Method of depositing barrier coatings by plasma assisted chemical vapour deposition

Abstract: Method for the simultaneous plasma assisted chemical vapor deposition of thin film material onto an elongated web of substrate material at a plurality of discrete spatially separated deposition zones (12). In order to accomplish said simultaneous deposition, the web of substrate material (10) is operatively positioned so as to assume a serpentine path of travel through a reduced pressure enclosure (1). By using an elongated linear applicator (4) as a source of microwave energy, a high rate of uniform deposition of said thin film material over a plurality of large areas of the web substrate material can be simultaneously achieved without heating of said web above the melting point thereof. In a preferred embodiment, the web of substrate material is formed of a low temperature, microwave transmissive synthetic plastic resin and the thin film material deposited thereupon forms a barrier coating for preventing oxygen diffusion therethrough.

Description: The present invention relates generally to a method of depositing high barrier coatings by plasma assisted chemical vapour deposition (PECVD), and more particularly to a method of PECVD for an oxygen and water vapour barrier coating onto a substrate using a precursor gas mixture. In a preferred embodiment of the present invention, a thin film material deposited upon the deposition surface of a web of substrate material is an ultra-thin, transparent barrier layer for inhibiting the diffusion of oxygen there through. Specific processing parameters are enumerated herein. BACKGROUND OF THE INVENTION One application of microwave energy is to efficiently create activated species from gaseous precursors for use in plasma treatment processes, such as semiconductor etching and thin film deposition. Previous microwave plasma deposition techniques, which illustrate the state of the art and highlight both the problems encountered in increasing the energy uniformity and the advantages provided by the microwave plasma generating structure and thin film deposition apparatus of the instant invention, will be discussed in the following paragraphs. Commonly assigned, U.S. Patent Nos. 4,517,223 and 4,504,518 to Ovshinsky, et al., both entitled "METHOD OF MAKING AMORPHOUS SEMICONDUCTOR ALLOYS AND DEVICES USING MICROWAVE ENERGY" describe processes for the deposition of thin films onto small area substrates in a low pressure, microwave glow discharge plasma. As specifically noted in these Ovshinsky, et al. patents, operation in the disclosed low pressure regimes not only eliminates powder and polymeric formations in the plasma, but also provides the most economic mode of plasma deposition. While these patents describe the revolutionary concept of operating at low pressure and high energy density utilizing microwave energy, i.e., operating at the substantial minimum of the modified Paschen curve, the problem of uniformity of the deposition of thin films over large areas remained addressed. Turning now to microwave applicators for large area substrates, commonly assigned U.S. Patent No. 4,729,341 of Fournier, et al., for "METHOD AND APPARATUS FOR MAKING ELECTROPHOTOGRAPHIC DEVICES" describes a low pressure microwave initiated plasma process for depositing a photoconductive semiconductor thin film on a large area cylindrical substrate using a pair of radiative waveguide applicators in a high power process. However, the principles of large area deposition described therein are limited to cylindrically shaped substrates, such as electrophotographic photoreceptors, and the teachings provided therein are not directly transferable to elongated, generally planar substrates. While workers in the field have disclosed methods of processing thin films utilizing the high power of microwave sustained plasmas, previously microwave plasma treatments have not been altogether appropriate for large surface area and/or low pressure deposition. This is because of the non-uniformity of the plasma over an enlarged or elongated substrate due to the non-uniformity of the energy initiating the plasma. One attempt to provide greater large area uniformity was the use of a slow wave microwave structure. A problem that is inherent in slow wave structures, however is the very rapid decline of microwave coupling into the plasma as a function of distance transverse to the microwave applicator. This problem has been addressed in the prior art by various structures that vary the spacing of the slow wave structure from the substrate to be processed. In this way the energy density at the surface of the substrate can be kept constant along the direction of movement of the substrate. For example, U.S. Patent No. 3,814,983 to Weissfloch, et al. for "APPARATUS AND METHOD FOR PLASMA GENERATION AND MATERIAL TREATMENT WITH ELECTROMAGNETIC RADIATION and U.S. Patent No. 4,521,717 to Kieser, et al., for "APPARATUS FOR PRODUCING A MICROWAVE PLASMA FOR THE TREATMENT OF SUBSTRATE IN PARTICULAR FOR THE PLASMA POLYMERIZATION OF MONITORS THEREON", both address this problem by proposing various spatial relationships between the microwave applicator and the substrate to be processed. More, particularly, Weissfloch, et al. discloses that in order to obtain the uniform electric field intensity necessary for a plasma of uniform power density along the full structure at an angle with respect to the substrate. It should be apparent, however, that inclination of the slow wave waveguide structure with respect to the substrate, to achieve uniformity, leads to an inefficient coupling of microwave energy into the plasma. Recognizing this deficiency, Kieser, et al. described that the conditions resulting from superposing of two energy inputs, i.e., two microwave applicators, can