LOW SURFACE ENERGY COATINGS

Abstract: The invention relates to a method and apparatus for providing an outer layer coating on a substrate such as an optical lens and the coating has low surface energy properties. The method of the invention involves the introduction of gas including hydrocarbon groupings into the vacuum chamber in which the substrates are optionally previously coated and thereby allows the provision of the coating to be an integral part of a coating process and also allows the coating apparatus provided to be utilised. The current method allows control of the application of the layer by using control apparatus already provided.

Description: Low Surface Energy Coatings The present invention relates to a method and apparatus for applying a layer of material to form a low surface energy coating and typically, but not exclusively a coating for a substrate in the form of an optical lens to provide an outer layer having a low surface energy. In the coating of substrates such as optical lens it is appreciated that in order to provide an outer surface of the same with the desired surface characteristic of low surface energy, an outer layer of suitable material is required to be applied once the conventional coating processes are comple¬ ted. The desired quality of the outer surface is that it has a low surface energy such that any material or liquids which contact with the surface during use of the lens can be relat¬ ively easily removed. If the surface energy of the outer surfaces of the lens is too high then material tends to "stick" to and be attracted to the surface. This tendency to stick causes materials such as liquids or grease to smear when attempts are made to remove the same. Repeated smearing leads to a reduction in the optical quality of the lens and is generally undesirable. The recognised method for measuring surface energy is to mea¬ sure the hydrophobic characteristic of the surface by analys¬ ing the contact angle of a drop of pure water on the surface. The shape of the water drop and the angle between the surface and the tangent of the water drop is called the contact angle and is related to the surface energy. It is desired that the water contact angle be greater than 70 degrees to be accept¬ able for optical use. In general, the greater the angle the lower the surface energy. By achieving the desired contact angle the lenses are more easily cleaned and the layer re¬ duces the friction of the outer surface and hence reduces the propensity for the lens to be scratched. This is an especial¬ ly important consideration with regard to plastic lenses. Conventionally, in order to apply the required coating onto the optical lens either of two methods are currently used. The first method requires the lens, once all other coating operations have been completed, to be dipped into a coating mixture and either wiped clean or then placed into an oven where the same are baked, typically for 10 minutes, at a temperature of 50 degrees Celsius. In this process when the lens are dipped care is required to ensure that the lens are evenly coated with the liquid coating. This possibility of uneven coating is exacerbated by the fact that the coating is typically applied by a manual operation, and the time re¬ quired for the dipping and baking process make this method undesirable to users. Furthermore the solutions which are used, age, even when refrigerated, and reach a point where the viscosity is so high that the layer applied to the lens is too thick and causes an optical change in the reflection colour which is not uniform and appears as a smear. Even the addition of a solvent to reduce the viscosity cannot prevent the eventual need to discard the material. The material used is also relatively expensive. An alternative method which is known is to prepare a liquid of a composition which, if applied to the lens to provide the outer coating, provides the low surface energy properties required. In this case the liquid is applied to the lens when the lens are held within the apparatus in which the previous optical coatings have been applied. The liquid is heated externally of the apparatus and the vapour produced passed through supply pipes into the chamber. When the vapour is provided a plasma source in the chamber is operated to crack the vapour into chemical groupings and the required groupings condense on the lens to provide the low surface energy coat¬ ing. However, once again, the time required for this coating operation is prohibitive as the liquid must first be heated to the required temperature to produce the vapour and the vapour simultaneously supplied to the chamber. Furthermore problems are caused by the vapour condensing in the supply means from the heated liquid to the chamber thus causing blockage and/or a reduction in the efficiency of cracking of the vapour which is produced. If the condensation becomes too great it is necessary for the supply means to be dismantled and cleaned and dried. The aim of the present invention is to provide a method and apparatus for providing a coating with low surface energy properties on optical lens efficiently and as an integral part of a lens coating process. The present invention provides a method for forming an outer coating having low surface energy characteristics on the surface of a substrate comprising placing the substrate in a chamber, forming a