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silicon nitride interface
Acta Materialia 53 (2005) 3147–3153 www.actamat-journals.com
A quantitative study of moisture adsorption in polyimide and its effect on the strength of the polyimide/silicon nitride interface A. Jain a, V. Gupta a
a,*
, S.N. Basu
b
Department of Mechanical and Aerospace Engineering, University of California Los Angeles, 38-137E, Engg IV Bldg, Los Angeles, CA 900951597, USA b Department of Manufacturing Engineering, Boston University, Brookline, MA 02446, USA
Received 10 September 2004; received in revised form 7 January 2005; accepted 11 February 2005 Available online 29 April 2005
Abstract Polyimide films on silicon nitride substrates were exposed to moisture under varying conditions of relative humidity, time and temperature. The moisture content of the films was measured by FTIR spectroscopy, and the polyimide/silicon nitride interface strength was measured at room temperature by a laser spallation technique. The moisture adsorption by polyimide films was analyzed using a diffusion model. Under the experimental conditions of this study, it was found that the rate of moisture adsorption was controlled the surface exchange reaction. For samples exposed at 38 C, the interface strength was found to decrease linearly with increasing interface moisture concentration. A critical interface moisture concentration was identified, where the strength is expected to go to zero. The interface strengths of all the measured samples were combined into one empirical equation that can be used as a basis to construct strength charts as a function of exposure conditions. Such strength charts should help in the development of more rational standards for handling packaged ICs during manufacturing and integration from the viewpoint of avoiding moisturerelated failures. 2005 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Interface strength; Moisture degradation; Electronic devices; Reliability; Adhesion
1. Introduction Electrical failures in packaged assemblies are a direct result of local delaminations at one or several of the many geometrically heterogeneous interfaces. Thus, maximizing adhesion at key interfaces and predicting its degradation when exposed to moisture and in situ temperature rise during processing and in service forms an integral part of any reliability exercise. Presently, the mechanical reliability is ascertained by carrying out an exhaustive set of accelerated humidity/temperature/time tests [1] during the development phase of integrated circuits (IC) and substrates individually and also after they *
Corresponding author. Tel.: +1 310 825 0223; fax: +1 310 206 2302. E-mail address: [email protected] (V. Gupta).
have been packaged together. The parameters for these tests are determined after calculating accelerated factors using extensive finite element codes; these are then applied to those used for testing the older generation of ICs for which the field data are already available. Unfortunately, with every new generation of IC, that still use the same buffer, passivation, and underfill epoxy materials as in the earlier devices, the empirical accelerated test procedures have to be virtually repeated at a great expense of time and delayed product introduction. Indeed, a more rational and efficient strategy is warranted. An alternative approach pursued in our group starts by first identifying the key interfaces where failures in packaged devices during processing and service have been observed. Next, such an interface is isolated in the planar form and its fundamental tensile strength mea-
1359-6454/$30.00 2005 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.actamat.2005.02.022
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sured using a novel laser spallation experiment that utilizes a laser-generated stress wave to separate the interface. The degradation in the interface tensile strength is determined next by exposing the interface to different levels and duration of moisture and temperature conditions. Finally, the tensile strength is directly correlated with the amount of moisture segregated at the interface by measuring the latter using the Fourier transform infrared spectroscopy (FTIR) technique. Thus, the amount of moisture in the interface can be considered as a criterion for determining the interface strength. Once such an interface strength chart is developed for varying moisture content, it can be used in conjunction with the already well developed simulation codes that are capable of predicting time-dependent stress concentrations, moisture accumulation, and temperature at any interfaces within the package assembly, for determining (a) whether the interface will be able to survive processing and system integration cycles during the design phase itself, prior to any IC fabrication and packaging; and (b) the level of interface strength needed at the time of manufacturing for ensuring a prescribed service life. This value can be essentially determined by reading the strength degradation corresponding to the calculated moisture accumulation over the desired service life from the charts, and then adding to it the largest stress concentration at the critical interface under the expected service loads, as calculated from the finite element method (FEM) codes. In addition to being more rational, reliable, and robust, the pursued methodology should result in significant time and cost savings compared with the exhaustive set of accelerated time/temperature/humidity tests that are presently employed by the industry. Several aspects of the above strategy, including the development of the laser spallation technique to measure the tensile strength of the interface, and strength charts as a function of varying humidity, temperature, and time, have already been developed and reported previously [2]. The aim of this paper is to demonstrate the last leg of the above strategy namely determining the interface strength data directly as a function of the moisture content at the interface. Although the experimental procedures are general and applicable to any material system, the results here are presented specifically for the Si/Si3N4/polyimide system.
2. Sample preparation Silicon substrates of Æ1 0 0æ orientation were chemically cleaned using the Piranha solution consisting of 5 parts sulfuric acid and 1 part hydrogen peroxide. The silicon wafers were placed in this solution for 5 min, removed and rinsed with deionized water. The wafers were then dried on a hot plate for 1–2 min at a temperature
Fig. 1. Cross-section of the sample.
between 120 and 150 C. A plasma enhanced chemical vapor deposition (PECVD) process was then used to deposit a 750 nm thick layer of silicon nitride. The PECVD reactor was a Plasmatherm 790 system. A polyimide precursor made by HD Microsystems with the commercial name PI-2545 was spin coated onto the nitride surface with the use of a Headway spin coater revolving at 2000 rpm for 30 s. The spin-coated resin was cured at 135 C for 2 h in a heating furnace to obtain a structurally stable film of thickness 2l, as measured with a Tencor Alpha-Step 200 profilometer. The cross-section of a typical sample is shown in Fig. 1. The samples were divided into three groups. Samples in Group 1 were kept at 38 C for 48 h at different relative humidity conditions ranging from 50% to 70% so as to study the effect of relative humidity on the interface strength. Group 2 samples were kept at 38 C and 60% relative humidity for exposure times ranging from 24 to 96 h. Finally, Group 3 samples were kept at 60% relative humidity for 24 h at temperatures ranging from 30 to 60 C. The amount of water trapped in the samples was measured using the FTIR technique. The same samples were subsequently tested for interface strength using the laser spallation technique. Both these measurement techniques are briefly discussed next.
