High aspect ratio inspection: The grand challenge (see cover)
01/01/2004
Defect detection is a rapidly moving technology that struggles to keep up with ITRS requirements. As the technology nodes shrink, the challenge of defect inspection follows due to the dramatic shrink of horizontal design rules with little to no reduction in the vertical film thickness. High aspect ratio (HAR) structures, typically contacts, vias, and trenches, are defined structures having depth-to-width ratios >3.
Most inspection technologies use various light sources and detectors that collect the light reflected off wafer and using filters and software algorithms, determine where background defects are. HAR structures amplify this problem because light does not have the chance to reflect out of the contact or via, making detection of subsurface defects in the contact and via impossible using conventional light sources and detectors. For HAR inspection (HARI) applications and at defect sizes below 100nm dia., defect detection is hampered by detection tools having low throughput and high cost-of-ownership. An economical solution must be found.
 Shown are key defect mechanisms and the hardest defects to detect given the limitations of the existing inspections tools. |
There are several types of yield limiting defects that exist in HAR structures. The illustration shows the key defect mechanisms and highlights the hardest defects to detect given the limitations of the existing inspection tools. The issues relate to signal-to-noise ratios influenced by roughness of edges and surfaces, and material similarities, especially at the bottom. Finding the presence of material at the bottom of a round feature made out of the same material is extremely difficult.
This review of inspection technologies has highlighted the point that there are only a few known methodologies that can supply a result for HARI. The only known technology to date is the e-beam SEM inspection that has throughput measured in hours/wafer instead of the wafers/hour that the industry needs. New emerging technologies include holographic imaging, e-beam (scattering or imaging), acoustic imaging techniques, x-ray imaging, and micro-SEM arrays. These novel approaches to HARI have been tried, but to date there are no scan speeds acceptable for implementation in manufacturing. HAR inspection needs have increased by 3–4× in the last three years as technology shrinks and films remain thick, so the need continues as a true challenge for the industry.
For more on HARI, see SST's November article on p. 49.
Michael Retersdorf, International Sematech/AMD