Capacitance measurements and k-value extractions of low-k films

We review test vehicles and methods that are commonly used for capacitance measurements of low-k films and the general procedure for k-value extractions. We demonstrate that a considerable loss of accuracy may occur if metal-insulator-semiconductor (MIS) planar capacitors are used in high frequency (HF) capacitance-voltage (CV) measurements leading to significant underestimation of the k-value. We show that the lack of accuracy is due to parasitic impedance at the backside connection with the Si substrate and we provide a model. The effect of the parasitic impedance can be minimized by reducing the area of the gate electrode. Alternatively, samples can be provided with an ohmic back contact by means of one of the practical fabrication methods that are described. Quasi-static (Q-S) CV measurements did not exhibit any variation related to backside connection. However, we show that Q-S CV measurements loose accuracy for plasma-damaged low-k films because of increased dielectric leakage. Finally, issues related to capacitance measurements in dry atmosphere are addressed. We show that long (∼hours) transients can take place for plasma-damaged low-k films because of the slow release of water from the material underneath the metal gate, which acts as a cap. As a consequence, extracted k-value can significantly depend on sample resident time in the measurement chamber and on gate dimensions.