JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 12, NO. 5, OCTOBER 2003 641

Cross-Linked PMMA as a Low-DimensionalDielectric Sacrificial Layer

W. H. Teh, Student Member, IEEE, Chi-Te Liang, Mark Graham, and Charles G. Smith, Associate Member, IEEE

AbstractA surface nanomachining fabrication process usingelectron beam cross-linked poly(methyl) methacrylate (PMMA)has been developed and characterized. PMMA with differentmolecular weights (MW 100 K, MW 495 K, MW 950 K) in anisolecasting solvent has been crosslinked with different electron beamirradiation levels ranging from 20C m2 to 240C m2. This is toinvestigate the quantifiable relationship between electron dose andits submicrometer remaining thickness after dissolving in acetone.This technique which uses electron beam lithography, offers ahigh resolution semi-three-dimensional (3-D) nanomachining ofthe sacrificial layer in a single run. Because of its low Youngsmodulus, it has been successfully integrated with nanoelectrome-chanical systems processing and has the advantage of producinglow-stress submicrometer thick structures with lateral dimensionsas low as, but not limited to 1 m. A fast dry release time from55 to 100 s using oxygen plasma ashing has been demonstratedfor a sacrificial layer aspect ratio of 125. This corresponds to anetch rate of about 0.6 m s at an average temperature of 40 C.The success of using cross-linked PMMA as a gate dielectric isdemonstrated by the fabrication of multilayered gated lateralquantum dot devices. Periodic and continuous conductanceoscillations arising from Coulomb charging effects are clearlyobserved in the transport properties at 50 mK. [1014]

Index TermsCross-linked PMMA, dielectric materials,electron beam lithography, quantum dot devices, surface nanoma-chining, titanium.

I. INTRODUCTION

A. Background

I N nano/microelectromechanical systems (NEMS/MEMS)processing, the use of sacrificial layers in between me-chanical layers is essential so that freedom in movement canbe achieved once the nano/micromechanical component isreleased. Many different types of sacrificial layers have beenused, including both metallic and nonmetallic layers. Formost MEMS developed using LIGA-like processes, metallicsacrificial layers such as copper, aluminum, titanium, andchromium have been used [1][5]. Nevertheless, nonmetallic

Manuscript received March 4, 2003; revised May 14, 2003. This work is sup-ported by Cavendish Kinetics Ltd. The work of C.-T. Liang was supported bythe NSC, Taiwan. Subject Editor N. de Rooij.

W. H. Teh is with the Cavendish Laboratory, University of Cambridge, Cam-bridge CB3 0HE, U.K. (e-mail: wht21@cam.ac.uk).

C.-T. Liang is with the Department of Physics, National Taiwan University,Taipei 106, Taiwan.

M. Graham is with the Cavendish Kinetics Ltd., 5223 LA-Hertogenbosch,The Netherlands.

C. G. Smith is with the Cavendish Laboratory, University of Cambridge,Cambridge CB3 0HE, U.K., and also with the Cavendish Kinetics Ltd., 5223LA-Hertogenbosch, The Netherlands.

Digital Object Identifier 10.1109/JMEMS.2003.817891

sacrificial layers such as silicon dioxide [6] and phosphoricglass [7] have also been reported.

There are several drawbacks in using these commonly usedsacrificial layers. In the case of the release mechanism, wet pro-cessing is typically used. The wet etching release process re-sults in the adhesion of the suspended microstructures. This iscaused by the large capillary and surface tension forces of liq-uids which induce stiction, rendering the devices to fail. As foron-chip integration, the release step often involves aggressiveetchants making the compatibility with conventional IC pro-cessing difficult. Due to requirements in etch selectivity be-tween the structural and sacrificial layers, a rather rigid limita-tion on the choice of the structural material is placed. This lim-itation is worsened when one uses wet release and bulk micro-machining techniques where crystal orientation and geometricalshapes are important considerations. Also, because of the im-portance placed in the residual stress of the mechanical layers,inertness at the interface between the structural and sacrificiallayers has to be ensured (with processing temperatures con-sidered) to avoid possible diffusion of ions and atomic speciesacross. Reaction between the structural and sacrificial layer mayresult in unwanted stress, which causes buckling. Coating oftypical sacrificial layer materials using thermal evaporation orsputtering is costly and time-consuming. Moreover, these mate-rials cannot be dissolved easily and cannot be patterned directly.

