Patterning processes combined metal masking and selective diamond growth to fabricate conductive diamond patterns on various substrates, allowing either the growth or nucleation surfaces to be applied as electrodes. These processes enable novel applications of diamond electrodes integrating diamond films into existing sensor systems and novel, temperature intolerant, polymer-based systems. A patterning process was initially developed for thermally oxidized silicon. Two nucleation (BEN and sonication seeding) and two growth (HFCVD and MPCVD) methods were evaluated. Feature dimensions and spacing down to 8 μm were obtained, having a minimal thickness of 1 μm. The films were high-quality polycrystalline diamond, as analyzed by Raman spectroscopy. As electrochemical sensors, the films detected dopamine (10 μM in PBS) with redox properties typical of microcrystalline diamond.
Attempts using BEN to selectively deposit diamond on insulating surfaces (alumina, high-temperature borosilicate glass) required metal coating of the back and sides of substrates. With alumina, adhesion problems prevented growth of complete films
(or patterns). With glass, interactions between the tungsten and substrate prevented etching of the mask, compromising the pattern.
Patterns on silicon dioxide were transferred to a polynorbornene polymer support with metal (Au, or Cr/Au/Cr) contacts to create the first diamond-on-polymer sensors - making the smooth, diamond nucleation surface the active electrode surface. The patterning process was scaled from ¼” chips to 3” wafers, to fabricate multi-electrode arrays (10 singly addressable pads). Sonication seeding was used to seed wafer-scale substrates due to limitations in implementing BEN with larger scale substrates.
As-fabricated, diamond-on-polymer electrodes from the wafer-scale process showed a highly capacitive dielectric response. XPS depth profiling revealed a SiOxCy layer on the electrode (diamond nucleation) surface, an issue introduced by the diamond transfer process (i.e. choice of nucleation method). After removing this insulating layer by fluorine plasma etch, electrochemical behavior more typical of a conductive diamond microelectrode was observed. Ferrocyanide oxidation was detected at 100pM (in PBS) using background-subtracted, fast scan cyclic voltammetry. These data confirmed that the plasma-treated nucleation surface was suitable as a microelectrode, showing the usefulness of these new patterning processes to develop novel diamond electrodes, e.g., in a polymer support.