Low-k Films Research Articles

Cu-Induced Dielectric Breakdown of Porous Low-Dielectric-Constant Film

Dielectric breakdown induced by Cu ion migration in porous low-k dielectric films has been investigated in alternating-polarity bias conditions using a metal–insulator–metal capacitor with Cu top metal electrode. The experimental results indicated that Cu ions migrated into the dielectric film under stress with positive polarity, leading to weaker dielectric strength and shorter time to failure (TTF). In the alternating-polarity test, the measured TTFs increased with decreasing stressing frequency, implying backward migration of Cu ions during reverse-bias stress. Additionally, compared with a direct-current stress condition, the measured TTFs were higher as the frequency was decreased to 10−2 Hz. The electric-field acceleration factor for porous low-k dielectric film breakdown in the alternating-polarity test was also found to increase. This Cu backward migration effect is effective when the stressing time under negative polarity is longer than 0.1 s.

Cryogenic etching of porous low-k dielectrics in CF3Br and CF4 plasmas

Low temperature etching of organosilicate low-k dielectrics in CF3Br and CF4 plasmas is studied. The chemical composition of pristine and etched low-k films was measured by Fourier transform infrared spectroscopy. Reduction of plasma-induced damage at low process temperature is observed. It is shown that the plasma damage reduction is related to protective effects of accumulated reaction products (CHxFyBrz, SiBrx after CF3Br, and CFx polymers after CF4 plasma). The reaction products could then be removed by thermal annealing for the pores to become empty. In the case of CF4 plasma, the thickness of CFx polymer increases with the temperature reduction, which is measured by ellipsometry. This polymer layer leads to a strong decrease in the diffusion rate of fluorine atoms and, as a consequence, to reduction of plasma-induced damage. Bromine containing reaction products are less efficient for low-k surface protection against the plasma damage.

Characterization of Low-k SiCOH Film Etching in Fluorocarbon Inductively Coupled Plasmas

Characterization of Low-<i>k</i> SiCOH Film Etching in Fluorocarbon Inductively Coupled Plasmas

Extrinsic time-dependent dielectric breakdown of low-k organosilicate thin films from vacuum-ultraviolet irradiation

In this work, the effect of vacuum ultraviolet (VUV) photon irradiation on the time-dependent dielectric breakdown (TDDB) of low-k organosilicate thin films was investigated, with particular emphasis on extrinsic TDDB (includes Cu migration effects). State-of-the-art low-k a-SiOC:H thin films were utilized because of their relevance as both an interlayer dielectric and as a candidate Cu capping-layer material. Synchrotron radiation was used to mimic VUV photon irradiation from processing plasmas without the presence of charged particles. TDDB characteristic lifetimes of the low-k a-SiOC:H dielectrics, before and after VUV photon exposure, were measured based on a Ti/a-SiOC:H/Cu metal-insulator-metal structure. The deterioration of extrinsic TDDB was observed in the film after exposure to VUV photons with 9 eV energy. The most notable degradation of the dielectric characteristic lifetime was found when the Cu electrode was used as an anode in the sample after 9.0 eV VUV photon exposure (photon fluence is 4.0 × 1015 photons/cm2). This is believed to be related to the Cu+ ions created by a VUV photon-assisted reaction. In the presence of an electric field, these Cu ions drift into the low-k dielectric and deteriorate TDDB performance.

BEOL Dielectric Processing for Cu-Low k Nano Interconnect- Impact of Plasma CVD Initial Transient Phenomena (ITP)

The Initial Transient Phenomena (ITP) during plasma CVD, its impact in nano thick film fabrication and methods to mitigate any negative impact from this phenomenon are presented in this paper. Plasma deposited dielectrics such as SiCOH, SiNx, SiCNx are regularly used for Cu low-k interconnect fabrication. For lightly porous low-k SiCOH films deposited with organosilicon and oxygen precursors, an initial C-Rich SiCOH is typically deposited at the interface due to ITP and the intrinsic plasma deposition chemistry. This carbon rich interface will weaken the film’s adhesion to the underlying surface and degrade the electrical properties of the nano-thick SiCOH dielectric. For SiCNx or SiNx dielectric films, local and global ITP charging effects on blanket and patterned surfaces have significant impacts on film’s quality and thickness control. Various novel process modifications were developed to mitigate this and to improve the plasma CVD dielectric film.

