Passive array test structure for cross-point memory characterization
An apparatus and method for testing two-terminal memory elements organized as a cross-point memory array. The apparatus allows functional testing of two-terminal memory elements organized as a cross-point memory array, and built in a short flow manufacturing process. The proposed apparatus substantially eliminates the use of any type of additional active or passive switches, selectors, or decoders. A large number of memory elements of various memory types including planar (two dimensional) or three dimensional memory structures can be tested without the need of manufacturing selectors or running the full flow process.
1. A method for parallel testing of a plurality of memory cells disposed in a plurality of memory arrays built using a semiconductor substrate, the method comprising:
creating a wafer containing the plurality of memory arrays, the wafer including at least one test memory array including a plurality of test memory cells, with each of the test memory cells having a word line and a bit line associated therewith, wherein the test memory array does not include a selector device associated therewith, and wherein the step of creating includes, after determining a selected plurality of test memory cells to test that is a subset of the plurality of test memory cells:
modifying a layout of connections within the test memory array between the selected plurality of test memory cells so that an unselected plurality of test memory cells is electrically disconnected from the selected plurality of test memory cells and the word lines and bit lines of the selected plurality of test memory cells are each connected to an associated test pad; and
modifying conductor layer patterns disposed above and below the selected plurality of test memory cells to minimize resistances between the selected plurality of test memory cells;
connecting a multi-channel parallel parametric tester to a selected plurality of test pads associated with the selected plurality of test memory cells;
using the multi-channel parallel parametric tester, grounding word lines associated with memory cells that are not within the selected subset plurality of test memory cells to test;
using the multi-channel parallel parametric tester, applying a predetermined test signal to each of the word lines associated with the selected plurality of test memory cells; and
using the multi-channel parallel parametric tester, measuring resistance values at each of the bit lines associated with the selected plurality of test memory cells.
2. The method according to claim 1 , wherein the step of creating further includes the step of creating a short flow connection to the test memory array, such that a lesser number of metal layers exist than compared to a standard memory array created on the wafer.
3. The method according to claim 2 , wherein the short flow connection includes only one metal layer on each of two opposite sides of the memory cell.
4. The method according to claim 2 , wherein the short flow connection includes only two metal layers on one side of the memory cell and one metal layer on the opposite side of the memory cell.
5. The method of claim 1 , further including the step of programming the subset plurality of memory cells to test using the multi-channel parallel parametric tester.
6. The method of claim 5 , wherein the multi-channel parallel parametric tester is connected to the test memory array structure via a probe card, and further including the step of controlling or limiting current flow using the probe card.
7. The method of claim 6 wherein the probe card contains at least one of passive or active elements to control or limit the current flow.
8. The method of claim 1 , wherein at least 128 memory cells are tested in parallel.
9. The method of claim 8 , wherein the test signal is one of a voltage sweep and a voltage pulse.
10. The method of claim 9 wherein the test signal is a voltage pulse, and wherein the pulses are applied in a Program/Erase sequence to check functionality of the test memory cells, with Read conditions between each pulse to check the status of the test memory cells.
11. The method of claim 9 wherein the test signal is a voltage pulse, and wherein the pulses are applied in a Program/Erase sequence, in a repetitive manner, to test endurance and reliability of the test memory cells.
12. The method of claim 1 wherein routing of the connections within the test memory array is optimized to balance routing resistance taking into account the positioning of the plurality of test pads to achieve resistances of word line connections being substantially equal, as well as resistances of bit line connections being substantially equal.
13. The method according to claim 1 wherein the modifying conductor layer patterns disposed above and below the selected plurality of test memory cells to minimize resistances includes widening certain of the wires.