Results from a 2nd production run of low temperature wafer-wafer bonded pad-diodes for particle detection

TL;DR

This paper reports on a second fabrication run of wafer-wafer bonded pad diodes for particle detection, confirming the interface acts as a heavily doped layer and analyzing leakage current behavior, advancing low-temperature bonding techniques.

Contribution

It demonstrates that low-temperature wafer-wafer bonding creates a heavily doped interface layer, validated by contamination-free fabrication results, and analyzes the electrical characteristics of the bonded diodes.

Findings

Bonding interface acts as heavily doped N++ layer

Leakage current behavior is consistent with depletion region generation

Contamination-free fabrication confirms initial interface properties

Abstract

We are investigating the use of low temperature wafer-wafer bonding in the fabrication of next-generation particle pixel detectors. This bonding technique could enable the integration of fully processed CMOS readout wafers with high-Z absorber materials, facilitating the creation of highly efficient X-ray imaging detectors. It might also facilitate the integration of structures embedded inside the wafer bulk, such as deep uniform gain layers. The bonding process results in a thin (nm-scale) amorphous layer at the bonding interface. To study the impact of this interface on detector operation, we fabricated simple wafer-wafer bonded pad diodes using high resistivity float-zone silicon wafers. Results from a first fabrication run of such diodes revealed that the presence of the bonding interface alters the depletion behaviour, with the interface acting as a heavily doped N++ layer. However, metal contamination of the bonding surfaces during fabrication compromised these results, making them unrepresentative of an ideal bonding interface. In this paper we present the results from a subsequent fabrication run, which does not exhibit this sort of metal contamination. These results confirm that the bonding interface behaves as a heavily doped N++ layer, even without contamination. Further, we discuss the reverse leakage current of the bonded samples, where current vs. voltage measurements show a behaviour which is consistent with current generated within the depletion region of the samples.