Theoretical and experimental studies on the measurement of the mechanical constitutive relation and the interfacial strength for thin polymeric film structures are presented. For the measurement of the constitutive relation, a new innovative testing technique based on the J-integral concept is introduced. This method measures the stressstrain relationship in the thickness direction, while the stress-strain relationship in the lateral direction can be measured with conventional techniques. For the measurement of interface strength the cut test and the ball rolling test were used to construct the failure criteria, based on the interface fracture mechanics concept. The interface between polyimide and glass (SiO2) has been tested and analyzed in detail. This shows that the toughness of the PMDA-ODA polyimide/glass interface was measured as 2.2 J/m2 in pure opening mode and as 15 J/m2 for pure sliding mode. It was also observed that the PMDA-ODA/SiO2 interface toughness is insensitive to temperature variations in the range of 20ºC — 250ºC.

Time dependent deformation of polymers must be considered in studies of deformation and fracture of polymer electronic packaging materials. In composites in which fabrication stresses are stored, the rate of plastic relaxation influences the long term mechanical stability. The practical need to predict mechanical performance of these materials from simple mechanical tests is the motivating factor in attempting to relate fundamental mechanisms of deformation to the stress relaxation behaviour. A review of some approaches based on activated rate processes and their application to some preliminary experiments onpolyimide is reported.

Copper with its high conductivity, specific heat and melting point (compared to Al), is being investigated as the interconnection metal for applications both on and off the chip. Such interconnection wirings will be seperated by the dielectric layers which could be either polymers or inorganic oxides like SiO2. In such applications an adhesion promoter, which may also work as a diffusion barrier, maybe used between the dielectric and the metal film. An investigation of the diffusion and interaction of Copper with such dielectrics and insulators has been carried out in the temperature range of 200 – 500 ºC. Specifically, interactions of Copper with SiO2'P—glass, Polyimidesiloxane and Magnesium are investigated. Results of these studies will be presented and discussed.

The fracture behavior of the Au(Cr)/PMDA-ODA interface was investigated using a stretch deformation method. The study has been focused on thc effects of metal dimension on the delamination behavior, the fracture morphology, and energy dissipation rate of the interface. The delamination behavior shows complex geometrical dependence, with changes observed in the magnitude of strain at failure as well as the progression of the fracture process. The fracture morphology and energy dissipation rate also show apparent dependence on the metal dimensions. The crack growth, as examined by the rate of energy dissipation, seems to consist of thrce stages: crack initiation, crack growth and post growth. Only the growth region exhibits a constant dissipation rate and the fracture energy determined in this stage is in good agreement with that obtained from energy balance. These results can be attributed to the change in the mechanical coupling between the metal and the polymer layers, which was evaluated using a finite element analysis with plastic deformation taken into aiccount.

Impedance spectroscopy has been used to study the electric properties of screen—printed polymer thick film (PTF) resistors over the frequency range 10 Hz — 500 MHz. With a suitable test pattern it was possible to separate the impedance response of the resistor/conductor interface from that of the resistive element itself. This proved to be useful when analysing, whether the manufacturing parameters have more influence on the properties of the interface or the bulk resistor. The main varied manufacturing parameters were curing method, curing temperature and the resistor/conductor interface (pure copper vs. silver treated copper). Furthermore, the test components were kept for 600 hours in a humid environment (85ºC / 85% RH), and after this the impedance measurements were repeated. Analysis of the effects of these manufacturing and environmental conditions on the electrical properties of PTF resistors are presented.

Subcritical crack growth at polymer/glass interfaces can occur due to a stress dependent chemical reaction at the crack tip at a crack driving force less than the critical fracture energy for catastrophic crack propagation. A four—point flexure apparatus coupled with an inverted microscope allows observation in situ of the subcritical crack growth at the polymer/glass interface. The specimen consists of soda—lime glass plates bonded together with epoxy acrylate. In the four—point flexure test, the strain energy release rate is independent of the interfacial crack length. This test technique allows for subcritical crack growth at the polymer/glass interface to be measured as a function of the applied crack driving force and relative humidity. The results are discussed in terms of a fracture mechanics model.

We present infrared spectroscopy as a means of characterizing polyimide structural changes occurring by RF argon ion sputtering. Samples of PMDA-ODA polyimide on chromium coated substrates have been sputter etched from initial thicknesses of 400 Å down to 10 Å. Relative intensity changes in imide vibrational absorption bands have been interpreted in terms of orientational reordering which occurs during the ion sputtering process. The appearance of a new vibration at 1580 cm–1 in the spectra of samples etched below 50 Å is assigned as the Elu mode of graphite corresponding to a surface damage layer which is of a graphitic form.

