Abstract
This paper describes an alignment technique applied to wafer stepper for sub-micron lithography. To obtain an alignment signal, a heterodyne interferometer is used. The detection plane is located at the conjugate position on the wafer surface to reduce an error caused by the tilt of wafer surface. Sensitibity of 8 nm is obtained stably with this system. In addition, a software method is applied to compensate the detection errors caused mainly by the differences of a complex amplitude refractive ratio between parallel and perpendicular electric vectors to the plane of incidence. With this alignment system, overlay accuracy of 0.07 μm (3σ) is obtained using target marks etched directly on silicate. Even when the target marks on the wafers have relatively rough surfaces, high overlay accuracy is obtained by introducing the software compensating method. For example, overlay accracy of 0.08 μm (3σ) is obtained using wafers overcoated with grained aluminium layer.
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