Analyzing Thick Film Multilayer Defects Using Acoustic Micro Imaging

Transcription

1 Analyzing Thick Film Multilayer Defects Using Acoustic Micro Imaging G. Harsányi 1, J. E. Semmens 2, S. R. Martell 2, L. Pércsi 1, and E. Tóth 3 1 Dept. of Electronics Technology Technical University of Budapest, Budapest, H-1521, Hungary Phone: Fax: Harsanyi@ett.bme.hu 2 Sonoscan Inc. 530 East Green St. Bensenville, Illinois Phone: Fax: info@sonoscan.com 3 Elektroprint Ltd. Budapest, 1061, Andrássy út 15, Hungary Abstract Acoustic Micro Imaging (AMI) has successfully been applied for detecting soldering and underfill failures when analyzing SMD, BGA, and Flip Chip mounting, as well as to find internal defects discontinuities, delaminations, and leakages in plastic packaged ICs. This paper highlights the possibilities of AMI in analyzing multilayer defects in thick film structures. Studying a model system of multilayer thick film, it has been found that C-SAM technique seems to be a good candidate for differentiating the location surface defects, subsurface bubbles, and short circuit locations, nondestructively. MCM-C LTCC and thick film multilayer structures are generally built up from relatively high-glass-content dielectric and conductive materials, causing one of the most important problems of the technology. In recent failure analysis investigations, it has been demonstrated that not only rude blistering effect or pinholes can cause short circuit failures leading to decreased yields, but ionic migration and dendritic growth through the melted glass electrolyte may occur. Short circuit locations remain generally undetected in conventional morphology studies of the surface performed with optical and scanning electron microscopy. Thus, these defects can only be analyzed by destructive methods, after polishing or cross-section preparation, even when the right position can be found only with difficulties. A special method has been developed by analyzing optical and acoustic multilevel picture for revealing the location of short circuit locations nondestructively. The latter finding was proved by performing conventional destructive investigation followed by Scanning Electron Microscopy (SEM) and electron microprobe analysis. This may be a relevant contribution for future multilayer failure analysis processes. Key words: Yield and Reliability of Thick Films, and Battery Effect. Acoustic Micro Imaging, Nondestructive Analysis, High Temperature Migration Mechanism, Failure Analysis, Blistering, The International Journal of Microcircuits and Electronic Packaging, Volume 22, Number4, Fourth Quarter 1999 (ISSN ) 388

2 Analyzing Thick Film Multilayer Defects Using Acoustic Micro Imaging 1. Introduction ity to gate and focus at specific levels, C-SAM is a powerful tool for analyzing the nature of any defect within a sample. Acoustic Micro Imaging (AMI) has successfully been applied for detecting soldering and underfill failures when analyzing SMD, 1.1. Background Acoustic Micro Imaging BGA, and Flip Chip mounting, as well as to find internal defects discontinuities, delaminations, and leakages in plastic packaged Acoustic Micro Imaging (AMI) utilizes high frequency ultrasound in scanning of samples to non-destructively detect intering multilayer defects in thick film structures. Studying a model ICs 1-4. This paper highlights the possibilities of AMI in analyznal discontinuities in materials and components 1-4. Ultrasound system of multilayer thick film, it has been found that the 210MHz is an elastic wave and as such interacts with the materials on a C-SAM technique seems to be a good candidate for differentiating the location surface defects, subsurface bubbles, and short micro size scale. The propagation behavior depends on the mechanical properties of the materials and can be expressed by the circuit locations, nondestructively. Although the ultrasound wavelength is supposed to be larger in metals and ceramics than in acoustic impedance. Ultrasound reflection properties at interfaces between different materials depend on their acoustic impedance water, a mm depth resolution seems to be achievable; thus, values 4. The acoustic impedance (Z) can be expressed as follows, several interface scans can be made within a thick film multilayer structure enabling a level-by-level image preparation. In a Z = q c where q is the density of the material and c the propagation speed single conductor/dielectric/conductor sandwich, a minimum a of the ultrasound waves. The reflection ratio (R, the ratio of the surface scan and a subsurface scan can be distinguished. This reflected and incoming amplitudes) depends on the difference can hardly be imagined with considerably lower ultrasound frequencies (around 100 MHz, which already enabled the evalua- between