G. Khatibi, B. Weiss, J. Bernardi & S. Schwarz Journal of Electronic Materials

Transcription

1 Microstructural Investigation of Interfacial Features in Al Wire Bonds G. Khatibi, B. Weiss, J. Bernardi & S. Schwarz Journal of Electronic Materials ISSN Journal of Elec Materi DOI /s

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3 Journal of ELECTRONIC MATERIALS DOI: /s Ó 2012 TMS Microstructural Investigation of Interfacial Features in Al Wire Bonds G. KHATIBI, 1,3 B. WEISS, 1 J. BERNARDI, 2 and S. SCHWARZ 2 1. Faculty of Physics, University of Vienna, Boltzmanngasse 5, 1090 Vienna, Austria. 2. Vienna University of Technology, USTEM, Wiedner Haupstr. 8-10, 1040 Vienna, Austria golta.khatibi@univie.ac.at In the present study the microstructure of ultrasonically bonded Al wires on AlSiCu and AlSi metallization was investigated by means of scanning electron microscopy, electron back-scattered diffraction, and high-resolution transmission electron microscopy techniques. Detailed microstructural investigations were conducted on samples in the as-bonded condition, subsequent to power cycling tests, and after long-time thermal exposure to reveal the temperature-dependent evolution of the interfaces and the metallization layer. Typical interfacial features were found to be ultrafine and nanoscaled grains of Al and Al 2 O 3, amorphous Al oxide particles, voids, and pores, with regions of high density of dislocations and dislocation loops within the larger grains of the wire and metallization. The observed interface features confirm the suggested mechanism of formation of bonding interface by emergence of submicron grains at the thin interfacial boundary between the metallic pair as a result of dynamic recrystallization and interdiffusion. While isothermal and/or thermomechanical cycling lead to strong grain growth in the metallization layer and the Al wire, the nanostructured interfacial regions mainly remain, indicating a high thermal stability and strength of the interface. Furthermore, evaluation of a large number of wire bonds prepared using standard bonding conditions showed the presence of a certain percentage of nonbonded areas and microstructural variations between the interconnects processed under nominally identical conditions. However, it was found that, if a sufficient effective bonding interface is provided, the long-time reliability of Al wire bonds is maintained due to the stability and strength of the nanostructured interface. Key words: Aluminum, ultrasonic bonding, interface, nanostructured grains, transmission electron microscopy, dynamic recrystallization, reliability INTRODUCTION Ultrasonic wire bonding with a coverage of about 95% is one of the most established and reliable interconnection technologies in electronic industry. Ultrasonic wedge bonding has been applied since (Received February 18, 2012; accepted July 23, 2012) about 50 years ago to a variety of fine wires of face-centered cubic (fcc) metals and thick Al wires, and its application field has recently been extended to thick Cu wires in power semiconductor devices. 1 Though the technological aspects of the ultrasonic bonding process are well established, the available explanations for the ultrasonic bonding mechanism are still more or less empirical. It is commonly accepted that wire bonding is a solid-state process taking place at low temperature. 2 5 As early as 1965, Neppiras 5 suggested that ultrasonic

