Microscopic origins of dry-etching damages in silicon large-scaled integrated circuits
revealed by electrically detected magnetic resonance
K. Uejima and T. Umeda
Citation: Applied Physics Letters 104, 082111 (2014); doi: 10.1063/1.4867070
View online: http://dx.doi.org/10.1063/1.4867070
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APPLIED PHYSICS LETTERS 104, 082111 (2014)

Microscopic origins of dry-etching damages in silicon large-scaled
integrated circuits revealed by electrically detected magnetic resonance
K. Uejima1,2,a) and T. Umeda1
1Institute of Applied Physics, University of Tsukuba, Tsukuba 305-8573, Japan
Renesas Electronics Corporation, Sagamihara 252-5298, Japan

2(Received 6 December 2013; accepted 11 February 2014; published online 27 February 2014)
We reveal microscopic structures of dry-etching damage defects in practical Si metal-oxidesemiconductor field effect transistors. Electrically detected magnetic resonance spectroscopy
identified interstitial defects of carbon (a split C-Si interstitialcy) and fluorine (a bond-centeredfluorine) as the major dry-etching damages, which survived even through high-temperature thermal
processes. In addition, we found other minor centers of carbon, fluorine, and possibly hydrogen
impurities. Our observation indicates that the observed defects became much more stable thanC 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4867070]
those in bulk silicon. V

Carbon and fluorine are the major impurities in siliconlarge-scale integration circuits (Si LSIs). They are intentionally introduced by co-implantation in order to form an abrupt
junction profile of Si metal-oxide-semiconductor field effecttransistors (MOSFETs).1,2 In the implanted regions, carbon
and fluorine atoms can trap excess interstitial Si atoms andstrongly suppress the transient enhanced diffusion of boron
and phosphorous dopants.1,2 They are also unintentionallyintroduced by dry etching.3,4 Fluorocarbon containing fluorine and carbon is one of the most popular gases for reactive
ion etching (RIE) of SiO2 and SiN films. The energetic fluxof carbon and fluorine damages the crystalline Si lattice andeventually causes the mixing of fluorine and carbon atoms
with the lattice.4 Controlling of such damage caused by RIE
is one of the major challenges for advanced scaled LSIs.
Both carbon and fluorine atoms can create lattice defects5,6
and may become the source of the leakage currents of p-njunctions.2,7 However, microscopic entities of such defects
are still unclear. Since Si MOSFETs are fabricated via a
number of LSI processes, the formation and survival of
defects inside MOSFETs are very complicated issues.
In this Letter, we reveal the microscopic structures of carbon- and fluorine-related defects remaining in practical Si
MOSFETs by means of electrically detected magnetic resonance (EDMR) spectroscopy.8 EDMR achieved electricaldetections of electron-spin-resonance (ESR) signals in thepractical devices. These defects were introduced via dryetching processes and survived even through hightemperature thermal processes. They could be reduced byshortening the etching time. This reduction of defects has also
the benefit of a low junction leakage current of MOSFETs.
Figure 1 shows the depth profiles of carbon and fluorine
atoms in Si substrates after an RIE process, which were measured by secondary ion mass spectrometry (SIMS). We compared the two processes. “Process A” included an etch-backof a SiO2 cover layer on Si substrate done by using a fluorocarbon RIE process. The etch-back was performed for a period 1.5-times longer than the etching time of just removing
the SiO2 film. Therefore, the Si substrate was exposed toa)E-mail: kazuya.uejima.kf@renesas.com

