Solid-State Electronics 43 (1999) 1997±2003
Nitrogen in ultra-thin gate oxides: its pro®le and functions
Chen Shou Mian*, Ip Suk-Yin Flora
Institute of Microeletronics, 11 Science Park Road, Singapore Science Park II, Singapore 117685, Singapore
Received 9 February 1999; accepted 20 April 1999
Abstract
The nitrogen composition pro®le in an ultra-thin gate oxide has been engineered through a multi-step oxidation
by rapid thermal process (RTP), and has been examined by the SIMS analysis. It is interesting to ®nd out that two
nitrogen composition peaks in an oxide ®lm have been formed using a three-step oxidation recipe (N2O+O2+N2O)
by RTP, which is inconsistent with that in literature. The correlation between the nitrogen pro®le and Qbd value of
a gate-oxide has been systematically studied. It shows that the Qbd value for an oxynitride increases with the
nitrogen incorporation at the Si/oxide interface, but reduces with the nitrogen incorporation in the oxide bulk or at
the oxide/poly-Si interface. Therefore, there is a balance in the choice of the Qbd value and the barrier to B
penetration. # 1999 Elsevier Science Ltd. All rights reserved.
Keywords: Nitrided oxide; Gate oxide; Rapid thermal process; Oxynitride
1. Introduction
As the feature sizes of MOS devices have aggressively scaled down into the deep submicron regime,
ultra-thin gate dielectrics with high reliability and
dual-poly gate electrodes become indispensable.
Conventional thermal SiO2 is inadequate for ULSI
devices below 0.25 mm due to several limitations, such
as the poor interface stability (resulting in degraded reliability) and a low barrier to boron penetration which
comes about when p+-polysilicon is used for pMOSFETs.
The degraded reliability of ultra-thin SiO2 is believed
to be due to an 020 AÊ thick structure transition layer
(STL) at the Si/SiO2 interface, resulting from the high
stress/strain associated with volume expansion of SiO2
[1]. Incorporating a small amount of nitrogen in the
* Corresponding author. Tel.: +65-770-5714; fax: +65-7701914.
E-mail address: [email protected] (Chen S.M.)
STL is supposed to reduce the amount of distorted
bonds by alleviating the strain, thus improving the
interface stability, such as the improved charge-toand
hot-carrier
reliability.
breakdown
(Qbd)
Furthermore, nitrogen in the gate-oxide acts as a good
barrier to boron penetration from p+-polysilicon [2,3].
However, nitrided oxides (or oxynitrides) also exhibit
some fatal demerits such as reduced carrier mobility
and enhanced electron trapping.
Traditionally, nitrogen can be introduced into a thin
oxide via high temperature nitridation of the oxide in
NH3, N2O or NO by furnace or rapid thermal process
(RTP) [1,4]. However, the N peak position in the oxide
is always located at the Si/oxide interface, no matter
which nitridation method or oxidation sequence has
been used. This kind of nitrogen pro®le in a gate-oxide
can prevent B penetration from p+-polysilicon into
silicon substrate (channel region), but cannot prevent
B di€using into the gate-oxide, where B acts as a trap
or charge and degrades the gate-oxide reliability. An
ideal nitrogen composition pro®le in a gate oxide
should have a nitrogen peak at the oxide/poly-Si inter-
0038-1101/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved.
PII: S 0 0 3 8 - 1 1 0 1 ( 9 9 ) 0 0 1 6 6 - 5
1998
Chen S.M., Ip S.-F. / Solid-State Electronics 43 (1999) 1997±2003
Fig. 1. The structure of a typical RTO recipe for the growth of nitrided oxide ®lms.
face to prevent B penetration from p+-poly and some
nitrogen at the Si/oxide interface to improve the interface stability. Great e€ort has been made to engineer
the N pro®le in an oxide and new process techniques
are also being explored [5]. However, it has not been
successful so far. Here, we demonstrate a technique to
engineer the N pro®le in an ultra-thin oxide via changing the oxidation sequence in O2 and N2O by RTP,
which was also named as RTO. An N pro®le close to
the ideal one has been formed and some new results
were found to be quite di€erent from those in literature.
