JOURNAL OF APPLIED PHYSICSVOLUME 94, NUMBER 11

1 DECEMBER 2003

Electrically detected magnetic resonance of ion-implantation damage
centers in silicon large-scale integrated circuits
T. Umedaa) and Y. Mochizukib)
Silicon Systems Research Laboratories, NEC Corporation, Sagamihara, 229-1198, JapanK. Okonogi and K. Hamada
Elpida Memory Incorporation, Sagamihara, 229-1198, Japan

共Received 5 May 2003; accepted 15 September 2003兲
We used electrically detected magnetic resonance to study the microscopic structure ofion-implantation-induced point defects that remained in large-scale Si integrated circuits 共Si LSIs兲.
Two types of defects were detected in the source/drain (n ⫹ -type兲 region of 0.25-␮m-gate-lengthn-channel metal oxide semiconductor field-effect-transistors on LSIs: 共i兲 a spin-1 Si dangling-bond
共DB兲 pair in divacancy–oxygen complexes 共DB–DB distance, R⬇0.6 nm); and 共ii兲 a series oflarger Si vacancies involving distant Si DBs (R⭓1.4 nm). These vacancy-type defects were much
more thermally stable in Si LSIs than those in bulk Si crystals. We suggested two physicalmechanisms for this enhanced stability: internal mechanical stress and oxygen incorporation in theactive regions of LSIs. After examining the relationship between the defects and current–voltage
characteristics, we concluded that these defects are distributed in the near-surface n ⫹ -type region
close to the gate and that they are the source of the gate-induced drain leakage currents. © 2003
American Institute of Physics. 关DOI: 10.1063/1.1623608兴
I. INTRODUCTION

give us microscopic information about defects equivalent tothat obtained using electron paramagnetic resonance 共EPR兲.
EPR has greatly improved our understanding of knowledge
about bulk-Si defects,6 so that EDMR should be equally useful. However, despite the potential of EDMR, its application
to Si devices has so far been confined to examining nonintegrated devices such as large-area Si p – n diodes7 or damaged
devices such as the structure of a metal oxide semiconductor
共MOS兲 after ␥-ray irradiation,8 in order to increase the defectsignal.
In this paper, we report EDMR measurements on asfabricated n-channel metal oxide semiconductor field-effecttransistors 共MOSFETs兲 on Si LSIs, which were made using a
0.25-␮m-rule process. The detection sensitivity to defects
can be greatly enhanced by using a reverse-bias EDMR
method and a carefully designed MOSFET array. These techniques make it possible to identify the microscopic structureof process-induced defects in device-quality samples. The
defects observed were two types of Si vacancies produced by
the ion implantation in the source/drain region of the MOSFETs. It was believed that these vacancy-type defects would
not remain in Si LSIs because of their low thermal stability
in bulk Si crystals. However, it should be emphasized that
regions in downsized devices are quite different from thosein a normal Si lattice, which makes the defects very stableeven at high temperatures 共⬃1000 °C兲. The physical mechanisms for this enhanced stability are discussed in this paper
based on our experimental results. We also examined therelationship between the defects and the I – V characteristicsof MOSFETs. Eventually, it was found that these defects
relate directly to the gate-induced drain leakage 共GIDL兲 current of MOSFETs. Since the GIDL constrains the scaling of
Si LSIs,9,10 the defects we observed are an important issue in
LSI technologies.

Ion implantation is a key technology in the fabrication oflarge-scale Si integrated circuits 共Si LSIs兲. This process always requires high-temperature annealing, typically at1000 °C, to recover the crystalline lattice, annihilate defects,
and activate implanted dopants. However, even after recovery annealing, some defects still remain, which is evident
from electrical measurements. For large defects such as dislocations, it is possible to obtain information directly by using transmission electron microscopy 共TEM兲 combined withfocused-ion-beam processing for LSIs.1 However, in studying small point defects, there were no methods that weredirectly applicable to LSIs. Characterizing these point defects is important because a number of failures in LSIs show
no detectable defects in TEM analyses, and are therefore
believed to relate to much smaller defects. With the current
trend towards downsizing device regions, such defects havebecome increasingly critical in terms of device performance.
Furthermore, the complicated processing steps used for LSIs
are resulting in the formation of more process-induced
defects.
One possible means of detecting point defects in LSIs is
the use of electrically detected magnetic resonance
共EDMR兲.2–5 Since this technique monitors the magnetic
resonance of defects via resonant changes in the device current, it is applicable to the small sample volumes in LSIs.
Moreover, EDMR selectively detects only defects along thecurrent path and thus related to device performance. It cana兲

