Download Improvement of Ultra-Thin Gate Oxide Reliability Using Fluorine and

亞 東 學 報
2006 年 5 月
亞 東 技
第 26 期
頁 7~12
術 學 院
Improvement of Ultra-Thin Gate Oxide Reliability Using Fluorine and Nitrogen
Implantation
Cheng-Chuan Huang
Chien-Nan Lee
Department of Electronics Engineering
Abstract
can improve the oxide reliability.
It has been reported that incorporation of fluorine in
The effects of fluorine and nitrogen incorporation
were
the gate oxide can improve device resistance to channel
investigated by implanting fluorine and nitrogen into
hot carrier degradation or radiation damage [3,4]. The
Poly gate or Si substrate. The transconductance (Gm)
effect of fluorine on gate dielectric integrity has also
and subthreshold swing (S.S) of fluorinated devices were
been studied. These studies have shown that fluorine
better than that of nitrided devices. For area dependence
incorporation in the gate dielectric provides no
of charge-to-breakdown, the fluorine atoms pile up at the
significant improvement on time-dependent dielectric
Poly-Si/SiO2
breakdown
on
ultra-thin
gate
oxide
interface
may
integrity
be
(GOI)
responsible
for
(TDDB).
Gate
oxide
with
also
exhibits
a
slightly
degradation in large gate area devices. It is observed that
incorporation
fluorine and nitrogen implantation into Si substrate prior
charge-to-breakdown
to oxidation can be used to obtain multiple oxide
unfluorinated gate oxides.
(Qbd)
value
fluorine
relative
lower
to
In the previously mentioned studies, fluorine was
thickness.
incorporated into the gate dielectric by various
Keyword: fluorine and nitrogen implantation、ultra-thin
techniques such as low energy fluorine ion implantation
gate oxide integrity、fluorinated, nitrided oxide
(reduce implantation damage) into the poly-silicon gate
、area dependence of charge-to-breakdown.
and driving in or dipping in HF solution before gate
oxide growth. For the tungsten polycide process, fluorine
I. Background and Motivation
is inadvertently introduced into the gate oxide from
chemical vapor deposition (CVD) W or WSi deposition
Down scaling of CMOS technology into sub-0.1 um
using WF6 gas.
regime requires high quality gate dielectric. Recently, as
system-on-chip (SOC) becomes the popular future trend
For ultra-thin gate MOSFETs have been realized at
of ULSI technologies, several studies have focused on
below 1.5 nm oxide thicknesses, a major obstacle to
the growth of multiple oxide thickness on a wafer, as
overcome is the high level of direct tunneling current
well as on improving gate oxide integrity (GOI) by
through the gate. One possible solution is to incorporate
implantation technique[1,2]. The nitrided and fluorinated
the use of a high-k film in the gate dielectric. High-k
oxides have received particular research interests since it
films allow the use of a physically thicker film while
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亞東學報 第 26 期
acting electrically as a thin dielectric. Silicon nitride is
with increase nitrogen dosage. On the other hand, the
an excellent choice for substitution in the gate stack
oxide thicknesses increase with fluorine incorporation
since it provides almost double the dielectric constant of
into Poly-Si gate or Si substrate. The variation of oxide
oxide. The nitrogen implantation into the gate electrode
thickness is large in this experiment. We use
is one method to obtain the nitride oxide [5].
Ellipsometer and F-N current fitting methods to
Under optimum design, nitrogen implantation into
determine the gate oxide thickness.
Poly-Si gate is effective in suppressing boron penetration
C-V Measurement: In the fluorine and nitrogen
without degrading performance of either p- or n-channel
incorporation into gate oxide and Si substrate. It will be
transistors.
caused oxide thickness variation. We use capacitance
and voltage measurement to monitor these change within
II. Experiment Description
the gate oxide.
MOS
n-MOSFET were
I-V Characteristics: The parameters of MOSFET can be
fabricated on 6-inch (100) oriented p-type Si wafers. The
extracted from the I-V curve. We define the threshold
process steps are described as below: (1) Well Formation:
voltage at Idsat = 0.5 uA per unit width (um), and linear
standard RCA cleaning; grow pad oxide at 925¢J in
transconductance Gm (Gm = dId/dVg), subthreshold
furnace; P-well implant BF2. (2) LOCOS oxidation:
swing S.S (S.S = 2.3*Id*dVg/dId) for Vd=0.1 V.
