Smart CMOS Sensors with Integrated Magnetic Concentrators

C. Schott, R. Racz, S. Huber Melexis n.v.

Bevaix, Switzerland csc@melexis.com

Abstract— The combination of Integrated Magnetic Concentrators (IMC) and modern CMOS Hall sensor technology extends the frontiers of magnetic sensing to new dimensions. Single-axis sensors are turned into multi-axis sensors while signal quality and resolution are enhanced by up to one order of magnitude. Integrated Magnetic Concentrators have a deep impact on sensor system design. On one hand, the material properties of the added magnetic layer and the tolerances arising from its deposition and structuring have to be taken into account. On the other hand multi-axis sensors offer the possibility to work with ratios of magnetic field components rather than with a single field component. This results in ageing and drift effects of sensor and magnet being virtually eliminated.

I. INTRODUCTION The idea of combining an on-chip magnetic layer

(IMC=Integrated Magnetic Concentrator) with silicon Hall devices is about 10 years old [1]. It was first used for precise current sensing and then for angular position sensing [2]. The manufacturing process is a low cost batch process, so that thousands of sensors are realized in a precise, reliable and low-cost way. First the ferromagnetic layer is bonded to the wafer and then it is structured by photolithographic etching, leaving only the desired structures on the surface. To date over 1 million such sensors have been commercially manufactured. First generation sensors only contained very basic analog features like amplification and filtering, later they were fitted with more complex architectures including dynamic offset compensation and parameter programming. With the introduction of IMC technology to modern deep submicron processes, such sensors are now equipped with powerful digital signal processing and interfacing making them very versatile for many applications. In this paper we first address some general properties of IMC Hall technology and then we illustrate its potential by two application examples.

II. FEATURES OF IMC-HALL

A. Multi-axis Sensing A Hall element which is positioned under the periphery

of an IMC is sensitive to two magnetic field components: a first one vertical to the chip surface, as if the IMC was

magnetically transparent, and a second one in the plane of the chip surface. Considering these two components we derive two rules for magnetic measurement with IMC:

a) the output voltages of two Hall elements under opposite edges of an IMC must be mutually subtracted to measure only the in-plane component and they must be added to measure only the vertical field component.

b) Hall elements can be placed under any point of the periphery of an IMC and they can therefore be used to measure any in-plane component.

Applying both rules, a three-axis Hall sensor can be implemented [3]. Generally speaking multi-axis sensing is the key to position measurement without errors from temperature drift and ageing effects of magnet and sensor. This comes from the fact that multi-axis sensing allows for position measurement by a change in field direction rather than by a change in field strength. Field direction in turn is derived from the ratio of measured field components. If the field strength changes, two components change to the same extent and their ratio remains unchanged. The same principle applies in analogy for sensitivity change as long as the measurement axes are matched. As we implement all three measurement axes within a few hundred microns on the same silicon crystal, the resulting matching is excellent.

B. Low Field Sensing Due to the high permeability of the IMC material, the

flux density in the vicinity of the IMC edge can be significantly increased compared to the applied external field. In such a way the IMC works as passive magnetic amplifier of up to a factor of about 10. This results in an increase of the signal-to-noise and signal-to-offset ratios.

C. Requires small Hall elements As the magnetic concentrators are usually about 20µm

thick and a few hundred µm long and wide, the regions on the chip where the IMC yields a high passive magnetic amplification are only a few micrometers in size (Figure 1). Therefore the Hall elements which are used must be very small to take fully advantage from this gain. Small Hall elements however suffer from higher offset and low-frequency noise, so that the sensor needs to be adapted. Such

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adaptation for example is the use of multiple Hall elements and of dynamic offset cancellation with higher frequency.

Figure 1 FEM Simulation of a 200µm Ø IMC disk with applied horizontal field. The colors are scaling the vertical field component under the disk. The strong field zone under the edge is only a few microns wide (blue). Therefore Hall elements used with IMC’s need to be very small

D. Hysteresis and Saturation Although the chosen IMC material is magnetically very

soft, it exhibits a residual magnetization effect in the micro-tesla range after exposure to a strong field. This so-called perming leads for low-field measurement applications to output errors which need to be compensated.

On the other hand saturates the IMC material at higher field, which leads for example to linearity errors in angle sensors [4]. Depending on the shape and dimensions the on-set of saturation ranges from sub-millitesla for narrow (few microns) and large (>1mm) structures up to several ten or hundreds of millitesla for small and compact structures.

