Thin-film multichip modules (i.e. MCM-D) can provide simple, low-cost packaging and interconnect options for interconnecting high-density, high-performance devices. The following is an overview of an MCM-D technology that can be implemented on top of several substrate materials. Tradeoffs will be discussed related to using different substrate materials and the corresponding implications from the assembly point of view. The MCM-D manufacturing process is reviewed and the subsequent reliability results are discussed.

Utilizing solution imidization, molecular weight and end group control techniques, soluble, fully cyclized polyimides with very high glass transition temperatures have been developed to meet high temperature applications. Mechanistic aspects are investigated for solution imidization by both the polyamic acid route and by the ester-acid route. Polyimides based on pyromellitic dianhydride and a 3F diamine exhibit glass transition temperatures of 420 °C. These polyimides are soluble in polar aprotic solvents and form tough, transparent films which demonstrate mechanical integrity and thermooxidative stability at 700 °F. Various processing routes are explored to demonstrate the viability of these materials in high temperature applications. Details of the synthesis and characterization of these materials will be provided.

Mullite (3Al2O3·2SiO2) has a low thermal expansion coefficient and a low dielectric constant making it a favorable material for substrate applications. Sintering of pure mullite ceramics is difficult, however, even above 1700°C. Thus, mullite-glass ceramics containing glass additives (Al2O3-MgO-SiO2 glass) which could be sintered at about 1600°;C were fabricated and their properties were investigated. The ternary system diagram of Al2O3-MgO-SiO2 shows that high SiO2 content glass depos its cristobalite at 200 to 270°C, which causes a substantial volume change, resulting in ceramic substrate cracking. Therefore it is particularly important to prevent crystallization of cristobalite from the glass phase in mullite-glass ceramics. The glass phase softens or partially fuses above 1600°C, and cristobalite formation in the glass phase occurs in the cooling process during firing. In order to obtain good substrates of mullite-glass ceramics, a higher temperature for sintering and faster cooling rate after firing are preferable. Analytical results by X-ray, SEM and EPMA show that mullite dissolves in the glass phase at a higher sintering temperature and more mullite crystallizes in the cooling process with a lower rate. The content of Al2O3 in the glass phase, therefore, increases with the increased sintering temperature and cooling rate, which may restrain crystallization of cristobalite. By adjusting of the composition of mullite and glass phase, mullite-glass ceramics with low dielectric constant (5.9), thermal expansion coefficient (3.5×10−6/°C) close to that of silicon chips, and high bending strength (210MPa) have been developed. These substrates made of mullite-glass ceramics are suitable for mounting silicon devices of computer processors.

We developed a photosensitive film using a polymer blend for use in applications that require a high density of interconnects. Our film can be used as a dielectric or a passivation layer in high density packaging technologies. We formed vias about 20 μm in diameter using our new materials.

In the GE High-Density Interconnect Process, thermoplastic polyetherimide adhesives with selectively variable glass transition temperatures (Tg's) are used as chip attach and overlay adhesive. Alternating layers of patterned metal and dielectric are then applied to fabricate the interconnect structure. Upper layer dielectrics are formed using a modified siloxane-polyimide that can be processed at temperatures below 200 °C. The unique materials requirements and materials development issues associated with this approach are discussed.

This article provides an overview on polymer/metal interconnection systems for microelectronics packaging. It will begin with a description of the functions of interconnection, and types of polymer/metal interconnection techniques available with tape automated bonding (TAB) being the most popular for chip connection applications. This is followed by examples of worldwide TAB applications and market outlook, TAB tape offerings and trends, as well as the entire TAB technology including tape types and fabrication, substrates interconnected by TAB, bonding techniques, TAB mounting options and TAB package encapsulation. Finally, future challenges on the most demanding fine-pitch TAB technology will be discussed.

