Thursday, February 26th, 2009
One year after this editor covered an SPIE panel on reference metrology and summarized the situation as “there is no more noise; there is only signal,” another lively panel at SPIE this evening discussed the need for not just precise but accurate critical dimension (CD) measurements in advanced IC lines. With smaller structures and reduced process margins, we’re in a era when 2D shapes must be controlled. We can still confidently say that if a 2D shape is within specifications that the process is in control – but how do we know if it is within specs?
Through-pitch CDSEM bias variation between two brands measuring poly-Si lines (source: Veeco)
Vladimir Ukrainstev, formerly of Veeco and now of Nanometrology International, showed Veeco data on through-pitch SEM bias variation on poly-Si lines from two different SEM brands (see figure). The bias difference can be 4nm, particularly at tight pitches and small CDs. AFM and TEM may aim toward accuracy to resolve the difference, but both techniques are slow and expensive and so some uncertainty in sampling seems inevitable. Audience expert Cyrus Tabery, of AMD and now with “The Foundry Company,” expressed the traditionally pragmatic fab view that precision has always been sufficient.
John Sturtevant, Mentor Graphics litho guru and new SPIE Fellow, said, ”In the old days precision was probably more important because it was easier to measure. The world that OPC engineers live in today has many variables, and accuracy is not what is causing them to fail at 32nm. It’s important, but we’re managing.” For example, he mentioned that we are currently able to get to 90% confidence levels with OPC models, and that’s a small but significant step.
Ofer Adan, CD-SEM expert for Applied Materials, strongly asserted the need for accuracy, “Let’s take roughness as a parameter, we need accurate roughness figures because if we have to figure everything out electrically then we take too much time to get to market.” Hidetoshi Morokama, CD-SEM expert for Hitachi, likewise expressed that we must try to get to accurate numbers for 2D structures despite known challenges. More about this panel and reference metrology will be covered in the next BetaSights Newsletter. –E.K.