3D failure analysis in depth profiles of sequentially ... - Semantic Scholar
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Microelectronics Reliability 47 (2007) 1595–1598 www.elsevier.com/locate/microrel
3D failure analysis in depth profiles of sequentially made FIB cuts C.N. Mc Auley a
a,*
, A. Rummel b, F.W. Keating a, S. Kleindiek
b
Xilinx, Logic Drive, Citywest Business Campus, Saggart, Co. Dublin, Ireland b Kleindiek Nanotechnik GmbH, Reutlingen, Germany Received 2 July 2007 Available online 20 August 2007
Abstract A new method of investigating structures below a surface in a dual beam microscope is presented. It comprises electrical measurements in depth profiles of sequential focused ion beam (FIB) cuts by the use of two or more nanomanipulators with plugged in probe needles. The sample is oriented such that the structures are observed with the electron beam while they are cut free with the FIB. The nanomanipulators are moved to contact the structures for examination. The FIB cut is extended step by step, and after each cut the nanomanipulators are repositioned and measurements of the new structures that appear in the FIB cut are made. The measurement series provide a three dimensional electrical characterization of the examined sample volume. � 2007 Elsevier Ltd. All rights reserved.
1. Introduction Failures in the lower layers of semiconductor devices are usually examined by removing the surface of a sample until the layer of interest followed by electrical probing on that layer [1,2] or by cutting out a part of the sample (lamella) with the focused ion beam (FIB) [3] and by analysing the lamella with a scanning transmission electron microscope (STEM) detector. Alternatively or in addition, the lamella is transferred to a transmission electron microscope (TEM) [4–7] for further analysis. In this paper, we propose an alternative method using nanomanipulators to perform electrical measurements of conductive structures like transistors, wires, and resistances between metal layers without removing the sample from the bulk material. The access to these structures of interest is usually done by mechanical polishing or milling with a FIB until the layer is reached where the failure is electrical isolated too. However, the failure is often extended over more than one layer, e.g. in array of vias contacting several metal layers.
*
Corresponding author. Tel.: +353 1 4615309; fax: +353 1 4640322. E-mail address: [email protected] (C.N. Mc Auley).
0026-2714/$ - see front matter � 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.microrel.2007.07.032
The purpose of this paper is to present a new method of investigating structures in a FIB cut by the use of two or more nanomanipulators. The electrical measurements can be done immediately after FIB milling. Moreover, this method can be used to get a three dimensional electrical characterization of the examined sample volume as the FIB cut can be extended step by step and the measurements are repeated after each new FIB cut. 2. System set-up An overview of the experimental set-up is given in the schematic drawings Figs. 1–3. The sample is orientated in such a way in a dual beam FIB that the structures are observed with the electron beam while they are milled with the FIB. The cross-sectional view shows the orientation of the FIB cut with respect to the ion and electron beam. Two nanomanipulators come from the sides of the FIB cut to contact the structures for examination. The FIB cut is extended step by step, and after each cut the nanomanipulators are repositioned and measurements of the new structures that appear in the cut are made. The needles are bent to around 45� in a way that they appear bended in ion beam images but only slightly bended in electron beam images. The degree of bending of the needle
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C.N. Mc Auley et al. / Microelectronics Reliability 47 (2007) 1595–1598
Electron beam view Ion beam view
Structures in the FIB cut for probing
Fig. 5. Side view of the system set-up.
Fig. 1. Cross-section of the FIB cut.
Probe needles
FIB cut
Cross Section View Metal lines and vias Fig. 2. Electron beam view of the FIB cut.
FIB cut
Probe needles
Step pattern Metal lines and vias Fig. 3. Overview of the FIB cut probing.
determines the ease of access to the FIB cut sample face. If the needle is not bent sufficiently the needles will not be able to make contact to the FIB milled face of the device. However, a larger bending angle reduces the visibility of the tip especially for very small contact areas.
Fig. 4. Top-down view of the system set-up.
