Al2O3-TiO2 nanolaminates for MEMS and optical applications: Combining the trimethylaluminum/water and titanium tetrachloride/water ALD processes

Nanolaminates of inorganic materials with adjustable optical, electrical and structural properties and with accurately controllable layer thicknesses are interesting for various applications in the manufacturing of microelectromechanical systems (MEMS) and optical devices. We have combined two well-known atomic layer deposition (ALD) processes, the trimethylaluminum/water process and the titanium tetrachloride/water process, to deposit nanolaminates of aluminum oxide and titanium dioxide. In the experiments reported here, the layer thicknesses were typically in the range 50-70 nm. Al2O3 and TiO2 single layers were deposited for reference. The experiments were made in a SUNALETM R-150 ALD reactor, manufactured by Picosun Oy, using 100 mm or 150 mm Si (100) wafers. The Al2O3-TiO2 nanolaminate process was shown to succeed in the temperature range 80–300°C studied. The Al2O3 and TiO2 films and the nanolaminates were under tensile stress of 200-800 MPa, the exact value depending on the material and growth temperature. According to scanning electron microscopy (SEM), the nanolaminate layers were highly conformal in high-aspect-ratio trenches (95%–99%, depending on temperature) even though the process parameters were not optimised. Atomic force microscopy (AFM) measurements indicated that the nanolaminate films grown in (22 Al2O3 cycles + 44 TiO2 cycles) x 16 nanolaminate cycles were smooth, with AFM rms roughness on the order of 0.2 nm. The smoothness of the nanolaminate films indicates that, at higher temperatures, the incorporation of amorphous Al2O3 layers between the TiO2 layers suppresses the crystal growth of TiO2; TiO2 films of a similar thickness were crystalline and rough. Optical measurements by spectroscopic reflectometry indicated that the refractive index of the nanolaminates grown at 150°C can be accurately adjusted between 1.6 and 2.4 by varying the TiO2 volume fraction. The refractive index was insensitive to the layer thicknesses, when the nanolaminate layer thickness was 2–20 nm (individual Al2O3 and TiO2 sub-layers 1–10 nm) and the total thickness was kept constant. Electrical measurements were made for simple Al/nanolaminate/Al capacitors, where the aluminium contacts were patterned by lithography and the nanolaminate film was not patterned. The Al2O3 and TiO2 sub-layer thicknesses were varied between 1 and 9 nm and the total film thickness was about 100 nm. Capacitance–voltage measurements indicated that the dielectric constant of the films can be adjusted, with the measured values in the range of about 9–30. No attempt was made to maximize the dielectric constant, and higher values should still be possible. Current–voltage measurements indicated that the nanolaminate films were leaky, with leakage current levels between those of pure Al2O3 and TiO2 films. Especially the Al2O3 sub-layer thickness affected the leakage current. According to our results, Al2O3-TiO2 nanolaminates from trimethyl aluminum and titanium tetrachloride reactants have many promising applications in MEMS and other technological areas due to their adjustable optical, electrical and structural properties combined with the wide range of possible deposition temperatures.