technology of anisotropic magneto resistive sensor on ......a real interest and meaningful....

Proceedings of the IConSSE FSM SWCU (2015), pp. SC.163–173 ISBN: 978-602-1047-21-7

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Technology of anisotropic magneto resistive sensor on silicon substrate

Slamet Widodo and Tony Kristiantoro

Research Center for Electronic & Telecommunications-LIPI

Jl. Cisitu No.21/154D, Komplex LIPI Sangkuriang, Phone: +62-22-2504660, Fax: +62-22-2504659, Bandung-40135, Indonesia

E-mail: [email protected]; [email protected]

Abstract In this paper will be described magnetoresistive sensor design using thin film technology. A thin layer of alloy (Ni-Cr) will be grown on silicon substrates by evaporation or DC-Sputtering. Also reported the results of the characterization of thin film coating deposition Ni80Cr20with and without magnetisation by SEM, EDS and GMR meter. By using photolithography and etching process, a layer of Ni-Cr alloy and then patterned to form some of the microstructure of the inter-connected into a bridge. By applying a magnetic field in the arms of the bridge structures which face each other, will create an asymmetry in the bridge. In this way, it is expected microstructure formed can serve as a magnetoresistive sensor that can detect magnetic fields below 5 mT, which are needed in navigation systems. Keywords AMR sensors, Nickel-chrome alloy (Ni80Cr20), titanium (Ti), Al/Au

1. Introduction

Magnetoresistive effect is the change in resistivity of a material (material) due to the magnetic field, which was discovered by Thomson (1856), but only the last 30 years who have a real interest and meaningful. Technology ferromagnetic thin film (thickness 10-50 nm) with the use of Anisotropic Magneto Resistive effect (AMR) to add to this technical improvement. In addition, the effects of Giant Magneto Resistive (GMR) has been found, on the basis of thin ferromagnetic films clutch (Baibich et al., 1988). The maximum resistivity changes up to 80% with the GMR effect, although with very high magnetic fields.

Sensor anisotropic magneto resistors (AMR) is a sensor which is generally suitable for the measurement of magnetic fields in the range up to 200μT. AMR sensors have high sensitivity, wide operating temperature, the sensor offset is more stable than the Hall sensor and a wide operating frequency range close to 10 MHz.

AMR sensors with high sensitivity can be applied to regulate traffic, contactless measurement of electric current, measurement and movement in the engine rotation speed, the Earth's magnetic field sensor, electronic compass and navigation systems. Currently, AMR sensors have increased importance in the automotive industry with measurement applications such as pedal position, wheel speed sensors for ABS (anti-block system) and the engine management system in which the sensor was used to measure the position of tenths of a millimeter and the angle of the crankshaft to the ignition timing electronic. Details about the latest types of AMR sensors can be found in other works (Dibbern, 1989) and on the websites maintained by manufacturers such as Philips, Honeywell and others (Fasching, 1994; Hauser & Fulmek, 1992).


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The need for sensors and transducers increased with many technological applications. Hrisoforou recently (Hauser et al., 2000) reviewed the magnetic effects in the physical sensors in the design and development at the International Workshop on Amorphous and nano structured Magnetic Materials (Iasi - Romania 2001).

In the development of Micro Technology Integrated Circuits (MTIC) studied magnetoresistive sensor developed micro, which is used as a magnetic field sensor such as the navigation system, servomotors, playback mechanism and other automated equipment. Solid state magnetic field sensors have advantages, such as the size and power compared to the coil, and a superconducting flux gate Quantum Interference Detectors (SQUID). As a physical phenomenon, this sensor is based on the anizotropic magnetoresistive effect of thin ferromagnetic layer, the deposit on mono crystal silicon substrate (Laimer & Kolar, 2000; Fulmek & Hauser, 1993).

Table 1. Comparison of magnetic sensors.

