Stack Gate Tec 4 Finfet
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IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 61, NO. 4, APRIL 2014 963
Stack Gate Technique for Dopingless Bulk FinFETsYi-Bo Liao, Student Member, IEEE , Meng-Hsueh Chiang, Senior Member, IEEE , Yu-Sheng Lai,
and Wei-Chou Hsu, Member, IEEE
Abstract— FinFETs have been made successfully for massmanufacturing on bulk and silicon-on-insulator wafers. Whenchoosing the bulk option, additional process steps are needed forsubstrate leakage suppression. Typically, heavy substrate dopingfor punchthrough stopping between the source and drain isused, but precise control of the doping profile to prevent its up-diffusion into the channel has been a challenging task, especiallyfor continuously shrinking device dimension. In this paper,we propose a stack gate structure with doping-free substratewhile punchthrough leakage can be suppressed. The proposedtechnique can be integrated in conventional gate-last high-kmetal gate process. Both polysilicon and metal gates are shownto be feasible in the proposed stack gate based on 3-D TCADsimulation. In addition, the stack gate structure without substratedoping is immune to its random dopant fluctuations.
Index Terms— Bulk FinFET, stack gate, substrate doping.
I. INTRODUCTION
AMONG emerging technologies, nonplanar transistors
such as double-gate FinFETs and trigate transistors have
stepped further into manufacturing [1], [2]. Because of reduced
short-channel effects in the multigate transistors, Moore’s law
continues to be extended from conventional device scaling
without the need of new substrate material. Typically, non-
planar structures are expected to be fabricated on silicon-on-
insulator (SOI) wafers for perfect isolation from substrate [3].
Alternatively, FinFET technology on bulk silicon wafer hasbeen shown to be feasible for volume production [1] though
requiring additional process steps.
Due to the issue of substrate leakage (similar to
punchthrough) in bulk-Si FinFETs, additional substrate doping
(punchthrough stopping, PTS) and thick isolation oxide (about
twice fin height) as in STI are needed [4]–[7]. However,
substrate doping implantation and thick isolation oxide growth
increase technology complexity, and usually require additional
thermal budget. Furthermore, the upward doping diffusion into
Manuscript received February 28, 2013; revised December 2, 2013 andJanuary 27, 2014; accepted February 10, 2014. Date of publication March 4,2014; date of current version March 20, 2014. This work was supported by theNational Science Council of Taiwan. The review of this paper was arrangedby Editor J. C. S. Woo.
Y.-B. Liao and W.-C. Hsu are with the Institute of Microelectronics, Depart-ment of Electrical Engineering, National Cheng Kung University, Tainan 701,Taiwan.
M.-H. Chiang is with the MS Degree Program on Nano-Integrated-CircuitEngineering, Department of Electrical Engineering, National Cheng KungUniversity, Tainan 701, Taiwan (e-mail: [email protected]).
Y.-S. Lai is with the National Nano Device Laboratories, Hsinchu 300,Taiwan.
Color versions of one or more of the figures in this paper are availableonline at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/TED.2014.2306012
Fig. 1. Conventional bulk-Si FinFET structure with (a) 3-D view and (b) 2-Dcross section.
the fin degrades undoped (or lightly doped) channel mobility
and the isolation oxide reduces the effective fin height at a
given aspect ratio (AR). In this paper, we propose an alterna-
tive and yet feasible approach for leakage suppression using
a stack gate structure without the need of additional substrate
doping and excessive oxide growth. We also investigate the
impact of the proposed technique on device performance
and provide an optimal design window using 3-D numerical
simulation.
