ENHANCED CATHODIC ARC SOURCE FOR ARC PLASMA DEPOSITION

Abstract

An improved cathodic arc source and method of DLC film deposition with a carbon containing directional-jet plasma flow produced inside of cylindrical graphite cavity with depth of the cavity approximately equal to the cathode diameter. The generated carbon plasma expands through the orifice into ambient vacuum resulting in plasma flow strong self-constriction. The method represents a repetitive process that includes two steps: the described above plasma generation/deposition step that alternates with a recovery step. This step provides periodical removal of excessive amount of carbon accumulated on the cavity wall by motion of the cathode rod inside of the cavity in direction of the orifice. The cathode rod protrudes above the orifice, and moves back to the initial cathode tip position. The said steps periodically can be reproduced until the film with target thickness is deposited. Technical advantages include the film hardness, density, and transparency improvement, high reproducibility, long duration operation, and particulate reduction.

Claims

1. A method of diamond-like carbon film deposition comprising: producing a carbon containing directional jet plasma flow with a cylindrical cathodic arc source, the cylindrical cathodic arc source comprising: a cylindrical graphite cathode and an anode formed from of a plurality of spaced baffles, wherein the cylindrical graphite cathode rod and the anode are separated by a shield, the shield further including an insulator tube with thin wall graphite bushing inside of the tube; a bent solenoidal magnetic filter following the cathodic arc source; a graphite cavity formed by extending the shield from the cathode top surface, creating a semi-confined space with a cavity orifice shaped identically to the cathode top surface.

2. A method of diamond-like carbon film deposition comprising: generating carbon arc plasma in the cavity; directing expansion of the plasma flow in form of macro-jet into ambient vacuum; moving the cathode rod inside of the shield in direction of the orifice protruding above the orifice, and back to the initial cathode tip position; and periodically reproducing the method until a film with target thickness is deposited.

3. The method of claim 1, wherein an arc discharge current of the cathodic arc source is higher than 600 A.

4. The method of claim 1, wherein the cavity orifice defines a diameter of approximately 5 mm to approximately 12 mm.

5. The method of claim 4, wherein the anode defines a diameter that is approximately equal to the diameter of the cavity orifice.

6. The method of claim 1, wherein the anode length does not exceed five times a diameter of the cavity orifice.

7. The method of claim 1, wherein the diameter of the bent solenoidal magnetic filter is approximately two to four times a diameter of the cavity orifice.

8. The method of claim 1, wherein the magnetic field strength inside of the bent solenoidal magnetic filter is approximately 1.5 to approximately 4 times the magnetic field strength sufficient to magnetize electrons.

9. The method of claim 1, wherein the magnetic field strength in the central area of the bent solenoidal magnetic filter ranges between approximately 400 Gauss and approximately 1200 Gauss.

10. The method of claim 8, wherein a current in the bent solenoidal magnetic filter solenoid is between approximately 400 Amps and approximately 800 Amps.

11. The method of claim 1, wherein the cathodic arc source and the bent solenoidal magnetic filter are operated in a pulsed mode, and an arc pulse starts after a filter coil current pulse began, and ends before the filter coil current pulse ends.

12. An apparatus for generation of directional carbon containing plasma flow in a cathodic arc source comprising: a cylindrical graphite cathode and an anode formed from of a plurality of spaced baffles, wherein the cylindrical graphite cathode rod and the anode are separated by a shield, the shield further including an insulator tube with thin wall graphite bushing inside of the tube; a bent solenoidal magnetic filter following the cathodic arc source; and a graphite cavity formed by extending the shield from the cathode top surface, creating a semi-confined space with cavity orifice shaped identical to the cathode top surface.

13. The apparatus of claim 12, wherein the source comprising motion mechanism of the cathode rod inside of the shield along the graphite rod axis in the direction of the cavity orifice protruding above the cavity orifice to a reference point and back to the initial cathode tip position.

14. The apparatus of claim 12, wherein a reference point for the graphite rod tip protruded position is determined by a laser beam directed in between the baffles of the anode crossing the cathode motion axis and a detector that controls the laser beam indicates a drop of intensity when the graphite rod crosses the laser beam propagation line.

