Boreman, Glenn D.

Gritz, Michael A.

Metzler, Meredith

Moser, Joel

Spencer, David

University of Central Florida (UCF): STARS (Showcase of Text, Archives, Research & Scholarship)

University of Central Florida STARS Faculty Bibliography 2000s Faculty Bibliography 1-1-2003 Fabrication of air bridges using electron beam lithography Michael A. Gritz University of Central Florida Meredith Metzler Joel Moser David Spencer Glenn D. Boreman University of Central Florida Find similar works at: https://stars.library.ucf.edu/facultybib2000 University of Central Florida Libraries http://library.ucf.edu This Article is brought to you for free and open access by the Faculty Bibliography at STARS. It has been accepted for inclusion in Faculty Bibliography 2000s by an authorized administrator of STARS. For more information, please contact STARS@ucf.edu. Recommended Citation Gritz, Michael A.; Metzler, Meredith; Moser, Joel; Spencer, David; and Boreman, Glenn D., "Fabrication of air bridges using electron beam lithography" (2003). Faculty Bibliography 2000s. 3786. https://stars.library.ucf.edu/facultybib2000/3786 Fabrication of air bridges using electron beam lithographyMichael A. Gritz, Meredith Metzler, Joel Moser, David Spencer, and Glenn D. BoremanCitation: Journal of Vacuum Science & Technology B: Microelectronics and Nanometer Structures Processing,Measurement, and Phenomena 21, 332 (2003); doi: 10.1116/1.1539062View online: https://doi.org/10.1116/1.1539062View Table of Contents: https://avs.scitation.org/toc/jvn/21/1Published by the American Institute of PhysicsARTICLES YOU MAY BE INTERESTED INFabrication of metallic air bridges using multiple-dose electron beam lithographyApplied Physics Letters 88, 202103 (2006); https://doi.org/10.1063/1.2204833Fabrication of large dimension aluminum air-bridges for superconducting quantum circuitsJournal of Vacuum Science & Technology B 31, 031601 (2013); https://doi.org/10.1116/1.4798399Fabrication and characterization of aluminum airbridges for superconducting microwave circuitsApplied Physics Letters 104, 052602 (2014); https://doi.org/10.1063/1.4863745Investigation into the charge distribution and barrier profile tailoring in AlGaN/GaN double heterostructures byself-consistent Poisson–Schrödinger calculations and capacitance–voltage profilingJournal of Applied Physics 91, 4387 (2002); https://doi.org/10.1063/1.1459604Direct three-dimensional patterning using nanoimprint lithographyApplied Physics Letters 78, 3322 (2001); https://doi.org/10.1063/1.1375006Determination of heterojunction band offsets by capacitance-voltage profiling through nonabrupt isotypeheterojunctionsApplied Physics Letters 46, 504 (1985); https://doi.org/10.1063/1.95572SHOP NOTESThese are ‘‘how to do it’’ papers. They should be written and illustrated so that the reader may easily follow whateverinstruction or advice is being given.Fabrication of air bridges using electron beam lithographyMichael A Gritza)School of Optics/CREOL, University of Central Florida, 4000 Central Florida Boulevard, Orlando,Florida 32816-2700Meredith Metzlerb)Cornell Nanofabrication Facility, Cornell University, Ithaca, New York 14853-5403Joel Moserc)Harvard Medical School, 25 Shattuck Street, Boston, Massachusetts 02115David Spencerd)Cornell Nanofabrication Facility, Cornell University, Ithaca, New York 14853-5403Glenn D. Boremane)School of Optics/CREOL, University of Central Florida, 4000 Central Florida Boulevard, Orlando,Florida 32816-2700~Received 18 June 2002; accepted 25 November 2002; published 31 January 2003!