STUDY OF FAILURE MODES OF MICROELECTRONIC PACKAGING MODULES BY HOLOGRAPHY QUASI PROJECTION MOIRE METHOD*

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1 ACTA MECHANCA SNCA (English Series), Vol.13, No.2, May 1997 The Chinese Society of Theoretical and Applied Mechanics Chinese Journal of Mechanics Press, Beijing, China Allerton Press, NC., New York, U.S.A. SSN STUDY OF FALURE MODES OF MCROELECTRONC PACKAGNG MODULES BY HOLOGRAPHY QUAS PROJECTON MORE METHOD* Shi Xunqing (~ijll~) Liu Baochen + (~J~) Dai Fulong + (E~) (Dept. of Power Eng., Beijing Univ. Of Aeronautics & Astronautics, Beijing , China) +(Department of Engineering Mechanics, Tsinghua University, Beijing , China) ABSTRACT: n this paper, a novel interferometric method with a wide range of sensitivities, called holography quasi projection moire, is proposed. t combines the features of the variated double projection moire method and the holographic interferometry method. This technique is used to study the failure modes of microelectronic packaging modules. KEY WORDS: holography, quasi projection moire, sensitivity, failure modes, packaging modules 1 NTRODUCTON With the development of microelectronic packages, quite a number of failures will occur during the period of packaging and service. n general, there are three causes for the failures of microelectronic packages: stress, electric field, and corrosion. Failures caused by stress can be studied in three main aspects: the mechanical properties of materials, the mechsaaism of fracture, and the failure modes. n the s~udy of failure modes, it is important to observe the whole failure process of packaging modules for various failure modes. However, the low sensitivity measurement method (e.g. shadow moire ) and the high sensitivity measurement method (e.g. holography) are only used to measure large and small deformation' respectively. There is a gap of sensitivity between the two techniques. Although the delamination and the buckle have been analyzed and studied by Suo [z] and Jensen [2] respectively, because of the difficulty in experimental techniques, there is very few experimental investigation, especially, on the quantitative measurement of the whole process of interface fracture. So an experimental method with a wide range of sensitivities is desirable. By the improvement on the projection moire method, a new interferometric method with a wide range of sensitivities, called holography quasi projection moire (HQPM), is presented. Based on this, the experimental investigation on various failure modes of microelectronic packaging modules is conducted. 2 HOLOGRAPHY QUAS PROJECTON MORE Optical system for HQPM moire is shown as Fig.1. t is a combination of the varied double projection moire method and holographic interferometry (H) method. Received 15 April 1996 * The project supported by the National Natural Science Foundation of China

2 180 ACTA MECHANCA SNCA (English Series) 1997 ~ ~ B, = ~1: / Fig.1 Optical system for HQPM: 1 -- laser, 2 -- specimen, 3 -- heating setup, 4 -- recording setup, BS -- cube, M1 ~Ma -- mirror, L -- lens, $1 '-,Sa-- beam enlarger, F1 -- filter, B1 Be -- splitter 2.1 Holographic nterferometry n Fig.l, 1 -+B1 --+M5 --+M6 "--+$ and 1 --+B1 -+B2 ---YS3 --~ 4 are the distance of object beam and that of reference beam, respectively. They form a H system for measuring out-of-plane displacement. According to the principle of hologram [a], the out-of-displacement recorded by the holographic plate can be written as NA d = (1) 2 where N is the fringe order, A is the wavelength of laser. From Eq.(1), the sensitivity of H is in the order of wavelength magnitude. When the deformation is large, the fringe pattern cannot be distinguished. 2.2 Quasi Projection Moire n Fig.l, 1 -+B1 --+B2 -~M~ -~Ms -+ (S1,F1) -~ (BS, M1,M2) --+L1 --+ (Ma,M4) --~ forms an optical system, called quasi projection moire (QPM). t is a variation of the double projection moire, it replaces the image of a real grating with a virtual grating created in space by the interference of two coherent beams of light. As shown in Fig.l, when the two mirrors (M1,M2) are paralleled to the opposite surfaces of cube (BS), the angle a included between the two coherent beams, S1,F1 -+BS-+Ma -+BS-+L (solid line) and S1,F -~BS-~M2 -~BS-+L1 (dotted line), is zero. f one of the mirror (M or M2 ) is set in a little rotation, the two coherent beams will be separated. According to the wave-front interence theory, the frequency of virtual grating formed by the two coherent beams is f--- 2 sin(~)a ~--~a (when a is very small) (2) f half of the virtual grating is reflected by the mirrors (M3, M4), and the reflected beam illuminates the surface of specimen at the same incident angle 8 as the other half of virtual grating in space, the relationship between the out-of-plane displacement and moire fringes can be written as N (3) 2f' tan 8