Present address: Research Center for Knowledge Communities, UniversityofTsukuba,
Tsukuba
305-8550,
Japan;
electronic
mail:umeda@slis.tsukuba.ac.jpb兲Author to correspondence should be addressed; electronic mail:y-mochizuki@az.jp.nec.com
0021-8979/2003/94(11)/7105/7/$20.00

7105© 2003 American Institute of Physics

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J. Appl. Phys., Vol. 94, No. 11, 1 December 2003

FIG. 1. 共a兲 Sample structure. 共b兲 Electrical configuration for EDMR measurements and schematic views of gate-bias (V G ) dependence of the sample.
Gray area represents the depletion layer.

II. EXPERIMENTAL PROCEDURE

Figure 1共a兲 shows the basic structure of the samples we
examined, which consisted of a parallel array of n-channel
MOSFETs with a gate length of 0.25 ␮m and a gate-oxide
thickness of 9 nm. The source/drain (n ⫹ -type兲 region wasformed by phosphorous ion implantations with a total doseof about 1013 cm⫺2 . Recovery annealing was carried out at
800⬃1000 °C. Under these conditions, the TEM images
showed no lattice defects. To obtain sufficient signal intensity from the device-quality samples, we integrated a numberof the MOSFETs into one sample so that the total source/
drain area increased to 34560 ␮m2. The LSI process used inthis study was the same as that used for dynamic random
access memories 共DRAMs兲, which has been optimized tominimize leakage currents in the n ⫹ – p junction 共junction
leakage currents, JLCs兲. Therefore, the density of defects has
also been minimized in our device regions. The lines of
MOSFET along the y-direction were isolated by shallow
trench isolations 共STIs兲. It is well known that these STIs
generate strong mechanical stress in specific device regions,depending on the STI pitch 共W兲. In general, decreasing W
causes a strong compressive stress in the MOSFET
regions,11,12 which frequently results in an appreciable increase in JLCs.12–14 This implies that STI-induced stress correlates with the formation of defects. To examine how thisstress influences defects and device performance, we prepared three samples with different STI pitches of 24, 2.4, and
0.8 ␮m. The existence of the STI-induced stress has been
confirmed in a STI structure similar to this one.15 The threesamples were fabricated on a common 8-in. wafer, so that theprocess for each one was completely identical. Finally, for
EDMR measurements, we cut small-sized chips 共0.5⫻0.5
cm2兲 from a wafer and made electrical contacts with Au-wirebonding.
The EDMR spectra were measured with a modified
X-band EPR spectrometer 共JEOL RE-2X兲, which we described previously.4,5 The sample was set in the center of a
TE011 cylindrical microwave cavity 共Q factor⫽18 000兲.
Since our samples were very small, we were able to apply amicrowave power of 200 mW to them with a minimum di-

FIG. 2. EDMR spectrum of our sample (W⫽0.8 ␮ m) measured under areverse-bias condition.

electric loss. Figure 1共b兲 shows the electrical configuration
for the EDMR. The rate of resonant current change (⌬I/I)
was monitored for the current 共I兲 flowing from the source/
drain (n ⫹ -type layer兲 to the p-type substrate of the MOSFETs; in other words, the n ⫹ – p junction current was used
for the EDMR. To amplify the resonant current changes, weused a magnetic-field-modulation technique operating at 320
Hz. The standard modulation width was 0.9 mT, but to enlarge the center part of the spectra, a width of 0.4 mT wasused. The measurement temperature was adjusted to 230 K
based on the following considerations. In principle, the spinresonance signal increases with decreasing temperatures, according to Curie’s law. However, at low temperatures, thermal generation–recombination processes via defects arelargely suppressed, and Si devices no longer show their normal I – V characteristics. Accordingly, an optimum temperature should be established for EDMR measurements, which
was 230 K in this study. To prevent the generation of photoexcited carriers, no illumination was applied.
III. RESULTS
A. Detection of defect signals and relationshipbetween the defects and I – V characteristics

