Diffraction Metrology Measures Overlays Down to 45 nm

Aerial imaging overlay technology has been a reassuringly familiar feature of semiconductor manufacturing for years, experiencing little change. The wafers are loaded, special box-in-box targets are examined using a white-light microscope, and the images are then processed to determine overlay error.

Although this has worked well, as we move toward implementing deep nanometer process levels there are several issues with image overlay tools. One of these has to do with the box-in-box structures that have been traditionally used, which tend to have large geometries and typically consist of bars microns in size that do not correspond to circuit feature sizes. Additionally, the box-in-box structures tend to have large areas of low feature density, which polish at different rates than the high-feature-density circuit areas. As increasingly smaller features are being produced, it is becoming more obvious that the behavior of these traditional box-in-box targets does not accurately reflect that of the circuit features themselves.

This imaging overlay uncertainty has been tolerable because process windows have not had to be too tight, and the available precision and accuracy have been adequate. Now, smaller geometries are resulting in accuracy issues that must be addressed in imaging overlay tools, particularly the tool-induced shift (TIS) metric. For imaging overlay tools, TIS can be on the order of several nanometers. At the 65 nm node, 2 or 3 nm of TIS is a significant measurement uncertainty.

The new overlay diffraction metrology technique promises to meet requirements down to the 45 nm node. (Source: Nanometrics)

Nanometrics (Milpitas, Calif.) is developing a new technology based on spectroscopy, rather than imaging, which it states is much more robust from a tool statistical performance standpoint. According to the company, the total measurement uncertainty for the technology is about six times smaller than the traditional one, and is capable of meeting the precision and accuracy requirements for advanced design nodes through 45 nm. This diffraction-based overlay metrology technology originates from R&D into optical critical dimension (OCD) metrology through scatterometry and diffraction methods. A major advantage of the diffraction technique is that targets can be produced to match circuit features — CDs on the order of the feature sizes of the process layer in question. Because the target's pitch is submicron as well, it provides the immediate benefit of looking at the overlay performance of features that closely resemble the circuit features.

Another benefit of the diffraction overlay technique is that the spectroscopic method is less susceptible to TIS. For the imaging overlay method, the goal is to map the target from the object plane to an image plane, and preserve the target's relative spatial information. This is difficult to do with residual aberrations such as coma and asymmetrical aberrations, third- and fifth-order distortions, etc. These residual aberrations directly affect features' spatial relationships in the target when it is transferred into the image plane. The diffraction technique collects light diffracted from the target and sends it into a spectrometer to have the spectra analyzed, so the spatial relationship of the photons in regard to the target is not critical. There is little sensitivity to aberrations in the optics in the spectroscopic technique, so it is not essential to have the purest quality optics to do the measurement. The ancillary benefit for the fab is that tool matching becomes simpler than it is for traditional stand-alone aerial imaging tools.

Real estate, however, is an issue for the new technology. Typical image overlay targets are 900 µm², and can be used for horizontal and vertical overlay registration measurements. With this system, overlay targets require separate targets for the X and Y registration value measurements. Also, it requires a different kind of target, which the device manufacturer will have to begin printing and get used to. However, techniques under development will allow the diffraction targets to shrink to typical box-in-box sizes.

The new technology appears to offer a high degree of repeatability and reproducibility. With it, TIS is reduced to the angstrom level, more than 10 times smaller than what is encountered with image overlay technology.

For additional information on inspection, measurement and test, go to www.semiconductor.net/imt.