Dual discharge modes operation for remote plasma

Abstract

Embodiments of the present technology may include a method of processing a semiconductor substrate. The method may include providing the semiconductor substrate in a processing region. Additionally, the method may include flowing gas through a cavity defined by a powered electrode. The method may further include applying a negative voltage to the powered electrode. Also, the method may include striking a hollow cathode discharge in the cavity to form hollow cathode discharge effluents from the gas. The hollow cathode discharge effluents may then be flowed to the processing region through a plurality of apertures defined by electrically grounded electrode. The method may then include reacting the hollow cathode discharge effluents with the semiconductor substrate in the processing region.

Description

FIELD

The present technology relates to semiconductor systems, processes, and equipment. More specifically, the present technology relates to systems and methods for reducing film contamination and improving device performance.

BACKGROUND

Integrated circuits are made possible by processes which produce intricately patterned material layers on substrate surfaces. Producing patterned material on a substrate requires controlled methods for removal of exposed material. Chemical etching is used for a variety of purposes including transferring a pattern in photoresist into underlying layers, thinning layers, or thinning lateral dimensions of features already present on the surface. Often it is desirable to have an etch process that etches one material faster than another facilitating, for example, a pattern transfer process. Such an etch process is said to be selective to the first material. As a result of the diversity of materials, circuits, and processes, etch processes have been developed with a selectivity towards a variety of materials.

Etch processes may be termed wet or dry based on the materials used in the process. A wet HF etch preferentially removes silicon oxide over other dielectrics and materials. However, wet processes may have difficulty penetrating some constrained trenches and also may sometimes deform the remaining material. Dry etches produced in local plasmas formed within the substrate processing region can penetrate more constrained trenches and exhibit less deformation of delicate remaining structures. However, local plasmas may damage the substrate through the production of electric arcs as they discharge or if a high enough selectivity is not achievable.

Thus, there is a need for improved systems and methods that can be used to produce high quality devices and structures. These and other needs are addressed by the present technology.

SUMMARY

A hollow cathode discharge may be a plasma discharge that increases the electron impact ionization rate at the center of a hollow cathode, a so-called virtual anode, by pendulum electrons emitted from a cathode surface under a particular pressure-diameter of hollow cathode (pD) condition. Generating a hollow cathode discharge for semiconductor processing, and possibly alternating generation of the hollow cathode discharge with generation of a glow discharge in the same processing chamber, may provide advantages in semiconductor processing.

Embodiments of the present technology may include a method of processing a semiconductor substrate. The method may include flowing gas through a cavity defined by a powered electrode. The method may further include applying a negative voltage to the powered electrode. Also, the method may include striking a hollow cathode discharge in the cavity to form hollow cathode discharge effluents from the gas. The hollow cathode discharge effluents may then be flowed to the processing region through a plurality of apertures defined by electrically grounded electrode. The method may then include reacting the hollow cathode discharge effluents with the semiconductor substrate in the processing region.

Embodiments may include a method of processing a semiconductor substrate, where the method may include flowing gas to a processing region. Additionally, the method may include striking a hollow cathode discharge to form hollow cathode discharge effluents from the gas. Furthermore, the method may include reacting hollow cathode discharge effluents with the semiconductor substrate in the processing region. The method may also include striking a glow discharge to form glow discharge effluents from the gas. The method may further include reacting glow discharge effluents with the semiconductor substrate in the processing region.

Embodiments of the present technology may include a system for processing a semiconductor substrate. The system may include a powered electrode electrically coupled with an electronic ballast. Additionally, the system may include a conical cavity defined by the powered electrode, where the conical cavity is characterized by a narrower diameter end and a wider diameter end. The system may further include a power supply electrically coupled with the electronic ballast, where the power supply is configured to deliver a negative voltage at a frequency below about 1 MHz to the powered electrode to strike a hollow cathode discharge in the conical cavity. The system may also include a gas inlet connected to the narrower diameter end of the conical cavity. Furthermore, the system may include an electrically grounded electrode defining a plurality of apertures, where the electrically grounded electrode is disposed closer to the wider diameter end of the conical cavity than the narrower end of the conical cavity. The electrically grounded electrode may have a first surface and a second surface, and both the first surface and the second surface may be substantially flat.

Embodiments of the present technology may provide improvements in semiconductor processing. A hollow cathode discharge may be more stable at higher operating pressures. Higher pressure processing may allow for increased densities of radicals, ions, and other species. Hollow cathode discharges may also increase the distance that electrons travel, and the increased travel distance may increase the probability that an electron would collide with a neutral atom to ionize the atom and generate another electron. The increased densities may result in faster etching of or depositing on a semiconductor substrate. Additionally, possibly as a result of increased densities or different chemistries, processing with a hollow cathode may allow for a more selective process than conventional processing.

The ionization processes in a hollow cathode may result in electrons remaining in a cavity space defined by an electrode and may result in less damage to the electrode, increasing process reliability and equipment uptime. The hollow cathode discharge may increase the amount of electron impact ionization (i.e., alpha process) and decrease the amount of ionization from secondary electrons (i.e., gamma process). The increase in the amount of electron impact ionization may also decrease damage of cathode and anode surfaces. Moreover, operating a plasma in two different discharge modes may permit increased precision, control, and/or flexibility in semiconductor processing. Embodiments of the present technology provide these and other advantages.

BRIEF DESCRIPTION OF THE DRAWINGS

A further understanding of the nature and advantages of the disclosed technology may be realized by reference to the remaining portions of the specification and the drawings.

FIG. 1 is a graphic showing a bipolar oscillating voltage, a cavity defined by a powered electrode, and resulting plasma location according to embodiments.

FIG. 2 is a graphic showing a unipolar oscillating voltage and resulting plasma according to embodiments.

FIG. 3 is a flow chart of a substrate processing method according to embodiments.

FIG. 4 is a flow chart of a substrate processing method according to embodiments.

FIG. 5 shows a substrate processing chamber according to embodiments of the present technology.

FIG. 6 shows a substrate processing chamber according to embodiments of the present technology.

FIG. 7 shows a showerhead of a substrate processing chamber according to embodiments of the present technology.

FIG. 8 shows a substrate processing system according to embodiments of the present technology.

FIG. 9 shows the voltage and current waveforms of plasma discharges according to embodiments.

FIG. 10 shows images of plasma discharges at various pressures for positive and negative applied voltages according to embodiments.

