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Mixed-mode interface toughness of wafer-level Cu–Cu bonds using asymmetric chevron test
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Journal of the Mechanics and Physics of Solids 56 (2008) 707–718 www.elsevier.com/locate/jmps
Mixed-mode interface toughness of wafer-level Cu–Cu bonds using asymmetric chevron test Rajappa Tadepallia, Kevin T. Turnerb, Carl V. Thompsona, a
Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, USA b Department of Mechanical Engineering, University of Wisconsin, Madison, WI 53706, USA Received 27 April 2007; received in revised form 14 July 2007; accepted 18 July 2007
Abstract Characterization of interfacial adhesion is critical for the development of wafer bonding processes to manufacture microsystems with high yield and reliability. It is imperative that the test method used in such adhesion studies corresponds to the loading conditions present during processing and operation of the devices. In most applications in which wafers and die are bonded, the interface experiences a combination of shear and normal loading (i.e. mixed-mode loading) with the relative magnitude of the Mode I and II components varying in different scenarios. In the current work, the toughness of Cu–Cu thermocompression bonds, which are of interest for the fabrication of three-dimensional integrated circuits, is analyzed using a bonded chevron specimen with layers of different thickness that allows for the application of interfacial loading with variable mode mixity. The phase angle (a function of the degree of mode mixity at the interface) is varied from 01 to 241 by changing the layer thickness ratio from 1 to 0.48. The Cu–Cu bond toughness increases from 2.68 to 10.1 J/m2, as the loading is changed from Mode I (pure tension) to a loading with a phase angle of 241. The energy of plastic dissipation increases with increasing mode mixity, resulting in the enhanced interface toughness. The Mode I toughness of Cu–Cu bonds is minimally affected by plasticity, and therefore, provides the closest estimate of the interfacial work of fracture under the bonding conditions employed. r 2007 Elsevier Ltd. All rights reserved. Keywords: Delamination; Fracture toughness; Layered material; Mechanical testing; Mixed-mode toughness
1. Introduction Wafer bonding is an important fabrication technique in microelectronics and microsystems technology, and has specific applications in advanced integrated circuits (Fan et al., 1999), microelectromechanical systems (MEMS) (Schmidt, 1998), and the manufacture of silicon-on-insulator (SOI) substrates (Celler and Cristoloveanu, 2003). Common wafer bonding techniques, such as direct bonding (Plo¨Xl and Kra¨uter, 1999), anodic bonding (Cozma and Puers, 1995), and thermocompression bonding (Fan et al., 1999; Eaton and Risbud, 1994), are widely used, but often require considerable process refinement when employed in specific device fabrication sequences. Corresponding author. Tel.: +1 617 253 7652; fax: +1 617 258 6749.
E-mail address: [email protected] (C.V. Thompson). 0022-5096/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jmps.2007.07.016
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Three-dimensional integrated circuit (3D IC) technology, which enables the vertical integration of stacked device layers (with their associated metal interconnects layers), has emerged as a key application of wafer bonding. 3D ICs are expected to have significantly reduced composite interconnect lengths, allowing for higher packing densities and smaller interconnect delay wiring-limited ICs (Rahman and Reif, 2000). Wafer bonding has emerged as the dominant route for the fabrication of 3D ICs, since it allows for low-temperature (o400 1C) processing, and stacking of individual 2D circuit layers created using conventional fabrication technology. This stacking approach results in higher throughput compared to sequential fabrication methods. Thermocompression bonding of Cu films has emerged as a particularly attractive option for creating 3D ICs, since Cu can serve as both the electrical (interconnect) and mechanical link between the individual device layers. Heterogeneous integration through wafer bonding requires an understanding of the mechanical integrity of the bonded interfaces. The interfaces in a vertically stacked integrated circuit can be severely loaded by a range of normal and shear stresses during fabrication and operation. For example, interfaces in bonded devices experience significant stresses due to thermal expansion mismatch and temperature cycling during fabrication as well as device operation. Furthermore, the interfaces can be subjected to shear stresses during chemical mechanical polishing, tensile stresses if pressurized flow cooling channels are integrated in the device, and complex loading during die-sawing. As a final example, intrinsic residual stresses incurred during device manufacture can be significant and can lead to metal voiding and delamination. All of these stresses can drive failure of bonded interfaces and result in both reduced yield and reliability. In order to develop models for the prediction of bond reliability that will enhance the design of 3D ICs, a systematic characterization of bond interfaces under different loading modes is essential. Mechanical testing methods for characterizing bonded interfaces typically either measure the bond strength, which is the stress that causes fracture, or the interface toughness, which is the mechanical energy per unit interfacial area at which delamination occurs. The bond strength, which is often measured in tensile pull tests (Farrens et al., 1993), is calculated as the average stress over the interface at fracture and is a strong function of specimen geometry and preparation (Vallin et al., 2005). The interface toughness, which is measured using fracture mechanics specimens, provides a direct measure of interfacial adhesion and is less sensitive to details of the specimen and, therefore, is a key quantity of interest. Common mechanical testing methods for quantitative interface toughness characterization include double-cantilever beam (DCB) (Maszara et al., 1988), four-point bend (Charalambides et al., 1989), and Mode I chevron tests (Hurd et al., 1995; Bagdahn et al., 2001). The loading conditions in fracture mechanics specimens may be pure tension (Mode I), pure shear (Mode II), or mixed-mode (Mode I+Mode II). The degree of mode mixity is quantified by the phase angle c, defined as rffiffiffiffiffiffiffi G II 1 c ¼ tan , (1) GI where GI and GII are Modes I and II strain energy release rates, respectively. The phase angle varies from 01 (pure Mode I) to 901 (pure Mode II). The measured interface toughness, also called the critical strain energy release rate, Gc, is affected by materials properties, such as the interface chemistry, the elastic-plastic constitutive behavior of adjacent materials, the microstructure, and the mode mixity near the crack tip. Generally, an increasing phase angle leads to a higher measured interface toughness, due to frictional effects in rough interfaces or energy dissipation through plasticity in ductile materials (Evans and Hutchinson, 1989; Tvergaard and Hutchinson, 1993). Since 3D ICs and other devices created by wafer bonding are subjected to different loading conditions during fabrication and operation, bond toughness values must be evaluated across a range of different mixedmode loading conditions. The aforementioned test methods encompass a wide range of loading conditions (Mode I—DCB, chevron; mixed-mode—four-point bend). The four-point bend test is extensively used for accurate, quantitative mixed-mode characterization of bonded interfaces (Dauskardt et al., 1998), including Cu–Cu bonds (Tadepalli and Thompson, 2003). While the four-point bend test allows for very reproducible measurements, sample preparation can be difficult and it is of limited use for the study of strong direct bonds between brittle materials (Turner et al., 2001). Also, the range of phase angles in a four-point bend specimen is
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typically 401–601 (Charalambides et al., 1989), which corresponds to 50% or greater shear loading. Therefore, the toughness measured in a four-point bend test always includes a significant plastic energy dissipation component in ductile materials and therefore provides little information on the ‘true’ interfacial toughness (or chemical work of adhesion). Mode I toughness measurement allows for the characterization of the interface with minimum plasticity contribution (Lane, 2003). The displacement-loaded double-cantilever test (also known as the ‘razor-blade’ or ‘Maszara’ test in the wafer bonding community) allows for quick Mode I interface characterization, with minimal specimen preparation. However, the DCB test is difficult to use on strong bonds and is error-prone, due to a strong dependence of toughness on crack length, which is difficult to accurately measure. Thus, there is a critical need for a robust test method that allows for both fast and easy specimen fabrication as well as the characterization of a variety of interfaces under a range of phase angles.
Fig. 1. (a) Schematic diagram of a chevron test specimen for characterization of interface toughness. (b) Top view of a chevron specimen showing details of the geometry.
