Differential thermal energy control for pattern effect suppression in rapid thermal annealing

Within-die process variability is a significant problem for advanced CMOS device manufacturing. One important contributor is local temperature non-uniformity during rapid thermal annealing (RTA). RTA with lamp heating provides the high heating and cooling rates needed to limit the thermal budget during annealing, but differences in the optical properties of various regions of the die can cause temperature variations known as the RTA pattern effect, which affects the shapes of doping distributions, causing large variations in device performance. Although the die layout can be modified with dummy features to reduce the pattern effect, this adds to the burden of design. This paper shows that modifying the way RTA tools are used can eliminate the need for dummy features. Although non-uniformity caused by local variations in the absorption of lamp energy can be eliminated by heating only the back-side of the wafer, this doesn´t reduce non-uniformity from local variations in the radiant heat loss. However, heating the wafer from both above and below allows the ratio of the heating power delivered to the pattern by the top lamps to be optimized so that non-uniform lamp power absorption precisely compensates for non-uniform heat losses, completely eliminating the pattern effect. Thermal simulations were used to demonstrate the principles of the approach and experiments with 1000°C spike anneals of patterned ion-implanted wafers confirmed the predicted behaviour. The method has been shown to be applicable to typical CMOS device structures.