JOURNAL OF ELECTRONIC MATERIALS
	ABSTRACTS Volume 26, Number 7, July 1997 This Month Featuring: Proceedings of the Symposium on Creep and Stress Relaxation in Miniature Structures, Regular Issue Papers, and the 1997 Electronic Materials Conference Program. View July 1997 Table of Contents.

SPECIAL ISSUE PAPERS

Harish D. Merchant, Gould Electronics, Eastlake, OH
Todd S. Gross, University of New Hampshire, Durham, NH
Proceedings and Special Issue Editors

Microstructure Coarsening During Thermo-Mechanical Fatigue of Pb-Sn Solder Joints
P.L. HACKE, A.F. SPRECHER, and H. CONRAD
Materials Science and Engineering Department, North Carolina State University, Raleigh, NC 27695-7907

KEY WORDS Coarsening, cracked area, phase size, plastic deformation kinetics, stress exponent, stress relaxation

Phase coarsening in as-cast solder joints produced by isothermal annealing and by thermo-mechanical cycling (TMC) (-30 to 130°C) was determined and its influence on the plastic deformation kinetics of 63Sn37Pb solder joints was investigated by stress relaxation (SR) tests. The static, isothermal coarsening kinetics were in accord with Senkov-Myshlyaev's theory. The rate of coarsening by TMC was appreciably greater than that by isothermal annealing. The SR curves following TMC exhibited two regions with a stress exponent n > 5 at high stresses (Region A) and n ~ 2-3 at low stresses (Region B). In both regions, the stress exponent varied in an irregular manner with the mean phase size. In Region A the plastic strain rate at a constant value of ⁿ ( = shear stress) was relatively independent of the average phase size, while in Region B the strain rate increased with increase in phase size. The activation energy in both regions appeared to be that for lattice diffusion. Possible reasons for the anomalous effects of phase size on the plastic deformation kinetics are offered.

Creep, Stress Relaxation, and Plastic Deformation in Sn-Ag and Sn-Zn Eutectic Solders
H. MAVOORI, J. CHIN, S. VAYNMAN, B. MORAN, L. KEER, and M. FINE
Northwestern University, Evanston IL 60208

KEY WORDS Creep, constitutive modeling, Sn-Ag eutectic, Sn-Zn eutectic, solders, stress relaxation

Because of the high homologous operation temperature of solders used in electronic devices, time and temperature dependent relaxation and creep processes affect their mechanical behavior. In this paper, two eutectic lead-free solders (96.5Sn-3.5Ag and 91Sn-9Zn) are investigated for their creep and stress relaxation behavior. The creep tests were done in load-control with initial stresses in the range of 10-22 MPa at two temperatures, 25 and 80°C. The stress relaxation tests were performed under constant-strain conditions with strains in the range of 0.3-2.4% and at 25 and 80°C. Since creep/relaxation processes are active even during monotonic tensile tests at ambient temperatures, stress-strain curves at different temperatures and strain rates provide insight into these processes. Activation energies obtained from the monotonic tensile, stress relaxation, and creep tests are compared and discussed in light of the governing mechanisms. These data along with creep exponents, strain rate sensitivities and damage mechanisms are useful for aiding the modeling of solder interconnects for reliability and lifetime prediction. Constitutive modeling for creep and stress relaxation behavior was done using a formulation based on unified creep plasticity theory which has been previously employed in the modeling of high temperature superalloys with satisfactory results.

Interfacial Sliding in Cu/Ta/Polyimide High Density Interconnects as a Result of Thermal Cycling
D.V. ZHMURKIN,¹ T.S. GROSS,¹, and L.P. BUCHWALTER²
1--Department of Mechanical Engineering, University of New Hampshire Durham, NH 03824. 2--IBM-Thomas J. Watson Research Laboratory, Yorktown Heights, NY 10598

KEY WORDS Atomic force microscope, interconnects, polyimide, stress relaxation, thermal deformation

A scanning probe microscope was used to observe thermally induced deformation of 1 µm thick Cu/Ta/polyimide test structures on Si. Relative height changes in arrays of parallel Cu and polyimide lines of various aspect ratios were examined in air at room temperature before and after a 25-350-25°C thermal cycle conducted in gettered nitrogen. Grain elevation and hillock formation at grain boundaries were observed on the Cu surface as a result of the thermal cycling. It was also observed that significant sliding occurs at the Cu/Ta interface with 1 µm wide Cu lines. Less or no sliding was observed at the interface with 10 µm wide Cu lines.

