Simplified process for producing nanoporous silica

Details Simplified process for producing nanoporous silica Simplified process for producing nanoporous silica

1998-07-07

2002-05-28

Ghyka, Alexander G. (Department: 2812)

Semiconductor device manufacturing: process

Coating of substrate containing semiconductor region or of...

Insulative material deposited upon semiconductive substrate

C438S750000, C438S790000

Reexamination Certificate

active

06395651

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to low dielectric constant nanoporous silica and to improved processes for producing the same on substrates suitable for use in the production of integrated circuits.
2. Description of the Prior Art
As feature sizes in integrated circuits approach 0.25 &mgr;m and below, problems with interconnect RC delay, power consumption and signal cross-talk have become increasingly difficult to resolve. It is believed that the integration of low dielectric constant materials for interlevel dielectric (ILD) and intermetal dielectric (IMD) applications will help to solve these problems.
Nanoporous Films
One material with a low dielectric constant is nanoporous silica, which, as a consequence of the introduction of air, which has a dielectric constant of 1, into the material via its nanometer-scale pore structure, can be prepared with relatively low dielectric constants (“k). Nanoporous silica is attractive because it employs similar precursors, including organic-substituted silanes, e.g., tetraethoxysilane (“TEOS”), as are used for the currently employed spin-on-glasses (“SOG”) and chemical vapor disposition (“CVD”) silica SiO
2
. Nanoporous silica is also attractive because it is possible to control the pore size, and hence the density, material strength and dielectric constant of the resulting film material. In addition to a low k, nanoporous silica offers other advantages including: 1) thermal stability to 900° C., 2) substantially small pore size, i.e, at least an order of magnitude smaller in scale than the microelectronic features of the integrated circuit), 3) as noted above, preparation from materials such as silica and TEOS that are widely used in semiconductors, 4) the ability to “tune” the dielectric constant of nanoporous silica over a wide range, and 5) deposition of a nanoporous film can be achieved using tools similar to those employed for conventional SOG processing.
Thus, high porosity in silica materials leads to a lower dielectric constant than would otherwise be available from the same materials in nonporous form. In an additional advantage for nanoporous silica, additional compositions and processes may be employed in nanoporous silica, relative to a denser material. Other materials requirements include the need to have all pores substantially smaller than circuit feature sizes, the need to manage the strength decrease associated with porosity, and the role of surface chemistry on dielectric constant and environmental stability.
Density (or the inverse, porosity) is the key parameter of nanoporous silica that controls the dielectric constant of the material and this property is readily varied over a continuous spectrum from the extremes of an air gap at a porosity of 100% to a dense silica with a porosity of 0%. As density increases, dielectric constant and mechanical strength increase but the pore size decreases and vice versa. This suggests that the density range of nanoporous silica must be optimally balanced between for the desired range of low dielectric constant, and the mechanical properties acceptable for the desired application.
Nanoporous silica films have previously been fabricated by a number of methods, without achieving significant practical or commercial success. For example, nanoporous silica films have been prepared using a mixture of a solvent and a silica precursor, which is deposited on a substrate, e.g., a silicon wafer suitable for producing an integrated circuit, by conventional methods, e.g., including spin-coating and dip-coating. The substrate optionally has raised lines on its surface and preferably has electronic elements and/or electrical conduction pathways incorporated on or within its surface. The as-spun film is typically catalyzed with an acid or base catalyst and additional water to cause polymerization/gelation (“aging”) and to yield sufficient strength so that the film does not shrink significantly during drying.
Another previously proposed method for providing nanoporous silica films was based on the observation that film thickness and density/dielectric constant can be independently controlled by using a mixture of two solvents with dramatically different volatility. The more volatile solvent evaporates during and immediately after precursor deposition. The silica precursor, typically partially hydrolyzed and condensed oligomers of TEOS, is applied to a suitable substrate and polymerized by chemical and/or thermal means until it forms a gel. The second solvent, called the Pore Control Solvent (PCS) is usually then removed by increasing the temperature until the film is dry. The second solvent is then removed by increasing the temperature. Assuming that no shrinkage occurs after gelation, the density/dielectric constant of the final film is fixed by the volume ratio of low volatility solvent to silica. EP patent application EP 0 775 669 A2, which is incorporated herein by reference, shows a method for producing a nanoporous silica film with uniform density throughout the film thickness.
Another method for producing nanoporous dielectrics is through the use of sol-gel techniques whereby a sol, which is a colloidal suspension of solid particles in a liquid, transforms into a gel due to growth and interconnection of the solid particles. One theory that has been advanced is that through continued reactions within the sol, one or more molecules within the sol may eventually reach macroscopic dimensions so that they form a solid network which extends substantially throughout the sol. At this point, called the gel point, the substance is said to be a gel. By this definition, a gel is a substance that contains a continuous solid skeleton enclosing a continuous liquid phase. As the skeleton is porous, the term “gel” as used herein means an open-pored solid structure enclosing a pore fluid.
Protecting the Surfaces of Nanometer Scale Pores
As the artisan will appreciate, a useful nanoporous film must meet a number of criteria, including having a dielectric constant (“k”) falling within the required value range, having a suitable thickness (“t”) (e.g., measured in angstroms), having an ability to effectively fill gaps on patterned wafers, and having an effective degree of hydrophobicity. If the film is not strong enough, despite achieving the other requirements, the pore structure may collapse, resulting in high material density and therefore an undesirably high dielectric constant. In addition, the surfaces of the resulting nano-scale pores carry silanol functional groups or moieties. Silanols, and the water that can be adsorbed onto the silanols, are highly polarizable and will raise the dielectric constant of the film. Thus, the requirement for hydrophobicity arises from the requirement for a reduced range of dielectric constant relative to previously employed materials.
Previous attempts to solve this problem and to provide hydrophobic nanoporous films free of silanols and adsorbed water have employed the process of silylation, which is the derivatization of a surface with a capping reagent, e.g., trimethylsilyl [TMS, (CH
3
)
3
SiO—]. However, previous silylation processes have not been successful in achieving the desired hydrophobic properties for nanoporous silica.
In one such failed method, the wet nanoporous silica film was subjected to the additional step of exposing the film to a liquid mixture of solvent and a surface modification agent suitable for silylating the pore surface, e.g., hexamethyldisilazane [HMDZ, (CH
3
)
3
SiNHSi(CH
3
)
3
]. The purpose of the solvent is to both carry the agent, e.g., HMDZ, inside the nano-scale pores and into the pore volume, as well as providing the additional advantage of lowering the surface tension of the pore fluid before drying, thus avoiding mechanical stresses on the pore structure. The aim was for the surface modification agent to render the surfaces of the nano-scale pore structures hydrophobic by capping the silanols in the film with a hydrophobic moiety, e.g., by means of the reactions

Affiliated with

Also associated with

Rating

Simplified process for producing nanoporous silica does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Simplified process for producing nanoporous silica, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Simplified process for producing nanoporous silica will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-2853270