15. The apparatus of claim 12, wherein the source comprising a feedback scheme that passes the detector signal to a controller that controls the cathode motion mechanism and returns the cathode tip to its initial position.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] FIG. 1 is a schematic view of the prior art miniaturized source; electrical connection of cathode-anode unit, and magnetic filter to a cathodic arc power supply.

[0015] FIG. 2 is a cross-section of the prior art miniaturized source with anode formed of a plurality of spaced baffles and open architecture filter.

[0016] FIG. 3A is a schematic view of a source in a disclosed embodiment, and FIG. 3B is a cross-section of the spatial distribution of the arc plasma flow within cathode-anode area with reference of cone angle of plasma flow to the prior art.

[0017] FIG. 4A shows a comparison of plasma flux distribution of cone angle of plasma flow distribution vs. cavity aspect ratio in side view.

[0018] FIGS. 4B and 4C show a comparison of a prior art source and a source in a disclosed embodiment, respectively.

[0019] FIG. 5 shows dependences of deposition rate at the wafer, particles density at the wafer, and graphite rod consumption (per 20 A thick film) vs. the cavity size-aspect ratio of depth to diameter. References to the prior art are shown.

[0020] FIG. 6 shows dependencies of deposition rate vs. the cavity size-aspect ratio of depth to diameter for different arc discharge current. Reduction of arc discharge current below 600 A value results in diminished magnetic self-pinching effect and deposition rate substantial reduction.

[0021] FIG. 7 is a schematic of a device that provides motion and position control of the graphite cathode during recovery stage—cleaning the cavity from excess of the accumulated carbon: motion is provided by linear motor with controller, and laser provides the cathode rod position feedback to the controller. Steps 1-4 of the recovery steps are shown in the figure.

[0022] FIG. 8 shows dependence of deposition rate vs. filter coil current in an embodiment of the disclosure with the source operating in hybrid mode—power to the filter is applied before arc discharge starts and completed after arc discharge ends.

[0023] FIG. 9 is a chart depicting an operating pattern for arc current and filter current according to an embodiment.

[0024] FIGS. 10A and 10B are charts of normalized deposition rate vs. pulses, according to an embodiment.

[0025] FIGS. 11A and 11B depict the improvement in deterioration characteristics of embodiments described herein.

DETAILED DESCRIPTION

[0026] FIG. 1 is a schematic diagram illustrating a miniaturized Pulsed Filter Cathodic Arc (PFCA) source with open architecture filter. As shown in FIG. 1, a miniaturized cathodic arc source 1 has a cathode 2 inside of anode 3. The cathodic arc source 1 can be located close to the entrance to filter 4, e.g. the surface of the cathode 2 is preferably located at a distance of about 0.5-2 times the inner diameter of the filter coil from the entrance. Cathodic arc source 1 may also include a focusing or injection solenoid 5 to increase the plasma output from the source into the filter. The filter can be formed from of a bent solenoidal coil with an open architecture, often with at least one of the following features: 1) additional field injection or focusing coils at the filter entrance to improve the plasma output from the source into the filter, and 2) coil turns having a flat cross-sections to promote macro-particles reflection out of the filter volume. The plasma from the filter is expended in direction of a substrate 6 where the film deposition occurs. An additional feature shown in the FIG. 1 is that the source 1 and filter 4 can be separated from the substrate 6 by particles blocking wall 7. Wall 7 functions as a macro-particle firewall which separates the macro-particle contaminated source and filter chamber 8 from a clean plasma process chamber 9. An opening 10 in wall 7 through which the filtered plasma can enter chamber 9 can be kept small. Ideally the wall 7 is very close to the end of the filter 4 so all the plasma passes through opening 10.

