Sensors and Actuators A72Ž. 1999 125±139 Fabrication of glassy carbon microstructures by soft lithography Olivier J.A. Schueller, Scott T. Brittain, George M. Whitesides ) HarÕard UniÕersity, Department of Chemistry and Chemical Biology, 12 Oxford Street, Cambridge, MA 02138, USA Received 22 April 1998; revised 10 August 1998; accepted 10 August 1998 Abstract This paper describes fabrication techniques for the fabrication of glassy carbon microstructures. Molding of a resin of polyŽ furfuryl alcohol. using elastomeric molds yields polymeric microstructures, which are converted to free-standing glassy carbon microstructures by heating Ž.T ;500±11008C under argon. This approach allows the preparation of macroscopic structures Ž several mm2. with microscopic features Ž.;2 mm . Deformation of the molds during molding of the resin allows preparation of curved microstructures. The paper also presents a method of incorporating these structures on a chip by masking and electroplating. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Glassy carbon; Microfabrication; Soft lithography 1. Introduction dielectrics are deposited and patterned onto the surface; subtractive processes are based on etching. The range of This paper describes the preparation of glassy carbon structures that can be generated by etching is constrained microstructures by a procedure in which the microstruc- by the requirement of some etching procedures for specific tures of polymeric precursors to carbon are first formed by crystalline orientation: curved or rounded structures may wx micromolding, and then converted to glassy carbon by heat be particularly difficult to make 4 . treatmentŽ. 500±11008C under an inert atmosphere. Re- We have recently developed a range of microfabrication markably, the overall structure of the polymeric shape is techniques for the preparation of microstructures in or- wx largely retained during the carbonization stage, and the ganic polymers 7,8 . We call these techniques, which are process yields structured, glassy carbon microstructures. based on the use of elastomeric polydimethylsiloxane These microstructures are mechanically stiff and electri- Ž.PDMS stamps or molds, soft lithography. Soft lithogra- wx cally conductivewx 1,2 . Large-area microstructures Ž;mm2 . phy includes microcontact printing Ž.mCP 9±12 , micro- wx with high aspect ratioŽ. 5±10:1 and fine surface detail molding in capillariesŽ. MIMIC 13,14 , microtransfer wx w x Ž.;mm can be prepared by molding in a single-step molding Ž.mTM 15 , and replica molding 16,17 . Many procedure. aspects of these techniques have been covered in recent wx This microfabrication approach is based on molding of articles and reviews 7,18 . Central to these techniques is a polymer followed by carbonizationwx 3 . This procedure is the use of PDMS molds. The PDMS molds are prepared radically different from the typical microfabrication tech- by casting PDMS on a bas-relief master patterned in niques used in the preparation of micromachined silicon- photoresist by photolithography. The most relevant soft based sensors and transducerswx 4,5 . Silicon technology can lithographic techniques in the work presented here are be divided into two broad classes: additive and subtractive MIMIC and mTM, in which a pattern of polymer is processeswx 6 . In additive processes, thin films of metals or prepared by molding. In addition to the familiar allotropic forms of carbon Ž.graphite, diamond, and fullerenes , there are numerous other types of carbon materials, typically prepared by controlled pyrolysis of organic precursorswx 3,19 . The ) Corresponding author. E-mail: [email protected] structural propertiesÐextent of graphitization; size of mi- 0924-4247r99r$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S0924-4247Ž. 98 00218-0 126 O.J.A. Schueller et al.rSensors and Actuators A72() 1999 125±139 crocrystalline domains; the relative orientation of these 2. Experimental procedures domainsÐof these carbon solids strongly affects their chemical, mechanical, and physical propertieswx 3 . The 2.1. Materials properties of these carbons depend on the starting organic materials and the processing procedures. Four successive The elastomeric molds were prepared from polydimeth- stages have been identified that describe the physico- ylsiloxaneŽ. PDMS, Sylgard 184, Dow Corning . The fur- chemical processes responsible for the conversion of a furyl alcohol-modified phenolic resin was obtained from polymer to carbon solidswx 3 . Molecules of solvent and Q.O. ChemicalsŽ. Furcarb LP-520, West Lafayette, IN . unreacted monomer are eliminated during the precar- Zinc chloride was obtained from Aldrich. The semi-bright bonization stage Ž.T-3008C . Heteroatoms Ž oxygen, nitro- nickel electroplating bath was purchased from Technic gen, halogens. are eliminated during the carbonization Ž.Cranston, RI , and used as received. TEM grids were stageŽ. 