Photodetectors designed for the Extreme Ultraviolet (EUV) range with the Aluminum Gallium Nitride
(AlGaN) active layer are reported. AlGaN layers were grown by Molecular Beam Epitaxy (MBE) on
Si(111) wafers. Different device structures were designed and fabricated, including single pixel
detectors and 2D detector arrays. Sensitivity in different configurations was demonstrated, including
front- and backside illumination. The latter was possible after integration of the detector chips with
dedicated Si-based readouts using high-density In bump arrays and flip-chip bonding. In order to avoid
radiation absorption in silicon, the substrate was removed, leaving a submicron-thin membrane of
AlGaN active layer suspended on top of an array of In bumps. Optoelectrical characterization was
performed using different UV light sources, also in the synchrotron beamlines providing radiation
down to the EUV range. The measured cut-off wavelength of the active layer used was 280 nm, with a
rejection ratio of the visible radiation above 3 orders of magnitude. Spectral responsivity and quantum
efficiency values
We report on the fabrication and characterization of solar blind Metal-Semiconductor-Metal (MSM) based
photodetectors for use in the extreme ultraviolet (EUV) wavelength range. The devices were fabricated in the AlGaN-on-
Si material system, with Aluminum Gallium Nitride (AlGaN) epitaxial layers grown on Si(111) by means of Molecular
Beam Epitaxy. The detectors' IV characteristics and photoresponse were measured between 200 and 400 nm. Spectral
responsivity was calculated for comparison with the state-of-the-art ultraviolet photodetectors. It reaches the order of 0.1
A/W at the cut-off wavelength of 360 nm, for devices with Au fingers of 3 μm width and spacing of 3 μm. The rejection
ratio of visible radiation (400 nm) was more than 3 orders of magnitude. In the additional post-processing step, the Si
substrate was removed locally under the active area of the MSM photodetectors using SF6-based Reactive Ion Etching
(RIE). In such scheme, the backside illumination is allowed and there is no shadowing of the active layer by the metal
electrodes, which is advantageous for the EUV sensitivity. Completed devices were assembled and wire-bonded in
customized TO-8 packages with an opening. The sensitivity at EUV was verified at the wavelengths of 30.4 and 58.4 nm
using a He-based beamline. AlGaN photodetectors are a promising alternative for highly demanding applications such as
space science or modern EUV lithography. The backside illumination approach is suited in particular for large, 2D focal
plane arrays.
We report on the results of fabrication and optoelectrical characterization of Gallium Nitride (GaN) based Extreme
UltraViolet (EUV) photodetectors. Our devices were Schottky photodiodes with a finger-shaped rectifying contact,
allowing better penetration of light into the active region. GaN layers were epitaxially grown on Silicon (111) by Metal-
Organic-Chemical Vapor Deposition (MOCVD). Spectral responsivity measurements in the Near UltraViolet (NUV)
wavelength range (200-400 nm) were performed to verify the solar blindness of the photodetectors. After that the
devices were exposed to the EUV focused beam of 13.5 nm wavelength using table-top EUV setup. Radiation hardness
was tested up to a dose of 3.3·1019 photons/cm2. Stability of the quantum efficiency was compared to the one measured
in the same way for a commercially available silicon based photodiode. Superior behavior of GaN devices was observed
at the wavelength of 13.5 nm.
We report on the fabrication of Schottky-diode-based Extreme UltraViolet (EUV) photodetectors. The devices were
processed on Gallium Nitride (GaN) layers epitaxially grown on 4 inch Silicon (111) substrates by Metal-Organic
Chemical Vapor Deposition (MOCVD). Cutoff wavelength was determined together with the spectral responsivity
measurements in the Near UltraViolet (NUV) range (200nm to 400nm). Absolute spectral responsivity measurements
were performed in the EUV range (5nm to 20nm) with the synchrotron radiation using the facilities of Physikalisch-
Technische Bundesanstalt (PTB), located at Berliner Elektronenspeicherring-Gesellschaft fuer Synchrotronstrahlung
(BESSY). The described work is done in the framework of the Blind to Optical Light Detectors (BOLD) project
supported by the European Space Agency (ESA).
