Observation of current polarity effect in stressing as-formed sub-micron Al-Si-Cu/TiW/TiSi sub(2) contacts

• Published in: Solid-state electronics 1999-01, Vol.43 (6), p.1031-1037
• Main Authors: Chen, Li-Zen, Hsu, Klaus Y-J
• Format: Article
• Language: English
• ISSN: 0038-1101

Formation of good silicide contacts becomes more important but difficult as the contact size continues shrinking toward the deep sub-micron regime. At the same time, higher current density, which may easily appear in small regions, could pose strong impact to the long-term reliability of sub-micron contacts. In this work, high current density stress experiments were conducted on the Al-Si-Cu /TiW/TiSi sub(2) contacts with the size ranging from 0.5x0.5 mu m super(2) down to 0.25x0.25 mu m super(2). The self-aligned silicide contacts were formed by using collimated sputtering, E-beam lithography, RTA, and RIE techniques. The silicide contacts were sintered at 400 degree C for 30 min. Cross-bridge Kelvin resistor structures were formed for electrical stressing and contact resistance measurement. One-way and two-way stressings were performed at high current density (approximately 10 super(7) A/cm super(2)) and the contact resistance was measured periodically at low current density during the stressing to monitor the evolution. It was found that the initial resistance of as-formed contacts was higher than expected. This is probably due to the difficulty of forming good interfaces in the small contact region by sputtering and that the sintering temperature may not be high enough to smear out the imperfection. The stressing was found to anneal the contacts. With electrons flowing from metal layer into the contact window, the contact resistance was reduced more efficiently than with reverse current of the same density. Stressed first by reverse current then by normal current, the resistance showed a two-step reduction with a significant transition at the switch of current polarity. For prolonged stressing, the contacts were gradually degraded and the reverse current induced more severe damage. These observations indicate strong electromigration effect at the small contacts.