Stock material or miscellaneous articles – Composite – Of silicon containing
Reexamination Certificate
1999-08-23
2002-06-25
Dawson, Robert (Department: 1712)
Stock material or miscellaneous articles
Composite
Of silicon containing
C428S448000, C438S761000, C438S763000, C438S782000, C438S787000, C427S096400, C427S387000, C427S421100, C427S430100
Reexamination Certificate
active
06410149
ABSTRACT:
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.
BACKGROUND OF THE INVENTION
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, that 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 organicsubstituted silanes, e.g., tetramethoxysilane (“TMOS”) and/or 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.
Nanoporous silica films have previously been fabricated by a number of methods. For example, nanoporous silica films have been prepared using a mixture of a solvent and a silica precursor, which is deposited on a substrate, eg., 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.
The internal pore surfaces of previously prepared nanoporous films are formed of silicon atoms which are terminated in a combination of any or all of the following species; silanol (SiOH), siloxane (SiOSi), alkoxy (SiOR), where R is an organic species such as, but not limited to, a methyl, ethyl, isopropyl, or phenyl groups, or an alkylsilane (SiR), where R is as defined previously. When the internal surface of the nanoporous silica is covered with a large percentage of silanols, the internal surface is hydrophilic and may adsorb significant quantities of atmospheric water. Even if the film is outgassed by heating before subsequent processing, the presence of the polar silanols can contribute negatively to the dielectric constant and dielectric loss. Previously employed methods for overcoming this limitation and rendering the internal pore surfaces of nanoporous silica less hydrophilic include reacting the internal surface silanols with surface modifying agents, including, for example, chlorosilanes or disilazanes. These reactions, which may be conducted in either liquid or gas phases, result in a (SiO)
4-x
SiR
x
[wherein x is an integer ranging from 1 to 3] surface which is normally hydrophobic and less polar than the silanol group it replaced.
However, all of the previously employed methods for producing nanoporous silica films used organic functional moieties to provide hydrophobicity. Although these carbon-containing nanoporous silica films (described, for example, in co-owned patent application Ser. No. 09/111,084, filed Jul. 7, 1998) the disclosure of which is incorporated by reference herein in its entirety) exhibit a number of advantages for semiconductor applications, they also have several potential disadvantages including:
1. Oxidation of the carbon content: During semiconductor processing, e.g., during plasma enhanced chemical vapor deposition (PECVD) and etching, following nanoporous silica film deposition, the presence of organic species can lead to problems such as high via resistance (i.e., the prospective integrated circuit is ruined by “poisoning” the interlayer connectors, due to oxidation of the carbon content of organic substituents, resulting in the deposition of undesirable residues from the etching process in the vias). (see, eg., R. J. Hopkins, T. A. Baldwin, S. K. Gupta, May 7-12, 1989
, ULSI Symposium, ECS
, Allied Signal) which may require additional process steps to rectify.
2. Added mass: For example, the addition of a trimethyl silyl entity (CH
3
)
3
Si as a replacement for a hydrophilic surface silanol adds significant mass to the nanoporous silica. All else being constant, the added mass can produce a significantly higher refractive index and dielectric constant which may be undesirable.
3. Strength: Normally, for semiconductor applications, one desires a material with both low dielectric constant and high strength. For nanoporous silica, these two properties must be balanced. For a given dielectric constant (refractive index/density), the density is fixed, at least for a specific chemical composition. With fixed density, the strength of the nanoporous silica is maximized by having the greatest fraction of solid within the skeleton of the film rather than as appended surface groups.
Thus, in view of the need for rapid competitive advances in the art of microprocessor fabrication, there remains a constant need in the art to improve upon previous methods and materials. In particular, there is a need to provide nanoporous silica films with hydrophobic pore surfaces, while minimizing the above described undesirable effects of organic surface moieties. In particular, it is strongly desired to provide such nanoporous silica films with reduced mass at the nano-scale pore surfaces. This later property will provide greater material film strength for a given desired dielectric constant. Thus, for all of these reasons, there remains a need in the art for methods and compositions for producing nanoporous films suitable for the production of integrated circuits that have all of the above-described desirable properties, while minimizing those previously indicated shortcomings of the art.
SUMMARY OF THE INVENTION
In order to solve the above mentioned problems and to provide other improvements, the invention provides new methods for effectively producing low dielectric constant nanoporous silica films having a desired range of dielectric constant significantly lower, or having greater strength at the same dielectric constant, than has previously been obtained, while simultaneously avoiding the shortcomings of previously known methods.
Surprisingly, the methods of the present invention are able to achieve this goal by producing nanoporous silica with pore surfaces on which most of the polar silanol (SiOH) functional groups have been replaced by hydrogen functional groups (SiH) and/or a combination of hydrogen functional groups and organic functional groups. The resulting novel pore surfaces also render the produced film somewhat hydrophobic. This is accomplished by employing suitable starting reagents and processes. In particular, the processes of the invention employ SiH and/or SiC (organic) species as surface modification agents, instead of exclusively relying upon surface modification agents based on silicon-hydrocarbon compounds, which have previously
Drage James
Hendricks Neil
Ramos Teresa
Smith Douglas M.
Wallace Stephen
Allied-Signal Inc.
Dawson Robert
Feely Michael J
Roberts & Mercanti LLP
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