DUAL-AXES CAPACITIVE INCLINOMETER / LOW-g ACCELEROMETER FOR AUTOMOTIVE APPLICATIONS

Daniel Lapadatu, Soheil Habibi, Bjarg Reppen, Guttorm Salomonsen, Terje Kvisteray SensoNor asa, Knudsrodveien 7, P.O.Box 196, N-3192 Horten, Norway

ABSTRACT This paper reports a dual-axes capacitive inclinometer / low-g acce- lerometer based on a novel feedback operation concept. The device consists of a sensor die and an ASIC, housed in a dual-in-line transfer moulded package for surface mounting. The sensor is fabri- cated by bulk micromachining of silicon and triple stack anodic bonding and is primarily an accelerometer sensitive to Earth's gra- vitational field. Through the dedicated ASIC, the device can be programmed to operate either as a low-g accelerometer or as an in- clinometer. The device is designed to have a programmable range up to !I 2 g for the accelerometer and up to f 40Q for the inclinome- ter; a n d a resolution of 0.61 mg and 0.1", respectively.

INTRODUCTION The excellent mechanical properties of silicon, together with the availability of an advanced silicon microtechnology and a potential- ly large market for such automotive applications as roll-over detec- tion, was the driving force for developing a low-cost, small-size, high-performance dual-axes inclinometer / low-g accelerometer. In- clinometers and low-g accelerometers for automotive applications have been reported earlier [I, 21, therefore the main focus of this paper will be on the novel "pulse-train'' feedback loop operation principle. An innovative pedestal suspension [3] was employed in order to minimise the effects of the package-related stress.

The sensing elements, one for each channel, consist of two identical silicon masses oriented at 90g with respect to each other, and suspended each by a pair of torsional springs to the fixed frame via a central pedestal, as shown in Fig. 1. In order to operate in a stable and well-controlled environment, the sensing elements are located inside a sealed cavity [4] realised by triple stack anodic bonding. Metallic electrodes deposited on the top glass wafer are used tci actuate electrostatically the inertial masses and to detect ca- pacitively their movement, on a time-share basis. The capacitors C l , due to construction reasons, are split in two halves, as it can be seen in Fig. 1. The design is, however, balanced as such that in the absence of mechanical inputs Cl = C2 = Co.

The device combines the advantages of capacitive detection [SI and electrostatic servo-drive with those of a digital processing, thus ensuring an excellent long time and temperature stability.

OPERATION PRINCIPLE In the following paragraphs, since the two channels are identical, the description will be restricted to the X-channel only.

When an "in plane" acceleration a, is present, an inertial force F, will generate a mechanical torque Ty that will tend to rotate the corresponding mass around its torsional springs (see Fig. 2):

Ty = A,,, F, = A,,,,, M . a,

where ,4,,, is the mechanical arm and M is the mass

0-7803-5998-4/01/%10.00 (32001 IEEE 34

1 - 1

Figure I . Top view (based on the actual layout) of the sensor. Legend: white - bonded areas; light grey - recessed areas; dark grey - etch through areas; black - metal on glass.

This mechanical torque has to generate a detectable angle, which corresponds to a capacitance difference CI - C2 of at least 35 aF (the lower detectability limit of the ASIC).

The ASIC can generate a "pulse train" consisting of low-level pulses (encoding '0) and high-level pulses (encoding 'l ') , as shown in Fig. 3. The high-level pulse, denoted by V,,, and its duration Tp are tuneable, around 1.0 V, respectively 2 ps. The total period of the pulses T, is fixed to 10 ps.

Based on the polarity of the signal C l - C2, the ASIC will send a '1' pulse to the smaller capacitor and a ' 0 to the larger one, thus generating an electrical torque counteracting the mechanical one. Note that, due to its internal construction, the ASIC always genera-

tes a complementary sequence of pulses on the two capacitors. This feedback procedure insures a dynamic equilibrium of the mass in a non-tilted position.

"--J - - -FX Figure 2. Cross-sectional geometry of the X-channel sensing element (not to scale).

