Patterning ITO Coated Glass Slides – Tech Note #202
The following procedure for etching ITO coated glass slides should only be performed while wearing appropriate safety apparel and eye protection, in an approved fume hood, under the supervision of a qualified scientist.
An aqueous solution of 20% HCl, 5% HNO3, plus a few drops of liquid detergent to promote wetting the ITO surface, is efficient in removing ITO. This solution should be mixed in an acid-resistant container in a fume hood that is certified for evacuation of acidic fumes. Use of this solution heated to 55 °C will typically etch 100 Ohm ITO coatings in 30 to 60 seconds. Since there is some variability in the morphology of the coatings, and more conductive coatings will be thicker, this will affect the time required to complete the removal of the ITO. If etching is carried out in a room temperature solution of the etchant, etching of a 100 Ohm coating will take from six to ten minutes. Changes in the purity or concentration of the acid solution as it is consumed by repeated etching will similarly affect the etch rate. It is important to remember that all ITO coatings are readily attacked by mineral acids and strong organic acids, and direct exposure to such agents risks loss of the coating.
Upon completing the etching process, the substrates should immediately be rinsed in a 10% aqueous solution of Na2CO3 to neutralize any acid remaining on the coated surface. Any subsequent process to remove resist used for generating patterns in the coating should be followed by a cleaning process, a series of deionized water rinses, and finally blowing dry with a clean, oil-free nitrogen or air source. This final step is important to avoid water-spotting, which may have unforeseen effects on any subsequent use of the etched coating.
In conjunction with an appropriate photo/screen resist, this procedure may be used to generate patterns in the ITO coating. We have found in our experience with both positive- and negative-acting photoresist chemistries, that adhesion of the resist to the ITO surface is best when applied to recently cleaned ITO substrates which have been subsequently baked for 30 minutes at 175 °C within 30 minutes prior to resist application. Heating the substrates in this fashion reduces the presence of moisture on the surface of the substrate, thus providing better adhesion of the photoresist coating. Storage of uncoated ITO in a N2 purged dry box will increase the time during which resist may be reliably applied to the ITO surface. In all cases, follow the directions of the manufacturer of the resist product being utilized to insure proper curing of the coating prior to exposure to the etching solution.