A dc plasma source for plasma material interaction experiments

Transcription

1 Plasma Sources Science and Technology Plasma Sources Sci. Technol. 23 (214) 2514 (11pp) doi:1.188/ /23/2/2514 A dc plasma source for plasma material interaction experiments T S Matlock, D M Goebel, R Conversano and R E Wirz University of California Los Angeles, 42 Westwood Plaza, Los Angeles, CA 995, USA tmatlock17@ucla.edu Received 3 September 213, revised 1 February 214 Accepted for publication 27 February 214 Published 24 March 214 Abstract A new device has been constructed for the investigation of interactions between engineered materials and a plasma in regimes relevant to electric propulsion and pulsed power devices. A linear plasma source, consisting of a hollow cathode, cylindrical anode, and axial magnetic field, delivers a 3 cm diameter beam to a biased target 7 cm away. The ion energy impacting the surface is controlled by biasing the sample from to 5 V below the local plasma potential. This paper discusses the major aspects of the plasma source design and presents measurements of the plasma parameters achieved to date on argon and xenon. Experiments show that splitting the gas injection between the hollow cathode and the anode region provides control of the discharge voltage to minimize cathode sputtering while providing ion fluxes to the target in excess of 1 21 m 2 s 1. Sputtering rate measurements on a non-textured molybdenum sample show close agreement with those established in the literature. Keywords: plasma interactions, sputtering, sputter deposition, molybdenum (Some figures may appear in colour only in the online journal) 1. Introduction The plasma interactions (Pi) facility is being developed at UCLA for the testing of advanced, micro-architecture materials [1] for applications in harsh environments, similar to that produced in electric propulsion (EP) and pulsed power systems. A 25 A hollow cathode plasma source is integrated with guiding solenoidal magnets running across a vacuum chamber to provide a roughly 3 cm diameter plasma to a downstream target material with target-directed, xenon or argon ion fluxes. Material coupons may be installed in the path of the plasma beam and biased to provide energetic ion bombardment, and then examined post-irradiation by removing them from the chamber and sending them to a separate laboratory for analysis, though a suite of in situ diagnostics is under development. The dynamic interactions between plasmas and materials far from equilibrium are a critical and currently not well understood aspect of EP and pulsed power device design. Facilities dedicated to the examination of phenomena arising when materials are exposed to extreme plasma environments are in ongoing use for research vital to magnetic confinement fusion [2 6] and hypersonic vehicles [7, 8] among other fields. The Pi facility is designed to investigate partially ionized plasmas in argon and xenon of moderate electron temperature (2 1 ev) and plasma density ( m 3 ) and overall conditions similar to those found in EP devices. In contrast to the more canonical experiments cited above, the interactions between EP plasmas and exposed materials have largely been investigated within the device of interest, where the device operation (and generated plasma) is intrinsically tied to the properties of the plasma-facing materials [9, 1]. The Pi facility is designed to minimize feedback between the plasma source and material under study in order to enable a less device-specific investigation of how material properties and surface architecture affect resilience to EP-relevant plasma bombardment. This paper presents the design and operational metrics of a plasma source intended to enable fundamental experiments to characterize both global and microscopic interactions of plasma material systems. The plasma provided must be simple enough to promote accurate numerical modeling and flexible enough to capture the range of phenomena occurring in the plasma regimes of interest. 2. Experimental apparatus The Pi facility at UCLA is comprised of a 1.8 m diameter by 2.8 m long vacuum chamber evacuated by dual cryopumps /14/ $ IOP Publishing Ltd Printed in the UK

2 Figure 1. (a) Pi facility sketch (top view) with cathode flow indicated by red arrow, anode flow indicated by blue arrows. (b) Pi facility in operation on xenon (front view). with a combined pumping speed of 5 l s 1 on argon, allowing base pressures as low as Torr. The plasma source is mounted on one flange of the chamber, diametrically opposed to the flange which supports the sample manipulator arm. Intermediate guide magnets are mounted to struts lying chordally across the bottom of the chamber. All components of the plasma source and delivery system lie within the vacuum chamber with small axial gaps between system elements allowing for the insertion of plasma diagnostics in a flexible manner. A sketch of the facility layout is given in figure 1(a) and a picture of plasma bombardment of a graphite target is shown in figure 1(b) Plasma source design The plasma source consists of a lanthanum hexaboride (LaB 6 ) hollow cathode coupled to a water cooled, cylindrical copper anode. The cathode has a 1.3 cm inner diameter LaB 6 insert inside a graphite cathode tube with a 1 cm aperture tungsten orifice plate. A graphite keeper electrode positioned around the cathode tube aids in starting the discharge and helps to protect the orifice plate and graphite