PLASMA ENHANCED THIN FILM DEPOSITION USING LIQUID PRECURSOR INJECTION

Abstract

The disclosure provides an apparatus for depositing poly(p-xylylene) onto a component. The apparatus comprises a deposition chamber configured to receive a component to be coated therein; an electrical power supply; a platen, disposed inside the deposition chamber and comprising an electrically conductive material, wherein the platen is electrically connected to the electrical power supply and configured to support the component; a monomer reservoir, configured to receive a monomer of poly(p-xylylene) therein; a monomer conduit extending between the monomer reservoir and the deposition chamber; and a heating means configured to heat the monomer reservoir and the monomer conduit to a temperature of between 25 and 250° C.

Claims

1-24. (canceled)

25. An apparatus for depositing poly(p-xylylene) onto a component, the apparatus comprising: a deposition chamber configured to receive a component to be coated therein; an electrical power supply; a platen, disposed inside the deposition chamber and comprising an electrically conductive material, wherein the platen is electrically connected to the electrical power supply and configured to support the component; at least one monomer reservoir, configured to receive a monomer of poly(p-xylylene) therein; a plurality of monomer conduits, each monomer conduit extending between the at least one monomer reservoir and the deposition chamber, and the plurality of monomer conduits being configured to inject the monomer into the deposition chamber at a plurality of locations; and a heating means configured to heat the at least one monomer reservoir and the plurality of monomer conduits to a temperature of between 25 and 250° C.; one or more monomer valves configured to selectively isolate the at least one monomer reservoir from the deposition chamber; an electronic controller configured to control the electrical power supply and the one or more monomer valves, wherein the electronic controller is configured to finish a coating cycle by deactivating the electrical power supply prior to, at the same to as or after closing the, or each, monomer valve.

26. The apparatus of claim 25, wherein the plurality of monomer conduits extend between a single monomer reservoir and the deposition chamber or the plurality of monomer conduits extend between a plurality of monomer reservoirs and the deposition chamber.

27. The apparatus of claim 25, wherein the heating means comprises a first heater configured to heat the monomer reservoir and a second heater configured to heat the monomer conduit, optionally wherein the first heater comprises a jacket heater disposed adjacent to the monomer reservoir and the second heater comprises a trace heater disposed adjacent to the monomer conduit.

28. The apparatus of claim 27, wherein the heating means is configured to heat the monomer reservoir to a temperature of at least 30° C., more preferably at least 40° C. or at least 45° C., and most preferably at least 50° C.

29. The apparatus of claim 27, wherein the heating means is configured to heat the monomer conduit to a temperature at least 1° C., at least 2.5° C., at least 5° C., at least 7.5° C. or at least 9° C. higher than the temperature of the monomer reservoir.

30. The apparatus of claim 25, wherein the apparatus comprises a mass flow controller disposed on the monomer conduit and configured to control the flow rate of the monomer into the deposition chamber.

31. The apparatus of claim 25, wherein the electrical power supply is configured to supply electrical power to the platen in a pulsed manner from when it is activated to when it is deactivated.

32. The apparatus of claim 31 wherein each pulse comprises or consists of: a) a first time period where electrical power is supplied to the platen, and b) a second time period when electrical power is not supplied to the platen.

33. The apparatus of claim 31, wherein the ratio of the first time period to the second time period is between 1:1 and 1:1,000, between 1:1.5 and 1:500, between 1:2 and 1:100, between 1:3 and 1:50, between 1:4 and 1:20, between 1:5 and 1:20 or between 1:10 and 1:12.

34. The apparatus of claim 31, wherein each pulse has a duration between 1 second and 1 hour, between 10 seconds and 30 minutes, between 20 seconds and 10 minutes, between seconds and 5 minutes, between 40 seconds and 2 minutes or between 50 seconds and 90 seconds.

35. The apparatus of claim 25, wherein the apparatus further comprises a feedstock reservoir configured to store a feedstock therein and a feedstock conduit extending between the feedstock reservoir and the deposition chamber.

