This paper presents an in-depth overview of the present status and novel developments in the field of plasma processing of low dielectric constant (low-k) materials developed for advanced interconnects in ULSI technology. The paper summarizes the major achievements accomplished during the last 10 years. It includes analysis of advanced experimental techniques that have been used, which are most appropriate for low-k patterning and resist strip, selection of chemistries, patterning strategies, masking materials, analytical techniques, and challenges appearing during the integration. Detailed discussions are devoted to the etch mechanisms of low-k materials and their degradation during the plasma processing. The problem of k-value degradation (plasma damage) is a key issue for the integration, and it is becoming more difficult and challenging as the dielectric constant of low-k materials scales down. Results obtained with new experimental methods, like the small gap technique and multi-beams systems with separated sources of ions, vacuum ultraviolet light, and radicals, are discussed in detail. The methods allowing reduction of plasma damage and restoration of dielectric properties of damaged low-k materials are also discussed.

Plasma processing of low-k dielectrics

Back end of the line dielectrics (BEOL) with low dielectric constants, so called low-k dielectrics, are needed for current and future integrated circuit technology nodes. However, an understanding of the defects which limit reliability and cause leakage currents for these films is not yet developed. We primarily utilize conventional electron paramagnetic resonance (EPR) and leakage current measurements to investigate amorphous hydrogenated carbon doped oxide (a-SiOC:H) dielectrics, the most important in current BEOL technology. The resonance measurements were complemented by transmission Fourier-transform infra-red spectroscopy, x-ray reflectivity, and Rutherford backscattering measurements. Various compositions of a-SiOC:H films were deposited on 300 mm diameter Si (100) wafers. They exhibit a wide range of dielectric constant, composition, and porosity. Variations in deposition method, process chemistry, and post deposition curing were also investigated. We observe a remarkable range of paramagnetic defect populations within the films. In a large subset of the films with similar defect structure, we observe a strong correlation between carbon dangling bond paramagnetic defect densities and leakage currents, especially at lower electric fields. This correspondence strongly suggests that, in this subset, defects observed by EPR are in a large part responsible for the leakage currents at low electric fields. In addition, the results suggest that the observed defects likely limit the dielectric reliability in problems such as time dependent dielectric breakdown and stress induced leakage current in many of these films. However, the EPR results are complex, and a simple universal correspondence between defect populations and leakage does not seem to be present.

Defect structure and electronic properties of SiOC:H films used for back end of line dielectrics

UV radiation during plasma processing affects the surface of materials. Nevertheless, the interaction of UV photons with surface is not clearly understood because of the difficulty in monitoring photons during plasma processing. For this purpose, we have previously proposed an on-wafer monitoring technique for UV photons. For this study, using the combination of this on-wafer monitoring technique and a neural network, we established a relationship between the data obtained from the on-wafer monitoring technique and UV spectra. Also, we obtained absolute intensities of UV radiation by calibrating arbitrary units of UV intensity with a 126 nm excimer lamp. As a result, UV spectra and their absolute intensities could be predicted with the on-wafer monitoring. Furthermore, we developed a prediction system with the on-wafer monitoring technique to simulate UV-radiation damage in dielectric films during plasma etching. UV-induced damage in SiOC films was predicted in this study. Our prediction results of damage in SiOC films shows that UV spectra and their absolute intensities are the key cause of damage in SiOC films. In addition, UV-radiation damage in SiOC films strongly depends on the geometry of the etching structure. The on-wafer monitoring technique should be useful in understanding the interaction of UV radiation with surface and in optimizing plasma processing by controlling UV radiation.

Prediction of UV spectra and UV-radiation damage in actual plasma etching processes using on-wafer monitoring technique

Electromigration lifetime was measured as a function of liner thickness for Cu/SiO2 interconnect structures. A significant increase in mean lifetime was observed for structures in which the liner thickness at the base of the test via was less than approximately 6 nm, with a current density <5 mA/μm2 in the power line connected to the test via. This is attributed to the continuous flow of Cu across the thin and possibly discontinuous liner at the base of the via. For extremely thin liner coverage, <1.4 nm at the base of the via and 2.5 at the bottom of the test line, the mean lifetime was observed to decrease as a rapid diffusion path was created which partially offset the beneficial effects of continuous flow. Failure distributions appeared to be trimodal and this was confirmed through failure analysis. In the case of thin liner coverage (<6 nm), early fails, which are typically characterized by slitlike voids at the via/line interface, were not observed.

Effect of liner thickness on electromigration lifetime

Electron swarm parameters (the electron drift velocity, the longitudinal diffusion coefficient, the ionization and attachment coefficients and the effective ionization coefficient) in pure CF3I were measured in the range of E/N over 140?1000?Td. These swarm parameters were analysed using a Boltzmann equation analysis, and a set of electron collision cross sections for the CF3I molecule was derived so that it was consistent with the measured electron swarm parameters in CF3I. The present set of electron collision cross sections of CF3I derived was used to calculate the limiting E/N values in CF3I?N2 mixtures to confirm that the results of the calculation agreed well with the recent experimental results.

https://iopscience.iop.org/article/10.1088/0022-3727/43/14/145202/pdf

Electron swarm parameters in CF3I and a set of electron collision cross sections for the CF3I molecule

The authors investigated the etching of grooves in low-k in Cu technology. Correlation between the line edge roughness (LER) and the time-dependent dielectric breakdown (TDDB) reliability for 100nm pitch Cu interconnects was investigated. They controlled LER by using various gases to etch multilayer photoresist. CF3I gas was found to reduce LER better than conventional gases such as CF4 and CHF3 because CF3I has higher etching selectivity of photoresist against spin-on glass film. The LER did not affect measures of electrical performance such as wiring resistance, capacitance, and leakage current, but did affect TDDB lifetime because, according to their simulation, the electric field was strongly enhanced at curvatures in the interconnects. The maximum electric field (Emax) was also determined to evaluate the effect of LER on TDDB lifetime. All their results show that CF3I etching is promising for creating reliable Cu interconnects with smaller pitches.

