Patent Application
20240412952
This invention relates generally to methods and systems for reliably delivering a controlled gas flow rate across a high voltage differential from inlet to outlet, such as in plasma ion sources, and to optionally return gas separately or simultaneously from vacuum pressures and high voltage to a pressure sensor at ground potential.
The delivery of a gas to equipment operating at high voltages can create possible high voltage breakdown and arcing through the gas line where the gas moves from a ground potential at the input to the gas line to a high potential at the output of the gas line to the equipment. An exemplary device where this can become a critical issue is with focused ion beam (FIB) tools. Focused ion beam (FIB) tools are used for nanometer scale precision material removal.
The benefits of FIB tools include nano-scale beam placement accuracy, a combined imaging and patterning system for accurate sample registration and pattern placement, and low structural damage of the area surrounding the removed volume. However, conventional FIB systems typically have a maximum removal rate of ˜5 μm3/s that limits their usefulness for removing volumes with dimensions exceeding 10 μm. Conventional FIB systems are further limited by the low angular intensity of the ion source, so at large beam currents the beam size dramatically increases from spherical aberration. For high beam currents and hence high removal rates of material, a high angular intensity is required, along with high brightness and low energy spread.
Focused ion beams are often referred to as ‘primary’ ion beams when used on secondary ion mass spectrometer (SIMS) systems. Here the term ‘primary’ is used to differentiate it from the secondary ion beam. As with FIB tools, SIMS tools use the focused primary ion beam for precise, sputter removal of atoms and molecules from a material surface. SIMS tools are primarily used to determine the spatial distribution of chemical constituents in the near surface region of a material. Oxygen primary ion beams are beneficial to enhance the yield of positively charged secondary ions from the sample material, and hence enhance the sensitivity of the SIMS measurement. Negatively charged oxygen ion beams, not only enhance the yield of positively charged secondary ions, but also result in minimal sample charge buildup when the sample is a dielectric material. The state-of-the-art SIMS instruments employ duoplasmatron ion sources to produce negative oxygen primary ion beams. Duoplasmatrons have insufficient brightness (˜40 Am−2sr−1V−1) and lifetime to produce the spatial resolution required for many SIMS applications. Duoplasmatrons also produce ion beams with a relatively large axial energy spread (˜15 eV), which is also problematic when endeavoring to produce high spatial resolution focused primary ion beams.
One solution is to use an inductively coupled plasma ion source. Inductively coupled plasma ion sources typically wrap an RF antenna about a plasma chamber. Energy is transferred by inductively coupling power from the antenna into the plasma. A RF power supply with 50 Ohm output impedance utilizes an impedance matching network so that the output of the RF supply can be efficiently coupled to the plasma which has an impedance substantially different than 50 ohms. To extract negative ions from the ion source, it is typically necessary to use a transverse magnetic field near the source aperture to modify the plasma to allow negative ions to leave the plasma and to separate unwanted electrons that are extracted with the negative ion beam.
Other applications for this type of plasma source include its use as the primary ion source for Ion Scattering Spectroscopy (ISS), focused and projection ion beam lithography, proton therapy, and high energy particle accelerators.
In all cases, a high-power density is deposited into the plasma from the antenna in order to create a high-density plasma, and the plasma is biased to a potential of a hundred Volts or more with respect to ground. An ion beam is extracted from a small aperture and is accelerated through a bias voltage to produce a fine beam of energetic ions.
One issue in the design of plasma ion sources is how to create an optimum magnetic field near the extraction aperture while a high voltage bias is applied to the plasma chamber. A second issue is how to provide radio frequency power to the antenna when the effective impedance of the plasma is very different than common RF power supply output impedances. This is especially problematic because the effective impedance of the plasma varies with plasma chamber gas pressure, gas composition, and the change of state during plasma ignition.
A third problem is how to introduce gas into a reactor chamber that is biased to a high voltage without having high voltage breakdown through the input gas line. It is this third problem that is the focus of the present invention.
