Accurate Matching for CDSEM on ADI Wafers

Abstract

There exist great difficulties and challenges in CD-SEM matching because of the destructive interaction between e-beam and resist. This paper presents a new cross sampling method which uses two separate locations of resist line and performs cross measurements by two separate tools in a special sequence. This sampling method together with a special designed calculation can eliminate the impact from both resist line shrinkage and measurement locations difference. It removes the error component with magnitude of second shrinkage in the conventional approach of matching, however a difference of second shrinkages from two tools is introduced although it is smaller than the second shrinkage itself. After introduce an exponential decay model of the resist shrinkages in multiple subsequent measurements to the sampling method, we could estimate first and second shrinkages for both CD-SEM tools with the measurement results from one of the above cross sampling plan, we are able to determine the real difference of measured CDs after the first measurement from these two CD-SEM tools, which cannot be obtained by direct measurements because of the memory of any previous measurement. IntroductionMatching evaluation between metrology tools in IC manufacturing is needed for accurate and consistent measurement of process. Critical dimension-scanning electron microscope (CD-SEM) has an important role in measuring critical dimension of pattern on wafer. The accuracy of measurement for critical dimension has large influence on process, so the CD-SEMs must be matched with each other within specified requirements.After etch inspection (AEI) wafer is commonly used in CD-SEM matching, because AEI wafer has high contrast, and no destructive puzzle such as shrinkage on after development inspection (ADI) wafer after exposure by e-beam. It is convenient to do match using AEI wafer. But some report [1][2] [3] shows that it is necessary to do matching for CD-SEM by ADI wafer. There are two main reasons: first, AEI wafer has higher contrast and resolution than ADI wafer when they are measured by CD-SEM, these advantage brings coarse matching result which could not reflect the inherent difference about the two CD-SEMs. Second, the shrinkage for photo resist on ADI wafer is sensitive to both time and intensity of e-beam while AEI wafer is not, matching on ADI wafer will reflect more sensitively whether two CD-SEMs are matched or not. Further more, in some conditions, matching between two CD-SEMs for ADI critical dimension (CD) has to be used for matching purpose, for example, when the ADI CD measurement for a BEOL process is transferred from one FEOL CD-SEM to another, AEI wafer could not be used because of Cu contamination.Shrinkage is an inherent shortcoming for ADI wafer to do matching between two CDSEMs, the measure on ADI wafer is destructive, so the true value of the CD on ADI