acoustic impedance values of the two materials at the interface, tion of solder joint failures). R = (Z 2 - Z 1 ) / (Z 2 + Z 1 ) The larger the impedance difference across the interface the larger 1.2. Background Processing Defects in the echo and the brighter the image. Ultrasound may be absorbed, Thick Film Multilayer Structures scattered and reflected whenever there is a change in material properties. Surface imperfections scatter the ultrasound and can Recently, in connection with the production of high-density be recognized as dark spots on surface images. Since ultrasound interconnection systems in integrated circuits and multichip is totally reflected at a vacuum or air gap, voids and delaminations inside the sample can be detected as reflecting areas. This with very high resolution and high reliability has come into the modules (MCMs), the claim for multilayer conductor-systems effect is widely used when investigating structures used in microelectronics forefront 5. The possibilities of integration are determined not only 1. On the other hand, there is no reflection when by the technological bases but also by the physical and chemical the same materials are present at the interface. processes that can cause resistive shorts between adjacent metallization stripes during processing and later on during the device There are several modes of operation. For the analysis of thick film multilayer structures a C-SAM, C-Mode Scanning Acoustic operation. Microscopy was applied, which is a pulse-echo (reflection type) The phenomenon of electrochemical migration has been well microscope that generates images by mechanically scanning a known for several decades as a process resulting in short circuit transducer in a raster pattern over the sample. A focused spot of failures during the operation of the electronic systems 6, 7. It can ultrasound is generated by an acoustic lens assembly. A frequency be defined as a transport of ions between two metallization stripes of 210 MHz was used for this particular application. The ultrasound is brought to the sample by a coupling medium, usually at high relative humidity levels. Electrodeposition also takes place under bias through an aqueous electrolyte formed on the surfaces deionized water or an inert fluid. The transducer alternately acts forming dendrites or dendrite-like deposits. Ultimately, such a as sender and receiver, being electronically switched between the deposit can lead to a short circuit in the device and can cause transmitter and receiver modes. The echos are separated according to the investigated depth based on a time-of-flight filtering liquid (usually water) to form an electrolyte, bias, and operating catastrophic failure. The preliminary conditions are a film of polar using an electronic gate; thus, it is possible to select a specific time 8. depth or interface to view. A very high-speed mechanical scanner is used to index the transducer across the sample and pro- The phenomenon is due to the fact that silver is anodically dis- Silver is famous for its inclination to form migratory shorts. duce images in tens of seconds. Both the lateral and the depth solved from its original location and redeposited as metal at a resolution is in the range of the applied wavelength; thus, it is cathodic site. The electrochemical deposition results in the formation of dendrite-like deposits 9. The electrochemical processes determined by the material and the frequency. A 210 MHz frequency ultrasound has a wavelength of cca. 7.5 µm in distilled are as follows, water. Anode: Ag e Ag + + e - In C-SAM images, the contrast changes compared to the Cathode: Ag + + e - e Ag background constitute the important information. Voids, cracks, On the other hand, studies on failure mechanisms during the disbonds, and delaminations provide high contrast and are easily distinguished from the background. Combined with the abilcome increasingly important. One of the well known yield limit- firing of high-resolution multilayer structures have recently be- The International Journal of Microcircuits and Electronic Packaging, Volume 22, Number 4, Fourth Quarter 1999 (ISSN ) 389

