Cost effective probing in high volume manufacture of micro LEDs
A wafer having μLEDs is inspected using cathodoluminescence microscopes. A fast scan is enabled by splitting the CL beam into several beams and sensing the beams with point detectors. Optical filters are inserted in the optical path upstream of the detectors, such that each detector senses a different frequency band. The signals are ratioed and the ratios are compared to expected reference. Regions of extreme value are identified and, if desired, a high resolution scan is performed on the regions or a sample of the regions. Viability score is calculated for each identified region.
1. A cathodoluminescence microscope for inspecting samples having a plurality of micro LED mesas formed therein, comprising:
an electron beam column having an electron source, a magnetic lens for focusing electrons emitted from the electron source to thereby form an electron beam, and a scanner scanning the electron beam over the sample;
a light objective collecting light emitted from the sample in response to the scanning of the electron beam and forming a light beam;
an imaging section comprising a focusing lens, splitter arrangement splitting the light beam into a plurality of split beams, a plurality of point detector, each positioned to receive one of the split beams, and a filter arrangement limiting the bandwidth of at least a subset of the plurality of split beams; and,
a controller receiving output signals from the plurality of point detectors and generating ratios of paired output signals of the plurality of point detector.
2. The cathodoluminescence microscope of claim 1 , wherein the point detectors comprise: a biased photodiode, avalanche photodiode, amplified photodiode or photomultiplier tube (PMT).
3. The cathodoluminescence microscope of claim 1 , further comprising a background filter blocking wavelengths below 400 nanometers.
4. The cathodoluminescence microscope of claim 1 , wherein the filter arrangement comprises at least one dichroic mirror.
5. The cathodoluminescence microscope of claim 1 , wherein the filter arrangement comprises a plurality of band-pass filters having different central frequency or different bandwidth.
6. The cathodoluminescence microscope of claim 1 , wherein the filter arrangement comprises at least one biased LED wafer.
7. The cathodoluminescence microscope of claim 1 , wherein the controller compares the ratios to reference ratios and identifies regions of the sample having ratios deviating from the reference ratios.
8. The cathodoluminescence microscope of claim 7 , wherein the controller further assigns viability scores to the regions.
9. A method for inspecting a wafer having a plurality of mesas for forming micro LED in each mesa, wherein each mesa having length of 100 microns or less, comprising:
placing the wafer in a CL microscope;
performing a first scan of the wafer at a first resolution to generate a plurality of CL beams;
using a plurality of single point detectors to simultaneously detect the plurality of CL beams, wherein each of the point detectors is arranged to receive a different partial spectrum than the remaining point detectors;
generating a plurality of detector ratios, wherein each of the detector ratios comprises a ratio of signals received from two of the point detectors; and
comparing the detected ratios to expected ratios to determine properties of the mesas.
10. The method of claim 9 , wherein the first resolution is set to generate at least one pixel per mesa and up to 10×10 pixels per mesa.
11. The method of claim 9 , wherein one of the plurality of point detectors is arranged to receive spectrum of wavelengths higher than background wavelengths.
12. The method of claim 9 , further comprising using the signals received from the plurality of detectors to reconstruct the spectral emission at each of the scanned mesas.
13. The method of claim 9 , further comprising using the signals received from the plurality of detectors to determine asymmetry of CL emission at each of the scanned mesas.
14. The method of claim 9 , further comprising using the signals received from the plurality of detectors to determine intensity distribution at each of the scanned mesas and comparing to total population intensity distribution determined from all of the scanned mesas.
15. The method of claim 9 , further comprising using the signals received from the plurality of detectors to assign the scanned mesas into a plurality of groups.
16. The method of claim 9 , further comprising:
using the signals received from the plurality of detectors to identify regions of interest on the wafer;
performing a second scan of the regions of interest at a second resolution, higher than the first resolution, to generate an inspection signal; and,
using the inspection signal to determine peak emission per wavelength within each scanned regions of interest.
17. The method of claim 16 , further comprising using secondary electron detector to form secondary electron image of each of the scanned regions of interest.
18. The method of claim 17 , further comprising using image processing to determine physical shape of each mesa within the scanned regions of interest.
19. The method of claim 17 , further comprising inspecting the secondary electron images to identify contaminants within the scanned regions of interest.
20. The method of claim 16 , further comprising assigning a viability score to each of the scanned regions of interest.