Metal−Dielectric Interface Toughening by Catalyzed Ring Opening in

Dec 7, 2009 - ABSTRACT We demonstrate a novel strategy for toughening metal-dielectric interfaces by catalyzed fissure of low-polarizability moieties ...
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Metal-Dielectric Interface Toughening by Catalyzed Ring Opening in a Monolayer Saurabh Garg,† Binay Singh,† Xinxing Liu,‡ Ashutosh Jain,† N. Ravishankar,† Leonard Interrante,‡ and Ganpati Ramanath*,† †

Materials Science and Engineering Department, Rensselaer Polytechnic Institute, Troy, New York 12180 and Chemistry Department, Rensselaer Polytechnic Institute, Troy, New York 12180

‡

ABSTRACT We demonstrate a novel strategy for toughening metal-dielectric interfaces by catalyzed fissure of low-polarizability moieties in an organosilane monolayer. Photoelectron spectroscopy and ab initio calculations show that sevenfold toughening of Cu-silica interfaces is due to Cu-catalyzed disilacyclobutane ring opening and bonding. Our findings open up possibilities for directly integrating metals with molecularly derived low permittivity dielectrics for applications without using an intermediary glue layer, for example, by incorporating strained moieties into polymer precursors. SECTION Surfaces, Interfaces, Catalysis

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ailoring adherent metal-dielectric interfaces, without using thick intermediary layers, is crucial for a variety of emerging applications such as wiring, packaging1 and laminates for high frequency-electronics,2 wireless communications and integrated circuits,3 coatings,4 and biomaterials.5 For example, tailoring adhesion between copper and a low dielectric permittivity (low k) interlayer and prohibiting interface chemical mixing is a key challenge in integrated circuit device wiring, where interfacial stability is essential for withstanding thermomechanical processing and critical for reliability and performance.6,7 The use of thick (e.g., >5 nm)8 intermediary adhesive layers is not a viable solution for emerging technology nodes because of the consequential increase in the effective resistivity and/or dielectric permittivity, which degrade device speed. Self-assembled molecular nanolayers (MNLs) provide an attractive alternative for toughening copper-silica interfaces9,10 and can inhibit chemical mixing through strong bonding between the terminal moieties of the MNL and the metal. Furthermore, this strategy provides a means to eliminate a separate glue layer altogether by incorporating moieties responsible for interfacial toughening and metal blocking into dielectric materials to obtain adherent metal-dielectric interfaces. Indeed, recent work11,12 has shown that low k materials derived from cyclolinear polycarbosilanes with 1,3-disilacyclobutane (DSCB) rings offer promise for realizing this exciting possibility. Curing cyclolinear polycarbosilanes results in DSCB ring opening and cross-linking, forming a hydrophobic carbosilane thermoset with k < 2.7, and curtails Cu transport across the Cu/CLPCS interface11 due to chemical immobilization of Cu, an attribute which could conceivably enhance interfacial adhesion as well. Here, we demonstrate seven-fold toughening of a coppersilica interface through metal-catalyzed ring opening and bonding by using a DSCB-terminated organosilane nanolayer

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at the interface. Such chemically induced interfacial toughening opens up new vistas for directly integrating metals with molecularly derived low k dielectrics, for example, by incorporating strained low k moieties into low k precursors. Furthermore, our approach of synthesizing short molecules amenable to self-assembly (e.g., organosilanes), with chosen low-polarizability termini for evaluating their effects on interfacial properties, is an attractive way to understand atomic-level mechanisms and identify moieties incorporating into molecular precursors of low k materials constituting the interface. We synthesized a DSCB-terminated organosilane (DSCBOS) by attaching DSCB rings using a butylene bridge to a triethoxysilane using Grignard chemistry (see Scheme 1 and the Experimental Section). Variable angle spectroscopic ellipsometry reveals a ∼0.8 ( 0.1 nm thick DSCBOS layer (Figure S1, Supporting Information), which is within 10% of the theoretical molecular length, indicating monolayer formation. Corelevel spectra acquired by X-ray photoelectron spectroscopy (XPS) at different takeoff angles θ (see Figure 1a) reveal that DSCBOS molecules assemble in an upright configuration on silica, with the DSCB rings pointing away from the surface. The surface specificity of the DSCB ring is evident from the emergence and intensity increase of the 101.2 eV Si 2p subband component corresponding to the Si-C bonds in DSCB rings with decreasing θ, contrary to the trend exhibited by the 103.4 eV Si 2p sub-band from Si-O-Si bonds9 in silica (see Figure 1b). Four-point bend tests on structures with Cu/MNL/SiO2 interfaces annealed to different Tanneal show that the Received Date: October 13, 2009 Accepted Date: November 20, 2009 Published on Web Date: December 07, 2009