Figure 2 shows a typical EDMR spectrum for oursamples. Two EDMR signals, which we called signals A andB, can be clearly observed. This spectrum was measured
using two special techniques that greatly increase the signal
intensity. Before analyzing the spectrum, we describe the detection conditions for these signals and clarify the relationship between them and the I – V characteristics of our devices.
A key point in the signal detection was the use of areverse-bias EDMR technique.5 Namely, we applied a reverse bias to the p – n junctions for taking the EDMR signals.
In previous studies using EDMR, either a forward bias or
photoexcitation of the p – n junction has been used to excite
a large current.2– 4 Under such conditions, however, only asmall fraction of the defect-induced recombination currents
is generated, as compared to diffusion currents. However,
reverse junction leakage currents can include a larger fraction of defect-induced currents.16 Among the various pro-

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J. Appl. Phys., Vol. 94, No. 11, 1 December 2003

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7107

FIG. 3. Reverse-bias dependence of the EDMR signal and device current ina sample (W⫽0.8 ␮ m). The signal intensity (⌬I/I) is calculated from thehigher-field-side peak height of signal B. Solid lines are guides to the eyes.

cesses that generate JLCs, capturing an electron by a paramagnetic defect in the depletion layer is spin dependent.17
The probability of this process occurring increases with thereverse bias due to tunneling of electrons from the Si valenceband to the defect.18 We therefore expected that signal detection would be more successful under a high reverse bias
(V R ), and in fact, the EDMR signals increased considerably
under these conditions 共Fig. 3兲. When V R is larger than 6 V,
the signal intensity (⌬I/I) easily exceeds 100⫻10⫺6 , which
is much larger than the maximum value of ⌬I/I for a forward bias (15⫻10⫺6 ). In addition, in our previous EDMR
measurements of a planar Si p – n diode 共forward biased, 4K, and under photoexcitation兲, the maximum ⌬I/I value wasno more than 10⫻10⫺6 . 4 Therefore, the detection sensitivity
was greatly enhanced by using a reverse-bias EDMR method
as compared to conventional forward-bias/photoexcitation
EDMR. As the reverse bias increased further, the JLC continued to increase, but the ⌬I/I decreased. This implies that
direct tunneling of carriers through the depletion layer
started to occur. When V R was over 10 V, the electric field of
the depletion layer increased to 1 MV/cm. In this regime, thetunneling and avalanche currents become dominant,16,18 andeventually the p – n junction undergoes a breakdown.
In addition to using a high reverse bias, controlling thedepletion layer with the gate bias is also necessary for signal
detection. As shown in Fig. 1共b兲, changing the gate voltage(V G ) causes the depletion layer to expand into the nearsurface region of either n ⫹ - or p-type Si. Therefore, the gatebias enabled us to focus on the region in which most of thedefects were located. Figure 4共a兲 shows the EDMR signal
intensity as a function of V G for the three samples. As isclear in this figure, the EDMR signals increased under negative V G for all our samples, demonstrating that the defects
were distributed in the near-surface n ⫹ -type region. This isquite reasonable, because ion implantation creates much
more damage in the n ⫹ -type 共i.e., high dose兲 region rather
than in the p-type region.

FIG. 4. 共a兲 Gate-bias dependence of the EDMR signal and device current,
where the lines are guides to eyes. The signal intensity is calculated from thehigher-field-side peak height of signal B. 共b兲 Estimation of the defectinduced current component in a total current at V G ⫽⫺4 V.

Together with the EDMR signals, the JLC increased exponentially under negative V G . This phenomenon is known
as gate-induced drain leakage 共GIDL兲.9 It has been concluded that the GIDL originates from band-to-band tunneling
and/or trap-assisted tunneling in the gate-induced nearsurface depletion layer.9,10 From the results of this study, it isclear that the observed defects are a source of trap-assisted
GIDL. Since the GIDL causes non-negligible leakage currents in the channel-OFF 共stand-by兲 state of MOSFETs, it is
an important issue for LSI performance.9 In particular, thecharge-retention time of DRAMs would be considerably reduced by such a GIDL.10 Thus, the defects we observed areof special significance in DRAM performance. Similarly to
Fig. 3, a high gate bias caused a decrease in ⌬I/I. Since thegate bias increased the electric field in the depletion region
共and hence the depletion layer expanded into the n ⫹ -type
region兲, this behavior can be explained by the occurrence ofthe direct tunneling.
In Fig. 4共a兲, it is also notable that ⌬I/I shows a drastic
increase as W decreases. In principle, ⌬I/I becomes larger if
the defect density of a sample increases.19 Therefore, it isreasonable to assume that defect density increases in anarrow-width MOSFET. This is in agreement with the increase in JLC 关Fig. 4共a兲兴, which also suggests an increase in
the defect density. To clarify this, using the ⌬I/I and I values

