Requirements and Challenges in Ion Implanters for Sub100nm CMOS Device Fabrication
Ukyo Jeong, Zhiyong Zhao*, Baonian Guo, Gongchuan Li, and Sandeep Mehta
Varian Semiconductor Equipment Associates,
35 Dory Road, Gloucester, MA 01930, USA
*AMD,
5204 East Ben White Blvd., M/S 608, Austin, TX 78741, USA
Abstract. As CMOS technology moves into sub-100nm regime, significance of non-planar structure effects have grown.
Traditional ion implant performed at off-critical angle exhibits shortfalls in device integration and performance merits.
Modern ion implanters have evolved to face challenges of on-axis implants. Ion beam incident angle control is one
essential requirement to manage the challenges. This paper describes number of implant applications in modern CMOS
fabrication on which required precision in angle control is estimated based on device measurement and TCAD
simulations.
Ion implant technology is also an area that received
an impact of non-planar structure effects. Ion beam
shadowing at the pattern edges of photoresist (PR)
mask or gate structure has become a limitation of
device scaling. Many of the ion implant applications
has moved to zero tilt process to avoid the non-planar
structure effects. This imposes great technical
challenges on ion implanter system technology.
INTRODUCTION
Until the CMOS device scale entered 0.25-micron
geometry, ion beam incident angle was not considered
as a critical factor in process control. The vertical
dimensions of CMOS device were small enough
compared to the lateral dimensions. The idea of
"planar" technology was that semiconductor circuit
could be printed onto a silicon wafer as if letters are
printed on papers. The heights of the device structure
were not an important factor in process, as if thickness
of the letter is not considered in paper printing. The
"planar" technology was one key aspect of CMOS
technology that enabled the success of semiconductor
today. However as a result of continued scaling in the
lateral dimensions of the device, modern CMOS
technology is no longer "planar" in its literal means.
Since many of the vertical dimensions were not
scalable or not as fast as lateral dimensions, continued
scaling was led to large aspect ratio of the devices.
MEDIUM CURRENT IMPLANTER
In traditional ion implant applications, most of the
Medium current (MC) ion implant process was
performed at tilt angle of the "magic" 7° with a range
of rotation angle between 22° and 45°. Lots of efforts
were put into process optimization of tilt and rotation
angles. As a result, depend on ion implanter systems
and the semiconductor devices, every fab seemed to
have their own best known set of angles that is known
to give best doping non-uniformity or best device
yield. The reason there is a better set of angles that the
other is because the control of the beam incident angle
is not perfect on implanter systems. In other words, if
ion beam angle is perfectly controlled without any
variation, any set of angles should give an equally
good doping uniformity.
The non-planar device structure effects impacted
many of the semiconductor process technology.
Damascene process using Chemical Mechanical
Polishing (CMP) planarization is an exemplary
technique
that
revolutionized
semiconductor
fabrication technology in the non-planar process era.
CP680, Application of Accelerators in Research and Industry: 17th Int'l. Conference, edited by J. L. Duggan and I. L. Morgan
© 2003 American Institute of Physics 0-7354-0149-7/03/$20.00
697
There are many reasons why angle control on
implanter systems is not perfect. Spinning disk scan
system is one large source of angle variation [1]. Beam
transportation with imperfect beam optics is another
source. Limits in mechanical precision of the tool
assembly are also a contribution. The "magic" set of
angles with 7° tilt was popular because it is not only
non-channeling angles but also one of the most
forgiving set of angles that are insensitive to angle
variations comes from those sources.
1600
ThermaWave units
1500
PR
p+ LOCOS
n+
1300
1200
1100
1000
900
800
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
Off-axis angle [degrees]
However, no matter how good doping uniformity it
can get with tilted implant, any beam incident other
than normal angle, casts shadow at the pattern edges
that causes a shift of the doping patterns. Well
isolation spacing is one of the direct impact of the ion
shadowing by non-zero tilt implant especially for those
have high space frequencies of alternating p and n well
layout such as fast SRAMs [2] and logic chips. Fig.1
illustrates the effect of implant tilt angle to the well
isolation distance.