tube end from ion bombardment from the cathode plume plasma. This size cathode is capable of running continuously at 25 A of discharge current [11, 12], and has been used in the Pi facility up to the maximum 125 A rating of the present discharge power supply. The anode has an approximately 6 cm inner diameter and is 3 cm long. A watercooled magnetic field coil positioned around the cathode and a solenoid coil wound on the anode provide an axial magnetic field that both confines the electrons to improve the ionization and guides the plasma to the target region. The magnetic field diverges from the cathode face to the anode solenoid region to better match the cathode exit diameter to the anode, and the axial strength can be selected in the range 1 5 G to optimize the discharge parameters for plasma generation and transport. Simulated magnetic field strengths and flux lines are shown in the appendix, for solenoid settings which maximized target current at a discharge condition in argon. The hollow cathode plasma source differs significantly from the previous plasma source (PISCES [13, 14]) developed for the purpose of plasma surface interactions experiments. In that case, a large area LaB 6 disk cathode coupled to a cylindrical anode produced the plasma in a quasi-reflex discharge mode. That source required large discharge power levels (1 5 kw) to generate a relatively large area hydrogen plasma (5 75 cm 2 ) capable of delivering a high flux to the target region downstream of the anode. The new Pi source design is optimized to produce a smaller-diameter argon or xenon plasma ( 1 cm 2 ) while operating at manageable discharge power levels in the 5 1 kw range. The system includes a gas injector positioned between the cathode and the anode to provide direct control of the discharge voltage at a given discharge current and cathode flow rate. This anode gas injector is also known to reduce the oscillation level in the cathode plume and the local production of energetic ions that tend to sputter the keeper face and anode wall [15, 16]. The area expansion ratio of the plasma from the hollow cathode orifice to the anode is on the order of 2 to 3, so the discharge is stable and well behaved over a large range in discharge current and plasma densities. In addition, the higher neutral gas pressure downstream of the cathode orifice in the new Pi, compared with the constant axial pressure in the flat disk PISCES cathode source, provides additional protection of the insert from contamination by metallic components that might be sputtered or evolved in the system. The plasma from the source region is coupled through two 1 cm diameter water-cooled chamber solenoids to the cooled target located inside the sample magnet coil. The plasma transport region through the chamber solenoids is intended to provide isolation between the target being sputtered and the plasma source. The ionization mean free path for particles sputtered from the target is typically less than the distance from the target to the anode, and the ions created from the target material then diffuse to the walls of the solenoid spools or fall back down the axial potential gradient in the plasma to the target region. 2

3 The plasma source components (anode and cathode) float relative to the vacuum facility and the grounded chamber solenoids. Because of the large area of the grounded solenoids, the plasma tends to float within an electron temperature (<1 ev) of the ground potential over the entire range of discharge currents. Since the bias applied to the target is made relative to chamber ground, this provides a stable reference potential from which the ions are accelerated through the sheath to the biased sample Source collisionality analysis. The long intermediate region between the plasma source and the material under test is one of the discerning features of the device design. This region is designed to be sufficiently long such that the particles sputtered from the target do not stream back to the anode region and contaminate the cathode or anode surfaces. For example, if the sputtered target material is ionized in the anode region then it is likely to accelerate toward a cathode surface causing contamination and limiting the life of the plasma source. The ability of the device to provide long duration plasma exposure to target materials relies on the ionization of sputtered target particles downstream of the anode, where they are likely to redeposit on the target (a process similar to the recycling which occurs at tokamak divertor targets) or grounded chamber magnet surfaces. This effectively isolates the plasma source and target regions of the device. It is convenient to refer to electrons within the facility as belonging to one of two classes. So-called primary electrons are those which enter the anode region with an energy close to the discharge voltage, gained through the potential difference between the cathode and the anode. Primary electrons are free to stream along the para-axial magnetic field, and in the absence of significant collisionality to randomize their velocities, may be treated as a mono-energetic population with negligible temperature. The second class of electrons, referred to as plasma electrons, are those which are created when a primary electron suffers an inelastic collision. An ionizing collision, for example, results in a low-energy electron being liberated from the atom and in a large drop in the energy