Porous SiOCH Films Research Articles

Comparison of Cobalt Integration with Various Dielectric Materials under Thermal and Electrical Stress

Cobalt (Co) is proposed to replace copper (Cu) as a conductor in the back-end-of-line (BEOL) interconnects of integrated circuits. In this study, the electric characteristics and reliability of the integration of Co with various dielectric films (SiO2, dense, and porous low-k SiOCH films) under thermal and electrical stress were compared. Thermal annealing repaired sputtering-Co-deposition-induced damage to the dielectric film but reduced the breakdown field and time-dependence-dielectric-breakdown (TDDB) times due to the diffusion of Co atoms. After annealing, the SiO2 film had the largest reductions in the breakdown field, TDDB failure time, and electric field acceleration factor, indicating that the diffusion of Co atoms dominates in the oxygen-rich surface. Under electrical stress, the drift of Co atoms favors the porous low-k film with the assistance of porosity. As a result, a barrier is required for Co metallization. To achieve barrier-free or barrier-less processing in Co metallization, the dense low-k film is the best option to integrate with Co.

Investigation of the impact of annealing temperature on characteristics of porous SiOCH films

Porous SiOCH films with different porosity and low dielectric constant (k) were prepared by sol-gel and spin coating using methyltrimethoxy-silane (MTMS) and 1, 2-bis (trimethoxysilyl) ethane (BTMSE) as precursors and cetyltrimethylammonium chloride (CTAC) as porogen. The impact of different annealing temperatures on the chemical composition, hydrophobicity, pore arrangement, and dielectric constant of porous SiOCH films was investigated. The experimental results show that the SiOCH films with MTMS and BTMSE as precursors contain -C-C-bond and terminal methyl (Si-CH3). After annealing at 400 °C, the chemical composition of the films is stable. The water contact angle (WCA) is larger than 90°, and the films have good hydrophobicity.

Comparison of Self-Assembled Monolayers Using 3-Aminopropyltrimethoxysilane and Decyltrimethoxysilane in Vapor Phase for Porous SiOCH Dielectrics

Self-assembled monolayers (SAMs) are the emerging materials to act as barriers in the back-end-of-line interconnects for advanced technological nodes. In this study, SAMs were formed on the porous SiOCH (p-SiOCH) films by using different precursors: 3-Aminopropyltrimethoxysilane (APTMS) or decyltrimethoxysilane (DTMOS), in the vapor phase. Effects of SAMs precursors on the electrical characteristics and reliability of p-SiOCH films were characterized and compared. Experimental results indicated that both SAMs derived from APTMS and DTMOS enhanced the breakdown field and time-dependent dielectric breakdown, provided Cu barrier capacity, and promoted adhesion with Cu. In particular, APTMS-SAMs had a larger improvement, but a larger increase in the dielectric constant was observed as compared to DTMOS-SAMs. Therefore, SAMs derived from APTMS are a promising candidate for sub-nanometer barrier application for advanced interconnects.

Comparison of Self-Assembled Monolayers on SiO2 and Porous SiOCH Dielectrics by Decyltrimethoxysilane Vapor Treatment

Self-assembled monolayers (SAMs) are emerging as materials that are candidates of barriers used in back-end-of–line interconnects of integrated circuits for future generations. In this study, SAMs were formed on the SiO2 and porous SiOCH (p-SiOCH) films by using decyltrimethoxysilane (DTMOS) precursor in vapor phase at a temperature of 100 °C. The effects of the formation of SAMs at the surfaces of SiO2 and p-SiOCH films on the electrical characteristics were characterized and compared. With O2 plasma irradiation, SAMs could successfully form on both SiO2 and p-SiOCH films, thereby enhancing the adhesion and dielectric breakdown field. In the p-SiOCH films, SAMs sealed the surface pores and had higher coverage, promoting the effectiveness of the Cu barrier. In the Cu/porous low-k integrated interconnects for advanced technological nodes, therefore, SAMs are promising emerging materials acting as a barrier and adhesive. On the other hand, for SiO2 films, SAMs weakened the barrier; however, they can act as an interfacial adhesion enhancer.

