Methyl Silsesquioxane Research Articles

A New Route to Monolithic Methylsilsesquioxanes: Gelation Behavior of Methyltrimethoxysilane and Morphology of Resulting Methylsilsesquioxanes under One-Step and Two-Step Processing

Polymerization of methyltrimethoxysliane (MTMS) results in methylsilsesquioxane (MSQ), which has found important applications in recent years including use as low-k dielectric materials in the semiconductor industry, superhydrophobic materials, monolithic columns, and hybrid matrixes for immobilizing proteins. For polycondensation of MTMS in ethanolic solutions, we report the sol−gel behavior under two different sets of conditions. First, we examined one-step polymerization over a wide range of pH and show that the initial pH is important in determining both the gelation behavior of MTMS-derived sols and the morphology of the resulting MSQ materials. In the one-step method, we obtained either transparent precipitates and/or macroscopically phase-separated resins when the pH was below the isoelectric point (IEP) of the silanols; either macroscopically phase-separated resins or macroporous monolithic gels with pH > IEP; and homogeneous solutions when the pH was close to the IEP. We also report on the use of...

Adhesion of Sputter‐Deposited Ta to Porous‐Methyl Silsesquioxane Pore Sealed with Molecular Caulk

The adhesion of sputter‐deposited Ta to porous methyl silsesquioxane (MSQ) was investigated with and without molecular caulk technology. Semi‐quantitative four‐point bending results and Rutherford backscattering spectrometry (RBS) data revealed that the adhesion of the Ta to porous‐MSQ is fragile, and adhesion failure occurs at or near the Ta/porous‐MSQ interface. The adhesion of Ta to porous‐MSQ can be improved by sealing the porous‐MSQ surface with a 40 Å thick molecular caulk layer composed of parylene N (PPX). The calculated adhesion energy for Ta/PPX/porous‐MSQ structure was about 10.5 J m–2. The molecular caulk layer improves the fracture toughness of the fragile porous‐MSQ interphase region due its controllable penetration into the porous material.

Characterization of Photosensitive Low-k Films Using Electron-Beam Lithography

Characteristics of photosensitive low- methylsilsesquioxane (MSQ) were investigated by the use of electron-beam lithography. Photosensitive low- MSQ makes it possible to realize via and trench structures for Cu damascene technology without dry etching processes in the multilevel interconnect integration of ultralarge scale-integrated circuits. In this paper the dependences of critical dimensions of developed patterns on exposure dose and hold time after exposure were investigated. Consequently the aspect ratio of the developed trench structure of 3.9 and the minimal feature size of were achieved.

Phase Separation Behavior of Poly(methyl methacrylate-co-dimethylaminoethyl methacrylate)/Methyl Silsesquioxane Hybrid Nanocomposites Studied by Dansyl Fluorescence

Ultralow dielectric constant (k) materials (k < 2.0) are generally prepared by the sacrificial macromolecular porogen route. Dansyl steady-state and time-resolved fluorescence have been used to study the phase separation behavior of methyl silsesquioxane (MSSQ) and poly(methyl methacrylate-co-dimethylaminoethyl methacrylate) [P(MMA-co-DMAEMA)] hybrid nanocomposites, which are promising candidates for spin-on, ultralow dielectric constant applications. Resins MSSQ-HI and MSSQ-LO, which differ mainly in the concentration of −SiOH (silanol) present in the uncured samples, are used to study the effect of endgroup functionality on the phase separation behavior of the hybrid nanocomposites. Fluorescence results reveal that, for MSSQ-LO resin, a single emission band located in the range of 428−456 nm is observed for P(MMA-co-DMAEMA) loading levels up to 20 wt %. With further increase in polymer loading, a second emission band located at higher emission energy ranging from 408 to 422 nm appears, and the amount of this higher energy band increases with polymer loading. By contrast, for MSSQ-HI, the composition-dependent high-energy band does not appear until polymer loading levels exceed 50 wt %. We infer information about local environment from the emission band position with the low-energy band associated with dansyl surrounded by MSSQ, and the composition-dependent high-energy band associated with dansyl surrounded by P(MMA-co-DMAEMA). Thus, our fluorescence results suggest that both increasing the polymer loading level and lowering the amount of silanol endgroup in the uncured MSSQ promote phase separation in the hybrid nanocomposites. This is evidenced by the increase in the proportion of the high-energy band, which we interpret as indicating that the probe molecules avoid the phase boundary region and seek a polymer-rich environment. This migration results in the formation of larger polymer domains and ultimately translates into larger size pores upon polymer burnout. Our interpretation is consistent with previous pyrene fluorescence, small angle neutron scattering, transmission electron microscopy, and small-angle X-ray scattering results.

