Physica B 308–310 (2001) 1169–1172

Defects related to DRAM leakage current studied byelectrically detected magnetic resonance
T. Umedaa,*, Y. Mochizukia, K. Okonogib, K. HamadabaSilicon Systems Research Laboratories, NEC Corporation, 34 Miyukigaoka, Tsukuba 305-8501, Japanb
Elpida Memory Incorporation, Sagamihara 229-1198, Japan

Abstract
The defects responsible for the charge leak of dynamic random access memory (DRAM) memory cells were revealed
by a novel type of electrically detected magnetic resonance measurements. The detection sensitivity was greatly
improved by using reverse-biased p–n junctions, which enabled us to specify the defect structure. Two types of defects
were identiﬁed as the origin of the leakage currents; the spin-1 Si dangling-bond (DB) pair in divacancy-oxygen
complexes (V2+O and V2+O2) or Si DBs of a diﬀerent kind from the well-known Si DB center (g ¼ 2:0055). It isnotable that such defects remain in the device structure in spite of their low thermal stability in Si bulk. r 2001 Elsevier
Science B.V. All rights reserved.
Keywords: DRAM; Leakage current; Defect; EDMR

1. Introduction
In the present and future Si ultralarge scale integrated
circuits (ULSIs), process-induced point defects become
increasingly crucial, due to downsizing of the device
regions as well as complicated processing steps. At
present, microscopic information on defects in ULSIs is
quite limited. One major reason is that the amount of
the defects is far below the detection limit of electron
paramagnetic resonance (EPR). Alternatively, electrically detected magnetic resonance (EDMR) technique
was developed (for a review see Ref. [1]) and a successful
detection of residual defects in Si planar p–n junctions
was demonstrated [2]. Since this technique monitors themagnetic resonance of defects via resonant changes inthe device current, it is applicable to small sample
volumes of ULSIs; moreover, only defects present along
the current path and thus related to the device
performance can be selectively detected. These advantages make it possible to approach a very few but
important defects in actual ULSIs.

We present an EDMR study on the nude n+–pjunctions of the dynamic random access memory(DRAM) cells. This type of memory device is one of
the most sensitive ULSIs to the inﬂuence of residual
defects. For DRAMs, leakage currents in reverse-biased
p–n junctions (junction leakage current, JLC) have to be
suppressed in order to keep the capacitor charges as longas possible. The defects in the n+–p junctions, however,act as an eﬃcient source of additional JLC. We thus
study the defects responsible for the JLC of DRAM
cells. In order to directly correlate the defects with the
JLC, the key issue is to measure EDMR under a reverse
bias, which was very occasionally reported in previous
literatures [3]. We carefully examined the reverse-bias
characteristic of EDMR and eventually found that thedetection under reverse bias is quite possible and evensurprisingly sensitive to defects. Based on such EDMR
experiments, we discuss the microscopic structures ofpoint defects that cause the JLC.

2. EDMR measurements on reverse-biased p–n junctions*Corresponding author. Tel.: +81-298-50-1548; fax: +81298-56-6138.
E-mail address: t-umeda@da.jp.nec.com (T. Umeda).

Fig. 1 shows the basic structure of the chip examined.
This sample mimics the DRAM cells fabricated by the0921-4526/01/$ - see front matter r 2001 Elsevier Science B.V. All rights reserved.
PII: S 0 9 2 1 - 4 5 2 6 ( 0 1 ) 0 0 9 2 8 - 0

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T. Umeda et al. / Physica B 308–310 (2001) 1169–1172

Fig. 1. Sample structure. The p- and n-type regions were
produced by the implantation of B and P in a p-type (1 0 0) Si
wafer.

