*Corresponding author.

Ultramicroscopy 71 (1998) 93—98

Micromachined aperture probe tip for multifunctionalscanning probe microscopy

Michael Abraham!, W. Ehrfeld!, Manfred Lacher!, Karsten Mayr!, Wilfried Noell!,*,Peter Guthner", J. Barenz#

! Institute of Microtechnology Mainz GmbH, Carl-Zeiss-Str. 18—20, D-55129 Mainz, Germany" Omicron Vakuumphysik GmbH, Idsteiner Str. 78, D-65232 Taunusstein, Germany

# Department of Experimental Physics, University of Ulm, Albert-Einstein-Allee 11, D-89069 Ulm, Germany

Abstract

The paper presents a new concept of a micromachined integrated sensor for combined atomic force/near-field opticalmicroscopy. The sensor consists of a microfabricated cantilever with an integrated waveguide and a transparentnear-field aperture tip. The advantage compared to the fiber-based near-field tips is the high reproducibility of theaperture and the control of the tip—sample distance by the AFM-channel. The aperture tip is fabricated in a reliable batchprocess which has the potential for implementation in micromachining processes of scanning probe microscopy sensorsand therefore leads to new types of multifunctional probes. For evaluation purposes, the tip was attached to an opticalfiber by a microassembly setup and subsequently installed in a near-field scanning optical microscope. First measure-ments of topographical and optical near-field patterns demonstrate the proper performance of the hybridprobe. ( 1998 Elsevier Science B.V. All rights reserved.

PACS: 07.79.Fc; 07.79.Lh; 42.79.Ag; 42.81.Wg

Keywords: Near-field optical microscopy; Atomic force microscopy; Micromachining; Integrated optics; Integratedprobes

1. Introduction

Information obtained by atomic force micro-scopy (AFM) reveals only the local topography ofthe sample. The near-field scanning optical micro-scope (NSOM) yields details of local optical prop-

erties: In order to measure the local absorption andtransmission of light, a tiny metallic aperture isplaced in the direct vicinity of the object, i.e. intothe optical near field.

Since the amplitude of the optical near field va-ries strongly with the spacing between the probe tipand the sample, a distance regulation for theNSOM tip is essential. One way to do this, is tocombine a NSOM with a well-established AFM,

0304-3991/98/$19.00 ( 1998 Elsevier Science B.V. All rights reserved.PII S 0 3 0 4 - 3 9 9 1 ( 9 7 ) 0 0 1 1 4 - 9

Fig. 1. Concept of a multifunctional AFM-NSOM: The sensoris based on a standard AFM setup with a beam-deflectiondistance regulation. An optical waveguide is integrated into thecantilever and optically connects the external optical fiber withthe integrated AFM-NSOM tip.

which also provides high topographical resolution.This type of microscopy is called AFM-NSOM [1].

Today most NSOM probes consist of speciallyprocessed fibers: first the fiber is tapered and thena metal — usually aluminum — is evaporated ata certain angle onto the rotating fiber end, thusforming the metallic aperture at the apex [2—7].These probes are difficult to produce in largenumbers and are inconvenient to handle. Thecommonly used distance regulation in NSOM isrealized by the so-called shear force mechanism,where the mechanical interaction between the par-allel to the surface vibrating fiber tip and thesurface is utilized [5,8]. The aperture formingevaporation process is difficult to control and com-monly results in deficiencies such as an undeter-mined shape and size of the opening and lighttransmitting pin holes on the side walls of the fiberprobe [2]. Additionally, it creates voluminousmetal clusters protruding irregularly from the aper-ture rim [2,4,7,9]. This generally leads to artifacts[10] in both the topographical and optical near-field image, because the point of shear force interac-tion (point of contact ) does not correspond to themiddle of the aperture area. Furthermore, the fiberprobe creates an unwanted gap between the glassend face and the sample surface causing an addi-tional optical barrier [2,4,9]. The relative through-put of the optical intensity, often referred to as theoptical transmission efficiency, decreases forNSOM probes with increasing resolution, typicallydown to a range of 10~5—10~7 [4,5]. A furtherdrawback of the fiber technique is the relativelyhigh operator effort for the preparation and selec-tion of the suited probes. In general, the operatorhas to characterize each probe prior to use if hewants to understand his pictures on a quantitativebasis.