Hygrothermal Effects upon Stress Relaxation in a Polyimide Film
BRIAN D. HARPER, JAYATHIRTHA M. RAO, VERNAL H. KENNER, and CARL H. POPELAR
The Ohio State University, Department of Aerospace Engineering, Applied Mechanics and Aviation Applied Mechanics Section, 155 W. Woodruff Ave., Columbus, OH 43210

KEY WORDS Film, hygrothermal, moisture, polyimide, stress relaxation, viscoelastic

Polyimide films are being used extensively in first and second level microelectronic packages. Despite their many advantages, the tendency of polyimide films to absorb moisture can pose serious reliability problems such as corrosion, internal shorts, delamination, loss in dimensional stability, and a reduction in mechanical performance. This paper presents the results of an investigation of the effects of moisture upon stress relaxation in Kapton^® polyimide films. Three relaxation frames were designed and built so that they could be placed side by side within an environmental chamber which controlled both temperature and relative humidity. Stress relaxation experiments were then conducted in triplicate at various combinations of temperature and relative humidity and the relaxation modulus was determined as a function of time, temperature, and moisture concentration. While the relaxation modulus was found to decrease with increasing temperature and moisture concentration, this plasticizing effect could not be described by the simplifying assumption of time-temperature-moisture superposition except for those results obtained at or below 50% relative humidity. Application of the time-temperature superposition principle to the results obtained at relative humidities higher than 50% produced several distinct master curves at constant moisture concentration. These curves had fundamentally different shapes which could not be superimposed by rigid horizontal shifting.

Thickness Dependent Mechanical Behavior of Submicron Aluminum Films
YOUNG-SEOK KANG and PAUL S. HO
Center for Materials Science and Engineering, The University of Texas at Austin, PRC/MER Mail Code R8650, Austin, TX 78712

KEY WORDS Dislocations, mechanical properties, thin films, yield strength

Mechanical behavior of thin metallic films has been investigated on aluminum films deposited on a flexible polyimide substrate. Aluminum thin films exhibit a higher tensile strength than bulk aluminum. As film thickness decreases from 480 to 60 nm tensile strength increases from 196 to 408 MPa. These mechanical behaviors are correlated with the microstructure and its evolution with the thickness of aluminum thin films. Films are consisted of fine columnar grains and average grain size increases monotonically with the film thickness. The volume fraction of (111)-textured grains increases and the dispersion of texture axis becomes narrow as the film thickens. The relative contributions of the film thickness, grain size, and texture to the strength of aluminum thin films are estimated using an empirical strengthening model. The result indicates that the high strength of aluminum thin films is due largely to their small grain size, followed by the strengthening due to the film thickness and texture.

Finite Element Modeling of Thermal Fatigue and Damage of Solder Joints in a Ceramic Ball Grid Array Package
BOR ZEN HONG
IBM Microelectronics Division, Hopewell Junction, NY 12533

KEY WORDS Ceramic ball grid array (CBGA), finite element, ratcheting, solder joint, thermal fatigue, viscoplastic

A nonlinear finite element model is presented for analyzing the cyclic and thermal fatigue loading and for viscoplastic damage characterization of the lead-tin (Pb-Sn) solder joints in a ceramic ball grid array (CBGA) surface mount package. An approach using a ⁱⁿ_eq-modified Coffin-Manson equation is proposed to estimate the fatigue life of the solder joints. The ⁱⁿ_eq represents a saturated equivalent inelastic strain range as determined by the finite element model. The present study shows that the predictied fatigue life and the associated damage mechanism of the solder joint agree reasonably well with the test data for the 18, 25, and 32 mm CBGA packages run at a cyclic temperature load of 0°C/100°C with a frequency of 1.5 cycles per hour. Analysis also shows that a preferred failure site is expected to occur in and around the Pb37-Sn63 solder attachment of the solder joint. A time-dependent (creep induced) damage mechanism is found to be more pronounced than the time-independent (plastic deformation) mechanism.