Energetics of porous amorphous low-k SiOCH dielectric films

The trend toward dimension reduction in electronic devices has driven the research and development of low dielectric constant materials to solve resistance capacitance delays and power consumption issues. Amorphous porous hydrogenated silicon oxycarbides form a class of materials that have even lower dielectric constants than other silicon oxycarbides, achieved by introducing organic functional groups as well as porosity to alter the structure and decrease the density of conventional silica. However, these films tend to decompose to SiO2 during thermal annealing and nano-electronic fabrication processes. This apparent instability leads to the need to investigate how porosity can change the fundamental thermodynamic stability of this class of materials. We used high temperature oxidative molten salt solution calorimetry and cryogenic heat capacity measurements to directly determine enthalpies of formation, heat capacities, and standard entropies of a series of well-characterized porous SiOCH films. Thus, a full thermodynamic dataset has been obtained for these porous low-k films. All of these samples are either found to be unstable or metastable at or even below 298.15 K with respect to their crystalline counterparts and gaseous products. Thus, there appears little chance of forming thermodynamically stable amorphous porous SiOCH films and their persistence in fabrication and use is controlled by kinetic rather than thermodynamic factors.

Electrical and reliability characteristics of dielectric stack with low dielectric constant SiCOH and capping SiCNH films

This work investigates the electrical characteristics and reliability of a dielectric stack with a low-dielectric-constant (low-k) SiOCH film and a capping SiCNH film. Two kinds of low-k SiOCH films were used: a dense low-k film without porosity and a porous low-k film with a porosity of 15.0. The deposition of the capping SiCNH layer on both dense and porous low-k SiOCH films increased the overall dielectric constant. The porous low-k SiOCH film exhibited a greater increase in the dielectric constant. Capping a SiCNH layer on a low-k SiOCH film can retard damage by O2 plasma and diffusion of Cu ions diffusion. Lager improvements in time-dependent-dielectric-breakdown and electromigration lifetimes were detected for the porous low-k SiOCH film.

Study of CO2 ashing for porous SiOCH film using 100 MHz/13.56 MHz dual frequency superimposed capacitive coupled plasma

CO2 plasma has advantages over O2 and N2/H2 plasma in low damage resist ashing processes for porous SiOCH films. To understand why CO2 plasma has advantages, the authors investigated the damage mechanisms in O2, N2/H2, and CO2 plasma using 100 MHz/13.56 MHz dual frequency superimposed capacitive coupled plasma. According to the results of pallet for plasma evaluation tests, O radical, vacuum ultraviolet light, and ions were the main cause of damage to the porous SiOCH films in the O2 plasma, the N2/H2 plasma, and the CO2 plasma, respectively. The authors believe that CO2 plasma has fewer O radicals in it than O2 plasma and is less intense than the light produced by a N2/H2 plasma. To suppress damage further, the authors propose controlling the O radicals by using pulse-modulated 100 MHz CO2 plasma.

Effects of Precursor Flow Rates on Characteristics of Low-K SiOC(H) Film Deposited by Plasma-Enhanced Chemical Vapor Deposition

In this study, low-dielectric constant (low-k) SiOC(H) films deposited using conventional plasma-enhanced chemical vapor deposition (PECVD) by varying flow rates of the deposition precursors were investigated through various characterization techniques. The deposition precursors were DEMS and ATRP, which acted as a network matrix and sacrificial porogen, respectively. Experimental results indicated that both DEMS and ATRT precursors influence the properties of resulting porous low-k SiOCH films. But matrix DEMS precursor causes greater impact. With an increase of DEMS flow rate, an enhanced cross-linking was formed, leading to a higher hardness, smaller pore size, better electric performance, and stronger dielectric breakdown strength. However, a higher dielectric constant (k) is the cost. The k values of the porous low-k SiOCH films with various ATRP flow rates (1700~2500 sccm) remained unchanged, although, the optimized electrical characteristics and reliability were obtained as ATRP flow rate was 2100 sccm.