High voltage (HV) breakdown in a gas feed line results when a conducting cascade of ionized gas forms. This results when individual charged particles cause impact ionization and photo ionization of neutral gas molecules often enough to sustain ionization, and progressively farther along the gas column until the full path length from HV to ground is bridged. HV breakdown is thus caused by both ionization by gas ions and by secondary electrons. Secondary electrons are produced by both the impact of ions onto the isolator walls, and from ionization of gas molecules, and by photons (primarily UV and vacuum UV). These photons are produced by the neutralization of ions impacting the isolator walls, and which in turn generate secondary electrons when these photons impact the isolator walls.
Previously known systems used both a high voltage isolator and a separate flow control device (e.g., a piezoelectric leak valve). In most valved cases, the valve must be located at the full beam voltage end of the HV isolator to maintain high pressure through the isolator, thus keeping the isolator's gas passage out of the middle pressure area of the Paschen curves dictating breakdown voltage versus line pressure. The control of a valve must also then be isolated across the high voltage gap. This is either an insulating control shaft for a mechanical valve, or a transformer isolation circuit for a piezoelectric valve. But this solution has drawbacks of added complexity and problems with gas flow stability and limited control of gas flow.
Accordingly, the need arises for new designs and methods for providing a high performance, reliable, and easily manufactured way to deliver a controlled gas flow rate at common pressures across a high voltage differential from inlet to outlet within environments having high potential differentials such as plasma ion sources (where doing so would improve the stability of ion current output). A further need exists for such designs and methods that can separately or simultaneously return gas from vacuum pressures and high voltage to a pressure sensor at ground potential.
A gas delivery conduit configured to deliver gas to a device comprises an input end maintained at a first potential into which a gas at a regulated pressure is delivered from a source, a pressure regulator coupled to the input end for delivering a regulated flow of gas to the conduit, and an output end maintained at a second potential through which the gas is delivered to the device, with a direction of gas flow moving through the conduit from the input end to the output end, wherein a gas flow rate at the output end is dependent upon the regulated pressure delivered at the input end. The potential difference between the first potential and second potential forms an electric field. A first plurality of electrically nonconductive conduit windings is disposed between the input end and output end and arranged such that the electric field running between the input end and output end runs substantially perpendicularly across the plurality of conduit windings and the direction of gas flow through the conduit.
In another aspect of the invention, a gas delivery conduit comprises a spool having upper and lower ends, with outer annular portions of the spool maintained at a first potential and inner annular portions of the spool maintained at a second potential, different from the first potential, such that a radial electric field exists between the first and second potentials. The conduit further includes an elongate microcapillary arranged in a spiral having a plurality of windings interposed between the upper and lower ends of the spool, with one of an input end or output end at the first potential and the other of the input end or output end at the second potential. The microcapillary is configured to transfer a gas along the microcapillary in a flow direction substantially perpendicular to the electric field.
In yet another aspect of the invention, a method is disclosed where gas is flowed within a gas line between input and output ends having a large difference in potential to mitigate possible high voltage breakdown and arcing through the gas line. The method comprises the steps of arranging a plurality of adjacent windings of the gas line such that a cross-section of the gas line is substantially perpendicularly to an electric field created by the difference in potential between the input and output ends. Gas is then flowed along the gas line substantially perpendicular to the electric field. The arrangement can either be in a flat spiral, a three-dimensional spiral formed about a conical structure, or a cylindrical spiral formed about a cylindrical structure.
The arrangement of the gas line running perpendicular to the electric field created between the input and output ends mitigates electric breakdown or arcing since the potential difference across the microcapillary walls is relatively small compared to the potential difference that would occur between one end and another of a gas line if the conduit were more aligned in the direction of the electric field. A curvature of the microcapillary further prevents any emission from moving far in a straight line around the bend in the gas line. The more windings, the less the potential difference between opposing walls of the microcapillary and a reduced chance of breakdown or arcing within the gas line.
These spool shapes allow the electrostatic field to be smoothly and slowly tapered between the ends of the microcapillary, which is one factor in obtaining the maximum permissible beam voltage. This is particularly important where the conduit is coupled to a plasma chamber that is isolated from ground for processing semiconductors or supplying a source of charged particles to a substrate. The spool shape also puts the great majority of the electric field vector perpendicular to the direction of the tube's internal gas passage, as is nearly so with a large, short tubing spiral. In the case of the flat spool, inner (HV) and outer (ground) electrodes are coated onto insulating material. In the case of a cone-like spool, the cylindrical outer metal shell of the system and inner metal gas outlet fitting are the electrodes, and the cone and electrodes can be shaped to produce an ideal slope to the electric field over the capillary length.
