Electrodes on a budget: Micropatterned electrode fabrication by wet chemical deposition
Original entry: Darren Yang, AP225, Fall 2010
W. Ebina, A.C. Rowata, D.A. Weitz, “Electrodes on a budget: Micropatterned electrode fabrication by wet chemical deposition,” Biomicrofluidics, 3, 3 (2009).
Micropatterned electrode, wet chemical deposition, microfluidic, microelectromechanical system
The authors present a very simple, inexpensive method for fabricating micropatterned electrodes. The fabricating process involves depositing a thin metal layer of controlled thickness using wet chemistry. This method also eliminates the need for expensive equipment typically required for metal deposition. They also demonstrated that the resulting deposited metal can be used to fabricate functional electrode.
Previously, the method to fabricate microelectrodes is by a method known as injection molding. In injection molding molten solder is injected into microfluidic channels and hardens upon cooling. The microfluidic channels providing a mold for the metal, micron-scale features can be achieved. However, this method produces electrodes that are separated from flow channels by a polydimethylsiloxane (PDMS) wall; this configuration is suitable for dielectrophoresis that requires only a field gradient, but direct current applications are not possible since the electrodes are not in contact with the aqueous phase. Moreover, all of the other available methods are expensive and require equipment that is typically housed in dedicated facilities. To reduce the cost and need for specialized equipments, the authors suggest a way to deposit metals for micropatterning that matches the simplicity and ease of soft lithography. This new technique involves the century-old chemistry of Tollens’ reaction to deposit a silver film of the desired thickness on glass. The metal can then be patterned using standard photolithography techniques to form features suitable for application in microelectromechanical systems (MEMS) or microfluidic applications.
Method and Results
First, a thin silver film is deposited by Tollens’ reaction. In the presence of a reducing sugar, aqueous ionic silver is reduced to metallic silver. To obtain even coverage and thickness of the metallic film, the precipitation proceeds gradually by adding the glucose solution dropwise and thoroughly mixing the deposition solution immediately after adding the glucose solution.
In the second step, Standard lithographic technique is used to micropattern the silver film. The silver-coated glass substrate is dehydrated on a hotplate and cooled. The substrate is spincoated with photoresist. A positive photomask is placed on the substrate and exposed to UV. The exposed substrate is developed in a sodium hydroxide-based developer for 1 min while swirling and rinsed with de-ionized water. Unprotected silver coating is etched away by sonicating the wet. The etchants are rinsed away with de-ionized water, and the protective photoresist layer is removed with acetone to reveal the patterned silver coating.
Figure 1. A schematic of the procedure.
Using this technique, the authors fabricate electrodes ranging in size from microns to hundreds of microns (Figure 2). The photolithography process limits the spatial resolution of the electrodes. Moreover, these silver films can be deposited on substrates of varying surface area: Spray-based electroless silvering than achieved in this work may facilitate uniform deposition over areas much greater. Thus, electrodes of varying heights and geometries can be rapidly fabricated in this simple, inexpensive way.
Figure 2. Wet-deposited silver films sustain micropatterning of micron-scale features. (a) Micropatterning after one deposition cycle; (b) and (c) micropatterning after three deposition cycles. Scale-bar: 10 um.
Soft Matter Connection
Majority of this work deals with chemical reaction, and at the end of the reaction the liquid reactants turn into solid phase such as the sliver deposite and the photoresist. Nonetheless, there is still a strong connection with soft matter. The concept of soft matter is important during the spincoating process. The film thickness of the silver and photoresist heavily depends not only on the spining rate but also the interfacial energy and surface tension between the liquid and solid phase. The film coating will not be thin and uniform without the favorable interaction between the two. For example, you cannot spin coat a thin layer of oil film on to the Hydrophile glass surface.