Surface and Coatings Technology 116–119 (1999) 662–665www.elsevier.nl/locate/surfcoat

Radio-frequency plasma production using a ladder-shaped antenna

Y. Kawai a, M. Yoshioka a, T. Yamane a, Y. Takeuchi b,*, M. Murata ba Interdisciplinary Graduate School of Engineering Sciences, Kyushu University, Kasuga, Fukuoka 816-8580, Japanb Nagasaki Research & Development Center, Mitsubishi Heavy Industries Ltd., Fukahori, Nagasaki 851-0392, Japan

Abstract

A VHF plasma was produced with a ladder-shaped antenna whose driving frequency ranges up to 200 MHz and the plasmaparameters were measured with a movable Langmuir probe. A very uniform plasma was achieved over 300 mm×300 mm with adriving frequency of 80 MHz. When the driving frequency was increased, the plasma density increased, amounting to1.6×1010 cm−3 at 200 MHz near the substrate, and the electron temperature decreased. © 1999 Published by Elsevier ScienceS.A. All rights reserved.

Keywords: Ladder-shaped antenna; Langmuir probe; Plasma CVD; Plasma parameter; VHF plasma

1. Introduction the high-speed deposition of a-Si:H. In order to achievea higher deposition rate of amorphous silicon films, wehave performed the experiments on the production of aThere is great interest in plasma chemical vapor

deposition (CVD) for the fabrication of hydrogenated large-area VHF plasma using a ladder-shaped antenna.Here we report the experimental results on VHF plasmaamorphous silicon (a-Si:H) in the field of solar cells and

thin film transistors (TFTs). Now industry has required production using a ladder-shaped antenna with a drivingfrequency up to 200 MHz and on the plasma parameterhigher deposition rates while maintaining high quality.

Recently, some researchers [1,2] succeeded in achieving measurements with a movable Langmuir probe.a high deposition rate of a-Si:H by increasing thefrequency (VHF ) of the RF power source, comparedwith that of 13.56 MHz. In most of the experiments [1– 2. Experimental apparatus5], the conventional two parallel plates were used as areactor. When the frequency of the RF power source is A schematic diagram of the experimental apparatusincreased, a lot of electrical problems [5,6 ] begin to is shown in Fig. 1. The system consists of a vacuumappear, e.g. the reactor needs to be treated as a transmis- chamber, a ladder-shaped antenna and an RF powersion line [6 ]. In general, it is very hard to obtain source. The vacuum chamber, 600 mm wide, 600 mmmatching between a reactor and a power source in the high and 400 mm deep, was electrically grounded. TheVHF range, i.e. a standing wave is generated on the electrode consists of a stainless steel ladder-shapedsurface of the parallel plates and on the feed lines and, antenna, 422 mm long and 422 mm wide (17 rods ofas a result, deposited films become non-uniform. Thus, 6 mm in diameter, spacing of 20 mm), which was posi-the development of a VHF plasma source for large-area tioned within the vacuum chamber. The distanceprocessing is one of the important topics in plasma between the ladder-shaped antenna and the substrateapplication. Furthermore, the plasma parameters, which was 40 mm. The RF power source used consists of anare necessary for understanding quantitatively plasma oscillator and a power amplifier. The RF power wasCVD process, were not measured in detail in such applied to four loading points on the ladder-shapedexperiments [1–5]. antenna via a matching box and was varied between 30

We developed a novel CVD device using an electrode and 150 W in the frequency range 13.56–200 MHz. Theof a ladder-shaped antenna [7–10] and succeeded [8] in gas used was pure H2 at a pressure of 20–300 mTorr

and a flow rate of 50–200 sccm. The plasma parameters* Corresponding author. were measured with a movable Langmuir probe inserted

0257-8972/99/$ – see front matter © 1999 Published by Elsevier Science S.A. All rights reserved.PII: S0257-8972 ( 99 ) 00295-9

663Y. Kawai et al. / Surface and Coatings Technology 116–119 (1999) 662–665

Fig. 1. Schematic diagram of the experimental setup.

in front of the substrate (Corning 7059 glass). The RFvoltage and current of the ladder-shaped antenna weremonitored with a voltage probe and a current probe,the frequency range of which was up to 50 MHz [9].The phase difference was found to be almost zero [9],which is different from a conventional reactor with adriving frequency 13.56 MHz.

3. Experimental results and discussion

In order to examine whether the ladder-shapedFig. 2. The dependence of the electron density ne on the RF drivingantenna is useful for VHF plasma production or not,frequency for different RF powers, where the pressure and gas flowthe plasma parameters were measured as a function ofrate are 100 mTorr and 50 sccm respectively.the driving frequency for different RF powers with a

movable Langmuir probe. It is well known that in thepresence of RF fields the V–I curve of the Langmuirprobe is deformed and does not provide correct plasmaparameters. Here, a filter was used to obtain a correctV–I curve. Furthermore, the plasma density (i.e. electrondensity) was estimated from the ion saturation currentbecause of the lesser influence of the RF fields on theV–I curve. The experimental results are shown in Figs. 2and 3, where the pressure and gas flow rate were100 mTorr and 50 sccm respectively. Fig. 2 indicates thatwhen the RF driving frequency is increased, the electrondensity ne increases and reaches ne=1.6×1010 cm−3 at200 MHz, which is ten times higher than that at Fig. 3. The dependence of the electron temperature Te on the RF driv-13.56 MHz. As described in Section 2, the plasma ing frequency for different RF powers, where the pressure and gas flow

rate are 100 mTorr and 50 sccm respectively.parameters were measured near the substrate, so that

