Patent Application
20090197014
The present invention relates to an apparatus and method for coating diamond on work pieces via hot filament chemical vapor deposition.
Hot filament chemical vapor deposition (“HFCVD”) is often used to coat diamond on a tool such as micro-drill or micro-router. In the HFCVD, a hot filament is used as a heating source for gases dissociation. On contacting the hot filament at 1800 to 2500 degrees Celsius, under tens of torrs, hydrogen or hydrocarbon gas such as methane, acetylene and acetic acid is dissociated into radicals such as hydrogen atoms and methyl. The radicals are chemically active. When the concentration of the methyl and hydrogen (CH4/H2) is about 0.5% to 6%, many active hydrogen atoms form carbon-hydrogen bonds on the diamond film to prevent graphite bonds (carbon-carbon bonds: C=C) from forming on the diamond film. Such graphite bonds would affect the quality of the diamond film. Thus, the diamond film is protected. Moreover, during the deposition of the carbon atoms, the hydrogen atoms react with hydrogen of the carbon-hydrogen bonds and form hydrogen gas so that the carbon atoms on the surface would become dangling bonds. When the methyl comes near, sp3 complex orbits will occur. If the temperature is retained at 800 to 1250 degrees Celsius on the surface of the tool, the diamond film will be formed smoothly.
However, because the diamond bonds are strong, when the carbon film is grown on the tool, the adhesion of the carbon film to the tool is weak. Moreover, because the hardness of the diamond film is high, and the thermal expansion coefficient is low, when the temperature returns to the room temperature from the high temperature for growing the diamond film on the tool, there will be intensive stress in the diamond film. When the diamond film is thicker the adhesion is worse. Therefore, the diamond film could easily be peeled from the tool.
According to U.S. Pat. No. 5,833,753, there is disclosed an array of hot filaments for growing diamond of a large area on a tool. It is however difficult to uniformly deposit diamond on a rough surface.
According to EPO Patent No. 254312 and U.S. Pat. No. 6,200,652, biases are provided between grid, a filament and a base to generate a plasma above the base to help diamond grow on the tool surface. It is however difficult to uniformly deposit diamond on a rough surface. Moreover, these techniques are not designed for metallic tools. The bonding of the diamond film to the tool cannot tolerate high-speed rotation although the double biases independently applied on filament and substrate is beneficial for the nucleation on a silicon substrate or a quartz tool. Therefore, the diamond film could easily be peeled from the tool.
Industrial micro-routers and micro-drills of diameter 3 mm to 0.1 mm are made of tungsten carbide with a micro-hardness of 2500 HV suitable for processing printed circuit boards. However, it is difficult to form diamond nuclides on such tool so that the number of the diamond nuclides is low and that the bonding of the coated diamond film to such tool is weak and the diamond film could easily be peeled from tool surface. Plasma could be used to help the forming of the diamond nuclides to enhance the bonding of the diamond nuclides to such tools. However, the particles in such plasma are highly energetic and could twist the diamond bonds or form graphite bonds by hitting the tool. Therefore, the diamond bonds would be stressed or replaced with the graphite bonds. That is, the bonding of the diamond film to such tools would be weak. Coated with diamond film, the life time of routers of diameter more than 3 mm could be extended for several times. However, a router, diameter smaller than 3 mm, could easily be broken on contacting a work piece in high speed machining. The cutting rate and tribology are obviously important as w ell as the bonding of the diamond films to the tools.
The present invention is therefore intended to obviate or at least alleviate the problems encountered in prior art.
The primary objective of the present invention is to provide an apparatus for firmly coated diamond on work pieces via hot filament chemical vapor deposition.
To achieve the foregoing objective, the apparatus includes a chamber. The chamber is water-cooled and the pressure is controllable therein. A valve is provided on the chamber. A pump is in connection with the valve for pumping air from the chamber. A pressure gauge is inserted in the chamber. A pressure regulator is connected to the pump valve on one side and connected to the pressure gauge on the other side for controlling the valve based on the reading of the pressure gauge. A grid includes vents defined therein. The grid is mechanically disposed in but electrically isolated from the chamber. A grid-bias power supply applies a bias to the grid for generating plasma. A workpiece holder includes rows of vents defined therein and rows of apertures defined therein for holding the work pieces. The rows of vents and the rows of apertures are arranged alternately. The holder is disposed in but isolated from the chamber. A holder-bias power supply provides a bias to the holder for generating the plasma. Stationary holding elements are mechanically disposed in but electrically isolated from the chamber. Movable holding elements are disposed in the chamber. Filaments are arranged in two tiers between the grid and the r holder. Each of the tiers includes rows. Each row of each tier of the filaments is supported by and between a related stationary holding element and a related movable holding element. A tension controller includes an end mechanically connected to but electrically isolated from a related movable holding element and another end connected to the chamber so that each tension controller is operable to horizontally move the related row of each tier of the filaments. A filament power supply energizes the filaments to heat up. A programmable temperature controller controls the filament power supply to make the filaments work at different temperatures in different periods. A piping extends in the chamber and includes tapering vents for spraying reaction gas upwards so that the reaction gas uniformly falls and is finally pumped out of the chamber by the pump. Different temperatures of workpiece can be controlled in nucleation and growth process of diamond film for enhancing adhesion and prolonging their life time.
