Apparatus and method for removal of surface oxides via fluxless technique involving electron attachement and remote ion generation

The present invention generally relates to a fluxless process for removing surface oxides. More specifically, the present invention relates to an apparatus and method comprising a fluxless reflow with electron adhesion in a remote ion generator and a fluxless process for soldering.

Reflow and soldering are important processing steps for soldering in the assembly of electronic components. As used herein, the term "reflow" refers to the process of melting and flowing solder previously applied on a substrate by applying an energy source such as thermal energy. As used herein, the term "soldering" refers to the process of joining at least two metal substrates with molten solder. A variety of different reflows and soldering processes include reflowing solder bumps used for wafer bumping, reflow soldering used in assembling surface mount electronic components, and wave soldering used in assembling insert mounting components, but It can be used in the assembly of electronic devices not limited to them.

Reflow soldering is a process used for outer lead bonding of surface mount components, in which the chips are properly transferred with the leads to the next level surface mount package. In the reflow soldering process, the solder paste is pre-printed on the circuit board and the components are mounted on the corresponding micro-regions of the circuit board. The soldered parts thus formed are then placed in a reflow furnace and passed through a heating and cooling zone. The solder bond between the component leads and the solder lands on the circuit board is formed by melting, wetting, and solidifying the solder paste. To ensure good wetting of the molten solder on the bonded surface, an organic flux is usually included in the solder paste to remove the initial surface oxides for both the solder and the base metal to remove the pre-solidified surface. Keep it clean. Most of the flux evaporates into the vapor phase during soldering, but the volatiles of the flux can cause problems such as the formation of voids in the solder joints and contamination of the reflow furnace. After soldering, flux residues that can cause corrosion and short circuits still remain on the circuit board.

Similarly, wave soldering is used for outer lead bonding, such as for assembling conventional through-hole mounting components. It can also be used for surface mount components by temporarily attaching the components on the circuit board with an adhesive prior to soldering. In both cases, circuit boards with inserted or temporarily attached components must be cleaned with liquid flux to remove oxides on the component leads and solder lands, followed by hot molten solder. I have to take a bath. The molten solder automatically wets the metal surface to be soldered, thus forming a solder bond. The molten solder in the bath has a high tendency to be oxidized and forms solder residue. Therefore, the surface of the solder bath must be cleaned frequently by mechanically removing the debris, increasing the operating cost and consumption of the solder. After soldering, the flux residue remains on the circuit board, causing the same problems described herein for reflow soldering.

Wafer bumping is a process used to create thick metal bumps on chip bond pads for inner reed bonding. The bumps generally have solder adhered onto the pads and then reflowed (referred to herein as first reflow) to alloy and change the shape of the solder bumps from a mushroom shape to a hemispherical shape. It is made by. The chip with the first reflow bump is "repelled" to correspond to the footprint of the solder-wet terminals on the substrate and then undergoes a second reflow to form a solder bond. These solder joints are referred to herein as inner reed bonds. High melting point solder (eg> 300 ° C) is typically used in wafer bumping. This is because subsequent assembly steps such as outer reed bonding, which proceeds using solder with a low melting point (for example, <230 ° C), can be performed without breaking the inner reed bond.

The shape of the solder bump after the first reflow is important. For example, a high bump height is preferred for better bonding and higher fatigue resistance. In addition, the bumps formed should preferably be substantially uniform to ensure flatness. A substantially uniform solder bump with a relatively high bump height is believed to be associated with the oxide-free bump surface during the first reflow. One approach to removing solder oxides during the first reflow of a wafer with solder bumps is to place an organic flux on the deposited solder bumps or in a solder paste mixture printed on the wafer to form the bumps. The application is to reflow the bumps in an inert environment so that the flux can effectively remove the initial oxides on the solder surface. However, this approach has drawbacks. Small voids may form in the solder bumps due to the decomposition of the flux. These voids not only reduce the electrical and mechanical properties of the solder bond formed, but also destroy the coplanarity of the solder bumped wafer and may affect subsequent chip bonding processes. Volatile substances in the decomposed flux can contaminate the reflow furnace, which can increase maintenance costs. In addition, flux residues often remain on the wafer, which can cause corrosion and reduce assembly performance.

In order to remove the flux residue from the above reflow and soldering treatment, a post-cleaning treatment using chlorofluorocarbon (CFC) as a cleaning agent can be adopted. However, post-cleaning adds a new treatment step and increases the manufacturing process time. Furthermore, the use of chlorofluorocarbons (CFCs) as cleaning agents is banned due to potential damage to the ozone layer that protects the planet. No-cleaning flux has been developed by reducing the residue with a small amount of activator, but there is a trade between the gain and loss of the amount of flux residue and the flux activity.

A good solution to all of the above problems, including the formation of voids, the formation of volatiles of flux, flux residues, and debris, is to reflow and replace organic flux for removing metal oxides. A reducing gas is used as the soldering environment. Such reflow and soldering techniques are referred to as "fluxless reflow" and "fluxless soldering". Of the various fluxless reflow and soldering methods, it is particularly attractive to use hydrogen as the reactive gas to reduce the base metal and oxides on the solder. Because it is a very clean process (the only by-product is water that can be easily ventilated from the furnace) and is compatible with an open and continuous soldering production line (H).₂Is non-toxic and has a flammability range of 4 to 75%). Therefore, hydrogen fluxless soldering has long been a technical goal.

One previously used hydrogen fluxless method for inner reed bonding was to utilize pure hydrogen for the reflow of solder bumped wafers at temperatures of 400-450 ° C. However, the flammability of pure hydrogen greatly limits its use. For soldering processes used in outer lead bonding such as reflow soldering and wave soldering, the main constraint on reducing surface oxides with hydrogen is in the normal processing temperature range (<250 ° C). This is especially true for solder oxides, which have an inefficient and slow reduction rate of metal oxides, and have a higher metal-oxygen bond strength than the oxides on the base metal to be soldered. This inefficiency of hydrogen is considered to be due to the poor reactivity of hydrogen molecules at low temperatures. Highly reactive radicals, such as monatomic hydrogen, are formed at temperatures well above the normal reflow and wave soldering temperature ranges. For example, pure H, which is effective in reducing tin oxide on tin-based solders.₂The temperature range of is higher than 350 ° C. Such high temperatures can damage the packaged electronic components or cause reliability problems. Therefore, highly reactive H₂Catalytic methods for facilitating the production of radicals, and thus effective ranges of hydrogen concentrations, and reductions in the treatment temperature for reducing surface oxides are sought after in the industry.