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J. Appl. Phys., Vol. 94, No. 11, 1 December 2003

FIG. 6. 共a兲 The V2 ⫹O2 center 共the origin of signal A兲. 共b兲 Defect systemcommon to spin-1 vacancy–oxygen complexes.

B. Origin of signal A „divacancy-oxygen complexes…

FIG. 5. Angular dependence of signal A. Solid lines represent the best-fit
pattern. In the gray area, signal A is not clear due to the overlapping of
signal B.

at V G ⫽⫺4 V, we estimated the intrinsic and defect-induced
components in the total JLC. We assumed that 共i兲 the intrinsic components were constant for the three samples and 共ii兲⌬I/I was in proportion to a fraction of the defect-induced
components in the total current I. The results are shown in
Fig. 4共b兲. This figure indicates that decreasing W causes anincrease in the defect-induced component, and therefore an
increase in the total JLC. A similar phenomenon was observed in other types of Si LSIs.10,12–14 In those experiments,
the JLCs increased with the scaling of the device, or with anincrease in process-induced stress. Accordingly, this phenomenon has been ascribed to stress effects, such as bandgap narrowing,13 formation of dislocations,14 or small
defects10,12 due to stress. Our results suggest that point defects relate to this phenomenon. We speculate that the reason
the formation of defects is dependent on a factor of W isconnected with the structure of the defects, which is considered subsequently.

Signal A is a EDMR signal that has not been reported inother EDMR studies. We presume that signal A may havebeen too weak to be detected in previous studies. This signal
splits into multiple lines 共Fig. 2兲 and shows a strong dependence on the magnetic-field 共B兲 orientation. Figure 5 plots
the resonant positions of signal A as a function of the rotationof B in the (01̄1) plane. One notable characteristic of thispattern is that the ratio between the splitting widths at
B储关100兴 and B储关011兴 is exactly 1:2. This strongly suggeststhat the signal splitting originates from a fine 共electron-spin–electron-spin兲 interaction, which is almost axially symmetricaround the 关011兴 direction. Previously, similar fine interactions were reported for a series of spin-1 vacancy-oxygen
complexes in radiation-damaged Si,20–23 such as V2 ⫹O2关Fig. 6共a兲兴. These complexes retain a pair of Si dangling
bonds 共DBs兲 that generate a fine interaction. Thus, we simulated the angular dependence of signal A by adapting thecommon defect system to spin-1 complexes 关Fig. 6共b兲兴. The
simulation and experimental data appeared to match very
well 共Fig. 5兲. Table I summarizes the best-fitting EPR parameters, in addition to the reported values for radiationdamaged Si. The most important parameter is the D 共fineinteraction兲 tensor. As can be seen in the table, the principal
values of D (D 1 to D 3 ) systematically decrease with thenumber of Si vacancies, in accordance with the R ⫺3 dependence 共R⫽separation between DBs兲 of the fine interaction.

TABLE I. EPR parameters for signal A and various spin-1 vacancy–oxygen complexes. The definition of these parameters is shown in Fig. 6共b兲. The SL1center corresponds to a photoexcited metastable state 共see Ref. 22兲. The g 2 axis in this table shows an approximate principal axis of g 2 . The principal D values
are expressed in mT. In Fig. 6共b兲, we simplified the principal axes of D 2 and D 3 to be 关 01̄1̄兴 and 关100兴, respectively, however, the reported axes were deviatedfrom these directions.
Defect 共label兲R (nm)

g1

g2

g3

g 2 axis

D1

D2

D3

Ref.