2. Experimental
Eight-inch notch prime wafers (p-type) were used for
oxidation. They received an RCA cleaning prior to
dielectric growth in order to strip the native oxide. All
thin oxide ®lms were grown in O2 and/or N2O at
10508C (for 45 AÊ oxides) or 10008C (for 35/30 AÊ oxides) via RTP (Applied Material's Centura). Two series
samples were produced. One was grown on blank
wafers for oxide thickness measurements and SIMS
analysis of the nitrogen pro®le in oxides. Another was
made for devices using a standard 0.25 mm (logic) process ¯ow, but only n+-poly was deposited on the gateoxide in order to eliminate the B penetration e€ect.
Five basic oxidation recipe structures have been used
as follows: (1) an one-step process, oxidation in pure
oxygen (O2), as a reference sample; (2) a two-step process with an initial oxidation in O2 followed by a nitridation in N2O (O2+N2O); (3) a two-step process with
an initial oxidation in N2O followed by a re-oxidation
in O2 (N2O+O2); (4) a three-step process with an initial oxidation in O2 followed by a nitridation in N2O,
then a re-oxidation in O2 (O2+N2O+O2); and (5) a
three-step process with an initial oxidation in N2O followed by a re-oxidation in O2, then an annealing in
N2O (N2O+O2+N2O). Fig.1 shows one example of a
recipe structure.
The oxide ®lm thickness has been measured by optiprobe 2600 of Thermal-Wave, cross-section HRTEM,
C±V plot in accumulation mode and Fowler±
Nordheim tunneling oscillation method (I±V curves).
Dynamic SIMS analysis from EVANS EAST has been
used to analyze the nitrogen depth pro®le in thin gate
oxide ®lms.
The gate injected (ÿVg bias) charge-to-breakdown
Qbd values were measured on MOS capacitors using
the constant current method. The gate oxide area in a
MOS capacitor was 1 10ÿ4 cm2 for Qbd measurements.
3. Results and discussion
The oxide thickness of 45 AÊ has been measured in
several ways, as follows. The optical thickness was
about 45 AÊ with a uniformity of 0.3% one sigma,
being measured by opti-probe. The electrical thickness
was about 54 AÊ measured by the C±V method in accumulation mode, and 49 AÊ calculated from the
Fowler±Nordheim tunneling I±V curves. HRTEM pictures showed a physical thickness of about 47 AÊ [6].
There was no signi®cant di€erence in C±V and I±V
plots among the gate oxides grown by di€erent recipes.
In general, the electrical properties of oxides can be
well characterized by the I±V/C±V plots. However, the
silicon/oxide interface property is sensitive to silicon
surface cleaning and other process conditions. For
industrial applications, Qbd measurement of the gate
oxide is a very common way to evaluate the oxide
Chen S.M., Ip S.-F. / Solid-State Electronics 43 (1999) 1997±2003
1999
Fig. 2. SIMS analysis of nitrogen and oxygen depth pro®les in an oxide. The oxide was grown by an initial oxidation in O2 (for 22
s) followed by a nitridation in N2O (for 37 s) at 10508C (O2+N2O). The optical thickness of the oxide ®lm was about 45 AÊ.
quality and process conditions. Hence, we will focus
on the Qbd evaluation of the gate oxide quality here.
Signi®cant di€erence in Qbd values has been observed
among oxides grown by di€erence recipes.
3.1. Nitrogen composition pro®les in oxides
SIMS analysis in Figs. 2 and 3 clearly shows that
the N peak position can be controlled by the sequence
Fig. 3. SIMS analysis of nitrogen and oxygen depth pro®les in an oxide. The oxide was grown by an initial oxidation in N2O (for
25 s) followed by a re-oxidation in O2 (for 57 s) at 10508C (N2O+ O2). The optical thickness of the oxide ®lm was about 45 AÊ.
