EEIER
`
`Microelectronic Engineering 50 (2000) 277—284
`
`www.clscviernl /locathmac
`
`Physical and chemical analysis of advanced interconnections using
`energy filtering transmission electron microscopy
`
`R. Panteli, J. Torres, P. Paniez, G. Auvert
`
`France Telecom CNET, BP 98, F-38243 Meylan, France
`
`Abstract
`
`IPR2016-01246
`
`In new interconnection process technology, very thin diffusion barriers are deposited in high aspect
`ratio contact holes fabricated using reactive ion etching (RIE). Physical and chemical characterization
`of difl‘hsion or contamination is increasingly challenging due to topography. The transmission electron
`microscopy (TEM) analysis technique is more extensively used to overcome this difliculty in
`association with the focused ion beam (FIB) specimen preparation technique [1]. Recently, we have
`shown the advantages of adding electron energy loss spectroscopy (EELS) to FIB—TEM analysis for
`chemical characterization [g]. In this paper, we present the newest technique to overcome this physical
`and chemical analysis challenge using FIB sample preparation allowing localized site specific thinning
`with thickness in the range of 100 nm and energy filtering transmission electron microscopy
`(EFI'EM) observations. The EFTEM technique allows high-resolution compositional mapping using
`
`specimen thinning and energy filtering
`Our newest method for interconnection analysis using focused ion beam
`transmission electron microscopy is presented. It is shown that using the site-specific capability and the controlled
`thinning efl'ect of the FIB in addition with the high spatial resolution of the EFTEM technique. fast chemical analysis of
`materials with nanometre spatial resolution can be obtained. This is the only method for the observation of very thin
`difl'ilsion barriers and interfaces in the presence of drastic topography. Application examples are given concerning firstly. the
`in-depth analysis of tungsten aluminum technology. barrier integrity and interdifliision of elements near interfaces and
`secondly. the surface contamination of copper in copper interconnection technology with high aspect ratio contacts. In this
`case. photoresist spin-coating is carried out prior to FIB thinning. This method is an alternative to surface analysis techniques
`and ofl'ers the best spatial resolution without topography limitations. © 2000 Elsevier Science BM All rights reserved.
`
`Keywords: Energy filtering transmission election microscopy: Chemical analysis: Focused ion beam: Microelectronics:
`Metal interconnections
`
`1. Introduction
`
`‘Corresponding author. Fax: + 33-4-7676—4379.
`E—mail address: roland.pantel@cnet.fi'ancetelecom E (R. Pantel)
`
`0167-9317/ 00/ $ — see front matter © 2000 Elsevier Science BM All rights reserved.
`PII: 50167—93l7(99)00293-2
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`imaging of core level ionization edges [3]. We show applications of FIB–EFTEM for bulk analysis of
`tungsten–aluminum technology interconnection, interface or diffusion barriers observation and surface
`contamination observation of contact etching in copper technology.
`
`2. Experimental details
`
`The specimens were thinned using the FIB technique as already described [2]. The FIB system is a
`MICRION model 9500 EX using a gallium ion beam of 50 keV maximum energy with a 5-nm
`minimum spot diameter. In the particular case of the contamination control in copper interconnection
`technology, a special technique was used to allow FIB cross section of contacts just after oxide
`etching (i.e. before barrier deposition and metal filling). A conventional I-line photoresist solution was
`spin-coated over the contacts and let to dry at room temperature in order to imbed and protect, the
`surface which is intended to be analyzed in the TEM, from the ion gallium irradiation.
`The thinned samples were observed at 200 keV electron energy using a TEM PHILIPS CM200 FEG
`equipped with an electron energy filter (Gatan Imaging Filter GIF200). The energy filtering equipment
`uses a magnetic prism and a set of electromagnetic lenses [3] allowing energy filtration of transmitted
`electrons (tunable energy loss and energy window). These electrons of particular energy are used to
`form, on a CCD camera, a magnified image of the thinned specimen with a resolution better than 1
`nm.
`
`3. Results and discussion
`
`3.1. Tungsten aluminum interconnection technology analysis
`
`The first series of FIB–EFTEM application examples concerns bulk analysis of tungsten, titanium
`nitride diffusion barrier and aluminum interconnection technology. The purpose of Fig. 1 is to
`illustrate the improvement due to EFTEM in the high-resolution physical imaging of materials.