3 ing the phenomena is the blistering 10,11 effect resulting in delamination of layers. Another phenomenon is the short circuit formation through pinholes that are also processing induced failure sites. However, investigations pointed out that short circuit failures could also be formed without the presence of pinholes or blistering. According to investigations published elsewhere 12, a metal migration similar to the above-described wet migration may be occurring in these latter cases through the melted glass electrolyte. Blistering usually happens if the multilayer structures consist of different conductor layers 13. If there is an electrode potential difference between the adjacent conductor layers and the glass component of the dielectric containing a lot of mobile ions at the peak temperature, the so-called battery effect will occur. This battery effect causes a redox reaction. For instance, if the conductor layers in the multilayer structure are Ag and Au, the redox reaction will result in the formation of mobile Ag + and O 2- ions, 4 Ag + + 2O 2- w 4 Ag + O 2 Since the standard electrode potential of the Au is V and Ag is +0.8 V, the positive electrode (anode) will be the gold. Consequently, the anions, in this case, the O 2- ions, are neutralized on the latter electrode. On the surface of the gold, gas evolution begins which can cause delamination or blistering. It can be demonstrated that oxygen bubbles occur on the anode due to the battery effect. According to experimental studies, this effect does not cause a big problem in this case when the anode is the top conductive layer 11. In such cases, the oxygen can easily move into the environment during the firing process without causing delamination. When the anode is in the bottom layer, the arising oxygen cannot escape from the structure and destroys the under-boundary surface. As a result, intensive blistering and delamination of the layers can be observed. During the blistering, even short circuits may be formed. One explanation is that the oxygen evolution may also result in the formation of pinholes inside the structures. That means, the blistering always should occur when the presence of short circuits is experienced. The other possible mechanism is a metal migration similar to the described wet migration through the melted glass electrolyte 12. The greatest problem in distinguishing these failure mechanisms is to find the location of short circuits that were formed without blisters. Short circuit locations remain generally undetected in conventional morphology studies performed with Optical and Scanning Electron Microscopy (SEM). Thus, they can only be analyzed by destructive methods, after polishing or cross-section preparation, when the right position can be found with difficulties. Studying a model system of multilayer thick film, it has been found that acoustic micro imaging (AMI) seems to be a good candidate for revealing the location of short circuit locations nondestructively. This may be a relevant result for future failure analysis processes. 2. Sample Preparation In the simplest multilayer structure, the thick film crossover structure was prepared as a model system for the examinations. The samples were produced by conventional thick film technology. The films were printed using 200-mesh stainless screen for each conductive layer and 200/325 mesh screen double printing was used for the dielectrics. The thickness of the conductive layers was approximately 20 µm. The thickness of the dielectric layer was between µm in order to avoid classical pinhole formation. Each layer was dried at 150 C. Only the bottom conductor layer and the dielectrics were fired in air in a conventional belt furnace, at a standard 850 C/10min profile. The top conductor layer was fired in a special experimental furnace. The temperature program of this furnace followed also the standard profile. An external bias was applied to the samples during this last firing step in order to stimulate the battery effect. This method proved to be advantageous for the following reasons. The polarity can be chosen externally. There is no need to multiple firing in order to achieve strong blistering. The leakage current can also be continuously monitored as a function of the temperature. The measuring set-up is shown in Figure 1. It was applied for this so called forced blistering in earlier studies 11,12,14. A Bottom conductor Alumina substrate U Top conductor Figure 1. The measuring set-up for forced battery effect. Figure 2 shows the microscope photography of a typical sample after firing. Surface imperfections can be clearly observed on the surface morphology. This particular sample showed a leakage current runaway during the firing and also short circuit could be measured after firing. In order to get direct proof of the existence of short circuit formation and high temperature migration mechanisms, an additional experimental procedure has been accomplished. In order to perform analytical investigation cross-sectional cuts on the samples, it is necessary to find the location of the short circuit. This task is practically impossible in a large area surface multilayer structure. Acoustic Micro Imaging technique enabled the researchers to find these locations non-destructively. The International Journal of Microcircuits and Electronic Packaging, Volume 22, Number4, Fourth Quarter 1999 (ISSN ) 390