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The bonded chevron specimen has been shown to provide an effective means for characterizing the interface toughness of wafer bonds (Hurd et al., 1995; Bagdahn et al., 2001). The specimen is simple to fabricate, amenable to miniaturization, and easy to test. Previous work (Hurd et al., 1995; Bagdahn et al., 2001) has used a pure Mode I test configuration, in which the layers are of equal thickness and have the same elastic properties. Fig. 1(a) shows a schematic of a chevron test specimen used for bonded interface characterization. The surface of one wafer or the interlayer is patterned in the form of a V-shaped notch prior to bonding. The specimen is glued between two studs that allow the specimen to be gripped and loaded in tension during the test. The sharp chevron tip serves as a stress-concentration point and enables characterization of both weak and strong bonds. Fig. 1(b) shows the top view of the chevron specimen. The loading line is different from the specimen edge due to the finite area over which the studs are glued. If the thickness or elastic properties of the two layers of the specimen are not the same, a mixed-mode loading is produced at the interface. Recently, finite element (FE) modeling was used to analyze chevron specimens with layers of different thicknesses, in order to provide the necessary analysis for use of such specimens for bond toughness measurements under a range of mixed-mode conditions (Tadepalli and Turner, 2007). The results of the analysis demonstrated that the phase angle (i.e. the degree of mode mixity) at the interface in the chevron specimen can be varied from 01 to 351 by changing the layer thickness ratio from 1 to 0.1. The FE results were fitted to expressions that allow the interface toughness to be calculated from experimental data. In this work, mixed-mode interface toughness of Cu–Cu bonds was experimentally characterized using asymmetric chevron test specimens, where the thickness ratios of the two layers (or wafers) were varied to achieve a range of mode mixities. Mode I toughness of Cu–Cu bonds was measured quantitatively for the first time using the symmetric chevron specimen. Since Mode I loading leads to minimum plastic energy dissipation, these chevron measurements yield a toughness value that is the closest to the true Cu-Cu chemical work of adhesion. 2. Experimental procedure 2.1. Test structure fabrication Thermally oxidized 100 mm-diameter Si (100) wafers, with nominal oxide thicknesses of 300 nm, were used as substrates for all thin film deposition and bonding studies. Two sets of wafers were fabricated for bonding studies. One set of wafers (set A) had patterned chevron structures, and the other set (set B) had patterned lines. All bonding pairs were comprised of one wafer from each set. The chevron structures had the following dimensions: overall length of 10 mm, width of B ¼ 10 mm, a notch angle of y ¼ 901, and an initial crack length of a0 ¼ 1 mm. The effective length is taken as W ¼ 9 mm as the studs that are glued to the specimen transfer the load to the specimen 1 mm from the edge. The thickness ratio, Z, is defined as the ratio between h2 and h1, with h2 corresponding to the thinner of the two layers. The thickness of the chevron wafers (set A) was kept constant at 475 mm. The thicknesses of wafers in set B were varied to induce mixed-mode loading. Four different thicknesses were used—525, 475, 335, and 230 mm. Table 1 summarizes the thickness ratios and phase angles of loading used in this study, based on the FE results in Tadepalli and Turner (2007). The chevron structures were patterned on oxidized wafers using standard positive photoresist. After patterning, the oxide film was etched using buffered oxide etch (BOE), followed by deep reactive ion etching (DRIE) of the Si to a depth of 10 mm. About 0.5 mm-wide alleys separated adjacent chevron structures to enable post-bonding dicing of individual specimens. After the creation of chevron structures, the wafers were cleaned in an O2 plasma asher, followed by a Piranha clean (3:1 H2SO4:H2O2). The SiO2 film on the front side of the wafer was then removed using BOE. The chevron wafers were then reoxidized to an oxide thickness of 300 nm, using thermal oxidation. The complementary set of wafers (set B) had patterned lines of varying widths. The line widths ranged from 2 to 250 mm. The ratio of the line width to line spacing was kept constant at one and therefore, the bonded area fraction was maintained at 0.5. Each set of lines covered a total width of 11 mm. Therefore, when the wafers from set B were bonded to the chevron wafers (set A), specimens with different line widths could be obtained, with each specimen containing lines of uniform width. A lift-off process was used to create the Cu line
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Table 1 Wafer thickness ratios used for chevron mixed-mode experiments. Phase angles are obtained from FE results in Tadepalli and Turner (2007) Wafer Thickness (Set A) (mm)
Thickness ratioZ
Phase angle-c
475 525 335 230
1.0 0.9 0.7 0.48
0 3.71 12.71 241
Set B wafer thickness ¼ 475 mm.
Fig. 2. (a) Schematic diagram of a bonded chevron test specimen with lines parallel to the direction of crack propagation. (Note that the ratio of the line width to the sample width is actually in the range of 0.0002–0.025). (b) Schematic cross-sectional view of a bonded specimen. (Note that the film and substrate thicknesses are not drawn to scale.)
structures. Following photoresist patterning, the wafers from set B were exposed to an O2 plasma for a short interval (1 min) to remove residual photoresist on the developed areas (lines). 30 nm of Ta and 400 nm of Cu were then successively deposited (without breaking vacuum) on both sets of wafers using e-beam evaporation, with Ta serving as both the adhesion promoter and diffusion barrier between Cu and SiO2. The extra metal was lifted off the set B wafers by soaking the wafers in NMP (N-Methyl-2-Pyrrolidone) for at least 1 h, followed by light ultrasonic agitation in NMP and deionized water. The wafers from sets A and B were then prepared for thermocompression bonding. Specific details on the bonding process and its optimization can be found in (Tadepalli and Thompson, 2003). Briefly, an acetic acidbased pre-bonding clean step was performed first to remove the native oxide on the copper surface. Then, the wafers were clamped together in a bond chuck on an Electronic VisionsTM EV 620 Aligner, with three 30 mmthick metal flaps separating the wafers. The patterned lines on the set A wafers were aligned to the chevron pattern such that the crack propagation would occur along the length of the lines, thereby presenting a homogeneous bond interface for crack growth, as shown in the schematic in Fig. 2a. Fig. 2b shows a schematic cross-section of the bonded stack. Following alignment, the bond chuck was transferred to an EV 501 bond chamber that was subjected to three pump-purge cycles, where the purge was done with forming gas (95% Ar, 5% H2). The wafers were then brought into contact and bonded for 1 h at 300 1C under a chamber vacuum of 1 mT. The bonding force was kept constant at 5000 N for all wafers, which corresponds to a bonding pressure of 2.12 MPa on the patterned Cu-Cu bond interface. After the bonded pair was cooled down to room temperature, it was removed from the chamber and annealed in a tube furnace in a forming gas ambient for 1 h at 300 1C. Individual specimens for chevron testing were cut from the bonded wafers using a diamond dicing saw. The first few cuts were made through the thickness of one wafer, such that the chevron wafer was exposed at the
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edges. These cuts were used to locate the alleys along which further cuts were made through the thickness of the total wafer stack to obtain individual test specimens. 2.2. Chevron testing Test specimens from bonded wafer-pairs were glued to aluminum studs using high-viscosity glue. Care was taken to prevent seepage of glue into the bonded interface. Approximately 20 specimens were tested for each geometric configuration. The specimens were selected to represent all regions on the bonded wafer. The glued specimens were then tested under tension in an InstronTM 8848 electromechanical universal testing machine. The specimens were loaded at a constant displacement rate of 0.12 mm/min and the load-displacement behavior was digitally recorded. A typical load-displacement curve for a chevron-notched specimen is shown in Fig. 3. After the initial elastic loading of the specimen, a crack initiates at the chevron notch and propagates along the bonded interface. The triangle-shaped bonded area provides an increasing interfacial area with increasing crack length and load, and initially results in stable crack propagation. At some crack length the propagation becomes unstable and catastrophic failure of the specimen occurs—this coincides with the point of maximum load in Fig. 3. For a uniformly bonded interface, the bond toughness can be calculated from the maximum load in the test, Fmax, using (Tadepalli and Turner, 2007): GC ¼
F 2max Y 2min , E B2 W
(2)
where Ymin is the minimum value of the geometry function, E is the plane strain modulus, and B and W are the width and length of the specimen, respectively (Fig. 1b). For the anisotropic silicon specimens used in the current work, the effective plane strain modulus in the [001] direction in the (110) plane (E ¼ 144:7 GPa) is used. FE analysis was used to determine the minimum value of the geometry function, which is dependent on the thickness of the bonded layers and crack geometry (Tadepalli and Turner, 2007). Ymin is a function of the initial crack length ratio, a0 ¼ a0/W, the total thickness of the specimen, htotal, and the thickness ratio, Z (Tadepalli and Turner, 2007): sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi htotal Z ð2:91Þ htotal ð2:91Þ . (3) Y min ¼ ð25:71 þ 85:53a0 Þ þ 1þZ 1þZ The above equation is a fit to parametric FE results and estimates Ymin to within 1.8% of the FE calculations across a range of thicknesses (from 0.2 to 1.35 mm), thickness ratios (from 0.1 to 1), and initial crack length ratios from (0.1 to 0.4). 5
FMAX
stable crack growth
Load (N)
4 complete specimen failure
3 2
elastic loading
1 0 0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
Displacement (mm) Fig. 3. Typical load-displacement curve from a bonded chevron test. The maximum load is used to calculate interface toughness.