While organic-based sacrificial materials such as photoresist[8], [9] and polyimide [10] have been used to circumvent mostof the problems above, the microstructures fabricated are typi-cally in the hundreds of micrometers range and do not employelectron beam lithography. This is a limiting factor in fabricatingNEMS devices for basic and applied research. However, the dryrelease technique in our process, which employs oxygen plasmais fully compatible with conventional silicon IC processing andeliminates stiction while extending structural material choice tomore exotic ones such as SiC, SiN, Al, and Ti [10]. Dendrinticmaterial such as hyperbranched polymer HB560 [11] has alsobeen reported as a high etch rate dry-release sacrificial layer al-lowing large-area and high aspect ratio release.

In this paper, we report a semi-3-D surface nanoma-chining fabrication process using electron beam cross-linkedpoly(methyl) methacrylate (PMMA) as a submicrometer thickinsulating sacrificial layer. This method has the merits ofthe organic-based sacrificial materials discussed above whileenabling the use of electron beam lithography to pattern thestructural and sacrificial layer in a single run. This is importantto provide flexibility and quick turn-around in the processingof NEMS devices. Because of its low Youngs modulus, it hasthe advantage of producing low-stress submicrometer thick

1057-7157/03$17.00 2003 IEEE

642 JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 12, NO. 5, OCTOBER 2003

Fig. 1. Monomer structure of poly(methyl) methacrylate (PMMA). Themolecular weights of PMMA in this study are MW 100 K, 495 K, and 950 Kin anisole casting solvent.

structures with lateral dimensions as low as, but not limitedto 1 . Also, the good insulating properties of cross-linkedPMMA enable unique low dimensional physics experiments tobe performed.

II. EXPERIMENTAL

Poly(methyl methacrylate) or PMMA is probably the highestresolution organic photoresist used for electron beam lithog-raphy. By exploiting the crosslinking properties of PMMA, weare able to electron lithographically pattern PMMA as the mouldfor our structural layers as well as selectively crosslink to var-ious degrees to be our sacrificial layer. This is perhaps one ofthe simplest way to fabricate prototype NEMS devices witha quick turn-around time and provides material compatibilitysince cross-linked PMMA is easily etched in oxygen plasma.Being a long chain molecule with a monomer structure shownin Fig. 1, it is characterized by a reduction of molecular weightowing to chain scission of the original molecule, making theexposed region more sensitive to dissolution in liquid devel-oper. This behavior does not extend to infinite dose and at higherexposure, crosslinking events dominate and the resist becomesless soluble [12]. Here, we attempt to characterize the quantifi-able relationship between electron beam irradiation level andthe resulting thickness of cross-linked PMMA after dissolutionin acetone and various developers.

A. Cross-Linked PMMA CharacterizationFor the characterization experiments, neat PMMA in an

anisole casting solvent with different molecular weights (MW100 K, MW 495 K, and MW 950 K) is used. Characterizationof each type of PMMA is performed by first spinning the resiston an n-doped Si substrate at 6000 rpm for 50 s and prebaking itfor 7 min at 150 before spinning another layer, subsequentlybaked at the same temperature for a further 30 min. Thereafter,the bilayer resist is selectively crosslinked by electron beamat different electron doses (20 to 240 ) using amodified Hitachi S800 SEM at an accelerating voltage of 25

kV. A Dektak surface profiler is used to measure the resultingdifference in remaining thickness after dissolving in 3 IPA:1 MIBK developer for 40 s followed by IPA rinse. Similarmeasurements were made for dissolution (if any) in acetonefor 7 min. followed by IPA rinse. Results are shown in Figs. 2and 3.