300±5008C . Rapid mass loss occurs during this obtained from Polysciences,Ž. Warrington, PA . The posi- stage, and a network of conjugated carbon systems, each tive photoresistsŽ. Microposit 1075 and 1110 were ob- electronically isolated from the other, is formed. Hydrogen tained from ShipleyŽ.Ž. Malborough, MA . The SU-8 50 atoms are eliminated during the carbonization stageŽ 500± photoresist was purchased from MicroChemŽ Newton, 12008C. The aromatic network becomes interconnected; MA. The silicon wafers were obtained from Silicon Sense as a result, permeability decreases and density, hardness, Ž.Nashua, NH . Glassy carbon disks Ž;13 mm diameter, 2 modulus, and electrical conductivity increase. Structural mm thick. were obtained from Atomergic Chemetals defects are gradually eliminated during the annealing step Ž.Farmingdale, NY . Ž.T)12008C3.wx At elevated temperature Ž.)25008C , some carbon solids 2.2. Procedures graphitizewx 3 . Carbon materials prepared by pyrolysis have been classified as either graphitizing carbonsŽ. soft carbons 2.2.1. Fabrication of PDMS molds or non-graphitizing carbonŽ. hard carbonswx 20 . The forma- The PDMS molds were prepared by casting the PDMS tion of a liquid mesophase at ;500±7008C enables subse- mixŽ. ratio 10:1 on photoresist thin films patterned by quent graphitization: molecules and aromatic networks are photolithography. The photoresist thin films were initially preorganized in liquid-crystal-like aggregates during this passivated by gas phase reaction withŽ tridecafluoro- stage. Graphitization may then take place within these 1,1,2,2-tetrahydrooctyl.Ž. -1-trichlorosilane HulsÈ Chemicals aggregates since chain stacking is favorable. in order to facilitate removal of the PDMS once cured. Glassy carbon is an example of non-graphitizing car- PDMS was cured for 1 h at 608C in an oven. The bon: the entangled structure of the crystallites prevents photoresist films were patterned by photolithography to extended crystal growth. It was independently prepared in give surface relief structures with thickness 10±150 mm. 1962 by Yamada and Sato from phenolic resinswx 21 , and The contact photomasks used in the photolithographic step Davidson from cellulosewx 22 . Glassy carbon is typically were prepared by rapid prototyping: a computer file of the prepared by slow pyrolysis at elevated temperatures of a design to be transferred to the photoresist was generated polymeric precursorŽ polyvinyl chloride, polyvinylidene using a drawing softwareŽ. Freehand 7.0 and printed di- chloride, polyacrylonitrile, cellulose, resins of phenol-for- rectly on a transparency using a commercial high resolu- maldehyde, or polyfurfuryl alcohol. under inert atmo- tion printer Ž.)3300 dpiwx 24 .1 This transparency was spherewx 23 . The structure of glassy carbon consists of then used as a contact photomask in the photolithographic long, randomly oriented microfibrilsŽ. 15±50 AÊ wide that step. This technique is rapid and inexpensive for making bend, twist and interlock to form robust interfibrillar nodes. test and prototype structures: it takes less than 24 h from Glassy carbon, also known as vitreous carbon, derives its conception of a design to casting of a PDMS mold using a name from a fracture behavior that is similar to that of master generated with that design, and costs less than glass. The low densityŽ 1.4±1.5 grcm3. of glassy carbon US$20 per transparency. relative to that of graphiteŽ 2.27 grcm3. and diamond Ž3.52 grcm3. indicates that it is porous. The pores are, 2.2.2. Fabrication of glassy carbon microstructures however, not connected, as reflected by the fact that glassy The substrates used for the preparation of glassy carbon carbon is impermeable to liquids and gaseswx 23 . microstructures consisted of silicon wafers coated with a This paper describes a procedure, based on soft litho- thin film of chromiumŽ. 400 AÊ prepared by electron beam graphy, used to prepare glassy carbon microstructures evaporation. Curing of the furfuryl alcohol-based resin wx1,2 . Microstructures with complex, 3D topology are pre- sented as a demonstration of the potential of this fabrica- tion technique. A method to connect these structures to a 1 surface is also described. Potential applications of glassy Economical mask-making procedures using desktop publishing is also described at http:rrmems.isi.edurarchivesrtoolsrPSMASK- carbon microstructures in microfabricated devices are then MAKERrby Ash Parameswaran, Simon Fraser University, Burnaby, BC, discussed. Canada. O.J.A. Schueller et al.rSensors and Actuators A72() 1999 125±139