3. Experimental procedures 3.1. Quantification of water content in samples The water content in all samples was quantified using standard FTIR spectroscopy. The IR beam was impinged from the polyimide side normal to the nitride/ polyimide interface, and the transmitted beam through the Si was analyzed. An unconditioned sample was used for obtaining the background spectrum. The amount of moisture present in a sample is directly proportional to the area under the water peak of the FTIR spectra. The basic relationship between IR intensity and concentration is given by the Beer–Lambert law, At = c Æ t Æ et, where At is the area under the peak, c is the concentration, t is the path length, and et = 1 · 105 cm2 mol1 L is the integrated molar absorption coefficient of water [3]. Fig. 2 shows a typical FTIR spectrum, which indicates the presence of water by the peak at 3337 cm1.
A. Jain et al. / Acta Materialia 53 (2005) 3147–3153
Fig. 2. A typical FTIR spectrum showing the presence of water absorption peak corresponding to 3337 cm1.
Fig. 3. FTIR water absorption peaks obtained from samples treated with varying preconditioning treatments. The area under each curve is proportional to the amount of water absorbed in the sample.
Fig. 3 shows the portion of the FTIR spectra with the water peak for Group II samples. The concentration of the moisture is directly proportional to the area under the curve and is calculated using the Beer–Lambert law. Fig. 3 clearly shows that the amount of water present in the samples is a direct function of the preconditioning treatment. 3.2. Measurement of interface strength The interface strength of all the samples was measured at room temperature using a laser spallation technique [4,8]. In this experiment, a 2.5 ns long Nd:YAG laser pulse is made to impinge over a 3 mm diameter area on a 0.5 lm thick aluminum film that is sandwiched between the back surface of a substrate disc (12–25 mm diameter and 1 mm thick) and a 10–20 lm thick layer of SiO2. The melting-induced expansion of aluminum under confinement generates a compressive stress pulse (with 1 ns rise time) directed towards the test coating that is deposited on the substrates front surface. The
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compression stress pulse reflects into a tensile wave from the coatings free surface and leads to its spallation (complete removal) at a sufficiently high amplitude. The incident laser fluence is controlled by changing the energy of the laser beam while keeping the illuminated area constant. Also, if spallation does not occur, the laser beam is focused onto a new spot in order to avoid fatigue effects. The interface tensile stress is obtained by measuring the transient displacement history of the coatings free surface (induced during pulse reflection) by using an optical interferometer. Besides having a resolution of only 0.2 ns in the single shot mode, the interferometer is capable of recording fringes from even optically rough surfaces. For a coating of density q and thickness h, the interface stress r is calculable from the measured transient velocity v(t) as: 1 h h rðh; tÞ ¼ qc v t þ v t ; ð1Þ 2 c c where c is the longitudinal stress wave velocity in the film. Interface strengths in a variety of systems, including those between metal, ceramic, and polymeric coatings and engineering substrates, have been measured. The spallation technique was preferred over the fourpoint bend [9], blister [10], double cantilever beam [11], and indentation [10] tests. Since interface decohesion is accomplished at an ultra high strain rate in the spallation test, all inelastic deformations within the separating components are essentially suppressed and, as shown earlier [5–8], the measured strengths are intrinsic to the interfacial microstructure, inclusive of defects, if any. While applying the spallation technique to characterize the multilayer system supported on a silicon chip, it was found that sufficient tensile stress needed for decohering interfaces could be generated without the use of the aluminum layer. So no Al layer was used in the present study. However, it was still necessary to cover the Si bare surface with the layer of waterglass of thickness 40–50 lm to generate sufficient stress amplitudes. The laser-heating diameter was kept at 3 mm to ensure one dimensionality of the stress wave over 95% of the area [12]. Since optical interferometery could not be performed directly on the bare polyimide surface because of its transparency, the stress in the samples was quantified by a three-part procedure. In the first step the critical laser fluence at which interface separation was observed was recorded. In the next step, the stress pulse in a bare Si wafer (without any test Si3N4 and polyimide test layers) bearing the exact same thickness, orientation, and waterglass layer thickness was recorded at the laser fluence at which interface decohesion was observed in the first step. The stress pulse was recorded using interferometery and following the procedures used in the basic spallation technique discussed above. Finally, this stress profile was used as an input to a finite element acoustic wave model, which calculates the tensile stress history at
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Table 1 Material properties used as input in the wave mechanics simulation Property
Si3N4
Polyimide
Si
Thickness (lm) Density (kg/m3) k (GPa) l (GPa)
0.75 3000 55.4 65
2.0 1400 3.5 0.87
750 2300 75 50
any interface in a given multilayer assembly when loaded normally by a propagating stress wave from the silicon side. Other inputs to the program include the thickness, density, and Lame constants, k and l, for each layer. Table 1 summarizes the properties used for the simulation used in this work. The same procedure was repeated for each sample.
