Philips Magnetoresistive Sensor manual Materials 10−8Ωm Δρ/ρ% ΙΙkΔ/m

Philips Semiconductors

Magnetoresistive sensors for									General
magnetic field measurement
This definition relates DU to a unit operating voltage.									requires a high sensor resistance R with a large area A,
The highest (HG) and lowest (Hmin) fields detectable by									since there are limits for power dissipation and current
the sensor are also of significance. The measuring range									density. The current density in permalloy may be very high
HG is restricted by non-linearity - if this is assumed at 5%,									(j > 106 A/cm2 in passivation layers), but there are weak
an approximate value for barber-pole sensors is given by:									points at the current reversal in the meander (see section
HG ≈ 0.5(H0 + Hx)								(11)	on sensor layout) and in the barber-pole material, with
									five-fold increased current density.
From this and equation (9) for signal voltage (UBP) for a									A high resistance sensor with U0 = 25 V and a maximum
									S results in a value of 2.5 x 10-3
barber-pole sensor, the following simple relationship can									0
									(A/m)-1for Su or, converted to flux density, S = 2000 V/T.
				≈ 0.5	⎛	Δρ⎞			T
be obtained: H		S						(12)	This value is several orders of magnitude higher than for a
	G		0		⎝	------	⎠
						ρ			normal Hall effect sensor, but is valid only for a much

narrower measuring range.

Other sensor types have a narrower range of linearity and therefore a smaller useful signal.

The lowest detectable field Hmin is limited by offset, drift and noise. The offset is nearly cancelled in a bridge circuit and the remaining imbalance is minimized by symmetrical design and offset trimming, with thermal noise negligible in most applications (see section on sensor layout). Proper film deposition and, if necessary, the introduction of a stabilization field will eliminate magnetization switching due to domain splitting and the introduction of ‘Barkhausen noise’.

Sensitivity S0 is essentially determined by the sum of the anisotropy (Hk), demagnetization (Hd) and bias (Hx) fields. The highest sensitivity is achievable with Hx = 0 and

Hd « Hk, although in this case S0 depends purely on Hk which is less stable than Hd. For a permalloy with a thickness greater than or equal to 20 μm, a width in excess of 60 μm is required which, although possible, has the drawback of producing a very low resistance per unit area.

The maximum theoretical S0 with this permalloy (at Hk = 250 A/m and Δρ/ρ = 2.5%) is approximately:

					⎛ A	⎞ 1	⎛ --------mV⎞
S		(max)	= 10	–4			⎝	V ⎠	(13)
					----		= 100--------------
	0				⎝ m⎠		⎛	------kA⎞
							⎝	m ⎠

For the same reasons, sensors with reduced sensitivity should be realized with increased Hd, which can be esti- mated at a maximum for a barber-pole sensor at 40 kA/m. A further reduction in sensitivity and a corresponding growth in the linearity range is attained using a biasing field. A magnetic shunt parallel to the magnetoresistor or only having a small field component in the sensitive direc- tion can also be employed with very high field strengths.

A high signal voltage Ux can only be produced with a sensor that can tolerate a high supply voltage Uo. This

Materials

There are five major criteria for a magnetoresistive material:

∙Large magnetoresistive effect Dr/r (resulting in a high signal to operating voltage ratio)

∙Large specific resistance r (to achieve high resistance value over a small area)

∙Low anisotropy

∙Zero magnetostriction (to avoid influence of mechanical stress)

∙Long-term stability.

Appropriate materials are binary and ternary alloys of Ni, Fe and Co, of which NiFe (81/19) is probably the most common.

Table 1 gives a comparison between some of the more common materials, although the majority of the figures are only approximations as the exact values depend on a number of variables such as thickness, deposition and post-processing.

Table 3 Comparison of magnetoresistive sensor materials

Materials	ρ (10−8Ωm)	Δρ/ρ(%)	ΙΙk(Δ/m)
NiFe 81:19	22	2.2	250
NiFe 86:14	15	3	200
NiCo 50:50	24	2.2	2500
NiCo 70:30	26	3.7	2500
CoFeB 72:8:20	86	0.07	2000

Δρ is nearly independent of these factors, but r itself increases with thickness (t ≤ 40 nm) and will decrease during annealing. Permalloys have a low Hk and zero magnetostriction; the addition of Co will increase Δρ/ρ, but

2000 Sep 06 18