Surface polymer coatings

Abstract

A plasma chamber (11) for coating a substrate with a polymer layer, the plasma chamber includes a first electrode set (14) and a second electrode set (14), the first and second electrode sets are arranged either side of a sample chamber for receiving a substrate, wherein the first and second electrode sets include plural electrode layers (141, 142) and wherein each electrode set includes plural radiofrequency electrode layers or plural ground electrode layers for coating polymer to each surface of a substrate.

Claims

1. A method for coating a substrate with a polymer layer, which method comprises: locating a first electrode set and a second electrode set within a plasma chamber, wherein each electrode set comprises an inner electrode layer and a pair of outer electrode layers and wherein the inner electrode layer is a radiofrequency electrode layer and the outer electrode layers are ground electrode layers respectively; placing a substrate between the first and second electrode sets so that an outer electrode layer of each electrode set faces the substrate; introducing a monomer into the plasma chamber; actuating each said radiofrequency electrode layer in order to activate a plasma; and exposing surfaces of the substrate to the plasma such that a polymer layer is deposited on each surface.

2. The method of claim 1, further comprising the step of initiating polymerization of the monomer by striking the plasma with the monomer to form the polymer layer.

3. The method of claim 1, further comprising the step of applying the polymer layer to a thickness of from 10 to 500 nm.

4. The method of claim 1, further comprising the step of applying the polymer layer to a thickness of from 10 to 200 nm.

5. The method of claim 1, further comprising the step of applying the polymer layer to a thickness of from 20 to 100 nm.

6. The method of claim 1, further comprising the step of applying the polymer layer to a thickness of from 40 to 70 nm.

7. The method of claim 1, further comprising drawing a fixed flow of the monomer into the plasma chamber using a monomer vapor supply system.

8. The method of claim 1, further comprising the step of allowing a vapor pressure of the monomer to stabilize in the plasma chamber before activating the plasma by actuating each said radiofrequency electrode layer.

9. The method of claim 1, further comprising the step of introducing the monomer into the plasma chamber in a first flow direction; and switching the flow after a predetermined time to a second flow direction.

10. The method of claim 1, further comprising the step of depositing the polymer layer such that it has a contact angle for water of 100 or more and/or an oil repellency level of 4 or more according to ISO14419 in a process time of approximately 2 minutes or less.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) In order that the invention may be more readily understood, it will now be described by way of example only and with reference to the accompanying drawings, in which:

(2) FIGS. 1A and 1B show schematic representations of a system according to the invention;

(3) FIG. 2A shows a plan view, FIG. 2B shows a side view and FIG. 2C shows a rear view of a radiofrequency electrode;

(4) FIG. 3A shows a front view, FIG. 3B shows a side view and FIG. 3C shows a plan view of a vacuum chamber according to the present invention;

(5) FIG. 4A shows a front view, FIG. 4B shows a side view and FIG. 4C shows a plan of another vacuum chamber according to the present invention;

(6) FIG. 5A and FIG. 5B show schematic representations of electrode set arrangements according to first (5A) and second (5B) embodiments of the invention;

(7) FIGS. 6A and 6B are photographs of a comparative test; and

(8) FIG. 7 is a graph of coating thickness against time.

DETAILED DESCRIPTION OF THE INVENTION

(9) With reference to FIG. 1A, an inventive plasma deposition system will now be described. The system, indicated generally at 1, comprises a vacuum chamber 11 in communication with input apparatus 12 via an input line 120 and an exhaust apparatus 13 via an output line 130. The input apparatus 12 comprises in flow order a cartridge 121, first 123 and second 125 canisters, a baraton 126, a mass flow controller 127 and first 128 and second 129 chamber inlet valves. The exhaust apparatus 13 comprises in flow order first 131 and second 132 pump valves, a throttle valve 133, a roots and rotary pump 134 and an exhaust valve 135.

(10) Within the vacuum chamber 11 there are four plasma electrode sets 14 arranged in stacked formation. Interposed between each plasma electrode set 14 is a sample tray 15. For purpose of clarity only a single sample tray 15 is shown in FIG. 1, the sample tray 15 being interposed between the lower pair of electrode sets 14. The space between adjacent electrode sets 14 is a sample chamber. In use, one or more PCBs are located on or within the sample tray 15. The sample tray 15 is subsequently positioned between a pair of electrode sets 14 within the vacuum chamber 11.