[0027] As shown in FIG. 2, cathodic arc plasma source 11 can have an anode 12 formed of a plurality of spaced baffles 13 which extend beyond the active cathode surface 14 of cathode 15. In other prior art sources where the anode is a cylindrical housing the cathode assembly, most macro-particles generated at the cathode are ejected at shallow angles with respect to the cathode surface, and either stick to the anode or are reflected back into the plasma. With the open baffle structure of anode 12 of source 11, most macro-particles pass through the gaps between baffles 13 and reflect off baffles 13 out of the plasma stream that enters the filter. Such an anode not only has an electrical function but serves as a pre-filter.

[0028] Source 11 is operated in pulsed mode at relatively high currents, about 1 kA or higher. The high arc current is typically used to operate the magnetic filter. Power consumption and associated heat load can thus be regulated via the arc pulse duty cycle and pulse length rather than arc current which would lower the filter field.

[0029] The cathode surface during operation is represented in prior art embodiments as ideally flush with respect to the end surface of the insulator ceramic around the graphite cathode. Erosion of the cathode surface is ideally counteracted by gradual advancement of the cathode. The gradual advancement can be performed by special feed mechanism coupled to cathode rod 15. In one embodiment, the feed mechanism (manual or motorized) advances the cathode at such speed that sustains the cathode position by compensating graphite erosion.

[0030] A consideration in determining the source performance is the plasma flow angular distribution. Even every arc spot on the cathode surface generates micro jets of plasma perpendicular to the cathode surface; overall plasma flow has conical or rather semispherical shape. (See FIG. 2, having cone angle 16). Probably, this is a result of integration of micro jets into divergent plasma flow caused by time averaging of instant distribution of micro jets, as well as rough cathode surface topology. Divergence of the plasma flow can result in substantial plasma losses due to difficulty to collect the generated plasma into the filter. Further, divergence of the plasma flow can result in excessive deposition of carbon on the anode and filter surfaces leading to degradation of the plasma flow intensity and shape (in turn deposition rate, and uniformity of the film deposited on a substrate). Divergence of the plasma flow can also result in deterioration of the films properties due to the generated ions average moment reduction, and as a result of this decrease of ions energy arriving on the substrate.

[0031] An improved cathodic arc source and method of this source operation is disclosed. A schematic of a disclosed embodiment is shown in FIG. 3A. Certain embodiments include cathode 17 separated from anode 18 by shield 19, which includes insulator tube with thin wall graphite bushing 20 inside of the tube, and the shield is extended from the cathode top surface forming a cavity 21, or semi confined space with outlet orifice shape similar to an exposed top surface of cathode 17, and a depth of cavity 21 approximately equal to a diameter of cathode 17.

[0032] Graphite bushing 20 represents a graphitic thin wall cylindrical insert, or graphite coating inside of the ceramic tube formed by carbon solution with carbon based binder with subsequent annealing.

[0033] Carbon plasma that is generated in the cavity expands through the orifice into ambient vacuum. We have found that several factors including the cavity geometry limit the plasma flux angular distribution 23. FIG. 3B depicts an enlarged view of cathode anode assembly. Confinement of arc discharge in the cavity results in higher heat load. Improvements to enhance cathode unit thermal capacity were made: changing alumina ceramic to beryllium oxide (BeO) ceramic 25 enhanced thermal conductivity 10×; changing cathode 26 diameter from 6 mm to 10 mm, resulting in ˜65% thermal conduction area increase; new anode collet design 27 to reduce interface thermal resistance. Cathode body 28 with coolant flow provides continuous heat removal.

[0034] The plasma flow cone angle is shown in FIG. 4. Conventional systems show an aspect ratio of approximately 0 and a cone angle of about 50-60° (as indicated in the encircled “A” data point, FIG. 4a), while systems as described and claimed herein exhibit a higher aspect ratio of between about 0.5 and 1, with a narrower cone angle of between about 15° and about 30°, in embodiments (as indicated in the encircled “B” data point, FIG. 4a). Optical images of the plasma flow distribution generated by the prior art source (FIG. 4b) and the presently disclosed source (FIG. 4c) are compared in FIG. 4. It is evident from the results that strong constriction of the plasma is observed when plasma was generated in the cavity by the source of the embodiment photographed in FIG. 4(b).