I I . CONVENTIONAL B UL K-SI F INFE TS
A. Device Structure and Simulation Methodology
The 3-D n-channel bulk-Si FinFET structure and cross
section are shown in Fig. 1(a) and (b), respectively. In our
simulation, the nominal SOI device with undoped (or lightly
doped) channel is based on the 11.9 nm node in ITRS [8] as
follows: L g = 10 nm, W Si = L g /2 = 5 nm, H Si = 12.5 nm
AR = 2.5), fin pitch = 16 nm (1.6 L g), equivalent oxide thick-
ness (EOT) = t ox (top gate oxide/side gate oxide) = 0.62 nm,
T ISO (isolation thickness on bulk-Si substrate) = 0.62 nm,
Na (substrate doping) = 1 × 1015 cm−3. The leakage cur-
rent ( I OFF) was set to meet high-performance specification(100 nA/ µm for V DS = 0.68 V) by gate work function
adjustment (4.35 eV). Uniform doping profile was assumed
in source/drain extension (SDE), and Gaussian doping profile
in 4.2 nm/decade was assumed in bulk-Si substrate for PTS
(if available). The doping depth of source/drain is around
22 nm. Effective channel length ( Leff ) is 11.41 nm, which is
measured between the source and drain when graded doping
concentration reaches 1019 cm−3. Note that, the channel
(or the fin) could be partially doped due to the bottom-up
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Fig. 2. Predicted I DS–V GS characteristics with PTS doping in differentdoping depths with inset showing different PTS doping depths extracted fromtheir Gaussian profiles, respectively.
Fig. 3. Electron distribution in bottom channel ( Z = 0) across the fin atV DS = 0.68 V and V GS = 0 V.
diffusion of PTS doping. Our work investigated the leak-
age issue associated with the bulk substrate and assess the
leakage suppression technique via the 3-D TCAD simulation
with Fermi–Dirac statistics, drift-diffusion transport using the
Philips unified mobility model, and the density-gradient quan-
tization model [9].
B. Leakage Current Issue and Suppression Approaches
When the buried oxide in SOI wafer is replaced with
bulk silicon, the leakage current increases due to excessive
carriers underneath the channel in bulk substrate. Previouswork showed that a leakage current suppression approach
in optimizing isolation oxide and PTS doping under the
channel [6]. Excessive carriers and punchthrough current
can be effectively suppressed by PTS doping. Fig. 2 shows
I DS–V GS characteristics with PTS doping in different doping
depths (T doping ≡ distance for the graded S/D doping profile
to distribute from its peak level to 1018 cm−3) with inset
showing different PTS doping depths extracted from their
Gaussian profiles. Leakage current is significantly suppressed
when doping depth is longer than 17 nm. However, threshold
voltage also changes when PTS doping diffuses into channel.
Fig. 4. Predicted I DS–V GS characteristics for SOI and bulk-Si FinFETs.Inset: predicted I OFF versus T ISO.
Fig. 3 shows the electron distribution in bottom channel
across the fin. The highest electron density occurs in the case
without any PTS. The electron density decreases as the PTSdoping depth increases. Fig. 4 shows I DS–V GS characteristics
for SOI and bulk-Si FinFETs with thick isolation oxide with-
out PTS. The predicted I OFF is still too high though it seems
to be limited when isolation oxide is thicker than 30 nm, as
shown in inset. The substrate PTS doping is most effective for
limiting equivalent I OFF to that of SOI. However, PTS requires
complicate and precise control of substrate doping profile
and additional thermal budget such that the universal process
for different applications on a chip could not be used [10].
The deep substrate implementation is usually not practical for
advanced nanometer-scale devices. A simplified and effective
process for leakage current suppression technique is needed.
III. PROPOSED S TACK G ATE T ECHNIQUE
In this section, we propose a stack gate structure with a
bottom layer of polysilicon gate while leaving the substrate
undoped. Such technique is shown to effectively suppress
the excessive carriers and hence reduces leakage current in
substrate.
