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Production of large diameter microwave plasma using an annular slot antenna Takayuki Ikushima, Yoshihiro Okuno, and Hiroharu Fujita Citation: Applied Physics Letters 64, 25 (1994); doi: 10.1063/1.110905 View online: http://dx.doi.org/10.1063/1.110905 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/64/1?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Characteristics of large-diameter plasma using a radial-line slot antenna J. Vac. Sci. Technol. A 24, 1421 (2006); 10.1116/1.2167983 Wave Propagation and Plasma Production in Planar Microwave Discharges Using a Slot Antenna of Rotating Mode AIP Conf. Proc. 669, 48 (2003); 10.1063/1.1593862 Production of a large diameter electron cyclotron resonance plasma using a multislot antenna for plasma application Rev. Sci. Instrum. 66, 5423 (1995); 10.1063/1.1146063 Large volume electron cyclotron resonance plasma generation by use of the slotted antenna microwave source J. Vac. Sci. Technol. A 13, 875 (1995); 10.1116/1.579845 Long plasma generation using microwave slot antennas on a rectangular waveguide Rev. Sci. Instrum. 65, 669 (1994); 10.1063/1.1145136 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 131.230.73.202 On: Fri, 19 Dec 2014 05:17:41

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Production of large diameter microwave plasma using an annular slot antennaTakayuki Ikushima, Yoshihiro Okuno, and Hiroharu Fujita Citation: Applied Physics Letters 64, 25 (1994); doi: 10.1063/1.110905 View online: http://dx.doi.org/10.1063/1.110905 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/64/1?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Characteristics of large-diameter plasma using a radial-line slot antenna J. Vac. Sci. Technol. A 24, 1421 (2006); 10.1116/1.2167983 Wave Propagation and Plasma Production in Planar Microwave Discharges Using a Slot Antenna ofRotating Mode AIP Conf. Proc. 669, 48 (2003); 10.1063/1.1593862 Production of a large diameter electron cyclotron resonance plasma using a multislot antenna for plasmaapplication Rev. Sci. Instrum. 66, 5423 (1995); 10.1063/1.1146063 Large volume electron cyclotron resonance plasma generation by use of the slotted antenna microwavesource J. Vac. Sci. Technol. A 13, 875 (1995); 10.1116/1.579845 Long plasma generation using microwave slot antennas on a rectangular waveguide Rev. Sci. Instrum. 65, 669 (1994); 10.1063/1.1145136

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

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Production of large diameter microwave plasma using an annular slot antenna

Takayuki Ikushima, Yoshihiro Okuno, and Hiroharu Fujita Department of Electrical Engineering, Saga University Honjo-machi 1, Saga 840, Japan

(Received 14 September 1993; accepted for publication 19 October 1993)

A new production method for a large diameter microwave plasma is proposed without magnetic coils. An annular sl6t antenna and two ring-typed permanent magnets are used for the generation of high density plasma in the circumference of a chamber with the plasma confinement and diffusing to the central region. The optimum arrangement of components in the device is examined for the production of the large diameter uniform plasma. The almost uniform electron density of about 4X 10” cms3 is realized, and the plasma with two electron temperatures is observed. The measure- ment with a directional ion energy analyzer reveals that ions are almost isotropic.

One of the current research topics in plasma processing techniques, especially in dry etching processes, is the devel- opment of plasma sources providing large diameter, high density, and uniform plasmas. Recently, various techniques have been proposed for the production of such plasmas using microwave powers. I-” The attractive feature of microwave discharges is that high density plasmas can be obtained at low pressures of 0.1-a few mTorr.ly2 In almost all devices proposed, magnetic fields are utilized under the various con- figurations. These devices are roughly divided into two cat- egories; those using magnetic~coils and those using perma- nent magnets. As is well known, electron cyclotron resonance (ECR) plasmas using magnet coils have been widely used.“s3 The devices advanced for the generation of large diameter plasmas have also been developed using mag- neti coils; e.g., with a Lisitano coil as a microwave launcher,4 or with a circular TEaI mode microwave converted from a TE,, rectangular mode,5 etc.