B. MEMS and NEMS FabricationFor the fabrication of simple MEMS and NEMS structures,

Ti/ /Ni multilayers was used as the structural materialwith cross-linked PMMA used as the sacrificial material. Asingle electron beam lithography step was used in crosslinkingPMMA in a semi-three-dimensional (3-D) way to fabricatethe nanometer scale sacrificial layers. This technique isschematically shown in Fig. 4. We start with a (100) antimonydoped n++ Si substrate (cleaned with hydrofluoric acid) withnominal resistivities measuring between 310 ohm cm. This isfollowed by multiple spin coatings [see Fig. 4(a)] of PMMA(MW 495 K) to set the desired gap, taking into account theresulting thickness reduction once selectively crosslinked byelectron beam with different irradiation levels [see Fig. 4(b)].To demonstrate the capability of semi-3-D patterning of thecross-linked PMMA in a single run, multiple steps were madenear the edges of the sacrificial layer using electron dosesranging from 60 to 100 followed by dissolution inacetone [see Fig. 4(c)]. For the structural patterning of NEMS,which requires better resolution, PMMA was spun on top ofthe cross-linked PMMA sacrificial layers to enable standarddose (2 9 depending on dimensions) electron beamlithography to be performed. As for the lower-resolution caseof MEMS fabrication and when parallel exposure is required,standard optical photoresist can be spun on, using opticallithography instead for the patterning of the structural layers.Finally, the structural materials were deposited and lifted offin acetone. The structural materials consist of 120180 nmthick evaporated Ti followed by magnetron sputtered 8 nmthick and 24 nm thick sputtered Ni. The final dry releasestep involves oxygen plasma ashing for 55 s150 s using amicrowave plasma stripper with gas flow at 100 sccm [seeFig. 4(d)].C. Multilayered Gated Quantum Dot Device Fabrication

The above technique of nanomachining submicrometer fea-tures in cross-linked PMMA using electron beam lithographyalso allows simple, yet unique fabrication of multilayer quantumdevices. By using cross-linked PMMA as an insulating layer, weare able to fabricate multilayered gated lateral quantum dot de-vices. There are three major steps in our fabrication technique.First, a split-gate device is made on a Hall bar using standardelectron beam lithography. Second, we spin the PMMA resiston the Hall bar at 8000 rpm for 40 s, which results in a 70nm thick PMMA layer. The solution-coated sample is subse-quently baked at 150 for 15 min. The central regions nearthe split-gate is heavily exposed by an electron beam whichgives a dose of 120 . In this case, there is a layer ofcross-linked PMMA which can be used as a gate dielectric. Fi-nally, three overlaying finger gates with joining pads are alignedand fabricated above the split-gate device. The inset to Fig. 8

TEH et al.: CROSS-LINKED PMMA AS A LOW-DIMENSIONAL DIELECTRIC SACRIFICIAL LAYER 643

Fig. 2. Comparison of the remaining thickness of cross-linked PMMA resist normalized to its initial spin-on thickness as a function of electron dose for (a) MW100 K (b) MW 495 K and (c) MW 950 K for different development steps.

644 JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 12, NO. 5, OCTOBER 2003

Fig. 3. Direct comparison of the normalized thickness of the remaining cross-linked PMMA resist after dissolving in acetone for different molecular weights.Inset shows an optical micrograph of a multiple-stepped cross-linked PMMA sacrificial layer machined by electron beam lithography in a single run.

shows the SEM picture of a typical device. The existence of thecross-linked PMMA acting as a dielectric allows us to indepen-dently control all the gates.

III. RESULTS AND DISCUSSION

A. Cross-Linked PMMA CharacterizationFig. 2 shows the influence of electron beam exposure levels

on the solubility of PMMA in 3 IPA: 1 MIBK developer andacetone for electron doses ranging from 20 to 240

. Fig. 2(a)(c) correspond to the characterization ofbilayer PMMA resist with molecular weights of 100 K, 495 Kand 950 K with initial spin-on thickness of 2065 , 3432 and3264 respectively.

One of the trends observed is the clear reduction of the thick-ness of the bilayer resist upon electron beam exposure as afunction of the electron dose. A reduction of approximately25% to 50% in thickness is shown with a higher reduction in-curred for higher electron doses. However, the rate of reduc-tion decreases as a function of dose and plateaus off at around90 , 110 and 120 for PMMA MW100 K,495 K, and 950 K respectively. This may be due to the evap-oration of the anisole casting solvent and is a function of thepre-exposure baking time. Upon further development in 3 IPA:1 MIBK developer and acetone, we note a further reduction inthickness. This is clear when we use high electron doses, whichcauses partial crosslinking to full crosslinking depending on thedose used. Moderate dose electron irradiation of the PMMAsimply degrades the resist and forms fragments of lower molec-

