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

Technical field The present invention generally relates to systems for chemical vapor deposition using plasma techniques, and more particularly to systems for coating piping in the field.

Background Art [0002] Often, the assembly of the various components of a piping system adversely affects the properties of the material used for the piping. For example, welding high-purity 316L stainless steel pipes used to supply semiconductor fabrication equipment with gas can change the properties of stainless steel. This change is caused by heating and the steam plume phenomenon, so that when the gas pipeline is filled with corrosive gas (e.g. SiCbH2), there is usually a thermal effect zone (welded area) in which corrosion damage is likely to occur. This also applies to the case of outlet pipelines, which are the path from pumps to scrubbers.

[0003] Many efforts have been made to improve the corrosion resistance of special metal alloys, e.g. stainless steel, by precisely determining chemical levels (e.g. 16 to 18% Cr in 316L stainless steel) and reducing admixture levels (e.g. less than 0, 03% S and C in 316L stainless steel), which remain after melting and cleaning. This requires specialized methods of steel production, such as VOD (Vacuum Oxygen Decarburization), VIM (Vacuum Induction Metlting) and remelting in a VAR (Vacuum Arc Remelting), which significantly increases costs. An additional problem with steel with a low level of contamination is that machinability, hardness and other important factors can be worsened. Expensive post-cutting machining processes, such as polishing or electrolytic polishing, often have to be carried out to meet the requirements of surface hardness and roughness, as defined by organizations, in particular the Semiconductor Equipment and Material Institute (SEMI). One of the solutions to these problems is coating a lower grade basic material with a high quality coating material having the desired mechanical, electrical or optical properties (e.g. high hardness and corrosion resistance). Usually, these types of properties can be found in metal, ceramic or diamond-like coatings.

[0004] Other expensive specialized alloys, such as Hastelloy and Inconel (both are federally registered trademarks of Huntington Alloys Corporation), are widely used for discharge piping, not only in the semiconductor industry, but also in the chemical processing industry in general. These alloys show high

- 2 temperature resistance and corrosion resistance. Also in this case, a cheaper base material can be used if a suitable surface coating is applied to the inner surface to be exposed to a corrosive environment.

[0005] The basic problem of using high-grade metals or metal coatings for corrosion resistance is that the assembly of components made of high-purity metal often involves welding of various components. As previously noted, the heat associated with the welding process can change the chemistry of the steel or coating. As a consequence of the differences in vapor pressure of the various components, some material will evaporate and settle again with the current on the surface. Such a change in chemistry can nullify the effort put in to produce steel that was supposed to be corrosion resistant, and proved to be the main source of corrosion damage that causes pollution and contamination problems, and ultimately can cause system failure and health and safety issues as leakage effect.

[0006] Prior art coating methods include chemical vapor deposition CVD (Chemical Vapor Deposition), physical vapor deposition PVD (Physical Vapor Deposition), plasma sputtering, electroplating and sol-gel technology. CVD and PVD methods provide the highest quality coatings in terms of purity, adhesion, uniformity and other properties. Both of these techniques require a special vacuum chamber, which makes coating of fully assembled components difficult. For pipelines for transporting corrosive material, the internal surface that is in contact with corrosive material must be coated. For very low pressure techniques such as PVD, where the pressure is below or near the molecular flow zone, the coating of internal surfaces has been limited only to pipes of large diameter and short length (low aspect ratio). Similarly, CVD techniques are limited to such applications because of the need for heat input for a chemical reaction, which can damage heat sensitive substrates. Plasma-supported CVD technique i.e. PECVD (Plasma Enhanced Chemical Vapor Deposition) can be used to the lower temperature required for the reaction, but then it becomes difficult to maintain plasma homogeneity inside the pipe and to prevent exhaustion of the source gas because it flows down the pipe.

[0007] The technique of implanting and depositing ion immersed in plasma PIIID (Plasma Immersion Ion Implantation and Deposition) appeared to be useful for coating the outer surfaces of complex shapes. The PIIID technique is performed by applying negative polarization to the workpiece, which will attract positive ions towards the workpiece if the plasma sheath is suitable. Improvements are also possible that can improve casing properties, such as adhesion or casing density, by ion bombarding the workpiece.

[0008] Methods have been described for coating the inner surfaces of pipes by which the source material to be used is inserted into the pipe and sprayed ionically or applied to the pipe by arcing. For example, US Patent 5,026,466 to Wesemeyer et al. Describes a method of inserting a cathode into a tube and applying material by arc discharge

- 3 cathodes for the inside of the tube. U.S. Patent 4,407,712 to Henshaw et al. Describes a hollow cathode with a high evaporation temperature metal source inserted into a tube, the cathode arc removing source material from the hollow cathode and coating the inner surface of the tube. This type of system has several disadvantages, including limitation only to large diameter pipes (due to the need to insert a hollow cathode tube, with appropriate heat shield and cooling tubes, into the tube to be coated), requiring complex systems for moving the anode and a hollow cathode tube through the tube, as well as generating macro-particles by the cathode arc.