4 Khatibi, Weiss, Bernardi, and Schwarz shear-type welding is solid-state bonding initiated by microslip, fretting, and adhesion. Since then, a number of experimental 6 10 and theoretical studies have been conducted to explain the mechanisms responsible for the formation of a very thin and stable bonding interface in a few milliseconds of reaction time. To date, ultrasonic softening effects are considered to be one of the key parameters. 14 Since the early 1990s, transmission electron microscopy (TEM) has been used to study the interfacial features of wire bonds and to understand the mechanism of ultrasonic wire bonding. 6 9 TEM investigations on the formation of substructure and interface of Al wire bonds on different substrates have shown that the microstructure of the interface region consists mainly of fine grains, amorphous regions, oxide particles, and also nonbonded areas. 6 8 It was suggested that the bonding interface is formed by the emergence of submicron grains at the thin interfacial boundary between the metallic pair as a result of dynamic recrystallization (DRX) and interdiffusion at rather low temperatures. 6 8 Investigation on thin AlSi1 bonds (25 lm) on Cu/Ni/Au pads also suggested the theory of DRX and ultrasonic softening as the major mechanism for ultrasonic bond formation. 10,15 Concerning the role of temperature during the ultrasonic bonding process, there exist controversial points of view. Thermocouple measurements 2 and liquid-nitrogen bonding 3 have shown that bonding occurs at low temperatures of about 80 C or less. In contrast, depletion of the dislocations in the grain boundaries of Al wire bonds was related to a temperature rise of about 270 C at the interface. 7 Formation of interfacial intermetallic phases, high concentration of alloying elements, and presence of crystalline c-al 2 O 3 in the interface were found as indications of the occurrence of local high temperatures and even melting during ultrasonic wire bonding. 16,17 This short overview presents the main findings related to the microstructure of the interface of ultrasonic wire bonds and the proposed mechanisms for bond formation. Though generally the observed interfacial features and proposed mechanisms are similar, there are still open questions to answer. On the other hand, these investigations comprise only characterization of the bonding interface of thin Al wires in the as-bonded condition. Due to their application field in power semiconductor devices, the long-time reliability of monometallic ultrasonic thick Al wire bonds is of great technological relevance, but the temperature-dependent evolution of the bonding interface has not been studied sufficiently. To our knowledge there exists a single TEM investigation with focus on the stability of the bonding interface during active thermal cyclic loading. 9 Thus, the focus of the present study is investigation of the temperature-dependent evolution of the interfacial features of Al wedge bonds at different Fig. 1. Overview of Al wedge bonds on Si chip. stages of their lifetime. Thick Al wedge bonds on AlSiCu and AlSi metallization pads as used in commercial power semiconductor modules were studied by means of scanning and TEM techniques. These investigations include samples in as-bonded condition, subsequent to an active thermal cycling procedure (power cycling), and after long-time thermal exposure. Finally, it has been attempted to arrive at a better understanding of the role of microstructure in the quality and long-time reliability of ultrasonic Al wire bonds. EXPERIMENTAL PROCEDURES Specimen Preparation Commercially available high-purity Al wires with diameter of 400 lm were ultrasonically bonded to a metallization layer on Si chips which were soldered onto direct copper bonded (DCB) substrates. The thickness of AlSiCu and AlSi metallization layers on the chips was about 3 lm, and bonding was conducted in ambient temperature using standard parameters as used for commercial insulated gate bipolar transistor (IGBT) modules (Fig. 1). Large Al wires are typically bonded at frequencies of 60 khz to 80 khz with ultrasonic power up to about 25 W and bonding forces up to about 1 kgf. 1 Investigations were performed on wire-bonded samples in as-bonded condition and after power cycling tests. Furthermore, additional investigations were conducted on wire bonds which were subjected to temperature storage at 135 C up to 1000 h. Crosssections of the bonded area and the interface regions of several bonds were examined using electron backscattered diffraction (EBSD) techniques with a Zeiss Supra 55 VP scanning electron microscope (SEM). Metallographic cross-sections of the samples were prepared by using SiC abrasive papers up to P-2000 grade. The samples were subsequently polished by 1-lm diamond paste and finished by a lubricant containing 0.02-lm colloidal Al 2 O 3 oxide particles. This specimen preparation method was sufficient to reveal the microstructure of the Al wire and

5 Author's personal copy Microstructural Investigation of Interfacial Features in Al Wire Bonds Fig. 2. Microstructure of Al wire bonds in different conditions: (a) EBSD image of the Al wedge bond in as-bonded condition, (b) EBSD image of an Al wedge bond out of a commercial IGBT module (isothermal temperature storage at 135 C/1000 h), (c) EBSD images of the microstructure of the wire (c1) and the wedge area (c3) of an Al wire bond after a power cycling test. SEM image of a wire bond separated from the substrate due to lift-off failure (c2). the interface in back-scattered electron (BSE) and EBSD modes. Representative locations of the interface region were selected and marked on the metallographic cross-sections of the wire bonds for further TEM investigations. TEM lamellae with thickness less than 200 nm and area of about 15 lm 9 10 lm were prepared from the interfacial region using a focused ion beam (FIB) device (FEI Quanta 200 3D DBFIB). TEM investigations were performed using a transmission electron microscope (FEI Tecnai F 20). RESULTS AND DISCUSSION SEM-EBSD Investigations of the Al Wire Bond In this section the global microstructure of the Al wire bonds in as-bonded, heat-treated, and powercycled conditions is presented in Fig. 2a, b, and c, respectively. Typical EBSD images of the wedge bonds represent the distribution of the grains and their orientation in relationship to their pretreatment. The respective inverse pole figure orientation

