- Email: [email protected]
3D-analysis of semiconductor structures by electron tomography
Microelectronic Engineering 84 (2007) 2707–2713 www.elsevier.com/locate/mee
3D-analysis of semiconductor structures by electron tomography H. Bender b
a,*
, O. Richard a, A. Kalio a, E. Sourty
b
a IMEC, Kapeldreef 75, BE-3001 Leuven, Belgium FEI, Achtseweg Noord 5, NL-5651GG Eindhoven, The Netherlands
Received 9 May 2007; accepted 21 May 2007 Available online 26 May 2007
Abstract The analysis of advanced nano-devices by the classical 2D imaging with transmission or scanning transmission electron microscopy suffers from projection effects over the sample thickness that result in e.g. blurring due to interfacial roughness or superposition of different structures. Electron tomography allows to overcome these problems. The method involves the acquisition of tilted series of 2Dimages, the accurate alignment of these images and the 3D volume reconstruction. Slicing in any direction through this volume yields sections through the device structure with resolution of a few nanometer. The methodology is discussed and illustrated with some case studies. Ó 2007 Elsevier B.V. All rights reserved. Keywords: Transmission electron microscopy; Electron tomography; Nano-devices
1. Introduction High resolution transmission (TEM) and scanning transmission (STEM) electron microscopy allow lateral resolutions at the atomic scale. These techniques are widely used for the detailed structural and chemical analysis of nano-materials and nano-structures. (S)TEM imaging finds applications after all kinds of semiconductor process steps and becomes crucial for the metrology of the advanced structures for which important details can often not be revealed anymore by scanning electron microscopy, e.g. for the metrology of thin layers (gate dielectrics, metal barriers, seed layers) and small structures (FINFET lines), the characterization of over and underetch, or the modified layers in etched low-k materials. The excellent resolution and generally good contrasts between different materials allow to study fine details. Except for metrology, also other information can be obtained, e.g. compositional analysis, phase identification, lattice defects, or strain. Limitations of TEM analysis arise due to the need for the preparation *
Corresponding author. Tel.: +32 16 281 304. E-mail address: [email protected] (H. Bender).
0167-9317/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2007.05.013
of very thin samples, possible electron beam damage in sensitive materials (low-k dielectrics) and projection effects. In conventional TEM and STEM the information is integrated along the viewing direction over the thickness of the TEM specimen and a 2D-image is obtained. If the material is uniform and the interfaces are perfectly flat and seen edge-on along the electron beam direction this is generally an acceptable situation. However, in typical semiconductor nano-device structures these conditions are rarely met as the interfaces show roughness due to the etching or deposition processes, and even in very thin TEM specimens (<50–100 nm) of nano-devices different materials are often superimposed throughout the TEM lamellae. Therefore, there is a growing need to expand the TEM capabilities from the classical 2D imaging and chemical analysis to a technique that allows 3D characterization. For biological applications electron tomography in TEM mode is introduced already decades ago [1] while for applications in materials science, electron tomography has only recently received a steeply growing interest [2–5]. This is stimulated by hardware and software developments that strongly facilitate the application of electron tomography, i.e. automated high accuracy stages, sample holders with
2708
H. Bender et al. / Microelectronic Engineering 84 (2007) 2707–2713
large tilt range, high angle annular dark field (HAADF) STEM-detectors, slow scan CCD cameras for TEM and EFTEM imaging, fast computers with automation software for acquisition, aligning the image series and reconstructing the volume. Electron tomography results of semiconductor devices are recently reported for Cu interconnects [3,6], via structures [5–8], transistors [5], memory cells [3], FINFET devices [9] and Si nanoparticles in oxide [10]. This paper shortly reviews the basics of electron microscopy and tomography and discusses some examples of electron tomography for the analysis of nano-devices. 2. Transmission electron microscopy: operating modes and contrast In standard transmission electron microscopy a wide electron beam interacts with the sample and the image is a direct representation of the exit wave magnified by the projection lens system. The image is formed with the unscattered transmitted electrons (bright field), the diffracted electrons (dark field) or the interference of the transmitted and a number of diffracted beams (high resolution imaging). For thin amorphous samples (e.g. dielectrics or biological materials) the bright field contrast is dominated by elastic scattering in the sample and scales mainly with the mass Æ thickness product of the sample. Upon tilting the sample the thickness will increase inversely with the cosine of the tilt angle and the contrast will scale accordingly. For crystalline materials the contrast strongly depends on the diffraction conditions, i.e. on the orientation of the crystal relative to the electron beam and changes strongly and irregularly upon tilting the crystals. Filtering the inelastically scattered electrons with a characteristic element-dependent energy loss in energy filtered TEM (EFTEM) results after subtraction of the background in a signal that is proportional to concentration Æ thickness product. However, this condition is only fulfilled for samples thinner than the inelastic mean free path k as for thicker samples the intensity drops due to multiple scattering events. The condition has still to be fulfilled at the maximum tilt angle used in the experiment, which practically limits the specimen thickness to <30– 50 nm. Diffraction effects are generally weak in elemental mappings. In scanning transmission electron microscopy the electron beam is focused in a small spot (sub-nm to nm range) and x–y scanned over the sample. The transmitted intensity of the unscattered (bright field), low angle scattered (dark field) and high angle scattered (high angle annular dark field, HAADF) are registered synchronously with the electron scan and generate the images. The HAADF signal consists mainly of elastically Rutherford scattered electrons. The intensity shows a strong materials dependence, i.e. Z1.7 and only weak diffraction effects, i.e. the product mass Æ thickness product will vary monotonously with sample tilt.
