Design and Implementation of High-Performance Tantalum Thin-Film Capacitors for Embedded Technologies

Embedding surface-mounted discrete capacitors into package substrates will result in more compact, efficient, and less-expensive electronic systems. Despite widespread awareness, embedded capacitor technology has not been widely used in electronic systems for the past two decades. The main reason is that embedded capacitors are still constrained by capacitance density, component size, and simplicity of integration into the package substrate. Tantalum (Ta) capacitors have the potential to provide the highest volumetric density among current capacitor technologies, but their bulky size and low-frequency stability limit their use in embedded technologies. In this article, a process for an ultrathin, high-density, thin-film capacitor (TFC) with excellent electrical properties using etched Ta foil as the anode is demonstrated. The equivalent series resistance (ESR) (7.5 m<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\Omega $ </tex-math></inline-formula>) and leakage current (0.266 nA/<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu $ </tex-math></inline-formula>F) of the Ta thin-film capacitor (TTFC) have been drastically reduced by modulating the chemical state of the Ta foil surface, optimizing the formation process and in situ treatment of the polymer cathode film. The capacitor (<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$45~\mu $ </tex-math></inline-formula>F, 10 V) has a volumetric density of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$9~\mu $ </tex-math></inline-formula>F/mm3 with a thickness of only <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$50~\mu $ </tex-math></inline-formula>m and has a capacitance retention of approximately 33% higher than that of the commercial polymer Ta capacitors (CPTCs) up to 100 kHz. Apart from these excellent attributes, TTFCs prepared by electrochemical etching have good compatibility with the package substrate, which makes the TTFCs a promising candidate for embedded capacitors.