Design and implementation of fast and sparsified readout for Monolithic Active Pixel Sensors
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This thesis focuses on the development of smart pixel readout architectures that should ultimately be targeted for the Micro-Vertex Detector (MVD) of the CBM (Compressed Baryonic Matter) experiment. The technical challenge of building a pixel detector for this experiment is to design particle sensors capable of meeting at the same time very strict requirements on both spatial resolution, time resolution and radiation hardness. The MVD is required to obtain data for the open charm physics programme of CBM. For a collision rate of 106 collisions/second, it can be shown that the required time resolution for the first MVD detector station, 5 cm away from the target, is around 2 μs, while the required spatial resolution becomes 5 μm (assuming a material budget of 300 μm Si). The detector has to withstand a non-ionizing radiation of 2 • 1014neq/cm2 and the material budget is about 0.2-0.3 % of radiation length. While bump-bonded hybrid detectors meet the requirements on time resolution and radiation hardness, they do not provide the required spatial resolution and material budget. Detectors based on Monolithic Active Pixel Sensors (MAPS), on the other side, have sufficiently high spatial resolution and low material budget, but they do not meet the requirements on time resolution and radiation hardness. To obtain a detector that has the superior spatial resolution and material budget of MAPS while at the same time providing the fast readout rate and high radiation tolerance of hybrid detectors, new technologies must therefore be explored. Increasing time resolution also means increasing the amount of raw data to transfer. With 2 μs frame readout time, and a typical pixel matrix size of 500,000 binary discriminated pixels, the raw data rate of each sensor becomes 250 Gbits/sec. To keep the amount of serial data links from the sensors at an acceptable level with respect to bonding and material budget, and to avoid overloading the data acquisition system, it is also necessary to embed data reduction or data compression functionalities within the sensor readout electronics. Towards fulfilling the requirements of the needed intelligent pixel detectors, numerous steps have been taken. At the IPHC laboratories of CNRS in Strasbourg, work has been done and is under progress to develop such detectors. The work is done step by step, starting with the most primitive and low bandwidth CMOS pixel sensors while making progress towards ever more complex and sophisticated detectors, with techniques like zero suppression and 3D integration - all techniques that will be explained in detail later in this thesis. In the first generation of IPHC MAPS based pixel sensors, the individual pixels were externally addressed and the pixel matrix was read out entirely . This means that even for detecting a few hits, the entire matrix had to be read out. In fact, as the pixel signal is at the same order as natural process variations, the pixel matrix had to be read twice and the hit signals extracted through offline correlated double sampling. In the second generation of chips, the frame readout time was reduced by embedding correlated double sampling and row-wise parallel readout, called rolling shutter operation. However, without any kind of data compression, such a reduction in the frame readout time comes at the expense of an ever increasing data rate. My contribution to the development of the MIMOSA26 sensor and the first 3D integrated detectors based on MAPS technology will be discussed in the following: Starting my work at IPHC in march 2008, the first steps had been taken towards embedding data reduction micro circuits in the periphery of the active pixel matrix. A prototype circuit, SUZE, interfacing two programmable pixel rows of 128 columns, had been designed, manufactured, and tested, proving the principle of zero suppression. The next step, where I did my first contribution, was then to build the zero suppression into an actual sensor, the MIMOSA26. As MIMOSA26 had 1152 columns, nine times more as those interfaced by SUZE, one had to completely redesign core parts of SUZE for remaining at an operating frequency of 100 MHz. MIMOSA26 was developed and manufactured for providing fast and sparsified readout for the EUDET-JRA1 beam telescope . Although being a large step forward in terms of reduced frame readout time (100 μs), the row-by-row processing of the entire matrix becomes a bottleneck for further reduction in frame readout time. With dual-sided readout and smaller feature size, the architectures based on MIMOSA26 are expected to provide frame readout times down to 25 μs. However, at some point it becomes difficult to decrease the frame readout time due to the inherited architectural limitations. The next step towards improving the time resolution of the monolithic active pixel detectors is therefore to develop a new architecture that utilizes the new degrees of freedom offered by 3D integration. With the first 3D detector prototypes developed at IPHC, two architectural approaches were followed: The first one, targeted at the inner layers of the International Linear Collider (ILC), was to power on all the pixels and provide the analog pixels with a digital tier containing timestamp circuitry (950 μs 25 ≈ 30 μs time resolution) and to read out the timestamp information