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Applied Physics Express
LETTERS • OPEN ACCESS
Impacts of plasma-induced damage due to UV
light irradiation during etching on Ge fin fabrication
and device performance of Ge fin field-effect
transistors
To cite this article: Wataru Mizubayashi et al 2017 Appl. Phys. Express 10 026501
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Applied Physics Express 10, 026501 (2017)
https://doi.org/10.7567/APEX.10.026501
Impacts of plasma-induced damage due to UV light irradiation during etching
on Ge fin fabrication and device performance of Ge fin field-effect transistors
Wataru Mizubayashi1*, Shuichi Noda2, Yuki Ishikawa1, Takashi Nishi1, Akio Kikuchi2,
Hiroyuki Ota1, Ping-Hsun Su3, Yiming Li3, Seiji Samukawa1,2,4, and Kazuhiko Endo1
1
National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8568, Japan
Institute of Fluid Science, Tohoku University, Sendai 980-8577, Japan
3
National Chiao Tung University, Hsinchu 300, Taiwan
4
Advanced Institute for Materials Research (AIMR), Tohoku University, Sendai 980-0811, Japan
2
*E-mail: w.mizubayashi@aist.go.jp
Received November 2, 2016; accepted December 12, 2016; published online January 11, 2017
We investigated the impacts of plasma-induced damage due to UV light irradiation during etching on Ge fin fabrication and the device performance
of Ge fin field-effect transistors (Ge FinFETs). UV light irradiation during etching affected the shape of the Ge fin and the surface roughness of the
Ge fin sidewall. A vertical and smooth Ge fin could be fabricated by neutral beam etching without UV light irradiation. The performances of Ge
FinFETs fabricated by neutral beam etching were markedly improved as compared to those of Ge FinFETs fabricated by inductively coupled
plasma etching, in which the UV light has an impact. © 2017 The Japan Society of Applied Physics
igh performance of Si complementary metal–oxide–
semiconductor (CMOS) devices has been realized
by their miniaturization. The gate length of advanced
CMOS field-effect transistors (CMOSFETs) has become
smaller than 20 nm. To improve the performance of nanometer-scale CMOSFETs, in addition to performing their
miniaturization, it is also necessary to replace the Si channel
with high-mobility materials such as Ge and III–V compounds.
In particular, since the carrier (electron and hole) mobility
in Ge is higher than that in Si. Ge is a promising highmobility channel.1–6) Furthermore, a multichannel is required
to improve the electrostatic control of the gate electrode of
Ge MOS FETs (MOSFETs). There are two main methods of
Ge fin fabrication: epitaxial growth of Ge from a SiGe=Si
substrate3) and conventional top-down etching.6) Fin structure
fabrication in Ge fin FETs (FinFETs) on Ge-on-insulator
(GeOI) substrates is usually performed by inductively coupled
plasma (ICP) etching. However, during ICP etching, the UV
light generated from the ICP and charge build-up by ionized
atoms cause plasma-induced damage. A concern here is that
such etching damage reduces the performance and reliability of
Ge-channel CMOS devices. In particular, since the thermal
resistance of Ge is very low compared to that of Si, the inability
to recover the damage by high-temperature thermal annealing
(≥1000 °C) is a critical issue.
In this work, we studied the impacts of plasma-induced
damage due to UV light irradiation during etching on Ge fin
fabrication and the device performance of Ge FinFETs.
Ge CMOS FinFETs were formed on (100) GeOI substrates. The Ge fin channel was formed by both conventional
ICP etching and neutral beam etching with pure Cl2 gas.
Essentially, neutral beam etching consists of an ICP source
and a high-aspect-ratio carbon aperture plate, wherein accelerated ions are effectively converted to an energy-controlled
neutral beam through charge transfer processes and most of
the UV light is shaded through absorption processes from the
ICP, as shown in Fig. 1.7,8) The neutral beam energy can be
controlled by applying RF bias power to the aperture plate.
Ge fin structures were etched with chemical vapor deposited-
H
SiO2 hard masks, which were patterned by electron beam
(EB) lithography and conventional ICP etching. A gate
dielectric of 3.0 nm HfO2=2.0 nm Al2O3 (equivalent oxide
thickness = 2.1 nm) was deposited by atomic layer deposition. Subsequently, a metal gate of 50 nm TiN was formed by
physical vapor deposition. The gate electrode was formed by
EB lithography and chemical etching. The source and drain
(S=D) of Ge n- and p-type FinFETs were formed by P+ and
BF2+ implantation, respectively, with a dose of 2.0 × 1015
cm−2 at 10 keV. After Al electrode metallization, forming gas
annealing was performed at 300 °C for 30 min in 3% H2.
