With the WAFERinspect AOI measuring system, Confovis is expanding its WAFERinspect product range with an AOI tool which combines defect inspection, defect review as well as 2D and 3D measurements in a single system. Defect detection and classification was implemented in cooperation with NeuroCheck GmbH.
Detection of defects down to sub-micron thanks to a new approach
For defect detection, the Confovis WAFERinspect AOI uses current computer architecture (1 TB memory) to compare golden samples (with a size of up to 500GB) in the memory with the current defect scan in real-time. Defect detection reaches a speed of up to 25 FPS because access times to the SSD hard disk as a limiting parameter are eliminated. Compared to currently established standard systems, the Confovis system enables a considerably higher resolution at the same process time, whereby even the smallest defects of a size down to 0.1 µ/px are detected on various surfaces and process steps in front end or back end.
Flexibility of implementation
Depending on the application, the entire wafer or individual chips can be taught as a “golden sample” for a die-by-die or reticle-by-reticle defect scan. The classification of the defects does not have to be carried out during set-up but can be carried out afterwards and separately based on the defects found.
Defect detection and classification has been implemented together with our partner NeuroCheck, a leading supplier of industrial image processing. A multi-stage process, which is also very robust compared with changes in contrast values, is used. An essential advantage of the NeuroCheck defect detection software is flexible adaptation of the inspection programs to changes in the production process, so that modifications in the program are not necessary. Instead, analysis tools can be enhanced as a function block in the program flow, which enables simple iterative adaptation of the test programs to the defects to be found via the GUI. In addition, all changes – including classifier training – can be carried out offline and locally on the user’s premises. This means it is not necessary to transfer sensitive data to a cloud to extend defect detection.
Artificial intelligence for challenging defects
Defect classification is carried out using artificial intelligence (AI) in conjunction with previously generated characteristics of the defects (such as number of pixels, aspect ratio, etc.). The basis for this is formed by a database, which is filled automatically by the defects found in the scan. The user only has to create categories and sort defects accordingly. The neural classifier is then trained and taught. Rejection thresholds can also be set for each class, ensuring that no defect remains undetected – particularly during the ramp-up phase.
For very small defects (0.1 µ/px) it is possible to identify and classify the defects exclusively with neural networks thanks to rapidly developing computer technology. The AI algorithms further facilitate the defect classification, especially when it comes to defects that cannot be detected robustly with classical methods (e.g. golden sample). For example, defects on non-recurring structures or if these show changing characteristics – whereby differences in contrast can lead to pseudo-defects.
Confocal 3D measurements for further analysis of the defects
If the possibilities of defect detection in 2D are not sufficient to reliably ascertain specific defects (e.g. in lens arrays or copper pillars), there is also the possibility of an advanced 3D defect analysis with WAFERinspect AOI. With the patented confocal 3D measuring method (Structured Illumination Microscopy) from Confovis, arrays can be analysed flat and vertically in the nanometre range for the required characterisation. Bumps can also be checked for defects and, if necessary, measured completely in 3D.
The 3D analysis is carried out automatically and directly after defect classification, whereby either all defects or only user-definable criteria can be measured in 3D. With Confovis WAFERinspect AOI, scratches, particles and other defects found in 2D (e.g. finest stress cracks) can be measured with just one system. Moreover the combination of 2D inspection and 3D measurements allows a more comprehensive evaluation of the defects.
One measuring system for defect inspection and metrology
In addition to defect detection, Confovis WAFERinspect AOI is a high-resolution 2D/3D measuring system. Confocal 3D measurements are typically carried out in two seconds (120 measurement planes with a z range of 20 µ and an accuracy of <4nm) – with a measurement result independent of the surface material to be measured. Line/space, VIAs and Overlay structures can also be measured with high accuracy in 2D. The Cognex Vision Library running in the background provides the user with intuitive and fast operation using easy modes. Existing Cognex recipes can also be processed from other systems in expert mode. Confovis WAFERinspect AOI is available as a fully automated system with SECS/GEM and EFEM application as well as laboratory equipment.
