Bridge configuration of piezoresistive devices for scanning force microscopes

ELSEVIER

Sensors

and Actuators

A 70 ( 1998)

SE6ORS ACT@ORS A _~ PHYSlCAL

88-91

Bridge configuration of piezoresistive devices for scanning force microscopes R. Jumpertz a, J. Schelten a-*, 0. Ohlsson b, F. Saurenbach’ ainstitutfiir

Schicht und Ionentechnik, Forschungszentrurn Jiilich GmbH, D-52425 Jiilich, Germany b Nanosensors GmbH, D-35578 Wetzlar, Germany ’ Sutface Imaging Systems GmbH, D-52134 Herzogenrath, Germany

Abstract The piezoresistive sensor components are directly integrated on commercially available cantilevers taken out of the production Iine of Nanosensors GmbH (0. Wolter, Th. Bayer, J. Greschner, J. Vat. Sci. Technol. B 9(2) (1991) 1353). They consist of four implanted semiconductor resistors that are connected to form a bridge.The processparameters have beenvaried to achieveultimatesensitivity.

Experimentally,avertical spatialresolutionof + 0.025nmwasachievedin scanningexperiments within abandwidthof 10mz. Furthermore, 0 1998ElsevierScienceS.A. /Jl suchpiezoresistivesensors with magnetictipsaresuccessfully usedfor magneticforce microscopy. rightsreserved. Kepwrds:

Scanning force microscopy; Piezoresistivity; Magnetic force microscopy; Bridge configuration

1. Introduction

The invention of the scanningelectron microscope(SEM) [ 1] triggered the development of a variety of other scanning probe microscopes.They can be distinguishedby the kind of physical propertiesbeing measured.The most important representativeis the scanningforce microscope(SFM) [ 21. The basic concept is that a one-side-fixedcantilever with a sharp tip at its free end is scannedacrossthe surface of a sample. Forcesacting betweenthe tip andthe surfaceinduce a deflection of the cantilever. Laterally, the resolution is limited by the curvature of the tip. Vertically, theresolutionis influenced by the smallestmeasurabledeflection of the cantilever. There are different methodsthat are widely usedto detect suchdeflections, e.g., optical methodsbasedon interferometry or reflection of laser light beams[ 31. The highest sensitivity achievable with theseoptical detectorsis + 0.005 run. Their disadvantageis that the external detectorsneedaprecise alignmentwith respectto the flexible and fragile cantilevers. The first integratedpiezoresistivedetector wasinvented by Tortoneseet al. [ 41. It consistsof one sensitiveresistoron a U-shapedcantilever connectedto an external bridge. Due to the use of a broad-area resistor and a quarter bridge, the vertical sensitivity is limited to only +OS nm in dynamic mode.Another group [5,6] developedapiezoresistivebridge * Corresponding

0924-4247/98/$

author.

on cantilevers with an attachedtip. However, attemptswere also reported [7] to integrate a tip on the cantilever during the processof the resistors. Within our development of a piezoresistive detector, emphasiswas placed on a reliable commercial processwith a high yield. This includesthe useof 4-in. silicon waferseach with 400 cantilevers andtips. On eachcantilever, a piezoresistive sensoris integrated, consistingof four semiconductor resistorsconnectedto form a bridge. Optionally, one of these resistorshasan additionalinsulatorand a gate electrode.With anaccurategatevoltage, thevalue of sucharesistoris changeable, and offers the possibility of an in situ ‘balanceable’ bridge configuration. To usethe areawith the higheststress, the bridge is positioned closedto the fixed end of the cantilever. In addition, the ful1 bridge with four sensitiveresistors yields the highest signal, twice as large as the signal of a quarter bridge [ 51.

