Nanoporous Silica as an Ultralow-k Dielectric

Extract

As feature sizes in integrated circuits approach 0.18 μm, problems with interconnect resistance-capacitance (RC) delay, power consumption, and crosstalk become more urgent. Integration of low-dielectric-constant (k) materials will partially mitigate these problems, but each candidate with k significantly lower than that of dense silica (k ∼ 4) suffers disadvantages. Current low-k commercialization emphasizes spin-on glasses (SOGs) and fluorinated SiO₂ with k > 3, and a number of polymers are under development with k in the range of 2–3. These suffer from potential problems including thermal stability, mechanical properties, low thermal conductivity, and reliability. For some low-k materials, a protective liner covering the conductor is necessary. Although the material k is often cited, the value of practical concern is the effective k, which may be quite different because of this protective liner. As feature sizes shrink, the presence of the liner becomes more problematic and necessitates even lower k materials.

Another approach employs nanoporous silica with k of ∼1–4. Porous silica has been classified as an aerogel (dried supercritically) or as a xerogel (dried by solvent evaporation). We use the term nanoporous silica since it captures the key material properties that may be independent of how the films are processed. The ultralow dielectric constant results from porosity incorporation. For a porous material, the dielectric constant is a combination of that of air (∼1) and of the solid phase. The variation of k with porosity (volume fraction of pores) appears in Figure 1.