Surface and Coatings Technology 174–175(2003) 33–39

0257-8972/03/$ - see front matter� 2003 Elsevier Science B.V. All rights reserved.doi:10.1016/S0257-8972(03)00523-1

Development of internal-antenna-driven large-area RF plasma sourcesusing multiple low-inductance antenna units

Y. Setsuhara *, T. Shoji , A. Ebe , S. Baba , N. Yamamoto , K. Takahashi , K. Ono , S. Miyakea, b c c d a a d

Department of Aeronautics and Astronautics, Graduate School of Engineering, Kyoto University,a

Yoshida-Honmachi, Sakyo-ku, Kyoto 606-8501, JapanDepartment of Energy Engineering and Science, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japanb

Preventure Program, Japan Science and Technology Corporation (JST), 2-1 Yamada-oka, Suita, Osaka 565-0871, Japanc

Joining and Welding Research Institute, Osaka University, 11-1 Mihogaoka, Ibaraki, Osaka 567, Japand

Abstract

Large-area and high-density radio frequency(RF) plasmas at 13.56 MHz have been produced by inductive coupling of internal-type low-inductance antenna units. The present study has been carried out to develop the basic discharge techniques which canbe applied to production of meter-scale large-area andyor large-volume plasma sources with high density for a variety of plasmaprocesses. The plasma source could be operated stably to attain plasma density as high as 1=10 cm at argon pressures of12 y3

approximately 1 Pa. It has been demonstrated that high plasma density can be obtained efficiently using the low-inductanceinternal antenna configuration with effectively suppressed electrostatic coupling. Discharge experiments in a meter-scale chamberdemonstrated uniform plasma production with densities as high as 6=10 cm at an argon pressure of 1.3 Pa and a RF power11 y3

of 4 kW.� 2003 Elsevier Science B.V. All rights reserved.

Keywords: Large-area plasma; Inductively-coupled RF plasmas; Internal-antenna configuration; Plasma-source ion implantation

1. Introduction

Trends in fabrications of microelectronics devices andflat panel displays toward substrate enlargement andhigh throughput require development of large-area plas-ma sources with high plasma density. Furthermore,development of large-volume plasma sources with highplasma density is desired for a variety of advancedsurface modifications including plasma-source ionimplantation(PSII) for conformal modification of indus-trial workpieces with complicated shape and large size.The plasma sources developed so far for production

of high-density andyor large-volume plasmas included.c. discharge plasmas using arrays of filamentsw1x,capacitively-coupled radio frequency(RF) plasmasw2x,electron cyclotron resonance plasmasw3–6x, heliconwave sourcesw7–10x, surface wave plasmas(SWP) w11xand inductively-coupled RF plasmas(ICP) w12x. How-ever, these plasma sources are constrained either by

*Corresponding author. Tel.:q81-75-753-5983; fax:q81-75-753-5980.

E-mail address: setsuhara@kuaero.kyoto-u.ac.jp(Y. Setsuhara).

rapid degradation of the filaments and the electrodes orby the dielectric window which is required to be thickenough to maintain mechanical strength against atmos-pheric pressure for scaling up of the sources. Here oneof the major problems associated with the dielectricwindow in SWPs and conventional ICPs for externalsupply of discharge power is degradation of the sourceefficiency, where the field strength for sustaining thedischarge tends to be reduced by the thick dielectricwindow.From this viewpoint, an internal-antenna configuration

to produce ICP has attracted great interests as a prom-ising candidate as an efficient high-density source, aswell as an alternative to avoid problems associated withdielectric windows in conventional sourcesw13x. Theinternal-antenna-driven ICPs were also studied as oneof the methods for enhancement of the planner magne-tron discharge to increase sputtered metal ionizationw14–16x.In the internal antenna configurations, however, it is

essential to minimize the electrostatic coupling of RFantenna voltages to the plasma, which results in anom-

34 Y. Setsuhara et al. / Surface and Coatings Technology 174 –175 (2003) 33–39

Fig. 1. Schematic illustration of LIA unit mounted on a vacuumflange.