4. Results and discussion It is expected that the moisture content at the polyimide/substrate interface will influence the interfacial strength. However, the FTIR spectroscopy study provides the integrated water concentration through the thickness of the polyimide film. Thus, it is first necessary to understand the correlation between the total water content in the polyimide film to the interfacial concentration. 4.1. Modeling moisture diffusion in polyimide The incorporation of moisture into the polyimide film can be modeled as two sequential kinetic steps; i.e., (i) moisture adsorption at the surface, and (ii) diffusion of the adsorbed moisture into the bulk of the polyimide film. It is always instructive to see if any one of these processes is kinetically dominant since the two steps lead to different time dependent rate laws. There have been several studies of the diffusion in polyimide [13–16]. These studies report the diffusion coefficient of moisture in polyimide, D, to be of the order of 109 cm2/s at room temperature (25 C). Taking the smallest exposure time, t, of 24 h in this study, the diffusion distance, x, estimated as (Dt)1/2 is 294 lm. It should be noted that since all the exposure temperatures in this study is greater than room temperature, the diffusion distances in all cases used in this study will be higher than this estimate. Since the thickness of the polyimide film, h, is 2 lm, the assumption h x is valid for all cases indicating that the diffusion profile in the film will always be flat at the end of every exposure when the interface strength is measured, and that the kinetics of surface adsorption is rate controlling since the moisture content is changing with exposure conditions indicating that saturation has not occurred. This flat profile mandates that the surface moisture content in the film, CS, is also the interface moisture content in the film, CI,
and that the measured integrated moisture content of the film is CSh. The kinetics of the surface adsorption process controlled by a flux of moisture from the gas phase to the solid, JS, defined as [17]: J S ¼ kðC 0 C S Þ;
ð2Þ
where k is the surface exchange coefficient, and C0 is the surface concentration of moisture in equilibrium with the moisture in the gas phase. This boundary condition leads to the well-known solution [17]: kt t C S ¼ C 1 ¼ C 0 1 e h ¼ C 0 1 e s : ð3Þ The time constant, s = h/k, relates to the time required for the surface concentration CS to reach the equilibrium value of C0. From the moisture adsorption data of the Group 2 samples where the total moisture adsorbed as a function of time is recorded at a constant temperature and relative humidity, it is possible to calculate the value of C0 and s using a best fit of Eq. (3) to the data. It should be noted that C0 is a function of the gas phase composition (related to % relative humidity) and temperature, while s (and therefore k) is a function of temperature alone. Fig. 4 shows such a best fit, for C0 = 0.0143 g/cm3 and s = 2.8 · 105 s at 38 C and 60% relative humidity. The dependence of Eq. (3) on the relative humidity and temperature can be calculated in the following manner. Since C0 corresponds to the surface concentration of moisture in the polymer in equilibrium with the moisture-containing atmosphere, the activity of moisture in the gas and solid phases should be equal. This implies that: cW X W ¼ pW ¼ pW ðsatÞ
%RH ; 100
ð4Þ
where cW is the activity coefficient of the dilute moisture–polymer solid solution (Henrys Law) as the poly-
Fig. 4. Variation of interface moisture content with exposure time for Group 2 samples at 38 C and 60% relative humidity. The line shows the best fit of Eq. (3) to the data.
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mer surface, XW is the mole fraction of moisture in the solid solution, pW is the partial pressure of moisture in the vapor phase, and pw(sat) is the saturated vapor pressure of moisture in the gas phase at the given temperature. The saturated partial pressure of water vapor as a function of temperature T is well known and can be expressed as [18]: log pW ðsatÞðatmÞ ¼ 3075=T 5:465 log T þ 0:279 103 T þ 862=T 2 þ 22:19: ð5Þ The mole fraction of moisture in the solid solution can be expressed as: XW ¼
C 0 MW P ; MW W C P
Fig. 6. Plot of the interface moisture concentration of Group 3 samples as a function of temperature after exposure for 24 h at 60% relative humidity.
ð6Þ
where MW and C stand for the molecular weight and concentration, and the subscripts W and P stand for the moisture and polymer phases, respectively. Assuming that the moisture and polymer solid solution follows a regular solution model, the temperature dependence of the moisture activity coefficient can be expressed as: 2
RT ln cW ¼ að1 X W Þ ;
ð7Þ
where a is the enthalpy of mixing of the two components. Since the concentration of moisture in the polymer surface is very small, XW 1, giving: a cW ¼ eRT : ð8Þ Similarly, the surface exchange coefficient k is a thermally activated kinetic constant which has an Arrhenius dependence of T given by: b
k ¼ k 0 eRT : Combining Eqs. (3)–(9) gives:
! b tk eRT MW W %RH a RT 0h P W ðsatÞe CI ¼ CP 1e : MW P 100
ð9Þ
ð10Þ
Eq. (10) predicts that at a constant T and exposure time t, CI (and thus the total moisture content) is propor-
tional to the percentage relative humidity. The excellent linear fit in Fig. 5 confirms this hypothesis for the Group 1 samples which were all exposed for 48 h at 38 C at different relative humidity conditions. For the Group 3 samples, the moisture absorption was measured as a function of temperature at fixed values of t and % relative humidity. The best fit of the experimental data to Eq. (10) gives a = 2.6 · 104 J/ mol K, b = 1.82 · 104 J/mol K, and k0 = 8.1 · 107 cm/s. These constants lead to some interesting physical insights. The negative value of a implies that the enthalpy of mixing of the moisture and polymer is negative, making water absorption by polyimide an exothermic process consistent with the literature [19]. Furthermore, the negative value of a implies that cW increases with temperature leading to a decrease in XW at a given moisture activity in the gas phase. Thus, the cW term predicts a decrease in moisture adsorption with increasing temperature. This is counterbalanced by an increase in pW(sat) and k with T, both of which predict an increase in the moisture concentration with temperature. The Group 3 samples show that under the conditions studied, the latter effect was more dominant than the former, leading to an increase in the moisture content with increasing temperature as seen in Fig. 6. 4.2. Interface moisture content/strength correlations
Fig. 5. The effect of relative humidity on interface moisture content for Group 1 samples exposed to 38 C and 48 h.
