A New Approach to Achieving High Granularity in Low-Gain Avalanche Detectors

The development of Low Gain Avalanche Detectors (LGADs) over the past five years has opened up the possibility of ultra-precise timing measurements (better than 30 psec) and high-frame rate sensing and imaging (in excess of 500 MHz) for X-rays and high-energy charged particles. However, due to the need to avoid electrical breakdown between neighboring pixel, conventional LGADs include inter-pixel isolation structures that limit the granularity to the 1x1 mm2 scale. Here, we propose the continue development of the Deep Junction LGAD (DJ- LGAD), a novel approach that will allow segmentation at the 10x10 µm2 scale while maintaining the precise timing resolution characteristic of LGADs. In the DJ-LGAD approach, a highly- doped junction is formed several microns below the surface of the sensor, isolating the high-field gain region from the surface electrode structure, and thus avoiding electrical breakdown between electrodes without the inclusion of inter-pixel termination structures. Two fabrication techniques to form the deep junction will be explored at the lead company (Cactus Materials): wafer-to- wafer bonding and epitaxial growth. During Phase-I work, a program of TCAD simulation was undertaken that allowed the conceptual DJ-LGAD idea to be developed into a fabricable planar (non-segmented) device with a stable buried junction. Fabrication parameters, including implantation dose and incidence angle, junction termination geometry (including guard-ring structure), annealing parameters, and junction width and depth, were established by the simulation studies. Preliminary fabrication results suggest that both wafer bonding and epitaxy are viable approach to fabricate the devices. Wafer bonding technique would be a transformational technology as it can enable detectors with dissimilar materials (III-V) along with silicon materials. I-V measurement from Phase I results suggest interface between wafers are electrically conductive. Further study is in progress to fabricate a fully planar DJ-LGAD device by the end of Phase I. The Phase-II project will build on this, refining the wafer-to-wafer bonding approach and developing the epitaxial method. TCAD simulation studies will be used to extend the planar design of the Phase-I prototype to a fully granular design, which will then be fabricated and characterized with X-ray and proton beams. Commercialization strategies will also be developed. Applications within the realm of pure science include 4D and next-generation silicon strip tracking for further exploration of the fundamental building blocks of the universe and the structure and behavior of nuclear matter. Applications within the realm of applied science and technology include medical tomography and imaging, optoelectronic communication, photon and materials science (X-ray imaging and diffraction) and accelerator development.