Signal A
V⫹O 共SL1兲V2 ⫹O 共A14)
V2 ⫹O2 (P2)V3 ⫹O 共P4)V3 ⫹O2 (P5)V3 ⫹O3 (A15)
V4 (P3)

0.38
0.58
0.58
0.77
0.77
0.77
0.96

2.008
2.01022.00902.00882.00822.00912.01152.00992.002
2.00572.00202.00192.00362.00502.00452.00102.010
2.00752.00932.00992.00662.00722.00852.0102

关 1̄1̄1̄兴关 01̄1̄兴关 1̄1̄1̄兴关 1̄1̄1̄兴关 01̄1̄兴关 01̄1̄兴关 01̄1̄兴关 1̄1̄1̄兴

⫾4.0
⫾21.9
⫾4.2
⫾4.3
⫾1.4
⫾1.6
⫾1.9
⫾0.84

⫿7.1
⫿46.8
⫿6.6
⫿7.4
⫿2.9
⫿3.2
⫿3.8
⫿1.64

⫾3.1
⫾24.9
⫾2.4
⫾2.9
⫾1.5
⫾1.6
⫾1.9
⫾0.80

Present
22
20, 21
20
20
20
20
23

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J. Appl. Phys., Vol. 94, No. 11, 1 December 2003

Our results are in good agreement with those obtained for thedivacancy plus one oxygen atom (V2 ⫹O, the A14 center兲 or
two oxygen atoms (V2 ⫹O2 , the P2 center兲.20,21 We ruled out
the possibility of an intrinsic divacancy (V2 ), because the V2
defect appears as a spin-1/2 center 共either G6 or G7 center, if
it is positively or negatively charged, respectively兲.24 Therefore, we concluded that signal A originated from a combination of A14 and P2 spectra. The atomic structure of the V2⫹O center is essentially the same as that of V2 ⫹O2 关Fig.
6共a兲兴, except that one Si–O–Si bond is replaced by an elongated Si–Si bond.20
This atomic structure is also consistent with the g tensor
data. As is seen in Table I, the g tensor of signal A is nearlyaxially symmetric around the principal axis of g 2 共i.e., g 1⬇g 3 ). This indicates that the DB pair is parallel to the g 2
axis, which is consistent with the model shown in Fig. 6. If
the DB pair is not parallel, as is the case for V1 and V3 , thisaxial symmetry is no longer observed. It is also worth notingthat the principal g values of signal A are quite similar to
those of a typical Si DB 共the P b and P b0 centers兲 at the
SiO2 /Si interface.4 This also supports our conclusion.
Since a Si DB has an unpaired electron, the generation–
recombination process by which a DB captures an electron isspin dependent. This spin-dependent process is only active if
the Si DB is located in the depletion layer. Therefore, theV2 ⫹O and V2 ⫹O2 centers are detectable only when they
are located in the depletion layer, which is consistent with
our arguments in Sec. A. It should be mentioned that whenthese Si DBs are located outside the depletion layer, they
make no contribution to the JLC as well as to the spindependent process.
C. Origin of signal B „large Si vacancies…

Signal B resembles the well-known Si DB signal 共gvalue, g⫽2.0055) observed in amorphous Si25 and in Si
crystals damaged by implantation,26 irradiation,8 or mechanical stress.27 The origin of the g⫽2.0055 signal has been
generally accepted as isolated Si DBs that are embedded randomly in an amorphous matrix.8,25–27 However, this picture
does not apply to our n ⫹ – p junctions, which were composed
entirely of a crystalline Si lattice. Signal B was observed in
our local-oxidation-of-Si-isolated Si p – n junction, which
was also composed of crystalline Si.4 Previously, we tentatively assigned the origin of signal B to an implantation damage center.4 However, we were unable to extract spectroscopic information for a weak signal B and were therefore
unable to clarify why isotropic Si DBs are embedded in crystalline lattice. In contrast with the previous work, the highintensity spectra in this study 关Fig. 7共a兲兴 clearly show thatsignal B is not isotropic and becomes sharp when B isaligned in the 关100兴 direction. This anisotropy is clear evidence that the Si DBs were not randomly oriented in our
samples. Furthermore, we can exclude the possibility of isolated DBs because their spectra are characterized by split
lines, as observed in the P b0 spectrum.4 Therefore, we consider that the relevant DBs interact with each other only
weakly 共i.e., R is large兲, resulting in the split lines being
smeared out by weak fine interactions.

Umeda et al.