ular weight and hence totally removed in the developer andacetone, i.e., standard electron beam lithography. However, athigher irradiation doses, the PMMA molecules crosslink witheach other to form a network of larger molecules. Dependingon the sensitivity (which depends on the molecular weight) ofthe PMMA resist used, different doses are needed to initiate fullcrosslinking. From Fig. 1, we estimate the electron dose to ini-tiate full crosslinking to be around 100 , 110 and120 for MW 100 K, 495 K, and 950 K, respectively. Also,the minimum amount of electron dose to start the PMMA tocrosslink is found to be 40 for both MW 100 K and 495 K,and 60 for the MW 950 K resist. The amount of absorbedenergy density, which is in general a complicated function ofposition within the resist film, results in partial dissolution ofPMMA in acetone if full crosslinking does not happen acrossits entire thickness. This explains the increase in the remainingthickness of the cross-linked PMMA resist from the onset ofcrosslinking to full crosslinking. Other factors that play a rolein determining the threshold dose for crosslinking include beamenergy, resist thickness, type of substrate, and the geometry ofthe irradiated pattern [12]. We see that the remaining thicknessof the cross-linked PMMA resist increases as a function of dose(beyond the threshold dose to crosslink) until it forms a plateau.The formation of this plateau is an indication that all of thePMMA molecules have been crosslinked. A slight decrease inthickness is observed beyond this, because of the intense ag-glomeration of the network of cross-linked PMMA moleculesand also, low volatile products which are formed during irradi-ation leave the polymer. Fig. 3 shows an overall comparison of

TEH et al.: CROSS-LINKED PMMA AS A LOW-DIMENSIONAL DIELECTRIC SACRIFICIAL LAYER 645

Fig. 4. General schematic diagram showing the fabrication process ofsuspended micromechanical structures using our developed cross-linkedPMMA semi-3-D surface nanomachining technique.

the normalized thickness of the remaining cross-linked PMMAresist after dissolving in acetone for MW 100 K, 495 K, and950 K. From the characterization results, we see that the thick-ness of the cross-linked PMMA can be modulated as a functionof the electron dose. This enables us to pattern nanometer scale3-D PMMA sacrificial layers using 2-D electron beam lithog-raphy pattern in a single run, after which the mechanical layercan be laid upon. The schematic of how this is done is shown inFig. 4. The inset of Fig. 3 shows an optical micrograph of a mul-tiple-stepped cross-linked PMMA sacrificial layer machined byelectron beam lithography in a single run.

B. MEMS and NEMS FabricationUsing the developed process, we successfully fabricated sus-

pended MEMS structures in the form of a cross micromem-brane. We use optical lithography for the patterning of the me-chanical structures and electron beam lithography for the sacrifi-cial layer. Fig. 5(a) shows an SEM micrograph of the Ti/ /Nimicromembrane before release. Here, the cross pattern was de-posited over a 100 100 cross-linked PMMA sacrificiallayer measuring 160 nm in thickness. Dry release of the mi-cromechanical structure, as seen in Fig. 5(b) has been achievedusing isotropic oxygen plasma ashing. The inset of Fig. 5(b) is4 magnified to reveal successful underetching of more than20 per side with an average sacrificial layer aspect ratio of,

Fig. 5. SEMs showing (a) a Ti/SiO /Ni micro-membrane with a visible160 nm thick cross-linked PMMA sacrificial layer underneath and (b) a fullyreleased Ti/SiO /Ni floating micromembrane after a 55-s isotropic ashing inoxygen plasma. The dry release yields lateral underetching of more than 20m per side with an average sacrificial layer aspect ratio of 125. The insetshown is 4 magnified.

but not limited to 125. Using cross-linked PMMA as the sac-rificial layer has the benefits of the dry release step being ex-tremely quick at an etch rate of about 0.6 (cross-linkedPMMA is easily ashed by oxygen plasma), simple, stiction-free,cheap and IC-compatible. The dry-release method avoids thelimitation placed on wet etchants, which have diffusion limi-tations and hence limited to etching short distances and timeconsuming [11]. This means that in most cases, access holesmay be avoided in design. The low temperature (40 ) involvedduring the ashing step is useful for compatibility in standardIC processing and also avoids accumulation of stress causedby temperature gradients. Because of the fact that cross-linkedPMMA is transparent, this allows easy alignment for the nextlayer should one decides to use optical lithography for the me-chanical layer. Fig. 5(b) also shows that the resulting structureshave extremely low stress and do not buckle into the 160 nm

