Improving the accuracy of overlay measurements through reduction in tool- and wafer-induced shifts

Abstract

The accuracy of overlay measurements is negatively impacted by asymmetries in the wafer targets and in the metrology system optics. These asymmetries lead to spurious shifts in the registration data which are referred to as tool induced shift (TIS) and wafer induced shift (WIS). In practice, there is always some interaction between the optics and the wafer, making it difficult to separate the errors into specific TIS and WIS components. As a result, the tool and wafer induced errors are usually confounded together and simply referred to as TIS. Overlay metrology systems typically attempt to quantify the TIS by measuring sample wafers at 0 and 180 degree orientations. The observed mean difference between measurements taken in these orientations is taken as an estimate of the TIS error. These TIS calibration values are often applied to all subsequent wafers of the same type, with the assumptions that (1) the tool contribution remains constant and (2) all wafers of the same type have identical asymmetries, and therefore identical contributions to TIS errors. In most case, the TIS error is further assumed to be constant across the entire wafer, even though processes such as chemical mechanical polishing are known to induce asymmetries which vary systematically across the wafer. One method to reduce measurement uncertainty would be to calibrate for TIS on each wafer and at every measurement site. This solution would drastically reduce the throughput of existing overlay metrology tools. We present two potential solutions to this problem, one involving modified system software, the other utilizing a unique new measurement methodology. The impact on throughput and improvement in overlay accuracy for each approach will be discussed, and data will be presented showing the advantages and drawbacks of each technique.