3 Vol.13, No.2 Shi Xunqing et al.: Failure Modes of Microelectronic Packaging Modules 181 where f = f cos ~ is the frequency of virtual grating on the surface of specimen. Substituting Eq.(2) into Eq.(3), we obtain NA w = 2a sin 0 (4) t is clear that there are not only the out-of-plane displacement of specimen, but also the lines of virtual grating recorded onto the holographic plate. Using the Fourier optical filtering system, the lines of virtual grating can be filtered, and good fringe pattern contrast can be obtained[4]. From Eq.(4), the sensitivity of the projection moire method is increased, while the range of sensitivity is enlarged. Thus, Eq.(1) and Eq.(4) can be used to measure the whole failure process of specimen varied from small to large deformation. 3 EXPERMENTAL STUDY OF FALURE MODES 3.1 Experimental Method nterface fracture of microeleetronic packaging modules deponds due to many factors, such as thickness and stiffness of the thin film, the strength of the bonded layer, the interracial fracture resistance, the flaw distribution, etc. n order to analyze systematically the influence of these factors, seven simulated microelectronic packaging specimens are employed in the experimental study. The details of various specimens and their geometry are shown in Table 1 and Fig.2, respectively. Table 1 Various specimens (unit: pro) Specimen No Fig.2 (a) (a) (a) (b) (c) (d) (d) Film Materials Cu Cu Cu Cu Cu TiO2 TiO2 tl Sub- Materials A1203 A1203 A1203 A1203 A1203 Cu Cu strate t x x x x Bonded Strength low low middle middle middle high high Flaw Shape / / / one-d two-d / / Loading Method heater heater heater heater heater electrify electrify Experimental spal- delami- delami- one two crush crush Usage lation nation nation dimen- dimenspal- spal- sional sional delami- delamicurve lation lation buckle buckle nation nation Two heating methods are applied with the view to simulate the practical service condition of microelectronic packaging modules. For the film/substrate bonded system as shown in Fig.2 (a) (b) (c), the ceramic substrate is made up into a heater which can produce a uniform thermal field. On the other hand, for the film/substrate bonded system as shown in Fig.2 (d), the voltage is directly applied across the metal to obtain large thermal load. The experimental procedures are as follows: first, the optical system is arranged as Fig.l; second, the small deformation is measured by H; third, when the deformation is larger and goes beyond the range of H, the QPM can be used to measure the deformation of the specimen by using its continuous sensitivities; finally, when the deformation exceeds over the limit of QPM, the microscope is employed to analyze qualitatively the last failure process of the specimen.

4 182 ACTA MECHANCA SNCA (English Series) 1997 (,q m 30-2o 5, tt ' "5- t tl (a) without flaw (b) one-dimensional flaw -~ ~ ",---20-',-,5" 03 R4 y,.4 "l.v - -J t~ t t~ (c) two-dimensional flaw (d) plating film/substrate bonded system Fig.2 Various simulated microelectronic packaging modules 3.2 Experimental Results (1) Spallation and curve From Eq.(1) and Eq.(4), eliminating the contribution of substrate, the out-of-plane displacements of specimen 1, 2, and 3 under various thermal load can be measured respectively, and plotted displacement vs temperature curves as shown in Fig.3. The critical temperatures of interface failure and the failure modes are shown in Table [ 4- specimen [.. spechnen ~ -specimen 3..4 ~: 40 30[..a. K:..g 20..,A" '0 3' '0 70 8'0 9'0 100 T~ Fig.3 Displacement vs temperature curves Fig.4 Specimen 1 curves at T - 60~

5 Vol.13, No Shi Xunqing et al.: Failure Modes of Microelectronic Packaging Modules Table 2 Failure modes of various specimens Specimen No Bonded Strength low low middle Failure Modes curve spallation spallation Temperature 55 ~ 70 ~ 85 ~ From Fig.3 and Table 2, the strength of film/substrate bonded system has a close relation with the bonded conditions and thickness of film. n general, the critical temperature of interface failure will rise with the increment of film thickness. When the film is very thin, because of th~ fact that the stiffness of film is very small, the fracture does not stop even though spallation occurs in the interface of specimen: n addition, the sudden curve fracture can be seen with the increase of temperature (Fig.4). This phenomenon is not observed in the other specimens with thick film. (2) Buckle For one-dimensional buckle, the film can be considered approximately as a strip plate with the width of 2b (Fig.5). According to the theory of composite beam, WhitcombiS] has established the relationship between the buckled deflection of film $ and the buckled critical stress a~ as follows (5) where er is the thermal stress, h the film thickness and ~ a proportional coefficient. For two-dimensional flaw, there is no complete theoretical solution yet. Considering the film as a round plate with the radius of R (Fig.6), under the condition of small buckled deflection, the approximate relationship between the central deflection of round plate 6 and #/a~ can be expressed as[b} 1 (r _ 1)]i/2h = = (6) where Cl = (1 + ul) (1 - v~). Assnmlng that there is a uniform pressure P, produced by the thermal stress and loaded on the interface of film/substrate, the associated critical pressure P0 can be written as p0=16 El h 3 --~ ( )2 (Z) where R is the radius of circular flaw, El is elastic modulus, ul is Poisson ratio. The fringe patterns of specimen 4 and 5 can be obtained using the HQPM method. Eliminating the contribution of substrate, the out-of-plane displacements under various thermal loads can be calculated by means of Eq.(1) and Eq.(4). Thereby, the theoretical and experimental curves 6/h,,, a/a~ of one-dimensional buckle can be obtained by Eq.(5) and HQPM method respectively (Fig,5). Also the theoretical and experimental curves 6/h,,, P/Po of twodimensional buckle are obtained by" Eq.(6), Eq.(?) and HQPM method respectively (Fig.6). n Fig.5, although the theoretical and experimental curves show similar tendency, it is obvious that experimental result is larger than theoretical analysis. n Fig.6, there is a great

