Diagnostics of ballistic electrons in a dc/rf hybrid capacitively coupled discharge

Xu, Lin; Chen, Lee; Funk, Merritt; Ranjan, Alok; Hummel, Mike; Bravenec, R.V.; Sundararajan, Radha; Economou, Demetre J.; Donnelly, Vincent M.

doi:10.1063/1.3062853

Cited by 46 publications

(66 citation statements)

References 18 publications

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“…Therefore, this kind of hybrid dc/RF plasma source has received increased attention recently in both industrial and academic fields. [11][12][13][14][15][16][17][18][19][20] When a negative dc voltage is applied to the electrode, the sheath structure is determined by the superposition of a dc sheath and an RF sheath. 12,13 Under the effect of the large potential drop in the dc/RF sheath, ions are attracted to bombard the dc electrode continuously, and abundant secondary electrons are thus emitted and accelerated toward the plasma.…”

Section: Introductionmentioning

confidence: 99%

Heating mode transition in a hybrid direct current/dual-frequency capacitively coupled CF4 discharge

Zhang

Wang

Bogaerts

2014

Journal of Applied Physics

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Computer simulations based on the particle-in-cell/Monte Carlo collision method are performed to study the plasma characteristics and especially the transition in electron heating mechanisms in a hybrid direct current (dc)/dual-frequency (DF) capacitively coupled CF4 discharge. When applying a superposed dc voltage, the plasma density first increases, then decreases, and finally increases again, which is in good agreement with experiments. This trend can be explained by the transition between the four main heating modes, i.e., DF coupling, dc and DF coupling, dc source dominant heating, and secondary electron dominant heating.

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Section: Introductionmentioning

confidence: 99%

Heating mode transition in a hybrid direct current/dual-frequency capacitively coupled CF4 discharge

Zhang

Wang

Bogaerts

2014

Journal of Applied Physics

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“…An example of the electron distributions measured in the experiment from Ref. [6] are included in Fig. 1.…”

Section: Rf-dc Discharge Configurationmentioning

confidence: 99%

“…The basic parameters were based on the experimental values of Ref. [6], namely p = 50 mTorr and = 2100 V. The EEPFs were simulated both with and without electron-induced secondary emission (SEE) from the powered and biased electrodes. The scatter plot in the top panel is for the case with SEE, and the dots situated away from the blue curve are due to elastic and inelastic reflections, accounted for in Vaughan's emission model (the envelope curve for the dots is − [ + ( )]).…”

Section: Effect Of Higher Harmonics On Evdf At the Rf Electrodementioning

confidence: 99%

“…Chung et al [5] showed how electron impact affects roughness of the line width in experiments where 193 nm photoresist films were exposed concurrently to ions, electrons, and UV flux. The RF-DC configuration where both the powered and DC-biased electrodes are co-planar with the substrate, with secondary electrons emitted from the electrode opposite the wafer, has also been studied extensively, both in experiments and by numerical means [6] [7] [8] [9] [10]. The simple physical picture is that the secondary electrons accelerated through the DC sheath form a beam (in the velocity space) which then interacts with the plasma and with the substrate positioned opposite to the biased electrode.…”

Section: Introductionmentioning

confidence: 99%

“…We use the experimental studies by Xu et al [6] and by Chen and Funk [10] as the basis for comparison with the numerical results. The paper begins with a description of the experimental configuration, and then the numerical model.…”

Section: Introductionmentioning

confidence: 99%

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Structure of the velocity distribution of sheath-accelerated secondary electrons in an asymmetric RF-dc discharge

Khrabrov

Kaganovich

Ventzek³

et al. 2015

Plasma Sources Sci. Technol.

