METHOD FOR FORMING A CARBON FILM

Abstract: A method for forming a carbon film includes the step of pulsing an arc (122) within a vacuum arc deposition apparatus (100) having a graphite source (115). The method includes pulsing a current signal (119) from an arc power supply (116). The current signal (119) is within a range of 100-300 amperes. The current signal (119) is pulsed at a power MOSFET switching circuit (200) to provide a pulsed arc signal (120). Pulsed arc signal (120) has a pulse width within a range of 0.5-2 microseconds and within a range of 20-30 % of the period of the pulsed arc signal (120). The pulsed arc signal (120) also has a rise time that is within a range of 5-20 nanoseconds.

Description: METHOD FOR FORMING A CARBON FILM Field of the Invention The present invention pertains to the area of cathodic arc depositions and, mere particularly, to cathodic arc depositions for the formation of carbon films. Background of the Invention The use of a cathodic vacuum arc deposition method for the formation of carbon films is known in the art. Also known is the use of a cathodic vacuum arc deposition method for the formation of field emissive carbon films. However, carbon films produced according to this prior art method are plagued with a high proportion of macroparticles. The arc creates a very high temperature and pressure environment at the carbon source, These conditions typically cause macroparticles to be expelled from the arc-receiving surface of the carbon source. To form uniform, smooth films it is desired to remove these macroparticles. In one prior art vacuum arc scheme, a filter bend is included in the deposition apparatus for the removal of macroparticles. A graphite source is vaporized and directed toward the filter bend. The filter bend includes an enclosed passageway that is bent and that is surrounded by magnetic coils. A magnetic field is formed within the passageway for directing charged carbon species around the bend. In theory, uncharged particles and heavy macroparticles are unable to be guided around the bend and consequently impinge upon the walls of the enclosure at the filter bend. The unflltered species are deposited onto a substrate to form the carbon film. This filtering scheme does not form carbon films having adequate uniformity and adequately low macroparticulate content. In another prior art vacuum arc scheme, the arc is periodically struck, rather than being continuous. In this scheme, a capacitor is periodically charged and discharged from the anode to the cathode. Each discharge results is striking of an arc. The periods during which the arc is off allows local cooling at the carbon source. By reducing the local temperature, the flux of macroparticles is reduced. However, this scheme suffers from extremely low deposition rates, which are on the order of ten angstroms per minute. The low deposition rates result from the slow charge up rate of the capacitor. Accordingly, there exists a need for an improved vacuum arc deposition method to produce carbon films having a low concentration of macroparticles. There also exists a need for an improved vacuum arc deposition method to produce carbon films having an improved deposition rate. Brief Description of the Drawings FIG. 1 is a simplified schematic illustration of a vacuum arc deposition apparatus useful for practicing the method of the invention; FIG. 2 is a graphical representation of a pulsed current signal in accordance with the invention; and FIG. 3 is a schematic illustration of a switching circuit useful for practicing the method of the invention. It will be appreciated that for simplicity and clarity of illustration, elements shown in the FIGURE have not necessarily been drawn to scale. For example, the dimensions of some of the elements are exaggerated relative to each other. Description of the Preferred Embodiments The invention is for an improved method for forming a carbon film. The invention reduces the content of macroparticles within a carbon film formed by a vacuum arc deposition technique. The reduction in the amount of macroparticles is achieved by reducing the arc life, which reduces local heating at the carbon source. The reduced local heating also improves the charge state distribution of the plasma. This improved plasma has a high degree of ionization and is, therefore, more efficient. A method in accordance with the invention further improves the deposition rates by utilizing a switching circuit that pulses high current at high frequencies, and by providing an arc current signal that has an improved, fast rise time. Illustrated in FIG. 1 is a simplified schematic representation of a vacuum arc deposition apparatus 100 useful for practicing the invention. Vacuum arc deposition apparatus 100 includes a vacuum chamber 110, an arc power supply 116, and a switch 118. Vacuum chamber 110 includes an anode 112 and a cathode 114. A graphite source 115 is provided at cathode 1 14. Graphite source 115 includes a solid piece of graphite, such as Poco-Graphite, Papy ex- Graphite, or the like. A deposition substrate 117 is provided at anode 112. Deposition substrate 117 includes an article, such as a glass wafer, a silicon wafer, and the like, upon which a carbon film is to be deposited. The negative terminal of arc power supply 116 is connected to cathode 114, and the positive terminal of arc power supply 1 16 is connected to an input terminal of switch 118. The operation of vacuum arc deposition apparatus 100 includes, first, initiating (striking) an arc 122, which is indicated by an arrow within vacuum chamber 1 10 in FIG. 1. Anode 112 includes a trigger 