be further improved if the two slow wave applicators are set at an angle to each other such that the planes normal to the medians of the applicators intersect at a straight line which extends parallel to the surfaces of the substrate to be treated and at right angles to the direction of travel of the substrate. Moreover, Kieser, et al. recommended that in order to avoid destructive interference of the wave field patterns of the two applicators, the applicators should be displaced from each other transversely of the direction of travel of the substrate by a distance equal to half of the space between the cross-bars of the waveguide. In this way the microwave field pattern is substantially suppressed. The problem of plasma uniformity and more particularly, energy uniformity was treated by J. Asmussen and his co-workers, for example in T. Roppel, et al. "LOW TEMPERATURE OXIDATION OF SILICON USING A MICROWAVE PLASMA DISC SOURCE", J. Vac. Sci. Tech. B-4 (January-February 1986) pp. 295-298 and M. Dahimene and J. Asmussen "THE PERFORMANCE OF MICROWAVE ION SOURCE IMMERSED IN A MULTICUSP STATIC MAGNETIC FIELD" J. Vac. Sci. Tech. B-4 (January-February 1986) pp. 126-130. In these, as well as other papers, Asmussen and his co-workers described a microwave reactor which they refer to as a microwave plasma disc source ("MPDS"). The plasma is reported to be in the shape of a disc or tablet, with a diameter that is a function of microwave frequency. A critical advantage claimed by Asmussen and his co-workers is that the plasma disc source is scalable with frequency: that is, at the normal microwave frequency of 2.45 gigahertz, the plasma disc diameter is 10 centimeters and the plasma disc thickness is 1.5 centimeters; but that the disc diameter can be increased by reducing the microwave frequency. In this way, the plasma geometry was said to be scalable to large diameters, potentially yielding a uniform plasma density over a large surface area. However, Asmussen, et al. only described a microwave plasma disc source which is operational at 2.45 gigahertz, where the plasma confined diameter is 10 centimeters and the plasma volume is 118 cubic centimeters. This remains far from being a large surface area. In order to provide for the deposition onto large area substrates, Asmussen, et al. proposed a system operational at the lower frequency of 915 megahertz, which would provide a plasma diameter of approximately 40 centimeters with a plasma volume of 2000 cubic centimeters. Furthermore, the deposited material quality and deposition rate is dependent on excitation frequency. The modulation of frequency to increase plasma dimensions compromises material quality and film deposition rate. Workers at Hitachi have described, for example in U.S. Patent No. 4,481,229 to Suzuki, et al., the use of electron cyclotron resonance (ECR) to obtain a high power plasma having a relatively high degree of uniformity over a limited surface area. However, the Hitachi patent does not teach, nor even suggest a method by which uniform large area plasmas may be achieved. Moreover, the use of ECR imposes the added requirement of highly uniform magnetic field structures in the microwave apparatus, and may be restricted in operation to only those very low pressure regimes where electron collision times are long enough to allow the ECR condition to be achieved. U.S. Patent Nos. 4,517,223 and 4,729,341 referred to above, describe the necessity of using very low pressures in very high microwave power density plasmas in order to obtain high deposition rates and/or high gas utilization. However, the relationship between high deposition rates, high gas utilization, high power density, and low pressure further limits the utility of slow wave structures and electron-cyclotron resonance methods. The limitations of the slow wave structure and of the electron-cyclotron resonance methods were obviated by the method and apparatus disclosed in commonly assigned U.S. Patent No. 4,893,584, to Doehler et al., "LARGE AREA MICROWAVE PLASMA APPARATUS". One particular application for the deposition of thin film coatings onto an elongated relatively wide web of substrate material is for the food packaging industry. More particularly, there has recently arisen a desire to deposit thin film oxygen and water vapor impermeable coatings atop an elongated polymeric web to be employed for packaging and giving extended shelf life to perishable foodstuffs. In order to accomplish this objective, researchers have previously developed thin film SiCO coatings and have even suggested the significance of controlling the hydrogen content of that thin film. More particularly, the importance of controlling the hydrogen content of prior art films for the purpose of, inter alia, depositing oxygen impermeable films has been discussed in commonly assigned U.S. Patent No. 4,737,379, the disclosure of which is incorporated hereinto by reference. As was noted therein, plasma deposited amorphous silicon hydrogen alloys, as well as alloys of silicon and hydrogen with one or more of carbon, oxygen and nitrogen suffered from various shortcomings. The hydrogen content was strongly dependent upon the substrate temperature, that content decreasing at high temperatures and increasing at low temperatures. The deleterious effect of hydrogenation on film properties, such as oxygen and water vapor permeability, is a direct consequence of hydrogen's role as a chemical bond terminator. As such, hydrogen can disrupt the connectivity of the chemical bond network of the deposited film, thereby reducing its average atomic coordination number. The solution preferred by said '379 patent was to eliminate the presence of hydrogen for the feedstock gases. This was at least partially due to the fact that thermally sensitive substrates, such