0.3-mm rule ULSI process. Each cell has an n+–pjunction with an adjacent switching metal-oxide-semiconductor-FET gate. The leakage current of the n+–pjunction is used for EDMR. Our EDMR setup is basedon a modiﬁed X-band EPR spectrometer [2]. The
measurement temperature was carefully determined asfollows. At low temperatures, the device current (I)decreases due to freeze out of carriers, while a low noiselevel of the resonant change (DI) is achieved due toreduction of the current noise. To gain the highest
signal-to-noise ratio and keep the normal currentvoltage characteristics, the measurement temperature
was set at 230 K. No illumination was done to preventthe generation of photo-excited carriers.
It has been believed that the JLC under reverse bias isunfavorable for EDMR because of the too low current
level. However, we found that the reverse JLC becomes
extremely sensitive to the defects when high reverse
voltages are applied to the junction (Fig. 2). The rate of
the resonant current change (DI=I) steeply increases
over 100 ppm above 5 V. This DI=I value is much higher
than observed under forward biases (30 ppm, independent of the forward bias). The reason why such anenhancement occurred will be simply because the reverse
JLC was dominated by the defect-induced leakage
currents. In contrast, p–n junctions under forward bias
or photo illumination generate only a small fraction ofrecombination currents related to the defects as compared to the diﬀusion current. Thus, EDMR under areverse bias has potential as a powerful technique toraise the defect detection sensitivity.
The location of defects can be known by changing thegate bias. Fig. 3(a) and (a0 ) schematically show a role of
the gate bias. When the gate is negatively biased withrespect to the p-type substrate, the channel region
becomes p-type. Further applying a high negative bias,
the p-type layer, and thus depletion layer, expands into
the near-surface n-type region (a0 ). The defects in the
newly formed depletion region can cause the JLC andEDMR. Actually, both of the JLC (I) and the resonantchange (DI) are found to increase exponentially with thegate bias [Fig. 3(b)]. This indicates that the defects are

Fig. 2. Reverse bias dependence of the resonant change in a
total current (DI=I). The magnitude of DI=I is shown in thepeak-to-peak intensity of a ﬁrst-derivative signal. The gate
voltage was set to be 3.2 V. VBD represents the breakdown
voltage.

Fig. 3. (a)F(a0 ) Schematic views of the gate-induced expansionof the depletion region. Crosses represent the defects. (b) Gatebias dependence of DI (solid circles) and I (open squares),measured at a reverse bias of 7.3 V.

located in the n-type side of the junction. Also, there is agood correlation between the JLC and DI:

3. Identiﬁcation of defects
A typical EDMR spectrum for the JLC is shown in
Fig. 4(a). The spectrum consists of two parts: one main
broad resonance at g (g value)=2.0055 and weak multilines at the both sides. The main broad signal resembles
to those from the well-known Si dangling bond (DB)T. Umeda et al. / Physica B 308–310 (2001) 1169–1172

observed in amorphous Si [4] and also in Si crystals
damaged by implantation [5], radiation [6,7], or
mechanical stress [8]. Their isotropic and single-peakfeatures are thought to indicate that the Si DBs are
randomly oriented and isolated from each other.
However, the same picture of the DBs cannot be applied
to the present case, because the n+–p junction of oursample is fully composed of crystalline Si lattice.
A similar DB signal has been observed in the othertype of planar Si p–n junctions which are also composedof crystalline Si lattice [2]. In Ref. [2], the DB signal wasso weak that detailed spectroscopic features could not be
resolved. On the other hand, very-low-noise spectra we
obtained [Fig. 4(b)] reveals that the present DB signal
really has anisotropy. As seen in the ﬁgure, when theexternal magnetic ﬁeld (B) is aligned to the [1 0 0]
direction, the peak moves toward lower magnetic ﬁeldand becomes slightly sharp. This anisotropy evidencesthat the Si DBs are not randomly oriented in our
samples. We consider that the observed anisotropy isrelated to the crystalline matrix of the relevant Si DBs,
which will be studied in detail elsewhere.
The weak multi-lines seen in Fig. 4(a) seem to be a
new EDMR signal. We tentatively call it signal A,
hereafter. In contrast to the main DB signal, the signal A
showed a strong B dependence. Fig. 5(a) shows its
angular dependence with respect to a magnetic ﬁeld
rotation in the ð0 1% 1Þ plane. One notable character ofthis pattern is that the ratio between the splitting widths
at B8½1 0 0 and B8½1 1 0 is just 1 : 2. This strongly
suggests that the signal splitting originates from the ﬁne(electron spin–electron spin) interaction which is nearly

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Fig. 4. (a) EDMR spectra for the JLC of the DRAM cells, and(b) the main signal. The magnitude of the JLC was 100 nA atreverse and gate voltages of 7.3 V and 4.2 V, respectively. The
spectra in (a) and (b) were measured using the magnetic ﬁeld
modulation of 0.9 and 0.4 mT, respectively.