When metals are deposited on polymers at low rates and elevated temperatures extended and non—uniform interfaces may form, the structural investigation of which requiresspecial techniques. In this paper the morphology of the silver—PMDA—ODA boundary formed during vapor deposition of the metal at 360 ºC has been investigated by means of cross—section TEM and a radiotracer technique in combination with ion—beam sputtering (IBS) for depth profiling. Ag was found to form clusters at the interface, but nearly spherical Ag particles up to diameters as large as 28 nm were also observed within the bulk of the polymer. The formation of Ag clusters is also reflected in the depth profiles, which, in addition, provide direct evidence for the diffusion o; singel Ag atoms deep into the polyimide. Diffusivities in the range of 10–21 – 10–18 m/s were determined for 300 ºC< T < 400 ºC. The Arrhenius plot is curved in accordance with a free—volume mechanism of diffusion. Cross—sectional TEM and low—energyIBS—depth profiling turned out to be an ideal combination for the investigation of extended metal—polymer interfaces.

Effects of RF sputtering on the hydrothermal stability of Cr/polyimide interfaceshave been studied using FTIR, XPS, and peel test. It has been found that RF sputter-treatment of polyimnide surface prior to metal deposition leads to an enhancement of adhesion through chemical bonding of the metal with polyimide resulting in cohesive polyimide failure. The RF sputter treatment of polyimide, however, simultaneously modifies the polyimide underneath its surface. The adhesion strength of the Cr/polyimide interfaceis degraded significantly upon exposure to a temperature/humidity (T/H) environment. It is suggested this degradation results from the hydrolysis of polyimide. The hydrolysis is facilitated by the presence of modified unstable polyimide near the interface. This degradation of adhesion strength can be minimized by converting the unstablemodified polyimide to a more stable state by re-curing the Cr/polyimide interfacebefore exposure to T/H environment.

Mechanical properties (tensile modulus, tensile strength, elongation at break), thermal properties (T8' CTE, thermo-decomposition temperature, and rate of weight loss) and electrical properties of Problmlde 414 cured films have been determined. The mechanical properties of Probimide 414 thin films are highly dependent on the hard-bake temperature, hard-bake time, and purge gas, but not dependent on the level of the exposure energy or the presence of 1% Irganox 1010 (w/w) as a stabilizer. At a hard-bake temperature of 350ºC and a nitrogen purge rate of 15 SCFH, Probimide 414 films showed excellent retention of the mechanical properties during extended heat treatment.

A method based on the bending beam technique has been developed to measure the mechanical stresses of fine lines confined by a dielectric layer. This method has been employed to determine the thermal stress of AI(2at%Cu) lines passivated by a quartz overlayer between room temperature and 400ºC. The effect of quartz confinement was analyzed by matching the thermal strains at the metal/quartz interfaces and imposing a mechanical equilibrium condition on the structure. The analysis enables us to deduce the triaxial stress components of metal and quartz from measurements of the substrate bending parallel and perpendicular to the length direction of the lines. Results of the measurementshow a substantial stress enhancement as a result of the confinement, with the stress level significantly higher than that of a passivatecd blanket film. The magnitude of the stress depends on the line geometry, the layer deposition conditions and the extent of plastic deformation during thermal cycling. Results of this measurement are consistent with those determined using X-ray techniques.

Corrosion processes affecting electronic devices are chemical and electrochemical in nature. Ionic contaminants, applied voltages, and relative humidity accelerate failure of integrated circuits. Although packaging materials often reduce the corrosion rate of the underlying circuitry, a complete understanding of the mechanism for the basic processes taking place at the coating/metallic circuitry interface is missing.

In this paper the role played by water and ionic contaminants on coated metallic substrates is considered. Transport properties of free standing polyimide membranes have been examined. The corrosion behavior of polyimide/Fe and polyimide/Al systems has been investigated using electrochemical polarization, and a.c. impedance measurements. For the AI/PI system underfilm degradation has been examined using x-ray photoelectron spectroscopy (XPS). Results indicate that the flux of ions is very small compared to that of oxygen and water and yet metallic failure can be ascribed mainly to the transport of ions across the packaging material. XPS showed that considerable degradation takes place at the metal/polyimide interface prior to visible corrosion or delamination.

A novel concept for improved environmental protection of integrated circuits uses thin, in-situ formed ceramic layers on the circuit surface. Metastable ceramic precursors, hydrogen silsesquioxane and silacyclobutane, convert to pure silica and amorphous silicon carbide dielectric films respectively upon processing at temperatures from 175 to 250ºC. These twolayer ceramic films were shown to protect silicon CMOS circuits from industry-recognized autoclave and HAST exposure environments for more than 400 hours whereas unprotected circuits always failed within 200 hours.