as plastics, could not be heated sufficiently to eliminate hydrogen bonding in the deposited thin films. This inability to drive off hydrogen produced thin films characterized by greatly deteriorated properties which limited the utility of said films. However, the recipes set forth in the '379 patent fail to provide a film which exhibits the type of oxygen and water vapor permeation characteristics demanded by the food packaging industry. Therefore, there is a need in the art for the deposition of thin film oxygen and water vapor impermeable flexible coatings atop a low temperature web of substrate material. This and other needs are met by the deposition method described hereinbelow. BRIEF SUMMARY OF THE INVENTION The present invention thus has as object a method of depositing, by plasma assisted chemical vapour deposition, an oxygen and water barrier coating atop a temperature sensitive substrate from a precursor gaseous mixture which includes at least a silicon containing gas and oxygen gas, according to present claim 1. Other objects of the present invention, as well as advantages and improvements of the present invention are further defined in the dependent claims. These and other objects and advantages of the instant invention will become apparent from the Detailed Description, the Drawings and the Claims which follow hereinafter. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic cross-sectional view of a first embodiment of the deposition apparatus used in the instant invention, specifically illustrating the elements operatively disposed therein for effecting the uniform microwave plasma deposition on an elongated substrate; this figure illustrates an embodiment of the instant invention having a single plasma deposition region, employing a single gas manifold and a single elongated microwave applicator; Fig. 2 is a schematic cross-sectional view of a second embodiment of the deposition apparatus used in the instant invention, this embodiment employing a single deposition region, a single elongated microwave applicator, and two gas manifolds, the first of which is adapted to introduce the pre-cursor gaseous mixture and the second of which is adapted to introduce a non-depositing gas mixture to effectuate plasma pre-treatment of the deposition surface of the elongated web of substrate material; Fig. 3 is a schematic cross-sectional view of a third embodiment of the deposition apparatus used in the instant invention, this apparatus employing two deposition regions, two elongated microwave applicators and two precursor gas mixture injection manifolds; Fig. 4 is a schematic cross-sectional view of a fourth embodiment of the deposition apparatus used in the present invention, this embodiment employing two elongated microwave applicators and three gas manifolds, two of which arc adapted to introduce precursor gaseous mixture into the deposition regions and a third adapted to introduce a non-depositing gas mixture for the plasma pre-treatment of the deposition surface of the elongated web of substrate material; and Fig. 5 is a cross-sectional view of a portion of the apparatus used in the instant invention, specifically depicting a radiative linear microwave applicator unit operatively attached interiorly of the deposition chamber. DETAILED DESCRIPTION OF THE INVENTION The method according to the instant invention is preferably carried out by an apparatus for the simultaneous plasma enhanced chemical vapor deposition of thin film material onto multiple portions of an elongated web of substrate material utilizing a linear microwave applicator. Deposition occurs within an evacuated deposition chamber. By maintaining the vessel, at sub atmospheric pressures, it is possible to operate the plasma at a pressure approximately that required for operation near the minimum of the modified Paschen curve. Low pressure operation also allows for a longer mean free path of travel for the plasma excited species, thereby contributing to overall plasma uniformity. In this way, it is possible for the microwave deposition apparatus to sustain a uniform plasma reaction upon a substrate disposed within the near field distance of the microwave source. Fig. 1, illustrates, in schematic cross-section, a first embodiment of a microwave deposition apparatus 1 for sustaining a substantially uniform microwave plasma over a relatively wide web of substrate material. As used herein, the term "wide" refers to a body having a width dimension greater than one microwave wavelength, and preferably greater than 30.5 cm (twelve inches). The apparatus 1 includes, inter alia, a vacuum vessel deposition chamber 2, the walls of which are preferably formed of a durable, corrosion resistant material such as stainless steel. The vacuum vessel deposition chamber 2 further includes a pump-down port 3 adapted for suitable connection to a vacuum pump for maintaining the interior of the vacuum vessel deposition chamber 2 at an appropriate sub-atmospheric pressure. The vacuum pump is further adapted to remove reaction products from the interior of said vessel 2. The vessel 2 further includes at least one process gas input line connected to a process gas input manifold 9, which manifold 9 is operatively disposed so as to uniformly distribute processed gases into the interior of said reaction vessel 2, and specifically into the plasma deposition region 12 thereof. The process gas input manifold 9 is operarively disposed between a pair of process gas containment means 11 and between at least two sections of the web of substrate material which define the deposition region 12. In combination, the web of substrate material and the process gas containment means 11 contain the process gas introduced by the manifold 9 into the plasma region 12 of the vacuum vessel deposition