Fluxless (dry) soldering is prior art using several techniques. One technique is to remove with a laser or heat the metal oxide to its vaporization temperature. Such treatments are typically carried out in an inert or reducing atmosphere to prevent reoxidation by the released contaminants. However, the melting point or boiling point of the oxide or base metal may be similar, and it may not be desirable to melt or vaporize the base metal. Therefore, it is difficult to carry out such laser treatment. Lasers are typically expensive and inefficient to operate and require linear aiming at the oxide layer. These factors limit the effectiveness of laser technology for most soldering applications.

Surface oxides are reactive gases at high temperatures (eg, H).₂) Chemically (eg, H)₂Can be reduced (to O). Inactive carrier (eg N₂), A mixture containing 5% or more of reducing gas is typically used. The reaction product (eg, H) is then₂O) is desorbed at high temperature, released from the surface, and carried away to the gas flow area. Typical processing temperatures exceed 350 ° C. However, this process may be slow and ineffective even at high temperatures.

The speed and effectiveness of the reduction treatment can be improved with more active reduced species. Such active species can be produced using conventional plasma techniques.

Gas plasmas at audible, radio, or microwave frequencies can be used to generate reactive radicals for surface deoxidation. In such processing, high intensity electromagnetic radiation is H₂, O₂,SCIENCE FICTION₆, Or other species, including fluorine-containing compounds, are used to ionize and dissociate into highly reactive radicals. Surface treatment can be performed at temperatures below 300 ° C. However, in order to obtain the optimum conditions for plasma formation, such processing is typically performed under vacuum conditions. Vacuum operation requires expensive equipment and must be performed as a slow batch process rather than a faster continuous process. In addition, the plasma is typically diffused and dispersed within the processing chamber, making it difficult to direct to a particular region. Therefore, reactive species cannot be used efficiently in this process. The plasma can also damage the processing chamber by sputtering, which can accumulate space charges on the dielectric surface and can damage the microcircuits. The microwave itself can also damage the microcircuit and it can be difficult to control the temperature of the substrate or component during processing. Plasma can also emit potentially dangerous ultraviolet light. Such processes also require expensive electrical equipment and consume significant power, thereby reducing the effectiveness of their overall cost.

U.S. Pat. No. 5,409,543 discloses a method of thermally dissociating molecular hydrogen under vacuum conditions using a thermal filament to produce reactive hydrogen species (ie, atomic hydrogen). There is. The energized hydrogen chemically reduces the surface of the substrate. The temperature of the hot filament can be between 500 ° C and 2200 ° C. An electrically biased grid is used to deflect or capture excess free electrons emitted from the thermal filament. The reactive species or atomic hydrogen is produced from a mixture containing 2% to 100% hydrogen in the Inactive Carrier Gas.

U.S. Pat. No. 6,203,637 (Patent Document 2) discloses a method of activating hydrogen using a discharge from a thermionic cathode. The electrons emitted from the thermionic cathode create a gas phase discharge that produces the active species. This release process is carried out in an independent or remote chamber containing the heated filaments. Ions and activated neutral species flow into the treatment chamber, chemically reducing the oxidized metal surface. However, such a hot cathode process requires vacuum conditions for optimum effectiveness and filament life. Vacuum operation requires expensive equipment that must be incorporated into the soldering conveyor belt system, thereby reducing the effectiveness of its overall cost.

Potier et al. Fluxless Soldering Under Activated Atmosphere at Ambient Pressure, Surface Mount International Conference, 1995, San Jose, CA (Non-Patent Document 1), and US Patent No. 6,146,503 (Patent Document 3), No. 6,089,445. Specification (Patent Document 4), No. 6,021,940 (Patent Document 5), No. 6,007,637 (Patent Document 6), No. 5,941,448 (Patent Document 7), No. 5,858,312. (Patent Document 8) and No. 5,722,581 (Patent Document 9) are H activated by discharge.₂(Or CH₄Or NH₃Explains methods for producing other reducing gases such as). The reducing gas is an inert carrier gas (N).₂) Generally present in "percentage level". The discharge is created using an AC voltage source of "several kV". The electrons emitted from the electrodes in the remote chamber produce excited or unstable seeds that are virtually free of charged seeds, thus allowing these seeds to flow to the substrate. The resulting process reduces the oxide on the base metal to be soldered at temperatures around 150 ° C. However, such remote discharge chambers require considerable equipment costs and are not easily updated with existing soldering conveyor belt systems. In addition, these processes are typically utilized to pretreat the metal surface before soldering rather than removing solder oxides.

U.S. Pat. No. 5,433,820 (Patent Document 10) describes a surface treatment method using discharge from a high voltage (1 kV to 50 kV) electrode at atmospheric pressure or plasma. The electrodes are placed closer to the substrate rather than in the remote chamber. Free electrons emitted from the electrodes create a reactive hydrogen radical, a plasma containing atomic hydrogen, which is then placed on a dielectric shield located on top of the oxidized substrate. Pass through the opening. This dielectric shield concentrates active hydrogen at specific surface locations that require deoxidation. However, such dielectric shields can change the electric field and accumulate surface charges that can interfere with accurate process control. The process described is only used to melt the surface of the base metal.

<patcit num="1"><text>U.S. Pat. No. 5,409,543</text></patcit><patcit num="2"><text>U.S. Pat. No. 6,203,637</text></patcit><patcit num="3"><text>U.S. Pat. No. 6,146,503</text></patcit><patcit num="4"><text>U.S. Pat. No. 6,089,445</text></patcit><patcit num="5"><text>U.S. Pat. No. 6,021,940</text></patcit><patcit num="6"><text>U.S. Pat. No. 6,007,637</text></patcit><patcit num="7"><text>U.S. Pat. No. 5,941,448</text></patcit><patcit num="8"><text>U.S. Pat. No. 5,858,312</text></patcit><patcit num="9"><text>U.S. Pat. No. 5,722,581</text></patcit><patcit num="10"><text>U.S. Pat. No. 5,433,820</text></patcit><nplcit num="1"><text>Fluxless Soldering Under Activated Atmosphere at Ambient Pressure by Potier et al., Surface Mount International Conference, 1995, San Jose, CA</text></nplcit>

<p> Therefore, it is an art of the art to provide an economical and efficient method for removing metal oxides from at least one component and / or solder surface at relatively low temperatures so as not to damage any electronic components. Needed in the field. There is a further need in the art to provide methods and equipment for fluxless soldering under conditions near ambient or atmospheric pressure so as not to cost money to purchase and maintain vacuum equipment. There is. In addition, there is a new need in the art to provide a fluxless soldering process using a nonflammable gas environment.</p>

