Plasma polymerization

Isne

Chapter 2: FUNDAMENTAL ASPECTS OF POLYMER AND PLASMA POL YMERIZA nON

2.2 Polymers II]

2.4.2 Plasma polymerization

Plasma polymen7<ltion IS a process in which Olganic materials are reacted in an ionizing gas environment to fonn cross-linked polymer films, An ultra Ihin film can be fonned by this process where thin films deposit directly on surfaces as compnsmg the vacuwn depOSItionof covalently bonded malenals In this process, the growth of low-molecular-weight molecules (Polymer) oceun; with lhe assistance of the plasma energy, which involves acli,'aled electrons, ions and radicals. The mechanisms of plasma polymerization and that of free radical polymerization have some simliarily but lhe fwtdamenllli processes are vastly different. lbe materials obtained by plasma pol)meri~.alion are SIgnificantly different from convenlional polymers and also different from most inorganic materials. Hence plasma polymenzallon should be considered as a method of [omung new types of materials rather titan a method of preparing convet1llonal polymers. This polymerization process covers a wide interdisciplirulI)' area of physics, chemistry, science of ,"terfaces and materials science and so on [lI-17J. Thus plasma pol)meri7.alion IS a versatile technique for the depOSItionof films with functional properties suitable for a wide range of modern applications.

Historically, it was known lhat electric discharge in a glass tube forms oily or polymer-like products at the surface of the electrodes and at the wall of the glass

Chapler2: FundamentalAspectsofPol)mcr and PlasmaPolymerization 22

tube. This undesirable deposit however had extremely important characteristICSthat Me sought after in Ihe modem technology of coll1ingthalls, i) excellent adhesion to suhstrate materials and Ii) strong resistance to mosl cheITllcais. To explain the reaction mechamsm, many invesligll10rsdiscussed lhe effects of discharge condnions such as polymerutltion time monomer pressure, discharge current, and discharge power and substrate temperature on the polymeriza\Ion rate

In many cases, polymers formed by plasma polymenallon show distinguished chemical composition, chemical and ph}SlCai properties from those formed by conventional polymerillltion, even the same monomer is used for (he two polymerizations. To appreciate !he uniqueness of plasma polymerization, it is useful to compare the steps necessary to obtain a good coating by a conventional coating process and by plasma polymerUation Coating a certain substrate with a conventIonal polymer, at least several steps are required (I) synthesis of a monomer, (2) polymerization of the monomer to form a polyrller, (3) preparation of coating solution, (4) cleaning, (5) application of the coating, (6) dry10g of the coating and (7) cunng of the coating. Polyrllers formed by plasma polpneriz<ltion aimed at such a coallng are 10most cases branched and cross-linked lI8-21]. Such pol)mers are also depend on (I) synthesis of a monomer, (2) Creation of plasma medlUIl1, (3) polymeriZatIon of the monomer to form a polymer, (4) cleaning, (5) application of the pol}mer film, and (7) curing of the film

The important fonus of plasma polymers are ultra thin films require special attention Many bulk properties of a polymer film, such as permeability, electric volume resiSlJ'lty, and dielectric constant may be considered material constants, a lbickness factor is included, and therefore these parameters do not change as the thickness of the film varies. lbis is true for a film as long as Its thickness is above a certain critical value, winch is O.05-0.ll-lm depending on parameters under consideration. As the thickness of the film decreases below the critical value, the constancy of such parameters ISno longer observed, probably due to increased contribution of flaws to the total film, which progressively increases as the thickness of the film decreases.

Pol)mers are formed, from solid or l1quid monomers, by plasma-induced polymeriztluon through essential chemical reaction that is bclieved to be conventional molecular polymerization thnt occurs with the influence of plasma

Chapter2: FundamentalAspectsof Polymerand PlasmaPol)meri'.llt;on 23

Thus, the capability of plasma polymeri.<allonto form an ultra thin film containing a minimal amount of flaws is high

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Fig.2,] Competitive ablution and pol)1nerlzatioJ1,scheme of glow discharge polymeri731l0n.