Figure 3. The ')pulse train" used to actuate electrostatically the two inertial masses.

The electrical forces on capacitor Cl and C2 are, respectively:

where CO is the capacitance in the non-tilted position, do is the air gap of the capacitors in the non-tilted positioni NI, Nz, N,, are the number of '1' pulses applied to capacitor C I , the number of '1' pul- ses applied to capacitor C2, and the total number of applied pulses. x is a factor (around 0.33) representing the fraction of the electrical energy that goes into the first torsional mode.

The total number of pulses N,, verifies the following equation:

N,, = N , + N~ = 214 ( 3 ) where the value 214 has been chosen in order to achieve the desired resolution.

If A,/, is the electrical arm (see Fig. 2) , the electrical torque is:

(4)

From the equality of the mechanical and electrical torques, and using Eqs. 1 through 4, one gets:

which means that the number of '1' pulses applied to Cl within a succession of N,, = 214 pulses is proportional to the input mecha- nical acceleration a,. The sensitivity, which is the collected factor in front of the acceleration in Eq. 5 , is tuneable within certain limits through parameters Vex and T,.

The output of the device is therefore a bit-stream that is conver- ted either to acceleration or inclination, depending on the selected mode of operation.

The range and resolution of the device are, according to Eq. 5:

(7)

where for the maximum range NI - N2 = N,, = 214, and for the resolution NI - N2 = 2 (due to the complementarity of the signals).

With the following numerical values for the excitation voltage, V,, = 1 V, the periods of time, T, = 2 ps and T, = 10 ps, the mecha- nical arm, A,,, = 15 pm, the electrical arm, A,[, = 460 pm, the rest capacitance, CO = 4.5 pF, the air gap of the capacitors, do = 1.0 pm, the mass, M = 1. kg, and using Eqs. 6 and 7, the range and resolution of the device are 10 g (k 5 g), respectively 0.61 mg.

The mechanical system is designed in such a way that when the resolution acceleration input is present, the mechanical torque is large enough to generate a capacitance difference CI - C2 > 35 aF, which was the lower limit that the ASIC can detect.

Due to tolerances in the fabrication process, there will be small asymmetries between capacitors Cl and C2, which will generate an offset in the feedback signal (pulse train). The offset is cancelled in the signal condioning ASIC. For each 35 aF of mismatch between the capacitors, one resolution acceleration unit is lost from the total range. For instance, if the mismatch is 214. 35 aF = 0.57 pF, then the entire available bit stream is used for offset compensation (by sen- ding a '1 11 l...' stream on one capacitor and a '0000 ...' stream on the other capacitor). If the total input range is 10 g and the specified operation range for the accelerometer is f 2 g, it is possible to com- pensate for offset up to & 3 g. This acceleration corresponds to a ca- pacitance mismatch of (3/5). 0.57 pF = 0.34 pF. For the inclinome- ter, a larger offset can be tolerated: rt 4OQ corresponds to rt 0.64 g, which implies that the system can compensate up to k 4.36 g, or 0.50 pF.

The closed-loop bandwidth is programmable to 1 or 32 Hz for the accelerometer, and to 1/16 Hz for the inclinometer. This low bandwidth is required in order to avoid resonance effects and is achieved by using atmospheric pressure inside the sealed cavity, which creates an overdamping regime.

The processing tolerances may introduce up to 3% mechanical cross-sensitivity to accelerations oriented perpendicular to the die surface. Cross-sensitivities between x and y axes are however com- pensated by the signal conditioning ASIC.

Secondary effects, such as bending of the, springs and masses under the electrostatic load, generate some non-linearities in the output signal. The achieved full-scale non-linearity for the current design was below 0.1%.

Several mechanical and electromechanical FEM and processing simulation rounds were used during the design phase of the sensor.

SIGNAL CONDITIONING ASIC The simplified block diagram for the device is presented in Fig. 4. The sensor die and the signal conditioning ASIC are coupled toge- ther, then housed inside a dual-in-line moulded epoxy package.