grow pad oxide; deposit nitride by LPCVD; define
TDDB (Time-Dependent Dielectric Breakdown): The
LOCOS pattern by RIE etch; channel stop implant; field
dependence of charge-to-breakdown under a constant
oxidation; remove the oxide by BOE; nitride stripped off
current injection ( Qbd ) is one of the TDDB reliability
by H3PO4. (3) Gate oxide fabrication: grow 30 nm
testing result. We use Qbd test to evaluate the fluorinated
sacrificial oxide; First Ion-implantation: (i) Threshold
and nitrided gate dielectrics. Qbd are defined as the
voltage adjustment BF2; (ii) Anti-punch through B; (iii)
product of gate injection current density and the time
F and N implant in the Si substrate. Remove sacrificial
until the voltage suddenly drops.
oxide by HF dip; gate dry oxide growth 4nm. (4) Poly-Si
z
The
capacitors
and
Results and Discussion
gate deposition: 200nm undoped polysilicon deposit by
We grown ultra-thin gate oxide with fluorine and
LPCVD; Second Ion-implantation: F and N implant into
nitrogen implantation into Poly-Si gate and Si substrate.
Poly-Si gate; define gate pattern. (5) Poly gate and
We observe clearly that the nitrogen implant dose
source/drain implantation. (6) Thermal activation by
increases, the resulting oxide thickness is reduced. On
furnace and RTP. (7) Contact hole: TEOS layer deposit
the contrary, oxide thickness increases with fluorine
by LPCVD; contact hold define. (8) Metallization: 1um
implant dose increase. Fig. 1 and Fig. 2. show gate oxide
Al-Si-Cu alloy deposit by PECVD; metal pads pattern
thickness (Tox) for O2 oxides as a function of fluorine or
define; metal sintering.
nitrogen dose. Note that we denote samples that received
substrate implant of fluorine or nitrogen with 1E14 or
III. Results and Discussion
z
1E15 cm-2 dose as FS1E14, NS1E14, FS1E15 and
NS1E15. And gate implant as FG1E14, NG1E14,
Measurement Techniques
Gate Oxide Thickness: The oxides grown on nitrogen
FG1E15 and NG1E15. From Fig. 1, only a small Tox
implanted Si substrate, the oxidation rates were reduced
increase is induced for FG samples. While for substrate
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Improvement of Ultra-Thin Gate Oxide Reliability Using Fluorine and Nitrogen Implantation
implant, Tox increase about 0.4 nm for FS1E15. From
This may be attributed to the substrate implantation that
Fig. 2, for NG sample, no noticeable Tox change is
received more implanted damage and too much fluorine
observed even with a large dose (1E15 cm-2). Bur for
will degrade the oxide reliability. Fig. 9. shows the
substrate implant, nitrogen is more effective in
proposed trapping mechanism for fluorine-enriched
suppressing Tox. Tox decreases by 0.4 nm for NS1E14
oxide. Fluorine displaces oxygen in Si-O-Si bond and
and 1.3 nm for NS1E15. Fig. 3. is the comparison of Tox
the dangling bond on the silicon atom acts as a trap.
using C-V and F-N current fitting methods; the trend of
Since the dangling bonds at the both interface would
increment and decrement of Tox is identical with C-V
cause interfacial traps. Bonding an nitrogen or fluorine
and I-V measurements. The high frequency C-V plot is
atoms to the dangling bond would move the state at least
shown in Fig. 4. It is clearly that different Tox show
0.5 eV outside the silicon bandgap. Because of the
different high frequency capacitance (Ch) values in
stronger bonding energy of the Si-F (5.73 eV) bond and
accumulation region. Fig. 5. shows proposed mechanism
Si-N bond compared to the Si-H bond (3.18 eV),
for additional oxide growth for fluorine-enriched oxide.
fluorine bonds at the Si-SiO2 interface should be more
First, F diffuses and bonds to dangling bonds and
immune to hot electrons.
weakened bonds in the silicon dioxide. After these
In the study, the fluorine and nitrogen implantation
interface regions have been saturated with F, additional
into gate electrode or substrate. Those will affect the
incorporation occurs primarily in the bulk. The F will
interface defect. It is interesting to discuss the area
then break the Si-O bonds and displace oxygen at these
dependence of
sites. Lastly, the free oxygen diffuses to the interfaces
oxides. Fig. 10. depicts the 50%-value of the intrinsic
and oxidizes additional silicon. The mechanisms of the
Qbd distribution as a function of the test gate area for
oxidation retardation, the Si-N bonds are formed at the
NG1E15 sample, the stress current is J= +0.3A/cm2,
first few monolayers of the thin gate oxides, which might
substrate injection. It is observed that for nitrided oxide,
be able to explain the large oxidation retardation.
the Qbd decreases with gate area increases and the trend
Qbd in the fluorinated and nitrided
Fig. 6. and Fig. 7. depicts the Gm and S.S. for
is the same with control sample. From Fig. 11. the Qbd
nMOSFETs with different implanted species and dose.
degradation of fluorinated oxide in large gate area device
The FG1E14 and NG1E14 show smaller Gm than pure
was worse than that in control sample. The exist of
oxide; fluorinated oxide is more effective in improving
fluorine atoms in large gate area devices may be the
Gm than nitrided oxide. The nMOSFETs with fluorine
factor of degrading Qbd value. We combine the effect of
and nitrogen implantation also have slight improvement
fluorine and nitrogen in large gate area, as shown in
in S.S.