DETAILED DESCRIPTION

Semiconductor patterning may involve patterning an upper layer along with an underlying layer. Semiconductor processing technology may also remove at least a portion of the upper layer while retaining the underlying layer. Conventional semiconductor processing technology may remove at least some of the upper layer but may still affect the underlying layer. The underlying layer may itself be etched partially away, thereby changing the initially patterned profile. Furthermore, the removal of the upper layer may also deposit contaminants on the underlying layer or affect the structural, electrical, or other properties of the underlying layer. Conventional processing may involve lower gas pressures and lower ion, radical, and electron densities. A lower concentration of reactive species may increase time needed for patterning a semiconductor substrate. Furthermore, conventional processing may not confine electrons to a space, and these electrons may collide with and damage electrodes or other portions of the chamber. Damage of electrodes or the chamber may reduce equipment availability and increase costs. Conventional processing may also require additional processing operations or equipment. These conventional methods may detrimentally degrade the performance of the semiconductor device. Embodiments of the present invention may provide improvements in semiconductor patterning technology.

Reference is now made to FIG. 1, which is a graphic showing a bipolar oscillating voltage used to drive a powered electrode and the plasma locations which have been found to occur during the positive and negative swings embodiments. A bipolar sinusoidal voltage is shown and may be symmetric around electrical ground. The bipolar sinusoidal voltage may represent a sinusoidally varying voltage (varying in time) applied to an electrode relative to a grounded electrode having a plurality of apertures. The powered electrode may not be planar and the grounded electrode may be planar according to embodiments. As shown, the plasma has been found to change positions within the remote plasma region between the powered electrode and the grounded electrode. The grounded electrode may be a perforated plate. The bipolar sinusoidal voltage may initially swing positive such that the powered electrode may be positively biased relative to the grounded electrode. During this time, the plasma may be located close to the grounded electrode. When the sinusoidal voltage swings negative, the powered electrode may be negatively biased relative to the grounded electrode and the plasma may switch to a position farther away from the grounded electrode. Other shapes of bipolar oscillating voltages may be used to drive the powered electrode and the locations of the two plasma excitations would be similar.

The non-planar electrode may be referred to as a concave electrode, a powered-electrode, or a hollow cathode when the interior surface is cup-shaped or conical as shown. A concave electrode may form a larger volume near the center of the remote plasma region. A perpendicular distance from the grounded electrode to a concave electrode may be greater near the center of the grounded electrode than a perpendicular distance from the perforated plate to the electrode near the edge. The remote plasma on the left of FIG. 1 may be referred to as “glow discharge mode” and the remote plasma on the right of FIG. 1 may be referred to as “hollow cathode mode.” The concave electrode may be negatively biased during the hollow cathode mode, which may direct ions away from the grounded electrode and may reduce sputtering effects. Visual inspection of both the concave electrode and the grounded electrode has shown less damage to the electrodes under hollow cathode discharge operation than glow discharge operation. The grounded electrode may be coated with nickel in embodiments. A substrate may be disposed below the grounded electrode during processing and substrates have been found to possess more micro-contamination (in the form of elevated particle count) in glow discharge mode relative to hollow cathode mode. The benefit may also arise from the location of the plasma relative to the grounded electrode and may result from the presence of the nickel coating.

FIG. 2 is a graphic showing a unipolar oscillating voltage according to embodiments. A bipolar sinusoidal voltage may be processed to switch the sign of the positive swing, in embodiments, so that the resulting unipolar oscillating voltage may possess two negative swings per period of the original bipolar sinusoidal voltage. The period of the unipolar oscillating voltage may be half the period of the original bipolar sinusoidal voltage. The unipolar oscillating voltage may still be a repetitive oscillating voltage but may not be referred to as a sinusoidal oscillating voltage anymore. The unipolar oscillating voltage may be applied to the concave electrode, and the plasma may predominantly form in the same portion of the remote plasma region during every peak of the unipolar oscillating voltage and not just half of the peaks. The bipolar sinusoidal voltage may be processed by passing the bipolar sinusoidal voltage through a diode bridge formed from any appropriate type of four diodes. High voltage bipolar sinusoidal voltages may be processed using IGBT (insulated gate bipolar transistor) diodes and low voltage bipolar sinusoidal voltages (less than 1-2 kV) may be processed using FET-style diodes. A full bridge rectifying circuit may eliminate the ability of the bipolar oscillating voltage to excite a plasma so only a portion of a rectification operation is performed on the bipolar sinusoidal voltage as shown in FIG. 2. Alternatively, the bipolar sinusoidal voltage may be processed with a more simplistic circuit (involving e.g. a single diode) to create a unipolar oscillating voltage with one negative swing for each period of the original bipolar oscillating voltage. In this case, the unipolar oscillating voltage may possess the same period as the bipolar sinusoidal voltage. Alternatively, the unipolar oscillating voltage may be formed by passing a DC voltage through a full bridge formed from any appropriate type of four switching devices.

Embodiments of the present technology may include a method of processing a semiconductor substrate. As shown in FIG. 3, method 300 may include may include flowing gas through a cavity defined by a powered electrode 302. The gas may include argon or helium, and the gas may include fluorinated or perfluorinated compounds. The gas may include semiconductor processing gases such as NF₃ or NH₃.

The cavity defined by the powered electrode may be conical, cylindrical, substantially conical, or substantially cylindrical. While a planar powered electrode opposite a planar grounded electrode may result in a constant distance between the two electrodes, a powered electrode with a cavity opposite a planar grounded electrode may provide varying distances between the two electrodes. These varying distances may allow for different diameters and different pD conditions.

Method 300 may further include applying a negative voltage to the powered electrode 304. The powered electrode may include nickel-coated aluminum without a dielectric coating. Applying the negative voltage may be through an electronic ballast. The electronic ballast may be a commercially available electronic ballast. The electronic ballast may have a power of between about 50 W and about 100 W, between about 100 W and about 200 W, between about 75 W and about 125 W, or about 60 W in embodiments. The electronic ballast may have a frequency of less than about 1 MHz. The frequency may be less than about 400 kHz, less than about 100 kHz, or less than about 60 kHz in embodiments. Too high a frequency may preclude the generation of a hollow cathode, even during an applied negative voltage. Frequencies less than the plasma frequency, which may be around 1 or 2 MHz, are believed to allow for the generation of the hollow cathode discharge. For example, under certain conditions, a frequency of 13.56 MHz was observed to not result in generation of a hollow cathode discharge during negative swings in voltage.