(11) Once the sample tray 15 is located within the vacuum chamber 11 the chamber 11 is evacuated and a gaseous monomer (or a bespoke mixture of monomers) is introduced. Plasma is then activated within the chamber 11 by energising the electrode sets 14. In the present invention the monomer is used to strike the plasma in order to initiate polymerisation of the monomer onto a surface of the PCB. This is in contrast to prior art methods, which utilise an additional gas to strike the plasma.

(12) FIG. 2 shows a radiofrequency electrode 20 in plan (a), side (b) and rear (c) views. The radiofrequency electrode 20 comprises a generally planar body formed from folded tubing 21. The tubing 21 may comprise a plurality of sections which are joined together by connectors 27. The tubing 21 is typically formed of a conductive metallic material such as aluminium, stainless steel or copper. The tubing 21 is hollow to allow for a temperature regulation fluid to be passed through the electrode 20 to regulate the plasma at a predetermined temperature. The tubing 21 comprises a series of bends 22 formed at regular intervals along the tubing length. The tubing 21 curves back on itself at each bend 22 by approximately 180. The tubing 21 has a diameter of approximately 10 mm and a wall thickness of approximately 2 mm. The distance between the tubing 21 before and after each bend 22 is approximately 5 times the diameter of the tubing 21.

(13) The tubing 21 is curved at each end so as to provide distal portions 25, 26 which are substantially orthogonal to the planar body. The distal portions 25, 26 may be connected to a fluid supply or egress line (not shown). Alternatively, the distal portions 25, 26 may be connected to the distal portions of adjacent or nearby electrodes.

(14) The electrode 20 further comprises a pair of connecting plates 23, 24 attached to the front and rear of the electrode 20 and adjacent the bends 22. The connecting plates 23, 24 provide both a means for attaching the electrode 20 to the inside of the vacuum chamber 11 and electrical contacts for applying a load thereto.

(15) Referring back to FIG. 1, examples of inventive processes will now be described. Initially, the chamber 11 is reduced to a base level vacuum, typically 20 mTorr, by means of the pump 134 with the first 131 and second 132 pump valves open and the first 128 and second 129 chamber inlets valves closed. A quantity of monomer is transferred from the cartridge 121 to the first canister 123 by means of a feed pump 122. Typically, sufficient monomer for a single day of processing is transferred at once. The monomers used are preferably in liquid form. Sufficient monomer required for a single process run is then transferred from the first canister 123 to the second canister 125 via a metering pump 124. The temperature of the second canister 125 and thus the monomer is raised, typically to between 130 and 150 C. in order to vaporise the monomer. The chosen temperature of the second canister 125 is dependent on the vapour pressure of the monomer, which is measured by the heated vacuum gauge 126.

(16) In alternative embodiments, solid or gaseous monomer may be used. In embodiments where the monomer is a solid then it may also vaporised, e.g. by heating in a canister. In embodiments where the monomer is a gas then there is typically no need for vaporisation.

(17) Once the target pressure within the vacuum chamber 11 is reached, typically between 40 to 50 C., the first pump valve 131 is closed and the first chamber inlet valve 128 is opened. Consequently, when the valve 138 is open, monomer vapour produced in the second canister 125 passes through the mass flow controller 127 and into chamber 11. The pressure within the chamber 11 is regulated at a working level of typically 10 to 1000 mTorr by either introduction of more monomer or regulation of the throttle valve 133, which is typically a butterfly valve.

(18) Once the pressure within the chamber 11 is stable, the electrode sets 14 are activated to generate plasma within the chamber 11. Thus, the monomer is activated and polymerisation occurs on a surface of the PCB. Preferred monomers comprise a high concentration of perfluorocarbon chains and are therefore highly reactive in the plasma. As such, polymerisation occurs rapidly even at low power and low monomer flow rates, typically 60 to 80 W and 30 to 50 standard cubic centimetres per min (sccm), respectively. Sufficient monomer is usually polymerised after approximately 60 to 120 seconds, to give a desired coating thickness of approximately 70 nm, depending on the power mode chosen.

