by Ed Korczynski, Senior Technical Editor
KLA-Tencor’s Surfscan SP2XP, released earlier this year, was initially release to inspect defects on bare wafers. Now the SURFmonitor haze analysis capability has added new image processing algorithms to allow for applications in IC manufacturing. SURFmonitor is an add-on option to either a SP2 or SP2XP darkfield/brightfield laser scattering surface inspection system.
Since any of the >300 sold SP2-family tools can scan an entire 300mm wafer surface in <1 minute, and since image processing is done by separate dedicated boxes to not impede throughput, there are ubiquitous applications throughout a fab to surf the haze and glean productive information from wafers.
A SURFimage file shows a wafer map with grayscale information for haze. In certain obvious cases, a characteristic signature may be seen in the haze map that indicates the root cause of a problem. For example, radially symmetric defects often track to a wafer chuck (see Fig. 1, above), while arcing scratches tend to originate with CMP steps, and some repeating defects may line-up with lithographic stepper shot positions. In such cases the raw SURFimage may be sufficient to point to the problem, but in other cases the signal of interest is still a bit lost in the noise. Streaks of chemical residues from a wet clean tool may be present on a wafer but difficult to resolve without further image processing (see Fig. 2, below).
To efficiently convert the image of a haze map into the “productive” information of a root-cause diagnosis, image filtering and processing is done by a SURFengine image computer located next to the SP2 tool, which recognizes and extracts the defects of interest, generate alarms, sorts and filters defects by type, and passes this information on to a yield management system for SPC charting and further troubleshooting. The SURFengine boxes in the fab are networked together using a remote SURFstation, for engineering analysis, access to x/y coordinates for DR-SEM and troubleshooting, and recipe writing. The raw haze maps are archived in the SURFengine for up to one year, allowing for retroactive data mining and trend tracking.
KLA-Tencor is positioning this new capability as a hybrid between defect inspection and metrology, and indeed it seems to overlap with both. In terms of extracting metrology information, the SURFmonitor has already demonstrated excellent correlation between haze maps and surface roughness, grain size, and thickness for certain thin films. The company claims >95% (typically 98%-99%) correlation between haze maps and atomic force microscopy (AFM) for grain-size measurements of hemispherical-grain poly, copper, and tungsten.
“Many 45nm and 32nm fabs are interested in monitoring surface morphology for a variety of applications,” explained Rahul Bammi, senior director of marketing at KLA-Tencor. For example, CMP processes require expensive consumables-sets, and having some capability to provide rapid-response to excursions allows for increased use of consumables without risking excessive work-in-progress. Being able to rapidly extract the signatures for CMP scratches or jitter from haze maps allows the CMP process to be run with longer time on a given consumables-set.
In developing immersion lithography processes, a characteristic residue water-mark has been found that is so very thin that it cannot be detected by an ellipsometer. The processed haze map could detect these “sub-threshold” defects on blanket test wafers, and those locations correlated perfectly with brightfield defect data from product wafers. “This eliminates accidents before they happen,” commented Bammi.
Another example of novel productive-information generated by SURFmonitor is the ability to detect pinholes in grown gate-oxide. One fab customer had experienced gate-oxide breakdowns as seen by electrical test results that tracked to DR-SEM pin holes in the oxide, yet they had no in-line method to detect these pin-holes. The SURFmonitor results after image processing correlated to the pin-holes detected by electrical test.
Overall, this new capability provides fundamentally new knowledge about what is happening on an advanced silicon wafer during manufacturing — and since knowledge is power, this adds tremendous power to what used to be “just defect inspection.” — E.K.