On the depletion behaviour of low-temperature covalently bonded silicon sensor diodes

TL;DR

This study investigates how low-temperature covalent wafer bonding affects charge collection in silicon sensors, revealing an asymmetric depletion behavior caused by an amorphous interface layer that impacts detector performance.

Contribution

It provides the first detailed analysis of the depletion behavior and charge collection limitations in bonded silicon sensors with amorphous interface layers.

Findings

Amorphous interface layer is about 3nm thick.

P side of bonded structure can be fully depleted.

N side cannot be fully depleted due to interface effects.

Abstract

Low temperature covalent direct wafer-wafer bonding allows for the fusion of multiple semiconductor wafers without any additional material at the bonding interface. In the context of particle pixel detectors this might provide an alternative to bump-bonding for joining sensors to readout chips. Previous investigations have shown that the amorphous layer formed at the interface during bonding is detrimental to charge propagation. To investigate the influence of the bonding interface on signal collection we have fabricated custom test structures by bonding high-resistivity N to high-resistivity P-type silicon wafers thus forming P-N junctions. Scanning transmission electron microscopy shows indeed the formation of ca. 3nm wide amorphous layer at the interface. Using a scanning transient current technique (TCT) setup we were able to record generated signals. Illuminating our sample with light of different wavelengths and from different sides, indicates that the P side of the bonded structures can be fully depleted, but not the N side. This indicates a strongly asymmetric depletion behaviour which we attribute to the presence of the bonding interface.