`Fig. 1 shows TEM bright field images of a contact thinned using FIB. This contact has intentionally
`been shifted with respect
`to the lower aluminum line in order to study the consequences of
`photolithography misalignment. Fig. 1a presents an unfiltered TEM image and Fig. 1b shows the same
`contact imaged using filtration on the zero loss energy. Fig. 1a corresponds to what is currently
`obtained in the classical TEM technique. All electrons are collected and, due to the chromatic
`aberration of TEM lenses, those who have loss energy are coarsely focused on the image. This effect
`induces a diffuse background, which affects contrast and resolution. On the contrary, in the image of
`Fig. 1b, only the electrons which have not lost energy are collected and the effect of chromatic
`aberration is suppressed. Small details, which are not visible in Fig. 1a, can now be observed in Fig.
`1b. Particularly the TiN anti-reflecting coating (ARC) on top of the aluminum can be seen as
`composed of three layers. The two lower layers are Ti and TiN sputter deposited; the upper layer is
`probably formed during the intermetal dielectric deposition.
`Another important purpose of energy filtering in the TEM is to allow elemental compositional maps
`with high spatial resolution. This is obtained using core level ionization edges imaging combined with
`electron background subtraction (i.e. three window method) [4]. In the resulting compositional image,
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`Fig. 1. TEM images of a tungsten contact presenting a negative overlap with respect to the lower aluminum line. (a)
`Unfiltered classical TEM bright field image. (b) Zero loss energy EFTEM image.
`
`the intensity roughly increases with atomic concentration: dark contrast for zero concentration and
`bright contrast for higher concentrations.
`Fig. 2a–c present respectively: the zero loss image, the compositional map of titanium (TiL
`ionization edge) and the compositional map of fluorine (F ionization edge). These images were
`K
`obtained on a contact such as that shown in Fig. 1. Despite the improved contrast and quality of the
`zero loss image, the gray level intensity is not directly related to the local chemical composition of the
`materials. Faintly contrasted interfaces may not be visible at low magnification such as in the ARC
`
`Fig. 2. EFTEM images of the same contact type as that presented in Fig. 1. (a) Zero loss energy EFTEM image. (b)
`Compositional map of titanium (Ti
`ionization edge). (c) Compositional map of fluorine (F ionization edge).
`K
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`L
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`layer as seen in Fig. 1. Also, the diffusion barrier, which is critical for the contact quality, could not
`be visualized on the image of Fig. 2a. On the contrary, the titanium map of Fig. 2b allows a clear
`visualization of the TiN barrier. This barrier is not continuous below the tungsten plug. The fluorine
`map of Fig. 2c shows fluorine accumulation on the bottom of the plug and below the anti-reflecting
`coating.
`Different gray level compositional maps can be combined in a color map. Three images can be
`easily mixed, each one representing one basic color component of the color map (i.e. green, red and
`blue). Fig. 3a shows a color map of the plug with oxygen in green, titanium in red, fluorine in blue,
`and Fig. 3b a color map of the same plug with nitrogen in green, titanium in red, oxygen in blue. Both
`images are obtained using color mixing of images similar to the one presented in Fig. 2b and c. Some
`complex layers are better interpreted than using only the separate images of Fig. 2b or c. In particular,
`at the left bottom of the contact, a three level layer composed of titanium nitride, a titanium fluorine
`rich compound and titanium is evidenced. In Fig. 3b, the TiN above and below the aluminum line
`appears in yellow, which is the addition of green and red. As a consequence of this analysis, the
`physical observation of the three layers of the TiN anti-reflecting coating, shown with high resolution
`in Fig. 1b, is confirmed and improved by the chemical analysis of each layer. In Fig. 3a, evidence is
`shown that the fluorine (in blue) is present where the diffusion barrier is discontinuous.
`The main advantage of the color compositional maps is that all materials and atomic elements are
`present in the same image and their localization with respect to each other is more evident.
`
`3.2. Copper interconnection technology analysis
`
`In this part we show that this volume analysis technique can be used as a surface analysis tool in
`the case of drastic surface morphology. Using a light material, as a protective layer, the surface is
`embedded and the application of this technique is illustrated on copper interconnections.