Sensor type: Min. B Max. B Frequency range Induction coils 100 fT unlimited 0.1 mHz–1 MHz

Hall sensors 10 nT 20 T 0–100 MHz Magnetoresistive Sensors 100 pT 100 mT 0–100 MHz

Fluxgates 10 pT 1 mT 0–100 MHz SQUIDs 5 fT 1000 nT 0–100 kHz

2. AMR sensors theory

AMR sensors are based on the theory of ferromagnetic complex process in a very thin film. Some of the effects that affect can be simplified in math. First, that the magnetization M in ferromagnetic materials always have a magnetic force magnitude saturation magnetization MS but just changed direction. Second, the theory of complex AMR effect (there is also the effect of isotropic MR used in the semiconductor layer) can be divided into two, namely, the relationship between the electrical resistivity and the direction of magnetization (magnetic force), and the relationship between the direction of magnetization (magnetic force) is applied in the an external magnetic field.

Anisotropic magnetoresistive effect

Anisotropic magnetoresistive effect is a distinct shift of energy levels of electrons with spin positive and negative, respectively under the influence of a magnetic field. This causes a shift in the Fermi level. To calculate this effect satisfactorily, by differentiating with experimental data (Dibbern, 1989). Therefore, the most important parameters are determined experimentally. It has been shown that the electrical resistance R can be derived by a simple theory of the angle Θ is the angle between the electric current density and magnetic force (see Figure 1(a)):

(1) In the above equation, ρ0, n and Δρ is a material constant, l is the length of the resistive

strip, b is the width, and d is, its thickness. In general,> l b >> d. R0, n is perpendicular to the magnetic barriers, and ΔR is the maximum change in resistance due to the magnetic field. For today in the direction of x, the voltage Ux is as follows:

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(2) It should be noted that the voltage Uy perpendicular to the direction of current flow.

Because of the similarity of the Hall effect, this effect is called planar Hall effect. In general the Hall effect, the voltage change because the magnetic field perpendicular to the film, because the planar Hall effect, the magnetic field is the same as the current flow. Planar Hall effect is rarely used for practical purposes because the voltages involved are very small.

(a) (b)

Figure 1. (a) The geometry of the strip to the magnetization M and the direction of the current I (b) Geometry of elliptically shaped thin films; axis is assumed to be in a state parallel to the x axis.

External magnetic field

Magnetization M in the film are in the direction of the total energy minimum. The most important energy involved is the external field energy, material anisotropy energy (magnetocrystalline anisotropy energy), and energy demagnetising (shape anisotropy energy). Most of the energy contribution depending on the direction. This means that the energy required to turn M into a given direction can be visualized by three-dimensional energy region (Fasching, 1994).

Spontaneous magnetization MS will be located in the direction of the minimum energy. Magnetocrystalline anisotropy energy region of the iron has six easy axis (ie, the minimum energy) towards the edge of the crystal cube. Nickel has eight easy axis in the diagonal of the unit cell volume. In addition, the total energy depends on the mechanical stress (Hauser & Fulmek, 1992) and geometry. Energy from permalloy (Ni: Fe 81:19) is more complicated. There are 16 easy axis. However, the constant Magnetostriktif near zero in permalloy, namely, the magnetic force has no effect on the dimensions of the crystal lattice. With a total anisotropy field H0 = 2K / μ0MS (the anisotropy constant K), the angle φ between M and the easy axis (x-direction) results for HX = 0 as:

(3) for –1 < Hy /H0 <1.

It should be noted that the voltage Uy perpendicular to the direction of current flow. Because of the similarity of the Hall effect, this effect is called planar Hall effect. In general the Hall effect, the voltage change because the magnetic field perpendicular to the film, because the planar Hall effect, the magnetic field is the same as the current flow. Planar Hall effect is rarely used for practical purposes because the voltages involved are very small.


