Thin Solid Films 395 (2001) 266–269

Charge-trapping defects in Cat-CVD silicon nitride films
T. Umedaa,*, Y. Mochizukia, Y. Miyoshib, Y. NashimotobaSystem Devices and Fundamental Research, NEC Corporation, Tsukuba, 305-8501, Japanb
Compound Semiconductor Device Division, NEC Corporation, Otsu, 520-0833, JapanAbstract
We show that Cat-CVD silicon nitride films contain more than 1019 cmy3 nitrogen-bonded Si dangling bonds, similarly to thecase for conventional CVD films. However, the charge-trapping behavior of the Cat-CVD films is found to be quite different, inspite of the same origin for the dominant defects. The difference is ascribed to a non-uniform distribution of defects that isstrongly depleted near the surface in Cat-CVD films. 䊚 2001 Elsevier Science B.V. All rights reserved.
Keywords: Silicon nitride; Cat-CVD; Metal-oxide-semiconductor structure; Electron paramagnetic resonance

1. Introduction
Silicon nitride (SiNx) prepared by low-temperature
Cat-CVD (catalytic-chemical vapor deposition) shows agood electrical insulating property which is comparableto high-temperature CVD films w1x. In view of its
application to passivation of high-speed GaAs devices,
it is important to characterize charge-trapping defects inthe films w2x. The fixed charges of defects greatly affectthe device performance through modification of theelectric-field distribution in devices. In a wide varietyof conventional SiNx films prepared either by plasmaenhanced CVD (PECVD) or low-pressure CVD(LPCVD), a high density ()1018 cmy3) of chargetrapping defects were commonly observed w3,4x. However, no measurements on charge-trapping defects havebeen reported for Cat-CVD SiNx. Thus, in this report,
charge-trapping defects in Cat-CVD SiNx films arestudied and also comparison with those of conventional
PECVD SiNx films.
2. Sample preparation and experimental
Our deposition system was described in a previous
report w5x. The deposition parameters were optimized to* Corresponding author. Tel.: q81-298-50-1548; fax: q81-29856-6138.
E-mail address: t-umeda@da.jp.nec.com (T. Umeda).

obtain the highest growth rate (7.5 nm y min) for massproduction. Cat-CVD SiNx films were deposited using
the process conditions given in Table 1. We also prepared a conventional PECVD SiNx film with the samegas source and similar growth temperature. Both films
were nearly stoichiometric (N y Sis1.1:1.2) and freefrom oxygen (-0.1 at.%). The hydrogen contents were
11 and 23 at.% for the Cat-CVD and PECVD films,
respectively w5x. In these films, no metal impurities weredetected by SIMS measurements.
In order to study the charge-trapping behaviors, wemade double dielectric Si-MOS (metal oxide semiconductor) samples (Fig. 1). The thin thermal-SiO2 layer
enables capacitance–voltage (C–V) analysis without disturbance from the insulator y Si interface states. The areal
density of the fixed charge was estimated from shifts inthe so-called midgap voltage of 1-MHz C–V curves. We
Table 1
Cat-CVD conditions
Parameters

Set points

Catalyzer temperature
Catalyzer-wafer distance
Substrate temperature
NH3 flow rate
SiH4 flow rate
Gas pressure

17008C
75 mm
3088C
200 sccm
1.2 sccm
20 Pa

0040-6090/01/$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved.
PII: S 0 0 4 0 - 6 0 9 0 Ž 0 1 . 0 1 2 7 9 - 2

T. Umeda et al. / Thin Solid Films 395 (2001) 266–269

267

Fig. 1. Si-MOS capacitor structure and configuration for carrier-injection experiments.

carried out carrier-injection experiments by applying abias stress to the capacitors (see Fig. 1). The bias stress
was set to be 60 and 40 V for the Cat-CVD and PECVD
samples, respectively, to avoid dielectric breakdown of
the capacitor and to maximize the midgap-voltage shift.
Electron paramagnetic resonance (EPR) was used toidentify the origin of the charge-trapping defects. EPR
spectra were taken with a JEOL X-band spectrometerusing 0.4-mT modulation at 100 kHz, a microwave
power of 0.01 mW, and a temperature of 20 K. The
metal parts of the MOS samples were removed for EPR
measurements. Furthermore, cleaved edges of the Si
substrates were etched by alkali solutions. This processcompletely eliminated an undesirable EPR signal of
gs2.0055 which arises from Si dangling bonds in theedge regions w6x. In etch-back experiments, the defectdensity was measured by room-temperature EPRmeasurements.
We also applied ultraviolet (UV) excitation to thesamples, using a deuterium lamp (3–7.5 eV). The
illumination was provided for 10–30 min at room
temperature.