Among ille many types of electric discharge, glow discharge is by far the most frequenlly used in plasma pol)1neri73tion Some oiller models were proposed based on IOn or electron bombardment The role of ion bombardment and pointed to a competition between etching and deposiuon processes in plasma polymeri~lItion was given hyYasuda [22] in Fig.2.L

2.4.3 An overnew of gas discharge plasma

Plasma polymerillltion take place

m

a low pressure (or low temperature) plasma that is provided by a glow discharge operaled in an organic gas or \'apor (monomer) lit low pressure between IWO electrodes, When a sufficient high potentIal difference IS applied bel",een two electrodes placed in a gas, the latter will break do",n into posilive iollS and electrons, giving rise to a gas discharge The mechanism of the gas

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Chapter 2: FundamenllliAspectsofPolymcr and PlllSmaPolymerization 24

breakdo\\~ can be explained as folio",.; a few electrotlS are emitted from the electrodes due to the omnipresent cosmic radiation,

However, when a potential difference is applied the electrons are accelerated by the electric field in front of the cathode and collide with the gas atoms. The most important colhsiotlS are the inelastic collisions leading to excitation and ioni7atlOn.

The excitation colhsions create nmv electrons and ions, The lons are accelerated by the electric field toward the cathode, where they release TIe\\' electrons by ion- induced secondary electron emission The electrons gi>-e rise to new ionirotion colliSIOns,creating new ions and electrons. These processes of electron emission at the cathode and ionization in the plasma make the glow discharge self-suatairung plasma.

Another important process in the glow discharge is the phenomenon of spnttering.

which occurs at suffiCIently hlgh voltage. When the ions and fast atoms from the plasma bombard the cathode, they not only release secondary' electrons, but also atoms of the cathode materials, ",hich are called sputtering, This is the basis of the use ofglo,", discharges for anal}1ical spectrochemisll). The ions can be detected with a mass spectrometer and the excited atoms or ions emit charactensllc photons, I'Ihich can be measured ""til opllcal emission spectrometry, AlternatIvely, the sputtered atoms can also diffuse through the plasma and they can be deposited on a substrate, thlS technique is used in materials technology e.g. for the deposition of thin

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Glow dlSChargeis characteri7'oo by the appearance of severallunllnoUS zones and by a constant pOlential difference between the electrodes independent of curren\. The relative sile of these zones varies with pressure and the distance between he electrodes, A typical distribullon is shown in Fig, 2.2(a), the distribution of potential among different zones is sho",n in Fig 2 2(b) As the pressure decreases the negative glow and the Faraday dark space expand at the expense of the positive glow, 'which may disappear altogether If the dlSCharge between the electrodes decreases, the positive glow diminishes while the size of the other zones remains intact.

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Chapter 2: FundamentalAspeclsofPolymerlllld Plasm. Poll'meri7ation 25

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Flg.2 2 Nonnal glow discharge; (8) the shaded areas are luminous, (b) distribulton of potential among luminous mnes

Basically the operation of the glow discl1arge depends critically on the role of the cathode dark space. In order to have a steady slale each electron emitted by the cathode must produce sufficienl ionization and excitation to affect the release of a sufficielll number of secondary electrons from the cathode upon Impact of the ions.

The role of the anode is to transform current from the glow dIscharge to the external circuil When there is no positive colurrm, the anode ,s usually in the Faraday dark space In this case, the anode fall of potential can be very small, or even negative, because ions and electrons diffuse togelher 10 the anode from the negative glow in such a way that charges neutrality ISmaintamed.

2.4.3.1 Dind current (de) glow discharge

Plasma polymerization process takes place usually in a low temperature generated by glow discharge. The space between the electrodes becomes visible when a glow discharge is established; the actual distnbutioll of light in the glow discharge is significant and is dependent on the currem-voltage chllTllCteristicsof the discharge [22-25J.