664 Y. Kawai et al. / Surface and Coatings Technology 116–119 (1999) 662–665

the electron density near the ladder-shaped antenna isconsidered to be higher than that given in Fig. 2. Thus,high-speed deposition of amorphous silicon films will beexpected by the VHF plasma produced with a ladder-shaped antenna. On the other hand, the electron temper-ature Te decreases above 80 MHz, independent of thepower, as shown in Fig. 3. The electron temperaturewas around Te#2 eV at 200 MHz. The decrease in theelectron temperature means a decrease in the plasmapotential, leading to a reduction of ion bombardmentand, as a result, high quality films will be deposited ona substrate, i.e. the VHF plasma is suitable for a high-speed deposition of amorphous silicon films with high Fig. 5. The dependence of the electron temperature Te on the gas pres-quality. When the RF power was increased, the electron sure for different driving frequencies, where the RF power and gasdensity increased while the electron temperature flow rate are 150 W and 50 sccm respectively.remained constant, as seen partly in Figs. 2 and 3.

We measured the plasma parameters as a function ofgas pressure for different RF driving frequencies. Thedependence of the electron density on the pressuredepended on the driving frequency. As shown in Fig. 4,the electron density is almost independent of the pressurebelow 60 MHz, but it begins to decrease above 80 MHzwith increasing the pressure. This result means that forhigher frequency plasma production it is necessary todecrease gas pressures. On the other hand, Fig. 5 showsthat the electron temperature is almost constant and asthe driving frequency is increased Te becomes low. Herethe RF power and gas flow rate were 150 W and 50 sccmrespectively.

Furthermore, plasma uniformity was examined by Fig. 6. Spatial distributions of the ion saturation current density Jis inthe horizontal direction for different driving frequencies, where themeasuring spatial profiles of the ion saturation currentpressure is 80 mTorr.of the Langmuir probe with a motor-driven device. The

horizontal distribution of the ion saturation currentdensity Jis is plotted in Fig. 6, where the power was kept further the plasma becomes non-uniform, although the

electron density increases and the electron temperatureat 100 W except for at 200 MHz. As described before,in this experiment the electron temperature was almost decreases. As seen in Fig. 6, nonetheless, the VHF

plasma of about 20% uniformity was produced withconstant, so that the ion saturation current is consideredto be proportional to the electron density. Fig. 6 shows 200 MHz. During this experiment a good matching was

obtained with the matching box, except at 200 MHzthat a very uniform plasma over 300 mm×300 mm wasachieved at 80 MHz. On increasing the driving frequency where a power of 30 W was reflected. Sansonnnens et al.

[5] pointed out that inhomogeneous thin film depositionis caused by non-uniform voltage distribution on thetwo parallel plates and that the voltage uniformity isimproved by changing the number and position of theRF connections. In this experiment, the results of thosestudies [5,6 ] will be necessary for the production of auniform 200 MHz plasma.

4. Summary

A VHF plasma was produced with a ladder-shapedantenna and the plasma parameters were measured witha movable Langmuir probe. When the driving frequencyFig. 4. The dependence of the electron density ne on the gas pressureof the RF power source was increased, the electronfor different driving frequencies, where the RF power and gas flow

rate are 150 W and 50 sccm respectively. density increased and reached 1.6×1010 cm−3 at

665Y. Kawai et al. / Surface and Coatings Technology 116–119 (1999) 662–665

[3] A.A. Howling, J.-L. Dorier, Ch. Hollenstein, U. Kroll, F. Finger,200 MHz, which is ten times higher than that obtainedJ. Vac. Sci. Technol. 10 (1992) 1080.at 13.56 MHz. The electron temperature decreased with

[4] W. Schwarzenbach, A.A. Howling, M. Fivaz, S. Brunner, Ch.increasing the driving frequency: Te#2 eV at 200 MHz. Hollenstein, J. Vac. Sci. Technol. 14 (1996) 132.A very uniform plasma was achieved over [5] L. Sansonnens, A. Pletzer, D. Magni, A.A. Howling, Ch. Hol-

lenstein, J.P.M. Schmitt, Plasma Sources Sci. Technol. 6 (1997)300 mm×300 mm with a driving frequency of 80 MHz.170.Thus, it is concluded that the ladder-shaped antenna is

[6 ] J. Kuske, U. Stephan, O. Steinke, S. Rohlecke, Mater. Res. Soc.useful for generation of a VHF plasma having theSymp. Proc. 377 (1995) 27.

advantage of high electron density. [7] M. Murata, Y. Takeuchi, E. Sasagawa, K. Hamamoto, Rev. Sci.Instrum. 67 (1996) 1542.

[8] M. Murata, Y. Takeuchi, S. Nishida, Vacuum 48 (1997) 34.[9] M. Murata, H. Mashima, M. Yoshioka, S. Nishida, S. Morita,References

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