Plasma induced damage mitigation in spin-on self-assembly based ultra low-k dielectrics using template residues

This paper describes an approach for the reduction of plasma-induced damage in self-assembly based porous ultra low-k organosilica dielectrics. The concept is based on retention of the partially decomposed sacrificial organic phase (template) into the pores of the low-k film during plasma exposure. The amount of the template residues can be controlled by varying the hard-bake process time. It is shown that those residues are uniformly distributed throughout the film in the form of pore wall coatings. After plasma processing, the remaining residues are removed by means of a UV cure. Plasma damage to the underlying organosilica matrix was assessed by exposure of the differently hard-baked low-k films to fluorine-rich Ar/SF6 plasma. The thickest coating, estimated to be around 0.4 nm, enables a nearly damage-free etch process without any carbon depletion or k-value degradation along with limited shrinkage induced by post-etch UV-curing (&amp;lt;4.5%). These results highlight the efficiency of a simple and scalable route for damage-free integration of highly porous self-assembly based low-k dielectrics.

Effect of Bridging and Terminal Alkyl Groups on Structural and Mechanical Properties of Porous Organosilicate Films

Chemical composition, pore structure and mechanical properties of porous organosilicate low-k films with terminal and bridging organic groups are studied. It is shown that BTMSE based low-k films with alkyl bridge between the Si atoms have non-uniform pore structure with internal voids (ink-bottle like pores) while the films with terminal alkyl groups have pores with cylindrical shape. All studied spin-on deposited films have better mechanical properties than PECVD films with similar chemical composition and porosity. The best mechanical properties showed alkyl bridged low-k films. Moreover, the Young's modulus increases with BTMSE content because of higher concentration of bridging bonds.

Molecular layer deposition using cyclic azasilanes, maleic anhydride, trimethylaluminum, and water

Molecular layer deposition (MLD) is used to grow hybrid organic–inorganic films based on two or more self-limiting surface chemical reactions. A four-step ABCD sequence using N-(2-aminoethyl)-2,2,4-trimethyl-1-aza-2-silacyclopentane (AZ), maleic anhydride (MA), trimethylaluminum, and H2O and a three-step ABC sequence consisting of AZ, MA, and H2O are reported for growing hybrid organic–inorganic thin films. The ABCD sequence exhibited self-limiting growth behavior with a constant growth rate of 3.5 Å/cycle at 100 °C, while the growth rate of the ABC sequence increased to 90 Å/cycle after approximately 50 cycles. The growth rate using the ABC chemistry is much larger than for MLD/ALD chemistries that involve exclusively surface reactions, and displayed a strong dependence on the substrate temperature. Fourier transform infrared measurements of the as-deposited films indicated -COOH functionalities in the ABC film, and provided evidence for the reaction mechanisms. These results indicate that precursor diffusion into the ABC MLD film plays a key role in the large growth rate. The density, roughness, and dielectric constant (κ) of the films are reported in the as-deposited state and after treatment in air at temperatures up to 400 °C. The as-prepared and thermally treated ABC films with densities &amp;lt;1 g/cm3 and κ &amp;lt; 3 may be useful in applications requiring porous and low-κ films.

Evaluation and criterion determination of the low-k thin film adhesion by the surface acoustic waves with cohesive zone model

The cohesive zone model (CZM) is introduced in the surface acoustic wave (SAW) technique to characterize the interfacial adhesion property of the low-k thin film deposited on the Silicon substrate. The ratio of the two parameters in the CZM, the maximum normal traction and normal interface characteristic length, is derived to evaluate the interfacial adhesion properties quantitatively. In this study, the adhesion criterion to judge the adhesion property is newly proposed by the CZM-SAW technique. The criterion determination processes of two kinds of film, dense and porous Black Diamond with different film thicknesses, are presented in this paper. The interfacial adhesion properties of the dense and porous Black Diamond films with different thicknesses are evaluated by the CZM-SAW technique quantitatively and nondestructively. The quantitative adhesion properties are obtained by fitting the experimental dispersion curves with maximum frequency up to 220MHz with the theoretical ones. Results of the nondestructive CZM-SAW technique and the destructive nanoscratch exhibit the same trend in adhesion properties, which means that the CZM-SAW technique is a promising method for determining the interfacial adhesion. Meanwhile, the adhesion properties of the detected samples are judged by the determined criterion. The test results show that different test film materials with different film thicknesses ranging from 300nm to 1000nm are in different adhered conditions. This paper exhibits the advantage of the CZM-SAW technique which can be a universal method to characterize the film adhesion.