First Pass Study of Surface Modified Porous Low-k by Ion Implantation for Zero Thickness Barrier Requirement of Cu/MSQ/Si Stacks in Copper Metallization Scheme

AbstractLow-κ dielectrics have to meet stringent requirements in material properties in order to be successfully integrated. A particularly difficult challenge for material development is to obtain a combination of low dielectric constant with good thermal and mechanical properties. Incorporation of low dielectric constant materials such as porous silica based materials as a replacement to conventional dielectrics like SiO2 and use of Cu metallization schemes has become a necessity as critical dimensions of devices decrease. This paper is focused on the challenges in developing materials with low dielectric constant but strong thermo mechanical properties. Thin films of Ultra-Low materials such as porous Methyl Silsesquioxane (MSQ) (κ=2.2) were implanted with argon 1 × 1016 cm-2 dose at energies varying from 20 to 50 keV at room temperature. This work shows that the surface hardness of the porous films can be improved five times as compared to the as-deposited porous films by implanting Ar with 1 × 1016 cm-2 doses at 20 keV, sacrificing only a slight increase (∼9%) in dielectric constant (e.g., from 2.2 to 2.4). The hardness persists after 4500C annealing. In this current work, an ion implantation strategy was pursued to create a SiO2-like surface on MSQ. The effects of implantation parameters on the barrier property and bulk stability of MSQ were then studied. The results reveal one possible route to attain the “zero barrier thickness” requirement for interconnects systems.

Mechanical Properties of Porous MSQ Films: Impact of the Porogen Loading and Matrix Crosslinking.

AbstractSemiconductor industry needs a continuously improvement of integrated circuits performance and an increase of integrated density on silicon. The 2004 ITRS Roadmap underlines the need of dielectric material for ILD with dielectric constant (k) lower than 3 for the 90 nm node and than 2.4 for the 45 nm node. In this work, porous films with k value lower than 2.2 were processed using a porogen approach. Firstly, a material composed of a methylsilsesquioxane (MSQ) matrix and of organic nanoparticles (called porogen) is deposited and baked. Then, this composite is thermally cured to allow the porogens degradation and matrix crosslinking. Different k values were obtained by varying the porogen loading in the composite. Mechanical properties of composite and porous films (before and after porogen removal respectively) were investigated using nanoindentation and FTIR analysis for different porogen loadings (between 0 and 40 %). The composite modulus is higher than the porous film modulus for high porogen loading. This result is interpreted in term of matrix crosslinking. Mechanical properties were also modelized using foam mechanical models. For high porosity level, the best Young modulus fitting is obtained with tetrakaidecaedric cells, which seems in good agreement with porosity morphology.

Observation of Intrusion Rates of Hexamethyldisilazane During Supercritical Carbon Dioxide Functionalization of Triethoxyfluorosilane Low-k Films

AbstractWater adsorption by porous low-k silica films results in increased dielectric constants and is often due to silanol groups on the pore surfaces. Reacting the silanols with silylating agents (e.g., hexamethyldisilazane (HMDS) and trimethylchlorosilane (TMCS)) in supercritical CO2 (SC-CO2) can increase film hydrophobicity and can remove adsorbed water. In porous methylsilsesquioxane (MSQ) films (average pore size ∼ 3-4nm), it has been determined that supercritical silylation reactions do not substantially penetrate beyond the film surface.1,2 In this work we have examined the supercritical penetration behavior of silylating agents in low-k films with larger pore sizes (5-10nm). The depth and extent of reactants was determined by in situ infrared spectroscopy (FTIR), and surface hydrophobicity was examined by contact angle experiments.

