Rotating dust ring in an RF discharge coupled with a DC-magnetron sputter source. Experiment and simulation.

Transcription

1 Rotating ust ring in an RF ischarge couple with a DC-magnetron sputter source. Experiment an simulation. K. Matyash a, M. Fröhlich b, H. Kersten a, G. Thieme b, R. Schneier c, M. Hannemann a, R. Hippler b a Institut für Nieertemperaturplasmaphysik Greifswal, Fr.-L.- Jahn-Str. 19, Greifswal, Germany b Institut für Physik, Ernst-Moritz-Arnt-Universität Greifswal, Domstr. 10a,17487 Greifswal, Germany c Max-Planck-Institut für Plasmaphysik, Teilinstitut Greifswal, Wenelsteinstr. 1, Greifswal, Germany Abstract During experiment on coating of ust grains trappe in an RF ischarge using a sputtering DC-magnetron source a rotating ust ring was observe an investigate. After the magnetron was switche on, the ust clou levitating above the RF-electroe forme a ring rotating as a rigi boy. Langmuir probe iagnostics was use for the measurement of plasma ensity an potential. It was iscovere that the coupling of the DCmagnetron source to the RF ischarge causes steep raial graients in electron ensity an plasma potential. The rotation of the ust ring is attribute to the azimuthal component of the ion rag force which appears ue to the azimuthal rift of the ions cause by crosse raial electric an axial magnetic fiels. In orer to get more insight into the mechanism of this ust ring rotation, the Particle-In-Cell simulation of a rotating ust clou was performe. The results of experiment an simulation are presente an iscusse.

2 1. Introuction Plasmas which, in aition to electrons, ions an neutrals, also contain microscopic particles of nanometer to micrometer size are calle usty (complex) plasmas. The ust particles in such plasmas gain a net electric charge. The sign an magnitue of the charge epens on the balance between ifferent charging processes. The absorption of electron an ion fluxes, thermo-, photo- an seconary electron emissions are the most typical mechanisms of particle charging in complex plasmas. In typical low-temperature laboratory plasmas absorption of electrons an ions is the ominant charging mechanism an ust grains get a net negative charge (typically few thousans of elementary charges for 1 µm particles) ue to the higher electron mobility. In a capacitive RF ischarge the gravitational force acting on the particles can be equilibrate by the electrostatic force acting in front of the lower electroe ue to a strong repulsive electric fiel in the RF sheath. In this case particles are trappe in the ischarge an form a clou levitating above the lower electroe. The ust particles interact with each other through the repulsive Coulomb potential, screene by the plasma electrons an ions. In the case of strong electrostatic coupling, i.e. when the energy of the interparticle interaction is large compare to the particle thermal energy, particles self-assemble into orere structures, known as plasma crystals [1-3]. Such strongly couple ust structures may serve as a unique moel system for stuying the physical processes in conense matter, such as phase transitions [4,5], waves an oscillations [6-8], occurrence of Mach cones [9,10], etc. During recent years the interest in usty plasmas has grown enormously ue to applie research relate to surface-processing technology [11-13]. Another important reason for the growing interest in usty plasmas is the possibility of generation an moification of microisperse powers with special properties. There is a wie fiel of applications for such particles [13, 14], i.e. the prouction of particles with a large specific surface for chemical catalysis, the moification of the optical properties of pigments an toners, or the enhancement of corrosion protection of particles. The investigations escribe in this paper were carrie out in a setup use for the coating of micro-isperse particles by means of a magnetron sputtering source [15]. Silicon oxie particles were trappe in the sheath above the electroe of an 2