0003-6951/2014/104(8)/082111/5/$30.00

fluorocarbon plasma. This kind of condition was conventionally used because it ensures a complete removal of the etching layer. To compare with “process A,” the other substrate
was subjected to “process B,” in which we used a reduced
RIE time9 less than half of the period used in “process A.”
Figure 1 demonstrates a drastic reduction in the incorporationof carbon and fluorine atoms when we used “process B.”
Following the SIMS analysis, we carried out EDMR
measurements on Si MOSFETs (samples A and B) subjected
to either “process A” or “process B.” The device structure isschematically shown in Fig. 2(a). Arrays of n-channel Si
MOSFETs (sub-100-nm gate length) were fabricated on 300mm-diameter Si(100) wafers. The boron concentration of thesubstrate was 1  1018 atoms/cm3. To pattern MOSFET
structures, the chemical vapor deposition (CVD) of a SiO2
cover film as well as the RIE process for etching back thecover film were performed. After that, arsenic and phosphorous atoms were highly doped over 1  1019 atoms/cm3 in thesource and drain regions by ion implantation. These dopants
were activated by rapid thermal annealing at over 1000  C.
This led to a thermal diffusion of the dopants, and the junction
depth extended to about 100 nm. The carbon and fluorine
impurities were also thermally diffused not only vertically but
also laterally toward the gate area, as is drawn in Fig. 2(a).
Figure 2(b) shows the EDMR spectra at 300 K of samples A and B for the same junction leakage currents (8 nA).
A strong signal was observed at 333.5 mT for sample A. We
named this as “Ci.” This signal was also found in sample B,
but it was so weak that we needed a longer accumulation
time. We checked the EDMR signal intensity (current
change due to ESR) as a function of the junction voltage andfound that the signal intensity was constant at 200 ppm forsample A when the voltage was under 1.75 V [see the inset
of Fig. 2(b)]. In comparison, it decreased to 50 ppm forsample B. This result indicates that there should be more
defects in sample A than in sample B. In fact, the junction
leakage currents at 1.75 V were also three times higher insample A (1.2 nA) than in sample B (380 pA). This EDMR
signal was not observable under forward-biased conditions.
Consequently, the observed EDMR signal is clearly related
to the source of the junction leakage currents.

104, 082111-1

C 2014 AIP Publishing LLC
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Appl. Phys. Lett. 104, 082111 (2014)

FIG. 1. Depth profiles of carbon andfluorine atoms in Si substrate measured
by SIMS. Substrates were subjected to
two types of dry etching (“process A”
and “process B”) by fluorocarbon gas
through a top SiO2 cover layer.
Process A was an accreted dry etch,
which is conventionally used for silicon device fabrications. Process B wasa controlled process. A 10-nm SiN film
was deposited at 650  C for SIMS
measurements after dry etching.

Figure 3(a) shows the details of the EDMR spectra in sample A and their angular dependence with respect to themagnetic-field rotation in the (011) plane. In this rotation, themajor crystal directions, [100], [111], and [011], correspond to0 , 54.8 , and 90 , respectively. We found two major EDMR
signals in Fig. 3(a). One is a stronger central signal, “Ci,” which
also appeared in Fig. 2(b). The second signal is a doubly split
signal, which we named “Fi.” We propose that “Ci” and “Fi”
arise from interstitial defects of carbon and fluorine, respectively. These atoms are the most dominant impurities, as evidenced in Fig. 1. The current model naturally accounts for the
drastic reduction of the two signals in sample B where theincorporation of carbon and fluorine was greatly suppressed.
Furthermore, the clear doublet hyperfine splitting of “Fi” isconsistent with that of 19F nuclear spin (naturalabundance ¼ 100%, nuclear spin I ¼ 1/2). In comparison, there
was no obvious hyperfine splitting for “Ci,” in spite of its
strong signal intensity. This supports the fact that “Ci” is acarbon-related defect because carbon includes only a very small
fraction of 13C isotope (natural abundance ¼ 1.1%, I ¼ 1/2).
The identifications of the origins of “Ci” and “Fi” as interstitial defects are based on their angular dependences. In
Fig. 3(a), we precisely fitted the EDMR spectra as displayedby the smooth overlapped lines from which we determinedsignal positions of “Ci” and “Fi” as well as of other weak signals. Finally, angular maps of all the observed EDMR centers
are summarized in Figs. 3(b) and 3(c). Solid lines in the figures represent simulated angular maps of each center, whichwere calculated by using the ESR parameters in Table I. In
the following sections, we will examine each defect in detail.
For main defect “Ci,” we found that its angular map issimilar to that of the famous G12 center.10 Therefore, we
judge that “Ci” is a close variation of G12. The G12 centerhas been identified as an isolated carbon interstitial in Si, andit has an atomic structure like shown in Fig. 4. The originalG12 center was found in carbon-doped highly resistive Si substrates after electron irradiation, and 13C-enriching experiments clearly revealed its sp2-bonding structure and an ideal
p-orbital of a C  Si3 unit (see Fig. 4).10 Its charge state is
þ1, namely, G12 is a Ci(þ) center.10 According to theoretical
calculations,5 the split C-Si interstitialcy shown in Fig. 4 is aground-state configuration for an isolated carbon atom in Si.
Even though a carbon atom can predominantly occupy substitutional sites, this form can capture a mobile Si self-interstitial(Sii) due to own strain field, resulting in the formation of split
C-Si interstitials.5

