An Accurate Ultra-low Current Measurement ASIC for Ionization Chamber Readout

CERN houses one of the most sophisticated pieces of machinery ever built by humans to carry out cutting-edge research in the field of particle physics. The experiments carried out at CERN can generate different kinds of ionizing radiation as a side effect. Using dedicated radiation monitors, the radiation protection (RP) group ensures that the generated radiation is contained within the allowable limit and thrusts to minimize the exposure which ensures the safety of the personnel and surrounding habitants. Ionization chambers are the most used sensors for continuous radiation monitoring for RP. This thesis explains the design, development, and characterization of an application specific integrated circuit (ASIC) that aims to replace the existing discrete component-based front end of ionization chambers used at CERN. The presented research started with the in-depth characterization of the first-generation ASICs designed in the RP group - UTOPIA. Measurements with pulsed radiation fields exposed the limitation of UTOPIA for efficient collection of charge pulses necessitating the need for a redesign with architectural modification. Apart from capacitating the analog front end for improved charge measurement, integrating the digital data processing section into the ASIC to create a mixed-signal, single-chip solution was also envisaged. UTOPIA ASICs were designed in AMS 350 nm technology which exhibited femtoampere leakages which were vital in achieving the ultra-low current measurement capability. The main hurdle faced in designing the second generation of the ASIC – ACCURATE, was identifying a suitable long-term replacement for the proven 350 nm node. A detailed literature review ascertained that no design is reported in technology nodes of 130 nm or finer that achieves femtoampere sensitivity and attains the dynamic range from femtoampere to microampere. Test structures made in 22FDX of GLOBALFOUNDRIES and TSMC 130 nm technology established techniques and architectures to attain the required dynamic range and sensitivity. Design of ACCURATE 2, which is the main focus of the research, is presented in detail. An architecture combining two current measurement methods, charge balancing current to frequency conversion and direct slope measurement method, was designed. It also incorporates all the digital logic along with the analog section to process the generated data and provide data storage and eventual transfer to an external system through a serial interface. The ASIC is successful in achieving a remarkable sensitivity of 200 aA and demonstrated a wide dynamic range from around -6 fA to -20 µA. The use of thick gate transistors in the leakage critical analog path, optimal feedback capacitance and three stepped progressive charge balancing with 500 fC, 1 pC, and 4 pC charge paths helped in achieving this performance. Prudent floorplanning utilizing Deep N-Well structures to minimize noise coupling along with guard rings and path length matching helped in attaining the sensitivity even with the mixed signal version. The ASIC occupies an area of 3.52 mm2 and reports a total power consumption of 17.4 mW. Characterization with the continuous current generated by different kinds of ionization chambers when exposed to radiation sources demonstrated the dose rate measurement of the ASIC from 5 µSv/h to 7.4 Sv/h. Dynamic characterization with pulsed radiation established the improvement of around 16% more charge collection by ACCURATE 2 compared with UTOPIA 2 for a charge pulse of 100 nC. The reason for the limitation in charge collection for charges above tens of nanocoulombs was identified to be caused by saturation of OTA with the high influx of charges beyond that could be compensated by the charge balancing block. The improvement achievable by increasing the time constant of the charge collection path was demonstrated. The designed ASIC thus improves the sensitivity, dynamic range and charge measurement efficiency of the existing systems and offers a first of its kind mixed-signal single chip solution for ionization chamber frontends. The ASIC can also be employed in applications such as biosensor readouts and device characterization which demands similar performance levels. ACCURATE 2 must be upgraded from a prototype to a certified reliable unit for the next generation of radiation monitors for CERN. The path culminating in ACCURATE 3 with different improvements and modifications is also laid out.