chapter 6 - Shodhganga

Surface Modification and Electrical Characterization of Porous Low-k Thin Films for Nanoelectronics Applications
CHAPTER 6
CONCLUSIONS AND FUTURE SCOPE
The interlayer dielectric (ILD) materials with lower dielectric constant are
needed to continue the increasing transistor density on single chip. The main aim of
present work carried out was to deposit and characterize the porous low-k thin films
in order to check their suitability for ILD applications in recent ULSI technology. The
porous low-k films have been deposited using sol-gel spin-on method on p-type Si
<100> substrate. The porosity is introduced in the film matrix in order to reduces
dielectric constant by two ways. The first way is to deposit silica based xerogel thin
films by sol- gel technique. The other way is by addition of porogen such as tween80
in the xerogel thin films. Further, the work was extended to dry the deposited films
by using liquid CO 2 supercritical drying method to enhance the porosity.
The present work was focused on the improvement of properties of deposited
low-k thin film like mechanical, electrical and hydrophobization of film surface. In
order to improve the properties of low-k thin films, the wet chemical surface
modification method was preferred. The surface modification was carried out using
trimethylchlorosilane (TMCS) as a surface modifying agent. To check of suitability of
low-k films as an ILD material for ULSI technology the electrical characterization of
film was carried out by fabricating Al/Low-k/Si MIS structure. The deposited and
surface modified low-k films are observed to have low dielectric constant value with
lower leakage current density.
According to international technology roadmap for semiconductor (ITRS)
2012 the ILD materials with k<2.3 are needed for 32nm and beyond technology node.
These ILD materials with lower k value are needed to minimize the RC delay, power
consumption and cross talk problems occurred in BEOL. Chapter 1 reflects the
various methods to reduce the dielectric constant of thin film. There are several
alternative low-k materials for ILD applications available in the market to fulfill the
current technology demands.
The different thin film deposition techniques are discussed in chapter 2. The
sol-gel technique has been used for deposition of low-k films considering their
advantages in order to introduce high porosity. The working principle of Sol- gel,
physical vapour deposition, chemical vapour deposition and other thin film deposition
methods along with different characterization techniques are elaborated in chapter 2.
Mr.Yogesh S. Mhaisagar, Department of Eelectronics, NMU, Jalgaon (2013)
6.1
Surface Modification and Electrical Characterization of Porous Low-k Thin Films for Nanoelectronics Applications
Clausius Mossotti equation describes the ‘k’ value reduction by incorporation
of low polarized bonds or by introducing porosity in low-k film matrix. But the
increase in porosity for the further reduction in k value simultaneously changes the
original material properties. The increase in percentage of porosity lowers the
mechanical strength of films. The poor mechanical strength leads to crack and
delamination in film during CMP. Incomplete removal of post ash in wet clean
process lefts contaminated Cu surface by forming byproducts such as CFx, CuFx,
CuO and Cu2 O. This contaminant then entrapped between dielectrics and barrier
which lead to leakage between metal lines and poor reliability. Further, active radicals
formed in plasma treatment method remove the carbon-containing hydrophobic
groups and makes the films surface hydrophilic. The hydrophilic surface causes the
absorption of moisture which further raises the k value. Thus the surface modification
process found to be beneficial to improve the surface properties and to overcome the
above problems. Therefore the integration of such porous low-k films in current
metallization process leads to different ILD reliability issues and degradation of k
value. The literature survey and advantages of surface modification to improve the
surface properties of low-k films are discussed in details in Chapter 3
Chapter 4 explains experimental process for the deposition of porous low-k
thin films using TEOS as a main precursor and ethanol as a solvent. HF was used as
an acid catalyst to accelerate the rate of hydrolysis reaction. The Tween 80 was used
as a porogen for the introduction of porosity in the film matrix. Further, the surface
modification of low-k films was carried out by using TMCS with Hexane solution.
The metallization of low-k thin films was also done by depositing Al metal on top of
film as upper electrode (area 2.48x10 -2 cm2 ) by thermal evaporation system. Also, the
ohmic contact was formed on backside of Si substrate to form MIS structure for
measurement of electrical properties.