3. Electron tomography The basic requirement for electron tomography is the assumption that the projected signal varies monotonously with a physical property of the sample. For amorphous materials it can be assumed that this condition is fulfilled in bright field TEM mode as the image contrast is dominated by elastic scattering and scales with the mass Æ thickness product. Therefore, for biological applications bright field TEM is commonly used for electron tomography. For materials science applications this mode is however not useful due to the diffraction contrast effects. The method of choice for nano-materials and nano-devices is therefore HAADF–STEM. The fact that high resolution STEM and HAADF-detectors became only more recently widely available on TEM-instruments is a major cause that electron tomography for materials science applications became popular only recently. Also inelastically scattered electrons analyzed with energy filtered TEM can be used for electron tomography both for biological and materials science applications [11,12]. The limitation on the specimen thickness is a major drawback for EFTEM-tomography which also requires longer acquisition times. In STEM mode mapping of the electron loss signal (EELS) as well as of the characteristic X-rays (EDS) can be considered [12]. As for EFTEMtomography also the EELS mapping will be limited to extremely thin samples, while for EDS-tomography long acquisition times will be needed. Non-standard tomography has also been reported for the study of p–n junctions with tomographic electron holography [13] and lattice defects in weak beam imaging mode [14]. The electron tomography experiment [2,3] consists of the acquisition of a series of 2D-images over a wide tilt range and with small angular increments, typically ±75° with 1° steps. The typical image shifts versus the tilt angle are pre-calibrated for the sample holder used. The software compensates for the major shifts and refocuses the images after each tilt step, so that in principle the data acquisition can be done fully automatically once the system is set-up. The subsequent data analysis involves the accurate alignment of the tilt series to compensate for the residual shifts between the images and to determine the direction of the tilt axis. The alignment is based on the cross-correlation between the images. To improve this process various kinds of filters can be applied to enhance the image features. Generally a better alignment can be obtained with markers. Typically nm-sized Au particles dispersed over the thinned specimen are used for this purpose. Several methods for the 3D-reconstruction are proposed working in Fourier or real space [2,3]. Most commonly used nowadays are back projection methods in real space. The direct back projection oversamples the low frequencies which results in blurring of the fine details in the object. This can be minimized by applying a weighting filter that reduces the low frequency contributions, a method known as weighted back projection (WBP). This calculation is relatively fast, but the result
H. Bender et al. / Microelectronic Engineering 84 (2007) 2707–2713
is generally rather noisy. In the simultaneous iterative reconstruction (SIRT) method the differences between the projection of the reconstructed volume and the original 2D-images are minimized in an iterative procedure. The contrasts in the SIRT-reconstruction are generally quite similar to the expectations based on standard (S)TEM imaging. The resolution for a ±90° tilt series is given by the Crowther equation [2,3]:
2709
The maximum tilt range during the acquisition of a tomography series is limited by the configuration of the polepieces of the objective lens of the microscope, the design of the sample holder and the preparation of the TEM
d ¼ pD=N with D the diameter of the reconstructed volume and N the number of 2D-images. The practical tilt range is generally only <±75°. The ‘‘missing wedge’’ gives rise to artifacts in the reconstruction, results in elongation along the optic axis z and loss of contrast at the outer boundaries of the reconstructed volume. Therefore, the resolution along the optic axis is degraded by a factor ez, which depends on the maximum tilt angle: sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi am þ sin am cos am d z ¼ d y ez ¼ d y am sin am cos am Based on these equations a theoretical resolution of 1–2 nm can be calculated for objects of 100 nm using 150 tilt steps. Reducing the missing wedge problem is possible by combining two tilt series obtained with nearly orthogonal tilt axes.
Fig. 1. Cylindrically shaped specimen prepared by FIB through a FINFET structure.
4. Sample preparation In situ lift-out in a focused ion beam/scanning electron microscope (FIB/SEM) system is a common method for the preparation of TEM specimens of nano-devices [15]. To minimize the projection effects for classical 2D TEM or STEM, the specimen thickness should be as small as possible. If the device dimensions are in the tens of nanometer range, this requirement poses strong constraints on the positioning accuracy of the preparation. As the exact position of the structures in nano-devices is usually not recognizable in the top view SEM image, one has to rely on the layout of the structure and on the cross-section SEM image during the preparation. Although the intermediate SEM images allow an accurate control of the progress of the milling towards the structure of interest, some uncertainty remains. Except for the limitations due to the SEM resolution, a major cause is the interpretation of the SEM-contrasts which are determined by the information depth which depends on the imaging and detection conditions, the thickness of the lamellae and the material properties. Fortunately, an important advantage of electron tomography is the possibility to select the slices of interest after the 3D-reconstruction. Therefore, the requirement on ultimately thin samples as a way to minimize projection effects, can be relaxed (except for EFTEM-tomography) and therefore the positioning accuracy during the preparation becomes less crucial.
Fig. 2. TEM (a) and HAADF–STEM (b) cross-section image of a Nisilicide FUSI gate with TiN/HfO2/interlayer gate stack.
2710
H. Bender et al. / Microelectronic Engineering 84 (2007) 2707–2713
Fig. 3. Tomography 3D-reconstruction of the Ni-silicide FUSI gate structure shown in Fig. 2: full gate structure (a) and detail of the upper silicide phase (b).
specimen. Dedicated sample holders are available nowadays that allow a ±75° tilt. For plan-parallel TEM lamellae prepared by polishing/ion milling or by the focused ion
beam lift-out method [15], the thickness will increase to infinity for further tilting. To overcome this problem, needle shaped specimens are considered which can be prepared by FIB (Fig. 1). Obviously it renders the needed accuracy of the site-specific FIB preparation from a one-dimensional to a two-dimensional problem so that even with advanced dual beam FIB/SEM systems the centering of small nanostructures in the needle is a tedious activity. For such needle shaped specimens a full ±90° tilt could be considered with dedicated sample holders [16] which will likely become widely available in the future. The absence of the missing wedge in tomography acquisitions with such specimens should result in an equal resolution in all specimen directions. The strong contrast of heavy materials (W, TaN, HfO2) in HAADF mode often overwhelms the details in areas with lighter composition (dielectrics, poly-Si). Therefore, whenever possible, removing the regions with heavy materials during sample preparation, e.g. by selective milling or use of cylindrical specimens is advantageous. 5. Applications Some applications of HAADF–STEM electron tomography on semiconductor device structures are discussed to illustrate the potential of the method.
