Μm Pixel Research Articles

Monolithic pixel sensor with 25 µm × 35 µm pixel size and high time resolution implemented in 180 nm technology

A monolithic pixel sensor with small collection electrode and partially depleted sensor diode named HVMAPS25 has been implemented in the 180 nm technology of TSI semiconductors. The pixel size is 25 µm × 35 µm. The pixel electronics contains a fast and low power charge sensitive amplifier, a comparator, a threshold tuning DAC and a digital block that measures the arrival time of the hit with 10 bit resolution and the signal amplitude (time over threshold) with 6 bit resolution. A deep p-well has been used for isolation of the pixel electronics from the sensor substrate. The building blocks of the chip, simulations and the measurement results will be presented.

Characterization of a 30 µm pixel size CLIP-based 3D printer and its enhancement through dynamic printing optimization.

Resolving microscopic and complex 3D polymeric structures while maintaining high print speeds in additive manufacturing has been challenging. To achieve print precision at micrometer length scales for polymeric materials, most 3D printing technologies utilize the serial voxel printing approach that has a relatively slow print speed. Here, a 30-µm-resolution continuous liquid interface production (CLIP)-based 3D printing system for printing polymeric microstructures is described. This technology combines the high-resolution from projection microstereolithography and the fast print speed from CLIP, thereby achieving micrometer print resolution at x103 times faster than other high-resolution 3D printing technologies. Print resolutions in both lateral and vertical directions were characterized, and the printability of minimum 30 µm features in 2D and 3D has been demonstrated. Through dynamic printing optimization, a method that varies the print parameters (e.g. exposure time, UV intensity, and dark time) for each print layer, overhanging struts at various thicknesses spanning 1 order of magnitude (25 µm - 200 µm) in a single print are resolvable. Taken together, this work illustrates that the micro-CLIP 3D printing technology, in combination with dynamic printing optimization, has the high resolution needed to enable manufacturing of exquisitely detailed and gradient 3D structures, such as terraced microneedle arrays and micro-lattice structures, while maintaining high print speeds.

Compact athermalized LWIR objective lens

A new compact optical passive athermalized objective lens is designed with a focal length 120 mm and F-number 1.1. Its full field of view is 12.2°. The geometrical aberrations due to the large aperture and the wide field of view are well corrected as well as the chromatic aberration. Hence, the maximum RMS radius of the spot diagram over the full field of view is less than 17 µm and the Strehl ratio is 0.95 at 20 °C. The MTF is nearly limited in diffraction over the full F.O.V. This good correction of aberrations makes the optical system gain high resolution and suitable to work with a focal plane array detector with 17-µm pixel pitch. This performance is stable over a long temperature range from − 40 °C to + 65 °C. Moreover, the optical system is relatively compact “140 mm.” The total transmittance (after absorption consideration) of the optical system is enhanced from 6.6 to 94.07% at λ = 10 µm along the optical axis upon applying an anti-reflection coating.

Full-color micro-display by heterogeneous integration of InGaN blue/green dual-wavelength and AlGaInP red LEDs.

A full-color micro-display via bonding of a InGaN blue/green dual-wavelength light-emitting diode (LED) array and a AlGaInP red LED array is demonstrated. The micro-display has a 120 µm pixel pitch, and each pixel consists of 40 µm × 120 µm red/green/blue (R/G/B) subpixels. The red LED array was integrated with the blue/green dual-wavelength LED array by Au/In flip-chip bonding to achieve full-color emission. Full-color images presented by the micro-display have high brightness and a wide color gamut. This heterogeneous integration technology using conventional LED materials shows the feasibility of a cost-effective approach for reliable high-performance full-color LED micro-displays in virtual reality (VR) and augmented reality (AR) devices.