6 Khatibi, Weiss, Bernardi, and Schwarz Fig. 3. Microstructure of the interface region of the Al wire bond (a) in as-bonded condition and (b) after temperature exposure (SEM- BSD image). maps are inserted in the figures. In these figures, a misorientation exceeding 2 was considered as a grain boundary without using different color codes for low- and high-angle grain boundaries. Figure 2a shows a typical example grain size distribution across the Al wedge bond representing the different degrees of material plastic flow induced by the highfrequency cyclic motion and force of the bonding tool. The original equiaxed grain structure of the Al wire with random texture and average grain size of about 10 lm is preserved at both free sides of the wedge, especially at the tail of the wire at the right-hand side. The area beneath the bonding tool exhibits the finest microstructure. The location of the most severe plastic deformation is in the immediate vicinity of the Si chip and expands with a halfelliptical shape toward the upper parts of the wedge. With increasing distance from the interface, the original size of the grains is more or less retained, indicating a gradient of plastic deformation beneath the bonding tool. During the various stages of the manufacturing process, such as soldering of the module to the base Fig. 4. (a) Overview of the bonding area showing the interfacial area and the grain size distribution of the metallization film in as-bonded condition (dark-field STEM image), (b) elemental line scan of the marked line in (a). plate and following treatments, IGBT modules are subjected to moderate temperatures which might alter the microstructure of the constituent parts. Depending on the specifications and reliability requirements, power semiconductor modules are subjected to a variety of thermal, mechanical, and electrical tests. Among others, long-term temperature storage and power cycling tests are considered as standard procedures for device qualification. An EBSD image of the cross-section of a wire bond prepared out of a commercial IGBT module and stored at 135 C for 1000 h is shown in Fig. 2b. At a homologous temperature of about 0.4 T m, considerable grain growth has occurred in the whole bonded region. The fine and deformed grains in the interface region are recovered, and grains with average diameter of about 36 lm and maximum size of 100 lm are formed in the wedge area. Figure 2c1, c3 shows EBSD images of the free end and the wedge area of a separated wire bond (Fig. 2c2) after a typical power cycling testing at T max = 150 C, DT j = 120 C, which failed after about 25,000 temperature cycles. These images were taken from microstructural cross-sections prepared from a failed wire bond. Figure 2c2 shows an

7 Microstructural Investigation of Interfacial Features in Al Wire Bonds Fig. 5. (a) Detailed TEM bright-field image of the bonding interface in the area along the marked line shown in Fig. 4a and SAD patterns of regions A, B, C (b, c, d, respectively). example of an Al wire bond which failed due to liftoff (SEM image). Separation of the wedge from the surface of the pad metallization occurs as a consequence of thermal mismatch between the silicon chip and the Al wire during the operational life of the devices. This type of failure is commonly known as wire bond lift-off and counts as one of the main failure modes in semiconductor devices. 1,18 During the temperature excursions of up to 150 C, current heating of the wire bond results in the recovery of the highly deformed inhomogeneous structure of the wedge and subsequent grain growth with increased number of thermal cycles. The final structure of the failed wire consists of a rather homogeneous microstructure of equiaxed grains with average grain diameter of 70 lm in the wedge area and 30 lm in the free end of the wire. The color shading of the grains in the bond foot and wire area is an indication of distortion-free and fully recovered grains with an almost dominant cubic texture of the free wire. Similar observations have been reported by Agyakwa et al. 19 in which the evolution of the microstructure of high-purity thick aluminum wire bonds during passive thermal cycling (up to T max of 190 C) and isothermal exposure (at 170 C and 350 C for different times) was studied by using FIB channeling technique. They related the simultaneous occurrence of recovery of dislocation structure, reordering of nonequilibrium grain boundaries, and grain coarsening of the Al wire bonds during annealing at low temperatures to the high plastic deformation due to the wire-bonding process. 19 SEM-BSE images of the interfacial region of wire bonds in as-bonded and heat-treated conditions are shown in Fig. 3a and b, which correspond to Fig. 2a and b, respectively. The interface is clearly visible as a fine line between the wire and metallization with pores and unbonded regions (broader dark areas and points). The microstructure of the as-bonded Al wire next to the interfacial line consists of fine grains or a cell-like substructure with

