ALD layers in MEMS fabrication

ALD technique (ALD = atomic layer deposition) offers thin films with properties that complement those obtained by more conventional thin-film growth techniques used in MEMS (microelectromechanical systems) fabrication, such as thermal oxidation, LPCVD, PECVD, sputtering and spin coating. Perhaps the most notable characteristic of ALD layers is the combination of relatively low (<300°C) processing temperatures with near-perfect layer conformality. This combination is not characteristic to the other layer deposition techniques, meaning that ALD fills an existing technological gap. Other obvious advantages come from the widening of the material selection available. For example, Al2O3 is an excellent insulator and exhibits efficient etch-stop properties in fluorine plasma, and TiO2 is a semi-insulating, high-index material. Ta2O5, in turn, offers excellent protection against aggressive chemical environments. The first reports of the use of ALD in MEMS date from 2002, and the area is rapidly developing. In this presentation, we will review the integration process of ALD Al2O3 and TiO2 processes in the MEMS fabrication in VTT's Micronova cleanroom. For successful integration of ALD layers in MEMS, many properties must be characterized, in addition to optimizing the properties and stability of the ALD process itself. Stress plays a central role in practically all MEMS designs. For patterning purposes, the chemical stability of the ALD layers needs to be known in various wet etches, and suitable selective patterning chemistries (wet and/or dry) need to be found. Electrical and optical properties of the ALD layers are often central for the use of ALD layers as functional or active layers. (Electrical properties of our ALD layers have been widely characterized, and are presented elsewhere - ALD 2009 presentation, submitted). In many MEMS designs, high-temperature steps follow ALD, and the response of ALD layers to such steps needs to be sufficiently understood. We will also introduce a new, functional MEMS device, visible-light Fabry-Perot filter, in whose fabrication ALD is utilized. In the design of this particular device, the possibilities offered by ALD have played a central, enabling role, and the ALD-layer stack functions as the active layer.