Electrical properties of sputter deposited SrTiO
3
gate dielectrics
Chih-Yi Liu, Tseung-Yuen Tseng
*
Department of Electronics Engineering and Institute of Electronics, National Chiao Tung University, Hsinchu 30050, Taiwan, ROC
Abstract
We report the electrical properties of gate dielectric SrTiO3(STO) thin films on Si substrates grown by radio-frequency
magne-tron sputtering. The interfacial layer between STO and Si degrades the performance of the gate dielectric. We used SiON as a sacrificial layer for incorporating nitrogen into Si substrate surface, which can retard the formation of interfacial layer during the high temperature growth of STO gate dielectric. The polycrystalline STO film grown on nitrogen incorporated Si substrate exhibited lower leakage current due to better interfacial properties and higher dielectric constant. The repeated spike heating technique was also employed to deposit polycrystalline STO film for improving the thermal uniformity of the wafer, which leads to lower gate leakage current. The processing parameters including working pressure, oxygen mass ratio and plasma power also show obvious effect on the electrical properties of the films.
#2003 Elsevier Ltd. All rights reserved.
Keywords:Electrical properties; Films; Gate dielectric; SrTiO3
1. Introduction
Following the device continuously scaling down, the
conventional SiO2gate oxide thickness will be less than
2 nmin the near future.1However, the use of ultrathin
SiO2 gate dielectric results in a number of issues,
including high gate leakage current, reduced drive cur-rent, reliability degradation, boron penetration, and the
necessity to grow ultra-thin and uniformSiO2 layer.
Any of these effects will fundamentally limit the
useful-ness of SiO2as a gate dielectric. According to the
fun-damental quantum mechanical law, the direct tunneling current increases exponentially with decreasing film thickness. The leakage current increases by one order of magnitude when each 0.2 nm thickness decreases. As
the SiO2thickness scales below 2 nm, the gate leakage
current will increase significantly due to direct tunnel-ing. Many high-k materials have been investigated as
potential replacements for SiO2to provide a physically
thicker filmfor reducing leakage current and improving gate capacitance. The physical thickness of such high-k material can be much larger to reduce the tunneling
current, so alternative gate dielectrics with high
dielec-tric constant are necessary. SrTiO3 (STO) thin films
have been proposed for applications in high charge sto-rage capacity devices, such as DRAM capacitors, because of their high dielectric constant. Recently, STO thin films have been successfully epitaxially grown on Si
by using molecular beam epitaxy at Motorola Labs2as
a gate dielectric of MOSFETs. On the other hand, STO films with low equivalent oxide thickness (EOT) and low leakage current can also be a potential insulating layer as in metal/ferroelectric/insulator/semiconductor (MFIS) structure, which is a basic structure for ferro-electric gate non-volatile ferroferro-electric memory applic-ation.3 In general, interfacial layer will naturally form
between high-k metal oxide materials and Si due to Si oxidation and inter-diffusion at high temperature. The resultant interfacial layer of low permittivity materials will limit the highest possible gate capacitance or the lowest achievable EOT. Several methods have been employed to reduce the interfacial reaction. Incorporat-ing nitrogen in silicon substrate has been used to grow ultra thin oxide because nitrogen can suppress the
growth of silicon oxide effectively.4 A
repeated-spike-heating technique has been proposed by Hong et al.5to
grow ultrathin oxide. The radiation heat absorption in two different temperature regions on a wafer could be
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* Corresponding author. Tel.: 5712121; fax: +886-3-5724361.
compensated by repeated-spike-heating technique,6 so
such technique can improve the thermal uniformity to
grow uniformSiO2. In the present study, we
incorpo-rated nitrogen into the silicon surface to alleviate the
reaction of the silicon surface into SiOx thin interface
layer during rf-sputtered deposition STO gate dielectric and to obtain better EOT value. The repeated-spike-heating technique was also employed to deposit STO films for improving the thermal uniformity of the wafer. Electrical characterizations of STO thin films were also performed. Besides the study of the interface improve-ment, the influence of the processing parameters including working pressure, oxygen mass ratio (OMR), and plasma power on electrical properties of STO thin films was also investigated.
