Selective conversion of polymer coatings to ceramics

A process for the selective conversion of a polymer coating to a ceramic material is disclosed. This process initially involves the provision of a polymer film which has been generated by R. F. plasma vapor phase polymerization of a monomer comprising an inorganic (i.e. silicon) or an organometallic constituent on a receptive substrate. The polymer is thereafter selectively exposed to a coherent or focused energy source (i.e. CO.sub.2 laser) at the appropriate wavelength and power output to effect in situ conversion of a polymer film to a ceramic deposit which is substantially devoid of carbonaceous impurities. This process is also unique for its ability to provide a ceramic deposit that is firmly adherent on a variety of receptive substrates. The degree of adherence is far superior to ceramic coatings derived by chemical vapor deposition (CVD) techniques. The process lends itself to the formation of ceramic patterns which have application in the microelectronics industry.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a process and to the products produced thereby. 
More specifically, this invention is directed to a process for the in situ 
generation of high purity ceramics on a variety of substrates. The 
starting materials utilized in this process comprise a polymer film which 
has been formed by R.F. plasma vapor phase polymerization of a monomer 
comprising inorganic or organometallic constituents. The conversion energy 
is supplied by selective exposure of the film to a coherent or focused 
energy source of the appropriate power output and wavelength and for the 
appropriate duration. 
2. Description of the Prior Art 
The methods for formation of ceramic powders and objects are well known to 
the prior art. The formation of a ceramic object is usually preceded by 
mixing a blend of refractory powders of varying particle sizes, the 
compression of the refractory mixture into a "green" body and the 
controlled firing or sintering of the green body for a prescribed 
interval. The resultant object will generally be porous and its physical 
properties characteristic of a ceramic. 
The prior art also discloses the preparation of sinterable ceramic powders 
from reactant gases which are rapidly heated by a CO.sub.2 laser, see 
Cannon et al, J. Am. Ceram. Soc. 65, pp. 324-330 (1982); and Cannon et al, 
Ibid., 65, 330-335. This article describes the decomposition of the 
reactant gases, thus, causing particles to nucleate and grow rapidly. This 
process reportedly permits the formation of ceramic particles essentially 
free from defects. This vapor phase method is reportedly superior to the 
more conventional furnace, R.F.-heated and arc-plasma-heated gas-phase 
synthesis techniques because of their less than ideal thermal profiles and 
because of the reaction zones of the equipment do not allow for 
distribution in nucleation rates and growth times. 
The prior art also discloses the preparation of ceramic coatings from R.F. 
plasma polymerized polymer film, C. L. Beatty, "Silicon Nitride and 
Silicon Carbide from Organometallic and Vapor Precursors: Ultrastructure 
Processing of Ceramics, Glasses and Composites", editor Hench, et al, 
Chapter 23, pp. 272-292, John Wiley & Son (New York, 1984). The Beatty 
paper also describes the conversion of such coatings to ceramics by 
conventional pyrolysis techniques. Unfortunately, the resultant ceramics 
produced by such techniques were contaminated with carbonaceous inclusion 
or oxygen and, thus, did not possess the degree of purity required for 
electronics components or other applications where crystalline purity is a 
must. 
OBJECT OF THE INVENTION 
It is the object of this invention to remedy the above as well as related 
deficiencies in the prior art. 
More specifically, it is the principal object of this invention to provide 
a method for the in situ formation of ceramic materials on a supporting 
substrate. 
It is another object of this invention to provide a method for a formation 
of a ceramic pattern on a supportive substrate having both gross and fine 
feature resolution. 
It is yet another object of this invention to provide a method for the 
selective patterning of ceramic materials in response to laser 
irradiation. 
It is still yet another object of this invention to provide a method for 
the selective conversion of polymeric films to ceramic materials by 
selective irradiation of such films with laser irradiation. 
Additional objects of this invention will include, the articles of 
manufacture produced in accordance with the foregoing methods. These 
articles of manufacture are characterized by their superior adherence to 
substrates, absence of inclusions or other physical defects (i.e. 
pinholes) and moisture and vapor barrier properties. 
