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edfas.org ELECTRONIC DEVICE FAILURE ANALYSIS

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NOVEMBER 2018 | VOLUME 20 | ISSUE 4

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DEPARTMENTS Early Life Automotive Electronics Failures and Their Root Causes Peter Jacob In automotive electronics, failures are frequently found that cannot be explained by the failure signatures of the defective devices. Most can be prevented by taking careful precautions in the design phase. The author describes several examples andconcludeswith somebasicprinciples for implementing a system level approach to FA. Advanced Packaging Fault Isolation Case Studies and Advancement of EOTPR Jesse Alton, Thomas White, and Martin Igarashi Electro optical terahertz pulse reflectometry (EOTPR) is now a well-established technique in failure analysis workflows and has been used to interrogate a wide range of distinctive architectures. This article describes a novel acoustic inspection tool oper- ating in the GHz band to extend both the lateral resolution and surface and subsurface sensitivity of acousticmicros- copy. Case studies illustrate the potential of GHz-SAM for inspection and analysis especially in high-reliability applications. Geolocation of Cu Wires During Sensitive IC Acid Decapsulation Michael Obein As IC packages have improved, the initial quality check of the assembly and any required failure analysis are criti- cal to continued IC progress. Among recent technological advancements is the use of copper wires, which need to be protected by stopping the acid decapsulation process at the right moment. 2 GUEST EDITORIAL Lee Knauss and Zhiyong Wang 38 ANADEF WORKSHOP REPORT Julien Perraud 40 ISTFA 2018 EXHIBITORS LIST 41 ISTFA 2018 EXHIBITORS SHOWCASE 42 EDFAS AWARDS 48 MASTER FA TECHNIQUES Dave Burgess High Resolution Acoustic GHz Microscopy Sebastian Brand, Michael Kögel, and Frank Altmann edfas.org ELECTRONIC DEVICE FAILURE ANALYSIS A RESOURCE FOR TECHNICAL INFORMATION AND INDUSTRY DEVELOPMENTS NOVEMBER 2018 | VOLUME 20 | ISSUE 4 24 30 4 16

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ABOUT THE COVER See page 61 for a description of the contest winners’ collage on the cover.

Author Guidelines Author guidelines and a sample article are available at edfas.org. Potential authors should consult the guidelines for useful informa- tion prior to manuscript preparation.

For the digital edition, log in to edfas.org, click on the “News/Magazines” tab, and select “EDFA Magazine.”

52 DIRECTORY OF FA PROVIDERS Rose Ring 54 PRODUCT NEWS Larry Wagner 58 TRAINING CALENDAR Rose Ring 60 LITERATURE REVIEW Mike Bruce 62 GUEST COLUMNIST Peter Jacob 64 ADVERTISERS INDEX

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ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 20 NO. 4 2 PURPOSE: To provide a technical condensation of information of interest to electronic device failure analysis technicians, engineers, and managers. Felix Beaudoin Editor/GlobalFoundries; felix.beaudoin@ globalfoundries.com Scott D. Henry Publisher Mary Anne Fleming Manager, Technical Journals Kelly Sukol Production Supervisor Joanne Miller Managing Editor ASSOCIATE EDITORS Nicholas Antoniou Nova Measuring Instruments Navid Asadi NOVEMBER 2018 | VOLUME 20 | ISSUE 4 A RESOURCE FOR TECHNICAL INFORMATION AND INDUSTRY DEVELOPMENTS ELECTRONIC DEVICE FAILURE ANALYSIS

EDITORIAL

ENGAGE WITH EDFAS Lee Knauss, FASM, Booz Allen Hamilton EDFAS President knauss_lee@bah.com Zhiyong Wang, Maxim Integrated EDFAS Immediate Past President zhiyong.wang@maximintegrated.com

T he continuously advancing semiconductor indus- try puts increasing demands on our profession to localize and analyze defects to increase yield and reduce failures in the field. From silicon to packag- ing, the feature size, density, and 3D nature of current fabrication methods are making FA increasingly more challenging. As a society of electronic device failure

University of Florida Guillaume Bascoul CNES France Michael R. Bruce Consultant David L. Burgess Accelerated Analysis Jiann Min Chin Advanced Micro Devices Singapore Edward I. Cole, Jr. Sandia National Labs Szu Huat Goh GlobalFoundries Singapore Martin Keim Mentor, A Siemens Business

analysts, we need to improve communication and engage across businesses, technologies, and academia to support our membership in addressing these ever-increasing challenges. Our professional society has long promoted the interaction between testing and failure analysis as indicated in the name of our major confer- ence. However, the representation of test-related topics has diminished substantially over the years. To address this deficit, we are co-locating with the International Test Conference (ITC) this year. Access to the ITC plenary, panel sessions, and exhibits will provide renewed opportunities for engage- ment with the test community and stimulate conversation that has not taken place through our conference venue for a long time. Strategic objectives under consideration going into 2019 are focused around engagement among people, our knowledge resources, and leading industry organizations. We arewriting this in advance of our fall EDFAS Board meeting, whichwill occur just before the start of ISTFA. Although our strategic objectives are not yet formalized, we want to make our society’s members aware of the draft version. Our hope is that you will engage us in discussions during the conference to confirmthe valueof theseobjectives andoffer further ideas to help shape them. EDFAS Strategic Objectives Draft • Engaging people ºº Establish discussion groups for year-long engagement, leveraging popular platforms ºº Renew development of local EDFAS chapters for more opportunities to connect throughout the year in local geographies ºº Improve engagement opportunities within ISTFA and through con- nections with other conferences, like ITC this year

Ted Kolasa Orbital ATK

Rose M. Ring Lam Research

Sam Subramanian NXP Semiconductors Paiboon Tangyunyong Sandia National Labs David P. Vallett PeakSource Analytical LLC Martin Versen University of Applied Sciences Rosenheim, Germany FOUNDING EDITORS Edward I. Cole, Jr.