of the ionizing electron while randomizing the direction in which both electrons leave the collision, thus creating two plasma electrons, in essence. These plasma electrons are approximated here as belonging to a Maxwellian distribution: v C λ C + = n e σ C+ v e. (1) The distance a sputtered carbon atom can travel in a background of ionizing electrons can be estimated using (1), where σ C+ is the ionization cross section for carbon, v e and n e the ionizing electron speed and density, v C the speed of the sputtered carbon, and with... denoting an ensemble average. In order for the majority of sputtered carbon atoms to be ionized before reaching the anode region roughly 7 cm away, the average density of plasma electrons in the long intermediate region must be greater than m 3 (assuming an electron temperature of 7 ev), using the calculated electron impact ionization cross sections from Kim and Desclaux [17] which we have integrated over a Maxwellian distribution. The above estimate assumes the sputtered carbon is ejected with an average axial speed of 7.3 km s 1 based on LIF measurements of carbon sputtered by 1.5 kv argon ions [18, 19] and gives a conservative upper bound for the electron density required to ionize sputtered atoms. Other materials, such as molybdenum and tungsten, will require much lower electron densities for the same ionization mean free path. Ground state ionization rates from the ADAS database [2] for molybdenum and tungsten are much higher than those calculated by us for carbon; with rates listed as m 3 s 1 and m 3 s 1, respectively, compared with m 3 s 1 for carbon. The ADAS rate coefficient for ionization of ground state carbon is listed as m 3 s 1 for comparison. The speed of the ejected particles may also be substantially lower for these refractory metals, due in part to an order of magnitude increase in mass over carbon. The axial speed of molybdenum sputtered by argon ions is expected to be around 1.7 km s 1, based on the assumption that the most probable energy is roughly half the sublimation energy (6.85 ev/atom for Mo [21]), leading to an electron density of only m 3 needed to ensure ionization in under 7 cm. The reverse situation, of material sputtered from the cathode contaminating the target, must also be considered. In order for carbon sputtered from the cathode keeper, for example, to be ionized before streaming past the anode, the plasma electron density in this region ought to be at least triple that estimated above (i.e. > m 3 ), neglecting ionization due to primary electrons in order to find an upper bound for the threshold density. Double probe measurements show the plasma density at the downstream exit of the anode region approaches m 3 during operation of the discharge at 85 A and 43 V on 2 sccm of xenon. The electron density between the anode exit and cathode is expected to exceed this measurement and may certainly lead to sputtered carbon ionization lengths less than the anode length, however, the peak on-axis plasma potential must occur downstream of most carbon ionization events in order to ensure recycling to the keeper. The location of the maximum on-axis plasma potential is not known at this stage of development and will change with the parameters of operation, but the conservative nature of the above estimates (e.g. neglects slowing of the sputtered carbon by high neutral gas pressures and the off-normal peak in sputtered particle angular distribution [22]) allows us to reasonably expect that cross-contamination may be avoided at the proper operating conditions. The preceding analysis demonstrates how the effective separation of the source and test region can be obtained, with the dense plasma and long intermediate section minimizing the possibility of cross-contamination of sputterants from one region to the other. Similar estimates may be made of the important collision scale lengths for plasma species in the device in order to gauge the relative importance of specific processes to the physics of source operation. A list of these lengths, and corresponding collision frequencies, are given in table 1, where momentum exchange collisions are listed by participating species abbreviated i j, with e for plasma electrons, p for primary electrons, i for singly charged xenon 3

4 Table 1. Collisional frequencies and mean free paths at the exit of the plasma source for operation on xenon. Collision type Frequency (MHz) Mean free path (cm) e n [23] e i [24] Ionization, e [25] i i [24].12.7 i n [26] Ionization, p [25] Slowing [24] ions and n for neutral xenon atoms. Electron impact ionization collisions are presented for primary electrons, with an assumed energy of 3 ev, and plasma electrons (by integrating available cross sections over a Maxwellian distribution), with an assumed temperature of 7 ev; ionization collision mean free paths are given for the electrons rather than neutrals. The effective slowing of primary electrons by the background plasma electrons is also presented, even though this process has a mean free path greater than twice the length of the device. Sources for the collision cross sections used are given in the table. Plasma and neutral densities of and m 3 are assumed, with the prior based on measurement and the latter based on a 2 sccm flow of xenon traveling through a 6 cm diameter tube with an ensemble speed of 14ms 1. An ion temperature of 1 ev is assumed (though the actual temperature may be significantly lower) for