Application Notes  Cleaning and Handling Recommendations  The following procedure for cleaning ITO should only be performed while wearing   appropriate safety apparel and eye protection, in a fume hood, under the   supervision of a qualified chemist.  Even with careful handling of the transparent conductive coated products which we   supply, we cannot guarantee them to be contaminant-free. When handling glass   products in our facilities, we use powder-free non-latex gloves, which in turn are   covered by nylon or polyester gloves to prevent transferring finger oils to the coated   surfaces. Parts are packaged with paper slips between to prevent the them from   rubbing against each other and possibly damaging the ITO coating. Over time,  organic contaminants may adsorb onto the metal oxide coating, and cleaning   becomes necessary.  To clean ITO products, we recommend using a 20% by weight solution of   ethanolamine in deionized water, heated to 80 ¡ÆC in an ultrasonic bath, within   an approved fume hood. Immersion of parts in this solution with ultrasonic   agitation for a period of 10 to 15 minutes is an efficient method for the removal   of any fingerprints, body oils or similar residual organic contaminants.   The solution is moderately caustic. Solution vapors can irritate mucous   membranes and can cause burns, and should be avoided. Following the   immersion cycle, the substrates should be removed and rinsed several times   with deionized water, and finally blown dry with a clean, oil-free nitrogen or air   source to avoid water spotting.  Alternate methods of cleaning include the use of solvent cleaning solutions in   conjunction with deionized water rinses, and the use of vapor degreasing systems.
	Patterning ITO Coatings  The following procedure for etching ITO should only be performed while wearing   appropriate safety apparel and eye protection, in an approved fume hood, under the  supervision of a qualified chemist.  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 ¡ÆC 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 ¡ÆC 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.
	Measuring Electrical Characteristics  We're occasionally asked about the "resistance" of the transparent conductive   products we offer. The resistance of these transparent, conductive, thin film   products is measured according to methods outlined in ASTM D 257 ?93, "DC   Resistance or Conductance of Insulating Materials." A thin film's electrical property   is generally characterized as surface resistance, Rs, which is the ratio of the dc  voltage applied to two electrodes on the surface of the sample to the current   between them. Since the effect of volume resistance is negligible, it is typically   ignored in films as thin as these. The surface conductivity is the reciprocal of the   surface resistivity. Surface resistivity, Ps, is the surface resistance, Rs, multiplied   by the ratio of specimen surface dimensions (width of two parallel electrodes   defining the current path divided by the distance between them) which converts   the measured resistance into a value that would be obtained if the electrodes  formed two opposing sides of a square. While specific resistance is quantified in   ohms, it is frequently referred to in "ohms/square." In this context, the size of the   square is immaterial and without dimension. The formula  Ps = Rs x W / L {P represents the greek letter, rho}  where Rs is the indicated resistance, W is the length of each of the electrodes,   and L is distance between them. Current will flow from one electrode to the other.   Assuming Rs for the example is 80 ohms, and that L = 2W (immediately below),   we would calculate that

 

Ps = Rs x W / L = 80 ohms x W / 2 W = 80 ohms / 2 = 40 ohms.  In comparison, measuring in the other direction (immediately above), our ohmmeter   should indicate Rs = 20 . Since we know that Ps is a ratio of the electrode length to  the distance between (and that the ratio for our example is now W = 2 L ), we would  correctly calculate the surface resistivity:  Ps = Rs x W / L = 20 ohms x 2 L / L = 20 ohms x 2 = 40 ohms  (Note that the dimensions for L and W cancel, regardless of the size of the sample   or units of measure, hence the dimensionless "ohms per square".)  Due to the morphological variability of these coatings, resistance will not be  isotropic, and measurable differences will be observed based upon the orientation   of the electrodes with respect to the coating. Since it is not always practical to   apply conductive bus materials to the thin film coating to make these   measurements, the probes of a digital volt meter, DVM, may be used to assess   the relative resistance of the coating, or simply to determine which side is coated.  Since this kind of measurement will include "fringing" effects, and will typically   include a significant contact resistance, you should expect to find a resistance   deviation of up to several times higher than if you were to perform the measurement   as described above.  To avoid damaging the thin film coating when determining which side is coated,   lightly touch the barrels of the probes to the same surface of a single-side coated   product. Testing the alternate side will quickly show one with a low resistance as   compared to the opposing side, which will exhibit an essentially infinite resistance.

Lead Attachment / Electrical Contacts  Establishing good electrical contact with the conductive film is essential, in order to   best utilize its properties. Unlike metal foils or meshes, an ITO or coating is only   200 Å to 2,000 Å thick, and point-of-contact probes, or devices such as alligator   clips may tend to cut through the coating rather than embed into it, as they would   in a metal sheet or foil. This produces a high resistance contact, which will limit the   current that can be carried by the coating and may also effect a significant voltage   drop.  Two better methods for attachment of leads involve the use of highly conductive   silver bus compositions. These can be applied to the ITO surface along an edge,   for example, and dried or cured, depending on whether the composition is a   thermoplastic or thermoset material. The resulting bus bar provides a highly   conductive layer in intimate contact with the transparent conductor.   The attachment of an alligator clip to the bus bar, while it may penetrate the   transparent conductor, will also make excellent contact with the bus material.   Since the bus material is several orders of magnitude (from 3 to 5 ) more  conductive than the transparent semiconductor, contact resistance is minimized  and any voltage drop a this interface becomes negligible.  An alternate method of attaching leads to the coatings also uses a conductive   silver bus composition, but in combination with fine wire (copper, silver, gold,   platinum) imbedded in the bus material before it is dried/cured. With careful   handling, this method is very effective. Care must be exercised in manipulating   the wire, once attached. One good method to employ is to wind a number of   turns of the wire around a pencil or rod of similar diameter, creating a helical   "spring" to take the stresses of subsequent handling of the wire as it is attached  to a current source. Should the lead break loose, it can be re-attached with an   additional droplet of bus material.  SPO also offers a special low reflection transparent conductive design that offers   first surface reflectivity of less than 0.5% across the visible spectrum and  resistivity of less than 40 ohms per square. This design is used on displays   and instrument windows that demand RFI/EMI shielding, excellent light  transmission, and superb contrast enhancement. This family of films is   extremely durable and stable under various temperature and humidity conditions.  All coating designs meet or exceed the severe durability requirements of  MIL-C-48497.