Low Temperature Wafer Bonding by Copper Nanorod Array Pei-I Wang, a, * ,z Sang Hwui Lee, a Thomas C. Parker, a Michael D. Frey, a Tansel Karabacak, b Jian-Qiang Lu, a and Toh-Ming Lu a a Center of Integrated Electronics, Rensselaer Polytechnic Institute, Troy, New York 12180, USA b Department of Applied Science, University of Arkansas at Little Rock, Little Rock, Arkansas 72204, USA A vast array of Cu nanorods with a diameter of 10–20 nm grown by an oblique angle deposition technique was utilized as an adhesive layer for bonding 200 mm Si wafers at low temperatures. The focus ion beam/scanning electron microscope images illustrate that the Cu nanorod array underwent coalescence readily upon a bonding temperature at 200°C. Upon 400°C, a dense Cu bonding layer with homogeneous structure was achieved. A fully dense bonding structure was also obtained upon a lower bonding temperature at 300°C followed by a postannealing at 400°C in a reducing ambient. © 2009 The Electrochemical Society. DOI: 10.1149/1.3075900 All rights reserved. Manuscript submitted December 5, 2008; revised manuscript received January 6, 2009. Published January 30, 2009. The continuing push for smaller and faster electronics has led to tremendous advances in the scale of circuit integration and packag- ing density. For decades, advances in device scaling and miniatur- ization directly resulted in increases in performance. Today, how- ever, developments in technology have pushed functional integration to such a high level that interconnect and packaging issues represent real barriers to further progress. Although a signifi- cant research effort has been expended on various planar ap- proaches, three-dimensional 3D technology is undoubtedly gaining momentum as a leading contender in the challenge to meet perfor- mance, cost, and size demands through this decade and beyond. 1 Wafer bonding can complement established 3D stacking approaches and makes them more efficient. In wafer bonding, device wafers are patterned individually to create semi-3D structures by microfabrica- tion and, subsequently, bonded together to form complex 3D struc- tures at the wafer level. Wafer bonding started in microelectrome- chanical systems MEMS as a process that helped form a part of a device and provided a first-level package. 2 From MEMS, wafer bonding is now advancing microelectronics 3 and optoelectronics. 4,5 Industry has shifted to Cu instead of the conventionally used Al as the interconnect metal because Cu has better conductivity and is less susceptible to electromigration. As such Cu is being particularly attractive as the bonding media for wafer bonding to achieve 3D integration. Copper bonding has been studied extensively. 6-8 The bonding mechanism in the existing studies primarily relies on atomic diffusion in solid state upon thermal annealing. Recent investigation of the annealing characteristics of Cu nano- rod arrays grown by an oblique angle deposition technique OAD showed that the disintegration of Cu nanorod arrays upon thermal annealing occurred at temperatures significantly lower than its bulk melting point. 9 The annealing characteristics of Cu nanorod arrays associated with their morphological changes were found to be fur- ther enhanced by the reduction of nanorod size. These results sug- gested the potential for the use of Cu nanorod arrays on low- temperature soldering/bonding applications. In this study, wafer bonding utilizing Cu nanorod arrays as the adhesive layer at 400°C or lower temperatures is investigated. The morphological changes and microstructures of the wafer-bonding structures were character- ized using focus ion beam/scanning electron microscope FIB/ SEM. The phenomenon associated with sintering temperature de- pression of such nanostructures owing to size effect and bonding pressure enhanced mass transport is discussed and elucidated. Experimental After RCA cleaning, a thermal oxide was grown to a thickness of 500 nm on 100 200 mm Si wafers. A 50 nm thick Ta film acted as adhesion layer for Cu and a 500 nm thick Cu film were depos- ited, in sequence, using a sputtering system without breaking vacuum in the deposition chamber. The base pressure was in the range of 10 -7 Torr. This process was followed by OAD of a 500 nm thick Cu nanorod layer in an electron-beam evaporator. The wafers were attached to a sample holder to maintain an angle of 85° be- tween the surface normal of the substrate and the surface normal of the evaporation source during the deposition. The base pressure dur- ing the deposition was 2  10 -7 Torr. The Cu rods were measured to be about 10–20 nm diam and 760 nm in length. Wafers with 500 nm thick blanket Cu films were also prepared for the compari- son between Cu film and Cu nanorod bonding. Wafers with either blanket films or Cu nanorods were immediately transferred from the deposition chamber to the wafer bonding chamber to minimize the surface oxidation in atmosphere. One set of wafers with Cu nanorod layers were preannealed in Ar-3%H 2 at 200°C for 1 hr prior to being subjected to bonding. Wafer bonding was carried out in an EV501 bonder EV Group. After N 2 purge, the chamber was evacuated to a base pressure of 2  10 -4 mbar and subsequently ramped at a rate of 32°C /min. Once the bonder temperature reached the set temperature, a uniform down force of 10 kN equivalent to 0.32 MPa across 200 mm diam wafers was applied and maintained for 1 h for the bonding process. The bonded wafers were unloaded after the chamber cooled down to room temperature. The mechanical integrity of the bonded wafer pairs was evalu- ated qualitatively, using a three-step back-side thinning evaluation grinding/polishing/wet-etching. One back side of the bonded Si wafer pair was ground and polished. The remaining Si layer was then removed using tetramethylammonium hydroxide TMAH di- luted with distilled water 2:1 at 90°C for 1–2 h. This recipe pro- vides an etching rate of 1 m/min for Si. TMAH etches Si selec- tively over Si oxide. All the wafer pairs did not disintegrate upon the back-side thinning process. The bond strength of the preliminary wafer bonding by Cu nanorod with a larger size of 50 nm diam has been evaluated using a razor blade insertion test, which showed that the wafer pairs bonded at 300°C passed the razor blade insertion test. 10 Cross-sectional microscopy was performed on the back-side thinned wafer pairs in a dual-beam system Zeiss Ultra 1540 XBeam, combining a FIB and a SEM at a 54° angle. By working at a coin- cidence of ion and electron beam, the system permits FIB milling and in situ SEM on a sample simultaneously. FIB milling was per- formed at 30 kV acceleration voltages and SEM was performed at 5 kV acceleration voltages. The crystallographic evolution of the samples was investigated using an X-ray diffractometer Bruker D8 Discover equipped with an area detector. A 1 mm diam collimator and a graphite monochro- mator were used to select the Cu K  radiation. The detector was placed 15 cm from the sample yielding a 2 and  tilt angle cov- erage of 35° for each frame. Frames were taken at 2 = 30, 60, * Electrochemical Society Student Member. z E-mail: wangp3@rpi.edu Electrochemical and Solid-State Letters, 12 4 H138-H141 2009 1099-0062/2009/124/H138/4/$23.00 © The Electrochemical Society H138 Downloaded 30 Jan 2009 to 128.113.122.45. Redistribution subject to ECS license or copyright; see http://www.ecsdl.org/terms_use.jsp