The chapter 5 illustrates the results of sol- gel deposited and surface modified
porous low-k thin film. The most intense absorption band observed around 1075 cm-1
in FTIR spectra due to asymmetric stretching confirmed the formation of Si-O-Si
network. Along with this the longitudinal optic (LO) mode observed at 1200 cm-1
confirms the presence of porosity in the film. The effect of addition of porogen
Tween80 on low-k thin film properties such as refractive index, thickness, density and
percentage of porosity were studied. The increase in porogen concentration shows the
decrease in density of films which forms the pore in film and hence increases the
Mr.Yogesh S. Mhaisagar, Department of Eelectronics, NMU, Jalgaon (2013)
6.2
Surface Modification and Electrical Characterization of Porous Low-k Thin Films for Nanoelectronics Applications
percentage of porosity. The dielectric constant of the films deposited using Tween80
porogen reduces the k value to 2. Further, formation of porosity in the film was
observed through AFM with an average pore size of 15nm. The electrical
characterization of deposited porous low-k films was carried out to determine the
dielectric constant and leakage current density of the films. The significant decrease
in leakage current density indicates that the generation of pores in the films due to
removal of high polarized Si-OH groups at higher annealing temperature. The leakage
current density of the film was found to be decresed from 6×10 -6 A/cm2 to 3.4×10-8
A/cm2 at 1MV/cm with high breakdown strength (< 3.5 MV/cm) . This obtained value
is considered as an acceptable leakage current density for the interconnect application.
In order to improve the properties of deposited low-k films, the surface
modification were carried out by wet chemical treatment using TMCS as a modifying
agent with hexane solvent. The bands around 2850 cm-1 to 2900 cm-1 observed in
surface modified films was due to the asymmetric and symmetric stretching of CH3 .
On the other hand the hydroxyl group appear around 3500cm-1 was found to be
decreased after surface treatment. The detailed study of change in chemical structure
after surface modification was also carried out by using TQ analyzer software
availabled with FTIR. By using this TQ analyzer the area of hydroxyl bonds and
incorporation of non-polar carbon groups were studied. The surface modified films
shows 50% reduction in –OH bands after surface modification. This reduction in –
OH bands and incorporation of -CH3 band revealed change in surface of films from
hydrophilic to hydrophobic nature. Further, the hydrophobicity of the films were
observed through the water contact angle measuremnt. The increased in water contact
angle from 85o to 110o confirmed the change in surface from hydrophilic to
hydrophobic.
The improvement in mechanical properties such as hardness and elastic
modulus of the thin films were determined from indentation load displacement data.
From the indenter load displacement data it is observed that, the surface modified
films show lower penetration depth than the as depeosited films. The decrease in
penetration depth indicate the improvement in the hardness of the film after surface
modification. The hardness of the film was found to be increased from 1.32 to 3.22
GPa. Further the Youngs modulus of the film was determined from loading and
unloading curve data. The film modified at higher TMCS concentration i.e at 20% in
hexane solution shows the higher Youngs modulus of 75 GPa. This significant
Mr.Yogesh S. Mhaisagar, Department of Eelectronics, NMU, Jalgaon (2013)
6.3
Surface Modification and Electrical Characterization of Porous Low-k Thin Films for Nanoelectronics Applications
improvement in mehcnaical properties of low-k films after surface modification
indicates the rearragnement of the film chemical structure which form stable and
strong Si-O-Si network. Further, the electrical data shows the lowering in k value
after surface modificaiton. The lowering in k value is due to the incorporation of nonpolar –CH3 groups.
In conclusion, the porous xerogel silica low-k thin film have been deposited
sucessfuly with
introduciton of porosity in the film matrix by using porogen
Tween80 and annealing at high temperature. Further improvement in low-k thin films
properties such as hydrophobic surface, lower k value and high mechanical strength
were achieved through wet chemcial surface modification method. The deposited
low-k thin filmsthrough this research work are suitable as an ILD material in ULSI
technology for nanoelectronics applications due to their low dielectric constant with
improved film properties.
The future scope of the work is explained in next section.
Mr.Yogesh S. Mhaisagar, Department of Eelectronics, NMU, Jalgaon (2013)
6.4
Surface Modification and Electrical Characterization of Porous Low-k Thin Films for Nanoelectronics Applications
Future scope
The new introduced ILD materials with lower k value must be physically and
chemically compatible for sucessful integraion in present IC fabrication technology.
This achievement can be obtain through the incorporation of high percentage of
porosity in silica based film matrix. Hence, the silica aerogel film deposited by sol- gel
spin-on technique followed by supercritical drying process can also be benificial with
its unique properties like > 90% of porosity and ultra low-k value ~1.1. Further, the
pore size and distribution of pores are critical issues for final implementation of lowk film for ILD applicaiton. Thus, the work can be extended for introduction of high
porosity where pores will have higher ordere of connectivity in film structure.
Further, the film must have sufficient modulus and high thermal stability to
overcome the issues occurred during porous low-k integration. The properties of lowk films can be imrpoved by rearranaging the chemical srtucture using surface
modification method. The super hydrophobic surface will improve the thermal
stability of low-k film. Silylation method can be used to achieve such super
hydrophobic surface of films. Further, the mechnical properties of low-k film can be
improved by UV treatment technique in conjunction with controlled curing time and
wavelength.
Mr.Yogesh S. Mhaisagar, Department of Eelectronics, NMU, Jalgaon (2013)
6.5

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