atmospheric pressure plasma for surface modification (wolf/atmospheric) || emerging and future...

12 Emerging and Future Atmospheric Plasma Applications

12.1 Solar and Other Alternative Energy Systems

The use of plasma surface modification technology in solar (photo-voltaic) cell manufacturing and within other alternative energy plat-forms has heretofore been used primarily in applications such as the deposition of amorphous hydrogenated silicon nitride (SiN) layers in a vacuum plasma-enhanced chemical vapor deposition (PECVD) process to create anti-reflection and surface (and bulk) passivation on thin-film solar cells, or the use of vacuum plasma etching in barrel-type reactors to perform edge isolation in some remaining fabrication processes. As photovoltaic cell manufacturing processes evolve, and with the added pressures of increasing hazardous chemical waste disposal costs, there has been interest in atmospheric plasma systems as efficient dry etching, surface cleaning and adhesion promotion process tools. This section examines these systems and details etch-ing, cleaning and bonding trial data confirming system efficacies.

The use of plasmas in the fabrication of photovoltaic cells is highly dependent upon the materials employed and the processing

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Atmospheric Pressure Plasma for Surface Modification by Rory A. Wolf

Copyright © 2013 Scrivener Publishing LLC

206 ATMOSPHERIC PRESSURE PLASMA FOR SURFACE MODIFICATION

cycle requirement. For example, vacuum plasmas are not suit-able for use in solar cell processing when high throughput on a continuous basis is required. Vacuum plasma chambers built for SiNx deposition are typically batch process related, but are also designed to work in a semicontinuous mode through the intermit-tent exchange of treatment materials within the vacuum chamber after the treatment is completed and one atmospheric pressure is returned. However, this process is still not economical for high-throughput plasma surface etching, cleaning and functionalization.

There are at least four major generations of photovoltaic cells whose materials define the application of plasma technology to their fabrication. They are:

1. Large-Area, Single Layer P-N Junction Diode -Typically made using a silicon wafer and the domi-nant technology in the commercial production of solar cells, accounting for more than 86% of the solar cell market.

2. Rigid and Flexible Thin-Film Solar Cells -Semiconductor deposition materials used include amorphous silicon, polycrystalline silicon, micro-crys-talline silicon, cadmium telluride, and copper indium selenide/sulfide. Typically, the top surface is low iron solar glass for rigid cells (a fluoropolymer for flexible cells), the encapsulant is crosslinkable ethylene-vinyl acetate (EVA), and the rear layer is a Tedlar-PET-Tedlar laminate (although glass, coated PET, or another bond-able polymeric film are also used).

3. Photoelectrochemical, Polymer and Nanocrystal Cells - Do not rely on a traditional p-n junction to separate photogenerated charge carriers. Polymer cell materials used include polyester (PET) foil, indium tin oxide (ITO) film, polyethylenedioxythiophene (PEDOT) and aluminum. Nanocrystalline cells use thin-film materials and are overlayed on a supporting matrix of conductive polymer or mesoporous metal oxide.

4. Composite (Hybrid)Photovoltaic Technology - One example is the use of polymers with nanoparticles to make a single multi-spectrum layer which can be stacked to make multi-spectrum solar cells.

EMERGING AND FUTURE ATMOSPHERIC PLASMA APPLICATIONS 207

Bulk silicon technologies, such as those employing wafer-based manufacturing, feature self-supporting wafers between 180-240 micrometers thick which are processed and then soldered together to form a solar cell module. Organic and polymer solar cells are built from thin films (typically 100 nm) of organic semiconductors such as polymers, and small-molecule compounds like polyphen-ylene vinylene, copper phthalocyanine (a blue or green organic pigment), and carbon fullerenes. Considering the wide range of materials employed to maximize solar efficiencies, the ability to integrate the completely continuous in-line manufacturing of rigid panel and flexible solar cells by utilizing a variable chemistry surface modification technique relative to complex material con-structions holds the prospect of significantly reducing manufactur-ing costs. Atmospheric pressure gas phase plasma technology will therefore become essential for future in-line manufacturing of solar cells if major reductions in fabrication costs are to be achieved.

Solar cell processes transferred to atmospheric pressure plasma processes are dry etching, surface cleaning, etching, and activation. Layer reductions using hydrogen-based atmospheric glow dis-charge plasmas are also an employed aspect of the technology.