3 asymmetric capacitive RF-ischarge. A DC-magnetron sputtering source mounte on top of the reactor was use to eposit metallic layers on the particles. In previous experiments [15] the following interesting effect was observe: when the magnetron was switche on, the ust forme a ring which rotate with a certain frequency [15]. This effect will be stuie in more etail. The raial profiles of plasma potential an electron ensity were measure for ifferent ischarge operation regimes by means of spatial-resolve Langmuir probe iagnostics. Using the measure plasma parameters the forces acting on the ust grains were estimate. In aition, 3D Particle-In-Cell simulation was use for qualitative stuy of the ust particle rotation in RF ischarge with axial magnetic fiel. 2. Experimental Setup The experiment has been performe in argon plasma generate in the reactor PULVA1 [15]. A schematic view of the experimental setup is shown in Fig. 1. The plasma glow is locate in the region between the planar aluminium RF-electroe (D = 130 mm) an the upper part of spherically shape reactor vessel (D = 400 mm) which serves as a groune electroe. The MHz RF-power is supplie by a generator (Dressler CESAR1310) in combination with an automatic matching network (Dressler VM700). The RF ischarge power was varie between 5 an 20 W. The turbopump (Pfeiffer TMU260C) which allows for a base pressure of 10-4 Pa is connecte to the vessel by a butterfly valve. The gas pressure of the argon working gas was varie between 3 an 22 Pa by using the valve an a flow controller (MKS). The planar DC-magnetron sputter source (von Arenne PPS 50) with aluminium cathoe is mounte on top of the reactor. The istance between cathoe an RF electroe is 75 mm. A magnetron power of W is supplie by a generator (Avance Energy MDX 500). The geometry of the magnetic fiel prouce by a cylinrical permanent magnet of the magnetron was analyze with a Hall probe. At the height of the levitating ust clou magnetic lines are irecte vertically an the fiel strength is B = 0.2 mt. In the experiment spherical silicon oxie particles with a iameter of 18 µm were use. Particles were injecte into the chamber through a power ropper an were trappe in the sheath above the RF electroe. In orer to confine the particles horizontally in raial irection a copper ring was place on the RF-electroe. 3

4 Dust particles were illuminate by a laser fan at 532 nm (Spectra Physics Millenia V) an were image from the sie port of reactor by a CCD camera (SBIG ST-6). The electron temperature T e an ensity n e as well as the plasma potential V p were measure by a movable RF-compensate Langmuir probe ( Smart Probe, Scientific Systems). The cylinrical probe mae of tungsten with raius R p = 0.05 mm an length L p = 7.2 mm is positione parallel to the powere electroe. The raially-resolve probe measurements were mae horizontally from one ege of the RF-electroe to another in 17 ifferent probe positions 25 mm above the electroe using a spatial step with of 5 mm. Before each spatial scan the probe was cleane by heating. 10 The electron temperature T e 1.5 ev an electron ensity n 10 cm -3 were e measure in the experiment. The particle charge Q was estimate to elementary charges, accoring to the moel for ust grain charging from [16, 17]. Figure 1. Schematic view of the experiment PULVA Experimental Results 4

5 During experiments involving particle coating using a magnetron, an interesting observation was mae: when the magnetron was turne on, the entire particle clou began to rotate, see Fig. 2. The frequency of the rotation for a pressure of 8 Pa an a magnetron power of 50 W was 0.15 Hz. After the magnetron was turne off, the rotation cease ue to the neutral rag. Similar observations of particle motion uner the influence of an axial magnetic fiel were mae earlier in [18, 19]. In Figs. 3-4 raial profiles of the electron ensity an plasma potential measure at two ifferent neutral gas pressures with an without magnetron operation are presente. As it can be seen in Fig. 3, uring magnetron operation the electron ensity in the center of the ischarge increases about twice compare to the regime when the magnetron is switche off. At the same time the electron ensity at the periphery remains roughly the same. The magnetron acts as a raially localize source of hot plasma. In the plasma potential profile (Fig. 4) we can see that the magnetron operation causes a raial potential graient at a position corresponing to the raius of the ust ring ( R 30 mm), especially for higher gas pressure. The resulting raial electric fiel in the presence of the vertical magnetic fiel from the permanent magnet of the magnetron causes an azimuthal rift of Ar + ions. The azimuthal ion rag, resulting from momentum transfer from ions to the ust grains is the riving force for this ust clou rotation. The only force which can counteract the azimuthal ion rag is the neutral gas friction. The balance between the azimuthal ion rag force an the neutral gas friction etermines the velocity of the ust ring rotation. The neutral friction force expression [20]: F n acting on the spherical grain is given by Fn = δmnnnvtnπ a R f rot, (1) 3 here m n is the mass of a neutral atom, n n is the neutral ensity, a is the grain raius, v tn = 8 ktn πm n is atom mean thermal velocity, δ 1 is the coefficient epening on the type of atom scattering an f rot is the frequency of the ust ring rotation. The ion rag force is etermine by the ion rift velocity in the raial E r an vertical E z electric fiels an the magnetic fiel B. Taking into account that for a 5

