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Performance Improvement of Nickel Salicided n-Type Metal Oxide Semiconductor Field
Effect Transistors by Nitrogen Implantation
View the table of contents for this issue, or go to the journal homepage for more 2002 Jpn. J. Appl. Phys. 41 L381
(http://iopscience.iop.org/1347-4065/41/4A/L381)
Jpn. J. Appl. Phys. Vol. 41 (2002) pp.L381–L383 Part 2, No. 4A, 1 April 2002
c
2002 The Japan Society of Applied Physics
Performance Improvement of Nickel Salicided
n-Type Metal Oxide Semiconductor Field Effect Transistors by Nitrogen Implantation
Tien-Sheng CHAO1,2and Liang Yao LEE2
1Department of Electrophysics, National Chiao Tung University, Hsinchu, Taiwan 30050, R.O.C. 2National Nano Device Laboratory, Hsinchu, Taiwan 300, R.O.C.
(Received January 11, 2002; accepted for publication February 18, 2002)
Nitrogen implantation was used to improve the performance of Ni-salicide process for n-type metal oxide semiconductor field effect transistors (MOSFETs). It is found that the driving current and transconductance of nMOSFETs increase with the nitrogen implantation. The hot carrier degradation of the nMOSFETs is significantly reduced as the nitrogen dosage increases. [DOI: 10.1143/JJAP.41.L381]
KEYWORDS: nickel, salicide, nitrogen implant
The silicide-related technology has been used in submicron devices to reduce the parasitic resistance in order to improve
the devices and circuit performance. Before 0.25µm nodes,
TiSi2 has been widely used in industry. However, the sheet
resistance of the TiSi2lines increases as line width decreases
that limits the application of TiSi2 salicide in 0.1µm
com-plementary metal oxide semiconductor (CMOS) applications
and beyond.1, 2)The NiSi salicide technique has been
devel-oped as an alternative to TiSi2 due to without such narrow
line effect.3)However, the problems encountered in the NiSi
silicidation process are anomalously large junction leakage current and sheet resistance degradation. Recently, using TiN
cap,4)nitrogen-doped,5)and Ti-cap6)techniques have been
re-ported to reduce junction leakage current. The improvement was found due to the formation of nitride layer, which pre-vents oxidation of silicide/silicon interface and suppresses in-terface roughness to a certain degree. In this paper, nitrogen implantation was used in the Ni-salicide processes to improve the n-type metal oxide semiconductor field effect transistors (MOSFETs’) performance. We found that devices with this nitrogen implantation technique show an improved perfor-mance, and a significant reduction of hot carrier degradation. Devices were fabricated on p-type (100) oriented Si wafers
with resistivity of 15–25·cm. Processes were followed the
standard nMOSFETs’ processes. After well formation, local oxidation of silicon (LOCOS), was used to form isolation of devices. Thickness of gate oxide and poly-Si gate was 4-nm
and 200-nm, respectively. The n+/p junction was formed
by As+ implantation with energy of 40 keV, to a dosage of
5× 1015ions/cm2. Then, nitrogen ion implantation was
con-ducted with energy of 15 keV, to dosage of 1× 1013, 1× 1014,
5× 1014, and 1× 1015ions/cm2. Thermal annealing was
car-ried out in a rapid thermal process (RTP), at 1050◦C for 20 s
in nitrogen. After cleaning, Ni-salicide processes were con-ducted. Nickel (25 nm) and titanium (5 nm) films were de-posited using sputtering method. All the samples were then
annealed to form the salicide process in the N2 ambient for
30 s at 600◦C in a RTP system. Unreacted metal was removed
by wet etching.
Figure 1 shows the Id–Vdcurves for 1µm nMOSFETs with
the Ni-salicide processes. The threshold voltages were almost the same for these devices. It is noted that devices with ni-trogen implantation exhibit an increase in drain-current, or
Id, than that of device without nitrogen implantation
(con-trol). As the nitrogen dosage increases to 5× 1014 and 1×
0.0 0.5 1.0 1.5 2.0 2.5 3.0 0 1 2 3 4 5 6 7 Vg-Vt=3V Vg-Vt=2V Vg-Vt=1V Vg-Vt=0V without N 2 + implant N 2 + 1x1013 ions/cm2 N2 + 1x1014 ions/cm2 N2 + 5x1014 ions/cm2 N 2 + 1x1015 ions/cm2
Drain current, Id (mA)
Drain Voltage, Vd (V)
Fig. 1. The Id–Vdcurves for 1µm nMOSFETs with and without nitrogen implantation in Ni-salicide processes.
1015ions/cm2, the Id slightly decreases. But, it is still higher
than control. The stability of the NiSi is improved by the
ni-trogen implantation and has reported in elsewhere.7) Figure
2 shows the resultant transconductance at maximum in
lin-ear region, gmmax, for these five samples. It exhibits the same
trend that nitrogen implantation increases the gm, but reduces
slightly when dosage is larger than 5× 1014ions/cm2. Figure
3 shows the resultant sheet resistivity, or Rs, with different
ni-trogen dosages for the NiSi n+/p junctions. This was done
us-ing the Kevin structure with line width of 4µm. It can be seen
that Rs increases slightly with nitrogen implantation.
