Uniformity Control of Ni Thin-Film Microstructures Deposited by Through-Mask Plating

Abstract

The thickness uniformity within a specimen and the cross-sectional profiles of electroplated individual Ni microstructures have been investigated as a function of the electroplating conditions. It was found that the uniformity and profiles of microstructures could be controlled by varying the process conditions. A uniform thickness distribution and microstructures with flat profiles could be obtained at optimal plating conditions of 8 mA/cm 2 and 60°C. Above this optimal plating current density, the microstructure has a rabbit-ears profile, and the thickness of a narrow microstructure is thicker than that of wide ones. Below this current density, the microstructure has a cap-like cross-sectional profile, and a narrow structure is thinner than wide ones. Increasing the plating temperature enhances the nonuniformity, whereas other process parameters have insignificant effects on it. The current crowding observed in patterned specimens is responsible for the rabbit-ears profile of individual microstructures, while a combination of the fluidic friction on the sidewall of the photoresist and the electrophoresis of the ions in the solution are believed to be responsible for the abnormal cap-like profile of individual microstructures and the thinning effect on narrow microstructures.Electrodeposition ͑or electroplating͒ has become one of the most important technologies for microelectronics and microelectromechanical systems ͑MEMS͒, as it produces high-quality metal films in a simple way at a low cost. Various metal and alloy films such as Ni, In, Cu, Au and NiFe, NiP, CoFe have been formed by electrodeposition. 1-3 A combination of photolithography technology and electrodeposition ͑so-called through-mask plating 4 ͒ makes it possible to fabricate microstructures suitable for microsensors and microactuators. 5-7 However, one of the problems associated with electrodeposition is the uniformity control of the electroplated films both in thickness and composition. 8,9 This affects the material properties and resulting performance of microcomponents.The thickness uniformity of electroplated metal films was intensively studied during the 1990s, especially for the microelectronics industry as Cu became the standard for interconnects in integrated circuits and also for the MEMS society as plated metal microstructures became popular microcomponents for sensors and actuators. The thickness variation of a Cu film on a Si wafer typically has a ''rabbit-ears'' shape distribution ͑it is also called a nodule shape͒, i.e., it is thinner in the center of the wafer and becomes thicker toward the edge. 9,10 The ''resistive seed layer'' model has been developed to explain the thickness variation on the wafer which is due to the voltage drop from the edge terminal contact to the center of a wafer. 9,10 For patterned wafer plating, the uniformity has an effect not only on the thickness variation across a wafer but also on the thickness of the individual microstructures, which develop rabbitears ͑or nodule͒ cross-sectional profiles. 11 The ''a...

Collapse-free thermal bonding technique for large area microchambers in plastic lab-on-a-chip applications

“…In this regard, it was found that the convex shape of the replicated substrate surface after bonding is resulted not by the pressure applied during thermal bonding but from the characteristic of the electroplating process. As mentioned above, the master mold II was fabricated by the electroplating process, which resulted in a concave shape of the microchamber positive relief of electroplated nickel on the nickel substrate, i.e., thinner in the center and thicker toward the edge, due to the current crowding effect during the electroplating (Luo et al 2005). Therefore, the replicated pattern of the microchamber area in the polymer substrates has the convex shape of bottom and top surfaces before and after the collapse-free thermal bonding.…”

Uniformity study of nickel thin-film microstructure deposited by electroplating

“…Current crowding should always exist for patterned wafer plating (Luo et al 2005). Both current crowding and cathodic polarizability are believed to be responsible for the thickness distribution.…”

“…Clearly this model can not explain the cap-like microstructure profiles. In order to explain this phenomenon, Luo et al (2005) proposed the fluidic friction and electrophoresis effects model to explain the thickness distribution of the specimen. The combination of fluidic friction and electrophoresis effects is believed to be responsible for this abnormal profile and thickness distribution.…”

Effect of temperature on nickel electrodeposition from a nickel sulfamate electrolyte

“…There are contradictory studies on the dependence of the interface width on the current density; references [14,15] state that the interface width increases at a low current density and decreases at a higher current density and the other reference states [16] vice versa. This is because the interface width used in the References [14][15][16] may be one at t \ t c , that is, the unsaturated interface width. In fact, in our experiment the unsaturated values of the interface width are observed in Fig.…”