Silicon Wafer Fracture Stress for Tracking Sensors in Particle Physics Experiments

TL;DR

This work investigates the intrinsic fracture stress of silicon strip sensor wafers used in the ATLAS ITk by performing four-point-bend tests on wafer halfmoons and full-size sensors. It demonstrates that fracture stress is strongly influenced by test orientation and geometry, with extension orientations yielding higher stresses than compression, and reveals significant intra-wafer inhomogeneity with limited correlation to standard QC metrics. Temperature effects are minor, suggesting the CTE mismatch-driven cracking in modules arises from local stress concentrations and defect distributions rather than fundamental material changes at cold temperatures. The findings guide module assembly practices and wafer selection to reduce cracking risk in ITk detectors, contributing to more reliable operation of the tracking system in high-energy physics experiments.

Abstract

For the construction of the ATLAS Inner Tracker strip detector, silicon strip sensor modules are glued directly onto carbon fibre support structures using a soft silicone gel. During tests at temperatures below \unit[-35]{$^{\circ}$C}, several of the sensors were found to crack due to a mismatch in coefficients of thermal expansion between polyimide circuit boards with copper metal layers (glued onto the sensor) and the silicon sensor itself. While module assembly procedures were developed to minimise variations between modules, cold tests showed a wide range of temperatures at which supposedly comparable modules failed. The observed variance (fracture temperatures between \unit[-35]{\textcelsius} and \unit[-70]{\textcelsius}) for supposedly comparable modules suggests an undetected variation between modules suspected to be intrinsic to the silicon wafer itself. Therefore, a test programme was developed to investigate the fracture stress of representative sensor wafer cutoffs. This paper presents results for the fracture stress of silicon sensors used in detector modules.

Figures (14)

• Figure 1: Wafer layouts for ATLAS ITk silicon strip sensors. The two square shapes at the top left are designed for the central part of the tracker ("barrel"). The other shapes were designed using different shapes and sizes for hermetic coverage in the forward part ("end-cap" Carlos). Depending on the geometry of the main sensor, the corresponding sensor halfmoon geometries show different shapes and sizes as well.
• Figure 2: Setup of a Long Strip module in a four-point-bender, with the strip side in extension (facing the bottom rollers). This configuration was also used for all halfmoons large enough to fit the standard geometry.
• Figure 3: Simulation of the stress corresponding to a given displacement for different cutoff geometries. While the results are similar for different geometries, different geometries were accounted for to ensure a correct conversion from load to corresponding stress.
• Figure 4: Fracture test results for halfmoons from randomly selected wafers at different temperatures and for different orientations. While the temperature was found to have only a minor impact on the fracture stress distribution, the orientation of the test pieces was found to affect the results significantly.
• Figure 5: Long Strip sensor wafer layout with the main sensor at the centre and four cutoffs ("halfmoons") surrounding it. The "East" halfmoon is located in an area of the wafer where the crystal lattice orientation is indicated by a straight cut on the wafer, leading to two long parallel edges on this halfmoon.