FIELD OF THE INVENTION
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The present invention relates to a method of depositing a th~n layer of material upon a surface of a substrate. More particularly, the present invention pertains to the use of gas discharge methods to deposit on surfaces, (low pressure plasma deposition) a thin film providing to the substrate impermeability to gases and vapors such as oxygen and ~ater.
BACKGROUND OF THE INVENTION
Plasma deposition has been used to deposit coatings for improving the barrier between the substrate on which this deposition is effected and the surrounding atmospheric or environmental conditions.
A plasma is a partially ionized gas or vapor containing ions, electrons and various neutral species, many of which are chemicaily highly reactive. This plasma state may be produced by strong electromagnetic fields, for example at radio or microwave frequency, and the resulting plasma-chemical reactions may be used, for example, to deposit thin film coatings.
There exists a very extensive body of literature on thin films prepared by plasma-enhanced chemical vapor deposition, which films are widely used in microelectronics technology, for example as passivation layers on account of their excellent barrier properties towards water molecules and alkali ions.
s~
US patent number 3,485,656 issued December 23, 1969, to Sterling et al., describes a method of direc-tly depositing an electrically insulating, amorphous, adherent solid layer of silicon nitride upon a surface of a substrate. This method is carried out with a radio frequency power source to establish the plasma.
In this paten-t, as well as in the literature, reference is sometimes made to the fact that the surface on which the layer is deposited, may be unheated. However, it is, in every case, added that it is desirable to heat the surface in order to improve the bonding within the layer to prevent water or OH
groups from being included in the layer. For example, in the above patent, it is stated that the layer of silicon nitride be deposited at a temperature less than 300C; but, a subsequent step of heat treating the layer at a -temperature of about 700C to 900C is recommended so that the silicon nitride layers be extremely hard, scratch and acid xesistant.
There are three important limitations with this type of plasma deposition procedure. F'irst, the use of a radio frequency plasma generator restricts it to relatively small surfaces or objects, i.e. surfaces or objects which may be placed between the electrodes forming the electric field required to generate the plasma. Second, the heating procedures of the prior art make lt Lmpossible to cleposit these protectlve layers on substrates which cannot bear these t:emperatures without deforming or decomposing. Third, deposltion rates by radio frequency plasma are inherently much slower than the same processes carried out at microwave frequency. For practical purposes, therefore, such as in a production line, radio frequency plasmas tend to be too slow, hence not cost effective.
OBJECTS AND STATEMENT OF THE INVENTION
It is therefore an object of the present invention to provide a method of depositing on large heat sensitive surfaces or objects, particularly polymeric surfaces or objects, a plasma-generated thin barrier film at substantially ambient temperature; the substrate structure is not altered by a heating step in the process, nor is its size or shape a limiting factor in achieving an efficient gas or vapor barrier.
This is accomplished by using electromagnetic energy in the microwave range for generating the plasma. This allows one to avoid the problems of plasma generators of the prior art, which are limited to a relatively smail plasma volume; furthermore, the substantially higher deposition rates which can be achieved with microwave plasmas permit much greater productivity, for example in a continuous production line process.
An appara-tus by means of which a large volume microwave plasma can be generated is des-cribed in the applicant's Canadian Patent 972,42 issued ~ugust 5, 1975.
The present invention is particularly useful in -the coa-ting of plastic objects having a relatively large size and complex configuration, such as polye-thylene terephtalate (PET) or poly-carbonate containers or bottles, which are light, cheap and non-breakable, compared with alternatives, (namely glass or multilayer plastics); however, their permeation rate, to oxygen for example, is excessively high if they are used as eontainers for perishables such as foods. The invention is also suitable for eoative polymerie surfaces based on polyacetal, polyamide, laminates of two or more polymers, and sueh polyblends, polyalloys, inter-penetrating network polymers as may from time-to-time find use in paekaging applieation.
It is also an objeet of the present invention to provide an effeetive plasma generated thin eoating for sueh plastie eontainers or bottles in order to reduee oxygen, earbon dioxide and water permeati.on and to thereby enhanee the life of the eontents of sueh containers and bottles.
It is a further objeet of the present i.nvention to provide an optieally transparent, thin .; .~ .
coating which confers greatly reduced overall permeabillty to a plastic object.
By depositing on these large complex surfaces, at ambient temperature, these plasma genera-ted thin barrier films, advantageous characteristics are achieved, namely: improved barrier properties, optical transparency, adequate ~lexibility and adherence of the coatings, and improved resisl.ance to abrasion.
