Enhancement of moisture resistance of spin-on low-k HSQ
films by hot wire generated atomic hydrogen treatment
Alka A. Kumbhar, Sunil Kumar Singh, R.O. Dusane *
Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology, Bombay, Mumbai-400 076, India
Abstract
Spin on hydrogen silsesquioxane (HSQ) is a material with low dielectric constant (k) and shows potential as intermetal dielectric (IMD)
layers for future VLSI circuits. One major challenge in the integration of these films is the moisture uptake with time, which degrades the
electrical performance and hence limits their application. In the present work, we show (under accelerated conditions) that the as deposited
films absorb moisture significantly which is reflected in the related signatures in the infrared (IR) spectroscopic data. Subsequently there is an
increase in the leakage current with a concurrent decrease in the electrical breakdown field. Upon treatment with atomic hydrogen generated
by a hot filament (T F = 1900 -C), drastic reduction in the moisture absorption is observed. Also there is almost a 2 orders of magnitude
reduction in the leakage current and no indication of any electrical breakdown within the range of applied field.
Keywords: Hot wire generated atomic hydrogen treatment; Surface passivation; Intermetal dielectric (IMD); Moisture absorption
1. Introduction
Continuous reduction in chip size and increase in chip
complexity have given rise to the need for multilevel
metallization. However, the RC delay contributed from
multilevel interconnects limits the performance of the chip.
Therefore, it is necessary to use a low-k material as
dielectric and high conductivity metal for interconnects, to
reduce this delay and increase the signal propagation speed
[1]. Among the various low-k dielectric materials [2],
hydrogen silsesquioxane (HSQ) shows a lot of promise for
implementation at the < 0.13 Am technology. The low-k
property can be achieved if the density of Si –H bonds are
maintained at a high level and the formation of – OH bond
due to the absorption of water in the film is minimized.
The Si –H bond in the as cured HSQ is sensitive to
temperature and moisture absorption since it tends to break
forming Si –OH. The electric and dielectric properties of
low-k IMD layers depend on their resistance moisture
uptake [3]. Moisture resistance is thus a reliability concern
for the long-term stability of the IMD layers even after
packaging.
In order to inhibit moisture absorption and the subsequent degradation of the properties we have carried out hot
wire generated atomic hydrogen treatment of the HSQ films.
To study the moisture resistance of these treated HSQ films,
the effect of moisture penetration was evaluated by
accelerated water treatment tests and subsequent determination of the I – V characteristics.
2. Experimental details
HSQ films were spin-coated on to p-type silicon wafers
and baked on a hot plate at 100 -C, 250 -C and 350 -C for
1 min each followed by furnace curing 1 h in nitrogen
ambient at 400 -C and 500 -C [4]. The HSQ films were
exposed to atomic hydrogen generated by hot wire (referred
to as hot wire generated atomic hydrogen (HWGAH)
treatment) in our hot wire CVD (HWCVD) chamber [5].
The treatment parameters are given in Table 1. For the
moisture reliability test, the as cured as well as the
330
Parameter
Value
Gas used
Gas flow
Pressure
Substrate temperature (T S)
Filament temperature (T F)
Time
H2
10 sccm
1 Torr
250 -C
1900 -C
30 min
HWGAH treated HSQ films were immersed in water at 80
-C for 3 h in a closed beaker (container). These conditions
were chosen for accelerated testing and are similar to the
pressure cooker test (PCT) in an earlier study [6]. The
chemical structure of the cured HSQ films before and after
the hot wire and water treatment was investigated by
Fourier transform infrared (FTIR) spectroscopy. The thickness of HSQ films was measured by Dektak II profilometer.
The dielectric constant (k) was extracted from the
capacitance – voltage (C – V) characteristics measured at
100 kHz frequency. The C –V characteristics and the
current – voltage (I – V) measurements were performed on
MIS structures fabricated as Aluminum/HSQ/c-Si.
3. Results and discussion
The HSQ film consists of a mixed cage/network structure
before thermal curing. Part of the cage structure in the film
is transformed to a network structure after thermal curing.
This structural transformation results in significant lowering
of the dielectric constant (k) down to 2.9. The associated
vibrations of the network/cage structure in the HSQ films
can be seen at 1070 (Si –O –Si network) and 1130 cm 1
(Si –O – Si cage) in its FTIR spectra (curve 1 as shown in
Fig. 1). Their respective bending bands appear at 830 and
880 cm 1 respectively while the band at 2250 cm 1
corresponds to the Si –H stretching mode [7].
Infrared Absorbance (a. u.)
Table 1
Parameters of hot wire hydrogen treatment of the HSQ films
Si-OH
0.6
(4)
Water treated HWGAH HSQ
0.4
0.0
(3)
HWGAH
0.2
(2)
Si-O (Network)
Si-O (cage)
Water treated HSQ
(1)
as cured HSQ
4000
3500
3000
Si-H
2500
2000
1500
1000
-1
Wavenumber (cm )
Fig. 2. FTIR spectra of the HSQ film and the HWGAH treated HSQ films
after water treatment.
Fig. 1 presents two major observations on the curing of
HSQ films. First, the peak at 1130 cm 1 (assigned to cage
like structure) decreases with increase in curing temperature
while the peak at 1070 cm 1 (assigned to the network
structure) increases with the curing temperature. Initially at
lower curing temperatures (100 – 350 -C), the peak at 1130
cm 1 is stronger than that at 1070 cm 1. At higher
temperatures > 350 -C, this trend changes, i.e. the intensity
of the 1130 cm 1 peak decreases while that of 1070 cm 1
increases. Thus at higher temperature, the bonding structure
of the HSQ film changes from cage like to a network
structure. Another important observation is that the intensity
of the Si – H peak (at 2250 cm 1), which appears to be the
same for low curing temperature, starts to decrease at 400 -C.
