General Appendices 1997 File under Discrete Semiconductors, SC17

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General Appendices 1997 File under Discrete Semiconductors, SC17

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General Appendices
1997 File under Discrete Semiconductors, SC17
Philips Semiconductors
Appendices
MAGNETORESISTIVE EFFECT Magnetoresistive sensors make fact that electrical resistance certain ferromagnetic alloys influenced external fields. This solid-state magnetoresistive effect, anisotropic magnetoresistance, easily realized using thin film technology, lends itself sensor applications. Resistance- field relation specific resistance anisotropic ferromagnetic metals depends angle between internal magnetization current according cos2
General
Figure shows geometry simple sensor where thickness much smaller than width which turn, less than length (i.e. With current flowing x-direction (i.e. then following equation obtained from equation cos2 with constant current voltage drop x-direction becomes:
Besides this voltage, which directly allied resistance variation, there voltage y-direction, given sincos This called planar pseudo Hall effect; resembles normal transverse Hall effect physically different origin. sensor signals determined angle between magnetization `length' axis and, rotates under influence external fields, these external fields thus directly determine sensor signals. assume that sensor manufactured such that e.a. x-direction that without influence external fields, only x-component 180°). energies have introduced when rotated external magnetic fields: anisotropy energy demagnetizing energy. anisotropy energy given crystal anisotropy field which depends material processes used manufacture. demagnetizing energy form anisotropy depends geometry this generally rather complex relationship, apart from ellipsoids where uniform demagnetizing field introduced. this case, sensor set-up Fig.1.
where resistivities perpendicular parallel quotient ||)/ called magnetoresistive effect amount several percent. Sensors always made from ferromagnetic thin films this major advantages over bulk material: resistance high anisotropy made uniaxial. ferromagnetic layer behaves like single domain distinguished direction magnetization plane called easy axis (e.a.), which direction magnetization without external field influence.
handbook, halfpage
MBH616
Fig.1 Geometry simple sensor.
where demagnetizing factor t/w, saturation magnetization induction constant Vs/Am.
1997
Philips Semiconductors
Appendices
General
Linearization shown, basic magnetoresistor square resistance-field (R-H) dependence, simple magnetoresistive element cannot used directly linear field measurements. magnetic biasing field used solve this problem, better solution linearization using barber-poles (described later). Nevertheless plain elements useful applications using strong magnetic fields which saturate sensor, where actual value field being measured, such angle measurement. this case, direction magnetization parallel field sensor signal described cos2 function. Sensors with inclined elements Sensors also linearized rotating current path, using resistive elements inclined angle shown Fig.2. actual device uses four inclined resistive elements, pairs each with opposite inclinations, bridge. magnetic behaviour such pattern more complicated determined angle inclination anisotropy, demagnetization bias field present). Linearity maximum 45°, which achieved through proper selection stabilization field (Hst) x-direction necessary some applications, this arrangement only works properly magnetization state.
field t/w(M0/m0) determines measuring range magnetoresistive sensor, given -(6) -cos where |Hy| components external field. simplest case voltages become:
(Note: then must replaced Hx/cos Neglecting constant part there main differences between magnetoresistive signal depends square Hy/H0, whereas Hall voltage linear ratio their maximum values L/w; Hall voltage much smaller most cases Magnetization thin layer magnetic field reality slightly more complicated than given equation (6). There solutions angle (with 180° Replacing 180° influence except change sign Hall voltage also that most linearized magnetoresistive sensors. Therefore, avoid ambiguity either short pulse proper field x-axis (|Hx| with correct sign must applied, which will switch magnetization into desired state, stabilizing field x-direction used. With exception advisable stabilizing field this case, values affected non-ideal behaviour layer restricted so-called `blocking curve'. minimum value depends structure sensitive layer order insufficient value will produce open characteristic (hysteresis) sensor. easy axis y-direction leads sensor higher sensitivity, then 1997
handbook, halfpage
MBH613
Fig.2
Current rotation inclined elements (current magnetization shown quiescent state).
Philips Semiconductors
Appendices
BARBER-POLE SENSORS number Philips' magnetoresistive sensors `barber-pole' construction linearize relationship, incorporating slanted strips good conductor rotate current. This type sensor widest range linearity, smaller resistance least associated distortion than other form linearization, well suited medium high fields.