At the same time, the long microcapillary is ideal for controlling gas flow, having the flow stability of a shorter capillary versus temperature, manufacturing precision, and aging. But it also allows a larger inside diameter (ID) for the required flow restriction. Where a short capillary requires few-micron or smaller internal diameters, a long capillary must use a larger ID, reducing its sensitivity to contamination by particles or adsorbed liquids, etc.
The foregoing and other objects, features and advantages of the invention will become more readily apparent from the following detailed description of a preferred embodiment of the invention that proceeds with reference to the accompanying drawings.
Preferably, the tubing used for the gas line within flat spiral 24 is of 0.5 meter or greater length and made of a flexible insulating material such as 0.1 to 3 mm outside-diameter (OD) fused silica and polyimide “microcapillary.” The microcapillary is wound into a spiral form, such as on a spool formed of an insulative materials to pack this length in a compact form. A preferable insulative material used throughout various parts of the delivery system is a polyether ether ketone (PEEK), which exhibits the following properties: high dielectric strength, good dimensional stability, and strength at up to 150° C. Alternate materials that can be used include glass, quartz, and polycarbonate or other structurally rigid insulators.
The tubing inside diameter (ID) is sized to provide the desired gas flow rate with an inlet pressure in an easily provided range. In application to plasma ion sources this ID could range from 10-200 μm, with about a 75 μm fitting most preferred. In a more specific implementation, a length of about 4-10 meters of 75 μm ID tubing is used for an inlet pressure range of 0.1 to 3 atmospheres absolute over the variety of gasses and flow rates desired.
A key point to this design is that using a long tube of the correct ID results in an ID that is larger than the flow-controlling diameter (aperture or capillary) of any alternative implementation using a conventional short tube and handles both the flow control/restriction task and high voltage isolation in a single component while providing better resistance to clogging from contaminants.
If a spool shape is used, an insulating material such as PEEK provides a form for a single layer pack of tubing, ideally spiraling on a spiraling path having a smaller diameter at the high voltage end of the spool to a larger spiraling diameter path at the ground potential end. While the preferred implementation is a flat pancake spool with a gap only wide enough to permit one layer of tubing to stack into a spiral as shown in
In the present preferred implementation, there are at least five windings of the flat spiral 24 and more preferably at least ten windings, with the most preferred implementation being approximately 4-10 meters of 360 μm OD of polyimide coated, fused silica tubing wound about 20-50 turns into a spool roughly 8 cm in diameter. The windings are preferably immediately adjacent to one another, although spacing between adjacent windings is possible despite potentially causing an increased potential difference between adjacent outer surfaces of the microcapillaries. This spool, and particularly flat spiral 24, is potted in a high voltage insulating compound 44 for use in air, or perforated and immersed in a dielectric liquid such as Galden™ Removable fittings 10, 22 and 36 connect this tubing to the low voltage outer end (gas supply or transducer) and to the high voltage (vacuum) inner end, respectively.
This potted implementation of the capillary spool puts the spool and inlet and outlet fittings in air, rather than in the Galden™ insulating fluid used by ion sources. This allows disassembly/assembly of the system without wetting the gas fittings with the fluid. To do this, the gas outlet to the central metal hub 40 (and thence to a FIB chamber) passes through a PEEK barrier plate 37, sealed with O-rings. The micro-capillary spool 26—sitting upon hub 40 and floating above barrier plate 37 to create a rising and falling airgap therebetween having an extended pathway—has surfaces shaped into concentric rings much like high voltage insulators on power lines (e.g., crenellation structures 50 and 80 described below). This increases the path length along the surface of the spool for high voltage conduction and helps mitigate arcing and corona discharge. This increases the maximum operating voltage before electrical breakdown (e.g., small or large failures). Additionally, these rings are sandwiched between similar, interlocking rings on the fluid barrier plate 37 below and a top hat cap (“shield”) 18 above, which can be interposed between the capillary and the dielectric cooling fluid. This also causes the direct (through air) arc path length to be increased nearly as much as the surface path length is.