Highly effective barriers against vapour and gas transmission have been obtained with plasma silicon nitride (P-SiN); plasma silicon oxide (P-SiO2); plasma silicon oxy-nitride (P-SiON); certain plasma polymers such as organo-silicons, copolymerized in plasma with oxygen; and certain halocarbons.
Examples P-SiN films were prepared in a large-volume microwave plasma apparatus using SiH4 and NH3 gas mixtures. Films were deposited on the following substrates: Mylar (trademark) polyethylene terephthalate, polyethylene, polypropylene and Lexan (trademark) polycarbonate~ (see Table Ia).
The left-hand side of Table la shows -the substrate t:ype, the fabrication conditions used to prepare the barrier coating, and the thickness of the latter. The right-hand side shows the flux of water 5~
vapor, which was measured using the procedure of ASTM
E96-53T for both the uncoated (Fo) and coated (F) polymers. Delta F, given by (Fo - F) X 100% is a meas-ure of the barrier's effect:iveness. Table lb provides similar examples for the case of oxygen.
Fabrica-tion parameters strongly affect the film quality, in particular, the flow rate ratio of SiH4 and NH3r The NH3 flow rate for all the series of samples used was 45 sccm, except sample 57 (30 sccm) and sample 58 (50 sccm) r while the SiH4 flow was varied from 7 to 20 sccm. The frequency of the power source, substrate temperature and pressure were kept constant at f= 2.45 GHz, Ts=25 C and p=0.3 Torr, respectively.
Plasma power was varied between 50 and 150 W and plasma volume varied from 1000 cm3 to 1500 cm3. The thickness of deposited film varied from 0.2 to l.O~um.
The best P-SiN coated samples showed a decrease in the water vapor flux up to 99.8%, and in oxygen flux up to 96.8%.
The values cited above are for illustration only, and are in no way intended to restrict the range of operating parameters. For example, much thinner films (less than O.l~um) have been found to constitute equally effective barriers if suitably prepared.
~5 ~able la MOISTURE PERMEATlON IN P--Si~ COATED POLYMERS
~lux (ghl12 day) # Substrate SiH4 NH3 Power Thickness Uhcoated Coated DeltaF
(sccm) (scom) (W) (A) (Fo) (F) (%) 3 Mylar(l) 7 45 100 5000 9.25 3.30 64.3 S Mylar(l) 1545 100 5000 9.25 1.30 85.9 6 Mylar(l) 1545 100 100009.25 4.80 48.1 8 Mylar(l) 2045 100 5000 8.80 5~30 39.8 11 PVC 15 451005000 4.10 1.50 63.4 12 Mylar(l) 1045 100 3000 8.80 1.30 85.2 17 Mylar(1) 1545 100 1000 8.30 5.80 30.1 18 Mylar(1) 1545 100 2000 8.30 5.60 32.5 19 Mylar(l) 1545 100 4000 8.30 2.60 68.7 22 Myla~(l) 1545 100 100008.30 3.60 56~6 23 Mylar(l) 1545 100 5000 8.30 1.80 78.3 24 ~ylar(1) 1545 100 5000 8.30 1.50 81.9 25 Mylar(l) 1545 100 5000 8.30 1.80 78.3 26 Mylar(l) 1545 100 5000 8.30 1.20 85.5 34 Mylar(2) 1545 100 5000 16.40 3.80 76.8 35 Mylar(2) 1545 120 5000 16.40 4.60 72.0 36 Mylar(2) 1545 80 5000 16.40 3.90 76.2 37 Mylar(2) 1545 100 5000 16.40 5.10 68.9 38 Mylar(3) 1545 100 5000 28.30 6.25 77.9 39 Mylar(3) 1545 70 5000 28.30 9.90 65.0 40 Mylar(3) 1545 150 5000 28.30 0.06 99.8 41 Mylar(3) 1145 120 5000 28.30 3.80 86.6 42 LRxan 15 451005000 72.7025.40 65.1 43 Lexan 10 451005000 72.7030.50 58.0 46 Lexan 15 451005000 72.7029.10 60.0 49 Lexan 15 45805000 72.7014.50 80.1 50 LRxan 15 451205000 72.7012.40 82.9 51 Lexan 15 451005000 72.7016.00 78.0 52 Mylar(3) 1545 100 5000 29.20 6.70 77.1 53 Mylar(3) 1545 100 3000 29.20 7.00 76.0 54 Mylar(3) 1545 100 5000 2g.20 6.40 78.1 55 Mylar(3) 1545 100 3000 29.20 5.30 81.8 57 Mylar(3) 1030 100 5000 29.20 10.30 64.7 58 Mvlar(3) 2050 100 5000 29.20 11.10 62.0 N ~ p = O.3 Torr ~ 6~9~
Table lb OXYGEN PER~EATION ~ASUREMENTS *
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SAMPLE# SUBSTRATE PRESSURE (02) TI~E PERMEABIL2TY
(psig) (h) ~cc/lOOin atm) . ~
uncoated Mylar(l) 50 1.6 26 Mylar(l) 50 72 O
24 Mylar(l) 50 48 0 uncoated Lexan 20 72.5 uncoated Lexan 20 71.5 50 Lexan 20 12 0 51 Lexan 20 12 0 50 Lexan 40 6.9 5] Lexan 40 10.4 uncoa-ted Mylar(3) 40 16.7 55 Mylar(3) 40 2.6 EXPERIMENTAL PROCEDUR
* Based on ASTM test method D-1434 The followi.ng is a :I.ist of possible thln film barrier materlals to be used in the present invention.