2. Experimental
Boron doped p-type silicon (100) wafers with 1–10 -cmresistivity were used as the starting substrates. After standard RCA clean, a 8-nmSiON filmwas grown on silicon wafer at 950C in pure N
2O gas as the sacrificial
oxide. For comparison, 8-nm SiO2was thermally grown
in pure O2ambient as the control sample. After SiON
and SiO2 films were removed by HF dip, the 20-nm
STO thin films were deposited on the above pre-treated substrates by using rf magnetron sputtering at a
sub-strate temperature of 500C at the same time. The
fab-rication flow chart is illustrated as Fig. 1(a). The
repeated-spike-heating and typical profiles used for
depositing STO films are shown inFig. 1(b). The
repe-ated-spike-heating method was set the temperature to
ramp up and down between 450 and 550 C, while the
typical profile was held at a constant temperature of
500 C. The rapid thermal annealing (RTA) was
per-formed in N2ambient to improve the quality of films.
To investigate the effect of the processing parameters such as working pressure, OMR, and plasma power on the electrical properties of the films, the other
parameters were fixed while one was varied. For the electrical measurement, Al top electrode with an area of
7.0104 cm2 was formed by thermal evaporation,
followed by electrode patterning using a wet litho-graphy process. Al was also used as backside electrode for obtaining ohmic contact. The capacitance–voltage (C–V) measurements were performed using a HP 4284A at 100 k to 1 M Hz. The current–voltage (I–V) measurements were recorded using a HP 4156A Semi-conductor Parameter Analyzer.
3. Results and discussion
After the removal of SiON and SiO2sacrificial oxide
by dipping in HF, the X-ray photoelectron spectroscopy (XPS) profiles of the silicon substrates were recorded. The XPS profiles are illustrated inFig. 2. It can be easily seen that nitrogen has been moderately incorporated onto the silicon substrate surface. The amount of
nitro-gen on the N2O pre-treatment wafer calculated from the
XPS measurement data is 0.895% atomic ratio. While the STO dielectric capacitance was measured by HP4284A at 100 k to 1 M Hz, the C–V curves were
found frequency dependence, as shown in Fig. 3. The
phenomena are due to extrinsic parasitic inductance and resistance. After being calibrated by the two-frequency four-element model,7illustrated in the insert of Fig. 3,
the calibrated C–V curves of the N2O pre-treatment and
control samples are shown in Fig. 4. We can observe
that the EOT of N2O pre-treatment sample is smaller
than that of the control sample. The quantum mechan-ical curve fitting8is also shown inFig. 4, fromwhich the
EOT is extracted to be 3.0 and 3.3 nmfor N2O
pre-treatment and control samples, respectively. The slight shift of the measured C–V curves in comparison with the theoretical curve is due to the interface trap density. Fig. 5 shows the leakage current densities of the N2O
pre-treatment sample and the control sample. The N2O
pre-treatment sample has leakage current density 2–3
order-magnitude lower than the control sample at the positive bias, while the leakage current of the N2O pre-treatment sample is little less than that of control sample
at the negative bias. The EOT of 500C deposited 20 nm
thick STO thin films on silicon substrates prepared with various working pressures, 5, 25, 35, and 45 mTorr, are
shown in Fig. 6. As expected, the growth rate of the
sputtered STO thin films decreases with increasing working pressure. Filmthicknesses were fixed at 20 nm through controlling the deposition time in order to eliminate effect of thickness on dielectric properties. The composition of STO films is influenced by the working pressure and may affect the dielectric constant of the films accordingly. This may be the main reason for the EOT decreasing with the increase of working pressure
for the samples indicated inFig. 6. We can also observe
that the N2O pre-treatment samples have lower EOT
than the control samples.Fig. 7shows the transmission
electron microscopy (TEM) images of the N2O
pre-treatment and the control samples with working
pressure of 45 mTorr, indicating that the interfacial
layers of N2O pre-treatment sample and the control
sample are 1.9 and 2.6 nm, respectively. Therefore, the
N2O pre-treatment sample with lower EOT is attributed
to its smaller thickness of interfacial layer between STO and Si. The variation of the leakage current density with
working pressure for control and N2O pre-treatment
samples is illustrated inFig. 8. The N2O pre-treatment samples have leakage current densities 2–3 order-mag-nitude lower than control samples at 3 V. This differ-ence may be due to the formation of the high band gap interfacial layer between STO and Si or the nitrogen can fill the dangling bonds at the interface leading to reduce the gate leakage current. The repeated spike heating technique was also employed to rf-sputtered STO film for improving the thermal uniformity of the wafer. After the STO thin films were deposited on Si substrate
at 500 C, RTA was performed in N
2 ambient at the
temperatures ranged from 650 to 800C to improve the
quality of the films. With the same EOT, the repeated spike heating samples have leakage current densities 1–2 orders of magnitude lower than typical samples at 3 V, as illustrated inFig. 9. The reason for the reduction
Fig. 4. Calibrated C–V curves of control and N2O pre-treatment
samples and their quantum mechanical (Q.M.) fitting. Fig. 2. XPS profiles of the silicon substrate surface after SiON and
SiO2sacrificial oxides removed by HF dip.