Reduction effect of line edge roughness on time-dependent dielectric breakdown lifetime of Cu/low-k interconnects by using CF3I etching

The feasibility of etching Cu/low-k interconnects by using a low global warming potential CF3I plasma was studied. Low-damage etching was done and porous SiOC (p-SiOC, k<2.6) film with low roughness was produced. Exposing p-SiOC film to CF3I plasma was found to suppress the decrease in the CH3 group and the increase in the k value compared to those of conventional CF4 and C4F6 plasmas. These effects are due to the low UV intensity and small amount of F radicals of CF3I plasma. The authors also found that the etching profile of CF3I plasma was comparable with that of CF4 plasma. Since the etching selectivity (p-SiOC∕ArF photoresist) of CF3I plasma is higher than that of CF4 plasma, the remaining photoresist thickness increases after etching, thus suppressing line edge roughness (LER). The decreased LER mitigated degradation of IV and time dependent dielectric breakdown characteristics in Cu interconnects. They also found that the roughness on the bottom surface of the p-SiOC trench was reduced. These benef...

Low-damage low-k etching with an environmentally friendly CF3I plasma

Extreme ultraviolet lithography (EUVL) is moving into the phase of the evaluation of integration for device fabrication.
This paper describes its applicability to the fabrication of back-end-of-line (BEOL) test chips with a feature size of hp 35
nm, which corresponds to the 19-nm logic node. The chips were used to evaluate two-level dual damascene
interconnects made with low-k film and Cu. The key factors needed for successful fabrication are a durable multi-stack
resist process, accurate critical dimension (CD) control, and usable overlay accuracy for the lithography process. A
multi-stack resist process employing 70-nm-thick resist and 25-nm-thick SOG was used on the Metal-1 (M1) and Metal-
2 (M2) layers. The resist thickness for the Via-1 (V1) layer was 80 nm. To obtain an accurate CD, we employed rulebased
corrections involving mask CD bias to compensate for flare variation, mask shadowing effects, and optical
proximity effects. With these corrections, the CD variation for various 35-nm trench and via patterns was about ± 1 nm.
The total overlay accuracy (|mean| ± 3σ) for V1 to M1 and M2 to V1 was below 12 nm. Electrical tests indicate that the
uses of Ru barrier metal and scalable porous silica are keys to obtaining operational devices. The evaluation of a BEOL
test chip revealed that EUVL is applicable to the fabrication of hp-35-nm interconnects and that device development can be accelerated.

Applicability of extreme ultraviolet lithography to fabrication of half pitch 35nm interconnects

The roughening mechanism of ArF photoresist during etching was investigated to find out why CF3I gas reduces the line edge roughness (LER) in the photoresist pattern better than CF4 gas. Since the plasma of reactive ion etching (RIE) consists of ultraviolet (UV) photons, radicals, and ions, the authors used a UV lamp and a neutral beam source for evaluating the effect of different plasma compositions on the photoresist roughness. The roughness was found not to increase only by UV photons or F radicals, but increase under the CF4 RIE plasma which has both UV photons and F radicals. A C–F modified layer was generated on the resist surface because the UV damaged CO bonds reacted with F radicals and the resist surface became softer and shrank. Since CF3I plasma has a lower UV intensity and fewer F radicals compared with CF4 plasma, the shrinkage on the sidewall of the photoresist was suppressed and resulted in a smaller LER when this plasma was used.

Mechanism of reducing line edge roughness in ArF photoresist by using CF3I plasma

Low-damage, high-rate, and highly selective low-k etching can be simultaneously satisfied using a plasma with an environmentally harmonized gas chemistry (CF3I). Such a CF3I plasma can drastically reduce the irradiation damage by ultraviolet (UV) photons during low-k etching, because the intensity of UV in CF3I plasma is much lower than that in conventional CF4 plasma. The etching selectivity of SiOCH to a photoresist can be drastically improved by using CF3I plasma because of reducing F radical generation. In addition, pulse-time-modulated CF3I plasma causes a drastic increase in the etching rate because a large amount of negative ions can be generated. These results show that CF3I plasma is a very promising candidate for low-damage and highly selective low-k etching.

Shuichi Saito

Papers

Reduction effect of line edge roughness on time-dependent dielectric breakdown lifetime of Cu/low-k interconnects by using CF3I etching

Low-damage low-k etching with an environmentally friendly CF3I plasma

Applicability of extreme ultraviolet lithography to fabrication of half pitch 35nm interconnects

Mechanism of reducing line edge roughness in ArF photoresist by using CF3I plasma

Environmentally harmonized CF3I plasma for low-damage and highly selective low-k etching