Fig. 4. Single slices from the 3D-reconstruction of Fig. 3 at the position where the Ni-rich phase extends till the bottom of the gate using WBP (a, d), and SIRT with 2 (b, e) and 20 (c, f) iterations seen along the optic axis (a–c) and along the wafer normal (d–f). The dashed line on (a) shows the position of the y slices shown in d–e; and the dashed line on (d) shows the position of the z slices in a–c.
H. Bender et al. / Microelectronic Engineering 84 (2007) 2707–2713
5.1. Fully silicided gate (FUSI) Fig. 2a shows a TEM cross-section image of a FUSI Nisilicide/TiN/HfO2 gate structure with the silicon substrate aligned along the [1 1 0] cross-section direction. Different grains are revealed at the top of the gate. The image contrast is strongly dominated by diffraction effects, i.e. grain orientations. Tilting the sample to optimize the diffraction contrast between the grains could be considered but then the interfaces between the different materials will be blurred as they will no longer be seen (nearly) edge-on. Identifying the possible presence of different silicide phases will require detailed high resolution or electron diffraction investigations and hence the possible presence of different phases is not immediately obvious from such images. The same structure imaged in HAADF–STEM mode (Fig. 2b) clearly reveals that the top of the gate is Ni-rich and moreover that the Ni-rich phase extends till the bottom of the gate. However, whether this deep extension is a local effect or occurs along the full gate line cannot be decided from the 2D-image. Tomography series are acquired with the sample mounted in two orientations with respect to the tilt axis, i.e. near the vertical and horizontal direction of the gate. The tilt range is 72° to 73° in 1° increments. The reconstructed volume of the tomogram acquired with rotation around the vertical axis is shown in Fig. 3. No gold markers are used in this case so that the alignment of the tilt series is based on cross-correlation between the images. Slicing down along the structure unambiguously shows that the upper Ni-rich phase only locally extends to the bottom of the gate. Reconstructions with different algorithms are compared in Fig. 4 which shows slices viewed along the z-axis at the position where the upper phase reaches the TiN and slices parallel with the Si substrate. The WBP reconstruction (Fig. 4a and d) is quite noisy compared to the SIRT results (Fig. 4b,c,e,f) but often shows better contrast, e.g. between the nitride spacer and the oxides. The SIRT calculation with only two iterations (Fig. 4b and e) shows an unacceptable smearing of the contrasts at the nitride spacer and at the edges of the source/ drain silicide. These artifacts diminish with increasing number of iterations (Fig. 4c and f). For the example shown, the calculation with WBP takes approximately 2 min, whereas the SIRT calculations require 10 min per iteration (on a Pentium-4, 3 GHz computer with 2 GB RAM). To minimize artifacts generally at least 10 SIRT iterations are required, resulting in a typical minimum calculation time of several hours for the reconstruction. On the slices parallel with the substrate (Fig. 4d–f), more artifacts are visible at the edges of the TEM sample. These are directly related to the missing wedge of the limited tilt range used for the acquisition.
2711
tilt range of 66° to 62° in 1° increments. The alignment is performed without gold markers and a 20 cycles SIRTreconstruction is applied. The detailed morphology of the different materials can be studied by stepping through the slices of the reconstructed volume, e.g. the shape of the keyhole in the W can directly be visualized whereas this information will be masked by the projection in a 2Dimage. The quantitative comparison of the results of different samples requires software tools that allow to perform automated metrology investigation in any given direction of, e.g. lengths or areas through the series of reconstructed slices [17]. The application of such automated procedures is hampered by the reconstruction artifacts (e.g. due to the missing wedge) and the criteria to determine the boundary between different materials. Surface rendering of the different materials can be done in, e.g. Amira software but some manual check/adjustment of the contours of the selected material at several positions in the series of slices remains necessary. Fig. 6 shows an example of the area of the W via calculated through the slices in x, y and z direction. The plot of the areas in the y direction yields information on the slope of the contact and shows severe narrowing of the via just under the Cu line. Similarly the dimensions of the other materials, e.g. the keyhole, can be studied.
5.2. Contacts Fig. 5 shows 3D slices through the reconstructed volume a Cu/W/NiSi contact structure. The acquisition is done in a
Fig. 5. 3D-reconstruction of a W contact between a silicided source and Cu metal line.
