Radiation Hardened Engineered Substrates for Time and Space Resolution

We propose to develop engineered substrates to improve radiation hardness and stability of pixel sensors by using wafer bonding and graded epitaxial techniques. We aim to produce a moderately doped (1E13-1E16) thin buried gain layer (~4-6 micron) using graded epitaxy on a high resistivity bonded substrate. Current state-of-the-art is a proposed sensor design of low to moderate gain (x10-50) reach through silicon avalanche diodes (LGADs) to achieve ~10 ps time resolution for collider experiments. The current generation of reach-through diodes suffers from large fractional dead area at the edges of the pixel (>50 micron), poor spatial resolution, and only moderate radiation hardness. The standard LGAD is a “reach through” diode - a deep moderately doped p “gain” implant though a shallow, highly doped n-type anode. When depleted, the ionized dopant atoms generate a gain field in the reach-through region. The field depends in density and distribution of dopants. In this process, radiation removes donors and creates acceptors, which modifies the field in the reach-through region, ultimately eliminating gain. A moderately doped thin buried (~4-6 micron) layer replacing a reach-through implant can address some of these problems. The epitaxial layer can be made well defined thinner gain layer and denser than the reach-through design with the same number of dopant atoms, hence improve effect of radiation. The substrate construction would be based on Cactus Materials, Inc. developed vacuum wafer bonding techniques with oxide free interface (silicon to silicon direct bonding) and commercially available standard epitaxy layer. The dopant concentration and depth of epitaxial layers will be varied along with bonding interface characterization to optimize the performance of the substrates (i.e. charge collection). The characteristics can be tuned by adjusting depth and density of the epitaxy. “AC coupled” buried layer will solve edge problem and make design simpler. In the AC coupled LGAD the prompt signal is coupled through a thin oxide layer to the pickup electrode. The sheet resistance of the anode implant and coupling capacitance has to be tuned to provide the proper output pulse shape, neighbor signal, and DC coupling to ground.