High Responsivity Thermopile Sensors Featuring a Mosaic Structure

This paper presents a detailed analysis of a micromachined thermopile detector featuring high responsivity and a versatile mosaic structure, based on 128 60 µm × 60 µm pixels connected in series and/or in parallel. The mosaic structure is based on the one employed for the thermal sensor known as TMOS, which consists of a CMOS-SOI transistor embedded in a suspended and thermally isolated absorbing membrane, released through microelectro mechanical system (MEMS) post-processing. Two versions of the thermopile detector, featuring different series/parallel connections, are presented and were experimentally characterized. The most performant of the two achieved 2.7 × 10 V/W responsivity. The thermopile sensors’ performances are compared to that of the TMOS sensor, adopting different configurations, and their application as proximity detectors was verified through measurements.

Automated multicolor mesoscopic imaging for the 3-dimensional reconstruction of fluorescent biomarker distribution in large tissue specimens.

Research in translational medicine often requires high-resolution characterization techniques to visualize or quantify the fluorescent probes. For example, drug delivery systems contain fluorescent molecules enabling in vitro and in vivo tracing to determine biodistribution or plasma disappearance. Albeit fluorescence imaging systems with sufficient resolution exist, the sample preparation is typically too complex to image a whole organism of the size of a mouse. This article established a mesoscopic imaging technique utilizing a commercially available cryo-microtome and an in-house built episcopic imaging add-on to perform imaging during serial sectioning. Here we demonstrate that our automated red, green, blue (RGB) and fluorescence mesoscope can generate sequential block-face and 3-dimensional anatomical images at variable thickness with high quality of 6 µm × 6 µm pixel size. In addition, this mesoscope features a numerical aperture of 0.10 and a field-of-view of up to 21.6 mm × 27 mm × 25 mm (width, height, depth).

Performance of a complementary metal-oxide-semiconductor sensor for laser guide star wavefront sensing

The adaptive optics systems of future Extremely Large Telescopes (ELTs) will be assisted with laser guide stars (LGS) which will be created in the sodium layer at a height of ≈90 km above the telescopes. In a Shack–Hartmann wavefront sensor, the long elongation of LGS spots on the sub-pupils far apart from the laser beam axis constraints the design of the wavefront sensor (WFS) which must be able to fully sample the elongated spots without undersampling the non-elongated spots. To fulfill these requirements, a newly released large complementary metal oxide semiconductor sensor with 1100 × 1600 pixels and 9 μm pixel pitch could be employed. Here, we report on the characterization of such a sensor in terms of noise and linearity, and we evaluate its performance for wavefront sensing based on the spot centroid variations. We then illustrate how this new detector can be integrated into a full LGS WFS for both the European Southern Observatory’s ELT and the Thirty Meter Telescope.

Demonstration of 1280 × 1024 long-wavelength infrared FPA with 10 μm pixel pitch based on InAs/GaSb type-II superlattice

1280 × 1024 format long-wavelength infrared focal plane array with 10 μm pixel pitch was fabricated based on InAs/GaSb type-II superlattice. At 70 K, the single-element device shows a 50% cutoff wavelength of 10.0 μm, peak quantum efficiency of 45.04% and peak detectivity of 1.87 × 1011cm·Hz1/2·W−1 at 8.22 μm, the FPA infrared imaging performed an average noise equivalent temperature difference of 28.9 mK at 300 K background (F/3 optics) with an integration time of 1.12 ms and applied bias of −50 mV. The operability and non-uniformity of the FPA were 99.87% and 2.90% respectively, and it exhibits a stable residual non-uniformity below 0.1% of the dynamic range.

Characterisation of a soft X-ray optimised CMOS Image Sensor

A prototype CMOS Image Sensor optimised for soft X-ray applications has been designed by the Centre for Electronic Imaging in partnership with Teledyne-e2v. The device features four different pixel variants (three variants of 40 μm pitch pixels, and one variant of 10 μm pixels) each covering a quarter of the 2 × 2 cm2 image area. The pixel designs feature fully depleted pinned photodiodes using reverse substrate bias and have been optimised for low noise operation, high responsivity and low image lag.The fabricated front-illuminated devices have been tested in a custom-built vacuum test setup at operating temperatures between -30°C and -40°C. The sensors feature less than 5 e- RMS readout noise and energy resolution of 142 eV at Mn-Kα (5.9 keV). The response to soft X-ray with different sensor parameters (e.g. pixel pitch, deep-depletion extension implant depth, and back-bias voltage) is also studied.