2712
H. Bender et al. / Microelectronic Engineering 84 (2007) 2707–2713
5.3. Memory cell Results obtained for an non-volatile memory (NVM) cell are compared in Fig. 7. A first sample is made including the nearby W contacts in the thinned lamella. Acquiring the series with a vertical tilt axis results in shadowing of the memory cell by the W plugs at high tilt axis, therefore in this configuration the maximum tilt range is limited by the structure to 75° to 75°. Moreover the W plug has a very bright contrast in HAADF–STEM mode which overwhelms the contrasts between the different light materials in the memory cell (Fig. 7a). Nevertheless details of the cell can be studied and, e.g. slicing through the device in z direction allows to study the roughness of the poly sidewalls in detail whereas this information is blurred by the projection in a classical 2D-image. To avoid the overlap with the W plugs the tilt axis can be chosen along the x-axis in the horizontal direction. This has however the drawback of overlap at high tilt angles with the substrate or with the FIB-Pt capping layer which has also a bright contrast. A more success-
Fig. 6. 3D surface rendering (a) of the W contact plug of Fig. 5 and area of the W contact plug calculated through the slices in x, y and z directions. (b) Z is the optic axis at 0° tilt.
ful approach is the removal of the W plugs by selective milling. This is applied to the same specimen which is reinvestigated afterwards. The slice at similar position (Fig. 7b) shows now better contrasts between the different materials so that, e.g. the liner layer over the cell can now clearly be distinguished. The milling of the W plugs resulted in some contamination on the faces of the specimen due to re-deposition and ion beam damage. This can be avoided by removing the plugs in an earlier state of the specimen preparation. A possible approach to remove the plugs is the preparation of needle shaped specimens (Fig. 7c). A tilt range of ±75° is used in this case and the HAADF–STEM contrast can now be fully optimized for the light materials in the memory cell. The cylindrically shaped specimens have the advantage of constant specimen thickness, i.e. constant
Fig. 7. Slices through a memory cell obtained with a specimen in which the W plugs are present (a), after selective milling of the W plug (b) and with a needle shaped specimen (c). On (c) the dark edges are the boundaries of the specimen which contains only the memory cell.
H. Bender et al. / Microelectronic Engineering 84 (2007) 2707–2713
2713
average contrast upon tilting. Due to their configuration the automatic focusing fails more easily than on plan-parallel specimens and also the cross-correlation for the alignment is less accurate. Therefore, the use of gold markers is more essential on such specimens.
be needed to avoid the missing wedge problem. Therefore, needle shaped specimens will be required which make sample preparation more critical.
6. Conclusions
[1] A.J. Koster, R. Grimm, D. Typke, R. Hegerl, A. Stoschek, J. Walz, W. Baumeister, J. Struct. Biol. 120 (1997) 276–308. [2] P.A. Midgley, M. Weyland, Ultramicroscopy 96 (2003) 413–431. [3] C. Ku¨bel, A. Voigt, R. Schoenmakers, M. Otten, D. Su, T.-C. Lee, A. Carlsson, J. Bradley, Microsc. Microanal. 11 (2005) 378–400. [4] M. Weyland, T.J.V. Yates, R.E. Dunin-Borkowski, L. Laffont, P.A. Midgley, Scripta Mater. 55 (2006) 29–33. [5] C. Ku¨bel, J. Ku¨bel, S. Kujawa, J.-S. Luo, H-M. Lo, J.D. Russell, AIP Conf. Ser. 817 (2006) 223–228. [6] P. Ercius, M. Weyland, D.A. Muller, L.M. Gignac, Appl. Phys. Lett. 88 (2006) 243116. [7] A. Takaoka, T. Hasegawa, K. Yoshida, and H. Mori, in: Proceedings 16th International Microscopy Conference (2006), 680. [8] S. Kudo, Y. Hirose, K. Fukumoto, S. Maegawa, J. Shimanuki, and Y. Inoue, in: Proceedings 16th International Microscopy Conference (2006), 1440. [9] O. Richard, A. Kalio, H. Bender, and E. Sourty, in: Proceeding 15th Conference on Microscopy of Semiconductor Materials, Cambridge, Springer Proceedings in Physics, 2007. [10] A. Yurtsever, M. Weyland, D.A. Muller, Appl. Phys. Lett. 89 (2006) 151920. [11] M. Weyland, P.A. Midgley, Microsc. Microanal. 9 (2003) 542–555. [12] G. Mo¨bus, R.C. Doole, B.J. Inkson, Ultramicroscopy 96 (2003) 433– 451. [13] A.C. Twitchett, T.H.V. Yates, R.E. Dunin-Borkowski, S.B. Newcomb, P.A. Midgley, J. Phys. Conf. Ser. 26 (2006) 29–32. [14] J. Sharp, J. Barnard, J. Tong, and P. Midgley, in: Proceedings 16th International Microscopy Conference (2006), 700. [15] L.A. Giannuzzi, B.W. Kempshall, S.M. Schwarz, J.K. Lomness, B.I. Prenitzer, F.A. Stevie, in: Introduction to Focused Ion Beams, Springer, 2005, pp. 201–228. [16] K. Jarausch, D. Leonard, and T. Kamino, in: Proceedings 16th International Microscopy Conference (2006), 687. [17] A. Kalio, O. Richard, E. Sourty, H. Bender, Proceeding 15th Conference on Microscopy of Semiconductor Materials, Cambridge, Springer Proceedings in Physics, 2007.
