Aerial Imaging Simplifies Mask Inspection

Mask inspection certainly
has not been made any easier by the continuing complexity being
designed into masks to enable them to print the desired features on
the wafer. Low k₁ lithography, for example, requires
aggressive OPCs, phase-shift masks, combined with extreme off-axis
illumination. These and other factors have combined to create major
inspection hurdles. Each generation of photomasks incorporates
increasingly sophisticated resolution enhancement techniques that
reduce the correlation between the pattern on the mask and that on
the printed wafer. This can present a problem for traditional
inspection systems because they can only produce an image of the
mask itself, and it has been a while since there was a connection
between the mask pattern and the wafer pattern sufficiently
significant to be used for reliable mask inspection.

Source: Applied Materials

Today, at SPIE’s Photomask Japan
conference held in Yokohama,
Applied Materials (San Jose, California), unveiled its new platform, the
Aera2 Mask Inspection system, which has been designed to deliver
what the company describes as “the fastest, most powerful
inspection and qualification solution for all advanced
photomasks.” The system uses aerial imaging technology and is
designed to meet mask inspection requirements for defect detection
of immersion and double patterning masks for the 45-nm node and
beyond. It is expected to address all mask shop inspection
applications, while providing over double the throughput of any
competing system.

The tool is touted as the
first mask inspection system that enables the user to immediately
see how a pattern on the mask will appear on the wafer; the
system’s software processes the defects detected on the mask
by estimating what effect they would have on the wafer. It then
filters out the large number of non-printing defects that some
other conventional mask inspection systems might label as being
printable. By emulating the optical system of 193 mm lithography
scanners and placing an image sensor in the wafer plane, the Aera2
system is capable of inspecting the mask under optical conditions
that are identical to those when it is exposed in an actual
stepper. This provides the user with a
“what-you-see-is-what-you-print” result.

Source: Applied Materials

The tool’s aerial
inspection technology also creates an entire set of enabling
capabilities that strengthen the link between the mask shop and
lithography cell by ensuring the printability of a mask before
delivery. The platform has an optional application, IntenCD, which
leverages the aerial imaging data to create high-precision,
high-density CD uniformity maps of an entire mask. These maps can
reveal subtle manufacturing defects, helping the mask maker to
fine-tune the mask manufacturing process.