646 JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 12, NO. 5, OCTOBER 2003

Fig. 6. Demonstration of semi-3-D surface nanomachining using cross-linkedPMMA sacrificial layer. SEMs showing (a) a Ti/SiO /Ni micropaddle structuredefined by electron beam lithography over a 400-nm-thick cross-linked PMMAsacrificial layer, (b) how the paddle support structures climb over the multipleedge steps modulated by different levels of electron dose, and (c) a fully dryreleased suspended paddle structure.

high gap. We believe that this may be caused by the inert in-terface between the sacrificial layer and the mechanical layer.Little or no interdiffusion of ionic/atomic species occurs, whichmay cause stress gradients upon release. Furthermore, the lowYoungs modulus of cross-linked PMMA (nominal PMMA hasa modulus of around 5 GPa) enables itself to deform, thus re-leasing the stress from the rigid mechanical layers which aredeposited on top of it.

Fig. 7. Other examples of microfabrication using cross-linked PMMAsurface nanomachining over a 400-nm gap. SEM micrographs of dry released(a) 1 m 11 m 130 nm Ti/SiO /Ni bridge and (b) 2 m 5 m 130nm Ti/SiO /Ni cantilever. Both insets show the corresponding unreleasedstructures.

Fig. 6 demonstrates some basic NEMS fabrication resultsusing electron beam lithography. Here, a semi-3-D patterningof the cross-linked PMMA sacrificial layer is performed usingelectron beam lithography in a single process step (as shown inFig. 4). The additional degree of freedom ( -direction) in pat-terning the sacrificial layer provides flexibility in mechanicaldesign, useful when designing nanomechanical filters, ribbedstructures and resonators. A micro-paddle made from 120 nmTi/8 nm /2 nm Ni measuring as low as 1 in lateral di-mensions and suspended 400 nm above the substrate is realized.Conformal deposition of the mechanical layer by thermal evap-oration over the patterned sacrificial layer results in step-ups atthe support. In particular, Fig. 6(b) shows an SEM micrographon how the micro-paddle support structures climb over the mul-tiple edge steps modulated by the different levels of electrondoses with Fig. 6(c) showing the final released low stress struc-ture.

Fig. 7 shows other examples of microfabrication usingcross-linked PMMA surface nanomachining over a 400nm gap. SEM micrographs of low stress and released 1

11 130 nm Ti/ /Ni bridge structure and 25 130 nm Ti/ /Ni cantilever structure are

shown in Fig. 7(a) and (b) respectively. In Fig. 7(b), a gradual

TEH et al.: CROSS-LINKED PMMA AS A LOW-DIMENSIONAL DIELECTRIC SACRIFICIAL LAYER 647

Fig. 8. Conductance measurements G(V ) for V = 0:482 V, V = 1:941 V and V = 1:776 V. (b) The oscillations without the backgroundwhich shows the beating pattern. The inset shows an SEM picture of a typical device. The brightest regions correspond to finger gates with joining pads, lyingabove the split-gate (SG), with an insulating layer of cross-linked PMMA in between.

step-up support can be made when one reflows the cross-linkedPMMA sacrificial layer at 150 for several minutes afterpatterning steps at the edge. This is to smoothen the stepsand to provide a good step coverage for the anchor of thecantilever. Besides the structural materials mentioned, othermetallic layers such as Al, Au, Ti, and electroplated Ni havebeen successfully tested as mechanical layers combined withcross-linked PMMA as a sacrificial layer.

The disadvantage of this simple technique is its limited reso-lution when using cross-linked PMMA as a negative resist. Thisis due to proximity effects, associated with backscattered andsecondary electrons exacerbated by the high dose exposure. Za-iler et al. [13] proposed a high-resolution technique involvingtwo exposures of the PMMA with the first stage involving stan-dard dose exposure and in the second stage these, up to this timeunexposed regions, are crosslinked by exposure at a high dose.

