Plasma Cleaner: Physics of PlasmaNature of PlasmaA plasma is a partially ionized gas consisting of electrons, ions and neutral atoms ormoleculesThe plasma electrons are at a much higher temperatures than the neutral gasspecies, typically around 104 K, although the plasma gas as a whole is at nearambient temperatureThe plasma electron density is typically around 1011 cm-3Plasma FormationAn RF oscillating electric field is generated in the gas region, either through the useof capacitive plates or through magnetic inductionAt sufficiently low pressures the combined effect of the electric field acceleration ofelectrons and elastic scattering of the electrons with neutral atoms or field linesleads to heating of the electronsWhen electrons gain kinetic energy in excess of the first ionization threshold in theneutral gas species, electron-neutral collisions lead to further ionization, yieldingadditional free electrons that are heated in turnPlasma-Surface InteractionThe energy of plasma electrons and ions is sufficient to ionize neutral atoms, breakmolecules apart to form reactive radical species, generate excited states in atomsor molecules, and locally heat the surfaceDepending on the process gases and parameters, plasmas are capable of bothmechanical work, through the ablative effect of kinetic transfer of electrons andions with the surface, and chemical work, through the interaction of reactive radicalspecies with the surfaceIn general, plasmas can interact with and modify a surface through severalmechanisms: ablation, activation, deposition, cross-linking and grafting see Plasma-Surface InteractionPlasma Cleaner: Overall Plasma AdvantagesSurface InteractionPlasma treatment only affects the near surface of a material; it does not changebulk material properties
Plasma cleaning leaves no organic residue, unlike many wet cleaning processes;under proper conditions, it can achieve complete contamination removal, resultingin an "atomically clean" surfacePlasma has no surface tension constraints, unlike aqueous cleaning solutions; it canclean rough, porous or uneven surfacesPlasma treatment occurs at near-ambient temperature, minimizing the risk ofdamage to heat-sensitive materialsProcess Flexibility & ConsistencyDepending on process gases and usage configuration, plasma treatment can beused for cleaning, activation, sterilization and general alteration of surfacecharacteristicsPlasma will react with a wide variety of materials; as such, plasma can also treatassemblies made of different materialsPlasma cleaning can treat odd-shaped parts with difficult surface geometriesPlasma treatment is highly reproducible; it is typically characterized by a greaterconsistency than chemical or mechanical processesLow Cost / Ease of UsePlasma processing is highly efficient, with short processing times, no drying stageand little energy consumedPlasma treatment helps to avoid process yield loss due to heat or solvent damagePlasma processing is easier to use and maintain than chemical or mechanicalprocesses; in addition, it requires no complicated chemical analysis or maintenancePlasma treatment frequently eliminates the need for solvents, along with theirongoing purchase and disposal costsUser & Environmental SafetyPlasma treatment eliminates safety risks associated with worker exposure todangerous chemicalsPlasma processing is contained within a vacuum chamber, with little or no directworker exposurePlasma processing operates at near-ambient temperatures with no risk of heatexposurePlasma treatment uses no harmful chlorinated fluorocarbons, solvents, or acidcleaning chemicalsThe EPA has classified most plasma processes as "green" environmentally friendlyprocesses
Plasma Cleaner: Plasma-Surface InteractionAblationPlasma ablation involves the mechanical removal of surface contaminants byenergetic electron and ion bombardmentSurface contamination layers (e.g. cutting oils, skin oils, mold releases) aretypically comprised of weak C-H bondsAblation breaks down weak covalent bonds in polymeric contaminants throughmechanical bombardmentSurface contaminants undergo repetitive chain scission until their molecular weightis sufficiently low for them to boil away in the vacuumAblation affects only the contaminant layers and the outermost molecular layers ofthe substrate materialArgon is often used for ablation; high ablation efficiency, no chemical reactivitywith the surface materialActivationPlasma surface activation involves the creation of surface chemical functionalgroups through the use of plasma gases - such as oxygen, hydrogen, nitrogen andammonia - which dissociate and react with the surfaceIn the case of polymers, surface activation involves the replacement of surfacepolymer groups with chemical groups from the plasma gasThe plasma breaks down weak surface bonds in the polymer and replaces themwith highly reactive carbonyl, carboxyl, and hydroxyl groupsSuch activation alters the chemical activity and characteristics of the surface, suchas wetting and adhesion, yielding greatly enhanced adhesive strength andpermanencyCrosslinkingCross-linking is the setting up of chemical links between the molecular chains ofpolymersPlasma processing with inert gases can be used to cross-link polymers and producea stronger and harder substrate microsurfaceUnder certain circumstances, crosslinking through plasma treatment can also lendadditional wear or chemical resistance to a materialDepositionPlasma deposition involves the formation of a thin polymer coating at the substratesurface through polymerization of the process gas
The deposited thin coatings can possess various properties or physicalcharacteristics, depending on the specific gas and process parameters selectedSuch coatings exhibit a higher degree of crosslinking and much stronger adherenceto the substrate in comparison to films derived from conventional polymerizationPlasma Cleaner: Plasma Process GasesGas Sources for Plasma Surface Cleaning and ModificationAirContamination Removal (chemical)Oxidation ProcessSurface ActivationO2Contamination Removal (chemical)Oxidation ProcessSurface Activation (wetting & adhesion)Etch (organics)N2Deposition (glass (w/ Si))Note: a special 'oxygen service' vacuum pump must be used in conjunction withO2 process gas in order to avoid risk of possible injury; inquire with HarrickScientific for detailsSurface ActivationDeposition (silicon nitride (w/ Si), metal nitride (w/ M))ArContamination Removal (ablation)CrosslinkingH2Contamination Removal (chemical)
Surface Modification (curing)Reduction Process (metal oxide)Deposition (metals (w/ M))Note: Extreme caution must be exercised when working with H 2 process gas inorder to minimize the risk of possible injury.Plasma Cleaner: Product FeaturesFeaturesCompact, tabletop unitAdjustable RF powerLow, Medium, and High power settingsTwo Plasma Cleaner models available:PDC-32G (110V); PDC-32G-2 (220V)Basic model with a 3" diameter by 7" long chamber and a removablecoverApplies a maximum of 18W to the RF coil, with no RF emissionSize: 8"H x 10"W x 8"DPDC-001(110V); PDC-002 (220V)Expanded model with a 6" diameter by 6" long chamber and an integralswitch for a vacuum pumpIts hinged cover features a magnetic closure and a viewing windowApplies a maximum of 30W to the RF coil, with no RF emissionSize: 11"H x 18"W x 9"DOptional quartz Plasma Cleaner chamberOptional flow mixer allows individually metered intakes for up to two differentgases and monitors the pressure in the chamberCompatible vacuum pump available
RequiresA vacuum pump with a minimum pumping speed of 1.4 m3/hr and a maximumultimate total pressure of 200 mtorrIncludes1/8" NPT needle valve to admit gases and control the pressurePyrex Plasma Cleaner chamberPower SettingsDescriptionPDC-32G or PDC-32G-2PDC-001 or PDC-002Input Power100W200WApplied to the RF CoilLow Setting680V DC 10 mA DC6.8W716V DC 10 mA DC7.16WMedium Setting 700V DC 15 mA DC 10.5W 720V DC 15 mA DC 10.15WHigh Setting720V DC 25 mA DC18W740V DC 40 mA DC29.6WPlasma Cleaner: Details of OperationNote: A detailed User's Manual is provided with all Harrick Scientific Plasma Cleanermodels.Principles of OperationThe sample is placed in the plasma vacuum chamberProcess gas(es) are admitted to the chamber at low flow rates (1-2 SCFH) usingeither a needle valve or the PlasmaFlo accessory and are kept at low pressure( 200-600 mTorr) through vacuum pumpingThe gases are subject to induced RF magnetic and electric fields generated by asolenoidal coil currentPlasma is generated through the subsequent RF/collisional heating of theelectrons in the gas
