Engineering
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In Depth Analyses of LEDs by a Combination of X-ray Computed Tomography (CT) and Light Microscopy (LM) Correlated with Scanning Electron Microscopy (SEM)
Chapters
Summary June 16th, 2016
A workflow for comprehensive micro-characterization of active optical devices is outlined. It contains structural as well as functional investigations by means of CT, LM and SEM. The method is demonstrated for a white LED which can be still be operated during characterization.
Transcript
The overall goal of this multimodal micro-characterization procedure is to provide a method for the location-dependent correlation of the data acquired by X-ray Computed Tomography, Light Microscopy and Scanning Electron Microscopy. This method can help answer key questions in the micro-characterization field such as failure analysis or reverse engineering of microelectronic devices. The main advantage of this technique is that optical properties of a sample may be linked to the microstructure and even to submicrometer structural details.
The implications of this technique extends toward prevention of device failure because optical defects or in homogeneities can definitively and traceably be linked to structural or electrical defects of the devices. So this method can provide insight into microelectronic devices. It can also be applied to other structural analysis such as characterization of composite materials.
Begin this procedure with simple preparation and CT measurement setup as described in text protocol. Select the region of interest or ROI by identifying the area not obscured by the measured object during one full rotation. In the measurement window with the live image press and hold the left mouse button and draw a red frame window.
Right-click on this window's frame to open a context menu. Then select Set as Observation Window. The frame's color will change to yellow and the observation window will be fixed in the measurement window.
Activate the software module Auto-scan Optimizer, through which nine images are taken before the actual scan of the sample. These are images are taken in 40-degree steps while rotating the sample. Simultaneously, activate the module Detector Shift routine.
The simultaneous activation of these two modules before starting the actual CT scan ensures correction for movements of the sample and for ring artifacts. Next, scan the sample by starting the Data Acquisition routine in the acquisition software. Transfer the reconstruction data to the CT Data Analysis software and align the sample in X-Y, X-Z and Y-Z planes using the simple registration function in the software.
Apply median filtering using a filter size of 3. For micro preparation embed LED in epoxy resin using transparent supports. Drill two small holes on opposite sides of the support and feed the silver wire through it.
At this point it's crucial the carefully position of the sample. If the deviation from the ideal position is defined by the CT scan as too big, electrical failures may result and the protocol will have to be started from the beginning. Position the LED by means of tightening or loosening the silver wire to align the front edge of the LED and the support.
Fill the ring with epoxy inside a silicon beaker pre-treated to ensure that it will not stick to the epoxy. Then, allow the epoxy to harden. Mechanically remove any excessive resin by grinding with coarse-abrasive paper.
Using a stereo microscope, visually ensure that the support and LED are aligned, then fix the LED embedded in the epoxy resin in a planer fashion to a simple holder for precision grinding. Use a grinder with abrasion measurement and remove the sample surface up to 100 microns from the targeted plane's position. After polishing, observe the smooth and scratch-free surface using a stereo microscope.
Clean the specimen with de-ionized water and cotton pads, then remove the water by rinsing with ethanol, and drying using a hair dryer. Melt the specimen in an appropriate sample holder for Correlative Light and Electron Microscopy, or CLEM for short. Ensure that the sample holder fixes the sample for use in LM, sputter coater and SEM.
Adjust the calibration marks to the same height as sample's surface. Ensure that the polished surface is parallel to the focal plane of the LM.Next, fix the sample holder on to the motorized X-Y stage of the LM.Connect the LED to the power supply. The power supply should operate in constant current mode.
Using the microscope software, calibrate the sample holder position on the X-Y stage by saving the position of calibration marks as reference points. Move the X-Y stage of the LM such that the ROI of the sample is in the field of view of the LM.Switch on the power supply and tune the LED emission. Switch off LM illumination and adjust the exposure time of the LM camera.
Obtain the LM image of the light distribution within the sample. If applicable, image luminescence together with other contrasts by activating LM Illumination and LED simultaneously. Save all LM images together with the corresponding stage position as described in the user manual.
Prepare for SEM analysis by sputter coating the sample and performing setup as described in the text protocol. Move the stage to show the ROI on the sample and perform SEM analysis in the same location as done for LM.Select Backscattered Electron Detector for material contrast. For this sample, choose an Electron Energy of 20 keV.
Set the aperture to 30 microns and position the sample at a working distance of 8.7 mm. Select Secondary Electron Detector for microstructure surface imaging. Then change to EDS Detector for element mapping.
For this sample, select an aperture of 60 microns to increase the beam current, and position the sample at a working distance of 9 mm. Representative results for the Micro-characterization of an electrically operable LED are shown here. Here is the specimen prepared for a CT scan, with chip surface of 1 squared mm.
Reconstructive CT volume information enables planning of cutting planes for micro-preparation. By carefully positioning the cross section of the sample, electrical operability after partial operation is ensured. Information of the luminescence behavior is gained by optical microscopy of the electrically-driven LED.
Blue emission of the semiconductor is well distinguished from red and yellow emission of the phosphors using this device. Overlay of the light microscopy images with SE Microgras through Correlated Microscopy enhances information depth. In this example, the microstructure of the two phosphors is revealed.
Further information may be gained by energy Dispersive X-ray Fluorescence Spectroscopy overlays. Once mastered the fluid device characterization including all models shown can be done in 48 hours if it's performed properly. While attempting this procedure it's important to remember to check repeatedly for device operability during cross section preparation.
Following this procedure, other methods like Fluorescence Microscopy can be performed in order to answer additional questions like excitation and emission behavior of the phosphors. This technique enables researchers in the field of micro-characterization to explore electro-optical devices in full function.
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