Fully Depleted CMOS Active Pixel Sensor on High Resistivity Silicon for Tracking of Minimum Ionizing Particles in High Energy Physics Experiments

The next generation of high energy physics experiments will require detectors for tracking Minimum Ionizing Particles (MIPs) at rates of >1000kHz/mm2 in a radiation environment with a neutron fluence up to 2x1016 neq/cm2. The high radiation dose must be tolerated by the sensors which must last over the experiment lifetime without repair or replacement. In addition the already achieved excellent tracking information of today’s pixel detectors must be extended to cover a larger area within established power budgets. This problem is aggravated by increasing particle density and trigger rate, which tend to increase power. The High Energy Physics Community (HEP) has long adopted low power CMOS ICs for reading out semiconductor tracking detectors. But the detector and readout IC for ongoing HEP experiments are fabricated separately and connected on a per chip basis through hybridization or fine pitch wire bonding. This approach suffers from high assembly cost and is not scalable towards high volume production. The detector technology that will provide a solution to these challenges is expected to leverage existing CMOS processes and mainstream semiconductor fabrication and packaging techniques. A monolithic detector, similar to commercial CMOS image sensors in smart phones, seems to be the most viable approach. In the HEP community these detectors are referred to as Monolithic Active Pixel Sensors (MAPS). In phase I we developed a pathfinder chip with a resolution of 256x256 pixels on 10micron pitch which is presently in fabrication as part of an SCI (Sensor Creations, Inc.) internal Multi Project Wafer (MPW) run. The sensor supports 2x2 binning into 20x20micron2 pixels, corresponding to a 128x128 resolution array. Analog as well as digital readout is supported for both resolutions. Digital information is generated by an end of column discriminator with programmable threshold. In phase II we will design a MAPS array with a resolution of 1024x1024 pixels on 20 micron pitch. The device will support low power onchip data reduction based on the OrthoPix architecture. The proposed device will achieve the targeted tracking rate of 100MHz/cm2. After the front side CMOS fabrication has been completed, wafers will be thinned and a common backside electrode is deposited. With a dedicated backside electrode, a strong depletion field can be created throughout the detection area leading to accurate charge collection with minimum lateral diffusion. As the accumulated radiation dose and associated device damage increase throughout the sensor lifetime, the backside bias can be increased independent of any bias constraints of the CMOS circuitry. By the end of phase II we will deliver a complete radiation tracking sensor system with a tracking rate of 100MHz/cm2. To our knowledge this will be the highest tracking rate achievable by any detector chip today. High energy physics experiments need stateofart, reliable, low cost sensors to detect and understand particles that make up matter. This proposal supports the development of such sensors through the use of everyday technology for commercial digital cameras. Key words: Imager, High Energy Physics, Detectors, Backside Illuminated, BSI, CMOS, Active Pixel Sensors, APS.