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Through Silicon Via (TSV) defect investigations using lateral emission microscopy
Microelectronics Reliability 50 (2010) 1413–1416
Contents lists available at ScienceDirect
Microelectronics Reliability journal homepage: www.elsevier.com/locate/microrel
Through Silicon Via (TSV) defect investigations using lateral emission microscopy C. Cassidy *, J. Teva, J. Kraft, F. Schrank austriamicrosystems AG, 30 Tobelbaderstrasse, Unterpremstätten 8141, Austria
a r t i c l e
i n f o
Article history: Received 30 June 2010 Accepted 19 July 2010 Available online 11 August 2010
a b s t r a c t Infra-red photoemission microscopy has been applied for the localization of defects in 3D integrated circuits containing Through Silicon Vias (TSVs). For these investigations, the familiar (planar) emission microscopy configuration was extended to allow imaging and emission microscopy on vertical TSV sidewalls, from versatile 3D viewpoints. Flexible viewing orientation was achieved by introducing an additional reflecting surface into the optical path. Precise alignment of the angle of incidence at the air–silicon interface, with sufficient accuracy to ensure no problematic refraction-related errors, was possible using this experimental set-up. Three examples are presented, showing defect localizations and underlying physical leakage mechanisms in TSV structures. Ó 2010 Elsevier Ltd. All rights reserved.
1. Background
1.2. IR photon emission microscopy
Defect localization is a key ingredient in successful semiconductor process development, attainment of high production yields and assurance of long-term reliability. Applied to 3D integrated devices, localization of defects presents numerous challenges; but also opportunities for optimisation or development of new analysis techniques [1–3]. This article is concerned with an adaptation of conventional emission microscopy to address failure localization in TSVs.
Emission microscopy is a ubiquitous technique for semiconductor failure analysis [7], usually performed in a fixed plan-view configuration, either from the device frontside or through the silicon substrate from the backside. In the current work, however, the experimental set-up is expanded to allow flexibility in viewing direction, with emphasis in this article on through-silicon lateral viewing of defective TSVs. This is extremely powerful when the line-of-sight to the defect is impeded from both frontside and backside viewpoints, and when the defect is located at an unknown depth on a vertically-oriented surface, as is true in this case.
1.1. 3D integration scheme and challenges for failure analysis Detailed information on the fabrication and structure of the investigated TSVs has been published previously [4,5]. For convenience, a short summary follows. The TSVs are cylindrical in shape, typically having dimensions of 100 lm diameter and 250 lm depth. These structures are not completely filled; rather the required insulating and conductive layers are deposited conformally on the sidewalls. With reference to this particular implementation of 3D integration, failure analysis challenges centre around: (i) stringent limits on tolerable sidewall leakage current, (ii) obstructed line-of sight to vertical TSV sidewalls, and (iii) very large TSV sidewall surface areas, relative to the (potentially very small) defects, with no lithographically-patterned features to act as a navigation guide in subsequent physical analysis [6]. Very precise localization of defects (with leakages down to a few pico-amperes with typical bias conditions), on blanket TSV sidewall surfaces, is necessary. * Corresponding author. Tel.: +43 3136 500 5943. E-mail address: [email protected] (C. Cassidy). 0026-2714/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.microrel.2010.07.116
2. Experimental details 2.1. Experimental set-up A Hamamatsu Phemos1000 emission microscope was used for the current work. Detailed equipment information has been published previously [6]. For TSV failure analysis, an illustration of the experimental setup can be seen in Fig. 1. The sample was positioned in a conventional horizontal position on the microscope stage, and suitable electrical bias applied continuously using probe needles. A reflecting surface was inserted into the optical path, to deflect the imaging and emitted radiation through the desired angle. To minimize any loss in intensity, an Au first-surface thin-film coating was utilized for reflection (having excellent reflectance characteristics over the IR wavelength range of interest [8]). To ensure adequate alignment of the optical path, relative to the polished Si sidewall (prepared with any desired orientation), an experimental system was designed that allowed precise tilting of the mirror, whilst
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Fig. 3. Detected counts, reflected from the polished Si sidewall, as a function of the vertical tilt of the reflecting surface (5 objective lens, NA = 0.14). Fig. 1. Illustration of TSV through-silicon lateral imaging and emission microscopy.
