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`Arsenic dopant mapping in State-of-the-art
`semiconductor devices using electron energy
`loss spectroscopy
`
`Article Micron October2009
`DO :10.1016/.m c on.2009.10.004 Sou ce: ubMed
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`c TAT ONS
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`14
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`2 authors:
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`READS
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`240
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`Germain Servanton
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`R. Pantel
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`STMicroelectronics
`14 PUBL CAT ONS 173 c TAT ONS
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`STMicroelectronics
`141 PUBL CAT ONS 2,255 c TAT ONS
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`Micron 41 (2010) 118–122
`
`Contents lists available at ScienceDirect
`
`Micron
`
`j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / m i c r o n
`
`Arsenic dopant mapping in state-of-the-art semiconductor devices using electron
`energy-loss spectroscopy
`
`Germain Servanton *, Roland Pantel
`
`STMicroelectronics, 850 rue Jean Monnet, F-38926 Crolles, France
`
`A R T I C L E I N F O
`
`A B S T R A C T
`
`Knowledge of the dopant distribution in nanodevices is critical for optimising their electrical
`performances. We demonstrate with a scanning transmission electron microscope the direct detection
`and two dimensional distribution maps of arsenic dopant in semiconductor silicon devices using
`electron energy loss spectroscopy. The technique has been applied to 40 45 nm high density static
`random access memory and to n p n BiCMOS transistors. The quantitative maps have been compared
`with secondary ion mass spectrometry analysis and show a good agreement. The sensitivity using this
`approach is in the low 1019 cm 3 range with a spatial resolution of about 2 nm.
`ß 2009 Elsevier Ltd. All rights reserved.
`
`Article history:
`Received 18 August 2009
`Received in revised form 2 October 2009
`Accepted 3 October 2009
`
`Keywords:
`STEM EELS
`Arsenic dopant mapping
`Silicon semiconductors
`CMOS
`BiCMOS
`
`1. Introduction
`
`The expansion of smart electronic systems for customer
`applications has been stimulated by the continuous progress of
`device performances and integration density. One of the key
`processes in optimising these silicon devices is to adapt the three
`dimensional dopant distribution to maximize the device electrical
`characteristics. This is obtained by combining the physical doping
`processes (ion implantation, thermal annealing) to elaborated
`computer simulations. However due to the low concentration level
`of impurities there is not up to now a simple technique that can
`be used to verify at the nanometre scale the actual dopant
`distribution. This is why the task of visualising the dopants in
`silicon nanodevices has been highlighted by the international
`technology roadmap for semiconductors (ITRS, 2009) as an
`objective to aid the continuous development of silicon devices.
`To face this challenge, numerous techniques have been developed
`and evaluated during the last ten years either with a nanometric
`resolution (Castell et al., 2003) or at the atomic scale (Voyles et al.,
`2002). For the analysis of electrically active dopants, the near field
`microscopy methods, i.e. scanning spreading resistance (SSRM) or
`capacitance (SCM) microscopy (Eybens et al., 2007) and electron
`holography (Twitchett et al., 2003; Cooper et al., 2007) have been
`shown to be the most promising techniques. For chemical dopant
`analysis, the three dimensional atom probe tomography (APT)
`
`* Corresponding author.
`E-mail address: germain.servanton@st.com (G. Servanton).
`
`0968-4328/$ – see front matter ß 2009 Elsevier Ltd. All rights reserved.
`doi:10.1016/j.micron.2009.10.004
`
`(Thompson et al., 2005, 2007) gives unsurpassed spatial resolution
`(nearly atomic) and sensitivity below 1019 cm 3. However for
`advanced CMOS technology nodes (45 and 32 nm) and inside high
`density circuits such as static random access memory (SRAM),
`none of these techniques are presently suitable. Indeed, the APT
`technique is limited by tip preparation difficulties added to
`insulator materials problems inside the SRAM devices (Ko¨ lling and
`Vandervorst, 2009). The direct detection of arsenic dopant using
`scanning transmission electron microscopy (STEM) energy dis
`persive X ray spectroscopy (EDX) has also been tested (Topuria
`et al., 2001) and was recently improved by lowering primary beam
`energy and applied in 45 nm n MOS transistors but with a limited
`resolution (Servanton et al., 2009, Submitted for publication).
