ELECTRON DEVICE LETTERS, VOL. XX, NO. YY, MONTH YEAR 1
Abstract—We report on the fabrication of Extreme Ultraviolet
(EUV) hybrid imagers with backside illuminated detector chip
based on aluminum gallium nitride (AlGaN) layers grown on
silicon and integrated with CMOS readout chip. The Focal Plane
Array (FPA) size is 256x256 pixels with 10 µm pixel-to-pixel
pitch. The devices were characterized at wavelengths from 300
nm down to 1 nm using synchrotron radiation. An upper cut off
wavelength of 280 nm was observed, as expected from the AlGaN
active layer composition (40% Al). Thus, the imagers have a high
rejection ratio of the near UV and visible radiation. Moreover, no
degradation due to proton irradiation was observed for 60 MeV
energy and 5·1010 protons/cm2 dose. Furthermore, devices with an
intentionally left thin silicon substrate layer were fabricated and
response only in the EUV range was observed. These results
demonstrate the possibility of achieving high resolution EUV
imaging with AlGaN-based FPAs, which is very promising for
high-end industrial, scientific and space applications.
Index Terms—AlGaN, backside illumination, hybrid imager,
ultraviolet imaging, EUV, flip-chip, high resolution
XTREME ultraviolet (EUV) imaging is crucial in solar
observations, as most phenomena on and near the surface
of the Sun can be observed in the low wavelength range (1 to
100 nm) . Silicon is the core of a majority of imagers used
in the current solar missions. However, wide bandgap
semiconductors, such as materials from the aluminum gallium
nitride (AlGaN) system can offer significant advantages,
Manuscript received March 29, 2011. This work was supported by
European Space Agency (ESA/ESTEC Contract #19947/06/NL/PM, BOLD).
P. E. Malinowski, P. Srivastava, R. Mertens and C. Van Hoof are with
imec, Leuven, Belgium, and the Katholieke Universiteit Leuven, Belgium
(phone: 0032-1628-1938; e-mail: email@example.com; firstname.lastname@example.org;
P. De Moor, J. John, K. Minoglou, are with imec, Leuven, Belgium (e-
mail: email@example.com; firstname.lastname@example.org; email@example.com).
J.Y. Duboz, F. Semond and E. Frayssinet are with CRHEA/CNRS,
Valbonne, France (e-mail: firstname.lastname@example.org;
A. BenMoussa and B. Giordanengo are with the Royal Observatory of
Belgium, Brussels (e-mail: Ali.BenMoussa@oma.be; email@example.com).
U. Kroth, A. Gottwald and C. Laubis are with the Physikalisch-Technisch
Bundesanstalt (PTB), Berlin, Germany (e-mail: Udo.Kroth@ptb.de;
provided the technology is mature enough to be considered
competitive [2-4]. First of all, AlGaN can have a tunable
energy gap, depending on the Al molar fraction, from 3.4 eV
up to 6.2 eV for 0% and 100% Al, respectively. This translates
to the cut-off wavelength in the UV range (365 nm down to
200 nm, respectively) and insensitivity to the visible and
infrared radiation, reducing the number of filters needed in the
system (e.g. a telescope). Furthermore, AlGaN compound is
known to be more radiation hard, which is significant
considering long mission lifetimes in harsh environments [5-
7]. Additionally, due to low leakage currents, AlGaN devices
can be operated at room temperature, reducing or eliminating
the need of complicated cooling systems. On Earth, EUV
imaging finds applications in industry, for example for
monitoring the beam in the EUV lithography stepper systems.
AlGaN-based hybrid imagers were demonstrated by several
groups [8-11]. Most of the devices used epitaxial layers grown
on sapphire, allowing illumination through the substrate, but
limiting the sensitivity range to Deep UV (lower transmission
limit around 150 nm). An AlGaN-on-Si-based imager with a
response down to the wavelength of 10 nm after removing the
silicon substrate with Reactive Ion Etching (RIE) was
presented by Reverchon et al. . Our group used a similar
approach, however, for arrays with higher resolution (10 µm
pitch ). Here, further synchrotron measurements and
proton irradiation are discussed. Device demonstrators
presented in this paper were developed in the framework of the
Blind to Optical Light Detectors (BOLD) project from the
European Space Agency (ESA), with the target application
onboard the planned Solar Orbiter mission.
