Pixel Device Research Articles

LePIX: First results from a novel monolithic pixel sensor

We present a monolithic pixel sensor developed in the framework of the LePIX project aimed at tracking/triggering tasks where high granularity, low power consumption, material budget, radiation hardness and production costs are a concern. The detector is built in a 90nm CMOS process on a substrate of moderate resistivity. This maintains the advantages usually offered by Monolithic Active Pixel Sensors (MAPS), like a low input capacitance, having a single piece detector and using a standard CMOS production line, but offers charge collection by drift from a depleted region and therefore an excellent signal to noise ratio and a radiation tolerance superior to conventional undepleted MAPS.Measurement results obtained with the first prototypes from laser, radioactive source and beam test experiments are described. The excellent signal-to-noise performance is demonstrated by the capability of the device to separate the peaks in the spectrum of a 55Fe source. We will also highlight the interaction between pixel cell design and architecture which points toward a very precise direction in the development of such depleted monolithic pixel devices for high energy physics.

Fast computation of Fresnel diffraction field of a three-dimensional object for a pixelated optical device

In this paper, a fast algorithm is proposed for accurate calculation of the scalar optical diffraction on a pixelated optical device used in the reconstruction process from a three-dimensional object that is formed by scattered sample points over the space. In computer-generated holography, fast and accurate calculation of the diffraction field is an important and a challenging problem. Therefore, several fast algorithms can be found in the literature. The accuracy of the calculations can be determined by the signal processing techniques and the numerical methods used in the calculation of diffraction fields. Furthermore, the quality of reconstructed objects can be affected by the properties of optical devices employed in the reconstruction process. For instance, the pixelated structure of those devices has a significant effect on the reconstruction process. Therefore, the pixelated structure of the display device has to be taken into account. Furthermore, fast calculation of the diffraction pattern can be a bottleneck in dynamic holographic content generation. As a solution to the problems, we propose a fast and accurate algorithm based on a precomputed one-dimensional kernel and scaling of that kernel for the computation of the diffraction pattern for a pixelated display.

High Spatial Resolution of Thin-Film-on-ASIC Particle Detectors

Thin-film-on-ASIC (TFA) detectors are monolithic pixel devices that consist of amorphous silicon detecting diodes directly deposited on readout electronics. This paper presents a characterization of the TFA spatial resolution using the electron-beam-induced current (EBIC) technique, in which pixel pads patterned in microstrips were swept by the beam. We measured the spatial resolution for different configurations and thicknesses of the TFA active layer with different beam energies, currents and sweep speeds. We observed that the generated electron-hole pairs are collected most efficiently when the beam is over the microstrips. This better collection efficiency gives a larger signal than off the strips, and thereby enabled us to distinguish strips as small as 0.6 μm wide which are spaced by 1.4 μm gaps. This high spatial resolution was obtained even though microvoids in the amorphous silicon layer-induced by an ASIC morphology as rough as 2 μm-were observed in the detector cross section, thus demonstrating the potential of the TFA architecture even with non-planar readout electronics.

55.5L: Late‐News Paper: A Theoretical Consideration of a Flat Panel Display based on Integrated Optical Devices

AbstractIn this paper, we introduce a novel display concept based on integrated optical components and discuss the feasibility of the device. The proposed concept is identified as photoluminescence based self‐emissive display. The main components of the system are pixel devices, optical switches, and a light coupler. The listed components are interconnected with optical waveguides, which are for the delivery of excitation light from a light coupler to individual pixel devices. Color expression is implemented by the combination of red, green, blue pixel devices which utilize color conversion materials for converting excitation light to corresponding color of light. Due to the nature of integrated optical device, the display system can have simple and efficient structure based on single substrate. Furthermore, this display has the potential to have attractive characteristics, such as high energy efficiency, flexibility and peerless transparency comparing with any other current flat panel displays, which would create diverse appealing applications.