Characterization of low light performance of a complementary metal-oxide semiconductor sensor for ultraviolet astronomical applications

Complementary metal-oxide semiconductor (CMOS) detectors offer many advantages over charge-coupled devices (CCDs) for optical and ultraviolet (UV) astronomical applications, especially in space where high radiation tolerance is required. However, astronomical instruments are most often designed for low light-level observations demanding low dark current and read noise, good linearity, and high dynamic range, characteristics that have not been widely demonstrated for CMOS imagers. We report the performance, over temperatures from 140 to 240 K, of a radiation hardened SRI 4k × 2k back-side illuminated CMOS image sensor with surface treatments that make it highly sensitive in blue and UV bands. After suppressing emission from glow sites resulting from defects in the engineering grade device examined, a 0.077 me − / s dark current floor is reached at 160 K, rising to 1 me − / s at 184 K, rivaling that of the best CCDs. We examine the trade-off between readout speed and read noise, finding that 1.43 e − median read noise is achieved using line-wise digital correlated double sampling at 700 kpix / s / ch corresponding to a 1.5 s readout time. The 15 ke − well capacity in high gain mode extends to 120 ke − in dual gain mode. Continued collection of photogenerated charge during readout enables a further dynamic range extension beyond 106 e − effective well capacity with only 1% loss of exposure efficiency by combining short and long exposures. A quadratic fit to correct for non-linearity reduces gain correction residuals from 1.5% to 0.2% in low gain mode and to 0.4% in high gain mode. Cross-talk to adjacent pixels is only 0.4% vertically, 0.6% horizontally, and 0.1% diagonally. These characteristics plus the relatively large (10 μm) pixel size, quasi 4-side buttability, electronic shutter, and sub-array readout make this sensor an excellent choice for wide field astronomical imaging in space, even at far-UV wavelengths where sky background is very low.

A laser powder bed fusion system for operando synchrotron x-ray imaging and correlative diagnostic experiments at the Stanford Synchrotron Radiation Lightsource.

Laser powder bed fusion (LPBF) is a highly dynamic multi-physics process used for the additive manufacturing (AM) of metal components. Improving process understanding and validating predictive computational models require high-fidelity diagnostics capable of capturing data in challenging environments. Synchrotron x-ray techniques play a vital role in the validation process as they are the only in situ diagnostic capable of imaging sub-surface melt pool dynamics and microstructure evolution during LPBF-AM. In this article, a laboratory scale system designed to mimic LPBF process conditions while operating at a synchrotron facility is described. The system is implemented with process accurate atmospheric conditions, including an air knife for active vapor plume removal. Significantly, the chamber also incorporates a diagnostic sensor suite that monitors emitted optical, acoustic, and electronic signals during laser processing with coincident x-ray imaging. The addition of the sensor suite enables validation of these industrially compatible single point sensors by detecting pore formation and spatter events and directly correlating the events with changes in the detected signal. Experiments in the Ti-6Al-4V alloy performed at the Stanford Synchrotron Radiation Lightsource using the system are detailed with sufficient sampling rates to probe melt pool dynamics. X-ray imaging captures melt pool dynamics at frame rates of 20 kHz with a 2 µm pixel resolution, and the coincident diagnostic sensor data are recorded at 470 kHz. This work shows that the current system enables the in situ detection of defects during the LPBF process and permits direct correlation of diagnostic signatures at the exact time of defect formation.

High-spatial resolution measurements with a GaAs:Cr sensor using the charge integrating MÖNCH detector with a pixel pitch of 25 μm

