Interval Scan Inspection Technique for Contact Failure of

http://dx.doi.org/10.5573/JSTS.2012.12.1.34
JOURNAL OF SEMICONDUCTOR TECHNOLOGY AND SCIENCE, VOL.12, NO.1, MARCH, 2012
Interval Scan Inspection Technique for Contact Failure
of Advanced DRAM Process using Electron BeamInspection System
J. H. Oh*, G. Kwon*, D. Y. Mun*, D. J. Kim*, I. K. Han*, H. W. Yoo*, J. C. Jo**, Y. Ominami**,
T. Ninomiya**, and M. Nozoe**
Abstract—We have developed a highly sensitive
inspection technique based on an electron beam
inspection for detecting the contact failure of a poly-Si
plugged layer. It was difficult to distinguish the
contact failure from normal landing plugs with high
impedance. Normally, the thermal annealing method
has been used to decrease the impedance of poly-Si
plugs and this method increases the difference of
charged characteristics and voltage contrast. However,
the additional process made the loss of time and
broke down the device characteristics. Here, the
interval scanning method without thermal annealing
was effectively applied to enhance the difference of
surface voltage between well-contacted poly-Si plugs
and incomplete contact plugs. It is extremely useful to
detect the contact failures of non-annealed plug
contacts with high impedance.
process toward a below 30 nm node. Electron beam (EB)
inspection based on scanning electron microscopy (SEM)
is useful to detect the hidden defects causing electrical
failure such as contact failure, under-layer electric short,
and leakages using voltage contrast [1-4]. It is difficult
for those defects to be detected by optical wafer
inspection tools. In particular, the inspection sensitivity
of electrical failures by EB inspection has a great
influence on the yield of a memory device. Fig. 1 shows
the schematics of an EB inspection system and image
process. The inspection sensitivity was affected by
various control factors such as incident beam current,
beam size, landing energy, and samples. The number of
electron scattering from the surface by optimized control
factors describes the accurate defect image such as very
small abnormal features and electrically incomplete
failures. Inspection images at an EB inspection system
are captured with a high-speed electron beam scanning
Index Terms—Electron beam inspection, contact failure,
voltage contrast, interval scanning
I. INTRODUCTION
The in-line inspection tool has been investigated to
support yield enhancement in a semiconductor fabrication
Manuscript received Apr. 29, 2011; revised Oct. 18, 2011.
* Hynix semiconductor Inc., San 136-1 Ami-ri Bubal-eub Icheon-si
Kyoungki-do 467-701, Korea
** Hitachi High-Technologies Corp., Central Research Laboratory,
Kokubunji, Tokyo 185-8601, Japan
E-mail : [email protected]
Fig. 1. Schematics of EB inspection system and image process.
JOURNAL OF SEMICONDUCTOR TECHNOLOGY AND SCIENCE, VOL.12, NO.1, MARCH, 2012
and detecting and then these images were compared with
a reference image by an image processor in real time.
The principle of voltage contrast of EB inspection is as
follows; a large EB current over a few of nAs is
irradiated to a device surface and the scattered signals
generate the difference of voltage contrast depended on a
electrical contact condition. The number of secondary
electrons are affected by the electrical field condition
caused by the surface charging voltage. Fig. 2 shows the
mechanism of voltage contrast depended on the structures.
When EB is irradiated on the surface of an incomplete
pattern, its charging voltage increases over that of normal
contact plugs. Because of the local electrical field which
is created by the difference of charging condition, the
number of secondary electrons, which are emitted on
each plug, are different between the normally opened
contact plug and disconnected plug. At a positively
charged condition, the brightness of a SEM image at a
disconnected defect becomes darker than that of a normal
plug because secondary electrons returned to wafer
surface by locally electrical fields. Meanwhile, the SEM
image by short and leakage defects between two plugs
becomes brighter than one of normal plugs because of
the low surface potential and additional electrons
supplied from bottom contact layers. In this way, voltage
contrast is strongly linked to the electrical potential of
each plug, which is related to resistive and dielectric
characteristics of each plug. It means that it is difficult to
inspect the high-impedance patterns such as nonannealed poly-Si plugs because of small difference of
charging characteristic between a well-plugged and
disconnected plug. Recently, in order to inspect an
interconnector as a landing plug poly-Si (LPP) and
storage node poly-Si contact (SNC), an annealing
Fig. 2. Mechanism of voltage contrast EB inspection at contact
plugs layer.