Other objectives, advantages and features of the present invention will become apparent from the following description referring to the attached drawings.
The present invention will be described via detailed illustration of the preferred embodiment referring to the drawings.
Referring to
The chamber 11 is water-cooled. The pressure is controllable in the chamber 11. The chamber 11 is connected to the pump 12 via a pipe. The pump 12 pumps gas out of the chamber 11 so that the pressure is reduced to several torrs in the chamber 11.
The pressure controller 13 is connected to a pressure gauge 3 on one side and connected to a valve 2 on the other side. The valve 2 is provided on the chamber 11 so that gas travels from the chamber 11 to the valve 2. The pressure gauge 3 is partially inserted in the vacuum 11. Based on the reading of the pressure gauge 3, the pressure controller 13 can adjust the valve 2 to control chamber pressure.
Referring to
The grid-bias power supply 15 is connected to the grid 14. The grid-bias power supply 15 provides a positive bias to the grid 14 to generate plasma. The positive bias is tens to hundreds of volts. The grid-bias power supply 15 may provide a direct current or direct current pulses.
Referring to
The holder-bias power supply 17 is connected to the holder 16. The holder-bias power supply 17 provides a negative bias to the holder 16 to generate plasma. The negative bias is negative tens of volts to lower than negative one hundred volts. The holder-bias power supply 17 may provide a direct current or pulsed direct current.
The filaments 18 may be made of tungsten, molybdenum, tantalum or any alloy of these metals so that the filaments 18 can be used to coat diamond on the work pieces 4 without having to be carbonized beforehand. The filaments 18 are arranged in two tiers each including rows. The distance between any two adjacent rows of the filaments 18 is equal to the distance between any two adjacent rows of the apertures 162. The distance is 1 to 2 cm. The tiers are located between the grid 14 and the holder 16. The distance between the upper tier and the grid 14 is 0.5 to 2 cm. The distance between the lower tier and the work pieces 4 is 0.3 to 1 cm. The distances are determined based on the sizes of the work pieces 4.
Each row of the upper tier of the filaments 18 and a related row of the lower tier of the filaments 18 are supported by and between two holding elements 181a and 181b. The holding elements 181a and 182b are made of molybdenum or any other metal of a high melting point. Each holding element 181a is up ported on a post 182 that is secured to the internal side of the chamber 11. The posts 182 may be replaced with mounts like the mounts 142. An isolating cap 183 is provided between each holding element 181a and a related post 182, thus isolating the filaments 18 from the chamber 11. Each holding element 181b is connected to a tension controller 184 attached to the chamber 11. The tension controllers 184 are operable to move the filaments 18 horizontally. There are grooves 185 each for receiving and guiding a related holding element 181b. The tension controller 184 may be embodied as a weight, a spring or a hydraulic cylinder.
The filament power supply 19 is connected to the filaments 18. The filament power supply 19 energizes the filaments 18 to heat up to 1800 to 2500 degrees Celsius.
The programmable temperature controller 20 is used together with at least one thermometer 201. The thermometer 201 is inserted through a selected one of the apertures 162. The thermometer 201 measures the temperature of an adjacent work piece 4 and accordingly sends a signal to the programmable temperature controller 20. The thermometer 201 may be an infrared thermometer. Based on the signal, the programmable temperature controller 20 sends a signal of 0 to 10 V or 4 to 20 mA to the filament power supply 19. A program controls the filament power supply 19 to heat the filaments 18 and then the work pieces 4 to different temperatures in different periods. The programmable temperature controller 20 may be a programmable logic controller or a computer so that it is not affected by plasma bombardment.
Preferably, many thermometers 201 are connected to the programmable temperature controller 20. Each thermometer 201 is located near a related work piece 4. The programmable temperature controller 20 may be used for programmable control over temperature differentials. Hence, if a filament 18 with a thermocouple nearby is broken, it can be replaced with at least one adjacent filament 18 with the other thermocouple nearby.