<p> The present invention provides some, if not all, of the needs of the art by providing a method for removing metal oxides from at least one component and / or solder surface without the need for flux. Satisfy. Specifically, in one embodiment of the invention, a method for removing metal oxides from the surface of at least one component, wherein the at least one component connected to a substrate is provided. Forming a target assembly in which the substrate has at least one potential selected from the group consisting of ground potentials or positive potentials; a gas mixture comprising a reducing gas, ion generation comprising the first and second electrodes. Passing through a vessel; generating electrons adhering to at least a portion of the reducing gas to supply sufficient voltage to at least one of the first and second electrodes to form a negatively charged reducing gas. To remove metal oxides from the surface of at least one component, which comprises contacting the target assembly with the negatively charged reducing gas to reduce the oxide on the at least one component. A way to do this is provided.</p><p> In another aspect of the invention, a fluxless soldering method is provided in which at least one component having a surface metal oxide and solder is soldered. This method provides at least one component connected to the substrate as a target assembly having at least one potential selected from the group consisting of ground potential or positive potential; reducing gas and carrier gas. A gas mixture comprising: a gas inlet, a gas outlet in which the gas inlet is in contact with the fluid, an anode, and a cathode located between the gas inlet and the gas outlet close to the target assembly. Passing through the ion generator; supplying sufficient energy to at least one of the cathode and the anode to generate electrons adhering to at least a portion of the reducing gas passing through the ion generator. It involves forming a negatively charged reducing gas at the gas outlet; and contacting the target assembly with the negatively charged reducing gas to reduce the metal oxide on at least one component. Consists of.</p><p> In a further aspect of the invention, an enclosure that defines a negatively charged ionic reducing gas generator, the inner hollow of which at least a portion of which comprises an anode connected to a first voltage level. And; with the gas inlet and the gas outlet through which the fluid communicates with the internal cavity; located in the internal cavity and located between the gas inlet and the gas outlet, with respect to the first voltage level. A negatively charged ionic reducing gas generator comprising a cathode connected to a second voltage level having is provided.</p><p> In yet another aspect of the invention, there is provided an apparatus comprising a first chamber and a second chamber to generate a negatively charged ionic reducing gas. The first chamber is at least two electrodes housed therein, with at least two electrodes to which a potential bias is applied between the electrodes; and a first gas inlet for receiving reducing gas; It has a gas outlet for releasing a negatively charged ionic reducing gas. The second chamber encapsulates the first chamber and has a second gas inlet for receiving carrier gas, the second gas inlet fluidly communicating with the first chamber, the reducing gas and the carrier. The gas forms a gas mixture in the second chamber.</p><p> These and other aspects of the invention will become apparent from the detailed description below.</p>

Methods and devices for removing metal oxides from at least one component and / or solder surface by irradiating with negatively charged hydrogen ions are disclosed herein. In certain embodiments, the irradiation can be performed before and / or during the reflow and soldering process. Negatively charged hydrogen ions react to reduce surface metal oxides. The present invention can be utilized by improving conventional reflow and soldering equipment such as, for example, a reflow machine used for inner reed bonding and reflow soldering, or a wave soldering machine used for outer reed bonding. The present invention also includes plating (ie, solder plating parts of a printed circuit board or metal surface to make them easier to handle for subsequent soldering), surface cleaning, brazing, welding, and silicon wafers. It can be applied to other treatments where removal of surface metal oxides such as, but not limited to, surface metal oxides formed during the treatment is desired. The removal of metal oxides using the methods and equipment of the present invention is equally applicable to the treatments described above or any other treatment desired to remove oxides without the need for organic flux.

The ion generators disclosed herein may be particularly suitable for certain embodiments, such as, for example, fluxless soldering of surface mount components, including flip chip assemblies. In these embodiments, electrons may penetrate across the component and accumulate downwards. This is because the parts may be composed of semi-conductive materials. The accumulation of electrons under the component can repel negatively charged active species in the reducing gas, which can change the efficiency of oxide removal. In order to improve this, the position of the ion generator can be adjusted according to the surface shape of the component. By using the ion generator disclosed herein, the negatively charged active species in the gas mixture can be uniformly distributed on the surface of the part to be treated, regardless of the surface shape. It is possible to optimize the operating conditions (eg, temperature, pressure and gas concentration) for forming negatively charged seeds without restrictions on the environmental conditions of the surface.

As used herein, the term "part" is generally used in silicon, silicon-coated silicon, aluminum-aluminum oxide, gallium arsenide, ceramics, quartz, copper, glass, epoxy, or electronic devices. With respect to parts composed of materials such as any suitable material. In certain embodiments, the component has solder placed on at least one of its surfaces. Exemplary solder compositions include, but are not limited to, fluxless tin-silver, fluxless tin-silver-copper, fluxless tin-lead, or fluxless tin-copper. However, the methods of the invention are suitable for a variety of different parts and / or solder compositions.

Although not intended to be bound by theory, when an energy source, such as a direct current (DC) voltage, is applied to at least one of the two electrodes housed in the remote ion generator, it causes an electric potential. Is generated from the negative bias electrode, from the gas phase between the two electrodes, or from a combination thereof, and the electrons drift toward the positive bias electrode along the electric field. In one preferred embodiment, the part to be deoxidized and / or the soldered part is placed on a grounded or positively biased substrate, in the immediate vicinity of the remote ion generator outlet. Placed in. A gas mixture consisting of a reducing gas and optionally a carrier gas is passed through an electric field created by the electrodes in the remote ion generator. During electron drift, some of the reducing gas forms negative ions by electron attachment, which then attach or adsorb to at least one component through the outlet of the ion generator. In this way, the adhered or adsorbed negatively charged ions can reduce the oxide present on the base metal and / or solder without the need for conventional flux. In certain preferred embodiments, the adsorption of the active species on the surface to be treated is facilitated by the opposite charges between the active species and the surface to be treated (eg, positive bias the target assembly).

In an embodiment in which the reducing gas contains hydrogen, the method of the present invention is considered to be carried out as follows. Dissociative adhesion: H₂ + e' H^- + H (I) Radioactive adhesion: e'+ H H^- + hν (II) Combination of (I) and (II): 2e'+ H₂ 2H^- + hν (III) Oxide reduction: 2H^- + MO M + H₂O + 2e'(M = solder / base metal) (IV) In these embodiments, the activation energy of oxide reduction using the electron attachment method of the present invention is lower than that of the method using molecular hydrogen. This is because the formation of atomic hydrogen ions using electron attachment excludes the energy associated with the bond breakdown of molecular hydrogen.

