Methods for testing unprogrammed OTP memory
Methods for testing unprogrammed single transistor and two transistor anti-fuse memory cells include testing for connections of the cells to a bitline by comparing a voltage characteristic of a bitline connected to the cell under test to a reference bitline having a predetermined voltage characteristic. Some methods can use test cells having an access transistor identically configured to the access transistor of a normal memory cell, but omitting the anti-fuse device found in the normal memory cell, for testing the presence of a connection of the normal memory cell to the bitline. Such a test cell can be used in a further test for determining the level of capacitive coupling of the wordline voltage to the bitlines relative to that of a normal memory cell under test.
1. A method for testing an unprogrammed one-time-programmable (OTP) memory cell, comprising:
precharging a first bitline and a second bitline to a first voltage;
activating a normal memory cell connected to the first bitline;
coupling a second voltage to the first bitline and the second bitline;
comparing a voltage characteristic of the first bitline to a predetermined voltage characteristic of the second bitline after the second voltage is coupled to the first bitline and the second bitline; and,
determining if the normal memory cell is defective in response to the comparing of the voltage characteristic of the first bitline to the predetermined voltage characteristic of the second bitline.
2. The method of claim 1 , wherein the first voltage is VSS and the second voltage is a positive voltage greater than VSS.
3. The method of claim 1 , wherein coupling includes driving the first bitline and the second bitline with a sense amplifier circuit.
4. The method of claim 1 , wherein comparing includes sensing a voltage level of the first bitline relative to the second bitline with a sense amplifier circuit.
5. The method of claim 4 , further including coupling a reference capacitance to the second bitline prior to coupling the second voltage to the second bitline, the reference capacitance being less than the capacitance of the normal memory cell.
6. The method of claim 5 , wherein determining includes determining that the normal memory cell is defective when the first bitline voltage is sensed to be greater than the second bitline voltage.
7. The method of claim 4 , wherein activating further includes activating a test memory cell connected to the second bitline.
8. The method of claim 7 , wherein the normal memory cell and the test memory cell are activated at the same time.
9. The method of claim 8 wherein determining includes determining that the normal memory cell is defective when the first bitline voltage is sensed to be greater than the second bitline voltage.
10. The method of claim 9 , wherein the normal memory cell adds a smaller capacitive load to the first bitline than the test memory cell adds to the second bitline.
11. The method of claim 8 , wherein the normal memory cell includes an access transistor and an anti-fuse device connected to the access transistor, and activating includes driving a cell plate voltage connected to the anti-fuse device to the first voltage and driving a wordline connected to the access transistor to a test voltage.
12. The method of claim 8 , wherein the normal memory cell includes a single transistor anti-fuse device having a variable thickness gate oxide, and activating includes driving a wordline connected to the single transistor anti-fuse device to a test voltage.
13. The method of claim 8 , wherein determining includes determining that the normal memory cell is defective when the second bitline voltage is sensed to be greater than the first bitline voltage.
14. The method of claim 13 , wherein the test memory cell has higher capacitive coupling of the second voltage to the second bitline than the normal memory cell capacitive coupling of the second voltage to the first bitline.
15. The method of claim 11 , wherein the test memory cell is identical to the access transistor.