COURSE 146C Laboratory Experiment 7 Soft Lithography Patterning Please read the lab procedure before your experiment References 1 Xia Y N and Whitesides G M Soft Lithography Annu Rev Mater Sci 28 153184 1998 2 Gerber R W and Oliver Hoyo M T Selective Etching via Soft Lithography of Conductive Multilayered Gold Films with Analysis of Electrolyte Solutions Journal of Chemical Education 85 1108 1111 2008 3 Campbell D J et al Replication and compression of bulk and surface structures with polydimethylsiloxane elastomer J Chem Educ 76 537 541 1999 1 Experiment 7 Soft Lithography Patterning I Introduction Soft lithography represents a non photo or e beam lithographic strategy based on self assembly and replica molding for carrying out micro and nanofabrication 1 It provides a convenient effective and low cost method for the formation and manufacturing of micro and nanostructures Soft lithography uses an elastomeric block with patterned relief structures on its surface to generate patterns and structures with feature sizes ranging from 30 nm to 100 m using this technique at least five techniques have been demonstrated microcontact printing replica molding microtransfer molding micromolding in capillaries and solvent assisted micromolding In this experiment we will learn the principle of soft lithography and the fabrication of poly dimethylsiloxane PDMS stamp We will use the microcontact printing technique to create patterned metal features e g electrodes on a substrate which can be further used for electrical measurements II PDMS Stamp An elastomeric block with patterned relief structures on its surface is the key to soft lithography PDMS is one of the most important materials for making the elastomeric block for soft lithography PDMS is a viscous liquid consists of long chain like polymer molecules They have very low glass transition temperatures and hence are fluids at room temperature These liquid materials can be readily converted into solid elastomers by adding a curing agent e g a mixture of a platinum complex and copolymers of methylhydrosiloxane and dimethylsiloxane and heat to a elevate temperature which lead to the formation of chemical bonds between polymer molecules cross linking The elastomeric block is prepared by molding as shown in Figure 1 The PDMS liquid will be poured over a master having relief structure on its surface then cured and peeled off The master is in turn fabricated using microlithographic techniques such as photolithography micromachining e beam writing or from available relief structures PDMS blocks having relief patterns on their surfaces can be used in a number of different processes for patterning for example in this experiment we will use stamps to print patterns of self assembled monolayers SAMs on metal film Figure 1 Schematic illustration of the procedure for fabricating PDMS stamps from a master having relief structures on its surface ref 1 2 III Making Metal Thin Films Metal thin films that are used for microcontact printing are typically made of gold and silver since they are easier to form SAMs Metal films can be prepared by thermal e beam evaporation sputtering electrochemical deposition and chemical deposition In this experiment we will use chemical reduction approach to make thin films of Ag on glass slides This process is called Tollens process which involves chemical reduction of silver ions to silver metal by using glucose C6H12O6 as the reducing agent We need to prepare two solutions A and B for the experiment Solution A contains a mixture of AgNO3 and NH4NO3 Solution B is NaOH and Solution C is glucose When solution A and B are mixed the hydroxide will react with ammonium ion to give free ammonia which will further react with silver ion to form diamminesilver I complex Ag NH3 2 Tollens reagent When solution C is added glucose will be oxidized to the form of gluconic acid C6H12O7 and reduce the silver I ion to silver metal The thickness of the film on glass slide depends on the time the glass slide is immersed in solution mixture IV Microcontact Printing of SAMs The concept of microcontact printing is to use the relief pattern on the surface of a PDMS stamp to form patterns of SAMs on the surfaces of substrates by contact Alkylthiol on Au and Ag surfaces is the best characterized and understood system of SAMs Both Au and Ag have strong affinity for sulfur When the thin film of thiol is bought into contact with Au and Ag metal surface covalent bond will be formed between the sulfur atom and metal atom on surface and the alkyl chains extend from the plane of the surface in a nearly all trans configuration to maximize the Van der Waals force between alkyl chains Figure 2 In microcontact printing PDMS stamp will be wetted with the thiol containing ink solution of hexadecanethiol in ethanol and is brought into contact with the surface of Ag on the glass slide for 10 20s The hexadecanethiol CH3 CH2 15SH transfers from the stamp to the Ag upon contact forms a hexadecanethiolate CH3 CH2 15S and generates patterns of SAMs on the surface of Ag Figure 2 Schematic procedures for Contact Printing of hexadecanethiol HDT on the surface of gold ref 1 SAMs that are 2 3 nm thick serve as resists to protect the underlying substrates effectively from attack by certain wet etchants For Ag metal film aqueous solutions containing K2S2O3 K3Fe CN 6 K4Fe CN 6 is an effective etchant The oxidizing agent of Fe CN 63 can be reduced to Fe CN 63 and oxidize the Ag to Ag I ion which will further form a stable dithiosulfatosilver I complex Ag S2O3 23 After etching process only the patterned features electrode in this case protected by SAMs remains on glass slide as shown in Figure 3 3 Figure 3 Optical image patterned electrodes on substrate ref 2 Procedure I Preparation of PDMS Pattern Stamp 1 Mix 15 ml PDMS base large syringe and 1 5 ml PDMS curing agent small syringe in a plastic cup Wipe up any spills with a dry paper towel 2 Stir the PDMS mixture thoroughly with glass rod Place the cup with PDMS into vacuum desiccators and evacuate it for at least 20 minutes until it is free of bubbles 3 Put a PDMS master in a petri dish and fix the master position 4 Put a cylinder plastic frame on the PDMS master Pour the PDMS carefully into the plastic frame and cover the dish 5 Cure the PDMS at 130 C for at least 20 mins 6 Peel the PDMS stand away from the master and plastic frame II Deposition of Metal Film on Substrates 1 Take a glass slide and petri dish Wash the glass