vacuum in the chamber, introducing a gas containing hydrocarbon groupings into the chamber, activating a plasma source in the chamber to split at least some hydro¬ carbon groupings from the gas and said groupings apply to the surface of the substrate to produce the low surface energy coating. Typically the substrate is an optical lens. Preferably sufficient gas is supplied and hydrocarbon group¬ ings split to provide a coating to a depth of 0.1-5 nano¬ metres. A coating of this depth will not substantially affect the optical quality of the lens. Typically the plasma source is any of an ion source, plasma source, unbalanced magnetron source or glow discharge means operable by any of d.c, a.c. or r.f. power means, mounted to be operable within the vacuum chamber to split the hydro¬ carbon groupings from the gas. Preferably the gas introduced into the chamber is a hydrocar¬ bon such as methane or propane. Secondary gases such as hydrogen, argon or oxygen may also be introduced into the chamber along with the hydrocarbon gas. Typically the gas is diluted, prior to final compression to atmospheric pressure by the vacuum pumps, in an inert gas such as nitrogen or argon or in air. A typical dilution range for methane in air is less than 5% by volume. Typically the time taken for application of the outer coating is between 1-5 minutes. The low surface energy property of the coating is measured by analysing the hydrophobic charac¬ teristics of the same. Typically the contact angle of a drop of pure water when methane is used is in the region of 76 -90 degrees and 68-72 degrees with propane. In a further aspect of the invention there is provided a method of coating an optical lens comprising the steps of introducing at least one optical lens into a chamber, said chamber containing at least one magnetron with a metal tar¬ get, and a plasma source, pulling a vacuum in the chamber, activating the magnetron to sputter a metal onto the lens surface, introducing oxygen into the chamber and activating the plasma source to chemically react the metal to provide the required optical coating on the lens, wherein the method comprises the further step of switching off the magnetrons. plasma source and oxygen supply, introducing a gas containing hydrocarbon groupings into the chamber and activating the plasma source to split at least some hydrocarbon groupings from the gas and deposit the same on the lens to form an outer surface coating having low surface energy characteris¬ tics. Typically the method includes activating two magnetrons, each having a target of different metal to produce a multi- layered optical coating on the lens prior to the introduction of the gas containing hydrocarbon groupings. In a further aspect of the invention there is provided appar¬ atus for the coating of optical lenses, said apparatus com¬ prising, a chamber, an optical lens holder within the cham¬ ber, a means for pulling a vacuum in the chamber, at least one magnetron with a metal target and a plasma source, drive means for moving optical lens on the holder past the magne¬ tron and plasma source and control means for activating the magnetron and plasma source, and wherein the apparatus fur¬ ther comprises a source for gas containing hydrocarbon group¬ ings and supply means for introducing the gas into the cham¬ ber. Typically the gas source is a conventional gas bottle and the supply means include a control device for controlling the supply of the gas from the bottle into the chamber. Preferab¬ ly the control device is, in turn, operated by the control means for the activation of the plasma source and magnetrons. Typically the control system is operated by a central compu¬ ter processor. The supply means preferably allow the gas to be diluted in an inert gas such as argon or nitrogen, or in air prior to final compression to atmospheric pressure by the vacuum pumps and the supply means includes a combustible gas sensor, flow meter or other means to monitor the level of dilution. The gas source and gas supply means can be included for use with conventional optical lens coating apparatus and opera¬ tion of the same is included as an integral part of the process for providing a complete coating of the optical lens. In a further aspect of the invention the method of applying an outer coating having low surface energy properties to optical lenses includes the further steps of applying a gas containing groupings other than hydrocarbon groupings to further increase the water contact angle achieved. In one instance a halogen containing gas such as CF4, C2F6, C.C14 is applied and creates a fluorinated or chlorinated surface. In an alternative instance a silicon containing gas such as tetraethoxysilane, tetramethoxysilane, hexamethyldisiloxane is applied to create a silicone layer. Specific embodiments of the invention are now disclosed with reference to the accompanying drawings wherein; Figure 1 illustrates a plan view of one embodiment of a chamber for the application of coatings to optical lens by sputter deposition of material; and Figure 2 illustrates a schematic representation of the compo¬ nents of the invention. Figure 3 illustrates a cross sectional view of an optical lens coated according to the present invention. Referring firstly to Figure 1 there is shown a representation of one type of chamber which is currently used for the appli¬ cation of coatings