2. Optimisation

The deflection of the cantilever inducesa stressat its surface, resulting in a change of the resistance.The transformation from the mechanical stressto an electrical signal is describedby the piezoresistive effect. A quantitative value is given by the K-factor appearingin AR/R = KS, where 8 is the strain causing the relative resistance change AR/R [S] . This

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Pi2 SO924-4247(98)00118-6

and fabrication of the bridge

R. Jumpertz

et al. /Sensors

factor depends on the doping concentration of the resistor and on the crystallographic orientation of the current in the resistor. In our work, we used p-doped resistors with a doping concentration of 4 * 1017 atoms/cm3 and a profile depth of 200 nm. Two resistors are oriented along the ( 1lO)-direction and the others perpendicular to these along the ( ll0) -direction. The ( 1 lO)-direction is also the long axis of the cantilever. In the case of a parallel orientation between the resistor and this axis, the stress induces an increase of the resistance, while in the case of the perpendicular orientation, the resistance is decreased by the same amount. Therefore, for a full bridge, the signal is twice as large as that of a quarter bridge. Furthermore, the bridge is positioned close to the fixed end of the cantilever, where the stress reaches its maximum. These conditions lead to a K-factor of 110. Besides this parameter optimisation for the process, it was important to preserve the curvature of the tip. Therefore, low oxidation processes, noncontact lithography methods, and wet etching techniques were used. Ultimately, tip radii below 20 nm remained. The SEM picture in Fig. 1 shows a cantilever with an integrated bridge. The curvature of the tip is 15 nm. The striplines connect the resistors with the bond pads located outside the cantilever. The cantilever has alength, a width and a thickness of 225 pm, 50 p,rn and 8 km, respectively. Electrically, the sensitivity is limited by the signal noise, i.e., the signal U, = U, *AR/R must exceed lJ,, with lJ,= d4kB TRAf +2qRAf U,

(1)

where ,& is the Boltzmann factor, 4 is the elementary charge, R is the resistance, Af is the bandwidth and tJ, the applied voltage. For a temperature of 300 K, a resistance of 8 k!& an applied voltage of 12 V, a bandwidth of 10 kHz avoiding the low frequency flicker noise, the thermal noise is about 1.8 pV. Assuming, U, = 3 U, as a practical limit, the minimum detectable deflection )jmin of the cantilever is + 0.01 nm. The value is calculated from 1 2L2 J4kBTAfR+2qRAfUo Ymin= E z

UO

(2)

where L is the length and d the thickness of the cantilever. G is a dimensionless geometry factor close to one. For this calculation, further values have been used: a length of 225 p,m, a thickness of 8 pm and a K-factor of 110. 3. Dynamic SFM In order to get the spatial resolution of the piezoresistive sensors experimentally, the cantilevers were integrated in a SFM developed by Surface Imaging Systems (SIS). In almost all applications, the dynamic mode was used, where the cantilever vibrates at its resonant frequency. This mode allows high scan speeds with an accurate lateral resolution, Therefore, in this work, only the dynamic mode was used. Fig. 2 shows a SFM picture of a HOPG (highly oriented pyroWic graphite) sutfacetakenwith apiezoresistivebtidge

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Fig. 1. SEM picture of a cantilever with an integrated bridge. The curvature of the tip at the free end of the cantilever is 15 nm. The cantilever has a length of 225 pm, a thickness of 7 km, and a width of 50 pm.

Fi

E:--

a-0 300 ‘loo 200 400 500 Fig. 2. SFM picture of a HOPG surface with a single (S) and a double (D) atomic step. The scanned area is 550 X 550 MI’,

sensor. The arrows indicate a single (S) and a double (D) atomic step. A profile cut perpendicular to the steps (Fig. 3) offers a more detailed analysis. Fig. 3 shows a step height of 0.72 nm for the double step and 0.34 nm for the single step. As reviewed in the literature [ 41, the exact height of an atomic step observed on HOPG is 0.3350 nm, in agreement with a d, i r lattice spacing, measured precisely in bulk material with diffraction experiments. In comparison to this value, the experimental results show only a deviation of 5%. The minimum detectable deflection of the cantilever is f 0.025 nm. This result is in a agreement with the theoretical minimum deflection of + 0.01 nm. A further optimisation of the process parameters may still improve the sensitivity. Fig. 4 shows a SFM picture of the surface of a silicon standard with very sharp etched tips in a periodic distance of 3 km. The tips have a curvature radius of 20 nm or less. The