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Chen S.M., Ip S.-F. / Solid-State Electronics 43 (1999) 1997±2003
Fig. 4. SIMS analysis of nitrogen and oxygen depth pro®les in an oxide. The oxide was grown by an initial oxidation in O2
(for 10 s) followed by a nitridation in N2O (for 30 s), then a re-oxidation in O2 (for 20 s) at 10508C (O2+ N2O+O2). The optical
thickness of the oxide ®lm was about 45 AÊ.
of O2/N2O oxidation. In Fig. 2, a thin SiO2 layer was
grown during the ®rst step of O2 oxidation, then the
second step of N2O annealing introduced N at the Si/
oxide interface. (The nitrogen `peak' in the ®rst 10 AÊ
of surface, as shown in all the SIMS ®gures, was a
measurement error in the SIMS analysis.) This result is
Fig. 5. SIMS analysis of nitrogen and oxygen depth pro®les in the oxide. The oxide was grown by an initial oxidation in N2O (for
18 s) followed by a re-oxidation in O2 (for 40 s), then an annealing in N2O (for 30 s) at 10508C (N2O+ O2+N2O). The optical
thickness of the oxide ®lm was about 45 AÊ.
Chen S.M., Ip S.-F. / Solid-State Electronics 43 (1999) 1997±2003
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Fig. 6. Qbd results for oxides produced via recipes in Figs. 2±5: (O2), (O2+N2O), (N2O+O2), (O2+N2O+ O2) and (N2O+
O2+N2O). The optical thickness of oxides was about 45 AÊ.
consistent with that in literature [7,8]. It is interesting
to note that, in Fig. 3, the N peak was shifted away
from the Si/oxide interface because a new layer of SiO2
was grown beneath the oxynitride layer during the second step of O2 re-oxidation. N peaked at about 15 AÊ
beneath the top surface, which will be at the oxide/
poly-Si interface if a MOS device is made.
In general, N is easier than O to di€use into Si and
Si±N bonds are easier than Si±O bonds to breakdown,
so during the growth of oxynitrides, the N peak used
to be located at the Si/oxide interface no matter the
N2O oxidation was the ®rst or last step. This has been
demonstrated in a furnace oxidation process [8].
However, since the RTP process has a very fast temperature ramp and a very short high temperature process time, oxygen may di€use through the thin
oxynitride layer to form a new SiO2 layer at the Si/
oxide interface during the re-oxidation of oxynitride in
O2. Most of the N in the oxynitride will not be
replaced by O and stay in the original oxynitride layer.
This phenomenon is similar to that appeared in RTP
annealing of implant dopants in silicon. Therefore, the
nitrided oxide with a `sandwich' structure can be produced via a multi-step O2 and N2O oxidation by RTP,
and the N pro®le in an oxide can be therefore engineered. This idea was further con®rmed by the results in
Figs. 4 and 5.
In Fig. 4, the third step of O2 re-oxidation introduced a very thin SiO2 layer beneath the oxynitride
layer, pushing the N peak slightly away from the Si/
oxide interface. One interesting and important obser-
vation here is that two nitrogen composition peaks
were formed in the oxide using recipe (5), as shown in
Fig. 5, which is very close to the expected ideal nitrogen composition pro®le in a gate oxide. One N peak is
at the Si/oxide interface (formed at step three) and
another near the top surface (formed at step one).
However, because of the broad distribution of N in
the oxide and the limitation of the oxide thickness,
two N peaks were overlap, giving a nearly ¯at N distribution in the oxide bulk. This result is inconsistent
with that predicted in literature, where the nitrogen
peak near the top surface (formed in step one) was
supposed to be replaced by the atomic oxygen during
the third step of N2O annealing [7,8]. The reason that
two nitrogen peaks can be formed in Fig. 5 is as
described above, we suppose.