General
dependence linear. fact this equation gives same linear dependence planar Hall-effect sensor, magnitude magnetoresistive sensor.
handbook, halfpage
MBH615
handbook, halfpage
,,,,,, ,,,,,, ,,,,,, ,,,,,, ,,,,,, ,,,,,,
Permalloy Barber pole
Magnetization
MBH614
-0.5
Fig.4 Fig.3 Linearization magnetoresistive effect with barber-poles (current magnetization shown quiescent state).
Calculated characteristic barber-pole sensor.
current takes shortest route high-resistivity gaps which, shown Fig.3, perpendicular barber-poles. Barber-poles inclined opposite direction will result opposite sign characteristic, making extremely simple realize Wheatstone bridge set-up. signal voltage Barber-pole sensor calculated from basic equation with 45°):
Barber-pole sensors require certain magnetization state. bias field several hundred generated sensing current alone, this sufficient sensor stabilization, neglected. most applications, external field applied this purpose.
Sensitivity high demagnetization, most applications field components z-direction (perpendicular layer plane) ignored. Nearly sensors most sensitive fields y-direction, with only having limited even negligible influence.
Definition sensitivity contains signal field variations DH), well operating voltage proportional U0): (10)
where constant arising from partial shorting resistor, amounting 0.25 barber-poles gaps have equal widths. characteristic plotted Fig.4 seen that small values relative
This definition relates unit operating voltage.
1997
Philips Semiconductors
Appendices
highest (HG) lowest (Hmin) fields detectable sensor also significance. measuring range restricted non-linearity this assumed approximate value barber-pole sensors given (11) From this equation signal voltage (UBP) barber-pole sensor, following simple relationship obtained: (12) Other sensor types have narrower range linearity therefore smaller useful signal. lowest detectable field Hmin limited offset, drift noise. offset nearly cancelled bridge circuit remaining imbalance minimized symmetrical design offset trimming, with thermal noise negligible most applications (see section sensor layout). Proper film deposition and, necessary, introduction stabilization field will eliminate magnetization switching domain splitting introduction `Barkhausen noise'. Sensitivity essentially determined anisotropy (Hk), demagnetization (Hd) bias (Hx) fields. highest sensitivity achievable with although this case depends purely which less stable than permalloy with thickness greater than equal width excess required which, although possible, drawback producing very resistance unit area. maximum theoretical with this permalloy 2.5%) approximately: (max)
General
since there limits power dissipation current density. current density permalloy very high A/cm2 passivation layers), there weak points current reversal meander (see section sensor layout) barber-pole material, with five-fold increased current density. high resistance sensor with maximum results value 10-3 (A/m)-1 converted flux density, 2000 V/T. This value several orders magnitude higher than normal Hall effect sensor, valid only much narrower measuring range. Materials There five major criteria magnetoresistive material: Large magnetoresistive effect Dr/r (resulting high signal operating voltage ratio) Large specific resistance achieve high resistance value over small area) anisotropy Zero magnetostriction avoid influence mechanical stress) Long-term stability. Appropriate materials binary ternary alloys which NiFe (81/19) probably most common. Table gives comparison between some more common materials, although majority figures only approximations exact values depend number variables such thickness, deposition post-processing. Table (13) Comparison magnetoresistive sensor materials (10-8m) 0.07 /(%) k(/m) 2500 2500 2000
Materials NiFe 81:19 NiFe 86:14 NiCo 50:50 NiCo 70:30 CoFeB 72:8:20
same reasons, sensors with reduced sensitivity should realized with increased which estimated maximum barber-pole sensor kA/m. further reduction sensitivity corresponding growth linearity range attained using biasing field. magnetic shunt parallel magnetoresistor only having small field component sensitive direction also employed with very high field strengths. high signal voltage only produced with sensor that tolerate high supply voltage This requires high sensor resistance with large area 1997
nearly independent these factors, itself increases with thickness will decrease during annealing. Permalloys have zero magnetostriction; addition will increase
Philips Semiconductors
Appendices
this also considerably enlarges small temperature coefficient required, NiCo alloys preferable. amorphous alloy CoFeB high slightly worse thermal stability absence grain boundaries within amorphous structure, exhibits excellent magnetic behaviour. APPENDIX SENSOR FLIPPING During deposition permalloy strip, strong external magnetic field applied parallel strip axis. This accentuates inherent magnetic anisotropy strip gives them preferred magnetization direction, that even absence external magnetic field, magnetization will always tend align with strips. Providing high level premagnetization within crystal structure permalloy allows stable premagnetization directions. When sensor placed controlled external magnetic field opposing internal aligning field, polarity premagnetization strips switched `flipped' between positive negative magnetization directions, resulting stable output characteristics.