PR
1400
Figure 2. TWU responses as a function of small off-axis
angle variations. Data were obtained from P+, 800keV,
3x1013cm-2 implants.
Figure 3 is SIMS doping profiles from the wafers
used for implants in Figure 2. The TWU changes
depicted Figure 2 is indeed due to the changes in peak
doping concentrations and depth of the profiles.
Halo doping has become one of the most critical
doping applications in scaled CMOS. It is usually
performed by large angle tilt implants using gates as
self aligned implant masks. The purpose of halo is to
form laterally non-uniform doping profile across the
channel to provide a counter measure to the threshold
voltage roll-off. As device scaled plunged into a severe
short channel region, halo became the most
responsible process in device parameter control.
PR
STI
Figure 1. Beam shadow of angle increase and well-to-well
isolation spacing as device geometry scales down
1.0E+18
The easiest way of avoiding implant shadowing
and pattern shift is completely eliminating the tilt
angle of the implant, namely zero tilt. However, true
zero tilt implant brings unprecedented challenges to
ion implanter design. A zero tilt implant to a <100>
wafer is directed to <001> axial channeling where ion
channeling behavior becomes extremely sensitive to
small variations in incident angle. Angle variations in
traditional implant systems are far excess of allowable
angle variation limit that meets doping uniformity
requirements.
+1.3 degrees
+0.8 degrees
+0.6 degrees
+0.4 degrees
+0.2 degrees
On-Axis
Concentration [cm-3]
1.0E+17
1.0E+16
1.0E+15
1.0E+14
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
Depth [um]
Figure 2 shows ThermaWave unit (TWU)
responses of a typical triple well formation implant P+,
800keV, 3x1013cm-2 as a function of small tilt angle
variations around the <001> channeling axis. 7% of
TWU change is observed per every 0.2° off-axis angle
change. In order to achieve 0.5% of doping nonuniformity that is widely accepted specification for
commercially available implant systems. Required
beam angle control precision is much less than 0.1°.
Figure 3. SIMS doping profiles obtained from P+, 800keV,
3x1013cm-2 implants.
Shown in Figure 4 are the effects of halo implant
parameter variation to the threshold voltages (Vth) of a
modeled device. A 45nm gate length device was
698
up beam lead to losses in dose and lateral junction
abruptness in microscopic scale [3].
modeled using TSUPREM 4 and MEDICI, satisfying
the ITRS 2001 requirements. The slopes shown in
Figure 4 indicate that a 0.5% shift in halo dose is
responsible for a 1.35mV shift in Vth. Achieving dose
uniformity and repeatability within 0.5 % is not a
difficult task for modern implanters. If the halo
implant angle was to be controlled within the Vth
variation allowance, the required angle control should
be less than 0.115°.
The result of angle offset in beam centroid, or in
other words, failure in beam alignment to the normal
direction wafer leads to an asymmetric shadowing of
the ion beam at the gate edges. Such a shadowed area
behaves like a gate side-wall spacer as if it has spacer
thickness equal to the beam angle offset tangent of the
gate structure thickness. Figure 5 shows simulated
doping profiles across the channel of a 100nm NMOS
after source/drain extension (SDE) implants with
offset angles from 0 to 4°.
0.40
Vt [V]
0.38
1.35mV per 0.5% dose shift
0.36
0.34
1.0E+21
0.32
-15%
-10%
-5%
0%
5%
Halo Dose Shift [%]
10%
1.0E+20
15%
Net Doping [cm-3]
0.30
0.40
Vt [V]
0.38
11.7mV per degree
or 1.35mV per 0.115°
0.36
tilt=0
tilt=1
tilt=2
tilt=3
tilt=4
1.0E+19
1.0E+18
0.34
1.0E+17
0.12
0.32
32
33
34
35
36
Halo Tilt Angle [degrees]
37
0.13
0.14
0.15
distance [um]
0.16
0.17
0.18
38
Figure 5. Lateral doping profile across the channel of a
100nm NMOS. Channel length increases as a function of
angle offset due to the shadow spacer effects.
