July/August 2007

Electronic Materials & Packaging

Imaging Spectroscopy for Microelectronics
Designing innovative LED structures with a Hyperspectral Imaging
Microscope System
By Bob Grietens, XenICs, and Jisk Holleman, University of Twente
Imaging spectroscopy has proved its worth in the acquisition of geological parameters from the air or by satellite in order, forexample, to answer environment-related questions such as relating to the water quality of lakes. On a microscopic scale, themethod can be applied to good advantage for multi-channel spectral analysis of light-emitting semiconductors in the design ofinnovative LED structures. For this purpose, XenICs has collaborated with the University of Twente in the development of a hyperspectral imaging microscope system (HIMS), which allows detailed examinations in the visible and near infrared range withhigh precision. Early measurement results on LED structures, achieved through the new imaging device, and an evaluation of
the hyperspectral scanner images using the X-Cube software clearly show the viability of this method for microelectronic applications.

Light Emitting Semiconductor Structures
Light emitting semiconductor structures have been used for along time in the form of LEDs which, due to their increasing
efficiencies, have made substantial inroads into illumination
technology, competing with traditional lighting systems as wellas enabling innovative illumination concepts. Despite their
obvious maturity they still have tremendous potential for further development. For example, at Twente University, thefaculty for Electrical engineering and the MESA+ Institute for
Nanotechnology have jointly investigated the impact of electrical fields on lateral LED structures as shown in Figure 1.

measurement and automation technologies. To understandthe basic mechanism of this, the structure must be modeled
and compared with the experimental results. The more precise and meaningful the results of this comparison are, themore accurate the modeling will be.

In this structure, a gate electrode of poly-silicon has beenplaced above the active, light emitting region. Operating thediode with constant current and a negative gate bias willcause it to shine brighter. Additionally, the intensity profile of
the radiating surface is significantly changed. Thus, the lightemission of an LED can be specifically varied via an MOS
gate. This enables novel applications in the illumination,

Hyperspectral Analysis
Basically, imaging spectroscopy, or hyperspectral analysis, isa happy fusion of spectroscopy and imaging processing. It isan outgrowth of the demands in the exploration of mineraldeposits and it has since proven to be an efficient exploration
tool from the air and by satellite in the most diverse tasks,
such as water quality analysis of Bavarian lakes, the ecology

Often the indication of mere intensity data across the various
surface elements is not sufficient: intensity must be complemented by the spectral distribution at the locations considered. For a solution of this problem, the methods of spectral
imaging on a micrometer scale are required.

Figure 1: Infrared image of an LED at 5mA forward current,a) illuminated from above, at 0.3msec integration timeb) without external illumination, at 0.1sec integration time.

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Electronic Materials & Packaging

July/August 2007

Figure 2: Evolutionary path from monochrome image through multi-spectral color capture to hyperspectral data cube.
of oyster banks in Brittany, France, the measurement of CO2
emissions from factory chimneys as well as desert formationin the South of Spain.
Imaging spectroscopy can be seen either as an extension ofclassical image processing or an enhancement of classicalspectroscopy. To begin with the former, let's take a look at
figure 2. The simplest case is using a black and white camerathat captures the grey-scale values of objects - yielding highsurface resolution but no spectral information (besides weighing the spectrum by its own spectral sensitivity curve). A color
camera, on the other hand, with three image sensors or asensor with Bayer color filter, will deliver a multi-spectral image with comparably high spatial resolution and three relatively broad-band color channels of red, green and blue, yetwith a relatively low spectral resolution.

Hyperspectral Microscope
The HIMS hyperspectral microscope was jointly developed by
XenICs and Twente University. It is to be used for research
work on light emitting semiconductor structures in the infrared(IR) region. Figure 3 shows its essential elements. The microscope's lens evidently is designed to work in the IR. A beamsplitter deflects about a third of the emission to a laterally positioned CMOS-camera, which serves as a tool to direct themicroscope and select the surface portion of interest on thetest object. The IR camera can be positioned on top of this,directly or together with a spectrograph, as Figure 3 indicates.
The IR camera and its InGaAs sensor array are cooled by aFinally, a spectral imaging system - it operates with just one
sensor and a tunable narrow-band filter placed in the opticalpath to select a frequency. Alternatively, it functions as a socalled push-broom scanner to perform, usually via mechanicalfeed of test object or spectrometer, a line-based scan. For
every pixel in every line, the spectrum is captured and stored.
Both methods use significantly more color channels. This is
why they are called hyperspectral: they deliver high spatial
resolution and, at the same time, high spectral resolution. The
measured data of the X- and Y-coordinates and the radiation
components at certain frequencies are located in a threedimensional data space (cube), as indicated in the upper rightof figure 2.