E. Tackling Process Imperfections For multi-axis IMC-Hall sensors, process imperfections

like mask misalignment and etching tolerances lead to mismatch in sensitivity and orthogonality between measurement axes. This results in errors on the output which need to be compensated. Figure 2 shows the angular non-linearity of a two-axis sensor uncalibrated, with offset calibration and additionally with sensitivity matching and orthogonality calibration. After those calibration steps the absolute accuracy of the angle sensor is better than 0.5º.

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Figure 2 Typical calibration steps of a 360º angle sensor: An initial error of ±3º is reduced to ±0.3º

III. UNIVERSAL POSITION SENSOR A first example for the versatility of IMC technology is

its application in a universal magnetic position sensor. Such a sensor is used in general in combination with a small magnet which translates or rotates with respect to the sensor. The sensor detects the change in field amplitude and/or field direction and generates a position signal from this change. A purely analog IMC three-axis position sensor in 0.8um standard CMOS technology was presented in [3]. An intelligent digital version of this sensor has also been recently developed [5]. This sensor features a straightforward digital architecture with AD converter, 16-Bit microcontroller, memory and analog and digital interfaces (Figure 3).

Figure 3 Block Schematic of the intelligent three-axis sensor realized in

0.35 µm CMOS technology

For each acquisition, the microcontroller selects via the multiplexer the channel of the triaxis magnetic front-end. The data of several acquisitions is then processed to generate the desired output signal. Even complex data processing like calculation of the arctangent function of the ratio of two magnetic field components is performed in fractions of a millisecond. By such, the absolute position angle of a rotating magnet is measured rapidly, precisely and reliably. Another application where all three magnetic field components are processed, is a very temperature stable two-axis magnetic joystick. The two spherical travel angles are here computed from two ratios of field components.

Within an application, the sensor typically contains some application-specific program code in the ROM and some user-programmable data in the EEPROM. Additional features are auto-gain control of the analog amplification and ADC overflow detection. Such features allow the input range of the AD converter to be fully used for best resolution and for failure conditions, like the absence of the source magnet, to be detected. The microcontroller can further be used to convert pre-processed data into a user-definable piecewise linear function or to calculate the time derivative of a signal, e.g. the rotation speed.

Figure 4 shows a photograph of the silicon die with the various electronic blocks and the IMC disk. The die is small enough to fit into a standard SO-8 package or as twin-die solution for total redundancy applications into a standard TSSOP-16 package.

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Figure 4 Photograph of the intelligent three-axis sensor and electron microscope magnification of the 200µm diameter IMC on it. The total chip

dimensions are about 2.6 x 1.8mm.

IV. ELECTRONIC COMPASS A second example for illustrating the performance of

IMC technology is the solid state electronic compass. For this application we take advantage from the IMC magnetic gain and the IMC multi-axis feature and we need to tackle hysteresis effects.

A. Analog Two-Axis Earth Field Sensor A first purely analog structure of an IMC based

electronic two-axis sensor in standard 0.8um technology for earth-magnetic field measurement was presented in [6]. The IMC structure is composed of five rings with a four-loop shaking coil wound around them (Figure 5). The Hall elements are placed under the rings close to the gap between them. This integrated IMC structure provides for a magnetic gain of about 8.

The block schematic of this chip is shown in Figure 6. The chip features a sensitivity of 8000V/T for X and Y and the analog signal needs to be averaged and processed by an external data acquisition system to reach a heading resolution of better than 1° in earth magnetic field. As all Hall elements are integrated into the same chip, they intrinsically feature good matching of sensitivity. Hall offset is eliminated by spinning current modulation.

Bond wires

Figure 5 Photograph of the 5-ring IMC structure with the four-loop shaking coil wound around it. The lower part of the loops is realized in

CMOS metal and its upper parts as wire bonding connections

The modulated signals are then multiplexed through the same amplification chain. After demodulation, the signals are sampled and held, offset corrected and buffered to the outputs. A programmable orthogonality correction is implemented by adding part of the X signal to Y and vice versa. By this procedure the measurement axes are rotated with respect to each other to finally form a perfectly orthogonal pair. Sensitivity correction is implemented similarly, by adding a small amount of X to X and/or of Y to Y.