l. Inorgani.c films l.l SLlicon compounds (made by co-reacting SiH4, also known as mono-silane, wi-th at least one other gas) P-SiN =(SiH4 + NH3 and/or N2) P-SiO =(SiH4 -~ N2O) P-Si.O:P =(SiH4 + N20 + PH3) P-SiON =(SiH4 + NH3 + N2O) a-SiC:H =(SiH4 + CHx) a-Si:H =(SiH4 alone or with an inert gas such as Ar, He, Ne or with hydrogen) Similar reactions can also be carried out using higher silanes (eg Si2H6) or their halogenated equivalents (eg SiF4, SiCl4, etc.) l.2 - metal compounds made by plasma deposition from a volatile organometallic compound (see Table 2):
- if reacted alone, these form metal-containing plasma polymers (see category 2.3 below) - if co-reacted with 2 or NH3, they can form oxides or nitrides, as in the case of silicon compounds above.
2. Organosilicones or organometallics A drawback of group 1.1 is that the principal reagent, silane, is dangerous (-toxic and flammable in 5 air). It may therefore be desirable to arrive at Si compound films starting from less dangerous compounds (organosilicones) (see table 3).
2.1 Organosilicon plasma polymers These result from reaction of the "monomer"
gas alone, or combined with an inert gas. (see table 3) 2.2 Silicon compounds (same list as in 1.1) These can be obtained by co-reacting a monomer with oxygen (which yields P-SiO) or with oxygen and ammonia or nitrogen (which yields P-SiON). The role of oxygen is to "burn" (i.e. oxidize to form CO2 and H2O) the organic part of the organosilicon monomer.
2.3 Organometallic plasma polymers Same comments as in 2.1 above, but using compounds from table 2.
2.4 Metal compounds Same comments as in 2.2 above, but using compounds from table 2.
3. Plasma polymers and diamond-like carbon 3.1 Plasma polymers These are thin films produced from volatile organic compounds (i.e. other than organosilicon or organometallic, which are treated in category 2 above~.
These may be:
hydrocarbons halocarbons other organics containing O, N, S...
One preferred embodiment of the invention is to produce thin plasma polymer equivalen-ts of known "conventional" barrier materials, namely:
vinylidene chloride e-thylene-vinyl alcohol (EVAL) others 3.2 Diamond-like carbon Under suitable conditions, hydrocarbons may be converted in the plasma to diamond-like carbon films (symbolized by a-C:H). This ma-terial is very dense and has good barrier characteristics.
Although the invention has been described above in relation to specific embodiments, it will be evident to a person skilled in the art that it may be refined and modified in various ways. It is therefore wished to have it understood that the present invention should not be limited :in interpretation except by the terms of the following claims.
'rable 2 The ~ollowing organometalllc Monomers and mixtures may be used for depositiny films as taught in the present invent~on.
MONOM~-R ~or mixture) S~ructural formula Copper phtalocyanine [Ph(CN)2]4Cu Acetylacetonatedimethylgold Me2AuOC(Me)=CHC(Me)0 Diethylzinc Et2Zn Diethylzinc/Diethylselenide Et2Zn/Et2Se Dimethylmercury Me2Hg Trimethylaluminium/Ammonia or Nitrogen Me3Al/NH3 or N2 Tri-sec-butyloxyaluminium (sec-BuO)3Al Tetramethylgermanium Me4Ge Tetramethylgermanium/Oxygen Me4Ge/02 Tetramethyltin Me4Sn Tetramethyltin/Oxygen Me4Sn/02 Tetramethyltin/Ammonia Me~Sn/NH3 Tetramethyltin/Nitrogen or Methane or Acetylene Me4Sn/N2 or CH4 or C2H2 Tetramethyltin/Propene Me4Sn/H2C=CHMe Tetrame-thyltin/Methylmetha-crylate Me4Sn/H2C=C(Me)CO2Me Tetramethyltin/Perfluoro-benzene Me4Sn/C6F6 Trimethyle-thyltin Me3SnEt Trimethylvinyltin Me3SnCH=CH2 ~$~
Table 2 (continued) Trlmethylethynyltin Me3SnC=CH
Butyltrivinyltin BuSn(CH=CH2)3 Tetramethyl]ead Me~Pb Pentaethoxytantalum (EtO)5Ta Trimethylblsmuth Me3Bi Dicyclopentadienylmangarlese ( 5 5)2 Ferrocene ( 5 5)2 Vinylferrocene 2 5 4 5 5 Cyclopentadienydicarbonyl-cobalt C5H5Co(CO)2 Allylcyclopentadienyl-palladlum H2C=cHcH2Pdc5 5 Table 3 The following Organosllicon Monomers and mixtures wlth other gases may be used for depositing films as taught in the present invention.
