Integrated Measurement System Does Macro Inspection, Overlay

A problem associated with inspection is that when a fab opens a box with a cassette of wafers and takes a cassette out for metrology to inspect a single wafer, some particles can land, not just on the wafer that is being measured, but on the others that remain in the box. It might be one particle per box or per lot, but it could have adverse effects on yield. With 20 to 30 masks in the process, even one particle per lot can become a major concern. Doing the measurement of these two important applications inside the track, without opening the box and removing the wafer twice — once for overlay and once for inspection — could eliminate the chance for incremental contamination that can occur with other methods.

Nova Measuring Instruments (Rehovoth, Israel) has introduced the NovaTrack, a dual-purpose integrated metrology system for photolithography overlay registration measurement and macro defect inspection. It measures the standard overlay registration target in multiple sites on a wafer with a 3 nm (3s) repeatability, enabling fully automated measurement without human intervention (Figure).

The system measures in parallel to the wafer processing sequence and does not affect lithography cell throughput. The whole-wafer macro defect inspection capability finds defects down to 40 µm. The system detects particles, coat- and develop-related defects, as well as edge bead removal (EBR) faults. These measuring modes can be used together or separately, depending on desired sampling rate and throughput. Developed to be integrated in the track, the tool measures each wafer after all the processing has been done, before returning it to the cassette. The process is fully integrated, affecting nothing, enabling the fab to determine — without moving the wafer to another inspection tool — whether the processes are tuned correctly. It also allows on-the-fly fine-tuning and optimization of the exposure matrix, improving lithography use.

The system is composed of four main modules. The measurement unit is installed in the track and contains the moving optics, focus mechanism, wafer chuck and wafer handling mechanism. Power, control signals and data signals all go to the control unit. Illumination is provided by a two-fiber-optic-directed light source, which transfers light to the measuring unit. The control unit, composed of the system's main computer and power supply, controls the real-time measurement sequence performed by the measurement unit. The operator runs the system from the operating console, a keyboard and monitor. •