Fig. 12. The Qbd shows less degraded. This is because
In order to examine the reliability of fluorinated and
for FGNS1E14 sample, it could be attributed to
nitrided oxide, we use the constant current stress
retardation of fluorine incorporation by nitrogen already
measurement to evaluate it. Fig. 8. depicts the charge to
present in the oxide for NS1E14 sample. So the
breakdown distribution as a function of implanted
concentration of fluorine of FGNS1E14 is lower than
species and dose. FG1E14 and NG1E14 show larger
that of FS1E14 sample. Previous results were stressing
Qbd. While FS1E14 and NS1E14 degrade Qbd. The
under the substrate injection that the damage should be
degradation of Qbd seen to more worse in FS1E15.
occurred at the Poly-Si/SiO2 interface. While for gate
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亞東學報 第 26 期
injection condition, the damage occurred at SiO2/Si
Fig. 2. Gate oxide thickness for O2 oxide as a function
interface, the area dependence of Qbd is not obvious for
of nitrogen dose.
FGNS1E14 sample, as shown in Fig. 13. We conclude
that fluorine pile up at the Poly-Si/SiO2 interface may be
responsible for Qbd degradation.
IV. Conclusion
We have studied the effect of fluorine and nitrogen
incorporation into poly gate and Si substrate on ultra-this
gate oxide integrity. The Gm and S.S. in fluorinated and
nitrided oxide were better than that of control samples.
Fig. 3. Gate oxide thickness for O2 oxide as a function
The fluorine at the Poly-Si/SiO2 interface may be
of fluorine and nitrogen dose. The data was obtained by
responsible for Qbd degradation in large gate area
C-V and F-N current fitting method.
devices. The clever manipulation of fluorine (to increase
Tox) and nitrogen (to decrease Tox) implantation into Si
substrate prior to oxidation can be used to obtain
multiple oxide thickness on the wafer.
Fig. 4. The high frequency C-V curve of samples with
fluorine and nitrogen incorporation into Si substrate.
Fig. 1.Gate oxide thickness for O2 oxide as a function of
fluorine dose.
Fig. 5. Proposed mechanism for additional oxide
growth for fluorine-enriched oxide. (a) F bonds to
dangling bonds at the interface, and weak bonds in the
bulk of the oxide. (b) Displace oxygen diffuses to the
interface and grows additional oxides.
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Improvement of Ultra-Thin Gate Oxide Reliability Using Fluorine and Nitrogen Implantation
Fig. 6. The transconductance (Gm) for samples with
Fig. 9. Proposed trapping mechanism for fluorine
fluorine and nitrogen implantation into the gate and Si
enriched oxide. (a) F displaces an oxygen in an Si-O-Si
substrate.
bond. (b) The dangling bond on the silicon atom acts as a
trap.
Fig. 7. The subthreshold swing (S.S.) for samples with
fluorine and nitrogen implantation into the gate and Si
Fig. 10. Area dependence of 50% Qbd value for nitrogen
substrate.
implanted the gate electrode. The stress current density
was J=+0.3 A/cm2.
Fig. 8. Charge to breakdown distribution for samples
with F and N implantation into the gate electrode and Si
Fig. 11. Area dependence of 50% Qbd value for
substrate.
fluorinated oxides. Stress current density J=+0.3 A/cm2.
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亞東學報 第 26 期
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Fig. 13. Area dependence of 50% Qbd value for fluorine
Cheng-Chuan Huang, “A study of Fluorine and
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利用氟與氮的離子佈植來改善超薄氧化層的可靠度
黃正權
李建南
電子工程系
摘要
利用離子佈植的方式將氟與氮植入到閘極氧化層以及矽基座，以探討氟與氮對閘極氧化層可靠度之效應。
在研究中發現，有氟摻雜在閘極氧化層的元件，其在電導、次臨界擺幅的表現上都比一般元件有些許改善。在
電荷累積崩潰分布的面積效應上，發現氟原子累積在多晶矽層與閘氧化層的介面之間，是造成大面積元件其電
荷累積崩潰分布嚴重衰退的原因。另外將氟與氮植入到矽基座再成長氧化層，可同時得到不同的氧化層厚度。
關鍵字：氟與氮離子佈植、超薄氧化層可靠度、氟化與氮化氧化層、電荷累積崩潰分布的面積效應
12