Method and system for coating internal surfaces of prefabricated process piping in the field

The present invention generally relates to plasma chemical vapor deposition systems, and more particularly to methods of in-situ coating of piping systems.

The work of assembling the various components of a plumbing system often adversely affects the properties of the materials used in the plumbing. For example, the welding operation of high-purity 316L stainless steel pipes used to supply process gas to semiconductor manufacturing equipment has the potential to alter the properties of this stainless steel. This change is due to heating and the vapor plume effect, which usually results in corrosive gases (eg, SiCl) in the gas line.₂H₂) Is filled, the heat-affected zones (welded areas) are likely to exhibit corrosion hazards. This also applies to the exhaust piping that forms the path from the pump to the washer.

By accurately defining the levels of the components (eg 16-18% Cr for 316L SS) and reducing the levels of impurities remaining after dissolution and purification (eg for 316L SS). Has made many attempts to improve the corrosion resistance of special metal alloys such as stainless steel with S and C) less than 0.03%. This requires specialized steel manufacturing methods such as VOD (Vacuum Oxygen Decarburization), VIM (Vacuum Induction Melting), and VAR (Vacuum Arc Remelting), which results in additional costs. A further problem with low-impurity steels is that they can adversely affect machinability, hardness, and other related considerations. Industry groups (especially SEMI (Semiconductor Equipment and Materials) In order to meet the hardness and surface roughness requirements specified by International)), costly machining such as vanishing and electropolishing must often be performed as a post-treatment. One solution to these problems is a relatively low quality basis with a high quality coating material with the desired mechanical, electrical and optical properties (eg, high hardness and corrosion resistance). This is a method of coating a material. Usually, these types of properties will be found in metal, ceramic, or diamond-like coatings.

Other expensive specialty alloys such as Hastelloy and Inconel (both of which are federal registered trademarks of Huntington Alloys Corporation) are commonly used in exhaust piping not only in the semiconductor industry but also in the general chemical processing industry. ing. These alloys have high temperature strength and corrosion resistance. In this case as well, it is possible to use a relatively inexpensive base material by applying an appropriate surface coating to the inner surface exposed to a corrosive environment.

One of the basic concerns when using high quality metals or metal coatings for corrosion resistance is that assembling high-purity metal components often involves welding various components. That is. As mentioned above, the heat associated with the welding process can change the chemical properties of the steel or coating. As a result of the different vapor pressures of the various components, some materials will vaporize and redeposit on the downstream surface. This change in chemistry has the potential to negate the efforts devoted in the manufacture of steel to confer corrosion resistance, which is a major cause of corrosion hazards that cause particle and contamination problems. It has been found to be the cause, and as a result, it is possible that system failures and health and safety issues will eventually result as a result of the leak.

Conventional coating methods include CVD (Chemical Vapor Deposition), PVD (Physical Vapor Deposition), plasma spray, electroplating, and sol-gel. Among these methods, CVD and PVD provide the highest quality thin films in terms of purity, adhesion, uniformity, and other properties. All of these techniques require the use of a dedicated vacuum chamber, making it difficult to coat assembled components. In the case of pipes carrying corrosive substances, the internal surface that comes into contact with the corrosive substances must be coated. In the case of very low pressure techniques such as PVD (in this case the pressure is below or near the molecular flow region), the coating on the inner surface is large in diameter and short in length (small). Limited to tubes (of aspect ratio). Similarly, in the case of the CVD method, its application is limited because the heat-sensitive substrate can be damaged due to the need to supply heat for the chemical reaction. PECVD (Plasma) It is possible to reduce the temperature required for the reaction by using Enhanced CVD), but in this case, maintaining a uniform plasma in the pipe and the source gas as it flows down the pipe. There is a difficulty in preventing the decrease in gas.

The PIIID (Plasma Immersion Ion Implantation and Deposition) method is said to be useful for coating the outer surface of complex shapes. PIIID is performed by applying a negative bias to the work piece, and if the plasma sheath is isometric, this will result in positive ions being attracted towards the work piece. There are also improvements that can be imparted to thin film properties such as adhesiveness and thin film density by ion irradiation of the work piece.