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Chapler2' FundamentalAspeclsofPolymcr and Plasma Polymerization

When a constant potential difference is apphed belween the cathode and anode, a continuous currenl will now through the discharge, giving rise 10a direct current (de) glow discharge. tn a de glow dIscharge the electrodes play an essenlial role for sustain10g the plasma by secondary' electron emisslOn. The potential difference applied betVvllenthe two electrodes is generally not equally distributed bel",een cathode and anode, bul it drops almost completely in the firsl millimeters in front of the cathode. However, for most of the other applicauons of dc glow discharges (spultering, deposition, chemical etching, analytical chemistry' etc,), the distance bet;veen cathode and anode is generally short, So normally a short anode zone is present beside cathode dark space and negative glow, where the slightly positi,'e plasma potential returns back \07.erOat the anode .

.A de glow V can opemle o,'er a wide range of discharge conditions. The pressure can vary from below 1 pa to atmosphenc pressure. The product of pressure and distance between the electrodes is a better parameler to characterize the discharge, For inslance, allower pressure, the distance bet\vcen cathode and anode should be longer to creale a discharge wiili properties comparable to these of high pressure wllh small distance, The discharge can operale in a rare gas (most often argon or helium) or 10a reactive gas (N;!,

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H" CH., Sill" SlF^4, etc.), as well as in a mixture of these gases 2.4.3.2 Alternating CUlTWlt(ac)glow discbarge

The mechanism of glow discharge generation will basically depend on the frequency of the alternation At low frequencies (60 H>:),the effect is simply to form de glow discharges of a1ternat1Ogpolanty. Ho",,"ver the frequency is higher than 60H7 the motion of ions can no longer follow the periodic changes in field polarity. But above 500 KHz ilie electrode never maintains lIS polarilY long enough to sweep all electrons or ions, originating at the opposite electrode, out of the inter-electrode volume, In this case the regeneration of electrons and ions thaI are lost to the walls and the electrodes takes place ""thin the body of the plasma The mechanism by which electrons pick up suffiCIent energy to cause bond dissociation or iOrUlation involves random collisions of electrons with gas molecules, ilie electron picking up an mcremenl of energy with each collision, A free electron m a vacurnu under the action of an alternating electric field oscillates with its velocity 90 oul of phase with the field, which obtains nO energy, on the average, from the applied field The

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Chapter2: FundamentalAspect. ofPol)IDer ""d PlasmaPolymerization 27

electron can gain energy ITom the field oruy as a consequence of elastic colliSIons with the gas atoms, as the electric field COnl'erts the electron's resulting random motion back to ordered oscillatory motion Because of its interactIon with the oscillatmg electnc field. the electron galllS energy on each collision UIllilit acquires enough energy to be able to make an melastic collision with a gas atom. In that case the process of these melastic collisions is termed volume ionization

Thus the transfer of energy from the eloctnc field to electrons at high frequencies is generall} accepted as that operallve m microwave discharges It has also been put forward as that applicable to the widely used rf of 13.56 MHz.

2.5 Glow Discharge Reactol1l [5)

Glow discharge reactor is the important part of plasma polymerization system.

Because reactor geometry influences the extent of charge particle bombardment on the growing films \\1iich affects the potential dIstribution in the system Different kinds ofreactors mcluding capacitively coupled and inductively coupled RF reactors, microwave, dual~Tr()deetc can be used for plasma polymenzation processes, The presence of insulating layers on the electrodes deflects plasma current into any surroundmg conducting areas and thus leads 10 gross plasma non-uniformity or plasma extinction Therefore, when insulating matenals are involved, ac power is usually employed so that power may pass through the insulator by capacitive coupling,

The moSI widely used reactor configurations for plasma pol}merization can he broadly di,ided in to three classes: i) internal electrode reactors, ii) external electrode reactors and electrodeless reactors

Reactors WIth internal electrodes have diITerent names, e,g, flat bed, parallel plates, planar. diode etc. Their main features are power supply, coupling system, vacuum chamber, RF driver electxode. grounded electrode, and eventually one or most substrate holders, Among the internal electrode arrangements a bell-jar-type reactor with parallel plate metal electrodes is most frequently used by using ac(1-50 kHz) and rf fields for plasma excitation.