Also, method 300 may include striking a hollow cathode discharge 306 in the cavity to form hollow cathode discharge effluents from the gas. Striking the hollow cathode discharge may occur at a pressure between about 1.5 Torr and about 10 Torr, between about 5 Torr and about 10 Torr, or between about 1.5 Torr and 5 Torr in embodiments. Pressures may vary depending on the composition of the gas and the pD condition.

In addition to striking a hollow cathode discharge, method 300 may include flowing the hollow cathode discharge effluents 308 to the processing region through a plurality of apertures defined by an electrically grounded electrode. The electrically grounded electrode may have a first surface and a second surface, both of which may be substantially flat or planar when not considering the apertures. The first surface may be the top surface, and the second surface may be the bottom surface. The grounded electrode may include nickel-coated aluminum without a dielectric coating. Method 300 may then include reacting the hollow cathode discharge effluents with the semiconductor substrate 310 in a processing region. The processing region may be separate from a plasma region, where the plasma is generated. The processing region and the plasma region may be divided by the electrically grounded electrode.

Method 300 may further comprise applying a positive voltage to the powered electrode. Applying the positive voltage may result in striking a glow discharge to form glow discharge effluents from the gas. Most of the glow discharge may be between the powered electrode and the electrically grounded electrode and not in the cavity. The glow discharge may be in a position or space different from the position or space of the hollow cathode. The glow discharge may be positioned farther away from the powered electrode and closer to the electrically grounded electrode than the hollow cathode. Method 300 may include striking a glow discharge without removing the semiconductor substrate from the processing region. Alternatively, the semiconductor substrate may be removed from the processing region after striking a hollow cathode discharge. Method 300 may include striking a glow discharge with a different semiconductor substrate in the processing region or the same semiconductor substrate in the processing region after an additional semiconductor processing operation. Accordingly, both hollow cathode discharge processing and glow discharge processing may occur in the same processing region, providing operational flexibility with the processing region.

As depicted in FIG. 4, embodiments may include a method 400 of processing a semiconductor substrate, where method 400 may include flowing gas 402 to the processing region. The gas may be any of the gases previously mentioned. Additionally, method 400 may include striking a hollow cathode discharge 404 to form hollow cathode discharge effluents from the gas. The hollow cathode discharge may be located in a first space in a conical cavity defined by a powered electrode. Striking the hollow cathode discharge may include applying a negative voltage to the powered electrode. Furthermore, method 400 may include reacting hollow cathode discharge effluents with the semiconductor substrate 406 in a processing region.

Method 400 may also include striking a glow discharge 408 to form glow discharge effluents from the gas. The glow discharge may be located in a second space between the powered electrode and an electrically grounded electrode. Striking the glow discharge may include applying a positive voltage to the powered electrode. The second space may be different from the first space. The second space may be located farther away from the powered electrode than the first space, and the second space may be shaped differently than the first space. The second space may be flatter in shape than the first space, while the shape of the first space may be defined more by the shape of the cavity than the second space. Method 400 may further include reacting glow discharge effluents with the semiconductor substrate 410 in the processing region.

Reacting both hollow cathode discharge effluents and glow discharge effluents with the same semiconductor substrate may enable better control of etching or deposition with more flexibility in tailoring ion, radical, electron, and other densities. Possibly as a result of the higher densities, higher pressures, or higher intensities, the composition of the hollow cathode discharge effluents may be different from the composition of glow discharge effluents. The hollow cathode discharge effluents may have a higher concentration of radicals than glow discharge effluents. In addition, the hollow cathode discharge effluents may contain different radicals in different proportions than in glow discharge effluents. These different concentrations and proportions may result in different reactions with the semiconductor substrate. Alternating between a hollow cathode discharge and a glow discharge may allow for better control of reaction chemistries over just a single discharge mode. Better control of reaction chemistries may result in higher selectivities or etch rates.

As shown in FIG. 5, embodiments of the present technology may include a system 500 for processing a semiconductor substrate. System 500 may include a powered electrode 502 electrically coupled with an electronic ballast. Additionally, system 500 may include a conical cavity 504 defined by powered electrode 502, where conical cavity 504 is characterized by a narrower diameter end 506 and a wider diameter end 508. The height of conical cavity 504 may be between about 5 cm and about 20 cm, between about 5 cm and about 10 cm, or between about 10 cm and about 20 cm in embodiments. The narrower diameter may be between about 0 cm and about 1 cm, between about 0 cm and about 0.5 cm, or between about 0.5 cm and about 1 cm in embodiments. The wider diameter may be between about 4 cm and about 10 cm, between about 4 cm and about 5 cm, or between about 5 cm and about 10 cm in embodiments.

System 500 may further include a power supply 510 electrically coupled with the electronic ballast, where power supply 510 is configured to deliver a negative voltage to powered electrode 502 to strike a hollow cathode discharge in conical cavity 504. The frequency of the voltage may be any of the frequencies previously described. System 500 may also include a gas inlet 512 connected to narrower diameter end 506 of the conical cavity 504.

Furthermore, system 500 may include an electrically grounded electrode 514 defining a plurality of apertures, where electrically grounded electrode 514 is disposed closer to wider diameter end 508 than narrower end 506 of conical cavity 504. The diameter of an aperture 526 may be between about 0.05 cm and about 0.10 cm, between about 0.05 cm and about 0.08 cm, or between about 0.08 cm and about 0.10 cm in embodiments. Electrically grounded electrode 514 may have a first surface and a second surface. The first and second surfaces may be substantially flat, or portions of the first and second surfaces exposed to gas and/or plasma may be substantially flat in embodiments. The first surface may be parallel to the second surface. Either or both of powered electrode 502 and electrically grounded electrode 514 may include a nickel-coated aluminum without a dielectric coating. Powered electrode 502 and electrically grounded electrode 514 may be separated by a dielectric 516.

System 500 may further include a high-speed intensified charge-coupled device (ICCD) camera 518 aligned to record time-resolved images of the hollow cathode discharge and a glow discharge. The time-resolved images may be taken through a viewport 520, which may be near the bottom of system 500. ICCD camera 518 may be aligned so that the camera can detect photons through the plurality of apertures in electrically grounded electrode 514. For instance, a path of a photon from a plasma 522 may be illustrated by dashed arrow 524. Photons may pass from cavity 504 through an aperture 526 in electrically grounded electrode 514 and through a process chamber 528. A semiconductor substrate may be placed in process chamber 528.