AlGaN layers were grown by Molecular Beam Epitaxy
(MBE) on 2 inch Si(111) wafers. After AlN nucleation layer
(40 nm), the Al concentration was graded down to 40% over
300 nm to provide a thin active layer of the detector. On top,
100 nm of Al0.4Ga0.6N was grown with Si doping, resulting in a
carrier concentration of 1019 cm-3 to provide a good ohmic
contact. The idea is to have the ohmic contact on the top of the
stack and the Schottky contact in an etched mesa. This unsual
inverted structure is motivated by the backside illumination
and the necessity to avoid absorption in a doped layer .
Array modules were fabricated together with single pixel
AlGaN-on-Si-based 10 µm pixel-to-pixel pitch
hybrid imagers for the EUV range
Pawel E. Malinowski, Jean Yves Duboz, Piet De Moor, Joachim John, Kyriaki Minoglou, Puneet
Srivastava, Fabrice Semond, Eric Frayssinet, Boris Giordanengo, Ali BenMoussa, Udo Kroth,
Alexander Gottwald, Christian Laubis, Robert Mertens, and Chris Van Hoof
ELECTRON DEVICE LETTERS, VOL. XX, NO. YY, MONTH YEAR 2
photodiodes with varying diameters. First the mesa was etched
by Cl2-based RIE. Then, Au Schottky pixels were deposited.
On the n-doped layer, a stack of Ti/Al/Mo/Au (10/40/25/50
nm, respectively) was deposited and annealed for 1 minute at
850°C. This ohmic contact was a common grid for the arrays
and a donut around the mesa for the single diodes. 100 nm of
SiO2 was used before depositing contact level of TiW/Ni/Au
(10/150/150 nm, respectively), acting as the Under Bump
Metallization (UBM) for the In bumps deposited on each pixel
of the array. The same UBM and In steps were performed on
the CMOS readout wafers. The readout had two Capacitive
Transimpedance Amplifier (CTIA) blocks, with one column of
the detector array fixed to one block and the remaining 255
columns connected to the other one through a switching
matrix. This allowed fabrication in a cost-effective, 0.35 µm
technology. Detector and readout chips were integrated by
flip-chip bonding and the Si substrate under AlGaN was
removed by SF6/C4F8 RIE with a hard mask (Al). The resulting
device had a 2.56x2.56 mm2 AlGaN membrane 400 nm thick
(250 nm only in MESA regions), supported by a 10 µm pitch
array of In bumps, with 100 dummy bumps on all sides of the
array to improve the mechanical stability (Fig. 1). After
processing, the imagers were packaged and wire-bonded.
In some samples, the RIE was stopped as soon as the first
openings in Si appeared (AlGaN layer was accessed). Since
the substrate was not backside polished, the etching was not
uniform, resulting in circular patterns of the initial openings
(Fig. 2). This was done to investigate the possibility of keeping
an additional layer on the back of the AlGaN active layer to
act as a backside contact or an integrated filter.
After front-side processing, the single diodes were measured
to assess the photodetector performance. A cut-off wavelength
of 280 nm was observed, with 3 orders of magnitude rejection
ratio of the 400 nm radiation. Selected devices were
characterized at PTB using the Metrology Light Source (MLS
), showing sensitivity down to the wavelength of 120 nm
(Fig. 3). High responsivity below 160 nm was attributed to the
contribution of photoemission from the Au Schottky electrode.
Synchrotron measurements of the complete imagers
revealed a spectral sensitivity similar to the one measured on
single diodes, with a different trend only below 160 nm.
Response was calculated by taking the signal from the active
pixels exposed to the incident radiation (Fig. 3a). Additionally,
sensitivity in the EUV range was verified in the Grazing
Incidence (GI) beamline of PTB at the BESSY II synchrotron,
allowing measurements down to the wavelength of 1 nm. The
EUV spectral sensitivity measured followed the shape of
irradiance measured with a reference diode (Fig. 3b).