36. The apparatus of claim 35, wherein the feedstock reservoir is configured to store: a feedstock configured to provide a diamond-like carbon (DLC) layer; a feedstock configured to provide a layer comprising a metal or metalloid; and/or a feedstock configured to provide an inorganic layer.

37. The apparatus of claim 25, wherein the electrical power supply is a direct current (DC) power supply or a radio-frequency electrical power supply.

38. The apparatus of claim 25, wherein the electrical power supply is configured to apply electrical power to the platen and/or the component at a power of between 0.0001 Watts/cm.sup.2 and 10 Watts/cm.sup.2, between 0.001 Watts/cm.sup.2 and 5 Watts/cm.sup.2, between 0.005 Watts/cm.sup.2 and 1 Watts/cm.sup.2 or between 0.01 and 0.5 Watts/cm.sup.2.

39. The apparatus of claim 25, wherein the apparatus comprises an electrode, wherein the electrode is electrically insulated from the platen, optionally wherein the deposition chamber defines the electrode and is connected to electrical ground or earth.

40. The apparatus of claim 25, wherein the apparatus comprises a vacuum pump configured to reduce the pressure of the deposition chamber to a pressure of less than 10 Torr, less than 1 Torr, less than 0.1 Torr, less than 50 mTorr, less than 40 mTorr, less than 30 mTorr, less than 20 mTorr, less than 10 mTorr, less than 5 mTorr or less than 1 mTorr.

41. The apparatus of claim 25, wherein the apparatus comprises an injector, configured to inject an additive and/or a carrier gas into the deposition chamber.

42. A method of depositing a poly(p-xylylene) layer onto a component, the method not requiring a high temperature cracking step, and comprising: supporting the component on a platen within a deposition chamber, wherein the platen comprises an electrically conductive material and is connected to an electrical power supply; heating a monomer of poly(p-xylylene) to a temperature of between 25 and 250° C., and thereby causing it to vaporise, and feeding the monomer along a plurality of monomer conduits to inject the monomer into the deposition chamber at a plurality of locations, wherein the plurality of monomer conduits are heated to a temperature of between 25 and 250° C.; activating the electrical power supply and thereby creating a plasma that surrounds the component and ionises and/or activates the monomer of poly(p-xylylene); and allowing the ionised and/or activated monomer of poly(p-xylylene) to deposit on the component and polymerise, and thereby form poly(p-xylylene) on the component.

43. The method of claim 42, wherein the monomer is a compound of Formula (I): ##STR00006## wherein each R.sup.1 is independently H, a C.sub.1-5 alkyl, a C.sub.1-5 alkoxy, a polymer group chain or a halogen; each R.sup.2 is independently H, a C.sub.1-5 alkyl, a halogen, CN or C(O)R.sup.4; each R.sup.3 is OH or a C.sub.1-5 alkoxy; and R.sup.4 is H, a C.sub.1-5 alkyl or a C.sub.1-5 alkoxy.

44. The method of claim 42, wherein the method comprises causing the electrical power supply to provide electrical power to the platen in a pulsed manner.

Description

[0161] For a better understanding of the invention, and to show how embodiments of the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which:—

[0162] FIG. 1 is a schematic diagram of an apparatus for depositing a film of poly(p-xylylene) polymer on a component;

[0163] FIG. 2 is a schematic diagram of a feedstock delivery apparatus;

[0164] FIG. 3 is a schematic diagram of an alternative feedstock delivery apparatus;

[0165] FIG. 4 is a graph showing the distance a component was placed from the input of the monomer and thickness of the deposited layers when different power levels were used;

[0166] FIGS. 5a and 5b are graphs showing the distance a component was placed from the input of the monomer and thickness of the deposited layers when a power level of 50 W was used and different amounts of hydrogen gas was present;

[0167] FIG. 6 is a graph showing the distance a component was placed from the input of the feedstock and thickness of the deposited layers when a power level of 200 W was used and the radio-frequency power supply was either on constantly or was pulsed;

[0168] FIGS. 7a and 7b are graphs showing the distance a component was placed from the input of the feedstock and the average deposition rate of the deposited layers across the length of the deposition process when a power level of 200 W was used and the radio-frequency power supply was either on constantly or was pulsed;

[0169] FIG. 8 is a graph showing the distance a component was placed from the input of the feedstock and the deposition rate of the deposited layers across the length of time the plasma was on when a power level of 200 W was used and the radio-frequency power supply was either on constantly or was pulsed; and

[0170] FIGS. 9a and 9b are graphs showing simulations of the concentration gradient of a poly(p-xylylene) monomer across a deposition chamber, the x and y axes distance in meters; FIG. 9a shows the initial conditions when the monomer is injected into the deposition chamber; and FIG. 9b shows the steady state conditions.

