SURFACE AND INTERFACE ANALYSISSurf. Interface Anal. 27, 296È298 (1999)

Mounting of Micromachined Diamond Tips andCantilevers

Ph. Niedermann,1* W. Ha� nni,1 S. Thurre, M. Gjoni,1 A. Perret,1 N. Skinner,1 P.-F. Indermu� hle,2U. Staufer2 and N. F. de Rooij21 CSEM, rue Jaquet-Droz 1, CH-2007 Neuchaü tel, Switzerland2 IMT, rue Jaquet-Droz 1, CH-2007 Neuchaü tel, Switzerland

Micromachined chemical vapour-deposited diamond tips are useful nanotools because of their high hardness, elec-trical conductivity and chemically inert nature. A technology is described to transfer pyramidal diamond tips oncantilevers to Pyrex substrates by anodic bonding in selected areas. This technology has been used to fabricate tipsfor individual handling and hybrid mounting on silicon cantilevers. Bond strengths have been quantiÐed and thedevices have been tested in a conducting atomic force microscopy experiment. Copyright John Wiley &( 1999Sons, Ltd.

KEYWORDS: diamond tips ; atomic force microscope probes ; conducting probes ; anodic bonding ; micromounting

INTRODUCTION

Diamond is a material with special useful properties fornanotools. Its hardness results in very robust tips,usable in scanning probe microscopy and nanomechani-cal testing, for example. Compared to natural diamond,chemical vapour-deposited (CVD) diamond has theadditional beneÐt that it can be made conductingthrough doping during deposition. Because the materialis inert and its surface does not form an insulatingbarrier, it can be used to fabricate electronic nano-probes.

When a CVD diamond coating technology with adense, non-destructive nucleation on silicon is com-bined with micromachining, precise replication isachieved and interesting devices are obtained. The fabri-cation of pyramidal diamond tips on cantilevers for usein AFM has been described in a previous publication.1

Here, we begin to explore a way to mount suchdevices on suitable supports for speciÐc applications.Pyramidal diamond tips together with patterneddiamond structures (cantilevers and supporting pads)have been fabricated and selectively transferred toPyrex substrates by anodic bonding. This technologyhas been used to remove tips individually from thePyrex wafer and to glue them in a controlled way onthin silicon cantilevers. This is motivated by the need tointegrate diamond tips with self-sensing cantilevers andarrays of such cantilevers that include actuators.Because of the high temperature of the CVD diamondprocess, direct integration with a complete micro-machining process is a major undertaking and hybridintegration after fabricating the circuits and the cantile-vers is of interest.

Individual handling, transferring and mounting (usingglue) of microscopic objects (such as small crystals) iscommon experimental practice, e.g. in crystallography,

* Correspondence to : CSEM, rue Jaquet-Droz 1,Ph. Niedermann,CH-2007 Neuchaü tel, Switzerland.

E-mail : philippe.niedermann=csem.ch

where di†erent transfer techniques are used. For thepresent application with scanning probe microscopy,however, bond strength is particularly important andthis has been characterized here.

TIP TRANSFER BY ANODIC BONDING

This is a generic, new technology allowing one tomicromachine diamond tips integrated with microstruc-tured planar diamond devices such as cantilevers.Bonding is performed via a special interlayer that canbe structured at will, deÐning areas where the diamondis bonded to the substrate and other areas where thediamond membrane is suspended freely.

The fabrication steps of this technology are illustratedin Fig. 1. Silicon wafers are thermally oxidized, open-ings are photolithographically deÐned and (111) crystalplane-delimited moulds are formed by etching in KOH[Fig. 1(a)]. Chemical vapour-deposited diamond layersof 1 lm thickness are deposited and photolitho-graphically patterned by plasma etching in oxygenusing silicon dioxide as a mask [Fig. 1(b)]. The diamondlayers are boron doped in situ and have a resistivity ofD0.02 ) cm. An interlayer for anodic bonding is

Figure 1. Tip and pedestal fabrication, and transfer by anodicbonding.

CCC 0142È2421/99/050296È03 $17.50 Received 15 September 1998Copyright ( 1999 John Wiley & Sons, Ltd. Accepted 1 December 1998

MICROMOUNTING OF DIAMOND TIPS AND CANTILEVERS 297

Figure 2. The SEM image of a demoulded, transferred tip. Barlength is 100 nm. The particles on the upper diamond plane in theimage are due to contamination build-up in the microscope. Theapex radius is estimated at 20 nm.

deposited and patterned [Fig. 1(c)]. The wafer isbonded to a Pyrex 7740 wafer by anodic bonding [Fig.1(d)].2 Finally, the silicon is removed by etching intetramethyl ammonium hydroxide (TMAH) [Fig. 1(e)].3

Sharp, clean tips such as the one in Fig. 2 have beenobtained. The contrast in the image is characteristic ofthe grain structure of the polycrystalline diamond Ðlmthat is clean and fully demoulded.

