Consequently, laser scatterometers are the current instrument of choice for particle sizing and counting in both production and research activities. However, because the nature of real particles varies and their differing composition and geometric shapes affect their scattering behaviors, the accuracy of the measurements reported by scatterometers is open to question. In addition, the roughness of final-polished silicon wafers also varies, and they may sometimes be far rougher than the wafers used for calibration. This difference in sizing could be attributable to several factors, including the weaker scattering intensity that results from the 6420's oblique incident angle and side detectors, and the orientation dependence of the scattering cross section of an irregularly shaped particle. The significant sizing difference for the LPDs designated numbers 7 and 8, in particular, indicates that reported LPD size is strongly dependent on the shape and orientation of the real particles or defects.
All results are in microns.
Even though it is less affected by surface roughness, the 6420 has a lower sensitivity specification than the 6200 does. In another experiment, a 6220 was used to characterize a wafer with a high level of roughness. The instrument may have picked up the background haze signal because the gain level and particle-size threshold were inappropriately set for this wafer. The above findings indicate the importance of wafer surface roughness for reliable measurement of LPDs by either the 6420 or the 6200. Haze pattern map of a 6-in.