Fig. 3. Measured capacitance of MOS capacitors with STO gate dielectric at three different frequencies. The inset is the equivalent
of leakage current in repeated spike heating samples might be due to the improvement of the thermal uniformity of the substrate. Such thermal uniformity may lead to the formation of uniform grain size films, which affects the properties of the STO films. Another possible reason may be due to the smaller interface state
density. Fig. 10indicates the normalized C–V curves of
typical and repeated-spike-heating samples. We can observe that the repeated-spike-heating sample has smaller interface trap density than the typical sample. The wagging and stretching of Si–O bond are tempera-ture dependent. Therefore, during the temperatempera-ture ramping up and down in repeated-spike-heating recipe, it is possible that the residual oxygen in films has more chance to fill the silicon dangling bonds and thus, better
interface has been formed. The C–V curves of 500 C
deposited STO thin films on silicon substrates with
various O2=ðO2þArÞ (OMR) ratios, 10, 20, 40, and
50%, are shown in Fig. 11. The accumulation
capaci-tance increases with increasing OMR up to 40%, and
then further increase of the OMR decreases the capaci-tance. In 10–40% OMR deposited films, the dielectric constant of STO films increases with the increasing OMR
(insert of Fig. 11), which is due to the enhanced
Fig. 6. Variation of EOT with working pressure for control and N2O
pre-treatment samples.
Fig. 8. Gate leakage current density at 3 V of N2O pre-treatment and
control samples prepared at various working pressures.
Fig. 7. TEM micrographs of (a) N2O pre-treatment STO thin film and
(b) control sample.
Fig. 9. Variation of gate leakage current density with EOT for typical and repeated-spike- heating samples.
Fig. 10. Normalized C–V curves of typical and repeated-spike-heating samples.
polarization after oxygen incorporation in the films. The decreased dielectric constant in 50% OMR depos-ited filmmay be attributed to thicker interfacial layer
and smaller grain size of the films. Fig. 12 shows the
high frequency C–V curves of MIS capacitors with various plasma powers deposited STO thin films. It is indicated that the accumulation capacitance increases with the increasing plasma power. Such higher capaci-tance is due to better crystallized films prepared at higher plasma power, which is confirmed by the XRD patterns as illustrated in the insert of Fig. 12. The kink observed in the C–V curve of 120 W STO filmmay be due to the trap resulting from the high plasma damage.
4. Conclusions
We have grown the SiON sacrificial oxide in pure
N2O ambient and then stripe it to incorporation
nitro-gen in the silicon surface. Nitronitro-gen incorporation method was carried out to improve the C–V and I–V
properties of the STO films. The EOT values of N2O
pre-treatment samples are 10–24% lower than those of the control samples while their leakage current densities also have 2–3 order-magnitude lower than those of control samples at the positive bias. The difference may be attributed to the incorporation of certain amount of
nitrogen onto the substrate surface during the N2O
pre-treatment to depress the formation of the interfacial layer and reduce the interface states during the high temperature growth of STO gate dielectrics. Besides, the repeated spike heating method was also studied to improve the film quality with the same EOT, this method can reduce the leakage current density 1–2 order-magnitude compared with normal samples, which may be due to resultant interface trap density reduction and uniformgrain size. The processing parameters, such as working pressure, OMR, and plasma power have significant influence on the electrical properties of STO thin films.
Acknowledgements
The authors acknowledge the financial support from the National Science Council of Republic of China under Contract No. NSC 90-2215-E009-100.
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Fig. 11. High frequency (100 kHz) C–V curves of MIS capacitors with various OMR STO films. The insert shows the dielectric constant of the films with various OMR.
