quantum interference devices. Based on Josephson effect: superconductive current flows through thin insulating layer that separates two superconductors (due to tunneling). If the current exceeds threshold value I m, a voltage drop occurs between superconductors, and AC current is generated: J s = J 0 sin(φ 0 t + φ), φ 0 = 2qV/h If external magnetic flux passes through the loop, the current will shift through the allowed modes: 2πnW o =LI, W o magnetic flux quantum; n number of quanta; L inductance of the loop; I I loop current. I m V (C) Andrei Sazonov 2005, 2006 14
SQUID is made of two Josephson junctions with 5-20 nm SiO 2 barrier. External circuit maintains the bias current I bias > I m. External magnetic flux results in the current oscillations that are dependent on the flux: I ~ cos(πφ/φ 0 ) Φ 0 = h/2q I = I max sin(φ + 2πf), f = 2qV/h I bias For V = 1 μv, f ~ 500 MHz. Advantage: high sensitivity. I 1 I 2 Threshold for SQUID: 1 ft Magnetic field of heart:50,000 ft Magnetic field of brain: a few ft Applications: biomedical electronics. (C) Andrei Sazonov 2005, 2006 15
Micromotors Eddy current Magnetic actuators Magnetostrictive Read/write heads (C) Andrei Sazonov 2005, 2006 16
Hard disk drive read/write head: HDD top view. Combined inductive/gmr read/write head. Writing is currently done by inductive head; reading by GMR head. Advantage of inductive head Advantage of GMR head stronger magnetic fields achievable. higher spatial resolution achievable. (C) Andrei Sazonov 2005, 2006 17
Inductive read/write head a device that combines a sensor and an actuator. HDD magnetic storage is based on magnetic domains spatial areas in magnetic coatings with aligned spins. Magnetic coatings are polycrystalline, ideally - with each crystal being a single magnetic domain. Single magnetic domain size is 0.1 μm. When magnetic domains are of that scale, their orientation may be influenced by orientation of neighboring domains. Therefore, coercive field in that case has to be high. That means, high magnetic field strength has to be generated by the writing head. Currently, only inductive heads can achieve the fields required to rewrite the information in high-performance HDDs. H gap = H head L head /L gap (C) Andrei Sazonov 2005, 2006 18
Inductive head: - based on the electromagnetic induction in the thin film coil; the gap is about 0.1-1μm. Fabrication: surface micromachining (thin film technology). Dimensions: ~100μm x 200μm. Gap size: 0.2 μm. Problem: at L gap 0.1 μm, ferromagnetic materials reach saturation. (C) Andrei Sazonov 2005, 2006 19
GMR heads are comprised of four layers of thin material sandwiched together into a single structure: Free Layer: the sensing layer made of a nickel-iron alloy. It is free to rotate in response to the magnetic patterns on the HD. Spacer: non-magnetic layer, typically made of copper. It magnetically separates free layer and pinned layer. Pinned Layer: Made of cobalt, it is held in a fixed magnetic orientation by virtue of its adjacency to the exchange layer. Exchange Layer: Made of an antiferromagnetic material (iron and manganese alloy), it fixes the pinned layer's magnetic orientation. (C) Andrei Sazonov 2005, 2006 20
Magnetoresistive read heads: -based on the change in electrical resistance due to carrier deflection by magnetic field (5-10%). - based on a tri-layer consisting of magnetoresistive (MR) material (a tri-layer of NiFe, FeMn, and Mo), non-magnetic film (SiO 2 ) and magnetic film (NiFe) shielded from outside fields (by metal films). When the head passes over a magnetic field of certain polarity (say, a "0" on the disk), the electron orbits in the free layer turn to be aligned with those of the pinned layer. This decreases electrical resistance in the entire head structure. When the head passes over a magnetic field of the opposite polarity ("1"), the electron orbits in the free layer rotate so that they are not aligned with those of the pinned layer. This causes an increase in the resistance of the overall structure. These structures are called spin valves because the electron orbit rotation is caused by spin interaction. If you imagine a plumbing pipe with a shut-off rotating valve, that's the general concept behind the name. Advantage over electromagnetic head: higher sensitivity and resolution (combined thickness of all layers 50 nm). (C) Andrei Sazonov 2005, 2006 21