ERC grant recipient Uwe Kraemer: “Fast-timing silicon is the future”

What is the goal of this ERC proposal?

“I am convinced that silicon pixel detectors with fast timing are the future of all experiments in particle physics. But compared to other areas of tracking detector development, not much fundamental research is being done in this field yet, although interest is growing. Work is being done on this at Nikhef, and we’ve already achieved some results there, but we’ve also run into limitations. We’re now going to tackle those with this new project.”

What is the crux of the matter?

“Improving so-called LGAD sensors—low-gain avalanche diodes. These are particle sensors with internal charge amplification that are highly suitable for fast timing, allowing for rapid detections. The trend is toward smaller pixels for better localization, but we’ve run into limitations in that regard in recent years.”

Can’t they be made even smaller?

“Not without problems. When a particle strikes a sensor, electrons are released in a thin layer and pushed toward the electrodes by an electric field, generating a signal. For spatial localization, each pixel must be electrically isolated. However, the heavily doped layer in LGAD can cause pixels to short-circuit with one another. The current method for creating the smallest separations involves cutting so-called slots into the silicon and filling them with an insulating material. But even then, there’s still a gap between the amplification layer and the slot to prevent breakdown within the silicon. This gap results in what we call a “no-gain” region at the edge of the pixel, where no charge amplification occurs.”

And does that interfere with detection?

“When we talk about the efficiency of 4D tracking, a blind spot arises. For tracking detectors, a sensitivity of 99 percent is the standard. If we require 99 percent efficiency in the same way for well-timed hits, our LGADs with slits currently achieve about 80 percent. That’s much better than with typical LGAD technologies, but too low to be practically usable.”

What can be done about this?

“Through what we call ‘enhanced lateral drift.’ Essentially, this involves creating electric fields around the slot by doping with impurity elements, allowing electrons to be moved toward the center of the pixel to ensure they pass through the amplification layer. Another method is to use this new doping region as an electrical contact. By applying a separate voltage, we can directly adjust the field strength. We’ve already demonstrated in simulations and with a proof-of-concept device that this should work.”

And in practice?

“That remains to be seen. In this project, we’ll first use simulations to develop the perfect formula for the semiconductor fabs that will actually produce the device. Next, it will come down to testing and characterizing prototypes and refining our formula based on these results. Our goal is to have functional LGAD sensors within five years with a spatial resolution of about 10 micrometers and a temporal resolution of 50 picoseconds.”

Will that happen at Nikhef?

“For the most part, yes. We have a fantastic laser lab where we can conduct many of the tests. In addition, beam tests are planned at CERN as soon as the accelerator there is back in operation.”

Where will the new sensors ultimately be used?

“The improved sensors for high-precision timing will arrive too late for the high-luminosity phase of the LHC. Experiments such as ALICE, ATLAS, and LHCb have already made their choices or will do so within the project’s timeframe and implement them in the coming years. These new techniques are relevant for future detector projects, such as a new CERN accelerator.”

Are there any other applications?

“These high-resolution, fast-timing sensors can also be used outside of particle physics, for example in live cameras for low-energy X-rays. Small, fast pixel sensors are useful in numerous applications.”

Back at Nikhef—does that feel good?

“Definitely. With this ERC grant, I can now truly build on my previous work.”

At the same desk where you were sitting six months ago?

“That’s already been arranged. It’s like coming home.”