As the performance of desk-top systems approaches 100 Mhz there is a need to develop a packaging technology capable of supporting high speed chip to chip interconnections. Polyimide based multichip modules are excellent packaging solutions that have minimum propagation delays and are cost effective. This paper presents the material properties and processing issues that need to be addressed in building reliable modules. A cost effective and reliable packaging solution is described in detail.

Aliphatic tetrafluoro-poly-p-xylylene (PA-f), has been evaluated as an interlayer dielectric and its properties reported. It has a lower dielectric constant (2.38) and higher thermal stability (480°C) than Parylene-n. The as-deposited films are of very low crystallinity. The crystallinity increases as the film is annealed. Thermo-Gravimetric Analysis has shown that these films loose weight at temperatures > 480°C. A shrinkage in the films of about 10% was observed when annealed in vacuum at a temperature of 425°C. The as-deposited films were measured to have low dielectric constant of 2.38 and a volume resistivity of 1.3×1016 ohm-cm. The refractive index in the optical wavelengths was measured to be 1.3 for as-deposited samples which increased with anneal temperatures. The stress levels observed are also lower (19 MPa) than PA-n (40 MPa) after annealing. Diffusion of Cu into PA-f is comparable with Cu diffusion in PMDA-ODA polyimide. Scanning Electron Microscopy of film cross-section shows microstructure change above temperatures of 350°C.

A series of polyimides prepared from BPDA-PDA and BTDA-PDA polyamic acid blends and the corresponding (BPDA-PDA)x-(BTDA-PDA)y copolymers have been investigated for the mechanical properties, the thermal expansion, and the residual stress in films cured on silicon wafers. To determine the effect of inclusion of a highly flexible chain segment into BPDA-PDA structure, a BPDA-PDA/BDAF copolyimide has also been characterized. The experimental data are correlated with the chemistry and the composition of the blends and the copolyimides.

A new wet patterning technology which is particularly well suited to moisture resistant polyimides is described. The key feature is a two step wet-etch with the photoresist patterned and the amic acid etched in sequential steps. A higher degree of sidewall control than previously attained is possible because the etchant can be tailored specifically for the polyamic acid. The etchants operate by the polarity shift mechanism conventionally used to etch polyamic acids with hydroxide bases, but instead use the weaker aliphatic amines as the active base component. Alcohols are used to adjust the polarity of the solvent.

Adding photoreactive substituents onto a biphenyldianhydride (BPDA) and p-phenylene diamine (PPD) poly(amic acid) precursor influences the onset of imidization and the development of anisotropy and crystalline morphology during processing and the resulting morphology and properties of the fully cured films. Amine salts shift the imidization to lower temperatures while the covalently bonded esters shift the imidization to slightly higher temperatures. Anisotropy of the photosensitive polyimides develops at higher cure temperatures compared to the poly(amic acid). The photoreactive substituents do not affect the crystal structure, but do influence the crystallite orientation and morphology. Out-of-plane coefficients of thermal expansion (CTEs), calculated from the volume expansivities of these systems, are an order of magnitude larger than the in-plane CTEs.

Uni-directional shrinkage sintering technology was studied to fabricate ceramic packaging substrates which meet requirements of very low dimensional tolerances (high dimensional accuracy). Three methods were considered. The first method used laminating of long fiber sheet materials and showed restraining effects on X, Y-directional (plane dimensional) shrinkage. However, the restricting ability was not sufficient for thicker laminates of the green sheets. The second method incorporated short fibers with a planar arrangement in green sheets, and some constraining effects on X, Y-directional shrinkage were observed with increasing short fiber content and modification of the fiber mixing method. But there was concurrent lowering of densification. The third method applied pressure lightly during sintering without using a mold. Control of the sintering shrinkage was relatively easy and X, Y-directional shrinkage of less than 0.1% was achieved by selecting a suitable pressure for the process. This method is very simple and easy to control, ensuring products of high dimensional accuracy.