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Magnetoresistive sensors

The calculation of the angle Θ between M and the easy axis and the dependence of the electrical resistance in the direction of M will be combined to evaluate the sensor. Also introduced a new resistance R0, p and R0:

(4)

(5) R0 is the average resistance, can be calculated using the AMR sensors simple

characterization of new parameters. Figure 2(a) illustrates the dependence of resistance on the angle between the current flow and magnetic force. Eqs. (2) and (4) lead to resistance R (Θ):

(6) Eq. (3) to calculate the resistance in dependence of the measured field Hy. Figure 2(b)

illustrates this dependence. For real measurement, the magnetic force F changed entirely the hard axis for a very strong field alone. Therefore, there is a smooth transition to the saturation resistance:

(7)

(8)

For |Hy| ≤ H0 and for |Hy|> H0. Resistance depends non-linearly on the external field. Furthermore, the sensitivity of dR/DHY very small area adjacent to the origin (and disappears entirely for Hy = 0). Further weakness of this setup is that the sign of Hy can not be determined because R is a function of Hy

2.

(a) (b) (c)

Figure 2. (a) Resistance in the x direction as a function of the angle Θ between the current I and the magnetic force M. (b) Resistance ferromagnetic thin films as a function of the transverse field Hy. (c) The current flows in a barber pole structure, and resistance R ferromagnetic thin films with a barber pole structure as a function of the transverse field Hy.


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Barber pole

In order to reduce losses, barber-pole structure is shown in the image above. Barber-pole structure consists of a series of strips of high electrical conductivity that forces the flow of current into a 45 ° angle to the x-axis. Figure 2(c) shows that the current path is distorted by the barber pole. The layout of the optimum width and spacing poles barber pole is important (Laimer & Kolar, 2000). Strip either did reduce the total resistance, they also reduce the active part of the surface where the resistance changes contribute to the sensor signal. Mathematically, a barber-pole represented by introducing additional angle ψ = 45 °, which is the angle between the axis and currents.

The angle Θ in this case:

(9) Barber-pole characteristics of AMR sensor 10 is formulated with the following equation:

(10) A graphical representation of a barber-pole characteristics of AMR sensor is shown in

Figure 2(c). For H0 < Hy/2, is quite linear with the non-linearity of less than 5%. This behavior applies only if the magnetic field spontaneously without outside in the positive x direction. Changes in resistance changes its sign if spontaneous magnetisation is flipped to the negative x direction. Reversing the spontaneous magnetic force can be used to determine the value of R0 accurate as the arithmetic mean value of the two resistance values before and after flipping.

In order to change the resistance to voltage changes without dc component, the sensor is realized as a Wheatstone bridge with four individual resistors. This approach shows one more advantage of the barber pole structure: With using barber pole under 45° and 135°, respectively, resistor with positive and negative ΔR in the linear range can be realized. In order to obtain the maximum output voltage, the resistor has two diagonally opposite pole barber below 45°, and the other two, below 135° (see Figure 3). This setup also compensates for the temperature dependence of the resistor.

Figure 3. Wheatstone bridge with four magnetoresistive devices. "+" Indicates a barber pole below 45°, and "-", below 135°.


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Sensitivity and measurement

The output voltage of the Wheatstone bridge can be explained by:

(11) The sensitivity of the sensor measurement results

(12) Thus the sensitivity can be improved by using a material that AMR effect with the

characteristics of high and low field H0. Linear behavior of the sensor with an error of less than 5% in the range of a-H0 / 2 for H0 / 2. It is possible, to improve the measurement range by applying a magnetic field compensation (ie, with compensation-null bridge). Since the sensor is always operating in the region of zero field, the non-linearity will have no effect. The maximum resolution in this case depends on the stability of the magnetic film.

In addition, the layout of the magnetoresistive element forming the Wheatstone bridge must be optimized. Demagnetising achieve homogeneous and small field, an elliptical shape AMR array is proposed [6]. By using the compensation coil integrator output, the sensor can be operated in zero magnetic field. Linear output response (V0 voltage versus applied field Ha) is comparable (resistor R) for the applicable compensation coils. Both compensation flips and coil conductor in the form of a thin layer mean deric, shown in Figure 4.

Figure 4. Electronic circuits with flip (Lf) and compensation (Lc) rolls (Hauset et al., 2000).