Since, both Group 1 and 2 samples were exposed to the same temperature (38 C), it is instructive to plot the interfacial strength of all Group 1 and 2 samples as a function of interface moisture concentration. Fig. 7 is such a plot, showing that at 38 C, the interface strength decreases linearly with temperature. A critical concentration, Ccrit, having a value of 1.49 · 102 g/cm3 can be identified as the interface concentration at which the strength is reduced to zero. The chemical structure of the as-spun polyimide precursor in Fig. 8 shows two open carbon rings. Since nitrogen in the Si3N4 has a lone pair of electrons, it can form hydrogen bonds at one of
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Fig. 7. The effect of moisture content on the interface strength for samples in Groups 1 and 2, which were all exposed at 38 C. A linear fit identifies Ccrit, an interface moisture concentration at which the strength will be reduced to zero.
the open ring sites of the precursor molecules, if available via direct contact. The degrading effect of increasing relative humidity content and hold times is a direct result of the competition between the water molecules that segregate the interfacial region and the nitrogen of the nitride in forming the hydrogen bonds with the polyimide precursor. Since hydrogen bonds are fairly weak they are constantly breaking and reforming even at ambient temperature. During baking, ring closure occurs on account of dehydrolysis and results in the formation of the stable polyimide molecule, as shown in Fig. 8. This dehydrolysis leads to a reduction of the number of hydrogen bonds between the Si3N4 and polyimide, leading to an irreversible detriment in interfacial strength with high temperature exposure, which is in addition to the effect of moisture segregation at the interface. Since dehydrolysis is a thermally activated process, the degradation in interfacial strength is expected to exhibit an Arrhenius dependence on temperature with an activation energy
Fig. 8. Proposed strength degradation mechanism at the silicon nitride/polyimide interface.
Fig. 9. Plot of the measured interface strength versus calculated interface strength using Eq. (11). The 45 line would match an exact fit between the measured and calculated strengths.
Q. Combining this temperature dependence with the experimentally measured linear interfacial concentration dependence (Fig. 7) of interfacial strength, S, gives a general expression: Q
S ¼ AðC crit C 1 ÞeRT :
ð11Þ
It should be noted that CI implicitly represents the exposure time and relative humidity in this expression. A best fit of Eq. (11) for all the samples in Groups 1, 2 and 3 shows A = 1384 MPa and Q = 7300 J/mol K. Fig. 9 shows a plot of experimental value of the interface strength S versus the calculated value of S based on Eq. (11). The figure shows the data points are reasonably close to the 45 line, which represents a perfect fit to the experimental data.
5. Conclusions Polyimide films on silicon nitride substrates were exposed to moisture under varying conditions of relative humidity, time and temperature. The moisture content of the films was measured by FTIR spectroscopy, and the polyimide/silicon nitride interface strength was measured at room temperature by a laser spallation technique. The moisture adsorption by polyimide films was analyzed using a diffusion model. Under the experimental conditions of this study, it was found that the rate of moisture adsorption was controlled the surface exchange reaction. For samples exposed at 38 C, the interface strength was found to decrease linearly with increasing interface moisture concentration. A critical interface moisture concentration was identified, where the strength is expected to go to zero. The interface strengths of all the measured samples were combined into one empirical equation (Eq. (11)), which can be used as a basis to construct strength charts as a function of exposure conditions.
A. Jain et al. / Acta Materialia 53 (2005) 3147–3153
Such strength charts should help in the development of more rational standards for handling packaged ICs during manufacturing and integration from the viewpoint of avoiding moisture-related failures. The current practice in industry for controlling moisture induced failures in IC packages is to dry the package by ‘‘baking’’ and to ship them to the assembly plants in desiccant bags [2]. After opening the bags, the IC components must be assembled within a specified (relatively short) time to avoid moisture-induced damages [7]. The standards relate to a factory environment of 30 C/60% relative humidity. If the environmental conditions at the factory are maintained at a lower temperature, the standards default to the same exposure times as in the case of 30 C/60% relative humidity. This specification does not justify the high cost of handling the IC packages. The strength charts, when used in conjunction with moisture transport models [8,9], can provide standards and schemes to properly de-rate (extrapolate) specifications for lower humidity conditions. Certainly, if the strength charts for key interfaces are available, standards for new and ever emerging ICs and packaging technologies can be developed without having to perform a new set of accelerated empirical tests, thus giving a significant cost saving.
Acknowledgments The work was supported by a NSF Grant No. ECS0000334, and a US Army Grant No. DAAD19-00-1049, for which we are grateful to Dr. David Stepp of that agency. The authors thank Professor Hicks and
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Steve Babayan of UCLA for giving access to the FTIR spectrometer, and Osman Anli at Boston University for his help with the simulations used in the diffusion and strength models.
