IEEE ELECTRON DEVICE LETTERS, VOL. 14, NO. 11, NOVEMBER 1993
530
Net Positive-Charge Buildup in Various
MOS Insulators Due to
High-field Stressing
R. M. Patrikar, R. Lal, and 3 . Vasi
Abstruct-The buildup of net positive charge with field stressing has been observed in all thermally grown oxides, viz. dry,
pyrogenic, reoxidized nitrided oxides (RNO), and reoxidized
nitrided pyrogenic oxides (RNPO). We observed a faster rate of
growth of net positive charge for dry oxides given a postmetalization anneal (PMA) in hydrogen than for those given a PMA
in nitrogen; the fastest growth of net positive charge, however,
was observed in pyrogenic oxides. It has been observed that in
dry and pyrogenic oxides the positive-charge growth as a function of time obeys a power law with time under the stress of
constant current or voltage. On the other hand, growth of
positive charge in RNO and RNPO shows a two-piece linear
growth of positive charge. These results suggest that positivecharge growth at high fields is related to both the hydrogen
concentration and its drift in the oxide.
D
IELECTRIC integrity of gate oxides is an important
consideration for the reliability of metal-oxide-semiconductor ( M O 9 devices. Apart from leakage and breakdown problems, charge buildup is an important consideration for reliable circuit operation. A well-established and
accepted observation is that there is net positive-charge
buildup at the Si-SiO, interface after high-field (HF)
stressing [ 11-[3]. Numerous models for explaining the
phenomenon of positive-charge generation in the MOS
oxide during H F stressing have been proposed [41, 151.
However the mechanism is not yet fully understood. We
have done a detailed study of net positive-charge generation for different stress conditions and for different Si0,based insulators grown by different techniques. These
results are presented in this letter.
The samples used in this study were metal gate MOS
capacitors with insulator thickness of 15 to 20 nm. The
substrate was 4-6 R . c m < 100 > p-type Si. The dry
oxide, of thickness 15.5 nm, was grown in O2 at 950°C
followed by a 20-min anneal in nitrogen at the same
temperature. Pyrogenic oxides, of thicknesses 16 nm, were
grown at 900°C with a fractional partial pressure of water
of 0.25. For reoxidized nitrided oxides (RNO) and reoxidized nitrided pyrogenic oxides (RNPO), the dry and
pyrogenic oxides grown under the conditions mentioned
above were nitrided by annealing in 100% ammonia at
Manuscript received July 26, 1993; revised September 7, 1993. This
work was supported by the Ministry of Human Resource Development
under its Thrust Area Programme in Microelectronics.
The authors are with the Department of Electrical Engineering,
Indian Institute of Technology, Bombay, Bombay 400076, India.
IEEE Log Number 9213213.
950°C for 30 min, and further reoxidized in dry oxygen at
950°C for 20 min. Each step was followed by a 5-min
anneal in nitrogen. The final thickness of the RNO and
RNPO insulators was about 19 nm. Capacitors of 0.75-mm2
area were made by electron beam evaporation of aluminum through a metal mask. The back oxide was etched
and aluminum was evaporated for the back contact. Postmetalization anneal (PMA) was performed at 450°C for 20
min in nitrogen, with some dry oxide samples annealed in
hydrogen.
The capacitors were stressed by two techniques: constant voltage and constant current. The oxide was stressed
for definite durations, and high-frequency C-V plots
(HFCV’s) were taken after each stress cycle. Fig. 1 shows
the flatband shifts as a function of time due to net
positive-charge buildup.
Fig. 2 shows HFCV curves for net positive-charge
buildup in pyrogenic oxide and dry oxide (PMA in nitrogen) before and after stressing at 1 p A for 38 s. Fig. 3
shows HFCV curves for net positive-charge buildup in
RNO and RNPO before and after stressing at 1 p A for
38 s.
Net positive-charge buildup in the oxide was observed
after all types of high-field stress in accumulation as well
as in inversion. The major observations are:
1) The net positive-charge buildup depends on the
applied field and charge fluence (total charge passed
through insulator/cm*) for all MOS insulators.