35
process is added to reduce the impedance of poly-Si [5-6].
The high surface voltage of the electron irradiated
contact hole with high impedance makes the high
potential barrier to be difficult to emit second electrons
from a surface of a device [7]. Poly-Si by a thermal
annealing effect was crystallized and it caused the
impedance of poly-Si to decrease [8]. According to an
experiment, the lowest impedance of poly-Si was formed
by the thermal annealing process at 900 °C. However, the
material property of poly-Si may be changed by thermal
annealing. Here, a new inspection method was studied to
inspect the advanced DRAM wafers without the loss of
process time and any destruction. This technique used the
charging and discharging characteristics for irradiating
time and waiting time by using interval scanning. It
enhances the low voltage contrast of non-annealed LPP
inspection.
1. New Inspection Method
Fig. 3 shows that the schematic diagrams of plug
structures and time dependent charging behavior by the
conventional and interval scan method. The normal plug
and disconnected plug have different electrical property,
impedance. This difference affects the charging and
Fig. 3. (a) Schematic diagram of poly-Si plug structures, (b)
Schematic diagram of charging mechanism at each scan method,
(c) Experimental result of the difference of SEM contrast
between normal plug and defective plug.
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J. H. OH et al : INTERVAL SCAN INSPECTION TECHNIQUE FOR CONTACT FAILURE OF ~
discharging characteristics of plugs during scanning. The
conventional method continuously scanning care area
onto wafer once did not make the clear difference of
surface voltage between a normal plug and a disconnected
plug. This phenomenon indicates that the difference of
voltage contrast between normal plugs and defects at an
EB inspection image will be very low. New inspection
method by applying proper interval time during
periodically scanning over the same area enhanced the
difference of surface voltage among materials and
electrically connected conditions. The increase of interval
time between 2 cycle scanning increased the difference
of surface voltage and brightness among normal plugs,
disconnected plugs and SiO2 as shown in Fig. 3(b) and
(c). The surface voltage of each structure was increased
by initial electron beam irradiation and then their
differences were widened because the plugged structures
with low impedance quickly discharged for interval time.
The detection sensitivity at an interval scan method by
applying interval time was much higher than one at a
conventional scan method.
II. EXPERIMENTAL RESULTS
The commercial EB inspection system (model: I6300H2, Hitachi High-technologies Co., Japan) was
utilized to evaluate a new scan method. It needed an
optimum detection conditions for the electrical failure of
a poly-Si plug into SiO2. In order to increase efficient SE
yield (δ), the landing energy and the beam current were
evaluated in the fixed conditions of 5 kV extract field
and 50 MHz scan frequency. These EB inspection factors
were optimized as 300 eV and 40 nA, respectively as
Fig. 5. SEM images of non-annealed poly-Si plugs by using (a)
Conventional scan, (b) Interval scan at the beam conditions of
300 eV, 40 nA and 5 kV.
shown in Fig. 4. The highest contrast and defect signal to
noise ratio (SNR) were related to detection sensitivity. The
scan pixel size of 25 nm at the Cell to Cell comparison
mode was applied at high resolution inspection.
Fig. 5 shows lowly magnified SEM images inspected
by a conventional scan method and an interval scan
method with the optimized beam conditions of 300 eV,
40 nA and 5 kV. The difference of voltage contrast
between normal plugs and incomplete contact plugs at
the interval time of 50 msec was more enhanced than at
the conventional method continuously scanning on the
surface of a wafer. A lot of dark defect images among
bright poly-Si plug images were clearly observed even at
non-annealed process as shown in Fig. 5(b). The proper
interval time related to surface charge condition of
materials caused to increase the difference of voltage
contrast between normal and abnormal contact. The
interval scan was very sensitive to electrical failure
detection of poly-Si interconnections and got an effective
voltage contrast image.
Fig. 6 shows the brightness level of SiO2 and poly-Si
depended on two scan methods. The contrast signal
Fig. 4. Graphs describe (a) The brightness of poly-Si and SiO2 surface depending on landing energy at beam current of 40 nA, (b)
The defect SNR depending on beam current at landing energy of 300 eV. Each evaluation used the fixed conditions of 5 kV extract
field and 50 MHz scan frequency.