2. Concept

To overcome the handicaps of the fiber probetips, an integrated optical NSOM-AFM sensor forvisible light operation has been developed. Theconcept provides a high optical efficiency as well asa high topographical resolution (Fig. 1). TheAFM-mode is achieved by a cantilever with an

attached probe tip. An optical wave guide integ-rated into the cantilever extends on one end intothe AFM-NSOM probe tip and on the other end toa standard optical fiber, thus connecting the sensorwith either a light emitter or detector dependingupon the user’s needs. The coupling between thefiber and the chip will be provided either by prismor butt coupling resulting into a “pig tailed” ready-to-use sensor. Using the described combination,topographical and optical information can be mea-sured at the same time.

The advantages of a device that is based on thinfilm technology are apparent, as the sensor canreliably be produced in arbitrarily large numbers.Although the development of the complete sensoris not yet accomplished, we can report on impor-tant results concerning the key components of thesensor.

3. The waveguiding cantilever

The first step was the development of a microfab-rication process which allows to integrate a single-mode waveguide into a cantilever beam. The sensorhad to fulfil both optical and mechanical require-ments. The optical signal transfered to or collectedfrom the sample should be mostly independent ofthe fiber to chip coupling and from surface impu-rities caused, e.g. by dust particles. Further, thecantilever dimensions should be ideal for AFMapplications. Therefore, we focused on a single-

94 M. Abraham et al. / Ultramicroscopy 71 (1998) 93—98

Fig. 2. Process steps for micromachining of the waveguiding cantilever without tip.

Fig. 3. Micrograph of the released waveguide cantilever with-out tip.

mode strip loaded rib waveguide which providesa very defined and stable field distribution. Thewaveguiding layer is made of SiO

xN

ywith a refrac-

tive index of nS*ON

"1.52 (Fig. 2). It is depositedby plasma enhanced chemical vapour deposition(PECVD) on top of a thermal SiO

2buffer layer

(n5)vS*O

"1.45). The topmost layer is PECVD SiO2

(n1vS*O

"1.46) and provides both the waveguidingrib structure and a protection against environ-mental influences. The rib width varies in the range2—5 lm. The propagation loss is as low as0.5 dB/cm.

The waveguiding cantilever is micromachinedwithout destroying the waveguide (Fig. 2). This isachieved by protecting the waveguide with a sput-tered chromium layer. The chromium is structuredin order to be used as a masking layer for theetching of the cantilever. The silicon compoundscan be etched by reactive ion etching (RIE) in oneetch process. Next the silicon underneath is aniso-tropically etched in potassium hydroxide (KOH).Finally the chromium is removed. Fig. 3 showsa micrograph of the released cantilever without thetip. The total thickness of the cantilever beam isapproximately 3—4 lm. The mechanical propertiesare adjusted by the cantilever dimension (width andlength), hence the spring constant varies between1 and 50 N/m.

4. The aperture tip

The NSOM tip as depicted in Fig. 4 containstwo particular components, a sharp apex equiva-lent to a probe tip used in atomic force microscopy(AFM) and a circular metallic aperture at a loca-tion slightly retracted from the tip end. The pro-truding apex and the core of the tip consist of Si

3N

4produced by PECVD. The radius of curvature of

the radially symmetric apex is approximately10—20 nm. Compared to SiO

2(n

S*O"1.46) the

transparent PECVD Si3N

4has a fairly high refrac-

tive index of nS*N

"2.35 at k"633 nm wavelength.The tip shape and its mechanical properties pro-vide qualities suitable for AFM applications. Theaperture is formed by an opaque aluminum coat-ing, masking the core completely and homogene-ously against stray light. The thickness of the Allayer varies from approximately 170 nm at the ap-erture to 300 nm at the base of the tip. In contrastwith cantilever-based probes [11,12] and com-monly used fiber probes, the Al coating evenlysurrounds the smooth surface of the conical Si

3N

4tip core. Consequently, an aperture is providedwhich has a well-defined size and an exact circularshape. Being 50—100 nm retracted from the Si

3N

4tip end, the metallic aperture rim is during scanningboth protected against damage and located at adefined position in reference to the point of contact.To date, aperture diameters can be reproduciblymicromachined in the range 120—150 nm, whereas

M. Abraham et al. / Ultramicroscopy 71 (1998) 93—98 95

Fig. 4. Micromachined aperture probe tip for NSOM: The aperture is formed by the opaque Al coating around the smooth surface ofthe protruding and transparent Si

3N

4tip core. The protrusion guarantees a defined and constant point of contact in reference to the

aperture area during scanning. Additionally, it protects the aperture rim against damage. The apex has a significantly smaller radius ofcurvature than standard fiber probe tips.

the minimal diameter achieved is approximately80 nm. The wave propagation limiting cutoff dia-meter d

#"j/2n, which detracts aperture NSOM tips

from high optical transmission efficiency, decreases inthe case of the high refractive index of Si

3N

4consider-

ably to a value of 140 nm at j"670 nm. Since cutoffand aperture diameter are nearly the same, theoptical transmission efficiency is significantly high-er for the micromachined NSOM probe tip de-scribed, in comparison to standard fiber probes.

