Passivation Thickness Research Articles

The Effects of Passivation Thickness and Initial Aluminum Line Stress on Electromigration Behavior

AbstractThe electromigration behavior of pure Al lines passivated with oxides of different thicknesses and passivation deposition temperatures was studied. The initial hydrostatic stress states of the passivated Al lines were modeled with finite element modeling (FEM), and, when possible, measured with X-ray diffraction. Conventional wafer-level electromigration tests showed a clear passivation thickness effect, but no detectable effect of initial stress on electromigration lifetimes. Increasing the passivation thickness increased the electromigration lifetimes, which has been observed by other researchers. However, in a sample set where the Al lines were covered with a thin (0. 1µm) oxide layer, the lifetimes were much longer than expected. Differences in the damage morphology and the failure mechanism between the thin and thicker oxides accounted for this unexpected result.

Finite element investigations of mechanical stress in metallization structures

For different double layer metallization structures the thermal-electrical-mechanical stress due to the mismatch of thermal expansion coefficients and elastic moduli was calculated by finite element analysis. A quantitative comparison between a conventional structure with small aluminum step coverage as well as an aluminum plug, a tungsten filled via structure and a structure with a barrier layer was done. The influence of applied current density, metallization length and passivation thickness and material was investigated for the tungsten filled via structure.

The Optimization of Passivation Layout Structure for Reliability Improvement of Memory Devices

In the present study, a 3-dimensional finite element model has been adopted to characterize the thermal stress distribution in memory devices with multilayered passivation coating. The simulation results detail how the distribution of the interfacial stress along the passivation and die premolding coating interface correlates with occurrence of the passivation crack. Furthermore, it is shown how the stress component which causes the passivation crack is determined by the geometry and the mechanical properties of the passivation layer. The simulation result shows that an increase of the passivation thickness contributes to its thermomechanical stability. The experimental work also indicates that a thickening of the passivation layer composed of a silicon nitride film with an oxide layer coating is a highly effective way to retard crack initiation due to shear stress. Particularly, the passivation layer with a ductile underlayer such as PSG (phosphosilicate glass) is essential for the reliability improvement of high density memory devices because this construction allows sufficient elasticity to compensate for thermal expansion mismatch between packaging materials and improves the step coverage.

Effect of Via Hole Profile on Electromigration Resistance for Via Chains

The electromigration resistance was studied for tapered and vertical via chains of 1.2 μm in diameter with conventional sputter deposited Al films. It was shown that the electromigration resistance of the tapered via chains with a slope angle of 55° is one order of magnitude lower than that of the vertical via chains with a slope angle of 85°. The electromigration resistance for the via chains decreases with decrease in the slope angle of the via hole. The activation energies of the electromigration resistance for the vertical and tapered via chains were 0.69 eV and 0.29 eV, respectively. The mean time-to-failure (MTF) for the tapered via chains with a single PSG passivation layer is at least two orders of magnitude greater than that with a dual SiN/PSG passivation layer. It was found that SiN passivation thickness is a significant factor affecting electromigration resistance for the via chain.

Thermal Stresses in Passivated Copper Interconnects Determined by X-Ray Analysis and Finite Element Modeling

ABSTRACTStresses in passivated copper lines were determined from strains measured using x-ray methods and compared to stresses calculated using finite element methods. Copper lines, 2 µm wide by 0.8 µm thick, were fabricated by DC magnetron sputtering and lift off patterning. The cross section of these lines was trapezoidal, with approximately 50° sidewalls. One set of samples was left bare, the others passivated with Si3N4 in several different thicknesses. Strain was determined using x-rays. Stress was calculated using elastic moduli adjusted for predominantly (111) fiber texture. Elastic-plastic finite element models provided comparisons of the effects of passivation thickness, rectangular and trapezoidal cross section, and mechanical anisotropy due to preferred texture on the hydrostatic stress in a line. A similar model for aluminum also provided a comparison case of stress as a function of passivation thickness.

Stress-Induced Voiding vs Temperature and Passivation Thickness IN Al-0.5%Cu-2%Si, AI-0.5%Cu AND Al-1%Si

ABSTRACTStress-induced voiding in microelectronics chips has been reported to exhibit a variety of dependencies on temperature and on passivation stress. The dependence of line failure is reported here at four different temperatures (150, 225, 285, and 315 ºC) for AI-0.5%Cu and, AI-0.5%Cu and AI-1%Si lines with passivation thicknesses (silicon oxide or silicon nitride) ranging from 0.1 to 2.5 times the metal thickness. Failure distributions change in a complex manner with changing passivation thickness, and with temperature for a specific passivation thickness, raising questions on the validity of using the conventional median time to failure (t50) and lognormal slope (σ) to project field failure rates from data generated by accelerated life testing.