(19) During the process, the direction of monomer flow through the chamber 11 is switched by control of the first 128 and second 129 chamber inlet valves and first 131 and second 132 pump valves. For example, for half the time the first chamber inlet valve 128 is open and the first pump valve 131 is closed (with the second chamber inlet valve 129 closed and second pump valve 132 open). For the remainder of the time, the second chamber inlet valve 129 is open and second pump valve 132 closed (with the first chamber inlet valve 128 closed and the first pump valve 131 open). This means that for half the time monomer flows from one side of the chamber 11 to another and for the remainder of the time vice versa. For example, for half the time monomer flows from the right to the left and for the remainder of the time monomer flows from the left to the right. The direction of monomer flow may be alternated one or more times during a single process run.

(20) FIG. 1B shows a view from the side of the chamber 11 of FIG. 1A. As will be appreciated the inlet 120 and outlet 130 lines are separate from each other. The inlet line 120 may be coupled to a distribution system 140 arranged to distribute gas across the chamber 11. The distribution system 140 may be integrated on or within the wall of the chamber 11 so that it can be maintained at the same temperature as the chamber 11. Further, in preferred embodiments the outlet line 130 is typically arranged to be closer to the door of the chamber 11 (rather than the rear of the chamber) to compensate for the fact that the intensity of the plasma tends to be higher at regions closer to the electrode connection plates 23.

(21) An alternative process will now be described. A fixed charge of monomer is transferred to the chamber 11 via the spatially separated ports 16, 17 whilst the inlet valves 128, 129 and pump valves 131, 132 are open. Once the chamber 11 is filled with monomer the pump valves 131, 132 are closed to allow the monomer to become evenly distributed within the chamber 11. Even distribution of the monomer results in a more evenly fragmented monomer in the plasma. The pump valves 131, 132 are then opened to allow the residual monomer to be extracted from the chamber 11 and the plasma initiated. This diffusion process is repeated several times, 1 to 20, preferably 3 to 7, for instance 5, and results in a more uniform coating of the surface and is considerably less sensitive to the orientation of the surface than conventional methods, making this process particularly well suited to three dimensional structures and vias, including microvias. Surface tension effects limit the smallest diameter of microvias that conventional wet chemistry can be used for. Plasma deposition, which is a dry gaseous process, completely eliminates the effects of surface tension, allowing coatings to be applied to even the smallest of microvias.

(22) In some embodiments of the invention it is preferable to introduce an inert gas to the chamber 11 with the monomer in order to prevent or at least inhibit fragmentation of the monomer.

(23) At the end of either process it is recommended for operator safety that the chamber inlet valves 128, 129 are closed and the chamber 11 pressure is reduced to base level to remove any residual monomer present. An inert gas such as nitrogen is introduced from a third canister 136 by opening valve 137. The nitrogen is used as a purge fluid and is pumped away with the residual monomer. After completion of the purge, the vacuum is removed and air is introduced into the chamber 11 until atmospheric pressure is achieved.

(24) After one or more process cycles it is recommended to purge the monomer supply line with inert gas. An inert gas line (not shown) can be connected to the or each canister to do this. It is preferable to purge the supply line straight to the pump (rather than via the chamber) so that the purge may be performed whilst the chamber is being loaded and/or unloaded.

(25) There are several possible ways of configuring the electrode layers of each electrode set 14. FIGS. 3 to 5 exemplify two possible configurations.

(26) FIGS. 3A to 3C show a first preferred arrangement in which four electrode sets 14 are located within a plasma chamber 11. The electrode sets 14 are identical to one another and therefore only one shall be described in detail. The electrode set 14 comprises an inner electrode layer 141 and a pair of outer electrode layers 142. The outer electrode layers 141 comprise a radiofrequency electrode of the type shown in FIG. 2 and the inner electrode layer 142 comprises a ground type electrode, e.g. a plate electrode. The three layers 141, 142 are coupled to form a distinct electrode. In use, load is only applied to the outer layers 142. FIG. 3B shows the same arrangement as FIG. 3A only in FIG. 3B sample trays 15 are also shown in the chamber 11.

