Quantum efficiency modeling for a thick back-illuminated astronomical CCD

TL;DR

This paper models the quantum efficiency and reflectivity of thick, back-illuminated astronomical CCDs, accounting for absorption in all films and fringing effects, and compares the model with experimental data.

Contribution

It introduces a comprehensive modeling approach for thick CCDs that includes absorption in all layers and fringing behavior, differing from standard thin-film optics models.

Findings

Model predictions agree well with experimental quantum efficiency data.

The approach effectively separates surface absorption from substrate absorption.

Reflectivity measurements on wafer samples support the model's validity.

Abstract

The quantum efficiency and reflectivity of thick, back-illuminated CCD's being fabricated at LBNL for astronomical applications are modeled and compared with experiment. The treatment differs from standard thin-film optics in that (a) absorption is permitted in any film, (b) the 200--500~$\mu$m thick silicon substrate is considered as a thin film in order to observe the fringing behavior at long wavelengths, and (c) by using approximate boundary conditions, absorption in the surface films is separated from absorption in the substrate. For the quantum efficiency measurements the CCD's are normally operated as CCD's, usually at $T = -140^\circ$C, and at higher temperatures as photodiodes. They are mounted on mechanical substrates. Reflectivity is measured on air-backed wafer samples at room temperature. The agreement between model expectation and quantum efficiency measurement is in general satisfactory.