2) The net positive charge buildup continues to increase
until breakdown when stressed in accumulation and in
inversion. We have found that most of the capacitors on a
wafer break down when the AV,, (flat band voltage shift
due to net positive charge) exceeds a certain limit AVfb,,,.
3 ) For dry oxides (with PMA done in hydrogen and
nitrogen) and pyrogenic oxides, an important observation
is that the flatband shift as a function of time obeys a
power law for both constant-voltage stressing as well as
constant-current stressing. We also found that using the
power law gives the minimum residue and minimum standard deviation in least square fits. Thus flatband shift as a
function of time of stressing can be written as
AV,,
= a th
(1)
where a and h are constants.
4) We found that the constants a and b depend upon
the applied field and process parameters like thickness
0741-3106/93$03.00
0 1993 IEEE
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PATRIKAR et al.: NET POSITIVE-CHARGE B U I L D U P I N VARIOUS MOS INSULATORS
<
531
e
.
1500,-
U
+
L
II)
01
1000
0
m
-
U
>
0
U
500
0
n
r
0
-
U
0
'
100
50
150
Time (seconds)
Fig. 1. Plots of AVrb versus time for the constant-current stress (1 pA)
for various insulators.
__--
Pyrogenic
Dry (PMA in
Np)
I
-
)O
t
-5
-4
-3
-2
-1
V o l t oge
Fig. 3. HFCV curves for positive-charge buildup in RNPO and RNO
before and after stressing at 1 p A for 38 in accumulation.
TABLE 1
VALUES
OF CONSTANTS
a AND b FOR VARIOUS
INSULATORS*
Type of Oxide
Power-Law Constants
U
b
57.75
0.539
42.38
0.674
29.50
0.848
~~
Dry oxide
(nitrogen PMA)
Dry oxide
(hydrogen PMA)
Pyrogenic oxide
*Stressed at 1 PA.
-5
-4
-3
-2
-1
1
2
Voltage
Fig. 2. HFCV curves for positive-charge buildup in pyrogenic oxide and
dry oxide (PMA in nitrogen) before and after stressing at 1 p A for 38 s
in accumulation.
and dielectric quality of the oxide. These constants are
listed in Table I for dry oxide with nitrogen PMA, dry
oxide with hydrogen PMA, and pyrogenic oxide.
5) In RNO and RNPO the positive charge initially
grows rapidly and then increases slowly until breakdown.
A piecewise linear model with two regions seems to fit the
data well.
6) The net positive-charge generation rate is much
faster in pyrogenic oxides and is much lesser in RNO and
in RNPO, compared to dry oxides.
The higher rate of generation in pyrogenic oxide and
dry oxides with hydrogen PMA compared to dry oxides
with nitrogen PMA suggests that positive-charge growth is
affected by hydrogen content in the oxide. Also positivecharge growth in nonnitrided oxides obeys a power law
with stress time, while growth in the RNO and RNPO
oxides does not show a power-law dependence. One dif-
ference between these two sets of insulators is the presence of a nitrogen-rich oxynitride layer near the Si-SiO,
interface in RNO and in RNPO, which is formed during
the nitridation step. This layer acts as a barrier for hydrogen motion in the oxide 161,171. This is the main structural
difference between these oxides as far as hydrogen is
concerned. This barrier could be the reason for the reduced rate of positive-charge growth in RNO and RNPO,
and also for the different nature of positive-charge growth
compared to other oxides. Thus not only is the concentration of hydrogen responsible for net positive-charge
buildup, but its drift under high-field stress affects the net
positive-charge buildup in MOS insulators.
This study shows that hydrogen-rich MOS oxides behave poorly under high-field stress, and would tend to give
reliability problems. On the other hand, RNO and RNPO,
possibly because of the hydrogen-blocking oxynitride layers, show small shifts under HF stress.
REFERENCES
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K. S. Krisch, B. .I.Gross and C. G. Sodini, “Positive-chargc
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IEEE ELECTRON DEVICE LETTERS. VOL. 14. NO. I I , NOVEMBER 1993
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Authorized licensed use limited to: INDIAN INSTITUTE OF TECHNOLOGY BOMBAY. Downloaded on November 7, 2008 at 01:29 from IEEE Xplore. Restrictions apply.