7109

FIG. 7. 共a兲 Angular dependence of signal B. Dashed and solid lines were
measured for B储关100兴 and B储关011兴, respectively. 共b兲 Simulated angular dependence of signal B for the B rotation in the (01̄1) plane, which was
calculated for a model of V7 ⫹ a DB pair. Solid and dashed lines correspondto simulations for parallel and nonparallel DB pairs, respectively.

An analogy with signal A indicates that these DBs areprobably involved in Si vacancies. In Fig. 8, the maximum
fine-splitting widths (D max) of the spin-1 Si vacancies areplotted as a function of R. For signal B, the D max value
should be less than the full width at half-maximum of theabsorption peak 共⫽1.5 mT兲. On the basis of the intersection
between 1.5 mT and the R ⫺3 dependence of D max , we estimated that R is larger than 1.4 nm. This R value approximately corresponds to the length of the V6 to V7 chain. In
Fig. 7共b兲, the angular dependence of signal B was simulated
by using a model consisting of V7 plus a DB pair, based on
the same defect system used earlier. The g tensor was set tothat for either V2 and V4 共a parallel DB pair兲 or V1 and V3 共anonparallel pair兲, as shown in Table I. We used D values ofD 1 ⫽⫾0.21, D 2 ⫽⫿0.37, and D 3 ⫽⫾0.16 mT, which were
calculated from their R ⫺3 dependence. The two simulation
patterns for the different g tensors are superimposed in Fig.

FIG. 8. Maximum fine-splitting width (D max) versus DB–DB distance 共R兲.
The solid line indicates the R ⫺3 dependence.

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FIG. 9. A defect model for signal B 共large Si vacancies including distant Si
DB pairs (R⭓1.4 nm) and probably oxidized兲. In this figure, R is 1.6 nm.

7共b兲. We should mention that this simulation was too simple
to account for all actual DB situations. However, it revealed
some essential features of signal B. First, due to complicatedfine splitting, a complete signal will form a single broad peak
for any rotation angle, as observed experimentally. Second,
the signal-narrowing effect observed at B储关100兴 has been tentatively reproduced because of the small D 3 value. Similar
results were also obtained when R was increased. Note thatsince there are a number of possible configurations of DBs
and vacancies, the signal broadening proceeds fully, resulting
in a complete single line for signal B. Consequently, we propose that signal B arises from a series of large Si vacancies(Vn ,n⭓6), which leave weakly interacting Si DBs (R⭓1.4 nm). These large vacancies are supposedly oxidized
during high-temperature processes. Figure 9 illustrates ourmodel for the origin of signal B. We suggest that they are asmall type of the oxide precipitates observed by TEM on
Czochralski 共CZ兲 Si wafers.28 Such oxygen precipitates areformed in the same temperature range used for recovery annealing of ion implantation processes,28 so similar defects
could be formed in Si LSIs.
IV. DISCUSSION: HOW CAN THE VACANCY-TYPE
DEFECTS REMAIN IN Si LSIs?

The microscopic origins of signals A and B have been
assigned to small and large Si vacancy-type defects, respectively. Since ion implantation creates a large number of Si
vacancies 共and also Si interstitials兲, this result seems to bereasonable. In our samples, signal B 共large vacancies兲 wasalways much more dominant than signal A (V2 ⫹O and V2⫹O2 ). This is also reasonable, because larger vacancies
show higher thermal stability, as observed in radiationdamaged Si.20,28
After implantation, however, our devices were subjected
to recovery annealing at 800⬃1000 °C, which is much higher
than the anneal-out temperature 共⬍400 °C兲 for V2 ⫹O andV2 ⫹O2 in bulk Si.20 In addition, the g⫽2.0055 signals usually disappear when implantation-damaged Si wafers are annealed in the appropriate temperature range.26 Therefore, Si

Umeda et al.

