Coating processes – Centrifugal force utilized
Reexamination Certificate
1999-10-04
2001-12-11
Beck, Shrive P. (Department: 1762)
Coating processes
Centrifugal force utilized
C427S425000, C427S397700
Reexamination Certificate
active
06329017
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to porous silica film with nanometer-scale porosity produced from solution precursors. More specifically, the present invention relates to mesoporous silica film from a solution containing a surfactant (surfactant templated) and the use of specific surfactants to template porosity with the characteristic pore size being defined by the surfactant micelle size. The present invention also relates to the use of dehydroxylation in combination with surfactant templated mesoporous silica films to obtain a dielectric constant less than 3 under ambient humid conditions.
As used herein, the term “silica” means a compound having silicon (Si) and oxygen (O) and possibly additional elements.
Further, as used herein, “mesoporous” refers to a size range which is greater than 1 nm, but significantly less than a micrometer. In general, this refers most often to a size range from just over 1.0 nm (10 angstroms) to a few tens of nanometers.
The term “stable” can mean an absolute stability, a relative stability or a combination thereof. Relative stability means that a dielectric constant increases no more than about 20% when a surfactant templated mesoporous film is taken from an equilibrated condition of 0.0% relative humidity or vacuum to an equilibrated condition of 50% relative humidity. Absolute stability means that the dielectric constant remains less than 3 under any conditions including humid conditions of at least 40% relative humidity.
The term “hydroxylated” encompasses partially and fully hydroxylated. The term “dehydroxylating” encompasses partial or total removal of hydroxyl groups from surface(s) of the surfactant templated mesoporous silica film.
BACKGROUND OF THE INVENTION
Porous silica films are potentially useful as low dielectric constant intermetal materials in semiconductor devices, as low dielectric constant coatings on fibers and other structures, and in catalytic supports. Most of the U.S. semiconductor industry is presently (1998) in the process of implementing interlevel dielectric films that are silica films, or derivatives of silica and silicates, or polymeric films, with less than 25% or no porosity with dielectric constant (k′) in the range of 3.0 to 4.0. Further reductions in dielectric constant are desired to improve the operating speed of semiconductor devices, reduce power consumption in semiconductor devices and reduce overall cost of semiconductor devices by decreasing the number of metallization levels that are required.
Since air has a dielectric constant of 1.0, the introduction of porosity is an effective way of lowering the dielectric constant of a film. In addition, because silica dielectrics have been a standard in microelectronic devices, silica films with porosity are very attractive to the semiconductor industry for advanced devices that require low dielectric constant materials. The feature size or design rule in the semiconductor interconnect will be sub-150 nm in ultralarge scale integration; and pore sizes to achieve lower dielectric constant (k<3) must be significantly smaller than the intermetal spacing.
Dielectric constant of porous films is dependent on the material and pore structure. For porous silica films for use in microelectronic devices, material and pore structure must result in uniform dielectric constants across the wafers and in different directions on the wafer. In general, isotropic material and pore structures are expected to provide the desired uniformity in film dielectric constant compared to anisotropic material and pore structures.
Also, low dielectric constant mesoporous films used commercially need to be prepared in a manner compatible with a semiconductor device manufacturing process line, for example spin coating. For large-area circular wafers, other coating techniques such as dip coating are not as convenient because dip coating requires masking of the backside to prevent contamination.
Surface topography is also very critical to fabrication of a multilevel interconnect structure. In the “damascene” process for copper interconnects intended for ultralarge scale integration on semiconductor chips, each dielectric layer is etched, following which copper is deposited, and the structure planarized by chemical-mechanical polishing (CMP). The initial planarity and the absence of surface texture in the low k dielectric film is very critical in maintaining planarity at each level of the interconnect.
Another important concern with porous dielectric films is mechanical integrity. Because of their fragility, it appears unlikely that porous films will be directly polished using conventional chemical-mechanical-polishing (CMP) equipment, but a dense “cap” layer of silica or another material on the porous low K film will be planarized. However, even with a cap layer, the porous low K material must have adequate stiffness, compressive and shear strengths, to withstand the stresses associated with the CMP process.
Silica films with nanometer-scale (or mesoporous) porosity may be produced from solution precursors and classified into two types (1) “aerogel or xerogel” films (aerogelixerogel) in which a random or disordered porosity is introduced by controlled removal of an alcohol-type solvent, and (2) “mesoporous” surfactant-templated silica films in which the pores are formed with ordered porosity by removal of a surfactant. Heretofore, the most successful demonstration of low dielectric constant silica films with dielectric constant of 3.0 or less has been with aerogel/xerogel-type porous silica films. However, disadvantages of aerogel/xerogel films include (1) deposition of aerogel/xerogel films requires careful control of alcohol removal (e.g. maintaining a controlled atmosphere containing solvent or gelling agent during preparation) for formation of the pore structure (2) the smallest pore size typically possible in aerogel/xerogel films falls in the size range of 10-100 nm, and (3) limited mechanical strength compared to dense selica films. These disadvantages have hindered implementation of these aerogel/xerogel porous silica films in semiconductor devices.
In order to obtain a porous film with a low dielectric constant of any material made by any process, it is necessary to minimize the number of hydroxyl groups in the structure, especially at pore surfaces. The dielectric films must be made hydrophobic in order for the electrical properties to be stable in humid air. Hydroxylated surfaces in porous silica films result in a dielectric constant exceeding that of dense silica (i.e. approximately 4.0). Physisorption of water molecules by hydroxylated surfaces can further increase the dielectric constant and effective capacitance of a mesoporous silica film. Physisorption of water molecules can be avoided by handling films in non-humid atmospheres or vacuum, or by minimizing exposure of films to humid conditions. Hydroxyl groups and physisorbed water molecules may be removed from silica surfaces at very high temperatures. C. J. Brinker and G. W. Scherer, in
Sol-Gel Science
, Academic Press, New York, N.Y. (1990) (Brinker et al. 1990) discuss thermal dehydroxylation of silica by exposure to very high temperatures of over 800° C. However, semiconductor devices with dielectric films and metal lines cannot usually be processed over about 500° C. Thus, other methods of dehydroxylation are needed for porous silica films on semiconductors.
E. F. Vansant, P. Van der Voort and K. C. Vrancken,
in Characterization and Chemical Modification of the Silica Surface
, Vol. 93 of Studies in Surface Science and Catalysis, Elsevier, New York, N.Y. (1995), and Brinker et al., 1990, cite procedures for hydroxylation of silica surfaces by fluorination or by treatment with silane solutions. Aerogel/Xerogel-type films have been dehydroxylated by both (a) fluorination treatment, and (b) a two-step dehydroxylation method of (1) initial silane solution treatment (e.g. trimethylchlorosilane or hexamethyidisilazane (HMDS) in a solvent), and then (2) following this solution treatment w
Baskaran Suresh
Birnbaum Jerome C.
Coyle Christopher A.
Domansky Karel
Fryxell Glen E.
Battelle (Memorial Institute)
Beck Shrive P.
Calcagni Jennifer
Marger & Johnson & McCollom, P.C.
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