Method for screening arrays of magnetic memories
View Patent ↗A testing method is described that applies a sequence external magnetic fields of varying strength to MRAM cells (such as those with MTJ memory elements) in chips or wafers to selectively screen out cells with low or high thermal stability factor. The coercivity (Hc) is used as a proxy for thermal stability factor (delta). In the various embodiments the sequence, direction and strength of the external magnetic fields is used to determine the high coercivity cells that are not switched by a normal field and the low coercivity cells that are switched by a selected low field. In some embodiment the MRAM's standard internal electric current can be used to switch the cells. Standard circuit-based resistance read operations can be used to determine the response of each cell to these magnetic fields and identify the abnormal high and low coercivity cells.
1. A method for screening MTJ cells in an MRAM device having a plurality of MTJ cells, the method comprising the steps of:
initializing the MTJ cells into a first group having a first state and a second group having a second state by driving a selected electric current through each of the MTJ cells;
applying an externally generated variable magnetic field to the MRAM device to switch a first subset of the first group to the second state and a first subset of the second group to the first state while leaving a second subset of the first group in the first state and a second subset of the second group in the second state; and
sorting each of the MTJ cells based upon the state thereof into a passing group consisting of the second subsets of the first and second groups or a rejected group consisting of the first subsets of the first and second groups of the MTJ cells.
2. A method for screening MTJ cells in an MRAM device having a plurality of MTJ cells, the method comprising the steps of:
initializing the MTJ cells into a first state;
applying a first externally generated magnetic field to the MRAM device to switch a first group of the MTJ cells into a second state and leave a second group of the MTJ cells in the first state;
switching a first subset of the first group of the MTJ cells back to the first state while leaving a second subset of the first group of the MTJ cells in the second state and the second group of the MTJ cells in the first state; and
sorting each of the MTJ cells based upon the state thereof into a passing group consisting of the second subset of the first group or a rejected group consisting of the second group and the first subset of the first group of the MTJ cells.
3. The method of claim 2 wherein each of the MTJ cells comprises a free magnetic layer, the free magnetic layers of the MTJ cells in the first state have a first magnetization direction, and the free magnetic layers of the MTJ cells in the second state have a second magnetization direction antiparallel to the first direction.
4. The method of claim 2 wherein the step of initializing the MTJ cells into a first state further comprises the steps of heating the MRAM device and applying a second externally generated magnetic field to the MRAM device to switch the MTJ cells into a first state.
5. The method of claim 2 wherein the step of switching a first subset of the first group of the MTJ cells back to the first state comprises the steps of heating the MRAM device and applying a third externally generated magnetic field to the MRAM device.
6. The method of claim 2 wherein the step of switching a first subset of the first group of the MTJ cells back to the first state comprises the step of heating the MRAM device.
7. The method of claim 2 wherein the step of switching a first subset of the first group of the MTJ cells back to the first state comprises the step of resistive heating the MTJ device by driving a selected electric current through each of the MTJ cells.
8. The method of claim 2 wherein the step of switching a first subset of the first group of the MTJ cells back to the first state comprises the step of applying a fourth externally generated magnetic field to the MRAM device.
9. The method of claim 8 wherein the strength and direction of the fourth externally generated magnetic field vary with time.
10. A method for screening MTJ cells in an MRAM device having a plurality of MTJ cells, the method comprising the steps of:
initializing the MTJ cells into a first state;
driving a selected electric current through each of the MTJ cells to switch a first group of the MTJ cells into a second state and a second group of the MTJ cells into the first state;
switching a first subset of the first group of the MTJ cells back to the first state while leaving a second subset of the first group of the MTJ cells in the second state and the second group of the MTJ cells in the first state by applying a first externally generated magnetic field to the MRAM device; and
sorting each of the MTJ cells based upon the state thereof into a passing group consisting of the second subset of the first group or a rejected group consisting of the second group and the first subset of the first group of the MTJ cells.
11. The method of claim 10 wherein each of the MTJ cells comprises a free magnetic layer, the free magnetic layers of the MTJ cells in the first state have a first magnetization direction, and the free magnetic layers of the MTJ cells in the second state have a second magnetization direction antiparallel to the first direction.
12. The method of claim 10 wherein the step of switching a first subset of the first group of the MTJ cells back to the first state further comprises the step of heating the MRAM device.
13. The method of claim 10 wherein the step of switching a first subset of the first group of the MTJ cells back to the first state further comprises the step of resistive heating the MTJ device by driving a selected electric current through each of the MTJ cells.
14. The method of claim 10 wherein the strength and direction of the first externally generated magnetic field vary with time.