2-4 April 2014, Cannes Côte d’Azur, France ©EDA Publishing/DTIP 2014 ISBN: 978-2-35500-028-7 Determination of material properties and failure using in-situ thermo-mechanical probe B. Arrazat 1 , S. Orellana 2,3 , C. Rivero 3 , P. Fornara 3 , A. Di-Giacomo 3 , S. Blayac 1 , P. Montmitonnet 2 , K. Inal 2 1 Ecole Nationale Supérieure des Mines de Saint-Etienne, CMP, 880, route de Mimet, 13541 Gardanne, France 2 Mines ParisTech, CEMEF UMR CNRS 7635, 1, rue Claude Daunesse, 06904 Sophia Antipolis Cedex, France 3 STMicroelectronics, TR&D, 190, avenue Célestin Coq, 13106 Rousset Cedex, France Abstract – A metallic in-situ stress sensor is modified to address electrical polarization and thus to locally heat this sensor by Joule effect. By coupling SEM electrical nano-probing with analytical modeling and multiphysics Finite Element Method (FEM), the thermo-mechanical properties are identified. As a result, a tensile stress state of 190 MPa, coefficient of thermal expansion of 22.5x10 -6 K -1 and thermal conductivity of 190 W/(K.m) are identified in the aluminum thin film in agreement with literature. Moreover, high current induces irreversible deformation and breaking. Using multiphysics FE model with identified thermo-mechanical properties, the failure of the sensor under electrical solicitation is investigated. The evolution of local temperature and mechanical deformation on different sensor designs allows the determination of the breaking location and condition. Keywords: Back-End of Line (BEoL), embedded sensor, thermo- mechanical properties, Joule effect, Finite Element Modeling, in-situ SEM nano-probing, failure mechanisms. I. INTRODUCTION In the past decade, the integrated circuits have strongly increased their performance due to the down-scaling of transistors [1]. The CMOS technology becomes more and more complex with higher integration density and introduction of new materials. For the metal interconnexion layers (Back End of Line - BEoL), metallic-intermetallic dielectric having heterogeneous thermo-mechanical properties are introduced [2]. The manufacturing thermal budget induces mechanical stress that can result in mechanical failures [3]. An in-situ sensor was developed in order to monitor the metal stress level [4-5]. Using standard CMOS BEoL processing on 8” wafer, aluminum thin film is patterned on dielectric layer. The stress sensor is composed by metallic arms and a rotating beam surrounded by dielectric. As the structure is released from its surrounded layer, the relaxation of residual stress induces a displacement of rotating beam. Using an analytical model [6], the measurement of this displacement allows the determination of residual stress in accordance with Finite Element Method (FEM) [7]. This approach is validated by experimental verification using X-ray technique and Stoney formula [8, 9]. This sensor turns out to be a local, fast and fine tool for material characterization under micron scale. More options are developed using electrical polarization. A series of rotating sensors are electrically connected in order to measure the electromigration-induced stress. Thus, a direct observation of stress gradients in aluminum interconnect metallization is done [10]. This sensor is also used to determine the thermo-mechanical properties (as thermal expansion and conductivity) [11]. It is locally heated by Joule effect (section II). Then, due to thermal expansion, the rotating beam will be displaced. By coupling experiments with FEM and analytical modeling, a gradual approach is developed to determine these parameters. Moreover, high current induces irreversible deformation and breaking [12]. The purpose of this paper is to investigate the failure of the sensor under electrical solicitation using predictive FEM [11]. For this raison, the FE thermo- electro-mechanical model is presented (section III). This model is used to study the local variation of the deformation and the temperature on the sensor (section IV). Thus, the location of the breaking is determined and a method to predict the breaking current is proposed. II. SENSORS The presented sensor is manufactured using standard CMOS process on 8” silicon substrate. An aluminum thin film, corresponding to sensor layer, is deposited (550 nm thick) and patterned on dielectric. The sensor (Figure 1) consists of two anchors (80x80 μm²), a rotating beam and arms (1 μm width and 30 μm length). The position of patterned rotating beam corresponds to a referential point (zero deviation, d=0, Figure 1). Figure 1. Sensor design (called “cross”).