THE INFLUENCE OF A LAYER OF OXIDE ON ELECTROMIGRATION PERFORMANCE OF AL/CU/SI METAL LINES

Abstract

The influence of a thin layer oxide on the electromigration behaviour of aluminum thin films is studied. The samples were fabricated as part of a CMOS process so that they are subjected to the same realistic processing conditions. The metallization of the test samples, consisting of Al/0.8wt%Si/0.5wt%Cu, was deposited by sputtering and the thin oxide films were deposited by Chemical Vapour Deposition (CVD). The metal line which is referred to as device under test (DUT) is a 8000µm long serpentine line passivated with Phosphosilicate glass (PSG) and silicon nitride. A constant stress current was applied across the metal at an elevated test temperature of 250 °C. The metallization is considered to have failed when its resistance increased by more than 20%. A theoretical model based on Black's equation is used to analyze the results. For metal tines used as control samples, the activation energy, E. was found to range from 0.74eV to 0.76eV while the current density exponent, n ranged from 2.3 to 2.4. These values are consistent with published results. It is found that an oxide layer in between the metal has an intrinsic influence on the median-time-to-failure (MTF). The structure has a 3000Å metal/500 Å oxide/3000 Å metal configuration. Two groups of failures are observed with about the same number of the units in each group. The early failure rate of the first group of samples is attributed to the localized high current density along the aluminum and it follows a lognormal distribution. The current density exponent, n of this group was determined to be 1.2. The characteristic feature of the failure is the removal of metals at the upper corners of the metal lines. The intrinsic failure rate of the second group of samples follows a lognormal distribution and fits into Black's equation having a current density exponent, n of 2. An improved MTF of the DUT is observed with the presence of an underlying layer of oxide in the metal. The underlying oxide was deposited prior to the sputtering of metal on the samples. The structure has a 500 Å oxide/6000 Å metal configuration. The MTF of the DUT with 500 Å oxide/6000 Å metal/500 Å oxide structure does not show further significant improvement. The overlaying oxide was deposited after the sputtering of metal on the samples. In conclusion, it is found that units with an oxide layer in-between the metal (which decreases the thickness of the metal) has a smaller MTF, those with an underlying oxide layer on metal and an underlying/overlaying oxide have an improved MTF of 1.4 times among the three different types of structures.