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= Adhesion layer impact on Au film stability with temperature =
= Adhesion layer impact on Au film stability with temperature =


Control of temperature is very important in several steps during micro and
Control of temperature is very important in several steps during micro and nanofabrication processes. For example, the general sequence of processing steps for a typical photolithography process is as follows:
nanofabrication processes. For example, the general sequence of processing
steps for a typical photolithography process is as follows: 1) substrate
preparation, 2) photoresist spin coating, 3) prebake, 4) photoresist exposure,
5) post-exposure bake, 6) photoresist development, 7) postbake and 8)
photoresist strip after pattern transfer.
Of the whole process, three steps (3, 5 and 8) involve the use of a heat
treatment on the sample for periods varying from one to 30 minutes (and
97


even hours for the case of very thick resists) and using temperatures ranging
1) substrate preparation,
from 100°C to 200°C.
As reported in Chapter 4, the modi�cation of the nanostructure of pure
Au in this temperature range is already quite pronounced and can have a big
impact on the fabrication and performances of nanodevices if this material
is used without adhesion layers. Furthermore, as observed in Chapter 5,
also the use of Ti and Cr adhesion layers to enhance adhesion of Au on the
substrate led to a change of the nanostructure and electrical performances
of the bilayer systems; however such in
uence did not seem as dramatic as
the change due to the increase in temperature. It is currently unclear if the
use of adhesion layers can have a positive impact on the stability of the Au
nanostructure at elevated temperatures: this is the main point addressed in
the following of this chapter.
In Section 6.2 a description of the experimental conditions is given, together
with the TKD analysis of the impact of Cr and Ti adhesion layers
on the Au nanostructure for di�erent temperatures; the results showed that
the continuity of the �lm was preserved up to 500°C using both adhesion
layers, but also that Cr and Ti had a di�erent impact on the �nal Au nanostructure.
Section 6.3 reports a preliminary study towards the understanding
of the Au grain coalescence, observed in Section 6.2, using the geometrical
concept of the Euler angles.


== Evolution of the Au nanostructure with temperature ==
2) photoresist spin coating,


Figure 6.1 shows the TKD maps of �lm nanostructure evolution for a di�erent
3) prebake, 4) photoresist exposure,
set of temperatures of a 20 nm pure Au �lm (left column), 2 nm Ti/20
nm Au (center column) and 2 nm Cr/20 nm Au (right column) bilayer
systems.


The nanostructure of pure Au at room temperature consisted of small
5) post-exposure bake,
grains having [100] and [110] crystal orientations with respect to the sample
surface orientation and larger grains having [111] orientation, as already
described in the previous chapters. The �lm started to dewet between 100°C
and 200°C. In general, the evolution of the nanostructure was in agreement
with the results reported in Chapters 3 and 4. When Ti was used as adhesion
layer, the nanostructure of Au at room temperature had a smaller grain size
and complete [111] orientation, as described in Subsection 5.2.1. When the
sample was treated with the same annealing conditions as pure Au, the e�ect
of the temperature on the nanostructure was very di�erent: the annealing
did not a�ect the continuity of the �lm up to 500°C and up to 200°C there
was little grain growth. From 400°C the grain size started to increase, but
not at the levels of pure Au. For the Cr/Au bilayer system, the Au �lm
had a slightly smaller average grain size than in the Ti/Au case. Due to
the low average grain size, a lot of grains were not properly indexed and are
displayed as black areas. The Au layer maintained a continuous morphology
up to 500°C also in this case, but the increase of the grain size was more
limited than in the Ti/Au sample.
A more quantitative analysis of the grain size increase for all three samples
was performed between the map at room temperature and the one
after the annealing at 200°C, since at such temperature the nanostructure
of Au was still continuous and the dewetting was not a�ecting the analysis
signi�cantly. The results are reported in Fig. 6.2.


The average grain size of the pure Au sample increased from 97 nm to
6) photoresist development,
105 nm in the evaluated temperature range (Fig. 6.2a), the one of the Ti/Au
sample from 45 nm to 56 nm (Fig. 6.2b) and the one of the Cr/Au sample
almost did not change, increasing from 34 nm to 36 nm (Fig. 6.2c).
At higher temperatures, the Au grain size increase was very di�erent
between the two adhesion layers. Figure 6.3a shows the variation for Ti/Au
between room temperature and 500°C: the plotted average grain size value
increased from 45 nm to 113 nm after the annealing. Figure 6.3b shows the
variation for Cr/Au: in this case the grain size increase is lower, varying from
34 nm to 44 nm, highlighting the higher nanostructure stability guaranteed
by Cr respect to Ti.