Sandia National Labs Lawrence C. Wagner LWSN Consulting Inc. GRAPHIC DESIGN Jan Nejedlik, www.designbyj.com

PRESS RELEASE SUBMISSIONS magazines@asminternational.org Electronic Device Failure Analysis™ (ISSN 1537-0755) is pub- lished quarterly by ASM International ® , 9639 Kinsman Road, Materials Park, OH 44073; tel: 800.336.5152; website: edfas. org.Copyright©2018byASMInternational.Receive Electronic Device Failure Analysis as part of your EDFAS membership. Non-member subscription rate is $135 U.S. per year. Authorizationtophotocopy itemsfor internalorpersonaluse, orthe internalorpersonaluseofspecificclients, isgrantedby ASM Internationalfor librariesandotherusersregisteredwith theCopyrightClearanceCenter(CCC)TransactionalReporting Service, provided that the base fee of $19 per article is paid directlytoCCC,222RosewoodDrive,Danvers,MA01923,USA. Electronic Device Failure Analysis is indexed or abstracted by Compendex, EBSCO, Gale, and ProQuest.

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ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 20 NO. 4

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ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 20 NO. 4 4 EDFAAO (2018) 4:4-12

1537-0755/$19.00 ©ASM International ®

HIGH RESOLUTION ACOUSTIC GHz MICROSCOPY Sebastian Brand, Michael Kögel, and Frank Altmann

Fraunhofer Institute for Microstructure of Materials and Systems, IMWS, Center for Applied Microstructure Diagnostics CAM Halle, Germany sebastian.brand@imws.fraunhofer.de

INTRODUCTION Nondestructive acoustic inspection techniques are widely used methods for defect localization. With their special ability to penetrate optically opaque materials, these techniques enable analysis and imaging through housing or encapsulation materials to sense irregulari- ties, which accompany potential defects and thus provide information on their 3D position inside the device. With this ability, acoustics-based techniques are also of high value for the quality screening often performed in the industrial manufacturing environment, especially for high-reliability applications, e.g., the automotivemarket, where a low failure rate is mandatory. However, the indirect access through intermediate materials goes along with a reduction in the techniques’ resolution capabilities, and artifacts can occur that are caused by interactions of the sensing rays on their propagation path through the sample. The result is a compromise between the nondestructive nature of the technique and its achievable lateral and axial resolution. Therefore, both the precision and sensitivity of a defect localization method need careful consideration, as they are highly relevant for guiding the subsequent physical preparation required to provide close access for imaging and analysis techniques that allow for superior lateral resolution. Inspection for delamination, cracks, and voids by scanning acoustic microscopy (SAM) is highly efficient because the contrast mechanism is based on the acoustic waves’ interaction with the material’s mechani- cal properties. This interaction also leads to phenomena such as scattering, reflection, and refraction. Therefore, voids, cracks, and delamination, which can be considered gaseous inclusions in the package, manifest as large gra- dients in the material’s mechanical properties. SAM can detect such material gradients with superior sensitivity. Ongoing advances in microelectronics technologies, such as 3D integration and system inpackage (SiP), enable significant improvements in performance and integration

density, resulting in increasingly complex systems while reducing a device’s spatial requirements. However, new failuremechanisms and failuremodes are inevitably con- nected to these novel technologies, challenging existing methods and tools for nondestructive defect detection and localization. Nonetheless, these tools and techniques are necessary both during process development and in high volume industrial manufacturing. Conventional acoustic microscopy that operates in the frequency band below 400 MHz is highly sensitive to the aforementioned defects, but lacks lateral resolution and sensitivity when the failure sites get too small. With the focus on high resolution failure analysis and metrology applications, a novel SAM device with the spectral range extending up to 2 GHz was developed in the Fraunhofer IMWS lab in collaboration with industry partners. This substantial increase in frequency leads to a significant decrease inwavelength, which combinedwith theapplicationof highly focusedacousticobjective lenses, would theoretically allow for lateral resolution capabili- ties of below1µm. [3] However, this increased lateral resolu- tion comes at the expense of the achievable penetration depth, which renders the technique highly sensitive to surface and subsurface areas. The method is therefore ideally suited for thin structures in the thickness range below 5 µm, which are highly relevant to current micro- electronic technologies. GHz-SAM Figure 1 shows a schematic of the gigahertz-scanning acoustic microscope (GHz-SAM). A signal, which is gen- erated either by an arbitrary waveform generator or a gated oscillator, is power amplified and directed to the acoustic transducer. There, the signal is converted into an elastic (mechanical) wave and submitted through a droplet of coupling medium (to reduce reflectivity) into the sample. The transducer contains a large numerical aperture acoustic lens that focuses the mechanical GHz waves into an extremely narrow spot of approximately

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0.8 µm in the lateral dimension and an axial length of 4-6 µm. The focus of the lens is located 80 µm in front of the transducer’s tip inside the cou- plant (distilled and degassed water is commonly used). On its pathway, the acoustic wave interacts with the elastic material properties and mass density of the propagation materials. Consequently, refraction, reflection, and scattering occur at any material boundaries the wave encounters. Fur- ther, additional interaction phenomena may occur inside the sample where the energy of the incident acoustic wave is partially converted into other wave modes that propagate inside the materials, at the surfaces, or at the interfaces between specific structures in the sample. Regardless, some of

ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 20 NO. 4

Fig. 1 Schematic of the GHz-SAM.

sub-micron voids on 3-µm-thick power metal lines. Additionally, the positive effect of incorporating a 40-nm TiN interlayer in between two AlCu metallization layers was investigated and confirmed. In addition to material composition, properties, and interactions in back end of line (BEOL) systems, void formation—and the critical volume that causes fatal fail- ures—greatlydependsonthemetal structuredesign. These structures need to be adapted based on current density and temperature stability requirements to ensure product reliability. The metal structures can be improved by opti- mizing the width of the metal lines and inserting an addi- tional interlayer material to form a sandwich-like struc- ture. This structure is known to lead to the suppression of large void formation and agglomeration. Samples investigated in this article were manufac- tured employing two different structures of the metal lines. The aluminum(AlCu)metallization had a dimension of 3 × 3 × 800 µm long. Because Ti and TiN are known to have properties that suppress hydrogen-related voiding, [5] TiN was added to the second set of samples. The second set contained a 40-nm thick TiN interlayer material

the incident acoustic waves' energy will return to the transducer, which depending on the mode can occur at different propagation times, as the wave velocities are mode-specific. Those signals are received by the transducer, convert- ed into an electrical signal, and low-noise amplified. To further increase the signal-to-noise ratio (SNR), the signal is preprocessed and then digitized for image formation. A recent extension of the GHz-SAM implemented a high- performance digitizer to capture the unprocessed radio frequency (RF) signals for further offline signal analysis and parametric imaging, particularly in the spectral domain. CASE STUDIES Several case studies have been published by the Fraunhofer IMWS research group to illustrate the success- ful application of GHz-SAM for defect inspection in fields where destructive slice and view analysis was the only alternative for detection and identification of interface defects or crackpropagationpaths. With its relatively large scan field of 2 × 2mmand a possible imaging resolution in the µmregime, superior sensitivity is combinedwith large

area screening and a contrast mecha- nism sensitive to material gradients.

CASE #1: DETECTION OF STRESS INDUCED VOIDS

In the first case study, GHz-SAM was employed to examine the for- mation, growth, and propagation of

Fig. 2 Schematics of samples containing thick AlCu lines with and without a TiN interlayer.

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TCT stress test. Using thresholding, voids were detected and marked as red pixels by a custom software tool. The micrographs in Fig. 7 were recorded from the reference sample, which did not receive the TCT. No voids can be observed in the reference samples. Figure 8 also con- tains a combination of GHz-SAM images and electron micrographs of FIB cross sections through selected voids of a sample that contained a TiN interlayer in the AlCu sandwich structure. Two features should be noted here. First, significantly fewer voids are found in samples containing a TiN layer even though they received the same TCT as the samples shown in Fig. 6. In Table 1, the results of a statistical

sandwiched in theAlCumetal lines (Fig. 2). Allmetallization layerswere deposited by physical vapor deposition (PVD). Two sets of sampleswere exposed toa thermal cycling test (TCT) mimicking the aging process under application con- ditions. This treatment consisted of periodically heating and cooling the samples between - 65° and 175°C for 1000 cycles, and was performed according to JESD22- A104E: Temperature Cycling standards, with the tem- perature range adapted according to AEC-Q100 Revision G. Also, an unaltered reference sample was investigated to assess each sample’s initial condition. As previously mentioned, the penetration depth of large numerical aperture acoustic lenses is significantly decreased compared to standard acoustic microscopy transducers. With an 80-µm focal length, penetration depth is limited to a fewmicrons beneath the surface. For this reason, sample preparation was required to provide backside access through 0-2 µm of the remaining silicon. To avoid sample preparation artifacts, provide a flat surface, and leave the AlCu lines unaltered, the acoustic inspection was performed through the backside as illus- trated in Fig. 3. Samplesweremounted on a dummywafer prior tomechanical treatment to ensure secure handling. The silicon substrate was then removed by highly parallel chemical mechanical polishing (CMP) down to the oxide layer acting as end point detection using a MultiPrep Polishing System (Allied High Tech Products, Rancho Dominguez, Calif.). Figure 4 shows an acoustic GHz micrograph recorded through the SiO 2 layer, according to Fig. 3. The AlCu power lines can be clearly resolved and voids in the metalliza- tion appear as bright spots. In Fig. 5, an acoustic GHz micrograph and a secondary electron (SE) micrograph of a focused ion beam (FIB) trench of the same area can be compared. Using the FIB, the surface of the AlCu lines

Fig. 3 Illustration of sample preparation and acoustic inspection.

Fig. 4 Acoustic GHz micrograph recorded through the SiO 2 layer after removing the Si die.

was milled to reveal the voids and confirm the findings obtained by GHz-SAM. Corresponding voids are marked by identically colored circles in Fig. 5. Wi t h t h i s pe r f o rman ce , GHz-SAM can be used to inves- tigate the influence of Ti or TiN interlayers on the formation of hydrogen triggered voids. Figure 6 contains two acoustic GHz micro- graphs recorded at two different defocus positions to place the imaging plane at adjacent depths within a sample that received the

Fig. 5 Left: Defocused GHz-SAM micrograph. Right: SE micrograph after exposing the AlCu lines by FIB. Corresponding voids are indicated by colored circles.