[0035] Performance improvement of the deposition source resulting from strong constriction of the plasma flow is shown in FIGS. 5A-5C. In certain embodiments, the plasma flow strong constriction occurs when the cavity aspect ratio (depth/diameter) is about 1. The following performance improvements over prior art sources were observed: (a) deposition rate more than 2 times increased; (b) particles densities about 15 times reduced; and (c) cathode consumption was reduced about 2 times. If the cavity aspect ratio is higher than approximately 1.2 the plasma can become unstable. Therefore, based on cathode consumption, deposition rate, and particle densities as shown in FIGS. 5A, 5B, and 5C, an ideal aspect ratio is about 1.

[0036] Another parameter that affects the plasma flow constriction is discharge current. In FIG. 6 an example of dependence of deposition rate vs. the cavity aspect ratio for variable discharge current is presented in an embodiment of the disclosure. It is clear from the graph that the strong effect is observed at discharge current higher than about 600 A. At discharge currents 600 A and below the effect of plasma flow constriction and process performance improvement is reduced.

[0037] FIGS. 7A-7D depict a system for producing a carbon containing directional jet plasma flow with a cylindrical cathodic arc source. In one embodiment, a repetitive process is disclosed comprising two steps: the above-described plasma generation/deposition step that alternates with a recovery step. The recovery step provides periodical removal of excessive amount of carbon accumulated on the cavity wall. The recovery step is shown in FIGS. 7A-7D. In the depicted embodiment, the source includes motion mechanism 29 of cathode 30 automatically controlled by a controller. Cathode 30 can be moved forth and back inside of shield 31 (e.g., in FIG. 7A cathode 30 is retracted within shield 31, in FIG. 7B cathode 30 is flush with shield 31). Cathode 30 can also be advanced outside of the shield 31 into the anode baffle area defined by baffles 33. A position of the cathode tip can be determined by detection of a beam 34 produced by laser 35. Laser detector 36 communicates with the controller 37 providing feedback for return of cathode 30 to the initial location. The recovery procedure can include four sub-steps: Sub-step 1: Initial Cathode position after deposition; Sub-step 2: Cathode motion—cleaning of C excess; Sub-step 3: Cathode position calibration; Sub-step 4: Return to the initial position. During these sub-steps the graphite rod is moved along the axis protruding above the orifice, and returned back to the initial position. This motion provides removal of evaporated graphite excess accumulated on the wall of the cavity while the graphite bushing surface still remains intact sustaining arc during the next run. A reproducible process can be sustained for a long time relative to conventional systems without breaking the chamber vacuum.

[0038] The magnetic filter can be operated at different, more optimal mode of operation as a result of “directional” plasma flow expanding from the cavity when compared to conventional systems. As mentioned previously, constriction of the plasma flow leading to directional or a macro jet like shape of the plasma so that most of the plasma is injected into the filter duct entry without need for a strong magnetic lens (by running a 1300-1500 A current in the filter solenoid, or using additional focusing solenoid 5) at the filter entry that collects diverged plasma flux typical for prior art source, such as the one shown with respect to FIG. 1. This allows the filter to operate by using a substantially decreased current through the filter coil (approximately 700 A in one disclosed embodiment vs. 1600 A in the prior art), as shown in FIG. 8. The reduced magnetic field inside of the coil is still strong enough to “magnetize” electrons and transmit plasma through the solenoidal filter, but at the same time prevents partial reflection of the plasma due to magnetic mirror effect. The plasma loss is substantially reduced at the entrance of the filter duct, while efficiency of transport through the filter remains the same. So, overall embodiments described herein are configured for film deposition rates that build up a wafer at a rate of about 2 times that of a conventional system.

[0039] In various embodiments, another improvement is related to utilization of hybrid mode of the source and filter operation shown in FIG. 9. The filter can be turned on before arc discharge starts and turned-off after discharge arc is ended (in prior art arc and filter pulses are typically synchronized). This mode of operation allows for reduced arc disturbance by filter coil current transitions at start/end of filter pulse; and better controls the filter coil thermal conditions.

