Optical Metrology

From micro to macro:
NIR sensors and imaging
spectroscopy – a perfect match
Bob Grietens, XenICs NV, Leuven, Belgium
Image capture in the near-infrared spectrum (NIR) at wavelengths up to
2.5 µm is gaining ever greater economic significance. This is especially true
in regard to imaging spectroscopy: hyperspectral microscopes, for example, can accelerate the design process of new kinds of LED structures; multispectral online inspection of recyclable materials ensures unmixed waste
separation [1]; and finally, in the arts, multispectral reflectography serves
as a tool for revealing the secrets of old masters and their works.
With the introduction of InGaAs as adetector material ten years ago, infrared
image capture moved out of the researchlaboratory and is now conquering more
and more industrial applications - even in
cost-sensitive areas. InGaAs array sensors
deliver good results in the near infrared up
to 1.7 µm, even at room temperature, andwith thermo-electrical (TE) cooling they canbe operated up to a wavelength of 2.5 µm– a real alternative to HgCdTe detectors.

1 Sensor array technology
The manufacture of InGaAs sensor
arrays is based on a metal-organic
chemical vapour deposition (MOCVD)
process, which lays down thin layersof InGaAs on substrates with matching crystal lattice constant, such as
InP. Due to the similarity with standard
silicon semiconductor manufacturing
processes, this can be done cost-effectively and at high yields. To form theback pixel electrodes, openings are
etched into the insulating glass layerusing optical lithography. After dicing
and a functional test of the sensor
chips, their pixel grids are interconnected with the readout circuitry.
Analogue or digital readout circuitry
and logic functions, on the other hand,are of course implemented in CMOS
technology. Therefore, sensor array andreadout circuitry must be produced indifferent technologies and then interconnected in some multipolar scheme.
Figure 1 shows five different ways of
doing this: first as a direct hybrid circuit, which connects the sensor arrayvia bump bonding (solder balls) with Photonik international · 2008/1

Figure 1: Various technologies for interconnecting a photoelectrical array sensor
with a readout circuit
the readout circuitry; second, as anindirect hybrid with array and readout
circuitry placed on a common connecting plate; third, as a sensor/readout
IC with wire interconnect for signalprocessing; fourth, as a three-dimensional structure carrying the sensor
array on the edges of a stack of thinreadout circuits (“Z technology”); andfinally, as a module with multipole
flip-chip interconnect between sensor
array and readout circuitry (“loopholetechnology”). The method used in theInGaAs sensors from XenICs uses flipchip bonding (bump bonding). Becausethe sensor and evaluation circuit lie on
top of each other, this allows the construction of especially compact cameras with minimum area usage.
As a protective measure, this chip combination is housed in a flat package witha window of NIR-transparent glass. The
module is then combined with infraredlens, connector elements, softwareand optional thermo-electric cooler
to form a complete camera system.
Originally published in German in Photonik 1/2007

Optical Metrology
Figure 2:
The path frommonochromeimage to multi­
spectral colour
capture to hyper­
spectral data cubeFigure 3:
Working principleof the spectral
camera: a combi­
nation of imag­
ing spectrograph(ImSpector fromSpectral Imaging)and matrix camera
2	Hyperspectral analysis
technology
Imaging spectroscopy or hyperspectral
analysis is a successful combination ofspectroscopy and image processing. As
shown in Figure 2, it can be seen as
an extension of classic image processing. In the most simple configuration,a black-and-white camera captures
a greyscale view of objects - at highspatial resolution but without delivering any spectral information. Next, acolour camera with three image sensors, or one sensor with a Bayer colour
filter, does considerably better because
it delivers a multispectral image at acomparably high spatial resolution plus
three fairly broadband colour channels, for example for red, green andblue; but it offers only a relatively
low spectral resolution. Finally, systems
that capture spectral images either
work with one sensor and a tunable
narrow-pass filter for frequency selection inserted into the optical path, orthey apply the so-called push-broom
scanner principle. A push-broom scanner, usually via mechanical feed of thetest object or spectrometer, performs aline scan of the object. For every image
element in a particular line, the spectrum is captured and then stored in a
pixel column of the detector.
Both methods depend on their large
number of colour channels, which is
why they are called “hyperspectral”:
They deliver high spatial as well as highspectral resolutions. The values for theX and Y coordinates and their spectral
Originally published in German in Photonik 1/2007

components are situated in a threedimensional data space (cube), as indicated in the top right of Figure 2.
NIR image sensors, due to their highresolution and broadband nature, arevery well suited for this type of hyperspectral analytical equipment – going
far beyond the mere imaging function
in a camera. Figure 3 shows the working principle of a direct-view spectral
camera with a grating as the dispersive
element. This combination of integrated spectrograph and monochromematrix camera basically functions asa line camera that fans out the spectrum. For each one of its sequentiallyorganized linear pixels, the spectrum isgenerated and captured as an intensity
pattern on the vertical axis (spectrumaxis) of the sensor array. The horizontal
axis is kept as the geometry axis.
By moving the object or the camera, atwo-dimensional image can be generated, which will yield certain properties
based on its spectral intensity distribu-Fig. 4: The structure of the hyperspec­
tral microscope at Twente Universitytion. This, in turn, can be used for classification purposes or for process control. But unlike using a spectrometer,
this spatial resolution method delivers
image information as well.
3 H yperspectral microscope
accelerates the design ofspecialty LEDs
Oftentimes, topically relevant researchand development projects act as drivers for innovations in related technicalequipment. At Twente University, forexample, ongoing research investigates
the impact of electrical fields on lateral
LED structures. This is done by placing
an additional gate electrode on top of
the active light-emitting region.
Now, operating this diode with constant current and negative bias at thegate electrode lets it shine brighter,and the intensity profile of the radiating surface also changes substantially.
Thus, the light emission of an LED canbe specifically varied via an MOS gate
– enabling innovative applications inillumination, measurement and automation technologies.
For a thorough understanding of theunderlying effects, such structures
must be modelled and compared withexperimental results. The more precise
and informative these comparative
measurements are, the more accurate
the modelling will be. Often it is not
sufficient to measure just the intensity
across the individual area elements:
The measured intensity must be complemented by the spectral distributionof the radiation at a sufficient spatial resolution. Solving this problemtherefore requires the methodology ofimaging spectroscopy in micrometers.
With this in mind, Twente University and XenICs have jointly developed
the mobile hyperspectral microscope(HIMS) shown in Figure 4. It is particularly well suited for research workon light-emitting semiconductor structures that radiate in the near infrared.
By means of a beam splitter, about athird of the emission is directed toward
a laterally positioned CCD camera,
which serves to align the microscopeand select the object’s surface ofinterest. The IR camera is mounted
on this structure with a transmissionspectrograph (Figure 3) from Finnish
manufacturer Specim preceding it. The
water-cooled (fan-free and light) camera head can be moved line by line
25 mm above the test object via astepper motor.
Photonik international · 2008/1 Optical Metrology

