TRANSDUCERS ’01 EUROSENSORS XVThe 11th International Conference on Solid-State Sensors and Actuators, Munich, Germany, June 10 – 14, 2001

A Longitudinally Vibrating Touch Probe SensorUsing PZT Thin Film Vibrator

Takefumi KANDA*, Minoru K. KUROSAWA**, and Toshiro HIGUCHI*

*The University of Tokyo, Graduate School of Engineering,7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan,

kanda@intellect.pe.u-tokyo.ac.jp, http://www.intellect.pe.u-tokyo.ac.jp**Tokyo Institute of Technology, Interdisciplinary Graduate School of Science and Engineering,

4259 Nagatsuta-cho, Midori-ku, Yokohama 226-8502, Japanhttp://www.awa.ae.titech.ac.jp

SUMMARY

In this paper, an improved touch probe sensor device forhigher sensitivity and low contact force is reported. Inorder to improve the resolution, we have evaluated thesensitivity and fabricated a miniaturized sensor, whichwas 3 mm long and had higher resonance frequency.The resonance frequency of the vibrator was 937 kHz.Evaluated sensitivity was 5.2x10-2 mV/nm. This valueequals 2.6 times larger than that of previous sensor andmeans that this sensor has sub-nano resolution.Miniaturization of the sensor device carried smallervibration operation and higher resolution.

Keywords: Touch probe sensor, PZT thin film,Hydrothermal method

INTRODUCTION

Our touch probe sensor was designed with the aim ofrealizing high resolution more than 0.5 nm, low contactforce under 500 nN, a wide scanning range in mm scalesquare, and quick scanning surface profile measurement.These features are advantageous for measuring nanostructure, for example, sub micron rule VLSI or microelectro mechanical systems (MEMS). Some kinds oflongitudinally vibrating touch probe sensors[1-4] forsurface profile measuring tools or scanning probemicroscopes (SPM) have been fabricated. Theschematic view of the surface profile measurement bythe probe sensor is shown in Fig. 1. The sensor consistsof a part longitudinal vibrating and an exponential hornwhich enlarges the amplitude at the contact point.

Although bending vibration is used for most AFMs, thelongitudinal vibration was used for our sensor in orderto maintain high mechanical Q value for high resolutionand high resonance frequency. Therefore this sensor isalso useful in liquid, and can realize the quick scanning.

In order to realize such a measurement, to achieve asensor which has high sensitivity for detection of thecontact is important. It can be considered to miniaturizethe sensor vibrator is effective to obtain highersensitivity. We have fabricated a sensor miniaturized,and evaluated the features.

PRINCIPLE

Resolution and Sensitivity

Our first probe sensor reported in MEMS 98[3] was rod-shaped and its resonance frequency was 116 kHz. InTransducers 99, we reported much smaller and flat-typesensor[4]. When the driving voltage was 3 Vp-p, theresonance frequency, vibration amplitude at theresonance, and mechanical Q value were 304.4 kHz, 126nmo-p, and 705. The sensitivity and resolution were2.0x10-2 mV/nm and 2.4 nm. Although this resolutiondepends on the sensitivity and the noise level, the noiselevel of pre-amplifier circuit is much larger than that ofvibrator. By reducing the noise in the circuit by usinglow-noise type operational amplifiers, the higherresolution up to 0.2 nm can be obtained. Although the

Scanning

Workpiece

Probe

Driving Signal

Surface Profile Image

Fig.1 Schematic view of the surface profilemeasurement using the probe sensor.

TRANSDUCERS ’01 EUROSENSORS XVThe 11th International Conference on Solid-State Sensors and Actuators, Munich, Germany, June 10 – 14, 2001

contact force was estimated to be 25 µN under 0.3 Vp-pdriving, it will be 300 nN by low noise circuit.

In addition, the sensitivity will be improved by thelarger piezoelectric constant of PZT film. This resultshows that the sensitivity is proportional to thepiezoelectric constant e31 when the piezoelectricconstant e31 is smaller than 6 C/m2[5]. From thecalculation based on the experimental data, thepiezoelectric coefficient of e31 of the longitudinaltransducer was estimated to be -0.2 C/m2[4]. This valueis smaller than one tenth of the constant of bulkmaterials, -3.1 C/m2 (calculated from the constants inref. 6). When the larger piezoelectric coefficient of thePZT thin film is obtained, the sensitivity will beimproved.

