Plasma enhanced chemical deposition for high and/or low...

Details Plasma enhanced chemical deposition for high and/or low... Plasma enhanced chemical deposition for high and/or low...

1998-12-16

2001-03-27

Beck, Shrive (Department: 1762)

Coating processes

Direct application of electrical, magnetic, wave, or...

Polymerization of coating utilizing direct application of...

C427S509000, C427S512000, C427S569000, C427S398100

Reexamination Certificate

active

06207238

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates generally to a method of making plasma polymerized films having a specified index of refraction. More specifically, the present invention relates to selecting certain monomers to obtain a desired index of refraction of a plasma polymerized polymer film via plasma enhanced chemical deposition with a flash evaporated feed source of a low vapor pressure compound.
As used herein, the term “(meth)acrylic” is defined as “acrylic or methacrylic”. Also, “(meth)acyrlate” is defined as “acrylate or methacrylate”.
As used herein, the term “cryocondense” and forms thereof refers to the physical phenomenon of a phase change from a gas phase to a liquid phase upon the gas contacting a surface having a temperature lower than a dew point of the gas.
BACKGROUND OF THE INVENTION
The basic process of plasma enhanced chemical vapor deposition (PECVD) is described in THIN FILM PROCESSES, J. L. Vossen, W. Kern, editors, Academic Press, 1978, Part IV, Chapter IV-1 Plasma Deposition of Inorganic Compounds, Chapter IV-2 Glow Discharge Polymerization, herein incorporated by reference. Briefly, a glow discharge plasma is generated on an electrode that may be smooth or have pointed projections. Traditionally, a gas inlet introduces high vapor pressure monomeric gases into the plasma region wherein radicals are formed so that upon subsequent collisions with the substrate, some of the radicals in the monomers chemically bond or cross link (cure) on the substrate. The high vapor pressure monomeric gases include gases of CH
4
, SiH
4
, C
2
H
6
, C
2
H
2
, or gases generated from high vapor pressure liquid, for example styrene (10 torr at 87.4° F. (30.8° C.)), hexane (100 torr at 60.4° F. (15.8° C.)), tetramethyldisiloxane (10 torr at 82.9° F. (28.3° C.) 1,3,-dichlorotetra-methyldisiloxane) and combinations thereof that may be evaporated with mild controlled heating. Because these high vapor pressure monomeric gases do not readily cryocondense at ambient or elevated temperatures, deposition rates are low (a few tenths of micrometer/min maximum) relying on radicals chemically bonding to the surface of interest instead of cryocondensation. Remission due to etching of the surface of interest by the plasma competes with the reactive deposition. Lower vapor pressure species have not been used in PECVD because heating the higher molecular weight monomers to a temperature sufficient to vaporize them generally causes a reaction prior to vaporization, or metering of the gas becomes difficult to control, either of which is inoperative.
The basic process of flash evaporation is described in U.S. Pat. No. 4,954,371 herein incorporated by reference. This basic process may also be referred to as polymer multi-layer (PML) flash evaporation. Briefly, a radiation polymerizable and/or cross linkable material is supplied at a temperature below a decomposition temperature and polymerization temperature of the material. The material is atomized to droplets having a droplet size ranging from about 1 to about 50 microns. An ultrasonic atomizer is generally used. The droplets are then flash vaporized, under vacuum, by contact with a heated surface above the boiling point of the material, but below the temperature which would cause pyrolysis. The vapor is cryocondensed on a substrate then radiation polymerized or cross linked as a very thin polymer layer.
The material may include a base monomer or mixture thereof, cross-linking agents and/or initiating agents. A disadvantage of the flash evaporation is that it requires two sequential steps, cryocondensation followed by curing or cross linking, that are both spatially and temporally separate.
According to the state of the art of making plasma polymerized films, PECVD and flash evaporation or glow discharge plasma deposition and flash evaporation have not been used in combination. However, plasma treatment of a substrate using glow discharge plasma generator with inorganic compounds has been used in combination with flash evaporation under a low pressure (vacuum) atmosphere as reported in J. D. Affinito, M. E. Gross, C. A. Coronado, and P. M. Martin, A Vacuum Deposition Of Polymer Electrolytes On Flexible Substrates. “Paper for Plenary talk in A Proceedings of the Ninth International Conference on Vacuum Web Coating”, November 1995 ed R. Bakish, Bakish Press 1995, pg 20-36, and as shown in
FIG. 1
a.
In that system, the plasma generator
100
is used to etch the surface
102
of a moving substrate
104
in preparation to receive the monomeric gaseous output from the flash evaporation
106
that cryocondenses on the etched surface
102
and is then passed by a first curing station (not shown), for example electron beam or ultra-violet radiation, to initiate cross linking and curing. The plasma generator
100
has a housing
108
with a gas inlet
110
. The gas may be oxygen, nitrogen, water or an inert gas, for example argon, or combinations thereof. Internally, an electrode
112
that is smooth or having one or more pointed projections
114
produces a glow discharge and makes a plasma with the gas which etches the surface
102
. The flash evaporator
106
has a housing
116
, with a monomer inlet
118
and an atomizing nozzle
120
, for example an ultrasonic atomizer. Flow through the nozzle
120
is atomized into particles or droplets
122
which strike the heated surface
124
whereupon the particles or droplets
122
are flash evaporated into a gas that flows past a series of baffles
126
(optional) to an outlet
128
and cryocondenses on the surface
102
. Although other gas flow distribution arrangements have been used, it has been found that the baffles
126
provide adequate gas flow distribution or uniformity while permitting ease of scaling up to large surfaces
102
. A curing station (not shown) is located downstream of the flash evaporator
106
.
In all of these prior art methods, the starting monomer is a (meth)acrylate monomer (
FIG. 1
b
). When R
1
is hydrogen (H), the compound is an acrylate and when R
1
is a methyl group (CH
3
), the compound is a methacrylate.
It is known that the monomer composition may be varied to selectively obtain a desired refractive index. Acrylated or methacrylated hydrocarbon chain compositions provide indices of refraction tightly grouped about 1.5. Bisphenyl A diacrylate has an index of refraction of 1.53. Degree of conjugation (number of carbon to carbon double or triple bonds or aromatic rings) generally increases index of refraction. For example, polyvinylcarbizone has an index of refraction of 2.1 or higher. However, multi-ring system compounds that are solids are not useful as a monomer in these systems. Addition of bromine may increase index of refraction as high as 1.7. Addition of fluorine may reduce index of refraction to as low as 1.3. However, bromine adds a brown color and tends to oxidize over time and fluorinated monomers have high vapor pressures, poor adhesion and high cost.
Therefore, there is a need for an apparatus and method for making plasma polymerized polymer layers at a fast rate but that is also self curing, and with selective index of refraction.
SUMMARY OF THE INVENTION
The present invention is an improved method of plasma polymerization wherein a monomer capable of providing a polymer with a desired index of refraction is cured during plasma polymerization.
The present invention may be (1) an apparatus and method for plasma enhanced chemical vapor deposition of low vapor pressure monomer or a mixture of monomer with particle materials onto a substrate, or (2) an apparatus and method for making self-curing polymer layers, especially self-curing PML polymer layers. From both points of view, the invention is a combination of flash evaporation with plasma enhanced chemical vapor deposition (PECVD) that provides the unexpected improvements of permitting use of low vapor pressure monomer materials in a PEDVD process and provides a self curing from a flash evaporation process, at a rate surprisingly faster than standard PECVD deposition rates.
Generally, the appa

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