SURFACE AND INTERFACE ANALYSISSurf. Interface Anal. 27, 299È301 (1999)

Micromachined SPM Probes with Sub-100 nmFeatures at Tip Apex

G. Schu� rmann1,* P. F. Indermu� hle,¤ U. Staufer1 and N. F. de Rooij11 Institute of Microtechnology, University of Neuchaü tel, CH-2007 Neuchaü tel, Switzerland

We have developed a fabrication process that allows thin Ðlms to be patterned at the end of a sharp tip. Using thismethod, many di†erent material contrasts can be achieved on the apex of a scanning probe microscope tip. Forexample, small apertures down to 30 nm have been patterned into an aluminium Ðlm covering quartz tips to formscanning near-Ðeld optical microscopy probes. Also, small well-deÐned metal electrodes can be fabricated on the tipapex, which can be used for electrochemical measurements. The tip structuring process is a batch fabricationmethod and is based on CMOS-compatible technologies, which means that it can be integrated easily into anexisting microfabrication process. Furthermore, it allows the patterning of a wide range of tip heights, materials orgeometries. Copyright 1999 John Wiley & Sons Ltd.(

KEYWORDS: NSOM; SNOM; aperture ; SPM; probes ; near-Ðeld ; microfabrication ; nanoelectrode ; nanotools ;nanotechnology

INTRODUCTION

The key component of every scanning probe micros-copy (SPM) probe is the SPM tip. General require-ments for SPM tips are a high aspect ratio for goodaccess to the sample and a small radius of curvature atthe tip apex for high-resolution imaging.1 Micro-fabrication of cantilevers with well-deÐned integratedtips was the key for the breakthrough of scanning forcemicroscopy.2 In order to meet the ever-growingdemands on the other high-resolution SPM sensors,new nanostructuring methods have to be found thatenable the mass production of multifunctional tips withreproducible properties.

In the Ðeld of scanning near-Ðeld optical microscopy,Davis and et al.3 proposed a micromachined silicon-based nanometre-scale photodiode. Others4 reportedthe microfabrication of scanning near-Ðeld opticalmicroscopy (SNOM) sensors consisting of hollow metaltips on silicon cantilevers. More recently, Noell et al.5have presented micromachined silicon nitride tips withan aluminium aperture.

In this paper, we report on a new method for themodiÐcation of silicon, quartz and pyrex micro-fabricated tips.

TIP APEX MODIFICATION

Before starting with the proper modiÐcation process,tips had to be prepared. The Ðrst material we workedon was silicon. This material is of particular interest

* Correspondence to : Institute of Microtechnology,G. Schu� rmann,University of Neuchaü tel, CH-2007 Neuchaü tel, Switzerland.

¤ Present address : ECE Department, University of CaliforniaDavis, Davis, CA 95616, USA.

because the realization of SPM Si tips and its photosen-sitive elements are well known. We Ðrst fabricated sharpsilicon atomic force microcopy (AFM) tips 15 lm inheight. They were etched in potassium hydroxide(KOH) according to a process developed by Wolter.6Also, 50 lm high quartz tips have been etched iso-tropically in hydroÑuoric acid. Using this etchingprocess7 one can produce sharp tips with a smoothsurface. These quartz tips will allow us to build smalllight sources or collect photons in the visible to UV fre-quency range. We also fabricated pyrex tips. Pyrex glassis an interesting material in microfabrication technologybecause it can easily be bonded to silicon by anodicbonding. However, the present etching process7 leads toa high surface roughness, which prevented us fromfurther processing the tips.

The tip structuring process is based on CMOS-compatible technologies, requires no lithography oralignment step and can easily be integrated into anexisting batch fabrication process. Furthermore, itallows tip apex modiÐcations on many di†erent tipheights, materials or geometries.

The working principle relies on a tip e†ect duringreactive ion etching (RIE) in a Ñuorine- and chlorine-containing plasma. Reactive ion etching is done in areactor where a plasma is sustained between two elec-trodes by a radiofrequency voltage. The di†erent mobil-ity of electrons and ions automatically induces a d.c.bias on the cathode, where the wafer is placed. Thereactive gas ions present in the etching chamber aretherefore accelerated towards the surface and react withthe substrate to form a volatile compound.8 Owing tothe small radius of curvature, the electric Ðeld is higheron the tip apex than on the side walls. We believe thatthis Ðeld enhancement is responsible for the higheretching rate observed on the tip apex. This e†ect allowssilicon nitride to be etched selectively only at the apexof a sharp tip. The Ðrst step in the modiÐcation processis therefore to coat the SPM tip with silicon nitride.Depending on the tip material, one can use chemical