Fig. 2. C–V curves for the Cat-CVD SiNx capacitor.

and the generated charges can be removed by UVexcitation. This observation gives clear evidence forcharge trapping at point defects. The density of thetrapped charge was approximately 6=1012 cmy2 forboth films. The actual density should be even larger,
because C–V measurements give an average density ofthe sheet charge which is weighted by the distance fromthe insulator y Si interface.
The transient behavior for charge trapping is shownin Fig. 3. Both the Cat-CVD and PECVD films reveal
similar transient behaviors: the charge trapping is immediately saturated within a period of 1 s, and more than
10% of injected carriers are efficiently captured at thedefects. These features are related to the nature of thecharge-trapping defects, and are discussed in the following section.

3. Results and discussion
3.1. Carrier-injection experiments
First, the charge trapping in Cat-CVD SiNx films isstudied by carrier-injection experiments on MOS structures. Fig. 2 shows C–V results for Cat-CVD MOS
capacitors. In as-grown samples, the C–V curve for theCat-CVD film (a dashed line) is quite similar to thatfor a PECVD film, and a small amount (1=1012 cmy2)of positive charges are commonly observed w7x.
However, after carrier injection due to a bias stress, adrastic increase in the density of fixed charge is observedin both films. In Fig. 2, the solid line shows C–V results
measured after a bias stress. As is seen in the figure, aflat-band voltage of the C–V curve is significantly
shifted, which clearly indicates the generation of a highdensity of fixed charge. We found that the observed
shifts are reversible with respect to stress-bias polarity

Fig. 3. Transient behavior for charge trapping (Cat-CVD: asgrown™q60 V stress; PECVD: as-grown™–40 V stress). The CatCVD film shows the higher density of the trapped charge as comparedto the PECVD film, which is due to the increase of positive chargein the as-grown Cat-CVD film w7x.

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T. Umeda et al. / Thin Solid Films 395 (2001) 266–269

3.2. Origin of charge-trapping defects — the K centers
EPR measurements can reveal the microscopic originof the defects mainly responsible for the charge trappingin the Cat-CVD films. Fig. 4 shows EPR spectra for(A) as-grown Cat-CVD and PECVD films and (B) for
those after UV excitation. As is clear in the figure, UV
excitation greatly increases the EPR signal. The spectra(C) show the UV-induced changes wi.e. (B)y(A)x,
which is clearly dominated by a single broad signal at
gs2.003 (solid lines). This signal corresponds to thewell-known signal of electrically-neutral K centers. TheK center is known as the dominant charge trap inconventional CVD SiNx films w3x, and our study demonstrates that it is also the case for the Cat-CVD films.
Based on detailed EPR analyses w4x, it is well establishedthat the K centers are nitrogen-bonded Si dangling bonds(N3^Si structure). We point out that the K signal isbroader in the Cat-CVD films (2.7 mT) as opposed tothe PECVD films (1.9 mT). This might reflect different
environments of the K centers in Cat-CVD SiNx.
The C–V and EPR results are consistently explained
based on the ‘negative-U’ nature of this defect. The
apparent absence of net charge in the as-grown films
means that the defect occupation is stabilized by nearly
equal numbers of negatively- and positively-charged K
centers which are both diamagnetic w3x. However, suchcharged K centers are converted to metastable neutral(paramagnetic) centers by UV excitation w3x, resulting
in increased EPR signal together with the removal oftrapped charge. The transient behavior is also consistent
with the nature of the K centers, because the Coulomb
attraction of charged K centers can enhance their capture
cross-section for injected carriers w3x.
In both as-grown spectra (A), a broad resonance at
gs2.003–2.006 is commonly observed, which is tentatively attributed to a combination of a neutral K signal(gs2.003) and a Si dangling-bond signal (gs2.0055