Advantages of using permanent magnets, on the other hand, are that the magnetic field can be applied in the local region, where plasmas can be efficiently generated if the electron cyclotron resonance (ECR) condition is satisfied, and that an almost magnetic free condition can be achieved on the substrates. Further, no electric power is required, and the space occupied by the devices would be saved. The ap- proaches to the production of large diameter uniform plas- mas using permanent magnets, therefore, are now intensively taken. Various sophisticated techniques have been proposed; e.g., with the multicusp magnetic fields,“-” or with the com- bination between the surface magnetic field and microwave electric field launched by slot antennas,‘l etc.

In the present work, a new production method using per- manent magnets is proposed as one possibility for a large diameter high density uniform microwave plasma. The mi- crowave power is launched into the circumference of a chamber by an annular slot antenna and the magnetic field of ring-typed permanent magnets confines the plasma and dif- fuses it to the central region of the chamber. Both the anten- nas and permanent magnets are arranged outside the chamber to minimize the contamination inside it. The apparatus of the plasma source proposed here is introduced, first of all, with the description about the optimum arrangement of the an- tenna and permanent magnets. The plasma properties such as

the density and temperature of electrons and the uniformity are investigated with probe measurements. Further, the ion behavior in this device is revealed by the measurement using a directional ion energy analyzer with the angular resolution of 10-2o,Q13 and the comparison with that in the conven- tional ECR microwave plasmas13’14 is briefly given in order to clarify the feature of the device proposed here.

The schematic diagram of the plasma source proposed is shown in Fig. 1. The microwave power (2.45 GHz, 400 W) is transferred to the annular slot antenna through a coaxial wave guide, and the microwave is launched from the antenna into the circumference of the cylindrical vacuum chamber (134 mm in inner-diameter and 97 mm in length) through a fused silica plate window (12 mm in thickness). Argon gas is used as a working gas at the pressure of 2 mTorr. The width of the annular slot is 10 mm (the inner and outer diameters are 110 and 130 mm, respectively). Two ring-typed ferrite permanent magnets are located just outside the chamber with facing same polarities at a separation of 4 mm, as shown in the figure. The inner and outer diameters of the magnets are 144 and 220 mm, respectively, and the thickness is 20 mm. The magnetic field provided by this arrangement confines the

fused silica plate

\

coaxial

I

waveguide

FIG. 1. Large diameter microwave plasma production device using annular slot antenna.

Appl. Phys. Lett. 64 (I), 3 January 1994 0003-6951194/64(1)/25/3/$6.00 8 1994 American institute of Physics 25 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

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-6-4 -2 0 2 4 6 r (cm)

FIG. 2. Profiles of ion saturation current I, in the radial direction I at various positions z from the window.

plasma generated in the circumference of the chamber and diffuses it toward the central region. The present configura- tion of the microwave launcher and the magnetic field could allow a generation of high density ECR plasmas. Since the permanent magnets used, however, provided the magnetic flux density below 875 G for 2.45 GHz inside the chamber, the ECR plasma was not realized. Nevertheless, the high density plasma is obtained in the present system, as will be mentioned later. The arrangements described above such as the width of annular slot antenna, and the number, and posi- tion of the magnets are based on the results from the prelimi- nary experiments where the plasma density and its unifor- mity were examined for various antenna widths and for various numbers of magnets at the different locations with respect to the silica window. The high density uniform plasma was confirmed to be satisfactorily generated for the above arrangement, as will be described below.

Figure 2 shows the profiles of ion saturation current li of a probe in the radial direction r at various positions z from the window (see Fig. 1) Here, the radial profiles were ob- tained by rotating the probe. Ii is a current collected by a highly negatively biased cylindrical probe (0.15 m m in di- ameter and 2.0 m m in length), which is almost proportional to the plasma density.15 It is seen that in the region of z<4 cm the density in the circumference is higher than that in the central region. This concave density profile near the window can be predicted from the configuration of this device men- tioned above. The density in the center increases while the density in the circumference decreases with the distance Z. At z--S cm, where the magnetic flux density abruptly de- creases, the density profile becomes almost flat, and at z>5 cm the profile becomes convex and the density decreases. The uniformity in Ii at z-5 cm is about 23.4% within the diameter of 8 cm.