The vacuum chambers can be made either of glass or of conductive materials, such as metal. In the case of bell-jar reactors, no particular care is taken for the grounded

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Chapler 2: FundamentalAspectsofPolymcr and PlasmaPolymerizalion 28

electrode apart from its area On the conlrary, Ute design and arrangement of dJe cathode require special aUenilon' a metallic shield surrounding the electrode highly improves the glow confinement inside interelectrodic space; electrode material and area greutly affect the extend of sputtering on the target

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Fig 2,3 Differenti}pes of reactor configuration used for plasma polymeri7ation.

In Ute current research, capacillvely coupled reactor (glow discharge plasma) system was used for the formation ofdJin films.

2.5.1 Capacitiwly coupled (ee) radio-frequency(11) discharge

If an ac voltage (Up to kHz) is used, the discharge is still basically of a dc Iype and each electrode really acts as a cathode and anode alternatively. The frequencies generally used for the alternating voltages are typically in Ihe rf range. Capacitive!y coupled (cc) discharge can also be generated by alternating voltages m another frequency range. Therefore, the term 'ae' dIscharges as opposed to dc discharges might be more appropriate. The term 'cc' refers to the way of coupling the input power into the dIscharges ie by means of two electrodes and their sheaths fonning a kind of capacitor. The occ r.f discharges which also results from the differences 10

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Chapter2, FundamcnlalAspect, of Polymcrand PlasllUlPolymeriz;alion 29

mass between electrollS and ions, is the phenomenon ofseIfbias. The ,elf-bIas or dc- bias is formed i) ",,'!lenboth electrodes differ in si~e and Ii) when IIcoupling capacItor is presenl between the rfpowcr supply and the electrode or when the electrode is non conductive (because It then acts as II capacitor) When II certain vohage IS applied over the capacitor formed by the electrodes, the ,'oltage over the plasma will initially have the same value as the applied voltage,

When the applied voltage is inilially positive the electrons will be accelerated toward the electrode, Hence the capacitor will be rapidly charged up by the electron current and the voltage over the plasma will drop, When lhe applied potential chenges polarity after one half-cycle, the ,'ollage over the plasma changes with the same amOlmt The capacitor will nOWbe charged up by the ion current and the voltage over the plasma will, therefore drop as well, but this second drop is less pronounced, because of the much lower mobility of the ions and hence the lower ion flux Al the next half-cycle, the applied polential, and hence also the voltage over the plasma, agam chenges polarity, The ,'oltage over the plasma drops again more rapidly, because the capacitor is again charged up by the electron flux, TItis process repeats itself, unlil the capacitor is finally suffiCIently negatively charged so thai the ion IlIld electron fluxes integrated over one rf-cycle, are equal to each other, This results in a time-averaged negative d,c. bias at the rf-powered electrode.

Because of the negative dc bias, the ions continue to be accelerated toward the rf- powered electrode, and they can, therefore cause sputtering of the rf-e1ectrode maleriaL In fact, the cc rf discharge often resembles II dc glow discharge with a similar subdivision in different regions, similar operating conditions, and ",ith similar processes occurring in the plasma

2.5.2 Inductively coupled (ic) glow discharges

In the inductively coupled source, the plasma chamber is mostly also surrounded by II coil. Simply speaking, the rf currents in the coil (inductive element) generate an rf magnetic flux, which penetrates the plasma region.

