Inorganic Hybrid Nanocomposites

Watson Research Center, Kitchawan Road/Route 134, Yorktown Heights, New York 10598. Received January 11, 2002. Revised Manuscript Received June 10, ...
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Structure and Interaction of Organic/Inorganic Hybrid Nanocomposites for Microelectronic Applications. 1. MSSQ/P(MMA-co-DMAEMA) Nanocomposites Q. R. Huang,† Willi Volksen,‡ Elbert Huang,§ M. Toney,‡ Curtis W. Frank,*,† and Robert D. Miller*,‡ Department of Chemical Engineering, Stanford University, Stanford, California 94305, IBM Almaden Research Center, 650 Harry Road, San Jose, California 95120, and IBM T. J. Watson Research Center, Kitchawan Road/Route 134, Yorktown Heights, New York 10598 Received January 11, 2002. Revised Manuscript Received June 10, 2002

Nanoporous methyl silsesquioxane (MSSQ), which is an important and promising candidate for spin-on ultralow dielectric constant applications, has been produced via the thermosetting of MSSQ, templated by a nanodispersed, thermally decomposable pore generator (porogen)poly(methyl methacrylate-co-dimethylaminoethyl methacrylate) [P(MMA-co-DMAEMA)]. Fourier transform infrared spectroscopy is used to study the interaction and structural changes of MSSQ/P(MMA-co-DMAEMA) nanocomposites as a function of curing temperature (ranging from 25 to 450 °C) and porogen loading (ranging from 0 to 70 wt %). We find that strong hydrogen-bonding interactions occur between the -OH end groups in MSSQ and the tertiary amino groups in P(MMA-co-DMAEMA) in films at 25 °C. An increase in cure temperature from 25 to 250 °C and finally to 450 °C transforms MSSQ from a material with many reactive end groups to a highly cross-linked structure. In addition, the amino substituent in P(MMA-co-DMAEMA) can act as a catalyst for the condensation and crosslinking of MSSQ. An increase of porogen loading to 70 wt % and a decrease in the silanol group concentration in MSSQ both hinder the formation of the -Si-O-Si- network. Finally, small-angle X-ray scattering (SAXS) results indicate that MSSQ resins initially having higher -OH end group concentrations ultimately generate smaller pores after the removal of porogens.

Introduction As feature sizes in microelectronic devices are driven to sub-100-nm dimensions, device performance will no longer scale as for past generations because of the substantial increase in interconnect delays (RC delay) caused by increased line resistance and capacitive coupling and cross talk between the smaller and more closely spaced metal lines. Current production features have already reached 150 nm and are projected to decrease to 100 nm by the year 2006, according to the National Technology Roadmap.1 Two approaches exist for mitigating an increase in RC delays: switching from aluminum to copper for the interconnect metallurgy to reduce the wiring resistance and replacing the traditional SiO2 (k ∼ 4.0) insulator with a significantly lower dielectric constant material to reduce the capacitance. The search for low dielectric constant materials has attracted much interest in recent years,2 and five Materials Research Society volumes focusing on low * To whom correspondence should be addressed. † Stanford University. ‡ IBM Almaden Research Center. § IBM T. J. Watson Research Center. (1) The National Technology Roadmap for Semiconductors; Semiconductor Industry Association: San Jose, CA, 1997. (2) (a) Maier, G. Prog. Polym. Sci. 2001, 26, 3. (b) Hedrick, J. L.; Carter, K. R.; Labadie, J. W.; Miller, R. D.; Volksen, W.; Hawker, C. J.; Yoon, D. Y.; Russell, T. P.; McGrath, J. E.; Briber, R. M. Adv. Polym. Sci. 1999, 141, 1. (c) Miller, R. D. Science 1999, 286, 421.