ICCD camera 518 may be in communication with an oscilloscope 530. The oscilloscope may read voltage and current from power supply 510. A control PC or computer 532 may be connected to ICCD camera 518 and/or oscilloscope 530. Images on ICCD camera 518 may then be recorded along with voltage/current information at that same time on oscilloscope 530.

Effluents from plasma 522 may also go through aperture 526 and process chamber 528 before going to an outlet 534 leading to a pump. System 500 may operate at any of the pressures and voltages previously described. Flowrates may vary from about 500 sccm to about 10 slm.

Exemplary Processing System

Processing chambers that may implement embodiments of the present invention may be included within processing platforms such as the CENTURA® and PRODUCER® systems, available from Applied Materials, Inc. of Santa Clara, Calif.

FIG. 6 is a substrate processing chamber 1001 according to embodiments. A remote plasma system 1010 may process a fluorine-containing precursor which then travels through a gas inlet assembly 1011. Two distinct gas supply channels are visible within the gas inlet assembly 1011. A first channel 1012 carries a gas that passes through the remote plasma system 1010 (RPS), while a second channel 1013 bypasses the remote plasma system 1010. Either channel may be used for the fluorine-containing precursor in embodiments. On the other hand, the first channel 1012 may be used for the process gas and the second channel 1013 may be used for a treatment gas. The lid (or conductive top portion) 1021 and a perforated partition 1053 are shown with an insulating ring 1024 in between, which allows a unipolar oscillating voltage to be applied to the lid 1021 relative to perforated partition 1053. Perforated partition 1053 may be an electrically grounded electrode. The unipolar oscillating voltage strikes a plasma in chamber plasma region 1020. The process gas may travel through first channel 1012 into chamber plasma region 1020 and may be excited by a plasma in chamber plasma region 1020 alone or in combination with remote plasma system 1010. If the process gas (the fluorine-containing precursor) flows through second channel 1013, then only the chamber plasma region 1020 is used for excitation. The combination of chamber plasma region 1020 and/or remote plasma system 1010 may be referred to as a remote plasma region herein. The perforated partition (also referred to as a showerhead) 1053 separates chamber plasma region 1020 from a substrate processing region 1070 beneath showerhead 1053. Showerhead 1053 allows a plasma present in chamber plasma region 1020 to avoid directly exciting gases in substrate processing region 1070, while still allowing excited species to travel from chamber plasma region 1020 into substrate processing region 1070.

Showerhead 1053 is positioned between chamber plasma region 1020 and substrate processing region 1070 and allows plasma effluents (excited derivatives of precursors or other gases) created within remote plasma system 1010 and/or chamber plasma region 1020 to pass through a plurality of through-holes or apertures 1056 that traverse the thickness of the plate. The showerhead 1053 also has one or more hollow volumes 1051 which can be filled with a precursor in the form of a vapor or gas (such as the fluorine-containing precursor) and pass through blind-holes 1055 into substrate processing region 1070 but not directly into chamber plasma region 1020. Showerhead 1053 is thicker than the length of the smallest diameter 1050 of the through-holes 1056 in embodiments. To maintain a significant concentration of excited species penetrating from chamber plasma region 1020 to substrate processing region 1070, the length 1026 of the smallest diameter 1050 of the through-holes may be restricted by forming larger diameter portions of through-holes 1056 part way through the showerhead 1053. The length of the smallest diameter 1050 of the through-holes 1056 may be the same order of magnitude as the smallest diameter of the through-holes 1056 or less in embodiments. Showerhead 1053 may be referred to as a dual-channel showerhead, a dual-zone showerhead, a multi-channel showerhead or a multi-zone showerhead to convey the existence of through-holes and blind-holes for introducing precursors.

Showerhead 1053 may be configured to serve the purpose of an ion suppressor as shown in FIG. 6. Alternatively, a separate processing chamber element may be included (not shown) which suppresses the ion concentration traveling into substrate processing region 1070. Lid 1021 and showerhead 1053 may function as a first electrode and second electrode, respectively, so that lid 1021 and showerhead 1053 may receive different electric voltages. In these configurations, unipolar oscillating electrical power may be applied to lid 1021, showerhead 1053, or both. For example, electrical power may be applied to lid 1021 while showerhead 1053 (and/or an ion suppressor) is grounded. The substrate processing system may include a unipolar oscillating voltage generator that provides electrical power to the lid 1021 or showerhead 1053 while the other is grounded. The voltage applied to lid 1021 may facilitate a uniform distribution of plasma (i.e., reduce localized plasma) within chamber plasma region 1020. To enable the formation of a plasma in chamber plasma region 1020, insulating ring 1024 may electrically insulate lid 1021 from showerhead 1053. Insulating ring 1024 may be made from a ceramic and may have a high breakdown voltage to avoid sparking Portions of substrate processing chamber 1001 near the capacitively-coupled plasma components just described may further include a cooling unit (not shown) that includes one or more cooling fluid channels to cool surfaces exposed to the plasma with a circulating coolant (e.g., water).

In the embodiment shown, showerhead 1053 may distribute (via through-holes 1056) process gases which contain fluorine, hydrogen, and/or plasma effluents of such process gases upon excitation by a plasma in chamber plasma region 1020. In embodiments, the process gas introduced into the remote plasma system 1010 and/or chamber plasma region 1020 may contain fluorine. The process gas may also include a carrier gas such as helium, argon, nitrogen (N₂), etc. Plasma effluents may include ionized or neutral derivatives of the process gas and may also be referred to herein as radical-fluorine referring to the atomic constituent of the process gas introduced.

Through-holes 1056 are configured to suppress the migration of ionically-charged species out of the chamber plasma region 1020 while allowing uncharged neutral or radical species to pass through showerhead 1053 into substrate processing region 1070. These uncharged species may include highly reactive species that are transported with less-reactive carrier gas by through-holes 1056. As noted above, the migration of ionic species by through-holes 1056 may be reduced, and in some instances completely suppressed. Controlling the amount of ionic species passing through showerhead 1053 provides increased control over the gas mixture brought into contact with the underlying wafer substrate, which in turn increases control of the deposition and/or etch characteristics of the gas mixture. For example, adjustments in the ion concentration of the gas mixture can alter the etch selectivity (e.g., the silicon:silicon nitride etch rate ratio).