~ ., . _ . . _ _ _ MONOM~R (or mixture) Structural Formula . _ . .
Tetramethylsilane Me4Si Tetramethylsilane/Oxygen Me4Si/02 Tetramethylsilane/Ammonia Me4Si/NH3 Tetramethylsilane/Hydrogen/
Argon Me4Si/H2/Ar Tetramethylsilane/Tetra-fluoromethane Me4Si/CF4 Ethyltrimethylsilane EtSiMe3 Vinyltrimethylsilane H2C=CHSiMe3 Vinyltrimethylsilane/Tetra-fluoromethane H2C=CHSiMe3/CF4 Ethynyltrimethylsilane HC-CSiMe3 Ethynyltri.methylsilane/
Tetrafluoromethane HC-CSiMe3/CF4 Allyltrimethylsilane H2C=CHCH2SiMe3 Trimethylchlorosilane Me3SiCl Phenylsilane PhSiH3 p-Chlorophenylsilane Cl-PhSiH3 Diphenylsilane Ph2SiH2 Hexamethyldisilane ( 3Si)2 ~is(trimethylsilyl)methane (Me3Si)2CH2 Disilybenzene (H3Si)2C6H4 ~6~
~a~le 3 (continued) Methyldimethoxysilane/Oxygen MeHSi(OMe)2/o2 Dimethyldimethoxysilane Me2Sl(OMe)2 Methyltrimethoxysilane MeSi(OMe)3 Methyltrimethoxysilane/Oxygen MeSi(OMe)3/02 Vinyltrimethoxysilane H2C=CHSi(OMe)3 Tetramethoxysilane (Me0)4si Vinyldimethylethoxysilane H2C=CHMe2SiOEt Vinyldimethylethoxysilane/ H2E=CHMe2SiOEt/
Styrene PhCH=CH2 Vinyltriethoxysilane H2C=CHSi(OEt)3 Ethynyltriethoxysilane HC-CSi(OEt)3 ~-Aminopropyltriethoxysilane 2 ( 2)3 ( )3 Tetraethoxysilane !Et0)4Si Tetraethoxysilane/Oxygen (Et0)4Si/02 Trimethyvinyloxysilane Me3SiOCH=CH2 Tetramethyldisiloxane (Me2SiH)20 Tetramethyldisiloxane/Oxygen (Me2SiH)202/02 Hexamethyldisiloxane (Me3Si)20 Hexamethyldisiloxane/Ammonia (Me3Si)20/NH3 Hexamethyldisiloxane/Vinyl-trimethylsilane (Me3Si)20/H2C=cHsiMe3 Hexamethyldisiloxane/Toluene (Me3si)2G/phMe Divinyltetramethyldisiloxane (H2C=CHSiMe2)20 Hexamethylcyclotrisiloxane (Me2Si0)3 Octamethylcyclotetrasiloxane (Me2Si0)4 Dimethylaminot:rimethylsilane Me2NSiMe3 Ta~le 3 (continued) Diethylami.notrimethylsilane Et2NSiMe3 Bis(dimethylam1no)methylsilane (Me2N)2SiHMe Bis(dimcthylamino)methyl-vinylsilane (Me2N)2SiMeCH=CH2 Hexamethyldisilazane (Me3Si)2NH
Hexamethyldisilazane/Nitro~en (Me3Si)2NH~N2 Hexamethyldisilazane/Toluene (Me3Si)2NH/PhMe Hexamethylcyclotrisilazane (Me2SiNH)3 Hexamethylcyclotrisilazane/
Ammonia (Me2SiNH)3/NH3 NN'Bis(dimethylsilyl)tetra-methylcyclodisilazane (Me2SiNSiHMe2)2 Hexamethyldis,lathiane (Me3Si)2S