Journal of ELECTRICAL ENGINEERING, VOL 57. NO 8/S, 2006, 171-174  *Vienna University of Technology, Faculty of Electrical Engineering and Information Technology, Institute of Sensor and Actuator Systems, Guss- hausstrasse 27-29/366, A-1040 Vienna, Austria, Email: hans.hauser@tuwien.ac.at Research supported by Schiebel Elektronische Geräte GmbH and by the Fonds zur Förderung der Gewerblichen Wirtschaft (FFF) under grant No. 3/9893. ISSN 1335-3632 © 2006 FEI STU MEASUREMENTS, TECHNOLOGY, AND LAYOUT OF SENSITIVE ANISOTROPIC MAGNETORESISTIVE SENSORS Hans Hauser* Günther Stangl* Michael Janiba* Ioanna Giouroudi* Magnetic measurements and noise calculation are presented for anisotropic magnetoresistive (AMR) sensors with advanced layout for improved sensitivity. The AMR effect is increased up to almost 4%. Depending on the sensor layout, a field measurement reso- lution of 380 pT seems feasible. Keywords: anisotropic magnetoresistance effect, thin permalloy film, demagnetizing factor, sensor noise 1 INTRODUCTION Related with the detection of weak magnetic fields, the anisotropic magnetoresistive (AMR) effect is utilized in many biomedical and industrial applications. Compared to giant magnetoresistance (GMR) and tunneling magneto- resistance (TMR) it offers the advantage of low hysteresis and low noise. Philips KMZ sensors consist of patterned NiFe thin film structures in a Wheatstone bridge configuration equipped with Barber-pole structures for output linearization [1]. As periodical flipping by a perpendicular field improves stability and reduces noise and hysteresis, these sensors have built-in flat flipping and feedback (compensation) coils so that they are ideally suited for magnetometers. Various sensor designs and electronic evaluation circuits have been developed to overcome temperature dependence, offset, and hysteresis, for example [2]. The resistivity ρ in the plane of a thin ferromagnetic film with uniaxial anisotropy varies with the angle between the current density and the spontaneous magnetization which is rotated by the applied field H α . The sensivity of a Wheatstone-bridge arrangement (supply voltage V s , bridge output voltage V b ) is k s b H R R V dH dV S 1 1 0 0 ∆ = = α and can be varied by the total anisotropy field H k = 2K/μ 0 M s . The effective anisotropy constant K, the spontaneous mag- netization M s , the average resistance R 0 , and the field de- pendent variation R are determined respectively by the ma- terial and geometry of the magnetoresistive element. 2 SENSOR TECHNOLOGY The magnetoresistive films have been deposited by DC cathode sputtering using a triode set-up. The silicon wafer substrate surface has been passivated by a 0.5-0.8 μm insu- lation layer, consisting either of a thermal silicon dioxide or a low stress silicon nitride deposited by a PECVD process at low temperature. The AMR effect of dc-sputtered Ni 81%-Fe 19% films has been increased up to  ρ / ρ = 3.93% at 50 nm thickness, close to the theoretical limit of about 4% [3]. The magneto- resistive films have been deposited by cathode sputtering (triode-process). The target is connected to a negative poten- tial of U(T) = -800 V and the substrate is biased by U(S) = - 60 V. The cathode current is I(C) = 43 A, the anode current is I(A) = 3.5 A, and the anode voltage is U(A) = +50 V against ground potential. The following parameters have been varied: Both target and substrate materials, the tempera- tures of target T(T) and substrate T(S), the distance a(T-S) between target and substrate, and the film thickness d. This value has been determined by resonance frequency meas- urements with an accuracy of better than 1%. 2 SENSOR LAYOUT In order to achieve a homogeneous and small demag- netizing field, an elliptical shape of the AMR array is pro- posed [4, 5]. Several layouts with barber-pole structures on rectangular permalloy strips of different width and separation distance (eg KMZ1010A in Fig.1: both width and distance 10 μm; H3: overall elliptical shape, half shown in Fig. 2) and small elliptical elements (E4060C, see Fig. 3) are compared. The total sensor area was 1×2 mm 2 for each layout. Fig. 1. Structural details of the KMZ1010A design; rectangular permalloy strips with width and distance 10 μm