ACERCA DEL CONCEPTO DE DESARROLLO

Hafnia layers were produced using tetrakis(dimethylamido) hafnium(IV) and Milli-Q water as the precursors. Tetrakis(ethylmethylamido) zirconium(IV) and Milli-Q water were used as the precursors for depositing zirconia layers. The hafnium precursor was heated to 75 ˚C and the zirconium source was heated to 130 ˚C; the water source was not heated. The hafnia layers were grown using a 0.15 s pulse of the hafnium source, a 0.015 s water pulse and 60 s purge times for each cycle. Zirconium layers were deposited using 0.2 s zirconium pulses, 0.015 s water pulses and 60 s purge times.

3.2.3.1 Characterization

3.2.3.1.1 Roughness

AFM images of hafnia and zirconia layers, deposited at 250 ˚C using 600 deposition cycles are presented in Figure 3.14.

Figure 3.14: AFM images of 20 nm thick hafnia (top) and zirconia (bottom) surfaces

The RMS roughness of the hafnia surfaces is 1.8 nm over an area of 1 µm2 _{while the}

roughness of the zirconia surface is 3.1 nm for the same area. The roughness of these surfaces, due to the propensity of these materials to become crystalline,94 is suitable for some surface studies, but is far from ideal. Certainly accurate force measurements at small separations are not possible. We investigated the deposition temperature of the layers on the surface roughness and found that it had no significant effect.

The roughness can be reduced by producing thinner films. Thin films of these material have low roughness, however, the van der Waals properties of such films is not representative of the properties of the bulk materials; the effective Hamaker constants

being low due to the influence of the silicon substrate. To produce surfaces with higher effective Hamaker constants, yet still smooth, we decided to deposit titania films overlaid with either hafnia or zirconia. Such layered structures will possess similar van der Waals properties to those of the bulk hafnia or zirconia while remaining smooth enough to provide accurate force measurements at small separations. To prepare such surfaces, 800-cycle titania layers were deposited at 80 ˚C then a 121-cycle layer of hafnia or a 141-cycle layer of zirconia was deposited over the titania layer. AFM images of these surfaces are shown in Figure 3.15.

Figure 3.15: AFM images of hafnia (top) and zirconia (bottom) grown onto titania films

The roughness of these surfaces are 0.53 nm and 0.8 nm for hafnia and zirconia respectively. The roughness of these is sufficiently low to be viable model surface for force measurements.

3.2.3.1.2 Film thickness

Due to the layered structure of these surfaces, it is important that the thickness of each layer is known accurately so that the calculations of the van der Waals force can be performed. XRR was used to measure these layer thicknesses. An example of the measurement of the titania/hafnia surface is shown in Figure 3.16

Figure 3.16: XRR measurement of 121 deposition cycles of hafnia grown onto 800 cycles of titania on a silicon wafer. The hafnia layer was found to be 12.16 nm thick.

Analysis of the fringes of the XRR measurement determined that the hafnia layer is 12.16 ± 0.02 nm using an SLD of 5.70 ± 0.13×10-5 Å-2 and a surface roughness of 0.77 ± 0.01 nm. The titania layer is 28.93 ± 0.02 nm thick, which was modeled with a SLD of 2.56 ± 0.06×10-5 Å-2 and an interface roughness between the titania and hafnia layers of 0.76 ± 0.01 nm. The SLDs used in the model indicates that the titania and hafnia films have an electron density similar to that for the bulk materials (SLD of titania = 3.08×10-5 Å-2 and SLD of hafnia = 6.40×10-5 Å-2)99. 10-7 10-6 10-5 10-4 10-3 10-2 10-1 100 R 0.5 0.4 0.3 0.2 0.1 Q_z

Similar measurements on zirconia/titania surfaces determined the zirconia layer was 1.45±0.04nm thick, using a SLD of 3.60±0.09×10-5 Å-2 and a roughness of 0.84 ± 0.01 nm. The titania layer was 30.05±0.01 nm thick using a SLD of 2.57±0.04×10-5 Å-2 and a boundary roughness of 0.54±0.02 nm. Again, the electron densities of these layers are similar to that of the bulk materials (SLD zirconia: 4.37×10-5 Å-2)99.

3.2.3.1.3 Crystal structure

The crystal structures of the hafnia and zirconia surfaces were examined using XRD. These measurements are presented in Figure 3.17.

Figure 3.17: XRD measurements of hafnia and zirconia films that have been grown onto a smooth titania film which was deposited onto a silicon wafer substrate with a native oxide layer.

The diffraction peaks observed in the XRD measurements are all consistent with the diffraction peaks for the silicon wafer substrate. The titania layers that were deposited at low temperature are amorphous and therefore present no diffraction peaks (see §3.2.2.1). The lack of diffraction peaks for the hafnia and zirconia layers is unexpected as studies of these materials prepared using ALD in the past have been seen to be crystalline.57 The lack of diffraction peaks for the hafnia and zirconia layers is likely the result of the layer being very thin and being trapped in an amorphous state; the films

101 102 103 104 105 106 107 Inte nsity (cps) 80 70 60 50 40 30 20 2θ (degrees) HfO2 ZrO2

3.2.3.1.4 Film stability

The stability of the hafnia and zirconia surfaces in an aqueous environment was determined using OR. Surfaces were prepared by depositing a 10 nm film of hafnia or zirconia onto silicon wafer with a 320 nm silica layer. Upon immersion into an aqueous environment, a stable baseline was achieved over a wide range of pH for both hafnia and zirconia surface. Therefore, these surfaces were deemed stable for surface force measurements.

3.2.3.2 Summary

Thick layers of hafnia and zirconia, which have bulk van der Waals properties are far too rough to be used for force measurements. However, by depositing these materials onto a titania layer, it is possible to produce smooth surfaces with similar properties to the bulk materials. These surfaces are stable in aqueous environments and therefore are suitable to use as model surfaces of hafnia and zirconia. The thickness of these layers needs to be measured accurately to enable the effective Hamaker constant between two surfaces or the sensitivity parameter for OR measurements to be accurately calculated.