satisfying the practical restrictions imposed by the limited working distances of the various objective lenses. This alignment capability was of central importance, to ensure that no apparent displacement of the emission site occurred as a result of refraction at the air–silicon interface. Whilst tilting of the sample itself, without using a mirror, might also be utilized to gain a direct line-of sight to the TSV sidewalls, using a mirror has the advantage of versatile angle selection, precise alignment, and no interference whatsoever with the mounting and (sometimes very delicate) micro-probing on the die surface. A sample prepared for side-view emission microscopy is shown in Fig. 2, from both planar and lateral viewpoints. 2.2. Optical path alignment Using this experimental set-up, it proved straightforward to align the optical path normal to the polished Si sidewall, with acceptable accuracy. For example, in Fig. 3, the detected intensity reflected from the Si sidewall, is shown as a function of mirror vertical tilt position (the same curves can similarly be plotted for the horizontal tilt). Clearly, the detected intensity goes through a maximum over a range of a few tenths of a degree (presumably when the reflecting surface is perfectly perpendicular to the optical path). A simple calculation (using the relevant angles, distances and refractive indices), shows that alignment accuracy over this angular range should be sufficient to prevent any appreciable refraction-induced displacement of the emission site. Note that best alignment angular sensitivity could be achieved by utilizing objective lenses with lower numerical aperture (i.e. smaller collection angle).
Fig. 2. A sample prepared for lateral emission microscopy: (a) plan-view optical micrograph and (b) cross-section view infra-red micrograph. The reflective metallization of the TSV, although still embedded in Si, can easily be discerned.
3. Results and discussion Having constructed the experimental system, and demonstrated that the necessary optical alignment precision could be obtained, this approach was applied to numerous defective TSVs. Three examples are presented: side-view emission microscopy and successful defect localization (Section 3.1), comparison of side-view emission microscopy with plan view focal series (Section 3.2), and exposed TSV defect emission microscopy (Section 3.3). 3.1. Case study 1 An example of TSV through-silicon emission microscopy, from a lateral viewpoint, is shown in Fig. 4a. In this case, an emission site has been detected very near the base of the TSV, near the bond interface. The core emitting area is ‘‘needle-shaped”, which may already give a direct insight into the defect mechanism causing the leakage. Note the defocus of the polished silicon sidewall and wafer bond interface, relative to the emission site. This highlights that the finite depth of field must again be considered in this configuration [6]. The position of best focus on the curved TSV sidewall must be determined, to maximize emission site resolution and signal-tonoise ratio; using a focal series if necessary. Fig. 4b shows a cross-sectional SEM micrograph through the defect identified in Fig. 4a, demonstrating that this emission microscopy approach enables successful physical FA and root cause determination. Further physical analysis (on this and other samples with the same failure signature, not shown) showed that this defect was indeed a needle-shaped particle lying between the Si substrate and the TSV metallization, thus interrupting the isolation oxide. This is consistent with the spatial distribution of the emission site, as shown in Fig. 4a, confirming the benefits of using this emission microscopy approach.
Fig. 4. (a) Reflected through-Si side-view infra-red micrograph showing an emission site on a TSV sidewall, near the base and (b) cross-sectional SEM image of the emitting defect shown in (a). The manufacturing defect is labelled ‘‘D”.
