Positron annihilation was used to probe vacancy-type defects in electrodeposited Cu films after nanosecond pulse laser annealing. For the as-deposited Cu film, we identified the coexistence of two different vacancy-type defects, vacancy clusters (such as V16) and monovacancy-type defects, coupled with impurities. An enlargement in the vacancy size was observed after the laser annealing process. The size of these defects was estimated to be close to V30, and such defects could not be formed by conventional furnace annealing. After furnace annealing at 400 °C, the size of the larger vacancy clusters decreased, but that of the smaller vacancies increased. The observed change in the sizes of vacancies is considered to be related to interactions between vacancies and impurities. The depth profile of the defects varied by changing the laser energy density and the number of laser shots. The impact of laser annealing on the vacancy-type defects was observed even after furnace annealing at 800 °C. Because the presence of point defects in electroplated Cu directly correlates with electromigration and grain growth, the ability of laser annealing to introduce large vacancy clusters in the localized region shows the potential of nanosecond laser annealing as a low-thermal budget process tool for back-end-of-line materials.

Recent progress in materials, processes and integration schemes to reduce line resistance (Line-R) of damascene Cu and alternative conductors (damascene Co and subtractive Ru) are reviewed, including (1) graphene/Co capped Cu to achieve both EM reliability and Line-R reduction (2) nanosecond laser anneal of Ru blanket films for subtractive-etched interconnects, (3) single damascene Cu, which is potentially one way to extend Cu to extreme nodes, and (4) Co/Cu composite integration to preserve Cu power rails. Finally, the technology shift from Cu to alternative conductors is discussed from the viewpoint of Line-R crossover.

Beams of nitrogen and hydrogen radicals were investigated as surface pre-treatment and process enhancement techniques for atomic layer deposition (ALD) of tantalum nitride barrier layer on a dense organosilicate (OSG) low k film. In-situ x-ray photoelectron spectroscopy (XPS) studies of the evolution of the low k surface chemistry revealed an initial transient growth region controlled mainly by the substrate surface chemistry. Pre-treatment of the low k surface with radical beams, particularly with nitrogen radicals, was found to enhance significantly the chemisorption of the TaCl5 precursor on the OSG surfaces. The enhancement was attributed to the dissociation of the weakly bonded methyl groups from the low k surface followed by nitridation with the nitrogen radicals. In the subsequent linear growth region, atomic hydrogen species was able to reduce the chlorine content under appropriate temperature and with sufficient purge. The role of the atomic hydrogen in this process enhancement is discussed.

The problem of k-value degradation (plasma damage) is a key issue for the integration, and it is becoming more challenging as the dielectric constant of low-k materials scales down. One way to circumvent this issue is temporarily conversion of low-k material from a porous to a dense state by filling the pores with a sacrificial polymer after the deposition and curing of the low-k material. A detailed process scheme for the pore stuffing and postetch polymer removal of PMMA is described in this work. The filling temperature was optimized according to the molecular weight of the PMMA. To remove the polymer after plasma-etching in a purely thermal environment, a temperature of at least 430 °C had to be applied. Annealing assisted by variable frequency microwaves could remove the polymer already at 380 °C and with a He–H2 afterglow plasma the polymer could be removed at 280 °C. Laser annealing allowed the removal at a stage temperature of 200 °C with an only surface-limited heating of about 500 °C and higher to prevent the FEOL structures from damage. This work presents the results of the detailed study of stuffing and unstuffing processes, discusses mechanisms, and provides background for a robust stuffing and polymer removal process for the plasma damage reduction in porous low-k dielectrics.

The damage induced by CO2 and O2 plasmas to an ultra low-k (ULK) dielectric film with a dielectric constant (κ) of 2.2 was investigated. The dielectric constant was observed to increase due to methyl depletion, moisture uptake, and surface densification. A gap structure was used to delineate the role of ions, photons and radicals in inducing the damage, where the experimental variables included an optical mask (MgF2, fused silica, and Si), a gap height, an inductively coupled plasma power source, a bias power on the bottom electrode, variable chamber pressure, and variable substrate temperature. The plasma radical density distribution inside the gap between the optical mask and the ULK film was simulated. The simulation was based on radical diffusion, reaction, and recombination inside the gap. The experimental results and the numerical simulation showed that the oxygen radicals played an important role in plasma induced damage which was found to be proportional to the oxygen radical density and enhanced byvacuum ultraviolet (VUV) photon radiation. Under certain experimental conditions, ion bombardment can induce surface densification and suppress radical diffusion. The role of UV and VUV photons in induced damage was investigated with Ar plasma using the gap structure and it was found that the photons can induce surface damage directly.

