N I S T Center for Neutron Research
Accomplishments and Opportunities 2001
Nanoporous Ultra-Low Dielectric Constant Materials
The next generation of interlayer dielectric materials for microelectronics must have an ultra-low dielectric constant of less than 2.0 to meet the National Technology Roadmap for Semiconductors (Refer to Reference 1). In addition, the material must have excellent thermal stability and mechanical properties (Refer to References 2 and 3). One route is to introduce nanometer sized pores into a material with an initially low dielectric constant (k ≈ 2 to 3). The presence of the pores lowers the dielectric constant of the nanoporous film as the dielectric constant of air is 1.0. One class of materials that is receiving significant attention is based on poly (methylsilsesquioxane) (P M S S Q) which has a dielectric constant between 2.7 and 2.8 (lower than conventional SiO2) and good thermal stability up to 500 °C (Refer to References 4 and 5). The dielectric constant is a function of the level of porosity and detailed knowledge of the morphology is required for implementation in electronic packaging.
A schematic of the synthesis process is shown in Figure 1. The materials are based on an inorganic matrix of P M S S Q containing a porogen (i.e., a labile pore generating material) based on a poly (methylmethacrylate) copolymer (co P M M A) as a template for creating the pores (Refer to References 6 and 7). A film is prepared by dissolving the P M S S Q matrix precursor material and the co P M M A porogen in a common solvent and spin casting onto a silicon substrate.
Graphics Caption FIGURE 1. Schematic showing the generation of nanoporous P M S S Q films.
Graphics Caption FIGURE 2. S A N S data taken during cure of a nanoporous P M S S Q thin film.
Graphics Caption FIGURE 3. Above, neutron reflectivity data (symbols) and best fit (lines) calculated from the scattering length density profiles shown below for a nanoporous P M S S Q thin film under various conditions.
The film is then slowly ramped up in temperature to 450 °C. During this temperature ramp, the matrix film fully cures by about 225 °C, resulting in microphase separation of the co P M M A polymer. Above about 350 °C the co P M M A polymer degrades leaving nanometer sized pores. Neutron scattering was used with a deuterated co P M M A polymer to understand the morphological development of the films during the cure. The use of the deuterated porogen provides contrast for neutron scattering in the hybrid (polymer containing) system that is not available with x-ray scattering.
In situ studies of the curing process by small angle neutron scattering (S A N S) were performed. To provide sufficient scattering signal, four or more samples on Si wafers were stacked to increase the scattering volume.
Figure 2 shows a set of S A N S data from a P M S S Q thin film containing 20 % molecular fraction of porogen at various cure temperatures. The scattered intensity varies as a function of curing temperature due to morphology changes occurring in the sample. For the as-spun materials there is a shoulder at high q from which a radius of gyration (Rg) of 12 Å was obtained by Guinier analysis. After heat treatment at 225 °C the shoulder in I(q) becomes more pronounced and shifts to about q = 0.06 Å-1. The shift to smaller q indicates a coarsening of the microphase separated porogen domain structure. A radius of gyration of Rg = 23 Å for the domains was obtained for the cured materials. The scattering curves do not change significantly between 225 °C and 300 °C, indicating that the morphology remains fixed until the degradation of the polymer at 450 °C. The dramatic decrease in intensity of the S A N S curve at 450 °C is due to a loss of neutron scattering length density contrast because of the degradation of the porogen and the formation of pores in the film.
To provide neutron contrast between the pores and the matrix, the fully cured sample was exposed to deuterated toluene. These data are also shown in Figure 2. The scattering is largely recovered after exposure to deuterated toluene, indicating that the porous structure is maintained with no collapse after degradation of the polymer.
Figure 3 shows a set of neutron reflectivity profiles and scattering length density ( S L D) profiles of a nanoporous P M S S Q thin film prepared with 20 % molecular fraction of porogen and deposited on a silicon substrate. The film was exposed to deuterated toluene liquid and vapor to examine solvent swelling. The film thickness is observed to increase upon exposure to both liquid and vapor. A higher porosity is observed in a 50 Å region next to the silicon surface. This is probably due to porogen surface segregation at the substrate interface and is in agreement with results from transmission electron microscopy.
 The National Technology Roadmap for Semiconductors: Technology Needs, SIA, Semiconductor Industry Association, 1997 Edition, p. 101.
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G. Y. Yang and R. M. Briber
Department of Materials and Nuclear Engineering
University of Maryland
College Park, MD 20742
E. Huang, H.-C. Kim, W. Volksen, and R. D. Miller
I B M Almaden Research Center
San Jose, CA 95120
Department of Materials Science and Engineering
State University of New York
Stony Brook, NY 11794
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