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When the interface is patterned with lines that are oriented parallel to the direction of crack propagation, the calculated GC value must be adjusted by the area fraction, Af: 1 G C jpatterned ¼ G C junpatterned . (4) Af From Eqs. (2) through (4), the interface toughness can be calculated from the maximum load measured in experiments and the geometry and elastic properties of the specimen. While residual stress is present in the specimen after bonding, the strain due to the thermal expansion mismatch will be mostly inelastically accommodated at the bonding temperature (Kobrinsky et al., 2001), such that the residual stress will be about 50 MPa. As such, the residual stress has a negligible effect on the bond toughness or mode mixity and is not accounted for when reducing the experimental data. After testing, the specimens were dismounted from the studs using acetone and the studs were reused. Optical microscopy, scanning electron microscopy (SEM), atomic force microscopy (AFM), and profilometry were then used to characterize the fracture surfaces. 3. Results 3.1. Bond toughness vs. mode mixity Chevron tests on bonded continuous Cu films (Af ¼ 1) resulted in the fracture of bulk Si in more than 60% of the specimens. Therefore, useful toughness measurements could not be performed in the majority of these specimens. The results discussed below will be exclusively focused on the patterned specimens, where the patterning led to a decrease in the global toughness and therefore most specimens exhibited stable crack propagation along a weak interface. For the range of line widths considered (2–250 mm), under conditions where crack growth occurred parallel to the lines, the Cu–Cu bond toughness was found to be independent of line width for all phase angles. For this configuration, an average value of toughness was calculated for each mode mixity and is plotted against phase angle in Fig. 4. Data from four-point bend tests of Cu–Cu bonds (Tadepalli and Thompson, 2003) created under the same bonding conditions is also plotted in Fig. 4. It is seen that the bond toughness monotonically increases with increasing phase angle. The maximum thermodynamic work of adhesion between two identical Cu surfaces is 2gCu–gGB, where gCu is the surface energy of Cu, and gGB is
Fig. 4. Cu–Cu bond toughness as a function of phase angle phase angle. In ascending order of phase angles, the first four points correspond to the thickness ratios in Table 1, and were obtained using asymmetric chevron test structures. Four-point bend test data from Tadepalli and Thompson (2003) is also plotted (c ¼ 421).
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the grain boundary energy of Cu. The surface energy of Cu at room temperature is 1.8 J/m2 and the grain boundary energy is gGB0.32gCu (Tyson and Miller, 1977). Therefore, the maximum contribution of the interface chemistry (atomic bonding) to the toughness is 3 J/m2, assuming atomically clean surfaces and no contributions due to surface roughness or plasticity. Hence, it can be deduced from Fig. 4 that other energy dissipation mechanisms contribute to the measured bond toughness under mixed-mode loading conditions. 3.2. Failure loci The locus of failure was found to be dependent on the mode mixity, with three different failure mechanisms/ locations observed—the Ta/SiO2 interface (adhesive), the Cu–Cu bonded interface, and bulk Cu (cohesive), as
Fig. 5. Modes of failure observed in mixed-mode chevron testing of Cu–Cu bonds. (a) Adhesive failure (Ta–SiO2 interface). (b) Cohesive failure (Cu–Cu bonded interface). (c) Cohesive failure (bulk Cu).
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Table 2 Variation in the failure loci with mode mixity. An increasing fraction of specimens exhibited crack deflection from the bonded interface as the mode mixity was increased Phase angle-c 0 12.71 241
% Adhesive failure
% Cohesive failure
% Mixed failure
33 75 90
17
33 25 10
0 0
Cu–Cu bond interface. Bulk Cu.
Fig. 6. SEM micrographs of fracture surfaces. (a) Mixed-failure mode—the initial failure mode at the chevron notch tip is adhesive before subsequently becoming cohesive. (b) Ta lines after fracture. (c) Mode I (c ¼ 0) Cu fracture surface. (d) Mixed-mode (c ¼ 12.71) Cu fracture surface. (e) Mixed-mode (c ¼ 241) Cu fracture surface. The fracture surface roughness increases with increasing mode mixity. The cohesive fracture path changes from 4(b)–(c) with increasing mode mixity.
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illustrated schematically in Fig. 5(a–c). Visual inspection via optical microscopy was used to differentiate between adhesive and cohesive failure mechanisms. Profilometry was used to distinguish between failure at the Cu–Cu interface and failure in the Cu film based on the relative thicknesses of the thin film stacks after failure. The percentage of samples that failed along these different paths varied with the mode mixity. There was no quantifiable difference in the fracture surfaces between c ¼ 01 and c ¼ 3.71. Therefore, only three different specimen loading conditions will be discussed–Mode I and near Mode I (c ¼ 01 and 3.71), c ¼ 12.71, and c ¼ 241. Table 2 shows the variation in the adhesive to cohesive failures observed in the specimens. The specimens tested under mixed-mode conditions had increasing amounts of adhesive failures, indicating crack deflection from the Cu–Cu bond interface to the Ta/SiO2 interface. Another difference between Mode I and mixed-mode failure loci was that the Mode I cohesive failures occurred along the bonded interface whereas the mixed-mode cohesive failures occurred within the Cu thin film, again confirming crack path deflection. Fig. 6 shows SEM images of the fracture surfaces. Fig. 6(a) shows a mixed failure locus, starting with Ta/SiO2 failure at the chevron notch tip followed by Cu–Cu interface failure. The fracture surface of the Ta appears smooth, as seen in Fig. 6(b). Mode I Cu–Cu bond failure is shown in Fig. 6(c). Cu grain structures can be observed in Fig. 6(c), indicating failure along the bonded interface. Under mixed-mode loading, the fractured surfaces are rougher (Figs. 6(d) and (e)), indicating greater plastic deformation and ductile failure.
Fig. 7. The 3 mm 3 mm 3D AFM images of fracture surfaces. RRMS ¼ RMS roughness in nm. (a) Cu surface just prior to bonding (RRMS ¼ 2.3 nm). (b) Mode I (c ¼ 0, Gc ¼ 2.68 J/m2, RRMS ¼ 6.52 nm). (c) Mixed mode (c ¼ 12.71, Gc ¼ 4.66 J/m2, RRMS ¼ 11.1 nm). (d) Mixed mode (c ¼ 241, Gc ¼ 10.1 J/m2, RRMS ¼ 21.3 nm).
