Nanoporous dielectric thin film formation using a...

Details Nanoporous dielectric thin film formation using a... Nanoporous dielectric thin film formation using a...

2000-01-20

2001-11-20

Meeks, Timothy (Department: 1762)

Semiconductor device manufacturing: process

Coating of substrate containing semiconductor region or of...

Insulative material deposited upon semiconductive substrate

C438S780000, C438S782000, C427S058000, C427S126300, C427S240000, C427S337000, C427S346000, C427S352000, C427S377000, C427S389000

Reexamination Certificate

active

06319852

ABSTRACT:

FIELD OF THE INVENTION
This invention pertains generally to precursors and deposition methods for low dielectric constant thin films on semiconductor substrates, and more particularly to deposition methods suited to aerogel thin film fabrication of nanoporous dielectrics.
BACKGROUND OF THE INVENTION
Semiconductor fabricators have used sol-gel techniques to produce dense thin films in semiconductors. The word sol-gel, however, does not describe a product but a reaction mechanism whereby a sol transforms into a gel. A sol is a colloidal suspension of solid particles in a liquid. One method of forming a sol is through hydrolysis and condensation reactions. These reactions cause a multifunctional monomer in a solution to polymerize into relatively large, highly branched particles. Many monomers suitable for polymerization are metal alkoxides. For example, a tetraethylorthosilicate (TEOS) monomer may be partially hydrolyzed in water by the reaction
Si(OEt)
4
+H
2
O→HO—Si(OEt)
3
+EtOH
Reaction conditions may be controlled such that, on the average, each monomer undergoes a desired number of hydrolysis reactions to partially or fully hydrolyze the monomer. TEOS which has been fully hydrolyzed becomes Si(OH)
4
. Once a molecule has been at least partially hydrolyzed, two molecules can then link together in a condensation reaction, such as
(OEt)
3
Si—OH+HO—Si(OH)
3
→(OEt)
3
Si—O—Si(OH)
3
+H
2
O
or
(OEt)
3
Si—OEt+HO—Si(OEt)
3
→(OEt)
3
Si—O—Si(OEt)
3
+EtOH
to form an oligomer and liberate a molecule of water or ethanol. The Si—O—Si configuration in the oligomer formed by these reactions has three sites available at each end for further hydrolysis and condensation. Thus, additional monomers or oligomers can be added to this molecule in a somewhat random fashion to create a highly branched polymeric molecule from literally thousands of monomers.
Through continued reactions, one molecule in the sol may eventually reach macroscopic dimensions so that it forms a network which extends 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, a gel can also be described as an open-pored solid structure enclosing a pore fluid. An oligomerized metal alkoxide, as defined herein, comprises molecules formed from at least two alkoxide monomers, but does not comprise a gel.
SUMMARY OF THE INVENTION
An ungelled precursor sol may be dip-coated or spin-coated onto a substrate to form a thin film on the order of several microns or less in thickness, gelled, and dried to form a dense film. The precursor sol often comprises a stock solution, a solvent, and a gelation catalyst. This catalyst typically modifies the pH of the precursor sol in order to speed gelation. In practice, such a thin film is subjected to rapid evaporation of volatile components, to the extent that the deposition, gelation, and drying phases may take place simultaneously as the film collapses rapidly to a dense film. Drying by evaporation of the pore fluid produces extreme capillary pressure in the microscopic pores of the wet gel. This pressure causes many pores to collapse and reduces the gel volume as it dries, typically by an order of magnitude or more.
A dried gel that is formed by collapsing and densifying a wet gel during drying has been termed a xerogel. U.S. patent application Ser. No. 08/247,195 to Gnade, Cho and Smith now U.S. Pat. No. 5,470,082 discloses a process for producing an aerogel thin film on a semiconductor substrate. An aerogel is distinguishable from a xerogel primarily by a manner of drying which largely avoids pore collapse during drying of the wet gel. This results in a substantially undensified thin film that can be fabricated with almost any desired porosity (thin films with greater than 90% porosity have been demonstrated). Such films have now been found to be desirable for a low dielectric constant insulation layer in microelectronic applications.
These techniques relate to fabricating inorganic dielectric (electrically nonconductive) materials. The inorganic porous dielectrics “aerogels” are preferably nanoporous having average pore sizes less than 500 nanometers (and more preferably less than 100 nanometers and still move preferably less than 25 nanometers). Nanoporous dielectrics are of particular interest in advanced semiconductor manufacturing. The nanoporous inorganic dielectrics include the nanoporous metal oxides, particularly nanoporous silica.
During conventional aerogel thin film formation, a catalyst such as ammonium hydroxide is typically mixed with the sol well prior to deposition. The catalyst typically operates to change the pH of the sol, thus initiating gelation. When a desired viscosity is reached prior to gelation, the sol is spun onto the wafer. Several properties of the film are related to its targeted density, including strength, pore size, and dielectric constant. Unfortunately, it has now been found that both density and film thickness are related to the viscosity of the sol at the time it is spun onto a wafer. Due to the rapid cross-linking which occurs in the film after addition of the catalyst, the apparent viscosity may change rapidly near the desired spin point. This apparent viscosity change may be so rapid that deposition time, catalyst concentration, and sol temperature may require exacting control to achieve a desired density and film thickness. The critical nature of this process not only makes it difficult to repeat, but nearly eliminates the possibility of sequential wafer processing at a common spin station.
The present invention generally provides a precursor sol and a method for deposition of aerogel thin films, e.g., for microelectronic applications in which the sol is not substantially gelling during deposition. In addition, sol viscosity is controlled and solvent evaporation is preferably either reduced or substantially halted during deposition.
For microelectronic applications, the precise control of film thickness and aerogel density are desirable. Several important properties of the film are related to the aerogel density, including mechanical strength, pore size and dielectric constant. It has now been found that both aerogel density and film thickness are related to the viscosity of the sol at the time it is spun onto a wafer. This presents a problem which was heretofore unrecognized. The problem being that with conventional precursor sols and deposition methods, it is extremely difficult to control both aerogel density and film thickness independently and accurately.
Aerogel thin films may be deposited on patterned wafers, e.g., over a level of patterned conductors. It has now been recognized that it may be useful to complete sol deposition before the onset of substantial gelation. This helps insure that gaps between such conductors remain adequately filled and that the surface of the gel remains substantially planar. To these ends, it is also desirable that no significant evaporation of pore fluid occur during gelation. Unfortunately, it is also desirable that the gel point be reachable as soon after deposition as possible to simplify processing, and the conventional method for speeding gelation of thin films is to allow evaporation to occur. It is recognized herein that a suitable precursor sol for aerogel deposition should allow control of film thickness, aerogel density, gap fill and planarity, and be relatively stable prior to deposition, yet allow rapid gelation that is controlled independently of deposition and evaporation.
A method has now been found which allows control of gelation to be performed substantially independent from evaporation of the pore fluid. In this method, the gelation catalyst is not included in the precursor sol. Instead, the catalyst is introduced during or after deposition of the sol on the substrate, thus removing the critical timing between sol spin-on and the onset of gelation. This independent introd

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