@DOI: 10.1116/1.1539062#I. INTRODUCTIONAn area of radio frequency~rf! technology has emergedbased on microelectromechanical systems~MEMS!. A defi-nition of a MEMS device is a miniature device or an array ofdevices combining electrical and mechanical components.1MEMS devices are designed and fabricated by techniquessimilar to those of very large scale integration~VLSI! andcan be manufactured by traditional batch processing meth-ods. The ease of manufacture and the increased developmentof VLSI MEMS devices allow engineers to miniaturize sen-sors and transducers. One example of such an advancementis in the MEMS accelerometer, now one of the largest singleMEMS applications with its incorporation in air bags. A rfMEMS device does not necessarily imply that the system isoperating at rf frequencies. In many MEMS devices andcomponents, the microelecromechanical operation is usedsimply for actuation or adjustment of another rf device. Oneexample of such a device would be a variable capacitor or asimple switch.For the air bridge shown in Fig. 1, the bottom metal con-tact forms a coplanar waveguide~CPW! transmission line. Arf switch has already been designed by using a distributedMEMS transmission line~DMTL !.2 For DMTL, the airbridges over the center conductor are operated in a parallelconfiguration. When a bias voltage is applied to the centerconductor of the CPW transmission line, charge distributesso that an electrostatic force occurs between the center con-ductor and the air bridge, which attracts the bridge downtoward the bottom contact. When the applied voltage reachesa threshold valueVth , the tensile force of the metallic airbridge can no longer withstand the electrostatic force, andthe metal falls abruptly to the bottom contact. When the volt-age is reduced, the metal bridge releases and returns to theoriginal position. In Ref. 2, DMTL air bridges were fabri-cated using photoresist as the sacrificial layer, which is thelayer removed toward the end of the fabrication process. Thephotoresist was removed by soaking the wafer in acetone.The wafer cannot be air dried because the surface tension ofthe liquid will pull the MEMS air bridges down. Therefore acritical point drying system is used to release the MEMS airbridges.3 A critical point drying system uses liquid CO2 toreplace the solvent. CO2 is employed because when it isbrought past its critical point the liquid immediately vapor-izes into a gas, thus alleviating any surface tension from asolvent.a!Electronic mail: mgritz@creol.ucf.edub!Electronic mail: metzler@cnf.cornell.educ!Electronic mail: moser@cnf.cornell.edud!Electronic mail: spencer@cnf.cornell.edue!Electronic mail: boreman@creol.ucf.edu FIG. 1. SEM micrograph of an air bridge.332 332J. Vac. Sci. Technol. B 21 „1…, Jan ÕFeb 2003 1071-1023Õ2003Õ21„1…Õ332Õ3Õ$19.00 ©2003 American Vacuum SocietyIn Ref. 2, a DMTL air bridge with a height of 1.5mm hasbeen used as a true-time-delay phase shifter in the rf portionof the spectrum. The phase shift is accomplished by applyinga single bias voltage to the center conductor of the CPWtransmission line. The applied voltage causes the height ofthe MEMS bridges to be decreased, increasing the capacitiveloading and decreasing the phase velocity. A major advan-tage to such a design is that only one control voltage isrequired. Our structure is a prototype for an infrared true-time-delay phase shifter. Using Refs. 2 and 4, an estimatedbridge height of 0.5mm is required because a significantlysmaller capacitance is required for use as an infrared-frequency phase shifter.In this work, we will describe the electron beam~e-beam!lithography methods used to fabricate air bridges with abridge height of 0.5mm without the need of a critical pointdryer. In our system, poly~methyl methacrylate! ~PMMA!was chosen as the sacrificial layer. The PMMA was removedwith oxygen plasma using a Branson barrel etcher as an iso-tropic reactive ion etcher~RIE!. The major advantage to ourmethod is in the simplification of the fabrication for the airbridges. By avoiding the critical point dryer, the PMMA sac-rificial layer can be etched away without any concern regard-ing maintenance of the proper solvent surface tension. Be-cause our system uses PMMA as the sacrificial layer, whichis a high resolution e-beam resist, this allows for the possi-bility of fabricating nano-DMTL air bridges.II. FABRICATION AND RESULTSThe air bridges were fabricated at the Cornell Nanofabri-cation Facility using a Cambridge/Leica EBMF 10.5 e-beamlithography system, with an on-chip resolution of 0.1mm.On each 10 mm by 10 mm Si chip ten devices were writtenwith a 730mm center-to-center spacing. Each DMTL devicehad 26 air bridges with a center-to-center spacing of 15mm.The dimensions