Synthesis and Characterization of Porogen Based Porous Low-k Thin Films

The present work is focused towards the lowering of the k value of deposited SiO2 thin films by varying solvent concentration i.e. ethanol in the range 4-10 ml. Porous low-k thin films were synthesized by using the sol-gel spinon technique. A non-ionic surfactant polysorbate 80 (Tween 80) was used as a porogen to generate the porosity in the film matrix. The lower values of refractive index and film density were measured to be 1.19 and 0.94 gm/cm3 respectively for 10 ml solvent concentration. Further, the lowest k value of 2.2 and highest porosity percentage of 58.5 % were obtained for the same film due to the dilution of coating solution at higher solvent concentration. The water contact angle of the film was observed to be increased to 106.3° which indicates the transformation of the deposited film surface from hydrophilic to hydrophobic. The change in chemical structure as an effect of solvent concentration is studied by using FTIR. From FTIR spectra the disappearance of Si-OH groups at higher solvent concentration reveals the increase in condensation rate. Overall in this study, the result shows the significant change in structural, chemical and optical properties of the deposited films at 10 ml solvent concentration. Such deposited porous thin films with lower k value and enhanced hydrophobicity can be used as an interlayer dielectric (ILD) for back end of line (BEOL) in CMOS technology.

Millimeter-Wave GaN HEMTs With Cavity-Gate Structure Using MSQ-Based Inter-Layer Dielectric

We have fabricated millimeter-wave gallium nitride high electron mobility transistors (GaN HEMTs) using methyl silsesquioxane (MSQ)-based inter-layer dielectric to suppress current collapse by enhancing the moisture resistance, and removed MSQ around the gate electrode for reducing parasitic capacitance. We clarified that the moisture resistance of conventional benzocyclobutene (BCB) is insufficient to suppress current collapse because water molecules easily permeate BCB, especially when using a cavity-gate structure. On the other hand, the moisture resistance of MSQ is very high because of the excellent hydrophobic property, and the current collapse due to moisture was effectively suppressed. So, improving the moisture resistance with hydrophobic low-k films plays a key role in reducing the current collapse of GaN HEMTs, especially when using a cavity-gate structure. Moreover, the parasitic capacitance of GaN HEMTs was successfully reduced by using the MSQ-cavity, and RF performance was improved by around 20%.

Optimization and upscaling of spin coating with organosilane monolayers for low-k pore sealing

For porous low-k film to be integrated into the next generation of interconnects, the pores need to be sealed against metal ions and barrier precursors. Self-assembled monolayers (SAMs) from organosilane precursor are spin coated onto 300mmk=2.2 low-k wafers. Two solvents, propylene glycol monomethyl ether acetate (PGMEA) and methanol with different dielectric constant of 8.3 and 20.1, are evaluated in terms of SAMs layer quality and sealing efficiency at coupon level. SAMs deposited from PGMEA show better sealing than SAMs deposited from methanol and therefore are selected for upscaling. Full wafer spin coating results show that a concentration of 0.05mM or below results in a partial coverage and a tilt angle as high as 70° from the backbone to the normal. Aggregation is observed for all tested concentrations and is worse for higher concentrations, which is possibly induced by the non-negligible presence of water in PGMEA solvents. In order to test the sealing efficiency of the SAMs layer against metal barrier precursors, MnN films by chemical vapor deposition (CVD) and TaNx/Ta (TNT) films by physical vapor deposition (PVD) are deposited on SAM coated low-k wafers. HfO2 is also deposited by Atomic layer deposition (ALD), which is not considered as a barrier but to test the sealing against ALD precursors. Depth profiling Rutherford Backscattering Spectrometry (RBS) measurements indicate an effective sealing of SAMs against CVD and ALD precursors but not against PVD barrier.