chamber 2. During operation, the web of substrate material is drawn from payoff roll 5 and guided by guide rollers 8 toward the deposition region. Guide rollers 8 may be variable tension rollers to relieve stress upon the web of substrate material as it passes through the deposition chamber. After passing along one or more guide roller 8, the web of substrate material is passed between the linear microwave applicator 4 and the process gas injection manifold 9. The web of substrate material is then passed around chill roller 7, again passing gas port 9, around additional guide rollers 8, and collected upon take-up roll 6. The sections of the web of substrate material extending between guide roller 8 and chill roller 7 create an enclosed plasma deposition region 12. The precursor gas mixture injected from gas manifold 9 interacts with the microwave energy supplied by the elongated microwave applicator 4 to create energized species in deposition region 12. Deposition therefore occurs on both the section of substrate material leading toward and moving away from chill roller 7. Because plasma deposition is inherently a high temperature process, temperature sensitive substrates must be intermittently cooled so that destruction thereof is avoided. This cooling is accomplished by chill roller 7. Cooling the web of substrate material allows for a longer overall exposure time to the plasma deposition. Tne apparatus of Fig. 1 allows for high deposition rates and good process gas utilization by allowing for deposition on at least two discrete portions of the web of substrate material on a single pass. The microwave deposition apparatus 1 further includes a microwave applicator unit 4 which extends at least partially into the interior of said vacuum vessel deposition chamber 2. The microwave applicator unit 4 is adapted to radiate microwave energy from a source thereof, into the interior of said vacuum vessel deposition chamber 2 for initiating and maintaining a plasma of the process gases introduced into said vessel 2 through a process gas manifold 9. The specifics of the microwave applicator unit 4 including the microwave applicator and microwave applicator isolating means will be discussed in detail hereinbelow. Turning now to Fig. 2, there is shown a schematic cross-sectional view of a second embodiment of the microwave deposition apparatus. This second embodiment is similar to the microwave deposition apparatus of the first embodiment (Fig. 1) with the exception of the addition of a deposition surface pretreatment zone. The instant inventors have surprisingly found that microwave pretreatment of the deposition surface of the web of substrate material enhances the barrier properties of the deposited thin film. This microwave pre-treatment is preferably a plasma pre-treatment and most preferably an argon plasma pre-treatment. The argon plasma pre-treatment of the deposition surface of the web of substrate material occurs in a plasma pre-treatment zone 13. The argon is injected into the plasma pre-treatment zone 13 by pre-treatment gas manifold 14. The injected argon interacts with the microwaves from the linear microwave applicator 4 thereby pretreating the deposition surface of the web of substrate material. In order to pass the deposition surface of the web of substrate material past pre-treatment zone 13, the guide rollers 8 on the payoff roll 5 side of the deposition apparatus 1 must be moved to the opposite side of the microwave applicator vis a vis the deposition apparatus of Fig. 1. In addition, another roller 15 must be added to the system. This additional roller 15 can be either a guide roller or a quench roller as required. It is hypothesized that this pretreatment process promotes the adherence of the coating to the web and possibly even affects the growth kinetics thereof. Turning now to Fig. 3, illustrated therein is a schematic cross-section of a third embodiment of the microwave deposition apparatus. This third embodiment is similar to the first embodiment of Fig. 1 except for the inclusion of an additional deposition region 12 which includes an additional chill roller 7, an additional gas manifold 9, an additional guide roller 8, and an additional linear microwave applicator 4. Also, the embodiment of Fig. 3 includes a microwave barrier 16 which reduces or eliminates the interaction of microwaves from the linear applicators 4. This embodiment provides for very high deposition rates, due to the dual deposition regions 12 which allow deposition on at least four discreet portions of the web of substrate material during a single pass. This higher deposition rate allows for faster throughput of the web of substrate material, for a given deposited thin film thickness. This higher throughput speed reduces the temperature rise of the web of substrate material at any given deposition zone by reducing the time of exposure to the deposition plasma and allowing the web of substrate material to pass over cooling rollers between depositions. Alternatively, the first deposition region of the microwave deposition apparatus of Fig. 3 may be used as a plasma pre-treatment region. In this instance, the deposition zone 12 which is closest to the pay off reel 5 would be used as a plasma pre-treatment zone 13, as in Fig. 2, by providing a non-depositing gas or gaseous mixture, such as argon, in the first gas manifold 9. This alternative embodiment would reduce any microwave losses encountered by the deposition apparatus of Fig. 2 due to the plasma pre-treatment microwave energy load by providing a separate linear microwave applicator for each of the plasma pre-treatment and plasma deposition zones. Turning now to Fig. 4, there is depicted therein a cross-sectional schematic representation of a fourth embodiment of the microwave deposition apparatus