A physical analysis was performed that involved crosssectional scanning electron microscopy (SEM) and transmission electron microscopy (TEM) observations. In the electrical measurement of the Ge FinFETs, to suppress charge
trapping in the high-k gate stack and self-heating during the
current–voltage (I–V ) measurement, a pulsed I–V measurement was performed for evaluating transconductance (gm).
Figure 1 shows a schematic illustration of the neutral beam
etching apparatus. In this neutral beam etching apparatus,
accelerated negative ions (Cl−, F −, etc.) are efficiently
neutralized by their passage through the carbon aperture.
The plasma consists mainly of negative and positive ions in
our pulse time modulated discharge.9) Both Cl+ and Cl− ions
can be accelerated alternately by the RF electric field applied
to the aperture plate. Since the neutralization efficiency of Cl−
ions (where electron detachment occurs by their collision
with the aperture sidewall) is much higher than that of Cl+
ions (where electron attachment occurs), the Cl− ions are
preferentially utilized by the application of negative DC bias
to the top electrode. The kinetic energy of the neutral beam
can be controlled by varying the RF bias power. There
are two advantages of the neutral beam etching process.
1) The wafer is not exposed to the UV light generated from
the plasma through the high-aspect-ratio carbon aperture
plate. 2) Ions are efficiently neutralized by their collision with
the carbon aperture plate. Under the neutral beam etching
condition adopted in this work, the neutralization efficiency
of Cl− ions is more than 95%.10) Thus, in neutral beam
Content from this work may be used under the terms of the Creative Commons Attribution 4.0 license. Any further distribution of
this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
026501-1
© 2017 The Japan Society of Applied Physics
Appl. Phys. Express 10, 026501 (2017)
W. Mizubayashi et al.
Cl2 Gas
-100V
ICP Source
㻰㻯
Time modulated
50μsec / 50μsec
13.56MHz
1 Pa
+ - + - + - +
- + - + - + + - + - + - +
㻾㻲
600kHz
Cl
Cl-
Cl
Cl-
Cl
Cl radical
Cl- ion
Cl-
High Aspect Ratio
Aperture Plate䠄AP䠅
㻾㻲
0.05䡚
0.1 Pa
Cl
Cl
Negative/Positive ion
plasma generated by
pulse time modulated
discharge
Cl neutral
beam
Cl
Effective neutralization
by electron detachment
from negative ions
Substrate
Exhaust
Etching Chamber
Neutral Beam Etching
Ge
Ge
(a)
8.0
Hard
Mask
(c)
Ge
X (nm)
Hard
Mask
ICP Etching
Ge
12.0
Glue
5 nm
RCH
4.0
100 nm
(b)
Fig. 2. Cross-sectional SEM images of Ge fin after (a) ICP etching and
(b) neutral beam etching.
(d)
12.0
Neutral Beam Etching
Ge
Glue
Ge
Y (n m)
8.0
(b)
5 nm
RCH
4.0
etching, the influences of the UV light and charge build-up
can be eliminated and defect-free etching can be realized.
We investigated the impact of plasma-induced damage due
to UV light irradiation during the etching process on Ge fin
fabrication. In our experiments, conditions other than the UV
light irradiation, such as the flux of the reactive species and the
physical bombardment energy, were kept the same in both
etching cases (i.e., ICP etching and neutral beam etching)
because the opening area ratio of the aperture plates was fixed
at 50% in both these cases. The Ge etch rate in ICP dry
etching was one order of magnitude larger than that in neutral
beam etching despite the use of completely identical chamber
configurations and ICP discharge conditions, with the exception of the aspect ratio of the aperture plates. Thus, we compared the Ge fin profiles at the same over-etching ratio of
30%. In ICP etching, the Ge fin has a trapezoidal shape, as
shown in Fig. 2(a). The trapezoidal shape indicates that a
horizontal etching reaction occurs at a constant etch rate
during the etching process. In neutral beam etching, a vertical
Ge fin can be formed without any side etching, which is in
contrast to the case of ICP etching, as shown in Fig. 2(b). This
result indicates that the Ge fin is etched only by the vertically
directional Cl neutral beam8) and the horizontal etching
reaction never occurs spontaneously. The trapezoidal shape
shown in Fig. 2(a) is a result of the horizontal etching reaction
induced by the UV light irradiation from the top by the ICP.