As described above, there are a significant number of solar tech-nology platforms, many of which are undergoing cost reductions and efficiency improvements to enable or extend their commercial viability. Cleaning and functionalizing the surface of flexible base films and foils in a continuous process prior to panel fabrication to

Minimum power density - thin films w/ft2/min.

Minimum power density

fi J rf^J' & 4* «* <? <f<f<f<f # * Figure 12.1 Minimum atmospheric pressure plasma power densities - PV thin films.


improve thin film adhesions and output efficiencies can be critical in achieving commercialization. Moreover, avoiding the use of wet chemical cleaning solutions in favor of "green" process techniques which do not generate VOCs or waste effluents can also signifi-cantly improve commercial returns.

Given the process benefits of APT above, an experimental study was performed by this author employing this continuous process. A microcosm of solar cell base materials were exposed to an APT process for the specified treatment purposes of optimizing inter-facial adhesion and improving solar cell output efficiency.

The treatment protocols identified the base plasma inert gas chem-istry, assisted by a reactive oxygen component, which was determined to optimize treatment results relative to the solar cell application. For example, specific peel adhesion benchmarks were targeted for PVC adhesion to a solvent-based adhesive. Relative to cleanliness benchmarks, prespecified low level organic particle contamination concentrations were established to optimize lamination adhesions. The required power densities applied to each protocol were prede-termined relative to the required surface effect by laboratory trials on commercial roll-to-roll and tangential atmospheric plasma surface treatment systems at the Enercon Industries pilot facility.

One specific experimental study employed polyimide film which was surface treated by APT at a power density of 20W/ft2/in using an argon/oxygen plasma. Surface tension was raised from its inherent level of 40 dynes/cm to water wettability, or 72 dynes/cm. Polyimide film was also treated using a reactive silane-based wet chemical primer treatment. Both surface treated films were lami-nated to aluminum foil and then subjected to a foil strain gauge test. The peel force results indicated a 22% improvement in bond-ing strength using APT surface modification vs. chemical primer pretreatment.

Metal foils are in widespread use in photovoltaic applications, and particularly with copper indium gallium selenide (CIGS) cells in the form of poly crystalline thin films. CIGS PV manufactur-ers require specific metal foil alloy formulations and dimensions, which are not uncommon formulations for metal foil providers. With the use of foil-based cells, copper and other materials replace silicon as the semiconductors. Key advantages of solar cells con-structed with flexible foils include their ability to withstand high temperatures during further processing, they experience low impact from evaporation, they are highly etchable, and they can


contain side electrodes which act as contacts for powering auxil-iary units. However, moisture transport, adhesion, and corrosion protection of PV module packaging materials relies in part upon clean foil surfaces for improving adhesion to glass and polymer (encapsulant, backsheet) component surfaces to prevent ingress and maximize efficiency. Atmospheric plasma pre-cleaning of foils in continuous roll-to-roll processes has been found to be a low cost, dry, and highly efficient method for removing organic surface contaminations from PV foils without the generation of chemical waste effluents compared to wet-cleaning processes. We will now examine these systems and detail etching, cleaning and bonding information confirming system efficacies.

Thin-film PV cells fabricated from CIGS technology, for example, have the potential to produce energy at a higher efficiency than crystalline Si and GaAs solar cell technologies. CIGS solar cells also have excellent chemical stability and a tolerance against high radia-tion. Because CIGS solar cell panels are manufactured from smaller cells which are connected by labor intensive welding processes, cost advantages materialize when CIGS thin films are deposited on metal foils by using continuous roll-to-roll manufacturing pro-cesses. This method of fabricating these lightweight PV cells enables flexible application of these cells to a much broader range of sup-ported surfaces. To further monolithic CIGS-based platforms with foils, new developments involving the use of glass as an insulating layer on foils offer the opportunity to support high processing tem-perature resistance (up to 550°C) and high dielectrics. Insufficient cleaning of these foils can suboptimize the glass-foil bond and create delamination during in-process thermal expansion, as well as pinholing effects.

Metals being integrated into thin-film PV systems include stain-less steel, aluminium, copper, iron, nickel, silver, zinc, molybde-num, stainless/copper alloy, copper/nickel alloy, and other alloys or multilayers. Economic preferences have elected aluminium, iron, copper, and alloys of these materials. Considering both perfor-mance and cost, aluminium, electroplated iron, and electroplated copper are ranked high. Surface etchings of these metal foils typi-cally employ Lewis acids and Bronsted acids. Specifically, copper is typically etched by ferric chloride, nitric acid, or sulfuric acid. Aluminium can be etched by caustic soda. The preferred foil thick-ness range for etching is roughly about 5-50 pm, with most PV foils found between 1-500 μπι.