to optical lens. It should be appreciated that chambers having differing geometries are equally relevant to the invention disclosed herein and that reference to ; gure 1 is for clarification purposes only. Figure 1 discloses apparatus for coating optical lens 2 held on a rotatable holder 4 within a chamber 6. The chamber is connected to a vacuum pump 8, the operation of which creates a vacuum in the chamber 6. At points around the walls of the chamber, apertures are provided to allow the inclusion of two magnetrons 10 and a plasma source 12. Each magnetron 10 contains a target of a metal and activation of the magne¬ trons causes sputtering of particles of metal into the cham¬ ber 6 and onto the optical lens 2 held on the rotating holder to coat the same. At the same time oxygen is supplied into the chamber and the plasma source, in this case an unbalanced magnetron, is operated to form a metal oxide coating on the optical lens. Thus alternate operation of the magnetrons 10 provides a multilayered metal coating on the surfaces of the optical lens and this method and apparatus is currently used to provide lenses with multilayered coatings to provide the required optical effect. At this stage the outer surface of the lenses have been found to be susceptible to smearing and to the gathering of dust and grease due to the relatively high surface energy of the surfaces. This is unacceptable and these lenses are then typically removed from the apparatus and dipped in a liquid outer coating mixture and baked to provide the required low surface energy outer coating. This represents an additional and wasteful coating operation. Figure 2 illustrates in schematic form the further apparatus and operations which are performed on the lenses in situ in the chamber and apparatus of Figure 1 according to the pre¬ sent invention. In the figure there is shown a plasma source 12 which is provided to operate in the vacuum chamber 6. After the lenses 2 have been coated using conventional appar¬ atus as disclosed in Figure 1 and using, in this case, metal targets in magnetrons, these magnetrons are stopped and the plasma source 12 activated to create a plasma 18 in the chamber. At this time gas supply means 19 are activated by the apparatus control system to supply a hydrocarbon gas/ air or inert gas mixture into the chamber 6. The gas supply means comprise a gas bottle 20 and a delivery control means 22 which may also comprise a gas monitor(not shown) . When the gas is introduced into the chamber 6 to be exposed to the plasma 18 at least some of the hydrocarbon groupings, here represented by the numeral 24, split from the gas. The separated groupings then settle on the walls of the chamber and the optical lens 2 to provide the required coating on outer surface of the lens. The duration of the method de¬ scribed above is determined on the volume and speed of gas supply and the required thickness of coating. Figure 3 illustrates a cross section of an optical lens 2 which has been coated in the manner disclosed above and the cross section or "stack" of coatings consists of a conven¬ tional multi layer anti reflective coating 26 applied by conventional methods and an outer "hydrophobic" layer 28 having the required low surface energy characteristics ap¬ plied by the methods and apparatus of this invention. Thus, in normal operation, once the multilayered coatings have been applied to the optical lenses to provide the re¬ quired optical effect, the control system of the apparatus serves to stop the plasma source and oxygen supply and the operation of the magnetrons 10. The gas containing hydrocar¬ bon groupings is then introduced into the chamber and the plasma source 12 activated to cause separation of at least some hydrocarbon groupings in the gas. The separated group¬ ings are then free to land on available surfaces in the chamber including the surfaces of the optical lenses held therein. Thus the outer surfaces of the optical lenses are coated with a relatively thin (typically 0.1-5 nanometres) coating of the hydrocarbon groupings. The coating applied provides lenses with outer surfaces which have sufficiently low surface energy characteristics to be significantly less susceptible to smearing and the attrac¬ tion of foreign matter. Tests have shown that the following operating parameter conditions of the method of the invention provide an outer coating on the lens which has the required hydrophobic test qualities without affecting the optical quality of the lens. The power of the plasma source is typically held between 15 and 25 Watts and the voltage at 600V. In this case methane is introduced at a pressure of 5 10(-3) mBar and the lens is exposed for around 1 minute. This has been found to provide a coating on the lenses which, when tested with a drop of pure water thereon, provides a water drop contact angle of 89 degrees thus achieving the required low surface energy char¬ acteristics. The apparatus and method of the current invention provides a means whereby an outer coating can be applied to optical lenses to reduce the surface energy of the surface to within required parameters and said coating can be applied as an integral part of the conventional coating process and appara- tus. Thus the quality and speed of application of this outer coating is significantly improved and can be controlled to provide an improved quality coating.

