Device reliability

Consider the ramifications when a sub-micron, high-aspect ratio contact will be filled in the conventional way using sputtered aluminum (see figure 1.2). When the step coverage is only minimally acceptable, the aluminum can still provide continuous conductance and electrical contact. In fact, Rc from such a contact, as measured from a Kelvin structure, can still be excellent under such conditions. Two problems, however, remain with this approach  

Although several attempts have been made to improve the step coverage of sputtered aluminum, the results have not been optima because other properties (such as electromigration resistance) of the aluminum were degraded. Clearly in ULSI there is a need for a contact/via planarization method. 

It is important to realize that in many designs the limit to integration is not a result of the density of the transistors and other chip components, but a result of the density of the metallization system. An often used solution is to incorporate a multi level metallization system (MLMS). In MLMS, up to four layers of aluminum, separated by dielectric layers, are incorporated to handle the needed interconnects. 

As pointed out in the previous section, excellent step coverage or 

once the contacts and via s are planarized not only is there a 

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Far instrumentation purposes, there are clear advantages in placing at least some of the electronic circuitry close to the sensor (see Section 15.1) in order to reduce pick-up noise. However, there may also be inherent advantages in operating transistors at low temperatures, such as increased switching speed or lower noise. A serious problem is the effect on device reliability of the stresses induced by thermal cycling. 

The increasing importance of multilevel interconnection systems and surface passivation in integrated circuit fabrication has stimulated interest in polyimide films for application in silicon device processing both as multilevel insulators and overcoat layers. The ability of polyimide films to planarize stepped device geometries, as well as their thermal and chemical inertness have been previously reported, as have various physical and electrical parameters related to circuit stability and reliability in use (1, 3). This paper focuses on three aspects of the electrical conductivity of polyimide (PI) films prepared from Hitachi and DuPont resins, indicating implications of each conductivity component for device reliability. The three forms of polyimide conductivity considered here are bulk electronic ionic, associated with intentional sodium contamination and surface or interface conductance. 

Stability for use in optical interconnects. In the near future, optoelectronic integrated circuits and optoelectronic multichip modules will be produced. Materials with high thermal stability will thus become very important in providing compatibility with conventional 1C fabrication processes and in ensuring device reliability. Polyimides have excellent thermal stability so they are often used as electronic materials. Furuya et al. introduced polyimide as an optical interconnect material for the first time. Reuter et al. have applied polyimides to optical interconnects and have evaluated the fluorinated polyimides prepared from 6FDA and three diamines, ODA (3), 2,2-bis(3-aminophenyl) hexafluoropropane (3,3 -6F) (4), and 4,4 -6F (2), as optical waveguide materials. 

Regarding device reliability, the stability of a-Si H under such conditions as high light intensity, high temperature, or operation in a vacuum should be confirmed. During fabrication, in addition, decreasing the deposition temperatures to 150°C or less would also be useful if a-Si H were deposited directly on a filter-integrating substrate. 

Device reliability is a combination of many different factors. Theee... 

Figures 2 and 3 show that the DRAM chip perforMance has been iMproved even though the chip functionality has increased for the accelerated tests used by the seMiconductor industry. The 85 C/85X RH results are better because of a coMbination of iMproveMents in the chip design, the Manufacturing procedures and the epoxy encapsu-lent. The teMperature cycle test results, however, were priMarily improved by converting to a "low stress" epoxy encapsulant. The im-proveMent in the pressure cooker and the 125 C operating life (Figure 3) was also due to a coMbination of iMproveMents, including those in the epoxy encapsulant. These iMproveMents in device reliability are especially reMarkable when it is realized that the chip susceptibility to contaminants and stress has increased tremendously due to the 60-fold increase to functionality.

One method to minimize leakage and improve device reliability is the use of chip overcoats. Silicone or polyimide chip overcoats... 

Several other types of encapsulants have been evaluated for use by the semiconductor industry. Polyphenylene sulfide, a thermoplastic, has the advantage of good thermal characteristics and a low viscosity for device encapsulation. In addition, since it is a thermoplastic, the molded runners can be reused. This improves the material utilization and reduces cost. Unfortunately, this material also contains a significant amount of impurities that caused device reliability problems and efforts to remove them have met with mixed success. 