the ion ion collisions presented in the table, while the ion neutral mean free path is calculated assuming the ions are streaming through the neutrals at the ion sound speed. The mean free paths listed in table 1 may be compared with the electron and ion Larmor radii in order to asses their degree of magnetization. The magnetic field strength near the anode is usually around 3 G, corresponding to an ion gyroradius of 1 cm (again using a presumed upper bound of 1 ev) and an electron gyroradius of.2 mm. This field strength corresponds to ion and electron Hall parameters, defined as the ratio of cyclotron to neutral collision frequency, of 1.7 and 47, respectively, indicating that electrons are highly magnetized, while the ions are in an intermediate magnetization regime where their motion is strongly affected by the magnetic field, though they are not well confined: ν e n T e/4.1, (2) 1+(T e /4) 1.6 ν e i = n e ln T 3/2 e, (3) ν i i = m 1/2 i n i ln T 3/2 i, (4) ν i n = 3.13e m 1/2 i n n α 1/2, 4ɛ (5) ν slowing = e4 ln ψn e, (6) 2πɛ 2 m2 e v3 p ( ) ne 1/2 ln = ln Te 3/2, (7) ψ = erf ( E p T e ) 2 E p πt e exp E p/t e. (8) The collision frequency equations used are reproduced in (2) (6). In these equations, n e is the density of Maxwellian electrons, ln is the Coulomb logarithm (given in SI in (7)), m i is the mass of the ionic species, T i the ion temperature, n n the neutral density, e is the elementary charge, α is the dipole polarizability of the neutral atom (taken as Fm 2 for xenon [27]), v p is the speed of the primaries, ɛ is the permittivity of free space, and ψ is a function of the primary electron energy, E p, and the Maxwellian electron temperature, T e. The function ψ is given in (8) where erf is the error function, which approaches unity at values of E p /T e greater than 1.2. Of the equations listed, only (2), which provides a fit to numerically integrated experimental cross sections, is specific to xenon. The form of (4) shown is based on the assumption, which we rely on throughout this article, that ions are singly charged. Validation of this assumption is left for future work, but we expect only minor corrections to arise from consideration of higher charges, due to the low electron energies involved. The electron electron collision frequency is identical to ν e i for singly charged ions. 3. Plasma source performance The discharge current voltage characteristics are presented for both xenon and argon in figure 2(a) at various flow splits between the cathode and the anode (through the ring injector). The discharge requires higher voltages during operation on argon, despite higher total volumetric flow rates (26 sccm Ar, compared with 2 sccm Xe), while a higher cathode-to-anode flow fraction is observed to lead to a decrease in discharge voltage at all discharge currents. A 3.8 by 5.1 cm coupon of Duocel vitreous carbon foam (1 PPI, 21 24% density) is used as a sample target material and biased 2 V below ground. The current collected by the target is shown in figure 2(b) as a function of the discharge current. The target current is observed to increase with discharge current in an approximately linear manner. Target currents obtained on argon are higher for all discharge currents when the majority of the flow is introduced through the anode ring injector rather than the cathode, even when normalized by the input power. Representative results showing the change in discharge parameters versus the anode mass flow rate are given in figure 3 at several values of the cathode flow rate for each gas species. In all cases an increase in anode flow allows operation at a lower discharge voltage for a given discharge current (95 A for the xenon data and 115 A for the argon settings shown). Effects from increasing the anode flow rate appear to diminish at high total flows. Ion currents at the target increase nearly linearly with anode flow rate according to the data of figure 3(b), with some saturation occurring a high total flow rates. The same general dependences of the discharge voltage on flow rate are observed when the anode flow rate is held constant and the cathode flow varied instead. This may be observed in 4

5 Figure 2. (a) Discharge voltage and (b) target current as a function of discharge current for different cathode and anode (in parenthesis) mass flow rates given in sccm. Figure 3. (a) Discharge voltage and (b) target current as a function of anode flow rate for different cathode mass flow rates given in sccm. figure 4(a), where again the discharge is able to operate at lower voltages when the neutral injection is supplemented. On the other hand, the effect of increased cathode flow rate on the target current, shown in figure 4(b), is dramatically different from that displayed in figure 3(b) for anode flow rate. Instead of increasing proportional to cathode flow rate, the target current tends to tail off at higher flows (with non-zero anode flow). In argon, for example, the target current reaches a maximum near 13 sccm and begins to decline as the cathode flow is increased further. The existence of target current maxima in figure 4(b) is expected to be due to the competing trends of increased plasma density with increased gas feed and the drop in energy available to electrons as the discharge voltage lowers affecting the ability of the plasma to bridge the neutral-dominated region between the anode and the target. Similar plots are given in figures 5(a) and (b), but