Samantha Lee

Dark current reduction in AlxGa1−xAs‐GaAs heterojunction diodes

The addition of a thin, high band‐gap P(N)‐type AlxGa1−xAs layer into a conventional AlyGa1−yAs‐GaAs N‐p (P‐n) heterojunction is found to significantly reduce the 2kT recombination current, thereby permitting the bulk diffusion current to dominate at current densities as low as ∼10−5 A/cm2. Experiment indicates that the 2kT current is due to surface‐recombination processes and originates in a very narrow region near the p‐n junction, presumably due to the small minority‐carrier surface‐diffusion length.

Effects of dielectric materials on electromigration failure

The effects of dielectric layers on electromigration failure were studied in situ using a high-voltage scanning electron microscope and at the wafer level using conventional accelerated testing. Several different passivation layers were deposited on wafers with A1 interconnect test structures. Prior to the deposition of the final dielectric, the wafers were processed identically and, whenever possible, simultaneously. Interconnects encapsulated with compliant polymer and very thin (0.1 μm) SiO2 layers demonstrated substantial lifetime extensions over those with more rigid (1 μm thick) SiO2 layers. Unpassivated lines behaved dramatically differently and failed much sooner than those covered with only 0.1 μm of SiO2. As expected, increasing the passivation thickness from 0.5 to 4 μm increased the electromigration lifetime for SiO2 covered specimens. The fabrication of silicon dioxide dielectrics using electron-cyclotron-resonance chemical-vapor deposition (CVD) and silicon nitride dielectrics via plasma-enh...

Stress-induced and electromigration voiding in aluminum interconnects passivated with silicon nitride

Void nucleation in aluminum interconnects passivated with silicon nitride was studied using a high voltage scanning electron microscope. Extensive stress-induced voiding was observed in these interconnects independent of the passivation thickness. Some of the stress-induced and electromigration-induced voids formed away from the interconnect sidewall in contrast to results from other studies of void nucleation in passivated aluminum lines. Nuclear reaction analysis measured large amounts of hydrogen in the aluminum films passivated with silicon nitride. Transmission electron microscopy showed a high density of nanometer-sized bubbles in the aluminum. These bubbles, which are thought to have formed from hydrogen that evolved from the silicon nitride layer during processing, served as nucleation sites for stress- and electromigration-induced voids.

Current‐voltage characteristics of AlxGa1−xAs Schottky barriers and p‐n junctions

Studies of temperature‐dependent current‐voltage (I‐V) characteristics of AlxGa1−xAs Schottky barriers and p‐n junctions indicate a gradual departure from theoretical behavior with decrease in temperature. The diode ideality factor n, which takes values between 1 and 2 at room temperature, increases to values between 1.5 and 4 at 94 K. It is shown that, if the ideality factor‐ambient temperature product is replaced by an appropriate effective temperature T*, the I‐V data appear to obey the theoretical formula at all temperatures. It is also shown that the 1‐kT I‐V characteristic of a p‐n diode can be described either by standard diffusion or thermionic emission theory. This enables one to determine the barrier heights of p‐n diodes from their 1‐kT I‐V plots.

Void phenomena in passivated metal lines: Recent observations and interpretation

Since the pioneering TEM work of Blech and Meieran thirty years ago, it has been evident that direct in-situ observations of electromigration in progress can provide information essential to the understanding of the phenomenon. Unfortunately, the practical difficulties of carrying out such observations have been so great that almost all experimental work on electromigration in passivated metal has been limited to lifetime measurements and post-mortem examination of failed specimens. Without an adequate qualitative picture, theoretical efforts at detailed modeling of the phenomenon have had limited success. In particular, the extremely large variance observed in life tests of virtually identical lines has not been satisfactorily explained. To make possible the necessary in-situ observations, Intel and Stanford have, for some years, been engaged in the development of specialized SEM technology leading to a High Voltage Scanning Electron Microscope (HVSEM) with complete computer control. A special computer g...

Stress-induced and electromigration voiding in nitride passivated Al interconnects

Void nucleation in nitride passivated aluminum interconnects was studied using High Voltage Scanning Electron Microscopy (HVSEM). Extensive stress-induced voiding was observed in the line regardless of passivation thickness. Some of the stress-induced and electromigration voids formed away from the interconnect sidewall, which is contrary to images presented in other studies of void nucleation. Transmission electron microscopy showed that hydrogen from the nitride layer forms bubbles in the aluminum. These bubbles act as pre-existing defects in the aluminum, reducing the barrier for void nucleation.

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

The 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.

Comparison of Stresses in Al Lines Under Various Passivations

Thermal stresses in pure Al lines passivated with a baseline 1000A oxide and additional passivations of 0.5μm oxide, 1μm polymer, or 0.5μm, 1μm, or 2μ nitride were analyzed. Results from finite element analysis and X-ray measurements were compared, and samples were examined in a high voltage SEM for stress voids. For unvoided samples, calculated and measured results showed good correlation, while results for the voided samples showed little correlation due to stress relaxation through voiding. Initial in-situ electromigration test results showed that electromigration voids can occur at stress void sites.

Comparisons of constraint effects on Al lines under various passivations

The average stress state and electromigration behavior of Al lines under 0.1μm oxide, 0.1μm oxide/1.0μm oxide, 0.1μm oxide/1.0μm nitride, or 0.1μm oxide/1.0μm polymer were analyzed. Thermal stresses calculated from finite element analysis and measured via x-rays were compared. A high voltage SEM was used to examine the stress samples for voiding and also to conduct in-situ electromigration testing on the various passivated lines. A reasonable correlation between the measured and calculated stresses was found for the unvoided samples, while results for the voided samples showed little correlation due to stress relaxation through voiding. Initial electromigration observations showed void behavior and failure modes varied under the different passivations.

Comparison of electromigration behavior in passivated aluminum interconnects

Electromigration void behavior in Al-0.1%Sc and Al-0.5%Cu alloys was compared by conducting tests in a High Voltage Scanning Electron Microscope (HVSEM). Wafer level test results showed that the median time to failure for the Al-Cu lines was ten times longer than that of the Al-Sc lines. In-situ tests of the two alloys showed a similarity in void nucleation times but differences in void movement between the two sample sets; the interactions of voids with Al2Cu theta precipitates in Al-Cu lines were especially interesting. From our studies, it appeared that the addition of scandium to Al does not have the proposed benefits for electromigration resistance, as suggested by previous studies.