Speaker
Description
Next-generation medical imaging modalities, particularly high-resolution time-of-flight positron emission tomography (TOF-PET), require photodetectors that combine excellent timing precision, high spatial granularity, and compatibility with advanced system integration. To address these needs, Fondazione Bruno Kessler (FBK) is developing silicon photomultipliers (SiPMs) specifically conceived for 2.5D and 3D integration with dedicated readout electronics through customized through-silicon via (TSV) solutions.
By overcoming the limitations of traditional wire bonding, this TSV-based approach enables high-density interconnections and finer detector segmentation, both of which are essential for precise clinical imaging. At the same time, it improves signal integrity by reducing parasitic output capacitance per channel, thereby preserving fast signal transitions and supporting the excellent coincidence time resolution (CTR) required in modern PET systems. The feasibility of this segmentation strategy is confirmed by the timing performance of micro-SiPM structures: experimental characterization shows that the single-photon time resolution (SPTR) remains substantially unchanged from a 7 × 7 SPAD matrix up to a 1 × 1 mm² active-area SiPM, with values close to those of a single 50 μm SPAD (≈25 ps FWHM) under blue-light excitation and high-frequency readout.
This integration strategy is currently implemented on FBK’s NUV-HD-MT (Near-Ultraviolet High-Density Metal-in-Trench) SiPM technology, which is optimized for timing applications and features an excellent SPTR, a photon detection efficiency (PDE) of about 65%, a dark count rate below 80 kcps/mm², and a direct crosstalk probability below 3% at 10 V excess bias. The same approach is expected to be extended to the more advanced NUV-DJ technology, recently demonstrated by FBK with even higher PDE while preserving excellent timing performance.
Two TSV solutions are under development. The first is a glass-less TSV, which supports segmentation below 0.5 mm pitch and removes the glass carrier wafer after thinning, enabling direct coupling between the scintillator and the SiPM and avoiding intermediate materials that could degrade timing performance or radiation tolerance. The second is the Single-Cell Access (SCA) TSV, which enables interconnection down to the microcell level. In this approach, the SiPM microcells are electrically isolated and can be regrouped on the backside through a redistribution layer into micro-SiPMs, such as 3 × 3 or 6 × 6 cell clusters, for applications requiring extremely fine granularity.
Comprehensive integration tests were carried out to validate the TSV process and backside interconnection. Laser-assisted ball deposition demonstrated nearly 100% yield at 500 μm pitch, with a resistance below 3 Ω per ball. A representative system-level application is the PETVision project, where glass-less TSV-integrated 12 × 6 mm² SiPM dies, each embedding a monolithic 2 × 4 array of 3 × 3 mm² channels, will be combined with a dedicated ICCUB ASIC in a 2.5D architecture to build 50 × 50 mm² photon detection modules with high fill factor and seamless tiling for next-generation TOF-PET.
| Track | FTMI |
|---|---|
| Presentation type | Oral |