Fig. 5. Relationship between the polarities of bias stress and trappedcharge. Solid and open circles are measured for the Cat-CVD andPECVD films, respectively.

w6,8x). We speculate that potential fluctuations and y or
unintentional UV irradiation generate metastable neutral
K centers in the as-grown films. We also speculate thatthe gs2.0055 centers are formed in the Si-rich regionsof our SiNx films, because they have a Si3^Si structure
w8x. Note that both centers are electrically neutral, whichis consistent with the observation of negligible net
charge in the as-grown films.
3.3. Unusual charge-trapping in Cat-CVD films — nonuniform distribution of the K centers
Although Cat-CVD and PECVD films have the same
dominant defects, their charge-trapping behavior was
found to be quite different. It appears as a striking
contrast in the polarity of the fixed charge (Fig. 5). In
PECVD samples, a negative bias stress (selectroninjection from the Al side) generates negative fixedcharge in the film. This behavior is quite reasonable,Fig. 4. UV-induced changes in EPR spectra. Two negative lines at both ends are Mn2q marker signals. The spectra were recorded for magnetic
field parallel to the w100x axis. The dashed lines indicate a signal from SiO2 –Si interface dangling bonds, which was confirmed by the angulardependence of the g factor.

T. Umeda et al. / Thin Solid Films 395 (2001) 266–269

because carrier injection from the Al side is dominant
in our MOS capacitors, from a potential barrier due tothe thermal SiO2 layer. Nevertheless, in Cat-CVD films,
a negative fixed charge is generated by a positive bias
stress, i.e. electron injection from the Si side (Fig. 5).
Namely, the charge trapping was largely suppressed for
carrier injection from the Al side. It means that the K
centers are strongly depleted near the surface of CatCVD films. In contrast, in the PECVD films, the K
centers appear to be rather uniformly distributed acrossthe thickness. With stronger bias stresses (;60 V),
compensation of net charge was observed pronouncedly
for the PECVD samples. This indicates that the charge
trapping in the PECVD films took place rather symmetrically from both the Al and Si sides. In both films,
positive fixed charge is appreciably compensated bynegative charge generated in the opposite side of thefilm (see Fig. 5). We speculate that this is due to low
injection efficiency for holes as compared to electrons.
To confirm the defect distribution across the thickness,
we performed etch-back experiments using buffered HF(BHF) treatments. BHF-etching rates were approximately 0.15 and 0.76 nm y s for Cat-CVD and PECVD films,
respectively, which were measured by the spectroscopic
ellipsometry. Before etching, the samples were excited
by UV light to neutralize the K centers completely.
Then, we measured the reduction in the EPR signal dueto etching. These profiles are shown in Fig. 6. Consistent
with the charge-trapping behavior, the defect density in
the Cat-CVD film is very small in 10 nm thickness
from the surface, and becomes larger with close proximity to the substrate. However, PECVD films show arather uniform distribution, which is in good agreementwith their charge-trapping behavior. From the etch-backexperiments, total K densities are estimated to be similar
for both films: from 1=1019 to 2=1019 cm–3 and from
1=1014 to 2=1014 cm–2, respectively. The number ofthe K centers is large enough to account for the fixedcharge densities observed in Figs. 2 and 3. These values
agree with typical figures (1018–1019 cm–3 w3x) for
various conventional CVD films.
4. Summary
The charge-trapping defects in Cat-CVD SiNx havebeen studied by means of C–V and EPR analyses. We269

Fig. 6. Defect density profiles estimated by etch-back experiments.

found that Cat-CVD films contain K centers (N3^Sidefect) as a dominant defect, in a manner similar to theconventional CVD SiNx films. The C–V and EPR results
were consistently interpreted in terms of negative-U
behavior of the K centers. The unusual charge trapping
was observed for Cat-CVD films, and is attributed to
the non-uniform distribution of the K centers in thefilms.
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