Radiationinduced interfacestate generation in reoxidized nitrided SiO2
V. Ramgopal Rao and J. Vasi
Citation: J. Appl. Phys. 71, 1029 (1992); doi: 10.1063/1.350390
View online: http://dx.doi.org/10.1063/1.350390
View Table of Contents: http://jap.aip.org/resource/1/JAPIAU/v71/i2
Published by the American Institute of Physics.
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Radiation-induced
interface-state
generation
in reoxidized
nitrided SKIa
V. Ramgopal Raoa) and J. Vasi
Department of EIectrical Engineering, Indian Institute of Technology, Bombay, Bombay 400 076, India
(Received 19 July 1991; accepted for publication 3 October 1991)
Reoxidized nitrided oxide is compared with nitrided oxides and dry Si02 for radiationinduced interface-state generation ( ADi,) and midgap voltage shifts (A Vmg). The suppression
of ADit, observed with heavy nitridation or reoxidation is explained in terms of the
trapped-hole recombination model together with the shifting of the location of the trapped
positive charge away from the Si interface. This model can also explain the effect of
nitrogen annealing on nitrided oxides.
It has been shown that thermal nitridation of SiO,
reduces charge trapping and suppresses interface-state generation due to ionizing radiation.te3 However, nitridation is
known to introduce a large number of electron traps and
also degrade the initial oxide quality.‘*5 Reoxidation of nitrided oxides is found to reduce the number of electron
traps while retaining or even improving the radiation performance of these oxides.6-9
Although many researchers have emphasized the suppfession of interface-state generation observed with heavy
mtridation or reoxidation of lightly nitrided oxides, no
good model has yet been proposed to explain the results
reported so far. The two-factor semiempirical model proposed by Hori, Iwasaki, and Tsuji” fails to explain the
effect of nitrogen annealing on nitrided oxides (NO).”
Here we present results on the radiation hardness of reoxidized nitrided oxides (RNO) and also explain the supression of interface-state generation as observed with heavy
nitridation or reoxidation on the basis of the trapped-hole
recombination model. This model also explains the effect of
nitrogen annealing of NO on interface-state generation.
Metal-oxide-semiconductor
(MOS) capacitors were
fabricated on p-type 4-6-0 cm { 100) silicon with Al as the
gate metal. A gate oxide 45 nm thick was grown in dry
oxygen at 1000 “C!. Nitridation was performed by in situ
annealing in 100% NH3 at 1050 “C for various times. Some
samples were reoxidized in situ in dry oxygen for various
times at 1050 “C. The tube was purged in nitrogen for 10
min between each step. The control oxide was annealed in
nitrogen for 10 min at the oxidation temperature. Capacitors of area 0.75 mm2 were detined with a metal mask and
postmetallization anneal was done in forming gas at 450 “C!
for 30 min. Ellipsometric measurements revealed that the
change in oxide thickness after nitridation and reoxidation
was less than 20%. The radiation was carried out without
bias using a Co6’ y-ray source with a dose rate of 120
krad (Si)/h. The midgap interface-state density (Dir,) was
calculated from the quasistatic measurement and the
midgap voltage ( Vma) was measured from high-frequency
capacitance-voltage (C-V) curves.
‘)Present address: Department of Electronics Engineering, K.I.T.S.,
Ramtek 441106, India.
1029
J. Appl. Phys. 71 (2), 15 January 1992
Before undertaking a detailed study of radiation-induced interface-state generation (AD,)
in RNO, we studied the process dependence of AD,
for RN0 at 880
krad(Si) and based on our results arrived at an optimum
RN0 condition. These results (not reported here) showed
that 30 min nitridation at 100% NH3 and 1050 “C followed
by a 60-min reoxidation at the same temperature gives the
best radiation hardness at 880 krad(Si). Our initial oxide
had a tied oxide charge Q? of 2 x 10”/cm2 and nitridation increased this to 9 X 10”/cm2. Subsequent reoxidation reduced Qf to 3 x 101’/cm2. Similarly, initial Di, for
the oxide was 2 X 10” cm ~-’ eV - i and nitridation increased this to 6 x 10” cm - 2 eV - ‘. Subsequent reoxidation only slightly altered this. All the radiation data presented here is for this optimized RNO. Each data point in
the plots is the average over five capacitors on the same
wafer.
Figures 1 and 2 show the dose dependence of midgap
voltage shift A Vms and ADifm, respectively, for NO, RNO,
and dry oxide. From Fig. 1 it can be seen that RN0 shows
the minimum charge trapping for total doses up to 4
Mrad (Si) . It is evident from Fig. 2 that dry oxide shows a
more than two-order increase in AD,. The saturation and
decreasing trend in ADif, for dry oxide may be an artifact
due to almost flat quasistatic C-Ps obtained at such high
total doses from which the ADi, was calculated. For NO
and RNO, quasistatic C-P”s did not show much distortion
even at high doses. Increasing nitridation time tends to
reduce ADi,. For total doses up to 2 Mrad( Si), heavy
nitridation seems to be better than reoxidation, but at
higher doses RN0 shows a saturation in AD,,. For the
case of NO, no such saturation was observed for either
ADi, or A V,,
The generation of interface states due to radiation has
been an area of active study, but no consensus has yet been
reached on the mechanism. Two of the most common
models are the hydrogen ion drift mode112,‘3 and the
trapped-hole recombination model.‘“16 The NO and RN0
have excess hydrogen incorporated in them, and on this
basis, increased ADi, should have been expected. On the
contrary, ADi, is suppressed for heavy nitridation and
reoxidation. This can be explained by invoking the
trapped-hole recombination model, according to which in-
0021-8979/92/141029-03$04.00
@I 1992 American Institute of Physics
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1029
4.0
TABLE I. Details of the type of oxide and the centroid values obtained
from the etch-back experiment.
r
Oxide
Oxide
3.0 -
Dry
NO
RN0
.