6 pressure p=8 Pa the mean free path for ion-neutral collisions λ 0.3 in mm is much smaller than the raial system size an smaller than the sheath length we can estimate the vertical u z, raial u r an azimuthal u α components of the ion rift velocity as: u u z r 2eE λ m z in i 2eE λ m r in i,, (2) u λ α in ur Rci, where e is the elementary charge, m i is the ion mass an R ci is the ion gyroraius. Here we assume that the azimuthal component of the ion rift velocity is cause by the bening of ion trajectories in the weak magnetic fiel. The vertical component of the electric fiel at the position of the ust clou can be estimate from the vertical force balance. In vertical irection the gravity force M g is balance by the electrostatic force QE z an the vertical component of the ion rag ( M is the particle mass an g is the gravitational acceleration). Neglecting M g the weak ion rag in comparison with the two other forces we obtain Ez Q. In experiment uring magnetron operation a vertical electric fiel E z at the height of the levitating ust clou was estimate to 130 V/cm. The ion rag force F i consists of two parts collection an orbital forces. The collection part is create by ions hitting the grain surface while the orbital part is ue to ions eflecte in the grain electric fiel. Using the expression for both parts from [21] we have: ( π 2 4π 2 π /2 ) F = m n v b + b Γ u (3) i i i s c here n is the ion ensity, i b c 2eV = a1 mv fl 2 s is the collection impact parameter, 2 2 V fl is the grain floating potential relative to the plasma, v = v + u is the mean s ti 6

7 eq ion velocity, v ti is the ion thermal velocity, bπ = m v /2 2 i s is the impact parameter for scattering angle π /2, from b c to Debye length λ D. b + λ 2 2 π /2 D Γ = 0.5ln 2 2 b π /2 + bc is the Coulomb logarithm integrate Figure 2. Particle clou before (left) an after (right) the magnetron was turne on. Figure 3. Raial electron ensity profile for ifferent pressures an magnetron powers 2.5 cm above RF electroe. 7

8 Figure 4. Raial plasma potential profile for ifferent pressures an magnetron powers 2.5 cm above RF electroe. The frequency of the ust ring rotation can be estimate from the balance of neutral friction (1) an the azimuthal component of ion rag (3). Using the estimation of the raial electric fiel E r 0.5 V/cm from probe measurements (Fig. 4) an assuming n i n at the ust clou position, we obtain e 2 f rot 10 Hz, which is about one orer of magnitue lower than the one observe in experiment. Recently the new approach for calculation of the ion rag force was propose by Khrapak et al. [22]. In this approach contribution of ions with impact parameter larger than Debye length is taken into account, which shoul give more accurate values than [21], especially in the case of slow rifting ions. Substituting the Coulomb logarithm in (3) with moifie Coulomb logarithm b b * π /2 D Γ = ln π /2 + λ from + a [22], we obtain rotation frequency about 30% higher, which shows a better agreement with the experimental results. 8

9 4. Particle-in-Cell moeling of rotating ust cluster In orer to get more insight into the behavior of the ust particles in a capacitive RF ischarge couple with a magnetron plasma source, we performe a qualitative simulation of such system using Particle-in-Cell moel. For this purpose a full 3D electrostatic Particle-in-Cell coe with Monte-Carlo Collisions (PIC MCC) was use [23]. In this coe ust particles are introuce in the PIC scheme as aitional charge species using Clou-in-Cell weighting formalism [24], thus no finite size effects for ust particles were accounte. In aition to the electrostatic force, the gravitational, neutral gas friction an ion rag forces were also consiere for the ust particles. Thus the equation of motion of a ust particle has a form: v Q = E + g β v + F i t M M, (4) here β is the normalize friction coefficient. Only the orbital component of ion rag force resulting from momentum transfer uring ion-ust Coulomb collisions was inclue in the simulation. The Coulomb collisions between ions an the ust particles were inclue in the simulation using a binary collision moel [25]. The collection component of the ion rag, ue to ions hitting particle surface was neglecte, as no finite size effects for ust particles were consiere in the simulation. The parameters of the simulation were chosen close to those use earlier for moeling of 3D ust crystal in RF methane plasma [23]. As a backgroun gas, 14-3 methane with a ensity n 4 = 7 10 cm an temperature = 500 K was CH TCH 4 use. The initial electron ensity an temperature were chosen as n e 0 = cm -3 an Te 0 = 20 ev respectively. Z The computational omain represents a 3D box with imensions: = = 16λ = 0.75 cm, X = Y = 8λ = 0.38 cm, where Z correspons to max D0 max max D0 the vertical irection an is the electroe spacing. The lower electroe at Z = is groune an the upper electroe at Z = 9 Z max 0 is powere with a sinusoial voltage with frequency f = ω 2π = MHz. The uniform magnetic fiel B = 14.4 mt RF RF was applie in the Z irection. At the electroes absorbing wall bounary conitions for the particles were applie. In the X an Y irections perioic bounary 9

