Boosting Quantum Efficiency in Silicon Sensors: A Leap Forward for X-ray Imaging

Quantum efficiency (QE) is a crucial factor in the performance of silicon sensors used in synchrotron and X-ray free-electron laser facilities. Researchers have improved the QE of silicon sensors by refining the design of the entrance window, increasing the QE for 250 eV photons from 0.5 to 6.2. This improvement has significant implications for various scientific and technological applications, including the Swiss Light Source and Swiss X-ray free-electron laser at the Paul Scherrer Institut. Despite these advancements, further research is needed to overcome limitations and continue improving silicon sensor technology.

What is Quantum Efficiency and Why is it Important in Silicon Sensors?

Quantum efficiency (QE) is a critical parameter in the performance of silicon sensors, particularly those used in synchrotron and X-ray free-electron laser facilities. These facilities rely on hybrid pixel detectors, which are favored for their large dynamic range, high frame rate, low noise, and large area. However, when it comes to energies below 3 keV, the performance of these detectors is often limited due to the poor quantum efficiency of the sensor and the difficulty in achieving single-photon resolution due to the low signal-to-noise ratio.

Quantum efficiency refers to the ability of a sensor to convert incoming photons into electrons. The higher the quantum efficiency, the better the sensor is at detecting light. In the context of silicon sensors used in X-ray detection, a high quantum efficiency means that the sensor can detect more X-ray photons, resulting in a clearer and more accurate image.

How Can the Quantum Efficiency of Silicon Sensors be Improved?

The researchers in the study focused on improving the quantum efficiency of silicon sensors by refining the design of the entrance window. This was achieved mainly by passivating the silicon surface and optimizing the dopant profile of the n-region. The researchers presented the measurement of the quantum efficiency in the soft X-ray energy range for silicon sensors with several process variations in the fabrication of planar sensors with thin entrance windows.

The results showed that the quantum efficiency for 250 eV photons increased from almost 0.5 for a standard sensor to up to 6.2 as a result of these developments. This is comparable to the quantum efficiency of backside-illuminated scientific CMOS sensors. The researchers also discussed the influence of the various process parameters on quantum efficiency and presented a strategy for further improvement.

What are the Applications of Silicon Sensors with Improved Quantum Efficiency?

The improved quantum efficiency of silicon sensors has significant implications for various scientific and technological applications. For instance, the Swiss Light Source (SLS) and Swiss X-ray free-electron laser (SwissFEL) at the Paul Scherrer Institut (PSI) have several beamlines working in the soft X-ray (SXR) energy range (200 eV-2 keV). The SXR energy regime covers the binding energies of the innermost electron shell (K-shell) of light elements, the second electron shell (L-shell) of 3d transition metals, and the third electron shell (M-shell) of rare earths.

These energies are relevant to the structural, physical, and electronic properties of condensed matter. Techniques such as resonant soft X-ray scattering (RSoXS) exploit the enhancement of scattering contrast and the high spatial resolution of SXR to reveal the nanostructure of thin films with light elements like organic semiconductors or polymers. Improved silicon sensors can enhance the performance of these techniques, leading to more accurate and detailed results.

What are the Limitations of Current Silicon Sensors and How Can They be Overcome?

Despite the advancements in silicon sensor technology, there are still limitations that need to be addressed. For instance, the poor quantum efficiency (QE) of standard silicon sensors and the low signal-to-noise ratio (SNR) obtained at low photon energies limit their suitability for use in the SXR energy range.

To overcome these limitations, researchers are exploring different strategies, including the use of monolithic detectors at SXR beamlines. These detectors, specifically the backside-illuminated charge-coupled device (CCD), have a pixel size that can be less than 20µm and noise as low as a few electrons. However, they also have their own shortcomings, such as a slow frame rate, radiation damage, the requirement for an electronic shutter, limited area, and a decrease in QE for X-ray energies larger than 2 keV due to the thin silicon thickness.

What is the Future of Silicon Sensor Technology?

The study presents a promising direction for the future of silicon sensor technology. By improving the quantum efficiency of silicon sensors, researchers can enhance their performance in various applications, from synchrotron and X-ray free-electron laser facilities to other areas that require high-quality imaging.

However, further research is needed to fully understand the influence of various process parameters on quantum efficiency and to develop strategies for further improvement. As the field continues to advance, we can expect to see more innovative solutions to the challenges currently faced by silicon sensor technology.