(27) FIGS. 4A to 4C show a second preferred arrangement in which four electrode sets 14 are located within a plasma chamber 11. The electrode sets 14 are identical to one another and therefore only one shall be described in detail. The electrode set 14 comprises an inner electrode layer 141 and a pair of outer electrode layers 142. The outer electrode layers 142 each comprise a ground electrode and the inner electrode layer 141 comprises a radiofrequency electrode layer of the type shown in FIG. 2. The three layers 141, 142 are coupled to form a distinct electrode. In use, load is only applied to the inner layer 141. FIG. 4B shows the same arrangement as FIG. 4A only in FIG. 4B sample trays 15 are also shown in the chamber 11.

(28) The electrode sets 14, 14 may be termed tri-electrodes.

(29) FIG. 5 shows simplified schematics of the first (a) and second (b) preferred arrangements of FIGS. 3 and 4, respectively. As will be appreciated the radiofrequency electrode layers are depicted by (+) and the ground electrode layers are depicted by ().

(30) The applicant has discovered that use of an electrode set arrangement of the type shown in FIGS. 3 to 5 further improves the uniformity of the deposited polymer coating.

(31) It is known that when halo hydrocarbon layers are deposited on a metal the free fluoride ions combine with metal ions to form a thin layer of metal fluoride. This metal fluoride functions as a flux. Thus, it is desirable to maximise the thickness of this layer in order to make subsequent soldering operations easier.

(32) One limitation on the thickness of this metal halide layer is the amount of free fluoride present in the plasma. However, because the monomers of the invention comprise high concentrations of perfluorocarbon chains the concentration of fluoride available to combine with the copper is greater than that observed in the prior art. Therefore, thicker layer of copper fluoride forms between the conductive tracks and the polymer coating.

(33) This flux has a number of advantages, including (i) removal of the coating to allow components to be soldered to the conductive tracks; (ii) removal of any contamination from the copper tracks; (iii) to prevent oxidation as the temperature is raised to the solder reflow point; and (iv) to act as an interface between the liquid solder and the cleaned copper tracks.

(34) It is not unusual for moisture and other gasses to be present within the structure of a PCB. If a polymeric coating is applied to a PCB, then this moisture becomes trapped and can cause various problems during soldering and also subsequently when the assembled PCBs are subjected to temperature variations. Trapped moisture may result in increased leakage currents and electromigration.

(35) It is essential to remove any trapped gases or moisture from the bare PCB; this also ensures good adhesion between the polymer coating and the PCB. Removal of trapped gasses or moisture can be carried out by baking the structure prior to placing it in a plasma chamber as in conventional conformal coating techniques. The invention described here enables this de-gassing, at least partially to be carried out in the same chamber as the precleaning, etching and plasma polymerization.

(36) The vacuum helps to remove moisture from the structure which improves the adhesion and prevents problems encountered in heat cycling during the lifetime of the products. The pressure range for degassing can be from 10 mTorr to 760 Torr with a temperature range from 5 to 200 C., and can be carried out for between 1 and 120 min, but typically for a few minutes.

(37) The degassing, activation and coating processes can all be carried out in the same chamber in sequence. An etching process can also be used to eliminate surface contamination of the copper prior to the activation and coating steps.

(38) The conductive tracks on the substrate may comprise any conductive material including metals, conductive polymers or conductive inks. Conductive polymers are hydrophilic in nature, resulting in swelling, which can be eliminated by applying the coating described herein.

(39) Solder resists are normally applied to PCB's during the manufacturing process, which serve to protect the metallic conductors from oxidation and to prevent the solder flowing up the metallic track, which would reduce the amount of solder in the joint. Solder resists also reduce the potential for solder shorts between adjacent conductors. Because the halocarbon polymer coating is only removed where flux is applied, a very effective barrier to corrosion is left across the rest of the board, including the metallic conductors. This action also prevents the solder flowing up the track during the soldering process and minimises the potential for solder bridges between conductors. Consequently, in certain applications, the solder resist can be eliminated.