Large aperture asymmetric Fabry Perot modulator based on asymmetric tandem quantum well for low voltage operation

Large aperture image modulators used as demodulator in receiver path are an important component for the use in three dimensional (3D) image sensing. For practical applications, low voltage operation and high modulation performance are the key requirements for modulators. Here, we propose an asymmetric Fabry-Perot modulator (AFPM) with asymmetric tandem quantum wells (ATQWs) for 3D image sensing. By using ATQWs for the AFPM design, the device operated at -4.25V, and the operating voltage was significantly lower by about 23% compared to -5.5V of a conventional AFPM with 8nm thick multiple QW with a single QW thickness (SQWs), while achieving high reflectivity modulation in excess of 50%. The performance of the fabricated devices is in good agreement with theoretical calculations. The pixelated device shows a high modulation speed of 21.8 MHz over a large aperture and good uniformity. These results show that AFPM with ATQWs is a good candidate as an optical image modulator for 3D image sensing applications.

Charged particle tracking with the Timepix ASIC

A prototype particle tracking telescope has been constructed using Timepix and Medipix ASIC hybrid pixel assemblies as the six sensing planes. Each telescope plane consisted of one 1.4 cm2 assembly, providing a 256x256 array of 55 micron square pixels. The telescope achieved a pointing resolution of 2.3 micron at the position of the device under test. During a beam test in 2009 the telescope was used to evaluate in detail the performance of two Timepix hybrid pixel assemblies; a standard planar 300 micron thick sensor, and 285 micron thick double sided 3D sensor. This paper describes a detailed charge calibration study of the pixel devices, which allows the true charge to be extracted, and reports on measurements of the charge collection characteristics and Landau distributions. The planar sensor achieved a best resolution of 4.0 micron for angled tracks, and resolutions of between 4.4 and 11 micron for perpendicular tracks, depending on the applied bias voltage. The double sided 3D sensor, which has significantly less charge sharing, was found to have an optimal resolution of 9.0 micron for angled tracks, and a resolution of 16.0 micron for perpendicular tracks. Based on these studies it is concluded that the Timepix ASIC shows an excellent performance when used as a device for charged particle tracking.

Micro-patterning of a conductive polymer and an insulation polymer using the Parylene lift-off method for electrochromic displays

We have developed a process to pattern the conductive polymer of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) film and the insulation polymer of the perfluoro polymer (Cytop) with a Parylene lift-off method for the purpose of fabricating a PEDOT:PSS-based electrochromic display device. Because conventional micro-patterning processes decrease the conductivity of organic electronic polymers due to the destructive solutions of photolithography, PEDOT:PSS and Cytop polymers were patterned by using the dry lift-off film of Parylene. Its patterning resolution of PEDOT:PSS was found to be as low as 20 µm. The insulation layer of Cytop was also patterned on the PEDOT:PSS pattern with the same resolution. This process was able to pattern the 300 µm wide wiring and 1 mm square pixels of PEDOT:PSS and the Cytop to cover the entire area except for the pixels constructed to form an electrochromic display. Finally, the fabricated 4 × 4 pixel device displayed a simple shape, a transverse line.

Optical Analysis of Small Pixel Liquid Crystal Microdisplays

A finite-element beam propagation method is applied to the optical analysis of small pixel liquid crystal (LC) (micro)displays. It is shown that the near-field profile is affected in these devices by diffraction, even for the ideal case of homogeneous pixels. The influence of the pixel size on the diffraction profile is shown and a fundamental contrast limit is demonstrated. The importance of LC materials with high birefringence is confirmed for small pixel LC devices as these improve the contrast for a fixed pixel size. Simulation of the pixel walk-off phenomenon in gray pixels due to anisotropy in the longitudinal direction illustrates the importance of including the full dielectric tensor in optical modeling of small pixel devices.

Thin pixel development for the SuperB silicon vertex tracker

The high luminosity SuperB asymmetric e + e − collider, to be built near the INFN National Frascati Laboratory in Italy, has been designed to deliver a luminosity greater than 10 36 cm −2 s −1 with moderate beam currents and a reduced center of mass boost with respect to earlier B-Factories. An improved vertex resolution is required for precise time-dependent measurements and the SuperB Silicon Vertex Tracker will be equipped with an innermost layer of small radius (about 1.5 cm), resolution of 10 – 15 μ m in both coordinates, low material budget ( < 1 % X0), and able to withstand a background rate of several tens of MHz/cm 2 . The ambitious goal of designing a thin pixel device with these stringent requirements is being pursued with specific R&D programs on different technologies: hybrid pixels, CMOS MAPS and pixel sensors developed with vertical integration technology. The latest results on the various pixel options for the SuperB SVT will be presented.

