A PLASMA POLYMERISATION METHOD FOR COATING A SUBSTRATE WITH A POLYMER

Abstract

A plasma polymerisation method for coating a substrate with a polymer layer, which method includes: providing a substrate to be coated within a plasma chamber; introducing a flow of a first polymer precursor to the plasma chamber; applying a power at a level greater than zero Watts (W) and converting the first polymer precursor to a first polymer precursor plasma; exposing the substrate to the first polymer precursor plasma; introducing a flow of a second polymer precursor to the plasma chamber; applying a power at a level greater than zero Watts (W) and converting the second polymer precursor to a second polymer precursor plasma; and exposing the substrate to the second polymer precursor plasma, wherein exposing the substrate to the first polymer precursor plasma forms a first polymer layer thereon and exposing the substrate to the second polymer precursor plasma forms a second polymer layer thereon, characterised by maintaining the power at a level greater than zero Watts (W) between exposing the substrate to the first polymer precursor plasma and exposing the substrate to the second polymer precursor plasma.

Claims

1. A plasma polymerisation method for coating a substrate with a polymer layer, which method comprises: providing a substrate to be coated within a plasma chamber; introducing a flow of a pre-treatment precursor to the plasma chamber; applying a power at a level greater than zero Watts (W) and converting the pre-treatment precursor to a pre-treatment precursor plasma; and exposing the substrate to the pre-treatment precursor plasma; introducing a flow of a first polymer precursor to the plasma chamber; applying a power at a level greater than zero Watts (W) and converting the first polymer precursor to a first polymer precursor plasma; exposing the substrate to the first polymer precursor plasma; introducing a flow of a second polymer precursor to the plasma chamber; applying a power at a level greater than zero Watts (W) and converting the second polymer precursor to a second polymer precursor plasma; and exposing the substrate to the second polymer precursor plasma, wherein exposing the substrate to the first polymer precursor plasma forms a first polymer layer thereon and exposing the substrate to the second polymer precursor plasma forms a second polymer layer thereon, characterised by maintaining the power at a level greater than zero Watts (W) between exposing the substrate to the pre-treatment precursor plasma and exposing the substrate to the first polymer precursor plasma and between exposing the substrate to the first polymer precursor plasma and exposing the substrate to the second polymer precursor plasma.

2. A plasma polymerisation method according to claim 1, wherein the power which converts the second polymer precursor to the second polymer precursor plasma is different from the power which converts the first polymer precursor to the first polymer precursor plasma.

3. A plasma polymerisation method for coating a substrate with a polymer layer, which method comprises: providing a substrate to be coated within a plasma chamber; introducing a flow of a first polymer precursor to the plasma chamber; applying a first power at a level greater than zero Watts (W) and converting the first polymer precursor to a first polymer precursor plasma; exposing the substrate to the first polymer precursor plasma; introducing a flow of a second polymer precursor to the plasma chamber; applying a second power at a level greater than zero Watts (W) and converting the second polymer precursor to a second polymer precursor plasma; and exposing the substrate to the second polymer precursor plasma, wherein the second power level is different from the first power level and wherein exposing the substrate to the first polymer precursor plasma forms a first polymer layer thereon and exposing the substrate to the second polymer precursor plasma forms a second polymer layer thereon, characterised by switching the power immediately from the first power level to the second power level and maintaining the power at the second power level.

4. A plasma polymerisation method according to claim 3, wherein the second power level is lower than the first power level such that the power is reduced immediately from the first power level to the second power level.

5. A plasma polymerisation method according to claim 1, including setting the pressure within the plasma chamber to a first polymer precursor operating pressure for converting the first polymer precursor to the first polymer precursor plasma and setting the pressure within the plasma chamber to a second polymer precursor operating pressure for converting the second polymer precursor to the second polymer precursor plasma.

6. A plasma polymerisation method according to claim 5, including changing the pressure from the first polymer precursor operating pressure to the second polymer precursor operating pressure without reducing the pressure to base pressure.

7. A plasma polymerisation method according to claim 5, including changing the pressure from the first polymer precursor operating pressure to the second polymer precursor operating pressure concurrent with introducing the second precursor to the plasma chamber.

8. A plasma polymerisation method according to claim 1, including reducing the flow of the first polymer precursor to the plasma chamber concurrent with increasing the flow of the second polymer precursor to the plasma chamber.

9. A plasma polymerisation method according to claim 1, wherein the second polymer precursor is different from the first polymer precursor.