FIG. 2. (a) Sample structure and EDMR setup. We activated junction currents between the n-type drain (or source) region and p-type substrate with an open
gate and monitored current changes due to ESR by using an EDMR detector synchronized to a magnetic-field modulation at 1.5 kHz. (b) Typical EDMR spectra of two samples prepared by the dry-etching processes of “process A” and “process B” shown in Fig. 1. The spectra were measured by using junction leakage currents of a  8-nA, 200-mW microwave at 9.46 GHz, and 0.5-mT-width magnetic-field modulation. The inset shows bias dependences of the current
changes (peak-to-peak) which were measured for the junction leakage currents from 50 pA to 75 nA.

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Appl. Phys. Lett. 104, 082111 (2014)

FIG. 3. (a) Angular dependence of EDMR spectra measured for a magnetic-field rotation in the (011) plane from the [100] direction (0 ) to the [011] direction(90 ). The spectra were measured for “sample A” by using the junction leakage currents of a  12-nA, 200-mW microwave at 9.46 GHz, and 0.25-mT-width
magnetic-field modulation. Two signals, labeled “Ci” and “Fi,” are clearly found, in addition to many other weak signals. Smooth lines overlapping the experimental spectra represent the results of our spectral simulations, where we used the Gauss, Lorentz, and Vogit functions for each signal. Typical full widths at
half maximum (FWHMs) were 0.4 mT for “Ci” and 0.2 mT for “Fi.” (b) and (c) Experimental (“þ” symbols) and simulated angular maps (solid lines) of theobserved EDMR centers. Darker and larger “þ” symbols represent larger experimental peaks. The “Fi” peaks are drawn more darkly for clarity. The simulations were performed by using the ESR parameters of “Ci” and “Fi” in Table I and those of G11,11 F1,13 AA9, and AA10.17