The notches observed on both sides of the Ge fin just under
the hard mask confirm a shadowing effect of the UV
RCH
X (nm)
(a)
BOX
1.2
0.8
0.4
0.0
0.0
100 nm
RCH
1.2
0.8
0.4
0.0
0.0
BOX
16.0
ICP Etching
16.0
Fig. 1. Schematic illustration of neutral beam etching apparatus.
Y (n m)
Fig. 3. Cross-sectional TEM images of Ge fin after (a) ICP etching and
(b) neutral beam etching. Surface roughness of Ge fin sidewall formed by
(c) ICP etching and (d) neutral beam etching as a function of fin height.
irradiation by the hard mask. Thus, a vertical and smooth
Ge fin can be formed by neutral beam etching without UV
light irradiation. Figures 3(a) and 3(b) respectively show
cross-sectional TEM images of a Ge fin after ICP etching
and neutral beam etching. It is clearly seen that smoother
sidewalls are attained by neutral beam etching without
UV light irradiation than by ICP etching. Next, to compare
quantitative roughness values, we analyzed them by digitizing the cross-sectional TEM images as shown in Fig. 3.
The surface roughness values in the channel height direction
[see Figs. 3(a) and 3(b)] were determined by the RMS of the
outermost line profiles, as shown in Figs. 3(c) and 3(d). The
026501-2
© 2017 The Japan Society of Applied Physics
Appl. Phys. Express 10, 026501 (2017)
W. Mizubayashi et al.
(a) ICP Etching
hν
Cl-
Cl-
hν
hν
Cl-
hν
Cl-
Cl-
hν
(b) Neutral Beam Etching
hν
Cl-
Cl-
Ge
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Ge
: Ge
: Ge
: Damage Region Due to UV Irradiation
Ge
Ge
Fig. 4. Schematic illustration of etching process in ICP etching and neutral beam etching.
Lg=500nm, T fin =54nm
-4 Ge pFinFET
Ge nFinFET
10
100
-5
10-6
10-7
10-8
Neutral
Beam
Etching
Vd
-0.5V
Vd
0.5V
10-9
10-10
10-11
|Id| ( μ A/μ m)
10
|Id| (A/μ m)
surface roughness in the channel height direction (RCH) on the
fin sidewall was 0.2 nm in the case of ICP etching and 0.1 nm
in the case of neutral beam etching. The sidewall surface of
the Ge fin formed by neutral beam etching is atomically flat.
Thus, the plasma-induced damage due to UV light was found
to have an impact on the surface roughness of the Ge fin
sidewall. UV light irradiation from the plasma to the surface is
isotropic, whereas ion irradiation is anisotropic. As a result,
UV light is mainly irradiated on the surface of the etched
sidewall. The penetration depth of UV light irradiation from
220 to 380 nm in Si has been estimated to be about 10 nm.11)
Since the extinction coefficient k of Ge in this wavelength
region is slightly smaller than that of Si,12) the penetration
depth of UV light irradiation in the case of Ge is larger than
that in the case of Si. This UV absorption region is heated or
directly attacked by the UV photons with energy much higher
than the bond energy of Ge, and the Ge bonds consequently
weaken or break. This is the reason why the damage layer is
formed by UV light irradiation. In conventional ICP etching
with UV light irradiation, the Ge bonds in the damage layer
(≥10 nm in depth) formed by UV light irradiation break or
weaken; therefore, the damage layer enhances the chemical
reactions. As a result, the etching process of Ge is followed by
multilayer etching in the damage layer deeper than 10 nm, as
shown in Fig. 4(a), rather than by atomic layer etching. On the
other hand, in the case of neutral beam etching without UV
light irradiation, the Ge surface is not damaged, and therefore,
a surface dangling bond is formed only on the atomic layer
and it undergoes a chemical reaction with the reactive species.
Then, atomic layer etching can be realized. The above
behaviors are related to the difference in the surface roughness
caused by the UV light irradiation. Next, to clarify the impact
of the plasma-induced damage due to UV light irradiation
during the etching process on the uniformity of Ge etching,
we examined the surface roughness of the Ge(110) substrate
after ICP etching and neutral beam etching by means of
atomic force microscopy. The etching conditions were the
Lg=500nm, Tfin=54nm
Vg-Vth=0~-1.0V,
Step=-0.2V
Vg-Vth=0~1.0V,
Step=0.2V
80 Ge pFinFET
Ge nFinFET
Neutral Beam
Etching
60
ICP Etching
40
20
0.05V
-0.05V
-0.8 -0.4 0.0 0.4 0.8 1.2
Vg (V)
(a)
0
-0.4
-0.2
0.0
Vd (V)
0.2
0.4
(b)
Fig. 5. (a) ∣Id∣–Vg characteristics of n- and p-type Ge FinFETs with
Lg = 500 nm and Tfin = 54 nm fabricated by neutral beam etching, as
measured by DC I–V method. (b) ∣Id∣–Vd and characteristics of n- and p-type
Ge FinFETs with Lg = 500 nm and Tfin = 54 nm fabricated by ICP etching
and neutral beam etching, as obtained by pulsed I–V measurement.
