Ž .Sensors and Actuators 73 1999 68–73

A novel silicon surface micromachining angle sensor

Jorg R. Kaienburg ), Ralf Schellin 1¨AutomotiÕe Equipment DiÕision 8, Department K8rSTZ, Robert Bosch Gmbtt, POB 1342, D-72703 Reutlingen, Germany

Received 27 April 1998; accepted 7 October 1998

Abstract

A novel angle sensor is reported: in contrast to other angle detection sensors, this sensor is completely based on silicon surfacemicromachining technology. Hence, its fabrication is fully compatible with the fabrication of other surface micromachined sensors, likeaccelerometers, yaw rate sensors, etc. The principle of operation of this sensor is based on the well-known Lorentz force in combinationwith a differential capacitance sensing technique yielding an accurate, contactless, and therefore, wear-free angle sensor. q 1999 ElsevierScience S.A. All rights reserved.

Keywords: Angle sensor; Surface micromachining technology; Lorentz force

1. Introduction

Due to the requirements on automotive safety and en-gine control systems, the number and complexity of sensor

w xapplications will increase rapidly in the next years 1 .Besides acceleration, yaw rate and pressure sensors, pre-cise and reliable angle measurement is important for auto-motive systems. Sensors used in the automotive markethave to be resistant to severe conditions. Because of theirwear-freeness, contactless angle sensors are preferred inthis case.

Sensors enabling such a contactless angle detection arealready available. These sensors can be realized usingdifferent signal transformation methods: capacitive pick-upw x w x2,3 , the anisotropic magnetoresistance effect 4–6 , the

w x w xHall effect 7,8 , a lateral bipolar magnetotransistor 9 , orw xoptical encoding 10–12 . But some of the transducer

Žprinciples do not allow a complete 3608-measurement e.g.,w x.Refs. 4–6 . A strong dependence on temperature is an-

Ž w x.other problem of some of those sensors e.g., Refs. 7–9 .The sensor discussed in this paper enables a precise,

contactless, and wear-free angle detection with a range ofw x3608 13 . Additionally, the dependence on temperature of

the sensing elements are negligible. The sensor is based on

) Corresponding author. Tel.: q49-7121-35-4102; Fax: q49-7121-35-4173; E-mail: joerg.kaienburg@rt.bosch.de

1 Tel.: q49-7121-35-4172; Fax: q49-7121-35-4173; E-mail:ralf.schellin@rt.bosch.de.

standard surface micromachining technology and was fab-ricated using the Robert Bosch Gmbtt surface microma-

w xchining foundry service 14 . Regarding industrial massproduction, this compatibility with other micromachinedsensors, like acceleration and yaw rate sensors, is a very

Ž .important feature as already mentioned above .

2. Operating principle

In Fig. 1, the sensing element of this sensor is depictedschematically. Basically, the sensing element consists of amovable polysilicon structure suspended by two torsion

Žbars forming a micromachined pendulum structure Figs. 1.and 2 . Obviously, position and orientation of the torsion

axis is defined by the torsion bars. As shown in Fig. 1, acurrent I, typically 1 mA, flows through the lower part lfb

of the pendulum. In order to achieve this well-defined pathof the current, a slit was introduced into one side of the

Ž .structure Fig. 2 . Hence, the current I flows exactly asshown in Fig. 1.

In the presence of an external, preferably horizontal andŽuniaxial magnetic field of flux density B here: about 160

.mT the resulting Lorentz force F leads to a deflection ofL

the pendulum structure. This deflection is described as thetorsional angle q . According to this deflection, both capac-

Ž .itors, C and C , change their values Fig. 3 . This can be1 2

detected capacitively using C and C as sensing capaci-1 2

0924-4247r99r$ - see front matter q 1999 Elsevier Science S.A. All rights reserved.Ž .PII: S0924-4247 98 00256-8

( )J.R. Kaienburg, R. SchellinrSensors and Actuators 73 1999 68–73 69

tors. These are formed by two electrodes underneath theŽpendulum structure and the pendulum itself Figs. 1 and

.2 . Using a special charge amplifier in combination with aŽ .lock-in amplifier Section 4 , the changes in the capaci-

tances are transformed into an electrical output signal. The™

angle a , enclosed by the magnetic field B and the current™I, is the quantity to be measured. It is changed by rotatingthe magnetic field around the z-axis.

An improvement of the sensing elements, e.g., lowerresistance, was achieved by attaching them to their torsion

Ž .bars from the inner side Fig. 2 . The torsion axis of thependulum structure shown in Fig. 2 is oriented vertically.The current is fed by the anchors and torsion bars. Theanchors are contacted electrically by burial conductorsŽ .LPCVD-Si, Fig. 4 . Both sensing capacitors C and C1 2

are of a rectangular shape and placed below the pendulumŽstructure on the right and left hand side, respectively Fig.

.2 .In Fig. 4, a detailed view of one torsion bar is shown.

