Specific Process Knowledge/Thin film deposition/Deposition of Gold/Adhesion layers: Difference between revisions
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== Nano-electronic devices == | == Nano-electronic devices == | ||
For nano-electronics applications, the reported results show that Ti and Cr | |||
form oxides with surface adsorbed water and free oxygen in the vacuum | |||
chamber of the physical vapor deposition system. To avoid oxidation of the | |||
adhesion layer, the chamber vacuum needs to be in UHV conditions and | |||
the sample must be baked to remove the surface adsorbed water. However, | |||
these baking temperatures are not compatible with e.g. lift-o� nanofabrication | |||
of nano-electronic devices that often utilize photo- or electron beam | |||
lithography resists which do not tolerate high baking temperatures. | |||
In the case of carbon nanotubes (CNTs), Ti uniformly coats the nanotube | |||
surface [133]. That implies that the partially oxidized Ti uniformly | |||
coats CNTs, which might lead to poor electrical contact to the nano-electronic | |||
device. However, it is unclear if this is true for other nano-electronics materials, | |||
because their surface chemistry is di�erent. 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 [134]. For MoS2 FETs, Radisavljevic showed | |||
that pure Au contacts out-performed Ti/Au contacts [135]. | |||
If an adhesion layer is required for mechanical stability, a less than 2-nmthin | |||
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-di�usion of Au and | |||
Cr and improve electrical contact between the nano-electronic material and | |||
the Au over-layer. Indeed, low temperature annealing is often used in nanoelectronic | |||
fabrication for improving electrical contact [135, 136]. Finally, | |||
an important implication is that better adhesion layers for nano-electronics | |||
might be metals with electrically conductive oxides such as ruthenium and | |||
iridium [137, 138]. | |||
= Adhesion layer impact on Au �film stability with temperature = | = Adhesion layer impact on Au �film stability with temperature = | ||