Electron tomography has strong potentials for the 3Danalysis of nano-scale device structures. The examples discussed show that by slicing through the reconstructed volume details can be revealed and more accurate metrology can be obtained than with standard 2D imaging. The variation of dimensions can be investigated across the volume, having the potential for statistical studies of, e.g. line widths or other critical parameters across the structures. The acquisition and analysis time for electron tomography experiments is much longer than for standard 2D imaging. Further improvements of the measurement procedure, including sample preparation, and data analysis methodology are on-going and will lead to better understanding/circumventing of the limitations and to reduction of the total acquisition and analysis time. Already nowadays improved software using the computer graphics allows a severe reduction of the calculation time. Still the data acquisition and the alignment of the series are time-consuming activities that make electron tomography much slower than the classical 2D TEM/STEM analysis. Other new algorithms, i.e. discrete tomography, are under development that can, in combination with prior knowledge about the material characteristics in the sample, for certain kinds of samples allow to reduce the number of images to be acquired. By slicing through the reconstructed volume the requirements for ultimately thin specimens can in principle be relaxed. On the other hand, to improve the resolution of the tomograms along the optic axis larger tilt ranges will
References
3D-analysis of semiconductor structures by electron tomography H. Bender b
a,*
, O. Richard a, A. Kalio a, E. Sourty
b
a IMEC, Kapeldreef 75, BE-3001 Leuven, Belgium FEI, Achtseweg Noord 5, NL-5651GG Eindhoven, The Netherlands
Received 9 May 2007; accepted 21 May 2007 Available online 26 May 2007
Abstract The analysis of advanced nano-devices by the classical 2D imaging with transmission or scanning transmission electron microscopy suffers from projection effects over the sample thickness that result in e.g. blurring due to interfacial roughness or superposition of different structures. Electron tomography allows to overcome these problems. The method involves the acquisition of tilted series of 2Dimages, the accurate alignment of these images and the 3D volume reconstruction. Slicing in any direction through this volume yields sections through the device structure with resolution of a few nanometer. The methodology is discussed and illustrated with some case studies. Ó 2007 Elsevier B.V. All rights reserved. Keywords: Transmission electron microscopy; Electron tomography; Nano-devices
1. Introduction High resolution transmission (TEM) and scanning transmission (STEM) electron microscopy allow lateral resolutions at the atomic scale. These techniques are widely used for the detailed structural and chemical analysis of nano-materials and nano-structures. (S)TEM imaging finds applications after all kinds of semiconductor process steps and becomes crucial for the metrology of the advanced structures for which important details can often not be revealed anymore by scanning electron microscopy, e.g. for the metrology of thin layers (gate dielectrics, metal barriers, seed layers) and small structures (FINFET lines), the characterization of over and underetch, or the modified layers in etched low-k materials. The excellent resolution and generally good contrasts between different materials allow to study fine details. Except for metrology, also other information can be obtained, e.g. compositional analysis, phase identification, lattice defects, or strain. Limitations of TEM analysis arise due to the need for the preparation *
Corresponding author. Tel.: +32 16 281 304. E-mail address: [email protected] (H. Bender).
0167-9317/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2007.05.013
of very thin samples, possible electron beam damage in sensitive materials (low-k dielectrics) and projection effects. In conventional TEM and STEM the information is integrated along the viewing direction over the thickness of the TEM specimen and a 2D-image is obtained. If the material is uniform and the interfaces are perfectly flat and seen edge-on along the electron beam direction this is generally an acceptable situation. However, in typical semiconductor nano-device structures these conditions are rarely met as the interfaces show roughness due to the etching or deposition processes, and even in very thin TEM specimens (<50–100 nm) of nano-devices different materials are often superimposed throughout the TEM lamellae. Therefore, there is a growing need to expand the TEM capabilities from the classical 2D imaging and chemical analysis to a technique that allows 3D characterization. For biological applications electron tomography in TEM mode is introduced already decades ago [1] while for applications in materials science, electron tomography has only recently received a steeply growing interest [2–5]. This is stimulated by hardware and software developments that strongly facilitate the application of electron tomography, i.e. automated high accuracy stages, sample holders with
2708
H. Bender et al. / Microelectronic Engineering 84 (2007) 2707–2713
large tilt range, high angle annular dark field (HAADF) STEM-detectors, slow scan CCD cameras for TEM and EFTEM imaging, fast computers with automation software for acquisition, aligning the image series and reconstructing the volume. Electron tomography results of semiconductor devices are recently reported for Cu interconnects [3,6], via structures [5–8], transistors [5], memory cells [3], FINFET devices [9] and Si nanoparticles in oxide [10]. This paper shortly reviews the basics of electron microscopy and tomography and discusses some examples of electron tomography for the analysis of nano-devices. 2. Transmission electron microscopy: operating modes and contrast In standard transmission electron microscopy a wide electron beam interacts with the sample and the image is a direct representation of the exit wave magnified by the projection lens system. The image is formed with the unscattered transmitted electrons (bright field), the diffracted electrons (dark field) or the interference of the transmitted and a number of diffracted beams (high resolution imaging). For thin amorphous samples (e.g. dielectrics or biological materials) the bright field contrast is dominated by elastic scattering in the sample and scales mainly with the mass Æ thickness product of the sample. Upon tilting the sample the thickness will increase inversely with the cosine of the tilt angle and the contrast will scale accordingly. For crystalline materials the contrast strongly depends on the diffraction conditions, i.e. on the orientation of the crystal relative to the electron beam and changes strongly and irregularly upon tilting the crystals. Filtering the inelastically scattered electrons with a characteristic element-dependent energy loss in energy filtered TEM (EFTEM) results after subtraction of the background in a signal that is proportional to concentration Æ thickness product. However, this condition is only fulfilled for samples thinner than the inelastic mean free path k as for thicker samples the intensity drops due to multiple scattering events. The condition has still to be fulfilled at the maximum tilt angle used in the experiment, which practically limits the specimen thickness to <30– 50 nm. Diffraction effects are generally weak in elemental mappings. In scanning transmission electron microscopy the electron beam is focused in a small spot (sub-nm to nm range) and x–y scanned over the sample. The transmitted intensity of the unscattered (bright field), low angle scattered (dark field) and high angle scattered (high angle annular dark field, HAADF) are registered synchronously with the electron scan and generate the images. The HAADF signal consists mainly of elastically Rutherford scattered electrons. The intensity shows a strong materials dependence, i.e. Z1.7 and only weak diffraction effects, i.e. the product mass Æ thickness product will vary monotonously with sample tilt.