This embodiment combines both the dual deposition regions 12 of the embodiment of Fig. 3 and the plasma pretreatment region 13 of the embodiment of Fig. 2. Therefore, this embodiment allows for both high deposition rate/high throughput and plasma pre-treatment of the deposition surface of the web of substrate material for enhancement thereof. As is illustrated in Fig. 5, the microwave applicator unit 4 comprises a substantially rectangular waveguide 42 having an open end portion 44 at the terminal end portion thereof extended into the vessel 2. The open end portion is adapted to avoid standing waves. It is to be understood that the applicator unit 4 may alternatively be sealed at the terminal end thereof. The waveguide means 42 includes a plurality of apertures formed through a wider face thereof. The apertures are dimensioned and spaced to provide for the uniform radiation of microwave energy therefrom. Returning now to Fig. 5, the microwave applicator unit 4 further comprises means 60 for isolating the microwave waveguide 42 from the deposition chamber 2. The isolating means 60 is preferably fabricated from a dielectric material which is substantially transparent to microwave energy. A preferred material from which said isolating means 60 may be fabricated is quartz, though it is to be understood than many other materials may be employed with equal success. The isolating means 60 should further be configured in a shape designed to optimize its ability to withstand forces due to the presence of a pressure differential. In this manner, the thickness of the isolating means may be minimized to provide for effective thermal cooling whereby high microwave power densities may be employed without deleteriously effecting the isolating means. To this end, a preferred shape for said isolating means is cylindrical or hemi-cylindrical as to encapsulate at least that portion of the waveguide 42 which extends into the vacuum vessel 2. A cylindrical or hemi-cylindrically shape is preferred to for example a planar shape, because a cylinder may be made thinner than a planar shape due to the strength inherent in a cylinder. Thus, a thin cylinder is capable of withstanding pressures which would require a much thicker planar plate. Additionally, a thick plate cannot be maintained at a uniform, relatively low temperature, as can a thin cylinder. Thus, a planar plate may be subject to thermal degradation, in a microwave plasma apparatus particularly at elevated power levels. By contrast, the thin cylindrical isolating means 60 may be uniformly cooled, and thus does not experience thermal degradation, and therefore does not impose practical limitations upon the amount of power which may be applied. Additionally, the waveguide 42 should be operatively disposed within the isolation means 60 and spaced from the circumferential wall thereof. So disposed, the waveguide 42 may extend partially through the vacuum vessel 2, without being directly exposed of the plasma regions 12 or 13 contained therein. The cylindrical isolating means 60 of Fig. 5 is configured so as to be coextensive with at least one dimension of the vacuum vessel 2, and protrude through at least a first and second wall portion of said vacuum vessel 2. The cylindrical isolating means 60 is fixed through the walls of the vacuum vessel 2 by means of two collar fittings 62 and 64 which are preferably mutably joined to the stainless steel vessel 2. Collar fitting 62 comprises an open end portion 66 extending from a connection flange 68 affixed directly to a side wall of the vacuum vessel 2, and includes an opening 70 co-extensive with the circumference of the cylindrical isolating means 60 and adapted to receive cylindrical isolating means 60. Open end portion 66 extends from said connection flange 68, and is adapted to receive at least two O-rings 72, 74, which O-rings 72, 74 are adapted to effect a vacuum and water barrier between the interior of said vacuum vessel 2 and outside ambient conditions. Between O-rings 72, 74 is disposed a cooling channel 73 through which a cooling medium such as water may be circulated to maintain the O-rings at a uniformly low temperature. The O-rings 72, 74 are preferably adapted to maintain the vacuum and water seal at substantially elevated temperatures, i.e., temperatures in excess of 100° Centigrade. The cylindrical isolating means 60 passes through opening 70, connection flange 68, and the open end portion 66. In this way, O-rings 72, 74 are urged against the outside circumference of said cylindrical isolating means 60. The compression of the O-rings 72, 74 against the cylindrical isolating means 60 results in an air tight, water tight seal. It is important to note that the location of O-rings 72, 74 is well outside the plasma region 12 of the apparatus 1. This is noteworthy because by keeping the O-rings out of the plasma region 12, they are not exposed to the excessive temperatures associated with microwave plasmas, i.e., temperatures in excess of 500° Centigrade. Had the O-ring seals been located within the plasma region, as shown in the aforementioned U.S. Patent No. 4,729,341, special (and costly) high temperature resistant seals would have been required, greatly increasing the complexity and cost of the apparatus 1. The cylindrical isolating means 60 may extend beyond the outside terminal edge of said open end portion 66. This portion of the cylindrical isolating means 60 must thus be equipped with microwave containment means 80. The microwave containment means 80 is typically fabricated from a metal microwave containment canister operatively affixed around the outside circumference of the cylindrical isolating means 60, and in electrical contact with said open end portion 66 by grounding fingers 82. The microwave containment canister is