Metal-Semiconductor-Metal photodiodes were fabricated on epitaxially grown AlxGa1-xN on Si(111). The Aluminium
content of the layers grown by means of molecular beam epitaxy (MBE) was 50, 80 and 100%, respectively. The
processing was performed by standard microelectronic fabrication techniques like photolithography, wet and dry etching
(RIE) and physical and chemical vapor deposition (PVD,CVD). The devices were characterized under illumination in a
wavelength range from 400 to 185nm to determine the cut-off wavelength defined by the band-gap energy. Typical
figures of merit like spectral responsivity R quantum efficiency &eegr; and specific detectivity D* have been extracted from
the measurement data.
The ultra-high density hybrid flip chip integration of an array of detectors and its dedicated readout electronics can be achieved with a variety of solder bump techniques such as pure Indium of Tin alloys, (In, Ni/PbSn), but also conducting polymers, etc. Particularly for cooled applications or ultra-high density applications, Indium solder bump technology (electroplated or evaporated) is the method of choice. The state-of-the-art of solder bump technologies that are to a high degree independent of the underlying detector material will be presented and examples of interconnect densities between 5e4/cm2 and 1e6/cm2 will be demonstrated.
For several classes of detectors, flip-chip integration is not allowed since the detectors have to be illuminated from the top. This applies to image sensors for EUV applications such as GaN/AlGaN based detectors and to MEMS-based detectors. In such cases, the only viable interconnection method has to be through the (thinned) detector wafer followed by a based-based integration. The approaches for dense and ultra-dense through-the-wafer interconnect "vias" will be presented.
Solar ultraviolet imaging instruments in space pose most demanding requirements on their detectors in terms of dynamic range, low noise, high speed, and high resolution. Yet UV detectors used on missions presently in space have major drawbacks limiting their performance and stability. In view of future solar space missions we have started the development of new imaging array devices based on wide band gap materials (WBGM), for which the expected benefits of the new sensors - primarily visible blindness and radiation hardness - will be highly valuable. Within this initiative, called “Blind to Optical Light Detectors (BOLD)”, we have investigated devices made of AlGa-nitrides and diamond. We present results of the responsivity measurements extending from the visible down to extreme UV wavelengths. We discuss the possible benefits of these new devices and point out ways to build new imaging arrays for future space missions.
Short wavelength infrared (SWIR) photovoltaic diode structures made of InGaAs material were grown on GaAs by means of molecular beam epitaxy. Growth quality and composition of the layers are determined by HRXRD. The electrical characterization is performed by Current-Bias characterization (proposal) and spectral resolved measurements to determine the resistance area product (R0A) and the spectral responsivity (R) of diodes. The processing is performed with standard photolithography and micro-structuring techniques aiming at the production of 1D and 2D infrared camera arrays. The diced IR sensor is flip chip assembled on a Silicon read out integrated circuit (ROIC). Linear arrays of 256 pixels with 25 μm pitch were fabricated as well as focal plane arrays (FPA) of 256 × 320 pixel with 30 μm pitch. Measures of electrical interconnection yield will be shown. Functionality is proven for different applications up to 2.5 μm wavelength.
Diode structures of short wavelength infrared (SWIR) InGaAs material were grown epitaxially on 3 inch GaAs substrates by molecular beam epitaxy. Despite the large lattice mismatch of 6% between In0.8Ga0.2As and GaAs the diode performance allows applications in spectroscopy and imaging. Photovoltaic diode characterization measures like R0A product and quantum efficiency were extracted from I-V curves. The layers are processed with standard photolithography and micro-structuring tools and finally flip-chip bonded on a silicon read out integrated circuit (ROIC). Linear arrays of 256 and 512 pixel with 25 mm pitch were fabricated as well as focal plane arrays (FPA) of 256 × 320 pixel with 30 mm pitch. Functionality is proven by using the assemblies in systems for spectroscopy and beam profiling up to 2.5 mm wavelength.