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Figure 2. Fracture toughness of Cu/MNL/SiO2 interfaces with DSCBOS (red) or HTES (green), annealed at different temperatures in vacuum. Schematic sketches capture the loading geometry and sandwich structure (inset) and the structures of DSCBOS and HTES (right), where Me denotes -CH3.

for 25 °C e Tanneal e 500 °C. Although HTES also toughens the interface for Tanneal g 300 °C, the fracture toughness is more than two-fold smaller than that observed for DSCBOS for 300 °C e Tanneal e 500 °C, pointing to the participation of the DSCB ring in interfacial toughening. Core-level spectra acquired from fracture surfaces reveal that as-prepared samples with DSCBOS delaminate at the Cu/ DSCBOS interface (see Figure 3 and Supporting Information Figure S3), pointing to weak bonding between Cu and DSCB rings. The fracture surface on the silica side of the thin film sandwich exhibits Si 2p sub-band components from Si-C in DSCB at 101.2 eV and from Si-O-Si at 103.4 eV (see Figure 3b). The other fracture surface shows only Cu signatures; the Si-C signature from DSCB is undetectable. In contrast, annealing over 100 °C e Tanneal e 500 °C results in delamination at the DSCBOS/silica interface. Silica fracture surfaces show a solitary 103.4 eV Si 2p band signature of Si-O-Si, with no DSCB or Cu signatures. Cu fracture surfaces exhibit a 102.8 eV Si 2p sub-band component from ethoxysilane moieties in DSCBOS (see Supporting Information Figure S4) along with the Si-C signature from DSCB, pointing to annealing-induced Cu-DSCB bonding. We note that structural changes in the MNL are unlikely during Cu sputter deposition because the 1-2 eV Cu ions and neutrals arriving at the surface during sputter deposition13 have about 5-10 times lower energy than that needed to disrupt the MNL.14,15 Although the Cu/DSCBOS interface is strengthened for Tanneal g 100 °C, we attribute the low fracture toughness for 100 °C < Tanneal < 300 °C to low-degree or incomplete Si-O-Si bridging at the DSCBOS/SiO2 interface, consistent with hydration-induced siloxane bridge weakening reported in the same temperature range for another organosilane/silica interface.9 Thus, the observed interface toughness increase for Tanneal > 300 °C is due to Cu bonding with DSCB moieties in combination with enhanced siloxane bridging known to occur above 300 °C.9,16 In order to further understand the Cu-DSCBOS bonding mechanism, we coated a DSCBOS film on an air-exposed copper surface and annealed it at T g 100 °C in vacuum. Our results reveal copper-oxide-catalyzed DSCB ring opening and bonding of the resultant moieties to copper. The 101.2 eV

Figure 1. (a) Core-level Si 2p band obtained by XPS at different takeoff angles θ (see left inset sketch). The right inset shows the Si 2p sub-band components from DSCB (red) and siloxane (blue) moieties in a spectrum obtained at θ =10°. (b) Integrated intensity of Si 2p sub-bands from silica and Si-C moieties in as-prepared DSCBOS, plotted as a function of θ. Scheme 1. DSCB-Terminated Organosilane 1,3-Disilacyclobutane1,4-Butanetriethoxysilane (DSCBOS) Synthesisa

a The DSCB ring (red), butylene linker (blue), and ethoxy silane head group (green) are highlighted in different colors.

DSCBOS-tailored interface toughens nearly seven-fold for Tanneal >300 °C (see Figure 2). As-prepared Cu/DSCBOS/ SiO2 interfaces exhibit a low average interface toughness of Greference = 2.1 ( 0.5 J/m2 (see Supporting Information Figure S2 for representative load-displacement curves). This toughness value is similar to that reported for pristine Cu/SiO2 interfaces10 and structures with a methyl-terminated organosilane MNL without a DSCB ring, hexyl triethoxysilane (HTES), at the interface, suggesting a lack of strong bonding in asprepared Cu/DSCBOS/silica interfaces. Cu/MNL/SiO2 interfaces annealed at 100 °C e Tanneal < 300 °C exhibit similar or marginally larger G ∼ 2-3 J/m2 for both DSCBOS and HTES interfacial nanolayers. For Tanneal g300 °C, however, DSCBOS-tailored interface toughness increases rapidly with the annealing temperature. For instance, G = 21.3 ( 0.5 J/m2 for Tanneal = 500 °C, which is a seven-fold higher toughness than the 3 ( 0.5 J/m2 measured for pristine Cu/SiO2 interfaces