[0040] In one example of carbon film deposition, the source parameters could be the following: cavity aspect ratio (depth/diameter)=1; cathode diameter=10 mm; double-bent filter; arc discharge current=1800 A; filter current=700 A; length of arc pulse=1 msec; length of filter coil=25 msec; frequency of pulses=5 Hz; and deposition/recovery steps repeating after every 1000 pulses.

TABLE-US-00001 TABLE Data Summary Technology Parameters Disclosure Prior Art Hardness, GPa, 500 A thick film 92* 80* Density, g/cm.sup.3, 200 A thick film   3.3   3.1 Stress, GPa, 200 A thick film  7.5-8.5  5-6 Absorption at λ = 800 nm, 5 * 10.sup.−2 16 * 10.sup.−2 100 (I-I.sub.o/I.sub.o) %, 20 A thick film Relative transport efficiency ~5  ~1  Particles per cm.sup.2, 20 A thick film ~1-2 ~20-25 *Measurements were performed by Hysitron

[0041] The data obtained by the above described embodiment source and prior art source are compared. It is seen from the table that mechanical and optical properties are noticeably improved; particle amounts on the surface of the deposited films are reduced; and losses of the plasma substantially decreased that resulted in plasma transport efficiency raise.

[0042] In another example illustrating the deposition rate reproducibility, an operation can be configured to illustrate long time source operation over the cathode life. In particular, the conditions of operation are as follows: the source parameters are the same as in the previous example; deposition rate is measured after every 10,000-15,000 pulses; and total amount of pulses performed during test was 150,000 pulses.

[0043] FIG. 10A depicts an reproducibility of an embodiment, compared to the significantly lower reproducibility of conventional systems as shown in FIG. 10B. Deposition rate as a function of pulses is substantially stable with a relatively lower variation from the mean in FIG. 9A, whereas normalized deposition rate decreases as a function of pulses in conventional systems and has a relatively higher variation from the trend line.

[0044] Accumulated carbon amount on the anode baffle ring is shown in the figure for a disclosed embodiment and a prior art source as well, in FIGS. 11A and 11B, respectively. Each of these images corresponds to an anode baffle ring after 150,000 pulses.

[0045] The example in FIG. 11A shows that the disclosed source does not degrade over life of the cathode. Rather, deposition rate of the carbon film remains stable while prior art source demonstrated deposition rate drop about 15-20%. Deposition rate reduction during operation as it is seen from anode baffles rings depicted in FIGS. 11A and 11B, respectively, is at least partially a result of much higher parasitic carbon accumulation in the prior art source than in the disclosed source (deposits of 2-3 mm vs. less than 0.4 mm). The accumulated carbon can physically block a portion of the plasma flow causing deposition rate change.

[0046] Overall it should be noted that various embodiments of the disclosed source provide generation and expansion of the constricted plasma flow from the cavity. It results in strong self-magnetic field formation and, in turn, the plasma self-pinching effect. A result of the plasma generation in the cavity are directional plasma flow within very narrow cone angle; plasma heating leading to intensive interaction of plasma and droplets that contributes to plasma density raise as well as graphite droplets/particles disappearance; ions acceleration that leads to higher sp3/sp2 ratio in the films. Disclosed embodiments of the source demonstrated particle reduction, deposition rate increase, reduction of graphite consumption per Angstrom of the deposited film; hardness, density, and transparency increase; and reduced parasitic carbon accumulation inside of the source.

[0047] From the forgoing description, it is clear that the disclosed embodiments provide an improved method and source for a high sp3/sp2 ratio carbon films. Enhanced aspects of the disclosed embodiments can include film hardness, density, and transparency improvement, high reproducibility, long duration operation, and particulate reduction.

[0048] All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, can be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. Each feature disclosed in this specification can be replaced by alternative features serving the same, equivalent, or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

[0049] For purposes of interpreting the claims for the present invention, it is expressly intended that the provisions of Section 112, sixth paragraph of 35 U.S.C. are not to be invoked unless the specific terms “means for” or “step for” are recited in the subject claim.