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Evidence of ductile failure is particularly apparent in the specimen loaded with the highest phase angle (241), where the failure surface is dimpled. AFM scans were used to analyze the roughness of the fractured surfaces (Fig. 7). For reference, the surface of Cu just prior to bonding is compared with the fractured surfaces (Fig. 7(a)). The RMS roughness increases with increasing mode mixity, indicating greater plastic deformation. These observations are consistent with the SEM images in Fig. 6. 4. Discussion The key result from this study is that the fracture toughness of Cu–Cu bonds increases significantly with the introduction of mixed-mode loading. As noted in the introduction, the interface fracture energy for ductile materials has two major contributions: the chemical work of adhesion G0 and the work of plastic dissipation Gp, so that GC ¼ G0+Gp. The work per unit area needed to create new surfaces by separating the interface, G0, is independent of the mode mixity (Tvergaard and Hutchinson, 1993). Therefore, the increase in GC with increasing phase angles can be attributed to the effect of plasticity. Under Mode I loading conditions, the measured fracture toughness, 2.6870.73 J/m2 is similar to the estimated ideal work of separation G0 (Section 3.1), although non-ideal surface conditions such as contamination and roughness may be present. Therefore, there is a minimal contribution of plasticity to the measured Mode I interface toughness. As suggested by Tvergaard and Hutchinson (1992), the limited development of a plastic zone could explain this minimal contribution of Gp to the Mode I toughness. As the mode mixity increases, the contribution of Gp to the fracture toughness significantly increases, indicating the creation of a fully developed plastic zone near the crack tip. At c ¼ 241, where the loading is still Mode I-dominated, the interface toughness is 3G0. Therefore, there is considerable plastic deformation of Cu before fracture. This behavior is qualitatively supported by SEM and AFM analyses of the fracture surfaces. As mode mixity increases, the roughness of the fractured surfaces increases, signifying enhanced plastic deformation. The crack path deviates as the loading is changed from pure Mode I to mixed mode, with an increasing number of specimens displaying failure away from the bonded interface and into the bulk Cu or Ta/SiO2 interface, indicating that the crack trajectory is a function of the phase angle of loading. Similar observations have been reported by Cao and Evans (1989). Hutchinson and Suo (1992) have reasoned that, in the case of the interface (Ta/SiO2) toughness being similar to that of the layer material (bonded Cu), the crack at the interface interacts with flaws in the layer material to nucleate microcracks. Mixed-mode loading results in the growth of these microcracks back towards the interface. Toughness measured using the symmetric chevron specimen under Mode I loading provides the closest estimate of the ‘true’ Cu–Cu bond toughness for bonds created under a particular set of bonding conditions. Mode I toughness is expected to be of importance in assessing the fundamental quality of chemical bonding as well as in prediction of debonding behavior under conditions of plastic deformation, since Gp is a function of G0 (Lane et al., 2000). 5. Summary Bonded chevron specimens with different layer thickness ratios have been used to investigate the mixedmode fracture toughness of Cu–Cu bonds created at 300 1C under an applied pressure of 2.12 MPa. The Cu–Cu bonded interfaces were analyzed under a range of loading conditions. In order to achieve this, the layer thickness ratios were varied from 1.0 to 0.48, to give loading phase angles ranging from 01 to 241. The interface toughness was found to significantly increase with increases in the phase angle, consistent with previous observations of mixed-mode ductile failure. Analyses of the fractured surfaces revealed highly ductile failure modes in the specimens loaded under mixed-mode conditions. The type of failure also varied from adhesive to cohesive as the mode mixity increased. Since there is little quantitative data on the interfacial adhesion of Cu–Cu bonds, the current study using an asymmetric chevron specimen not only demonstrates the utility of the approach to characterizing the effects of mixed-mode conditions on interface toughness, it also provides important fundamental information for assessment of fabrication strategies for 3D integrated circuits. Since the Mode I toughness was found to have
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a negligible plastic work component, it can be used as a measure of the true work of separation for wafer-level Cu–Cu bonds. Mode I Cu–Cu toughness is expected to be a useful metric for the selection of an optimized bond process, particularly under conditions for which the bond experiences pure tensile loading. This study also shows that the interpretation of bond toughness measurements requires detailed knowledge of the loading conditions employed in the test method. The asymmetric chevron test can be used to predict the yield and reliability of bonded structures, such as 3D ICs, that are subjected to a range of different loading conditions during fabrication and use. References Bagdahn, J., Petzold, M., Plo¨Xl, A., Wiemer, M., 2001. Measurement of the local strength distribution of directly bonded silicon wafers using the micro-chevron test. Electrochem. Soc. Proc. 99 (35), 218–223. Cao, H.C., Evans, A.G., 1989. An experimental study of the fracture resistance of biomaterial interfaces. Mech. Mater. 7 (4), 295–304. Celler, G.K., Cristoloveanu, S., 2003. Frontiers of silicon-on-insulator. J. Appl. Phys. 93 (9), 4955–4978. Charalambides, P.G., Lund, J., Evans, A.G., McMeeking, R.M., 1989. A test specimen for determining the fracture resistance of biomaterial interfaces. J. Appl. Mech. 56 (1), 77–82. Cozma, A., Puers, R., 1995. Characterization of the electrostatic bonding of silicon and Pyrex glass. J. Micromech. Microeng. 5 (2), 98–102. Dauskardt, R.H., Lane, M., Ma, Q., Krishna, N., 1998. The adhesion and debonding of multi-layer thin film structures. Eng. Fract. Mech. 61 (1), 141–162. Eaton, W.P., Risbud, S.H., 1994. Silicon wafer-to-wafer bonding at To200 1C with polymethylmethacrylate. Appl. Phys. Lett. 65 (4), 439–441. Evans, A.G., Hutchinson, J.W., 1989. Effects of non-planarity on the mixed-mode fracture resistance of biomaterial interfaces. Acta Metall. Mater. 37 (3), 909–916. Fan, A., Rahman, A., Reif, R., 1999. Copper wafer bonding. Electrochem. Solid State Lett. 2 (10), 534–536. Farrens, S.N., Roberds, B.E., Smith, J.K., Hunt, C.E., 1993. Analysis of bond characteristics in Si direct-bonded materials. Proc. Second Int. Symp. Semicond. Wafer Bonding: Sci. Technol. Appl. 4, 81–95. Hurd, D.S., Caretta, R., Gerberich, W.W., 1995. An experimental fracture mechanics study of a strong interface: the silicon/glass anodic bond. J. Mater. Res. 10 (2), 387–400. Hutchinson, J.W., Suo, Z., 1992. Mixed-mode cracking in layered materials. Adv. Appl. Mech. 29, 63–191. Kobrinsky, M.J., Thompson, C.V., Gross, M., 2001. Diffusional creep in damascene Cu lines. J. Appl. Phys. 89 (1), 91–98. Lane, M., 2003. Interface fracture. Annu. Rev. Mater. Res. 33, 29–54. Lane, M., Vainchtein, A., Gao, H., Dauskardt, R.H., 2000. Plasticity contributions to interface adhesion in thin-film interconnect structures. J. Mater. Res. 15 (12), 2758–2769. Maszara, W.P., Goetz, G., Caviglia, A., McKitterick, B., 1988. Bonding of silicon wafers for silicon on insulator. J. Appl. Phys. 64 (10), 4943–4950. Plo¨Xl, A., Kra¨uter, G., 1999. Wafer direct bonding: tailoring adhesion between brittle materials. Mater. Sci. Eng. R-Rep. 25 (1–2), 1–88. Rahman, A., Reif, R., 2000. System-level performance evaluation of three-dimensional integrated circuits. IEEE Trans. VLSI Syst. 8 (6), 671–678. Schmidt, M.A., 1998. Wafer-to-wafer bonding for microstructure formation. Proc. IEEE 86 (8), 1575–1585. Tadepalli, R., Thompson, C.V., 2003. Quantitative characterization and process optimization of low-temperature bonded copper interconnects for 3-D integrated circuits. In: Proceedings of the IEEE 2003 International Interconnect Technology Conference, pp. 36–38. Tadepalli, R., Turner, K.T., 2007. A chevron specimen for the measurement of mixed-mode interface toughness of wafer bonds. Eng. Fract. Mech, to appear in Engineering Fracture Mechanics. Turner, K.T., Ayon, A.A., Choi, D., Miller, B., Spearing, S.M., 2001. Characterization of silicon fusion bonds using a four-point bend specimen. In: MRS Proceedings: Materials Science of Microelectromechanical Systems (MEMS) Devices III 657, EE631–636. Tvergaard, V., Hutchinson, J.W., 1992. The relation between crack growth resistance and fracture process parameters in elastic-plastic solids. J. Mechs. Phys. Solids 40 (6), 1377–1397. Tvergaard, V., Hutchinson, J.W., 1993. The influence of plasticity on mixed-mode interface toughness. J. Mech. Phys. Solids 41 (6), 1119–1135. Tyson, W.R., Miller, W.A., 1977. Surface free energies of solid metals. Surf. Sci. 62 (1), 267–276. Vallin, O¨., Jonsson, K., Lindberg, U., 2005. Adhesion quantification methods for wafer bonding. Mater. Sci. Eng. R-Rep. 50 (4–5), 109–165.
