Silicon pinhole strip defects and their impact on ATLAS Inner Tracker HV current measurement

TL;DR

This work investigates pinhole defects in ITk strip modules and their impact on HV-return leakage current measurements used to monitor sensor health under HL-LHC conditions. It combines a analytical pinhole model with empirical observations from BNL and SCIPP to characterize how pinholes alter AMAC HV-return readouts and to develop methods for locating pinholes via ABC bias tuning and light-induced gain measurements. The authors also demonstrate that pinholes can be artificially created through bonding damage and propose QC procedure adjustments—such as turning off ABC power during IV scans and starting 0V-offset measurements at a small negative voltage—to preserve accurate leakage-current diagnostics during module testing. The findings show pinholes do not pose a long-term risk to ITk module testing when these detection and mitigation strategies are adopted, while providing diagnostic tools linking pinholes to bonding damage and potential sensor cracking.

Abstract

In preparation for the High-Luminsoity LHC (HL-LHC), the ATLAS detector will undergo major detector upgrades, including the replacement of the current Inner Detector with the new all-silicon Inner Tracker (ITk). The ITk consists of a pixel detector close to the beamline surrounded by a large-area strip detector. During detector production, the electrical properties of silicon sensors and readout electronics must be characterized through a series of quality control (QC) and quality assurance tests. These tests ensure any defect is captured at the earliest possible stage. One such defect, called a pinhole, occurs when the strip implant and the metal readout electrode are shorted through the intermediary dielectric layer. Notably, the introduction of pinholes during module assembly and pinhole effects on completed modules, especially on leakage current measurement circuitry, have never been studied. In this paper, we investigate the effect of such connections on the sensor leakage current measurements of completed modules and introduce new ways to locate pinholed strips. With minor modifications to testing procedures, such defects are shown not to impede module testing or performance.

Figures (15)

• Figure 1: (a) Exploded view of an ITk short-strip barrel module CERN-LHCC-2017-005. (b) Cross section of an ITk strip sensor. Aluminum strips (blue) are AC-coupled via dielectric (yellow) to n$^+$ implant strips in a p-type bulk. Leakage current travels through the implant strips into bias resistors connected to the bias ring (red).
• Figure 2: AMAC circuit for measuring leakage current traveling through the HV-return line.
• Figure 3: Model of a strip, ABC channel input, and AMAC HV-return channel. A pinhole is represented as a short across the dielectric, represented by $C_C$. $R_b$ is the bias resistor.
• Figure 4: Microscope image of several channels on module BNL-PPB-MLS-244 where bonds were removed before re-bonding. The channels were initially bonded off-center, making contact with the silicon passivation layer and potentially causing a pinhole connection.
• Figure 5: IV curves taken on an endcap module (TRIUMF$\_$R5$\_$0007) built in Vancouver and tested at BNL with (a) ABCs powered and (b) ABCs unpowered. Though with powered ABCs the module appears to exhibit a sudden increase in current at 100V characteristic of early breakdown, the current is actually constant at 0 nA prior to this increase, indicating the AMAC was simply saturating low at lower bias voltages. This behavior disappears when ABCs are switched off.