Background capabilities of pixel detectors for double beta decay measurements

We discuss the possible use of a progressive detection technique based on pixel detectors for the study of double beta decay (ββ) processes. A series of background measurements in various environments (surface laboratory, underground laboratory, with and without Pb shielding) was performed using the TimePix silicon hybrid pixel device. The pixel detector response to the natural background and intrinsic background properties measured by a low-background HPGe detector are presented.

An all optically driven integrated deformable mirror device

We demonstrate a technique for actuating micromirrors vertically cascaded on wafer fused GaAs-GaP photodiodes. Unlike traditional actuation schemes, the electrostatic drive of the individual capacitive actuators is addressed optically in this device. Vertical mirror displacements of up to 500 nm were observed using interferometry while addressing the photodetectors with a 5 mW optical signal. Microlenses were used to address a 900 pixel device with patterned conductive pillars and thin film load resistors for each actuator-detector element. This approach can enable realization of faster and denser adaptive optics wave front corrector arrays.

Radiation effects in silicon-on-insulator transistors with back-gate control method fabricated with OKI Semiconductor 0.20 μm FD-SOI technology

Bonded silicon-on-insulator (SOI) wafers have the capability of realizing monolithic pixel devices, where the silicon resistivity is optimized separately for the electronics and detector parts. Using UNIBOND wafers, we are developing monolithic pixel devices fabricated with OKI Semiconductor 0.20 μm FD-SOI technology. A set of PMOS and NMOS transistors were irradiated with protons in order to investigate the total ionization dose effect in transistor operation. We evaluated also the devices with a back-gate control electrode added underneath the buried oxide layer. Primary radiation effect appears in transistor threshold shifts, which can be explained by charge traps in the oxide layers and charge states created at the silicon–oxide boundaries. We discuss the possibility of TCAD simulation for evaluation of the charge densities.

Investigation of multistack InAs/InGaAs/GaAs self-assembled quantum dots-in-double-well structures for infrared detectors

The authors report the InAs/InGaAs/GaAs/AlGaAs quantum dots-in-double-well (D-DWELL) design, which has a lower strain per DWELL stack than the InAs/InGaAs/GaAs DWELLs thereby enabling the growth of many more stacks in the detector. The purpose of this study is to examine the effects of varying the number of stacks in the double DWELL detector on its device performance. The structures are grown by solid source molecular beam epitaxy on GaAs substrates. After fabrication of single pixel devices, a series of device measurements such as spectral response, dark current, total current, and responsivity were undertaken and the photoconductive gain and the activation energies were extracted. The goal of these experiments is not only to optimize the device performance by optimizing the number of stacks but also to investigate the transport properties as a function of the number of stacks.

A generic readout environment for prototype pixel detectors

Pixel detectors for experimental particle physics research have been implemented with a variety of readout formats and potentially generate massive amounts of data. Examples include the PSI46 device for the Compact Muon Solenoid (CMS) experiment which implements an analog readout, the Fermilab FPIX2.1 device with a digital readout, and the Fermilab Vertically Integrated Pixel device. The Electronic Systems Engineering Department of the Computing Division at the Fermi National Accelerator Laboratory has developed a data acquisition system flexible and powerful enough to meet the various needs of these devices to support laboratory test bench as well as test beam applications. The system is called CAPTAN (Compact And Programmable daTa Acquisition Node) and is characterized by its flexibility, versatility and scalability by virtue of several key architectural features. These include a vertical bus that permits the user to stack multiple boards, a gigabit Ethernet link that permits high speed communications to the system and a core group of boards that provide specific processing and readout capabilities for the system. System software based on distributed computing techniques supports an expandable network of CAPTANs. In this paper, we describe the system architecture and give an overview of its capabilities.

