Imaging and analysis of submicron single-crystal aluminum whiskers in Al/Cu/Si deposited films

Abstract

The formation of stress-relieving hillocks, also referred to as single-crystal whiskers is a well-known occurrence in the preparation of metallic thin films. In the manufacture of Application Specific Integrated Circuits, devices at the wafer level are often processed through the first metallic thin film process, coated with a plasma enhanced oxide (PEO), then stockpiled either at the manufacturer's site or the customer's to await final option-level programming. Whisker growth has been detected on wafers retrieved from the stockpile inventory. This paper describes the techniques employed for initial detection, subsequent sample preparation, and the composition analysis of submicron diameter whiskers in Al/Cu/Si thin films on inventoried product wafers.First indications of whisker growth are increased surface defect levels observed during high power optical microscope sample inspection. Typical appearance of the whiskers, as viewed with an optical microscope, are shown in Figure 1. Structures thought to be surface particles are later imaged in the SEM and confirmed to be oxide-covered whiskers, as shown in Figure 2. Imaging of the nonconductive oxide layer is accomplished via a Field Emission SEM.