[0009] Known methods have the disadvantage that they must be carried out at a remote location in a special vacuum chamber. This excludes the possibility of coating all welded sections of the long course of the corrosive gas line, or completing the discharge pipe after welding. U.S. Patent 4,714,589 to Auwerda et al. Describes coating the inside of a pipe by plasma-activated gas mixture deposition, but this method is limited to electrically insulating pipes and coatings, and also uses a sophisticated system to move the microwave source along the outside of the pipe. A less complex concept is sought.

Summary of the invention [0010] The invention is defined in claims 1 and 18, respectively. Particular embodiments of the invention are set out in the dependent claims.

[0011] The method of the present invention makes it possible to coat the inner surface of thick-walled or thin-walled pipes ("workpiece"), which is carried out in the field (on site) by using the workpiece itself as a deposition chamber. Before applying the coating material, the workpiece can be welded or otherwise assembled with other components (e.g. adjacent thick-walled or thin-walled pipes) of a larger pipe system in which the workpiece is to function. The term "in the field" here means a place remote from the place of manufacture of the workpiece to be coated, and a place of assembly of this workpiece with other components of the pipe system.

[0012] This method involves introducing the source gas from the gas supply subsystem connected to the first anode at the input of a complete piping system. It also includes connecting a pumping subsystem to a second anode at the discharge end of the welded pipe system and connecting a polarization voltage system so that the workpiece is negatively polarized and the anodes are grounded, these anodes being separated from the conductive pipe by insulating spacers. The gas flow and pumping speed are adjusted so that the pressure in the workpiece provides the hollow cathode with bias voltage. The pressure is such that the average free electron path is slightly smaller than the diameter of the tube, which causes the electrons to oscillate across the tube, resulting in numerous ionization collisions and more intense plasma. This is an improvement over the known PECVD technique, in which the plasma is generated outside the workpiece, causing ionization losses, because the gas flows through the pipe so that a smaller layer settles near the exit of the workpiece. However, thanks

The invention achieves a more evenly ionized plasma over the entire length of the workpiece, and thus a more even deposition is obtained.

[0013] Optical sensors and Langmuir probes are located at the anode connections at the gas inlet and at the pump ends. These sensors are used to monitor plasma intensity so that information regarding the plasma intensity level of the hollow cathode is routed back to the control system.

[0014] This method makes it possible to coat the inner surfaces of thick-walled pipes or thin-walled pipes in the field or at the location of the coating services, much easier and at a lower cost than the remote coating of smaller sections in the vacuum deposition chamber. Furthermore, this method can be carried out without having to insert the source metal electrodes into the workpiece and without the need for complicated systems for moving the tube or cathode. In a preferred embodiment, the method is implemented by using the workpiece as a PECVD deposition chamber. Prior to the coating operation, the workpiece should be welded and mounted to adjacent components of the target piping system, such adjacent components will also be coated or heated where the assembly process requires heating (e.g. welding) of the workpiece.

In principle, any metal, ceramic or carbon DLC (Diamond Like Karbon) coating that has the desired hardness and corrosion resistance (e.g. TiN, CrN etc.) can be used. However, non-toxic gas is used for field coatings. In a preferred embodiment of the invention, a diamond gas-like precursor of the DLC-like carbon coating, such as methane, acetylene or toluene, is used as the source gas. DLC enables hard, corrosion resistant and low friction coating. The properties of such a coating can be selected by adjusting the hybridization coefficients of the sp3 (diamond), sp2 (graphite) and sp1 (linear) bond in the coating. The hydrogen content will also affect the coating properties. Usually, the highest sp3 factor (closest to diamond) is obtained when using methane, but it also produces a lower deposition factor for larger carbon particles, as well as higher compressive stress, which limits the coating thickness to about 5000 A. Addition of some admixtures semiconductor (e.g. silicon or silicon oxide) to the DLC substrate will improve thermal stability and can reduce compressive stress. An organic based precursor such as hexamethyldisiloxane (C6H1₈Si2O) can be mixed with the hydrocarbon precursor (s) to introduce these dopants.

[0016] The coating properties can therefore be modified by choosing a precursor gas or layered coatings can be deposited. For example, if in some case (e.g. for very rough welds) it is necessary to deposit a thick coating, the process can be modified by depositing a thin layer based on methane, and then using a higher deposition factor for precursors with lower stress, such as toluene, or by bombardment with higher energy ions to increase adhesion and reduce tension. The exchange between the desired mechanical, electrical or optical properties of the coating and the deposition factor and stress for given precursors and bond hybridizations can be optimized for a given process.