C. Multilayered Gated Quantum Dot Device FabricationFig. 8 shows the excellent dielectric properties of low-di-

mensional cross-linked PMMA and how it can be used to fab-ricate quantum devices. By applying negative voltages on F1,F3, and the split-gate (SG), we are able to electrostatically de-fine a lateral quantum dot device. When we vary the appliedvoltage on the second finger gate (F2), we observe periodicand continuous oscillations superimposed on resonant feature,as shown in Fig. 8(a). Decreasing has two effects. First,

it depletes the electrons with the quantum dot, causing contin-uous conductance oscillations due to Coulomb charging effects.Second, it also reduces the number of transmitted one-dimen-sional (1-D) channel through the dot. The latter effect gives riseto the slowly varying background FabryProt type resonanteffects [14]. Fig. 8(b) shows the beating pattern after subtrac-tion of the background. Our experimental results clearly demon-strate the success of using cross-linked PMMA as a gate dielec-tric. These gated quantum structures can be incorporated withNEMS to reveal many interesting physical phenomena and po-tential applications. The excellent insulating properties of cross-linked PMMA have also been exploited for various low-dimen-sional physics experiments [13], [15].

IV. CONCLUSION

New sacrificial materials for surface nanomachining usingelectron beam lithography has been investigated and charac-terized. Particularly, cross-linked PMMA is used to fabricatesemi-3-D sacrificial layers in the nanometer scale using elec-tron beam lithography in a single process step. The cross-linkedPMMA sacrificial layer is easy to coat, simple to fabricate, easyto dissolve by dry-etching, offers direct high resolution pat-terning, flexible, compatible with different metallic materials,possesses good dielectric properties and offers low temperaturepostprocessing of MEMS.

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Free standing and highly low stress microstructures suchas suspending membranes, paddles, bridges and cantileversare successfully released over a submicrometer gap usingthis technique. It offers quick turnaround development forNEMS/MEMS devices and is easily accessible to universitiesand small companies.

The ability to nanomachine semi 3-D submicrometer featurestogether with the excellent dielectric properties of cross-linkedPMMA, are useful in investigating physical phenomena. This,integrated alongside with NEMS will potentially create novelmechanical quantum devices in the future.

ACKNOWLEDGMENT

W. H. Teh would like to thank IME for his scholarship and J.K. Luo for many helpful discussions.

REFERENCES[1] B. Lchel, A. Maciossek, M. Knig, H. J. Quenzer, and H.-L. Huber,

Galvanoplated 3D structures for micro systems, Microelectron. Eng.,vol. 23, pp. 455459, 1994.

[2] P. M. Zavracky, S. Majumder, and N. E. McGruer, Micromechanicalswitches fabricated using nickel surface micromachining, J. Microelec-tromech. Syst., vol. 6, pp. 39, Mar. 1997.

[3] C. Burbaum, J. Mohr, and P. Bley, Fabrication of capacitive accelera-tion sensors by the LIGA technique, Sens. Actuators A, Phys., vol. 27,pp. 559563, 1991.

[4] A. B. Frazier, C. H. Ahn, and M. G. Allen, Development of micro-machined devices using polymide-based processes, Sens. Actuators A,Phys., vol. 45, pp. 4755, 1994.

[5] A. Maciossek, B. Lchel, H. Quenzer, B. Wagner, S. Shulze, and J.Noetzel, Galvanoplating and sacrificial layers for surface microma-chining, Microelectron. Eng., vol. 27, pp. 503508, 1995.

[6] S. Evoy, D. W. Carr, L. Sekaric, A. Olkhovets, J. M. Parpia, and H.G. Craighead, Nanofabrication and electrostatic operation of single-crystal silicon paddle oscillators, J. Appl. Phys., vol. 86, no. 11, pp.60726077, 1999.

[7] R. T. Howe, Surface micromachining for microsensors and microactu-ators, J. Vac. Sci. Technol. B, vol. 6, no. 6, pp. 18091813, 1988.

[8] Z. Cui and R. A. Lawes, A new sacrificial layer process for the fabrica-tion of micromechanical systems, J. Micromech. Microeng., vol. 7, pp.128130, 1997.