Methyl depletion and subsequent moisture uptake have been found to be the primary plasma damages leading to dielectric loss in porous organosilicate (OSG) low-k dielectrics. A vacuum vapor silylation process was developed for dielectric recovery of plasma damaged OSG low-k dielectrics. The methyl or phenyl containing silylation agents were used to convert the hydrophilic -OH groups to hydrophobic groups. Compared with Trimethylchlorosilane (TMCS) and Phenyltrimethoxysilane (PTMOS), Dimethyldichlorosilane (DMDCS) was found to be more effective in recovering surface carbon concentration and surface hydrophobicity. But the carbon recovery effect was limited to the surface region. Alternatively, UV radiation with thermal activation was applied for dielectric recovery of plasma damaged OSG low-k dielectrics. The combined UV/thermal process was found to be efficient in reducing −OH, physisorbed water, and C=O bonds. The dielectric constant was recovered within 5% of the pristine sample and the leakage current was also much reduced. Aging test in air showed that no moisture retake was observed, indicating the repaired film was stable.

Implementation of air-gaps in the trench dielectric levels has been demonstrated as a potential effective solution to further reduction of the capacitance coupling in the Cu/low-k interconnects.[1-4] In this paper, the critical issue of mechanical stability in such air-gap interconnect structures during thermal processing and under chip packaging interaction (CPI) is investigated using 3D multilevel finite element analysis (FEA) models. Introduction Implementation of air-gaps in the trench dielectric levels has been demonstrated as a potential effective solution for dielectric scaling beyond 32nm with an achievable dielectric constant below 2.0.[1-4] However, air-gap interconnect confronts serious challenges concerning its structural integrity and mechanical stability. Bridging low-k cap (or hard mask) was observed to collapse over wide gaps during thermal decomposition of the gapforming material.[2,4] Crack initiation in keyhole-shaped gaps [5] formed by etchback and nonconformal refill schemes can also be a potential reliability concern. As packaging assembly exerts additional stresses to the fragile interconnect structures, chip packaging interaction (CPI) has been recognized as a serious reliability issue for air-gap structures. In this paper, we analyzed the mechanical stability issues of air-gap interconnect structures during thermal processing and subsequent packaging assembly using 3D multilevel finite element analysis (FEA) models. Structural and Mechanical Stability of as-Processed Air-gap Interconnects A 3D five-level finite element model was developed to simulate processinduced stresses in an air-gap interconnect (see Figure 1). To catch the representative features, FSG was used in the first two metal levels and the global level. Air-gaps were implemented in levels three and four. With varying line widths and dielectric materials, only slight reduction of the stress levels was observed in Cu wires as a result of air-gap implementation. This is not unexpected, as Cu wires are primarily confined by the silicon substrate. The introduction of air-gaps only further weakens the already weak confinement of Cu wires by the interlevel dielectrics. These results indicate that air-gap implementaMechanical Stability of Air-gap Interconnects Xuefeng Zhang, Suk-Kyu Ryu, Rui Huang, Paul S. Ho, Junjun Liu, Dorel Toma University of Texas, Austin; Tokyo Electron US Holdings. PRINT E-MAIL

The mechanical stability of air-gap interconnect structures during thermal processing and under chip packaging interaction (CPI) were investigated using 3D multilevel finite element analysis (FEA) models. Low k cap delamination from the Cu barrier during thermal processing, channel cracking of the bridging cap and dielectric overlayers, and interface delamination under packaging were identified as the main concerns of mechanical stability and reliability for air-gap implementation. Simulation results revealed that the delamination driving force depends very much on the gap width to cap thickness ratio, the channel cracking issue in the dielectric overlayers can be managed in the presence of constraints from adjacent Cu wires, and the introduction of air-gaps significantly increases the interfacial delamination probability under the outermost solder bumps.

The O2 plasma damage of porous OSG films was investigated by examining the origin of the dielectric loss and using a CH4 plasma treatment for dielectric recovery. To study the dielectric loss, a combination of analytical techniques including spectroscopic ellipsometry (SE) and Fourier transform infrared spectroscopy (FTIR) was used and the Kramers-Kronig dispersion relation was applied to deduce the contribution from the polarization components. The results indicated that the dielectric loss is dominated by the dipole contribution in comparison with the electronic and the ionic polarizations. To trace the origin of the dipole contribution, quantum chemistry calculations were performed to evaluate the dipole moment of various molecular clusters derived from the basic unit of the MSQ material as a result of plasma interaction. The uptake of physisorbed water molecules was found to be primarily responsible for the dipole moment increase and the dielectric loss. The dielectric recovery by CH4 plasma treatment was investigated using a combination of x-ray, electron spectroscopy and ellipsometry techniques together with contact angle measurements. After CH4 plasma treatment, the surface carbon concentration and surface hydrophobicity were found to be partially recovered. A gradient densified surface layer was found on top of the damaged OSG film containing certain amounts of C=C and Si-CH3 or C-C bonds. The C-V hysteresis and the leakage current were reduced, confirming a partial removal of Si-OH bond and Si dangling bond. XPS depth profiling revealed that the dielectric recovery was limited mainly to the surface region of about 10 nm.