of the bridges were 8mm long by 8 mmwide, which is shown in Fig. 2. There were four separatelayers which were fabricated in the following order:~1! alignment marks,~2! CPW transmission lines,~3! PMMA sacrificial layer, and~4! Au bridge.The first step in the fabrication process was to write a set ofalignment marks. The alignment marks were made of Aubecause the EBMF uses a backscatter detector to detect sec-ondary electrons for precise alignment, which means that anelement with a highZ number is more effective.Both local and global alignment marks were written. Theglobal marks were used to correct for any rotation errorswhile loading the substrates into the chuck, while the localmarks were used to correct for any stage drift when movingfrom field to field. By using the two sets of alignment marksextremely precise alignment was possible. A similar align-ment scheme was employed in Ref. 5, which had an overlayaccuracy of less than 100 nm. Another alignment schemeused high contrast alignment marks, which were fabricatedby RIE into Si.6 For our alignment scheme, the thickness ofthe Au was varied to find the best overlay accuracy. Wefound the overlay accuracy to be better than 25 nm when 250nm of Au was used.The next step is the fabrication of the CPW transmissionlines, which are the bond pads and the center conductor ofthe CPW@see Fig. 3~a!#. The width of the center conductor is4 mm with 1 mm gaps. These dimensions were chosen for adesign capacitance of 20 fF in the unbiased state. The pro-cess used to fabricate the CPW transmission lines was a tra-ditional liftoff process using a bilayer resist profile. FirstPMMA–MAA was spun onto the wafer at 3000 rpm, whichhas a higher sensitivity. The substrates were baked on a hot-plate set to 170 °C for 10 min to remove any solvents. Usingan interferometer the resist thickness was measured at 305nm. After the resist thickness was measured, the second layerwas spun onto the substrates. For this layer PMMA was spunonto the substrates at 3500 rpm. Next the substrates werebaked on a hotplate set to 170 °C for 10 min. This yielded atotal resist thickness of 460 nm. The patterns were written ata beam current of 30 nA with a dose of 150mC/cm2. Afterthe exposure, the substrate was developed in methylisobu-tylketone~MIBK !:isopropyl alcohol~IPA! 1:1 for 1 min. Af-ter development the substrates were rinsed with isopropanoland dried with N2 . The next step was to remove any un-wanted resist, which may have been left behind after thedevelopment but not seen under an optical microscope. Anysuch resist layer was removed by oxygen plasma in a Bran-son barrel etcher.7 This was done each time immediately af-ter the development of the substrates. After the developmentand the oxygen plasma cleaning of the substrates, a 10 nmthick adhesion layer of titanium and a 100 nm thick layer ofAu was e-beam evaporated onto the substrate. The unwantedresist and metallic films were lifted off using methylenechloride.The next step in the fabrication process was to fabricatethe PMMA sacrificial layer@see Fig. 3~b!#. This is accom-plished by first exposing the area where the Au bridge isdesired, which will pattern the base of the bridge. The di-FIG. 2. Schematic of a single DMTL air bridge with dimensions of 8mm by8 mm.333 Gritz et al. : Fabrication of air bridges using electron beam lithography 333JVST B - Microelectronics and Nanometer Structuresmensions for the base of the bridge were 8mm by 8 mm.Then the PMMA is overexposed, which causes the resist tocrosslink, in the area where the PMMA will act as the sacri-ficial layer. The dimensions for the sacrificial layer were 8mm by 400mm. In order to accomplish this, PMMA resistwas spun on the wafer at 3000 rpm. The substrates werebaked on a hotplate set to 170 °C for 10 min to remove anysolvents. Using an interferometer the resist thickness wasmeasured at 550 nm. The first set of patterns, which were thebase of the bridges, was written at a beam current of 30 nAwith a dose of 200mC/cm2. The next set of patterns, whichis the sacrificial layer, was written at a beam current of 30 nAwith a dose of 10 000mC/cm2. After the exposures, the sub-strate was developed in MIBK:IPA 1:1 for 2 min. After thedevelopment, an oxygen plasma etch was performed for 1min. Then a 50 nm layer of chromium was thermally evapo-rated onto the substrate to help protect the PMMA from be-ing washed away by the acetone during the liftoff process ofthe gold bridge@see Fig. 3~c!