Bulk and Surface Conduction in CVD SiO2 and PSG Passivation Layers

Abstract

Measurements of electrical conduction in the bulk of chemical vapor deposited (CVD) SiO.2 and phosphosilicate glass (PSG) passivation layers typically used for over-me~al IC protection are reported. Moisture uptake increases the bulk conductivity, and the moisture effects depend on phosphorus content. The bulk current thermal activation energy of CVD SiO.~ (0.6 eV) is constant with varying moisture exposure. Before moisture exposure, phosphorus-containing glass has an activation energy of 0.8 eV, and it decreases with moisture exposure. Surface conductivity is also reported as a function of relative humidity. Conductivity values are related to IC leakage currents, and it is suggested that bulk currents in PSG should not contribute to aluminum corrosion in the absence of defects, such as cracks or pinholes.SiO~ and phosphosilicate glass (PSG) are important materials in silicon device passivation (1). Thin layers of PSG (hundreds of angstroms) have been described as useful for Na-gettering under gate metal of MOS devices (2-4). Thin layers must be used to avoid instabilities due to PSG polarization (3, 5-7). For overmetal passivation of IC's, thicker layers (about 104fl,) can be used (since polarization of the PSG is unimportant in this geometry). Generally, over-metal layers are deposited by chemical vapor deposition (CVD) (8-11 ). The phosphorus addition lowers intrinsic stress over aluminum metal and provides Na-gettering capability (12), which is particularly useful in plastic packaged devices. In addition, the passivating glass over metal provides important scratch protection during the device production process (13), and serves as insulating protection against loose conducting particles in hermetic packages (14).The bulk and surface electrical properties of CVD passivation over metal are important in understanding Na-gettering (4, 15) device leakage as affected by lateral charge spreading (16-21), metal corrosion (22-25), and moisture effects (24,25).In this report, measurements of bulk electrical conductivity as a function of water uptake and temperature, and surface conductivity as a function of relative humidity (RH) are reported for CVD SiO2 and CVD PSG. The data are used in estimating electrical and moisture effects typical for IC's. Sample PreparationThe glass layers were deposited on degenerate ntype (0.01 ohm-cm) silicon wafers at 450~ at a growth rate of about 1000 A/min to a total thickness of 1 ~m, using a reactor described by Kern (11,26). Phosphorus concentrations, determined by etch rate analysis, are 0, 4.8, and 8.5 weight per cent (w/o) P in the glass. No postdeposition heat-treatments were applied. * Electrochemical Society Active Member.

Techniques for measuring the integrity of passivation overcoats on integrated circuits

“…(3) critical comparison of the performance of the new methods; (4) examples of applications; and (5) recommended methods, techniques, and test procedures.…”

An investigation of the impact of a Ti barrier metal on the thermal stress field in passivated aluminium lines and vias in VLSI systems using finite element modelling approach

“…As a result, reliability problems increasingly become associated with the metal lines rather than the active devices. The inherent lack of reliability may result from corrosion (Koelmans 1974, Comizzoli 1976, Wada et al 1986), electromigration (Black 1969, Ghate 1983, Fischer and Neppl 1984 or the stress-migration phenomenon (Curry et al 1984, Klema et al 1984, Hinode et al 1990, Korhonen et al 1991, Kusaka et al 1996. The last of these, stress-migration, is fast becoming the dominant failure mechanism, especially as metal line widths reach submicron dimensions.…”

Characteristics of a Surface Conductivity Moisture Monitor for Hermetic Integrated Circuit Packages

“…This enables an alternate type of in-situ vehicle operating via surface adsorption rather than volume-effect diffusion. Effects of condensed misture on surface electrical [16][17] conduction of oxides and glasses have been studied.A sensor operating on the principle of changes in surface conductivity as water condenses onto an oxide surface has been described.18 A surface conductivity type sensor has also been employed in a study of herrmetic package leak rates.19This paper describes a non-glassivated in-situ sensor wherein the water-sensitive surface is composed of an interdigitated pattern of aluminum metal stripes.The surface conductivity of the pattern increases as water condenses onto its surface and peaks at the temperature where condensation ceases. Sirnce negligible anounts of foreign ions are present to cause stray surface leakage currents, a sensor of this type is a mre dependable moisture detector than a glass surface.…”

“…This enables an alternate type of in-situ vehicle operating via surface adsorption rather than volume-effect diffusion. Effects of condensed misture on surface electrical [16][17] conduction of oxides and glasses have been studied.…”