Journal of the Mechanics and Physics of Solids 56 (2008) 707–718 www.elsevier.com/locate/jmps
Mixed-mode interface toughness of wafer-level Cu–Cu bonds using asymmetric chevron test Rajappa Tadepallia, Kevin T. Turnerb, Carl V. Thompsona, a
Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, USA b Department of Mechanical Engineering, University of Wisconsin, Madison, WI 53706, USA Received 27 April 2007; received in revised form 14 July 2007; accepted 18 July 2007
Abstract Characterization of interfacial adhesion is critical for the development of wafer bonding processes to manufacture microsystems with high yield and reliability. It is imperative that the test method used in such adhesion studies corresponds to the loading conditions present during processing and operation of the devices. In most applications in which wafers and die are bonded, the interface experiences a combination of shear and normal loading (i.e. mixed-mode loading) with the relative magnitude of the Mode I and II components varying in different scenarios. In the current work, the toughness of Cu–Cu thermocompression bonds, which are of interest for the fabrication of three-dimensional integrated circuits, is analyzed using a bonded chevron specimen with layers of different thickness that allows for the application of interfacial loading with variable mode mixity. The phase angle (a function of the degree of mode mixity at the interface) is varied from 01 to 241 by changing the layer thickness ratio from 1 to 0.48. The Cu–Cu bond toughness increases from 2.68 to 10.1 J/m2, as the loading is changed from Mode I (pure tension) to a loading with a phase angle of 241. The energy of plastic dissipation increases with increasing mode mixity, resulting in the enhanced interface toughness. The Mode I toughness of Cu–Cu bonds is minimally affected by plasticity, and therefore, provides the closest estimate of the interfacial work of fracture under the bonding conditions employed. r 2007 Elsevier Ltd. All rights reserved. Keywords: Delamination; Fracture toughness; Layered material; Mechanical testing; Mixed-mode toughness
1. Introduction Wafer bonding is an important fabrication technique in microelectronics and microsystems technology, and has specific applications in advanced integrated circuits (Fan et al., 1999), microelectromechanical systems (MEMS) (Schmidt, 1998), and the manufacture of silicon-on-insulator (SOI) substrates (Celler and Cristoloveanu, 2003). Common wafer bonding techniques, such as direct bonding (Plo¨Xl and Kra¨uter, 1999), anodic bonding (Cozma and Puers, 1995), and thermocompression bonding (Fan et al., 1999; Eaton and Risbud, 1994), are widely used, but often require considerable process refinement when employed in specific device fabrication sequences. Corresponding author. Tel.: +1 617 253 7652; fax: +1 617 258 6749.
E-mail address: [email protected] (C.V. Thompson). 0022-5096/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jmps.2007.07.016
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Three-dimensional integrated circuit (3D IC) technology, which enables the vertical integration of stacked device layers (with their associated metal interconnects layers), has emerged as a key application of wafer bonding. 3D ICs are expected to have significantly reduced composite interconnect lengths, allowing for higher packing densities and smaller interconnect delay wiring-limited ICs (Rahman and Reif, 2000). Wafer bonding has emerged as the dominant route for the fabrication of 3D ICs, since it allows for low-temperature (o400 1C) processing, and stacking of individual 2D circuit layers created using conventional fabrication technology. This stacking approach results in higher throughput compared to sequential fabrication methods. Thermocompression bonding of Cu films has emerged as a particularly attractive option for creating 3D ICs, since Cu can serve as both the electrical (interconnect) and mechanical link between the individual device layers. Heterogeneous integration through wafer bonding requires an understanding of the mechanical integrity of the bonded interfaces. The interfaces in a vertically stacked integrated circuit can be severely loaded by a range of normal and shear stresses during fabrication and operation. For example, interfaces in bonded devices experience significant stresses due to thermal expansion mismatch and temperature cycling during fabrication as well as device operation. Furthermore, the interfaces can be subjected to shear stresses during chemical mechanical polishing, tensile stresses if pressurized flow cooling channels are integrated in the device, and complex loading during die-sawing. As a final example, intrinsic residual stresses incurred during device manufacture can be significant and can lead to metal voiding and delamination. All of these stresses can drive failure of bonded interfaces and result in both reduced yield and reliability. In order to develop models for the prediction of bond reliability that will enhance the design of 3D ICs, a systematic characterization of bond interfaces under different loading modes is essential. Mechanical testing methods for characterizing bonded interfaces typically either measure the bond strength, which is the stress that causes fracture, or the interface toughness, which is the mechanical energy per unit interfacial area at which delamination occurs. The bond strength, which is often measured in tensile pull tests (Farrens et al., 1993), is calculated as the average stress over the interface at fracture and is a strong function of specimen geometry and preparation (Vallin et al., 2005). The interface toughness, which is measured using fracture mechanics specimens, provides a direct measure of interfacial adhesion and is less sensitive to details of the specimen and, therefore, is a key quantity of interest. Common mechanical testing methods for quantitative interface toughness characterization include double-cantilever beam (DCB) (Maszara et al., 1988), four-point bend (Charalambides et al., 1989), and Mode I chevron tests (Hurd et al., 1995; Bagdahn et al., 2001). The loading conditions in fracture mechanics specimens may be pure tension (Mode I), pure shear (Mode II), or mixed-mode (Mode I+Mode II). The degree of mode mixity is quantified by the phase angle c, defined as rffiffiffiffiffiffiffi G II 1 c ¼ tan , (1) GI where GI and GII are Modes I and II strain energy release rates, respectively. The phase angle varies from 01 (pure Mode I) to 901 (pure Mode II). The measured interface toughness, also called the critical strain energy release rate, Gc, is affected by materials properties, such as the interface chemistry, the elastic-plastic constitutive behavior of adjacent materials, the microstructure, and the mode mixity near the crack tip. Generally, an increasing phase angle leads to a higher measured interface toughness, due to frictional effects in rough interfaces or energy dissipation through plasticity in ductile materials (Evans and Hutchinson, 1989; Tvergaard and Hutchinson, 1993). Since 3D ICs and other devices created by wafer bonding are subjected to different loading conditions during fabrication and operation, bond toughness values must be evaluated across a range of different mixedmode loading conditions. The aforementioned test methods encompass a wide range of loading conditions (Mode I—DCB, chevron; mixed-mode—four-point bend). The four-point bend test is extensively used for accurate, quantitative mixed-mode characterization of bonded interfaces (Dauskardt et al., 1998), including Cu–Cu bonds (Tadepalli and Thompson, 2003). While the four-point bend test allows for very reproducible measurements, sample preparation can be difficult and it is of limited use for the study of strong direct bonds between brittle materials (Turner et al., 2001). Also, the range of phase angles in a four-point bend specimen is
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typically 401–601 (Charalambides et al., 1989), which corresponds to 50% or greater shear loading. Therefore, the toughness measured in a four-point bend test always includes a significant plastic energy dissipation component in ductile materials and therefore provides little information on the ‘true’ interfacial toughness (or chemical work of adhesion). Mode I toughness measurement allows for the characterization of the interface with minimum plasticity contribution (Lane, 2003). The displacement-loaded double-cantilever test (also known as the ‘razor-blade’ or ‘Maszara’ test in the wafer bonding community) allows for quick Mode I interface characterization, with minimal specimen preparation. However, the DCB test is difficult to use on strong bonds and is error-prone, due to a strong dependence of toughness on crack length, which is difficult to accurately measure. Thus, there is a critical need for a robust test method that allows for both fast and easy specimen fabrication as well as the characterization of a variety of interfaces under a range of phase angles.
Fig. 1. (a) Schematic diagram of a chevron test specimen for characterization of interface toughness. (b) Top view of a chevron specimen showing details of the geometry.