EXAMPLE 1—POLY(P-XYLYLENE) POLYMER DEPOSITION APPARATUS

[0171] FIG. 1 is a simplified schematic diagram showing an apparatus 2 configured to deposit a film of poly(p-xylylene) polymer on an electrically conductive component 4. The apparatus comprises a deposition chamber 6 comprising a metallic housing 8. As shown in FIG. 1, the metallic housing 8 is earthed. The inside of the deposition chamber 6 may be accessed via a loading door 10, allowing the component 4 to be placed in the chamber 6 and subsequently removed therefrom.

[0172] When it is placed in the chamber 6, the component 4 may be disposed on a platen 12, which holds the component 4 above the base 14 of the deposition chamber 10 and electrically connects the component 4 to a radio-frequency electrical power supply 16. The radio-frequency power supply 16 used by the inventors operates at a frequency of 13.56 MHz, as this is an industrial, scientific and medical (ISM) radio band, and so will not disrupt radio communication. The component 4 is electrically insulated from the metallic housing 8 due to an insulating material 18 being disposed between the platen 12 and the housing 8.

[0173] The platen 12 may comprise a metallic rod 20 with a resilient metallic clip (not shown) disposed thereon. The metallic clip comprises spaced apart flanges joined by a connecting portion. A portion of the component 4 can slot between the flanges and the platen is thereby able to support the component 4. The metallic clip is sized so as to contact as little of the component 4 as possible, and typically contacts less than 1% of the surface of the component.

[0174] The apparatus 2 further comprises a delivery system 22, configured to deliver a vaporised monomer, such as a vaporised poly(p-xylylene) monomer, to the deposition chamber 6. For simplicity, elements of the delivery system 22 have been omitted from FIG. 1. A more complete representation of the delivery system 22 is provided in FIG. 2.

[0175] The delivery system 22 comprises a monomer reservoir 24 configured to receive a poly(p-xylylene) monomer therein. A monomer conduit 26 extends between the deposition chamber 6 and the monomer reservoir 24. A jacket heater 28 (only shown in FIG. 1) is disposed adjacent to the monomer reservoir 24 and a tracer heater 29 (again, only shown in FIG. 1) is disposed adjacent to the monomer conduit 26. The jacket heater 28 and trace heater 29 are controlled by a heating control unit 30. A liquid mass flow controller (MFC) 32 is disposed on the monomer conduit 26 and is controlled by a liquid MFC control unit 34. A first monomer valve 36 is disposed on the monomer conduit 26 between the MFC 32 and the monomer reservoir 24, and selectively isolates the monomer reservoir 24 from the rest of the delivery system 22. A second monomer valve 38 is disposed on the monomer conduit 26 between the MFC 32 and the deposition chamber 6, and selectively isolates the delivery system 22 from the deposition chamber 6.

[0176] The delivery system 22 further comprises a gas reservoir 40, configured to store a pure inert gas, such as nitrogen, therein. A gas conduit 42 extends between the gas reservoir and the monomer conduit 26, intersecting the monomer conduit 26 between the first monomer valve 36 and the MFC 32. A gas valve 44 is disposed on the gas conduit 42, and selectively isolates the gas reservoir 40 from the rest of the delivery system 22.

[0177] The delivery system 22 further comprises a connector 46 configured to be connected to a vacuum source and/or a helium leak tester. A connector conduit 48 extends between the connector 46 and the monomer conduit 26, and intersects the monomer conduit 26 between the first monomer valve 36 and the MFC 32. A connector valve 50 is disposed on the connector conduit 48.

