Computer Failures: A Nonproblem?
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Computer Failures: A Nonproblem?
con, and particularly of SiO2, is relatively low compared with that of metals. As a second alternative, one could use gold, which has somewhat better conductivity than aluminum and substantially better resistance against electromigration. Nevertheless, the electronics industry is at present reluctant to replace aluminum metallizations for silicon devices by gold because of cost (!), a lack of suitable processing procedures, and particularly because gold, once it has diffused into silicon, may cause deep-lying recombination centers for occurred in September 1981 after about 20,000 operating hours at about 90°C junc- the current carriers. A barrier layer between gold and silicon has to be inserted, theretion temperature and 2 x 105 A cm2 current fore, which also improves the adhesion of density and were caused by electromigrathe gold on the substrate. Barrier layers are, tion-induced aluminum extrusions through incidentally, widely used, even in the case the protective overcoat.) Each microelecof Al metallizations, and should therefore tronic company of repute employs a pernot deter the usage of gold. manent staff of scientists and engineers who monitor the electromigration-related A third alternative for replacing alufailures and investigate new (and old) minum is pure copper, which shares with avenues which are intended to alleviate this gold good conductivity and acceptable persistent "nonproblem." Indeed, failure electromigration resistance. However, rates are actually "normalized." One copper may also introduce carrier recomdefines one failure in 109 device hours bination centers in silicon unless a barrier (number of devices multiplied by operating layer is inserted between the two materihours) to be one FIT. As an example, for als. Additionally, copper causes processmost consumer products manufacturers ing and corrosion problems which are not strive for about 20-50 FITs over about yet completely under control. 100,000 hours of operation. One has to keep Fourth, experiments have shown that in mind, however, that failures in microaluminum can be continued to be used if electronic devices are of statistical nature. different deposition methods are applied. This means that one computer chip may fail One of these processing procedures is a just one day beyond the warranty period technique in which some of the aluminum whereas others give up several years later. atoms are ionized by electron bombardment while they are still in the vapor state. Can something be done about this? In principle, yes. Electromigration is essen- The aluminum ions are then accelerated tially a materials problem. Materials need to by an electric field toward the substrate on which they are deposited. Substantial be employed for these interconnects improvements in electromigration resiswhich have an extremely high resistance tance (one order of magnitude compared to electromigration (concomitant with a to Al) have been achieved with this "selfgood electrical conductivity). Some of ion enhanced physical vapor
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