Figure 3: Construction of the hyperspectral microscope atTwente University.

©Copyright 2007 RNR Associates, England. All rights reserved.

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July/August 2007

Electronic Materials & Packaging

ries the PC for the control and evaluation programs, as well asa chiller for the camera's water cooling.
The spectrograph, of Finnish manufacturer Specim, operates
in transmission mode - it doesn't require any movable mechanical parts. The dispersion element is a grating with a highpermeability of more than 50% and a spectral resolution of5nm in the NIR area from 900 to 1700 nm. The IR lens of themicroscope projects the surface to be measured onto thespectrograph, which performs a spectral dispersion of theimpending light and projects the spatial axis as well as theperpendicular spectral axis via a coupling lens onto an NIR
surface sensor.

Figure 4: High-resolution, water cooled InGaAs FPA camera ofthe XenICs XEVA Series.
water-cooled Peltier element. This offers at least two advantages: it works within the camera head without needing a fan.
For the user, this lowers the vibration level., and the camera
itself is of lighter weight. After all, it is to be moved up to25mm across the test object by a microfeed with stepping
motor to do a line scan.
To be more flexible in its applications, the hyperspectral microscope is constructed as a movable unit. In its base, it car-NIR Cameras from XenICs
XenICs is one of the few vendors specializing in NIR image
capture and cameras for extremely diverse applications. As aspin-off from IMEC, the leading European research center for
microelectronics and nanotechnologies located in Leuven,Belgium, XenICs can draw on a multitude of patented technologies and know-how transfers from its shareholder IMEC.
The software-configurable NIR digital camera (Figure 4) usedin the HIMS, with its InGaAs focal plane array covers the standard wavelength area from 900 to 1700nm. Pixel pitch is
30µm at a pixel availability of more than 98 %.
One of the development goals of the hyperspectral micro-Figure 5: Software environment of the hyperspectral microscope for direct capture and spectral measurement.

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Electronic Materials & Packaging

July/August 2007

Figure 6: Measurement results for one given point of the LED in Figure 1 (in the crosshairs).

scope was to achieve the same geometric resolution in both
X- and Y directions. While pixel pitch determines X-axis resolution, the resolution of the Y axis is tracked by appropriately
driving the micro-feed step motor. The camera head communicates with a PC via standard data interface such as USB 2.0
for the control signals, and Cameralink or a parallel LVDS bus
for the 14-bit image data. The camera is quickly integrated todifferent application environments through its standard lens Cmount and spectrometer mount.
A Multitude of Measuring FunctionsThe hyperspectral microscope is universally applicable in theexamination of light emitting semiconductors. Figure 5 shows
two different measurement setups for direct surface captureand spectral analysis through the spectrograph. Software support in the case of light emitters is provided by the X-Control
graphical user interface, which comes with every camera. XControl enables direct access to various camera settings,
such as integration time or operating temperature. It also contains software tools such as two-point uniformity correction
and bad-pixel replacement.
The laterally mounted CMOS camera permits adjusting andselecting the scan area for the hyperspectral examination,
which is done by a parameter definition. In the configuration
shown on the right this is a very time consuming task. Here,too, X-Control is used for parameter adjustment whereas theX-Cube software tool controls the scan procedure und generates a three-dimensional data set of X-, Y-, and spectral coordinates in *.RAW format. Finally the evaluation is performed
within ENVI (Environment for visualizing images), which con-tains finely tuned and proven programs as well as thousandsof options.
First Results of an LED Examination
One of the first captures performed by the hyperspectral microscope is shown in Figure 6: a spectral analysis of the LEDstructure according to Figure 1 in the ENVI environment. At
first in three-channel false-color rendering, followed by fourfold magnification and selection of an appropriate analysis
coordinate and finally the spectrum profile. It is clearly visiblethat a three-channel multi-spectral analysis has only a very
marginal meaning.
OutlookThis examination principle is applicable to all materials andstructures that fit under a microscope and are in any way
marked and identified by specific spectral information. The
method's quick and meaningful results can substantially contribute to a shortening of the design procedure of new productintroductions and thus accelerate the pace of innovation.
Contacts
BOB GRIETENS is CEO of XenICs (Leuven, Belgium)
e-mail: bob.grietens@xenics.be, Tel: +32 (16) 389900
www.xenics.com
JISK HOLLEMAN is an associate professor in the electricalengineering department at the University of Twente(Enschede, the Netherlands)e-mail: J.Holleman@utwente.nl.

©Copyright 2007 RNR Associates, England. All rights reserved.

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