with the total mass flow rate into the system held constant (at 26 sccm for argon, and 2 sccm for xenon) while the ratio of anodeto-cathode flow is altered. Figure 5(b) reveals the somewhat unexpected trend of dramatic gains in target current at high anode flow fractions. The target current obtained is limited by the ability of the discharge power supply to support the high voltages necessary at low cathode flow rates. Another practical limit to the useful anode flow fraction is the increased cathode wear expected when the cathode flow is low and ions born in the anode region impact cathode potential surfaces with higher energies. Higher anode voltages can also be expected to lead to higher fractions of multiply charged ions (the desirability of which depends on the experiment). It is clear figures 4(b) and 5(b) that the addition of an auxiliary neutral flow near the anode allows us to attain significantly higher target currents than can be delivered with cathode flow alone. It should also be noted that the steady increase in discharge voltage at high anode-tocathode flow fractions in figure 5(a) is expected to be driven by a dearth in neutrals in the cathode rather than an excess of neutrals near the anode, as figure 3(a) shows that an increase in anode flow alone helps to lower the discharge voltage. Constant magnet settings are used in the experiments described above for each gas species. The magnetic field is 3% stronger at the cathode when operating on argon rather than xenon, and 2% weaker in the anode region. Finiteelement simulations of the field near the target surface suggest it is roughly 2 G during operation on either gas. 5

6 Figure 4. (a) Discharge voltage and (b) target current as a function of cathode flow rate for different anode mass flow rates given in sccm. Figure 5. (a) Discharge voltage and (b) target current as a function of anode-to-cathode flow rate ratio for constant total mass flow rates. All discharge voltage measurements are made at the power supply and reduced to account for an average resistance in the cabling between the supply and the electrodes of 27 m. Changes in the cabling resistance with heating are not monitored and are estimated to lead to a discharge voltage uncertainty of 1.35 V, corresponding to a 5% variability in the resistance. Accuracy of the target current and target floating potential measurements are limited by the resolution of the power supply, resulting in uncertainties of ±5 ma and ±.5 V. 4. Plasma profiles A preliminary survey of the plasma has been undertaken using a planar Langmuir probe to measure plasma density, temperature and potential. The molybdenum probe is 4 mm in diameter with ceramic paste insulating the downstream side and an alumina probe holder. Measurements displayed here are performed with the probe 8 mm upstream of the target material surface and translated laterally and vertically by motor driven ball-screw stages. The target is biased to 2 V while a Kepco BOP 1-1M power supply is used to bias the planar probe to 1 V for measurements of the spatial distribution of ion saturation current in front of the target. A representative ion current distribution is shown in figure 6(a). Integrating the ion current density shown in figure 6(a) roughly yields a total of 1 A and about.54 A in the area projected to the target surface, while the actual current collected by the target is.26 A. The reason for this discrepancy is not yet known, but is expected to be due in large part to an increase in the probe collection area by the sheath [28], with the coarseness of the data collection mesh also contributing to uncertainty in integration. A finer spatial scan of the ion flux across the vertical center of the target is shown in figure 6(b) for the same discharge conditions. An ion current profile taken at the same discharge conditions used for the sputter yield measurements described below is also presented in figure 6(b). Standard deviations in ion current collected within a 1.27 cm radius of the sample center are below 11% for argon and below 7% for xenon. 5. Sputter yield measurement Accurate sputter yield measurement is a critical benchmark for any plasma surface interaction testbed and as such we have attempted to validate the Pi facility using materials with wellknown sputter rates. To this end, a flat molybdenum sample (initial mass and thickness of g and.254 mm) was inserted in the Pi target holder and its weight loss measured after a 1 h exposure to a previously characterized plasma. The target is covered by a Mo plate with a 2.54 cm diameter aperture, constricting the plasma exposure on the sample to a 6

7 Vertical Position, y (cm) Ion Flux, Γ i (m 2 s 1 ) x Ion Flux, Γ i (m 2 s 1 ) 2.5 x Xe Ar Lateral Position, x (cm) Lateral Position, x (cm) Figure 6. Ion flux measured 8 mm upstream of a 2 V biased target (a) across the target plane (b) along the vertical center (Y = ). The discharge is operated at 112 A and 73 V with a cathode flow of 15 sccm Ar and zero auxiliary anode flow. Operation on xenon at 1 A and 4 V with the cathode and the anode flows of 6 and 14 sccm is shown in (b) along with a smoothing spline used for sputter yield estimates. known area with limited edge effects. The discharge is run at 1 A with the cathode and the anode flow rates of 6 sccm and 14 sccm xenon, respectively. Time-averaged values of discharge voltage, target current and background pressure of 39.8 ±.4 V, 27 ± 11 ma, and (1. ±.26) 