George L. Schnable

Aluminum metallization—Advantages and limitations for integrated circuit applications

The advantages and limitations of aluminum metallization are reviewed and compared with other systems used for integrated circuits. Metallization system properties of particular importance are summarized, including initial physical and chemical properties of the system which define potential performance and reliability considerations. The special requirements for MOS arrays and for multilevel-metallized integrated circuits are described. Recently available knowledge of aluminum metallization process technology and of metallization-related failure mechanisms is reviewed, and new results of experimental studies are presented. It is concluded that aluminum will continue to be the most widely used metallization material, not only for single-level metallized integrated circuits, but also for multilevel LSI arrays.

Failure mechanisms in large-scale integrated circuits

A study was made of the factors affecting the reliability of large-scale integrated (LSI) circuits. Particular attention was given to the effect on array reliability of the additional processing steps required to obtain multilevel metalization. The additional significant steps are low temperature deposition of a second dielectric layer on metalized LSI wafers, etching of vias through the second dielectric, and second level metalization. Consideration was also given to the effect of other trends in LSI, such as the use of smaller geometry and closer spacings. Possible new failure modes in multilevel arrays are: 1) shorts or increased leakage through or along deposited second layer dielectrics, 2) opens or increased series resistance in conductors, and 3) silicon surface effects. A review of literature on LSI was supplemented by results of a number of concurrent LSI programs and experimental studies. Test vehicles were designed to provide fundamental information about each of the categories of failure. Data obtained using these test vehicles to evaluate the multilevel metalized array structures are presented. A discussion concerning the merits and limitations of the specially designed test vehicles for process development, process control, and reliability tests is given. It is concluded that with the proper in-process controls, tests and screens, LSI arrays will be substantially more reliable per function accomplished than are conventional integrated circuits.

Advantages of vapor-plated phosphosilicate films in large-scale integrated circuit arrays

The use of low temperature phosphosilicate glass (PSG) films which are compatible with aluminum-metallized integrated circuits, has been found to possess significant merits in LSI applications. PSG used on integrated circuits fabricated in production has resulted in improvements in fabrication yields and device performance and reliability. The properties and effects of PSG which contribute to these benefits have been determined and studied. They include hardness, ability to getter alkali ions, effect on immobile charge density, ability to quench fast states, low stress, resistance to cracking, low pinhole density, and effect on electromigration. The deposition system used to deposit the PSG films and the phosphorus content of the films are also discussed.

Mechanisms of Contact Failure in Semiconductor Devices

A study of the mechanisms of failure in ohmic and expanded contacts, including metal-semiconductor contacts and bonds to metalization in semiconductor devices, indicates that within a number of systems degradation or catastrophic failure has been produced by diffusion which produces solid solutions and/or compound formation. A great many systems are thermodynamically capable of interaction, but the important consideration is not whether they interact at all but whether the reaction proceeds at a rate which will result in metallurgical changes which adversely affect device reliability. The material systems studied are used or are considered for use in the fabrication of semiconductor devices. The systems include Ag-Cr, Ag-Ni, Ag-Pb, Ag-Sn, Al-Au, Al-Cr, Al-Ni, Al-Si, Al-SiO, Al-SiO2, Al-(AlxSiOy), Al-Ta, Al-Ti, Au-Cr, Au-Mo Au-Ti, Cr-Cu, Cr-Ta, Al-(Ni-Cr), Au-(Ni-Cr), Au-(Sb-Sn), Ti-Al-Si and Ti(Pt-Si).

Some reliability considerations pertaining to LSI technology

This paper discusses the trends that have developed in LSI, and the requirements for refinements in process technology, closer process control, and improvements in design that are considered to be essential for the full realization of high LSI reliability. Particular attention is paid to advanced multilevel-structure processing and to the use of specially designed test vehicles for purposes of process improvement and process control.

Metalization and Bonds - A Review of Failure Mechanisms

Materials and techniques used for metalization and bonding of silicon devices, particularly integrated circuits, are reviewed, and available data on the reliability of various structures, and on failure mechanisms, are discussed. Metalization systems considered include Al, Mo-Au and Ti-Pt-Au. Bonding methods considered include thermocompression and ultrasonic wire bonding, plus face-down bonding and soldering techniques. Metalization and bond failures are the principal modes of failure in integrated circuits. At high stress levels, inherent failure modes are most significant; these inherent failure modes are not, however, necessarily a factor in reliability at rated stress levels. Manufacturing defects are a major cause of failures at rated stress levels. At present there are no indications of a significant reliability advantage for any bonding or metalization system for general integrated circuit usage. It is predicted that Al will continue to be the most widely used metalization system, with an increasing usage of ultrasonic bonding and face-down bonding techniques.

The Effects of Insulator Surface-Ion Migration on MOS and Bipolar Integrated Circuits

The effects of mobile ions at insulator surfaces on integrated circuits are reviewed. The published literature is reviewed and recent experimental results are presented. Recent data are given on inversion voltages and surface recombination velocities both in regions under aluminum metal and in regions not covered by metal. Other data include measurements of surface conductivity on different types of surfaces, and a demonstration of the existence of mobile ions at an interface between two insulator layers. Techniques are described for minimizing surface-ion effects on integrated circuits.

Development of large-area 200-volt planar voltage-variable capacitance diodes

A research and development program resulted in fabrication of high-capacitance voltage-variable capacitance diodes for electronic tuning. Devices were fabricated by epitaxial and planar technology, with the diode prepared by diffusion into n-type silicon to approximate an abrupt junction. Objective specifications required devices with capacitance of 250 pF and 1000 pF (-8V), breakdown voltage V_{(BR)} > 200 V, capacitance change ratio of > 5.6 (-4 to -200 V), and quality factor Q > 200 (10 MHz). The principal problem was the V_{(BR)} limitation of planar diodes, which results in part from the tendency of thermally oxidized n-type Si to form an accumulation layer in the Si at the Si-SiO 2 interface. The planar process was refined to achieve large-area 200-V planar diodes, while maintaining the other essential diode characteristics, by the introduction of modifications to lower the electrical field of the p-n junction at the surface. Both structural modifications and processing changes were found to result in an increase in the level of V_{(BR)} of planar diodes.

The Impact of reliability requirements on LSI technology

The reliability advantages of LSI will be compared with the adverse reliability effects in LSI fabrication technology, noting that LSI circuits will be more reliable per function accomplished than conventional integrated circuits, but less reliable in terms of packaged device failure rates. New techniques for insuring in-process control during fabrication will be reviewed, and the application of such techniques to reliability improvement will be described.