alous rise of plasma potential. Furthermore, consideringthe antenna designs for the ICP production to satisfythe requirement for enlargement of plasma sourcestoward a meter scale, increase of an antenna inductanceand hence increase of an RF-voltage amplitude arisingat the antenna terminals cannot be avoided with increas-ing source size, when the source employs a large loop-shaped antenna. These constraints associated withlarge-area andyor large-volume ICP sources may besolved by employing the antenna configurations whichallow low-voltage operation of the ICPs.In our previous studiesw17–20x, we demonstrated the

feasibility of obtaining high-density plasmas with sup-pressed electrostatic coupling by the low-voltage oper-ation of the internal antenna, which was approached by(i) the employment of antenna configurations withreduced inductance, which is roughly proportional tothe square of turn numbers and the area size of the loop,(ii) minimization of the RF-voltage amplitude by theantenna termination with a blocking capacitor and(iii )dielectric isolation of the antenna conductor from plas-ma. In these works, we studied internal-antenna-drivenICPs for development of large-area plasma sources byusing the double-half-loop antenna configuration(non-loop shaped geometry) for lowering the total inductanceof the antenna system instead of using the conventionalloop-shaped configuration. Even with the double-half-loop antenna configuration, however, increase of anantenna inductance cannot be avoided with increasingsource size.As a novel technology to achieve low-voltage opera-

tion of ICPs suitable for producing large-area andyorlarge-volume source development, we propose an inter-nal-antenna configuration with ‘multiple low-inductanceantenna(LIA ) units’. Our proposal of the source con-figuration is based on the principle to use multiple ICPunits with LIA, which allows the low-voltage operationto suppress electrostatic coupling while maintainingprimary inductive coupling.Properties of argon plasmas sustained in 300 mm

source are reported to exhibit capability of high-densityplasmas. Furthermore, uniform high-density plasma pro-ductions are demonstrated in a meter-scale chamber.

2. Experimental

The ICP sources studied in the present study employLIA units, schematically illustrated in Fig. 1. The LIAunit consists of a U-shaped antenna conductor, which isseparately mounted on a vacuum flange. The antennaconductor is made of copper, which is fully coveredwith ceramics material for the dielectric isolation fromthe plasma.Experimental setup for feasibility test of ICPs sus-

tained with a set of the LIA units is schematically shownin Fig. 2. Four LIA units with a 55 mm width and a

100 mm height were mounted on the top flange of thedischarge chamber and were coupled to a 3 kW RFpower generator at 13.56 MHz via a matching network.Each LIA unit was connected in parallel to the matchingnetwork. The discharge chamber had a 300 mm innerdiameter and a 155 mm height, which was connected toa diffusion chamber made of stainless-steel vessel witha 310 mm inner diameter and a 280 mm height. On thedischarge chamber wall, uniformly-spaced alternating-polarity Sm–Co magnets were mounted both on the topflange and the side wall. This arrangement yielded line-cusp fields with azimuthally distributed 16 poles on thetop flange and radially distributed 18 poles on the sidewall.Examinations of large-area plasma production were

performed in a rectangular discharge chamber with aninner area size of 910=780 mm and an effective height2

(from top flange surface to bottom plane) of 370 mm,as schematically shown in Fig. 3. Ten LIA units with a150 mm width and a 150 mm height were mounted onthe four sides of the rectangular chamber at an axialdistance from the top flange of 125 mm; two LIA unitson each of the shorter sides(780-mm sides) and threeLIA units on each of the longer sides(910-mm sides).Plasma productions were performed using four RFpower generators, which were connected in parallel to

35Y. Setsuhara et al. / Surface and Coatings Technology 174 –175 (2003) 33–39

Fig. 2. Schematic diagram of experimental apparatus for RF plasma production in 300-mm diameter discharge chamber.

the LIA units mounted on each side of the chamber(Fig. 3b).For both the experiments,z axis is taken along the

axis of the discharge system as illustrated in Figs. 2 and3, so that thezs0 position lies in the inner surface ofthe top flange and the positivez values are taken in top-to-bottom direction. Plasma parameters were measuredusing a cylindrical Langmuir probe. For the experimentwith the 300-mm chamber(Fig. 2), the probe tip waslocated at an axial position ofzs230 mm at the centerof the radial position. For the experiment with the meter-scale rectangular chamber(Fig. 3), the probe tips wereinserted both axially and radially from the top flangeand the side flange, respectively. The axial probe waslocated at a position ofzs220 mm(95 mm below theantenna plane) at the center of the radial position. Thebase pressure of the discharge systems evacuated with aturbomolecular pump was 2=10 Pa and the plasmasy3