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Commonly in failure analysis, additional inspection methods like mechanical or FIB cross sectioning com- bined with subsequent SEM imaging are used. However, these methods only provide a detailed local assessment of the interfaces across a certain cross section and do not provide complete information on the full bonding area.

analysis of the voiding behavior are provided. Samples with and without a TiN-interlayer were investigated and voids were detected and analyzed. Results suggest that the presence of a TiN-interlayer significantly reduces the number of voids. The second feature worth noting is that some of the voids in Fig. 8 appear bright at - 3 µmdefocus and dark at - 10 µm and vice versa. These voids are at different depths in the stack, as shown in the electron micrographs. The colors of frames placed around the electronmicrographs correspond to themarker lines in the acoustic images above in Fig. 8. This shows that GHz-SAM has the potential not only to detect stress-induced voids, but also to assign a void to its axial position in the layer stack. From the results, it is concluded that in the sand- wich structuremost voids occur in the AlCu layer adjacent to the substrate, as can be seen in Fig. 8. The secondcase studypresents the capabilityof acous- tic GHzmicroscopy for inspectingwire bond interfaces. [10] Despitealternatives suchas laminateor flipchippackages, classical wire bonding is still essential formicroelectronic packaging. An assessment of the quality and reliability of the wire bond interconnects is usually conducted by pull and shear tests of ball bonds and wedges. However, those destructive mechanical test methods only provide an integral information about the required forces and the sites of the interconnect breakdown, but do not allow direct access to information about the condition of the bond interfaces themselves. Moreover, for modern (fine pitch) Cuwire bonding [1] and sensitive padmetallizations, the pull and shear values strongly depend on the applied decapsulation process, which is a prerequisite for provid- ing access to the wire bonds. [7,9] CASE #2: INSPECTION OF WIRE BOND INTERFACES

ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 20 NO. 4

Fig. 7 GHz-SAM images of the reference sample (not con- taining a TiN interlayer; samples did not receive a TCT). No voids have been detected.

Fig. 6 GHz-SAM images of two samples (not containing a TiN interlayer). Redmarking indicates voids detected by the custom analysis software.

Fig. 8 GHz-SAM images andSEM images of FIBcross sections through voids in different depths.

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ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 20 NO. 4 8 Sample indent # of voids per 100 µm Size of void appearance [µm 2 ] REF 0 0.00 ± 0.00 S1 144.6 2.54 ± 2.7 S2 207.9 2.1 ± 2.5 S3 12.49 1.1 ± 0.8 S4 15.1 1.13 ± 1.17 Without TiN layer With TiN layer

Table 1 Results of void detection based on the acoustic GHz microscopy data plus electrical resistance. The acoustic focus was approximately placed in the axial center of the metallization.

through 25-µm Cu bond wires on an unstructured Cu metallization. For comparison, the same chip was also investigated by conventional acoustic microscopy at 300 MHz. Figure 10 shows the results of both the SAM and the GHz-SAM analyses on the wire bond interfaces. The leftmost image was obtained at 300 MHz by conventional SAM. The graph in the horizontal center is the result of GHz-SAM imaging, recorded at the same interconnects as imaged by conventional SAM. The rightmost graph shows amagnification of a region in the center image (GHz-SAM). It can be noted that the sensitivity and resolution of the conventional SAM even at 300 MHz is too poor to resolve the structures in the interface between the bondwire and the pad metallization. Although, the GHz-SAM reveals a highdegree of inhomogeneity in thebonds at the right and the bottom bond at the left. The three bonds that appear bright in the conventional SAM image are delaminated across most of the bonding area. However, at the right

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As an alternative approach to ball bond quality assess- ments, the intermetallic phase formation between ball and pad can be analyzed. While the selective removal of the ball bonds iswidely established (e.g., for Auor Cuwires on Al pads), [11] thismethod naturally cannot be applied to mono-metallic interconnects formed by the upcoming package technologies (e.g., Cu to Cu wire bonding). Conventional high frequency backside SAM can be employed for analyzing bond interfaces of various geom- etries. [6] Although, the resolution achievable using con- ventional ultrasonic transducers in the frequency range of up to 230 MHz is approximately 20 µm depending on the numerical aperture of the lens and the required analysis depth inside the specimen. However, this is not nearly sufficient for a precise and accurate assessment of wire bond interconnects, especially considering today’s small wire bonds with diameters of a few tens of micrometers. To provide access for the GHz-SAM to image the bonding interface, the lead-frame and die attach materi- als such as solder or glue were removed by employing standard wet chemical etching. Even though GHz-SAM inspection is possible through 1-4 µmof remaining Si, with the samples investigated here all remaining Si was removed by employing a selective SF 6 -based plasma etching process stopping at the oxide layers of the BEOL stack. The reason for this approach is that imperfectly prepared surfaces containing scratches or surface warpage lead to unwanted artifacts in the acoustic micrographs. Special effort and care must be taken to prepare perfectly flat Si surfaces of thinned dies. A schematic of the samples’ cross sections before and after prepara- tion is provided in Fig. 9. The red arrow indicates the direction of acoustic inspection. The sample housing was a standard PG-LQFP-176 package. Electrical interconnection was obtained

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Fig. 9 (a) Schematic of a common plastic package with ball bond interconnects bonded on a pad or power metallization. (b) Toprovide access for backsidehigh- resolution SAM analysis, the die was exposed and thinned. Red arrow indicates direction of acoustic inspection/ analysis.

