February_EDFA_Digital

edfas.org 3 ELECTRONIC DEV ICE FA I LURE ANALYSIS | VOLUME 24 NO . 1 EDFAAO (2022) 1:3-10 1537-0755/$19.00 ©ASM International ® DETERMINATION OF INDENTER CRACK PROBABILITY ON MULTILAYER STACKS USING AN ACOUSTIC EMISSION TEST METHOD Marianne Unterreitmeier and Oliver Nagler Infineon Technologies AG marianne.unterreitmeier@infineon.com INTRODUCTION Acoustic emission (AE) testing is one of the most effi- cient and versatile techniques used for nondestructive testing (NDT) of materials. It can detect, localize, and monitormaterial defects during the entire system lifetime without any destruction of the specimen. During solid material fracturing, such as due to a mechanical load, the stored elastic energy is released suddenly, producing acoustic shock waves. For this purpose, AE sensors are physically coupled to the surface of the specimen todetect elasticwaves, whichare radiated spherically fromthe loca- tion of a spreading defect. The volume and surface waves propagate with ultrasonic frequencies (20 kHz – 2 MHz) at lowamplitudes and short durationsmeasuredby piezo- electric sensors. [1] This method is widely used in various science and industry applications to study the formationandgrowthof cracks, delamination, or plastic deformation in mechani- cal structures. A special field of application for the AE test method includes semiconductor devices, which can be damaged during handling, testing, or dicing. During semiconductor wafer probing each device under test (DUT) is mechanically contacted on its con- nection pads by elastic contact springs, or probes, of a probe card, and electrically connected to the automatic test equipment (ATE). The induced mechanical contact stress, however, canpartially exceed the permissible yield. Brittle isolation layers belowthepadmay fracture, causing cracks, delamination, or deformation. In the past, there were only destructive optical meth- ods to detect cracks in brittle insulation layers. This kind of failure analysis, which is only applicable after wafer testing, is time consuming, destructive, and partly inac- curate. To detect material defects in multilayer semi- conductor structures in real-time during wafer probing, an innovative process based on AE testing was recently developed at Infineon Technologies AG. [1,2] This article explains the AE test method and crack- detectionprocess first. Examplesof applications for various semiconductor multilayer structures are described, and a new prediction model for the crack probability is pre- sented, which can be applied for probing-induced failures of complex semiconductor pad stacks. This method is faster, more accurate, and widely applicable compared to classical failure analysis techniques. ACOUSTIC EMISSION TEST METHOD APPLIED ON WAFER PROBING During semiconductor wafer probing, elastic probes are pressed on the pad surface of integrated circuits (ICs), generating regions of highmechanical stress in thin-layer structures below the pad (see Fig. 1). At critical stress conditions, cracks in brittle layers are induced, releasing acoustic shock waves moving further into the body and onto the surface of thewafer. Probing induced inter-metal layer dielectric (IMD) cracks have sizes down tonanometer dimensions with very small released crack energy. This requires very high sensitivity and resolution of the AE measurement system and elimination of disturbances. [1] The elastic waves caused by acoustic events during contacting are transmitted into an AE sensor, which is normally glued below the chip backside (see Fig. 2, left). Most types of probes used for wafer probing generate a lateral force in addition to a vertical force. The lateral scrubbing motion, however, causes an acoustic noise which overlays with a possible oxide crack. Therefore, instead of a conventional spring-like cantilever needle, a rigid vertically arranged needle (indenter) is used, acting exclusively perpendicular to the test structure during the contact without causing a lateral scrub (see Fig. 2, right).