1 4 Torr (corrected for xenon) are obtained over the course of material sputtering. The target is biased 2 V below ground, while the plasma potential measured 8 mm upstream of the guard plate is 3 ± 2.5 V. Plasma potential is measured using the planar Langmuir probe described above and the relation, φ pl = T e (1/2 +ln[ m i /2πm e ]) + φ fl, with the stated uncertainty due mainly to the accuracy of T e, which is measured here as 5.8 ±.4 ev. This relation for plasma potential, φ pl, based on electron temperature, T e (in ev), floating potential, V fl, and the ion-to-electron mass ratio, m i /m e, assumes a twocomponent plasma (i.e. no high-energy primaries or lowenergy secondaries) exists near the probe surface. Integrating the current collected from a prior run at the same discharge conditions over the 2.54 cm diameter exposed area leads to a particle current estimate of 1.11(+.34/.21)1 18 ions s 1 impinging on the sample, with the uncertainty evaluated by integrating the current including one standard deviation of the measurement (2 samples are obtained at 2 khz for each position). The result of a 6 min plasma exposure was a loss of 81.8 ±.5 mg, corresponding to a sputter yield (with no correction for redeposition of sputtered atoms) of.132(+.47/.28) atoms/ion. As noted in the previous section, sputtered molybdenum atoms are readily ionized in the plasma near the target and may by guided back by the plasma potential distribution to redeposit on the target. Mass loss measurements are then only an indicator of the net sputter yield, which is the difference between the actual sputter rate and redeposition rate on the target. Redeposition is accounted for by Doerner et al using geometric arguments along with spectroscopic measurements of the sputtered molybdenum energy and ionization mean free path [29]. Their calculations yield a correction factor of 1.7, multiplied by the measured net yield to find the gross sputter yield, at conditions very similar to the present experiment (target radius, electron temperature and electron density are close to those found here). This correction factor assumes that the sputtered molybdenum is ejected from the surface with a cosine distribution while (as noted by Doerner) the actual sputter yield is expected to be primarily at angles near 45 off normal for the conditions here of low-energy ions (2 ev) with normal incidence [22]. Here we present a similar analysis of redeposition rates that extends to include the angular and energy distributions of sputtered particles. We frame our analysis on the assumption that (almost) all sputtered particles that are ionized somewhere within the projected area of the target surface are entrained by the electric field to be redeposited on the target. Taking a spherical coordinate system fixed at some radial location on the target surface, r, we can find the maximum velocity angle from surface normal, θ max, for a sputtered particle of speed v to travel a distance λ and still be within the projected target area using (9) for a given azimuthal angle ψ (on the target surface plane) sin θ max (r,v,ψ)= r cos(ψ) + r 2 cos 2 (ψ) (r 2 Rt 2 ). λ(r, v) (9) Here we are taking the target radius, R t, as the maximum radial distance from axis a sputtered particle can be ionized and still redeposit on the target, however, an ion-confining radial electric field often exists in cylindrical plasmas which may effectively funnel particles ionized at higher radii back to the target area. This effect is disregarded for now, but is easily handled by replacing R t in (9) with some larger R eff. The characteristic distance a sputtered particle travels before being ionized is calculated assuming a Maxwellian distribution of plasma electrons (with density n e and temperature T e ) using (1), where the cross section for electron impact ionization of the sputtered particle σ +,isa 7

8 function of relative velocity of the electron and sputtered atom. Equation (1) may equivalently be expressed as λ = v/(n e (r)r + (v)). Integrating over the theoretical total ionization cross-sections of ground state and metastable molybdenum from Kwon et al we find the atomic velocitydependent ionization rate is well fit by R + = v v (m 3 s 1 ) when T e = 5eV[3]. λ(r, v) v = ( ) 3/2 me n e (r) exp ( m ec 2 ). c v σ + ( c v ) d 3 c 2πT e 2T e (1) We assume that the distribution of sputtered particle velocities can be split into angular and energy-dependent functions as f(v) = f (v)h(θ) so the fraction of the axial flux of sputtered particles from a given location that redeposit on the target can be calculated by (11) below, where the radial dependence enters through θ max. An upper bound on the integration over v is introduced to account for the ability of fast sputtered particles to continue a roughly ballistic trajectory despite the plasma electric field and avoid entrainment back to the target. Here we set v max equivalent to 2 ev, which is roughly half the discharge voltage (note that for this small range of energies the ionization rate is essentially independent of atom velocity): vmax f (v)v 3 dv 2π θmax h(θ) cos(θ) sin(θ)dθdψ Ɣ(r) = f (v)v 3 dv 2π π/2 h(θ) cos(θ) sin(θ) dθ dψ. (11) In the present experiment, the surface area being sputtered that can contribute to redeposition on the target area includes both the target itself and the molybdenum guard plate. The guard plate is a square with 5.8 cm sides, but is considered as a disk of radius R g = 5.8/ π cm