A method of coating the inner surface of a tube by inserting the source material to be applied into the tube and then sputtering or arching it onto the tube is disclosed. For example, U.S. Pat. No. 5,026,466 granted to Wesemeyer et al. Describes a method of inserting a cathode into a tube and arching the cathode material inside the tube. U.S. Pat. No. 4,407,712 granted to Henshaw et al. Describes a hollow cathode with a high vaporization temperature metal source inserted into a tube, in which case a cathode arc removes the source material from the hollow cathode. And the inner surface of the tube is coated. This type of configuration is limited to large diameter tubes (due to the need to insert a hollow cathode tube with the associated heat shield and cooling tube inside the tube to be coated). It has several drawbacks, including the complex configuration requirements for the movement of the anode and hollow cathode through the tube, and the generation of macroparticles by the cathode arc.<patcit num="1"><text>U.S. Pat. No. 5,026,466</text></patcit><patcit num="2"><text>U.S. Pat. No. 4,407,712</text></patcit>

<p> Known types of methods have the disadvantage that they must be performed in a dedicated vacuum chamber at a remote location. As a result, it is not possible to coat the entire welded section of a series of corrosive gas lines or exhaust pipes after the weld is complete. U.S. Pat. No. 4,714,589, granted to Auwerda et al., Describes the coating inside tubes by the deposition of gas mixtures by plasma, but this method is limited to electrically insulating tubes and coatings, and It also requires a complex system for moving the microwave source along the outside of the tube. There is a need for an easier method.<patcit num="3"><text>U.S. Pat. No. 4,714,589</text></patcit></p>

<p> According to the method according to the invention, the coating of the inner surface of a pipe or tube (workpiece) can be performed in the field (in its original place) by using the work piece itself as a deposition chamber. The work piece can be welded or assembled to other components of the higher level piping system (eg, adjacent pipes and tubes) to which the work piece will function before applying the coating material. The phrase "on site" is defined herein as a location away from the manufacturing site of the workpiece to be coated and close to the assembly site of the workpiece on other components of the piping system. Has been done.</p><p> The method involves introducing source gas from the gas supply subsystem connected to the first anode at the inlet of the completed piping system. In addition, this method involves connecting the pumping subsystem to the second anode at the exhaust end of the welded piping system, and in a state where the anode is separated from the conductive piping by an insulating spacer. It also includes the step of connecting the voltage bias system so that it is negatively biased and the anode is grounded. With the application of the voltage bias, the gas flow and pumping speed are adjusted so that the pressure in the work piece provides a hollow cathode in the work piece. This pressure causes electrons to oscillate across the tube as the mean free path of the electrons is slightly below the diameter of the tube, resulting in a large number of ionization collisions and a relatively strong plasma. There is. In this method, plasma is generated outside the workpiece, resulting in ionization loss as the gas flows through the tube, resulting in reduced thin film deposition from the workpiece towards the outlet PECVD according to the prior art. It provides improvements to the law. In comparison, the present invention provides a relatively uniformly ionized plasma along the length of the work piece, thereby providing a relatively uniform deposition.</p><p> An optical detector and a Langmuir probe are provided for the anode connection at the gas inlet and the end of the pump. By monitoring the intensity of the plasma using these detectors, information about the intensity level of the hollow cathode plasma is fed back to the control system.</p><p> According to this method, coating the inner surface of a pipe or tube at a much easier and cheaper cost in the field or coating service location compared to coating a relatively small section in a remote vacuum deposition chamber. Can be executed. The method is also feasible without the requirement of inserting a source metal electrode into the work piece or the need for complex configurations for the movement of the tube or cathode. In a preferred embodiment, the method is realized by using the work piece as a PECVD deposition chamber. Prior to this coating method, the work piece must be welded or assembled to the adjacent components of the intended piping system (in this case, do these adjacent components also need to be coated? Alternatively, this assembly process requires heating (eg, welding) of the workpiece).</p><p> Basically, it is possible to apply any metal, ceramic, or DLC coating that has the desired properties in hardness and corrosion resistance (eg TiN, CrN, etc.). However, when the coating is applied in the field, non-toxic gas is used. In a preferred embodiment, DLC (Diamond-Like) such as methane, acetylene, or toluene Carbon) Precursor is used as the source gas. DLC is said to provide a coating that is hard, resistant to corrosion, and has low friction. The properties of this thin film can be adapted by adjusting the sp3 (diamond), sp2 (graphite), and sp1 (linear) bonding hybridization ratios within the thin film. The hydrogen content will also affect the properties of the thin film. Normally, the highest sp3 ratio (most diamond-like) is obtained by methane, which results in relatively low deposition rates and relatively large compressive stresses compared to relatively large carbon gram molecules. The thickness of the thin film is limited to about 5000 Å. Adding a particular dopant (eg, silicon or silicon oxide) to the DLC matrix can improve thermal stability and reduce compressive stress. Hexamethyldisiloxane (C₆H₁₈Si₂These dopants can be introduced by mixing organic-based precursors such as O) with one or more hydrocarbon precursors.</p><p> Therefore, it is possible to match the thin film characteristics by selecting the precursor gas, or it is possible to deposit a plurality of laminated thin films. For example, in a particular process (eg, very coarse welding), if it is necessary to have a thickly deposited coating, the deposition of a thin layer based on methane is relative to the subsequent deposition rate. The above process is modified by the use of a pre-mouse (eg, toluene), which is relatively large and relatively low in stress, or by ion irradiation with a relatively large energy to improve adhesion and reduce stress. be able to. The trade-off between the desired mechanical, electrical, or optical thin film properties, deposition rate, and stress in a given precursor and bonding hybridization can be optimized for a given process.</p><p> The advantage of the present invention is that the adhesiveness and density of the thin film can be improved by using the advantages of the ion irradiation of the PIIID method described above. In a preferred embodiment, this is achieved by applying a negative pulsed DC bias to the workpiece in relation to the grounded anode. Since the DLC coating is an insulator, the use of short pulses (1-20 microseconds) prevents the accumulation of excessive positive charge on the coating. This charge is offset when the plasma sheath collapses in the off-cycle. The work piece or coating surface is irradiated with strong positive ions generated by the hollow cathode in the work piece. The energy of this ion can be controlled by the magnitude and pressure of the applied voltage (the higher the pressure, the more collisions will occur, resulting in a lower energy at a given voltage).</p><p> A further advantage of the present invention is that it is possible to match the quality of the thin film deposited on the inner surface of the welded work piece by using a multi-step process. Introducing a sputtering gas such as Ar in the first stage of this method, followed by 1x10^-3Torr (preferably 1x10^-4It is also possible to pre-clean the surface of the work piece by pumping down to Torr) pressure. Contaminants on the inner surface of the work piece are removed by sputtering when a negative DC pulse is applied. A carbon layer can then be formed inside the steel by performing the second step using carbon injection. This layer improves the adhesion of DLC. This is done by increasing the magnitude of the bias above 5 kV. At this stage, care must be taken for small diameter tubes so that the size of the plasma sheath does not exceed the radius of the tube. The formula for the minimum radius of the cylinder where the sheaths do not overlap is as follows.</p><p><maths num="1"><img file="JP5043657B2_D0001.tif" /></maths></p><p> Where V is the magnitude of the voltage and n is the plasma density.</p><p> This injection step is followed by a DLC deposition step using the aforementioned methane, acetylene, or toluene precursor. In this process step, by reducing the DC pulse voltage (eg, 100V-10kV), instead of injecting, thin film deposition is provided. Argon is also mixed with the carbon-containing precursor during these coating steps. In one embodiment of the invention, the coating material was continuously sputtered from the inlet portion of the tube and sputtered in order to control the sputtering vs. deposition rate, thereby providing a relatively uniform coating to the tube. The material is pulled toward the rear end of the tube by the flow velocity. Homogeneity is also controlled by the duty cycle of the DC pulse so that the source gas can be replenished and flow down the tube when the pulse is "off". Those skilled in the art will appreciate that uniformity is also controlled by the choice of gas flow rate and pumping rate.</p>

