LETTER pubs.acs.org/NanoLett
Field Mapping with Nanometer-Scale Resolution for the Next Generation of Electronic Devices David Cooper* and Francisco de la Pe~na CEA-LETI, MINATEC Campus, 17 rue des Martyrs, 38054 Grenoble Cedex 9, France
Armand Beche and Jean-Luc Rouviere CEA-INAC, MINATEC Campus, 17 rue des Martyrs, 38054 Grenoble Cedex 9, France
Germain Servanton, Roland Pantel, and Pierre Morin ST Microelectronics, 850 Rue Jean Monnet, F38926 Crolles, France ABSTRACT: In order to improve the performance of today’s nanoscaled semiconductor devices, characterization techniques that can provide information about the position and activity of dopant atoms and the strain fields are essential. Here we demonstrate that by using a modern transmission electron microscope it is possible to apply multiple techniques to advanced materials systems in order to provide information about the structure, fields, and composition with nanometer-scale resolution. Off-axis electron holography has been used to map the active dopant potentials in state-of-the-art semiconductor devices with 1 nm resolution. These dopant maps have been compared to electron energy loss spectroscopy maps that show the positions of the dopant atoms. The strain fields in the devices have been measured by both dark field electron holography and nanobeam electron diffraction. KEYWORDS: Off-axis electron holography, semiconductors, dopant mapping, strain mapping, specimen preparation
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he past few years have seen great advances in the power of transmission electron microscopes (TEM). The use of Cs aberration correctors is now widespread, and imaging with spatial resolutions as low as 50 pm is possible.1 With electron energy loss spectroscopy (EELS), the measurement of chemistry and bonding at an atomic scale has been shown2 and even the detection of hydrogen by annular bright field scanning transmission electron microscopy has been demonstrated.3 More recently the 3D atomic imaging of nanoparticales has been reported.4 In the future, the development of semiconductor devices will indeed involve the observation and control of individual atoms.5 For the current generations of semiconductors in development, tools that can provide two-dimensional field maps with nanometerscale resolution from real devices are required. During the development of new semiconductor technologies, it is important to be able to obtain information about the distribution of dopant atoms and active dopants in the source, drain, and gate regions. In addition, information about the strain that is now routinely applied to the channel is required. Knowledge of these properties allows the electrical performances of these devices to be simulated and understood. TEM is becoming a more easy to use and versatile tool, allowing multiple techniques to be applied to a single TEM specimen in order to gain an insight into the structure, composition, and nanofields that are present, all within a reasonable amount of time. In this paper we show for the first r 2011 American Chemical Society
time how off-axis electron holography and EELS can be combined to provide maps of both the active dopants potentials and atomic dopant concentrations in the same 40-nm-gate nMOS device. Dark-field electron holography and nanobeam electron diffraction (NBED) have been performed to map the strain that is applied to the conduction channel using a nitride contact etch stop linear (CESL). In addition, structural information about these specimens has been acquired by using conventional imagecorrected TEM. Due to the excellent versatility and stability of the latest generation of TEMs, it is possible to extract a wealth of useful and quantitative information during the same microscope session. For example, Figure 1 demonstrates what is now possible using a single TEM specimen. Figure 1a shows a conventional high-angle annular dark field scanning transmission electron microscopy (HAADF STEM) image of a arsenic-doped nMOS device. This image is formed using an annular detector that collects electrons which have been elastically scattered to provide massthickness contrast. In particular, it is sensitive to the atomic number, Z, so heavier elements appear brighter and the structure and dimensions of the device are revealed. Figure 1b shows an aberration corrected Received: May 28, 2011 Revised: September 22, 2011 Published: October 05, 2011 4585
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Figure 1. (a) A STEM image of the nMOS specimen showing the structure and the different materials that are present. (b) A Cs corrected TEM image of the specimen focusing on the thickness of the SiO layer. (c) An arsenic map acquired by EELS. (d) A dopant potential map acquired by off-axis electron holography. (e, f) Strain maps that have been acquired by dark field electron holography for the εxx and εzz directions, respectively. Panels cf all use the same length scale.