In the pure Au and Ti/Au samples, was clearly visible with a visual
7) postbake,
inspection of the maps that the grain growth proceeded through grain coalescence
 
(marked by white circles in the maps of Fig. 6.4). However, the
8) photoresist strip after pattern transfer.
mechanism of growth looks slightly di�erent between the two samples. In
 
pure Au, the growth proceeds trough the coalescence between the smaller
Of the whole process, three steps (3, 5 and 8) involve the use of a heat treatment on the sample for periods varying from one to 30 minutes (and even hours for the case of very thick resists) and using temperatures ranging from 100°C to 200°C.The modification of the nanostructure of pure Au in this temperature range is already quite pronounced and can have a big impact on the fabrication and performances of nanodevices if this material is used without adhesion layers. Furthermore, also the use of Ti and Cr adhesion layers to enhance adhesion of Au on the substrate led to a change of the nanostructure and electrical performances of the bilayer systems; however such in uence did not seem as dramatic as
[100] and [110] grains and the larger and more energetically stable [111]
the change due to the increase in temperature. It is currently unclear if the use of adhesion layers can have a positive impact on the stability of the Au nanostructure at elevated temperatures: this is the main point addressed in the following.
grains, as already described in Subsection 4.4.2. In the Ti/Au sample instead,
 
the coalescence takes place between [111] grains, since they are the
Figure 6.1 shows the TKD maps of �lm nanostructure evolution for a di�erent set of temperatures of a 20 nm pure Au �lm (left column), 2 nm Ti/20 nm Au (center column) and 2 nm Cr/20 nm Au (right column) bilayer systems. The nanostructure of pure Au at room temperature consisted of small grains having [100] and [110] crystal orientations with respect to the sample surface orientation and larger grains having [111] orientation, as already described in the previous chapters. The �lm started to dewet between 100°C and 200°C. In general, the evolution of the nanostructure was in agreement with the results reported in Chapters 3 and 4. When Ti was used as adhesion layer, the nanostructure of Au at room temperature had a smaller grain size and complete [111] orientation, as described in Subsection 5.2.1. When the sample was treated with the same annealing conditions as pure Au, the e�ect of the temperature on the nanostructure was very di�erent: the annealing did not a�ect the continuity of the �lm up to 500°C and up to 200°C there was little grain growth. From 400°C the grain size started to increase, but not at the levels of pure Au. For the Cr/Au bilayer system, the Au �lm had a slightly smaller average grain size than in the Ti/Au case. Due to the low average grain size, a lot of grains were not properly indexed and are displayed as black areas. The Au layer maintained a continuous morphology
only ones present in the nanostructure. The grain size increase in the Cr/Au
up to 500°C also in this case, but the increase of the grain size was more limited than in the Ti/Au sample. A more quantitative analysis of the grain size increase for all three samples was performed between the map at room temperature and the one after the annealing at 200°C, since at such temperature the nanostructure of Au was still continuous and the dewetting was not a�ecting the analysis signi�cantly. The results are reported in Fig. 6.2.
sample is due to a mechanism most likely similar to the one of Ti/Au, as also
 
this sample only [111] Au grains are present; however the grain coalescence
The average grain size of the pure Au sample increased from 97 nm to 105 nm in the evaluated temperature range (Fig. 6.2a), the one of the Ti/Au sample from 45 nm to 56 nm (Fig. 6.2b) and the one of the Cr/Au sample almost did not change, increasing from 34 nm to 36 nm (Fig. 6.2c). At higher temperatures, the Au grain size increase was very di�erent
was not immediately visible due to the higher stability of the Au nanostructure.
between the two adhesion layers. Figure 6.3a shows the variation for Ti/Au between room temperature and 500°C: the plotted average grain size value increased from 45 nm to 113 nm after the annealing. Figure 6.3b shows the variation for Cr/Au: in this case the grain size increase is lower, varying from 34 nm to 44 nm, highlighting the higher nanostructure stability guaranteed by Cr respect to Ti.
The next section presents the introduction of the geometrical concept
 
of the Euler angles in relation to a preliminary and non-conclusive study
In the pure Au and Ti/Au samples, was clearly visible with a visual inspection of the maps that the grain growth proceeded through grain coalescence (marked by white circles in the maps of Fig. 6.4). However, the mechanism of growth looks slightly di�erent between the two samples. In pure Au, the growth proceeds trough the coalescence between the smaller[100] and [110] grains and the larger and more energetically stable [111] grains, as already described in Subsection 4.4.2. In the Ti/Au sample instead, the coalescence takes place between [111] grains, since they are the only ones present in the nanostructure. The grain size increase in the Cr/Au sample is due to a mechanism most likely similar to the one of Ti/Au, as also this sample only [111] Au grains are present; however the grain coalescence was not immediately visible due to the higher stability of the Au nanostructure.
of the coalescence process observed here, having the �nal aim of a better
comprehension of its mechanism.
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