Temperature-Induced Apparent Mass Changes Observed during Quartz Crystal Microbalance Measurements of Atomic Layer Deposition

Abstract

The quartz crystal microbalance (QCM) is a valuable in situ probe of surface chemistry and film growth during atomic layer deposition (ALD). Unfortunately, the QCM is sensitive to both mass and temperature effects that complicate the interpretation of QCM measurements. To characterize the temperature effects, QCM measurements are performed at 170 °C in a hot-wall, ALD flow reactor using pulses of inert and other unreactive probe gases that simulate reactant dosing during ALD. The difference between the probe gas temperature and the QCM sensor temperature is shown to cause instantaneous positive or negative apparent mass transients during the gas pulse. In addition, there is a net apparent mass drift after the gas pulse. The apparent mass transients and apparent mass drifting can lead to misinterpretation of ALD surface chemistry and produce error in measured ALD growth rates. Temperature-induced apparent mass changes are shown to be affected by the temperature profile in the ALD flow reactor before the QCM sensor. Changes in the gas flux during the ALD dosing sequence, changes in the type of dosing gas, and adiabatic cooling of the dosing gas can also produce temperature-induced apparent mass changes. The temperature effects on the QCM sensor are also demonstrated during Al2O3 ALD growth using Al(CH3)3 and H2O. Experimental methods using unreactive probe gases, such as SF6, are developed to tune the temperature profile of the ALD flow reactor to minimize the temperature-induced apparent mass changes.