P25 978-1-4799-5288-5/14/$31.00 c ⃝ 2014 IEEE 249 Electromigration Induced Resistance Increase in Open TSVs W. H. Zisser * , H. Ceric *† , J. Weinbub * , and S. Selberherr * * Institute for Microelectronics, TU Wien, Vienna, Austria † Christian Doppler Laboratory for Reliability Issues in Microelectronics at the Institute for Microelectronics Email: {zisser|ceric|weinbub|selberherr}@iue.tuwien.ac.at ABSTRACT Through silicon vias are the components in three- dimensional integrated circuits, which are responsible for the vertical connection inside the dies. In this work we present studies about the reliability of open through silicon vias against electromigration. A two-step approach is followed. In the first step the stress development of a void free structure is analyzed by means of simulation to find the locations, where voids due to stress are most probably nucleated. In the second step, voids are placed in the through silicon vias and their evolution is traced including the increase of resistance. The resistance raises more than linearly in time and shows an abrupt open circuit failure. Simulations were carried out for different currents and fitted to Black’s equation. These results are in good agreement with results of time accelerated electromigration tests. I. I NTRODUCTION Three-dimensional (3D) integration is a promising ap- proach for the development of systems with higher perfor- mance. Interconnections for 3D integrated circuits, though, include components not used in planar 2D architectures, such as through silicon vias (TSVs). Open TSVs introduced in [1] are a TSV concept in which the cylindrical structure is coated, rather than entirely filled with a conducting metal. Fig. 1 shows the upper part of the TSV including the feeding interconnection. The advantage of this technology is the ability to reduce the stress originating from the mismatched thermal expansion coefficients between the substrate and the TSV. The reliability of interconnects in integrated circuits is an important issue in microelectronics. Therefore the various degradation processes and their impact on the different compo- nents have to be evaluated. One of those processes of particular importance is electromigration (EM), which is essentially the flux of material due to current flow. On the atomistic level EM is the impulse transfer from the conducting electrons to the ionized metal atoms. This degradation process can be divided into two phases. During the first phase the flux of the material leads to the build-up of intrinsic stress. As the stress reaches a certain threshold value, voids can form especially at those locations, where the adhesion of the interconnect metal and the surrounding material is reduced. The formation of a void is the beginning of the second phase. During this second phase, the void grows and migrates, which results in a continuous increase of the interconnect resistance, leading to failure after a certain threshold value is reached. Here we investigate the EM reliability issues of the open TSV technology. Fig. 1. Profile view of the TSV structure: Aluminium in yellow, tungsten in green and substrate and seal layer in red. The tungsten cylinder is shortened to 10% of the real length. For the simulation a segment of the upper part is chosen, where the aluminium ring, and the substrate are removed (see Fig. 2). II. THEORETICAL BACKGROUND The TSV geometry considered is shown in Fig. 1. The geometrical dimensions given in [1] are used. Here, the tung- sten, shown in green, forms a hollow cylinder closed on the bottom side. Below that an aluminium plate is placed on which a solder pump is mounted to connect to other wafers. On the top side, an aluminium layer (shown in yellow) forms a second hollow cylinder, which overlaps with the inside, upper part of the tungsten cylinder wall. The upper side of the aluminium connects to the planar interconnect structure by a round plate as shown in Fig. 1. These open TSVs are different compared to the traditional copper TSVs which have their cylinders completely filled. In order to model EM, two important microscopic forces must be considered to determine the material transport. The first is the so called direct force (  F direct ), caused by the local electric field acting on the ionic atoms in the metal. The second is called the wind force (  F wind ), which is caused by the electrons scattered by the atoms in the metal [2]. The sum of these two forces determines the total force, as  F =  F direct +  F wind =(Z d + Z w )e  E = Z * e  E, (1) where Z d and Z w are the so called direct valence and wind valence, respectively, and Z * is the effective valence, which describes the sensitivity to EM.  E is the electrical field and e is the elementary electron charge.