`In this paper we demonstrate a significant progress in
`quantitative arsenic dopant mapping by using electron energy
`loss spectroscopy (EELS) in STEM mode. The STEM EELS technique
`consists of focusing and scanning an intense electron probe on a
`sample. The direct ionization of core electrons from the atoms
`cause an energy loss in the detected electrons which can then be
`quantified as these energy losses are specific to different atom
`species. The improvements presented come from the high electron
`doses per pixel without saturating the detector using a low
`primary electron beam energy (120 keV instead of 200 keV); i.e.
`below the knock on damage threshold for silicon (Servanton et al.,
`2009, Submitted for publication). In addition, the higher electron
`collection efficiency (about 80%) with high current (8 nA) using
`STEM EELS makes this method of dopant profiling more suitable
`than STEM EDX (2% efficiency with a current lower than 1 nA to
`avoid the detector saturation).
`Importantly, STEM EELS is
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`G. Servanton, R. Pantel / Micron 41 (2010) 118–122
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`119
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`compatible with focused ion beam (FIB) TEM lamella preparation
`that enables a large thickness range to be examined (from 30 to
`200 nm). Such thin lamella can be extracted from high density
`32 nm SRAM circuits. The STEM EELS arsenic maps can be
`quantified and the sensitivity is estimated to be near the low
`1019 cm 3 range. The pixel sampling is high for reasonable
`experimental times (4000 pixels/h) and the spatial resolution is
`around 2 nm.
`Here we present experimental data from n MOS transistors
`integrated in high density circuits (45 40 nm SRAM) and will show
`that exact arsenic distribution limit can be obtained around the
`CMOS source/drain and LDD regions. The examination of 90 nm
`bipolar transistor compatible MOS (BiCMOS) (Avenier et al., 2008)
`show high arsenic fluctuations in the arsenic doped silicon
`emitter. Finally, quantitative STEM EELS measurements have been
`compared to secondary ion mass spectrometry (SIMS) experiments
`which show good agreement.
`
`2. Experimental details
`
`These experiments have been performed using an FEI S Twin
`TEM TECNAI F20 equipped with a field emission gun (FEG) electron
`source and a Gatan energy filter GIF200 with 1K  1K CCD camera.
`The samples were prepared using FIB milling (Gallium ions
`accelerated at 30 keV) with a final cleaning stage at 5 keV. During
`STEM EELS mapping, the sample drift is actively compensated
`every minute using a cross correlation with an initial STEM image.
`The EELS experiments, method and data processing principles for
`arsenic signal extraction and arsenic doped silicon quantification
`are described in references (Pantel et al., 2008; Servanton et al.,
`2009, Submitted for publication).
`
`3. Results
`
`3.1. Arsenic dopant maps in 45 40 nm CMOS transistors
`
`Our first example shows that STEM EELS is compatible with
`state of the art semiconductor nodes, such as the analysis of high
`density 40 nm SRAM (0.3 mm2 bitcell area). Fig. 1(a) presents a
`
`Fig. 2. As-L2,3 EELS signals extracted from three single pixels (Si(As 0%), Si(As 0.15%)
`and Si(As 0.25%)).
`
`TEM bright field image of a FIB cross section of n MOS transistors.
`The TEM contrast reveals only materials with different composi
`tions (nickel silicide, tungsten) but not the arsenic dopants inside
`silicon, as their relative concentration is small. Fig. 1(b) presents an
`arsenic concentration distribution map, for the same sample,
`obtained using STEM EELS. The colour map, acquired during 2 h for
`140  60 pixels, clearly shows areas with different arsenic
`concentrations such as the low doped drains (LDDs), the source/
`drain junction below the nickel silicide, the gate side walls and the
`top of spacers. The two dimensional quantitative distribution,
`
`Fig. 1. (a) TEM bright field image of 40 nm n-MOS transistors in a SRAM device. (b)
`Arsenic concentration distribution obtained using STEM EELS for the same SRAM
`device (140  60 pixels; 2 h acquisition time; colour scale: undoped silicon is green,
`arsenic-doped silicon is red–yellow).
`
`Fig. 3. (a) Arsenic concentration distribution obtained using STEM EELS in a 45 nm
`SRAM with high implantation process (150  60 pixels, acquisition time 2.2 h). (b)
`Arsenic concentration line profile extracted below the gate in Fig. 2 from LDD (A) to
`LDD (A0). The channel length for this n-MOS transistor is 46 nm.