3.2. Qbd measurements
The Qbd results for oxides produced via the above
recipes (Figs. 2±5) are shown in Fig. 6, together with
that of a control sample (O2). Compared with the control sample (O2), oxynitrides grown via (O2+N2O)
and (O2+N2O+O2) have higher Qbd values, indicating
that incorporation of a small amount of nitrogen at
the Si/oxide interface did improve the interface quality,
and, therefore, improves the reliability of oxides. The
oxide grown via (O2+N2O+O2) has a tighter Qbd distribution than that via (O2+N2O). This is supposed to
be due to the third step of re-oxidation in O2, which
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Chen S.M., Ip S.-F. / Solid-State Electronics 43 (1999) 1997±2003
Fig. 7. Qbd results for oxides produced via recipes: (O2), (N2O +O2), (N2O+O2+N2O) and (O2+N2O+O2). The optical thickness
of oxides was about 35 AÊ.
possibly smoothed the interface and removed some
defects in oxide, just like a step of thermal annealing.
Qbd values of the sample (N2O+O2) are much smaller than that of the control sample (O2), suggesting
that nitrogen in the oxide bulk or near oxide/poly-Si
interface degraded the oxide reliability. However, the
nitrogen at the oxide/poly-Si interface is important to
prevent boron penetration from the p+-poly gate, so
Fig. 8. Qbd results for oxides produced via recipes: (O2+N2O) and (N2O+O2). The optical thickness of oxides was about 30 AÊ.
Chen S.M., Ip S.-F. / Solid-State Electronics 43 (1999) 1997±2003
there is a balance in the choice of the Qbd value and
the ecient barrier to B penetration. The oxide grown
via (N2O+O2+N2O) has higher Qbd value than that
via (N2O+O2) because the third step of N2O annealing introduced some nitrogen at the Si/oxide interface.
From the point view of Qbd values, the recipe
(O2+N2O+O2) seems to be the best to produce a
good quality oxide. However, its nitrogen peak was
still located at the Si/oxide interface, which was not an
ideal nitrogen pro®le to prevent B penetration. On the
other hand, the Qbd value for the oxide grown via
(N2O+O2+N2O) seems to be not very high, but it
had a better nitrogen pro®le, as shown in Fig. 5, to
prevent B penetration. Hence, if the recipe
(N2O+O2+N2O) is optimized to have improved Qbd
values, it might produce an ideal gate oxide with both
a high barrier to B penetration (more nitrogen at the
oxide/poly-Si interface) and high enough Qbd values as
required by the oxide reliability.
Similar results to that in Fig. 6 have also been
observed when the oxynitride thickness reduced to 35
and 30 AÊ, as shown in Figs. 7 and 8, respectively.
However, the di€erence in Qbd values among oxynitrides grown by di€erent recipes was narrowed. The
reason might be that the nitrogen pro®le was narrowed
to having only one peak in the ultra-thin gate oxide.
The N peak position can be only slightly adjusted by
changing the sequence of O2 and N2O oxidation,
because the oxide ®lm was so thin that it becomes
almost an interface between poly-Si and substrate Si.
However, the di€erence in Qbd values can be still
observed as the oxide thickness was as thin as 30 AÊ, as
shown in Fig. 8. The Qbd value for 35 AÊ gate-oxide
grown via (N2O+O2+N2O) has been much improved
compared with that for 45 AÊ, and was even larger than
that via (O2+N2O+O2), as shown in Fig. 7. The possible reasons are that the two nitrogen peaks in Fig. 5
had been overlapped to one peak when the oxide
thickness scaled down, and that more nitrogen had
been introduced during (N2O+O2+N2O) oxidation
than the (O2+N2O+O2) process when the oxidation
time/temperature reduced.
2003
4. Conclusions
It can be concluded that the nitrogen composition
pro®le in oxides can be engineered through a multistep O2/N2O RTP oxidation. The ideal N pro®le in a
gate oxide, i.e. N peaks at the oxide/poly-Si interface
and tails at the Si/oxide interface, can be obtained by
optimizing the RTO recipe, such as recipe (5) in this
study. The Qbd results for oxynitrides show that its
value increases with the nitrogen incorporation at the
Si/oxide interface, but reduces with the nitrogen incorporation in the oxide bulk or at the oxide/poly-Si
interface. Therefore, there is a balance in the choice of
the Qbd value and the barrier to B penetration.
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