MicroLEDs Optical testing of μLEDs on a wafer and in modules

Efficient, simultaneous testing of thousands μLEDs on a wafer or in modules within a few seconds

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.

The challenge in MicroLED in-process inspection

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.

Our solution: Camera-based LumiTop 4000 2D system

With the camera-based LumiTop 4000 2D system, Instrument Systems offers a measurement solution that forestalls the effects of a temperature drift during measurement due to its high measurement speed. In addition, the procedure is synchronized with the current source of the μLED and begins immediately upon switching on the LED array. Compared to traditional measurement methods that measure each LED of the array individually, the simultaneously measuring LumiTop system is many times faster. Thanks to the spectral extension of the system with Instrument Systems’ high-precision CAS 140D spectroradiometer, it also delivers highly accurate readings due to absolute system calibration. Instrument Systems is a test laboratory accredited to ISO 17025 and guarantees traceable measured values with known accuracy.

The LumiTop 4000 has a resolution of 12 MP and can detect the smallest of defects and inhomogeneities - also on a wafer. Thanks to a 100 mm macro lens, the camera enables fast parallel inline analysis of all μLEDs on a wafer at a single test station and is particularly well suited to μLED wafer testing. With a field of view (FoV) of approx. 1.0 by 1.4 cm it can measure many thousands of μLEDs in parallel with a minimum pixel size as low as 30 micrometers.

We will be pleased to provide advice for your planning in order to configure the ideal measuring concept.

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