Surface cleaning of these foils by wet cleaning processes for thin-film solar cells will employ deionized water and tenside surfactants in a relatively lengthy processing sequence. The total system cycle time can be from 10-15 minutes, and is dependent upon the processes employed. The typical wet cleaning system footprint is eight meters in length, 3-4 meters in width (depending upon the foil width) and three meters in height. Surface cleaning via atmospheric plasma techniques reduces organic contamination on the surface in the form of residues, antioxidants, carbon resi-dues and other organic compounds. Oxygen-based atmospheric plasmas in particular are effective in removing organics whereby monoatomic oxygen (Ο+, O-) reacts with organic species resulting in plasma volatilization and removal.

Relative to cleanliness benchmarks, prespecified low-level organic particle contamination concentrations were established by this author in another study to optimize lamination adhesions. The specified plasma gas mixtures applied to each protocol were predetermined relative to the required surface effect by laboratory trials on commer-cial continuous roll-to-roll and tangential atmospheric plasma sur-face treatment systems at the Enercon Industries pilot facility. The minimum power densities required to achieve a goniometer surface contact angle of <5 degrees averaged between 76 W/m 2 /min . and 120 W/m 2 /min. The contact angle benchmark, although nonquan-titative, is an effective indicator of relative surface cleanliness. An idealized, perfectly clean metal PV foil surface would have a contact angle of 0°, which is impossible to obtain in laboratory air. A contam-inated foil would have a high contact angle, such as 90° or more. The effective plasma discharge area was 38 mm in width with a length corresponding to the foil web width. The atmospheric plasma treat-ment was conducted at a foil conveyance speed of 10 mpm.

As another key indicator of cleaning performance, peel adhe-sion tests were conducted by the author on these foil materials at the same minimum power densities to achieve <5 degree contact angles. Improvements in peel adhesion were significant. Values ranged from 41 percent to nearly 71 percent using the argon-oxygen mixture for all trials.

Atmospheric plasma cleaning protocols that offer real opportu-nities for in-line continuous improvements in surface cleanliness as a dry cleaning process are gaining early adoption within thin-film PV foil cleaning operations. The key drivers for this trend of conver-sion from chemical wet cleaning processes include the following:


• Continuous, roll-to-roll process • Significantly lower production floor footprint • Significantly lower capital cost • No VOCs or chemical effluent disposal costs • No water supply costs

Solar power is a technology of the present and the future. However, successful commercialization of low cost, high effi-ciency fabrications is highly dependent upon fabrication methods which employ continuous processing techniques. One major issue encountered in solar cell construction is the adhesion of thin-film solar cells on polyimide substrates. This author evaluated the adhe-sion promotion potential of variable chemistry atmospheric plasma surface modification against wet primer chemistry on a polyimide-based substrate to determine their comparative bonding strengths to aluminum foil. It was apparent that APT is a viable continuous and environmentally friendly process alternative to batch plasma and surfactant-based surface modification protocols.

12.2 Energy Storage Technologies

Considering energy storage applications for atmospheric pres-sure plasmas, the application which has become an early adopter of these plasmas is the lithium ion battery manufacturing sector. This is due to the fact that enhanced surface properties of separa-tor and foil components of lithium ion batteries are achieved using atmospheric plasma technology. Specific outcomes from this adop-tion include the creation of high performance and cost effective surface characteristics of foils and separators using in-line, continu-ous atmospheric plasma technology for practical applications in lithium-ion polymer batteries. The modified separator membranes utilizing the atmospheric plasma modification process showed sig-nificantly improved wettability and electrolyte retention. Plasma treatment of foils leads to high surface tension and cleaning of surface contaminations to improve interfacial adhesion with coat-ings. The profiles of research in this field suggests that significant cost reduction in lithium-ion battery fabrication can be achieved and battery performance enhanced by applying in-line, continuous atmospheric plasma surface modification techniques.


Minimum APT power density - base w/m2/min.

140

Stainless/ copper alloy

Copper/ nickel alloy

Stainless steel

Figure 12.2 Minimum atmospheric pressure plasma power densities - foils.