The thermal curing chemistry of a fluorinated polyamic acid based on 6FDA (hexafluoroisopropylidene bis(phthalic anhydride)) and APBP (4,4′-bis(4-aminophenoxy)biphenyl) was studied by thermal-IR spectroscopy. Anhydride formation was observed at intermediate cure temperatures and maximized at approximately 220°C. The degree of anhydride formation was affected by the solvent, being least in 2-methoxyethyl ether and increasing in the solvent order: 2-methoxyethyl ether < NMP < 2-(2-ethoxy)ethoxyethanol. In addition to the back reaction of amic acid to anhydride and amine, at least one additional mechanism of anhydride formation is observed. The onset of the second mechanism of anhydride formation is coincident with the onset of imidization, which leads us to propose that water generated by imidization can react with anhydride during the curing process, before escaping from the film, to form diacid. Cyclization of diacid to anhydride is proposed as the second mechanism of anhydride formation.

The stress at room temperature in many typical polymer coatings used for microelectronic dielectrics is near 40 MPa. Polymer coatings derived from divinylsiloxane bisbenzocyclobutene (BCB), (CAS 117732-87-3), have a stress level of ca. 38 MPa after a conventional thermal oven cure. In this work, a novel approach based on curing the BCB polymeric network in the vitrified state has been used to achieve far field stresses of 24 MPa in the final coatings. This represents a 36% reduction in the far field stress of BCB coatings cured at 250 °C. The cure methodology and compatibility with device process design are discussed.

Mechanical properties of thin-film polymers often dictate the mechanical integrity and performance of microelectronic assemblies. For example, excessive interfacial stress between the polymer and signal via induced by thermal mismatch will lead to delamination of the interface or via cracking. This type of problem becomes more aggravated as dimensions of assemblies continue to shrink. In order to define the optimal design variables and process windows, the mechanical properties of polymer thin films throughout the complete process cycle must be carefully characterized. This paper will first review the experimental techniques commonly adopted in the mechanical measurements of thin-film polymers. Then, it will focus on the most difficult part of the mechanical characterization which is along the out-of-plane (Z) direction. Experimental set-up using the capacitance gauge will be introduced. Critical mechanical properties of polymers will be discussed. For the purpose of illustration, two polymers are chosen as examples. A BPDA-PDA polyimide is used to demonstrate mechanical characterization before melting whereas a PTFE-SiO2 composite is chosen to demonstrate the rheological characterization after melting. Before melting, mechanical behaviors such as the stress-strain curve, creep, relaxation, coefficient of thermal expansion, strain-rate dependence, and temperature dependence are discussed. After melting, the rheological behavior and the effect of aspect ratio measured by a squeezing flow experiment are addressed. Constitutive descriptions are proposed. The functional dependency between stress (pressure), strain (displacement), time, and temperature are characterized. A comparison between the numerical simulations and the experimental measurements is also presented.

Oxygen and O2+CF4 RIE showed faster etch rate of PQ-100 film than nonoxygen containing RIE, and caused rough surface morphology. Ti shows good adhesion to the PQ-100 film because of Ti-O and possible Ti-N compound formation at the interface. No diffusion of Ti and Ti-containing precipitates were observed at the Ti/PQ interface even at temperatures of 250 °C. In contrast to the Ti/PQ interface, Cu showed very poor adhesion to the PQ film because of weak chemical bonding. Cu reaction compounds were not observed at the interface at the 250 °C annealing. Ti adhesion to the PQ-100 film was good for control, RIE modified, and water-boiled cases. Initial studies suggest a reduction in peel strength at 250 °C annealing, although this topic must be further addressed to understand the exact mechanism.

A laser based surface scanning technique was utilized to measure the polyimide coated silicon wafer curvature resulting from thermal cycling and mismatch, Meanwhile, mechanical properties of polyimide thin film were characterized by DMA, TMA and tensile test. Based on the obtained material properties, A FEA model was developed to analyze the experimental results -reasonable correlation was obtained.

