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Effect of Electrode Composition on Performance of Sputter Deposited MIM Capacitors on LTCC Substrates
Keywords: LTCC, Thin Film Capacitor, Non-linear
Low Temperature Co-fired Ceramic (LTCC) substrates are widely used for packaging Radio Frequency (RF) and high frequency applications. As such, LTCC substrates are used to create Multi-Chip Modules (MCM) for mounting and interconnecting semiconductor integrated circuits and surface mount passive components such as capacitors and resistors. Utilizing thin film capacitors rather than surface mounted devices offers packaging miniaturization and integration advantages. In addition, process steps and weaker links under aging and extreme environment situations such as solder connections can be eliminated. However, LTCC substrates have much greater surface roughness than silicon wafers, making it imperative that the thin film deposition technique produce highly conformal films. Sputtering is a commonly used technique for physical vapor deposition and is usable with a wide variety of substrates and deposition materials. It yields films with good thickness uniformity, conformality, purity, and adhesion. The effect of various electrode materials on the performance of thin film capacitors was investigated. The configuration for the capacitors was a metal-insulator-metal (MIM) design with 500 nm thick electrodes and 1,000 nm thick dielectric. Characterization of the devices was by DC current-voltage measurements and impedance analysis. The thin film capacitors were fabricated via DC magnetron sputtering on DuPont 951 LTCC substrates using Al2O3 as the dielectric and three types of metal electrode configurations: 1) a single metal type was used for both electrodes; 2) a bi-layer metallic electrode consisting of a thin coating of Al between Cu and the dielectric; and 3) a two metal electrode system with Al as one electrode and Cu as the other electrode. The electrodes and dielectric deposition were defined using physical masks. Area of the capacitors varied between 0.0225 mm2 and 0.5625 mm2 and were measured optically to verify resulting dimensions. Metals used in the single material electrode case were Al, Cu, Ag, Au, and Pt to evaluate the effect of the material interplay at the interface between the electrode and the dielectric composition on capacitor performance. In the second case of a bi-layer metallic electrode, the dependence of the capacitor performance on the thickness of a thin Al layer deposited between the bulk Cu electrode and the dielectric was studied. The third case contrasts the top and bottom positions of the dissimilar Al and Cu electrode materials. Characterization of the capacitor performance was done by DC current-voltage measurements and AC impedance analysis. Results show a marked improvement in device performance when aluminum is in direct contact with the dielectric on LTCC substrates. The most noticeable performance difference across the configuration variations was in the yield of functioning capacitors. Those with Al electrodes or layers in contact with the dielectric had the highest yield of functioning capacitors. There were also differences in breakdown strengths of the capacitors dependent on the electrode composition with Al electrodes exhibiting higher breakdown strengths than those with Cu electrodes in contact with the dielectric. Other researchers have studied the effect that the electrode work function has on the dielectric performance in thin film capacitors. Further studies are in work to determine the cause of the performance improvement for the case with aluminum in direct contact with the dielectric.
Daniel S. Krueger, Engineer Fellow
Kansas City, MO

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