(40) In order to further demonstrate features of the invention, reference is made to the following Examples.

Example 1

(41) An experiment was run to coat a substrate using the parameters of Table 1.

(42) TABLE-US-00001 TABLE 1 Process parameters according to a first example Parameter Value Liquid Monomer Supply (LMS) Temperature_canister 130-150 C. Temperature_LMS 140-150 C. Plasma Chamber Dimensions 700 730 960 mm Temperature wall 40-50 C. Electrodes & Generator Plasma RF/ground Power 60-80 W Frequency 13.56 MHz Frequency mode cw Monomer 1H,1H,2H,2H-Perfluorooctyl methacrylate flow 10-50 sccm Pressure Base pressure 10-20 mTorr Work pressure 45-55 mTorr Oleophobicity Level 5 (ISO 14419-2010)

Example 2

(43) A further experiment was run to coat a substrate using the parameters of Table 2.

(44) TABLE-US-00002 TABLE 2 Process parameters according to a second example Parameter Value Liquid Monomer Supply (LMS) Temperature_canister 130-150 C. Temperature_LMS 130-150 C. Chamber System 700 730 960 mm Temperature_wall 40-50 C. Electrodes & Generator Plasma RF/ground Power 60-100 W Frequency 13.56 MHz Frequency mode pulsed (10.sup.2-10.sup.4 Hz; duty cycle 0.1-20%) Precursor 1H,1H,2H,2H-Perfluorodecyl acrylate flow 10-50 sccm Pressure Base pressure 10-20 mTorr Work pressure 40 mTorr Throttle valve 20-30% Oleophobicity Level 8 (ISO 14419-2010)

(45) Results

(46) 1. Oil and Water Repellence

(47) The water contact angle is used to measure the hydrophobicity or wettability from a surface.

(48) The oleophobicity level is measured according ISO 14419-2010.

(49) TABLE-US-00003 TABLE 3 Oil and water repellence test data Precursor C.sub.3F.sub.6 Ex 1 Ex 2 Contact angle 90-100 100-110 110-120 Oleophobic level 3 5 8 Process parameters Time (minutes) 10 2 2 Work pressure 50 50 40 (mTorr) Power (W) 500 75 75 Flow (sccm) 100 25 25

(50) It is clear from Table 3 that the hydrophobicity, as measured by the contact angle, is higher in the cases of the invention than for the prior art precursor. It is further established that the coatings of the invention have a higher oleophobicity than the prior art coating. It is also noteworthy that the process time for the coatings of the invention, power and flow rate were all lower in developing the coatings of the invention than in the prior art case.

(51) Table 3 also demonstrates that coatings formed from precursors having perfluorocarbon backbones comprising eight carbons, such as 1H,1H,2H,2HPerfluorodecyl acrylate, have higher oleophobicity and hydrophobicity than coatings formed from precursors having perfluorocarbon backbones comprising six carbon atoms, such as 1H,1H,2H,2HPerfluorooctyl methacrylate.

(52) Surprisingly, however, the Applicant has found that optimum coatings are deposited when the mode (i.e. continuous wave or pulsed) is selected according to the following key (Table 4).

(53) TABLE-US-00004 TABLE 4 Mode optimisation key perfluorocarbon chain length C2-C6 C7+, e.g. C7-C8 monomer type acrylate cw pulsed methacrylate cw pulsed

(54) 2. Deposition Rate

(55) To demonstrate the deposition rate of different coatings, the coating thickness was measured with ellipsometry after a certain treatment time on coated glass plates. The results are shown in Table 5 below.

(56) TABLE-US-00005 TABLE 5 Deposition rate test data Precursor C.sub.3F.sub.6 Ex 1 Ex 2 Thickness (nm) 28.4 29.9 35.8 Process time 7 1 1 (minutes) Process parameters Work pressure 50 50 40 (mTorr) Power (W) 500 75 75 Flow (sccm) 100 25 25

(57) The process time is approximately seven times higher for C.sub.3F.sub.6 than for the coatings of the invention.