References [1] Feng Y, Raju VR, Suhir E. EEP-21. New York: ASME; 1977. p. 89. [2] Gupta V, Hernandez R, Charconnet P. J Mater Sci Eng A 2001;317:256. [3] Libowitzky E, Rossman GR. Am Mineralogist 1997;82:1111. [4] Gupta V, Yuan J, Pronin AN. J Adhes Sci Technol 1994;8(6):713. [5] Yuan J, Gupta V. Acta Metall Mater 1995;43(2):781. [6] Yuan J, Gupta V, Kim M. Acta Metall Mater 1995;43(2):769. [7] Gupta V, Wu J, Pronin AN. J Am Ceram Soc 1997;80(12):3172. [8] Pronin AN, Gupta V. J Mech Phys Solids 1998;46(3):389. [9] Evans AG. J Mater Sci Eng A 1989;107:227. [10] Mittal KL, editor. Adhesion measurement of thin films, and bulk coatings, ASTM STP-640; 1978. p. 5. [11] Gupta V, Argon AS, Cornie JA. J Mater Sci 1989;24:2031. [12] Argon AS, Cornie JA, Gupta V, Lev L, Parks DM. Mater Sci Eng A 1997;237:224. [13] Ree M, Han H, Gryte CC. High Perf Polym 1994;6:321. [14] Denton DD, Day DR, Priore DF, Senturia SD. J Electron Mater 1985;14(2):119. [15] Okamato K-I, Tanihara N, Watanabe H, Tanaka K, Kita H, Nakamura A, et al. J Polym Sci 1992;30:1223. [16] Huang J, Cranford RJ, Matsuura T, Roy C. J Appl Polm Sci 2003;87:2306. [17] Crank J. The mathematics of diffusion. Oxford: Clarendon Press; 1956. [18] Gaskell DR. Introduction to thermodynamics of materials. Washington (DC): Taylor & Francis; 1995. [19] Crank J, Park JS. Diffusion in polymers. London: Academic Press; 1968.
A quantitative study of moisture adsorption in polyimide and its effect on the strength of the polyimide/silicon nitride interface A. Jain a, V. Gupta a
a,*
, S.N. Basu
b
Department of Mechanical and Aerospace Engineering, University of California Los Angeles, 38-137E, Engg IV Bldg, Los Angeles, CA 900951597, USA b Department of Manufacturing Engineering, Boston University, Brookline, MA 02446, USA
Received 10 September 2004; received in revised form 7 January 2005; accepted 11 February 2005 Available online 29 April 2005
Abstract Polyimide films on silicon nitride substrates were exposed to moisture under varying conditions of relative humidity, time and temperature. The moisture content of the films was measured by FTIR spectroscopy, and the polyimide/silicon nitride interface strength was measured at room temperature by a laser spallation technique. The moisture adsorption by polyimide films was analyzed using a diffusion model. Under the experimental conditions of this study, it was found that the rate of moisture adsorption was controlled the surface exchange reaction. For samples exposed at 38 C, the interface strength was found to decrease linearly with increasing interface moisture concentration. A critical interface moisture concentration was identified, where the strength is expected to go to zero. The interface strengths of all the measured samples were combined into one empirical equation that can be used as a basis to construct strength charts as a function of exposure conditions. Such strength charts should help in the development of more rational standards for handling packaged ICs during manufacturing and integration from the viewpoint of avoiding moisturerelated failures. 2005 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Interface strength; Moisture degradation; Electronic devices; Reliability; Adhesion
1. Introduction Electrical failures in packaged assemblies are a direct result of local delaminations at one or several of the many geometrically heterogeneous interfaces. Thus, maximizing adhesion at key interfaces and predicting its degradation when exposed to moisture and in situ temperature rise during processing and in service forms an integral part of any reliability exercise. Presently, the mechanical reliability is ascertained by carrying out an exhaustive set of accelerated humidity/temperature/time tests [1] during the development phase of integrated circuits (IC) and substrates individually and also after they *
Corresponding author. Tel.: +1 310 825 0223; fax: +1 310 206 2302. E-mail address: [email protected] (V. Gupta).
have been packaged together. The parameters for these tests are determined after calculating accelerated factors using extensive finite element codes; these are then applied to those used for testing the older generation of ICs for which the field data are already available. Unfortunately, with every new generation of IC, that still use the same buffer, passivation, and underfill epoxy materials as in the earlier devices, the empirical accelerated test procedures have to be virtually repeated at a great expense of time and delayed product introduction. Indeed, a more rational and efficient strategy is warranted. An alternative approach pursued in our group starts by first identifying the key interfaces where failures in packaged devices during processing and service have been observed. Next, such an interface is isolated in the planar form and its fundamental tensile strength mea-
1359-6454/$30.00 2005 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.actamat.2005.02.022
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A. Jain et al. / Acta Materialia 53 (2005) 3147–3153
sured using a novel laser spallation experiment that utilizes a laser-generated stress wave to separate the interface. The degradation in the interface tensile strength is determined next by exposing the interface to different levels and duration of moisture and temperature conditions. Finally, the tensile strength is directly correlated with the amount of moisture segregated at the interface by measuring the latter using the Fourier transform infrared spectroscopy (FTIR) technique. Thus, the amount of moisture in the interface can be considered as a criterion for determining the interface strength. Once such an interface strength chart is developed for varying moisture content, it can be used in conjunction with the already well developed simulation codes that are capable of predicting time-dependent stress concentrations, moisture accumulation, and temperature at any interfaces within the package assembly, for determining (a) whether the interface will be able to survive processing and system integration cycles during the design phase itself, prior to any IC fabrication and packaging; and (b) the level of interface strength needed at the time of manufacturing for ensuring a prescribed service life. This value can be essentially determined by reading the strength degradation corresponding to the calculated moisture accumulation over the desired service life from the charts, and then adding to it the largest stress concentration at the critical interface under the expected service loads, as calculated from the finite element method (FEM) codes. In addition to being more rational, reliable, and robust, the pursued methodology should result in significant time and cost savings compared with the exhaustive set of accelerated time/temperature/humidity tests that are presently employed by the industry. Several aspects of the above strategy, including the development of the laser spallation technique to measure the tensile strength of the interface, and strength charts as a function of varying humidity, temperature, and time, have already been developed and reported previously [2]. The aim of this paper is to demonstrate the last leg of the above strategy namely determining the interface strength data directly as a function of the moisture content at the interface. Although the experimental procedures are general and applicable to any material system, the results here are presented specifically for the Si/Si3N4/polyimide system.
