Intense boundary emission destroys normal radio-frequency plasma sheath

Abstract

Current models of capacitively coupled plasma indicate that external bias is mainly consumed by oscillating sheathes which shield external field. We report first evidence that strong boundary emission destroys normal radio-frequency (RF) sheath and establishes a new RF plasma where external bias is consumed by bulk plasma instead of sheathes. This produces ion confinement and intense RF current in bulk plasma, combined with unique particle and energy balance. Proposed model offers new method for ion erosion mitigation, wave mode conversion and a reaction rate control technic in low pressure plasma processing.A capacitively coupled plasma (CCP) is naturally formed by applying radio-frequency (RF) voltage to electrodes. Understanding RF plasma properties is of fundamental interests and is essential in numerous applications 1-7 . Contemporary models of CCP discharge assume that the bulk plasma is well isolated from the boundaries by oscillating sheathes and applied bias mainly rests on sheathes instead of bulk plasma. Conduction current prevails displacement current in bulk plasma and field in bulk plasma is weak. But in this letter we will show that the bias can be primarily consumed by bulk plasma and electric field in plasma center needs not to be shielded by sheathes, due to intense boundary emission.Numerous studies have been done regarding boundary emission in CCP, but its influences are mostly assumed to be unessential 4,[8][9][10][11][12][13][14][15] . The steady flux of ion produces a surface emission flux = due to ion-induced secondary electron emission (SEE), with the emission coefficient. Under low pressure, secondary electrons (SEs) are lost before remarkable ionizations occur and their impact is less significant than that of hot electrons generated by stochastic heating 16 . Higher pressure incurs transition to γ mode where particle balance is modified by ionizations of SEs 10 . Yet the structure of plasma, i.e. bulk plasma connected by Bohm presheath and Child-law sheath, is supposed to be untouched and mean wall potential remains negative relative to plasma 17-21 .In this letter, we show that boundary emission can bring very strong disturbance and restructure entire RF plasma, which happens when is greater than averaged plasma electron flux 〈 〉 . It is well known that a space-charge limited (SCL) sheath is formed if > near floating boundary, such as thermionic emitter, emissive probe, Hall thruster, etc [22][23][24] . In this case, normal Child-law sheath still presents between plasma and the potential dip called virtual cathode (VC) near boundary, so dynamics of plasma are not essentially modified 25 . Recent works reported that a floating SCL sheath can become unstable due to cold ion trapping 26,27 . But no one has studied a RF plasma with intense boundary emission. Below we shall first show with simulation how RF plasma properties behave *

Vlasov simulation of the emissive plasma sheath with energy-dependent secondary emission coefficient and improved modeling for dielectric charging effects

“…The first term in RHS of Equation ( 17) is hence negative. The derivative 𝜕𝛾 𝑒 𝜕𝜑 𝑠ℎ is further expressed as follows combining Equations ( 10), (13): (18) Note that 𝜎 𝑤𝑎𝑙𝑙 < 0 in the present analyses. If a constant plasma electron temperature is assumed, which is true for the present simulation but may vary in practical discharges, the RHS of Equation ( 18) should be positive since 𝜕𝜎 𝑤𝑎𝑙𝑙 𝜕𝜑 𝑠ℎ < 0.…”

“…PIC model where self-consistent plasma-neutral collisions are implemented. A comparison of the kinetic simulation and PIC simulation was recently performed for the capacitively-coupled plasma subject to strong electron emission from the boundary (13,35), where discrepancies are more obvious at high neutral pressure levels as the kinetic model does not implement realistic electron-neutral collisions and ionization sources. Similar comparison between kinetic and PIC model of the SEE coefficient charging effect are expected for DC or RF plasma conditions.…”

“…In this section, employed simulation algorithms and typical simulation results are presented. The simulation model is inspired by previous Vlasov-Poisson solvers [11,15], and a basic version of the code was used in our recent simulations [13,20,21]. The employed 1D1V kinetic simulation code is based on the following kinetic equation:…”

“…More recent studies suggested that the SCL sheath cannot remain stable if cold ions are generated by charge exchange collisions in the sheath [11]. The emissive sheath is difficult to be directly accessed by conventional probe diagnostics due to its small size, so sheath theories are frequently validated against numerical simulations [12][13][14].…”

“…Simulation setup In this section, employed simulation algorithms and typical simulation results are presented. The simulation model is inspired by previous Vlasov-Poisson solvers (11,15), and a basic version of the code was used in our recent simulations (13,20,21). The employed 1D1V kinetic simulation code is based on the following kinetic equation: Here the subscript s represents species, 𝑚 𝑠 , 𝑞 𝑠 , 𝑣 𝑠 are mass, charge and velocity of the species, 𝐸 is the electric field and the RHS is the collision-source term.…”