10 conitions were applie, both for particles an the potential. The neutral gas was treate as a fixe backgroun with constant ensity an temperature. Only the charge particle ynamics was followe. For the sake of simplicity, only Coulomb collisions between charge species an electron-impact ionization of methane were consiere in the simulation. A gri with spacing x = y = z = λ 0 2 = cm an time step 11 t 0.2 ωpe 7 10 s was use in the simulation. In orer to spee up the = = simulation, reuce masses of the ions an ust particles were use. The ionelectron mass ratio was set to m + m = The mass of the ust particles was CH4 e chosen as M m + = 18560, which gives the mass of the ust particle about 9 CH4 orers of magnitue smaller than in the experiment. In orer to compensate for the ecrease mass of the ust particles an match the ratio of gravitational force to electrostatic force acting on particles in the RF sheath, the gravitational acceleration was increase by a factor of charge of the ust particles Q D 9 10 compare to the real value. A constant 4 = 5 10 e was assume (no charging processes were accounte for). The viscosity coefficient was set to β t = , giving a characteristic time of particle slowing-own ue to neutral gas friction τ β 8 = = s. Such artificial moification of the system parameters as compare with the experiment was necessary to accelerate the simulation an to achieve an acceptable computation time. The calculations were carrie out on a Linux cluster with 16 AMD Athlon MP processors in approximately one month. In the simulation, the plasma was sustaine self-consistently ue to electron impact ionization of the neutral gas by the electrons accelerate in the applie RF voltage. In orer to reach equilibrium ischarge conitions, the amplitue of the RF voltage U RF was automatically ajuste uring the simulation using a feeback control loop [26]. In orer to inclue in the simulation the effect of magnetron plasma source use in the experiment, an aitional source of hot plasma was ae. Electrons an ions with a Maxwellian istribution with T es = 20 ev an T is = 1 ev were introuce in the ring-shape region in the center of the system: 6λ 0 Z 10λ R1 X X0 + Y Y0 R 2. Here R1 = 1. 5λD 0 an D D, ( ) ( ) R2 = 2λD 0 the center of the ring. are inner an outer raii an X0 = 4λD 0, Y0 = 4 λd 0 - coorinates of 10

11 In the beginning of the simulation, the ust particles were ranomly introuce into the center of the system. Gravitation force the particles to move own to the lower electroe, until it was balance by the electrostatic force in the sheath. After about 10 4 s particles forme a clou levitating about 2. 5λ D 0 above the lower electroe. In Fig. 5 we plot the ust layer levitating over the lower electroe of the ischarge. When the aitional hot plasma source in the center of the ischarge was activate, a rotation of the outer ring of the particle clou was observe. The perio of rotation was T = s. In Fig.6 we plot the horizontal potential profile in the system s miplane an at the position of the ust layer. At the miplane of the ischarge we can istinguish the ring-like maximum of the potential ue to the source of hot plasma applie in the moel. At the position of the ust layer the potential has a plateau in the center an shows a steep raial graient at the periphery, where the ust particles are locate. The resulting raial electric fiel confines the ust particles within the system balancing the particles repulsion in the horizontal irection. This electric fiel provies a raial acceleration to the ions. In the presence of an axial magnetic fiel ion trajectories are bene in azimuthal irection, creating an azimuthal ion flux. In Fig 7. we plot the vector map of the horizontal component of the ion mean velocity at a horizontal section at the position of the ust layer. For parameters use in the simulation the ion Larmor raius is of the orer of the system with R ci v = ih ωci 7λ D 0. Here v ih is the horizontal (perpenicular to the magnetic fiel) component of the ion velocity. Thus at the position of the outer ring of the ust particles the azimuthal component of the ion rift velocity is of the same orer of magnitue as the raial component. The rectangular shape of the computational omain an the applie perioic bounary conitions result in a rather complicate pattern of the ion velocity which shows no axial symmetry. The azimuthal component of the ion rag force resulting from the azimuthal ion flux in the presence of crosse electric an magnetic fiel causes the rotation of the ion clou observe in the simulation. The neutral friction force, balancing the ion rag, etermines the spee of rotation. The perio of rotation can be estimate as T 2π R βm F = iα. Here R is the raius of the outer ust ring an F iα is the azimuthal component of the ion rag force. Calculating the orbital component of the ion rag force accoring to [21] we obtaine the perio of rotation for the outer ust 11