fabricated so as to be coextensive with that portion of cylindrical isolating means 60 which extends beyond the open end portion 66. Additionally, the microwave containment means 80 further includes an open end portion 84 over which is disposed a metallic microwave blocking mesh 86 adapted to contain stray microwaves therewithin. The mesh 86 is also adapted to allow for the passage of a flux of cooling air through the cylindrical isolating means 60. Alternatively, and as is illustrated in phantom in Fig. 5, the opened end portion 84 of the microwave containment canister 80 may be attached to a dummy load adapted to absorb excess microwave radiation. This embodiment is particularly useful at high power levels, where excess reflected microwave energy causes reflection modes which may degrade the uniformity of the microwave plasma. The vacuum vessel 2 is further adapted to receive the cylindrical isolating means 60 through at least a second wall portion thereof, preferably the wall portion opposite the wall portion upon which collar fitting 62 is mounted. Collar fitting 64 is disposed on said opposite wall position, substantially in line with collar fitting 62. Collar fitting 64 comprises an open end portion 90 extended from a connection flange 92. Connection flange 92 is affixed directly to the opposite wall position and includes an opening 94 co-extensive with the circumference of the cylindrical isolating means 60. The open end portion 90 extends from the connection flange 92, and is adapted to receive at least two O-rings 96, 98 which are adapted to effect a vacuum and water barrier between the vacuum vessel 2 and the surrounding ambient conditions. Between O-rings 96, 98 is disposed a cooling channel 97, through which a cooling medium such as water may be circulated so as to maintain the O-rings at a uniformly low temperature. The O-rings 96, 98 like O-rings 72, 74 are adapted to withstand elevated temperatures. The cylindrical isolating means 60 is passed through opening 94, through connection flange 92 and open end portion 90 whereby O-rings 96, 98 are urged against the outside circumferential edge of said cylindrical isolating means 60. The compression of said O-rings is the mechanism by which an air tight, water tight seal is effected. Also, O-rings 96, 98, like O-rings 72, 74, are well out of the plasma region 20, and therefore not subjected to degradation thereby. By effecting an air-tight, water tight seal around the outer circumference of the cylindrical isolating means 60, it is possible to maintain the vacuum vessel 2 at substantially sub-atmospheric pressures, while the interior of the cylindrically isolating means 60 is maintained at atmospheric pressures, and is directly exposed to ambient conditions. This is in fact an advantage in the operation of the apparatus 1. By maintaining the vacuum vessel at sub-atmospheric pressures, it is possible to operate the apparatus 1 at a pressure approximating that required for operation near the minimum of a modified Paschen curve. Additionally, the low pressures allow for a longer mean free path of travel for the plasma species, thereby contributing to overall plasma uniformity. Since the interior of the cylindrical isolating means 60 is exposed to ambient conditions, a flux of cooling air may be maintained therethrough so as to prevent the excessive heating which may be associated with microwave plasmas. Alternatively, a microwave transmissive cooling medium such as silicon oil may be circulated within the cylinder to maintain a uniform, cool temperature. The cylindrical isolating means 60, extending beyond the open end portion 90 must be sheathed within a metallic microwave containment means 100, of the type described hereinabove. The microwave containment means 100 is disposed adjacent to connection plate 102 which affects a connection between the microwave waveguide 42, and a source of microwave energy. Another form of substrate pretreatment, which may be used with or without plasma pretreatment, is a thermal bake-out of the substrate material. This bake-out may be accomplished by providing a heat generator (i.e. coil, lamp etc.) in the apparatus such that the substrate passes nearby before any plasma pretreatment or deposition occurs. See, for example, heater 300 in Fig. 1. This bake-out of the substrate is thought to drive off water from the bulk of the substrate, thereby reducing the water vapor transmission rate of the coated product. Finally, although enclosure of the deposition plasma region 12 within the folds of the web of substrate material 10 is highly effective in preventing the deposition of coating material onto the quartz isolation tube, a web of microwave transmissive material on payout roller 200 (Fig. 1) may be operatively positioned interiorly of said deposition chamber 1 so as to slowly move about the surface of the isolation means of the linear microwave applicator 4 and be wound about the take-up roller 201. When the microwave transmissive web is coated with an excess thickness of depositing barrier material, the rollers are indexed and a clean portion of the web is allowed to collect the unwanted buildup. In this manner, even at extremely high rates of deposition, no coating is deposited onto the outer surface of the isolation means (the quartz tube). Accordingly, the deposition process can proceed for long periods of time and is specifically adapted for volume production. EXAMPLE 1 (Comparative) A microwave deposition apparatus 1 as described hereinabove with reference to Fig. 1 was employed to fabricate a clear silicon based thin film oxygen permeation barrier layer coating upon an elongated web of polyester substrate material. The exact steps by which the above described apparatus 1 was used to fabricate such coatings will be detailed hereinbelow. An