Structural Studies of High-Performance Low-k Dielectric Materials Improved by Electron-Beam Curing

With the use of a newly developed electron beam (EB) curing process, an advanced methylsilsesquioxane (MSQ) low-k dielectric (LKD) film of k=2.9 was developed. It is noteworthy that the EB curing process can drastically improve the mechanical strength of LKD film and reduces the thermal budget without increasing the k value. The X-ray absorption fine structure (XAFS) study on the LKD was conducted to clarify the structural change upon EB curing. The structure of the film was compared with those of two different types of other MSQ films, the ladder-network structure and the random-network structure, and a chemical vapor deposition (CVD) film. The Si–O–Si bond angle and Si–O (Si–C) bond length were determined by fitting the Fourier transformed extended X-ray absorption fine structure (EXAFS) spectra. Si–O–Si bond angle of LKD film was found to be between those of the ladder and the random structure, which are 135° and 147°, respectively. The X-ray absorption near-edge structure (XANES) spectra of LKD film revealed two broad features corresponding to a mixture of the two structures. In contrast, Si–O–Si angles of the EB-cured LKD film and the CVD film were similar, and the XANES features of both films were almost identical with those of the random structure. The electronic structure as determined from XANES spectra was also discussed by comparing three-dimensional-linkage models obtained by ab initio calculations. We confirmed that the EB curing process of LKD film causes a drastic structural change. The change from the mixture of ladder and random structures to the completely random structure was caused by C–H bond breaking followed by the formation of new polymer-like clusters with C–C bonds.

Compatibility of high pressure cleaning mixtures with a porous low dielectric constant film: A positronium annihilation lifetime spectroscopic study

High pressure CO2 based cleaning mixtures have recently been proposed as an environmentally benign approach for postplasma etch residue removal. These mixtures must remove etch residues without damaging the low-k dielectric film that will be used to isolate interconnect structures in future generation devices. In this work, the compatibility of a CO2-based mixture with a porous low-k film is evaluated. Positronium annihilation lifetime spectroscopy (PALS) is used to monitor the change in pore size and film chemistry in a porous methyl silsesquioxane film after treatments under several different elevated pressure conditions. Spectroscopic ellipsometry and infrared spectroscopy are used to complement the PALS technique in order to better understand cleaning mixture effects on the dielectric film. CO2–TMAHCO3–methanol mixtures cause negligible changes in pore dimensions and bulk composition of the film. The high pressure treatments cause a small decrease in positronium formation which may be attributed to co...

CVD Boron Carbo-Nitride as Pore Sealant for Ultra Low-K Interlayer Dielectrics

AbstractBlanket porous methyl silsesquioxane (pMSQ) films on a Si substrate were studied with the intent to seal the pores and prevent penetration of a metallic precursor during barrier deposition. The blanket pMSQ films studied were approximately 220 nm thick and had been etched and ashed. When tantalum pentafluoride (TaF5) is exposed to an unsealed pMSQ sample, X-ray photoelectron spectroscopy (XPS) depth profiling and secondary ion mass spectroscopy (SIMS) depth profiling reveal penetration of Ta into the pores all the way to the pMSQ / Si interface. Boron carbo-nitride films were grown by thermal chemical vapor deposition (CVD) using dimethylamine borane (DMAB) precursor with Ar carrier gas and C2H4 coreactant. These films had a stoichiometry of BC0.9N0.07 and have been shown in a previous study to have a k value as low as 3.8. BC0.9N0.07 films ranging from 1.8 to 40.6 nm were deposited on pMSQ and then exposed to TaF5 gas to determine the extent of Ta penetration into the pMSQ. Ta penetration was determined by XPS depth profiling and sometimes SIMS depth profiling. XPS depth profiling of a TaF5 / 6.3 nm BC0.9N0.07 / pMSQ / Si film stack indicates the attenuation of the Ta signal to &lt; 2 at. % throughout the pMSQ. Backside SIMS of this sample suggests that trace amounts of Ta (&lt; 2 at. %) are due to knock-in by Ar ions used for sputtering. An identical film stack containing 3.9 nm BC0.9N0.07 was also successful at inhibiting Ta penetration even with a 370°C post-TaF5 exposure anneal, suggesting the stability of BC0.9N0.07 to thermal diffusion of Ta. All BC0.9N0.07 films thicker than and including 3.9 nm prevented Ta from penetrating into the pMSQ.

