Recent advances in IC device encapsulants and polymeric materials have made high reliability VLSI plastic packaging a reality. High performance silicone gel possesses excellent electrical and physical properties for IC protection. With their intrinsic low modulus and soft gel-like nature, silicone gels have become very effective encapsulants for the delicate larger chip size and wire-bonded VLSI chips. Recent studies indicate that adequate IC chip surface protection with high performance silicone gels in plastic packaging could possibly replace conventional ceramic: hermetic packaging. This paper will review some potential IC encapsulants. Special focus will be placed on the high performance silicone gel, its chemistry, and its application as a VLSI device encapsulant.

This article describes recent studies of the chemistry of adhesion between thin (d ≥ 11 Å) polyimide films and silver and copper substrates, and the structural changes in the polymer when polyamic acid is imidized to polyimide. The thin polyamic acid films were formed by vapor phase deposition of 1,2,4,5-benzenetetracarboxylic anhydride (PMDA) and 4,4-oxydianiline (ODA) under high vacuum conditions and subsequent imidization by heating in vacuum. Both ODA and PMDA are at least partially dissociated upon adsorption onto clean copper and silver and with increasing film thicknesses react to form the polyimide precursor, polyamic acid. Heating to T ≥ 425 K leads to polymerization to form polyimide films which are thermally stable to about 700 K. Polyimide films with mean thicknesses as small as 1.1 nm have been fabricated in this way and their bonding to the substrate as determined by x-ray photoemission studies is su marized. Infrared reflection absorption data gives further evidence that the polyimide bonds to the substrate via fragmented PMDA. Changes in the surface topography and molecular structure of the films during imidization are demonstrated by scanning tunneling micrographs and infrared reflection absorption data.

Polyethylene thin films were deposited by the ionized cluster beam (ICB) method. The dielectric, resistivity, and breakdown field measurements showed that the ICB polyethylene films have excellent properties as an electrical insulator. The characteristics of Au/polyethylene/Si MIS diodes and MISFErs indicated that the ICB method can control the film-substrate interface property. The SIMS and ESCAan alyses showed that the ICB films have pure and stable chemical structure.

Some results of simultaneous measurement of polyimide etch rates, temperature, and O− and F− atom emission intensities in CF4/O2 plasma discharges are described. Several process variables have been investigated both during steady and unsteady plasma reactor operation. Atomic compositions on the polymer surface have been determined as a function of time of exposure to the plasma using ESCA, and the C IS spectra apalyzed to identify the various C-O and C-F bonds.

The cracking and decohesion of thin films can be characterized by critical values of a non-dimensional parameter governed by the residual stress, the film thickness and a fracture resistance. This article describes the status of understanding concerning the magnitude of this number for various types of adherent film on either brittle or ductile substrates. Important effects of elastic properties, substrate thickness and yield strength are described.

This research relates electrochemical failure of encapsulated microelectronics to surface moisture and to surface impurities. It draws heavily on “osmotic blistering”, a phenomenon known to produce corrosive failure of coated/painted metals [1].

Leakage currents were measured on aluminum comb specimens as a function of relative humidity (RH) and temperature. We studied both bare combs and combs encapsulated with polysiloxane. 9.05 Vdc bias was used. We performed “contamination-by-design” experiments by deliberately introducing known amounts of NaCl, CaCl2 and sucrose onto the comb surface. Results were compared with corresponding data taken on well-cleaned specimens.

Our principal findings are

(1) Under dry conditions (RH<1%), small leakage currents are observed, ranging between 1-10 pA, which are insensitive to surface contamination levels. This implies that solid surface impurities Rer se do not promote electrochemical IC failure.

(2) In extremely moist environments (RH>99%), surface-contaminated samples exhibit large leakage currents, ranging between 1 and 10 pA, that are roughly proportional to surface loading.

(3) Different surface chemical compounds produce leakage-current steps at specific RH values, corresponding to solid-to-saturated solution transitions. For example, CaCI2 exhibits a transition at 21% RH; NaCl at 75% RH. At RH values above the transition, aqueous droplets, or vacuoles, were observed at surface sites occupied by solid deposits. The RH location of the transitions is largely unaffected by the presence or absence of polymer encapsulant. Leakage current steps were typically four to six orders of magnitude. The size of the step change varied between bare and encapsulated samples, and with surface loadings.