Figure 4. Effects of the implant parameter variations to the
threshold voltage on a typical 100nm node NMOS device
HIGH CURRENT IMPLANTER
As a result of asymmetric shadow spacer in offset
angle SDE implant, device develops different
electrical parameters when source and drain electrodes
are swapped. The parametric skew between before and
after the swapping is plotted in Figure 6 as a function
of the offset angle.
Control of beam incident angle consists of two
separate tasks. One is aligning the target wafer to the
centroid of the beam direction. The other is ensuring
that the flights of ions within the beam have the same
direction of flights.
IDSAT skew
Unlike MC implanters, beam incident angle control
is even more difficult for high current (HC)
implanters. For the scaled device applications, there
are two requirements for HC implanters to meet
simultaneously. One is fulfilling the doping depth
requirements for ultra shallow junction (USJ) and the
other is doing it with a production worthy throughput.
From an angle control perspective, those contradicting
requirements of low energy and large current beam,
characterize the challenge imposed on traditional HC
implanter systems. Because it requires containing
more mass of slowly moving charged particles in a
beam envelope as small as possible in its size.
Coulomb pressure built in highly compacted beam
blows up the envelope resulting in a compromise of
the beam angle control. Beam shadows cast by blown-
10%
30.0
8%
25.0
20.0
6%
15.0
4%
10.0
2%
Vt Skew [mV]
0.30
5.0
0%
0.0
0.0
1.0
2.0
3.0
4.0
Beam off-set angle [degrees]
Figure 6. Effects of beam angle offset to Vth and IDSAT on
a 100nm NMOS
699
In order to control the IDSAT skew below 1.5%, the
beam angle offset should not exceed 0.5°. Such a high
precision of angle control was demonstrated on a
Varian HC implanter VIISta 80 which tackled HC
implanter angle control challenges by wide ribbon
beam technology and closed loop angle control system
[4].
REFERENCES
1. U. Jeong, J.-Y. Jin, and S. Mehta, "Devices dictate
control of implant-beam incident angle," Solid State
Technology, Oct. 2001
2. T. Yamashita, M. Kitazawa, Y. Kawasaki, H. Takashino,
T. Kuori, Y. Inoue, and M. Inuishi, “Advanced
retrograde well technology for 90-nm node embedded
static random access memory using high-energy parallel
beam,” April, 2002, Jpn. J. Appl. Phys. Vol. 41, Part 1,
No. 4B, 2399, (2002).
CONCLUSION
3. U. Jeong, S. Mehta, C. Campbell, R. Lindberg, Z. Zhao,
B. Cusson, and J. Buller, "Effects of Beam Incident
Angle Control on NMOS Source/Drain Extension
Applications," 14th International Conference on Ion
Implantation Technology, Taos, New Mexico, USA, Sep.
22-27, 2002 (to be published)
Highly precise angle control is required for ion
implant applications in scaled CMOS. MC implanters
should be capable of less than 0.1° of angle control in
order to perform uniform doping at zero tilt well
implants and to suppress the device parametric
variation successfully at halo applications. For HC
implanters, less than 0.5° of angle control is required
from the device symmetry perspectives.
4. C. Campbell, C. Radovanov, J. Olson, J. Cummings, R.
Lindberg, D. Smatlak, T. Callahan, "Beam Angle
Control on the VIISta 80 Ion Implanter," 14th
International Conference on Ion Implantation
Technology, Taos, New Mexico, USA, Sep. 22-27, 2002
(to be published)
In order to achieve such level of angle control in
ion implanter systems, an angle variation free scan
system makes a prerequisite. Because tilt and rotation
angle variations coming from traditional spinning disk
scan system alone is more than twice larger than the
largest allowable angle variations dictated by device
requirements.
700