Stress Relaxation in Molding Compounds
VERNAL H. KENNER, BRIAN D. HARPER, and VLADIMIR Y. ITKIN
Department of Aeronautical Engineering, Applied Mechanics, and Aviation, 155 West Woodruff Avenue, The Ohio State University, Columbus, OH 43210

KEY WORDS Electronic packaging, molding compounds, stress relaxation, viscoelastic properties

Stress relaxation tests have been carried out on an epoxy-based molding compound at temperatures ranging from ambient to 260°C. It is found that horizontal shifting produces coherent master curves which are different for different levels of imposed strain, i.e., the material behavior is nonlinear. The use of constant strain rate tensile test data to augment stress relaxation data in forming the broadest viscoelestic representation of the material is discussed.

Stress Relaxation and Creep in an Impression Test
JAMES C.M. LI
Materials Science Program, Department of Mechanical Engineering, University of Rochester, Rochester, NY 14627

KEY WORDS Creep, dislocation dynamics, impression test, interfacial sliding, LiF, Sn-Pb alloy, solder, stress relaxation, superplasticity

A localized stress relaxation technique is described suitable for testing miniature structures and components. The actual size of the area that can be tested depends on the size of the indenter and the sensitivity of the load measurement device. However, the indenter should not be so small that macroscopic theory of deformation cannot be used. The technique described includes a quantitative treatment of the data which can be handled by a computer. Extensive data collection can be made so that many deformation parameters can be examined by using this simple test.

Stress Relaxation and Creep of 12 to 35 um Copper Foil
H.D. MERCHANT
Gould Electronics, Eastlake, OH 44095

KEY WORDS Copper, creep, electrodeposit, foil, stress relaxation

Stress relaxation and creep of the electrodeposited and rolled copper foils, 12-35 µm thick, are investigated near yield stress and near room temperature. The stress relaxation does not obey a logarithmic time law; the creep appears to follow a power function. These deviations from the expected logarithmic behavior are thought to be caused by very small grain size, unstable non-equilibrium defect structure and extensive micropore population (vacancies and vacancy clusters) typical of the electrodeposit. Relaxation and creep are significantly lower for the rolled (than for the electrodeposited) foil. Decreasing the electrodeposit thickness has an effect of enhancing relaxation and creep, attributable to a limited nucleation on the cathode surface and consequent generation of microvoids between growth clusters in the vicinity of the substrate. The foil thickness effect on creep and stress relaxation is not observed for the rolled foil, which is prone to embrittlement and stiffening at about 323K.

Tensile, Creep, and ABI Tests on Sn5%Sb Solder for Mechanical Property Evaluation
K. LINGA MURTY,¹ FAHMY M. HAGGAG,² and RAO K. MAHIDHARA³
1--North Carolina State University, Raleigh, NC 27695-7909. 2--Advanced Technology Corporation, 661 Emory Valley Road, Suite A, Oak Ridge, TN 37830. 3--Currently with Tessera Inc., 3099 Orchard Dr., San Jose, CA 95134

KEY WORDS Chemical diffusion, creep, deformation mechanisms, dislocation glide, dislocation pipe diffusion, grain-boundary diffusion, lead-free, self-diffusion, solder, tensile properties

Sn5%Sb is one of the materials considered for replacing lead containing alloys for soldering in electronic packaging. We evaluated the tensile properties of the bulk material at varied strain-rates and temperatures (to 473K) to determine the underlying deformation mechanisms. Stress exponents of about three and seven were observed at low and high stresses, respectively, and very low activation energies for creep (about 16.7 and 37.7 kJ/mole) were noted. A maximum ductility of about 350% was noted at ambient temperature. Creep tests performed in the same temperature regime also showed two distinct regions, albeit with slightly different exponents (three and five) and activation energy (about 54.4 kJ/mole). Ball indentation tests were performed on the shoulder portions of the creep samples (prior to creep tests) using a Stress-Strain Microprobe (Advanced Technology Corporation) at varied indentation rates (strain-rates). The automated ball indentation (ABI) data were at relatively high strain-rates; however, they were in excellent agreement with creep data, while both these results deviated from the tensile test data. Work is planned to perform creep at high stresses at ambient and extend ABI tests to elevated temperatures.