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short circuits generatedby the metal deposition, the bridge areawasprotected by a stencilduring the evaporation of Co. The interaction between the ferromagnetic tip and the stray field of the magnetic samplesleads to a deflection of the cantilever. Thus, the SFM becomesa magnetic force microscope(MFM) . Fig. 5 showsthe MFM imageof the bit structure of a magnetic hard disk. The single-bit information consistsof an emanating (white arrow) and a penetrating (dark arrow) part of the magnetic stray field. Therefore, on one sideof the bit, the tip is repulsed,and on the other sideit

I :

0

l#

Fig. 3. Profile cut perpendicular cerning Fig. 2.

m

33

403

to the single and double atomic step con-

I

I

tlm

0 1 OUOO Fig. 5. MFM picture of a magnetic hard disk. The bit structure has a density of lo6 bits/cm’. The arrows indicate the emanating (light) and penetrating (dark) stray fields of the sample.

Fig. 4. SFM picture of a standard sample with sharp-etched tips in a periodic distance of 3 pm. The curvatures of the tips are guaranteed to be less than 20 nm.

imaging of suchsurfacesallows to deduceinformation about the quality of the tips of the usedcantilever. It waspossible to image the silicon standard tips with a 20 nm sharpness, and therefore the curvature radius of the cantilever’s tip must be 20 nm or even better.

4. Modifications

for magnetic force measurements

In order to make the cantilevers sensitive to magnetic forcesthey were covered with a 50-nmthin Co-b. To avoid

0

1 moo

Fig. 6. MFM picture of a polycrystalIine Co surface. The domain structure shows a certain degee of ordering. In some places dust particles disturbed the magnetic signal.

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completly different ordering is observed. In MFM pictures with smaller scanned areas of 2.5 X2.5 pm*, a lateral resolution of magnetic structures of less than 100 nm was determined.

5. Conclusion and outlook

a-0

l

I

I

I

I

I

1000

7.000

3000

4000

5000

6000

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Fig. 7. MFM picture of a polycrystalline Co surface. The domain structure differs from that in Fig. 6, but also shows a certain degree of ordering.

is attracted. In the MF’M picture (Fig. 5), these areas are represented by the light- and dark-coloured regions. This interpretation assumes that the magnetisation of the tip is oriented towards the surface of the sample. Another example for a MFM application with the piezoresistive sensors is given in Fi g. 6. The picture shows a surface of apolycrystalline Co layer. Due to the polycrystallineordering, the domain structure of a Co layer changes from place to place over the whole sample because of the uniaxial magnetic anisotropy of cobalt. Nevertheless, in small scanned areas of 20 X 20 pm2 (Fig. 6), there is obviously a certain observable degree of short-range ordering. Fig. 7 shows a further MFM picture of the same scanned area, but at another place on the same Co-sample. Due to the polycrystallinity, a

SFM measurements with the piezoresistive bridge sensors showed a high spatial resolution of f0.025 nm even in dynamic modes. The simplicity of the sensor system leads to a very compact equipment and a simple electronic readout. More than 100 sensors were tested and showed the same sensitivity, as well as the same remaining curvature of the tips below 20 run. This indicates that a reliable fabrication process was achieved. Furthermore, a simple magnetic modification of the tip allows the use of the piezoresistive sensors for MFM measurements. A lateral resolution of magnetic structures in the range of 100 run was achieved. In order to open the field of applications the use of the piezoresistive sensors for measurements in liquids and UlIV as well as for tribological properties is being prepared.

References 111 G. 121 G.

Binnig, H. Rohrer, Helv. Phys. Acta 55 (1982) 726. Binnig, CF. Quate, Ch. Gerber, Phys. Rev. Lett. 56 (1986) 930. H.-J. Giintherrodt, Scanning Force Microscopy II, [31 R. Wiesendanger, Springer, 1992. [41 M. Tortonese, R.C. Barrett, C.F. Quate, Appl. Phys. Lett. 62 (1993) 834. [51 T. Gotszalk, I. Rangelow, P. Dumania, P. Grabiec, SPIE 2879 ( 1996) 66-69. 161 T. Gotszalk, I. Rangelow, P. Dumania, P. Grabiec, SPIE 2880 ( 1996) 264. L. Hadjiiski, I.W. Rangelow, Thin Solid [71 R. Linneman, T. Gotszalk, Films 264 (1995) 159-164. 181 Y. Kanda, IEEE Trans. Electron Devices 64 (1982) ED29.