With reference to FIG. 1, a conductive pipe (ie, a workpiece) 10 is shown connected to a system that includes a gas supply subsystem 12 and a process control subsystem 14. Although the work piece is shown as a single piece, it may be an assembly of a plurality of tubes or pipes. The assembly must be in a state where all welding and assembly steps have been completed and must be leak tested prior to the coating process described below. Easily available non-toxic carbon-containing gases such as methane and acetylene in the first gas supply container<u style="single">18</u>Supplied from. By using this gas, a DLC (Diamond-Like Carbon) coating is formed inside the work piece. Argon (or other sputtering gas) in the second gas supply container<u style="single">16</u>By supplying from, "pre-purification" by plasma on the pipe surface and mixing of Ar and carbon-containing gas are realized.

By using the DC pulsed power supply 20, a negative bias is applied to the workpiece 10. By using this bias, (a) plasma is generated between the cathode and the grounded anode, (b) the ionized reaction gas is attracted to the surface to be coated, and (c) density and stress. Controlling thin film uniformity by achieving ion irradiation of the thin film to improve thin film properties such as levels and (d) adjusting the duty cycle to allow replenishment of source gas in the "off" portion of the cycle. Has been realized. In this case, the work piece functions as a cathode, and there are grounded anodes 22 and 24 at both ends on the opposite side of the work piece. The inlet insulator 26 separates the inlet anode 22 from the workpiece, and the outlet insulator 28 electrically insulates the workpiece from the grounded anode 24 at the outlet end.