8 Khatibi, Weiss, Bernardi, and Schwarz Fig. 6. Magnification of a region between points A and B in Fig. 5a with dislocation loops in the wire bond and near the grain boundary (see region A). diameter in the range of 1 lm to2lm and submicron grains in the metallization layer (Fig. 3a). About 5 lm to 10 lm above this region, larger grains with a substructure consisting of a high density of cells can be identified. Figure 3b presents the interfacial area of the heat-treated wire bond in which the extremely fine structure is replaced by larger grains in which a certain degree of distortion can still be recognized. A considerable degree of grain growth is also observed in the AlSi metallization layer. The thickness of the metallization layer in Fig. 3b is not uniform anymore and depends on the local rate of grain growth and the final size of Al grains, which vary from about 1 lm to5lm. TEM Investigations of the Bond Interface As-Bonded Condition Figure 4a presents a dark-field STEM image of the FIB section prepared from the Al wedge in the as-bonded condition. At the lowest part of the image, next to the silicon chip, the Al metallization film with average grain size of about 0.5 lm to2lm is observed. At a distance of about 3 lm above the chip, which corresponds to the nominal thickness of the Al metallization, a narrow line consisting of dispersed particles, extremely fine grains, and intermittent pores is revealed, which is assumed to be the original bonding interface. The upper boundary of the interfacial band appears as a wavy boarder line which comprises grain boundaries of the Al wire. The line scans of elements Al and O (marked line) show an oxygen peak and a slight Fig. 7. (a) Overview of the FIB lamella. (b) Interface of the bonding area of the wedge prepared from a commercial IGBT (STEM images). reduction in Al at a distance of about 3.5 lm from the Si substrate, which can be related to the presence of Al oxides and/or voids in the interfacial line (Fig. 4b). The detailed microstructure of the interfacial band with width of about 1 lm consisting of submicron grains along with a few voids is shown in Fig. 5a. Though an oxygen peak was identified in the line scan of this location (Fig. 4b), the grains are mostly identified as Al, as shown in the corresponding selected-area diffraction (SAD) patterns for this region (Fig. 5b d). The size of the extremely fine recrystallized grains which are located below a layer of larger, almost dislocation-free grains in the region is on the order of several nanometers up to a few 100 nm. The dislocation substructure inside the individual nanograins varies from low to high density, suggesting different stages of recovery subsequent to grain formation. A clear high-angle grain boundary separates the nanostructured region from the original wire bond. A detailed image of the region marked A in Fig. 5a is shown in Fig. 6 and reveals a high density of dislocation loops within one of the Al grains of the wire bond.

9 Microstructural Investigation of Interfacial Features in Al Wire Bonds Fig. 8. (a) Detail of region B with interfacial voids (bright-field TEM image), (b) elemental line scan of the area, (c and d) SAD pattern of the region in the vicinity of voids. There exist controversial opinions concerning the origin of dislocation loops in TEM specimens prepared by FIB. Generally, formation of dislocation loops in Al can be caused by irradiation, cyclic deformation, or DRX. Small dislocation loops (<20 nm) in Al interconnects were considered as artifacts generated by Ga + ion irradiation and clustering of vacancies during FIB preparation. 20 The loops with diameter larger than 20 nm were found to originate from cyclic loading of thin Al lines similar to those observed in bulk aluminum samples. 20 Formation of a high density of dislocation loops in aluminum as a result of cyclic hardening at high frequencies (20 khz) has been reported by Hoffelner and Weiss 21 and Peslo. 22 However, presence of dislocation loops in Al wire bonds is mostly related to dynamic recovery or recrystallization process as reported by Krzanowski and Murdeshwar. 7,8 In brief, interfacial nanoscaled grains and dislocation loops observed in the interface of aluminum wire bonds justify ultrasonically induced DRX in aluminum, which in turn counts as a reasonable explanation for the mechanism of solid-state bonding at low temperatures in a very short duration of time. Generally, dynamic recovery and recrystallization are processes that occur during metal forming as a result of high plastic deformation at temperatures above 0.5 T m. As previously mentioned, with the exception of a few investigations, it is commonly believed that the temperature increase at the bonding interface during ultrasonic bonding is rather moderate. 2 4 Formation of interfacial nanocrystalline grains as a result of DRX at low temperature in ultrasonic bonds can be argued based on the wellknown ultrasonic softening effect as suggested by Langenecker. 14 Their experiments showed that concurrent application of ultrasonic irradiation and force results in a dramatic decrease in the flow curve at ambient temperature which is comparable to the high-temperature flow behavior of metals (e.g., 600 C for Al). Furthermore, regarding the DRX process in Al, controversial opinions have been presented. Some studies show that aluminum with high stacking fault energy is prone to dynamic recovery rather than DRX. 23 The rapid rate of recovery is suggested to be due to the capability of Al for easy dislocation climb and cross-slip, resulting in early formation of a recovered stable dislocation arrangement during thermomechanical processing and a lack of the