2. Sample preparation Silicon substrates of Æ1 0 0æ orientation were chemically cleaned using the Piranha solution consisting of 5 parts sulfuric acid and 1 part hydrogen peroxide. The silicon wafers were placed in this solution for 5 min, removed and rinsed with deionized water. The wafers were then dried on a hot plate for 1–2 min at a temperature
Fig. 1. Cross-section of the sample.
between 120 and 150 C. A plasma enhanced chemical vapor deposition (PECVD) process was then used to deposit a 750 nm thick layer of silicon nitride. The PECVD reactor was a Plasmatherm 790 system. A polyimide precursor made by HD Microsystems with the commercial name PI-2545 was spin coated onto the nitride surface with the use of a Headway spin coater revolving at 2000 rpm for 30 s. The spin-coated resin was cured at 135 C for 2 h in a heating furnace to obtain a structurally stable film of thickness 2l, as measured with a Tencor Alpha-Step 200 profilometer. The cross-section of a typical sample is shown in Fig. 1. The samples were divided into three groups. Samples in Group 1 were kept at 38 C for 48 h at different relative humidity conditions ranging from 50% to 70% so as to study the effect of relative humidity on the interface strength. Group 2 samples were kept at 38 C and 60% relative humidity for exposure times ranging from 24 to 96 h. Finally, Group 3 samples were kept at 60% relative humidity for 24 h at temperatures ranging from 30 to 60 C. The amount of water trapped in the samples was measured using the FTIR technique. The same samples were subsequently tested for interface strength using the laser spallation technique. Both these measurement techniques are briefly discussed next.
3. Experimental procedures 3.1. Quantification of water content in samples The water content in all samples was quantified using standard FTIR spectroscopy. The IR beam was impinged from the polyimide side normal to the nitride/ polyimide interface, and the transmitted beam through the Si was analyzed. An unconditioned sample was used for obtaining the background spectrum. The amount of moisture present in a sample is directly proportional to the area under the water peak of the FTIR spectra. The basic relationship between IR intensity and concentration is given by the Beer–Lambert law, At = c Æ t Æ et, where At is the area under the peak, c is the concentration, t is the path length, and et = 1 · 105 cm2 mol1 L is the integrated molar absorption coefficient of water [3]. Fig. 2 shows a typical FTIR spectrum, which indicates the presence of water by the peak at 3337 cm1.
A. Jain et al. / Acta Materialia 53 (2005) 3147–3153
Fig. 2. A typical FTIR spectrum showing the presence of water absorption peak corresponding to 3337 cm1.
Fig. 3. FTIR water absorption peaks obtained from samples treated with varying preconditioning treatments. The area under each curve is proportional to the amount of water absorbed in the sample.
Fig. 3 shows the portion of the FTIR spectra with the water peak for Group II samples. The concentration of the moisture is directly proportional to the area under the curve and is calculated using the Beer–Lambert law. Fig. 3 clearly shows that the amount of water present in the samples is a direct function of the preconditioning treatment. 3.2. Measurement of interface strength The interface strength of all the samples was measured at room temperature using a laser spallation technique [4,8]. In this experiment, a 2.5 ns long Nd:YAG laser pulse is made to impinge over a 3 mm diameter area on a 0.5 lm thick aluminum film that is sandwiched between the back surface of a substrate disc (12–25 mm diameter and 1 mm thick) and a 10–20 lm thick layer of SiO2. The melting-induced expansion of aluminum under confinement generates a compressive stress pulse (with 1 ns rise time) directed towards the test coating that is deposited on the substrates front surface. The
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compression stress pulse reflects into a tensile wave from the coatings free surface and leads to its spallation (complete removal) at a sufficiently high amplitude. The incident laser fluence is controlled by changing the energy of the laser beam while keeping the illuminated area constant. Also, if spallation does not occur, the laser beam is focused onto a new spot in order to avoid fatigue effects. The interface tensile stress is obtained by measuring the transient displacement history of the coatings free surface (induced during pulse reflection) by using an optical interferometer. Besides having a resolution of only 0.2 ns in the single shot mode, the interferometer is capable of recording fringes from even optically rough surfaces. For a coating of density q and thickness h, the interface stress r is calculable from the measured transient velocity v(t) as: 1 h h rðh; tÞ ¼ qc v t þ v t ; ð1Þ 2 c c where c is the longitudinal stress wave velocity in the film. Interface strengths in a variety of systems, including those between metal, ceramic, and polymeric coatings and engineering substrates, have been measured. The spallation technique was preferred over the fourpoint bend [9], blister [10], double cantilever beam [11], and indentation [10] tests. Since interface decohesion is accomplished at an ultra high strain rate in the spallation test, all inelastic deformations within the separating components are essentially suppressed and, as shown earlier [5–8], the measured strengths are intrinsic to the interfacial microstructure, inclusive of defects, if any. While applying the spallation technique to characterize the multilayer system supported on a silicon chip, it was found that sufficient tensile stress needed for decohering interfaces could be generated without the use of the aluminum layer. So no Al layer was used in the present study. However, it was still necessary to cover the Si bare surface with the layer of waterglass of thickness 40–50 lm to generate sufficient stress amplitudes. The laser-heating diameter was kept at 3 mm to ensure one dimensionality of the stress wave over 95% of the area [12]. Since optical interferometery could not be performed directly on the bare polyimide surface because of its transparency, the stress in the samples was quantified by a three-part procedure. In the first step the critical laser fluence at which interface separation was observed was recorded. In the next step, the stress pulse in a bare Si wafer (without any test Si3N4 and polyimide test layers) bearing the exact same thickness, orientation, and waterglass layer thickness was recorded at the laser fluence at which interface decohesion was observed in the first step. The stress pulse was recorded using interferometery and following the procedures used in the basic spallation technique discussed above. Finally, this stress profile was used as an input to a finite element acoustic wave model, which calculates the tensile stress history at
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Table 1 Material properties used as input in the wave mechanics simulation Property
Si3N4
Polyimide
Si
Thickness (lm) Density (kg/m3) k (GPa) l (GPa)
0.75 3000 55.4 65
2.0 1400 3.5 0.87
750 2300 75 50
any interface in a given multilayer assembly when loaded normally by a propagating stress wave from the silicon side. Other inputs to the program include the thickness, density, and Lame constants, k and l, for each layer. Table 1 summarizes the properties used for the simulation used in this work. The same procedure was repeated for each sample.