Lithium-ion batteries (LIBs) are rechargeable battery designs in which lithium ions move from the negative electrode to the positive electrode during discharge, and back when charging. There are wide variations in LIBs relative to the chemistries used and their perfor-mance. Principally speaking, the cells of LIBs utilize an intercalated lithium compound as the electrode material instead of metallic lith-ium. During a LIB discharge, lithium ions (Li+) carry current from the negative to the positive electrode through the nonaqueous elec-trolyte and separator (see Figure 12.1). During charging, an electrical power source applies a higher voltage than that produced by the bat-tery, forcing the current to pass in the reverse direction. The lithium ions migrate from the positive to the negative electrode, where they become embedded (intercalated) within the porous electrode material.

From a construction standpoint, LIBs are primarily composed of an anode, a cathode, and electrolyte. The anode is typically con-structed from carbon (mostly graphite), the cathode is composed of a metal oxide such as lithium cobalt oxide, and the electrolyte is a lithium salt in an organic solvent. The electrolyte is a mixture of organic carbonates such as ethylene carbonate or diethyl carbonate containing complexes of lithium ions [98]. Because pure lithium is very reactive with water and will form lithium hydroxide and liber-ate hydrogen gas, a nonaqueous electrolyte is typically used within a sealed container to exclude water from the battery package.

The separator is critical to the performance of LIBs. Its primary function is to enable the transport of ions between the electrodes, and to prevent contact between the two electrodes. The design and fabrication of separators ultimately controls power density


and cycle longevity. Among its key performance characteristics, extremely high wettability in electrolyte solution will directly influ-ence and facilitate LIB performance.

Separators can be constructed of a single layer of fiberglass cloth, or a flexible plastic film made from nylon (polyamide, or PA), poly-ethylene (PE), polypropylene (PP), or a triple layer of PP-PE-PP. Functionally, it must be thin and porous to allow charged ions to pass through it without any impediment. The material used must also be resistant to the potential penetration of metal burrs or den-drite growths from the electrode plates, or from any surface con-tamination emanating from the electrode coating to prevent the potential for short circuits between the electrodes. The separator material must also be resistant to high operating temperatures so as to prevent melting of the separator fibers which could reduce or block its porosity or change its penetration resistance. LIB cells can have a discharge operating range of between -40°C and +70°C.

Structurally, these separator materials will not readily absorb polar solvent-based electrolyte solutions with high dielectric con-stants, such as ethylene carbonate (EC), propylene carbonate (PC), and γ-butyrolactone (GBL), because of their hydrophobic surfaces with low surface energy, and because they have a poor ability to retain the electrolyte solutions [99, 100]. Surface modification of polyolefin separator substrates which does not perforate the mem-brane structure is required to make them hydrophilic.

Both the anode and cathode of LIBs have a composite structure. Most LIBs currently in production will incorporate a carbonaceous coating to a copper foil substrate which acts as the anode, or nega-tive electrode. The polymer-based binder will typically be Teflon or a polyvinylidene dichloride (PVdF) which is coated on the cop-per foil, designed as the current collector. These copper foils will have an extremely fine grain structure and low surface roughness profile. As such, uniform adhesion of carbonaceous coatings to this smooth surface requires the elimination of organic and oxide con-taminations. During treatment, copper dendrites may be deposited onto the foil surface to enable bonding to these binder resins for uniform coating in LIB applications. However, interference with surface adhesion due to low molecular weight particle contami-nation requires surface pre-cleaning to promote binder adhesion, as well as for post-process coatings applied to prevent oxidation during shipping, storage, lamination, and post-bake cycles. As pro-filed within solar cell manufacturing processes above, atmospheric plasma treatment devices allow for completely homogenous


surface modification and cleaning of LIB foils without filamen-tary discharges because a uniform and homogenous high-density plasma at atmospheric pressure and low temperature is produced.

A LIB separator is typically a polypropylene microporous mem-brane made from a beta-nucleated precursor, and has an electri-cal resistance of less than 30 ohms-inches per mil, and a puncture strength of greater than 400 grams-force per mil. There are four pri-mary variables which can affect surface modification of LIB separa-tors by the atmospheric plasma treatment process, namely: 1) the substrate gsm, 2) prior substrate surface pretreatment, 3) surface post-treatment power density (power setting relative to the dis-charge assembly length, line speed, and power level), and 4) sur-face treatment chemistry (the type and proportion of chemistries).