[0017] An advantage of this invention is that the previously mentioned benefits of ion bombardment in the PIIID technique can be used to improve coating adhesion and compactness. This is done in a preferred embodiment by applying polarization with negative DC pulses to the workpiece relative to the grounded anode. Because the DLC coating is an insulator, short pulses (1-20 microseconds) are used to prevent excessive accumulation of positive charge on the coating. The charge is compensated when the plasma sheath disappears during the zero cycle. The workpiece or coated surface is bombarded with positively charged ions generated by the hollow cathode inside the workpiece. The ion energy can be controlled by the amount of voltage supplied and the pressure (higher pressure causes more collisions, which results in lower energy at a given voltage).

[0018] Another advantage of this invention is that a multi-step process can be used to customize the properties of the coating deposited on the inner surface of the workpiece being welded. The workpiece surface can also be pre-cleaned by introducing sputtering gases such as Ar in the first method step, then draining to a pressure of 0.133 Pa (1 x 10^-3 Torr) or, preferably, up to 0.133 Pa (1 x 10 "⁴ Torr). Impurities on the internal surface of the workpiece are sprayed when a negative DC voltage pulse is applied. The second stage can then be carried out using carbon implantation to form a subsurface carbon layer in the steel. This layer improves DLC adhesion. This is done by increasing the polarization voltage to over 5kV. Care should be taken at this stage for small diameter pipes so that the size of the plasma sheath does not become larger than the radius of the pipe. The formula for the smallest cylinder radius for which the casings do not overlap is as follows:

d - Ι^4ε7 y en where V is the magnitude of the voltage, and the density of the plasma.

[0019] After this implantation step, a DLC deposition step is carried out using the above-mentioned methane, acetylene or toluene precursors. The DC pulse voltage is lowered at this stage of the process to achieve thin film deposition instead of implantation (e.g. 100 V - 10 kV). During these coating steps, argon is also mixed with carbon-containing precursors. In one embodiment of the invention, to control the spraying depending on the deposition factor, and thus to provide a more uniform coating over the entire length of the pipe, the coating material is sprayed continuously at the inlet portion of the pipe, from where it is sucked by the flow velocity towards the rear end of the pipe. Uniformity is also controlled by the duty cycle of the DC pulses in such a way that when the pulse is "zero", it is allowed to replenish the source gas and flow down the pipe. One skilled in the art will recognize that uniformity can also be controlled by selecting the gas flow rate and pumping speed.

Brief Description of the Drawings [0020] Fig. 1 is a functional view of the field coating apparatus according to one embodiment of the invention.

[0021] Fig. 2 is a functional view of a second embodiment of the field coating apparatus according to the invention.

[0022] Fig. 3 is a flow diagram of process steps for carrying out the invention.

Detailed Description [0023] Referring to Fig. 1, there is shown a pipe, or "workpiece," connected to a system that includes a gas supply subsystem 12 and a process control subsystem 14. The workpiece is illustrated as a single piece, but it can be a thick wall or thin wall pipe assembly. The pipe assembly should have all stages of welding and assembly completed and should be tested for leakage before the coating process to be described below. An easily available, non-toxic carbon-containing gas such as methane or acetylene is introduced through the first gas supply container 16. This gas is used to form a diamond-like carbon (DLC) coating inside the workpiece. Argon (or other sputtering gas) is fed from a second gas supply container 18 to allow "pre-cleaning" the surface of the pipe and mixing Ar and carbon containing gas.

[0024] The pulsed DC voltage source 20 is used for the negative polarization of the workpiece 10. This polarization is used to (a) generate plasma between the cathode and the grounded anode, (b) attract ionized active gas to the surfaces to be coated, (c) allow coating ion bombardment to improve coating properties such as density and stress levels and (d) allowing the uniformity of the coating to be controlled by regulating the duty cycle in such a way as to permit replenishment of source gas during the "zero" portion of the cycle. Here, the workpiece functions as a cathode, and at the opposite ends of the workpiece there are grounded anodes 22 and 24. The input insulator 26 separates the input anode 22 from the workpiece, while the output insulator 28 electrically insulates the workpiece from the grounded anode 24 at the output end.