10 Khatibi, Weiss, Bernardi, and Schwarz Fig. 9. (a) STEM image of the overview of the FIB lamella prepared from the interface region of an Al wedge bond of a power-cycled module (not failed), (b) corresponding line scan of the interface between the wire and metallization layer. internal energy required for further DRX and formation of new grains. 23 In contrast, DRX was observed in Al single crystals subjected to tensile and compression tests at 0.57T 24 m and reported as the mechanism responsible for formation of nanostructured grains in aluminum in special metal-forming processes such as various methods of severe plastic deformation 25 and also during friction stir welding. 26 Consequently it can be assumed that, through the ultrasonic irradiation of the aluminum wire and the substrate, the driving force required for dislocation motion and rearrangement is considerably reduced, which promotes the formation of a new interface between the bonding pair by a DRX process. Nevertheless, in our opinion further investigations are required to clarify the origin of dislocation loops in ultrasonic Al wire bonds. Wire Bond Subjected to Isothermal Annealing The evolution of the microstructure of the interface of the Al wire bond after temperature storage at 135 C for 1000 h is presented in Fig. 7a, b. An overview of the FIB lamella and the magnified STEM images present clear grain coarsening of the AlSi metallization, which consists of one or two grains across the thickness due to the long-time temperature exposure (lower side of the image). A single, fully dislocation-free Al grain can be observed at the upper side of the image, while some of the grains of the metallization layer show a certain amount of distortion. This considerable grain coarsening at the rather low annealing temperature can be related to the previous high plastic deformation of the wire bond providing the required energy for the recovery of the substructure and a subsequent grain growth. The interfacial region can be defined through a transition between the single Al grain from the wire side and two distinct regions of the metallization layer with different microstructures. The left half of the interface is fully bonded and can be characterized by a crossover from a single Al grain to the smaller grains of the metallization. The right side of the interface consists of a highly distorted area with large voids (B) and a nonbonded area with width of about 0.5 lm and length >1 lm (marked C ). The thickness of the metallization layer at this location is about 3 lm, while at the fully bonded side the interface seems to be moved towards the Al wire with a thickness of about 4.5 lm. The grains marked A and D show tiny dots that are similar to those features identified as dislocation loops in the as-bonded wire (Fig. 6). A magnified image of the fully distorted interfacial region marked B with extremely fine nanostructured grains, moiré fringes, and voids is shown in Fig. 8a. It is somehow surprising that, in spite of the strong grain coarsening in the metallization film and the wire, the interfacial nanoscaled grains are still locally present. An elemental line scan along the dashed line marked in Fig. 8 reveals a clear drop of Al and a peak of oxygen that is not only directly related to the rather large voids and pores but also to the presence of Al oxide particles (Fig. 8b). SAD patterns obtained from different points at the periphery of sites B and C were identified as fine crystalline Al 2 O 3 grains (Fig. 8c) and Al (Fig. 8c, d). The high concentration of Al oxide in the vicinity of the voids and pores in the interface of the wire bonds can be explained by the principle of the ultrasonic bonding process. The local conglomeration of oxide particles, voids, and fine grains indicates that, during bonding, finely crushed surface oxide layers are not totally removed from the interface, remain as a disperse phase at certain sites, and inhibit the atomic contact between the clean Al Al surfaces, leaving rather large nonbonded areas. The thermal stability of interfacial nanograins which are observed in coexistence with the oxide particles may be also related to the pinning of the grain boundaries. However, the origin of crystalline Al oxides in the interface is not fully clear. It is commonly known