C. Cassidy et al. / Microelectronics Reliability 50 (2010) 1413–1416
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3.2. Case study 2 In this case, a TSV sidewall emission was first detected using a conventional plan-view configuration (Fig. 5a), thereby identifying the azimuthal position of the defect in the cylindrical TSV. A coarse focal series yielded maximum emission intensity at approximately 210 lm below the surface of the upper wafer (see Fig. 5b), indicating that the physical defect was located at approximately this depth (although with the benefit of hindsight, the variation of maximum intensity as a function of depth is unusual [6]). Upon preparing the sample and performing side-view emission microscopy, an entirely different picture emerged (Fig. 5c), however. A vertical stripe of emitting area, almost the full length of the TSV was observed (albeit with a peak emission intensity at approximately 210 lm below the surface; and therefore consistent with the planar focal series). Based on this information, the leakage path could again be located with physical analysis (Fig. 5d). Along a long vertical scratch or crack in the isolation oxide, TSV metallization came into direct contact with the tips of the Si sidewall ‘‘scallops”. This example directly highlights the invaluable insight which may be obtained by viewing the emission surface directly, rather than relying on the conventional plan-view configurations. 3.3. Case study 3 In this case, the investigation was extended to include emission microscopy from an exposed TSV defect cross-section. This type of exposed cross-sectional approach has been reported previously (e.g. see [9], albeit in quite a different application). Having first localized the defect using through-Si side-view investigations (as shown in Sections 3.1 and 3.2), cross-section preparation was performed (in such a way that the probe points and electrical path to the defect were still available at all times). This cross-section, shown in Fig. 6a clearly shows that the manufacturing defect is a particle between the Si substrate and the TSV metallization (marked ‘‘D” in Fig. 6a). Spectroscopy (energy dispersive X-ray spectroscopy, not shown) indicated that the particle is Si, or Si-rich. Note that later sequential sectioning, destructively through the full defect, did not yield any location at which direct contact between the defect particle, and the Si
Fig. 6. TSV defect cross-sectional emission microscopy: (a) cross-sectional SEM micrograph, showing the defect but prior to severing of the electrical connection, (b) cross-sectional IR micrograph showing direct photon emission from the exposed defect, and (c) cross-sectional IR micrograph showing an expanded view of region A in Fig. 6b, with optimised acquisition settings.
substrate, could be seen. However, with the defect exposed in cross-section at an intermediate point (as in Fig. 6a), but before the defect electrical connection was severed, the cross-section process was paused to allow emission microscopy directly from the exposed defect. Fig. 6b and c thus show IR emissions obtained directly from the exposed defect. The detected irradiance proved to be extremely intense, relative to that obtained through the silicon. These IR micrographs verify beyond any doubt that there is a leakage current path from the TSV to bulk Si at this location,1 even if no physical contact to the Si substrate was observed with physical analysis. It is interesting to consider the specific emission volumes in Fig. 6c. Here it can be seen that the emissions are localized into two distinct regions. These regions correspond well to the peaks associated with the ‘‘Bosch scallops” in the region of the defect (marked 1 and 2 in Fig. 6a). It is of course reasonable that such regions might suffer significant leakage as a result of the locally thinner oxide, coupled with the enhanced electric field strength owing to the geometry of the etched Si scallops, highlighting that a direct metal-silicon junction may not be necessary for substantial leakage to be evident nevertheless. This type of exposed cross-section analysis is very helpful in practical terms during physical FA, to check the proximity to the (potentially nanoscale) defect. As a final note, analysis of the emission spatial and spectral distribution from such exposed defects could potentially yield very useful information on the mechanisms of current transfer through defect junctions, assisting greatly in developing strategies for elimination or tailoring screening measures for such defect mechanisms. 4. Summary and conclusions
Fig. 5. Various perspectives on an emitting defect: (a) plan view emission microscopy (emission best focus), (b) plan view focal series – detected counts as a function of focal plane position, (c) side-view emission microscopy, and (d) crosssection SEM image through the vertical line indicated by (c), showing a direct contact from Si substrate to TSV metallization.
Emission microscopy has been extended from conventional planar configurations to allow flexible viewing orientations for 3D integrated devices. The employed approach allowed the sample under investigation to be maintained in a convenient fixed position, with bias applied in a conventional manner. The required angles of incidence were achieved by reflection of the imaging and emitted radiation. Given the fact that the TSVs are embedded in silicon, a suitable experimental set-up and alignment procedure was developed to ensure no refraction-induced displacement of the emission site occurred. Three examples were presented, highlighting the effectiveness of this approach in 1 Note that previous work has shown that TSV sectioning can be performed without introducing new leakage currents, even down to pico-ampere levels [5].
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locating TSV defects and understanding their underlying origins and behaviour. Whilst this article has considered IR photon emission microscopy applied to TSV fault localization, there is no reason why this approach cannot be followed with other optical techniques, applied to various 3D integrated technologies. Furthermore, whilst metallic reflection has been explicitly discussed in this article, failure analysis of TSVs in this manner can also utilize deliberate refraction and/or Total Internal Reflection (TIR) to give further flexibility in accessing the desired surface region of the vertical TSV sidewalls. Acknowledgements Assistance from Mr. Alois Schaden (austriamicrosystems AG), in preparation of a customized mirror mounting system, is sincerely acknowledged. Funding by the European Commission under project HELIOS (photonics electronics functional integration on CMOS), FP7224312, is also gratefully acknowledged.