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Figure 3. High-resolution Si 2p bands from (a) copper and (b) silica fracture surfaces of Cu/DSCBOS/SiO2 structures tested after annealing to different temperatures. The sub-band components from Si-C (red) from the DSCB ring and Si-O (blue) from either DSCB or silica are indicated. Schematic sketches capture salient aspects of the fracture location and DSCB ring opening at Cu/DSCBOS/SiO2 interfaces; Me denotes -CH3. For as-prepared structures and Tanneal < 100 °C, fracture occurs at the Cu/DSCBOS interface, while for Tanneal g 100 °C, fracture occurs at the DSCBOS/SiO2 interface.

DSCB ring band intensity decreases relative to that of SiO-Si at 102.6 eV, indicative of ring- opening via Si-C bond cleavage (see Figure 4) and DSCBOS anchoring onto the copper surface. Annealing HTES on Cu desorbs and/or degrades the organosilane, as is evident from the absence of Si 2p signatures (see Supporting Information Figure S5), again underscoring the importance of the DSCB termini for bonding with the air-exposed copper surface. Our observation of DSCB ring opening at 100 °C is significantly lower than the ∼200250 °C range reported previously,17 suggesting that the copper surface catalyzes ring opening. Annealing also reduces the surface oxide, as seen from the disappearance of the ∼943 and ∼963 eV 2p shakeup bands18 from Cu(II) and the emergence of the ∼335 eV LMM Auger signature of Cu(0).18 Such molecularly induced copper surface oxide reduction could be exploited for cleaning and passivation of Cu wiring, for example, to inhibit metal diffusion,6 and to decrease surface-scattering-induced resistivity.11 Since products from DSCB ring opening do not exhibit unique spectral signatures, we carried out ab initio density functional theory calculations of a DSCBOS molecule bonded covalently to silica to obtain insights into the nature of Cu-DSCB interactions. Our calculations were based on the B3LYP gradient-corrected functional and used 6-31þG* basis sets and the Gaussian 03 code,19 which are well-suited to accurately capture long-range interactions such as hydrogen bonding. We considered a single DSCBOS molecule bonded covalently to silica and relaxed to a ground state of Eref =0 eV and hence do not address intermolecular interactions. Our

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Figure 4. Core-level Cu 2p (left) and Si 2p bands (right) from DSCBOS films coated onto an air-exposed copper surface before (red) and after (blue) annealing at 100 °C in vacuum. The Si-O (brown) and Si-C (green) sub-bands of Si 2p from DSCBOS are shown. Cu LMM Auger peaks are also shown (middle). Schematic sketches depict copper reduction by oxygen scavenging and concomitant DSCB ring opening. Me denotes -CH3.

calculations reveal that Cu-Si bond formation lowers the system energy by ∼1.5 eV (see Figure 5), corroborating that copper can catalyze the cleavage of the strained Si-C bonds in the DSCB ring. The presence of oxygen further decreases

