MicroLEDs Optical testing of μLEDs on a wafer and in modules

MicroLEDs are renowned for being an exacting new display technology. They are smaller than 100 micrometers and have exceptional optical properties that enable the manufacture of displays with a broad color gamut, high contrast and extremely high resolution. Also new adaptive front-lighting systems (AFS) use MicroLED arrays with several 10,000 individual light sources. This enables precise control of the light beam, as each MicroLED can be controlled individually.

Particularly in production, these exceptional optical properties present new challenges for the optical quality tests required at every stage of production, i.e. including wafer level. For efficient testing of thousands of μLEDs on a wafer, the test procedure must be parallelized. The manufacturer is thus required to simultaneously contact or optically stimulate as many μLEDs as possible. In addition, the optical inspection should be fast and accurate, and performed synchronous to the production flow. This functions only with a 2D measurement using specifically calibrated instruments to prevent measurement errors.

The major challenge in quality inspection of high-performance LED modules, as used in e.g. AFS systems, is the immediate onset of temperature rise when switched on. The resulting heat causes a drop in performance and leads to a color shift. Due to the latter, measurement systems for quality control of uniformity, brightness and color of the arrays are confronted with combined challenges: the measurement must take place quickly and nevertheless very accurately, so that the temperature independence of the measured values has no influence.