121 proximate to graphite source 115 for forming and transmitting arc 122. After arc 122 is initiated, a current signal 119 is generated from arc power supply 116. Current signal 119 is fed to an input terminal of switch 118. Current signal 119 is pulsed at switch 118 to produce a pulsed current signal 120 at the output terminal of switch 118. Pulsed current signal 120 is fed to anode 112. Pulsed current signal 120 provides the current for pulsing arc 122. In this manner a pulsed arc signal is formed at graphite source 115 for producing a carbon plasma. Illustrated in FIG. 2 is a graphical representation of pulsed current signal 120 in accordance with the invention. Pulsed current signal 120 has periodic pulses 123. Each of pulses 123 has a pulse width, PW, which corresponds to the duration of the pulse for currents at and above 70% of the maximum pulse current, i^. Each of pulses 123 also has a rise time, RT, which corresponds to the time required to increase the current signal from a baseline current value, i^, to the maximum pulse current, i max . The period, P, of pulsed current signal 120 is also indicated in FIG. 2. In accordance with the invention, the values of the pulse width and of the ratio of the pulse width to the period are predetermined to provide reduced local heating at graphite source 115. A reduction in the residence time of arc 122 at a site on graphite source 115 reduces the local heat load delivered to that site. This reduced local heating provides the benefit of a reduction in the number, the size, and the width of the size distribution of graphite macroparticles emitted from that site. The reduced local heating also provides the benefit of an improved charge state distribution of the plasma. Further in accordance with the invention a large maximum pulse current and a short rise time are provided for realizing a high deposition rate. The pulse width is within a range of 0.25 - 100 microseconds, preferably within a range of 0.25 - 10 microseconds, and most preferably within a range of 0.5 - 2 microseconds. The pulse width is also predetermined to be within a range of 10 - 50% of the period of pulsed current signal 120, preferably within a range of 20 - 30% of the period of pulsed current signal 120. The short duration of pulses 123 limit the heating time, and the time between pulses 123 allows for heat dissipation at graphite source 115. The ratio of the pulse width, PW, to the period, P, of pulsed current signal 120 is established by the duty cycle of arc power supply 116. The value of maximum pulse current, i ^ ^, is within a range of 20 - 500 amperes, preferably within a range of 100 - 300 amperes. These high current values promote ease of restriking of the arc and also provide a high flux of plasma ions from graphite source 115. The rise time, RT, is less than 100 nanoseconds, preferably less than 50 nanoseconds, and most preferably within a range of 5 - 20 nanoseconds. This fast rise time improves plasma flux during each of pulses 123 and improves the deposition rate of the carbon film at deposition substrate 1 17. Illustrated in FIG. 3 is a power MOSFET switching circuit 200 for implementing the function of switch 118. Power MOSFET switching circuit 200 is capable of providing high frequency pulsing of high current signals. It is further capable of realizing fast rise times. Power MOSFET switching circuit 200 includes an first input terminal 210, to which current signal 119 is fed. Power MOSFET switching circuit 200 further includes a second input terminal 214, to which a drive signal 217 is fed. Drive signal 217 defines the waveform for pulsed current signal 120. Pulsed current signal 120 is transmitted from an output terminal 212 of power MOSFET switching circuit 2 00 . Power MOSFET switching circuit 200 has a predriver transistor 216, which is connected in series with four modules 225. Modules 225 are connected in parallel to one another. For ease of understanding, only one of modules 225 is represented in FIG. 3 within a dashed box. Each of modules 225 includes speed up capacitors 222, current limiting resistors 226, a driver field effect transistor (FET) 228, transient suppression diodes 218, filter capacitors 220, pulldown resistors 224, and output power MOSFETs 230. These components are connected in the manner illustrated in FIG. 3. Output power MOSFETs 230 provide fast switching times and excellent power efficiency. In the embodiment of FIG. 3, each of output power MOSFETs 230 includes a 200 volt FET. While the power MOSFET switching circuit has been described in accordance with the arrangement, those skilled in the art will recognize that other arrangements are possible for practicing the invention. In summary, a method for forming a carbon film in accordance with the invention provides carbon films that have low macroparticulate content and improved uniformity. A method according to the invention further improves the deposition rate of these carbon films by utilizing a switching circuit that pulses high current at high frequency and that provides an arc current signal that has a short rise time. A method according to the invention also improves the ionization characteristics of the plasma by reducing local heating at the graphite source. While I have shown and described specific embodiments of the present invention, further modifications and improvements will occur to those skilled in the art. I desire it to be understood, therefore, that this invention is not limited to the particular forms shown and I intend in the appended claims to cover all modifications that do not depart from the spirit and scope of this invention.