How-ever, as the dosage is larger than 5× 1014ions/cm2, the R
s
increases significantly. This can explain the reduction of Id
as the nitrogen dosage is larger than 5 × 1014ions/cm2, as
shown in Fig. 1. The slight increase on Rscould be explained
by the nitrogen diffusion upward into the NiSi grain
bound-aries.8) The too much stuffing nitrogen at grain boundaries
will increase the barrier-height; this results in an increase of
Rs of NiSi n+/p junctions. It has been reported that the
ni-trogen implantation on narrow active lines (0.5µm to 1 µm)
results in a decrease on sheet resistance. This reduction of the
resistivity exhibits an increase (8%) of Id for pMOSFETs.9)
Since we did not find the reduction of Rs for the n+/p NiSi
L382 Jpn. J. Appl. Phys. Vol. 41 (2002) Pt. 2, No. 4A T.-S. CHAOand L. Y. LEE 200 250 300 350 400 13 14 14 15 Transconductance , gm max (uA/V) N 2 + implant (ions/cm2) 1x10 w/o implant 1x10 5x10 1x10
Fig. 2. The maximum transconductance for 1µm nMOSFETs with and without nitrogen implantation in Ni-salicide processes.
3.5 3.6 3.7 3.8 3.9 4.0 13 14 14 15
Sheet resistance, Rs (ohm/sq)
N
2
+ implant(ions/cm2)
w/o implant 1x10 1x10 5x10 1x10
Fig. 3. The sheet resistance for 1µm nMOSFETs with and without nitro-gen implantation in Ni-salicide processes.
junctions, it needs further investigation for this increase of
Id. The subthreshold swing, or S. S. and interface defect
den-sity, or Dit, were measured. We found that the S. S. decreases
monotonically from 90 to 88 mV/dec as the nitrogen dosage
increases to 5× 1014ions/cm2. The D
itshows the same trend
from 3×1012(without nitrogen) to 2×1012cm−2·eV−1(with
5× 1014nitrogen ions/cm2). This implies the interface
prop-erty of the SiO2/Si is improved by this nitrogen implantation,
resulting in the increase of the Idand gm. This is reasonable
since nitrogen atoms stuffed in poly-Si grain boundaries will retard the diffusion of Ni atoms during the salicide process.
Besides, the diffused nitrogen at the interface of SiO2/Si will
improve the interfacial property, and also the reliability under stressing, as we will show in followings.
Figure 4 shows the shift of drain current (percent, %)
un-der the hot carrier stressing at Isub.max. (at Vg = 2 V and
Vd = 5 V). It is noted that the shift of drain current reduces significantly as the nitrogen dosage increases. The device with
0 200 400 600 800 1000 1200 -14 -12 -10 -8 -6 -4 -2 0 without N 2 + implant N 2+ implant 1x1013ions/cm2 N 2 + implant 1x1014ions/cm2 N 2 + implant 5x1014ions/cm2 N 2+ implant 1x1015ions/cm2
Drain Current Shift, ` Id/Id (%)
Stress Time (sec)
Fig. 4. The shift of drain current (%) for 1µm nMOSFETs with and without nitrogen implantation in Ni-salicide processes under stressing at
Vg= 2 V and Vg= 5 V. 0 200 400 600 800 1000 1200 -0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 without N2+ implant N2 + implant 1x1013ions/cm2 N2 + implant 1x1014ions/cm2 N2 + implant 5x1014ions/cm2 N2+ implant 1x1015ions/cm2
Threshold Voltage Shift, GVt (V)
Stress Time (sec)
Fig. 5. The shift of threshold voltage for 1µm nMOSFETs with and without nitrogen implantation in Ni-salicide processes under stressing at
Vg= 2 V and Vg= 5 V.
higher nitrogen implantation shows the smallest shift of Id.
Figure 5 shows the shift of threshold voltage. The control device without nitrogen implantation shows the largest shift on threshold voltage, and the shift decreases as the nitrogen dosage increases. This implies that the nitrogen implantation can improve the resistance to hot carrier stressing. Since ni-trogen were implanted in the salicide processes, that means poly-Si and the S/D regions were all implanted with the ni-trogen. Theses nitrogen atoms will diffuse and pile up at the oxide/silicon interface. The nitrogen bonds reduce the stress
at the interface,10)which enhances the resistivity to hot carrier
stressing in return.
In summary, we have developed a nitrogen implanta-tion technique on the Ni-salicide for nMOSFETs. From this scheme, we found that the nMOSFETs’ performance was im-proved, including drain current, transconductance, especially the resisting capability in hot carrier degradation.
This work was supported by National Science Council Taiwan, Republic of China under the contract No. NSC89-2215-E-317-009.
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