Nanoporous silica may hold solution for better dielectrics

02/01/1997

Nanoporous silica may hold solution for better dielectrics

AlliedSignal Advanced Microelectronics Materials, Santa Clara, CA, and NanoPore Inc., Albuquerque, NM, announced in September the formation of a new joint venture, Nanoglass. The 50-50 venture combines AlliedSignal`s expertise in thin films, application development, and sales with NanoPore`s nanoporous silica technology. Nanoglass has developed a new process to deposit thin films of nanoporous silica for interlevel dielectrics (ILD) applications. Initial work will focus on optimization and characterization of the proprietary films, which offer thermal stability and SiO2 composition. A limited number of alpha site partners will be selected from leading IC makers. Broader sampling is slated to begin in 3Q97.

Figure 1. Variation of predicted dielectric constant (solid line) and measured values (squares) as a function of porosity. (Courtesy of Douglas Smith, Nanoglass.)

Decreasing feature sizes (=0.25 microns) are introducing increasing problems with interconnect RC delay, power consumption, and crosstalk. The use of material with low dielectric constants (k) will partially alleviate the problem. Despite the introduction of organic and inorganic polymers with k ranging from 2.2-3.5, a significant improvement in circuit performance demands k =2. These materials also suffer from potential problems associated with low thermal stability, poor mechanical properties, sample outgassing, and lack of long term reliability.

Nanoporous silica, sometimes known as silica aerogel or xerogel, has a low dielectric constant, high thermal stability, small pore sizes, and other properties that make it ideal for ILD thin films. An aerogel is dried supercritically, while a xerogel is dried by solvent evaporation. Even though the advantages of using nanoporous silica have been known for a long time, its usage has been limited by difficult processing conditions.

The initial deposition of nanoporous silica films is similar to other spin-on materials and some polymer dielectrics. If the film is immediately dried such that insufficient bonds form to enable gelation, the material is unconstrained during drying and will yield a planarized, dense film. Formation of nanoporous silica require gelation of the film to yield a continuous network spanning the entire fluid before it dries. Gelation occurs after precursor deposition, either due to changes in the precursor immediately before deposition or by evaporation after deposition.

In traditional methods, formation of low-density silica for either bulk or thin-film applications usually results in large drying stresses, which cause shrinkage until the matrix can resist the pore fluid capillary pressure. Supercritical processing can effectively eliminate the vapor-liquid menisci that cause shrinkage. The technique, however, requires high pressures (>60 bar).

Development of most low-k materials emphasizes the use of spin-on-glasses (SOGs) and fluorinated plasma CVD SiO2. Nanoporous silica employs similar precursors and has the added benefit of tight control over pore size as well as size distribution.

In addition to the low dielectric constant, other advantages of nanoporous silica are:

 thermal stability up to 900?C,

 pore size of dimensions much smaller than microelectronics features,

 material (silica) and precursors that are widely used in the semiconductor industry,

 ability to "tune" dielectric constant over a wide range, and

 deposition using similar tools as employed for conventional SOG processing.

Density (or the inverse, porosity) affects the dielectric constant, mechanical strength, and pore size of nanoporous silica (Fig. 1). Douglas Smith, a professor at the University of New Mexico, founder of NanoPore, and co-founder of Nanoglass LLC, suggested that the optimum density range for semiconductor applications is not the very low range associated with k~1 but rather, the higher densities that yield higher strength and smaller pore size.

Figure 2. NanoGlass nanoporous silica process flow. (Courtesy of Douglas Smith, Nanoglass.)

At a conference on "Advanced Metallization and Interconnect Systems for ULSI Applications, 1996," Smith announced the introduction of a simplified process to deposit thin films of nanoporous silica for ILD applications. Nanoglass has developed a gel that is strong enough to resist the capillary pressure resulting from drying directly from the gel solvent. It allows independent control of the film thickness and density/dielectric constant, which was a major drawback of older processes. Using the new process, these materials can be deposited using a conventional spin-coating system and can be cured and processed at atmospheric pressure. The number of process steps has been considerably reduced when compared to previous methods and the total process time is on the order of minutes (Fig. 2). The process consists of five simple steps: deposition, aging, drying, annealing, and surface modification (for hydrophobic product only).

Nanoporous silica films may be reproducibly manufactured with typical SOG spin-on processing conditions, and the resulting k value is significantly less than competing materials. They have demonstrated a shelf life greater than six months. Depending on the density (or porosity), k can be tailored between 1.4 and 2.0.

Questions concerning this new technology can be directed to Douglas Smith ([email protected]) or Neil Hendricks ([email protected]). - M.Y.M.L.