were produced in argon at pressures in the range 0.67–1.3 Pa.Peak-to-peak RF current applied to the antenna(I )rf

was also measured to evaluate a power transfer efficien-cy (h), which is defined as the ratio of the net RFpower absorption in the plasma(P ) to the input RFnet

power from the power generator(P ) w21x; i.e. hsrf

P yP . The power transfer efficiency,h, is expressednet rf

as hsr y(r qr ), where r and r are the equivalentp p c p c

plasma resistance and the circuit loss, respectively, and(r qr ) is the series loading resistance. The powerp c

transfer efficiencyh can be measured by measuring theRF currentsw22x.

3. Results and discussion

3.1. Plasma properties in 300-mm chamber

Power dependence of the plasma density sustained in300-mm chamber(Fig. 2) is shown in Fig. 4 for argonplasmas sustained at a pressure of 1.1 Pa. The plasmaswere stably sustained and the plasma density increasesalmost linearly with increasing RF power, where noanomalous arcing was observed even at the input RFpower as high as 3 kW. Dense argon plasma as high as1=10 cm was obtained at RF powers of)2.5 kW.12 y3

These results demonstrate that high-density plasmas ofthe order of 10 cm can be obtained even with an12 y3

inductive near field locally induced along the internalantenna with a sufficiently reduced inductance. Further-more, demonstration of the high-density plasma produc-tion using the multiple LIA units proposed in the present

36 Y. Setsuhara et al. / Surface and Coatings Technology 174 –175 (2003) 33–39

Fig. 3. Schematic diagram of experimental apparatus for RF plasmaproduction in meter-scale rectangular discharge chamber(inner size910=780 mm); (a) side view,(b) top view.2

Fig. 4. Variation of plasma density as a function of input RF powerfor argon plasmas sustained in the 300-mm chamber at a pressure of1.1 Pa.

Fig. 5. Power transfer efficiency as a function of input RF power forargon plasmas sustained in the 300-mm chamber at a pressure of 1.1Pa.

study is significant for providing wide range of flexibil-ity in designing the large-scale sources with high plasmadensity, since the LIA unit can be installed almost atany location of the discharge chamber wall, which isnot limited by the dielectric window.In order to examine the efficiency of the plasma

source, the antenna RF current was measured to evaluatethe power transfer efficiency(h). Fig. 5 shows thevariation of the power transfer efficiency for argonplasmas as a function of the input RF power. Highlyefficient plasma production is performed; i.e. the effi-ciency as high as 90% is achieved in wide range of theinput RF power. It is quite remarkable that even at lowinput power-100 W the plasmas can be sustained withits efficiency as high as 80%.

The RF voltages measured at the high-voltage end ofthe antenna conductor are plotted in Fig. 6 as a functionof the input RF power. The peak-to-peak voltage of theterminal RF voltage is measured to be less than 600 V,which is markedly smaller than those obtained with thehemispherical multi-turn antenna(;8 kV) w23x and thedouble half-loop antenna(;2 kV) w17x.The reduction in the antenna terminal voltage is

expected to exhibit an effect on suppression of thepotential in the plasma, since the antenna voltage is thesource term. Here the electrostatic voltage to the plasma

37Y. Setsuhara et al. / Surface and Coatings Technology 174 –175 (2003) 33–39

Fig. 6. Variation of peak-to-peak antenna RF voltage as a function ofinput RF power for argon plasmas sustained in the 300-mm chamberat a pressure of 1.1 Pa.

Fig. 7. Floating potential(d.c. component and oscillation component)for argon plasmas sustained in the 300-mm chamber at a pressure of1.1 Pa.

Fig. 8. Variation of plasma density as a function of input RF powerfor argon plasmas sustained in the meter-scale chamber at pressuresof 0.67 and 1.3 Pa.

is considered to be applied as a portion of the antennavoltage used up in the sheath region after subtractingthe potential drop through the insulator surrounding theantenna conductor. The floating potential measured forargon plasmas is compared in Fig. 7 as a measure forevaluating the electrostatic coupling. Both DC(tempo-rally averaged) and oscillation (fluctuating at 13.56MHz) components of the floating potential are signifi-cantly small. These results demonstrate that the plasmageneration regime using LIA units allows low-voltageoperation of ICP, which is especially significant forCVD process to obtain high-quality films with markedlyreduced plasma damage.