Figure 5: Significant spectrum content
of a point of the LED in the crosshairs
The spectrograph’s dispersion element
is a grating with more than 50%
transparency and a spectral resolutionof 5 nm in the NIR range from 900
to 1700 nm. The microscope’s IR lensprojects the area to be measured on
the test object onto the entry slit of
the spectrograph. There, the incoming
light is spectrally dispersed and thespatial axis as well as the orthogonal
spectral axis are coupled out througha lens onto the NIR array sensor.
One of the first images taken with this
hyperspectral microscope is shown in
Figure 5. It shows the spectral analysisof an LED structure in the ENVI (environment for visualizing images) realm.
To the left is a three-channel false-colour depiction, with an arbitrarily selected analysis coordinate. To the right isthe pertinent spectral profile. It can beclearly seen that a mere three-channel“multispectral” analysis conveys only
very sketchy and limited information.
This examination method is applicable
to all materials and structures that canbe pictured through a microscope andare somehow characterised by theirspectral information. The instantaneous and informative results yieldedby this method can help accelerate
the designing of new products andquicken the pace of innovation.
4	Secrets of old masters revealed
by hyperspectral analysis
Besides their obvious industrial applications, NIR array sensors can also be
advantageous in the scientific realm,
as an art-related example will prove.

For a long time, infrared images have
served as analytical tools in art history,
in the restoration and authenticationof art works, or to assign anonymousworks to specific periods.
Moreover, infrared analysis is often
used to determine whether there areunderdrawings present in oil and tempera paintings (Figure 6). Due to its
low IR absorption, longer-wave radiation offers a simple and, above all,non-destructive method of “lookingthrough” the upper layers of a painting to recognize and evaluate anyunderlying structures, drafts or previous versions.
Underdrawings are outlines lying
directly beneath a painting, often
done in charcoal. At a later stagethese drawings are mostly hidden by
the final layer of paint. Painters use
underdrawings in their own individual
ways: from simple perspective layouts
to detailed sketches. Underdrawings
can thus provide deeper insights into
the creative process of the artist.
It very much depends on the absorptive
and reflective behaviour (Figure 7) of
the underdrawing and the overlying
layers of paint whether there is sufficient optical contrast to distinguishbetween them. Since the absorptionof most materials depends heavily on
the frequency, modern analysis procedures are tending toward using several
different spectral ranges by using multispectral reflectography. They evaluate
digital images taken in the realm of visible light, in the NIR between 0.8 und
1.1 µm, as well in the extended infrared
between 1.3 und 2.2 µm. Ideally, theseimages are overlaid on the computerand processed with appropriate algorithms to enhance contrast and uncover
any underdrawings for analysis.
When selecting a suitable image sensor for taking such digital IR pictures,therefore, the first priority is to makesure it offers sufficient sensitivity in theNIR range. At the same time, however,
the often difficult operating conditions
for such imaging tasks must be met,
and this requires
a light, low-cost
camera which iseasy to use.

Figure 7: Reflec­tion and absorp­
tion of light in
the paint layer ofa picture [3] Photonik international · 2008/1

Figure 6: The painting “La Jocondeimpudique” by Pierre Gilou as an
example of a possible underdrawingof the bosom, arms and hands of theMona Lisa [2]5 Conclusion
Thanks to their inherently advantageous properties such as uncooled or
TE-cooled operation, low operating
costs, high sensitivity, small dimensions
and high usage flexibility, InGaAs NIR
image sensors enable innovative solutions in the most diverse areas of application. Thus they are establishing newtrends as well as new benchmarks formore economical processes in design,
manufacturing, process control andservices.
References:[1]	Bob Grietens, Spektrale Schatzsuche im NIR mit
ungekühlten InGaAs-Kameras, Photonik 1/2006,p. 78-79.
[2]	Jean Margat, Le mythe de la Joconde, ISBN 28289-0547-0, p. 45.
[3]	Chad Weiner, Improved Acquisition Technique ofUnderdrawings in Oil Paintings using IR-Reflecto­
graphy, www.cis.rit.edu/research/thesis/bs/1999/
weiner/thesis.html

Author contact:Bob Grietens
CEOXenICs NV
Ambachtenlaan 443001 Leuven
Belgium
Tel. +32/16/389900
Fax +32/16/389901
eMail: bob.grietens@xenics.com
Internet: www.xenics.com

Originally published in German in Photonik 1/2007