#.The final step in the fabrication of the air bridges is tocreate the Au bridge on top of the PMMA sacrificial layerusing a liftoff process@see Fig. 3~d!#. First PMMA ~PMMA–MAA ! resist was spun onto the wafer at 3500 rpm. Thesubstrates were baked on a hotplate set to 115 °C for 30 min.The temperature was lowered to 115 °C because bothPMMA and PMMA–MAA will re-flow at approximately120 °C. Using an interferometer the resist thickness wasmeasured at 560 nm. After the resist thickness was measured,the second layer was spun onto the substrates. For this layerPMMA was spun onto the substrates at 3500 rpm. Next thesubstrates were baked on a hotplate set to 115 °C for 30 min.This yielded a total resist thickness of 761 nm. The patternswere written at a beam current of 30 nA with a dose of200mC/cm2. The substrate was developed in MIBK:IPA 1:1for 2 min. After an oxygen plasma etch was performed for 1min, 10 nm of chromium and 250 nm of Au was e-beamevaporated onto the substrates. The unwanted resist and me-tallic films were lifted off using acetone. Acetone was pref-erable to methylene chloride because methylene chloride wasfound to be too aggressive. Next the substrates were placedin a chrome etchant for 20 s to remove the chromium layer.Once the chromium was removed, a resist strip was doneusing oxygen plasma on a Branson barrel etcher@s e Fig.3~e!#. A barrel etcher is an isotropic etch, which means itetches equally in all directions. The etch time was set to 4min in order to ensure that all the PMMA under the Aubridge was etched away. After the etching of the PMMA, theair bridges were examined using a scanning electron micro-scope~SEM!. Micrographs taken from the SEM can be seenin Fig. 1, clearly indicating an air bridge.III. SUMMARYThe fabrication of air bridges has been demonstrated us-ing electron beam lithography using PMMA as the sacrificiallayer. PMMA has a high resolution when used as an e-beamresist. This process provides the possibility for fabrication ofnano-MEMS air bridges. Furthermore this work demon-strates the fabrication of air bridges with a Branson barreletcher instead of a critical point dryer, which significantlyadvances the ease of fabrication for MEMS devices in gen-eral. This process could be developed to allow for MEMStuners, MEMS passive elements, or MEMS phase shifters tobe monolithically integrated onto the same chip.ACKNOWLEDGMENTSThis work was performed in part at the Cornell Nanofab-rication facility ~a member of the National NanofabricationUsers Network!, which is supported by the National ScienceFoundation under Grant No. ECS-9731293, Cornell Univer-sity, and industrial affiliates. This material is based upon re-search supported by NASA Grant No. NAG5–10308.1J. Bryzek, K. Peterson, and W. McCulley, IEEE Spectrum May, 20~1994!.2N. S. Barker and M. G. Rebeiz, IEEE Trans. Microwave Theory Tech.2,1881 ~1998!.3G. T. Mulhern, D. S. Soane, and R. T. Howe, International Conference onSolid-State Sensors and Actuators, June 1993, Vol. 296.4C. Fumeaux, M. A. Gritz, I. Codreanu, W. L. Schaich, F. J. Gonza´lez, andG. Boreman, Infrared Phys.41, 271 ~2000!.5C. A. Berseth, B. Dwir, I. Utke, H. Pier, A. Rudra, V. P. Iakovlev, and E.Kapon, J. Vac. Sci. Technol. B17, 3222~1999!.6M. N. Webster, A. H. Verbruggen, H. Jos, J. Romijn, P. M. A. Moors, andS. Radelaar, Microelectron. Eng.17, 37 ~1992!.7M. N. Websteret al., Microelectron. Eng.23, 441 ~1994!.FIG. 3. Details of the fabrication process for the air bridges.334 Gritz et al. : Fabrication of air bridges using electron beam lithography 334J. Vac. Sci. Technol. B, Vol. 21, No. 1, Jan ÕFeb 2003

Fabrication of air bridges using electron beam lithography

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Fabrication of air bridges using electron beam lithography

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