same as those in the fin fabrication described above. Ge(110)
corresponds to the plane direction of the fin sidewall. The
average surface roughness (Ra) of Ge(110) was 0.27 nm after
ICP etching (not shown). On the other hand, Ra of Ge(110)
after neutral beam etching was 0.16 nm, which is almost the
same as Ra of the initial Ge(110) substrate (not shown). This
means that neutral beam etching without UV light irradiation
can uniformly etch Ge. Further, the surface roughness of
Ge(110) after neutral beam etching is consistent with the
results in Figs. 3(c) and 3(d) (not shown). It was found that in
the case of Ge etching, the UV light affected the surface
roughness as well as the uniformity of the etching. Thus, to
ensure the fabrication of a damage-free vertical and smooth
fin, it is necessary to perform the etching process without UV
light irradiation, for example, by neutral beam etching.
We investigated the influence of UV light irradiation
during the fin fabrication on the performance of n- and p-type
Ge FinFETs. Figure 5(a) shows the ∣Id∣–Vg characteristics of
n- and p-type Ge FinFETs with a gate length (Lg) of 500 nm
fabricated by neutral beam etching, as measured by the DC
026501-3
© 2017 The Japan Society of Applied Physics
Appl. Phys. Express 10, 026501 (2017)
gm,max at Vd=0.5V (μ S/μ m)
Ge nFinFET
Lg=500nm
Neutral Beam
Etching
gm,max at Vd=-0.5V (μ S/μ m)
150
200
150
W. Mizubayashi et al.
Ge pFinFET
Lg=500nm
Neutral Beam
Etching
100
100
ICP Etching
50
0
20 30 40 50 60 70 80 90 100
Fin Thickness T fin (nm)
ICP Etching
50
20 30 40 50 60 70 80 90 100
Fin Thickness T fin (nm)
(a)
(b)
gm,max at |Vd|=0.5V (μ S/μ m)
Fig. 6. Maximum gm (gm,max) at Vd = ±0.5 V as a function of fin thickness
in n-type (a) and p-type (b) Ge FinFETs fabricated by ICP etching and
neutral beam etching.
200
160
gm max/SS=m
=1.37
0.72 0.55
Neutral Beam
Etching
120
Ge FinFET
80
40
ICP Etching
Lg=500nm
Tfin=54nm
nFinFET
pFinFET
60 80100
300
500
SS at |Vd|=0.5V (mV/dec)
Fig. 7. gm,max as a function of SS for n- and p-type Ge FinFETs fabricated
by ICP etching and neutral beam etching.
I–V method. The n- and p-type Ge FinFETs fabricated by
neutral beam etching without UV light irradiation exhibit
excellent ∣Id∣–Vg characteristics. However, Ion for n- and
p-type Ge FinFETs obtained in the DC I–V measurement
is estimated to be less than 10% of that obtained in the pulsed
I–V measurement, which is due to charge trapping in the
high-k gate stack and self-heating during the I–V measurement. To suppress these effects, we precisely estimated the
on current (Ion) and the maximum gm (gm,max) by the pulsed
I–V method. In Fig. 5(b), it is clearly seen that for both n- and
p-type FinFETs, the ∣Id∣–Vd characteristics of the Ge FinFET
fabricated by neutral beam etching are markedly improved as
compared to those of the FinFETs fabricated by ICP etching.
The difference between the ∣Id∣ improvement ratios of the
nFinFET and pFinFET fabricated by neutral beam etching is
related to the difference in the carrier masses in these
FinFETs. Since the electron mass is lighter than the hole
mass, the electrons in the nFinFET are easily affected by the
etching damage. Figure 6 shows the maximum gm (gm,max)
at Vd = ±0.5 V as a function of fin thickness (Tfin) in a Ge
nFinFET (a) and a Ge pFinFET (b) fabricated by ICP etching
and neutral beam etching. gm,max for the Ge FinFET fabricated by neutral beam etching is two times that for the
nFinFET, and 10% higher for the pFinFET than those of
the FinFETs fabricated by ICP etching, regardless of Tfin.