In an experimental design developed by this author, the levels employed with these parameters are found below:

• Substrate: Spunbonded polypropylene sheets • Pretreatment: None • Power Density: 16 W / ft2 / min • Post-Treatment: By plasma, on both sides of the

substrate • Treat Chemistry: 1) Air (ambient) plasma and 2) nitro-

gen plasma

The use of an "air plasma" and a nitrogen-based plasma were chosen for two primary reasons:

• Air plasma trials would represent a commonly used and economical in-line roll-to-roll corona discharge treatment protocol. It is a highly filamentary discharge, and these micro-arcs were theorized to have more of a tendency to find a path to ground rather than directly modify the fiber surfaces and overall structure to improve breathability.

• Nitrogen-based plasma trials would represent an in-line roll-to-roll surface treatment protocol which was theorized to not only envelop the surface fibers and structure of nonwovens with a high density plasma, but also because the nitrogen as an relatively abun-dant and inexpensive gas would increase surface wet-tability and breathability following treatment.


The Enercon atmospheric plasma treatment (APT) system applied to this experimental is described as follows:

• Individual hand sheets of 20 gsm, 50 gsm and 75 gsm spunbonded polypropylene were taped to a carrier web for conveyance through a 60" wide atmospheric plasma system.

• Power density and operating speeds were established on the basis of Enercon experience in plasma treat-ment of nonwoven substrates for multiple industry segments.

• Web and sample material conveyance through the plasma treater created a plasma exposure of <1/10 of a second on each side.

• Treated materials were removed from the carrier web and packaged for testing.

The study discovered that atmospheric plasma treatment within an inert (N2) gas atmosphere using oxygen as the reactive gas mixture component significantly increased MVTR and structure breathability.

12.3 Aviation and Aerospace Applications

Within the aeronautics space industry, the application of atmospheric pressure plasmas can be seen in both the development of leading edge military craft, as well as in the maintenance and upkeep of these vehicles. It has been estimated that greater than $4 billion con-stitutes the value of aircraft and their components which are rein-spected and/or repaired any given year. A large majority of these projects require that these steel, copper, titanium, aluminum, polymer and composite surfaces must be cleaned prior to painting or coating. As is current practice in component repair facilities, parts are cleaned using solvents such as methyl ethyl ketone, methyl isobutyl ketone, denatured isopropol alcohol, and hexane. As envi-ronmental laws continue to evolve, protocols involving solvents are slowly being prohibited for use in these critical applications due to VOC and effluent emissions. More importantly, military specifica-tions (mil specs) are being revised to set performance standards for new process cleaning approaches for these components. One such


process now includes atmospheric pressure plasma technology, specifically reacting either helium or argon with oxygen to oxidize and vaporize hydrocarbon deposits prior to decoration or identifi-cation marking. Additionally, the process of organic volatilization does not require VOC-generating liquid chemical reactants to cata-lyze the cleaning process at the localized treatment area.

Another expanding aeronautical application of atmospheric plasmas involves the activation of carbon fiber composites for com-mercial and military aircraft components. Many of these composite structures also incorporate laminations of dissimilar, high perfor-mance materials which can also be very nonpolar. Atmospheric plasma regimes have been quite capable of activating carbon composite, PTFE, Kevlar and Kapton surfaces, for example, for improved interlay er adhesions.

As an application example where atmospheric plasmas are use-ful beyond cleaning and adhesion promotion in aeronautics, let us consider the lift performance of an airfoil. As the angle of attack increases, the airfoil's performance will be limited by its ability to maintain boundary layer air attachment to the suction surface. When air pressure gradients become sufficiently large, the boundary layer air will lose momentum and separate from the airfoil surface. To re-energize the boundary layer air and re-establish attachment, it has been customary to use vortex generators or pneumatic jets. More recently, an atmospheric plasma regime has been successfully applied to impart momentum to the boundary layer for the same result [101-106]. It has also been employed to create boundary layer separation control with airfoils and turbine blades to increase lift and to lower drag. In this application, the methodology employs a plasma in the boundary layer where there is a possibility of sepa-ration. Excited ions and electrons impart momentum through col-lisions with the air molecules, forcing them to remain attached to the airfoil. The AC-based system utilizes a dielectric-covered lower electrode and an exposed top electrode. Applied voltage can range up to 5 kV, and frequency up to 10 kHz.