[0025] The turbine pump 30 and the vacuum pump 32 draw gas from inside the workpiece 10 through the exit end. The pressure controller 34 receives information from the optical probe and Langmuir probe, which is positioned in such a way that the optical probe has a plasma path and the Langmuir probe touches the plasma. These two probes detect plasma intensity and produce information indicating the level of this intensity. This information is used by the controller to determine the correct setting of the adjustable flow member 40, which may be a valve. The setting should be such that the pressure inside the workpiece 10 establishes a state in which the average free electron path is slightly smaller than the inside diameter of the workpiece, which causes electron oscillations and increased ionization collisions through the "elongated cathode" effect. Thus, more intense plasma is generated inside the workpiece. Because the average free

- the electron path increases as the pressure drops, it is necessary to lower the pressure as the diameter of the pipe increases. For example, a quarter inch (6.35 mm) gas line will produce hollow cathode plasma at approximately 26.6 Pa (200 mTorr), while a pump outlet line with a diameter of four inches (101.6 mm) will produce plasma at pressure around 1.6 Pa (12 mTorr). Approximate values ​​are given here to show the general tendency to reduce pressure for a larger diameter, but the pressure range may vary significantly and deviate from these values, while the hollow cathode plasma may still be maintained.

[0026] The degree of ionization or the intensity of the plasma is important for the PIIID technique to be effective because only the ionized gas is accelerated by the plasma shield to the workpiece. The hollow cathode effect provides a more intense plasma than would be possible with DC or RF plasma. This increase in intensity is available without the complications of other intensive plasma generation systems, such as magnetic or microwave plasma sources, which would be very difficult to adapt to internal surfaces, especially for "field" applications. This process also eliminates the need for separate workpiece heating 10. The optical probe and Langimur probe are located at the anode end connections to monitor when the intensity of the hollow cathode is properly generated.

[0027] The computer software controller is connected to a pulsed DC power source or a pressure controller 34. In addition, the computer software controller is capable of generating and sending control signals via interface cable 44 to the gas supply subsystem 12 to manage operations.

[0028] Another embodiment of the invention is shown in Fig. 2. Here, the system has been modified to operate on RF power, the workpiece 10 being placed in an insulating jacket 46 with RF shielding 48. Numbers from Fig. 1 were also used for the system of Fig. 2, where the components are identical. Impulse power source from Fig. 1 has been replaced with RF 47, while the anode must be modified so that the surface area of ​​the anode is larger than that of the cathode. Furthermore, a barrier capacitor (in the appropriate network) must be added to allow the induction of a negative voltage on the cathode. This voltage is determined by the relationship Vp / V_g = (A._g/ Ap) q, where p in the subscript is the supply electrode (workpiece 10), agw in the subscript is the grounded anodes 22 and 24, with q varying from 1.25 to 2.5. Alternatively, DC pulses can overlap the RF power supply, eliminating the need to induce a negative cathode voltage.

[0029] One embodiment of the process flow will be described with reference to Figures 1 and 3. In step 50, the workpiece is assembled to other components of the piping system, so that the workpiece does not need to be heated after the interior coating process is completed. Thus, all welding steps associated with the workpiece are completed to provide coating material inside the workpiece. As previously noted, although the workpiece has been shown as a single tube, it can be a set of tubes or components. What's more, the workpiece can have

- 8 a certain number of tracks, and this does not have to be an uncomplicated workpiece shown with a single inlet and a single outlet.

[0030] In step 52, pre-cleaning takes place. The pre-cleaning may be the introduction of a sputtering gas, such as argon, from the first gas supply container 16. This pre-cleaning can be initiated after pumping to 0.133 Pa (1 x 10 "³ Torr) or, preferably, less than 0.133 Pa (1 x 10 "⁴ Torr). Impurities on the inner surface of the workpiece are blown out when a negative DC pulse is applied from the power source 20.

[0031] In some cases, it is possible to use the optional carbon implantation step 54. Carbon implantation forms a subsurface layer of carbon in the workpiece material, which may be stainless steel. This layer improves adhesion to DLC and other materials. Coal implantation is performed at a higher polarization voltage than other stages of the coating process. The correct polarity is one that exceeds 5 kV. Care should be taken at this stage for small diameter pipes so that the dimensions of the plasma sheath do not become larger than the radius of the workpiece.

[0032] After the optional implantation step 54, in step 56, at least one precursor is introduced into the workpiece 10. Acceptable precursors contain methane, acetylene or toluene. At this stage of the process, the DC pulse voltage is lowered to ensure thin film deposition instead of implantation. The use of DC pulse voltage is shown as step 58 in Fig. 3. During the coating step, argon may be mixed with carbon-containing precursors as indicated in step 60.

[0033] At step 62, the coating parameters are dynamically adjusted during the coating process. The probes provide information that can be used by the computer software regulator 42 and pressure controller 34 to maintain various parameters within tolerance ranges. Thus, the factors that determine the pressure inside the workpiece can be adjusted as needed, or the size of the pulse bias duty cycle can be adjusted as needed.