Fig. 4 shows a top down view, Fig. 5 a side view of the sample mounted on precision TEM t-type tool stub used for parallel polishing on the stage of a dual beam FIB at its coincidence and eucentric point for FIB milling and the two nanomanipulators in place to enable electrical probing. The photo of Fig. 4 is taken from the direction where the ion beam comes from, the stage is tilted 52�. 3. Test measurement A first measurement was performed to test this new technique. As test object intermetal via connections of a 90 nm CMOS device was chosen. The nanomanipulators were positioned on both ends of the via and the resistance of the structure was measured to be 387 X (see Fig. 6). The measurement shows the easy access to structures that are vertically extended with respect to the sample surface. As no change was made in the orientation or the position of the sample, which was located at the eucentric height and coincidence point for SEM and FIB in a dual beam system, it can be shown that this new technique provides a three dimensional electrical characterization of the examined sample volume by making sequential FIB cuts. Repositioning is done within some minutes as the nanomanipulators need to be moved away and towards the sample only a few microns to make a electrical measurement after a new cut due to the close positioning of the manipulators to the FIB cut trench.
Fig. 6. SEM image of the two nanomanipulators probing a via between two metal layers on a 90 nm process device.
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C.N. Mc Auley et al. / Microelectronics Reliability 47 (2007) 1595–1598
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Moreover, it is possible to resharpen or clean the tips with the ion beam as the ion beam looks at the contact area between the structure in the FIB cut and the tip. This increases the time to perform experiments without opening the SEM chamber.
nipulators were positioned on both ends of the via and the resistance of the structure was measured to be 257 (see Fig. 8).
4. Application to a 65 nm flip chip device
5.1. Probing on a metal finger capacitor
A Virtex 5 FPGA Flip chip device (see Fig. 7) was deprocessed from the back side [8,9] to allow access for the nanomanipulators to probe the switch capacitor (SC) reference circuit for generating Vrefp and Vrefn on chip. The circuits have been created using 0.25 lm thick oxide MOSTs which are required for chip I/O interfacing to the 65 nm 1 v digital VLSI [10]. The sample was analysed in a dual beam FIB equipped with two nanomanipulators set-up opposite to each other on a door mount adapter. A FIB cut was made in area of interest on the SC reference circuit (see Fig. 7) and the inter metal via connections of a 65 nm technology device was measured. The nanoma-
Fig. 9 shows a backside FIB cut of a milled cross-section of a metal finger capacitor (see Fig. 7) being probed between the interconnect between metal 11 and metal 9. The resistance was measured to be 412 X between the interconnect. The total resistance between the probes consist of the individual resistances that are connected in parallel between the probed metal layers and the contact resistances. The latter can be avoided if four point measurement is done. Typical resistances between two tips including cabling are 30–60 X.
Fig. 7. IR image of metal 1 showing location of cuts on system monitor on Virtex 5 65 nm FPGA.
Fig. 8. Probing on an intermetal connection of the SC reference circuit after initial FIB cut.
5. 3D failure analysis on sequential FIB cuts
5.2. 3D electrical analysis As the SC reference circuit is extended vertically and horizontally, sequential cuts must be made starting from one side of the SC reference circuit. Measurements are made after each cut until the end of the SC Reference circuit is reached. This is demonstrated in Figs. 10 and 11 for two additional cuts. After having probed on the first FIB cut (Fig. 8) the manipulators are lifted up some microns as and the FIB cut is extended for about 100 nm such that new details of the SC reference circuit are becoming visible. The manipulators are then moved down again to the SC reference circuit and a measurement can be performed (Fig. 10). The complete process can be done within minutes. This technique is repeated (probing after a further cut is seen in Fig. 11) until a three dimensional electrical characterization of the SC reference circuit is done. A main advantage is that no movement of the stage is needed – it remains at a tilted angle of 52�. In addition,
Fig. 9. Two point probing on the metal finger capacitor.
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between the sample and the tips. At first, because the sample (and therefore any contamination on the surface of the cut) is cut off with the ion beam after each measurement. Secondly, the tips can be cleaned and re-sharpened with the ion beam just at the contact point. This technique can be applied to other types of analysis like capacitive measurements and EBIC analysis, where a three dimensional image of the p-n-junctions can be made. Acknowledgement Special thanks to Device Analysis Laboratory Xilinx Ireland and Kleindiek Nanotechnik for their technical support and assistance in developing this technique. Fig. 10. Probing on the SC reference circuit after the second FIB cut.