elongated roll of 0.5 mil (12 micron) thick 30 cm wide polyester was used as the substrate for the instant example. The roll of polyester substrate was loaded onto the payout roller of apparatus 1 described hereinabove with reference to Fig. 1. After placing the substrate onto the payout roller 5, the apparatus 1 was closed and sealed so as to effect an air tight seal between the interior of the vessel and the surrounding ambient conditions. The interior of the vessel was then evacuated to a pressure of approximately 20 to 25 millitorr. Thereafter, the interior of said chamber was purged in an argon gas environment for a period of approximately one-half hour. After purging for approximately one-half hour, the chamber was evacuated to a background pressure of approximately 3 to 4 millitorr. Thereafter, preferred feedstock gases were introduced to the interior of said chamber through said input gas manifold 9 according to the following recipe: GAS FLOW RATE SiH 4 180 SCCM CO 2 500 SCCM O 2 500 SCCM Ar 400 SCCM A general range of gases for this four component mixture would be about 100 to 200 SCCM SiH 4 , 300-700 SCCM O 2 , 300-700 SCCM CO 2 and 300-700 SCCM Ar. After initiating the flow of these feedstock gases into the interior of the vacuum vessel, a microwave plasma was initiated at a frequency of 2.45 GHz and at a power of approximately 5 kilowatts. The elongated web of substrate material was passed through the deposition region 12 of the apparatus 1 at a rate of about 20 m/min. During the operation of the microwave plasma, a flux of cooling air was directed through the cylindrical isolating means 60 so as to maintain a uniform low temperature. Thereafter, microwave plasma was extinguished, and the flow of process gases to the vacuum vessel 2 was terminated. After extinguishing the plasma and terminating the flow of process gases to the vacuum vessel 2, the interior of the reaction vessel was purged with argon, and the reaction vessel was vented to ambient conditions. Thereafter, the reaction vessel was opened and the substrate was removed for examination which revealed a uniform, clear, silicon based, thin film, oxygen permeation barrier coating on the web of substrate 10. Tests indicate that the deposited film has the chemical composition SiO x , where x=1.8-2. Oxygen permeation tests on a coated substrate having a 200 Å thin film deposited thereon show an O 2 permeation of 1 cm 3 /m 2 /day at room temperature and 0% relative humidity. In order to obtain the impressive oxygen and water vapor impermeability behavior described herein, it was necessary to devise a method of eliminating hydrogen from the deposited film taking into account the low melting point of the PET substrate. In this regard, both the effect of increased oxygen concentration in the precursor gaseous mixture was studied and the chemical bonds responsible for water vapor diffusion were studied by conducting IR spectroscopy on sample films deposited on crystalline silicon wafers. Table A summarizes the relationship of Si-H concentration versus water vapor transmission and the precursor mixture. A perusal of Table A will demonstrate that the Si-H concentration and the water vapor transmission decreases with the increase of oxygen concentration in the precursor gaseous mixture. It is, of course, well established that Si-H bonds readily react with water. Accordingly, it is not surprising that Si-H bonds in the deposited films will absorb, react with and create a transmission pathway for the water vapor. The conclusion to be drawn is that the relatively poor water vapor permeation properties shown in samples 1 and 2 were due to the presence of Si-H bonds in the films and that increased oxygen concentration in the precursor mixture eliminated those Si-H bonds and increased the impermeability of the films. Note that the further increase of oxygen concentration in the precursor beyond that indicated in Sample 3, failed to improve the water barrier properties. Of course, the plasma treatment process detailed hereinabove further improved the barrier properties of the deposited film. SAMPLE No. FEEDSTOCK GASES (SCCM) Si-H BOND CONTENT H 2 O VAPOR TRANS. RATE (gm/m 2 -24 hrs.) SiH 4 CO 2 Ar O 2 38°C, 100% RH 1 220 750 450 275 0.5% 26 2 220 400 450 400 Trace 11 3 150 500 450 500 None 7 EXAMPLE 2 A microwave deposition apparatus 1 as described hereinabove with reference to Fig. 2 was employed to fabricate a second clear silicon based thin film oxygen permeation barrier layer coating upon an elongated web of polyester substrate material. A roll of 0.5 mil (12 micron) thick 30 cm wide polyester was again used as the substrate. The roll of polyester substrate was loaded onto the payout roller of apparatus 1 described hereinabove with reference to Fig. 2. After placing the substrate onto the payout roller 5, the apparatus 1 was closed and sealed so as to effect an air tight seal between the interior of the vessel and the surrounding ambient conditions. The interior of the vessel was then evacuated to a pressure of approximately 20 to 25 millitorr. Thereafter, the interior of said chamber was purged in an argon gas environment for a period of approximately one-half hour. After purging for approximately one-half hour, the chamber was evacuated to a background pressure of approximately 3 to 4 millitorr. Thereafter, feedstock gases were introduced to the interior of said chamber through said input gas manifold 9 according to the following recipe: GAS FLOW RATE SiH 4 300 SCCM O 2 900 SCCM He 100 SCCM A general range for this three component mixture would be about 200-400 SCCM SiH 4 , 800-1000 SCCM O 2 , and 0-200 SCCM He. Additionally, 400 SCCM of Ar was introduced to the plasma pretreatment zone 13 through gas manifold 14. After initiating the flow of these gases into the interior