Coating apparatus – Control means responsive to a randomly occurring sensed... – Condition of coated material
Reexamination Certificate
2000-05-23
2002-04-02
Crispino, Richard (Department: 1734)
Coating apparatus
Control means responsive to a randomly occurring sensed...
Condition of coated material
C118S689000, C118S690000, C118S691000, C118S699000, C118S052000, C118S075000
Reexamination Certificate
active
06364953
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method and to an apparatus for making an aerogel film which is porous, exhibits a low dielectric constant and low signal loss, and may be used as a dielectric layer in a high-frequency circuit and an insulating interlayer in a semiconductor device such as an LSI. The method and the apparatus can minimize variations in film characteristics, such as thickness, porosity, and dielectric constant.
In particular, the present invention relates to a method and to an apparatus for making an aerogel film having stable qualities which can minimize variations in film characteristics, such as thickness, porosity, and dielectric constant, among a plurality of substrates with aerogel films from one production lot when these substrates with gel films are produced by a simultaneous supercritical or sub-critical drying process so as to control gelation reactions in the wet-gel films.
2. Description of the Related Art
Microwave circuits for portable phones, satellite broadcasts, and telecommunications employ integrated circuits using microstrips instead of conventional waveguides or coaxial cables.
With reference to
FIG. 1A
, a single-layered microstrip includes a substrate
1
, a base layer
2
formed on the substrate
1
by metallization or the like, a dielectric layer
3
formed on the base layer
2
, and a conductive microstrip line
4
formed on the dielectric layer
3
. With reference to
FIG. 1B
, a multilayered microstrip including a plurality of the single-layered microstrips shown in
FIG. 1A
is also used.
Fluoride glass and alumina ceramics have been used for the dielectric layers
3
and the insulating interlayer
5
of such microstrips. In recent years, however, dielectric materials having smaller dielectric constants and smaller signal losses have been required. Japanese Unexamined Patent Application Publication No. 8-228105 discloses a porous ceramic meeting such a requirement.
Japanese Unexamined Patent Application Publication No. 9-213797 discloses a method for making a semiconductor device. In the method, a wet-gel film is formed on a substrate, and a solvent contained in the wet gal film is evaporated by a supercritical or sub-critical drying process to form an aerogel film. Next, the aerogel film is patterned. In more detail, a stock solution containing a metal alkoxide (tetraethoxysilane), a solvent (ethanol), water, and a catalyst is spin-coated onto a rotating substrate. Silicon alkoxide in the stock solution is hydrolyzed to form silicon hydroxide. The silicon hydroxide forms a network of siloxane bonds containing the residual solvents by condensation. That is, a wet-gel film of a silica skeleton containing the solvents is formed on the substrate. When the wet-gel film is dried by a supercritical or sub-critical drying process, a silica aerogel having a low dielectric constant is formed by replacement of the residual solvent with air.
Herein, “supercritical drying” means that a solvent or water is in a supercritical state in which the solvent or water is present as both gas and liquid. The supercritical drying state is achieved under high-temperature and high-pressure conditions which is above the critical point of the solvent. In the supercritical drying process, no gas-liquid interface is formed in the gel. Thus, no stress due to capillary force is applied to the gel skeleton. As a result, the solvent remaining in the gap of the network skeleton can be substantially removed without shrinkage of the wet gel. “Sub-critical drying” means drying performed in a sub-critical state which is a high-temperature and high-pressure state near the supercritical state. In the sub-critical drying, the solvent can also be substantially removed without shrinkage of the wet gel. The resulting dried gel (aerogel) film still maintains the network structure of the wet-gel film, and is a porous material having a significantly high porosity and a significantly low relative permittivity. Accordingly, aerogels are expected to be used as new materials for the dielectric layer and the insulating interlayer.
Silicon alkoxide, however, is readily condensed in the presence of a catalyst, that is, is rapidly gelated. Thus, production of aerogel films has the following disadvantages.
When a stock solution containing silicon alkoxide, water, alcohol, and a catalyst is prepared for a wet-gel film in an amount sufficient to coating a plurality of substrates and is spin-coated on these substrates, elapsed times from the preparation of the stock solution to the spin coating differ between these substrates. Since condensation or gelation proceeds in the stock solution immediately after the stock solution is prepared, the viscosity of the stock solution increases over time. When the spin coating is performed at a constant rotation rate, the thickness of the film coated on the substrate increases as the viscosity of the stock solution increases. That is, the thickness of the film on a later substrate is larger than that on an earlier substrate. As the gelation proceeds further, a thin-film cannot be formed on the substrate due to extremely high viscosity. Moreover, the water and the catalytic component in the stock solution will be evaporated during the coating, and thus the gelation will not proceed further. As a result, a high-quality wet-gel film cannot be formed on the substrate.
A possible method for solving the above problem is preparation of stock solutions for the plurality of substrates so that the elapsed times from the preparation to the coating of the stock solutions are the same. Since the viscosities of these solutions are the same during the coating steps, the thicknesses of the films can be equal to each other by fixing the rotation of the substrate.
When the degrees of gelation differ between the substrates at the start of the supercritical or sub-critical drying process, the resulting aerogel films have different porosities even if the wet-gel films have the same thickness.
As a result, the aerogel films do not have uniform, reproducible qualities. For example, aerogel films may be mass-produced by a method shown in
FIGS. 2A
to
2
D. Stock solutions having the same viscosity or the same degree of gelation are applied onto substrates
10
to prepare substrates
12
with wet-gel films
11
having the same thickness, and then these substrates
12
are placed into a holder
13
(hereinafter, a substrate
12
with a wet-gel film is referred to as a “wet-gel-film-substrate”). After a predetermined number (six in
FIG. 2C
) of wet-gel-film-substrates
12
is placed into the holder
13
, the holder
13
is transferred into a hermetically sealed container
14
for supercritical or sub-critical drying. In this case, for example, the first wet-gel-film-substrate
12
a is subjected to the supercritical or sub-critical drying after a longer holding time compared to the last wet-gel-film-substrate
12
f
. That is, the wet-gel-film-substrates
12
a
,
12
b
, . . . ,
12
f
in the holder
13
have different holding times before the supercritical or sub-critical drying. Thus, the degrees of gelation of the wet-gel films on these wet-gel-film-substrates
12
a
,
12
b
, . . . ,
12
f
differ from each other at the start of the supercritical or sub-critical drying process, even if the applied stock solutions have the same viscosity or even if the gelation reaction starts after the stock solutions are applied onto the substrates. Such a difference in the degrees of gelation causes differences in porosities of the silica aerogel films obtained by the supercritical or sub-critical drying. Accordingly, the aerogel films obtained by the same supercritical or sub-critical drying process have different properties, e.g., dielectric constant and hydrophobicity. The quality of the aerogel films produced in one lot is, therefore, not uniform.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a method and an apparatus for making an aerogel film having uniform qualities by a supercritical or
Fukumoto Yoshito
Inoue Ken-ichi
Kawakami Nobuyuki
Kinoshita Takashi
Suzuki Kohei
Crispino Richard
Kabushiki Kaisha Kobe Seiko Sho.
Reed Smith LLP
Shortsle Kevin P.
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