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To avoid oxidation of Ti or Cr that is in physical contact with the nano-electronic materials, one solution is to avoid these materials completely. Indeed, some of the best performing CNT devices are made without the use of adhesion layers, as e.g. Pd which is directly used. '''If an adhesion layer is required for mechanical stability, a less than 2nm thin Cr layer is recommended and Ti must be avoided'''. This is because the partially oxidized Ti might form a barrier between the nano-electronic material and the Au over-layer, with a consequent deterioration of the electron transport performances. Because of the single-layer morphology due to the Cr-Au alloy formation, the alloy will make electrical and physical contact to the nano-electronic material, despite the chrome oxide content. Furthermore, a low temperature annealing will enhance inter-diffusion of Au and Cr and improve electrical contact between the nano-electronic material and the Au over-layer.
To avoid oxidation of Ti or Cr that is in physical contact with the nano-electronic materials, one solution is to avoid these materials completely. Indeed, some of the best performing CNT devices are made without the use of adhesion layers, as e.g. Pd which is directly used. '''If an adhesion layer is required for mechanical stability, a less than 2nm thin Cr layer is recommended and Ti must be avoided'''. This is because the partially oxidized Ti might form a barrier between the nano-electronic material and the Au over-layer, with a consequent deterioration of the electron transport performances. Because of the single-layer morphology due to the Cr-Au alloy formation, the alloy will make electrical and physical contact to the nano-electronic material, despite the chrome oxide content. Furthermore, a low temperature annealing will enhance inter-diffusion of Au and Cr and improve electrical contact between the nano-electronic material and the Au over-layer.


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


[[Image:section under construction.jpg|150px]]
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: 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


allic bond with gold. 5 nm to 10 nm thick of Cr or Ti is commonly used and it is important to deposit Cr or Ti and then immediately Au. If the vacuum chamber is opened in between, the surface of Cr or Ti will get oxidized and that will give a poor adhesion. If a gold layer needs to be deposited directly on Silicon, then native oxide has to be removed by deep in diluted HF and immediately load the evaporation chamber. And after the deposition, the wafer has to be heated op to get some Au-Si diffusion which improves the adhesion.
even hours for the case of very thick resists) and using temperatures ranging
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.
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