dielectric constant materials have appeared.3 A number of insulating materials, including both organic and inorganic-based, are currently under consideration as replacements for SiO2. Organic low-k materials include polyimides, heteroaromatic polymers, poly(aryl ether)s, fluoropolymers, and hydrocarbon polymers.2a Inorganic low-k systems include a variety of silica-like materials. Although several promising candidates, such as SiLK (Dow)4 and organosilicates,5 have achieved k < 3, among fully dense candidates only highly fluorinated polymers, such as poly(tetrafluoroethylene), reach the ultralow-k target (k < 2.2).6 However, fluoropolymers have inadequate thermal stability for current integration procedures, and there are concerns regarding HF evolution during processing and reactions with the metallurgy. For nonfluorinated materials, dielectric constants below (3) Low-Dielectric Constant Materials; Materials Research Society Symposia Proceedings, Pittsburgh, PA; Materials Research Society, 1st, 1995, Vol. 381; 2nd, 1996, Vol. 443; 3rd, 1997, Vol. 476; 4th, 1998, Vol. 511; 5th, 1999, Vol. 565. (4) (a) Townsend, P. H.; Martin, S. J.; Godschalx, J.; Romer, D. R.; Smith, D. W., Jr.; Castillo, D.; DeVries, R.; Buske, G.; Rondan, N.; Froelicher, S.; Marshall, J.; Shaffer, E. O.; Im, J.-H.; Mater. Res. Soc. Symp. Proc. 1997, 476, 9. (b) Martin, S. J.; Godschalx, J. P.; Mills, M. E.; Shaffer, E. O.; Townsend, P. H. Adv. Mater. 2000, 12, 1769. (5) Kim, S. M.; Yoon, D. Y.; Nguyen, C. V.; Han, J.; Jaffe, R. L. Mater. Res. Soc. Symp. Proc. 1998, 511, 39. (6) (a) Brandrup, J.; Immergut, E. H. Polymer Handbook, 3rd ed.; Wiley: New York, 1989; p. V1. (b) Rosenmeyer, C. T.; Wu, H. Proc. Mater. Res. Soc. 1996, 427, 463. (c) Rosenmeyer, C. T.; Bartz, J. W.; Hammes, J. Proc. Mater. Res. Soc. 1997, 476, 231.

10.1021/cm020014z CCC: $22.00 © 2002 American Chemical Society Published on Web 08/13/2002

Nanocomposites for Microelectronic Applications

2.2 will be inaccessible without introducing air voids into the film. Since air has a dielectric constant of 1.01, the controlled incorporation of air voids into the matrix becomes an attractive approach to significantly decrease the dielectric constant of polymeric materials.7,8 Several classes of porous materials, either organic2b,9 or inorganic,8,10 have been explored for ultralow-k applications. Mesoporous silica with ordered morphologies prepared through a templated sol-gel process followed by calcination can reach the ultralow-k target.10 However, silica is intrinsically hydrophilic, causing problems with water moisture adsorption, and the high porosities (>50%) necessary to reach ultralow dielectric targets further degrade the mechanical properties. Although the hydrophobicity of the silica can be improved by reacting silanols with chlorotrimethylsilane (TMCS),10b,c this increases the cost of integration. One of the most promising routes to ultralow-k materials is the production of nanoporous structures from low molecular weight organosilanes where cross-linking is templated by nanodispersed, thermally decomposable porogens. One particularly promising organosilicate matrix is methyl silsesquioxane (MSSQ) with the general formula (MeSiO3/2)n, a material that is intrinsically hydrophobic with low water uptake ( 250 °C) the ≈1030-cm-1 band dominates the IR spectra. It is known that caged structures show a strong Si-O-Si stretching absorption in the high-frequency region from 1115 to 1150 cm-1.19 However, Wang et al.16a,c have suggested that silicon end groups that contain nonbridging oxygen atoms (Onb, i.e., an oxygen not bonded to two silicon atoms such as would be expected in SiOH end groups) can also absorb in the high-frequency range. Short -SiO- chains will necessarily contain a higher proportion of Onb units. Cross(18) (a) Baney, R. H.; Itoh, M.; Sakakibara, A.; Suzuki, T. Chem. Rev. 1995, 95, 1409. (b) Brown, J.; Vogt, J.; Katchman, A.; Eustance, J. W.; Kiser, K. M.; Krantz, K. W. J. Am. Chem. Soc. 1960, 82, 6194. (c) Brown, J.; Prescott, P. I. J. Am. Chem. Soc. 1964, 86, 1402. (d) Frye, C. L.; Klosowski, J. M. J. Am. Chem. Soc. 1971, 93, 4599. (e) Brown, J. F., Jr. J. Polym. Sci.: Part C 1963, 1, 83. (19) (a) Voronkov, M. G.; Lavrent’yev, V. I. Top. Curr. Chem. 1982, 102, 199. (b) Bornhauser, P.; Calzaferri, G. J. Phys. Chem. 1996, 100, 2035.