High quality extended-wavelength InGaAs (80% In) has been grown heteroepitaxially on 3'-GaAs substrates by means of Molecular Beam Epitaxy (MBE). In contrast growth on InP, sensor manufacturing on GaAs substrates can be readily scaled to 6'. An additional benefit is the larger cut-off of the 80%- In material (2.5μm vs. 1.7μm). The photodiode layer design was optimized for maximum improvement of dark current with temperature. A high-uniformity and high- yield process was developed to manufacture bondable 128 pixel 50 μm pitch linear array sensors. To this end, a dedicated mesa-type process was developed assuring high uniformity in dark current and photoresponse. The sensors were designed for top-illumination. Sensors have been characterized electrically and optically at the intended thermo-electrical operation temperature (250K). The homogeneity of dark current and photoresponse was also evaluated at 250 K. The improvement in dark current as a function of temperature has been studied in more detail, yielding a zero bias resistance of approximately 2.5 MΩcm2 at 77K. A linear array has been bonded to a commercial CMOS read-out circuit and the functionality is shown. The sensor is suitable for spectroscopic and imaging applications in the short wavelength infrared (SWIR) region. Work is underway to develop sensors with smaller pitch and up to 512 pixels.
We report on In(80%)GaAs line scan sensors with 128 pixels on 50 micrometers pitch for use as thermo-electrically-cooled spectroscopic sensor in the short-wave infrared (1 - 2.5 micrometers ).
Infrared detector arrays can be divided in two distinct classes: hybrid (and typically photon) detectors and monolithic (and generally thermal) detectors. Hybrid detectors involve flip-chip integration of the detector array and the readout chip, require cooling and thus cause substantial system cost. Monolithic detectors do not suffer this system overhead and most notably the microbolometer thermal detectors allow ambient operating temperature. IMEC focuses on III-V (InGaAs, InAs and InAsSb) short-wave and mid-infrared detector arrays for hybrid integration on one side and surface micromachined uncooled polySiGe microbolometer arrays on the other hand. Progress in both types of detector systems is reported.
Short-wave IR and mid-IR photovoltaic detector arrays consisting of In(Ga)As and InAsSb were realized. Maximum array size is 256 X 256 elements on a 25 micron pitch. The layers were grown on 3' semi-insulating GaAs substrates by MBE thereby avoiding the need for substrate removal by wafer thinning after hybridization. A reliable and uniform detector process using improved wet-etching has been developed. The citric-acid based etch has been optimized for minimum underetch such that high fill factor is achieved even with a mesa-type process. Typical RoA products at room temperature are within a factor of 2 of the theoretical limit for bulk leakage currents. The hybridization with silicon readout circuits consisted of Si-postprocessing by electroless plating or lithographic definition of Ni/Au, indium bump electroplating on the III-V chip and flip-chip integration with individual indium bumps. The indium bump process resulted in 13 micron diameter solderbumps which allows pixel pitches below 20 micron.
Polycrystalline Silicon Germanium is a useful material for CMOS compatible uncooled IR bolometer manufacturing due to its excellent material characteristics such as low stress and high TCR. However, for IR imaging applications, fast and yet sensitive detectors are required. We managed to combine these two contrasting characteristics by fabricating very thin devices. This was only possible thanks to a new release technique based on vapor HF at elevated wafer temperatures, and to structural stiffness enhancement of the devices by applying U-profiles. Furthermore, a performant and low cost on-chip vacuum package has been developed. The combination of these features is applied in linear arrays of bolometers which are read-out by a dedicated noise reduction circuit using an MCM board.
Progress in the development of narrow gap IV-VI-on-Si technology for IR sensor arrays is reviewed. Epitaxial Pb1-xSnxSe layers, about 4 micrometers thick, are grown by molecular beam epitaxy onto 3 inch Si(111) substrates. An intermediate CaF2 buffer layer of only 2 nm thickness was employed for compatibility reasons in most cases, direct growth without buffer layer, however, is possible. Material quality is improved by proper growth conditions and annealing. Threading dislocation densities as low as 106 cm-2 are obtained in samples with 3 X 3 cm2 size after proper anneal. It seems that glissile threading dislocations sweep out across the edge of the samples, and, in addition, such dislocations are able to react with sessile ones and transform them to glissile. IR photodiodes with much higher resistance area products can be obtained which approach the theoretical limit in a certain temperature range with such improve material quality. If the Pb/Pb1-xSnxSe IR Schottky-barrier sensors are described with a model which allows fluctuations of the barrier height, the saturation of the resistance-area products at low temperatures as well as ideality factors > 1 are explained as well.