(4) Variable temperature studies, performed at constant external water vapor, exhibited step decreases in leakage current at temperatures corresponding to saturated solution to solid transitions.

(5) Sucrose, a nonelectrolyte, exhibited a leakage current step similar to those observed with CaCl 2 and NaCl. The size of the sucrose step change was significantly less than that observed with the electrolytes.

(6) Electrochemical attack patterns varied among the different chemical compounds. For example, as shown in Fig. i, CaCl 2 exhibited anodic attack on alternate metallization lines. NaCl produced attack on all metallization lines.

A full report of this work will appear elsewhere [2].

We have studied the reactions occurring for the deposition of chromium onto pyromellitic dianhydride -4,4′-oxydianiline (PMDA-ODA) polyimide for a wide range of metal coverages, using X-ray photoelectron spectroscopy as well as solution studies involving model compounds. A multi-step, coverage-dependent, mechanism is proposed which includes an initial electron transfer from Cr to PMDA-ODA. followed by formation of Cr-O and and Cr-N polymer-bound intermediates, At higher metal coverage, Cr-oxides and Cr-nitrides are formed which may activate the polymer for subsequent formation of Cr-carbide. No evidence was obtained in support of a model involving initial formation of a π-arene complex between chromium atoms and PMDA-ODA. In the course of this work we have shown that similarities exist between the chemistry of metal atoms in condensed phases and the reactions occurring between metal atoms and surfaces.

XPS data obtained during the initial stages of vapor deposition of chromium atoms onto a PMDA-ODA polyimide substrate is presented. Utilizing the results of molecular orbital calculations it is emphasized that the data is consistent with the formation of a charge transfer complex between the deposited metal atoms and the PMDA unit of the polymer.

The adhesion between metals and polymers is normally weak because they have drastically different electronic properties and interfacial bonding between metals and organic polymers is not readily affected by conventional coating techniques. The ion beam enhanced deposition (IBED) approach to improve adhesion focuses on the promotion of non-equilibrium “interdiffusion” and the creation of nucleation sites at the interfaces by low energy ion bombardment before and during metal deposition onto the polymer substrate.

The results of our work show a significant improvement of adhesion at the metal (Au)/polymer (Teflon) interface.

A Nuclear Reaction Analysis (NRA) for Hydrogen technique was used to analyze the hydrogen concentration near Plasma Enchanced Chemical Vapor Deposition (PECVD) silicon nitride-polyimide interfaces at various nitride-deposition and polyimide-polymer-curing temperatures. The CF4 + O2 (8% O2) plasma-etch-rate variation of PECVD silicon nitride films deposited on polyimide appeared to correlate well with the variation of hydrogen-depth profiles in the nitride films. The NRA data indicate that hydrogen-depth-profile fluctuation in the nitride films is due to hydrogen diffusion between the nitride-polyimide interfaces during deposition. Annealing treatment of polyimide films in a hydrogen atmosphere prior to the nitride film deposition tends to enhance the hydrogen-depth-profile uniformity in the nitride films, and thus substantially reduces or eliminates variation in the nitride plasma-etch rate.

The evolution of optical lithography to higher resolution patterning with step-and-repeat exposure methods has resulted from the precise fabrication of higher numerical aperture lenses, more precisely engineered and controlled mechanical and electrical subsystems, and new lenses, systems components and light sources which enable efficient printing at shorter wavelengths. Optical resist technology is undergoing a parallel evolution which is sharpening this capability even further. This paper outlines resist materials and processes at the leading edge of photolithography which will enable printing near and perhaps at the theoretical resolution limits. Subtopics discussed include resists sensitive to 248.4 nm excimer laser radiation, planarization methods, multilevel resist materials, and surface-functionalized plasma-developed resists. Finally, some conjecture about the practical resolution limits of optical lithography is presented.

Among the variety of techniques available today for interlevel dielectric planarization, techniques based on spin-on glass (SOG) films are relatively attractive because of process simplicity and minimal equipment requirements. This paper reviews the materials and processes of SOG technology as it is applied in dielectric planarization. The SOG planarization schemes employ SOG films either as a permanent part of the interlevel insulation layer or as a sacrificial layer in the ‘etch-back” techniques. The properties of the various types of comnircially available SOG materials are discussed in relation to their functional and processing characteristics.

