Influence of Additive Inclusion on Hardness of Electroplated Al Film with Nanometer-Level Flatness from Chloroaluminate Ionic Liquid
Authors: Al Farisi Muhammad Salman , Tsukamoto Takashiro , Tanaka Shuji 
Published: December 2020

In this study, we report the influence of additive inclusion on the hardness of an Al film electrochemically deposited from the chloroaluminate ionic liquid. The additive inclusion drastically increased the hardness as well as the surface flatness. The measured elastic modulus and mechanical hardness of Al on Au seed layer were 71.9 GPa and 10.3 GPa, respectively.

Introduction

Electrochemical deposition (ECD) process is widely used for micro-electro mechanical system (MEMS) fabrication and packaging to form a relatively thick metallization. The process is often selected owing to its low-cost, high deposition rate and availability to tune various properties of the resulting deposit. Its application spans from the structural material of sensors [1] to wafer-level bonding [2].
The emergence of ionic liquid as a solvent has allowed ECD of various materials which are impossible to reduce in water, including Al. Additive inclusion has been introduced to reduce the surface roughness of the resulting deposit [4]. On the other hand, material properties are also of importance for designing MEMS based on the film. However, the influence of additive inclusion on the material property has not been investigated yet. Therefore, this study aims to close the gap.

Experimental method

The ECD process was performed on a Si substrate with Au or Pt seed layer using a commercially available AlCl3–1-ethyl-3-methylimidazolium chloride ([EMIm]Cl) (3:2) ionic liquid electrolyte. A 0.5 g/L 2-chloronicotinyl chloride was introduced as the additive to improve the surface roughness. The deposition was carried out using direct current (DC) at 10 mA/cm2 current density under room temperature for 60 min, which resulted in around 7 μm thickness of deposits. To avoid the degradation of the ionic liquid solvent, the deposition was performed inside a glove box with continuous flow of an inert and dry N2 gas.

Results and discussions

Scanning electron micrographs (SEMs) of the deposited films are shown in Fig. 1 (a), (b) and (c). The difference of surface roughness between films deposited in electrolyte without and with the additive inclusion is quite significant. Fig. 1 (d) shows the quantitative comparison. The surface roughness was reduced to a few nm, which is 2 orders magnitude less than that of without additive. The additive inclusion also introduces a 15% reduction of the deposition rate.
The influence of the additive inclusion in the electrolyte to the mechanical properties of the resulting film is shown in Fig. 1 (e). The hardness and elastic modulus of the deposited films were characterized by nanoindentation using a diamond tetrahedron indenter, i.e. Berkovich tip. The indentation depth was maintained to be more than 100 nm and less than 10% of the film thickness to avoid the influence of the native oxide layer and the substrate.
The evaluation results are compared to previously reported values of bulk Al and thin films deposited using other techniques [5,6]. Compared to the bulk, thin films have a higher nanoindentation hardness. This might be partly attributed to the grain size refining, which is in agreement with the Hall-Petch relation. The nanoindentation hardness became further higher for films deposited with additive inclusion. This significant strengthening can also be partly attributed to the grain size refining [4].
The elastic modulus of the electrodeposited Al film on the Au seed layer without additive inclusion and the film deposited on the Pt seed layer with additive inclusion were significantly lower compared to other conditions. The significantly low elastic modulus regardless of high hardness can be caused by the existence of pores as well as other defects in the grain boundaries, including vacancies, dislocations, impurities and the local distribution of strain [7]. The difference in the deposition mechanism between the Au and Pt seed layers is to be investigated in the future.

Conclusion

In this study, the evolution of mechanical properties of the electrodeposited Al films with the inclusion of additive has been investigated. Enhancement of nanoindentation hardness was observed, mainly due to the grain boundary hardening. This result is useful as a design guide for future MEMS application of the film.

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Figure 1