Miniaturization

It can be considered that the miniaturization of thesensor vibrator is also effective to obtain highersensitivity. To estimate the dependence on thedimensions of the vibrator, we estimated the sensitivityof a longitudinal vibrator sensor as shown in Fig. 2.Electrodes are positioned on each side, one is fordriving, and the other is for pickup. The length, width,and thickness of the Ti substrate and those of the PZTfilm are l, b, t1, and t2, respectively. PZT thin film layersare present on both surfaces of the Ti substrate. Theratio between l, b, and t1 is 40:10:1. The thickness of thePZT film, t2, was constant at 3 µm, since the thickness ofthe film was kept constant at about 3 µm when the PZTthin film was deposited using the hydrothermalmethod[7-9]. Figure 3 shows the relationship betweenthe sensitivity of the sensor and the vibrator length whenthe ratio of dimensions of the vibrator was fixed. Whenthe dimensions are miniaturized, the sensitivity of thesensor was improved.

EXPERIMENTS

Structure

The structure of the sensor is shown in Figs. 4 and 5.The length of the sensor vibrator was 3.0 mm and thewidth was 0.3 mm. The thickness of the Ti substrate was0.1 mm. PZT thin film was deposited by a hydrothermalmethod[7-9] on the titanium substrate. The thickness ofthe film was about 3 µm each side. The width at the tipof the vibrator was 0.1 mm. The vibration amplitude atthe tip of the vibrator was enlarged by the exponentialhorn. The step-up ratio of the exponential horn was 1.7.

l

b

Top View

Driving Source

Pickup

PZT Thin Film: 3 µm

Au Electrode

Ti Substrate: t 1

Cross Section

Fig.2 Vibrator for the evaluation of the sensitivity.

0.01

0.1

1

10

0 2 4 6 8 10Length (mm)

Vol

tage

/Am

plitu

de S

ensi

tivity

(mV

/nm

)

Fig.3 Relationship between the sensitivity of thesensor and length of the vibrator when theratio of dimensions are fixed.

Fig.4 Photo of the touch probe sensor: thelength and width of the vibrator were 3.0mm and 0.3 mm.

TRANSDUCERS ’01 EUROSENSORS XVThe 11th International Conference on Solid-State Sensors and Actuators, Munich, Germany, June 10 – 14, 2001

The vibration was excited and detected by PZT thin filmon each side of the vibrator.

Vibration Amplitude

To estimate the vibration amplitude, the vibrationvelocity was measured. Figure 6 shows the vibrationamplitude at the tip of the vibrator. When the drivingvoltage was 1.0 Vp-p, the resonance frequency, vibrationvelocity at the resonance frequency and mechanical Qvalue were 937 kHz, 2.4x10-2 m/s and 394. From thesevalues, the vibration amplitude at the resonancefrequency was 4.1 nmo-p.

Figure 7 shows the relationship between the drivingvoltage and the vibration amplitude at the resonancefrequency. The relationship has the linearity.

Sensitivity

The sensitivity can be obtained by using the equivalentcircuit[4]. Figure 8 shows the equivalent circuit of thesensor. Lump element circuit components of L, 1/C, R,A1 and A2 are equivalent mass, equivalent elasticitymodules, equivalent viscosity coefficient, and force

factors in the driving piezo element and that in thepickup piezo element. Cd1 and Cd2 indicate thecapacitance in the driving element and that in the pickupelement due to the PZT film's ferroelectricity. By usingthese parameters, the sensitivity P can be described as[4]

2

2

21

2

1

1

+

=

drd RC

AC

AP

ωEstimated equivalent circuit elements are summarized inTable 1. From these elements, the sensitivity of thesensor was 5.2x10-2 mV/nm. This value equals 2.6 timeslarger than that of previous as shown in Table 1.

CONCLUSION

In order to improve the resolution, we have evaluatedthe sensitivity and fabricated miniaturized sensor. Thesensor vibrator was 3 mm long and the resonancefrequency of the vibrator was 937 kHz. The vibration

-+

-+

V~

-+

Amplifier circuit

Pick-up

Vibrator length 3.0 mm

Vibrator width 0.3 mm(Ti Base)

Contact Point

Supporting Part

Au Electrode

Drive

Pickup

Driving Source

Pickup

PZT Thin Film: 3 µmAu Electrode

Ti Base: 100 µm(Reference Electrode)

Cross Section

Fig.5 Structure of the probe sensor.