JOURNAL OF SEMICONDUCTOR TECHNOLOGY AND SCIENCE, VOL.12, NO.1, MARCH, 2012
37
Fig. 6. Brightness level of SiO2 and poly-Si plugs in SEM
images obtained by conventional scan and interval scan.
between SiO2 and poly-Si plugs in interval scan increased
8 times higher than a signal in a conventional scan.
The inspected area was analyzed by scanning
transmission electron microscopy (STEM: Hitachi HD2300) so as to confirm the reality of the detected defect
signal. Fig. 7 shows the high resolution EB inspection
image with the dark defect spot and the STEM image on
its area. After the micro-sampling and thinning process
of the dark defect using focused ion beam (FIB: Hitachi
FB-2100), the incomplete contact defects were observed
by using dark filed (DF) STEM as shown in Fig. 7(b).
The image brightness of DF-STEM depends on element
mass or density and the dark image in circle area was
described by void characteristic. Based on the image, the
diameter of the middle section of the incomplete plug
was much smaller than one of other plugs. The bottom
nitride deposited inside of a contact hole was not clearly
etched because of a small CD contact hole and then a
Poly-Si plug was easily not formed at the bottom of a
Fig. 8. Defect maps and inspection SEM images detected by
(a,c) Conventional method, (b,d) Interval scan method.
hole. Therefore, it made incomplete connection with a Si
active area. Other plugs except a single void defect area
were well connected with Si active areas as shown in
STEM images. Here, it was clarified that the constriction
at the middle of plug induced a void defect and the dark
voltage contrast image in SEM showed a real defect
electrically disconnected. The effect of an interval scan
method was also verified to inspect a storage node
contact (SNC) layer with poly-Si plugs. The detection
sensitivity by an interval scan method at a SNC layer was
similar to a LPP layer.
Fig. 8 shows defect maps and inspection images
obtained by using a non-annealed SNC structure.
Focusing on a defect distribution trend, the high defect
density at the edge was significantly observed in the
interval scan EB inspection. And, the defect density
detected by the interval scan method was improved 5
times than one by the conventional scan method. In case
of the interval scan method, a single dark spot image
clearly shows the incomplete contact as discussed in Fig.
2. However, the defect signal detected by a conventional
scan method was not clear because of the low difference
of voltage contrast between normal and abnormal plugs.
As a result, the resolution and sensitivity of defect
detection dramatically were improved.
III. CONCLUSIONS
Fig. 7. (a) Review image of EB inspection, (b) DF-STEM
image of LPP layer. The poly-Si plug did not contact with an
active Si area because of a void defect in a solid line circle.
We have developed a novel EB inspection technique
to effectively detect electrical contact failures. It is
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J. H. OH et al : INTERVAL SCAN INSPECTION TECHNIQUE FOR CONTACT FAILURE OF ~
difficult to inspect high-impedance patterns like nonannealed poly-Si plugs because of the similar difference
of charging characteristic between a well-contacted plug
and an incomplete one. The proper interval time during
cycling an electron beam scan enhanced the difference of
surface voltage and voltage contrast between normal
plugs and incomplete contact plugs without reducing the
impedance of poly-Si plugs. The inspection result shows
that the signal between SiO2 and poly-Si plug by a new
method applying 50 msec interval was increased 8 times
higher than the signal by the conventional method. This
result indicates the potential of effective in-line monitoring
of electrical contact failures at an inspection step with
even high impedance material.
ACKNOWLEDGEMENT
The authors are grateful to Tomohiro Tamori in Hitachi
High-Technologies Corporation for technical support.
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J. H. Oh has worked as a semiconductor
photolithography engineer in Hynix
Semiconductor Inc. since 1995. At
that time, he was charged with CDSEM Tools. In present, he has
worked at Metrology & Inspection
Technology Team. He is currently in
charge of development of inspection and analysis
applications based on electron beam inspection
technology.
G. Kwon received the B.S. degree in
Electronic engineering in 2001, the
M.S. degree in micromachining
engineering from Korea University,
Korea in 2003 and the Ph.D in
Nanotechnology
from Hanyang
University, Seoul, Korea in 2010. He
has ever joined to develop optical- and bio-devices based
on micro-electromechanical system (MEMS) as a
researcher at Korea Institute of Science and Technology
(KIST) from 2003 to 2004. In present, he has worked at
Metrology & Inspection Technology Team in Hynix
Semiconductor Inc., Korea since 2010. He is currently in
charge of the development of inspection and analysis
applications based on electron beam technology.