A. Stack Gate Structure and Conceptual Process
Based on the previous discussion, the excessive carriers
underneath the channel in the substrate are the main source
causing the leakage current. If we can find some way to
reduce the electron density underneath the channel, I OFF is thengreatly lowered. To achieve this, two types or gate material
are adopted in a stack structure [Fig. 5(f)]. The additional
bottom gate can be made of p-type polysilicon (or equivalent
metal gate) for nFETs with finite T BG. Using this structure, the
substrate region close to the fin can be in a near-accumulation
condition simply by adjusting gate work function.
Based on high-k metal-gate process (HKMG) [1], [11], the
stack gate process is compatible with gate-last approach
(Fig. 5). In etching process for silicon fins, requirements of
actual fin height ( H Si), isolation oxide (T ISO), and bottom gate
thickness (T BG) must be considered as they are all related in
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LIAO et al.: STACK GATE TECHNIQUE FOR DOPINGLESS BULK FinFETs 965
Fig. 5. Conceptual process flow of the stack gate bulk FinFET.
Fig. 6. Predicted I DS–V GS characteristics with different T BG’s at(a) V DS = V DD and (b) V DS = 50 mV. Inset: electron distribution withstack gate structure.
technology. Following the fin formation, isolation oxide and
dummy poly-Si gate are formed and patterned. After S/D for-
mation, the dummy poly-Si gate should be removed. To avoid
potential damage on the high-k dielectric, one shall use high-
k last process (as used in gate-last HKMG process) such that
the high-k dielectric is formed after removal of dummy poly-Sigate. Finally, dual-metal gate is formed, resulting in two layers
of gate. An interfacial cap layer [12], [13] could be employed
after high-k dielectric deposition to prevent the damage during
the etch back step.
B. Stack Gate With Polysilicon Gate
Using our proposed stack gate (or gate engineering) with
thick isolation oxide (T ISO = 30 nm), Fig. 6(a) and (b) show
I DS–V GS characteristics with different T BG’s, as compared
with conventional bulk-Si and SOI counterparts. The stack gate
is advantageous in leakage current suppression because the
Fig. 7. Leakage current versus bottom gate thickness (T BG) in differentisolation oxides (T ISO) with labeled SOI for reference.
p-type-doped (NPoly−Si = 1019 cm−3) bottom gate lowers
the electron density due to improved electric field control
over the lower fin region. Leakage current is suppressed
significantly, and the proposed bulk FinFET can now com-pete with SOI when T BG is thicker than 10 nm. Note that,
the bottom gate adds up to the etched fin height ( H etch =
H Si + T BG + T ISO) while the fin height is restricted in an
available fin aspect ratio. We will discuss the design window
regarding H Si, T BG, and T ISO for the stack gate later. Inset of
Fig. 6(b) shows electron distribution with T ISO of 30 nm and
T BG of 10 nm. Excessive electron carrier density in lower fin
is suppressed by the bottom gate. To include T BG in the total
etched fin height ( H etch) without reducing the channel height
( H Si), a thin isolation oxide is needed in stack gate structure.
Fig. 7 shows the predicted I OFF versus T BG characteristics
in different T ISO’s with stacked poly-Si and metal gates. The
thickness of bottom gate plays an important role in suppressing
leakage current whereas the thickness of isolation oxide does
not affect the leakage current significantly. As the substrate
leakage suppression is not sensitive to isolation oxide thickness
variation, the required fin etching is flexible in our proposed
technique.
C. Stack Gate With Metal Gate
Besides poly-Si gate, metal gate can be also used in pro-
posed stack gate. In Fig. 7, bottom metal gate (electrode)
is included with assumed work function near valence band
(5.29 eV), which is equivalent to p-type poly-Si gate. Leakage
current suppression is even more effective than the case withpoly-Si gate because polysilicon-gate depletion does not occur
in metal gate. Besides, wide range of isolation oxide thickness
is acceptable, enabling the use of thin T ISO for the concern of
required fin height. Note that, the isolation oxide thickness
could influence intrinsic gate capacitance [6].