Probe measurements revealed that electrons were com- posed of two temperature groups. Thus, the semilog plots of the probe characteristics were clearly represented by two straight lines with different slopes. Figure 3 shows the radial profiles of the density and temperature of electrons at z=5 cm. Here, n, and neh denote the densities of low (T,,J and

I ---JO -2 0 2 4 6

r (cm)

FIG. 3. Radial profiles of the density and temperature of electrons at z=5 cm. net and neh denote the densities of low (Td and high (Teh) electron temperature groups, respectively.

high (Teh) electron temperature groups, respectively, which were obtained according to the procedure described in Ref. 16. As shown in this figure, the density n,, of the low elec- tron temperature (T,,=3-4 eV) group is about 4X101’ cm~ -3, while neh of the high temperature (T,h=8-10 eV) is 0.6-l x 101” cmm3. The uniformity in n, which is the den- sity of major component, is about 2 12% within the diameter of 8 cm. The discrepancy with the uniformity in Ii mentioned above is ascribed to the variation of electron temperature in the radial direction. Thus, in the strict sense, li is propor- tional to the ion density and to the square root of the electron temperature.15 In addition to this fact, two electron groups with different temperatures exist, as described above. In a practical sense for the application to etching processes, the uniformity in Ii would be more meaningful rather than that of the electron density.

The behavior of the two electron groups with different temperatures, however, should clarify the discharge charac- teristics in the present device. The densities and temperatures of these electron groups were measured at various distances z from the window on the center axis. The results revealed that the major component in the vicinity of the window was the group with the high temperature. The density neh de- creased with z and became negligibly small compared with that of the low temperature group II, at z>6 cm, where the magnetic flux density was very small. The density of the group with low temperature, iz,, on the contrary, increased abruptly from z=2 to 4 cm, and at 2>4 cm the density of about 4X101’ cmW3 was maintained. This fact indicates that high energy electrons are generated near the microwave source and they contribute to ionizations. In addition to this fact, a magnetic filtering effect17 could not allow the high energy electrons to flow into the downstream or central re- gion.

The isotropy of ions in the energy space has been exam- ined using a directional ion energy analyzer which was con- structed with a capillary plate as a first grid (channel length of 0.4 mm, hole diameter of 10 pm, and a pitch of 12 pm), a second grid (100 mesh/in.*, tungsten), and a collector.12”3

26 Appl. Phys. Lett., Vol. 64, No. 1, 3 January 1994 Ikushima, Okuno, and Fujita This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

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Q ion q

directional analyzer

FIG. 4. Ion directional energy E with respect to the ground, and the energy spread A.E. Insets show schematic diagrams of a directional ion energy analyzer for the definition of 6, and the first derivative of the analyzer characteristics for E and BE.

The analyzer was rotated about an axis perpendicular to the center axis at z=5 cm where an almost uniform plasma was obtained as mentioned above. The collector current cl,)-second grid voltage (V,) characteristics and the first de- rivative (dI,/dV,) were obtained at various angles 8 with respect to the center axis. Figure 4 shows the grid voltage E giving a peak of the first derivative and the energy spread AE. Here, E corresponds to the typical drift or directional ion energy with respect to the ground, and AE is defined as the half-width at the l/e (e: the base of natural logarithms) in the first derivative, possibly being proportional to the ion temperature. The insets show schematic diagrams of the di- rectional analyzer for the definition of 6, and of the first derivative for E and AE. It can be seen in this figure that the ion energy distribution is roughly isotropic. Thus, the relative discrepancies in E and in AE, which indicate the anisotro- pies in the ion flow and in the ion temperature, respectively, seem to be small. For 19=180”, the values might be influ- enced by the shadow of the analyzer. In a conventional ECR plasma device with a divergent magnetic field, an ion beam flows from the source to the downstream region owing to the

potential drop along the divergent field lines.‘32’4 In the present device, on the contrary, considerable ion beam was not observed. This would be due to the fact that the magnetic flux density provided by permanent magnets abruptly de- creases with the distance from them and an almost magnetic free condition can be achieved in the central region. In fact, the plasma potential was maintained almost constant at z<lO cm on the center axis.

As mentioned above, the device proposed here would be useful for large diameter high density uniform plasma. A production of uniform plasma with much larger diameter un- der stronger magnetic field will be tried in the near future.

The authors would like to thank H. Sato for technical assistance.

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Appl. Phys. Lett., Vol. 64, No. 1, 3 January 1994 Ikushima, Okuno, and Fujita 27 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

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