between the bunch train. The analog pixels could then be turned off completely during the dead time to save power. To provide this architecture with a faster data transmission using a minimum of bonded wires, a readout control circuit with 8b10b serial transmission was designed and implemented. The other approach, required for continuous beam experiments like CBM, continues with rolling shutter operation, but the time for processing each row is reduced by taking advantage of 3D technology to embed a discriminator into each pixel. As the bottleneck of rolling shutter operation still is the fixed line processing time, the next measure to reduce frame readout time is parallellization by splitting into rolling shutter segments operated in parallel. To arrive at a frame readout time as low as 2 μs, required for CBM, and two orders of magnitude lower than the so far achieved frame readout times, the recently submitted 3D integrated rolling shutter architecture was chosen as a starting point for continuing the 3D detector development in the direction necessary for this experiment. Already interfacing a binary discriminated rolling shutter circuit with excellent line processing time, an important part of this work would then be to prevent the digital readout from lagging behind this improved rolling shutter circuitry, and similar measures of parallellization were therefore proposed for the digital zero-suppression circuitry. Instead of keeping the zero-suppression circuitry in the periphery, where it has to process lines one by one at the same speed as that of the rolling shutter, a new and pixelized structure is proposed, where the zero suppression is distributed to the individual pixels, and with the pixel rows extracting their hit information in parallel. To combine the sequential rolling shutter operation of a submatrix with a parallel search for hit patterns, each submatrix splits into two halves, where the rolling shutter injects hits in one half while the zero suppression circuitry extracts pattern information in the other half. The proposed design has been verified through simulation. Compared to the zero suppression circuitry of MIMOSA26, the proposed new readout is superior with respect to scaling with a higher hit density, and it does not have the limitations of a maximum number of hits to extract from each row, only a maximum number of hits to extract from the entire matrix. The thesis is organized into six chapters, with the last chapter containing a summary and an outlook: Chapter 1 starts with a general description of different solid state detectors and their integration with electronics, like hybrid and monolithic, followed by a description of the MAPS architectures and the strategy chosen to provide these sensors with faster readout. Chapter 2 gives a discussion on the state of the art of readout architectures for pixel detectors. It also gives a discussion on the readout electronics of the latest MAPS-based sensors and their requirements, especially focusing on the concepts of zero suppression and clusterisation. Chapter 3 presents the MIMOSA26 pixel architecture, with its fast line processing and sparsified readout. It also presents the work that has been done with creating a test environment for verifying and validating the MIMOSA26 design prior to submission, and it presents the future perspectives of adapting this architecture to new experiments. While chapter 3 presents the last achievements in 2D MAPS, chapter 4 introduces the new techniques of 3D integration. Vertical 3D integration offers the possibilities of using the best features from different processes, and to increase the amount of logic per pixel for providing functionalities previously available only in hybrid detectors. With 3D integration, it is also possible to split rolling-shutter operated MAPS into smaller segments that are operated in parallel, thus avoiding the rolling shutter from becoming a bottleneck for the time resolution. The same chapter also introduces the three first 3D chips designed at IPHC. The first two chips are 3-tier designs with data driven readout or rolling shutter operated readout, respectively. The third chip, which is described in more detail, is a twotier prototype chip, targeted for ILC, which is implemented with time stamping and delayed readout, and with an 8b10b readout interface for fast serial transmission. In chapter 5, a conceptual design is presented for a 3D integrated detector that meets the requirements of CBM. In the proposed design, the zero-suppression techniques of the rolling-shutter based 2D MAPS is combined with the new features of the first 3D integrated circuits. Rolling-shutter operation is still utilized to save power, but the matrix is split into segments to meet the requirements of time resolution. The zero-suppression circuitry that was placed in the periphery of the 2D matrices has been distributed into the pixels. By splitting each submatrix at the digital tier into two halves that are read out at each their time interval, the sequential rolling-shutter operation of rows in the analog tier is combined with parallel token-injection in the rows of the digital tier. Through segmentation and parallellization, the performance of the readout electronics is raised to a level where it is possible to arrive at 2 μs frame readout time and thus meet the time resolution requirements of CBM.
PublisherThe University of Bergen
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