Hybrid Pixel Sensors Research Articles

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.

Recent development on CMOS monolithic active pixel sensors

Vertex Detectors for future high energy physics experiments will need to fulfill more stringent requirements with respect to presently operating tracking detectors. Existing pixel devices (such as hybrid pixel sensors and charge-coupled-devices) have yet to strike an appropriate balance among granularity, material budget, radiation tolerance and readout speed. On the contrary, CMOS Monolithic Active Pixel Sensors (MAPS) are a promising candidate for application in future tracking devices: they incorporate on the same substrate the readout electronics and an extremely thin active radiation detector. CMOS MAPS have been developed by several groups in the last few years, following the readout scheme already adopted for imaging applications, achieving very good tracking performance for charged particles. This paper will review the main feature of CMOS sensors and will give an overview of the main R&D activities in this field. In particular, recent progress made in the implementation of an innovative CMOS MAPS sensor, with on-pixel continuous-time analog signal processing and data sparsification capabilities will be presented. This new device incorporates the potential for thin detector fabrication, typical of MAPS sensors, and the flexibility and high functional density featured by hybrid pixel sensors.

The Hybrid Pixel Sensors and Read-Out Electronics

Hybrid pixel sensors (detectors) have shown to be a valid alternative to other types of Xray imaging devices due to their high sensitivity, linear behavior and wide dynamic range, and low noise. One important feature of these devices is the fact that detectors and readout electronics are manufactured separately. The charge created by the interaction of X-ray photons in the sensor is very small and has to be amplified in a low-noise circuit before any further signal processing. The signal induced on the electrodes of the sensor is transferred to the readout chip, where it is integrated in a charge sensitive amplifier. The issue reviews on physical principles of operation and design of the hybrid pixel sensors developed on the basis of the silicon CMOS and GaAs MESFETtechnologies. The authors have designed GaAs charge sensitive amplifiers for hybrid pixel detectors and show the results of a simulation.

First detective quantum efficiency measurement of 500 μm silicon hybrid pixel sensor with photon counting readout for X-ray medical imaging

We report the performances of a 500 μm pixellated silicon sensor bonded to the photon counting chip Medipix2 [1]. In order to perform an absolute characterization of our detector, we measured both the pre-sampling MTF and NPS with respect to the International standard IEC-62220-1. From those data we have been able to extract the Detective Quantum Efficiency (DQE) and hence to assess the suitability of our detector for X-ray medical imaging purpose. Due to poor absorption of the Si at 70 kV the DQE peaked at 0.06 for null frequency. Nevertheless, these results are very promising since thicker Si or more absorbing material such as GaAs will soon be available.

A chip removal facility for indium bump bonded pixel detectors

Yield maximization in multichip hybrid pixel sensors is a crucial issue in large volume productions planned for future High-Energy Physics experiments. Bump bonding process optimization can guarantee statistical single bump failure rates at the acceptable level of 10–100 ppm; nevertheless, systematic effects connected to process repeatability can affect the functionality of a full chip in a module to a much larger extent. Because of this, the reversibility of the bonding procedure has been investigated. A feasibility study on single chip assemblies for the ATLAS experiment has been successfully completed, proving the possibility of reworking. As a result of it, a dedicated facility has been conceptually designed, engineered and commissioned. The characteristics of the facility in terms of motion, temperature and tensile strength control are outlined, together with the main results.

Monolithic active pixel sensors with in-pixel double sampling operation and column-level discrimination

Monolithic active pixel sensors constitute a viable alternative to hybrid pixel sensors and charge coupled devices for the next generation of vertex detectors. Possible applications will strongly depend on a successful implementation of on-chip hit recognition and sparsification schemes. The task is tough, first because of very small signal amplitudes (/spl sim/ mV), which are of the same order of magnitude as natural dispersions in the transistor threshold voltages, secondly because of the limitation to use only one type of transistor over the sensitive area. This paper presents a 30/spl times/128 pixel prototype chip, featuring fast, column parallel signal processing. The pixel concept combines in-pixel amplification with double sampling operation. The pixel output is a differential current signal proportional to the difference between the reference level and the charge collected. The readout of the pixel is two-phase, matching discrimination circuitry implemented at the end of each column. Low-noise discriminators feature autozero functionality. The details of the chip design are presented. Difficulties, encountered in the first attempt to address on-line hit recognition, are reported. Performances of the pixel and discriminator blocks, determined in separate measurements, are discussed. An important part of this paper consists of results of first tests performed with soft X-rays from a /sup 55/Fe source.

Hybrid active pixel sensors and SOI inspired option

Two novel pixel sensor concepts for future linear collider applications are presented in this paper: a hybrid pixel sensor characterized by a layout improving the single point resolution and a monolithic detector inspired by silicon on insulator (SOI) technology. The results of charge collection studies for the first prototypes of hybrid pixel sensors with interleaved pixels are reported and the new detector test structures are introduced. The technology and the readout architecture design for SOI sensors are also discussed.

Hybrid pixel detector development for the linear collider vertex detector

In order to fully exploit the Physics potential of future e+ e- linear collider, a Vertex Detector providing high resolution track reconstruction is required. Hybrid Silicon pixel detectors are an attractive option for the sensor technology due to their read-out speed and radiation hardness but have been so far limited by the achievable single point resolution. A novel layout of hybrid pixel sensor with interleaved cells to improve the spatial resolution has been developed. The characterisation of the first processed prototypes is reported.

High-resolution hybrid pixel sensors for the e +e − TESLA linear collider vertex tracker

In order to fully exploit the physics potential of a future high-energy e +e − linear collider, a Vertex Tracker, providing high-resolution track reconstruction, is required. Hybrid silicon pixel sensors are an attractive option, for the sensor technology, due to their read-out speed and radiation hardness, favoured in the high-rate environment of the TESLA e +e − linear collider design, but have been so far limited by the achievable single point space resolution. In this paper, a conceptual design of the TESLA Vertex Tracker, based on a novel layout of hybrid pixel sensors with interleaved cells to improve their spatial resolution, is presented.