by M. David Levenson, Editor-in-Chief, Microlithography World
April 16, 2008 – Applied Materials re-entered the mask inspection tool market at this year’s Photomask Japan by unveiling the Aera2 stepper emulation mask inspection system. If it performs as advertised, it could enable even more aggressive RET strategies, lower reticle costs, and help make chip yield more predictable at 45nm and 32nm.
Unlike familiar photomask inspection tools, which are basically high-resolution microscopes that image the detailed mask structure, the Aera2 captures the image that would have been projected onto the wafer by an exposure tool. Because of the pervasive use of resolution enhancement technology (RET), today’s mask designs are full of very fine features that contribute to image fidelity while not actually exposing photoresist. Rather than inspecting all the edges of all of these structures, the Aera2 compares the aerial image at the wafer with what would be needed for proper yield.
“With the Aera2, you see what you print,” commented Mark Wagner, head of the company’s mask inspection division.
Aerial image measurement tools related to the Aera2 are not new — capturing the light diffracted from a mask as would an exposure tool, but with lenses that magnify the image on a CCD type detector rather than de-magnifying it on photoresist. The numerical aperture thus is 1/4 that of the exposure tool, far less than that of a high-resolution mask inspection system. The exposure wavelength (193nm these days) must be used for illumination, and inspecting the entire mask requires scanning and processing the images quite rapidly. Small mask defects cause subtle localized changes in the brightness of the captured aerial images which then must be identified as significant or not.
According to Wagner, conventional mask inspection systems find themselves inundated with non-printing nuisance defects that arise from irregularities in sub-resolution assist features, etc. Separating the real printing defects from the others then requires aerial image simulation and a defect review step, whereas the Aera2 gives the result directly. Wagner claimed the Aera2 shows twice the throughput of competing technology resulting in the shortest time to decision.
The Aera2 also can perform a survey to capture the mask contribution to CD uniformity by measuring the brightness of every feature on the mask — 5 × 108 measurements done in parallel with the conventional mask inspection. Subtle variations in mask processing are known to cause feature dimensions to vary by a few critical nanometers across the reticle. The IntenCD measurements on the Aera2 reveal ~1% nonuniformities on minimum CD features in regions as small as 10&mum × 10μm, according to Wagner, facilitating improved mask process control (and possibly downstream correction).
Wagner noted that the first shipment of the Aera2 was one year ago and there are now several in the field (one replacing an older version at a major MPU manufacturer), and orders are backlogged.
Since the aerial images now have simpler geometry than the OPC mask designs, the time may have come for aerial image inspection to replace microscopic mask measurement. Pixelated masks, contact reticles with intersecting assist slots for random logic, and “loopy” inversion designs have all proved challenging for traditional systems to inspect, leading to loosened defect inspection criteria or regions labeled “DNI” for “do not inspect.” One result has been uncertainty in chip yield as uncorrected mask anomalies become “killer” defects, but another has been over-priced and over-specified reticles. If the Aera2 performs as advertised, it may enable even more aggressive RET strategies, lower reticle costs, and help make chip yield more predictable at 45nm and 32nm. — M.D.L.