4. Results and discussion It is expected that the moisture content at the polyimide/substrate interface will influence the interfacial strength. However, the FTIR spectroscopy study provides the integrated water concentration through the thickness of the polyimide film. Thus, it is first necessary to understand the correlation between the total water content in the polyimide film to the interfacial concentration. 4.1. Modeling moisture diffusion in polyimide The incorporation of moisture into the polyimide film can be modeled as two sequential kinetic steps; i.e., (i) moisture adsorption at the surface, and (ii) diffusion of the adsorbed moisture into the bulk of the polyimide film. It is always instructive to see if any one of these processes is kinetically dominant since the two steps lead to different time dependent rate laws. There have been several studies of the diffusion in polyimide [13–16]. These studies report the diffusion coefficient of moisture in polyimide, D, to be of the order of 109 cm2/s at room temperature (25 C). Taking the smallest exposure time, t, of 24 h in this study, the diffusion distance, x, estimated as (Dt)1/2 is 294 lm. It should be noted that since all the exposure temperatures in this study is greater than room temperature, the diffusion distances in all cases used in this study will be higher than this estimate. Since the thickness of the polyimide film, h, is 2 lm, the assumption h x is valid for all cases indicating that the diffusion profile in the film will always be flat at the end of every exposure when the interface strength is measured, and that the kinetics of surface adsorption is rate controlling since the moisture content is changing with exposure conditions indicating that saturation has not occurred. This flat profile mandates that the surface moisture content in the film, CS, is also the interface moisture content in the film, CI,
and that the measured integrated moisture content of the film is CSh. The kinetics of the surface adsorption process controlled by a flux of moisture from the gas phase to the solid, JS, defined as [17]: J S ¼ kðC 0 C S Þ;
ð2Þ
where k is the surface exchange coefficient, and C0 is the surface concentration of moisture in equilibrium with the moisture in the gas phase. This boundary condition leads to the well-known solution [17]: kt t C S ¼ C 1 ¼ C 0 1 e h ¼ C 0 1 e s : ð3Þ The time constant, s = h/k, relates to the time required for the surface concentration CS to reach the equilibrium value of C0. From the moisture adsorption data of the Group 2 samples where the total moisture adsorbed as a function of time is recorded at a constant temperature and relative humidity, it is possible to calculate the value of C0 and s using a best fit of Eq. (3) to the data. It should be noted that C0 is a function of the gas phase composition (related to % relative humidity) and temperature, while s (and therefore k) is a function of temperature alone. Fig. 4 shows such a best fit, for C0 = 0.0143 g/cm3 and s = 2.8 · 105 s at 38 C and 60% relative humidity. The dependence of Eq. (3) on the relative humidity and temperature can be calculated in the following manner. Since C0 corresponds to the surface concentration of moisture in the polymer in equilibrium with the moisture-containing atmosphere, the activity of moisture in the gas and solid phases should be equal. This implies that: cW X W ¼ pW ¼ pW ðsatÞ
%RH ; 100
ð4Þ
where cW is the activity coefficient of the dilute moisture–polymer solid solution (Henrys Law) as the poly-
Fig. 4. Variation of interface moisture content with exposure time for Group 2 samples at 38 C and 60% relative humidity. The line shows the best fit of Eq. (3) to the data.
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mer surface, XW is the mole fraction of moisture in the solid solution, pW is the partial pressure of moisture in the vapor phase, and pw(sat) is the saturated vapor pressure of moisture in the gas phase at the given temperature. The saturated partial pressure of water vapor as a function of temperature T is well known and can be expressed as [18]: log pW ðsatÞðatmÞ ¼ 3075=T 5:465 log T þ 0:279 103 T þ 862=T 2 þ 22:19: ð5Þ The mole fraction of moisture in the solid solution can be expressed as: XW ¼
C 0 MW P ; MW W C P
Fig. 6. Plot of the interface moisture concentration of Group 3 samples as a function of temperature after exposure for 24 h at 60% relative humidity.
ð6Þ
where MW and C stand for the molecular weight and concentration, and the subscripts W and P stand for the moisture and polymer phases, respectively. Assuming that the moisture and polymer solid solution follows a regular solution model, the temperature dependence of the moisture activity coefficient can be expressed as: 2
RT ln cW ¼ að1 X W Þ ;
ð7Þ
where a is the enthalpy of mixing of the two components. Since the concentration of moisture in the polymer surface is very small, XW 1, giving: a cW ¼ eRT : ð8Þ Similarly, the surface exchange coefficient k is a thermally activated kinetic constant which has an Arrhenius dependence of T given by: b
k ¼ k 0 eRT : Combining Eqs. (3)–(9) gives:
! b tk eRT MW W %RH a RT 0h P W ðsatÞe CI ¼ CP 1e : MW P 100
ð9Þ
ð10Þ
Eq. (10) predicts that at a constant T and exposure time t, CI (and thus the total moisture content) is propor-
tional to the percentage relative humidity. The excellent linear fit in Fig. 5 confirms this hypothesis for the Group 1 samples which were all exposed for 48 h at 38 C at different relative humidity conditions. For the Group 3 samples, the moisture absorption was measured as a function of temperature at fixed values of t and % relative humidity. The best fit of the experimental data to Eq. (10) gives a = 2.6 · 104 J/ mol K, b = 1.82 · 104 J/mol K, and k0 = 8.1 · 107 cm/s. These constants lead to some interesting physical insights. The negative value of a implies that the enthalpy of mixing of the moisture and polymer is negative, making water absorption by polyimide an exothermic process consistent with the literature [19]. Furthermore, the negative value of a implies that cW increases with temperature leading to a decrease in XW at a given moisture activity in the gas phase. Thus, the cW term predicts a decrease in moisture adsorption with increasing temperature. This is counterbalanced by an increase in pW(sat) and k with T, both of which predict an increase in the moisture concentration with temperature. The Group 3 samples show that under the conditions studied, the latter effect was more dominant than the former, leading to an increase in the moisture content with increasing temperature as seen in Fig. 6. 4.2. Interface moisture content/strength correlations
Fig. 5. The effect of relative humidity on interface moisture content for Group 1 samples exposed to 38 C and 48 h.