12.4 Electronic Device Fabrication

The relentless drive to higher performance and greater miniatur-ization, coupled with the need to incorporate more functionality in electronic goods is putting continuous pressure on electronics


designers and manufacturers to increase the packaging and inter-connect densities of their electronics assemblies. This has resulted in semiconductor components having greater numbers of inter-connects and smaller packaging profiles. In order to accommodate these packages, and higher density interconnects, all the features on a PCB, such as track widths and hole diameters are all having to become smaller as well. As evidence of this, PCBs with layer counts from one layer to 50+ layers are being produced. Circuit board sizes range from less than the size of a fingernail to as large as a dinner table. Copper foil can reach thickness as thin as 1000 angstroms. Circuit features in some applications are discernable only with a microscope with metal traces ranging down to less than 10 micron. Consequently, the performance demands on PCB laminates are also increasing significantly and there is a move to introduce new laminate materials that can meet these challenges.

As a result of this trend, traditional manufacturing methods for pretreating new generation components of these layered boards and removing residue are no longer as cost effective. More specifi-cally, process flows which employs low-pressure (vacuum) plasma chambers has traditionally been labor intensive in its use to provide surface treatment and cleaning of PCBs of historical configurations. When considering plasma treatment in vacuum, the low pressure levels in vacuum coating chambers allow the generation of a uni-form plasma, usable for effective treatment of many surfaces. As such, the technology is also widely used for web-coating applica-tions and for the treatment of 3D objects like automotive bumpers. Although the uniformity of the plasma allows for high treatment levels, these applications require cycle times which may not meet new equations for worldwide competitiveness.

Innovations in PCB material technology, such as those which offer higher thermal resistance, and the use of smaller geometries as portrayed above, have led to an increased requirement for plasma processing at higher speeds and at multiple steps in the manu-facturing cycle. Multilayer PCBs with high-density interconnects require designs with finer pitch, and the use of new material tech-nologies with high yield potential. Laminates which include those based on cyanate esters, allylated polyphenylene ethers, and the so-called BT-epoxy and tetrafunctional epoxy systems may resolve the coefficient of thermal expansion and speed issues, but new impasses to productivity can arise during the board manufacturing process where the use of traditional processes is limited. As another


example, it is known that chemical processing with high pH per-manganate solutions is limited due to the inability of the fluid to fully penetrate the small vias found in multilayer PCBs. It is also known that wet chemistry approaches which involve acid etching have difficulties etching polyimide dielectric materials.

Certainly, complex PCB constructions with ever-increasing lay-ers and circuit densities benefit from plasma treatment for descum, desmear, removal of carbon-based organics, PTFE activation, and surface preparation. Plasma processing of various board types with high aspect ratio holes and various via configurations using vacuum plasma technologies have been historically sufficient. However, although consumption costs are moderate with vacuum plasma systems, initial capital costs can be substantial as the need for increased processing capacities are considered.

The high functionality of a uniform plasma discharge in vacuum has driven many efforts to establish a uniform glow discharge at atmospheric pressure, making this technology applicable to pro-cesses at atmospheric pressure and hence avoiding expensive vac-uum equipment. Efforts have been reported from groups around the world. A stable glow discharge free of filamentary streamers has been developed using noble gases, whose high metastable phases allow the steady glow discharge. In order to reduce the consump-tion of these gases to a minimum the gas is injected directly into the discharge gap (between the PCB component and the discharge electrode). Doing it this way also allows the injection of other treat-ment gases, which get highly ionized in the discharge and which address the specific chemistry required for a particular PCB appli-cation. Depending on the gas or combination of gases, a different reaction can occur on the sample surface.

Atmospheric plasma glow discharges are being used as a dry etch process for the removal of drill smear and for etchback. Desmearing holes refers to the removal of a small amount of epoxy resin from the hole barrel, including any that may have been smeared across the copper interface during drilling. The smear on the copper sur-face, if not removed, will prevent interconnection between it and the electroless copper which is to be plated in the hole barrel. Etchback, performed less frequently on standard materials due to the advances in the performance of desmear chemistries and the subsequent relaxation of most specifications, is the removal of a significant amount of epoxy resin and glass fibers (as much as 1-3 mils) that leaves the copper interface protruding into the hole. The


protruding copper surface allows a large surface area for the inter-connection with the subsequent copper plating and the surfaces exposed by the removal of epoxy that could not be smeared during drilling.

Kapton® and Teflon® activation has also been accomplished by atmospheric plasma processing. For example, atmospheric plasma has increased the surface energy of Teflon from 18 dynes/cm to levels which provide excellent lamination and wettability for plat-ing through holes without use of wet chemicals. In fact, all PTFE materials require plasma activation to change the surface energy for electroless copper adhesion.