Figure 7 shows gm,max as a function of the subthreshold swing
(SS) for n- and p-type Ge FinFETs fabricated by ICP etching
and neutral beam etching. For both the n- and the p-type Ge
FinFETs, gm,max=SS = m in the case of neutral beam etching is
superior to that in the case of ICP etching. Furthermore, since
there is no plasma-induced damage due to UV light irradiation in neutral beam etching, the interface state density (not
shown) and surface roughness [Fig. 3(d)] are lower than those
in the case of ICP etching. This is the reason for the improved
gm,max and SS for the n- and p-type Ge FinFETs fabricated by
neutral beam etching (Figs. 6 and 7).
In summary, we investigated the impacts of plasmainduced damage due to UV light irradiation during etching
on Ge fin fabrication and the device performance of Ge
FinFETs. We found that the plasma-induced damage due
to UV light irradiation affects the shape of the Ge fin and
the surface roughness of the fin sidewall. A vertical and
atomically flat fin can be realized by neutral beam etching
without UV light irradiation. The performances of n- and
p-type Ge FinFETs fabricated by neutral beam etching are
greatly improved as compared to those of FinFETs fabricated
by conventional ICP etching, which is affected by UV light.
This means that the plasma-induced damage due to UV light
irradiation has an impact on the device performances. Thus,
it is concluded that damage-free fabrication is essential for
the fin etching process of future Ge CMOS FinFETs.
Acknowledgment This work was partly funded by the ImPACT Program
of the Council for Science, Technology and Innovation (Cabinet Office,
Government of Japan).
1) K. Ikeda, M. Ono, D. Kosemura, K. Usuda, M. Oda, Y. Kamimuta, T.
Irisawa, Y. Moriyama, A. Ogura, and T. Tezuka, VLSI Symp. Tech. Dig.,
2012, p. 165.
2) R. Zhang, P.-C. Huang, J.-C. Lin, M. Takenaka, and S. Takagi, IEDM Tech.
Dig., 2012, p. 371.
3) M. J. H. van Dal, G. Vellianitis, G. Doornbos, B. Duriez, T. M. Shen, C. C.
Wu, R. Oxland, K. Bhuwalka, M. Holland, T. L. Lee, C. Wann, C. H.
Hsieh, B. H. Lee, K. M. Yin, Z. Q. Wu, M. Passlack, and C. H. Diaz, IEDM
Tech. Dig., 2012, p. 521.
4) C. H. Lee, T. Nishimura, T. Tabata, C. Lu, W. F. Zhang, K. Nagashio, and
A. Toriumi, IEDM Tech. Dig., 2013, p. 32.
5) H. Wu, W. Wu, M. Si, and P. D. Ye, IEDM Tech. Dig., 2015, p. 16.
6) Y.-J. Lee, F.-J. Hou, S.-S. Chuang, F.-K. Hsueh, K.-H. Kao, P.-J. Sung,
W.-Y. Yuan, J.-Y. Yao, Y.-C. Lu, K.-L. Lin, C.-T. Wu, H.-C. Chen, B.-Y.
Chen, G.-W. Huang, H. J. H. Chen, J.-Y. Li, Y. Li, S. Samukawa, T.-S.
Chao, T.-Y. Tseng, W.-F. Wu, T.-H. Hou, and W.-K. Yeh, IEDM Tech.
Dig., 2015, p. 382.
7) S. Samukawa, K. Sakamoto, and K. Ichiki, J. Vac. Sci. Technol. A 20, 1566
(2002).
8) E.-T. Lee, S. Noda, W. Mizubayashi, K. Endo, and S. Samukawa, Proc.
16th Int. Conf. Nanotechnology, 2016, p. 816.
9) S. Samukawa, Appl. Surf. Sci. 192, 216 (2002).
10) K. Endo, S. Noda, M. Masahara, T. Kubota, T. Ozaki, S. Samukawa, Y.
Liu, K. Ishii, Y. Ishikawa, E. Sugimata, T. Matsukawa, H. Takashima, H.
Yamauchi, and E. Suzuki, IEDM Tech. Dig., 2005, p. 840.
11) S. Samukawa, B. Jinnai, F. Oda, and Y. Morimoto, Jpn. J. Appl. Phys. 46,
L64 (2007).
12) E. D. Palik, Handbook of Optical Constants of Solids (Academic Press,
New York, 1985).
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