`
`Page 3 of 6
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`G. Servanton, R. Pantel / Micron 41 (2010) 118–122
`
`Fig. 4. (a) STEM EELS arsenic concentration distribution showing a magnified area
`covering the gate and drain of a n-MOS transistor in 45 nm SRAM device (80  80
`pixels, acquisition time 1.5 h). The arsenic distribution limit is drawn (dot line).
`(b) Arsenic concentration profile (raw data) extracted from the map of Fig. 3(a)
`along BB0.
`
`when compared with device processing models is as expected. The
`experimental results show that the transistor gates are uniformly
`doped. Additionally, it can be seen that the spacers have played
`their role by protecting the LDD (first implantation) during the
`second source/drain implantation. The smooth arsenic extension
`limit (pixels where arsenic concentration is near to zero) shown
`in Fig. 1(b) (left) indicates a high quality dopant implantation
`process. The EELS background extraction that has been optimised
`for low arsenic doped silicon (Servanton et al., 2009, Submitted for
`publication) fails in heavy alloys (NiSi, SiGe) which are displayed in
`black in the following figures.
`Fig. 2 displays typical As L2,3 EELS signals extracted from three
`single pixels (Si(As 0%), Si(As 0.15%) and Si(As 0.25%)). The
`background extrapolation and the signal windows are shown (150
`and 260 eV width, respectively). For the Si(As 0.15%), the total
`number of electrons integrated in the As L2,3 window is about
`700  260, i.e. 1.8  105 electrons. This suggests that the signal
`p
`over noise determined by the number of detected electrons
` 4:102) is high for Si As 0.15% and will stay
`1:8  105
`(S=N
`acceptable even if one decreases the dose by a factor of ten.
`However, the main noise comes from the background extrapola
`
`Fig. 5. (a) TEM bright field image of n–p–n 90 nm BiCMOS. (b) Arsenic concentration
`distribution map obtained using STEM EELS in the same area: (150  60 pixels,
`acquisition time 2.2 h). (c) Arsenic concentration profile extracted along CC0.
`
`tion error. The background extraction precision depends on the
`sample quality (possible Ga contamination due to FIB preparation)
`and on the EELS detector. Cumulated with the As/Si sensitivity
`factor around 14.0  0.3, we estimated the error bar in the arsenic
`detection to be 0.05% (see displays in Figs. 3b, 4b, 5c and 6c).
`The second application shows the analysis of a 45 nm SRAM
`with an extremely high implantation dose which has been
`processed in order to study the instability of the nickel silicide
`(commonly called encroachment) (Imbert et al., 2009). The process
`was stopped after the silicide formation. Fig. 3(a) shows a STEM
`EELS arsenic concentration distribution map acquired during 2.2 h
`for 150  60 pixels. On the right of the figure, a high concentration
`of arsenic separated from the source/drain is detected and the
`arsenic distribution limit shows an unexpected arsenic extension
`(see black arrow in Fig. 3(a)). This is probably due to arsenic
`segregation at silicon defects that were created at the end of range
`(EOR) implantation. These defects observed using TEM imaging on
`the same sample (Imbert et al., 2009) are not symmetric around
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`concentration profiles can be extracted such as indicated by BB0 in
`Fig. 4(b). The point where the arsenic concentration curve crosses
`zero (near 1.1019 at cm 3) is about the arsenic extension limit. By
`using this process, the As dopant extension limit has been drawn
`(dot line) in Fig. 4(a).
`
`3.2. Arsenic dopant maps in 90 nm BiCMOS transistors
`
`STEM EELS arsenic maps can be applied to any device integrating
`arsenic doped mono or poly crystalline silicon and in particular to
`BiCMOS transistors. Fig. 5(a) presents a TEM bright field image of
`such a n p n bipolar device (the emitter is arsenic doped silicon and
`the base is Si0.78Ge0.22 boron doped). In the emitter part, the contrast
`shows a grain boundary between the mono crystalline silicon at
`centre and the poly crystalline silicon to the right. While the arsenic
`distribution cannot be seen in this image, Fig. 5(b) shows the STEM
`EELS arsenic map, acquired during 2.2 h for 150  60 pixels, which
`corresponds exactly to the same area than Fig. 5(a). The emitter
`is highly doped but, as it was thought to be uniform following
`SIMS measurements, strong concentration variations are revealed.