Following Faraday's law,

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Chapter 2: FlIDdamcnl8lAsp"ct, of Polymerand PlasmaPolymerizallon 30

the time-varying magnetie flux density induces a solenoidal r.f., electric field, ",Inch accelerates the free electrons and sustains the discharge [26, 27].

Basically, lwo different coil configurations can be distinguished in inductive dIscharges for processing applications, i.e. cylindrical and planar. tn the first configuration, a coil is ",oWld around !he discharge chamber, as a helil', In the second wnfiguration, which is more commonly used for materials processing, a flat helix or spiral is wound from near the axis to near the outer radius of the discharge chamber, separated from the discharge region by a dielectnc. Advantages of the lalter are reduced plasma loss and belter ion generation efficiency; dIsadvantage is the higher spuller-contamination, Uv-da!nage and heatmg of neutrals at the substrate, Mullipole permanent magnets can be used around the process chamber circumference 10 increase radial plasma uniformity. The planar coil can also be moved close 10 the wafer surface, resulting m near-planar source geometr)', having good uniformity properties, even In the absence of multipole confinement.

/I should be mentioned that the coupling in Ie plasma is generall} not purely inductive, but has a capacitive component as well, through the wall of the reactor.

Indeed, v,-henan mductive coupling IS used. deposition on the wall is often observed to follow a pattern matching the shape of the coil, This is an mdlcation of locali7.ed stronger electric fields on the '.•..alls, sho",'ing that the coupling is at least partly capacilIVewaugh the walls of lhe reactor.

It is mentioned that inductively coupled plasma are not only used as materials processing discharges, but they are also appl1ed m other fields, although in totally different operating regimes, So ic plasmas are the most popular plasma sources in plasma spectrochemIStry.

2.6 Plasma PolymeriZlltion MN':hanism

The mechanism ofreac/lon by which plasma polymerilation occurs is qulte complex and cannol be specifically descnbed for the general case Operational parameters such as monomer flow rale, pressure frequency, and power affect the depositIon rale and structure of the plasma film, The electrons or a10ms generated by partIal ionization of the molecules are the principle sources for trlmSrerring enerb'y ITom the eleclnc field to the gas in all glow discharges [28, 29]'

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Chapter2' FundamentalAspcclSof Polymerand PlasmaPolymenzation

In plasma polymenzation, free electrons gain energy from an imposed electrical field and then transfer the energy to neutral gas molecules, which lead to the fonnation of many chemically reactive species, By applying greater power to the rf source, the energy per unit mass of the monomer is increased and may bring about changes in the fragmentation process, As a result, free radicals may become entrapped in the plasma-polymeriI.ed film and increase in concentration with increaslOg rfpower, The deposition of polymer fIlms in low-pressure plasma is a complex phenomenon involving reactions, which occur both in the plasma phase and at the surfaces bounding the plasma Plasma polymerization can be represented by the following schematics where I and k are numbers ofrepeaung units and M' is a reactive SpecIes, which can be positive and negative ions, a free rad1caI, or excited molecule Since the radical concentration IS very high, the combination of radical fragments predominates:

Initiation:

Mk --7 M\

Propagation: M',+ M\ --7M;+<

Termination: M.;+Ml --7 M;+k

Due to high concentrahon of radicals produced in the plasma polymeri7.ation process, termination reactions tend to dominate over propagation reactions. [n the plasma polymerizauon, the product of a reaction is subjected to a repeated initiation process in the presence of plasma, The phenomenon of radical combination does not stop polymer propagation reactions became the combined molecules can also be attacked through a continua] cycle of initiation, combmation, and reinitiation.

In plasma polymerization, deposition rales and polymer film denSIties have been shown to vary Wtth substrate temperature and discharge power. Some authors have observed that deposition rate decreases with increasing substrale temperature, Polymeric films produced by plasma polymeri7.ation have branched and cross-linked structures and are difficult to dissolve in organic solvents. Their structure is irregular and amorphous and there may he no distinction bern'cen the main chain and branches,

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