Fig. 5. (a) Angular dependence of the signal A. In the gray area, this signal is not clearly resolved due to the overlapped main signal.
(b) The microscopic origins of the signal A.

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T. Umeda et al. / Physica B 308–310 (2001) 1169–1172

axial symmetric around the [0 1 1] direction. Previously,
similar ﬁne interactions were reported for a series of thespin-1 vacancy-oxygen complexes in radiation damaged
Si [9–12]. Such complexes retain a pair of Si DBs, which
generate the ﬁne interaction. The solid lines in Fig. 5(a)are calculated by adopting the same defect system
known for the divacancy plus one oxygen (V2+O, theA14 center) [10] or that plus two oxygen atoms (V2+O2,
the P2 center) [9]. The best ﬁt between the calculationand experimental data was obtained with averaged EPRparameters of the two centers. Therefore, we conclude
that the signal A originates from a combination of theP2 and A14. The microscopic origins of this signal are
given like Fig. 5(b). The oxygen incorporation possibly
occurs during the dopant implantation through surface
oxides and/or the thermal oxidation process.
Since our chips never recieved radiation damages orother intentional damages, it is reasonable to consider
that the present defects are created by the dopant(phosphorous) implantation. This assignment is also
supported by the fact that they are present in the n+type region, which was formed by the ion implantation.
After the implantation, our chips were subjected toannealing at a temperature of higher than 8001C. This ismuch higher than the anneal-out temperature for V2+O
and V2+O2 (o4001C) observed in radiation damaged Si[9]. Also the DB signal (g ¼ 2:0055) usually disappears
when implantation damaged Si wafers are annealed at
the relevant temperature range [5]. Therefore, ourobservation indicates that the thermal annihilation of
the defects is suppressed in the device structures. The
device regions are usually surrounded by the substratesurface and isolation structures. These may inhibit theannihilation of the defects.
For the Si DB pairs in V2+O and V2+O2, it isreasonable to treat their DBs individually, because of
the large separation between the DBs [9]. Also the DB
center of g ¼ 2:0055 can be regarded as a single-DB
system [2–4]. Thus, we discuss the EDMR mechanism
using a simple system, which consists of one carrier and
one unpaired electron (an electrically neutral Si DB) in
reverse-biased junctions. This system allows only one
primary spin-dependent process in which the DB
captures a valence-band electron in p-type Si. On theother hand, the carrier generation via the DB (electron
emission to the conduction band of n-type Si) is not a
spin-dependent process [13]. Therefore, the appearanceof the EDMR signals is supposedly a consequence of theelectron tunneling from p-type Si to the DB’s levels.
Such a tunneling probability naturally increases with thereverse voltage. Besides, the relatively high reverse
voltage is necessary to extend the depletion region into

the n+-region where the DBs are present. We speculatethat the drastic signal increase at high reverse voltages is
attributed to these factors.
The actual reverse bias for keeping the capacitor
charges is typically 1 or 2 V, which is lower than used for
EDMR measurements. At such low voltages, theelectron tunneling discussed above is largely suppressed.
However, the carrier generation via the DB’s levels caneﬀectively contribute to the JLC. Therefore, in order tosuppress the defect-induced JLC, it is necessary to
remove the DBs from the n+-region. This will beachieved by optimizing the ion-implantation process andthe subsequent annealing.

4. Summary
We have found implantation-induced point defects inthe n+–p junctions of DRAM cells by means of a novel
EDMR technique. The EDMR results clearly showedthat such defects are responsible for the JLC in thereverse-biased junctions. The defect structures wereassigned to be Si DBs or the spin-1 Si DB pairs of thedivacancy-oxygen complexes (V2+O and V2+O2). We
pointed out that the thermal annihilation of the defects
is suppressed in the device structure than in the bulk,
which indicates a signiﬁcance of probing actual device
regions.

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