TDI Line Scan Cameras for Photovoltaics, Wafer Inspection and Crack Analysis

Schäfter+Kirchhoff delivers a series of TDI line scan cameras which achieve high measurement and scan velocities in low-illumination applications. Cameras using Time Delayed Integration (TDI) technology have a sensitivity about 100 times greater than conventional line scan cameras. Application areas are, e.g., the examination of highly reflecting surfaces (e.g., silicon wafers) with dark field illumination, the detection of micro-cracks in solar cells,and DNA analysis in Biophotonics. The principle of TDI technology relies on the timedelayed multiple exposure of a moving object by using a TDI sensor that consists of a parallel array of line sensors. At the end of one period of exposure, the accumulated charges in each line sensor are shifted to the next line. A further period of exposure produces more accumulated charges, which are then added to the already existing charges and the new sum is again shifted to the next line.

Multiple Exposures

In practice, the process is greatly accelerated by moving the object to be scanned across the field of the camera to produce a series of multiple exposures. For each exposure period, the object has moved far enough that the next sensor line is not only ready for exposure but has already been filled with the accumulated charges from the previous sensor lines. Finally, after multiple periods of exposure, the sum of all lines is output as a video signal, both highly amplified and speeded up. Schäfter+Kirchhoff delivers TDI line scan cameras with GigE or CameraLink interfaces and with 1024, 2048 or 4096 pixels for a maximum pixel frequency of 120 MHz.