[0178] The first and second monomer valves 36, 38, the gas valve 44 and the connector valve 50 are all solenoid valves and are connected to a control system (not shown). Connecting the connector conduit 48 to a helium leak tester allows a leak check to be conducted. Alternatively, connecting the connector conduit 48 to a vacuum source allows a pulse/purge regime to be carried.

[0179] The apparatus further comprises a turbo pump 52 and a pump conduit 54 extending between the turbo pump 52 and the deposition chamber 6. A pump valve 56 is disposed on the pump conduit 54. The pump valve 56 may be a gate valve or a throttle valve. The gate valve may be a manual gate valve or a pneumatic gate valve. The throttle valve may be an automatic pressure controller (APC) valve. An APC valve can automatically set its position in order to adjust a set pressure (ascertained from a pressure gauge in the main chamber) via electronics.

[0180] Finally, the apparatus 2 also comprises a gas source 58 and a further gas conduit 60 extending between the gas source 58 and the deposition chamber 6, with a valve 62 disposed thereon. This allows the selective injection of a gas into the deposition chamber 6.

[0181] To coat a component 4 with a film of poly(p-xylylene) polymer, a user first loads a poly(p-xylylene) monomer into the reservoir 24. The user also loads the component 4 into the deposition chamber 6, positing it on the platen 12.

[0182] The user can also place a small amount of an adhesion promotion agent, such as A-174, in the deposition chamber 10. The adhesion promotion agent can be provided in an open container, such as a petri dish. The amount of adhesion promotion agent required would depend upon the size of the component 4, but the inventors have typically used about 3 ml. It should be noted, that the use of a plasma, as described below, enhances the reactivity of the monomers and activates the surface of the component 4. Accordingly, the adhesion of the poly(p-xylylene) polymer is stronger than was possible previously. Accordingly, the adhesion agent may not be required. The user then closes the loading door 10, thereby hermetically sealing the deposition chamber 6, and ensures that the pump valve 56 is open and then activates the turbo pump 52 to cause the pressure within the deposition chamber 6 to reduce to lower than 10.sup.−3 Torr. This causes the adhesion agent, if present, to evaporate and coat the inside of the deposition chamber 6 and component 4.

[0183] The heating control unit 30 then activates the jacket heater 28 to heat the reservoir 24 to a temperature between 50 and 100° C. The heating control unit also activates the trace heater 29 to heat the monomer conduit 26 to a temperature of about 10° C. higher than the monomer reservoir 24. The temperature gradient ensures that the monomer does not condense in the monomer conduit 26.

[0184] If either of the monomer valves, 36, 38 are closed then they would be opened, either manually or automatically, prior to, at the same time or after activating the heaters 28, 29.

[0185] Heating of the reservoir causes the poly(p-xylylene) monomer to vaporise. The MFC controls the flow of the vaporised poly(p-xylylene) monomer from the reservoir 24, along the monomer conduit 26 and into the deposition chamber 6. The desired flow rate may depend on a variety of factors. The results discussed below were obtained using a flow rate of 2.5 sccm.

[0186] The user may turn-on the radio-frequency electrical power supply 16. The electrical power delivered by the radio-frequency electrical power supply 16 is typically 0.1 Watts/cm.sup.2. Due to the metallic housing 8 of the deposition chamber 6 being grounded, it acts as a virtual electrode, and a plasma is created around the component 4. The plasma ionises and/or activates the monomers, typically causing them to become positively charged. The plasma activates the surface of the component 4. The ionised monomers are attracted to the component 4, deposit thereon and polymerise to form a poly(p-xylylene) polymer coating.

[0187] The radio-frequency electrical power supply 16 may be left on throughout the process. Alternatively, in some embodiments the radio-frequency electrical power supply 16 may supply power in a pulsed manner, as discussed in more detail below.

[0188] During deposition, additives can be added to the deposition chamber by injection from the gas source 58. These additives could include a hydrocarbon, such as acetylene, and/or an organometallic compound, such as tetraethyl orthosilicate (TEOS), and/or titanium isopropoxide (TIPP). The additives may be injected into the deposition chamber 10 throughout the deposition process so they are disposed throughout the coating to add functionality. Alternatively, they may be added at selected times to produce a multi-layer coating.