Since the bottom gate thickness and isolation oxide may
vary in actual processing, it is essential to assess I OFF sensi-
tivity to T BG. Comprehensive analyses of leakage suppression
with stacked polysilicon and metal gates suggest that any T BG
thicker than 10 nm is appropriate. The fin height requirement
is comparable with that of conventional bulk FinFETs because
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Fig. 8. Leakage current versus bottom-gate work function withT ISO = 20 nm.
thick isolation oxide is not necessary in stack gate structure
with T BG of 10 ∼ 12 nm. To reach maximum available fin
height, the isolation oxide should be controlled at 20 nm or
below. Based on the proposed approach, the bulk FinFET
technology without the need of extra substrate doping is
feasible.
IV. DESIGN I NSIGHT AND D ESIGN M ETHODOLOGY
As the stack gate structure can be a solution to resolve the
common leakage issue in bulk FinFETs, another question is
what range of the gate work function is necessary. In addition,
considering random dopant fluctuations (RDFs), is the stack
gate free of RDFs without PTS? To answer these questions, in
this section, we discuss in depth and analyze the mechanism
of leakage suppression in the stack gate structure.
A. Design Insight in Gate Work Function
High gate doping is required in polysilicon gate to reach
metal-like properties with a desired gate work function sub-
jected to its type (n or p). For the metal gate, setting a proper
work function of bottom gate (BG) is crucial. Fig. 8 shows
the leakage current versus gate work function of bottom gate
with T ISO = 20 nm. Leakage suppression is insensitive to
T ISO variation and the work function is not necessarily near
valance band as suggested by p-type polysilicon gate. For
the complementary p-channel FET with gate work function
of 4.92 eV (midgate), the stack gate approach also shows
similar leakage suppression capability. Thus, a single midgatematerial for the bottom gate could be applied for both n- and
p-FET. On the other hand, two complementary top gate
materials are needed in stack gate structure as in conventional
CMOS technology. Several gate material options such as TiN
(4.6 eV), TaN (4.0 eV), WN (4.5 eV), and MoN (5.0 eV)
are available, which are also covered in the range for bottom
gate material. Besides, the gate work function could be varied
by material properties including orientation, grain size, and
process [15].
To understand the insight into the impact of the bottom
gate, one can assume that the stack gate is composed of two
Fig. 9. (a) 2-D cross section along z − y. (b) Predicted I DS–V GS character-istics of stack gate structure with T ISO = 20 nm.
separated structures: a pure SOI FinFET on top with uniform
S/D doping (Tr-1) and a bulk-Si FinFET down below with
Gaussian S/D doping (Tr-2), as shown in Fig. 9(a). Fig. 9(b)
shows I DS–V GS characteristics with 20 nm isolation oxide.
The leakage current of stack gate structure is modeled with
two weighted transistors. Higher leakage current of the two
transistors dominates the total leakage current. Fig. 9(b) shows
the relationship between Tr-1 and Tr-2. The Tr-2 representsconventional planar MOSFETs doping profile. In the bulk
FinFET portion (Tr-2) without PTS doping, the punchthrough
issue is inevitable. Additional substrate doping is needed to
reduce leakage. As also can be observed in Fig. 9(b), when
the work function of bottom gate (BG) increases, the leakage
current from the Tr-2 portion of the stack gate is suppressed
while another Tr-1 portion provides SOI FinFET-comparable
drive current.
Note that, the intrinsic capacitance (C gg) could be affected
when we use PTS doping or additional bottom gate. The
gate capacitance (2.24 × 10−17 F) of stack gate structure
(T BG = 8 nm) is higher than that of PTS bulk FinFETs
(1.83 × 10−17 F). Thus, the intrinsic delay of stack gate
structure is 23.3% worse than that of PTS bulk FinFET, as
listed in Table I. However, the intrinsic delay of the proposed
stack gate could still be improved by changing T BG, as shown
in Fig. 10.
