| Improved Thermal Stability of Copper Vias Using a Cyclical Stress Test† |
|
|
| Keithley Instruments | |
| Jan 31 2008 | |
 Login first to download. Via stress migration (VSM) occurs when the relaxation of thermal stresses in copper interconnects causes defects to form under a via [1]. Tests for this phenomenon usually involve measurements of the change in via resistance after an extended bake of 100 to 1000 hours at fixed temperature. The long cycle time for VSM testing delays the feedback loop for process optimization. Isothermal VSM tests also depend on the choice of optimum stress temperature. An alternative VSM test is proposed that cycles temperature through the region of largest creep rate. A12-hour cyclic test was found to give larger via resistance shifts than a 50-hour isothermal test. Further improvements in the VSM test were obtained by tracking smaller resistance shifts to improve the statistics of the failure rate and reduce wafer-wafer variations. Detection of small via resistance shifts required an electrical test that could measure shifts of less than 0.002&without excessive current. The improved via stress migration test has been applied to copper/low-κstructures to identify a new type of failure mode associated with resist poisoning. |