3. Electron tomography The basic requirement for electron tomography is the assumption that the projected signal varies monotonously with a physical property of the sample. For amorphous materials it can be assumed that this condition is fulfilled in bright field TEM mode as the image contrast is dominated by elastic scattering and scales with the mass Æ thickness product. Therefore, for biological applications bright field TEM is commonly used for electron tomography. For materials science applications this mode is however not useful due to the diffraction contrast effects. The method of choice for nano-materials and nano-devices is therefore HAADF–STEM. The fact that high resolution STEM and HAADF-detectors became only more recently widely available on TEM-instruments is a major cause that electron tomography for materials science applications became popular only recently. Also inelastically scattered electrons analyzed with energy filtered TEM can be used for electron tomography both for biological and materials science applications [11,12]. The limitation on the specimen thickness is a major drawback for EFTEM-tomography which also requires longer acquisition times. In STEM mode mapping of the electron loss signal (EELS) as well as of the characteristic X-rays (EDS) can be considered [12]. As for EFTEMtomography also the EELS mapping will be limited to extremely thin samples, while for EDS-tomography long acquisition times will be needed. Non-standard tomography has also been reported for the study of p–n junctions with tomographic electron holography [13] and lattice defects in weak beam imaging mode [14]. The electron tomography experiment [2,3] consists of the acquisition of a series of 2D-images over a wide tilt range and with small angular increments, typically ±75° with 1° steps. The typical image shifts versus the tilt angle are pre-calibrated for the sample holder used. The software compensates for the major shifts and refocuses the images after each tilt step, so that in principle the data acquisition can be done fully automatically once the system is set-up. The subsequent data analysis involves the accurate alignment of the tilt series to compensate for the residual shifts between the images and to determine the direction of the tilt axis. The alignment is based on the cross-correlation between the images. To improve this process various kinds of filters can be applied to enhance the image features. Generally a better alignment can be obtained with markers. Typically nm-sized Au particles dispersed over the thinned specimen are used for this purpose. Several methods for the 3D-reconstruction are proposed working in Fourier or real space [2,3]. Most commonly used nowadays are back projection methods in real space. The direct back projection oversamples the low frequencies which results in blurring of the fine details in the object. This can be minimized by applying a weighting filter that reduces the low frequency contributions, a method known as weighted back projection (WBP). This calculation is relatively fast, but the result
H. Bender et al. / Microelectronic Engineering 84 (2007) 2707–2713
is generally rather noisy. In the simultaneous iterative reconstruction (SIRT) method the differences between the projection of the reconstructed volume and the original 2D-images are minimized in an iterative procedure. The contrasts in the SIRT-reconstruction are generally quite similar to the expectations based on standard (S)TEM imaging. The resolution for a ±90° tilt series is given by the Crowther equation [2,3]:
2709
The maximum tilt range during the acquisition of a tomography series is limited by the configuration of the polepieces of the objective lens of the microscope, the design of the sample holder and the preparation of the TEM
d ¼ pD=N with D the diameter of the reconstructed volume and N the number of 2D-images. The practical tilt range is generally only <±75°. The ‘‘missing wedge’’ gives rise to artifacts in the reconstruction, results in elongation along the optic axis z and loss of contrast at the outer boundaries of the reconstructed volume. Therefore, the resolution along the optic axis is degraded by a factor ez, which depends on the maximum tilt angle: sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi am þ sin am cos am d z ¼ d y ez ¼ d y am sin am cos am Based on these equations a theoretical resolution of 1–2 nm can be calculated for objects of 100 nm using 150 tilt steps. Reducing the missing wedge problem is possible by combining two tilt series obtained with nearly orthogonal tilt axes.
Fig. 1. Cylindrically shaped specimen prepared by FIB through a FINFET structure.
4. Sample preparation In situ lift-out in a focused ion beam/scanning electron microscope (FIB/SEM) system is a common method for the preparation of TEM specimens of nano-devices [15]. To minimize the projection effects for classical 2D TEM or STEM, the specimen thickness should be as small as possible. If the device dimensions are in the tens of nanometer range, this requirement poses strong constraints on the positioning accuracy of the preparation. As the exact position of the structures in nano-devices is usually not recognizable in the top view SEM image, one has to rely on the layout of the structure and on the cross-section SEM image during the preparation. Although the intermediate SEM images allow an accurate control of the progress of the milling towards the structure of interest, some uncertainty remains. Except for the limitations due to the SEM resolution, a major cause is the interpretation of the SEM-contrasts which are determined by the information depth which depends on the imaging and detection conditions, the thickness of the lamellae and the material properties. Fortunately, an important advantage of electron tomography is the possibility to select the slices of interest after the 3D-reconstruction. Therefore, the requirement on ultimately thin samples as a way to minimize projection effects, can be relaxed (except for EFTEM-tomography) and therefore the positioning accuracy during the preparation becomes less crucial.
Fig. 2. TEM (a) and HAADF–STEM (b) cross-section image of a Nisilicide FUSI gate with TiN/HfO2/interlayer gate stack.
2710
H. Bender et al. / Microelectronic Engineering 84 (2007) 2707–2713
Fig. 3. Tomography 3D-reconstruction of the Ni-silicide FUSI gate structure shown in Fig. 2: full gate structure (a) and detail of the upper silicide phase (b).
specimen. Dedicated sample holders are available nowadays that allow a ±75° tilt. For plan-parallel TEM lamellae prepared by polishing/ion milling or by the focused ion
beam lift-out method [15], the thickness will increase to infinity for further tilting. To overcome this problem, needle shaped specimens are considered which can be prepared by FIB (Fig. 1). Obviously it renders the needed accuracy of the site-specific FIB preparation from a one-dimensional to a two-dimensional problem so that even with advanced dual beam FIB/SEM systems the centering of small nanostructures in the needle is a tedious activity. For such needle shaped specimens a full ±90° tilt could be considered with dedicated sample holders [16] which will likely become widely available in the future. The absence of the missing wedge in tomography acquisitions with such specimens should result in an equal resolution in all specimen directions. The strong contrast of heavy materials (W, TaN, HfO2) in HAADF mode often overwhelms the details in areas with lighter composition (dielectrics, poly-Si). Therefore, whenever possible, removing the regions with heavy materials during sample preparation, e.g. by selective milling or use of cylindrical specimens is advantageous. 5. Applications Some applications of HAADF–STEM electron tomography on semiconductor device structures are discussed to illustrate the potential of the method.