Claims: CLAIMS 1. A method for forming a coating having low surface energy characteristics on the surface of a substrate, comprising placing a substrate in a chamber, forming a vacuum in the chamber, introducing a gas containing hydrocarbon groupings into the chamber and activating a plasma source in the cham¬ ber to split at least some hydrocarbon groupings from the gas and at least some of said groupings apply to the surface of the substrate to produce the low surface energy coating. 2. A method according to claim 1 wherein the substrate is an optical lens and sufficient gas is supplied and hydrocarbon groupings split to provide a coating to a depth of 0.1-5 nanometres. 3. A method according to claim 1 wherein the plasma source is any of an ion source, plasma source, unbalanced magnetron source or glow discharge means operable by any of d.c, a.c. or r.f. power means. 4. A method according to claim 1 wherein the gas introduced into the chamber is a hydrocarbon such as methane or propane. 5. A method according to claim 4 wherein the gas is diluted, in an inert gas or air. 6. A method according to claim 5 wherein the gas is methane and the dilution range for methane in air is less than 5% by volume. 7. A method according to claim 6 wherein the contact angle of a drop of pure water on the surface when methane is used is in the region of 76 -90 degrees. 8. A method according to claim 5 wherein the contact angle of a drop of pure water on the surface when propane is used is 68-72 degrees. 9. A method of coating an optical lens comprising the steps of introducing at least one optical lens into a chamber, said chamber containing at least one magnetron with a metal tar¬ get, and a plasma source, pulling a vacuum in the chamber, activating the magnetron to sputter a metal onto the lens surface, introducing oxygen into the chamber and activating the plasma source to chemically react the metal to provide the required optical coating on the lens, wherein the method comprises the further step of switching off the magnetrons, plasma source and oxygen supply, introducing a gas containing hydrocarbon groupings into the chamber and activating the plasma source to split at least some hydrocarbon groupings from the gas and deposit the same on the lens to form an outer coating. 10. A method according to claim 9 wherein two magnetrons are activated, each having a target of different metal to pro¬ duce a multilayered optical coating on the lens prior to the introduction of the gas containing hydrocarbon groupings. 11. A method according to any of the preceding claims wherein the method includes the further step of applying a gas containing groupings other than hydrocarbon groupings. 12. A method according to claim 11 wherein a halogen con¬ taining gas such as CF4, C2F6, C.C14 is applied and creates a fluorinated or chlorinated surface. 13. A method according to claim 11 wherein a silicon contain¬ ing gas such as tetraethoxysilane, tetramethoxysilane, hexa- methyldisiloxane is applied to create a silicone surface layer. 14. Apparatus for the coating of optical lenses, said appara¬ tus comprising, a chamber, an optical lens holder within the chamber, a means for pulling a vacuum in the chamber, at least one magnetron with a metal target and a plasma source, drive means for moving optical lens on the holder past the magnetron and plasma source and control means for activating the magnetron and plasma source, and wherein the apparatus further comprises a source for gas containing hydrocarbon groupings and supply means for introducing the gas into the chamber. 15. Apparatus wherein the gas source is a conventional gas bottle and the supply means include a control device for controlling the supply of the gas from the bottle into the chamber. 16. Apparatus according to claim 15 wherein the control device is in turn operated by the control means for the activation of the plasma source and magnetrons. 17. Apparatus according to any of claims 14-16 wherein the supply means allow the gas to be diluted in an inert gas, or in air prior to final compression to atmospheric pressure by the vacuum pumps. 18. Apparatus according to any of claims 14-17 wherein the supply means includes a combustible gas sensor, flow meter or other means to monitor the level of dilution. 19. Apparatus as hereinbefore described with reference to the accompanying drawings.

Inventors: SOUTHWORTH PAUL, SPENCER ALARIC GRAHAM, WALLS JOHN MICHAEL, WAUGH ALLEN ROBERT, WHITE NORMAN HENRY, SOUTHWORTH, PAUL, SPENCER, ALARIC, GRAHAM, WALLS, JOHN, MICHAEL, WAUGH, ALLEN, ROBERT, WHITE, NORMAN, HENRY
Applicants: APPLIED VISION LTD, SOUTHWORTH PAUL, SPENCER ALARIC GRAHAM, WALLS JOHN MICHAEL, WAUGH ALLEN ROBERT, WHITE NORMAN HENRY, SOUTHWORTH, PAUL, SPENCER, ALARIC, GRAHAM, WALLS, JOHN, MICHAEL, WAUGH, ALLEN, ROBERT, WHITE, NORMAN, HENRY
Application year: 1995
Main classification: C23C16
IPC codes: C23C8, C23C16, G02B1