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The bonded chevron specimen has been shown to provide an effective means for characterizing the interface toughness of wafer bonds (Hurd et al., 1995; Bagdahn et al., 2001). The specimen is simple to fabricate, amenable to miniaturization, and easy to test. Previous work (Hurd et al., 1995; Bagdahn et al., 2001) has used a pure Mode I test configuration, in which the layers are of equal thickness and have the same elastic properties. Fig. 1(a) shows a schematic of a chevron test specimen used for bonded interface characterization. The surface of one wafer or the interlayer is patterned in the form of a V-shaped notch prior to bonding. The specimen is glued between two studs that allow the specimen to be gripped and loaded in tension during the test. The sharp chevron tip serves as a stress-concentration point and enables characterization of both weak and strong bonds. Fig. 1(b) shows the top view of the chevron specimen. The loading line is different from the specimen edge due to the finite area over which the studs are glued. If the thickness or elastic properties of the two layers of the specimen are not the same, a mixed-mode loading is produced at the interface. Recently, finite element (FE) modeling was used to analyze chevron specimens with layers of different thicknesses, in order to provide the necessary analysis for use of such specimens for bond toughness measurements under a range of mixed-mode conditions (Tadepalli and Turner, 2007). The results of the analysis demonstrated that the phase angle (i.e. the degree of mode mixity) at the interface in the chevron specimen can be varied from 01 to 351 by changing the layer thickness ratio from 1 to 0.1. The FE results were fitted to expressions that allow the interface toughness to be calculated from experimental data. In this work, mixed-mode interface toughness of Cu–Cu bonds was experimentally characterized using asymmetric chevron test specimens, where the thickness ratios of the two layers (or wafers) were varied to achieve a range of mode mixities. Mode I toughness of Cu–Cu bonds was measured quantitatively for the first time using the symmetric chevron specimen. Since Mode I loading leads to minimum plastic energy dissipation, these chevron measurements yield a toughness value that is the closest to the true Cu-Cu chemical work of adhesion. 2. Experimental procedure 2.1. Test structure fabrication Thermally oxidized 100 mm-diameter Si (100) wafers, with nominal oxide thicknesses of 300 nm, were used as substrates for all thin film deposition and bonding studies. Two sets of wafers were fabricated for bonding studies. One set of wafers (set A) had patterned chevron structures, and the other set (set B) had patterned lines. All bonding pairs were comprised of one wafer from each set. The chevron structures had the following dimensions: overall length of 10 mm, width of B ¼ 10 mm, a notch angle of y ¼ 901, and an initial crack length of a0 ¼ 1 mm. The effective length is taken as W ¼ 9 mm as the studs that are glued to the specimen transfer the load to the specimen 1 mm from the edge. The thickness ratio, Z, is defined as the ratio between h2 and h1, with h2 corresponding to the thinner of the two layers. The thickness of the chevron wafers (set A) was kept constant at 475 mm. The thicknesses of wafers in set B were varied to induce mixed-mode loading. Four different thicknesses were used—525, 475, 335, and 230 mm. Table 1 summarizes the thickness ratios and phase angles of loading used in this study, based on the FE results in Tadepalli and Turner (2007). The chevron structures were patterned on oxidized wafers using standard positive photoresist. After patterning, the oxide film was etched using buffered oxide etch (BOE), followed by deep reactive ion etching (DRIE) of the Si to a depth of 10 mm. About 0.5 mm-wide alleys separated adjacent chevron structures to enable post-bonding dicing of individual specimens. After the creation of chevron structures, the wafers were cleaned in an O2 plasma asher, followed by a Piranha clean (3:1 H2SO4:H2O2). The SiO2 film on the front side of the wafer was then removed using BOE. The chevron wafers were then reoxidized to an oxide thickness of 300 nm, using thermal oxidation. The complementary set of wafers (set B) had patterned lines of varying widths. The line widths ranged from 2 to 250 mm. The ratio of the line width to line spacing was kept constant at one and therefore, the bonded area fraction was maintained at 0.5. Each set of lines covered a total width of 11 mm. Therefore, when the wafers from set B were bonded to the chevron wafers (set A), specimens with different line widths could be obtained, with each specimen containing lines of uniform width. A lift-off process was used to create the Cu line
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Table 1 Wafer thickness ratios used for chevron mixed-mode experiments. Phase angles are obtained from FE results in Tadepalli and Turner (2007) Wafer Thickness (Set A) (mm)
Thickness ratioZ
Phase angle-c
475 525 335 230
1.0 0.9 0.7 0.48
0 3.71 12.71 241
Set B wafer thickness ¼ 475 mm.
Fig. 2. (a) Schematic diagram of a bonded chevron test specimen with lines parallel to the direction of crack propagation. (Note that the ratio of the line width to the sample width is actually in the range of 0.0002–0.025). (b) Schematic cross-sectional view of a bonded specimen. (Note that the film and substrate thicknesses are not drawn to scale.)
structures. Following photoresist patterning, the wafers from set B were exposed to an O2 plasma for a short interval (1 min) to remove residual photoresist on the developed areas (lines). 30 nm of Ta and 400 nm of Cu were then successively deposited (without breaking vacuum) on both sets of wafers using e-beam evaporation, with Ta serving as both the adhesion promoter and diffusion barrier between Cu and SiO2. The extra metal was lifted off the set B wafers by soaking the wafers in NMP (N-Methyl-2-Pyrrolidone) for at least 1 h, followed by light ultrasonic agitation in NMP and deionized water. The wafers from sets A and B were then prepared for thermocompression bonding. Specific details on the bonding process and its optimization can be found in (Tadepalli and Thompson, 2003). Briefly, an acetic acidbased pre-bonding clean step was performed first to remove the native oxide on the copper surface. Then, the wafers were clamped together in a bond chuck on an Electronic VisionsTM EV 620 Aligner, with three 30 mmthick metal flaps separating the wafers. The patterned lines on the set A wafers were aligned to the chevron pattern such that the crack propagation would occur along the length of the lines, thereby presenting a homogeneous bond interface for crack growth, as shown in the schematic in Fig. 2a. Fig. 2b shows a schematic cross-section of the bonded stack. Following alignment, the bond chuck was transferred to an EV 501 bond chamber that was subjected to three pump-purge cycles, where the purge was done with forming gas (95% Ar, 5% H2). The wafers were then brought into contact and bonded for 1 h at 300 1C under a chamber vacuum of 1 mT. The bonding force was kept constant at 5000 N for all wafers, which corresponds to a bonding pressure of 2.12 MPa on the patterned Cu-Cu bond interface. After the bonded pair was cooled down to room temperature, it was removed from the chamber and annealed in a tube furnace in a forming gas ambient for 1 h at 300 1C. Individual specimens for chevron testing were cut from the bonded wafers using a diamond dicing saw. The first few cuts were made through the thickness of one wafer, such that the chevron wafer was exposed at the
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edges. These cuts were used to locate the alleys along which further cuts were made through the thickness of the total wafer stack to obtain individual test specimens. 2.2. Chevron testing Test specimens from bonded wafer-pairs were glued to aluminum studs using high-viscosity glue. Care was taken to prevent seepage of glue into the bonded interface. Approximately 20 specimens were tested for each geometric configuration. The specimens were selected to represent all regions on the bonded wafer. The glued specimens were then tested under tension in an InstronTM 8848 electromechanical universal testing machine. The specimens were loaded at a constant displacement rate of 0.12 mm/min and the load-displacement behavior was digitally recorded. A typical load-displacement curve for a chevron-notched specimen is shown in Fig. 3. After the initial elastic loading of the specimen, a crack initiates at the chevron notch and propagates along the bonded interface. The triangle-shaped bonded area provides an increasing interfacial area with increasing crack length and load, and initially results in stable crack propagation. At some crack length the propagation becomes unstable and catastrophic failure of the specimen occurs—this coincides with the point of maximum load in Fig. 3. For a uniformly bonded interface, the bond toughness can be calculated from the maximum load in the test, Fmax, using (Tadepalli and Turner, 2007): GC ¼
F 2max Y 2min , E B2 W
(2)
where Ymin is the minimum value of the geometry function, E is the plane strain modulus, and B and W are the width and length of the specimen, respectively (Fig. 1b). For the anisotropic silicon specimens used in the current work, the effective plane strain modulus in the [001] direction in the (110) plane (E ¼ 144:7 GPa) is used. FE analysis was used to determine the minimum value of the geometry function, which is dependent on the thickness of the bonded layers and crack geometry (Tadepalli and Turner, 2007). Ymin is a function of the initial crack length ratio, a0 ¼ a0/W, the total thickness of the specimen, htotal, and the thickness ratio, Z (Tadepalli and Turner, 2007): sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi htotal Z ð2:91Þ htotal ð2:91Þ . (3) Y min ¼ ð25:71 þ 85:53a0 Þ þ 1þZ 1þZ The above equation is a fit to parametric FE results and estimates Ymin to within 1.8% of the FE calculations across a range of thicknesses (from 0.2 to 1.35 mm), thickness ratios (from 0.1 to 1), and initial crack length ratios from (0.1 to 0.4). 5
FMAX
stable crack growth
Load (N)
4 complete specimen failure
3 2
elastic loading
1 0 0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
Displacement (mm) Fig. 3. Typical load-displacement curve from a bonded chevron test. The maximum load is used to calculate interface toughness.