10. A plasma polymerisation method according to claim 1, wherein the first polymer precursor is a polymer precursor monomer comprising a metal element, a metalloid element or a combination thereof.

11. A plasma polymerisation method according to claim 10, wherein the metal element is selected from the group consisting of Al, Fe, Co, Ni, Cu, Zn, Ag, Sn, Au or any combination thereof.

12. A plasma polymerisation method according to claim 10, wherein the metalloid element is selected from the group consisting of B, Si, Ge, As, Sb, Te, Po or any combination thereof.

13. A plasma polymerisation method according to claim 1, wherein the second polymer precursor is a polymer precursor monomer consisting of non-metal elements.

14. A plasma polymerisation method for coating a substrate with a polymer layer, which method comprises: providing a substrate to be coated within a plasma chamber; introducing a flow of a pre-treatment precursor to the plasma chamber; applying a first power at a level greater than zero Watts (W) and converting the pre-treatment precursor to a pre-treatment precursor plasma; exposing the substrate to the pre-treatment precursor plasma; introducing a flow of a first polymer precursor to the plasma chamber; applying a second power at a level greater than zero Watts (W) and converting the first polymer precursor to a first polymer precursor plasma; and exposing the substrate to the first polymer precursor plasma, wherein exposing the substrate to the first polymer precursor plasma forms a first polymer layer thereon, characterised by maintaining the power at a level greater than zero Watts (W) between exposing the substrate to the pre-treatment precursor plasma and exposing the substrate to the first polymer precursor plasma.

15. A plasma polymerisation method according to claim 14, wherein the second power level is different from the first power level.

16. A plasma polymerisation method according to claim 14, including switching the power immediately from the first power level to the second power level and maintaining the power at the second power level.

17. A plasma polymerisation method according to claim 16, wherein the second power level is lower than the first power level such that the power is reduced immediately from the first power level to the second power level.

18. A plasma polymerisation method according to claim 1, including setting the pressure within the plasma chamber to a pre-treatment precursor operating pressure for converting the pre-treatment precursor to the pre-treatment precursor plasma and setting the pressure within the plasma chamber to a first polymer precursor operating pressure for converting the first polymer precursor to the first polymer precursor plasma.

19. A plasma polymerisation method according to claim 18, wherein the pressure is changed from the pre-treatment precursor operating pressure to the first polymer precursor operating pressure without reducing the pressure to base pressure.

20. A plasma polymerisation method according to claim 18, including changing the pressure from the pre-treatment precursor operating pressure to the first polymer precursor operating pressure concurrent with introducing the first polymer precursor to the plasma chamber.

21. A plasma polymerisation method according to claim 14, including reducing the flow of the pre-treatment polymer precursor to the plasma chamber concurrent with increasing the flow of the first polymer precursor to the plasma chamber.

22. A substrate comprising a surface having a polymer coating formed thereon by a plasma polymerisation method according to claim 1.

23. A substrate according to claim 22, wherein the surface thereof includes a metal element, a metalloid element or a combination thereof prior to having the polymer coating deposited thereon.

Description

DETAILED DESCRIPTION OF INVENTION

[0143] Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying figures.

[0144] FIG. 1 illustrates a plasma polymerisation method according to the prior art;

[0145] FIG. 2 illustrates a plasma polymerisation method according to an embodiment of the invention; and

[0146] FIG. 3 shows a graph comparing results of shortcut tests carried out on PCBs that have been coated with polymers using methods according to the prior art (3a) and the invention (3b).

[0147] FIG. 2 outlines a plasma polymerisation method for coating a substrate with a polymer layer according to the present invention, where (a) is the absolute pressure (mTorr) within a plasma chamber as a function of time (minutes); (b) is the power (wattage) applied to an electrode set located within the plasma chamber as a function of time (minutes); and (c) is the flow rate (sccm) of a plasma precursor(s) into the plasma chamber as a function of time (minutes).