to simplify analysis. The gross sputter yield, γ, can then be estimated from the measured net sputter yield, γ net, using the radial distribution of incident ion flux, Ɣ i (r): Rt γ net Ɣ i (r)rdr γ = Rt Ɣ i (r)r dr R g (12) Ɣ i (r)ɣ(r)r dr Equation (12) is easily solved numerically with knowledge of the sputtered particle velocity distribution. Linear cascade theory of particle sputtering leads to ejected particles with an energy distribution function, attributed to Thompson [31], of g T (E, θ) E cos θ (E + E b ), (13) 3 where E b is the surface binding energy which is taken as 6.82 ev for molybdenum [32]. The energy distribution function can be split as before to g(e, θ) = g (E)h(θ) and converted to velocity space by the relation g (E) de = 2πv 2 f (v) dv, yielding f v(v 2 + vb 2) 3. Use of the Thompson distribution results in (11) reducing to Ɣ(r) = 8v b 3π 2 vmax 2π v 4 (v 2 + v 2 b )3 (1 cos3 (θ max )) dψ dv. (14) The measured incident ion flux profile is fit to a radial distribution function of the form Ɣ i = Ɣ i J (2.448r/R w ), where R w is the radius at which the flux goes to zero (taken as 5.8 cm here to match the solenoid inner radius) and J is the zero order Bessel function of the first kind, while maintaining the net ion current to the target found by integrating the measured profile (Ɣ i = m 2 s 1 ). The electron temperature is assumed constant across the target (at 5 ev) to yield an electron density profile of the same shape, n e = 2Ɣ i J (2.448r/R w )/ T e /m i. The electron density and ion flux are inserted in (1) and (12), resulting in a gross sputter yield estimate of.145 atoms/ion, a roughly 1% increase over the net yield. The low redeposition fraction estimated by this method is a result of the high energy, most probably, of (13) corresponding to a long mean free path for ionization (3.6 cm on axis) and thus a small solid angle subtending the target area. The energy distribution function of Thompson is known to be inaccurate at low incident ion energies (i.e. sub-kev), where narrower distributions peaked at lower energies are often found [33, 34]. Stepanova and Dew suggest an alternative energy distribution at low incident energy based on theoretical considerations, shown in (15) below, where m t is the target atom mass, a 2 m t /(4m i ), E max is the maximum energy for a sputtered atom and E i is the incident ion energy [34]. The maximum sputtered particle energy is (E i /E th )E b, where E th is the threshold incident energy for sputtering to occur, taken as 46.8 ev for Xe + on Mo [32] g S (E, θ) E (E + E b ) 3 exp ( 1 E + E b E max + E b [ ( mi (E cos a ) ] θ + E b ) m t E i ). (15) To continue the simple analysis we neglect the angular dependence of the energy distribution in (15) as a second order effect and instead maintain h(θ) = cos θ. The energy distribution of Stepanova and Dew then leads to a gross sputter yield estimate of.34 atoms per ion (2.3γ net ). The assumption of a cosine distribution of sputtered particles also breaks down at low incident energies. To capture this effect we use a rough model of the angular sputter yield of molybdenum as h(θ) = cos θ(1 + 2 sin θ), which has the desired properties of a peak yield 45 off normal and a local minimum at [22]. This more realistic angular distribution leads to a gross sputter yield estimate of.264 atoms/ion (2γ net ). A comparison of this value and several gross sputter yields found in the literature is made in table 2, where the average value of tabulated yields is.24 ±.7 atoms/ion, showing close agreement with our adjusted measurement. The analysis presented allows a simple evaluation of the degree to which redeposition affects sputter yield measurements obtained through mass loss. An accurate redeposition evaluation, however, requires an accurate energy distribution function for sputtered particles, for which we have relied on theory-based curve fits, and an accurate evaluation of the ionization frequency (n e R + ). For example, a 25% increase in the electron density over the value assumed here results in an estimate of nearly double the sputter yield. This analysis neglects collisions of the sputtered particles with neutral atoms 8

9 Plasma Sources Sci. Technol. 23 (214) 2514 Table 2. Gross sputter yield (atoms/ion) for Xe+ on Mo at 2 ev from various sources, with signifying a model output. Present Rosenberg [35] Yamamura* [32] Doerner [29] Kolasinski [36] Tartz* [37] (a) (b) (c) (d) Figure 7. SEM images of Mo surface post-exposure showing (a) the interface of the exposed (lower left) and guarded (upper right) regions of the sample at 68 and (b) 456 magnification. (c) Exposed portion of sample at 929 and (d) guarded portion of sample at 346 magnification. and that exposed to the plasma, such as those of figures 7(a) and (b), show a lighter, more textured surface where the sample was guarded from ion bombardment. Energy dispersive spectroscopy (EDS) analysis of the sample in the exposed region indicates a surface composition identical (within.6% by weight) to that of a pristine molybdenum sample similarly analyzed confirming a lack of significant contamination of the target by the plasma source. There is not a discernable lanthanum peak in the energy spectrum of the exposed sample obtained by EDS. (neutral xenon densities are lower at the target than the source where the mean free path is long), magnetic field effects (Mo+ gyroradius is Rt ), and takes the ionization mean free path based on ne (r) at the surface rather than a more complicated, angle-dependent, path length integral value of ne. We also neglect the incomplete ionization of sputtered Mo flux in one ionization mean free path, but note that though we erroneously count the 36% of particles that are not ionized within λ as redeposited we simultaneously discount the particles that are ionized in a distance <λ that would exit the projected target area after traveling λ. The molybdenum sample was imaged with a scanning electron microscope (SEM) after the sputter yield experiment, the results of which are shown in figure 7. SEM images of the interface between the guarded portion of the sample 6. Conclusion A facility has been designed, constructed and operated that is intended to provide a stable plasma to a downstream 9