Similar approaches were taken one step further in the MCM silicon substrate curvature measurement. In a MCM package with silicon substrate, epoxy adhesive, and ceramic package, substrate warpage was developed in a thermal cycle due to thermal mismatch between the substrate and the package and coupling effect linked by epoxy adhesive. Three different substrate curvature measurement techniques were applied to identify the substrate curvature and epoxy thin film properties were also well characterized. A 3D FEA model incorporating with the epoxy material properties was developed to analyze the substrate warpage and investigate an optimal package design.

Several electronics packaging schemes involve polymer/inorganic interfaces, including: dual-in-line packages, tape automated bonding and multilayer interconnects. Typically, the thermal expansion coefficients are disparate, so these interfaces often cause high stress. Therefore, a phenomenological model describing transient stresses in spin-coated polyimide films was developed. The model is based on linear viscoelastic theory, and it accounts for shrinkage caused by solvent evaporation and imidization, viscoelastic relaxation, and thermal expansion mismatch. Strains have been defined from three mechanisms: thermal expansion mismatch, chemical curing, and solvent evaporation. Stress is, then, calculated by using the Classical Maxwell Model with one element. The concept of free volume is used throughout the model to estimate viscosity, modulus, and other quantities related to calculating strains. Model predictions for stress as a function of temperature during film cure and thermal cycling are fit with experimental data obtained from a bending beam apparatus.

Stress has been estimated by using the thin film approximation of the Timoshenko bilayer stress equation. Experimental data agree well with wafer bowing stress measurements. Although the technique does not yet take into account changing polyimide thickness during curing, the results still show qualitative curing dynamics. This preliminary study revealed good agreement between predicted and observed effects of material properties on stresses developed during cure and thermal cycling. Specificially, an unexpected high in-cure stress was observed for a standard low CTE polyimide. High stresses during curing can be as detrimental to an electronics device as high stress during device operation, so this technique may be useful when screening polyimides and/or prescribing curing schedules. Future work will improve the predictive capability of the model.

The adhesion of glassy or semicrystalline polymers to themselves or to other polymers is often poor. Polymers gain their strength from the mechanical interconnection between different chains. This interconnection can occur either by the use of long chains that entangle with each other or by chemically joining the chains by cross-linking. A strong polymer-polymer interface is crossed by many chains that are mechanically coupled into the material on both sides of the interface. For cross-linked systems these coupling chains must be cross-linked into both sides. If the polymers are not crosslinked, the coupling chains are formed by interdiffusion or welding. Interdiffusion can occur only if the system is heated to a temperature that allows significant chain mobility and if the interconnecting chains are sufficiently miscible in both polymers. For some systems, such as polyimides, the mobility required can either be generated by just part curing the materials before joining or by reversing the completed cure of a thin surface layer before joining and completing the cure. The interconnecting chains do not have to be the same material as the polymers being joined and, for joining dissimilar polymers, can be block copolymers where each block is miscible in just one of the relevant polymers.

We describe an approach using thin films on metals as models to study interfacial interactions. Thin films of biphenyl-tetracarboxylic dianhydride (BPDA) - p-phenylene diamine (PPD) based polyimide on Cr and Au surfaces have been investigated using infrared reflection absorption spectroscopy (IRRAS) and x-ray photoelectron spectroscopy (XPS). IRRAS was used for in-situ monitoring of polymer curing processes. Cured polyimide thin films on metals were characterized ex-situ by both XPS and IRRAS. Anhydride was observed to be an intermediate during chemical transformation of polyamic acid to polyimide. Imidization processes were found to be completed after curing at 250°C for thin films. Strong chemical modification of polyimide thin films resulting from interaction with Cr substrates were observed by infrared spectroscopy. These observations were consistent with angle-resolved XPS data showing a different C 1s emission spectrum of polyimide-Cr interfacial region, compared to the polyimide spectrum.