Synthesis and characterization of porous silsesquioxane dielectric films

The silsesquioxane (SSQ) films with low dielectric constant have been successfully synthesized by covalently binding a thermally decomposable porogen [poly(amidoamine), PAMAM] to a host polymer (hydrogen methyl silsesquioxane, HMSQ) via a coupling agent. The decomposition behavior of the porogen as well as the thermal and dielectric properties of the host polymer heat-treated in different atmospheres have been studied and compared. The dielectric properties of the HMSQ–PAMAM porous films have been investigated as a function of porogen concentration. An average dielectric constant about 2.06 could be obtained with leakage current density on the order of 10 -7 A/cm 2 for a film with 20-wt.% loading of the PAMAM polymer.

Preliminary evaluation of interfacial stability of surface modified porous methyl silsesquioxane by ion implantation for copper metallization scheme

Thin films of ultra-low- κ materials such as porous methyl silsesquioxane (MSQ) ( κ=2.2) were implanted with argon 1×10 16 cm −2 dose at energies varying from 20 to 50 keV at room temperature. This work shows that the surface hardness of the porous films can be improved five times as compared to the as-deposited porous films by implanting Ar with 1×10 16 cm −2 doses at 20 keV, sacrificing only a slight increase (∼9%) in dielectric constant (e.g., from 2.2 to 2.4). The hardness persists after 450 °C annealing. In this current work, an ion implantation strategy was pursued to create a SiO 2-like surface on MSQ. The effects of implantation parameters on the barrier property and bulk stability of MSQ were then studied. The results reveal one possible route to attain the “zero barrier thickness” requirement for interconnects systems.

Chemical Bonding Structure of Low Dielectric Constant Si:O:C:H Films Characterized by Solid-State NMR

Organosilicon films (Si:O:C:H) have the potential to replace as a lower dielectric constant microelectronic interconnection layer. The structure of Si:O:C:H films was characterized by and solid-state nuclear magnetic resonance (NMR) and also by X-ray photoelectron and Fourier transform infrared spectroscopies. Spin-coated hydrogen silsesquioxane, methyl silsesquioxane, and surface-modified nanoporous silica films were observed to be more homogeneous in structure than films grown by chemical vapor deposition (CVD). The CVD films were deposited from mixed with either tetramethylsilane, a mixture of tetramethylsilane and silane, trimethylsilane, or methylsilane. The various gas mixtures altered the degree of oxidation and hydrogenation in the resulting films, confirming that the composition of Si:O:C:H can be readily adjusted by CVD. Of all the characterization methods, the NMR was most readily able to distinguish differences between the Si:O:C:H films, resolving up to twelve distinct bonding environments. These same bonding configurations are also found in bulk organosilicate glasses. In all films studied, NMR reveals that methyl is the primary bonding configuration for carbon and no -bonded carbon was detected. © 2004 The Electrochemical Society. All rights reserved.

Etching of low-k materials in high density fluorocarbon plasma

Low dielectric constant materials (low-k) are used as interlevel dielectrics in integrated circuits. This paper concerns the etching process of these materials in high density plasma with the aim to provide some insights concerning the etch mechanisms. Materials studied are methylsilsesquioxane (MSQ) polymers, either dense (SiOC) or containing 40% of porosity (porous SiOC). Amorphous hydrogenated silicon carbide (SiC) material, used as hard mask and/or etch stop layer, is also investigated. Etch is performed in an inductively coupled reactor using fluorocarbon gases, which have proven to be very successful in the etch of conventional SiO 2 . First, etching with hexafluoroethane (C 2 F 6 ) is performed. Although etch rates are high, etch selectivities with respect to SiC are weak. So, oxygen, argon, and hydrogen are added to C 2 F 6 with the aim of improving selectivities. The best selectivity is obtained for the C 2 F 6 /H 2 (10%-90%) mixture. To understand etch rate and selectivity variations, plasma analyses by optical emission spectroscopy are correlated to surface analysis using X-Ray Photoelectron Spectroscopy (XPS). In general, atomic fluorine concentration in the plasma explains the etch rate, while the presence of a fluorocarbon layer on the surface is well correlated to the selectivity. To ensure that the etch process does not affect materials properties, and particularly their dielectric constant, bulk analysis by Fourier Transformed Infra-Red spectroscopy and images by Scanning Electron Microscopy have also been carried out.

