CMOS Monolithic Active Pixel Sensors (MAPS) direct electron detectors

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CMOS Monolithic Active Pixel Sensors (MAPS) Direct Electron Detectors

Monolithic active pixel sensor (APS) is a complementary metal–oxide–semiconductor (CMOS) chip with a sensitive, doped epitaxial layer. When high-energy electrons pass through this layer, they produce many low-energy electrons that diffuse toward sensor diodes, where they are collected and read out via CMOS electronics. Following commercialization, APS direct electron detectors have been widely adopted. APS detectors offer high sensitivity and fast readout speeds, although they have relatively limited dynamic range. For high-efficiency imaging, single “electron counting” is commonly applied to images recorded with APS detectors. This method requires numerous pixels and relatively low electron doses to reduce the electron density per image to around 0.1 electrons per pixel per frame, as higher densities hinder the localization of individual electron strikes. When these conditions are met, electron counting can enhance the efficiency of 4D-STEM experiments. Notably, due to the relatively simple design of APS detector pixels, these detectors often feature a large number of pixels, which helps lower electron density per pixel.

Figure 6. Schematic of MAPS CMOS detector. This detector's structure and functioning are as follows:

Pixel Spacing: The pixel spacing within the detector is controlled by the spacing between the diodes, which are formed by N-well doped areas (indicated in blue).

Layer Division: The MAPS structure is divided into three primary layers:
- Passivation and Heavily Doped Wells: The top layer includes both the protective passivation layer and regions with heavily doped wells.
- Sensitive Layer: This central layer consists of a lightly doped epilayer, which is responsible for detecting incident electrons by generating electron-hole pairs.
- Heavily Doped Substrate: The final layer is a heavily doped substrate, which supports the structure and plays a role in electron backscattering.

Electron Track and Backscattering: An electron track from an incident electron is illustrated in the schematic, highlighting a significant issue in non-backthinned detectors, where electrons may scatter back from the substrate layer. This backscattering contributes to signal noise and can reduce image clarity.

Electron Collection: Mobile electrons, generated by electron-hole pair excitations within the sensitive layer, are collected via diffusion. This collection process is facilitated by reverse-biased N-well diodes, which gather the electrons diffusing within the epilayer.

MAPS

Figure 6. Schematic of MAPS CMOS detector. [1]

[1] McMullan, G., Chen, S., Henderson, R., & Faruqi, A. R. (2009). Detective quantum efficiency of electron area detectors in electron microscopy. Ultramicroscopy, 109(9), 1126–1143, https://doi.org/10.1016/j.ultramic.2009.04.002.

Practical Electron Microscopy and Database

An Online Book, Second Edition by Dr. Yougui Liao (2006)

CMOS Monolithic Active Pixel Sensors (MAPS) Direct Electron Detectors