72 IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 27, NO. 1, FEBRUARY 1999

Heating Modes in Capacitively Coupled RFPlasmas Observed with Emission Spectroscopy

C. M. O. Mahony and W. G. Graham

Abstract—It is demonstrated that spatio-temporally resolvedemission studies of a capacitively coupled gaseous electronicsconference reference cell discharge can be used to determinechanges in the heating mechanisms in such discharges.

Index Terms— Optical spectroscopy, plasma measurement,power measurement, RF discharges.

I T is common to produce plasmas with electrode voltagedriven at 13.56 MHz i.e., with a period of 74 ns. Fast

imaging techniques, with a few nanoseconds resolution cantherefore be used to study the temporal dependence of plasmaemission through the RF cycle. These images elucidate thebehavior of electrons with sufficient energy to excite the gasand, particularly when combined with space resolved mea-surements, provide an insight into dominant electron heatingmechanisms.

There have been previous spatially and temporally resolvedmeasurements of emission in RF plasmas [1 and referencestherein], the present measurements were made in a gaseouselectronics conference (GEC) reference cell [2] with a fast-gated 2 ns) intensified charge coupled device (ICCD). TheGEC reference cell consists of two 102 mm diameter elec-trodes separated by 25.4 mm. The upper, aluminum electrodeand the stainless steel vacuum vessel were grounded. Theplasma was energized by a 13.56 MHz power supply attachedto the lower, stainless steel electrode. Our Ar and Himagesspan a pressure and power matrix from 7–133 Pa and 3–75 W,respectively. We quote rms plasma power and time varyingplasma power calculated from the driven electrode currentand voltage determined using standard GEC reference celltechniques [2].

Time integrated and time resolved data were obtained byimaging the central plasma onto the fast-gated ICCD witha variable focal length f 4.5 telephoto lens positioned 1.1m from the plasma center and 250 mm from the ICCD. Atest image was placed at the electrode centre and the lensfocus adjusted to give the sharpest ICCD image, this resultedin a spatial resolution of 1 mm. The depth of field of theimage represented 18 mm and vignetting of the image by theelectrodes was a negligible effect in the present measurements.Abel inversion of plasma images showed emission to peak atthe central plasma axis.

For time resolved data a rectangular region including bothelectrodes and extending15 mm about the central axis was

Manuscript received July 7, 1998; revised November 16, 1998.The authors are with the Department of Pure and Applied Physics, The

Queen’s University of Belfast, Belfast, BT7 1NN, Northern Ireland (e-mail:c.mahony@qub.ac.uk; b.graham@qub.ac.uk).

Publisher Item Identifier S 0093-3813(99)02674-0.

imaged. Filters (750 20 nm for Ar and 656 11 nm forH ) positioned between the plasma and ICCD were used toselect Ar 4s-4p and H3p-2s emission lines from these shortlifetime states (21.5 and 15 ns, respectively).

The ICCD gate was triggered by the output of a delayedpulse generator which was synchronized to a reference timingsignal from the RF matching unit. Software automaticallyincremented the pulse delay by 2 ns to record data for theeach gating step, and the process repeated for two RF cycles.Thus features in our images repeat every 74 ns. The time andspace resolved images were accumulated within 120 s. Thetotal accuracy of the imaging system with respect to the drivenelectrode voltage phase was determined to be2 ns. Themeasured emission signal was deconvoluted with the radiativelifetime of the excited state to derive excitation images.

In Ar usually only a single spatial emission maximum isobserved above the driven electrode, this is shown in thesimple time averaged emission image for 10 Pa, 35 W inFig. 1(a). Time resolved images show that bulk excitationpeaks [1] occur at a specific RF phase. In Fig. 1(b) and (c) (13Pa, 5 W ), typical of our low pressure, low power data peakis at 33 ns. It is difficult as yet to distinguish the contributionsto this peak resulting from the low pressure collisionlessheating mechanism, described as either a stochastic [3] ora pressure heating process [4], and the collisional, Ohmicheating process. At increased pressure and power (Fig. 1(d),133 Pa, 35 W) the time-resolved excitation again shows bulkexcitation peaks at 40 ns and an additional feature at 65 nsindicating gamma mode heating. This is caused by ion-inducedsecondary electron emission from the driven electrode. Thechange in the time at which the bulk excitation occurs isconsistent with changes in the time varying plasma power [5].

There have been no specific studies of heating mechanismsin RF H plasmas. However in Fig. 1(e) (33 Pa, 35 W), thereis clear evidence in Hof the bulk (25 ns) and possibly gamma(71 ns) excitation observed in Ar, but in addition there is near-electrode excitation (13 ns). We have previously [1] shown thatthis excitation is associated with sheath reversal and occurswhen electrons are accelerated toward the driven electrodeduring a short negative excursion of the sheath potential.Fig. 1(f) shows the time variation of plasma power for thesame conditions. The correlation with gamma regime timing isas yet inconclusive but there is clearly good phase correlationbetween the observed excitation and power input for the bulkand near electrode excitation.

REFERENCES

[1] C. M. O. Mahony, R. Al Wazzan, and W. G. Graham, “Sheath dynamicsobserved in a 13.56 MHz-driven plasma,”Appl. Phys. Lett.vol. 71, no.

0093–3813/99$10.00 1999 IEEE

IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 27, NO. 1, FEBRUARY 1999 73

(a) (b)

(c) (d)

(e) (f)

Fig. 1. (a) Argon 750.4 nm: plasma image, 10 Pa (75 mtorr), 8 W. (b) Argon 750.4 mn: time resolved excitation, 13 Pa (100 mtorr), 5 W. (c) Argon 750.4mn: time resolved excitation, 13 Pa (100 mtorr), 5 W. (d) Argon 750.4 mn: time resolved excitation, 133 Pa (1 torr), 35 W. (e) Hydrogen 656.3 nm: timeresolved excitation, 33 Pa (250 mtorr), 35 W. (f) Hydrogen: time resolved plasma power, 33 Pa (250 mtorr), 35 W.

5, pp. 608–610, 1997.[2] P. J. Hargis Jr.et al., “The GEC RF reference reactor: A parallel-plate

radio frequency system to study plasma processing discharges,”Rev.Sci. Instrum., vol. 65, no. 1, pp. 140–154, 1994.

[3] V. A. Godyak and R. B. Piejak, “Abnormally low electron energy andheating-mode transition in a low-pressure argon RF discharge at 13.56

MHz,” Phys. Rev. Lett., vol. 65, no. 8, pp. 996–999, 1990.[4] M. M Turner, “Pressure heating of electrons in capacitively-coupled RF

discharges,”Phys. Rev. Lett., vol. 75, no. 7, pp. 1312–1315, 1995.[5] C. M. O. Mahony and W. G. Graham, “Correlation of time resolved

optical and electrical measurements in capacitive RF discharges,” un-published.