References [1] Yang Y, Bender H, Arstila K, Swinnen B, Verlinden B, De Wolf I. Detection of failure sites by focused ion beam and nano-probing in the interconnect of threedimensional stacked circuit structures. Microelectron Reliab 2008;48:1517–20. [2] Gagnard X, Mourier T. Through Silicon Via: from the CMOS imager sensor wafer level package to the 3D integration. Microelectron Eng 2010;87:470–6. [3] Infante F, Perdu P, Lewis D. Magnetic microscopy for 3D devices: defect localization with high resolution and long working distance on complex system in package. Microelectron Reliab 2009;49:1169–74. [4] Kraft J, Hueber A, Carniello S, Schrank F, Wachmann E. Passivation integrity investigations for through wafer interconnects. In: Proceedings – IRPS; 2008. p. 506–9. [5] Cassidy C, Kraft J, Koppitsch G, Brandlhofer E, Steiner M, Schrank F, et al. Characterization and failure analysis of wafer bonded devices and unfilled Through-Silicon-Vias (TSVs). In: Proceedings – ISTFA; 2008. p. 368–74. [6] Cassidy C, Renz F, Kraft J, Schrank F. Depth-resolved photoemission microscopy for localization of leakage currents in Through Silicon Vias (TSVs). In: Proceedings – IPFA; 2009. p. 735–40. [7] Boit C. Fundamentals of photon emission (PEM) in silicon – electroluminescence for analysis of electronic circuit and device functionality. Microelectronics failure analysis desk reference, 5th ed.; 2004. p. 356–68. [8] Hecht E. Optics. Addison Wesley; 2001. [9] Wang Z. A study of low leakage failure mechanism of X7R multiple layer ceramic capacitor (MPCC). In: Proceedings – ISTFA; 2006. p. 142–46.
Contents lists available at ScienceDirect
Microelectronics Reliability journal homepage: www.elsevier.com/locate/microrel
Through Silicon Via (TSV) defect investigations using lateral emission microscopy C. Cassidy *, J. Teva, J. Kraft, F. Schrank austriamicrosystems AG, 30 Tobelbaderstrasse, Unterpremstätten 8141, Austria
a r t i c l e
i n f o
Article history: Received 30 June 2010 Accepted 19 July 2010 Available online 11 August 2010
a b s t r a c t Infra-red photoemission microscopy has been applied for the localization of defects in 3D integrated circuits containing Through Silicon Vias (TSVs). For these investigations, the familiar (planar) emission microscopy configuration was extended to allow imaging and emission microscopy on vertical TSV sidewalls, from versatile 3D viewpoints. Flexible viewing orientation was achieved by introducing an additional reflecting surface into the optical path. Precise alignment of the angle of incidence at the air–silicon interface, with sufficient accuracy to ensure no problematic refraction-related errors, was possible using this experimental set-up. Three examples are presented, showing defect localizations and underlying physical leakage mechanisms in TSV structures. Ó 2010 Elsevier Ltd. All rights reserved.
1. Background
1.2. IR photon emission microscopy
Defect localization is a key ingredient in successful semiconductor process development, attainment of high production yields and assurance of long-term reliability. Applied to 3D integrated devices, localization of defects presents numerous challenges; but also opportunities for optimisation or development of new analysis techniques [1–3]. This article is concerned with an adaptation of conventional emission microscopy to address failure localization in TSVs.
Emission microscopy is a ubiquitous technique for semiconductor failure analysis [7], usually performed in a fixed plan-view configuration, either from the device frontside or through the silicon substrate from the backside. In the current work, however, the experimental set-up is expanded to allow flexibility in viewing direction, with emphasis in this article on through-silicon lateral viewing of defective TSVs. This is extremely powerful when the line-of-sight to the defect is impeded from both frontside and backside viewpoints, and when the defect is located at an unknown depth on a vertically-oriented surface, as is true in this case.