The Short and Long Term Properties of a Liquid Crystalline Polymer at Elevated Temperatures: Characterization and Modeling
A. SAIGAL,¹ V. DIFILIPPO,¹ and M.A. ZIMMERMAN²
1--Department of Mechanical Engineering, Tufts University, Medford, MA 02155. 2--Lucent Technologies, 1600 Osgood Street, North Andover, MA 01845

KEY WORDS Arrhenius equation, creep, liquid crystalline polymer, stress dependence, tensile properties, Xydar G930

Tensile and short term (24 h) creep tests were performed on Xydar G930, a liquid crystalline polymer (LCP) with 30 wt.% glass filler, at temperatures and stress levels ranging from room temperature to 175°C and 0.3 fraction ultimate tensile strength (UTS) to 0.8 fraction UTS, respectively. Temperature was found to have an affect on the short term tensile properties. The resulting strain vs time creep curves showed the expected dependence of creep strain on temperature and stress level. Creep compliance curves were derived from the creep curves and showed distinctively nonlinear viscoelastic behavior at all stress levels and temperatures. Creep compliance was found to follow a power law in time. The power law was used to model the stress dependence of creep and the Arrhenius equation was employed to model the temperature dependence up to 120°C. A significant reduction in creep resistance was observed at 175°C. Time-temperature-stress-superposition was used to show that the material followed power law behavior up to 1000 h.

Micro-Indentation Relaxation Measurements in Polymer Thin Films
D.M. SHINOZAKI and Y. LU
Department of Mechanical and Materials Engineering, The University of Western Ontario, London, Ontario, Canada N6A 5B9

KEY WORDS Dynamic mechanical test, microindentation, polymer films, polystyrene, viscoelastic

A micro-indenter consisting of a piezo-electric driven flat cylindrical punch has been used to measure the dynamic mechanical properties of polystyrene films as thin as 50 µm. The measured viscoelastic response was sensitive to the bonding of the polystyrene to an underlying silicon substrate for films which were thinner than one indenter diameter. The instrument therefore was shown to have practical use in measuring the dynamic mechanical response of polymer films, and the strength of bonding between disparate materials.

Impression Recovery of Amorphous Polymers
FUQIAN YANG, SHENGLIANG ZHANG, and JAMES C.M. LI
Materials Science Program, Department of Mechanical Engineering, University of Rochester, Rochester, NY 14627

KEY WORDS Creep, dimensional recovery, dimensional stability, impression recovery, indentation recovery, PC, PMMA, polymer

The impression made by a loaded cylindrical flat-end punch on a polymer surface was observed to disappear with time at the same temperature after the load was removed. The depth-time relation appeared to obey second order kinetics. The temperature dependence of the rate constant shows two consecutive processes with activation energies, 906 kJ/mole (between 143 and 147°C) and 91 kJ/mole (between 150 and 160°C) for PC (Tg is at about 146°C) and 440 kJ/mole (between 104 and 110°C) and 95 kJ/mole (between 115 and 140°C) for PMMA (Tg is at about 112°C). These activation energies are probably associated with the motion and annihilation of conformational or structural defects of opposite signs. This kind of shape memory phenomena may be general for all amorphous polymers.

REGULAR ISSUE PAPERS

KEY WORDS C-V profile, molecular beam epitaxy (MBE), modeling, pseudomorphic high electron mobility transistor (p-HEMT)

The effect of parasitic series resistances on electrochemical capacitance-voltage (EC-V) profiling is simulated numerically for Al_xGa_1-xAs/In_yGa_1-yAs pseudomorphic high electron mobility transistor (p-HEMT) structures. The actual EC-V measurement is simulated numerically by reconstructing the charge distribution from an intrinsic distribution calculated from a self-consistent k . p model. The calculated charge distribution then forms the basis for examining the possible profiles an EC-V measurement would produce when parasitics are taken into account. From a simple lumped-circuit model, it is shown that parasitic resistances distort the shape of the charge distribution by changing the relative heights of the -layer and channel charges. Hysteresis effects may also occur and may be accompanied by a change in material type from n to p even though the material is known to be n-type over the entire range of measurement. Additionally, an undesirable dependence of the charge profile on the probe signal frequency is found.