TEM image of the sample, the improvement of the delocalization from the Cs correction allows the thickness of the SiO2 layer to be measured quickly and easily. In Figure 1c the atomic concentration of arsenic dopants acquired by STEM EELS can be seen. Figure 1d shows a dopant potential map and panels e and f of Figure 1 show strain maps in the specimen for the in-plane, εxx, and growth, εzz, directions, all acquired using electron holograms. Off-axis electron holography is a powerful technique that can be used to map the nanofields that are now present in semiconductor devices. These can be in the form of dopant potentials or the strain that is now routinely introduced in order to boost the mobility of the carriers in the conduction channel.6 A coherent electron beam is transmitted through a specimen and is interfered with a reference that passes through vacuum by using a charged wire, known as a Moellenstedt biprism.711 The resulting interference pattern, called the hologram, contains information about the phase of the electrons which can be reconstructed. As the phase of an electron is sensitive to the changes in electrical potential, it can be used to measure the position of the electrically active dopants.12 Dark field electron holography can be used to recover the strain fields in nanoscale materials;13 here a diffracted beam for the required set of lattice planes is selected by using an objective aperture. The electrons in the diffracted beam are interfered with the electrons that have passed through an unstrained reference region with the same crystallographic orientation as the region of interest. This forms a dark field electron hologram from which a phase image, known as a displacement field, can be reconstructed. The strain can then be recovered from the phase image by using a geometrical phase analysis (GPA) algorithm.14 Despite the advances in the performance of modern microscopes, specimen preparation is the key to successful characterization. To locate a 40 nm gate transistor in a 300 mm wafer is a formidable task, and the use of a dual-beam focused ion beam (FIB) tool allows parallel-sided specimens to be extracted from a region of interest. The specimens were prepared by FIB milling at a low operating voltage and were finished by back side milling to prevent the differential milling under the differently metallized surface regions. A problem for dopant profiling of FIB prepared samples is the presence of a damaged crystalline near-surface layer where the dopants are not active; this artifact is strongly
dependent on the method used to prepare the specimens and also the dopant concentration.15 As the concentrations of dopants in modern semiconductor specimens are now very high, these artifacts can be minimized and thinner specimens can be examined. Conversely, for strain measurements by dark field electron holography, the principal problem is getting enough electron counts into the diffracted beam. The key to electron holography is to maximize the number of electron counts while preserving the fringe contrast.16 By use of the stability of a modern electron microscope, electron holograms can be acquired for more than a minute compared to typical acquisition times of a few seconds to improve the signal-to-noise ratio in the reconstructed phase images.17 This stability also allows specimens to be examined that are thicker than usual for dark field electron holography. As a compromise between dopant and strain measurements the specimen examined here had a final crystalline thickness of 180 nm. The HAADF STEM, NBED, and electron holography experiments were performed using a probe corrected FEI Titan TEM operated at 200 kV; however the corrector was not used. All of these experiments were performed on the same sample during the same session on the TEM. To obtain information only about the dopants when performing electron holography, the specimen is tilted to a weakly diffracting condition. Figure 2a shows an electron hologram acquired using conventional off-axis electron holography. Here a weak Lorentz lens is used instead of the conventional objective lens to provide a field-of-view of 360 nm with a fringe spacing of 2.2 nm. The spatial resolution of the reconstructed phase image is typically three times the fringe spacing. The ultimate spatial resolution is limited by the coherence of the electron beam; as the voltage on the biprism is increased, the fringe spacing will decrease, but so will the contrast. To preserve the hologram contrast, it is preferable to use 6 CCD pixels to sample each fringe. Thus the spatial resolution is also limited by required field of view due to the finite number of CCD pixels. For a thin specimen of constant thickness which is homogeneous in the direction of the electron beam, the phase of an electron, Δϕ can be related to the built in potential, Vbi in the specimen by the formula Δϕ = CEVbitactive, where CE is a constant and tactive is the crystalline specimen 4586
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Figure 2. (a) A conventional medium resolution off-axis electron hologram acquired using a weak Lorentz lens to provide a large field of view. (b) A potential map of the 40 nm gate nMOS device clearly showing the presence of dopants and the electrical junctions. (c) Potential profiles from the regions indicated in (b). (d) A potential map of the specimen acquired using the conventional objective lens and a field of view of 75 nm. The spatial resolution determined by the hologram fringe spacing is 1.0 nm. (e) An arsenic map of the specimen acquired by EELS. (f) A magnified region of the EELS map under the gate. (g) Arsenic concentration profiles acquired from across the doped region indicated in (e). A 1% arsenic concentration corresponds to a dopant concentration of 5 1020 cm3.