The aim of this project is to determine the imaging capabilities of a 25 μm pixel pitch GaAs:Cr sensor of 500 μm thickness bump-bonded to the charge integrating MÖNCH 03 readout chip (also called GaAs-MÖNCH assembly) and to assess the possibility to improve the spatial resolution by applying a position interpolation algorithm developed at PSI. Measurements were performed at the TOMCAT beamline of the Swiss Light Source (SLS) using photon beams in the energy range of 10–30 keV. The imaging experiments indicate the possibility to enhance the spatial resolution of the detector beyond its actual physical pixel pitch. We have quantified the spatial resolution of a GaAs-MÖNCH assembly by means of the modulation transfer function (MTF), achieving 10 μm at 10 keV and 12 μm at 20 keV photon energies. By applying a modified interpolation algorithm, a spatial resolution of ∼5 μm was obtained for 16 keV when binning to 2.5 μm virtual pixels, while with the silicon-MÖNCH assembly, we achieved a spatial resolution of 3.5 μm, which serves as gold standard. The results are promising because they open new possibilities to perform imaging measurements using the GaAs-MÖNCH assembly at photon energies above 15 keV, where silicon sensors suffer from a diminishing quantum efficiency.

Development of a Fringe Printer With 0.35 μm Pixel Pitch

The development of computational devices can provide computer-generated holograms (CGH) of over 100 Giga-pixel easily. The performance improvement of the graphical processing unit makes calculations faster with a normal personal computer. In contrast, the output device of CGH is not reported much. Since CGH is a fringe pattern, high resolution and fine pixel pitch are required for the output device. We have been developing a direct fringe printer, which consists of a laser, an X-Y stage, an SLM, and optical parts available on the market. Our previous report presented a liquid crystal panel, which was used for SLM that had full HD resolution and 7 μm pixel pitch. Since the Pixel pitch of the liquid crystal panel is not small enough for CGH, the optical setup works to demagnify the pixel pitch of the printed fringe pattern. To record high resolution CGH, the calculated fringe pattern is split and exposed in tiles by the X–Y stage. The results showed a 0.44 μm pixel pitch and over 100 Gpixel CGHs. However, to output more high-quality CGH, the development of pixel pitch and resolution is very important. In this paper, we review the optical system of a fringe printer that achieved an output of 0.35 μm pixel pitch CGH. We also investigate the performance of the new fringe printer.

Do Anti-tuberculosis Drugs Reach Their Target?─High-Resolution Matrix-Assisted Laser Desorption/Ionization Mass Spectrometry Imaging Provides Information on Drug Penetration into Necrotic Granulomas.

Tuberculosis (TB) is characterized by mycobacteria-harboring centrally necrotizing granulomas. The efficacy of anti-TB drugs depends on their ability to reach the bacteria in the center of these lesions. Therefore, we developed a mass spectrometry (MS) imaging workflow to evaluate drug penetration in tissue. We employed a specific mouse model that─in contrast to regular inbred mice─strongly resembles human TB pathology. Mycobacterium tuberculosis was inactivated in lung sections of these mice by γ-irradiation using a protocol that was optimized to be compatible with high spatial resolution MS imaging. Different distributions in necrotic granulomas could be observed for the anti-TB drugs clofazimine, pyrazinamide, and rifampicin at a pixel size of 30 μm. Clofazimine, imaged here for the first time in necrotic granulomas of mice, showed higher intensities in the surrounding tissue than in necrotic granulomas, confirming data observed in TB patients. Using high spatial resolution drug and lipid imaging (5 μm pixel size) in combination with a newly developed data analysis tool, we found that clofazimine does penetrate to some extent into necrotic granulomas and accumulates in the macrophages inside the granulomas. These results demonstrate that our imaging platform improves the predictive power of preclinical animal models. Our workflow is currently being applied in preclinical studies for novel anti-TB drugs within the German Center for Infection Research (DZIF). It can also be extended to other applications in drug development and beyond. In particular, our data analysis approach can be used to investigate diffusion processes by MS imaging in general.