(58) 3. Uniformity of Coating for Single and Plural Electrodes

(59) A conventional electrode set up was established with a single electrode per electrode set. In such conventional configurations the top side of the substrate or the side facing towards the RF has a thicker coating formed thereon than the obverse face or face pointing to the ground electrode.

(60) TABLE-US-00006 TABLE 6 Uniformity test data 1H,1H,2H,2H-Perfluorooctyl Precursor methacrylate electrodes/set Single.sup.1 Multiple.sup.2 Thickness: Top (nm) 58.9 64.2 Bottom (nm) 25.3 66.1 Process parameters: Process time (minutes) 2 2 Work pressure (mTorr) 50 50 Power (W) 75 160 Flow (sccm) 25 25 .sup.1A single electrode system is one conventionally used in the prior art .sup.2A multiple electrode system is that as shown in the accompanying figures

(61) As can be seen in the above Table 6, the data demonstrates that the electrode configuration of the invention leads to markedly more consistent coverage on both surfaces of the substrate.

(62) 4. Coating Uniformity According to the Precursor

(63) In order to determine the coating uniformity, process parameters were optimised for a prior art substance (C.sub.3F.sub.6) and a coating of the invention (1H,1H,2H,2HPerfluorodecyl acrylate).

(64) The minimum standard deviation for the prior art substance was 25%. The standard deviation for the coating of the invention was 6.75%.

(65) The coating of the invention was applied at a lower power than that of the prior art (ca. five times less). It was also coated with a lower treatment time. The graph of FIG. 7 demonstrates that the uniformity for coatings of the invention is higher when process times are shorter (the error bars at long process times are larger than at short process times).

(66) 5. Solderability from Plasma Coated PCB

(67) Different coating thicknesses were evaluated regarding the solderability. For coatings of the invention (e.g. 1H,1H,2H,2HPerfluorodecyl acrylate) (in pulsed or continuous power mode) the PCB joints soldered well. It was found that a wide range of coating thicknesses, in this experiment from 10 to 170 nm, showed good solderability.

(68) 6. Corrosion Resistance

(69) To test the corrosion resistance, a single gas verification test was used that had been developed as a quick and effective method of evaluating gold and nickel coatings on copper. The samples were placed in a chamber that had been filled with H.sub.2SO.sub.3 and the chamber was then placed in an oven at 40 C. After 24 hours the samples were removed from the chamber and photographed. The samples were replaced in the chamber which was refilled with a fresh charge of H.sub.2SO.sub.3. The chamber was put back in the oven and the temperature increased to 45 C. The chamber was kept at this temperature for a further four days, when some limited corrosion started to appear on the polymer coated samples. Further photographs of the samples were taken at the end of the test.

(70) The result shows that after 24 hours the ENIG-reference PCB was showing sufficient corrosion to make it unusable whereas the coatings of the invention (Examples 1 and 2 above) plasma treated samples showed no signs of corrosion. After a further four days, the ENIG reference sample was heavily corroded with large areas of copper oxide and nickel showing through. By contrast, the coatings of the invention (Examples 1 and 2 above) showed no corrosion at all or just some tiny spots. In this experiment different precursor types, different coating thickness as well as different power modes (continuous or pulsed) showed the same excellent results.

(71) Reference is made to FIG. 6 which shows the ENIG reference (FIG. 6A) and plasma coating of the invention (FIG. 6B) after the test.

(72) 7. Wettability

(73) The wettability of the Example 1 plasma coated PCB was shown according to following method IPC JSTD-003B.

(74) The test coupons were dipped in solder paste: SnPb at 235 C. or SAC305 at 255 C. Test fluxes 1 and 2 were used for SnPb and SAC305 respectfully. Samples were used as a baseline tested at 90 degrees incident to the solder pot. The immersion depth was 0.5 mm and the dwell time in the solder pot was 10 seconds. A series of samples were additionally exposed to 8h of 72 C./85% R.H. in a humidity chamber.

(75) All of the test coupons showed good wetting and robustness of the coating.

(76) The current invention demonstrates that the use of reactive monomers is effective in providing improved coatings. Moreover, the use of low powers and fast process times provides homogeneous coatings with excellent performance characteristics.