The lower-powered microwave signals used by communication transmitters are usually produced by solid-state devices. The Gunn diode is an example. When supplied with voltage from a well-regulated power supply these devices reliably produce a few watts of microwave signal. 

The Center for Chemical Process Safety (CCPS) of the American Institute of Chemical Engineers has published a comprehensive and authoritative guide to quantitative risk analysis (AlChE, 2001). The CCPS has also collected extensive data on device reliability. 

A clean surface is essential for device reliability and performance [183], It becomes critical as the dimensions of devices become smaller and smaller as a result of ever-increasing integration and complexity. It has been estimated that over 50% of yield losses in integrated circuit (IC) fabrication are due to microcontamination [184], Today, a typical process flow for advanced ICs consists of 300 to 500 steps, 30% of which are wafer cleaning steps [185]. The need for wafer cleaning can be separated into three areas (1) preparation of the wafer surfaces for oxidation, diffusion, deposition, and... 

Si (epitaxial) precipitation in contact areas to silicon [Hirashita et al.140). The main problem here is an increase in contact resistance, especially in small contacts, thus leading to device reliability problems. 

The nature of the interaction between water and the polymers is important because absorbed water can adversely affect thermal, electrical and mechanical properties of the polymer. Moisture absorption increases the dielectric constant, (5.6) and dielectric loss, (7) and has been related to device reliability problems. (8) Water-induced plasticization causes hygroscopic expansion, lowering of Tg, and degradation of mechanical properties. (9)... 

With the consistent increase in computer memory capacity (from 16K to 4 MM) the requirement of higher purity epoxy resins becomes essential since the circuitry of high capacity memory chips has to be much denser and finer which will render it more susceptible to corrosion failure. Table I indicates the resin purity requirement over recent years. Figure 1 demonstrates the relationship between resin purity and device reliability the higher the purity the longer the expected device life. Inorganic halides of resins are less than 5 ppm. 

Figure 1. Bias pressure cooker-device reliability test, cresol-epoxy-novolac-based molding compounds.

With the introduction of low total chloride epoxy resins (chlorine content <700 ppm), the wire bond failure due to the chlorine impurities in the resin has become much less prominent than that due to the bromine from the fire retardant additives in the molding compound. The chlorine content in a typical molding compound is less than 150 ppm while the bromine content is 0.75-1.25% (12,500 ppm). A release of bromine upon heating from resin, as experienced above, certainly will have a great detrimental effect on device reliability. 

Epoxy molding compounds, used to encapsulate microelectronic devices, contain bromine to provide flame retardancy to the package. This bromine, typically added as tetrabromo bisphenol-A or its epoxy derivative, has been found to contain many hydrolyzable bromides. These bromides, along with the presence of chloride impurities, are detrimental to the life of the electronic component. Bromine especially has been suspected (proven) to cause wire bond failure when subjected to moisture and/or high temperatures. With the addition of a more thermally and hydrolytic stable bromine compound, flame retardancy does not have to be compromised to increase the device reliability. Stable brominated cresol epoxy novolac, when formulated into a microelectronic encapsulant, increases the reliability of the device without sacrificing any of the beneficial properties of present-day molding compounds. 

Brominated compounds, where the bromine is in the meta position to the phenolic hydroxyl, have been shown to be more hydrolytically and thermally stable than ortho-brominated compounds such as TBBA (4). These stable bromine compounds can be incorporated in the CEN molecule and formulated into a molding compound. These molding compounds provide increased device reliability without sacrificing any of the beneficial properties of present molding compounds. 

Table I shows the typical analytical properties of stable brominated CEN and the standard high purity resin blend of CEN (QUATREX 3430) and the epoxy of TBBA (QUATREX 6410). This resin blend of CEN and the epoxy of TBBA was mixed in a ratio that corresponds with what is typically used in microelectronic encapsulants. The stable bromine CEN was synthesized to match the bromine content of the resin blend. The total bromine content of the resin blend and the stable bromine CEN is 7% however, the hydrolyzable bromide impurities of the stable bromine CEN is much lower than that of the standard resin blend. This low content of hydrolyzable bromide, along with the low chloride content, contributes to an increase in device reliability...

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