Fig. 4. Single slices from the 3D-reconstruction of Fig. 3 at the position where the Ni-rich phase extends till the bottom of the gate using WBP (a, d), and SIRT with 2 (b, e) and 20 (c, f) iterations seen along the optic axis (a–c) and along the wafer normal (d–f). The dashed line on (a) shows the position of the y slices shown in d–e; and the dashed line on (d) shows the position of the z slices in a–c.
H. Bender et al. / Microelectronic Engineering 84 (2007) 2707–2713
5.1. Fully silicided gate (FUSI) Fig. 2a shows a TEM cross-section image of a FUSI Nisilicide/TiN/HfO2 gate structure with the silicon substrate aligned along the [1 1 0] cross-section direction. Different grains are revealed at the top of the gate. The image contrast is strongly dominated by diffraction effects, i.e. grain orientations. Tilting the sample to optimize the diffraction contrast between the grains could be considered but then the interfaces between the different materials will be blurred as they will no longer be seen (nearly) edge-on. Identifying the possible presence of different silicide phases will require detailed high resolution or electron diffraction investigations and hence the possible presence of different phases is not immediately obvious from such images. The same structure imaged in HAADF–STEM mode (Fig. 2b) clearly reveals that the top of the gate is Ni-rich and moreover that the Ni-rich phase extends till the bottom of the gate. However, whether this deep extension is a local effect or occurs along the full gate line cannot be decided from the 2D-image. Tomography series are acquired with the sample mounted in two orientations with respect to the tilt axis, i.e. near the vertical and horizontal direction of the gate. The tilt range is 72° to 73° in 1° increments. The reconstructed volume of the tomogram acquired with rotation around the vertical axis is shown in Fig. 3. No gold markers are used in this case so that the alignment of the tilt series is based on cross-correlation between the images. Slicing down along the structure unambiguously shows that the upper Ni-rich phase only locally extends to the bottom of the gate. Reconstructions with different algorithms are compared in Fig. 4 which shows slices viewed along the z-axis at the position where the upper phase reaches the TiN and slices parallel with the Si substrate. The WBP reconstruction (Fig. 4a and d) is quite noisy compared to the SIRT results (Fig. 4b,c,e,f) but often shows better contrast, e.g. between the nitride spacer and the oxides. The SIRT calculation with only two iterations (Fig. 4b and e) shows an unacceptable smearing of the contrasts at the nitride spacer and at the edges of the source/ drain silicide. These artifacts diminish with increasing number of iterations (Fig. 4c and f). For the example shown, the calculation with WBP takes approximately 2 min, whereas the SIRT calculations require 10 min per iteration (on a Pentium-4, 3 GHz computer with 2 GB RAM). To minimize artifacts generally at least 10 SIRT iterations are required, resulting in a typical minimum calculation time of several hours for the reconstruction. On the slices parallel with the substrate (Fig. 4d–f), more artifacts are visible at the edges of the TEM sample. These are directly related to the missing wedge of the limited tilt range used for the acquisition.
2711
tilt range of 66° to 62° in 1° increments. The alignment is performed without gold markers and a 20 cycles SIRTreconstruction is applied. The detailed morphology of the different materials can be studied by stepping through the slices of the reconstructed volume, e.g. the shape of the keyhole in the W can directly be visualized whereas this information will be masked by the projection in a 2Dimage. The quantitative comparison of the results of different samples requires software tools that allow to perform automated metrology investigation in any given direction of, e.g. lengths or areas through the series of reconstructed slices [17]. The application of such automated procedures is hampered by the reconstruction artifacts (e.g. due to the missing wedge) and the criteria to determine the boundary between different materials. Surface rendering of the different materials can be done in, e.g. Amira software but some manual check/adjustment of the contours of the selected material at several positions in the series of slices remains necessary. Fig. 6 shows an example of the area of the W via calculated through the slices in x, y and z direction. The plot of the areas in the y direction yields information on the slope of the contact and shows severe narrowing of the via just under the Cu line. Similarly the dimensions of the other materials, e.g. the keyhole, can be studied.
5.2. Contacts Fig. 5 shows 3D slices through the reconstructed volume a Cu/W/NiSi contact structure. The acquisition is done in a
Fig. 5. 3D-reconstruction of a W contact between a silicided source and Cu metal line.
2712
H. Bender et al. / Microelectronic Engineering 84 (2007) 2707–2713
5.3. Memory cell Results obtained for an non-volatile memory (NVM) cell are compared in Fig. 7. A first sample is made including the nearby W contacts in the thinned lamella. Acquiring the series with a vertical tilt axis results in shadowing of the memory cell by the W plugs at high tilt axis, therefore in this configuration the maximum tilt range is limited by the structure to 75° to 75°. Moreover the W plug has a very bright contrast in HAADF–STEM mode which overwhelms the contrasts between the different light materials in the memory cell (Fig. 7a). Nevertheless details of the cell can be studied and, e.g. slicing through the device in z direction allows to study the roughness of the poly sidewalls in detail whereas this information is blurred by the projection in a classical 2D-image. To avoid the overlap with the W plugs the tilt axis can be chosen along the x-axis in the horizontal direction. This has however the drawback of overlap at high tilt angles with the substrate or with the FIB-Pt capping layer which has also a bright contrast. A more success-
Fig. 6. 3D surface rendering (a) of the W contact plug of Fig. 5 and area of the W contact plug calculated through the slices in x, y and z directions. (b) Z is the optic axis at 0° tilt.