Fig. 11. Probing on the SC reference circuit after the third FIB cut.
before and after making a FIB cut the manipulators must be lifted up and repositioned only some microns. 6. Conclusions A new method of characterizing a sample electrically in three dimensions is proposed and demonstrated. Direct probing on a FIB cut is a quick and easy method for the measurement of structures that are extended horizontally and vertically underneath a surface. The experimental set-up significantly reduces contact resistances that arise through electron beam radiation
References [1] Zimmermann Gunnar, Mueller Stefan. 90 nm technology SRAM soft fail analysis using nanoprobing and junction stain TEM. In: Proceedigns of the 32th ISTFA; 2006. p. 512. [2] Sibileau F, Ali C, Giret C, Faure D. SRAM cell defect isolation methodology by sub micron probing technique. Microelectron Reliab 2005;45:1562–7. [3] Langford RM, Petford-Long AK. Preparation of transmission electron microscopy cross section specimens using focused ion beam milling. J Vac Sci Technol A 2001;19(5). [4] Herlinger LR, Chevacharoenkul S, Erwin DC. TEM sample preparation using a focused ion beam and a probe manipulator. Proceedings of the 22nd international symposium for testing and failure analysis. ISTFA 96. Materials Park (OH): ASM International; 1996. p. 199–205. [5] Kempshall BW, Schwarz SM, Giannuzzi LA. In-situ FIB lift-out for site specific TEM specimen preparation of grain boundaries and interfaces. In: ICEM 15, Durban; 2002. [6] Burkhardt Claus, Kleindiek Stephan, Gnauck Peter, Bihr Johannes, Nisch Wilfried. In-situ lift-out of TEM lamellae using a compact and precise micromanipulator. Poster, EFUG 2002, Igea Marina Bellaria (Rimini), Italy, Monday 7 October 2002. [7] Moore TM. The total release method for fib in-situ TEM sample preparation. Microsc Today 2005;13(4):40–2. [8] Rowlette Jeremy A, Di Battista Michael, Fortuna Seth, Livengood Richard H. Microwave signal propagation in backside focused ion beam (FIB) fabricated interconnects. In: Proceedings of the 28th ISTFA; 2002. p. 559. [9] Thompson MA, Richardson C, Le Roy E, Lundquist T, Thompson WB. Coaxial photon-in technology enables direct navigation to buried nodes on planarized surfaces including silicon. In: Proceedings of the 28th ISTFA; 2002. p. 409. [10] Quinn PJ, van Roermund AHM. Switched-capacitor techniques for high-accuracy filter and ADC design. Springer; 2007.
Microelectronics Reliability 47 (2007) 1595–1598 www.elsevier.com/locate/microrel
3D failure analysis in depth profiles of sequentially made FIB cuts C.N. Mc Auley a
a,*
, A. Rummel b, F.W. Keating a, S. Kleindiek
b
Xilinx, Logic Drive, Citywest Business Campus, Saggart, Co. Dublin, Ireland b Kleindiek Nanotechnik GmbH, Reutlingen, Germany Received 2 July 2007 Available online 20 August 2007
Abstract A new method of investigating structures below a surface in a dual beam microscope is presented. It comprises electrical measurements in depth profiles of sequential focused ion beam (FIB) cuts by the use of two or more nanomanipulators with plugged in probe needles. The sample is oriented such that the structures are observed with the electron beam while they are cut free with the FIB. The nanomanipulators are moved to contact the structures for examination. The FIB cut is extended step by step, and after each cut the nanomanipulators are repositioned and measurements of the new structures that appear in the FIB cut are made. The measurement series provide a three dimensional electrical characterization of the examined sample volume. � 2007 Elsevier Ltd. All rights reserved.
1. Introduction Failures in the lower layers of semiconductor devices are usually examined by removing the surface of a sample until the layer of interest followed by electrical probing on that layer [1,2] or by cutting out a part of the sample (lamella) with the focused ion beam (FIB) [3] and by analysing the lamella with a scanning transmission electron microscope (STEM) detector. Alternatively or in addition, the lamella is transferred to a transmission electron microscope (TEM) [4–7] for further analysis. In this paper, we propose an alternative method using nanomanipulators to perform electrical measurements of conductive structures like transistors, wires, and resistances between metal layers without removing the sample from the bulk material. The access to these structures of interest is usually done by mechanical polishing or milling with a FIB until the layer is reached where the failure is electrical isolated too. However, the failure is often extended over more than one layer, e.g. in array of vias contacting several metal layers.
*
Corresponding author. Tel.: +353 1 4615309; fax: +353 1 4640322. E-mail address: [email protected] (C.N. Mc Auley).
