IEEE ELECTRON DEVICE LETTERS, VOL. 22, NO. 9, SEPTEMBER 2001
447
Nanoscale Ultrathin Body PMOSFETs With Raised
Selective Germanium Source/Drain
Yang-Kyu Choi, Daewon Ha, Member, IEEE, Tsu-Jae King, Senior Member, IEEE, and Chenming Hu, Fellow, IEEE
Abstract—Nanoscale ultrathin body (UTB) p-channel MOSFETs with body thickness down to 4 nm and raised source and
drain (S/D) using selectively deposited Ge are demonstrated for
the first time. Devices with gate length down to 30 nm show
high drive current, low off current, and excellent short-channel
behavior. Mobility enhancement and threshold-voltage shift due
to the quantum confinement of inversion charge in the ultrathin
body are investigated.
Index Terms—Mobility enhancement, nano-CMOS, quantum
confinement of inversion charge, raised S/D, selective Ge,
threshold voltage shift, ultrathin body.
Fig. 1.
TEM cross section of a 3 nm ultrathin body (UTB) MOSFET.
I. INTRODUCTION
A
N ULTRATHIN body (UTB) MOSFET with raised source
and drain (S/D) structure has been proposed to suppress
short-channel effects and improve device performance [1]–[3].
Selective deposition of Ge is an attractive technique for producing raised S/D because germane provides an in-situ clean
for the removal of native oxide [4]. In addition, a low thermal
budget is adequate to activate dopants in Ge, which is attractive for compatibility with future high- gate-dielectric materials and metal gate. The raised Ge S/D process is fully CMOS
compatible and compared to previous raised S/D process[1], it
is much simpler and results in lower parasitic overlap capacitance.
The ultrathin body and adjacent silicon dioxide (gate and
buried oxide) layers form a thin potential well, which causes
subband splitting (between two-fold and four-fold valleys of the
conduction band and between light and heavy hole subbands
of the valence band) [5]. As a result, mobility enhancement is
expected in n-channel and p-channel UTB devices when the
channel thickness is reduced below the inversion-layer thickness [5], [6].
The quantum confinement also results in a smaller density of
states, so that more energy-band bending is required to attain a
desired inversion-charge density as compared to a bulk-Si device. Thus, an increase in threshold voltage is also expected for
ultrathin body devices [7], [8]. In this work, UTB raised Ge S/D
MOSFETs are demonstrated to yield excellent short-channel
performance, for p-channel devices with gate length down to
Manuscript received May 3, 2001; revised June 19, 2001. This work was supported in part by the DARPA AME Program under Contract N66001-97-1-8910
and the SRC under Contract 2000-NJ-850. The review of this letter was arranged
by Editor A. Chatterjee.
The authors are with the Department of Electrical Engineering and Computer Science, University of California, Berkeley, CA 94720 USA (e-mail: ykchoi@eecs.berkeley.edu).
Publisher Item Identifier S 0741-3106(01)07740-0.
30 nm. The effects of quantum confinement on carrier mobility
and threshold voltage are studied.
II. DEVICE FABRICATION
The process flow used in this work is similar to that used for
UTB NMOSFETs reported in [2] except that TiN capped with
poly-Si was used as the gate electrode to provide a mid-gap
was
work-function in the present work. Body thickness
measured with Nanometrics 210 XP Scanning UV and calibrated with TEM. After gate patterning and sidewall spacer formation, 50 nm Ge was selectively deposited onto the ultrathin
Si in the S/D areas by conventional LPCVD to achieve the selfaligned raised S/D structure shown in Fig. 1. The S/D regions
cm at 15 keV) folwere doped by ion implantation (
lowing by a 650 C, 20 s RTA. Selectively deposited Ge, 3 nm
ultrathin body, 2.1 nm gate oxide, and 3 nm TiN metal gate can
be seen in the TEM micrograph in Fig. 1.
III. RESULTS AND DISCUSSION
Fig. 2 shows the – characteristics of a 30 nm
device
cm . The
with 4 nm -type doped UTB
V. A more negative (on
threshold voltage, is
nm) is obtained because of
shift caused by quantum confinement of inversion charge with TiN gate. High drive current
A m at
V is seen in Fig. 2(a), and
pA m is seen in Fig. 2(a). Fig. 3 shows
low off current
V
subthreshold swing and DIBL
V versus
where typical
is 30 nm to 190 nm
is 4 nm to 8 nm. Clearly
is an important factor
and
is larger than
determining the short-channel effect. If
4, the UTB device can meet the subthreshold swing criterion,
mV/dec, and DIBL
V V.
The effective hole mobility was extracted from the linear
–
data with a compensation of series resistance, from
several devices at each body thickness. The higher mobility
0741–3106/01$10.00 © 2001 IEEE
448
IEEE ELECTRON DEVICE LETTERS, VOL. 22, NO. 9, SEPTEMBER 2001
Fig. 4. Effects of quantum confinement on carrier mobility and threshold
voltage. (a) Extracted hole mobility versus UTB thickness and (b) measured
threshold voltage shift for NMOS and PMOS devices versus UTB thickness.
Measured UTB MOSFET. (a) I –V and (b) I –V characteristics, for
L = 30 nm and T = 4 nm.
Fig. 2.
were due to the change of dopant charge in the body, the NMOS
and PMOS curves in Fig. 4 would occur in the same direcfor
tion—but this is clearly not the case. The increase in
NMOS and PMOS devices is therefore attributed to the quantum
confinement of inversion charge.
IV. CONCLUSION
Fig. 3.
Dependence of subthreshold and DIBL on L =T
ratio.
is obtained with lower body doping concentration and higher
drive current with a relatively thick gate oxide (2.1 nm) in
nm regime. Fig. 4(a) shows the hole-mobility dependence on UTB thickness with the maximum, minimum and
average mobility values indicated. The data show that mobility
down to 5 nm, and then
decreases with body thickness
increases as the body thickness decreases further. This trend
is similar to that expected for n-channel UTB FETs [4]. The
UTB thickness starts to limit the extent of the wave function
corresponding to the heavy hole band when the inversion-layer
nm, so
thickness is decreased in the range nm
that the mobility gradually decreases. The enhancement in
nm is caused by an increase in
mobility seen for
the fraction of light holes in the lowest subband, which has
higher mobility than the other subbands. Since the quantization
( ,
), the bands are
energy is proportional to
decreases. It results in
shifted away from each other as
decrease in effective mass and inter-bands scattering rate and
an enhancement in mobility.
Fig. 4(b) shows the dependence of threshold voltage on
for n-channel (NMOS) and p-channel (PMOS). If the
shifts
Nanoscale ultrathin-body p-channel MOSFETs with gate
length down to 30 nm and the body thickness down to 4 nm
have been successfully fabricated and they show excellent device performances. A selective Ge deposition process is used to
form self-aligned raised S/D regions with low thermal-budget.
Enhancement in hole mobility is experimentally observed for
body thickness below 5 nm. Increases in threshold voltage due
to the quantum confinement effect of inversion charge with
decreasing body thickness are observed experimentally. The
threshold voltage depending on body thickness must be taken
into account in the design of UTB devices, including thin-body
double-gate MOSFETs.
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