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precipitated by adding hexane, and the liquid was transferred to another flask by cannulation. After solvent and chlorosilane removal, we obtained 5.5 g, corresponding to an 80% yield of 95% pure (determined by gas chromatography) 1,3-disilacyclobutane-1,4-butanetriethoxysilane (DSCBOS) by vacuum distillation. DSCBOS boils at ∼100 °C at 0.6 mmHg and exhibits the following 1H NMR signatures (ppm): -0.2-0.1 (multiplet, 4H, SiCH2Si), 0.12-0.62 (multiplet, 9H, SiCH3), 0.58-0.89 (multiplet, 4H, SiCH*2CH2CH2 CH*2Si), 1.281.65 (multiplet, 4H, CH2(CH*2)2CH2), 3.60-4.22 (multiplet, 6H, OCH*2CH3), 1.05-1.22 (triplet, 9H, OCH2CH*3). 13C NMR (ppm): 0.52 (singlet, SiC*H2Si), 1.09-1.41 (triplet, SiCH3), 9.10 (singlet, C*H2SiO), 1.28-1.65 (triplet, OCH2C*H3), 3.60-4.22 (doublet, CH2(CH*2)CH2), 57.17 (singlet, OC*H2CH3). 29Si NMR (ppm): 3.10 and 3.62 (SiCH2Si), -44.25 (SiO). Molecular Nanolayer Formation, Mechanical Tests, and Fracture Surface Spectroscopy. Molecular nanolayers (MNLs) of the DSCBOS were self-assembled by dipping n-type Si(001) wafers capped with a 85 nm thick thermal oxide in a 5 mM solution of DSCBOS in toluene for 150 min, adapting the procedure described in detail elsewhere.10 In order to measure the fracture toughness of the Cu/MNL/ SiO2 interface using four-point bend mechanical tests, we prepared dummy-Si/epoxy/Ta/Cu/MNL/SiO2/Si(001) structures. We sputter- deposited 50 nm thick Cu and 150 nm thick Ta layers successively onto the MNL/SiO2/Si(001) structures without vacuum break. The Ta layer was used to prevent failure at the weak Cu-epoxy interface to measure the toughness of the Cu/MNL/SiO2 interface. We annealed the Ta/Cu/MNL/SiO2/Si(001) structures at 100 °C e Tanneal e 500 °C for 30 min in a 2  10-7 Torr vacuum before being bonded face-to-face with a dummy silicon wafer and diced into 40 mm  5 mm pieces. Four-point bend tests were carried out at a 0.01 μm s-1 strain rate in a high-stiffness micromechanical system as described elsewhere.10 The interface toughness G was determined by applying a linear-elastic fracture model to the first plateau of the load-displacement curve that corresponds to crack propagation along the weakest interface.7 The test involves crack propagation at a 43° phase angle that describes the shear/normal stress ratio, and the measured fracture energy could contain, and does not distinguish between, contributions from bond breaking, molecular stretching, and plasticity in adjoining films. The XPS spectra were acquired using a PHI 5400 instrument with a Mg KR beam and 23.5 eV pass energy and were corrected for charging by using the adventitious C 1s peak at 285 eV as a reference.

Figure 5. System energy for molecules with DSCB rings on silica, determined by first-principles density functional theory calculations, for Cu-catalyzed ring opening with or without oxygen (see schematics). The Cu atom represents the metal overlayer, and Me denotes -CH3. All energies are referenced to the molecule with an unopened DSCB ring.

the system energy by nearly four-fold to ∼6.3 eV/molecule through the formation of Cu-O-Si bridges. The expected 101.3 eV sub-bands from Cu-Si bonds are within experimental uncertainty of the ∼101.2 eV signatures of DSCB rings, precluding their distinction by XPS (see Supporting Information Figure S6). However, the importance of oxygen from the surface copper oxide is supported by the results of DSCB annealing on Au surfaces not showing ring opening at T=100 °C (see Supporting Information Figure S7). If oxygen is key to low-temperature ring opening, strong bonding between opened DSCB ring moieties and copper in buried Cu/DSCBOS/silica interfaces (where the inner surface of Cu is not oxidized) points to the role of residual moisture present in the DSCBOS monolayer. Upon annealing, we expect the interface to be comprised of a hybrid organic/inorganic layer, consistent with organosilane polymers showing weight loss above 400 °C and beginning to form amorphous silicon-oxycarbide above 600 °C.20 In summary, we have demonstrated a novel approach to toughen metal-dielectric interfaces by metal-catalyzed ring opening in a subnanometer-thick layer of a newly synthesized DSCB-terminated organosilane. Annealing thin film structures with the organosilane monolayer at copper-silica interfaces yields seven-fold higher fracture toughness than exhibited by unfunctionalized interfaces. Our results indicate that the bonding of DSCB ring moieties and copper, combined with siloxane bridging, is the toughening mechanism. The concept of catalytic fissure of strained rings and their subsequent anchoring with the metal could be applied to other materials systems. Moreover, incorporating strained rings into polymeric precursors could provide a means to directly integrate polymer-derived dielectrics with metals without a separate interfacial layer.

SUPPORTING INFORMATION AVAILABLE Additional data showing the raw data on mechanical properties and fracture surface spectroscopy. This material is available free of charge via the Internet at http://pubs.acs.org.

Experimental Section Synthesis of 1,3-Disilacyclobutane (DSCB)-1,4-Butanetriethoxysilane. The Grignard reagent, prepared by dropwise addition of 1-chloro-1,3,3-dimethyl-1,3-disilacyclobutane (3.28 g, 0.02 mol) to 0.02 mol of freshly prepared 1,4-bis(bromomagnesio)butane at 0 °C, was mixed with chlorotriethoxysilane (4.75 g, 0.024 mol) at 0 °C. The byproduct salts were

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AUTHOR INFORMATION Corresponding Author: *To whom correspondence should be addressed. E-mail: ramanath@ rpi.edu.

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ACKNOWLEDGMENT This work was supported by the NSF

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through DMR 0519081 and the New York State Foundation for Science, Technology and Innovation. (19)

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