New methods of probing beyond the limits of optical penetration have been developed using sensitive laser modulated reflectance techniques. Coupled with this subsurface defect detection method is a new surface topography imaging method. Together, these techniques nondestructively produce submicron-resolution images of surface and subsurface inhomogeneities in microelectronic structures. The measurables of these techniques are related to current reliability subjects. In interconnect metalization processes, defects such as stress- and electromigration-induced voids, precipitates of silicon or copper, sidewall notches, and hillocks and pits are images with these techniques. Sequential stressand-probe nondestructive testing allows the growth rate and other dynamical processes to be documerited. In silicon substrates, metal contamination and crystallographic defects are detected. In this talk, the measurement techniques are described and examples of their usage for VLSI reliability improvement are related.

Czochralski (CZ) grown Si wafers, specially prepared with unusually high carbon content (ranging from 3 to 7 ppma), were subjected to a rapid thermal processing (RTP) treatment at 1050°C for 60s. Synchrotron white beam x-ray topography in transmission geometry was used to study defect structures in these Si wafers, both prior and subsequent to this RTP treatment. Observations of both the partial relaxation of the strain fields of precipitates and widespread nucleation and propagation of dislocations accompanying RTP are presented and discussed. Results are contrasted with those from parallel studies previously conducted on low carbon content wafers.

“Shadow Step Structures” (3S) [1] are pre-patterned mesa-type geometries which are electrically self-isolating when overlayed with conducting thin films. This structure has been successfully implemented to generate CV dot structures. This CV dot structure eliminates the need for screen or photomasking after the oxide growth, thus eliminating manual handling, poor definition of dots due to “shadow screens”, and eliminates the possibility of contamination from screens and/or chemicals. This technique is very useful to qualify metal coaters as opposed to conventional techniques which involve patterning after metal deposition and has the advantage of fast turnaround and inherent cost savings. Measurement fidelity has been demonstrated via a large statistical sampling and is shown to give CV results consistently more accurate and superior to CV structures generated using the conventional techniques of metal shadow screens or photo masking.

Accurate moisture measurements in microelectronic assemblies are crucial in assessing reliability of integrated circuits (ICs). We describe the fabrication and use of a silicon-based device for evaluation of moisture barrier coatings. The capacitive moisture sensors use oxidized porous silicon (OPS) as the sensing element. Porous silicon (PS) is formed by anodization of Si in hydrofluoric acid (HF). Oxidation of PS in oxygen produces OPS, which is also porous if an appropriate starting microstructure and oxidation treatment are selected. Metallization layers on the OPS and the wafer back complete the capacitor structure. The capacitance of OPS sensors is functionally related to the moisture content of the surrounding atmosphere. For example, the capacitance of one sensor changed from 4 nF/cm2 when exposed to a moisture level of 300 ppm by volume (ppmv) to 36 nF/cm2 at 10,000 ppmv. In addition to this excellent sensitivity, capacitor response to step changes in moisture is rapid and reversible. The sensors are also rugged, as demonstrated by their consistent performance during accelerated testing at 85% relative humidity and 140°C. A silicon nitride IC moisture barrier coating was deposited on sensors of this type after normal ceramic dual in-line packaging. Sensor operability after coating deposition was confirmed following intentional introduction of a pinhole into the moisture barrier coating.

In this paper, the use of corona discharges to investigate and monitor surface ion failure mechanisms in devices is reviewed. The physical mechanisms resulting in surface ioninduced failures are first described. Then, some of the traditional design features and testing methods to minimize these failures are summarized. After presenting the general features of surface charge deposition on devices by a corona discharge, several examples of the application of this technique for design verification, process improvement and control, and failure analysis are given.

The present trend in the silicon IC industry is toward the replacement of AlSi metallization by AlCu(Si). However, the addition of Cu to Al(Si) increases the sensitivity of the metallization to moisture corrosion. Total reflection X-Ray Fluorescence (TXRF) with a detection limit in the 1012 atoms/cm2 range is used to measure the residual Cl on Al(Cu) as a function of processing. The results indicate that significant Cl can remain even after standard ash and chemical solvent resist removal. Furthermore, a need is identified to study some unexpected sources of Cl (e.g., air) and the chemistry of its removal from Al films, and this can be now be done easily using TXRF.

The dielectric properties of substrate materials constitute key parameters that determine the selection, reliability and design of sophisticated modern electronic devices. Two important factors affecting the reliability of electronic devices are the adsorption of moisture from the atmosphere, and contaminants like airborne particles (dust), flux residues, gaseous pollutants, residues from rinsing and cleaning unit operations etc. Any variation in the dielectric properties due to moisture and/or contaminants would be detrimental to the performance of an electronic package.

Among the inorganic materials, alumina represents an optimal combination of electrical properties for many low and high-frequency applications. In this paper, the dielectric constant of thin-film alumina substrates has been measured over a wide frequency range (100 Hz to 3 GHz) as a function of relative humidity. The changes in the dielectric constant when the alumina substrates are contaminated with different contaminants like CuC12, CuSO4, organic acids etc. and exposed to various relative humidity levels for different lengths of time have also been measured. The effect on the dielectric constant when the contaminated substrates are treated with surfactants like oleic acid and dodecyl amine and exposed to different relative humidity levels is also presented. An important observation is that at a relative humidity level of approximately 70% there is a sudden increase in the dielectric constant of the contaminated alumina substrates. Results are also presented for the measured moisture adsorption using Thermo-Gravimetric Analysis in pure alumina, alumina contaminated with CuCI2 and contaminated alumina treated with dodecyl amine. It is shown that the measured dielectric constant is consistent with the amount of moisture adsorption that decreases on treatment with the surfactant.

We have investigated the pitting corrosion of Al, Al-Cu, and Al-Si thin films in dilute hydrofluoric acid solution. Additions of Cu and, to a lesser extent, Si cause an increase in open circuit potentials (OCP), pit densities and pit sizes as compared to pure Al. Pits in Al-Cu alloys are associated with Cu depleted grain boundaries which are galvanically coupled to adjacent θ-phase precipitates. Pits formed in Al-Si alloys are associated with the Al matrix which is galvanically coupled to Si nodules. θ-Al2Cu has different electrochemical characteristics than Al even though both maintain a thin Al surface oxide. θ-Al2Cu has a more positive OCP, above the average pit repassivation potential for pure Al. Since θ-Al2Cu supports cathodic reactions at enhanced rates compared to pure Al, its presence raises the potential of the adjacent pure Al grain boundary to values near its average breakdown potential and above its average repassivation potential. Metastable pitting is thus promoted.

A specimen preparation technique for the study of surface reactions and thin-film effects in aluminum nitride ceramic substrates is described. The technique uses specially prepared electron-transparent thin-foil substrates. These substrates can be examined directly in the electron microscope without the need for any further specimen preparation.