4 Analyzing Thick Film Multilayer Defects Using Acoustic Micro Imaging 3a Figure 2. Microscopic photograph of the thick film sample. 3. Acoustic Micro Imaging Investigations During the failure analysis, the samples were examined with the new non-destructive inspection, AMI. The advantage of this technique is that information can be received not only from the surface of the sample, but also from the subsurface regions. Surface and subsurface defects are represented by spots with different contrast on the images, the nature of which can be vary among several possibilities; surface blistering, groves, pinholes, subsurface delaminations, bubbles, and even metallic grains causing short circuit. The systematic correlation analysis on surface and subsurface acoustic micrographs, with optical microscopy and SEM images could reveal the origin of the various spot types on the images. Acoustic images were generated on the sample shown in Figure 2 using the interface scan technique. Using this technique, the ultrasound echo image is filtered at the depth of interest. For the surface scan image, the ultrasound echo reflected by the surface of the sample was collected. For the subsurface scan image the ultrasound reflected from just below the top of the sample was screened. Surface and subsurface acoustic images of the sample are demonstrated in Figure 3.a and Figure 3.b, respectively. 3b Figure 3. Acoustic micro images of the thick film sample: a) surface scan, b) subsurface scan images. A careful pattern analysis of the three images indicated the following conclusions, There are strong, dark spots on the surface scan image that correspond to small surface imperfections clearly shown also in the optical image. These imperfections are represented in the subsurface image as dark circles with bright centers (Spot type 1). There are gray spots on the surface image that are not visible optically. These features are represented again as circles on the subsurface image, clearly indicating that they are subsurface defects (Spot type 2). There are some light gray spots both on the surface and subsurface images that cannot be recognized in the optical image (Spot type 3). The International Journal of Microcircuits and Electronic Packaging, Volume 22, Number 4, Fourth Quarter 1999 (ISSN ) 391

5 4. Discussion of Results As it was mentioned in the introduction, the reflected intensity is a function of the acoustic impedance difference between the two materials of the interface. The larger the impedance difference across the interface the larger the echo and the brighter the image. Ultrasound may be absorbed, scattered and reflected whenever there is a change in material properties (as indicated in Figure 4). Surface imperfections scatter the ultrasound and can be recognized as dark spots on surface images. Since ultrasound is totally reflected at a vacuum or air gap, voids and delaminations inside the sample can be detected as bright areas. This effect is widely used when investigating structures used in microelectronics 1-4. On the other hand, there is no reflection when the same materials are present at the interface. According to these results, the spots on the images of the investigated thick film sample can be interpreted as follows. Subsurface bubbles often cause surface imperfections that can be recognized optically. Ultrasound is scattered on these imperfections; thus, dark spots represent them on surface images. All kind of subsurface bubbles, even those that do not cause large surface defects, reflect the ultrasound. Therefore, they give bright spots when the ultrasound is focused onto the subsurface region. Subsurface short circuits, metallic grains do not cause large reflection. Thus features represented by light gray spots on both surface and subsurface images (mainly recognizable on the former, but without giving bright spot on the latter one) may be identified as short circuit locations. 5. Verification by Scanning Electron Microscopy Analysis Ultrasound Substrate Ultrasound (a) Investigation was performed with the aid of the Scanning Electron Microscopy (SEM) and energy dispersive X-ray analysis. For the experiments, the same sample was used as for the nondestructive Acoustic Micro Imaging. The top electrode was removed by polishing. The preparation of the samples was performed with great care in order to avoid polishing the material of the upper conductive layer into the dielectric. A two-step polishing was performed, followed by an ultrasonic cleaning. The SEM investigation was performed exactly on those areas of the sample where the locations of the short circuits were hypothesized by the acoustic images. Tiny bright spots could be recognized inside the dielectric layer at these locations, as shown by Figure 5. Substrate Bubble (b) Ultrasound Substrate Short circuit (c) Figure 4. Ultrasound reflection phenomena on multilayer defects: a) scattering on surface imperfections, b) reflection on subsurface bubbles, c) no reflection on short circuit sites. Figure 5. SEM photograph of a metallic grain causing short circuit that was found at the location of Spot type