F
? l.O-
//-----*------m
o.oo+
I
0
Mrads
I
4
3
5
(Si)
FIG. 1. Midgap voltage shifts AV,, as a function of dose for 45nm oxide
nitrided and reoxidized at 1050“C! for different times: ( X ) dry oxide;
(0) 5-min nitridation; (0) 15-min nitridation; (A) 30-min nitridation;
(m) 30-min nitridation and 60-min reoxidation (RNO); (0) 30-min
nitridation and 60-min nitrogen anneal.
terface states are generated when electrons recombine with
holes trapped near the interface. The model, appropriately
modiied for NO and RNO, is as follows: The original dry
oxide contains a few strained Si-0 bonds near the Si interface formed during the oxidation of silicon. For light
nitridations, the peak nitrogen concentration [N], is right
at the Si/Si02 interface5”7y18 and this increases the number
of strained Si--0 bonds at the interface. A strained S&O
bond can act as a hole trap.r4 With radiation, these traps
get filled by the radiation-generated holes and for light
nitridation conditions we therefore have a large number of
1o13 -
/-\
Oxide
1o12 9
5
P
E
NO
.& 10” 0
a
ldOl
0
RN0
I
I
I
I
1
2
3
4
Dose,
Mrads
I
5
(Si)
FIG. 2. Midgap interface-state density generation hD,, as a function of
dose for 45-nm oxide nitrided and reoxidized at 1050°C for different
times, as in Fig. 1.
1030
dry 0,, 1000“C
as in dry t
100% NHr, 105O”C, 30 min
as in NO +
Oz, 105O”C, 60 min
Thickness
(nm)
Centroid
(nm)
45
6
49
17
51
44
RN0
1
Dose,
Processcondition
J. Appl. Phys., Vol. 71, No. 2, 15 January 1992
trapped holes right at the Si/Si02 interface. These trapped
holes on capturing electrons (which could be radiation
generated) give rise to a large AD,, more than that for
dry oxide as observed by Lo et al.” for lightly nitrided
oxides. The mechanism for trapped holes getting converted
to interface states could be similar to that proposed by
Wang and co-workers16 for dry oxides.
With heavy nitridations there is a shifting of [NIP away
from the Si interface due to the oxygen liberated as a
byproduct of the nitridation reaction.5’17-‘9 With reoxidation, too, a similar shifting occurs due to inter-facial
reoxidation.‘0P20 This reduces the strained Si-0 bonds at
the interface and hence the interfacial hole traps.11s21The
hole trapping for these cases takes place away from the Si
interface. Hence there are fewer hole traps near the interface that can get converted to interface states upon electron
capture. Furthermore, the presence of nitrogen in the dielectric away from the Si interface for heavy nitridations or
reoxidation counters the compressive stress that is present
near the SiOJSi interface before nitridation, thereby reducing the effective strain at the SiO,N/Si
interface.2’
This allows the reoxidation to take place with less strain
than the original oxidation resulting in an interface with
fewer hole traps. The few near-interfacial hole traps that
are generated during the reoxidation produce ADi, at low
doses, but for increasing doses AD,
saturates rapidly.
This can be clearly seen from our results and also from
similar results reported by Dunn.’ Nitrogen annealing of
NO does not show any suppression of AD,.”
This is
expected because with nitrogen annealing we do not have
any oxygen incorporation at the interface and hence no
shifting of [N], away from the Si interface.
We verified that the trapped-hole recombination model
is applicable to NO and RN0 by looking for the centroid
of trapped charge after irradiation using the etch-back
technique.z2 The oxides were irradiated to 3 Mrad(Si) and
etched in a HF solution to remove 4 nm each time.
AV,, was measured after every etch. The remaining oxide
thickness was measured ellipsometrically.
The results,
summarized in Table I, show that for the oxide the trapped
charge centroid is very close to the Si interface, and for NO
17 nm from the Si interface. For RN0 we found the centroid at 44 nm which is even further away from Si interface. The values of the centroid are accurate to about +3
nm. This explains the suppression of interface-state generation observed with heavy nitridation or reoxidation.
We also did constant-current stressing (CCS) for NO
V. Ramgopal Rao and J. Vasi
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1030
and RN0 to look for electron trapping. Stressing was done
in accumulation at 0.065 ,uA for up to 4000 s (total fluence
of 2.1 x lO”/cm’).
C-V measurements were taken by
interrupting the stress periodically. Our results for CCS are
similar to those reported earlier’ with the difference that
we observed a negative V,,,s shift (corresponding to positive charge trapping) for our RN0 condition as against the
positive shift reported earlier. NO, however, showed a
large positive shift (negative charge trapping). These CCS
results prove two points. First, our optimized RN0 process has reduced the electron trapping to a low value and,
second, the shifting of the trapped charge centroid from
the Si interface for RN0 is*mainly due to the shifting of
trapped hole charge and not due to a possible compensating electron trapping effect.23 However, for nitrided oxide
the shifting of the trapped charge centroid could partly be
due to electron trapping present in these oxides.“3
In summary, we have found that RN0 shows very
little ADit, up to 4 Mrad(Si). The trapped-hole recombination model can be used to explain the suppression of
ADi, for heavy nitridation and reoxidation.
The support of the Department of Electronics, Government of India, is gratefully acknowledged.
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V. Ramgopal Rao and J. Vasi
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1031