3.2. Plasma properties in meter-scale chamber

Power dependence of the plasma density sustained inthe meter-scale rectangular chamber(Fig. 3) is shownin Fig. 8 for argon plasmas sustained at pressures of0.67 and 1.3 Pa. Here the horizontal axis denotes thetotal RF power supplied from all of the RF powergenerators. The plasmas were stably sustained and theplasma density increases almost linearly with increasingRF power, similarly to the results obtained in the 300-mm chamber. Dense argon plasma as high as 6=1011

cm was obtained at the total RF powers of 4 kW.y3

Considering that the ratio of the plasma volumes in themeter-scale chamber to the 300-mm one is approximate-ly 6–8 times, significantly higher density is obtained inthe meter-scale chamber for the equivalent RF powers,which is possibly due to less ratio of wall loss in thelarger chamber than in the small chamber. Here theplasma volumes were estimated from the Langmuir

probe measurements performed in axial and radial direc-tions. The electron temperatures did not vary consider-ably in the range of total RF power of 1–4 kW(2.3–2.8 eV for 0.67 Pa and 2.1–2.2 eV for 1.3 Pa).Fig. 9 shows the plasma potential measured as a

function of the total RF power. With increasing RFpower, the plasma potential is found to decrease. Recal-ling that the electrostatic voltage to the plasma isconsidered to be applied as a portion of the antennavoltage used up in the sheath region after subtractingthe potential drop through the insulator surrounding the

38 Y. Setsuhara et al. / Surface and Coatings Technology 174 –175 (2003) 33–39

Fig. 9. Variation of plasma potential as a function of input RF powerfor argon plasmas sustained in the meter-scale chamber at pressuresof 0.67 and 1.3 Pa.

Fig. 10. Variation of potential drop from the plasma potential to thefloating potential as a function of input RF power for argon plasmassustained in the meter-scale chamber at pressures of 0.67 and 1.3 Pa.

antenna conductor, the decrease in the plasma potentialwith increasing RF power could be dominated by thelowering of the impedance in the sheath region due tothe increase in the plasma density as shown in Fig. 8.For applications to the film formations by plasma

CVD on the non-conductive substrates such as glass,the energy of ion bombardment during film growth canbe evaluated by the potential drop from the plasmapotential to the floating potential, which is shown inFig. 10. As expected from the fact that the electrontemperature was measured to be kept at low values(2.3–2.8 eV for 0.67 Pa and 2.1–2.2 eV for 1.3 Pa)and did not vary significantly in the range of total RFpower of 1–4 kW, the potential drops are found to bemaintained at considerably small values and their vari-ation with increasing RF power is insignificant. Thisfeature of the low-voltage potential formation is consid-ered to be very significant for CVD preparation of high-quality films with considerably reduced plasma damage,such as in the case of polycrystalline silicon filmformation. Furthermore, together with the feature ofplasma density which increases almost linearly withincreasing RF power, these results suggest that higherdeposition rate can also be feasible without sufferingsignificant degradation of the film quality due to iondamage.Variation of the radial density distributions with axial

distance from the antenna plane are shown in Fig. 11for the argon plasmas sustained at a pressure of 1.3 Paat a total RF power of 4 kW. For obtaining a uniformdensity profile in a down-stream region, it is essentiallyimportant to achieve hollow type distribution at the axialposition near the antenna, since the side walls of the

chamber are considered to contribute only to the plasmaloss, which tends the radial density distribution to acenter-peaked profile. Though the source configurationis not optimized yet, uniformity of the radial distributionbecomes better with increasing distance from the anten-na plane to the down-stream region. In the presentexperimental setup, fairly uniform radial distribution canbe obtained at a distance of 180–200 mm below theantenna plane.