Atmospheric plasmas are being employed to remove photore-sist residue that can remain after developing fine pitch circuitry on panels and inner layers. Currently, photoresist is stripped from the outer layers using wet chemistries, and often in the same bath or spray chamber as the inner layers. Although resist stripping is a one-tank operation, both the developer and the stripper have short bath lives (often measured in hours), and these operations generate a large volume of waste process fluid. With the appropriate amount of dwell or exposure time, utilizing atmospheric plasma processes can significantly reduce, if not eliminate, the generation of waste stripping fluids.

There are many construction variations with regard to printed circuit boards. For instance, the functionality of a PCB is directly related to the thickness of the copper laminated to the surfaces. The amount of current carried by the board dictates the thickness of this copper foil. Normally the thickness of the copper foil is standard. Also, you can choose between different board types for material and the number of layers. For reviewing the performance of atmo-spheric plasma in removing photoresist, a construction utilizing two layers of copper, one each side of the board, was integrated into an experimental protocol conducted by this author. The nonthermal atmospheric plasma glow discharge system employed to clean the copper layer of resist created higher levels of hydrophilicity as levels of reactive oxygen were increased relative to the plasma-generating carrier gas. Contact angles below 5% were measured, indicating that low molecular weight organics were effectively removed from the copper surface as a priming step prior to lamination. It is impor-tant to note that the high density, RF-generated plasma field used to treat the copper layers was contained within a continuous design, meaning that processes such as resist removal by plasma treatment


may be achieved through continuous conveyance of either sheet or web forms of PCB construction layers. Because such designs can be scaled to meet productivity objectives, increasing power density gas chemistry flow rates will directly enable increased processing speeds. This is diversely different from traditional batch (cham-bered) plasma cleaning systems which advocate a two-stage treat-ment process which can take up to one hour of cycle time per batch.

12.5 Air Purification Applications

Large scale atmospheric pressure nonthermal plasmas are being developed to address exhaust gases from diesel generators and engines which contain levels of soot and high densities of NOx

which has high residual oxygen concentrations, preventing certain catalytic conversions common for gasoline engines. The two pri-mary issues pertaining to the exhaust emissions of diesel engines are ΝΟχ and soot. Primary to recent efforts, as alluded to above, is the NOx component where approximately 1,000 ppm of NOx can be emitted under 1,000 kVa. It is well known that improvements in ΝΟχ removal can be achieved with the inclusion of ethylene and other hydrocarbons to the exhaust gas [107]. Other catalysts for ΝΟχ removal with atmospheric plasmas can also include vana-dium oxide, platinum, titanium oxide, zeorite, cordierite, alumina (aluminum oxide), tungsten oxide, and titanium barium oxide. Within the United States, researchers at Los Alamos, the Ford Motor Company, and at McMaster University have been exploring the use of these catalysts [108]. Siemens in Europe has been investigating chemical processes which combine atmospheric plasmas with vari-ous catalysts to remove NOx and incorporate ceramic filtration to remove soot.

Odor control has also been a focus area of application for non-thermal atmospheric plasmas. There are a number of prime sources for air odors, such as methyl mercaptan, ammonia, hydrosulfide, acetaldehide, trimethylamine, methyl sulfide, and others which are released at manufacturing, physical transfer between different modes of transportation, at transfer to storage containers, and at end use. Nonthermal plasma processes have been found capable of decomposing over 90% of these air-borne contaminants. In one appli-cation, a processing air flow volume of 72,000 m 3 /hr is managed by a large particulate filter, followed by a pulsed atmospheric plasma


reactor, and then a fine particulate filter composed of activated car-bon. The total power consumption is 28.8 kW [109]. The plasma reactor initially reduces the volume of odor-causing particulates to 5% of its original concentration by vaporizing the organic content, and the activated carbon filter removes the remaining odor. An air-based plasma will generate ozone and that ozone has refreshed the activated carbon. Similar systems have been installed at food manu-facturing facilities, and at sewage disposal/purification locations.

12.6 Medical Engineering

Within the life sciences there is continuing interest in the use of new technologies to enhance progress in medical product and process engineering. Over recent years, adaptations of technologies devel-oped in microelectronics, in material science and nanotechnology have been key to the development of new treatments within broad-ranging medical applications. One such trend in technological adaptation concerns plasma technology.

The use of plasma within the medical field is becoming inte-grated within surface modification, bio-decontamination, and the therapeutic space. Within plasma surface modification and plasma bio-decontamination, atmospheric plasmas are used to modify surfaces and tools to improve medical device bio-performance for direct therapeutic purposes.