Sufficient energy to generate electrons from the cathode, the gas phase between the two electrodes, or a combination thereof is supplied to at least one of the electrodes, preferably the cathode. In certain preferred embodiments, the energy source can be an electrical energy source such as an alternating current (AC) or direct current (DC) voltage source. Other energy sources, such as thermal energy sources, electromagnetic energy sources, or light energy sources, can also be used alone or in combination. The energy source can be constant or pulsed. In one embodiment of the invention, the cathode is connected to the voltage source at the first voltage level and the anode is connected to the voltage source at the second level. The difference in voltage levels creates a potential bias. One of the first or second voltage levels can be zero, indicating that either the cathode or the anode is grounded.

In order to produce negatively charged ions by electron adhesion, it is necessary to generate a large amount of electrons. In this regard, electrons can be generated by various methods such as, but not limited to, cathodic emission, gas discharge, or a combination thereof. Among these electron generation methods, the choice of method depends mainly on the energy level and efficiency of the generated electrons. For embodiments in which the reducing gas comprises hydrogen, electrons with an energy level close to 4 eV may be preferred. In these embodiments, such low energy levels of electrons can be generated by cathodic emission and / or gas discharge. The generated electrons then drift from the cathode to the anode that produces the space charge. Space charge provides an electron source for producing negatively charged ions as the reducing gas passes through at least two electrodes.

For embodiments relating to electron generation by cathode emission, these embodiments include field emission (referred to herein as cold emission), heat emission (referred to herein as hot emission), and thermal field emission. , Photoelectron emission, and electron or ion beam emission.

Field emission involves applying a sufficiently strong electric field between the cathode and anode to overcome the energy barrier and emit electrons from the cathode surface. In one preferred embodiment, a DC voltage is applied between the cathode and anode with a large surface curvature at a voltage of 0.1-50 kV or 1-30 kV. In these embodiments, the distance between the electrodes can be 0.1-30 cm or 0.5-5 cm.

On the other hand, heat release involves exciting the electrons in the cathode by high temperatures and separating the electrons from the metal bonds of the cathode material. In certain preferred embodiments, the temperature of the cathode can be 800-3500 ° C or 800-1500 ° C. The cathode is directly heated by passing an energy source such as AC or DC current through the cathode; an electrically insulated high temperature surface that is heated by preheating the heating member, IR radiation, and gas mixture to a temperature above the desired temperature of the cathode. It can be heated and / or maintained at a high temperature by a variety of methods, such as, but not limited to, indirect heating such as contacting the cathode surface with; or a combination thereof.

Thermal field emission is a mixture of field emission and heat emission methods that apply both electric and high temperatures. Therefore, thermal field emission can require lower electric fields and lower cathode temperatures by producing the same amount of electrons as compared to pure field emission and pure heat emission. Thermal field emission minimizes the difficulties faced with pure field emission, such as contamination on the emission surface tends to reduce electron emission, and high restrictions on the flatness and uniformity of the emission surface. Can be done. In addition, thermal field emission can avoid problems related to thermal emission, such as the high likelihood of a chemical reaction occurring between the emission electrode and the gas phase. In embodiments using thermal field emission, the temperature of the cathode can be 3500 ° C or 150-1500 ° C from the ambient temperature. In these embodiments, the electric field can be 0.01-30 kV or 0.1-10 kV.

In certain preferred embodiments, a heat release or thermal field emission mechanism is used to generate electrons in a reflow or soldering process. In these embodiments, the hot cathode used in any of these mechanisms can also act as a heat source for the gas mixture passing between the anodes in the ion generator. In certain embodiments, the temperature of the gas mixture in the ion generator is the reflow and soldering temperature or so that the thermal energy required to heat the gas for reflow and soldering can be reduced. Can be in the vicinity. In other embodiments, the temperature of the gas mixture in the ion generator may be relatively higher than the temperature of the reflow and soldering process. In this embodiment, H is used as the reducing gas.₂In some cases, such as when using, the electron attachment process can be facilitated within the ion generator. This is because the formation of negatively charged hydrogen ions due to electron adhesion is an endothermic reaction. The temperature of the negatively charged ionic reducing gas at the outlet of the ion generator can be lowered to the reflow and soldering temperatures, for example, by diluting the processing gas with a large amount of furnace top gas. As long as negatively charged ions are formed, the ions can repel each other by something like a charge, thereby reducing the tendency to recombine at reduced temperatures.

In certain embodiments of the invention, electron generation is achieved by a combination of cathode emission and corona discharge methods. In these embodiments, an energy source, such as a DC voltage, is applied between the two electrodes and can generate electrons from both the cathode (low or high temperature) and the gas (corona discharge) near the chip. Keep the corona discharge as small as possible to increase the efficiency of forming negatively charged hydrogen ions due to electron adhesion and to improve the life of the cathode chip by minimizing the collision of positive ions on the cathode surface. Is preferable.

In certain preferred embodiments of the cathode emission mechanism, the voltage applied across the two electrodes can be constant or pulsed. The frequency of the voltage pulse is 0 to 100 kHz. Figures 1a and 1b give diagrams of voltage pulses for the cathode and anode, respectively. In these embodiments, it is considered that the pulse voltage may be preferable to the constant voltage in order to improve the electron emission amount and reduce the tendency of gas phase discharge.

For embodiments relating to the generation of electrons by gas discharge, these embodiments can include a variety of avalanche discharges, including thermal discharges, photodischarges, and glow discharges, arc discharges, spark discharges, and corona discharges. In these embodiments, the electrons are generated by gas phase ionization. In some embodiments of gas phase ionization, the gas phase comprises a reducing gas and an inert gas, a voltage source is applied between the two electrodes, electrons are generated from the inert gas between the two electrodes, and then the anode. Drift to the positive bias electrode such as. During this electron drift, some of these electrons can attach to the reducing gas molecules and form negatively charged ions due to the electron attachment. In addition, some positive ions are also generated by gas phase ionization and then drift to a negative bias electrode such as the cathode and are neutralized on the electrode surface.