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Low-pressure capacitively-coupled discharges with additional DC bias applied to a separate electrode are utilized in plasma-assisted etching for semiconductor device manufacturing. Measurements of the electron velocity distribution function (EVDF) of the flux impinging on the wafer, as well as in the plasma bulk, show a thermal population and additional peaks within a broad range of energies. That range extends from the thermal level up to the value for the "ballistic" peak corresponding to the bias potential. The non-thermal electron flux has been correlated to alleviating the electron shading effect and providing etch-resistance properties to masking photoresist layers. "Middle-energy peak electrons" at energies of several hundred eV may provide an additional sustaining mechanism for the discharge. These features in the electron velocity (or energy) distribution functions are possibly caused by secondary electrons emitted from the electrodes and interacting with two high-voltage sheaths: a stationary sheath at the DC electrode and an oscillating, self-biased sheath at the powered electrode. Since at those energies the mean free path for large-angle scattering (momentum relaxation length) is comparable to, or exceeds the size of the discharge gap, these "ballistic" electrons will not be fully scattered by the background gas as they traverse the inter-electrode space. We have performed test-particle simulations where the features in the EVDF of electrons impacting the RF electrode are fully resolved at all energies. An analytical model has been developed to predict existence of peaked and step-like structures in the EVDF. These features can be explained by analyzing the kinematics of electron trajectories in the discharge gap.Step-like structures in the EVDF near the powered electrode appear due to accumulation of trapped electrons during a part of the RF cycle and their subsequent release. Trapping occurs when the RF sheath voltage exceeds the applied bias, and is decreasing. The step structures are formed by secondary electrons originating from the DC-biased surface (which also produce a peak near the energy equal to the bias potential). Additional peaks, at lower energies, are formed by the electrons emitted from the RF electrode and eventually escaping to it. The latter electrons can be grouped according to the number of bounces between the sheaths during their residence time in the discharge. Each of such groups may give rise to an individual peak in the distribution. The trap-and-release theory developed in this paper provides a convincing explanation for the observations of the ballistic and "middle energy peak" electrons measured in experiments.

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Particle‐in‐cell simulation of electron and ion energy distributions in dc/rf hybrid capacitively‐coupled plasmas

2013

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A Particle‐in‐Cell simulation with Monte Carlo collisions was used to study electron and ion energy distributions (IEDs) in low‐pressure (2.67 Pa) direct‐current (dc)/radio‐frequency (rf) hybrid capacitively‐coupled Ar plasmas. One electrode (dc/rf electrode) of the parallel plate diode was powered by a 13.56 MHz source, and a negative dc bias voltage, whereas the opposite (substrate) electrode was grounded. Secondary electrons emitted from the dc/rf electrode accelerated in the adjacent sheath and entered the plasma, yielding a high‐energy tail of the electron energy distribution. For given dc bias voltage, the plasma density increased as the secondary electron emission yield due to ion bombardment increased. A fraction of the secondary electrons were energetic enough to overcome the sheath potential barrier on the substrate electrode and bombard the substrate. The electron angular distribution on the substrate electrode had a peak of directional electrons superimposed on a typical cosine distribution. The mean energy and angular spread of directional electrons could be controlled by varying the dc bias voltage. However, as the dc bias became more negative, the dc/rf sheath expanded at the expense of the bulk plasma, reducing the plasma density, in agreement with published data. The IED on the substrate electrode exhibited a dominant bimodal feature with multiple shoulder peaks due to ion‐neutral charge exchange collisions. The average ion energy decreased as the dc voltage became more negative, also in agreement with data. Pulsing the plasma power enhanced the tail of the electron energy distribution in the early activeglow (power ON), and yielded a distinct ballistic electron flux on the substrate with energy equal to the applied dc bias. © 2013 American Institute of Chemical Engineers AIChE J, 59: 3214–3222, 2013

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Diagnostics of ballistic electrons in a dc/rf hybrid capacitively coupled discharge

Cited by 46 publications

References 18 publications

Heating mode transition in a hybrid direct current/dual-frequency capacitively coupled CF4 discharge

Heating mode transition in a hybrid direct current/dual-frequency capacitively coupled CF4 discharge

Structure of the velocity distribution of sheath-accelerated secondary electrons in an asymmetric RF-dc discharge

Particle‐in‐cell simulation of electron and ion energy distributions in dc/rf hybrid capacitively‐coupled plasmas

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