The medical device industry in the USA is wide-ranging, with approximately $55 billion allocated for equipment. Among this equipment are surface modification technologies to serve medical applications where polymers are required to react properly with the biological environment in which they are employed. Since biocom-patibility involves the interface between the device and the biologi-cal environment, surface modification techniques have been critical to solving adhesion issues and avoiding costly changes of materials in medical device engineering. Atmospheric surface modification techniques such as air plasma, flame plasma and chemical plasma discharges have evolved as engineering of new medical device materials has progressed. Their evolutionary purpose, however, has remained consistent - to positively affect medical polymers such as high molecular weight polyethylene in a localized manner to produce useful results, such as increasing the hydrophilic nature of the surface or crosslinking functional groups to the surface.


Many intravascular devices, such as balloon catheters, are assem-bled by adhesive bonding of polyethylene components. Chemical surface activation or mechanical surface roughening techniques provide only modest bonding performance, with bond failures noted after as few as eight repetitive inflations. Following atmo-spheric plasma treatment, up to 40 repetitions are achievable.

Release agents and other contaminations on molded and formed medical parts have routinely impeded the performance of medical adhesives dramatically, and are effectively being addressed with surface pretreatments such

as air plasmas, flame plasmas and atmospheric chemical plasmas. The agents are effectively displaced and/or vaporized with atmo-spheric plasmas. The use of low polarity polyolefins in the manu-facturing of medical device assemblies such as catheters, syringes, tubings and other components are therefore being suitably cleaned of molding and forming organic contaminations, and functional-ized using atmospheric air and flame plasmas for improved adhe-sive adhesion.

The use of atmospheric plasmas for bio-decontamination was also among the first protocols adopted by the medical industry. One such application involved the use of argon-based plasmas for surgical coagulation which was primarily based upon introducing lethal plasma discharge effects on living systems. New pathways of growth in the field involve application of atmospheric plasma for therapeutic, non-lethal stimulation of patient cells and tissues. More specifically, RF-based atmospheric plasmas applied directly to chronic wounds has generated antimicrobial surface treatment without damage to surrounding tissue, as well as providing stim-ulative effects for tissue regeneration. Skin disease treatments, cell generation and tissue engineering, are also being trialed with atmospheric plasmas with promising results. However, the leading applications of atmospheric pressure plasma devices as of this writ-ing include the inactivation of microbial organisms, cultured cell proliferation, and the regeneration of living tissues.

From a surface bio-decontamination perspective, cold atmo-spheric plasmas are capable of inactivating gram-negative and gram-positive bacteria which can form biofilms on medical surfaces such as glass, polymer, and stainless steel. Applying the appropriate gas type, along with power source current and frequency, has achieved bacteria inactivation reductions of over six log (orders of magni-tude) in less than 60 seconds [110]. Scanning electron micrographs


of Bacillus Subtilis spores on a membrane filter after exposure to cold atmospheric plasmas have, for example, shown cellular dena-turation. Even greater inactivation and destruction efficiencies are seen with gram-negative bacteria such as E. coli. Kinetic studies attribute the resistivity of bacteria to atmospheric plasma inactiva-tion to the integrity of the bacteria's membrane wall. More specifi-cally, gram-positive bacteria tend to be more resistive to plasma inactivation than gram-negative bacteria because of their thicker membrane. The environments within which the microbials grow will also influence their resistance to atmospheric plasmas. For example, bacteria which form biofilms will have improved resis-tance by producing a three-dimensional polysaccharide structure in which to embed themselves [111]. However, cold atmospheric plas-mas have been engineered along with specific protocols to success-fully inactivate gram-negative, gram-positive and biofilm-forming bacteria. Since it is known that cold atmospheric plasmas can pro-duce well over one-half quadrillion chemical reactive plasma spe-cies per cubic millimeter, these plasmas are effective in inactivating common hospital-borne bacteria.

Atmospheric plasmas are also being applied to destroy and remove proteins in a misfolded form from medical surfaces, inclu-sive of surgical instruments. The significance of this relates to the inefficiencies of standard sterilization procedures which are inca-pable of removing prion contamination found with surgical instru-ments. Areas on medical surfaces contaminated by prions and treated by atmospheric plasmas have shown stark reductions of carbon and oxygen species, as opposed to untreated regions which normally have significant carbon and oxygen trace lines. At its current state, cold atmospheric plasmas can reduce protein levels below twenty femtomoles.

atmospheric pressure plasma for surface modification (wolf/atmospheric) || emerging and future...

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