Recent Publications

The intention of publications made by Atotech is to share the results of our extensive research and development on a non commercial basis. With these presentations we aim to provide in-depth process information and inform about the latest investigations and results in each technology field. It should enable the reader to gain a complete and comprehensive understanding of our process solution and of general phenomena encountered in the electroplating industry.

This segment presents the recent papers from the business technology team Semiconductor.

Nickel–Palladium Bond Pads for Copper Wire Bonding
Horst Clauberg, Bob Chylak, Kulicke and Soffa Ind. Inc., USA, Petra Backus, Atotech Deutschland GmbH, Germany

The semiconductor packaging industry is undergoing a step-change transition from gold to copper wire bonding brought on by a quadrupling of gold cost over the last 8 years. The transition has been exceptionally rapid over the last 3 years and virtually all companies in the industry now have significant copper wire bonding production. Among the challenges to copper wire bonding is the damage to bond pads that had been engineered for wire bonding with the softer gold wire. This paper presents an extensive evaluation of electroless NiPd and NiPdAu bond pads that offer a much more robust alternative to the standard Al pad finish. These NiPd(Au) bond are shown to outperform Al in virtually all respects: bond strength, bond parameter window, lack of pad damage and reliability.

Download complete paper here: pdf file (File size 530 kB)

Investigation of Cu Plating and Additive Interactions in the Presence of Fe2+/Fe3+ Redox Couple
Igor Volov, Alan C. West, Columbia University, New York, USA; Tadashi Saito, Atotech Japan K.K.

The impact of the ferric/ferrous (Fe3+/Fe2+) redox couple on the behavior of polyethylene glycol (PEG) and bis-(3-sulfopropyl) disulfide (SPS) during copper electrodeposition is investigated. The Fe3+/Fe2+ couple is used in plating of copper onto printed circuit boards and may have advantages when implemented for on-chip metallization of copper. Experiments show that in the presence of Fe3+/Fe2+ the suppressing behavior of PEG does not change; in contrast, the accelerating activity of SPS increases when Fe3+/Fe2+ is present in a copper-plating bath. Furthermore, potentiostatic experiments suggest that SPS interacts with Fe2+ ions in the bulk electrolyte, probably to produce 3-mercapto-1-propane sulfonic acid (MPS) at low ppb levels.

Download complete paper here: pdf file (File size 1.400 kB)

SEM cross sections of TSV filling (diameter = 5 µm, height = 25 µm) keeping all conditions the same except for Fe2+/Fe3+ redox mediator concentration and the anode: (a) Cu anode, no Fe3+ ions; (b) inert anode, 0.47 g/L of Fe3+; (c) (b) inert anode, 2.0 g/L of Fe3+.
High aspect ratio through-wafer copper interconnects after the dissolution of silicon. (a) Diameter 15 µm, pitch size 35 µm, aspect ratio 15. (b) Diameter 30 µm, pitch size 80 µm, aspect ratio-12.

Fabrication of High Aspect Ratio 35 µm Pitch Through-Wafer Copper Interconnects by Electroplating for 3-D Wafer Stacking
Pradeep Dixit, Jianmin Miao, Micromachines Centre, School of Mechanical and Aerospace Engineering, Nanyang Technological University, Singapore; Robert Preisser, Atotech Deutschland GmbH, Germany

The demands of fabricating cheaper, smaller, and lighter electronic products offering better performance and increased functionalities are continuously growing. The number of electronic devices on a single chip is rapidly increasing, and thus initiating the need of multilevel interconnections. According to International Technology Roadmap for Semiconductors (ITRS) 2005, integrated circuit (IC) chip size will be in the order of 30 nm by 2010. Such a nanosized IC will carry more than 100 million transistors, which will further require more than 100,000 I/Os for next level packaging. To accommodate such a huge number of I/Os in constantly reducing package size, novel approaches, such as wafer stacking, are needed to fabricate interconnects having a pitch size as low as 20 µm.

Download complete paper here: pdf file (File size 265 kB)

 

Next Generation Nickel-Based Bond Pads Enable Copper Wire Bonding
Bob Chylak, Jamin Ling, Horst Clauberg, Kulicke and Soffa Ind. Inc., USA, Tom Thieme, Atotech Deutschland GmbH, Germany

Copper wire bonding has huge cost advantages over gold wire bonding. As a result, low pin count, heavy wire applications have already been converted to copper wire and many companies are in high volume production. Recently, with the price of gold skyrocketing, conversion of high pin count (>250 I/O), high performance applications to Cu has dramatically accelerated.

These high performance devices are increasingly relying on low-k materials under the bond pads. Unfortunately, the 33% greater hardness of Cu compared to gold places even greater stress on these inherently fragile materials. This can result in difficulties with pad damage and cratering of the underlying structures. Advancements have been made to copper wire bonders, tools, and wire that have resolved many of these issues and made fine pitch copper wire bonding feasible.

Ni-based bond pads have emerged to solve the pad damage problem. Nickel is about 50% harder than copper and four times harder than aluminum so that it provides greater protection against the higher stress resulting from Cu ball bonding, as well as damage during probe. This is especially beneficial for devices with low-k active circuitry under the bond pad. NiPd, NiPdAu, and/or NiAu have demonstrated their great robustness to receive the Cu wire bonding with a huge wire bonding window without any splash and with excellent reliability.

Download complete paper here: pdf file (File size 440 kB)

The Interactions between SNAGCU Solder and NI(P)/AU, NI(P)/PD/AU UBMS
Jui-Yun Tsai, Josef Gaida, Gerhard Steinberger, Albrecht Uhlig, Atotech Deutschland GmbH, Germany

The metallurgical reactions between the Sn3.5Ag0.5Cu solder and two different UBM struc¬tures, Ni(P)/Au and Ni(P)/Pd/Au, were studied.

Two kinds of Ni(P) layers, medium-P and high-P content, were selected. The reliability of the solder joints were characterized by ball shear test after multiple reflows. It was found that the reaction products at the interface were the same in the all systems after multiple reflows. However, the addition of Pd layer between Ni(P) and Au played strong effects on morphology of compounds and strength of joints as well. The morphology of the compounds were needle-shaped in the Ni(P)/Au system and were layer-shaped in the Ni(P)/Pd/Au system. The needle-shaped compounds became the weak points during the ball shear test.

Therefore, the major fracture mode of ball shear test was between the roots of needle-shaped compounds and Ni(P)/Au UBMs. In the Ni(P)/Pd/Au system, the major fracture mode was inside the solder. The thickness of Ni3P in high-P content Ni(P) layer is thicker than that in medium-P content Ni(P) layer.

Download complete paper here: pdf file (File size 590 kB)