The batch fabrication of NSOM tips can basi-cally be applied to any substrate material, providedthe process does not damage the surface. For pro-cess optimization the technology has been appliedto silicon and Pyrex wafers. A 4.5 lm thick Si

3N

4layer is first deposited by PECVD onto the substra-te. After performing a photolithography step, single6]6 lm2 photoresist pads with a thickness of1.2 lm remain. Then the wafer is isotropically dryetched in a CF

4plasma, which leads to sharp Si

3N

4tips [13]. Next the tips are coated with a 300 nm Allayer. In the last step the Al is selectively removedat the apex of the tip resulting in the small aperture.

5. Experimental results

Several experiments have been carried out withthe microfabricated NSOM probe tips in order tocollect information on the optical and mechanical

properties. In the first experiment the probe tips wereproduced on a transparent Pyrex wafer and illu-minated by different lasers from their base (Fig. 5).

From the top one could only see small illumin-ated dots at the location of the apertures. Theoptical transmission efficiency of NSOM tips withaperture diameters of approximately 100 nm was10~3—10~4 at j"670 nm. Comparable fiber-basedprobes usually obtain much lower optical transmis-sion efficiencies in the range 10~5—10~6[5]. Nextthis arrangement was installed into a fiber-basedNSOM. The fiber probe was scanned over themicromachined NSOM tips, while the topographyand optical near-field signal were recorded simulta-neously. This experiment proved that the novel tipsare free of pin holes and that the FWHM of therecorded optical intensity corresponds exactly withthe aperture diameter measured in the SEM image.

For optical and mechanical evaluation purposes,the NSOM tips were combined with an opticalfiber so that they could serve as a modified fiberprobe in a standard fiber-based NSOM setup. Torealize this arrangement the NSOM tips were firstmicromachined onto a membrane and a cantileverbeam, respectively, both being made of Si

3N

4on

silicon substrate. Then the wafer was placed intoa special microassembly setup, where a 80 lm thinand cleaved optical fiber was adjusted with its coreright underneath the tip base (Fig. 6). Sub-sequently, the fiber was glued to the membrane or

96 M. Abraham et al. / Ultramicroscopy 71 (1998) 93—98

Fig. 5. Micrograph and transmission image of a NSOM tip array. The tips are microfabricated onto a Pyrex wafer and have an apertureof approximately 200 nm. Illuminated from their bottom the tips show light transmission only at the location of the aperture. Therefore,no pin holes are present at the side walls of the tip.

Fig. 6. Hybrid NSOM probe: first the NSOM tip was microfabricated onto a cantilever beam. Next the fiber core was aligned directlyunderneath the tip base and glued to the beam. At the end the excessing parts of the mounting structure were carefully removedmechanically.

cantilever beam, respectively. The parts of the tipmounting that extended the fiber rim were carefullymechanically removed. Then the hybrid probe wasinstalled into a fiber-based NSOM setup. In thefirst measurements the probe was scanned overa non-transparent test pattern [14]. In the topogra-phy image the measured dimensions of the patterncorresponded exactly to the given reference values.In a further arrangement, a standing evanescentwave was generated on a prism surface by theinterference of two counterpropagating beams ofone Ar-ion laser, which were totally reflected inter-nally on the inner side of the mentioned prismsurface [6,15]. Fifty line scans were performed inthe optical near field at the same y-position withthe hybrid NSOM probe and subsequently aver-aged. The interference pattern could be revealedwith a good fringe visibility (Fig. 7). The experi-ments prove that the combination of a microfab-ricated tip and a macroscopic fiber can be used as

a hybrid NSOM probe, which is basically compat-ible with all fiber-based NSOM setups.

