Reliability Studies on the 2-Level Al/SiO2/Al System

The reliability of the 2-level aluminum/silicon dioxide/aluminum system has been investigated. Test structures were fabricated using two types of aluminum films, deposited on unheated substrates (Al-I) and on 200°C substrates (Al-II). Two types of insulation layers were used: SiO2 from oxidation of silane, and rf sputtered SiO2 with an overlay of silane SiO2. Scanning electron microscopy was used to examine the topography of the crossovers and via-holes. More hillocks were seen with Al-I films than with Al-II films. Via-hole resistance and the incidence of intralevel opens and interlevel shorts were determined. A comparative evaluation of the four structures indicated that Al-II films are more desirable for first-level metallization. The test structures with Al-II films, subjected to 150°C storage for 1000 hours, exhibited no increase in the interlevel resistance and a failure rate of one interlevel short per 12000 crossovers, each 0.5 mil (13 μm) square. Temperature cycling of these structures from -65°C to 150°C for 500 cycles developed no interlevel shorts and resulted in a decrease of the interlevel resistance. These observations show a failure rate of less than one interlevel short per 13200 crossovers in 500 cycles. Process problems related to the reliability of 2-level structures are discussed.