Plasma-Enhanced Atomic Layer Deposition Sealing Property on Extreme Low-k Film with k = 2.0 Quantified by Mass Metrology

We have investigated plasma-enhanced atomic layer deposition (PEALD) SiN pore-sealing film formation and diffusion behavior on highly porous SiOCH films. Mass measurement revealed the diffusion of the precursor used in PEALD into pores of SiOCH films, which was enhanced for higher-porosity SiOCH films. The diffusion of the precursor into the pores was reduced by applying UV-assisted restoration treatment before the pore-sealing process, which helped the formation of hermetic pore-sealing films. The results indicated that a 1-nm-thick SiN film was sufficient to seal the surface of the restored SiOCH film with k = 2.0. It was found that the decrease in k due to the pore-sealing deposition was as small as 0.02. The results indicated that the sequential application of UV-assisted restoration and PEALD-SiN pore sealing is a promising method of introducing the use of highly porous SiOCH films with k = 2.0 into interconnect integration.

Material and Structure Designs for Reliable Quad-Flat-Package for Scaled-Down Ultralarge-Scale Integrations With Porous Low-$k/{\rm Cu}$ Interconnects

Reliability of a quad-flat-package (QFP) with a circuit-under-pad (CUP) structure is investigated for Cu interconnects with porous low dielectric constant (low-k) films in scaled-down ultralarge-scale integrations. The following experimental factors are discussed: 1) low-k material properties and their stacking structures; 2) CUP structure; and 3) mold compound material properties. The QFP characteristics are analyzed after chip dicing and Ag wire bonding, as well as after molding. Higher adhesion strength of porous low-k film to SiCN cap dielectrics and rigid Cu-anchored CUP structure can achieve highly reliable QFP packaging. A lower coefficient of thermal expansion (CTE) of the molding compound is also found to be effective in eliminating low-k delamination during thermal cycle test because it can reduce the stress at the cracking position. The adhesion-promoting porous SiOCH film with the Cu-anchored CUP in a low-CTE mold is a promising system to realize a reliable QFP with no low-k delamination, passing electrical tests after the pressure-cooker test and high-temperature storage test.

Etching Enhancement Followed by Nitridation on Low-k SiOCH Film in Ar/C5F10O Plasma

The etching rates of low-dielectric-constant (low-k), porous SiOCH (p-SiOCH) films were increased by nitrogen-added Ar/C5F10O plasma etching in dual-frequency (60 MHz/2 MHz)-excited parallel plate capacitively coupled plasma. Previously, perfluoropropyl vinyl ether [C5F10O] provided a very high density of CF3+ ions [Nagai et al.: Jpn. J. Appl. Phys. 45 (2006) 7100]. Surface nitridation on the p-SiOCH surface exposed to Ar/N2 plasma led to the etching of larger amounts of p-SiOCH in Ar/C5F10O plasma, which depended on the formation of bonds such as =C(sp2)=N(sp2)– and –C(sp)≡N(sp).

Impact of Hydrocarbon Control in Ultraviolet-Assisted Restoration Process for Extremely Porous Plasma Enhanced Chemical Vapor Deposition SiOCH Films withk= 2.0

We investigated the effects of UV-assisted restoration on porous plasma-enhanced chemical vapor deposition (PECVD) SiOCH films with k = 2.0 and 2.3 having high porosities. By applying the UV-assisted restoration to O2-plasma-damaged films with k = 2.0 and 2.3, the recovery of the k-value was observed on the k = 2.3 film in proportion to –OH group reduction. However, the k = 2.0 film did not show recovery in spite of –OH group reduction. We found that hydrocarbon content in the k = 2.0 film was significantly increased by the UV-assisted restoration compared with the k = 2.3 film. According to these findings, we optimized the UV-assisted restoration to achieve improved controllability of the hydrocarbon uptake in the k = 2.0 film and confirmed the recovery of the k-value for O2-plasma-damaged film. Thus, adjusting the hydrocarbon uptake was crucial for restoring extremely porous SiOCH film.

Impact of Hydrocarbon Control in Ultraviolet-Assisted Restoration Process for Extremely Porous Plasma Enhanced Chemical Vapor Deposition SiOCH Films with k = 2.0

We investigated the effects of UV-assisted restoration on porous plasma-enhanced chemical vapor deposition (PECVD) SiOCH films with k = 2.0 and 2.3 having high porosities. By applying the UV-assisted restoration to O2-plasma-damaged films with k = 2.0 and 2.3, the recovery of the k-value was observed on the k = 2.3 film in proportion to –OH group reduction. However, the k = 2.0 film did not show recovery in spite of –OH group reduction. We found that hydrocarbon content in the k = 2.0 film was significantly increased by the UV-assisted restoration compared with the k = 2.3 film. According to these findings, we optimized the UV-assisted restoration to achieve improved controllability of the hydrocarbon uptake in the k = 2.0 film and confirmed the recovery of the k-value for O2-plasma-damaged film. Thus, adjusting the hydrocarbon uptake was crucial for restoring extremely porous SiOCH film.

