The precise targeting of critical dimension (CD) features on photolithographic masks is an essential part of the mask production process. It is straight forward that the usual decrease of specification numbers can only be achieved using cutting edge CD Metrology tools. That also implies that the most advanced CD tool might change from node to node and over time mask houses accommodate a small variety of different tools. Therefore, it is an important task of current mask metrology to ensure accurate matching and calibration and also to transfer these standards precisely over time.
Here, we investigate the influences of the photolithographic mask material and the resist type on critical dimension measurements utilizing one Atomic Force Microscope (AFM), two CD-Scanning electrical microscopes (CD-SEM) by different suppliers and one optical CD tool. Simulating usual mask house strategies we defined one CD tool as golden tool and measured a 700 A chrome mask on it. This reference measurement was then repeated on all other tools and each of them was matched to the golden tool using standard procedures. Once matching was achieved 5 other masks were measured on all tools with exactly the same settings as the reference measurements. In all we varied the material COG, Mosi193, Mosi248, Chrome thickness 700A and 1000A and different resists. We do observe that calibration within the CD SEM tool class works very well for linearity, but with detectable offset in the range of a couple of nm for different reference masks used. Cross-calibration on the other hand from optical CD to CD SEM tools shows significant differences for process variations, layer thickness and different materials. These findings strongly point out that first of all cross calibration is extremely difficult with current metrology tools and can not be utilized for high end products with the necessary precision. And secondly, even matching within tool classes is material dependent which has to be considered for accurate tool to tool matching.