X-Ray Visionaries

Developed for use in high energy particle physics, applications for KEK’s new SOIPIX technology can also extend to medical imaging.

Medical challenge

The medical community is looking to tools developed for particle physics to solve a host of diagnostic and therapeutic challenges. On the diagnostic side, digital X-ray systems with semiconductor radiation detectors (in which sensor arrays replace film) offer greater anatomical precision, higher sensitivity, higher speed (so lower radiation dosage) as well as more functional information (such as density). Doctors would also like to see X-ray systems integrated with complementary systems such as MRI, and in vivo imaging techniques. On the therapeutic side, small, low-power, radiation-hardened detectors could be leveraged in real-time, integrated, tunable dosimetry for radiotherapy. But as always, the bottom line is cost.

Design challenge

In conventional bulk CMOS technology, X-ray imaging sensors and associated readout electronics are typically produced separately, then bump-bonded. They need to be smaller, cheaper, faster, higher performance and lower power.

SOI solution

KEK* scientists, working with an international team, have developed a new detector technology for high energy particle physics, which they call silicon-on-insulator pixel (SOIPIX).

In KEK’s SOI solution, the sensor and its associated electronics are produced together on a single chip, separated by just a very thin, insulating Buried OXide (BOX) layer.

The ingenuity of the KEK detector design is that it uses the silicon substrate as the sensor. This arrangement reduces the parasitic capacitance in the device, and speeds the readout. As is always the case with SOI, the insulator reduces leakage current, resulting in a faster, lower-power device.

Beyond particle physics, SOI advantages specific to the SOIPIX detector are also applicable in the context of medical X-ray sensors:

  • insulation between the pixels protects them from electrical signals generated by neighboring pixels – so the pixels can be smaller and more sensitive, for higher performance imaging in a smaller system
  • the thickness of the sensor can be adjusted – so it can fit in a smaller package
  • operation over a much broader temperature range – heat dissipation is a major challenge in the design of radiographic systems
  • radiation hardness – making them suitable for medical X-ray and radiotherapeutic systems
  • lower power – which particularly benefits moves to smaller, mobile or in vivo systems
SOIPIX is a technology that combines the sensor and read-out electronics on a single semiconductor device. (Courtesy: KEK)

SOIPIX is now KEK’s largest and best-funded international project, with 16 partners including Fermilab, Lawrence Berkeley, and several European institutions.  Research is ongoing, exploring next steps such as vertical integration. OKI Semiconductor provides its industry-leading expertise in FD-SOI design, and fabricates the chips in multi-project wafer (MPW) runs.

Part of this article was adapted from a feature on the KEK website, www.kek.jp. Many thanks to Dr. Yasuo Arai, leader of the KEK SOI group, for his help.

*KEK is Japan’s High Energy Accelerator Research Organization.

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