1.1. 3D integration scheme and challenges for failure analysis Detailed information on the fabrication and structure of the investigated TSVs has been published previously [4,5]. For convenience, a short summary follows. The TSVs are cylindrical in shape, typically having dimensions of 100 lm diameter and 250 lm depth. These structures are not completely filled; rather the required insulating and conductive layers are deposited conformally on the sidewalls. With reference to this particular implementation of 3D integration, failure analysis challenges centre around: (i) stringent limits on tolerable sidewall leakage current, (ii) obstructed line-of sight to vertical TSV sidewalls, and (iii) very large TSV sidewall surface areas, relative to the (potentially very small) defects, with no lithographically-patterned features to act as a navigation guide in subsequent physical analysis [6]. Very precise localization of defects (with leakages down to a few pico-amperes with typical bias conditions), on blanket TSV sidewall surfaces, is necessary. * Corresponding author. Tel.: +43 3136 500 5943. E-mail address: [email protected] (C. Cassidy). 0026-2714/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.microrel.2010.07.116
2. Experimental details 2.1. Experimental set-up A Hamamatsu Phemos1000 emission microscope was used for the current work. Detailed equipment information has been published previously [6]. For TSV failure analysis, an illustration of the experimental setup can be seen in Fig. 1. The sample was positioned in a conventional horizontal position on the microscope stage, and suitable electrical bias applied continuously using probe needles. A reflecting surface was inserted into the optical path, to deflect the imaging and emitted radiation through the desired angle. To minimize any loss in intensity, an Au first-surface thin-film coating was utilized for reflection (having excellent reflectance characteristics over the IR wavelength range of interest [8]). To ensure adequate alignment of the optical path, relative to the polished Si sidewall (prepared with any desired orientation), an experimental system was designed that allowed precise tilting of the mirror, whilst
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C. Cassidy et al. / Microelectronics Reliability 50 (2010) 1413–1416
Fig. 3. Detected counts, reflected from the polished Si sidewall, as a function of the vertical tilt of the reflecting surface (5 objective lens, NA = 0.14). Fig. 1. Illustration of TSV through-silicon lateral imaging and emission microscopy.
satisfying the practical restrictions imposed by the limited working distances of the various objective lenses. This alignment capability was of central importance, to ensure that no apparent displacement of the emission site occurred as a result of refraction at the air–silicon interface. Whilst tilting of the sample itself, without using a mirror, might also be utilized to gain a direct line-of sight to the TSV sidewalls, using a mirror has the advantage of versatile angle selection, precise alignment, and no interference whatsoever with the mounting and (sometimes very delicate) micro-probing on the die surface. A sample prepared for side-view emission microscopy is shown in Fig. 2, from both planar and lateral viewpoints. 2.2. Optical path alignment Using this experimental set-up, it proved straightforward to align the optical path normal to the polished Si sidewall, with acceptable accuracy. For example, in Fig. 3, the detected intensity reflected from the Si sidewall, is shown as a function of mirror vertical tilt position (the same curves can similarly be plotted for the horizontal tilt). Clearly, the detected intensity goes through a maximum over a range of a few tenths of a degree (presumably when the reflecting surface is perfectly perpendicular to the optical path). A simple calculation (using the relevant angles, distances and refractive indices), shows that alignment accuracy over this angular range should be sufficient to prevent any appreciable refraction-induced displacement of the emission site. Note that best alignment angular sensitivity could be achieved by utilizing objective lenses with lower numerical aperture (i.e. smaller collection angle).
Fig. 2. A sample prepared for lateral emission microscopy: (a) plan-view optical micrograph and (b) cross-section view infra-red micrograph. The reflective metallization of the TSV, although still embedded in Si, can easily be discerned.