[0148] The method may involve: [0149] pumping down the plasma chamber to a base pressure and allowing the pressure to stablise (0 to 19 minutes); [0150] introducing a pre-treatment precursor 1′ to the plasma chamber and increasing the pressure to a pre-treatment precursor operating pressure (19 to 24 minutes); [0151] pre-treating a substrate by applying a power of approximately 300 W to convert the pre-treatment precursor 1′ to a pre-treatment precursor plasma and exposing the substate to the pre-treatment precursor plasma (30 to 36 minutes); [0152] reducing the flow of the pre-treatment precursor 1′ to zero concurrent with increasing the flow of a first polymer precursor 2′ (36 to 48 minutes); [0153] reducing the power from approximately 300 W to approximately 200 W (36 minutes); [0154] reducing the pressure from the pre-treatment precursor operating pressure to a first polymer precursor operating pressure without reducing the pressure to base pressure (36 to 48 minutes); [0155] depositing a first polymer layer on the substrate by applying a power of approximately 200 W to convert the first polymer precursor 2′ to a first polymer precursor plasma and exposing the substate to the first polymer precursor plasma to form a first polymer layer thereon (48 to 55 minutes); [0156] reducing the flow of the first polymer precursor 2′ to zero concurrent with increasing the flow of a second polymer precursor 3′ (55 to 70 minutes); [0157] increasing the power from approximately 200 W to approximately 240 W (55 to 70 minutes); [0158] reducing the pressure from the first polymer precursor operating pressure to a second polymer precursor operating pressure without reducing the pressure to base pressure (55 to 70 minutes); [0159] depositing a second polymer layer on the substrate by applying a power of approximately 240 W to convert the second polymer precursor 3′ to a second polymer precursor plasma and exposing the substate to the second polymer precursor plasma to form a second polymer layer thereon (70 to 76 minutes); [0160] reducing the flow of the second polymer precursor 3′ to zero concurrent with increasing the flow of a third polymer precursor 4′ (76 to 86 minutes); [0161] reducing the power from approximately 240 W to approximately 125 W (76 minutes); [0162] reducing the pressure from the second polymer precursor operating pressure to a third polymer precursor operating pressure without reducing the pressure to base pressure (76 to 86 minutes); [0163] depositing a third polymer layer on the substrate by applying a power of approximately 125 W to convert the third polymer precursor 4′ to a third polymer precursor plasma and exposing the substate to the third polymer precursor plasma to form a third polymer layer thereon (86 to 103 minutes); [0164] switching off the power and the flow of the third polymer precursor 4′ (103 minutes); and [0165] pumping down the plasma chamber to a base pressure and allowing the pressure to stablise (103 to 116 minutes).

[0166] Whilst the plasma chamber is at base pressure the chamber and any associate tubing may be purged with an inert gas to remove any residual precursors, following which the plasma chamber may be aerated to allow removal of all substances therefrom.

[0167] By ensuring that the power does not reduce to zero W during the course of the method means that a plasma state (even if only a weak plasma state) is maintained within the plasma chamber which can have the effect of reducing contamination on the substrate and the polymer layers. Advantageously, adhesion of the polymer layers to the substrate—in particular the conductive tracks of a PCB—is improved when compared to prior art methods.

[0168] It has further been determined that switching the power immediately from a first power level to a second power level and then maintaining the power at the second power level can be beneficial, especially in instances where the second power level is required to be lower than the first power level. A power level is typically reduced when a polymer precursor having greater reactivity than the present precursor is introduced to the plasma chamber. In reducing the power immediately—rather than gradually over time—less undesirable fragmentation of the polymer precursor being introduced is observed.

[0169] Moreover, ensuring that the pressure does not reduce to base pressure during the course of the method has been found to further reduce contamination and condensation on the substrate and the polymer layers, thereby further improving adhesion of the polymer layers to the substrate when compared to prior art methods.

[0170] Furthermore, introducing the polymer precursors to the plasma chamber for a period of time concurrently can achieve a polymer coating having a composition that varies through its thickness. For example, in embodiments where the polymer coating is formed from two discrete polymer precursor, the base of the polymer coating (i.e. closest to the substrate) may comprise polymer formed predominantly from the first polymer precursor 2′, the surface of the polymer coating (i.e. furthest from the substrate) may comprise polymer formed predominantly from the second polymer precursor 3′ and the region between the base and the surface may comprise polymer formed from a mixture of the first and second polymer precursors 2′, 3′. The concentration of polymer formed from the first polymer precursor 2′ may decrease gradually moving towards the surface and the concentration of polymer formed from the second polymer precursor 3′ may increase gradually moving towards the surface.

[0171] Whilst included in the method of FIG. 2, it is to be appreciated that the pre-treatment step and deposition of the third polymer layer are optional.