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When the interface is patterned with lines that are oriented parallel to the direction of crack propagation, the calculated GC value must be adjusted by the area fraction, Af: 1 G C jpatterned ¼ G C junpatterned . (4) Af From Eqs. (2) through (4), the interface toughness can be calculated from the maximum load measured in experiments and the geometry and elastic properties of the specimen. While residual stress is present in the specimen after bonding, the strain due to the thermal expansion mismatch will be mostly inelastically accommodated at the bonding temperature (Kobrinsky et al., 2001), such that the residual stress will be about 50 MPa. As such, the residual stress has a negligible effect on the bond toughness or mode mixity and is not accounted for when reducing the experimental data. After testing, the specimens were dismounted from the studs using acetone and the studs were reused. Optical microscopy, scanning electron microscopy (SEM), atomic force microscopy (AFM), and profilometry were then used to characterize the fracture surfaces. 3. Results 3.1. Bond toughness vs. mode mixity Chevron tests on bonded continuous Cu films (Af ¼ 1) resulted in the fracture of bulk Si in more than 60% of the specimens. Therefore, useful toughness measurements could not be performed in the majority of these specimens. The results discussed below will be exclusively focused on the patterned specimens, where the patterning led to a decrease in the global toughness and therefore most specimens exhibited stable crack propagation along a weak interface. For the range of line widths considered (2–250 mm), under conditions where crack growth occurred parallel to the lines, the Cu–Cu bond toughness was found to be independent of line width for all phase angles. For this configuration, an average value of toughness was calculated for each mode mixity and is plotted against phase angle in Fig. 4. Data from four-point bend tests of Cu–Cu bonds (Tadepalli and Thompson, 2003) created under the same bonding conditions is also plotted in Fig. 4. It is seen that the bond toughness monotonically increases with increasing phase angle. The maximum thermodynamic work of adhesion between two identical Cu surfaces is 2gCu–gGB, where gCu is the surface energy of Cu, and gGB is
Fig. 4. Cu–Cu bond toughness as a function of phase angle phase angle. In ascending order of phase angles, the first four points correspond to the thickness ratios in Table 1, and were obtained using asymmetric chevron test structures. Four-point bend test data from Tadepalli and Thompson (2003) is also plotted (c ¼ 421).
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the grain boundary energy of Cu. The surface energy of Cu at room temperature is 1.8 J/m2 and the grain boundary energy is gGB0.32gCu (Tyson and Miller, 1977). Therefore, the maximum contribution of the interface chemistry (atomic bonding) to the toughness is 3 J/m2, assuming atomically clean surfaces and no contributions due to surface roughness or plasticity. Hence, it can be deduced from Fig. 4 that other energy dissipation mechanisms contribute to the measured bond toughness under mixed-mode loading conditions. 3.2. Failure loci The locus of failure was found to be dependent on the mode mixity, with three different failure mechanisms/ locations observed—the Ta/SiO2 interface (adhesive), the Cu–Cu bonded interface, and bulk Cu (cohesive), as
Fig. 5. Modes of failure observed in mixed-mode chevron testing of Cu–Cu bonds. (a) Adhesive failure (Ta–SiO2 interface). (b) Cohesive failure (Cu–Cu bonded interface). (c) Cohesive failure (bulk Cu).
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Table 2 Variation in the failure loci with mode mixity. An increasing fraction of specimens exhibited crack deflection from the bonded interface as the mode mixity was increased Phase angle-c 0 12.71 241
% Adhesive failure
% Cohesive failure
% Mixed failure
33 75 90
17
33 25 10
0 0
Cu–Cu bond interface. Bulk Cu.
Fig. 6. SEM micrographs of fracture surfaces. (a) Mixed-failure mode—the initial failure mode at the chevron notch tip is adhesive before subsequently becoming cohesive. (b) Ta lines after fracture. (c) Mode I (c ¼ 0) Cu fracture surface. (d) Mixed-mode (c ¼ 12.71) Cu fracture surface. (e) Mixed-mode (c ¼ 241) Cu fracture surface. The fracture surface roughness increases with increasing mode mixity. The cohesive fracture path changes from 4(b)–(c) with increasing mode mixity.
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illustrated schematically in Fig. 5(a–c). Visual inspection via optical microscopy was used to differentiate between adhesive and cohesive failure mechanisms. Profilometry was used to distinguish between failure at the Cu–Cu interface and failure in the Cu film based on the relative thicknesses of the thin film stacks after failure. The percentage of samples that failed along these different paths varied with the mode mixity. There was no quantifiable difference in the fracture surfaces between c ¼ 01 and c ¼ 3.71. Therefore, only three different specimen loading conditions will be discussed–Mode I and near Mode I (c ¼ 01 and 3.71), c ¼ 12.71, and c ¼ 241. Table 2 shows the variation in the adhesive to cohesive failures observed in the specimens. The specimens tested under mixed-mode conditions had increasing amounts of adhesive failures, indicating crack deflection from the Cu–Cu bond interface to the Ta/SiO2 interface. Another difference between Mode I and mixed-mode failure loci was that the Mode I cohesive failures occurred along the bonded interface whereas the mixed-mode cohesive failures occurred within the Cu thin film, again confirming crack path deflection. Fig. 6 shows SEM images of the fracture surfaces. Fig. 6(a) shows a mixed failure locus, starting with Ta/SiO2 failure at the chevron notch tip followed by Cu–Cu interface failure. The fracture surface of the Ta appears smooth, as seen in Fig. 6(b). Mode I Cu–Cu bond failure is shown in Fig. 6(c). Cu grain structures can be observed in Fig. 6(c), indicating failure along the bonded interface. Under mixed-mode loading, the fractured surfaces are rougher (Figs. 6(d) and (e)), indicating greater plastic deformation and ductile failure.
Fig. 7. The 3 mm 3 mm 3D AFM images of fracture surfaces. RRMS ¼ RMS roughness in nm. (a) Cu surface just prior to bonding (RRMS ¼ 2.3 nm). (b) Mode I (c ¼ 0, Gc ¼ 2.68 J/m2, RRMS ¼ 6.52 nm). (c) Mixed mode (c ¼ 12.71, Gc ¼ 4.66 J/m2, RRMS ¼ 11.1 nm). (d) Mixed mode (c ¼ 241, Gc ¼ 10.1 J/m2, RRMS ¼ 21.3 nm).
