Hideaki Kobayashi

Mask pattern quality assurance based on lithography simulation with fine pixel SEM image

We evaluated the accuracy of the simulation based on mask edge extraction for mask pattern quality assurance. Edge extraction data were obtained from SEM images by use of TOPCON UR-6080 in which high resolution (pixel size of 2nm) and fine pixel SEM image (8000 x 8000 pixels) acquisition is possible. The repeatability of the edge extraction and its impact on wafer image simulation were studied for a normal 1D CD prediction and an edge placement error prediction. The reliability of the simulation was studied by comparing with actual experimental exposure results with an ArF scanner. In the normal 1D CD prediction, we successfully obtained good repeatability and reliability. In 65nm node, we can predict a wafer CD with the accuracy of less than 1 nm using the simulation based on mask edge extraction. In the edge placement error prediction mode, the simulation accuracy is ~5 nm including edge extraction repeatability and the uncertainty of lithography simulation model. The simulation with edge extraction more accurately predicts the resist pattern at line-end in which the actual mask pattern may be varied from the mask target (CAD) than a conventional simulation in which CAD is used as a mask pattern. This result supports the view that the wafer simulation with edge extraction is useful for mask pattern quality assurance because it can consider actual mask pattern shape.

A photomask defect evaluation system

Photomasks are currently inspected based on the standard of defect size. A shortcoming of this standard is that the type of defects which do not impact on a wafer, could be detected as impermissible defects. All of them are subject to repair works and some of them require further inspection by AIMS. This is one of the factors that are pushing down the yield and the turnaround time (TAT) of mask manufacturing. An effective way to improve this situation will be to do the repair works selectively on the defects that are predicted to inflict a functional damage on a wafer. In this report, we will propose a defect evaluation system named ADRES (Advanced Photomask Defect Repair Evaluation System), featuring a function to extract edges from a mask SEM image to be passed on to a litho-simulation. A distinctive point of our system is the use of a mask SEM image with a high resolution.

Reticle SEM specifications required for lithography simulation

We investigated the specifications of scanning electron microscope required for the lithography simulation based on the edge data extracted from an actual reticle pattern in the assurance of reticle pattern in which two-dimensional optical proximity correction is applied. Impacts of field of view, positioning error and image distortion on a lithography simulation were studied experimentally. For the reticle pattern assurance in hp90, the field of view of larger than 16 μm squares, the positioning error within +/- 1 μm and the magnification error of less than 0.3% are needed. Under these conditions, wafer image can be predicted with sufficient accuracy by the simulation.