A Note on Terminology:
The electrical properties of the ITO coating are given as surface resistivity, measured in ohms per square. In some disciplines, people refer to this as sheet resistance; but should not be confused with "bulk resistivity”. To determine the actual resistance of a slide, it is necessary to do a short calculation. For example, if the resistivity is 30 to 60 ohms per square, then the resistance of the coating on a slide 1 x 3 inches (25 x 75 mm), measured along the length, is 90 to 180 ohms. This is determined by dividing the length of the slide by the width of the slide, in our example dividing 3 by 1, or 75 by 25, and multiplying that result (3 in our example) by the resistivity. No matter what units you use to measure the dimensions, the resistance of the coating is the same.
1) How do I select the right coating for me?
One should select the coating thickness on the basis of the desired coating resistivity. One can select the appropriate coating from our conversion chart that relates coating thickness to resistivity.
2) Is it easy to tell which side of the slide has the coating?
This is one of the most often-asked questions of those who are first-time purchasers of the coated slides. There are actually two ways, but the easiest to tell if the side has been coated is to hold it up to the light. There should be a purple tint, which can be seen right to the very edge of the glass. If you can only see it part way to the edge, the ITO coating is the other side. Of course, one can also attach a multimeter to one of the sides. There will be no reading if there is no coating; and there will be a reading if it is the side with the coating.
3) Can I get Anti Reflective or Index Matching Coatings?
Yes, these are special order options that are available. Both will aid in increased transmittance of the coating. Anti-reflective (AR) coating is applied to the side opposite to the ITO coating. Even with an index matching coating, the transmittance can be increased even more, maximizing the transmittance. Index matching coating (IMC) consists of a layer of quartz that is applied to the top of the already applied ITO coating. Depending on just what wavelength is being used, the transmittance can be increased from slightly to significantly. Do all users need an index matching coating or an AR coating? The answer is No. But for some of our customers, since transmittance is one of those things in life that we say "never enough", the possibility of getting these additional coatings is highly attractive for their work.
A) Index matching coating, which consists of a layer of quartz that is applied to the top of the already applied ITO coating. Depending on just what wavelength is being used, the transmittance can be increased from slightly to significantly. For smaller quantities (ones that involve less than 1500 sq. inches, money can be saved if you can use the ITO coating at one of the standard ITO resistivities.
B) Anti-reflective (AR) coating, which is applied to the side opposite to the ITO coating. Even when the index matching coating has already been applied, the transmittance can be increased even more, in order that there be the maximum transmittance.
Either of these coatings involves a special order and requires additional time and expense.
4) Is there an SiO2 passivation layer present under the ITO coating?
While certain products used in research are seen by the typical user as being generic, that is not the case for ITO coatings. The SPI Supplies Brand of ITO coated substrates are most certainly not like any other. Using a proprietary technology SPI Supplies ITO coatings are not applied over an SiO2 passivation layer and the absence of such a passivation layer can have important advantages to many users.
Because of the uniqueness of our process, there is no leaching of alkali oxides into liquid crystal systems, and therefore there is no need for a passivation layer. The absence of this costly step results in a lower cost production process and at the same time, results in an incremental increase in transmittance. SPI Supplies can of course, provide ITO coatings over an SiO2 layer, but it will also add to the final price and would be treated as a special order.
5) How do I clean my ITO-coated Slides?
Each SPI Supplies Brand ITO coated slide arrives individually wrapped in a special protective paper selected to keep the slide clean and damage free during shipment and storage. However, there could be times when the user would feel more comfortable giving the slide a cleaning treatment prior to use, and the recommended cleaning solvent is isopropyl alcohol. This solvent should not injure the ITO coating but with prolonged contact, it could start to dissolve a bus-bar if present. Under no circumstances should you try cleaning with alkali solutions. Dilute sodium hydroxide solutions are commonly used for patterning ITO.
We would also recommend a high quality lint-free cotton wiper such as our own SPI-Wipes™ Lint-Free Cotton Wipers. In any case, do not use any acid for cleaning, even a weakly acid agent, because it will definitely damage the ITO coating irreparably.
6) Can I submit my own substrates for coating?
We are very happy to receive customer supplied substrates for ITO coating. Included in our pricing quotes for such a service is the proper cleaning and preparation of the substrates prior to coating. This includes a special proprietary solvent cleaning followed by a plasma cleaning to promote adhesion of the coating to the substrate. Let us know your requirements being sure to tell us the outside dimensions of the items to be coated and also the resistivity wanted, remembering that using one of our standard resistivities will result in the lowest cost to you. This also means that we can apply over the ITO coating an index matching layer designed to increase the transmittance. And additionally, we can apply a proprietary antireflective (AR) coating on the opposite face to increase the transmittance even further. Please e-mail us for such special order quotes.
7) Can I cut down larger coated pieces to smaller sizes?
Some of our customers are at times tempted to purchase a large piece of ITO-coated glass, with the idea of scribing and then breaking down the glass to smaller sizes. Life should only be that simple! Unfortunately, when this is done, there is sufficient disruption to the ITO coating at the edges that it is never a good result. We always advise cutting the glass down to the desired size and then coat the cut pieces. This does result in additional handling, and higher costs but one is assured of a much higher quality result.
8) Should I get a busbar?
In order to gain the benefit of the ITO coating, one does need the ability to make electrical connections from the coating to the power source. This can be done by the experimenter or the slides can be purchased with the busbar already applied and ready to use.
There are a number of different qualities of the busbar that one could apply. As a standard product, we offer the slides with a silver line silk screened on at the edge (or elsewhere is desired) that exhibits, when cured and dried, exhibits a surface resistivity of better than 0.2 ohms / square. The busbar is typically three micrometers in thickness with a three millimeter width.