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4.2 Polymer Deposition

The deposition of conducting polymers can be achieved in several ways as discussed in prior chapters.  If the conducting polymer is soluble it can be placed in solution and deposited by spray coating, spin coating, or drop coating.  Spray coating is accomplished by dispersing the solution into a fine mist using compressed nitrogen and an air brush.  During spin coating, the substrate is spun at a high rate of speed while the solution is poured onto it.  The solution is then spread uniformly across the substrate where the solvent evaporates quickly leaving behind a very smooth film.  One limitation of this technique is that if the viscosity of the solution is small, only a thin conducting polymer layer is possible.  Drop coating is a simple technique in which drops of the solution are pipetted onto the substrate and the solvent is allowed to evaporate.  This technique produces polymer films that are quite rough compared with spray coated and spin coated films.  One other limitation to depositing soluble conducting polymer films is that the polymer must be soluble in a solvent which is compatible with the high temperature superconductor.  Water soluble polymers as well as acidic solutions are obviously not possible due to the reactive nature of the cuprate materials as discussed in Chapter 2.  Solvents such as acetonitrile, toluene, or tetrahydrofuran (THF) have been found to be compatible with cuprate phases.  If the conducting polymer can be dissolved in a compatible solvent, then it is possible to deposit it on to the superconductor.  If electrochemical doping or chemical doping is done via a solution method to change the oxidation state of the conducting polymer, the choice of solvents must again be considered.  This solvent must not dissolve the conducting polymer.  One successful method for depositing poly(3-hexylthiophene) onto a high temperature superconductor²² is to spray coat a THF solution of poly(3-hexylthiophene).  Then, the polymer is electrochemically doped in an acetonitrile solution. 

The other deposition method employed is the electrochemical polymerization of monomer species, particularly polypyrrole due to its stability.  The polymerization of polypyrrole onto high temperature superconductor thin films was discussed in Chapter 3.  A few simple modifications must be made for their deposition onto patterned microbridges.  First, the contact areas must be masked off with Apiezon wax (Varian) which is easily removed with toluene at a later stage.  Also, the electrical connection needs to be made at the corner of the thin film so that the entire microbridge is immersed in the monomer solution.