SUMMARY OF THE INVENTION 
The above and related objects are achieved by initially providing a polymer 
film which has been formed on a supporting substrate by R.F. plasma vapor 
phase polymerization techniques. This polymer film is then selectively 
converted to ceramic materials by exposure thereof to laser irradiation at 
the appropriate wavelength and power output. The duration of exposure is 
sufficient to effect the conversion of the film, in the exposed areas, to 
an essentially pure crystalline ceramic deposit. This ceramic deposit is 
localized and faithfully conformed to the distribution of laser energy 
within the polymer film. The ceramic deposit can be made to conform to any 
pattern of laser energy distribution and, thus, has obvious application in 
the fabrication of printed circuit boards and other SMT devices.

DESCRIPTION OF THE INVENTION INCLUDING PREFERRED EMBODIMENTS 
The process of this invention provides a unique route for the fabrication 
of high purity, ceramic coatings having excellent bonding to a supportive 
substrate. This process is also unique in its ability to form such 
coatings without the common types of physical defects (i.e. pinholes) 
which plague the more traditional chemical vapor deposition processes. 
Initially, a polymer film is formed on the supporting substrate by an R.F. 
plasma vapor phase polymerization process. The method of generation of 
this polymer film is believed to be critical to the process of this 
invention. In the preferred embodiments of this invention, the film is 
generated by R.F. plasma vapor phase polymerization of a "silicon 
containing monomer" onto a supportive substrate. The phrase "silicon 
containing monomer" as used herein, is intended as descriptive of a 
compound containing both carbon and silicon atoms which can be converted 
by radio frequency energy to free radicals or to an ionized gas. The film 
fabricated in this manner is of a controlled thickness and distribution 
upon the supportive substrate. 
Following formation of the polymer film from the silicon containing 
monomer, the film is selectively exposed to a coherent or focused energy 
source (i.e. representative conversion energy sources, electron beams; 
X-ray; CO.sub.2 laser @ 10.6 .mu.m; Nd: YAG laser @ 106 .mu.m, or 532 nm 
or 355 nm or 266 nm; Ar--ion laser or excimer lasers at various 
wavelengths in the ultraviolet region of the electromagnetic spectrum) at 
the appropriate wavelength and power output. The duration of exposure is 
sufficient to effect rapid conversion of the exposed regions of the 
polymer film to ceramic material. This conversion occurs in situ and 
without alteration of the polymer film in the areas which have been 
shielded from similar irradiation. The crystalline composition of the 
ceramic material thus produced will, of course, vary with the elemental 
composition of the polymer. In one of the preferred embodiments of this 
invention, an R.F. plasma polymerized film is derived from 
hexamethylcyclotrisilazane. The silicon is, thus, present in the recurring 
structure units of the polymer. The selection of conversion energy source 
is dictated by the absorption characteristics of this polymer film. In 
this instance, a CO.sub.2 laser (.lambda.=10.6 .mu.m) was the conversion 
energy source most appropriate to the task. The physical appearance of the 
resultant ceramic is indicative of a highly pure crystalline structure. 
The infrared spectrum of the ceramic deposit and its color closely 
resembles the infrared spectrum of pure silicon carbide or silicon nitride 
crystals (depending upon the precursor material). 
The silicon containing monomers which can be used in the formation of R.F. 
plasma vapor phase polymerized coatings and films include the silazanes 
and silanes of the following formulae: 
##STR1## 
wherein R.sub.1, R.sub.2, R.sub.3 and R.sub.4 are independently selected 
from the group consisting of hydroxyl, aryl or alkyl of 1 to 5 carbon 
atoms. 
In the preferred embodiments of this invention, R.sub.1, R.sub.2, R.sub.3 
and R.sub.4 are the same; and, are either methyl or ethyl. The preferred 
compounds suitable for use in the process of this invention, thus, 
include: hexamethylcyclotrisilazane, hexaethylcyclotrisilazane, 
hexamethyldilazane, hexaethyldisilazane, trimethyl silane, triethyl 
silane, tetramethyl silane or tetraethyl silane. 