6 Analyzing Thick Film Multilayer Defects Using Acoustic Micro Imaging The result of the X-ray analysis is demonstrated in Figure 6. Accordingly, it contains PdAg, exactly the material of the bottom electrode, which could not originate from the polishing process, since the top electrode was pure Ag. Thus, a real short circuit was found by the SEM investigation that proves the hypothesis about the spots on the acoustic images. Visually detectable surface defects give dark spots in acoustic surface images. Subsurface bubbles, which are not always visually detectable, can be detected as bright spots on subsurface acoustic images. Gray spots that are available on both images may indicate the location of short circuits. C) Verification of the results: Scanning electron microscopy and electron microprobe analysis have proved the presence of a short circuit, the location of which was designated by a preliminary combined optical/ami analysis, as described in the present work. Acknowledgments Figure 6. Energy dispersive X-ray of the metallic grain. This research has been supported by the European Commission in the frame of INCO Copernicus Project No. ERBIC 15CT , as well as by the following projects in Hungary: MKM FKFP 0254/1997 and OTKA F007365, T Conclusions References A) C mode Acoustic Micro Imaging (using 210MHz ultrasound) has been found a good tool not only for analyzing package leakage and solder joint failures, but also for screening failure sites of multilayer structures. The depth resolution of the high ultrasound frequency enabled to prepare surface-scan and subsurface-scan images within a single thick film conductor/insulator/conductor layer system. Combining this information with optical microscopic images, the failure sites of different nature (surface imperfections, bubbles, and short circuit sites) can be localized and distinguished non-destructively for the further analysis. An appropriate automated inspection system based on pattern recognition can be also used for failure screening in the production. The technique has demonstrated good success. A case study of thick film multilayer structure has demonstrated this statement, but the method is much more general. In general, the main ideas can be adopted for any multilayer system. B) The case study: One main yield-limiting factor of LTCC and thick film systems is a short circuit formation during the firing. This mechanism does not always appear to be in correlation with the blistering, although both of them are caused by the battery effect. The model system was a single thick film conductor/insulator/conductor layer structure on alumina substrate. Acoustic Micro Imaging combined with optical inspection seems to be a good tool for failure analysis indicating not only blistering but short circuit locations as well. Accordingly, the following statements can be concluded, 1. J. E. Semmens, S. R. Martell, and L. W. Kessler, Evaluation of Flip Chip Interconnects Using Acoustic Microscopy For Failure Analysis And Process Control Applications, Proceedings of the 1995 International Conference on Multichip Modules, Denver, Colorado, MCM 95, April 19-21, pp , J. E. Semmens, S. R. Martell, and L. W. Kessler, Analysis of BGA and Other Area Array Packaging Using Acoustic Micro Imaging, Proceedings of the 1 st Pan Pacific Microelectronics Symposium, Honolulu, Hawaii, Febr.uary 6-8, pp J. E. Semmens and L. W. Kessler, Acoustic Analysis of Failure Modes in Flip Chip Devices, Proceedings of the 2 nd Pan Pacific Microelectronics Symposium, Maui, Hawaii, January 28-30, pp , S. Ong, S. Tan, and K. Tan, Acoustic Microscopy reveals IC Packaging Hidden Defects, Proceedings of the 1997 Electronic Components and Technology Conference, Singapore, October pp , Zs. Illyefalvi-Vitéz, M. Ruszinkó, J. Pinkola, Recent Advancements in MCM/L Imaging and Via Generation by Laser Direct Writing, Proceedings of the 48 th Electronic Components and Technology Conference, ECTC 98, Seattle, Washington, May 25-28, pp , R. C. Benson, B. M. Romenesko, J. A. Weiner, B. H. Nall, and H. K. Charles, Metal Electromigration Induced by Sol- 393

