Toshikazu Umatate

Enhanced Global Alignment For Production Optical Lithography

In wafer steppers, the alignment of an exposure field to the reticle being imaged is known to affect the success or failure of that fields 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 NSRs computer constructs a mathematical model of the wafers 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.

New advanced lithography tools with mix-and-match strategy

Nikon has developed cutting-edge lithography tools, and its product lineup encompasses all exposure wavelengths. They are: the NSR-S307E ArF scanner for the 90nm node; the NSR-S207D KrF scanner for the 110nm node; the NSR-SF130 i-line stepper for the middle layer and the new concept NSR-SF200 KrF stepper, which offers unparalleled productivity and cost performance. In addition, a powerful support system is provided, the Lithography Equipment Engineering System, which will allow its customers to use all of these exposure tools simultaneously and derive the maximum benefit of the mix-and-match strategy. The use of this system will increase the uptime and enable their combined performance to exceed that of a stand-alone tool.Latest actual performance data from each of the tools and the result of the optimization performed using application software will be reported.

Management of pattern generation system based on i-line stepper

A Device mask of 180nm generation was fabricated by Photomask Repeater system and the performance of it proved to be high by the results of fabricated mask. Great margins between the results of the fabricated mask and specifications suggest that lower graded masks can be used as master masks. From this point of view, error budgets were estimated about CD uniformity and pattern placement. The required specifications for master mask were estimated for 180nm and 130nm lithography. In CD uniformity the specification is 50nm(3?) for 180nm and 30nm(3?) for 130nm lithography. In pattern placement the specification is 75nm(3?) for 180nm and 50nm(3?) for 130nm lithography. In defect size the specification is lOOOnm for 180nm and 900nm for 130nm lithography. The requirements of master mask are rather rough even for 130nm lithography and enough realistic.

Photomask repeater strategy for high quality and low cost reticle fabrication

The severe mask specification makes mask cost increase drastically. Especially, the increase in the mask cost deals ASIC businesses a fatal blow due to its small chip volume per product. Pattern writing cost has always occupied the main part of the prime mask cost and the emphasis of this is still increasing. This paper reports on a Photomask Repeater strategy to be a solution for reducing mask cost in pattern writing, comparing with conventional EB system.

Improvement of photomask repeater for 130-nm lithography

Device masks for 180nm lithography was fabricated by PR system. These masks were verified by device yields comparing with masks written by other conventional systems. There were no differences in device yields between PR system and conventional system. Fine analysis of CD error was carried out for enhancement of CD uniformity to apply Photomask Repeater to 130nm lithography. It revealed that major CD error function is global CD error. By optimizing exposure dose of each shot to compensate global distribution, global CD error was reduced from 7.9nm to 5.5nm. Finally, CD uniformity of 8nm was achieved. PR system can afford to fulfill the requirement of CD uniformity for 130nm lithography. Simultaneously, the result of fine analysis indicates excellence of PR system in littleness of random error.

New photomask pattern generation method based on i-line stepper

New pattern generation system, Photomask Repeater, based on i-line stepper has been developed. This system can transfer device patterns from master masks onto a photomask plate with 22mm field size. To print a chip larger than the 22mm field, stitching technology has been developed. Critical dimension error in the region where fields are stitched is the key issue of this technology. Quantification of critical dimension deviation induced by field misplacement was carried out by calculation. Introducing exposure dose gradation, it was reduced less than 1.5nm. From measurements of a real exposed mask this technique proved to be able to stitch fields seamlessly. Major two specifications, pattern placement accuracy and critical dimension uniformity, were evaluated. Both specifications required for 150nm photomasks were fully satisfied. Availability of the photomask repeater to memory device and system on chip is discussed.