Optical deep trench inspection keeps DRAMs out of the red

by Ed Korczynski, Senior Technical Editor

Manufacturing of deep trenches poses great challenges, especially the etching of deep, high-aspect ratio (>20:1) bottle-shaped cavities into the silicon substrate. In particular, the sidewalls of high-aspect ratio trenches may collapse during the isotropic wet etch step, so a fast and non-destructive technique is needed to inspect for such damage. Vistec Semiconductor Systems GmbH, Weilburg, Germany, has worked with Qimonda in Dresden to develop a new in-line optical inspection technique for deep-trench DRAM structures.

Typical trench metrology methods including scatterometry, wafer mass measurement, model-based IR spectroscopy, and atomic force microscopy are all limited in wafer throughput. By using an automated macro defect inspection system (Vistec’s LDS3300) with special illumination and optimized analysis software, the visualisation and detection of merged trenches in high-volume trench-DRAM production became possible. The tool scans full 300mm wafers at speeds of up to 130 wafers/hour, fast enough that 100% inspection can be used for excursion monitoring.

The tool uses very strong broadband strobe lights, which are synchronized to the x/y-stage and with that to the wafer. The angle of incidence of the strobe lights is optimized such that correctly formed trenches reflect almost nothing, while merged trenches reflect red light (see figure, below). Special analysis software translates the strength of the reflected signal into pseudo colour data. With that, the setup of a threshold for automatic detection of merged trench problems is possible.

The results are claimed to be highly repeatable and reliable, providing substantial data for process control and optimization while lowering overall costs. Qimonda in Dresden says that it has helped speed up both learning cycles in development as well as production ramps, and response time to excursions was shortened. Vistec is selling the system worldwide. — E.K.


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