# Etching ITO on PET without damage

I am planning to use some of this to construct a transparent touch panel for a prototype of mine. It is PET plastic coated in Indium Tin Oxide.

My plan is as follows:

1. Use some of this UV-cure dry film, a transparency, and a fluorescent light fixture placed very close to it for a while to cure it (not sure if enough UV there at the right wavelength...I may get one of those UV LED nail spa cure things).
2. Develop the film, remove excess with water.
3. Use some of this 32% HCl to etch the ITO.
4. Remove the film.
5. Enjoy my completed touch screen.

The problem is, I'm not really a chemistry person. My background is electrical engineering and my experience is limited to one semester in a cleanroom class where we did a lot of SiO2 etching (BOE) and some aluminum and nickel etching.

My questions are as follows:

1. Will dry film work well for this application? I worry that since it is designed for use with PCBs, made of the relatively stable FR-4, the etchant and remover provided with the kit I linked to will be too harsh and damage the PET or ITO
2. This paper seems to note that $\ce{HCl}$ is quite slow at $20^{\circ}$C for etching ITO. I estimate the molarity of the solution above is probably between 8-12M, giving me an etch rate of ~2-4angstroms/sec. I'm not sure what the thickness of the film is on the plastic, so I could be there a while. How can I determine the thickness of the ITO given its sheet resistance of 50 ohms? (What I'm really asking is how to determine the resistivity so I can solve for the thickness of the film.) I don't have access to anything fancy like a 4-point probe.
3. This chemical information sheet states that PET plastic does great with $\ce{HCl}$ at 10%, but terribly when "concentrated" (whatever that means). Am I going to destroy the PET during this process?
4. Is my plan fundamentally flawed? Is it likely not going to work? Do I need to use a different photoresist or etchant (aqua regia is terrible to PET according to that info sheet)?

I'm not too concerned with overetching or overdeveloping since the pattern doesn't need to be incredibly precise (this is a one-off thing and the contacts are around a half-inch square each).

Assume that I don't have access to a laser for ablation, fancy deposition equipment, or any complex measurement equipment (I've got a multimeter and oscilloscope). I can acquire basic glassware through amazon and such AFAIK.

My lab has a lot of experience with ITO-coated PET, but unfortunately I don't think anyone's tried etching it. We typically etch ITO-coated glass with a mixture of hydrochloric acid/water/nitric acid (4:2:1 by volume) which is fairly fast at room temperature, going through a few µm in about 10 minutes and mask with a positive photoresist.

1. Will dry film work well for this application?

Because the resolution you need isn't great, it should work fine, so long as the film adheres properly to the ITO. Underetching is a possibility, but if the features are big enough it should be okay. The big issue here is trying to use a normal fluorescent light to expose the film. This is probably hopeless; you're better off finding a proper UV lamp or even using the sun.

2. How can I determine the thickness of the ITO given its sheet resistance of 50 ohms?

The mixture I mention above is fairly quick, but either way, I wouldn't worry about the precise film thickness (which may actually vary somewhat depending on the batch/supplier). The easy way to deal with this is to simply pull the sample out of the etchant occasionally and test the resistance with a multimeter in a region where the ITO should be etched away. On PET, the conductivity should be negligible when the ITO is gone.

3. Am I going to destroy the PET during this process?

The datasheet you linked refers to the use of PET containers for storage of chemicals—it may actually last a few hours without significant damage, but I haven't tried it.

4. Is my plan fundamentally flawed?

I think it's worth a shot. You will need to handle the ITO sheets fairly delicately—the ITO can crack if the sheet is flexed too much, but I don't see this method as being totally unworkable, though you may have issues with the quality of etching you'll get.

As a final note, I wouldn't worry about getting special glassware. For measuring simple ratios and etching/developing baths, kitchen quality Pyrex dishes and measuring cups work just fine for this kind of thing. We keep stacks of them in the cleanroom because they're nice and cheap.