Design of a fully CMOS compatible 3-μm size color pixel

The Transverse Field Detector (TFD) is a recently proposed CMOS pixel device for color imaging applications without the need of physical color filters. The color detection capability relies on the generation of a suitable electric field configuration in a depleted region of the Silicon active layer. In this paper a newly designed, structurally different version of the TFD is proposed, through which a three-color passive pixel can be implemented with a width reduced to 3 μm in a CMOS standard 90 nm technology. Experimental results on a prototypal device are also reported.

Radiation Resistance of SOI Pixel Devices Fabricated With OKI 0.15 $\mu {\rm m}$ FD-SOI Technology

Silicon-on-insulator (SOI) technology is being investigated for monolithic pixel device fabrication. The SOI wafers by UNIBOND allow the silicon resistivity to be optimized separately for the electronics and detector parts. We have fabricated pixel detectors using fully depleted SOI (FD-SOI) technology provided by OKI Semiconductor Co. Ltd. The first pixel devices consisting of 32times32 matrix with 20 mum times 20 mum pixels were irradiated with 60Co gamma's up to 0.60 MGy and with 70-MeV protons up to 9.3times10 60Co p/cm2. The performance characterization was made on the electronics part and as a photon detector from the response to reset signals and to laser. The electronics operation was affected by radiation-induced charge accumulation in the oxide layers. Detailed evaluation of the characteristics changes in the transistors was separately carried out using transistor test structures to which a wider range of irradiation, from 0.12 kGy to 5.1 MGy, was made with 60Co gamma's.

High Performance, Low-Dose Radiography Systems for Space and Remote Site Applications

Medical imaging applications using X- and gamma-rays have reached outstanding levels of complexity and performance, thanks to technology achievements in the fields of radiography, tomography and high resolution synchrotron devices. The operability of the related instrumentation resides in the availability of highly qualified specialists, as well as of medical doctors for diagnoses and treatment. This is a problem for personnel working in difficult ambient conditions as can be found in remote sites like Arctica, Antarctica and under-populated, desolated or mountainous regions on Earth. Astronauts, during long permanence at the Space Station or on future mission to Mars are potentially subject to risks such as traumas, fractures or diseases which would require semi-automated, easy-to-handle application of X-ray radiographic devices. For this purpose, the possible use of imaging instrumentation based on solid state detectors is discussed with special regard to CdZnTe pixel devices, now available with sub-mm resolution. The use of these semiconductor detectors will also allow to perform image diagnostics at much lower doses compared to the current plate-based radiographic techniques.

A coated pixel device TimePix with micron spatial resolution for UCN detection

A position sensitive detector for ultra-cold neutrons (UCNs) has been developed. Here we present the first experimental results and compare them with Monte-Carlo simulations. The silicon pixel device “TimePix” with a spatial resolution below 1 μm for strongly ionizing particles has been coated with a conversion layer of 6LiF for the detection of UCN. A spatial resolution of 5.3 μm has been measured for UCN. An optimized setup with 10B as conversion layer can achieve a spatial resolution below 3 μm.

Mode purities of Laguerre–Gaussian beams generated via complex-amplitude modulation using phase-only spatial light modulators

We investigate output mode purities of Laguerre-Gaussian (LG) beams generated from four typical simultaneous amplitude and phase modulation methods with phase-only spatial light modulators (SLMs). Numerical simulations supposing the practical SLM, i.e., stepwise phase modulation with a pixelated device, predict an output mode purity of beyond 0.969 for the LG beams of less than radially and azimuthally fifth order. Experimental results of generating LG beams are also shown to demonstrate the effects of the simultaneous phase and amplitude modulation.

Multiplexing schemes for an achromatic programmable diffractive lens

A multiplexed programmable diffractive lens, displayed on a pixelated liquid crystal device under broadband illumination, is proposed to compensate for the severe chromatic aberration that affects diffractive elements. The proposed lens is based on multiplexing a set of sublenses with a common focal length for different wavelengths. We consider different types of integration of the optical information (spatial only, temporal only and hybrid spatial-temporal) combined with a proper selection of the spectral bandwidth. The properties and limits of the achromatic programmable multiplexed lens are described. Experimental results are presented and discussed.