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The semiconductor logic and memory circuits currently being produced with 193nm immersion lithography wafer scanners exhibit feature sizes as small as 45 nm. This is only possible with sophisticated photomask designs based on reticle enhancement techniques (RET). As the costs of a mask rise exponentially with shrinking and more complex features, “see what prints”-type mask metrology based on 193nm excimer lasers becomes essential. Traditional inspection methods such as atomic force microscopy or interferometry are feature size limited or cannot account for different scanner conditions.
Laser Used:IndyStar Series
Other Compatible Lasers:
In order to determine whether a mask circuit prints correctly onto the wafer, the scanner conditions are simulated in the metrology tool’s optical path (see fig. 2). Whereas the scanner uses 4:1 demagnification to transfer the mask pattern, advanced metrology tools such as the AIMS tool or the PHAME tool from Zeiss SMS use very large magnification to image the mask pattern to a CCD chip for inspection. Therefore, the 193nm output of the IndyStar evaluates the performance of the mask under realistic exposure conditions. In case of phase imaging as shown in fig 2 the electrical field in the image plane is measured and converted into accurate phase information about the mask.
Both the AIMS and the PHAME tools, built around the 193nm IndyStar excimer laser, provide the necessary optical output characteristics to duplicate the scanner conditions at a fraction of the scanner costs. As each measurement requires several laser shots the kilohertz repetition rate of the IndyStar is essential in meeting the tool’s throughput demand. Billions of shots of component lifetimes of the IndyStar provide sufficiently low costs of ownership. IndyStar based AIMS and PHAME tools are fully compatible to 193nm immersion lithography scanners down to the 32nm node.
Fig. 1 - Photo Masks with Resolution Enhancement Techniques Enable Ever Smaller Circuit Dimensions.
Click on image for larger view.
Fig. 2 - Beam Path of the latest 193nm Mask Metrology Tool PHAME by Zeiss SMS with resulting Phase Image.
High-end photo masks utilizing reticle enhancement techniques such as optical proximity correction or phase shift features have become an enabling technology in today’s lithography down to the 45nm node. 193nm mask metrology tools using scanner-relevant parameters safeguard process quality.