Corrosion control of microelectronic devices which operate in high humidity is essentialfor reliability. A selective encapsulation method based on the electrochemical synthesisof polyphenylene—oxide (PPO) film has been investigated. The monomer is electrochemically polymerized and simultaneously deposited on the device surface to form an adherent coating. Electrical properties of PPO coatings on gold substrates subjected to water immersion and voltage bias were measured. The influence of coating thickness, adhesion promoter, and substrate cleanliness were investigated and optimized. In the early stage of biased immersion, coating failure was mainly caused by defects at the metal—polymer interface, whereas failure after extended testing was due to the slow degradation of polyphenylene—oxide in an aqueous media.

Reduction in the feature sizes of electronic chips is forcing a commensurate decrease inthe sizes of the various structures in packaging so that the assumption that the materials of the structures have bulk properties is suspect, especially near the interfaces. Historically, packaging has not received research and development as sophisticated as thatof the chip, so a collection of techniques specific to packaging technology has not beendeveloped. The National Institute of Standards and Technology (NIST) has identified the need for and initiated efforts to develop and refine such techniques. This paper will discuss the current NIST program and plans which were developed in conjunction with industry and other government agencies. Initial attention is given to measurements of the electrical, thermal, mechanical, and chemical properties of all types of materials used in the packaging.

The state of stress for a uniform coating away from the edges reduces to that of plane stress, two in-plane normal stresses, and an in-plane shear stress. For this state, the interface between the coating and the substrate is totally stress free. Since the substrate and the coating are not interacting mechanically, an internal section of the substrate can be removed creating a tensioned drum-like membrane without altering the stress state. Holographic interferometry of vibrationally excited membranes is used to evaluate the stress. Using this technique, up to thirty vibrational modes can be obtained. This high degree of redundancy enables one to determine the one shear and two normal stresses that act in the plane of the coating. The only physical property requires is the coating density. The density is obtained from commonly reported literature values. Simple variations on the membrane vibration scheme, e.g., cutting the membrane to create a uniaxially tensioned ribbon, enables one to determine the in-plane Poisson's ratio and shearmodulus.

In separate but related experiments on commercially made free-standing films with residual orientation, the above techniques, combined with special free and axially constrainedcompressibility experiments should enable all of the Poisson's ratios and elasticmoduli for an orthotropic material (nine elastic constants) to be determined. Methods for measuring the state of stress and the elastic constants are required to predict the state of stress in complex coating geometries.

For a soft coating on a hard substrate the overall hardness of the coating/substrate system increases substantially when the indenter penetrates into the substrate sincethe substrate can now directly support some of the indenter load. A model for the overall hardness is developed by accounting for the indentation load shared by the coating andsubstrate. This model accounts for the additional load supported by the coating due to the pile-up of coating material underneath the indenter. The model predicts the overall hardness as a function of coating and substrate bulk hardness and coating thickness. Comparison of the model to experimental data for two polymer coatings (epoxy and epoxy acrylate) on soda-lime glass substrates shows good agreement.

An approach to measuring the mechanical properties of thin films with in—plane and thickness dimensions comparable to those of films in VLSI electronic devices and in multi—chip modules is being developed. The guiding principles are to utilize as much as possible the technology of microelectronic fabrication and to make the design downward—scalable in size so that eventually the very smallest films and structures can be tested. The necessary components have been designed; we are in the process of building and testing them.

The specimen and associated instrumention are fabricated by semiconductor techniques on a 50 mm—diameter silicon wafer. Piezoresistors arranged as strain—sensitive Wheatstone bridges are fabricated by diffusion. One bridge, located on a grip section, is the load cell. After opening contact windows and metallization, the thin—film specimen is deposited. Then the silicon wafer is patterned and etched, leaving the specimen hanging between two instrumented grip sections linked by flexible silicon springs. Relative displacement of the grip sections is imposed by either a micrometer or a micro—stepper motor and measured by a scanned laser beam and by additional strain—sensitive bridges.

A simple model for the formation of residual stresses in thin films deposited on elastically strained substrates was derived and experimentally tested. In the experiments, Cu thin films were deposited on elastically stretched nickel substrates. These Cu films were2 to 4.m thick and were deposited through vapor phase evaporation or electroplating. The loads applied during the deposition were then relaxed, and the total stress in both the film and the substrate were monitored (by x-ray diffraction) during this relaxation. It was seen that the final (residual) film stresses were significantly different for bothdeposition methods. The causes of such differences are discussed.

A continuous indentation test has been used to evaluate the difference in adhesion between copper films deposited on oxidized silicon substrates with different transition metalinterlayers. Variations in adhesion between samples with interlayers of Al,Cr,Ni,Ti and V correlated with the free energy of formation of the particular metal oxide present.