Poly(methyl silsesquioxane)/amphiphilic block copolymer hybrids and their porous derivatives: Poly(styrene‐block‐acrylic acid) and poly(styrene‐block‐3‐trimethoxysilylpropyl methacrylate)

AbstractPorous poly(methyl silsesquioxane) (PMSSQ) films were prepared from PMSSQ/amphiphilic block copolymer (ABC) hybrids, and this was followed by spin coating and multistep baking. The ABCs were poly(styrene‐block‐acrylic acid) (PS‐b‐PAA) and poly(styrene‐block‐3‐trimethoxysilylpropyl methacrylate) (PS‐b‐PMSMA), which were synthesized by living polymerization. The chemical bonding between the ABCs and PMSSQ resulted in significant differences in the morphologies and properties of the hybrids and their porous derivatives. Both intramolecular and intermolecular hydrogen bonding existed in the PMSSQ/PS‐b‐PAA hybrid and led to macrophase separation. Through the modification of the chemical structure from the poly(acrylic acid) segment to PMSMA, covalent bonding between PMSSQ and PMSMA occurred and prevented the macrophase separation and initial pyrolysis of the ABC. Modulated differential scanning calorimetry results also suggested a significant difference in the miscibility of the two hybrid systems. The chemical bonding resulted in higher retardation of the symmetry‐to‐nonsymmetry SiOSi structural transformation for PMSSQ/PS‐b‐PMSMA than for PMSSQ/PS‐b‐PAA according to Fourier transform infrared studies. The pore size of the nanoporous thin film from the PMSSQ/PS‐b‐PMSMA hybrid was estimated by transmission electron microscopy to be less than 15 nm. The refractive index and dielectric constant of the prepared porous films decreased from 1.354 to 1.226 and from 2.603 to 1.843 as the PS‐b‐PMSMA loading increased from 0 to 50 wt %, respectively. This study suggests that chemical bonding in hybrid materials plays a significant role in the preparation of low‐dielectric‐constant nanoporous films. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 4466–4477, 2004

Adhesion studies of low- k silsesquioxane

As the dimension of IC devices continues to shrink, interconnection delay becomes a limiting factor to increase device speed. Therefore, it is necessary to use low-dielectric-constant (low- k) materials to reduce the capacitance between the metal interconnection. In this work, two kinds of low- k silisesquioxane have been investigated: Methylsilsesquioxane (MSQ) and Hydrogen silisesquioxane (HSQ). The adhesion between silicon nitride (Si 3N 4) and low- k MSQ or HSQ has been examined. The adhesion strength can be improved by various plasma treatments on top of the silisesquioxane. The adhesion strength between MSQ and Si 3N 4 can be increased by NH3 or H2 plasma treatment. On the other hand, the adhesion strength between HSQ and Si 3N 4 is only increased by NH3 treatment effectively. Surface properties and electrical characterization of low- k silisesquioxane have also been examined after various plasma treatments.

Investigation of structure, thermal and oxygen plasma stability of mesoporous methylsilsesquioxane films by X-ray reflectivity and small angle scattering

The structural properties of the methylsilsesquioxane (MSQ) based mesoporous low dielectric constant (low- k) film JSR LKD have been characterized by specular X-ray reflectivity (SXR) and small angle X-ray scattering (SAXS). Using SXR, the as cured LKD material was found to have a mass density of 0.996 g/cm 3 and an estimated porosity of 54%. From SAXS, the typical pore size was determined to be 2.5 nm. However, larger pores with diameter of ∼13 nm were also deduced to be present. Structural changes in LKD after isochronal annealing in nitrogen and oxygen plasma treatment were studied by SXR. The density of LKD films decreased with annealing temperature up to 400 °C and a single layer model was adequate for SXR fitting. For annealing above 500 °C and for oxygen plasma treatment, a bilayer model consisting of a denser upper layer and a bottom layer was necessary for data fitting. The mechanisms behind the development of this bilayer structure are discussed and compared with reference to Fourier transform infrared (FTIR) spectra.