3. Results and discussion Having constructed the experimental system, and demonstrated that the necessary optical alignment precision could be obtained, this approach was applied to numerous defective TSVs. Three examples are presented: side-view emission microscopy and successful defect localization (Section 3.1), comparison of side-view emission microscopy with plan view focal series (Section 3.2), and exposed TSV defect emission microscopy (Section 3.3). 3.1. Case study 1 An example of TSV through-silicon emission microscopy, from a lateral viewpoint, is shown in Fig. 4a. In this case, an emission site has been detected very near the base of the TSV, near the bond interface. The core emitting area is ‘‘needle-shaped”, which may already give a direct insight into the defect mechanism causing the leakage. Note the defocus of the polished silicon sidewall and wafer bond interface, relative to the emission site. This highlights that the finite depth of field must again be considered in this configuration [6]. The position of best focus on the curved TSV sidewall must be determined, to maximize emission site resolution and signal-tonoise ratio; using a focal series if necessary. Fig. 4b shows a cross-sectional SEM micrograph through the defect identified in Fig. 4a, demonstrating that this emission microscopy approach enables successful physical FA and root cause determination. Further physical analysis (on this and other samples with the same failure signature, not shown) showed that this defect was indeed a needle-shaped particle lying between the Si substrate and the TSV metallization, thus interrupting the isolation oxide. This is consistent with the spatial distribution of the emission site, as shown in Fig. 4a, confirming the benefits of using this emission microscopy approach.
Fig. 4. (a) Reflected through-Si side-view infra-red micrograph showing an emission site on a TSV sidewall, near the base and (b) cross-sectional SEM image of the emitting defect shown in (a). The manufacturing defect is labelled ‘‘D”.
C. Cassidy et al. / Microelectronics Reliability 50 (2010) 1413–1416
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3.2. Case study 2 In this case, a TSV sidewall emission was first detected using a conventional plan-view configuration (Fig. 5a), thereby identifying the azimuthal position of the defect in the cylindrical TSV. A coarse focal series yielded maximum emission intensity at approximately 210 lm below the surface of the upper wafer (see Fig. 5b), indicating that the physical defect was located at approximately this depth (although with the benefit of hindsight, the variation of maximum intensity as a function of depth is unusual [6]). Upon preparing the sample and performing side-view emission microscopy, an entirely different picture emerged (Fig. 5c), however. A vertical stripe of emitting area, almost the full length of the TSV was observed (albeit with a peak emission intensity at approximately 210 lm below the surface; and therefore consistent with the planar focal series). Based on this information, the leakage path could again be located with physical analysis (Fig. 5d). Along a long vertical scratch or crack in the isolation oxide, TSV metallization came into direct contact with the tips of the Si sidewall ‘‘scallops”. This example directly highlights the invaluable insight which may be obtained by viewing the emission surface directly, rather than relying on the conventional plan-view configurations. 3.3. Case study 3 In this case, the investigation was extended to include emission microscopy from an exposed TSV defect cross-section. This type of exposed cross-sectional approach has been reported previously (e.g. see [9], albeit in quite a different application). Having first localized the defect using through-Si side-view investigations (as shown in Sections 3.1 and 3.2), cross-section preparation was performed (in such a way that the probe points and electrical path to the defect were still available at all times). This cross-section, shown in Fig. 6a clearly shows that the manufacturing defect is a particle between the Si substrate and the TSV metallization (marked ‘‘D” in Fig. 6a). Spectroscopy (energy dispersive X-ray spectroscopy, not shown) indicated that the particle is Si, or Si-rich. Note that later sequential sectioning, destructively through the full defect, did not yield any location at which direct contact between the defect particle, and the Si
Fig. 6. TSV defect cross-sectional emission microscopy: (a) cross-sectional SEM micrograph, showing the defect but prior to severing of the electrical connection, (b) cross-sectional IR micrograph showing direct photon emission from the exposed defect, and (c) cross-sectional IR micrograph showing an expanded view of region A in Fig. 6b, with optimised acquisition settings.