10 target, while avoiding coupling between the plasma material interactions at the target and the operation of the upstream plasma source. A simplified plasma environment local to the test article is desired for investigation, both experimentally and numerically, of the response of micro-architectured materials to propulsion-relevant plasmas. The design of the plasma source has been discussed and contextualized by scale length arguments in section 2.1. The configuration described here has provided desirable operation, but is as yet unoptimized. The use of four independent solenoids and two flow injection paths allows substantial flexibility during plasma source operation, though component dimensions and spacings are based on best estimates and a desire for diagnostic accessibility during this developmental stage. Further testing, coupled with numerical modeling, will be necessary to reach a more finalized design, but the measurements described herein have shown that the initial operation of the source is near the desired levels. A modeling effort is currently underway to aid in optimization of the flow injection locations and distribution, magnetic field profile, and device dimensions. Ion currents of up to.5 A were achieved on a 2 cm 2, negatively biased graphite target plate, at a discharge power near 7.5 kw. Such high target currents were enabled by the addition of a auxiliary flow injector placed between the cathode and the anode, providing a source of cold neutrals directly to the main ionization region. Ion fluxes of over 1 21 m 2 s 1 on argon were measured at the target, where spatial Langmuir probe scans reveal an ion beam size of roughly 2.5 cm diameter. A sputter yield measurement was performed on an asreceived sample of molybdenum that matched results in the literature well. The accuracy of sputter yields obtained in the Pi facility through mass loss may be augmented in the future by spectroscopic measurements that could aid in estimating the rate of material redeposition on sample surfaces. Some discrepancy exists in the ion currents measured through spatial scans with a Langmuir probe and those collected by the biased target, the resolution of which should also increase the accuracy of sputter yield estimates. Several important plasma parameters remain to be captured before testing on micro-architectured materials can begin in earnest. The ion energy distribution, and ion species fractions need to be identified at the intended discharge conditions in order to accurately assess sputtering rates and important mechanisms. Spatial profiles of the plasma potential and electron energy distribution will be necessary to properly account for interactions in the near-surface plasma. The acquisition of such data is the focus of ongoing work, which also includes an investigation of the plasma fluctuation characteristics. Acknowledgments This work is funded by the US Air Force Office of Scientific Research under grant nos FA and FA and the UCLA School of Engineering and Applied Sciences. The authors would like to thank Professor Nasr Ghoniem for support of this effort and David Rivera for providing the SEM images and EDS analysis. Appendix. Magnetic field Figure A1. (a) Maxwell simulation of magnetic field utilized for operation on argon with close-ups of (b) the target region and (c) the anode region. References [1] Sharafat S, Aoyama A, Williams B and Ghoniem N 213 Development of micro-engineered textured tungsten surfaces for high heat flux applications J. Nucl. Mater. 442 S32 8 [2] Hirooka Y et al 199 A new plasma surface interactions research facility: PISCES-B and first materials erosion experiments on bulk-boronized graphite J. Vac. Sci. Technol. A [3] VanEckHJNet al 27 Pre-design of Magnum-PSI: a new plasma wall interaction experiment Fusion Eng. Des [4] Uckan T 1987 Plasmamaterials interactions test facility Rev. Sci. Instrum [5] Wright G M, Whyte D G and Lipschultz B 29 Measurement of hydrogenic retention and release in molybdenum with the DIONISOS experiment J. Nucl. Mater [6] Naujoks D, Fussman G and Meyer H 1998 I(U)-characteristics of the plasma generator PSI-1: experiment and theory Contrib. Plasma Phys [7] Bottin B, Paris S, Van Der Haegen V and Carbonaro M 1999 Experimental and computational determination of the VKI plasmatron operating envelope 3th Plasmadynamics and Lasers Conf. (Norfolk, VA, 1999) number 367 [8] Owens W P, Uhl J, Dougherty M, Lutz A, Meyers J and Flecther D G 1999 Development of a 3 kw inductively coupled plasma torch facility for aerospace material testing 3th Plasmadynamics and Lasers Conf. (Norfolk, VA) 1

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