11 Microstructural Investigation of Interfacial Features in Al Wire Bonds Fig. 10. (a) Overview of the interface region (bright-field TEM image), (b) detail of the interfacial region at point D, and (c e) SAD patterns of regions A, B, and C, respectively. that Al surface oxide is a thin, uniform, and amorphous layer at temperatures up to about 300 C. 27 Snijders et al. 28 studied the development of the thin aluminum oxide films on Al substrates as a function of oxidation temperature, time, and oxide thickness and found that the oxide films grown at 373 K were fully amorphous, that above 573 K a mixture of amorphous and crystalline structure was observed, whereas the thickest oxide films grown at 773 K primarily consisted of crystalline c-al 2 O 3. It can be concluded that formation of crystalline Al oxides as a result of long-time temperature exposure at 135 C is rather unlikely. At this point it may be hypothesized that formation of crystalline Al oxide nanoscaled grains is related to high temperatures which may have occurred during the ultrasonic bonding process, similar to findings of Brodyanski et al. 16 Wire Bond Subjected to Power Cycling Test In this section, the microstructural features of an Al wire bond subsequent to an active thermal cycling test with T max = 155 C, DT j = 120 C after about 25,000 temperature cycles are presented in Figs The overview STEM image presented in Fig. 9a includes the AlSiCu metallization layer at the lower part, a closed bonding interface which is revealed as a bright line above the metallization, and the remnants of the Al wire with thickness of about 1 lm to 1.5 lm at the upper part of the FIB cut. Similar to the heat-treated sample, it can be observed that temperature exposure results in considerable grain growth of the metallization layer. In this case, the AlSiCu metallization consists of one to two grains across the thickness with fine Al 2 Cu precipitates within the grains or partly at the grain boundaries. Another interesting point is the increased thickness of the metallization layer to about 6 lm and the formation of rather large voids in this area. The observed increase in thickness and voiding in the metallization layer can be related to the degradation mechanisms of thin film due to thermal and electrical stresses. Relaxation of thermomechanical stresses and electromigration result in formation of hillocks, voids, and preferential grain growth in thin films. 29 During pulsed operation in high-power semiconductor devices,

12 Khatibi, Weiss, Bernardi, and Schwarz Fig. 11. (a) Bright-field TEM images of point D of the interface region with moiré fringes, (b) SAD patterns of this point (all identified as Al rings). thermally induced compressive stress cycles, originated from the mismatch of thermal expansion coefficient between Al and Si, result in reconstruction of the metallization layer. Degradation appears as increased surface roughness by formation of extrusions, cracking, and voiding of the films. 18 Since the present case concerns the layer beneath the bonded area, the mechanisms of degradation due to electrical and thermal loading may be altered due to the suppression effect of the wire bond. While formation of surface extrusions may be hindered at this area, grain growth seems to be extremely promoted due to the bonding process. The cross-sections of the films show local thickening similar to those features observed in interconnect line in which voiding occurs concurrently with hillock formation due to conservation of mass. The interface can be clearly defined as the location at which a drop in Al and peaks of oxygen and silicon are observed in the elemental line scan (Fig. 9a, line 1-1) of the interfacial region, as marked by an arrow in Fig. 9a. A magnified brightfield TEM image of the marked area is shown in Fig. 10a, which reveals the interface as a band with width of about 1 lm between the metallization layer (marked C ) and the remnant of the wire (marked A ). The interface consists of somewhat elongated grains separated by low-angle grain boundaries, with rather low dislocation density and tiny dots which are dispersed across almost the whole area above the metallization layer. The characteristic recrystallized nanoscaled interfacial crystals seem to have vanished after the power cycling test (Fig. 10a, b). Figure 10b provides a closer look at the grain marked E in Fig. 10a and reveals the moiré fringes that form the low-angle grain boundaries in the interfacial band (Fig. 10b). The corresponding SAD patterns of the regions marked with A, B, and C are presented in Fig. 10c e. In addition to Al (113) and Al (202) reflections, Al 2 O 3 rings were identified in the SAD pattern of the region, indicating that the microstructure of this area consists of a dispersion of nanoparticles of Al 2 O 3 crystals in the aluminum matrix. The inner and outer diffraction rings correspond to (110) and (113), respectively. Region B consists as well of Al crystallites, identified as Al (113) and Al (335) with an Al 2 O 3 diffuse ring indicating the presence of amorphous Al oxides in the interfacial band. The diffraction pattern of area C reveals a crystalline structure consisting of Al and Al 2 O 3 in the metallization layer near the interfacial borderline (Fig. 10e). A magnified image of point D marked in Fig. 10a shows several moiré fringe domains or grains embedded in the matrix, some of which are highlighted with dashed lines (Fig. 11a). Moiré fringes are produced by the superposition of two lattices with similar or near-similar directions at areas transparent to the incident electron beam. The sharp closed nature of a series of Al diffraction rings from this point corresponds to the nanoscaled grains with size of about 10 nm (Fig. 11b). Investigation of the different sites of the interfacial area confirmed an abundant presence of these domains or grains in the interfacial area. It is interesting to note that, in spite of the rather long power cycling time and peak temperature of 155 C, thermally stable nanoscaled grains are still present and the interface region is not fully recrystallized. Again the thermal stability of the nanostructured interfacial grains can be related to the presence of fine dispersed oxide particles in this region which provide a high-strength interface during the power cycling process. A comparison of the microstructural observations of the interface of the Al wire bonds subjected to power cycling tests in the present study with findings of Onuki et al. 9 reveals some interesting similarities. They report that, subsequent to the power cycling tests (T max = 120 C, DT j = 85, K, N = 27,500), a strong interface is formed that is characterized by a