vacancy-type defects appear to be much more thermallystable in device structures than in bulk Si crystals.
We propose two physical mechanisms for this phenomenon. One is a mechanical stress due to the structure of thedevice. As was seen in Fig. 4共a兲, the STI structure strongly
influences the density of residual defects in specific device
regions. When MOSFET width 共W兲 is narrowed, compressive stress increases monotonically and becomes several
hundred MPa for W in the submicron range.11,12 Generally,
under such compressive stress, Si vacancies become thermally stable because the tensile stress induced around a Si
vacancy cancels the compressive strain of the Si lattice.29
Although this energy gain 共⫽pressure coefficient of the vacancy formation energy, ⌬E f ) has not been determined forvacancy–oxygen complexes, the simplest example of a Si
monovacancy (V1 ) reveals that ⌬E f ⫽⫺0.1 eV at a compressive stress of 600 MPa.29 Even with this small ⌬E f , V1
is about three times 关 ⬇exp(⌬E f /kBT), where k B is the Boltzmann constant兴 more stable at T⫽800 °C and 600 MPa, ascompared to a zero-stress situation. It is thus possible that
more Si monovacancies remain in a narrower MOSFET, resulting in more Si vacancy-type defects. The key factor in
the effect of stress is how large the ⌬E f value is, which isdetermined mainly by how large the defect-induced stressis.29 If this defect-induced stress is larger for vacancy-oxygen
complexes than for monovacancies, the stress effect will become more significant than suggested earlier.
The other important factor is the concentration of oxygen atoms. Studies on oxide precipitates have suggested thatonce some Si vacancies are oxidized, they form a thermallystable nucleus for growing large oxide precipitates.28 The
EPR studies have confirmed more directly that Si vacancies(V1 to V3 ) become increasingly stable when they couple
with more oxygen atoms.20 Our devices were fabricated in ahigh-purity epitaxially grown Si layer, so that the initial oxygen concentration should have been much smaller than thatof CZ Si wafers 共which involve a supersaturated concentration of oxygen atoms during the crystal growth兲. In our
samples, however, oxygen incorporation could occur in thenear-surface regions during oxidation and implantation
through scattering oxides. It is thus likely that the oxygen
concentration is high only in the near-surface region of thedevices and, therefore, that Si vacancies are thermally stablein such a region. This is consistent with our experimental
result that the defects observed were distributed in the nearsurface region 共Sec. III A兲. Recently, a similar phenomenon
was found by using positron-annihilation measurements on
separation by implanted oxygen 共SIMOX兲 Si wafers.30 In
this case, vacancy–oxygen complexes were detected in nearsurface regions that contained a two-orders higher concentration of oxygen atoms (1020 cm⫺3 ) than a typical value for
CZ Si wafers. These complexes were stable even in processes at 1300 °C. Interestingly, their sizes were tentatively
estimated to be V1 ⬃V2 (⫽V2 ⫹oxygen atoms) or larger thanV6 , which is quite consistent with our microscopic data.

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Umeda et al.

J. Appl. Phys., Vol. 94, No. 11, 1 December 2003

In our samples, only Si vacancies were observed and no
Si interstitials 共self-interstitials兲 were found. Basically, ion
implantation creates Frenkel pairs of vacancies and interstitials. The question then arises: Why were the self-interstitials
missing? We speculate as follows: 共i兲 self-interstitials aremuch more mobile than vacancies,31 which means they may
be captured by gettering centers outside the device region;
共ii兲 self-interstitials are not stabilized by coupling with oxygen atoms, but are strongly stabilized by boron atoms in thep-type Si,31 which means they may not be thermally stable inour n-channel MOSFETs. For these reasons, Si vacancies
may be the dominant defects in our devices.

V. SUMMARY

We carried out EDMR studies on the microscopic structure of the ion-implantation damage centers that remain in
as-fabricated Si LSIs. Using special EDMR techniques thatwere extremely sensitive to defects, we detected two types of
implantation damage defects in the source/drain (n ⫹ -type兲
region of 0.25-␮m-gate-length n-channel MOSFETs on LSIs.
One defect was identified as a spin-1 Si DB pair (R⬇0.6 nm) in a divacancy–oxygen complex (V2 ⫹O and V2⫹O2 ). The other was attributed to a series of larger Si vacancies including distant Si DBs (R⭓1.4 nm). These large
vacancies may be oxidized to form small types of oxide precipitates. Our experiments suggested that these vacancy-type
defects are thermally stable in scaled-down devices, as compared to those in bulk Si crystals. The physical origins of thisenhanced stability were attributed to the following causes: 共i兲
internal lattice stress due to the structure of devices 共in particular, STI structures兲 raises the stability of defects by canceling the lattice stress caused by defect-induced stress, andthat 共ii兲 a high concentration of oxygen atoms in the nearsurface regions of devices strongly stabilizes the vacancytype defects. It is also notable that these defects relate directly to the GIDL current of MOSFETs, which is an
important issue for the performance of Si LSIs.

7111

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