Fig. 10 Comparison of acoustic micrographs of ball bonds formed from a 25-µm Cu bond wire, recorded through the back- side access. Images were recorded by (a) conventional 300 MHz transducer with 2.5 mm focal length and (b) ultra-high resolution 1 GHz acoustic lens with 80 µm focal length. (a) (b) (c)

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sides of all three bonds, adhesion is higher, likely still allowing for some electrical contact. As mentioned, the inspection of the entire bonding interface would not be possible by any other technique. This is because selective etching would not stop inside a mono-metallic material system and physical removal of the wires impact the structure of the bond interface, leading to severe artifacts. Figure 11 shows electronmicro- graphs of FIB-prepared cross sections through two wire bonds. In the upper image, the bond interconnects the wire and themetallization of the pad. A beginning delami- nation is pointed to by the yellow arrow on the right. The wire bond in the lower image shows a clear delamination between pad and wire, also indicated by yellowmarkers. CASE # 3: GHz-SAM INSPECTION OF TSVs OPERATED IN TIME-RESOLVED MODE One of the most promising technologies for forming electrical interconnections involves direct routing through the Si material by through-silicon vias (TSVs), which are manufactured in a complex processing sequence. Vias are filled with W or Cu and insulated to the surrounding Si by a thin oxide layer. As 3D integration keeps evolving, applicable inspection techniques for assessment and failure analysis are urgently required. The present case study investigated the applicability of GHz-SAM for the inspection of the integrity of fillings in TSVs. [2] The samples analyzed were Cu-filled TSV structures in a Si-matrix with 5 µm diameter and 50 µm in length. In prior GHz-SAM experiments, differences seen be- tween TSVs in acoustic GHz-micrographs were not always related to physical deviation in the fillings or the insula- tion liner, but likely resulted from surface topography. Therefore, to improve the reliability of the technique, the GHz-SAM has been adapted according to Fig. 12. When operating the GHz-SAM in time-integrated mode, a supe- rior signal-to-noise ratio is obtained by an analog prepro- cessor. However, any spectral content, which potentially holds the information about the defects, is removed from the acoustic signals. For time-resolved data acquisition, the GHz-SAM was supplemented by an arbitrary waveform genera- tor and a 10 GS/s digitizer with 3 GHz input bandwidth (BW) and a 14-bit resolution to acquire the unprocessed acoustic data received by the acoustic lens. Whereas in conventional acoustic imaging only the pure peak amplitude is used for image formation, processing the RF data allows for signal analysis and the extraction of advanced signal parameters for non-destructive inspec- tion and parametric imaging. The signals obtained from

the TSV samples have been transformed into the spectral domain and the power spectra were split into individ- ual frequency containers. Imaging was then performed

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Fig. 11 FIB cross sections prepared at wire bonds showing (a) good and (b) insufficient adhesion according to SAM analysis (Fig. 10). A continuous irregular interface between ball and pad metallization is only observed for the “fail” ball bond (b), whereas only one edge is slightly affected for the “intact” ball bond (a).

Fig. 12 Signal chainof theacousticGHzmicroscope including custom adaptations for extending the GHz-SAM to time-resolved acquisition and fixed-phase excitation (experimental setup).

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ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 20 NO. 4 10 using the spectral energy in a spe- cific frequency band, as illustrated in Fig. 13. Because even minor changes in signal shape can be recognized with a high amount of certainty when evalu- ating the distribution of the spectral power, [9] extraction of even weak features from a noisy signal becomes possiblewith spectral analysis. For the experimental work of this study, the microscope was operated at a central

Fig. 13 Signal transformation into spectral domain and split-spectrum imaging.

frequency of 1 GHz with a bandwidth of approximately 10%. For excitation of the acoustic waves, burst pulses of 10 ns length were applied to the highly focused acoustic lens with an aperture opening angle of 100° and a radius of curvature (ROC) of 80 µm. Figure 14 contains two acoustic GHz micrographs recorded in time-integrated mode. The resulting images show a high SNR and individual TSVs can be identified. The two micrographs have been recorded at two differ- ent defocus positions. For the upper graph, the acoustic lens was defocused by - 12 µm, while the bottom graph was defocused by - 14 µm (Fig. 14). It should be empha- sized that the defocus does not directly translate into the imaging depth inside the sample. Due to the large differences in the acoustic wave velocities and the large opening angle of the acoustic lens, the focal spot of the acoustic beam is only pushed slightly beneath the surface. The images show that TSVs can appear with different contrast and that the contrast dynamics can change with the amount of defocus. In the upper image in Fig. 14 the acoustic focus is near the surface, so features 1-2 µm below or at the surface will contribute to the imaging contrast. When defocusing the acoustic lens, the contrast of surface and near surface features will change more noticeably than the contrast caused by features that are deeper inside the solid. [4] The acoustic micrograph shown on the right of Fig. 14was recorded at a defocus of - 14 µm, and a change in contrast in comparison to the left-hand image can be seen. The row of TSVs indicated by colored markers in Fig. 14 were further investigated by time-resolved GHz-SAM (presented in Fig. 15) and subsequent physical preparation plus high-resolution SEM imaging. Figure 16 shows electron micrographs of a FIB- prepared cross section through the TSVs that are indi- cated by colored markers in Fig. 14 and whose acquired time domain data were subjected to a split spectrum analysis. The graph at the top in Fig. 16 shows a cross

section through the three TSVs on the left side of the acoustic micrographs. The color marking corresponds to the colors in Fig. 14. The yellow-marked TSV contains only minor voids in the upper 2-3 µm region. However, these small voids significantly contribute to the contrast in the acoustic images recorded in time-integrated mode

Fig. 14 Acoustic GHz-micrographs of a TSV-sample recorded in time-integrated mode. Top: Acoustic GHz image recorded with the acoustic lens defocused by –12 µm. Bottom: Acoustic micrograph recorded at –14 µm defocus.

Fig. 15 Results of time-resolved acoustic GHz microscopy and split-spectral analysis. From the acquired echo signals, the acoustic micrograph was computed and displayed in gray values. Signals were analyzed using split spectrum processing (SSP). Results of the SSP were then superimposed onto the acoustic micrograph. Top: Results of the SSP at 910 MHz. Bottom: Results of the SSP at 955 MHz.