`Fig. 5(c) shows a profile of arsenic distribution along the line
`indicated CC0. Six maxima are observed whose variations are more
`than a factor two. Arsenic depletion near the bottom centre of the
`emitter is observed surrounded by two maxima. It is likely that the
`arsenic atoms might be poorly incorporated in the silicon mono
`crystalline central regions during the epitaxy process as these
`dopants tend to concentrate at the defects in the lateral areas. After
`annealing, the arsenic has then diffused to the grain boundaries and
`to the external interfaces.
`Fig. 6(a) shows a TEM bright field image of a different BiCMOS
`transistor. Silicon crystal planar defects are observed in the quasi
`mono crystalline part of the emitter. The arsenic STEM EELS map
`presented in Fig. 6(b) (2 h acquisition time for 130  60 pixels)
`highlights arsenic segregation at
`these stacking faults. The
`emitter base interface shows a high arsenic concentration as
`confirmed in Fig. 6(c) which presents the arsenic concentration
`profile extracted along DD0. This profile is compared to SIMS
`analysis which has been carried out on large test structure found
`on the same BiCMOS wafer. The measured profile validates our
`STEM EELS quantification method. An arsenic concentration peak
`is detected at the emitter base interface in the BiCMOS, however
`this is not seen in the large test structures observed by SIMS; this is
`certainly due to size related effects during the interface process
`cleaning.
`
`4. Conclusion
`
`In this paper we have presented a solution for two dimensional
`quantitative mapping of arsenic doped areas in nanometer scale
`semiconductor silicon devices. The method can be applied using
`standard FIB TEM lamella preparation and is compatible with the
`analysis of high density SRAM 32 45 nm (0.2 0.3 mm2 bit cell area).
`The spatial resolution is 2 nm, which suggests that this method will
`still be compatible with future 32 22 nm technologies. As indicated
`by the single line profiles (without lateral averaging), the signal to
`noise ratio is excellent and the sensitivity limit is in the low 1019 cm 3
`range. This spatial resolution and sensitivity could be improved using
`new electron source generation (high brightness (Freitag et al., 2008)
`or probe Cs aberration corrected (Krivanek et al., 2008)). However
`probe size reduction should be coupled with sample holder stability
`and drift correction improvement if one wants to really gain in spatial
`resolution. It is reasonable to think that 1 nm resolution and
`1  1019 cm 3 sensitivity are the limits since for a 100 nm thick
`sample the number of arsenic atoms present in the probed volume is
`about the unity. The presented technique opens a new field of
`possibilities for controlling semiconductor fabrication processes.
`
`Fig. 6. (a) TEM bright field image of another n–p–n 90 nm BiCMOS showing stacking
`faults at the emitter edges. (b) Arsenic concentration distribution map obtained
`using STEM EELS in the same area (130  60 pixels, acquisition time 2 h). (c)
`Comparison of an arsenic concentration profile extracted along DD0 and SIMS profile
`obtained in larger test structure from the same wafer.
`
`the source (drain) due to implantation angle. Arsenic segregation
`on same defects were also observed using STEM EDX (Servanton
`et al., 2009, Submitted for publication). As indicated on the left of
`Fig. 3(a), a line profile can be extracted in the n MOS channel from
`LDD (A) to LDD (A0). The result is displayed in Fig. 3(b), with a
`chemical channel length estimation of 46 nm. Note that this profile
`presents the raw data (no smoothing) and that the detectable
`arsenic level is below 0.1%, i.e. approximately 1  1019 at cm 3.
`Fig. 4(a) shows a STEM EELS arsenic distribution map, acquired
`during 1.5 h for 80  80 pixels, zoomed on to one single n MOS
`transistor extracted from a 45 nm SRAM identical to the one shown
`in Fig. 3(a). In this case, the poly crystalline silicon gate seems to be
`arsenic rich at the bottom. This map is quantitative and dopant
`
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`G. Servanton, R. Pantel / Micron 41 (2010) 118–122
`
`Concerning the BiCMOS transistors, the low temperature used for the
`emitter epitaxy and the interface process cleaning induces variable
`arsenic incorporation that can be checked using STEM EELS. For CMOS
`transistors, arsenic distributions are also difficult to simulate without
`comparison to experimental data. For example anomalies related to
`implantation defects can be detected in n MOS transistors. The
`optimisation of silicon devices using simulations, process changes
`and electrical tests can now be improved by the contribution of the
`two dimensional arsenic mapping which will permit faster feedback
`along the semiconductor fabrication chain.