thickness containing active dopants. Figure 2b shows a potential map of the device calculated from the reconstructed phase image; the doped regions are clearly observed. As the original hologram has a fringe spacing of 2.2 nm, the spatial resolution in the potential map is 6.6 nm. To improve the spatial resolution, holograms have also been acquired using the conventional objective lens. Until recently this approach was not used, as the achievable field-of-view was often smaller than the size of the specimens that were being examined. Figure 2d shows a potential map of the specimen with a field-of-view of 75 nm and a spatial resolution of 1 nm. Here the original phase image was reconstructed from an electron hologram with a fringe spacing of 0.33 nm that was acquired for 64 s. The gate overlap distance, δL which is an important parameter, has been measured as 4 nm, which is consistent with the expected value and validates the parameters used in the device simulations. As off-axis electron holography is typically performed away from a zone axis in order to reduce the effects of diffraction in the phase image, it is important to discuss the differences in the spatial resolution imposed by the fringe spacing and the spatial resolution imposed by the specimen tilt. For the experimental results shown here, the
specimen was tilted by 0.5° in the β-direction which is indicated in Figure 2a. For a 180 nm thick specimen this corresponds to a smearing of the information of 1.5 nm. There was no specimen tilt used in the α direction. As either tilt axis can be chosen, the spatial resolution imposed by the specimen tilt can be reduced for a particular direction. Figure 2e shows a quantitative arsenic map of the sample that has been acquired by STEM EELS. The spatial resolution is 2 nm and the detection limit is better than 1 1019 cm3.18 The arsenic maps were acquired using a FEI Osiris TEM equipped with an X-FEG high-brightness gun operated at 120 kV to reduce knock on damage. The spectra were acquired over a specimen area of 380 by 140 nm and sampled by 180 by 65 pixels, leading to a spatial resolution of 2.1 nm in the map. As a consequence, 12000 individual spectra were acquired at a speed of 0.3 s each, leading to an acquisition time of 1 h for the arsenic map. Principal components analysis (PCA) was used to improve the signal-to-noise ratio of the technique, and curve fitting was used for quantification19,20 The magnified EELS map shown in Figure 2f reveals that the arsenic has diffused 4 nm under the gate, 4587
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Figure 3. (a) A dark field electron hologram of the 40 nm nMOS specimen for the εxx direction and (b) the corresponding strain map. As only diffracted electrons are collected, only the crystalline regions of the specimen are visible in the strain map. (c) Strain profiles from the region indicated in (b) acquired by both dark field electron holography and NBED. (d) A dark field electron hologram for the εzz direction and (e) the corresponding strain map. (f) Strain profiles from the region of interest indicated in (e) acquired by both dark field electron holography and NBED.