[0189] Once the desired coating thickness has been reached the user can stop the process. This can be achieved by deactivating the radio-frequency power supply 16, deactivating the pump 52 and closing the second monomer valve 38 and/or the MFC 32. The pump valve 56 may also be closed. The deposition chamber 6 can then be purged with air or nitrogen and brought up to atmospheric pressure. The user can then open the deposition chamber 10 and retrieve the coated component 4.

[0190] If a complete shutdown of the apparatus is required, the heaters 28, 29 can also be deactivated. Alternatively, the heaters 28, 29 may be left on. This ensures that the apparatus 2 is ready to coat another component more quickly that would otherwise be the case.

EXAMPLE 2—LIQUID PRECURSOR DEPOSITION APPARATUS WITH A MODIFIED DELIVERY SYSTEM

[0191] In some embodiments the delivery system 22 may comprise multiple subunits. For instance, FIG. 3 shows a delivery system 22 comprising two subunits 64a, 64b. Each subunit 64a, 64b comprises a monomer reservoir 24a, 24b, a monomer conduit 26a, 26b, a liquid MFC 32a, 32b and first and second monomer valves 36a, 36b, 38a, 38b, as described in example 1.

[0192] The delivery system 22 comprises a primary gas conduit 66 which is configured to connect a gas source 40 with the subunits 64a, 64b. A primary gas valve 68 is disposed on the primary gas conduit 66, and is configured to isolate the gas source 40 from the reset of the delivery system. Each subunit 64a, 64b comprises a secondary gas conduit 42a, 42b which extends between the primary gas conduit 66 and the respective monomer conduits 26a, 26b, intersecting the respective monomer conduits 26a, 26b between the first monomer valve 36a, 36b and the MFC 32a, 32b. A secondary gas valve 44a, 44b is disposed on each secondary gas conduit 42a, 42b, and selectively isolates the respective subunit 64a, 64b from the gas reservoir 40.

[0193] The delivery system 22 further comprises a primary connector conduit 70 which is configured to connect the subunits 64a, 64b to a connector 46 configured to be connected to a vacuum source and/or a helium leak tester. A primary connector valve 72 is disposed on the primary connector conduit 70, and is configured to isolate the connector 46 from the reset of the delivery system. Each subunit 64a, 64b comprises a secondary connector conduit 48a, 48b which extends between the primary connector conduit 70 and the respective monomer conduits 26a, 26b, intersecting the respective monomer conduits 26a, 26b between the first monomer valve 36a, 36b and the MFC 32a, 32b. A secondary connector valve 50a, 50b is disposed on each secondary connector conduit 48a, 48b, and selectively isolates the respective subunit 64a, 64b from the connector 46.

[0194] While not shown in the Figure, it will be appreciated that the delivery system 22 shown in FIG. 3 would also comprise a heater 28 disposed adjacent to the monomer reservoirs 24a, 24b and the monomer conduits 26a, 26b, and is controlled by a heating control unit 30.

EXAMPLE 3—LIQUID PRECURSOR DEPOSITION APPARATUS FOR USE WITH AN INSULATING COMPONENT

[0195] The apparatus 2 discussed in Examples 1 and 2 could be modified to deposit a layer from a liquid precursor on a component comprising an electrically insulating material. Such a component would be thin and substantially flat. The exact thickness of the component may vary. For instance, it is noted that the inventors have successfully used this method to coat components thicknesses between 2 and 3 cm. It will be appreciated that thicker components could be coated if a stronger electrical field were used.

[0196] The apparatus would be as described in Examples 1 and 2 except that the platen would define a flat platform configured to receive the component thereon. The apparatus would coat the exposed side of the component, and the component would then need to be repositioned to expose the other side thereof. The repositioning could be carried out manually by a user or automatically by equipment disposed within the deposition chamber.