0026-2714/$ - see front matter � 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.microrel.2007.07.032
The purpose of this paper is to present a new method of investigating structures in a FIB cut by the use of two or more nanomanipulators. The electrical measurements can be done immediately after FIB milling. Moreover, this method can be used to get a three dimensional electrical characterization of the examined sample volume as the FIB cut can be extended step by step and the measurements are repeated after each new FIB cut. 2. System set-up An overview of the experimental set-up is given in the schematic drawings Figs. 1–3. The sample is orientated in such a way in a dual beam FIB that the structures are observed with the electron beam while they are milled with the FIB. The cross-sectional view shows the orientation of the FIB cut with respect to the ion and electron beam. Two nanomanipulators come from the sides of the FIB cut to contact the structures for examination. The FIB cut is extended step by step, and after each cut the nanomanipulators are repositioned and measurements of the new structures that appear in the cut are made. The needles are bent to around 45� in a way that they appear bended in ion beam images but only slightly bended in electron beam images. The degree of bending of the needle
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C.N. Mc Auley et al. / Microelectronics Reliability 47 (2007) 1595–1598
Electron beam view Ion beam view
Structures in the FIB cut for probing
Fig. 5. Side view of the system set-up.
Fig. 1. Cross-section of the FIB cut.
Probe needles
FIB cut
Cross Section View Metal lines and vias Fig. 2. Electron beam view of the FIB cut.
FIB cut
Probe needles
Step pattern Metal lines and vias Fig. 3. Overview of the FIB cut probing.
determines the ease of access to the FIB cut sample face. If the needle is not bent sufficiently the needles will not be able to make contact to the FIB milled face of the device. However, a larger bending angle reduces the visibility of the tip especially for very small contact areas.
Fig. 4. Top-down view of the system set-up.
Fig. 4 shows a top down view, Fig. 5 a side view of the sample mounted on precision TEM t-type tool stub used for parallel polishing on the stage of a dual beam FIB at its coincidence and eucentric point for FIB milling and the two nanomanipulators in place to enable electrical probing. The photo of Fig. 4 is taken from the direction where the ion beam comes from, the stage is tilted 52�. 3. Test measurement A first measurement was performed to test this new technique. As test object intermetal via connections of a 90 nm CMOS device was chosen. The nanomanipulators were positioned on both ends of the via and the resistance of the structure was measured to be 387 X (see Fig. 6). The measurement shows the easy access to structures that are vertically extended with respect to the sample surface. As no change was made in the orientation or the position of the sample, which was located at the eucentric height and coincidence point for SEM and FIB in a dual beam system, it can be shown that this new technique provides a three dimensional electrical characterization of the examined sample volume by making sequential FIB cuts. Repositioning is done within some minutes as the nanomanipulators need to be moved away and towards the sample only a few microns to make a electrical measurement after a new cut due to the close positioning of the manipulators to the FIB cut trench.
Fig. 6. SEM image of the two nanomanipulators probing a via between two metal layers on a 90 nm process device.
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Moreover, it is possible to resharpen or clean the tips with the ion beam as the ion beam looks at the contact area between the structure in the FIB cut and the tip. This increases the time to perform experiments without opening the SEM chamber.
nipulators were positioned on both ends of the via and the resistance of the structure was measured to be 257 (see Fig. 8).
4. Application to a 65 nm flip chip device
5.1. Probing on a metal finger capacitor
A Virtex 5 FPGA Flip chip device (see Fig. 7) was deprocessed from the back side [8,9] to allow access for the nanomanipulators to probe the switch capacitor (SC) reference circuit for generating Vrefp and Vrefn on chip. The circuits have been created using 0.25 lm thick oxide MOSTs which are required for chip I/O interfacing to the 65 nm 1 v digital VLSI [10]. The sample was analysed in a dual beam FIB equipped with two nanomanipulators set-up opposite to each other on a door mount adapter. A FIB cut was made in area of interest on the SC reference circuit (see Fig. 7) and the inter metal via connections of a 65 nm technology device was measured. The nanoma-
Fig. 9 shows a backside FIB cut of a milled cross-section of a metal finger capacitor (see Fig. 7) being probed between the interconnect between metal 11 and metal 9. The resistance was measured to be 412 X between the interconnect. The total resistance between the probes consist of the individual resistances that are connected in parallel between the probed metal layers and the contact resistances. The latter can be avoided if four point measurement is done. Typical resistances between two tips including cabling are 30–60 X.