The above monomers can be polymerized by R.F. plasma vapor phase 
polymerization techniques onto a suitable, supportive substrate. The films 
prepared in the foregoing manner are highly cross-linked and are of a 
uniform thickness. This ability to control uniformity of the resultant 
film is possible even where the surface upon which the film is deposited 
is irregular. In brief, the formation of such films involves R.F. plasma 
induced vapor phase polymerization (preferably utilizing power levels 
sufficiently low to enable substrate and the reactor environment to remain 
at ambient temperature) to induce formation/conversion of the monomer into 
free radicals. These free radicals can then combine in random fashion to 
form a highly cross-linked polymer film on a receptive substrate. The R.F. 
plasma polymerization technique which is suitable for use in the 
fabrication of such films is described in the open literature, see for 
example Beatty, "Silicon Nitride and Silicon Carbide from Organometalic 
and Vapor Precursors: Ultrastructure Processing of Ceramics, Glasses and 
Composites", editor Hench, et al, Chapter 23, pp. 272-292, John Wiley & 
Son (New York, 1984) (which is hereby incorporated by reference in its 
entirety). 
As noted above, the R.F. plasma vapor phase polymerization is directed to 
effect formation of a film or coating on a receptive substrate. The 
cross-linked nature of the polymer, in addition to the in situ formation 
upon the substrate, insures the creation of an extremely intimate and firm 
bond between the substrate and polymer. Substrates which can provide a 
receptive surface for such polymer film formation include various metal 
surfaces, synthetic surfaces and ceramic surfaces (i.e. ceramics which 
have been generated by the process of this invention). The surface of 
these substrates need not be specially prepared other than to insure that 
the polymer receptive surface thereof is essentially free of contaminants 
(i.e. grease or dirt). This can be accomplished by exposing the substrate 
to the R.F. plasma prior to introduction of the monomer reactants. 
In a number of the preferred embodiments of this invention, the substrate 
can provide a fugitive support; that is, once the polymer film has been 
formed, or once the polymer film has been selectively converted to 
ceramic, the substrate can be chemically or physically etched away thereby 
leaving a self-support film, or a ceramic structure which has been derived 
from the polymer film. It is also contemplated that a plurality of R.F. 
plasma polymer films may be prepared from different materials and formed 
upon one another. Each of the films would contain a ceramic precursor 
different from the other. Accordingly, the laminate could be subjected to 
different conversion energies thereby sequentially converting each film, 
thereby producing a ceramic composite. This process also lends itself to 
the formation of coatings possessing varying gradients of materials within 
the ceramic. This gradient effect is achieved by progressively changing 
the content of the reactant composition during the plasma polymerization 
phase of the process. 
As noted above, in the preferred embodiments of this invention, the polymer 
film containing the ceramic precursor (which is formed by R.F. plasma 
induced polymerization), is selectively exposed to an emission from a 
conversion energy source, the amount of energy impinging upon the film 
being sufficient to effect its selective conversion to a ceramic. The 
amount of energy is carefuly modulated to effect such conversion without 
generation of free carbon or graphite particle inclusions. Carbon dioxide 
lasers are effective for this purpose, where the polymer film has been 
prepared from silicon containing monomers (i.e. trisilazanes, disilazanes 
and silanes). The energy absorption characteristics of these materials are 
closely matched to the laser emissions of a carbon dioxide laser. The 
power output of the laser and the duration of exposure are controlled in 
order to effect the both rapid selective conversion of the films prepared 
from these monomers to the corresponding ceramic. The resultant ceramic 
has an infrared absorption spectrum which closely approximates pure 
silicon carbide crystals. 
As noted above, the R.F. plasma polymerized films can also be converted to 
the corresponding ceramic by other conversion energy sources (i.e. 
electron beam, X-rays, Nd: YAG lasers, excimer laser, Ar--ion lasers). 
The conditions prevailing during such conversion will of course be 
dependent upon the energy source. Ordinarily, the selective irradiation of 
the R.F. plasma polymerized film with laser energy is carried out under 
carefully controlled conditions in order to insure that the resultant 
ceramic is free from contaminants. In the preferred embodiments of this 
invention, such control over the conversion process is effected by the 
performance of such conversion in an inert environment, (i.e. a 
non-oxidizing environment, such as argon or nitrogen atmosphere). 
