|
In wafer steppers, the alignment of an exposure field to the reticle being imaged is known to affect the success or failure of that field's circuit(s). Because of this relationship between alignment accuracy and device yield, much emphasis is placed on obtaining and consistently maintaining an alignment accuracy within tight design rules. In current wafer steppers, alignment options can be placed in two primary categories: Global Alignment techniques and Site-by-Site Alignment techniques. During Global Alignment, selected areas of the wafer undergo alignment. The grid of all exposure field positions is generated according to the information obtained during the selected alignments. Step-and-Repeat exposure of all fields is then performed according to this grid. In Site-by-Site Alignment (a.k.a. Field-by-Field Alignment), the stepper performs a step/align/expose sequence on each exposure field until all fields on the wafer are printed. While Site-by-Site alignment can result in greater overlay accuracy, Global Alignment is primarily used on all but the most critical of alignment levels. This is due to the much higher throughput that Global Alignment allows. Nikon has developed and implemented an original technique that achieves a throughput nearing that of most standard Global Alignment methods, while maintaining an overlay accuracy consistent with a Site-by-Site alignment scheme. This method, called Enhanced Global Alignment (EGA), utilizes the Laser Step Alignment (LSA) system of a Nikon Step-and-Repeat (NSR) system to measure the offsets of several selected fields. The NSR's computer constructs a mathematical model of the wafer's exposure field grid according to the measured offsets. The model includes six components that may contribute to overlay error: 1)Translation in X 2)Translation in Y 3) Scaling in X 4)Scaling in Y 5) Wafer Rotation 6) Pattern Orthogonality Error All exposure field addresses are determined during the EGA. Thus, there is no further alignment once the Step-and-Repeat exposure sequence begins. This sequence is performed on each wafer. The wafer model constructed by EGA relies heavily on the offsets measured by LSA. While the LSA system has a wide dynamic range (over which no variation in offset measurement accuracy is seen), the alignment mark topography must be constructed such that it can be detected by the LSA system. Since different processes may yield differing alignment topographies, an alignment mark size found to be optimum for one device manufacturer may not be best suited to another manufacturer with a different process. Optimization studies have focused on mark design variables that affect the LSA, while monitoring the process variables encountered and their effects. Discussions of these variables, including current recommendations, will be detailed.
|
