Specific Process Knowledge/Thin film deposition/Deposition of Gold/Adhesion layers: Difference between revisions
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= Adhesion layer effect on Au thin films = | = Adhesion layer effect on Au thin films = | ||
== Effect on Au | == Effect on Au film morphology, grain size and texture == | ||
From the cross section pro�le of the 2-Au sample (Fig. 2a), it is clear that Au, when deposited directly on SiO2, in order to reduce the interface energy, forms a nanoparticle-like layer in contrast to a continuous �lm. The diameter of the nanoparticles is about 10 nm. The nanoparticle morphology | |||
is con�rmed by AFM analysis (Fig. 5.2b). The RMS surface roughness was | |||
measured to be 2.4 nm. For comparison, the RMS roughness of the SiO2 | |||
substrate was 0.3 nm. When the thin-�lm nominal thickness is increased to | |||
20 nm (20-Au sample, Fig. 5.2c), the Au layer becomes continuous, but a | |||
certain degree of surface roughness is still present due to grain coalescence. | |||
The AFM image (Fig. 5.2d) shows an RMS surface roughness of 1.0 nm, | |||
which is signi�cantly lower than the 2-Au sample. | |||
To investigate the crystal orientation of the metal thin-�lms, TKD was | |||
used (Fig. 5.3a). The nanostructure of the 20-Au �lm has a bimodal grain | |||
size distribution (Fig. 5.3b). While the smaller grains have di�erent crystal | |||
orientations, the large grains (blue color) all have [111] orientation, as | |||
already observed in Chapters 3 and 4. | |||
Microstructural evolution and growth of metal thin-�lms deposited by | |||
physical vapor deposition on amorphous dielectric substrates has been reported | |||
following island growth [13]. The �rst thin-�lm 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 di�usion and grain boundaries (GB) motion. During this process, the | |||
island growth is driven by the minimization of surface and interface energy. | |||
Fig. 5.4a represents two Au islands having di�erent crystal orientations. In | |||
particular they have the (111)- and (100)-facets, respectively, parallel to the | |||
substrate surface. The growth of these islands is dependent on their orientation | |||
(Fig. 5.4b): for Au, which has a face-centered cubic unit cell, the (100) | |||
surface has a higher surface energy than the (111) surface, and to minimize | |||
energy, the (100) surface grows faster. Islands having (100) or (110) facets | |||
parallel to the substrate grow faster in the vertical direction, while islands | |||
with (111) facets parallel to the substrate grow faster laterally. The result | |||
is a continuous �lm, having laterally larger and | |||
atter islands with [111] | |||
crystal direction, and laterally smaller but taller islands with [100] and [110] | |||
orientations [106]. | |||
From the TEM bright �eld cross section analysis of the 2-Ti/2-Au | |||
sample, it is observed that 2 nm of Ti forms a continuous layer below the Au | |||
layer (Fig. 5.5a). The Au layer is continuous over the Ti, indicating that Ti | |||
is responsible for an interface energy decrease, acting as an adhesive. The | |||
AFM analysis of the same sample con�rmed the continuous structure, and | |||
an RMS surface roughness of 0.8 nm was measured (Fig. 5.5b). The 2- | |||
Ti/20-Au sample also shows a smoother �lm compared to the 20-Au sample | |||
(Fig. 5.5c), and the AFM analysis shows an RMS surface roughness of 0.5 | |||
nm (Fig. 5.5d), very similar to the one of the Si substrate. | |||
In contrast to the case with the Ti adhesion layer, TEM bright �eld | |||
analysis of the 2-Cr/2-Au sample shows a single continuous layer (Fig. 5.6a). | |||
The AFM RMS surface roughness is 1.2 nm (Fig. 5.6b). Increasing the | |||
nominal Au thickness to 20 nm for the 2-Cr/20-Au sample, the �lm still | |||
presented a single-layer morphology (Fig. 5.6c), while the RMS surface | |||
roughness decreased to 0.6 nm (Fig. 5.6d). | |||
The samples with 20 nm Au layer were analyzed with TKD. The addition | |||
of the adhesion layer had in both cases a profound impact on grain size and | |||
orientation of the Au �lm, as visible in Fig. 5.7a and 5.7c. The image shows | |||
small grains mainly oriented in the [111] crystal direction, with an average | |||
grain size of 40 nm for Ti (Fig. 5.7b) and 36 nm for Cr (Fig. 5.7d). 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. | |||
The smaller grain size is attributed to an enhanced wetting of the deposited | |||
Au promoted by the adhesion layer. The enhanced wetting behaviour | |||
increases the number of nucleation sites compared to the pure Au | |||
�lm case, where Au is evaporated directly onto the SiO2 surface. This eventually | |||
leads to a much denser nucleation of the Au grains, which at the | |||
same time facilitates the inter-di�usion of Au atoms. The enhanced wetting | |||
might be due to the formation of Ti-Au and Cr-Au bonds | |||
The very dominant [111] crystal orientation observed implies a decrease | |||
of the energy barrier for the formation of the energetically most favourable | |||
Au crystal structure. This is promoted by the denser nucleation and stronger | |||
inter-di�usion of Au atoms described above. In contrast to the pure Au case, | |||
all the grains have the same di�usion rate of the (111) exposed planes, and | |||
therefore the grains grow with a narrow grain size distribution. Since the �lm | |||
has been deposited at room temperature, the system did not have enough | |||
energy to overcome the energy barrier for grain coalescence, hence resulting | |||
in grains with a small average size. For the Cr case, also small grains with | |||
[100] and [110] crystal orientations were detected. This might suggest that | |||
for Au on Cr the [111] orientation is energetically less favored with respect | |||
to the [100] and [110] orientations compared to the Ti/Au case. | |||
== Bilayer chemical composition and elemental distribution == | == Bilayer chemical composition and elemental distribution == | ||
== Adhesion layer effect on bilayer thin- | == Adhesion layer effect on bilayer thin-film electrical resistivity == | ||
= Adhesion layer model = | = Adhesion layer model = | ||