September 14, 2009 - KLA-Tencor Corp. has unveiled its Teron 600 Series mask defect inspection systems. A programmable scanner-illumination capability and improvements in sensitivity and computational lithography power over the company's current platform, the TeraScan XR, enable the new system to address a major transition in mask design at the 2Xnm logic (3Xnm half-pitch memory) node.

The industry is currently evaluating several lithography technologies for use at 2Xnm:  spacer pitch splitting (SPS)/pattern cutting (PC), true pitch splitting (PS), inverse lithography (ILT)/ source mask optimization (SMO), as well as EUV. With many fabs investigating all of these technologies simultaneously, they need a single reticle inspection system that can handle all of them, explained Dan Lopez, senior director of marketing of the the company's reticle and photomask inspection division.

Lopez articulated the challenges for each lithography technology that the new platform addresses. For example, with EUV the challenges are small linewidths, a reflective imaging technique, no pellicles to protect the mask, and multilayer defects, along with other new defect mechanisms. In particular, to handle multilayer defects, the company designed a completely new optics bench for the new platform, the details of which are proprietary.

The challenges for advanced double-patterning lithography (DPL) at 2Xnm include critical CDs that are formed between two masks, mask defects that are created by the contribution from two masks, and a mask contribution to the overlay budget that is ~35% (KLA-Tencor's estimate). To address these, the Teron platform offers: die-to-database inspection, multilayer database processing, context-sensitive defect detection, and intensity CDU (iCDU) that enables correlation to mask CD-SEM results and provides more detail for mask process development. The iCDU technology gives a more complete picture of mask quality which is important for managing overlay and CD distribution, Lopez told SST -- the iCDU technology is experimental, but the Teron 600 can support it if end users need it, he added. Because iCDU measures a million points, it enables a lithographer to see how the CD is varying across the plate in a highly localized way. Whether or not lithographers can use such information to make corrections in the mask process development remains open.

With ILT/SMO come the challenges of small feature sizes, and primary features and assist features are similar in size. This last point leads to difficulties with defect impact assessment -- i.e., how would a lithographer know if a mask defect is important because one can't tell the difference between a primary and an assist feature. And the illumination pattern used with SMO is very complex, "we start to see illumination patterns that aren't simple and the mask doesn't look like a mask," explained Lopez. "You don't see the 1:1 correlation between the mask and the wafer." So with ILT/SMO, it's no longer straightforward what an important defect might be on the mask. To address these issues, the new reticle inspection platform is able to simultaneously inspect with transmitted and reflected light, and context-sensitive defect detection is also essential (i.e., are you inspecting an assist feature or a primary feature). Also important is having a programmable scanner source model, to be able to predict what the illumination and the mask will "do" at the wafer, said Lopez. And wafer plane inspection -- a technology recently introduced by KLA-Tencor -- is also needed; this technology takes the two inputs, scanner illumination and the mask image, and does a rigorous lithography simulation to predict what will happen on the wafer.

The company says the Teron 600 platform has successfully inspected prototype reticles created for ILT/SMO, double-patterning lithography (DPL), and EUV (masks and blanks). The system is engineered to be extendible to potential 1Xnm optical solutions as well. The new platform can also work with the TeraScan 500 Series reticle defect inspection systems in a mix-and-match strategy. According to Lopez, the company is now shipping beta tools. -- D.V.