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Meeting the demands of high-throughput semiconductor inspection


Maximilian Klammer and Klaus Riemer share how colour line scan CMOS cameras are enabling high-throughput inspection for semiconductor and PCB manufacturing


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emiconductor geometries and printed circuit board (PCB) components are


becoming smaller, while the demands on quality testing and control are growing. Because of the minute structures


involved, optical inspection in modern semiconductor manufacturing occurs in the micrometre range. Te need to detect defects and contamination at this scale and at high throughput places the highest demands on inspection systems. Extremely high-resolution colour line scan cameras with CMOS technology have been developed to accomplish this.


CMOS technology Line scan cameras are suitable for image-capturing tasks where fast- moving large areas or flat objects must be captured and analysed with high optical resolution. In a CCD line scan camera, the pixels of the sensor are arranged in a single line. Te readout cycle is adapted to the throughput speed of the inspected objects or surface areas. Te whole two-dimensional image is then created by joining the individual lines together. CCD sensors have long been


the technology of choice in line scan imaging because of their image quality, high sensitivity, low noise, low photo-response non- uniformity (PRNU), high dynamic range, and signal-to-noise ratio. Tese benefits are essential in line scanning because of the high-speed image acquisition and reduced number of photosensitive cells, which means exposure is limited. New-generation CMOS line


scan sensors now offer similar image quality to CCDs, coupled with benefits such as higher readout speed and flexible readout modes. Unlike single-line CCDs,


flawless product yield. Terefore, manufacturers test products multiple times, starting with bare wafers and ending with inserting the chips into the IC packages. As the wafer moves along the


production line, components are added, increasing its complexity. Yield-reducing defects must therefore be identified as quickly and as early as possible while also reducing overall inspection time. Tis results in more inspection points at each process step in the production line, with the inspection system required to produce the scan results more quickly. Inspection throughput is critical to the fab’s overall production.


With time delay integration, the same point on the object is captured by all pixels of the same column as the object passes below the sensor. The signals of all pixels are added up to increase sensitivity


these sensors have several lines of pixels, allowing for different operation modes depending on an application’s requirements. One key benefit of a multiline sensor is time delay integration (TDI). TDI consists of adding up the values of adjacent pixel lines synchronously with the motion of the object below the camera. Te same section of the object is imaged by several lines, and adding up the signals of all these lines increases the signal intensity compared with a single line sensor. Tis solution has a much better signal-to-noise ratio than an equal increase in gain. Te combination of specially


adapted line illumination with CMOS line scan cameras results in a powerful system optimally matched to the application. Te illumination system homogeneously illuminates a line on the object with a very high luminosity, whereby the light colour and the illumination geometry can be precisely adapted to the image capture requirements. Profitability in semiconductor manufacturing relies on the level of


18 IMAGING AND MACHINE VISION EUROPE APRIL/MAY 2023


Faster semiconductor inspection Scanning and illumination techniques offer a solution by providing faster and more efficient semiconductor inspection. As a rule, wafer inspection is performed with dark-field illumination and coaxial bright-field illumination. PCBs can be inspected using a combination of diffuse light and bright-field illumination – the diffuse light is used primarily for 3D-shaped components to eliminate gloss and reflections while the bright-field lighting is used to inspect the substrate itself. New-generation cameras


feature multichannel illumination functionalities that enable different types of illumination in one scan. Acquiring up to four different images in one scan provides much more information and improves defect detection. Multifield imaging enables the generation of HDR images from up to four lines captured with different settings. Tis allows details to be detected in both bright and dark areas in the image. Both can be combined to


increase detection reliability and shorten inspection time. Te trend toward miniaturisation


continues for PCBs and the components mounted on them. Tis leads to so-called panelisation, whereby several PCBs are manufactured on one panel. Each PCB on the panel is identified by a unique barcode, and thus rapidly inspecting the PCBs is correspondingly complex and demanding. For example, during the visual inspection, existing solder defects, such as interruptions, solder bridges, solder short circuits, or excess solder, must be identified, in addition to component defects such as lifted solder and missing or misplaced components. A colour line scan camera system can perform these inspection tasks with great accuracy because of its high operating speed and lateral resolution. Oxidised copper wires on PCBs must also be detected, yet monochrome systems cannot reliably identify oxidised areas. However, high-performance colour line scan cameras such as the allPIXA evo from Chromasens, combined with high-performance illumination, excel at this task. Examples from semiconductor


and PCB manufacturing show that high-performance colour line scan cameras can play an important role as central components in 100% real-time inspection processes. Te image quality and resolution performance of multiline full- colour CMOS line sensors result in a particularly high application potential for camera systems. Fast and accurate colour measurements and rapid 3D inspection are additional future applications. O Maximilian Klammer is an R&D Manager andKlaus Riemer a Project Manager at Chromasens


Chromasens’ allPIXA evo allows up to four lighting configurations to be combined in one scan. Three different light configurations are shown here


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