0

1

2

3

4

5

900 920 940 960 980 1000Frequency (kHz)

Vib

ratio

n A

mpl

itude

(nm

o-p)

Resonance Frequency: 937 kHzMechanical Q factor: 394

Fig.6 Measurement result of the vibration amplitude atthe tip of the vibrator: Driving voltage was 1.0Vp-p.

y = 4.299xR2 = 0.9996

0

5

10

15

20

25

0 1 2 3 4 5Driving Voltage (Vp-p)

Vib

ratio

n A

mpl

itude

(nm

o-p)

Fig.7 Relationship between the driving voltage thevibration amplitude.

. (1)

TRANSDUCERS ’01 EUROSENSORS XVThe 11th International Conference on Solid-State Sensors and Actuators, Munich, Germany, June 10 – 14, 2001

amplitude at the resonance was 4.1 nm when the drivingvoltage was 1.0 Vp-p. Evaluated sensitivity was 5.2x10-2

mV/nm. This value equals 2.6 times larger than that ofprevious sensor. Miniaturization of the sensor devicecarried small vibration operation and higher resolution.

ACKNOWLEDGEMENTS

This work was supported by the Grant-in-aid for generalscientific research of the Ministry of Education, Culture,Sports, Science and Technology, and by the Proposal-Based New Industry Creative Type Technology R&DPromotion Program from the New Energy and IndustrialTechnology Development Organization (NEDO) ofJapan, and by the Grant-in-aid for Research Fellowshipfor Young Scientists of the Japan Society for thePromotion of Science.

The authors would like to thank Mr. Yasui of TheUniversity of Tokyo for valuable advice on and hisassistance with the hydrothermal method of depositingthe PZT thin film.

REFERENCES

[1] S. M. Harb, M. Vidic, Resonator-based touch-sensitive probe, Sensors and Actuators A, Vol. 50,pp. 23-29, 1995.

[2] M. Nishimura, K. Hidaka, and M. Teraguti,Proceedings of annual spring meeting of the JSPE,1994, pp.795-796 (in Japanese).

[3] T. Kanda, T. Morita, M. K. Kurosawa, and T.Higuchi, “A rod-shaped vibro touch sensor usingPZT thin film,” IEEE Trans. on Ultrason.Ferroelectric and Freq. Cont., 46(4), pp. 875-882,1999.

[4] T. Kanda, T. Morita, M. K. Kurosawa, and T.Higuchi, "A flat type touch probe sensor usingPZT thin film vibrator," Sensors and Actuators A,vol. 83, pp.67-75, 2000.

[5] T. Kanda, T. Morita, M. K. Kurosawa, and T.Higuchi, "Estimation of Resolution and ContactForce of a Longitudinally Vibrating Touch ProbeSensor using Lead Zirconate Titanate (PZT) Thin-Film Vibrator," Jpn. J. Appl. Phys., vol.40, part 1,no.5B, 2001 (to be published).

[6] B. Jaffe, W. R. Cook and H. Jaffe: PiezoelectricCeramics (Academic Press, London, 1971) p.146.

[7] K. Shimomura, T. Tsurumi, Y. Ohba and M.Daimon, “Preparation of lead zirconate titanatethin film by hydrothermal method,” Jpn. J. Appl.Phys., 30(9B), pp. 2174-2177, 1991.

[8] T. Morita, T. Kanda, M. Kurosawa, T. Higuchi:“Single Process to Deposit Lead Zirconate Titanate(PZT) Thin Film by Hydrothermal Method”, Jpn.J. Appl. Phys., No.36 5B, pp. 2998-2999,1997.

[9] T. Kanda, M. K. Kurosawa, H. Yasui and T.Higuchi, "Performance of Hydrothermal PZT Filmon High Intensity Operation," Sensors andActuators A, vol. 89, pp.16-21, 2001.

RmCmLm

Cd2Cd1

1:A1 A2:1

Vin Vout

Fig.8 Equivalent circuit of the touch probe sensor.

Table 1: Estimated Equivalent Circuit ElementsPrevious[4] Miniaturized

Resonancefrequency

[kHz]304 937

LmEquivalentmass [kg] 2.0x10-6 1.8x10-6

Rm

Equivalentviscoelasticloss [Ns/m]

5.4x10-3 2.7x10-2

Cm

Equivalentcompliance

[m/N]1.4x10-7 1.6x10-8

A1Force factor

[N/V] 8.7x10-5 5.5x10-5

Cd2

Dampedcapacitance

[F]5.5x10-9 1.1x10-9