`Fig. 4 shows EFTEM pictures of a contact hole opened in SiO using a selective SiO –SiN plasma
`2
`2
`etching process. The etching process has stopped in the SiN layer above the copper line. In the zero
`
`Fig. 3. EFTEM compositional color maps of the same contact as that presented in Fig. 2. (a) Fluorine (blue), titanium (red)
`and oxygen (green). (b) Oxygen (blue), titanium (red), nitrogen (green) and TiN appears yellow (green plus red).
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`Fig. 4. EFTEM images of a contact hole opened in SiO using a selective SiO –SiN plasma etching process. (a) Zero loss
`2
`2
`energy EFTEM image. (b) Compositional color map of oxygen (blue), nitrogen (red) and copper (green).
`
`loss image presented in Fig. 4a no surface contamination is evidenced either on the bottom or on the
`contact sides. The compositional color map presented in Fig. 4b (nitrogen in red, oxygen in blue and
`copper in green) confirms this result, except for some traces of nitrogen on the bottom left of the
`contact hole.
`Fig. 5a shows a zero loss EFTEM picture of another contact after SiN removal using plasma
`etching. Dark residues are now observed at the bottom and on the edges of the contact hole. These
`residues are composed mainly of copper as shown on the EFTEM compositional color map of Fig. 5b
`in which copper is in green color, oxygen in blue and nitrogen of the SiN layer in red.
`Fig. 6 presents EFTEM pictures of a similar contact than in Fig. 5 but after wet cleaning using
`hydrofluoric acid. Fig. 6a presents the zero loss image showing the removal of the residues observed
`in Fig. 5a. The copper removal is confirmed on the color compositional map of Fig. 6b.
`The analysis presented here is typically an experimental problem which could be solved using
`surface analysis techniques: Auger, electron spectroscopy for chemical analysis (ESCA) or secondary
`
`Fig. 5. EFTEM pictures of the same contact hole as that shown in Fig. 4, but after SiN removal using plasma etching. (a)
`Zero loss energy EFTEM image. (b) Compositional color map of oxygen (blue), nitrogen (red) and copper (green).
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`Fig. 6. EFTEM pictures of a contact hole similar to that shown in Fig. 5 but after wet cleaning using hydrofluoric acid. (a)
`Zero loss energy EFTEM image. (b) Compositional color map of oxygen (blue), nitrogen (red) and copper (green).
`
`ion mass spectroscopy (SIMS). However, due to critical topography shown in this example, such
`experiment could not be carried out successfully. Moreover the spatial resolution (1 nm) obtained
`using EFTEM is more than one order of magnitude better then the spatial resolution of any surface
`analysis technique.
`Finally, to observe the entire configuration of the copper interconnection, we use the FIB–EFTEM
`bulk analysis. Fig. 7 shows compositional color map of the interconnection structure after deposition
`of the diffusion barrier, CVD copper contact filling and top copper removal using mechanical
`chemical polishing. The TiN barrier, which is difficult to observe using classical TEM contrast, is
`clearly visualized in the EFTEM color image. This TiN barrier is continuous and has a uniform
`thickness. The compositional map is modulated in intensity with crystal orientation, which changes
`the electron diffraction conditions.
`
`4. Conclusion
`
`In this communication, we present a powerful method for volume and surface analysis applied to
`the observation of interconnections. We show that the EFTEM technique allows a great improvement
`of spatial resolution and contrast of the TEM physical material imaging. In addition the EFTEM
`technique allows a specific chemical information with nanometre spatial resolution using elemental
`mapping. This is valuable for the observation of very thin diffusion barriers. The barrier integrity and
`interdiffusion of elements near interfaces can be observed. Contamination control of high aspect ratio
`contacts can be performed using photoresist spin-coating prior to FIB thinning. This method is an
`alternative to surface analysis techniques and offers better spatial resolution without topography
`limitations.
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`Fig. 7. EFTEM compositional color map of a complete copper interconnection structure: nitrogen (blue), titanium (red) and
`copper (green).
`
`Acknowledgements
`
`This work was performed within a cooperation between CNET France Telecom, CEA LETI and
`SGS Thomson Microelectronics.
`
`References
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