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potentially allows for resonance effects. In the current study, these voids have not been investigated and will be subject to future work. SUMMARY AND CONCLUSIONS The current article describes a novel acoustic inspec- tion tool operating in the GHz band to extend both the lateral resolution and surface and subsurface sensitivity of acousticmicroscopy. Employing elasticwaves, acoustic microscopy investigates the mechanical and structural properties of materials on a microscopic level. GHz-SAM is therefore a valuable extension of conventional SAM for applications in particular fields of research and industrial applications, including microelectronic failure analysis and qualitymonitoring in production environments. This article also presented a selection of case studies illustrat- ing the potential of GHz-SAM to perform investigations on materials that have not been covered so far, as both a standalone application, but also as a complementing tool for guiding subsequent physical preparation for further high-resolutionmicrostructural analyses. The case studies illustrate the applicability and potential of GHz-SAM for the inspection and analysis of stress-induced voiding in power-metal lines (3 µm), inspection and 2D analysis of wire bond interfaces, and analysis of fillings in TSVs. ACKNOWLEDGMENTS This work has been (partly) performed in the project SAM3, where the German partners are funded by the German Bundesministerium für Bildung und Forschung (BMBF) under contract No 16ES0348. SAM3 is a joint project running in the European EUREKA EURIPIDES and CATRENE programs. The authors also acknowledge financial support fromthe EuropeanUnion’s Horizon 2020 research and innovation program, which has funded the Metro43D project under grant No 688225. The authors thank IngridDeWolf for providing the TSV samples for case study #3, G. Vogg for providing the wire bond samples in case study #2, and R. Portius for provid- ing the AlCu powermetal structures used in case study #1. REFERENCES 1. B.K. Appelt, A. Tseng, C.H. Chen, and Y.S. Lai: “Fine Pitch Copper Wire Bonding in High Volume Production,” Microelectron. Reliab., 2011, 51 (1), p. 13-20. 2. S. Brand, M. Kögel, F. Altmann, I. DeWolf, A. Khaled, M. Moore, E. Strohm, and M. Kolios: “Acoustic and Photoacoustic Inspection of Through-Silicon Vias in the GHz-Frequency Band,” Proc. 43rd InternationalSymposiumforTestingandFailureAnalysis(ISTFA), 2017. 3. S. Brand, M. Petzold, P. Czurratis, J.D. Reed, M. Lueck, A. Huffman, J.M. Lannon, and D.S. Temple: “High Resolution Acoustical Imaging of High-Density-Interconnects for 3D-Integration,” Proc. 61st IEEE Electronic Components and Technology Conference (ECTC), 2011, p. 37-42.

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(Fig. 14) allowing a clear differentiation from the TSVs on the right in that row (red marker). When inspecting the parametric images (Fig. 15) of the results of the split- spectrum analysis performed at 910 and 955 MHz, the appearance of this TSV (yellow marker) differs slightly from the three defect-free TSVs on the right (red marker). The green-marked TSV (in the upper right graph) in Fig. 16 contains a void at a depth of 3-10 µmbeneath its surface. In time-integratedmode, this TSV appears slightly brighter at both defocus positions making a clear differentiation difficult. However, the SSP results of the time-resolved data show a substantially increased intensity in the fre- quency containers at 910 and 955 MHz compared to the TSVs in the red marker. These results suggest that a spectral analysis of the acoustic signals of TSVs acquired in time-resolved mode increases the sensitivity toward filling defects like voids. However, since the penetration depth is limited to 6-10 µm, it is expected that for voids at larger depths, the direct compressional wavewill not be sufficiently suscepti- ble. Unpublishedobservations encourage the assumption that TSVs act as waveguides when their physical dimen- sions are on the order of the acoustic wavelength, which Fig. 16 Electronmicrographsof anFIB-preparedcross section through the TSVs indicated by colored markers in Fig. 14. Top: Left three TSVs (with corresponding color marking) in Fig. 14. The yellow-marked TSV (left) containsminor voids in the upper 2-3 µm region. The green-marked TSV (right) contains a void at a depth of 3-10 µm beneath its surface. Bottom: TSVs in the red marker of Fig. 14. These TSVs show no defect and have a similar appearance in the acoustic GHz micrographs.

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ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 20 NO. 4 12 ABOUT THE AUTHORS

4. A. Briggs and O. Kolosov: “Acoustic Microscopy, in Monographs on the Physics and Chemistry of Materials,” second edition, Oxford University Press, 2009. 5. P.A. Flinn, A.S. Mack, P.R. Besser, and T.N. Marieb: “Stress-Induced Void Formation in Metal Lines,” MRS-Bulletin, 1993, 18 (12), p. 26-35. 6. S.M. Kay: “Modern Spectral Estimation,” Prentice-Hall Signal Processing Series, ISBN: 978-0135985823. S. Murali, N. Srikanth: “Acid Decapsulation of Epoxy Molded IC Packages with Copper Wire Bonds,” IEEE Trans. Electron. Packag. Manuf., 2006, 29 (3), p. 179-183. 7. S. Murali, N. Srikanth: “Acid Decapsulation of Epoxy Molded IC Packages with Copper Wire Bonds,” IEEE Trans. Electron. Packag. Manuf., 2006, 29(3), p. 179-183. 8. M. Poschgan, J. Maynollo, M. Inselsbacher: “InvertedHigh Frequency Scanning Acoustic Microscopy Inspection of Power Semiconductor Devices,” Microelectron. Reliab., 2012, 52 (9-10), p. 2115-2119.