While the μCHI spool is capable of being immersed in dielectric fluid, it can also be located in air with attention to normal HV design issues. This allows the gas fittings to be out of the coolant, allowing the spool and gas fittings to be changed without risk of gas system contamination.
Top half 28 of spool is configured as a circular disk with a series of concentric annular ridges or crenellations 46 disposed on a top surface 48 thereof. These crenellations 46 mate with complimentary crenellation structures 50 formed on an underside of the device framework 16, with a narrow rising and falling airgap 52 therebetween as shown in
A series of metallic screws 56 extend through apertures 58 formed in the spool top half 28 and couple the top half 28 together with the spool bottom half 30, with the microcapillary flat spiral 34 and potting material 44, captured between them to form the spool assembly 26. As will be explained further below, the metallic screws 56 electrically couple inner conductively coated annular portions 62 (
The potential difference between the inner ring 62, 64 and the outer ring 72, 74 causes a radial electric field 77 passing from the area of high potential (the microcapillary output 34 and inner ring 62, 64) to the area of lower (e.g., ground) potential (the microcapillary input 32 and outer ring 72, 74). Although the microcapillary flat spiral 24 is preferably embedded in the non-conductive central ring portion 76 between these two sets of conductive inner and outer rings, the electric field 77 will still pass through the windings of the microcapillary and affect the behavior of the gas passing through it.
It is desirable to control the electric fields through the tubing length deterministically and to prevent a high voltage differential from appearing across the short tubing lengths extending from the spool to the inlet and outlet fittings. To do this, conductive surfaces are used on the ends of the spool, connected to the High Voltage and ground potentials, respectively. This is not necessary but is expected to be beneficial in the maximum voltage that can be applied reliably without high voltage breakdown. Painted on carbon coatings are used presently for this but it could be done with metal components or layers. For the pancake spool this means an inside radius of the spool is coated conductively and connected to the high voltage, and an outside radius of the spool is coated and connected to ground.
The μCHI spool can be wound with two microcapillary tubes (in interlaced spirals), both connected to the outlet of the gas. The second tube can return to a gas pressure sensor at ground potential, allowing direct measurement of the gas outlet pressure. This has heretofore been only unreliably inferred from a downstream pressure in a vacuum system which is affected by several other factors and measured by notoriously inaccurate high vacuum sensors, such as cold cathode gauges. The capillary returned pressure is much higher, is not ionized, is at room temperature and has fewer things to cause composition changes. It could be measured by a capacitive manometer sensor, making the reading gas composition independent.
The easily controlled flow resulting from μCHI's extreme stability, monotonicity (no control reversals), resolution (no stick-slip behavior) and repeatability should make automatic control of the optimum plasma source chamber pressure more practical, which would be a great benefit for users of ion beam sources.
The longer length and small OD results in better HV isolation (higher maximum voltage) for several reasons. The tube ID being smaller than the gas spaces within other HV isolator designs, and the large number of turns, results in a lower voltage step across the gas (e-field direction, across the ID). Thus, lower energy is imparted on charged particles 98 which accelerate across the space before impact with the wall. This results in fewer and/or lower energy secondary charged particles 100 and photons 102 from those charges to start a conduction cascade. Smaller internal diameter than alternative HV isolators also results in a shorter distance along the gas passage before a secondary photon intercepts a wall.
That is,
The μCHI design requires no valve, and as a result the flow rate is affected by the inlet pressure, gas composition, and temperature, and essentially nothing else. The fused silica tubing used is extremely stable, long lived and has a low thermal expansion coefficient. The problem of adjusting the flow rate is “exported” to the ground potential end (inlet side), where the pressure can be controlled by feedback from a pressure or flow sensor. This can be a commercial electronic pressure regulator or mass flow controller, of which there are many to choose from. The physics of gas flow through a capillary is inherently not affected much by temperature because of balanced effects on viscosity, density, and speed of sound. The calculated and measured temperature coefficient of flow rate is roughly 0.2%/C. The net result is a maximized gas flow stability, monotonicity, resolution, and repeatability.
Having described and illustrated the principles of the invention in a preferred embodiment thereof, it should be apparent that the invention can be modified in arrangement and detail without departing from such principles. We claim all modifications and variation coming within the spirit and scope of the following claims.