[9] M. Bartek and R. F. Wolffenbuttel, Dry release of metal structuresin oxygen plasma: Process characterization and optimization, J. Mi-cromech. Microeng., vol. 8, pp. 9194, 1998.

[10] A. Bagolini, L. Pakula, T. L. M. Scholtes, H. T. M. Pham, P. J. French,and P. M. Sarro, Polyimide sacrificial layer and novel materials for post-processing surface micromachining, J. Micromech. Microeng., vol. 12,pp. 385389, 2002.

[11] H.-J. Suh, P. Bharathi, D. J. Beebe, and J. S. Moore, Dendritic materialas a dry-release sacrificial layer, J. Microelectromech. Syst., vol. 9, pp.198205, June 2000.

[12] J. S. Greeneich, Developer characteristics of poly-(Methyl Methacry-late) electron resist, J. Electrochem Soc., vol. 122, pp. 970976, 1975.

[13] I. Zailer, J. E. F. Frost, V. Chabasseur-Molyneux, C. J. B. Ford, andM. Pepper, Crosslinked PMMA as a high-resolution negative resistfor electron beam lithography and applications for physics of low-di-mensional structures, Semicond. Sci. Technol., vol. 11, pp. 12351238,1996.

[14] C. G. Smith, M. Pepper, H. Ahmed, J. E. F. Frost, D. G. Hasko, R. New-bury, D. C. Peacock, D. A. Ritchie, and G. A. C. Jones, Fabry-Perotinterferometry with electron waves, J. Phys. Condens. Matter, vol. 1,pp. 90359044, 1989.

[15] C.-T. Liang, M. Y. Simmons, C. G. Smith, G.-H. Kim, D. A. Ritchie,and M. Pepper, Multilayered gated lateral quantum dot devices, Appl.Phys. Lett., vol. 76, no. 9, pp. 11341136, 2000.

W. H. Teh (S99) received the B.Eng. degree (1stHons.) in electrical engineering from Universityof Malaya in 1999, the S.M. degree in advancedmaterials from Singapore-Massachusetts Instituteof Technology Alliance program in 2000, and iscurrently working towards the Ph.D. degree at theCavendish Laboratory, University of Cambridge,U.K.

Before his Ph.D. work, he has helped in charac-terizing TaN/Cu interconnect modules for 8 wafersat the Institute of Microelectronics, Singapore.

Since then, he has been involved in CMOS-compatible MEMS involvingIMS-CHIPS, Germany and Cavendish Kinetics Ltd., The Netherlands.Recently, he developed a 3-D lithography technique based on two-photon-ab-sorption at IBM Zurich Research Laboratory, under Dr. U. Duerig of theMicro/Nano-mechanics group. His areas of interest include carbon nan-otubes-MEMS integration, MEMS-based electron devices, and basic NEMSresearch based on low-temperature measurements.

Mr. Teh was the recipient of the Shell Scholarship for his undergraduatestudies, the SMA scholarship and IME book-prize award for being the top stu-dent/graduating valedictorian for his S.M. degree, and the IME/NSTB scholar-ship for his Ph.D.

Chi-Te Liang received the B.Sc. degree in physicsfrom National Taiwan University, Taiwan and thePh.D. degree in physics from Cambridge University,U.K.

He is currently an Associate Professor in thePhysics Department at National Taiwan University,Taiwan. His current interests include quantumHall-insulator transitions, spintronics, and MEMS.

Mark Graham received the M.A. and Ph.D. degreesin physics from the University of Cambridge, U.K.

He has previously worked in the fields of micro-electronics and MEMS, and is currently involved inthe development of single-electron acoustoelectriccurrent devices.

Charles G. Smith (A00) received the physics degreefrom the University of St. Andrews, Scotland, in 1982and the Ph.D. degree in physics from the Universityof Cambridge, U.K., in 1987.

He became a lecturer in the University of Cam-bridge Physics Department in 1995 and was madea Reader in nanoelectronic devices in 2001. Heis part-time CTO of Cavendish Kinetics Ltd, TheNetherlands, a company developing CMOS compat-ible MEMS/NEMS devices. He is also a Reader innanodevices at the University of Cambridge, Physics

Department, U.K. He has contributed to nine patents and over 90 publications.

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