Details of OperationThe plasma vacuum chamber door has an o-ring quick disconnect seal for easyaccess to the chamberThe vacuum pump is connected to an outlet at the back of the reaction chamberThe needle valve can be used to break the vacuum gently, to control thepressure or to admit a special gas for plasma processingThe RF power level can be adjusted by means of a three-way selector switchThe plasma will emit a characteristic glow, which visibly indicates the successfulgeneration of the plasma stateThe temperature change of a substrate during plasma treatment is minimalSurface Cleaning / ModificationThe interaction between the plasma and the surface is determined by:The nature of the substrate and surface contaminant layersThe process gases usedThe pressure and flow rate of the gasesThe RF power level & length of sample exposureFor surface cleaning, a few seconds exposure, following pump down of thechamber and formation of plasma, is often adequateSurface cleanliness can be tested most easily by observing the wettability of thesample: on a clean surface, water drops will not bead, but will spread out in auniform filmPlasma Cleaner Applications: Plasma CleaningPlasma CleaningConventional cleaning methods often fail to completely remove surface films,leaving a thin contamination layer; additionally, solvent cleaning typically leaves asurface residuePlasma cleaner use exposes the surface to a gas plasma discharge, gently andthoroughly scrubbing the surfacePlasma cleaning will remove non-visible oil films, microscopic rust or othercontaminants that typically form on surfaces as a result of handling, exposure orprevious manufacturing or cleaning processes; additionally, plasma cleaning doesnot leave a surface residue
A plasma cleaner can treat both a wide variety of materials - including plastics,metals and ceramics - as well as complex surface geometriesA plasma cleaner is most commonly used prior to adhesive bonding both to cleanaway loosely held contaminant residues and to activate the surface for increasedbonding strengthFigures 1 and 2 below show the ATR spectra, respectively, of Ge and Si substrates prior toand following surface contaminant removal via plasma cleaning.Figure 1. ATR spectra (qave 45 , N 20) of a Ge surface before and after plasma cleaning with aHarrick plasma cleaner. The lower trace shows Ge coated with a thick (about I micron) film ofphotoresist (AZ111). The upper trace shows same surface after fifteen minutes of plasma cleaningwith 0 2 , indicating Ge is restored to its original organic free condition with the photoresist strippedoff the surface.
Figure 2. Harrick plasma cleaner hydrocarbon removal from the surface of a silicon ATR plate(60 reflections, q 45 ). The C-H band (bottom trace), representing 10% absorption, iscompletely eliminated (top trace) after one minute exposure to an air plasma.Plasma Cleaner Applications: PolymersSurface Cleaning of PolymersPlasma ablation mechanically removes contaminant layers through energeticelectron and ion bombardment of the surface - see AblationPlasma surface cleaning removes surface contaminants, unwanted surface finishfrom polymers and weak boundary layers which may be present in certainprocessed polymersSurface Restructuring of PolymersThe breaking of polymer surface bonds by plasma ablation using an inert gasleads to the creation of polymeric surface free radicalsA surface free radical can rebond in its original polymeric structure, it can bondwith an adjoining free radical on the same polymeric chain, or it can bond with anearby free radical on a different polymeric chain - see Crosslinking
Such polymer surface restructuring can improve surface hardness, as well astribological and chemical resistanceSurface Alteration of PolymersThe breaking of polymer surface bonds by plasma ablation leads to the creationof polymeric surface free radicalsThe bonding of these surface free radicals with atoms or chemical groups fromthe plasma leads to the replacement of surface polymer functional groups withnew functional groups, based upon the chemistry of the plasma process gas see ActivationTypical polymer functional groups formed through plasma surface activation andgrafting include: amine amino-carboxyl, carboxyl hydroxyl and fluorinationcarbonylSuch polymer surface alteration can modify the chemical properties of thesurface while leaving the bulk properties unchangedSurface Deposition of PolymersPlasma deposition involves the formation of a thin polymer coating on thesubstrate surface through polymerization of the process gasIf a process gas comprised of more complex molecules, such as methane orcarbon tetrafluoride, is employed, these may undergo fragmentation in theplasma, forming free radical monomers; these in turn bind to the surface andrecombine into deposited polymeric layersThese polymer thin-film coatings can dramatically alter the permeation andtribological properties of the surface - see DepositionPlasma Cleaner: Plasma-Surface InteractionAblationPlasma ablation involves the mechanical removal of surface contaminants byenergetic electron and ion bombardmentSurface contamination layers (e.g. cutting oils, skin oils, mold releases) aretypically comprised of weak C-H bondsAblation breaks down weak covalent bonds in polymeric contaminants throughmechanical bombardmentSurface contaminants undergo repetitive chain scission until their molecularweight is sufficiently low for them to boil away in the vacuumAblation affects only the contaminant layers and the outermost molecularlayers of the substrate materialArgon is often used for ablation; high ablation efficiency, no chemical reactivitywith the surface material
