Within the field of photolithography, Mask Inspection is a process of identifying defects before they cause problems.

In all photolithography, somewhat like a Xerox machine, a pattern is transferred from a perfect original to many copies. Here, the original is called a reticle or a mask and the pattern being copied is one layer of a circuit layout. Those copies are then developed, etched, and processed in layer upon layer of tiny wires and transistors to become a working computer chip. Any defects, dust, scratches, or imperfections on the original potentially end up on the copies and ruin them. So the original has to be 'perfect'—both in the underlying substrate, and the printed pattern.

The most advanced photolithography under development today is called Extreme Ultraviolet Lithography (EUVL). For EUVL, the masks start out as perfect, flat mirrors, and the patterns are applied on the top surface as a light-absorbing layer. An EUV mirror is formed on an atomically flat, glass surface, onto which a special, multilayered reflective coating is applied. The coating, made from alternating layers of molybdenum and silicon (Mo/Si) is tuned to give peak reflectivity for just one color of EUV light.

It doesn't take much to ruin a perfect reticle. The smallest sizes of defects that can cause problems are on the order of a few tens of nanometers wide—just a few hundred atoms. The defects can be small particles that have fallen onto the surface and stayed there; or they can be trapped under the reflective multilayer coating, causing a disturbance in the perfect layer structure. Pits and dimples are also a serious concern. The process of finding defects, and potentially correcting them, is called Mask Inspection.

It takes an extraordinarily sensitive tool to find such small defects. For example, finding a 40-nm-wide defect on a six-inch (15 cm) reticle, is like a looking for a single errant hair in a super-clean area the size of a big city block. Several commercials tools are currently used, and new ones with greater sensitivity are under development. In order for such an inspection tool to work, it has to be able to find all of the defects that are large enough or severe enough to disrupt the pattern when printing. The sensitivity of the tools are compared against each other in round-robin cross-correlation testing, and, most importantly, the inspection results are compared with printing tests.

An LBNL-LLNL collaboration, funded by SEMATECH, has created a unique mask-inspection tool called the SEMATECH Berkeley Actinic Inspection Tool (AIT), at LBNL's Advanced Light Source. Years ahead of commercially available tools, the AIT uses EUV, "actinic," light to inspect masks. It operates like a high-resolution EUV microscope with scanning and imaging modes.

 AIT Preview:

Learn more about The SEMATECH Berkeley Actinic Inspection Tool (AIT).