12 ring T simulation. Rotating ust ring in RF ischarge couple with DC-magnetron sputter source s, which is in goo agreement with the value observe in the Thus, using the Particle-in-Cell simulation we have shown the possibility of a rotation of the ust clou in the capacitive RF ischarge ue to an azimuthal rift of ions in crosse electric an magnetic fiels. In our moel the raial electric fiel was prouce ue to the applie localize source of hot plasma. Figure 5. Dust layer levitating over the lower electroe of the capacitive RF ischarge. 12

13 Figure 6. Potential profile at the horizontal section Z = 8λD 0 (top), corresponing to the miplane of the system an at Z = 2. 5λD 0 (bottom) at the position of the ust layer. 13

14 Figure 7. Vector iagram of horizontal components of CH 4 + ion velocities at the horizontal section Z = 2. 5λD 0, corresponing to the position of the ust layer. The length of the vector is proportional to the velocity magnitue. 6. Conclusion The investigations escribe were carrie out in a setup use for coating of micro-isperse particles by means of magnetron sputtering. Silicon oxie particles were trappe in the sheath above the electroe of an asymmetric capacitive RFischarge. A DC-magnetron sputtering source mounte on top of the reactor was use to eposit metallic layers on the particles. It was iscovere that when the magnetron was switche on, the ust isc forme a ring rotating with a constant frequency. In orer to get information about the plasma behavior in case of a working magnetron source the raial-resolve Langmuir probe iagnostics was applie. The 14

15 raial profiles of plasma potential an electron ensity were measure for ifferent ischarge operation regimes. The probe measurements reveale that the magnetron acts as a localize heating source causing raial electric fiel at the ust clou position. This raial electric fiel combine with the axial magnetic fiel from the permanent magnet of the magnetron prouces the azimuthal rift of plasma ions. The azimuthal component of the ion rag force, arising from momentum transfer from ions to ust particles, is the riving force for the rotation of the ust ring. The balance between the azimuthal component of the ion rag force an the neutral gas friction etermines the frequency of this ust ring rotation. The 3D Particle-In-Cell simulation was use to qualitatively investigate the ust particle behavior in a RF ischarge with axial magnetic fiel. The simulation has shown that the raial electric fiel resulting from the applie localize heating source combine with an axial magnetic fiel gives rise to the azimuthal ion rag which causes the rotation of the ust clou trappe in the ischarge. 7. References [1] J. H. Chu an I. Lin, Direct observation of Coulomb crystals an liquis in strongly couple rf usty plasmas, Phys. Rev. Lett. 72 (1994) [2] H. Thomas, G. E. Morfill, V. Demmel, J. Goree, B. Feuerbacher, an D. Möhlmann, Plasma crystal: Coulomb crystallization in a usty plasma, Phys. Rev. Lett. 73 (1994) 652. [3] A. Melzer, T. Trottenberg, an A. Piel, Experimental etermination of the charge on ust particles forming Coulomb lattices, Phys. Lett. A 191 (1994) 301. [4] A. Melzer, A. Homann, A. Piel, Experimental investigation of the melting transition of the plasma crystal, Phys. Rev. E 53 (1996) [5] G. E. Morfill, H. M. Thomas, U. Konopka, M. Zuzic, The plasma conensation: Liqui an crystalline plasmas, Phys. Plasmas 6 (1999)

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