Finite-element simulations of coupling capacitances in capacitively coupled pixel detectors

Capacitively coupled hybrid silicon pixel-detector assemblies are under study for the vertex detector at the proposed future CLIC linear electron-positron collider. The assemblies consist of active CCPDv3 sensors, with 25 μm pixel pitch implemented in a 180 nm High- Voltage CMOS process, which are glued to the CLICpix readout ASIC, with the same pixel pitch and processed in a commercial 65 nm CMOS technology. The signal created in the silicon bulk of the active sensors passes a two-stage amplifier, in each pixel, and gets transferred as a voltage pulse to metal pads facing the readout chip (ROC). The coupling of the signal to the metal pads on the ROC side proceeds through the capacitors formed between the two chips by a thin layer of epoxy glue. The coupling strength and the amount of unwanted cross coupling to neighbouring pixels depends critically on the uniformity of the glue layer, its thickness and on the alignment precision during the flip-chip assembly process. Finite-element calculations of the coupling capacitances were performed, based on a detailed 3-dimensional implementation of the geometrical layout for various alignment parameters. We present the simulation setup and results of coupling and cross-coupling ca- pacitance extractions for the current chip versions (CCPDv3 and CLICpix), as well as results for new chips with optimised designs (C3PD and CLICpix2).