Mechanical Behavior of Methyl-Silsesquioxane Based Nano Porous Low-K Film Using by Ultraviolet Curing Method

Mechanical Behavior of Methyl-Silsesquioxane Based Nano Porous Low-K Film Using by Ultraviolet Curing Method

Effect of H2/He plasma treatment on porous low dielectric constant materials

Porous low-k films, deposited using a plasma-enhanced chemical vapor deposition (PECVD) porogen-based process, were treated by H2/He plasma at varied temperatures ranging from 25°C to 350°C in this study. Two different plasma treatment reactors: capacitive coupling plasma (CCP) and remote plasma (RP) systems, were used to compare the impacts of H2/He plasma treatment on the porous low-k film. Experimental results indicate that H2/He plasma treatment in CCP system caused a larger degradation in low-k film's properties due to the physical-chemical reaction by H ions, radicals, and VUV photons as compared to that in RP system with only presence of H radicals. Additionally, the operation temperature of H2/He plasma treatment operated in the CCP and RP systems resulted in an adverse effect. H2/He plasma-treated low-k films operated in CCP system at elevated temperatures exhibited a large scission of Si–CH3 groups, a large increase in k-value and leakage current, and a large degradation in breakdown field and TTF. However, the damage induced by H2/He plasma in RP system was alleviated by increasing the operation temperature. Additionally, as the operation temperature is raised to 350°C, the positive effects with the reduction of k-value and the improvement of reliability were detected in H2/He plasma-treated low-k films. These enhanced properties are attributed to the removal of carbon-based porogen residues by He/H2 plasma in RP system at high temperatures. Consequently, remote He/H2 plasma treatment operated at high temperatures is a damage-free process for porous low-k films as it is used to strip resist during the fabrication of interconnects.

Influence of porosity on electrical properties of low-k dielectrics irradiated with vacuum-ultraviolet radiation

During plasma processing, low-k dielectrics are exposed to high levels of vacuum ultraviolet (VUV) radiation emitted from the plasma. The porous structure of these materials makes them more sensitive to modification because of their low density and consequently deep penetration of active species into the film. Here, we investigate the changes to electrical properties of porous low-k dielectrics as a function of porosity after VUV irradiation. Organosilicate low-k films of porosities between 30% and 50% were exposed to synchrotron VUV radiation at 8 eV with a fluence of approximately 5 × 1014 photons/cm2. Capacitance-voltage measurements showed an increase in the dielectric constant along with a flat-band voltage shift. FTIR results show methyl depletion as well as water uptake after VUV treatment. These show that deterioration of the electrical properties after VUV exposure and the degree of damage are found to be higher for the more porous films.

The effect of vacuum ultraviolet irradiation on the time-dependent dielectric breakdown of organosilicate dielectrics

In this work, the effect of vacuum ultraviolet (VUV) exposure on the time-dependent dielectric breakdown (TDDB) properties of porous low-k films was investigated. Synchrotron irradiation was used to simulate the VUV photon irradiation from processing plasmas without any particle flux. The synchrotron flux varies with the wavelength, so the irradiation time was chosen to produce the same fluence at various photon energies. The deterioration of TDDB and generation of negative mobile charge were observed in the film after exposure to the VUV photons with 9 eV or higher energy. These effects were not observed in the films exposed with 7-eV photon energies or less. The creation of paramagnetic defects was observed with the ESR measurement and believed to be the reason for TDDB degradation. Depletion of carbon and breakage and rearrangement of the Si-O-Si structure were observed and believed to be the reason for mobile charge generation and the change in TDDB, chemical, and mechanical properties.

In-situ surface and interface study of atomic oxygen modified carbon containing porous low-κ dielectric films for barrier layer applications

The surface treatment of ultralow-κ dielectric layers by exposure to atomic oxygen is presented as a potential mechanism to modify the chemical composition of the dielectric surface to facilitate copper diffusion barrier layer formation. High carbon content, low-κ dielectric films of varying porosity were exposed to atomic oxygen treatments at room temperature, and x-ray photoelectron spectroscopy studies reveal both the depletion of carbon and the incorporation of oxygen at the surface. Subsequent dynamic water contact angle measurements show that the chemically modified surfaces become more hydrophilic after treatment, suggesting that the substrates have become more “SiO2-like” at the near surface region. This treatment is shown to be thermally stable up to 400 °C. High resolution electron energy loss spectroscopy elemental profiles confirm the localised removal of carbon from the surface region. Manganese (≈1 nm) was subsequently deposited on the modified substrates and thermally annealed to form surface localized MnSiO3 based barrier layers. The energy-dispersive X-ray spectroscopy elemental maps show that the atomic oxygen treatments facilitate the formation of a continuous manganese silicate barrier within dense low-k films, but significant manganese diffusion is observed in the case of porous substrates, negatively impacting the formation of a discrete barrier layer. Ultimately, the atomic oxygen treatment proves effective in modifying the surface of non-porous dielectrics while continuing to facilitate barrier formation. However, in the case of high porosity films, diffusion of manganese into the bulk film remains a critical issue.