Interface evolution and pinch-off mechanism of droplet in two-phase liquid flow through T-junction microfluidic system

This work has explored interface evolution and pinch-off mechanism of the droplet formation in two-phase flow through cross-flow microfluidic device. The two-dimensional mathematical model equations have been solved using the finite element method under the squeezing regime (Cac< 10−2) for wide range of flow rates (0.1 ≤ Qr ≤ 10) and fixed contact angle (θ = 135°). The droplet formation process has been classified into various instantaneous stages as initial, filling, squeezing, pinch-off and stable droplet through microscopic visualization of interface evolution in phase profiles. The dynamics of interface, and point pressure in both phases is further gained and discussed. Maximum pressure in the continuous phase varied linearly with Qr. The droplet pinch-off mechanism has been thoroughly elucidated by determining the local radius of the curvature (Rc,min) and neck width (2r) during the squeezing and pinch-off stages. At the pinch-off point, both Rc,min and 2r are non-linearly related to Qr. Further, the topological dynamics of interface has been explored by analyzing the Laplace pressure (pL), acting on the interface curvature, evaluated using (a) pressure sensors in both phases, pL = pdp − pcp, (b) local radius of curvature pL=σ1∕Rf+1∕Rr, and (c) minimum radius of curvature, pL=σ1∕Rc,min. The insights obtained from the present work can reliably be used in designing the model and prototypes of microfluidic devices for generating monodispersed droplets in emulsions, and the droplet breakup mechanism would help accurate prediction of the pinch-off moment. The proposed knowledge provides detailed insights of the interface evolution and droplet pinch-off to a precision of 10 μs and resolution of 10 μm, equivalent to experimental flow visualization with a high-speed (105 fps) and high-resolution (10 μm pixel size) camera.

Performance of the FASTPIX Sub-Nanosecond CMOS Pixel Sensor Demonstrator

Within the ATTRACT FASTPIX project, a monolithic pixel sensor demonstrator chip has been developed in a modified 180 nm CMOS imaging process, targeting sub-nanosecond timing measurements for single ionizing particles. It features a small collection electrode design on a 25 micron thick epitaxial layer and contains 32 mini matrices of 68 hexagonal pixels each, with pixel pitches ranging from 8.66 to 20 micron. Four pixels are transmitting an analog output signal and 64 are transmitting binary hit information. Various design variations are explored, aiming at accelerating the charge collection and making the timing of the charge collection more uniform over the pixel area. Signal treatment of the analog waveforms, as well as reconstruction of time and charge information, is carried out off-chip. This contribution introduces the design of the sensor and readout system and presents the first performance results for 10 μm and 20 μm pixel pitch achieved in measurements with particle beams.

Characterizing Enzyme Cooperativity with Imaging SAMDI-MS.

This paper describes a method that combines a microfluidic device and self-assembled monolayers for matrix-assisted laser desorption/ionization mass spectrometry (SAMDI) mass spectrometry to calculate the cooperativity in binding of calcium ions to peptidylarginine deiminase type 2 (PAD2). This example uses only 120 μL of enzyme solution and three fluidic inputs. This microfluidic device incorporates a self-assembled monolayer that is functionalized with a peptide substrate for PAD2. The enzyme and different concentrations of calcium ions are flowed through each of eight channels, where the position along the channel corresponds to reaction time and position across the channel corresponds to the concentration of Ca2+ . Imaging SAMDI (iSAMDI) is then used to determine the yield for the enzyme reaction at each 200 μm pixel on the monolayer, providing a time course for the reactions. Analysis of the peptide conversion as a function of position and time gives the degree of cooperativity (n) and the concentration of ligand required for half maximal activity (K0.5 ) for the Ca2+ - dependent activation of PAD2. This work establishes a high-throughput and label-free method for studying enzyme-ligand binding interactions and widens the applicability of microfluidics and matrix-assisted laser desorption/ionization mass spectrometry (MALDI) imaging mass spectrometry.

A wide-field micro-computed tomography detector: micron resolution at half-centimetre scale.

Ideal three-dimensional imaging of complex samples made up of micron-scale structures extending over mm to cm, such as biological tissues, requires both wide field of view and high resolution. For existing optics and detectors used for micro-CT (computed tomography) imaging, sub-micron pixel resolution can only be achieved for fields of view of <2 mm. This article presents a unique detector system with a 6 mm field-of-view image circle and 0.5 µm pixel size that can be used in micro-CT units utilizing both synchrotron and commercial X-ray sources. A resolution-test pattern with linear microstructures and whole adult Daphnia magna were imaged at beamline 8.3.2 of the Berkeley Advanced LightSource. Volumes of 10000 × 10000 × 7096 isotropic 0.5 µm voxels were reconstructed over a 5.0 mm × 3.5 mm field of view. Measurements in the projection domain confirmed a 0.90 µm measured spatial resolution that is largely Nyquist-limited. This unprecedented combination of field of view and resolution dramatically reduces the need for sectional scans and computational stitching for large samples, ultimately offering the means to elucidate changes in tissue and cellular morphology in the context of larger, whole, intact model organisms and specimens. This system is also anticipated to benefit micro-CT imaging in materials science, microelectronics, agricultural science and biomedical engineering.