Progress in the development of narrow gap IV-VI-on-Si technology for infrared sensor arrays is reviewed. Epitaxial Pb1-xSnxSe layers, about 4 micrometer thick, are grown by molecular beam epitaxy onto 3 inch Si(111) substrates, and employing an intermediate CaF2 layer of only 2 nm thickness for compatibility reasons. Material quality is improved by proper growth conditions and annealing. Threading dislocation densities as low as 106 cm-2 are obtained in samples with 3 by 3 cm2 size after proper anneal. It seems that glissile threading dislocations sweep out across the edge of the samples, and, in addition, such dislocations are able to react with sessile ones and transform them to glissile. Infrared photodiodes with much higher resistance area products can be obtained which approach the theoretical limit in a certain temperature range with such improved material quality. If the Pb/Pb1-xSnxSe infrared Schottky-barrier sensors are described with a model which allows fluctuations of the barrier height, the saturation of the resistance-area products at low temperatures as well as ideality factors very much greater than 1 are explained, too.
A demonstrational thermal imaging LWIR camera system is described which is based on photovoltaic Pb1-xSnxSe-on-Si infrared sensor arrays. Epitaxial Pb1-xSnxSe layers, about 2 micrometer thick, are grown by molecular beam epitaxy onto 3 inch Si(111) substrates, and employing an intermediate CaF2 buffer layer of only 2 nm thickness for compatibility reasons. Linear arrays with 256 pixels on 50 micrometer centers are fabricated in the layers with a batch photolithographic technique. Cut- off wavelength is about 10 - 10.5 micrometer at operating temperatures of 80 - 120 K, and quantum efficiencies greater than 60%. The sensors operate near BLIP. The read-out electronics chips contain, for each sensor, an integrator with a low noise JFET input transistor, correlated multiple sampling, and a sample and hold amplifier. They are wire- bonded to the sensor array. The JFET input transistors allow it to amplify from much lower source impedances (down to less than 10 kOhm) than with CMOS design without adding significant noise. Infrared sensors with lower impedances, operation at higher temperature, or sensors with longer cut-off wavelengths can therefore be used.
IV-VI materials (PbTe, PbSe, Pb1-xSnxSe) are grown by molecular beam epitaxy onto Si(111) and Si(100) substrates. Device quality layers are obtained on Si(111), if a very thin CaF2, or a stacked CaF2/BaF2 buffer layer is employed. For these layers, thermal mismatch strain relaxation occurs by glide along the main {100} glide planes inclined by 54 degrees with respect to the surface. Cumulative plastic deformation on temperature cycling up to 500% was observed even at cryogenic temperatures, while the structural quality of the layers was only slightly adversely affected by such extreme plastic deformations. Interaction probabilities between moving dislocations were estimated to be below 10-5. Epitaxial growth of IV-VI materials directly on Si (without buffer layer) is possible, too. However, the structural quality is inferior. On Si(111), PbSe with either (100), (111) or mixed (111) and (100) orientation, depending on growth temperature, is obtained. Single oriented (100)PbSe layers result on Si(100). Strain relaxation occurs via the same mechanisms as for layers grown with buffer layers: On Si(111), strain relaxes by slip, while on Si(100), both relaxation by plastic deformation as well as by cracking is observed. The latter occurs for layers thicker than about 0.5 micrometers only.
Photovoltaic narrow-gap IV-VI (lead chalcogenide) infrared sensor arrays on Si substrates have the potential for low-cost infrared focal-plane arrays. The arrays can be bump bonded to readout multiplexers, or be grown on prefabricated active Si substrates containing the whole readout circuits. Sensitivities are similar to that of Hg1-xCdxTe, but processing procedures are much less demanding. This is because the structural quality of even heavily lattice-mismatched IV-VI layers is adequate to fabricate devices with good sensitivities, because 2- to 4-μm layer thickness suffices, and because good homogeneity in ternary Pb1-xSnxSe for the 8- to 12-μm range is much easier to obtain than in Hg1-xCdxTe. New results are presented on the molecular beam epitaxal growth of the layers, including a very thin CaF2 buffer needed for compatibility reasons, and a new photolithographic patterning technique suited for full wafer processing has been developed to fabricate the sensor arrays. First thermal images using these chips are demonstrated.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.