The glass used to manufacture LCD's comes overcoated with an ITO (Indium Tin Oxide) layer. We image this ITO layer to form the pattern of digits and annunciators you will eventually see on the finished display. The ITO has a uniform sheet resistance, which is determined at the time the glass is manufactured.
The resistance of the glass is usually in a range between 60 and 180 ohms per square, as measured with a four point probe. A number of customers have asked us, "Ohms per square what?" Ohms per square inch? Ohms per square meter? The answer is Ohms per square anything, or just Ohms per square. The following derivation, which is valid for any very thin film or material, shows how it is measured.
With a four point probe technique, the resistance is measured by passing a fixed known current through two points, and measuring the voltage at two other points, as shown in the drawing below. In practice, the four points are usually colinear, but that is not really a requirement.
The constant current is "injected" on the leftmost pin, and "withdrawn" at the rightmost pin, and the voltage is then measured across the two center pins. If the distance between the pins is equal, "a" in our drawing, the following derivation applies. As will be shown, if the current is fixed to the value of 4.53 ma, the voltage reading on our voltmeter will in fact be equal to the resistance value of the indium.
In the following equations, E is the Electric field and j is the current density.
By controlling the input current to the fixed known value of 4.53 ma, the sheet resistance is equal to the voltage across the two test points, and as is seen in the derivation, the units of distance involved have dropped out of the equation.