Characterization of process-induced damage in Cu/low-k interconnect structure by microscopic infrared spectroscopy with polarized infrared light

Microscopic Fourier-transform infrared (FT-IR) spectra are measured for a Cu/low-k interconnect structure using polarized IR light for different widths of low-k spaces and Cu lines, and for different heights of Cu lines, on Si substrates. Although the widths of the Cu line and the low-k space are 70 nm each, considerably smaller than the wavelength of the IR light, the FT-IR spectra of the low-k film were obtained for the Cu/low-k interconnect structure. A suitable method was established for measuring the process-induced damage in a low-k film that was not detected by the TEM-EELS (Transmission Electron Microscope-Electron Energy-Loss Spectroscopy) using microscopic IR polarized light. Based on the IR results, it was presumed that the FT-IR spectra mainly reflect the structural changes in the sidewalls of the low-k films for Cu/low-k interconnect structures, and the mechanism of generating process-induced damage involves the generation of Si-OH groups in the low-k film when the Si-CH3 bonds break during the fabrication processes. The Si-OH groups attract moisture and the OH peak intensity increases. It was concluded that the increase in the OH groups in the low-k film is a sensitive indicator of low-k damage. We achieved the characterization of the process-induced damage that was not detected by the TEM-EELS and speculated that the proposed method is applicable to interconnects with line and space widths of 70 nm/70 nm and on shorter scales of leading edge devices. The location of process-induced damage and its mechanism for the Cu/low-k interconnect structure were revealed via the measurement method.

Comparison of Various Low Dielectric Constant Materials

In order to reduce resistance-capacitance (RC) delay of back-end-of-line (BEOL), a low-dielectric constant (low-k) material with the dielectric constant (k) below 4.0 has been introduced to be used as an insulator of Cu integration from 130 nm technological node. In this study, the electrical and reliability characteristics of various commercial low-k dielectric films with k value ranging from 3.6~2.5 deposited using plasma-enhanced chemical vapor deposition (PECVD) were investigated. In addition to FSG and dense OSG low-k dielectric films, porous OSG (P-OSG) films with two different sacrificial organic porogen precursors (alpha terpinene (ATRP) and cyclooctane (C8H16) were also compared. Although P-OSG films have a low k value, their resistance to the process integration is relatively low, resulting in a higher increase of k value and a degraded reliability performance as compared to OSG film. Hence, it is doubtable to use porous OSG films with a higher porosity in the following advanced technology nodes. Furthermore, the sacrificial porogen precursor used for porous OSG film deposition influences the film’s property. The P-OSG film for ATRP precursor has a better mechanical, electrical, and reliability characteristics as compared to that for C8H16 precursor in this study.

Effect of Cu Diffusion on Electrical and Reliability Characteristics of Low Dielectric Constant Dielectrics

The interaction between copper (Cu) interconnect and dense or porous low dielectric constant films (low-k) under thermal annealing was investigated in this study. A fabricated Metal-Oxide-Semiconductor (MOS; Cu/low-k/Si) capacitor was used to study the electrical and reliability characteristics of low-k films under Cu diffusion. The experimental results show that Cu diffusion depth increased with the annealing temperature and annealing time in both dense and porous low-k films. Moreover, the Cu diffusion rate in the porous low-k films is faster and is dominated by the annealing temperature, so that a large increase in the electrical properties was observed. Furthermore, the increase in the dielectric constant and the dielectric breakdown failure time were found to be proportional to the Cu diffusion depth for both dense and porous low-k films. Therefore, the Cu diffusion depth can be an indication for the fast reliability evaluation in the future Cu/low-k interconnects applications.