`The authors would like to thank Dr. Alain Chantre, Pascal
`Chevalier and Gregory Avenier for fruitful discussions and for
`supplying BiCMOS transistors. We are also grateful
`for the
`assistance of Dr. David Cooper for English corrections.
`
`References
`
`Avenier, G., et al., 2008. 0.13 mm SiGe BiCMOS technology for mm-wave applica-
`tions. BCTM Proc. 89–92.
`Castell, M.R., Muller, D.A., Voyles, P.M., 2003. Dopant mapping for the nanotech-
`nology age. Nat. Mater. 2, 129.
`Cooper, D., et al., 2007. Quantitative dopant profiling of laser annealed FIB-prepared
`silicon n–p junctions with nm-scale resolution. Appl. Phys. Lett. 91, 143501.
`Eybens, P., Vandervorst, W., Alvarez, D., Xu, M., Fouchier, M., 2007. In: Kalinin, S.,
`Gruverman, A. (Eds.), Scanning Probe Microscopy. Springer, pp. 31–87.
`Freitag, B., et al., 2008. First performance measurements and application results of a
`new high brightness Schottky field emitter for HR-S/TEM at 80–300 kV accel-
`eration voltage. Microsc. Microanal..
`
`Imbert, B., et al., 2009. Nickel silicide encroachment formation and characteriza-
`tion. Microelectron. Eng. (online in June 2009).
`International Technology Roadmap for Semiconductors, 2009. See http://public.
`itrs.net for details.
`Kolling, S., Vandervorst, W., 2009. Failure mechanisms of silicon-based atom-probe
`tips. Ultramicroscopy 109 .
`Krivanek, O.L., et al., 2008. An electron microscope for the aberration-corrected era.
`Ultramicroscopy 108 .
`Pantel, R., Clement, L., Rubaldo, L., Borot, G., Dutartre, D., 2008. Analytical STEM
`comparative study of the incorporation of covalent (Ge) or heterovalent (As)
`atoms in silicon crystal. Microscopy of Semiconducting Materials, vol. 120.
`Springer.
`Servanton, G., Pantel, R., Juhel, M., Bertin, F., 2009. Two-dimensional quantitative
`mapping of arsenic in nanometer-scale silicon devices using STEM EELS-EDX
`spectroscopy. Micron 40, 543.
`Servanton, G., Pantel, R., Juhel, M., Bertin, F., Submitted for publication. STEM EDX
`applications for arsenic dopant mapping in nanometer-scale silicon devices.
`Proceedings of the 16th Conference Microscopy of Semiconducting Materials,
`March 2009.
`Thompson, K., Booske, J.H., Larson, D.J., Kelly, T.F., 2005. Three-dimensional atom
`mapping of dopants in Si nanostructures. Appl. Phys. Lett. 87, 052108.
`Thompson, K., Flaitz, P.L., Ronsheim, P., Larson, D.J., Kelly, T.F., 2007. Imaging of
`arsenic cottrell atmospheres around silicon defects by three-dimensional atom
`probe tomography. Science 317, 1370.
`Topuria, T., James, E.M., Browning, N.D., 2001. Direct atomic scale characterization
`of interfaces and doping layers in field-effect transistors. Appl. Phys. Lett. 79,
`132.
`Twitchett, A.C., Dunin-Borkowski, R.E., Hallifax, R.J., Broom, R.F., Midgley, P.A., 2003.
`Off-axis electron holography of electrostatic potentials in unbiased and reverse
`biased focused ion beam milled semiconductor devices. J. Microsc. 214 .
`Voyles, P.M., Muller, D.A., Grazul, J., Citrin, P.H., Gossmann, H.-J.L., 2002. Atomic-
`scale imaging of individual dopant atoms and clusters in highly n-type bulk Si.
`Nature 416 (826).
`
`
`
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