D. Y. Mun received the B.S. degree in
Electronic Material from Kwangwoon
University, Seoul, Korea, in 2003. He
has worked in Hynix Semiconductor
Inc. since 2004. He experienced as an
application engineer in optic Inspection
for 5 years. Since 2009, he has been
in charge for the development of e-beam inspection
applications.
JOURNAL OF SEMICONDUCTOR TECHNOLOGY AND SCIENCE, VOL.12, NO.1, MARCH, 2012
39
D. J. Kim received the B.S. degree in
Materials engineering from the
University of Korea Aerospace, Seoul,
Korea, in 2000. He has worked in
Hynix Semiconductor since 2003. He
is charge of the optical inspection
tool to increase the performance of
the defect detection ability and control the noise by
process and wafer environment.
J. C. Jo got the B.S degree in physics
from Ajou University. From 2000 to
2003, he had worked as directly e-beam
lithography and photolithography
Engineer at DAWOO semiconductor
and EONCOM company. He joined
Hitachi High technologies Korea Co.,
Ltd. In 2005 and has worked as application engineer of ebeam inspection and analysis tool (nano prober and FIB
etc.). He can be reached by e-mail at jo-jaecheol@
hitachi-hitec-kr.com.
I. K. Han received the B.S. degree in
Ceramic Engineering from Yonsei
University in 1983, the M.S. degree
in MBA from Yonsei University in
2011. He has worked in Hynix
Semiconductor Inc. ever since 1985
and is presently a head of Inspection
& Metrology- Infra group in the Fab. manufacturing
devision of Hynix Semiconductor Inc. . His current
research interests include new electronic materials,
processing, and metrology of nanoscale silicon devices
memories.
Y. Ominami received the B.S.
degree and M.S. degree in quantum
science and engineering at Hokkaido
University, Japan, in 2000 and 2002
respectively, and the Ph.D. degree in
devision of quantum science and
engineering at Hokkaido University,
Japan, in 2008. He is currently in charge of developments
in SEM and FIB at Hitachi High-Technologies Corporation,
Japan. From 2004 to 2006, he was with the Center for
Nanostructures at Santa Clara University in USA, where
he was focusing on the development of carbon nanofiber
(CNF) interconnects using SEM, FIB, and scanning
transmission electron microscopy (STEM). From 2006 to
2008, he joined Catalysis Research Center (CRC) at
Hokkaido University, where he is developing novel
electron microscopy techniques for the characterization
of nanomaterials. His main research interests include
electron microscopy, nanomaterial science such as
nanotube and catalysts.
H. W. Yoo received the B.S. degree
in chemical engineering from the
University of Inha, incheon, Korea in
1987. He has worked in Hynix
Semiconductor since 1988. He joined
at a position of increasing responsibility
in the areas of process engineering
and yield management. And he has had a responsibility
to manage the Metrology and Inspection Team at Fab
Manufacturing Devision since 2006. In recent years, he
is a leader of TSC (Technical Steering Community) in the
field of metrology and inspection, in which he is
directing the research of leading next generation devices
as well as new memory.
T. Ninomiya joined Hitachi, Ltd. In
1988, and now works at the Electron
Beam System Design Department,
Electronic Device System Business
Group. He is currently engaged in
the development and design of
advanced EB inspection system. Mr.
Ninomiya is a member of JSAP, and can be reached by email at [email protected].
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J. H. OH et al : INTERVAL SCAN INSPECTION TECHNIQUE FOR CONTACT FAILURE OF ~
M. Nozoe joined Hitachi, Ltd. in
1986, and experienced semiconductor
yield management through inspection
and analysis for 9 years. From 1997
to 2004, she was at Hitachi Central
Research Laboratory. From 2004 to
2010,
she
engaged
in
the
development of new application for inspection and
analysis using EB at Hitachi High-Technologies.
Currently, she is charging of the management of
advanced technologies for semiconductor process control
system, Electronic Device System Business Group. She
is a member of The Japan Society of Applied Physics
(JSAP), and can be reached by e-mail at [email protected].