Stress-Induced Voiding Vs Temperature and Passivation Thickness IN AI-0.5%Cu-2%Si, AI-0.5%Cu AND Al-i%Si

AbstractStress-induced voiding in microelectronics chips has been reported to exhibit a variety of dependencies on temperature and on passivation stress. The dependence of line failure is reported here at four different temperatures (150, 225, 285, and 315 °C) for AI-0.5%Cu-2%Si, AI-0.5%Cu and AI-1%Si lines with passivation thicknesses (silicon oxide or silicon nitride) ranging from 0.1 to 2.5 times the metal thickness. Failure distributions change in a complex manner with changing passivation thickness, and with temperature for a specific passivation thickness, raising questions on the validity of using the conventional median time to failure (t50) and lognormal slope (σ) to project field failure rates from data generated by accelerated life testing.

Simulation of the effect of thin film microstructure on current and temperature distributions in very large scale integrated metallization structures

A two‐dimensional finite element program is used to solve current and temperature distributions in inhomogeneous aluminum and tungsten metal lines deposited over very large scale integrated topography. Microstructure depictions of both aluminum and tungsten lines are obtained using the process simulator simbad. Density information is then used to calculate conductivity profiles as input to the current and temperature solver simcat. Experimental results are used to calculate and correct for the cooling in the third dimension of the simulation and results are presented regarding the effect of substrate and passivation thickness on line heating. Two examples of the effect of topography and microstructure on the flow of current and heating within metal lines are presented. Aluminum and tungsten lines are deposited over a series of trenches of varying sidewall angle, and the line temperature and maximum current stress are plotted as a function of this angle. The lower density of the tungsten deposited on the s...

Thermal stresses in aluminum lines bounded to substrates

Thermal stresses in unpassivated and passivated aluminum thin-film lines bonded to substrates have been calculated using finite-element techniques. For the unpassivated lines, only elastic behavior was considered. The stresses were found to increase with increasing linewidth and with increasing substrate rigidity. Edge effects were found to become nonnegligible for narrow lines. For passivated lines, allowed to deform either elastically or in elastic perfectly plastic manner, the stresses increase with decreasing linewidth, with increasing passivation thickness, and with increasing passivation stiffness. Stresses in elastic perfectly plastic lines are slightly lower and have flatter profiles than stresses in purely elastic lines. >

Finite Element Calculations of Thermal Stresses in Passivated and Unpassivated Lines Bonded to Substrates

ABSTRACTThermal stresses in passivated and unpassivated aluminum lines bonded to substrates have been calculated using the MARC finite element code. Stresses in the unpassivated lines increase with increasing line width, with increasing substrate rigidity, and with decreasing distance from the substrate. Edge effects become significant for small line widths. Stresses in passivated lines increase with decreasing aspect ratio, with increasing passivation thickness, and with increasing Young's modulus of the passivation. Allowing plastic deformation to occur in the lines homogenizes and reduces the stresses. The calculated stresses in passivated lines are compared to recent measurements of stresses in encapsulated aluminum lines. The measured values are about a factor of four smaller than the calculated stresses, indicating that some deformation process besides plasticity, such as voiding or cracking, must have occurred to relieve the stresses.

Effects of composition and structure on electromigration kinetics in aluminum-alloy thin films

Variations in time to failure data for electromigration processes have shown the importance to conductor design of factors that include composition, grain size, and passivation thickness. Traditionally, these variables have been studied separately. An understanding of the combined effects of these variables is necessary to design reliable conductors. Temperature-ramp resistance analysis to characterize electromigration (TRACE) has been applied to sample groups in which composition, passivation thickness, and grain size were varied. The results of these experiments provide the kinetic parameters for electromigration, and from these values, a calculation is proposed that allows an estimate of the conductors’ resistance to electromigration. The results suggest that the predominant factor controlling electromigration rates is the passivation thickness.

The effect of passivation thickness on the electromigration lifetime of Al/Cu thin film conductors

Although it has become common practice to cover thin film conductors with tough passivation layers, the reliability of the underlying conductor has not been well established. Most investigators have observed an increase in lifetime for stripes covered with glass as compared to uncovered stripes, but have disagreed as to the extent and the cause of this effect. A research program was undertaken in which the electromigration lifetime of Al/Cu conductors was studied as a function of passivation thickness. Stripes covered with 1.5 μm of glass lasted longer than those which were not covered. A further increase in lifetime was found for samples with thicker glass, but a change in failure mode was also observed. All uncovered stripes and all stripes covered with thin glass failed by open circuit, whereas those with thick glass failed by short circuit, as well. Electromigration-induced compressive stresses are believed to be responsible for these observations.

The relationship among electromigration, passivation thickness, and common-emitter current gain degradation within shallow junction NPN bipolar transistors

In very large-scale integration, the trend towards smaller horizontal dimensions and shallower doping profiles within NPN bipolar devices aggravates degradation mechanisms affecting common-emitter current gain (beta). It is experimentally shown for shallow junction NPN bipolar transistors operating in the conduction mode that the degradation of beta is electromigration related. Increasing the passivation thickness simultaneously improves metal conductor lifetime and aggravates beta degradation. Both increases are attributed to the electromigration-induced compressive stress. The complete recovery of beta after a 400 °C anneal indicates that the electromigration-induced compressive stress is relieved by creep in an aluminum-based metallization. The experimental results for beta degradation are consistent with an existing theory that takes into account the effects of stress on the effective mass of light and heavy holes.