HEXITEC 2 × 2 tiled hard X-ray spectroscopic imaging detector system

HEXITEC is a spectroscopic imaging X-ray detector technology developed at the STFC Rutherford Appleton Laboratory for X-ray and γ-ray spectroscopic imaging applications. Each module has 80 × 80 pixels on a 250 μm pixel pitch, and has been implemented successfully in a number of applications. This paper presents the HEXITEC 2 × 2 detector system, a tiled array of 4 HEXITEC modules read out simultaneously, which provides an active area of 16 cm2. Systems have been produced using 1 mm thick Cadmium Telluride (CdTe) and 2 mm thick Cadmium Zinc Telluride (CdZnTe) sensor material. In this paper the system and data processing methods are presented, and the performance of the systems are evaluated. The detectors were energy calibrated using an 241Am sealed source. Three types of charge sharing correction were applied to the data-charge sharing addition (CSA), charge sharing discrimination (CSD), and energy curve correction (ECC) which compensates for energy lost in the inter-pixel region. ECC recovers an additional 34 % of counts in the 59.5 keV peak in CdTe compared to the use of CSD; an important improvement for photon-starved applications. Due to the high frame rate of the camera system (6.3 kHz) an additional End of Frame (EOF) correction was also applied to 6.0 % of events to correct for signals that were readout whilst the signal was still forming. After correction, both detector materials were found to have excellent spectroscopic performance with a mean energy resolution (FWHM) of 1.17 keV and 1.16 keV for CdZnTe and CdTe respectively. These results successfully demonstrate the ability to construct tiled arrays of HEXITEC modules to provide larger imaging areas.

Simulated sensor characterization for virtual clinical trials in mammography and digital breast tomosynthesis

Virtual clinical trials in X-ray breast imaging permit to compare different technical solutions and imaging modalities at reduced costs, involved personnel, reduced times and reduced radiation risks to patients. In this context, the detector characteristics (spatial resolution, noise level and efficiency) play a key role for an appropriate generation of simulated images. The project AGATA proposes to compute images as dose deposit maps in a detector layer of defined materials. Simulated images are then post-processed on the basis of suitable comparison between intrinsic characteristics of real and simulated detectors. With this scope, as first step for the post-processing manipulations, we evaluated the presampled modulation transfer function (MTF), the detector-response function and the noise power spectrum (NPS) of the simulated detectors. Two detectors were simulated: (1) 0.20 mm-thick a-Se direct flat panel with 70 µm pixel pitch and (2) CsI(Tl) indirect flat panel with 100 µm pixel pitch and scintillator layer 0.25 mm thick. In addition, the impact of simulating the de-excitation processes (Auger emission and fluorescence) was explored. Simulated detector characteristics were evaluated for W/Rh spectra between 25 kV and 31 kV. The in-silico platform used a Monte Carlo software based on Geant4 toolkit (vers. 6). First, the simulation and tracking of electrons generated from photoelectric or Compton interactions was shown to have neglectable influence on the pixel values for the explored spectra, with the produced electrons presenting short ranges with respect to the pixel dimension. In the case of the CsI detector, which has fluorescence energies higher than those of the simulated X-ray photons, the deexcitation processes have not noticeable influence on the calculated pixel values. On the other hand, the MTF of the a-Se detector resulted slightly lower when the fluorescence is simulated in the detector materials, due to the dose spread derived from the fluorescence photons, which can travel far from the initial ionization interaction. Regarding the a-Se detector, the noise power spectrum resulted lower with simulated deexcitation.