Proceedings Article | 17 October 2008
KEYWORDS: Inspection, Photomasks, Signal detection, Acquisition tracking and pointing, Airborne remote sensing, Defect detection, Modulation, Diffraction, Image enhancement, Defect inspection
Memory chips, now constituting a major part of semiconductor market, posit a special challenge for
inspection, as they are generally produced with the smallest half-pitch available with today's
technology. This is true, in particular, to photomasks of advanced memory devices, which are at the
forefront of the "low-k1" regime. In this paper we present a novel photomask inspection approach,
that is particularly suitable for low-k1 layers of advanced memory chips, owing to their typical dense
and periodic structure. The method we present can produce a very strong signal for small mask
defects, by suppression of the modulation of the pattern's image. Unlike dark-field detection,
however, here a single diffraction order associated with the pattern generates a constant "gray"
background image, that is used for signal enhancement. We define the theoretical basis for the new
detection technique, and show, both analytically and numerically, that it can easily achieve a
detection line past the printability spec, and that in cases it is at least as sensitive as high-resolution
based detection. We also demonstrate this claim experimentally on a customer mask, using the
platform of Applied Material's newly released Aera2TM mask inspection tool. The high sensitivity
demonstrates the important and often overlooked concept that resolution is not synonymous with
sensitivity. The novel detection method is advantageous in several other aspects, such as the very
simple implementation, the high throughput, and the relatively simple pre- and post-processing
algorithms required for signal extraction. These features, and in particular the very high sensitivity,
make this novel detection method an attractive inspection option for advanced memory devices.