As noted above, the conversion process of this invention can produce 
composite ceramic materials. This can be effected by sequentially 
converting a polymer film to a ceramic and then forming a second polymer 
film over the ceramic and repeating the conversion process. Alternatively, 
a series of R.F. plasma polymerized films can be prepared one upon the 
other. Each of the films which are formed in this manner would be prepared 
from different ceramic precursor materials. The selection of such 
different materials would be based upon their energy absorption 
characteristics. For example, a series of films could be formed, one upon 
the other, and then the conversion effected with laser energy of defined 
wavelengths. In the context of this invention, this would be achieved by 
simply irradiating the top layer with activation energy within its unique 
area of absorption. Upon conversion of this top polymer film to the 
corresponding ceramic, the remaining film would be removed by selective 
chemical etching techniques. The underlying film would then be converted 
to its corresponding ceramic by activation with yet another energy source 
which would be uniquely matched to the ceramic precursor of the underlying 
polymer film. The process of etching and conversion could be repeated 
depending upon the number of contiguous polymer films which are initially 
prepared. The resulting composite would, thus, contain a variety of 
ceramic materials, presumably in some cooperative relationship. This 
technique could be used to prepare various masking devices. For example, 
one of the ceramic materials which could be obtained in the foregoing 
fashion, could be absorptive of certain imaging energies. In this manner, 
a high resolution mask could be formed without going through the laborious 
process of stenciling, etching and chemical vapor deposition, as is common 
in the manufacture of masks for the microelectronics industry. Such high 
resolution ceramic patterns can also be formed concurrent with the plasma 
polymerization of the monomers. This would involve combining the 
conversion energy source (i.e. laser) and the plasma irradiation step in a 
single continuous process to achieve a laser assisted R.F. plasma 
patterned ceramic film deposition. 
The examples which follow provide a number of specific illustrations of the 
preferred embodiments of this invention. The apparatus and techniques used 
in these examples are standard or hereinbefore described. Parts and 
percentages listed in these examples are by weight unless otherwise 
stipulated. 
EXAMPLE I 
The following procedure was used to form ceramic deposits upon a series of 
substrates. Initially, a substrate was cleaned (degreased), placed in an 
R.F. plasma reaction chamber and the chamber evacuated until the pressure 
within reduced to about 15 torr. The plasma was then started and the 
substrate exposed thereto for a period of about 30 minutes in order to 
insure that its surface was free from contamination. The monomer, 
hexamethyltrisilazane, was then introduced into the chamber and the 
pressure within the chamber allowed to rise to about 200 torr. The radio 
frequency of the chamber was maximized at 50 watts and the polymerization 
of the monomer allowed to proceed for about 1 hour. The amount of monomer 
introduced into the chamber is determined empirically to yield a polymer 
coating on the substrate of approximately one (1) micron. Polymer coating 
thickness can also be influenced by a number of factors, including the 
R.F. power level, reaction time and monomer pressure within the 
polymerization chamber. 
The polymer coated substrate prepared in the above manner is now removed 
from the plasma chamber and placed in a vacuum chamber so as to insure 
adequate control over the environmental conditions during selective 
exposure of the surface of the polymer coating to conversion energies. The 
preferred energy source used in this conversion was a grating tuned 
continuous wave carbon dioxide laser which operates over a wavelength 
range of from about 9 to 11 microns with a maximum power output of 60 
watts. An alternative source of conversion energy used in this process was 
an excimer laser operating at 308 nm. 
Typical irradiation time used in the conversion process ranged from 10-60 
seconds depending upon the power setting. The selection of source of 
conversion energy, the power of this energy source, the efficiency of 
absorption of the energy by the polymer film and the duration of exposure 
are carefully matched, based upon empirical testing, to selectively effect 
conversion of the polymer to a corresponding ceramic that is exposed to 
this conversion without the formation of carbonaceous inclusions. The 
ceramic thus produced is essentially pure crystalline material. 
The foregoing process was used with varying degrees of success in the 
formation of a ceramic deposition upon the following substrates: 
stainless steel 
brass 
glass 
quartz 
mylar 
silicon wafers 
The best results were obtained utilizing a stainless steel substrate. This 
is believed attributable to one or more of the following factors: (a) the 
quality of the R.F. plasma polymerized polymer coating in this substrate; 
(b) the bonding characteristics of the coating to the substrate; and/or 
(c) the ability of the substrate to rapidly and uniformly dissipate heat 
generated during the conversion process. 
The foregoing description and examples have been provided to illustrate 
some of the preferred embodiments of this invention. As is evident, the 
invention described herein is not to be limited by such disclosure, but 
rather is set forth in the claims which follow.