of the vacuum vessel, microwave plasmas were initiated at a frequency of 2.45 GHz and at a power of approximately 6 kilowatts for Ar plasma pretreatment and 5 kilowatts for plasma deposition. The elongated web of substrate material was passed through the plasma pretreatment region 13 and deposition region 12 of the apparatus 1 at a rate of about 20 m/min. During the operation of the microwave plasma, a flux of cooling air was directed through the cylindrical isolating means 60 so as to maintain a uniform low temperature. Thereafter, microwave plasma was extinguished, and the flow of process gases to the vacuum vessel 2 was terminated. After extinguishing the plasma and terminating the flow of process gases to the vacuum vessel 2, the interior of the reaction vessel was purged with argon, and the reaction vessel was vented to ambient conditions. Thereafter, the reaction vessel was opened and the substrate was removed for examination which revealed a uniform, clear, silicon based, thin film, oxygen permeation barrier coating on the web of substrate 10. Tests indicate that the deposited film has the chemical composition SiO x , where x=1.8-2. Permeation tests on a coated substrate having a 150 Å thin film deposited thereon show an O 2 transmission rate of less than 1 cm 3 /m 2 /day at room temperature and 0% relative humidity and a water vapor transmission rate of less than 1 g/m 2 /day at 38 °C and 100% relative humidity. While the specific examples of the method of depositing high barrier coatings by microwave plasma enhanced CVD recited herein above are described with reference to flat rolls of substrate material and the apparatus of the instant invention, it should be noted that the present method is useful for deposition of high barrier coating onto substrates of any shape, by any machine which is capable of deposition onto the desired substrate. Other types of substrates which would benefit from a coating of high barrier material would be those such as plastic bottles, jars or other plastic containers in which the material to be stored is sensitive to water or oxygen. Of specific interest is coating the interior of plastic bottles or jars for storage of food or beverages. As long as the basic microwave deposition steps are present (i.e. such as thermal bake-out, plasma pretreatment, microwave PECVD of high barrier coating etc.), the shape and/or size of the substrate and/or portion of the substrate to be coated is irrelevant.

Claims: Method of depositing, by plasma assisted chemical vapor deposition, an oxygen and water vapor barrier coating atop a temperature sensitive substrate from a precursor gaseous mixture which includes at least a silicon-containing gas and an oxygen-containing gas; said method characterized by the steps of: performing a plasma pretreatment of said low temperature substrate prior to the microwave deposition step; and introducing a sufficient flow rate of oxygen gas (O 2 ) as oxygen-containing gas into the precursor mixture to eliminate the formation of silicon-hydrogen bonds in the deposited coating. The method of claim 1, including the further improved step of thermally baking said temperature sensitive substrate prior to the plasma pretreatment step. The method of claim 1, wherein said plasma pretreatment of said substrate is an argon plasma pretreatment. The method of claim 1, including the further step of employing an elongated, flexible polyester film as the temperature sensitive substrate. The method of claim 1, including the further step of employing a plastic bottle as the temperature sensitive substrate. The method of claim 5, including the further step of depositing said barrier coating onto the interior surface of said plastic bottle substrate. The method of claim 1, including the further step of employing a plastic jar as the temperature sensitive substrate. The method of claim 7, including the further step of depositing said barrier coating onto the interior surface of said plastic jar substrate. The method of claim 1, including the further step of depositing said coating atop the substrate to a thickness of 100 to 500 Angstroms to avoid cracking said coating when said substrate flexes. The method of claim 9, including the further step of depositing the barrier coating so as to have an oxygen transmission rate of less than 1.0 cm 3 /m 2 /day at room temperature and 0% relative humidity and a water vapor transmission rate of less than 3 g/m 2 /day at 38°C and 100% relative humidity. The method of claim 1, including the further step of forming the precursor gaseous mixture of SiH 4 , CO 2 , O 2 and Ar. The method of claim 11, including the further step of introducing a precursor mixture having a ratio of 1 part SiH 4 to 9 parts of the other gases. The method of claim 12, including the further step of introducing a precursor mixture of about 100 to 200 SCCM SiH 4 , 300-700 SCCM O 2 , 300-700 SCCM CO 2 and 300-700 SCCM Ar. The method of claim 9, including the further step of depositing the barrier coating so as to have an oxygen transmission rate of less than 1.0 cm 3 /m 2 /day at room temperature and 0% relative humidity and a water vapor transmission rate of less than 1 g/m 2 /day at 38°C and 100% relative humidity. The method of claim 1, including the further step of forming the precursor gaseous mixture of SiH 4 , O 2 and He. The method of claim 15, including the further step of introducing a precursor mixture of about 200-400 SCCM SiH 4 , 800-1000 SCCM O 2 , and 0-200 SCCM He.

Inventors: DOTTER BUDDIE R II, HASEGAWA WATARU, IZU MASATSUGU, OVSHINSKY STANFORD R, DOTTER, BUDDIE R., II, HASEGAWA, WATARU, IZU, MASATSUGU, OVSHINSKY, STANFORD, R.
Applicants: ENERGY CONVERSION DEVICES INC, ENERGY CONVERSION DEVICES, INC.
Application year: 1993
Main classification: C23C16
IPC codes: C23C16