Chemical bond modification in porous SiOCH films by H2 and H2/N2 plasmas investigated by in situ infrared reflection absorption spectroscopy

The modification of porous low-dielectric (low-k) SiOCH films by ashing plasma irradiation and subsequent exposure to air was investigated by in situ characterizations. Porous blanket SiOCH film surfaces were treated by a H2 or H2/N2 plasma in a 100-MHz capacitively coupled plasma reactor. The individual or combined effects of light, radicals, and ions generated by the plasmas on the chemical bonds in the porous SiOCH films were characterized using an in situ evaluation and by in situ Fourier-transform infrared reflection absorption spectroscopy (IR-RAS). In situ IR-RAS analysis revealed that the number of Si-OH, Si-H, and Si-NH2 bonds increased while the number of Si-CH3 bonds decreased during exposure to a H2 or H2/N2 plasma. Subsequent air exposure increased the number of Si-OH bonds by modifying Si-O-Si structures. The experimental results indicate that light emitted from a H2 or H2/N2 plasma can break Si-CH3 and Si-O-Si bonds and thereby generate dangling bonds. Radicals (e.g., NxHy and H radicals) can break Si-CH3 and Si-O-Si bonds and Si-NH2, Si–H, and Si-OH bonds could be formed. Si-NH2, Si-H, and dangling bonds react with moisture in the air cause the formation of Si-OH bonds. The dehydroxylation reaction on Si-OH was found to be the origin of Si-O-Si network structures. The mechanism of the degradation of porous low-k SiOCH films induced by a H2 or H2/N2 plasma is discussed based on the in situ characterization results.

The effect of VUV radiation from Ar/O2 plasmas on low-k SiOCH films

The degradation of porous low-k materials, like SiOCH, under plasma processing continues to be a problem in the next generation of integrated-circuit fabrication. Due to the exposure of the film to many species during plasma treatment, such as photons, ions, radicals, etc, it is difficult to identify the mechanisms responsible for plasma-induced damage. Using a vacuum beam apparatus with a calibrated Xe vacuum ultraviolet (VUV) lamp, we show that 147 nm VUV photons and molecular O2 alone can damage these low-k materials. Using Fourier-transform infrared (FTIR) spectroscopy, we show that VUV/O2 exposure causes a loss of methylated species, resulting in a hydrophilic, SiOx-like layer that is susceptible to H2O absorption, leading to an increased dielectric constant. The effect of VUV radiation on chemical modification of porous SiOCH films in the vacuum beam apparatus and in Ar and O2 plasma exposure was found to be a significant contributor to dielectric damage. Measurements of dielectric constant change using a mercury probe are consistent with chemical modification inferred from FTIR analysis. Furthermore, the extent of chemical modification appears to be limited by the penetration depth of the VUV photons, which is dependent on wavelength of radiation. The creation of a SiOx-like layer near the surface of the material, which grows deeper as more methyl is extracted, introduces a dynamic change of VUV absorption throughout the material over time. As a result, the rate of methyl loss is continuously changing during the exposure. We present a model that attempts to capture this dynamic behaviour and compare the model predictions to experimental data through a fitting parameter that represents the effective photo-induced methyl removal. While this model accurately simulates the methyl loss through VUV exposure by the Xe lamp and Ar plasma, the methyl loss from VUV photons in O2 plasma are only accurately depicted at longer exposure times. We conclude that other species, such as oxygen radicals or ions, may play a major role in chemical modification at short times near the surface of the material, while VUV photons contribute to the majority of the damage in the bulk.

