High Precision Charge Detection Frontend Electronics

Abstract

This Ph.D. dissertation focuses on designing high-precision readout frontends for low energy charge detection in scanning electron microscopy (SEM), achieving a time resolution of 2.5 ns, a detection error rate below 6 ppm, and power consumption under 400 μW. Novel techniques at both the system and circuit levels were developed to enhance operational accuracy and meet the target specifications. Two prototypes were presented and experimentally tested to demonstrate the effectiveness of these techniques.
Chapter 1 introduces the motivation, research objectives, and organization of the thesis, highlighting the advancements in SEMs for nanometer-resolution imaging and the challenges posed by high scanning speeds. It emphasizes the need for sensitive detectors and low-noise, power-efficient readout electronics, which often conflict. The main research question is defined as developing a frontend readout architecture with power consumption below 500 μW, time resolution of 2.5 ns, and an electron count error under 10 ppm. To address this, the thesis employs a systematic study and iterative design process, resulting in two novel readout frontend architectures. The chapter also outlines the structure of the thesis, covering the operating principles of the PIN diode, design details, experimental evaluations, and conclusions.
Chapter 2 provides a detailed review of the target application specifications, focusing on the design and requirements for detecting weak charge signals with high precision and time resolution. It critically analyzes the current state-of-the-art readout frontends, highlighting their strengths and inherent limitations, particularly in terms of noise performance, time resolution, and power consumption. This chapter also introduces the concept of short and open circuit readout modes for PIN-diodes, offering insights into their potential advantages for addressing the challenges identified in the existing systems.
Chapter 3 presents the design of readout solutions for the short circuit operation mode of PIN-diodes, critical for BSE detection in electron microscopy. It examines the use of a preamplifier to create a virtual ground, effectively simulating a zero-impedance load and ensuring accurate charge transfer. The chapter further explores the analog frontend components: preamplifier, signal shaping filters, and threshold discriminators. Signal shaping filters, both passive and active high-pass types, are discussed for their role in signal optimization by reducing noise and improving signal clarity. Additionally, the threshold discriminator design is analyzed for both filter types, emphasizing the importance of accurate signal discrimination to minimize detection errors. A key focus is the tradeoff between power consumption, noise performance, and detection accuracy, with each stage's design detailed to ensure optimal performance and signal integrity in the short circuit mode.
Chapter 4 explores readout solutions for the open circuit mode of PIN-diodes, focusing on high sensitivity, low power consumption, and signal integrity. It highlights challenges like charge pileup and saturation, proposing solutions such as a reset mechanism and dynamic comparators. The chapter discusses an advanced frontend architecture with offset compensation and active capacitor matching for improved accuracy. Periodic sampling at 800 MHz minimizes timing misalignments, balancing power efficiency and reliability for high-resolution, high-rate applications.
Chapter 5 discusses the experimental setup and qualification of the proposed readout architectures. The device under test (DUT), a 40 nm CMOS chip with short and open circuit mode readout matrices, is tested to validate its ability to detect and digitize charge signals within the specified power budget. The test includes evaluating performance across gain, noise, bandwidth, and threshold levels, using a programmable detector emulating circuit (DEC) to simulate charge signals. The setup features a FPGA-based Data Acquisition Board (DAB) for signal monitoring and a test PCB to run experimental qualifications.
Chapter 6 concludes the thesis by highlighting the development of advanced readout frontends for high-precision charge detection, achieving improved time resolution, accuracy, and power efficiency. The proposed designs, optimized for short circuit and open circuit modes, demonstrate excellent performance. This chapter also proposes and discusses some aspects of this work that could be explored for further improvements.