Similarly to Sii, Ci is also mobile, which is evident fromthe wider extension of SIMS profiles for carbon (Fig. 1). In theprevious ESR study on irradiated bulk Si,10 the original G12
center was easily reduced by a low-temperature anneal at 60  C
and was converted into the G11 center [positively charged carbon interstitial-substitutional carbon pair, CiCs(þ)].10,11 The
G11 center is formed by combining two mobile carbon atoms.
The G11 center was supposedly observed in our devices
because an angular-map simulation of G11 seems to fit with apart of the experimental data in Fig. 3(b). However, the signal
intensity of G11 was much smaller than that of “Ci”
(G12-type), despite the fact that our devices went through thehigh-temperature thermal processes (over-1000  C activation
anneal for seconds as well as over-400  C anneal for hours).
We suggest that a large amount of excess Sii in MOSFETs1,2
causes the dominant formation of the G12-type centers[C þ Si]i rather than the G11 centers (Ci þ Cs). Also strongly
strained Si lattice by the surrounding structures12 may affectthe thermal stability of the defects.8 In fact, our “Ci” center
exhibited slightly different ESR parameters from the originalG12 center (Table I). Therefore, we distinguish “Ci” from theoriginal G12 center in this paper.
The “Fi” center is the secondary major defect in our devices. Its noticeable hyperfine splitting is clearly anisotropic (Fig.
3) and is in a similar range (4.8–8.2 mT) to the case of 19F
hyperfine splittings (4.2–6.3 mT) of the F0 center (FV2 defect).13
The F0 center is a primitive type of fluorine-vacancy clusters(FnVm defects) and is only a defect that revealed the detectable19
F hyperfine splitting in Si. However, the 19F hyperfine splittings of “Fi” and F0 exhibited such different angular dependences that we should consider opposite types of atomic structures
for each center. We assign “Fi” to a bond-centered fluorine(Fig. 4) on the basis of the following reasons:
We first consider a crucial difference in their symmetries.
In the F0 center, a fluorine atom forms a Si-F bond inside a V2
space.6,13 Since this bond is nearly along the original Si-Si
bond, the 19F hyperfine splitting of F0 showed approximately ah111i axial symmetry.13 Consequently, the hyperfine splitting
was maximized in the [111] direction (55 ) and was minimized in the normal direction to [111] (90  55 ¼ 35 ).13 On

TABLE I. ESR parameters of “Ci” and “Fi” EDMR centers and G12 ESR centers. The spin Hamiltonian of these centers is given by H ¼ lBSgB þ SAI 
gnbnIB,10,13 where g is a g (gyromagnetic) tensor of an electron spin and A is a hyperfine tensor (in mT) for each nuclear spin. The g tensor is defined by threeprincipal g factors and three principal axes (X, Y, and Z) of the tensor. The hyperfine tensor is also expressed in the same manner.
Principal Z axis

Principal values
CenterTensorX

Y

Z (//)

h

Ci
G12

g
g

2.00522.00620.63

2.00242.0020
5.20

90
90
0

Fi

A(13C)g
A(19F)

2.00622.00680.67
2.0044
4.8

4.8

8.2

u

Axis

90
[011]
90
[011]
0
[0
11]
Not uniquely determined
120
145
?[111]

OriginRef.

A variation of G12
C-Si split interstitial, Ci(þ)

Present
Ref. 10

Bond-centered fluorine, Fbc(þ)

Present

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K. Uejima and T. Umeda

FIG. 4. Atomic models for the dry-etching damages of “Ci” and “Fi.” “Ci”
is a C-Si split interstitialcy or Ci(þ), which is a variation of the G12 center
observed in bulk Si.10 “Fi” is a bond-centered fluorine or Fbc(þ), which was
found in this study. The principal Z (//) axes of the g and A tensors of “Ci”
and “Fi” are also shown.

the contrary, it was minimized in the 55 direction for “Fi”
[Fig. 3(b)], indicating that the symmetry axis of the 19F hyperfine splitting of “Fi” is perpendicular to the h111i axis. This curious symmetry is reasonably explained by a bond-centeredfluorine configuration. As shown in Fig. 4, a bond-centered fluorine forms a Si-F-Si bond along the h111i axis. A
non-bonding orbital of fluorine should be set to be perpendicular to the Si-F-Si bond, owing to the orthogonal relationship in
the wave functions. Then, an unpaired electron in this orbital
exhibited a h111i-normal axial symmetry for the 19F hyperfine
splitting. The same relationship is also seen in the case of G12
and “Ci,” where an unpaired electron occupied a non-bonding
p-orbital perpendicular to a sp2-bonding plane (see Fig. 4).
In Fig. 3(b), we simulated an angular map of “Fi.”
However, a lot of the simulated lines was “missing” in theexperimental data. Such phenomenon was known in bulk
crystals which were intentionally strained by 10 to 90 MPa.14
The strain causes a preferential orientation of defects and theselection of observable ESR lines.10,11,14 Since there is astronger strain (up to 600 MPa) inside Si MOSFETs,12 it
may be reasonable to observe the “missing lines” in our
EDMR centers. The occurrence of the “missing lines” wasalso reported in other MOSFETs8,15 and in interface systems(Si-SiO2 systems).16 In the latter case, an interface lowered thesymmetry of the system, and in consequence, some orientations of a defect were prohibited.16 This situation seems plausible because “Fi” may be located near the Si-SiO2 interface.
We note that in Figs. 3(b) and 3(c), the “missing lines” may bealso observed in angular maps of “Ci” and other centers.
Due to the “missing lines,” we could not uniquely determine the angular-map of “Fi.” However, a number of ourangular-map simulations based on the least-square fitting
algorithm always converged to a similar result where theA(19F) tensor was given as in Table I. The A|| axis, which represents the direction of a 2p-orbital of fluorine, pointed
approximately to the center of another Si-Si bond, as shownin Fig. 4.
Another strong support for the model of a bond-centeredfluorine is based on a theoretical consideration. For an isolated fluorine in Si, ab initio calculations6 predicted that abond-centered fluorine is the most stable configuration whenAppl. Phys. Lett. 104, 082111 (2014)

