Metrology for Monitoring and Detecting Process Issues in a TSV Module

Summary The early capture of process issues or drifts for all the operationsin a process module is important to avoid material scrapping. Investi-gating the quality of the incoming material (e.g. the FEOL module(s)in case of a via-middle TSV flow) as well as the outgoing material(for a typical 3D flow, numerous process steps follow in the thinningand stacking modules after the TSV module processing is completed,both of which can be severely impacted when, for instance, there is anon-uniformity in TSV depth present) is important for cost-effectiveprocessing. Aside from capturing issues that have occurred in a givenprocess step, it will be essential to have methods available for failureanalysis in order to find the root cause. 22 An overview of possiblemethods to investigate process results is given in Table I, including Table I. Overview of metrology. The list should not be interpreted as exhaustive. Technique / Reference Advantages / Remarks DisadvantagesFIB-SEM 20,24 ◦ Good resolution of defects → Capture‘voids’/defects in a metal matrix (e.g. platingdefects, post-anneal ‘void’)◦ Cross-sectioning by FIB time-consuming forTSV →Slow feedback and limited statistics◦ Destructive◦ Failure analysis: Cross-sectioning in awell-defined plane possibleX-ray transmission (XRT) 36,37 ◦ Non-destructive◦ Fast feedback◦ Many (statistics !) + specific structures in diecan be inspected◦ Some type of devices and layers (e.g. organic)may be sensitive to radiation◦ Resolution limited (e.g. plating defect in smallO TSV difficult to resolve)Mass measurement ◦ Fast feedback → Early capture of issues possible ◦ Only 1 data point per wafer◦ Sensitive for detecting full-wafer depositionand removal◦ No information on process step uniformity◦ In case of deviations, other metrology needed forfinding root causeOptical TSV depth measurement ◦ Non-destructive ◦ Small-diameter, high aspect ratio TSV structures◦ Many positions per wafer possible → Wafer-level difficult to accessuniformity of etch processScanning Acoustic Microscopy(SAM) 6,7,28,29◦ non-destructive ◦ Time consuming when inspecting many dies on a◦ Detection of plating ‘voids’ possible (depth scan) wafer (limited field of view)◦ Specific structures in die can be inspectedScanning Electron Microscopy (SEM) ◦ Measurement of CD-litho and CD-etch ◦ Time consuming when inspecting many dies on a◦ Specific structures in die can be inspected wafer (limited field of view)Atomic Force Microscopy (AFM) /Profilometry◦ non-destructive ◦ Indirect imaging of plating ‘voids’◦ Specific structures in die can be inspected