and this value is in agreement with both the electron holography results and the simulations. It is important to discuss the complementarity between EELS and electron holography. Profiles A and B in Figure 2c show that the step in electrical potential across the gate varies as a function depth in the substrate. Profile A shows a step in potential of 0.42 V across the gate, whereas in profile B, 0.52 V is measured. This variation of electrical potential under the gate is an important property of the device and it can be observed by electron holography. However, the EELS profiles A0 and B0 shown in Figure 2g and extracted from the same location in the device as A and B suggest that no arsenic dopants are present. This is because, for the acquisition times used here, the EELS maps are insensitive to the very low dopant concentrations that can influence the electrical potentials in this region. One of the problems with electron holography when used for dopant profiling is quantification. The device simulations suggest that this step in potential should be 0.65 V across profile A; however, it is difficult to take quantitative values from these potential maps due to artifacts that are introduced in the specimens during preparation. Progress is being made in understanding these problems with the aim of being able to extract information about the active dopant concentrations in the future.15 Presently, if care is taken during the experiments, it is reasonable to draw conclusions about the active dopants from variations in the measured potentials. In the EELS profile A0 , very high concentrations of arsenic, in the range 48%, are present in the source and drain regions of the transistor in order to provide Ohmic contacts. Profile, B0 shows that the arsenic concentration deeper in the devices is more regularly distributed at 3% or 1.5 1021 cm3. This corresponds more closely to the heterogeneity of the potential map seen in the potential profiles, A and B. This is partly due to
the solubility limit of arsenic in silicon which is just over 3%;21 therefore the measured potential should not vary in these regions of the devices, as the excess dopants above the solubility limit will not be active. In addition, classical semiconductor theory shows that the electrical potential is not very sensitive to small changes of very high dopant concentrations. In the regions under the gate, it is the LDD implants that effectively control the performance of the device. Here the dopant concentrations are typically an order of magnitude lower than those seen in the source and drain regions and, as a consequence, electron holography is well adapted for measuring the large changes of electrical potential that are present here. Additionally, it is possible to see the arsenic that is present in the gate structure, such as the spacers which have been implanted during device processing. The devices examined here were not exactly the same. The device examined by electron holography was from an array of nine, each spaced by 220 nm, and the device examined by EELS was from an array spaced by 180 nm. This was due to human error in identifying individual devices in a specimen that contained many arrays of similar devices. However, the experiments were performed on the same TEM lamella that were prepared at the same time with the same specimen thickness. This capability to quantitatively extract the atomic dopant concentrations and then to make direct comparisons to the electrical gate width measured by holography will be extremely useful in semiconductor research. Just as for the dopants, the direct measurement of strain in these nMOS devices is important in order to understand their performance. Although it is possible to simulate the effects of the CESL films, it is difficult to take into account the effects of dopant implantation and annealing on the strain that is present in the channel. To map the strain, dark field electron holograms were acquired using a (002) diffracted beam for the growth direction, εzz and a (020) beam for the in-plane direction, εxx. Strain 4588
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Nano Letters mapping by NBED with nanometer-scale spatial resolution is an additional recent development in electron microscopy.22 Here, a three condenser lens system was used to provide a 6 nm fwhm electron beam with a convergence angle of 0.2 mrads. The beam was scanned over the region of interest, and the shift of the diffracted spots was used to determine the strain relative to a reference pattern. Figure 3a shows a dark field electron hologram that has been acquired for 64 s in order to maximize the number of electron counts with a fringe spacing of 2.2 nm for the εxx direction. The strain map in Figure 3b shows an expanded lattice parameter; thus the regions underneath the electrical gate are under tensile strain. Figure 3c shows a quantitative strain profile that has been acquired from the region indicated in panel b and averaged over only 4 nm. For CESL strained devices, low values of strain are expected and therefore an excellent sensitivity of the strain measurement is required which is not easily accessible when using traditional techniques such as GPA.25 A value of strain of only 0.2% has been measured which is consistent with the NBED profile and simulations. This corresponds to a gain of ∼7% in the electron mobility which is significant in nMOS devices. Figure 3d shows a dark hologram for the εzz direction and panel e shows the corresponding strain map. The