EXAMPLE 4—DEPOSITION OF MULTIPLE LAYERS ONTO A COMPONENT

[0197] The apparatus describes in Example 1 was used to deposit multiple layers, including a layer of a poly(p-xylylene) polymer, onto multiple components. The components were 1 cm×1 cm silicon witness samples. They were allocated through the entire length of the chamber. These components were used as they allow a thickness measurement of a deposited layer to be obtained. The experiment was conducted three different times with the power supplied to the component by the radio-frequency power supply varied each time. No carrier gas was used in these experiments.

[0198] The thickness of the multilayers on the components was measured as a function of the distance from the monomer input, and the results are shown in FIG. 4.

[0199] The advantages associated depositing a poly(p-xylylene) polymer layer onto a component using a liquid monomer feedstock are:

[0200] 1. It is easy to set and maintained a flow rate of the monomer into the deposition chamber. This ensures that a steady deposition rate may be achieved.

[0201] 2. It is possible to use multiple injection sites. This enables the deposition of a poly(p-xylylene) polymer layer onto components with more complex architecture than was previously possible.

[0202] 3. A high temperature cracking step is not required. This significantly lowers the required temperature for deposition.

EXAMPLE 5—DEPOSITION OF LIQUID PRECURSOR WHEN USING HYDROGEN CARRIER GAS

[0203] The apparatus describes in the Example 1 was used to deposit a layer from a liquid precursor on a component. The experimental details were as described in Example 4, except the power supplied to the component by the radio-frequency power supply was kept constant at 50 W and for five of the six experiments hydrogen was injected into the deposition chamber at a specific flow rate.

[0204] The results can be seen in FIGS. 5a and 5b. The results show that the most uniform deposition without plasma is associated with a carrier gas flow rate of 10 sccm. The linear approximations in 5b show that even with this improvement of uniformity, the maximum throwing distance is limited to at best about 1 m (i.e. the point where the line crosses the x axis).

[0205] One way to improve uniformity over greater distances would be to use multiple injection points for the monomer throughout the deposition chamber. Alternatively, or additionally, the plasma could be pulsed. This strategy was investigated by the inventors and is discussed below.

EXAMPLE 6—INVESTIGATING THE EFFECT OF PLASMA PULSING

[0206] The apparatus described in the Example 1 was used to deposit a layer from a liquid precursor onto components. The components were as described in Example 4.

[0207] The power supplied to the component by the radio-frequency power supply was kept constant at 200 W. No carrier gas was used in these experiments. For the first experiment the plasma was maintained constantly for 10 minutes. For the following four experiments the plasma was pulsed on and off at regular intervals as indicated in table 1.

TABLE-US-00001 TABLE 1 Different deposition conditions used Symbol Length of Length of time Length of time in deposition with plasma on with plasma off FIGS. 6 Pulsed process/ for each pulse/ for each pulse/ and 7 plasma? minutes seconds seconds .Math. No 10 — — .diamond-solid. Yes 20 10 20 Yes 10 10 50 Yes 20 10 50 .circle-solid. Yes 20  5 55

[0208] As can been seen from FIGS. 6 and 7, using the plasma in short bursts allowed the inventors to obtain a stable deposition rate across the chamber and very uniform coating. Extrapolating the results suggests that the use of 5:55 on:off pulse would only have a variation in deposition rate of about 0.5 nm/min up to a length of 5 m away from the input, see FIG. 7b.

[0209] FIG. 8 shows the deposition rate per second the plasma is on. Samples subjected to a pulsed plasma with a ratio of 5:55 on:off were found to have the same deposition rate across the chamber as a sample subjected to a constant plasma disposed directly under the inlet.

[0210] The inventors conducted a simulation of the precursor concentration gradient along the length of the chamber, and the results are shown in FIG. 9. In particular, FIG. 9a shows the monomer concentration across the deposition chamber. As can be seen in the Figure, the concentration of the monomer is highly variable across the chamber. This explains why a plasma which is applied constantly produces a non-uniform coating.

[0211] FIG. 9b shows the concentration gradient after the chamber has reached steady state. This shows that over time the monomer will spread throughout the deposition chamber, and the gradient is removed resulting in a uniform coating being obtained when a pulsed plasma is used.