2_2_paperTATNALUM
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Low Voltage CV Loss in Tantalum Capacitors Y. Freeman1, P. Lessner1, E. Dickey2, J. Li2, J. Koenitzer3, L. Mann3, Q. Chen1, T. Kinard1, J. Qazi1
1. Kemet Electronics Corporation, 2835 Kemet Way, Simpsonville, SC 29681. yurifreeman@kemet.com Phone: (864) 228-4064 2. Center for Dielectric Study (CDS), Pennsylvania State University, University Park, PA 16802. Ecd10@psu.edu, Phone: (814) 865-9067 3. Cabot Corporation, 157 Concord Road, Billerica, MA 01821. Larry_Mann@cabot-corp.com
Miniaturization and reducing operating voltage are constant trends in many fields of modern electronics such as computers and telecommunications. To follow these trends, the surface area (A) of anodes in tantalum (Ta) capacitors is constantly increasing and thickness (t) of the dielectric, anodic oxide film of Ta, is constantly decreasing. This results in increased capacitance C = kA/t, where k is permittivity of the dielectric. To increase A, finer Ta powders with smaller primary particles are used.1-3 To decrease t, lower formation voltages
(V) are used since the thickness of the anodic oxide is directly proportional to formation voltage with the coefficient a ≈ 1.6 nm/V for anodizing at room temperature.4
Specific charge CV per unit of volume or weight characterizes efficiency of the capacitors. Taking into consideration that t = aV, provides CV/A = k/a. Since k and a do not change with formation voltage in the anodic oxide of Ta, CV/A should remain constant at different formation voltages. At the same time, CV roll-off with increasing formation voltage was presented in literature.5,6 This high voltage CV loss was attributed to consumption of the necks connecting sintered powder particles and closure of the pores between the particles by growing anodic oxide film. The neck-pore model is in good agreement with the experimental data on high voltage CV loss.
CV roll-off with reducing formation voltage below approximately 12 V was also presented in literature.2,7 This low voltage CV loss is limiting further miniaturization of Ta capacitors and is often accompanied by d.c. leakage increase. It was speculated that low voltage CV loss was related to the thermal oxide on the Ta surface prior to anodizing; however, the nature of this phenomenon remained unclear. In this paper transmission electron microscopy
(TEM) of sintered and formed Ta anodes was combined with electrical measurements to understand this phenomenon. A physical model was developed which explains low voltage CV loss in Ta capacitors and is in good agreement with the experimental data.
Experimental and Instrumentation
Fabrication of Ta anodes was performed with 40,000 µC/g and 150,000 µC/g Ta powders. The coarser Ta powder with 2.7 µm average primary particles was pressed into 100 mg pellets with a 5.5 g/cc green density and then sintered in vacuum at 1350°C for 20 min. The finer Ta powder with 0.9 µm average primary particles was pressed into 9 mg pellets with a 5.5 g/cc green density and then sintered in vacuum at 1250°C for 10 min. A Ta lead wire was embedded into the pellets during their pressing. Sintered Ta anodes were then anodized in an aqueous solution of 0.1 wt. % phosphoric acid at 80° C. The anodizing included a constant current stage when the formation voltage increased from zero to a set value, and a constant voltage stage when the current gradually decayed with time. The formation voltages varied from 2 V to 12 V. Heat treatment of Ta anodes either before or after their anodizing was performed in air at 300o C for 1 hour.
Thin sections for transmission electron microscopy (TEM) were prepared by vacuum impregnating Ta anodes with an ultra low viscosity resin system (LR WhiteTM), followed by curing at 65° C. Electron transparent slices were then cut using a Reichert Jung Ultracut E microtome. Thin sections were examined using a JEOL 2010 TEM operated at 200 kV or a 300 kV high resolution Hitachi H9500 TEM. For oxide layer thickness measurements multiple photomicrographs were taken at the same magnification from different areas for each sample.
Electrical testing of Ta anodes was performed at room temperature in a liquid electrolyte consisting of 25 wt. % phosphoric acid and a Ta cathode. Capacitance measurements were performed at 120 Hz and no bias using QuadTech 7600 Precision LCR Meter.
Results and Discussion
Fig. 1 presents CV/g dependence on formation voltage for 40,000 µC/g and 150,000 µC/g Ta powders. Fig. 1 also shows relative CV/g loss vs. CV/g at 12 V. This loss was calculated as [CV/g(12V) – CV/g(V)]/CV/g(12V).