As mentioned above, for cathode emission, electrons are emitted from an electrode that can act as a cathode. For FIGS. 2a-2i, the electrodes are, for example, coaxial cable 2a, rods with pointed tips, combs or rods 2c with multiple pointed tips, screen or wire mesh 2d, loose coils 2e, in a row. It can have a variety of geometries, such as a comb 2f, a bundle of coaxial cables or filaments 2g, a rod 2h with a sharp tip protruding from its surface, or a plate 2i with a knurled surface. Additional geometries can include combinations of the above geometries such as plates or rods with surface protrusions, rods wrapped in windings or filaments, coils of coaxial cables, and the like. Multiple electrodes can be used that can be placed in parallel series or in an intersecting grid. In certain embodiments, such as those involving field emission, the cathode is preferably made from a geometry with a large surface curvature, such as a plurality of sharp tips, such as the geometry shown in FIG. Maximize the electric field near the electrode surface. As shown in FIG. 3, the electrode 1 has a series of coaxial cables 2 inside a groove on the electrode surface, along with a plurality of chips 3 protruding from its surface.

The electrode material acting as a cathode is preferably composed of a conductive material having a relatively low electron emission energy or work function. The material also preferably has a high melting point and relatively high stability under treatment conditions. Examples of suitable materials include metals, alloys, semiconductors, and oxides coated or deposited on conductive substrates. Further examples are high temperature alloys such as tungsten, graphite and nickel-chromium alloys, as well as BaO and Al deposited on conductive substrates.₂O₃Including, but not limited to, metal oxides such as.

Electrodes that act as anodes are composed of conductive materials such as metal or any of the other materials described herein. The anode can have a variety of different geometries depending on the application, such as any of the geometries described herein. The anode is grounded or connected to a voltage level that has a positive bias with respect to the cathode. In order to avoid neutralization of the negatively charged ions at the anode, the anode has an insulating layer such as a ceramic or glass layer on top of the conductive material, among the negatively charged ions at the anode. It is possible to prevent sexualization.

In another embodiment, the application of a magnetic field during the generation of electrons can minimize the neutralization of negatively charged ions to the anode surface. In this embodiment, a magnetic field can be created, for example, by providing one or more magnetic coils or external magnetic sources on either the inside or outside of the ion generator and / or the anode tube. Whether one or more magnetic coils are outside or inside the anode tube, an electric field can move electrons outside the tube, and a magnetic field moves electrons inside the tube. Can be restricted. The strength of the magnetic field created can vary, but is generally 0.1-5,000 Wb / m.²Or 500 ~ 2,000Wb / m²is there. In certain embodiments, a higher magnetic field can reduce the radius of the spiral path for ion movement. In these embodiments, it may be necessary that the radius of the spiral path of the ions be less than the radius of the anode tube.

In certain embodiments of the invention relating to thermal field emission, the cathode or emission electrode can comprise a segmented assembly such as the electrode shown in FIG. In this regard, the core 10 of the emission electrode can be made of a metal with high electrical resistance and can have a plurality of chips 11 exiting the core 10. The chip 11 can be made from a conductive material having a relatively low electron emission energy or work function, such as any of the materials disclosed herein. The core can be heated by passing an energy source (not shown) such as an AC or DC current directly through the core 10. Heat is transferred from the core to the chip 11 by heat conduction. The hot core can be encapsulated in an insulating material 12 having multiple chips 11 exposed to the outside of the enclosure, which are then inserted into the support frame, thereby split into segments as shown. An assembly is formed. The segmented assembly can allow thermal expansion of the core during operation. In this arrangement, electrons can be generated from the hot chip 11 by applying a potential between the cathode and the anode.

In another preferred embodiment of the invention relating to thermal field emission, the temperature of the emission electrode can be increased by indirect heating. This can be achieved by using a heating cartridge as the core of the emission electrode. The surface of the heating cartridge can be made of a conductive material such as metal that is electrically insulated from the heating member inside the cartridge. Multiple dispersed emission chips can be mounted on the surface of the heating cartridge to facilitate electron emission. The cartridge can be heated by passing an energy source such as an AC or DC current through a heating member inside the cartridge. By applying a negative voltage bias to the surface of the cartridge associated with the second electrode, electrons can be emitted from the dispersion chip of the cartridge. To create a voltage bias in this arrangement, the second electrode can be grounded so that the cartridge can be negatively biased, or the cartridge can be grounded so that the second electrode can be positively biased. be able to. In some embodiments, the latter case is between two electrical circuits, one with AC or DC current along the heating member and the other with a high voltage bias between the cartridge surface and the second electrode. It may be preferable to eliminate the potential interference of the. In these embodiments, the hot cartridge electrodes can also act as a heat source for the gas mixture to achieve the temperatures required for the reflow and soldering process.

As mentioned above, a gas mixture consisting of reducing gas is passed through a remote ion generator that houses at least two electrodes. The reducing gas contained in the gas mixture can produce active species that form gaseous oxides by reacting with the following categories: 1) essentially reduced gases, and 2) metal oxides. It can be classified into one or more of the gases that can be produced, or 3) gases that can produce active species that form liquid or aqueous oxides by reaction with metal oxides.

The first category of gas, that is, essentially a reducing gas, includes any gas that thermodynamically acts as a reducing on the oxide to be removed. An example of an essentially reducing gas is H₂, CO, SiH₄, Si₂H₆, Formic acid, such as alcohols such as methanol, ethanol, etc., and some acid vapors having the following chemical formula (III).<chemistry num="1"><img file="JP4846987B2_D0001.tif" /></chemistry>In formula (III), the substituent R can be an alkyl group, a substituted alkyl group, an aryl or a substituted aryl group. As used herein, the term "alkyl" includes linear, branched or cyclic alkyl groups containing preferably 1 to 20 carbon atoms, or more preferably 1 to 10 carbon atoms. This also applies to alkyl moieties contained in other groups such as haloalkyl, alkaline reels or aralkyl. The term "substituted alkyl" refers to heteroatoms such as O, N, S or halogen atoms; OCH₃OR (R is C_1-10Alkyl or C_6-10Aryl); C_1-10Alkyl or C_6-10Aryl; NO₂; SO₃R (R is C_1-10Alkyl or C_6-10Aryl); or NR₂(R is H, C_1-10Alkyl or C_6-10Applies to alkyl moieties having substituents containing (aryl). The term "halogen" as used herein includes fluorine, chlorine, bromine and iodine. As used herein, the term "aryl" includes a 6-12 membered carbocycle with aromatic properties. As used herein, the term "substituted aryl" refers to heteroatoms such as O, N, S or halogen atoms; OCH₃OR (R is C_1-10Alkyl or C_6-10Aryl); C_1-10Alkyl or C_6-10Aryl; NO₂; SO₃R (R is C_1-10Alkyl or C_6-10Aryl); or NR₂(R is H, C_1-10Alkyl or C_6-10Contains an aryl ring having a substituent containing (aryl). In certain preferred embodiments, the gas mixture contains hydrogen.

