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sheet resistance is nothing but resistivity/thickness.... it is measured by taking readings from 4 points of the material... i dont know exactly how it is done....
Usually, to figure out a resistance of a given material, you need the cross sectional area of the material, length, and the resistivity. Sheet resistance is a way to simplifying the calculation by assuming H is uniform and mix it into the resistivity. Once you have the sheet resistance, you just need W & L to know that the resistance between two edges are.
Sheet resistance is given as number of squares (Ohms/Square). Example: See picture below. You have a doped poly in shape of a rectangle with 1um width and 2um length. Let's say your sheet resistance is 10ohms/square. If you want resistance between edge A & edge B, you have two squares, so your resistance is 20Ohms. If you want to measure from edge C & D, you have 1/2 square (Think about this for a while...) so your total resistance is 5ohms.
For sheet resistance, it does not matter how big the square is. So, the above examples are just as valid for 1m,2m as well as 1um & 2um. Also, lots of people get tripped up on the 1/2 square thing.... sort of imagine that you have a big square made up of 4, 1umx1um squares. If you have edge D as the bottom edge of this big square, you can see that edge C would be in the mid point of this square, giving you 1/2 square.
To measure sheet resistance, take a resistance measurement between two edge then figure out how many squares you have between those two points. R/square gives you the sheet resistance.
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