Thermal Oxidation of Ni and Co Alloys Formed by Electroless Plating
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0914-F05-09
Thermal Oxidation of Ni and Co Alloys Formed by Electroless Plating Jeff Gambino1, Igor Ivanov2, Steve Mongeon1, Ed Adams1, Scott Hazel1, Dave Meatyard1, Phil Pokrinchak1, Fen Chen1, Pat DeHaven3, Artur Kolics2, and Marina Polyanskaya2 1 IBM Microelectronics, Essex Junction, VT, 05452 2 Blue29, Inc., Sunnyvale, CA, 94089 3 IBM Microelectronics, Hopewell Junction, NY, 12533
ABSTRACT The thermal oxidation of Co alloy and Ni alloy thin films on top of Cu, has been studied as a function of film composition. Four different alloys were studied; CoWP, CoWB, CoMoB, and NiMoB. For the Co alloys, significant oxidation is observed after an air anneal above 350oC, accompanied by Cu diffusion through the film to the surface. In contrast, for the NiMoB films, there is minimal oxidation of the surface even after a 400oC anneal in air, and Cu diffusion through the film is greatly reduced. These results indicate that Ni-based barriers may be desirable in applications where self-aligned metal layers are used on top of Cu interconnects as a replacement for dielectric caps such as SiN and SiCN. INTRODUCTION Copper interconnects have gained wide acceptance in the microelectronics industry due to improved resistivity and reliability compared to Al interconnects [1]. Recently, self-aligned metal caps, such as CoWP, have been investigated as a replacement for dielectric caps on top of the Cu, such as SiCN and SiN [2,3,4,5,6]. The self-aligned metal caps provide improved electromigration lifetime and a lower effective dielectric constant compared to dielectric caps [2,3]. For CMOS image sensors, the self-aligned metal cap also eliminates problems with reflections caused by dielectric capping layers [4]. The Co alloys and Ni alloys can be formed selectively on Cu using electroless plating [7,8]. However, it is still unclear whether a CoWP cap by itself (with no dielectric cap) can provide an adequate barrier during oxidizing processes such as dielectric deposition and resist strip. During the deposition of oxide-based dielectrics by plasma enhanced chemical vapor deposition (PECVD), the CoWP will be briefly exposed to an oxidizing ambient at temperatures of 350 to 400oC. Both Cu [9,10] and Co [11,12] are known to oxidize at low temperatures (< 400oC), whereas there is no significant oxidation of Ni below 400oC [13]. In this study, Co alloys or Ni alloys deposited on Cu are exposed to oxidation between 350oC and 400oC, and the resulting microstructure is characterized. It is shown that for Co alloys, Co oxides or Cu oxides grow on the surface, accompanied by significant diffusion of Cu through the Co alloy. In contrast, for the NiMoB alloy, there is minimal oxidation and minimal diffusion of Cu through the alloy. Therefore, Ni alloys may be a better choice than Co alloys as a replacement for the dielectric cap.
EXPERIMENT Samples for the oxidation experiments were formed on either blanket wafers or patterned wafers. In both cases the starting substrates were 200 mm Si wafers. Blanket films were formed on substrates consisting
