Abstract
Integrated circuits (ICs) with Cu based interconnects are facing various challenges
such as electromigration, metallization issues associated with high aspect ratio lines
and vias (vertical interconnect access), high current density requirements for future
interconnects, heat dissipation in interconnects etc., with miniaturization. Therefore,
the need of new materials for IC interconnects is at present emphasized. Carbon
nanotubes (CNTs) have evolved as a potential candidate due to their exceptional
properties such as high current carrying capacity, ballistic conduction, extraordinary
strength and bottom-up growth behaviour. Vertically aligned CNT bundles have been
suggested for via applications. For the fabrication of CNT based vias, low temperature
efficient growth of high quality and high density CNTs on metal lines are required.
However, the growth rate and quality of CNTs is largely degraded when the growth is
conducted below 700oC using conventional CVD techniques. A suitable technique
enabling the growth of CNTs in a buried catalyst arrangement is required in order to
exploit the true bottom-up growth behaviour of CNTs in high aspect ratio vias. The
prevention of the catalyst from oxidation upon exposure to the atmosphere and
improved adhesion of CNTs with substrate (which reduces the reliability) are needed.
Here, a novel state-of-the-art photo-thermal chemical vapour deposition
(PTCVD) system is used for the low temperature (below 450oC) growth of high
quality carbon nanotubes on metallic layers for the interconnect via applications. The
CNT growth on TiN (50 nm)/Al (10 nm) metallic layers is studied as function of
growth temperature, catalyst material and catalyst thickness where 3 nm of Fe film as
a catalyst material is found to be the best as compared with the other two commonly
used catalyst materials which are Ni and Co. The achieved growth rate of 8 μm/min is
the highest when compared with the other reports of low temperature grown CNTs. A
low temperature growth process for the production of highest quality (ID/IG = 0.13)
vertically aligned carbon nanotubes on TiN film is demonstrated. The dependence of
the CNT growth profile on the thickness of TiN film provides a unique opportunity to
fine tune the various parameters of CNTs such as quality, growth rate, diameter,
density etc. High quality catalysed growth of CNTs on Ti/Cu/TiN metallic structure
has been achieved. The effect of catalyst heat treatment time and temperature is
studied. By rapidly reaching 659oC at the top surface of the samples, the CNTs with
lengths in excess of 1 μm were grown in less than 4 min of total process time.
The resistance of a single 100 × 100 μm2 CNT based via is calculated as 0.42 _, which is
one of the best values reported for CNT based via. A new ‘protected catalyst’
technique for the growth of CNTs is demonstrated where a capping layer of TiN at the
top of Fe catalyst is used to protect the catalyst. The protected catalyst scheme can be
a breakthrough in many potential application areas of CNTs as it simultaneously
protects catalyst from oxidation, improves the surface adhesion of CNTs and allows
the etching of SiO2 in a buried catalyst scheme while keeping the catalyst unharmed.
The fabrication and characterisation of CNT based vias in a chain structure on
TiN and Ti/Cu/TiN metallic layers is demonstrated. All processes resulted in the
growth of vertically aligned and dense (1010 cm-2) CNTs in all vias giving 100% yield.
The current-voltage plots show ohmic behaviour for all structures and resistance of
the structures increases linearly with increasing number of vias. The resistance values
of 23 _ and 91 _ (resistivity = 1.1 × 10-2 _-cm) for 10x10 μm2 and 5x5 μm2 vias
obtained for CNT based vias show more than a factor of two improvement as
compared with the recently published reports. The improvement can be attributed to
the high quality of the CNTs. The CNT via structures of protected catalyst scheme
exhibit a significant improvement in resistance of the structures both in terms of the
resistance of metal strips (100 _) and via resistances (21 _ and 84 _ for 5x5 μm2 and
10x10 μm2 vias). The growth of CNTs in deep vias is demonstrated. The vias of 3.5 to
20 μm depth are fabricated in polyimide (PI-5878G) film coated on n-Si substrate.
The CNT growth of up to 50 μm height in the vias is demonstrated which can be used
in TSVs. Length controlled growth of carbon nanotubes in 20 μm deep vias is
demonstrated. Levelling of the CNTs with the surface of the sample and filling of the
via-holes with photoresist is demonstrated using etch-back techniques.