Atomic layer deposition (ALD) of ultra-thin barrier layers is a key process for implementation of Cu/low k interconnects. Low k dielectric surfaces are generally characterized by weak surface bonds which have to be properly activated for barrier formation. In this study, we investigated the surface activation of low k dielectrics by atomic hydrogen, nitrogen and ammonia beams for ALD of Ta/TaN barrier layers. In-situ x-ray photoemiision spectroscopy (XPS) together with differential FTIR were used to examine the beam modification of the surface and the subsequent growth on the low k surface. The evolution of the low k surface chemistry revealed an initial transient growth region controlled mainly by the substrate surface chemistry. The initial chemisorption was found to be through formation of Ta-O bonds on organosilicates (OSG) and charge transfer complexes on aromatic SiLK. This led to a significant slower initial nucleation on the OSG low k surface without beam activation. The atomic beams, particularly the nitrogen and amine radials, were found to serve as effective reducing agents in initiating and enhancing the precursor chemisorption. This led to an improvement of the barrier film quality and an increase of the deposition rate at a temperature compatible with low k dielectrics (< 400C). In the subsequent linear growth region, atomic hydrogen species was able to reduce the chlorine content under appropriate temperature and with sufficient purge. The effect of porosity in low k dielectrics on barrier formation will be discussed.

A mechanistic study was performed to investigate plasma damage and CFL, recovery of porous carbon-doped oxide (CDO) low k surfaces. First the nature of damage was examined for different plasma treatments in a standard RIE chamber then followed by a study using a downstream hybrid plasma source with separate ions and atomic radicals to investigate their respective roles in the plasma process. Plasma damage was found to be a complicated phenomenon involving both chemical and physical effects, depending on chemical reactivity and the energy and mass of the plasma species. Moisture uptake after plasma damage was found to be a major reason to cause dielectric constant increase. The CFL plasma treatment was found to be promising in repairing oxygen ashing damages by formation of a carbon-rich polymer layer. However, sp2 carbons on the top polymer layer seemed to limit the penetration of plasma CH4 and thus full recovery of low k damage.

During an O 2 plasma ashing process, carbon depletion and subsequent moisture uptake caused increase of keff and the leakage current in an organosilicate (OSG) low-k dielectric. For dielectric restoration, additional CH 4 plasma treatment on the O 2 plasma ashed OSG low-k dielectric was investigated using angle resolved x-ray photoelectron spectroscopy (ARXPS), XPS depth profiling, x-ray reflectivity (XRR), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, and contact angle goniometer. After CH 4 plasma treatment on the O 2 plasma ashed OSG, the surface carbon concentration and surface hydrophobicity were partially recovered. A dense surface layer containing C=C bonds was found to have formed on the top of the damaged OSG. The C-V hysteresis and the leakage current were reduced as a result of the CH 4 plasma treatment. XPS depth profiling revealed that the recovery effect was limited to the surface region.

Plasma damage to low k dielectric materials was investigated from a mechanistic point of view. Low k dielectric films were treated by plasma Ar, O-2, N-2/H-2, N-2 and H-2 in a standard RIE chamber and the damage was characterized by Angle Resolved X-ray Photoelectron Spectroscopy (ARXPS), X-Ray Reflectivity (XRR), Fourier Transform Infrared Spectroscopy (FTIR) and Contact Angle measurements. Both carbon depletion and surface densification were observed on the top surface of damaged low k materials while the bulk remained largely unaffected. Plasma damage was found to be a complicated phenomenon involving both chemical and physical effects, depending on chemical reactivity and the energy and mass of the plasma species. A downstream hybrid plasma source with separate ions and atomic radicals was employed to study their respective roles in the plasma damage process. Ions were found to play a more important role in the plasma damage process. The dielectric constant of low k materials can increase up to 20% due to plasma damage and we attributed this to the removal of the methyl group making the low k surface hydrophilic. Annealing was generally effective in mitigating moisture uptake to restore the k value but the recovery was less complete for higher energy plasmas. Quantum chemistry calculation confirmed that physisorbed water in low k materials induces the largest increase of dipole moments in comparison with changes of surface bonding configurations, and is primarily responsible for the dielectric constant increase.

Future microprocessor technologies will require interlayer dielectric (ILD) materials with a dielectric constant (κ-value) less than 2.5. Organosilicate glass (OSG) materials must be nanoporous to meet this demand. However, the introduction of nanopores creates many integration challenges. These challenges include 1) integrating nanoporous films with low mechanical strength into conventional process flows, 2) managing etch profiles, 3) processinduced damage to the nanoporous ILD, and 4) controlling the metal/nanoporous ILD interface. This paper reviews research to maximize mechanical strength by engineering optimal pore structures, controlling trench bottom roughness induced by etching and understanding its relationship to pore size, repairing plasma damage using silylation chemistry, and sealing a nanoporous surface for barrier metal (liner) deposition.

This letter reports a study of the porosity effect on material properties of methylsilsesquioxane films, including the dielectric constant, thermal conductivity, and thermal stress behavior. In a porosity range from 0% to 50%, both the dielectric constant and thermal conductivity decreased with increasing porosity and no significant change was observed at the percolation point where pores became interconnected. In comparison, the stress–temperature slope also decreased with porosity, but as the porosity approached the percolation point, the slope showed a large drop of 40%, indicating a significant degradation of the thermomechanical properties due to percolation of pores. Assuming the coefficients of thermal expansion remain at 17 ppm/°C within the porosity range, the change in the stress–temperature slope corresponds to a decrease of the biaxial modulus from 7 to 5 GPa around the percolation point.