The recent demonstrations of manufacturable multilevel Cu metallization have heightened interest to integrate Cu and low-K dielectrics for future integrated circuits. For reliable integration of both materials, Cu may need to be encapsulated by barrier materials since Cu ions (Cu+) might drift through low-K dielectrics to degrade interconnect and device integrity. This paper addresses the use of electrical testing techniques to evaluate the Cu+ drift behavior of low-K polymer dielectrics. Specifically, bias-temperature stress and capacitance-voltage measurements are employed as their high sensitivities are well-suited for examining charge instabilities in dielectrics. Charge instabilities other than Cu+ drift also exist. For example, when low-K polymers come into direct contact with either a metal or Si, interface-related instabilities attributed to electron/hole injection are observed. To overcome these issues, a planar Cu/oxide/polymer/oxide/Si capacitor test structure is developed for Cu+ drift evaluation. Our study shows that Cu+ ions drift readily into poly(arylene ether) and fluorinated polyimide, but much more slowly into benzocyclobutene. A thin nitride cap layer can prevent the penetration.

Moisture can cause a host of reliability problems at interfaces including interface debonding. Two mechanisms can be identified. First, moisture at an interface can reduce the interface bonding strength dramatically by altering the chemical bonds. Second, when an interface with a crack or a crack-like defect is under tensile stresses, stress corrosion may allow crack growth at stresses much lower than critical fracture would require. To avoid wet interfaces, wafers should be briefly baked or exposed to a plasma in situ before the next film deposition step. However, moisture can also reach interfaces by diffusion along interfaces from unprotected edges during a wet process, such as CMP, or during storage in the ambient. In this work, the effect of moisture induced interface strength reduction was utilized to determine the diffusion distance. By using a mechanical peel technique, the diffusivity of moisture along the interface between Al and a poly(arylene ether) based low-K material (PAE2) was measured to be 4–6 μm2/s. Stress corrosion was studied using a special 4-point bend technique so that both strain energy release rate and crack velocity can be obtained. It was found that the mechanism of stress corrosion at this interface is more complicated compared to that in a bulk material: while the chemical reaction took place at the crack tip, moisture diffusion was also occurring along the interface ahead of the crack tip, preconditioning the interface. There appeared to be a region that kinetics was limited by interfacial moisture diffusion and reaction, from which the reaction time for interface weakening was estimated to be ∼ 10 seconds. It was also found that even for samples saturated with moisture, the relative humidity of the test environment was still very important.

We investigated the aluminum wiring reliability of fluorinated amorphous carbon (a-C:F) interlayer dielectrics (ILD) using electromigration tests at the wafer level under accelerated stress conditions with current density ranging from 25–32 MA/cm2 and a the substrate temperature of 300 K. The a-C:F film is one of the low-k organic materials with a dielectric constant of 2.5. The thermal conductivity of the a-C:F film (0.108 W/m·K) is about one order lower than that of SiO2 (1.2 W/m.K). We found that joule heating effect is enhanced by the lower thermal conductivity of a-C:F and that the wiring lifetime for a-C:F ILD is about one order lower than that for SiO2 ILD under high current stress. However, when the wiring lifetime is plotted as a function of the wiring temperature, the wiring lifetimes for both a-C:F ILD and SiO2 ILD became almost the same. The degradation of the wiring lifetime for a-C:F ILD is explained by the increase of the wiring temperature which is caused from joule heating. Moreover, the activation energy of the electromigration for a-C:F ILD has the same value as that of SiO2 LD at a temperature.

The effect of the post plasma treatment on the dielectric properties and reliability of fluorine doped silicon oxide (SiOF) films was studied. Also, the thermal stability of a Cu/WN interconnect system with SiOF intermetal dielectrics was examined by RTA. The surface roughness of SiOF films increased with the increasing plasma treatment power due to ion bombardment effect during the plasma treatment. As the plasma treatment power increased, the dielectric constant increased from 3.16 to 3.43, while the change in the relative dielectric constant of the plasma treated films by the boiling treatment was decreased in magnitude. Furthermore, the chemical properties of the plasma treated SiOF films near the top layer tend to resemble those of thermal oxides by the plasma treatment of sufficient power because of the reduction in the Si-F bonding in the films. In the case of Cu/WN/SiOF/Si multilayer structure, surface oxidation and densification due to the plasma treatment seemed to play an important role in protecting the interdiffusion between SiOF and metal interconnects.

