Photonics

Improving fringe projection 3D shape measurement

Scientists leverage Mikrotron CoaXPress camera to develop telecentric system

21.03.2022 - Based on a novel measurement algorithm and the Mikrotron EoSens 12CXP+ area scan camera, scientists in Taiwan developed a system to reconstruct a 3D model in a continuous scan mode instead of the conventional approach of taking three full-field fringe images sequentially in pause mode.

Optical 3D shape measurement is rapidly gaining importance in metrology applications thanks to its capability to accurately measure and reconstruct 3D moving objects such as silicon wafers, printed circuit boards, or flat panel displays.

Several production-worthy 3D measurement methods are available today, all offering varying tradeoffs between speed and accuracy. Two of the most commonly deployed methods are triangulation, which is the process of determining a point in 3D space given its projections onto two or more images, and fringe projection, which encodes depth via the deformation of a known fringe pattern projection.

To improve the performance of fringe projection 3D shape measurement, scientists at the Industrial Technology Research Institute in Taiwan recently developed a system based on a novel measurement algorithm and the Mikrotron EoSens 12CXP+ area scan camera. The scientists flexibly programmed the camera’s region of interest (ROI) to contain selectable rows to simultaneously acquire three fringe images with 120° phase shifts. Because only the pixel information within the ROI was stored in the memory, this approach reduced the data volume and processing, therefore greatly accelerating area scan acquisition.

This 3D measurement system features three segments: optical imaging, projection, and object space. The optical imaging system consists of a telecentric lens (Net-Gmbh L-VS-LTC) with selectable magnification, and the Mikrotron camera offering a resolution of 4096 (H) × 3072 (V) pixels, an image field with a diagonal of 32.6 mm, and 4.5 μm × 4.5 μm pixels, which correspond to the lateral sampling for the selectable magnification. If needed, the camera can be set to a smaller ROI so that only the pixel information within the ROI is stored in the memory, and the frame rate can be bumped up to a staggering 10,100 fps with an active sensing area of 4096 (H) × 50 (V) pixels, made possible by the CoaXPress high-speed interface. The projection system has a telecentric lens and 250W metal halide light source with lightline lenses that can project a sinusoidal fringe with each bright stripe being parallel. Finally, the object space is made up of a high-precision stage with both x and y axial movement range of 300 mm.

One of the things that makes the system unique is that sufficient information is retained to reconstruct a 3D model in a continuous scan mode instead of the conventional approach of taking three full-field fringe images sequentially in pause mode. This improves the measurement frame rate up to several hundred-fold. In addition, the hardware can be flexibly adjusted compared with a three-row line scan camera as long as the ROI contains the signal of the three rows with 120° phase shifting. Experiments showed that the fringe projection system provided good linearity between the calibration and optical data with excellent accuracy. A reflectometry sensor was implemented to provide a correction offset to the bump height in a protection layer in a subsequent high-speed inspection.

In the future, the scientists plan to test a fringe projecting module with a selectable fringe density and adjustable projecting angles to optimally introduce the phase-shifting algorithm to bump measurements for a wider range of heights and pitches. They also plan to focus on a measurement solution for higher density ‘microbumps’ only a few micrometers in diameter to meet advanced packaging market needs.

Reference: Ku Yi-Sha, Po-Yi Chang, Han-Wen Lee, Chun-Wei Lo, Yi-Chang Chen, and Chia-Hung Cho: Metrology for Measuring Bumps in a Protection Layer Based on Phase Shifting Fringe Projection, Appl. Sci. 12(2), 898; DOI: 10.3390/app12020898

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