Fig. 7. IR image of metal 1 showing location of cuts on system monitor on Virtex 5 65 nm FPGA.
Fig. 8. Probing on an intermetal connection of the SC reference circuit after initial FIB cut.
5. 3D failure analysis on sequential FIB cuts
5.2. 3D electrical analysis As the SC reference circuit is extended vertically and horizontally, sequential cuts must be made starting from one side of the SC reference circuit. Measurements are made after each cut until the end of the SC Reference circuit is reached. This is demonstrated in Figs. 10 and 11 for two additional cuts. After having probed on the first FIB cut (Fig. 8) the manipulators are lifted up some microns as and the FIB cut is extended for about 100 nm such that new details of the SC reference circuit are becoming visible. The manipulators are then moved down again to the SC reference circuit and a measurement can be performed (Fig. 10). The complete process can be done within minutes. This technique is repeated (probing after a further cut is seen in Fig. 11) until a three dimensional electrical characterization of the SC reference circuit is done. A main advantage is that no movement of the stage is needed – it remains at a tilted angle of 52�. In addition,
Fig. 9. Two point probing on the metal finger capacitor.
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between the sample and the tips. At first, because the sample (and therefore any contamination on the surface of the cut) is cut off with the ion beam after each measurement. Secondly, the tips can be cleaned and re-sharpened with the ion beam just at the contact point. This technique can be applied to other types of analysis like capacitive measurements and EBIC analysis, where a three dimensional image of the p-n-junctions can be made. Acknowledgement Special thanks to Device Analysis Laboratory Xilinx Ireland and Kleindiek Nanotechnik for their technical support and assistance in developing this technique. Fig. 10. Probing on the SC reference circuit after the second FIB cut.
Fig. 11. Probing on the SC reference circuit after the third FIB cut.
before and after making a FIB cut the manipulators must be lifted up and repositioned only some microns. 6. Conclusions A new method of characterizing a sample electrically in three dimensions is proposed and demonstrated. Direct probing on a FIB cut is a quick and easy method for the measurement of structures that are extended horizontally and vertically underneath a surface. The experimental set-up significantly reduces contact resistances that arise through electron beam radiation
References [1] Zimmermann Gunnar, Mueller Stefan. 90 nm technology SRAM soft fail analysis using nanoprobing and junction stain TEM. In: Proceedigns of the 32th ISTFA; 2006. p. 512. [2] Sibileau F, Ali C, Giret C, Faure D. SRAM cell defect isolation methodology by sub micron probing technique. Microelectron Reliab 2005;45:1562–7. [3] Langford RM, Petford-Long AK. Preparation of transmission electron microscopy cross section specimens using focused ion beam milling. J Vac Sci Technol A 2001;19(5). [4] Herlinger LR, Chevacharoenkul S, Erwin DC. TEM sample preparation using a focused ion beam and a probe manipulator. Proceedings of the 22nd international symposium for testing and failure analysis. ISTFA 96. Materials Park (OH): ASM International; 1996. p. 199–205. [5] Kempshall BW, Schwarz SM, Giannuzzi LA. In-situ FIB lift-out for site specific TEM specimen preparation of grain boundaries and interfaces. In: ICEM 15, Durban; 2002. [6] Burkhardt Claus, Kleindiek Stephan, Gnauck Peter, Bihr Johannes, Nisch Wilfried. In-situ lift-out of TEM lamellae using a compact and precise micromanipulator. Poster, EFUG 2002, Igea Marina Bellaria (Rimini), Italy, Monday 7 October 2002. [7] Moore TM. The total release method for fib in-situ TEM sample preparation. Microsc Today 2005;13(4):40–2. [8] Rowlette Jeremy A, Di Battista Michael, Fortuna Seth, Livengood Richard H. Microwave signal propagation in backside focused ion beam (FIB) fabricated interconnects. In: Proceedings of the 28th ISTFA; 2002. p. 559. [9] Thompson MA, Richardson C, Le Roy E, Lundquist T, Thompson WB. Coaxial photon-in technology enables direct navigation to buried nodes on planarized surfaces including silicon. In: Proceedings of the 28th ISTFA; 2002. p. 409. [10] Quinn PJ, van Roermund AHM. Switched-capacitor techniques for high-accuracy filter and ADC design. Springer; 2007.