At high curing temperatures > 400 -C, the Si – H bonds are
easily broken which causes generation of dangling bonds
and hence absorption of moisture in the films. Thus,
1.0
Infrared Absorbance (a. u.)
Si-O (cage)
(4)
0.6
Annealed at 400°C for 1h
Water and HWGAH HSQ
(3)
0.4
Baked at 350°C for 1min
Absorbance
Si-O (Network)
0.8
Annealed at 500°C for 1h
Baked at 250°C for 1min
HWGAH
0.2
Baked at hot plate 100°C for 1min
Si-O (cage)
0.0
Si-O (Network)
(2)
Water treated HSQ
(1)
-0.2
Si-H
as cured HSQ
As deposited
-0.4
1600
2400
2000
1600
1200
800
Wavenumber (cm-1)
Fig. 1. FTIR spectra of the HSQ films cured at different temperatures.
1400
1200
1000
800
600
Wavenumber (cm-1)
Fig. 3. FTIR spectra in the range 1600 – 600 cm 1 of as cured and hot wire
hydrogen treated HSQ film after 3 h of water treatment.
Leakage Current density (A/cm2)
331
(2)
1.25x10-6
HSQ water treated
1.25x10-7
HSQ
HWGAH HSQ/water treated
1.25x10-8
(1)
(4)
HWGAH HSQ (3)
1.25x10-9
1.25x10-10
0.0
0.2
0.4
0.6
0.8
1.0
Electric Field (MV/cm)
Fig. 4. Leakage current density as a function of electric field of the as
deposited (curve 1), as deposited but after water treatment (curve 2),
HWGAH treated (curve 3) and HWGAH treated but after water treatment
(curve 4).
increasing the curing temperature for the HSQ films causes
reduction in the Si –H (2250 cm 1) bond density and in turn
increases the Si –O – Si (1070 cm 1) bond densities (see Fig.
1 annealed at 400 -C for 1 h). The appearance of the Si – H
stretching mode at 2250 cm 1 is characteristic of the HSQ
supplied by Dow Corning Inc. This material is a derivative of
SiO2 in which one of the four oxygen atoms bonded to every
Si atom is replaced by hydrogen. The higher electronegativity of oxygen atom is responsible in shifting the
Si– H stretching mode from 2000 to 2250 cm 1.
When this film is subjected to boiling water treatment,
the film not only absorbs water vapor but also undergoes
significant changes in its chemical bonding. Fig. 2 shows
the FTIR spectra of the as-deposited HSQ film and the
HWGAH treated HSQ films after water treatment. Curve 2
of Fig. 2 depicts these changes very clearly. We see that a
broad band at 3350 cm 1 appears which is characteristic of
water absorption. The integrated area of this band gives the
relative amount of moisture absorbed by the HSQ film
which is 7.5 for the as cured HSQ films as against 5.8 for
the HWGAH treated HSQ films (curve 4). Interestingly the
FTIR spectrum of the HWGAH treated HSQ film is similar
to that of the untreated HSQ film (curve 3). This indicates
that the HWGAH treatment does not affect the cage to
network ratio in the HSQ film and that the dielectric
property remains essentially unaffected. Fig. 3 shows the
same results in an expanded IR range (1600 – 600 cm 1). It
should be noticed that a small but significant change occurs
in the region 800– 900 cm 1 which reflects the bending
mode characteristics of Si – O –Si, after water treatment of
the as cured HSQ films (curves 1 and 2). This can be
explained on the basis of the following reaction
Si H þ HOH ¼ Si OH þ H2 :
The significant increase in both the k value and the leakage
current could be attributed to these differences. However
when the HWGAH treated film is water treated these changes
in the bending modes are not seen (curves 3 and 4). The
formation of Si – OH after moisture pickup seems to be
responsible for the deterioration of the dielectric properties.
Fig. 4 shows the leakage current density of HSQ films and
HWGAH films before and after boiling water treatment. The
leakage current density reduces by more than one order of
magnitude by the hot wire hydrogen treatment of the as
deposited films (curve 3). This may be due to the passivation
of the surface dangling bonds by the active hydrogen radical
provided by the hot wire. Even after moisture absorption the
HWGAH treated HSQ film show a reduced (one order)
leakage current density as compared to the as cured HSQ
film. Also the as cured HSQ films without HWGAH
treatment undergo an electrical breakdown at 0.75 MV/cm
when they absorb moisture. On the other hand we do not
observe any such breakdown up to a field of 1 MV/cm in the
case of HWGAH treated films even after hot water treatment.
4. Conclusion
As cured HSQ films tend to absorb moisture, which
eventually leads to the degradation of their dielectric
property. Exposure of these films to the hot wire generated
hydrogen enhances their resistance to moisture absorption
and helps to reduce their leakage current by more than one
order of magnitude without affecting the dielectric property.
The atomic hydrogen treated films do not reveal any
discernible change in the IR spectrum, which clearly
indicates that the atomic hydrogen generated by hot wire
modifies the HSQ only on the surface and hence is not
detectable in FTIR measurement. However the enhancement
in the performance is significant enough to make this
process applicable in the ULSI backend technology.
Acknowledgements
This work was carried out under the project supported by
DST, Govt. of India (SR/FTP/PS-41/2000).
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