In recent years, copper thick film materials have gained rapid acceptance in ceramic multilayer interconnect boards because of their ability to meet advanced packaging requirements at reasonable cost. With high volume production, however, problems such as opens, shorts, blisters and porosity have been experienced. Many of these failures may be attributed to undesirable materials interactions caused by reducing conditions which can be caused by incomplete burnout of thick film organics during firing.

This paper considers the interactions in current copper thick film material systems. Primary emphasis will be placed on chemical interactions that occur in both ideal and non-ideal conditions during firing. Failure modes will be related to chemistry and processing. New systems designed to overcome processing sensitivity will be discussed.

Materials and processes used for high functionality hybrid circuit packages based on ceramic substrates are described. Emphasis is on hybrid circuits used for telecommunications and related applications. Examples include packages utilizing thin film technology, thick film technology and combinations of the two technologies. Various thick and thin multilayer approaches to achieve high interconnection density are discussed. These employ a variety of metal conducting systems in conjunction with glasses and/or polymeric materials as dielectric insulating layers.

In order to examine the use of a compliant cantilever structure as a contact scheme for a Multi-chip Interconnection System (MIS), multi-layer (metals and SiO2) cantilever beams were fabricated utilizing standard silicon processing and micromachining technologies. The mechanical behavior and electrical characteristics of the beams were investigated in order to establish their optimum dimensions for use in the MIS. During the course of this study, a new mechanical testing method for thin films has also been developed, which makes use of the same cantilever beam structure and a “Nanoindenter.” The Young's modulus and yield strength of thermally grown SiO2 and Au were measured using this technique.

We have previously described a microstructural model for electromigration in thin film metallisation, based on the concept of a stress-driven backwards flux opposing the forwards electromigration flux. The model can be used to calculate the build-up of stress in a metallisation track subject to arbitrary variations in electromigration flux, and to predict both the location of electromigration damage and incubation times before its appearance. In this paper we present an experimental study of electromigration behaviour in thin aluminium films on a niobium substrate and relate the results obtained to the model.

Electromigration damage mechanisms around lithographically introduced notches in unannealed and annealed test tracks are studied.

Microstructural characterization of thin film (Au-Pt-Ti) and thick film (Mo-Mn) metallization on AIN substrates has been performed using Transmission Electron Microscopy (TEM), Analytical Electron Microscopy (AEM), Convergent Beam Electron Diffraction (CBED) and Auger Electron Spectroscopy (AES). The reaction mechanisms for both types of metallization methods are proposed. In particular, the microchemical and morphological nature of grain boundary penetration and precipitation within the AIN near the metallization interface has been examined.

A new organoaluminum polymer (EtAINH)n(Et2AlNH2)m·AlEt3 derived from triethylaluminum and ammonia, is thermoplastic at elevated temperatures and a glassy solid at ambient temperature. As a thermoplastic it can be processed in certain shapes, solidified, cured and transformed to dense aluminum nitride with retention of its shape. Aluminum nitride fibers are prepared by melt spinning the polymer, pyrolyzing in ammonia and at high temperature in nitrogen. The AlN microstructure forms as very fine particles at 400–600°C, coarsens at higher temperature, and densifies at 1600–1800 °C into polycrystalline AlN with submicron grains. Mechanical strength, thermal expansion and dielectric constant are consistent with bulk ceramic values. Initial thermal conductivity deduced from composite measurements is 82 W/m°K in fibers containing 0.5 to 1.0 percent oxygen.

There are three predominant interconnection technologies in use today for silicon integrated circuits: wire bonding; TAB (tape automated bonding); and C4 (controlled collapse chip connection). This paper briefly reviews each of these technologies for their strengths and weaknesses but focuses especially on wire bonding for single chip VLSI applications.

Although wire bonding has been in widespread use ever since the invention of the small scale integrated circuit, the technique is still applicable for today's much larger and denser chips. With leadcounts up to 250 or even higher, many challenges are presented for equipment accuracy and speed and in package design, where novel techniques often are required to prevent the package from being the limiting factor in chip interconnection. These challenges are discussed in some detail.

The paper concludes with a discussion of the future direction for mechanical connections as influenced by the technical and cost requirements of both the chip and the system in which it resides.