Claims: CLAIMS 1. A method for forming a carbon film comprising the steps of: providing a graphite source (115) within a vacuum arc deposition apparatus (100); forming between the graphite source (115) and an anode (112) of the vacuum arc deposition apparatus (100) an arc (122); and pulsing the arc (122) to define a pulsed arc signal (120) having a pulse width being within a range of 0.25 - 100 microseconds and further being within a range of 10 - 50% of the period of the pulsed arc signal (120). 2. The method for forming a carbon film as claimed in claim 1, wherein the step of pulsing the arc (122) comprises the step of pulsing the arc (122) to define a pulsed arc signal (120) having a pulse width within a range of 0.25 - 10 microseconds. 3. The method for forming a carbon film as claimed in claim 2, wherein the step of pulsing the arc (122) comprises the step of pulsing the arc (122) to define a pulsed arc signal (120) having a pulse width within a range of 0.5 - 2 microseconds. 4. The method for forming a carbon film as claimed in claim 1, wherein the step of pulsing the arc (122) comprises the step of pulsing the arc (122) to provide a pulsed arc signal (120) having a pulse width within a range of 20 - 30% of the period of the pulsed arc signal (120). 5. The method for forming a carbon film as claimed in claim 1 , wherein the step of forming an arc (122) comprises the step of forming an arc (122) having an arc current within a range of 20 - 500 amperes. 6. The method for forming a carbon film as claimed in claim 5, wherein the step of forming an arc (122) comprises the step of forming an arc (122) having an arc current within a range of 100 - 300 amperes. 7. The method for forming a carbon film as claimed in claim 1, wherein the step of pulsing the arc (122) comprises the step of pulsing the arc (122) to define a pulsed arc signal (120) having a rise time of less than 100 nanoseconds. 8. The method for forming a carbon film as claimed in claim 7, wherein the step of pulsing the arc (122) comprises the step of pulsing the arc (122) to define a pulsed arc signal (120) having a rise time of less than 50 nanoseconds. 9. The method for forming a carbon film as claimed in claim 8, wherein the step of pulsing the arc (122) comprises the step of pulsing the arc (122) to define a pulsed arc signal (120) having a rise time within a range of 5 - 20 nanoseconds. 10. The method for forming a carbon film as claimed in claim 1, wherein the step of pulsing the arc (122) comprises the steps of: connecting an input terminal (210) of a power MOSFET switching circuit (200) t- an output terminal of an arc power supply (116); connecting an output terminal (212) of the power MOSFET switching circuit (200) to the anode (112) of the vacuum arc deposition apparatus (100); and pulsing a current signal (119) from the arc power supply (116) at the power MOSFET switching circuit (200) to provide a pulsed current signal (120) to the anode (112). connecting an input terminal (210) of a power MOSFET switching circuit (200) to an output terminal of an arc power supply (116); connecting an output terminal (212) of the power MOSFET switching circuit (200) to the anode (112) of the vacuum arc deposition apparatus (100); and pulsing a current signal (119) from the arc power supply (116) at the power MOSFET switching circuit (200) to provide a pulsed current signal (120) to the anode (112).

Inventors: JOHNSON SCOTT V, JOHNSON, SCOTT, V.
Applicants: MOTOROLA INC, MOTOROLA INC.
Application year: 1998
Main classification: C23C14
IPC codes: C01B31, C23C14, H01J37