In-Plane Sensitive Magnetoresistors as a Hall Device ^†

Abstract

A novel coupling of a pair of identical two-contact (2C) magnetoresistors transformed into an in-plane sensitive Hall device is presented. The ohmic contacts are cross-linked, also adding a load resistor bridge, providing for constant current mode of operation and eliminating the inevitable parasitic offset. This silicon configuration, apart from its simplified layout, has linear and odd output voltage as a function of the magnetic field and current. The quadratic and even magnetoresistance in the two parts of this innovative device is completely compensated, which ensures high measurement accuracy alongside with identification of the magnetic field polarity. The experimental prototypes feature sensitivity of 110 V/AT. The mean lowest detected magnetic induction B at supply current of 3 mA over frequency range f ≤ 100 Hz at a signal-to-noise ratio equal to unity is B_min ≈ 10 μT. The high performance and the complete electrical, temperature and technological matching of the parts of this unusual Hall device make it very promising for many practical applications.

1. Introduction

The well-known 4C in-plane sensitive Hall devices (HD) are widely used in numerous configurations as universal sensors for practical application and last, but not least—to compensate the main disadvantage in differential output transducers—the offset, frequently through dynamic cancellation techniques. In these architectures, one outer and one inner contact are the supply and the other two contacts are the output [1,2,3,4,5,6,7,8,9]. By switching the input and output electrodes of the Hall plate and summing-up algebraically the two obtained voltages, the offset can be significantly decreased [5,7,8]. These sensors, however, feature some drawbacks, such as: different values of the individual Hall potentials, generated on the two output terminals in a homogeneous magnetic field, output non-linearity, and offset. All these disadvantages result from the low inherent layout symmetry with asymmetry of the current lines with respect to the output terminals. Therefore, non-uniform Lorentz force action in different sections of the current paths is available. The described defects reduce the measurement accuracy. In this paper, a novel in-plane symmetrical HD based on a two magnetoresistor structures overcoming the mentioned disadvantages is presented.

2. Sensor Design and Operation Principle

The new configuration contains two identical n-Si regions established in parallel to one another. On the top surfaces, two ohmic contacts are formed—C₁, C₂ and C₃, C₄, respectively, Figure 1. A deep surrounding p-ring is also implemented, reducing the surface current’s spreading and confines the transducer region into the substrate bulk.

The magnetic field B is parallel to the long sides of the contacts. The pair of terminals are cross-coupled, C₂–C₃, whereas, between C₁ and C₄, high-impedance equal in value resistors R₁ = R₂ and a low-impedance trimmer r are connected. The inevitable offset V_H(0) ≠ 0 is fully compensated by trimmer r. The experimental prototype has been implemented using part of the processing steps applied in bipolar IC technology. The carrier’s concentration in the n-Si substrate is n ~ 4.3 x 10¹⁵ cm^-3. Similarly to [9], four masks are employed. The size of contacts C₁…C₄ is 50 x 10 μm², and the width of the p-ring on the chip surface is about 20 μm. The penetration of the current trajectory into the n-Si substrate reaches a depth of 30–40 μm. The novel in-plane sensitive Hall device is in hybrid realization (resistor elements R₁, R₂ and r are discrete).

Components I_C1,2 and I _C4,3 flow in the substrate when the supply ES is switched. The planar ohmic contacts C₁ … C₄ represent equipotential planes, to which, in the absence of external magnetic field B, B = 0, currents I_C1, I_C2, and I_C3, I_C4 respectively flow perpendicularly to the upper surfaces of the slabs, deeply penetrating into the bulk. The current lines I_C1,2 and I _C4,3 in the other parts of the two substrates are parallel to their upper surfaces. Therefore, the two trajectories I_C1,2 and I_C4,3 are curvilinear. As a result of the uniformity of the two substrates, as well as of contacts C₁ …C₄, the two currents I_C1,2 and—I _C4,3 are equal in magnitude and opposite in sign. If, as a result of technological imperfections, mechanical strain and stress during chip capsulation, temperature gradients and the like [4,5,6], at output V_H(B) of the device, at field B = 0, offset V_H(B = 0) ≠ 0 appears, notwithstanding that load resistors R₁ and R₂ are equal, R₁ = R₂. Varying the value of the low ohmic trimmer r, full compensation of the negative offset at differential output is achieved, V_H(B = 0) = 0. The resistors R₁ and R₂ are at least by one order greater than the effective resistance between the ohmic contacts C₁ …C₄.

The application of the measured magnetic field B in parallel to the long sides of contacts C₁ …C₄, Figure 1, results in deformation of current lines I_C1,2 and—I _C4,3 along the entire length of the non-linear trajectories, i.e. the electric symmetry of the trajectories is disturbed. This is due to the action of Lorentz force F_L, F_L = qV_dr x B, where q is the electron’s elementary charge, and V_dr is the vector of the mean drift velocity of the electrons in the substrates. Therefore, as a result of Lorentz deflection, depending on the specific directions of currents I_C1,2 and—I _C4,3 and of the magnetic field ± B, the trajectories shrink and expand, respectively. For this reason, in the bulks and on the surfaces with planar contacts C₁ …C₄, two effects are generated simultaneously. One of them is the quadratic even magnetoresistance MR ~ B², resulting in increase of the length of the trajectories of the current [4,6]. The other sensor mechanism is the linear and odd Hall effect V_H ~ ± B, resulting from the additional non-equilibrium charges on the surfaces with contacts C₁ and C₂, and C₃ and C₄, respectively. A key specific of the new solution is that the two substrates interact functionally and constitute an integrated sensor system, notwithstanding that the transducer zones are separated from one another.

The chosen original cross-coupling of the two pairs of magnetoresistors and the differential output V_H formed by the two load resistors R₁ and R₂ accomplishes synphase suppressing (compensation) of the parasitic quadratic voltage MR ~ B². Thus, the linear and odd Hall voltage V_H ~ ± B is the output signal of the new in-plane sensitive Hall device.

The output generates information simultaneously for the value of induction B and the direction (sign) of the magnetic vector ± B. The linear output Hall voltage V_C1,4(B) ≡ V_H(B) in Figure 1 increases significantly the metrological accuracy of the device.

3. Results

The output characteristics V_H(B) of the Hall configuration are presented in Figure 2a. The linear and odd output V_H(B) identifies the magnetic field polarity ± B. The non-linearity does not exceed 0.5% within the range +0.6 T ÷ −0.6 T, the sensitivity is SR ≈ 110 V/AT. The individual sensitivities on the two contacts C₁ and C₄ of the HD assessed by potentials V_C1(B) and V_C4(B), respectively, are equal, S_RI ≈ 55 V/AT, Figure 2b. The quadratic and even magnetoresistance MR ~ B² on the two terminals C₁ and C₄ is fully compensated.

The measured power spectral density of the internal noise V_C1,4(0) ≡ V_H(0) is shown in Figure 3, the supply current I_s is a parameter, T = 20 °C. The mean lowest detected magnetic induction B at supply current of 3 mA over a frequency range f ≤ 500 Hz at a signal-to-noise ratio equal to unity is B_min ≈ 10 μT.

4. Conclusions

The innovation of the arrangement and the original connection with one current source accomplish a magnetosensitive device with new properties—linear and odd voltage is generated at its differential output. For the first time, using two separate non-linear magnetoresistors, a linear Hall sensor is realized. The universality of the proposed HD and the simplified technological realization, as well as the promising results shows that it may be suitable for many industrial applications.