[0172] It is also to be appreciated that the invention is not to be in any way limited by the specified flow rates, powers, pressures and/or timings of the described examples. These parameters are merely explanatory and may differ depending on factors such as any one or more of the volume of the plasma chamber, the chemistry of the precursors, the thickness of the desired coating(s) and so forth.

[0173] For a plasma chamber having a volume of 0.282 m.sup.3 the parameters may fall within the below ranges.

[0174] The plasma deposition method may have an overall time of from approximately 5 minutes to approximately 600 minutes.

[0175] Plasma polymerisation may be continuous wave or pulsed wave. Whether continuous wave or pulsed wave plasma is used depends on various factors such as the chemistry of the precursors, the volume and/or the design of the plasma chamber.

[0176] The applied power may be from approximately 5 W to approximately 2000 W.

[0177] The precursor operating pressures may be from approximately 2 mTorr to approximately 150 mTorr, preferably approximately 2 mTorr to approximately 100 mTorr.

[0178] In order to demonstrate that the inventive method is an improvement over known methods electrical shortcut tests were carried out. The shortcut tests involved immersing a polymer-coated PCB in artificial sweat solution, applying a voltage (5V) across the polymer coating and continuously measuring the current at the conductive tracks of the PCB for 900 seconds.

[0179] FIG. 3a is a plot of the measured current (mA) versus time (seconds) for a PCB having a polymer coating deposited thereon according to the prior art method of FIG. 1. FIG. 3b is a corresponding plot for a PCB having a polymer coating deposited thereon according to the inventive method defined by claim 1.

[0180] The applied polymer coatings had the same thickness of 1 μm for comparative purposes. Shortcut tests were conducted twice on each PCB and mean values for the measured currents were determined and used to plot the graphs.

[0181] It is clear from FIG. 3 that the polymer coating deposited using the inventive method (which corresponds to plot 3b) is less conductive through its thickness than the polymer coating deposited using the prior art method (which corresponds to plot 3a). In other words, the polymer coating deposited using the inventive method is more electrically resistant than the polymer coating deposited using the prior art method. It is considered by the inventors that this improvement in electrical resistivity is due to better adhesion of polymer to the substrate by virtue of maintaining a plasma state inside the plasma chamber when depositing the polymer layers.

[0182] The described example are of polymer coatings that have been deposited on PCBs, although it has been determined that the inventive methods can also improve adhesion of polymers to other substrates that include inorganic species, such as other components having metallic surfaces, e.g. batteries.

[0183] When used herein, the term “organic polymer” is intended to mean a polymer which consists of non-metal elements. Such organic polymers do not include any metal elements and/or metalloid elements.

[0184] When used herein, the term “inorganic polymer” is intended to mean a polymer which includes at least one metal element or metalloid element.

[0185] When used herein, the term “metalloid element” is intended to mean elements of the periodic table that are selected from the group consisting of B, Si, Ge, As, Sb, Te and Po.

[0186] When used herein, the term “non-metal element” is intended to mean elements of the periodic table that are selected from the group consisting H, He, C, N, O, F, Ne, P, S, Cl, Ar, Se, Br, Kr, I, Xe and Rn.

[0187] When used herein, the term “metal element” is intended to mean elements of the periodic table that do not fall within the definitions of “metalloid element” and “non-metal element”.

[0188] When used herein, the term “base pressure” is intended to refer to the lowest pressure that a plasma chamber can be pumped down to without any gases flowing. It is to be appreciated that base pressures can vary from plasma chamber to plasma chamber since the value is dependent on various factors, such as the size of the plasma chamber, the configuration of the plasma chamber, the efficiency of the vacuum pump, leaks associated with the plasma chamber and so forth.

[0189] The term “immediately” when used herein to describe a change in power level is intended to mean that the power level is instantly switched from one power level to another power level without stepping through any intermediate power levels. In other words, the immediate switch would be represented by a vertical line when viewed on a plot of power (y-axis) versus time (x-axis), e.g. FIG. 2(b) at approximately 36 minutes and 76 minutes.

[0190] When used herein, the terms “comprises” and “comprising” and variations thereof mean that the specified features, steps or integers are included. The terms are not to be interpreted to exclude the presence of other features, steps or components.

[0191] The features disclosed in the foregoing description, or the following claims, or the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for attaining the disclosed result, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof.

[0192] Although certain example embodiments of the invention have been described, the scope of the appended claims is not intended to be limited solely to these embodiments. The claims are to be construed literally, purposively, and/or to encompass equivalents.