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ITO is used as the conductive layer in a touchscreen. If you would replace ITO in a capacitive touchscreen, would it ideally have a lower or high relative permittivity? The relative permittivity (dielectric constant) is proportional to the capacitance.

A capacitance touchscreen measures the change in capacitance.

So if you would replace ITO with another conductive and transparent material, would it be useful to have the dielectric constant and therefore the capacitance as low as possible, so the Signal-To-Noise ratio for the change in capacitance would be higher?

Or would it be more useful to have the capacitance as high as possible, so the screen can store more charge?

If you increase the capacitance, wouldn't the relative change in capacitance/electric field be smaller and make the sensor less accurate?

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The role of ITO is not to be capacitive, but conductive.

ITO is not between the electrodes, but is an electrode instead. Improving the ITO's conductivity is important to assure the lowest electrical resistance between the ITO and your finger, thus optimizing the discharge current, and thus the signal.

Improving the signal (and hence the sensitivuty), can indeed be achieved by increasing the dielectric constant between the ITO and your finger (or whatever you are using). Another approach that is used to improve the signal, is using one of those touch pens. These pens have a very small "electrode" area (the tip), and thus it is necessary to have a battery to create a higher potential between the ITO and the tip, so that the discharge current is high enough (without actually improving the capacitance).

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  • $\begingroup$ Why would increasing the dielectric constant, improve the sensitivity, as the background capacitance is already in the order of a few nano farads and the capacitance of a finger is a few pico farads, if you increase the background capacitance even more, wouldn't it become less sensitive? $\endgroup$
    – CapaSense
    Commented Jan 15, 2017 at 17:03
  • $\begingroup$ @CapaSense What do you mean by background capacitance? What I meant by increased sensitivity was that increasing the capacitance between the screen and your finger, the area of your finger that needs to be in contact with the screen to make a detectable signal is smaller, and therefore spatially more accurate. $\endgroup$
    – cinico
    Commented Jan 15, 2017 at 17:59
  • $\begingroup$ The question is, if the dielectric constant of the material increases, wouldn't you see that as parasitic capacitance, which is seen as an offset in the sensor. If baseline capacitance is larger,the changes in the capacitance when the object approaches the touch sensing system can be difficult to detect. $\endgroup$
    – CapaSense
    Commented Jan 15, 2017 at 18:14
  • $\begingroup$ @CapaSense Oh, I understand now what you meant. You are right to assume that if the dielectric constant of the ITO increases, its capacitance would increase. A higher capacitance, electrically connected in series with the capacitance from your finger, would lead to a parasitic signal. However, since the ITO is highly conductive the voltage drop across the material will be very low, thus leading to a very small capacitance. The final answer should be, "while a lower dielectric constant would, in theory, improve the performance of ITO, it really doesn't matter from a practical point of view". $\endgroup$
    – cinico
    Commented Jan 15, 2017 at 19:04
  • $\begingroup$ @CapaSense I should add that the interface morphology is much (much!) more relevant to a "parasitic" capacitance than the dielectric constant of the material. This is because the higher the effective area of the interface, the higher the capacitance. Think as supercapacitors work: the high capacitance doesn't come from a high dielectric constant of the electrode material, but from it high surface area. $\endgroup$
    – cinico
    Commented Jan 15, 2017 at 19:08

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