Figure 6 Block schematics of the analog two-axis electronic compass realized in 0.8 µm CMOS technology.

To compensate for hysteresis and perming effects, we implemented the “magnetic shaking” principle on IMC ring structures here for the first time (Figure 7).

The 20mA current pulse for the shaking is provided by an external current source. A short current pulse is sent through the center of each ring and generates a circular magnetic field that “shakes” the magnetic domains inside the ring and aligns them in a closed circular way. Thus the flux lines are bound within the IMC and do not create magnetic offset anymore. The method requires current pulses of only a few mA with a duration of just microseconds making the magnetic shaking method suitable for even battery powered applications.

IMCDAC Amplifier

Chain Regulator ADC

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ROMDual Output Stage

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Figure 7 Magnetic Shaking: The IMC ring structure gets magnetized by an external field (a). After taking away the field, a residual magnetization

remains and leads to offset (b). By applying a current pulse of a few mA for a few us,(c), the flux lines are closed inside the ring and magnetic offset is

eliminated (d)

The comparison of remaining magnetization (perming) without and with magnetic shaking is plotted in Figure 8.

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Figure 8 Measurement of the magnetic hysteresis for a field sweep between –500uT and+500uT. Without shaking the hysteresis is almost

20uT, whereas with shaking it is reduced to below 1uT.

Thus shaking provides an efficient method of reducing material related magnetization effects to less than 0.5uT, so that the direction of the earth magnetic field can be measured with better than 1°.

B. Digital Three Axis Earth Field Sensor Based on this analog version, a second generation

prototype was developed in 0.35µm standard CMOS technology. Whereas the IMC front end was kept the same, the signals of all three axes (X,Y and Z) are now on-chip converted to digital data and output via an SPI interface (Figure 9). The SPI interface also allows for access to a configuration register, where such parameters as analog gain, ADC resolution, clock speed, and also several test modes can be accessed. An automatic mode provides for cyclic time

slicing acquisition between the three axes X, Y and Z every 10ms.

Figure 9 Block schematics of the digital three-axis electronic compass realized in 0.35 µm CMOS technology

The 12-bit AD converter is a state-of-the-art two stage converter with an integrating Σ∆ first stage for the 6 MSBs and an algorithmic second stage which delivers the 6 LSBs. The default analog gain of the system is 10’000 which yields a full scale range of 100µT for X and Y and 800µT for Z.

The shaking mechanism of this chip merely requires an external capacitor of a few ten nano farads which is charged by an internal current source with a current of a few microamps. The discharge of the capacitor which generates the shaking current for the IMC is triggered via an external pin by a host microcontroller.

V. CONCLUSION Based on IMC technology, intelligent multi-axis Hall

sensors for position measurement and earth field sensing are presented. We demonstrate that the inherent drawbacks of IMC technology related to process and material can be overcome by efficient means in sensor architecture so that the technology’s potential is fully exploited. Field direction measurement makes such sensors robust, digital signal processing makes them versatile and intelligent. All this combined in a small chip manufactured by standard CMOS technology leads to the perspective of a bright future for IMC technology.

REFERENCES [1] US Patent 5942895 [2] R.S. Popovic, C. Schott, P. Drljaca, R. Racz, , “A new CMOS Hall

angular position sensor”, tm – Technisches Messen, 68, June 2001, pp. 286-291

[3] C. Schott, R. Racz, S. Huber, “CMOS three axis Hall sensor and joystick application”, Proceedings of IEEE Sensors, Vienna, Austria, 24-27 Oct. 2004. vol. 2, pp. 977 - 980

[4] P. Drljaca, M. Demierre, C. Schott, R.S. Popovic, “Nonlinear Effects in magnetic angular position sensor with integrated Flux Concentrator”, Proc. of the 23rd Intern. Conf. on Microelectronics (MIEL 2002), VOL 1, Nis, Yugoslavia, 12-15 MAY, 2002

[5] V. Hiligsmann, P. Riendeau, “Monolithic 360 degrees rotary position sensor IC”, Proceedings of IEEE Sensors, Vienna, Austria, 24-27 Oct. 2004, vol. 3, pp. 1137 – 1142

[6] R. Racz, C. Schott, S. Huber, “Electronic compass sensor”, Proceedings of IEEE Sensors, Vienna, Austria, 24-27 Oct. 2004. vol. 3, pp. 1446 - 1449

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