Accurate wafer-level color testing of LEDs will drive adoption of white solid state lighting
A Solution with a Problem
Energy-efficient, low cost white solid state lighting would be much more widely adopted by the lighting industry if the color output could be consistently controlled. Unfortunately, testing of LED dies at the wafer level under “cold” conditions does not map to the color output at true operating temperatures once integrated into an LED package. KLA-Tencor is poised to revolutionize the market with a high throughput, precision hot test tool capable of sorting LEDs into bins with perfectly matched color output at actual operating conditions – and it’s based on an Ocean Optics spectrometer.
A Complex Simulation
Many factors contribute to the color distribution within a given product batch of white LEDs, affecting the ratio of blue to red and green light in an LED-phosphor package. Consistent color delivery from a white light LED array requires exact characterization of the die before packaging. These include particle size distribution, morphology, and even particle shapes according to Rayleigh or Mie scattering theory. Also contributing variations are the nonuniformity in phosphor mixing concentrations, hot spots present in each InGaN electroluminescent pump die, variations in forward voltage and efficiency (and therefore temperature) from die to die, as well as variations in overall phosphor binder layer thickness and therefore phosphor temperature distributions and electroluminescent conversion efficiency from blue to green and red wavelengths.
This is further complicated by a strong temperature dependence of both die and phosphor performance and the presence of large thermal gradients between materials under operating conditions. While the InGaN film may operate at 85 °C and higher, the phosphor temperature can reach 120 °C where it meets the high index, low thermal conductivity silicone lens. Neither testing under external heat nor models to extrapolate performance from cold operation has been accurate in predicting color output under actual operating conditions.
Local Action with Global Impact
A new patent-pending approach will change all that. It uses precise, local laser heating of each die/phosphor/lens on a 2D wafer combined with electroluminescence of the InGaN film to replicate the exact thermal conditions and gradients to be experienced by the packaged product. Amazingly, this can be done for a range of possible customer operating conditions, allowing LEDs to be classified for multiple use conditions and picked as needed for minimal waste while providing unprecedented consistency in color output.
Move Quickly, Before It’s Gone …
The precision laser heating occurs within a millisecond, and needs to be measured just as quickly. To be indistinguishable to the human eye, each color measurement needs to be accurate to within ±0.005 units in x– and y– coordinates. Our Maya LSL spectrometer is up to the challenge, providing the low stray light necessary to achieve high color measurement accuracy simultaneously with high throughput for rapid measurements. The resulting tool will be capable of testing 180,000 units per hour (UPH) – far greater than any other test tool on the market!
The Future Looks Bright
Use of this novel hot testing method at the wafer will vastly improve product consistency, enabling white solid state lighting as a truly viable option for energy conservation. And the fact that it does so at reduced manufacturing cost and with less waste makes it a solution with real global impact. Watch for this new product at www.kla-tencor.com.
Optical System – Color
- Maya LSL spectrometer (360–825 nm)
- Calibrated light source
- Optical patch cord fiber