CCC 0142È2421/99/050299È03 $17.50 Received 15 September 1998Copyright ( 1999 John Wiley & Sons, Ltd. Accepted 1 December 1998

300 G. SCHU� RMANN ET AL .

Figure 1. Top view of a silicon KOH tip covered with siliconnitride. The nitride has been selectively removed on the tip apex.The width of the ridge is estimated to be Á70 nm.

vapour deposited nitride or plasma-enhanced chemicalvapour deposited nitride. Then the coated tips areexposed to the plasma in the RIE reactor until thenitride is removed at the apex (Fig. 1). This takes typi-cally 1 min of etching time for 100 nm of nitride. Ifover-etched, the nitride will next be attacked also on theedges of the tip, followed immediately by the side walls.This patterned nitride layer can be used afterwards as amask for further processing steps of the SPM probe.

MICROFABRICATED SNOM TIPS

The Ðrst step is the fabrication of a sharp tip and the tipmaterial can be silicon, silicon dioxide, silicon nitride oreven glass. Then a thin aluminium layer of D50 nmthickness is evaporated on the tip [Fig. 2(a)]. A 100 nmthick layer of silicon nitride is plasma-enhanced chemi-cal vapour deposited over the aluminium layer [Fig.2(b)]. The silicon nitride layer is then patterned bymeans of the previously described tip e†ect in the RIEreactor [Fig. 2(c)]. In the Ðnal step, the silicon nitride isused as a mask to selectively wet-etch the protrudingaluminium in a standard Al-etching solution, leaving asmall aperture on the apex of the tip [Fig. 2(d)]. Theshape of the aperture is determined by the tip geometry.

Figure 3(a) shows a silicon tip after the nitride has

Figure 2. The fabrication process, of a SNOM tip aperture.

been selectively removed. In Fig. 3(b) the same tip canbe seen after the aluminium etch. This Ðgure shows evi-dence that the small cusp in Fig. 3(a) was actuallyformed by the underlying aluminium Ðlm, because it isremoved by the Al-etchant that does not attack thesilicon nitride. Figure 4 is a top view of such as aperturestructured on a 15 lm high silicon AFM tip. Usingexactly the same process, apertures of 30 nm diameterwere also achieved on 50 lm high quartz tips (Fig. 5).

PLATINUM NANOELECTRODE

A silicon tip was Ðrst coated with a 50 nm thick plati-num layer. Then silicon nitride was plasma-enhancedchemical vapour deposited on top of the platinum andpatterned according to the tip e†ect RIE, resulting in asmall protruding platinum apex (Fig. 6). If the siliconnitride layer is thick enough, the protruding platinumapex is isolated electrically from the base. This smalland well-deÐned electrode can be used for electrochemi-cal measurements,9 for example.

Figure 3. A SEM photograph of the same tip after selective etching of the nitride (a) (the tip apex is still covered with aluminium) and afteran aluminium wet-etch (b). It can be seen clearly that only the very apex of the tip has been affected. The arrow on the left image points tothe Al-cusp which is removed during wet-etching.

Surf. Interface Anal. 27, 299È301 (1999) Copyright ( 1999 John Wiley & Sons, Ltd.

MICROMACHINED SPM PROBES 301

Figure 4. Top view of the aperture in an aluminium film coveringa 15 lm high silicon tip. To enhance the contrast of the SEMimage, the silicon nitride has been completely removed from thetip. The aperture is estimated to be 50 nm in diameter.

Figure 5. Top view of a 30 nm diameter aperture structured intoan aluminium film on a 50 lm high quartz tip. Silicon nitride hasbeen completely removed from the tip.

Figure 6. A SEM micrograph of the apex of a platinum micro-electrode. The arrow points to the protruding platinum apex,which is Á100 nm long and 50 nm wide.

CONCLUSION

We have developed a new micromachining process thatallows the apex of microfabricated SPM tips to bebatch processed on a nanometre scale. The processrelies on a tip e†ect in an RIE plasma and is fullyCMOS compatible. It is simple because it contains nolithography step or alignment step. Furthermore, it isnot sensitive to tip height or tip material. Using thisprocess, SNOM apertures of 30 nm diameter have beenstructured in aluminium covering an SPM tip. Isolatedplatinum nanoelectrodes also can be fabricated easily.

Acknowledgements

We are grateful to H. Heinzelmann and W. Noell for useful dis-cussions, and to the technical sta† of IMT for the technical assistance.This work was supported by the Swiss Priority Program MINAST.

REFERENCES

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Copyright ( 1999 John Wiley & Sons, Ltd. Surf. Interface Anal. 27, 299È301 (1999)