Electromigration reliability is a key concern for implementation of low dielectric constant (k) materials for on-chip interconnects. To address this problem, we have carried out a comparative study of electromigration performance for Al(0.5 wt% Cu) line structures formed with TEOS-based SiO2 and a low k polymeric material, poly(arylene ether) (PAE). The test structures consist of 800 μm long single-level ines. Resistometric measurements were performed to determine electromigration lifetime. To supplement lifetime measurements, TEM was used to measure overall grain size and electron back-scatter diffraction (EBSD) was employed to determine the local grain orientation and grain boundary characteristics in the line structure. In addition, SEM was used to examine void morphology. Microstructural analysis revealed a larger grain size and an enhanced (111) texture in PAE-passivated lines compared to TEOS-passivated lines. The differences can be attributed to a high-emperature process used to cure the PAE dielectric. The overall results obtained in this study show reasonably good electromigration performance for low k dielectric interconnects.

Polymers are currently considered as a possible alternative to silicon dioxide in the fabrication of interlevel dielectrics. To penetrate mainstream semiconductor device fabrication polymers have to meet a number of requirements regarding their long-term stability. One aspect is the mechanical stability of integrated polymer films under changing climatic conditions. In the present work, the impact of ambient moisture on the mechanical properties of thin polymer films (PI, BCB, and PFCB) was investigated. The sorption of water molecules in these materials typically causes an anisotropic volume expansion, resulting in increased mechanical film stress if the film is physically constrained by adjacent inorganic structures. Especially polyimides show both considerable moisture uptake and large changes in the mechanical film stress, while BCB and PFCB are virtually insensitive to ambient moisture. In the paper, experimental data (water uptake, in-plane swelling, out-of-plane swelling) are presented and discussed in detail.

We have investigated the time dependent reliability of a-C:F films, and found that SiO 2 or SiN cover layers on a-C:F films peeled off partially after about a week when the a-C:F film was grown from C4F8. and annealed in N2 or grown from C4F8+CH4. The interfaces between a-C:F and SiN have a mixing layer indicating a radical reaction at the interface. An a-C:F film grown from C4F8+CH4 has greater water-absorption than a film grown from C4F8 Thus, hydrolysis and radical formation after annealing may cause degradation of the interface. We found that hydrogen annealing effectively decreases the dielectric constant and suppresses such time-dependent degradation.

Ab initio configuration interaction calculations have been previously used to account for the relatively large decreases in the static dielectric constant of Si-O-F alloys with low alloy concentrations of F-atoms, ∼ 22%for F concentrations of ∼ 10 at.%. The present study addresses the stability of these alloy films and carbon-fluorine films with respect to attack of Si-F bonds or C-F bonds by water molecules. The present calculations show that the reaction: H20 + 2 Si-F - 2 HF + Si-O-Si is exothermic by about 0.7 eV. However, the reaction of H20 + 2 C-F - 2 HF + CO-C is calculated to be endothermic by 1.6 eV. Our calculations focus on the reaction energetics and geometries as a function of the distance between the F-atoms of the Si-F and C-F groups and water molecule.

The effects of RF plasma precleaning and polyimide curing conditions on the peel strength have been studied. polyimide precursors of BG-2480 (Toray) and PI-2611 (Du Pont) were spincoated and cured under the various conditions. Cured polyimide substrates were in-situ Ar+ RF plasma cleaned prior to metal deposition. Al-2%Si or Al-0.5%Cu-1%Si thin films were deposited onto polyimide substrates using DC magnetron sputtering.

The peel strength was enhanced by RF plasma treatment. The Al/modified PI specimen failed cohesively in the polyimide. The polyimide curing condition strongly affects the peel strength in the Al/modified PI system.