At one end, the torsion bar is attached to an anchor and atŽthe other end, it is fixed to the sensing element meshed

.structure . Below the torsion bar, a burial conductor, towhich the anchor is contacted electrically, is visible.

3. Theoretical considerations

The mechanical behaviour of one sensing element isdefined by three torques.

Ž .i Due to the Lorentz force

™ ™ ™F s l I=B 1Ž .L f b

the torque™™< < < <M s r =F A IBsin a 2Ž . Ž .L L L

™results and deflects the pendulum structure, where r isL

Fig. 1. Schematic view of one sensing element.

Ž 2 .Fig. 2. Top view of one sensing element size: 950=750 mm .

the lever of the Lorentz force and a the quantity to bemeasured.

Ž .ii Electrostatic forces between both pairs of electrodesyield another deflecting torque. In the linear region, it canbe written as

™ ™™< < < <M s r = F yF Aq , 3Ž .ž /el el el ,1 el ,2

™ ™where F and F are the electrostatic forces betweenel ,1 el,2

™Ž .the electrodes of both capacitors C , C and r is the1 2 el

lever of the electrostatic forces. This torque can be ne-glected in cases of potential differences in the order of 10mV between the electrodes. Therefore, the current shouldbe limited to several milliamperes.

Ž .iii According to the torsions of both torsion bars therepulsive torque

M s2k q 4Ž .q tb

is given, where k is the torsional stiffness of one torsiontb

bar and is determined by its mechanical properties anddimensions. The torsional stiffness can be influenced byaltering the width w and length l of the torsion bar; it istb tb

w xexpressed by 15 :

b w3tb

k s h G , 5Ž .tb tb3 ltb

where G is the shear modulus, h is the height of thetb

torsion bar, and b is a factor depending on the cross-sec-tion of the torsion bar. For a beam of a rectangular

Ž .cross-section, width w and height h )w b istb tb tb

defined as follows:

`192 w 1 2 iy1 hŽ .tb tbbs1y tanh p .Ý5 5 ž /h 2 wp 2 iy1Ž .tb tbls1

6Ž .

( )J.R. Kaienburg, R. SchellinrSensors and Actuators 73 1999 68–7370

Fig. 3. Electromechanical transducer principle of one sensing element.

In the linear region, the condition of equilibrium

M sM qM fM 7Ž .q el L L

yields:

ltbq a A IBsin a . 8Ž . Ž . Ž .3wtb

C , C , and their difference C yC change their values1 2 1 2

according to a change of q and a , respectively:

ltbC yC A IBsin a . 9Ž . Ž .1 2 3wtb

The difference C yC can be transformed into a voltage1 2Ž .output signal which can be measured easily Section 4 .

w xUsing only one pendulum angles ag y908; q908

Ž .can be detected definitely Fig. 7 . Adding a second sens-ing element identical to the first and mounted mutually

Ž .perpendicular in plane a 3608-measurement can be per-formed. This arrangement of two pendulum structuresleads to a 908-phase shift between the output signals ofboth pendulum structures. In Fig. 12, the output signals ofsuch an arrangement are shown. They are very suitable fora following processing.

4. Circuit description

Fig. 5 depicts a schematic view of a sensor with itsreadout circuit. The sensor contains two pendulum struc-

Fig. 4. Scanning electron microscope photo of one torsion bar.

Ž .tures A, B mounted mutually perpendicular. The neces-sary current is provided by using a DC-bias U . Further-DC

more, both sensing elements are supplied with an AC-volt-age U which is used as a driving high frequency voltageACŽ f 4 f , where f is the resonant frequency of theAC 0 0

.pendulum structure :

U sU qU 10Ž .in DC AC .

The signals are processed by the use of two chargeamplifiers and one differential amplifier per structure.Hence, C yC and C yC , respectively, can be1,A 2,A 1,B 2,B

transformed into output voltages

ltbU A IBsin a U 11Ž . Ž .out ,A ,B AC .3wtb

A lock-in amplifier can be used to demodulate U .out,A,B

5. Technology

Several prototypes were fabricated using the RobertBosch Gmbtt surface micromachining foundry service. Ontop of a 6Y Si-wafer an insulating SiO -layer is grown2

thermally followed by the deposition of a poly-Si layerŽ . w xLPCVD 16 . This layer is structured to be used as lower

Ž .electrodes and conductors Fig. 6a . Then, a second oxide,Ž .the sacrificial layer, is grown and structured Fig. 6b . A

thick poly-Si layer is deposited in an epitaxial reactorŽ .so-called EPIPOLY . On top of this EPIPOLY aluminum

Žis sputtered and patterned to form bonding pads metalliza-.tion, Fig. 6c Then, the EPIPOLY is trenched with excel-

lent aspect ratios in depth. Subsequently, the sacrificialŽ .layer is etched by the use of a vapor HF Fig. 6d . Next,

Fig. 5. Block diagram of the sensor and its circuit.