In embodiments, the number of through-holes 1056 may be between about 60 and about 2000. Through-holes 1056 may have a variety of shapes but are most easily made round. The smallest diameter 1050 of through-holes 1056 may be between about 0.5 mm and about 20 mm or between about 1 mm and about 6 mm in embodiments. There is also latitude in choosing the cross-sectional shape of through-holes, which may be made conical, cylindrical or combinations of the two shapes. The number of blind-holes 1055 used to introduce unexcited precursors into substrate processing region 1070 may be between about 100 and about 5000 or between about 500 and about 2000 in different embodiments. The diameter of the blind-holes 1055 may be between about 0.1 mm and about 2 mm.

Through-holes 1056 may be configured to control the passage of the plasma-activated gas (i.e., the ionic, radical, and/or neutral species) through showerhead 1053. For example, the aspect ratio of the holes (i.e., the hole diameter to length) and/or the geometry of the holes may be controlled so that the flow of ionically-charged species in the activated gas passing through showerhead 1053 is reduced. Through-holes 1056 in showerhead 1053 may include a tapered portion that faces chamber plasma region 1020, and a cylindrical portion that faces substrate processing region 1070. The cylindrical portion may be proportioned and dimensioned to control the flow of ionic species passing into substrate processing region 1070. An adjustable electrical bias may also be applied to showerhead 1053 as an additional means to control the flow of ionic species through showerhead 1053.

Alternatively, through-holes 1056 may have a smaller inner diameter (ID) toward the top surface of showerhead 1053 and a larger ID toward the bottom surface. In addition, the bottom edge of through-holes 1056 may be chamfered to help evenly distribute the plasma effluents in substrate processing region 1070 as the plasma effluents exit the showerhead and promote even distribution of the plasma effluents and precursor gases. The smaller ID may be placed at a variety of locations along through-holes 1056 and still allow showerhead 1053 to reduce the ion density within substrate processing region 1070. The reduction in ion density results from an increase in the number of collisions with walls prior to entry into substrate processing region 1070. Each collision increases the probability that an ion is neutralized by the acquisition or loss of an electron from the wall. Generally speaking, the smaller ID of through-holes 1056 may be between about 0.2 mm and about 20 mm. In other embodiments, the smaller ID may be between about 1 mm and 6 mm or between about 0.2 mm and about 5 mm. Further, aspect ratios of the through-holes 1056 (i.e., the smaller ID to hole length) may be approximately 1 to 20. The smaller ID of the through-holes may be the minimum ID found along the length of the through-holes. The cross-sectional shape of through-holes 1056 may be generally cylindrical, conical, or any combination thereof.

FIG. 7 is a bottom view of a showerhead 1053 for use with a processing chamber according to embodiments. Showerhead 1053 corresponds with the showerhead shown in FIG. 6. Through-holes 1056 are depicted with a larger inner-diameter (ID) on the bottom of showerhead 1053 and a smaller ID at the top. Blind-holes 1055 are distributed substantially evenly over the surface of the showerhead, even amongst the through-holes 1056 which helps to provide more even mixing than other embodiments described herein.

An exemplary patterned substrate may be supported by a pedestal (not shown) within substrate processing region 1070 when fluorine-containing plasma effluents arrive through through-holes 1056 in showerhead 1053. Though substrate processing region 1070 may be equipped to support a plasma for other processes such as curing, no plasma is present during the etching of patterned substrate, in embodiments.

A plasma may be ignited either in chamber plasma region 1020 above showerhead 1053 or substrate processing region 1070 below showerhead 1053. A plasma is present in chamber plasma region 1020 to produce the radical-fluorine from an inflow of the fluorine-containing precursor. A unipolar oscillating voltage (shifted or otherwise transformed to generally confine to one polarity) is applied between the conductive top portion (lid 1021) of the processing chamber and showerhead 1053 to ignite a plasma in chamber plasma region 1020 during deposition. The unipolar oscillating voltage applied to lid 1021 is shifted such to not center about the potential of showerhead 1053. A unipolar oscillating voltage power supply generates a unipolar oscillating frequency of less than or about 1,000 kHz, less than or about 500 kHz, less than or about 300 kHz or between 1 kHz and 200 kHz according to embodiments.

The top plasma may be left at low or no power when the bottom plasma in the substrate processing region 1070 is turned on to either cure a film or clean the interior surfaces bordering substrate processing region 1070. A plasma in substrate processing region 1070 is ignited by applying the unipolar oscillating voltage between showerhead 1053 and the pedestal or bottom of the chamber. A cleaning gas may be introduced into substrate processing region 1070 while the plasma is present.

The pedestal may have a heat exchange channel through which a heat exchange fluid flows to control the temperature of the substrate. This configuration allows the substrate temperature to be cooled or heated to maintain relatively low temperatures (from room temperature through about 120° C.). The heat exchange fluid may comprise ethylene glycol and water. The wafer support platter of the pedestal (aluminum, ceramic, or a combination thereof) may also be resistively heated to achieve relatively high temperatures (from about 120° C. through about 1100° C.) using an embedded single-loop embedded heater element configured to make two full turns in the form of parallel concentric circles. An outer portion of the heater element may run adjacent to a perimeter of the support platter, while an inner portion runs on the path of a concentric circle having a smaller radius. The wiring to the heater element passes through the stem of the pedestal.

The chamber plasma region and/or a region in a remote plasma system may be referred to as a remote plasma region. In embodiments, the radical precursors (e.g. radical-fluorine) are formed in the remote plasma region and travel into the substrate processing region where they may individually react with chamber walls or the substrate surface. Plasma power may essentially be applied only to the remote plasma region, in embodiments, to ensure that the radical-fluorine (which may also be referred to as plasma effluents) are not further excited in the substrate processing region.

In embodiments employing a chamber plasma region, the excited plasma effluents are generated in a section of the substrate processing region partitioned from a deposition region. The deposition region, also known herein as the substrate processing region, is where the plasma effluents mix and react to etch the patterned substrate (e.g., a semiconductor wafer). The excited plasma effluents may also be accompanied by inert gases (in the exemplary case, argon). The substrate processing region may be described herein as “plasma-free” during etching of the substrate. “Plasma-free” does not necessarily mean the region is devoid of plasma. A relatively low concentration of ionized species and free electrons created within the remote plasma region do travel through pores (apertures) in the partition (showerhead/ion suppressor) due to the shapes and sizes of through-holes 1056. In some embodiments, there is essentially no concentration of ionized species and free electrons within the substrate processing region. In embodiments, the electron temperature may be less than 0.5 eV, less than 0.45 eV, less than 0.4 eV, or less than 0.35 eV in substrate processing region 1070 during excitation of a remote plasma. The borders of the plasma in the chamber plasma region are hard to define and may encroach upon the substrate processing region through the apertures in the showerhead. In the case of an inductively-coupled plasma, a small amount of ionization may be effected within the substrate processing region directly. Furthermore, a low intensity plasma may be created in the substrate processing region without eliminating desirable features of the forming film. All causes for a plasma having much lower intensity ion density than the chamber plasma region (or a remote plasma region, for that matter) during the creation of the excited plasma effluents do not deviate from the scope of “plasma-free” as used herein.