it is positively charged, while a tetrahedral interstitial configuration is the most favorable in the negative charge state.
Judging from the fact that the coexisting defect of “Ci” is in
the positive charge state, the same charge state should be
expected for “Fi.” Accordingly, the bond-centered fluorine is
the most probable from a theoretical view point.
Furthermore, positive charge states of “Ci” and “Fi” areconsistent with the distribution of carbon and fluorine.
According to the SIMS results, these impurities should be
distributed in the sub-surface region. Since our EDMR monitored the junction leakage currents from the depletion region[a white area in Fig. 2(a)], the location of the observed “Ci”
and “Fi” centers is most likely within the depletion layer
near or beneath the gate area, which is the edge of p-type
region.
From the 19F hyperfine constants (A//  8.2 mT,
A?  4.8 mT, Table I) and the linear-combination-of-atomic-orbital analysis,13 we estimated 2%–3% of thewave-function distribution on a bond-centered fluorine atom.
This situation is quite similar to the case of a bond-centeredhydrogen atom (the AA9 center).17 In the AA9 center, ahyperfine splitting of 1H revealed only 1.6% of thewave-function distribution on 1H. Instead, its wave function
rather extended over the two Si neighbors by 21%  2, causing a noticeable 29Si hyperfine signature of AA9.17 Although
we could not resolve the 29Si hyperfine splitting of “Fi,” thesimilar distribution would be expected for a Si-F-Si bond.
In addition to the major defects “Ci” and “Fi”, one canstill find non-fitted experimental data in Figs. 3(b) and 3(c),which suggests the presence of other minor defects in our
MOSFETs. We supposed that they are formed from carbon,
fluorine, oxygen, and hydrogen, which are possible impurities introduced through the LSI processes. They have minor
contributions to the junction leakage current, judging fromtheir small signal intensities.
First, we point out that an angular map between “Ci”
and G11 seemed to account for the F1 center (FnV5).13 The
formation of F1 is quite reasonable because it is a final typeof FnVm defects after annealing.13 The FnVm defects preventthe diffusion of fluorine atoms and lead to a sharper depth
profile of fluorine.1,6,13 The SIMS profile in Fig. 1(b) showssuch a behavior.
Second, angular maps observed in the higher-magneticfield side (337.5–339.0 mT) or low g factors (g ¼ 1.994–2.003)
might be fitted to hydrogen-related defects such as AA9[Fig. 3(b)] and AA10 [Fig. 3(c)]. Both centers were found in
proton-implanted Si substrates.17 The AA10 center wasobserved only in Czochralski-grown silicon, and it may be acomplex of oxygen, carbon, and hydrogen impurities.17
However, in bulk crystals, they were only stable below 200 K.
Thus, we tentatively assume the enhancement of their thermal
stability in Si MOSFETs. In fact, we observed the enhanced
stability of “Ci” (G12) in our devices, even though they vanished in 300–350 K isochronal annealing when they were inbulk samples.10,11
In summary, using EDMR, we revealed the presence ofcarbon- and fluorine-interstitial defects (C-Si split interstitialcy and bond-centered fluorine) in practical Si
MOSFETs. They were formed in the source/drain area andthe edge of the gate area by fluorocarbon RIE andThis article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
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K. Uejima and T. Umeda