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Evidence of ductile failure is particularly apparent in the specimen loaded with the highest phase angle (241), where the failure surface is dimpled. AFM scans were used to analyze the roughness of the fractured surfaces (Fig. 7). For reference, the surface of Cu just prior to bonding is compared with the fractured surfaces (Fig. 7(a)). The RMS roughness increases with increasing mode mixity, indicating greater plastic deformation. These observations are consistent with the SEM images in Fig. 6. 4. Discussion The key result from this study is that the fracture toughness of Cu–Cu bonds increases significantly with the introduction of mixed-mode loading. As noted in the introduction, the interface fracture energy for ductile materials has two major contributions: the chemical work of adhesion G0 and the work of plastic dissipation Gp, so that GC ¼ G0+Gp. The work per unit area needed to create new surfaces by separating the interface, G0, is independent of the mode mixity (Tvergaard and Hutchinson, 1993). Therefore, the increase in GC with increasing phase angles can be attributed to the effect of plasticity. Under Mode I loading conditions, the measured fracture toughness, 2.6870.73 J/m2 is similar to the estimated ideal work of separation G0 (Section 3.1), although non-ideal surface conditions such as contamination and roughness may be present. Therefore, there is a minimal contribution of plasticity to the measured Mode I interface toughness. As suggested by Tvergaard and Hutchinson (1992), the limited development of a plastic zone could explain this minimal contribution of Gp to the Mode I toughness. As the mode mixity increases, the contribution of Gp to the fracture toughness significantly increases, indicating the creation of a fully developed plastic zone near the crack tip. At c ¼ 241, where the loading is still Mode I-dominated, the interface toughness is 3G0. Therefore, there is considerable plastic deformation of Cu before fracture. This behavior is qualitatively supported by SEM and AFM analyses of the fracture surfaces. As mode mixity increases, the roughness of the fractured surfaces increases, signifying enhanced plastic deformation. The crack path deviates as the loading is changed from pure Mode I to mixed mode, with an increasing number of specimens displaying failure away from the bonded interface and into the bulk Cu or Ta/SiO2 interface, indicating that the crack trajectory is a function of the phase angle of loading. Similar observations have been reported by Cao and Evans (1989). Hutchinson and Suo (1992) have reasoned that, in the case of the interface (Ta/SiO2) toughness being similar to that of the layer material (bonded Cu), the crack at the interface interacts with flaws in the layer material to nucleate microcracks. Mixed-mode loading results in the growth of these microcracks back towards the interface. Toughness measured using the symmetric chevron specimen under Mode I loading provides the closest estimate of the ‘true’ Cu–Cu bond toughness for bonds created under a particular set of bonding conditions. Mode I toughness is expected to be of importance in assessing the fundamental quality of chemical bonding as well as in prediction of debonding behavior under conditions of plastic deformation, since Gp is a function of G0 (Lane et al., 2000). 5. Summary Bonded chevron specimens with different layer thickness ratios have been used to investigate the mixedmode fracture toughness of Cu–Cu bonds created at 300 1C under an applied pressure of 2.12 MPa. The Cu–Cu bonded interfaces were analyzed under a range of loading conditions. In order to achieve this, the layer thickness ratios were varied from 1.0 to 0.48, to give loading phase angles ranging from 01 to 241. The interface toughness was found to significantly increase with increases in the phase angle, consistent with previous observations of mixed-mode ductile failure. Analyses of the fractured surfaces revealed highly ductile failure modes in the specimens loaded under mixed-mode conditions. The type of failure also varied from adhesive to cohesive as the mode mixity increased. Since there is little quantitative data on the interfacial adhesion of Cu–Cu bonds, the current study using an asymmetric chevron specimen not only demonstrates the utility of the approach to characterizing the effects of mixed-mode conditions on interface toughness, it also provides important fundamental information for assessment of fabrication strategies for 3D integrated circuits. Since the Mode I toughness was found to have
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a negligible plastic work component, it can be used as a measure of the true work of separation for wafer-level Cu–Cu bonds. Mode I Cu–Cu toughness is expected to be a useful metric for the selection of an optimized bond process, particularly under conditions for which the bond experiences pure tensile loading. This study also shows that the interpretation of bond toughness measurements requires detailed knowledge of the loading conditions employed in the test method. The asymmetric chevron test can be used to predict the yield and reliability of bonded structures, such as 3D ICs, that are subjected to a range of different loading conditions during fabrication and use. References Bagdahn, J., Petzold, M., Plo¨Xl, A., Wiemer, M., 2001. Measurement of the local strength distribution of directly bonded silicon wafers using the micro-chevron test. Electrochem. Soc. Proc. 99 (35), 218–223. Cao, H.C., Evans, A.G., 1989. An experimental study of the fracture resistance of biomaterial interfaces. Mech. Mater. 7 (4), 295–304. Celler, G.K., Cristoloveanu, S., 2003. Frontiers of silicon-on-insulator. J. Appl. Phys. 93 (9), 4955–4978. Charalambides, P.G., Lund, J., Evans, A.G., McMeeking, R.M., 1989. A test specimen for determining the fracture resistance of biomaterial interfaces. J. Appl. Mech. 56 (1), 77–82. Cozma, A., Puers, R., 1995. Characterization of the electrostatic bonding of silicon and Pyrex glass. J. Micromech. Microeng. 5 (2), 98–102. Dauskardt, R.H., Lane, M., Ma, Q., Krishna, N., 1998. The adhesion and debonding of multi-layer thin film structures. Eng. Fract. Mech. 61 (1), 141–162. Eaton, W.P., Risbud, S.H., 1994. Silicon wafer-to-wafer bonding at To200 1C with polymethylmethacrylate. Appl. Phys. Lett. 65 (4), 439–441. Evans, A.G., Hutchinson, J.W., 1989. Effects of non-planarity on the mixed-mode fracture resistance of biomaterial interfaces. Acta Metall. Mater. 37 (3), 909–916. Fan, A., Rahman, A., Reif, R., 1999. Copper wafer bonding. Electrochem. Solid State Lett. 2 (10), 534–536. Farrens, S.N., Roberds, B.E., Smith, J.K., Hunt, C.E., 1993. Analysis of bond characteristics in Si direct-bonded materials. Proc. Second Int. Symp. Semicond. Wafer Bonding: Sci. Technol. Appl. 4, 81–95. Hurd, D.S., Caretta, R., Gerberich, W.W., 1995. An experimental fracture mechanics study of a strong interface: the silicon/glass anodic bond. J. Mater. Res. 10 (2), 387–400. Hutchinson, J.W., Suo, Z., 1992. Mixed-mode cracking in layered materials. Adv. Appl. Mech. 29, 63–191. Kobrinsky, M.J., Thompson, C.V., Gross, M., 2001. Diffusional creep in damascene Cu lines. J. Appl. Phys. 89 (1), 91–98. Lane, M., 2003. Interface fracture. Annu. Rev. Mater. Res. 33, 29–54. Lane, M., Vainchtein, A., Gao, H., Dauskardt, R.H., 2000. Plasticity contributions to interface adhesion in thin-film interconnect structures. J. Mater. Res. 15 (12), 2758–2769. Maszara, W.P., Goetz, G., Caviglia, A., McKitterick, B., 1988. Bonding of silicon wafers for silicon on insulator. J. Appl. Phys. 64 (10), 4943–4950. Plo¨Xl, A., Kra¨uter, G., 1999. Wafer direct bonding: tailoring adhesion between brittle materials. Mater. Sci. Eng. R-Rep. 25 (1–2), 1–88. Rahman, A., Reif, R., 2000. System-level performance evaluation of three-dimensional integrated circuits. IEEE Trans. VLSI Syst. 8 (6), 671–678. Schmidt, M.A., 1998. Wafer-to-wafer bonding for microstructure formation. Proc. IEEE 86 (8), 1575–1585. Tadepalli, R., Thompson, C.V., 2003. Quantitative characterization and process optimization of low-temperature bonded copper interconnects for 3-D integrated circuits. In: Proceedings of the IEEE 2003 International Interconnect Technology Conference, pp. 36–38. Tadepalli, R., Turner, K.T., 2007. A chevron specimen for the measurement of mixed-mode interface toughness of wafer bonds. Eng. Fract. Mech, to appear in Engineering Fracture Mechanics. Turner, K.T., Ayon, A.A., Choi, D., Miller, B., Spearing, S.M., 2001. Characterization of silicon fusion bonds using a four-point bend specimen. In: MRS Proceedings: Materials Science of Microelectromechanical Systems (MEMS) Devices III 657, EE631–636. Tvergaard, V., Hutchinson, J.W., 1992. The relation between crack growth resistance and fracture process parameters in elastic-plastic solids. J. Mechs. Phys. Solids 40 (6), 1377–1397. Tvergaard, V., Hutchinson, J.W., 1993. The influence of plasticity on mixed-mode interface toughness. J. Mech. Phys. Solids 41 (6), 1119–1135. Tyson, W.R., Miller, W.A., 1977. Surface free energies of solid metals. Surf. Sci. 62 (1), 267–276. Vallin, O¨., Jonsson, K., Lindberg, U., 2005. Adhesion quantification methods for wafer bonding. Mater. Sci. Eng. R-Rep. 50 (4–5), 109–165.