In the first part of a duty-cycle, the capacitance inbalance is sampled by the 16 kHz sigma-delta sensor interface that will gene- rate a one-bit high frequency serial digital signal. This signal is fed- back as electrostatic servo-drive to the sensing element during the

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next part of the duty-cycle. The same sensor interface output signal is also low-pass filtered and decimated in the following block to give a. low frequency multi-bit output with the desired resolution.

V~~ p ASIC

VOUTl

Parallel to serial

PROM

Internal STATUS Diagnosis - U)

Figune 4. Block diagram of the low-g accelerometer.

The next block, "offset and gain calibration", contains logic to compmsate for sensor and interface circuitry offset, gain errors and temperature dependencies. Individual sensor coefficients are stored in a PROM. This block also compensates for cross-sensitivities bet- ween ;c and y axes. Thus the output of either axis is independent of temperature and the input signal on the other axis. Temperature is intematlly measured by a bandgap reference module.

The device can be programmed to operate in the "inclinometer" mode, by activating the "Arcsine" block, which converts the accele- ration signals into inclination.

The low frequency digital signal is passed to a D/A converter to supply the two ratiometric analog output signals'VOUT, one for each channel. For specific applications, the signal is also available in its digital form at the TXD port, clocked by TXCK.

The results of several monitoring internal tests (such as opera- ting or temperature range exceeded, parity errors in reading the PROM, overflow in the A/D converter) are also available as the digital STATUS output.

A n oscillator and voltage reference blocks, although not indica- ted in Fig. 4, are also components of the dedicated ASIC.

Thc ASIC was designed by Nordic VLSI as, and fabricated in a SGS-Thompson HCMOSSLA process, which insures a 5 V power supply. The entire device has a maximum power consumption of 25 mW, of which 8 mW are in the digital areas.

MIL-A models were used during the design phase of the ASIC and sensing element for the electromechanical simulation of the en- tire system.

FABRICATION PROCESS OF THE SENSOR The fabrication sequence of the sensing element consists of the fol- lowing main processing steps:

a) Phosphorous implantations and drive-in to define the n-type wells and masses into ap-type silicon substrate;

b) Boron implantation and drive-in to define the p-type buried feed-throughs into the n-wells, used to pass the electrical signals in and out of the future sealed cavity;

c) Epitaxial growth of an n-type layer, to define the thickness of the future springs and to bury the feed-throughs;

d) Recess dry etching to define the future air gap of the capaci- tors and the required space-gap for future press-contacts;

e) Boron implan'tation and drive-in to define the p-type surface conductors into the n-epi layer, used to contact the underlying buri- ed feed-throughs;

f) Aluminium deposition and patteming to define the lower part of the future press-contacts;

g) Anisotropic TMAH etching from the backside, to define the thin membrane and the inertial masses, by means of the 4-electrode electrochemical etch stop technique [ 6 ] ;

h) Perforation of the thin membrane by dry etching in order to define the torsional springs;

i) Several metallic layers deposition and patteming steps on the top glass wafer in order to define the electrodes of the capacitors, the screening electrodes, the wire-bond pads and the upper part of the press-contacts;

j) Anodic bonding [7] between the structured silicon wafer and the top and bottom glass wafers in order to form the sealed cavity.

Figure 5. Top view of the sensing element prior to anodic bonding.

electrode glass

\ \, pedesta' oxide stopper press-contact

spring

buried 'sealed cavity feed-through

glass

Figure 6. Cross-section through the sensing element. Not to scale. Not all the layers, features and structures are indicated.

Fig. 5 shows a picture of the X-channel of the sensing element prior to anodic bonding. A generic cross-section through the final sensing element is illustrated in Fig. 6. A picture of the sensor die after the completion of the processing sequence is shown in Fig. 7.

The wire-bond pads are located on the top-glass wafer, in order to minimise the parasitic capacitances. The largest contribution to the parasitics is given by the buried feed-throughs.

The conductors are transferred to and from the silicon and glass wafers by using the "press-contacts", structures realised by pressing and squeezing together - during the anodic bonding step - at least two metallic layers, one of which being mandatory a soft material, like aluminium.