B. Impact of RDFs
RDFs play an important role when transistor size scales
down to nanometer regime [16]. Impact of RDFs has been
evaluated via 3-D atomistic simulation [9]. FinFET design
in RDFs has also been proposed [17]. Fig. 11(a) and (b)
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TABLE I
COMPARISON OF D IFFERENT L EAKAGE S UPPRESSION APPROACHES
Fig. 10. Intrinsic delay versus bottom gate thickness.
Fig. 11. Predicted I DS–V GS characteristics with RDFs for (a) stack gatestructure and (b) single gate with PTS doping.
shows I DS–V GS characteristics for stack gate structure and
single gate case with PTS doping in 100 samples with
RDFs, respectively. The PTS-doped bulk FinFET is more
degraded as the additional random doping source comesfrom PTS.
Fig. 12 shows I ON– I OFF plot of the 100 samples with RDFs.
Stack gate structure shows much less variation from RDFs as
σV t is much smaller than that of the PTS-doped counterpart.
Improved immunity to RDFs is another advantage of the
stack gate approach. Fig. 13 shows comparison of I DS–V GS
characteristics using different leakage suppression approaches.
The leakage current of the case (Bulk, T ISO = 30 nm) with
undoped substrate and thick isolation oxide is not sufficiently
suppressed. Stack gate structure not only shows great leakage
suppression as PTS-doped approach does, but also provides
Fig. 12. Statistical variation in I ON and I OFF for different structures.
Fig. 13. Comparison of I DS–V GS characteristics using different leakagesuppression approaches.
steeper subthreshold slope and comparable drive current due
to better gate controllability as the lower part of the fin is
surrounded by bottom gate.
V. CONCLUSION
A stack gate structure was proposed to suppress leakage
current effectively without extra doping and without com-
promising the effective fin height. The proposed technique
is compatible with CMOS applications when using gate-last
HKMG process. Both polysilicon and metal gates were shown
to be feasible in the proposed stack gate. Furthermore, I OFF is
comparable with the case on SOI substrate. By properly
choosing the work function of bottom gate, I OFF can be even
lower than that of the SOI counterpart. Since PTS doping is
not needed in the proposed technique, good immunity to RDFs
was shown.
ACKNOWLEDGMENT
The authors are grateful to the National Center for
High-Performance Computing, National Chip Implementation
Center and National Nano Device Laboratories for technical
support.
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Yi-Bo Liao (S’12) received the B.S. and M.S.degrees in electronics engineering from the NationalIlan University, I-Lan, Taiwan, in 2006 and 2008,respectively. He is currently pursuing the Ph.D.degree with the Department of Electrical Engineer-ing, Institute of Microelectronics, National ChengKung University, Tainan, Taiwan.
Meng-Hsueh Chiang (S’97–M’01–SM’07)received the B.S. degree in electrical engineeringfrom the National Cheng Kung University, Tainan,Taiwan, and the M.S. and Ph.D. degrees in electricaland computer engineering from the University of Florida, Gainesville, FL, USA, in 1992, 1995, and2001, respectively.
He is a Faculty Member with the Departmentof Electrical Engineering, National Cheng KungUniversity.
Yu-Sheng Lai received the B.S. degree in electricalengineering from Fun Chia University, Taichung,Taiwan, and the M.S. and Ph.D. degrees from theInstitute of Electronics Engineering, National TsingHua University, Hsinchu, Taiwan, in 1995, 1997, and2007, respectively.
He is with the National Nano Device Laboratories,National Applied Research Laboratories, Hsinchu.
Wei-Chou Hsu (M’87) received the Ph.D. degreefrom the National Cheng Kung University, Tainan,Taiwan.
He is with the Department of Electrical Engi-neering, Institute of Microelectronics, AdvancedOptoelectronic Technology Center, and the Collegeof Electrical Engineering and Computer Science,National Cheng Kung University.