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linking initiated by condensation of SiOH units will increase the length of the -SiO- chains and decrease the absorption due to nonbridging oxygens. The authors have also proposed the absorbance ratio of the lower frequency/higher frequency Si-O-Si bands as a measure of cross-linking as a function of curing temperature. Although it is tempting to associate the disappearance of the higher frequency Si-O-Si stretching band and the corresponding increase in the band at lower frequencies (1000-1080 cm-1) with the loss of caged structures and the formation of an extended ladder structure, this conclusion should not be based on IR data alone.18e Neither is the increase in the ratio of the lower/ higher frequency Si-O-Si absorption bands inconsistent with the conversion of cages and chain ends into more ladder-like structures. At this point, it seems reasonable that the cured samples (450 °C) will contain a mixture of cages, partial cages, ladders, and linear and branched chains, although very few reactive chain ends. Two more features are also obvious in Figure 5: (1) The broad O-H stretching band at ≈3400 cm-1 and the Si-OH stretching vibrational band at ≈930 cm-1 disappear at curing temperatures higher than 150 °C, which corresponds to the onset of cross-linking. (2) The carbonyl absorption of the porogen at ≈1730 cm-1 ultimately completely disappears at curing temperatures higher than 400 °C. The absorbance of the Si-OH stretching vibrational mode around 930 cm-1 also mirrors the population of unreacted Si-OH groups. Thus, the disappearance of the Si-OH associated bands at T ) 150 °C in Figure 5 indicates that the condensation of Si-OH groups is advanced at this temperature. This is also the temperature region where the resin modulus begins to increase significantly (dynamic mechanical analysis data), and shows a progressive increase in storage modulus above 150 °C.8a The fact that the thermal decomposition temperature for the porogen is significantly higher than the cross-linking temperature of the MSSQ matrix is an important feature for prevention of the collapse of the nanoporous film due to capillary pressures during the decomposition of the porogen. The role of -SiOH end groups in the initial MSSQ resin on the phase separation process and the final MSSQ structure can also be studied by FTIR. Figure 6 shows the FTIR spectra of nanocomposites of P(MMAco-DMAEMA) with MSSQ-HI (Figure 6a) and MSSQLO (Figure 6b) cured at T ) 450 °C. At this temperature, all of the P(MMA-co-DMAEMA) is decomposed (no carbonyl absorption remains at 1730 cm-1). Although the chemical compositions of cured pure MSSQ-HI and MSSQ-LO are the same after burnout (MeSiO1.5)n, nanoporous MSSQ-LO retains a larger proportion of caged structures than nanoporous MSSQ-HI based on the stronger IR absorption around 1130 cm -1 in the former for initial porogen loadings higher than 15 wt %. As a result, the absorbance ratio (1030/1130) is greater for MSSQ-HI, and this difference becomes greater with increasing porogen loading. For example, at 70 wt % P(MMA-co-DMAEMA) loading level, the absorbance ratio of the cross-linked to cage structures is 1.04 for MSSQ-LO compared with 1.27 for MSSQHI.

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Figure 8. FTIR spectra of MSSQ-HI/P(MMA-co-DMAEMA) nanocomposites cured at T ) 110 °C.

Figure 6. FTIR spectra of nanocomposites of P(MMA-coDMAEMA) with MSSQ-HI (6a) and MSSQ-LO (6b) at different porogen loading levels cured at 450 °C.

Figure 7. Thermal gravimetric analysis (TGA) of P(MMAco-DMAEMA) (solid line) and MSSQ-HI/P(MMA-co-DMAEMA) (80:20 w/w) nanocomposites (dashed line).

Decomposition of Porogen with Curing. The thermal decomposition of P(MMA-co-DMAEMA) alone (solid line) and in the nanocomposite (dashed line) can also be monitored by thermal gravimetric analysis (TGA), as shown in Figure 7. The thermal decomposition of P(MMA-co-DMAEMA) occurs in two distinct steps. It begins to lose weight (20-25%) already at temperatures around 200 °C. This weight loss is associated with the loss of DMAEMA comonomer, as verified by mass spectral analysis. The weight loss in this temperature range amounts to ≈25 wt %, which is roughly comparable to the copolymer loading level. Mass spectral (MS) analysis of the volatiles produced in this temperature