The turbo pump 30 and the vacuum pump 32 draw gas from the inside of the workpiece 10 through the outlet end. The pressure controller 34 receives information from the optical probe and the Langmuir probe, and these probes are arranged so that the optical probe has a visual field direction into the plasma and the Langmuir probe is in contact with the plasma. ing. The two probes detect the intensity of the plasma and generate information indicating this intensity level. The controller uses this information to determine the appropriate configuration for the adjustable flow member 40, which may be a valve. This setting establishes a pressure in the workpiece 10 where the mean free path of electrons is slightly below the internal diameter of the workpiece, causing an increase in electron oscillation and ionization collisions due to the "hollow cathode" effect. Must be. As a result, a relatively strong plasma is generated in the work piece. Since the electron mean free path increases as the pressure decreases, it is necessary to reduce the pressure as the diameter of the pipe increases. For example, a 1/4 inch (6.35 mm) diameter gas line produces a hollow cathode plasma at a pressure of about 200 mTorr, and a 4 inch (101.6 mm) diameter pump exhaust duct produces a plasma at a pressure of about 12 mTorr. Will generate. However, these are approximate values to show the general tendency that the larger the diameter, the lower the pressure, and the pressure range can vary significantly from these values and can still maintain the hollow cathode plasma. it can.

Since only the ionized gas is accelerated in the plasma sheath into the work piece, the degree of ionization or plasma intensity is important for the PIIID method to be effective. The hollow cathode effect provides a stronger plasma than can be obtained by other methods within a DC or RF plasma. This increase in intensity is due to other means for producing strong plasmas, such as magnets and microwave plasma sources, which are very difficult to implement on internal surfaces (especially in "field" applications). It is available without complexity. The process also eliminates the need to separately heat the work piece 10. Optical and Langmuir probes are placed at the end connections of the anode to monitor when a strong hollow cathode is properly produced.

The computer software control unit 42 is shown connected to the DC pulsed power supply 20 and the pressure controller 34. The control unit by the computer software can also generate a control signal to control the operation and transmit it to the gas supply subsystem 12 via the interface cable 44.

FIG. 2 shows another embodiment of the present invention. In this case, the system has been modified to operate with RF power and the workpiece 10 is housed in an insulator blanket 46 with an RF shield 48. The reference code of Figure 1 is applied to the system of Figure 2 with the same components. The pulsed power supply of FIG. 1 has been replaced by the RF feeder 47 and the anode configuration must be modified so that the anode region is larger than the cathode. Also, an induced negative voltage on the cathode must be allowed by adding a blocking capacitor (in the matching circuit). This voltage is V_p/ V_g= (A_g/ A_p) Q, in which case the subscript p represents the power electrode (workpiece 10) and the subscript g represents the grounded anodes 22 and 24, where q represents the grounded anode 22 and 24. It varies between 1.25 and 2.5. Alternatively, by superimposing a DC pulse on the RF power instead, it would be possible to eliminate the need for an induced negative voltage on the cathode.

An embodiment of the process flow will be described with reference to FIGS. 1 and 3. At step 50, the work piece is assembled to the other components of the piping system so that the work piece does not need to be heated after the internal coating process is complete. As a result, all welding steps associated with the work piece are completed in order to apply the coating material to the inside of the work piece. As mentioned above, the work piece is shown as being a single tube, but the work piece may be an assembly of multiple tubes or parts. Further, instead of a simple work piece having a single inlet and a single outlet as shown in the figure, a plurality of paths may exist through the work piece.

At stage 52, pre-cleaning is being performed. The pre-purification may be the introduction of a sputtering gas such as argon from the first gas supply container 16. Pre-purification is 1x10^-3Torr (or preferably 1x10)^-4Can be started after pumping down to (less than Torr). Contaminants on the inner surface of the work piece are removed by sputtering when a negative DC pulse is applied by the power source 20.

In some applications, the optional step 54 of injecting carbon can be used. Carbon injection forms a carbon layer within the material of the work piece, which may be stainless steel. This layer improves adhesion to DLC and other materials. Carbon injection is provided by a greater bias than that applied at other stages of the coating process. A suitable bias is above 5 kV. Care must be taken at this stage for small diameter tubes so that the size of the plasma sheath does not exceed the radius of the work piece.

Following the optional carbon injection step 54, at step 56, at least one precursor is introduced into the workpiece 10. Precursors that satisfy the conditions include methane, acetylene, or toluene. In this process stage, the DC pulsed voltage is reduced to provide thin film deposition rather than injection. The application of this DC pulsed voltage is shown in step 58 of FIG. In the coating step, argon can be mixed with the carbon-containing precursor, as shown in step 60.

At step 62, the coating parameters in the coating process are dynamically adjusted. Information available to the computer software controller 42 and pressure controller 34 to keep the various parameters within their permissible range is provided by the probe. As a result, it is possible to adjust the factors that determine the pressure in the work piece as needed, or, if necessary, the magnitude of the pulsed bias and the duty cycle.

<figref num="1">It is a functional diagram of the field-implementation type coating apparatus by one Example of this invention.</figref><figref num="2">It is a functional diagram of the 2nd Example of the field-implementation type coating apparatus by this invention.</figref><figref num="3">The process flow of each stage of implementing the present invention is shown.</figref>