9. Y.Y. Tan, M. Ng, J.L. Khoo, C.H. Tan, C.D. Silva, and K.S. Sim: “A Microwave Plasma Dry Etch Technique for Failure Analysis of Cu and PdCuWire Bonds Strength,” Proc. 20th IEEE International Symposium on the Physical and Failure Analysis of Integrated Circuits (IPFA), 2013, p. 153-157. 10. G. Vogg, T. Heidmann, et al.: “Scanning Acoustic GHz-Microscopy versus Conventional SAM for Advanced Assessment of Ball Bond and Metal Interfaces in Microelectronic Devices,” Microelectron. Reliab., Proc. 26th European Symposium on Reliability of Electron Devices, Failure Physics and Analysis, 2015, 55, p. 1554-1558. 11. F.W. Wulff, C.D. Breach, D. Stephan, S.B. Saraswati, and K.J. Dittmer: “Characterization of Intermetallic Growth in Copper and Gold Ball Bonds on Aluminum Metallization,” Proc. 6th IEEE Electronics Packaging Technology Conference (EPTC), 2004, p. 348-353.

SebastianBrand is a senior scientist at the Fraunhofer IMWS, Germany, where he leads a research, development, and application team for non-destructive defect localization in the field of failure analysis and metrology in microelectronics. Sebastian holds a Ph.D. in electrical engineering from the University of Magdeburg, Germany. In 2004 and 2005 he joined the University of Toronto and Ryerson University in Toronto as a post-doctoral fellow working in the field of cancer research with a focus on ultrasonically-based methods for early detection of treatment responses. Sebastian has 18 years of experience in the field of acoustics in life and material sciences and authored more than 65 publications. His current research extends fromacousticmethods (scanning acousticmicroscopy)

over lock-in-thermography to magnetic micro-imaging where he and his team undertake R&D of non-destructive defect localization and characterization to address challenges arising from novel technologies, such as 3D integration.

Frank Altmann received his diploma in physics from the Technical University, Dresden. He is head of the diagnostics of semiconductor technologies research group at Fraunhofer IMWS-CAM. His main research field is failure analysis on Si- and III/V-based electronic devices. He currently works on newdevelopments of advanced defect localization, site-specific preparation, and physical failure analysis techniques for new3D packaging technologies. He has authoredmore than 50 publications, has five patents, and lectures in the master’s program in mechatronics, industrial engineering, and physical engineering at the University of Applied Sciences Merseburg.

Michael Kögel graduated from the University of Applied Sciences in Leipzig in 2015 with a degree in electrical engineering and information technology. Hewrote hismaster’s thesis at Fraunhofer IMWS- CAM, onmethods and techniques for exploring spectral extractionof Rayleighwaves on solid interfaces with GHz-SAM. Since 2015, Kögel has worked as an engineer for failure analysis of semiconductor components at IMWS-CAM and is member of the team for non-destructive defect localization in the field of failure analysis and metrology in microelectronics. His experience includes application and development for scanning acousticmicroscopy, lock-in-thermography andmagnetic current imaging.

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ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 20 NO. 4 14 • Engaging knowledge resources ºº Develop fully indexed and searchable content while preserving traditional media ♦ ♦ ISTFA papers ♦ ♦ Desk Reference (updated content in the works) ♦ ♦ EDFA magazine ºº Reevaluate our educational offerings—e.g., short courses and tutorials, and their delivery methods ºº Explore the possibility of providing online tools and calculators to assist with daily FA • Engaging the industry ºº Continue exploring the FA roadmap ºº Consider corporate memberships Our opportunity to focus on these three engagement pillars will be facilitated through our parent society, ASM International, and our members. Many of these opportunities will be made possible by advancements to our website. ASM’s Digital Transformation initiative will drive changes to our website needed to accomplish these objectives. Look for these improvements to be rolled out over the next couple of years. We have already seen progress in improved e-commerce capabilities including easier onlinemembership renewal, the creation of ASMConnect (an online collaboration tool), and an improved association management system. Our membership is imperative to these initiatives. We are a volunteer-driven society and member input is critical to our initiatives. Regular feedback will ensure that discussion groups, roadmap planning, EDFA magazine, ISTFA, and more provide value that will engage and grow our membership. During the past two years, the board and our member volunteers have made a significant impact. Initiatives include growing our ISTFA programming and adding new tutori- als, designing two awards to recognize member contributions, and creating new board positions to engage students and help us reach international markets. If you’re not already involved, please consider volunteering and becoming a part of making EDFAS successful. While you attend ISTFA, take some time to think about what EDFAS means to you and how it could best serve your professional needs, and share your thoughtswith us. Your boardmemberswill be at the event, wearing designated ribbons on our badges. So stop us and let us know your thoughts or send us your ideas via email. You can find us on the EDFAS website, edfas.org. Most of all, we hope that you will engage with the presenters, colleagues from across the industry, and vendors on the exhibit floor to solve some of your toughest problems and enhance your skills and performance. GUEST EDITORIAL CONTINUED FROM PAGE 2 EDFAS Strategic Objectives Draft (continued)

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ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 20 NO. 4 16 EDFAAO (2018) 4:16-22

1537-0755/$19.00 ©ASM International ®

EARLY LIFE AUTOMOTIVE ELECTRONICS FAILURES AND THEIR ROOT CAUSES Peter Jacob EMPA Swiss Federal Laboratories for Materials Science &Technology Electronics & Reliability Center, Dübendorf, Switzerland peter.jacob@empa.ch

But how far do these condition changes influence the electronics? The following list describes a few typical changes: • New rubber tubes in the engine compartment are isolating—but when street salt becomes attached to them, they become electrostatic dissipative over time. • Noise pulses from electric motor commutators decrease when the carbon contact pads are ground in toward the round shape of the commutator (Fig. 1). • Contact behavior in relays and switches degrades due to vibration, corrosive gases, and fretting or curing. • Corrosion appears and dendrites form within LEDs. • Shorts arise frommetal filaments falling into active electronics.