holography and NBED profiles shown in Figure 3f indicate that the region in the conduction channel has a reduction in the lattice parameter of 0.2%, this compensation is consistent with the tensile strain that is measured in the εxx direction. The thin TEM specimens will relax.23 From finite element simulations, the experimentally measured value of strain has been shown to have been reduced by around 10% in calibration specimens when compared to bulk material;24 however the relaxation is expected to be much less for real device specimens.13 The presence of strain in these devices will strongly affect the mobility of the carriers. It does not, however, significantly effect the electrical potentials. Therefore it is not useful to directly compare the potential and strain maps. However, it is important when mapping specimens that are both doped and strained that the presence of highly doped regions do not lead to artifacts in the strain maps. However, for the concentrations that are used in these devices, the active dopants make only a very small contribution to the strain maps as the measured phase change that results from the strain is typically an order of magnitude higher than the contribution from the active dopants. It has been demonstrated that by dark holography, a precision of 0.02% can be achieved compared to around 0.06% for a NBED profile.22,24,25 The main advantages of using NBED are that the strain profiling is performed down a zone axis removing the projection effects in transmission, that information about all of the lattice planes are collected simultaneously, and that there is no need to have a reference region either close or well-aligned to the region of interest making it more versatile. We have shown that by using a state-of-the-art TEM, it is possible to obtain information about the structure, the dopant potentials and dopant concentrations, and the strain from a single TEM specimen. The EELS maps show the atomic concentrations and the strain maps are fully quantitative. The potential maps obtained by electron holography can give information about the active dopants in the specimens. The high dopant concentrations reduce the artifacts that are present in dopant profiling by holography and also permit their detection by EELS. The stability of the TEM permits electron holograms to be acquired for long time periods which allow dopant and strain maps to be obtained with excellent signal-to-noise ratios. The
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values of strain measured by dark holography and NBED are consistent with each other. These measured properties have been used to improve the simulations that are used to understand the electrical performance of the device. The latest generation of TEMs permit significant improvements, such as the incorporation of high-brightness coherent electron sources for electron holography and EELS and the use of high sensitivity energy dispersive detectors (EDX) for chemical mapping. Three-dimensional information about the uniformity of the structure of the devices can now be obtained by taking an automated tilt series of the device by using HAADF STEM. In addition, progress is being made on 3D dopant potential profiling26 and chemically resolved electron tomography.27 In summary, the complexity of modern nanostructured materials requires precise control of their properties which can only be achieved through accurate characterization. By taking advantage of both the stability and flexibility of a modern TEM, it is now possible to quantitatively map structure, chemistry, strain, and electrical properties with nanometer-scale resolution which will be of benefit to all fields of microelectronics, metallurgy, and beyond. Of course, our future challenges are to realize Feynmann’s dream and to visualize all of these properties at an atomic scale in three dimensions.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT This work has been funded by the Recherche Technologie de Base (RTB) and UTTERMOST programmes. The experiments were performed on the Nanocharacterisation platform at MINATEC. ’ REFERENCES (1) Erni, R.; Rossell, M. D.; Kisielowski, C.; Dahmen, U. Phys. Rev. Lett. 2009, 102, 096101. (2) Muller, D.; Kourkoutis, L.; Murfitt, M.; Song, J. H.; Hwang, H. Y.; Silcox, J.; Dellby, N.; Krivanek, O. L. Science 2008, 319, 1073–1076. (3) Ishikawa, R.; Okunishi, E.; Sawada, H.; Kondo, Y.; Hosokawa, F.; Abe, E. Nat. Mater. 2011, 10, 278–281. (4) Van Aert, S.; Batenburg, K.J.; Rossell, M.D.; Erni, R.; Van Tendeloo, G. Nature 2011, 470, 374–377. (5) Koenrad, P.M.; Flatte, M. E. Nat. Mater. 2011, 10, 91–100. (6) Ghani, T.; et al. IEDM Tech. Dig. 2003, 11.6.1–11.6.3. (7) Lichte, H. Ultramicroscopy 1986, 20, 293–304. (8) Tonomura, A. Rev. Mod. Phys. 1987, 59, 639–669. (9) Lichte, H.; Formanek, P.; Lenk, A.; Linck, M.; Matzeck, C.; Lehmann, M.; Simon, P. Annu. Rev. Mater. Res. 2008, 37, 539–588. (10) McCartney, M. R.; Smith, D. J. Annu. Rev. Mater. Res. 2008, 37, 729–767. (11) Midgley, P. A.; Borkowski, R.E. Dunin Nat. Mater. 2009, 8, 271–280. (12) Rau, W. D.; Schwander, P.; Baumann, F. H.; Hoppner, W.; Ourmazd, A. Phys. Rev. Lett. 1999, 82, 2614–2617. (13) Hytch, M.; Houdellier, F.; Hue, F.; Snoeck, E. Nature 2008, 453, 1086. (14) Hytch, M.; Snoeck, E.; Kilaas, R. Ultramicroscopy 1998, 74, 131. (15) Cooper, D.; Ailliot, C.; Truche, R.; Hartmann, J.; Barnes, J.; Bertin, F. J. Appl. Phys. 2008, 104, 064513. (16) Lichte, H. Ultramicroscopy 2008, 108, 256–262. 4589
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