13 Microstructural Investigation of Interfacial Features in Al Wire Bonds sharp interfacial line and also sites with a very thin amorphous oxide layer with thickness of 10 nm to 200 nm. They suggest that, under power cyclic conditions, the fine-grained interfacial layer is recovered and a subsequent grain growth results in the formation of a bonding interface with good quality. The partial formation of the amorphous oxide layer was related to a process in which amorphous phase in the fine grain layer or the layer with high dislocation density was pushed out to the interface by grain growth or the recovery of strains during power cycle testing. They conclude that the investigated ultrasonic Al wire bonds, which were mainly composed of a closed and void-free bonding interface, remain very stable after power cycling tests. 9 CONCLUSIONS Detailed characterization of the microstructure of thick Al wire bonds on AlSiCu and AlSi metallization was conducted on samples in as-bonded condition, subsequent to power cycling tests, and after thermal exposure. Typical interfacial features were found to be extremely fine grains of Al and Al 2 O 3 in the range of a few hundred nanometers down to scales below 10 nm, amorphous Al oxide particles, voids, and pores. These features were found in the interfacial region irrespective of pretreatment conditions. Furthermore, within the Al grains in the wire and metallization, regions of high density of dislocations loops were found. The origin of these dislocations loops is not fully clear and can be related to DRX and/or high-frequency cyclic loading induced due to the motion of the bonding tool. The presence of fine Al oxide crystallites at the interface might be an indication of occurrence of local high temperature (>500 C) during bonding. The recrystallized extremely fine-grained structure of the interface was preserved even after longtime temperature exposure. Nanoscaled Al grains or domains with size of a few nanometers could be observed in the interface of power-cycled wire bond. The stability of the nanostructured interfacial grain structure at higher temperatures is an indication of the high strength of the bonding interface. A temperature- and time-dependent transition of the microstructure of the metallization layer from fine to coarse grains along with a local increase of thickness was observed. This phenomenon was related to void formation and/or abnormal growth of individual grains of the Al metallization. An evaluation of a large number of ultrasonic Al wire bonds prepared under standard bonding conditions showed the presence of a certain percentage of nonbonded areas and microstructural variation in all samples. However, failure of Al wire bonds in semiconductor devices occurs mainly due to crack growth above the interface and fatigue of the Al wire. 18,30 This failure mode indicates that the high strength of the nanostructured bonding interface is maintained even after thermomechanical loading and long-time thermal exposure. ACKNOWLEDGEMENTS The authors acknowledge the financial support of the Austrian Research Agency (FFG) and Technology Agency of the City of Vienna (ZIT) in the framework of the Comet program, Project No We would like to thank A. Steiger-Thrisfeld for preparing the FIB samples and T. Licht for interesting discussions. REFERENCES 1. G. Harman, Wire Bonding in Microelectronics, 3rd ed. (New York: McGraw-Hill, 2010), p K.C. Joshi, Weld. J. 50, 840 (1971). 3. G.G. 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