The second category of reducing gases, although not reducing in nature, produces active species such as H, C, S, H', C'and S'by the dissociative attachment of electrons on the gas molecule. Includes any gas that can form a gaseous oxide by the reaction of the active species with the metal oxide to be removed. An example of this type of gas is NH₃, H₂S, for example CH₄, C₂H₄, Acid vapors with formula (III), and organic vapors with formula (IV) below, but not limited to C₁~ C₁₀Contains hydrocarbons.<chemistry num="2"><img file="JP4846987B2_D0002.tif" /></chemistry>In the chemical formulas (III) and (IV), the substituent R can be an alkyl group, a substituted alkyl group, an aryl group, or a substituted aryl group.

The third category of gas is not reducing in nature, but the dissociative attachment of electrons on the gas molecule can form active species such as F, Cl, F'and Cl', and said active species. Includes any gas capable of forming a liquid or aqueous oxide by reaction with a metal oxide. An example of this type of gas is CF₄,SCIENCE FICTION₆, CF₂Cl₂, HCl, BF₃, WF₆, UF₆, SiF₃, NF₃, CClF₃And contains fluorine and chlorine containing gases such as HF.

In addition to including one or more of the above categories of reducing gases, the gas mixture may further contain one or more carrier gases. The carrier gas can be used, for example, to dilute the reducing gas, or to dilute the reactive gas, or to provide collision stabilization. The carrier gas used in the gas mixture can be any gas that has a lower electron affinity than the reducing gas in the gas mixture. In certain preferred embodiments, the carrier gas is an inert gas. An example of a suitable inert gas is N₂, Ar, He, Ne, Kr, Xe and Rn, but not limited to them.

In one preferred embodiment, the gas mixture comprises hydrogen as a reducing gas and nitrogen as a carrier gas due to its relatively low cost and environmental friendliness of exhaust gas emissions. In these embodiments, the gas mixture comprises 0.1-100 vol%, preferably 1-50 vol%, or more preferably 0.1-4 vol% hydrogen. A hydrogen content of less than 4% is preferred, making the gas mixture nonflammable.

In some embodiments, the gas mixture is passed through an ion generator at a temperature of 3500 ° C., or 150-1500 ° C., from ambient temperature to form the active species. After passing through the ion generator, the gas mixture can then be lowered to processing temperatures such as reflow and soldering temperatures due to surface deoxidation. The pressure of the ion generator can be 1 to 20 atmospheres or 1 to 5 atmospheres. The pressure of the negatively charged ionic reducing gas at the outlet of the ion generator is 1 atm before entering the furnace or processing area using, for example, a flow limiting orifice, back pressure regulator, flow regulator, or similar means. Can be lowered to.

As mentioned above, the parts or workpieces from which oxides should be removed and / or the soldered parts or workpieces are preferably placed in the immediate vicinity of the gas outlet of the ion generator. The distance between the outlet and the top surface of the part can be 0.1-30 cm or 0.5-5 cm. In one preferred embodiment of the invention, the component can be placed on a substrate to provide a target assembly. The substrate can be grounded or have a positive bias potential. In other embodiments, the component can be placed between the outlet of the ion generator and a substrate that is grounded or has a positive bias potential.

In certain embodiments, the remote ion generator and / or component (or target assembly) can be moved. In this regard, the remote ion generator can be in a fixed position and the part can be moved, the remote ion generator can be moved and the part can be in a fixed position, or the ion generator Both parts move. The movement can be vertical, horizontal, radial or along an arc.

FIG. 5 gives, for example, a diagram of one embodiment of the invention used in reflow soldering. The apparatus comprises a dryer or furnace 20 typically having heating / cooling regions arranged in different compartments along the central axis of the dryer / furnace 20. A remote ion generator 21 with at least two electrodes (not shown) is inserted inside the dryer / furnace 20. The ion generator 21 further has a gas inlet 22 and a gas outlet 23. A grounded movable belt 24 made of a conductive material such as metal carries one or more parts 25 such as electronic devices on a printed circuit board, and these parts are on one or more of the parts. It is temporarily connected to each other by a pre-printed solder paste and passes through the dryer / furnace 20 as well as the heating and cooling regions. The gas mixture 26 composed of nitrogen and hydrogen reducing gas is introduced into the ion generator 21 through the gas inlet 22, and the energy source (not shown) acts as a cathode and an anode contained therein. Applies to at least one of them. The gas mixture 26 reacts with the charge between the cathode and the anode due to the electrons generated at the cathode site to become a negatively charged ionic reducing gas 27 with respect to the reducing gas, preferably hydrogen, and passes through the gas outlet 23. To do. The gas outlet 23 is in the immediate vicinity of part 25. The negatively charged ionic reducing gas 27 reduces any metal oxide present on the surface of the component and solder, thereby significantly strengthening the solder bond. The solder paste melts in the heating region of the dryer / furnace 20, wets the surface of the part, and solidifies again in the cooling region of the dryer / furnace 20 to form a soldered product. This eliminates the need for flux at all and avoids solder defects caused by oxides or flux residues.

FIG. 6 gives an example of one embodiment of the remote ion generator 30 of the present invention. The ion generator 30 comprises at least two electrodes, a cathode 31 having the geometry of the coil and a metal anode 32 comprising the walls of the device. The cathode 31 and anode 32 are connected to an external energy source (not shown). The metal anode 32 further comprises a ceramic liner 33 disposed on its surface, as shown. The ion generator 30 further has a gas inlet 34 and a gas outlet 35. The geometry of the gas inlet 34 and the gas outlet 35 can vary with respect to each other to affect the flow velocity of the gas mixture (not shown) passing through the ion generator 30. A gas mixture (not shown) containing a reducing gas and optionally a carrier gas is passed through the ion generator 30. An energy source (not shown), such as a DC voltage, is passed through the cathode 31 and anode 32, thereby creating an electric field and generating electrons in the cathode 31. The electrons generated from the cathode 31 drift in the direction of the electric field. The electrons attach to at least a portion of the reducing gas, which produces a negatively charged reducing ion gas (not shown). The negatively charged reducing ion gas reduces the surface oxide of the component (not shown) through the gas outlet 35.

FIG. 7 provides a diagram for another embodiment of the remote ion generator of the present invention. The ion generator 40 has two gas inlets 41 and 42 flowing into the two concentric chambers 43 and 44, ensuring safety for processing concentrated and reduced gases such as hydrogen. Concentrated hydrogen gas passes through the gas inlet 42 to the main chamber<u style="single">44</u>to go into. Main chamber<u style="single">44</u>Is a secondary chamber purged by a carrier gas such as nitrogen that enters through inlet 41<u style="single">43</u>It is surrounded by. The pressure of the concentrated hydrogen flow is the secondary chamber<u style="single">43</u>The gas flow rate ratio of the concentrated hydrogen stream to the nitrogen stream is adjusted to a level where the total hydrogen concentration in the mixture of the two streams is 4 vol% or less.