4. Summary

We performed investigations on the internal-antenna-driven ICP production using the newly developed LIAunits. The plasma source could be operated stably evenat RF input powers of up to 3 kW to attain densitiesapproaching as high as 1=10 cm at argon pressures12 y3

approximately 1 Pa with simultaneous achievement ofthe suppression of the electrostatic coupling as well asthe power transfer efficiency as high as 90%. Plasmaproductions in the meter-scale rectangular chamberresulted in achievements of density as high as 6=1011

cm with low-voltage potential formation. The exper-y3

iments have exhibited that this type of ICPs driven bythe internal LIA units is quite attractive as a dense low-potential plasma source for a variety of plasma processesfrom CVD, etching and ashing applications used infabrications of microelectronics devices and flat paneldisplays to advanced PVD processes including PSII.Another advantage of the internal-antenna-driven ICP isexpected in its flexibility of the source design.

39Y. Setsuhara et al. / Surface and Coatings Technology 174 –175 (2003) 33–39

Fig. 11. Radial distribution of the plasma density at various axial distance from the antenna plane for argon plasmas sustained in the meter-scalechamber at a pressure of 1.3 Pa at a total RF power of 4 kW.

Acknowledgments

The authors from Kyoto University would like tothank Japan Society for the Promotion of Science(JSPS)for partially supporting the investigations under theauspices of Grant-in-Aid for Scientific Research.

References

w1x J.N. Matossian, J. Vac. Sci. Technol. B 12(1994) 850.w2x B.P. Wood, I. Henins, R.J. Gribble, et al., J. Vac. Sci. Technol.

B 12 (1994) 870.w3x K. Suzuki, S. Okudaira, N. Sakudo, I. Kanomata, Jpn. J. Appl.

Phys. 16(1977) 1979.w4x S. Matsuo, Y. Adachi, Jpn. J. Appl. Phys. 21(1982) L4.w5x M. Misina, Y. Setsuhara, S. Miyake, J. Vac. Sci. Technol. A

15 (1997) 1922.w6x M. Misina, Y. Setsuhara, S. Miyake, Jpn. J. Appl. Phys. 36

(1997) 3629.w7x R. Boswell, Plasma Phys. Contr. Fusion 26(1984) 1174.w8x J.Q. Zhang, Y. Setsuhara, S. Miyake, B. Kyoh, Jpn. J. Appl.

Phys. 36(1997) 6894.w9x S. Miyake, Y. Setsuhara, K. Shibata, M. Kumagai, Y. Sakawa,

T. Shoji, Surf. Coat. Technol. 116–119(1999) 11.

w10x Y. Setsuhara, Y. Sakawa, T. Shoji, M. Kumagai, S. Miyake,Surf. Coat. Technol. 142–144(2001) 874.

w11x M. Moisan, C. Baudry, P. Leprince, IEEE Trans. Plasma Sci.PS-3(1975) 1004.

w12x Hopwood, Plasma Sources Sci. Technol. 1(1992) 109.w13x T. Shirakawa, H. Toyoda, H. Sugai, Jpn. J. Appl. Phys. 29

(1990) L1015.w14x S. Miyake, Y. Setsuhara, J.Q. Zhang, M. Kamai, B. Kyoh,

Surf. Coat. Technol. 97(1997) 768.w15x Y. Setsuhara, M. Kamai, S. Miyake, J. Musil, Jpn. J. Appl.

Phys. 36(1997) 4568.w16x M. Yamashita, Y. Setsuhara, S. Miyake, M. Kumagai, T. Shoji,

J. Musil, Jpn. J. Appl. Phys. 38(1999) 4291.w17x Y. Setsuhara, S. Miyake, Y. Sakawa, T. Shoji, Jpn. J. Appl.

Phys. 38(1999) 4263.w18x S. Miyake, Y. Setsuhara, Y. Sakawa, T. Shoji, Vacuum 59

(2000) 472.w19x S. Miyake, Y. Setsuhara, Y. Sakawa, T. Shoji, Surf. Coat.

Technol. 131(2000) 171.w20x Y. Setsuhara, S. Miyake, Y. Sakawa, T. Shoji, Surf. Coat.

Technol. 136(2001) 60.w21x T. Shoji, Y. Sakawa, S. Nakazawa, K. Kadota, T. Sato, Plasma

Sources Sci. Technol. 2(1993) 5.w22x J.Q. Zhang, Y. Setsuhara, T. Ariyasu, S. Miyake, J. Vac. Sci.

Technol. A 14(1996) 2163.w23x M. Tuszewski, J.T. Scheuer, R.A. Adler, Surf. Coat. Technol.

93 (1997) 203.