Since, both Group 1 and 2 samples were exposed to the same temperature (38 C), it is instructive to plot the interfacial strength of all Group 1 and 2 samples as a function of interface moisture concentration. Fig. 7 is such a plot, showing that at 38 C, the interface strength decreases linearly with temperature. A critical concentration, Ccrit, having a value of 1.49 · 102 g/cm3 can be identified as the interface concentration at which the strength is reduced to zero. The chemical structure of the as-spun polyimide precursor in Fig. 8 shows two open carbon rings. Since nitrogen in the Si3N4 has a lone pair of electrons, it can form hydrogen bonds at one of
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Fig. 7. The effect of moisture content on the interface strength for samples in Groups 1 and 2, which were all exposed at 38 C. A linear fit identifies Ccrit, an interface moisture concentration at which the strength will be reduced to zero.
the open ring sites of the precursor molecules, if available via direct contact. The degrading effect of increasing relative humidity content and hold times is a direct result of the competition between the water molecules that segregate the interfacial region and the nitrogen of the nitride in forming the hydrogen bonds with the polyimide precursor. Since hydrogen bonds are fairly weak they are constantly breaking and reforming even at ambient temperature. During baking, ring closure occurs on account of dehydrolysis and results in the formation of the stable polyimide molecule, as shown in Fig. 8. This dehydrolysis leads to a reduction of the number of hydrogen bonds between the Si3N4 and polyimide, leading to an irreversible detriment in interfacial strength with high temperature exposure, which is in addition to the effect of moisture segregation at the interface. Since dehydrolysis is a thermally activated process, the degradation in interfacial strength is expected to exhibit an Arrhenius dependence on temperature with an activation energy
Fig. 8. Proposed strength degradation mechanism at the silicon nitride/polyimide interface.
Fig. 9. Plot of the measured interface strength versus calculated interface strength using Eq. (11). The 45 line would match an exact fit between the measured and calculated strengths.
Q. Combining this temperature dependence with the experimentally measured linear interfacial concentration dependence (Fig. 7) of interfacial strength, S, gives a general expression: Q
S ¼ AðC crit C 1 ÞeRT :
ð11Þ
It should be noted that CI implicitly represents the exposure time and relative humidity in this expression. A best fit of Eq. (11) for all the samples in Groups 1, 2 and 3 shows A = 1384 MPa and Q = 7300 J/mol K. Fig. 9 shows a plot of experimental value of the interface strength S versus the calculated value of S based on Eq. (11). The figure shows the data points are reasonably close to the 45 line, which represents a perfect fit to the experimental data.
5. Conclusions Polyimide films on silicon nitride substrates were exposed to moisture under varying conditions of relative humidity, time and temperature. The moisture content of the films was measured by FTIR spectroscopy, and the polyimide/silicon nitride interface strength was measured at room temperature by a laser spallation technique. The moisture adsorption by polyimide films was analyzed using a diffusion model. Under the experimental conditions of this study, it was found that the rate of moisture adsorption was controlled the surface exchange reaction. For samples exposed at 38 C, the interface strength was found to decrease linearly with increasing interface moisture concentration. A critical interface moisture concentration was identified, where the strength is expected to go to zero. The interface strengths of all the measured samples were combined into one empirical equation (Eq. (11)), which can be used as a basis to construct strength charts as a function of exposure conditions.
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Such strength charts should help in the development of more rational standards for handling packaged ICs during manufacturing and integration from the viewpoint of avoiding moisture-related failures. The current practice in industry for controlling moisture induced failures in IC packages is to dry the package by ‘‘baking’’ and to ship them to the assembly plants in desiccant bags [2]. After opening the bags, the IC components must be assembled within a specified (relatively short) time to avoid moisture-induced damages [7]. The standards relate to a factory environment of 30 C/60% relative humidity. If the environmental conditions at the factory are maintained at a lower temperature, the standards default to the same exposure times as in the case of 30 C/60% relative humidity. This specification does not justify the high cost of handling the IC packages. The strength charts, when used in conjunction with moisture transport models [8,9], can provide standards and schemes to properly de-rate (extrapolate) specifications for lower humidity conditions. Certainly, if the strength charts for key interfaces are available, standards for new and ever emerging ICs and packaging technologies can be developed without having to perform a new set of accelerated empirical tests, thus giving a significant cost saving.
Acknowledgments The work was supported by a NSF Grant No. ECS0000334, and a US Army Grant No. DAAD19-00-1049, for which we are grateful to Dr. David Stepp of that agency. The authors thank Professor Hicks and
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Steve Babayan of UCLA for giving access to the FTIR spectrometer, and Osman Anli at Boston University for his help with the simulations used in the diffusion and strength models.
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