FIG. 10 gives an example of an ion generator 50 further including a magnetic coil 51. The device 50 shows the magnetic coil 51 as the magnetic field source, but it is expected that other sources of magnetic field besides the coil can be considered here. The magnetic coil 51 can be present in the internal volume of the anode 54 as shown, outside the anode 54 or on the anode 54, or in various other arrangements. The ion generator 50 has at least two electrodes, a cathode assembly 52 having a "comb" cathode emitter 52 (like the electrode arrangement shown in FIG. 2c) and located within the cathode holder 53, and an apparatus. Consists of including the anode 54, which comprises the wall of. The cathode emitter 52 is composed of a conductive material such as any of the materials disclosed herein. The cathode holder 53 is made of an insulating material. However, in other embodiments, the cathode holder 53 can consist of a conductive material, an insulating material, a semiconductor material, or a combination thereof. In certain embodiments, the cathode emitter chip 58 can reach the internal volume of the anode 54 through multiple perforations in the wall of the anode 54. Alternatively, the chip 58 can be outside the internal volume of the anode 54, such as the region between the cathode emitter and the anode 54.

The cathode emitter 52, anode 54, and magnetic coil 51 are connected to an external energy source as shown. In certain embodiments, the metal anode 54 can consist of one or more layers of conductive material, an insulating material that is at least partially coated with the conductive material, or a variety of other configurations. The ion generator 50 further has a gas inlet 56 and a gas outlet 57. The geometry of the gas inlet 56 and the gas outlet 57 can vary with respect to each other to affect the flow velocity of the gas mixture (not shown) passing through the ion generator 50. A gas mixture (not shown) containing a reducing gas and optionally a carrier gas is passed through the ion generator 50. Energy such as a DC voltage over source, is applied between the cathode emitter 52 and anode 54, thereby creating an electric field, the cathode emitter 52 to generate electrons. A magnetic field generated by passing an electric current through the coil 51 or other magnetic source can assist the induction of electrons from the ion generator 50 to the gas outlet 57. The size and number of perforations 55 on the anode 54, the number of turns on the magnetic coil, the outline, number and angle of the conductive chip 58 on the cathode emitter 52, the gap between the chip of the cathode emitter 52 and the surface of the anode 54, two Parameters such as, but not limited to, the voltage between the electrodes, the current of the magnetic coil 51, and the geometry of the anode 54 can achieve varying amounts of electrons.

The methods disclosed herein can be used in multiple areas of electronic assembly in addition to soldering, such as, for example, surface cleaning, plating, brazing, welding, and reflowing wafers with solder bumps. In one embodiment, the invention is used for the reflow of solder bumped wafers, such as the method provided in US Application No. 10 / 425,405, co-pending filed April 28, 2003. , This patent application has been assigned to the assignee of the present invention. In one particular embodiment, this method can be used to reduce surface oxides of metals such as copper oxide formed during silicon wafer processing. Such oxides may be formed as a result of various wet treatment steps used to form microelectronic devices on wafers, such as chemical mechanical flattening. These surface oxides reduce the yield of the device and the reliability of the device. This method allows the surface oxides to be removed in a sufficiently dry and environmentally friendly manner that does not require the use of aqueous reducing agents. Moreover, since this method can be performed at relatively low temperatures, it does not significantly affect the device's thermal budget during processing. In contrast, at higher temperatures, the diffusion of dopants and oxides tends to reduce the yield and reliability of the device, which in turn reduces the performance of the device. Since this method can also be carried out on a single wafer, it can be integrated with other single wafer processes, thereby providing better compatibility with other manufacturing processes.

The present invention will be described in more detail with reference to the following examples, but it should be understood that the present invention is not considered to be limited thereto.

[Example 1] The first experiment was performed using a lab-scale tube furnace with a downward metal rod with a sharp tip (see Figure 2b for cathode geometry) inserted near the center of the furnace. The sample used was a fluxless tin-lead solder premold (melting point 183 ° C) on a grounded copper plate (anode), which was loaded inside the furnace and N₂Medium 5% H₂It was heated to 250 ° C under the gas flow of. When the sample temperature reached equilibrium, a DC voltage was applied between the cathode and the grounded sample (anode), gradually increasing to about 2 kV with a current of 0.3 mA. The distance between the two electrodes was about 1 cm. The pressure was the surrounding atmospheric pressure.

The solder was found to be sufficiently wet on the copper surface. Achieving good wetting of the fluxless solder on the copper surface without applying voltage is pure H at such low temperatures.₂Even can never be done. Because pure H is effective in removing tin oxide on tin based solder₂This is because the temperature of is higher than 350 ° C. Therefore, this result shows that the electron adhesion method is H.₂Has proven to be effective in facilitating fluxless soldering.

Small tests have shown that negatively charged hydrogen ions are much more reactive than natural hydrogen gas molecules. Therefore, by using this new approach, hydrogen reduction of oxides on solder and base metals can be greatly facilitated. Fluxless soldering of hydrogen under ambient pressure (N₂Medium 5 vol% H₂) Is reduced to the normal soldering temperature range (&lt;250 ° C).

[Example 2] By using the field emission mechanism using the same setting conditions as in Example 1, several cathode materials were investigated for hydrogen fluxless soldering with electron adhesion support. The results of the survey are shown in Table 1.

As shown in Table 1, the best results were obtained by using a Ni / Cr cathode. It gave the highest melting efficiency and therefore the shortest wetting time. A possible reason is that the Ni / Cr cathode produces a relatively large amount of electrons and has a suitable electron energy level compared to other cathode materials.

<tables num="1"><img file="JP4846987B2_D0003.tif" /></tables>

[Example 3] H of electron adhesion support using a remote ion generator₂To demonstrate the feasibility of fluxless soldering, experiments were performed with remote ion generators made using downward Ni / Cr cathode rods with sharp tips. The anode consisted of a copper plate covered with a ceramic layer. The electric field applied between the two electrodes was about 2 KV / cm. The remote ion generator was set in front of the test sample, in other words, near the gas inlet of the furnace. The distance between the remote ion generator and the test sample was about 2-4 cm. A test sample consisting of a Sn / Pb solder premold (melting point 183 ° C) on a copper plate was grounded. A furnace loaded with a remote ion generator and a test sample, H₂And N₂Gas mixture (about 5 vol% H)₂It was found that the solder began to wet on the copper at about 220 ° C when purged and heated in).

