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image:Picture7.png|Fig. 4: TEM cross section images and 300x300 nm AFM images of the 2nm Cr/2nm Au sample (a-b) and of the 2nm Cr/20nm Au sample (c-d). </gallery>
image:Picture7.png|Fig. 4: TEM cross section images and 300x300 nm AFM images of the 2nm Cr/2nm Au sample (a-b) and of the 2nm Cr/20nm Au sample (c-d). </gallery>


To investigate the crystal orientation of the metal thin-films, tramsimission Kikuchi diffraction was used (Fig. 5a) (see [[LabAdviser/CEN/Nova NanoSEM 600/Transmission Kikuchi diffraction|Transmission Kikuchi diffraction]] for more information). The nanostructure of the 20nm Au film has a bimodal grain size distribution (Fig. 5b). While the smaller grains have different crystal orientations, the large grains (blue color) all have [111] orientation.
To investigate the crystal orientation of the metal thin-films, tramsimission Kikuchi diffraction was used (Fig. 5a) (see [[LabAdviser/314/Microscopy 314-307/SEM/Nova/Transmission Kikuchi diffraction|Transmission Kikuchi diffraction]] for more information). The nanostructure of the 20nm Au film has a bimodal grain size distribution (Fig. 5b). While the smaller grains have different crystal orientations, the large grains (blue color) all have [111] orientation.


Microstructural evolution and growth of metal thin-films deposited by physical vapor deposition on amorphous dielectric substrates follows island growth. The first thin-film growth step is the nucleation of small islands once the activation barrier and the critical nuclei size have been overcome. It is followed by a second step of island growth, during which the impinging atoms contribute to increase island size. The third step, usually happening simultaneously with step 2, is island coalescence, where a strong driving force is present for coarsening through surface atom diffusion and grain boundaries (GB) motion. During this process, the island growth is driven by the minimization of surface and interface energy.
Microstructural evolution and growth of metal thin-films deposited by physical vapor deposition on amorphous dielectric substrates follows island growth. The first thin-film growth step is the nucleation of small islands once the activation barrier and the critical nuclei size have been overcome. It is followed by a second step of island growth, during which the impinging atoms contribute to increase island size. The third step, usually happening simultaneously with step 2, is island coalescence, where a strong driving force is present for coarsening through surface atom diffusion and grain boundaries (GB) motion. During this process, the island growth is driven by the minimization of surface and interface energy.
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The samples with the adhesion layers and 20 nm Au layer were also analyzed. The addition of the adhesion layer had in both cases a profound impact on grain size and orientation of the Au film, as visible in Fig. 7a and 7c. The image shows small grains mainly oriented in the [111] crystal direction, with an average grain size of 40 nm for Ti (Fig. 7b) and 36 nm for Cr (Fig. 7d). During the collection of the TKD maps, only the Kikuchi patterns produced by the electron scattering from the Au layer were recorded and indexed. Cr and Ti layers did not contribute to the pattern formation due to two main reasons: i) they are too thin to produce enough scattered electrons and ii) they are
The samples with the adhesion layers and 20 nm Au layer were also analyzed. The addition of the adhesion layer had in both cases a profound impact on grain size and orientation of the Au film, as visible in Fig. 7a and 7c. The image shows small grains mainly oriented in the [111] crystal direction, with an average grain size of 40 nm for Ti (Fig. 7b) and 36 nm for Cr (Fig. 7d). During the collection of the TKD maps, only the Kikuchi patterns produced by the electron scattering from the Au layer were recorded and indexed. Cr and Ti layers did not contribute to the pattern formation due to two main reasons: i) they are too thin to produce enough scattered electrons and ii) they are
worse electron scattering centers tahn Au because they have a lower atomic number.
worse electron scattering centers than Au because they have a lower atomic number.


[[File:Picture8.png|300px|center|thumb|Fig. 7: TKD IPFZ maps of the growth direction and grain size distributions of the 2-Ti/20-Au sample (a-b) and of the 2-Cr/20-Au sample (c-d).]]
[[File:Picture8.png|300px|center|thumb|Fig. 7: TKD IPFZ maps of the growth direction and grain size distributions of the 2-Ti/20-Au sample (a-b) and of the 2-Cr/20-Au sample (c-d).]]
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= Adhesion layer model =
= Adhesion layer model =


Based on the results obtained above, a revised adhesion layer model for Ti/Au and Cr/Au thin-film systems is proposed. The model is intended as an improvement in the description of the layer-layer interaction compared to the model described in Fig. 1, with the aim of helping the thin-film engineering of nanodevices
Based on the results obtained above, a revised adhesion layer model for Ti/Au and Cr/Au thin-film systems is proposed. The model is intended as an improvement in the description of the layer-layer interaction compared to the model described in Fig. 1, with the aim of helping the thin-film engineering of nanodevices.


The adhesion layer model is shown in Fig. 14. The growth of both adhesion layers starts with the deposition of the atoms on the substrate (1) and the subsequent formation of an amorphous layer (2). During the deposition, both Ti and Cr get partially oxidized by oxygen and water molecules present on the substrate surface and in the deposition chamber. The adhesion layer acts as a wetting layer for Au, reducing the nucleation energy barrier and increasing the number of nucleation sites compared to the case where Au is directly evaporated onto the SiO2 surface. The enhanced wetting is due to the formation of Ti-Au and Cr-Au chemical bonds. This leads to the formation of a continuous film having i) monodisperse grain size and ii) the energetically most favorable [111] crystal orientation for Au (3). For the Cr/Au system there is an extra step: the inter-diffusion between Cr and Au to form a Cr-Au alloy (4). Such diffusion is limited to a thickness of 2-3 nm for samples prepared at room temperature.
The adhesion layer model is shown in Fig. 14. The growth of both adhesion layers starts with the deposition of the atoms on the substrate (1) and the subsequent formation of an amorphous layer (2). During the deposition, both Ti and Cr get partially oxidized by oxygen and water molecules present on the substrate surface and in the deposition chamber. The adhesion layer acts as a wetting layer for Au, reducing the nucleation energy barrier and increasing the number of nucleation sites compared to the case where Au is directly evaporated onto the SiO2 surface. The enhanced wetting is due to the formation of Ti-Au and Cr-Au chemical bonds. This leads to the formation of a continuous film having i) monodisperse grain size and ii) the energetically most favorable [111] crystal orientation for Au (3). For the Cr/Au system there is an extra step: the inter-diffusion between Cr and Au to form a Cr-Au alloy (4). Such diffusion is limited to a thickness of 2-3 nm for samples prepared at room temperature.
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