In Depth Analyses of LEDs by a Combination of X-ray Computed Tomography (CT) and Light Microscopy (LM) Correlated with Scanning Electron Microscopy (SEM)

Summary

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.

Abstract

In failure analysis, device characterization and reverse engineering of light emitting diodes (LEDs), and similar electronic components of micro-characterization, plays an important role. Commonly, different techniques like X-ray computed tomography (CT), light microscopy (LM) and scanning electron microscopy (SEM) are used separately. Similarly, the results have to be treated for each technique independently. Here a comprehensive study is shown which demonstrates the potentials leveraged by linking CT, LM and SEM. In depth characterization is performed on a white emitting LED, which can be operated throughout all characterization steps. Major advantages are: planned preparation of defined cross sections, correlation of optical properties to structural and compositional information, as well as reliable identification of different functional regions. This results from the breadth of information available from identical regions of interest (ROIs): polarization contrast, bright and dark-field LM images, as well as optical images of the LED cross section in operation. This is supplemented by SEM imaging techniques and micro-analysis using energy dispersive X-ray spectroscopy.

Introduction

This article demonstrates the potential and advantages of a combination of X-ray computed tomography (CT) with correlative light and electron microscopy (CLEM) for the exemplary in depth characterization of light emitting diodes (LED). With this technique it is possible to plan the micro preparation of the LED in such a fashion that while a cross section can be imaged microscopically the electrical functionality is preserved in the remainder of the specimen. The procedure has several unique features: firstly, the planned micro preparation by aid of the rendered volume of the entire sample obtained by CT; secondly, the observation of the LED by light microscopy (LM) wi....

Protocol

1. Sample Preparation for X-ray Computed Tomography (CT)

1. Glue sample (LED cf. materials section) to a 2 mm Ø hollow carbon fiber bar of appropriate length using hot-melt adhesive.
2. Adjust position of specimen by using a hot-air gun if needed. Fix the sample in the CT-sample chamber using the three-jaw chuck.

2. CT Measurement Setup

1. Perform warmup and centering procedures according to the control software of the X-ray tube.
   NOTE: Use tube-control software of manufacturers' CT and standard protocol as specified by the supplier (cf. materials section).
2. Calibrat....

Results

The characterized LED is shown in Figure 1. It is a white emitting LED with a Chip size of 1 x 1 mm² and a partially ceramic luminescent color converter. Gluing the LED in a slightly slanted position onto a carbon fiber bar avoids CT artifacts caused by sample symmetry (Figure 2). Results of the CT measurement allow for planning the position of the cross section of the sample, and ensure electrical operability after partial abrasion (Fi.......

Discussion

The advantages of this multimodal approach consist in the location-dependent correlation of the acquired data. The multimodal approach described here should be contrasted in subsequent analyses with each technique separately. For example, luminescence properties visible in LM can be linked to compositions as detected using SEM/EDS. The volume information obtained by CT can be extended with in depth analyses of cross-sections prepared in a targeted fashion. CT data also enable fast location of possible areas of interest i.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
X-ray Computer Tomograph	General Electric	not applicable	type: nanotom s research edition
acquisition software	General Electric	not applicable	phoenix Datos| x2 acquisition and corresponding manual
reconstruction software	General Electric	not applicable	phoenix Datos| x2 acquisition and corresponding manual
rendering software	Volume Graphics	not applicable	VGStudio Max 2.2 and corresponding manual
grinder (manual)	Struers	5296327	Labopol 21
sample holder	Struers	4886102	UniForce
grinder (automated)	Struers	6026127	Tegramin 25
epoxy resin/hardener	Struers	40200030/40200031	Epoxy fix resin / Epoxy fix hardener
Ethanol	Struers	950301	Kleenol
Light Microscope	Zeiss	not applicable	Axio Imager M2m
Electron Microscope	Zeiss	not applicable	Sigma
CLEM software	Zeiss	not applicable	Axio Vision SE64 Rel.4.9 and corresponding manual
CLEM sample holder	Zeiss	432335-9101-000	Specimen holder CorrMic MAT Universal B
SEM Adapter for CLEM sample holder	Zeiss	432335-9151-000	SEM Adapter for Specimen holder CorrMic MAT Universal B
sputter coater	Quorum	not applicable	Q150TES
EDS detector	Röntec	not applicable	X-Flash 1106
solder	Stannol	535251	type: HS10
LED	Lumileds	not applicable	LUXEON Rebel warm white, research sample