[Example 4] The idea of using a hot cathode (thermal field emission) with an electric field to increase the efficiency of electron emission was tested using a Ni / Cr coaxial cable (0.004 inch in diameter) hanging in the center of a vertically oriented tubular furnace. Proven. This cable was heated by AC power and connected to the negative electrode side of the DC power source to provide a hot cathode. In the same furnace, again, a grounded metal plate was hung parallel to the hot cable to provide the anode. The gap between the anode and cathode was 1.43 cm. Cathode temperatures were measured at room temperature or at various temperatures from ambient temperature to 650 ° C using thermocouples in contact with the cathode surface. During the experiment, a nitrogen stream was maintained in the furnace to generate electrons from the hot cathode.

Emission currents at different cathode temperatures are given in Figure 8 as a function of the DC voltage applied between the two electrodes. FIG. 8 shows that when the temperature of the cathode rises above the ambient temperature above 200 ° C, the emission current of the cathode increases considerably. The largest increase in emission current occurred below 400 ° C.

This example was conducted to investigate the effectiveness of the thermal field emission method for generating electrons. A 3 mm diameter graphite rod with multiple 1 mm long machined chips protruding from its surface acted as a cathode, which had a geometric shape similar to that shown in Figure 2h. Each of the protruding machined chips had a tip angle of 25 degrees. Graphite rod 5% H₂And 95% N₂In the gas mixture of, it was heated to about 400 to 500 ° C by resistance heating using an AC power source. A DC voltage source of 5 KV was applied between the graphite cathode and a copper plate acting as an anode and having a gap of 1.5 cm between the graphite cathode. It was shown that all the chips on the graphite rod were brightened, which allowed them to generate electrons uniformly from the dispersed chips on the graphite rod. Without heating the graphite rod, there was no electron generation from the cathode or arc generation between one of the chips and the anode plate. This demonstrates that the combination of the use of a cathode with a large number of chips and the elevated temperature, i.e., the thermal field emission method, is effective in obtaining uniform electron generation from an integrated emission system. are doing.

[Example 6] This example shows two machined Als like the electrodes shown in Figure 4.₂O₃This was done using a horizontally fixed 0.04 inch diameter nickel-chromium alloy heating wire between the refractory plates. A series of five nickel-chromium alloy release wires, each with a sharp tip (12.5 degrees) at one end of the wire, protrudes straight from the nickel-chromium heating wire and is between the two refractory plates. Arranged vertically in. Nickel-chromium heating wire and chips, 5% H₂And 95% N₂It was heated to about 870 ° C using an AC power source in the gas mixture of. A DC voltage of 2.6 KV was applied between the cathode and a copper plate that acted as the anode and had a 6 mm gap between the two electrodes. All five chips were brightened and the total current emission reached 2.4mA. Without heating the wire, there was no electron generation from the cathode or arc generation between one of the chips and the anode plate. Similar to Example 5, Example 6 demonstrates that thermal field emission provides uniform electron generation. Moreover, because of the higher temperature of the cathode, it also increases the amount of electron production at a given potential.

[Example 7] This example demonstrates the performance of a hot emitting electrode with a large number of chips on the surface where the hot electrode is achieved by indirect heating and the voltage applied between the two electrodes is a pulse. The emission electrode had a heating cartridge as its core. The enclosure of the cartridge is made of stainless steel that is electrically insulated by the heating members inside the enclosure. Six nickel / chrome wires, each with two sharp tips (tip angle 12.5 degrees) extending 90 degrees from each other, were inserted into the grooves distributed on the cartridge surface as shown in Figure 3. 5% H₂And 95% N₂The cartridge was heated to a temperature of 800 ° C. by passing an AC current through the heating member in an atmosphere containing. A pulsed DC voltage source was applied between the surface of the emission electrode and the grounded copper electrode. The gap between the two electrodes is about 1 cm.

FIG. 9 shows that the emission current increases as the amplitude and frequency of the voltage pulse increase.

Although the present invention has been described in detail and with reference to these examples, it will be appreciated by those skilled in the art that various modifications and improvements can be made in the present invention without departing from the spirit and scope of the invention. It will be clear.

<figref num="1a">The voltage pulses for the cathode and anode are illustrated.</figref><figref num="1b">The voltage pulses for the cathode and anode are illustrated.</figref><figref num="2a">It is a schematic of the various electrode designs of the present invention.</figref><figref num="2b">It is a schematic of the various electrode designs of the present invention.</figref><figref num="2c">It is a schematic of the various electrode designs of the present invention.</figref><figref num="2d">It is a schematic of the various electrode designs of the present invention.</figref><figref num="2e">It is a schematic of the various electrode designs of the present invention.</figref><figref num="2f">It is a schematic of the various electrode designs of the present invention.</figref><figref num="2g">It is a schematic of the various electrode designs of the present invention.</figref><figref num="2h">It is a schematic of the various electrode designs of the present invention.</figref><figref num="2i">It is a schematic of the various electrode designs of the present invention.</figref><figref num="3">An example is provided for one embodiment of a emission electrode with a plurality of tips.</figref><figref num="4">An example is provided for one embodiment of a emission electrode having a segmented assembly.</figref><figref num="5">An example of one embodiment of the present invention illustrating distant ion generation is provided.</figref><figref num="6">An example is provided for one embodiment of the ion generator of the present invention.</figref><figref num="7">Further examples are provided for one embodiment of the ion generator of the present invention.</figref><figref num="8">A graph representing [voltage] vs. [current for various cathode temperatures] is provided using one embodiment of the apparatus of the present invention.</figref><figref num="9">[Emission current] vs. [Frequency and amplitude of pulse voltage applied between two electrodes] using one embodiment of the apparatus of the present invention is shown.</figref><figref num="10a">A disassembled side view of an additional embodiment of the ion generator of the present invention is provided.</figref><figref num="10b">A detailed view of the anode for an additional embodiment of the ion generator of the present invention is provided.</figref><figref num="10c">A detailed view of a cathode holder for an additional embodiment of the ion generator of the present invention is provided.</figref><figref num="10d">A detailed view of a cathode emitter for an additional embodiment of the ion generator of the present invention is provided.</figref>

Code description

20 dryer / furnace 21 Remote ion generator 22 Gas inlet 23 Gas outlet 24 Movable belt 25 parts 26 gas mixture 27 Ionic reducing gas