Effectiveness of Dimethyl Carbonate and Dipivaloyl Methane Chemicals for Internal Repair of Plasma-Damaged Low-k Films

We propose the use of methylating chemicals to repair plasma-damaged sites in low-k films. Dimethyl carbonate (DMC) and dipivaloyl methane (DPM) were selected as the methylating chemicals to supply methyl (CH3) groups into the bulk of damaged porous SiOCH films. The absorption of water into the damaged films was fully suppressed by the DMC repair treatment. The relative dielectric constant, which was increased from the value of 2.4 in the as-grown porous SiOCH film to 3.1 by plasma processes, recovered to 2.6 owing to the DMC treatment. In the case of the DPM repair treatment, the relative dielectric constant showed an almost full restoration from a value of 3.8 in the damaged film. To discuss the chemical reaction during the DMC repair treatments, SiO2 and the damaged porous SiOCH films subjected to the treatments were analyzed using time-of-flight secondary ion mass spectrometry and Fourier-transform infrared spectroscopy. The results can be explained using the model showing that the methyl and methoxy groups generated via the decomposition of DMC molecules react with silicon and oxygen sites in the films.

H2/N2 plasma damage on porous dielectric SiOCH film evaluated by in situ film characterization and plasma diagnostics

This study investigates the mechanism of H2/N2 plasma ashing damage of porous SiOCH films. Porous SiOCH films were treated by a H2/N2 plasma using a 100-MHz capacitively coupled plasma etcher. The impact of ions, radicals, and vacuum ultraviolet radiation on the porous SiOCH films was investigated using in situ bulk analysis techniques such as spectroscopic ellipsometry and Fourier-transform infrared spectroscopy and ex situ film characterization techniques such as dynamic secondary ion mass spectrometry and x-ray photoelectron spectroscopy. In addition, plasma analysis including vacuum ultraviolet absorption spectroscopy was performed. The film characterization and plasma analysis show that the extraction of methyl by H radicals was enhanced by light while N radicals were responsible for inhibit the extraction of Si-CH3 bonds by forming nitride layer. The H2/N2 plasma damage mechanism is discussed based on characterization of the film and plasma diagnostics.

Mechanism of plasma-induced damage to low-k SiOCH films during plasma ashing of organic resists

Plasma-induced damage to porous SiOCH (p-SiOCH) films during organic resist film ashing using dual-frequency capacitively coupled O2 plasmas was investigated using the pallet for plasma evaluation method developed by our group. The damage was characterized by ellipsometry and Fourier-transform infrared spectroscopy. Individual and synergetic damage associated with vacuum ultraviolet (VUV) and UV radiation, radicals, and ions in the O2 plasma were clarified. It was found that the damage was caused not only by radicals but also by synergetic reactions of radicals with VUV and UV radiation emitted by the plasmas. It is noteworthy that the damage induced by plasma exposure without ion bombardment was larger than the damage with ion bombardment. These results differed from those obtained using an H2/N2 plasma for resist ashing. Finally, the mechanism of damage to p-SiOCH caused by O2 and H2/N2 plasma ashing of organic resist films is discussed. These results are very important in understanding the mechanism of plasma-induced damage to p-SiOCH films.

Influence of oxygen plasma treatment on properties of Methyl-BCN film

Methyl-BCN film as a low dielectric material has been investigated by our group for Cu/low-k interconnection. We studied the ashing characteristics of Methyl-BCN films using oxygen plasma. As a contrast, porous-SiOCH films were also treated by oxygen plasma with the same conditions. The change of composition ratio of Methyl-BCN film is less than that of porous-SiOCH film after oxygen plasma treatment. There is no evident change in each bond of Methyl-BCN film after treatment, either. FT-IR analysis was carried out to investigate the chemical bonds of Methyl-BCN films and porous-SiOCH films with and without oxygen plasma treatment. The methyl groups of Methyl-BCN film were more stable than that of porous-SiOCH film.

Crosslinking of porous SiOCH films involving Si–O–C bonds: Impact of deposition and curing

This paper focuses on the properties of nanoporous SiOCH thin films deposited using a porogen approach by plasma enhanced chemical vapor deposition. The impact of deposition temperature, porogen loading and porogen removal treatment is investigated using Fourier transform infrared spectroscopy, solid-state nuclear magnetic resonance analysis, and electrical and mechanical measurements. This work shows that a higher deposition temperature allows limiting the film shrinkage during the porogen removal treatment and leads to the best compromise in term of electrical and mechanical properties. Beside, the effect of Si–O–C bonds on the enhancement of mechanical properties is promoted since a typical crosslinking mechanism is highlighted in case of ultraviolet curing.