6 184 4 ~ experimental value [, theoretical value ACTA MECHANCA SNCA (English Series) 1997 ' + experimental value.. theoretical value ~2 9 " \ / "~ d ~ -- ~:-- /., z / CuWi,2,,.4,/',. / 9 y 0 Fig.5 (a] unbuckbd (b] buckled r - 2r--.4]., r- (a) unbuckled (b) buckled (7 % P Po 6/h ~ a/c% curves of specimen 4 Fig.6 6/h ~ P/Po curves of specimen F difference between experimental value and approximate theoretical solution obtained under the condition of linear elastic deformation and small buckled deflection. This error is due to two important reasons. First, the stress caused by thermal load is not uniform. There is no ideal linear relationship between the buckled temperature T/To of film and the uniform pressure P/Po. Second, the adverse effect of elastic mismatch of materials in film/substrate system is significant [6], and is not negligible. For one-dimensional condition, the buckle doesn't start at the same time from anywhere of one-dimensional flaw in the length direction (x-direction), but from somewhere else (Fig.7). The same phenomenon occurs in the width direction (y-direction). However, there is a difference in cracking between left and right crack-tip, which remains unless the interface fracture occurs. For two-dimensional condition, at the beginning of buckle, the fringe is not an ideal circle due to the fact that the prefabricated flaw is not an ideal circle. t will approach toward a circle in shape with the increase of thermal load (Fig.8). Fig.7 Typical holographic pattern for specimen 4 under 10~ Fig.8 Typical moire pattern for specimen 5 under 50~ (f = 30 1/mm) (3) Cracking and delamination To obtain a larger load, specimen 6 is loaded by electrifying. Then one side of specimen 6 (y direction, Fig.2)in the marked range is placed vertically under a microscope for the observation of the interfacial fracture process. The experimental result shows that when thermal load is small, the interface is perfect. However, for larger load, decohesion appears in the weak area of film/substrate bonded system. With the increase of thermal load, in the area the crack will propagation, which is analogous to laceration in metal cutting and there is a blunt plastic-zone in the crack-tip. When the crack propagation reaches a certain level,

7 Vol.13, No.2 Shi Xunqing et al.: Failure Modes of Microelectronic Packaging Modules 185 the delamination will appear in the whole film/substrate interface (Fig.9). (4) Crush As mentioned in above, specimen 7 is loaded by electrifying. Then the surface of specimen 7 (z direction, Fig.2) in marked range is placed horizontally under a microscope to observe the surface fracture process. The experimental result shows that when thermal load is small, the surface is perfect. When the load reaches a critical value, there are many little bulges on the surface of film, known as a collective buckling phenomenon. These bulges spread over wide areas on the increase of thermal load. However, the magnitude of every bulge does not grow up clearly. This indicates that the buckle of film is sensitive to load changing under the condition of small load. With the increase of load, sudden crush will occur in some bulges (Fig.10). t spreads over all around and produces larger range fracture. Fig.9 Delamination fracture of specimen 6 (=6A) Fig. 10 Crush fracture of specimen 7 (=4.5A) 4 CONCLUSON From the above experiment, it was found that HQPM is an effective method for studying the failure modes of microelectronic packaging modules. The experimental results show that when the mechanical strength of film/substrate bonded system is weak, in the interface there will be decohesion, spallation, and delamination. f the strength of film is far less than that of the substrate, the film will curve suddenly after delamination. f the film is ductile and the substrate is brittle, the film will buckle from the substrate after cracking, delaminating along the interface crack-tip due to the flaw. Otherwise, the film will crush on the substrate. REFERENCES 1 Suo Z. Delamination specimens for orthotropic materials. J Appl Mech, 1990, 57:627~634 2 Jensen HM. Mixed modes fracture analysis of the blister test. nt J Solids Structures, 1990, 26: 1099~ Dai FL. Modern optical measurement mechanics. Beijing: Science Press, ~42 4 Shi XQ. Optical micro-measurement techniques and its applications to experimental investigation of microelectronic packages. (Ph,D. Thesis). Beijing: Tsinghua University, ~100 5 Hutchinson JW, Suo Z. Mixed mode cracking in layered materials. Advance in Appl Mech, 1991, 28:10,-,18 6 Evans AG. Residual stresses ans cracking in brittle solids bonded with thin ductile layer. Acta Metall, 1988, 36:2 037~2 046