When a 300mm wafer is vacuum mounted onto the chuck of a scanner, it needs to be flat to within about 16nm over a typical exposure field, for wafers intended for 28nm node devices.1 A particle as small as three microns in diameter, attached to the back side of the wafer—the dark side, if you will—can cause yield-limiting defects on the front side of the wafer during patterning of a critical layer. The impact of back side particles on front side defectivity becomes even more challenging as design rules decrease.
Studies have shown that a relatively incompressible particle three microns in diameter or an equivalent cluster of smaller particles, trapped between the chuck and the back surface of the wafer, can transmit a localized height change on the order of 50nm to the front side of the wafer.2 With the scanner’s depth-of-focus reduced to 50nm for the 28nm node, the same back side particle or cluster can move the top wafer surface outside the sweet spot for patterning. The CD of the features may broaden locally; the features may be misshapen. The result is often called a defocus defect or a hotspot (Figure 1). These defects are frequently yield-limiting because they will result in electrical shorts or opens from the defective feature to its neighbors.
A particle on the back side of the wafer may remain attached to the wafer, affecting the yield of only that wafer, or it may be transferred to the scanner chuck, where it will create similar defects on the next wafer or wafers that pass through the scanner.
At larger design nodes, back side defects were not much of an issue. The scanner’s depth of focus was sufficient to accommodate a few microns of localized change in the height of the top surface of the wafer. At larger design nodes, then, inspection of the back side of the wafer was performed only after the lithography track and only if defects were found on successive wafers, indicating that the offending particle remained on the scanner chuck, poised to continue to create yield issues for future wafers. In this case corrective measures were undertaken on the track to remove any suspected contamination. The track was re-qualified by sending another set of wafers through it and looking for defectivity at the front side locus of the suspected back side particle. This reactive approach was economically feasible for most devices throughout volume production of 32nm devices.
At the 28nm node, however, lithography process window requirements are such that controlling back side particles requires a more proactive approach. Advanced fabs now tend to inspect the wafer back side before the wafer enters the scanner, heading off any potential yield loss. Scanner manufacturers are also encouraging extensive inspection of the back side of wafers before they enter the track. As we see what lithography techniques unfold for the 16nm, 10nm nodes and beyond, it’s entirely possible that 100% wafer sampling will become the best-known method.
As with inspection of the front side of the wafer, sensitivity to defects of interest (DOI) and the ability to discriminate between DOI and nuisance events are important. Even though particles need to be two to three microns in diameter before they have an impact on front side defectivity, the inspection system ought to be able to detect sub-micron defects, since small defects can agglomerate to form clusters of critical size. Sub-micron sensitivity is beneficial for identifying process tool issues based on the spatial signature of the defects—while high-resolution back side review enables imaging of localized defects, so that appropriate corrective actions can be taken to protect yield. Sub-micron sensitivity also serves to extend the tool’s applicability for nodes beyond 28nm.
Rebecca Howland, Ph.D., is a senior director in the corporate group, and Marc Filzen is a product marketing manager in the SWIFT division at KLA-Tencor.
Check out other Process Watch articles: “The Dangerous Disappearing Defect,” “Skewing the Defect Pareto,” “Bigger and Better Wafers,” “Taming the Overlay Beast,” “A Clean, Well-Lighted Reticle,” “Breaking Parametric Correlation,” “Cycle Time’s Paradoxical Relationship to Yield,” and “The Gleam of Well-Polished Sapphire.”
1. Assuming 193nm exposure wavelength, NA = 1.35 and K2 = 0.5, then depth of field = 50nm. Normally 30% of the DOF is budgeted for wafer flatness.
2. Internal studies at KLA-Tencor.