Low dielectric constant nanoporous poly(methyl silsesquioxane) using poly(styrene-block-2-vinylpyridine) as a template

Low dielectric constant nanoporous poly(methyl silsesquioxane) (PMSSQ) was prepared through the templating of an amphiphilic block copolymer, poly(styrene-b-2-vinylpyridine) (PS-b-P2VP). The experimental and theoretical studies suggest that the intermolecular hydrogen bonding interaction is existed between the PMSSQ precursor and PS-b-P2VP. The result of modulated differential scanning calorimeter (MDSC) indicates the miscible hybrid of the PMSSQ precursor/PS-b-P2VP. The miscible hybrid and the narrow thermal decomposition of the PS-b-P2VP lead to nanopores in the prepared films from the results of transmission electronic microscopy (TEM), atomic force microscopy (AFM), and small angle X-ray scattering (SAXS). The effects of the loading ratio and the PS block volume ratio (fPS: 0.74, 0.46 and 0.35) on the morphology and properties of the prepared nanoporous PMSSQ films were investigated. The AFM and TEM studies suggest that the uniform pore morphology should be prepared from a modest porogen loading level for the optimum intermolecular hydrogen bonding. The PS-b-P2VP with a smaller fPS requires a higher loading level to obtain the uniform pores. The refractive index and dielectric constant of the prepared nanoporous films could be tuned by the loading ratio in the range of 1.361–1.139 and 2.359–1.509, respectively. However, both properties are independent of the fPS. The prepared study demonstrates the control of the morphology and properties of the nanoporous films through the polymer structure.

Statistical Study for Electromigration Reliability in Dual-Damascene Cu Interconnects

This paper summarizes results from recent studies on electromigration (EM) reliability of Cu dual-damascene interconnects using a statistical approach. First, mass transport in Cu damascene structure was discussed, where the activation energies for oxide, porous MSQ (methylsilsesquioxane), and organic polymer dielectrics were found to be in the range of 0.8-1.0 eV, suggesting mass transport is dominated by interfacial diffusion at the Cu and SiN/sub x/ cap-layer interface regardless of the dielectric material. Then the impact of low-k inter-layer dielectrics on EM characteristics was investigated by measuring the critical product of current density and conductor length, (jL)/sub c/. Compared with oxide, the weaker mechanical strength of low-k dielectrics reduces (jL)/sub c/ due to less confinement and a smaller back flow stress gradient /spl Delta//spl sigma//L in Cu/low-k interconnects. Extrinsic failure due to interfacial delamination was observed in Cu/organic polymer interconnects, which caused further decrease in (jL)/sub c/. Complementing EM tests, Monte Carlo simulation was developed based on the weakest link approximation to separate the bimodal failure distribution into two individual lognormal distributions and deduce the characteristics of the weak-mode (early) and the strong-mode failures. Failure analysis using FIB confirmed the bimodal failure behavior with void formation at the cathode via bottom found to be responsible for the early failures.

Molecular Caulking: A Pore Sealing CVD Polymer for Ultralow k Dielectrics

Porosity has been introduced in existing low-k interlayer dielectrics to further reduce their dielectric constant. It is desirable to deposit a metallic layer on top of the porous dielectric by chemical vapor deposition (CVD). However this presents the challenge of preventing the precursor from penetrating into the porous dielectric and depositing metal within this insulating layer. In the present paper a low-k CVD polymer capping (Molecular Caulking™) is deposited at room temperature onto the porous ultralow k dielectric methyl silsesquioxane. Experiments show that the Molecular Caulking prevents precursor penetration during subsequent metallorganic CVD. In addition, while the Molecular Caulking itself slightly penetrates into the methyl silsesquioxane, it does not appreciably increase surface roughness or film dielectric constant. © 2004 The Electrochemical Society. All rights reserved.