region shows a weak parent mass at m/e 157 (DMAEMA) and fragments at m/e 71 and 58. The fragmentation pattern is very similar to that of the nitrogeneous monomer. Surprisingly, in this temperature region, the polymer does not completely degrade, although there is evidence for the loss of some DMAEMA. Complete degradation of the polymer does, however, occur at temperatures above 340 °C where MS studies indicate the formation of MMA, detected by a monomer mass at m/e 100 and confirmed by its characteristic fragmentation pattern. The char yield after heating to 450 °C in this process is around 2-3%. Although P(MMA-coDMAEMA) is nanodispersed in MSSQ-HI, we do not see a significant improvement in thermal stability in hybrid nanocomposites. Catalytic Effect of P(MMA-co-DMAEMA). Figure 8 shows the FTIR spectra of MSSQ-HI/P(MMA-coDMAEMA) nanocomposites cured at T ) 110 °C as a function of porogen loading. At this temperature, most of the initial Si-OH remains in the sample of pure MSSQ-HI; however, addition of P(MMA-co-DMAEMA) significantly decreases the amount of IR-detectable silanol (at ≈3300 and 930 cm-1) functionality at this temperature. This was not the case for other nonbasic type porogens, for example, star-shaped or hyperbranched PCL and PPGs.8 This result suggests that the tertiary amino group in P(MMA-co-DMAEMA) can serve as a catalyst for the silanol condensation reaction. Amine catalysis of silanol condensation and chain extension is well-known,20 and improved mechanical properties have been reported in amine-cured MSSQ resins.21 Effect of Initial Silanol Concentration on Pore Size. SAXS data were circularly averaged and fit to the local monodisperse approximation (LPA) model proposed by Pedersen.22 Here, we consider each pore as a hard sphere, and we treat interconnected pores with the LPA model. The number-density size distribution of the (20) Brinker, C. J.; Scherer, G. W. Sol-Gel Science: The Physics and Chemistry of Sol-Gel Processing; Academic Press: San Diego, 1990. (21) (a) Carter, K. R.; Cook, R. F.; Harbison, M. A.; Hawker, C. J.; Hedrick, J. L.; Kim, S. M.; Liniger, E. G.; Miller, R. D.; Volksen, W.; Yoon, D. Y. U.S. Patent 5,953,627, 1999. (b) Carter, K. R.; Cook, R. F.; Harbison, M. A.; Hawker, C. J.; Hedrick, J. L.; Lee, V. Y.; Liniger, E. G.; Miller, R. D.; Volksen, W.; Yoon, D. Y. U.S. Patent 6,177,360 B1, 2001. (22) Pedersen, J. S. J. Appl. Crystallogr. 1994, 27, 595.

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Figure 9. Pore size distributions of nanoporous MSSQ-LO (solid line) and MSSQ-HI (dashed line) at 35 wt % P(MMAco-DMAEMA) loading level determined by SAXS.

pores can be described as a linear combination of a set of basic functions given by N

N(R) )

∑ anBn(R) n)1

(2)

where an are coefficients and Bn are the Log-Normal functions. The SAXS cross section dσ(q)/dΩ is given by

∫0∞N(R)Φ(q,R) dR

dσ (q) ) ∆F2 dΩ

(3)

where ∆F is the excess scattering-length density of the pores, q is the amplitude of the scattering vector, which is equal to 4π sin θ/λ, where θ is half the scattering angle and λ is the wavelength of the radiation, and Φ(q, R) is the form factor of the pore with size R. For spherical pores, the form factor is

Φ(q,R) ) 9V02[sin(qR) - qR cos(qR)]2/(qR)6 (4) where V0 is the volume of a sphere with radius R. We use the linear least-squares method to fit the measured intensities to the model intensities given by eq 3. Figure 9 shows the pore size distribution of nanoporous MSSQ-HI and MSSQ-LO produced from 35 wt % porogen loading as determined by SAXS. At this loading level, the average pore radius achieved for porous MSSQ-LO is about 3 times larger than that of porous MSSQ-HI. In addition, the pore size distribution for MSSQ-LO is also much broader than that observed for the MSSQ-HI resin. Surface Morphology. The effect of the initial level of SiOH functionality in the resin on the final surface morphology of the nanoporous MSSQ films was studied by tapping mode AFM. Figure 10 shows the AFM height images of porous MSSQ derived from MSSQ/P(MMAco-DMAEMA) blends using the two resins. Each of the films was cured at 450 °C for 2 h before measurement. This temperature is sufficient to volatilize virtually all of the porogen, as indicated by the TGA result shown in Figure 7. Two features are apparent in Figure 10: First, with the increase of porogen loading from 10 to 50%, the surface roughness (RMS) for both samples increases. Second, the surface roughness for the porous MSSQ-LO sample increases 4-fold (from 0.53 to 2.1 nm) with increasing porogen loading compared to a much

smaller increase (