The Properties of Pulse-Plated Copper Derived from Additive-Bearing, Low-Metal Ion Concentration Electrolytes

Abstract

In semiconductor devices, copper metallization patterns are created using a combination of photolithography and electroplating. Photolithography, which uses light to transfer geometric patterns from a mask onto a substrate, is a multi-step process that is costly, time-consuming, and entails the use of harmful chemicals. A recent electrodeposition process called Electrochemical nano and micro Fabrication by flow and Chemistry (EnFACE) was developed to enable single-step, direct, mask-less pattern transfers onto metallic substrates. EnFACE offers the possibility of replacing photolithography and avoid the difficulties associated with the process. For typical copper electrolytes, the use of pulsed current and plating additives are known to improve deposit properties such as grain structure, mechanical strength, and throwing power. However, EnFACE uses an acid-free, additive-free, low-conductivity plating electrolyte containing low concentrations of metal salts (0.1 M CuSO4), a composition which significantly differs from the conventional copper plating electrolytes. Since the chemistry of this electrolyte is relatively new, a majority of its properties still needs to be discovered and understood. In this work, we report on the influence of current modulation (i.e., pulsed-current) on the properties of Cu electrodeposits obtained from additive-bearing, low metal salt concentration electrolyte. Copper films were plated from 0.1 M CuSO4 electrolyte containing different concentrations of plating additives (i.e., 0%, 17%, 33%, 50%, 100%, and 200% of the industry recommended concentration) using direct (DC) and pulsed current (PC) modulations. The operating pulse parameters used were θ = 0.25 at a t p of 100ms and the actual plating time was set for a deposit thickness of 25 um. The mechanical properties of deposits were characterised using the tensile test (UTM), while electrical properties were assessed using the four-point probe. Grain characteristics and morphology was investigated using scanning electron microscope (SEM), electron backscattered diffraction (EBSD), and X-ray diffraction (XRD). Results of mechanical tests indicate that the PC-plated films possessed higher yield strength, tensile strength and ductility than its DC-plated counterparts (see Fig.1). Similarly, electrical measurements confirmed that pulse plated films have better conductivity than the DC plated ones. These changes in properties were traced to the changes in microstructure induced by the application of pulsed current modulation during plating. Finally, the use of pulsed current offers the added benefit of reducing the required amount of additives to obtain the desired deposit properties. The optimum additive concentration was 50% of the recommended value when using DC plating; while it was only 33% in PC plating. Figure 1