There are two definitions of the sensitivity of AMR sensors in bridge arrangement: 1.

S0 = two / DHY / UB, and 2 × SU = Umax two / DHY / UB. The advantage of the second definition is that it also takes into account the maximum energy dissipation (Pmax = UB, MAX2 / R) on the sensor. The supply voltage can not be done suddenly becomes high. For a comparison of the sensitivity of the sensor, use the first definition in this paper, as is done by most of the literature.

3. Metodology

The design of this sensor includes a sensor system design, NiCr alloy deposition with DC-Sputtering method with and without the influence of a magnetic (magnetization) and characterization. In the first stage is to design AMR sensors. Here is the explanation.


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Materials AMR

Magnetoresistive film materials consist of: • high coefficient �� / � � on high. • Low temperature dependence of �. • low anisotropy field Hk. • Low Coersivitas • zero magnetostriction • Have a high stability.

The materials most suitable is a compound or alloy (Alloy) of elements such as Ni, Fe, Co Especially the material of Permalloy Ni81 / Fe19 or alloy Ni80Cr20. By using magnetoresistive anisotropy effects to design a magnetoresistive microsensor with a thin layer of permalloy to detect the small magnetic field: 0.1 to 5 mT.

Barber pole

To reduce losses, barber pole structure consists of a series of strips of high electrical conductivity by forcing the flow of current into a 45 ° angle to the x-axis. The layout of the optimum width and barber pole-pole distance is important [7]. Mathematically, a barber-pole represented by introducing additional angle ψ = 45 °, which is the angle between the axis and currents.

Linearized

Magnetoresistive effect can be linearized by depositing aluminum lines (Barber-Pole), at the top of the strip (line) permalloy at 45� angle with the axis of the strip (see Figure 5). Aluminum has a higher conductivity than permalloy, Barber-Pole effect is to rotate the direction of flow at an angle of 45� and effectively change the angle of rotation of the magnetization relative to the flow of � � for � � � � 45�.

Figure 5. linearization magnetoresistive effect.

Sensitivity and measurement range

Thus the sensitivity can be improved by using a material that AMR effect with the characteristics of high and low field H0. This sensor shows a linear state with an error of less than 5% in the range of a-H0 / 2 for H0 / 2. It is possible to increase the range of measurements to apply a magnetic field compensation (ie, with compensation-null bridge). Since the sensor always operates zero field region, the non-linearity will have no effect. The maximum resolution in this case depends on the stability of the magnetic film. In addition, the layout of the magnetoresistive element forming the Wheatstone bridge must be optimized.


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4. Results and discussion

4.1 Design AMR sensor

From the methodology of research that has been described previously, the following are the results of AMR sensor design to be created (see Figure 6). From Figure 6 has been designed AMR sensor is composed of four layers, namely a layer of thin film alloy Ni80Cr20, barber pole layer, a layer of titanium, and the bonding pad. Figure 6 shows the Layer 1 in the form of alloy layer (Ni80Cr20), Layer 2. Barber Pole (Al / Au), Layer 3. Layer Titanium (Ti) and Layer 4. Bonding Pad (Ag / Au).

Layer 1. Nickel-Chrom alloy

(Ni80Cr20)

Layer 3. Lapisan Titanium (Ti)

Layer 2. Barber Pole (Al/Au)

Layer 4. Bonding Pad (Ag/Au)

Figure 6. Design of AMR sensor consists of 4 layer.

The four layers are printed on a silicon wafer substrate with a diameter of 3 inches.

Figure 7 shows the location of the AMR sensor arrangement in a 3-inch silicon wafer can be made 2 AMR sensor devices.

Figure 7. AMR Sensor Layout in Wafer Silicon (Si) with a diameter of 3 inches.


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4.2 Characterization of Thin Film Coating Alloy Ni80Cr20

4.2.1 Alloy Thin Film Deposition by Sputtering method

In Figure 8, Ni80Cr20 deposition process with the DC-sputtering method, the thickness will increase linearly with increasing time.