Bond pads of 200 ] 200 lm were manually probedfor adhesion strength, and were found to sustain at least0.1È1 N shear force.

INDIVIDUAL, HYBRID MOUNTING OF TIPS

A controlled mounting procedure of individual tips oncantilevers has been developed. Diamond tips weredesigned on a 60 ] 60 lm pedestal held by a thin arm,as depicted in the inset of Fig. 3. In order to handlethese small structures, a sharp, electrolytically etchedmetal tip is coated at its end with a small amount ofacrylic glue, as is done for gluing nanotubes to tips.4This tip is positioned on the diamond pedestal (withouttouching the diamond tip), holding it down, and thesupport arm is broken o†. The diamond pedestal is then

Figure 3. Picking up an individual tip and gluing it on a cantile-ver. The inset shows schematically the layout of a tip on a ped-estal, held by a thin arm and fixed to the Pyrex substrate in theshaded area.

lifted o† and brought down on the cantilever, which hasbeen coated with a thermally curing, electrically con-ducting glue (Epotek 4110 or Epotek 417). The metaltip with the acrylic glue is lifted o† while a second tipholds down the diamond pedestal, ensuring good align-ment with the cantilever.

A typical mounted tip is shown in Fig. 4, glued on a0.6 lm silicon cantilever. In the SEM image of Fig. 4(b)a contrast is again seen that is characteristic of a clean,smooth, fully demoulded CVD diamond surface. Adhe-sion is strong, and in shear tests the resistance of thebond between these diamond pedestals and Al pads wasfound to be 0.09 ^ 0.05 N. The resistance of the bondremained unchanged after repeated thermal cycling toliquid nitrogen temperature. Adhesion was similarlystrong on bare silicon.

This mounting procedure is well controlled becausemicrometre screw-driven xÈyÈz translation tables areused for manipulating the metal tips as well as forapplying the glue.

Tips glued in this manner were tested in a conductingatomic force microscopy (AFM) experiment on carbonmicroelectrodes at a constant force of the order of 10nN. At this low force, the ability to obtain conductingAFM images is a good indicator of the cleanliness ofthe tips. The amorphous carbon microelectrodes aredeÐned by 5 lm circular openings in a 300È500 nminsulating diamond-like carbon overlayer. The imagesof Fig. 5 show that a current Ñows relatively homoge-neously on the surface of a microelectrode, although atendency has been observed for the conductivity to be

Figure 4. Cantilever with glued tip.

Copyright ( 1999 John Wiley & Sons, Ltd. Surf. Interface Anal. 27, 296È298 (1999)

298 Ph. NIEDERMANN ET AL .

Figure 5. Conducting AFM images on 5 Ã5 mm (a, b) and 1 Ã1 mm (c, d) scan ranges : (a, c) topography; (b, d) simultaneously measuredcurrent images. Voltages applied and current scales are 3 V, 1 lA and 1 V, 10 nA in (b) and (d), respectively. In (c), the grey scale of thetopography is 50 nm.

higher in the valleys between the submicrometre granu-lar topographic features.

In conclusion, a new technology to selectively transfermoulded diamond microstructures to Pyrex wafers hasbeen presented, which will be used in future to fabricatediamond AFM cantilevers on chips. This technology

has been used to microfabricate tips on small pedestalsthat were then removed individually in a controlled wayfrom a Pyrex wafer and glued to thin silicon cantilevers.This procedure will be useful for prototyping andmounting diamond tips on self-sensing silicon cantile-vers.

REFERENCES

1. Ph. Niedermann, W. Ha� nni, D. Morel, A. Perret, N. Skinner,P.-F. Indermu� hle, N. F. de Rooij and P.-A. Buffat, Appl . Phys.A66, S31 (1998).

2. G. Wallis and D. I. Pomerantz, J . Appl . Phys. 40, 3946 (1969).3. U. Schnakenberg, W. Benecke and P. Lange, in Digest of Tech-

nical Papers , 6th Int . Conf . on Solid-State Sensors and Actu-ators (Transducers ‘91), San Francisco, CA, USA, June 24–28,1991, p. 815, IEEE, New York, (1991).

4. H. Dai, J. H. Hafner, A. G. Rinzler, D. T. Colbert and R. E.Smalley,Nature 384, 147 (1996).

Surf. Interface Anal. 27, 296È298 (1999) Copyright ( 1999 John Wiley & Sons, Ltd.