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Figure 7. The sensor die after completion of processing.

In order to prevent the sticking of the silicon movable parts on- to the top glass during the anodic bonding step, screening electro- des that surrounded all the active electrodes were employed. The screening electrodes, the epi-layer and the inertial masses were con- nected to the same electrical potential through press-contacts.

Oxide stoppers, placed in the corners of the inertial masses and always under the screening electrodes, were used to prevent acci- dental short-circuits between the active electrodes and the masses, as well as to limit the vertical displacement of the movable parts.

Since the requirements for the pads and press-contacts (soft me- tal to allow wire-bonding and squeezing) were different from those for the electrodes (hard and hillocks-free metal, to prevent sticking and short-circuits), a special multi-layer metallic stack has been de- veloped, which is now the subject of a patent application.

In order to minimise the effects of the packaging and tempera- ture stress, an innovative pedestal suspension [3] has been designed and employed. The pedestal is a single-sided clamped beam, the surface of which is anodically bonded to the top glass wafer, with a specific geometry that allows the mechanical decoupling between the sensing inertial masses and the frame of the sensor.

MEASUREMENTS The sensor element has been characterised prior to assembly by CV measurements. The capacitance of all channels was measured while sweeping the DC voltage applied between the active electrode and the ground (inertial masses and screening electrodes).

7.0 T - T - I - - T - T - T ~ ~ - ~ - - . ~ - - - I --T--T-l

6.5

5.0

4.5 0 1 2 3 4 5 6 7 8 9 1 0 1 1 1 2 1 3 1 4 1 5

v [vl Figure 8. Typical CV measurements for the sensor die.

Fig. 8 shows typical CV measurements, indicating the follo- wing features: the influence of the p-n junction parasitic capacitan- ces (the parabolic decreasing trend), the torsional and the bending pull-in around 1.2 V, respectively 13.5 V. The absence of sticking and post release sticking was put in evidence by the presence of the torsional pull-in.

The capacitance and pull-in voltage values were indirectly pro- viding information on the actual air-gap and mechanical sensitivity.

-

1.0 r----

0.5 . _ _ 5 0.0

I

N k -0.5

-1 .o -8.0 -6.0 -4.0 -2.0 0.0 2.0 4.0 6.0 8.0

Input acceleration [g] I Figure 9. Typical output of one channel of the sensor (acceleration on the other channel was in this case set to zero).

The full device, after assembling together the sensor and the ASIC, has been characterised by using an ACUTRONIC 'rotating table, able to apply both DC and AC acceleration signals, simulta- neously in two directions. Fig. 9 shows a typical plot of the digital pulse-count signal as a function of the x-axis oriented acceleration, prior to the calibration routines.

CONCLUSION A. new inclinometer / low-g accelerometer has been successfully fabricated. The device, based on a novel, digital feedback operation concept, with several programable features, is suitable for automo- tive applications.

REFERENCES 1. SCA600 Series Inclinometer, Data sheet, VTI HAMLIN Oy, Finland (2000).

2. ADXL202 Low Cost S g Dual Axis Accelerometer with Digital Output, Data sheet, Analog Devices, Norwood, USA (1998). 3. D. Lapadatu, T. Kvisteray, H. Jakobsen, "Micromechanical Device", European Patent Application, EP 99308589.3 (1999).

4. H. Jakobsen, T. Kvisteray, "Sealed Cavity Arrangement Method", United States Patent, #5591679 (1997).

5. R. Puers, "Capacitive sensors: when and how to use them", Sensors and Actuators A , 37-38 (1993), pp. 93-105.

6. D. Lapadatu, G. Kittilsland, M. Nese, S.M. Nilsen, H. Jakobsen, "A model for the etch-stop location on reverse-biased pn junctions", Sensors and Actuators, A 66 (1998), pp. 259-267.

7. A. Cozma, "Development of Wafer Bonding Techniques with Application to Self-Testable Silicon Pressure Sensors", P h D . Thesis, K.U. Leuven, Leuven, Belgium, (1999), pp. 23-72.

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