ActivationPlasma surface activation involves the creation of surface chemical functionalgroups through the use of plasma gases - such as oxygen, hydrogen, nitrogenand ammonia - which dissociate and react with the surfaceIn the case of polymers, surface activation involves the replacement of surfacepolymer groups with chemical groups from the plasma gasThe plasma breaks down weak surface bonds in the polymer and replacesthem with highly reactive carbonyl, carboxyl, and hydroxyl groupsSuch activation alters the chemical activity and characteristics of the surface,such as wetting and adhesion, yielding greatly enhanced adhesive strengthand permanencyCrosslinkingCross-linking is the setting up of chemical links between the molecular chainsof polymersPlasma processing with inert gases can be used to cross-link polymers andproduce a stronger and harder substrate microsurfaceUnder certain circumstances, crosslinking through plasma treatment can alsolend additional wear or chemical resistance to a materialDepositionPlasma deposition involves the formation of a thin polymer coating at thesubstrate surface through polymerization of the process gasThe deposited thin coatings can possess various properties or physicalcharacteristics, depending on the specific gas and process parameters selectedSuch coatings exhibit a higher degree of crosslinking and much strongeradherence to the substrate in comparison to films derived from conventionalpolymerizationPlasma Cleaner Applications: BiomaterialsSterilizationPlasma sterilization treatment is gaining growing acceptance for disinfecting andsterilizing medical devicesPlasma treatment offers the potential for simultaneous cleaning and sterilizationof medical instrumentsPlasma sterilization is particularly appropriate for medical or dental implants anddevices that are sensitive to the high temperature, chemical or irradiative
environments associated with autoclaving, EtO or gamma sterilization,respectivelyAdhesion PromotionMany biomaterials have a low to medium surface energy, making it difficult toeffectively apply adhesives or coatingsPlasma surface activation leads to the formation of surface functional groupsthat increase surface energy and improve interfacial adhesion for biomaterialbondingWetting PropertiesMost untreated biomaterials have poor wettability (hydrophilicity)Plasma surface treatment has been used to enhance or decrease the wettingcharacteristics on a wide variety of biomaterialsSurfaces may be rendered hydrophilic through plasma activation, and may berendered hydrophobic through plasma deposition of thin filmsLow-Friction & Barrier CoatingsSome silicones and polymers such as polyurethanes have a typically highcoefficient of friction against other surfacesPlasma coating deposition of a lower coefficient of friction polymer coating yieldsa more lubricious surface for biomaterials applicationsPlasma coating deposition can also be used to form thin, dense barrier coatingsthat decrease permeability to liquids or vapors for biomaterials applicationsBiocompatibilityBiomaterials that come in contact with blood or protein require special surfacetreatments to enhance biocompatibilityPlasma activation of biomaterial surfaces prepares them for cell growth orprotein bonding; additionally, biomaterial surfaces may also be modified todecrease the bonding of proteinsBiomaterials with modified surfaces exhibit improved "biocompatibility",including enhanced cell adhesion, improved cell culture surfaces, non-foulingsurfaces and promotion of selective protein adsorptionhttp://www.harricksci.com/plasma.cfm
Plasma Cleaner: Physics of Plasma Nature of Plasma A plasma is a partially ionized gas consisting of electrons, ions and neutral atoms or molecules The plasma electrons are at a much hi
Plasma Etching Page 2 OUTLINE Introduction Plasma Etching Metrics – Isotropic, Anisotropic, Selectivity, Aspect Ratio, Etch Bias Plasma and Wet Etch Summary The Plasma State - Plasma composition, DC & RF Plasma Plasma Etching Processes - The principle of plasma etching, Etching Si and SiO2 with CF4
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