In Figure 9 of the deposited Ni80Cr20 this, the argon gas pressure of 15 mTorr produces Ni80-Cr20 deposition is relatively higher, compared to the pressure of 25 mTorr.

The higher the argon gas pressure, the slower the speed deposition, even at pressures above 130 mTorr argon gas deposition process would be slow, because of the partial material settles back on the cathode due to the diffusion process.

The deposition speed will increase with the increase in power, especially at 100–300 watts, but for the addition of 350–450 watts of power, the increase in speed deposition shows nearly constant value.

Figure 8. The film thickness curve Ni80Cr20 against time.

Figure 9. Characteristic curves for Ni80Cr20 pendeposisian speed.

Figure 10. The result of the measurement by means of GMR meter MR on Thin Film Coatings sample Ni80Cr20 with deposition time of 30 minutes.


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Characterization by SEM and EDS

Figure 11. Curve EDS of Thin Film Coatings Ni80Cr20 results with DC-Sputtering deposition.

Figure 12. Thin Film Surface Morphology Ni80Cr20 Results DC-Sputtering with 10 minutes and 40 000X magnification.

Figure 13. Morphology of Thin Film Coating Growth Ni80Cr20 Results DC-Sputtering with 10 minutes and 40 000X magnification

5. Conclusion

AMR sensor has been designed consisting of four layers (layer) is a layer-1: alloy Ni80Cr20, layer-2: barber pole, 3 layers: titanium, and layer 4: bonding pad.


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In addition it has done a thin layer deposition process Ni80Cr20 with DC-Sputtering method with and without the influence of a magnetic (magnetization). Deposition process is carried out variable thickness vs. deposition time resulting linear deposition. Also has done a variable power (watts) vs. deposition speed at various argon gas flow (Ar). In Thin Film coating Ni80Cr20 within the magnetic field (magnetic rod is brought 2) the results of magnetism higher than without the influence of the magnet. Ni80Cr20 material was measured by a GMR meter showed a fairly good magnetic properties (Figure 10).

The results of characterization by EDS indicated deposited silicon substrate with a thin film layer Ni80Cr20 and morphology Ni80Cr20 thin layer with a thickness ranging from about 50 nm.

References Thomson, W. (1857). On the electro-dynamic qualities of metals: Effects of magnetization on the

electric conductivity of nickel and of iron. Proceedings of the Royal Society of London, 8, 546–550.

Baibich, M.N., Broto, J.M. , Fert, A., Nguyen Van Dau, F., Petroff, F., Etienne, P., Creuzet, G., Friederich, A., & Chazelas, J. (1988). Giant magnetoresistance of (001)Fe/(001)Cr magnetic superlattices. Phys. Rev. Lett., 61, 2472.

Dibbern, U. (1989). Magnetoresistive sensors. In Gopel, W., Hesse, J., & Zemel J.N. (Eds.), Sensors, Vol. 5, Magnetic sensors (Vol. Editors.: Boll, R., & Overshott, K. J.), 342–379. Weinheim: VCH.

Fasching, G.M. (1994). Werkstoffe für die elektrotechnik. New York: Springer. Hauser, H., & Fulmek, P. (1992). The effect of mechanical stress on the magnetization curves of Ni-

and FeSi-single crystals at strong fields. IEEE Trans. Magn., 28, 1815–1825. Hauser, H, Stangl, G., Chabicovky, R., & Riedling, K. (2000). Magnetoresistive sensors. TU Wien,

Gusshausstrasse 27/366, A-1040 Vienna, Austria. Laimer, G., & Kolar, J.W. (2000). Design and experimental analysis of a DC to 1 MHz closed loop

magnetoresistive current sensor. Swiss Federal Institute of Technology (ETH) Zurich. Fulmek, P., & Hauser, H. (1993). Simulation of the magnetization process in anisotropic ferromagnetic

materials by energy areas. Elsevier Studies in Applied Electromagnetics in Materials, 5, 327–330.

technology of anisotropic magneto resistive sensor on ......a real interest and meaningful....

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