INTRODUCTION Inautomotive electronics, failures are frequently found that cannot be explained by the failure signatures of the defective devices. In deeper investigations, it turns out that a superimposition of impact factors, which never can be represented by the usual qualification testing, caused the failure. Numerous cases presented the opportunity to be examined in depth, including system-based analysis and failure anamnesis (similar to a complete medical history) in root cause evaluations. This article describes several examples and concludes with general principles for how to proceed with these types of failures. POST FAB STRESS CONDITION CHANGES AND SUPERIMPOSITIONS When a car leaves the factory, operational conditions immediately start to change significantly: • Dirt from the street and environmental dust, humid- ity, and water penetrate into structures, even if they were assumed to be sealed. • Continuous vibrations and mechanical shocks apply. • Outgassing from rubber seals, tires, and plastics find their way into electronic circuits. • Carbon contact pads for numerous electric (servo) motors (for instance, for window lifts, seat or mirror adjustment, or ventilation) begin to form themselves to the round shape of the commutator. • Metal punching burrs peel off and sometimes fall into electronic circuitry, causing shorts. • High thermal cycles impact the sealing of capsuled electronics and optoelectronics, e.g., LEDs. • Radio signals from internal or external data or radio transmissions surround the vehicle.

Fig. 1 Carbon contacts at the commutator of an electric motor. Top: New carbon pieces, planar contact, submitting high level pulses. Middle: After the shape adaption of the carbon pieces to the round collector, the pulse level reduces. Bottom: The pulses can be easily suppressed by a small capacitor.

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ESD UNDER OPERATION Mechanically experienced car technicians know that electrical shocks may occur—even through rubber gloves—when sandblasting rusty metal parts. Each time particles or liquids are sputtered, electrostatic charge separation takes place and high voltage is generated. Striking visual examples include lightning at volcano exhausts (Fig. 2) and Kelvin generators. [1,2] In a car, such situations frequently occur within the engine and gear box, where oil is sputtered at high speed in the presence of many sensors (e.g., Hall or pressure sensors), which are electrically on. If an electrostatic discharge (ESD) strike happens, the leakage path opened will be under the full short current from the power supply, which means that the failure path will suffer artefactual burning and show a typical electrical overstress (EOS) signature in later failure analysis. CMOS circuitry is especially affected by this phenom- enon, because in the case of ESD (or extremely high exter- nal E-fields), both nFET and pFET channels may open at the same time, thus opening direct Vdd-Vss short paths. This has even been observed in modern car electronics when lightning strikes close to an operating vehicle— creating numerous EOS signatures in several electronic components. If the car had been in a parked position and electrically off, the failures would have been significantly less. If the electric field froma lightning strike opens both nFET and pFET channels for only a short time, it doesn’t matter as long as the CMOS circuitry is in the off state and not electrically powered. The next example describes a constructionweakness. Returns of fairly new cars were observed, usually at a mileage of less than 10,000 km. Within the engine control unit, amixed signal device controlling a sensor unit failed. The semiconductor device clearly generated an EOS sig- nature. At first, a sound failure anamnesis indicated that the returns came from countries without a speed limit and after the control unit was repaired, the failure never occurred again. Inspection revealed that the unit had beenmounted between rubber tubeswithout any ground connection. A contactless electrostatic voltage probe was used to measure the charge of the unit’s metal case (Fig. 3). At a speed of less than about 150 km/h, no failure was observed, but at faster runs, suddenly high charge was measured—above the 3kV limit of the instrument. This voltage was generated by the unit’s extremely high air/petrol drop flow speed. Due to the missing ground (GND) connection, the high voltage directly arrived at the mixed signal device and killed it within a short time. After

• Partial memory loss occurs due to noise and EMI within the onboard supply system. EARLY LIFE FAILURE EXAMPLES Following are some more detailed examples of early life failures. CROSS TALKING In this first example, about 100-200 electric motors are evaluated. Many of them are classic DC motors with carbon contacts to the commutator as described previ- ously. If they send inductive pulses into the power line, these pulses can be coupledwithin the cable tree feeding the signal lines. If the inductance behind is at a high level, for example, in the engine’s starter motor or themagnetic switch for it, the inductive responsemight become critical to sensitive electronics, even with capacitive coupling. In addition to this coupling, electromagnetic interference (EMI) also needs to be considered in a modern car, with special attention given to tire pressure measurement. For such measurement, independent radio frequency (RF) transmitters are mounted within the tires, sending small pulse RF signals to an onboard receiver. In many cases, this transmission is done within industrial, sci- entific, and medical (ISM) frequencies, very often in the 433MHz ISMband; this band iswithin the busy 70-cmham radio frequency band and close to other 70-cm users as taxi transceivers and those from fire brigades, police, and ambulance services. If these µW-signals have to compete with neighboring high power signals of licensed radio amateurs (around50W) or other transmitters, interference or destruction of the highly sensitive receiver can occur. Therefore, a careful considerationof interference suppres- sion measures is mandatory for modern cars.

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Fig. 2 Volcanic eruption of the Sakurajima volcano. Elec- trostatic charge is generated by separation of par- ticles. Courtesy of Martin Rietze.

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