Imagers with the Si openings were first investigated under
the optical microscope. It was observed that the remaining
substrate is transparent in the visible light in most of the area
(Fig. 4b), indicating a thickness of maximum several
micrometers. Under illumination with higher wavelengths, the
imager response was observed only for pixels corresponding to
the area where the substrate was completely etched, as can be
seen in Fig. 4c for 240 nm (selected openings are highlighted).
This was expected, since the transmission of Si in this range is
close to zero. However, under illumination with the
wavelength of 1 nm, the same imager showed response in most
pixels, indicating that the remaining substrate was thin enough
to allow sufficient penetration of the incident radiation into the
AlGaN active layer. This corresponded well to the increased
transmission of the substrate at this wavelength (~40% for the
thickness of 5 µm and 90% for 1 µm ). Furthermore, signal
from majority of pixels in the imager suggested good
interconnect yield. The non-uniformity of response can be
attributed to many factors, such as front-side processing of the
detector chip, interconnects, thickness of the remaining
substrate and shape of the incident beam.
Selected photodiodes and imagers were also irradiated with
60 MeV protons up to a dose of 5·1010 pr/cm2. For single pixel
devices, degradation of neither the dark current nor the
spectral responsivity was observed. Furthermore, the imager
showed the same insensitivity to proton irradiation.
Interestingly, some increase of the dark signal of the readout
Fig. 2. (Color online) Hybrid imager with a thin silicon layer left on top of
the AlGaN active layer: (a) conceptual schematic; and (b) top view optical
microscope picture with the 1-mm-wide substrate frame around the FPA.
imager response / a.u.
wavelength / nm
diode responsivity / A/W
imager response / a.u.
wavelength / nm
power / W
Fig. 3. Response of the imager calculated from the images acquired at the
PTB synchrotrons with two beamlines: (a) Normal Incidence (NI, 115 –
330 nm, MgF and SiO2 filters), compared to a diode from the same wafer;
and (b) Grazing Incidence (GI, 1 – 33 nm).
Fig. 1. Cross-section schematic of the hybrid imager with AlGaN-on-Si FPA
(top) integrated with CMOS readout (bottom). Image not to scale.
ELECTRON DEVICE LETTERS, VOL. XX, NO. YY, MONTH YEAR 3
was observed in the samples where no detector chip was
bonded. This shows that 1) Si is sensitive to irradation, 2)
AlGaN is not (or less) sensitive to irradiation 3) the AlGaN
layer protects the Si ROIC from proton irradiation which
validates the hybrid array approach for irradiation hardness.
Backside illuminated hybrid imagers with an AlGaN-on-Si
detector chip and a CMOS readout chip integrated with high
density interconnects (10 µm pixel pitch) were fabricated.
Sensitivity down to the wavelength of 1 nm was observed. A
thin layer of remaining silicon substrate blocked radiation for
wavelengths higher than approximately 20 nm but allowed
EUV imaging. Response at 1 nm allowed to verify the
interconnect yield and demonstrated the feasibility of high
resolution AlGaN imagers
applications. Operation with a thin backside layer is promising
for the next generation imagers, where the remaining silicon
could be substituted by another material, e.g. a backside ohmic
contact (for a vertical pixel structure) or an integrated filter
(for better suppression of higher wavelengths and other
incident particles). AlGaN imagers are a promising alternative
for the commonly used silicon devices.
for the low wavelength
Authors would like to acknowledge the support of the
European Space Agency (ESA-BOLD #19947/06/NL/PM).
Gratitude is expressed to D. Frederickx (imec) for packaging
and T. Torfs and Y. Creten (imec) for the readout design.
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and Technology database:
Fig. 4. (Color online) Imagers with a thin silicon layer: optical microscope
images, top view of (a) middle 128x128 pixels area; (b) zoom into the edge
of one opening; and images corresponding to the area in (a): (c) 240 nm and
(d) 1 nm. Brighter pixels correspond to higher response.