General
more magnetization rotates towards therefore becomes easier flip sensor into corresponding stable position `-x' direction. This means that smaller field sufficient cause flipping action seen Fig.6, transverse field strengths (0.5 kA/m) sensor characteristic stable positive values reverse field approximately kA/m required flip sensor. However higher values kA/m), sensor will also flip smaller values kA/m). Also illustrated this figure noticeable hysteresis effect; also shows that permalloy strips flip same rate, flipping action instantaneous.
MLC131
(mV) kA/m
kA/m (kA/m)
MLC130
handbook, halfpage
(mV)
(kA/m)
Fig.6
Sensor output `Vo' function auxiliary field
reversal sensor characteristics
Fig.5 Sensor characteristics.
sensitivity sensor reduces auxiliary field increases, which seen Fig.6 more clearly Fig.7. This because moment imposed magnetization directly opposes that resulting reduction degree bridge imbalance hence output signal given value
field required flip sensor magnetization (and hence output characteristic) depends magnitude transverse field greater this field, 1997
Philips Semiconductors
Appendices
General
handbook, full pagewidth
(mV)
MLC132
kA/m kA/m kA/m
(kA/m)
Fig.7 Sensor output `Vo' function transverse field
Safe Operating ARea (SOAR) determined magnetoresistive sensors, within which sensor will flip, depending number factors. higher auxiliary field, more tolerant sensor becomes external disturbing fields (Hd) with kA/m greater, sensor stabilized disturbing fields long does irreversibly demagnetize sensor. negative much larger than stabilising field sensor will flip. This effect reversible, with sensor returning normal operating mode again becomes negligible (see Fig.8). However higher greater reduction sensor sensitivity generally recommended have minimum auxiliary field that ensures stable operation, generally around kA/m. SOAR also extended values long transverse field less than kA/m. also recommended apply large positive auxiliary field before first using sensor, which erases residual hysteresis
handbook, halfpage
(kA/m)
,,,,,, ,,,,,, ,,,,,, ,,,,,, ,,,,,, ,,,,,, ,,,,,,
MLC133
SOAR
(kA/m)
Fig.8
SOAR KMZ10B sensor function auxiliary field `Hx' (MLC133).
1997
Philips Semiconductors
Appendices
APPENDIX SENSOR LAYOUT Philips' magnetoresistive sensors, permalloy strips formed into meander pattern silicon substrate. With KMZ10 (see Fig.9) KMZ51 series, four barber-pole permalloy strips used while KMZ41 series simple elements. patterns used different these three families sensors every case,
General
elements linked same fashion form four arms Wheatstone bridge. meander pattern used KMZ51 more sophisticated also includes integrated compensation flipping coils (see chapter weak fields); KMZ41 described more detail chapter angle measurement.
handbook, full pagewidth
,,,,,,,,,, ,,,,, ,,,,,, ,,,,, ,,,,,, ,,,,, ,,,,,, ,,,,, ,,,,, ,,,,,, ,,,,, ,,,,,, ,,,,,, ,,,,, ,,,,, ,,,,,, ,,,,, ,,,,,,
MBC930
Fig.9 KMZ10 chip structure.
1997
Philips Semiconductors
Appendices
General
pair diagonally opposed elements barber-poles +45° strip axis, with second pair -45°. resistance increase pair elements external magnetic field matched equal decrease resistance second pair. resulting bridge imbalance then linear function amplitude external magnetic field plane permalloy strips normal strip axis. This layout largely eliminates effects ambient variations (e.g. temperature) individual elements also magnifies degree bridge imbalance, increasing sensitivity. Fig.10 indicates further trimming resistors (RT) which allow sensors electrical offset trimmed down zero during production process.
MLC129
handbook, halfpage
Fig.10 KMZ10 KMZ11 bridge configuration.
1997

General Appendices 1997 File under Discrete Semiconductors, SC17

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