6. Integration

The next step is the integration of the tip and thecantilever structure. The key question to resolve ishow to bend the light propagation from the canti-lever waveguide into the aperture tip with highefficiency using a simple process. The realization ofsuch structures is a fundamental problem in integ-rated optics. In a first approach we have testeda very simple design for such a “mirror”. We madeuse of a strong refractive index gradient towardsthe tip in the tip base region (Fig. 8). Since the tipcore has a considerably higher refractive index ofnS*N

"2.35, light couples from the waveguidingstructure (n

%&&"1.49) into the tip. The short avail-

able length for the coupling provided by the tip

M. Abraham et al. / Ultramicroscopy 71 (1998) 93—98 97

Fig. 7. Optical near-field measurement of a standing evanescentwave generated by the interference of two totally reflectedcounterpropagating beams of an Ar-ion laser. The dots depictthe measurement and the solid line shows a fit. The ratio be-tween the amplitude of the modulation and the average power is0.148. The measurement was performed with the hybrid probe.

Fig. 8. Coupling between waveguide and uncoated Si3N

4tip:

(a) waveguide structure and optical setup; the buffer layer wasthinned underneath the tip, (b) video image of light radiation oftip through a 50] microscope objective.

base length makes an effective coupling difficult. Toenhance the coupling, the top SiO

2buffer layer was

thinned directly underneath the tip base. Fig. 8shows the first coupling results between a wave-guide and an uncoated Si

3N

4tip.

7. Conclusion

In conclusion, we have introduced a micro-machined NSOM probe tip with mostly predefin-able mechanical and optical properties that over-comes the uncertainities of standard fiber probes.The microfabrication of the NSOM probe tipshould be easily implementable in more compre-hensive micromachining processes of scanning

probe microscopy (SPM) sensors. Arrays of suchNSOM-sensors can by easily fabricated. This opensa wide range of applications such as high-densityoptical data storage or high throughputnanolithography tools.

Acknowledgements

The authors thank the German Federal Ministryof Education, Science, Research and Technology(BMBF) and Omicron Vakuumphysik GmbH forfinancial support. For useful discussions we wouldlike to thank T. Zetterer of the Institute of Micro-technology Mainz GmbH. Special thanks go toThomas Sulzbach who performed the calculationsand the micromachining of the waveguiding canti-lever. We gratefully acknowledge the microas-sembling of the hybrid fiber probe performed byMarkus Rawert and Jorg Diebel.

References

[1] D.W. Pohl, Thin Solid Films 264 (1995) 250.[2] A. Harootunian, E. Betzig, M. Isaacson, A. Lewis, Appl.

Phys. Lett. 49 (1986) 674.[3] M. Isaacson, J.A. Cline, H. Barshatzky, J. Vac. Sci. Tech-

nol. B 9 (1991) 3103.[4] G.A. Valaskovic, M. Holton, G.H. Morrison, Appl. Opt.

34 (1995) 1215.[5] G. Tarrach, M.A. Bopp, D. Zeisel, A.J. Meixner, Rev. Sci.

Instr. 66 (1995) 3569.[6] A.J. Meixner, D. Zeisel, M.A. Bopp, G. Tarrach, Opt. Eng.

34 (1995) 2324.[7] B. Hecht, H. Bielefeld, L. Novotny, Y. Inouye, D.W. Pohl,

Phys. Rev. Lett. 77 (1996) 1889.[8] E. Betzig, P.L. Finn, J.S. Weiner, Appl. Phys. Lett. 60

(1992) 2484.[9] T. Matsumoto and M. Ohtsu, Near Field Optics-3, EOS

Optical Meeting Vol. 8, 1995, pp. 107—108.[10] B. Hecht, H. Bielefeld, Y. Inouye, D.W. Pohl, J. Appl. Phys,

to be published[11] A.G.T. Ruiter, M.H.P. Moers, N.F. van Hulst, M. de Boer,

J. Vac. Sci. Technol. B 14 (1996) 597.[12] M. Radmacher, P.E. Hillner, P.K. Hansma, Rev. Sci. Instr.

65 (1994) 2737.[13] A. Ruf, Ph.D. Dissertation, Technical University, Darm-

stadt, Germany, 1996, p. 67.[14] Test pattern provided by Topometrix GmbH, Darmstadt,

Germany: It consists of round pads, which are 250 nm highand have a diameter of 1.9 lm.

[15] A.J. Meixner, M.A. Bopp, G. Tarrach, Appl. Opt. 34 (1995)7995.

98 M. Abraham et al. / Ultramicroscopy 71 (1998) 93—98