7 der Flux Residue in Hybrid Microcircuits, IEEE Transactions on Components, Hybrids and Manufacturing Technology, CHMT, No. 4, pp , H. M. Naguib and B. K. Maclaurin, Silver Migration and the Reliability of Pd/Ag Conductors in Thick-Film Dielectric Crossover Structures, IEEE Transactions on Components, Hybrids, and Manufacturing Technology, CHMT, No. 2, pp , M. V. Coleman and A. E. Winster, Silver Migration in Thick Film Conductors and Chip Attachment Resins, Microelectronics Journal, No. 4, pg. 23, G. Harsányi, New Type Short Circuits at Fritless Thick Film Conductors Formed from Reduced Oxides, Proceedings of The International Symposium on Microelectronics, ISHM 92, San Francisco, California, October 19-21, pp , T. Gilles, Q. Reynolds, and J. Steinberg, A New Generation Multilayer System: Low Cost and High Reliability, Hybrid Circuits, No. 21, January J. V. Manca, L. De Schepper, W. De Ceuninck, M. D Olieslager, and L. M. Stals, In-Situ Electrical Measurements on Thick Film, Hybrid Circuits, No. 31, pp. 5-11, L. Pércsi and G. Harsányi, Processing Induced Short Circuit Failures in LTCC MCM-C Structures, International Journal of Microcircuits and Electronic Packaging, Vol. 20, No. 2, B. Sjoeling and S. Turvey, The Material Science of a Non Warp, Hermetic, Non Battery Effect, Crystallizing Dielectric for Simplification Multilayer Hybrid Circuitry Construction, Hybrid Circuits, No. 29, pp , J. V. Manca, L. De Schepper, W. De Ceuninck, M. D Olieslager, and L. M. Stals: In-Situ Failure Detection in Thick Film Multilayer Systems, ESREF 94 Proceedings, pp , About the authors 394 Gábor Harsányi, Ph.D., is presently Associate Professor and Head of the Sensors Laboratory at the Department of Electronics Technology, Technical University of Budapest. He graduated in Electrical Engineering in 1981 and received his Ph.D in Microelectronics Technology. He spent several years in industry conducting research projects on chemical microsensors. His current research activity focuses on sensors in biomedical applications, polymers in sensorics, fiber optic sensors, and reliability physics of high-density electronic interconnection systems. He spent half a year fellowship at the Florida International University in Dr. Harsányi has an international reputation in the field of microelectronics and sensorics, and is very active professionally in international conferences and societies: he was President of the Hungarian Chapter of IMAPS (International Microelectronics And Packaging Society) in 1993/94; he received a Best Paper of Session Award at the International Symposium on Microelec- The International Journal of Microcircuits and Electronic Packaging, Volume 22, Number4, Fourth Quarter 1999 (ISSN ) tronics (Orlando, 1991); he is member of the Technical Program Committee of European Microelectronics Conferences, and became a Fellow of IMAPS in He has been involved in various international projects, including PHARE-ACCORD, NATO- ARW, COBASE (a NSF program), INCO-COPERNICUS (a European Union funded program), and others. His scientific research, publication, as well as the related coordinating and traveling activity is also supported by several Hungarian national institutions, such as the National Committee for Technological Development (OMFB), the National Scientific Research Fund (OTKA), the Ministry for Culture and Education (MKM), and the Foundation for the Hungarian Higher Education and Research (AMFK). He has authored more than 100 research publications and technical papers in international journals and conference proceedings books, co-edited several books, as well as authored one monograph Polymer Films in Sensor Applications published by Technomic Publishing Co., Inc. in In various sensor application topics, he has also conducted several high level international professional seminars and courses sponsored by IMAPS, Europractice, and Technomic Publishing Co., Inc. Levente Pércsi received his M.Sc. Degree in Electrical Engineering from the Technical University of Budapest in He was a Ph.D. student at the Department of Electronics Technology at the same University from 1994 to Currently, he is working on his final thesis. His main interests are the research and development of laser power detectors and the examination of metal migrations in various structures applied in MCMs. He joined Montana Ltd. as a project manager. Mr. Pércsi has been a member of IMAPS since Endre Tóth, dr. univ., graduated and received his M.Sc. from the Technical University of Budapest. He worked seven years in Hungarian electronics manufacturing and assembling industry. From 1964, he has been teaching and performing research work in the field of printed wiring boards, soft soldering, board assembling, and multichip modules. Dr. Tóth has been engaged in environmental protection issues in electronics industry for more than twenty years. He has published over 100 papers and has been active in JEC/TC-52, TC-90 projects. He is the Editor in Chief of the Hungarian journal Microtechnics, Electronics Technology and he is also member of the organization committees of several international scientific conferences.