Test key and method for monitoring semiconductor wafer

1. A test key, comprising:

a first testing unit having a first diode-to-conductive layer area ratio, wherein the first testing unit comprises a first conductive layer and at least one first conductive via electrically connected to the first conductive layer; and

a second testing unit having a second diode-to-conductive layer area ratio smaller than the first diode-to-conductive layer area ratio, wherein the second testing unit comprises a second conductive layer and at least one second conductive via electrically connected to the second conductive layer, a quantity of the at least one first conductive via is different from a quantity of the at least one second conductive via.

2. The test key according to claim 1 , wherein an area of the first conductive layer is smaller than an area of the second conductive layer.

3. The test key according to claim 2 , wherein the area of the first conductive layer in one level is smaller than the area of the second conductive layer in one level.

4. The test key according to claim 2 , wherein each of the first conductive layer and the second conductive layer comprises at least one sub conductive layer, a total area of the at least one sub conductive layer of the first conductive layer is smaller than a total area of the at least one sub conductive layer of the second conductive layer.

5. The test key according to claim 2 , comprising a dielectric layer, wherein at least one of the first conductive layer and the second conductive layer comprises a plurality of the sub conductive layers separated from each other by the dielectric layer.

6. The test key according to claim 1 , wherein each of the first testing unit and the second testing unit comprises a conductive layer and a diode unit electrically connected to each other, the first diode-to-conductive layer area ratio and the second diode-to-conductive layer area ratio are calculated based on areas of doped regions of the diode units and the conductive layer of the first testing unit and the second testing unit, respectively.

7. The test key according to claim 6 , wherein each of the first testing unit and the second testing unit further comprises a contact electrically connected between the conductive layer and the diode unit, the doped regions of the diode units are in contact with the contacts and have one of a p-type conductivity and a n-type conductivity.

8. The test key according to claim 6 , comprising a semiconductor substrate, wherein the doped regions of the diode units are in the semiconductor substrate, the conductive layers are on the semiconductor substrate.

9. The test key according to claim 1 , wherein the quantity of the at least one first conductive via in contact with the same first conductive layer is different from the quantity of the at least one second conductive via in contact with the same second conductive layer.

10. The test key according to claim 1 , wherein the at least one first conductive via is disposed on the first conductive layer, the least one second conductive via is disposed on the second conductive layer.

11. A method for monitoring a semiconductor wafer, comprising:

providing a test key comprising testing units having different diode-to-conductive layer area ratios; and

a testing step comprising individually providing a voltage to the testing units of the test key to test the semiconductor wafer with a sequence of decreasing diode-to-conductive layer area ratio until the semiconductor wafer is damaged.

12. The method for monitoring the semiconductor wafer according to claim 11 , wherein the semiconductor wafer is damaged after being tested with one of the testing units, the one of the testing units has a spec diode-to-conductive layer area ratio, the method further comprises using the spec diode-to-conductive layer area ratio for defining a design rule of process window.

13. The method for monitoring the semiconductor wafer according to claim 11 , wherein the semiconductor wafer is damaged by an arcing phenomenon.

14. The method for monitoring the semiconductor wafer according to claim 11 , wherein the test key comprises:

a first testing unit having a first diode-to-conductive layer area ratio; and

a second testing unit having a second diode-to-conductive layer area ratio smaller than the first diode-to-conductive layer area ratio, wherein the testing step comprises:

providing the voltage to the first testing unit to test the semiconductor wafer; and

then providing the voltage to the second testing unit to test the semiconductor wafer.