substrate, could be seen. However, with the defect exposed in cross-section at an intermediate point (as in Fig. 6a), but before the defect electrical connection was severed, the cross-section process was paused to allow emission microscopy directly from the exposed defect. Fig. 6b and c thus show IR emissions obtained directly from the exposed defect. The detected irradiance proved to be extremely intense, relative to that obtained through the silicon. These IR micrographs verify beyond any doubt that there is a leakage current path from the TSV to bulk Si at this location,1 even if no physical contact to the Si substrate was observed with physical analysis. It is interesting to consider the specific emission volumes in Fig. 6c. Here it can be seen that the emissions are localized into two distinct regions. These regions correspond well to the peaks associated with the ‘‘Bosch scallops” in the region of the defect (marked 1 and 2 in Fig. 6a). It is of course reasonable that such regions might suffer significant leakage as a result of the locally thinner oxide, coupled with the enhanced electric field strength owing to the geometry of the etched Si scallops, highlighting that a direct metal-silicon junction may not be necessary for substantial leakage to be evident nevertheless. This type of exposed cross-section analysis is very helpful in practical terms during physical FA, to check the proximity to the (potentially nanoscale) defect. As a final note, analysis of the emission spatial and spectral distribution from such exposed defects could potentially yield very useful information on the mechanisms of current transfer through defect junctions, assisting greatly in developing strategies for elimination or tailoring screening measures for such defect mechanisms. 4. Summary and conclusions
Fig. 5. Various perspectives on an emitting defect: (a) plan view emission microscopy (emission best focus), (b) plan view focal series – detected counts as a function of focal plane position, (c) side-view emission microscopy, and (d) crosssection SEM image through the vertical line indicated by (c), showing a direct contact from Si substrate to TSV metallization.
Emission microscopy has been extended from conventional planar configurations to allow flexible viewing orientations for 3D integrated devices. The employed approach allowed the sample under investigation to be maintained in a convenient fixed position, with bias applied in a conventional manner. The required angles of incidence were achieved by reflection of the imaging and emitted radiation. Given the fact that the TSVs are embedded in silicon, a suitable experimental set-up and alignment procedure was developed to ensure no refraction-induced displacement of the emission site occurred. Three examples were presented, highlighting the effectiveness of this approach in 1 Note that previous work has shown that TSV sectioning can be performed without introducing new leakage currents, even down to pico-ampere levels [5].
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locating TSV defects and understanding their underlying origins and behaviour. Whilst this article has considered IR photon emission microscopy applied to TSV fault localization, there is no reason why this approach cannot be followed with other optical techniques, applied to various 3D integrated technologies. Furthermore, whilst metallic reflection has been explicitly discussed in this article, failure analysis of TSVs in this manner can also utilize deliberate refraction and/or Total Internal Reflection (TIR) to give further flexibility in accessing the desired surface region of the vertical TSV sidewalls. Acknowledgements Assistance from Mr. Alois Schaden (austriamicrosystems AG), in preparation of a customized mirror mounting system, is sincerely acknowledged. Funding by the European Commission under project HELIOS (photonics electronics functional integration on CMOS), FP7224312, is also gratefully acknowledged.
References [1] Yang Y, Bender H, Arstila K, Swinnen B, Verlinden B, De Wolf I. Detection of failure sites by focused ion beam and nano-probing in the interconnect of threedimensional stacked circuit structures. Microelectron Reliab 2008;48:1517–20. [2] Gagnard X, Mourier T. Through Silicon Via: from the CMOS imager sensor wafer level package to the 3D integration. Microelectron Eng 2010;87:470–6. [3] Infante F, Perdu P, Lewis D. Magnetic microscopy for 3D devices: defect localization with high resolution and long working distance on complex system in package. Microelectron Reliab 2009;49:1169–74. [4] Kraft J, Hueber A, Carniello S, Schrank F, Wachmann E. Passivation integrity investigations for through wafer interconnects. In: Proceedings – IRPS; 2008. p. 506–9. [5] Cassidy C, Kraft J, Koppitsch G, Brandlhofer E, Steiner M, Schrank F, et al. Characterization and failure analysis of wafer bonded devices and unfilled Through-Silicon-Vias (TSVs). In: Proceedings – ISTFA; 2008. p. 368–74. [6] Cassidy C, Renz F, Kraft J, Schrank F. Depth-resolved photoemission microscopy for localization of leakage currents in Through Silicon Vias (TSVs). In: Proceedings – IPFA; 2009. p. 735–40. [7] Boit C. Fundamentals of photon emission (PEM) in silicon – electroluminescence for analysis of electronic circuit and device functionality. Microelectronics failure analysis desk reference, 5th ed.; 2004. p. 356–68. [8] Hecht E. Optics. Addison Wesley; 2001. [9] Wang Z. A study of low leakage failure mechanism of X7R multiple layer ceramic capacitor (MPCC). In: Proceedings – ISTFA; 2006. p. 142–46.