subsequent annealing processes, and they caused the junction leakage currents of the MOSFETs. We also found othertypes of minor dry-etching damages of carbon, fluorine, andhydrogen impurities, such as G11 (dicarbon interstitial), F1(FnV5 cluster), and possibly AA9 (hydrogen interstitial) or
AA10 (hydrogen-related defects). Surprisingly, they survived inside MOSFETs even after high-temperature thermal
processes exceeding their annihilation temperatures in bulk
samples. Our EDMR observations suggest an environmental difference between the insides of bulk silicon and LSIs.
1

M. N. Kham, I. Matko, B. Chenevier, and P. Ashburn, J. Appl. Phys. 102,
113718 (2007).
2
G. Roll, S. Jakschik, M. Goldbach, T. Mikolajick, and L. Frey, inProceedings of the European Solid-State Device Research Conference(IEEE, 2010), p. 329.
3
H. Komeda, M. Sato, A. Ishihama, K. Sakiyama, and T. Ohmi, inProceedings of the 3rd International Symposium on Plasma ProcessInduced Damage (IEEE, 1998), p. 88.
4
M. Wang and M. J. Kushner, J. Vac. Sci. Technol., A 29, 051306 (2011).
5
F. Zirkelbach, B. Stritzker, K. Nordlund, J. K. N. Lindner, W. G. Schmidt,
and E. Rauls, Phys. Rev. B 84, 064126 (2011).

Appl. Phys. Lett. 104, 082111 (2014)6

M. Diebel and S. T. Dunham, Phys. Rev. Lett. 93, 245901 (2004).
J. Lee, C. Cho, J. Lee, S. Shin, J. Lee, D. Kwak, K. Lee, B. Roh, T.
Chung, and K. Kim, in Proceedings of 32th European Solid-State Device
Research Conference (IEEE, 2002), p. 571.
8
T. Umeda, Y. Mochizuki, K. Okonogi, and K. Hamada, J. Appl. Phys. 94,
7105 (2003).
9
Since the reduced RIE time was shorter than the period necessary for thecomplete etching, we must be careful of an incompleteness of the etching,when we adopt “process B.”
10
G. D. Watkins and K. L. Brower, Phys. Rev. Lett. 36, 1329 (1976).
11
L. W. Song, X. D. Zhan, B. W. Benson, and G. D. Watkins, Phys. Rev. B
42, 5765 (1990).
12
S. Ito, H. Namba, K. Yamaguchi, T. Hirata, K. Ando, S. Koyama, S.
Kuroki, N. Ikezawa, T. Suzuki, T. Saitoh, and T. Horiuchi, Tech. Dig. of
the Int. Electron Devices Meeting (IEEE, 2000), p. 247.
13
T. Umeda, J. Isoya, T. Ohshima, S. Onoda, N. Morishita, K. Okonogi, andS. Shiratake, Appl. Phys. Lett. 97, 041911 (2010).
14
M. Stalova, Identification of Defects in Semiconductors, Semiconductors
and Semimetals Vol. 51B, edited by R. K. Willardson and E. R. Weber
(Academic Press, 1998).
15
M. S. Brandt, R. T. Neuberger, and M. Stutzmann, Appl. Phys. Lett. 76,
1467 (2000).
16
A. Stesmans, B. Nouwen, and V. V. Afanas’ev, Phys. Rev. B 58, 15801(1998).
17
Yu. V. Gorelkinski and N. N. Nevinnyi, Physica B 170, 155 (1991).
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