Fluorescence microscopy is a highly sensitive laboratory diagnostic method for examining the smallest sample quantities in the development of pharmaceuticals, for medical diagnostics and other “life sciences” research and development fields. To detect the fluorescent light, a research team from the CiS Research Institute developed highly sensitive low-noise blue-violet avalanche photodiodes with very low noise. Avalanche photodiodes (APD) have a very high sensitivity for very weak light signals due to the internal amplification effects. APDs available on the market are spectrally designed for the green to red spectral range. For the blue to (ultra)violet range, no highly efficient and low-noise APDs exist as yet. In addition, blue and violet photons have a significantly smaller penetration depth into the silicon. In conventional APDs, therefore, only a small part of the internal amplification zone is used. In order to still be able to measure blue and violet light, an excessive electrical bias voltage is required, which in turn causes increased background noise in the output signals. This results in unfavorable signal-to-noise ratios of the detected signals, which can increase measurement times and/or limit the detection limits of the measurement system.
The BVAPD project focused on a technological development that creates a very flat near-surface amplification zone within the silicon component. The key element of the component development was a precisely optimized course of the gain zone near the surface of the silicon, designed for the targeted spectral range. For this purpose, an optimized depth profile of impurity atoms (doping) was designed and implemented, which is both very flat and at the same time stress-resistant. In this way, the gain zone can be limited to the range of the actual light penetration depth and excessive operating voltages (compared to the state of the art) are no longer required. In addition, the quantum efficiency is increased by a technological optimization of this conversion zone and an anti-reflective coating. The operating voltage has also been reduced by a clever combination of mask design and doping. Further noise sources are suppressed by a trench structure around the active areas.
The developed BVAPDs with moderate electrical operating voltages (around 210 – 220 V) are suitable for the spectral range from approx. 300 nm to 550 nm (maximum around 400 nm) and achieve an internal amplification factor of well over 100. This provides device and sensor developers with a new component that can be used to make low-light effects (such as fluorescence) measurable.
The research and development work described was funded by the Federal Ministry of Economic Affairs and Climate Action (BMWK) in the research project “Highly sensitive low-noise blue-violet APD” (BVAPD).
Funding code: 49MF200098