ful approach is the removal of the W plugs by selective milling. This is applied to the same specimen which is reinvestigated afterwards. The slice at similar position (Fig. 7b) shows now better contrasts between the different materials so that, e.g. the liner layer over the cell can now clearly be distinguished. The milling of the W plugs resulted in some contamination on the faces of the specimen due to re-deposition and ion beam damage. This can be avoided by removing the plugs in an earlier state of the specimen preparation. A possible approach to remove the plugs is the preparation of needle shaped specimens (Fig. 7c). A tilt range of ±75° is used in this case and the HAADF–STEM contrast can now be fully optimized for the light materials in the memory cell. The cylindrically shaped specimens have the advantage of constant specimen thickness, i.e. constant
Fig. 7. Slices through a memory cell obtained with a specimen in which the W plugs are present (a), after selective milling of the W plug (b) and with a needle shaped specimen (c). On (c) the dark edges are the boundaries of the specimen which contains only the memory cell.
H. Bender et al. / Microelectronic Engineering 84 (2007) 2707–2713
2713
average contrast upon tilting. Due to their configuration the automatic focusing fails more easily than on plan-parallel specimens and also the cross-correlation for the alignment is less accurate. Therefore, the use of gold markers is more essential on such specimens.
be needed to avoid the missing wedge problem. Therefore, needle shaped specimens will be required which make sample preparation more critical.
6. Conclusions
[1] A.J. Koster, R. Grimm, D. Typke, R. Hegerl, A. Stoschek, J. Walz, W. Baumeister, J. Struct. Biol. 120 (1997) 276–308. [2] P.A. Midgley, M. Weyland, Ultramicroscopy 96 (2003) 413–431. [3] C. Ku¨bel, A. Voigt, R. Schoenmakers, M. Otten, D. Su, T.-C. Lee, A. Carlsson, J. Bradley, Microsc. Microanal. 11 (2005) 378–400. [4] M. Weyland, T.J.V. Yates, R.E. Dunin-Borkowski, L. Laffont, P.A. Midgley, Scripta Mater. 55 (2006) 29–33. [5] C. Ku¨bel, J. Ku¨bel, S. Kujawa, J.-S. Luo, H-M. Lo, J.D. Russell, AIP Conf. Ser. 817 (2006) 223–228. [6] P. Ercius, M. Weyland, D.A. Muller, L.M. Gignac, Appl. Phys. Lett. 88 (2006) 243116. [7] A. Takaoka, T. Hasegawa, K. Yoshida, and H. Mori, in: Proceedings 16th International Microscopy Conference (2006), 680. [8] S. Kudo, Y. Hirose, K. Fukumoto, S. Maegawa, J. Shimanuki, and Y. Inoue, in: Proceedings 16th International Microscopy Conference (2006), 1440. [9] O. Richard, A. Kalio, H. Bender, and E. Sourty, in: Proceeding 15th Conference on Microscopy of Semiconductor Materials, Cambridge, Springer Proceedings in Physics, 2007. [10] A. Yurtsever, M. Weyland, D.A. Muller, Appl. Phys. Lett. 89 (2006) 151920. [11] M. Weyland, P.A. Midgley, Microsc. Microanal. 9 (2003) 542–555. [12] G. Mo¨bus, R.C. Doole, B.J. Inkson, Ultramicroscopy 96 (2003) 433– 451. [13] A.C. Twitchett, T.H.V. Yates, R.E. Dunin-Borkowski, S.B. Newcomb, P.A. Midgley, J. Phys. Conf. Ser. 26 (2006) 29–32. [14] J. Sharp, J. Barnard, J. Tong, and P. Midgley, in: Proceedings 16th International Microscopy Conference (2006), 700. [15] L.A. Giannuzzi, B.W. Kempshall, S.M. Schwarz, J.K. Lomness, B.I. Prenitzer, F.A. Stevie, in: Introduction to Focused Ion Beams, Springer, 2005, pp. 201–228. [16] K. Jarausch, D. Leonard, and T. Kamino, in: Proceedings 16th International Microscopy Conference (2006), 687. [17] A. Kalio, O. Richard, E. Sourty, H. Bender, Proceeding 15th Conference on Microscopy of Semiconductor Materials, Cambridge, Springer Proceedings in Physics, 2007.
Electron tomography has strong potentials for the 3Danalysis of nano-scale device structures. The examples discussed show that by slicing through the reconstructed volume details can be revealed and more accurate metrology can be obtained than with standard 2D imaging. The variation of dimensions can be investigated across the volume, having the potential for statistical studies of, e.g. line widths or other critical parameters across the structures. The acquisition and analysis time for electron tomography experiments is much longer than for standard 2D imaging. Further improvements of the measurement procedure, including sample preparation, and data analysis methodology are on-going and will lead to better understanding/circumventing of the limitations and to reduction of the total acquisition and analysis time. Already nowadays improved software using the computer graphics allows a severe reduction of the calculation time. Still the data acquisition and the alignment of the series are time-consuming activities that make electron tomography much slower than the classical 2D TEM/STEM analysis. Other new algorithms, i.e. discrete tomography, are under development that can, in combination with prior knowledge about the material characteristics in the sample, for certain kinds of samples allow to reduce the number of images to be acquired. By slicing through the reconstructed volume the requirements for ultimately thin specimens can in principle be relaxed. On the other hand, to improve the resolution of the tomograms along the optic axis larger tilt ranges will
References