Masahiko Yasuda

Stability and calibration of overlay and focus control for a double patterning immersion scanner

To achieve the 2 nm overlay accuracy required for double patterning, we have introduced the NSR-S620D immersion scanner that employs an encoder metrology system. The key challenges for an encoder metrology system include its stability as well as the methods of calibration. The S620D has a hybrid metrology system consisting of encoders and interferometers, in XY and Z. The advantage of a hybrid metrology system is that we can continuously monitor the position of the stage using both encoders and interferometers for optimal positioning control, without any additional metrology requirements or throughput loss. To support this technology, the S620D has various encoder calibration functions that make and maintain the ideal grid, and control focus. In this paper we will introduce some of the encoder calibration functions based on the interferometer. We also provide the latest performance of the tool, with an emphasis on overlay and focus control, validating that the NSR-S620D delivers the necessary levels of accuracy and stability for the production phase of double patterning.

Double patterning lithography study with high overlay accuracy

Double patterning (DP) has become the most likely candidate to extend immersion lithography to the 32 nm node and beyond. This paper focuses on experimental results of 32nm half pitch patterning using NSR-S620D, the latest Nikon ArF immersion scanner. A litho-freeze-litho (LFL) process was employed for this experiment. Experimental results of line CDU, space CDU, and overlay accuracy are presented. Finally, a budget for pitch splitting DP at the 22 nm half pitch is presented.

Advanced alignment optical system for DUV scanner

Advanced scanners need an extremely high accuracy wafer alignment system, and nowadays it is also necessary that the alignment marks occupy a smaller area in order to expand the available area for IC patterns. Therefore, narrower lines with a smaller pitch must form the alignment marks. In this paper, a higher Numerical Aperture (NA) and lower aberration alignment optical system are studied for these requirements. At first the small alignment marks are shown, and suitable NA in the optical system is then discussed. As a result, the necessity for higher NA is shown. As for low aberration, the necessary specification of wavefront aberration is discussed. Assuming it is possible to suitably select the NA and the illumination NA in the optical system, the results of simulation -- that simulate image signals and perform image processing -- are reported. These results show the optical system that has aberration causes position shift, so that the specification of wavefront aberration is estimated in order that the position shifts may be sufficiently small. To make sure that with such a strict specification the system will be possible, a trial optical system has been made. Finally the techniques of manufacturing and the results of evaluation are reported.

TTR (through the reticle) alignment system with photoresist ablation technique

Various alignment methods for a semiconductor exposure tool have been proposed and developed. Especially, the TTR (through the reticle) alignment technique has been expected as the ideal system since the direct measure between a reticle and a wafer through the projection lens has no baseline error. However, it requires that an alignment illumination be a single wavelength of the exposure light because of the chromatic aberration of the projection lens. The strong absorption by the resist and the BARC (bottom anti reflective coating) weakens the alignment signal intensity, and the interference fringe in the resist by the single wavelength sacrifices the precise position detection. Such difficulty in signal detection has blocked the TTR system from becoming realized. We tried to address this problem by peeling the resist and BARC on alignment marks. To peel the resist and BARC, we performed elective ablation using a laser ablation method with the Q-switch Nd YAG laser. The laser-ablated alignment marks on some process wafers were measured by the TTR alignment system. The signal waves with enough contrast were measured over all wafers and the satisfied alignment accuracy was examined.