( )J.R. Kaienburg, R. SchellinrSensors and Actuators 73 1999 68–73 71

Fig. 6. Process used in the Robert Bosch surface micromachining foundryservice.

the sensors are sealed on wafer level using a special capwafer. Finally the sensors are diced.

6. Experimental results

Several sensors with different sensitivities were realizedand characterized. In Fig. 7, an output signal of onependulum structure is compared with the simulation re-sults, whereas in Fig. 8 various output signals of different

Fig. 7. Theory vs. experiment—angle measurement with one pendulumstructure.

Fig. 8. Comparison of output signals of different sensing elements withdistinct sensitivities.

Ž .Fig. 9. Sensitivity vs. length l normalized to l .tb tb,0

sensing elements are compared to each other. Apart from aslight offset, both curves match very well in Fig. 7. Thedifferences in magnitude especially at af"908 are causedby a mismatch in the readout circuit and by the imperfectsymmetry of the pendulum structures. These deviationsbetween theoretical and experimental results increased forhigher sensitivities. Therefore, the pendulum structureshave to be designed highly symmetrical to obtain the bestmatching.

If only one sensing element is used, definite measure-w xments of angles ag y908; q908 are possible. As ex-

pected, the maximum resolution was observed at as08 as

Ž .Fig. 10. Sensitivity vs. width w normalized to w .tb tb,0

Fig. 11. Comparison of distinct output signals of the same sensingelement which is fed with different currents.

( )J.R. Kaienburg, R. SchellinrSensors and Actuators 73 1999 68–7372

Table 1Ž . Ž .SNR and resolution A a vs. current I Da s18, a s08

w x Ž . Ž . w xI mA SNR a A a 8

1 36:1 0.0282 75:1 0.0133 127:1 0.008

operating point. The resolution of the sensor can be im-proved by either increasing the mechanical sensitivity or

Ž Ž ..by using a higher current Eq. 11 . Fig. 8 comparesdifferent output signals of sensing elements with distinctsensitivities. Corresponding to Fig. 8, the following resolu-

Žtions were measured: 0.0258, 0.0298, and 0.0358 operating.point: as08, Is1 mA .

On the one hand, it is possible to tune the sensitivity ofone pendulum structure by tailoring the dimensions of thetorsion bars. A variation of w is useful for a coarsetb

Ž Ž ..tuning Eq. 5 . To enable a fine tuning of the sensitivity,it is recommended to vary the length l of the torsiontb

bars. Both possibilities of tuning were investigated experi-mentally. Fig. 9 shows the sensitivity E with respect toa

changes of l . The proportionality E A l is obvious. Intb a tb

Fig. 10, the dependence of E on the width w is shown.a tb

As expected, a wy3-dependence was found. It must betb

remarked that both variations lead to a change of theresulting resistance of the sensing elements. This effect canbe compensated by either adapting the bias U to gain aDC

constant current or by increasing the resulting resistanceby the use of additional resistors.

On the other hand, varying the current offers a furtherpossibility of tuning the sensors’ sensitivities. As predicted

Ž .by Eq. 11 , the measured output signals are proportionalto I. In Fig. 11, distinct output signals of the same sensingelement carrying different currents are shown. As ex-pected, an increase of the current I leads to a higheroutput signal and to a higher SNR, respectively. It must beremarked that no change of the noise was observed while

Ž .varying the current. The resulting resolutions Das18

are listed in Table 1.Fig. 12 depicts the output signals of a 3608-sensor. Both

Ž .curves show the expected shape of a cosine U andout,A

Fig. 12. Full range output signals of a 3608-sensor.

Ž .sine U , respectively U is of a nearly perfectout,B out,B

sinusoidal shape with deviations from the theory explainedabove. In contrast to U , U is of a less perfectout,B out,A

shape. This is due to a certain mismatch in the read-outcircuit. The matching of the experimental results with thetheory can be improved by eliminating mismatches in thereadout circuit and, additionally, by optimizing the symme-try of the pendulum structures.

7. Conclusions

A new angle detection sensor is presented. Severaltypes of this sensor were realized using surface microma-

Žchining technology Robert Bosch Gmbtt surface microma-.chining foundry service . Different resolutions were ob-

tained by different designs of different sensitivities andcurrents, respectively. The best resolution we measured is

Ž .0.0088 operating point: as08, Is3 mA .Although the natural range of detectable angles of one

w xsensing element is limited to ag y908; q908 , an ex-pansion to a 3608-sensor can be achieved easily by mount-ing a second and identical sensing element mutually per-

Ž .pendicular in plane . Then, both output signals show aphase shift of 908 which is very suitable for a furthersignal processing in angle detection applications. Thisarrangement of two sensing elements was tested success-fully.

Acknowledgements

The authors would like to thank Dr. B. Maihofer and¨M. Lutz for many useful suggestions. Also, the authors arevery grateful to Dr. K. Marx for simulating the magneticfield and to the wafer fab-team for the fabrication of thesesensors.

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