The piping 21 includes a trunk and branches extended from the trunk. Each of the branches includes vents 211 defined therein. Each of the vents 211 tapers while extending from the axis of the related branch. The vents 211 spray reaction gas upwards. Then, the reaction gas uniformly falls. Finally, the pump 12 pumps the reaction gas out of the chamber 11. The piping 21 may be T-shaped, cruciform or multi-directional for example. The reaction gas includes hydrogen, methane, acetylene, ethane, benzene and alcohol for example.
Referring to
At 52, the workpieces 4 are inserted in the apertures 162 of the carrier 16 of the apparatus 1. The pump 12 reduces the pressure to a desired value in the chamber 11. From the vents 211 of the piping 21, gas that contains 2% of methane/hydrogen is introduced into the chamber 11 so that the pressure is 20 torrs in the chamber 11.
A first stage of the coating process lasts for 30 minutes under the control of the programmable temperature controller 20. In the first stage of the coating process, the filament power supply 19 energizes the filaments 18 to heat the workpieces 4 so that the temperature of the work pieces 4 is raised to 750 to 800 degrees Celsius.
In a second stage of the coating process, a positive bias of 100 V is exerted on the grid 14 while a negative bias of −60 V is exerted on the holder 16. The work pieces 4 are cleaned in this environment of hydrogen plasma for 10 to 30 minutes so that impurity is removed from the work pieces 4, i.e., the work pieces 4 are cleaned. At the same time, the temperature of the work pieces 4 is increased to 900 to 980 degrees Celsius from 750 to 800 degrees Celsius.
At 53, in a third stage of the coating process, the grid 14 is provided with a positive bias of 30 to 200 V while the holder 16 is provided with a negative bias of −30 to −150 V. The temperature of the work pieces 4 is retained at 900 to 980 degrees Celsius. Into the chamber 11 is introduced other gas including 0.5% to 4% of methane/hydrogen so that the pressure reaches 20 torrs in the chamber. With the power supplies 15 and 17, discharge occurs between the grid 14 and the filaments 18, which are grounded, to generate plasma. When the filaments 18 emit electrons to the grid 14 at high temperature, the electrons partially ionized hydrogen, methane and these radicals. These ions are attracted by work pieces and impinged on then which are provided with the negative bias. These ions help and enhance the production of diamond includes on the work pieces 4. After 3 minutes to 3 hours, the power supplies 15 and 17 are turned off, the temperature of the work pieces 4 drops to 800 to 880 degrees Celsius. The methane may be replaced with acetylene, ethane, benzene and/or alcohol.
At 54, in a fourth stage of the coating process, the grid 14 is grounded while the work pieces 4 are grounded or floating. The temperature of the work pieces 4 drops gradually to 780 degrees Celsius from 800 to 880 degrees Celsius. Gas including 0.5% to 4% of methane is introduced into the chamber 11 so that the pressure becomes 10 to 50 torrs in the chamber 11. By reducing the temperature steadily, i.e., in a gradient, re-composition occurs and deposit diamond of 1 to 10 micrometers on the work pieces 4. The time required for growing the diamond film is 5 to 20 hours. After the growth the supply of the methane is stopped. In tens of minutes, the temperature of the work pieces 4 is reduced to the room temperature so that the work pieces 4 can be removed from the apparatus. The methane may be rep laced with acetylene, ethane, benzene and/or alcohol.
The temperature of the work pieces 4 can be retained at 950 to 1000 degrees Celsius for a period of time for the diamond film growing and reduced to and retained at 780 to 830 degrees Celsius for another period of time when the diamond film continues to grow.
As discussed above, the temperature of the work pieces 4 does not exceed 1000 degrees Celsius. With the special pre-processing and the plasma-enhanced deposition, the structure of the resultant diamond is similar to that of natural diamond. The tribology and adhesion are increased so that the usage and industrial applicability of the diamond film are increased. The stress of the diamond film is reduced. The friction on the work pieces 4 is reduced. Therefore, the work pieces 4 can be used to process printed circuit boards at high rotational speeds without the risk of breach. The life of a tungsten carbide router coated with diamond of 5 to 8 micrometers is more than 6 times as long as that of a tungsten carbide router without diamond film coating.
The present invention has been described via the detailed illustration of the preferred embodiment. Those skilled in the art can derive variations from the preferred embodiment without departing from the scope of the present invention. Therefore, the preferred embodiment shall not limit the scope of the present invention defined in the claims.