The fluorine-containing precursor) may be flowed into chamber plasma region 1020 at rates between about 5 sccm and about 500 sccm, between about 10 sccm and about 300 sccm, between about 25 sccm and about 200 sccm, between about 50 sccm and about 150 sccm or between about 75 sccm and about 125 sccm in embodiments.

The flow rate of the fluorine-containing precursor into the chamber may account for 0.05% to about 20% by volume of the overall gas mixture; the remainder being carrier gases. The fluorine-containing precursor are flowed into the remote plasma region but the plasma effluents have the same volumetric flow ratio, in embodiments. A purge or carrier gas may be initiated into the remote plasma region before that of the fluorine-containing gas to stabilize the pressure within the remote plasma region.

Plasma power applied to the remote plasma region can be a variety of frequencies or a combination of multiple frequencies. In the exemplary processing system the plasma is provided by unipolar oscillating power delivered between lid 1021 and showerhead 1053. The energy is applied using a capacitively-coupled plasma unit. The remote plasma source power may be between about 10 watts and about 3000 watts, between about 20 watts and about 2000 watts, between about 30 watts and about 1000 watts, or between about 40 watts and about 500 watts in embodiments.

Substrate processing region 1070 can be maintained at a variety of pressures during the flow of carrier gases and plasma effluents into substrate processing region 1070. The pressure within the substrate processing region is below or about 50 Torr, below or about 30 Torr or below or about 20 Torr. The pressure may be above or about 0.1 Torr, above or about 0.2 Torr, above or about 0.5 Torr or above or about 1 Torr in embodiments. Lower limits on the pressure may be combined with upper limits on the pressure to obtain embodiments.

In one or more embodiments, the substrate processing chamber 1001 can be integrated into a variety of multi-processing platforms, including the Producer™ GT, Centura™ AP and Endura™ platforms available from Applied Materials, Inc. located in Santa Clara, Calif. Such a processing platform is capable of performing several processing operations without breaking vacuum. Processing chambers that may implement embodiments of the present invention may include dielectric etch chambers or a variety of chemical vapor deposition chambers, among other types of chambers.

Embodiments of the etching systems may be incorporated into larger fabrication systems for producing integrated circuit chips. FIG. 8 shows one such system 1101 of etching, deposition, baking and curing chambers according to embodiments. In the figure, a pair of FOUPs (front opening unified pods) 1102 supply substrate substrates (e.g., 300 mm diameter wafers) that are received by robotic arms 1104 and placed into a low pressure holding areas 1106 before being placed into one of the wafer processing chambers 1108a-f. A second robotic arm 1110 may be used to transport the substrate wafers from the low pressure holding areas 1106 to the wafer processing chambers 1108a-f and back. Each wafer processing chamber 1108a-f, can be outfitted to perform a number of substrate processing operations including the dry etch processes described herein in addition to cyclical layer deposition (CLD), atomic layer deposition (ALD), chemical vapor deposition (CVD), physical vapor deposition (PVD), etch, pre-clean, degas, orientation and other substrate processes.

The wafer processing chambers 1108a-f may include one or more system components for depositing, annealing, curing and/or etching a dielectric film on the substrate wafer. In one configuration, two pairs of the processing chamber (e.g., 1108c-d and 1108e-f) may be used to deposit dielectric material on the substrate, and the third pair of processing chambers (e.g., 1108a-b) may be used to etch the deposited dielectric. In another configuration, all three pairs of chambers (e.g., 1108a-f) may be configured to etch a dielectric film on the substrate. Any one or more of the processes described may be carried out on chamber(s) separated from the fabrication system shown in different embodiments.

The substrate processing system is controlled by a system controller. In an exemplary embodiment, the system controller includes a hard disk drive, and a processor. The processor contains a single-board computer (SBC), analog and digital input/output boards, interface boards and stepper motor controller boards. Various parts of CVD system conform to the Versa Modular European (VME) standard which defines board, card cage, and connector dimensions and types. The VME standard also defines the bus structure as having a 16-bit data bus and a 24-bit address bus.

System controller 1157 is used to control motors, valves, flow controllers, power supplies and other functions required to carry out process recipes described herein. A gas handling system 1155 may also be controlled by system controller 1157 to introduce gases to one or all of the wafer processing chambers 1108a-f. System controller 1157 may rely on feedback from optical sensors to determine and adjust the position of movable mechanical assemblies in gas handling system 1155 and/or in wafer processing chambers 1108a-f. Mechanical assemblies may include the robot, throttle valves and susceptors which are moved by motors under the control of system controller 1157.

In an exemplary embodiment, system controller 1157 includes a hard disk drive (memory), USB ports, and a processor. System controller 1157 includes analog and digital input/output boards, interface boards and stepper motor controller boards. Various parts of multi-chamber processing system 1101 which contains substrate processing chamber 1001 are controlled by system controller 1157. The system controller executes system control software in the form of a computer program stored on computer-readable medium such as a hard disk, a floppy disk, a flash memory thumb drive, or a network drive. Other types of memory can also be used. The computer program includes sets of instructions that dictate the timing, mixture of gases, chamber pressure, chamber temperature, RF power levels, susceptor position, and other parameters of a particular process.

A process for etching, depositing or otherwise processing a film on a substrate or a process for cleaning chamber can be implemented using a computer program product that is executed by the controller. The computer program code can be written in any conventional computer readable programming language: for example, 68000 assembly language, C, C++, Pascal, Fortran, or others. Suitable program code is entered into a single file, or multiple files, using a conventional text editor, and stored or embodied in a computer usable medium, such as a memory system of the computer. If the entered code text is in a high level language, the code is compiled, and the resultant compiler code is then linked with an object code of precompiled Microsoft Windows® library routines. To execute the linked, compiled object code the system user invokes the object code, causing the computer system to load the code in memory. The CPU then reads and executes the code to perform the tasks identified in the program.