Strain Relaxation in Silicon Ion Molecular Beam Epitaxy on Silicon (001) Substrates During Aging
P. ROZENAK
Practical Engineering College of Beer Sheva, Department of Mechanical Engineering, Ben Gurion University of the Negev, P.O. Box 45, Beer Sheva 84100, Israel

KEY WORDS Molecular beam epitaxy (MBE), Si, strained epitaxial films, transmission electron microscopy (TEM), x-ray diffraction (XRD)

Silicon strained epitaxial films were grown on Si (001) substrates by low energy ion beam assisted molecular beam epitaxy. Films grown in the range of 450-550°C with concurrent Ar⁺ ion bombardment (100 eV) were characterized using x-ray diffraction and transmission electron microscopy and found to be dislocation free and ununiformly strained. During aging, the strained layers stay stable until 500°C. Relaxation of most of the strain occurred at temperatures of 500-650°C. At higher aging temperatures, the strained layers relaxed by the formation of dense dislocation structures.

Material Properties of Pb_1-xSn_xSe Epilayers on Si and Their Correlation with the Performance of Infrared Photodiodes
A. FACH, J. JOHN, P. MüLLER, C. PAGLINO, and H. ZOGG
Thin Film Physics Group, Institute of Quantum Electronics, Swiss Federal Institute of Technology, ETH-Teil Technopark CH-8005 Zürich, Switzerland

KEY WORDS Buffer layer, CaF2, dislocation densities, PbSnSe, Si substrates

Hall mobilities and resistance area products R_oA of infrared diodes in epitaxial Pb_1-xSn_xSe layers on CaF₂ covered Si(111) substrates were correlated with threading dislocation densities . The low temperature saturation Hall mobilities were entirely determined by and proportional to their mean spacing 1/ . For the photodiodes, the R_oA values at low temperatures were inversely proportional to . A model where each dislocation in the active area of the diodes causes a shunt resistance correctly describes the results, the value of this resistance for a single dislocation is 1.2 G for PbSe at 85K. The dislocation densities were in the 2 x 10⁷ to 5 x 10⁸ cm^-2 range for the 3-4 µm thick as-grown layers. Higher R_oA values are obtainable by lowering these densities by thermal annealing, which sweeps the threading ends of the misfit dislocations to the edges of the sample.

Microwave Modulated Photoluminescence in Doped GaAs
C.E. INGLEFIELD,¹ M.C. DE LONG,¹ P.C. TAYLOR,¹ and W.A. HARRISON²
1--Department of Physics, University of Utah, Salt Lake City, UT 84112. 2--Department of Applied Physics, Stanford University, Stanford, CA 94305

KEY WORDS Carrier thermalization, doping, gallium arsenide, microwave modulation, photoluminescence

Microwave modulated photoluminescence (MMPL) is a spectroscopy wherein a sample is placed in the electric field maximum of a microwave cavity and is simultaneously subjected to continuous optical pumping and chopped microwave electric fields. In this work, MMPL has been performed on GaAs epilayers with n- and p-type doping ranging from 10¹⁵ to 10²⁰ cm^-3. The MMPL spectra can be experimentally separated into two categories, fast and slow. The interaction is predominately faster in the lightly doped samples and slower in the heavily doped samples. These observations are consistent with a more-or-less constant rate of lattice heating per free carrier through free carrier energy relaxation. The magnitude of the temperature modulation through free carrier thermalization with the lattice, and hence the amplitude of the slower component of the MMPL, can be explained in terms of theoretical estimates for the rate of energy loss of the accelerated free carriers.

Activation Energies of Intermetallic Growth of Sn-Ag Eutectic Solder on Copper Substrates
D.R. FLANDERS, E.G. JACOBS, and R.F. PINIZZOTTO
Materials Science Department, University of North Texas, P.O. Box 5308, Denton, TX 76203-0308

KEY WORDS Activation energy, Cu6Sn5, Cu3Sn, diffusion kinetics, intermetallic growth, Pb-free solder

Intermetallic phases formed along a Sn-Ag eutectic solder/Cu interface during solid-state aging have been characterized and the activation energies of Cu₃Sn and Cu₆Sn₅ growth have been calculated. Diffusion couples consisting of Cu/96.5Sn-3.5Ag/Cu were aged at 110 to 208°C for 0 to 32 days. After aging, the Cu/solder interfaces were examined using scanning electron microscopy and energy dispersive x-ray spectroscopy. The growth rate constants for each intermetallic layer were calculated assuming a simple parabolic diffusion-controlled growth model. The activation energy for Cu₃Sn growth is 0.73 eV/atom and the activation energy for Cu₆Sn₅ growth is 1.11 eV/atom.