The interface between a user and the controller may be via a touch-sensitive monitor and may also include a mouse and keyboard. In one embodiment two monitors are used, one mounted in the clean room wall for the operators and the other behind the wall for the service technicians. The two monitors may simultaneously display the same information, in which case only one is configured to accept input at a time. To select a particular screen or function, the operator touches a designated area on the display screen with a finger or the mouse. The touched area changes its highlighted color, or a new menu or screen is displayed, confirming the operator's selection.

Example 1

In this example, plasmas were generated in a system, where the total height of conical cavity defined by powered electrode is 10 cm. The diameter at the top of cavity was 0.5 cm, and the diameter at the bottom of cavity was 5 cm. Apertures in electrically grounded electrode have a diameter of 0.08 cm. Both the powered electrode and the electrically grounded electrode were made of nickel-coated aluminum without any dielectric coating. The setup was a direct-current type resistive plasma device, without a phase difference between voltage and current.

A commercial electronic ballast with a maximum power of 100 W was used to generate a plasma. The commercial electronic ballast in this example was an electronic ballast for household fluorescent lamps. The electronic ballast had an output frequency of 60 to 100 kHz and a 0 to 10 V control for dimming. The voltage was measured with a 100:1 voltage probe (PPE2 KV, LeCroy®), and the current was measured with a current probe (P6021, Textronix®). The voltage-current (V-I) waveforms were recorded to an oscilloscope (WaveRunner® 640zi, LeCroy®). Time-resolved discharge images were measure with a high-speed intensified charge-coupled device (ICCD), Andor®).

Argon was introduced from the top of the powered electrode. The flow of argon was controlled by a mass flow controller. Argon at 500 sccm maintained the pressure at 1.5 Torr. Increasing the argon flowrate to 8 slm increased the pressure to 27 Torr.

Example 2

From the system setup in Example 1, the peak voltages were 707 V and −290 V, as shown in FIG. 9(a). The positive and negative peak currents were 270 mA and −398 mA, respectively, as shown in FIG. 9(b). The voltage waveform was biased around 200 V. Pressure was varied between 1.5 Torr and 27 Torr. The voltage bias decreased with increasing pressure. In the positive half cycle, as the pressure increased, the voltage decreased and the current increased.

At 1.5 Torr, in the positive half cycle of the period with the powered electrode being an anode, the peak current was about 30 percent lower than that in the negative half cycle when the powered electrode was a cathode, even though the actual applied voltage was about 2.4 times higher. The total charge transferred during positive and negative cycles were 1.69 C and 1.65 C, respectively, which was only different by about 2.5%. The results imply that the discharge phenomena of the positive and negative half cycles may be different.

As shown in FIG. 9(a), when pressure increased to 8.8 Torr, peak voltage in the positive half cycle decreased. The magnitude of the peak voltage in the positive half cycle was about equal to the magnitude of the peak voltage in the negative half cycle. With current, as shown in FIG. 9(b), when pressure increases to 8.8 Torr, peak current in the positive half cycle increased. The magnitude of the peak current in the positive half cycle was about equal to the magnitude of the peak current in the negative half cycle. Both the current and voltage swings in the positive and negative cycles became more similar and balanced at this increased pressure. And at this increased pressure, the intensities of the glow discharge and hollow cathode discharge became similar. This example shows that varying the pressure may affect the characteristics of each discharge mode.

Example 3

Time-resolved images from discharges generated in the system in Example 2 are shown in FIG. 10. The exposure time for these images was 0.5 μs. The images from the glow discharge appear different than the images from the hollow cathode discharge. The hollow cathode discharge images were observed to be more intense than the glow discharge images.

At 8.8 Torr, the glow discharge images show strong emissions in the ten o'clock direction of the grounded electrode. This observation, along with the electrical characteristics at this pressure in Example 2, suggests that a hollow cathode discharge may be present at a pressure of 8.8 Torr with a pD of about 0.7 Torr-cm (D=0.08 cm). As can be seen in the image, this discharge is not uniform, and may not be desirable for semiconductor processing.

The hollow cathode discharge mode in the negative half cycle was sustained up to 8.8 Torr and became more intense with increasing pressure. Because the diameter of the intense light emission from the hollow cathode discharge is about 2 cm, pD for stable operation of this mode should be about 3 to 20 Torr-cm for the electrode configuration in Example 1. Stable operation of the glow discharge mode would be at pressures at less than 2.4 Torr, with the glow discharge at 4.1 Torr showing some non-uniformities. As the pressure was increased to 27 Torr (not shown in FIG. 10), discharges in both the positive half cycle and the negative half cycle became unstable and started to extinguish.

As used herein “substrate” may be a support substrate with or without layers formed thereon. The patterned substrate may be an insulator or a semiconductor of a variety of doping concentrations and profiles and may, for example, be a semiconductor substrate of the type used in the manufacture of integrated circuits. Exposed “silicon” of the patterned substrate is predominantly Si but may include minority concentrations of other elemental constituents (e.g. nitrogen, oxygen, hydrogen, carbon). Exposed “silicon nitride” of the patterned substrate is predominantly Si₃N₄ but may include minority concentrations of other elemental constituents (e.g. oxygen, hydrogen, carbon). Exposed “silicon oxide” of the patterned substrate is predominantly SiO₂ but may include minority concentrations of other elemental constituents (e.g. nitrogen, hydrogen, carbon). In some embodiments, silicon oxide films etched using the methods disclosed herein consist essentially of silicon and oxygen.

The term “precursor” is used to refer to any process gas which takes part in a reaction to either remove material from or deposit material onto a surface. “Plasma effluents” describe gas exiting from the chamber plasma region and entering the substrate processing region. Plasma effluents are in an “excited state” wherein at least some of the gas molecules are in vibrationally-excited, dissociated and/or ionized states. A “radical precursor” is used to describe plasma effluents (a gas in an excited state which is exiting a plasma) which participate in a reaction to either remove material from or deposit material on a surface. “Radical-fluorine” is a radical precursor that contains fluorine but may contain other elemental constituents. The phrase “inert gas” refers to any gas which does not form chemical bonds when etching or being incorporated into a film. Exemplary inert gases include noble gases but may include other gases so long as no chemical bonds are formed when (typically) trace amounts are trapped in a film.

The terms “gap” and “trench” are used throughout with no implication that the etched geometry has a large horizontal aspect ratio. Viewed from above the surface, trenches may appear circular, oval, polygonal, rectangular, or a variety of other shapes. A trench may be in the shape of a moat around an island of material. The term “via” is used to refer to a low aspect ratio trench (as viewed from above) which may or may not be filled with metal to form a vertical electrical connection. As used herein, a conformal etch process refers to a generally uniform removal of material on a surface in the same shape as the surface, i.e., the surface of the etched layer and the pre-etch surface are generally parallel. A person having ordinary skill in the art will recognize that the etched interface likely cannot be 100% conformal and thus the term “generally” allows for acceptable tolerances.

In the preceding description, for the purposes of explanation, numerous details have been set forth in order to provide an understanding of various embodiments of the present technology. It will be apparent to one skilled in the art, however, that certain embodiments may be practiced without some of these details, or with additional details.

Having disclosed several embodiments, it will be recognized by those of skill in the art that various modifications, alternative constructions, and equivalents may be used without departing from the spirit of the embodiments. Additionally, a number of well-known processes and elements have not been described in order to avoid unnecessarily obscuring the present technology. Accordingly, the above description should not be taken as limiting the scope of the technology.

Where a range of values is provided, it is understood that each intervening value, to the smallest fraction of the unit of the lower limit, unless the context clearly dictates otherwise, between the upper and lower limits of that range is also specifically disclosed. Any narrower range between any stated values or unstated intervening values in a stated range and any other stated or intervening value in that stated range is encompassed. The upper and lower limits of those smaller ranges may independently be included or excluded in the range, and each range where either, neither, or both limits are included in the smaller ranges is also encompassed within the technology, subject to any specifically excluded limit in the stated range. Where the stated range includes one or both of the limits, ranges excluding either or both of those included limits are also included.

As used herein and in the appended claims, the singular forms “a”, “an”, and “the” include plural references unless the context clearly dictates otherwise. Thus, for example, reference to “an aperture” includes a plurality of such apertures, and reference to “the gas” includes reference to one or more gases and equivalents thereof known to those skilled in the art, and so forth.

Also, the words “comprise(s)”, “comprising”, “contain(s)”, “containing”, “include(s)”, and “including”, when used in this specification and in the following claims, are intended to specify the presence of stated features, integers, components, or operations, but they do not preclude the presence or addition of one or more other features, integers, components, operations, acts, or groups.

Claims

1. A method of processing a semiconductor substrate, the method comprising: flowing a gas through a cavity defined by a powered electrode;applying a negative voltage to the powered electrode;striking a hollow cathode discharge in the cavity to form hollow cathode discharge effluents from the gas;flowing the hollow cathode discharge effluents to a processing region through a plurality of apertures defined by an electrically grounded electrode; andreacting the hollow cathode discharge effluents with the semiconductor substrate in the processing region, wherein: applying the negative voltage comprises applying the negative voltage through an electronic ballast, andthe electronic ballast has a frequency of less than about 1 MHz.

2. The method of claim 1, wherein the method further comprises: applying a positive voltage to the powered electrode;striking a glow discharge to form glow discharge effluents from the gas, wherein most of the glow discharge is between the powered electrode and the electrically grounded electrode and not in the cavity;flowing glow discharge effluents to the processing region through the plurality of apertures; andreacting the glow discharge effluents with the semiconductor substrate in the processing region.

3. The method of claim 1, wherein the gas comprises argon or helium.

4. The method of claim 3, wherein the gas further comprises fluorinated compounds.

5. The method of claim 1, wherein the gas comprises NF3 or NH3.

6. The method of claim 4, wherein the cavity is conical.

7. The method of claim 6, wherein the electrically grounded electrode has a first surface and a second surface, and the first surface and the second surface are substantially flat.

8. The method of claim 1, wherein the cavity is cylindrical.

9. The method of claim 7, wherein striking the hollow cathode discharge comprises striking the hollow cathode discharge at a pressure between about 1.5 Torr and about 10 Torr.

10. The method of claim 9, wherein the powered electrode comprises nickel-coated aluminum without a dielectric coating and the electrically grounded electrode comprises nickel-coated aluminum without a dielectric coating.

11. The method of claim 10, wherein the electronic ballast has a power of about 60 W.

12. A method of processing a semiconductor substrate, the method comprising: flowing a gas to a processing region;striking a hollow cathode discharge to form hollow cathode discharge effluents from the gas;reacting hollow cathode discharge effluents with the semiconductor substrate in the processing region;striking a glow discharge to form glow discharge effluents from the gas; andreacting the glow discharge effluents with the semiconductor substrate in the processing region, wherein: striking the hollow cathode discharge comprises applying a negative voltage to a powered electrode through an electronic ballast, andthe electronic ballast has a frequency of less than about 1 MHz.

13. The method of claim 12, wherein striking the hollow cathode discharge comprises striking the hollow cathode discharge in a first space in a conical cavity defined by the powered electrode.

14. The method of claim 13, wherein striking the glow discharge comprises striking the glow discharge in a second space between the powered electrode and an electrically grounded electrode, wherein the second space is different from the first space.

15. The method of claim 12, wherein striking the glow discharge further comprises applying a positive voltage to the powered electrode.

16. A system for processing a semiconductor substrate, the system comprising: a powered electrode electrically coupled with an electronic ballast;a conical cavity defined by the powered electrode, wherein the conical cavity is characterized by a narrower diameter end and a wider diameter end;a power supply electrically coupled with the electronic ballast, wherein the power supply is configured to deliver a negative voltage at a frequency below about 1 MHz to the powered electrode to strike a hollow cathode discharge in the conical cavity;a gas inlet connected to the narrower diameter end of the conical cavity; andan electrically grounded electrode defining a plurality of apertures, wherein:the electrically grounded electrode is disposed closer to the wider diameter end of the conical cavity than the narrower diameter end of the conical cavity,the electrically grounded electrode has a first surface and a second surface,the first surface and the second surface are substantially flat, andthe first surface is parallel to the second surface.

17. The system of claim 16, wherein the system further comprises a high-speed intensified charge-coupled device (ICCD) aligned to record time-resolved images of the hollow cathode discharge and a glow discharge.

18. The system of claim 16, wherein the powered electrode comprises nickel-coated aluminum without a dielectric coating and the electrically grounded electrode comprises nickel-coated aluminum without a dielectric coating.

19. The method of claim 1, wherein the electronic ballast has a power between about 50 W and about 100 W.

20. The method of claim 2, wherein the negative voltage and the positive voltage are applied as sinusoidal oscillating voltages.